Distillation process to separate mixtures containing three or more components

Abstract: The heat requirement of conventional distillation processes which separate feed mixtures containing three or more components is reduced by sending a liquid bottoms stream 20 (or gaseous overhead stream 230) containing two or more components from an earlier column to a subsequent column and returning a vapor stream 227 (or liquid stream 337) between the same locations in the two columns. A portion 24 (234) of the liquid bottoms stream 20 (overhead vapor stream 230) is at least partially vaporized (condensed) prior to feeding to the second column or heat is added (removed) from the second column between the feed location of the entire liquid stream 20 (gaseous stream 230) and the bottom (top) of the second column.

Description: The objective of this invention is to provide efficient distillation schemes to separate feed mixtures containing three or more components into product streams each enriched in one of the components. Generally the objective of a process engineer designing a distillation scheme is to make it more efficient by reducing the heat requirement of the distillation columns within the distillation scheme. However, it is well known that in a ternary or three component separation, attempts to reduce the heat requirement leads to the need for more heat utility at a higher temperature i.e. more heat has to be supplied by the higher temperature heat source. For an above ambient temperature distillation, this could mean that heat can be saved only if more higher temperature steam is used. A higher temperature steam is more expensive. Therefore, there is a need for distillation schemes to decrease heat requirement without a need for any additional higher temperature heat utility or to allow more flexibility in adjusting the temperatures at which heat is supplied to the distillation columns. This invention fulfills this long desired need for improving the efficiency of multicomponent distillation without increased demand for more expensive utilities. SU-A-1214721 (published 28th February 1986) discloses the separation of hydrocarbons in which both the heavier fraction and a lighter fraction are fed from a first distillation column to a second distillation column. The lighter fraction is preheated and fed to the second column at a location below the heavier portion. Optionally a portion of the heavier fraction can be added to the lighter portion prior to the preheating. In the accompanying drawings: Figures 1 through 7 are drawings depicting various prior art distillation processes to separate mixtures containing three or more components and Figures 8 through 13 are drawings depicting various embodiments of the present invention's distillation process to separate mixtures containing three or more components. Consider the separation of a ternary mixture ABC into three product streams each enriched in one of the components. A is the most volatile component and C is the least volatile component. To separate a ternary mixture ABC into almost pure components it is required that a distillation scheme use two distillation columns. Such distillation schemes are well known in the art. There are five well known such schemes: direct sequence, indirect sequence, side rectifier, side stripper and thermally coupled columns. Each of the schemes will now be described in detail: Figure 1 shows a direct sequence scheme. A feed mixture containing components A, B and C (stream 10) is fed to the first column (Col. 1) having a condenser (Con. A) and a reboiler (Reb. BC) where it is distilled to produce A-enriched product (stream 70) from the top. The liquid from the bottom of this column (stream 20) is primarily a binary mixture composed of components B and C. This BC liquid stream is split into two streams. A first portion (stream 22) is fed to the second column (Col. 2). The second portion (stream 24) is boiled and fed as stream 26 to the bottom of the first column (Col. 1). B-enriched product (stream 80) and C-enriched product (stream 90) are produced from the second column (Col. 2) having a condenser (Con. B) and a reboiler (Reb. C). A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column (Col. 2) to provide boil-up. Figure 2 shows an indirect sequence scheme. Distillation of the feed mixture (stream 10) in the first column (Col. 1) having a condenser (Con. AS) and a reboiler (Reb. C) produces C-enriched product (stream 90) from the bottom and a primarily binary vapor mixture AB (stream 30) from the top. A portion of this saturated vapor stream is fed (stream 32) to the second column (Col. 2). Another portion (stream 34) is condensed and sent as reflux (stream 36) to the first column (Col. 1). A-enriched product and B-enriched product (streams 70 and 80 respectively) are produced from the second column (Col. 2) having a condenser (Con. A) and a reboiler (Reb. B). Figure 3 shows a side rectifier scheme wherein the feed mixture ABC (stream 10) is distilled in the first column (Col. 1) having a condenser (Con. A) and a reboiler (Reb. C) to produce A-enriched product (stream 70) from the top and C-enriched product (stream 90) from the bottom. A portion of the C-enriched bottoms liquid is boiled (stream 92) and returned to the column (Col. 1) to provide boil-up. The component of intermediate volatility, B, is collected (stream 80) from the top of the second column (Col. 2) (also known as a side rectifier) having a condenser (Con. B). Notice that the second column (Col. 2) does not have a reboiler at the bottom and instead it is fed by a vapor (stream 50) which is withdrawn from a location below the feed of the first column (Col. 1). This vapor stream is primarily a binary mixture consisting of components B and C. The liquid (stream 52) from the bottom of the second column (Col. 2) is sent to the first column (Col. 1) at the same location as where the vapor (stream 50) was removed from the first column (Col. 1). This thermal coupling between the two columns reduces the number of reboilers. As compared to the schemes in Figures 1 and 2, the number of reboilers is reduced by one, and the total number of reboilers and condensers used are three vs. four. Figure 4 shows a side stripper scheme which is similar to Figure 3 (corresponding streams and equipment use the same identification) except that the feed to the second column (Col. 2) (now known as side stripper) is a liquid (stream 60), product B (stream 80) is collected at the bottom of the second column (Col. 2) instead of the top and the second column (Col. 2) has a reboiler (Reb. B) but no condenser. The liquid stream is withdrawn from the first column (Col. 1) from a location which is above the feed location to the first column (Col. 1) and is primarily a binary mixture composed of components A and B and is fed to the top of the second column (Col. 2). The vapor (62) from the top of the second column (Col. 2) is returned to the first column (Col. 1) resulting in the thermal coupling between the two columns. Notice that as compared to the scheme in Figure 2, one less condenser is used in Figure 4. Figure 5 shows a thermally coupled columns scheme which uses two thermal couplings between the first and second columns, thereby eliminating both the reboiler and condenser in the second column (Col. 2). The thermal coupling at the bottom of the second column (Col. 2) is the same as the one shown in Figure 3 and at the top is the same as the one shown in Figure 4 (corresponding streams and equipment use the same identification). B-enriched product (stream 80) is collected from an intermediate location of the second column (Col. 2). Notice that due to two-thermal coupling, the total number of reboilers and condensers is reduced by two. By now it is well known that the schemes with thermal coupling (shown in Figures 3-5) require less heat input than the ones without thermal coupling (Figures 1 and 2) (Minimum Energy Requirements of Thermally Coupled Distillation Systems, Z. T. Fidkowski and L. Królikowski, AIChE Journal pages 643-653, volume 33, 1987). The heat requirement in the reboiler (Reb. C) of side rectifier in Figure 3 is less than the total heat requirement in both reboilers ((Reb. BC and Reb. C) of Figure 1. Similarly, total heat input in the reboilers (Reb. B and Reb. C) for the side stripper configuration in Figure 4 is less than the total heat input in the reboilers (Reb. B and Reb. C) of indirect sequence in Figure 2. Of the five schemes, the thermally coupled configuration in Figure 5 requires the least heat input in its reboiler. While the heat demand decreases with thermal coupling, it comes at a cost of more expensive utilities. For example, the thermally coupled scheme in Figure 5 requires that all the hot utility be available at the highest temperature and all the cold utility be available at the coldest temperature. In a direct sequence scheme (Figure 1), some heat is added to the BC reboiler (Reb. BC) and some to C reboiler (Reb. C). The temperature of the BC reboiler (Reb. BC) is lower than the C reboiler (Reb. C) which implies that the heat source for the BC reboiler (Reb. BC) can be at a lower temperature than the heat source for the C reboiler (Reb. C). On the other hand, all the heat input for thermally coupled column (Figure 5) is to the C reboiler (Reb. C) and the total heat source has to be at the higher temperature. Similarly, in direct sequence scheme (Figure 1), some heat is removed in the B condenser (Con. B) which is warmer than the A condenser (Con. A). This implies that a cold utility used for the B condenser (Con. B) can be warmer (and hence cheaper) than the cold utility used for the A condenser (Con. A). On the other hand, thermally coupled columns require that all the heat be removed by the more expensive cold utility in the A condenser (Con. A). This effect of more expensive utilities is also observed when side rectifier (Figure 3) and side stripper (Figure 4) configurations are compared with schemes in Figure 1 and 2. This prompted Hohmann et al. to state "thermal integration by direct vapor coupling will reduce the heat load on a network while increasing the relative temperatures of the sources (hot utility) and sinks (cold utility) required" (E. C. Hohmann, M. T. Sander and H. Dunford; A New Approach to the Synthesis of Multicomponent Separation Schemes, Chem. Eng. Commun. volume 17, pages 273-284, 1982). Therefore, the major challenge is how to reduce the total heat demand without too much compromising of the temperatures of the utilities. For schemes such as the ones shown in Figures 1 and 2, suggestions have been made in the literature to further reduce the demand of the more expensive utility by trading some of this demand with less expensive utility (For example, see "Two-Feed Distillation: Same Composition Feeds with Different Enthalpies", by P. C. Wankat and D. P. Kessler in Ind. Eng. Chem. Res., volume 32, pages 3061-3067, 1993). In this suggestion, when a liquid (vapor) stream containing two or more components is boiled (condensed) in a reboiler (condenser), rather than feeding just one of the saturated liquid or the saturated vapor feed to the next column, it is desired that both the streams be fed to the next columns. An example for the direct sequence configuration of Figure 1 is shown in Figure 6. (Corresponding streams and equipment in Figures 1 and 6 use the same identification.) Now a portion of the saturated vapor exiting reboiler (Reb. BC) is sent as a second feed (stream 28) to the second column (Col. 2). The total quantity of the feed to the second column (Col. 2) in Figure 6 is identical to the one in Figure 1. However, by transferring a portion of the feed as vapor to the second column (Col. 2), heat requirement for the C reboiler (Reb. C) is decreased but the heat input in the BC reboiler (Reb. BC) is increased by the same quantity. Therefore, the total heat input is unchanged but more of it can now be provided by using a lower temperature heat source. The corresponding solution for Figure 2 is shown in Figure 7. (Corresponding streams and equipment in Figures 2 and 7 use the same identification). Now a portion of the condensed stream from the AB condenser (Con. AB) is fed as a second feed (stream 38) to the second column (Col. 2). By having a portion of the feed to the second column (Col, 2) as saturated liquid, demand for the condensing duty in the cold condenser (Con. A) is decreased but the condensing duty need in the warmer condenser (Con. AB) is increased by the same quantity. Once again more of the cold utility at warmer temperature can be used but the total need for cold utilities remains unchanged. Clearly, there is a need for alternative solutions which can reduce the total heat demand while providing some flexibility to decrease the need for warmer hot utility and/or colder cold utility. The same challenge exists when mixtures containing more than three components are distilled to produce product streams each enriched in one of the components. The reason being that the distillation schemes used to distill mixtures with more than three components are made up of the ternary subschemes shown in Figures 1 through 7. Therefore deficiencies of the ternary subschemes are also carried to the distillation of mixtures containing a greater number of components. Some known examples of four and five component distillation schemes can be found in a paper by Agrawal (Synthesis of Distillation Column Configurations for a Multicomponent Separation, Ind. Eng. Chem. Res., volume 35, pages 1059-1071, 1996). For the distillation of binary mixtures, use of an intermediate reboiler or an intermediate condenser is well known to improve the efficiency of the distillation. (For example, see a paper by Z. T. Fidkowski and R. Agrawal, Utilization of Waste Heat Stream in Distillation, Ind. Eng. Chem. Res. volume 34, pages 1287-1293, 1995.) However, it is also known that use of an intermediate reboiler does not decrease the overall heat required for binary distillation but decreases the heat input in the bottom reboiler by the amount of heat added to the intermediate reboiler. The present invention is a technique to reduce the heat requirement of conventional distillation processes which separate feed mixtures containing three or more components. The technique provides a great deal of flexibility in adjusting the temperatures of the required utilities including a scenario where the heat requirement is reduced without a need for additional higher temperature (and more costly) heat utility. In this technique, when a liquid bottoms stream (or gaseous overhead stream) containing two or more components is sent from an earlier column to a subsequent column, then a return vapor (or liquid) stream is implemented between the same locations in the two columns. The present invention also provides the possibility to reduce the overall heat demand without changing the temperature of any of the utilities and while keeping the demand for the highest temperature hot utility and the lowest temperature cold utility at their minimum values. The distillation of a feed stream in general and the distillation of a ternary feed stream in particular includes at least the following two general steps: (a) feeding the feed stream to a first distillation column and (b) removing a liquid (gaseous) stream enriched in the heavier (lighter) components of the feed stream from the first column at a location below (above) the feed location of the feed stream [generally the bottom (top) of the first column], feeding a first portion thereof to a second column, at least partially vaporizing (condensing) a second portion thereof and returning the at least partially vaporized (condensed) second portion to the first column as boil-up (reflux). The present invention is a technique to reduce the heat requirement of the above process as it relates to a feed containing three or more components. The technique comprises removing a gaseous (liquid) stream from the second column at the feed location of the liquid (gaseous) stream and feeding it to the first column at the removal location of the liquid (gaseous) stream. The heat (refrigeration) that was formerly supplied to the first column via the at least partial vaporization (condensation) of the second portion of the liquid (gas) in step (b) is now more efficiently supplied to the second column via at least partially vaporizing (condensing) a portion of the liquid (vapor) stream prior to feeding it to the second column at least one separation stage [generally a couple of stages] below (above) the feed location of the remainder of the liquid (gaseous) stream. Thus, according to the main process aspect of the present invention, there is provided a process for the distillation of a feed stream containing three or more components into product streams enriched in the respective components, said process comprising: (a) feeding the feed stream to a first distillation column; (b) removing a liquid (or gaseous) stream enriched in the heavier (lighter) components of the feed stream from the first column at a location below (above) the feed location of the feed stream and feeding a first portion of the liquid (gaseous) stream to a second column, partially vaporizing (condensing) a second portion of the liquid (gaseous) stream and subsequently feeding the at least partially vaporized (condensed) second portion to the second column at least one separation stage below (above) the feed location of the first portion; (c) removing a gaseous (liquid) stream from the second column at the feed location of the first portion of the liquid (gaseous) stream and feeding the gaseous (liquid) stream to the first column at the removal location of the liquid (gaseous) stream. In one apparatus aspect, the present invention also provides an apparatus for the distillation of a feed stream containing three or more components into product streams enriched in the respective components by a process of the invention, said apparatus comprising: a first distillation column; a second distillation column; conduit means for feeding the feed stream to the first distillation column; conduit means for removing a liquid stream enriched in the heavier components of the feed stream from the first column at a location below the feed location of the feed stream; conduit means for feeding a first portion of the liquid stream to the second column; heat exchange means for partially vaporizing a second portion of the liquid stream; conduit means for feeding the at least partially vaporized second portion from the heat exchange means to the second column at least one separation stage below the feed location of the first portion; and conduit means for removing a gaseous stream from the second column at the feed location of the first portion of the liquid stream and feeding the gaseous stream to the first column at the removal location of the liquid stream. In another apparatus aspect, the present invention further provides an apparatus for the distillation of a feed stream containing three or more components into product streams enriched in the respective components by a process of the invention, said apparatus comprising: a first distillation column; a second distillation column; conduit means for feeding the feed stream to the first distillation column; conduit means for removing a gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream; conduit means for feeding a first portion of the gaseous stream to the second column; heat exchange means for partially condensing a second portion of the gaseous stream; conduit means for feeding the at least partially condensed second portion from the heat exchange means to the second column at least one separation stage above the feed location of the first portion; and conduit means for removing a liquid stream from the second column at the feed location of the first portion of the gaseous stream and feeding the liquid stream to the first column at the removal location of the gaseous stream. Preferably, the at least partially vaporized (condensed) second portion in step b is fed to the second column at a location where the composition of the vapor (liquid) phase matches with the composition of the vapor (liquid) phase of the second column. In one process embodiment of the present invention, the process comprises: (a) feeding the feed stream to a first distillation column; (b) removing a liquid stream enriched in the heavier components of the feed stream from the first column at a location below the feed location of the feed stream, feeding a first portion of the liquid stream to a second column, partially vaporizing a second portion of the liquid stream and subsequently feeding the at least partially vaporized second portion to the second column at least one separation stage below the feed location of the first portion; and (c) removing a gaseous stream from the second column at the feed location of the first portion of the liquid stream and feeding the gaseous stream to the first column at the removal location of the liquid stream. The process can further comprise: (d) removing a second gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream; feeding a first portion of the second gaseous stream to the second column, partially condensing a second portion of the second gaseous stream and subsequently feeding the at least partially condensed second portion to the second column at least one separation stage above the feed location of the first portion: and (e) removing a second liquid stream from the second column at the feed location of the first portion of the second gaseous stream and feeding the second liquid stream to the first column at the removal location of the second gaseous stream. Alternatively, the process can further comprise: (d) removing a second gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream, feeding the entire second gaseous stream to the second column without any intervening condensation step and removing heat from the second column between the feed location of the second gaseous stream and the top of the second column; and (e) removing a second liquid stream from the second column at the feed location of the entire second gaseous stream and feeding the second liquid stream to the first column at the removal location of the second gaseous stream. In another process embodiment of the present invention, the process comprises: (a) feeding the feed stream to a first distillation column; (b) removing a gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream; feeding a first portion of the gaseous stream to a second column, partially condensing a second portion of the gaseous stream and subsequently feeding the at least partially condensed second portion to the second column at least one separation stage above the feed location of the first portion; and (c) removing a liquid stream from the second column at the feed location of the first portion of the gaseous stream and feeding the liquid stream to the first column at the removal location of the gaseous stream. The invention has particular application to the distillation of a feed gases which is a mixture selected from a mixture comprising benzene, toluene and xylene; a mixture comprising nitrogen, carbon monoxide and methane; a mixture comprising nitrogen, oxygen and argon. a mixture comprising three or more components selected from C 1 to C 5 alcohols; and a mixture comprising three or more components selected from C 1 to C 5 hydrocarbons. The present invention is best illustrated with respect to specific embodiments thereof such as those shown in Figures 8 through13. Figure 8 shows one embodiment as applied to Figure 1 where transfer of a liquid enriched in the heavier components of the feed stream is involved. (Corresponding streams and equipment in Figures 1 and 8 use the same identification.) The heavier components-enriched liquid stream 20 is the bottoms liquid from the first column (Col. 1). This stream is primarily composed of heavier components B and C and a portion thereof (stream 22) is fed to the second column (Col. 2). A vapor (stream 27) is withdrawn from the second column (Col. 2) at the same location as the feed location for stream 22 and fed to the bottom of the first column (Col. 1). Only a portion of the heavier components-enriched liquid is transferred to the second column (Col. 2), the remaining portion (stream 24) is at least partially vaporized (preferably totally vaporized) in a reboiler (Reb. BC) and the resulting stream 25 is fed at least one separation stage (generally a couple of stages) below the feed point of stream 22. The preferred feed location for stream 25 is such that the composition of the vapor phase matches with the composition of the vapor phase in the column. Now consider embodiments of the present invention where transfer of a gas enriched in the lighter components of the feed stream is involved. Figure 9 shows one such embodiment as applied to Figure 2. (Corresponding streams and equipment in Figures 2 and 9 use the same identification.) The lighter components-enriched vapor stream is overhead stream 30 from the first column (Col. 1). This stream is primarily composed of lighter components A and B. A portion of the lighter components-enriched vapor stream 30 is fed as stream 32 to the second column (Col. 2). A liquid stream 37 is withdrawn from the second column (Col. 2) at the same location as the feed location for stream 32 and fed to the top of the first column (Col. 1). In this particular embodiment where only a portion of the lighter components-enriched vapor is transferred to the second column (Col. 2), the remaining portion (stream 34) is at least partially condensed (preferably totally condensed) in condenser Con. AB and the resulting stream 35 is fed at least one separation stage (generally a couple of stages) above the feed point of stream 32. The preferred feed location for stream 35 is such that the composition of the liquid phase matches the composition of the liquid phase in the column. Figure 10's embodiment is a combination of Figures 8 and 9 (corresponding streams and equipment use the same identification) in that both the heavier components-enriched liquid and the lighter components-enriched vapor streams are transferred from the first column (Col. 1) to second column (Col. 2). The first column (Col. 1) produces lighter components-enriched vapor stream 30 from the top, and all of this stream is eventually fed to the second column-I (Col. 2) as per Figure 9. The first column (Col. 1) also produces heavier components-enriched liquid stream 20 and this stream is then fed to the second column-II (Col. 2) as per Figure 8. As shown in Figure 10, product B can be removed as either a vapor (stream 80a) or liquid (stream 80b). Reboiler Reb. B and condenser Con. B between the two second columns (Col. 2) are optional. The concepts taught for the distillation of ternary mixtures can be easily extended to mixtures containing more than three components. They will now be illustrated for the distillation of four-component mixtures. Figures 11 through 13 show a conventional scheme on the left side and the corresponding improved scheme according to the present invention on the right side. Figure 11 a shows a known direct sequence scheme to separate a four component feed mixture ABCD (stream 10) into four product streams each enriched in one of the components. In this mixture, the relative volatility follows the alphabetical order i.e., A is the most volatile and D is the least volatile. The first column (COL. 1) separates the feed mixture into product A (stream 70) and a liquid stream 20 at the bottom which is rich in heavier components. Stream 20 is predominantly composed of B, C and D. A portion of this stream (stream 22) is sent to the second column (COL. 2) and another portion (stream 24) is boiled and fed as stream 26 to the bottom of the first column (COL. 1). The second column (COL. 2) separates the heavier components-enriched stream 22 from the first column (COL. 1) into product B (stream 280) and even more heavier components-enriched liquid stream 220 at the bottom. This even more heavier components-enriched liquid stream is primarily composed of components C and D. A portion of stream 220 is sent as feed (stream 222) to the third column (COL. 3) and another portion (stream 224) is boiled and fed as stream 226 to the second column (COL. 2). The third column (COL. 3) separates the even more heavier components-enriched liquid stream 222 from the second column (COL. 2) into product C (stream 390) and product D (stream 395). An inspection of Figure 11a reveals that there are two places where the present invention can be applied. They both occur when a heavier components-enriched liquid stream is transferred from an earlier column to the next column. Thus in Figure 11b, the portion of the heavier components-enriched liquid stream 20 which is boiled (stream 24) is not sent to the bottom of the first column (COL. 1) but sent to an intermediate location of the second column (COL. 2) as stream 25. The boil-up to the first column (COL. 1) is provided by a vapor stream 227 which is withdrawn from the same location of the second column (COL. 2) as the feed point of the heavier components-enriched liquid stream 22. Similar modification is made for the even more heavier components-enriched liquid stream 220. Thus boiled stream 225 is sent to the third column (COL. 3) and the boil-up to the second column (COL. 2) is provided by stream 327 which is withdrawn from the same location of the third column (COL. 3) as the feed location for lighter components-enriched vapor stream 222. Figure 12a shows another scheme to separate a four component mixture ABCD (stream 10) into four streams each enriched in one of the components. As in Figure 11, the first column (COL. 1) again separates the feed mixture into product A (stream 70) and a heavier components-enriched liquid stream 20, a portion of which (stream 22) is sent to the second column (COL. 2) and another portion (stream 24) of which is boiled and fed as stream 26 to the bottom of the first column (COL. 1). The second column (COL. 2) produces product D (stream 295) at the bottom and a lighter components-enriched vapor (stream 230) at the top, a portion of which (stream 234) is condensed and returned as reflux to the top of the column as stream 236 and another portion (stream 232) which is fed to the third column (COL. 3). The third column (COL. 3) separates the lighter components-enriched vapor stream 230 from the second column (COL. 2) into product B (stream 380) and product C (stream 390). Applying the present invention to both streams 22 and 232 in Figure 12a results in Figure 12b. The heavier components-enriched liquid stream 20 from the bottom of the first column (COL. 1) is primarily composed of components B, C and D. A portion of it is directly sent to the second column (COL. 2) (stream 22) and another portion is boiled. The boiled stream 25 is not sent as boil-up to the first column (COL. 1) but is sent to an intermediate location of the second column (COL. 2). The boil-up for the first column (COL. 1) is provided by vapor stream 227 which is withdrawn form the second column (COL. 2) at the same location as the feed location for heavier components-enriched liquid stream 22. The lighter components-enriched vapor stream 230 from the top of the second column (COL. 2) is primarily composed of components B and C. A portion (stream 232) of it is directly sent to the third column (COL. 3) and another portion is condensed. The condensed stream 235 is not sent as reflux to the second column (COL. 2) but is sent to an intermediate location of the third column (COL. 3). The reflux for the second column (COL. 2) is provided by liquid stream 337 which is withdrawn from the third column (COL. 3) at the same location as the feed location for lighter components-enriched vapor stream 232. Figure 13a shows a scheme where the first column (COL. 1) does not produce any product stream directly form the feed mixture ABCD (stream 10). Instead, the first column (COL. 1) produces both the lighter components-enriched vapor stream 30 (primarily composed of A and B) from the top and the heavier components-enriched liquid stream 20 (primarily composed of C and D) from the bottom. A portion of vapor stream 30 (stream 34) is condensed and returned as reflux (stream 36) while another portion (stream 32) is fed to the second column (COL. 2) where it is separated into product A (stream 270) at the top and product B (stream 280) at the bottom. Similarly, a portion of liquid stream 20 (stream 24) is condensed and returned as boil-up (stream 26) while another portion (stream 22) is fed to the third column (COL. 3) where it is separated into product C (stream 390) at the top and product D (stream 395) at the at the bottom. Applying the present invention to both streams 22 and 32 in Figure 13a results in Figure 13b. Condensed stream 36 is not sent as reflux to the first column (COL. 1) but is sent to an intermediate location of the second column (COL. 2) as stream 35. The reflux for the first column (COL. 1) is provided by liquid stream 237 which is withdrawn from the second column (COL. 2) at the same location as the feed location for lighter components-enriched vapor stream 32. Similarly, boiled stream 26 is not sent as boil-up to the first column (COL. 1) but is sent to an intermediate location of the third column (COL. 3) as stream 25. The boil-up for the first column (COL. 1) is provided by vapor stream 327 which is withdrawn from the third column (COL. 3) at the same location as the feed location for heavier components-enriched liquid stream 22. From the above discussion, application of the present invention to the separation of mixtures containing more than four components is obvious. Some examples of feed streams which the present invention is applicable to include nitrogen/oxygen/argon mixtures, benzene/toluene/xylene mixtures, nitrogen/carbon monoxide/methane mixtures, any combination of 3 or more components from C 1 to C 5 alcohols or any combination of 3 or more components from C 1 to C 5 hydrocarbons As noted previously, a prevailing wisdom in chemical engineering literature is that in order to decrease heat requirement of a distillation system, more heat is needed at higher temperatures. Some examples of such configurations are shown in Figures 1, 3, and 5. Configurations in Figures 3 and 5 require less heat than the configuration in Figure 1, however, they demand that all the heat be supplied at one high temperature in reboiler Reb. C. This means that a more expensive heat source would be needed to achieve distillation in Figures 3 and 5. Some other solutions such as the one in Figure 6 do not change the total heat demand but shift more heat from the higher temperature to the lower temperature. Thus, total heat requirements for configurations in Figure 1 and 6 are same but in Figure 6 less heat is supplied in reboiler Reb. C and more in reboiler Reb. BC. The present invention breaks the existing paradigm by not only reducing the total heat demand but also by providing a great deal of flexibility in temperatures at which heat is provided. As shown in the following Example, Figure 8's particular embodiment of the present invention requires less total heat than prior art Figures 1 and 6 with only slightly more heat at higher temperature of reboiler Reb. C. The following example is offered to demonstrate the efficacy of the present invention. Example Separation of a three component mixture containing 33.3% A, 33.3% B and 33.4% C into pure components was considered. The relative volatility of A with respect to C was taken to be 10 and of B with respect to C was taken to be 5. All three components were taken to have same latent heat. For each scheme calculations were done to achieve separation with minimum possible vapor flow. In other words, the total boil-up need was at its minimum. This is achieved by having columns pinched at feed locations. This allowed a fair comparison of heat duties between different schemes. The results are listed in Table 1. The basis of calculation in this table is feed flow of 1 mole. x is mole fraction in liquid phase and y is mole fraction in vapor phase; subscript indicates the mole fraction of the component under consideration. SL stands for saturated liquid while SV stands for saturated vapor. Calculations were first done for the base flowsheet shown in Figure 6. The needed heat duty in reboiler Reb. BC is to vaporize 1.353 moles of mixture containing 50% B (stream 24). The amount of liquid boiled in reboiler Reb. C is 0.167 moles (stream 92). The total vapor produced is 1.52 moles. Then calculations were done according to present invention for the flowsheet in Figure 8. The amount of liquid vaporized in reboiler Reb. BC is now decreased to 1.103 moles (streams 24 and 25), and the amount of liquid vaporized in reboiler Reb. C has increased to 0.359 moles (stream 92). As compared to prior art flowsheet in Figure 6 the total boil-up has decreased nearly 4% from 1.520 to 1.462. Calculations were also done for the prior art process shown in Figure 3. It is observed that all the boil-up for this process is provided in one reboiler Reb. C. The total vapor flowrate for this process is the same as for the present invention in Figure 8. However, the temperature of the reboiler Reb. C is warmer than that in the reboiler Reb. BC. Mixture BC contains a large fraction of B and component B is more volatile than C; this makes mixture BC vaporize at much lower temperature than component C. As a result, the heat duty for reboiler Reb. BC can be supplied by a heat source that is at a lower temperature than the heat source for reboiler Reb. C. A lower temperature heat source is cheaper than the higher temperature heat source. Therefore, the benefit of Figure 8's embodiment of the present invention as compared to scheme in Figure 3 is that a large fraction (about 75%) of the total heat can be provided by a cheaper lower temperature heat source. When the process in Figure 8 is compared with the conventional process in Figure 6, it is observed that while a lower temperature heat source can now be used for reboiler Reb. BC (stream 24 has higher mole fraction of B in Figure 8), more heat is needed in reboiler Reb. C. It means that while total demand for heat has decreased, a tradeoff for heat source has taken place. Some heat is needed at an even lower temperature (in reboiler Reb. BC) and more heat is needed at a higher temperature (in reboiler Reb. C). Example Calculations for Various Figures Stream Number and its Condition FIGURE 6 FIGURE 8 Flow 1.687 1.687 20 x B 0.5 0.768 State SL SL Flow 1.353 1.103 24 x B 0.5 0.768 State SL SL Flow 1.103 25 y B - 0.768 State SV Flow 1.02 26 y B 0.5 - State S V Flow 1.02 27 y B - 0.943 State SV Flow 0.167 0.359 92 y C 1 1 State SV SV Flow 925 y C - - State Total Vapor Flow From All Reboilers 1.520 1.462

Claims: A process for the distillation of a feed stream containing three or more components into product streams enriched in the respective components, said process comprising: (a) feeding the feed stream to a first distillation column; (b) removing a liquid or gaseous stream enriched respectively in the heavier or lighter components of the feed stream from the first column at a location respectively below or above the feed location of the feed stream and either: when said liquid or gaseous stream is a liquid stream enriched in the heavier components and removed at a location below the feed location of the feed stream, feeding a first portion of the liquid stream to a second column, partially vaporizing a second portion of the liquid stream and subsequently feeding the at least partially vaporized second portion to the second column at least one separation stage below the feed location of the first portion, removing a gaseous stream from the second column at the feed location of the first portion of the liquid stream and feeding the gaseous stream to the first column at the removal location of the liquid stream or when said liquid or gaseous stream is a gaseous stream enriched in the lighter components and removed from the first column at a location above the feed location of the feed stream, feeding a first portion of the gaseous stream to a second column, partially condensing a second portion of the gaseous stream and subsequently feeding the at least partially condensed second portion to the second column at least one separation stage above the feed location of the first portion, removing a liquid stream from the second column at the feed location of the first portion of the gaseous stream and feeding the liquid stream to the first column at the removal location of the gaseous stream. A process as claimed in Claim 1, wherein said at least partially vaporized or at least partially condensed second portion is fed to the second column at a location where the composition of the vapor or liquid phase respectively matches with the composition of the vapor or liquid phase respectively of the second column. A process as claimed in Claim 1 or Claim 2, said process comprising: (a) feeding the feed stream to a first distillation column; (b) removing a liquid stream enriched in the heavier components of the feed stream from the first column at a location below the feed location of the feed stream, feeding a first portion of the liquid stream to a second column, partially vaporizing a second portion of the liquid stream and subsequently feeding the at least partially vaporized second portion to the second column at least one separation stage below the feed location of the first portion; and (c) removing a gaseous stream from the second column at the feed location of the first portion of the liquid stream and feeding the gaseous stream to the first column at the removal location of the liquid stream. A process as claimed in Claim 1 or Claim 2, said process comprising: (a) feeding the feed stream to a first distillation column; (b) removing a gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream; feeding a first portion of the gaseous stream to a second column, partially condensing a second portion of the gaseous stream and subsequently feeding the at least partially condensed second portion to the second column at least one separation stage above the feed location of the first portion; and (c) removing a liquid stream from the second column at the feed location of the first portion of the gaseous stream and feeding the liquid stream to the first column at the removal location of the gaseous stream. A process as claimed in Claim 3 which further comprises: (d) removing a second gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream; feeding a first portion of the second gaseous stream to the second column, partially condensing a second portion of the second gaseous stream and subsequently feeding the at least partially condensed second portion to the second column at least one separation stage above the feed location of the first portion; and (e) removing a second liquid stream from the second column at the feed location of the first portion of the second gaseous stream and feeding the second liquid stream to the first column at the removal location of the second gaseous stream. A process as claimed in Claim 3 which further comprises: (d) removing a second gaseous stream enriched in the lighter components of the feed stream from the first column at a location above the feed location of the feed stream, feeding the entire second gaseous stream to the second column without any intervening condensation step and removing heat from the second column between the feed location of the second gaseous stream and the top of the second column; and (e) removing a second liquid stream from the second column at the feed location of the entire second gaseous stream and feeding the second liquid stream to the first column at the removal location of the second gaseous stream. A process as claimed in any one of the preceding claims, wherein the feed gas is a mixture selected from a mixture comprising benzene, toluene and xylene; a mixture comprising three or more components selected from C 1 to C 5 alcohols; and a mixture comprising three or more components selected from C 1 to C 5 hydrocarbons. A process as claimed in any one of Claims 1 to 6, wherein the feed gas is a mixture comprising nitrogen, carbon monoxide and methane or a mixture comprising nitrogen, oxygen and argon. An apparatus for the distillation of a feed stream containing three or more components into product streams enriched in the respective components by a process as defined in Claim 1, said apparatus comprising: a first distillation column (Col. 1); a second distillation column (Col. 2); conduit means (10) for feeding the feed stream to the first distillation column (Col. 1); conduit means (20) for removing a liquid stream enriched in the heavier components of the feed stream from the first column (Col. 1) at a location below the feed location of the feed stream; conduit means (24) for feeding a first portion of the liquid stream to the second column (Col. 2); heat exchange means (Reb. BC) for partially vaporizing a second portion of the liquid stream; conduit means (25) for feeding the at least partially vaporized second portion from the heat exchange means ((Reb. BC) to the second column (Col. 2) at least one separation stage below the feed location of the first portion; and conduit means (27) for removing a gaseous stream from the second column (Col. 2) at the feed location of the first portion of the liquid stream and feeding the gaseous stream to the first column (Col. 1) at the removal location of the liquid stream. An apparatus for the distillation of a feed stream containing three or more components into product streams enriched in the respective components by a process as defined in Claim 1, said apparatus comprising: a first distillation column (Col. 1); a second distillation column (Col. 2); conduit means (10) for feeding the feed stream to the first distillation column (Col. 1); conduit means (30) for removing a gaseous stream enriched in the lighter components of the feed stream from the first column (Col. 1) at a location above the feed location of the feed stream; conduit means (34) for feeding a first portion of the gaseous stream to the second column (Col. 2), heat exchange means (Con. AB) for partially condensing a second portion of the gaseous stream; conduit means (35) for feeding the at least partially condensed second portion from the heat exchange means (Con. AB) to the second column (Col. 2) at least one separation stage above the feed location of the first portion; and conduit means (37) for removing a liquid stream from the second column (Col. 2) at the feed location of the first portion of the gaseous stream and feeding the liquid stream to the first column (Col. 1) at the removal location of the gaseous stream. Use of an apparatus as claimed in Claim 9, wherein said at least partially vaporized second portion is fed to the second column at a location where, in operation, the composition of the vapor phase matches with the composition of the vapor liquid phase of the second column. Use of an apparatus as claimed in Claim 10, wherein said at least partially condensed second portion is fed to the second column at a location where, in operation, the composition of the liquid phase matches with the composition of the liquid phase of the second column. An apparatus as claimed in Claim 9 which further comprises: conduit means (30) for removing a second gaseous stream enriched in the lighter components of the feed stream from the first column (Col. 1) at a location above the feed location of the feed stream; conduit means (34) for feeding a first portion of the second gaseous stream to the second column (Col. 2); heat exchange means (Con. AB) for partially condensing a second portion of the second gaseous stream; conduit means (35) for feeding the at least partial condensed second portion to the second column (Col. 2) at least one separation stage above the feed location of the first portion; and conduit means (37) for removing a second liquid stream from the second column (Col. 2) at the feed location of the first portion of the second gaseous stream and feeding the second liquid stream to the first column (Col. 1) at the removal location of the second gaseous stream. An apparatus as claimed in Claim 9 which further comprises: conduit means for removing a second gaseous stream enriched in the lighter components of the feed stream from the first column (Col. 1) at a location above the feed location of the feed stream; conduit means for feeding the entire second gaseous stream to the second column (Col. 2) without any intervening condensation step; means for removing heat from the second column (Col. 2) between the feed location of the second gaseous stream and the top of the second column; and conduit means for removing a second liquid stream from the second column (Col. 2) at the feed location of the entire second gaseous stream and feeding the second liquid stream to the first column (Col. 1) at the removal location of the second gaseous stream.

Inventors: AGRAWAL RAKESH, FIDKOWSKI ZBIGNIEW TADEUSZ, AGRAWAL, RAKESH, FIDKOWSKI, ZBIGNIEW TADEUSZ
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1998
Main classification: B01D3
IPC codes: B01D3, F25J3