FR2993571A1 - METHOD OF DESULFURIZING A GASOLINE - Google Patents

Abstract

The present invention relates to a process for treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans. The process comprises the following steps: a) a demercaptation step is carried out by adding at least a portion of the mercaptans to the olefins by contacting the gasoline with at least a first catalyst; b) the gasoline resulting from step a) is separated into a light intermediate gasoline cut and an intermediate heavy gasoline cut; c) a stream of hydrogen is introduced and at least the second cut of intermediate heavy gasoline from step b) in a catalytic distillation column comprising at least one reaction zone including at least one second catalyst in sulphide form comprising a second support, at least one Group VIII metal and a Group VIb metal, in order to carry out the decomposition of the sulfur compounds in HS, d) at least one final light gasoline fraction comprising H S and a heavy gasoline fraction desulphurized from the catalytic distillation column is removed, the gasoline fraction light end being discharged at a point above the reaction zone and the desulfurized heavy gasoline fraction being discharged at a point below the reaction zone.

Description

The present invention relates to a process for the deep desulphurization of a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans while minimizing hydrogen consumption and preserving the octane number.

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

Pretreatment by hydrotreatment (hydrodesulfurization) of feeds sent to catalytic cracking leads to FCC gasolines typically containing less than 100 ppm of sulfur. These hydrotreatment units, however, operate in severe conditions of temperature and pressure, which implies a high consumption of hydrogen and a high investment. In addition, the entire charge must be desulfurized, resulting in the processing of very large load volumes. It is therefore necessary, in order to meet the sulfur specifications, to post-treat by hydrotreating (or hydrodesulfurization) the catalytic cracking gasolines. When this post-treatment is carried out under conventional conditions known to those skilled in the art, it is possible to further reduce the sulfur content of the gasoline. However, this method has the major disadvantage of causing a very significant drop in the octane number of the FCC gasoline, due to the saturation of olefins during the hydrotreatment.

US Pat. No. 4,131,537 teaches the advantage of splitting the gasoline into several cuts, preferably three, depending on their boiling point, and of desulfurizing them under conditions which may be different and in the presence of a catalyst. comprising at least one Group VIB and / or Group VIII metal. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three cuts, and when the cut having intermediate boiling points is treated under mild conditions. French Patent FR 2,785,908 teaches the advantage of splitting the gasoline into a light fraction and a heavy fraction and then performing a specific hydrotreatment of light gasoline over a nickel-based catalyst, and a hydrotreating heavy gasoline on a catalyst comprising at least one Group VIII metal and / or at least one Group VIb metal.

Among the possible routes for producing fuels with a low sulfur content, that which has been widely adopted consists in specifically treating the sulfur-rich gasoline bases by hydrodesulfurization processes in the presence of hydrogen. The traditional processes desulphurize the species in a non-selective manner by hydrogenating a large part of the mono-olefins, which generates a strong loss in octane number and a high consumption of hydrogen. The most recent processes, such as the Prime G + (trademark) process, make it possible to desulphurize the olefin-rich cracking species, while limiting the hydrogenation of the mono-olefins and consequently the loss in octane number. Such methods are for example described in patent applications EP 1077247 and EP 1174485.

As taught in patent application EP 1077247, it is advantageous to carry out, prior to the hydrotreating step, a step of selective hydrogenation of the feedstock to be treated. This first hydrogenation step essentially consists of selectively hydrogenating the diene compounds (diolefins), to increase the saturated light sulfur compounds by increasing their weight (by increasing their molecular weight), that is to say sulfur compounds whose point of boiling is less than the boiling point of the thiophene, such as methanethiol, ethanethiol, since it allows then to produce by simple distillation and without loss of octane a desulfurized light gasoline fraction composed of a large amount of olefins.

The step of hydrodesulfurization of cracking gasolines which contain mono-olefins consists in passing, on a transition metal sulphide catalyst, the charge to be treated mixed with hydrogen, in order to convert the sulfur compounds. in hydrogen sulfide (H2S). The reaction mixture is then cooled to condense the gasoline.

The gaseous phase containing excess hydrogen and H2S is separated and the desulfurized gasoline is recovered. The residual sulfur compounds generally present in the desulphurized gasoline can be separated into two distinct families: the non-hydrogenated sulfur compounds present in the feed on the one hand, and the sulfur compounds formed in the reactor by so-called recombination side reactions. Among the latter family of sulfur compounds, the major compounds are the mercaptans resulting from the addition of the H2S formed in the reactor on the mono-olefins present in the feedstock. Mercaptans of the chemical formula R-SH where R is an alkyl group, are also referred to as recombinant mercaptans, and generally represent between 20% weight and 80% by weight of the residual sulfur in the desulphurized species. The selective hydrogenation step described in patent application EP 1077247 is indispensable in order to prevent the gradual deactivation of the selective hydrodesulfurization catalyst, to avoid a gradual plugging of the reactor by the formation of polymerization gums on the surface of the catalysts. catalysts or in the reactor and to avoid too rapid clogging of the heat exchangers. The saturation of the diolefins is generally not necessary for the end use of the desulphurized gasoline. This selective hydrogenation step, which is essential only for the proper functioning of the process of the patent application EP 1077247, has the drawback of inducing an unnecessary consumption of hydrogen linked to the saturation of the diolefins of the feedstock. The hydrogenation of the diolefins being carried out under severe conditions, it is generally accompanied by a low hydrogenation of olefins which further increases the hydrogen consumption and induces a loss of octane. Finally, in addition to the hydrogen consumed, hydrogen is also lost at the head of the distillation situated between the selective hydrogenation stage and the selective hydrodesulfurization step of the process of patent EP 1077247, insofar as a Excess hydrogen is usually required to convert almost all diolefins in the first stage. Also known is US Pat. No. 6,984,312, which discloses a process for treating a light catalytic cracked gasoline (about C5-175 ° C) containing olefins, diolefins, mercaptans and heavy organic sulfur compounds. The process employs a first thioetherification step in which the mercaptans are reacted with the diolefins of the feed in the presence of a thioetherification catalyst to form sulfides. The gasoline which has undergone this first stage is then sent to a distillation column where it is fractionated into a light sulfur-depleted fraction and a heavy fraction containing the sulphides formed in the first stage as well as the heavy sulfur-containing organic compounds initially present in the first stage. essence to be treated. The heavy fraction is then treated in the presence of hydrogen and cracked heavy naphtha in a reactive distillation zone containing a hydrodesulfurization catalyst. The cracked heavy naphtha is recycled to the reactive distillation zone so that the distillation column can be operated at high temperature while maintaining a liquid fraction in the catalyst bed. The process described in US Pat. No. 6,984,312 makes it difficult to obtain a light fraction of the gasoline that meets the future specifications for sulfur, ie the upper limit for total sulfur in gasoline is 50 ppm (weight ), or even 30 or 10 ppm weight (without post-treatment of this light fraction). Indeed, the elimination of mercaptans is by addition to the diolefins of the feed with a thioetherification catalyst based on nickel or palladium. However, this type of catalyst also catalyzes the selective hydrogenation of diolefins. Thus the two reactions are competing on diolefins and this results in a limited conversion of mercaptans by thioetherification. The light gasoline produced at the top of the distillation column of the process described in US Pat. No. 6,984,312 can therefore contain a large fraction of light mercaptans present in the initial charge. This is why a post-treatment of the light fraction by hydrodesulfurization is necessary in order to further reduce the sulfur content of the light gasoline fraction. The process described in US Pat. No. 6,984,312 also has the disadvantage of being able to treat only a light gasoline and not a total gasoline. It has indeed been demonstrated that a total gasoline (whose boiling range generally ranges between 20 ° C and 230 ° C) can not be effectively treated in a thioetherification reactor because of the high contents. sulfur which is a poison thioetherification catalysts, including nickel catalysts (see US 7,638,041 patent) or palladium (cf patent US5,595,634).

An object of the invention is therefore to provide a method for producing a gasoline, from a broad spectrum of gasoline type in terms of boiling point range, low sulfur content, that is, having a sulfur content of less than 50 ppm by weight and preferably less than 30 ppm or 10 ppm by weight, while limiting the hydrogen consumption and the octane number loss. SUMMARY OF THE INVENTION there is provided a method of treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans, said process comprising the steps of: a) performing a demercaptation step by adding at least a portion mercaptans on the olefins by bringing the gasoline into contact with at least one first catalyst, at a temperature of between 50 and 250 ° C., at a pressure of between 0.4 and 5 MPa and an LVVH of between 0.5 and and 10 am-1, the first catalyst both in the sulphide form and comprises a first support, at least one metal selected from group VIII and at least one metal selected from the group VIb of the periodic table of elements, the weight%, expressed as the oxide equivalent of the metal selected from the group VIII relative to the total weight of catalyst, is between 1 and 30% and the weight%, expressed as the oxide equivalent of the metal selected from the group VIb, is between 1 and 30% relative to the total weight of catalyst; b) a fractionation of the gasoline from step a) is carried out in a distillation column in at least a first cut of intermediate light gasoline having a total sulfur content lower than that of the starting gasoline and a second intermediate heavy gasoline cut containing most of the starting sulfur compounds; C) introducing a flow of hydrogen and at least the second intermediate heavy gasoline fraction resulting from step b) into a catalytic distillation column comprising at least one reaction zone including at least one second catalyst in sulphide form comprising a second support, at least one group VIII metal and a group VIb metal, the conditions in the catalytic distillation column being chosen so as to bring into contact in the presence of hydrogen the intermediate heavy gasoline resulting from the step; b) with the second catalyst in order to carry out the decomposition of the sulfur-containing compounds into H 2 S, d) removing from the catalytic distillation column at least one final light gasoline fraction comprising H 2 S and a desulphurized heavy gasoline fraction, the final light fuel fraction being discharged at a point above the reaction zone and the desulphurized heavy gasoline fraction being discharged at a point if killed below the reaction zone. The process according to the invention implements a first step a) in which the sulfur compounds of the mercaptan type (R-SH) are converted into heavier sulfur compounds by reaction with the olefins present in the gasoline to be treated. The demercaptation reactions according to the invention are characterized by elimination of mercaptans on olefins: either by direct addition to the double bond to produce sulphides with a higher boiling point or by a hydrogenolysing route: the hydrogen present in the The reactor will produce H2S by contact with a mercaptan which will add directly to the double bond of an olefin to form a heavier mercaptan, ie at the higher boiling point. This route is however a minority in the preferred conditions of the reaction. This first step of increasing mercaptans reaches very high conversions (> 90% and very often> 95%) because the demercaptation reactions are selectively carried out on the olefins which are generally present at high levels. The lighter mercaptans are the most reactive in this step a). On the other hand, H2S, if it is present in the feed, is converted into mercaptan (which itself can be converted) by addition to the olefins by means of the catalyst with the selected conditions. This makes it possible to avoid the presence of H2S in the overhead gases of step b). which very largely contain the unreacted hydrogen in step a), which is interesting to recycle in step a). This recycling, made possible by the absence of H2S in these gases, makes it possible to further reduce the hydrogen consumption of step a), which is an advantage of the process according to the invention. The demercaptation reactions are carried out preferentially on a catalyst comprising at least one element of group VIII (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996), at least one element of the Vlb group (group 6 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and a support. Before contacting the feedstock to be treated, the catalyst undergoes a sulphurization step. The catalyst performs the desired demercaptation reactions only in its sulfide form. Sulfurization is preferably carried out in a sulforeductive medium, that is to say in the presence of H 2 S and hydrogen, in order to convert the metal oxides to sulphides such as, for example, MoS 2 and Ni 3 S 2.

According to the invention, the process comprises a step b) of fractionation of the effluent from step a) of demercaptation which is carried out in a fractionation column (or splitter). The column is configured to split the gasoline into at least two sections, namely: an intermediate light fuel cut having a total sulfur content lower than that of the starting gasoline and an intermediate heavy gasoline cut containing the major part of the starting sulfur compounds and the sulfur products generated in step a). The distillation column of step b) can also be configured to function as a depentanizer or dehexanizer. Preferably, the cutting point of the distillation column of step b) is chosen so as to avoid the entrainment of thiophene in the intermediate light gasoline cut. The sulfur compounds with a boiling point lower than that of thiophene (ie 84 ° C.) are converted in step a) of demercaptation and are thus driven to the intermediate heavy cut, the combination of steps a) and b) allows to obtain a light intermediate gasoline cut with a very low sulfur content. The intermediate light fuel cut generally has a total sulfur content of less than 50 ppm by weight, preferably less than 30 ppm or even less than 10 ppm and contains at least all of the C5 olefins, preferably the C5 compounds and at least 20% by weight of olefins C6. The recovery of a large part of the olefins from the feedstock in the intermediate light gasoline fraction makes it possible to significantly improve the selectivity of the process with respect to the hydrogenation of the olefins and to avoid overconsumption of hydrogen since the olefins are not not redirected to the selective hydrodesulfurization section and therefore have no risk of being hydrogenated. The intermediate heavy gasoline fraction generally contains the hydrocarbons having, for example, a boiling point greater than 84 ° C., the heavy sulfur compounds (of the thiophene, sulphide and disulfide families) initially present in the gasoline to be treated, as well as the compounds substantially sulfide-type sulfur that formed during step a) by adding mercaptans to the olefins.

According to the invention, the process comprises a step c) of hydrodesulfurization of the heavy fraction resulting from the fractionation step b). This treatment is carried out in a distillation column in which is incorporated a catalytic reaction zone, also called "catalytic column".

This step c) consists in desulfurizing the feedstock of the catalytic column by contact with hydrogen injected into the column and a hydrodesulfurization catalyst. The distillation column is thus configured to work under operating conditions which make it possible to react the sulfur compounds of the feed (mercaptans, sulfides and thiophene compounds) with hydrogen to form H2S. Simultaneously with step c), a step d) is carried out in the catalytic distillation column in which the feedstock consisting of at least the second intermediate heavy fraction resulting from the fractionation step b), optionally in admixture with a mixture, is separated. recycling stream, in at least two fractions, namely a final desulfurized light gasoline fraction from the decomposition of sulfur compounds and a heavy desulphurized fraction. The final light gasoline fraction is recovered at the top of the catalytic column with the H2S 15 produced by the desulfurization and the unreacted hydrogen while the heavy desulphurized fraction is generally discharged in the lower part or at the bottom of the catalytic column. . Optionally a complementary desulfurized gasoline cut can be withdrawn using a side withdrawal at a point between the supply and the bottom of the column. The final light gasoline cut with H2S and unreacted hydrogen in the catalytic column is then condensed to separate the incondensables from the liquid phase. Part of this final light fuel cut is withdrawn while another is recycled to the column as internal reflux. Optionally, the process according to the invention also comprises a step of recycling all or part of the desulfurized heavy gasoline fraction in the catalytic distillation column. When this optional recycling is used, a booster and a purge of the recycling stream can also be implemented. According to one embodiment in which the recycling of the desulphurized heavy gasoline fraction is total and that there is no additional withdrawal, the fractionation operated by the catalytic column is only used to allow the recirculation of the section. desulphurised heavy gasoline. In this case, the final light gasoline fraction withdrawn at the top of the catalytic distillation column consists of the second intermediate heavy gasoline cut from step b). The heavy desulphurized cut which is drawn off at the bottom of the column is in this case constituted by a cut of hydrocarbons added by means of a booster and whose boiling point is in a temperature range higher than the second gasoline cut. heavy intermediate from step b). This external hydrocarbon fraction which is then recycled in a loop in the catalytic distillation column serves to maintain a liquid phase at the bottom of the catalytic column in order to operate the column at higher temperature to desulfurize the heavier sulfur molecules which are also the most difficult to convert. The bottom of the catalytic column operating at high temperature is the zone of the catalytic bed most sensitive to deactivation by deposition of coke or gums. This heavier cut is preferably of the paraffinic type and serves as a solvent for washing the coke and the gums which are deposited at the bottom of the catalytic column. This washing is essential to obtain very good cycle times of the catalytic bed. This is all the more true that the charge of the catalytic column has a large amount of unsaturated compounds of the diolefin type. This embodiment is particularly advantageous for limiting the hydrogen consumption of step a) since a step of hydrogenation of gum and coke precursors (in particular the diolefin type compounds) is no longer necessary.

According to another embodiment in which there is no recycling of the desulphurized heavy gasoline fraction, the intermediate heavy gasoline fraction resulting from the fractionation step b) is separated into at least two sections, each of these cuts being desulfurized. In this configuration, the two cuts recovered at the outlet of the catalytic column are directly recoverable for the gasoline pool.

According to another embodiment, the catalytic distillation column is operated so as to separate the second intermediate heavy gasoline fraction resulting from the fractionation step b) into at least two desulphurized sections. This embodiment also implements a booster with an external heavy hydrocarbon cut. This hydrocarbon supplement is recycled in said catalytic distillation column to maintain a liquid phase at the bottom of the catalytic column. In this case, the two desulphurized cuts from the second intermediate heavy gasoline cut are withdrawn respectively at the top (final light fraction) and with a lateral withdrawal (complementary desulfurized gasoline cut) and the heavy desulfurized cut which is withdrawn. in the background is the heavy recycled cup.

In the context of the invention, it is also possible to carry out partial recycling. An advantage of the process according to the invention lies in the fact that it is not necessary to desulphurize the light fraction of the gasoline resulting from the fractionation step b) because almost all the sulfur compounds of the mercaptan type have were converted into compounds of higher molecular weight in step a), so that they are trained in the heavy gasoline fraction. This petrol cut has a low sulfur content and a good octane number and does not require post-treatment.

Hydrogenation reactions are not required in step a). Hydrogen, if used, serves essentially to maintain a hydrogenating surface state of the catalyst so as to provide a high yield on the demercaptation reactions. The process according to the invention is therefore not penalized by the low pressures and implies a reduced consumption of hydrogen, which is an advantage of the process according to the invention.

Another advantage of the process is that the first two steps can be performed at the same pressure (with the pressure drop close) because step a) requires little or no hydrogen, which is also the case of step b). The lack of necessity of the dedienization reaction during step a) is also favorable in terms of hydrogen consumption, because no or little hydrogen is consumed during this step. This iso-pressure operation of steps a) and b) makes it possible to recycle the overhead gas from the column of hydrogen-rich step b) to the demercaptation reactor of step a) when the catalyst of the step a) must have a hydrogenating surface state suitable for high conversions of demercaptation. This recycling makes it possible to reduce the hydrogen consumption of step a) and makes it possible to avoid the loss of this hydrogen towards the fuel gas network. This hydrogen does not normally contain H 2 S because it is not produced by the catalyst used in step a) under the selected conditions. This H2S can even be converted in step a) in case of presence in the load.

An advantage of the process according to the invention lies in the fact that the catalyst and the operating conditions used during stage a) make it possible, unlike the thioetherification reactors as described in the prior art, to treat a total gasoline (ie C5- 220 ° C) with a high sulfur content. Treating a total gasoline cut with this catalyst is particularly advantageous for maintaining a liquid phase in step c) when high desulphurization conversions are targeted, ie when the catalytic column is operating at a high temperature. The use of a catalytic column and not of a conventional fixed bed hydrodesulfurization reactor allows a continuous rinsing of the catalytic zone by the liquid reflux inside the column. This rinsing of the catalytic zone makes it possible to reduce the coking of the catalyst and thus to lengthen the cycle time of the hydrodesulfurization catalyst of step c). This rinsing of the catalytic zone also makes it possible to wash the gums which can be formed by polymerization of the diolefins. The hydrogen partial pressure is also reduced compared to a conventional fixed bed hydrodesulfurization, which is favorable to avoid the hydrogenation side reactions of olefins which generate both a hydrogen overconsumption and a loss of hydrogenation index. octane. Another advantage in connection with the use of a catalytic column for carrying out the selective hydrodesulfurization is that the continuous upward flow of hydrogen allows entrainment of the H2S produced by the hydrodesulfurization reactions and thus contributes to limiting the formation of recombinant mercaptans by adding hydrogen sulphide to the olefins still present. DETAILED DESCRIPTION OF THE INVENTION The subject of the present invention is a process for the desulfurization of a gasoline having a limited sulfur content from a gasoline, preferably originating from a catalytic cracking, coking or visbreaking unit. The gasoline can be a so-called total cracking gasoline (05-220 ° C) or a gasoline whose final boiling point is less than or equal to 210 ° C (light gasoline).

According to the invention, the gasoline first undergoes a step a) of transforming the sulfur compounds, essentially the lighter mercaptans of the gasoline, on the olefins in order to increase their molecular weight. The method further comprises a second step b) which comprises passing all or part of the gasoline from step a) in a fractionation column also called "splitter". This sequence makes it possible to obtain a light fraction whose sulfur content has been lowered without a significant decrease in the olefin content, even at high desulphurization rates, and without it being necessary to treat this light species by means of an additional hydrodesulphurization section or using methods to restore the octane number of gasoline. The process according to the invention thus makes it possible to provide a light gasoline fraction, which can be directly sent to the gasoline pool, the total sulfur content of which is less than 50 ppm, preferably less than 30 ppm, or even less than 10 ppm. ppm, depending on the amount of sulfur initially present and the chemical nature of the sulfur compounds. The process according to the invention also comprises a step c) of hydrodesulfurization of the heavy fraction resulting from the fractionation step b). This treatment is carried out in a distillation column in which is incorporated a catalytic reaction zone, also called "catalytic column". This second step consists in desulfurizing this first heavy fraction by contact with hydrogen on the catalytic bed. The catalytic distillation column is configured to work under operating conditions which simultaneously make it possible: - to react the sulfur compounds of the feed (mercaptans, sulphides and thiophene compounds) with hydrogen to form H2S - to separate the feedstock containing at least the intermediate heavy gasoline fraction 20 from the fractionation step b) into at least two fractions, namely a final light gasoline fraction depleted in sulfur compounds to a very low sulfur content, and which contains the most of the olefins and a desulphurized heavy cut also with very low sulfur content. As used herein, the term "catalytic column" refers to an apparatus in which the catalytic reaction and the separation of the products takes place at least simultaneously. The equipment employed may comprise a distillation column equipped with a catalytic section, in which the catalytic reaction and the distillation take place simultaneously. It may also be a distillation column in association with at least one reactor disposed within said column and on a wall thereof. The internal reactor may be operated as a vapor phase reactor or as a liquid phase reactor with a co-current or countercurrent liquid / vapor circulation.

The implementation of a catalytic distillation column has the advantages, compared to the implementation of a single fixed bed reactor operated in the gas phase, to allow a continuous rinsing of the catalytic zone by the liquid reflux inside. of the column. This rinsing of the catalytic zone makes it possible to reduce the coking of the catalyst and thus to lengthen the cycle time of the hydrodesulfurization catalyst. The hydrogen partial pressure is also reduced compared to a conventional fixed bed hydrodesulfurization, which is favorable to avoid the hydrogenation side reactions of olefins which generate both a hydrogen overconsumption and a loss of hydrogenation index. octane. The use of a catalytic column also makes it possible to control the reaction while promoting an exchange of the evolved heat; the heat of reaction can be absorbed by the heat of vaporization of the mixture. The gasoline to be treated The process according to the invention makes it possible to treat any type of sulfur-containing gasoline cut, preferably a petrol cut from a catalytic cracking unit, whose range of boiling points typically extends. since about the boiling points of hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up to about 250 ° C, more preferably from about the boiling points of hydrocarbons with 5 carbon atoms up to about 220 ° C. The method according to the invention therefore also applies to a gasoline cut which has been previously stabilized, that is to say to a petrol cut which has been removed hydrocarbons having a carbon number of less than 6 or 5. The process according to the invention can also treat a "light" gasoline feedstock having a final boiling point lower than those mentioned above, such as, for example, less than or equal to 210 ° C., less than or equal to 180 ° C. C, less than or equal to 160 ° C or even lower than or equal to 145 ° C. The sulfur content of catalytic cracked gasoline (FCC) gasoline cuts depends on the sulfur content of the FCC-treated feedstock, the presence or absence of FCC feedstock pretreatment, and the endpoint of the feedstock. chopped off. Generally, the sulfur contents of the entirety of a petrol cut, in particular those coming from the FCC, are greater than 100 ppm by weight and most of the time greater than 500 ppm by weight. For gasolines with end points higher than 200 ° C., the sulfur contents are often greater than 1000 ppm by weight, and in some cases they may even reach values of the order of 4000 to 5000 ppm by weight. For example, gasoline from catalytic cracking units (FCC) contain, on average, between 0.5% and 5% by weight of diolefins, between 20% and 50% by weight of olefins, between 10 ppm and 0.5 % by weight of sulfur, generally less than 300 ppm of mercaptans. Mercaptans are generally concentrated in the light ends of gasoline and more precisely in the fraction whose boiling point is below 120 ° C. It should be noted that the sulfur compounds present in the gasoline may also comprise heterocyclic sulfur compounds, such as, for example, thiophenes, alkylthiophenes or benzothiophenes. Step a) Increasing the mercaptans with olefins This step consists in transforming the light sulfur compounds of the mercaptan family, that is to say the compounds which at the end of the fractionation step b) would be found in the light gasoline, heavier sulfur compounds that are entrained in the intermediate heavy gasoline fraction during the fractionation step b). During this step a) a demercaptation reaction takes place which sees the addition of mercaptans to the olefins of the feedstock in the presence of a catalyst. Typically the mercaptans that can react in step a) are as follows (non-exhaustive list): methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, iso-propyl mercaptan, iso-butyl mercaptan, tert-butyl mercaptan, n-butyl mercaptan, sec-butyl mercaptan, iso-amyl mercaptan, n-amyl mercaptan, α-methylbutyl mercaptan, ethylpropyl mercaptan, n-hexyl mercaptan, 2- mercapto-hexane. The demercaptation reaction is carried out preferentially on a catalyst comprising at least one group VIII element (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996), at least one element of the Vlb group (group 6 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and a support. The group VIII element is preferably chosen from nickel and cobalt and in particular nickel. The group VIb element is preferably selected from molybdenum and tungsten and very preferably molybdenum.

The catalyst support is preferably selected from alumina, nickel aluminate, silica, silicon carbide, or a mixture of these oxides. Alumina is preferably used, and even more preferably pure alumina. Preferably, a support having a total pore volume measured by mercury porosimetry of between 0.4 and 1.4 cm 3 / g and preferably between 0.5 and 1.3 cm 3 / g is used. The specific surface of the support is preferably between 70 m 2 / g and 350 m 2 / g. According to a preferred variant, the support is a cubic gamma alumina or delta alumina. The catalyst used in step a) generally comprises: a support consisting of gamma alumina or surface delta of between 70 m 2 / g and 350 m 2 / g, a content by weight of oxide of the element of group VIb between 1 and 30 (3/0 weight relative to the total weight of the catalyst, - a weight content of oxide of the element of group VIII between 1 and 30 (3/0 weight relative to the total weight of the catalyst, a sulphidation rate of the metals constituting said catalyst at least equal to 60%, a molar ratio between the non-noble metal of group VIII and the metal of group VIb is between 0.6 and 3 mol / mol, In particular, it has been found that the performances of the catalysts are improved when the catalyst has the following characteristics: a support consisting of surface gamma-alumina of between 180 m 2 / g and 270 m 2 / g-the weight content of oxide of the group VIb element in oxide form is between 4 and 20% by weight relative to the total weight of catalyst, preferably between 6 and 18% by weight; the content of Group VIII metal expressed in oxide form is between 3 and 15% by weight and preferably between 4% by weight and 12% by weight relative to the total weight of catalyst; the molar ratio between the non-noble metal of group VIII and the metal of group VIb is between 0.6 and 3 mol / mol and preferably between 1 and 2.5 mol / mol. A preferred embodiment of the invention corresponds to the use of a catalyst containing a content by weight of nickel oxide (in the form NiO) of between 4 and 12%, a content by weight of molybdenum oxide (in MoO3 form) included between 6% and 18% and a nickel / molybdenum molar ratio of between 1 and 2.5, the metals being deposited on a support consisting solely of alumina and the sulphidation rate of the metals constituting the catalyst being greater than 80%.

The catalyst according to the invention may be prepared using any technique known to those skilled in the art, and in particular by impregnation of the elements of groups VIII and VIb on the selected support. After introduction of the elements of groups VIII and VIb, and possibly shaping of the catalyst, it underwent an activation treatment. This treatment generally aims to transform the molecular precursors of the elements in the oxide phase. In this case it is an oxidizing treatment but a simple drying of the catalyst can also be carried out. In the case of an oxidizing treatment, also known as calcination, this is generally carried out under air or under dilute oxygen, and the treatment temperature is generally between 200 ° C. and 550 ° C., preferably between 300 ° C. C and 500 ° C. After calcination, the metals deposited on the support are in oxide form. In the case of nickel and molybdenum, the metals are mainly in the form of MoO3 and NiO. Before contacting with the feedstock to be treated, the catalysts undergo a sulphurization step. Sulfurization is preferably carried out in a sulforeductive medium, that is to say in the presence of H 2 S and hydrogen, in order to convert the metal oxides to sulphides such as, for example, MoS 2 and Ni 3 S 2. Sulfurization is carried out by injecting onto the catalyst a stream containing H2S and hydrogen, or a sulfur compound capable of decomposing into H 2 S in the presence of the catalyst and hydrogen. Polysulfides such as dimethyl disulphide are H 2 S precursors commonly used to sulphurize catalysts. The temperature is adjusted so that the H2S reacts with the metal oxides to form metal sulfides. This sulphurization may be carried out in situ or ex situ (inside or outside the reactor) of the demercaptation reactor at temperatures between 200 and 600 ° C and more preferably between 300 and 500 ° C.

Step a) can be carried out without adding hydrogen to the reactor, but preferably it is injected with the feed so as to maintain a hydrogenating surface state of the catalyst suitable for high conversions in demercaptation. Typically, step a) operates with a flow rate ratio H2 / charge flow rate of between 0 and 25 Nm 3 of hydrogen per m 3 of feedstock, preferably between 0 and 10 Nm 3 of hydrogen per m 3 of feedstock. very preferred between 0 and 5 Nm3 of hydrogen per m 3 of filler, and even more preferably between 0.5 and 2 Nm3 of hydrogen per m 3 of filler.

The entire charge is usually injected at the reactor inlet. However, it may be advantageous in some cases to inject a fraction or the entire charge between two consecutive catalytic beds placed in the reactor. This embodiment makes it possible in particular to continue operating the reactor if the inlet of the reactor is clogged by deposits of polymers, particles, or gums present in the load.

The gasoline to be treated is brought into contact with the catalyst at a temperature of between 50 ° C. and 250 ° C., and preferably between 80 ° C. and 220 ° C., and even more preferably between 90 ° C. and 200 ° C. C, with a liquid space velocity (LHSV) of between 0.51-1-1 and 10 h ', the unit of the liquid space velocity being the liter of charge per liter of catalyst per hour (1 / 1h). The pressure is between 0.4 MPa and 5 MPa and preferably between 0.6 and 2 MPa and even more preferably between 0.6 and 1 MPa. At the end of step a) the gasoline treated under the conditions set out above, has a reduced mercaptans content. Generally, the gasoline produced contains less than 50 ppm by weight of mercaptans, and preferably less than 10 ppm by weight. Light sulfur compounds whose boiling point is lower than that of thiophene (84 ° C) are generally converted to more than 80% or more than 90%. The olefins are not or very little hydrogenated, which allows to maintain a good octane output of step a). The hydrogenation rate of the olefins is generally less than 2%.

Step b) of separation into an intermediate light gasoline cut and an intermediate heavy gasoline cut The separation step b) is preferably carried out by means of a conventional distillation column also called "splitter". This fractionation column makes it possible to separate an intermediate light gasoline fraction containing a low content of sulfur compounds and an intermediate heavy gasoline fraction preferably containing most of the sulfur compounds initially present in the initial gasoline. This column generally operates at a pressure of between 0.1 and 2 MPa and preferably between 0.6 and 1 MPa. It should be noted that this pressure can be substantially the same (at the pressure drop by) as that prevailing in the reactor of step a). This iso-pressure operation of steps a) and b) makes it possible to recycle the overhead gas from the column of hydrogen-rich step b) to the demercaptation reactor of step a) (when the catalyst of the step a) must have a hydrogenating surface state suitable for high conversions of demercaptation). This recycling makes it possible to reduce the hydrogen consumption of step a) and makes it possible to avoid the loss of this hydrogen towards the fuel gas network. This hydrogen does not normally contain H 2 S because it is not produced by the catalyst used in step a) under the selected conditions. The number of theoretical plates of this separation column is generally between 10 and 100 and preferably between 20 and 60. The reflux ratio, expressed as the ratio of the liquid flow rate in the column divided by the distillate flow rate expressed in kg / h, is generally less than unity and preferably less than 0.8. The intermediate light gasoline obtained after separation b) generally contains at least all of the C5 olefins, preferably the C5 compounds and at least 20% of the C6 olefins. The cutting point of the column is often determined so that the thiophene is not drawn into the intermediate light gasoline cut. Thus, the intermediate heavy gasoline cut has an initial point distillation range located around 84 ° C. This initial point may be higher depending on the sulfur content expected in the intermediate light gasoline cut and may be about 100 ° C or up to 120 ° C. Alternatively, the distillation column is configured to allow lateral withdrawal of an intermediate gasoline cut, that is to say a gasoline cut whose boiling points is between the final boiling point. light intermediate gasoline and the initial boiling point of intermediate heavy gasoline. Said intermediate gasoline can then be treated by hydrodesulphurization in a dedicated reactor and then mixed with the intermediate light gasoline. Step c) and d) of hydrodesulphurization in a catalytic column The desulphurization reaction of step c) is a hydrodesulphurization reaction carried out by passing the feedstock, in the presence of hydrogen which is injected into said column, on at least one catalyst at least partly in sulphide form comprising at least one group VIII element, at least one group VIb element and optionally phosphorus, at a temperature between 210 ° C and 350 ° C, preferably between 220 ° C and 320 ° C. The pressure at the top of the column is generally maintained between about 0.1 and about 4 MPa, preferably between 1 and 3 MPa. The ratio H2 flow / charge rate in the column is between 25 to 400 Nm3 per m3 of liquid charge and preferably between 40 and 100 Nm3 per m3 of liquid charge. The element of group VIII is preferably cobalt or nickel, and the element of group VIb is generally molybdenum or tungsten. Combinations such as molybdenum cobalt or molybdenum nickel are preferred. The metal content of group VIII, expressed as oxide, is generally between 0.5 and 25% by weight, preferably between 1 and 10% by weight relative to the weight of the catalyst. The metal content of group VIb, expressed as oxide, is generally between 1.5 and 60% by weight, preferably between 3 and 50% by weight relative to the weight of catalyst.

Preferably, when the catalyst is of cobalt-molybdenum type, the cobalt content, expressed as oxide, is generally between 0.5 and 15% by weight, and even more preferably between 2 and 5% by weight; the molybdenum content, expressed as oxide, is between 1.5 and 60% by weight and even more preferably between 5 and 20% by weight.

Preferably, when the catalyst is of the nickel-molybdenum type, the nickel content, expressed as oxide, is generally between 0.5 and 25% by weight, and even more preferably between 5 and 25% by weight; the molybdenum content, expressed as oxide, is between 1.5 and 30% by weight and even more preferably between 3 and 20% by weight.

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 mixed with alumina or silica-alumina, and may be originally in the form of small-diameter extrudates or spheres. The catalyst must have in the column a form of structure suitable for catalytic distillation in order to act both as a catalytic agent to carry out the reactions but also as a material transfer agent in order to have separation stages available on along the bed. To minimize the hydrogenation of the olefins of the treated feed, it is advantageous to preferentially use a cobalt molybdenum-type catalyst in which the molybdenum density, expressed in% by weight of MoO 3 per unit area, is greater than 0.07 and preferably greater than 0.12. The catalyst according to the invention preferably has a specific surface area of less than 250 m 2 / g, more preferably less than 230 m 2 / g, and very preferably less than 190 m 2 / g.

If it is desired to obtain a good conversion in hydrodesulfurization and both in hydrogenation of olefins (in particular at the bottom of the catalytic column when using the recirculation of a heavy gasoline fraction), it is advantageous to use preferentially a nickel molybdenum catalyst. In this case, the catalyst according to the invention preferably has a specific surface area of between 70 and 250 m 2 / g. The deposition of the metals on the support is obtained for all methods known to those skilled in the art such as, for example, the dry impregnation, by excess of a solution containing the metal precursors. Said solution is chosen so as to solubilize the metal precursors in the desired concentrations. In the case of the synthesis of a CoMo catalyst, for example, the molybdenum precursor may be molybdenum oxide, ammonium heptamolybdate. For cobalt, mention may be made, for example, of cobalt nitrate, cobalt hydroxide and cobalt carbonate. The precursors are generally dissolved in a medium allowing their solubilization in the desired concentrations. This can be, depending on the case, carried out in an aqueous medium and / or in an organic medium. Phosphorus may be added as phosphoric acid. In the context of the invention, it is possible to implement more than one catalytic bed in the reaction zone, for example two separate catalyst beds, separated from each other by a space. It is also possible in certain configurations that the catalytic column comprises more than one catalytic bed which are loaded with different catalysts, especially when it is desired to use an effective combination of the different properties of the cobalt-molybdenum and nickel-type catalysts. molybdenum. In another configuration of the process according to the invention, the catalytic bed of the column may be located only above the feed or only below. Preferably, the column has one or more catalytic beds covering at least a portion of both the area above the feed and the area below the feed. The operation of the catalytic column induces the simultaneous presence of vapor and liquid in the reaction zone. A large portion of the vapor is hydrogen, with the remainder being a portion of the vaporized charge and hydrogen sulphide. As with any distillation, there is a temperature gradient in the system so that the lower end of the column comprises the compounds whose boiling point is higher than that of the upper end of the column. The distillation makes it possible to separate the compounds present in the feed by difference in boiling temperature. The heat of reaction possibly generated in the catalytic column is removed by vaporization of the mixture on the distillation plate concerned. Consequently, the thermal profile of the column is very stable and the catalytic reactions which take place on the bed do not disturb its operation. Likewise, this stability of the thermal profile makes it possible to have stable reaction kinetics since they are isothermal on each separation stage, the temperatures being dependent solely on the liquid-vapor equilibrium of the separation stages and the control of the pressure of the column.

The catalytic distillation column is configured so as to work under operating conditions that make it possible to separate the feedstock consisting of at least the second intermediate heavy fraction resulting from the fractionation step b) into at least two fractions, namely a fraction final desulfurized light gasoline from the decomposition of sulfur compounds and a heavy desulphurized fraction. The final light gasoline fraction is recovered at the top of the catalytic column with the H2S produced by the desulfurization and the unreacted hydrogen while the heavy desulphurized fraction is withdrawn at the bottom of the catalytic column. Optionally a complementary desulphurized gasoline cut can be withdrawn by means of a side withdrawal at a point between the feed and the bottom of the column. Preferably, the final light gasoline fraction is cooled together with the H2S produced by the desulfurization reactions and the unreacted hydrogen to a temperature generally below 60 ° C. in order to condense the hydrocarbons. . The gas phases (mainly containing the product H 2 S and unreacted hydrogen) and the liquid hydrocarbon phase (ie the final recoverable light gasoline fraction) are separated in a separator. Part of this final light fuel cut is transferred to the gasoline pool while another is recycled back to the column as internal reflux. The internal reflux is both useful for carrying out the distillation of the feedstock and also allows for permanent washing of the catalyst. The downward flow of the flow of liquid from the column makes it possible to clean the coke catalyst and the gums that may form, particularly because of the presence of highly unsaturated compounds, diolefin or acetylenic types, in the feedstock. This makes it possible to reduce the deactivation of the catalyst and consequently to ensure a high cycle time.

The gaseous phase rich in H2S and hydrogen can be sent for example into an amine absorber to purify and recover the hydrogen for recycling in the process.

Optionally, the process according to the invention also comprises a step of recycling in the catalytic column all or part of the desulfurized heavy gasoline fraction which is withdrawn at the bottom of said catalytic column. When this optional recycling is used, a booster and a purge of the recycling stream can also be implemented. Alternatively, the recycling stream may also comprise an external hydrocarbon cut (fed via the booster) whose initial boiling point is greater than or equal to that of the intermediate heavy gasoline cut. This external hydrocarbon cut is withdrawn at the bottom of the catalytic distillation column and recycled loop in said column.

Various configurations of the method according to the invention are indicated below, the list of these configurations being not exhaustive. In a first configuration, the feed entering step c) which consists of the intermediate heavy gasoline cut from step b) is fractionated into at least two gasoline cuts. In this case, the final light fuel cut and heavy desulphurized cut from step c) and d) are directly recoverable for the gasoline pool. This configuration is preferably used when the gasoline cut injected in step a) at the input of the whole of the sequence is a total gasoline, that is to say whose range of boiling points typically extends from about the boiling points of the hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up to about 250 ° C, or preferably from about the boiling points of the hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up to about 220 ° C, or more preferably from about 5 hydrocarbon boiling points to about 220 ° C. It can nevertheless also be used with a light gasoline, that is to say with a boiling point of less than 210 ° C. In this configuration, the catalyst bed is preferably composed of a single bed of cobalt molybdenum catalyst. The final light gasoline cut recovered at the head and the heavy desulfurized cut recovered at the bottom are desulfurized gasoline cuts to a very low sulfur content, that is to say having a sulfur content of less than 50 ppm by weight and preferably lower than 50 ppm by weight. at 30 ppm or 10 ppm weight. The final light fuel cut is generally a cut whose range of boiling points typically extends from about the initial point of the catalytic column charge generally between 80 ° C-120 ° C, up to about 145 ° C. C 5 or preferably up to about 160 ° C, or more preferably up to about 180 ° C. The desulphurized heavy cut is generally a cut whose range of boiling points typically extends from about the end point of the final light gasoline cut to about the end point of the catalytic column charge, generally between 220 ° C and 250 ° C. The two cuts recovered at the outlet of the catalytic column 10 can then be mixed and then sent to a stripper in order to eliminate the last traces of H2S solubilized to be finally intended for the gasoline pool. In a second configuration, the feedstock of step c) comprises the intermediate heavy gasoline cut resulting from step b) and a recycling of all or part of a desulphurized heavy cut recovered at the bottom of the catalytic column. This configuration is particularly used when the gasoline cut injected in step a) at the entry of the whole sequence is a light gasoline having a final boiling point below 220 ° C., such as, for example, less than or equal to at 210 ° C, less than or equal to 180 ° C, less than or equal to 160 ° C or even lower than or equal to 145 ° C. However, when the intermediate heavy gasoline fraction recovered at the bottom of the distillation column of step b) has a final boiling point of about 180.degree. C., it is desirable to use a top-up boiling point. external heavy gasoline as a recycle to maintain a liquid phase in the catalyst column at the selected operating conditions. This recycling also makes it possible to increase the catalyst washing rate in the background, which is favorable when the feedstock of the hydrodesulfurization step contains precursors of coke and gums. Preferably, a simple top-up of the heavy cut is made in the recycle stream and said heavy cut is recycled loop in the column. This external recycle gasoline cutoff typically has a distillation range from the end point of the batch to be treated, i.e., in a range of from 145 ° C to about 210 ° C for this configuration, up to at a temperature between about 180 ° C and 240 ° C. This external recycling gasoline cut may for example be a desulfurized cracked heavy gasoline cut. The heavy external recycling cup must contain a low content of unsaturated compounds in order to be used as a solvent for optimal catalyst washing. In this configuration, the catalytic column will preferably contain two catalytic beds located respectively above and below the feed. Preferably, a catalyst having both hydrodesulphurization and hydrogenation properties will be fed to the bottom of the catalytic column. A catalyst comprising at least one group VIII element and at least one at least one element of the VIb group in sulphide form, wherein preferably the group VIII element is nickel and the element of the group VIb is molybdenum. The catalyst in the upper zone is, on the other hand, preferably a cobalt molybdenum catalyst. In a third configuration, three different cuts are withdrawn from the catalytic column: a final cut of light gasoline desulphurized withdrawn at the top of the column and 15 recoverable towards the gasoline pool; a desulphurized heavy cut withdrawn at the bottom of the column, most of which will be recycled to the catalytic distillation column; and a complementary desulfurized gasoline fraction withdrawn at a point between the withdrawal of the final desulphurized light gasoline and the withdrawal at the bottom of the column. In a preferred manner, this cut is drawn off at a point situated between the supply of the column and the withdrawal at the bottom of the column. This configuration is preferably implemented when the petrol fraction injected in step a) at the input of the whole sequence is a total gasoline, that is to say whose range of boiling points typically extends from about boiling points of hydrocarbons with 2 or 3 carbon atoms (C2 or C3) to about 250 ° C, or preferably from about boiling points of hydrocarbons with 2 or 3 carbon atoms. carbon (C2 or C3) up to about 220 ° C, or more preferably from about the boiling points of the 5-carbon hydrocarbons up to about 220 ° C. This configuration is also preferred when the catalytic column must operate at very high conversions (high desulphurization), therefore at high temperature, especially when the end point of the cut treated in the process according to the invention is particularly high and the load contains therefore heavy sulfur compounds, thiophene type or benzothiophenic, difficult to desulfurize.

The recycling of the heavy fraction in the catalytic column makes it possible, in spite of the high temperature, to maintain a liquid phase in the column and also makes it possible to increase the catalyst washing rate in the bottom. In fact, operation at a higher temperature promotes the formation of coke and gums by polymerization of diolefins in the feed, in particular at the bottom of the column, where the temperatures are the highest. Preferably, a top-up of an external hydrocarbon cut is also added to the recycling loop. This external heavy cut typically has a distillation range of from 220 ° C to 270 ° C, preferably from 220 ° C to 250 ° C. This heavy cut is generally a heavy cracked cut resulting from the fractionation of the FCC such as a LCO (Light Cycle Oil), that is to say a cut resulting from catalytic cracking and boiling in a temperature range greater than that of the gasoline) or a kerosene cut or straight-run diesel fuel cut. In this configuration, the catalytic column will preferably contain two catalytic beds located respectively above and below the feed. The catalyst of the catalytic zone situated at the bottom of the catalytic column will preferably be a nickel molybdenum type catalyst. The catalyst located in the upper zone is preferably a cobalt molybdenum type catalyst allowing a good selectivity of the hydrodesulfurization reactions relative to those of hydrogenation of olefins in order to maintain the octane number of the treated feedstock.

The final light gasoline cut recovered at the head and the complementary desulphurized gasoline cut recovered by the side rack are desulfurized gasoline cuts having a low sulfur content, ie having a sulfur content of less than 50 ppm by weight and preferably lower than 50 ppm by weight. at 30 ppm or 10 ppm weight. The final light gasoline cut is generally a cut whose range of boiling points typically extends from about the initial point of the treated feedstock in the catalyst column (generally between 80 ° C and 120 ° C), up to about a temperature above 145 ° C, or preferably up to about a temperature above 160 ° C, or more preferably up to about 180 ° C. The complementary desulphurized gasoline cut is generally a cut whose range of boiling points typically extends from about the end point of the final light gasoline cut to about the end point of the second intermediate heavy gasoline cut from step b), that is to say up to about a temperature between 210 ° C and 230 ° C. The two gasoline fractions recovered at the end of the catalytic column (final light gasoline and complementary desulfurized gasoline) can then be mixed and sent to a stripper in order to eliminate the last traces of solubilized H2S and finally be stored in the gasoline pool. BRIEF DESCRIPTION OF THE DRAWINGS These aspects as well as other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, reference being made to the drawings of the figures, in which: FIG. diagram of the process according to the invention; FIG. 2 shows a second diagram of the method according to the invention; FIG. 3 shows a third diagram of the method according to the invention; FIG. 4 shows a fourth diagram of the method according to the invention. Generally, similar elements are denoted by identical references in the figures.

FIG. 1 shows a first diagram of the process according to the invention for the treatment of a gasoline feedstock comprising in particular olefins, diolefins and sulfur compounds of the mercaptan type and of the thiophenic family with a view to supplying several fractions of the gasoline having a total sulfur content of less than 50 ppm by weight, preferably less than 30 ppm by weight, or even less than 10 ppm by weight. According to the process, the gasoline feedstock to be treated is optionally sent with an additional hydrogen in a demercaptation reactor 2 through a feed line 1. The reactor 2 comprises a catalytic section provided with a catalytic bed specifically chosen in order to perform the selective addition of mercaptans to olefins in order to increase their molecular weight. The reactor is preferably a fixed catalytic bed reactor which operates in a three-phase or two-phase system and one of the phases (the catalyst) is solid. The demercaptation reactions are generally conducted at a temperature between 50 and 250 ° C, at a pressure of between 0.6 and 2 MPa and a liquid space velocity of between 0.5 hr-1 and 10 hr-1. The effluent from step a) of demercaptation is then sent to a fractionation column 4 also called "splitter" through line 3. The fractionation column 4 is configured and operated to separate a light intermediate gasoline cut containing a small fraction of sulfur and an intermediate heavy gasoline fraction containing most of the sulfur initially present in the gasoline to be treated. This column generally operates at a pressure of between 0.1 and 2 MPa and preferably between 0.6 and 1 MPa. The number of theoretical plates of this fractionation column is generally between 10 and 100, and preferably between 20 and 60. The reflux ratio, expressed as the ratio of the liquid traffic in the column divided by the distillate flow expressed in kg / h, is generally less than 1 and preferably less than 0.8. The intermediate light gasoline obtained at the end of the separation generally contains at least all the C5 olefins, preferably the C5 compounds and at least 20% of the C6 olefins. Generally, this light fraction has a very low sulfur content, ie less than 50 ppm by weight, preferably less than 30 ppm by weight, or even less than 10 ppm by weight. It is not necessary to post-treat the light cut before using it as a gasoline base.

As shown in FIG. 1, the intermediate light gasoline fraction extracted from the head of the fractionation column via line 5 is cooled through an exchanger 6 and then transferred into a gas / liquid separator 9. A gas effluent containing the compounds incondensable, mainly hydrogen, is withdrawn at the top of the separator through the pipe 9 while the liquid gasoline fraction is drawn off in the bottom by the line 10, a portion of which serves to supply the gasoline pool (via the pipe 11) and a another part corresponds to the reflux of distillation. The intermediate heavy gasoline fraction which is withdrawn at the bottom of the fractionation column 4 and which contains the majority of the sulfur products including those generated during the demercaptation step a) serves as a feedstock for the third stage of the process according to the invention. 'invention. With reference to FIG. 1, the intermediate heavy gasoline fraction is sent via line 13 to a catalytic distillation column 14 provided with a reaction section 15 comprising at least one catalytic bed. The catalyst is selected according to the invention for its ability to decompose sulfur compounds into H2S in the presence of hydrogen in a selective manner with respect to the hydrogenation of olefins to maintain the octane number of the feedstock. The hydrodesulfurization catalyst is implemented in its sulfurized form and comprises a porous support, at least one group VIII element and at least one group VIb element. Preferably, the catalyst used in the process according to the invention in its configuration corresponding to FIG. 1 is of cobalt molybdenum type.

In order to carry out the catalytic conversion of the sulfur compounds, hydrogen is supplied via line 16. The catalytic distillation column 14 is configured so as to fractionate said intermediate heavy gasoline in at least two fractions, namely a section final desulfurized heavy gasoline and a final desulfurized light gasoline cut. The two final desulfurized light gasoline and final desulfurized heavy gasoline coupons can then be sent to a stripper in order to remove the last traces of solubilized H2S (not shown). As shown in FIG. 1, the intermediate heavy gasoline resulting from step b) is brought into contact in the reaction section 15 with hydrogen, supplied via line 16, and a hydrodesulfurization catalyst in order to achieve the conversion of sulfur compounds to H2S. Concomitant with the conversion reaction is a fractionation of the intermediate heavy gasoline producing a final light gasoline cut comprising H2S resulting from the decomposition of the sulfur compounds. The final light gasoline is withdrawn at the top of the distillation column via line 17. The final light gasoline distilling at the top of the column accompanied by the H2S formed following the desulfurization reactions and the unreacted hydrogen in the column is then cooled by means of a heat exchanger 18 and then sent through the pipe 19 in a gas / liquid separator 20 where are separated a gaseous effluent comprising essentially hydrogen and H2S (via the pipe 21 ) and a desulfurized liquid essence. The desulphurized liquid gasoline is then divided into two fractions, a fraction of which is recycled to the distillation column 14 in order to ensure reflux and another fraction that can be used in a gasoline pool after possibly passing through an H2S stripper. The overhead gases can be sent to an amine absorption unit to separate the hydrogen from the hydrogen sulfide to purify the hydrogen for possible recycling. The process according to the invention as shown in FIG. 1 essentially concerns the treatment of a total gasoline cut. A second embodiment of the method according to the invention is shown in FIG. 2. This embodiment differs essentially from the first embodiment in that there is a recycling flow from the bottom section of the catalytic column to the feed. of said column. With reference to FIG. 2, a fraction of the effluent withdrawn via line 25 is mixed with intermediate heavy gasoline via line 26 and thus recycled to the catalytic distillation column. An extra heavy external cut is made via line 27 in this recycling stream. Purge 29 is provided on this circuit. The recycling of the heavy fraction at the inlet of the catalytic column makes it possible, in spite of the high temperature of the bottom of the column, to maintain a liquid phase in the column and also makes it possible to increase the rate of washing of the catalyst in the bottom. This washing of the gums and coke formed due to the presence of highly unsaturated compounds in the intermediate heavy gasoline cut ensures a good high conversion cycle time for the catalysts used. The catalytic column will preferably contain two catalytic beds located respectively above and below the feed. The catalytic zone catalyst located at the bottom of the catalytic column will preferably be a nickel-molybdenum type catalyst. The catalyst located in the upper zone is preferably a cobalt molybdenum type catalyst. A third embodiment of the method according to the invention is shown in FIG. 3. This embodiment differs essentially from the second embodiment in that there is a withdrawal of a complementary fraction of heavy desulphurized gasoline via the pipe. 28 at a point between the supply and the bottom of said column. A fourth embodiment of the method is shown in FIG. 4. This embodiment incorporates the features of the embodiment of FIG. 1 and adds to fractionation step b) in the distillation column 4 a lateral withdrawal of a gasoline cut whose boiling range extends in the range between the final boiling point of the intermediate light gasoline and the initial boiling point of the intermediate heavy gasoline. With reference to FIG. 4, a gasoline cut is withdrawn laterally from the distillation column 4 via the line 25. The withdrawal point via the line 25 is placed in the column at a level between the overhead withdrawal via the line 5 and racking at the bottom of the column. Preferably, the withdrawal is carried out above the feed introduction level via line 3 in column 4. This gasoline cutoff is sent to a dedicated hydrodesulfurization unit 26, in order to convert, in the presence of hydrogen, especially mercaptans and thiophene-type compounds present in said H2S cut. Unit 26 is composed of an enclosure comprising at least one bed of hydrodesulfurization catalyst. Preferably, the hydrodesulfurization catalyst comprises at least one support, at least one group VIII element (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and at least one an element of group VIB (group 6 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996). Preferably, the catalyst has a density of Group VIB elements per unit area of the support included (limits included) between 2.104 and 18.104 g of Group VIB element oxides per m 2 of support, preferably included (limits included) between 3.104 and 16.104 g of Group VIB element oxides per m 2 of support, even more preferably (including limits) between 3.104 and 14.104 g of Group VIB element oxides per m 2 of support, so very preferred (inclusive limits) between 4.104 and 13.104 g of Group VIB element oxides per m 2 of support.

The content, expressed with respect to the total weight of catalyst, of elements of group VIB is preferably comprised (limits included) between 1 and 20% by weight of oxides of group VIB elements, more preferably (inclusive) between 1.5 and 18% by weight of Group VIB element oxides, very preferably (limits included) between 2 and 15% by weight of Group VIB element oxides, even more preferably (limits included) between 2.5 and 12% by weight of Group VIB oxides. Preferably the group VIB element is molybdenum or tungsten or a mixture of these two elements, and more preferably the group VIB element consists solely of molybdenum or tungsten. The element of group VIB is very preferably molybdenum. The content, expressed relative to the total weight of catalyst, of elements of group VIII of the catalyst according to the invention is preferably comprised (limits included) between 0.1 and 20% by weight of oxides of group VIII elements, preferably included (limits included) between 0.2 and 10% by weight of oxides of group VIII elements and more preferably included (limits included) between 0.3 and 5% by weight of group VIII elements oxides . Preferably, the group VIII element is cobalt or nickel or a mixture of these two elements, and more preferably the group VIII element consists solely of cobalt or nickel. The group VIII element is very preferably cobalt. The molar ratio of Group VIII elements to Group VIB elements is generally included (limits included) between 0.1 and 0.8, preferably (inclusive) between 0.2 and 0.6, more preferably included) between 0.3 and 0.5.

The hydrodesulfurization catalyst may further include phosphorus. The phosphorus content is preferably included (limits included) between 0.1 and 10% by weight of P2O5, more preferably (inclusive) between 0.2 and 5% by weight of P2O5, very preferably (limits included). between 0.3 and 4% by weight of P2O5, even more preferably (limits included) between 0.35 and 3% by weight of P2O5, relative to the total weight of the catalyst.

When phosphorus is present, the phosphorus molar ratio on the element of group VIB is generally greater than or equal to 0.25, preferably greater than or equal to 0.27, more preferably included (limits included) between 0.27. and 2, even more preferably included (inclusive) between 0.35 and 1.40, very preferably included (inclusive) between 0.45 and 1.10, even more preferably included (inclusive) between 0.45 and 1.0 or even included (limits included) between 0.50 and 0.95. The catalyst support is a porous solid selected from the group consisting of: alumina, silica, silica alumina or even titanium or magnesium oxides used alone or in admixture with alumina or silica alumina. It is preferably chosen from the group consisting of: silica, the family of transition aluminas and silica alumina, very preferably, the support consists essentially of at least one transition alumina, that is to say it comprises at least 51% by weight, preferably at least 60% by weight, very preferably at least 80% by weight, or even at least 90% by weight of transition alumina. The specific surface of the support used for the preparation of the catalyst, before incorporation of the elements of groups VIB and VIII and optionally shaped and heat treated, is generally less than 200 m 2 / g, preferably less than 200 m 2 / g. 170 m 2 / g, even more preferably less than 150 m 2 / g, very preferably less than 135 m 2 / g, or even less than 100 m 2 / g and even less than 85 m 2 / g. port can be prepared using any precursor, any method of preparation and any shaping tool known to those skilled in the art. Examples In a fixed flow-through bed reactor are charged 1000 cc of NiMo catalyst 8/8 on nickel aluminate support in the form of 2-4 mm spheres. This is firstly sulphurated by injection for 4 h at VVH = 2h-1, at 350 ° C. and 2.5 MPa of a heptane feed containing 4% DMDS under a flow rate of 500 N hydrogen. liters / liters. Under these conditions, the DMDS decomposes to H2S and allows the sulphidation of the catalyst. The feedstock used for the test is an FCC gasoline with an initial boiling point PI = 2 ° C and a boiling point PF = 208 ° C. The operating conditions are the following: - P = 1.0 MPa - T = 100 ° C - VVH = 3h-1 - H2 / HC = 2 N liters / liters A speciation analysis of the sulfur compounds gives us: Compounds Charge (ppm ) Effluent (ppm) RSH C1-03 83 2 Sulfur excluding RSH C1-03 857 938 Total 940 940 It is found that under these conditions a conversion of 97.6% is achieved on mercaptans from Cl to C3. These mercaptans are the sulfur compounds most likely to end up in the light fraction of gasoline after distillation.

Chromatographic analysis of the feedstock and effluent gives the following results for the hydrocarbon families: Compounds Charge Effluent Parrafins (%) 29.0 28.9 Olefins (%) 50.0 49.8 Naphthenes (%) ) 8.8 8.9 Aromatics (%) 12.2 12.2 C5 diolefins (%) 0.31 0.26 MAV (mg / g) 12.1 11.6 It can be seen that olefins have almost no not been hydrogenated between the inlet and the outlet of the reactor. The octane number has not been degraded. In addition, the chromatographic method used makes it possible to identify the C5 diolefins, which come out of the olefin family. These diolefins are: isoprene, 1,3-cispentadiene and 1,3-transpentadiene. Their conversion is about 17% in the reactor. Finally, the measurement of the MAV (Maleic Anhydride Value) tells us about the amount of highly unsaturated compounds present in the effluents of exit. It can be seen that the reactor effluents have a MAV very close to the feedstock. The effluent from the reactor is then separated in a batch distillation column. The effluent is charged to a 100L reboiler heated by resistors while condensation is provided at the top of the column with glycol-added water to prevent light losses. The water containing glycol is at a temperature of 15 ° C.

The column has a diameter of 10 cm and is filled with packing (multiknit pads) over a height of 2 m. The separation is done with a reflux ratio of 15. The separation pressure is the atmospheric pressure. When the top thermocouple reaches the temperature of 65 ° C and the bottom temperature is around 90 ° C, the distillation stops. The target cutting point is 65 ° C. This batch distillation makes it possible to carry out a separation on a pilot scale reproducing the industrial splitter having the following characteristics: 40 theoretical plates; Reflux pressure: 0.9 MPa; Reflux ratio: 0.9; Cutting point: 65 ° C. The light gasoline fraction recovered at the top of the column represents 32.8% by weight of the initial gasoline.

The characteristics of the products recovered at the head (after stripping of the formed H2S) and at the bottom are as follows: Top cut Bottom section Recovery rate (%) 32.8 67.2 PI (° C) 2.0 63 , 0 PF (° C) 64.8 208.0 Paraffins (%) 33.4 26.7 Olefins (%) 65.0 42.4 Naphthenes (%) 0.9 12.8 Aromatic (%) 0.5 17.9 MAV (mg / g) 5.2 14.7 RSH C1-C3 (ppm) 6 0 Sulfur excluding RSH C1-C3 (ppm) 4 1394 Total sulfur (ppm) 10 1394 The combination of the demercaptation reactor with a splitter thus makes it possible to recover an intermediate light fuel cut having a very low sulfur content. The heavy gasoline fraction from the bottom of the splitter is then sent to the catalytic distillation column. This so-called intermediate heavy fuel cut has a MAV a little higher than the load due to the separation. The cut of intermediate heavy gasoline is injected into a catalytic distillation column 5 cm in diameter and 12 m in height.

This column is loaded with 0.75 kg of cobalt and molybdenum-based hydrodesulfurization catalyst supported on sulfide alumina. This catalyst contains 3% by weight of cobalt in oxide form and 10% by weight of molybdenum in oxide form. The feed is injected in the presence of hydrogen so that 70% by weight of the catalyst is below the feed level. The catalytic distillation column operates with the following operating conditions: Column pressure: 1.6 MPa Headboard temperature: 270 ° C Bottom bed temperature: 315 ° C Hydrogen ratio over feed rate: 100 Nm 3 / m3 20 - Reflux ratio: 2 The results on the head cut (after H2S degassing) and bottom are as follows: Head cut Bottom cut Recovery rate (%) 86.4 13.6 Pl (° C) 61.2 142.3 PF (° C) 147.0 208.2 Paraffins (%) 43.3 34.7 Olefins (%) 28.7 16.1 Naphtthenes (%) 11.7 19, 9 Aromatics (%) 16.3 29.3 Total Sulfur (ppm S) 34 3 HDO (%) 36.1 38.9 HDS (%) 97.2 98.4

Claims (15)

REVENDICATIONS1. A process for treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans, said process comprising the following steps: a) performing a demercaptation step by adding at least a portion of the mercaptans to the olefins by contacting the gasoline with at least a first catalyst, at a temperature between 50 and 250 ° C, at a pressure of between 0.4 and 5 MPa and with a liquid space velocity (LHSV) of between 0.5 and 10 h -1, the first catalyst being in sulphide form and comprising a first support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic table of elements, the weight expressed in oxide equivalent of the metal selected from group VIII relative to the total weight of catalyst, is between 1 and 30% and the weight%, expressed as oxide equivalent of the metal selected in the group e Vlb is between 1 and 30% relative to the total weight of catalyst; b) a fractionation of the gasoline from step a) is carried out in a distillation column in at least a first cut of intermediate light gasoline having a total sulfur content lower than that of the starting gasoline and a second cut intermediate heavy gasoline containing the major part of the sulfur compound of departure; c) introducing a flow of hydrogen and at least the second intermediate heavy gasoline fraction resulting from step b) in a catalytic distillation column (14) comprising at least one reaction zone (15) including at least one second catalyst in sulphide form comprising a second support, at least one group VIII metal and a group VIb metal, the conditions in the catalytic distillation column being chosen so as to bring into contact in the presence of hydrogen the intermediate heavy gasoline of step b) with the second catalyst in order to carry out the decomposition of the sulfur compounds into H 2 S, d) the catalytic distillation column is removed from at least one final light gasoline fraction comprising H 2 S and a fraction of desulphurized heavy gasoline, the final light gasoline fraction being discharged at a point above the reaction zone and the desulfurized heavy gasoline fraction being discharged at a point s below the reaction zone. 2. Method according to claim 1, wherein the contacting with the second catalyst in step c) is carried out at a pressure of between 0.1 and 4 MPa, preferably between 1 and 3 MPa, and at a temperature of between 210 and 350 ° C, preferably between 220 and 320 ° C. 3. A process according to claim 1 or 2, wherein the first catalyst comprises nickel and molybdenum, with a weight of nickel, expressed as oxide, based on the total weight of the catalyst is between 1 and 30% and % by weight of molybdenum, expressed as oxide, relative to the total weight of catalyst is between 1 and 30%. 4. Process according to claim 3, in which the weight of nickel, expressed as oxide, relative to the total weight of catalyst, is between 4 and 12% and the weight of molybdenum, expressed as oxide, relative to the total weight of catalyst is between 6 and 18% 5. Process according to one of the preceding claims, wherein the second catalyst comprises cobalt and a group VIb metal selected from molybdenum and tungsten. 6. Method according to one of the preceding claims, wherein the second catalyst comprises a weight% of Group VIII metal, expressed as oxide, relative to the total catalyst weight between 0.5 and 15%. 7. Method according to one of the preceding claims, wherein the second catalyst comprises a metal content of the group VIb, expressed in% by weight relative to the total weight of catalyst, between 1.5 and 60%. 25 8. Method according to one of claims 5 to 7 wherein the second catalyst comprises cobalt and molybdenum. 9. Method according to one of the preceding claims wherein the fraction of desulfurized heavy gasoline from step c) is partly recycled by being introduced into the catalytic distillation column (14). 10. A method according to one of the preceding claims wherein is carried a top of a hydrocarbon fraction as a recycle stream in the catalytic distillation column (14) so as to maintain a liquid phase in the bottom of said column. 11. The method as claimed in claim 1, in which step a) is carried out at a pressure of between 0.6 and 1 MPa and with a flow rate ratio of H2 / charge rate of between 0.5 and 2. Nm3 of hydrogen per m3 of charge. 12. Method according to one of the preceding claims wherein the distillation column comprises a first and a second catalytic bed respectively comprising a cobalt-molybdenum type catalyst and a nickel-molybdenum type catalyst, the first catalyst being disposed above of the second catalyst. 13. Method according to one of the preceding claims wherein in step b) is further carried out a side withdrawal by a line (25) whose withdrawal point is disposed in the column at a level between the withdrawal in head and withdrawal at the bottom of the distillation column, a gasoline cut whose boiling temperature range extends in the interval between the final boiling point of the first intermediate light gasoline cut and the d-point initial boiling of the intermediate heavy gasoline cut. 20 14. The method of claim 13, wherein the gasoline cut off laterally is treated in a catalytic hydrodesulfurization unit in the presence of hydrogen. 15. Process according to one of the preceding claims, in which the treated gasoline is obtained from a catalytic cracking unit.