FR2885137A1 - PROCESS FOR THE DESULFURATION OF OLEFINIC ESSENCES - Google Patents

Abstract

The invention relates to a process for the production of gasoline with a low sulfur content and a controlled olefin content, comprising an oligomerization step of an olefinic feedstock (step i), a step (ii) of mixing the gasoline branched olefinic product produced in step (i) to a gasoline rich in sulfur and olefins, a step (iii) of hydrodesulphurization of this mixture and the separation of the H2S formed (step iv).

Description

The production of reformulated species meeting the new standards

  In particular, the environment requires that their sulfur content be significantly reduced. Thus, current and future environmental standards require refiners to lower the sulfur content in the gasoline pool to

  values less than or equal to 50 ppm in 2005 and 10 ppm as of 1 January 2009 within the European community. In addition, European refiners must also meet the standard for olefinic compounds, the content of which must be less than 18% by volume since 2000.

  Io Of the petrol bases used in refineries, cracking gasoline makes a strong contribution to sulfur and olefins in commercial gasoline. Essentially cracking means a cut from a coking unit, visbreaking, steam cracking or catalytic cracking (FCC) whose range of boiling points typically extends from that of hydrocarbons with 5 carbon atoms up to about 250 C.

  Catalytic cracking gasolines, which can constitute 30 to 50% by volume of the gasoline pool, have high olefin and sulfur contents. Sulfur present in reformulated gasoline is generally attributable, more than 90%, to gasoline from catalytic cracking. Desulphurisation of species and mainly FCC species is therefore of obvious importance for achieving standards.

  PRIOR ART: Hydrotreating or hydrodesulphurization of catalytic cracking gasolines, when carried out under the conditions conventionally known to those skilled in the art, makes it possible to reduce the sulfur content of the cut. However, this process has the major disadvantage of causing a very significant drop in the octane number of the cut, due to the hydrogenation or saturation of a large portion, or even all of the olefins under the conditions hydrotreatment.

  It has therefore been proposed methods for deep desulfurization of FCC gasolines while maintaining the octane number to an acceptable level.

  These processes are, for the most part, based on the principle of selective hydrodesulfurization aimed at converting sulfur compounds into H 2 S, while limiting the hydrogenation of olefins to paraffins, a transformation which induces a significant loss of octane. For example, patent FR 2,797,639 describes a process for the selective desulfurization of gasolines which makes it possible to produce olefinic gasolines that meet the current sulfur content standards, while minimizing the loss of octane during hydrodesulfurization.

  In addition, oligomerization processes have been developed to produce high octane gasolines from hydrocarbon fractions containing olefins of 3 to 6 carbon atoms. These processes use an acid catalyst which makes it possible to oligomerize the olefins present in the feedstock to isoolefins containing from 6 to 12 carbon atoms and having a high octane number. These processes make it possible to produce a gasoline with a high octane number and no sulfur content. However, these essences consist of nearly 100% by weight of olefins.

  There are numerous industrial processes for the production of gasolines by oligomerization from olefins of 3 to 6 carbon atoms, which are described in particular in the book "Acid-base catalysis", C. Marcilly, ed. Technip, vol2. p 475-495.

  In addition, the patent WO2005019391 proposes a process for the desulfurization of olefinic gasoline without significant loss of octane based on the production of an isolefinic species of 8 to 12 carbon atoms from a sulfur species, and the selective hydrodesulfurization of this isolefinic species to produce a desulfurized gasoline without loss of octane. However, this process requires treating, on an acidic catalyst, an olefinic gasoline containing sulfur and whose end point can be high, which can affect the stability of said catalyst and cause its deactivation. Indeed, the presence of compounds that may affect the stability of acidic catalysts such as sulfur compounds, polyunsaturated compounds and nitrogen compounds are generally concentrated in the heavy fractions of gasoline and in particular the fractions containing predominantly olefins from 7 to 12 carbon atoms.

  The present invention provides a process for producing a controlled sulfur and olefin gasoline, the desulphurization and hydrogenation being accomplished with a minimized decrease in octane number.

  SUMMARY OF THE INVENTION: The invention relates to a process for the production of low sulfur content gasoline with controlled olefin content comprising at least the following steps: a first step of contacting the an olefinic feedstock with an acid catalyst under conditions capable of causing oligomerization of the olefins present in the feedstock to produce a branched olefinic gasoline; - a step (ii) of mixing the branched olefinic gasoline produced in step ( i) a gasoline rich in sulfur and olefins, - a step (iii) of treatment of the mixture thus constituted, in the presence of hydrogen on a hydrodesulfurization catalyst for converting the sulfur present in the feedstock into H2S and at least partially hydrogenating the olefins, - a step (iv) for separating the H2S formed.

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

  DETAILED DESCRIPTION OF THE INVENTION

  The invention relates to a production scheme for low sulfur gasoline with controlled olefin content. The gasoline produced also has a high octane number.

  A hydrocarbon fraction rich in olefins, preferably containing predominantly (that is to say at least 40% by weight, preferably at least 70% by weight and very preferably at least 90% by weight), hydrocarbons having 3 to 6 carbon atoms per molecule (also called C3-C6 cuts, or C3, C4, C5, C6 slice mixtures) is treated on an acid catalyst to produce a gasoline comprising a larger number of carbons, via an oligomerization reaction for example dimerization and / or trimerization. It is indeed advantageous to treat only the hydrocarbon fractions containing the shortest olefins on the acid catalyst. The olefinic fractions containing hydrocarbons having 7 or more carbon atoms concentrate the compounds capable of deactivating acidic catalysts, such as basic nitrogen compounds, sulfur compounds or polyunsaturated compounds. On the other hand, olefins containing 7 or more carbon atoms 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 from trimerization reactions containing more than 12 carbon atoms. It is advantageous to separate this heavy fraction which concentrates the unsaturated and optionally polyunsaturated products before hydrogenation of the C6-C12 cut. The separation of the C12 + heavy fraction is conventionally carried out by distillation. The C12 + heavy fraction can be treated in a hydrotreating or hydrogenation unit to be upgraded as a base for aviation fuel also called kerosene, or for diesel.

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

  3o said hydrocarbon fraction rich in olefins (also called olefinic feedstock) 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 feedstock treated in the oligomerization section (step i) can be derived from catalytic cracking, coking or steam-cracking units. It is generally preferable to subject the feedstock to pretreatment steps, prior to step i), in order to reduce the content of polyunsaturated compounds, sulfur compounds and nitrogen compounds. These pretreatment make it possible to maximize the cycle time of the acid catalysts. i0

  Indeed, the diolefins present in the feed are likely to cause deactivation of the catalyst by polymerization and formation of gums. The polyunsaturated compounds can be extracted for recovery for petrochemical applications. This extraction is generally carried out by liquid liquid extraction or extractive distillation using polar solvents. For example, butadiene can be extracted from the C4 cut from steam cracking using solvents such as acetonitrile, furfural or other polar solvents. Another possibility is to convert the diolefins to olefins by selective hydrogenation. In order to maximize the lifetime of the acid catalyst used to oligomerize the olefins, the content of polyunsaturated compounds of the feedstock entering the oligomerization unit is preferably less than 1% by weight and more preferably less than 0. , 5% weight.

  Nitrogen compounds often have a basic character and thus neutralize catalytic acid sites. The basic compounds likely to be present in the C3 - C6 cuts are mainly amines. The nitrogen compounds, if they are present in said feedstock, can be extracted by a washing step with water or acid washing, or by a selective hydrogenation step as described in patent FR 2,850,113. The nitrogenous compounds to be extracted are not only the basic nitrogen compounds, known to deactivate the acidic catalysts by neutralization of acid sites such as amines, but also non-basic nitrogen compounds such as nitriles which can also cause deactivation of the acid catalysts.

  However, it has been observed by the inventors that non-basic nitrogen compounds such as nitriles can also cause deactivation of acid catalysts because they can, under certain conditions be converted into basic compounds such as amines and neutralize acidic sites.

  The sulfur compounds present in C3-C6 cuts are mainly mercaptans. Mercaptans can react with olefins on acid catalysts to form sulphides which are basic in nature. The sulphides thus formed can therefore degrade the activity of the acid catalysts.

  The mercaptans can be removed either by means of an extractive oxidation treatment which makes it possible to extract them in the form of disulphide such as the MEROX (registered trademark) method, or by adding addition to the olefins on a metal catalyst according to a reaction of thioetherification. The content of sulfur compounds in the form of mercaptans or sulphides of the olefinic feedstock of the oligomerization unit is preferably less than 100 ppm by weight, and more preferably less than 50 ppm by weight.

  In the case where the feed contains thiophene compounds and more particularly thiophene or methylthiophenes, these compounds are capable of reacting with a fraction of the olefins present in the C3-C6 olefinic cut to form heavier alkylated thiophene compounds. These compounds can therefore optionally be removed from the branched olefinic gasoline by simple distillation.

  The catalysts used in step i) are acid catalysts preferably selected from the group consisting of: supported inorganic acids such as phosphoric acids on silica, sulfonic acids on polymers such as ion exchange resins, inorganic oxides such as aluminas, amorphous silica-alumina, zeolites.

  The compounds that are to be produced must be branched to maximize their octane number. For this purpose, it is preferable to use acid catalysts having a mean pore diameter large enough to allow the diffusion of highly branched molecules. The acid catalyst used preferably has an average pore diameter greater than 0.5 nm and more preferably greater than 0.7 nm. An amorphous silica-alumina type catalyst or an ion exchange resin is preferably used.

  In the particular case where the catalyst is an amorphous silica-alumina, it is preferable to dry the olefinic feedstock before contacting with the catalyst in order to obtain a feedstock containing less than 100 ppm of water, and preferably less than 50 ppm. of water. The reactor is operated at a temperature of between 50 ° C. and 250 ° C., a VVH of between 0.5 hr and 5 hr -1, and a pressure such that the reaction mixture is maintained in the liquid phase in the reactor.

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

  The joint treatment of branched olefin gasoline and cracking gasoline makes it possible to treat the two gasoline bases in the same reaction section, which limits the investments to be made in the refinery. Moreover, it has surprisingly been found that the joint treatment of these two sections makes it possible to limit the octane loss associated with the hydrogenation of the olefins during the hydrodesulfurization and hydrogenation stage.

  The mixture consisting of branched olefinic gasoline and cracking gasoline is therefore treated in a hydrodesulfurization 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 very preferably at least 15% by weight of branched olefinic gasoline with respect to sum of the two essences.

  It is advantageous to extract cracking gasoline from the light fraction containing the 5 and 6 carbon olefins. Indeed, this fraction contains essentially saturated sulfur compounds in the form of mercaptans and therefore does not require a severe treatment of desulfurization. Mercaptans can be extracted mainly in two ways. The cracking gasoline may be treated in a selective hydrogenation and thioetherification reactor, for example in that which is described in patent FR 2,797,639, and then distilled in order to separate the light fraction containing the hydrocarbons at 5 and possibly 6 carbon atoms and sulfur-free. Another solution is to treat the light fraction in an extractive MEROX type of process known to those skilled in the art and which makes it possible to selectively extract the mercaptans.

  The hydrodesulfurization step is carried out by contacting the gasoline mixture to be treated with hydrogen on a catalyst capable of converting the organic sulfur compounds into H 2 S. However, it is advantageous to use a catalyst and operating conditions making it possible to control the degree of hydrogenation of the olefins according to a process known as selective hydrodesulfurization.

  The operating conditions of the hydrodesulfurization reactor are those typically used to selectively desulphurize olefinic species. The operation is carried out, for example, at a temperature of between 220 ° C. and 350 ° C., under a pressure preferably of between 0.1 and 5 MPa, more preferably between 1 MPa and 3 MPa.

  The space velocity is preferably between about 0.5 h -1 and 20 h -1 (expressed as the volume of liquid gas to be desulphurized per volume of catalyst per hour), and more preferably between 1 h -1 and 10 h -1 and very preferably between 2 h -1 and 8 h -1.

  The ratio of the hydrogen flow rate to the gasoline flow to be desulphurized is preferably between 50 liter / liter and 800 liter / liter, and more preferably between 100 liter / liter and 400 liter / liter.

  The hydrodesulfurization reactor contains at least one hydrodesulphurization catalyst to which it preferably 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 content of group VIII element expressed as oxide is preferably between 0.5% w / w and 15% w / w and more preferably between 0.7% w / w and 10% w / w. The metal content of group VIb is preferably between 1.5% by weight and 60% by weight and more preferably between 2% by weight and 50% by weight.

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

  In order to control the hydrogenation of the olefins, it is advantageous to use a support comprising alumina, preferably at least 70% by weight of alumina, more preferably at least 80% by weight of alumina, the specific surface area of which is less than 200 m 2 / g, preferably less than 150 m 2 / g, and very preferably less than 100 m 2 / g.

  The porosity of the catalyst before sulfurization is preferably such that the average pore diameter is greater than 20 nm and more preferably between 20 and 100 nm. The surface density of the Group VIb metal is preferably between 2 × 10 -4 and 40 × 10 -4 gram of oxide of said metal per m 2 of support, more preferably between 4 × 10 -4 and 16 × 10 -4 g / m 2.

  The Group VIB and VIII elements being active in hydrodesulphurization in their sulfurized form, the catalyst generally undergoes a sulphurization step before it is brought into contact with the feedstock to be treated. Generally this sulphurization is obtained by a heat treatment of the solid during which it is brought into contact with a decomposable sulfur compound and hydrogen sulphide generator. Alternatively, the catalyst can be directly contacted with a gaseous stream comprising hydrogen sulfide. This step may be carried out ex situ or in situ, ie inside or outside the hydrodesulfurization reactor.

  Optionally, before contacting the feedstock, the sulfurized catalyst is also subjected to a carbon deposition step so as to deposit a not insignificant carbon content, preferably less than or equal to 2.8% by weight. This carbon deposition step aims at improving the selectivity of the catalyst by preferentially poisoning the hydrogenating activity of the catalyst. Very preferably the carbon content deposited is between 0.5 and 2.6% by weight. This carbon deposition step may be carried out before, after, or during the step of sulphurizing the catalyst.

  The selective hydrodesulfurization can be carried out in a reactor comprising a catalyst as described previously in this application. However, it may be advantageous to carry out this step using a sequence of several catalysts and possibly in several reactors.

  According to a particular embodiment of the invention, a step of finishing the hydrodesulfurization, interposed between step iii) and step iv) in the presence of a catalyst preferably comprising at least one element selected from the elements group VIII and deposited on a porous support, such as for example alumina or silica. The group VIII element content is preferably between 1 and 60% by weight and more preferably between 2 and 20% by weight. The metal is introduced as a metal oxide, then it is sulphurized before use. This finishing step is mainly used to decompose saturated sulfur compounds such as mercaptans or sulphides, which are contained in the effluent of a first desulfurization step. When present, the finishing step is preferably carried out at a temperature higher than the first desulfurization step.

  According to another particular embodiment of the invention, the finishing step may be carried out on a hydrodesulfurization 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 content of group VIII element expressed as oxide is preferably between 0.5% w / w and 10% w / w and more preferably between 0.7% w / w and 5% w / w. The metal content of the group VIb is preferably between 1.5% by weight and 50% by weight and more preferably between 2% by weight and 20% by weight.

  The porous support is preferably selected from the group consisting of: silica, alumina, silicon carbide or a mixture of these constituents. To minimize the hydrogenation of the olefins, it is advantageous to use an alumina-based support whose specific surface area is less than 200 m 2 / g, preferably less than 150 m 2 / g, and very preferably less than 100 m 2. m2 / g.

  The porosity of the catalyst before sulfurization is preferably such that the average pore diameter is greater than 20 nm and preferably between 20 and 100 nm. The surface density of the group VIb metal is preferably between 2.10 -4 and 40 10 -4 gram of oxide of said metal per m 2 of support, preferably between 4.10-4 and 16. 10-4 g / m 2.

  The catalyst of this finishing step preferably has a catalytic activity of between 1% and 90%, indeed between 1% and 70%, and preferably between 1% and 50% of the catalytic activity of the main hydrodesulfurization catalyst. (step iii), measured in terms of conversion. When present, this finishing step is preferably carried out at a temperature above the temperature of the main desulfurization step.

  The desulfurization steps are generally carried out in fixed catalyst bed reactors which may comprise several catalyst beds separated by a cold fluid injection zone called a cooling zone for controlling the rise in temperature along the reactor.

  At the end of the hydrodesulfurization step, the gasoline is cooled and condensed. The H2S resulting from the transformation of the organic sulfur compounds is separated with the excess of hydrogen from the gasoline (step iv). The H2S dissolved in the gasoline is preferably removed by stripping.

  EXAMPLE 1 (Comparative): a) Production of a branched gasoline: A C4 olefinic cut called al from a catalytic cracking unit whose characteristics are shown in Table 1 is treated on an acidic catalyst composed of a silica alumina amorphous marketed by Axens under the reference IP 501. In addition, this cut contains 3 ppm of sulfur and 1 ppm of nitrogen. The operating conditions are as follows: the temperature is 140 ° C., the VVH is 1 h -1 and the pressure is 6 MPa The reaction product is distilled to recover dimers and trimers containing 6 to 12 carbon atoms The so-produced gasoline a2 contains more than 90 wt% olefins, more than 90 wt% of which are branched olefins, and has an octane number calculated by averaging the wt. engine octane number and the search octane number of 90.1 This average is generally called FON This gasoline is therefore a branched olefinic gasoline as described in the invention.

13 Table 1

  0/0 weight Propane 0.2 Isobutane 34.1 Isobutene 13.9 butene 1 13.4 Butadiene 0.3 n-butane 9.8 butene 2 trans 14.5 butene 2 cis 10.0 C5 + 3.8 This essence a2 is hydrogenated according to a hydrogenation process not according to the invention, in the presence of a hydrogenation catalyst containing nickel deposited on alumina, in order to hydrogenate a fraction of the olefins and to be able to incorporate the gasoline and hydrogenated in a commercial gasoline mixture, whose overall olefin content is limited to 18%.

  Gasoline a3 obtained after partial hydrogenation contains 45% by volume of olefins and has a FON octane number of 90.5. Hydrogenation of olefinic gasoline therefore has little effect on the octane number.

  b) obtaining a Desulphurized Catalytic Cracking Gasoline: A catalytic cracking gasoline containing 810 ppm of sulfur, whose cutting points are 60 ° C. to 170 ° C. and whose FON octane number is 86.4. is treated on a hydrodesulphurization catalyst containing 2.5% by weight of cobalt and 12% by weight of molybdenum expressed in% of oxide, on a surface-specific alumina support of 145 m 2 / g. The gasoline is desulfurized under the following conditions: the temperature is 280 C, the pressure of 2 MPa, the VVH of 3 h-1, and the ratio of the hydrogen flow rate on the gasoline flow to be desulphurated is 300 liters per liter. The desulfurized gasoline (32 produced contains 12 ppm sulfur, 19.8% olefin volume and its FON octane number is 82.2.

  (c) mixture of gasolines: 2.5 liters of gasoline a3 and 7.5 liters of gasoline are mixed in order to produce gasoline intended to be introduced into the gasoline pool. The gasoline 81 thus produced contains 26.1% olefins, 10 ppm sulfur and has a FON octane number of 84.2.

  EXAMPLE 2 (according to the invention): 2.5 liters of branched olefinic gasoline a2 and 7.5 liters of gasoline (31, obtained using the processes described in Example 1, are mixed and then treated on a catalyst The hydrodesulphurization catalyst is identical to that used in Example 1. The gasoline is desulfurized under the following conditions: the temperature is 280 ° C., the pressure 2 MPa, the VVH 3 h -1 , and the ratio of the hydrogen flow rate on the gasoline flow to be desulphurized is 300 liters per liter of gasoline The desulphurized gasoline produced 82 contains 25.9% olefin volume, 9 ppm sulfur, and its FON octane number is 85.

  The joint hydrodesulfurization treatment of olefinically-amine gasoline and cracking gasoline (Example 2) thus makes it possible to produce a desulfurized gasoline whose FON octane number is better than when the two gasolines are hydrogenated separately (Example 1). .

Claims (17)

  The invention relates to a process for the production of gasoline with a low sulfur content, with a controlled olefin content, comprising at least the following steps: a first stage (i) consisting in bringing an olefin feedstock into contact with an acid catalyst under conditions capable of causing oligomerization of the olefins present in the feedstock to produce a branched olefinic gasoline; - a step (ii) of mixing the branched olefinic gasoline produced in step (i) with a gasoline rich in sulfur and olefins, a step (iii) of treatment of the mixture thus constituted, in the presence of hydrogen on a hydrodesulphurization catalyst for converting the sulfur present in the feed into H2S and hydrogenate at least partially olefins, - a step (iv) of separation of the H2S formed.   2. A process according to claim 1 wherein the branched olefinic gasoline obtained in step i) is distilled to recover a cut containing predominantly hydrocarbons having 5 to 12 carbon atoms per molecule.   3. Method according to one of the preceding claims wherein the olefinic feedstock treated in step i) undergoes before said step pretreatment steps, in order to reduce the content of polyunsaturated compounds, sulfur compounds and nitrogen compounds.   4. Process according to one of the preceding claims, in which the olefinic feedstock treated in stage i) contains less than 100 ppm of sulfur and preferably less than 50 ppm of sulfur in the form of mercaptans or sulphides.   5. Method according to one of the preceding claims wherein the olefinic feed treated in step i) is a species containing predominantly hydrocarbons having 3 to 6 carbon atoms per molecule.   6. Method according to one of the preceding claims wherein the catalyst used in step i) is an acid catalyst selected from the group consisting of: inorganic acids supported such as phosphoric acids on silica, sulfonic acids on polymers such as ion exchange resins, mineral oxides such as aluminas, amorphous silica-aluminas, zeolites. io   7. Process according to claim 6, in which the acid catalyst used is of the amorphous silica-alumina type and has an average pore diameter greater than 0.5 nm.   8. The process of claim 6 wherein the acid catalyst used is an ion exchange resin.   9. Method according to one of the preceding claims wherein the step iii) hydrodesulfurization is carried out at a temperature between 220 C and 350 C, at a pressure preferably between 0.1 and 5 MPa, with a speed The space flow is between about 0.5 hr and 20 hr and with a hydrogen flow rate ratio on the gasoline flow to be desulphurized between 50 liter / liter and 800 liter / liter.   10. Method according to one of the preceding claims wherein the hydrodesulfurization catalyst comprises at least one group VIII element and a Group VIb element, deposited on a porous support.   11. A process according to claim 10 wherein the content of group VIII element expressed as oxide is between 0.5 wt.% And 15 wt.% And the metal content of group Vlb is between 1.5 wt.% And 60 wt. % weight   12. Method according to one of claims 10 or 11 wherein said support comprises alumina and has a specific surface area of less than 200 m 2 / g.   13. Method according to one of claims 10 to 12 wherein the surface density of the metal group Vlb is between 2.104 and 40.10-4 gram of oxide of said metal per m2 of support.   14. Process according to one of Claims 10 to 13, in which the hydrodesulfurization catalyst undergoes a sulphurization step before it is put in contact with the charge to be treated.   15. Method according to one of the preceding claims wherein a finishing step of hydrodesulphurization is interposed between step iii) and step iv).   16. The method of claim 15 wherein the catalyst used in said finishing step is a catalyst comprising at least one member selected from Group VIII elements and deposited on a porous support.   17. The process according to claim 15 wherein the catalyst used in said finishing step has a catalytic activity of between 10% and 70% of the catalytic activity of the main hydrodesulfurization catalyst.