Method for Desulfurising Olefin Motor Gasoline

Abstract

The invention concerns a process for producing gasoline with a low sulphur content and a controlled olefins content, comprising a step for oligomerizing an olefinic feed (step i), a step ii) consisting of mixing the branched olefinic gasoline produced in step i) with a gasoline rich in sulphur and olefins, a step iii) for hydrodesulphurization of said mixture, and separating the H 2 S formed (step iv)).

1 . A process for producing gasoline with a low sulphur content and with a controlled olefins content, comprising at least the following steps:
 a first step i) consisting of bringing a gasoline containing mainly hydrocarbons containing 3 to 6 carbon atoms per molecule into contact with an acid catalyst under conditions which may cause oligomerization of the olefins present in the feed to produce a branched olefinic gasoline;   a step ii) consisting of mixing the branched olefinic gasoline produced in step i) with a gasoline which is rich in sulphur and olefins;   a step iii) for treating the mixture so constituted in the presence of hydrogen on a hydrodesulphurization catalyst to transform the sulphur present in the feed into H 2 S and to at least partially hydrogenate the olefins;   a step iv) for separating the H 2 S formed.   
   
   
       2 . A process according to  claim 1 , in which the branched olefinic gasoline obtained in step i) is distilled to recover a cut containing mainly hydrocarbons containing 5 to 12 carbon atoms per molecule. 
   
   
       3 . A process according to  claim 1 , in which the gasoline treated in step i) undergoes pre-treatment steps before said step to reduce the amount of polyunsaturated compounds, sulphur-containing compounds and nitrogen-containing compounds. 
   
   
       4 . A process according to  claim 1 , in which the gasoline treated in step i) contains less than 100 ppm of sulphur and preferably less than 50 ppm of sulphur in the form of mercaptans or sulphides. 
   
   
       5 . A process according to  claim 1 , in which the catalyst used in step i) is an acid catalyst selected from the group constituted by: supported inorganic acids such as phosphoric acids on silica, sulphonic acids on polymers such as ion exchange resins, mineral oxides such as aluminas, amorphous silica aluminas or zeolites. 
   
   
       6 . A process according to  claim 5 , in which the acid catalyst used is of the amorphous silica alumina type and has a mean pore diameter of more than 0.5 nm. 
   
   
       7 . A process according to  claim 5 , in which the acid catalyst used is an ion exchange resin. 
   
   
       8 . A process according to  claim 1 , in which hydrodesulphurization step ii) is carried out at a temperature in the range 220° C. to 350° C., at a pressure which is preferably in the range 0.1 to 5 MPa, with a space velocity in the range from about 0.5 h −1  to 20 h −1  and with a ratio of the flow rate of hydrogen to the flow rate of the gasoline to be desulphurized in the range 50 litre/litre to 800 litre/litre. 
   
   
       9 . A process according to  claim 1 , in which the hydrodesulphurization catalyst comprises at least one group VIII element and at least one group VIB element, deposited on a porous support. 
   
   
       10 . A process according to  claim 9 , in which the amount of the group VIII element, expressed as the oxide, is in the range 0.5% by weight to 15% by weight and the amount of group VIB element is in the range 1.5% by weight to 60% by weight. 
   
   
       11 . A process according to  claim 9 , in which said support comprises alumina and has a specific surface area of less than 200 m 2 /g. 
   
   
       12 . A process according to  claim 9 , in which the surface density of the group VIB metal is in the range 2×10 −4  to 40×10 −4  grams of the oxide of said metal per m 2  of support. 
   
   
       13 . A process according to  claim 9 , in which the hydrodesulphurization catalyst undergoes a sulphurization step before it is brought into contact with the feed to be treated. 
   
   
       14 . A process according to  claim 1 , in which a hydrodesulphurization finishing step is interposed between step iii) and step iv). 
   
   
       15 . A process according to  claim 14 , in which the catalyst used in said finishing step is a catalyst comprising at least one element selected from group VIII elements and deposited on a porous support. 
   
   
       16 . A process according to  claim 14 , in which the catalyst used in said finishing step has a catalytic activity in the range 1% to 70% of the catalytic activity of the principal hydrodesulphurization catalyst.