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Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study
Andrzej Kraslawski and Ilkka Turunen (Editors) Proceedings of the 23rd European Symposium on Computer Aided Process Engineering – ESCAPE 23, June 9-12, 2013, Lappeenranta, Finland © 2013 Elsevier B.V. All rights reserved. 7
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study Fernando I. Gomez-Castro,a Juan G. Segovia-Hernandez,a Salvador HernandezCastro,a Vicente Rico-Ramirez,b Zeferino Gamiño-Arroyo,a Irene CanoRodrígueza a
Universidad de Guanajuato, Campus Guanajuato, Division de Ciencias Naturales y Exactas, Departamento de Ingeniería Química, Noria Alta S/N. Guanajuato, Guanajuato, 36050, Mexico b Instituto Tecnologico de Celaya, Departamento de Ingenieria Quimica, Av. Tecnologico y Av. Antonio Garcia Cubas S/N. Celaya, Guanajuato, 38010, Mexico
Abstract On the last years, there have been proposals for using supercritical conditions to produce biodiesel fuel from vegetable oils and/or animal fats without a catalyst. Different schemes have been proposed, the most popular consisting on the use of supercritical methanol as reactant. Other alternatives involve the use of methyl acetate or acetic acid as reactants. The potential of those processes may be established in terms of their total annual cost and environmental impact. Thus, in this work, the production of biodiesel fuel by using different reactants is studied. Four processes are considered: the one step supercritical methanol process (Saka process), the two steps supercritical methanol process (Saka-Dadan process), a process with methyl acetate as reactant and a process with acetic acid as reactant. Possible flowsheets for the reaction and separation stages are proposed. The processes are analyzed and compared in terms of energy consumption, pollutant emissions and total annual costs. It has been observed that, in terms of energy, the one step methanol process has the lowest energy requirements. Nevertheless, a higher temperature for the steam supplied is required; thus, that process has high values of CO2 emissions. Furthermore, methyl esters are obtained at higher temperatures, which may have a negative impact on its quality. Keywords: Biodiesel production, supercritical processes, costs analysis.
1. Introduction Among the alternatives for reducing the environmental impact of the energy-demanding activities of human kind, liquid biofuels (bioethanol and biodiesel) have received considerable attention on the last decades, since they are renewable fuels with low pollutant emissions when burned. Biodiesel fuel consists on mixtures of alquil esters, and it is considerated as a clean alternative for petroleum diesel. There are different methods to produce biodiesel, being the homogeneous catalysis one of the most used nowadays. Nevertheless, to overcome the limitation of the homogeneous catalysts (e.g., sensibility of the catalysts), other alternatives have been studied recently, as using homogeneous catalysts or supercritical alcohols to perform the transesterification/esterification reactions. Among the supercritical processes, the use of supercritical methanol has been considered as a convenient method since it requires few process equipments and generates only few by-products. Furthermore, the use of supercritical methanol may result in higher reaction rates, having the capacity to deal
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with both, clean or used oils as feedstock. Studies have taken place for the production of biodiesel with supercritical methanol in one-step (Kusdiana and Saka, 2001) and twosteps (Saka, 2005) processes, where the two-steps process (also known as Saka-Dadan process) requires lower temperature and pressure for the reactions than the one-step process (Saka process). Both schemes have glycerol as main by-product. Nevertheless, it has been stated that an excess on the production of glycerol may result in a considerable reduction on its value for selling (Johnson and Taconi, 2007). Thus, different alternatives have been proposed, among them using methyl acetate (Saka and Isayama, 2009) or acetic acid (Saka et al., 2010) as reactants, by which triacetin is obtained as by-product. This by-product (triacetin) can be used as a component of biodiesel fuel, together with the obtained alquil esters. Supercritical processes are characterized by their high reaction rates and their capacity to handle with low-quality oils as raw material. Nevertheless, those advantages may be not enough to have a costeffective process. Thus, it is important to consider the energetic input required to perform the supercritical reactions and the separation of the by-products obtained, together with the total annual cost of the processes. Furthermore, it is desirable for the processes having a low environmental impact, which may be measured in terms of the emissions of pollutant gases due to the production of the biofuel. Thus, in this work, the production of biodiesel fuel by using different high-pressure, high temperature technologies is studied. Four processes are considered: the one step supercritical methanol process, the two steps supercritical methanol, a process with methyl acetate as reactant and a process with acetic acid as reactant. Possible flowsheets for the reaction and separation stages are proposed. The study is carried out using Aspen Plus. The different alternatives are analyzed and compared in terms of energy consumption, pollutant emissions due to the energy requirements and total annual costs, in order to state the advantages and disadvantages of each alternative and decide, based on environmental and costing criteria, which alternative is more promissory.
2. Process Simulation The analysis of the different processes has been performed by rigorous simulation in Aspen Plus. For all the production alternatives, the raw material has been considered as a mixture of triolein (70 mol%) and oleic acid (30 mol%), which are important components of several vegetable oils. A feed flow rate of 10 kmol/h has been used as basis for the simulations. In the next sub-sections, a description of the analyzed processes is presented. 2.1. One Step Methanol Process Regarding the one step methanol process, the main reaction consists on the transesterification of the triolein and the simultaneous esterification of the oleic acid by supercritical methanol treatment (about 42 mol/mol of vegetable oil). On the transesterification reaction (1), methyl oleate is obtained as main product and glycerol as by-product, while on esterification (2) methyl oleate is the main product and water is the by-product. Here reactions take place at 450 bar and 350°C. Kinetic models considered for the simulation and rate constants are shown on Table 1. Once the reaction has taken place, the excess of methanol is separated in a low-pressure distillation column, followed by a decanter to separate the methyl oleate from the water/glycerol mixture, which is then treated in a second distillation system to obtain glycerol (99.5 mass%). For the purification section, the thermodynamic model
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study
9
UNIFAC for liquid-liquid-vapor systems has been used. The whole process is shown in Figure 1. Table 1. Kinetic models and parameters
Reaction rate
Kinetic parameters (ki in s-1)
Process
Reaction
One Step Methanol
1 2
k1 = 0.0190 (350°C) k2 = 0.0280 (350°C)
Two Steps Methanol
3 4
k3 = 0.0028 (270°C) k4 = 0.0029 (270°C)
Methyl acetate
5 6
k5 = 0.0020 (350°C) k6 = 0.0700 (350°C)
Acetic acid
7 8
k7 = 0.8000 (300°C) k8 = 0.0029 (270°C)
Figure 1. One-step methanol process. 2.2. Two-Steps Methanol Process On this process, a first reaction step occurs by hydrolysis of the triolein (reaction 3, about 338 mol of water/mol of vegetable oil), where oleic acid and glycerol are obtained. The reaction occurs at 7 MPa and 270°C. Then, a phase separation occurs on a decanter, where a fatty stream is obtained and enters to a esterification reactor, where it is transformed into methyl oleate and water by treatment with methanol (about 28 mol/mol of vegetable oil) at 7 MPa and 270°C (reaction 4). Then, purification of methyl oleate and recovery of methanol takes place at low pressure. The second stream from the decanter consists mainly on glycerol and water; this stream enters to a purification step where glycerol is obtained with a purity of 99.5mass%. The VLE of the two purification steps has been modeled with the UNIFAC-LL thermodynamic model. A schematic representation of the process is shown in Figure 2.
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Figure 2. Two-steps methanol process. 2.3. Methyl Acetate Process There are two simultaneous reactions occurring on this process. By treatment with methyl acetate (about 42 mol/mol of vegetable oil), triolein is transformed into methyl oleate and triacetin, while oleic acid reacts with methyl acetate to obtain methyl oleate and acetic acid. The reactions occurs at 20 MPa and 350°C. Then, the excess of methyl acetate and acetic acid are recovered on a low-pressure flash equipment. Finally, the biofuel mixture (methyl oleate/triacetin) is further purified on a distillation column. For the purification section, the thermodynamic model UNIFAC-LL has been used. The process is depicted on Figure 3.
Figure 3. Methyl acetate process. 2.4. Acetic Acid Process The acetic acid process consists on a first treatment of the triolein contained on the raw material with acetic acid (about 38 mol/mol of vegetable oil) at 20 MPa and 300°C (reaction 7), on which oleic acid and triacetin are obtained as products. Then, by washing with water, acetic acid and trioacetin are separated from oleic acid. Oleic acid is then treated with methanol (about 34 mol/mol of vegetable oil) in a second reactor, at 17 MPa and 270°C (reaction 8), obtaining methyl esters. Kinetic models considered for the simulation and rate constants are shown in Table 1. The stream leaving the esterification reactor goes to a purification step to reach the methyl oleate purity and recovery the excess methanol. The UNIFAC-LL model has been used for this section.
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study
11
The stream leaving the washing goes to a distillation column where the triacetin is obtained (97 mol%), and then the acetic acid is recovered in an azeotropic column, using isobutyl-acetate as separation agent. For this last column, the NRTL-HOC thermodynamic model has been used. The process is shown In Figure 4.
Figure 4. Acetic acid process.
3. Results Total energy requirements (QT), emissions of carbon dioxide and total annual costs for the four analyzed processes are shown on Table 2. Carbon dioxide emissions are calculated from total energy requirements, according to the methodology by Gadalla et al. (2005), and are due to the fuel burned to produce the steam that provides energy to the processes. Natural gas has been here considered for the production of steam. Total annual cost (TAC) is computed following Guthrie´s method (Turton et al., 2009). It can be seen that, in terms of energy, the one-step methanol process presents the lowest total energy requirements among the four analyzed processes, followed by the methyl acetate process. Nevertheless, in terms of CO2 emissions, it can be seen that the two-steps methanol process shows the lowest emissions, while the one-step methanol and the methyl acetate processes have the highest emissions among the four studied processes. This is due to the higher temperature requirements of the one-step methanol and the methyl acetate processes, which implies the use of a higher temperature steam and, as a consequence, the burning of more fuel to obtain that steam. Table 2. Energy requirements, CO2 emissions and TAC for the studied processes. Process
QT (MJ/h)
CO2 emissions (kg/h)
TAC (USDx103/year)
One-step methanol Two-steps methanol Methyl acetate Acetic acid
28,917 129,099 31,644 203,738
23,461 10,556 34,735 16,698
10,061.7 13,175.9 12,354.8 44,479.3
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In terms of total annual cost, it can be seen that TAC is not considerably different for the one-step and two-steps methanol processes and the methyl acetate process. Here the increasing on the TAC for the one-step methanol and the methyl acetate processes (despite of their lower energy requirements) is due to the need of using special material (such as Ni alloys) for supporting the high operation pressure, while the two-steps methanol process may consider the use of stainless steel for the high pressure, high temperature reaction. Furthermore, another important consideration to be made is that it has been reported that, for temperatures higher than 300°C, biodiesel may show thermal inestabilities and isomerize into its trans-form, then reducing the yield (Imahara et al., 2008). Thus, the milder conditions of the two-step methanol process are desirable; nevertheless, this process will have a TAC 24% higher than that corresponding to the one-step process.
4. Conclusions A comparative analysis for four high temperatures, high pressure processes for the production of biodiesel fuel has been presented. Simulations have been performed to assess the energy required to achieve high yields and purities of the product and byproducts. It has been found that, in terms of energy requirements, the process taking place in a single reaction step requires lower quantities of energy. Nevertheless, due to its higher temperature and pressure requirements, it shows higher CO2 emissions when compared with the processes in two steps. In terms of total annual costs, the one-step methanol process shows the lowest TAC. Nevertheless, the possibility of isomerization at the temperature required for that process must be considered. By the other hand, the acetic acid process has shown to be an expensive process with high energy requirements; this is due mainly to the difficulty to separate the binary mixture wateracetic acid. Thus, even though two-steps methanol process has high energy requirements, it represents the best alternative since it has the lowest environmental impact and represents no risk of isomerization, requiring an investment only slightly higher that the one-step processes.
References D. Kusdiana, S. Saka, 2001, Kinetics of Transesterification in Rapeseed Oil to Biodiesel Fuel as Treated in Supercritical Methanol, Fuel, 80, 5, 693-698. D.T. Johnson, K.A. Taconi, 2007, The Glycerin Glut: Options for the Value-Added Conversion of Crude Glycerol Resulting from Biodiesel Production, Environmental Progress, 26, 4, 338-348. H. Imahara, E. Minami, S. Hari, S. Saka, 2008, Thermal Stability of Biodiesel in Supercritical Methanol, Fuel, 87, 1, 1-6. M.A. Gadalla, Z. Olujic, P.J. Jansens, M. Jobson, R. Smith, 2005, Reducing CO 2 Emissions and Energy Consumption of Heat-Integrated Distillation Systems, Environmental Science & Technology, 39, 17, 6860-6870. R. Turton, R.C. Bailey, W.B. Whiting, J.A. Shaeiwitz, 2009, Analysis, Synthesis, and Design of Chemical Processes, 3rd ed., Prentice Hall, U.S.A. S. Saka, 2005, Biodiesel Fuel Production by Supercritical Methanol Technology, Journal of the Japan Institute of Energy, 84, 5, 413-419. S. Saka, Y. Isayama, 2009, A New Process for Catalyst-Free Production of Biodiesel Using Supercritical Methyal Acetate, Fuel, 88, 7, 1307-1313. S. Saka, Y. Isayama, Z. Ilham, X. Jiayu, 2010, New Process for Catalyst-Free Biodiesel Production Using Subcritical Acetic Acid and Supercritical Methanol, Fuel, 89, 7, 1442-446.
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study Fernando I. Gomez-Castro,a Juan G. Segovia-Hernandez,a Salvador HernandezCastro,a Vicente Rico-Ramirez,b Zeferino Gamiño-Arroyo,a Irene CanoRodrígueza a
Universidad de Guanajuato, Campus Guanajuato, Division de Ciencias Naturales y Exactas, Departamento de Ingeniería Química, Noria Alta S/N. Guanajuato, Guanajuato, 36050, Mexico b Instituto Tecnologico de Celaya, Departamento de Ingenieria Quimica, Av. Tecnologico y Av. Antonio Garcia Cubas S/N. Celaya, Guanajuato, 38010, Mexico
Abstract On the last years, there have been proposals for using supercritical conditions to produce biodiesel fuel from vegetable oils and/or animal fats without a catalyst. Different schemes have been proposed, the most popular consisting on the use of supercritical methanol as reactant. Other alternatives involve the use of methyl acetate or acetic acid as reactants. The potential of those processes may be established in terms of their total annual cost and environmental impact. Thus, in this work, the production of biodiesel fuel by using different reactants is studied. Four processes are considered: the one step supercritical methanol process (Saka process), the two steps supercritical methanol process (Saka-Dadan process), a process with methyl acetate as reactant and a process with acetic acid as reactant. Possible flowsheets for the reaction and separation stages are proposed. The processes are analyzed and compared in terms of energy consumption, pollutant emissions and total annual costs. It has been observed that, in terms of energy, the one step methanol process has the lowest energy requirements. Nevertheless, a higher temperature for the steam supplied is required; thus, that process has high values of CO2 emissions. Furthermore, methyl esters are obtained at higher temperatures, which may have a negative impact on its quality. Keywords: Biodiesel production, supercritical processes, costs analysis.
1. Introduction Among the alternatives for reducing the environmental impact of the energy-demanding activities of human kind, liquid biofuels (bioethanol and biodiesel) have received considerable attention on the last decades, since they are renewable fuels with low pollutant emissions when burned. Biodiesel fuel consists on mixtures of alquil esters, and it is considerated as a clean alternative for petroleum diesel. There are different methods to produce biodiesel, being the homogeneous catalysis one of the most used nowadays. Nevertheless, to overcome the limitation of the homogeneous catalysts (e.g., sensibility of the catalysts), other alternatives have been studied recently, as using homogeneous catalysts or supercritical alcohols to perform the transesterification/esterification reactions. Among the supercritical processes, the use of supercritical methanol has been considered as a convenient method since it requires few process equipments and generates only few by-products. Furthermore, the use of supercritical methanol may result in higher reaction rates, having the capacity to deal
8
F. I. Gomez-Castro et al.
with both, clean or used oils as feedstock. Studies have taken place for the production of biodiesel with supercritical methanol in one-step (Kusdiana and Saka, 2001) and twosteps (Saka, 2005) processes, where the two-steps process (also known as Saka-Dadan process) requires lower temperature and pressure for the reactions than the one-step process (Saka process). Both schemes have glycerol as main by-product. Nevertheless, it has been stated that an excess on the production of glycerol may result in a considerable reduction on its value for selling (Johnson and Taconi, 2007). Thus, different alternatives have been proposed, among them using methyl acetate (Saka and Isayama, 2009) or acetic acid (Saka et al., 2010) as reactants, by which triacetin is obtained as by-product. This by-product (triacetin) can be used as a component of biodiesel fuel, together with the obtained alquil esters. Supercritical processes are characterized by their high reaction rates and their capacity to handle with low-quality oils as raw material. Nevertheless, those advantages may be not enough to have a costeffective process. Thus, it is important to consider the energetic input required to perform the supercritical reactions and the separation of the by-products obtained, together with the total annual cost of the processes. Furthermore, it is desirable for the processes having a low environmental impact, which may be measured in terms of the emissions of pollutant gases due to the production of the biofuel. Thus, in this work, the production of biodiesel fuel by using different high-pressure, high temperature technologies is studied. Four processes are considered: the one step supercritical methanol process, the two steps supercritical methanol, a process with methyl acetate as reactant and a process with acetic acid as reactant. Possible flowsheets for the reaction and separation stages are proposed. The study is carried out using Aspen Plus. The different alternatives are analyzed and compared in terms of energy consumption, pollutant emissions due to the energy requirements and total annual costs, in order to state the advantages and disadvantages of each alternative and decide, based on environmental and costing criteria, which alternative is more promissory.
2. Process Simulation The analysis of the different processes has been performed by rigorous simulation in Aspen Plus. For all the production alternatives, the raw material has been considered as a mixture of triolein (70 mol%) and oleic acid (30 mol%), which are important components of several vegetable oils. A feed flow rate of 10 kmol/h has been used as basis for the simulations. In the next sub-sections, a description of the analyzed processes is presented. 2.1. One Step Methanol Process Regarding the one step methanol process, the main reaction consists on the transesterification of the triolein and the simultaneous esterification of the oleic acid by supercritical methanol treatment (about 42 mol/mol of vegetable oil). On the transesterification reaction (1), methyl oleate is obtained as main product and glycerol as by-product, while on esterification (2) methyl oleate is the main product and water is the by-product. Here reactions take place at 450 bar and 350°C. Kinetic models considered for the simulation and rate constants are shown on Table 1. Once the reaction has taken place, the excess of methanol is separated in a low-pressure distillation column, followed by a decanter to separate the methyl oleate from the water/glycerol mixture, which is then treated in a second distillation system to obtain glycerol (99.5 mass%). For the purification section, the thermodynamic model
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study
9
UNIFAC for liquid-liquid-vapor systems has been used. The whole process is shown in Figure 1. Table 1. Kinetic models and parameters
Reaction rate
Kinetic parameters (ki in s-1)
Process
Reaction
One Step Methanol
1 2
k1 = 0.0190 (350°C) k2 = 0.0280 (350°C)
Two Steps Methanol
3 4
k3 = 0.0028 (270°C) k4 = 0.0029 (270°C)
Methyl acetate
5 6
k5 = 0.0020 (350°C) k6 = 0.0700 (350°C)
Acetic acid
7 8
k7 = 0.8000 (300°C) k8 = 0.0029 (270°C)
Figure 1. One-step methanol process. 2.2. Two-Steps Methanol Process On this process, a first reaction step occurs by hydrolysis of the triolein (reaction 3, about 338 mol of water/mol of vegetable oil), where oleic acid and glycerol are obtained. The reaction occurs at 7 MPa and 270°C. Then, a phase separation occurs on a decanter, where a fatty stream is obtained and enters to a esterification reactor, where it is transformed into methyl oleate and water by treatment with methanol (about 28 mol/mol of vegetable oil) at 7 MPa and 270°C (reaction 4). Then, purification of methyl oleate and recovery of methanol takes place at low pressure. The second stream from the decanter consists mainly on glycerol and water; this stream enters to a purification step where glycerol is obtained with a purity of 99.5mass%. The VLE of the two purification steps has been modeled with the UNIFAC-LL thermodynamic model. A schematic representation of the process is shown in Figure 2.
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F. I. Gomez-Castro et al.
Figure 2. Two-steps methanol process. 2.3. Methyl Acetate Process There are two simultaneous reactions occurring on this process. By treatment with methyl acetate (about 42 mol/mol of vegetable oil), triolein is transformed into methyl oleate and triacetin, while oleic acid reacts with methyl acetate to obtain methyl oleate and acetic acid. The reactions occurs at 20 MPa and 350°C. Then, the excess of methyl acetate and acetic acid are recovered on a low-pressure flash equipment. Finally, the biofuel mixture (methyl oleate/triacetin) is further purified on a distillation column. For the purification section, the thermodynamic model UNIFAC-LL has been used. The process is depicted on Figure 3.
Figure 3. Methyl acetate process. 2.4. Acetic Acid Process The acetic acid process consists on a first treatment of the triolein contained on the raw material with acetic acid (about 38 mol/mol of vegetable oil) at 20 MPa and 300°C (reaction 7), on which oleic acid and triacetin are obtained as products. Then, by washing with water, acetic acid and trioacetin are separated from oleic acid. Oleic acid is then treated with methanol (about 34 mol/mol of vegetable oil) in a second reactor, at 17 MPa and 270°C (reaction 8), obtaining methyl esters. Kinetic models considered for the simulation and rate constants are shown in Table 1. The stream leaving the esterification reactor goes to a purification step to reach the methyl oleate purity and recovery the excess methanol. The UNIFAC-LL model has been used for this section.
Alternatives for the Production of Biodiesel by Supercritical Technologies: A Comparative Study
11
The stream leaving the washing goes to a distillation column where the triacetin is obtained (97 mol%), and then the acetic acid is recovered in an azeotropic column, using isobutyl-acetate as separation agent. For this last column, the NRTL-HOC thermodynamic model has been used. The process is shown In Figure 4.
Figure 4. Acetic acid process.
3. Results Total energy requirements (QT), emissions of carbon dioxide and total annual costs for the four analyzed processes are shown on Table 2. Carbon dioxide emissions are calculated from total energy requirements, according to the methodology by Gadalla et al. (2005), and are due to the fuel burned to produce the steam that provides energy to the processes. Natural gas has been here considered for the production of steam. Total annual cost (TAC) is computed following Guthrie´s method (Turton et al., 2009). It can be seen that, in terms of energy, the one-step methanol process presents the lowest total energy requirements among the four analyzed processes, followed by the methyl acetate process. Nevertheless, in terms of CO2 emissions, it can be seen that the two-steps methanol process shows the lowest emissions, while the one-step methanol and the methyl acetate processes have the highest emissions among the four studied processes. This is due to the higher temperature requirements of the one-step methanol and the methyl acetate processes, which implies the use of a higher temperature steam and, as a consequence, the burning of more fuel to obtain that steam. Table 2. Energy requirements, CO2 emissions and TAC for the studied processes. Process
QT (MJ/h)
CO2 emissions (kg/h)
TAC (USDx103/year)
One-step methanol Two-steps methanol Methyl acetate Acetic acid
28,917 129,099 31,644 203,738
23,461 10,556 34,735 16,698
10,061.7 13,175.9 12,354.8 44,479.3
12
F. I. Gomez-Castro et al.
In terms of total annual cost, it can be seen that TAC is not considerably different for the one-step and two-steps methanol processes and the methyl acetate process. Here the increasing on the TAC for the one-step methanol and the methyl acetate processes (despite of their lower energy requirements) is due to the need of using special material (such as Ni alloys) for supporting the high operation pressure, while the two-steps methanol process may consider the use of stainless steel for the high pressure, high temperature reaction. Furthermore, another important consideration to be made is that it has been reported that, for temperatures higher than 300°C, biodiesel may show thermal inestabilities and isomerize into its trans-form, then reducing the yield (Imahara et al., 2008). Thus, the milder conditions of the two-step methanol process are desirable; nevertheless, this process will have a TAC 24% higher than that corresponding to the one-step process.
4. Conclusions A comparative analysis for four high temperatures, high pressure processes for the production of biodiesel fuel has been presented. Simulations have been performed to assess the energy required to achieve high yields and purities of the product and byproducts. It has been found that, in terms of energy requirements, the process taking place in a single reaction step requires lower quantities of energy. Nevertheless, due to its higher temperature and pressure requirements, it shows higher CO2 emissions when compared with the processes in two steps. In terms of total annual costs, the one-step methanol process shows the lowest TAC. Nevertheless, the possibility of isomerization at the temperature required for that process must be considered. By the other hand, the acetic acid process has shown to be an expensive process with high energy requirements; this is due mainly to the difficulty to separate the binary mixture wateracetic acid. Thus, even though two-steps methanol process has high energy requirements, it represents the best alternative since it has the lowest environmental impact and represents no risk of isomerization, requiring an investment only slightly higher that the one-step processes.
References D. Kusdiana, S. Saka, 2001, Kinetics of Transesterification in Rapeseed Oil to Biodiesel Fuel as Treated in Supercritical Methanol, Fuel, 80, 5, 693-698. D.T. Johnson, K.A. Taconi, 2007, The Glycerin Glut: Options for the Value-Added Conversion of Crude Glycerol Resulting from Biodiesel Production, Environmental Progress, 26, 4, 338-348. H. Imahara, E. Minami, S. Hari, S. Saka, 2008, Thermal Stability of Biodiesel in Supercritical Methanol, Fuel, 87, 1, 1-6. M.A. Gadalla, Z. Olujic, P.J. Jansens, M. Jobson, R. Smith, 2005, Reducing CO 2 Emissions and Energy Consumption of Heat-Integrated Distillation Systems, Environmental Science & Technology, 39, 17, 6860-6870. R. Turton, R.C. Bailey, W.B. Whiting, J.A. Shaeiwitz, 2009, Analysis, Synthesis, and Design of Chemical Processes, 3rd ed., Prentice Hall, U.S.A. S. Saka, 2005, Biodiesel Fuel Production by Supercritical Methanol Technology, Journal of the Japan Institute of Energy, 84, 5, 413-419. S. Saka, Y. Isayama, 2009, A New Process for Catalyst-Free Production of Biodiesel Using Supercritical Methyal Acetate, Fuel, 88, 7, 1307-1313. S. Saka, Y. Isayama, Z. Ilham, X. Jiayu, 2010, New Process for Catalyst-Free Biodiesel Production Using Subcritical Acetic Acid and Supercritical Methanol, Fuel, 89, 7, 1442-446.