Abstract:
A process for separating para-xylene from a plurality of xylene isomers, wherein the process introduces at a first feed point a first mixed xylene stream comprising a plurality of xylene isomers into a first adsorptive separation unit to produce a first para-xylene enriched stream and a first raffinate stream, and introduces a second mixed xylene stream comprising a plurality of xylene isomers into a second adsorptive separation unit to produce a second raffinate stream. The process feeds both the first raffinate stream and the second raffinate stream into a raffinate column. The process further introduces an extract stream from the second adsorptive separation unit into a first input of a split extract column comprising an internal partition defining a first distillation zone and a second distillation zone.

Description:
FIELD OF THE INVENTION 
       [0001]    The disclosure relates to a process for the formation and adsorptive separation of a select xylene isomer, preferably para-xylene, from a feed stream containing a mix of aromatic and non-aromatic hydrocarbons. More specifically, the disclosure relates to increasing the efficiency of para-xylene production by eliminating the need to vaporize the product stream from the isomerization process. Most specifically, the disclosure relates to a para-xylene process comprising multiple adsorptive separation units, each using a different desorbent, to eliminate fractional distillation of the isomerized product stream, and a split fractionation column to consolidate multiple fractionation columns. 
       BACKGROUND OF THE INVENTION 
       [0002]    Para-xylene, an aromatic hydrocarbon, is an important intermediate which 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. 
         [0003]    The production of para-xylene is practiced commercially in large-scale facilities and is highly competitive. Concerns exist not only about the effective conversion of feedstock through one or more of isomerization, transalkylation and disproportionation to produce para-xylene and effective separation of para-xylene from the resultant mixture of C8 aromatic isomers, but also with the energy costs and capital costs associated with such processes. 
         [0004]    In prior art processes, C9 aromatics are separated from the C8 aromatics, i.e., xylene isomers, by fractional distillation. This requires heating of the admixture to vaporize the C8 and lighter aromatics. A large portion of the isomerization stream must be vaporized to accomplish the C9 separation because the stream is generally composed primarily of C8 and lighter aromatics. This separation requires a substantial amount of energy and associated cost. After the C9 aromatic removal, the C8-containing stream is then recycled into the adsorptive separation unit. 
         [0005]    Accordingly, it would be an advance in the state of the art to provide a process for the production of para-xylene, including separation and isomeric formation from an admixture of C8 aromatic isomers, that removes the need to vaporize the isomerized stream for removal of C9 aromatics, thereby lowering operational expenses, in the form of energy consumption, and capital expenditures, in the form of required processing equipment and the size of such processing equipment. It would be a further advance in the state of the art to share a fractionation column between the aromatic conversion process and the xylene separation process, thereby reducing capital costs. 
       SUMMARY OF THE INVENTION 
       [0006]    A process for separating para-xylene from a plurality of xylene isomers is presented. The process introduces at a first feed point a first mixed xylene stream comprising a plurality of xylene isomers into a first adsorptive separation unit to produce a first para-xylene enriched stream and a first raffinate stream, and introduces a second mixed xylene stream comprising a plurality of xylene isomers into a second adsorptive separation unit to produce a second raffinate stream. 
         [0007]    The process feeds both the first raffinate stream and the second raffinate stream into a raffinate column. The process further introduces an extract stream from the second adsorptive separation unit into a first input of a split extract column comprising an internal partition defining a first distillation zone and a second distillation zone. The split extract column comprises a first input for a first distillation zone, a second input for a second distillation zone, a first output for the first distillation zone, and a second output for the second distillation zone. 
         [0008]    A fractional distillation column is presented. The factional distillation column comprises a housing, a partition disposed within the housing, where that partition defines a first distillation zone and a second distillation zone. The fractional distillation column further comprises a first input port formed to extend through the housing and into the first distillation zone, a first output port formed to extend outwardly from the first distillation zone and through the housing, a second input port formed to extend through the housing and into the second distillation zone, and a second output port formed to extend outwardly from the second distillation zone and through the housing. 
         [0009]    An apparatus for separating para-xylene from a plurality of xylene isomers is presented. The apparatus comprises a first adsorptive separation unit, a second adsorptive separation unit, a raffinate column in fluid communication with both the first adsorptive separation unit and the second adsorptive separation unit. The apparatus further comprises Applicant&#39;s fractional distillation column in fluid communication with the second adsorptive separation unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram of a prior art process; 
           [0011]      FIG. 2  is a diagram of one embodiment of the disclosed process having a split fractionation column where light impurities are removed early in the process and a drag stream is fed into an aromatic conversion assembly; and 
           [0012]      FIG. 3  is a schematic view of one embodiment of Applicant&#39;s split fractionation column. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]    Para-xylene is typically recovered from a mixed aromatic hydrocarbon fraction derived from various sources such as catalytic reforming by liquid-liquid extraction and/or fractional distillation. The para-xylene is commercially separated from a feed stream that typically contains all three xylene isomers, namely ortho-xylene, meta-xylene, and para-xylene. The para-xylene, or other desired isomer, is separated by either fractional crystallization or adsorptive separation or a combination of these two techniques. Adsorptive separation is generally preferred as it has a significantly higher single pass recovery (˜97%) relative to crystallization separation (˜65%). 
         [0014]    A typical adsorptive separation process first involves the separation of C8 aromatic hydrocarbons, including ortho-xylene, meta-xylene, para-xylene, and ethylbenzene, from heavier aromatic hydrocarbons (i.e., C9+) and non-aromatic hydrocarbons through fractional distillation. 
         [0015]    Those skilled in the art will appreciate that the designator “CX” refers to a compound comprising X carbon atoms, “CX+” refers to a compound comprising X or greater carbon atoms, and “CX−” refers to a compound comprising X or fewer carbon atoms. 
         [0016]    The para-xylene isomers are then separated from the C8 isomer admixture using a simulated countercurrent moving-bed (SMB) adsorptive separation unit. This simulation is performed using established commercial technology wherein an adsorbent, commonly a solid zeolitic material, is held in place in one or more cylindrical adsorbent chambers. The positions at which the streams involved in the process enter and leave the chamber(s) are slowly shifted along the height of the chamber(s). Normally there are at least four streams (feed, desorbent, extract and raffinate) employed in this procedure and the location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals in a step-wise manner. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. 
         [0017]    A typical chamber has a single line for each bed. The flow into or out of a particular line, as the case may be, is controlled by a rotary valve. The shifting of streams along the bed simulates movement of the absorbent in a direction opposite the flow of liquid, even though the absorbent is fixed in place within the chamber. SMB chambers are also well suited for high volume production because the input and output streams have nearly constant compositions throughout simulated motion of the absorbent material in the bed. 
         [0018]    A typical SMB unit recycles a heavy desorbent, such as para-diethylbenzene, to separate high purity para-xylene from the other C8 isomers. Para-diethylbenzene is a C10 aromatic that is separated from para-xylene by fractional distillation. 
         [0019]    The admixture of non-para-xylene isomers from the adsorptive separation unit is subjected to catalytic isomerization to reestablish an equilibrium amount of para-xylene isomers in the admixture. In addition to para-xylene and other C8 isomers, the isomerized stream typically contains an amount of C9+ aromatics, which will accumulate in the desorbent and, therefore, must be removed. 
         [0020]    Processes for isolating a desired isomer of xylene without the vaporization of the full isomerized product stream are presented. The process comprises two adsorptive separation units. The first unit utilizes a heavy desorbent and the second unit utilizes a light desorbent. Those skilled in the art will appreciate that a desorbent used in combination with a simulated moving bed absorbent system facilitates removal of an absorbed material from the adsorbent bed. This being the case, a useful desorbent will have an affinity for the desired material, i.e., para-xylene, that is substantially the same as the affinity of the adsorbent bed for that desired material. 
         [0021]    As used herein, the terms heavy and light are generally in reference to the boiling point of the desorbent relative to the C8 aromatics, namely, ortho-, meta-, para-xylene and ethylbenzene. 
         [0022]    In certain embodiments, the heavy desorbent is selected from the group consisting of para-diethylbenzene, para-diisopropylbenzene, tetralin, and the like, and combinations thereof. In certain embodiments, toluene and the like can be used as the light desorbent. The para-diethylbenzene has a higher boiling point than the C8 aromatic isomers and, as such, the para-diethylbenzene is the bottoms (i.e., heavy) product when separated from the C8 isomers in a fractional distillation column. Similarly, toluene has a lower boiling point than the C8 aromatic isomers and, as such, the toluene is the overhead (i.e., light) product when separated from the C8 isomers in a fractional distillation column. 
         [0023]    Unlike prior art processes, Applicant&#39;s process comprises feeding a stream of material containing a desired xylene isomer formed in an isomerization unit into a second adsorptive separation unit, as opposed to being fed back into a fractional distillation column. An extract stream from the second adsorptive separation unit, rich in the desired xylene isomer, is fed back into a first adsorptive separation unit for isolation of the desired xylene isomer. The process is presented in greater detail below. 
         [0024]    Referring to  FIG. 1 , a diagram  100  of a prior art process for the production of para-xylene is depicted. A feed stream  102  enters a xylene fractionation unit  104 . The feed stream typically contains ortho-, meta-, and para-xylene isomers and may also contain quantities of ethylbenzene, toluene, C8 cycloalkanes, alkanes, and hydrocarbons having more than eight carbon atoms per molecule. 
         [0025]    The xylene fractionation unit  104  is a fractional distillation column. The xylene fractionation unit  104  divides the incoming stream into an overhead stream  106  comprising the C8 and lighter aromatics, including the xylene isomers, ethylbenzene, and toluene, and a bottoms  108  and one or more side cut streams (not shown) comprising C9+ aromatics. 
         [0026]    Table 1 recites an example composition for feed stream  102 . 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 para-xylene 
                 10-20  
                 wt % 
               
               
                   
                 Total C8 Aromatics 
                 25-60  
                 wt % 
               
               
                   
                 ethylbenzene 
                 10-20  
                 wt % 
               
               
                   
                 toluene 
                 0.5-2.0  
                 wt % 
               
               
                   
                 C9+ 
                 25-30  
                 wt % 
               
               
                   
                 Nonaromatic hydrocarbons 
                 &lt;0.5  
                 wt % 
               
               
                   
                 Nitrogen 
                 1.0  
                 mg/kg 
               
               
                   
                 Sulfur 
                 1.0  
                 mg/kg 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    The overhead stream  106  enters the separation assembly  110 , where the input stream  106  is separated into a raffinate stream  114 , a toluene stream  116 , and a para-xylene stream  118 . The raffinate stream  114  has been substantially depleted of para-xylene but contains other C8 aromatics, including ortho-xylene, meta-xylene and ethylbenzene. 
         [0028]    Within the separation assembly  110 , the stream  106  enters an adsorptive separation unit  112 . The adsorptive separation unit  112  separates the incoming stream  106  into a raffinate stream  120  and an extract stream  122 . The adsorptive separation unit  112  typically comprises two SMB chambers and a rotary valve. Each individual chamber typically has 12 beds. A bedline connects each bed to the rotary valve. The rotary valve controls the flow of material into or out of each SMB chamber in a step-wise manner to create a simulated moving bed. 
         [0029]    A heavy desorbent, typically para-diethylbenzene, is used to facilitate the separation of the raffinate stream  120  and extract stream  122 . The raffinate stream  120  comprises ethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent and any heavies. Heavies are hydrocarbons with a boiling point greater than that of the C8 aromatic isomers and include C9+ aromatics. The extract stream  122  comprises para-xylene diluted with desorbent and light ends. Light ends are hydrocarbons with a boiling point below that of the C8 aromatic isomers and include toluene and other C7− aromatics. 
         [0030]    The raffinate stream  120  is directed to a raffinate column  142 . The raffinate column  142  is a fractional distillation column that divides the incoming stream  120  into (i) an overhead raffinate stream  114  comprising ethylbenzene, meta-xylene, and ortho-xylene and (ii) a bottoms stream  124  comprising desorbent and any heavies. The bottoms stream  124  is recycled back to the adsorptive separation unit  112  through combined stream  130 . The overhead raffinate stream  114  is directed to an isomerization unit  136 . 
         [0031]    The extract stream  122 , comprising xylene isomers and ethylbenzene, enters extract column  126 . Extract column  126  is a fractional distillation column that divides the incoming stream  122  into (i) an overhead stream  128  comprising para-xylene and toluene and (ii) a bottoms stream  132  comprising desorbent and heavies. The bottoms stream  132  containing desorbent is recycled back to the adsorptive separation unit  112  through combined stream  130 . Heavies in the combined desorbent stream  130  may be removed by directing a slipstream of the desorbent into a small desorbent rerun column. 
         [0032]    The overhead stream  128 , comprising para-xylene and toluene, enters finishing column  134 . Finishing column  134  is a fractional distillation column that divides the incoming stream  128  into (i) an overhead toluene stream  116  and (ii) a bottoms para-xylene stream  118 . The bottoms para-xylene stream  118  contains the final desired product. 
         [0033]    The raffinate stream  114  comprising ethylbenzene, meta-xylene, and ortho-xylene enters an isomerization unit  136 . Catalysts in the isomerization unit  136  reestablish an equilibrium mixture of the ortho-, meta-, and para-xylene isomers and convert the ethylbenzene into xylenes and/or benzene. 
         [0034]    Nonaromatic compounds in the raffinate stream  114  are cracked to light ends and removed in stream  138  along with any benzene. The isomerization process also creates quantities of C9+. The output stream  140  comprises an equilibrium mixture of xylene isomers as well as quantities of C9 aromatics and unreacted ethylbenzene. The output stream  140  is recycled back into the xylene fractionation unit  104 . 
         [0035]    The C9 aromatics produced during isomerization are separated from the C8 isomers in the xylene fractionation unit  104 . While the C9 aromatics are only a very small portion of the stream, the entire C8 fraction must be vaporized to accomplish this separation. 
         [0036]      FIG. 2  illustrates embodiment  200  of Applicant&#39;s apparatus and process. 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, C8 cycloalkanes, alkanes, and hydrocarbons having more than eight carbon atoms per molecule. 
         [0037]    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 aromatics. The most light ends and C7− fractions are removed in a debutanizer and fractional distillation column, respectively. The feed stream  202 , comprising a C8+ fraction, enters the xylene fractionation unit  204 . In one embodiment, the feed stream  202  comprises about 23 weight percent para-xylene. 
         [0038]    The xylene fractionation unit  204  is a fractional distillation column. The xylene fractionation unit  204  divides the incoming stream into an overhead stream  206  comprising the C8− aromatics, including the xylene isomers, ethylbenzene, and toluene, and a bottoms stream  208  and one or more side cut streams (not shown) comprising C9+ aromatics. 
         [0039]    The overhead stream  206  from xylene fractionation unit  204 , and bottoms 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 . In certain embodiments, the heavy desorbent is selected from the group consisting of para-diethylbenzene, para-diisopropylbenzene, tetralin, and the like, and combinations thereof. The raffinate stream  218  comprises ethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent. The extract stream  220  comprises para-xylene diluted with desorbent. 
         [0040]    In one embodiment, adsorptive separation unit  216  comprises an SMB assembly and a rotary valve. In other embodiments, the adsorption separation unit  216  comprises an SMB assembly and one or more rotary valves, one or more computing device operated valves, or a combination thereof. In one embodiment, the SMB assembly comprises a single physical chamber. In one embodiment, the physical chamber includes 24 beds. In one 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 one embodiment, each physical chamber includes less than 12 beds. In one embodiment, each physical chamber includes more 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. 
         [0041]    As a mixture of xylene isomers is fed into adsorptive separation unit  216 , and flows downwardly, the mixture of xylene isomers contacts a solid adsorbent within the chamber. The zeolitic adsorbents disposed in adsorptive separation unit  216  have 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. 
         [0042]    In certain embodiments, the selectivity of the adsorbent in the adsorptive separation unit  216  for C7− aromatics is very close to that of para-xylene. As such, the C7− aromatics exit the adsorptive separation unit  216  by way of extract stream  220 . 
         [0043]    The extract stream  220  enters the extract column  224 . Extract column  224  is a fractional distillation column that separates the incoming stream  220  into (i) an overhead para-xylene stream  226  comprising para-xylene, C7− aromatics, and light ends and (ii) a bottoms stream  228  comprising a heavy desorbent fraction, such as para-diethylbenzene, a C10 aromatic. The bottoms stream  228  is recycled back to the adsorptive separation unit  216  through combined stream  230 . 
         [0044]    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  comprising an equilibrium mixture of xylene isomers. 
         [0045]    In one embodiment, the light desorbent toluene is used to facilitate the separation of the raffinate stream  236  and extract stream  238 . In another embodiment, a light desorbent other than toluene is used to facilitate the separation of the raffinate stream  236  and extract stream  238 . The raffinate stream  236  comprises ethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent. The extract stream  238  comprises para-xylene diluted with desorbent. 
         [0046]    In one embodiment, adsorptive separation unit  234  comprises an SMB assembly and a rotary valve. In other embodiments, the adsorption separation unit  234  comprises an SMB assembly and one or more rotary valves, one or more computing device operated valves, or a combination thereof. In one embodiment, the SMB assembly comprises a single physical chamber. In one embodiment, the physical chamber includes 24 beds. In one 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 one embodiment, each physical chamber includes less than 12 beds. In one embodiment, each physical chamber includes more 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. 
         [0047]    As a mixture of xylene isomers is fed into adsorptive separation unit  234 , and flows downwardly, the mixture of xylene isomers contacts a solid adsorbent within the chamber. The zeolitic adsorbents disposed in adsorptive separation unit  234  have 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. 
         [0048]    The raffinate stream  236  enters a raffinate column  222  at a third location  276 . 
         [0049]    The extract stream  238  and the output  278  from aromatic conversion unit  280  are fed into split extract column  210 , a split fractional distillation column, at a first input  290  and second input  288 , respectively. 
         [0050]    The split extract column  210  separates the input streams into an overhead stream  212  at third output port  296  and a bottoms stream  214  and  282  at first output port  294  and second output port  292 , respectively. The overhead stream  212 / 242  comprises primarily toluene. In one embodiment, stream  212 / 242  comprises also comprises C7− aromatics and light impurities. The bottoms stream  214  comprises C8 aromatic isomers, including a high concentration of para-xylene. The bottoms stream  282  comprises C8 aromatic isomers. In one embodiment, the bottoms stream  282  also comprises C9+ aromatics introduced by way of aromatics conversion unit  280 . In one embodiment, the bottoms stream  282  has a lower concentration of para-xylene than does bottoms stream  214 . The light desorbent, in one embodiment, toluene, is recycled in a light desorbent loop  212 ,  232 ,  238 . 
         [0051]    In one embodiment, a slipstream  242  is extracted from the overhead stream  212 . 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 impurities in the light desorbent loop. In one embodiment, slipstream  242  comprises high purity toluene. In one embodiment, slipstream  242  comprises toluene and light impurities from the feed stream  202 . 
         [0052]    Raffinate column  222  is a fractional distillation column that separates the raffinate stream  236  and  218 , each comprising para-xylene depleted C8 aromatic isomers diluted with light and heavy desorbent, respectively, into a C8 aromatic isomer stream  244 , a light desorbent stream  246 , and a heavy desorbent stream  248 . The C8 aromatic isomer stream  244  exits the raffinate column  222  at a second location  274 . 
         [0053]    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 heavies have the highest boiling point and are, as such, extracted as a net bottoms 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 . 
         [0054]    In one embodiment, the C8 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 . 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. 
         [0055]    The C8 aromatic isomer stream  244  comprising 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. 
         [0056]    Nonaromatic compounds in the C8 aromatic isomers stream  244  are cracked to light ends and removed in stream  252  along with any benzene co-product created. The isomerization process may also create small quantities of C9 and heavier aromatics. In one embodiment, the output stream  254  comprises an equilibrium mixture of xylene isomers. In one embodiment, the output stream  254  comprises small quantities of C9+ aromatics. In one embodiment, the output stream  254  comprises unreacted ethylbenzene. 
         [0057]    In one embodiment, the output stream  254  comprises about 1.5 weight percent ethylbenzene or less. The isomerized output stream  254  enters adsorptive separation unit  234 . 
         [0058]    In certain embodiments, certain C9+ aromatics may be introduced by isomerization unit  250  and accumulate in the heavy desorbent loop  230 ,  220 / 218 ,  228 / 248 . In certain configurations of the raffinate column  222 , any C9 aromatics will accumulate in the isomerization loop  254 ,  236 ,  244 . In other configurations of the raffinate column  222 , any C9 aromatics will accumulate in the heavy desorbent loop  230 ,  220 / 218 ,  228 / 248 . In yet other configurations of the raffinate column  222 , any C9 aromatics will accumulate in both the isomerization loop and the heavy desorbent loop. 
         [0059]    In different embodiments, one or more drag streams are used to prevent the accumulation of C9+ aromatics 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  comprises primarily heavy desorbent along with the C9 aromatic and heavy 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 bottoms stream  270 . The bottoms stream  270  comprises 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 . 
         [0060]    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  comprises a mixture of ortho-xylene, meta-xylene, ethylbenzene along with the C9 aromatic and heavy 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 . 
         [0061]    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 C9 aromatics and heavies from para-diethylbenzene. 
         [0062]    In one embodiment, the aromatic conversion unit  280  converts the incoming stream  262 , comprising a mixture of toluene and C9+ aromatics, into an output stream  278  comprising 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 C9 aromatic isomers to xylene isomers through the migration of methyl groups between methyl-substituted aromatics. Benzene produced in the aromatic conversion assembly is extracted in an additional stream (not shown). 
         [0063]    The output stream  278  is fed into the split extract column  210 , which separates the output stream  278  into an overhead stream comprising toluene and a bottoms stream  282  comprising C8+ aromatics. The bottoms stream  282  is fed back into the xylene fractionation column  204  to separate the C8 aromatics into stream  206  and the C9+ aromatics into stream  208 . 
         [0064]    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 . 
         [0065]    The finishing column  272  separates the overhead stream  226  from extract column  224  into an overhead stream  298  comprising C7− aromatics, and a bottoms stream  276  comprising high purity para-xylene. 
         [0066]    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  or in extract column  224 . In some embodiments, the para-ethyltoluene is removed from the para-xylene product using techniques known in the art. 
         [0067]    In one embodiment, the bottoms para-xylene stream  276  comprises about 95.0 weight percent para-xylene. In one embodiment, the bottoms para-xylene stream  276  comprises about 99.2 weight percent para-xylene. In one embodiment, the bottoms para-xylene stream  276  comprises about 99.7 weight percent para-xylene. In one embodiment, the bottoms para-xylene stream  276  comprises about 99.9 weight percent para-xylene. In one embodiment, the bottoms para-xylene stream  276  comprises greater than about 99.9 weight percent para-xylene. 
         [0068]      FIG. 3  illustrates Applicant&#39;s split fractionation column  210 . Split fractionation column  210  comprises housing  302 , an upper portion  320 , and a lower portion  322 . Upper portion  320  includes trays  303  that are similar to a prior art fractionation column. Lower portion  322  is divided lengthwise by partition  308 , which extends upwardly from the bottom  332  of column  302  to top portion  334 , to create lower distillation zones  324  and  326 . Each of the lower distillation zones  324  and  326  have trays  304  and  306 , respectively. 
         [0069]    Input  278  is introduced into lower distillation zone  324 . Input  278  comprises a mixture of C8 aromatics, C9+ aromatics, and toluene. 
         [0070]    Input  238  is introduced into the lower distillation  326 . Input  238  comprises a mixture of C8 aromatics and toluene. In one embodiment, input  238  comprises a high concentration of para-xylene. 
         [0071]    In one embodiment, the para-xylene concentration of input  238  is higher than the para-xylene concentration of input  278 . Combining the material from input  278  and  238  into a single stream for fractional distillation in a prior art column (i.e., a column without the upper and lower portions  320  and  322 ) would result in the undesirable dilution of the high purity material in input  238 . In addition, such combining would introduce undesirable C9+ aromatics into the process of  FIG. 2 . 
         [0072]    The partition  308  prevents the liquid in the lower distillation zones from mixing. The partition  308  extends upwardly from the bottom  332  of column  302  to top portion  334 . The top 332 of partition  308  is determined by a particular application, but is selected to prevent the mixing of the bottoms liquid products  282  and  214 . 
         [0073]    The lighter components (i.e., those with a lower boiling point) introduced in inputs  278  and  238  vaporize at the temperature of the lower portion  322 . As such, the lighter components travel upwardly in column  302 . In one embodiment, the toluene is extracted as a liquid at a side cut tray (not shown) and light ends are extracted as a vapor at the overhead stream  212 / 242 . In another embodiment, the toluene is extracted as a vapor in the overhead stream and condensed to form a liquid stream. 
         [0074]    In one embodiment, the stream  212 / 242  comprises high purity toluene. In one embodiment, the stream  212 / 242  comprises toluene and light impurities. 
         [0075]    The heavier components (i.e., those with a higher boiling point) will remain in liquid form and, therefore, will remain in the lower distillation zone  324 , if introduced by way of input  278 , or will remain in lower distillation zone  326 , if introduced by way of input  238 . As such, the heavy fractions of the input  278  and input  238  will remain segregated in the lower portion  322 . 
         [0076]    In one embodiment, the bottoms product  282  is a mixture of C8 aromatics, C9+ aromatics, and heavies. As shown in  FIG. 2 , the bottoms product  282  is fed into a xylene fractionation column  204 . 
         [0077]    In one embodiment, the bottoms product  214  comprises a stream of high purity para-xylene. As shown in  FIG. 2 , the bottoms product  214  is fed into an adsorption separation unit  234 . 
         [0078]    The number of trays  303 ,  304  and  306  in each of the upper portion  320 , lower distillation zone  324 , and lower distillation zone  326 , respectively, vary with the particular product inputs and desired outputs, as would be appreciated by those skilled in the art. In one embodiment, the number of trays in lower distillation zone  324  is different than the number of trays in lower distillation zone  326 . In one embodiment, the number of trays in lower distillation zone  324  is the same as the number of trays in lower distillation zone  326 . 
         [0079]    The locations of inputs  278  and  238  on column  302  are selected to prevent any mixing of the heavy fractions across partition  308 . In one embodiment, the partition  308  extends  4  trays above the highest of feed trays  328  and  330 . In one embodiment, the partition extends greater than 4 trays above the highest of feed trays  328  and  330 . In one embodiment, the partition extends less then 4 trays above the highest of feed trays  328  and  330 . 
         [0080]    As those skilled in the art will appreciate, Applicant&#39;s split fractionation column  300  may be used to separate any two input streams that share a common light fraction but different heavy fractions. 
         [0081]    By incorporating Applicant&#39;s split fractionation column  302  into the apparatus and process of  FIG. 2 , the need for a separate toluene fractionation column in communication with aromatic conversion unit  280  is negated. Rather, the split fractionation column  210  is used to extract the toluene from a mixture of heavies for both the xylene separation process and for the aromatic conversion process. 
         [0082]    Referring to the foregoing paragraphs, this invention is described in preferred embodiments in the following description with reference to the Figures, in which like numerals represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0083]    The described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the above description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
         [0084]    While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example, although some aspects of separating para-xylene has been described, those skilled in the art should readily appreciate that functions, operations, decisions, etc., of all or a portion of each step, or a combination of steps, of the series of steps described may be combined, separated into separate operations or performed in other orders. Moreover, while the embodiments are described in connection with various illustrative processes, one skilled in the art will recognize that the methods and processes described herein can be embodied using a variety of techniques. Furthermore, disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s). The scope of the invention should be determined with reference to the pending claims along with their full scope or equivalents, and all changes which come within the meaning and range of equivalency of the claims are to be embraced within their full scope.