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Separative reactors for integrated production of bioethanol and biodiesel
Computers and Chemical Engineering 34 (2010) 812–820
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Computers and Chemical Engineering journal homepage: www.elsevier.com/locate/compchemeng
Separative reactors for integrated production of bioethanol and biodiesel Anton A. Kiss ∗ AkzoNobel Research, Development & Innovation, Process & Product Technology Velperweg 76, 6824 BM, Arnhem, The Netherlands
a r t i c l e
i n f o
Article history: Received 24 July 2009 Received in revised form 7 September 2009 Accepted 10 September 2009 Available online 16 September 2009 Keywords: Reactive distillation Green catalysts Solid acid/base Sustainable biofuels FFA FAEE
a b s t r a c t Conventional integration of bioethanol and biodiesel plants employs the use of anhydrous ethanol in the biodiesel production process. The problem is that the production of anhydrous bioethanol is very energydemanding, especially due to the azeotropic distillation required to producing high purity ethanol. The use of hydrous ethanol in the biodiesel production is preferable but unfeasible in conventional processes due to the equilibrium limitations and the economic penalties caused by the additional process steps. To solve this problem, this study proposes a novel energy-efficient integrated production of biodiesel from hydrous bioethanol. The key to success is a novel setup that combines the advantages of using solid catalysts with the integration of reaction and separation. This integrated process eliminates all typical catalyst-related operations, and efficiently uses the raw materials and the reactor volume in a separative reactor that allows significant savings in the capital and operating costs. Rigorous simulations embedding experimental results were performed using computer aided process engineering tools – such as AspenTech Aspen Plus – to design the separative reactor and evaluate the overall technical feasibility of the process. The RD column was simulated using the rigorous RADFRAC unit with RateSep (rate-based) model, and explicitly considering three phases balances. Sensitivity analysis was used to determine the optimal range of the operating parameters. The main results are given for a plant producing 10 ktpy biodiesel (>99.9%wt) from hydrous bioethanol (96%wt) and waste vegetable oil with high free fatty acids content (∼100%), using solid acids as green catalysts. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The governmental restrictions on discharge of green-house gasses associated with the steep changes in fossil fuel prices, shifted the worldwide trend to focus on renewable energy sources. Biodiesel is an alternative renewable fuel, currently produced from vegetable oils, animal fats or even recycled waste cooking-oils from the food industry (Buczek & Czepirski, 2004; Encinar, Gonzalez, & Rodriguez-Reinares, 2005; Kulkarni & Dalai, 2006). Compared to regular diesel, biodiesel is an environmental friendly fuel, with better combustion and reduced emissions of greenhouses gases, while sulfur is practically absent (Bowman, Hilligoss, Rasmussen, & Thomas, 2006). Due to its properties similar to regular diesel, biodiesel can be used in most modern diesel engines, in pure form or blended with petroleum diesel at any concentration (Balat & Balat, 2008). Fatty esters, the main components of biodiesel, are currently produced by (trans-)esterification of tri-glycerides and free fatty acids, followed by several neutralization and purification steps. However, most traditional methods suffer from drawbacks related
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to the use of liquid acid/base catalysts, leading to major economical and environmental penalties (Knothe, Gerpen, & Krahl, 2005; Narasimharao, Lee, & Wilson, 2007). In spite of the technical problems (Meher, Vidya Sagar, & Naik, 2006), as well as the social and sustainability issues (Kumar & Sharma, 2005; Puhan, Vedaraman, Rambrahamam, & Nagarajan, 2005), the biodiesel production rate maintained a tremendous increase during the past ten years, mostly in Asia, US, and Western Europe—as illustrated by Fig. 1. Although, biodiesel consists typically of fatty acid methyl esters – as methanol is the cheapest alcohol used at industrial scale – other alcohols such as ethanol may be employed as well. Bioethanol is the most important biofuel today, currently produced by fermentation of sugar derived from various crops such as sugar cane, corn and sugar beet (Kamm, Gruber, & Kamm, 2006; Liu & Tanaka, 2006; Mielenz, 2001; Solomon, Barnes, & Halvorsen, 2007). Just as other biofuels, ethanol can be blended with gasoline and used in existing or optimized flexi-fuel engines. Moreover, it burns cleaner than gasoline, producing less CO, CO2 and NOx emissions (Blottnitz & Curran, 2007). Fig. 2 shows that bioethanol is also experiencing a very fast growth on the global scale, particularly in Brazil and United States. Remarkably, Brazil gets more than 30% of its transport fuels from sugar cane bioethanol (SRI Consulting, 2009). The production of anhydrous bioethanol is very energydemanding – a major reason being the azeotropic distillation
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ations, no waste streams, as well as reduced capital and operating expenditures. The process design of a fatty acid ethyl esters plant described here is based on experimental results, integrated in rigorous simulations performed using AspenTech Aspen Plus as a computer aided process engineering tool (Aspen Technology, 2009a, 2009b). 2. Biodiesel production processes
Fig. 1. Biodiesel production worldwide, per region.
Fig. 2. Bioethanol production per country—Top 5.
required to producing pure ethanol – hence various process integrations were proposed in order to reduce the energy requirements (Cardona & Sanchez, 2007; Pfeffer, Wukovits, Beckmann, & Friedl, 2007). Since bioethanol can be used in the biodiesel production, many bioethanol plants integrate nowadays biodiesel production. According to SRI Consulting (2009), such an integrated bioethanol and biodiesel plant was developed by Dedini and it is being used since 2006 by Barralcool Mill (State of Mato Grosso, Brazil). Fig. 3 illustrates the current bioethanol–biodiesel process integration as well as the new model proposed in this work. Obviously, it would be much more convenient to use hydrous ethanol in the biodiesel production, but this is currently unfeasible in conventional processes due to equilibrium limitations and economic penalties caused by the additional processing steps. This study makes a brief overview of existing biodiesel production processes and the associated benefits and drawbacks, and proposes a novel separative reactor using hydrous bioethanol over solid acid catalysts. This integrated solution simplifies the overall process and brings significant benefits, such as: high conversion and selectivity, elimination of conventional catalyst-related oper-
Nowadays, the most widespread manufacturing technologies use homogeneous catalysts, in batch or continuous processes where both reaction and separation steps can create bottlenecks. The literature study reveals the following processes currently in use at pilot or industrial scale: 1. Batch processes. These allow high flexibility with respect to composition of the feedstock. The trans-esterification is performed using an acid or base catalyst (Lotero et al., 2005; Narasimharao et al., 2007). Nevertheless, the equipment productivity is low and the operating costs are high (Van Gerpen, 2005). Moreover, the use of liquid catalyst has severe economical and environmental penalties (Hanna, Isom, & Campbell, 2005). 2. Continuous processes combine the esterification and transesterification steps, allowing higher productivity. However, most of these processes are still plagued by the disadvantages of using homogeneous catalysts (Vicente, Martinez, & Aracil, 2004) although solid catalysts emerged in the last decade (Dale, 2003; Dossin, Reyniers, Berger, & Marin, 2006; Kiss, Dimian, & Rothenberg, 2006a; Kiss, Dimian, & Rothenberg, 2006b; Kiss, Rothenberg, Dimian, & Omota, 2006). Nevertheless, integrated processes based on reactive distillation have been also reported (He, Singh, & Thompson, 2006; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006; Suwannakarn, Lotero, Ngaosuwan, & Goodwin, 2009). Moreover, an innovative process – known as ESTERFIP-HTM – was developed for the trans-esterification with methanol by the French Institute of Petroleum. The process is based on heterogeneous catalyst based on zinc and aluminum oxides and it is currently being applied in commercial plants (Bournay, Casanave, Delfort, Hillion, & Chodorge, 2005). However, it requires relatively high temperature (210–250 ◦ C) and pressure (30–50 bar). 3. Supercritical processes were developed to solve the problem of miscibility of oil and alcohol that hinders the kinetics of transesterification, as well as to take advantage of not using a catalyst at all. However, the operating conditions are severe (T > 240 ◦ C, p > 80 bar) and therefore require special equipment (Cao, Han, & Zhang, 2005; He, Wang, & Zhu, 2007). 4. Hydrolysis and esterification processes. These are simpler processes as the glycerides are hydrolyzed first to fatty acids that are esterified in a second step to fatty esters (Kusdiana & Saka, 2004; Minami & Saka, 2006). Such processes have become very
Fig. 3. Integrated bioethanol–biodiesel plant: present (left) and future (right).
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Table 1 Pro/Con of the acid/base catalysts tested for (trans-)esterification. Catalyst type
Benefits
Drawbacks
Ion-exchange resins (Nafion, Amberlyst)
Very high activity Easy regeneration Very high activity Super acid sites Controlable acidity and hydrophobicity High activity Thermally stable Water tolerant
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
Low temperatures High yield, reusable High yield, short times Low temperatures Short reaction times
Long reaction times High reactants ratio High reactants ratio Long reaction times Incomplete yields
TungstoPhosphoric Acid (H3 PW12 O40 ) TPA-Cs (Cs2.5 H0.5 PW12 O40 ) Zeolites (H-ZSM-5, Y and Beta) Sulfated metal oxides (zirconia, titania, tin oxide) Niobic oxide (Nb2 O5 ) Calcium oxide/CaO Calcium methoxide/Ca(OMe)2 Calcium ethoxide/Ca(OEt)2 Li-dopped zinc oxide/ZnO KF loaded on Eu2 O3
attractive and gain market share due to obvious advantages. High purity glycerol is obtained as by-product of hydrolysis step. Tailored properties of the biodiesel fuel are possible using esterification (Knothe, 2005). Moreover, the esterification step can be performed using solid acid catalysts (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006) in an integrated reactiveseparation setup (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Dimian, & Rothenberg, 2008; Kiss, Rothenberg, et al., 2006; Kiss, 2009; Omota, Dimian, & Bliek, 2003). The usage of heterogeneous catalysts avoids the neutralization and washing steps, leading to a simpler and more efficient process. 5. Enzymatic processes have low energy requirements, as the reaction is carried out at mild conditions—ambient pressure and a temperature of 50–55 ◦ C. However, due to the lower yields and the long reaction times the enzymatic processes can still not compete with other processes at industrial scale (Demirbas, 2008; Demirbas & Balat, 2006; Van Gerpen, 2005). 6. Hydro-pyrolysis processes employ a fundamentally different chemical route as compared to the previously described manufacturing methods. Tri-glycerides are converted to fuel by hydrogenation followed by pyrolysis. The key difference is that the fuel product (second-generation biodiesel) is a mixture of long-chain hydrocarbons instead of the conventional fatty esters. The process is known as NExBTL (biomass to liquid) and it was invented by the Finnish company Neste Oy (www.nesteoil.com). While it has clear advantages, this process requires more complex equipment and implies the availability of a low-cost hydrogen source. 3. Problem statement As shown in the previous section, several biodiesel production methods exist, each of them having associated optimal operating parameters and various downstream processing steps. Nonetheless, much of the available literature emphasizes the base catalyzed route (Demirbas, 2008; Knothe, 2005). The problem with using liquid acid or base catalysts is that they require neutralization, separation, washing, recovery, and salt waste disposal operations with serious economical and environmental penalties. Nowadays, the surplus of waste vegetable oil (wvo) or animal fat available at industrial scale would allow production of low-cost biodiesel from bioethanol. For example, in Brazil alone, more than 350 millions liters of biofuel are produced annually from animal fat (SRI Consulting, 2009). The problem with the animal fat or waste oil, is that it becomes useless within 24 h 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 all these problems, we propose a sustainable integrated process based on the esterification of FFA’s with hydrous bioethanol in a separative reac-
tor using solid acids. When required, this step can be followed by the trans-esterification of the remaining tri-glycerides (TG) using solid base catalysts. 1. R-COOH + EtOH ↔ R-COO-Et + H2 O (esterification) 2. TG + 3 EtOH ↔ 3 RCOO-Et + Gly (trans-esterification) The integrated separative reactor proposed in this work is able to shift the chemical equilibrium and drive the esterification reaction to completion by continuously removing the fatty acid ethyl esters and water by-product. The raw materials consist of waste-oil or animal fat – mainly a mixture of free fatty acids – and a cheap alcohol such as hydrous bioethanol (96%wt C2 H5 OH, 4%wt H2 O). A key feature of this work is the novel integration mode of the bioethanol and biodiesel plants, the replacement of anhydrous ethanol by its hydrous azeotrope leading to significant reduction of the biodiesel and bioethanol production costs. Table 1 presents an overview of the available solid acid and base catalysts for biodiesel production by (trans-)esterification. In this work we selected the metal oxides (sulfated zirconia) as water-tolerant solid acid catalysts for FFA esterification (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Calcium ethoxide may be also employed as solid base catalyst for the trans-esterification of the remaining tri-glycerides (Liu, Piao, Wang, & Zhu, 2008a; Liu, Piao, Wang, & Zhu, 2008b). 4. Simulation methods Depending on the composition of any oil/fat feedstock used in the biodiesel production process, there is an associated blend of fatty acid esters in the final biodiesel product, with major consequences on the biodiesel properties (Encinar et al., 2005; Knothe, 2005). The simulation of a biodiesel production process can be performed using one of the available simulation methods illustrated in Table 2: rigorous, shortcut or hybrid method. Each of these methods has clear advantages but also specific drawbacks. Moreover, the requirements in terms of input data can differ considerably and it has a great impact as the information reflects directly in the results. Obviously, the quality of output is determined by the quality of the input, as illustrated by the ‘garbage in–garbage out’ concept. For example, if the property method is improperly selected, the result of the simulation will most likely be inaccurate. Similarly, if incorrect kinetic data is used as input, or some key chemicals are missing from the list of components, the output of the simulation is unlikely to be any informative. The rigorous method is favored due to the accurate results, but it is virtually not feasible in practice due to the amount of input data required. On the other hand, the shortcut method is very handy due to its quickness in making results available, but it provides merely
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Table 2 Simulation methods for biodiesel production: requirements, benefits and drawbacks. Rigorous method
Shortcut method
Hybrid method
Properties for all species.
Single or reduced list of fatty acid/ester/TG.
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).
Easy optimization of reaction and separation.
Simple model.
High fidelity model. Usable for many plants. Easy comparison for various feedstocks.
Fast simulations. Easy-to-build mass and energy balance. No data needed for all species present.
Optimization possible for reaction and separation. Certain ability to compare various feedstocks. Better model fidelity. Fast simulations for RTO.
Slow simulations and convergence problems.
No comparison possible for various feedstocks.
Expensive measurements. Limited RTO and model based control usage.
Low-fidelity model. Less ability to use RTO.
Requirements
Benefits
Drawbacks
low-fidelity models with very limited practical applications. Therefore, for practical reasons, the hybrid approach gives the best results as it combines the advantages of both rigorous and shortcut methods, while limiting the overall disadvantages due to the synergistic effect. The following AspenTech Aspen Plus simulations use kinetic parameters previously determined experimentally (Dimian, Bildea, Omota, & Kiss, 2009; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006), but the fatty components were lumped into one fatty acid and its corresponding fatty ester compound—according to the following chemical reaction: RCOOH + C2 H5 OH ↔ R-COO-C2 H5 + H2 O. Dodecanoic acid/ester was selected as lumped component due to the availability of experimental results, kinetics and VLLE parameters for this system (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). The assumption of lumping components is very reasonable since fatty acids and their corresponding fatty esters have similar properties, as shown hereafter. Note that this approach has been successfully used in the past to simulate other biodiesel production processes, based on reactive distillation for example (Dimian et al., 2009; Dimian, Omota, & Bliek, 2004; Kiss, Dimian, et al., 2006a, 2006b; Kiss et al., 2008; Kiss, Rothenberg, et al., 2006; Omota et al., 2003; Steinigeweg & Gmehling, 2003; von Scala, Moritz, & Fassler, 2003). 5. Results and discussion The integrated reactive distillation process was designed according to previously reported process synthesis methods for reactive separations (Noeres, Kenig, & Gorak, 2003; Schembecker
Short list of VLL data and BIP’s for components. Reduced list of kinetic parameters, few reactions.
More effort to build component list and get kinetic parameters. More work to find VLL data and regress BIP’s.
& Tlatlik, 2003). The properties of the fatty components were determined experimentally, or estimated using state-of-theart contribution methods such as UNIFAC—Dortmund modified. UNIQUAC parameters are also available in Aspen Plus (Aspen Technology, 2009a, 2009b). Note that the phase equilibrium depends on temperature, pressure, as well as reactants ratio: 1. LE, one liquid phase, at T < 100 ◦ C and excess of alcohol. Water by-product remains in the reaction mixture, and the conversion is low due to lower reaction rate. 2. LLE, two liquid phases at T < 100 ◦ C and stoichiometric ratio of alcohol. Liquid separation is undesirable due to the catalyst deactivation in the presence of water. 3. VLE, vapor–liquid at T > 100 ◦ C and continuous removal of water (open system). 4. VLLE, vapor–liquid–liquid equilibrium at T > 100 ◦ C, P > 1 bar (closed system). Again, the water phase separation is not desirable for the activity of the catalyst. Obviously, the most convenient case for the reactive zone is VLE—one liquid and one vapor phase at elevated temperatures leading to higher reaction rates, and with continuous removal of water by-product hence shifting the equilibrium towards ester formation. In the following simulations, the rigorous VLL equilibrium was used in order to correctly determine the stages on which phase separation may occur. Fig. 4 (left) shows the residue curve map (RCM) for the ethanol–water–acid ternary mixture. The presence of the ethanol–water azeotrope does not hinder the biodiesel production
Fig. 4. Residue curve map (RCM) and two-liquid (LL) phase envelope of the mixture ethanol–water–acid.
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Fig. 5. Vapor pressure of the most common fatty acids vs temperature.
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. The VLL envelope of the same mixture is illustrated in Fig. 4 (right). Remarkably, phase splitting may occur if the molar fraction of water exceeds 0.3 in the liquid phase. Obviously, the presence of free water is not desired as it may lead to catalyst deactivation (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). However, as shown later by the simulation results, this is not the case in the reactive zone of the column. The analysis of the RCM also shows that quantitative water removal can be obtained by means of a reflux of fatty acid instead of alcohol (Dimian et al., 2009; Omota et al., 2003). In this case the process should be fed with a stoichiometric amount of alcohol. As a result, the column behaves rather as a reactive absorption than reactive distillation (Dimian et al., 2009; Kiss, 2009). This finding was previously confirmed experimentally in a laboratory column equipped with Katapak packing hosting a solid acid resin (Steinigeweg & Gmehling, 2003). Moreover, as confirmed by the results hereafter, it was observed that using an excess of alcohol does not improve significantly the conversion of fatty acid. Vapor pressure is perhaps one of the most important properties with a critical effect in modeling reactive separations. Figs. 5 and 6 show the vapor pressure of the most common fatty acids and fatty acid ethyl esters. At ambient pressure the boiling points are relatively high, exceeding 300 ◦ C. These values are in line with previous reported data (Poling, Prausnitz, & O’Connell, 2001; Yuan, Hansen, &
Fig. 6. Vapor pressure of the most common fatty acids ethyl esters.
Zhang, 2005). 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 a small amount (∼0.2%) of ethanol in the bottom product. Kinetic data for esterification of dodecanoic acid with various alcohols is available from previous work (Dimian et al., 2009; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Since the reverse hydrolysis reaction is negligible, a simple kinetic expression can be used for simulation purposes. The reaction rate is given by r = k·Wcat ·CAcid ·CAlcohol , where k = A·exp(−Ea /RT). Note that by changing the weight amount of catalyst used (Wcat ) the reaction rate can be similar for different catalysts. In this particular case, the kinetic constant is given by k = 9.6 × 104 ·exp(−35,600/RT)–meaning that the catalyst is about 20% less active as compared to the reaction with methanol. This is in line with previous findings that the reaction rate decreases with the increased chain length of the alcohol, in the order: methanol > ethanol > propanol . . . 2-ethyl hexanol (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Fig. 7 presents the flowsheet of an integrated biodiesel production process based on a reactive distillation column (RDC) as the key unit for esterification or pre-treatment of free fatty acids. This flowsheet can be optionally extended by adding a second step for the trans-esterification of tri-glycerides, as illustrated in Fig. 8. The process proposed here (Fig. 7) was designed, simulated and analyzed using AspenTech Aspen Plus. The rigorous RADFRAC unit with
Fig. 7. Flowsheet of biodiesel production by catalytic reactive distillation.
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Fig. 8. Flowsheet of a two-step process for biodiesel production.
RateSep (rate-based) model – explicitly considering the three phase balances on all stages – was used for the simulation of the RDC. The top part of the reactive distillation column is the rectifying section, while the bottom part is the stripping section. The reactive zone is located in the mid-part of the column, where the catalyst is present and the reactions take place. The reactants can be fed as liquids at the boiling point (hydrous ethanol) or at the reaction temperature (free fatty acids). Several heat exchangers are required to pre-heat the reactants, and cool down the products. Further heatintegration is certainly possible, but this is out of the scope of the present study. 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 is introduced in the bottom of reactive zone, thus creating a counter-current VL 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 the top, then separated in a decanter from which only the fatty acids are recycled to the column. The aqueous outlet of the decanter is recovered at high purity; thus it is reusable as industrial water on the same site. 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 cooled down and stored as biodiesel product. Alternatively (case shown in Fig. 8), the flashed stream may be send to a trans-esterification reactor to further convert the remaining triglycerides to fatty esters. However, in this work we consider only
the worst case scenario, in terms of maximum processing capacity of the column, with 100% FFA in the feedstock. The production rate of the plant described here is 10 ktpy fatty acid ethyl esters (FAEE)—equivalent to 1250 kg/h biodiesel (>99.9%wt). Note that hydrous bioethanol (96%wt) is used as raw material in the process. The complete mass and energy balance is given in Table 3, while the main process design parameters – such as column size and catalyst loading – are listed in Table 4. The productivity of this separative reactor is over 20 kg fatty ester per kg catalyst per hour. 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 (RECACID and REC-ALCO) is not significant, representing only ∼0.5% of the biodiesel production rate. As a consequence, the non-linear behavior that may be caused by recycles can be avoided (Kiss, Bildea, & Dimian, 2007; Kiss, Bildea, Dimian, & Iedema, 2005). Fig. 9 shows the liquid and vapor composition profiles in the reactive distillation column. The concentration of fatty acids decreases while the concentration of fatty esters increases from the top to the bottom. Similarly, the ethanol concentration decreases while water concentration increases from the bottom to the top of the column. The temperature and reaction rate profiles in the RDC are presented in Fig. 10. Remarkably, the reaction rate exhibits a maximum in the middle of the column—where the reactive zone is located. 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 balance of a 10 ktpy biodiesel process based on reactive-distillation.
Temperature K Pressure atm Vapor Frac Mass Flow kg/h Volume Flow l/min Enthalpy Gcal/h Mass Flow kg/h ETHANOL ACID WATER ESTER-E Mass Frac ETHANOL ACID WATER ESTER-E
F-ACID
F-ALCO
TOP
REC-ACID
REC-ALCO
418.1 1.036 0 1094.921 22.986 −0.892
352.2 1.036 0 264.202 6.156 −0.394
508.2 1.017 0 1257.457 28.967 −0.885
372.7 0.987 0 116.366 2.127 −0.411
303.1 1 0 7.27 0.139 −0.006
373.2 1 0 7.458 0.157 −0.006
303.1 1 0 1250 23.774 −1.035
303.1 1 0 109.096 1.839 −0.413
0 1094.921 0 0
253.568 0 10.635 0
2.636 <0.001 0.019 1254.803
trace 1.097 109.216 6.052
0 1.095 0.124 6.051
0.873 trace 0.009 6.576
1.763 <0.001 0.01 1248.226
trace 0.002 109.092 0.001
trace 0.009 0.939 0.052
0 0.151 0.017 0.832
0.117 41 ppb 0.001 0.882
0.001 132 ppb 8 ppm 0.999
trace 17 ppm 1 12 ppm
0 1 0 0
0.96 0 0.04 0
BTM
0.002 131 ppb 15 ppm 0.998
FAEE
WATER
818
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Fig. 9. Liquid and vapor composition profiles in the reactive distillation column.
Table 4 Design parameters for simulating the reactive distillation column. Parameter
Value
Units
Total number of theoretical stages Number of reactive stages Column diameter HETP Valid phases Volume liquid holdup per stage Mass catalyst per stage Catalyst bulk density Reflux ratio (mass) Boil-up ratio (mass) Condensor duty Reboiler duty Fatty acid conversion Fatty acid feed (heated at 145 ◦ C) Bioethanol feed (azeotrope at boiling point) Production of biodiesel (FAEE) Separative reactor productivity
15
–
10 (from 3 to 12) 0.4 0.5 VLL 5.5
– m m – L
6 1050 0.1 0.075 78 82 >99.99 1095
kg kg/m3 kg/kg kg/kg kW kW % kg/h
264
kg/h
1250
kg/h
20.8
kg FAEE/kg cat/h
Fig. 10. Temperature and reaction rate profiles in the reactive distillation column.
Fig. 11. Purity of RDC products as function of the reflux ratio.
One final question that may arise is: what is the consequence of using the azeotrope feed instead of anhydrous ethanol? Ethanol is consumed by the chemical reaction with the fatty acids hence the composition of the reaction mixture changes from high content of alcohol to lower fractions of ethanol. Therefore there is no crossing through the azeotropic composition towards pure ethanol. On the contrary, there is a dilution of the ethanol that is consumed by the chemical reaction with the water by-product formed in the esterification. For this reason, there is no significant consequence since water is also formed in the reaction but at the same time continuously removed from the separative reactor. The simulation results presented in this study confirm that indeed using hydrous ethanol in this process is feasible. Despite recent progress in understanding the feasibility, design, and control of reactive distillation, the conceptual design of reactive distillation may lead to several different process configurations and operating parameters. In this work, sensitivity analysis was used as a powerful tool to evaluate the optimal range of the operating parameters: reflux ratio, reactants ratio, feed temperature, decanting temperature, flashing pressure, and recycle rates. Note that the design variables can improve the efficiency of the reactive distillation if selected within a design optimization framework (Dalaouti & Seferlis, 2006). The most significant results of the sensitivity analysis are shown in Figs. 11 and 12. Remarkably, the reflux has no significant influence on the purity of the bottom product (Fig. 11). A minimum reflux ratio of 0.05 kg/kg is required in order to obtain high purity top product. The nominal value of 0.1 kg/kg was selected in this
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Fig. 12. Purity of RDC products as function of the reactants ratio.
work. Note that high reflux rates are not recommended as returning the water by-product to the column is not favorable to the chemical equilibrium. The optimal molar ratio of the reactants (alcohol:acid) is the stoichiometric value of one, as illustrated by Fig. 12. If this ratio is higher than one (i.e. excess of alcohol) then the fatty acid is completely converted to fatty ester, but the excess of ethanol becomes a significant impurity in the top stream and thereafter in the water by-product. On the contrary, when the ratio is less than one (i.e. excess of fatty acids) then the purity of water by-product remains high, but the conversion of fatty acid is incomplete—hence the bottom product contains unreacted fatty acid that can not be removed from the final product by simple flashing. Since the separation of fatty acids from fatty esters is more difficult than the separation of fatty acids from water, this situation should be avoided. In practice, using a very small excess of ethanol (∼0.5%) or an efficient control structure that can ensure the stoichiometric ratio of reactants (Dimian et al., 2009), is sufficient for the complete conversion of the fatty acids and prevention of the difficult separation previously mentioned. 6. Conclusions The innovative process proposed in this work allows a feasible integration of bioethanol and biodiesel plants. While previous reports available in the open literature focused on the esterification of fatty acids with methanol, this is – to the best of our knowledge – the first study about using bioethanol in a reactive distillation setup based on solid catalysts. Unlike conventional integration, the separative reactor proposed in this work makes use of hydrous bioethanol in the biodiesel production—thus avoiding the costly azeotropic distillation required to producing anhydrous ethanol. Moreover, no hydrous ethanol is returned to the bioethanol plant for purification due to the use of reactants in stoichiometric ratio—hence the excess of alcohol is avoided. Overall, due to these novel features the integration of bioethanol and biodiesel plants is less energy intensive and therefore more cost-effective. Previous experimental results on solid catalysts were integrated in the process development using computer aided engineering tools such as AspenTech Aspen Plus. Due to the integrated design of the separative reactor and the use of solid catalysts, the number of downstream processing steps in the biodiesel production is significantly reduced. The major benefits of this sustainable process are: • Energy effective integration of biodiesel and bioethanol plants, by using hydrous ethanol as raw material, and consequently avoiding costly azeotropic distillation.
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• Flexible integrated reactor suitable for a large range of fatty raw material with 20–100% FFA content, such as: frying oils, animal tallow, tall oil, waste vegetable oil (wvo). • Simple and robust process with no soap formation, no catalystrelated waste streams, and sulfur-free biodiesel as solid acids do not leach into the product. • Effective use of the integrated separative reactor volume leading to high unit productivity. Efficient use of the raw materials: complete conversion and high selectivity, stoichiometric reactants ratio, FFA conversion to fatty 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.
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Noeres, C., Kenig, E. Y., & Gorak, A. (2003). Modelling of reactive separation processes: Reactive absorption and reactive distillation. Chemical Engineering & Processing, 42, 157–178. Omota, F., Dimian, A. C., & Bliek, A. (2003). Fatty acid esterification by reactive distillation. Part 1: Equilibrium-based design. Chemical Engineering Science, 58, 3159–3174. Pfeffer, M., Wukovits, W., Beckmann, G., & Friedl, A. (2007). Analysis and decrease of the energy demand of bio-ethanol production by process integration. Applied Thermal Engineering, 27, 2657–2664. Poling, B. E., Prausnitz, J. M., & O’Connell, J. P. (2001). The properties of gases and liquids (5th ed.). McGraw-Hill. Puhan, S., Vedaraman, N., Rambrahamam, B. V., & Nagarajan, G. (2005). Mahua (Madhuca Indica) seed oil: A source of renewable energy in India. Journal of Scientific & Industrial Research, 64, 890–896. Schembecker, G., & Tlatlik, S. (2003). Process synthesis for reactive separations. Chemical Engineering & Processing, 42, 179–189. Solomon, B. D., Barnes, J. R., & Halvorsen, K. E. (2007). Grain and cellulosic ethanol: History, economics, and energy policy. Biomass Bioenergy, 31, 415–425. SRI Consulting (2009). Latin American Developments in Renewable Fuels and Chemicals, Platinum Seminar Series. Steinigeweg, S., & Gmehling, J. (2003). Esterification of a fatty acid by reactive distillation. Industrial & Engineering Chemistry Research, 42, 3612–3619. Suwannakarn, K., Lotero, E., Ngaosuwan, K., & Goodwin, J. G. (2009). Simultaneous free fatty acid esterification and triglyceride transesterification using a solid acid catalyst with in situ removal of water and unreacted methanol. Industrial & Engineering Chemistry Research, 48, 2810–2818. Van Gerpen, J. (2005). Biodiesel processing and production. Fuel Processing Technology, 86, 1097–1107. Vicente, G., Martinez, M., & Aracil, J. (2004). Integrated biodiesel production: A comparison of different homogeneous catalyst systems. Bioresource Technology, 92, 297–305. von Scala, C., Moritz, P., & Fassler, P. (2003). Process for the continuous production of fatty acid esters via reactive distillation. Chimia, 57, 799–801. Yuan, W., Hansen, A. C., & Zhang, Q. (2005). Vapor pressure and normal boiling point predictions for pure methyl esters and biodiesel fuels. Fuel, 84, 943–950.
Contents lists available at ScienceDirect
Computers and Chemical Engineering journal homepage: www.elsevier.com/locate/compchemeng
Separative reactors for integrated production of bioethanol and biodiesel Anton A. Kiss ∗ AkzoNobel Research, Development & Innovation, Process & Product Technology Velperweg 76, 6824 BM, Arnhem, The Netherlands
a r t i c l e
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Article history: Received 24 July 2009 Received in revised form 7 September 2009 Accepted 10 September 2009 Available online 16 September 2009 Keywords: Reactive distillation Green catalysts Solid acid/base Sustainable biofuels FFA FAEE
a b s t r a c t Conventional integration of bioethanol and biodiesel plants employs the use of anhydrous ethanol in the biodiesel production process. The problem is that the production of anhydrous bioethanol is very energydemanding, especially due to the azeotropic distillation required to producing high purity ethanol. The use of hydrous ethanol in the biodiesel production is preferable but unfeasible in conventional processes due to the equilibrium limitations and the economic penalties caused by the additional process steps. To solve this problem, this study proposes a novel energy-efficient integrated production of biodiesel from hydrous bioethanol. The key to success is a novel setup that combines the advantages of using solid catalysts with the integration of reaction and separation. This integrated process eliminates all typical catalyst-related operations, and efficiently uses the raw materials and the reactor volume in a separative reactor that allows significant savings in the capital and operating costs. Rigorous simulations embedding experimental results were performed using computer aided process engineering tools – such as AspenTech Aspen Plus – to design the separative reactor and evaluate the overall technical feasibility of the process. The RD column was simulated using the rigorous RADFRAC unit with RateSep (rate-based) model, and explicitly considering three phases balances. Sensitivity analysis was used to determine the optimal range of the operating parameters. The main results are given for a plant producing 10 ktpy biodiesel (>99.9%wt) from hydrous bioethanol (96%wt) and waste vegetable oil with high free fatty acids content (∼100%), using solid acids as green catalysts. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The governmental restrictions on discharge of green-house gasses associated with the steep changes in fossil fuel prices, shifted the worldwide trend to focus on renewable energy sources. Biodiesel is an alternative renewable fuel, currently produced from vegetable oils, animal fats or even recycled waste cooking-oils from the food industry (Buczek & Czepirski, 2004; Encinar, Gonzalez, & Rodriguez-Reinares, 2005; Kulkarni & Dalai, 2006). Compared to regular diesel, biodiesel is an environmental friendly fuel, with better combustion and reduced emissions of greenhouses gases, while sulfur is practically absent (Bowman, Hilligoss, Rasmussen, & Thomas, 2006). Due to its properties similar to regular diesel, biodiesel can be used in most modern diesel engines, in pure form or blended with petroleum diesel at any concentration (Balat & Balat, 2008). Fatty esters, the main components of biodiesel, are currently produced by (trans-)esterification of tri-glycerides and free fatty acids, followed by several neutralization and purification steps. However, most traditional methods suffer from drawbacks related
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to the use of liquid acid/base catalysts, leading to major economical and environmental penalties (Knothe, Gerpen, & Krahl, 2005; Narasimharao, Lee, & Wilson, 2007). In spite of the technical problems (Meher, Vidya Sagar, & Naik, 2006), as well as the social and sustainability issues (Kumar & Sharma, 2005; Puhan, Vedaraman, Rambrahamam, & Nagarajan, 2005), the biodiesel production rate maintained a tremendous increase during the past ten years, mostly in Asia, US, and Western Europe—as illustrated by Fig. 1. Although, biodiesel consists typically of fatty acid methyl esters – as methanol is the cheapest alcohol used at industrial scale – other alcohols such as ethanol may be employed as well. Bioethanol is the most important biofuel today, currently produced by fermentation of sugar derived from various crops such as sugar cane, corn and sugar beet (Kamm, Gruber, & Kamm, 2006; Liu & Tanaka, 2006; Mielenz, 2001; Solomon, Barnes, & Halvorsen, 2007). Just as other biofuels, ethanol can be blended with gasoline and used in existing or optimized flexi-fuel engines. Moreover, it burns cleaner than gasoline, producing less CO, CO2 and NOx emissions (Blottnitz & Curran, 2007). Fig. 2 shows that bioethanol is also experiencing a very fast growth on the global scale, particularly in Brazil and United States. Remarkably, Brazil gets more than 30% of its transport fuels from sugar cane bioethanol (SRI Consulting, 2009). The production of anhydrous bioethanol is very energydemanding – a major reason being the azeotropic distillation
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ations, no waste streams, as well as reduced capital and operating expenditures. The process design of a fatty acid ethyl esters plant described here is based on experimental results, integrated in rigorous simulations performed using AspenTech Aspen Plus as a computer aided process engineering tool (Aspen Technology, 2009a, 2009b). 2. Biodiesel production processes
Fig. 1. Biodiesel production worldwide, per region.
Fig. 2. Bioethanol production per country—Top 5.
required to producing pure ethanol – hence various process integrations were proposed in order to reduce the energy requirements (Cardona & Sanchez, 2007; Pfeffer, Wukovits, Beckmann, & Friedl, 2007). Since bioethanol can be used in the biodiesel production, many bioethanol plants integrate nowadays biodiesel production. According to SRI Consulting (2009), such an integrated bioethanol and biodiesel plant was developed by Dedini and it is being used since 2006 by Barralcool Mill (State of Mato Grosso, Brazil). Fig. 3 illustrates the current bioethanol–biodiesel process integration as well as the new model proposed in this work. Obviously, it would be much more convenient to use hydrous ethanol in the biodiesel production, but this is currently unfeasible in conventional processes due to equilibrium limitations and economic penalties caused by the additional processing steps. This study makes a brief overview of existing biodiesel production processes and the associated benefits and drawbacks, and proposes a novel separative reactor using hydrous bioethanol over solid acid catalysts. This integrated solution simplifies the overall process and brings significant benefits, such as: high conversion and selectivity, elimination of conventional catalyst-related oper-
Nowadays, the most widespread manufacturing technologies use homogeneous catalysts, in batch or continuous processes where both reaction and separation steps can create bottlenecks. The literature study reveals the following processes currently in use at pilot or industrial scale: 1. Batch processes. These allow high flexibility with respect to composition of the feedstock. The trans-esterification is performed using an acid or base catalyst (Lotero et al., 2005; Narasimharao et al., 2007). Nevertheless, the equipment productivity is low and the operating costs are high (Van Gerpen, 2005). Moreover, the use of liquid catalyst has severe economical and environmental penalties (Hanna, Isom, & Campbell, 2005). 2. Continuous processes combine the esterification and transesterification steps, allowing higher productivity. However, most of these processes are still plagued by the disadvantages of using homogeneous catalysts (Vicente, Martinez, & Aracil, 2004) although solid catalysts emerged in the last decade (Dale, 2003; Dossin, Reyniers, Berger, & Marin, 2006; Kiss, Dimian, & Rothenberg, 2006a; Kiss, Dimian, & Rothenberg, 2006b; Kiss, Rothenberg, Dimian, & Omota, 2006). Nevertheless, integrated processes based on reactive distillation have been also reported (He, Singh, & Thompson, 2006; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006; Suwannakarn, Lotero, Ngaosuwan, & Goodwin, 2009). Moreover, an innovative process – known as ESTERFIP-HTM – was developed for the trans-esterification with methanol by the French Institute of Petroleum. The process is based on heterogeneous catalyst based on zinc and aluminum oxides and it is currently being applied in commercial plants (Bournay, Casanave, Delfort, Hillion, & Chodorge, 2005). However, it requires relatively high temperature (210–250 ◦ C) and pressure (30–50 bar). 3. Supercritical processes were developed to solve the problem of miscibility of oil and alcohol that hinders the kinetics of transesterification, as well as to take advantage of not using a catalyst at all. However, the operating conditions are severe (T > 240 ◦ C, p > 80 bar) and therefore require special equipment (Cao, Han, & Zhang, 2005; He, Wang, & Zhu, 2007). 4. Hydrolysis and esterification processes. These are simpler processes as the glycerides are hydrolyzed first to fatty acids that are esterified in a second step to fatty esters (Kusdiana & Saka, 2004; Minami & Saka, 2006). Such processes have become very
Fig. 3. Integrated bioethanol–biodiesel plant: present (left) and future (right).
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Table 1 Pro/Con of the acid/base catalysts tested for (trans-)esterification. Catalyst type
Benefits
Drawbacks
Ion-exchange resins (Nafion, Amberlyst)
Very high activity Easy regeneration Very high activity Super acid sites Controlable acidity and hydrophobicity High activity Thermally stable Water tolerant
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
Low temperatures High yield, reusable High yield, short times Low temperatures Short reaction times
Long reaction times High reactants ratio High reactants ratio Long reaction times Incomplete yields
TungstoPhosphoric Acid (H3 PW12 O40 ) TPA-Cs (Cs2.5 H0.5 PW12 O40 ) Zeolites (H-ZSM-5, Y and Beta) Sulfated metal oxides (zirconia, titania, tin oxide) Niobic oxide (Nb2 O5 ) Calcium oxide/CaO Calcium methoxide/Ca(OMe)2 Calcium ethoxide/Ca(OEt)2 Li-dopped zinc oxide/ZnO KF loaded on Eu2 O3
attractive and gain market share due to obvious advantages. High purity glycerol is obtained as by-product of hydrolysis step. Tailored properties of the biodiesel fuel are possible using esterification (Knothe, 2005). Moreover, the esterification step can be performed using solid acid catalysts (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006) in an integrated reactiveseparation setup (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Dimian, & Rothenberg, 2008; Kiss, Rothenberg, et al., 2006; Kiss, 2009; Omota, Dimian, & Bliek, 2003). The usage of heterogeneous catalysts avoids the neutralization and washing steps, leading to a simpler and more efficient process. 5. Enzymatic processes have low energy requirements, as the reaction is carried out at mild conditions—ambient pressure and a temperature of 50–55 ◦ C. However, due to the lower yields and the long reaction times the enzymatic processes can still not compete with other processes at industrial scale (Demirbas, 2008; Demirbas & Balat, 2006; Van Gerpen, 2005). 6. Hydro-pyrolysis processes employ a fundamentally different chemical route as compared to the previously described manufacturing methods. Tri-glycerides are converted to fuel by hydrogenation followed by pyrolysis. The key difference is that the fuel product (second-generation biodiesel) is a mixture of long-chain hydrocarbons instead of the conventional fatty esters. The process is known as NExBTL (biomass to liquid) and it was invented by the Finnish company Neste Oy (www.nesteoil.com). While it has clear advantages, this process requires more complex equipment and implies the availability of a low-cost hydrogen source. 3. Problem statement As shown in the previous section, several biodiesel production methods exist, each of them having associated optimal operating parameters and various downstream processing steps. Nonetheless, much of the available literature emphasizes the base catalyzed route (Demirbas, 2008; Knothe, 2005). The problem with using liquid acid or base catalysts is that they require neutralization, separation, washing, recovery, and salt waste disposal operations with serious economical and environmental penalties. Nowadays, the surplus of waste vegetable oil (wvo) or animal fat available at industrial scale would allow production of low-cost biodiesel from bioethanol. For example, in Brazil alone, more than 350 millions liters of biofuel are produced annually from animal fat (SRI Consulting, 2009). The problem with the animal fat or waste oil, is that it becomes useless within 24 h 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 all these problems, we propose a sustainable integrated process based on the esterification of FFA’s with hydrous bioethanol in a separative reac-
tor using solid acids. When required, this step can be followed by the trans-esterification of the remaining tri-glycerides (TG) using solid base catalysts. 1. R-COOH + EtOH ↔ R-COO-Et + H2 O (esterification) 2. TG + 3 EtOH ↔ 3 RCOO-Et + Gly (trans-esterification) The integrated separative reactor proposed in this work is able to shift the chemical equilibrium and drive the esterification reaction to completion by continuously removing the fatty acid ethyl esters and water by-product. The raw materials consist of waste-oil or animal fat – mainly a mixture of free fatty acids – and a cheap alcohol such as hydrous bioethanol (96%wt C2 H5 OH, 4%wt H2 O). A key feature of this work is the novel integration mode of the bioethanol and biodiesel plants, the replacement of anhydrous ethanol by its hydrous azeotrope leading to significant reduction of the biodiesel and bioethanol production costs. Table 1 presents an overview of the available solid acid and base catalysts for biodiesel production by (trans-)esterification. In this work we selected the metal oxides (sulfated zirconia) as water-tolerant solid acid catalysts for FFA esterification (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Calcium ethoxide may be also employed as solid base catalyst for the trans-esterification of the remaining tri-glycerides (Liu, Piao, Wang, & Zhu, 2008a; Liu, Piao, Wang, & Zhu, 2008b). 4. Simulation methods Depending on the composition of any oil/fat feedstock used in the biodiesel production process, there is an associated blend of fatty acid esters in the final biodiesel product, with major consequences on the biodiesel properties (Encinar et al., 2005; Knothe, 2005). The simulation of a biodiesel production process can be performed using one of the available simulation methods illustrated in Table 2: rigorous, shortcut or hybrid method. Each of these methods has clear advantages but also specific drawbacks. Moreover, the requirements in terms of input data can differ considerably and it has a great impact as the information reflects directly in the results. Obviously, the quality of output is determined by the quality of the input, as illustrated by the ‘garbage in–garbage out’ concept. For example, if the property method is improperly selected, the result of the simulation will most likely be inaccurate. Similarly, if incorrect kinetic data is used as input, or some key chemicals are missing from the list of components, the output of the simulation is unlikely to be any informative. The rigorous method is favored due to the accurate results, but it is virtually not feasible in practice due to the amount of input data required. On the other hand, the shortcut method is very handy due to its quickness in making results available, but it provides merely
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Table 2 Simulation methods for biodiesel production: requirements, benefits and drawbacks. Rigorous method
Shortcut method
Hybrid method
Properties for all species.
Single or reduced list of fatty acid/ester/TG.
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).
Easy optimization of reaction and separation.
Simple model.
High fidelity model. Usable for many plants. Easy comparison for various feedstocks.
Fast simulations. Easy-to-build mass and energy balance. No data needed for all species present.
Optimization possible for reaction and separation. Certain ability to compare various feedstocks. Better model fidelity. Fast simulations for RTO.
Slow simulations and convergence problems.
No comparison possible for various feedstocks.
Expensive measurements. Limited RTO and model based control usage.
Low-fidelity model. Less ability to use RTO.
Requirements
Benefits
Drawbacks
low-fidelity models with very limited practical applications. Therefore, for practical reasons, the hybrid approach gives the best results as it combines the advantages of both rigorous and shortcut methods, while limiting the overall disadvantages due to the synergistic effect. The following AspenTech Aspen Plus simulations use kinetic parameters previously determined experimentally (Dimian, Bildea, Omota, & Kiss, 2009; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006), but the fatty components were lumped into one fatty acid and its corresponding fatty ester compound—according to the following chemical reaction: RCOOH + C2 H5 OH ↔ R-COO-C2 H5 + H2 O. Dodecanoic acid/ester was selected as lumped component due to the availability of experimental results, kinetics and VLLE parameters for this system (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). The assumption of lumping components is very reasonable since fatty acids and their corresponding fatty esters have similar properties, as shown hereafter. Note that this approach has been successfully used in the past to simulate other biodiesel production processes, based on reactive distillation for example (Dimian et al., 2009; Dimian, Omota, & Bliek, 2004; Kiss, Dimian, et al., 2006a, 2006b; Kiss et al., 2008; Kiss, Rothenberg, et al., 2006; Omota et al., 2003; Steinigeweg & Gmehling, 2003; von Scala, Moritz, & Fassler, 2003). 5. Results and discussion The integrated reactive distillation process was designed according to previously reported process synthesis methods for reactive separations (Noeres, Kenig, & Gorak, 2003; Schembecker
Short list of VLL data and BIP’s for components. Reduced list of kinetic parameters, few reactions.
More effort to build component list and get kinetic parameters. More work to find VLL data and regress BIP’s.
& Tlatlik, 2003). The properties of the fatty components were determined experimentally, or estimated using state-of-theart contribution methods such as UNIFAC—Dortmund modified. UNIQUAC parameters are also available in Aspen Plus (Aspen Technology, 2009a, 2009b). Note that the phase equilibrium depends on temperature, pressure, as well as reactants ratio: 1. LE, one liquid phase, at T < 100 ◦ C and excess of alcohol. Water by-product remains in the reaction mixture, and the conversion is low due to lower reaction rate. 2. LLE, two liquid phases at T < 100 ◦ C and stoichiometric ratio of alcohol. Liquid separation is undesirable due to the catalyst deactivation in the presence of water. 3. VLE, vapor–liquid at T > 100 ◦ C and continuous removal of water (open system). 4. VLLE, vapor–liquid–liquid equilibrium at T > 100 ◦ C, P > 1 bar (closed system). Again, the water phase separation is not desirable for the activity of the catalyst. Obviously, the most convenient case for the reactive zone is VLE—one liquid and one vapor phase at elevated temperatures leading to higher reaction rates, and with continuous removal of water by-product hence shifting the equilibrium towards ester formation. In the following simulations, the rigorous VLL equilibrium was used in order to correctly determine the stages on which phase separation may occur. Fig. 4 (left) shows the residue curve map (RCM) for the ethanol–water–acid ternary mixture. The presence of the ethanol–water azeotrope does not hinder the biodiesel production
Fig. 4. Residue curve map (RCM) and two-liquid (LL) phase envelope of the mixture ethanol–water–acid.
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Fig. 5. Vapor pressure of the most common fatty acids vs temperature.
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. The VLL envelope of the same mixture is illustrated in Fig. 4 (right). Remarkably, phase splitting may occur if the molar fraction of water exceeds 0.3 in the liquid phase. Obviously, the presence of free water is not desired as it may lead to catalyst deactivation (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). However, as shown later by the simulation results, this is not the case in the reactive zone of the column. The analysis of the RCM also shows that quantitative water removal can be obtained by means of a reflux of fatty acid instead of alcohol (Dimian et al., 2009; Omota et al., 2003). In this case the process should be fed with a stoichiometric amount of alcohol. As a result, the column behaves rather as a reactive absorption than reactive distillation (Dimian et al., 2009; Kiss, 2009). This finding was previously confirmed experimentally in a laboratory column equipped with Katapak packing hosting a solid acid resin (Steinigeweg & Gmehling, 2003). Moreover, as confirmed by the results hereafter, it was observed that using an excess of alcohol does not improve significantly the conversion of fatty acid. Vapor pressure is perhaps one of the most important properties with a critical effect in modeling reactive separations. Figs. 5 and 6 show the vapor pressure of the most common fatty acids and fatty acid ethyl esters. At ambient pressure the boiling points are relatively high, exceeding 300 ◦ C. These values are in line with previous reported data (Poling, Prausnitz, & O’Connell, 2001; Yuan, Hansen, &
Fig. 6. Vapor pressure of the most common fatty acids ethyl esters.
Zhang, 2005). 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 a small amount (∼0.2%) of ethanol in the bottom product. Kinetic data for esterification of dodecanoic acid with various alcohols is available from previous work (Dimian et al., 2009; Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Since the reverse hydrolysis reaction is negligible, a simple kinetic expression can be used for simulation purposes. The reaction rate is given by r = k·Wcat ·CAcid ·CAlcohol , where k = A·exp(−Ea /RT). Note that by changing the weight amount of catalyst used (Wcat ) the reaction rate can be similar for different catalysts. In this particular case, the kinetic constant is given by k = 9.6 × 104 ·exp(−35,600/RT)–meaning that the catalyst is about 20% less active as compared to the reaction with methanol. This is in line with previous findings that the reaction rate decreases with the increased chain length of the alcohol, in the order: methanol > ethanol > propanol . . . 2-ethyl hexanol (Kiss, Dimian, et al., 2006a, 2006b; Kiss, Rothenberg, et al., 2006). Fig. 7 presents the flowsheet of an integrated biodiesel production process based on a reactive distillation column (RDC) as the key unit for esterification or pre-treatment of free fatty acids. This flowsheet can be optionally extended by adding a second step for the trans-esterification of tri-glycerides, as illustrated in Fig. 8. The process proposed here (Fig. 7) was designed, simulated and analyzed using AspenTech Aspen Plus. The rigorous RADFRAC unit with
Fig. 7. Flowsheet of biodiesel production by catalytic reactive distillation.
A.A. Kiss / Computers and Chemical Engineering 34 (2010) 812–820
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Fig. 8. Flowsheet of a two-step process for biodiesel production.
RateSep (rate-based) model – explicitly considering the three phase balances on all stages – was used for the simulation of the RDC. The top part of the reactive distillation column is the rectifying section, while the bottom part is the stripping section. The reactive zone is located in the mid-part of the column, where the catalyst is present and the reactions take place. The reactants can be fed as liquids at the boiling point (hydrous ethanol) or at the reaction temperature (free fatty acids). Several heat exchangers are required to pre-heat the reactants, and cool down the products. Further heatintegration is certainly possible, but this is out of the scope of the present study. 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 is introduced in the bottom of reactive zone, thus creating a counter-current VL 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 the top, then separated in a decanter from which only the fatty acids are recycled to the column. The aqueous outlet of the decanter is recovered at high purity; thus it is reusable as industrial water on the same site. 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 cooled down and stored as biodiesel product. Alternatively (case shown in Fig. 8), the flashed stream may be send to a trans-esterification reactor to further convert the remaining triglycerides to fatty esters. However, in this work we consider only
the worst case scenario, in terms of maximum processing capacity of the column, with 100% FFA in the feedstock. The production rate of the plant described here is 10 ktpy fatty acid ethyl esters (FAEE)—equivalent to 1250 kg/h biodiesel (>99.9%wt). Note that hydrous bioethanol (96%wt) is used as raw material in the process. The complete mass and energy balance is given in Table 3, while the main process design parameters – such as column size and catalyst loading – are listed in Table 4. The productivity of this separative reactor is over 20 kg fatty ester per kg catalyst per hour. 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 (RECACID and REC-ALCO) is not significant, representing only ∼0.5% of the biodiesel production rate. As a consequence, the non-linear behavior that may be caused by recycles can be avoided (Kiss, Bildea, & Dimian, 2007; Kiss, Bildea, Dimian, & Iedema, 2005). Fig. 9 shows the liquid and vapor composition profiles in the reactive distillation column. The concentration of fatty acids decreases while the concentration of fatty esters increases from the top to the bottom. Similarly, the ethanol concentration decreases while water concentration increases from the bottom to the top of the column. The temperature and reaction rate profiles in the RDC are presented in Fig. 10. Remarkably, the reaction rate exhibits a maximum in the middle of the column—where the reactive zone is located. 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 balance of a 10 ktpy biodiesel process based on reactive-distillation.
Temperature K Pressure atm Vapor Frac Mass Flow kg/h Volume Flow l/min Enthalpy Gcal/h Mass Flow kg/h ETHANOL ACID WATER ESTER-E Mass Frac ETHANOL ACID WATER ESTER-E
F-ACID
F-ALCO
TOP
REC-ACID
REC-ALCO
418.1 1.036 0 1094.921 22.986 −0.892
352.2 1.036 0 264.202 6.156 −0.394
508.2 1.017 0 1257.457 28.967 −0.885
372.7 0.987 0 116.366 2.127 −0.411
303.1 1 0 7.27 0.139 −0.006
373.2 1 0 7.458 0.157 −0.006
303.1 1 0 1250 23.774 −1.035
303.1 1 0 109.096 1.839 −0.413
0 1094.921 0 0
253.568 0 10.635 0
2.636 <0.001 0.019 1254.803
trace 1.097 109.216 6.052
0 1.095 0.124 6.051
0.873 trace 0.009 6.576
1.763 <0.001 0.01 1248.226
trace 0.002 109.092 0.001
trace 0.009 0.939 0.052
0 0.151 0.017 0.832
0.117 41 ppb 0.001 0.882
0.001 132 ppb 8 ppm 0.999
trace 17 ppm 1 12 ppm
0 1 0 0
0.96 0 0.04 0
BTM
0.002 131 ppb 15 ppm 0.998
FAEE
WATER
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Fig. 9. Liquid and vapor composition profiles in the reactive distillation column.
Table 4 Design parameters for simulating the reactive distillation column. Parameter
Value
Units
Total number of theoretical stages Number of reactive stages Column diameter HETP Valid phases Volume liquid holdup per stage Mass catalyst per stage Catalyst bulk density Reflux ratio (mass) Boil-up ratio (mass) Condensor duty Reboiler duty Fatty acid conversion Fatty acid feed (heated at 145 ◦ C) Bioethanol feed (azeotrope at boiling point) Production of biodiesel (FAEE) Separative reactor productivity
15
–
10 (from 3 to 12) 0.4 0.5 VLL 5.5
– m m – L
6 1050 0.1 0.075 78 82 >99.99 1095
kg kg/m3 kg/kg kg/kg kW kW % kg/h
264
kg/h
1250
kg/h
20.8
kg FAEE/kg cat/h
Fig. 10. Temperature and reaction rate profiles in the reactive distillation column.
Fig. 11. Purity of RDC products as function of the reflux ratio.
One final question that may arise is: what is the consequence of using the azeotrope feed instead of anhydrous ethanol? Ethanol is consumed by the chemical reaction with the fatty acids hence the composition of the reaction mixture changes from high content of alcohol to lower fractions of ethanol. Therefore there is no crossing through the azeotropic composition towards pure ethanol. On the contrary, there is a dilution of the ethanol that is consumed by the chemical reaction with the water by-product formed in the esterification. For this reason, there is no significant consequence since water is also formed in the reaction but at the same time continuously removed from the separative reactor. The simulation results presented in this study confirm that indeed using hydrous ethanol in this process is feasible. Despite recent progress in understanding the feasibility, design, and control of reactive distillation, the conceptual design of reactive distillation may lead to several different process configurations and operating parameters. In this work, sensitivity analysis was used as a powerful tool to evaluate the optimal range of the operating parameters: reflux ratio, reactants ratio, feed temperature, decanting temperature, flashing pressure, and recycle rates. Note that the design variables can improve the efficiency of the reactive distillation if selected within a design optimization framework (Dalaouti & Seferlis, 2006). The most significant results of the sensitivity analysis are shown in Figs. 11 and 12. Remarkably, the reflux has no significant influence on the purity of the bottom product (Fig. 11). A minimum reflux ratio of 0.05 kg/kg is required in order to obtain high purity top product. The nominal value of 0.1 kg/kg was selected in this
A.A. Kiss / Computers and Chemical Engineering 34 (2010) 812–820
Fig. 12. Purity of RDC products as function of the reactants ratio.
work. Note that high reflux rates are not recommended as returning the water by-product to the column is not favorable to the chemical equilibrium. The optimal molar ratio of the reactants (alcohol:acid) is the stoichiometric value of one, as illustrated by Fig. 12. If this ratio is higher than one (i.e. excess of alcohol) then the fatty acid is completely converted to fatty ester, but the excess of ethanol becomes a significant impurity in the top stream and thereafter in the water by-product. On the contrary, when the ratio is less than one (i.e. excess of fatty acids) then the purity of water by-product remains high, but the conversion of fatty acid is incomplete—hence the bottom product contains unreacted fatty acid that can not be removed from the final product by simple flashing. Since the separation of fatty acids from fatty esters is more difficult than the separation of fatty acids from water, this situation should be avoided. In practice, using a very small excess of ethanol (∼0.5%) or an efficient control structure that can ensure the stoichiometric ratio of reactants (Dimian et al., 2009), is sufficient for the complete conversion of the fatty acids and prevention of the difficult separation previously mentioned. 6. Conclusions The innovative process proposed in this work allows a feasible integration of bioethanol and biodiesel plants. While previous reports available in the open literature focused on the esterification of fatty acids with methanol, this is – to the best of our knowledge – the first study about using bioethanol in a reactive distillation setup based on solid catalysts. Unlike conventional integration, the separative reactor proposed in this work makes use of hydrous bioethanol in the biodiesel production—thus avoiding the costly azeotropic distillation required to producing anhydrous ethanol. Moreover, no hydrous ethanol is returned to the bioethanol plant for purification due to the use of reactants in stoichiometric ratio—hence the excess of alcohol is avoided. Overall, due to these novel features the integration of bioethanol and biodiesel plants is less energy intensive and therefore more cost-effective. Previous experimental results on solid catalysts were integrated in the process development using computer aided engineering tools such as AspenTech Aspen Plus. Due to the integrated design of the separative reactor and the use of solid catalysts, the number of downstream processing steps in the biodiesel production is significantly reduced. The major benefits of this sustainable process are: • Energy effective integration of biodiesel and bioethanol plants, by using hydrous ethanol as raw material, and consequently avoiding costly azeotropic distillation.
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• Flexible integrated reactor suitable for a large range of fatty raw material with 20–100% FFA content, such as: frying oils, animal tallow, tall oil, waste vegetable oil (wvo). • Simple and robust process with no soap formation, no catalystrelated waste streams, and sulfur-free biodiesel as solid acids do not leach into the product. • Effective use of the integrated separative reactor volume leading to high unit productivity. Efficient use of the raw materials: complete conversion and high selectivity, stoichiometric reactants ratio, FFA conversion to fatty 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.
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