TAPPI JOURNAL - August 2012

W W W. TA P P I . O R G

JOURNAL

THE PA P E R A N D PAC K AG I N G I N D U ST R I E S ’ TECHNICAL RESOURCE

AUGUST 2012 | VOL. 11 NO.8

LIME KILN

9

Increasing lime production while decreasing kiln pluggage through installation of the first LimeFlash lime kiln feed system in North America Peter W. Hart, Gary W. Colson, Michael H. Clapper, Brandon M. Pollet, and William K. Doughty

17 Biorefinery implementation for recovery debottlenecking at existing pulp mills – Part II: Techno-economic evaluation BIOREFINERY

Hakim Ghezzaz, Luc Pelletier, and Paul R. Stuart

BIOREFINERY

The sulfite mill as a sugarflexible future biorefinery

27

Lisa X. Lai and Renata Bura

PAPER PHYSICS

Fiber interaction with a forming fabric

39

Jingmei Li And Sheldon I. Green

CONTAMINANT ANALYSIS

Optical analysis of ink and other contaminants in process waters Antti Haapala, Mika Körkkö, Elisa Koivuranta, and Jouko Niinimäki

Progress in Paper Recycling

51

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T E C H N I CA L S E R V I C E S

August 2012 TABLE OF CONTENTS

WWW.TAPPI.ORG

VOL. 11 NO. 8

JOURNAL

Vice President, Operations Eric Fletty Editorial Director Monica Shaw +1 770 367-9534

5

EDITORIAL



Monica Shaw



Paul Wiegand: Environmental expert

[email protected] [email protected]

PRESS Operations Manager Lisa Stephens [email protected] +1 770 209-7313 | fax: +1 770 446-6947 Webmaster Karen Roman +1 770 209-7416

[email protected]

9

LIME KILN

TJ EDITORIAL BOARD

Increasing lime production while decreasing kiln pluggage through installation of the first LimeFlash lime kiln feed system in North America

James W. Atkins, Atkins, Inc., [email protected], +1 908 806-8689



Peter W. Hart, Gary W. Colson, Michael H. Clapper, Brandon M. Pollet, and William K. Doughty



17 BIOREFINERY Biorefinery implementation for recovery debottlenecking at existing pulp mills – Part II: Techno-economic evaluation



Hakim Ghezzaz, Luc Pelletier, and Paul R. Stuart

27 BIOREFINERY The sulfite mill as a sugar-flexible future biorefinery



Lisa X. Lai and Renata Bura

39 PAPER PHYSICS Fiber interaction with a forming fabric



Jingmei Li And Sheldon I. Green

Raimo J. Alén, University of Jyvaskyla, [email protected], +358-14-2602562

Terry L. Bliss, Ashland, Inc., Wilmington, DE [email protected], +1 302 995-3523 Brian N. Brogdon [email protected], +1 678 581-9114 Dave Carlson, Carlson Consulting [email protected], 1-847-323-2685 Jere W. Crouse, JWC Consulting, [email protected], +1 608 362-4485 Mahendra Doshi, Progress in Paper Recycling, [email protected], +1 920 832-9101 William S. Fuller, [email protected], +1 253 279-0250 Peter W. Hart, Westvaco Corp., [email protected], +1 409 276-3465 Carl J. Houtman, [email protected], + 1 608 231-9445 Norman Lifshutz, JEFF Journal, [email protected], +1 643 242-7304 Arthur Ragauskas, Georgia Institute of Technology [email protected], +1 404 894-9701 Scott Rosencrance, Kemira Chemicals [email protected], +1 770 429-2753 Nick G. Triantafillopoulos, OMNOVA Solutions Inc. [email protected], +1 330 794-6249 Paul Wiegand, NCASI [email protected], +1 919 941-6417

Featuring Exclusive Content from

Progress in Paper Recycling 51 CONTAMINANT ANALYSIS Optical analysis of ink and other contaminants in process waters



Antti Haapala, Mika Körkkö, Elisa Koivuranta, and Jouko Niinimäki

Junyong Zhu, USDA Forest Products Laboratory [email protected], +1 608 231-9520

Join TAPPI today! TAPPI JOURNAL is a free benefit of TAPPI membership, and is only available to members. To join TAPPI, to renew your TAPPI Membership, or to learn about other valuable benefits, visit www.tappi.org. TAPPI, 15 Technology Parkway S., Norcross, GA, 30092, publishes TAPPI JOURNAL monthly. ATTENTION PROSPECTIVE AUTHORS: All papers published are subject to TAPPI JOURNAL’s peer-review process. Not all papers accepted for review will be published. Before submitting, check complete author guidelines at http://www.tappi.org/s_tappi/doc.asp?CID=100&DID=552877. Statements of fact and opinions expressed are those of individual authors. TAPPI assumes no responsibility for such statements and opinions. TAPPI does not intend such statements and opinions or construe them as a solicitation of or suggestion for any agreed-upon course of conduct ­or concerted action of any sort.

»ON THE COVER: ANDRITZ LimeFlash technology for lime kilns is a single-level feed system for increasing capacity of conventional lime mud drying kilns and long kilns (p. 9). Image courtesy of ANDRITZ.

Copyright 2012 by TAPPI, all rights reserved. For copyright permission to photocopy pages from this publication for internal or personal use, contact Copyright Clearance Center, Inc. (CCC) via their website: www.copyright.com. If you have questions about the copyright permission request process, please contact CCC by phone at +1 978 750-8400. To obtain copyright permission to use excerpts from this publication in another published work, send your specific request in writing to Editor, TAPPI JOURNAL, 15 Technology Parkway S., Norcross, GA 30092, USA; or by fax to +1 770 446-6947. Send address changes to TAPPI, 15 Technology Parkway S., Norcross, GA 30092, USA; Telephone +1 770 446-1400, or FAX +1 770 446-6947. www.tappi.org

AUGUST MONTH 2012 2007 | TAPPI JOURNAL

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Constantly proving and improving — Our promise to the paper industry. At Ashland, we believe some of the best chemistry happens outside the laboratory. This occurs in paper mills all over the world when we engage our customers, talk about their needs and together conceive of the chemical innovations that will solve their problems. We also believe good chemistry happens inside the laboratory. In our strategically located customer applications laboratories, we combine our knowledge and understanding of papermaking to create the chemistries you require today and those you will need tomorrow. Listening to our customers is the foundation on which we build our partnerships. Together, there’s no end to the solutions we can devise. Because when you ask the right questions, great things are never far behind. See how good chemistry can work for you at ashland.com

® Registered trademark, Ashland or its subsidiaries, registered in various countries ™ Trademark, Ashland or its subsidiaries, registered in various countries * Trademark owned by a third party © 2011, Ashland AD-11366

Guest Editorial MONICA SHAW | EDITORIAL DIRECTOR [email protected]

Paul Wiegand: Environmental expert Editor’s Note: This month, we continue our series of columns profiling the background and expertise of TJ’s editorial board members. These volunteer members spend long hours overseeing the peer review process and contributing to TJ. TAPPI would like to express its gratitude to the editorial board members for their hard work and willingness to share their expertise with the forest products community. – Monica Shaw, Editorial Director

F

or over 20 years, TAPPI Journal (TJ) editorial board member Paul Wiegand has worked on solid waste and effluent quality studies at NCASI − the National Council for Air and Stream Improvement. This independent, non-profit research institute has focused on environmental topics of interest to the forest products industry since 1943 and now has more than 75 member companies throughout the U.S. and Canada. Recognized as a leading source of data on environmental issues affecting the industry, NCASI’s program areas include air quality, water quality, ecological assessment, chemical reporting, forestry and climate change. After earning a B.S. in Pulp and Paper Engineering from Western Michigan University in Kalamazoo, MI, Paul’s efforts in the environmental field started as a process engineer and environmental supervisor in 1984 at a southeastern integrated kraft pulp and paper mill. In 1990, he joined NCASI as a research engineer in the organization’s Northern Regional Center in Kalamazoo. From there, he moved to NCASI’s West Coast Regional Center located in Corvallis, OR, as regional manager. While in Oregon, he earned his M.S. in Civil and Environmental Engineering from Oregon State University. Paul, who is currently vice president of water quality, is one of several NCASI staff members who have lent their environmental expertise to TJ over the years by serving as an editorial board member. He is now responsible for the water and aquatic biology

Paul Wiegand and friend on the water at Idaho’s Salmon River. programs at NCASI, working from the organization’s corporate office in Research Park Triangle, NC. In this capacity, he coordinates programs related to wastewater treatment, water quality, effluent toxicity, total maximum daily loads for pollutants, the significance of mill effluent discharges to receiving waters, and sustainable management of water resources. In addition to publishing in scientific journals, Paul has contributed to more than 30 NCASI publications and reports. A member of TAPPI since 1990, he became a TJ editorial board member in 2007. The position, Paul says, keeps him in touch with the latest environmental research, particularly that occurring outside of North America, and also serves as a vehicle for building and maintaining professional relationships via the peer-review process. Since joining the editorial board, he notes a continuing shift in the research focus of the environmental field. “While the changes have been progressive beyond the years I have served on the editorial board, more conventional research associated with air, water, and solid waste management has diminished, while reAUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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search on topics related to environmental sustainability has increased considerably,” said Paul.

Industry challenges and prospects Paul joined TAPPI because he views it as the premier venue for non-competitive technical information exchange on pulp and paper topics. In addition to serving on the TJ editorial board, he has assisted in planning for the environmental track of the PEERS Conference for many years. From Paul’s perspective, maintaining the exchange of technical information through TAPPI and other means is both the industry’s biggest challenge and its biggest opportunity. “It is no secret that resources dedicated to process and environmental research have decreased in recent years, yet the need for continual improvement remains,” said Paul. “I think that the industry is finding ways to address new challenges, but many of those involved with innovative solutions are not always taking the time to share their non-competitive findings. “Creativity feeds off itself, and the more ideas, inves-

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“Creativity feeds off itself, and the more ideas, investigations, successes, and failures are shared, the more everyone can take advantage of opportunities to use and extend science-based solutions.” – PAUL WEIGAND

tigations, successes, and failures are shared, the more everyone can take advantage of opportunities to use and extend science-based solutions.” On a personal note, Paul enjoyed volunteering as a baseball, basketball and soccer coach as his three children grew up and recommends it as a highly rewarding way of influencing young people and building great memories. He has also appreciated the perspective of moving around the “big triangle” of Michigan, Oregon and North Carolina, where “climate, politics and geology make each place unique.” TJ

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TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

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PEER-REVIEWED

LIME KILN

Increasing lime production while decreasing kiln pluggage through installation of the first LimeFlash lime kiln feed system in North America PETER W. HART, GARY W. COLSON, MICHAEL H. CLAPPER, BRANDON M. POLLET, and WILLIAM K. DOUGHTY

ABSTRACT: The Evadale, TX, USA, mill recently engaged in a major lime kiln and recausticizing upgrade. The centerpiece of the project was to upgrade the existing lime kiln feed system to a new LimeFlash feed system, a trademarked technology from Andritz. The feed system in Evadale is the first installation in North America of this technology, and the second installation in the world. The feed system increased the capacity of the lime mud dryer (LMD) fed kiln from 350 tons/day of lime product to 480 tons/day of kiln product, with less than 3% carbonate. The system mixes hot flue gasses with the lime mud before the gas enters the feed end housing, which allows the kiln to operate at a higher feed end temperature without plugging and lime splitting. In startup and low capacity situations, lime mud is fed directly into the kiln rotating part, which eliminates potential plugging problems. Startup and operational experience, along with selected environmental performance, is reviewed. Application: The use of the new LimeFlash kiln feed system allows a mill to substantially increase the amount of lime production for a given volume of existing lime kiln. The new feed system can potentially support production increases or the elimination of older kilns without the need to purchase an entire new lime kiln and supporting equipment.

I

n kraft pulp mills, lime kilns are used to convert lime mud (CaCO3) to lime (CaO) for reuse in the causticizing plant. The process requires a large amount of heat (6-10 GJ/ton CaO production), which is supplied to the kiln mainly by burning fossil fuels such as fuel oil and natural gas. When lime kilns are routinely started and stopped to accommodate variable white liquor production rates, the heat demand on a global basis can change from around 6 to 7 GJ/ton of lime to 11 or 12 GJ/ton of lime produced. The Evadale, TX, USA, mill has maintained and operated three lime kilns. The two older kilns routinely operated with a combined production rate of 183 tons/day of lime. The No. 7 kiln average production rate was 350 tons/day. The No. 7 kiln was built in 1995 and utilized an early generation lime mud dryer (LMD) feed system. The two older kilns use traditional feed systems with chain sections. Recently, the mill shut down one paper machine and reduced the pulp demand from roughly 1900 tons/day to 1400 tons/day. Lime demand has significantly decreased to the point where the mill needs to run the No. 7 kiln full time and one of the other kilns only part time. As a result of routinely heating up the kiln, the global fuel demand for the part-time kiln operation has become costly and excessive. Additionally, the mill sought to reduce manpower by permanently shutting down the old kiln and associated liquor works. In an effort to reduce natural gas consumption and improve the overall operation of the mill, a study was under-

taken to increase the production rate of the No. 7 kiln to a level that would allow the mill to shut down both old kilns. In general, a production rate of about 460–475 tons/day of lime would be required. As the No. 7 kiln limit without plugging was roughly 350 tons/day, this required a 36% increase in lime production. Recent literature on upgrading kiln capacity has frequently dealt with less modern kilns [1], so the suggestions and solutions presented were not applicable to the Evadale No. 7 kiln. In an effort to determine the best method of upgrading the No. 7 kiln capacity, the different rate limiting factors for the existing kiln were examined. RATE LIMITING FACTORS FOR EXISTING KILN

Kiln volume Typically, lime kiln production is limited by the volume of the kiln inside the refractory. Higher production rates require a larger processing area inside the kiln. Older, inefficient kilns usually were oversized. Newer kilns utilize higher efficiency internal components and product coolers to reduce the kiln volume and improve the energy efficiency of the kiln. Generalized rules of thumb for kiln sizing suggest that conventional, chain section kilns without product coolers would require roughly 95 to 110 ft3/ton of lime produced [2]. Frequently, for new or retrofit kilns, the volumetric loading has been reduced to 70–80 ft3/ton of lime [3]. Kilns with modern flash driers may require only 50–60 ft3/ton of lime produced. Table I shows volumetric production factor conAUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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LIME KILN Production Factor m3/MTPD (ft3/STPD)

Relative Heat Rate at 75% Feed Solids MM Btu/MT (MM Btu/ST)

Relative Power Consumption at 75% Feed Solids kWh/MT (kWh/ST)

3.09 (100+)

7.7 (7.0)

74 (67+)

Existing long kiln retrofitted with modern internals

2.25 – 2.4 (73 - 78)

6.6 (6.0)

69 (63)

New long kiln with modern internals, product cooler, and electrostatic precipitator (ESP)

2.16 – 2.32 (70 - 75)

6.2* (5.6*)

50 (45)

Kiln with modern internals, product cooler, external suspension drying system, and ESP

1.7-1.85 (55-60)

6.06 (5.5)

55 (50)

System Type

Conventional long kiln

* Value corrected from original source with data supplied by Jack Leichliter of Andritz.

I. Kiln volumetric production factor [4]. MT = metric tons; ST = short tons; MTPD = MT/day; STPD = ST/day.

siderations for different types of conventional, retrofit, and modern kilns [4].

Process control instrumentation The use of a multivariable, multipredictive control system has been shown to improve kiln operation in mill scale settings [5]. Typical production rate increases on the order of 5%–6% have been reported with the implementation of a high quality advanced control system. Lime kiln specific fuel consumption has also been reported to decrease as a direct result of advanced lime kiln control. Unfortunately, the Evadale No. 7 kiln already had an advanced control system installed. A potential problem with the advanced control system in use at the mill was the measurement of residual oxygen in the exiting flue gas. The extremely dusty conditions in the sampling area periodically plugged the sampling probe and required blowing out to unplug the probe. During these times, the oxygen content value was estimated at its last measured value, instead of obtaining an actual measurement. As a result, while it might have been possible to improve the kiln operation, the improvement would be nowhere near the 35%-plus production values required for success of the project.

feed system pluggage associated with increased production rates resulted in a significant limitation to capacity improvement. It was not possible to simply increase the feeding rate of the LMD kiln, as increased throughput resulted in an increasing feed end temperature in the kiln. Typically, when an LMD fed kiln feed end temperature reaches about 700°C (1300°F), the LMD tends to plug, thus limiting production in the kiln. The Evadale No. 7 kiln also experienced carbon monoxide (CO) burning in the feed ducts. The CO resulted from incomplete combustion of the fuel due to low oxygen levels in the kiln. Air leakage into the LMD feed system resulted in monitored oxygen levels that were higher than the actual level of oxygen in the kiln at the feed end, causing CO combustion in the LMD housing. CO burning in the LMD helped contribute to pluggage problems in the LMD feed system. F ig u r e 1 shows the impact of CO burning in the kiln. With this feed

Chains Typically, modern LMD type kilns do not use chains to help dry the mud. In the 2010 Lime Kiln Survey, of the 22 LMD type kilns responding, only five of the kilns utilized chains to assist in drying the mud [6]. In general, it was decided that chain addition would not be the best idea for the No. 7 kiln, as the use of chains would have the potential to increase the amount of dusting in the kiln and result in a net decrease in production [7].

Feed system pluggage In addition to the volumetric limitations of the existing kiln, 10

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

1. Impact of carbon monoxide (CO) burning in the lime mud dryer (LMD) feed end of the kiln results in increased feed system pluggage. Left axis is ppm of CO in the feed end of the kiln, and the right axis is the cyclone inlet pressure in inches of water pressure gage. The x-axis is a 24-h span of operation.

LIME KILN end temperature as a constraint, the kiln was only capable of producing about 350 tons/day of quality product without experiencing plugging problems. NEW LIME KILN FEED SYSTEM A potential solution to the feed system pluggage problem was to alter the feeding system of the kiln. The Evadale mill opted to install a new LimeFlash feed system on the No. 7 kiln. The new system, trademarked and supplied by Andritz (Graz, Austria), reduces the volumetric constraint of the kiln to 50–60 ft3/ton of lime produced. The new feed system is the first of its type installed in North America and only the second installed in the world. This system allows much higher feed end temperature, resulting in a significant increase in production. Back end temperature is controlled through the addition of wet lime mud. Wet mud is introduced directly into the gas stream as the gas comes out of the kiln, before the mud enters the ductwork and is flash dried. This forces the sticky fume phase of mud drying to occur inside the kiln shell instead of in the duct; thus, the ductwork does not plug up. The new feed system basically keeps the sticky dust out of the ducts. The LimeFlash feed system routinely operates at 850°C (1550°F) with no pluggage issues. Under routine operation, lime mud falls down the chute to the LMD feed screw. The mud falls off the end of the screw and is sucked into the LMD riser duct and up to the cyclone. Any feed that does not enter the LMD falls directly to the kiln. Dry kiln feed enters the kiln feed screw from the cyclone and is conveyed to the kiln. Under low temperature and low production rate scenarios, the mud falls down the chute to the LMD feed screw. The mud overflows from the LMD feed screw to the lower kiln feed screw, and the wet mud goes directly to the kiln. Fig ur e 2 shows details of the new feed system. As a result of installing this system, the kiln production rate has increased to 480 tpd of quality lime, with no operational issues. The new feed system can still plug and result in decreased production. Typical reasons for pluggage include feeding mud that is too wet to the LMD. When excessively wet mud is fed to the kiln, it can accumulate in the bottom of the duct and bake to a hard buildup, which can impair production. Another method of plugging the new feed system is allowing the gas temperature to exceed 700°C without mud being present. This condition forms a coating on the ductwork and leads to pluggage. Another potential plugging problem results when mud is fed to the LMD without enough gas velocity to convey it to the cyclone. As with the old kiln feed system, CO burning in the LMD ductwork can also lead to plugging problems. The new feed system is constructed much more tightly than the old feed system, thus minimizing air leaks and faulty oxygen readings. Finally, if the blasters are not working, the kiln also has a potential to plug. To date, no pluggage problems have occurred in the upgraded kiln.

2. The new LimeFlash feed system installed on the No. 7 kiln. Lime mud is fed into top screw and dropped into the kiln at the inlet to the throat of the ductwork. The lime is flash dried as it is sucked up the duct to the cyclone (not shown on top of larger duct), then dropped down to the lower screw to be deposited into the kiln for calcining.

Mechanical design The mechanical design of the new feed system consists of a rectangular shaped riser duct with a large radius to give extended time for lime mud drying before hitting the duct surface. The larger radius results in less plugging. The feed system utilizes a two screw feeding system and incorporates an adjustable cooling water spray system. It has seven blasters to keep the curve area of the feed duct clean. The riser duct is supported on rails for easy pulling-out, and the elbow contains a removable single-piece wear plate. The kiln feed screw has solid flights at the end to minimize leakage through the screw. ADDITIONAL PRODUCTION CONSTRAINTS There are additional variables that impact kiln production. The most critical parameter in addition to kiln volume would be the inside unit diameter, which affects velocities through the unit. With the installation of the new feed system, blowing dust out of the kiln from high air velocity will be the ultimate bottleneck to the kiln production rate. Kiln diameter has a direct impact upon flue gas velocity out of the kiln. The AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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LIME KILN

3. Impact of the No. 7 kiln feed system on the hours of operation for the No. 1 kiln.

kiln has a diameter of 246 ft in length and a shell diameter of 11 ft 10 in. The amount of brick insulation in the kiln is another factor that inherently impacts internal kiln diameter and also plays a role in production capacity and fuel economy. Excess combustion air and air leaking into the kiln through the feed system ductwork is another source of kiln production limitations. Excess combustion air and leaking air have the potential to increase flue gas velocity and carry excessive amounts of dust into the precipitators. Finally, fuel type, flame shape, and luminosity also impact kiln performance. For instance, when a kiln burns natural gas instead of fuel oil, the back end temperature of the kiln is typically about 50°F higher [2]. UPGRADED KILN GOALS AND PERFORMANCE The goal of the project was to be able to permanently shut down the No. 1 lime kiln. The new feed system on the No. 7 kiln was started on May 1, 2010. The upgraded kiln was run through a month long startup curve while several minor operational issues were addressed. Initial problems with short term draft spikes in the elbow were resolved in June 2010 when the temperature control spray nozzle was replaced with a different nozzle design and operating procedures were refined.

The main operational problem that the mill has experienced has been a tendency for the kiln to ring whenever the kiln is run for an extended period of time at a low production rate. Ringing does not appear to be an issue when the kiln is base loaded at full production (99% of the time). Three ringing events have occurred that resulted in downtime. Each time, the kiln had to run with a very low production rate prior to the ringing. The slow production rate was driven by the rest of the mill running a very low pulp production rate and a full white liquor inventory. A minimum production rate has been established for the kiln in an effort to avoid ring formation. When the system has to run below the minimum rate, load is pulled off the kiln. Fig ur e 3 shows the impact of the No. 7 kiln feed system replacement upon the operational hours of the No. 1 lime kiln. Since startup of the No. 7 kiln feed system, the hours of operation of the No. 1 kiln have substantially decreased to the point where it is not required at design pulp production rates. The monthly improvement in kiln operation as the operators worked through the startup learning curve is shown in Table I I . Production rate, availability, and lime quality have all improved since the initial startup of the kiln feed system. The new system has routinely allowed the production of 480 tpd of quality lime, with production spurts up to 520 tpd of lime product. Paper production has continued to increase above the project goal, and the kiln is now the mill production limit. Opportunities to further increase the kiln production rate will need to be pursued or the No. 1 kiln will have to be restarted to meet the higher-than-expected paper production goals.

Environmental impacts The kiln upgrade was not an environmental project per se. However, environmental performance criteria had to be met and maintained with the increased production from the kiln. One environmental benefit of the project was the installation of a newly designed Andritz burner with primary air on the inside of the flame. Adding the primary air on the inside of the flame results in a better mixing of the staged air. Improved mixing of the air and fuel lowers NOx emissions. The new kiln feed and burner system has contributed to a significant reduction in NOx emissions compared to the old LMD feed system and the old burner.

Average Production Rate (TPD)

mm Btu/Ton

Average Carbonate in Product (%)

Uptime (%)

Past performance

385

6.0

2.9

97.0

May 2010

409

6.2

4.3

93.7

June 2010

432

6.1

3.8

96.6

July 2010

482

6.1

2.8

99.6

II. No. 7 kiln performance post-startup through July 29, 2010. 12

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

LIME KILN STARTUP AND OPERATIONAL ISSUES One system that did not perform satisfactorily was the continuous emission monitoring system for CO and O2 for the kiln exit. The system consisted of a unique design (serial no. 1), using a laser gas analyzer system. The issue was pluggage of the side stream sample line in which the analyzer was installed. The sampling point for this monitor has been moved to the outlet of the cyclone. A redesigned temperature control spray nozzle was installed in June 2010 due to the typical flow being below the nozzle’s design range for optimal atomization. During the October 2010 outage, it was discovered that the refractory on the side wall of the LMD elbow was failing. Temporary repairs had to be made before startup. A new elbow with improved plating was installed during the Spring 2011 mill outage and has prevented this problem from reoccurring. TJ ACKNOWLEDGEMENTS The Andritz team of Kimmo Peltonen, Robbie Diaz, and Jack Leichliter and their support team were instrumental in the construction and startup of this project. Their efforts in diagnosing and correcting the elbow refractory problem were also important to the ultimate success of the project. Without their ABOUT THE AUTHORS The Evadale, TX, mill needed to increase production on a new, larger, energy efficient kiln in order to eliminate the maintenance, operating cost, and manpower associated with operation of two older kilns. The current work was undertaken to improve the cost effectiveness of the mill. The new LimeFlash kiln feed system, trademarked and supplied by Andritz, builds upon existing lime mud drying technology. The new system effectively extends the calcining section of the kiln by performing all of the mud drying in flight inside the ductwork and cyclone. The most difficult aspect of this research was identifying the technology. As this lime kiln feed system is the first installation in North America and only the second installation in the world, there was some concern about the viability of the project. There were also some scale-up issues and materials of construction issues associated with this project.

Hart

Colson

Clapper

help and the assistance of the Evadale mill operators, the No. 7 lime kiln upgrade would not have anywhere close to successful as it has been. LITERATURE CITED 1. Ellis, M., Manning, R., Tran, H., et al., Int. Chem. Recovery Conf., TAPPI PRESS, Atlanta, GA, USA, 2010, p. 141. 2. Adams, T.N., “Lime Reburning,” in Pulp and Paper Manufacture Vol. 5: Alkaline Pulping (T.M. Grace and E.W. Malcolm, Eds.; M.J. Kocureck, Series Ed.), TAPPI PRESS, Atlanta, 1989, p. 603. 3. Venkatesh, V., “Lime Reburning: The Rotary Lime Kiln,” in Chemical Recovery in the Alkaline Pulping Process (R.P. Green and G. Hough, Eds.) 3rd edn., TAPPI PRESS, Atlanta, 1992, p. 158. 4. Hanson III, G.M. and Gencarelli, T., Int. Chem. Recovery Conf., TAPPI PRESS, Atlanta, 2010, p. 150. 5. McDermott, T., Solutions People, Processes Pap. 88(11): 41(2005). 6. Francey, S., Tran, H., and Berglin, N., TAPPI J. 10(8): 19(2011). 7. Websdale, O., Downing, B., and Tran, H.N., Pulp Pap. Can. 110(8): 39(2009).

The most surprising thing we discovered from this project was the ability of a modern kiln to operate with a volumetric production rate of less than 55 ft 3/ ton of lime, which is an astounding achievement. The back end temperatures are over 700°C, and the system operates without plugging. Current technology suggests that modern lime kilns require 60-75 ft3 of kiln capacity per ton of lime produced. With the new feed system, this number has been successfully reduced to less than 55 ft 3/ton of lime. The new feed system will allow other mills to increase production on modern kilns and perhaps improve their energy efficiency. Our next step is to optimize the kiln production and long term operability. Hart is manager, New Technology, for MeadWestvaco Corporation in Atlanta, GA, USA. Colson is engineering manager, Clapper is senior manager, and Pollet is recovery process engineer for MeadWestvaco Corporation in Evadale, TX, USA. Email Hart at [email protected].

Pollet

Doughty

AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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6/28/12 12:57

PEER-REVIEWED

BIOREFINERY

Biorefinery implementation for recovery debottlenecking at existing pulp mills – Part II: Techno-economic evaluation HAKIM GHEZZAZ, LUC PELLETIER,

and

PAUL R. STUART

ABSTRACT: The evaluation and process risk assessment of (a) lignin precipitation from black liquor, and (b) the near-neutral hemicellulose pre-extraction for recovery boiler debottlenecking in an existing pulp mill is presented in Part I of this paper, which was published in the July 2012 issue of TAPPI Journal. In Part II, the economic assessment of the two biorefinery process options is presented and interpreted. A mill process model was developed using WinGEMS software and used for calculating the mass and energy balances. Investment costs, operating costs, and profitability of the two biorefinery options have been calculated using standard cost estimation methods. The results show that the two biorefinery options are profitable for the case study mill and effective at process debottlenecking. The after-tax internal rate of return (IRR) of the lignin precipitation process option was estimated to be 95%, while that of the hemicellulose pre-extraction process option was 28%. Sensitivity analysis showed that the after tax-IRR of the lignin precipitation process remains higher than that of the hemicellulose pre-extraction process option, for all changes in the selected sensitivity parameters. If we consider the after-tax IRR, as well as capital cost, as selection criteria, the results show that for the case study mill, the lignin precipitation process is more promising than the near-neutral hemicellulose pre-extraction process. However, the comparison between the two biorefinery options should include long-term evaluation criteria. The potential of high value-added products that could be produced from lignin in the case of the lignin precipitation process, or from ethanol and acetic acid in the case of the hemicellulose pre-extraction process, should also be considered in the selection of the most promising process option. Application: This work highlights the potential economic benefits of implementing emerging biorefinery technologies for recovery debottlenecking, and indicates how dissimilar process options can be evaluated and compared.

I

ncreasingly, industrial analysts agree that the forest biorefinery comprises a promising opportunity for forestry companies, through which revenues can be diversified outside traditional pulp and paper products. The transformation of pulp and paper mills into some form of forest biorefinery is considered by some as an inevitable alternative. It potentially gives mills opportunities to maintain their core business while developing new business activities [1-4]. It is clear that the complete transformation of pulp mills into integrated forest biorefineries must be achieved incrementally over the coming years, since biorefinery technologies are still emerging and are at various phases of development. During the implementation of the biorefinery, the forest industry should seek to improve existing process operations and competitive position. In our case study, (a) lignin precipitation from black liquor by acidification using CO2, and (b) the near-neutral hemicellulose pre-extraction process, were identified as suitable for recovery cycle debottlenecking and the subsequent production increase in an existing semichemical pulp and paper mill. The selection of the biorefinery technology to be imple-

mented in an existing mill is a critical decision for the longterm development of the mill. For this case study, the choice between lignin precipitation from black liquor and the nearneutral hemicellulose pre-extraction process is important for the pulp mill to succeed in its transformation to an integrated forest biorefinery (IFBR). According to Van Heiningen [3], the transition from a pulp and paper mill to an IFBR will be enabled by economic returns from ongoing pulp and paper production, while producing additional bioproducts and renewable energy. Chambost et al. [5] recommend incremental biorefinery implementation in an existing pulp mill for existing process operation improvement in the first phase, but the selected technology must also meet longer-term, more ambitious objectives in later implementation phases when the pulp and paper mill is totally transformed to an IFBR. In Part I of this paper [6], it was shown that the potential for recovery debottlenecking and the possible paper production increase by implementing lignin precipitation from black liquor or hemicellulose pre-extraction processes are significant. The objectives of Part II of this paper are to perform an economic evaluation of the processes and to identify their relative strengths and weaknesses. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

17

BIOREFINERY TECHNO-ECONOMIC EVALUATION The techno-economic evaluation of the two biorefinery process options was performed in the context of process debottlenecking in order to achieve (a) production increase and (b) pulp quality improvement by lowering pulp yield. The incremental costs in the existing process and additional revenues from the increase of paper production were considered in addition to the biorefinery operating costs and revenues.

Scenarios definition The case study mill is a sodium carbonate-based semichemical pulp and paper mill producing 600 oven dried (o.d.) metric tons/day of paper. The paper is produced from 65% virgin pulp produced from a mixture of hardwood (pulp yield ~85%) and 35% recycled fiber. Two biorefinery implementation scenarios were selected and are described in Part I of this paper [6]. In the case of the lignin precipitation option, the maximum

paper production capacity attainable by keeping the pulp yield constant (690 o.d. metric tons/day of paper) was considered. In this scenario, the lignin precipitation rate is equal to 39.1% of the total lignin in black liquor. For the hemicellulose pre-extraction process option, it was shown that it has more potential for process debottlenecking. Paper production could increase to 720 o.d. metric tons/day, and pulp yield could decrease from 85% to 79%. In this scenario, the degree of hemicellulose extraction is equal to 10.2% of dry wood. METHODOLOGY The economic evaluation of the two biorefinery process options was performed using the following steps. Table I summarizes the overall assumptions and evaluation basis.

Mass and energy balances First, a simulation model of the existing process was built

Overall Assumptions Lignin precipitation process • There are no negative impacts of lignin precipitation on the existing process operations. • Lignin dissolved in black liquor has the same behavior as kraft lignin. • Produced lignin is used at the mill for fossil fuel replacement in boilers. • Key information for mass balance are taken from reference [12]. Hemicellulose pre-extraction process • There are no negative impacts of the hemicellulose pre-extraction process on the properties of the produced paper. • Key information for mass and energy balances are taken from reference [10]. Investment Costs • Scaling equation for total installed equipment costs estimation of lignin precipitation process is taken from reference [9]. • Scaling equations for total installed equipment costs estimation of the major sections of hemicellulose pre-extraction process are taken from references [10,11]. • Additional direct investment costs are equal to 9% of the total installed equipment costs. • Indirect investment costs are equal to 50% of the total installed costs. • Contingency factor of 30% of total installed costs was applied. • Working capital assumed to be 3% of the project investment cost. Operating Costs • Operating time: 350 days/year (8400 h). • Key information for biorefinery variable costs are taken from reference [10] in case of hemicellulose pre-extraction process and from reference [13] in case of lignin precipitation process. • Fixed costs of the existing process are assumed to remain constant after increasing paper production. • Overhead costs assumed to remain constant after biorefinery implementation. • No additional operating manpower is required (existing mill personnel assumed to be able to handle biorefinery also). • Maintenance: 2% of total installed costs. • Insurance: 1.5% of total installed costs. • Raw material and product prices are provided by the studied mill or taken from literature. Other Economic Parameters • • • • • • • • •

100% equity investment. Investment depreciation: 7 years- 200% declining balance depreciation method. Economic life: 20 years. Investment path: full investment in first year (2012). The price of paper is not increased after its improvement by lowering the pulp yield. Inflation rate: 3%. Income tax rate: 40%. Working capital is not recovered. There is no residual value of the investment after 20 years.

I. Assumptions and economic evaluation basis. 18

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

BIOREFINERY using WinGEMS version 5.3 software (Metso Automation; Helsinki, Finland) and validated by mill personnel. Then, simple models of lignin precipitation from black liquor and hemicellulose extraction prior to pulping were implemented in the simulation model. The outputs of the simulation and published data about the biorefinery options were used to establish the mass and energy balances of the integrated biorefineries (lignin precipitation or hemicellulose pre-extraction process integrated into the existing process).

that there is no need for drying the lignin before its combustion. In the case of the hemicellulose pre-extraction process, the revenues come from selling the produced ethanol, separated acetic acid, and furfural. It was assumed that there is no increase in the selling price of the paper when its quality is improved by lowering the pulp yield.

Profitability After-tax internal rate of return (IRR) is used as a measure of the profitability. It is calculated according to Eq. (2):

Investment costs The case study mill is characterized by process sections having overcapacity, and no additional investments are needed in the existing pulping processes for either biorefinery process scenario. Investment costs were thus calculated only for the lignin precipitation or the hemicellulose pre-extraction process. Since the objective is to evaluate and to compare the two process options at the pre-feasibility level, the capacity-factored method was used for investment costs evaluation. The method gives relatively quick and sufficiently accurate estimates [7,8] by providing a cost estimate of a new plant based on the cost of a similar plant of a known capacity, according to Eq. (1):

(1) where Cn is the cost of the new plant, C is the cost of the similar plant, fe is the ratio between the cost indexes related to the costs Cn and C, R is the ratio between the capacities of the two plants, and x is the exponent factor. The method was applied to calculate the installed equipment costs of the lignin precipitation process and of major process sections of the hemicellulose pre-extraction process using literature scaling equations [9-11]. After the calculation of installed equipment costs, factors were used to estimate additional direct and indirect costs, contingency, and working capital.

Operating costs The estimation of operating variable costs for the two biorefinery process options and for the existing pulp and paper mill is based on the mass and energy balances. Incremental variable costs for paper production are calculated as the difference between the variable costs before and after production increase and pulp yield lowering.

Revenues Total revenues for both considered scenarios consist of the revenues coming from the biorefinery products and from selling the incremental production of paper. In the case of the lignin precipitation process, the produced lignin is assumed to be used at the mill as a fossil fuel (oil) replacement. Therefore, the revenues are also derived from fossil fuel savings. It was assumed that lignin is produced at 70% solids content and

(2) where NPV is the net present value, t is the plant life, and NCFt is the net cash flow at time t.

Sensitivity analysis Sensitivity analysis was performed for both process options. Uncertain and sensitivity parameters were selected according to variable cost and revenue breakdowns. Sensitivity analysis was also conducted considering uncertain parameters which can significantly affect the profitability of the process options. The sensitivity variables were varied inside an expected interval of variation. RESULTS AND DISCUSSION The results of the economic evaluation are summarized in Table II. The investment costs in the two processes are lower compared to those published in reference studies [9,14], due mainly to the lower capacities of the two processes. The lignin precipitation process produces only 3980 metric tons/year of lignin with 70% solid content, while hemicellulose pre-extraction process produces only 1.4 million gal/year of ethanol. The investment costs in the lignin precipitation process are much lower than those needed for the hemicellulose pre-extraction process. Also, in the hemicellulose pre-extraction process case, the extraction step processes all the feedstock chips, and subsequent hemicellulose pre-extraction process steps process high flow rate dilute streams. On the other hand, with the lignin precipitation process, only a fraction of black liquor has to be processed if the precipitation yield of lignin is high. Incremental variable costs in the existing process constitute a substantial part of the total operating costs for both scenarios. Wood chips, OCC, steam, and electricity are the main cost items. This is not a surprising result given the increase in paper production. Revenue from the incremental production of paper is the main revenue in both scenarios. Revenues from produced lignin represent only 4% of the total revenues, while revenues from hemicellulose pre-extraction process represent 28% of the total revenues. The revenues from the hemicellulose pre-extraction process are greater because three bioproducts are manufactured (ethanol, acetic acid, and furfural). Both biorefinery options are profitable, and the lignin preAUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

19

BIOREFINERY Techno-Economic Results Investment costs (US$) • Total installed equipment cost − TIEC • Additional direct costs − ADC (9% of TIEC) • Total installed cost − TIC (TIEC+ADC) • Indirect costs − IC (50% of TIC) • Project contingency − PC (30% of TIC) • Total capital investment − TCI (TIC+IC+PC) • Working capital − WC (3% of TCI) • Total Project Investment TPI (TCI+WC) Incremental operating cost breakdown (%) • Wood chips • Old corrugated containers (OCC) • Electricity • Steam • Other variable costs for paper production increase • Variable costs in biorefinery • Fixed costs Incremental revenue breakdown (%) • Paper • Biorefinery products Profitability • After-tax IRR (%)

Lignin Precipitation

2 809 253 3 062 1 531 919 5 512 165 5 677

Hemicellulose Pre-extraction

000 000 000 000 000 000 000 000

18 1 19 9 5 35 1 36

134 632 767 883 930 580 067 647

000 000 000 000 000 000 000 000

29 27 15 14 13 01 01

31 14 11 15 09 16 04

96 04

72 28

95

28

II. Economic evaluation results. Selected Sensitivity Variables

Justification of the Choice

Basis for Selecting the Interval of Variation

Lignin Precipitation

Hemicellulose Pre-extraction

• Paper selling price • Ethanol selling price • Acetic acid selling price

Important sources of revenues (see revenues breakdown in Table II)

Historical data and market trends

ü

ü ü ü

• Wood chips cost • OCC price

Important costs (see operating costs breakdown in Table II)

Historical data and market trends

ü ü

ü ü

• Steam cost

Uncertainty related to which fuel is used for steam production

Fraction of the amount of steam produced by fuel oil boiler

ü

ü

• Increase in selling price of paper after pulp yield lowering

No information available about improvements in paper quality after pulp yield lowering

Values have been assumed

• Investment costs

The evaluation method gives estimations with accuracy between -50% to +100%

0% to 100% of the expected value of the installed equipment costs

ü

• M  aximum rate of lignin precipitation

• P  ossible paper production increase depends on lignin precipitation rate • Many parameters may limit the precipitation rate of lignin

Values of lignin precipitation rates have been assumed

ü

III. Sensitivity variables. 20

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

ü

ü

BIOREFINERY cipitation process option has an unusually high IRR. The integration of the two options enables a production increase without an increase in the fixed operating costs and overheads; therefore, operating margins of paper production are substantially improved.

Sensitivity analysis The sensitivity analysis variables are summarized in Table III. The revenues in the two scenarios come mainly from selling the incremental production of paper; therefore, the paper selling price affects the profitability of the two process options. Ethanol and acetic acid prices have historically been volatile and this would impact the profitability of the hemicellulose pre-extraction process option. Wood chips and OCC costs are the main variable costs for both scenarios and according to historical data, the OCC price is volatile. The cost of steam is highly dependent on the type of fuel used. In the case study mill, only 20% of the steam is produced by fossil fuel boilers. By analyzing historical data, it was found that in some operating conditions, a larger quantity of steam is produced by the fossil fuel boilers and since the price of fuel is volatile, it makes the cost of steam an uncertain variable. In the case of the scenario considering the hemicellulose pre-extraction process, an increase in the selling price of paper after lowering the pulp yield is expected. It is, however, difficult to estimate this increase in price because there is no specific information available on how the properties of the paper will be improved if the pulp yield is lowered and, at the same time, hemicellulose is pre-extracted. Therefore, the increase in paper price is an uncertain parameter in the hemicellulose pre-extraction process scenario. Another parameter strongly affecting the profitability is the investment cost. The investment cost calculation has an accuracy of -50% to +100% [7]. Also, the capacities of the lignin precipitation and hemicellulose pre-extraction processes in this study are very low compared to the capacities in reference studies, which can lead to underestimation of the investment costs. The last selected sensitivity parameter that may affect the profitability of the lignin precipitation option is the maximum precipitation rate of lignin. It has been shown in Part I of this paper that the attainable paper production increase depends on the precipitation rate of lignin. However, many factors can affect the maximum precipitation rate of lignin. Among these parameters are the minimum pH achievable for black liquor and the maximum decrease of black liquor viscosity that is tolerated by the existing system. This makes the maximum rate of lignin precipitation an uncertain parameter. Sensitivity analysis graphs of the two process options are shown in Figs. 1 and 2. The slope of each curve gives an indication of how much the profitability is dependent on the corresponding parameter. The upper bound of each curve represents an optimistic scenario, whereas the lower bound represents a pessimistic scenario. The profitability of the two options depends strongly on the cost of wood, the selling price of paper, the price of OCC, and the cost of steam, and

1. Sensitivity analysis: Profitability of lignin precipitation process option as a function of sensitivity parameters (variable operating costs and selling prices or products).

2. Sensitivity analysis: Profitability of hemicellulose preextraction process option as a function of sensitivity parameters (variable operating costs and selling prices or products).

to a lesser extent, on the prices of biorefinery products. In the case of the hemicellulose pre-extraction process, the profitability is slightly sensitive to the change in the ethanol and acetic acid selling prices. Fig ur e 3 shows that the profitability of the hemicellulose pre-extraction process option can be strongly affected by the increase of the selling price of paper after lowering the pulp yield. If, for example, the selling price of paper increases by 20 US$/ton, the after-tax IRR will increase from 28% to 39%. Figur e 4 shows that the profitability of the lignin precipitation option is more sensitive to changes in investment costs compared to the hemicellulose pre-extraction process option. For example, if the investment costs are 50% higher than estimated, after-tax IRR of lignin precipitation option decreases from 95% to 66%, whereas the after-tax IRR of hemicellulose pre-extraction process option decreases from 28% to 19%. Fig ur e 5 shows that the profitability of the lignin precipitation process option is dependent on the maximum preAUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

21

BIOREFINERY

3. Profitability of hemicellulose pre-extraction process option as function of paper selling price increase after lowering pulp yield.

4. Profitability of lignin precipitation and hemicellulose preextraction process options as function of the investment cost increase.

cipitation rate of lignin. It is an expected result, knowing that the increase in paper production is related to the lignin precipitation rate (see Part I of this paper [6]). For a precipitation rate of 34.5%, paper production is increased only by 13% (78 o.d. metric tons/day), which decreases the after-tax IRR of the option to 87%. However, if the maximum achievable precipitation rate of lignin is 45.7%, then paper production can be increased by 18% (108 o.d. metric tons/day) and aftertax IRR of the option increases to 100%.

Comparison between lignin precipitation and hemicellulose pre-extraction process options Some important criteria for comparing the two biorefinery process options are presented in Table I V. The comparison includes results reported in Part I of this paper [6]. For the hemicellulose pre-extraction process, several of the process steps are not yet developed to commercial scale. However, in the context of the case study mill, the implementation 22

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

5. Profitability of lignin precipitation process option as function of maximum rate of lignin precipitation.

of the lignin precipitation process (as it has been developed for kraft mills) presents high risks and significant possible negative impacts on the existing process. The impacts of the hemicellulose pre-extraction process on the existing process are expected to be smaller. The hemicellulose pre-extraction process has a higher potential for process debottlenecking and production increase compared to the lignin precipitation process. The hemicellulose pre-extraction process can also lead to paper quality improvement by lowering the pulp yield. The lignin precipitation process produces lignin with high solids content and high purity, which is assumed to be used at the mill for fossil fuel replacement. However, this lignin could also be used as a feedstock for higher added-value products manufacturing. In the case of the hemicellulose pre-extraction process, two main products are produced: ethanol and acetic acid. The revenues from the hemicellulose pre-extraction process products represent around 28% of the total revenues, and this allows the mill to diversify its revenues to some extent. Lignin produced by the lignin precipitation process was assumed to be used in the mill, and there is no market risk; whereas the hemicellulose pre-extraction process products are to be sold on the market, and difficulties may arise for marketing these products with such small product volumes. The lignin precipitation option has a lower capital investment than the hemicellulose pre-extraction process and this is the main reason why the after-tax IRR of the process option is significantly higher than the after-tax IRR of the hemicellulose pre-extraction process option. Lignin precipitation and hemicellulose pre-extraction are emerging process technologies and investment in the two processes can be considered as risky, so a higher profitability is needed compared to traditional debottlenecking alternatives. Sensitivity analysis showed that several parameters can strongly affect the profitability of the two projects. The lower bound of after-tax IRR of the two process options occurred when the total investment was increased by 100%. In this

BIOREFINERY Lignin Precipitation

Hemicellulose Pre-extraction

• Process maturity

Higher

Lower

• Negative impact on the existing process

Higher

Lower

• Maximum production increase

15%

20%

• Pulp yield lowering

n/a

From 85% to 79%

Lignin

Ethanol, acetic acid, furfural

Criteria of Comparison

• Bioproducts • Investment cost

5 677 000

36 647 000

• Base case after-tax IRR

95%

28%

• % revenues from bioproducts

4%

28%

• Lower bound of after-tax IRR according the sensitivity analysis

51%

14 %

IV. Comparison between the two process options.

case, the after-tax IRR of the lignin precipitation option decreases from 95% to 51%, while the after-tax IRR of the hemicellulose pre-extraction process option decreases from 28% to 14%. The sensitivity analysis showed that the profitability of the lignin precipitation process option remains higher than that of the hemicellulose pre-extraction process option, whatever the changes in the selected sensitivity parameters. CONCLUSIONS The economic evaluation and comparison of lignin precipitation from black liquor by acidification with CO2 and near-neutral preextraction processes for recovery debottlenecking of an existing semichemical pulp and paper mill have been performed. The results show that for the case study mill, the two biorefinery process options are profitable in the context of existing process debottlenecking. The base case after-tax IRR of the lignin precipitation process option was calculated to be 95%, while the after-tax IRR of the hemicellulose pre-extraction process option was 28%. The sensitivity analysis showed that after tax-IRR of the lignin precipitation process option remains higher than that of the hemicellulose pre-extraction process option, whatever the changes in the selected sensitivity parameters. If we consider only the value of after-tax IRR and the importance of investment costs as selection criteria, the results show that in the context of the case study mill, the lignin precipitation process is more promising compared to the hemicellulose pre-extraction process. However the comparison between the two biorefinery options should include the implementation risks and the long-term evaluation criteria. The potential of high value-added products that could be produced from lignin in the case of the lignin precipitation process or from ethanol and acetic acid in the case of the hemicellulose pre-extraction process should be considered for the selection of the most promising process option. IMPLICATIONS The hemicellulose pre-extraction process enables a greater increase in paper production compared to the lignin

precipitation process. The hemicellulose pre-extraction process can potentially be more suitable to the mill if the strategy of the mill is to stay competitive in the same core business. However, the biorefinery should be considered as a strategic investment, allowing forestry companies to diversify their revenues. For this, lignin precipitation can potentially be more promising because it enables transformation of the existing mill into an integrated forest biorefinery. Produced lignin can be used for energy purposes in the short term and as a feedstock for higher value-added bioproducts in the long term. The lignin precipitation process also has a higher short-term IRR, and this provides opportunity for the mill to recover project investment costs in a short time and helps the mill to progress in biorefinery implementation towards high addedvalue products. In this case study, results show that the capacities of the two biorefinery process options are very small. The production capacity of ethanol and acetic acid in the hemicellulose pre-extraction process are low compared to existing petroleum or corn-based ethanol plants. It may be a challenge for the mill to sell such small quantities of products at an attractive market price. However, the amount of hemicellulose pre-extraction sugars could potentially be appropriate as a building-block chemical for high-value added bioproduct manufacturing (e.g., xylose for xylitol). In conclusion, the comparison between the two biorefinery options should consider short-term criteria such as capital investment, profitability, technology maturity, technology and market risks, and long-term criteria by considering, for example, the potential of producing high valueadded products in the future when the technologies and bioproduct markets have matured. TJ ACKNOWLEDGEMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Environmental Design Engineering Chair at École Polytechnique de Montréal, and Norampac, A Division of Cascades Inc. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

23

BIOREFINERY LITERATURE CITED 1. Thorp, B., Pulp Pap. 79(11): 35(2005). 2. Stuart, P., Pulp Pap. Can. 107(6): 13(2006). 3. Van Heiningen, A., Pulp Pap. Can. 107(6): 38(2006). 4. Amidon, T.E. and Liu, S., Biotechnol. Adv. 27(5): 542(2009). 5. Chambost, V., McNutt, J., and Stuart, P.R., Pulp Pap. Can. 109(7/8): 19(2008). 6. Ghezzaz, H., Pelletier, L., and Stuart, P.R., TAPPI J. 11(7): 17(2012). 7. Dysert, L.R., Cost Eng. 45(6): 22(2003). 8. Peters, M.S., Timmerhaus, K.D., and West, R.E., Plant Design and Economics for Chemical Engineers, MacGraw-Hill, Boston, 2003, p. 242 9. Laaksometsa, C., Axelsson, E., Berntsson, T., et al., Clean Technol. Environ. Policy 11(1): 77(2009). 10. Mao, H., “Technical evaluation of a hardwood biorefinery using the ‘near-neutral’ hemicellulose extraction process,” Master’s thesis, University of Maine, Orono, ME, USA, 2007. 11. Mitchell, J., “Production of ethanol from hardwood,” Master’s thesis, University of Maine, Orono, 2006. 12. Ohman, F., Theliander, H., Tomani, P., et al., pat. US2008/0047674 A1 (February, 2008). 13. Tomani, P., Berglin, N., and Axegård, P., TAPPI Eng., Pulping Environ. Conf., TAPPI PRESS, Atlanta, GA, USA, 2009, p. 2347. 14. Mao, H., Genco, J.M., Yoon, S.H., et al., J. Biobased Mater. Bioenergy 2(2): 177(2008).

ABOUT THE AUTHORS This work comprises the second part of a case study considering the evaluation of the implementation of lignin precipitation from black liquor by acidification with CO2 and the near-neutral hemicellulose pre-extraction processes in an existing semichemical pulp mill. The potential and process risk assessment of the two biorefinery processes for existing process debottlenecking and pulp production increase was described in the first part of the case study. This second part of the paper focuses on the techno-economic evaluation of the two biorefinery processes in the context of process debottlenecking for production increase and pulp quality improvement. It was found that for the case study mill, the two biorefinery process options are profitable, and that the after-tax IRR of the lignin precipitation option is high. However, the comparison between the two debottlenecking options should also consider other criteria such as the amount of capital investment, technology maturity, technology and market risk. During the development phase of biorefinery technologies and due to growth of the bioproduct markets, pulp and paper mills should take advantage from the progress that

24

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

Ghezzaz

Pelletier

Stuart

has been made in the area of biorefinery processes in order to improve their existing process operations and their competitiveness. Ghezzaz was a Master’s student in the NSERC Design Chair at École Polytechnique de Montréal in Montréal, QC, Canada, at the time of this study and today works for Cascades Canada- R&D Division, Kingsey-Falls, QC. Stuart is chairholder of the NSERC Design Engineering Chair and professor of Chemical Engineering at École Polytechnique de Montréal. Pelletier is mill manager of Norampac-Cabano, a Division of Cascades Canada in Cabano, QC. Email Stuart at [email protected].

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BIOREFINERY

The sulfite mill as a sugar-flexible future biorefinery LISA X. LAI

and

RENATA BURA

ABSTRACT: The production of single- and mixed-sugar streams and their conversion to bioproducts were studied, using sulfite pulping streams as feedstocks. Sulfite pulp, sludge, and spent sulfite liquor are concurrently generated alongside of bleached pulp, and the pulping process renders pretreatment of solid streams unnecessary. Streams were converted separately; however, due to their low production volume, solid and liquid streams were also combined as a means to increase the quantity of starting feedstock. Spent sulfite liquor, comprising mostly monomeric hexose and pentose sugars, was directly fermented to ethanol and xylitol with Candida guilliermondii. Single-sugar streams were generated through hydrolysis of pulp and sludge in water, followed by fermentation to ethanol with Saccharomyces cerevisiae. Mixed-sugar streams were generated through both separate hydrolysis and fermentation and simultaneous saccharification and fermentation of pulp and sludge in spent sulfite liquor using S. cerevisiae. The best single-sugar source was obtained by hydrolysis of pulp in water, which produced 78.8 g/L of glucose after 96 h. The glucose concentration from hydrolysis of sludge in water was lower (33.5 g/L). Both of these streams were easily converted to ethanol, with yields of 77.8% and 76.2%, respectively. Hydrolyzability of solids was the limiting factor in separate hydrolysis and fermentation conversion of pulp and sludge in water, but hydrolyzability of sludge was not affected when mixed with spent sulfite liquor. Application: Pulp, sludge, and 14% spent sulfite liquor can be processed in a variety of ways to produce a sugar platform for making biofuels and biochemicals. This study elucidates flexibility as a major advantage of process schemes for converting these feedstocks into a sugar platform for future sulfite mill-based biorefineries.

F

uture biorefineries are likely to follow the model of a sugar platform as a feedstock for production of chemicals such as bioethanol, biobutanol, and many others [1]. Such a sugar source, however, is currently unavailable. An ideal sugar source must be inexpensive, readily available year-round, and relatively consistent in composition. Current biological conversion processes in the United States use starch because use of lignocellulosic biomass is not yet economically feasible. Although biomass to biochemical and biofuel pilot plants exist in the United States (e.g., POET, Mascoma, Gevo, and ZeaChem), large-scale facilities are yet to be established [2]. In the meantime, coupling biochemical production with an existing industry, such as sulfite pulping, can help to meet feedstock demands. This model has a number of advantages. First, sugar streams from sulfite pulping are already generated concurrently with pulp, so there is no additional cost associated with their acquisition [3]. Second, some of these potential sugar streams, such as sludge, are considered waste products to sulfite mills and represent a disposal cost that can be mitigated if they are instead converted to a bioproduct [4]. Finally, pretreatment is not needed for these sugar streams because they are mostly delignified by the pulping process [4]. Previous research has examined the bioconversion of sludge and red liquor to ethanol. Kang et al. [5] examined simultaneous saccharification and fermentation (SSF) of sludges using Saccharomyces cerevisiae (ATCC 200062 [ATCC; Manassas, VA, USA], NREL-D5A) and simultaneous

saccharification and cofermentation (SSCF) using recombinant Escherichia coli (ATCC 55124, KO11; ATCC) capable of xylose to ethanol fermentation. Respective ethanol yields of 79.5% and 80.1% of the theoretical maximum were obtained with primary sludge using 3% (w/v) consistency and 15 FPU/g (filter paper units per gram cellulose) enzyme loading [5]. Helle et al. [6] examined ethanol fermentation from spent sulfite liquor (SSL) by genetically modified S. cerevisiae (259ST) capable of xylose fermentation and a naturally occurring strain adapted to SSL (Tembec T2, Tembec Inc.; Témiscaming, QC, Canada). Yields of up to 90% were achieved [6]. Zhang et al. examined SSCF of sludge using recombinant xylose-fermenting Zymomonas mobilis 8b and S. cerevisiae RWB222. At an enzyme loading of 10 FPU/g cellulose and 17% solids, SSCF yielded ethanol concentrations as high as 45 g/L [7]. These published works indicate that ethanol production from sludge and SSL has been examined. Aside from ethanol, however, these materials can alternatively be converted to single- or mixed-sugar streams, from which a host of biochemicals can be produced using the model of a sugar platform [1]. Our objective was to examine the potential of generating single- and mixed-sugar streams from sulfite pulp, sludge, and SSL for bioproduct conversion via separate hydrolysis and fermentation (SHF) and SSF. By applying a variety of process schemes, the results from this study can help future biorefineries identify the best scenarios for producing sugar-stream precursors from which biochemical can be made. Without bioconversion, sludge and SSL can require expensive and enAUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

27

BIOREFINERY

1. Potential process schematic for a sulfite mill biorefinery based on a flexible sugar platform from which biochemicals can be made (SSL = spent sulfite liquor).

ergy-intensive treatment techniques before disposal, so their conversion to higher-value products is both desirable and economically beneficial. Pulp is itself a highly valued product, so its conversion to biochemicals is less economically desirable, although conversion to a high-value bioproduct may become desirable in the future if there is an excess of sulfite pulp on the market. In the future sulfite mill biorefinery, pulp may be used for paper products or as a sugar source for high-end biochemicals (Fig. 1). Sludge, or combined sludge and SSL streams, may be used for biochemicals. Such flexibility would allow the composition and concentration of sugar streams to be tailored to final product requirements. Also, reconfiguration of the traditional bioconversion scheme using these three pulping streams has not been well studied. Combining low-volume streams, such as sludge and SSL, offers the possibility of creating a larger feedstock quantity and potentially a more economically feasible sugar source for biochemical production. High-consistency, low-enzymeloading SHF of each stream was explored separately; however, SHF and SSF of combined feedstocks, derived by mixing SSL with either of the two solid streams, were also analyzed. Mixing offers the potential to produce highly concentrated sugar streams without using large quantities of enzyme, as would be the case in traditional, single-feedstock enzymatic hydrolysis. METHODS

Raw materials Ammonia-based sulfite pulping at Kimberly-Clark in Everett, WA, USA, produces wood pulp for direct sale or conversion to tissue products. Byproducts include primary clarifier sludge and spent sulfite liquor. Pulp is produced at a nominal rate of 500 a.d. metric tons/day. Under the right conditions, up to 90 a.d. metric tons/day could be provided for hydrolysis [8]. Current practice at Kimberly-Clark is to dry, press, and store pulp when it is not being used for paper production. Sludge is produced at an average rate of 40 dry tons/day, and SSL is produced at 450 tons dry solids/a.d. metric ton at 14% solids [8]. Sludge is dewatered and burned as hog fuel onsite, and SSL is evaporated and burned to recover sulfur dioxide (SO2) and heat. Collaborators at Kimberly-Clark provided us with the pulp, sludge, and SSL used in this study. All materials were primarily derived from softwoods. The mill also produces a hardwood grade, so pulp and sludge contained a small, unknown amount of hardwood fiber. Pulp was 28

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

collected from the mill’s prebleach washers and had not been treated with chlorine dioxide (ClO2). Sludge was collected from primary clarifiers before introduction to aerobic bacteria and contained a mixture of pulping fines and rejects, tissue mill sludge, and boiler house effluent. SSL was taken directly from brownstock washers at 14% solids (g solids/g SSL) and had not been evaporated. Solids were washed with 10 times their mass in water and stored at -20°C until use. Moisture content was 77.3% for pulp and 75% for sludge. Spent sulfite liquor was stored at 4°C until use.

Compositional analysis A modified TAPPI method T-222 om-98 “Acid insoluble lignin in wood and pulp” was used to gravimetrically analyze insoluble lignin and photometrically analyze soluble lignin, as previously described by Ewanick and Bura [9]. Soluble monomeric and oligomeric carbohydrate content was determined using modified NREL LAP TP-510-42623 “Determination of sugars, byproducts, and degradation products in liquid fraction process samples,” as previously described [9].

Enzymatic hydrolysis and fermentation We explored process designs by which pulp, sludge, and SSL could be converted to a sugar platform from which biochemicals could be produced. F i g u r e 2 shows the resulting schematic of converting pulp and sludge to ethanol and SSL to ethanol and xylitol. Separate hydrolysis and fermentation was used to convert the three streams separately, and SSF was used to convert SSL fortified with each of the two solid streams in one step. Spent sulfite liquor was fermented to ethanol and xylitol. Saccharification Hydrolysis was carried out in 125-mL (50-mL reaction volume) Erlenmeyer flasks in triplicate on washed solid materials. Solids were enzymatically hydrolyzed at 10% (w/v) consistency in both water and SSL with pH adjusted to 4.8. Flasks were incubated at 50°C and 150 rpm in an orbital shaker (New Brunswick Scientific; Enfield, CT, USA). Enzymes added were cellulase at 5 FPU/g cellulose (Spezyme, Genencor; Palo Alto, CA, USA) and β-glucosidase at 10 CBU/g (cellobiose units per gram cellulose) (Novozymes 18, Novozymes; Bagsverd, Denmark). For controls, the same amount of enzyme added to pulp and sludge flasks was respectively added to flasks containing plain SSL. Samples of 1 mL volume were taken periodically over 48 h, boiled at 100°C for 5 min to denature enzymes, and stored at -20°C until high performance liquid chromatography (HPLC) analysis. Fermentation (S. cerevisiae) Before fermentation, S. cerevisiae (ATCC 96581; ATCC, Manassas, VA, USA) isolated from spent sulfite liquor [10] was streaked onto YPD agar plates comprising 10 g/L each of yeast extract, peptone, and dextrose, and allowed to grow for 48 h. Yeast were propagated and harvested as previously

BIOREFINERY 30°C and 150 rpm for 72 h in an orbital shaker. Samples of 1 mL volume were taken periodically and centrifuged at 10000 rpm for 5 min. Supernatant was collected through 0.22-μm syringe filters and stored at -20°C, while pellets were washed with water, resuspended in 1 mL water, and optical-density measured at 600 nm to determine cell concentration. Fermentation (Candida guilliermondii) Fermentation was also performed on original SSL and hydrolyzed SSL spiked with synthetic xylose up to a 30 g/L total concentration using C. guilliermondii (ATCC 201935; ATCC) to demonstrate xylitol production. This was done in an identical manner as fermentation with S. cerevisiae, except that the nutrients added in this case were 5 g/L urea, 1.7 g/L yeast nitrogen base, and 1 g/L yeast extract. Two controls containing 30 g/L each of glucose and xylose were handled in the same manner.

Spent sulfite liquor

2. Experimental design of converting pulp, sludge, and SSL to bioproducts via fermentation, separate hydrolysis and fermentation (SHF), and saccharification and fermentation (SSF). In SHF, solid streams are combined with either water or SSL. In SSF, solid streams are combined with SSL and converted in one step.

described [9]. After completion of hydrolysis, the remaining liquid hydrolysate was boiled at 100°C to denature enzymes and vacuum filtered through filter paper. The resulting filtrate was collected and nutrients were added in the form of (NH4)2HPO4 (diammonium phosphate) at 2 g/L, Na2SO4 (sodium sulfate) at 0.2 g/L, and NaNO3 (sodium nitrate) at 2 g/L. Hydrolyzed SSL and a solution of 10 g/L each of glucose, galactose, and mannose in water were also treated in this manner to use as controls. The pH was adjusted to 6.0 with 50% (w/w) NaOH (sodium hydroxide) and S. cerevisiae was added at a concentration of 5 o.d. g/L. Fermentation was done in a 50 mL volume in 125-mL Erlenmeyer flasks and incubated at

Spent sulfite liquor containing the same nutrient concentrations as in SHF was adjusted to pH 5.5 with 50% (w/w) NaOH. The solution was fortified with pulp or sludge at 10% consistency. Enzymes and S. cerevisiae were added at 5 FPU/g cellulose (10 CBU/g β-glucosidase) and 5 g/L, respectively. Samples were run in triplicate in 100-mL volume at 37°C in an orbital shaker for 72 h. A control containing 10 g/L each of galactose, glucose, and mannose was prepared and analyzed identically.

Analysis of sugars and ethanol Monosaccharides were quantified using a Dionex (Sunnyvale, CA, USA) HPLC (ICS-3000) system fitted with a Carbopac PA1 column as previously described by Ewanick and Bura [9]. Ethanol, xylitol, acetic acid, furfural, and hydroxymethylfurfural (HMF) were measured using a Shimadzu (Columbia, MD, USA) Prominence LC HPLC system fitted with a Rezex RHM H+ (Phenomenex; Torrance, CA, USA) column [9]. Calculation of standard curves and standard deviation was done in Microsoft Excel. RESULTS AND DISCUSSION

Chemical composition of pulp, sludge, and SSL Table I shows the chemical composition of pulp and sludge in terms of polysaccharides, lignin, and ash. Both solid streams were mainly made up of glucan (pulp 88.6%, sludge 70.2%),

Glucan (%)

Xylan (%)

Mannan (%)

AIL1 (%)

ASL2 (%)

Ash (%)

Pulp

88.6

2.8

6.6

3.3

1.6

0.2

Sludge

70.2

3.1

4.4

12.3

0.7

9.9

1 2

AIL = acid insoluble lignin. ASL = acid soluble lignin.

I. Percent composition of solid streams based on TAPPI T 222 om-98 “Acid insoluble lignin in wood and pulp.” AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

29

BIOREFINERY Ara (g/L)

Gal (g/L)

Glu (g/L)

Xyl (g/L)

Man (g/L)

Total 5-C Sugars (g/L)

Total 6-C Sugars (g/L)

Monomer

1.2

3.4

5.1

7.6

14.0

8.8

22.5

Oligomer

0.11

0.40

0.92

0.04

3.86

0.15

5.18

Monomer % of total

91.5

98.6

84.7

99.5

78.4

98.4

81.3

1

Acetic acid 5 g/L, HMF 0.11 g/L, furfural 0.16 g/L.

II. Sugar composition of SSL, oligomer percent of total based on NREL LAP TP-510-42623 “Determination of sugars, byproducts, and degradation products in liquid fraction process samples” and concentration of fermentation inhibitors. 1 (Ara = arabinose; Gal = galactose; Glu = glucose; Xyl = xylose; Man = mannose).

contained a small amount of acid insoluble lignin (pulp 3.3%, sludge 12.3%), and very minimal acid soluble lignin (pulp 1.6%, sludge 0.7%). Pulp contained more glucan and less total lignin than sludge, an expected outcome of the pulping process. Mannan was the next most abundant polysaccharide (pulp 6.6%, sludge 4.4%) for both streams due to their derivation from mostly softwoods. As expected, neither stream contained a detectable amount of arabinan or galactan. Sludge contained significantly more ash than pulp (sludge 9.9%, pulp 0.2%), a result of being partially sourced from boiler house effluent. Table I I shows the monomeric and oligomeric sugar composition of SSL. Because softwood hemicellulose is mainly made up of galactoglucomannans and arabino-4-O-methylglucuronoxylans [11], the most abundant sugar was mannose (14.0 g/L monomers), followed by xylose (7.6 g/L monomers). Glucose (5.1 g/L) was the next most abundant sugar in SSL, followed by minimal amounts of galactose (3.4 g/L) and arabinose (1.2 g/L). All five sugars were mostly in monomeric form (83%), making SSL a readily available mixed-monosaccharide source (Table II). Acetic acid, 5-hydroxymethylfurfural (5HMF), and furfurals were also present in SSL at concentrations of 5.0, 0.11, and 0.16 g/L, respectively. An unevaporated SSL stream containing 31.3 g/L total sugar was used for this study; however, this can be concentrated onsite up to 97 g/L total sugar if a higher sugar concentration is desired [8]. The sugar composition of SSL was similar to that of softwood SSL fermented to ethanol by Helle et al. [6]. Due to its high glucan content and minimal lignin and ash, sulfite pulp, if successfully hydrolyzed, can be expected to produce a clean, single-sugar stream (glucose). Sludge may produce similar results, though its higher lignin content would be expected to reduce hydrolysis yields compared with pulp. Because SSL has higher concentrations of both hexoses and pentoses, it is already an excellent source of mixed sugars, most of which are in monomeric form.

Fermentation of SSL (mixed-sugar stream) The high concentration of monomeric sugars in SSL was demonstrated through its compositional analysis; it was next used as a mixed-sugar stream for ethanol and xylitol production to test the feasibility of its use as a feedstock for multiple 30

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

(a)

(b)

3. (a) Fermentation of spent SSL to ethanol and xylitol using 5 g/L C. guilliermondii at 30°C. (b) Fermentation of SSL, spiked with synthetic xylose, to ethanol and xylitol using 5 g/L C. guilliermondii at 30°C. (Ara = arabinose; Gal = galactose; Glu = glucose; Xyl = xylose; Man = mannose; AA = acetic acid; EtOH = ethanol; XOH = xylitol).

product fermentation. The theoretical yield of microbial ethanol fermentation is 51% of starting glucose mass, while the theoretical yield for xylose to xylitol fermentation is 91% [12]. F i g u r e 3 shows fermentation of SSL using C. guilliermondii, which ferments hexoses to ethanol and xylose to xylitol. Glucose was consumed after 21 h, followed by mannose and galactose at 33 h. Xylose was fully consumed

BIOREFINERY

4. High consistency (10% w/v) hydrolysis of pulp and sludge in both water and spent sulfite liquor with 5 FPU/g cellulose enzyme loading.

after 72 h, and ethanol and xylitol were produced concurrently. Ethanol concentration peaked at 8.8 g/L after 24 h, which corresponded to 81.6% of the theoretical yield. Xylitol yield reached 2.13 g/L after 72 h, or 37.1% of the theoretical maximum. In the fermentation shown in Fig. 4 , the original SSL, which has low native xylose content (7.6 g/L), was spiked with synthetic xylose to demonstrate the full potential of xylose to xylitol fermentation using C. guilliermondii. Glucose was consumed after 10 h, followed by mannose and galactose at 54 h. Complete consumption of approximately 30 g/L xylose required 120 h. Ethanol concentration reached 9.5 g/L after 72 h, corresponding to 89.5% of the theoretical yield. Xylitol yield was 10.5 g/L after 96 h, or 40.3% of the theoretical yield. The sequence of sugar consumption observed in both fermentations was consistent with that of Tembec T1 (Tembec Inc.) and a galactose-assimilating S. cerevisiae isolate (Y-1528) used by Keating et al. [13] during mixed synthetic sugar fermentation. Ethanol in both instances was within or above the range reported for fermentation of untreated SSL by genetically modified S. cerevisiae 259ST, which was 64%–84% [14]. It was consistent with the highest yields achieved by Helle et al. [6] using S. cerevisiae strain Tembec T2 (Tembec Inc.) (85%) and 259ST (90%) to ferment detoxified SSL [6]. Also, ethanol yields from spiked and unspiked SSL were higher than that of a synthetic sugar mixture run concurrently as a control (77%, unpublished data). This may be explained by the presence of inhibitors in SSL, acetic acid in particular, which in low concentrations have been reported to enhance fermentation by S. cerevisiae [15]. Xylitol yields for both SSL fermentations were lower than that of a mixed synthetic sugar control (47%, unpublished data), an expected result, as mixed 5- and 6-C sugar fermentation with C. guilliermondii has previously been reported to produce lower xylitol yields than single-sugar fermentation [16]. Xylitol yields may improve by propagating yeast in xylose

media, or fermenting from SSL streams with a higher xylose:glucose ratio if xylitol is the only desired product [17]. In addition, 43% and 73% of the initial acetic acid was consumed in original and spiked SSL, respectively, demonstrating that C. guilliermondii can concurrently detoxify SSL during fermentation (Figs. 3 and 4). Acetic acid has been found to be inhibitory to yeasts due to its diffusion across the cell membrane, which lowers the pH within the cytoplasm and affects enzymatic activity [18]. It has also been found that the presence of acetic acid may have a greater effect on xylose to xylitol conversion than that of HMF or furfural [19]. Concentrations of HMF and furfural were initially low and completely consumed after 3 h. These observed yields and minimal interference by inhibitory compounds demonstrate the use of SSL as an excellent mixed-sugar stream for producing higher-value bioproducts, exemplified by ethanol and xylitol.

SHF of pulp and sludge (single- and mixed-sugar streams) The compositional analysis of pulp and sludge were shown to contain high amounts of glucan, so the hydrolyzability of these two solid streams was examined next to determine the feasibility of their conversion to glucose streams, via hydrolysis in water, and mixed-sugar streams, via hydrolysis in SSL (Fig. 2). Preliminary hydrolysis work was done by first varying the solids consistency, then the enzyme loading, to determine the most desirable conditions for SHF, based on yield. Solids were enzymatically hydrolyzed at 2%, 10%, and 15% (w/v) consistency and 10 FPU/g cellulose enzyme loading. In both cases, hydrolysis at 10% consistency yielded high glucose concentrations (87 g/L for pulp, 62 g/L for sludge; data not shown) after 24 h. Next, enzyme loadings of 1, 2, 5, 10, and 15 FPU/g cellulose were used to hydrolyze 2% (w/v) consistency slurries of each solid. Here, 5 FPU/g cellulose resulted in reasonably high glucose yields (18 g/L for pulp, 14 g/L for sludge) after 24 h without sacrificing enzyme cost. Therefore, SHF experiments were all performed at 10% (w/v) solids consistency and 5 FPU/g cellulose enzyme loading. Pulp and sludge were each hydrolyzed in both water and in SSL with pH adjusted to 4.8. Hydrolysis in SSL was done to demonstrate a method of boosting the amount of sugar available for bioproduct conversion. Also, these two streams were combined to increase the quantity of total feedstock, as the volume of each stream individually was too low to capture economic feasibility. Fig ur e 5 shows the hydrolysis of pulp and sludge in water and SSL. Hydrolysis of pulp in SSL resulted in 49% cellulose to glucose conversion (48.5 g/L), compared with hydrolysis of pulp in water, where 80% cellulose to glucose conversion was achieved, corresponding to 78.8 g/L glucose (Fig. 5, Table I I I ). Conversely, hydrolysis of sludge in SSL yielded 52.4% cellulose to glucose conversion (40.8 g/L), compared with 43% (33.5 g/L) for sludge in water. The seemingly low cellulose to glucose conversion rates were due to the use of relatively high solids consistency (10% w/v), in combination with low enzyme loading (5 FPU/g cellulose). AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

31

BIOREFINERY

5. Fermentation of pulp hydrolysates using 5 g/L S. cerevisiae at 30°C (EtOH = ethanol).

For sludge, the addition of a liquid sugar stream did not appear to have an effect on its hydrolysis, but the same was not found for pulp. With water and with SSL, pulp hydrolyzed more completely than did sludge, an expected result due to the higher lignin and ash content of sludge, which has been found by Kang et al. [5] to inhibit hydrolysis enzymes. In SSL, however, sludge hydrolysis was not inhibited, while pulp hydrolyzed more completely in water. Previously, Moritz and Duff [4] reported that sulfite sludge hydrolyzed more rapidly in 100% SSL, at a rate of 0.60 g/L/h, compared with SSL diluted to 50% with water, at 0.36 g/L/h. Helle et al. [20] found that SSL inhibited cellulase enzyme activity during hydrolysis of knot rejects combined with SSL. The observed result for pulp supported these previously published findings regarding SSL inhibition of fermentation. Pulp hydrolyzed more completely in the absence of SSL; however, the result for sludge was to the contrary. One possible explanation for this difference is that endproduct inhibition may have had a greater effect on pulp due to its higher glucan content and greater ease of hydrolyzation. Varga et al. [21] found that product inhibition began to occur

at 15 g/L glucose and some glucose was already present in SSL added to the solid streams. A possible explanation for these results is that the combined stream of pulp in SSL may have reached the point of inhibition more rapidly than sludge in SSL due to pulp’s greater susceptibility to enzyme attack. The seemingly null effect of SSL on the hydrolysis of sludge may be explained by interactions between the ash in sludge and compounds in SSL, though this hypothesis has yet to be tested. Kang et al. [5] reported a buffering action of the CaCO3 (calcium carbonate) present in the ash of kraft mill sludge, which made pH control difficult. This effect was neutralized through the addition of acetic acid [5]. One possible explanation for the observed result is that the presence of acetic acid in SSL may have mitigated the negative effect of sludge ash on hydrolysis. Fermentation of hydrolysates generated by the four streams used previously was performed to demonstrate the possibility of converting hydrolyzed single- and mixed-sugar streams to high-value products, exemplified by ethanol. For solid feedstocks mixed with SSL, hydrolysis data were corrected to exclude the monomeric sugars in SSL and any hydrolysis of its oligomeric sugars. Fig ur e 6 shows the fermentation of pulp hydrolysates by S. cerevisiae, which uses hexoses for ethanol production. For pulp in water, glucose was fully consumed after 6 h. For pulp in SSL, total hexose concentration dropped to 1.7 g/L after 6 h but was never fully exhausted due to incomplete consumption of galactose (Fig. 6). Ethanol yield as a percentage of theoretical yields was 77.8% (28.3 g/L) for pulp in water and 76.5% (19.9 g/L) for pulp in SSL, similar to that of a concurrently fermented mixed-hexose control (13.0 g/L, 78.2%, data not shown). Fig ur e 7 shows the fermentation of sludge hydrolysates. For sludge in water, glucose was fully consumed after 4 h. Hexoses were maximally consumed after 6 h for sludge in SSL. Again, galactose was never fully consumed. Ethanol yields of the two sludge hydrolysates as a percentage of the theoretical maximum were similar, 76.2% (10.9 g/L) for sludge in water and 73.1% (18.6 g/L) for sludge in SSL, and again were close to those of the synthetic sugar control (Fig. 7). These results were comparable to those of Helle et al. [20], where consistent ethanol yields of around 80% of the Pulp/H2O

Pulp/SSL

Sludge/H2O

Sludge/SSL

Hydrolysis

Glucose concentration (g/L)

78.8

48.5

33.5

40.8

Fermentation

Ethanol concentration (g/L)

28.3

19.9

10.9

18.6

SHF

Original biomass to ethanol yield (%)

62.2%

37.7%

32.8%

38.3%

SSF

Original biomass to ethanol yield (%) Ethanol concentration (g/L)

56.1%

50.0%

31.9

23.7

Glucose concentrations for mixed feedstock hydrolysis (pulp or sludge with SSL) are corrected to exclude the monomeric sugars in SSL and any hydrolysis of its oligomers. Total process yields for SHF and SSF were calculated as hydrolysis (%) yield multiplied by fermentation (%) yield.

1

III. Glucose and ethanol yields shown in concentration (g/L), and percentage of theoretical maximum ethanol yield for SHF and SSF.1 32

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

BIOREFINERY SSF of pulp and sludge

6. Fermentation of sludge hydrolysates using 5 g/L S. cerevisiae at 30°C (EtOH = ethanol).

7. SSF of SSL fortified with pulp and sludge using 5 FPU/g cellulose enzyme loading and 5 g/L S. cerevisiae at 37°C (EtOH = ethanol).

theoretical maximum were observed using a naturally occurring S. cerevisiae strain (259A) on hydrolyzed softwood reject knots, with and without SSL mixed in. Fermentation yields were similar for pulp in both cases, but a much higher hydrolysis yield for pulp in water contributed to a significantly higher total SHF process yield and 8.4 g/L more of ethanol, as compared with pulp in SSL (Table III). Conversely, for sludge, final ethanol concentration was 7.7 g/L greater for fermentation of sludge in SSL compared with sludge in water, due to the presence of additional sugars in the liquid stream. Fermentation yields were consistent between all four hydrolysates and the hexose control, demonstrating that fermentation did not account for differences in final ethanol concentration. Instead, ethanol yield is more likely to be improved by (1) improving cellulose to glucose conversion during hydrolysis and (2) increasing the initial availability of sugars by, in some cases, mixing solid and liquid sugar streams.

Simultaneous saccharification and fermentation of SSL fortified with pulp and sludge was investigated as another means of combining low-volume streams to produce mixed sugars for bioconversion. As previously discussed, end-product inhibition may have limited the hydrolyzability of solids, and of pulp in particular. Consequently, this could have had a negative effect on fermentation yields due to a lower starting sugar concentration. To benefit from the additional sugars made available by mixing solid and liquid feedstocks, SSF may be advantageous due to the continuous removal of sugars as they are being generated, mitigating the effect of end-product inhibition. What is not known is whether the benefit brought on by minimizing end-product inhibition can overcome the apparent inhibitory effect of SSL on hydrolysis reported in previous research [4, 20, 22]. Figure 7 shows SSF of both pulp and sludge in SSL in terms of sugar consumption and ethanol production by S. cerevisiae. Simultaneous saccharification and fermentation of pulp in SSL produced 56.1% (31.9 g/L) of the theoretical ethanol yield, and sludge in SSL produced 50% (23.7 g/L). Total process yield as a percentage of the theoretical maximum improved dramatically in comparison to SHF of the same mixed streams, from 37.7% for pulp in SSL and 38.3% for sludge in SSL (Table III). Moritz and Duff [4] reported a similar improvement in ethanol yield during SSF when sludge was mixed with SSL but noted that a longer residence time was required, because all hexoses in SSL had to be consumed before significant hydrolysis could occur. We did not observe this in our study, possibly because a more dilute batch of SSL was used. The reproducibility of fermentation yields by the strain of S. cerevisiae used in this study was demonstrated in the fermentation data associated with SHF (Figs. 5 and 6). Fermentation yields ranged only from 73.1% to 77.8% (Table III). Because the reproducibility of S. cerevisiae fermentation of these sugar streams has already been demonstrated, the increase in ethanol yield witnessed during SSF can only be attributed to improvements in the hydrolysis step. Thus, it appeared that the improvement in hydrolysis brought on by minimizing end-product inhibition was greater than the inhibitory effect of SSL addition. Simultaneous saccharification and fermentation of mixed solid and liquid feedstocks can be a viable option for generating mixed sugars from low-volume streams generated at the sulfite mill. For pulp, mixing with SSL and doing SSF appeared to detrimentally affect hydrolysis, compared with SHF results on the same stream. However, mixing sludge with SSL and performing SSF appeared to beneficially affect hydrolysis. CONCLUSION Single- and mixed-sugar streams were generated from sulfite pulp, sludge, and SSL in this study. Due to its composition of both hexoses and pentoses in mostly monomeric form, SSL was shown to be an excellent mixed-sugar stream. Fermentation of SSL with C. guilliermondii produced high AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

33

BIOREFINERY ethanol yields (89.5%), and xylitol yields consistent with that of a synthetic sugar control (40.3%). Pulp proved to be an excellent feedstock for single-sugar generation by producing a clean glucose stream conducive to fermentation with hexose metabolizing organisms. SHF of pulp in water resulted in the highest overall ethanol yield during fermentation for that feedstock (62.2%, 28.3 g/L) during fermentation with S. cerevisiae (Table III). Mixing pulp with SSL during SHF severely decreased yields (37.7%, 19.9 g/L). Conversely, and unexpectedly, the best use of sludge found in this study, in terms of yield, was as a supplement to the mixed-sugar stream available from SSL. Mixing sludge with SSL during SSF produced the highest ethanol yield (50.0%, 23.7 g/L) during S. cerevisiae fermentation (Table III). For mixed feedstocks, SSF improved yields for both pulp and sludge. Fermentation yields were consistent across all streams, demonstrating that hydrolysis, not fermentation, is the rate-limiting step of SHF and SSF. TJ ACKNOWLEDGEMENTS The authors thank Doug Asbe, Wally Sande, and Jeff Ross of Kimberly-Clark for providing the feedstocks used in this study, and for their valuable expertise; Shannon Ewanick, Azra Vajzovic, and Robert Lin for their assistance with laboratory work; and Kimberly-Clark and NSF-IGERT for providing funding for this project.

LITERATURE CITED 1. Clark, J. and Deswarte, F., Introduction to Chemicals from Biomass, Wiley, West Sussex, 2008. 2. IEA Task 39. Commercializing Liquid Biofuels from Biomass. Available [online] http://www.task39.org/. 3. Rydholm, S.A., Pulping Processes, Krieger, Malabar, FL, USA, 1985. 4. Moritz, J. and Duff, S., Appl. Biochem. Biotechnol. 57–58(1): 689(1996). 5. Kang, L., Wang, W., and Lee, Y., Appl. Biochem. Biotechnol. 161(1): 53(2010). 6. Helle, S.S., Lin, T., and Duff, S.J.B., Enzyme Microb. Technol. 42(3): 259(2008). 7. Zhang, J. and Lynd, L.R., Biotechnol. Bioeng. 107(2): 235(2010). 8. Sande, W., personal communication. 9. Ewanick, S. and Bura, R., Bioresour. Technol. 102(3): 2651(2011). 10. Linden, T., Peetre, J., and Hahn-Hägerdal, B., Appl. Environ. Microbiol. 58(5): 1661(1992). 11. Fengel, D. and Wegener, G., Wood: Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin, 1989. 12. Barbosa, M.F.S., de Medeiros, M.B., de Mancilha, I.M., et al., J. Ind. Microbiol. Biotechnol. 3(4): 241(1988). 13. Keating, J.D., Robinson, J., Cotta, M.A., et al., J. Ind. Microbiol. Biotechnol. 31(5): 235(2004). 14. Helle, S.S., Murray, A., Lam, J., et al., Bioresour. Technol. 92(2): 163(2004). 15. Keating, J.D., Panganiban, C., and Mansfield, S.D., Biotechnol. Bioeng. 93(6): 1196(2006).

ABOUT THE AUTHORS The current lack of an abundant and inexpensive feedstock is one of the biggest bottlenecks impeding the widespread commercialization of lignocellulose-based biofuels. In this paper, we examined the use of novel feedstocks that are already being produced in sulfite mills: pulp, sludge, and spent sulfite liquor (SSL). While ethanol production from similar pulp mill product streams has been studied, combining solid and liquid feedstocks has not been well examined; for example, fortifying SSL with sludge. Also, the production of single and mixed stream sugars for co-product fermentation from sulfite mill product streams has not been well examined. It was known from the beginning that, due to the absorption properties of pulp and sludge, sufficient mixing of these solid feedstocks in solution during hydrolysis would be difficult to achieve. In order to accomplish this, various solids consistencies were tested, and a particular consistency was chosen that allowed for both sufficient mixing and high sugar concentration following hydrolysis. The most surprising finding from this research was that, despite the presence of fermentation inhibitors in SSL, the hydrolysability of sludge was not affected 34

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

by the addition of SSL during simultaneous saccharification and fermentation (SSF). Mills may use this information to deLai Bura cide how to utilize their excess product streams to produce biofuels and other valuable bioproducts. Our next step will involve a techno-economic analysis of biochemical production from sulfite mill product streams. Lai is a graduate research assistant, Department of Plant Biology, University of Illinois, Urbana, IL, USA. Bura is assistant professor in natural products chemistry and Denman Professor in bioresource science and engineering, School Environmental and Forest Sciences, College of Environment, University of Washington, Seattle, WA, USA. Email Bura at [email protected].

BIOREFINERY 16. Lee, H., Sopher, C.R., and Yau, K.Y.F., J. Chem. Technol. Biotechnol. 65(4): 375(1996). 17. da Silva, D.D.V. and de Almeida Felipe, M.d.G., J. Chem. Technol. Biotechnol. 81(7): 1294(2006). 18. Palmqvist, E. and Hahn-Hägerdal, B., Bioresour. Technol. 74(1): 25(2000). 19. Vajzovic, A., “Novel approach to improve xylose to xylitol biological conversion by Candida guilliermondii and Rhodotorula mucilaginosa,” Symp. Biotechnology for Fuels and Chemicals, 33rd , Seattle, WA, USA, 2011, Poster Session 1-26. 20. Helle, S.S., Petretta, R.A., and Duff, S.J.B., Enzyme Microb. Technol. 41(1–2): 44(2007). 21. Varga, E., Szengyel, Z., and Réczey, K., Appl. Biochem. Biotechnol. 98-100(1): 73(2002). 22. Smith, M.T., Cameron, D.R., and Duff, S.J.B., J. Ind. Microbiol. Biotechnol. 18(1): 18(1997).

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IMERYS PIGMENTS FOR PAPER & PACKAGING

THE SHAPE OF THINGS TO COME

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Experts in Biomass Supply and Demand Needed A message from the leadership of the TAPPI Biomass Supply, Demand & Handling Subcommittee Regardless of the cellulosic biorefinery platform, the success of all cellulosic biorefineries will be dependent on a steady and sustainable supply of and reasonable cost for, well cleaned, cellulosic biomass delivered to the biorefinery. The TAPPI Biomass Supply, Demand & Handling Subcommittee was formed in early 2011 to focus on various aspects of the biomass supply chain. The intent of this subcommittee is to provide a venue for interested parties to have a free exchange of state-of-the-art technical ideas on these important topics. We have identified a few subcommittee projects, and are looking for volunteers to join the subcommittee and assist in working on these or other projects. If you are interested in joining this subcommittee, please contact Mary Ann Cauthen ([email protected]) at TAPPI.

BIOMASS SUPPLY SHORT COURSE – This course would address issues in cost-effective delivery and preparation of biomass materials (wood and non-wood).  Bob Hurter, Bill Fuller, and Desmond Smith are currently working on educational outcomes and identifying speakers.

STANDARDS FOR BIOMASS SAMPLING AND TESTING METHODS – We are looking for

1-2 individuals to head up this area, plus others to contribute.  This group would first work to identify needs and evaluate gaps between existing standards and identified needs.  A survey of needs has been suggested as a first step.  If you are interested in this area, or know someone that would be, please encourage them to contact Mary Ann Cauthen ([email protected]) and 1) join this subcommittee if not currently a member, and 2) indicate their interest to work on this project.

LINKEDIN GROUP – We are also investigating a TAPPI LinkedIn group for biomass supply, demand, and handling issues.  If you are interested in helping on this project, please let Mary Ann Cauthen ([email protected]) know.

SEPTEMBER 19TH SUBCOMMITTEE WEBINAR – We are scheduling a webinar to update the

subcommittee on efforts in these areas.  Please plan to attend on Wednesday, September 19th at 1:00 – 2:00 PM.  Subcommittee Chair - Bob Hurter, HurterConsult Subcommittee Co-Chair – Bill Fuller, FRM Consulting

PEER-REVIEWED

PAPER PHYSICS

Fiber interaction with a forming fabric JINGMEI LI

and

SHELDON I. GREEN

ABSTRACT: During sheet forming, the structure of the forming fabric leaves wire marks on the pulp mat. Paper nonuniformity caused by the wire mark can lead to ink nonuniformity in printing. We investigated wire mark numerically through simulations of the interaction of individual fibers with a forming fabric. In the simulations, the flow field through the forming fabric was taken to be that of single-phase water flow without disturbance of fibers. A particle level simulation method was applied to simulate the motion of fibers in the flow through a single layer sine-wave fabric. A hundred fibers of random initial distribution were placed into the flow above the fabric. Those fibers were advected onto the fabric, forming a fiber mat. The surface roughness of the resulting fiber mat was then calculated. The results show that during the initial formation, topographic wire mark is caused partially by fiber bending and partially by the geometry of the fabric. For the specific fibers and sinusoidal forming fabric considered, more than 50% of topographic wire mark is the result of geometry, with the remainder attributed to fiber bending. Fabrics with different geometries (e.g., different filament pitches or a nonsinusoidal geometry) will have different relative influences from geometry and fiber bending.

Application: This paper provides a theoretical basis for understanding topographic wire mark.

W

ire mark is present on the wire side of the paper web and replicates to some degree the geometry of the forming fabric. It is especially noticeable in the surface topography and light transmittance characteristics of paper [1]. In the finished paper, wire mark may be either topographic (a three-dimensional paper surface caused by the knuckles and holes of the fabric) or hydrodynamic (nonuniform mass distribution in the sheet caused by the nonuniform mass flux through the fabric). Wire mark is known to affect the printing quality of paper [2,3] and is of interest for this reason. In this study, we mainly focus on the mechanism of topographic wire marking. Wire mark can be characterized as a very weak systematic variation in the distribution and orientation of structural elements in paper [4]. Most research on wire mark and paper structure has used image analysis. Several image acquisition techniques have been used to assess printing paper quality, including desktop scanners, profilometry, X-ray microtomography [5,6], and scanning electron microscopy [7,8]. Danby [2,3] studied the effect of forming fabric structure on sheet formation and wire mark. He concluded that density variations on the sheet surface, resulting from the knuckles of the yarns in the forming fabrics, are the main cause of poor printing quality. He showed that the knuckles in a fabric create light areas in the printed sheet, and areas in the sheet that print darkly correspond to the holes in the fabrics. Helle [9] studied the initial paper web formation on a theoretical basis. The optimum geometric shape of the “frames” formed between crossing strands in the forming surface of the wire was calculated based on beam theory. Vakil et al. [10] studied hydrodynamic wire mark. They simulated the flow through forming fabrics and averaged the velocity field over areas comparable to the projected area of a fiber; the resulting average is related

to the amount of fiber that should accumulate in different regions of the fabric during the initial stages of dewatering. Several parameters exist to characterize the roughness of a three-dimensional surface. The quadratic mean of the deviation from the mean Sq is one [8,11]:

(1) where ha = the average height of the paper surface, and h (m,n) = the surface height at (m,n). To study how fiber interacts with a forming fabric, the motion of flexible fibers must be simulated. Because of the inherent complexity of flexible particles, numerical simulations have been applied to study the motion of such particles in a flow field. One such numerical approach is particle-level simulation [12-17]. In particle-level simulations, flexible particles are modeled as a chain of rigid particles connected by flexible links. Yamamoto and Matsuoka [12,13] modeled the flexible particles as chains of interconnected spherical beads linked by springs, interacting with a prescribed fluid flow through viscous drag forces. Ross and Klingenberg [14] modeled fibers as a chain of spheroids connected by ball and socket joints, which reduced the computational time by eliminating the iterative constraints for connectivity of segments. In those works, fiber motions in a free flow field were simulated. Lindstrom and Uesata [16,17] simulated a semidilute suspension of fibers in shear flow. The contact force between fibers was prescribed based on the velocity and position of fibers in contact. Vakil and Green [18] studied fiber interaction with a cylinder in two dimensions. They determined the region of fiber “hang up” on the cylinder as a function of several dimensionless numbers. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

39

PAPER PHYSICS In all these works, the creeping flow assumption was used to determine the hydrodynamic force on the fiber. In this paper, we study topographic wire mark by investigating fiber interaction with a forming fabric. Particle-level simulations have been used to describe the motion of flexible fibers through the fabric. Unlike the previous works, we did not use the creeping flow assumption. Rather, we calculated hydrodynamic force based on the lift and drag coefficient of cylindrical sections in a flow field. One-way coupling of fiber and flow was considered; the flow field was assumed to be the single phase flow through the fabric, and that fibers were assumed not to interact with one another. A sine-wave (2 × 2 shed) single-layer forming fabric was used in the simulations. Simulations of the flow through the fabric were conducted using ANSYS Fluent (Canonsburg, PA, USA) software. Single fibers with different orientations and positions were released in the flow upstream of the forming fabric; their deposition on the fabric surface created a simulated lightweight fiber mat. The surface roughness of the simulated fiber mat, which is related to the wire mark on the fiber mat, was then calculated as a function of the fiber rigidity, fiber length, and other parameters. METHODS In the particle-level simulation method, the fiber is modeled as a chain of particles connected by flexible joints (Fig. 1). For segment i, ri is the vector pointing to its center, and ci is the vector along its axis. The governing equations of motion for the fiber include kinematic equations and dynamic equations. For each segment in the fiber, connectivity, linear momentum, and angular momentum equations were calculated. (Details of the governing equations are shown in the Appendix). By some manipulation of the governing equations, the translational velocity and angular velocity for each segment can be solved directly by the following equations:

(2) (3) where ωi is the angular velocity of segment i, and r i is the translational velocity of segment i. F(c) is the contact force, and T(c) is the contact torque. N1, N2, N3, N4, M1, M2, and M3 are matrices associated with the position and properties of the particles, as well as the velocity and properties of the flow field (see Appendix). When the fiber moves in a nonconstrained flow, the contact force is zero. In this case, the equations can be simplified as follows:

(4)

40

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

1. Schematic of a discretized fiber and neighboring segments i and i+1.

2. Schematic of a fiber in contact with a forming fabric.

(5) When in contact, a forming fabric usually provides multiple support points over the length of the fiber (Fig. 2 ). Because of the hydrodynamic force, the contact force normal to the contact surface is high compared with the tangential force, so the tangential force cannot overcome the frictional force and make the fiber slide. Therefore, when the fiber is in contact with the forming fabric, the segments in touch with the surface are assumed to be nonsliding. In this case, the contact forces are added to the equations as unknown variables, and the segments in touch with the surface with zero velocity are added as boundary conditions. The hydrodynamic force of a spheroid in a creeping flow is a linear function of the relative velocity of the particle. In our simulations, the Reynolds number of the flow was much higher than that for creeping flow. Therefore, creeping flow equations were not suitable in these cases. Vakil and Green [19] simulated the flow field around a cylinder and calculated the hydrodynamic force on the cylinder at different angles and aspect ratios. A series of formulas for drag and lift coefficients at different conditions were obtained by curve-fitting the simulation results. The hydrodynamic forces at a moder-

PAPER PHYSICS ate Reynolds number were calculated using the following equations, based on the drag coefficient CD and lift coefficient CL:

(6) (7)



(8) (9)



(11)

The deflection of beams under uniform force was calculated in simulations. The beam was modeled as a number of rigid particles connected with flexible joints. The force was applied to the center of each segment. Fig ur e 3 shows that as the number of segments increases, the numerical results approach the theoretical results. The numerically computed beam deflection was within 3% of the small deflection beam theory result when the number of segments was 20 or more.

Flow through the forming fabric Fig ur e 4 shows the sine-wave single layer forming fabric geometry that we studied. The diameter of the filaments, d,

where d is the unit directional vector along the axis of the cylinder; δ is an identity matrix of size 3; U∞ and U are the velocity of the flow and the particle, respectively; Ur is the magnitude of the relative velocity of the particle; L and D are the length and the diameter of cylinder; and θ is the angle between the axis of cylinder and its relative velocity vector. RESULTS AND DISCUSSION

Code validation C++ code has been written to simulate the motions of a fiber and its interaction with the forming fabric. The code was verified by comparison with two analytical results. Jeffery [20] studied the equations of motion of a neutrally buoyant ellipsoid suspended in a flow field, where the inertia force was neglected. According to Jeffery’s theory, the axis of an isolated, rigid, neutrally buoyant ellipsoid in a uniform simple shear flow at low Reynolds number moves on one of a family of closed periodic orbits, the center of the particle moving with the velocity of the undisturbed fluid at that point. The dimensionless period of rotation of the ellipsoid in a simple shear flow depends merely on the aspect ratio of the particle (Eq. [10]):



3. Comparison of simulation and theory for the maximum deflection of a cantilever beam.

(10)

. where T is the period of rotation, γ is the shear rate, and rp is the aspect ratio of the ellipsoid. Numerical simulations of a spheroid motion in a shear flow were conducted to compare with Jeffery’s equation. The hydrodynamic force for creeping flow conditions was used in the simulations. The periods of rotation calculated based on simulations and Eq. (10) were compared. The numerical results matched the theoretical solution well, with a difference of less than 0.03%. Small deflection beam theory describes the deflection of beams under a small force. For a cantilever beam with uniform load q, the maximum deflection ω is a function of the beam flexural rigidity EI and beam length L and q:

4. The geometry of the sine-wave forming fabric. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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PAPER PHYSICS

6. Z-velocity contour in a plane d/4 above the forming fabric.

5. The computational domain and boundary conditions.

is 150 μm, and the gap between two filaments is 200 μm. We also studied a second sine-wave fabric with twice the gap (400 µm). The flow field through the forming fabric was simulated using Fluent software. Fig ur e 5 shows the computational domain and boundary conditions. The computational fluid dynamics mesh was generated in ANSYS Gambit (Canonsburg, PA, USA) and imported into ANSYS Fluent. Fig ur e 6 shows the z-velocity contour in a plane d/4 upstream of the forming fabric. In this figure, the bulk velocity through the fabric is 0.5 m/s. The flow field data was imported into the written code to simulate the motion of fibers in the vicinity of the fabric.

Individual fibers interacting with the fabric In the simulations, a single fiber with a given initial position was released into the flow upstream of the forming fabric. Different initial positions, X0, and orientations, θ, were considered (Fig. 7 ). Unless otherwise stated, fibers used in the simulations had lengths of 1 mm and diameters of 40 μm. Previous researchers have measured the bending stiffness, EI, of pulp fibers to be between 10 -10 and 10 -13 Nm2 [21-23]. For this reason, we considered bending stiffnesses in this range (and as small as 10 -14 Nm2) in the simulations. After the fiber is released, it rests atop the forming fabric. From the fiber geometry, the average height of the fiber can be computed, measured relative to the plane flush with the top of the fabric. Fig ur e 8 shows that the average height of the fiber did not vary much with EI; the three curves for different EI have similar trends. Therefore, the geometry of the forming fabric is primarily responsible for the fiber mat topography. The standard deviation of fiber height decreases as the 42

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

7. A schematic of the fiber position.

fiber becomes more rigid (Fig. 9 ). For fibers centered on one fabric knuckle (i.e., Xo = 0), but with variable θ, because the height in the center is fixed, the average height of fibers varies substantially with EI and θ, except for very rigid fibers (Figs. 10 and 11). The cases shown in Fig. 8 and Fig. 10, respectively, are situations in which the geometry of the forming fabric and fiber bending contribute most to the change of the fiber average height. Therefore, in general, fiber bending and the geometry of the forming fabric are responsible for the topographical wire mark.

Lightweight fiber mat To understand topographical wire mark in papermaking, we studied the initial stage of sheet forming. Simulations were conducted to simulate a number of fibers deposited on the

PAPER PHYSICS

8. Average height of fibers at different initial positions (Xo).

9. Standard deviation of fiber height at different initial positions (Xo).

10. Average height of fibers at different orientations (θ).

11. Standard deviation of fiber height at different orientations (θ).

12. Standard deviation of the surface height of the fiber mat made from fibers with different lengths.

top of the forming fabric. Fibers with initially randomly distributed positions and orientations were simulated. The fibers move independently of other fibers. The surface roughness of the fiber mat is represented by the standard deviation of the surface height, given by Eq. (1). The standard deviations of the surface height of the fiber mat made from different numbers of fibers were compared. The standard deviation changes only slightly (2.5% difference) from 100 fibers to 200 fibers; therefore, in the simulations, 100 fibers give statistically robust values of mat surface roughness. Fibers with different lengths were simulated to study the effect of fiber length on surface roughness. The EI of each fiber was fixed at 10 -12 Nm2. Fig ur e 1 2 shows that as the fiber length increased, the value of standard deviation decreased. However, in actual papermaking, paper made from short fibers is smoother than paper made from longer AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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PAPER PHYSICS

13. Standard deviation of the surface height of the fiber mat made from fibers with different bending stiffness (EI).

14. Standard deviation of the surface height of the fiber mat made at different drainage velocity.

fibers. The reason for the difference might be that, in the simulations, only the initial formed fiber mat is calculated, without accounting for the pressing and drying sections. We believe that pressing might change the topography of short fiber mats more than long fiber mats because short fibers might be less constrained by other fibers in the mat. Furthermore, it is very difficult to vary just one pulp property, independent of other properties; thus, for example, fiber length is correlated with fiber stiffness, so the smooth paper made from short fibers might be a function of their different stiffness. Fiber mats made from fibers with different initial orientations (random and machine-direction oriented) were compared. The standard deviation did not change much between the two orientations (30.3 μm for randomly oriented and 33.0 μm for machine-direction oriented). For the sine-wave forming fabric, fiber orientation did not affect the surface smooth44

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

ness significantly. We believe this result would not apply for nonsymmetric forming fabrics. Fig ur e 13 shows the surface roughness of the fiber mat for randomly distributed fibers, as a function of the fiber bending stiffness. When EI was 10 -10 Nm2 or larger, the fiber was very rigid and the deflection of the fiber was negligible. Therefore, a standard deviation of surface height of 18 μm results solely from the geometry of the forming. Fibers used in papermaking have EI in the range of 10 -10 Nm2 to 10 -13 Nm2, for which the corresponding standard deviation of surface height varies from 18 μm to 37 μm. Thus, for the simple sine-wave forming fabric and realistic fibers, 50% or more of the surface roughness is caused by the geometry of the forming fabric. Fiber mats also were formed at different drainage velocities. In these simulations, the stiffness of fibers was fixed at 10 -12 Nm2. When the velocity increased from 0.1 m/s to 1 m/s, the standard deviation of the mat surface height increased (Fig. 14 ). The increased deviation is a consequence of the greater fiber bending, caused by the increased hydrodynamic force acting on fibers at higher flow velocities. CONCLUSION For this report, we studied topographical wire mark numerically. We simulated the physical interaction between the fibers, the flow field, and the forming fabric. The numerical methods used were verified by comparison with Jeffery’s equation and beam theory. The simplest fabric geometry, a sine-wave single layer fabric, was used in the simulations. We simulated the motion of a single fiber with different given initial positions and orientations. A lightweight fiber mat was formed in simulations from 100 fibers, which initially were randomly distributed on top of the forming fabric. The surface roughness of the fiber mat was calculated and compared as a function of fiber EI and length. For fixed EI, increasing the fiber length decreased pulp mat surface roughness, owing to the multiple support points over the length of each fiber. For fixed fiber length, increasing the EI decreased the surface roughness, owing to the reduced bending of fibers into fabric surface holes. Surface roughness also was calculated as a function of drainage velocity. As drainage velocity increased, so did fiber bending, and therefore, mat surface roughness increased. We concluded that topographical wire mark is caused partially by fiber bending and partially by the geometry of the fabric. For the simple sine-wave single layer fabric considered, more than 50% of topographical wire mark was attributed to the geometry of the forming fabric, and the remainder resulted from fiber bending. Because of the complexity of the fiber mat interaction with the forming fabric, numerous simplifications were made in the simulations, including uniform fibers, simple fabric geometry, negligible interaction between fibers, etc. We plan to follow up on this research by carrying out measurements of the topography of fiber mats, and comparing the surface roughness of the formed mat with numerical results. TJ

PAPER PHYSICS ACKNOWLEDGEMENTS The authors appreciate financial support received from AstenJohnson Inc. and the Natural Sciences and Engineering Research Council of Canada. LITERATURE CITED

where m is the mass of the segment, F(h) is the hydrodynamic force, F (c) is the contact force, Xa is the internal force at the joint a, S is the connectivity matrix, and N is the number of segments. The equation of angular momentum is:



1. Helle, T., J. Pulp Pap. Sci. 14(4): 91(1988).

(14)

2. Danby, R., Pulp Pap. Can. 87(8): 69(1996). 3. Danby, R., Pulp Pap. Can. 95(1): 48(1994). 4. Helle, T., EUCEPA Conf. Proc., 22nd , ATICELCA, Milan, 1986, p. 1. 5. Samuelsen, E.J., Gregersen, Ø., Houen, P., et al., J. Pulp Pap. Sci. 27(2): 50(1999). 6. Chinga, G., Johnsen, P.E.R.O., Dougherty, R., et al., J. Microsc. 227(3): 254(2007).

where T(h) is the hydrodynamic torque, T(c) is the contact torque, and c is connectivity vector. Υa is the internal torque at the joint.a, which includes the twisting torque and bending moment. H is the time derivative of angular momentum, and can be extended as:



7. Reme, P., Johnsen, P., and Helle, T., J. Pulp Pap. Sci. 28(4): 122(2002). 8. Ashori, A., Raverty, W.D., Vanderhoek, N., et al., Bioresour. Technol. 99(2): 404(2008). 9. Helle, T., TAPPI J. 71(1): 112(1998). 10. Vakil, A., Olyaei, A., and Green, S.I., Nord. Pulp Pap. Res. J. 24(3): 342(2009). 11. Vernhes, P., Bloch, J., Mercier, C., et al., Appl. Surf. Sci. 254(22): 7431(2008). 12. Yamamoto, S. and Matsuoka, T., J. Chem. Phys. 98(1): 644(1993).

(15)

where I is the tensor of inertia, and w is the angular velocity. The hydrodynamic force and hydrodynamic torque of a prolate spheroid in an unbounded creeping flow can be expressed as:



(16) (17)

13. Yamamoto, S. and Matsuoka, T., J. Chem. Phys. 102(5): 2254(1995). 14. Ross, R.F. and Klingenberg, D.J., J. Chem. Phys. 106(7): 2949(1997). 15. Schmid, C.F. and Klingenberg, D.J., Phys. Rev. Lett. 84(2): 290(2000). 16. Lindstrom, S.B. and Uesaka, T., Int. J. Eng. Sci. 46(9): 858(2008). 17. Lindstrom, S.B. and Uesaka, T., Phys. Fluids 19(11): 3307(2007).

In Eq. (17), i and j are the index of direction, and U∞, Ω∞, E∞ are the velocity, vorticity, and rate of strain of the ambient flow, respectively. The resistance tensors are defined by:

18. Vakil, A. and Green, S.I., Int. J. Multiphase Flow 37(2): 173(2011). 19. Vakil, A. and Green, S.I., Comput. Fluids 38(9): 1771(2009). 20. Jeffery, G., Proc. R. Soc. London, Ser. A 102: 161(1922). 21. Tam Doo, P.A. and Kerekes, R.J., Tappi J. 64(3): 113(1981) 23. Yan, D. and Li, K., J. Mater. Sci. 43(8): 7210(2008) 24. Ning, Z. and Melrose, J.R., J. Chem. Phys. 111(23): 10717(1999). 25. Kim, S., and Karrila, S.J., Microhydrodynamics, Principle and Selected Applications, Dover Publications, Mineola, NY, USA, 2005.

APPENDIX As shown in Fig. 1, if the fiber is inextensible, system connectivity for the fiber can be described as:

where X A, YA, XC, YC, and YH are only functions of eccentricity e, and d is the unit directional vector along major axis of the spheroid.



(13)

(20) (21)



(12)

In Eq. (12), r is the vector to the center of spheroids, and c is the vector along the major axis of spheroids. The equation of momentum for particle i is given as:



(19)



22. Yan, D. and Li, K., J. Mater. Sci. 43(8): 2869(2008).



(18)





(22)



(23)



(24) AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

45

PAPER PHYSICS (25)



(26) Here, a and b are semi-major axis and semi-minor axis of the spheroid. Y represents the internal resistance torque at joints, which includes torsional torque Yt and bending moment Yb. According to Bernoulli-Euler Law, the bending moments can be given by:



(30) where Δt is the time step of discretization and n is the number of time steps. By some manipulation of the governing equations, the translational and rotational equations of motion may be cast in the form of Eqs. (2) and (3), where in those equations, N1, N2, N3, N4, M1, M2, and M3 are matrices associated with position and properties of fiber, velocity and properties of flow field. The matrices are as follows:

(27)

where θ is the angle of bending in radiant, and θe is the equilibrium bending angle. EI is the flexural rigidity, and nb is the unit vector normal to the plane of bending. For a uniform fiber under torsion, the torsional torque Yt can be written as:



(28)

where ϕ is the angle of torsion in radiant, and ϕ e is the equilibrium torsion angle. GJ is the torsional rigidity, and nt is the unit vector normal to the plane of torsion. Eqs. (2) and (3) can be discretized in time as



where A′ i = Ai + miI3×3Δt and A′ = A′i. and are antisymmetrical matrices of vector di and bi. The details are given by Ross and Klingenberg [14].

(29)

ABOUT THE AUTHORS This study resulted from trying to understand the influence of forming fabric geometry on the topographic wire mark in paper. In our previous research, we studied how simulated wood fibers interact with a single cylinder, which is representative of one filament in a forming fabric. We extended that earlier research to consider interaction with a realistic fabric geometry. Fiber interaction with a forming fabric is an example of a fluid-structure interaction. The physics of such interactions is complex. We addressed it by decoupling the fluid mechanics from the fiber structure response to the fluid mechanical forcing. The fiber deformations we predict here (and that we confirm in a follow-up paper) are much larger than the topographic wire mark observed in pressed and dried paper. It therefore is apparent that pulp mat pressing and drying not only dewaters but also 46

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

substantially smooths paper. Paper machine fabric manufacturers can use this information to design and recommend better fabrics for mills. Li Experimental confirmation will be the next step in this research.

Green

Li is a doctoral student and Green is professor, University of British Columbia, Department of Mechanical Engineering, Vancouver, BC, Canada. Email Green at [email protected].

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business sense. (His High-Speed Video Audit can show all the details, if you like.) Proof can be found at Gold East, where Martin has worked closely with the mill on the application of Compozil Fx. Today, Gold East’s huge, advanced machines are running even faster and even better. Put simply, Compozil is how we make papermaking everywhere smoother. Inspiring paper all over the world is our inspiration. Meet us at eka.com.

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eyes. Careful preparation and planning is vital, but practical experience of chemicals, processes, machines and people near and far guides the work in the lab and makes Eka’s innovations truly safe to use. And every time he’s on site, more ideas are born. All of them with one purpose: making your whole papermaking process that much more competitive. Inspiring paper all over the world is our inspiration. Meet us at eka.com.

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CONTAMINANT ANALYSIS Progress in Paper Recycling

Optical analysis of ink and other contaminants in process waters ANTTI HAAPALA, MIKA KÖRKKÖ, ELISA KOIVURANTA,

and

JOUKO NIINIMÄKI

ABSTRACT: Analysis methods developed specifically to determine the presence of ink and other optically active components in paper machine white waters or other process effluents are not available. It is generally more interesting to quantify the effect of circulation water contaminants on end products. This study compares optical techniques to quantify the dirt in process water by two methods for test media preparation and measurement: direct process water filtration on a membrane foil and low-grammage sheet formation. The results show that ink content values obtained from various analyses cannot be directly compared because of fundamental issues involving test media preparation and the varied methodologies used to formulate the results, which may be based on different sets of assumptions. The use of brightness, luminosity, and reflectance and the role of scattering measurements as a part of ink content analysis are discussed, along with fine materials retention and measurement media selection. The study concludes with practical tips for case-dependent measurement methodology selection. Application: Taking account of fines retention on measurement media and parameters such as scattering makes a significant difference on how reliable the results of process water quality analysis can be considered.

I

n mill processes, the accumulation of detrimental particles in process waters has been shown to have an impact on the quality of the end product, including pitch components, inks and other contaminants [1-8]. The long-term impact of small-scale contaminants can be severe because of the closure rate and solid buildup in water loops, both of which can have adverse effects on runnability and product quality [9-18]. The appearance of paper products relies on optical measurements, because they depict the way the end-user perceives the product quality. The assessment of optical parameters (brightness, whiteness, opacity, color, etc.) has been thoroughly reviewed by many literature sources [1,19-23]. The characteristics of pulp components can be measured from process waters using conventional methods. The content of solid and colloidal substances is commonly evaluated based on filtration or evaporation methods, wood extractives, and stickies with extraction methods, turbidity, organic carbon analysis, or even flow cytometry [24-30]. Analysis methods depicting the amount of ink and other components present that have specific adverse effects on the optical appearance of paper are not available. Such a method would be required to analyze the efficiency of different process water purification schemes on paper quality and paper machine cleanliness [2,15-18,31]. The amount of ink is often measured from pulp because of its near infrared (NIR) light absorption [32-35], either as the effective residual ink concentration (ERIC) at 950 nm or a similar measurement of residual ink (RI) at 700 nm. These methodologies have previously been adapted to measure ink content from mixtures of process water fines and fibers. For example, recent studies have used opaque sheets of 1.2 g

(60 g/m2) formed from different pulp and process water mixes on varying screens [31,36] using recirculation [2] or made on filter paper [2,3] to increase fine materials retention. Similarly, ink has been measured using an opaque 0.2 g (210 g/m2) pad filtered on high-retention membrane foil [37]. Measurement of the scattering coefficient, low-grammage sheets of approximately 30 g/m2 was achieved on 1-2 µm filter paper from virgin fibers and process waters [17,37,38]. Despite the variety of methods currently available, no single, well-defined protocol exists. There are open questions related to measurement of media preparation and the utilization of measured scattering coefficient. This study is designed to clarify the complexities involved when assessing the effect white water contaminants have on the optical appearance of paper. Different optical measurement methodologies are used and compared for their benefits and limitations in measuring process water fine materials. Additionally, the role of parameters such as ISO brightness (457 nm), luminosity (557 nm), or intrinsic reflectance value (700 nm) are discussed. As a practical conclusion, suitable methods for different white water analysis requirements are proposed based on their operational characteristics. MATERIALS AND METHODS

Determination of optical properties Preparation of pads on a membrane Direct filtration of process water was used to prepare pads on a membrane. The method was comparable to normal pad preparation [37], except the pads were prepared by filtrating process water equivalent to 0.2 g of dry mass on a membrane foil (Sartorius ø 50 mm, pore size 0.45 µm; Sartorius AG, Goettingen, Germany), resulting in a relative pad grammage AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

51

CONTAMINANT ANALYSIS Progress in Paper Recycling

of approximately 210 g/m2. The pH was not adjusted. The prepared pads were considered optically opaque based on earlier studies [37,39]. The drainage rate for samples differed in the range of 10-15 min, depending on the process water solids content. Pads on membrane foils were dried in a desiccators. Optical properties were measured from pulp pads using an L&W Elrepho spectrophotometer (Lorentzen & Wettre; Kista, Sweden). The results were compared to membranes on which 100% deinked pulp (DIP) was filtered. The reference pulp optical properties were: luminosity, 64.4%; reflectance at 700 nm, 68.4%; ISO brightness, 60.1%; and ink content, 433 ppm. Preparation of low-grammage sheets on filter paper Sheets with a grammage of 30 g/m2 were prepared by mixing a constant amount of fibers (hyperwashed virgin supercalendered grade denoted as HWSC) with process water to ensure strength and slight transparency (opacity < 97%) of the prepared sheet. The pH was not adjusted. The fibers used for mixing were obtained by hyperwashing unprinted SC grade material. The mixing ratio of 70% fibers and 30% process water fines was kept constant for all the experiments and calculated based on dry masses of 0.82 g for sheets of 165 mm by 165 mm. Hence, 0.57 g of fibers and a volume of process water containing 0.25 g of solids were mixed and diluted with deionized water to approximately 250 ml. Sheets were prepared in a sheet mold on filter paper (Munktell 00H; Munktell Filter AB, Falun, Sweden) having a porosity of 1-2 µm for high retention. Drainage rate for the prepared 250 ml samples differed within the range of 5 to 10 min. A more detailed description of the method is given in [37]. The results were compared to low grammage sheets of 100% hyperwashed SC pulp. The optical properties of HWSC reference pulp were: luminosity, 75.4%; reflectance at 700 nm, 87.0%; ISO-brightness, 70.4%; and ink content measured as 36 ppm, although the virgin pulp contained no ink. Optical measurements Optical brightness and luminosity were tested according to SFS-ISO 2470 “Paper, board and pulps. Measurement of diffuse blue reflectance factor (ISO brightness)” and TAPPI T 527 om-02 “Color of paper and paperboard (d/0, C/2).” A test media absorption coefficient (k) was determined using the scattering coefficient (s) and reflectance (R∞) of an opaque stack, according to Eq. (1). The scattering coefficient was measured from sheets according to Eq. (2). A constant scattering value of 42 m2/kg, given in INGEDE Method 2 “Measurement of optical characteristics of pulps and filtrates from deinking processes,” was also used. Reference measurements were taken from the pads and sheets without addition of white water. All measurements were performed at 700 nm.









(1)

(2)



where k is the absorption coefficient at 700 nm [m2/kg], s the scattering coefficient at 700 nm [m2/kg], w is the grammage of the sheet [kg/m2], R0 is the reflectance against a black cavity at 700 nm, R∞ is the reflectance of the stack at 700 nm. Analyzed white waters The process waters studied comprised three process waters collected from mill sites and prepared model water samples. Water sample 1 had components from both thermomechanical pulp (TMP) and DIP in a feed ratio of 35%/65%, denoted here as WW1. Water samples 2 and 3 came from mills that run 100% deinking pulp-based processes. This series is denoted as WW2 and WW3. The samples were treated with flotation to obtain varied levels of ink and fine material. The pH, suspended solids content, and the amount of inorganic ash from all solid materials present in the process waters were analyzed (SFS-EN 872 “Determination of suspended solids,” 2005; and ISO 1762 “Paper, board and pulps – determination of residue [ash] on ignition at 525°C,” 2001). The results are presented in Table I . Analyzed model waters Prepared model waters involved the dissolution of varied fine materials into a water suspension solely to monitor the effect of changes in scattering properties on the ink content determination. The fines selected were typically used grades of ground calcium carbonate (GCC) and kaolin, obtained as commercial products, and TMP fines that were collected by ID

pH

Suspended Solids [%]

Ash 525°C [%]

WW1

7.5

0.2 – 2.2

52.7 – 70.2

WW2

8.0

0.5 – 1.2

55.7 – 69.4

WW3

8.2

2.0 – 3.6

73.5 – 76.3

I. The pH and solids content of the mill process waters studied. Solution No.

GCC [%]

Kaolin [%]

TMP Fines [%]

1

-

-

100

2

-

100

-

3

100

-

-

4

33

33

33

5

25

25

50

II. Mixing ratio of fine material in prepared process water solutions. 52

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

CONTAMINANT ANALYSIS Progress in Paper Recycling

Printed

Attached Ink Content in HW ONP [ppm]1

Fiber 1

No

73

Fiber 2

Yes

202

Fiber 3

Yes

422

Fiber 4

Yes

632

Fiber Material #

1

Pulp ink contents measured from low-grammage sheets.

III. The components of test pulps used as fiber fractions in the scattering coefficient measurement studies.

rinsing and thickening them from pulp through a 150 mesh wire screen. Mean particle sizes of fillers were 7.9 μm and 5.0 μm for kaolin and GCC respectively, as determined with a laser diffractometer (Beckman Coulter LS 13320; Beckman Coulter, Fullerton, CA, USA). From these, five different mixtures were prepared, as shown in Table I I , where the proportions are shown on a dry mass basis. The prepared model waters contained only fillers and

fines. Their ink content was varied by using fiber material that contained fixed levels of ink. The sheet preparation was similar to the earlier process [37], but pulp from printed newspaper was used in addition to the same unprinted paper. Four pulps were prepared by oven drying at 60°C and hyperwashing the pulped ONP (cold-set web-offset printed old newsprint, HW ONP) samples with varying pulping times to obtain distinctly differing levels of attached ink content that were then mixed with different mixtures of fillers, as shown in Table II. The prepared pulps are summarized in Table I I I . RESULTS

Effect of water dirt content on the optical appearance of pulps The impact of circulation water contaminants was compared directly from pads with the optical properties of a typical DIP and indirectly as their potential to darken or stain pure HWSC pulp in the case of low-grammage sheets. Fig ur es 1-4 illustrate the measured ISO-brightness, luminosity, reflectance at 700 nm, and ink content of white waters (WW1-WW3), which

1. Measured ISO brightness of white water fine materials in comparison to reference pulps. On the left are the values measured from opaque pads in comparison to 100% deinked pulp (DIP), and on the right, the darkening of low-grammage sheets (30% white water fine components) with respect to the hyperwashed virgin supercalendered (HWSC) grade is shown.

2. Measured luminosity of white water fine materials in comparison to reference pulps. On the left are the values measured from opaque pads in comparison to 100% DIP. On the right, darkening of low-grammage sheets (30% white water fine components) with respect to HWSC is shown. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

53

CONTAMINANT ANALYSIS Progress in Paper Recycling

3. Measured reflectance of white water fine materials in comparison to reference pulps. On the left are the values measured from opaque pads in comparison to 100% DIP. On the right, darkening of low-grammage sheets (30% white water fine components) with respect to HWSC is shown.

4. Measured ink content of white water fine materials in comparison to reference pulps at 700 nm using a constant scattering of 42 m2/kg. On the left are values measured from opaque pads in comparison to 100% DIP. On the right, darkening of low-grammage sheets (30% white water fine components) with respect to HWSC is shown.

are also given in Table I. Both methods are shown to provide applicable results in the evaluation of the optical properties of white water fine materials. As seen from Fig. 1, the white water solid components can be characterized differently, depending on the measurement media. A direct comparison of values is possible only from the pads on the left, showing that the brightness of the process water solid components is lower than that of the pulp pads. From the pads, the WW’s 2 and 3 appear to have 5 to 8 percentage points lower brightness than DIP. WW1 has 14 to 18 percentage points lower ISO brightness. On the right, the mixes of waters and HWSC pulp have notably lower brightness values than the hyperwashed pulp itself. This observed decrease in brightness of the sheets remains a more indicative result because of the presence of fibers in addition to white water solids; however, differences in white water composition can be noted from either measurement method. The comparison given in Fig. 2 on measured luminosity shows an analogous effect to the aforementioned brightness values – mixing 54

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

the “dirty” water fractions in HWSC pulp lowers the optical properties of the mixes. The values obtained from pads are also lower for white water components than for DIP. The amount of dirt in the water samples in comparison to pulps gives a similar result to brightness. The difference is most pronounced in the case of WW1, which has 13 to 20 percentage points lower luminosity than DIP. Corresponding figures for WW2 and WW3 are 4 to 7 and 7 to 9 percentage points, respectively. In the case of sheets, the fine materials in WW1 lower the observed luminosity by 15 to 21 percentage points; those in WW2, by 10 to 12 percentage points; and those in WW3, by 13 to 15 percentage points. Reflectance at 700 nm shows a trend similar to the previously mentioned parameters, although it is more sensitive to inks. Measurement from opaque pads in Fig. 3 (left) shows that the white water fine components reduce the measured reflectance by 12 to 27 percentage points in the case of WW1, 4 to 8 percentage points in the case of WW2, and 8 to 10 percentage points in the case of WW3. When compared with

CONTAMINANT ANALYSIS Progress in Paper Recycling

sheets, the difference appears to be 17 to 25 percentage points in the case of WW1, 14 to 17 percentage points for WW2, and approximately 20 percentage points for WW3. The selectivity of ink content analysis (Fig. 4) makes the white water samples differ most significantly. White waters clearly appear to contain substantially more ink than DIP (433 ppm), changing the levels from 700 to 1370 in WW1, from 450 to 630 ppm in WW2, and from 580 to 630 ppm in WW3. On the right, HWSC has practically no ink; however, sheets containing 30% white water solids clearly show a level of pulp contamination rising up to 920 ppm for WW1. The ink content analysis is most selective in showing the differences between dark white water solid components other than fiber fines. However, the results given in Fig. 4 do not take into account the changes in their scattering properties. Differences in the ink content analysis are studied in the following section.

Effect of the scattering coefficient on the measured ink content Substantial changes were noted in the NIR scattering of sheets, depending on the content of different components. Here, the ink content of samples 1-4 was kept constant by the use of model waters, including fiber components from the same base paper as unprinted (Fiber 1) and cold set printed ONPs (Fibers 2-4) with recorded levels of attached inks, while the scattering properties of pulp-fines mixes were changed (Tables II-III). The scattering coefficients of the furnishes studied, comprising the unprinted fiber fraction, the added fiber fines (TMP), and the mineral fillers (kaolin clay and ground calcium carbonate [GCC]), are shown individually and in the mass weighed proportions used in Fig. 5. The alternative constant scattering level is also given as a reference. Results are presented with error bars at 7.6% based on the TAPPI repeatability ratio given in [38]. As seen, constant levels of scattering

5. The change in sheet scattering coefficient values measured at 700 nm with different fines fractions (30%) from model suspensions and different pulps as fiber fractions (70%) of the sheet. Error bars of 7.6% were used, as given in [38].

would in these cases give a poor correlation to the optical scattering properties of the suspensions when the fine materials’ content of the white water changes, which would also lead to incorrect ink content estimation. The ink content obtained from model waters using a constant scattering coefficient value of 42 m2/kg is shown in F ig. 6 (left), with error bars at 5.1% based on [37]. The insensitivity to changes in fines scattering properties (Table II) is shown to have a considerable effect on the perceived ink content. Obtained values should be constant. The use of constant scattering implies that the ink content changes, although this is not the case. Taking the measured scattering into account allows a more accurate analysis of the ink content of the water sample, shown in Fig. 6 (right), where the ink levels appear to stay constant for each solution within the given error limits.

6. The effect of different fines fractions from model suspensions on the observed ink content of sheets. Residual ink measurements are made with 700 nm from sheets using the constant scattering coefficient of 42 m2/kg (left) and the measured scattering coefficients (right). Error bars of 5.1% were used, as given in [37]. AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

55

CONTAMINANT ANALYSIS Progress in Paper Recycling DISCUSSION

Optical characterization of white water fine materials Optical properties measured from membrane pads (left sides of Figs. 1-3) show the optical properties of the white water fine components when compared with 100% DIP. Direct comparisons can be made between the obtained brightness, luminosity, and reflectance at 700 nm. Similarly, the difference in white water solids differs when mixed with clean fiber and measured from sheets. However, in this case, a comparison between the obtained values cannot be made directly. The sheet method allows the comparison of different white waters to determine their relative purity, but it does not give absolute values for the properties of white water fine materials. The importance of measured NIR scattering is not apparent from Fig. 4, where constant scattering is used, but the model waters analyzed in Fig. 5 show a remarkable difference. White water fines components can vary significantly in nature (Fig. 4). Figure 5 shows that constant scattering values are unable to take into account the changes in white water fine materials’ content and scattering properties (15 units). As seen from Eq. (1), scattering affects light absorption and ink content significantly. For example, the use of a constant scattering coefficient of 42 kg/m2 when the true value would be 55 kg/m2 would yield a 30% bias in

the measurement. This effect has previously been shown to apply to deinked pulps [37,38,40]. In practice, if white waters contain fines with high scattering properties, they can also hold substantial amounts of ink without necessarily appearing “dirty.” In contrast, if the fines present in process waters have poor scattering properties, even small amounts of ink can lower the observed brightness and luminosity. The same reasoning also applies to reflectance, even if it is measured with wavelengths most sensitive to inks. Figure 6 illustrates this effect and shows how the mixes of white waters and pulps are observed differently when determining ink content with either constant or measured scattering coefficients. The pulp mixes had constant ink contents, but the use of constant scattering was unable to compensate for the changes occurring in the composition of white water fine materials. The use of measured scattering values took these changes into account and provided a statistically accurate measure of the white water ink content, as previously shown for deinked pulps [38,40].

Methods and test media The comparison of methods and test media is complicated by the number of available methodologies for pulping analysis. Most commonly, measurements have been made from sheets or opaque pads, but the specifics of their preparation are only

STRENGTHS

WEAKNESSESS

A: Easy and fairly simple method, results easily comparable.

A: P  oor retention of fines because of the 25 µm filter paper aperture. Indirect measurement because of the presence of pulp.

B: R  esults easily comparable, good retention possible because of recirculation.

B: V  ery time-consuming (multiple sheets needed) and requires some monitoring of fine material retention. Indirect measurement because of the presence of pulp.

C: Easy and fairly simple method, results easily comparable.

C: P  oor retention of fines because of the 11 µm filter paper aperture. Indirect measurement because of the presence of pulp.

D: O  ffers good retention of fines because of the 1-2 µm filter paper aperture and can be used to determine scattering coefficient changes for each sheet (opacity < 97%).

D: Indirect measurement because of the presence of pulp.

E: Q  uick and easy measurement from sample to result. High retention because of 0.45 µm membrane aperture. Can be used to compare optical properties directly.

E: None.

OPPORTUNITIES

THREATS

A: S  heets of 60 g/m2 also usable for study of mechanical sheet properties.

A: D  oes not account for possible changes in scattering properties of white water fine materials.

B: S  heets of 60 g/m2 also usable for study of mechanical sheet properties.

B: D  oes not account for possible changes in scattering properties of white water fine materials.

C: S  heets of 60 g/m2 also usable for study of mechanical sheet properties.

C: D  oes not account for possible changes in scattering properties of white water fine materials.

D: B  est for comparative studies between mills and for other research purposes.

D: If fiber source is hyperwashed, there is less fiber fines in the measurement sheet than in papermaking, thus highlighting the optical effect of white water solids.

E: S  ufficient for day-to-day monitoring purposes on a production line.

E: D  oes not account for possible changes in scattering properties of white water fine materials.

V. SWOT analysis of recently applied methodologies for optical characterization of white waters. 56

TAPPI JOURNAL | VOL. 11 NO. 8 | AUGUST 2012

CONTAMINANT ANALYSIS Progress in Paper Recycling

explained in broad terms. Their grammage is often not clearly discussed. Additionally, the retention, media characterization, and use of constant scattering can lack in accuracy. The following SWOT (strengths, weaknesses, opportunities, and threats) analysis (Table V) depicts some recent methodologies used for the optical analysis of white water properties. The abbreviations used are: A: 1.2 g (60 g/m2) sheet made on a 500-mesh wire [31]; B : recirculated 1.2 g (60 g/m2) sheet [2]; C: 1.2 g (60 g/m2) sheet on 11 µm filter paper [2,3], based on a similar method using a 20 µm wire screen [36]; D: lowgrammage (30 g/m2) sheet made on 1-2 µm filter paper [17,37,41]; and E : 0.2 g (210 g/m2) pad filtered on a 0.45 µm membrane foil [37].

Remarks on test media and analysis Only a few qualities of the measurement media were important in the analysis: the retention of fine materials and the opaque nature of the test media. If the small particles that account for most of the ink and colloidal pitch are not retained in the media, the representativeness of the measurement remains poor [42]. The use of a small pore-size filter can be considered beneficial since this does not unreasonably prolong the drainage time. The use of white water recycling to obtain high retention can be problematic because it has been observed that, in practice, the true equilibrium is rarely reached in both ash and ink. Multiple sheets should be prepared and ashed to verify the equilibrium, which further adds to both the time and trouble taken for analysis. Opaque pads are proven to have high retention of small ink particles and can be considered beneficial, whether made on paper or membranes [39,41]. Similarly, if the changes in fines scattering cannot be determined, the analysis can become insensitive to ink content. Estimation of scattering is possible only from translucent sheets, but this often limits the amount of white water fine material that can be deposited onto the sheet without rupturing its structure prior to the measurement. In practice, some fiber components are needed to enforce the sheet matrix where they will contribute to the optical measurement result [37]. As the ratio of white water solids and pulp changes, the scattering also changes, but in a nonlinear manner [43]. Hence, the correct assessment of the absolute scattering of white water solid components is not possible using this method. However, low-grammage sheets can detect changes in scattering that can be related to changes in fine materials to obtain a measure of ink content change within process waters. As a practical conclusion, it appears that the best way to monitor the changes in the properties of white water fine material is to produce opaque pads on small, pore-size membranes and compare either ISO brightness, luminosity, or reflectance values between white water samples. When ink content is of specific interest, the NIR scattering coefficient of the white water fine materials should be obtained from lowgrammage sheets.

CONCLUSIONS The easiest and fastest way to analyze process water optical appearance is by making opaque pads on membrane foil to ensure high retention of small ink particles. This result represents the sum effect of all the components present in suspension and can be compared to pulp luminosity, ISO brightness, or reflectance. If the ink content is of specific interest, methods that can provide a quantitative measure of the NIR scattering coefficient are needed. This is achieved by production of sufficiently low-grammage sheets that meet the opacity requirement from mixes of process water and selected reference pulp. Hyperwashed, unprinted fibers provide one suitable matrix for these sheets, although DIP can also be applied. The results obtained can then be compared to pulp sheets to measure the difference caused by ink particles to the ERIC measured between pulp sheets and white water sheets. TJ ACKNOWLEDGEMENTS The authors wish to thank Tommi Niskanen, Ossi Laitinen, and Jani Österlund for their efforts in the analyses and practical method development. LITERATURE CITED 1. Hubbe, M.A., Pawlak, J.J., and Koukoulas, A.A., BioResources 3(2): 627(2008). 2. Ricard, M., Dorris, G., Lapointe, C., et al., PTS-CTP Deinking Symp., PTS, Munich, 2008, Conf. CD. 3. Ricard, M. and Dorris, G., PAPTAC Annu. Meet., 93rd, CPPA, Montreal, Canada, 2007, p. B251. 4. Rundlöf, M., Htun, M., Höglund, H., et al., J. Pulp Pap. Sci. 26(9): 308(2000). 5. Luukko, K., “Characterization and properties of mechanical pulp fines,” Ph.D. thesis, Helsinki University of Technology, Helsinki, Finland, 1999. 6. Retulainen, E., Luukko, K., Fagerholm, K., et al., Appita J. 55(6): 457(2002). 7. Springer, A.M., Dullforce, J.P., and Wegner, T.H., TAPPI J. 68(4): 78(1985). 8. Wearing, J.T., Barbe, M.C., and Ouchi, M.D., J. Pulp Pap. Sci. 11(4): 113(1985). 9. Garver, T.M., Xie, T., and Boegh, K.H., TAPPI J. 80(8): 163(1997). 10. Garver, T.M. and Smith, G., TAPPI Paper Summit, TAPPI, Atlanta, GA, USA, 2002, Conf. CD. 11. Blanco, A., Miranda, R., Negro, C., et al., TAPPI J. 65(1): 19(2007). 12. Kröhl, T., Lorenˇcak, P., Gierulski, A., et al., Nord. Pulp Pap. Res. J. 9(1): 26(1994). 13. Vähäsalo, L., Degerth, R., and Holmbom, B., Pap. Tech. 44(1): 45(2003). 14. Kallio, T. and Kekkonen, J., Tappi J. 4(10): 20(2005). 15. Hubbe, M.A., J. Pap. Technol. 48(2): 31(2007). 16. Hubbe, M.A., J. Pap. Technol. 48(3): 23(2007). 17. Haapala, A., Körkkö, M., Kemppainen, K., et al., BioResources 5(4): 2153(2010). AUGUST 2012 | VOL. 11 NO. 8 | TAPPI JOURNAL

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18. Ben, Y. and Dorris, G., Pulp Pap. Can. 105(11): 28(2004).

32. Jordan, B.D. and Popson, S.J., J. Pulp Pap. Sci. 20(6): J161(1994).

19. Corte, H., Perception of the Optical Properties of Paper, The Fundamental Properties of Paper Related to Its Uses (F. Bolam, Ed.), British Paper and Board Industry Federation, London, UK, 1976, pp. 626-658.

33. Jordan, B. and O’Neill, M., J. Pulp Pap. Sci. 20(12): J371(1994).

20. Brandon, C.E., Properties of Paper, Pulp and Paper Chemistry and Chemical Technology (J.P. Casey, Ed.) 3rd edn., Wiley-Interscience, New York, 1981, pp. 1715-1972.

35. Vahey, D.W., Zhu, J.Y., and Houtman, C.J., Prog. Pap. Recycl. 16(1): 3(2006).

21. Popson, S.J. and Malthouse, D.D., Measurement and Control of the Optical Properties of Paper, 2nd edn., Technidyne Corp., New Albany, IN, USA, 1996. 22. Pauler, N., Paper Optics, AB Lorentzen & Wettre, Kista, Sweden, 2002, pp. 17-37. 23. Vaarasalo, J., Pulp and Paper Testing, (J-E. Levlin and L. Söderhjelm, Eds.), Fapet Oy, Jyväskylä, Finland, 1999, Chap. 8, pp. 163-181. 24. Ricard, M. and Dorris, G., PAPTAC Annu. Meet., 93rd, CPPA, Montreal, Canada, 2007, p. B263. 25. Alexandersson, T., “Water reuse in paper mills. Measurements and control problems in biological treatment,” Lic. thesis, Lund University, Lund, Sweden, 2003.

34. Körkkö, M., Haapala, A., Mäkinen, L., et al., TAPPI PEERS, TAPPI PRESS, Atlanta, 2011, Conf. CD.

36. Rundlöf, M., Htun, M., Höglund, H., et al., J. Pulp Pap. Sci. 26(9): 301(2000). 37. Haapala, A., Körkkö, M., Laitinen, O., et al., TAPPI Eng. Pulping Environ. Conf. Proc., TAPPI PRESS, Atlanta, 2009, Conf. CD. 38. Körkkö, M., Haapala, A., Liimatainen, H., et al., Appita J. 64(1): 71(2011). 39. Bennington, C.P.J. and Wang, M-H., J. Pulp Pap. Sci. 27(10): 347(2001). 40. Körkkö, M., Laitinen, O., Haapala, A., et al., Tappi J. 10(6): 17(2011). 41. Haapala, A., Körkkö, M., Kemppainen, K., et al., Res. Forum Recycl., 9th, TAPPI PRESS, Atlanta, 2010, Conf. CD.

26. King, C.A., Solutions People, Processes Pap. 2(4): 48(2002).

42. Körkkö, M., Haapala, A., Mäkinen, L., et al., Res. Forum Recycl., 9th, TAPPI PRESS, Atlanta, 2010, Conf. CD.

27. MacNeil, D., Sarja, T., Reunanen, M., et al., Prof. Papermaking (1): 10(2006).

43. Middleton, S.R., Desmeules, J., and Scallan, A.M., J. Pulp Pap. Sci. 20(8): 231(1994).

28. Koskinen, J., Sung, D., Kazi, F., et al., TAPPI J. Online Exclusives 2(4): 2(2003). 29. Banerjee, S., U.S. pat. 6,841,390 (Jan. 11, 2005). 30. Haynes, R.D., TAPPI PEERS, TAPPI PRESS, Atlanta, 2011, Conf. CD. 31. Dionne, Y., Ricard, M., Dorris, G., et al., Res. Forum Recycl., 8th, TAPPI PRESS, Atlanta, 2007, Conf. CD.

ABOUT THE AUTHORS We chose to research this topic because proposed methods to measure the optical quality of white water fines remained unclear in literature. We focused on the measurement of scattering coefficient and its importance, which has been shown to have an influence on the determination of ink content from Haapala pulps. This study complements earlier studies by introducing different sample preparation methods and assessing their differences and usability regarding the influence of white water quality on pulp and end-product properties. The most difficult aspect of the study was to prepare pads and sheets consistently, because applicable TAPPI and INGEDE methods are not available for analyzing fines properties from water samples. It was interesting to observe how much ink and other fine materials were rinsed out from prepared sheets if prepared on, for example, wire screen without filter paper. Moreover, the lack of previous studies in this area made the work genuinely interesting and novel by nature. 58

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Körkkö

Koivuranta

Niinimäki

Paper and board manufacturers using deinked material would benefit from this study by understanding how significantly the circulation waters may affect their processes, and also that selection of measurement method certainly impacts reliability and results. Antti Haapala is a post-graduate research fellow, Körkkö is a researcher, Koivuranta is a researcher, and Niinimäki is a professor at the Fibre and Particle Engineering Laboratory, University of Oulu, Oulu, Finland. Email Haapala at [email protected]

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