Process for enhanced propylene yield from cracked hydrocarbon feedstocks and reduced benzene in resulting naphtha fractions

Abstract

A process in which a catalytic cracking unit is operated to crack a hydrocarbon feedstock in a manner to enhance light olefin yields. The accompanying benzene-containing naphtha product stream is further processed through a benzene selective membrane to provide a low content benzene stream. Refiners frequently operate their cracking units to optimize light olefin yields, e.g. propylene, in response to needs in the petrochemical industry, and it has been discovered that units operated in this manner frequently produce naphtha containing increased amounts of benzene. The method of this invention therefore allows one to operate the unit when it is desired to optimize light olefin yields, yet at the same time produce a naphtha yield having a low benzene content. The invention is particularly useful when the cracking unit utilizes pentasil zeolites at increased concentrations to enhance light olefins yield.

1 . A catalytic cracking process comprising:
 (a) introducing a hydrocarbon feedstock into a reaction zone of a catalytic cracking unit which feedstock is characterized as having: an initial boiling point from about 120° C. with end points up to about 850° C.;   (b) catalytically cracking said feedstock in said reaction zone employing a cracking catalyst, temperature, catalyst-to-oil ratio, pressure, steam dilution and space velocity such that an olefin yield from the unit is enhanced to produce a product comprising olefin, naphtha and benzene, wherein the product comprises about 6 to about 20% propylene based on the weight of the hydrocarbon feedstock introduced in step (a) above;   (c) fractionating the product into at least a light olefin-containing fraction, and a benzene-containing naphtha fraction;   (d) recovering the light olefin-containing fraction;   (e) contacting the benzene-containing naphtha fraction with a membrane having a sufficient flux and selectivity to separate a benzene-enriched permeate fraction and a benzene-deficient naphtha retentate fraction, said benzene-enriched permeate fraction being enriched in benzene compared to the retentate;   (f) recovering the benzene-deficient naphtha retentate fraction; and   (g) routing the benzene-enriched permeate fraction for further processing.   
     
     
         2 . A catalytic cracking process according to  claim 1 , wherein the cracking catalyst in step (b) comprises a pentasil zeolite crystal content in the range of 2 to about 20% by weight of the catalyst. 
     
     
         3 . A catalytic cracking process according to  claim 1 , wherein the cracking catalyst in step (b) comprises a catalyst additive comprising ZSM5 in the range of 10 to about 80% by weight of the catalyst additive. 
     
     
         4 . A catalytic cracking process according to  claim 1 , wherein the benzene-containing naphtha fraction in step (c) comprises 0.6 to about 3% by weight benzene. 
     
     
         5 . A catalytic cracking process according to  claim 1 , wherein the benzene-containing naphtha fraction in step (c) is light cat naphtha having a boiling point in the range of 50° to 105° C., and comprises 1.2 to about 6% by weight benzene. 
     
     
         6 . A catalytic cracking process according to  claim 2 , wherein the benzene-containing naphtha fraction in step (c) is light cat naphtha having a boiling point in the range of 50° to 105° C., and comprises 1.2 to about 6% by weight benzene. 
     
     
         7 . A catalytic cracking process according to  claim 1 , wherein the process is a deep catalytic cracking process, and the conditions in the reaction zone of step (a) are as follows: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Temperature, ° C. 
                   505-575 
                 
                     
                   Cat./Oil 
                    9 to 15 
                 
                     
                   Pressure, atmospheres 
                   0.7 to 1.5 
                 
                     
                   Steam Dilution, wt % of feed 
                   10 to 30 
                 
                     
                   WHSV, hr −1   
                   0.2-20  
                 
                     
                   Pentasil crystal content, 
                   10-75 
                 
                     
                   % by weight catalyst 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         8 . A catalytic cracking process according to  claim 1 , wherein the membrane in step (e) comprises a member selected from the group consisting of polyimide, polyurea-urethane, polysiloxane and combinations thereof. 
     
     
         9 . A catalytic cracking process according to  claim 2 , wherein the membrane in step (e) comprises a member selected from the group consisting of polyimide, polyurea-urethane, polysiloxane and combinations thereof. 
     
     
         10 . A catalytic cracking process according to  claim 5 , wherein the membrane in step (e) comprises a member selected from the group consisting of polyimide, polyurea-urethane, polysiloxane and combinations thereof. 
     
     
         11 . A catalytic cracking process according to  claim 1 , where in the benzene-containing naphtha fraction is contacted with the membrane under pervaporation conditions. 
     
     
         12 . A catalytic cracking process according to  claim 1 , wherein the benzene-deficient naphtha retentate comprises less than 1% by weight to about 100 ppm benzene. 
     
     
         13 . A catalytic cracking process according to  claim 1 , wherein the benzene-deficient naphtha retentate comprises less than 0.6% by weight to about 100 ppm benzene. 
     
     
         14 . A catalytic cracking process according to  claim 1 , wherein the benzene-enriched permeate comprises 1 to about 10% by weight benzene. 
     
     
         15 . A catalytic cracking process according to  claim 1  further comprising fractionating the benzene-containing naphtha fraction from step (c) to remove C 5  fractions prior to contacting the benzene-containing naphtha fraction with a membrane in accordance with step (e). 
     
     
         16 . A method of producing light olefins and low benzene-containing naphtha in a catalytic cracking unit from a hydrocarbon feedstock, the method comprising:
 (a) selecting catalyst, temperature, catalyst-to-oil ratio, pressure, steam dilution and/or space velocity to enhance olefin yield in the catalytic cracking unit;   (b) contacting the feedstock with cracking catalyst in the unit, thereby producing a product comprising olefin, naphtha and benzene;   (c) fractionating the product into at least a light olefin-containing fraction, and a benzene-containing naphtha fraction;   (d) recovering the light olefin-containing fraction;   (e) contacting the benzene-containing naphtha fraction with a membrane having a sufficient flux and selectivity to separate a benzene-enriched permeate fraction and a benzene deficient naphtha retentate fraction, said benzene-enriched permeate fraction being enriched in benzene compared to the retentate;   (f) recovering the benzene deficient naphtha retentate fraction; and   (g) routing the benzene-enriched permeate fraction for further processing.   
     
     
         17 . A method according to  claim 16 , wherein the benzene-containing naphtha fraction in step (c) has a boiling point in the range of 50° C. to about 220° C., and the fraction comprises 0.6 to about 3% by weight benzene. 
     
     
         18 . A method according to  claim 16 , wherein the benzene-containing naphtha fraction in step (c) has a boiling point in the range of 50° C. to about 105° C., and the fraction comprises 1.2 to about 6% by weight benzene. 
     
     
         19 . A method according to  claim 16 , wherein the product from step (b) comprises 6 to about 20% by weight propylene based on the weight of the feedstock in step (a). 
     
     
         20 . A method according to  claim 16 , wherein the membrane in step (e) comprises a member selected from the group consisting of polyimide, polyurea-urethane, polysiloxane and combinations thereof. 
     
     
         21 . A method according to  claim 16  wherein the benzene-containing naphtha fraction is contacted with the membrane under pervaporation conditions. 
     
     
         22 . A method according to  claim 16 , wherein the benzene-deficient naphtha retentate comprises less than 1% by weight to about 100 ppm benzene. 
     
     
         23 . A method according to  claim 16 , wherein the benzene-deficient naphtha retentate comprises less than 0.6% by weight to about 100 ppm benzene. 
     
     
         24 . A method according to  claim 18 , wherein the benzene-deficient naphtha retentate comprises less than 1% by weight to about 100 ppm benzene. 
     
     
         25 . A method according to  claim 19 , wherein the benzene-deficient naphtha retentate comprises less than 1% by weight to about 100 ppm benzene. 
     
     
         26 . A method according to  claim 16 , wherein the feedstock is contacted with a catalyst comprising ZSM5. 
     
     
         27 . A method according to  claim 16 , wherein the feedstock in step (b) is contacted with a catalyst comprising 2 to about 20% by weight pentasil crystal. 
     
     
         28 . A method according to  claim 19 , wherein the feedstock in step (a) is contacted with a catalyst comprising 2 to about 20% by weight pentasil crystal. 
     
     
         29 . A method according to  claim 27 , wherein the pentasil is ZSM-5. 
     
     
         30 . A method according to  claim 28 , wherein the pentasil is ZSM-5. 
     
     
         31 . A method according to  claim 16 , wherein the cracking catalyst in step (b) comprises a catalyst additive comprising ZSM5 in the range of 10 to about 80% by weight of the catalyst additive. 
     
     
         32 . A method according to  claim 19 , wherein the cracking catalyst in step (b) comprises a catalyst additive comprising ZSM5 in the range of 10 to about 80% by weight of the catalyst additive.