Patent Publication Number: US-11040304-B1

Title: Method of recovering paraxylene in a pressure swing adsorption unit with varying hydrogen purge flow direction

Description:
The present teachings relate generally to processes for recovering paraxylene, and in particular, to processes utilizing pressure swing adsorption for recovering paraxylene. 
     BACKGROUND 
     Paraxylene is a chemical intermediate that is oxidized to form terephthalic acid, which is a precursor to polyester. 
     Paraxylene is typically manufactured and recovered from streams comprising “mixed xylenes.” In the industry, mixed xylenes refer to a narrow boiling distillation heart cut of C8 aromatic hydrocarbons comprising the three xylene isomers orthoxylene, metaxylene, and paraxylene, as well as the structural isomer ethylbenzene. Mixed xylenes may also contain non-aromatic compounds with boiling points close to the xylenes. These mainly comprise C9 paraffins and naphthenes. Mixed xylenes generally also contain low levels of toluene and C9 and higher aromatics present due to their imperfect separation in the distillation towers used to produce the mixed xylenes heart cut. Mixed xylenes are typically obtained from a reformate of the refinery catalytic reformer unit or another unit used to produced mixed xylenes, such as a non-selective toluene disproportionation (TDP) unit, a selective toluene disproportionation (STDP) unit, a non-selective or selective toluene alkylation unit, a toluene/aromatic C9-plus transalkylation (TA) unit or an aromatic C9-plus transalkylation unit. 
     Paraxylene manufacturing units typically have three sections in a recycle loop: 1) a reaction section comprising a xylene isomerization catalyst and an ethylbenzene conversion catalyst; and 2) a fractionation section for separating byproducts produced in the reaction section and/or present in the fresh feed; and a 3) a paraxylene recovery section for recovering paraxylene from a mixture of xylene isomers and ethylbenzne. A recycle returns a paraxylene-lean stream formed as a reject filtrate in the paraxylene recovery section to the reaction section. 
     The isomerization catalyst returns a paraxylene-lean stream to its near equilibrium ratio of 1:2:1 (paraxylene:metaxylene:orthoxylene). The ethylbenzene conversion catalyst is also present because it is not practical to remove ethylbeneze by distillation because its boiling point is very close to the xylene isomers. Thus, ethylbenzene must be converted to xylenes or to byproducts that can be easily separated by distillation to prevent its build-up in the loop. For example, ethylbenzene isomerization-type catalysts (also known as naphthene pool catalysts) have the ability to convert a portion of the ethylbenzene to xylene isomers via C8 naphthene intermediates. Ethylbenzene dealkylation-type catalysts convert ethylbenzene primarily via reaction with hydrogen to form benzene and ethane. Ethylbenzene transalkylation-type catalysts convert ethylbenzene primarily by the transfer of the ethyl group to another ethyl benzene or to a xylene. 
     All of these catalysts produce by-products from the ethylbenzene conversion reactions and/or side reactions that must be separated in the fractionation section. These by-products include benzene, toluene, and C9-plus aromatics. The fractionation zone also removes C9-plus aromatics and other heavies present in the feed. 
     Two known methods for recovering paraxylene in the paraxylene recovery section are crystallization and selective adsorption. Selective adsorption processes include the UOP Parex process described in R A Meyers (editor) Handbook of Petroleum Refining Processes, Third Edition (2004) and the Axens Eluxyl process described in G Ash, et al, Oil and Gas Technology, 49 (5), 541-549 (2004). However, crystallization is often preferred to selective adsorption because it leads to overall process energy savings. Although xylene isomers and ethylbenzene have undesirably similar boiling points (making distillation difficult), they have dramatically different melting points. Pure paraxylene freezes at 56° F. (13° C.), pure metaxylene freezes at −54° F. (−48° C.), pure orthoxylene freezes at −13° F. (−25° C.) and pure ethylbenzene freezes at −139° F. (−95° C.). 
     In a typical crystallization zone for recovering paraxylene, liquid paraxylene is crystallized from a feedstream comprising the xylene isomers and ethylbenzene. The paraxylene is generally caused to crystallize by cooling the feedstream to a temperature below the freezing point of the paraxylene but preferably above the freezing point of the other components in the feedstream. More particularly, the temperature is selected to seek to optimize the crystallization of paraxylene, for example by selecting a temperature at which paraxylene freezes but which is above the eutectic temperature (the eutectic temperature is the temperature at which a xylene isomer other than paraxylene begins to co-crystallize). The paraxylene-metaxylene and paraxylene-orthoxylene eutectic temperatures can be close depending on the composition within the crystallizer, so either metaxylene or orthoxylene may be the first isomer to begin to co-crystallize. For non-selective feedstocks, the eutectic temperature is typically around −88° F. (−67° C.) to around −94° F. (−70° C.). 
     The low temperatures required to crystallize paraxylene from xylene mixtures are typically achieved by a cascaded vapour compression refrigerant system using a Deep Refrigerant. A Deep Refrigerant is defined as one for which it is generally not possible, or not economic, to compress its vapour or gas to a pressure level where it can be condensed by air or water cooling. Ethylene is a Deep Refrigerant, because its critical temperature is 49° F. (9.5° C.), and its critical pressure is 50.76 bar. Thus, for most places on earth, for at least part of the year, ethylene is a gas above its critical temperature at ambient temperature, and it is not possible to condense ethylene via air or water cooling. When used as a refrigerant, ethylene is usually condensed by transferring heat to a High Level Refrigerant. A High Level Refrigerant is defined as one for which it is possible to condense its vapour against air or water. Thus, a cascaded ethylene/propylene, ethylene/propane, or ethylene/ammonia refrigeration system can be used to achieve the low temperatures required for paraxylene crystallization. 
     Effluent from the crystallization zone contains paraxylene solids dispersed in a mother liquor, and it will typically therefore be necessary to separate these solids in one or more solid-liquid separation devices, such as centrifuges. Separation of the effluent produces a filtrate and a relatively paraxylene-rich cake. The cake obtained by separating the effluent from the crystallization stage contains paraxylene crystals with adhered mother liquor that contains ethylbenzene, other xylene isomers, unrecovered paraxylene and other components of the feedstream. To improve the purity, the cake is typically further processed in one or more reslurry zones in which the cake is equilibrated with a diluent stream comprising liquid paraxylene to provide a slurry. The reslurry effluent is separated in a solid-liquid separator to form a relatively pure paraxylene solid product and a filtrate that may be recycled or used in other parts of the process. 
     Another method for recovering paraxylene from mixed xylenes is known as pressure swing adsorption and is disclosed, for example, in U.S. Pat. Nos. 6,573,418, 6,600,083, 6,627,783, 6,689,929, and 7,271,305. In a pressure swing adsorption unit, a vapor phase containing mixed xylenes is fed at elevated temperature and pressure to a bed of fixed adsorbent containing a selective molecular sieve. Paraxylene and ethylbenzene are preferentially adsorbed to the sieve. The remaining stream is rich in metaxylene and orthoxylene and passes out of the pressure swing adsorption unit. The pressure is then lowered and paraxylene and ethylbenzene are desorbed to form a paraxylene and ethylbenzene rich effluent stream. This effluent may be then sent to a crystallization zone for recovery of the paraxylene. 
     Prior pressure swing adsorption units were effective in separating and recovering paraxylene. However, one of the variable costs of the prior methods was to compress hydrogen for circulation to the pressure swing adsorption unit. There remains a need to develop cost efficient processes for manufacturing paraxylene. 
     SUMMARY 
     According to one aspect of the invention, a process for the recovery of a paraxylene product from a mixture of C8 aromatic hydrocarbons is provided. The process includes feeding a C8-rich aromatic hydrocarbon mixture to a pressure swing adsorption zone to form a paraxylene-rich stream and a paraxylene-lean stream. The pressure swing adsorption zone is adapted to adsorb and desorb paraxylene based on the cycling of pressure in the zone. The cycling of pressure in the zone uses a first hydrogen purge that travels in the cocurrent direction relative to the xylene feed in the zone and a second hydrogen purge that travels in the countercurrent direction relative to the xylene feed in the zone. Intermediate hydrogen purges traveling in either the cocurrent or countercurrent direction relative to the xylene feed may also be utilized. 
     Other aspects of the invention will be apparent to those skilled in the art in view of the description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows a process flow diagram for manufacturing paraxylene according to one prior art method. 
         FIG. 1 b    shows a process flow diagram for manufacturing paraxylene in accordance with one embodiment of the present invention. 
         FIG. 2  shows a process flow diagram of a pressure swing adsorption zone in accordance with one embodiment of the present invention 
         FIG. 3  shows a process flow diagram of a paraxylene recovery zone in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     By way of general introduction, a process for recovering paraxylene is disclosed. The process includes a pressure swing adsorption zone for separating paraxylene and ethylbenzene from metaxylene and orthoxylene. The present invention improves the recovery and purity of the paraxylene rich stream leaving the pressure swing adsorption zone. This results in cost savings, reducing impure recycles around the unit thus saving energy and reducing undesirable xylene losses across reactors in the design. 
     Referring now to  FIG. 1 a    and  FIG. 1 b   , a process for the recovery of paraxylene product is shown generally at  10 .  FIG. 1 a    illustrates a prior art process for manufacturing paraxylene, and  FIG. 1 b    illustrates a process in accordance with the present invention. A fresh feed  12  including mixed xylenes is fed to a fractionation zone including a xylene recovery distillation column  20  for separation of C8-rich aromatic hydrocarbon mixture from other components. In one embodiment, the mixed xylene fresh feed comprises paraxylene, orthoxylene, metaxylene, as well as the structural isomer ethylbenzene. In other embodiments, the fresh feed also comprises C7 and C9+ aromatic compounds, as well as non-aromatic compounds such as C9 paraffins and naphtenes. Typically, the mixed xylene fresh feed  12  is formed as reformate of a refinery catalytic reformer unit, or another unit used to produced mixed xylenes, such as a non-selective toluene disproportionation (TDP) unit, a selective toluene disproportionation (STDP) unit, a non-selective or selective toluene alkylation unit, a toluene/aromatic C9-plus transalkylation (TA) unit or an aromatic C9-plus transalkylation unit. The mixed xylene fresh feed  12  is typically at least 90 wt % mixed xylenes. In some embodiments, the mixed xylene fresh feed  12  is at least 95 wt %, 98 wt %, or 99 wt % mixed xylenes. 
     In some embodiments, a second fresh feed mixed xylene containing stream  14  is also fed the column  20 . The second mixed xylene containing stream  14  is typically a heavier cut of reformate containing a higher concentrations of C9+ compounds, and is fed lower on the column  20  than the first fresh feed  12 . The second mixed xylene stream  14  typically contains at least 10 wt % of C9+ compounds. In some embodiments, the mixed xylene fresh feed  12  is at least 15 wt %, 20 wt %, 25 wt %, 35 wt %, or 50 wt % of C9+ compounds. 
     At least one other xylene containing stream resulting from recycle loops in the process  10  is fed to the column  20 . In the embodiment shown, two such feeds  16 ,  18  are shown. Those skilled in the art will appreciate that other configurations of the recycled feed are also possible. The xylene containing feed stream  16  and xylene containing feed stream  18  typically contain proportionally less ethylbenzene than the mixed xylene feed streams  12 ,  14 . The feed streams  16  and  18  further comprise benzene. In some embodiments, the feed streams contain at least 1 wt % or 2 wt % benzene. In other embodiments, feed stream  16  contains at least 5 wt % benzene. 
     The xylene recovery column  20  is configured to separate the feed streams  12 ,  14 ,  16 ,  18  into one or more streams comprising a C8-rich aromatic hydrocarbon mixture, a stream containing C7-compounds, and a stream containing C9+ compounds. In the embodiment shown in  FIG. 1 a    or  FIG. 1 b   , a first sidedraw stream  22  comprises a liquid phase C8-rich aromatic hydrocarbon mixture, while a second sidedraw stream  24  comprising a vapor phase C8-rich aromatic hydrocarbon mixture. The first sidedraw stream  22  is withdrawn at location on the column above the second sidedraw stream  24 . The feed stream  18  is introduced to the column above the vapor phase sidedraw stream  24  so that gaseous components in the feed stream  18  do not exit through the vapor phase sidedraw stream  24 . The liquid phase sidedraw phase is pressurized by pump  23 . The vapor phase sidedraw stream is condensed by condenser  26  and the resulting condensate is pressured by pump  27 . The pressurized condensate of the vapor phase sidedraw stream and the pressurized liquid phase sidedraw stream are combined to form a combined C8-rich aromatic hydrocarbon mixture stream  28 . 
     An overhead product stream  30  is withdrawn from the top of the column  20  and comprises C7-compounds including benzene, toluene, and ethane. The overhead product stream  30  is partially condensed by condenser  32  and the condenser effluent is separated into liquid and gaseous components in flash drum  34 . The liquid phase is partially returned to the column  30  as a reflux stream  36   b  and partially removed from the process via stream  36   a . The gaseous components are removed from the process as a light co-product stream  38 . 
     A bottoms product stream  40  is removed from the bottom of the column  20  and comprises C9+ compounds including trimethylbenzene and methylethylbenzene. A portion of the bottoms product is recovered as a bottoms co-product stream  42 , while another portion  44  of the bottoms product stream  40  is reboiled by reboiler furnace  46  and returned to the column  20 . The reboiler furnace  46  provides for the elevated temperature of the column  20  which operates in a temperature gradient, for example, between 500° F. (260° C.) and 50° F. (10° C.) and a pressure of 15-80 psia. 
     In the prior art process shown in  FIG. 1 a   , the combined C8-rich aromatic hydrocarbon mixture stream  28  is fed to a paraxylene recovery zone  72 . However, in the present invention illustrated in  FIG. 1 b   , at least a portion of the C8-rich aromatic hydrocarbon mixture  28  recovered from the fractionation zone is pre-heated by furnace  50  and one or more heat exchangers (not shown) and delivered via line  51  to a pressure swing adsorption zone  52 . In the pressure swing adsorption zone  52 , the C8-rich aromatic hydrocarbon mixture is fed at elevated temperature and pressure to a bed of fixed adsorbent containing a selective molecular sieve. Paraxylene and ethylbenzene are preferentially adsorbed to the sieve. The remaining stream is rich in metaxylene and orthoxylene and passes out of the pressure swing adsorption unit as paraxylene-lean stream  54 . The partial pressure is then lowered and paraxylene and ethylbenzene are desorbed to form a paraxylene-rich and ethylbenzene-rich effluent stream  56 . The configuration and operation of the pressure swing adsorption zone is more fully described below and in reference to  FIG. 2 . Stream  56  is then sent to a condenser  57  and a flash drum  62  where a C8 rich stream  70  is formed along with a hydrogen rich stream  64 . Stream  64  is then sent to compressor  66  to be compressed and sent back to pressure swing adsorption zone  52 . 
     Pressurized hydrogen purge gas streams are fed through lines  60   a  and  60   b  from compressor  66  to the pressure swing adsorption zone  52 . In one embodiment, a first hydrogen purge gas, through line  60   a , is fed cocurrent to the feed direction with stream  54  as its destination. In another embodiment, the first hydrogen purge, through line  60   a , is fed cocurrent to the feed direction with stream  51  as its destination. The feeding of the first hydrogen purge through  60   a  cocurrent to the feed takes advantage of the fact that at the end of the feed step the material near where the feed was exiting the bed is enriched in metaxylene and orthoxylene. By having a first cocurrent hydrogen purge, this remaining metaxylene and orthoxylene can be removed while minimizing the paraxylene and ethylbenzene being lost thus increasing the purity and recovery of the paraxylene and ethylbenzene and improving the overall performance of the pressure swing adsorption section. 
     In one embodiment, a second hydrogen purge is fed, through line  60   b , countercurrent to the feed direction with stream  56  as its destination. In another embodiment, an intermediate hydrogen purge is introduced between the first and second hydrogen purge in a direction cocurrent to the feed through line  60   a . In another embodiment, an intermediate hydrogen purge is introduced between the first and second hydrogen purge in a direction countercurrent to the feed through line  60   b . The paraxylene-rich and ethylbenzene-rich effluent  70  exiting the flash drum  62  is fed to a paraxylene recovery zone  72 . In one embodiment, a C8-rich aromatic hydrocarbon mixture feed  74  to the paraxylene recovery zone  72  comprises a second portion of the combined C8-rich aromatic hydrocarbon mixture stream  28  exiting the column  20  and bypasses the pressure swing adsorption unit  52 . In one embodiment, the second feed  74  comprises at least 10 wt % of the combined paraxylene-rich and ethylbenzene-rich stream  28 . In other embodiments, the second feed  74  comprises at least 20 wt %, at least 30 wt %, at least 40 wt %, at least 50 wt %, at least 60 wt %, at least 70 wt %, at least 80 wt %, or at least 90 wt % of the combined C8-rich aromatic hydrocarbon mixture stream  28 . 
     The paraxylene recovery zone  72  operates to produce a paraxylene product  76  and to recycle a paraxylene-lean stream  78  for further processing. In one embodiment, the paraxylene recovery zone  72  is configured to recover paraxylene product through a selective adsorption process. In another embodiment, the paraxylene recovery zone  72  is configured is configured as to recover paraxylene through a crystallization process. One particular crystallization process is described below in reference to  FIG. 3 . 
     The paraxylene-lean stream  54  exiting the pressure swing adsorption unit  52  is fed to an isomerization reactor  80 . The isomerization reactor  80  is a packed bed reactor containing a bed of an isomerization catalyst for converting metaxylene and orthoxylene to paraxylene at an approximately equilibrium ratio of 1:2:1 (paraxylene:metaxylene:orthxylene). In one embodiment, hydrogen  82  is added to the paraxylene-lean stream  54  upstream of the isomerization reactor  80 . In another embodiment, the paraxylene-lean  54  contains enough hydrogen after exiting the pressure swing adsorption unit  52  that make-up hydrogen is not added to the feed  54 . In the embodiment shown, the isomerate stream  91  is fed to a condenser  120  and a flash drum  122 . where a C8 rich stream  126  is formed along with a hydrogen rich stream  124 . Stream  124  is then sent to compressor  66  to be compressed and sent back to pressure swing adsorption zone  52 . Bottom stream  126  is sent to pump  128 . Then all or a portion of the pump effluent may be sent via stream  130  directly to furnace  50  or all or a portion of the pump effluent may be sent via stream  132  to drum  94 . 
     The second paraxylene-lean stream  78  exiting the paraxylene recovery zone  72  is mixed with fresh hydrogen  84  and pre-heated with a furnace  86  and/or one or more heat exchangers (not shown). The preheated mixture  88  is fed to the additional isomerization reactor  90 . The isomerization reactor  90  contains a isomerization catalyst for converting metaxylene and orthoxylene to paraxylene at an approximately equilibrium ratio of 1:2:1 (paraxylene:metaxylene:orthxylene). In some embodiments, the isomerization reactor  90  also contains an ethylbenzene conversion, catalyst such as dealkylation catalyst for converting ethylbenzene to benzene and ethane. Suitable isomerization catalysts and ethylbenzene catalysts are disclosed, for example, in U.S. Pat. Nos. Re 31,782, 4,899,011, and 6,518,472. 
     In the embodiment shown, the isomerate stream  92  from the additional isomerization reactor  90  is fed to a high temperature separator  94  where the stream is flashed. A liquid-rich phase bottom stream from the high temperature separator  94  is one of the xylene containing feed stream  18  to the column  20 . A vapor-rich phase stream  95  exiting high temperature separator  94  is sent to a low temperature separator  96  where the vapor-rich phase stream is flashed. A liquid-rich phase stream exiting the low temperature separator  96  is another of the xylene containing feed stream  16  fed to the column. A vapor phase stream  98  exiting the low temperature separator  96  comprises hydrogen, ethane, and other light components and may be recycled and used as a source for streams  82  and/or  84  or be used for fuel. 
       FIG. 2  shows one embodiment of the pressure swing adsorption zone  52  according to the present invention. The pressure swing adsorption zone  52  comprises one or more vessels  100   a ,  100   b ,  100   c ,  100   d , containing a paraxylene selective adsorbent. In the embodiment shown, there are four vessels, but those skilled in the art will recognize that other configurations are also possible, such as configurations with different numbers of vessels, e.g. 3 vessels, 5 vessels, 6 vessels, 7 vessels, 8 vessels, etc. In one embodiment, the paraxylene selective adsorbent is a non-acidic, medium pore, molecular sieve. In one embodiment, the molecular sieve is of the MFI structure type and the process is operated in the vapor phase at elevated temperatures and pressures wherein the temperature is substantially isothermal. Adsorbents useful in the present invention are based on molecular sieves that selectively adsorb paraxylene within the channels and pores of the molecular sieve while not effectively adsorbing metaxylene and orthoxylene C 8  isomers (i.e., total exclusion of the larger metaxylene and orthoxylene or having much slower adsorption rates compared to paraxylene). 
     Molecular sieves are ordered porous crystalline materials, typically formed from silica, alumina, and phosphorus oxide (PO 4 ) tetrahedra, that contain a crystalline structure with cavities interconnected by channels. The cavities and channels within the crystalline structure are uniform in size and may permit selective separation of hydrocarbons based upon molecular dimensions. Generally, the term “molecular sieve” includes a wide variety of natural and synthetic crystalline porous materials which typically are based on silica tetrahedra in combination with other tetrahedral oxide materials such as aluminum, boron, titanium, iron, gallium, and the like. In these structures networks of silicon and elements such as aluminum are cross-linked through sharing of oxygen atoms. Substitution of elements such as aluminum or boron for silicon in the molecular sieve structure produces a negative framework charge which must be balanced with positive ions such as alkali metal, alkaline earth metal, ammonium or hydrogen. Molecular sieve structures also may be formed based on phosphates in combination with other tetrahedrally substituted elements such as aluminum. 
     Adsorbents useful in this invention should not possess catalytic isomerization or conversion activity with respect to the C 8  aromatic feedstream. Thus, suitable molecular sieves should be non-acidic. If an element such as aluminum or gallium is substituted in the molecular sieve framework, the sieve should be exchanged with a non-acidic counter-ion, such as sodium, to create a non-acidic sieve adsorbent. 
     Examples of molecular sieves suitable as adsorbents useful in this invention include zeolitic materials containing pore dimensions in the range of 5 to 6 angstroms (10 meter), typically 5.1 to 5.7 angstroms, and preferably 5.3 to 5.6 angstroms, as measured in cross axes of the pore. This range typically is referred to as “medium pore” and typically contains 10-ring tetrahedra structures. Typical examples of medium pore molecular sieves include those with MFI and MEL framework structures as classified in Meier and Olson, “Atlas of Zeolite Structure Types,” International Zeolite Association (1987), incorporated herein by reference in its entirety. A small pore molecular sieve, such as A zeolite, which contains 8-ring structures does not have a sufficiently large pore opening to effectively adsorb para-xylene within the sieve. Most large pore molecular sieves, such as mordenite, Beta, LTL, or Y zeolite, that contain 12-ring structures do not adsorb para-xylene selectively with respect to ortho- and meta-xylenes. However, several 12 ring structures, having a smaller effective pore size, for example due to puckering, are potentially useful in the invention, such as structure types MTW (e.g., ZSM-12) and ATO (e.g., ALPO-31). 
     Specific examples of molecular sieves include ZSM-5 (MFI structure type) and ZSM-11 (MEL structure type) and related isotypic structures. Since suitable adsorbents should not be catalytically reactive to components in the feedstream, the preferable adsorbent useful in this invention is silicalite (MFI structure type), an essentially all silica molecular sieve, which contains minimal amounts of aluminum or other substituted elements. Typically, the silica/alumina ratio of suitable silicalite is above 200 and may range above 1000 depending on the contaminant level of aluminum used in the sieve&#39;s preparation. Other MFI and MEL sieves may be use to the extent they are made non-catalytically active. Other potentially useful adsorbents include structure types MTU, FER EUO, MFS, TON, AEL, ATO, NES, and others with similar pore sizes. 
     A molecular sieve which is not catalytically reactive will typically exhibit less than 10% conversion of paraxylene to metaxylene and orthoxylene, and in some embodiments, less than 5%, and in other embodiments less than 1%, at the temperature of operation for the process of the invention. 
     The C8-rich aromatic hydrocarbon mixture enters pressure swing adsorption zone  52  through xylene header  102  and is introduced into vessels  100   a ,  100   b ,  100   c ,  100   d  through respective feed control valves  102   a ,  102   b ,  102   c ,  102   d . The first hydrogen purge, from  60   a , enters the vessels  100   a ,  100   b ,  100   c ,  100   d  through hydrogen header  106  and high pressure hydrogen feed control valves  106   a ,  106   b ,  106   c ,  106   d  respectively. The second hydrogen purge gas, from  60   a , enters the vessels  100   a ,  100   b ,  100   c ,  100   d  through hydrogen header  112  and hydrogen feed control valves  112   a ,  112   b ,  112   c ,  112   d , respectively. The third hydrogen purge gas, from  60   b , enters the vessels  100   a ,  100   b ,  100   c ,  100   d  through hydrogen header  104  and hydrogen feed control valves  104   a ,  104   b ,  104   c ,  104   d , respectively. 
     The pressure swing adsorption zone  52  also comprises a paraxylene and ethylbenzene collection header  108  and a set of outlet control valves  108   a ,  108   b ,  108   c ,  108   d  for removing a paraxylene-rich and ethylbenzene-rich stream  56  from each of the vessels  100   a ,  100   b ,  100   c ,  100   d ,  100   e ,  100   f , respectively. The pressure swing adsorption zone  52  also comprises a metaxylene and orthoxylene collection header  110  and a set of outlet control valves  110   a ,  110   b ,  110   c ,  110   d ,  110   e ,  110   f  for removing the first paraxylene-lean stream  54  from the vessels  100   a ,  100   b ,  100   c ,  100   d , respectively. The pressure swing adsorption zone  52  also comprises a feed recirculation header  114  which can direct flow back to the xylene header  102  through a set of control valves  114   a ,  114   b ,  114   c ,  114   d.    
     The vessels  100   a ,  100   b ,  100   c ,  100   d  in the pressure swing adsorption zone  52  are operated in a sequence of operations, the sequence of operations in each vessel being offset in time from the sequence of operations in the other vessels such that the vessels operate together in a pseudo-continuous manner. 
     The sequence of operations are now described with reference to the first vessel  100   a . All the valves are controlled automatically by a control system (not shown). The valves are maintained closed unless they are described as being opened below for a particular operation. 
     In the first operation, designated “FEED”, C8-rich aromatic hydrocarbon stream is introduced through feed header  102  and feed control valve  102   a  to vessel  100   a  at elevated pressure. The paraxylene and ethylbenzene molecules adsorb to the adsorbent, while the metaxylene and orthoxylene molecules are blown through the bed and leave the process through the outlet control valve  110   a  and the metaxylene and orthoxylene collection header  110 . 
     In the second operation, designated “HP 1 ,” (hydrogen purge  1 ) the high pressure purge gas, from  60   a  ( FIG. 1 b   ), is fed through header  106  and control valve  106   a  to sweep the bed. This hydrogen displaces some of the metaxylene and orthoxylene left in the void space of the bed and continues to flow out through control valve  110   a  and header  110  after the FEED operation is complete. This allows the paraxylene and ethylbenzene to be extracted in a later operation without being contaminated by metaxylene and orthoxylene. 
     In the third operation, designated “HP 2 ”, (hydrogen purge  2 ) the high pressure purge gas, from  60   a  ( FIG. 1 b   ), is fed through header  112  and  112   a  to sweep the bed. This hydrogen displaces the remaining metaxylene and orthoxylene left in the void space of the bed. In the act of removing the last remaining metaxylene and orthoxylene some paraxylene and ethylbenzene also begins to be displaced from the bed. Thus, the HP 2  step leaves the bed through valve  114   a  to the feed recirculation header  114  and back to the xylene feed header  102 . This allows the remaining metaxylene and orthoxylene to be removed from the bed without losing the paraxylene and ethylbenzene that go with it to the metaxylene and orthoxylene collection header  110 . This improves the recovery and purity of the final products. 
     In the fourth operation, designated “HP 3 ” (hydrogen purge  3 ), the hydrogen stream, from  60   b  ( FIG. 1 b   ), is fed through hydrogen header  104  and valve  104   a  into vessel  100   a  at the desorption pressure and sweeps the bed. This sweeping of the bed drops the partial pressure of paraxylene and ethylbenzene. This causes desorption of paraxylene and ethylbenzene from the adsorbent. The paraxylene and ethylbenzene rich stream then leaves the bed via valve  108   a  and on to the paraxylene and ethylbenzene collection header  108 . 
     All four vessels  100   a ,  100   b ,  100   c ,  100   d  go through this same cycle of four operations, but at any given time, each vessel is at a different stage of the cycle. The system is designed and operated such that one of vessels is always in the FEED operation so that the feed to the pressure swing adsorption zone  52  as a whole is constant. Table 1 illustrates one embodiment of a sequence of the four operations for the pressure swing adsorption zone  52  in which plurality of vessels operate together in a pseudo-continuous manner. The Table illustrates four time periods and shows which operation is being performed in each vessel at each time period. A typical time period is from 5 seconds to about 120 seconds. Those skilled in the art will recognize that the sequence in Table 1 is exemplary and other sequences are also possible to carry out the invention. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Sequence of Pressure Swing Adsorption Operations 
               
            
           
           
               
               
               
               
               
            
               
                 Time Period 
                 Vessel a 
                 Vessel b 
                 Vessel c 
                 Vessel d 
               
               
                   
               
               
                 1 
                 Feed 
                 HP3 
                 HP2 
                 HP1 
               
               
                 2 
                 HP1 
                 Feed 
                 HP3 
                 HP2 
               
               
                 3 
                 HP2 
                 HP1 
                 Feed 
                 HP3 
               
               
                 4 
                 HP3 
                 HP2 
                 HP1 
                 Feed 
               
               
                   
               
            
           
         
       
     
       FIG. 3  illustrates one embodiment of the paraxylene recovery zone  72  in accordance with the present invention. The paraxylene-rich and orthoxylene-rich effluent  70  enters a crystallization zone comprising a first crystallization zone comprising a first crystallizer  202  and a second crystallization zone comprising a second crystallizer  216 . In one embodiment, the effluent  70  comprises at least 25 wt % paraxylene. In other embodiments, the effluent  70  comprises less than 75 wt %, less than 85 wt %, or less than 95 wt % paraxylene. The first crystallizer  202  is temperature controlled to operate to solidify paraxylene in the effluent  70 . In one embodiment, the first crystallization zone operates at a temperature greater than −40° F. (−40° C.). In another embodiment, the first crystallization zone operates at a temperature greater than −30° F. (−34.4° C.). The first crystallization zone typically operates a temperature between 40° F. (4.4° C.) and −40° F. (40° C.). The first crystallizer effluent  204  is withdrawn from the first crystallizer  202  and sent to a first solid-liquid separator  206 . The first solid-liquid separator  206  separates a paraxylene-lean filtrate stream  208  from a first paraxylene-rich cake stream  210 . One portion  212  of the paraxylene-lean filtrate stream may be recycled to the first crystallizer  202 , while another portion  214  of the paraxylene-lean filtrate stream is sent to a second crystallizer  216 . The second crystallizer  216  is temperature controlled to further solidify any remaining paraxylene. The second crystallization zone operates at a temperature less than the first crystallization zone. In one embodiment, the second crystallization zone operates at a temperature less than −70° F. (−56.7° C.). In another embodiment, the second crystallization zone operates at a temperature less than −90° F. (−67.8° C.). In another embodiment, the second crystallization zone operates at a temperature less than −110° F. (−78.9° C.). The second crystallization zone typically operates at a temperature between −30° F. (−34.4° C.) and −130° F. (−90° C.). The second crystallizer effluent  218  is withdrawn from second crystallizer  216  and introduced to a second solid-liquid separator  220 . In one embodiment, the effluent  218  exiting the second crystallizer is colder than −95° F. (−70.6° C.). The second solid-liquid separator  220  separates a second paraxylene-rich cake  222  from a second paraxylene-lean filtrate stream  224 . A portion  228  of the filtrate stream  224  may be recycled to the crystallizer  216 , while another portion  78  of the paraxylene-lean filtrate stream  224  is recycled for further processing as described above and in reference to  FIG. 1   b.    
     The first and second solid-liquid separator  202 ,  216  may be any solid-liquid separation devices known in the art, such as centrifuges, rotary pressure filters, rotary vacuum filters, or filter columns. In one particular embodiment, the first solid-liquid separator  206  comprises a pusher centrifuge and the second solid-liquid separator  220  comprises a screen bowl centrifuge. In one embodiment, the second solid-liquid separator  220  removes an additional paraxylene-lean filtrate  226  before withdrawing the second paraxylene-lean filtrate  224 . The additional filtrate  226  is higher in paraxylene concentration than the second paraxylene-lean filtrate  224  and is recycled to the second crystallizer  216 . 
     The first paraxylene-rich cake  210  and the second paraxylene-rich cake  222  enters one or more reslurrying zones for removing any remaining impurities. The embodiment of  FIG. 3  shows two reslurrying zones, each having a reslurry drum  224 ,  240 . The paraxylene-rich cake  210  from the first solid-liquid separator is fed to either or both of the first reslurry drum  224  and the second reslurry drum  240  through streams  226  and  242 , respectively. The second paraxylene-rich cake  222  is fed to the first reslurry drum  224 . The paraxylene-rich cake(s) are reslurried in the first reslurry drum  224  with reslurrying fluids to remove impurities from the paraxylene crystals and the effluent  231  from the first reslurry drum  224  is sent to the third solid-liquid separator  232 . The third solid-liquid separator  232  separates the effluent  231  into a third paraxylene-rich cake  238  and a third paraxylene-lean filtrate stream  234 . A portion  228  of the third paraxylene-lean filtrate stream  234  is recycled to the first reslurry drum  224  as a reslurrying fluid, and another portion  236  may be recycled to the first crystallizer  202  for further recovery of paraxylene. 
     The third paraxylene-rich cake  238  is fed to a second reslurry drum  240  for further reslurrying with one or more reslurrying fluids for removing impurities from the paraxylene crystals. The effluent  248  from the second reslurry drum  240  is fed to a fourth solid-liquid separator  250 . The fourth solid-liquid separator  250  separates the effluent  248  into a fourth paraxylene-rich cake  252  and a fourth paraxylene-lean filtrate stream  254 . A portion  244  of the fourth paraxylene-lean filtrate stream  254  is recycled to the second reslurry drum  244  as a reslurrying fluid, and another portion  230  of the fourth paraxylene-lean filtrate stream  254  may be recycled to the first reslurry drum  224  for use as a reslurrying fluid. 
     The third and fourth solid-liquid separator  232 ,  250  may be any solid-liquid separation devices known in the art, such as centrifuges, rotary pressure filters, rotary vacuum filters, or filter columns. The fourth solid-liquid separator  250  may also be a wash column. Suitable filter columns are disclosed, for example, in U.S. Pat. Nos. 7,812,206, 8,211,319, and 8,530,716, and 8,962,906, Suitable wash columns are disclosed, for example, in U.S. Pat. Nos. 4,734,102 and 4,735,781. In one particular embodiment, the third solid-liquid separator  232  comprises a pusher centrifuge and the fourth solid-liquid separator  250  comprises a pusher centrifuge. In one embodiment, the fourth solid-liquid separator  250  removes an additional paraxylene-lean filtrate  246  before withdrawing the fourth paraxylene-lean filtrate  254 . The additional filtrate  246  is higher in paraxylene concentration than the fourth paraxylene-lean filtrate  254  and is recycled to the second reslurry drum  240 . 
     The fourth paraxylene-rich cake  252  is fed to a melt drum  256 . The fourth paraxylene-rich cake is completely melted and a paraxylene product stream  76  is recovered. A portion  258  of the melted paraxylene may be recycled to the fourth solid-liquid separator  250  in order to wash impurities from the cake. In one embodiment, the paraxylene product  76  is at least 99 wt % paraxylene. In other embodiment, the paraxylene product is at least 99.5 wt %, 99.6 wt %, 99.7 wt %, or 99.8 wt % paraxylene. 
     The use of a pressure swing adsorption zone with an additional isomerization zone allows for less total mass being fed to the paraxylene recovery zone, because a significant portion of the metaxylene and orthoxylene in the system is recycled through stream  54  ( FIG. 1 b   ). In one embodiment, the ratio of the total mass of the paraxylene-rich stream entering the paraxylene recovery zone to the total mass of the paraxylene-rich product stream is less than 6. In other embodiments, the ratio of the total mass of the paraxylene-rich stream entering the paraxylene recovery zone to the total mass of the paraxylene-rich product stream is less than 5, less than 4, less than 3, or less than 2. In other embodiments, the ratio of the total mass of the recycle stream  78  ( FIG. 1 b   ) to the total mass of the paraxylene-rich product stream  76  is less than 5, less than 3, or less than 2. The feed to the paraxylene recovery zone also contains a higher concentration of paraxylene compared to systems not having a pressure swing adsorption zone. This is because the pressure swing adsorption zone allows for paraxylene concentrations greater than the equilibrium concentration resulting from the isomerization reaction. 
     According to another aspect of the invention, a method for retrofitting a system for recovering paraxylene is provided. According to the retrofitting method, the pressure swing adsorption zone  52  ( FIG. 1 b   ) is added to a pre-existing system ( FIG. 1 a   ) not having a pressure swing adsorption zone. At least a first portion  51  of the combined C8-rich aromatic hydrocarbon mixture stream  28  is routed to the pressure swing adsorption zone  52  to form a paraxylene-rich intermediate stream  56  (which is flashed in drum  62  to form stream  70 ) before being fed to the paraxylene recovery zone  72 . The retrofit method may also comprise adding the secondary isomerization zone  80  to a pre-existing system where there was no previous secondary isomerization zone  80 . The retrofit method may also include adding the bypass stream  74  so that a second portion of the combined C8-rich aromatic hydrocarbon mixture stream  28  routes directly to the paraxylene recovery zone  72 , bypassing the pressure swing adsorption zone  52 . The amount of C8-rich aromatic hydrocarbon mixture stream bypassed through bypass stream  74  is dependent upon the throughputs of the pressure swing adsorption zone  52  and the pre-existing equipment. In one embodiment, the pre-existing equipment does not have to be re-sized as a result of the retrofit, which allows increased recovery of paraxylene without significant capital expenditures. By enriching the combined stream  28  in paraxylene prior to its delivery to the paraxylene recovery zone and adding isomerization capacity, the retrofit method allows for increased recovery of paraxylene product compared to the pre-existing system. In one embodiment, the amount of a paraxylene product recovered by the retrofitted system increases without increasing the throughput of the primary isomerization zone  90 . In another embodiment, the amount of paraxylene product recovered increases without increasing the amount of hydrogen fed to the system. In another embodiment, the amount of paraxylene product recovered increases without increasing the amount of the refrigeration duty of the crystallization zone. In another embodiment, the amount of paraxylene product recovered increases without increasing the amount of the furnace duty  86  of the primary isomerization zone. In another embodiment, the amount of paraxylene product recovered increases without increasing the amount of the furnace duty  46  of the fractionation zone. 
     The foregoing detailed description and the accompanying drawings have been provided by way of explanation and illustration, and are not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. 
     It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims can, alternatively, be made to depend in the alternative from any preceding claim—whether independent or dependent—and that such new combinations are to be understood as forming a part of the present specification.