WO2014013154A1 - Method of petrol desulphurisation - Google Patents

Description

 METHOD OF DESULFURIZING A GASOLINE

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 their concentration in aromatics (especially benzene) and in sulfur be significantly increased. 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. including at least a 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 1 174485.

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 sulphide of hydrogen (H ₂ S). The reaction mixture is then cooled to condense the gasoline. The gaseous phase containing the excess hydrogen and H ₂ S is separated and the desulfurized gasoline is recovered.

The residual sulfur compounds generally present in the desulfurized 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 secondary recombination reactions. Among the latter family of sulfur compounds, the major compounds are the mercaptans resulting from the addition of H ₂ S formed in the reactor on mono-olefins present in the feedstock. The mercaptans of chemical formula R-SH where R is an alkyl group, are also called recombinant mercaptans, and generally represent between 20% by weight and 80% by weight of the residual sulfur in the desulphurized species.

The selective hydrogenation step described in the patent application EP 1077247 is essential in order to prevent the progressive deactivation of the selective hydrodesulphurization catalyst, to avoid a gradual plugging of the reactor by the formation of polymerization gums on the surface of the catalysts or in the reactor and to avoid too fast 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 excess of hydrogen consumption 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 step and the selective hydrodesulfurization step of the process of patent EP 1077247, insofar as an excess Hydrogen is generally 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 (approximately C5-175 ° C) containing olefins, diolefins, mercaptans and heavy organic sulfur compounds. The process implements a first thioetherification step in which the mercaptans are react 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 the 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 tralée effectively in a thioetherification reactor because of high levels sulfur which is a poison of thioetherification catalysts, especially nickel catalysts (see US Pat. No. 7,638,041) or palladium-based (see US Pat. No. 5,595,634).

An object of the invention is therefore to propose a method for producing a gasoline, from a broad spectrum of gasoline type in terms of boiling temperature range, to low sulfur content, that is to say 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 loss of octane number of the invention

 To this end, there is provided a process for treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans, said process comprising the following steps:

a) a step of demercaptation is carried out by adding at least a portion of the mercaptans to 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 between 0.4 and 5 MPa and an LVVH between 0.5 and 10 h ^-1 , the first catalyst being in sulphide form and comprises a first support, at least one metal selected from group VIII and at least one metal selected in the group VIb of the periodic table of the elements, the weight, expressed as 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 the oxide equivalent of metal selected from group VIb is between 1 and 30% based on 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) 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, 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 step b) with the second catalyst in order to carry out the decomposition of sulfur 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 desulfurized heavy gasoline fraction, the final light gasoline fraction being discharged at a point above of the reaction zone and the desulfurized heavy gasoline fraction being discharged at a point below the reaction zone.

The method 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 higher boiling sulphides

or by a hydrogenolysing route: the hydrogen present in the reactor will produce H ₂ S 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 lightest mercaptans are the most reactive in this step a).

On the other hand, H ₂ S, 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 H ₂ S in the overhead gases of step b). which very largely contain unreacted hydrogen in step a), which is interesting to recycle in step a). This recycling, made possible by the absence of H ₂ S 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 preferably carried out on a catalyst comprising at least one metal 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 Group VIb metal (Group 6 of the new Periodic Handbook of Chemistry and Physics, 76th Edition, 1995-1996) and a carrier.

Before contacting with the charge 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 hydrocarbons having, for example, a boiling point greater than 84 ° C., the heavy sulfur compounds (of the family of thiophenes, sulphides, disulfides) initially present in the gasoline to be treated. as well as sulfur compounds essentially of the sulphide type which were formed during step a) by addition of 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 that make it possible to react the sulfur compounds of the feed (mercaptans, sulphides and thiophene compounds) with hydrogen to form H ₂ S. Simultaneously with the step c) in the catalytic distillation column, a step d) is carried out in which the feedstock consisting of at least the second intermediate heavy fraction resulting from the fractionation step b), optionally mixed with a recycling stream, is separated into 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 H ₂ S 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 column. catalytic.

 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 fuel cut accompanied by H ₂ S 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 cut drawn 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 valued 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 to higher molecular weight compounds in step a), so that they are entrained 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 specific to high conversions of demercaptisation. 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 H ₂ S 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 H ₂ S produced by the hydrodesulfurization reactions and thus contributes to limit the formation of recombinant mercaptans by adding hydrogen sulphide to the olefins still present. Detailed description of the invention

The present invention relates to a process for the desulfurization of a gasoline having a limited sulfur content from a gasoline, preferably from a catalytic cracking, coking or visbreaking unit. Gasoline can be an essence of total cracking (C5-220 ° 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 method 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. , 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 that simultaneously allow:

• reacting sulfur compounds in the feed (mercaptans, sulphides and thiophene compounds) with hydrogen to form H ₂ S

Separating the feedstock containing at least the intermediate heavy gasoline fraction resulting 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, which contains most of the olefins and a heavy desulphurized cut also with very low sulfur content.

In the context of the present application, the term "catalytic column" denotes 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 inside 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 essence to treat

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 from about the points of contact. boiling of the 2- or 3-carbon hydrocarbons (C2 or C3) to about 250 ° C, more preferably from about 5-carbon hydrocarbon boiling points 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 method according to the invention can also treat a so-called "light" gasoline charge 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 180C, lower or 160 ° C or less 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 feed, the presence or absence of FCC feed pretreatment, and the end point of the cut. . 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 mercaptans with olefins

This step consists in transforming the light sulfur compounds of the mercaptan family, that is to say the compounds which, after the fractionation step b), would be in the light gasoline, into heavier sulfur compounds. which 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 mercaptans which can react during step a) are the following (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, oc-ethylpropyl mercaptan, n-hexyl mercaptan, 2-mercapto hexane.

The demercaptation reaction is preferably carried out on a catalyst comprising at least one metal 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 metal of the group Vlb (group 6 of the new periodic table Handbook of Chemistry and Physics, 76th edition, 1995-1996) and a support. The Group VIII metal is preferably chosen from nickel and cobalt and in particular nickel. The metal of group VIb 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 delta with a specific surface area of between 70 m ² / g and 350 m ² / g

 A content by weight of oxide of the group VIb metal of between 1 and 30% by weight relative to the total weight of the catalyst,

 A content by weight of oxide of the group VIII metal of between 1 and 30% by weight relative to the total weight of the catalyst,

A sulphurization 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 gamma-alumina with a specific surface area of between 180 m ² / g and 270 m ² / g

 The oxide weight content of the metal of group VIb 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 NiO form) of between 4 and 12%, a content by weight of molybdenum oxide. (in MoO ₃ form) 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 degree of sulfurization 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 metals of groups VIII and VIb on the selected support.

After introduction of the metals of groups VIII and VIb, and possibly shaping of the catalyst, it undergoes an activation treatment. This treatment generally aims to convert the molecular precursors of the metals into 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. 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 MoO ₃ 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 H ₂ S 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 H ₂ S 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 H ₂ / charge rate of between 0 and 25 Nm ³ of hydrogen per m ³ of filler, preferably between 0 and 10 Nm ³ of hydrogen per m ³ charge, very preferably between 0 and 5 Nm ³ of hydrogen per m ³ of filler, and even more preferably between 0.5 and 2 Nm ³ 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., more preferably still between 90 ° C. and 200 ° C. liqiide with a space velocity (LHSV) of from 0.5 hr ^"1 to 10 ^h", the unit of the liquid space velocity being per liter of feed per liter of catalyst per hour (l / lh). 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 entrained in 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 of intermediate light fuel and the initial boiling point of intermediate heavy gasoline. Said intermediate gasoline can then be treated by hydrodesulfurization in a dedicated reactor and then mixed with the intermediate light gasoline.

Step c) and d) hydrodesulfurization in a catalite column

The desulfurization 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, onto at least one catalyst at least partly in sulphide form comprising at least a Group VIII metal, at least one Group VIb metal 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 H ₂ flow / charge rate in the column is between 25 to 400 Nm ³ per m ³ of liquid charge and preferably between 40 and 100 Nm ³ per m ³ of liquid charge.

The group VIII metal is preferably cobalt or nickel, and the group VIb metal 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. The said solution is chosen so as to solubilize the precursors of metals 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 catalytic 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 stage of separation, the temperatures being dependent only 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 fuel cut is recovered at the top of the catalytic column with the H ₂ S produced by the desulfurization and the unreacted hydrogen while the heavy desulfurized fraction is withdrawn 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. Preferably, the final light gasoline section is cooled with the H ₂ S produced by the desulfurization reactions and the unreacted hydrogen to a temperature generally below 60 ° C in order to condense hydrocarbons. The gas phases (mainly containing the produced H ₂ S and the 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 the implementation of the distillation of the feedstock and also allows a 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 H ₂ S and in 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 heavy gasoline cut desulfurized 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 valued for the gasoline pool. This configuration is preferably used when the gasoline cut injected in step a) at the inlet of the whole 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 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, 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 recapitulated cups at the outlet of the catalytic column can then be mixed and then sent to a stripper in order to remove the last traces of H ₂ S solubilized to be finally intended for the gasoline pool.

In a second configuration, the feed of step c) comprises the intermediate heavy gasoline cut 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 entrance of the entire sequence is a light gasoline having a final boiling point of less than 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 stage b) has a final boiling point of approximately 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 catalytic column under the selected operating conditions. This recycling also makes it possible to increase the rate of washing of the catalyst in the bottom, which is favorable when the feed 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 recycling gasoline cutoff typically has a distillation range from the end point of the batch to be treated, that is to say in a range from 145 ° C to about 210 ° C for this configuration, up to a temperature of 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 catalytic column having a catalyst having properties of both hydrodesulfurization and hydrogenation. A catalyst comprising at least one Group VIII metal and at least one Group VIb metal in sulphide form, wherein the group VIII metal is preferably nickel and the Group VIb metal is molybdenum. The catalyst located in the upper zone is, on the other hand, preferably a cobalt molybdenum type catalyst.

In a third configuration, three different cuts are withdrawn from the catalytic column:

 - a final cut of light gasoline desulphurised drawn at the top of the column and recoverable towards the gasoline pool;

 a heavy desulfurized cut drawn off at the bottom of the column, the bulk of which will be recycled to the catalytic distillation column;

 - And a complementary desulfurized gasoline cut drawn at a point between the withdrawal of the final light gasoline desulphurized and racking 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 gasoline cut injected in step a) at the input of the entire sequence is a total gasoline, that is to say whose range of boiling points extends. typically from about 2 to 3 carbon atoms (C2 or C3) boiling points up to about 250 ° C, or preferably from about 2 to 3 carbon atom boiling points of boiling point; (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. Indeed a higher temperature operation 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 cuts recovered at the catalytic column outlet (final light gasoline and complementary desulfurized gasoline) can then be mixed and sent to a stripper in order to eliminate the last traces of H ₂ S solubilized to be finally stored in the gasoline pool. Brief description of the figures

 These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, with reference to the drawings of the figures, in which:

 FIG. 1 shows a first diagram of the method 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 a supplement of 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 carry out the selective addition of the mercaptans to the 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 carried out at a temperature of between 50 and 250 ° C., at a pressure of between 0.6 and 2 MPa and a liquid space velocity of between 0.5 h ^-1 and 10 h 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 gasoline cut intermediate 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 incondensables, mainly hydrogen, is withdrawn at the top of the separator through the pipe 9 while the liquid gasoline fraction is drawn off at the bottom by the line 10, part of which serves to feed the gasoline pool (via the pipe 1 1) and another part corresponds to the reflux of the distillation.

 The intermediate heavy gasoline fraction which is drawn off at the bottom of the fractionation column 4 and which contains most of the sulfur products including those generated during the demercaptation stage 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 in H ₂ S in the presence of hydrogen selectively 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 metal and at least one Group VIb metal. 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 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 final desulfurized heavy gasoline cut and a final desulfurized light gasoline cut. The two final desulfurized light gasoline and final desulfurized heavy gasoline fractions can then be sent to a stripper in order to remove the last traces of solubilized H ₂ S (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 the sulfur compounds to H ₂ S. Concomitant with the conversion reaction is a fractionation of the intermediate heavy gasoline producing a final light gasoline fraction comprising H ₂ S resulting from the decomposition of the sulfur compounds. . The final light gasoline is withdrawn at the top of the distillation column through line 17.

The final light gasoline distilling at the top of the column accompanied by the H ₂ S 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 by the pipe 19 in a gas / liquid separator 20 where are separated a gaseous effluent comprising essentially hydrogen and H ₂ S (via the pipe 21) and a desulphurized liquid gasoline. 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 column of catalytic distillation. 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 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 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.

A third embodiment of the method according to the invention is shown in FIG. 3. This embodiment differs essentially from the second mode in that there is a withdrawal of a complementary heavy duty gasoline fraction via the line 28. at a point between the supply and the bottom of said column.

A fourth embodiment of the process is shown in FIG. 4. This embodiment takes up the characteristics 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 petrol cut is withdrawn laterally from distillation column 4 via line 25. The draw-off point via line 25 is placed in the column at a level between the overhead withdrawal via 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, in particular mercaptans and thiophene-type compounds present in said H ₂ S section. 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 metal (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and at least one a metal of group Vlb (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 metals per unit area of the support included (limits included) between 2.10 ^-4 and 18.10 ⁴ g of Group VIb metal oxides per m ² of support, preferably inclusive) between ³ × ^{10 -4} and 16 × 10 ⁴ g of Group VIb metal oxides per m ² of support, even more preferably included (limits included) between 3 × 10 ⁴ and 14 10 ^-4 g of metal oxides of the group VIb per m ² of support, very preferably included (limits included) between 4.10 ^-4 and 13.10 ⁴ g of Group VIb metal oxides per m ² of support.

 The content, expressed relative to the total weight of catalyst, of Group VIb metals is preferably between 1 and 20% by weight of Group VIb metal oxides, more preferably (limits included) between 1 and 20% by weight. 5% and 18% by weight of Group VIb metal oxides, very preferably (limits included) between 2 and 15% by weight of Group VIb metal oxides, even more preferably (inclusive) between 2, 5 and 12% by weight of Group VIb metal oxides. Preferably, the group VIb metal is molybdenum or tungsten or a mixture of these two metals, and more preferably the group VIb metal consists solely of molybdenum or tungsten. The metal of group VIb is very preferably molybdenum.

 The content, expressed relative to the total weight of catalyst, of Group VIII metals of the catalyst according to the invention is preferably comprised (limits included) between 0.1 and 20% by weight of Group VIII metal oxides, preferably (inclusive ranges) between 0.2 and 10% by weight of Group VIII metal oxides and more preferably (inclusive) between 0.3 and 5% by weight of Group VIII metal oxides. Preferably, the group VIII metal is cobalt or nickel or a mixture of these two metals, and more preferably the group VIII metal consists solely of cobalt or nickel. The Group VIII metal is very preferably cobalt.

The molar ratio of Group VIII metals to Group VIb metals 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 (inclusive) between 0.1 and 10% by weight of P ₂ O ₅ , more preferably (inclusive) between 0.2 and 5% by weight of P ₂ O ₅ , very preferably ( inclusive) between 0.3 and 4% by weight of P ₂ 0 ₅ , even more preferably (inclusive) between 0.35 and 3% by weight of P ₂ 0 ₅ , based on the total weight of the catalyst.

 When the phosphorus is present, the phosphorus molar ratio on the metal 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. It may optionally consist solely of a transition alumina.

The specific surface of the support used for the preparation of the catalyst, before incorporation of the metals of groups VIb and VIII and optionally shaped and heat treated, is generally less than 200 m ² / g, preferably less than 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. The carrier may be prepared using any precursor, method of preparation and any shaping tool known to those skilled in the art.

Examples

In a fixed flow-down bed reactor are charged 1000 cc of a NiMo 8/8 catalyst on a nickel aluminate support in the form of 2-4 mm spheres. This is firstly sulphurated by injection for 4 h at VVH = 2 h ^-1 , at 350 ° C. and 2.5 MPa of a heptane feed containing 4% DMDS at a flow rate of 500 N hydrogen. liters / liters. Under these conditions, the DMDS decomposes to H ₂ S 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 as follows:

 - P = 1.0 MPa

 - T = 100 ° C

- VVH = 3h ^"1

- H ₂ / HC = 2 N liters / liter

A speciation analysis of sulfur compounds gives us:

It is found that under these conditions a conversion of 97.6% is achieved on the mercaptans going from C1 to C3. These mercaptans are the sulfur compounds most likely to end up in the light fraction of gasoline after distillation.

The chromatographic analysis of the feedstock and the effluent gives the following results for the hydrocarbon families:

It is found that the olefins have hardly 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 is found that the reactor effluents have a MAV very close to the load

The effluent from the reactor is then separated in a distillation column in batch mode.

The effluent is charged to a 100L reboiler heated by resistors while the condensation is provided at the top of the column with water added with glycol in order to avoid 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 plateaus;

 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 H ₂ S formed) and at the bottom are the following: Cup of head Cup of bottom

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 with a very low sulfur content. The heavy gasoline cutoff 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 alumina in sulphide form. 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 head pressure: 1, 6 MPa

- Headboard temperature: 270 ⁰ C

 - Bottom bed temperature: 315 ° C

- Hydrogen ratio on flow of load: 100 Nm ³ / m ³

 - Reflux ratio: 2

The results on head cutting (after outgassing of H ₂ S) and background are as follows:

Cup of head Cup of bottom

Recovery rate (%) 86.4 13.6

PI (° C) 61, 2,142.3

PF (° C) 147.0 208.2

Paraffins (%) 43.3 34.7

Olefins (%) 28.7 16.1

Naphthenes (%) 1 17 19.9

Aromatic (%) 16.3 29.3

Total sulfur (ppm S) 34 3

H DO (%) 36.1 38.9

HDS (%) 97.2 98.4

Claims

1. 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 comprises 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 first catalyst having the characteristics following: A support consisting of gamma-alumina having a specific surface area of between 180 m ² / g and 270 m ² / g;  "The group VIb metal content expressed in oxide form is between 4 and 20% by weight relative to the total weight of catalyst;  The metal content of group VIII, expressed as oxide, is between 3 and 15% by weight relative to the total weight of catalyst;  The molar ratio between the Group VIII metal and the Group VIb metal is between 0.6 and 3 mol / mol; 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 with the Group VIII metal content expressed in oxide of between 0.5 and 25% by weight relative to the weight of the catalyst and with the group VIb metal content, expressed as oxide between 1.5 and 60% by weight relative to the weight of catalyst, the conditions in the catalytic distillation column being chosen so as to bring into contact in the presence of hydrogen the intermediate heavy gasoline from step b) with the second catalyst for effecting the decomposition of the sulfur compounds into H ₂ S, the contacting with the second catalyst being carried out at a pressure of between 0.1 and 4 MPa and at a temperature of between 210 and 350 ° C; d) removing from the catalytic distillation column at least one final light gasoline fraction comprising H ₂ S and a desulfurized heavy gasoline fraction, the final light gasoline fraction being discharged at a point above of the reaction zone and the desulphurized heavy gasoline fraction being discharged at a point below the reaction zone. 2. The method of claim 1, wherein the contacting with the second catalyst in step c) is carried out at a pressure of between 1 and 3 MPa and at a temperature between 220 and 320 ° C. 3. Process according to claims 1 or 2, wherein the first catalyst comprises nickel and molybdenum and in which the weight of nickel, expressed as oxide, relative to the total weight of catalyst is between 4 and 12% and the % by weight of molybdenum, expressed as oxide, relative to the total weight of catalyst is between 6 and 18% 4. Method according to one of the preceding claims, wherein the second catalyst comprises cobalt and a Group VIb metal selected from molybdenum and tungsten. 5. 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 1 and 10%. 6. Method according to one of the preceding claims, wherein the second catalyst comprises a weight% of Group VIb metal, expressed as oxide, relative to the total weight of catalyst, of between 3 and 50%. 7. Method according to one of claims 4 to 6, wherein the second catalyst comprises cobalt and molybdenum. 8. Method according to one of the preceding claims wherein the desulfurized heavy gasoline fraction from step c) is partly recycled by being introduced into the catalytic distillation column (14). 9. Process according to one of the preceding claims, in which an addition of a hydrocarbon fraction serving as a recycle stream is carried out in the catalytic distillation column (14) so as to maintain a liquid phase in the bottom of said column. . 10. Process according to one of the preceding claims, in which step a) is carried out at a pressure of between 0.6 and 1 MPa and with a flow ratio of H ₂ / charge flow rate of between 0.5 and 2. Nm ³ of hydrogen per m ³ of charge. 1 1. Process 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 the second catalyst . 12. Method according to one of the preceding claims wherein in step b) is further carried out a lateral withdrawal by a line (25) whose draw-off point is arranged in the column at a level between the top racking. and the withdrawal at the bottom of the distillation column, of 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 dash point. initial boiling of the intermediate heavy gasoline cut. 13. The method of claim 12, wherein the gasoline cut off laterally is treated in a catalytic hydrodesulfurization unit in the presence of hydrogen. 14. Method according to one of the preceding claims wherein the treated gasoline is from a catalytic cracking unit.