EP1370627B1 - Method for producing petrol having a low sulphur content - Google Patents

Abstract

The invention relates to a method for producing petrol having a low sulphur content comprising at least one selective hydrogenation of diolefins, optionally at least one transformation stage, preferably an increase stage, of light sulphur compounds present in petrol, at least one fractioning of the petrol obtained into at least two fractions, light petrol and heavy petrol, then an optional transformation stage, preferably alkylation or adsorption of sulphur compounds and a one-stage desulphurization treatment of at least one part of the heavy fraction.

Description

The invention relates to a method for producing low sulfur gasoline comprising hydrogenation, fractionation, a step of converting sulfur compounds and desulfurization. This method makes it possible to upgrade a gasoline cutoff optionally further comprising hydrocarbons having two three or four carbon atoms, reducing the total sulfur content of said cut to very low levels compatible with current or future specifications. This desulphurization is also carried out without a substantial reduction in fuel efficiency and minimizing the reduction of the octane number.

Prior 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.

Hydrotreating (hydrodesulphurisation) of the feedstock sent to catalytic cracking leads to gasolines typically containing 100 ppm of sulfur. The catalytic cracking feed hydrotreating units, however, operate under severe temperature and pressure conditions, which implies a high hydrogen consumption and a high investment. In addition, the entire charge must be desulfurized, resulting in the processing of very large load volumes.

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

On the other hand, the patent US-A-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 metal group VIB and / or group VIII. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three cuts, and when the intermediate boiling cut is treated under mild conditions.

The patent application EP-A-0 755 995 discloses a FCC gasoline desulfurization process comprising at least two steps. The first is a catalytic hydrodesulfurization at a temperature between 200 and 350 ° C, with a desulphurization rate of between 60 and 90% and is carried out in the presence of a filler containing less than 0.1% by volume of sulfide. hydrogen (H2S). The second, and optionally the following, are also catalytic hydrodesulfurization steps operated between 200 and 300 ° C and in the presence of a feed comprising less than 0.05% by volume of H2S. The desulfurization rate is between 60 and 90% in this step. In this process, the concentration of H 2 S must be kept at a very low level. In the case of recycling the excess hydrogen, it is therefore generally necessary to remove the H2S, for example by means of an amine absorption step, after the second step and the following, so that the recycle gas contains less than 0.1% H2S volume. It is also preferred to remove the H2S between the first and second stages in order to respect the maximum content of H2S entering the second stage (0.05% volume). For gasoline loaded with sulfur, such an elimination is therefore necessary, taking into account the desulphurisation rate greater than 60% in the first stage.

The patent application EP-A-0 725 126 discloses a method for hydrodesulfurizing a cracking gasoline in which the gasoline is separated into a plurality of fractions comprising at least a first fraction rich in easily desulfurized compounds and a second fraction rich in difficult to desulphurize compounds. Before carrying out this separation, it is first necessary to determine the distribution of the sulfur compounds by means of analyzes. These analyzes are necessary to select the equipment and the separation conditions.

In this application it is thus indicated that a light fraction of cracking gasoline sees its olefin content and octane drop significantly when it is desulfurized without being fractionated. On the other hand, the fractionation of the said light fraction in 7 to 20 fractions, followed by analyzes of the sulfur and olefin contents of these fractions, makes it possible to determine the fraction or fractions richest in sulfur compounds which are then desulphurized simultaneously or separately. and mixed with the other desulfurized fractions or not. Such a procedure is complex and must be reproduced with each change in the composition of the gasoline to be treated.

In the French patent 2,785,908 , it is taught the interest of splitting the gasoline into a light fraction and a heavy fraction then to perform a specific hydrotreatment of light gasoline on a nickel-based catalyst, and a hydrotreatment of the heavy gasoline on a catalyst comprising at least one Group VIII metal and / or at least one Group VIb metal.

It has also been proposed, for example in the patent US-A-5,290,427 methods for hydrotreating gasolines by fractionating the gasoline, then introducing the fractions at different levels of a hydrodesulfurization reactor and converting the desulfurized fractions to a zeolite ZSM-5. to compensate for the loss of octane recorded by means of isomerization. This isomerization is accompanied by a cracking of gasoline towards lighter compounds.

In these processes, the gasolines to be treated generally have an initial point greater than 70 ° C, and again it is necessary to treat the light gasoline separately (fraction corresponding to the compounds of boiling point between C5 hydrocarbons having 5 carbon atoms and 70 ° C), for example by means of softening.

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

As a result, the olefins present in a relatively large amount in the heavy fraction are partly saturated during the hydrotreatment. To compensate for the drop in the octane number related to the hydrogenation of olefins, the patent advocates cracking zeolite ZSM-5 which produces olefins, but at the expense of yield. In addition, these olefins can recombine with the H2S present in the medium to reform mercaptans. It is then necessary to perform additional softening or hydrodesulfurization.

The patent application WO 00/15319 discloses a process for simultaneously fractionating and treating a light naphtha. In this process, the light cut contains mercaptans generally ranging from methyl mercaptan to hexyl mercaptan. These sulfur compounds are removed from the light fraction only in the case where the fractionation column contains a hydrodesulfurization section at the top of the column. In the absence of this section, it is therefore not possible to eliminate the mercaptans which either end up in the desulfurized gasoline when the light fraction is recombined with the heavy desulphurized fraction, or can be eliminated with the whole of the light fraction, which causes a loss of the gasoline yield after desulphurization. ,

The patent US 6,083,379 describes a process for the desulphurization and improvement of the octane number of gasolines comprising a fractionation of the gasoline into at least two sections, the treatment of the light fraction in the presence of a zeolite, a fractionation of the light fraction thus treated, the mixture of heavy obtained during the two fractionation steps and the hydrodesulfurization of the mixture of these fractions.

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

The patent US 5,968,346 discloses a method of hydroconversion of a hydrocarbon feedstock for removing impurities such as compounds comprising heteroatoms. This method comprises a first step of hydroconversion of the entire charge, followed by a separation of the liquid and the vapor present in the effluent of this first step and a contact of the gas with a liquid. The mixture of the two liquid fractions resulting from the contacting and the fractionation is then treated in a second hydroconversion stage in the presence of a catalyst.

Summary of the invention

The present invention relates to a process for the production of gasolines with a low sulfur content, which makes it possible to recover the whole of a sulfur-containing gasoline fraction, preferably a gasoline catalytic cracking or coking gasoline fraction (coking according to English terminology). , or pyrolysis, or visbreaking (visbreaking according to English terminology), and reduce sulfur levels in said gasoline cut at very low levels, without significant reduction in gasoline yield while minimizing the decrease in the index of octane due to the hydrogenation of olefins. The feedstock of the process according to the invention may also optionally comprise, in addition to a gasoline cut, a C4 'cut comprising hydrocarbons containing two, three or four carbon atoms.

• a) au moins une hydrogénation sélective des dioléfines présentes dans l'essence initiale.
• b) au moins une étape de transformation chimique des composés soufrés légers présents dans l'essence, de préférence cette transformation chimique vise à alourdir, c'est-à-dire à augmenter le poids moléculaire dedits composés soufrés, de préférence essentiellement des composés soufrés présentant un point d'ébullition inférieur à celui du thiophène. Cette étape peut être éventuellement réalisée simultanément à l'étape a sur tout ou partie de essence initiale, dans le même réacteur ou un réacteur différent. Elle peut également être effectuée de façon séparée sur tout ou partie de l'essence hydrogénée à l'étape a. Elle peut enfin être réalisée simultanément à l'étape c de fractionnement décrite ci-après. Dans cette étape, le thiophène et les composés thiophèniques sont peu transformés.
• c) au moins un fractionnement de l'essence obtenue à l'étape a ou b en au moins deux fractions (ou coupes), une fraction légère de préférence pratiquement dépourvue de soufre et contenant les oléfines les plus légères de l'essence initiale (essence légère ou fraction légère), une fraction intermédiaire et une fraction lourde dans laquelle de préférence la majeure partie des composés soufrés initialement présents dans l'essence initiale est concentrée (essence ou fraction lourde).
• d) une étape de transformation des composés soufrés différente de l'étape b. Il s'agit de préférence d'une étape d'alkylation ou d'adsorption des composés soufrés choisis dans le groupe constitué par le thiophène, les composés thiophéniques et les mercaptans, de préférence les mercaptans ayant 1 à 6 atomes de carbone, présents dans au moins une coupe obtenue à l'étape c, de préférence dans la coupe légère et/ou dans au moins une coupe intermédiaire. Les mercaptans éventuellement présents sont soit formés à l'étape a et/ou b, soit présents dans l'essence initiale et non convertis à l'étape a et/ou b.
• f) un traitement de désulfuration en une étape d'au moins une partie de la fraction lourde, issue du fractionnement à l'étape c ou de l'étape d de transformation des composés thiophèniques et éventuellement des mercaptans, en présence d'au moins un catalyseur d'hydrodésulfuration ou d'un absorbant.
• g) éventuellement une étape de mélange de la fraction légère issue de l'étape c ou d, et éventuellement d'au moins une fraction intermédiaire, issue de l'étape c ou d, avec la fraction lourde désulfurée issue de l'étape f.

The process according to the invention is a process for producing gasoline with a low sulfur content, from a gasoline cut containing sulfur (initial gasoline). It includes at least the following steps:

• a) at least one selective hydrogenation of the diolefins present in the initial gasoline.
• b) at least one step of chemical transformation of the light sulfur compounds present in the gasoline, preferably this chemical conversion aims to increase, that is to say to increase the molecular weight of sulfur compounds, preferably substantially sulfur compounds having a boiling point lower than that of thiophene. This step may optionally be carried out simultaneously in step a on all or part of initial gasoline, in the same reactor or a different reactor. It can also be carried out separately on all or part of the hydrogenated gasoline in step a. Finally, it can be carried out simultaneously with the fractionation step c described below. In this step, thiophene and thiophene compounds are poorly processed.
• c) at least one fractionation of the gasoline obtained in step a or b in at least two fractions (or cuts), a light fraction preferably substantially free of sulfur and containing the lighter olefins of the initial gasoline ( light gasoline or light fraction), an intermediate fraction and a heavy fraction in which most of the sulfur compounds initially present in the initial gasoline are concentrated (gasoline or heavy fraction).
• d) a step of transforming the sulfur compounds different from step b. It is preferably a step of alkylation or adsorption of sulfur compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans, preferably mercaptans having 1 to 6 carbon atoms present in at least one cut obtained in step c, preferably in the light cut and / or in at least one intermediate cut. The mercaptans possibly present are either formed in step a and / or b, or present in the initial gasoline and not converted to step a and / or b.
• f) a desulfurization treatment in one step of at least a portion of the heavy fraction, resulting from fractionation in step c or step d of transformation of thiophene compounds and optionally mercaptans, in the presence of at least a hydrodesulfurization catalyst or an absorbent.
• g) optionally a step of mixing the light fraction resulting from step c or d, and possibly at least one intermediate fraction, resulting from step c or d, with the heavy fraction desulphurized resulting from step f .

Preferably, all of the heavy gasoline desulphurized from step f is mixed with the light gasoline from step c or d, without separation of the liquid and the gas contained in the heavy gasoline after desulphurization, optionally a simple stripping with an inert gas can be carried out to remove H2S from the totally desulfurized heavy gasoline. In certain specific cases, the recovery of light gasoline, desulphurised heavy gasoline, and possibly at least one intermediate gasoline, is carried out separately. It is then unnecessary to perform step g.

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

Detailed Description the invention:

It is described in the present invention a method for obtaining a gasoline preferably from a catalytic cracking unit, coking or deionization. visbreaking and having a limited sulfur content in which the gasoline is first subjected to a selective hydrogenation treatment diolefins, then optionally a step of processing the lighter sulfur compounds of gasoline which should after fractionation be in the light species so that they are essentially in the heavy fraction after the fractionation step of the process according to the invention. The thus treated gasoline then undergoes a fractionation in at least two cuts. Optionally, at least one fraction resulting from the fractionation step, preferably the light fraction or an intermediate fraction, may be treated in a step of conversion of the sulfur compounds chosen from the group consisting of thiophene, thiophene compounds and mercaptans. . Said step is preferably an alkylation or adsorption step.

The heavy gasoline is treated in a desulfurization section, preferably in the presence of a hydrodesulfurization catalyst or an absorbent. No desulfurization of the light fraction is necessary in the process according to the invention, since most of the sulfur compounds initially present in the gasoline are found in the heavy fraction and optionally in the intermediate fraction or fractions after the steps of hydrogenation, transformation of light sulfur compounds (step b), fractionation (step c), and transformation of sulfur compounds including thiophene and mercaptans, in particular unconverted residual mercaptans and / or formed in steps a and b (step d); ).

This sequence makes it possible ultimately to obtain a desulphurized gasoline without significant reduction in the olefin content or octane number, even for high desulphurization rates, and this without it being necessary to treat light gasoline. using a hydrodesulphurization or softening section, or using methods to restore the octane number of gasoline. Thanks to this process, high desulphurization rates are achieved under reasonable operating conditions specified below.

The fractionation point of the gasoline is preferably limited in order to avoid the presence of thiophene in light gasoline. Since the latter forms azeotropes with a certain number of hydrocarbons, only C5 olefins and a small part of the C6 olefins can be separated into the light gasoline. too much thiophene fraction in this section. In this case, it is advantageous to add to the process according to the invention a step d of transformation of thiophene and more generally of thiophene compounds, for example by means of an adsorption section or an alkylation reactor coupled to the fractionation section or integrated in said section according to a detailed mode in the following description.
In order to make it possible to recover a larger fraction of the olefins present in the light gasoline while limiting the sulfur content of this fraction without additional treatment, it is preferably proposed to treat the feed in step b and / or d in conditions and on catalysts which make it possible to convert the sulfur-containing compounds, preferably the light sulfur-containing compounds, into higher-boiling sulfur-containing compounds found after separation, optionally in at least an intermediate fraction or in heavy gasoline These intermediate and / or heavy cuts can then be desulfurized.

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.
The method according to the invention is particularly applicable when high desulphurization rates of gasoline are required, ie when the desulphurized gas must contain at most 10% of the sulfur of the initial gasoline and possibly at most 5% or even more than 2% of the sulfur of the initial gasoline which corresponds to desulfurization rates greater than 90% or even greater than 95 or 98%.

• a) au moins une étape effectuée par passage de la charge, constituée de préférence par l'ensemble de la coupe essence, sur un catalyseur permettant d'hydrogéner sélectivement les dioléfines de l'essence sans hydrogéner les oléfines.
• b) au moins une étape consistant à passer tout ou partie de l'essence initiale ou de l'essence hydrogénée à l'étape a, de préférence l'ensemble de l'essence initiale ou hydrogénée à l'étape a, sur un catalyseur permettant de transformer au moins en partie les composés soufrés légers (par exemple: ethylmercaptan, propyl mercaptan), par réaction avec tout ou partie des oléfines, en composés soufrés plus lourds. Cette étape est de préférence réalisée simultanément à l'étape a en passant par exemple l'essence initiale sur un catalyseur capable à la fois d'hydrogéner les dioléfines et de transformer les composés soufrés légers, de préférence avec les oléfines, en composés soufrés plus lourds, ou sur un catalyseur distinct mais permettant de réaliser cette transformation dans le même réacteur que l'étape a. Il est éventellement possible d'observer sur certains types de charges une formation de certains mercaptans à l'issu de l'étape a ou b, cette formation est probablement due à une hydrogenolyse des disulfures de poids moléculaire élevé.
• c) au moins une étape visant à séparer l'essence initiale en une essence légère une fraction intermédiaire et une essence lourde. Le point de coupe de l'essence légère et de l'essence lourde est déterminé afin de limiter la teneur en soufre de l'essence légère et de permettre son utilisation dans le pool essence de préférence sans post traitement supplémentaire.
• d) une étape de transformation des composés soufrés, de préférence d'alkylation ou d'adsorption des composés choisis dans le groupe constitué par le thiophène, les composés thiophéniques et éventuellement les mercaptans présents dans au moins une coupe obtenue à l'étape c, de préférence dans la coupe légère et/ou dans au moins une coupe intermédiaire.
• f) un traitement de désulfuration en une étape d'au moins une partie de la fraction lourde issue du fractionnement à l'étape c ou de l'étape d de transformation des composés soufrés notamment thiophèniques en présence d'au moins un catalyseur d'hydrodésulfuration ou d'un absorbant. L'essence lourde et/ou au moins une essence intermédiaire ainsi désulfurée peut ensuite être éventuellement strippée (c'est-à-dire qu'un courant gazeux, de préférence contenant un ou des gaz inertes est passé au travers de cette essence), afin d'éliminer l'H2S éventuellement produit lors de la désulfuration.
• g) éventuellement une étape de mélange de la fraction légère issue de l'étape c ou d avec éventuellement au moins une fraction intermédiaire issue de l'étape c, d et avec la fraction lourde désulfurée issue de l'étape f. Ces fractions peuvent également être valorisées séparément sans être mélangées.

The method according to the invention comprises at least the following steps:

• a) at least one step carried out by passing the feed, preferably consisting of the entire gasoline cut, on a catalyst for selectively hydrogenating the diolefins of the gasoline without hydrogenating the olefins.
• b) at least one step of passing all or part of the initial gasoline or hydrogenated gasoline in step a, preferably all of the initial gasoline or hydrogenated in step a, on a catalyst at least partially converting light sulfur compounds (eg ethyl mercaptan, propyl mercaptan), by reaction with all or part of the olefins, into heavier sulfur compounds. This step is preferably carried out simultaneously in step a, for example by passing the initial gasoline over a catalyst capable of both hydrogenating the diolefins and converting the light sulfur compounds, preferably with the olefins, into sulfur compounds more heavy, or on a separate catalyst but to achieve this transformation in the same reactor as step a. It is possible to observe on certain types of fillers the formation of certain mercaptans at the end of step a or b, this formation is probably due to a hydrogenolysis of disulphides of high molecular weight.
• c) at least one step of separating the initial gasoline into a light gasoline an intermediate fraction and a heavy gasoline. The cutting point of light gasoline and heavy gasoline is determined in order to limit the sulfur content of light gasoline and to allow its use in the gasoline pool preferably without additional post treatment.
• d) a step of transformation of the sulfur compounds, preferably of alkylation or adsorption of the compounds selected from the group consisting of thiophene, the thiophene compounds and optionally the mercaptans present in at least one cut obtained in step c, preferably in the light section and / or in at least one intermediate section.
• f) a desulfurization treatment in one step of at least a portion of the heavy fraction resulting from fractionation in step c or step d of transformation of sulfur compounds, especially thiophenic compounds, in the presence of at least one catalyst; hydrodesulfurization or absorbent. The heavy gasoline and / or at least one intermediate gas thus desulphurized may then optionally be stripped (that is to say a gaseous stream, preferably containing one or more inert gases, is passed through this gasoline), to remove any H2S produced during the desulphurization.
• g) optionally a step of mixing the light fraction resulting from step c or d with possibly at least one intermediate fraction resulting from step c, d and with the heavy desulphurized fraction resulting from step f. These fractions can also be valued separately without being mixed.

• hydrogénation des dioléfines (étape a) : lorsque l'étape b est absente, ou lorsque l'étape a est effectuée simultanément à l'étape b de transformation des composés soufrés légers.
• transformation des composés soufrés légers (étape b).
• transformation des composés soufrés tels que le thiophène, les composés thiophèniques et les mercaptans (étape d).
• désulfuration de la fraction lourde (étape f).

According to a variant of the process according to the invention, it is possible to associate at least one reaction section with the fractionation column. Said said reaction section or sections then operate on at least one fraction taken from the inside of the fractionation column and the effluent from the reaction section is returned to the fractionation column. The reaction section or sections thus coupled to the fractionation column of step c may be chosen from the group consisting of the reaction sections of the following steps:

• hydrogenation of diolefins (step a): when step b is absent, or when step a is carried out simultaneously with step b of transformation of light sulfur compounds.
• transformation of light sulfur compounds (step b).
• transformation of sulfur compounds such as thiophene, thiophene compounds and mercaptans (step d).
• desulfurization of the heavy fraction (step f).

Such devices comprising a fractionation column associated with an external reactor and that can be used in the process according to the invention have been described, for example, for applications in the field of refining and petrochemistry in patents. US5,1777,283 , US 5,817,227 and US 5,888,355

According to other variants of the process according to the invention, it is also possible to use a reactive column instead of the fractionation column, that is to say to place at least one of said reaction sections in the column of fractionation (internal column reaction section), preferably in a zone where the reagent concentration is maximum. For example, in the case of step d of transformation of the thiophene compounds, the reaction section will preferably be placed in a zone having the maximum concentration of these compounds. Since the light fraction of the gasoline does not require a desulfurization treatment, when a desulfurization section is internal to the fractionation column, the said reaction section will generally not be placed at the top of the column.
According to a preferred variant of the process according to the invention, the reaction section internal to the column is chosen from the group consisting of the following reaction sections: hydrogenation (step a), transformation of light sulfur compounds (step b), transformation of sulfur compounds such as thiophene, thiophene compounds and optionally mercaptans (step d), desulfurization of the heavy fraction (step f).

In a very preferred embodiment of the process according to the invention, the reaction section is placed in the middle of a fractionation column, so as to treat the compounds having intermediate boiling points, that is to say the compounds which can constitute a intermediate cut and which are recovered alone or with the heavy fraction at the bottom of the column, at the end of the fractionation step The heavy fraction is then treated in an external reactor associated or not with the fractionation column.

Such reactive columns are known to those skilled in the art and have been described, for example, in patents or patent applications. US5,368,691 , US5,523,062 , FR 2.737 131 , FR 2,737,132 , EP-A-0461 855 .

Another variant of the process according to the invention consists both in using a reactive column comprising at least one reaction section and an external reactor coupled or not to said column. Such variants are for example described in the patent application WO00 / 15319 .
The variants described above are only illustrations of the possible variants of the method according to the invention. The method according to the invention can indeed be implemented by combining reaction sections (steps a, b, d, or f) or associated with the fractionation column of step c, or internally (s). to said column, either exteme (s) and uncoupled (s) to said column in that the effluent of said reaction section (s) (s) is not recycled to the fractionation column.

One of the advantages 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. The transformation of the sulfur and thiophene compounds (steps b and d) makes it possible to considerably reduce the content of sulfur compounds in the light cut and optionally at least one intermediate cut, and generally to recover most of these compounds in the heavy fraction, and optionally in the intermediate fraction or fractions.

Steps b and d are distinguished inter alia by the fact that the conversion of the thiophene compounds is generally less than 60% by weight, or even less than 40% by weight in step b, whereas the conversion or adsorption of said compounds is more often greater than 80% by weight, preferably greater than 90% by weight, very preferably greater than 95% by weight in step d. In fact, step b essentially increases the weight of light mercaptans.

This operation is carried out while maintaining most of the olefins in the light fraction, possibly in at least one intermediate fraction that does not require extensive desulfurization. The content of sulfur compounds of the light fraction thus obtained is generally less than 50 ppm, preferably less than 20 ppm, more preferably less than 10 ppm and very preferably less than 5 ppm.

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

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

hydrogenation of diolefins (step a):

Hydrogenation of dienes is a step which eliminates, before hydrodesulphurization, almost all the dienes present in the gasoline cutter containing sulfur to be treated. It preferably takes place in the first step (step a) of the process according to the invention, generally in the presence of a catalyst comprising at least one metal of group VIII, preferably chosen from the group consisting of platinum, palladium and nickel , and a support. For example, a catalyst based on nickel or palladium deposited on an inert support, such as, for example, alumina, silica or a support containing at least 50% alumina, will be used.
The pressure employed is sufficient to maintain more than 60%, preferably 80%, and more preferably 95% by weight of the gasoline to be treated in the liquid phase in the reactor; it is most generally between about 0.4 and about 5 MPa and preferably greater than 1 MPa, more preferably between 1 and 4 MPa. The hourly space velocity of the liquid to be treated is between about 1 and about 20 h ^-1 (volume of filler per volume of catalyst per hour), preferably between 2 and 10 h ^-1 , very preferably between 3 and 8 h ^-1 . The temperature is most generally between about 50 and about 250 ° C, and preferably between 80 and 220 ° C, and more preferably between 100 and 200 ° C, to ensure sufficient conversion of the diolefins. Very preferably it is limited to 180 ° C. The ratio of hydrogen on charge expressed in liters is generally between 1 and 50 liters per liter, preferably between 3 and 30 liters per liter, more preferably between 8 and 25 liters per liter.
The choice of operating conditions is particularly important. It will operate most generally under pressure and in the presence of a quantity of hydrogen in small excess relative to the stoichiometric value necessary to hydrogenate the diolefins. The hydrogen and the feedstock to be treated are injected in ascending or descending streams into a reactor preferably comprising a fixed bed of catalyst.
Another metal may be associated with the main metal to form a bimetallic catalyst, such as, for example, molybdenum or tungsten. The use of such catalytic formulas has for example been claimed in the patent FR 2,764,299 . The catalytic cracked gasoline may contain up to a few weight percent of diolefins. After hydrogenation, the diolefin content is generally reduced to less than 3000 ppm, or even less than 2500 ppm and more preferably less than 1500 ppm. In some cases, it can be obtained less than 500 ppm. The diene content after selective hydrogenation can even if necessary be reduced to less than 250 ppm.
According to a particular embodiment of the process according to the invention, the hydrogenation stage of the dienes takes place in a catalytic hydrogenation reactor which comprises a catalytic reaction zone traversed by the entire charge and the quantity of hydrogen necessary to effect the desired reactions.

transformation of light sulfur compounds (step b):

This step consists of transforming the light compounds of sulfur. That is to say the compounds that after the separation step b would be in the light gasoline, heavier sulfur compounds driven in heavy gasoline. Preferably, the light compounds transformed have a boiling point lower than that of thiophene. This transformation is preferably carried out on a catalyst comprising at least one element of group VIII (groups 8, 9 and 10 of the new, periodic classification), or comprising a resin. The choice of catalyst is made in particular to promote the reaction between light mercaptans and olefins, which leads to heavier mercaptans.

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

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

- separation of the essence into at least two fractions (step c):

• une fraction légère contenant une teneur en soufre résiduelle limitée, de préférence inférieure à environ 200 ppm, de manière plus préférée inférieure à 100 ppm, de manière très préférée inférieure à environ 20 ppm, et permettant d'utiliser cette coupe sans effectuer d'autre traitement visant à diminuer sa teneur en soufre, sauf éventuellement l'étape d de traitement du thiophène et/ou des composés thiophéniques,
• une fraction intermédiaire,
• une fraction lourde dans laquelle de préférence la majeure partie du soufre initialement présent dans la charge est concentrée.

According to a first variant of the process according to the invention, the essence is split into two fractions:

• a light fraction containing a limited residual sulfur content, preferably less than about 200 ppm, more preferably less than 100 ppm, very preferably less than about 20 ppm, and allowing this cup to be used without any other treatment to reduce its sulfur content, except possibly step d of treatment of thiophene and / or thiophene compounds,
• an intermediate fraction,
• a heavy fraction in which preferably most of the sulfur initially present in the feed is concentrated.

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

This column generally operates at a pressure of between 0.1 and 2 MPa and preferably between 0.2 and 1 MPa. The number of theoretical plates of this separation column is generally between 10 and 100 and preferably between 20 and 60. The reflux 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 light gasoline obtained after 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 low sulfur content, ie it is not generally necessary to treat the light cut before using it as fuel.

• une coupe dite légère (coupe L) dont les points d'ébullition sont de préférence inférieurs à environ 60°C. Cette température est donnée à titre indicatif, il s'agit en fait de la température maximale pour laquelle la teneur en thiophène est inférieure à 5 ppm,
• au moins une coupe dite lourde (coupe H1) dont les points d'ébullition sont (à titre indicatif) supérieurs à environ 60°C.

The essence is first preferably fractionated into at least two sections having the following properties:

• a so-called light section (L-section) whose boiling points are preferably less than about 60 ° C. This temperature is given as an indication, it is in fact the maximum temperature for which the thiophene content is less than 5 ppm,
• at least one so-called heavy cup (H1 section) whose boiling points are (for information only) greater than about 60 ° C.

The light cut L is preferably injected into a liquid gas separation tank in order to separate the unconsumed hydrogen and the H 2 S, formed during step a and / or b, from olefins generally having from 5 to 7 carbon atoms. carbon.

• une coupe intermédiaire I2 dont les points d'ébullition à titre d'exemple sont au minimum de 60°C et au maximum d'environ 120°C voire environ 160°C. Cette coupe peut être traitée à l'étape d du procédé selon l'invention.
• une coupe plus lourde H2 dont les points d'ébullition sont généralement supérieurs à environ 160°C ou à environ 120°C.

The so-called heavy cut H1, ie the cut whose temperatures are greater than approximately 60 ° C., is then sent to a distillation column or any other separation process capable of separating this cut into at least two sections:

• an intermediate section I2 whose boiling points by way of example are at least 60 ° C and at most about 120 ° C or about 160 ° C. This section can be treated in step d of the process according to the invention.
• a heavier H2 cup whose boiling points are generally greater than about 160 ° C or about 120 ° C.

Heavy cut H2 whose boiling temperatures are generally greater than about 160 ° C or about 120 ° C is fed to f) desulphurization step.

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

Intermediate section I2 whose boiling points are exemplified between about 60 ° C and about 120 ° C or about 160 ° C can be sent to a sulfur compound conversion unit according to step d.

After step d, the sections 12 may again be fractionated into an intermediate section 13 and a heavy section H3, especially when step d is an alkylation step of the thiophene compounds. The H3 cut thus obtained may optionally be mixed with the H2 cut, preferably before desulfurization.

- Transformation of sulfur compounds, especially thiophene compounds, by alkylation (step d):

Step d is a step of transformation of the sulfur compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans present in the light cut and / or in at least one intermediate fraction.

This step consists of passing the light fraction and / or possibly at least one intermediate fraction resulting from the fractionation (step c) over a catalyst having an acid function which makes it possible to carry out the addition of the sulfur compounds in the form of mercaptans to the olefins and the alkylation reaction of thiophene and thiophene derivatives with these same olefins. The operating conditions are adjusted to achieve the desired conversion with conversion rates of thiophene and / or thiophenics and / or light mercaptans, preferably mercaptans having from 1 to 6 carbon atoms, greater than 80% by weight, preferably greater than 90% by weight, very preferably greater than 95% by weight. Other compounds such as COS or CS2 may optionally also be adsorbed or converted.

To minimize the oligomerizing activity of the acid catalyst used, the gasoline may be additive of a compound known to inhibit the oligomerizing activity of acid catalysts such as alcohols, ethers or water.

This transformation can be carried out, for example, according to the procedures and using the diagrams described in French patent applications no. 00/08113 , and no 00/10233 for steps a to d of the process according to the invention, or by combining steps a to c of the process according to the invention and as described above with step d detailed as follows.

In the process according to the invention, the light cut or intermediate cut obtained in step c is treated in a section for converting by alkylation the compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans. The thiophenic compounds contained in the 60 ° C.-160 ° C. section will react with conversion levels of greater than 80% by weight, preferably greater than 90% by weight, with the olefins to form thiophene alkyls according to the following reaction for thiophene. :

Some or all of the benzene may also be removed by alkylation with the olefins. These compounds of higher molecular weight are mainly characterized by higher boiling temperatures than they had before alkylation. Thus, the theoretical boiling temperature, which is 80 ° C., is displaced towards 250 ° C. for the thiophene alkyls, and this reaction therefore generally leads to a heavier gasoline, especially in the case where the gasoline fraction and / or the starting gasoline are light.

This alkylation step is carried out in the presence of an acid catalyst. This catalyst may be indifferently a resin, a zeolite, a clay, any functionalized silica or any silico-aluminate having acidity, or any grafted support of acidic functional groups. The ratio of the charge volume injected onto the catalyst volume is between 0.1 and 10 titer / liter / hour and preferably between 0.5 and 4 titer / liter / hour. More precisely, this alkylation step is carried out in the presence of at least one acidic catalyst selected from the group consisting of silicoaluminates, titanosilicates, mixed titanium alumina, clays, resins, mixed oxides obtained by grafting with least an organosoluble or water-soluble organometallic compound (selected from the group consisting of alkys and / or alkoxy metals of at least one element such as titanium, zirconium silicon, germanium, tin, tantalum, niobium, etc.) on at least one oxide such as alumina (gamma, delta, eta, alone or as a mixture), silica, silica aluminas, titanium silicas, zirconia silicas or any other solid having any acidity. A particular embodiment of the invention may consist in using a physical mixture of at least two of the above catalysts in proportions varying from 95/5 to 5/95, preferably from 85/15 to 15185 and very preferably from 70/30 to 30/70 ".

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

The olefin molar ratio on thiophene compounds is between 0.1 and 1000 mol / mol, preferably between 0.5 and 500 mol / mol.

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

The effluent from step d, of transformation of the sulfur compounds, preferably alkylation or adsorption may optionally be mixed at least in part with a heavy cut from the fractionation in step c.

The effluent from step d of transformation of the sulfur compounds can also optionally be sent to a new fractionation unit to be separated into two fractions, an untreated or optionally desulfurized intermediate fraction without being mixed, and a heavy fraction which is preferably mixed with the heavy fraction from step d before being desulphurized in step f,

• une coupe 60°C-180°C ou 60°C-100°C (coupe D2) dépourvue de tout composé thiophénique qui est recueillie,
• une coupe dont les températures d'ébullition sont supérieures à 180°C ou 100°C (coupe D3).

For example, the effluent D1 resulting from an alkylation can be separated into:

• a section 60 ° C-180 ° C or 60 ° C-100 ° C (cut D2) devoid of any thiophene compound which is collected,
• a section whose boiling temperatures are above 180 ° C or 100 ° C (cut D3).

Step d may advantageously be carried out on the light fraction resulting from step c. Indeed, the alkylation of the light mercaptans present in this fraction then facilitates the desired removal of the sulfur compounds, but also makes it possible to reduce the vapor pressure (RVP or Reid Vapor Pressure index according to the Anglosaxon terminology) of the final desulphurized gasoline. .

All of the effluent D1 from said alkylation unit (step d) or the D3 cut resulting from the fractionation after alkylation may be preferably mixed at least in part with a heavy cut (for example the H2 cut) and sent to the desulfurization section of step f.

- Desulfurization of the heavy fraction (step f):

This step may for example be a hydrodesulphurization step carried out by passing the heavy gasoline, in the presence of hydrogen, over a catalyst comprising at least one element of group VIII and / or at least one element of group VIb at least portion in sulphide form, at a temperature between about 210 ° C and about 350 ° C, preferably between 220 ° C and 320 ° C, at a pressure generally between about 1 and about 4 MPa, preferably between 1.5 and 3 MPa. The space velocity of the liquid is between about 1 and about 20 h ^-1 (expressed as volume of liquid per volume of catalyst per hour), preferably between 1 and 10 h ^-1 , very preferably between 3 and 8 h -1. The ratio H ₂ / HC is between 100 to 600 liters per liter and preferably between 300 and 600 liters per liter.

The content of Group VIII metal expressed as oxide is generally between 0.5 and 15% by weight, preferably between 1 and 10% by weight. The metal content of group VIb is generally between 1.5 and 60% by weight, preferably between 3 and 50% by weight.

The element of group VIII, when present, is preferably cobalt, and the element of group VIb, when present, is usually molybdenum or tungsten. Combinations such as cobalt-molybdenum are preferred. The catalyst support is usually a porous solid, such as for example an alumina, a silica-alumina or other porous solids, such as, for example, magnesia, silica or titanium oxide, alone or in mixture with alumina or silica-alumina. To minimize the hydrogenation of the olefins present in heavy gasoline, it is advantageous to preferentially use a catalyst in which the density of molybdenum, expressed in% by weight of MoO 3 per unit area, is greater than 0.07 and preferably greater than 0.07. at 0.10. The catalyst according to the invention preferably has a specific surface area of less than 190 m 2 / g, more preferably less than 180 m 2 / g, and very preferably less than 150 m 2 / g.

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

The catalyst is preferably used at least in part in its sulfurized form. The introduction of sulfur may occur before or after any activation step, that is, calcination or reduction. Preferably, no catalyst oxidation step is performed when the sulfur or a sulfur compound has been introduced on the catalyst. The sulfur or a sulfur compound can be introduced ex situ, that is to say outside the reactor where the process according to the invention is carried out, or in situ, that is to say in the reactor used for process according to the invention. In the latter case, the catalyst is preferably reduced under the conditions described above, then sulfided by passing a feed containing at least one sulfur compound, which once decomposed leads to the fixation of sulfur on the catalyst. This charge may be gaseous or liquid, for example hydrogen containing H ₂ S, or a liquid containing at least one sulfur compound.

In a preferred manner, the sulfur compound is added to the ex situ catalyst . For example, after the calcination step, a sulfur compound may be introduced onto the catalyst in the presence of possibly another compound. The catalyst is then dried and then transferred to the reactor for carrying out the process according to the invention. In this reactor, the catalyst is then treated in hydrogen to convert at least a portion of the main metal sulfide. A procedure which is particularly suitable for the invention is that described in the patents FR-B-2 708 596 and FR-B-2,708,597 .

The desulfurization of heavy gasoline can also be carried out by means of an absorber comprising an absorbent mass, for example based on zinc oxide. Said desulfurization can also be carried out by means of a combination between a hydrodesulfurization section and an absorber; preferably located after the hydrodesulfurization section.

For heavy gasoline, the desulfurization section preferably comprises only one absorber and / or one reactor. Said reactor contains only one type (in terms of chemical formulation) of hydrodesulfurization catalyst, possibly arranged in several separate beds. Preferably said catalyst is cobalt-based, most preferably it is a catalyst comprising cobalt and molybdenum or tungsten. Preferably, the catalyst employed in this section is sulfided.

On the other hand, the optional treatment section of at least one intermediate section can comprise several reactors possibly associated with one or more absorbers. Said section may comprise two reactors in series, possibly with a separation of the gas containing H2S and the liquid between the two reactors. It is also possible to use two different catalysts arranged in at least two beds inside said reactors, optionally with an intermediate addition of hydrogen (also called quench according to English terminology). For example, it is possible to use a catalyst comprising cobalt and molybdenum or tungsten associated with a catalyst comprising nickel. Preferably, said catalysts are sulphurized.

Figure 1 illustrates some preferred variants of the method according to the invention. In all these variants, the charge is admitted via line 1 and mixed with hydrogen arriving via line 2. The mixture 3 is introduced into the reactor 4 containing a catalyst for selective hydrogenation of diolefins (step a). The effluent from this reactor is introduced into a reactor 7, optionally after the addition of hydrogen via line 6. The reactor 7 contains a catalyst which makes it possible to weigh up light sulfur compounds, such as light mercaptans, by reaction with olefins (step b). Efftuent 8 from the reactor 7 is introduced into a fractionation column (9) to achieve a separation of the gasoline in at least 2 sections: a light cut which is untreated, and a heavy cut 13 which is desulfurized in a reactor 20 containing a hydrodesulphurization catalyst, after mixing with hydrogen taken up by line 19. The heavy desulfurized fraction 21 is mixed with the light fraction to yield the desulfurized gasoline 22.

According to another variant described in FIG. 1, the fractionation column 9 makes it possible to separate the effluent 8 from the reactor 7 in 4 sections: an untreated light section 10, a first intermediate section 11 introduced into a reactor 11 alkylation 14 which allows the alkylation of thiophene, thiophenic compounds and optionally mercaptans and whose effluent is returned to the fractionation column 9 via line 15, a second intermediate fraction 12 treated in the presence of hydrogen brought via line 16 in a reactor containing a hydrodesulphurization catalyst, and a heavy cut 13 which is also desulphurized in the presence of hydrogen taken via line 19 in a reactor 20 containing a hydrodesulfurization catalyst. The desulphurized heavy fraction 21, and the desulfurized intermediate fraction 18 are mixed with the light fraction to yield the desulfurized gasoline 22.

In summary, according to one variant, the process according to the invention is a process for producing gasoline with a low sulfur content, comprising at least the following steps: at least one selective hydrogenation of the diolefins present in the initial gasoline (step a) at least one step of transforming the light sulfur compounds present in the gasoline (step b), at least one fractionation (step c) of the gasoline obtained in step a or b into at least fractions a light fraction a fraction intermediate and a heavy fraction a step (step d) of transformation of the thiophene compounds and mercaptans present in at least one cut obtained in step c, a desulphurization treatment in one step (step f) of at least a portion of the heavy fraction resulting from the fractionation in step c.

According to a preferred variant of the process according to the invention, steps a and b are carried out simultaneously in the same reactor.

Preferably, steps b and / or d make it possible to increase the molecular weight of the sulfur compounds, in order to separate them mainly in the heavy fraction of step c.

Preferably, the heavy gasoline is desulfurized in step f in the presence of a hydrodesulphurization catalyst or an absorbent, and more preferably the heavy gasoline desulfurized in step f is stripped by means of an inert gas.

The process according to the invention may also comprise a step g of mixing the light fraction resulting from step c or d and optionally at least one fraction intermediate resulting from stage c or d with the heavy desulfurized fraction resulting from stage f.

According to a preferred variant, the process according to the invention comprises at least one internal column-forming editorial section selected from the group consisting of the following reaction sections: hydrogenation (step a), transformation of light sulfur compounds (step b), transformation sulfur compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans (step d), and desulfurization of the heavy fraction (step f).

According to another preferred variant, at least one reaction section of the process according to the invention is coupled to the column and selected from the group consisting of the following reaction sections: hydrogenation (step a), transformation of light sulfur compounds (step b), transformation of the sulfur compounds selected from the group consisting of thiophene, thiophene compounds and mercaptans (step d), and desulfurization of the heavy fraction (step-f).

The effluent from step d of transformation of the sulfur compounds may optionally be mixed at least in part with a heavy cut from the fractionation in step c. Said effluent may also be sent to a new fractionation unit to be separated into two fractions, an untreated or optionally desulfurized intermediate fraction without being mixed, and a heavy fraction which is preferably mixed with the heavy fraction resulting from step c before being desulphurized in step f.

The following examples illustrate the invention: Example 1 (comparative).

A catalytic cracking gasoline whose characteristics are shown in Table 1 is separated into two fractions (step c), a light fraction whose cutting point corresponds to a temperature of 63 ° C and a heavy fraction. The light fraction represents 25% by weight of the starting gasoline and contains 88% of the compounds olefins having 5 carbon atoms which were present in the starting gasoline and 23% of the olefins having 6 carbon atoms.
The characteristics of the separation column are as follows: theoretical plates, reflux tank pressure = 5 MPa, supply temperature 100 ° C.

The characteristics of the light fraction and the heavy fraction are presented in Table 1. catalytic cracked gasoline light fraction heavy fraction total essence PI-63 63-220 ° C IND.REFRACTION at 20 ° C 1.43 1.38 1.45 VOLUMIC MASS 15/4 0.76 0.67 0.79 IBr (g / 100g) 78 166 48 MAV (mg / g) 6.4 5.2 6.8 TOTAL NITROGEN (mg / l) 40 2.7 50 RON 93.0 95.8 92.0 MY 80.3 84.1 78.9 SULFUR NON MERCAPTANS (mg / Kg) 1509 37 2000 MERCAPTANS (mg / kg) 58 158 24 TOTAL SULFUR (mg / Kg) 1567 195 2024

At the end of this separation, the light fraction has a content of sulfur, mercaptan and diolefin such that it is no longer necessary to carry out additional treatment of this fraction before using it.

The heavy fraction is subjected to hydrodesulfurization on a catalyst in isothermal tubular reactor. The catalyst is obtained by impregnation "without excess solution" of a transition alumina, in the form of beads, with a specific surface area of 130 m ² / g and a pore volume of 0.9 ml / g, with a solution aqueous solution containing molybdenum and cobalt in the form of ammonium heptamolybdate and cobalt nitrate. The catalyst is then dried and calcined under air at 500 ° C. The cobalt and molybdenum content of this catalyst sample A is 3% CoO and 14% MoO3. The catalyst is previously sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock containing 2% by weight of S in the form of dimethylsulphide diluted in n-heptane.
The operating conditions for desulfurization are as follows: VVH = 4h-1, H2 / HC = 300l / l, P = 2 MPa. The temperature of the reaction zone is 280 ° C. The characteristics of the effluents obtained are presented in Table 2. Table 2: Characteristics of heavy gasoline after hydrodesulfurization Heavy gas Desulphurized heavy gas S total ppm 2024 182 S RSH ppm 24 80 Ibr gBr / 100g 48 33 RON 92.0 86.5 MY 78.9 75.8 H2S% vol 0.2

The desulphurized heavy gasoline is then mixed with the light gasoline whose composition is given in Table 1. The gasoline thus constituted contains 185 ppm of sulfur including 100 ppm of mercaptans. Such gasoline will therefore require additional treatment before use.

Example 2

The gasoline whose characteristics are described in Example 1 is subjected to a hydrogenation treatment of diolefins (step a) under conditions such that the sulfur-containing light compounds present in the feed are partly converted into heavier compounds (step b performed simultaneously in step a).
This treatment is carried out in a continuous reactor operating in updraft. The catalyst is an HR945 catalyst marketed by the company Procatalyse. The reaction is carried out at 160 ° C. under a total pressure of 13 bar, with a space velocity of 6 h ^-1 . The H2 / feed ratio, expressed in liters of hydrogen per liter of feed and 10.

The characteristics of the effluent after hydrogenation of diolefins and conversion of light compounds present in gasoline are presented in Table 3. Catalytic cracking gasoline total essence Gasoline after treatment IND REFRACTION at 20 ° C 1.44 1.43 VOLUMIC MASS 15/4 0.78 0.77 IBr (g / 100g) 78 75.5 MAV (mg / g) 6.4 0.2 TOTAL NITROGEN (mg / l) 40 40 RON 93.0 92.2 MY 80.3 79.9 SULFUR NON MERCAPTANS (mg / Kg). 1509 1561 MERCAPTANS (mg / kg) 58 6 TOTAL SULFUR (mg / Kg) 1567 1567

At the end of this treatment, the essence is separated into two fractions (step c) under the conditions described in Example 1. The characteristics of the two fractions obtained are specified in Table 4. Table 4: Characteristics of light and heavy cuts after fractionation (step c) Catalytic cracking gasoline light fraction heavy fraction total essence PI-63 63-220 ° C IND REFRACTION at 20 ° C 1.44 1.38 1.45 VOLUMIC MASS 15/4 0.78 0.67 0.79 IBr (g / 100g) 75.5 158 48 MAV (mg / g) 0.2 0.0 0.3 TOTAL NITROGEN (mg / l) 40 2.7 50 RON 92.2 92.8 92.0 MY 79.9 82.9 78.9 SULFUR NON MERCAPTANS (mg / Kg) 1561 31 2071 MERCAPTANS (mg / kg) 6 2 7 TOTAL SULFUR (mg / Kg) 1567 33 2078

At the end of this separation, the light fraction contains a content of sulfur, mercaptan and diolefin such that it is no longer necessary to carry out additional treatment of this fraction before using it.

The heavy fraction is subjected to hydrodesulfurization on the catalyst A of Example 1 in an isothermal tubular reactor. The catalyst is previously sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock containing 2% by weight of S in the form of dimethylsulphide diluted in n-heptane.
The operating conditions for desulfurization are the following: WH = 4 h -1, H 2 / H 2 = 300 l / l, P = 2 MPa. The temperature of the reaction zone is 280 ° C. The characteristics of the effluents obtained are presented in Table 5. Table 5: Characteristics of heavy gasoline after hydrodesulfurization Heavy gas Desulphurized heavy gas S total ppm 2078 185 S RSH ppm 7 90 Ibr gBr / 100g 48 33 RON 92.0 86.5 MY 78.9 75.8 H2S% vol 0.2

The desulphurized heavy gasoline is then mixed with the untreated light gasoline whose composition is given in Table 4. The desulphurized gasoline thus constituted contains 147 ppm of sulfur including 68 ppm of mercaptans.

Claims (9)

1. Process for the production of gasoline with a low sulfur content that comprises at least the following stages:

   - at least one selective hydrogenation of diolefins that are present in the starting gasoline (stage a),

   - at least one stage for transformation of light sulfur-containing compounds that are present in the gasoline (stage b) aiming at increasing the molecular weight of the sulfur-containing compounds, said transformation being carried out on a catalyst that comprises at least one element of group VIII

   - at least one fractionation (stage c) of the gasoline that is obtained in stage b into at least a light fraction an intermediate fraction and a heavy fraction,

   - a stage (stage d) for transformation of sulfur-containing compounds consisting in running at least one intermediate fraction that is issued from fractionation (stage c) over a catalyst that has an acid function that makes it possible to carry out the addition of sulfur-containing compounds in the form of mercaptans over olefins and the alkylation reaction of thiophene and thiophenic derivatives by these same olefins,

   - a desulfurization treatment in one stage (stage f) of at least a portion of the heavy fraction that is obtained from the fractionation in stage c,

   and wherein the effluent that is obtained from stage d for transformation of the sulfur-containing compounds is sent into a new fractionation unit to be separated into two fractions, an intermediate fraction that is not treated or optionally desulfurized without being mixed, and a heavy fraction that is mixed with the heavy fraction that is obtained from stage c before being desulfurized in stage f.
2. Process according to claim 1, wherein the light fraction is treated in a stage for transformation of sulfur-containing compounds according to stage d.
3. Process according to claim 1 or 2, wherein stages a and b are carried out simultaneously in the same reactor.
4. Process according to one of claims 1 or 2, wherein stage b makes it possible to increase the molecular weight of said sulfur-containing compounds.
5. Process according to one of claims 1 to 4, wherein the heavy gasoline is desulfurized in stage f in the presence of a hydrodesulfurization catalyst or an absorbent.
6. Process according to any of claims 1 to 5, wherein the desulfurized heavy gasoline in stage f is stripped by means of a cover gas.
7. Process according to one of claims 1 to 6 that also comprises a stage g for mixing the light fraction that is obtained from stage c or d and optionally at least one intermediate fraction that is obtained from stage c or d with the desulfurized heavy fraction that is obtained from stage f.
8. Process according to one of claims 1 to 7, wherein at least one reaction section is inside of the column and is selected from the group that consists of the following reaction sections: hydrogenation (stage a), transformation of light sulfur-containing compounds (stage b), transformation of sulfur-containing compounds that are selected from the group that consists of thiophene, thiophenic compounds, and mercaptans (stage d), and desulfurization of the heavy fraction (stage f).
9. Process according to one of claims 1 to 8, wherein at least one reaction section is coupled to the column and selected from the group that consists of the following reaction sections: hydrogenation (stage a), transformation of light sulfur-containing compounds (stage b), transformation of sulfur-containing compounds that are selected from the group that consists of thiophene,

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