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 H2S formed (step iv)).

Description

The production of reformulated gasoline satisfying new environmental regulations necessitates substantially reducing their sulphur content. Current and future environmental standards constrain refiners to reduce the sulphur content in the gasoline pool to values of 50 ppm or less in 2005 and 10 ppm on the first of January 2009 within the European Community. Further, European refiners must also satisfy the standard regarding the olefinic compounds content, which must be lower than 18% by volume from 2000.

Of gasoline bases used in refineries, cracked gasolines supply a large proportion of the sulphur and olefins in a commercial gasoline. The term “cracked gasoline” means a cut from a coking unit, a visbreaking unit, or a steam cracking or fluid catalytic cracking (FCC) unit wherein the boiling point range is typically from that of hydrocarbons containing 5 carbon atoms to about 250° C.

Catalytic cracked gasoline, which may constitute 30% to 50% by volume of the gasoline pool, has a high olefin and sulphur content. More than 90% of the sulphur in reformulated gasoline is generally attributable to the gasoline from catalytic cracking. Desulphurization of gasoline, and principally FCC gasoline, is thus of clear importance in satisfying these standards.

PRIOR ART

Hydrotreatment or hydrodesulphurization of catalytic cracked gasoline, when carried out under conditions which are conventional to the skilled person, can reduce the sulphur content of the cut. However, that process has the major disadvantage of causing a very large drop in the octane number of the cut due to hydrogenation or saturation of a large proportion or even all of the olefins under hydrotreatment conditions.

Thus, processes have been proposed which can substantially desulphurize FCC gasoline while keeping the octane number to an acceptable level. Those processes are primarily based on the principle of selective hydrodesulphurization aimed at transforming sulphur-containing compounds into H₂S, while limiting the hydrogenation of olefins to paraffins, which transformation induces a large octane number loss. As an example, French patent FR-A-2 797 639 describes a process for selective desulphurization of gasoline which can produce olefinic gasoline which satisfies current sulphur content standards while minimizing octane number loss during hydrodesulphurization.

Further, oligomerization processes have been developed to produce gasoline with a high octane number from hydrocarbon fractions containing olefins containing 3 to 6 carbon atoms. Those processes use an acid catalyst which can oligomerize the olefins present in the feed into iso-olefins containing 6 to 12 carbon atoms having a high octane number. Those processes can produce a gasoline with a high octane number which is depleted in sulphur. However, such gasoline is almost 100% by weight olefin.

There are a number of industrial processes for producing gasoline by oligomerizing olefins containing 3 to 6 carbon atoms which are described in the work “Catalyse acido-basique” [Acido-basic catalysis], C Marcilly, pub. Technip, vol 2, p 475-495.

Further, International patent application WO-A-2005/019391 proposes a process for desulphurizing olefinic gasolines without a significant loss of octane number based on the production of an iso-olefinic gasoline containing 8 to 12 carbon atoms from a sulphur-containing gasoline, and selective hydrodesulphurization of that iso-olefinic gasoline to produce a desulphurized gasoline with no octane number loss. However, that process necessitates treating, on an acid catalyst, an olefinic gasoline containing sulphur with an end point which may be high, which can deleteriously affect the stability of said catalyst and cause its deactivation. The presence of compounds which may deleteriously affect the stability of acid catalysts, such as sulphur-containing compounds, polyunsaturated compounds and nitrogen-containing compounds, are generally concentrated in the heavy fractions of the gasoline and in particular the fractions containing mainly olefins containing 7 to 12 carbon atoms.

The present invention proposes a process for producing a gasoline with a controlled sulphur and olefins content, desulphurization and hydrogenation being carried out with a minimal reduction in octane number.

SUMMARY OF THE INVENTION

The invention concerns 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 an olefinic feed 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 H₂S and to at least partially hydrogenate the olefins;
  • a step iv) for separating the H₂S formed.

In the context of the invention, the term “branched olefinic gasoline” means a gasoline comprising at least 50% by weight of branched olefins, preferably at least 60% of branched olefins and more preferably at least 65% by weight of branched olefins. Among these branched olefins, iso-olefins are primarily present, but not uniquely present.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a scheme for producing gasoline with a low sulphur content and with a controlled olefins content. The gasoline produced also has a high octane number.

A hydrocarbon cut which is rich in olefins, preferably containing mainly (i.e. at least 40% by weight, preferably at least 70% by weight and more preferably at least 90% by weight) hydrocarbons containing 3 to 6 carbon atoms per molecule (also termed C₃-C₆ cuts, or mixtures of C₃, C₄, C₅ and C₆ cuts) is treated on an acid catalyst to produce a gasoline comprising a larger number of carbon atoms, via an oligomerization reaction, for example dimerization and/or trimerization. It is advantageous only to treat hydrocarbon fractions containing the shortest olefins on the acid catalyst. The olefinic fractions containing hydrocarbons containing 7 carbon atoms or more concentrate compounds which may deactivate acid catalysts, such as basic nitrogen-containing compounds, sulphur-containing compounds or polyunsaturated compounds. Further, olefins containing 7 carbon atoms or more form, by oligomerization, branched olefins containing more than 14 carbon atoms, which are too heavy to be incorporated into the gasoline cut.

In addition, the branched olefinic gasoline produced by oligomerization of the olefinic fraction may contain heavy oligomerization products originating from trimerization reactions containing more than 12 carbon atoms. Advantageously, this heavy fraction is separated, to concentrate the unsaturated products and any polyunsaturated products before hydrogenation of the C₆-C₁₂ cut. The heavy C₁₂+ fraction is conventionally separated by distillation. The heavy C₁₂+ fraction may be treated in a hydrotreatment unit or hydrogenation unit for upgrading as a base for aviation fuel, also termed kerosene, or for gas oil.

The oligomerized gasoline obtained thus preferably includes mainly hydrocarbons containing 5 to 12 carbon atoms per molecule (C₅-C₁₂ cut).

Said olefin-rich hydrocarbon cut (also termed the olefinic feed) is preferably an olefinic gasoline comprising at least 50% by weight, preferably at least 60% by weight of olefins and more preferably at least 70% by weight of olefins.

The olefinic feed treated in the oligomerization section (step i) may derive from catalytic cracking, coking or steam cracking units. It is generally preferable for the feed to undergo pre-treatment steps prior to step i) to reduce the amount of polyunsaturated compounds, sulphur-containing compounds and nitrogen-containing compounds. These pre-treatments may maximize the cycle time of the acid catalysts.

The diolefins present in the feed may cause deactivation of the catalyst by polymerization and gum formation. The polyunsaturated compounds may be extracted for upgrading for petrochemicals applications. This extraction is generally carried out by liquid-liquid extraction or extractive distillation using polar solvents. As an example, butadiene may be extracted from the C₄ cut derived from steam cracking using solvents such as acetonitrile, furfural or other polar solvents. Another possibility consists of transforming the diolefins into olefins by selective hydrogenation. To maximize the service life of the acid catalyst used to oligomerize the olefins, the amount of polyunsaturated compounds in the feed entering an oligomerization unit is preferably less than 1% by weight and more preferably less than 0.5% by weight.

Nitrogen-containing compounds usually have a basic nature and thus neutralize catalytic acid sites. The primary basic compounds which may be present in C₃-C₆ cuts are amines. Nitrogen-containing compounds, if present in said feed, may be extracted by a water washing or acid washing step, or by a selective hydrogenation step as described in FR-A-2 850 113. The nitrogen-containing compounds to be extracted are not only basic nitrogen-containing compounds, which are known to deactivate acid catalysts by neutralizing acid sites, such as amines; non basic nitrogen-containing compounds such as nitrites may also cause deactivation of acid catalysts.

However, the inventors have observed that non basic nitrogen-containing compounds such as nitrites may also cause deactivation of acid catalysts as under some conditions they may be transformed into basic compounds such as amines and then neutralize the acid sites.

The sulphur-containing compounds present in C₃-C₆ cuts are principally mercaptans. Mercaptans may react with olefins on acid catalysts to form sulphides which have a basic nature. The sulphides formed may thus degrade the activity of acid catalysts.

The mercaptans may be eliminated either using an extractive oxidation treatment to extract them in the form of the disulphide, such as the MEROX process (trade mark) or by rendering them heavier by addition to olefins on a metal catalyst by dint of a thioetherification reaction. The amount of sulphur-containing compounds in the form of mercaptans or sulphides in the olefinic feed for the oligomerization unit is preferably less than 100 ppm by weight, more preferably less than 50 ppm by weight.

In the case in which the feed contains thiophenes, more particularly thiophene or methylthiophenes, these compounds may react with a fraction of the olefins present in the olefinic C₃-C₆ cut to form heavier alkylated thiophene compounds. These compounds may thus optionally be eliminated from the branched olefinic gasoline by simple distillation.

The catalysts used in step i) are acid catalysts which are preferably 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.

The compounds to be produced must be branched to maximize their octane number. To this end, acid catalysts with a mean pore diameter which is sufficiently large to allow the diffusion of highly branched molecules are preferably used. The acid catalyst used preferably has a mean pore diameter of more than 0.5 nm, more preferably more than 0.7 nm. Preferably, an amorphous silica alumina or an ion exchange resin is used.

In the particular case in which the catalyst is an amorphous silica alumina, it is preferable to dry the olefinic feed before bringing it into contact with the catalyst to obtain a feed containing less than 100 ppm of water, preferably less than 50 ppm of water. The reactor is operated at a temperature in the range 50° C. to 250° C., an HSV in the range 0.5 h⁻¹ to 5 h⁻¹, and a pressure such that the reaction mixture is maintained in the liquid phase in the reactor.

The branched olefinic gasoline derived from the oligomerization step contains more than 90% by weight of olefins. To reduce the olefins content, this gasoline must be partially hydrogenated. It has been discovered that it is particularly advantageous to hydrogenate this gasoline, preferably the C₅-C₁₂ fraction, along with desulphurization of the sulphur rich and olefin-rich cracked gasoline. The branched olefinic gasoline is thus mixed with the cracked gasoline which is to be desulphurized (step ii)).

The joint treatment of the branched olefinic gasoline and cracked gasoline allows the two gasoline bases to be treated in the same reaction section, thereby limiting investment in the refinery. Further, it has surprisingly been discovered that the joint treatment of these two cuts can limit the octane number loss linked to hydrogenation of olefins during the hydrodesulphurization and hydrogenation step.

The mixture constituted by the branched olefinic gasoline and the cracked gasoline is thus treated in a hydrodesulphurization step (step iii)). Preferably, this mixture comprises at least 5% by weight of branched olefinic gasoline, more preferably at least 10% by weight of branched olefinic gasoline and highly preferably at least 15% by weight of branched olefinic gasoline with respect to the sum of the two gasolines.

It is advantageous to extract the light fraction containing olefins containing 5 or 6 carbon atoms from the cracked gasoline. This fraction essentially contains saturated sulphur-containing compounds in the form of mercaptans and thus does not necessitate severe desulphurization treatment. The mercaptans may principally be extracted in two manners. The cracked gasoline may be treated in a selective hydrogenation and thioetherification reactor, for example in that which is described in FR-A-2 797 639, then distilled to separate the light fraction containing hydrocarbons containing 5 and possibly 6 carbon atoms and free of sulphur. Another solution consists of treating the light fraction in an extractive MEROX (trade mark) type process, known to the skilled person, which allows mercaptans to be selectively extracted.

The hydrodesulphurization step is carried out by bringing the mixture of gasoline to be treated into contact with hydrogen on a catalyst which can transform organic sulphur-containing compounds into H₂S. However, it is advantageous to use a catalyst and operating conditions which can control the degree of hydrogenation of olefins using a process denoted selective hydrodesulphurization.

The operating conditions for the hydrodesulphurization reactor are those typically used to selectively desulphurize olefinic gasolines. As an example, a temperature in the range 220° C. to 350° C. is used, at a pressure which is preferably in the range 0.1 to 5 MPa, more preferably in the range 1 MPa to 3 MPa.

The space velocity is preferably in the range from about 0.5 h⁻¹ to 20 h⁻¹ (expressed as the volume of liquid gasoline to be desulphurized per volume of catalyst per hour), more preferably in the range 1 h⁻¹ to 10 h⁻¹ and highly preferably in the range 2 h⁻¹ to 8 h⁻¹.

The ratio of the hydrogen flow rate to the flow rate of the gasoline to be desulphurized is preferably in the range 50 litre/litre to 800 litre/litre, more preferably in the range 100 litre/litre to 400 litre/litre.

The hydrodesulphurization reactor contains at least one hydrodesulphurization catalyst which preferably comprises at least one group VIII element and at least one group VIB element, deposited on a porous support.

The group VIII element is preferably iron, cobalt or nickel. The group VIB element is preferably molybdenum or tungsten. The amount of group VIII element, expressed as the oxide, is preferably in the range 0.5% by weight to 15% by weight and more preferably in the range 0.7% by weight to 10% by weight. The amount of group VIB element is preferably in the range 1.5% by weight to 60% by weight, more preferably in the range 2% by weight to 50% by weight.

The porous support is preferably selected from the group constituted by: silica, alumina, silicon carbide or a mixture of these constituents.

To control olefin hydrogenation, it is advantageous to use a support comprising alumina, preferably at least 70% alumina, more preferably at least 80% by weight alumina, with a specific surface area of less than 200 m²/g, preferably less than 150 m²/g, more preferably less than 100 m²/g.

The porosity of the catalyst before sulphurization is preferably such that the mean pore diameter is more than 20 nm and more preferably in the range 20 to 100 nm. The surface density of the group VIB metal is preferably in the range 2×10⁻⁴ g to 40×10⁻⁴ g of the oxide of said metal per m² of support, more preferably in the range 4×10⁻⁴ g to 16×10⁻⁴ g/m².

Since the group VIB and VIII elements are active in hydrodesulphurization in their sulphurized form, the catalyst generally undergoes a sulphurization step before bringing it into contact with the feed to be treated. Generally, said sulphurization is achieved by heat treating the solid during which it is brought into contact with a decomposable sulphur-containing compound which can generate hydrogen sulphide. Alternatively, the catalyst may be brought into direct contact with a gas stream comprising hydrogen sulphide. This step may be carried out ex situ or in situ, i.e. inside or outside the hydrodesulphurization reactor.

Optionally, before contact with the feed, the sulphurized catalyst also undergoes a carbon deposition step to deposit a non-negligible amount of carbon, preferably 2.8% by weight or less. This carbon deposition step is intended to improve the selectivity of the catalyst by preferentially poisoning the hydrogenating activity of the catalyst. Highly preferably, the amount of deposited carbon is in the range 0.5% to 2.6% by weight. This carbon deposition step may be carried out before, after or during the catalyst sulphurization step.

Selective hydrodesulphurization may be carried out in a reactor comprising a catalyst as described above in the present application. However, it may be advantageous to carry out this step using a series of a plurality of catalysts, optionally in a plurality of reactors.

In a particular implementation of the invention, a hydrodesulphurization finishing step may be carried out, inserted between step iii) and step iv), in the presence of a catalyst preferably comprising at least one element selected from group VIII elements and deposited on a porous support such as alumina or silica. The amount of the group VIII element is preferably in the range 1% to 60% by weight, more preferably in the range 2% to 20% by weight. The metal is introduced in the form of the metal form then it is sulphurized before use. This finishing step is principally carried out to decompose saturated sulphur-containing compounds, such as mercaptans or sulphides, which are contained in the effluent from a first desulphurization step. When it is present, the finishing step is preferably carried out at a higher temperature than that for the desulphurization step.

In another particular implementation of the invention, the finishing step may be carried out on a hydrodesulphurization catalyst which comprises at least one group VIII element and a group VIB element deposited on a porous support.

The group VIII element is preferably iron, cobalt or nickel. The group VIB element is preferably molybdenum or tungsten. The amount of group VIII element, expressed as the oxide, is preferably in the range 0.5% by weight to 10% by weight, more preferably in the range 0.7% by weight to 5% by weight. The amount of group VIB metal is preferably in the range 1.5% by weight to 50% by weight, more preferably in the range 2% by weight to 20% by weight.

The porous support is preferably selected from the group constituted by: silica, alumina, silicon carbide or a mixture of these constituents. To minimize olefin hydrogenation, it is advantageous to use a support based on alumina with a specific surface area of less than 200 m²/g, preferably less than 150 m²/g and more preferably less than 100 m²/g.

The porosity of the catalyst before sulphurization is preferably such that the mean pore diameter is more than 20 nm, preferably in the range 20 to 100 nm.

The surface density of the group VIB metal is preferably in the range 2×10⁻⁴ g to 40×10⁻⁴ g of the oxide of said metal per m² of support, preferably in the range 4×10⁻⁴ g to 16×10⁻⁴ g/m².

The catalyst in this finishing step preferably has a catalytic activity in the range 1% to 90%, or even in the range 1% to 70%, preferably in the range 1% to 50% of the catalytic activity of the principal hydrodesulphurization catalyst (step iii)), measured in terms of conversion. When present, this finishing step is preferably carried out at a temperature which is higher than that for the principal desulphurization step.

The desulphurization steps are generally carried out in reactors with a fixed bed of catalyst which may comprise a plurality of catalyst beds separated by a cold fluid injection zone termed a cooling zone to control the temperature rise along the reactor.

At the end of the hydrodesulphurization step, the gasoline is cooled and condensed. The H₂S derived from transforming the organic sulphur-containing compounds is separated from the gasoline with the excess hydrogen (step iv)). H₂S dissolved in the gasoline is preferably eliminated by stripping.

EXAMPLE 1

Comparative

a) Production of a Branched Gasoline

An olefinic C₄ cut denoted α1 from a catalytic cracking unit the characteristics of which are shown in Table 1 was treated on an acid catalyst composed of an amorphous silica alumina sold by Axens with reference IP501. Further, this cut contained 3 ppm of sulphur and 1 ppm of nitrogen. The operating conditions were as follows: the temperature was 140° C., the HSV was 1 h⁻¹ and the pressure was 6 MPa. The reaction product was distilled to recover dimers and trimers containing 6 to 12 carbon atoms derived from reaction of the butenes. The gasoline produced, denoted α2, contained more than 90% by weight of olefins of which more than 90% by weight was branched olefins, and had an octane number, calculated by taking the mean of the motor octane number and the research octane number of 90.1. This mean is generally termed the FON. This gasoline was thus a branched olefinic gasoline as described in the invention.

TABLE 1
Weight %
Propane        0.2
Isobutane      34.1
Isobutene      13.9
1-butene       13.4
Butadiene      0.3
n-butane       9.8
Trans 2-butene 14.5
Cis 2-butene   10.0
C5+            3.8

This α2 gasoline was hydrogenated using a hydrogenation process which was not in accordance with the invention, in the presence of a hydrogenation catalyst containing nickel deposited on alumina, to hydrogenate a fraction of the olefins and to be able to incorporate the hydrogenated gasoline into a mixture of commercial gasoline with an overall olefin content limited to 18%.

The α3 gasoline obtained after partial hydrogenation contained 45% by volume of olefins and had a FON octane number of 90.5. Thus, hydrogenation of the olefinic gasoline had little effect on the octane number.

b) Production of a Desulphurized Catalytic Cracked Gasoline

A catalytic cracked gasoline β1 containing 810 ppm of sulphur with cut points of 60° C. and 170° C. and with an FON octane number of 86.4 was treated on a hydrodesulphurization catalyst containing 2.5% by weight of cobalt and 12% of molybdenum, expressed as the % of oxide, on an alumina support with a specific surface area of 145 m²/g. The gasoline was desulphurized under the following conditions: the temperature was 280° C., the pressure was 2 MPa, the HSV was 3 h⁻¹, and the ratio of the flow rate of hydrogen to the flow rate of gasoline to be desulphurized was 300 litres per litre. The desulphurized gasoline β2 produced contained 12 ppm of sulphur, 19.8% by volume of olefins and its FON octane number was 82.2.

c) Mixing Gasolines

2.5 litres of gasoline α3 and 7.5 litres of gasoline β2 were mixed to produce a gasoline for introduction into the gasoline pool. The gasoline 61 produced contained 26.1% by volume of olefins, 10 ppm of sulphur and had a FON octane number of 84.2.

EXAMPLE 2

In Accordance with the Invention

2.5 litres of branched olefinic gasoline α2 and 7.5 litres of gasoline β1 obtained using the processes described in Example 1 were mixed then treated on a hydrodesulphurization catalyst. The hydrodesulphurization catalyst was identical to that used in Example 1. The gasoline was desulphurized under the following conditions: the temperature was 280° C., the pressure was 2 MPa, the HSV was 3 h⁻¹, and the ratio of the flow rate of hydrogen to the flow rate of the gasoline to be desulphurized was 300 litres per litre of gasoline. The desulphurized gasoline produced, δ2, contained 25.9% by volume of olefins, 9 ppm of sulphur and had a FON octane number of 85.

Joint hydrodesulphurization of branched olefinic gasoline and cracked gasoline (Example 2) can thus produce a desulphurized gasoline with a better FON octane number than when the two gasolines are hydrogenated separately (Example 1).

Claims

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 H2S and to at least partially hydrogenate the olefins;

a step iv) for separating the H2S 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 m2/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 m2 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.