- Email: [email protected]
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation
Antonio Espuña, Moisès Graells and Luis Puigjaner (Editors), Proceedings of the 27th European Symposium on Computer Aided Process Engineering – ESCAPE 27 October 1st - 5th, 2017, Barcelona, Spain © 2017 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/B978-0-444-63965-3.50181-1
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation Devrim B. Kaymaka* a
Istanbul Technical University,Ayazaga Campus, Department of Chemical Engineering, Maslak, Istanbul, 34469, Turkey [email protected]
Abstract Bioethanol is one of the most promising alternatives among sustainable biofuels. Methods such as extractive distillation columns are used for the production of bioethanol, because the binary azeotrope of ethanol-water mixture limits the ethanol purity achievable with conventional separation techniques. Since these methods comprise several disadvantages in terms of solvents usage, capital and energy costs, alternative methods are investigated in the industry. This study focuses on the steady-state design and dynamic controllability of a new configuration including a pre-concentrator column and a reactive distillation column to overcome these problems. Steady-state process is simulated using Aspen Plus and Aspen Dynamics is used for dynamic simulations. This configuration is capable to produce ethylene glycol with 99.4 mole% purity as a second product besides bioethanol with 99.7 mole% purity. Dynamic results show that a stable base-level regulatory control is possible for this configuration. Both product purities are held at their design specification against disturbances such as change in production rate handle and feed composition. Keywords: Bioethanol purification, reactive distillation column, process control
1. Introduction Economic and environmental concerns force the industry to focus on alternative renewable energy sources, and bioethanol is considered as one of the most promising alternatives for sustainable biofuels. According to the current international standards, there is a maximum allowed water content for bioethanol. However, a binary azeotrope of ethanol-water mixture limits the maximum ethanol purity achievable with the traditional separation techniques. There are several methods in the literature to break the azeotrope and purify the bioethanol such as distillation and pervaporation, pressure swing distillation, dividingwall distillation, reverse osmosis membrane pretreatment and extractive distillation (Hoch and Espinosa, 2008; Mulia-Soto and Flores-Tlacuahuac, 2011; Kiss and Ignat, 2012, Kanchanalai et al., 2013, Errico et al., 2013a). To overcome the disadvantages of energy intensive distillation methods, industry looks for alternative methods based on process intensification, and reactive distillation columns combining the reaction and separation units into a single equipment is a well-known example of process intensification. The aim of this study is designing and controlling a process including a reactive distillation column to achieve bioethanol with international standards starting from fermentation broth.
1076
D.B. Kaymak
Figure 1. Flow diagram for the four-column configuration (Errico et al., 2013a)
2. Problem statement Processes including extractive distillation columns are the most widely used methods in case of large scale production of bioethanol. Different configurations of this process have been studied in the literature (Errico et al., 2013a, Errico et al., 2013b). Figure 1 illustrates the well-known four-column extractive distillation configuration. The problem is that this configuration presents relatively high energy cost, since it consists of two energy intensive separation steps to reach the purity target; the pre-concentration step and ethanol dehydration step. In addition, a separation agent must be used to break the azeotrope, and this component should be recovered during the process, which results in a higher capital cost. On the other hand, there have been papers published in the literature on the design and control of the ethylene glycol reactive distillation columns where ethylene oxide and water reacts to produce high purity ethylene glycol (Al-Arfaj and Luyben, 2002; Zhu et al., 2009). Recently, a modified configuration of the ethylene glycol reactive distillation column has been proposed in the literature where ethanol-water azeotrope is fed into the system instead of pure water (Tavan and Hosseini, 2013; An et al., 2014). Based on these reactive distillation column studies in the literature, design and control of a two-column process including a pre-concentrator and a reactive distillation column is investigated in order to solve the above-mentioned problems of extractive distillation configurations. The main advantage of this process is that there is no need for a separation agent to break the ethanol-water azeotrope. In addition, high purity ethylene glycol is produced as a co-product besides the dehydrated ethanol.
3. Process studied 3.1. Process Design A fermentation broth with a flowrate of 1700 kmol/h including 5 mole % ethanol and 95 mole % water is fed to the pre-concentrator column. The excess of water leaves the column from the bottoms stream, while a mixture close to azeotropic composition, i.e. containing 85 mole % ethanol and 15 mole % water, is removed from the distillate stream and fed to the reactive distillation column. The second feed stream of the reactive distillation column is ethylene oxide. An ethylene oxide and water molar feed ratio of 1.0 is employed. Bioethanol with a mole purity of 0.997 is obtained from the distillate stream, while ethylene glycol with a mole purity of 0.994 is removed from the bottoms stream.
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation
1077
Table 1. Design parameters of the configuration C1
C2
Number of stages
44
17
Feed stage locations
30
12Mix / 15EG
Pressure (atm)
1
4.5
Reflux ratio (molar)
2.42
2.29
Diameter (m)
1.36
0.92
Condenser duty (kW)
3754.37
2756.56
Reboiler duty (kW)
4912.10
2674.72
The reaction kinetics of ethylene glycol formation is taken from Tavan and Hosseini (2013), and given by Ethylene Oxide + Water → Ethylene Glycol
(1)
r(kmol m-3 s-1) = 3.15 x 1012 exp[-9547/T] XEO XWater
(2)
The steady-state simulations have been performed by the process simulator Aspen Plus. The NRTL method has been applied to evaluate the activity coefficients. In the preconcentrator column, the reflux ratio is varied to achieve the desired design specification which is the ethanol mole purity of 0.85 in the distillate stream. In the reactive column, an ethanol mole recovery of 0.999 and an ethanol mole purity of 0.997 in distillate stream are used as design specifications, and the distillate rate and reflux ratio are varied to achieve these specifications, respectively. The sensitivity analysis is used to choose the operating parameters minimizing the reboiler heat duty such as pressure, number of trays, feed tray locations of azeotropic mixture and ethylene oxide. The design parameters for both columns are reported in Table 1. 3.2. Process Control In this study, decentralized multi-loop control systems employing several single-input, single-output (SISO) feedback controllers are considered. Control structure of the configuration is given in Figure 2. The fresh feed to the pre-concentrator column is flow controlled and used as the production rate handle. The reflux drum levels of both columns are controlled by manipulating distillate flowrates. The base levels of both columns are controlled by manipulating bottoms flowrates. The reflux ratios are held constant in each column at their nominal values using a ratio control. The operating pressures of all columns are controlled by manipulating the corresponding condenser duties. Reboiler heat duty of the pre-concentrator column is manipulated to control the temperature of tray 37. The temperature of tray 16 in reactive distillation column is controlled by manipulating the corresponding reboiler duty. Tray locations for temperature control are selected based on the steady-state temperature profiles of columns given in Figure 3. Since there is only a small section in the reactive column with a sufficient temperature change and the rest of the column exhibits a flat temperature profile, it is not possible to use a second temperature controller in this column. Thus, the composition of ethylene oxide in its feed tray (x15,EO) is controlled by manipulating the feed flowrate of ethylene oxide to satisfy the stoichiometric balance between reactants of the reaction.
1078
D.B. Kaymak '7 X X
'7
'7
Figure 2. Control structure of the configuration All level control loops are proportional-only controllers with a controller gain of 2. PI controllers are used for temperature and composition control loops. These control loops are tuned by using the closed loop ATV method and the tuning parameters are calculated using Tyreus-Luyben settings. A 3 minute dead-time is used in the composition analyzer, while the temperature control loops include a 1 minute dead-time. It should be noticed that all the valves are half open at steady state conditions. The robustness of the control structures is demonstrated by subjecting the processes to disturbances in production rate handle and feed composition.
4. Results and discussion The variables in Figure 4 and Figure 5 are given as deviation variables. The top row figures are for ethanol and ethylene glycol mole fractions at the top and bottoms of the reactive column, respectively. The second row figures illustrate the change in the controlled variables of the reactive column, while the bottom row figures give the results of the corresponding manipulated variables. Figure 4 illustrates the performance of the control structure to ±20% step changes in the production rate handle F. It is seen that all controlled variables settle down into their setpoints in ~5 hours. Results demonstrate that both ethanol and ethylene glycol purities in the product streams of reactive distillation column are also recovered, and settle down into their steady-state values. However, it is observed that the maximum transient deviation of the ethanol purity increases up to 2 mol% in the case of negative 20% step change.
Figure 3. Steady-state temperature profiles: (A): C1, (B): C2
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation
1079
Figure 4. Closed-loop results to ±20% step changes in the production rate handle The response of the control structure to the changes in the feed composition is given in Figure 5. The results illustrate that the product purities return back to their desired values in less than 6 h. Similar to the results of step changes in the production rate handle, relatively large transient deviations are observed in the ethanol product purity compared to those of ethylene glycol product purity.
Figure 5. Closed-loop results to the changes in the feed composition
1080
D.B. Kaymak
5. Conclusions In this study, the steady-state design and dynamic controllability of an alternative process configuration for the purification of bioethanol is presented. The configuration studied is a two-column process including a pre-concentrator column and a reactive distillation column. Thus, the number of columns in the separation sequence is reduced compared to the well-known four-column process, which results in a significant decrease in the capital cost. In addition, a second product, ethylene glycol, is obtained besides the main product, bioethanol, as the result of reaction between water and ethylene oxide. Dynamic results indicate that stable base-level regulatory control is possible for the studied design configuration. Both bioethanol and ethylene glycol purities could be held at their design specification against disturbances studied, although some deviations are observed during the transient behavior. Besides, it is found that a composition controller is necessary in the reactive distillation column because of the flat temperature profile.
References M.A. Al-Arfaj, W.L. Luyben, 2002, Control of ethylene glycol reactive distillation column, AICHE Journal, 48, 905-908. W. An, Z. Lin, J. Chen, J. Zhu, 2014, Simulation and analysis of a reactive distillaiton column for removal of water from ethanol-water mixtures, Industrial & Engineering Chemistry Research, 53, 6056-6064.
M. Errico, B.G. Rong, G. Tola, M. Spano, 2013, Optimal synthesis of distillation systems for bioethanol separation: Part 1. Extractive distillation with simple columns, Industrial & Engineering Chemistry Research, 52, 1612-1619. M. Errico, B.G. Rong, G. Tola, M. Spano, 2013, Optimal synthesis of distillation systems for bioethanol separation: Part 2. Extractive distillation with complex columns, Industrial & Engineering Chemistry Research, 52, 1620-1626. P.M. Hoch, J. Espinosa, 2008, Conceptual design and simulation tools applied to the evolutionary optimization of a bioethanol purification plant, Industrial & Engineering Chemistry Research, 47, 7381-7389. P. Kanchanalai, R.P. Lively, M.J. Realff, Y. Kawajiri, 2013, Cost and energy savings using an optimal design of reverse osmosis membrane pretreatment for dilute bioethanol purification, Industrial & Engineering Chemistry Research, 52, 1113211141. A.A. Kiss, R.M. Ignat, 2012, Innovative single step bioethanol dehydration in an extractive dividing-wall column, Separation and Purification Technology, 98, 290-297. J.F. Mulia-Soto, A. Flores-Tlacuahuac, 2011, Modeling, simulation and control of an internally heat integrated pressure-swing distillation process for bioethanol separation, Computers & Chemical Engineering, 35, 1532-1546. Y. Tavan, S.H. Hosseini, 2013, A novel integrated process to break the ethanol/water azeotrope using reactive distillation – Part I: Parametric study, 118, 455-462. F. Zhu, K. Huang, S. Wang, L. Shan, Q. Zhu, 2009, Towards further internal heat integration in design of reactive distillation columns – Part IV: Application to a high-purity etylene glycol reactive distillaiton column, Chemical Engineering Science, 64, 3498-3509.
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation Devrim B. Kaymaka* a
Istanbul Technical University,Ayazaga Campus, Department of Chemical Engineering, Maslak, Istanbul, 34469, Turkey [email protected]
Abstract Bioethanol is one of the most promising alternatives among sustainable biofuels. Methods such as extractive distillation columns are used for the production of bioethanol, because the binary azeotrope of ethanol-water mixture limits the ethanol purity achievable with conventional separation techniques. Since these methods comprise several disadvantages in terms of solvents usage, capital and energy costs, alternative methods are investigated in the industry. This study focuses on the steady-state design and dynamic controllability of a new configuration including a pre-concentrator column and a reactive distillation column to overcome these problems. Steady-state process is simulated using Aspen Plus and Aspen Dynamics is used for dynamic simulations. This configuration is capable to produce ethylene glycol with 99.4 mole% purity as a second product besides bioethanol with 99.7 mole% purity. Dynamic results show that a stable base-level regulatory control is possible for this configuration. Both product purities are held at their design specification against disturbances such as change in production rate handle and feed composition. Keywords: Bioethanol purification, reactive distillation column, process control
1. Introduction Economic and environmental concerns force the industry to focus on alternative renewable energy sources, and bioethanol is considered as one of the most promising alternatives for sustainable biofuels. According to the current international standards, there is a maximum allowed water content for bioethanol. However, a binary azeotrope of ethanol-water mixture limits the maximum ethanol purity achievable with the traditional separation techniques. There are several methods in the literature to break the azeotrope and purify the bioethanol such as distillation and pervaporation, pressure swing distillation, dividingwall distillation, reverse osmosis membrane pretreatment and extractive distillation (Hoch and Espinosa, 2008; Mulia-Soto and Flores-Tlacuahuac, 2011; Kiss and Ignat, 2012, Kanchanalai et al., 2013, Errico et al., 2013a). To overcome the disadvantages of energy intensive distillation methods, industry looks for alternative methods based on process intensification, and reactive distillation columns combining the reaction and separation units into a single equipment is a well-known example of process intensification. The aim of this study is designing and controlling a process including a reactive distillation column to achieve bioethanol with international standards starting from fermentation broth.
1076
D.B. Kaymak
Figure 1. Flow diagram for the four-column configuration (Errico et al., 2013a)
2. Problem statement Processes including extractive distillation columns are the most widely used methods in case of large scale production of bioethanol. Different configurations of this process have been studied in the literature (Errico et al., 2013a, Errico et al., 2013b). Figure 1 illustrates the well-known four-column extractive distillation configuration. The problem is that this configuration presents relatively high energy cost, since it consists of two energy intensive separation steps to reach the purity target; the pre-concentration step and ethanol dehydration step. In addition, a separation agent must be used to break the azeotrope, and this component should be recovered during the process, which results in a higher capital cost. On the other hand, there have been papers published in the literature on the design and control of the ethylene glycol reactive distillation columns where ethylene oxide and water reacts to produce high purity ethylene glycol (Al-Arfaj and Luyben, 2002; Zhu et al., 2009). Recently, a modified configuration of the ethylene glycol reactive distillation column has been proposed in the literature where ethanol-water azeotrope is fed into the system instead of pure water (Tavan and Hosseini, 2013; An et al., 2014). Based on these reactive distillation column studies in the literature, design and control of a two-column process including a pre-concentrator and a reactive distillation column is investigated in order to solve the above-mentioned problems of extractive distillation configurations. The main advantage of this process is that there is no need for a separation agent to break the ethanol-water azeotrope. In addition, high purity ethylene glycol is produced as a co-product besides the dehydrated ethanol.
3. Process studied 3.1. Process Design A fermentation broth with a flowrate of 1700 kmol/h including 5 mole % ethanol and 95 mole % water is fed to the pre-concentrator column. The excess of water leaves the column from the bottoms stream, while a mixture close to azeotropic composition, i.e. containing 85 mole % ethanol and 15 mole % water, is removed from the distillate stream and fed to the reactive distillation column. The second feed stream of the reactive distillation column is ethylene oxide. An ethylene oxide and water molar feed ratio of 1.0 is employed. Bioethanol with a mole purity of 0.997 is obtained from the distillate stream, while ethylene glycol with a mole purity of 0.994 is removed from the bottoms stream.
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation
1077
Table 1. Design parameters of the configuration C1
C2
Number of stages
44
17
Feed stage locations
30
12Mix / 15EG
Pressure (atm)
1
4.5
Reflux ratio (molar)
2.42
2.29
Diameter (m)
1.36
0.92
Condenser duty (kW)
3754.37
2756.56
Reboiler duty (kW)
4912.10
2674.72
The reaction kinetics of ethylene glycol formation is taken from Tavan and Hosseini (2013), and given by Ethylene Oxide + Water → Ethylene Glycol
(1)
r(kmol m-3 s-1) = 3.15 x 1012 exp[-9547/T] XEO XWater
(2)
The steady-state simulations have been performed by the process simulator Aspen Plus. The NRTL method has been applied to evaluate the activity coefficients. In the preconcentrator column, the reflux ratio is varied to achieve the desired design specification which is the ethanol mole purity of 0.85 in the distillate stream. In the reactive column, an ethanol mole recovery of 0.999 and an ethanol mole purity of 0.997 in distillate stream are used as design specifications, and the distillate rate and reflux ratio are varied to achieve these specifications, respectively. The sensitivity analysis is used to choose the operating parameters minimizing the reboiler heat duty such as pressure, number of trays, feed tray locations of azeotropic mixture and ethylene oxide. The design parameters for both columns are reported in Table 1. 3.2. Process Control In this study, decentralized multi-loop control systems employing several single-input, single-output (SISO) feedback controllers are considered. Control structure of the configuration is given in Figure 2. The fresh feed to the pre-concentrator column is flow controlled and used as the production rate handle. The reflux drum levels of both columns are controlled by manipulating distillate flowrates. The base levels of both columns are controlled by manipulating bottoms flowrates. The reflux ratios are held constant in each column at their nominal values using a ratio control. The operating pressures of all columns are controlled by manipulating the corresponding condenser duties. Reboiler heat duty of the pre-concentrator column is manipulated to control the temperature of tray 37. The temperature of tray 16 in reactive distillation column is controlled by manipulating the corresponding reboiler duty. Tray locations for temperature control are selected based on the steady-state temperature profiles of columns given in Figure 3. Since there is only a small section in the reactive column with a sufficient temperature change and the rest of the column exhibits a flat temperature profile, it is not possible to use a second temperature controller in this column. Thus, the composition of ethylene oxide in its feed tray (x15,EO) is controlled by manipulating the feed flowrate of ethylene oxide to satisfy the stoichiometric balance between reactants of the reaction.
1078
D.B. Kaymak '7 X X
'7
'7
Figure 2. Control structure of the configuration All level control loops are proportional-only controllers with a controller gain of 2. PI controllers are used for temperature and composition control loops. These control loops are tuned by using the closed loop ATV method and the tuning parameters are calculated using Tyreus-Luyben settings. A 3 minute dead-time is used in the composition analyzer, while the temperature control loops include a 1 minute dead-time. It should be noticed that all the valves are half open at steady state conditions. The robustness of the control structures is demonstrated by subjecting the processes to disturbances in production rate handle and feed composition.
4. Results and discussion The variables in Figure 4 and Figure 5 are given as deviation variables. The top row figures are for ethanol and ethylene glycol mole fractions at the top and bottoms of the reactive column, respectively. The second row figures illustrate the change in the controlled variables of the reactive column, while the bottom row figures give the results of the corresponding manipulated variables. Figure 4 illustrates the performance of the control structure to ±20% step changes in the production rate handle F. It is seen that all controlled variables settle down into their setpoints in ~5 hours. Results demonstrate that both ethanol and ethylene glycol purities in the product streams of reactive distillation column are also recovered, and settle down into their steady-state values. However, it is observed that the maximum transient deviation of the ethanol purity increases up to 2 mol% in the case of negative 20% step change.
Figure 3. Steady-state temperature profiles: (A): C1, (B): C2
Design and Control of a Separation Process for Bioethanol Purification by Reactive Distillation
1079
Figure 4. Closed-loop results to ±20% step changes in the production rate handle The response of the control structure to the changes in the feed composition is given in Figure 5. The results illustrate that the product purities return back to their desired values in less than 6 h. Similar to the results of step changes in the production rate handle, relatively large transient deviations are observed in the ethanol product purity compared to those of ethylene glycol product purity.
Figure 5. Closed-loop results to the changes in the feed composition
1080
D.B. Kaymak
5. Conclusions In this study, the steady-state design and dynamic controllability of an alternative process configuration for the purification of bioethanol is presented. The configuration studied is a two-column process including a pre-concentrator column and a reactive distillation column. Thus, the number of columns in the separation sequence is reduced compared to the well-known four-column process, which results in a significant decrease in the capital cost. In addition, a second product, ethylene glycol, is obtained besides the main product, bioethanol, as the result of reaction between water and ethylene oxide. Dynamic results indicate that stable base-level regulatory control is possible for the studied design configuration. Both bioethanol and ethylene glycol purities could be held at their design specification against disturbances studied, although some deviations are observed during the transient behavior. Besides, it is found that a composition controller is necessary in the reactive distillation column because of the flat temperature profile.
References M.A. Al-Arfaj, W.L. Luyben, 2002, Control of ethylene glycol reactive distillation column, AICHE Journal, 48, 905-908. W. An, Z. Lin, J. Chen, J. Zhu, 2014, Simulation and analysis of a reactive distillaiton column for removal of water from ethanol-water mixtures, Industrial & Engineering Chemistry Research, 53, 6056-6064.
M. Errico, B.G. Rong, G. Tola, M. Spano, 2013, Optimal synthesis of distillation systems for bioethanol separation: Part 1. Extractive distillation with simple columns, Industrial & Engineering Chemistry Research, 52, 1612-1619. M. Errico, B.G. Rong, G. Tola, M. Spano, 2013, Optimal synthesis of distillation systems for bioethanol separation: Part 2. Extractive distillation with complex columns, Industrial & Engineering Chemistry Research, 52, 1620-1626. P.M. Hoch, J. Espinosa, 2008, Conceptual design and simulation tools applied to the evolutionary optimization of a bioethanol purification plant, Industrial & Engineering Chemistry Research, 47, 7381-7389. P. Kanchanalai, R.P. Lively, M.J. Realff, Y. Kawajiri, 2013, Cost and energy savings using an optimal design of reverse osmosis membrane pretreatment for dilute bioethanol purification, Industrial & Engineering Chemistry Research, 52, 1113211141. A.A. Kiss, R.M. Ignat, 2012, Innovative single step bioethanol dehydration in an extractive dividing-wall column, Separation and Purification Technology, 98, 290-297. J.F. Mulia-Soto, A. Flores-Tlacuahuac, 2011, Modeling, simulation and control of an internally heat integrated pressure-swing distillation process for bioethanol separation, Computers & Chemical Engineering, 35, 1532-1546. Y. Tavan, S.H. Hosseini, 2013, A novel integrated process to break the ethanol/water azeotrope using reactive distillation – Part I: Parametric study, 118, 455-462. F. Zhu, K. Huang, S. Wang, L. Shan, Q. Zhu, 2009, Towards further internal heat integration in design of reactive distillation columns – Part IV: Application to a high-purity etylene glycol reactive distillaiton column, Chemical Engineering Science, 64, 3498-3509.