Abstract:
Split-shell fractionation columns and associated processes for separating aromatic hydrocarbons are provided by using, a split-shell fractionation column includes a housing shell having a first height and a partition having a second height and disposed within the housing shell. The partition includes first and second vertically oriented baffles separated by a gap region, a seal plate connecting top ends of the baffles, a first input port formed to extend through the partition for the introduction of a gas into the gap region, and a first output port formed to extend outwardly from a bottom of the gap region and through the housing shell. The partition defines a first distillation zone and a second distillation zone within the housing shell.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Division of copending application Ser. No. 13/715,774 filed Dec. 14, 2012, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The technical field relates generally to chemical separation processes and associated apparatus. More particularly, the disclosure relates to processes for the separation of an aromatic hydrocarbon isomer, for example a xylene isomer, from a feed stream containing a mix of aromatic and non-aromatic hydrocarbons using a split-shell fractionation column. 
     BACKGROUND 
     Aromatic hydrocarbons find a plurality of uses in various chemical synthesis industries. In one non-limiting example, para-xylene is an important intermediate aromatic that finds wide and varied application in chemical syntheses. Upon oxidation, para-xylene yields terephthalic acid. Polyester fabrics and resins are produced from a polymer of ethylene glycol and terephthalic acid. These polyester materials are used extensively in a number of industries and are used to manufacture such items as, for example, clothing, beverage containers, electronic components, and insulating materials. 
     In prior art processes, C 9  aromatic hydrocarbons are separated from C 8  aromatic hydrocarbons, for example xylene isomers, by fractional distillation. This requires heating of the admixture to vaporize the C 8  and lighter aromatic hydrocarbons. A large portion of the isomerization stream must be vaporized to accomplish the C 9  separation because the stream is generally composed primarily of C 8  and lighter aromatic hydrocarbons. After the C 9  aromatic removal, the C 8 -containing stream is then recycled into an adsorptive separation unit. Multiple, large fractionation columns are often required to accomplish these process steps. As such, this separation process requires a substantial amount of energy and associated capital costs. 
     The production of aromatic hydrocarbon isomers, including for example para-xylene, is practiced commercially in large-scale facilities and is highly competitive. A never-ending drive exists to decrease the energy costs and capital costs yet increase the effectiveness associated with the conversion of a feedstock through one or more of isomerization, transalkylation, and disproportionation to produce select isomers and separate the select isomers from the resultant mixture of C 8  aromatic isomers. 
     Accordingly, it is desirable to provide processes for the production of particular aromatic isomers, including the separation of such isomers from an admixture of C 8  and C 9  aromatic isomers, that lowers operational expenses, particularly energy consumption. In addition it is desirable to provide processes for the production of particular aromatic isomers that lowers capital expenditures, in the form of processing equipment and the size of such processing equipment. Further, it is desirable to provide split-shell fractionation columns for use in such processes. These and other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     Split-shell fractionation columns and associated processes for separating aromatic hydrocarbons are provided herein. In one exemplary embodiment of the present disclosure, a split-shell fractionation column includes a housing shell having a first height and a partition having a second height and disposed within the housing shell. The partition includes first and second vertically oriented baffles separated by a gap region, a seal plate connecting top ends of the baffles, a first input port formed to extend through the partition for the introduction of a gas into the gap region, and a first output port formed to extend outwardly from a bottom of the gap region and through the housing shell. The partition defines a first distillation zone and a second distillation zone within the housing shell. 
     In another exemplary embodiment of the present disclosure, a process for separating aromatic hydrocarbons includes the steps of introducing a first stream including a plurality of aromatic hydrocarbons into a first distillation zone of a split-shell fractionation column and introducing a second stream including a plurality of aromatic hydrocarbons into a second distillation zone of the split-shell fractionation column. The first and second distillation zones are defined by a partition within the split-shell fractionation column. The partition has a gap region located therein. The process further includes the step of separating the first stream into a first overhead product and a first bottom product. The first bottom product includes a first liquid that collects at a bottom portion of the first distillation zone. The process further includes the step of separating the second stream into a second overhead product and a second bottom product. The second bottom product includes a second liquid that collects at a bottom portion of the second distillation zone. Still further, the process includes the step of draining the first liquid, the second liquid, or a combination thereof in the gap region through a first outlet port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The split-shell fractionation column and its associated processes will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic view of an exemplary embodiment of a split-shell fractionation column in accordance with the present disclosure; and 
         FIG. 2  is a diagram of an embodiment of a process for separating aromatic hydrocarbons employing a split-shell fractionation column as in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     Para-xylene, or any other select aromatic hydrocarbon isomer as may be desired, is typically recovered from a mixed aromatic hydrocarbon fraction derived from various sources such as catalytic reforming of petroleum by adsorptive separation, liquid-liquid extraction, and/or fractional distillation. The select aromatic isomer is then separated from that fraction, which typically contains all three xylene isomers, namely ortho-xylene, meta-xylene, and para-xylene. The para-xylene, or other desired isomer, is separated from the fraction to isolate the desired isomer. 
       FIG. 1  illustrates an exemplary embodiment of a split-shell fractionation column  210 . Split-shell fractionation column  210  includes housing shell  102 , an upper portion  120 , and a lower portion  122 . The housing shell  102  has a height  155 . Upper portion  120  includes a plurality of column trays  103 . Column trays  103  may be implemented as conventional fractionation column trays as are well-known in the art. Lower portion  122  is divided lengthwise by a partition  108 , which extends upwardly from housing shell  102  of bottom portion  122 , to create lower distillation zones  124  and  126 . The partition has a height  154 . Height  154  of the partition  108  is less than height  155  of the housing shell  102 . The partition  108  prevents the liquid bottom product, described below, in the lower distillation zones from mixing. Each of the lower distillation zones  124  and  126  have trays  104  and  106 , respectively, that are conventional distillation column trays that have been shaped to accommodate the wall of the partition  108  and the housing shell  102 . Each of the distillation zones  124  and  126  produce a different liquid bottom product (bottom product streams  214  and  282 ) from different feed streams fed thereto (input streams  238  and  278 ) to the fractionation column  210  by virtue of the partition  108  separating the distillation zones  124  and  126 . The partition  108  prevents the liquid bottom products in the lower distillation zones from mixing. 
     A potential problem with some prior art split-shell arrangements is leakage across the partition, in particular in the column “sump” near the bottom of the column where the liquid bottom product inventories are kept separate. Leakage across the partition, which sometimes occurs due to imperfect welding joints or metal fatigue, could contaminate the liquid bottom products, which could erode or eliminate the benefits of the split-shell design. This is particularly important if one of the liquid bottom products is a product for sale instead of an intermediate stream to be recycled for further processing. The presently disclosed split-shell fractionation column  210  addresses this potential leakage. 
     With continued reference to  FIG. 1 , input stream  278  is introduced into lower distillation zone  124 . Input stream  278 , in one embodiment, includes a mixture of C 8  aromatic hydrocarbons, C 9  and higher aromatic hydrocarbons (hereinafter referred to as C 9 + aromatic hydrocarbons), and toluene, for example. Input stream  238  is introduced into the lower distillation zone  126 . Input stream  238 , in one embodiment, includes a mixture of C 8  aromatic hydrocarbons and toluene, for example. In one embodiment, input stream  238  includes a relatively high concentration of para-xylene. That is, in one embodiment, the para-xylene concentration of input stream  238  is higher than the para-xylene concentration of input stream  278 . Of course, these compositions are merely non-limiting examples and are provided to illustrate the operation of column  210 . 
     Combining the hydrocarbons from input streams  278  and  238  into a single stream for fractional distillation in a prior art column (i.e., a column without the partition  108 ) would result in the undesirable dilution of the high purity material in input stream  238 . In addition, such combining would introduce undesirable C 9 + aromatic hydrocarbons into subsequent processes. The split-shell design allows the two input streams to be distilled separately, while still using only a single distillation column instead of two separate distillation columns, thereby reducing capital expenditures and operational energy costs. 
     In the operation of the split-shell fractional column  210 , the lighter components (i.e., those with a lower boiling point, for example some C 8  components and lighter components) introduced to the column  210  via inputs  278  and  238  vaporize at the temperature of the lower portion  122 . As such, these lighter components travel upwardly in column  210 . In one embodiment, the toluene present (C 7 ) is extracted as a liquid at a side cut tray (not shown) and light hydrocarbons such as C 6  and lower hydrocarbons (hereinafter referred to as C 6 − hydrocarbons) are extracted as a vapor at an overhead stream  212 / 242 . In another embodiment, the toluene is extracted as a vapor in the overhead stream  212 / 242  and condensed to form a liquid stream. In one embodiment, the stream  212 / 242  includes high purity toluene. In another embodiment, the stream  212 / 242  includes toluene and light hydrocarbons (C 6 −). 
     The heavier components (i.e., those with a higher boiling point, for example some C 8  components and heavier components) will remain in liquid form and, therefore, will remain in the lower distillation zone  124 , if introduced by way of input stream  278 , or will remain in lower distillation zone  126 , if introduced by way of input stream  238 . As such, the heavier fractions of the input stream  278  and input stream  238  will remain segregated in the lower portion  122 . In one embodiment, the bottom product stream  282  is a mixture of C 8  aromatic hydrocarbons, C 9 + aromatic hydrocarbons, and heavier hydrocarbons. In one embodiment, the bottom product stream  214  is a stream including primarily para-xylene. Again, these non-limiting, exemplary stream components are provided merely to illustrate the operation of the split-shell  210  column in one embodiment. 
     The number of trays  103 ,  104 , and  106  in each of the upper portion  120 , lower distillation zone  124 , and lower distillation zone  126 , respectively, vary with the particular product input streams and desired output streams, as will be appreciated by those skilled in the art. In one embodiment, the number of trays in lower distillation zone  124  is different than the number of trays in lower distillation zone  126 , for example the number of trays in distillation zone  124  may be greater than the number of trays in distillation zone  126 , or via versa. In an alternative embodiment, the number of trays in lower distillation zone  124  is the same as the number of trays in lower distillation zone  126 . 
     The locations of input streams  278  and  238  on split-shell fractionation column  210  are selected to prevent any mixing of the heavy (C 8 +) constituents across partition  108 . In one embodiment, the partition  108  extends  4  trays above the highest of the feed trays (not shown), trays  128  and  130  being the highest trays in distillation zones  124 ,  126 , respectively. Feed trays, as used herein, refer to the first trays encountered by input streams  238  and  278  upon entry into the column  210 . In one embodiment, the partition extends greater than 4 trays above the highest of the feed trays. In one embodiment, the partition extends less than 4 trays above the highest of the feed trays. As used herein, with reference to fractional distillation columns, the term “above” refers to a location in or on the column such that liquid inserted at the location will flow down toward the reference point. Similarly, the term “below” refers to a location in or on the column such that liquid inserted at the location will flow down away from the reference point. 
     Greater detail is now provided regarding the partition  108  as shown in  FIG. 1 . The partition  108  is designed so as to mitigate the risk of leakage and cross-contamination of the liquid bottom products. The partition includes two vertical baffles  141 ,  142  with a gap  143  between these baffles. Any leakage of liquid bottom products through the baffles  141 ,  142  is collected in a space  143  between the baffles  141 ,  142 , and can be removed by periodic or continuous draining from the column  210  via line  145 . 
     The partition  108  further includes a seal plate  144  at the top of the partition  108  to create a closed system, effectively a “pressure vessel” within the fractionation column  210 . The partition space  143 , in one embodiment, is operated at a lower pressure than the column  210  such that any and all leakage through either baffle  141 ,  142  flows from the relatively higher pressure column  210  into the partition space  143 . Nitrogen or another suitable inert vapor is used to maintain the pressure inside the partition space  143 , and is introduced via line  146 . A drain connection is provided to remove liquid accumulation in the bottom of the partition space  143  via line  145 . 
     The exemplary fractionation column  210 , and in particular the “pressure vessel” therewithin, is made by welding two vertical baffle plates  141 ,  142  into the fractionation column  210  with a gap, which in one embodiment can be sized from about 25 mm to about 50 mm, for example from about 30 to about 40 mm, between the two plates  141 ,  142  to create a partition  108  with space  143  therein. Each plate  141 ,  142  is welded along the vessel shell so that there is no direct fluid communication between the opposite sides of the baffle plates  141 ,  142 . The top of the baffle plates  141 ,  142  are welded to the cover plate  144  that effectively closes the gap and prevents fluid communication between the inside of the partition and the fractionation column  210 . A vapor inlet connection is made to introduce nitrogen or appropriate inert vapor to maintain an operating pressure within the partition space  143  that is lower than the operating pressure of the column. An outlet connection with an output port is made at the bottom of the partition space  143  to remove any liquid that may accumulate inside the partition space  143  due to unintended leakage through the baffles  141 ,  142 , for example due to an imperfect weld connection. 
     ILLUSTRATIVE EXAMPLE 
     The following example is merely provided to illustrate one possible implementation of a split-shell fractionation column in a broader aromatic hydrocarbon processing system. As such, the form and content of the various material streams are intended to serve only as a non-limiting example for the skilled artisan to better understand the operation thereof. 
       FIG. 2  illustrates an embodiment  200  of an aromatic hydrocarbon processing system that includes split-shell fractionation column  210 , described above. A feed stream  202  enters a xylene fractionation unit  204 . In one embodiment, the feed stream  202  contains ortho-, meta-, and para-xylene isomers. In one embodiment, the feed stream  202  contains quantities of ethylbenzene, toluene, C 8  cycloalkanes, alkanes, and hydrocarbons having more than eight carbon atoms per molecule. In one embodiment, the feed stream  202  is a result of hydrotreating naphtha to remove any sulfur and nitrogen contaminants and the subsequent catalytic reforming where paraffins and naphthenes in the decontaminated naphtha are converted to aromatic hydrocarbons. Most C 7 − fractions are removed in a debutanizer and fractional distillation column, respectively. 
     The feed stream  202 , including a C 8 + fraction, enters the xylene fractionation unit  204 . In one embodiment, the feed stream  202  includes about 23 weight percent (wt %) para-xylene. The xylene fractionation unit  204  is a fractional distillation column. The xylene fractionation unit  204  divides the incoming stream into an overhead stream  206  including the C 8 − aromatic hydrocarbons, including the xylene isomers, ethylbenzene, and toluene, a bottom product stream  208 , and one or more side cut streams (not shown) including C 9 + aromatic hydrocarbons and any C 7 − fractions present in the feed stream  202 . 
     The overhead stream  206  from xylene fractionation unit  204 , and a bottom product stream  214  from extract column  210 , enter adsorptive separation unit  216  at a first feed input  286  and a second feed input  284 , respectively. Adsorptive separation unit  216  separates the incoming streams  206  and  214  into a raffinate stream  218  and an extract stream  220 . In one embodiment, the heavy desorbent para-diethylbenzene is used to facilitate the separation of the raffinate stream  218  and extract stream  220 . The raffinate stream  218  includes ethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent. The extract stream  220  includes para-xylene diluted with desorbent. 
     In one embodiment, adsorptive separation unit  216  includes a simulated moving bed (SMB) assembly and a rotary valve. The SMB assembly includes a single physical chamber. In one embodiment, the physical chamber includes 24 beds. In an alternative embodiment, the physical chamber includes less than 24 beds. In another embodiment, the SMB assembly includes two physical chambers. In one embodiment, each physical chamber includes 12 beds. In an alternative embodiment, each physical chamber includes more or less than 12 beds. In one embodiment, the physical chambers have an unequal number of beds. A bed line connects each bed in the SMB assembly to the rotary valve. The rotary valve controls the flow of material into or out of the SMB assembly in a step- wise manner to create a simulated moving bed and to flush the bed lines between flows of differing materials. 
     As a mixture of xylene isomers is fed into adsorptive separation unit  216 , and flows downwardly under the force of gravity, the mixture of xylene isomers contacts a solid, zeolitic adsorbent within the chamber. The zeolitic adsorbent disposed in adsorptive separation unit  216  has an affinity for para-xylene. As the mixture of xylene isomers flows over the solid adsorbent, the para-xylene is selectively adsorbed into the adsorbent while the other isomers continue to travel downward in the chamber in the bulk liquid. In certain embodiments, the selectivity of the adsorbent in the adsorptive separation unit  216  for C 7 − aromatic hydrocarbons and lighter hydrocarbons is very close to that of para-xylene. As such, the C 7 − aromatic hydrocarbons and lighter hydrocarbons exit the adsorptive separation unit  216  by way of extract stream  220 . The extract stream  220  enters the extract column  224 . Extract column  224  is a fractional distillation column that separates the incoming stream  220  into an overhead para-xylene stream  226  including para-xylene, C 7 − aromatic hydrocarbons, and lighter hydrocarbons and a bottom product stream  228  including a heavy desorbent fraction, such as para-diethylbenzene (a C 10  aromatic hydrocarbon). The bottom product stream  228  is recycled back to the adsorptive separation unit  216  through combined stream  230 . 
     Light desorbent enters adsorptive separation unit  234  by way of combined stream  232 . Adsorptive separation unit  234  separates an incoming stream  254  into a raffinate stream  236  and an extract stream  238 . Stream  254  is an isomerized stream from isomerization unit  250  including an equilibrium mixture of xylene isomers. In one embodiment, the light desorbent toluene is used to facilitate the separation of the raffinate stream  236  and extract stream  238 . The raffinate stream  236  includes ethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent. The extract stream  238  includes para-xylene diluted with desorbent. 
     In one embodiment, adsorptive separation unit  234  includes an SMB assembly and a rotary valve. In one embodiment, the SMB assembly includes a single physical chamber. In one embodiment, the physical chamber includes 24 beds. In an alternative embodiment, the physical chamber includes less than 24 beds. In one embodiment, the SMB assembly includes two physical chambers. In one embodiment, each physical chamber includes 12 beds. In an alternative embodiment, each physical chamber includes more or less than 12 beds. In one embodiment, the physical chambers have an unequal number of beds. A bed line connects each bed in the SMB assembly to the rotary valve. The rotary valve controls the flow of material into or out of the SMB assembly in a stepwise manner to create a simulated moving bed and to flush the bed lines between flows of differing materials. 
     As a mixture of xylene isomers is fed into adsorptive separation unit  234 , and flows downwardly under the force of gravity, the mixture of xylene isomers contacts a solid, zeolitic adsorbent within the chamber. The zeolitic adsorbent disposed in adsorptive separation unit  234  has an affinity for para-xylene. As the mixture of xylene isomers flows over the solid adsorbent, the para-xylene is selectively adsorbed into the adsorbent while the other isomers continue to travel downward in the chamber in the bulk liquid. The raffinate stream  236  enters a raffinate column  222  at a third location  276 . The extract stream  238  and the output  278  from aromatic conversion unit  280  are fed into the split-shell extract column  210  (thereby become input streams  238 ,  278  to the column  210 ), which was described in greater detail above with regard to  FIG. 1 , at a first input port  290  and second input port  288 , respectively. The split-shell column  210  separates the input streams into the previously described overhead stream  212  at third output  296  and the previously described bottom product streams  214  and  282  at first output port  294  and second output port  292 , respectively. As previously noted, the overhead stream  212 / 242 , in one embodiment, includes primarily toluene. In one embodiment, stream  212 / 242  includes also includes C 7 − aromatic hydrocarbons and lighter hydrocarbon impurities. The bottom product stream  214  includes C 8  aromatic hydrocarbon isomers, including a high concentration of para-xylene (as compared to stream  282 ). The bottom product stream  282  includes C 8  aromatic hydrocarbon isomers. In one embodiment, the bottom product stream  282  has a lower concentration of para-xylene than does bottom product stream  214 . The light desorbent, in one embodiment toluene, is recycled in a light desorbent loop  212 ,  232 ,  238 . In one embodiment, a slipstream  242  is extracted from the overhead stream  212 / 242 . In one embodiment, slipstream  242  prevents the accumulation of additional toluene introduced into the desorbent loop from the feed stream  202 . In one embodiment, slipstream  242  prevents the accumulation of light hydrocarbon impurities in the light desorbent loop. In one embodiment, slipstream  242  includes high purity toluene. In one embodiment, slipstream  242  includes toluene and light hydrocarbon impurities from the feed stream  202 . 
     Raffinate column  222  is a fractional distillation column that separates the raffinate stream  236  and  218 , each including para-xylene depleted C 8  aromatic hydrocarbon isomers diluted with light and heavy desorbent, respectively, into a C 8  aromatic hydrocarbon isomer stream  244 , a light desorbent stream  246 , and a heavy desorbent stream  248 . The C 8  aromatic hydrocarbon isomer stream  244  exits the raffinate column  222  at a second location  274 . The light desorbent along with any light impurities have the lowest boiling point and are, as such, extracted as a net overhead stream  246 . The heavy desorbent along with any heavy hydrocarbons (C 9 +) have the highest boiling point and are, as such, extracted as a net bottom product stream  248 . The ortho-xylene, meta-xylene, and ethylbenzene have an intermediate boiling point and are, as such, extracted at a sidecut tray. The heavy desorbent is recycled in a heavy desorbent loop  230 ,  220 / 218 ,  228 / 248 . In one embodiment, the C 8  aromatic isomer stream  244  exits the raffinate column  222  at a location below that of raffinate stream  236  and above that of raffinate stream  218 . In one embodiment, the raffinate stream  236  enters raffinate column  222  at a location on the column where the composition within the column  222  is similar to the composition in stream  236 . In one embodiment, the raffinate stream  218  enters raffinate column  222  at a location on the column where the composition within the column  222  is similar to the composition in stream  218 . 
     The C 8  aromatic hydrocarbon isomer stream  244  including meta-xylene, ortho-xylene, and ethylbenzene enters an isomerization unit  250 . Catalysts in the isomerization unit  250  reestablish an equilibrium mixture of the ortho-, meta-, and para-xylene isomers. In one embodiment, the catalyst is an ethylbenzene dealkylation catalyst, which converts ethylbenzene to a benzene co-product. In one embodiment, the catalyst is an ethylbenzene isomerization catalyst, which converts the ethylbenzene into additional xylene isomers. Non-aromatic compounds in the C 8  aromatic hydrocarbon isomers stream  244  are “cracked” (C—C bonds broken) to lighter hydrocarbons and removed in stream  252  along with any benzene co-product created. The isomerization process may also create small quantities of C 9  and heavier aromatic hydrocarbons. In one embodiment, the output stream  254  includes an equilibrium mixture of xylene isomers. In one embodiment, the output stream  254  includes small quantities of C 9 + aromatic hydrocarbons. In one embodiment, the output stream  254  includes unreacted ethylbenzene. In one embodiment, the output stream  254  includes about 1.5 wt. % ethylbenzene or less. The isomerized output stream  254  enters adsorptive separation unit  234 . 
     In certain embodiments, some C 9 + aromatic hydrocarbons may be introduced as a result of the isomerization of ortho-xylene, meta-xylene, and ethylbenzene at isomerization unit  250 . Any C 10 + hydrocarbons will accumulate in the heavy desorbent loop  230 ,  220 / 218 , 228 / 248 . In certain configurations of the raffinate column  222 , any C 9  aromatic hydrocarbons will accumulate in the isomerization loop  254 , 236 , 244 . In other configurations of the raffinate column  222 , any C 9  aromatic hydrocarbons will accumulate in the heavy desorbent loop  230 ,  220 / 218 ,  228 / 248 . In yet other configurations of the raffinate column  222 , any C 9  aromatic hydrocarbons will accumulate in both the isomerization loop and the heavy desorbent loop. In different embodiments, one or more drag streams are used to prevent the accumulation of C 9 + aromatic hydrocarbons in the process. In one embodiment, if accumulation occurs in the heavy desorbent loop, a drag stream  264  is withdrawn from the desorbent loop by way of stream  230 . Stream  230  includes primarily heavy desorbent along with the C 9  aromatic and heavier hydrocarbon impurities. The drag stream  264  is fed into a fractional distillation column  266 , which separates the drag stream  264  into an overhead stream  268  and a bottom product stream  270 . The bottom product stream  270  includes high purity para-diethylbenzene, which is returned to the desorbent loop by way of stream  230 . In one embodiment, the amount of material withdrawn in drag stream  264  is about 1 to about 20 volume percent of stream  230 . In another embodiment, if accumulation occurs in the isomerization loop (i.e.,  254 ,  236 ,  244 ), a drag stream  262  is withdrawn from the isomerization loop by way of raffinate stream  244 . Stream  262  includes a mixture of ortho-xylene, meta-xylene, ethylbenzene along with the C 9  aromatic and heavier hydrocarbon impurities. In one embodiment, the amount of material in the drag stream  262  is about 1 to about 20 volume percent of the raffinate stream  244 . In yet another embodiment, if the accumulation occurs in both the isomerization loop and the heavy desorbent loop, drag streams  262  and  264  are both used. In other embodiments, no drag streams are used. In other embodiments, the impurities are extracted by another process known in the art capable of separating C 9  aromatic hydrocarbons and heavier hydrocarbons from para-diethylbenzene. 
     In one embodiment, the aromatic conversion unit  280  converts the incoming stream  262 , including a mixture of toluene and C 9 + aromatic hydrocarbons, into an output stream  278  including an equilibrium mixture of xylene isomers, ethylbenzene, and toluene. The aromatic conversion unit  280  facilitates catalytic disproportionation reactions, which convert toluene into a mixture of benzene and xylene isomers. The aromatic conversion unit  280  also facilitates catalytic transalkylation reactions, which convert a blend of toluene and C 9  aromatic isomers to xylene isomers through the migration of methyl groups between methyl-substituted aromatic hydrocarbons. Benzene produced in the aromatic conversion assembly is extracted in an additional stream (not shown). The output stream  278  is fed into the split-shell extract column  210  of the present disclosure (described in greater detail above with regard to  FIG. 1 ), which separates the output stream  278  into an overhead stream including toluene and a bottom product stream  282  including C 8 + aromatic hydrocarbons. The bottom product stream  282  is fed back into the xylene fractionation column  204  to separate the C 8  aromatic hydrocarbons into stream  206  and the C 9 + aromatic hydrocarbons into stream  208 . The overhead toluene stream from the split extract column  210  is split into streams  212  and  242 . Stream  242  is recycled back into the aromatic conversion unit  280  for transalkylation. Stream  212  is part of the light desorbent loop  212 ,  232 ,  238 . 
     The finishing column  272  separates the overhead stream  226  from extract column  224  into an overhead stream  274  including C 7 − aromatic hydrocarbons and lighter hydrocarbons, and a bottom product stream  276  including high purity para-xylene. In certain embodiments, para-ethyltoluene, structurally similar to para-xylene, may be introduced into the process by the isomerization unit  250 . In some embodiments, the para-ethyltoluene is separated from the para-xylene in the adsorptive separation unit  216 , in the extract column  224 , or in the finishing column  272 . In some embodiments, the para-ethyltoluene is removed from the para-xylene product using techniques known in the art. In one embodiment, the bottom para-xylene stream  276  includes about 95.0 wt. % para-xylene. In one embodiment, the bottom para-xylene stream  276  includes about 99.2 wt. % para-xylene. In one embodiment, the bottom para-xylene stream  276  includes about 99.7 wt. % para-xylene. In one embodiment, the bottom para-xylene stream  276  includes about 99.9 wt. % para-xylene. In one embodiment, the bottom para-xylene stream  276  includes greater than about 99.9 wt. % para-xylene. 
     Accordingly, an improved split-shell fractionation column has been described. The improved column beneficially mitigates the risk of leakage and cross-contamination of liquid bottom products at the bottom of the split-shell fractionation column. Furthermore, the improved column desirably reduces capital and energy costs by combining two fractionation columns into a single column that has a common overhead distillate product but dissimilar liquid bottom products. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the processes without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of this disclosure.