Running Batch Distillation in a Column with a Middle Vessel

Oct 15, 1996 - Centre for Process Systems Engineering, Imperial College of Science, Technology and Medicine,. London ... is made up of a large reboile...
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Running Batch Distillation in a Column with a Middle Vessel Massimiliano Barolo,* G. Berto Guarise, Sergio A. Rienzi, and Antonio Trotta Istituto di Impianti Chimici, Universita` di Padova, via Marzolo 9, I-35131 Padova PD, Italy

Sandro Macchietto Centre for Process Systems Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BY, U.K.

The practical feasibility of batch distillation in a column with a middle vessel is analyzed. Experimental runs have been carried out in a pilot plant scale continuous distillation column, whose setup has been arranged in such a way as to allow batch operations to be carried out. Practical guidelines are provided for the startup and the operating sequence of the batch column. The implementation of different control configurations is discussed, and the effect of some operating parameters on column performance is shown by experiments. Introduction Batch processing is becoming more popular as the chemical process industries move toward manufacturing fine and specialty chemicals, where flexibility is a key issue due to the frequent change of product demand. Batch distillation columns are inherently flexible, as a single column can separate many different components from a multicomponent feed or multiple cuts with different product specifications from a binary feed. Thus, the use of batch distillation is becoming increasingly important for the separation and purification of high-value chemicals in many chemical, food, and pharmaceutical processes. Traditionally, the most popular kind of batch column is the so-called “regular” or “rectifying” column, which is made up of a large reboiler, to which all the feed is charged, and of a rectifying section from whose top cuts of different compositions are removed. Less frequently, an “inverted” or “stripping” batch column is preferred, for example when the amount of the light component in the feed charge is small and the products are to be recovered at high purity (Sørensen and Skogestad, 1995); in this column the feed is charged to the top vessel, and the products are withdrawn from the bottom, so that a smaller reboiler can be used. Yet a different configuration for a batch column can be considered, as was mentioned by Robinson and Gilliland back in 1950. Similarly to a continuous column, this kind of batch column (which we call “complex” hereafter) is made up of a rectifying section and a stripping section, with a feed tray in the middle. The liquid feed is charged to an intermediate vessel, and a liquid stream is continuously recycled between the feed/withdrawal tray and the feed vessel. Liquid streams may be continuously withdrawn from the top and the bottom of the column. Since the feed is charged to the middle vessel, a smaller reboiler can be used as compared to the one of a regular batch column. The use of such a column for the separation of binary and multicomponent mixtures was analyzed by Bortolini and Guarise (1970) by means of McCabe-Thiele diagrams. The complex batch column gained renewed attention after the work of Hasebe et al. (1992), who apparently rediscovered the process. Mujtaba and Macchietto * To whom correspondence should be addressed. Phone: +39 49 8275473. FAX: +39 49 8275461. E-mail: max@ polochi.cheg.unipd.it.

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(1992, 1994) performed comparative studies between conventional and complex columns under optimal operation. A theoretical analysis of the dynamic behavior of a complex column has been presented by Davidyan et al. (1994) and by Meski and Morari (1995) using a simplified column model with no plate holdup and an infinite number of stages in both column sections. Safrit et al. (1995) examined the potential of a middle vessel column to separate azeotropic mixtures. Barolo et al. (1996) compared two different strategies for complex column operation, and they considered some practical aspects for column design and control. The use of multiple heat-integrated complex columns was suggested by Hasebe et al. (1995) as an alternative to a train of continuous columns for the separation of multicomponent systems. This kind of “multivessel” batch column has been further studied by Skogestad et al. (1995), who considered the potential of this configuration with respect to a conventional batch column and proposed a strategy for product composition control. Although the number of theoretical and simulation papers on the complex batch columns have increased steadily in the past few years, there has not been a parallel increase in the published experimental work. As a matter of fact, to date no experimental evidence of the features of a single complex batch column has been reported in the open literature. This may somewhat reduce the attractiveness for industrial implementation of such a process. This paper is intended to bridge this gap. The dynamics and the operation of a pilot plant complex batch column are analyzed by experiments. The main characteristics of the complex column are shown by comparison with the behavior of a conventional rectifying batch column. Guidelines are provided for the startup and the operating sequence of a complex column; the implementation of different control configurations is discussed, and the effect of some operating parameters on column performance is shown. Reference is made to the separation of a binary mixture, so that the temperature profile gives a univocal representation of the composition profile in the column. Experimental Apparatus As was mentioned in the Introduction, a complex batch column can be equipped with a smaller reboiler compared to that of a regular batch column. This is an interesting issue, because a conventional continuous © 1996 American Chemical Society

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Pressure: steam, column pressure drop Differential holdup (on a differential pressure basis): reflux drum and bottom sump Composition measurement is provided by off-line chromatographic analysis. Pneumatic control valves are installed in the feed, distillate, bottoms, and steam lines, while the withdrawal line is equipped with a needle manual valve. The data acquisition and control system is centered on a 486-PC, which communicates with the plant via the standard RS-232C serial port. The sampling period was set to 8 s in all the experimental runs presented. The results reported here refer to the separation of an ethanol/water mixture. It is well known that this system exhibits a homogeneous azeotrope in the ethanol-rich concentration interval. Thus, in the range of compositions considered, the relative volatility of the system ranges from ∼1 to ∼11. The composition specification on each product will be generally intended as the upper bound of the composition of the relevant impurity in that product. All the results presented refer to a configuration for which the feed is fed to the downcomer of tray 17 (numbering from the bottom up), while the liquid is withdrawn from the nonperforated area of tray 16 (Figure 1a). By comparison with the results obtained switching the feed point two trays above (downcomer of tray 19) while retaining the same withdrawal tray location, it was verified that there is no appreciable channeling effect due to the adopted feed/withdrawal tray configuration. The steam supply to the reboiler was held constant throughout all the experimental runs. Results and Discussion Figure 1. (a) Schematic of the pilot plant; (b) detail of the withdrawal tray (tray 16).

column can be arranged in such a way as to be operated in a complex batch mode, thus increasing significantly the plant flexibility. The pilot plant distillation column considered in this study (Figure 1a) was designed and utilized for atmospheric continuous separations (Barolo et al., 1994; Baratti et al., 1995). The plant comprises a distillation column (0.3 m diameter; 30 sieve trays; 0.2 m tray spacing; total height 9.9 m approximately), a vertical steam-heated thermosiphon reboiler, and a horizontal shell-and-tube water-cooled total condenser. The reflux drum has a maximum capacity of about 40 L, while proper operation of the reboiler requires the total holdup in the column bottom to be in the range of 40-90 L. The feed vessel has a maximum capacity of 500 L; adequate mixing of the liquid in the vessel is provided by means of a recirculation pump. Additional information about the plant layout is given in the above-cited papers. The plant can be easily switched from continuous to complex batch operations by manipulating valve V1 (Figure 1a). Figure 1b shows a detail of the pipe arrangement on the withdrawal tray (see Barolo et al. (1996) for details). On-line measurement of the following variables is provided: Temperature: selected trays along the column, feed, reflux, steam, cooling water (inlet and outlet) Flow rate (on a volumetric basis): feed, distillate, bottoms, steam, cooling water, liquid withdrawal from the column

Pure Components. It is well-known (Bortolini and Guarise, 1970; Treybal, 1970) that by operating at total reflux a conventional rectifying batch column with a relatively high number of trays, it is possible to split the feed charge (of given composition) between the top and bottom holdups, in such a way as to obtain very pure light and heavy products at the end of the batch. According to the work of Sørensen and Skogestad (1994), this mode of operation can be regarded as a particular case of a cyclic operation, where only one cycle is performed. The total reflux operation is particularly advantageous because neither the yield nor the quality of the products is influenced by variations in the heating rate or interruption of the distillation; also, no product changeovers are required during the distillation, so that column operation is easier. However, incorrect estimation of the feed composition may result in an erroneous choice of the set point of the reflux drum holdup, so that part of the light component may contaminate the heavy product or vice versa, and some feedback adjustment of the holdups is needed in order to keep the products on specification. Yet this adjustment can be avoided by using a column with a middle vessel. Suppose that it is required to separate a given feed in a distillate containing not more than 6 wt % water and a residue with an ethanol content less than 1 wt %, and suppose that the expected feed amount and composition are such that 14 kg of liquid needs to be kept in the reflux drum. If a conventional (i.e., without a middle vessel) total-reflux rectifying distillation of the given charge is made, a considerable amount of ethanol is found in the reboiler: although the steady state is not completely attained for the column bottom, the analysis of the temperature profiles in the lower part

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Figure 2. Effect of the activation of the recycling streams: (a) temperature profile in the column (the numbers on the curves indicate tray number); (b) composition profile of the top and bottom products; (c) profiles of the top and bottom holdups.

of the column (Figure 2a, t < 82 min) indicates the presence of ethanol, as is confirmed by the composition profiles (Figure 2b, t < 82 min). This means that the feed charge was actually richer in ethanol than expected. This would call for a reduction of the holdup in the reboiler, which can be accomplished by increasing the set point of the reflux drum level controller. However, if the feed vessel is connected (Figure 2, t > 82 min, withdrawal rate 120 L/h) and the reflux drum and reboiler holdups remain the same (i.e., the total liquid holdup inside the column does not change), the temperature (i.e., composition) profile along the column changes completely. Due to the fact that the recycling streams constrain the compositions of the feed tray and

of the feed vessel to be close, the ethanol is completely removed from the stripping section of the column and it is moved up to the feed vessel. It should be noted that if a thermosiphon reboiler is used, at the end of the batch the bottom product is contaminated by the liquid that drains from the trays. Obviously, this problem does not arise if a kettle reboiler is employed. This mode of operation of the complex column should be referred to as total reflux and total reboil operation; however, we shall refer to it as to a total reflux operation in the remainder of the paper. It is also important to notice that, regardless of the mode of operation, when the middle vessel is connected an additional level controller needs to be set on (Barolo et al., 1996). In the total reflux operation of Figure 2, a bottom level controller was switched on at t ) 82 min. Since the withdrawal rate is satisfactorily constant, a simple proportional controller was used in order to drive the set point of the feed flow controller, as is sketched in Figure 3a. However, a proportional-integral controller can be used if a very tight control of the bottom level is required and mild oscillations in the feed rate are tolerated. Two Cuts of Different Component Purities. One frequent objective of the batch distillation of a binary mixture is to obtain a given component (namely, the ethanol) at two different degrees of purity (namely, the water content in the products) out of a single batch; supplementary restrictive conditions may be specified (e.g., minimum recover of one cut, maximum batch time, maximum losses of the desired component), leading to interesting optimal control problems, which will not be addressed in this paper. If a complex column is used, the lighter cut is obtained from the top of the column while the heavier cut is recovered in the middle vessel at the end of the batch. The startup sequence of the complex column can be outlined as follows. First, the reboiler is charged with a given amount of feed, which will depend on the reboiler capacity, on the estimated total tray holdup, and on the desired reflux drum and bottom holdup set points; the remaining feed charge is stored in the middle vessel. Then cooling water is fed to the condenser and the reboiler is heated (Figure 4a, t ) 0). The reflux is returned to the column as soon as a sufficient holdup in the reflux drum has built up (Figure 4a, t ) 14 min). In this phase the middle vessel is not connected and only the top level controller is switched on. This situation proceeds until the fluid dynamic regime has established in the column, that is, until all the trays have been filled with liquid; this occurrence can be detected by checking the total pressure drop in the column. When all the trays have been sealed, the bottom level controller is switched on, so that the feed is introduced into the column (Figure 4a, t ) 21 min). At this time, the liquid withdrawal from the withdrawal tray can be started (Figure 4a, t ) 22 min). The set points for the top and bottom holdups can be determined by off-line optimization (Barolo et al., 1996). If these set points have been chosen correctly (and, of course, if the column has enough trays), then the operation can proceed at total reflux until the products in the reflux drum and in the middle vessel are on specification. However, likewise in conventional batch distillation, incorrect estimation of the feed composition may lead to off-specification products (i.e., too diluted products).

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Figure 3. Control schemes for the complex column: (a) total reflux operation; (b) finite reboil ratio operation with bottom level controlled by feed rate; (c) finite reboil ratio operation with bottom level controlled by bottoms rate.

Figure 4. Startup procedure and total reflux operation of the complex column (control scheme as in Figure 3a); profiles of (a) flow rates, (b) temperatures, (c) holdups, and (d) amount of water in the top and middle vessel products. Amount charged to the feed vessel: 132 L.

Increasing the purity of the lighter cut requires diminishing the reflux drum holdup. As in continuous distillation, when the column is operated at total reflux the steady state reflux drum composition is univocally related to the temperature profile in the rectifying section of the column (unless multiple steady states are expected). Thus, the reduction of the reflux drum holdup (i.e., the increase of the lighter cut purity) can be accomplished either directly (by cascading the reflux drum level controller to a rectifying section temperature controller as was proposed by Bortolini and Guarise (1970)) or indirectly (by letting the temperature controller directly manipulate the reflux rate as suggested by Skogestad et al. (1995)). In either case, the control is straightforward. A similar approach can be adopted for controlling the purity of the product in the middle vessel. However, a

direct control of the middle vessel level is not possible on the pilot plant, since the measure of the middle vessel level was not available on line (Figure 1a). Thus, the middle vessel holdup (i.e., the purity of the heavier cut) can be controlled at total reflux by changing (in some feedback fashion) the reboiler level. As is shown in Figure 4c, at t ) 86 min the set point of the bottom level controller was step-increased from 60 to 70 kg; the bottom temperature profile (Figure 4b) indicates that this eventually resulted in the transfer of 10 kg of water from the middle vessel to the bottom of the column, so that the ethanol concentration of the heavier cut was increased (Figure 4d). Note that in this case there is not enough temperature gradient in the stripping section of the column (a pinch point is located at the column bottom); thus, the composition of the heavier cut cannot be inferred from temperature measurements, and a

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Figure 5. Fixed (and finite) reboil ratio operation of the complex batch column (control scheme as in Figure 3b) at two bottoms rates (solid and broken lines): profiles of (a) flow rates, (b) temperatures, (c) holdups, and (d) amount of water in the middle vessel product.

criterion for estimating the composition of the liquid in the middle vessel is required if on-line composition measurement is not provided. In practice, the transfer of water from the middle vessel to the bottom of the column is limited by the capacity of the reboiler. This is a typical problem which may arise when a continuous column is “switched” to complex batch operations. In such case, the water needs to be removed from the column bottom. This is accomplished by setting the reboil ratio to a finite value. In the run represented in Figure 5 (solid line), the removal of water from the column bottom was started at zero time with a bottoms rate of about 77 L/h (Figure 5a), according to the control scheme of Figure 3b. The bottom level controller promptly increased the feed rate in order to compensate for the reduction of the bottom level (Figure 5a,c, solid lines); however, note that during the bottoms removal, the bottom level decreased appreciably because a proportional-only bottom level controller was employed. If it is required to avoid any losses of ethanol from the bottom of the column, the bottoms removal can be stopped according to some temperature criterion; for example, the bottoms flow was arrested (i.e., operation was switched back to total reflux) when the temperature on tray 8 fell below 100 °C (Figure 5b, ∆t ) 13 min, solid line). Thus, the bottom level increased till the set point was reached (∆t ) 45 min), and the amount of water in the feed vessel decreased accordingly, as was verified by the composition measurements (Figure 5d, solid line). If a large amount of water needs to be separated, the water removal can be carried out in steps. When the new steady state is attained, or when a specified temperature profile has been built back in the column

(T12 > 100 °C in Figure 5b), the bottoms withdrawal can be restarted. This kind of operation somewhat resembles the cyclic operating policy described by Sørensen and Skogestad (1994), in that it is characterized by repeating total reboil, dumping, and filling-up periods. Clearly, the number and duration of cycles (i.e., the reboil ratio and the removal stopping criterion) are a matter of optimization; as an example of the effect of reboil ratio on the duration of each cycle and on the temperature dynamics, the dashed lines in Figure 5 refer to a run in which only the bottoms rate was changed (broken lines) with respect to the base case (solid lines). Alternatively, during the water removal period the control scheme can be set as in Figure 3c, that is, the bottom level control can be transferred to the bottoms rate, while the ratio of the feed rate to the withdrawal rate is step-increased to some optimal value (Figure 6). A simple on-off bottom level controller was used in the run represented in Figure 6; the water removal was allowed only when the bottom level was higher than the set point; the same temperature criterion as in Figure 5 was used in order to start and to stop the removal of water. As is seen from Figure 6c, this control structure results in a better control of the bottom level. This can be an advantage if the reboiler is small and proper reboiler operation is guaranteed only in a narrow holdup range. Note that the duration of each removal cycle is shorter than in the previous run considered (that is, less water is removed during each cycle), but the number of cycles is increased (that is, less time is required in order to reestablish the desired temperature profile in the column). These results indicate that when an optimiza-

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Figure 7. Effect of feed/withdrawal rates on batch time at total reflux. Temperature profiles at two withdrawal rates: (a) 117 L/h; (b) 188 L/h.

Figure 6. Variable reboil ratio operation of the complex batch column (control scheme as in Figure 3c): profiles of (a) flow rates, (b) temperatures, and (c) bottom holdup.

tion of column operation is performed, one should take into account which control configuration will be adopted in practice. The effect of the increasing purity of the middle vessel product is shown in Figure 6b: as long as the ethanol concentration in the middle vessel is increasing, the temperature drop on tray 12 during the water removal period (finite reboil ratio operation) becomes faster and faster, while the time required to raise this temperature to 100 °C (total reflux operation) increases steadily. This is a clear indication that the difficulty of separation is increasing. Effect of the Feed/Withdrawal Rates. Barolo et al. (1996) showed by simulation that the performance of a complex batch column is significantly affected by the feed/withdrawal rates, in that an increase of the feed rate to the column determines a decrease of the batch time.

Figure 7 shows the experimental verification of this fact for the total reflux mode of operation. The figure refers to two different batches of the same feed (in terms of feed amount and feed composition) under total reflux and with two different withdrawal rates: 117 (Figure 7a) and 188 L/h (Figure 7b). The temperature profiles in the stripping section indicate clearly that an increase in the withdrawal rate (with matching increase in the feed rate) does shorten the batch time. Increasing the feed rate results in a faster renewal of the middle vessel content. Thus, the same steady state is obtained, but in a shorter time. A comparison of the temperature profiles of the rectifying section is harder, since in the range of ethanol-rich mixtures the temperature sensitivity is much smaller. Conclusions The experimental results reported indicate the practical feasibility of batch distillation in a column with a middle vessel. It has been shown that a continuous column can be easily switched to carry out complex batch operations, thus increasing significantly the plant flexibility with a minimum investment costs. Two kinds of operations have been considered: total reflux operation and finite reboil ratio operation. Regardless of the mode of operation, when the middle vessel is connected the control of two holdups in the column is necessary. In practice, at total reflux the reflux drum and the bottom holdups can be easily controlled by two level controllers that manipulate the reflux and the feed rates, respectively; thus, indirect control of the middle vessel holdup is achieved. Opera-

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tion at total reflux can be profitable when it is required to recover the mixture components at a very high degree of purity. Also, operation is simple, because no product changeovers are necessary. When an impurity needs to be removed from the column bottom (finite reboil ratio operation), two alternative control strategies are possible. One control configuration is easier to implement, in the sense that it basically retains the same structure used for total reflux operation (only a bottoms flow controller is added in order to keep the reboil ratio to the specified, perhaps optimal, value); however, the control of the bottom level during the impurity removal may not be satisfactory if a proportional-only level controller is used. Alternatively, a more direct control structure can be used during the impurity removal period; during this phase, the control of bottom level is transferred to the bottoms rate, and the ratio of the feed rate to the withdrawal rate is step-increased (to the prespecified optimal value); this strategy allows an easier control of the bottom level. Which configuration should be adopted is a matter of optimization. The removal of one impurity from the column can be carried out in steps, if it is required to avoid any losses of the desired component. The advantageous effect of increasing the feed/ withdrawal rates has been demonstrated. Clearly, feed and withdrawal valve rangeabilities limit the maximum flow rates achievable. The primary aim of this work has been to demonstrate the practical feasibility of complex batch distillation, to provide some practical guidelines for the startup and the operation of the column, to show how the column can be run under different operating strategies, and to clarify the effect of a key operating parameter on the column performance. However, the study has been limited to a binary mixture. Moreover, only “open-loop” operations has been considered, and no concern has been given to the optimal operation of the column. This calls for more work in the area of on-line estimation of product composition (especially for multicomponent mixtures of any relative volatility), in order to provide closed-loop product composition control; the practical implementation of optimal operating strategies is another important area which needs to be investigated in detail. Acknowledgment The financial support granted to this work by Italian CNR (Progetto Strategico Tecnologie Chimiche Innovative) and MURST (ex-40%) is gratefully acknowledged. One of the authors (S.M.) was supported by EPSRC (UK). The authors thank Ing. Andrea Morandini for his help in carrying out the experimental runs.

Literature Cited Baratti, R.; Bertucco, A.; Da Rold, A.; Morbidelli, M. Composition Estimator for Binary Distillation Columns. Application to a Pilot Plant. Chem. Eng. Sci. 1995, 50, 1541. Barolo, M.; Guarise, G. B.; Rienzi, S. A.; Trotta, A. Nonlinear Model-Based Startup and Operation Control of a Distillation Column: An Experimental Study. Ind. Eng. Chem. Res. 1994, 33, 3160. Barolo, M.; Guarise, G. B.; Ribon, N.; Rienzi, S.; Trotta, A.; Macchietto, S. Some Issues in the Design and Operation of a Batch Distillation Column with a Middle Vessel. Comput. Chem. Eng. 1996, 20, S37. Bortolini, P.; Guarise, G. B. A New Practice of Batch Distillation. (in Italian) Quad. Ing. Chim. Ital. 1970, 6, 150. Davidyan, A. G.; Kiva, V. N.; Meski, G. A.; Morari, M. Batch Distillation in a Column With a Middle Vessel. Chem. Eng. Sci. 1994, 49, 3033. Hasebe, S.; Abdul Aziz, B. B.; Hashimoto, I.; Watanabe, T. Optimal Design and Operation of Complex Batch Distillation Column. Proceedings IFAC Workshop on Interactions Between Process Design and Process Control, London (UK), September 1992; Pergamon Press: Oxford, U.K., 1992. Hasebe, S.; Kurooka, T.; Hashimoto, I. Comparison of the Separation Performances of a Multi-Effect Batch Distillation System and a Continuous Distillation System. Preprints IFAC Symposium DYCORD+’95, Elsinore (Denmark), June 1995; Danish Automation Society: Copenhagen, 1995; pp 249-254. Meski, G. A.; Morari, M. Design and Operation of a Batch Distillation Column with a Middle Vessel. Comput. Chem. Eng. 1995, 19, S597. Mujtaba, I. M.; Macchietto, S. Optimal Operation of Reactive Batch Distillation. Presented at the AIChE Annual Meeting, Miami Beach, November 1992; Paper 135g. Mujtaba, I. M.; Macchietto, S. Optimal Operation of Multicomponent Batch DistillationsA Comparative Study Using Conventional and Unconventional Columns. Preprints IFAC Symposium ADCHEM’94, Kyoto (Japan), May 1994; Pergamon Press: Oxford, U.K., 1994; pp 415-420. Robinson, C. S.; Gilliland, E. R. Elements of Fractional Distillation; McGraw Hill Book Co.: New York, 1950. Safrit B. T.; Westerberg, A. W.; Diwekar, U.; Wahnschafft, O. M. Extending Continuous Conventional and Extractive Distillation Feasibility Insights to Batch Distillation. Ind. Eng. Chem. Res. 1995, 34, 3257. Skogestad, S.; Wittgens, B.; Sørensen, E.; Litto, R. Multivessel Batch Distillation. Presented at the AIChE Annual Meeting, Miami Beach, November 1995; Paper 184i. Sørensen, E.; Skogestad, S. Optimal Operating Policies of Batch Distillation with Emphasis on the Cyclic Operating Policy. Proceedings IFAC Symposium PSE’94, Kyongju (Korea), 1994; Korean Institute of Chemical Engineering, Seoul, Korea, 1994; pp 449-456. Sørensen, E.; Skogestad, S. Comparison of Inverted and Regular Batch Distillation. Preprints IFAC Symposium DYCORD+’95, Elsinore (Denmark), June 1995; Danish Automation Society: Copenhagen, 1995; pp 141-146. Treybal, R. E. A Simple Method for Batch Distillation. Chem. Eng. 1970, Oct 5, 95.

Received for review May 13, 1996 Revised manuscript received September 3, 1996 Accepted September 4, 1996X IE960268S X Abstract published in Advance ACS Abstracts, October 15, 1996.