Abstract:
A separation membrane is described. The separation membrane comprises a porous inorganic membrane, the pores of the inorganic membrane being coated with a polybenzoxazole polymer coating. Methods of making the separation membrane and methods of separating xylenes using the separation membrane are also described.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to separation membranes and more particularly to high temperature stable membranes, methods of making high temperature stable membranes, and methods of using high temperature stable membranes. 
       BACKGROUND OF THE INVENTION 
       [0002]    When xylenes are subjected to isomerization, they form mixtures of para-xylene, ortho-xylene and meta-xylene. At the temperatures at which xylene isomerization is typically conducted, para-xylene forms approximately 24% of the equilibrium mixture, ortho-xylene about 23%, and meta-xylene about 53%. The C8 aromatic ethylbenzene is usually also present in a mixture of xylenes. Although each of these products has commercial value, the highest demand is for the para-xylene isomer. 
         [0003]    Commercial processes for recovering para-xylene involve the selective removal of para-xylene by selective crystallization or sorption. One embodiment of a combined xylene separation process is illustrated in  FIG. 1 . The feed  5 , having previously been stripped of all materials lighter than C8 aromatics, enters the C8 distillation column  10  and is separated into a stream  15  of C8 hydrocarbons and a stream  20  of C9+hydrocarbons. The stream  15  of C8 hydrocarbons is sent to a sorption or crystallization zone  25  where it is separated into a stream  30  of highly pure para-xylene and a stream  35  containing ortho-xylene, meta-xylene, and ethyl benzene. Stream  35  is sent to an isomerization zone  40  where the xylenes are isomerized to an equilibrium xylene mixture, and some portion of any ethylbenzene present is either dealkylated (to benzene and ethylene) or converted to a xylene. Stream  45  which is a mixture of equilibrium xylenes and unconverted ethylbenzene is then recycled to the C8 distillation column  10 . 
         [0004]    The recycle loop typically contains separation operations (not shown) downstream of the isomerization zone, including, but not limited to, one or more of a toluene splitter to remove toluene and lighter hydrocarbons from the xylenes, and a xylene splitter to remove a portion of the o-xylene and heavier hydrocarbons from the C8 hydrocarbons as a separate product. In most commercial processes, other components are present in the recycle loop, such as saturates that boil in the C8 aromatic range or other components formed in the isomerization zone, such as hydrocarbons lighter than C8, hydrocarbons heavier than C8, and naphthenes. 
         [0005]    It would be desirable to improve the separation of xylene isomers. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the invention is a separation membrane. In one embodiment, the separation membrane comprises a porous inorganic membrane, the pores of the inorganic membrane being coated with a polybenzoxazole polymer coating. 
         [0007]    Another aspect of the invention is a method of making a separation membrane. In one embodiment, the method comprises dissolving a polybenzoxazole precursor in a solvent to form a solution. A porous inorganic membrane is coated with the solution. The coated porous membrane is heated to form a polybenzoxazole polymer coated porous membrane. 
         [0008]    Another aspect of the invention is a method of separating xylenes. In one embodiment, the method comprises contacting a vapor or liquid stream comprising para-xylene, and at least one of ortho-xylene, meta-xylene, and ethylbenzene with a porous inorganic membrane, the pores of the inorganic membrane being coated with a polybenzoxazole polymer coating, forming a membrane permeate stream enriched in para-xylene, ethylbenzene, or both, and a membrane retentate stream enriched in ortho-xylene, meta-xylene, or both. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an illustration of a separation process including a crystallization or sorption zone and an isomerization zone. 
           [0010]      FIG. 2  is an illustration of one embodiment of a process using the porous inorganic membrane modified with a polybenzoxazole polymer. 
           [0011]      FIG. 3  is an illustration of another embodiment of a process using the porous inorganic membrane modified with a polybenzoxazole polymer. 
           [0012]      FIG. 4  is an illustration of still another embodiment of a process using the porous inorganic membrane modified with a polybenzoxazole polymer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The high temperature stable membranes can be used for xylene separation in xylene isomerization processes, and combined isomerization and crystallization or adsorptive separation processes. The high temperature stable membrane may be used in processing effluents from processes producing C8 aromatics including toluene disproportionation units, reforming units, C6 to C9 transalkylation units, and steam cracking units. The membranes are stable up to an operating temperature of about 500° C., are insoluble in organic solvents, and have a high flux (or permeance) for xylene separation. 
         [0014]    The high temperature stable membrane is a porous inorganic membrane modified with a polybenzoxazole (PBO) polymer. The porous inorganic membrane incorporates a layer of PBO polymer on the inside wall of the pores of the separation surface. The PBO polymer enhances the membrane&#39;s selectivity compared to the unmodified porous inorganic membrane. 
         [0015]    The inorganic membrane can be made of any suitable porous inorganic material, including, but not limited to, silica, metals such as stainless steel, alumina (including α-alumina, γ-alumina, and transition alumina), ceramics, molecular sieves, or combinations thereof. The selection of the material will depend on the conditions of the separation as well as the type of porous structure formed. The porous inorganic membranes can have different geometries, including, but not limited to, disks, tubes, hollow fiber, and the like. 
         [0016]    The pore size is generally less than about 1000 nm, or less than about 500 nm, or less than about 400 nm, or less than about 300 nm, or less than about 200 nm, or less than about 100 nm, or less than about 50 nm, or in a range of about 0.5 nm to about 50 nm. 
         [0017]    The PBO polymer can be derived from PBO precursors including, but not limited to, poly(hydroxyl imide), poly(hydroxyl amic acid), poly(hydroxyl amide), or mixtures thereof. The PBO precursors are soluble in solvents. Suitable solvents include, but are not limited to, organic solvents. Suitable organic solvents include, but are not limited to, 1-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, 1,3-dioxolane, dichloromethane, or combinations thereof. 
         [0018]    The porous inorganic membrane modified with a polybenzoxazole (PBO) polymer can be made using the following process. The porous inorganic membrane is cleaned and dried. One surface of the porous inorganic membrane is immersed in a PBO precursor solution for about 30 seconds to about 5 minutes. The excess solution on the surface of the porous inorganic membrane can be removed, and the surface carefully cleaned. The porous inorganic membrane with the PBO precursor can be dried at about room temperature under vacuum, followed by drying at about 100-300° C. under vacuum. The porous inorganic membrane can then be heated to about 350° C. to about 500° C., or about 400° C. to about 450° C. for 5 minutes to 2 hours to convert the PBO precursor polymer inside the pores of the porous inorganic membrane to form the high temperature stable PBO polymer. 
         [0019]    The porous inorganic membrane modified with PBO can be used to enrich at least a portion of the recycle stream from an isomerization zone. The enriched stream, when combined with the remaining feed to a sorption or crystallization zone, will improve the efficiency of the selective sorption or crystallization zone because the feed will contain a higher concentration of para-xylene. Advantageously, the porous inorganic membrane modified with PBO can separate para-xylene and ethyl benzene from ortho-xylene and meta-xylene. The membrane permeate is enriched in para-xylene and ethyl benzene, and the residue or retentate is enriched in ortho-xylene and meta-xylene. By “enriched in para-xylene” it is meant that greater than about 50 wt %, or greater than about 60 wt %, or greater than about 70 wt %, or greater than about 80 wt %, or greater than about 90 wt %, or greater than about 95 wt % of the para-xylene present in the stream entering the porous inorganic membrane modified with PBO is recovered in the permeate stream. By “enriched in ortho-xylene and meta-xylene,” it is meant that greater than about 50 wt %, or greater than about 60 wt %, or greater than about 70 wt %, or greater than about 80 wt %, or greater than about 90 wt %, or greater than about 95 wt % of the ortho-xylene and meta-xylene present in the stream entering the porous inorganic membrane modified with PBO is recovered in the retentate stream. 
         [0020]    The membrane has high permeate flux. The selectivity for para-xylene and ethyl benzene over ortho-xylene and meta-xylene can be relatively low, while still providing a substantial process benefit because any material which can be sent back to the isomerization zone without going through the fractionation and separations zones reduces the capital and energy costs. 
         [0021]    Although the entire xylene recycle stream can be subjected to the membrane separation, only a portion of the stream can be sent, if desired. In some embodiments, only about 10 to 50 percent by volume of the stream (preferably an aliquot portion) is sent to the membrane, with the remainder going to a xylene column for recycle to the selective para-xylene removal unit operation. The membrane separation is operated to recover at least about 50 wt %, at least about 60 wt %, at least about 70 wt %, or at least about 75 wt %, or at least about 80 wt %, or at least about 85 wt %, or at least about 90 wt %, or at least about 95 wt % of the para-xylene in the slip stream. Thus, the increase in the feed to the isomerization zone, as well as the downstream unit operations such as strippers and deheptanizers, as a result of the retentate being combined with the effluent from the para-xylene recovery unit operation is minimized. 
         [0022]    In one embodiment, the porous inorganic membrane modified with PBO can be used in a combined isomerization and sorption or crystallization process, as illustrated in  FIG. 2 . A feed stream  105  enters the C8 distillation column  110  where it is separated into a stream  115  of C8 hydrocarbons and a stream  120  of C9+hydrocarbons. Stream  120  can be further processed, if desired. The stream  115  of C8 hydrocarbons is sent to the membrane separation zone  130 . In the membrane separation zone  130 , stream  115  is separated by the porous inorganic membrane modified with PBO into permeate stream  135  enriched in para-xylene and ethyl benzene, and retentate stream  140  enriched in ortho-xylene and meta-xylene. 
         [0023]    In a typical process, the permeate stream  135  contains about 97 wt % of the para-xylene (about 97 wt % recovery) from stream  115 , and about 95 wt % of the ethyl benzene (about 95 wt % recovery) from stream  115 . Retentate stream  140  contains about 94 wt % of the ortho-xylene and about 94 wt % of the meta-xylene from stream  115 . 
         [0024]    Para-xylene enriched stream  135  can then be sent to a sorption or crystallization zone  145  where it is separated into stream  150 , which is highly pure para-xylene (e.g., about 99.7%), and stream  155 , which contains ethyl benzene and some ortho- and meta-xylene. 
         [0025]    Stream  155  is sent to isomerization zone  160  where it is isomerized to produce an equilibrium mixture of xylene isomers. Some conversion of ethylbenzene either to benzene and ethylene or to xylene is also accomplished in the isomerization zone. Retentate stream  140  can also be sent to the isomerization zone, if desired. Stream  165 , which is an equilibrium mixture of xylene isomers and unconverted ethylbenzene, can be recycled back to the C8 distillation column  110 . 
         [0026]    Because of the separation that takes place in the membrane separation zone  130 , the sorption or crystallization zone  145  can be smaller than the sorption or crystallization zone  25  in the process shown in  FIG. 1 . 
         [0027]      FIG. 3  illustrates another embodiment of a process incorporating the porous inorganic membrane modified with PBO. The feed  205  enters C8 distillation column  210  where it is separated into a stream  215  of C8 hydrocarbons and a stream  220  of C9+hydrocarbons. The stream  215  of C8 hydrocarbons is sent to the sorption or crystallization zone  225  where it is separated into a highly pure para-xylene product stream  230  and stream  235  containing ortho-xylene, meta-xylene, and ethyl benzene. Stream  235  is sent to an isomerization zone  240  where it is isomerized to produce a stream  245  of a mixture of xylenes. Stream  245  is sent to the membrane separation zone  250 . In the membrane separation zone  250 , stream  245  is separated by the porous inorganic membrane modified with PBO into permeate stream  260  and retentate stream  255 . Retentate stream  255 , which is enriched in m-xylene and o-xylene, is recycled to the isomerization zone  240 . Permeate stream  260 , which is enriched in p-xylene and ethylbenzene, is recycled to the C8 distillation column  210 . In some embodiments, stream  260  may be sent directly to sorption or crystallization zone  225 . In some embodiments, a slip stream from stream  255  may be sent to C8 distillation column  210  to prevent the build up of heavy components in the isomerization zone. Stream  255  returns m-xylene and o-xylene directly back to the isomerization zone without needing to pass through the C8 distillation column or the selective p-xylene extraction zone. As a result, these units can be smaller than the sorption or crystallization zone  25  in the process shown in  FIG. 1 . 
         [0028]    In the embodiment illustrated in  FIG. 4 , the feed  305  is sent to C8 distillation column  310  where it is separated into C8 stream  315  and C9+stream  320 . Stream  315  is sent to the sorption or crystallization zone  325  and separated into the highly pure para-xylene stream  330  and stream  335  containing ortho-xylene, meta-xylene, and ethyl benzene. Stream  335  is sent to a membrane reactor  340  in which the porous inorganic membrane coated with PBO forms a shell around the catalyst performing the isomerization. The PBO coated membrane is preferentially permeable to para-xylene and ethylbenzene. As the para-xylene and ethylbenzene are preferentially removed from the active area of the catalyst, the meta-xylene and ortho-xylene concentrations rise, allowing continued conversion of the meta-xylene and ortho-xylene to para-xylene. This higher conversion per pass of meta-xylene and ortho-xylene to para-xylene in the equilibrium limited xylene isomerization reaction reduces the recycle stream, allowing the C8 fractionation column  310  and sorption or crystallization zone  325  to be smaller than the analogous units in  FIG. 1 . Permeate stream  345  from the membrane reactor  340  is recycled to the C8 distillation column  310 . In some embodiments, permeate stream  345  may be sent directly to sorption or crystallization zone  325 . In some embodiments, a slip stream from the membrane reactor  340  may be sent to C8 distillation column  310  to prevent the build up of heavy components in the membrane reactor. 
       EXAMPLE 
       [0029]    A porous ceramic membrane disk having a diameter of 39.0 mm, a thickness of 2.0 mm, and 180 nm pores (available from ECO Ceramics BV) was used. 
         [0030]    The porous ceramic membrane disk was cleaned by rinsing with 2-propanol and water to remove surface impurities and dried at 110° C. for 24 hours in a vacuum oven. One surface of the porous ceramic membrane was immersed in a PBO precursor solution for 30 seconds. One PBO precursor solution was a solution of poly(hydroxyl imide) in 1-methyl-2-pyrrolidone, and another was poly(hydroxyl amic acid) in 1-methyl-2-pyrrolidone. The excess solution on the surface of the ceramic membrane was removed, and the surface was carefully cleaned. The ceramic membrane with the PBO precursor was dried at about room temperature under vacuum, followed by drying at about 200° C. under vacuum. The membrane was then heated to about 400° C. for 30 minutes to convert the PBO precursor polymer inside the pores of the ceramic membrane to form the high temperature stable PBO polymer. 
         [0031]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.