FR2804966A1 - Process for the production of low sulfur gas oil fuels by a multi-stage catalytic dehydrosulfurization including a stage of partial elimination of hydrogen sulfide by stripping - Google Patents

Abstract

The partial removal of sulfur as hydrogen sulfide after the first stage gives a reduced partial pressure of hydrogen sulfide in subsequent stages with improved functioning of the catalyst. The process comprises :- a) A primary intensive hydrodesulfurization stage in which the gas oil cut and hydrogen are passed over a catalyst arranged in a fixed bed ; the catalyst comprises a mineral support, 0.5 - 40% of a group VIB metal or compound and 0.1 - 30% of a non-noble metal or compound of group VIII ; b) A second stage in which a gaseous fraction is recovered containing part of the H 2S contained in the effluent from the first stage, and an effluent depleted in H 2S ; and c) At least one third stage in which part of the H 2S-depleted effluent from (b) and H 2 is passed over a similar catalyst. The amount of catalyst used in the first stage is about 5 - 50% of the total amount of catalyst used in the process.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to the field of fuels for engines <B> to </ B> internal combustion. It relates more particularly to the manufacture of a fuel for engine <B> to </ B> ignition compression. In this field the invention relates to a method of transforming a diesel fuel cut to produce a fuel <B> to </ B> high cetane number, deeply desulfurized. In the current legislative framework of the majority of industrialized countries, the fuel used in engines must contain a sulfur content <B> to <B> 500 parts per million by weight (ppm). In some countries there are currently no standards imposing a maximum level of aromatics and nitrogen. However, it can be seen that several countries or states, like Sweden and California, and in particular certain diesel fuel classes in Sweden, already have to answer <B> to </ B> very strict specifications. Thus, in this country, diesel fuel of class <B> II </ B> must not contain more than <B> 50 </ B> ppm of sulfur and that of class <B> 1 </ B> more than <B> 10 </ B> ppm sulfur. Currently in Sweden Class 111 diesel fuel must contain <500> <BR> <BR> <BR> <BR> Similar limits are also <B> to </ B> meet for selling this type of fuel in California. The new environmental standards for the refinery's storage area for this type of fuel (this storage area is called by the skilled person <B> </ B> diesel pool <B>) </ B> for <B > 2005 </ B> require that the sulfur content of gas oils be reduced to <B> 50 </ B> ppm or even <B> 30 </ B> ppm. Other specifications could also relate to the aromatics content, the cetane number, the density or the final distillation point. STATE OF THE PRIOR ART For the purposes of the present description, the term "gas oil" refers to cuts of this type originating from the straight-run (SR) according to the English name) of a crude oil, that cuts of this type from different conversion processes and in particular those from the catalytic cracking process.

 Hydrogenation <B> If </ B> u ration is the essential refining process to bring these products to the required sulfur content. <B> It <b> has <B> already </ B> been proposed methods of hydrodesulphurization of conventional diesel gasoids in one step, because using a single catalytic bed. A brief description of such a process can be found, for example, in Hydrocarbon Processing, September 1984, page 70, or in Ullmann's Encyclopedia of Clinical Chemistry, Vol. A18. </ B> > 65-66. </ B> The conversion of the hydrocarbons in the reaction zone is then carried out in the presence of a certain partial pressure of H2S, essentially due to the desulfurization reactions. However, the presence of H2S on the hydrotreating catalyst has the effect of very significantly slowing the hydrodesulfurization reactions. These processes were sufficient as long as the desired sulfur contents in the final product were not too low (up to <300-500 ppm). For higher desulfurization levels, the inhibitory effect of H2S becomes critical.

This is why so-called two-step process diagrams using two catalytic beds have been proposed. A device for stripping the effluent at the outlet of the first bed thus makes it possible to eliminate most of the H 2 S, and the second catalyst bed thus operates with a low partial pressure of H 2 S, with a better desulfurizing activity.

Patent US Pat. No. 5,292,428 proposes a process for hydrotreating hydrocarbon feeds including gas oils, comprising two or more catalytic zones, with removal of H2S at the outlet of the first zone and addition of fresh hydrogen in the second reactor. Removal of the thus formed H2S is generally accomplished by means of an amine wash device. The activity of the catalyst in the second catalytic zone is thus improved because of the lower partial pressure of H2S. However, to achieve high levels of desulphurisation, sufficient to meet the most stringent sulfur specifications <B> (50 </ B> ppm or <B> 30 </ B> ppm), it is necessary to adopt as early as beginning of the cycle of severe operating conditions by increasing the temperature and the operating pressure and / or by adopting VVH (volume of charge per volume of catalyst and per hour) sufficiently low. Increasing the cycle start temperature can be detrimental in terms of cycle time. The operating pressure can be increased only within reasonable limits for reasons of economy of the process. Finally, for a given unit of capacity, operating <B> at </ B> a lower VVH means the use of a larger volume of catalyst, which implies an additional operating cost. <B> It </ B has also been described for example in the French patent application FR-A-2,757,532 a two-stage process using in the second step a catalyst containing a noble metal of group VIII, allowing a very deep desulphurization of diesel cuts. However, this process has a certain disadvantage because of the use of a noble metal in the second stage, linked on the one hand to the cost of such a catalyst and on the other hand to its sensitivity. B> to </ B> the hydrogen sulfide whose content must be limited to the maximum <B> to </ B> the output of the first stage if one wishes to obtain a lifetime of this reasonable second stage catalyst .

DESCRIPTION OF THE INVENTION Surprisingly, it has been discovered a process for producing kerosenes and / or gas oils <B> at </ B> with a very low sulfur content, making it possible to improve the efficiency of the operation of the catalyst by playing on the one hand on the partial pressure of H2S, and by optimizing the distribution of the residence times (and thus the volumes of catalysts) in the different catalytic zones. This process makes it possible to obtain a deep desulphurization with a lower hydrogen consumption than in the processes according to the prior art, which is another very important advantage for the refiner always <B> to </ B> the research of low processes consuming hydrogen, which is a valuable commodity at the refinery level. More specifically in the process according to the invention, the hydrocarbon fraction is typically a kerosene and / or a gas oil, whose initial boiling point is between approximately <B> 150 </ B> and <B> 250 OC, < And the final boiling point is between about 300 and 400. The process according to the invention uses two hydrodesulphurisation zones each containing at least a hydrodesulphurization catalyst bed containing on a support at least one group VIII non-noble metal associated with at least one Group VIB metal.

Thus, in its widest form of implementation, the hydrodesulphurization process of a kerosene and / or diesel fuel cut of the present invention comprises at least one of: a first step a) of deep hydrodesulfurization in which said gas oil fraction and hydrogen are passed over a fixed-bed catalyst comprising on a mineral support at least one metal or group VIB metal compound of the periodic table; elements in an amount expressed by weight of metal relative to the weight of the finished catalyst of approximately <B> 0.5 to </ B> 40%, at least one non-noble metal or non-noble group VIII metal compound of said periodic classification in an amount expressed by weight of metal relative to the weight of the finished catalyst of approximately <B> 0.1 to 30% </ B> and <B> b) </ B> at least a second stage <B > b) </ B> in which a gaseous fraction containing at least a part of the hydrogen is recovered sulfide (H2S) contained in the total effluent of said first step and a hydrogen sulfide depleted effluent, c) at least a third step c) in which at least a portion <B> of effluent depleted in hydrogen sulfide from step <B> b) </ B> and hydrogen on a catalyst arranged in fixed bed identical or different from that used in step a) comprising on a mineral support at least a metal or metal compound of group VIB of the Periodic Table of Elements in an amount expressed by weight of metal relative to the weight of the finished catalyst of approximately <B> 0.5 to </ B> 40 <B>%, At least one non-noble metal or non-noble metal compound of group VIII of said periodic classification in an amount expressed by weight of metal relative to the weight of the finished catalyst of approximately <B> 0.1 to 30 < %, Said process being characterized in that the amount of catalyst used in the first step is from about 5% to about 50% by weight of the total amount of catalyst used in said process.

The first step a) of deep hydrodesulfurization is usually carried out in a reaction zone comprising at least one fixed bed of catalyst. This zone may contain several catalyst beds identical or different from each other. Similarly, step c) is usually carried out in a reaction zone comprising at least one fixed bed of catalyst. This zone may contain several catalyst beds identical or different from each other. The different catalytic zones can be arranged in different reactors. Several catalytic zones, <B> to </ B>, with the exception of the first one, can be integrated into one and the same reactor.

The amount of catalyst used in the first step (step a is preferably from about 10% to about 40% by weight of the total amount of catalyst used in said and more preferably this amount will be from about 15% to about 30% by weight of the total amount of catalyst used in said process.

In order for the catalyst bed (s) in the reaction zone of step c) to remain at the sulphurized state, the H2S concentration at the inlet of this second catalytic zone is maintained at <B>. </ B> a sufficient level, playing on the level of hydrogen sulfide removal in step <B> b). </ B> <b> <b> </ b> recovery step a liquid feedstock depleted of hydrogen sulphide and a gaseous fraction containing at least a portion of the hydrogen sulphide contained in the total effluent of step a) may be used by any means known to man of the art. <B> A </ B>, this recovery can be carried out of a gaseous fraction containing at least a part of the hydrogen sulphide contained in the total effluent of step a) by stripping or entrainment ( also called stripping according to the English terminology) by at least one gas containing hydrogen under substantially the same pressure <B> to </ B> that prevailing in the first step and <B> at </ B> a temperature of about <B> 100 OC to </ B> approximately 450 <B> OC </ B> under conditions of formation of a gaseous effluent strip containing hydrogen and hydrogen sulfide. This recovery can also be carried out for example by expansion (flash) of the total effluent from step a). According to a particular embodiment of the invention, the gaseous fraction recovered in the <B> b) </ B> stage containing hydrogen sulphide is sent to an at least partial elimination zone of the hydrogen sulphide which it contains, from which purified hydrogen is recovered, which is recycled to the inlet of the deep hydrodesulphurization stage a). In this zone Hydrogen purification, <B> to </ B> from the gaseous mixture containing hydrogen and hydrogen sulphide from the at least partial elimination zone of the hydrogen sulphide, is usually carried out according to one or the other of the conventional techniques well known to those skilled in the art and in particular by prior treatment of this gas mixture with a solution of approximately <B> 25 </ B> MPa and preferably about <B> 1 </ B> MPa <B> to </ B> approximately <B> 10 </ B> MPa. According to this embodiment, when the adsorption zone comprises two reactors, one reactor is used to treat the gas while the other is in the course of regeneration or replacement of the material it contains allowing the drying and desulfurization of the reactor. gaseous mixture entering the said zone. <B> A </ B> the outlet of this additional treatment the content of hydrogen sulphide in the gas is usually <B> 1 </ B> ppm by weight and often of the order a few tens of ppb by weight.

The operating conditions of step a) usually comprise a temperature of about 240 ° C to about 420 ° C, a total pressure of about 2 MPa <B> to <50 ° C. About 20 MPa and a liquid charge hourly space velocity of about <B> 0.1 to </ B> about <B> 5 </ B> and that of step c) usually comprise a temperature of From about 240 OC to about 420 OC, a total pressure of about 2 MPa <B> to about 20 MPa and a liquid charge space velocity at most about <B> to </ B> about the hourly liquid charge space velocity of step a).

 The catalyst (s) used in the different catalytic zones are hydrodesulfurization catalysts. These catalysts may be conventional catalysts such as those described in the prior art and for example one of those described by the applicant in the French patent applications FR-A-2197966, FR-A-2583813 or in the patent document. EP <B> 297949. </ B> It is also possible to use commercial catalysts, such as, for example, those sold by PROCATALYSE. These catalysts each comprise at least one metal or a group VIB metal compound and / or at least one non-noble metal or a group VIII non-noble metal compound, on a suitable mineral support.

The catalyst support is generally a porous solid. This support is usually chosen from the group formed by alumina, silica, silica-aluminas, zeolites, magnesia, TiO 2 titanium oxide and the mixtures of at least two of these mineral compounds. Alumina is very commonly used.

The Group VIB metal is usually selected from the group consisting of molybdenum and tungsten, and the Group VIII metal is usually selected from the group consisting of nickel, cobalt and iron and most often from the group consisting of nickel and cobalt. Combinations such as NiMo or CoMo are typical. According to a preferred embodiment, the catalyst used in step a) and that used in step c) each comprise molybdenum or a molybdenum compound in an amount expressed by weight of metal relative to the weight of the finished catalyst of about 2 <B> at 30% </ B> and a metal or a metal compound selected from the group consisting of nickel and cobalt in an amount expressed by weight of metal based on the weight of the finished catalyst of about <B > 0.5 to 15%. </ B> Most often in step a) and in step c) a catalyst comprising as metal of group VIII of nickel and as metal of group VIB of molybdenum is used.

According to a preferred embodiment, the catalyst used in step a) and that used in step c) each further comprise at least one element selected from the group consisting of silicon, phosphorus and boron or one or more compounds of this or these elements.

In another embodiment, the catalysts employed in step a) and in step c) each comprise at least one halogen. Usually the amount of halogen is from about 0.1 to about 15% by weight based on the weight of the finished catalyst. Halogen is often selected from the group consisting of chlorine and fluorine and in a particular form the catalysts employed will contain chlorine and fluorine.

The temperature of the different catalytic zones is preferably between <B> 260 </ B> and 400 <B> OC, </ B> and more preferably between <B> 280 </ B> and <B> 390 OC. < The operating pressures used are <B> of </ B> preferably between 2 MPa and <B> 15 </ B> MPa and preferably between 2 MPa and <B> 10 </ B> MPa.

The overall hourly space velocity or global VVH (volume of charge per volume of catalyst per hour) is between <B> 0, 1 </ B> and <B> 10 </ B> h '. In the case where the process comprises two catalytic zones, the distribution of the residence times in the catalytic zones is such that the residence time in the first catalytic bed represents at most <50% </ b>. B> overall residence time.

EXAMPLES Example <B> 1 </ B> (Comparative) A gas oil, consisting of half straight-run gas oil and half LCO (catalytic cracked gas oil) is treated in a one-step process with a single catalyst bed in a reactor. The characteristics of this diesel are mentioned in table <B> 1. </ B>

200 cm.sup.2 of catalyst sold by the company Procatalyse under the reference HR 448 containing nickel and molybdenum <B> at </ B> are available at levels within the ranges indicated in steps a) and c) of the process according to US Pat. 'invention. After activation of the catalyst by sulfurization, the reactor is maintained under a total pressure of <B> 50 </ b> bar <B> g (1 </ B> bar <B> g </ B> equals <B> to 0.1 </ B> Mpa) and <B> to </ B> a temperature of 340 ° C. The diesel fuel charge is injected by the bottom of the reactor with a VVH of <B> 1 </ b> h- 1. A corresponding amount of hydrogen <B> at </ B> a ratio of 1-12 / Charge of 400 <B> 1/1 </ B> is injected <B>, </ B> the mixture charge and hydrogen passing through the catalytic bed in upward flow.

Table <SEP><B> 1 </ B>
<tb> Main <SEP> characteristics <SEP> of <SEP> the <SEP> load <SEP> diesel
<tb> Density <SEP> 1514 <SEP> (gicm) <SEP><B> 0.8984 </ B>
<tb> Sulfur <SEP> (ppm <SEP> wt) <SEP><B> 15900 </ B>
<tb> Distillation <SEP><B> ASTM <SEP> D86 </ B>
<tb> period <SEP><B> 5 <SEP>% <SEP> 228 <SEP><> C </ B>
<tb> period <SEP><B> 50 <SEP>% <SEP> 278 <SEP>'> C </ B>
<tb> point <SEP><B> 95 <SEP>% <SEP> 359 <SEP> OC </ B> In these conditions, the sulfur content of the product gas oil stabilizes <B> at </ B> a value <B> 60 </ B> ppm, a desulfurization rate of <B> 99.62 </ B> The hydrogen consumption by weight over <B> at </ B> the load is <B > 1.30%. </ B>

Example 2 (Comparative) The same gas oil was treated using the same catalyst under the same conditions but with an injection of the feedstock from the bottom of the reactor with a VVH of <B> 0.55 </ b> h-1. Under these conditions, the sulfur content of the product gas oil stabilizes <B> to </ B> a value of <B> 10 </ B> ppm, ie a desulfurization rate of 99.94 <B>%. </ B> The hydrogen consumption by weight with respect to <B> at </ B> the load is <B> 1.70%. </ B> In addition to the penalty due <B> to </ B> the low VVH used a sharp increase in hydrogen consumption compared to <B> to </ B> the implementation according to the example <B> 1 </ B> which is penalizing from the point of view industrial.

Example <B> 3 </ B> (Comparative) The same feedstock is now treated as in Example <B> 1, </ B> in a two-stage process with a single catalyst bed in two successive reactors. A stripping device between the two beds makes it possible to eliminate the H2S produced on the first bed.

This time, two beds of the same catalyst HR 448, which are both subjected to the same conditions of temperature and pressure (total pressure <B≥50 </ B> bar <B> g, </ B> T <B ≥ </ B> 340 "C. The hydrogen injected into the two catalytic beds is such that the ratio H2 <B> / </ B> Charge is in both cases 400 <B> 1/1. </ B>

The first bed consists of <B> 150 </ B> CM3 HR 448 catalyst, while the second bed consists of <B> 50 </ b> cm3 HR 448 catalyst. The charge rate is 200 cm3 / h, a VVH of <B> 1.33 </ B> h- 'on the first bed and 4 h-1 on the second bed. The residence time is globally <B> 1 </ B> hour, as in example <B> 1. </ B> The process diagram with intermediate stripping of H2S between the two catalyst beds allows in the operating conditions specified above to obtain a gasoline <B> at 30 </ B> ppm sulfur, a desulfurization rate of <B> 99.81%. </ B> The consumption of hydrogen by weight by ratio <B> to </ B> the load is, <B> 1.30%. </ B> Operating under these conditions we see a consumption of hydrogen substantially identical <B> to </ B> that of example <B> 1 </ B> with a slightly improved desulfurization rate compared to <B> to </ B> that obtained by operating under the conditions specified in example <B> 1. </ B>

Example 4 Process According to the Invention The same diesel fuel charge as in the preceding examples is now treated according to the same process as in Example 3, and under the same conditions of pressure, temperature, load and ratio flow H2 <B> / </ B> Load. The same overall volume of catalyst HR 448 is used (200 cm) but with a different distribution between the two beds, which now include each <B> 100 </ B> CM3.

Under these conditions, the overall residence time on all two beds is <B> 1 </ B> hour, with a residence time of <B> 0.5 </ B> hour on each of the beds ( 2 hr VVH on each of the beds).

This particular arrangement of the two catalytic beds makes it possible to reach a sulfur content in the final product of <B> 15 </ B> ppm, ie a <99.91% <B> desulfurization rate. The hydrogen consumption by weight relative to <B> at </ B> the load is <B> 1.30%. </ B> Compared <B> to </ B> the example <B> 3 </ B> there is a clear improvement in the desulfurization rate without increasing the hydrogen consumption Example <B> 5: </ B> Process according to the invention The conditions are here identical <B> to </ B> those of Example 4, <B> to </ B> with the exception of the catalyst volumes which are respectively <B> 50 </ B> CM3 for the first bed and <B> 150 </ B> CM3 for the second bed. For an overall residence time of <B> 1 </ B> hour, the residence time is <B> 0.25 </ B> hour on the first bed and <B> 0.75 </ B> time on the second bed.

 This particular arrangement of the two catalytic beds makes it possible to obtain a gasoline <B> at 7 </ B> ppm of sulfur, a desulfurization rate of <B> 99.96%. </ B> The consumption of hydrogen in weight compared to <B> to </ B> the load is <B> 1.30 </ B> Thus, the process according to the invention is advantageous for carrying out gas oil desulfurization treatments. <B> It </ B> becomes particularly advantageous for achieving low sulfur levels in diesel, less than <B> 30 </ B> ppm (Example 4) or even less than <B> 10 </ B> ppm (example <B> 5). </ B> To obtain a given specification of sulfur, the process according to the invention allows steps <B> to </ B> higher VVH, and thus a saving of catalyst of interest for the operator. In addition, the hydrogen consumption remains constant which is particularly interesting for the operator.

Claims (1)

<B> <U> CLAIMS </ U> </ B> <B> 1 - </ B> A method for hydrodesulfurizing a kerosene and / or diesel cut comprising at least a first step a) of hyd DETECTION <B> IF </ B> a deep ration in which said gas oil cup and hydrogen are passed over a catalyst arranged in a fixed bed comprising on a mineral support at least one metal or metal compound of group VIB of the periodic classification of the elements in an amount expressed by weight of metal with respect to the weight of the finished catalyst of from approximately <B> 0.5 to </ B> 40 <B>%, </ B> at least one non-noble metal or non-noble group VIII metal compound of said periodic table in an amount expressed by weight of metal based on the weight of the finished catalyst of approximately <B> 0.1 to 30% </ B> and # <B> b) At least a second subsequent step in which a gaseous fraction containing at least a portion of the hydrogen sulphide contained in the total effluent is recovered. of said first step and an effluent depleted of hydrogen sulfide, # c) at least a third step c) in which at least a portion of the effluent depleted in hydrogen sulfide from step <B> b) <is passed through / B> and hydrogen on a catalyst arranged in fixed bed identical or different from that used in step a) comprising on a mineral support at least one metal or metal compound of group VIB of the periodic table of the elements in an amount expressed by weight of metal relative to the weight of the finished catalyst of from <B> 0.5 to <B>%, </ B> at least one non-noble metal or compound <B> of Non-noble group VIII metal of said periodic classification in an amount expressed by weight of metal relative to the weight of the finished catalyst of approximately <B> 0.1 to 30%, said process being characterized in that the amount of catalyst used in the first step is from about <5% to about <B > 50% </ B> by weight of the total amount of catalyst used in said process. The process of claim 1 wherein the amount of catalyst used in the first step is from about 10% to about 40% by weight. the total amount of catalyst used in said process. <B> 3 - </ B> The process according to claim 1, wherein in step b) the recovery of a gaseous fraction containing at least a portion the hydrogen sulphide contained in the total effluent of step a) is carried out by stripping with at least one gas containing hydrogen under a pressure substantially identical to that prevailing in the first stage and <B> to </ B> a temperature of about <B> 100 OC to </ B> about 450 <B> OC </ B> under conditions of forming a stripping gaseous effluent containing hydrogen and hydrogen sulphide and a liquid charge depleted of hydrogen sulphide. The process according to claim 1 wherein in step b) the recovery of a gaseous fraction containing at least one part of the hydrogen sulphide contained in the total effluent of step a) is carried out by expansion of the total effluent from step a). <B> 5 - </ B> Method according to one of claims <B> 1 to </ B> 4 wherein the operating conditions of step a) comprise a temperature of about 240 <B> OC to < About 420 <B> OC, </ B> a total pressure of about 2 MPa <B> to </ B> about 20 MPa and a liquid charge space velocity of about <B> 0.1 about <B> 5 </ B> and that of step c) a temperature of about 240 <B> OC to </ B> about 420 <B> OC, </ B> a pressure total of about 2 MPa <B> to </ B> about 20 MPa and a liquid charge space velocity at most equal <B> to </ B> about the liquid charge space velocity of step a) . <B> 6 - </ B> A process as claimed in any of claims 1 to 5 wherein the catalyst used in step a metal or group VIB metal compound selected from the group consisting of molybdenum and tungsten and at least one metal or group VIII metal compound selected from the group consisting of nickel, cobalt and iron. <B> 7 - </ B> The process according to one of claims <B> 1 to 6 </ B> in which the catalyst used in step a) and that used in step c) each comprise molybdenum or a molybdenum compound in an amount expressed by weight of metal based on the weight of the finished catalyst of about 2 <B> to 30% </ B> and a metal or a metal compound selected from the group consisting of nickel and cobalt in an amount expressed by weight of metal based on the weight of the finished catalyst of approximately <B> 0.5 to 15%. </ B> <B> 8 - </ B> Process according to one of the Claims <B> 1-7 </ B> wherein the catalyst used in step a) and that used in step c) each comprise as group VIII metal of nickel and as molybdenum group VIB metal. <B> 9 - </ B> The process according to one of claims <B> 1 to 8 </ B> in which the catalyst used in step a) and that used in step c) each further comprise at least one element selected from the group consisting of silicon, phosphorus and boron or one or more compounds of this or these elements. <B> 10 - </ B> Method according to one of claims <B> 1 to 9 </ B> in which the support of the catalysts employed in step a) and in step c) are independently selected one of the other in the group formed by alumina, silica, silica-aluminas, zeolites, magnesia, TiO 2 titanium oxide and mixtures of at least two of these mineral compounds. <B> 11 - </ B> Method according to one of claims <B> 1 to 10 </ B> in which the catalysts used in step a) and in step c) each comprise at least one halogen . 12-. Process according to one of claims 1 to 11 wherein the catalysts employed in step a) and in step c) each comprise an amount of halogen of about <B> 0, 1 to <B> 15% </ B> by weight based on the weight of the finished catalyst. <B> 13 - </ B> Method according to one of claims <B> 1 to </ B> 12 the catalysts employed in step a) and in step c) each comprise at least one halogen selected in the group formed by chlorine and fluorine. 14- Method according to one of claims <B> 1 to 13 </ B> the catalysts employed in step a) and in step c) each comprise chlorine and fluorine. <B> 15 - </ B> Method according to one of claims <B> 1 to </ B> 14 wherein the gaseous fraction recovered in step <B> b) </ B> containing hydrogen sulphide is sent to an at least partial elimination zone of the hydrogen sulphide it contains, from which purified hydrogen is recovered and recycled to <B> to </ b>. / B> the entry of step a) of hyd rodsulfésulf u ration deep.