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Separation of ternary mixtures in a batch distillation column with side withdrawal
Computers and Chemical Engineering 28 (2004) 643–650
Separation of ternary mixtures in a batch distillation column with side withdrawal D. Demicoli∗ , J. Stichlmair Lehrstuhl für Fluidverfahrenstechnik, Technische Universität München, Boltzmannstr. 15, Garching D-85748, Germany
Abstract In this paper we introduce a novel operation policy for the separation of zeotropic mixtures via batch distillation. The novel policy is based on feasibility studies of a batch distillation column provided with a side withdrawal. The process is illustrated via computer-based simulations of the purification of an intermediate-boiling component (1-propanol) from a mixture containing a low boiler (ethanol) and a high boiler (1-butanol). Furthermore, the effects of the most important process and design parameters are investigated in detail. A comparison to the middle vessel column is provided at the end of the paper. © 2004 Elsevier Ltd. All rights reserved. Keywords: Batch distillation; Cyclic operation; Side withdrawal; Zeotropic mixtures
1. Introduction Batch distillation is a very efficient and advantageous unit operation for the separation of multicomponent mixtures into pure components. Due to its flexibility and low capital costs, batch distillation is becoming increasingly important in the fine chemical and pharmaceutical industries. Nevertheless, there are intrinsic disadvantages associated with the conventional batch distillation process (Fig. 1a). These are: long batch times, high temperatures in the charge vessel and complex operation. Hence, alternative processes and operation policies, which have the potential to overcome these disadvantages, are being extensively investigated. Four of the available configurations of batch distillation are shown in Fig. 1. In the regular batch distillation column, the charged is loaded to the column’s sump at the beginning of the process. It is then heated to its boiling point, and the products are withdrawn sequentially from the column’s head in order of increasing boiling point. In the case of the inverted batch distillation column (Fig. 1b), the charged is loaded to the column’s head. The products are withdrawn sequentially from the column’s bottom in order of decreasing boiling point. Sørensen and Skogestad (1996) compared these batch distillation column configurations for the separation of bi∗ Corresponding author. Tel.: +49-89-289-16507; fax: +49-89-289-16510. E-mail address: [email protected] (D. Demicoli).
0098-1354/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compchemeng.2004.02.009
nary mixtures. They concluded that the inverted column was found to yield the shortest batch time for separations where the light component is present in small amount in the feed. In a later work, Sørensen and Prenzler (1997) investigated the cyclic, or closed operation, of a batch distillation column for the separation of binary mixtures. Here, the column was operated at total reflux. The light and heavy boilers were accumulated in the top and bottom vessels, respectively. The main advantage of this configuration was found to be its easy operation since the column did not need an advanced control loop. Later, Warter and Stichlmair, 2002; Warter, Demicoli, and Stichlmair (2002) and Warter, Demicoli, and Stichlmair (2003) suggested the application of the closed operation of the middle vessel column (Fig. 1c) for the separation of ternary mixtures. The investigation was based on both simulation and experimental results. It was found that the operation was much simpler than the corresponding process in the regular column. This was due to simpler handling of the liquid fractions (no distillate off-cuts were required) and, furthermore, the novel process required a mere monitoring the temperature profile in the column. As suggested by Wittgens, Litto, Sørensen, & Skogestad, 1996, multi-component mixtures can be separated in the multi-vessel distillation column. This might also be operated in closed operation. The three groups pointed out that the main disadvantage of the cyclic operation is that the exact composition of the charge needs to be known a priori. Hence, closed operation with a
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Nomenclature a b B ˙ B c D ˙ D ˙ L M RB RL SP V˙ ˙ W x
low boiling component intermediate boiling component bottom fraction flow rate of bottom product (mol/s) high boiling component distillate fraction flow rate of distillate product (mol/s) liquid flow rate (mol/s) middle vessel fraction reboil ratio reflux ratio side product accumulator vapour flow rate (mol/s) flow rate of withdrawal stream (mol/s) molar fraction
Greek letter α relative volatility
temperature control loop was suggested for the three column configurations. In this paper we introduce a novel process for the separation of ternary mixtures via cyclic operation of a batch distillation column provided with a side withdrawal (Fig. 1d). This consists of a distillation column equipped with sump and distillate vessels, to which the charge is loaded at the beginning of the process (Demicoli and Stichlmair, 2003). A liquid stream of required concentration is withdrawn from the middle of the column. In the first part of the paper the feasibility of the process is discussed. From this, an operational policy is derived and it is illustrated by simulations for
the purification of an intermediate boiler (1-propanol) from a mixture containing light (ethanol) and heavy (1-butanol) impurities. The main operational and design parameters are discussed in the third part of the paper. Finally, this is compared to the middle vessel column. 2. Modelling Two different rigorous models for the distillation columns were written. In both models, the following assumptions were made: equilibrium stage column, constant liquid holdup on each stage, negligible vapour holdup, perfect mixing, ideal gas. Furthermore, to avoid the high index problem (Pantelides, Gritsis, Morison, & Sargent, 1988), it had to be assumed that on each stage the energy dynamics were infinitely fast. In this case the energy balance on each stage could be written as algebraic equations. The resulting mathematical problem is a system of differential and algebraic equations which was solved using the commercial process simulator gPROMS® from Process Systems Enterprise. In one model, the liquid phase was assumed to be ideal, i.e. the vapour–liquid equilibrium was written in terms of constant relative volatilities. In the second model, the properties of the liquid phase and the vapour–liquid equilibrium were calculated using the property data base Multiflash from Infochem Computer Services ltd. The activity coefficients for the ternary mixture ethanol/1-propanol/1-butanol were calculated using the Wilson E correlation.
3. Feasibility The column shown in Fig. 1d can be visualised as an inverted batch distillation column placed on top of a regular
Fig. 1. Different column types: (a) regular; (b) inverted; (c) middle vessel; (d) novel batch distillation column with side withdrawal.
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4. Process The process will be illustrated by considering the separation of a ternary zeotropic mixture 1-propanol (intermediate boiler), ethanol (low boiler) and 1-butanol (high boiler). Based on the previous feasibility analysis, a process sequence consisting of two steps was postulated. The concentration profile in the vessels and that of the accumulated side product are shown in Fig. 3. 4.1. Close operation mode After the charge is loaded to the two vessels, the column is operated at total reflux, i.e. no liquid is withdrawn from the withdrawal stage. The light and heavy boilers will accumulate in the top and bottom vessels, respectively. During this phase, the concentration of the intermediate boiling component on the withdrawal stage is increasing until it reaches its specification. 4.2. Open operation mode
Fig. 2. Concentration path in sump and distillate vessels and in withdrawal stream at very high number of stages and very high reflux and reboil ratios.
batch column, the two being connected at the withdrawal stage. Hence, feasibility studies for the regular and inverted batch distillation columns may be applied to the novel process, provided that the concentration of the withdrawal tray lies on the column’s profile. Therefore, it is possible to obtain pure intermediate-boiling product (b) from an infinite column operated at infinite reflux ratios, only if the distillate and sump vessels do not contain the heavy and the low-boiling components, respectively. This is also demonstrated by Fig. 2, which shows the simulated concentration path of the liquid phase on the withdrawal tray and in the top and bottom vessels. Here an equimolar mixture of ethanol, 1-propanol and 1-butanol was separated in a column having a very high number of stages and being operated at close to total reflux conditions. It is clearly visible that, as long as the top and bottom vessels contain heavy and light boiler respectively, the concentration of the intermediate boiler on the withdrawal tray is smaller than one.
When the concentration of the intermediate boiling component on the withdrawal tray reaches its specification, it can be withdrawn from the side withdrawal. During this phase, the side withdrawal stream conceptually divides the column into two sections. The column section above the side stream is similar to an inverted distillation column, while the lower section can be visualised as a regular distillation column. As can be seen from the concentration profile (Fig. 4), the upper column section is needed to purify 1-propanol from ethanol, while in the lower column section the product is separated from 1-butanol. Hence, the reflux ratio (RL ) of the lower column is used to control the heavy boiling impurity in the withdrawal stream. Analogously, the reboil ratio (RB ) of the inverted column is used to control the low-boiling impurity. The internal reflux and reboil ratios are related to the flow ˙ through the mass balance rate of the withdrawal stream (W) around the withdrawal stage: ˙ W 1 1 ˙ ˙ ˙ ˙ W = LU − LL = V − RL ; − RL (1) = ˙ RB R V B ˙ U ; RL = L ˙ L /V˙ ; L ˙ L the liquid flow rate where RB = V˙ /L ˙ U the liquid flow rate in the in the lower column section; L upper column section; V˙ the vapour flow rate. At the end of the process, the internal reflux ratios become equal to unity and the flow of the withdrawal stream equal ˙ = 0 (Fig. 5). to zero, i.e. RL = RB = 1, W To further purify the light and heavy boiler from the remaining intermediate boiler, an off-cut can be collected. This can be than added to the charge to the next batch. 5. Effect of the operational parameters The analysis of the process parameters was performed by systematically varying one parameter at a time and studying
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Fig. 4. Concentration profile along the column, under composition control (number of stages is increasing from bottom to top of the column). Full lines: relative time = 0.4; dotted lines: relative time = 0.9.
the changes on the process outputs. In the following paragraphs the results of such analysis are presented and discussed. In this section the liquid phase was assumed to be an ideal liquid mixture. 5.1. Composition of the charge To study the effect of the composition of the charge, equal amounts of charges of different compositions were processed in the same column operated in closed loop. The separation was carried out in the shortest time when the charge was rich in the intermediate boiling component (Fig. 6). This was due to the fact that both the light/intermediate and the intermediate/heavy separations, at the beginning of the process, could be carried out at low reflux and reboil ratios (Fig. 5) for feeds rich in intermediate boiler. On the other hand, if the charge contains low amounts
Fig. 3. (a) Batch distillation column with side withdrawal. Open operation with controller loops; (b) concentration path in sump and distillate vessels and in withdrawal stream, under composition control. Fig. 5. Effect of the initial compositions of the charge on the internal reflux and reboil ratios.
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Fig. 7. Effect of the reflux and reboil ratios on the concentration and recovery of the intermediate-boiling product. Fig. 6. Effect of the initial compositions of the charge on the duration of the process.
of the intermediate boiler, both the light/intermediate and the heavy/intermediate separations would require high reflux and reboil ratios. This is in agreement with the results obtained by Sørensen and Skogestad (1996) in their comparative studies on the regular and inverted batch columns. For feeds containing low amounts of intermediate boiler, the specified purity of the intermediate component becomes earlier infeasible, hence, the recovery drops significantly and the process time decreases. Therefore, our investigations were limited to the case in which the feed was much richer in the intermediate boiler than in light and heavy boilers. In such cases the relative content of the extreme boilers plays a minor role and influences mainly the duration of the start-up of the process i.e. the closed operation mode. 5.2. Effect of reflux and reboil ratios Since the first process step is a total reflux operation, during which the light boiler is separated from the heavy-boiling component, the analysis of the effect of different reflux ratios can be limited to the second process step. During this analysis, the feed was charged to the top and bottom vessels and the total reflux operation was interrupted when the concentration of the top vessel contained no heavy boiler and the bottom vessel no light boiler. During the second process step, the reflux and reboil ratios were set to constant values. The simulation was interrupted when one of the vessels was empty. In Fig. 7, the concentration of the intermediate boiler and its molar amount accumulated in the side product are plotted against the relative distillation time, for different values of the reboil and reflux ratios. This plot shows that to achieve high recovery and high purity in the intermediate product, high reflux and reboil ratios are required. On the other hand,
increasing the reflux and reboil ratios increases the duration of the process. It can be further observed that at the end of the process, the applied constant reflux and reboil ratios are not able to maintain the purity of the intermediate product. Hence, it is evident that a constant reflux-reboil ratio policy is not optimal. Additionally, to study the interactions between the two column sections, the reboil ratio was varied while the reflux ratio was kept constant. It is observed that the amount of high boiling impurity in the side product increases with decreasing reboil ratio, while that of the low boiler changes very slightly. Furthermore, the concentration of the heavy boiler in the upper column section considerably increases with decreasing reboil ratio, indicating a strong interaction between the upper and lower column sections. 5.2.1. Operation in closed loop In closed loop operation the reflux and reboil ratios are manipulated variables and, therefore, cannot be considered as design variables. In this case the operation parameters are the set points to the two controllers. The control loop of the upper column section controls the composition in the liquid phase of the low boiler a two stages above the withdrawal stage, while the lower control loop controls the composition of the high boiling impurity c two stages below the withdrawal stage. Hence, the concentration of impurities in the withdrawal stream increases with increasing set points, while the batch time increases with decreasing set point. This is due to the fact that higher reflux and reboil ratios are required to reach the lower set-points, i.e. high purity b. Set-points lower than the concentration reachable at infinite reboil and reflux ratios are unfeasible. 5.3. Termination criteria for the first process step Increasing the duration of the first process step reduces the concentration of the light boiler present in the sump of
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Fig. 8. Effect of the duration of the close operation mode on the concentration on the withdrawal tray, in the distillate and sump vessels.
the column and that of the heavy boiler in the top vessel at the beginning of the second step (Fig. 8). Hence, with increasing duration of the first process step, the concentration of the intermediate boiler in the middle of column at the beginning of the second process step increases. This leads to an increased concentration of the middle-boiling product and to an increased recovery of the low and high-boiling products. Nevertheless, very long times are required to obtain pure intermediate-boiling component on the withdrawal tray, hence, the best termination criterion should be found by process optimisation. 6. Effects of the geometric parameters The geometric parameters of the process are identified as the total number of stages and the position of the withdrawal tray. 6.1. Number of stages The total number of stages was varied while the position of the withdrawal feed was kept in the middle of the column and the composition controllers were placed two stages below and two stages above the withdrawal tray. The set points to the two controllers were not varied during this investigation. Hence, the concentration profile around the withdrawal tray was fixed by the two control loops and the composition of the intermediate boiler was independent of the number of stages.With increasing number of stages, lower reflux ratios are required to achieve high purity b hence, the recovery rate of the intermediate boiler increases and the batch time decreases. The concentration of b in the top and sump vessels at which the process becomes infeasible decreases with increasing number of stages. Hence, both the recovery of b (σ b ) and the purity of the light and heavy boilers increase with increasing number of stages.
Fig. 9. Effect of the position of the withdrawal tray on the recovery, αab = αbc .
6.2. Position of withdrawal tray The position of the withdrawal stage determines the relative size of the two column sections. Hence, the optimal position of the withdrawal tray would depend on the relative volatilities αab and αbc . If, for instance, the separation of the intermediate boiler from the low boiler is more difficult than that from the high boiler (i.e. αab < αbc ), then a longer upper column section would be required and the position of the withdrawal tray would shift downwards, and vice versa. In the case that the two separations are similar, then the best position of the withdrawal point would be in the middle of the column as shown in Fig. 9.
7. Considerations on control In the description of the process given in the previous section, the compositions of the impurities are used as control variables in the simulations. That is, the control loop in the upper column section controls the composition of the light impurity four stages above the withdrawal stage, and that in the lower column section controls the composition of the heavy-boiling impurity four stages below the withdrawal stage (Fig. 3a). This choice allows for a partial decoupling of the control loops, since the composition of the low boiler in the upper column section is only slightly dependent on the reflux ratio in the lower column section. This is illustrated in Fig. 10a. Here, the response of controller in the lower column section is plotted for closed loop operation (black lines), and for a malfunctioning the upper control loop (grey lines), while the lower control is in closed loop. The response of controller in the lower column section is effected only very slightly by the control action of the upper column section even when this is malfunctioning.
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Fig. 11. Comparison of the energy demand of a batch distillation column with a middle vessel column (grey surface) and batch distillation column with a side withdrawal (black surface), as a function of the relative volatilities and the concentration of the intermediate boiler in the charge.
Fig. 10. (a) Effect of a malfunctioning controller signal in the upper column section on the response of the controller in the lower column section: (i) close loop operation (dotted line); (ii) malfunctioning controller signal (full lines) (b) Effect of a malfunctioning controller signal in the upper column section on the temperature in the upper and lower column sections.
On the other hand, choosing the temperature as control variable introduces stronger coupling between the control loops. This is due to the fact that if, for instance, the reboil ratio of the upper column section were too low, then too much light boiler would reach the lower column section, decreasing the temperature in both sections (Fig. 10b). The controller in the lower column section would then react by decreasing the reflux ratio of the lower column section and hence, more of the high-boiling impurity would be withdrawn with the side product.
8. Comparison to the middle vessel batch distillation column For the separation of zeotropic ternary mixtures, batch distillation in a middle vessel column was shown to be a
very advantageous process alternative to the regular batch distillation process. In this comparison the charge to the middle vessel column was loaded to middle vessel, and the column was operated in closed loop. In Fig. 11 the energy demand (at equal number of stages and separation task) of the two columns is plotted as a function of the concentration of the intermediate boiler in the charge (containing equal amounts of light and heavy boilers), and the relative volatilities between the light and intermediate boiler (αab ) and that between the intermediate and heavy boiler (αbc ). Hence, for very difficult separations (low relative volatility), the process with the side withdrawal is energetically more advantageous than that in the middle vessel column. On the other hand, for higher values of the relative volatilities, the side withdrawal column is energetically advantageous only for mixtures very rich in the intermediate boiler. This can be explained by the structure of the two columns as shown in Fig. 1. The batch distillation column with a middle vessel can be visualised as a regular batch distillation column placed on top of an inverted column. Hence, this column should be advantageous for charges poor in intermediate boiler since the low-boiling component is separated from the intermediate boiler in a rectifying section (upper section) and the heavy-boiling component is separated from the intermediate boiler in an inverted column (lower column section). This situation is inverted in the case of the side withdrawal column. Here, the intermediate-light boiler separation is carried out in an inverted column, while the intermediate-heavy boiler separation is carried out in a regular column.
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Furthermore, in the middle vessel column, the two column sections are separated from each other by a huge hold-up (in comparison to the column’s hold-up), hence, the two sections are decoupled (Farschman & Diwekar, 1998). Unfortunately, this is not the case of the side withdrawal column. In this paper it was shown that the two column sections interact and, hence, the design of the control loops is more complex. Nevertheless, if the required high purity product is the intermediate boiler, the column with side withdrawal allows direct control of its purity, while the column with the middle vessel allows controlling the purity of the light and heavy components (impurities).
Appendix A
9. Conclusion
References
In this paper we introduced a novel operational policy for the purification of an intermediate boiling component via the cyclic operation of a batch distillation column with a side withdrawal. The feasibility of the process was investigated by considering the novel column configuration as an inverted batch distillation column placed over a regular batch column. A novel operating strategy, based on the feasibility studies, was developed and verified by computer-aided simulations. Furthermore, the influence of most important parameters on the performance of the process was systematically investigated. Finally, the novel process was compared to the batch distillation column with a middle vessel column. It was shown that in spite of several problems, the novel process has the potential to introduce improvement over the batch distillation with a middle vessel. Practically, the process can be carried out in a distillation column very similar to a middle vessel column comprising three vessels: at the top, in the middle and at the sump. In this case, the middle vessel would be exclusively used as a product accumulator. This further shows that a middle vessel column introduces increased plant flexibility with respect to a conventional batch distillation column.
Demicoli, D., & Stichlmair, J. (2003). Novel operational strategy for the separation of ternary mixtures via cyclic operation of a batch distillation column with side withdrawal. Computer Aided Chemical Engineering, 11, 629–634. Farschman, C. A., & Diwekar, U. (1998). Dual composition control in a novel Batch distillation column. Industrial and Engineering Chemistry Research, 37, 89–96. Pantelides, C. C., Gritsis, D., Morison, K. S., & Sargent, R. W. H. (1988). The mathematical modelling of transient systems using differential-algebraic equations. Computers and Chemical Engineering, 12(5), 449–454. Sørensen, E., & Prenzler, M. (1997). A cyclic operating policy for batch distillation—theory and practice. Computers and Chemical Engineering, 21(Suppl.), S1215–S1220. Sørensen, E., & Skogestad, S. (1996). Comparison of regular and inverted batch distillation. Chemical Engineering Science, 51(22), 4949–4962. Warter, M., & Stichlmair, J. (2002). Batch distillation in middle vessel columns. Chemie Ingenieur Technik, 74(9), 1195–1206. Warter, M., Demicoli, D., & Stichlmair, J. (2002). Batch distillation of zeotropic mixtures in a column with a middle vessel. Computer Aided Chemical Engineering, 10, 385–390. Warter, M., Demicoli, D., & Stichlmair, J. (2003). Operation of a batch distillation column with a middle vessel: experimental results for the separation of zeotropic and azeotropic mixtures. Chemical Engineering Proceedings, 43(3), 263–272. Wittgens, B., Litto, R., Sørensen, E., & Skogestad, S. (1996). Total reflux operation of multivessel batch distillation. Computers and Chemical Engineering, 20(Suppl.), S1041–S1046.
Column data
Stages per section Initial feed (mol) Initial condenser holdup (mol) Initial middle vessel holdup Initial reboiler holdup (mol)
Column with side withdrawal
Middle vessel column
14 6000 3000 – 3000
14 6000 0 5550 50
Separation of ternary mixtures in a batch distillation column with side withdrawal D. Demicoli∗ , J. Stichlmair Lehrstuhl für Fluidverfahrenstechnik, Technische Universität München, Boltzmannstr. 15, Garching D-85748, Germany
Abstract In this paper we introduce a novel operation policy for the separation of zeotropic mixtures via batch distillation. The novel policy is based on feasibility studies of a batch distillation column provided with a side withdrawal. The process is illustrated via computer-based simulations of the purification of an intermediate-boiling component (1-propanol) from a mixture containing a low boiler (ethanol) and a high boiler (1-butanol). Furthermore, the effects of the most important process and design parameters are investigated in detail. A comparison to the middle vessel column is provided at the end of the paper. © 2004 Elsevier Ltd. All rights reserved. Keywords: Batch distillation; Cyclic operation; Side withdrawal; Zeotropic mixtures
1. Introduction Batch distillation is a very efficient and advantageous unit operation for the separation of multicomponent mixtures into pure components. Due to its flexibility and low capital costs, batch distillation is becoming increasingly important in the fine chemical and pharmaceutical industries. Nevertheless, there are intrinsic disadvantages associated with the conventional batch distillation process (Fig. 1a). These are: long batch times, high temperatures in the charge vessel and complex operation. Hence, alternative processes and operation policies, which have the potential to overcome these disadvantages, are being extensively investigated. Four of the available configurations of batch distillation are shown in Fig. 1. In the regular batch distillation column, the charged is loaded to the column’s sump at the beginning of the process. It is then heated to its boiling point, and the products are withdrawn sequentially from the column’s head in order of increasing boiling point. In the case of the inverted batch distillation column (Fig. 1b), the charged is loaded to the column’s head. The products are withdrawn sequentially from the column’s bottom in order of decreasing boiling point. Sørensen and Skogestad (1996) compared these batch distillation column configurations for the separation of bi∗ Corresponding author. Tel.: +49-89-289-16507; fax: +49-89-289-16510. E-mail address: [email protected] (D. Demicoli).
0098-1354/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compchemeng.2004.02.009
nary mixtures. They concluded that the inverted column was found to yield the shortest batch time for separations where the light component is present in small amount in the feed. In a later work, Sørensen and Prenzler (1997) investigated the cyclic, or closed operation, of a batch distillation column for the separation of binary mixtures. Here, the column was operated at total reflux. The light and heavy boilers were accumulated in the top and bottom vessels, respectively. The main advantage of this configuration was found to be its easy operation since the column did not need an advanced control loop. Later, Warter and Stichlmair, 2002; Warter, Demicoli, and Stichlmair (2002) and Warter, Demicoli, and Stichlmair (2003) suggested the application of the closed operation of the middle vessel column (Fig. 1c) for the separation of ternary mixtures. The investigation was based on both simulation and experimental results. It was found that the operation was much simpler than the corresponding process in the regular column. This was due to simpler handling of the liquid fractions (no distillate off-cuts were required) and, furthermore, the novel process required a mere monitoring the temperature profile in the column. As suggested by Wittgens, Litto, Sørensen, & Skogestad, 1996, multi-component mixtures can be separated in the multi-vessel distillation column. This might also be operated in closed operation. The three groups pointed out that the main disadvantage of the cyclic operation is that the exact composition of the charge needs to be known a priori. Hence, closed operation with a
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Nomenclature a b B ˙ B c D ˙ D ˙ L M RB RL SP V˙ ˙ W x
low boiling component intermediate boiling component bottom fraction flow rate of bottom product (mol/s) high boiling component distillate fraction flow rate of distillate product (mol/s) liquid flow rate (mol/s) middle vessel fraction reboil ratio reflux ratio side product accumulator vapour flow rate (mol/s) flow rate of withdrawal stream (mol/s) molar fraction
Greek letter α relative volatility
temperature control loop was suggested for the three column configurations. In this paper we introduce a novel process for the separation of ternary mixtures via cyclic operation of a batch distillation column provided with a side withdrawal (Fig. 1d). This consists of a distillation column equipped with sump and distillate vessels, to which the charge is loaded at the beginning of the process (Demicoli and Stichlmair, 2003). A liquid stream of required concentration is withdrawn from the middle of the column. In the first part of the paper the feasibility of the process is discussed. From this, an operational policy is derived and it is illustrated by simulations for
the purification of an intermediate boiler (1-propanol) from a mixture containing light (ethanol) and heavy (1-butanol) impurities. The main operational and design parameters are discussed in the third part of the paper. Finally, this is compared to the middle vessel column. 2. Modelling Two different rigorous models for the distillation columns were written. In both models, the following assumptions were made: equilibrium stage column, constant liquid holdup on each stage, negligible vapour holdup, perfect mixing, ideal gas. Furthermore, to avoid the high index problem (Pantelides, Gritsis, Morison, & Sargent, 1988), it had to be assumed that on each stage the energy dynamics were infinitely fast. In this case the energy balance on each stage could be written as algebraic equations. The resulting mathematical problem is a system of differential and algebraic equations which was solved using the commercial process simulator gPROMS® from Process Systems Enterprise. In one model, the liquid phase was assumed to be ideal, i.e. the vapour–liquid equilibrium was written in terms of constant relative volatilities. In the second model, the properties of the liquid phase and the vapour–liquid equilibrium were calculated using the property data base Multiflash from Infochem Computer Services ltd. The activity coefficients for the ternary mixture ethanol/1-propanol/1-butanol were calculated using the Wilson E correlation.
3. Feasibility The column shown in Fig. 1d can be visualised as an inverted batch distillation column placed on top of a regular
Fig. 1. Different column types: (a) regular; (b) inverted; (c) middle vessel; (d) novel batch distillation column with side withdrawal.
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4. Process The process will be illustrated by considering the separation of a ternary zeotropic mixture 1-propanol (intermediate boiler), ethanol (low boiler) and 1-butanol (high boiler). Based on the previous feasibility analysis, a process sequence consisting of two steps was postulated. The concentration profile in the vessels and that of the accumulated side product are shown in Fig. 3. 4.1. Close operation mode After the charge is loaded to the two vessels, the column is operated at total reflux, i.e. no liquid is withdrawn from the withdrawal stage. The light and heavy boilers will accumulate in the top and bottom vessels, respectively. During this phase, the concentration of the intermediate boiling component on the withdrawal stage is increasing until it reaches its specification. 4.2. Open operation mode
Fig. 2. Concentration path in sump and distillate vessels and in withdrawal stream at very high number of stages and very high reflux and reboil ratios.
batch column, the two being connected at the withdrawal stage. Hence, feasibility studies for the regular and inverted batch distillation columns may be applied to the novel process, provided that the concentration of the withdrawal tray lies on the column’s profile. Therefore, it is possible to obtain pure intermediate-boiling product (b) from an infinite column operated at infinite reflux ratios, only if the distillate and sump vessels do not contain the heavy and the low-boiling components, respectively. This is also demonstrated by Fig. 2, which shows the simulated concentration path of the liquid phase on the withdrawal tray and in the top and bottom vessels. Here an equimolar mixture of ethanol, 1-propanol and 1-butanol was separated in a column having a very high number of stages and being operated at close to total reflux conditions. It is clearly visible that, as long as the top and bottom vessels contain heavy and light boiler respectively, the concentration of the intermediate boiler on the withdrawal tray is smaller than one.
When the concentration of the intermediate boiling component on the withdrawal tray reaches its specification, it can be withdrawn from the side withdrawal. During this phase, the side withdrawal stream conceptually divides the column into two sections. The column section above the side stream is similar to an inverted distillation column, while the lower section can be visualised as a regular distillation column. As can be seen from the concentration profile (Fig. 4), the upper column section is needed to purify 1-propanol from ethanol, while in the lower column section the product is separated from 1-butanol. Hence, the reflux ratio (RL ) of the lower column is used to control the heavy boiling impurity in the withdrawal stream. Analogously, the reboil ratio (RB ) of the inverted column is used to control the low-boiling impurity. The internal reflux and reboil ratios are related to the flow ˙ through the mass balance rate of the withdrawal stream (W) around the withdrawal stage: ˙ W 1 1 ˙ ˙ ˙ ˙ W = LU − LL = V − RL ; − RL (1) = ˙ RB R V B ˙ U ; RL = L ˙ L /V˙ ; L ˙ L the liquid flow rate where RB = V˙ /L ˙ U the liquid flow rate in the in the lower column section; L upper column section; V˙ the vapour flow rate. At the end of the process, the internal reflux ratios become equal to unity and the flow of the withdrawal stream equal ˙ = 0 (Fig. 5). to zero, i.e. RL = RB = 1, W To further purify the light and heavy boiler from the remaining intermediate boiler, an off-cut can be collected. This can be than added to the charge to the next batch. 5. Effect of the operational parameters The analysis of the process parameters was performed by systematically varying one parameter at a time and studying
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Fig. 4. Concentration profile along the column, under composition control (number of stages is increasing from bottom to top of the column). Full lines: relative time = 0.4; dotted lines: relative time = 0.9.
the changes on the process outputs. In the following paragraphs the results of such analysis are presented and discussed. In this section the liquid phase was assumed to be an ideal liquid mixture. 5.1. Composition of the charge To study the effect of the composition of the charge, equal amounts of charges of different compositions were processed in the same column operated in closed loop. The separation was carried out in the shortest time when the charge was rich in the intermediate boiling component (Fig. 6). This was due to the fact that both the light/intermediate and the intermediate/heavy separations, at the beginning of the process, could be carried out at low reflux and reboil ratios (Fig. 5) for feeds rich in intermediate boiler. On the other hand, if the charge contains low amounts
Fig. 3. (a) Batch distillation column with side withdrawal. Open operation with controller loops; (b) concentration path in sump and distillate vessels and in withdrawal stream, under composition control. Fig. 5. Effect of the initial compositions of the charge on the internal reflux and reboil ratios.
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Fig. 7. Effect of the reflux and reboil ratios on the concentration and recovery of the intermediate-boiling product. Fig. 6. Effect of the initial compositions of the charge on the duration of the process.
of the intermediate boiler, both the light/intermediate and the heavy/intermediate separations would require high reflux and reboil ratios. This is in agreement with the results obtained by Sørensen and Skogestad (1996) in their comparative studies on the regular and inverted batch columns. For feeds containing low amounts of intermediate boiler, the specified purity of the intermediate component becomes earlier infeasible, hence, the recovery drops significantly and the process time decreases. Therefore, our investigations were limited to the case in which the feed was much richer in the intermediate boiler than in light and heavy boilers. In such cases the relative content of the extreme boilers plays a minor role and influences mainly the duration of the start-up of the process i.e. the closed operation mode. 5.2. Effect of reflux and reboil ratios Since the first process step is a total reflux operation, during which the light boiler is separated from the heavy-boiling component, the analysis of the effect of different reflux ratios can be limited to the second process step. During this analysis, the feed was charged to the top and bottom vessels and the total reflux operation was interrupted when the concentration of the top vessel contained no heavy boiler and the bottom vessel no light boiler. During the second process step, the reflux and reboil ratios were set to constant values. The simulation was interrupted when one of the vessels was empty. In Fig. 7, the concentration of the intermediate boiler and its molar amount accumulated in the side product are plotted against the relative distillation time, for different values of the reboil and reflux ratios. This plot shows that to achieve high recovery and high purity in the intermediate product, high reflux and reboil ratios are required. On the other hand,
increasing the reflux and reboil ratios increases the duration of the process. It can be further observed that at the end of the process, the applied constant reflux and reboil ratios are not able to maintain the purity of the intermediate product. Hence, it is evident that a constant reflux-reboil ratio policy is not optimal. Additionally, to study the interactions between the two column sections, the reboil ratio was varied while the reflux ratio was kept constant. It is observed that the amount of high boiling impurity in the side product increases with decreasing reboil ratio, while that of the low boiler changes very slightly. Furthermore, the concentration of the heavy boiler in the upper column section considerably increases with decreasing reboil ratio, indicating a strong interaction between the upper and lower column sections. 5.2.1. Operation in closed loop In closed loop operation the reflux and reboil ratios are manipulated variables and, therefore, cannot be considered as design variables. In this case the operation parameters are the set points to the two controllers. The control loop of the upper column section controls the composition in the liquid phase of the low boiler a two stages above the withdrawal stage, while the lower control loop controls the composition of the high boiling impurity c two stages below the withdrawal stage. Hence, the concentration of impurities in the withdrawal stream increases with increasing set points, while the batch time increases with decreasing set point. This is due to the fact that higher reflux and reboil ratios are required to reach the lower set-points, i.e. high purity b. Set-points lower than the concentration reachable at infinite reboil and reflux ratios are unfeasible. 5.3. Termination criteria for the first process step Increasing the duration of the first process step reduces the concentration of the light boiler present in the sump of
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Fig. 8. Effect of the duration of the close operation mode on the concentration on the withdrawal tray, in the distillate and sump vessels.
the column and that of the heavy boiler in the top vessel at the beginning of the second step (Fig. 8). Hence, with increasing duration of the first process step, the concentration of the intermediate boiler in the middle of column at the beginning of the second process step increases. This leads to an increased concentration of the middle-boiling product and to an increased recovery of the low and high-boiling products. Nevertheless, very long times are required to obtain pure intermediate-boiling component on the withdrawal tray, hence, the best termination criterion should be found by process optimisation. 6. Effects of the geometric parameters The geometric parameters of the process are identified as the total number of stages and the position of the withdrawal tray. 6.1. Number of stages The total number of stages was varied while the position of the withdrawal feed was kept in the middle of the column and the composition controllers were placed two stages below and two stages above the withdrawal tray. The set points to the two controllers were not varied during this investigation. Hence, the concentration profile around the withdrawal tray was fixed by the two control loops and the composition of the intermediate boiler was independent of the number of stages.With increasing number of stages, lower reflux ratios are required to achieve high purity b hence, the recovery rate of the intermediate boiler increases and the batch time decreases. The concentration of b in the top and sump vessels at which the process becomes infeasible decreases with increasing number of stages. Hence, both the recovery of b (σ b ) and the purity of the light and heavy boilers increase with increasing number of stages.
Fig. 9. Effect of the position of the withdrawal tray on the recovery, αab = αbc .
6.2. Position of withdrawal tray The position of the withdrawal stage determines the relative size of the two column sections. Hence, the optimal position of the withdrawal tray would depend on the relative volatilities αab and αbc . If, for instance, the separation of the intermediate boiler from the low boiler is more difficult than that from the high boiler (i.e. αab < αbc ), then a longer upper column section would be required and the position of the withdrawal tray would shift downwards, and vice versa. In the case that the two separations are similar, then the best position of the withdrawal point would be in the middle of the column as shown in Fig. 9.
7. Considerations on control In the description of the process given in the previous section, the compositions of the impurities are used as control variables in the simulations. That is, the control loop in the upper column section controls the composition of the light impurity four stages above the withdrawal stage, and that in the lower column section controls the composition of the heavy-boiling impurity four stages below the withdrawal stage (Fig. 3a). This choice allows for a partial decoupling of the control loops, since the composition of the low boiler in the upper column section is only slightly dependent on the reflux ratio in the lower column section. This is illustrated in Fig. 10a. Here, the response of controller in the lower column section is plotted for closed loop operation (black lines), and for a malfunctioning the upper control loop (grey lines), while the lower control is in closed loop. The response of controller in the lower column section is effected only very slightly by the control action of the upper column section even when this is malfunctioning.
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Fig. 11. Comparison of the energy demand of a batch distillation column with a middle vessel column (grey surface) and batch distillation column with a side withdrawal (black surface), as a function of the relative volatilities and the concentration of the intermediate boiler in the charge.
Fig. 10. (a) Effect of a malfunctioning controller signal in the upper column section on the response of the controller in the lower column section: (i) close loop operation (dotted line); (ii) malfunctioning controller signal (full lines) (b) Effect of a malfunctioning controller signal in the upper column section on the temperature in the upper and lower column sections.
On the other hand, choosing the temperature as control variable introduces stronger coupling between the control loops. This is due to the fact that if, for instance, the reboil ratio of the upper column section were too low, then too much light boiler would reach the lower column section, decreasing the temperature in both sections (Fig. 10b). The controller in the lower column section would then react by decreasing the reflux ratio of the lower column section and hence, more of the high-boiling impurity would be withdrawn with the side product.
8. Comparison to the middle vessel batch distillation column For the separation of zeotropic ternary mixtures, batch distillation in a middle vessel column was shown to be a
very advantageous process alternative to the regular batch distillation process. In this comparison the charge to the middle vessel column was loaded to middle vessel, and the column was operated in closed loop. In Fig. 11 the energy demand (at equal number of stages and separation task) of the two columns is plotted as a function of the concentration of the intermediate boiler in the charge (containing equal amounts of light and heavy boilers), and the relative volatilities between the light and intermediate boiler (αab ) and that between the intermediate and heavy boiler (αbc ). Hence, for very difficult separations (low relative volatility), the process with the side withdrawal is energetically more advantageous than that in the middle vessel column. On the other hand, for higher values of the relative volatilities, the side withdrawal column is energetically advantageous only for mixtures very rich in the intermediate boiler. This can be explained by the structure of the two columns as shown in Fig. 1. The batch distillation column with a middle vessel can be visualised as a regular batch distillation column placed on top of an inverted column. Hence, this column should be advantageous for charges poor in intermediate boiler since the low-boiling component is separated from the intermediate boiler in a rectifying section (upper section) and the heavy-boiling component is separated from the intermediate boiler in an inverted column (lower column section). This situation is inverted in the case of the side withdrawal column. Here, the intermediate-light boiler separation is carried out in an inverted column, while the intermediate-heavy boiler separation is carried out in a regular column.
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Furthermore, in the middle vessel column, the two column sections are separated from each other by a huge hold-up (in comparison to the column’s hold-up), hence, the two sections are decoupled (Farschman & Diwekar, 1998). Unfortunately, this is not the case of the side withdrawal column. In this paper it was shown that the two column sections interact and, hence, the design of the control loops is more complex. Nevertheless, if the required high purity product is the intermediate boiler, the column with side withdrawal allows direct control of its purity, while the column with the middle vessel allows controlling the purity of the light and heavy components (impurities).
Appendix A
9. Conclusion
References
In this paper we introduced a novel operational policy for the purification of an intermediate boiling component via the cyclic operation of a batch distillation column with a side withdrawal. The feasibility of the process was investigated by considering the novel column configuration as an inverted batch distillation column placed over a regular batch column. A novel operating strategy, based on the feasibility studies, was developed and verified by computer-aided simulations. Furthermore, the influence of most important parameters on the performance of the process was systematically investigated. Finally, the novel process was compared to the batch distillation column with a middle vessel column. It was shown that in spite of several problems, the novel process has the potential to introduce improvement over the batch distillation with a middle vessel. Practically, the process can be carried out in a distillation column very similar to a middle vessel column comprising three vessels: at the top, in the middle and at the sump. In this case, the middle vessel would be exclusively used as a product accumulator. This further shows that a middle vessel column introduces increased plant flexibility with respect to a conventional batch distillation column.
Demicoli, D., & Stichlmair, J. (2003). Novel operational strategy for the separation of ternary mixtures via cyclic operation of a batch distillation column with side withdrawal. Computer Aided Chemical Engineering, 11, 629–634. Farschman, C. A., & Diwekar, U. (1998). Dual composition control in a novel Batch distillation column. Industrial and Engineering Chemistry Research, 37, 89–96. Pantelides, C. C., Gritsis, D., Morison, K. S., & Sargent, R. W. H. (1988). The mathematical modelling of transient systems using differential-algebraic equations. Computers and Chemical Engineering, 12(5), 449–454. Sørensen, E., & Prenzler, M. (1997). A cyclic operating policy for batch distillation—theory and practice. Computers and Chemical Engineering, 21(Suppl.), S1215–S1220. Sørensen, E., & Skogestad, S. (1996). Comparison of regular and inverted batch distillation. Chemical Engineering Science, 51(22), 4949–4962. Warter, M., & Stichlmair, J. (2002). Batch distillation in middle vessel columns. Chemie Ingenieur Technik, 74(9), 1195–1206. Warter, M., Demicoli, D., & Stichlmair, J. (2002). Batch distillation of zeotropic mixtures in a column with a middle vessel. Computer Aided Chemical Engineering, 10, 385–390. Warter, M., Demicoli, D., & Stichlmair, J. (2003). Operation of a batch distillation column with a middle vessel: experimental results for the separation of zeotropic and azeotropic mixtures. Chemical Engineering Proceedings, 43(3), 263–272. Wittgens, B., Litto, R., Sørensen, E., & Skogestad, S. (1996). Total reflux operation of multivessel batch distillation. Computers and Chemical Engineering, 20(Suppl.), S1041–S1046.
Column data
Stages per section Initial feed (mol) Initial condenser holdup (mol) Initial middle vessel holdup Initial reboiler holdup (mol)
Column with side withdrawal
Middle vessel column
14 6000 3000 – 3000
14 6000 0 5550 50