10th International Symposium on Process Systems Engineering - PSE2009 Rita Maria de Brito Alves, Claudio Augusto Oller do Nascimento and Evaristo Chalbaud Biscaia Jr. (Editors) © 2009 Elsevier B.V. All rights reserved.
Versatile Biodiesel Production by Catalytic Separative Reactors Anton A. Kiss AkzoNobel Research, Development & Innovation, Process & Product Technology, Arnhem, The Netherlands,
[email protected]
Abstract This study proposes an integrated biodiesel production via a two-step process that combines the advantages of using solid acid and base catalysts with the integration of reaction and separation. Such an integrated separative reactor is flexible to treat any range of free fatty acids present in the fatty raw material. Computer aided process engineering tools such as AspenONE are used for process design and simulation of a plant producing 10 ktpy biodiesel from animal fat and bio-ethanol. Keywords: reactive distillation, green catalysts, solid acid / base, biofuels
1. Introduction The recent steep increase in fossil fuel prices associated with governmental restrictions on discharge of green-house gasses shifted the worldwide trend to focus on renewable energy sources. Biodiesel is a very popular renewable fuel, currently produced from vegetable oils, animal fat or even recycled waste cooking-oil from the food industry.1,2 Due to its properties similar to petrodiesel, biodiesel can be used in pure form, or may be blended with petroleum diesel at any concentration, in most modern diesel engines. As a green fuel, biodiesel that has many advantages over conventional petrodiesel: it is safe, renewable, non-toxic and biodegradable, it contains insignificant amounts of sulfur and its increased lubricity extends the life of diesel engines. In addition, it has a high cetane number (above 60 compared to 40 for petrodiesel), a high flash point (>130°C) and it emits ~70% fewer hydrocarbons, ~80% less CO2, and ~50% less particles.1-3 Biodiesel is a mixture of fatty esters, currently produced by (trans-)esterification of triglycerides and free fatty acids, followed by several neutralization and purification steps. However, all the traditional methods suffer from drawbacks related to the use of liquid acid/base catalysts, heading to major economical and environmental penalties, especially considering the recent boost of the international biodiesel production rate. Worldwide, the production of biodiesel increased tremendously during the past 10 years, mostly in Asia, US, and Western Europe with Germany, France, Austria, Spain and UK among top consumers (Figure 1). This work proposes a novel two-step biodiesel production process bases on reactiveseparation using solid acid/base catalysts, thus simplifying the overall process and bringing significant benefits: high conversion and selectivity, elimination of conventional catalyst-related operations, no waste streams, as well as reduced capital investment and operating costs. The process design of the 10 ktpy fatty acid ethyl esters plant described here is based on experimental results, integrated in rigorous simulations performed using AspenTech AspenONE as computer aided process engineering tool.
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16000 14000 12000 10000 8000 6000 4000 2000 0
Biodiesel production per region (ktons / year)
Biodiesel consumption in EU, 2007
North America Central & South America Western Europe Central & Eastern Europe Asia
13%
51%
5% 5% 6%
20%
2000
2002
2004
2006
2008
Germany France Austria UK Spain Others EU
Figure 1. Biodiesel production per region (left), and biodiesel consumption in EU (right).
2. Problem statement All conventional biodiesel production methods have associated optimal operating parameters and downstream processing steps, although much of the available literature emphasizes the base catalyzed route.1-3 Traditional biodiesel processes employ liquid catalysts, such as H2SO4 or KOH.3 The problem is that such catalysts demand neutralization, separation, washing, recovery, and salt waste disposal operations with serious economical and environmental consequences. Nowadays, the surplus of waste oil available at industrial scale would allow production of very cheap biodiesel – a key benefit in the energy market. For example, in Brazil alone, more than 350 millions litres of biofuel are produced annually from animal fat. The problem with the animal fat or waste-oil, is that it becomes useless within 24 hours since it turns so acidic due to the increased free fatty acids (FFA) content, that it is more appropriate for making soap than for biodiesel. To solve these problems, we propose a sustainable two-step process based on the esterification of FFA’s in a separative reactor using solid acids,4,5 followed by trans-esterification of the remaining tri-glycerides (TG) using conventional or solid base catalysts. 1: R-COOH + EtOH ↔ R-COO-Et + H2O (esterification) 2: TG + 3 EtOH ↔ 3 RCOO-Et + Gly
(trans-esterification)
The integrated reactive distillation equipment proposed in this work is able to shift the chemical equilibrium and drive the esterification reaction to completion by continuously removing the fatty esters products and water by-product.6,7 The raw materials consist of waste-oil or animal fat – mainly a mixture of free fatty acids – and a light alcohol, such as methanol or (bio-)ethanol. A key feature of this work is the replacement of anhydrous ethanol by its hydrous azeotrope, thus leading to further reduction of production costs. Table 1 presents an overview of the available solid acid and base catalysts for biodiesel production by (trans-)esterification.4-6 In this work we selected the metal oxides as solid acid catalysts for FFA esterification (first step) and calcium ethoxide as solid base catalyst for the trans-esterification of the remaining tri-glycerides (second step).
Versatile Biodiesel Production by Catalytic Separative Reactors
3
Table 1. Advantages and disadvantages of the acid/base catalysts tested for (trans-)esterification.
Catalyst type Ion-exchange resins (Nafion, Amberlyst) TPA (H3PW12O40) TPA-Cs (Cs2.5H0.5PW12O40) Zeolites (H-ZSM-5, Y and Beta) Sulfated metal oxides (zirconia, titania, tin oxide) Niobic oxide (Nb2O5) Calcium oxide / CaO Calcium methoxide / Ca(OMe)2 Calcium ethoxide / Ca(OEt)2 Li-dopped zinc oxide / ZnO KF loaded on Eu2O3
Benefits Very high activity Easy regeneration Very high activity Super acid sites Controlable acidity and hydrophobicity High activity Thermally stable Water tolerant Low temperatures High yield, reusable High yield, short times Low temperatures Short reaction times
Drawbacks Low thermal stability Possible leeching Soluble in water Low activity per weight Small pore size Low activity Deactivates in water, but not in organic phase Average activity Long reaction times High reactants ratio High reactants ratio Long reaction times Incomplete yields
3. Simulation methods The simulation methods available are given in Table 2. Each method has important benefits but also certain drawbacks and the requirements can differ significantly. The amount of data required by the rigorous method is practically not feasible in practice while the shortcut method leads to low-fidelity models only, with limited applications. For practical reasons, the hybrid approach gives the best results. In this work the experimentally determined kinetic parameters were used 5,6 but the fatty components were lumped into one fatty acid/ester compound, according to the reaction: R-COOH + EtOH ↔ R-COO-Et + H2O
Drawbacks
Benefits
Requirements
Table 2. Simulation methods for biodiesel production: requirements, benefits and drawbacks.
Rigorous method
Shortcut method
Hybrid method
Properties for all species. VLL data and BIP’s for all pairs of components. Kinetic parameters for all reactions possible.
Properties for single fatty acid/ester/tri-glyceride. VLL data for the system ester/glycerol/alcohol. Asumed conversion (no kinetic parameters). Simple model. Fast simulations. Easy-to-build mass and energy balance. No data needed for all species present. No comparison possible for various feedstocks. Low-fidelity model. Less ability to use RTO.
Single or reduced list of fatty acid/ester/TG. Short list of VLL data and BIP’s for components. Reduced list of kinetic parameters, few reactions. Optimization possible for reaction and separation. Certain ability to compare various feedstocks. Better model fidelity. Fast simulations for RTO. More effort to build component list and get kinetic parameters. More work to find VLL data and regress BIP’s.
Easy optimization of reaction and separation. High fidelity model. Usable for many plants. Easy comparison for various feedstocks. Slow simulations and convergence problems. Expensive measurements. Limited RTO and model based control usage.
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4. Results and discussion The properties of the fatty components were determined experimentally, or estimated using state-of-the-art contribution methods such as UNIFAC – Dortmund modified.5-9 Figure 2 (left) shows the residue curve map (RCM) for the ethanol-water-glycerol ternary mixture. The presence of the ethanol-water azeotrope does not hinder the biodiesel production process, since ethanol is a reactant and not a high purity product. Moreover, as water by-product from the esterification reaction further dilutes the ethanol, its hydrous azeotrope can be used directly as a lower-cost feedstock. Vapor pressure is perhaps one of the most important properties with a critical effect in modeling reactive separations. Figure 2 (right) shows the vapor pressure of most common fatty acids and esters. At ambient pressure the boiling points are relatively high, exceeding 300 °C. Although high purity products are possible by reactive distillation, the high temperature in the reboiler – caused by the high boiling points, is in conflict with the thermo-stability of the biodiesel product. However, this problem can be avoided by working at lower pressure or by allowing ethanol in the bottom product. Figure 3 presents the flowsheet of a two-step biodiesel production process based on a reactive distillation column (RDC) as the key unit for esterification or pre-treatment of free fatty acids (FFA). The process proposed here was rigorously simulated and optimized using AspenTech AspenONE. The production rate considered for the plant designed in this work is 10 ktpy fatty acid ethyl esters (FAEE). Note that the kinetic parameters used in the simulation were previosly reported in the open literature.5,6 The RDC is operated in the temperature range of 100–250 °C, at ambient pressure. Out of the 15 stages of the integrated unit, the reactive zone is located in the middle of the column (10 stages). The fatty acid is pre-heated then fed as hot liquid on top of the reactive zone while a stoichiometric amount of alcohol introduced in the bottom of the reactive zone, thus creating a counter-current V-L flow regime over the middle reactive section. The reflux ratio is very low (RR=0.1) as returning water to the column is detrimental to the chemical equilibrium. Water by-product is removed in top, then separated in a decanter from which only the fatty acids are recycled to the column while water is recovered at high purity and hence reusable as industrial water on the same site. 10
Residue Curve Map: ethanol-water-glycerol
Vapor pressure / bar
Mole fraction ethanol
1
0.8
0.6
0.4
1
0.1
E-LAURIC E-MYRISTIC E-PALMITIC E-STEARIC E-OLEIC E-LINOLEIC E-LINOLENIC TG-LAURIC TG-MYRISTIC TG-PALMITIC TG-STEARIC TG-OLEIC TG-LINOLEIC TG-LINOLENIC
Fatty esters
Tri-glicerides
0.01
0.2
0 0
0.2
0.4
0.6
Mole fraction water
0.8
1
0.001 100
200
300 Temperature / C
400
500
Figure 2. RCM ethanol-water-glycerol (left), Vapor pressure of fatty esters vs temperature (right).
Versatile Biodiesel Production by Catalytic Separative Reactors
FFA-TG
TOP
1
HEX1
5
F-FAT
REC-ACID
DEC
RDC HEX2
WATER
B-DIESEL
REC-ALCO
F-ALCO
ALCO
FAEE-TG BTM
FLASH
COOLER
RX
SEP
R-OUT
ALCO
1. Esterification of FFA 2. Trans-esterification of TG
2
GLYCEROL
Figure 3. Flowsheet of biodiesel production by catalytic reactive distillation.
The fatty esters are delivered as high-purity bottom product of the RDC. The hot product is flashed first to remove the traces of ethanol, then it is send to the transesterification reactor to further convert the remaining tri-glycerides to fatty esters. In this work we considered the worst case scenario, with 100% FFA in the feedstock. The mass and energy balance is given in Table 3. High purity products are possible, the purity specifications exceeding 99.9%wt for the final biodiesel product (FAEE stream). Note that the total amount of the recycle streams (REC-ACID and REC-ALCO) is not significant, representing only ~0.5% of the total biodiesel production rate. Figure 4 (left) shows the liquid composition profiles in the reactive distillation column. The concentration of fatty acids decreases while the concentration of fatty esters increases from the top to bottom. Similarly, the ethanol concentration decreases while water concentration increases from bottom to top. The temperature and reaction rate profiles in the RDC are presented in Figure 4 (right). As expected, the reaction rate exhibits a maximum in the middle of the column, in the reactive zone. Moreover, the concentration of water is low in the reactive zone, hence the catalyst activity is not affected. Nevertheless, the concentration of reactants is relatively high and the temperature is sufficiently high to allow high reaction rates and complete conversion. Table 3. Mass and energy balance of a 10 ktpa biodiesel production process, based on RD. F-ACID Temperature K Pressure atm Vapor Frac Mass Flow kg/hr Volume Flow l/min Enthalpy Gcal/hr Mass Flow kg/hr ETHANOL ACID WATER ESTER-E Mass Frac ETHANOL ACID WATER ESTER-E
F-ALCO
BTM
TOP REC-ACID
REC-ALCO
FAEE
WATER
372.6 1 0 7.312 0.154 -0.006
303.1 1 0 1250 23.774 -1.035
303.1 1 0 109.094 1.839 -0.413 0 0.008 109.086 0
418.1 1.036 0 1094.918 22.986 -0.892
352.2 507.9 1.036 1.017 0 0 264.202 1257.312 6.156 28.953 -0.394 -0.886
372.7 0.987 0 109.246 1.983 -0.405
303.1 1 0 0.152 0.003 0
0 1094.918 0 0
253.568 2.622 0 0 10.635 0.029 0 1254.661
0 0.115 109.089 0.043
0 0.107 0.003 0.042
0.857 1.765 0 0 0.013 0.016 6.442 1248.219
0 0.001 0.999 0
0 0.702 0.02 0.278
0.117 0 0.002 0.881
0 1 0 0
0.96 0 0.04 0
0.002 0 0 0.998
0.001 0 0 0.999
0 0 1 0
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Molar fraction 0
0.2
0.4
0.6
Temperature / °C 0.8
1
0
0
50
100
150
200
250
0
3
Temperature
3
Water Acid
6
6
Stage
Reaction rate
9
9
12
Alcohol
12
Ester
15
15 0
0.2
0.4
0.6
Molar fraction
0.8
1
0
1 2 3 4 Reaction rate / kmol/hr
5
Figure 4. Profiles in RDC: liquid composition (left), temperature and reaction rate (right).
5. Conclusions An innovative two-step biodiesel process based on separative reactors using solid catalysts was developed in this study using computer aided engineering tools such as AspenTech AspenONE. The novel two-step process proposed here improves considerably the biodiesel production and reduces drastically the number of downstream processing steps. The major benefits of this sustainable process are: • Flexible integrated reactor suitable for a large range of fatty raw material with up to 100% FFA content, such as: frying oils, animal tallow, tall oil, waste vegetable oil. • Straightforward and robust process with no soap formation, no catalyst-related waste streams, and sulfur-free biodiesel as solid acids do not leach into the product. • Effective use of the integrated reactor volume leading to high unit productivity. • Efficient use of the raw materials: complete conversion and high selectivity, stoichiometric reactants ratio, FFA conversion to esters and not to soap waste. • Reduced equipment costs, with up to ~40% savings on the total investment costs. • Competitive operating costs due to the integrated design and the elimination of conventional steps: handling of homogeneous catalyst and corrosive solutions, separation and disposal of salts, waste water treatment, excess alcohol recovery.
References 1. 2. 3. 4. 5. 6. 7. 8. 9.
M. Balat, H. Balata, 2008, Energy Conversion and Management, 49, 2727. M.G. Kulkarni, A. K. Dalai, 2006, Industrial & Engineering Chemistry Research, 45, 2901. K. Narasimharao, A. Lee, K. Wilson, 2007, J. Biobased Materials & Bioenergy, 1, 19. T. Okuhara, 2002, Chemical Reviews, 102, 3641. A. Kiss, A. C. Dimian, G. Rothenberg, 2006, Advanced Synthesis & Catalysis, 348, 75. A. Kiss, G. Rothenberg, A. C. Dimian, F. Omota, 2006, Topics in Catalysis, 40, 141. A. Kiss, A. C. Dimian, G. Rothenberg, 2008, Energy & Fuels, 22, 598. S. Steinigeweg, J. Gmehling, 2003, Ind. Eng. Chem. Res., 42, 3612. C. Dimian, F. Omota, A. Bliek, 2004, Chem. Eng. & Proc., 43, 411.
