Multicomponent Distillation Columns with Partitions and Multiple

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Multicomponent Distillation Columns with Partitions and Multiple Reboilers and Condensers Rakesh Agrawal† Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501

Single distillation column structures with partitions and multiple reboilers and condensers are suggested for the separation of a multicomponent feed. The total number of reboilers and condensers required for such structures is generally equal to the number of components in the feed. In this case, each partitioned distillation zone recovers one of the components and has either a reboiler or a condenser. This enables control of the liquid and vapor flow-rate ratios in each partitioned distillation zone, resulting in better control of product purities. These structures have heat duties and thermodynamic efficiencies similar to those for the known side-stripper and side-rectifier configurations. Introduction A multicomponent distillation column with a vertical partition to distill a ternary mixture was introduced by Wright.1 This configuration is shown in Figure 1 and is often referred to as a divided wall column.2 The feed mixture ABC is fed to the distillation zone on one side of the partition in the distillation column. In this figure and in the rest of this paper, components in a mixture are rank listed according to their relative volatility; i.e., for feed mixture ABC, A is the most volatile component and volatility decreases in successive order, with C being the least volatile. A liquid mixture AB primarily consisting of components A and B is introduced at the top of the partitioned distillation zone on the other side of the partition. A binary vapor mixture BC primarily consisting of components B and C is introduced at the bottom of the same zone. Component B is withdrawn from an intermediate location of the column, and components A and C are recovered from the ends of the column. The advantage of this partitioned column is that a ternary mixture can be distilled into pure product streams with only one distillation column, one reboiler, and one condenser. This reduces the cost of separation and is found to be attractive for some applications.3,4 Single shell distillation columns with more than one vertical partition were introduced by Kaibel5 to distill mixtures containing four or more components. Using multiple vertical partitions, four or more pure product streams could be recovered from one distillation column. In these configurations, sharp splits between components of intermediate volatility are used across distillation sections. For a four-component mixture ABCD, Christiansen et al. have pointed out serious operational difficulties with Kaibel’s columns.6 These result from one of the distillation sections being fed with a B-rich stream at the top and a C-rich stream at the bottom but having no “designated” separation task. In an actual operation, this may make it difficult to produce B and C streams of high purity. Christiansen et al. suggested a modification to the four-component Kaibel column to eliminate the distillation section with no “designated” separation task.6 They introduced a horizontal partition † Telephone: (610) 481-4689. Fax: (610) 481-6748. E-mail: [email protected].

Figure 1. Single partitioned distillation column of Wright.1

between the bottom of the distillation section producing B and the top of the section producing C. This isolated the two sections so that no material was allowed to pass between them. Thermal communication between the two sections was allowed by condensing vapor from the top of the C-producing section against vaporizing liquid at the bottom of the B-producing section. For a four-component mixture, Christiansen et al. have suggested equivalent single shell distillation column structures derived from the two known fully thermally coupled distillation schemes.6-9 Each of these single shell column structures uses two vertical partitions. For the structure derived from the sequential fully coupled column arrangement,8 one of the partitions has an opening at an intermediate height to allow transfer of liquid and vapor BC mixtures between the two neighboring zones. In actual operation, it will be difficult

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to control the amounts of liquid and vapor BC mixtures transferred across such an opening. For the equivalent single shell column structure derived from the satellite column arrangement,9 no such transfer of intermediate BC mixtures is required. For this case, two equivalent single shell column structures, one with two parallel partitions and one with three walls located in a triangular geometry, were suggested. For the four-component feed mixtures studied, it was found that, with the same total number of distillation stages, the equivalent single shell column structure derived from the satellite column arrangement required the least amount of heat duty as compared to other known single shell column structures or to a prefractionator scheme.6 The same single shell column was found to have the lowest total annualized cost for the separation of an alkane mixture.10 A general method to draw a single shell distillation column using only one reboiler and one condenser for a multicomponent mixture with any number of components has also been suggested.11 In this suggestion, a column with only radial partitions or a column with cylindrical inner core and radial partitions was found to provide a satisfactory solution. For the multicomponent single distillation column structures with partitions described in the open literature, all of the heat for distillation is provided through only one reboiler located at the bottom of the distillation column. Similarly, all of the heat is rejected through one condenser located at the top of the distillation column. This seems to have been done for two main reasons; first, to reduce capital cost by minimizing the number of reboilers and condensers and, second, from the belief that a fully thermally coupled configuration uses substantially less energy than all other alternatives. While it is true that a fully thermally coupled configuration typically requires significantly less heat than the conventional direct and indirect schemes, for a large range of feed conditions its heat duty can be quite similar to other thermally coupled column schemes, such as the side-rectifier or side-stripper.12 Figure 2 shows the percentage difference between the minimum total boilup of a fully thermally coupled configuration and a side-rectifier or a side-stripper for a ternary liquid feed mixture. These figures were drawn for pinched distillation columns with the assumption of constant relative volatility and equimolal latent heat. Both RA and RB are relative volatilities of A and B with respect to component C. It is observed from parts a and c of Figure 2 that the fully thermally coupled configuration has a much lower heat duty for cases when the relative volatility of A with respect to B (RA/RB) is similar to the relative volatility of B with respect to C (RB). For such applications, the side-rectifier and side-stripper have heat duties comparable to the fully thermally coupled configuration only when B is in small quantities in the feed mixture. However, when the two relative volatilities (RA/RB and RB) are dissimilar, it is readily observed from parts b and d-f of Figure 2 that for a wide range of feed compositions the total boilup for the side-rectifier or side-stripper is close to that of a fully thermally coupled configuration. Furthermore, a fully thermally coupled configuration receives all of the heat at the highest temperature and rejects it at the lowest temperature. As a result, the thermodynamic efficiency of a fully thermally coupled configuration can often be inferior to that of other configurations.13 This is the

Figure 2. Percent difference between the minimum total boilup of a fully thermally coupled configuration relative to a side-rectifier or a side-stripper.

primary reason that argon is distilled from air using a side-rectifier and not a fully thermally coupled configuration. (Argon’s volatility is intermediate between the major components of air: nitrogen and oxygen.) Thus, there is a need to consider a multicomponent single distillation column with partitions to mimic the other thermally coupled structures, such as the side-rectifier and side-stripper. Such structures may use multiple reboilers and condensers and receive and/or reject heat at several temperature levels. These concepts can lead to higher thermodynamic efficiencies, especially for cryogenic distillations. Use of a multicomponent single distillation column with a vertical partition and containing either one reboiler and two condensers or two reboilers and one condenser has been suggested in the patent literature.14 In each case, the vertical partition starts at the multicomponent feed location. When the vertical partition is located in the rectifying section, all of the vapor from the stripping section is sent to one side of the vertical partition. The vapor need on the other side of the vertical partition is met from the feed stream. This requires that the feed must have a reasonable vapor content. The liquid streams from both sides of the partition are sent to the stripping section. Similarly, when the vertical partition is located in the stripping section, all of the liquid from the rectifying section is sent to one side of the vertical partition. The liquid need on the other side of the partition is met from the feed stream. This requires that the feed be a liquid stream. Generally, it will be very difficult for either of these configurations to produce from a ternary mixture all three components as pure product streams. The component of intermediate volatility will be enriched with

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Figure 3. Side-stripper configurations: (a) traditional and (b) equivalent single partitioned distillation column.

either the most volatile component or the least volatile component. Both of these configurations with a vertical partition do not mimic thermally coupled structures, such as the side-rectifier or side-stripper. A number of studies of the design and control of a ternary single distillation column with partitions are available.2,15-17 Abdul Mutalib and Smith reported an operability study from a ternary distillation pilot plant using such a column.17 In this study, the manipulation of the vapor split between two parallel sections of the column was found to be impractical. The liquid split between the two sections was also left uncontrolled. Therefore, such a system was found to be only useful for those applications where optimum performance is not very sensitive to variations in the vapor and liquid splits. A system in which the vapor and liquid splits were not controlled can be expected to lack operating flexibility. Furthermore, this problem could become worse for feed mixtures containing more than three components because liquid and vapor splits would be required among more than two partitions. It will be shown later that the use of additional reboilers and condensers facilitates better control of the liquid-tovapor ratios in each of the partitioned distillation sections, thus providing the needed operating flexibility. From the field of cryogenic air distillation, independent suggestions have been made to incorporate an argon-producing side-rectifying column inside the lowpressure column.18-20 In one suggestion, a cylindrical column is located coaxially in the middle section of the low-pressure column and argon is distilled in the annular space between the coaxially located column and the outer wall of the low-pressure column.19 In another suggestion, incorporation of the side-rectifying column in the low-pressure column is made in a manner that would be easier to design, fabricate, and operate.20 In all of these cryogenic air distillation designs, the vapor from the top of the argon-producing distillation section is condensed to provide reflux to this distillation section. Incorporation of a small-diameter distillation column through the bottom of a larger diameter column to fractionate a multicomponent stream is also known in

the patent literature.21,22 A pure stream of intermediate boiling range material is produced from the annular space between the two columns that functions as a stripping section with a reboiler. Despite some of these activities in the past literature, no systematic method to draw such structures is available in the open literature. The purpose of this paper is to systematically introduce multicomponent single distillation column structures with partitions and with multiple reboilers and condensers. Structures that are equivalent to the sidestripper and side-rectifier configurations will be discussed first. This will be followed by a general discussion of a number of possible partitioned distillation column structures that become available through the use of multiple reboilers and condensers. Multicomponent Distillation Columns with Partitions and Multiple Reboilers and Condensers First consider partitioned distillation columns with additional reboilers and condensers for a ternary mixture. The side-stripper scheme and an equivalent partitioned column configuration are shown in Figure 3. In this case, a reboiler is used on each side of the partition. This provides a means to control the vapor flow on each side of the partition to better maintain product purities during operation. Means to achieve proper distribution of liquid flow on each side of the partition would still be needed. The side-rectifier scheme and an equivalent partitioned column configuration are shown in Figure 4. Because a condenser is used on each side of the partition, the vapor as well as the liquid flows on each side of the partitions can be individually controlled. The vapor flow can be controlled by use of a valve on a vapor line preceding a condenser or through the control of the temperature difference between the hot and cold fluids within a condenser. Once again, as compared to a partitioned column with only one reboiler and one condenser, control of the product purities will be quite straightforward.


Figure 4. Side-rectifier configurations: (a) traditional and (b) equivalent single partitioned distillation column.

As compared to conventional ternary distillation configurations that use two distillation columns and a total of four reboilers and condensers, the single distillation configurations with vertical partitions of Figures 3b and 4b require one less distillation column and one less reboiler or condenser. Furthermore, their total heat duties will generally be much less than those of the conventional direct or indirect split configurations. This implies that not only is the number of reboilers and condensers reduced but the total heat-transfer area is lower as well. This will lead to lower capital as well as operating cost when compared to the direct or indirect split configurations. Compared to a side-stripper or a side-rectifier, one less distillation column is needed. Furthermore, for a liquid feed, the vapor flow is now uniform along the height of the distillation column. Therefore, it is expected that while retaining the good control and lower heat demand features of the original side-stripper and side-rectifier configurations, the new single distillation column structures with vertical partitions could also be cheaper. Compared to the partitioned column of Figure 1, the new structures in Figures 3b and 4b require an additional reboiler or condenser. Furthermore, the column structure in Figure 1 is equivalent to a fully thermally coupled ternary distillation configuration and therefore will have the lowest heat duty. However, as shown in Figure 2, for a large range of feed compositions and relative volatilities, the heat duty of a side-rectifier or a side-stripper configuration is similar to that of the fully thermally coupled configuration. For such feed conditions, the vapor flow and therefore the diameter of the distillation column for the new distillation structures in Figures 3b and 4b will be similar to those of the column in Figure 1. Also, the total heat-transfer area needed by all of the reboiler and condensers would be similar to those in Figure 1. Therefore, for such feeds, the improved operability will come at only a marginal increase in cost, resulting from an increase in the number of reboilers or condensers. Furthermore, when the second law efficiencies are important, configurations with an additional reboiler or condenser may produce an operating cost advantage, resulting from their ability either to supply a portion of the heat duty at a lower

temperature (Figure 3b) or to reject heat at a higher temperature (Figure 4b). For such applications, it is possible that a distillation column with a partition and an additional reboiler or condenser may provide a lower overall cost option than the configuration in Figure 1. Therefore, on the basis of operability and economic considerations, the proposed configurations in Figures 3b and 4b may provide an attractive alternative for some applications. Single distillation column configurations with vertical partitions and multiple reboilers and condensers can be proposed for any multicomponent feed. However, for feeds containing more than three components, the number of such possible configurations becomes quite large. Some configurations for a four-component mixture are illustrated in Figures 5-7. Figure 5a shows a single distillation column with two partitions and a total of four reboilers and condensers. The feed is introduced to the distillation column in a space between the two partitions. A partitioned distillation zone receives a binary mixture AB at the top of its partition and produces a B-rich stream from the bottom. Similarly, a binary mixture CD is fed to another partitioned distillation zone at the bottom of the associated partition, and a C-rich product is recovered from the top. Because of the use of a reboiler at B and a condenser at C, the vapor flow rates in each of the Band C-producing partitioned distillation zones can be controlled. This allows for better control of all of the product purities. Alternatively, the partitions in the distillation column of this figure can be created through the use of three radial walls or with an inner core and two radial walls.6,11 In the equivalent three distillation column thermally coupled configuration, a side-stripper will be needed to recover B and a side-rectifier to recover C.9 Therefore, an alternate equivalent structure can also be drawn as shown in Figure 5b. This structure with some heat transfer between the condensing C stream and the boiling B stream was suggested earlier in the literature.6 If the number of separation stages needed in the partitioned distillation zones recovering the B and C product streams is not large, then the configuration in Figure 5b will require a smaller diameter column than the configuration in Figure 5a. On the other hand,

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Figure 5. Two equivalent single distillation column structures with partitions for a four-component separation.

if the number of such separation stages needed is large, then the configuration in Figure 5a would give a distillation column of lower height. As an aside, it is interesting to note that the structures of Figure 5 could also be used for a ternary feed ABC. In such a case, the partitioned distillation zone in parts a and b of Figure 5, which is shown to receive mixture CD, will instead receive binary mixture BC, and now the condenser on top of this zone will produce B. The bottom reboiler shown to boil D will boil C, and component C will be produced from the bottom of the column. Now two B streams are produced, one from the bottom of a partitioned distillation zone and the other from the top of a partitioned distillation zone. This configuration is analogous to the use of both a sidestripping and a side-rectifying column, along with the feed column. Returning to four-component mixtures, some additional single column configurations with a total of four reboilers and condensers are shown in Figure 6. The difference between the configurations of Figures 5a and 6a lies in the manner by which C is recovered. In Figure 6a, the C-recovering partitioned zone is fed with a ternary mixture ABC at the top and has a reboiler at the bottom. This configuration has three reboilers and only one condenser. (Alternatively, it would be possible to draw a single distillation column structure with three condensers and one reboiler for component D. In such a structure, B-recovering partitioned zone would be fed at the bottom with a ternary mixture BCD and the C-recovering partitioned zone with a binary mixture CD.) The structure in Figure 6b can be derived from the structure in Figure 6a by modifying the recovery scheme for B. Now the feed to the partitioned zonerecovering component B is a binary mixture BC from the partitioned zone recovering component C. Figure 6c is analogous to Figure 6b, except that component D is removed first in Figure 6b, whereas in Figure 6c,

component A is removed first. Both parts b and c of Figure 6 are examples of structures where not all of the partitioned distillation zones are in direct communication with the distillation zone receiving the main feed. The interesting configuration of Figure 6d results when the two structures of parts b and c of Figure 6 are combined in a single distillation column. In this configuration, two condensers are shown for component B. However, these two duties could be combined in a single condenser. In one option, the vapors from each of the B-producing partitioned zones may be combined and then condensed in one condenser. Similarly, the two reboilers for component C could also be combined as one reboiler. This will keep the total number of reboilers and condensers to four. In another option, it may be possible to combine the two B-producing partitioned zones into one, as the outer annular zone of a cylindrical distillation column, for example. This will naturally lead to only one condenser for B. The same could be done for the production of C. The structure in Figure 6d is expected to have a lower total heat demand than those shown in parts b and c of Figure 6. It is also possible to draw a four-component distillation column structure where the total number of reboilers plus condensers is three. Two such structures are shown in parts a and b of Figure 7. In these schemes, one of the sections for recovering B and C does not have either a reboiler or a condenser and the component is recovered from behind a partition, similar to the configuration for a ternary mixture shown in Figure 1. Consequently, it may be difficult to control the purity of the B-rich stream for the configuration shown in Figure 7a. The same may be true for the C-rich stream in Figure 7b. These structures may be, however, useful in applications where the purity constraint is not stringent on the B-rich or the C-rich product streams, respectively. Some features of the structures in parts a and b of Figure 7 can be combined to produce other


Figure 6. Four-component single distillation column structures with partitions: (a-c) with a total of four reboilers and condensers; (d) structures of b and c combined in a single distillation column.

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Figure 7. Four-component single distillation column structures with partitions: (a and b) with a total of three reboilers and condensers; (c) with multiple product streams.

complex distillation column structures, such as the one shown in Figure 7c. Now there are two B-rich and two C-rich streams with the possibility that the purity of each stream could be different. Because of complex nature of this structure, it is not expected to be of much practical use. Throughout this paper, columns with partitions are represented with uniform diameters. This may be practical if the numbers of separation stages in neighboring distillation zones are comparable to one another. In situations where one of these zones needs many more separation stages than its neighbors, however, it may be more economical to terminate the neighboring zones and change the diameter of the column shell. Such decisions will depend on the feed composition, relative volatilities, and manufacturing costs. For feeds with four or more components, schemes have been suggested where other distillation columns are used together with a distillation column with partitions.4,10 Lestak and Collins investigated the fractionation of natural gas liquids by retaining the demethanizer and deethanizer columns of the original direct sequence and combining the operation of depropanizer and debutanizer columns into one partitioned column.4 Savings were achieved by reducing the number of distillation column shells, reboilers, and condensers. Similar ideas could be used to modify other well-known multicomponent distillation configurations by combining the functions of only some of the distillation columns into a partitioned distillation column using multiple reboilers and condensers, together with ancillary columns of standard design. A few examples are shown in Figure 8 for a four-component separation. In Figure 8a, the direct sequence is modified by using the ternary structure of Figure 3b. Similarly, the indirect sequence can be modified as shown in Figure 8b. A modification of the indirect sequence using the ternary structure of

Figure 4b is shown in Figure 8c. Clearly, many such modified schemes are possible. For any given application, the challenge is to find a modified scheme that would maximize the economic benefit while retaining the required level of operating flexibility. Conclusions Single distillation column structures with partitions and multiple reboilers and condensers are suggested for the distillation of a multicomponent feed. The total number of reboilers and condensers required for such structures is generally equal to the number of components in the feed. In this case, each of the partitioned distillation zones recovers one of the feed components and has either a reboiler or a condenser. This allows for better control of liquid and vapor flow-rate ratios in each partitioned distillation zone, resulting in better control of the product purities. For an n-component mixture, when n is greater than 3, it is also possible to draw single distillation column structures with partitions where the total number of reboilers and condensers are greater than 2 but less than n. As compared to conventional schemes that use n - 1 distillation columns and a total of 2(n - 1) reboilers and condensers, these structures decrease the number of distillation column shells to one and the total number of reboilers and condensers by at least n - 2. These single distillation column structures are drawn by analogy to side-stripper and side-rectifier configurations. As a result, their heat duty demands will be as low as those of the side-stripper and side-rectifier configurations. Therefore, as compared to conventional direct and indirect split configurations, not only are the number of distillation columns, reboilers, and condensers reduced but the total heat duty is also lower. When compared to a fully thermally coupled scheme, the total


Figure 8. Some four-component schemes with only two distillation columns.

heat duty of these new single distillation column structures would be higher. However, for a large number of feed conditions, this difference may be small so that the total heat duties would be similar. For such feed conditions, when compared to the well-known single distillation column structures with only one reboiler and one condenser, the proposed single distillation column structures with multiple reboilers and condensers may be attractive because of an increase in operability with only a marginal increase in equipment and operating costs. Furthermore, the use of multiple reboilers and condensers in the proposed single distillation column structures will often enable them to be

more thermodynamically efficient than single distillation column structures with only one reboiler and one condenser. Acknowledgment Numerous discussions with D. M. Herron and Dr. Z. T. Fidkowski of Air Products and Chemicals, Inc., are gratefully acknowledged. A careful review of this paper by Dr. Keith B. Wilson is appreciated. Literature Cited (1) Wright, R. O. Fractionation Apparatus. U.S. Patent 2,481,134, 1949.

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(2) Triantafyllou, C.; Smith, R. The Design and Optimisation of Fully Thermally Coupled Distillation Columns. Trans. Inst. Chem. Eng. 1992, 70, Part A, 118. (3) Auer, H.; Eiden, U.; Schuch, G.; Thiel, J. Separation of a Ternary Mixture Containing Salt Using a Divided Wall Column. AIChE Annual Meeting, Los Angeles, CA, 1997. (4) Lestak, F.; Collins, C. Advanced Distillation Saves Energy and Capital. Chem. Eng. 1997, July, 72. (5) Kaibel, G. Distillation Columns with Vertical Partitions. Chem. Eng. Technol. 1987, 10, 92. (6) Christiansen, A. C.; Skogestad, S.; Lien, K. Partitioned Petlyuk Arrangements for Quaternary Separations, IChemE Symp. Series, 1997, No. 142, p 745. (7) Christiansen, A. C.; Skogestad, S.; Lien, K. Complex Distillation Arrangements: Extending the Petlyuk Ideas. Comput. Chem. Eng. 1997, 21, Suppl., S237. (8) Sargent, R. W. M.; Gaminibandara, K. Optimum Design of Plate Distillation Columns. Optimization in Action; Dixon, L. W. C., Ed.; Academic Press: New York, 1976; p 267. (9) Agrawal, R. Synthesis of Distillation Column Configurations for a Multicomponent Separation. Ind. Eng. Chem. Res. 1996, 35, 1059. (10) Du¨nnebier, G.; Pantelides, C. C. Optimal Design of Thermally Coupled Distillation Columns. Ind. Eng. Chem. Res. 1999, 38, 162. (11) Agrawal, R. More Operable Fully Thermally Coupled Distillation Column Configurations for Multicomponent Distillation. Trans. Inst. Chem. Eng. 1999, 77, Part A, 543. (12) Agrawal, R.; Fidkowski, Z. T. New Thermally Coupled Schemes for Ternary Distillation. AIChE J. 1999, 45, 485. (13) Agrawal, R.; Fidkowski, Z. T. Are Thermally Coupled Distillation Columns Always Thermodynamically More Efficient

for Ternary Distillation? Ind. Eng. Chem. Res. 1998, 37, 3444. (14) Ognisty, T. P.; Manley, D. B. Partitioned Distillation Column. U.S. Patent 5,755,933, 1998. (15) Wolff, E. A.; Skogestd, S. Operation of Integrated ThreeProduct (Petlyuk) Distillation Columns. Ind. Eng. Chem. Res. 1995, 34, 2094. (16) Halvorsen, I. J.; Skogestad, S. Optimizing Control of Petlyuk Distillation: Understanding the Steady-State Behaviour. Comput. Chem. Eng. 1997, 21, Suppl., S249. (17) Abdul Mutalib, M. I.; Smith, R. Operation and Control of Dividing Wall Distillation Columns. Trans. Inst. Chem. Eng. 1998, 76, Part A, 308. (18) Muto, Y.; Ken, I.; Hyugaji, T.; Mihara, T.; Shi, H. Rectification Towers. U.S. Patent 3,298,673, 1967. (19) Wong, K. K.; Billingham, J. F.; Bonaquist, D. P.; Arman, B.; Drnevich, R. F.; Shah, M. M. Annular Column for Cryogenic Rectification. U.S. Patent 5,946,942, 1999. (20) Agrawal, R.; Herron, D. M.; Choe, J. S. Process for Distillation of Multicomponent Fluid and Production of an ArgonEnriched Stream from a Cryogenic Air Separation Process. U.S. Patent Application pending. (21) DeGraff, R. R. Fractionation Apparatus Having Plural, Integral and Concentric Fractionating Units. U.S. Patent 3,844,898, 1974. (22) DeGraff, R. R. Fractionation Apparatus Having Two Integral and Concentric Fractionating Units. U.S. Patent 3,959,085, 1976.

Received for review March 14, 2000 Accepted March 25, 2001 IE000315N

Multicomponent Distillation Columns with Partitions and Multiple

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