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The control of heterogeneous azeotropic distillation column in industry considering entrainer ratio in reflux
Process Systems Engineering 2003 B. Chen and A.W. Westerberg (editors) 9 2003 Published by Elsevier Science B.V.
1094
The control of heterogeneous azeotropic distillation column in industry considering entrainer ratio in reflux Shoushi Yamamoto,
Minoru Yoneda,
Yasuaki Yabuki
Science & Technology Research Center, Mitsubishi Chemical Corporation, Yokkaichi, Mie 510-8530, Japan
Abstract In this paper, the control of heterogeneous azeotropic distillation of acetic acid + water with n-butyl-acetate as an entrainer is studied in an industrial distillation column. Generally, this column forms heterogeneous liquid phase not in the decanter but also in the upper part of the column. Deliberations should be paid to keep a heterogeneous liquid phase in the upper part of the column and consider the ratio of entrainer reflux flow rate and water reflux flow rate, otherwise the operation ends up at higher reflux and boilup rate and goes through a catastrophic change from the lower impurity to the higher impurity. To overcome this problem, the behavior of this column is investigated by using dynamic simulation, which is developed for this work. The control system is also developed and employed in an industrial application. Boilup steam flow rate is reduced with maintaining impurity at low level. Keywords hetrogeneous azeotropic distillation, dynamic simulation, industrial process, water/entrainer ratio in reflux 1. INTRODUCTION There have been many studies and reports about heterogeneous azeotropic distillation column [1][2][3]. Most of the literatures on the topic are focused on process design and system characteristics. There have been relatively few reports on the control of heterogeneous azeotropic distillation column, especially entrainer ratio in reflux is concerned. Typical distillation column has two degrees of freedom which determine overhead and bottom product quality- they are the material balance and the energy input. The distillation colunm in this study has a third degree of freedom, entrainer ratio in reflux. Shinskey pointed out heterogeneous azeotropic distillation column is unstable in one liquid phase and needs to keep two liquid phases region by adjusting the entrainer ratio in reflux[4]. However, entrainer-rich mode leads to an unstable situation and is prone to be upset. In addition, its mode uses much more energy than should be required to make the separation. In this work, the control of heterogeneous azeotropic distillation column is investigated by considering entrainer ratio in reflux. The control system is designed based on simulation results and validated in an industrial process.
1095
_
~
~decanter
4 ~eam ~
~"
aceticacid
Table 1. Data sheet tray number 27 feed tray 9 feed flowrate 50 t/h water feed compornent acetic acid n-b-acetate reflux flow rate water entrainer distillate flowrate 5.5 t/h bottom flowrate 39 t/h boil up steam 5.5e6 keal/h
12 wt% 82 wt% 6 wt% 0.7 t/h 17.8 t/h
Figure 1. Process overview 2.PROCESS OVERVIEW
The schematic of the distillation column in this study is shown in Figure l. The column separates a two-component mixture of water and acetic acid. N-butyl-acetate, by-product from the process, is employed as an entrainer and boils over water and acetic acid. Water is obtained from the heavy phase in the decanter, and the acetic acid is withdrawn from the bottom of the column. Three component mixture results in a liquid-liquid equilibrium for a wide range of compositions. A liquid phase becomes heterogeneous not only in the decanter but also in the upper part of the column. Three manipulated variables are available, i.e. entrainer reflux flow rate, water reflux flow rate and boilup steam flow rate. There are upper limits of impurity at the top and bottom specification, because water and acetic acid are used in other reaction processes. The impurity level of acetic acid is less than 1% at the top. The impurity level of water is less than 1% at the bottom. The impurity level of n-butyl-acetate is also imposed at the bottom and kept less than 10%. Main disturbances come from variation of feed rate and feed composition. Entrainer-rich mode leads to an unstable situation. Therefore, operators have to adjust the balance of water and entrainer in reflux and boilup steam flow rate. However, they found it difficult to maintain impurity level at the top and the bottom. At first, single-loop control strategy (3x3) was employed with most sensitive manipulate variable to control each impurity level at the bottom and top. But this strategy ended up interactions among impurities and operations at higher reflux and boilup rate. 3. BEHAVIOR OF COMPOSITION CHANGE To analyze the behavior of composition change in the column, the dynamic simulation model was developed for this study. The assumptions of the physical model were listed in the appendix. Figure 2 shows the influence on impurity at the top and the bottom with the number of two liquid phases trays in the column. As the number of two liquid phases trays decrease, the
1096
Figure 2. Impurity of top/bottom vs. 2-phases trays
Figure 3a.3b. 0.6
2-phases trays influence for temperatures in column -
- .
-
8.0
water feed i+0.5%
7.8 ,~ I----1
0.55
. . . . . . .
~
. . . . . . . . i
7.6
i i i
t~
i
t~
7.4
i . . . . . . . .
0.5 t t !
!
n-bu lyl--ac~{tate
t
0.45
P
i i ._..+__ i
7.2
=
i
!
i
I
I
7.0
*60 mh
Figure 4
The behavior of composition at the bottom against the disturbance
impurity at the top (acetic acid) increases. Altematively, as the number of two liquid phases trays increase, the impurity of bottom (water, n-butyl-acetate) increase. To maintain under specifications of each impurity, the number of two liquid phases trays should be kept around 3 "~12.
Figure 3a,3b show the influence on temperature in columns with the number of two liquid phases trays in the column. In figure 3a, temperatures on the tray are plotted with three cases, which are 2 phases tray = 0,10,15. Figure 3b shows the difference of the temperature on the tray for each case based on the temperature with 2-phases tray = 10. Figure 3b indicates temperatures on tray between tray 13 and tray 18 is sensitive to the variation of the number of 2-phase tray. In other words, the variation of the number of two liquid phases tray can be judged to observe these temperatures on trays.
1097 If the impurity at the top and the number of two liquid phases trays are kept almost constant, two impurities at the bottom (water, n-butyl-acetate) changes symmetry like on the both ends of seesaw. Figure 4 indicates the behavior of composition at the bottom in open loop when the disturbance of feed composition is injected (water composition increased by 0.5%). From the study of dynamic simulation above, the behavior of composition change is summarized in the following. 9 The number of two liquid phases trays should be kept between 3"--12 to maintain the impurity level at the bottom and top. The number of the two liquid phases is sensitive to mid temperature of column. The control of mid temperature of column is key to maintaining the number of two liquid phases. 9 If the number of two liquid phases trays are constant, two impurities (water and n-butyl acetate) at the bottom change symmetry like on the both ends of seesaw. Therefore, either of water or n-butyl acetate is kept constant under the condition of two liquid phases trays constant, then the other can be also kept constant. 4. DESIGN OF NEW CONTROL SYSTEM
Control system is designed based on the simulation study. The new control system is schematically drawn in figure 5. The number of two liquid phases is controlled by manipulating the total reflux flow rate (sum of water reflux and entrainer reflux) and boilup steam flow rate. Then, the impurities at the top and bottom are kept at low level. As for water/entrainer balance in column, soft sensors are developed to detect the compositions at the bottom. Prediction model is built based on temperatures on the trays by using PLS (Partial Least Squares) methods. The accuracy of soft sensor is shown in figure 6(a), 6(b). Water/entrainer ratio in total reflux is controlled in respond to the amount of n-butyl acetate of bottom.
water/entrainer ratio
,o..
~-~) Figure 5.
~
i=....,o, -'~
aceticacid
New Control System
1098
Figure 6. Soft sensor prediction vs. actual
Figure 7. New Control system against the disturbance 5. THE RESULT OF CONTROL Figure 7(a) indicates the behavior of impurities at the bottom against the disturbance in an industrial distillation column. The ratio of e n t r a i n e r to water in reflux is adjusted to m a i n t a i n the impurities at the bottom (Figure 7(b)). From the standpoint of energy saving, total reflux flow rate can be minimized by setting the temperature T2 to the upper limit, then boilup steam flow rate is also lowed. Figure 8 shows boilup steam flow rate is decreased by 3%. The new control system is implemented successfully.
Figure 8. Reduction of total reflux and boilup steam flow rate
1099 6. CONCLUSION In this work, the control of heterogeneous azeotropic distillation of acetic acid water with n-butyl-acetate is studied in an industrial application by using dynamic simulation, which is developed for this work. The study shows the number of two liquid phases trays in column affect impurities of the top and the bottom very much. And the variation of two liquid phases trays is able to detect by observing mid column temperatures(13 ~ 18trays). Two impurities at the bottom change symmetry like on the both ends of seesaw under the condition the number of two liquid phases trays constant. New control system is designed based on simulation study, and validated in an industrial distillation column. Two liquid phases trays region is controlled by total reflux (sum of water reflux and entrainer reflux) flow rate and boil up steam flow rate. Impurities at the bottom (n-butyl-acetate) is controlled by changing water/entrainer ratio in total reflux flow rate. The automated control enables us to decrease burden of operators fairly. This control strategy makes it easier to meet the plant load change on the demand from the market and expands chances of business decision-making. APPENDIX
A physical model is developed under the following assumptions. 9Perfect mixing both in vapor phase and in liquid phase of decanter, each stage and reboiler 9Ideal stage, Ideal gas, Negligible vapor holdup 9Boilup steam totally condenses in reboiler and all latent heat is used for boilup. 9Liquid-liquid equilibrium (LLE) equations are calculated in decanter and Vapor-liquid-liquid equilibrium (VLLE) equations are considered in the column. REFERENCES
[1] Maurizio, R.; Michael, F.D., Dynamic of heterogeneous azeotropic distillation columns, AIChE J, 36, 1 (1990). [2] Nikolaos, B.; George, A. M.; Manfred, M., Multiple steady state in heterogeneous azeotropic distillation, Ind. Eng. Chem. Res. , 35(1996). [3] I-L.Chein; C. J. Wang; D. S. H. Wong, Dynamic and control of a heterogeneous azeotropic distillation column: Conventional control approach, Ind. Eng. Chem. Res., 38(1999). [4] Shinskey,F.G., Distillation control for productivity and energy conservation, l Sted., McGraw-Hill, New York(1981). [5] Shoushi, Y. Patent No.2001-281837, Control method of distillation column, Japan (2001). [6] Soemantri, W.; Warren, D.S., Azeotropic distillation, AIChE J, 42, 1 (1996).
1094
The control of heterogeneous azeotropic distillation column in industry considering entrainer ratio in reflux Shoushi Yamamoto,
Minoru Yoneda,
Yasuaki Yabuki
Science & Technology Research Center, Mitsubishi Chemical Corporation, Yokkaichi, Mie 510-8530, Japan
Abstract In this paper, the control of heterogeneous azeotropic distillation of acetic acid + water with n-butyl-acetate as an entrainer is studied in an industrial distillation column. Generally, this column forms heterogeneous liquid phase not in the decanter but also in the upper part of the column. Deliberations should be paid to keep a heterogeneous liquid phase in the upper part of the column and consider the ratio of entrainer reflux flow rate and water reflux flow rate, otherwise the operation ends up at higher reflux and boilup rate and goes through a catastrophic change from the lower impurity to the higher impurity. To overcome this problem, the behavior of this column is investigated by using dynamic simulation, which is developed for this work. The control system is also developed and employed in an industrial application. Boilup steam flow rate is reduced with maintaining impurity at low level. Keywords hetrogeneous azeotropic distillation, dynamic simulation, industrial process, water/entrainer ratio in reflux 1. INTRODUCTION There have been many studies and reports about heterogeneous azeotropic distillation column [1][2][3]. Most of the literatures on the topic are focused on process design and system characteristics. There have been relatively few reports on the control of heterogeneous azeotropic distillation column, especially entrainer ratio in reflux is concerned. Typical distillation column has two degrees of freedom which determine overhead and bottom product quality- they are the material balance and the energy input. The distillation colunm in this study has a third degree of freedom, entrainer ratio in reflux. Shinskey pointed out heterogeneous azeotropic distillation column is unstable in one liquid phase and needs to keep two liquid phases region by adjusting the entrainer ratio in reflux[4]. However, entrainer-rich mode leads to an unstable situation and is prone to be upset. In addition, its mode uses much more energy than should be required to make the separation. In this work, the control of heterogeneous azeotropic distillation column is investigated by considering entrainer ratio in reflux. The control system is designed based on simulation results and validated in an industrial process.
1095
_
~
~decanter
4 ~eam ~
~"
aceticacid
Table 1. Data sheet tray number 27 feed tray 9 feed flowrate 50 t/h water feed compornent acetic acid n-b-acetate reflux flow rate water entrainer distillate flowrate 5.5 t/h bottom flowrate 39 t/h boil up steam 5.5e6 keal/h
12 wt% 82 wt% 6 wt% 0.7 t/h 17.8 t/h
Figure 1. Process overview 2.PROCESS OVERVIEW
The schematic of the distillation column in this study is shown in Figure l. The column separates a two-component mixture of water and acetic acid. N-butyl-acetate, by-product from the process, is employed as an entrainer and boils over water and acetic acid. Water is obtained from the heavy phase in the decanter, and the acetic acid is withdrawn from the bottom of the column. Three component mixture results in a liquid-liquid equilibrium for a wide range of compositions. A liquid phase becomes heterogeneous not only in the decanter but also in the upper part of the column. Three manipulated variables are available, i.e. entrainer reflux flow rate, water reflux flow rate and boilup steam flow rate. There are upper limits of impurity at the top and bottom specification, because water and acetic acid are used in other reaction processes. The impurity level of acetic acid is less than 1% at the top. The impurity level of water is less than 1% at the bottom. The impurity level of n-butyl-acetate is also imposed at the bottom and kept less than 10%. Main disturbances come from variation of feed rate and feed composition. Entrainer-rich mode leads to an unstable situation. Therefore, operators have to adjust the balance of water and entrainer in reflux and boilup steam flow rate. However, they found it difficult to maintain impurity level at the top and the bottom. At first, single-loop control strategy (3x3) was employed with most sensitive manipulate variable to control each impurity level at the bottom and top. But this strategy ended up interactions among impurities and operations at higher reflux and boilup rate. 3. BEHAVIOR OF COMPOSITION CHANGE To analyze the behavior of composition change in the column, the dynamic simulation model was developed for this study. The assumptions of the physical model were listed in the appendix. Figure 2 shows the influence on impurity at the top and the bottom with the number of two liquid phases trays in the column. As the number of two liquid phases trays decrease, the
1096
Figure 2. Impurity of top/bottom vs. 2-phases trays
Figure 3a.3b. 0.6
2-phases trays influence for temperatures in column -
- .
-
8.0
water feed i+0.5%
7.8 ,~ I----1
0.55
. . . . . . .
~
. . . . . . . . i
7.6
i i i
t~
i
t~
7.4
i . . . . . . . .
0.5 t t !
!
n-bu lyl--ac~{tate
t
0.45
P
i i ._..+__ i
7.2
=
i
!
i
I
I
7.0
*60 mh
Figure 4
The behavior of composition at the bottom against the disturbance
impurity at the top (acetic acid) increases. Altematively, as the number of two liquid phases trays increase, the impurity of bottom (water, n-butyl-acetate) increase. To maintain under specifications of each impurity, the number of two liquid phases trays should be kept around 3 "~12.
Figure 3a,3b show the influence on temperature in columns with the number of two liquid phases trays in the column. In figure 3a, temperatures on the tray are plotted with three cases, which are 2 phases tray = 0,10,15. Figure 3b shows the difference of the temperature on the tray for each case based on the temperature with 2-phases tray = 10. Figure 3b indicates temperatures on tray between tray 13 and tray 18 is sensitive to the variation of the number of 2-phase tray. In other words, the variation of the number of two liquid phases tray can be judged to observe these temperatures on trays.
1097 If the impurity at the top and the number of two liquid phases trays are kept almost constant, two impurities at the bottom (water, n-butyl-acetate) changes symmetry like on the both ends of seesaw. Figure 4 indicates the behavior of composition at the bottom in open loop when the disturbance of feed composition is injected (water composition increased by 0.5%). From the study of dynamic simulation above, the behavior of composition change is summarized in the following. 9 The number of two liquid phases trays should be kept between 3"--12 to maintain the impurity level at the bottom and top. The number of the two liquid phases is sensitive to mid temperature of column. The control of mid temperature of column is key to maintaining the number of two liquid phases. 9 If the number of two liquid phases trays are constant, two impurities (water and n-butyl acetate) at the bottom change symmetry like on the both ends of seesaw. Therefore, either of water or n-butyl acetate is kept constant under the condition of two liquid phases trays constant, then the other can be also kept constant. 4. DESIGN OF NEW CONTROL SYSTEM
Control system is designed based on the simulation study. The new control system is schematically drawn in figure 5. The number of two liquid phases is controlled by manipulating the total reflux flow rate (sum of water reflux and entrainer reflux) and boilup steam flow rate. Then, the impurities at the top and bottom are kept at low level. As for water/entrainer balance in column, soft sensors are developed to detect the compositions at the bottom. Prediction model is built based on temperatures on the trays by using PLS (Partial Least Squares) methods. The accuracy of soft sensor is shown in figure 6(a), 6(b). Water/entrainer ratio in total reflux is controlled in respond to the amount of n-butyl acetate of bottom.
water/entrainer ratio
,o..
~-~) Figure 5.
~
i=....,o, -'~
aceticacid
New Control System
1098
Figure 6. Soft sensor prediction vs. actual
Figure 7. New Control system against the disturbance 5. THE RESULT OF CONTROL Figure 7(a) indicates the behavior of impurities at the bottom against the disturbance in an industrial distillation column. The ratio of e n t r a i n e r to water in reflux is adjusted to m a i n t a i n the impurities at the bottom (Figure 7(b)). From the standpoint of energy saving, total reflux flow rate can be minimized by setting the temperature T2 to the upper limit, then boilup steam flow rate is also lowed. Figure 8 shows boilup steam flow rate is decreased by 3%. The new control system is implemented successfully.
Figure 8. Reduction of total reflux and boilup steam flow rate
1099 6. CONCLUSION In this work, the control of heterogeneous azeotropic distillation of acetic acid water with n-butyl-acetate is studied in an industrial application by using dynamic simulation, which is developed for this work. The study shows the number of two liquid phases trays in column affect impurities of the top and the bottom very much. And the variation of two liquid phases trays is able to detect by observing mid column temperatures(13 ~ 18trays). Two impurities at the bottom change symmetry like on the both ends of seesaw under the condition the number of two liquid phases trays constant. New control system is designed based on simulation study, and validated in an industrial distillation column. Two liquid phases trays region is controlled by total reflux (sum of water reflux and entrainer reflux) flow rate and boil up steam flow rate. Impurities at the bottom (n-butyl-acetate) is controlled by changing water/entrainer ratio in total reflux flow rate. The automated control enables us to decrease burden of operators fairly. This control strategy makes it easier to meet the plant load change on the demand from the market and expands chances of business decision-making. APPENDIX
A physical model is developed under the following assumptions. 9Perfect mixing both in vapor phase and in liquid phase of decanter, each stage and reboiler 9Ideal stage, Ideal gas, Negligible vapor holdup 9Boilup steam totally condenses in reboiler and all latent heat is used for boilup. 9Liquid-liquid equilibrium (LLE) equations are calculated in decanter and Vapor-liquid-liquid equilibrium (VLLE) equations are considered in the column. REFERENCES
[1] Maurizio, R.; Michael, F.D., Dynamic of heterogeneous azeotropic distillation columns, AIChE J, 36, 1 (1990). [2] Nikolaos, B.; George, A. M.; Manfred, M., Multiple steady state in heterogeneous azeotropic distillation, Ind. Eng. Chem. Res. , 35(1996). [3] I-L.Chein; C. J. Wang; D. S. H. Wong, Dynamic and control of a heterogeneous azeotropic distillation column: Conventional control approach, Ind. Eng. Chem. Res., 38(1999). [4] Shinskey,F.G., Distillation control for productivity and energy conservation, l Sted., McGraw-Hill, New York(1981). [5] Shoushi, Y. Patent No.2001-281837, Control method of distillation column, Japan (2001). [6] Soemantri, W.; Warren, D.S., Azeotropic distillation, AIChE J, 42, 1 (1996).