Treatment of Produced Water from Unconventional Resources by ...

IPTC 17481 Treatment of Produced Water from Unconventional Resources by Membrane Distillation J. Minier-Matar, A. Hussain, A. Janson, S. Adham, ConocoPhillips Copyright 2014, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Doha, Qatar, 20–22 January 2014. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435

Abstract Unconventional resources (Shale gas/oil) use significant volumes of water for hydraulic fracturing (fracking). While some of the water used is fresh groundwater, there are more environmental pressures to use brackish water sources for fracking. This brackish water may need to be treated to lower the saturation levels and to allow mixing of field chemicals. Unconventional resources also produced high volume of flow-back water (produced water). This produced water (PW) contains high levels of total dissolved solids (TDS) and desalination may be needed to allow recycling or reuse of this water source. Membrane Distillation (MD) is an innovative process that can desalinate highly saline waters (30,000–100,000 mg/L TDS) more effectively than reverse osmosis. As a proof of concept, bench-scale MD testing were performed on brackish and produced water samples (30,000 mg/L-60,000 mg/L TDS) obtained from Texas. Results have shown excellent TDS rejection (99.9 %) on all the water samples that were tested without impacting membrane’s flux performance. To evaluate the O&M and scale up issues, two one m3/day MD pilot units are currently operating side by side at a local desalination plant in Doha. Brine from the thermal desalination plant was used as representative high salinity water (70,000 mg/L), similar salinity levels could be found in brackish groundwater and/or flow-back water. It was assumed that all other contaminants that could cause membrane fouling (such as suspended oil, solids, organics, microorganisms) will be removed in a pretreatment step prior to MD. Preliminary results showed that the pilot units were successful in completely removing salt. Flux was very stable for more than 2 weeks. However, it was concluded that pretreatment is critical for stable performance of the MD units. This presentation will provide up to date data on MD bench and pilot-scale performance with O&M issues and projected cost estimates.

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1. Introduction Produced Water (PW) is the highest volume liquid waste stream generated by the petroleum industry. Figures published in 2011 [1] and 2007 [2] show that globally, between 70 and 100 billion barrels of PW were generated. Currently, the majority of PW generated from shale gas play are re-injected into deep well injection and therefore, the treatment facilities are mostly designed to remove dispersed oil and grease (O&G) and suspended solids (SS), to avoid formation plugging. PW treatment methods applied in the petroleum industry are historically limited to physical separation technologies such as the API separator, coalescers or hydrocyclones. These technologies are, in most cases, not capable of producing an effluent compatible with water quality standards for beneficial recycling in the petroleum industry itself or reuse in, for example, irrigation or other industrial processes. A combination of factors including new regulations, geological restrictions and local water scarcity, is putting enormous pressure on the petroleum industry operators to find new ways of treating and managing PW that promotes water conservation and sustainability. Furthermore, for fracturing operations in shale gas fields, large volumes of water (as high as 15,000 m 3/well) are also required to drill the wells and to make up the fracturing liquids [3]. With proper treatment, there is an opportunity for treated PW and flowback water to replace these significant volumes of water extracted from lakes, rivers and aquifers in production operations. Increasing efforts are being developed by the petroleum industry to develop and adopt advanced technologies that are capable of further treating PW to produce an effluent compatible with water quality standards for beneficial reuse. This will ultimately lead to significant water conservation benefits in several growing oil and gas key sectors: R R R

Where PW is currently being injected in wells, the injection volumes will be reduced, diminishing environmental impacts; If wastewaters generated in water-intense operations (e.g. unconventional and oil sands resources) are treated and reused, it will dramatically reduce the volumes of fresh water required; There is growing evidence that enhanced oil recovery may become more efficient and productive if the injected water is treated with advanced technologies to meet specific salinity levels.

However, treating PW and producing a good quality effluent is challenging. PW characteristics can vary considerably. For instance, Total Dissolved Solids (TDS) can vary from >100,000 mg/L in Flowback waters from shale gas wells to less than 3,000 mg/L in PWs from coal bed methane (CBM) wells [4,5]. However, as a broad generalization the constituents that offer the greatest concerns from a treatment standpoint are organic content (in particular dissolved fraction) and salinity. Inorder to treat high saline and organic contents, an innovative process called Membrane Distillation (MD) is presented in the research paper. 1.1 Membrane Distillation MD is a mass transfer process driven by a partial vapor pressure difference due to a temperature gradient across the hydrophobic porous membrane as shown in Fig. 1[6]. A temperature difference of 10 °C–20 °C between the warm and cold streams can be sufficient to produce distilled water at the right conditions. The following are the benefits of MD compared to RO:  Distilled water quality  Product quality not affected by high salinity  Utilizes low grade waste heat  Lower capital cost with the use of inexpensive plastics as construction material  Can achieve desired salt rejection in single pass, where RO process requires multiple passes. The main challenge is limited experience in full scale plants. There are four main module configurations [7] in the MD process. These are:  Direct Contact MD (DCMD)  Air Gap MD (AGMD)  Sweeping Gas MD (SGMD)  Vacuum MD (VMD)

Figure 1 Membrane Distillation

Most of the bench-scale work done on the MD process evaluation has been DCMD but for field/commercial applications the focus has been on AGMD and VMD configurations. There are pros and cons to the various configurations listed above [8], which are currently being investigated by researchers and technology providers. As for membrane configurations used in MD,

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the Plate and Frame is the most widely applied design. However, the most promising configuration for large scale applications can be the hollow fiber membrane design, which is currently under research and development [9]. Bench scale Membrane Distillation (MD) experiments were carried out using different feed streams.The research paper addresses the feasibility of using MD for treatment of high salinity brines from one of the unconventional play in Texas. The results of the MD pilot unit of capacity 1 m3/d operated under high saline condition were also investestigated. 2. Materials and Methods 2.1 Bench Scale Direct Contact Membrane Distillation Unit The photo of the bench scale experimental set up and the schematic diagram of the MD unit are shown in Figures 2 and 3, respectively. Flat sheet membranes with an active area of 0.014 m2 were used in the study. The membrane was sandwiched between the feed and distillate plates. Air operated pumps (Model: P025 Wilden, USA) were used to deliver feed and distillate water to the membrane. Deionized water was used as the cold stream to initiate the distillation process. Pressure and temperature on the inlet and outlet streams of the membrane on both sides were measured using pressure transducers (Model: PX309-030GI, Omega Engineering, UK) and thermo resistance RTDs (Model: RTD-NPT-72-E, Omega Engineering, UK). The data was accessed by digital display meter (Model: DP25B-E-230-A, Omega Engineering, UK). The temperature of the feed and distillate water was controlled by two refrigerated / heating circulator (Model: F32-MC, Julabo, Germany). The flux was determined by measuring the weight of the distillate using weighing balance (Model: VWR# 97035-640, Mettler Toledo) over active membrane area and time. The feed concentration was maintained constant by adding nanopure water in the feed tank to replace water lost as distillate. The data was acquired using a National Instruments data acquisition hardware (Chasis Model cDAQ9188; Module Model: NI-9219, National Instruments, US). A serial server (Model: NI ENET 232, National Instruments, USA) was used to acquire the weight from the balance. The data was stored and processed using LABView data acquisition software. The feed and the distillate temperatures were kept at 70°C and 30°C in most of the experiments. The feed and the distillate flow rates were kept at 1.8 L/min.

Figure 2: Picture of bench scale MD unit

Figure 3: Schematic diagram of bench scale MD unit

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2.2 Membrane Distillation flux Theoretically, the distillate flux is directly proportional to vapor pressure difference between hot and cold side [10]. The proportionality constant (C) is related to membrane structural parameter which relates to the pore size, porosity, tortuosity and thickness of the membrane [10]. C is also called as global mass transfer coefficient. The vapor pressure (Pv) difference is calculated based on Antoine equation [11] J = C (Pv,hot – Pv,cold) The experimental flux, J in LMH is calculated based

(1)

J = q/A

(2)

Where

q = distillate flow rate, l/h

A =

membrane area, m2

2.3 Membrane Table 1 provides membrane specifications of commercial membranes A, B and C Table 1 Membrane specifications Membrane

Pore size (µm)

Thickness (mm)

Water entry pressure (PSI)

Membrane description

A

0.2

0.23-0.31

>50

Active layer : PTFE Backing : PP

B

0.2

0.23-0.31

NA

Active layer : PTFE Backing : PP

C

0.2

0.12-2.0

Active layer : PTFE

>50

Backing : PP

2.4 Chemical Analysis The chemical compositions of the seawater and brines were analyzed using several analytical methods as listed in Table 2. Table 2 Analytical methods Parameter Calcium

Method

Instrument

Separation by IonPac® CS12A - 4 x 250 mm column with EGC III MSA eluent and 4 mm CSRS® 300 suppressor; eluent flowrate 1 mL/min; ions measured by conductivity detector

chromatography Ion 3000, Dionex)

(ICS

Separation by IonPac® AS19 - 4 x 250 mm column with EGC III KOH eluent and 4 mm ASRS® 300 suppressor; eluent flowrate 1 mL/min, ions measured by conductivity detector

Ion chromatography 3000, Dionex)

(ICS

Alkalinity, HCO3

Titration using 0.1N HCl and a pH electrode

Metrohm auto-titrator Titrando (Titrando 857)

Conductivity

Immersing conductivity probe in the sample

Orion meter

pH

Immersing pH probe in the sample

Orion 3 Star pH meter

Magnesium Sodium Potassium Chloride Sulphate

3

Star

conductivity

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3. Results & Discussion 3.1 Effect of salt concentration MD experiments were conducted to evaluate the impact of salt concentration on the performance of the MD process. For these experiments, synthetic brines were prepared (deionized water with various sodium chloride concentrations). The flux remained constant at approximately 25 LMH and there was no significant difference in flux at lower concentration. However, when the salt concentration was increased above 70 g/L slightly lower flux values (20 LMH) were obtained, which may be related to vapor pressure variations and difference in water viscosity that could impact the thermal conditions at the membrane boundary layer. These results are consistent with the literature [12].

Figure 5: Effect of salt concentration on MD performance. (hot/cold flow rate: 1.8L/min)

3.2 MD performance on brine from thermal desalination plant MD experiments were also conducted using the brine collected from a full-scale thermal desalination plant operating in Qatar. Figure 6 show the performance of membranes A, B & C operating on this brine. The fluxes, although different for the individual membranes, were stable and no significant flux decline was observed. It should be noted that the brine was already dosed with antiscalant, which is part of the pretreatment process for the feed seawater at the full scale plant. The salt rejection by all membranes was also over 99.99%. Table 3 provides the salt rejection data for Membrane B as an example.

Figure 6: Performance of different MD membranes in brine. (hot/cold flow rate: 1.8L/min)

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Table 3: Characteristics of thermal desalination brine / distillate concentration (Membrane B) Thermal Desalination brine

Distillate

Salt Rejection (%)

Conductivity

Parameter

µS/cm

Unit

100,000

1

>99.99

TDS

mg/l

71,981

99.99

Calcium

mg/l

718

99.99

Magnesium

mg/l

3,120

99.99

Sodium

mg/l

22,604

99.99

Potassium

mg/l

787

99.99

Alkalinity, HCO3

mg/l

242

0.1

>99.99

Chloride

mg/l

39,274

99.99

Sulphate

mg/l

5,116

99.99

3.3 MD performance on produced water from unconventional shale play The produced water used in the test were collected from ConocoPhillips’s operations in Texas, US. The produced water was moderate in salinity and organic content (47,761 mg/L and 496 mg/L TOC). The produced water was prefiltered using 0.8 micron Nylon filter. The plot in Figure 7 shows that the flux was stable throughout the test for both membranes, indicating that no severe scaling and/or fouling occurred on the surface of the membrane. Membrane B yielded a higher flux than membrane C (33LMH vs 28LMH). The membranes achieved comparable levels of TDS and TOC rejection. Table 4 provides the salt rejection data for Membrane C as an example.

Figure 7: Performance of different MD membranes in Produced Water. (hot/cold flow rate: 1.8L/min)

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Table 4: Characteristics of Produced water / distillate concentration (Membrane B) Produced water

Distillate

Salt Rejection (%)

Conductivity

Parameter

µS/cm

Unit

72,400

209

99.7

TDS

mg/L

47,761

99.7

Calcium

mg/L

3,416

99.9

Magnesium

mg/L

249

99.9

Sodium

mg/L

14,294

99.9

Potassium

mg/L

239

99.9

Chloride

mg/L

27,442

0.4

>99.9

Sulphate

mg/L

12

0.2

98.7

TOC

mg/L

496

34

92.4

3.4 Screening of MD pilot technology The research team decided to conduct a testing program to benchmark various MD technologies in the field. Hence, a consortium consisting of the ConocoPhillips GWSC, Qatar University (QU), and Qatar Electricity & Water Company (QEWC) was formed to evaluate the feasibility of testing the MD process at pilot scale for desalination of high saline brines. The knowledge captured from this program will help optimize the MD system operation, address scale-up issues and develop cost estimates for technology deployment. Table 5 present the various MD technology vendors that were contacted to submit proposals towards the field testing program. The team selected two technologies for pilot testing namely Xzero and memsys. Xzero and memsys operates under air gap and vacuum air gap mode. The capacity of each pilot unit was 1 m3/d. Table 5: Various MD Technology Providers

Technology developer

Technology promoter

Membrane configuration

MD type

Country

Fraunhofer ISE

SolarSpring

Spiral wound

Air gap

Germany

Scarab

Xzero

Flat sheet

Air gap

Sweden

TNO

Memstill

Proprietary

Air gap

Netherland

Memsys GmbH

Memsys

Flat sheet

Vacuum Air gap

Germany

Keppel

N/A

Flat sheet

Direct contact

Singapore

3.5 Desalination of high saline brine The high saline brine (TDS > 70,000 mg/L) was tested in the memsys pilot unit at local QEWC desalination plant. The flux remained constant around 4.2 LMH. The experiment was carried out for 12 days. Flux decline was not observed as shown in Figure 8. The system was able to desalinate a feed conductivity of ~ 100,000 µS/cm to a distillate conductivity of 10 µS/cm. The salt removal was greater than 99.99%. The feed and distillate characteristics are shown in Table 6. The details of the pilot plants, its operations and optimization will be published in future publications.

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1000000

6

100000

5

10000

4

1000

Flux Brine conductivity Feed conductivity

3

100

Distillate Conductivity

2

10

1

1

0

0

1

2

3

4

5

6

7

Days

8

9

10

11

12

Conductivity, µS/cm

Distillate Flux, LMH

8

0

Figure 8: Performance of memsys pilot unit under high saline brine conditions

Table 6: Characteristics of thermal desalination brine / distillate concentration (Membrane B) Thermal Desalination brine

Distillate

Salt Rejection (%)

Conductivity

Parameter

µS/cm

Unit

94,925

22

99.98

TDS

mg/l

71,031

6

99.99

Calcium

mg/l

616

0.7

99.88

Magnesium

mg/l

2,068

99.99

Sodium

mg/l

21,333

2

99.99

Potassium

mg/l

779

99.97

Chloride

mg/l

40,988

2

99.99

Sulphate

mg/l

4,344

1.2

99.97

4. Conclusion Below is a summary of the study’s main conclusions:  At salt concentrations up to 70 g/L, membrane flux (25 LMH) is typically not affected by salt concentration. At salinities above 70 g/L, a 20% flux decline was observed (to 20 LMH). Under all salinities tested, no membrane fouling was noted and the flux was stable.  MD is feasible to desalinate the brines from thermal desalination plants and can consistently produce a high quality distillate (conductivity< 10 µS/cm).  The two MD membranes were able to generate a stable flux and yield high salt rejections for produced water from unconventional play  Under pilot conditions, stable membrane performance was observed for more than 12 days operating under higher salinity conditions.

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5. References 1.

Produced Water Market - Opportunities in the oil, shale and gas sectors in North America. Global Water Intelligence Publication, 2011 2. B. Ferro, M. Smith - Global Onshore and Offshore Water Production. Exploration & Production – Oil & Gas review, 2007 3. P. Horner, B. Halldorson, J. Slutz - Shale Gas Water Treatment Value Chain – A Review of Technologies, including Case Studies. SPE Annual Technical Conference and Exhibition, Colorado, USA, October 2011 4. D. Williams - Turning water into oil: Desalination: a process to enhance world oil resources. IDA World Congress, Perth, Australia, September 2011 5. F. Ahmadun, A. Pendashteh, L. Abdullah, D. Biak, S. Madaeni, Z. Abidin - Review of technologies for oil and gas produced water treatment. Journal of Hazardous Materials, 170 (2009) 530–551 6. M. S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, Journal of Membrane Science, 285 (1-2) (2006) 4 - 29. 7. A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: A comprehensive review, Desalination, 287 (2012) 2 – 18. 8. K. W. Lawson, D. R. Lloyd, Membrane distillation, Journal of Membrane Science 124 (1997) 1 - 25. 9. Xi. Yang, R. Wang, A. G. Fane, Novel designs for improving the performance of hollow fiber membrane distillation modules, Journal of Membrane Science 384 (2011) 52 – 62. 10. M. Khayet, Membranes and theoretical modeling of membrane distillation: A review, Advances in Colloid and Interface Science 164 (2011) 56 – 88. 11. S. Al-Obaidani, E. Curcio, F. Macedonio, G. D. Profio, H. Al-Hinai, E. Drioli, Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation. Journal of Membrane Science 323 (2008) 85-98. 12. D. Winter, J. Koschikowski, M. Wieghaus, Desalination using membrane distillation: Experimental studies on full scale spiral wound modules. Journal of Membrane Science 375 (2011) 104-112.

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