Patent Application: US-201414525205-A

Abstract:
for an n - component mixture , an array of new distillation columns is disclosed with vertical partitions that allow independent control of the vapor flowrates in each partitioned zone , while operating the columns to produce constituent product streams . specifically , all such more operable columns with vertical partitions for ternary and quaternary feed mixtures are illustrated . for a ternary feed , through extensive computation , the minimum heat duty for each of the new columns is same as for the ftc configuration . the new columns with vertical partitions become even more attractive when the vapor split between column sections must be controlled within a narrow range . finally , it is disclosed how a new column with vertical partition drawn for an n - component mixture can be adapted to distil feed mixtures that contain more than n - components .

Description:
for the purposes of promoting an understanding of the principles of the present disclosure , reference will now be made to the embodiments illustrated in the drawings , and specific language will be used to describe the same . it will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended . presented herein are new dividing wall columns that are more operable than the tc - tc column and operate with approximately the same minimum heat duty as the tc - tc column ( or configuration ). following are brief definitions of key terms used in the present disclosure . a , b , c , d , e denote components with volatilities decreasing in the same order that are present in a feed mixture abcde . a is the most volatile component and e is the least volatile component and b , c and d are components of intermediate volatility with b being more volatile than c , and c being more volatile than d . the feed mixture for separation using the invention described here may be from the group but are not limited to benzene / toluene / xylene mixtures , nitrogen / oxygen / argon mixtures , nitrogen / carbon mono - oxide / methane mixtures , combinations of three or more components from c1 to c5 alcohols , and hydrocarbon mixtures , the mentioned hydrocarbon mixtures could be any from the set of pentane / hexane / heptane , isopentane / pentane / hexane , butane / isopentane / pentane , isobutene / n - butane / gasoline , and combinations of at least three components from c1 to c10 hydrocarbons or c 4 isomers . further , the invention could be used to separate benzene from pyrolysis gasoline , c7 + aromatics from c7 + olefin / paraffin or for separations by distillation as described in references 23 to 42 . streams are named according to the components they predominantly contain . for example bc is a stream that is primarily a mixture of components b and c , but may contain traces or acceptable concentrations of other components . similarly , a product stream a may contain acceptable concentration of other components but will primarily be rich in a . more volatile component means , for a given feed , any feasible stream that contains the most volatile component of the feed . for feed abc , more volatile component implies any one of ab or a . for feed abcd , more volatile component implies any one of abc or ab or a . for feed abcde , more volatile component implies any one of abcd or abc or ab or a . intermediate volatile component means , for a given feed , any feasible stream that is lean in the most volatile and the least volatile components of the feed . for feed abc , intermediate volatile component implies b . for feed abcd , intermediate volatile component implies any one of bc or b or c . for feed abcde , intermediate volatile component implies any one of bcd or bc or cd or b or c or d . less volatile component means , for a given feed , any feasible stream that contains the least volatile component of the feed . for feed abc , less volatile component implies any one of bc or c . for feed abcd , less volatile component implies any one of bcd or cd or d . for feed abcde , less volatile component implies any one of bcde or cde or de or e . a stream enriched in the more volatile component ( less volatile component ) means , in such a stream , the ratio of the flow rate of the more volatile component ( less volatile component ) to the flow rate of the least volatile component ( most volatile component ) present in the feed is higher than the corresponding value for the feed . a stream depleted in the more volatile component ( less volatile component ) means , in such a stream , the ratio of the flow rate of the more volatile component ( less volatile component ) to the flow rate of the least volatile component ( most volatile component ) present in the feed is lower than the corresponding value for the feed . a stream enriched ( depleted ) in the intermediate volatile component means , in such a stream , at least one of the two ratios — the ratio of the flow rate in the stream of the intermediate volatile component , to the flow rate in the stream of the feed &# 39 ; s least volatile component , or the ratio of the flow rate in the stream of the intermediate volatile component , to the flow rate in the stream of the feed &# 39 ; s most volatile component , is higher ( lower ) than in the feed . in general , depending on the context , a product stream enriched in the less volatile component may contain components in addition to the least volatile component , but these additional components are the ones whose volatility is adjacent to the least volatile component . for example , for a feed mixture abcd , where d is the least volatile component , a stream enriched in the less volatile component may contain either one of the following : d , or cd , or bcd . similarly , depending on the context , a stream enriched in the intermediate volatile component may contain one or more components of intermediate volatilities . for example , for the feed mixture abcd , the stream enriched in the intermediate volatile component may be any one of the following : b , c , or bc . likewise , depending on the context , a product stream enriched in the more volatile component may contain components in addition to the most volatile component , but these additional components are the ones whose volatility is adjacent to the most volatile component . for example , for a feed mixture abcd , a stream enriched in the more volatile component may contain either one of the following : a , or ab , or abc . vertical partition means , any physical separation that is used to prevent the exchange of mass between the two sides of the vertical partition inside the distillation column . fig5 a and 5 b show the top view of the distillation column when the vertical partition is a dividing wall and concentric cylinder respectively . fig5 c and 5 d show the top view of two possible arbitrarily shaped vertical partitions that could be used in a distillation column . fig6 a , 6 b , 6 c , and 6 d respectively show the front views of the distillation columns in fig5 a , 5 b , 5 c , and 5 d . the shapes of the distillation columns shown in the figures are not to be construed as implying that only cylindrical distillation columns are possible . rather , the distillation column &# 39 ; s diameter may vary at any location along the height of the column . similarly , partitions are a partition is any physical separation of any shape , which prevents the liquid and vapor of its two sides from mixing inside the distillation column . use of a vertical partition divides a column shell into a least two zones , each on either side of the vertical partition . in this case , the zone on any one side of the partition is the region between the vertical partition and the column shell on that side . a zone has separation stages for liquid - vapor contact . thus , in fig1 , a vertical partition provides two zones z1 and z2 and there are separation stages in each of the zones . when a column contains more than one vertical partition , then there are additional zones between the adjacent vertical partitions and also between the column shell and some of the vertical partitions that are adjacent to the column shell . in our invention , a zone is fed with a stream at an intermediate location . this stream may be the given feed mixture for distillation . alternatively , this stream may be another stream derived from the feed mixture . in this case , the derived stream is withdrawn from another distillation zone . generally , when it is said that a stream is withdrawn from a location in a column or a zone which is below the withdrawal or feed location of another stream , then it means that there is one or more separation stages between the two locations . same is true when a stream is fed above a location where another stream is either fed or withdrawn . similarly , when it is said that a stream is fed or withdrawn from an intermediate location of a zone or a distillation column , it is meant that there are one or more separation stages above as well as below the location under consideration . by separation stages , it is meant mass transfer contact devices such as trays , structured or random packing , etc . a distillation column operated in an advantageous manner means , the operation of the distillation column accompanied by a reduction in total heat duty in the reboilers of the distillation column or attainment of desired product purities . cooling medium means , a stream of sufficiently low temperature used in a heat exchanger that allows heat exchange with a process stream to reduce the process stream &# 39 ; s enthalpy content . heating medium means , a stream of sufficiently high temperature used in a heat exchanger that allows heat exchange with a process stream to increase the process stream &# 39 ; s enthalpy content . streams of similar composition means , in such streams , the ratio of each component &# 39 ; s flowrate in one stream to the total flowrate of that stream is similar to the corresponding values in every other stream . streams of dissimilar composition means , in such streams , the ratio of at least one component &# 39 ; s flowrate in one stream to the total flowrate of that stream is not equal to the corresponding values for every other stream . distillation configurations with liquid transfers between distillation columns are easier to operate and control than configurations with vapor transfers between distillation columns . based on this fact , for the distillation of a ternary mixture , agrawal proposed the three configurations of fig7 a , 7 b , and 7 c , which are more operable than the tc - tc configuration . further , based on physical reasoning , he proposed that the configurations of fig7 a , 7 b , and 7 c have the same overall minimum vapor requirement as the tc - tc configuration , and hence , are equivalent to the tc - tc configuration . through modeling and extensive computation , we confirm this equivalence . furthermore , this is disclosure refers to the configurations of fig7 a , 7 b , and 7 c as the l - tc , tc - l and l - l configurations , respectively . for example , the l - tc configuration is named so because of the liquid transfer at submixture ab and thermal coupling link at submixture bc . the low heat duty requirement of the l - tc configuration , for example , can be partially understood by the fact that , with the same vapor that enters distillation column 1 at the bottom , for separating feed abc into ab and bc , some a is also distilled from the top of the first column . hence , this quantity of a is absent from the system for separation in column 2 , which potentially reduces the heat duty requirement of this column . a similar analysis can be extended to understand the low heat duty requirements of the tc - l and l - l configurations . in fig8 a , 8 b , and 8 c , the new , more operable dividing wall column versions of the l - tc , tc - l and l - l configurations are introduced : the l - tc , tc - l and l - l columns . note that the same names will be used later when the same structures are used for higher component separations . a distinct feature of all the dividing wall columns of fig8 a , 8 b , and 8 c is that the liquid transfers associated with the submixtures ab and bc that are explicitly shown , are made around the vertical partition . this is achieved by collecting the liquid of desired quantity from an intermediate location of one zone ( z 1 ), and then feeding it to an intermediate location of the other zone ( z 2 ), on the other side of the vertical partition . an example of such a liquid transfer is shown for the l - tc column in fig9 . the liquid flows can be managed either through gravitational head or use of pumps . valves in the liquid lines ( not shown in the figure ) could be used to manipulate the liquid split from collection pot 1 . there is no vapor exchange between the two intermediate locations of the two parallel zones of the vertical partition . thus , the vertical partitions are continuous . it should be noted that even though a vertical partition is continuous , the liquid that is to be transferred around the vertical partition to the other side of the column could be transferred through a pipe that will penetrate through the vertical partition to the other side , but no vapor is exchanged between the two locations . in this case , the end effect will be the same as shown in fig9 . such constructions in all the new dividing wall columns eliminates the constraint that the pressure drop in the two parallel zones , on either side of the vertical partition , be equal . this feature of the new dividing wall columns , as will be seen , makes them more operable than the conventional tc - tc column . the l - tc column , like the tc - tc column , has one vapor split at the bottom of the vertical partition . however , the two condensers 103 at a in the l - tc column can be artificially used to create a desired pressure drop in zones z 1 and / or z 2 of the vertical partition . this can be achieved by either placing a valve in the piping before the condenser 103 , or , by controlling the inlet temperature of the cooling medium within each of the condensing heat exchangers . for example , assume the condensing fluid in both the condensers 103 of the l - tc column to be pure benzene , the outlets to be pure saturated liquid benzene and an approach temperature of the pure saturated liquid benzene with respect to the cooling medium to be 10 ° c . in both condensers 103 . at 1000 mm hg , benzene condenses at approximately 90 ° c ., which means the inlet cooling medium for both condensers 103 is at 80 ° c . however , if the inlet cooling medium temperature of one condenser 103 is raised by 10 ° c ., to 90 ° c ., maintaining the same approach temperature , benzene condenses at approximately 100 ° c ., 1050 mm hg in this condenser 103 . thus , this increase in pressure at the outlet saturated liquid benzene of this condenser 103 results in reduced pressure drop across the respective zone and hence , reduced split of vapor through the respective zone at the bottom of the partition . simultaneously , the inlet temperature of the cooling medium in the other condenser 103 may also have to be appropriately modified for achieving desired vapor splits . alternatively , the heat exchanger may be designed to be a submersible heat exchanger , whereby , submergence of the passage for the condensing fluid can be controlled to tailor the active area through which most of the heat transfer takes place . this will control the condensing temperature , and hence the pressure of the condensing fluid . the control of the pressure at the top of either of the zones z 1 or z 2 of the vertical partition will tailor the pressure drop across that zone , and hence the vapor flowrate through that zone . thus , the l - tc column offers an indirect control on the vapor split at the bottom of the vertical partition . interestingly , the tc - l and l - l columns have no vapor splits . the two reboilers 101 at c can be used to operate each section in the two parallel zones , on either side of the vertical partition , at the desired l / v ratios . it is worth noting that , in the case of the l - l column , the two parallel zones can be operated like two independent distillation columns , which may give the configuration more flexibility and freedom to operate . a dividing wall column somewhat similar to the l - l column in fig8 c can be found in fig5 ( a ) and 11 ( c ) of the paper by ho et al . their figures are exactly replicated herein in fig1 a and 10 b . from the tc - tc column of fig5 , they arrive at the dividing wall columns of fig1 a and 10 b by fictitiously extending the vertical partition to the top and bottom of the distillation column . they represent these fictitious extensions to the vertical partition in their paper by dotted lines as shown in fig1 a and 10 b . thus , in the dividing wall columns of fig1 a and 10 b , only the part of the vertical partition that also exists in the tc - tc column of fig5 is real and the rest is fictitious and does not exist . likewise , in their dividing wall column , the liquid transfers of submixtures ab and bc are also fictitious . they use such a fictitious dividing wall column concept only to develop certain analytical expressions , and hence the improved operability of our columns is neither identified nor applicable here . the l - tc and tc - l columns use one more heat exchanger , and the l - l column uses two more than the tc - tc column . arrangements can be made to each dividing wall column of fig8 to reduce the total number of heat exchangers to two . one possible arrangement of the l - tc , tc - l and l - l columns with one reboiler 101 and condenser 103 is shown in fig1 a , 11 b , and 11 c . in the l - tc column of fig1 a , cooling utility of sufficiently low temperature is used as a common condensing medium to simultaneously condense pure a vapor collected from both the parallel zones , z 1 and z 2 of the vertical partition . to achieve this , the heat exchanger has two separate passages for the vapor collected from the two zones . the condenser heat exchanger is designed so that the condensing fluid in each of the passages can achieve its own desired approach temperature to the cooling medium temperature . this can be implemented in several possible ways . each passage can be designed with different active surface area to tailor the approach temperature . alternatively , the passage for the cooling medium can also be divided into two . the flowrate and inlet temperature of the cooling medium for each of the passages may be independently controlled to allow for differences in the temperature of the condensing fluids . likewise , in the tc - l column of fig1 b , pure c liquid collected from the two parallel zones , z 1 and z 2 of the vertical partition , is fed to two separate passages in the reboiler 101 . a common heating medium of sufficiently high temperature is used to simultaneously vaporize the liquid in the two passages . similar to the condenser heat exchanger for l - tc column , the vapor boilup rate in each of the passages of the reboiler 101 can be controlled to provide the desired split of vapor flow between the vertical partition &# 39 ; s zones z 1 and z 2 . in the l - l column of fig1 c , the condenser 103 and reboiler 101 arrangements , respectively of fig1 a and 11 b , are used . fig1 a and 12 b and 12 c show an alternate arrangement for the l - tc , tc - l and l - l columns with one reboiler 101 and condenser 103 . in the l - tc column of fig1 a , a throttling valve is provided in the vapor line leaving the vertical partition &# 39 ; s zone z 1 ( assuming that the top of zone z 1 is at a higher pressure than the top of zone z 2 ) to reduce the pressure of the vapor to that leaving the vertical partition &# 39 ; s zone z 2 . the combined vapor is condensed in a single heat exchanger . a part of the condensed pure liquid a is withdrawn as product , while the rest is used as reflux to the two zones . the reflux to zone z 1 of the vertical partition is pumped . alternatively , the condenser heat exchanger could be located at such a height that liquid reflux to the vertical partition &# 39 ; s zone z 1 could be fed under gravitational head . in the tc - l column of fig1 b , a pump is provided in the liquid line leaving the vertical partition &# 39 ; s zone z 2 ( assuming that the bottom of zone z 2 is at a lower pressure than the bottom of zone z 1 ) to increase the pressure of the liquid to that leaving the vertical partition &# 39 ; s zone z 1 . the combined liquid is boiled in the reboiler 101 , and used for boil - up to the two zones . a throttling valve is used in the vapor line entering zone z 2 of the vertical partition for reducing the pressure . alternatively , the bottom of the column with respect to the reboiler 101 inlet could be located at such a height to allow liquid drain from zone z 2 of the vertical partition via gravitational head without the use of a pump . the l - l column of fig1 c uses the condenser 103 and reboiler 101 arrangements of fig1 a and 12 b respectively . in fig1 a , 12 b , and 12 c , for the purpose of illustration , the throttling valves and pumps are shown before / after streams that enter / leave one of the two parallel zones . in general , depending on the pressure in the two parallel zones of the dividing wall column , the pump and the throttling valves may be switched between either zone . furthermore , the configurations shown in fig1 a , 12 b , and 12 c can be suitably modified to use compressors ( either in combination with pumps or independently ), resulting in alternate single reboiler 101 and condenser 103 arrangements . under the assumptions of ideal mixtures and constant molar flow conditions , through modeling and extensive computation , we have observed that the total minimum vapor requirement for the tc - tc , l - tc , tc - l and l - l distillation columns are equal . it is also observed for a number of feed conditions that the range of ratio of splits of vapor in zones z 1 and z 2 of the vertical partition are also equal in all the four distillation columns . there is yet another flexibility of the l - tc , tc - l and l - l columns of fig8 , which is missing from the tc - tc column of fig3 . once physically built , they also allow operation in the side rectifier and side stripper modes . for example , in the l - l column , if no liquid bc is transferred across the vertical partition , with only the liquid ab transfer , the column could produce b from the bottom of the vertical partition &# 39 ; s zone z 2 , as shown in fig1 a . in this case , no b may be produced from an intermediate location of the vertical partition &# 39 ; s zone z 2 . this will be analogous to the operation of a side stripper . in an alternate case , as shown in fig1 b , where liquid bc is transferred but no liquid ab is transferred , b could be produced from the top of the vertical partition &# 39 ; s zone z 2 of the l - l column , leading to the operation similar to a side rectifier . thus , a l - l column , once built , can be operated as a fully thermally coupled column / side rectifier / side stripper . similarly , without the liquid ab transfer across the vertical partition , the l - tc column could be operated in the side rectifier mode of fig4 a , while the tc - l column , without the liquid bc transfer , could be operated in the side stripper mode of fig4 b . this added flexibility can be quite advantageous , as , for certain feed conditions , a side rectifier or a side stripper may be thermodynamically more efficient than the fully thermally coupled tc - tc configuration . conversely , a dividing wall column already built on a plant to operate in the side rectifier / side stripper mode can be suitably modified to operate as a l - tc / tc - l column respectively . generally , the overall cost saving from dividing wall columns significantly increases with the number of components in the feed . in this section , it is demonstrated that the disclosed method can also be easily used to draw new more operable dividing wall columns for the distillation of feeds containing more than three components . the method is illustrated by drawing new , more operable , standalone dividing wall columns that separate a four - component feed into four pure products . the focus will be on the dividing wall columns derived from the fully thermally coupled configuration . similar dividing wall columns for higher number of components can be easily drawn . the fully thermally coupled four - component configuration is shown in fig1 a . the equivalent three - column configuration with all liquid transfers and only one vapor transfer between distillation columns , as shown in fig1 b , was suggested by agrawal . the two configurations , in terms of minimum heat duty requirement , are expected to be equivalent . a dividing wall version of the configuration in fig1 b is shown in fig1 a . the only vapor split present in this dividing wall column at the intermediate location of submixture bc , can be controlled by the condensers 103 and reboilers 101 at a and d respectively , as discussed earlier for the ternary feeds . a simplified version of the dividing wall column in fig1 a is shown in fig1 b , with three parallel zones and only liquid splits . the dividing wall column in fig1 b differs from that in fig1 a only due to the mode of transfer of submixture bc . we expect the minimum heat duty requirement of the dividing wall column in fig1 b to be comparable to that in fig1 a . each zone in fig1 b may be operated akin to a separate distillation column . in contrast to the one and zero vapor splits in fig1 a and 15 b , the dividing wall version of the four - component fully thermally coupled column ( not shown here ) has three vapor splits , all of which are unregulated during operation . at least eighteen variants of the dividing wall column of fig1 b ( or 15 a ) can be drawn by introducing thermal coupling links at different submixtures . four such more operable variants are shown in fig1 . while the dividing wall columns of fig1 a and 16 b have no vapor splits , the dividing wall columns of fig1 c and 16 d respectively have one and two . the vapor splits in the fig1 c and 16 d can be controlled by the condensers 103 at a . furthermore , the dividing wall column of fig1 d is equivalent to the one shown in fig1 . the two dividing wall columns of fig1 and 16 d differ only due to the location where the feed mixture is fed to the dividing wall column , however the sequence of separations in both the columns is the same . similarly , the dividing wall columns referred to in this patent application could also refer to any of their equivalent dividing wall columns . while we have shown the more operable dividing wall columns derived from the fully thermally coupled configuration , other such more operable dividing wall columns derived from the various well - known four component configurations can be drawn , which may offer other benefits . an example dividing wall column , similar in skeleton structure to the one in fig1 d , but equivalent to the four - component satellite column is shown in fig1 . a feature of this dividing wall column , that is absent in the rest of the dividing wall columns introduced so far , is that the intermediate volatility products b and c are produced from the intermediate locations of two different partitioned zones . furthermore , as shown earlier for three - component dividing wall columns in fig1 a - 11 c and 12 a - 12 c , the total number of condenser and reboiler heat exchangers in each of the presented four - component dividing wall columns can be reduced . in this work , we focused on the new , more operable dividing wall columns derived primarily from the fully thermally coupled multicomponent configurations . however , using the concept proposed by agrawal , any thermal coupling link can be converted to a liquid only transfer . such a liquid transfer can be incorporated in a dividing wall column as explained in this paper . furthermore , our proposed method can also be easily applied to feeds containing more than four components . application of the new n - component dividing wall structures to feeds with more than n - components : the disclosed new n - component skeleton dividing wall structures presented earlier can be easily adapted to separate a multicomponent feed containing more than n components . in such cases , product streams enriched in different components will be produced . however , the possible product streams and the number of operating modes increase rapidly with the number of components in the feed . any of these operating modes can be included within a larger flowsheet that separates multicomponent mixtures into component product streams . we will first illustrate the adaptation of the various operating modes of the l - tc , tc - l and l - l columns , originally drawn for the distillation of a ternary feed , to a quaternary feed mixture , abcd . then , as a generalization of our approach , a quinary mixture will be distilled using one of our quaternary skeleton dividing wall structures . the l - tc , tc - l and l - l columns have two submixture transfers from intermediate locations , one above the feed and the other below the feed ( ab and bc in the earlier studied three - component case ). when a quaternary feed mixture abcd is distilled in these columns , there are two possible submixtures , abc or ab , which could be transferred from an intermediate location above the feed . similarly , from an intermediate location below the feed , the two possible submixture transfers are bcd or bc . this implies that , for each of the three vertical partitioned columns , we have four possible combinations of the two submixtures . fig1 ( a ) - 19 ( l ) shows these combinations . some interesting observations can be made from fig1 ( a )- 19 ( l ) . when compared to the tc - tc column for separating abcd ( not shown ), the l - tc , tc - l and l - l columns of fig1 ( a )- 19 ( l ) , apart from better vapor split control , offer an additional flexibility to produce two different products from the top or / and bottom of the column . for example , in the l - tc and l - l columns of fig1 ( a ) and 19 ( c ) , stream a can be produced as a product from the top of one zone , while stream ab may be produced as a product from the top of the other . similarly , in tc - l and l - l columns of fig1 ( b ) and 19 ( c ) , one has an option to produce stream d from the bottom of one zone , and cd from the bottom of the other zone . furthermore , in some of the dividing wall columns of fig1 ( a )- 19 ( l ) , one sidedraw stream may be withdrawn from zone z 2 , if desirable , instead of two . an interesting case emerges in fig1 ( j ) through 19 ( l ) , where all the products may be produced with high purity . the sequence of component splits / separations shown in fig1 ( j ), 19 ( k ) and 19 ( l ) , using the tc - tc column , has been known in the past . the use of our new l - tc , tc - l and l - l columns instead , allows for a better control of vapor flow on each side of the vertical partition . this makes it easy to control the production of pure b and c product streams from an operating plant . also , the column may now be operated closer to its designed optimal heat duty . based on the observations made for quaternary mixtures , the various operating modes of the l - l column to separate a quinary mixture are shown in fig2 ( a )- 20 ( h ) . some of the intermediate withdrawal streams from zone z 2 of these distillation columns may be eliminated , if desired . it is clear that the concept can also be applied to l - tc and tc - l columns . the dividing wall columns of fig2 can be further extended to produce streams of pure products . as an example , extensions to dividing wall columns of fig2 ( b ) and 20 ( e ) to produce streams of pure products are shown in fig2 ( a ) and 21 ( b ) . these dividing wall columns have been obtained by adding an extra distillation zone to those in fig2 ( b ) and 20 ( e ) . the intermediate volatility components , b , c and d are produced from the ends of the new zone in these dividing wall columns . interestingly , the skeleton partition structure with three parallel zones in the dividing wall columns of fig2 ( a ) and 21 ( b ) is the same as that shown in fig1 ( b ) , a standalone dividing wall column for separating a quaternary feed mixture . this generalizes our concept of using the n - component skeleton dividing wall partition structure , and adapting it for the distillation of a mixture with more than n components . 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