WO2002072739A1

WO2002072739A1 - Method for producing low-sulphur petrol

Info

Publication number: WO2002072739A1
Application number: PCT/FR2002/000350
Authority: WO
Inventors: Quentin Debuisschert; Denis Uzio; Jean-Luc Nocca; Florent Picard
Original assignee: Institut Francais Du Pretrole
Priority date: 2001-03-12
Filing date: 2002-01-29
Publication date: 2002-09-19
Also published as: CA2440188C; FR2821851B1; BR0208047A; EP1370629B1; ATE502992T1; MXPA03008217A; KR20030080089A; BR0208047B1; JP2004527611A; KR100813777B1; EP1370629A1; DE60239524D1; BRPI0216116B1; CA2440188A1; FR2821851A1

Abstract

The invention relates to a method for producing low-sulphur petrol comprising at least one sulphur compound transformation step consisting of the alkylation or adsorption of the sulphur compounds and/or in increasing the weight of the light sulphur compounds; at least one step involving treatment in the presence of an acid catalyst; and at least one step consisting in desulphurising at least one part of the petrol. According to the invention, the method can also optionally comprise at least one step involving the selective hydrogenation of diolefins and, optionally, at least one fractionation of the petrol obtained into at least two fractions: light petrol and heavy petrol.

Description

PROCESS FOR PRODUCING LOW SULFUR GASOLINE

The invention relates to a process for the production of gasoline with a low sulfur content comprising at least one step of transformation of the sulfur compounds consisting in an alkylation or adsorption of the sulfur compounds and / or in a weighting of the light sulfur compounds, at least one step treatment in the presence of an acid catalyst and at least one desulfurization treatment of at least part of the heavy fraction. This process makes it possible to enhance a gasoline cut possibly comprising also hydrocarbons with two three or four carbon atoms, by reducing the total sulfur content of said cut to very low levels compatible with current or future specifications. This desulfurization is moreover carried out without appreciable reduction in the gasoline yield and while minimizing the reduction in the octane number.

Prior art:

The production of reformulated gasolines meeting the new environmental standards notably requires that their concentration of olefins be reduced slightly, but significantly their concentration of aromatics (especially benzene) and sulfur. The catalytic cracked gasolines, which can represent 30 to 50% of the gasoline pool, have high olefin and sulfur contents. The sulfur present in reformulated gasolines is attributable, to almost 90%, to catalytic cracking gasoline (FCC, "Fluid Catalytic Cracking" or catalytic cracking in a fluidized bed). Desulfurization (hydrodesulfurization) of gasolines and mainly FCC gasolines is therefore of obvious importance for the achievement of specifications.

The hydrotreatment (hydrodesulfurization) of the feed sent to catalytic cracking leads to gasolines typically containing 100 ppm of sulfur. The units for hydrotreating catalytic cracking feeds, however, operate under severe temperature and pressure conditions, which presupposes a significant consumption of hydrogen and high investment. In addition, the entire charge must be desulphurized, which entails the treatment of very large charge volumes.

Hydrotreating (or hydrodesulfurization) of catalytic cracking gasolines, when carried out under conventional conditions known to those skilled in the art, makes it possible to reduce the sulfur content of the cut. However, this process has the major drawback of causing a very significant drop in the octane number of the cut, due to the saturation of all of the olefins during the hydrotreatment.

On the other hand, US Pat. No. 4,131,537 teaches the advantage of dividing petrol into several cuts, preferably three, according to their boiling point, and of desulfurizing them under conditions which can be different and in the presence of a catalyst comprising at least one metal from group VIB and / or group VIII. It is stated in this patent that the greatest benefit is obtained when the gasoline is divided into three cuts, and when the cut having intermediate boiling points is treated under mild conditions.

Patent application EP-A-0 755 995 describes a process for desulfurization of FCC gasolines comprising at least two steps. The first is a catalytic hydrodesulfurization at a temperature between 200 and 350 ° C, with a desulfurization rate between 60 and 90% and is carried out in the presence of a feed containing less than 0.1% by volume of sulfide hydrogen (H2S). The second, and possibly the following ones, are also catalytic hydrodesulfurization stages carried out between 200 and 300 ° C. and in the presence of a feed comprising less than 0.05% by volume of H2S. The desulfurization rate is between 60 and 90% in this step. In this process, the H2S concentration must be kept at a very low level. In the event of recycling of the excess hydrogen, it is therefore generally necessary to eliminate IΗ2S, for example by means of an absorption step with amines, after the second and subsequent steps, so that the recycle gas contains less 0.1% volume of H2S. It is also preferred to eliminate IΗ2S between the first and the second stage in order to respect the maximum H2S content at the input of the second stage (0.05% volume). For gasoline loaded with sulfur, such elimination is therefore necessary given the desulfurization rate greater than 60% in the first stage

Patent application EP-A-0 725 126 describes a process for hydrodesulfurization of a cracked gasoline in which the gasoline is separated into a plurality of fractions comprising at least a first fraction rich in compounds easy to desulfurize and a second fraction rich in compounds difficult to desulfurize. Before carrying out this separation, it is first necessary to determine the distribution of the sulfur-containing compounds by means of analyzes. These analyzes are necessary to select the apparatus and the separation conditions.

In this application it is thus indicated that a light fraction of cracked gasoline sees its olefin content and its octane number drop significantly when it is desulfurized without being fractionated. On the other hand, the fractionation of said light fraction into 7 to 20 fractions followed by analyzes of the sulfur and olefin contents of these fractions makes it possible to determine the fraction (s) richest in sulfur compounds which are then desulfurized simultaneously or separately and mixed other desulfurized or non-desulfurized fractions. Such a procedure is complex and must be repeated each time the composition of the essence to be treated is changed.

In French patent 2,785,908, it is taught the advantage of fractionating the gasoline into a light fraction and a heavy fraction and then carrying out a specific hydrotreatment of the light gasoline on a catalyst based on nickel, and a hydrotreatment heavy petrol on a catalyst comprising at least one metal from group VIII and / or at least one metal from group Vlb.

It has also been proposed, for example in US Pat. No. 5,290,427, methods for hydrotreating gasolines which consists in fractionating gasoline, then in introducing the fractions at different levels of a hydrodesulfurization reactor and in converting the desulfurized fractions on a ZSM-5 zeolite, in order to compensate for the loss of octane recorded by means of isomerization. This isomerization is accompanied by a cracking of the gasoline towards lighter compounds. In these processes, the gasolines to be treated generally have an initial point greater than 70 ° C., and it is necessary to treat the light gasoline separately (fraction corresponding to the compounds with a boiling point comprised between the C5 hydrocarbons with 5 atoms of carbon and 70 ° C) for example by means of a softening.

US-A-5 318 690 proposes a process comprising a fractionation of the gasoline and a softening of the light gasoline, while the heavy gasoline is desulphurized, then converted on a zeolite ZSM-5 and desulphurized again in mild conditions. This technique is based on a separation of the crude gasoline so as to obtain a light cut preferably practically devoid of sulfur compounds other than mercaptans. This makes it possible to treat said cut only by means of a softening which removes the mercaptans.

As a result, the olefins, present in a relatively large amount in the heavy cut, are partially saturated during the hydrotreatment. To compensate for the fall in the octane number linked to the hydrogenation of olefins, the patent recommends cracking on zeolite ZSM-5 which produces olefins, but to the detriment of the yield. In addition, these olefins can recombine with the H ₂ S present in the medium to reform mercaptans. It is then necessary to carry out additional softening or hydrodesulfurization.

Patent application WO 00/15319 describes a method making it possible to simultaneously carry out the fractionation and the treatment of a light naphtha. In this process, the light cut contains mercaptans generally ranging from methyl-mercaptan to hexyl-mercaptan. These sulfur compounds are eliminated from the light fraction only in the case where the fractionation column contains a hydrodesulfurization section at the top of the column. In the absence of this section, it is therefore not possible to eliminate the mercaptans which are either found in the desulfurized gasoline when the light fraction is recombined with the heavy desulfurized fraction, or can be eliminated with the whole of the light fraction, which results in a loss of gasoline yield after desulphurization.

US Pat. No. 6,083,379 describes a process for the desulphurization and improvement of the octane number of gasolines comprising fractionation of the gasoline into at at least two cuts, the treatment of the light fraction in the presence of a zeolite, a fractionation of the light fraction thus treated, the mixing of the heavy fractions obtained during the two stages of fractionation and the hydrodesulfurization of the mixture of these fractions.

Patent application WO 94/22980 describes a gasoline desulfurization process comprising a fractionation into two sections, the heaviest section is desulfurized in a hydrodesulfurization reactor and then treated in the presence of an acid catalyst which makes it possible to compensate for the loss of octane. The lightest cut is also desulphurized by means of a non-hydrogenating extraction of the mercaptans.

US Pat. No. 5,968,346 describes a process for hydroconversion of a hydrocarbon charge, making it possible to remove impurities such as compounds comprising heteroatoms. This process comprises a first step of hydroconversion of the entire feed, followed by a separation of the liquid and the vapor present in the effluent from this first step and of contacting the gas with a liquid. The mixture of the two liquid fractions resulting from the contacting and the fractionation is then treated in a second hydroconversion stage in the presence of a catalyst.

Summary of the invention:

The present invention relates to a process for producing essences with a low sulfur content, which makes it possible to recover the whole of a petrol cut containing sulfur, preferably a petrol cut of catalytic cracking or coking (coking according to English terminology). , or pyrolysis, or visbreaking (visbreaking according to English terminology), and reduce the sulfur contents in said gasoline cut to very low levels, without appreciable reduction in gasoline efficiency while minimizing the decrease in the index octane due to the hydrogenation of olefins. The feedstock of the process according to the invention may also optionally comprise, in addition to a gasoline cut, a C4 ^" cut comprising hydrocarbons with two, three or four carbon atoms. The process according to the invention is a process for producing gasoline with a low sulfur content, from a gasoline cut containing sulfur (initial gasoline). It includes at least the following steps:

A) at least one stage of transformation of the sulfur compounds present in the gasoline. According to a variant, this transformation is a chemical conversion aimed at weighing down, that is to say increasing the molecular weight of said sulfur-containing compounds, preferably essentially sulfur-containing compounds having a boiling point lower than that of thiophene. It essentially consists of the addition of mercaptans to the olefins. This step can possibly be carried out simultaneously with step D of selective hydrogenation and on all or part of the initial gasoline, in the same reactor or a different reactor. It can also be carried out separately on all or part of the essence. It can also be carried out simultaneously with the fractionation step described below when it is present. In this step, the thiophene and the thiophene compounds are little or not transformed. According to another variant of the process according to the invention, the step of transformation of the sulfur compounds is a step of alkylation or of adsorption of the sulfur compounds chosen from the group consisting of thiophene, thiophene compounds and mercaptans, preferably mercaptans having 1 to 6 carbon atoms. It is also possible to combine said variants, that is to say to carry out both an increase in light mercaptans and an alkylation or adsorption of thiophene, thiophene compounds and mercaptans. This step A is arranged before or after the step or steps B and / or D and / or E described below when they are present. It can also be carried out simultaneously, that is to say for example in a reactive column.

B) at least one treatment step in the presence of an acid catalyst located before or after steps A, B, C, D and / or E, or simultaneously with step C. The treatment in the presence of an acid catalyst makes it possible to obtain an effluent having a higher octane number than the charge of this stage. The reactions that occur during this step are essentially cracking and / or isomerization reactions which lead to branched olefins or paraffins contributing to an increase in the octane number. Any type of acid catalyst can optionally be used, but a catalyst comprising an alumino-silicate and more preferably a zeolite will preferably be used.

C) at least one desulfurization treatment of at least part of the gasoline treated in step A and / or B, and optionally fractionated in step E or hydrogenated in step D in the presence of at least a hydrodesulfurization catalyst or an absorbent or an agent allowing an extractive softening to be carried out.

edé according to the invention can also further comprise:

D) optionally at least one selective hydrogenation of the diolefins located before or after steps A, B, or E.

E) optionally at least one fractionation of the gasoline obtained in step A or B into at least two fractions (or cuts), a light fraction preferably practically devoid of sulfur and containing the lightest olefins of the initial gasoline (light petrol or light fraction), and a heavy fraction in which preferably the major part of the sulfur compounds initially present in the initial petrol is concentrated (petrol or heavy fraction). It is also possible to separate the gasoline obtained in step A or B or directly the untreated feed into more than two fractions, that is to say for example a light fraction, at least an intermediate fraction and a fraction heavy.

F) optionally a step of mixing the light fraction obtained after one of steps A, D or E and optionally at least one intermediate fraction from one of steps A, B, C or E with the heavy fraction desulfurized from one of steps A, B, C, or E. In this step, preferably, all of said heavy desulfurized fraction is mixed with said light fraction, without separation of the liquid and the gas contained in the heavy petrol after desulfurization, optionally a simple stripping with an inert gas can be carried out to remove I éliminer2S from the heavy petrol totally desulfurized. In certain specific cases, the recovery of light petrol, desulfurized heavy petrol, and possibly at least one intermediate petrol, is carried out separately. It is then unnecessary to carry out this step F.

The charge of the process according to the invention is a gasoline cutter containing sulfur, preferably a gasoline cutter produced by a catalytic cracking, coking or visbreaking or pyrolysis unit, the range of boiling points of which extends typically from about the boiling points of hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up to about 250 ° C, preferably from about the boiling points of hydrocarbons with 5 carbon atoms (C2 or C3 ) up to approximately 220 ° C. The end point of the gasoline cut depends on the refinery from which it comes and market constraints, but generally remains within the limits indicated above.

Detailed description of the invention: A process is described in the present invention which makes it possible to obtain a gasoline preferably obtained from a catalytic cracking, coking or visbreaking or pyrolysis unit and having a limited sulfur content. Said method comprises at least one step of transformation of the sulfur compounds consisting of an alkylation or adsorption of the sulfur compounds and / or a weighting of the light sulfur compounds, at least one step of treatment in the presence of an acid catalyst and at least one treatment desulphurization of at least part of the gasoline. The process according to the invention can also optionally comprise at least one step of selective hydrogenation of the diolefins and optionally at least one fractionation of the gasoline obtained into at least two fractions: light gasoline and heavy gasoline.

The simultaneous presence of stages A of transformation of the sulfur-containing compounds and B of acid treatment makes it possible to obtain, in stage C, very thorough desulfurization of petrol without excessive reduction in petrol yield and while maintaining a high octane number.

Thanks to this process, significant desulfurization rates are reached, under reasonable operating conditions specified below.

The possible fractionation point of the gasoline is preferably limited in order to limit the presence of sulfur-containing compounds, in particular thiophene compounds in light gasoline, in particular in the case where the latter are not transformed by alkylation in step A In particular, the thiophene which forms azeotropes with a certain number of hydrocarbons cannot be easily separated from the C5 and / or C6 olefins.

In the case where a fractionation is present, to allow recovery of a light fraction comprising a significant proportion of the light olefins present in the gasoline while limiting the sulfur content of this fraction, it is preferred to treat the feed under conditions and on catalysts which make it possible to transform the sulfur-containing compounds (step A). Thus, the light sulfur and / or thiophenic compounds can be transformed into sulfur compounds with higher boiling points which may be found, after the optional separation, in at least one intermediate fraction or in heavy gasoline. These intermediate and or heavy cuts can then be easily desulfurized. It is also possible to transform the sulfur-containing compounds by means of stage A of the process according to the invention arranged after fractionation. In this case, a more preferred variant consists in placing an additional fractionation section after step A and in sending at least one cut, preferably the heavy cut, resulting from this fractionation to the acid treatment step (step B). and / or desulphurization (step C).

The hydrotreatment of gasolines and in particular hydrodesulfurization is carried out in the presence of hydrogen under conditions such that at least some of the olefins present in the feed to be hydrotreated are hydrogenated. This leads to a more or less significant drop in the octane number of the petrol cut. The method according to the invention makes it possible, by means of step A, to limit the content of sulfur-containing compounds in the light fraction of the feed. It therefore preferably allows, that is to say when a fractionation is present, to avoid hydrodesulfurization of this fraction or to carry out desulfurization under mild conditions, for example a softening. However, it is sometimes preferred not to fractionate the gasoline before desulphurization, or even to treat only certain cuts resulting from a fractionation in stage A. In addition, there remain olefins in the intermediate and heavy fractions of l gasoline which can be hydrogenated during the desulfurization stage. For all these reasons, when a very high octane number is sought, it is advantageous to compensate for the decrease in the octane number recorded during the desulfurization by means of an acid treatment step in the presence of a acid catalyst, such as for example a zeolite.

The sulfur content of gasoline cuts produced by catalytic cracking (FCC) depends on the sulfur content of the feed treated with the FCC, on the presence or not of a pretreatment of the feed of the FCC, as well as on the end point of the cut. . Generally, the sulfur contents of an entire gasoline cut, in particular those originating from the FCC, are greater than 100 ppm by weight and most of the time greater than 500 ppm by weight. For gasolines having end points greater than 200 ° C., the sulfur contents are often greater than 1000 ppm by weight, they can even in certain cases reach values of the order of 4000 to 5000 ppm by weight.

The process according to the invention applies in particular when high rates of desulphurization of the petrol are required, that is to say when the desulphurized petrol must contain at most 10% of the sulfur of the initial petrol and possibly at most 5% or even at most 2% of the sulfur of the initial gasoline which corresponds to desulfurization rates higher than 90% or even higher than 95 or even higher than 98%.

It applies more particularly when these desulfurization rates must be obtained while limiting the reduction in the research and motor octane indices. Thus, the research and motor octane numbers (octane number according to English terminology and hereinafter called ON) of the gasoline obtained after treatment by means of the process according to the invention have a value generally close to or greater than that of l untreated gasoline. Preferably, the gasoline resulting from the process according to the invention has a research or motor octane number such as ON> (ON) ₀ -1, more preferably ON> (ON) _o -0.7, very preferably ON> (ON) _o -0.5, and - even more preferably ON> (ON ) ₀ , (ON) ₀ representing the value of the research or motor octane number of the load (initial or untreated petrol). The method according to the invention comprises at least the following steps:

A) This step consists of at least one step of transformation of the sulfur compounds present in the gasoline. According to a variant, this transformation is a chemical conversion aimed at weighing down, that is to say increasing the molecular weight of said sulfur compounds, preferably essentially sulfur compounds having a boiling point lower than that of thiophene. This step can possibly be carried out simultaneously with step D of selective hydrogenation and on all or part of the initial gasoline, in the same reactor or a different reactor. It can also be carried out separately on all or part of the essence. It can optionally be carried out simultaneously with the fractionation step described below when it is present. In this step, the thiophene and the thiophene compounds are little or not transformed. According to another variant of the process according to the invention, the stage of transformation of the sulfur compounds is a stage of alkylation or of adsorption of the sulfur compounds chosen from the group consisting of thiophene, thiophene compounds and mercaptans, preferably mercaptans having 1 to 6 carbon atoms. It is also possible to combine said variants, that is to say to carry out both an increase in light mercaptans and an alkylation or adsorption of thiophene, thiophene compounds and mercaptans. Said variants can then be carried out in the same reactor or in two different reactors arranged one after the other, and between which is possibly interposed one or more other reaction sections (according to steps B, C or D) or fractionation. This step A can also be arranged before or after steps B, D, and / or optionally E described below, but is preferably located before step C of desulfurization. B) at least one step of treatment in the presence of an acid catalyst located before or after steps A, C, D and / or E or carried out simultaneously with step C. The treatment in the presence of an acid catalyst allows '' obtain an effluent having a higher octane number than the charge of this stage. The reactions which take place during this stage are essentially cracking and / or isomerization reactions which lead to branched olefins or paraffins contributing to an increase in the octane number. Any type of acid catalyst can optionally be used, but a catalyst comprising an alumino-silicate and more preferably a zeolite will preferably be used.

edé according to the invention can also further comprise:

D) optionally at least one selective hydrogenation of the diolefins located before or after steps A, B, C or E.

E) optionally at least one fractionation of the initial gasoline, or of the gasoline obtained in step A or B or D, into at least two fractions (or cuts), a light fraction preferably practically free of sulfur and containing the lighter olefins of the initial gasoline (light gasoline or light fraction), and a heavy fraction in which preferably the major part of the sulfur compounds initially present in the initial gasoline is concentrated (gasoline or heavy fraction). It is also possible to separate the gasoline obtained in step A or B or directly the untreated filler in more than two fractions, that is to say for example a light fraction, at least an intermediate fraction and a fraction heavy. F) optionally a step of mixing the light fraction obtained after one of steps A, B, C, D or E (steps A to E) preferably one of steps A, B, C or E, and more preferably from step A or C, or after an additional fractionation and optionally at least one intermediate fraction from one of steps A to E, preferably from step A or from step C or from an additional fractionation, with the desulfurized heavy fraction resulting from one of steps A to E, preferably from step A or from step C. In this step, preferably, the all of said heavy desulphurized fraction is mixed with said light fraction, without separation of the liquid and of the gas contained in the heavy petrol after desulphurization, optionally a simple stripping with an inert gas can be carried out to remove IΗ2S from the heavy petrol completely desulphurized . In certain specific cases, the recovery of light gasoline, heavy desulfurized gasoline, and possibly at least one intermediate gasoline, is carried out separately. It is then unnecessary to carry out this step F.

According to a variant of the process according to the invention, it is possible to associate at least one reaction section with a fractionation column according to step E. Said or said reaction sections then operate on at least one fraction sampled inside the fractionation column and the effluent from the reaction section is returned to said column. The reaction section or sections thus coupled to the fractionation column of step E can be chosen from the group constituted by the reaction sections of the following steps: transformation of the sulfur-containing compounds (step A), acid treatment (step B), desulfurization ( step C), hydrogenation of the diolefins (step D) possibly carried out simultaneously with step A of transformation of the sulfur-containing compounds.

Such devices comprising a fractionation column associated with an external reactor and which can be used in the process according to the invention have for example been described for applications in the field of refining and petrochemistry in US Patents 5,1777,283, US 5,817,227 and US 5,888,355 According to other variants of the process according to the invention, it is also possible to use a reactive column in place of the fractionation column, that is to say to place at least one of said reaction sections in the column of fractionation (reaction section internal to the column), preferably in an area where the reagent concentration is maximum. Thus, for example, in the case of stage A of transformation of light sulfur and / or thiophenic compounds, the reaction section will preferably be placed in an area having the maximum concentration of these compounds.

According to a preferred variant of the process according to the invention, the reaction section internal to the column is chosen from the group consisting of the following reaction sections: transformation of the sulfur-containing compounds (step A), acid treatment (step B), desulfurization (step C ), hydrogenation of the diolefins (step D) possibly carried out simultaneously with step A of transformation of the sulfur-containing compounds.

Such reactive columns are known to those skilled in the art and have for example been described in patents or patent applications US 5,368,691, US 5,523,062, FR 2,737 131, FR 2,737,132, EP-A-0461 855.

Another variant of the process according to the invention consists both in using a reactive column comprising at least one reaction section and an external reactor coupled or not to said column. Such variants are for example described in patent application WO 00/15319.

The variants described above are only illustrations of the possible variants of the method according to the invention. The method according to the invention can indeed be implemented by combining reaction sections either associated with the fractionation column, or internal to said column, or external and uncoupled to said column in the sense that the effluent from said reaction section (s) is not recycled to the fractionation column.

In the process according to the invention, when the fractionation according to step E is present, most of the olefins are maintained in the light fraction, possibly in at least one intermediate fraction which does not require excessive desulphurization. The sulfur compound content of the light fraction obtained after fractionation is generally less than 100 ppm, preferably less than 30 ppm, more preferably less than 20 ppm and very preferably less than 10 ppm. When this is not the case, the light fraction is preferably sent to at least one additional section for processing the sulfur compounds according to step A, so as to make its sulfur compound content less than 50 ppm.

Another advantage lies in the fact that the residual content of sulfur-containing compounds in the desulfurized petrol by means of the process according to the invention is particularly low, and that the octane number of the petrol is maintained at a high level.

The steps of the method according to the invention are described in more detail below.

- transformation of sulfur compounds (step A):

According to a first variant, this step consists in transforming the light sulfur compounds. That is to say the compounds which, after a possible separation, would be found in light petrol, in heavier sulfur compounds entrained in heavy petrol. Preferably, the transformed light compounds have a lower boiling point than that of thiophene. This first variant essentially consists of an addition of the mercaptans to the olefins to form heavier sulfur compounds and is preferably carried out on a catalyst comprising at least one element from group VIII (groups 8, 9 and 10 of the new periodic classification), or comprising a resin. The choice of catalyst is carried out in particular so as to promote the reaction between the light mercaptans and the olefins, which leads to heavier mercaptans.

This step can optionally be carried out at the same time as the hydrogenation step D, when the latter is present. For example, it may be particularly advantageous to operate, during the hydrogenation of the diolefins, under conditions such that at least a portion of the compounds in the form of mercaptan are transformed. Thus a certain reduction in the mercaptan content is obtained. To do this, we can use the hydrogenation procedure dienes described in patent application EP-A-0 832 958, which advantageously uses a palladium-based catalyst, or that described in patent FR 2 720 754.

Another possibility is to use a catalyst based on nickel identical or different from the catalyst of stage D, such as for example the catalyst recommended in the process of patent US-A-3,691,066, which makes it possible to transform the mercaptans (butylmercaptan) into heavier sulfur compounds (sulfides). Another possibility for carrying out this step consists in hydrogenating at least part of the thiophene to thiophane, the boiling point of which is higher than that of thiophene (boiling point 121 ° C). This step can be carried out on a catalyst based on nickel, platinum or palladium. In this case the temperatures are generally between 100 and 300 ° C and preferably between 150 and 250 ° C. The H2 / feed ratio is adjusted between 1 and 20 liters per liter, preferably between 3 and 15 liters per liter, to further promote, if possible, the desired hydrogenation of the thiophene compounds and minimize the hydrogenation of the olefins present in the feed. The space velocity is generally between 1 and 10 h ^"1 , preferably between 2 and 4 h ^{" 1} and the pressure between 0.5 and 5 MPa, preferably between 1 and 3 MPa.

According to a second variant, this step consists in passing the feed, optionally hydrogenated in step D or possibly a cut resulting from the fractionation in step E, preferably the light fraction and / or possibly at least one intermediate fraction originating from the fractionation (step E) on an absorbent or on a catalyst having an acid function which essentially makes it possible to carry out the alkylation reaction of thiophene and thiophene derivatives with olefins the addition of sulfur-containing compounds in the form of mercaptans to the olefins. The operating conditions are adjusted to carry out the desired transformation with conversion or adsorption rates of thiophene and / or thiophenics and / or light mercaptans, preferably mercaptans having from 1 to 6 carbon atoms, greater than 80% by weight, preferably greater than 90% by weight, very preferably greater than 95% by weight. Other compounds such as COS or CS2 can optionally also be adsorbed or converted. To minimize the oligomerizing activity of the acid catalyst optionally used, the essence can be added with a compound known to inhibit the oligomerizing activity of acid catalysts such as alcohols, ethers or water. This transformation can for example be carried out according to the procedures and using the diagrams described in French patent applications No. 00/08113, and No. 00/10233.

According to this variant of step A, the optionally hydrogenated gasoline or preferably the light cut and / or the intermediate cut possibly obtained in step E is treated in a section making it possible to transform, preferably by alkylation or adsorption, the compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans. In the case of an alkylation, the thiophene compounds contained in the 60 ° C-160 ° C cut will react with conversion rates greater than 80% by weight, preferably greater than 90% by weight, with the olefins to form alkyls thiophenes according to the following reaction for thiophene:

Some or all of the benzene can also be removed by alkylation with olefins. These higher molecular weight compounds are mostly characterized by higher boiling temperatures than they had before alkylation. Thus the theoretical boiling temperature which is 80 ° C is shifted to 250 ° C for thiophene alkyls.

This alkylation step is carried out in the presence of an acid catalyst. This catalyst can be either a resin, a zeolite, a clay, any functionalized silica or any silico-aluminate having an acidity, or any support grafted with acid functional groups. The ratio of the charge volume injected over the catalyst volume is between 0.1 and 10 liters / liter / hour and preferably between 0.5 and 4 liters / liter / hour. More specifically, this alkylation step is carried out in the presence of at least one acid catalyst chosen from the group consisting of silicoaluminates, titanosilicates, mixed alumina titanium, clays, resins, mixed oxides obtained by grafting of at least one organosoluble organosoluble or water-soluble compound (chosen from the group consisting of alkys. and / or alkoxy. metals of at least one element such as titanium, silicon zirconium, germanium, tin, tantalum, niobium ...) on at minus an oxide such as alumina (gamma, delta, eta forms, alone or as a mixture) silica, silica-aluminas, silica-titanium, silica-zirconia or any other solid having any acidity. A particular embodiment of the invention may consist in using a physical mixture of at least two of the above catalysts in proportions varying from 95/5 to 5/95, preferably from 85/15 to 15/85 and very preferably from 70/30 to 30/70.

The temperature for this stage is generally between 10 and 350 ° C. depending on the type of catalyst or the strength of the acidity. Thus for an acidic resin of ion exchange type, the temperature is generally between 50 and 150 ° C. preferably between 50 and 120 ° C.

The molar ratio of olefin to thiophene compounds is between 0.1 and 1000 mole / mole, preferably between 0.5 and 500 mole / mole.

The operating pressure of this step is generally between 0.1 and 3 MPa and preferably such that the charge is in liquid form under the temperature and pressure conditions, ie at a pressure greater than 0.5 MPa.

The effluent from step A, for the transformation of sulfur-containing compounds, preferably from alkylation or adsorption, can optionally be mixed at least in part with a heavy cut resulting from the fractionation in step E.

The effluent from this step of transforming sulfur compounds can also optionally be sent to a new fractionation unit to be separated into two fractions, an untreated intermediate fraction or optionally desulphurized without being mixed, and a heavy fraction which is preferably mixed with the heavy fraction resulting from stage E before being desulphurized in stage C.

For example the effluent D1 resulting from an alkylation can be separated into: - a cut 60 ° C-180 ° C or 60 ° C-100 ° C (cut D2) devoid of any thiophenic compound which is collected, - a section with boiling temperatures above 180 ° C or 100 ° C (section D3) containing thiophene compounds.

When it is an alkylation, step A can optionally be advantageously carried out on the light fraction resulting from step E. In fact, the alkylation of the light mercaptans present in this fraction then facilitates the desired elimination sulfur compounds, but also reduces the vapor pressure (RVP index or Reid Vapor Pressure according to English terminology) of the final desulfurized gasoline.

All of the effluent D1 from said alkylation unit or the cut D3 from fractionation after alkylation can preferably be mixed at least in part with a heavy cut (for example the cut H2) and sent to a desulphurization section according to step C.

- treatment in the presence of an acid catalyst (step B):

This step compensates for the loss of octane due to the step of desulfurization of the total gasoline or of a gasoline cut (step C). It is placed either before said desulfurization step C, or after said step. It can optionally be implemented on all of the gasoline before the fractionation step and before or after the step of transformation of the sulfur compounds (step A). Preferably, it is located after fractionation (step E) when it is present, and more preferably just before or just after the desulfurization step (step C). It is finally possible to carry out step B simultaneously with step C, either by placing an acid catalyst and a hydrodesulfurization catalyst in the same reactor, or by using a bifunctional catalyst comprising an active phase in desulfurization as described in step C, dispersed on a support comprising an acid solid such as for example a zeolite. When the desulphurization step is carried out in two separate reactors, it is sometimes preferred to have step B of acid treatment between the two reactors of step C. Indeed, the second desulphurization reactor makes it possible to remove the mercaptans optionally formed in step B by recombination of the olefins formed in this step with sulfur compounds. This second desulfurization step can then be a catalytic hydrodesulfurization or a softening, for example an extractive softening.

When the acid treatment step is placed after the desulfurization step, the acid catalyst chosen for the acid treatment step has a controlled acidity so as to limit the degree of cracking of the hydrocarbons. Therefore, the cracking reaction is essentially limited to compounds with a low octane number.

Preferably, this acid treatment stage achieves a limited cracking of the paraffins of low octane number to form products of higher octane number. The reactions involved are on the one hand the cracking of parrafines having the highest boiling points into paraffins of lower boiling point, and the cracking of paraffins of low octane number. Olefins are also formed during these reactions, as well as branched parrafins via isomerization reactions. All of these reactions therefore make it possible to compensate for the reduction in the octane number due to the desulfurization step, or even in certain cases to increase this index beyond the index of the cut or of the gasoline. before desulfurization.

In order to limit the deactivation of the catalyst by deposition of hydrocarbons, hydrogen is generally injected into the acid treatment reactor. It is also possible to admit excess hydrogen from a previous step (for example a desulphurization step) at the entrance to this step, possibly with additional hydrogen provided by a separate supply.

The operating conditions for this step are generally as follows: the temperature is generally between 140 ° C and 500 ° C, preferably between 170 ° C and 480 ° C; the pressure is generally between 0.5 and 15 MPa, preferably between 1 and 10 MPa, more preferably between 2 and 8 MPa; the space velocity expressed in volume of feed per volume of catalyst and per hour is generally between 0.3 and 15 h -1, preferably between 0.5 and 10 h -1; the volume ratio of hydrogen to hydrocarbon charge is generally between 0 and 1000 liters per liter, preferably between 10 and 800 liters per liter, more preferably between 20 and 500 liters per liter.

Among the preferred catalysts for this stage, mention may be made of silicates, aluminosilicate, borosillicates, gallosilicates, and generally metallosilicates. Among the aluminosilicates, those belonging to the family of ZSM (for example ZSM-5, ZSM-12, ZSM 22) or MCM (for example MCM-22) are preferred, as well as the zeolite Y, the mordenite, the zeolite beta or the faujasites. In order to regulate the acidity of these materials, they can optionally be at least partially dealuminated, or synthesized in a fluoride medium.

The catalysts of this stage can also comprise an active metal in hydrogenation, so as to facilitate the elimination of the compounds likely to poison the catalyst and notably its acid function. Preferably, a metal from group VIII of the old periodic classification (groups 8, 9, 10 of the new classification) is used, for example a noble metal or nickel.

- desulphurization (step C):

This step can for example be a hydrodesulfurization step carried out by passing heavy or intermediate gasoline, in the presence of hydrogen, over at least one catalyst comprising at least one element from group VIII and / or at least one element from group Vlb at least partly in sulphide form, at a temperature between about 210 ° C and about 350 ° C, preferably between 220 ^C C and 320 ° C, under a pressure generally between about 1 and about 4 MPa, preferably between 1.5 and 3 MPa. The space velocity of the liquid is between approximately 1 and approximately 20 h ^-1 (expressed in volume of liquid per volume of catalyst and per hour), preferably between 1 and 10 h -1, very preferably between 3 and 8 h The H ₂ / HC ratio is between 100 to 600 liters per liter and preferably between 300 and 600 liters per liter. The content of group VIII metal expressed as oxide is generally between 0.5 and 15% by weight, preferably between 1 and 10% by weight. The metal content of group Vlb is generally between 1.5 and 60% by weight, preferably between 3 and 50% by weight.

The element of group VIII, when it is present, is preferably cobalt, and the element of group Vlb, when it is present, is generally molybdenum or tungsten. Combinations such as cobalt-molybdenum are preferred. The catalyst support is usually a porous solid, such as for example an alumina, a silica-alumina or other porous solids, such as for example magnesia, silica or titanium oxide, alone or in combination. mixture with alumina or silica-alumina. The catalyst according to the invention preferably has a specific surface of less than 190 m2 / g, more preferably less than 180 m2 / g, and very preferably less than 150 m2 / g.

After introduction of the element (s) and possibly shaping of the catalyst (when this step is carried out on a mixture already containing the basic elements), the catalyst is in a first activated stage. This activation can correspond either to an oxidation then to a reduction, or to a direct reduction, or to a calcination only. The calcination step is generally carried out at temperatures ranging from approximately 100 to approximately 600 ° C. and preferably between 200 and 450 ° C., under an air flow. The reduction step is carried out under conditions which make it possible to convert at least part of the oxidized forms of the base metal to metal. Generally, it consists in treating the catalyst under a stream of hydrogen at a temperature preferably at least equal to 300 ° C. The reduction can also be carried out in part by means of chemical reducers.

The catalyst is preferably used at least in part in its sulfurized form. The introduction of sulfur can occur before or after any activation step, that is to say calcination or reduction. Preferably, no oxidation step of the catalyst is carried out when the sulfur or a sulfur-containing compound has been introduced onto the catalyst. The sulfur or a sulfur-containing compound can be introduced ex situ, that is to say outside the reactor where the process according to the invention is carried out, or in situ, that is to say same reactor or in two different reactors arranged in series. Preferably said catalysts are sulfurized.

One of the preferred modes of desulfurization of at least one intermediate or heavy cut or of the total gasoline consists, for example, of operating in two stages: a first stage essentially carrying out the hydrogenation of the unsaturated sulfur compounds and a second stage carrying out a decomposition of the saturated sulfur compounds, according to one of the procedures taught in patent application EP-A-1 031 622.

It is also possible, preferably for cuts that are less rich in sulfur compounds, in particular the light cut obtained after the optional fractionation step, to carry out a simple softening by means of any softening process known to those skilled in the art. It is thus possible to use a softening process by air oxidation, optionally in the presence of copper, or alternatively by an extractive process by liquid / liquid contact, for example with a sodium hydroxide solution. Other processes for the catalytic conversion of mercaptans to disulfide can also be used, in combination if necessary with additional fractionation after softening.

- hydrogenation of diolefins (optional step D):

The hydrogenation of dienes is an optional step which makes it possible to eliminate, before hydrodesulfurization, almost all of the dienes present in the gasoline fraction containing sulfur to be treated. It preferably takes place in the first step of the process according to the invention, or possibly on at least one of the sections obtained just after fractionation in step E. It is generally carried out in the presence of a catalyst comprising at least one metal of the group VIII, preferably chosen from the group consisting of platinum, palladium and nickel, and a support. A catalyst based on nickel or on palladium deposited on an inert support, for example such as alumina, silica or a support containing at least 50% alumina, will be used.

The pressure used is sufficient to maintain more than 60%, preferably 80%, and more preferably 95% by weight of the gasoline to be treated in the liquid phase in the reactor; it is most generally between approximately 0.4 and approximately 5 MPa and preferably greater than 1 MPa, more preferably between 1 and 4 MPa. The hourly space velocity of the liquid to be treated is between approximately 1 and approximately 20 h ~ 1 (volume of charge per volume of catalyst and per hour), preferably between 2 and 10 h ^~ 1, very preferably between 3 and 8 h "^''. The temperature is most generally between about 50 and about 250 ° C, and preferably between 80 and 220 ° C, and more preferably between 100 and 200 ° C, to ensure sufficient conversion of the diolefins Very preferably it is limited to 180 ° C. The hydrogen to charge ratio expressed in liters is generally between 1 and 50 liters per per liter, preferably between 3 and 30 liters, more preferably between 8 and 25 liters The choice of operating conditions is particularly important, and will generally be carried out under pressure and in the presence of a quantity of hydrogen in slight excess relative to the stoichiometric value necessary for hydro generate the diolefins The hydrogen and the feedstock to be treated are injected in ascending or descending currents into a reactor preferably comprising a fixed bed of catalyst. Another metal can be combined with the main metal to form a bimetallic catalyst, such as for example molybdenum or tungsten. The use of such catalytic formulas has for example been claimed in patent FR 2 764 299. The catalytic cracking gasoline can contain up to a few% by weight of diolefins. After hydrogenation, the content of diolefins is generally reduced to less than 3000 ppm, even less than 2500 ppm and more preferably less than 1500 ppm. In some cases it can be obtained less than 500 ppm. The content of dienes after selective hydrogenation can even be reduced to less than 250 ppm if necessary. According to a particular embodiment of the process according to the invention, the hydrogenation step of the dienes takes place in a catalytic hydrogenation reactor which comprises a catalytic reaction zone traversed by the entire charge and the quantity of hydrogen necessary to carry out desired reactions.

- separation of the gasoline into at least two fractions (optional step E): According to a first variant of the method according to the invention, the gasoline is optionally divided into two fractions:

a light fraction containing a limited residual sulfur content, preferably less than approximately 100 ppm, more preferably less than approximately 50 ppm, very preferably less than approximately 20 ppm, and allowing this cut to be used without performing any other treatment aimed at reducing its sulfur content, except possibly a transformation of the sulfur compounds according to step A, or a simple softening. - A heavy fraction in which preferably most of the sulfur initially present in the feed is concentrated.

This separation is preferably carried out by means of a conventional distillation column also called a splitter. This fractionation column must make it possible to separate a light fraction of the gasoline containing a small fraction of the sulfur and a heavy fraction preferably containing the major part of the sulfur initially present in the initial gasoline.

This column generally operates at a pressure between 0.1 and 2 MPa and preferably between 0.2 and 1 MPa. The number of theoretical plates of this separation column is generally between 10 and 100 and preferably between 20 and 60. The reflux rate, expressed as being the ratio of the liquid flow rate in the column divided by the distillate flow rate expressed in kg / h, is generally less than one and preferably less than 0.8.

The light gasoline obtained at the end of the separation generally contains at least all of the C5 olefins, preferably the C5 compounds and at least 20% of the C6 olefins. Generally, this light fraction has a low sulfur content, that is to say that it is not generally necessary to treat the light cut before using it as fuel. According to a second variant of the process according to the invention, the essence is divided into at least 3 fractions: a light fraction, a heavy fraction and at least an intermediate fraction.

In the case where the transformation of the thiophene compounds according to step A is present in the process according to the invention, the essence is preferably fractionated into at least two cuts having the following properties:

- a so-called light cut (cut L), the boiling points of which are preferably less than about 60 ° C. This temperature is given as indicative, it is preferably the maximum temperature for which the thiophene content is less than 5 ppm,

- at least one so-called heavy cut (cut H1) whose boiling points are greater than approximately 60 ° C.

The light section L is preferably injected into a liquid gas separation flask in order to separate the unconsumed hydrogen, and possibly IΗ2S present, from the olefins generally having 5 to 7 carbon atoms. It can then optionally be treated according to step C, preferably by means of a simple softening.

The so-called heavy cut H1, that is to say the cut whose temperatures are above about 60 ° C., is sent to a distillation column or any other separation process capable of separating this cut into at least two cuts:

an intermediate section 12, the boiling points of which, for example, are at least 60 ° C and at most about 120 ° C or even about 160 ° C. This cut can be treated in step A and optionally B and / or C of the process according to the invention. - a heavier H2 cut whose boiling points are generally higher than approximately 160 ° C or approximately 120 ° C.

The heavy fraction H2, the boiling temperatures of which are generally greater than approximately 150 ° C. or even approximately 180 ° C., is sent to stage C of desulphurization.

In another version of the process according to the invention, it is also possible to directly split the essence into at least three cuts, a light cut (L), at least one intermediate cut (12) and at least one heavy cut (H2) having the properties described above.

The intermediate section 12, the boiling points of which are between approximately 60 ° C and approximately 120 ° C or approximately 160 ° C can be sent to a sulfur compound transformation unit according to step A. After step A, the sections 12 can again be split into an intermediate section 13 and a heavy section H3, in particular when step A is a step of alkylation of the thiophenic compounds. The H3 cut thus obtained can optionally be mixed with the H2 cut, preferably before desulfurization.

Two possible variants of the method according to the invention are illustrated in FIGS. 1 and 2. In the variant of FIG. 1, the method according to the invention does not include a fractionation step. The feed 1 is first of all optionally and preferably sent to the hydrogenation unit 2 according to step D. This reaction section can optionally comprise a catalyst capable of both hydrogenating the diolefins (step D) and d '' weighing down sulfur compounds, in particular light mercaptans by addition to olefins (step A, first variant). The effluent from this section 2 or the feed, when the section 2 is absent, is sent via line 3 to a reaction section 4 allowing the alkylation of the thiophene compounds (step A, second variant). The effluent from section 4 is then sent via line 5 to a section 6 containing a hydrodesulfurization catalyst. The desulphurized effluent from this section (7) is finally sent to a reaction section containing an acid catalyst (section 8) and allowing essentially a slight cracking and other reactions described according to step B, in order to increase the octane number of the desulfurized gasoline. The hydrogen necessary for sections 2, 6 and 8 can be supplied respectively by lines 10, 12 and 13. It is optionally possible to supply via line 11 a hydrocarbon fraction rich in olefins from another process, to facilitate the alkylation of thiophene compounds in section 4. Effluent 9 from this treatment can be used in a gasoline pool because it has a low content of sulfur compounds and a high octane number.

In the variant of FIG. 2, the method according to the invention comprises a fractionation step. The feed 1 is first optionally and preferably sent to the hydrogenation unit 2 according to step D. This reaction section can optionally comprise a catalyst capable of both hydrogenating the diolefins (step D) and d '' weighing down sulfur compounds, in particular light mercaptans (step A). The effluent from this section 2 or the feed, when section 2 is absent, is sent via line 3 to a reaction section 4 allowing to carry out the alkylation of the thiophene compounds (step A). The effluent from section 4 is then sent via line 5 to a fractionation section 6 (step E) in three sections: a light fraction untreated and recovered via line 12, an intermediate fraction recovered via line 13 and then treated , a heavy fraction recovered via line 7 and then treated. The treatment of the intermediate fraction (13) consists first of all of an acid treatment (stage B) in section 14, then in a hydrodesulfurization in the presence of a catalyst in section 16 (stage C) of the effluent from of section 14 (effluent 15). The heavy cut treatment consists of hydrodesulfurization in the presence of a catalyst in section 8 (step C), then in an acid treatment of the desulfurized effluent 9 in section 10 (step B). The hydrogen required for sections 2, 8, 10, 14 and 16 can be supplied respectively via lines 19, 21, 22, 23 and 24. It is also possible to supply via line 20 a hydrocarbon cut rich in olefin derived of another process, to facilitate the alkylation of the thiophene compounds in section 4. The intermediate and heavy cuts thus desulphurized and having a high octane number thanks to the acid treatment are recovered respectively via lines 17 and 11 and mixed with the light cut 12 to form a desulfurized gasoline 18 having a high octane number.

In summary, the invention relates to a process for the production of gasoline with a low sulfur content comprising at least one stage of transformation of the sulfur-containing compounds (stage A), at least one stage of treatment in the presence of an acid catalyst (stage B ) and at least one desulfurization treatment of at least part of the gasoline (step C). Said method can optionally also comprise at least one step for the selective hydrogenation of diolefins (step D). it can optionally also further comprise at least one step of fractionating the gasoline (step E) into at least two fractions. It can also optionally further comprise at least one fractionation of the gasoline into at least three fractions.

According to a preferred mode of the process according to the invention, the stage of transformation of the sulfur-containing compounds (stage A) comprises an alkylation or an adsorption of the sulfur-containing compounds. According to a more preferred mode, said step consists essentially of an alkylation of the thiophene compounds. According to another mode preferred, the stage of transformation of the sulfur-containing compounds (stage A) consists essentially of an addition of the mercaptans on the olefins. According to a very preferred mode, the step of transformation of the sulfur-containing compounds (step A) comprises both an alkylation of the thiophene compounds and an addition of the mercaptans to the olefins carried out in two separate reaction sections. Optionally, the step of weighing down the sulfur-containing compounds (step A) is carried out in the same reaction section as the step of hydrogenation of the diolefins (step D).

The method according to the invention can also comprise a step of mixing (step F) the light fraction obtained after one of steps A to E and optionally at least one intermediate fraction from one of steps A to E with the desulfurized heavy fraction from one of steps A to E.

According to a preferred variant of the process according to the invention, the heavy petrol is desulfurized in the presence of at least one hydrodesulfurization catalyst or of an absorbent. According to another variant, the heavy gasoline is desulfurized in two hydrodesulfurization reactors arranged in series and comprising a hydrodesulfurization catalyst. In this case, the H2S is preferably stripped between the two hydrodesulfurization reactors. According to another variant of the process according to the invention, an acid treatment step (step B) is inserted between the two hydrodesulfurization reactors. Optionally, the heavy gasoline desulfurized by means of any of the variants described is stripped by means of an inert gas.

Optionally, at least one reaction section of the process according to the invention is internal to the fractionation section and chosen from the group consisting of the following reaction sections: transformation of the sulfur-containing compounds (step A), acid treatment (step B), desulfurization ( step C), hydrogenation of the diolefins (step D) possibly carried out simultaneously with step A of transformation of the sulfur-containing compounds.

Optionally, at least one reaction section is coupled to the fractionation section and chosen from the group consisting of reaction sections following: transformation of sulfur compounds (step A), acid treatment (step B), desulfurization (step C), hydrogenation of diolefins (step D) possibly carried out simultaneously with step A of transformation of sulfur compounds.

According to one of the preferred variants of the process according to the invention, at least one effluent from step A of transformation of the sulfur compounds is sent to a second fractionation section to be separated into two fractions, an untreated intermediate fraction or optionally desulfurized (step C), and a heavy fraction which is preferably mixed with the heavy fraction from the first fractionation section (step E) or from step B of acid treatment, before desulfurization in step C.

Claims

CLAIMS:

1. A process for the production of gasoline with a low sulfur content comprising at least one stage of transformation of the sulfur-containing compounds (stage A), at least one stage of treatment in the presence of an acid catalyst (stage B) and at least one treatment desulphurization of at least part of the gasoline (step C)

2. Method according to claim 1 further comprising at least one step of selective hydrogenation of diolefins (step D)

3. Method according to one of claims 1 or 2 further comprising at least one step of fractionating the gasoline (step E) into at least two fractions.

4. Method according to one of claims 1 to 3 further comprising at least one fractionation of the gasoline into at least three fractions.

5. Method according to one of claims 1 to 4 wherein the step of transforming the sulfur compounds (step A) comprises an alkylation or an adsorption of the sulfur compounds

6. The method of claim 5 wherein said step consists essentially of an alkylation of thiophene compounds.

7. The method of claim 5 wherein the step of transforming sulfur compounds (step A) consists essentially of adding mercaptans to the olefins.

8. The method of claim 5 wherein the step of transforming sulfur compounds (step A) comprises both an alkylation of thiophene compounds and an addition of mercaptans on the olefins carried out in two separate reaction sections.

9. Method according to one of claims 2 to 4, 7 and 8 wherein the step of weighing down sulfur compounds (step A) is carried out in the same reaction section as the step of hydrogenation of diolefins (step D ).

10. Method according to one of claims 3 to 9 further comprising a step of mixing (step F) the light fraction obtained after one of steps A to E and optionally at least one intermediate fraction from one of steps A to E with the desulfurized heavy fraction from one of steps A to

E.

11. Method according to one of claims 3 to 10 wherein the heavy gasoline is desulfurized in the presence of at least one hydrodesulfurization catalyst or an absorbent.

12. Method according to one of claims 11 wherein the heavy gasoline is desulfurized in two hydrodesulfurization reactors arranged in series and comprising a hydrodesulfurization catalyst.

13. The method of claim 12 wherein IΗ2S is stripped between the two hydrodesulfurization reactors.

14. The method of claim 12 wherein an acid treatment step (step B) is interposed between the two hydrodesulfurization reactors.

15. Method according to any one of claims 1 to 14 wherein the heavy desulfurized gasoline is stripped by means of an inert gas.

16. Method according to one of claims 3 to 15 wherein at least one reaction section is internal to the fractionation section and chosen from the group consisting of the following reaction sections: transformation of the sulfur-containing compounds (step A), acid treatment ( stage B), desulfurization (stage C), hydrogenation of the diolefins (stage D) possibly carried out simultaneously with stage A of transformation of the sulfur-containing compounds.

17. Method according to one of claims 3 to 16 wherein at least one reaction section is coupled to the fractionation section and chosen from the group consisting of the following reaction sections: transformation of the sulfur-containing compounds (step A), acid treatment ( stage B), desulfurization (stage C), hydrogenation of the diolefins (stage D) possibly carried out simultaneously with stage A of transformation of the sulfur-containing compounds.

18. Method according to one of claims 3 to 17 wherein at least one effluent from step A of transformation of the sulfur compounds is sent to a second fractionation section to be separated into two fractions, an untreated intermediate fraction or optionally desulfurized (step C), and a heavy fraction which is preferably mixed with the heavy fraction from the first fractionation section (step E) or from step B of acid treatment, before desulfurization in step C.