Method for sweetening an olefinic petrol of sulphide-type compounds

Abstract

A process reducing sulfides R1-S-R2, with R1 and R2 methyl or ethyl, in a gasoline containing diolefins, mono-olefins and sulphur: A) contacting gasoline in mixture with a light gasoline cut recycled from C) and hydrogen in a reactor with catalyst A at least one VIb metal and at least one non noble group VIII metal on a support, producing effluent having diolefins and sulfides R1-S-R2, with R1 and R2 methyl or ethyl radicals lower than that that of the starting gasoline; B) the effluent from A) is sent into a fractionating column separating at the top a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule; C) recycling a part of the light gasoline from B) to the reactor of A) with a recycle ratio 0.1 to 0.7.

Claims

1. A process for reducing the content of sulfide compounds of formula R1-S-R2, with R1 and R2 being methyl and/or ethyl radicals, in a gasoline containing diolefins, mono-olefins and sulfur, said process consisting of: A) contacting an FCC gasoline having a boiling point between 0 and 280 C. in a mixture with a light gasoline cut recycled from C) and hydrogen in a reactor with catalyst A comprising at least one metal of group VIb and at least one non noble metal of group VIII deposited on a support, A) being carried out at a temperature in the reactor of 60 C. to 150 C. with a liquid hourly space velocity of 1 h.sup.1 to 10 h.sup.1, a pressure of 0.5 to 5 MPa and with a ratio by volume of added hydrogen/mixture of gasolines of 1 to 40 normal liters of hydrogen per liter of the mixture of gasolines vol/vol, producing an effluent having a content of diolefins and a content of sulfide compounds of formula R1-S-R2, lower than that that of the starting gasoline; B) fractionating said effluent originating from A) in a fractionating column, separating t the top of the column a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom of the column a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule, treating the heavy gasoline cut in a hydrodesulfurization unit in the presence of hydrogen; and C) recycling a part of the light gasoline cut originating from B) into the reactor of A) with a recycle ratio of 0.1 to 0.7, in which catalyst A comprises: a content of oxide of the metal of group VIb of 4 to 20% by weight with respect to the total weight of the catalyst, a content of oxide of the metal of group VIII comprised between 4 to 15% by weight with respect to the total weight of the catalyst, a degree of sulfurization of catalyst metals at least equal to 60%, a molar ratio between the non noble metal of group VIII and the metal of group VIb of 0.6 to 8 mol/mol, and a specific surface area of the catalyst of 30 to 300 m.sup.2/g.

2. The process according to claim 1, in which the density of the metal of group VIb per unit area of the catalyst is less than or equal to 110.sup.3 gram of oxides of the metal of group VIb per m.sup.2 of catalyst.

3. The process according to claim 1, in which the metal of group VIb of catalyst A is molybdenum or tungsten.

4. The process according to claim 1, in which the metal of group VIII of catalyst A is nickel, cobalt or iron.

5. The process according to claim 1, in which the metal of group VIII of catalyst A is nickel and the metal of group VIb of catalyst A is molybdenum.

6. The process according to claim 1, in which the gasoline originates from catalytic cracking or thermal cracking, a coking process, a visbreaking process or a pyrolysis process.

7. The process according to claim 3, in which the metal of group VIb of catalyst A is molybdenum.

8. The process according to claim 4, in which the metal of group VIII of catalyst A is nickel.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE illustrates a flow sheet of the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) The other characteristics and advantages of the invention will become apparent on reading the description which follows, given by way of illustration and non limitatively, and with reference to the FIGURE which is a flow sheet of the process according to the invention.

(3) The feedstock of hydrocarbons which is capable of being treated by the process according to the invention is a gasoline of the olefinic type containing diolefins, mono-olefins, and sulphur-containing compounds in particular in the form of mercaptans and light sulphides. Within the scope of the invention, by the term compounds of light sulphide type is meant the compounds of formula R1-S-R2 where R1 and R2 are selected from the methyl (CH.sub.3) and ethyl (C.sub.2H.sub.5) radicals. Thus, the lightest sulphide present in the olefinic gasoline is dimethylsulphide.

(4) The present invention finds its application for treating gasolines originating from conversion processes, and in particular gasolines (alone or in a mixture) originating from catalytic cracking or thermal cracking, a coking process, a visbreaking process or a pyrolysis process. The feedstocks of hydrocarbons to which the invention relates have a boiling temperature generally comprised between 0 C. and 280 C., preferably comprised between 15 C. and 250 C.

(5) The gasoline treated by the process according to the invention generally contains between 0.5% and 5% by weight of diolefins, between 20% and 55% by weight of mono-olefins, between 10 ppm and 1% by weight of sulphur and has a content of light sulphide compounds of formula R1-S-R2 where R1 and R2 are selected from the methyl (CH.sub.3) and ethyl (C.sub.2H.sub.5) radicals is generally comprised between 1 and 150 ppm by weight of sulphur.

(6) Preferably the gasoline which is capable of being treated originates from a Fluid Catalytic Cracking unit. A mixture of gasolines originating from a Fluid Catalytic Cracking unit with one or more gasolines originating from another conversion process can also be treated.

(7) With reference to the FIGURE, a feedstock of hydrocarbons of the gasoline type is treated in a mixture with a recycled light gasoline cut in a first catalytic step. Thus, the gasoline feedstock supplied through line 1 and the recycling flow originating from line 9 are sent through line 1 into a reactor 2 and in which the mixture is brought into contact with hydrogen (supplied through line 3) and catalyst A which in particular makes it possible to selectively hydrogenate the diolefins to olefins. The reactor 2 can be a reactor with a fixed or mobile catalytic bed, preferably fixed. The reactor can comprise one or more catalytic beds.

(8) The quantity of hydrogen injected is such that the ratio by volume (added H.sub.2/mixture of gasolines) is comprised between 1 and 40 normal litres of hydrogen per litre of the mixture of gasolines (vol/vol), and preferably comprised between 1 and 5 normal litres of hydrogen per litre of the mixture of gasoline (vol/vol). Too large an excess of hydrogen can lead to a strong hydrogenation of the mono-olefins and as a result, a reduction in the octane number of the gasoline. All of the feedstock is generally injected into the inlet of the reactor. However, it can be advantageous in certain cases to inject a part or all of the feedstock between two consecutive catalytic beds placed in the reactor. This embodiment makes it possible in particular to continue to operate the reactor if the inlet of the reactor or the first catalytic bed becomes clogged by deposits of polymers, particles, or gums present in the feedstock.

(9) The mixture of gasolines and hydrogen are brought into contact with catalyst A at a temperature comprised between 60 C. and 150 C. and preferably comprised between 80 and 130 C., with a liquid hourly space velocity (LHSV) comprised between 1 h.sup.1 and 10 h.sup.1, the liquid hourly space velocity unit being expressed in litres of the mixture of gasolines per hour per litre of catalyst (L/h/L, i.e. h.sup.1). The pressure is adjusted so that the reaction mixture is in the majority in liquid form in the reactor. The pressure is generally comprised between 0.5 MPa and 5 MPa and preferably comprised between 1 and 4 MPa.

(10) As indicated in the FIGURE, a reaction effluent is drawn off from the reactor 2 through line 4. This effluent has a lower content of diolefins compared to the gasoline to be treated because of the selective hydrogenation reaction that it has undergone. The effluent originating from the hydrogenation reactor 2 has a temperature close to the average temperature of the reactor 2 and generally greater (typically from 1 to 3 C.) than that of the feedstock at the inlet of the reactor 2, given that the selective hydrogenation reaction of the diolefins is exothermic.

(11) It has been observed that the effluent obtained after step A) also has a lower content of compounds of the light sulphide type. These compounds however have not undergone a hydrodesulphurization (HDS) reaction, as the effluent obtained after step A) does not contain H.sub.2S. Without being bound by any theory, the catalyst used in step A) makes it possible to react the light sulphide compounds described above with the olefins of the feedstock in order to form sulphide compounds with a higher molecular weight which are capable of being separated from the effluent by distillation. It should be noted that this catalytic step is different from a hydrodesulphurization (or HDS) catalytic step in which the sulphur-containing compounds are converted to H.sub.2S and to hydrocarbons by contact with a catalyst having hydrogenolyzing proprieties. The hydrodesulphurization is generally operated at a temperature comprised between 200 and 400 C., with an added H.sub.2/gasoline feedstock ratio by volume comprised between 100 to 600 normal litres of hydrogen per litre of gasoline (vol/vol), at a total pressure comprised between 1 MPa and 3 MPa and with a liquid hourly space velocity (LHSV) comprised between 1 h.sup.1 and 10 h.sup.1.

(12) Catalyst A utilized in the process according to the invention comprises at least one metal of group VIb (group 6 according to the new notation of the periodic table of the elements: Handbook of Chemistry and Physics, 76th edition, 1995-1996) and at least one non noble metal of group VIII (groups 8, 9 and 10 according to the new notation of the periodic table of the elements: Handbook of Chemistry and Physics, 76th edition, 1995-1996) deposited on a support.

(13) Preferably catalyst A is used in the sulphurized form. Preferably, the degree of sulphurization of catalyst A is at least 60%.

(14) The sulphurization of the catalyst can be carried out in a sulphur reducing medium, i.e. in the presence of H.sub.2S and hydrogen, in order to convert the metal oxides to sulphides such as for example, MoS.sub.2 and Ni.sub.3S.sub.2. The sulphurization is for example carried out by injecting onto the catalyst a flow containing H.sub.2S and hydrogen, or a sulphur-containing compound capable of decomposing into H.sub.2S in the presence of the catalyst and hydrogen. The polysulphides such as dimethyldisulphide are precursors of H.sub.2S commonly used for sulphurizing the catalysts. The temperature is adjusted so that the H.sub.2S reacts with the metal oxides in order to form metal sulphides. This sulphurization can be carried out in situ or ex situ (inside or outside) of the reactor of step A) at temperatures comprised between 200 and 600 C., and more preferentially between 300 and 500 C.

(15) An element is considered as substantially sulphurized when the molar ratio between the sulphur (S) present on the catalyst and said element is preferably at least equal to 60% of the theoretical molar ratio corresponding to the total sulphurization of the element considered:
(S/element).sub.catalyst0.6(S/element).sub.theoretical

(16) with:

(17) (S/element).sub.catalyst=molar ratio between sulphur (S) and the element present on the catalyst

(18) (S/element).sub.theoretical=molar ratio between the sulphur and the element corresponding to the total sulphurization of the element to sulphide.

(19) This theoretical molar ratio varies according to the element considered: (S/Fe).sub.theoretical=1 (S/Co).sub.theoretical=8/9 (S/Ni).sub.theoretical=2/3 (S/Mo).sub.theoretical=2/1 (S/W).sub.theoretical=2/1

(20) When the catalyst comprises several metals, the molar ratio between the S present on the catalyst and all the elements is preferably at least equal to 60% of the theoretical molar ratio corresponding to the total sulphurization of each element to sulphide, the calculation being carried out in proportion to the relative mole fractions of each element.

(21) For example, for a catalyst comprising molybdenum and nickel with a respective mole fraction of 0.7 and 0.3, the minimal molar ratio (S/Mo+Ni) is given by the relationship:
(S/Mo+Ni).sub.catalyst=0.6{(0.72)+(0.3(2/3)}

(22) Very preferably, the degree of sulphurization of the metals will be greater than 80%. Preferably the sulphurization is implemented on the metals in the form of oxide without carrying out a prior step of reducing the metals. In fact, it is known that the sulphurization of reduced metals is more difficult than the sulphurization of metals in the form of oxides.

(23) Catalyst A according to the invention can have the following characteristics: a content of oxide of the metal of group VIb is comprised between 4 and 20% by weight with respect to the total weight of the catalyst, a content of oxide of the metal of group VIII is comprised between 4 and 15% by weight with respect to the total weight of the catalyst, a degree of sulphurization of the metals constituting said catalyst is at least equal to 60%, a molar ratio between the non noble metal of group VIII and the metal of group VIb is comprised between 0.6 and 8 mol/mol, a specific surface area of the catalyst is comprised between 30 and 300 m.sup.2/g.

(24) Catalyst A preferably has a content by weight of the element of group VIb in the oxide form comprised between 6 and 18%, preferably comprised between 8 and 12% and yet more preferably comprised between 10 and 12% by weight with respect to the weight of catalyst. The metal of group VIb is preferably selected from molybdenum and tungsten. More preferably, the metal of group VIb is molybdenum.

(25) Catalyst A also contains a metal of group VIII preferably selected from nickel, cobalt and iron. More preferably, the metal of group VIII is nickel. The content of the metal of group VIII expressed in the form of oxide is comprised between 4 and 12% by weight and preferably comprised between 6 and 10% by weight and of yet more preferably between 6 and 8% by weight with respect to the weight of catalyst.

(26) The molar ratio between the non noble metal of group VIII and the metal of group VIb is comprised between 0.6 and 8 mol/mol and preferably comprised between 1 and 3 mol/mol. The density of the metal of group VIb, expressed as being the ratio between said content by weight of oxide of the metal of group VIb and the specific surface area of the catalyst, is less than or equal 10.sup.3 g/m.sup.2, preferably comprised between 10.sup.4 and 10.sup.3 g/m.sup.2. Preferably the density of metal of group VIb is comprised between 4 and 6.10.sup.4 g/m.sup.2. Thus, for example in the specific case where the catalyst comprises 11% by weight of molybdenum oxide with respect to the weight of catalyst and has a specific surface area of 219 m.sup.2/g, while the density of molybdenum, expressed as being the ratio between the content by weight of molybdenum oxide (MoO.sub.3) and the specific surface area of the catalyst, is equal to (0.11/219) i.e. 5.10.sup.4 g/m.sup.2.

(27) The specific surface area of the catalyst A is preferably comprised between 100 and 300 m.sup.2/g and more preferably comprised between 150 and 250 m.sup.2/g. The specific surface area is determined according to the standard ASTM D3663.

(28) Preferably, the catalyst A has a total pore volume measured by mercury porosimetry greater than 0.3 cm.sup.3/g, preferably comprised between 0.4 and 1.4 cm.sup.3/g and preferentially comprised between 0.5 and 1.3 cm.sup.3/g. The mercury porosimetry is measured according to the standard ASTM D4284-92 with a wetting angle of 140, with a device of the model Autopore III trade mark Micromeritics.

(29) The support of the catalyst A is preferably selected from alumina, nickel aluminate, silica, silicon carbide, or mixture thereof. Preferably, alumina is used.

(30) According to a variant, the support of the catalyst A is constituted by cubic gamma alumina or delta alumina.

(31) According to a particularly preferred variant, the catalyst A is an NiMo catalyst on alumina. Catalyst A according to the invention can be prepared using any technique known to a person skilled in the art, and in particular by impregnation of the elements of groups VIII and VIb on the selected support. This impregnation can for example be carried out according to the method known to a person skilled in the art by the term dry impregnation, in which exactly the quantity of desired elements is introduced in the form of salts soluble in the solvent selected, for example demineralized water, so as to fill the porosity of the support as precisely as possible.

(32) After introducing the elements of groups VIII and VIb, and optionally forming the catalyst, the latter is subjected to an activation treatment. This treatment generally has the aim of converting the molecular precursors of the elements to the oxide phase. In this case it is an oxidizing treatment, but a simple drying of the catalyst can also be carried out. In the case of an oxidizing treatment, also called calcination, the latter is generally implemented under air or under diluted oxygen, and the treatment temperature is generally comprised between 200 C. and 550 C., preferably between 300 C. and 500 C.

(33) After calcination, the metals deposited on the support are in the form of oxide. In the case of nickel and molybdenum, the metals are found principally in the form of MoO.sub.3 and NiO. Preferably, catalyst A is used in the sulphurized form i.e., it has undergone a sulphurization activation step after the oxidizing treatment.

(34) According to the invention, the effluent originating from the reactor 2 is drawn off from said reactor and sent through line 4 into a dedicated fractionating column 5 in order to provide at least one cut of light gasoline 6 (or Light Cracked Naphtha (LCN)) which is drawn off at the top of column 5 and a cut of heavy gasoline 7 (Heavy Cracked Naphtha (HCN)) which is recovered at the bottom of column 5. It should be noted that this fractionation step is carried out subsequent to the catalytic step A) in a column dedicated only to the separation unlike an implementation in a catalytic distillation column where the catalytic reaction and the separation are carried out concomitantly.

(35) The cut point of the fractionating column is selected so that the cut of light gasoline contains hydrocarbons (essentially olefins) having less than six carbon atoms and a low content of compounds of the light sulphide type and that the cut of heavy gasoline has a significant quantity of sulphur-containing compounds such as mercaptans, the compounds of the thiophene family and sulphides and hydrocarbons (essentially olefins) having six or more than six carbon atoms (C6+). Typically, the cut point is adjusted so that the cut of light gasoline has a boiling temperature comprised between 5 C. and 70 C., preferably comprised between 5 C. and 65 C. As regards the cut of heavy gasoline, it can have a boiling temperature comprised between 60 C. and 280 C., preferably comprised between 65 C. and 280 C. A person skilled in the art knows that the separations of hydrocarbons are imperfect and, as a result, a certain overlapping of the boiling points of the light and heavy cuts can occur close to the cut point. Typically, the light gasoline cut has a total sulphur content less than 15 ppm, preferably less than 10 ppm by weight and a content of light sulphides less than 10 ppm by weight of sulphur.

(36) According to the invention, the cut of light gasoline 6 thus produced by the fractionation, which is rich in olefins (therefore with a high octane number) and depleted in sulphur-containing compounds, including the light sulphides, is divided into two flows 8 and 9, after optional removal of the hydrogen and stabilization. As indicated in the FIGURE, the flow 8 is sent to the gasoline pool for the formulation of gasoline type fuel. This light gasoline cut depleted in sulphur does not generally require additional hydrodesulphurization treatment.

(37) As regards the complementary flow 9, according to the invention, it is recycled into the reactor 2. The recycling ratio of the light gasoline cut depleted in sulphur is defined as being the ratio of the flow 9 to the flow 6, is comprised between 0.1 and 0.7 and preferably comprised between 0.3 and 0.6.

(38) The heavy gasoline cut 7 which contains the majority of the organo-sulphur compounds including the sulphides, is advantageously treated in a hydrodesulphurization (HDS) unit comprising a reactor 10 equipped with a catalyst bed having hydrogenolyzing properties. The HDS catalyst can comprise at least one metal of group VIb, for example molybdenum, and at least one metal of group VIII, for example cobalt, deposited on a support. Reference can be made in particular to documents EP 1 369 466 and EP 1 892 039 by the applicant which describe HDS catalysts.

(39) The operating conditions allowing a hydrodesulphurization in the presence of hydrogen of the heavy gasoline cut are: a temperature comprised between approximately 200 and approximately 400 C., preferentially comprised between 250 and 350 C.; a total pressure comprised between 1 MPa and 3 MPa, preferentially comprised between 1 MPa and approximately 2.5 MPa; an added H.sub.2/gasoline feedstock ratio by volume comprised between 100 and 600 normal litres of hydrogen per litre of gasoline (vol/vol); and a liquid hourly space velocity (LHSV) comprised between 1 h.sup.1 and 10 h.sup.1, preferentially comprised between 2 h.sup.1 and 8 h.sup.1.

(40) The desulphurized heavy gasoline cut, after removal of the H.sub.2S formed by the catalytic hydrodesulphurization reaction and stabilization, can then be sent to the gasoline pool and/or to the diesel pool depending on the requirements of the refiner.

(41) The process according to the invention can thus be incorporated into a hydrodesulphurization unit as a pretreatment step of the gasoline before the hydrodesulphurization step itself.

EXAMPLES

Example 1 (Comparative)

(42) Table 1 presents the general characteristics of a gasoline which has been treated according to the invention. The MAV is the Maleic Anhydride Value and gives an indication of the content of conjugated diolefins (precursor compounds of gums) in the gasolines.

(43) TABLE-US-00001 TABLE 1 Characteristics of the gasoline Composition of the gasoline Unit Value Density at 15 C. g/cm.sup.3 0.718 MAV g/100 g 15 Content of elemental sulphur % m/m 0.192 Content of light sulphides Dimethyl sulphide ppm S 1.8 Methyl-ethyl-sulphide ppm S 4.5 Content of olefins % m/m 49.0 Simulated distillation T 5% C. 23 T 95% C. 179 RON of the gasoline 87.2 MON of the gasoline 76.6

(44) The gasoline is treated in the presence of catalyst A in a selective hydrogenation reactor.

(45) Catalyst A is a catalyst of NiMo type on gamma alumina. The contents of metals are respectively 7% by weight NiO and 11% by weight MoO.sub.3 with respect to the total weight of the catalyst, i.e. an Ni/Mo molar ratio of 1.2. The specific surface area of the catalyst is 230 m.sup.2/g. Prior to its use, the catalyst A is sulphurized at atmospheric pressure on a sulphurization bench under an H.sub.2S/H.sub.2 mixture constituted by 15% by volume of H.sub.2S to 1 L/g.Math.h of catalyst and at 400 C. for two hours. This protocol makes it possible to obtain a degree of sulphurization greater than 80%.

(46) Table 2 shows the operating conditions implemented as well as the results of conversion of the light sulphides during the selective hydrogenation step.

(47) TABLE-US-00002 TABLE 2 Dimethyl-sulphide Methyl-ethyl-sulphide added residual Content Content LHSV H.sub.2/HC Temperature Pressure MAV ppm by Conversion ppm by Conversion Example h.sup.1 NL/L C. MPa mg/100 g weight S % weight S % 1 1.5 5 140 2.5 0.6 0.9 50% 3.0 33%

(48) The effluent of the selective hydrogenation step is sent into a fractionating column in order to separate at the top of the column a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom of the column a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule.

(49) The contents of dimethyl-sulphide and methyl-ethyl-sulphide in the light gasoline cut are respectively 4.5 and 6.3 ppm by weight, i.e. a total content of sulphides of 10.8 ppm by weight.

Example 2 (According to the Invention)

(50) The gasoline of Example 1 is treated in the process according to the invention with a recycle ratio of 0.5.

(51) Taking into account the recycling, the feedstock treated in the presence of catalyst A in the selective hydrogenation reactor has a content of dimethyl-sulphide and methyl-ethyl-sulphide of respectively 2.1 and 4.4 ppm by weight.

(52) Catalyst A is a catalyst of NiMo type on gamma alumina. The contents of metals are respectively 7% by weight NiO and 11% by weight MoO.sub.3 with respect to the total weight of the catalyst, i.e. an Ni/Mo molar ratio of 1.2. The specific surface area of the catalyst is 230 m.sup.2/g. Prior to its use, the catalyst A is sulphurized at atmospheric pressure on a sulphurization bench under an H.sub.2S/H.sub.2 mixture constituted by 15% by volume of H.sub.2S to 1 L/g.Math.h of catalyst and at 400 C. for two hours. This protocol makes it possible to obtain a degree of sulphurization greater than 80%.

(53) TABLE-US-00003 TABLE 3 Dimethyl-sulphide Methyl-ethyl-sulphide added residual Content Content LHSV H.sub.2/HC Temperature Pressure MAV ppm by Conversion ppm by Conversion Example h.sup.1 NL/L C. MPa mg/100 g weight S % weight S % 2 1.8 5 140 2.5 0.6 1.1 46% 3.0 31%

(54) The effluent of the selective hydrogenation step is sent into a fractionating column in order to separate at the top of the column a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom of the column a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule.

(55) The contents of dimethyl-sulphide and methyl-ethyl-sulphide in the light gasoline cut produced are respectively 3.3 and 3.8 ppm by weight, i.e. a total content of sulphides of 7.1 ppm by weight.

Example 3 (According to the Invention)

(56) The gasoline of Example 1 is treated in the process according to the invention with a recycle ratio of 0.33.

(57) Taking into account the recycling, the feedstock treated in the presence of catalyst A in the selective hydrogenation reactor has a content of dimethyl-sulphide and methyl-ethyl-sulphide of respectively 2.0 and 4.5 ppm by weight.

(58) Catalyst A is a catalyst of NiMo type on gamma alumina. The contents of metals are respectively 7% by weight NiO and 11% by weight MoO.sub.3 with respect to the total weight of the catalyst, i.e. an Ni/Mo molar ratio of 1.2. The specific surface area of the catalyst is 230 m.sup.2/g. Prior to its use, the catalyst A is sulphurized at atmospheric pressure on a sulphurization bench under an H.sub.2S/H.sub.2 mixture constituted by 15% by volume of H.sub.2S to 1 L/g.Math.h of catalyst and at 400 C. for two hours. This protocol makes it possible to obtain a degree of sulphurization greater than 80%.

(59) TABLE-US-00004 TABLE 4 Dimethyl-sulphide Methyl-ethyl-sulphide added residual Content Content LHSV H.sub.2/HC Temperature Pressure MAV ppm by Conversion ppm by Conversion Example h.sup.1 NL/L C. MPa mg/100 g weight S % weight S % 3 1.65 5 140 2.5 0.6 1.0 49% 3.0 33%

(60) The effluent of the selective hydrogenation step is sent into a fractionating column in order to separate at the top of the column a light gasoline cut containing hydrocarbons having less than 6 carbon atoms per molecule and at the bottom of the column a heavy gasoline cut containing hydrocarbons having 6 and more than 6 carbon atoms per molecule.

(61) The contents of dimethyl-sulphide and methyl-ethyl-sulphide in the light gasoline cut produced are respectively 3.7 and 4.6 ppm by weight, i.e. a total content of sulphides of 8.3 ppm by weight.