Process for desulphurization of a gasoline

Abstract

A process for treatment of gasoline comprising diolefins, olefins and sulphur-containing compounds including mercaptans. The process comprises: a) demercaptanization by addition of a portion of the mercaptans onto the olefins by bringing gasoline into contact with a first catalyst; b) separating the gasoline obtained from a) into a light gasoline cut and a heavy gasoline cut; c) introducing a stream of hydrogen and the second heavy gasoline cut obtained from b) into a distillation column comprising a reaction zone including a second catalyst in the sulphide form comprising a second support, a metal from group VIII and a metal from group VIb, to decompose the sulphur-containing compounds to form H 2 S; d) evacuating at a point located above the reaction zone at least one final light gasoline fraction comprising H 2 S, and at a point located below the reaction zone a desulphurized heavy gasoline fraction from the catalytic distillation column.

1 . A process for the treatment of a gasoline comprising diolefins, olefins and sulphur-containing compounds including mercaptans, said process comprising the following steps:
 a) carrying out a step for demercaptanization by addition of at least a portion of the mercaptans onto the olefins by bringing gasoline into contact with at least one first catalyst, at a temperature in the range 50° C. to 250° C., at a pressure in the range 0.4 to 5 MPa and with a liquid hourly space velocity (LHSV) in the range 0.5 to 10 h −1 , the first catalyst being in the sulphide form and comprising a first support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic classification of the elements, the first catalyst having the following characteristics:
 a support constituted by a gamma alumina with a specific surface area in the range 180 m 2 /g to 270 m 2 /g; 
 the quantity of metal from group VIb, expressed in the oxide form, is in the range 4% to 20% by weight with respect to the total catalyst weight; 
 the quantity of metal from group VIII, expressed in the oxide form, is in the range 3% to 15% by weight with respect to the total catalyst weight; 
 the molar ratio between the metal from group VIII and the metal from group VIB is in the range 0.6 to 3 mol/mol; 
   b) in a distillation column, carrying out fractionation of the gasoline obtained from step a) into at least one first intermediate light gasoline cut with a total sulphur content below that of the starting gasoline and a second intermediate heavy gasoline cut containing the major proportion of the starting sulphur-containing compounds;   c) introducing a stream of hydrogen and at least the second intermediate heavy gasoline cut obtained from step b) into a catalytic distillation column ( 14 ) comprising at least one reaction zone ( 15 ) including at least one second catalyst in the sulphide form comprising a second support, at least one metal from group VIII and a metal from group VIb, with the quantity of metal from group VIII, expressed as the oxide, in the range 0.5% to 25% by weight with respect to the weight of catalyst and with the quantity of metal from group VIb, expressed as the oxide, in the range 1.5% to 60% by weight with respect to the weight of catalyst, the conditions in the catalytic distillation column being selected so as to bring the intermediate heavy gasoline obtained from step b) into contact, in the presence of hydrogen, with the second catalyst in order to decompose the sulphur-containing compounds into H 2 S, contact with the second catalyst being carried out at a pressure in the range 0.1 to 4 MPa and at a temperature in the range 210° C. to 350° C.;   d) evacuating at least one final light gasoline fraction comprising H 2 S and a desulphurized heavy gasoline fraction from the catalytic distillation column, the final light gasoline fraction being evacuated at a point located above the reaction zone and the desulphurized heavy gasoline fraction being evacuated at a point located below the reaction zone.   
     
     
         2 . The process according to  claim 1 , in which the contact with the second catalyst in step c) is carried out at a pressure in the range 1 to 3 MPa and at a temperature in the range 220° C. to 320° C. 
     
     
         3 . The process according to  claim 1 , in which the first catalyst comprises nickel and molybdenum and in which the % by weight of nickel, expressed as the oxide, with respect to the total catalyst weight is in the range 4% to 12% and the % by weight of molybdenum, expressed as the oxide, with respect to the total catalyst weight is in the range 6% to 18%. 
     
     
         4 . The process according to  claim 1 , in which the second catalyst comprises cobalt and a metal from group VIb selected from molybdenum and tungsten. 
     
     
         5 . The process according to  claim 1 , in which the second catalyst comprises a % by weight of metal from group VIII, expressed as the oxide, with respect to the total catalyst weight in the range 1% to 10%. 
     
     
         6 . The process according to  claim 1 , in which the second catalyst comprises a % by weight of metal from group VIb, expressed as the oxide, with respect to the total catalyst weight in the range 3% to 50%. 
     
     
         7 . The process according to  claim 4 , in which the second catalyst comprises cobalt and molybdenum. 
     
     
         8 . The process according to  claim 1 , in which a portion of the desulphurized heavy gasoline fraction obtained from step c) is recycled by being introduced into the catalytic distillation column ( 14 ). 
     
     
         9 . The process according to  claim 1 , in which a makeup of a hydrocarbon cut is made, acting as a recycle stream into the catalytic distillation column ( 14 ) so as to maintain a liquid phase in the bottom of said column. 
     
     
         10 . The process according to  claim 1 , in which step a) is carried out at a pressure in the range 0.6 to 1 MPa and with a flow rate of H 2 /flow rate of feed ratio in the range 0.5 to 2 Nm 3  of hydrogen per m 3  of feed. 
     
     
         11 . The process according to  claim 1 , in which the distillation column comprises a first and a second catalytic bed respectively comprising a catalyst of the cobalt-molybdenum type and a catalyst of the nickel-molybdenum type, the first catalyst being disposed above the second catalyst. 
     
     
         12 . The process according to  claim 1 , in which in step b), an additional withdrawal is carried out, as a side stream via a line ( 25 ) the withdrawal point of which is disposed in the column at a level located between the overhead withdrawal and the withdrawal from the bottom of the distillation column, of a gasoline cut the boiling temperature range for which is in the range from the final boiling point for the first intermediate light gasoline cut and the initial boiling point of the intermediate heavy gasoline cut. 
     
     
         13 . The process according to  claim 12 , in which the gasoline cut withdrawn as a side stream is treated in a catalytic hydrodesulphurization unit in the presence of hydrogen. 
     
     
         14 . The process according to  claim 1 , in which the treated gasoline is obtained from a catalytic cracking unit.