Process for selective sulfur removal from FCC naphthas using zeolite catalysts

Abstract

A process for the removal of sulfur compounds from FCC catalytic naphtha which is simple to implement and is economical in operation operates by selectively hydrofining the cracked naphtha to remove sulfur without sacrificing octane (hydrodesulfurization typically around 85%) followed by a downstream alkylation which will remove residual sulfur from the gasoline boiling range. The alkylation is carried out using a solid, acidic molecular sieve catalyst under mild conditions to shift sulfur species from the lighter, olefin-rich portions of the naphtha to the heavy, olefin-poor gasoline. Separation of the heavy portion of the treated product followed by catalytic hydrodesulfurization or some other means (e.g. sorption) removes the sulfur from the heavy portion without losing significant octane value. Some of the heavy compounds formed in this process can drop out of the gasoline range, into the kero/light diesel range; this stream can be hydrotreated without penalty since octane is not a requirement for the middle distillate fuels.

1 . A process for the removal of sulfur compounds from a catalytically cracked petroleum naphtha feed comprising organic sulfur compounds and olefins, which comprises: 
 Selectively hydrodesulfurising the cracked naphtha under conditions and with a catalyst which is effective to convert organic sulfur in the naphtha feed to inorganic form with a selective, limited degree of hydrogenation of olefinic compounds,    Subjecting the hydrodesulfurized naphtha to a fixed bed alkylation step in the presence of a solid molecular sieve alkylation catalyst to alkylate organic sulfur compounds in the hydrodesulfurized naphtha with the olefins contained in the naphtha;    Fractionating the hydrodesulfurized, alkylated naphtha to remove alkylated sulfur compounds which boil above the gasoline boiling range.    
     
     
         2 . A process according to Paragraph  1  in which the hydrodesulfurized naphtha is subjected to alkylation over a zeolite alkylation catalyst comprising an intermediate pore size zeolite.  
     
     
         3 . A process according to Paragraph  2  in which the hydrodesulfurized naphtha is subjected to alkylation over a zeolite alkylation catalyst comprising ZSM-5 or ZSM-12.  
     
     
         4 . A process according to Paragraph  1  in which the hydrodesulfurized naphtha is subjected to alkylation over a zeolite alkylation catalyst comprising a zeolite of the MWW family.  
     
     
         5 . A process according to Paragraph  4  in which the zeolite of the MWW family comprises MCM-22 or MCM-49.  
     
     
         6 . A process according to Paragraph  1  in which additional benzene from outside sources is added to the alkylation step.  
     
     
         7 . A process according to Paragraph  1  in which the hydrodesulfurized, alkylated naphtha is fractionated into a light fraction and a heavy fraction with the heavy fraction being subjected to a hydrodesulfurized step to remove sulfur compounds.  
     
     
         8 . A process according to Paragraph  7  in which the heavy fraction, after being hydrodesulfurized to remove sulfur compounds, is recombined with the light fraction.  
     
     
         9 . A process according to  claim 1  in which the desulfurization during the selective hydrodesulfurization process is not greater than 90%.  
     
     
         10 . A process according to  claim 1  in which the desulfurization during the selective hydrodesulfurization process is not greater than 80%.  
     
     
         11 . A process according to  claim 1  in which the sulfur removal from the 190° C.− fraction of the hydrodesulfurized naphtha is at least 50%.  
     
     
         12 . A process according to  claim 1  in which the sulfur removal from the 190° C.− fraction of the hydrotreated naphtha is at least 75%.  
     
     
         13 . A process according to  claim 1  in which the sulfur removal from the 190° C.− fraction of the hydrodesulfurized naphtha is at least 85%.  
     
     
         14 . A process according to  claim 1  in which the boiling range conversion of the 190 C− (375 F−) portion of the hydrodesulfurized naphtha to the 190° C.+ (375° F.+) portion is less than 20%.  
     
     
         15 . A process according to  claim 1  in which the selective naphtha hydrodesulfurization process is carried out in the presence of a supported cobalt-molybdenum catalyst which comprises a porous inorganic refractory support, about 1 to 10 wt. % MoO 3  and about 0.1 to 5 wt. % CoO; and having 
 a Co/Mo atomic ratio of about 0.1 to 1.0; and    a median pore diameter of about 60 to 200 Å; and    a MoO3 surface concentration of about 0.5×10 −4  to 3×10 −4  g·m −2  MoO3; and    an average particle size diameter of less than about 2.0 mm.    
     
     
         16 . A process according to  claim 15  in which the hydrodesulfurization catalyst has a metals sulfide edge plane area from about 7600 to 2800 μmol oxygen/g MoO3 as measured by oxygen chemisorption.  
     
     
         17 . A process according to  claim 15  in which the hydrodesulfurization is such that the inlet temperature of the feedstock to the reaction unit is below the dew point of the feedstock and 100% of the feedstock becomes vaporized in the catalyst.  
     
     
         18 . A process according to  claim 15  in which the liquid hourly space velocity of the cracked naphtha over the hydrodesulfurization catalyst is from about 0.5 hr−1 to about 15 hr −1 .  
     
     
         19 . A process according to  claim 15  in which the hydrodesulfurization catalyst has a median pore diameter of from 80 to 150 Å.  
     
     
         20 . A process according to  claim 15  in which the hydrodesulfurization catalyst has a MoO 3  surface concentration of about 1×10 −4  to 2×10 −4  g/m 2  MoO 3 .