EP1138749B1 - Gasoline desulphurisation process comprising the desulphurisation of heavy and intermediate fractions from a fractionation into at least three cuts - Google Patents

Description

The production of reformulated species meeting the new environmental standards requires, in particular, that the concentration of olefins be reduced slightly, but significantly the concentration of aromatics (especially benzene) and sulfur. Catalytic cracking gasolines, which can represent 30 to 50% of the gasoline pool, have high olefin and sulfur contents. The sulfur present in the reformulated gasolines is attributable, to nearly 90%, to the catalytic cracking gasoline (FCC, "Fluid Catalytic Cracking" or catalytic cracking fluidized bed). The desulphurisation (hydrodesulphurisation) of gasolines and mainly FCC species is therefore of obvious importance for the achievement of specifications. In addition to catalytic cracking gasoline, other gasolines, such as gasoline directly derived from the distillation of crude oil, or petroleum products of conversion (coker, steam cracker or others) can contribute significantly to sulfur in the petrol.

Hydrotreating (hydrodesulphurisation) of the feedstock sent to catalytic cracking leads to gasolines typically containing 100 ppm of sulfur. However, the hydrotreating units of catalytic cracking feeds operate under severe conditions of temperature and pressure, which implies a high investment. In addition, the entire feedstock of the catalytic cracking process must be desulfurized, resulting in the treatment 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 a significant portion of the olefins during the hydrotreatment.
The separation of light gasoline and heavy gasoline before hydrotreatment has already been claimed in US Pat. No. 4,397,739. This type of separation makes it possible to separate a light, olefin-rich, sulfur-containing section. low, which will no longer be compatible with future specifications and a rich heavy fraction depleted in olefins and containing a significant proportion of the sulfur of the initial gasoline. In this patent, there is claimed a method of hydrodesulphurization of gasolines comprising a fractionation of gasoline into a light fraction and a heavy fraction and a specific hydrodesulphurization of heavy gasoline, but no solution is proposed to eliminate the sulfur present in light gasoline.

On the other hand, in US-A-4 131 537 is taught the interest of fractioning the gasoline in several sections, preferably three, depending on their boiling point, and desulfurize under conditions that may be different and in the presence of a catalyst comprising at least one Group VIB and / or Group VIII metal. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three cuts, and when the cut having intermediate boiling points is treated under mild conditions.

Patent Application EP-A-0 725 126 describes a process for the hydrodesulphurization of a cracking gasoline in which the gasoline is separated into a plurality of fractions comprising at least a first fraction rich in compounds which are easy to desulphurize and a second fraction rich in compounds difficult to desulphurize. Before carrying out this separation, it is necessary to first determine the distribution of the sulfur-containing products 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 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 desulfurized simultaneously or separately and mixed. 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 application No. 98/14480, it is taught the interest of splitting the gasoline into a light fraction and a heavy fraction and then to perform a specific hydrotreatment of the light gasoline on a nickel-based catalyst , and hydrotreating 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 US Pat. No. 5,290,427, methods for hydrotreating gasolines consisting of splitting the gasoline and then introducing the fractions at different levels of a hydrodesulfurization reactor and converting the desulfurized fractions to a ZSM-5 zeolite to compensate for the recorded octane loss by means of isomerization.

In these processes, the species to be treated generally have an initial point greater than 70 ° C, and again it is necessary to treat the light petrol separately. (fraction corresponding to compounds of boiling point between C5 (hydrocarbon with 5 carbon atoms) and 70 ° C) for example by a softening.
US-A-5,318,690 proposes a process comprising fractionation of gasoline and sweetening of light gasoline, while heavy gasoline is desulfurized, then converted to ZSM-5 and desulphurized again under conditions sweet. This technique is based on a separation of crude gasoline so as to obtain a light cut practically 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 heavy cut contains a relatively large amount of olefins which are partly saturated during 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 H ₂ S present in the medium to reform mercaptans. It is then necessary to perform additional softening or hydrodesulfurization.

Summary of the invention

The present invention relates to a process for the desulphurisation of gasoline, that is to say a process for the production of gasolines with a low sulfur content, which makes it possible to recover the totality of a charge (generally a petrol fraction) containing sulfur, preferably a gasoline cut resulting from a catalytic cracking process, and reduce the sulfur content in said gasoline cut to very low levels, without substantially reducing the gasoline yield, and while minimizing the decrease in gasoline content. octane number due to the hydrogenation of olefins. In addition, the process according to the invention makes it possible to restore at least part of the possible octane losses due to the hydrogenation of the olefins by reforming one of the fractions of the previously desulphurized species.

• a1) au moins une hydrogénation sélective des dioléfines et des composés acétylèniques présents dans la charge,
• a2) au moins une étape visant à augmenter le poids moléculaire des produits soufrés légers présent dans la charge ou l'effluent de l'étape a1.
  Cette étape peut être éventuellement réalisée simultanément à l'étape a1 sur au moins une partie de la charge, dans le même réacteur ou un réacteur différent. Elle peut également être effectuée de façon séparée sur au moins une partie de la charge hydrogénée à l'étape a1.
• b) au moins une séparation (également appelée ci-après fractionnement) de l'essence obtenue à l'étape a1 ou a2 en au moins trois fractions (ou coupes), une fraction légère contenant les oléfines les plus légères de l'essence initiale (essence légère ou fraction légère), une fraction lourde dans laquelle la majeure partie des composés soufrés initialement présents dans l'essence initiale est concentrée (essence ou fraction lourde), et au moins une fraction intermédiaire présentant une teneur en oléfines et une teneur en aromatiques relativement faibles et donc un indice d'octane bas par rapport aux fractions légères et lourdes de cette essence.
• c1) au moins un traitement de l'essence lourde séparée à l'étape b sur un catalyseur permettant de décomposer ou d'hydrogéner au moins partiellement les composés soufrés insaturés, notamment les composés soufrés cycliques, voire aromatiques tels que par exemple les composés thiophéniques, en se plaçant dans des conditions telles que l'hydrogénation des oléfines sur ce catalyseur est limitée. La fraction lourde peut éventuellemment être mélangée avant ou après cette étape c1, avec au moins une partie d'une fraction intermédiaire issue de l'étape b de séparation et préférentiellement non désulfuré.
• c2) l'étape c1 est éventuellement suivie par une étape c2 de traitement de l'effluent de l'étape c1 sur un catalyseur qui permet de décomposer les composés soufrés et plus préférentiellement les composés soufrés saturés linéaires et/ou cycliques, avec une hydrogénation limitée des oléfines.
• d) au moins une étape visant à baisser de façon significative la teneur en soufre et en azote d'au moins une des fractions intermédiaires. Cette étape de désulfuration et déazotation est accompagnée de préférence d'une hydrogénation pratiquement totale des oléfines présentes dans cette fraction. La fraction ainsi obtenue est alors traitée par reformage catalytique afin d'augmenter l'indice d'octane de la (des) dîte(s) coupe(s) intermédiaire(s).
• e) éventuellement une étape e de mélange d'au moins deux fractions dont au moins une a subit un traitement de désulfuration à l'étape c1 et éventuellement c2 et/ou à l'étape d.

The process according to the invention, as defined in the claim of claim 1, is a process for producing low sulfur gasoline from a sulfur-containing feedstock. It includes at least the following steps:

• a1) at least one selective hydrogenation of the diolefins and acetylenic compounds present in the feed,
• a2) at least one step for increasing the molecular weight of light sulfur compounds present in the feedstock or effluent of step a1.
  This step may possibly be carried out simultaneously in step a1 on at least part of the feed, in the same reactor or a different reactor. It can also be carried out separately on at least part of the hydrogenated feedstock in step a1.
• b) at least one separation (also hereinafter referred to as fractionation) of the gasoline obtained in step a1 or a2 in at least three fractions (or cuts), a light fraction containing the lighter olefins of the initial gasoline (Light gasoline or light fraction), a heavy fraction in which the major part of the sulfur compounds initially present in the initial gasoline is concentrated (gasoline or heavy fraction), and at least one intermediate fraction having an olefin content and a content of aromatics relatively low and therefore a low octane number compared to the light and heavy fractions of this species.
• c1) at least one treatment of heavy gasoline separated in step b on a catalyst for at least partially decomposing or hydrogenating unsaturated sulfur compounds, in particular cyclic or even aromatic sulfur compounds such as, for example, thiopheneic compounds; , under conditions such that the hydrogenation of olefins on this catalyst is limited. The heavy fraction may optionally be mixed before or after this step c1, with at least a portion of an intermediate fraction from the separation step b and preferably not desulfurized.
• c2) step c1 is optionally followed by a step c2 for treating the effluent of step c1 on a catalyst that makes it possible to decompose the sulfur compounds and more preferably the linear and / or cyclic saturated sulfur compounds, with a hydrogenation limited olefins.
• d) at least one step of significantly reducing the sulfur and nitrogen content of at least one of the intermediate fractions. This desulfurization and denitrogenation step is preferably accompanied by a substantially total hydrogenation of the olefins present in this fraction. The fraction thus obtained is then treated by catalytic reforming in order to increase the octane number of the intermediate cup (s) cut (s).
• e) optionally a step e of mixing at least two fractions, at least one of which undergoes a desulfurization treatment in step c1 and optionally c2 and / or in step d.

The catalytic treatments of steps c1 and / or c2 can be carried out either in a single reactor containing the two catalysts, or in at least two different reactors. When the treatment is carried out using two reactors, the latter two are preferably placed in series, the second reactor preferably treating integrally the effluent at the outlet of the first reactor, preferably without separation of the liquid and the gas between first and second reactor. It is also possible to use several reactors in parallel or in series for one and / or the other of the steps c1 or c2.
Furthermore, a step e is preferably carried out after step d, this step consists of mixing the separated essences in step b, whether or not they have undergone desulfurization treatments.

The feedstock of the process according to the invention is generally a gasoline cutter containing sulfur, such as, for example, a cut resulting from a coking, visbreaking, steam cracking or catalytic cracking (FCC) fraction. . Said filler preferably consists of a gasoline cut from a catalytic cracking unit, whose range of boiling points typically extends from about the boiling points of hydrocarbons containing 5 carbon atoms (C ₅ ). up to about 250 ° C. This gasoline can possibly be composed of a significant fraction of gasoline of other origins such as the essences directly resulting from the atmospheric distillation of the crude oil (essence straight run) or process of conversion (essence of coker or steam-cracking for example ). 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 process for obtaining a gasoline containing sulfur compounds, preferably from a catalytic cracking unit, wherein the gasoline first undergoes a selective hydrogenation treatment of diolefins and acetylenic compounds, then optionally a step to weigh down the lighter sulfur compounds, possibly present in the gasoline and which should, in the absence of this step, be in the light gasoline after fractionation, at least one separation of gasoline into at least three fractions, a treatment of at least one of the intermediate fractions to desulphurize and significantly nitrogenize this cut before it undergoes a catalytic reforming treatment, a treatment of heavy gasoline optionally mixed with at least a portion of one of the intermediate fractions, by means of a catalyst known to promote the conversion of unsaturated sulfur compounds present in gasoline, such as for example thiophenic compounds, into sulfur-saturated compounds such as thiophane and mercaptans, and then optionally a second catalyst promoting the selective conversion of saturated sulfur compounds, linear or cyclic already present in the heavy cut or produced during the previous treatment. The heavy and intermediate fractions thus treated, and optionally the light gasoline fraction can then be mixed in order to obtain a desulfurized gasoline.

This sequence makes it possible to ultimately obtain a desulphurized gasoline with a control of the olefin content or the octane number, even for high desulphurization rates. By this method, significant hydrodesulphurization rates are achieved under reasonable operating conditions specified below. Moreover, by optimizing the cut points of the intermediate fractions and selecting those which are sent to the catalytic reforming stage, it is possible to minimize the benzene content of the final gasoline (for example at contents below 5%). % weight in the final mixture of the desulfurized gasoline fractions), to control the olefin content, and to maintain high research and engine octane values.

• une fraction légère qui contient la fraction principale des molécules à 5 atomes carbone (C₅) et une fraction significative des molécules à 6 atomes de carbone (C₆) initialement présentes dans l'essence à traiter. Cette fraction est caractérisée par une concentration importante en oléfines et une très faible teneur en soufre,
• une première fraction intermédiaire composée essentiellement (c'est-à-dire à plus de 60 % poids de préférence plus de 80 % poids) de molécules à six atomes de carbone (C₆) et d'une partie des molécules présentant 7 atomes de carbone (C₇) ainsi que la plus grande partie des composés soufrés ayant un point d'ébulition voisin de celui de l'azéotrope qu'il forme avec des hydrocarbures, à environ 20 % près. Cette fraction est de préférence mélangée à l'essence lourde avant hydrodésulfuration (étapes c1 et c2) ou après décomposition ou hydrogénation au moins partielle des composés soufrés insaturés (étape c1) mais avant décomposition des composés soufrés saturés (étape c2), afin d'être désulfurée dans des conditions permettant de limiter l'hydrogénation des oléfines. Cette essence n'est de préférence pas envoyée au reformage catalytique car elle contient un ensemble de composés qui lors du traitement de reformage, conduiraient à la formation de benzène. Ces composés sont connus par l'homme du métier comme étant des "précurseurs de benzène" et peuvent par exemple être le méthylcyclopentane, le cyclohexane, le n-hexane ou le benzène lui-même. Lorsque la législation locale le permet, cette coupe intermédiaire peut toutefois éventuellement être envoyée vers l'unité de traitement de la deuxième coupe intermédiaire;
• une deuxième fraction intermédiaire qui est désulfurée et déazotée sur un catalyseur d'hydrotraitement conventionnel afin d'éliminer la quasi totalité du soufre et de l'azote initialement présents dans cette coupe (c'est-à-dire d'abaisser leur teneur à moins de 5 ppm, de préférence moins de 1 ppm). Ce traitement s'accompagne de l'hydrogénation pratiquement totale des oléfines de cette coupe, qui permet d'abaisser la teneur en oléfine à des valeurs de préférence inférieures à 10% poids, et de manière plus préférée inférieures à 5 % poids. Cette coupe est ensuite traitée sur une catalyseur de reformage catalytique permettant l'isomérisation et la déhydrocyclisation des paraffines et des naphtènes avec formation de paraffines branchées et de composés aromatiques.
• la coupe lourde, de préférence mélangée avec la première fraction intermédiaire, est désulfurée dans des conditions définies et au moyen d'un enchaînement de catalyseurs permettant d'atteindre des taux de désulfuration élevés tout en limitant le taux d'hydrogénation des oléfines et donc la perte en octane.

The sulfur species contained in the feedstocks treated by the process of the invention may be mercaptans or heterocyclic compounds, such as, for example, thiophenes or alkylthiophenes, or heavier compounds, for example benzothiophene or dibenzothiophene. . When a gasoline containing light sulfur compounds is split into two slices, a light olefin-rich slice and a heavy olefin-depleted slice, the light sulfur compounds (eg ethyl mercaptan, propyl mercaptan and optionally thiophene) may be partly or wholly present in the light essence. It is then often necessary to apply additional treatment to this light fraction to reduce the sulfur content it contains. In a conventional manner, this treatment is for example an extractive softening which makes it possible to eliminate from gasoline the light compounds of sulfur present in the form of mercaptan. In addition to the fact that this treatment necessarily increases the cost of the operation, it is operational only if the sulfur is in the form of mercaptan. The fractionation point of the gasoline must therefore be preferentially limited so as not to cause the presence of thiophene in light gasoline. The latter forms indeed azeotropes with a number of hydrocarbons, it can separate in the light gasoline that the C ₅ olefin and a small portion of C ₆ olefins otherwise result in a too large fraction of thiophene in this section.
In the process according to the invention, 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 feedstock after a first step of selective hydrogenation under conditions and on catalysts which make it possible to transform the light sulfur compounds into higher sulfur compounds with a higher boiling point so that they are found after the separation step, in the higher fractions. heavy. The gasoline is then fractionated into at least three slices: a light fraction which contains a significant fraction of the olefins initially present in the gasoline to be treated but a very small amount of sulfur compounds, at least one intermediate fraction which is desulfurized and denitrogenated, then treated on a reforming catalyst and a heavy fraction which is desulfurized under defined conditions and by means of a catalyst or a chain of catalysts allowing to reach high desulfurization rates while limiting the hydrogenation rate of the catalysts. olefins and thus the loss in octane. In order to minimize the benzene content of the final gasoline, one of the more preferred embodiments of the invention is to treat the feed under conditions and on catalysts which make it possible to convert the light sulfur compounds into sulfur-containing compounds. higher boiling found after the separation step, in the heavier fractions. The essence is then separated into 4 sections:

• a light fraction which contains the main fraction of the molecules with 5 carbon atoms (C ₅ ) and a significant fraction of the molecules with 6 carbon atoms (C ₆ ) initially present in the gasoline to be treated. This fraction is characterized by a high concentration of olefins and a very low sulfur content,
• a first intermediate fraction composed essentially (that is to say more than 60% by weight, preferably more than 80% by weight) of molecules with six carbon atoms (C ₆ ) and a part of the molecules having 7 atoms of carbon (C ₇ ) as well as most of the sulfur compounds having a boiling point close to that of the azeotrope it forms with hydrocarbons, to about 20%. This fraction is preferably mixed with the heavy gasoline before hydrodesulfurization (steps c1 and c2) or after decomposition or at least partial hydrogenation of the unsaturated sulfur compounds (step c1) but before decomposition of the saturated sulfur compounds (step c2), in order to to be desulfurized under conditions which make it possible to limit the hydrogenation of the olefins. This gasoline is preferably not sent to catalytic reforming because it contains a set of compounds which during the reforming treatment would lead to the formation of benzene. These compounds are known to those skilled in the art as being "benzene precursors" and may be for example, methylcyclopentane, cyclohexane, n-hexane or benzene itself. When local legislation permits, this intermediate cut may however possibly be sent to the processing unit of the second intermediate cut;
• a second intermediate fraction which is desulfurized and denitrogenated on a conventional hydrotreating catalyst to remove substantially all of the sulfur and nitrogen initially present in this cut (i.e., to lower their content unless 5 ppm, preferably less than 1 ppm). This treatment is accompanied by the substantially total hydrogenation of the olefins in this section, which makes it possible to lower the olefin content to values preferably of less than 10% by weight, and more preferably less than 5% by weight. This section is then treated on a catalytic reforming catalyst for the isomerization and dehydrocyclization of paraffins and naphthenes with the formation of branched paraffins and aromatic compounds.
• the heavy fraction, preferably mixed with the first intermediate fraction, is desulphurized under defined conditions and by means of a series of catalysts which make it possible to achieve high desulfurization rates while limiting the degree of hydrogenation of the olefins and therefore the octane loss.

Thus, when the light, intermediate and heavy cuts are mixed after the desulfurization treatments according to the invention, the search or motor octane loss, expressed as the difference between the average value (RON + MON) / 2 observed in this mixture and the average value (RON + MON) / 2 of the initial charge, is limited to less than 2 octane points, preferably less than 1.7 octane points, and more preferably less than 1.5 octane points, and very preferably less than 1 octane point. In some cases, the average value (RON + MON) / 2 of the desulfurized gasoline by means of the process according to the invention can even decrease by less than 0.5 octane point relative to the average value (RON + MON ) / 2 of the load, or even increase by at least 0.5 point.

The sulfur content of catalytic cracked gasoline (FCC) gasoline cuts depends on the sulfur content of the FCC treated feed as well as 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%.

• a1) au moins une étape consistant à faire passer la charge, constituée de préférence par l'ensemble de la coupe essence, sur un catalyseur permettant d'hydrogéner sélectivement les dioléfines et les composés acétyléniques de l'essence sans hydrogéner les oléfines,
• a2) éventuellement au moins une étape optionnelle consistant à faire passer au moins une partie de l'essence initiale ou de l'essence hydrogénée à l'étape a1, de préférence l'ensemble de l'essence initiale ou hydrogénée à l'étape a1, sur un catalyseur permettant de transformer au moins en partie les composés soufrés légers (par exemple : ethylmercaptan, propyl mercaptan, thiophène) avec au moins une partie des dioléfines ou oléfines en composés soufrés plus lourds. Cette étape est de préférence réalisée simultanément à l'étape a1 en passant par exemple l'essence initiale sur un catalyseur capable à la fois d'hydrogéner les dioléfines et acétyléniques et de transformer les composés soufrés légers et une partie des dioléfines ou des oléfines en composés soufrés plus lourds, ou sur un catalyseur distinct mais permettant de réaliser cette transformation dans le même réacteur que l'étape a1. Il est éventellement possible d'observer sur certains types de charges une augmentation de la teneur en mercaptans à l'issue de a1) ou a2), cette augmentation de la teneur en mercaptans est probablement due à une hydrogenolyse des disulfures de poids moléculaire élevé. Au cours de cette étape, l'ensemble des composés soufrés légers, c'est à dire les composés dont le point d'ébulition est inférieur à celui de thiophène, peuvent être transformé. On peut citer parmis ces composés le CS₂, le diméthylsulfure, le methyléthylsulfure ou le COS.
• b) au moins une étape visant à séparer l'essence initiale en au moins une essence légère (fraction légère) au moins une essence intermédiaire (fraction intermédiaire) et une essence lourde (fraction lourde). Le point de coupe de l'essence légère 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 aucun traitement supplémentaire, notamment sans désulfuration. Le point de coupe de l'essence intermédiaire est généralement conditionné par les restrictions qu'impose le procédé de reformage dans lequel cette dernière sera traitée. Une des configurations préférée consiste à fractionner l'essence afin d'obtenir une fraction légère, une fraction lourde, et deux essences intermédiaires : une première essence intermédiaire principalement constituée de composés à six atomes de carbone, qui est ensuite de préférence mélangée à la fraction lourde de l'essence, de préférence avant l'étape c1 ou éventuellement entre le traitement de saturation des composés soufrés insaturés (étape c1) et la décomposition de ces composés (étape c2) et une seconde essence intermédiaire constituée principalement des molécules contenant 7 ou 8 atomes de carbone (C₇ ou C₈) qui est traitée dans l'étape d.
• c1) au moins une étape comprenant le traitement d'au moins une partie de l'essence lourde et éventuellement d'au moins une partie des coupes intermédiaires sur un catalyseur permettant la transformation d'au moins une partie des composés soufrés insaturés présents dans ladite charge, tels que par exemple les composés thiophéniques, en composés soufrés saturés, tels que par exemple les thiophanes (ou thiacyclopentane) ou les mercaptans, selon une succession de réactions décrites ci-après :
  
  La réaction de décomposition totale avec libération d'H₂S est également possible et accompagne généralement de façon significative les réactions de saturation des composés soufrés insaturés.
  Cette réaction d'hydrogénation peut être réalisée sur tout catalyseur favorisant ces réactions tel que par exemple un catalyseur comprenant au moins un métal du groupe VIII et/ou au moins un métal du groupe Vlb, de préférence au moins en partie sous la forme de sulfures. Quand un tel catalyseur est utilisé, les conditions opératoires sont ajustées afin de pouvoir hydrogéner au moins en partie les composés insaturés, tels que les composés thiophéniques, tout en limitant l'hydrogénation des oléfines.
• c2) après ce traitement il peut être réalisé éventuellement au moins une étape c2 dans laquelle les composés soufrés saturés présents dans l'essence initiale ou obtenus à l'issue de la réaction de saturation (étape c1) sont convertis en H₂S selon les exemples de réactions :
  
  Ce traitement peut être réalisé sur tout catalyseur permettant la conversion des composés saturés du soufre (principalement les composés de type thiophanes ou de type mercaptans). Il peut par exemple être effectué sur un catalyseur à base d'au moins un métal du groupe VIII de l'ancienne classification périodique (groupes 8, 9 ou 10 de la nouvelle classification périodique).
  L'essence lourde ainsi désulfurée est ensuite 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 produit lors de la hydrodésulfuration. L'essence légère séparée à l'étape b et l'essence lourde désulfurée à l'étape c1 et/ou à l'étape c2 peuvent ensuite soit éventuellement être mélangées et envoyées dans le pool essence de la raffinerie, soit valorisées séparément sans être mélangées.
• d) le traitement d'au moins une des coupes intermédiaires par un procédé visant à éliminer la quasi totalité des composés soufrés et azotés de cette fraction, et préférentiellement à hydrogéner la totalité des oléfines, puis à traiter l'effluent ainsi hydrotraité sur un catalyseur de reformage qui permet l'isomérisation et la déshydrocyclisation des paraffines.
• e) éventuellement une étape e de mélange d'au moins deux fractions dont au moins une a subi un traitement de désulfuration à l'étape c1 et éventuellement c2 et/ou à l'étape d.

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

• a1) at least one step of passing the feed, preferably consisting of the entire gasoline cut, on a catalyst for selectively hydrogenating the diolefins and acetylenic compounds of the gasoline without hydrogenating the olefins,
• a2) optionally at least one optional step of passing at least a portion of the initial gasoline or hydrogenated gasoline to step a1, preferably all of the initial gasoline or hydrogenated in step a1 , on a catalyst for converting at least partially light sulfur compounds (eg ethyl mercaptan, propyl mercaptan, thiophene) with at least a portion of the diolefins or olefins into heavier sulfur compounds. This step is preferably carried out simultaneously in step a1, for example by passing the initial gasoline over a catalyst capable of both hydrogenating the diolefins and acetylenics and of transforming the light sulfur compounds and a part of the diolefins or olefins into heavier sulfur compounds, or on a separate catalyst but allowing this transformation to be carried out in the same reactor as step a1. It may be possible to observe on certain types of charges an increase in the mercaptan content at the end of a1) or a2), this increase of the mercaptan content is probably due to a hydrogenolysis of the disulphides of high molecular weight. During this step, all light sulfur compounds, ie compounds whose boiling point is lower than that of thiophene, can be converted. Among these compounds, mention may be made of CS ₂ , dimethylsulphide, methylethylsulphide or COS.
• (b) at least one step of separating the initial gasoline into at least one light gasoline (light fraction) at least one intermediate gasoline (intermediate fraction) and one heavy gasoline (heavy fraction). The cutting point of the light gasoline is determined in order to limit the sulfur content of the light gasoline and to allow its use in the gasoline pool, preferably without any additional treatment, in particular without desulfurization. The cutting point of the intermediate gasoline is generally conditioned by the restrictions imposed by the reforming process in which it will be processed. One of the preferred configurations is to split the gasoline to obtain a light fraction, a heavy fraction, and two intermediate species: a first intermediate gasoline mainly composed of six-carbon compounds, which is then preferably mixed with the heavy fraction of the gasoline, preferably before step c1 or optionally between the saturation treatment of the unsaturated sulfur compounds (step c1) and the decomposition of these compounds (step c2) and a second intermediate gasoline consisting mainly of molecules containing 7 or 8 carbon atoms (C ₇ or C ₈ ) which is treated in step d.
• c1) at least one step comprising treating at least a portion of the heavy gasoline and optionally at least a portion of the intermediate cuts on a catalyst allowing the transformation of at least a portion of the unsaturated sulfur compounds present in said filler, such as for example thiophenic compounds, saturated sulfur compounds, such as for example thiophanes (or thiacyclopentane) or mercaptans, according to a succession of reactions described below:
  
  The total decomposition reaction with release of H ₂ S is also possible and generally accompanies significantly the saturation reactions of the unsaturated sulfur compounds.
  This hydrogenation reaction can be carried out on any catalyst promoting these reactions such as, for example, a catalyst comprising at least one Group VIII metal and / or at least one Group VIb metal, preferably at least partly in the form of sulphides. . When such a catalyst is used, the operating conditions are adjusted so as to be able to hydrogenate at least partly unsaturated compounds, such as thiophene compounds, while limiting the hydrogenation of olefins.
• c2) after this treatment, at least one step c2 may optionally be carried out in which the saturated sulfur compounds present in the initial gasoline or obtained after the saturation reaction (step c1) are converted into H ₂ S according to examples of reactions:
  
  This treatment can be carried out on any catalyst allowing the conversion of saturated sulfur compounds (mainly thiophane or mercaptan type compounds). It can for example be carried out on a catalyst based on at least one metal of group VIII of the old periodic classification (groups 8, 9 or 10 of the new periodic classification).
  The heavy gasoline thus desulfurized is then optionally stripped (that is to say that a gaseous stream, preferably containing one or more inert gases is passed through this gasoline), in order to eliminate the H ₂ S produced during hydrodesulfurization. The light gasoline separated in step b and the heavy gasoline desulphurized in step c1 and / or in step c2 can then be optionally mixed and sent to the refinery gasoline pool, or recovered separately without being mixed.
• d) treating at least one of the intermediate slices by a process for eliminating substantially all the sulfur and nitrogen compounds of this fraction, and preferably to hydrogenate all the olefins, then treating the effluent thus hydrotreated on a catalyst reforming process which allows the isomerization and dehydrocyclization of paraffins.
• e) optionally a step e of mixing at least two fractions, at least one of which has undergone a desulfurization treatment in step c1 and optionally c2 and / or in step d.

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

Hydrogenation of diolefins and acetylenics (step a1):

Hydrogenation of dienes is a step that eliminates, before hydrodesulfurization, almost all the dienes present in the petrol cut. containing sulfur to be treated. It preferably takes place in the first step (step a1) 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 formed by 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 3 and 10 h ^-1 , very preferably between 4 and 8 h ^-1 . The temperature is most generally between about 50 and about 250 ° C, and preferably between 80 and 230 ° C, and more preferably between 150 and 200 ° C, to ensure sufficient conversion of the diolefins. Very preferably it is limited to at most 180 ° C. The hydrogen / charge ratio expressed in liters is generally between 5 and 50 liters per liter, preferably between 8 and 30 liters per liter.
The choice of operating conditions is particularly important. The operation will generally be carried out under pressure and in the presence of a quantity of hydrogen in small excess relative to the stoichiometric value necessary for hydrogenating the diolefins and acetylenics. 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 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 into heavier compounds (step a2):

This optional step consists in transforming the light compounds of the sulfur, which at the end of the separation step b would be in the light gasoline in the absence of this step. It 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. In the presence of this catalyst, the light sulfur compounds are converted into heavier sulfur compounds, entrained in heavy gasoline.

This optional step may possibly be performed at the same time as step a1. For example, it may be particularly advantageous to operate, during the hydrogenation of diolefins and acetylenics, under conditions such that at least a portion of the mercaptan compounds are converted. Thus some reduction of the mercaptan content is obtained. For this purpose, it is possible to use the diene hydrogenation procedure described in patent application EP-A-0 832 958, which advantageously uses a catalyst based on palladium, or that described in patent FR 2 720 754.
Another possibility is to use a nickel-based catalyst that is identical to or different from the catalyst of step a1, such as, for example, the catalyst recommended in the process of US-A-3,691,066, which makes it possible to transform the mercaptans. (butyl mercaptan) to heavier sulfur compounds (methyl thiophene).
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 1 to 20 liters per liter, preferably 2 to 15 liters per liter, to allow the desired hydrogenation of the thiophene compounds while minimizing 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. During this step, and whatever the method used, some of the light sulfur compounds such as sulfides (dimethyl sulfide, methyl-ethylsulfide), CS ₂ , COS can also be converted.
Another possibility for carrying out this step is to pass the gasoline 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 to carry out the reaction. alkylation of thiophene by these same olefins. It is for example possible to pass the gasoline to be treated on an ion exchange resin, such as a sulfonic resin. The operating conditions will be adjusted to achieve the desired transformation while limiting the olefin oligomerization parasitic reactions. The operation is generally carried out in the presence of a liquid phase, at a temperature of between 10 and 150 ° C. and preferably between 10 and 70 ° C. The operating pressure is between 0.1 and 2 MPa and preferably between 0.5 and 1 MPa. The space velocity is generally between 0.5 and 10 h ^-1 and preferably between 0.5 and 5 h ^-1 . In this step the conversion rate of mercaptans is generally greater than 50% and the conversion rate of thiophene is generally greater than 20%.

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

- Separation of the essence in at least three fractions (step b):

• une fraction légère présentant une teneur en soufre résiduelle de préférence limitée à 50 ppm, de manière plus préférée limitée à 25 ppm, et de manière très préférée limitée à 10 ppm, et permettant de préférence d'utiliser cette coupe sans autre traitement visant à diminuer sa teneur en soufre,
• au moins une fraction intermédiaire présentant une teneur en oléfines et une teneur en aromatiques relativement faibles,
• une fraction lourde dans laquelle la majeure partie du soufre, initialement présent dans la charge, est concentrée.

In this step the essence is split into at least three fractions:

• a light fraction having a residual sulfur content preferably limited to 50 ppm, more preferably limited to 25 ppm, and very preferably limited to 10 ppm, and preferably allowing to use this cup without further treatment to reduce its sulfur content,
• at least one intermediate fraction having a relatively low olefin content and aromatic content,
• a heavy fraction in which the majority 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 fraction of sulfur, at least an intermediate fraction composed mainly of compounds having from 6 to 8 carbon atoms and a heavy fraction containing most of the sulfur initially. present in the initial essence.

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, of the column expressed by the ratio between the liquid flow rate in the column and the flow rate of the charge. is generally less than unity and preferably less than 0.8, when these flow rates are measured in kilograms per hour (kg / h).

The light gasoline obtained after the separation generally contains at least all the C ₅ olefins, preferably the C ₅ compounds and at least 20% of the C ₆ olefins. Generally, this light fraction has a low sulfur content (for example less than 50 ppm), ie it is not generally necessary to treat the light cut before using it as fuel. However, in some extreme cases, softening of light gasoline may be considered.

Hydrogenation of Unsaturated Compounds of Sulfur (Step c1)

This step, which applies to the heavy gasoline optionally mixed with at least a portion of an intermediate fraction obtained at the end of the separation step b. Preferably this intermediate fraction is composed essentially (that is to say more than 60% by weight, preferably more than 80% by weight) of C ₆ or C ₇ molecules as well as of most of the sulfur compounds. having a boiling point close to that of the azeotrope of thiophene with paraffins, to about 20%. This step consists in converting at least a portion of the unsaturated sulfur compounds such as the thiophene compounds into saturated compounds, for example thiophanes (or thiacyclopentanes) or in mercaptans, or optionally at least partially hydrogenolysing these unsaturated sulfur compounds to form H ₂ S.
This step may for example be carried out by passing the heavy fraction, optionally mixed with at least a portion of an intermediate fraction on a catalyst comprising at least one element of group VIII and / or at least one element of group VIb at least part in sulphide form, in the presence of hydrogen, at a temperature between about 200 ° C and about 350 ° C, preferably between 220 ° C and 290 ° 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 H ₂ / HC ratio is between 50 to 600 liters per liter and preferably between 300 and 600 liters per liter.
In order to achieve, at least in part, the hydrogenation of the unsaturated sulfur compounds of the gasoline according to the process of the invention, at least one catalyst, comprising at least one group VIII element (Group 8 metals), is generally used. 9 and 10 of the new classification, i.e. iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium or platinum) and / or at least one Group VIb element (Group 6 metals of the new classification, ie chromium, molybdenum or tungsten), on a suitable support.

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 the 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. In order to minimize the hydrogenation of the olefins present in heavy gasoline, it is advantageous to preferentially use a 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 0.07. 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 carried out under conditions making it possible to convert at least a part of the oxidized forms of the metal of group VIII and / or group VI b to the metallic state. 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 oxidation step is performed after the sulfur or a sulfur compound has been introduced onto the catalyst.
It is therefore for example preferable when the catalyst is sulphurized after drying not to calcine the catalyst, against a reduction step may optionally be carried out after the sulphidation.

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 the 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 patents FR-B-2,708,596 and FR-B-2,708,597.

In the process according to the invention, the conversion of the unsaturated sulfur compounds is greater than 15% and preferably greater than 50%. At the same time, the degree of hydrogenation of the olefins is preferably less than 50%, more preferably less than 40%, and very preferably less than 35%, during this step.

In the process according to the invention, the species treated in stage c1 may optionally contain at least part of at least one intermediate fraction obtained in stage b. For example it may be advantageous to treat in this step a fraction of the gasoline that is not desirable to send in the catalytic reforming process.

The effluent resulting from this first hydrogenation step is then optionally sent to step c2 which makes it possible to decompose saturated sulfur compounds into H ₂ S.

- Decomposition of compounds saturated with sulfur (stage c2):

The feedstock of this step consists either of only the effluent from step c1, or of a mixture comprising the effluent of step c1 and at least a portion of at least one intermediate fraction. Preferably this intermediate fraction is composed essentially (that is to say more than 60% by weight, preferably more than 80% by weight) of C ₆ or C ₇ molecules as well as of most of the sulfur compounds. having a boiling point close to that of the azeotrope of thiophene with hydrocarbons to 20%.

In this step, the saturated sulfur compounds are converted to the presence of hydrogen on a suitable catalyst. The decomposition of the non-hydrogenated unsaturated compounds in step c1 can also take place simultaneously. This transformation is carried out without substantial hydrogenation of the olefins, ie during this step the amount of hydrogenated olefins is generally limited to less than 20% by volume relative to the olefin content of the gasoline. initial, and preferably limited to 10% by volume relative to the olefin content of the initial gasoline.

The catalysts which may be suitable for this stage of the process according to the invention, without this list being limiting, are catalysts generally comprising at least one basic element (metal) chosen from Group VIII elements, and preferably chosen from group formed by nickel, cobalt, iron, molybdenum, tungsten. These metals can be used alone or in combination, they are preferably supported and used in their sulfurized form. It is also possible to add at least one promoter to these metals, for example tin. Catalysts comprising nickel, or nickel and tin, or nickel and iron, or cobalt and iron, or cobalt and tungsten are preferably used. Said catalysts are more preferably sulfurized, and very preferably presulfided in situ or ex situ. The catalyst of step c2 is preferably of a different nature and / or composition than that used in step c1.
The base metal content of the catalyst according to the invention is generally between about 1 and about 60% by weight, preferably between 5 and 20% by weight, and very preferably between 5 and 9% by weight. Preferably, the catalyst is generally shaped, preferably in the form of beads, pellets, extrudates, for example trilobes. The metal may be incorporated into the catalyst by deposition on the preformed support, it may also be mixed with the support before the shaping step. The metal is generally introduced in the form of a precursor salt, generally soluble in water, such as for example nitrates, heptamolybdates. This mode of introduction is not specific to the invention. Any other mode of introduction known to those skilled in the art may be suitable.

The supports of the catalysts used in this stage of the process according to the invention are generally porous solids chosen from refractory oxides, such as, for example, aluminas, silicas and silica-aluminas, magnesia, as well as titanium and zinc oxide, these latter oxides can be used alone or mixed with alumina or silica-alumina. Preferably, the supports are transition aluminas or silicas whose specific surface is between 25 and 350 m ² / g. The natural compounds, such as, for example, kieselguhr or kaolin, may also be suitable as supports for the catalysts used in this step of the process.

According to a preferred method of preparing the catalyst, after introduction of at least one metal or precursor of said metal and possibly forming the catalyst, the catalyst is in a first activated step. This activation can correspond to either an oxidation, then a reduction, or a reduction after drying without calcination, or only to a calcination. The calcination step when present is generally carried out at temperatures of from about 100 ° C to about 600 ° C and preferably from 200 ° C 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 the metallic state. Generally, it consists of treating the catalyst under a flow of hydrogen at a temperature 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 partly in its sulfurized form, which has the advantage of minimizing the risks of hydrogenation of unsaturated compounds such as olefins or aromatic compounds during the starting phase. The introduction of sulfur can occur between different activation steps. Preferably, no 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 presulfided 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 optionally dried and then transferred to the reactor used to carry out the process of the invention. In this reactor, the catalyst is then treated with hydrogen in order to transform at least one part of the base metal and possibly another sulphide metal. A procedure which is particularly suitable for the invention is that described in patents FR-B-2,708,596 and FR-B-2,708,597.
After sulfurization, the sulfur content of the catalyst is generally between 0.5 and 25% by weight, preferably between 4 and 20% by weight and very preferably between 4 and 10% by weight. The hydrodesulfurization carried out during this step c2 is intended to convert to H ₂ S saturated sulfur compounds of gasoline which have already undergone at least one prior hydrogenation of unsaturated sulfur compounds in step c1. It makes it possible to obtain an effluent that meets the desired specifications in terms of sulfur compound content. The gasoline thus obtained shows only a slight loss of octane (decrease of the research and / or engine octane number).

The treatment for decomposing the saturated sulfur compounds from step c1 of the process is carried out in the presence of hydrogen, with a catalyst comprising at least one base metal selected from the group formed by nickel, cobalt, iron, molybdenum, tungsten, used alone or in a mixture with one another, at a temperature of between about 100 ° C. and about 400 ° C., preferably between about 150 ° C. and about 380 ° C., more preferably between 210 ° C. and 360 ° C, and very preferably between 220 ° C and 350 ° C, at a pressure generally between about 0.5 and about 5 MPa, preferably between 1 and 3MPa, more preferably between 1.5 and 3 MPa. The space velocity of the liquid is between about 0.5 and about 10 h ^-1 (expressed as volume of liquid per volume of catalyst per hour), preferably between 1 and 8 h ^-1 . The H ₂ / HC ratio is adjusted according to the desired hydrodesulphurization rates in the range of from about 100 to about 600 liters per liter, preferably from 20 to 300 liters per liter. All or part of this hydrogen may possibly come from step c1 (unconverted hydrogen) or a recycling of the hydrogen not consumed in steps a1, a2, c2 or d.

It has been found that the implementation of this second catalyst in this step, under particular operating conditions, makes it possible to decompose the saturated compounds, contained in the effluent from step c1, into H2S. This implementation makes it possible to achieve a high overall level of hydrodesulfurization at the end of all the steps of the process according to the invention, while minimizing the octane loss resulting from the saturation of the olefins, because the conversion of the olefins during step c1 is generally limited to at most 20% by volume of the olefins, preferably at most 10% by volume.

- Hydrotreatment of at least one intermediate cut (step d):

This treatment of at least one of the intermediate sections aims to eliminate substantially all the sulfur and nitrogen compounds of this fraction and to treat the effluent thus hydrotreated on a reforming catalyst for the isomerization and dehydrocyclization of paraffins. This step applies to at least a portion of an intermediate fraction obtained in step b.

It consists in treating said fraction on a catalyst or a set of catalysts for desulphurizing and totally denaturing the fraction in question, that is to say to obtain a fraction containing a sulfur and nitrogen content preferably less than 5 ppm, and more preferably less than 1 ppm by weight by conversion of sulfur compounds or nitrogen respectively to H ₂ S and ammonia.

This step is generally carried out by passing the fraction over at least one conventional hydrotreating catalyst under conditions which make it possible to eliminate sulfur and nitrogen. Catalysts which are particularly suitable are, for example, catalysts based on a Group VIII metal such as cobalt or nickel and a Group VI metal, such as tungsten or molybdenum. Typically, without these conditions being limiting, this treatment can be carried out on a type HR 306 or HR448 catalyst sold by the company Procatalyse, at a temperature generally between 250 and 350 ° C., an operating pressure generally of between 1 and 5 MPa, preferably between 2 and 4 MPa, and a space velocity generally between 2 and 8 h ^-1 (expressed in volume of liquid filler per volume of catalyst and per hour). During this treatment, almost all the olefins present in this fraction are hydrogenated.

The effluent thus obtained is cooled, the decomposition products are then separated by means of any technique known to those skilled in the art. For example, washing processes, stripping or extraction methods may be mentioned.

The effluent corresponding to one of the desulphurized and de-nitrogenated intermediate fractions is then treated on a catalyst or a catalyst sequence allowing reforming of the said fraction, that is to say to carry out at least partly the dehydrogenation of the saturated cyclic compounds. isomerization of paraffins and dehydrocyclization of parafines present in the treated intermediate fraction. This treatment aims at increasing the octane number of the fraction considered. This treatment is done by means of a conventional catatalytic reforming process. For example, it may be advantageous to use "fixed bed" or "bed" methods for this purpose. mobile ", that is to say processes in which the catalyst is respectively arranged in a fixed bed or on the contrary fluidized and possibly circulated in at least one reactor and in an external circulation loop optionally comprising other reactors and / or at least one regenerator In the process used, the desulfurized effluent is brought into contact with a reforming catalyst, generally based on platinum supported on alumina, at a temperature of between 400 ° C. and 700 ° C. with a hourly space velocity (kg of treated feedstock per hour and per kg of catalyst) between 0.1 and 10. The operating pressure can be between 0.1 and 4 MPa.A part of the hydrogen produced during the different reactions can be recycled in a ratio of between 0.1 and 10 moles of hydrogen per mole of filler.

Figure 1 shows an embodiment of the method according to the invention. In this example, an essence cut (initial essence) containing sulfur is introduced via line 1 into a catalytic hydrogenation reactor 2, which makes it possible to selectively hydrogenate the diolefins and / or the acetylene compounds present in said gasoline cut (step a1 of the process). The effluent 3 of the hydrogenation reactor is sent to a reactor 4, which comprises a catalyst capable of converting the light sulfur compounds with diolefins or olefins into heavier sulfur compounds (step a2). The effluent 5 from the reactor 4 is then sent to the fractionation column 6, which makes it possible to separate the gasoline into 3 fractions (step b).

The first fraction obtained is a light cut 7. This light cut preferably comprises less than 50 ppm of sulfur and does not require desulfurization, since the light sulfur compounds present in the initial gasoline have been converted into heavier compounds at the same time. step a2.

A second fraction 8 (intermediate fraction) is obtained which is first sent to a catalytic desulphurization reactor 10, then via line 11 to a catalytic reforming reactor (step d).

A third fraction (heavy fraction) is obtained via line 9. This cut is first treated in a reactor 14 on a catalyst for converting at least a portion of the unsaturated sulfur compounds present in the feed into saturated sulfur compounds (step c1). The effluent from the reactor 14 is sent to the reactor 16 (step c2) which contains a catalyst promoting the decomposition in H ₂ S saturated sulfur compounds initially present in the load and / or formed in the reactor 14.

The light section 7 and the effluent 13 (from the reforming reactor 14) and the effluent 17 (from the decomposition reactor 13) are mixed to form the desulfurized gasoline 18 (step e).

According to other preferred embodiments, presented in FIG. 1, it is also possible to send at least part of the non-desulphurized intermediate fraction (line 8), either via line 19 and then mixed with heavy fraction 9 towards the reactor 14 (step c1), either via the line 20 and then mixed with the effluent 15 to the reactor 16 (step c2).

The following examples illustrate the invention.

Example 1 (comparative)

A catalytic cracking gasoline whose characteristics are summarized in Table 1 is treated with the objective of reaching a specification of the refinery outlet gasoline pool such that the sulfur content is less than 10 ppm, which requires reducing the Sulfur content of a gasoline from a catalytic cracking unit to less than 20 ppm by weight.

The gasoline is separated into three sections in a light section, the distillation range of which is between 35 ° C and 95 ° C, an intermediate cut of which the distillation range is between 95 ° C and 150 ° C and a cut the distillation range is between 150 ° C and 250 ° C.

The sulfur content of light gasoline, which represents 38% of the total gasoline, is 210 ppm by weight.

The intermediate and heavy cuts are treated on a HR306 catalyst of the company Procatalyse. The catalyst (20 ml) is first sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., in contact with a feed consisting of 2% of sulfur in the form of dimethyl disulphide in n-heptane. . The desulfurization step is carried out at 300 ° C. under 35 bar with an H2 / HC of 150 I / I and a VVh of 3 h ^-1 . Under these treatment conditions the effluents obtained after stripping of the H ₂ S contain 1 ppm of sulfur. Mixing these two desulphurized cups with the lightest cut leads to a gasoline containing 81 ppm by weight of sulfur.

Example 2

The gasoline from a catalytic cracking unit whose characteristics are described in Example 1 is subjected to a hydrogenation treatment of diolefins under conditions in which the sulfur-containing light compounds present in the feed are partly converted into more complex compounds. heavy (steps a1 and a2 simultaneous).
This treatment is carried out in a reactor operating continuously and in updraft. The catalyst is based on nickel and molybdenum (HR945 catalyst marketed by the company procatalysis). The catalysts are first sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock consisting of 2% of sulfur in the form of dimethyl disulphide in n-heptane. The reaction is carried out at 160 ° C. under a total pressure of 1.3 MPa, with a space velocity of 6 h ^-1 . The H2 / feed ratio, expressed in liters of hydrogen per liter of feed and 10.
The gasoline is then separated into two slices, one having a distillation range between 35 ° C and 80 ° C and representing 29% volume and the other distilling between 80 ° C and 240 ° C representing 71% volume of the product. essence cut. The sulfur content of light gasoline is 22 ppm by weight.
The heavy gasoline is subjected to hydrodesulphurization on a series of catalysts in an isothermal tubular reactor. The first catalyst (catalyst A, step c1) 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 an 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 sample is 3% CoO and 10% MoO3.
The second catalyst (catalyst B, step c2) is prepared from a transition alumina of 140 m 2 / g in the form of beads 2 mm in diameter. The pore volume is 1 ml / g of support. 1 kilogram of support is impregnated with 1 liter of nickel nitrate solution. The catalyst is then dried at 120 ° C and calcined under a stream of air at 400 ° C for one hour. The nickel content of the catalyst is 20% by weight.
25 ml of catalyst A, and 50 ml of catalyst B, are placed in the same hydrodesulfurization reactor, so that the feedstock to be treated (heavy fraction) first meets the catalyst A (step c1) and then the catalyst B (step c2). An effluent sampling zone resulting from step c1 is provided between the catalysts A and B. The catalysts are first sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., contact of a filler consisting of 2% sulfur as dimethyl disulphide in n-heptane.

The operating conditions of the hydrodesulfurization are as follows: VVH = 1.33 h ^-1 relative to the entire catalytic bed H2 / HC = 360 l / l, P = 1.8 MPa. The temperature of the catalytic zone comprising catalyst A is 260 ° C, the temperature of the catalytic zone containing catalyst B is 350 ° C. The product obtained contains 19 ppm of sulfur.
The desulfurized product is recombined with light gasoline. The measurement of the sulfur content of the gasoline thus obtained leads to a content of 20 ppm by weight. It has a RON of 88.1 and a MON of 79.6, which is a loss of (RON + MON) / 2 compared to the 2.2 point load. The olefin content of this species is 22% vol.

Example 3 (according to the invention):

• l'une présentant un intervalle de distillation entre 35°C et 80°C et représentant 28 % vol et présentant une teneur en soufre de 20 ppm poids;
• une deuxième coupe distillant entre 80°C et 95°C et représentant 10 % volume de l'essence de de départ et contenant 250 ppm poids de soufre;
• une troisième coupe distillant entre 95°C et 150°C, représentant 30 % volume de l'essence de départ et contenant 1000 ppm poids de soufre. Le RON et le MON de cette coupe sont respectivement de 90 et 79.
• une quatrième coupe distillant entre 150°C et 240°C représentant 32 % volume de l'essence de départ et contenant 4600 ppm poids de soufre.

The gasoline from a catalytic cracking unit whose characteristics are described in Example 1 is subjected to a hydrogenation treatment of diolefins under conditions in which the sulfur-containing light compounds present in the feed are partly converted into more complex compounds. heavy (steps a1 and a2 simultaneous).
This treatment is carried out in a reactor operating continuously and in updraft. The catalyst is based on nickel and molybdenum (HR945 catalyst marketed by the company procatalysis). The catalysts are first sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock consisting of 2% of sulfur in the form of dimethyl disulphide in n-heptane. The reaction is carried out at 160 ° C. under a total pressure of 1.3 MPa, with a space velocity of 6 h ^-1 . The H2 / feed ratio, expressed in liters of hydrogen per liter of feed and 10.
The essence is then separated into four sections:

• one having a distillation range between 35 ° C and 80 ° C and representing 28% vol and having a sulfur content of 20 ppm by weight;
• a second cut distilling between 80 ° C and 95 ° C and representing 10% volume of the starting gasoline and containing 250 ppm by weight of sulfur;
• a third section distilling between 95 ° C and 150 ° C, representing 30% volume of the starting gasoline and containing 1000 ppm by weight of sulfur. The RON and MON of this section are respectively 90 and 79.
• a fourth cut distilling between 150 ° C and 240 ° C representing 32% volume of the starting gasoline and containing 4600 ppm weight of sulfur.

The gasoline is heavy and mixed with the second section and is subjected to hydrodesulfurization on a series of catalysts in isothermal tubular reactor. The first catalyst (catalyst A, step c) 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 an 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 sample is 3% CoO and 10% MoO3.
The second catalyst (catalyst B, step d) is prepared from a transition alumina of 140 m 2 / g in the form of beads 2 mm in diameter. The pore volume is 1 ml / g of support. 1 kilogram of support is impregnated with 1 liter of nickel nitrate solution. The catalyst is then dried at 120 ° C. and calcined under a stream of air at 400 ° C for one hour. The nickel content of the catalyst is 20% by weight.
25 ml of catalyst A, and 50 ml of catalyst B, are placed in the same hydrodesulphurization reactor, so that the charge to be treated (heavy fraction) first meets the catalyst A (step c) and then the catalyst B (step d). An effluent sampling zone resulting from step c is provided between catalysts A and B. The catalysts are first sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., contact of a filler consisting of 2% sulfur as dimethyl disulphide in n-heptane.

The operating conditions of the hydrodesulphurization are as follows: VVH = 1.33 h ^-1 relative to the entire catalytic bed H2 / HC = 360 I / I, P = 1.8 MPa. The temperature of the catalytic zone comprising catalyst A is 260 ° C, the temperature of the catalytic zone containing catalyst B is 350 ° C. The product obtained contains 37 ppm of sulfur.

The third section is treated with an HR306 catalyst from Procatalyse. The catalyst (20 ml) is first sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350 ° C., in contact with a feed consisting of 2% of sulfur in the form of dimethyl disulphide in n-heptane. . The desulfurization step is carried out at 300 ° C. under 3.5 MPa with a H2 / HC of 150 I / I and a VVH of 3 h ^-1 . Under these treatment conditions, the effluent obtained after stripping the H2S contains less than 1 ppm of sulfur. The olefin content is 0.9% by volume and the octane values are 68.7 for the RON and 68.3 for the MON. The gasoline obtained is then treated on a reforming catalyst CR201 marketed by the company Procatalyse. The catalyst (30 ml) is first reduced to 500 ° C under a stream of hydrogen before use. The reforming treatment is carried out at 470 ° C. under a pressure of 7 bar. The ratio H2 / HC is 500 l / l. The VVH is 2 h ^-1 .
The effluent is stabilized by removal of compounds having less than 5 carbon atoms. The reformate obtained, which represents 86% of the treated gasoline fraction, has a sulfur content of less than 1 ppm by weight, an RON of 97 and a MON of 86.

The fractions from the different treated sections are remixed. The sulfur content is 20 ppm by weight. The average value (RON + MON) / 2 of the total desulphurized gasoline increased by 1.3 points compared to that of the starting gasoline. In addition, the hydrogen produced during the catalytic reforming step can be used for the hydrotreatment reaction sections which is an obvious advantage of the process.

Claims (8)

1. Process for production of petrol with a low sulphur content from a feed-stock containing sulphur comprising at least the following steps:

   a1) at least selective hydrogenation of the diolefins and of the acetylene compounds contained in the feed-stock,

   a2) at least one step situated before step b intended to increase the molecular weights of the light sulphur-containing products present in the feed-stock and/or the effluent from step a1,

   b) at least separation of the effluent obtained on completion of step a1 or a2 into at least three fractions, a light fraction practically free from sulphur and containing the lightest olefins, a heavy fraction in which the major part of the sulphur-containing compounds initially present in the initial petrol is concentrated, and at least one intermediate fraction having a relatively low content of olefins and aromatics,

   c1) at least treatment of the heavy petrol separated in step b on a catalyst in the presence of hydrogen, the said step consisting of transforming at least a part of the unsaturated sulphur compounds into saturated compounds, while limiting conversion of the olefins to at most 20% by volume, and of at least partial hydrogenolysis of these unsaturated sulphur-containing compounds to form H₂S, this step is followed by stripping of the desulphurized heavy petrol in order to remove the H₂S produced,

   d) at least one step of desulphurization and denitrogenation of at least one intermediate fraction, followed by catalytic reformation.
2. Process as describe in one of claims 1, also comprising, before the said stripping, a step c2 of treatment of the effluent from step c1, the saturated sulphur compounds being transformed during the said step c2 in the presence of hydrogen and on a catalyst which also permits decomposition of the unsaturated compounds not hydrogenated in step c1.
3. Process as described in any one of claims 1 to 2, also comprising a step e of mixture of at least two fractions at least one of which has been desulphurized in step c1 and optionally c2 and/or in step d.
4. Process as described in any one of claims 1 to 3, in which a part of at least one intermediate fraction obtained in step b is mixed with the heavy fraction from step b before step c1.
5. Process as described in any one of claims 1 to 3, in which a part of at least one intermediate fraction obtained in step b is mixed with the effluent from step c1.
6. Process as described in any one of claims 1 to 5, in which step d of desulphurization and denitrogenation is accompanied by total hydrogenation of the olefins.
7. Process as described in any one of claims 1 to 6, in which the feed-stock is a petrol cut from a catalytic cracking unit.
8. Process as described in any one of claims 1 to 7, in which step b comprises separation of the effluent obtained on completion of step a1 into four fractions: a light fraction, a heavy fraction and two intermediate fractions, and in which one of the intermediate fractions is treated in step d and the other mixed with the heavy fraction separated in step b and then treated in step c1 and/or c2.

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