EP1659163B1 - process for converting saturated sulfur compounds in a hydrocarbon fraction containing few or no olefins - Google Patents

Description

Technical field of the invention

The invention relates to a process for softening and desulfurizing hydrocarbon fractions generally containing less than 1% by weight of olefins, and sulfur mainly in the form of mercaptans. The process described in the present invention is particularly suitable for the treatment of hydrocarbon fractions derived from atmospheric distillation, such as, for example, gasoline, kerosene or light gas oil fractions.
These hydrocarbon fractions contain varying amounts of mercaptans which give them a corrosive and malodorous acid character. The solutions generally used to treat these mercaptans consist of oxidizing the mercaptans to disulfides, and then separating the latter, which are heavier, generally by distillation.
The present invention provides a technical solution for softening the fractions to be treated by converting the mercaptans to sulphides, and optionally to desulfurize the fractions to be treated by separating the sulphides thus formed.

The process consists in mixing the hydrocarbon fraction to be treated with, on the one hand, another hydrocarbon fraction containing unsaturated compounds, and on the other hand with hydrogen, then on passing the mixture over a metal sulphide catalyst in optimized conditions to promote the addition of mercaptans to the unsaturated compounds to form sulphide sulfur compounds. The sulfur compounds thus transformed have a boiling point higher than their initial temperature and can then be separated to recover a sulfur-depleted hydrocarbon fraction.

Prior art

The reduction of the mercaptan content in the hydrocarbon fractions directly from atmospheric distillation is generally carried out by a so-called "softening" process.
Many softening processes do not implement the reduction of the overall sulfur content, but only the dimerization of the mercaptans to disulfides to meet specifications. This oxidation of mercaptans is generally carried out catalytically using oxygen as the oxidizing agent.
On the other hand, the extractive softening processes make it possible to subtract the sulfur from the feedstock. They consist in contacting the distillate to be treated with a basic aqueous solution (most often based on sodium hydroxide) which makes it possible to recover the mercaptans. The basic solution is then regenerated by catalytic oxidation of sodium mercaptides to disulfides.
The US Patent 3,574,093 describes the implementation of such a process for gasolines and cuts lighter (C3-C4). Many improvements have been made to the process, such as the use of aliphatic amines in small amounts in sodium hydroxide solutions ( US Patents 2,546,345 and US 2,853,431 ). However, the method requires a very large volume of solution and many extraction stages, which limits its interest.
The use of metal hydroxides is not limited to liquid-liquid extraction. The teaching of US Patent 2,808,365 shows us that an "alkaline solid" obtained by the action of sodium hydroxide on calcium carbonate can serve as a catalyst for the oxidation of mercaptans in the presence of epoxides in small amounts in the charge.
Other carriers, such as activated carbons, have been impregnated with a basic solution to act as catalysts (patent GB 763625 ).
Many oxidizing molecules are used in supported form to effect the oxidation of mercaptans. However, they have the disadvantage of leaving most often undesirable traces of metals in the sweetened effluent.
The US Patent 2,255,394 the use of copper in its CuCl ₂ form, whereas softening is carried out in the presence of oxygen by the addition of organometallic type Et ₂ Ni directly into the charge according to the US Patent 3,053,756 .
Catalysts based on CrO ₃ or more recently Cobalt phthalocyanine are also used. The advantage of the cobalt phthalocyanine process is that it can be carried out in the supported phase for simple softening, or in the basic aqueous phase in its extractive variant.
The list is not exhaustive and other metals have been tested, always with the aim of oxidizing the mercaptans disulfide.

The softening of the hydrocarbon fractions may also be carried out by addition reaction of the mercaptans to the diolefins.
These reactions are in particular envisaged in the case of the treatment of gasolines containing both mercaptans and unsaturated hydrocarbon compounds, such as, for example, FCC (Fluid Catalytic Cracking) gasolines in which the unsaturated compounds, which are most often olefinic in nature, are widely present.
The patent US 2003/0094399 A1 describes a process in which a distillation column is used in which the conversion of mercaptans to sulphides is carried out at the top of the column in contact with a hydrogenation catalyst involving a Group VIII metal.
The Patent FR 2821851 A1 also describes a process for adding mercaptans to olefins by means of a catalyst comprising at least one Group VIII element.
The same type of reactions is envisaged in the US Patent 5,659,106A on an acid catalyst such as a sulfonated resin. In this case, the reaction is carried out in the absence of hydrogen.

Non-catalytic solutions are also presented in the literature.
For example, in the US Patent 2,694,034 a softening method involving unsaturated compounds is developed. A saturated mercaptan-rich naphtha is treated by adding an olefinic compound (from 1 to 10% by weight) in the presence of a phenylene diamine inhibitor, at a rate of 0.0001% to 1% weight. After a sufficient storage period, the mercaptan concentration of the mixture meets the specifications.

In conclusion, there is no solution proposed in the literature that makes it possible to transform mercaptans to soften or even desulfurize the hydrocarbon fractions obtained from atmospheric distillation by catalytic routes other than the oxidation of mercaptans to disulfides.
Solutions based on the addition of mercaptans to unsaturated compounds are only described for hydrocarbon fractions containing large amounts of olefins such as catalytic cracking or thermal cracking gasolines. The present invention provides a simple solution for converting or even separating saturated sulfur compounds present in the hydrocarbon fractions obtained from atmospheric distillation.

Brief description of the invention

The process described in the present invention provides a solution for partially softening or partially desulfurizing hydrocarbon fractions that are free of olefins or contain low levels of unsaturated compounds. The term hydrocarbon fractions containing little or no olefins, fractions having less than 5% olefins, and preferably less than 1% of olefins.
The process described in the present invention is particularly suitable for the treatment of hydrocarbon fractions derived from atmospheric distillation, such as, for example, gasoline, kerosene or light gas oil fractions.
The process consists in mixing, with the hydrocarbon fraction to be treated, hydrogen and possibly another hydrocarbon fraction containing olefinic olefins. This olefinic fraction is generally derived from cracking processes such as the FCC, the steam cracker, or a coking unit. Hydrogen can come from any source in the refinery. The quantities of hydrogen required are generally low enough not to require the complementary implantation of a dedicated hydrogen production unit. The resulting mixture must be such that on the one hand the molar ratio H 2 / olefins is between 0.03 and 2, preferably between 0.05 and 1, so more preferably between 0.2 and 0.8, and secondly the molar ratio olefins / mercaptans is between 5 and 5000 and preferably between 10 and 1000, more preferably between 80 and 600, or even between 150 and 400 .
The mixture corresponding to the two preceding conditions is then injected into a reactor containing a catalyst capable of reacting the saturated sulfur compounds such as the mercaptans present. To be effective, the reaction must be carried out in the presence of hydrogen.

According to a preferred embodiment of the invention, the non-olefinic feedstock treated is a gasoline fraction resulting from atmospheric distillation, the boiling point of which is less than 250.degree. C., and preferably less than 220.degree. But in some cases which are still part of the field of the invention, the charge to be treated may itself contain a certain amount of olefins to which it will be necessary to add only a quantity of hydrogen determined to be in the range of ratio H2 olefins defined above, namely between 0.03 and 2, and preferably between 0.05 and 1.
In cases where it is also necessary to add to the feedstock to be treated a certain amount of olefins, the latter will generally come from an olefinic gasoline such as for example a cracking gasoline generally from a catalytic or thermal cracking unit.

The process for converting the mercaptans contained in a hydrocarbon feed containing less than 1% olefins according to the invention may comprise an additional step of separation of the sulfur compounds formed during the reaction step, so as to produce an effluent containing less than 50% by weight of the sulfur compounds present in the feed, and a cut containing the majority of the sulfur compounds.

The process according to the invention thus makes it possible to convert the mercaptans into heavier sulfur compounds with a conversion rate generally of at least 50% by weight.

Detailed description of the invention

• Une première étape dite de mélange, qui consiste à mélanger la charge hydrocarbonée à traiter et contenant les mercaptans avec une certaine quantité d'hydrogène et éventuellement avec une fraction oléfinique.
• Une seconde étape dite réactionnelle, qui consiste à faire réagir le mélange résultant de la première étape sur un catalyseur comportant au moins un métal du groupe VIII de manière à transformer les mercaptans en sulfures. On appellera dans la suite du texte cette réaction transformation des mercaptans en sulfures.

La charge hydrocarbonée à traiter contient généralement moins de 1% poids d'oléfines et plus de 50 ppm poids de soufre principalement sous forme de mercaptans.
La présente invention s'applique plus particulièrement au traitement des coupes essences directement issues de la distillation atmosphérique qui sont généralement pratiquement dépourvues d'oléfines et riches en composés soufrés saturés. Mais elle peut s'appliquer à d'autres charges tels que des distillats contenant également peu ou pas d'oléfines.
Par composés soufrés saturés, on entend les composés soufrés appartenant aux familles des mercaptans ou sulfures.
Les températures d'ébullition de la charge à traiter sont inférieures à 350°C, et de préférence inférieure à 250°C, ce qui correspond généralement à une coupe essence.The invention can be defined as a process for treating a hydrocarbon feedstock, generally a gasoline containing little or no olefins, generally less than 5% of olefins, and typically less than 1% of olefins, so as to to transform and possibly eliminate the sulfur compounds that it contains, in particular mercaptans. The method according to the invention comprises at least 2 steps.

• A first so-called mixing step, which consists in mixing the hydrocarbon feedstock to be treated and containing the mercaptans with a certain amount of hydrogen and optionally with an olefinic fraction.
• A second so-called reaction step, which consists in reacting the mixture resulting from the first step on a catalyst comprising at least one Group VIII metal so as to convert the mercaptans to sulphides. In the rest of the text, this reaction will be called the transformation of mercaptans into sulphides.

The hydrocarbon feedstock to be treated generally contains less than 1% by weight of olefins and more than 50 ppm by weight of sulfur, mainly in the form of mercaptans.
The present invention is more particularly applicable to the treatment of gasoline cuts directly from atmospheric distillation which are generally substantially free of olefins and rich in saturated sulfur compounds. But it can be applied to other fillers such as distillates also containing little or no olefins.
The term "saturated sulfur compounds" means sulfur compounds belonging to the families of mercaptans or sulphides.
The boiling temperatures of the feedstock to be treated are less than 350.degree. C., and preferably less than 250.degree. C., which generally corresponds to a gasoline cut.

The olefinic fractions present in the refinery are generally derived from cracking units such as catalytic cracking, thermal cracking or steam cracking.

The boiling temperatures of this olefinic fraction are generally below 250 ° C.
However, it is preferable to use hydrocarbon fractions containing olefins with 4, 5 or 6 carbon atoms in order to improve the yield of the weighting reaction of the sulfur compounds.
Hydrogen is also most often from the refinery. Hydrogen substantially free of H ₂ S is used because this compound can react with olefins to form undesirable sulfur compounds. The hydrogen may come either from a specific hydrogen production unit or, for example, from the catalytic reforming unit of the gasolines.

The reaction step consists of passing the mixture formed at the end of the mixing step on a catalyst operated in a fixed bed, under optimized operating conditions.
The catalyst used in the reactor is a catalyst comprising at least one Group VIII metal deposited on an inert support based on porous metal oxide. Preferably, the support is made of alumina, silica, titanium oxide or contains at least 50% alumina.
Another Group VIb metal may be associated with the Group VIII metal to form a bimetallic catalyst.
The amount of Group VIII metal in oxide form is between 1% and 30% by weight. The amount of metal of group Vlb in oxide form is between 0% and 20% by weight.
Nickel-based catalysts or catalysts based on mixed nickel and molybdenum or tungsten oxide are preferably used.
Typically, the catalyst used in the reaction stage contains between 1 and 30% by weight of NiO and between 0% and 20% by weight of MoO ₃ .
Before injection of the charges to be treated, the catalyst may undergo a preliminary sulphurization step in order to convert the metal oxides to sulphides.
Sulfurization is carried out in the presence of H ₂ S, either directly injected in a mixture with hydrogen, or generated in situ in the reactor by hydrogenolysis of a sulfur compound, so that the metal sulphidation rate of the catalyst is greater than 50%, and preferably greater than 90%.
The temperature of the reactor is generally between 100 ° C. and 250 ° C., and preferably between 140 ° C. and 200 ° C.
The reactor is operated at a pressure of between 0.5 MPa and 5 MPa, and preferably between 1 MPa and 3 MPa, and at a VVH between 1 h ^-1 and 10 h ^-1, and preferably between 1, 5 h ^-1 and 8 h ^-1 .
The flow of hydrogen is generally adjusted to obtain a molar ratio between hydrogen and olefins of between 0.03 and 2 and preferably between 0.05 and 1.
Under these conditions, and unexpectedly, the saturated sulfur compounds present in the feed are converted into saturated sulfur compounds of higher boiling point.
The saturated compounds belong to the families consisting of mercaptans, sulphides and CS ₂ .
The conversion is measured by the conversion rate of mercaptans into heavier compounds, ie having a higher boiling point.
Moreover, the molar ratio between the olefins and hydrogen is optimized in order to limit the deactivation of the catalyst by the carbon deposition from the olefinic compounds.
In the absence of hydrogen, the catalyst undergoes a strong deactivation, and the conversion rate of the mercaptans is significantly reduced.

At the end of the reaction stage, the effluent is depleted in light saturated sulfur compounds and more particularly in mercaptans.
The degree of conversion of the mercaptans containing 1 to 4 carbon atoms is generally between 50% and 100%. The fraction thus produced is therefore softened in the sense well known to those skilled in the art.

A third step can be carried out optionally if one seeks to reduce the sulfur content of the feedstock to be treated.

This third step, known as separation, consists in separating the sulfur compounds formed during the reaction stage from the hydrocarbon effluent of said reaction stage.
The separation step may consist of any method capable of achieving this separation.
However, it will be preferred to use a physical separation method based on the boiling temperatures of the compounds to be separated, such as, for example, a simple flash, or distillation in a distillation column.
In this case, the light fraction recovered at the top of the column contains the majority of the saturated hydrocarbon fraction, and is depleted in sulfur compounds and in mercaptans.
The heavy fraction recovered at the bottom of the column concentrates the sulfur compounds formed during the reaction step. This fraction can be treated in a hydrodesulfurization unit to extract the sulfur. The implementation of the separation step subsequent to the reaction step, therefore allows to desulfurize the hydrocarbon feedstock to be treated without resorting to a conventional hydrodesulphurization process for at least a portion of the hydrocarbon effluent.

Examples

A series of tests was conducted in a pilot unit loaded with 100 cm ³ of catalyst.
The catalyst used contains nickel and molybdenum on an alumina support (catalyst marketed under the reference HR845 by the company Axens). Before the charge is injected, the catalyst is sulphurated with an H ₂ + H ₂ S mixture at 350 ° C. During the tests, the temperature, the pressure and the space velocity are kept constant at 180 ° C., 2.5 MPa and 4 h ^{-1 respectively} for all the tests carried out.

Example 1 (according to the prior art)

In this example, a gasoline A of atmospheric distillation of a crude oil is injected into a reactor in the absence of hydrogen.

At the outlet of the reactor, the effluent B1 is separated by distillation into two fractions with a cutting point corresponding to a temperature of 100 ° C. The two fractions obtained are denoted LCN1 (light fraction) and HCN1 (heavy fraction). The characteristics of the different species are grouped together in Table 1. <b> Table 1 </ b> Characteristics Essence A Effluent B1 LCN1 HCN1 T _5% - T _95% 25-125 25-125 25-100 100-125 Yield per fraction (%) 100 100 74.6 25.4 Density 15/4 0.702 0.705 0.68 0.763 Total sulfur (ppm) 300 300 135 783 Sulfur mercaptan (ppm) 160 129 135 111 Olefin / RSH (mol / mol) 2.4 - - - H ₂ / Olefin (mol / mol) 0 - - - Gas Chromatography (% wt) paraffins 67.4 67.4 74.6 47.4 olefins 0.1 0.1 0.1 0.0 naphthenes 26.2 26.2 23.4 34.0 aromatic 6.3 6.3 1.9 18.6

The presence of the catalyst makes it possible to reduce the mercaptan content by 20%. However, the mercaptan content of the LCN1 cut remains high, and the distribution of the mercaptans between the LCN1 cuts and the HCN1 shows that the observed conversion equally affects the light mercaptans and the heavy mercaptans, of at least five carbon atoms. . In addition it is observed that the organic sulfur content remains unchanged during the treatment on the catalyst.

The absence of hydrogen and a very low content of unsaturated compounds in the feed does not make it possible to obtain a substantially desulphurized light cut.

However, the sulfur of this fraction is found exclusively in the form of mercaptans, which has the consequence of concentrating the non-mercaptan sulfur in the heavy fraction.

Example 2 (according to the invention)

Gasoline A is mixed with an olefinic cracking gasoline C obtained from a catalytic cracking unit before being injected into the reactor in the presence of hydrogen at the rate of 5 liters of hydrogen per liter of feedstock. Gasoline C represents 10% by weight of the mixture noted as D1 gasoline . The gasoline D1 mixed with hydrogen is injected onto the catalyst.

At the outlet of the reactor, the effluent B2 is separated by distillation into two fractions with a cutting point corresponding to a temperature of 100 ° C. The two fractions obtained are denoted LCN2 (light fraction) and HCN2 (heavy fraction). Table 2 presents the characteristics of species A, C and D1. The characteristics of the different sections are summarized in Table 3. <b> Table 2 </ b> Characteristics Essence A Gasoline C Essence D1 T _50% - T _95% 25-125 25-215 25-215 Density 15/4 0.702 0.746 0.707 Total sulfur (ppm) 300 177 279 Sulfur mercaptan (ppm) 160 8 151 Olefins / RSH (mol / mol) 2.4 16430 91 Gas Chromatography (% wt) paraffins 67.4 28.9 63.5 olefins 0.1 34.5 3.6 naphthenes 26.2 7.6 24.4 aromatic 6.3 29 8.5 Characteristics Essence D1 B2 effluent LCN2 HCN2 T _5% - T _95% 25-215 25-215 25-100 100-215 Fraction / Gasoline D1 (% _w ) 100 100 71.5 28.5 Density 15/4 0.707 0.708 0.683 0.779 Total sulfur (ppm) 279 279 34 893 Sulfur mercaptan (ppm) 151 64.5 34 141 Olefins / RSH (mol / mol) 91 - - - H ₂ / Olefin (mol / mol) 0.71 - - - Gas Chromatography (% wt) paraffins 63.5 63.6 72.3 43 olefins 3.6 3.5 3.2 4.1 naphthenes 24.4 24.4 22.7 28.4 aromatic 8.5 8.5 1.9 24.6

The simultaneous presence of olefins and hydrogen in the D1 mixture reduces the initial mercaptan content by 57%. The conversion is three times greater than that observed in Example 1. In addition, the distribution of mercaptans between LCN2 sections and HCN2 is largely modified, the light mercaptans contained in the LCN2 section are preferentially converted.

Example 3 (according to the invention)

Gasoline A is mixed with gasoline C before being injected into the reactor in the presence of hydrogen at 5 liters of hydrogen per liter of feed. Gasoline C represents 20% by weight of the mixture noted as D2 gasoline .

At the outlet of the reactor, the effluent B3 is separated by distillation into two fractions with a cutting point corresponding to a temperature of 100 ° C. The two fractions obtained are denoted LCN3 (light fraction) and HCN3 (heavy fraction). Table 4 presents the characteristics of species A, C and D2. Those of the different sections are grouped together in Table 5. <b> Table 4 </ b> Feature Essence A Gasoline C Essence D2 T _5% - T _95% 25-125 25-215 25-215 Density 15/4 0.702 0.746 0.715 Total sulfur (ppm) 300 177 270 Sulfur mercaptan (ppm) 160 8 134 Olefins / RSH (mol / mol) 2.4 16430 196 Gas Chromatography (% wt) paraffins 67.4 28.9 59.8 olefins 0.1 34.5 6.9 naphthenes 26.2 7.6 22.5 aromatic 6.3 29 10.8 Characteristics Essence D2 Effluent B3 LCN3 HCN3 T _50% - T _95% 25-215 25-215 25-100 100-215 Fraction / Gasoline D2 (% _w ) 100 100 68.5 31.5 Density 15/4 0.715 0.717 0.685 0.788 Total sulfur (ppm) 270 270 32 789 Sulfur mercaptan (ppm) 134 71 32 156 Olefins / RSH (mol / mol) 196 - - - H ₂ / Olefin (mol / mol) 0.37 - - - Gas Chromatography (% wt) paraffins 59.8 59.9 69.7 39.4 olefins 6.9 6.8 6.6 7.2 naphthenes 22.5 22.5 21.9 23.8 aromatic 10.8 10.8 1.8 29.6

Increasing the amount of olefins injected does not significantly improve performance, the conversion of mercaptans stagnates around 50%. The H ₂ / Olefin ratio is decreased, which favors catalyst deactivation problems.

Example 4 (according to the invention)

Gasoline A is mixed with gasoline C before being injected into the reactor in the presence of hydrogen at a rate of 10 liters of hydrogen per liter of feedstock.
Gasoline C representing 10% of the mixture which is noted the essence D3.

At the outlet of the reactor, the effluent B4 is separated by distillation into two fractions with a cutting point corresponding to a temperature of 100 ° C. The two fractions obtained are denoted LCN4 (light fraction) and HCN4 (heavy fraction). Table 6 groups the characteristics of the different species. <b> Table 6 </ b> Characteristics Essence D3 Effluent B4 LCN4 HCN4 T _5% - T _95% 25-215 25-215 25-100 100-215 Fraction / Gasoline D1 (% _w ) 100 100 71.6 28.4 Density 15/4 0.707 0.704 0.682 0.78 Total sulfur (ppm) 279 279 32 901 Sulfur mercaptan (ppm) 151 63 32 141 Olefins / RSH (mol / mol) 91 - - - H ₂ / Olefin (mol / mol) 1.43 - - - Gas Chromatography (% wt) paraffins 63.5 63.8 72.4 43.4 olefins 3.6 3.2 3 3.8 naphthenes 24.4 24.4 22.7 28.4 aromatic 8.5 8.5 1.9 24.6

The presence of unsaturates and hydrogen are two factors favorable to the conversion of mercaptans. However, the H ₂ / olefins ratio must also be optimized to limit the deactivation of the catalyst, the hydrogen consumption and the saturation of the olefins.

Claims (13)

1. A method of converting mercaptans contained in a hydrocarbon feed containing less than 5 % by weight of olefins, comprising at least two stages:

   - a first stage of mixing the feed to be treated with an amount of hydrogen, and possibly an amount of olefinic cut, the resulting mixture being characterized by a H₂/olefin molar ratio ranging between 0.03 and 2, preferably between 0.05 and 1, and an olefin/mercaptan molar ratio ranging between 5 and 5000, preferably between 10 and 1000, and

   - a second stage of reaction of the mixture formed at the end of the first stage on a supported catalyst containing at least one group VIII metal, the method allowing the mercaptans to be converted to heavier sulfur compounds with a conversion rate of at least 50 %.
2. A mercaptan conversion method as claimed in claim 1, wherein said hydrocarbon feed contains less than 1 % by weight of olefins.
3. A mercaptan conversion method as claimed in any one of claims 1 or 2, wherein the reaction stage is carried out at a temperature ranging between 100°C and 250°C, preferably between 140°C and 200°C.
4. A mercaptan conversion method as claimed in any one of claims 1 or 2, wherein the reaction stage is carried out at a pressure ranging between 0.5 MPa and 5 MPa, preferably between 1 MPa and 3 MPa.
5. A mercaptan conversion method as claimed in any one of claims 1 or 2, wherein the reaction stage is carried out at a velocity ranging between 1 h^-1 and 10 h^-1 preferably between 1.5 h^-1 and 8 h^-1.
6. A mercaptan conversion method as claimed in any one of claims 1 to 5, wherein the feed to be treated is a straight-run gasoline.
7. A mercaptan conversion method as claimed in any one of claims 1 to 6, wherein olefins are provided by adding to the feed to be treated a certain amount of catalytically cracked gasoline.
8. A mercaptan conversion method as claimed in any one of claims 1 to 7, wherein the mercaptan-depleted effluent from the reaction stage is separated from the sulfur compounds formed during said reaction stage so as to produce an effluent containing less than 50 % sulfur compounds present in the feed, and a cut containing the major part of the sulfur compounds.
9. A mercaptan conversion method as claimed in claim 8, wherein the separation stage is selected from the group consisting of flash or distillation.
10. A mercaptan conversion method as claimed in any one of claims 1 to 7, wherein the catalyst used in the reaction stage also contains at least one group VIb metal.
11. A mercaptan conversion method as claimed in any one of claims 1 to 7, wherein the catalyst used in the reaction stage contains between 1 % and 30 % by weight of group VIII metal in oxide form, and between 0 % and 20 % by weight of group VIb metal in oxide form.
12. A mercaptan conversion method as claimed in any one of claims 1 to 7, wherein the catalyst used in the reaction stage contains between 1 % and 30 % by weight of NiO, and between 0 % and 20 % by weight of MoO₃.
13. A mercaptan conversion method as claimed in any one of claims 1 to 7, wherein the catalyst used in the reaction stage is first subjected to a sulfurization stage so that the proportion of metal present in sulfide form is above 50 %, preferably above 90 %.