Polysulfone membranes for aromatics/saturates separation
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. Ser. No. 550,912, filed Jul. 11, 1990, now abandoned. 
    
    
     BACKGROUND 
     The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community and is the subject of numerous patents. 
     U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene. 
     U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e. aromatic, unsaturated, saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, e.g., naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.). 
     U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons, e.g., aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain non-porous cellulose ester membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid. 
     U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation. 
     The present invention relates to a process for the separation of aromatics from saturates. Compared to distillation, membrane permeation can lead to considerable energy savings. A membrane can separate a mixture of aromatics and saturates, e.g., a heavy cat naphtha, into a high-octane, mainly aromatic permeate and a high-cetane, mainly saturated retentate. Both permeate and retentate are more valuable than the starting heavy cat naphtha. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for separating mixtures of aromatics and non-aromatics into aromatic-enriched and non-aromatic-enriched streams by contacting the aromatics/non-aromatics mixture with one side of a polysulfone membrane, and selectively permeating the aromatic components of the mixture through the membrane. Polysulfones are commercially available. Examples are Udel, having the following formula ##STR1## and Victrex, having the following formula ##STR2## 
     Crosslinked polysulfone membranes can also be used for the aromatics/non-aromatics separation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the present invention, membranes are used to separate a mixture of aromatics and non-aromatics into an aromatic-enriched fraction and a non-aromatic-enriched fraction. 
     The membranes are useful for the separation of aromatics from saturates in petroleum and chemical streams, and have been found to be particularly useful for the separation of large substituted aromatics from saturates as are encountered in heavy cat naphtha streams. Other streams which are also suitable feed streams for aromatics from saturates separation are intermediate cat naphtha streams boiling at 93°-160° C. light aromatics content streams boiling in the C 5  -150° C. range, light catalytic cycle oil boiling in the 200°-345° C. range as well as streams in chemical plants which contain recoverable quantities of benzene, toluene, xylenes (BTX) or other aromatics in combination with saturates. The separation techniques which may successfully employ the membranes of the present invention include perstraction and pervaporation. 
     Perstraction involves the selective dissolution of particular components contained in a mixture into the membrane, the diffusion of those components through the membrane and the removal of the diffused components from the downstream side of the membrane by the use of a liquid sweep stream. In the perstractive separation of aromatics from saturates in petroleum or chemical streams, the aromatic molecules present in the feedstream dissolve into the membrane film due to similarities between the membrane solubility parameter and those of the aromatic species in the feed. The aromatics then permeate (diffuse) through the membrane and are swept away by a sweep liquid which is low in aromatics content. This keeps the concentration of aromatics at the permeate side of the membrane film low and maintains the concentration gradient which is responsible for the permeation of the aromatics through the membrane. 
     The sweep liquid is low in aromatics content so as not to itself decrease the concentration gradient. The sweep liquid is preferably a saturated hydrocarbon liquid with a boiling point much lower or much higher than that of the permeated aromatics. This is to facilitate separation, as by simple distillation. Suitable sweep liquids, therefore, would include, for example, C 3  to C 6  saturated hydrocarbons and lube basestocks (C 15  -C 20 ). 
     The perstraction process is run at any convenient temperature, preferably as low as possible. 
     The choice of pressure is not critical since the perstraction process is not dependent on pressure, but on the ability of the aromatic components in the feed to dissolve into and migrate through the membrane under a concentration driving force. Consequently, any convenient pressure may be employed, the lower the better to avoid undesirable compaction, if the membrane is supported on a porous backing, or rupture of the membrane, if it is not. 
     If C 3  or C 4  sweep liquids are used at 25° C. or above in liquid state, the pressure must be increased to keep them in the liquid phase. 
     Pervaporation, by comparison, is run at generally higher temperatures than perstraction and relies on vacuum on the permeate side to evaporate the permeate from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. As in perstraction, the aromatic molecules present in the feed dissolve into the membrane film, migrate through said film and emerge on the permeate side under the influence of a concentration gradient. Pervaporation separation of aromatics from saturates can be performed at a temperature of about 25° C. for the separation of benzene from hexane but for separation of heavier aromatic/saturate mixtures, such as heavy cat naphtha, higher temperatures of at least 80° C. and higher, preferably at least 100° C. and higher, more preferably 120° C. and higher should be used. Temperatures of about 200° C. have been successfully used with crosslinked polysulfone membranes of the present invention, the maximum upper limit being that temperature at which the membrane is physically damaged. Vacuum on the order of 1-50 mm Hg is pulled on the permeate side. The vacuum stream containing the permeate is cooled to condense out the highly aromatic permeate. Condensation temperature should be below the dew point of the permeate at a given vacuum level. 
     The membrane itself may be in any convenient form utilizing any convenient module design. Thus, sheets of membrane material may be used in spiral wound or plate and frame permeation cell modules. Tubes and hollow fibers of membranes may be used in bundled configurations with either the feed or the sweep liquid (or vacuum) in the internal space of the tube or fiber, the other material obviously being on the other side. 
     When the membrane is used in a hollow fiber configuration with the feed introduced on the exterior side of the fiber, the sweep liquid flows on the inside of the hollow fiber to sweep away the permeated highly aromatic species, thereby maintaining the desired concentration gradient. The sweep liquid, along with the aromatics contained therein, is passed to separation means, typically distillation means, however, if a sweep liquid of low enough molecular weight is used, such as liquefied propane or butane, the sweep liquid can be permitted to simply evaporate, the liquid aromatics being recovered and the gaseous propane or butane (for example) being recovered and reliquefied by application of pressure or lowering of temperature. 
     Polysulfone films can be obtained by preparing a solution in a suitable solvent, e.g. chloroform, casting on a glass plate or a porous support, adjusting the thickness with a casting knife and drying the membrane first at room temperature, then at high temperature, e.g., 120° C. Crosslinking is effected by thermal treatment, possibly in the presence of suitable additives. The obtained membranes necessarily result in homogeneous and non-porous. The distinction between non-porous (dense) and porous membranes is well known in the art (see Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 15, p. 102-104, John Wiley and Sons). 
     The present invention will be better understood by reference to the following examples which are offered by way of illustration and not limitation. 
     In the following examples, membranes are used to separate aromatics from saturates in a pervaporation apparatus. The pervaporation apparatus is a cell, separated into two compartments by a porous metal plate, on which the membrane is supported. During a pervaporation experiment the aromatics/saturates mixture is circulated through the upper compartment at the desired temperature. The lower compartment is kept at reduced pressure. The permeate is collected in a trap cooled with dry ice-acetone or dry ice-isopropanol and periodically analyzed by gas chromatography. The feed contains 20 weight % isooctane, 10% toluene, 30% n-octane and 40% p-xylene. The following examples illustrate the invention. 
     EXAMPLE 1 
     40 g of Bisphenol-A polysulfone was dissolved in 240 g of chloroform, then 0.38 g of cupric acetylacetonate was added. The solution was cast on glass plates. The membranes so obtained were allowed to dry in a slow stream of nitrogen, then they were removed from the glass plates by immersing in warm water. After drying, square pieces were cut, the sides were clamped between stainless-steel holders which lay on small tables covered with Gore-tex (porous Teflon) sheets. The distance between the membranes and the Gore-tex sheets was about 1/2 inch. Weights attached to the clamps kept the membranes suspended above the Gore-tex sheet. One of the membranes was subjected to the following heating cycle: 
     
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120° C. 17 hours  in N.sub.2                                       
160° C. 24 hours  in air                                           
230° C. 140 hours in air                                           
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     At this point the membrane was insoluble in chloroform, i.e., it was crosslinked. A pervaporation experiment carried out in the unit described above gave the following results: 
     
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Temperature                                                               
           Separation Factor                                              
                            Normalized Flux                               
°C. For Toluene/n-Octane                                           
                            (Kg.μM/M.sup.2 /D)                         
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150        10               250                                           
170        8.5              540                                           
190        6.5              960                                           
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     EXAMPLE 2 
     0.40 g of elemental sulfur and 120 g of dimethylacetamide were put into a jar containing a magnetic bar. Heat was applied to help dissolution. When all sulfur was dissolved, 40 g of the polyethersulfone, commercially produced by ICI under the name of Victrex 5200, was added. When everything was dissolved, four membranes were cast, using a casting knife with a 10-mil opening. One of the membranes was put into a 120° C. oven swept with nitrogen for 2 hours, then the temperature was brought to 300° C. and air replaced nitrogen. After a total of 24 hours at 300° C. the membrane was insoluble in dimethylacetamide. In a pervaporation experiment using the feed described above, the membrane showed a toluene/n-octane selectivity of about 11 and a normalized flux of about 100 Kg.μM/M 2  /D at 210° C. 
     EXAMPLE 3 
     0.04 g of elemental sulfur was dissolved in 20 g of CH 2  Cl 2 , then 4 g of Bisphenol-A polysulfone was added. When everything was dissolved, membranes were cast on glass plates. 
     One of the membranes was heated at 300° C. for 12 hours, after which it was insoluble in CH 2  Cl 2 . 
     The membrane was tested in a slightly different way from that described above. The feed was a mixture of 50 wt % toluene and 50 wt % n-octane. The permeate was analyzed by mass spectroscopy. The following table gives the results. 
     
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Temperature                                                               
           Separation Factor                                              
                            Normalized Flux                               
°C. For Toluene/n-Octane                                           
                            (Kg.μM/M.sup.2 /D)                         
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150        9.3              283                                           
175        6.5              624                                           
200        4.6              1170                                          
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