Batchwise extractive distillation in a column with a middle vessel

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Computers and Chemical Engineering Supplement (1999) 5915-5918 C> 1999 Elsevier Science Ltd. All righ ts reserved

Pergamon

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S009 8-135~l99fOOO 80-0

Batchwise Extractive Distillation in a Column with a Middle Vessel M. Warter, J. Stichlmair Lehrstuhl fur Fluidverfahrenstechnik, TV Milnchen , Boltzmannstr. 15, D-85748 Garching/Munchen, Germany, email: [email protected] .de Abstract - Batchwise extractive distillation is a well-known process for the separation of azeotropic mixtures. We studied novel modifications of the middle vessel column and did rigorous simulat ions of a batchwise extractive distillation using different types of batch columns . The results show the advantages of a new modification of the middle vessel column and of the middle vessel column in general for the batchwise extractive distillation.

INTRODUCTION Batch distillation is a very efficient unit operation for the separation of multicomponent mixtures into pure components due to the low investment cost and the high purity of products. However, if the mixture exhibits azeotropes distillation becomes very difficult since liquid and vapor have the same concentration at the azeotrope and, therefore, no driving force for the separation exists at this point. There are several possibilities for separating azeotropic mixtures by continuous (Stichlmair and Fair (1998) , Stichlmair and Herguijuela (1992» and batch wise distillation processes (Dilssel and Stiehlmair (1995), Dussel (1997». One of these is the combination of batch distillation and absorption, that is called batchwise extractive distillation. Generally, a regular batch column is used for the batchwise extractive distillation (e.g. Yatirn et al. (1993), Warter et al. (1997), Lelkes et al. (1998». Only few publications deal with the usage of a middle vessel column for the batchwise extractive distillation, although it has many advantages, for instance, the simultaneous recovery of the entrainer during the process (Safrit et al. (1995), Safrit and Westerberg (1997». The middle vessel column itself can be modified in different ways. The first part of the paper deals with the general structure and the different modifications of the middle vessel column. This includes three novel modifications which have not been investigated so far. The second part of the paper presents a batchwise extractive distillation of ethanol/water using one of the novel modifications of the column and explains the differences to processes with conventional middle vessel columns. In the third part of the paper a comparison of three middle vessel columns and the regular batch column concerning the entrainer and energy demand and the temperature of the liquid in the feed vessel during the process is shown. It reveals the advantages of the novel column modification and of the middle vessel column in general for the batchwise extractive distillation.

DIFFERENT MODIFICATIONS OF THE MIDDLE VESSEL COLUMN The middle vessel column is a combination of a regular batch column and an inverted batch column (see Fig. I). It was originally proposed by Robinson and Gilliland (1950). The separation section of this column is devided into a rectifying and a stripping section with the feed vessel in between. Therefore, it is possible to obtain the light and heavy fractions D and B simultaneously from the top and bottom of the column , while an intermediate boiling fraction M may be recovered at the end of a process from the middle vessel. II)

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Fig. I: Regular batch column (1), inverted batch column (2) and the general structure of a middle vessel column (3). The liquid and vapor streams L and V leaving the rectifying and stripping sections of the column can be directed in different ways. The liquid stream L leaving the upper column section can either be fed to the middle vessel or passed by to the lower column section (see Fig. I). The vapor stream V leaving the lower column section can also be fed to the middle vessel or passed by to the upper column section. This leads under the assumption that at least one stream is fed to the middle vessel to three basic modifications A, Band C of the middle vessel column (see Fig. 2).

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BATCHWISE EXTRACTIVE DISTILLATION WITH A NOVEL MODIFICATION OF THE I\IIDDLE VESSEL COLUMN The batch wise extractive distillation with a middle vessel column of modificat ion A and Cv has been presented by Safrit et al. (1995) and .Hilmen et al . (1997). Fig. 4 shows the process with a middle vessel column of modification B for the separation of ethanol a and water b which form a minimum, azeotrope. Ethylene glycol is used as entrainer e. Fig. 2: Basic modifications A, D and C of the middle vessel column . Two more special cases BL and Cv exist when in column B the liquid stream or in column C the vapor stream leaving the middle vessel are set zero (see Fig. 3).

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At the beginning of the process the starting mixture F is fed into the vessel Y·I together with an intermediate fraction D2 from the previous process cycle. Since the minimum azeotrope has the lowest boiling point it would be the top product DI of the column . To overcome the azeotrope the high boiling entrainer e is fed close to the top of the column (see process step one in Fig. 4). The entrainer absorbs the water from the rising vapor making pure ethanol the top product Dl. At the bottom of the column the high boiling entrainer e is recycled and fed into the entrainer vessel Y-2. During the first process step the liquid in the middle vessel depletes in ethanol and enriches in water. MIa characterizes the liquid fraction in the middle vessel at the beginning and M1(Q at the end of the first process step (see Fig. 4 and 5). Only small amounts of entrainer accumulate in the middle vessel.

Fig. 3: Special cases DL und Cv of the middle vessel columns Band C. Beside of these modifications of the middle vessel column it is possible to operate the column by splitting the liquid stream L (see Fig. I). In this case some of the liquid stream is fed to the middle vessel and the rest to the other column section . So the column is operated in a condition between that of modification A and B. The same can be done with the vapor stream V. Without the assumption that at least one stream is fed to the middle vessel there exist three more modifications D, D L and Do, in which the two the column leaving streams L and V are directly fed to the other column section. In this cases the column has the character of a continuous (Do) or discont inuous (D and Dd distillation column and should not be called a middle vessel column any more. Furthermore it should be noted that all modifications shown here can be transfered to every vessel of a multivessel column. There are many publications concerning the middle vessel columns of modification A or Cv. But no publications have been found which report on columns of modification B, BL or C, althou gh there exist promising applications. One application for the middle vessel column of modification B is the batchwise extractive distillation.

Fig. 4: Batchwise extractive distillation of ethanol and water with ethylene glycol as entrainer using a middle vessel column of modification B. In the second process step remaining parts of ethanol and ethylene glycol are removed from the middle vessel in order to get pure water as product. In this process step the upper column section is operated without entrainer feeding as a conventional rectifying column with an azeotropic mixture D2 as overhead fraction. The fraction D2 can be recycled to the starting mixture of the next process cylce. The residuum M2(Q in the middle vessel at the end of the second process step is pure water (see Fig. 4 and 5).

Computers and Chemical Engineering Supplement (1999) S915-S918

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Fig. 5 : Concentration path of the liquid in the middle vessel during the process for middle vessel columns of modifications A and B. In the process with a column of type B the loaded entrainer coming from the upper column section is directly fed into the lower column section where it is recovered. Therefore, the concentration of the liquid in the middle vessel during the process is nearly on the binary edge of the concentration triangle between ethanol and water (see modification B in Fig. 5). In a middle vessel column of modification A or C v the the loaded entrainer is fed to and mixed with the liquid in the middle vessel and, in turn , the liquid in the middle vessel enriches during the process in ethylene glycol (see modification A in Fig. 5). The accumulation of entrainer in the feed vessel during the process with modification A or Cv increases the amount of entrainer necessary for the separation and the temperature of the liquid in the vessel during the process. Furthermore, the dilution of the entrainer before it is fed to the lower column section makes the recovery of the entrainer more difficult. This can result in a higher energy demand of the separation.

ADVANTAGES OF THE MIDDLE VESSEL COLUl\IN FOR THE BATCHWISE EXTRACTIVE DISTILLATION A comparison of the batch wise extractive distillati on using a regular batch column with the process using different modifications of the middle vessel column reveals the advantages of the middle vessel column concerning the entrainer demand, the temperature in the feed vessel during the process and, furthermore, the energy demand . The investigation was performed for the separation of an equirnolar ethanol/water feed. The product concentration was 99,5 mol-% for both and the recovery rates 90% and 98% respectively. The columns had a total number of equilibrium stages of 19. The middle vessel columns had 16 stages in the

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upper and 3 in the lower section of the column. The entrainer had a concentration of 98% ethylene glycol and 2% water and was fed at constant flowrate onto the second stage of the column . The concentration of the intermediate fraction in the second process step was 86% ethanol. The columns were operated with constant product composition withdrawal. The middle vessel columns of modification A and C v were operated in a very simple way by heating the bottom of the column with the same power as the regular batch column . No heat was added or removed in the middle vessel in this case. In the middle vessel column of modification B the sum of the heat added at the bottom of the column and in the middle vessel was kept constant and the liquid stream leaving the middle vessel optimized. The entrainer flowrate was fitted for every process. The entrainer demand of a batchwise extractive distillation with a regular batch column is significantly higher than that with a middle vessel column. The reason for this is that the conventional process with a regular batch column is devided in two process steps (see Warter et al. (1997» in which the entrainer can be recovered only at the end of the process and used in the next process cycle. In a middle vessel column the entrainer is recovered during the process and recycled immediately. ' 2S ~--------------~

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Fig. 6: Maximum amount of entrainer accumulating in the feed vessel during the process for different columns. The amount of entrainer necessary for a separation depends on the holdup on the plates and to a high degree on the amount of entrainer accumulated in the feed vessel (see Fig. 6). The amount of entrainer accumulated in the feed vessel of a middel vessel column of modification B is the lowest one because in this column configuration the downcoming loaded entrainer is directly fed to the lower column section and not to the feed vessel. The amount of high boiling entrainer in the feed vessel has a big influence on the temperature of the liquid in the feed vessel. Therefore, the temperatures in a process with a middle vessel column are significant lower than that with a regular batch column . This can be important for substances which tend to decomposite at high temperatures. The 10-

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Computers and Chemical Engineering Supplem ent (1999) S915-5918

west temperature can be achieved with modification B in which nearly none entrainer accumulates in the feed vessel (see Fig. 7).

batch column. The best results were obtained with a middle vessel column of modification B. NOTATION

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Fig. 7: Temperature in the feed vessel during the process for different columns. The investigation also shows that the energy demand of the process specified with a middle vessel column is comparable to that of a regular batch column (Co), lower (A) or considerably lower (B). The energy reduction results mainly from the heat integration of the two separations carried out simultaneously in the upper and lower section of the middle vessel column. This heat integration is not possible in a regular batch column.

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Fig . 7: Energy demand of the process for different columns. It should be noted that the energy demand of the middle vessel columns could be further optimized by e.g. controling the concentration of the liquid in the feed vessel. SUl\IMARY In this paper the general structure and three novel modifications of the middel vessel column are shown. One of these novel modifications is feeding the liquid stream from the upper column section directly into the lower column section. The batchwlse extractive distillation process for this column (modification B) and the differences to other middle vessel column modifications are explained. A comparison of different column types of the batchwisc extractive distillation shows that with a middle vessel column the temperature in the feed vessel, the entrainer and energy demand of the proceses are significantly lower than with a regular

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feed comoonents bottom fraction distillate entrainer feed liauid stream intermediate fraction oressure time vaoor stream vessel orocess steo condition at the beginning/end of a process step REFERENCES

Dilssel, R., (1997), Zerlegung azeotroper Gemische durch Batch-Rektifikation, Fortschr.-Ber. VDI Reihe 3 Nr. 476, VDI Verlag Dllssel, R. and Stichlmair, J., (1995), Separation of Azeotropic Mixtures by Batch Distillation Using an Entrainer, Computers chern . Engng., 19, SuppI., S 113-S 118 Hilmen, E. K., Skogestad, S., Doherty, M. F. and Malone, M. F., (1997), Integrated Design, Operation and Control of Batch Extractive Distillation with a Middle Vessel, presented at the AIChE Annual Meeting 1997, Los Angeles, USA, paper No. 20lh Lelkes, Z., Lang, P., Benadda, B. and Moszkowicz, P., (1998) , Feasibility of Extractive Distillation in a Batch Rectifier, AIChE Journal, 44 , 810 -822 Robinson, C. S. and Gilliland, E. R., (1950), Elements of Fractional Distillation, 4 th ed. McGraw Hill , New York Safrit, B. T., Westerberg, A. W., Diwekar, U. and Wahnschafft, O. M., (1995), Extending Continuous Conventional and Extractive Distillation Feasibility Insights to Batch Distillation, Ind. Eng. Chern. Res., 34, 3257-3264 Safrit , B. T. and Westerberg, A. W., (1997), Improved Operational Policies for Batch Extractive Distillation Columns, Ind. Eng. Chern . Res., 36, 436-443 Stichlrnair, J. and Fair, J. R., (1998), Distillation Principles and Practices, J. Wiley, New York Stichlmair, J. and Herguijuela, J. R., (1992), Separation Regions and Processes of Zeotropic and Azeotropic Temary Distillation , AIChE J., 38 (10) , 1523-1535 Warter, M., Dilssel, R. and Stichlrnair, J., (1997), Separation of Azeotropic Mixtures by Batchwise Extractive Distillation, Inst. of chern . Eng ., Symp. Ser. No. 142 Vol. 2,705-714 Yatirn, H., Moszkowicz, P., Otterbein, M. and Lang, P., (1993), Dynamic Simulation of a Stitch Extractive Distillation Process, Computers Chern. Engng., 17, Suppl., S57-562