Patent Publication Number: US-2022227688-A1

Title: Hexane as a by-product of isomerization unit using a dividing wall column

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage application of International Application No. PCT/IB2020/000379, filed May 14, 2020, which claims priority to U.S. Provisional Patent Application No. 62/848,217 filed on May 15, 2019, the contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Field of the Invention 
     High-purity hexane is a light distillate product with a very narrow boiling range. It is used as a solvent in vegetable oil extraction processes, polymer processes, and in the drug and pharmaceutical industries. A special boiling point (“SBP”) product, usually consisting of hydrocarbons with between 5 and 10 carbon atoms and having a distillation range between 55 and 155° C., is also a light distillate used in the paint industry. 
     Background Information 
     Traditionally, hexane and SBP product are produced by a solvent extraction process.  FIG. 1  illustrates a conventional solvent extraction process flow scheme  100  for producing hexane and SBP by carrying out extraction of naphtha cut (initial boiling point “IBP”—140° C.) using a solvent. Solvent and naphtha are fed to an extraction column  102 . The solvent selectively extracts aromatics from the naphtha, producing a low aromatics content stream called raffinate. Raffinate from extraction column  102  is fed to a raffinate wash column  104  where the raffinate is water washed to remove traces of solvent from the raffinate. The dearomatized naphtha, so obtained, is then treated in a mercaptan removal unit  106  to meet a sulfur specification. The dearomatized naphtha is then fractionated in a series of three splitter columns  108 ,  110 ,  112  to produce the desired hexane and SBP cuts. While the process of  FIG. 1  does produce hexane, the quality is inferior. For example, the benzene and sulfur content of hexane produced by the solvent extraction process of  FIG. 1  is high (max. 500 ppm wt. and 5 ppm wt. respectively). 
     Processing via isomerization saturates benzene and upgrades the octane of the light naphtha fraction (&lt;80° C. boiling point).  FIG. 2  illustrates a conventional isomerization process flow scheme  200 . As shown in  FIG. 2 , feed and recycle gas  202  from a recycle gas compressor (RGC)  204 , is preheated in a reactor feed-effluent exchanger  206  to a desired temperature before being routed to a series of reactors, for example reactor one (“RI”)  208  and reactor two (“RII” 210), in which saturation of aromatics and conversion of normal paraffins to iso-paraffins takes place. Gas and liquid in the reactor effluent are separated in a product separator  212 . Gas  214  from the product separator  212  is recycled back, using the RGC  204 , to a reaction section after adding makeup hydrogen by using a makeup gas (MUG) compressor  216 . Liquid, from the product separator  212 , is routed to a stabilizer  218  for stabilization by removal of gas and liquefied petroleum gas (LPG) from the liquid. The stabilized isomerate is then split in a De-Iso-Hexanizer (DIH) column  220  to produce isomerate, meeting the specification of octane etc. Isomerate, thus produced, is a blend component of the refinery gasoline pool. The isomerization process flow scheme  200  produces only isomerate as the desired product. 
     SUMMARY 
     The present invention relates to a dividing wall column system for producing hexane, the dividing wall column system comprising: 
     a dividing wall column comprising a dividing wall that divides the dividing wall column at least partially into a first side and a second side, with one of the first and second sides preferably configured to operate as a deisohexanizer column and the other side of the first and second side preferably configured to operate as a hexane column to produce hexane. 
     Preferably, during operation of the dividing wall column system, the production of high-purity hexane involves taking a narrow cut of a hexane rich stream (32-45 wt % n-C 6 ). Since the feed consists of many components (e.g., C 5 , C 6  paraffins, C 6  isoparaffins and C 6  naphthenes) with similar relative volatility, the process to produce high-purity hexane is quite energy intensive. The use of dividing wall column (“DWC”) technology significantly improves the viability of this process by enabling the separation to take place in the same column shell by avoiding back mixing of the heaviest components with the middle boiling components. Due to the segregation of the column, an adequate number of trays are available on each side to facilitate an efficient separation of the components. As compared to a two column process scheme, the DWC scheme requires less energy and less equipment for the same separation. Hence, a DWC improves profitability for high-purity hexane production. In particular, the operating costs are about 20 to 70% lower than those of a solvent extraction process, such as one shown in  FIG. 1 . 
     In accordance with a particular preferred embodiment of the present invention, the dividing wall column system further comprises a hexane polishing unit connected with the dividing wall column so that the hexane produced in the dividing wall column is transferred into the hexane polishing unit, wherein the hexane polishing unit comprises a hexane polishing reactor for hydrogenating at least a part of the benzene included in the produced hexane. A hexane polishing unit means in accordance with the present invention any unit being configured to hydrogenate at least a part of the benzene included in the hexane being produced in the dividing wall column so as to reduce the benzene content of the hexane to a desired value. Thus, the hexane polishing unit can also be designated as benzene saturation unit or benzene hydrogenation unit. The hydrogenation itself takes place in the hexane polishing reactor, whereas the hexane polishing unit preferably comprises in addition to the hexane polishing reactor devices for feeding hydrogen into the unit, for mixing the hydrogen with the hexane being produced in the dividing wall column, for preheating this mixture before being fed into the hexane polishing reactor and the like. 
     In view of this, it is preferred that the hexane polishing unit further comprises a mixer for mixing the hexane being produced in the dividing wall column and hydrogen, wherein the mixer is arranged upstream of the hexane polishing reactor. 
     In a further development of embodiments of the present invention, it is proposed that the hexane polishing unit further comprises a stripper column for separating lights from the hexane, wherein the stripper column is preferably arranged downstream of the hexane polishing reactor. 
     In order to adjust an optimal temperature for the hexane/hydrogen-mixture introduced into the hexane polishing reactor, it is proposed that the hexane polishing unit further comprises one or more heat exchangers. 
     In accordance with a further preferred embodiment of the present invention, the dividing wall of the dividing wall column does not extend over the whole height of the dividing wall column, so that the dividing wall column comprises a portion with the first side and the second side being divided by the dividing wall arranged therebetween, and one or two further portions being not divided by the dividing wall. Each of the first side and the second side preferably comprises, independently from each other, 10 to 70 theoretical stages, wherein the one or two further portions preferably comprise in sum 10 to 50 theoretical stages. Preferably, the dividing wall is arranged vertically in the dividing wall column. 
     More preferably, each of the first side and the second side comprises, independently from each other, 30 to 60 theoretical stages and the one or two further portions comprise in sum 30 to 40 theoretical stages. 
     In accordance with one embodiment of the present invention, the number of theoretical stages of the first side and the number of theoretical stages of the second side are the same. 
     In accordance with an alternative embodiment of the present invention, the number of theoretical stages of the first side and the number of theoretical stages of the second side are the different. It is preferred in this embodiment that one of the first and second side has 20 to 30 theoretical stages more than the other of the two sides. For instance, good results are obtained, when the first side has 10 to 80 theoretical stages and the right side has 10 to 40 theoretical stages or vice versa. 
     The dividing wall can be located anywhere in the column, such as, for example, in a top section ( FIG. 6 ), a bottom section ( FIG. 7 ), or a middle section ( FIG. 8 ). During operation, the process uses the DWC to produce a light isomerate fraction and a heavy isomerate fraction as the top and bottom products, respectively. A high-purity n-C 6  product can be withdrawn as the middle cut. As set out above, the process produces similar product specifications with much lower energy costs as compared to a two-column distillation process scheme for the same feed. 
     In accordance with a first particular preferred embodiment of the present invention, the dividing wall column is a DWC with a top dividing wall configuration, i.e. the dividing wall is a (preferably vertically arranged) top dividing wall positioned in the top portion of the dividing wall column, with the first side being a first fractionation section and the second side being a second fractionation section. Due to the presence of a dividing wall in the top of the column, the top portion of the column has two independent rectifying sections (i.e., on either side of the dividing wall) with a common stripping section. Each rectifying section is preferably equipped with an independent overhead system. Similarly, the stripping section (i.e., the bottom section below the dividing wall) is preferably equipped with a thermosiphon reboiler. The invention is similar in operation to a two-column separation sequence comprising a DIH column and a hexane column in which high-purity hexane is the middle cut. In the rectifying zone on the feed side, due to pre-fractionation the lightest boiling components are collected as lights at the top. The middle boiling components (mainly a mix of C 6s ) and the heaviest components (mainly i-C 6  and heavier) move towards the bottom of the column. The heating duty provided by the reboiler helps to move the middle boiling components up the other side of the top dividing wall. The middle boiling components are eventually concentrated at the top on this side of the DWC., Preferably, the DWC that includes a top wall, the DWC has two independent overhead systems, one on either side. 
     Moreover, it is preferred that the dividing wall is placed in this embodiment between the first and second side, seen from top to bottom of the dividing wall column, between the theoretical stage  1  (i.e. the uppermost part of the dividing wall column, which is the first stage of the dividing wall column) and the theoretical stage  100  of the dividing wall column in the first side, i.e. the dividing wall extends from the top of the dividing wall column to the last theoretical stage of the first side, which is preferably theoretical stage  100  seen from the first side of the dividing wall column. If the second side of the dividing wall column comprises the same number of theoretical stages as the first side, this corresponds to the theoretical stage  100  seen from the second side of the dividing wall column. However, if the second side of the dividing wall column comprises a different number of theoretical stages as the first side, this corresponds to whatever is the last theoretical stage of the second side. 
     Preferably, the feed is placed in the top column embodiment on the first side between the theoretical stage  20  and the theoretical stage  60  of the first side. Since the dividing wall extends from the uppermost part of the dividing wall column until the last theoretical stage of the first side, this corresponds to between the theoretical stage  20  and the theoretical stage  60  of the dividing wall column. 
     Moreover, it is preferred in this embodiment that the dividing wall column system further comprises a first overhead condenser in fluid communication with the first fractionation section and a second overhead condenser in fluid communication with the second fractionation section. 
     It is also preferred that the dividing wall column system further comprises a reboiler in fluid communication with the bottoms section of the dividing wall column. 
     In accordance with a second particular preferred embodiment of the present invention, the dividing wall column is a DWC with a bottom dividing wall, i.e. the dividing wall is a (preferably vertically arranged) bottom dividing wall positioned in the bottom portion of the dividing wall column, with the first side being a first fractionation section and the second side being a second fractionation section. Unlike a DWC with a top dividing wall, a DWC with a bottom dividing wall has two independent stripping sections (i.e., on either side of the dividing wall) with a common rectifying section (i.e., the top section above the dividing wall). The feed is introduced on a pre-fractionation side of the DWC, wherein the heaviest boiling components are separated at a bottom of the DWC. The lightest boiling components are recovered at a top of the DWC. The middle boiling components are concentrated at the bottom on the other stripping section and removed as a separate product. Preferably, in the DWC with a bottom dividing wall, each stripping section is equipped with a thermosiphon reboiler. The DWC has a common rectifying zone with a single overhead system. 
     Preferably, the dividing wall is placed in this embodiment between the first and second side, seen from top to bottom of the dividing wall column, between the theoretical stage  20  and the last theoretical stage (i.e. the lowermost part) of the dividing wall column. 
     It is further preferred that the dividing wall column system further comprises a first reboiler in fluid communication with the first fractionation section, and a second reboiler in fluid communication with the second fractionation section. Good results are in particular obtained, when the first reboiler is a thermosiphon reboiler and/or the second reboiler is a thermosiphon reboiler. 
     In addition, it is preferred that the dividing wall column system further comprises a common rectifying section in fluid communication with the top portion of the dividing wall column. 
     In accordance with a third particular preferred embodiment of the present invention, the dividing wall column is a DWC with a middle dividing wall configuration, i.e. the dividing wall is a (preferably vertically arranged) middle dividing wall positioned in the middle portion of the dividing wall column, with the first side being a first fractionation section and the second side being a second fractionation section. Due to the presence of a dividing wall in the middle of the column, the middle of the column has two independent sections (i.e., on either side of the dividing wall) with common top and bottom sections (i.e., the sections above and below the middle dividing wall). The top and bottom sections are equipped with an overhead system and a reboiler, respectively. 
     Preferably, the dividing wall is placed in this embodiment between the first and second side, seen from top to bottom of the dividing wall column, between the theoretical stage  20  of the dividing wall column and the last theoretical stage of the first side of the dividing wall column. 
     Moreover, it is preferred that the feed is placed in this embodiment on the first side between the theoretical stage  30  of the dividing wall column and the theoretical stage  60  of the dividing wall column. The hexane stream outlet is preferably placed on the second side between the theoretical stage  40  of the dividing wall column and the theoretical stage  60  of the dividing wall column. 
     Good results are in particular obtained, when the dividing wall column system further comprises a thermosiphon reboiler configured to receive a bottoms product from the dividing wall column and an overhead system configured to receive a lights product from the dividing wall column. 
     In accordance with a further aspect, the present invention relates to a method of producing hexane, wherein the method comprises producing, as a byproduct from a C 5/6 -isomerization unit, hexane using a dividing wall column system as described above. Byproduct means in this connection less than 20 vol.-% of the sum of all product streams. 
     In particular, it is preferred that embodiments of the method comprise the steps of feeding a stable isomerate feed to the first side of the dividing wall column; of producing a hexane feed from the second side of the dividing wall column; feeding the hexane feed to the hexane polishing unit. More specifically, the hexane feed and hydrogen are preferably fed to a mixer of the hexane polishing unit to form a hexane-hydrogen mixture. 
     Preferably, the feed is the isomerate stream produced in an isomerization unit and preferably a C 5 -C 6 -isomerization unit. The feed can e.g. comprise mainly C 4 -C 7 -hydrocarbons, such as C 4 -C 7 -n-paraffins, isoparaffins, naphthenes and aromatics. However, it can also be a narrow C 5 -C 6 -cut, such as one comprising C 5 -C 6 -n-paraffins, isoparaffins, naphthenes and aromatics. 
     In accordance with a further preferred embodiment of the present invention, the method further comprises the steps of preheating the hexane-hydrogen mixture; and of feeding the preheated hexane-hydrogen mixture to a polishing reactor of the hexane polishing unit. 
     Good results are in particular obtained, when the method further comprises the step of feeding an output stream from the polishing reactor to a stripper column of the hexane polishing unit for separating lights from the hexane, wherein the stripper column is preferably arranged downstream of the hexane polishing reactor. 
     In order to further increase the efficiency of the method, it is proposed in a further development of the present invention that in the method the preheating comprises the steps of exchanging heat between the hexane-hydrogen mixture and the feed from the stripper column in a first heat exchanger; and exchanging heat between the hexane-hydrogen mixture and the output stream from the polishing reactor in a second heat exchanger. 
     An exemplary system and process, according to embodiments of the disclosure, is directed to the production of SBP (55-115° C.) as a blend of the side cut of the DWC with light isomerate and a heavier naphtha cut from the isomerization feed splitter. 
     An exemplary system and process, according to embodiments of the disclosure, is directed to the variable operating cost for the production of hexane, as a byproduct, from an isomerization unit. The operating cost, per ton of hexane produced, from isomerization unit is 20 to 70% or even 90% lower than the cost of hexane produced by a solvent extraction process, thereby appreciably reducing CO2 emissions. Moreover, preferably the hexane has an improved quality (such as improved with respect to sulfur, benzene and n-hexane content) compared to a quality of hexane produced by a solvent extraction process and/or the hexane meets specifications for use in food, pharmaceutical, and polymer processes. 
     In all embodiments of the disclosure, a column overhead pressure on either side of the dividing wall of the DWC is maintained via a pressure controller on the respective overhead vapor product lines. The overhead vapor (on each side in a top DWC) is condensed using air-cooled exchangers and collected in an overhead receiver. 
     An exemplary system and process, according to embodiments of the disclosure, includes a DWC with a top dividing wall. The two top halves on either side of the top dividing wall of the DWC receive reflux from respective overhead condensers. Preferably, the temperature in the top of the DWC is cascaded to a reflux flow control loop to allow control over a quality of the product. This control philosophy prevents the heavier components from going to the top of the DWC. Similarly, the heaviest bottom product flow rate (on each side in a bottom DWC) is controlled by cascading with a level control loop in the lower section. 
     An exemplary system and process, according to embodiments of the disclosure, is directed toward production of high purity hexane as a byproduct from an isomerization unit. Stabilized isomerate is split in the DWC to co-produce hexane from the second side of the column. The hexane, so produced, is treated in a polishing unit comprising of an adsorbent section and a stripper section. Hexane produced from the isomerization unit has a benzene content of &lt;3 ppm wt., a sulfur content of &lt;0.5 ppm wt. (passing the test for poly cyclic aromatics (PCA)), and an n-hexane content of &gt;40%. The quality of hexane produced, as a byproduct from isomerization unit, is much superior to that produced by the traditional solvent extraction process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail hereinafter with reference to the drawings. 
         FIG. 1  represents a process flow scheme for production of hexane in accordance with the prior art; 
         FIG. 2  represents the process flow scheme of an isomerization unit for the production of only isomerate as the desired product in accordance with the prior art; 
         FIG. 3  represents the conventional process scheme of a high-purity hexane column in conjunction with a deisohexanizer in accordance with the prior art; 
         FIG. 4  represents the concentration profile inside a conventional deisohexanizer column in accordance with the prior art; 
         FIG. 5  is a graph illustrating a concentration profile inside a conventional hexane column in accordance with the prior art; 
         FIG. 6  illustrates a process scheme using a DWC with top dividing wall in accordance with embodiments of the disclosure; 
         FIG. 7  illustrates a process scheme using a DWC with a bottom dividing wall in accordance with embodiments of the disclosure; 
         FIG. 8  illustrates a process scheme using a DWC with a middle dividing wall in accordance with embodiments of the disclosure; and 
         FIG. 9  illustrates a process scheme for a hexane polishing unit in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     In the instant disclosure, an isomerization unit deisohexanizer (“DIH”) column is a dividing wall column (DWC) and is used to produce hexane, as a byproduct, along with the main product of isomerate. An n-hexane rich product (about 32-45 wt % n-C 6 ) is obtained from the isomerization unit DIH column. Other C 6  components (e.g., 2-methylpentane, 3-methylpentane and methylcyclopentane) make up the rest of the product. Besides high-purity n-hexane, the other products from the isomerization unit column are light isomerate (mainly i-C 5 ) and heavy isomerate (mainly i-C 6 ). 
     Conventionally, high-purity hexane can be obtained by distillation in a deisohexanizer column followed by a hexane column.  FIG. 3  illustrates a prior art system  300  for producing high-purity hexane. System  300  includes a DIH column  302  and a hexane column  304 . Bottoms from DIH column  302  are fed to hexane column  304 . Hexane column  304  produces high-purity hexane and heavy isomerate. System  300  has certain disadvantages. For example, the boiling points of C 6  components are very close to that of the non-desirable components (e.g., C 5  paraffins and naphthenes). To obtain good separation, the process of system  300  requires a significant number of trays (leading to a bigger column) as well as high reboiling energy. 
     Systems with two columns also have the problem of back-mixing of a concentrated hexane stream within the DIH column. Thus, the energy spent in concentrating the hexane stream to higher purity levels is lost due to the back-mixing of hexane with the heavy isomerate at the bottom of the column. The concentration profiles of light isomerate, hexane, and heavy isomerate fractions in the DIH column are shown in  FIG. 4 . Additional energy is spent in the hexane column (see  FIG. 5 ) to separate the hexane from the heavy isomerate, thereby reducing an overall energy efficiency of the process. 
     A solution to this thermodynamic problem is to separate the hexane from the heavy isomerate at the peak of its concentration within DIH column  302  to optimize an energy requirement of system  300 . Furthermore, since two columns are required for the process, capital costs increase due to additional equipment and bigger plot space. For such applications, a DWC concept can be applied to provide an alternative solution. 
     A DWC combines operations of the two columns (e.g., DIH column  302  and hexane column  304 ) into a single column thereby lowering both the capital and energy (operating) costs by approximately 20-30%. In general, dividing wall columns are broadly classified into three types based on the location of a wall disposed with the DWC. The wall can be located in top section, a middle section, or a bottom section. In a DWC scheme, three (or four) products are typically withdrawn from the DWC: a lightest cut and a heaviest cut are withdrawn at the top and bottom, respectively, of the DWC; and a middle cut is obtained from the DWC as a side cut. In a majority of DWCs in operation worldwide, the dividing wall is present in a middle section of the DWC. In DWCs, a location of the dividing wall primarily dictates the movement of vapor within the column and can affect a quality of the separation. The dividing wall in a DWC leads to the splitting of the top (or bottom or middle) half of the column into two separate columns, which produces two high-purity products at the top (or bottom or middle). Top, bottom, and middle dividing walls are shown in  FIGS. 6, 7, and 8 , respectively. A feed (e.g., stable isomerate) is introduced on one side of the dividing wall (pre-fractionation) and the side cut is withdrawn from the other side (main fractionation). The process scheme is similar to that of a direct or indirect sequence of a two-column conventional separation. 
     The systems of  FIGS. 6-8  have several advantages. For example,  FIG. 6  illustrates a top DWC system  600  that includes a top DWC  602 . A top dividing wall  604  divides a top section  606  of top DWC  602  into a first side  608  and a second side  610 . Top dividing wall  604  extends from a top of top DWC  602  and terminates above a bottoms section of DWC  602 . By incorporating top dividing wall  604  at the top of top DWC  602 , first side  608  and second side  610  remain isolated from one another with no chance of contamination or back-mixing. Because first side  608  and second side  610  are two parallel sections created in a single column, a higher number of trays are available to achieve better fractionation within the same column. This tends to reduce the final height of the column by lowering the number of trays required. Lastly, first side  608  and second side  610  operate independent of each other. One side can operate as a rectification section while the other side can operate as the absorption (or rectification) section, with independent controls on each side. In this type of DWC, there are two separate overhead systems, a first overhead system  612  and a second overhead system  614 . Each overhead system  612 ,  614  can include, for example, a heat exchanger (e.g., an air-cooled heat exchanger) and an overhead receiver. 
     First side  608  of top DWC  602  receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Top DWC  602  outputs light isomerate as a lights product from first side  608  and high-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) as a lights product from second side  610 . A portion of the light isomerate can be returned to first side  608  as reflux and the remainder can be collected as a portion of total isomerate produced by top DWC system  600 . Top DWC  602  also outputs heavy isomerate as a bottoms product. A portion of the bottoms product can be returned to top DWC  602  after passing through a reboiler  616  and the remainder can be output with the remainder of light isomerate as the other portion of the total isomerate output by top DWC  602 . The heating duty provided by the reboiler helps to move the middle boiling components up the other side of the top dividing wall. 
       FIG. 7  illustrates a bottom DWC system  700  that includes a bottom DWC  702 . Bottom DWC  702  works on a similar principle as that of top DWC  602  and includes a bottom dividing wall  704 . Bottom dividing wall  704  extends from a bottom of bottom DWC  702  and divides a bottom section  703  of the bottom DWC  702  into a first side  706  and a second side  708 . Compared to top DWC system  600 , bottom DWC  702  includes two bottom reboilers, a first reboiler  710  and a second reboiler  712 , and a common rectifying section  714 . Both sides  706 ,  708  in bottom DWC  702 , are controlled independent of each other. 
     First side  706  of bottom DWC  702  receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Bottom DWC  702  outputs light isomerate as a lights product from the top of bottom DWC  702  and high-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) as a bottoms product from second side  708 . A portion of the light isomerate can be returned to the top of bottom DWC  702  as reflux from common rectifying section  714  and the remainder can be collected as a portion of total isomerate produced by bottom DWC system  700 . Bottom DWC  702  also outputs heavy isomerate as a bottoms product from first side  706 . A portion of the heavy isomerate can be returned to first side  706  after passing through first reboiler  710  and the remainder can be output with the remainder of light isomerate as the other portion of the total isomerate output by bottom DWC  702 . 
       FIG. 8  illustrates a middle DWC system  800  that includes a middle DWC  802 . Middle DWC  802  works on a similar principal as that of top DWC  602  and bottom DWC  702  and includes a middle dividing wall  804 . Middle dividing wall  804  extends the length of a middle portion  803  of middle DWC  802  and divides middle DWC  802  into a first side  806  and a second side  808  to pre-fractionate the feed and concentrate the middle boiling components on the second side  808  to produce a high-purity product. Middle dividing wall  804  does not extend all the way to the top or bottom of middle DWC  802 . Middle DWC system  800  also includes a reboiler  810  and a rectifying section  812 . As in the top and bottom DWCs  602 ,  702 , there is no back-mixing of the feed and the side cut. This results in an efficient separation with lesser consumption of reboiling duty. 
     First side  806  of middle DWC  802  receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Middle DWC  802  outputs light isomerate as a lights product from a top of middle DWC  802 . A portion of the light isomerate can be returned to the top of middle DWC  802  as reflux and the remainder is output as a portion of the total isomerate produced by middle DWC  802 . Middle DWC  802  outputs heavy isomerate as a bottoms product. A portion of the heavy isomerate can be returned to the bottom of middle DWC  802  after passing through reboiler  810  and the remainder is output as the other portion of the total isomerate produced by middle DWC  802 . High-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) is produced as a side cut from second side  808 . 
       FIG. 9  illustrates a system  900  for processing hexane in a polishing unit (polisher) comprising a polishing reactor  902 , a mixer  904 , three heat exchangers  906 ,  908 ,  910  and a hexane stripper column  912 , to ensure that benzene content of the hexane is in a desired range. In some embodiments, the hexane is fed from an isomerization unit DWC. In the polishing unit, hydrogen and raw hexane are preheated before being routed through an adsorbent. The hydrogen and raw hexane are mixed in the mixer  904  prior to the preheating. The preheating can include heating from one or more heat exchangers. For example, as illustrated in  FIG. 9 , preheating can be accomplished via heat exchangers  906 ,  908 , and  910 . Heat exchanger  906  uses an output from the hexane stripper column  912  to heat the hydrogen/raw hexane feed. Heat exchanger  908  uses an output of the polishing reactor  902  to heat the hydrogen/raw hexane feed. Heat exchanger  910  uses an additional heat source (e.g., an upstream feed) to heat the hydrogen/raw hexane feed. In various embodiments, one or more of heat exchangers  906 ,  908 , and  910  are optional. After adsorption, lights from polishing reactor  902  are separated from hexane in a hexane stripper column  912 . The hexane from hexane stripper column  912  can be used as a heat source in heat exchanger  906 , collected as an end product, or used as part of a downstream process. 
     The advantage of producing hexane, as a byproduct from isomerization unit, is that its quality is much superior to that produced by the traditional solvent extraction process. Hexane produced from isomerization unit meets specifications for food, pharmaceutical, and polymer grade hexanes. Additionally, the cost of production of hexane, as a byproduct from isomerization unit, is much lower than the cost of hexane produced by the solvent extraction process. 
     EXAMPLES 
     Tables 1-6 below demonstrate various operating parameters for conventional processes and systems and processes and systems of the instant disclosure that utilize DWCs. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Performance of Conventional Design versus Dividing Wall Column Design 
               
            
           
           
               
               
            
               
                   
                 DWC Design 
               
            
           
           
               
               
               
               
            
               
                   
                 Top 
                 Bottom 
                 Middle 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Conventional 
                 Dividing 
                 Dividing 
                 Dividing 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Items 
                   
                 DIH 
                 Hexane 
                 Wall 
                 Wall 
                 Wall 
               
               
                 Columns 
                 Units 
                 Column 
                 Column 
                 Column 
                 Column 
                 Column 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Feed 
                 kg/hr 
                 67,955 
                 67,955 
                 67,955 
                 67,955 
               
               
                 Light Isomerate 
                 kg/hr 
                 43,262 
                 42,474 
                 43,303 
                 43,237 
               
               
                 Heavy Isomerate 
                 kg/hr 
                 11,993 
                 12,862 
                 11,934 
                 12,106 
               
               
                 High-purity Hexane 
                 kg/hr 
                 12,700 
                 12,700 
                 12,700 
                 12,700 
               
               
                 n-Hexane content 
                 wt % 
                 39.4 
                 35.2 
                 38.2 
                 37.9 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Reboiler Duty 
                 MMkcal/hr 
                 13.5 
                 9.1 
                 16.4 
                 16.7 
                 16.4 
               
               
                 Condenser Duty 
                 MMkcal/hr 
                 16.8 
                 9.2 
                 19.8 
                 20.1 
                 19.7 
               
            
           
           
               
               
               
               
               
               
            
               
                 Energy Savings 
                 % 
                 — 
                 27.4 
                 26.1 
                 27.4 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Material Balance of Conventional DIH Colum + Hexane Column 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 High- 
                   
               
               
                 Stream 
                   
                   
                 Light 
                 purity 
                 Heavy 
               
               
                 Description 
                 Units 
                 Feed 
                 Isomerate 
                 Hexane 
                 Isomerate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Flowrate 
                 kg/hr 
                 67,955 
                 43,262 
                 12,700 
                 11,993 
               
               
                 Composition 
               
               
                 profile 
               
               
                 H2 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C3− 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C4 
                 wt. % 
                 0.29 
                 0.46 
                 0.00 
                 0.00 
               
               
                 Paraffins 
               
               
                 i-Pentane 
                 wt. % 
                 8.40 
                 13.19 
                 0.00 
                 0.00 
               
               
                 n-Pentane 
                 wt. % 
                 2.68 
                 4.21 
                 0.00 
                 0.00 
               
               
                 C5 
                 wt. % 
                 0.60 
                 0.94 
                 0.00 
                 0.00 
               
               
                 Naphthenes 
               
               
                 C6 
                 wt. % 
                 58.82 
                 80.44 
                 40.66 
                 0.06 
               
               
                 i-Paraffins 
               
               
                 Hexane 
                 wt. % 
                 8.24 
                 0.69 
                 39.36 
                 2.50 
               
               
                 C6 
                 wt. % 
                 16.08 
                 0.07 
                 19.97 
                 69.69 
               
               
                 Naphthenes 
               
               
                 Benzene 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C7 
                 wt. % 
                 2.22 
                 0.00 
                 0.00 
                 12.57 
               
               
                 Paraffins 
               
               
                 C7 
                 wt. % 
                 2.68 
                 0.00 
                 0.01 
                 15.17 
               
               
                 Naphthenes 
                   
                   
               
               
                 Total 
                 wt. % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
            
               
                 Reboiler 
                 MMkcal/ 
                 13.5 + 9.1 
               
               
                 Duty 
                 hr 
               
               
                 Condenser 
                 MMkcal/ 
                 16.8 + 9.2 
               
               
                 Duty 
                 hr 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Material Balance of Top Dividing Wall Column 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 High- 
                   
               
               
                 Stream 
                   
                   
                 Light 
                 purity 
                 Heavy 
               
               
                 Description 
                 Units 
                 Feed 
                 Isomerate 
                 Hexane 
                 Isomerate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Flowrate 
                 kg/hr 
                 67,955 
                 42,474 
                 12,700 
                 12,862 
               
               
                 Composition 
               
               
                 profile 
               
               
                 H2 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C3− 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C4 
                 wt. % 
                 0.29 
                 0.46 
                 0.00 
                 0.00 
               
               
                 Paraffins 
               
               
                 i-Pentane 
                 wt. % 
                 8.40 
                 13.44 
                 0.00 
                 0.00 
               
               
                 n-Pentane 
                 wt. % 
                 2.68 
                 4.29 
                 0.00 
                 0.00 
               
               
                 C5 
                 wt. % 
                 0.60 
                 0.96 
                 0.00 
                 0.00 
               
               
                 Naphthenes 
               
               
                 C6 
                 wt. % 
                 58.82 
                 80.35 
                 44.66 
                 0.48 
               
               
                 i-Paraffins 
               
               
                 Hexane 
                 wt. % 
                 8.24 
                 0.46 
                 35.21 
                 7.21 
               
               
                 C6 
                 wt. % 
                 16.08 
                 0.04 
                 20.13 
                 66.41 
               
               
                 Naphthenes 
               
               
                 Benzene 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C7 
                 wt. % 
                 2.22 
                 0.00 
                 0.00 
                 11.74 
               
               
                 Paraffins 
               
               
                 C7 
                 wt. % 
                 2.68 
                 0.00 
                 0.00 
                 14.16 
               
               
                 Naphthenes 
                   
                   
               
               
                 Total 
                 wt. % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
            
               
                 Reboiler 
                 MMkcal/ 
                 16.4 
               
               
                 Duty 
                 hr 
               
               
                 Condenser 
                 MMkcal/ 
                 17.0 + 2.8 
               
               
                 Duty 
                 hr 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Material Balance of Bottom Dividing Wall Column 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 High- 
                   
               
               
                 Stream 
                   
                   
                 Light 
                 purity 
                 Heavy 
               
               
                 Description 
                 Units 
                 Feed 
                 Isomerate 
                 Hexane 
                 Isomerate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Flowrate 
                 kg/hr 
                 67,955 
                 43,303 
                 12,700 
                 11,934 
               
               
                 Composition 
               
               
                 profile 
               
               
                 H2 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C3− 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C4 
                 wt. % 
                 0.29 
                 0.46 
                 0.00 
                 0.00 
               
               
                 Paraffins 
               
               
                 i-Pentane 
                 wt. % 
                 8.40 
                 13.28 
                 0.00 
                 0.00 
               
               
                 n-Pentane 
                 wt. % 
                 2.68 
                 4.25 
                 0.00 
                 0.00 
               
               
                 C5 
                 wt. % 
                 0.60 
                 0.95 
                 0.00 
                 0.00 
               
               
                 Naphthenes 
               
               
                 C6 
                 wt. % 
                 58.82 
                 80.33 
                 40.70 
                 0.07 
               
               
                 i-Paraffins 
               
               
                 Hexane 
                 wt. % 
                 8.24 
                 0.69 
                 38.22 
                 2.51 
               
               
                 C6 
                 wt. % 
                 16.08 
                 0.04 
                 21.08 
                 69.53 
               
               
                 Naphthenes 
               
               
                 Benzene 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C7 
                 wt. % 
                 2.22 
                 0.00 
                 0.00 
                 12.64 
               
               
                 Paraffins 
               
               
                 C7 
                 wt. % 
                 2.68 
                 0.00 
                 0.00 
                 15.25 
               
               
                 Naphthenes 
                   
                   
               
               
                 Total 
                 wt. % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
            
               
                 Reboiler 
                 MMkcal/ 
                 12.0 + 4.7 
               
               
                 Duty 
                 hr 
               
               
                 Condenser 
                 MMkcal/ 
                 20.1 
               
               
                 Duty 
                 hr 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Material Balance of Middle Dividing Wall Column 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 High- 
                   
               
               
                 Stream 
                   
                   
                 Light 
                 purity 
                 Heavy 
               
               
                 Description 
                 Units 
                 Feed 
                 Isomerate 
                 Hexane 
                 Isomerate 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Flowrate 
                 kg/hr 
                 67,955 
                 43,237 
                 12,700 
                 12,106 
               
               
                 Composition 
               
               
                 profile 
               
               
                 H2 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C3− 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C4 
                 wt. % 
                 0.29 
                 0.46 
                 0.00 
                 0.00 
               
               
                 Paraffins 
               
               
                 i-Pentane 
                 wt. % 
                 8.40 
                 13.20 
                 0.00 
                 0.00 
               
               
                 n-Pentane 
                 wt. % 
                 2.68 
                 4.21 
                 0.00 
                 0.00 
               
               
                 C5 
                 wt. % 
                 0.60 
                 0.94 
                 0.00 
                 0.00 
               
               
                 Naphthenes 
               
               
                 C6 
                 wt. % 
                 58.82 
                 80.40 
                 40.87 
                 0.07 
               
               
                 i-Paraffins 
               
               
                 Hexane 
                 wt. % 
                 8.24 
                 0.69 
                 37.94 
                 4.22 
               
               
                 C6 
                 wt. % 
                 16.08 
                 0.09 
                 20.68 
                 68.69 
               
               
                 Naphthenes 
               
               
                 Benzene 
                 wt. % 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 C7 
                 wt. % 
                 2.22 
                 0.00 
                 0.22 
                 12.25 
               
               
                 Paraffins 
               
               
                 C7 
                 wt. % 
                 2.68 
                 0.00 
                 0.28 
                 14.76 
               
               
                 Naphthenes 
                   
                   
               
               
                 Total 
                 wt. % 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
            
           
           
               
               
               
            
               
                 Reboiler 
                 MMkcal/ 
                 16.4 
               
               
                 Duty 
                 hr 
               
               
                 Condenser 
                 MMkcal/ 
                 19.7 
               
               
                 Duty 
                 hr 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Comparison of Quality of Hexane produced by the Solvent 
               
               
                 Extraction Process vs from Isomerization Unit 
               
            
           
           
               
               
               
            
               
                   
                 Solvent Extraction Process 
                 Isomerization Unit 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 n-hexane 
                 % wt. 
                   
                 &gt;40 
               
               
                 Sulfur 
                 mg/kg 
                 1.0-5.0 
                 &lt;0.5 
               
               
                 Benzene 
                 mg/kg 
                 130-240 
                 &lt;3.0 
               
               
                   
               
            
           
         
       
     
     The term “substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” “generally,” “around,” and “about” can be substituted with “within [a percentage] of” what is specified, where the percentage includes 0.1, 1, 5, and 10 percent. 
     The foregoing outlines features of several embodiments so that those skilled in the art can better understand the aspects of the disclosure. Those skilled in the art should appreciate that they can readily use the disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the disclosure, and that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the disclosure. The scope of the invention should be determined only by the language of the claims that follow. The term “comprising” within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open group. The terms “a,” “an” and other singular terms are intended to include the plural forms thereof unless specifically excluded.