FR2790000A1 - PROCESS FOR PRODUCING LOW SULFUR CONTENT - Google Patents

Abstract

Process for the production of low sulfur gasoline comprising at least the following two stages: a) a stage of hydrogenation of the unsaturated sulfur compounds, b) a stage of decomposition of the saturated sulfur compounds and optionally a preliminary stage of pretreatment of the feed such as selective hydrogenation of dienes.

Description

The present invention relates to a process for the production of

  low sulfur content, which makes it possible to recover the totality of a gasoline fraction containing sulfur, to reduce the total sulfur contents of said gasoline fraction to very low levels, without a substantial reduction in gasoline yield, and minimizing the decrease octane number due to the hydrogenation of olefins. 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 of aromatics (especially benzene) and sulfur be significantly increased. Catalytic cracking gasolines, which can represent

  at 50% of the gasoline pool, have high olefin and sulfur contents.

  The sulfur present in the reformulated gasolines is attributable, at nearly 90%, to catalytic cracking (FCC), the "fluid catalytic cracking" or the fluid desulphurisation (hydrodesulphurisation) of the gasolines. mainly FCC species is therefore of obvious importance to

the achievement of specifications.

  Hydrotreating (hydrodesulphurisation) of the feedstock sent to catalytic cracking leads to gasolines typically containing 100 ppm of sulfur. However, the hydrocracking units of catalytic cracking feeds operate in severe conditions of temperature and pressure, which assumes a major investment effort. In addition, the entire charge must be desulfurized, which

  leads to 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

hyd rotirement.

  The separation of light gasoline and heavy gasoline before hydrotreatment has already been claimed in US Pat. No. 4,397,739. In this patent, there is claimed a process for the hydrodesulfurization of gasolines comprising a fractionation of the gasoline. gasoline into a light fraction and a heavy fraction and a specific hydrodesulfurization of the heavy fraction. On the other hand, in US Pat. No. 4,131,537 it is taught that it is advantageous to fractionate the gasoline in several cuts, preferably three, depending on their boiling point, and to desulphurize them in conditions which may be different and in the presence of a catalyst comprising at least one Group VIB and / or Group VIII metal. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three sections and when the cut exhibiting

  Intermediate boiling points are 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 sulfur-containing products by means of analyzes.

  equipment and 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 repeated whenever the composition of

the essence to be treated.

  In the French patent application No. 98/14 480, 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. In French Patent Application No. 98/02 944, there is described a process for treating catalytic cracking gasolines comprising the sequence of 2 steps: a mild hydrotreatment with optional stripping of the H 2 S produced, followed by removal of the mercaptans. This process makes it possible to almost completely eliminate the mercaptans during the second stage, but the overall rate of hydrodesulfurization at the end of the two stages remains limited, especially when operating with a recycling of

  unconsumed hydrogen and optionally containing hydrogen sulfide (H2S).

  It has also been proposed, for example in US Pat. No. 5,290,427, hydrotreatment processes for gasolines consisting in fractionating the gasoline, then in desulfurizing the fractions and in converting the desulphurized fractions on a zeolite ZSM-5.

  to compensate for the loss of octane recorded by means of isomerization.

  US-A-5,318,690 proposes a process with gasoline fractionation and softening of the light fraction, while the heavy fraction is desulphurized, then converted to ZSM-5 and desulphurized again under mild conditions. 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 that 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 recommends a

  cracking on 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 a softening or

  additional hydrodesulphurization.

  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 all of a gasoline fraction containing sulfur, preferably a gasoline catalytic cracking cut, and to reduce sulfur contents in said gasoline cut at very low levels, without substantially reducing the gasoline yield while minimizing the decrease in the octane number

  due to the hydrogenation of olefins.

  The process according to the invention is a process for the production of gasoline with a low sulfur content from a petrol cut containing sulfur. In the process according to the invention, it is not necessary to fractionate the feedstock, which is therefore preferably constituted by the entire gasoline cut. This is a technical and economical advantage over most of the methods described in the prior art. The process according to the invention comprises at least one treatment of the feedstock on a first catalyst making it possible to at least partially hydrogenate unsaturated sulfur compounds, in particular cyclic or even aromatic sulfur compounds such as, for example, thiophene compounds by placing them in conditions where the hydrogenation of the olefins is limited on this catalyst, then a second treatment on a second catalyst for decomposing linear and / or cyclic saturated sulfur compounds, with a

  limited hydrogenation of olefins.

  The two catalytic treatments can be carried out either in a common reactor with a sequence of the two catalysts or in two different reactors. It is also desirable in some cases to add a pretreatment step, preferably catalytic, aimed at hydrogenating the diolefins of the

  charge before the first step of the process according to the invention.

  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 5 carbon atoms (C5) up to about 250 C. The final point of the gasoline cut depends on the refinery from which it comes and

  market constraints, but generally remains within the limits indicated above.

  For this type of gasoline, a chromatographic analysis of the sulfur compounds shows that the various compounds described below are inter alia methanethiol, ethanethiol, propanethiol, thiophene, thiacyclobutane, butanethiol, pentanethiol, and the like. , 2-methylthiophene, 3-methylthiophene,

  thiacyclopentane, 2-methylthiacyclopentane, 2-ethylthiophene, 3-

  ethylthiophene, 2-5-dimethylthiophene, 3-methylthiacyclopentane, 2,4-

  dimethylthiophene, 2,3-dimethylthiophene, 2,5-dimethylthiacyclopentane, 3,3-

  dimethylthiacyclopentane, 3,4-dimethylthiophene, 2,3-dimethylthicyclopentane, 2-isopropyl thiophene, 3-isopropylthiophene, 3-ethyl-2-methylthiophene,

  thiophenol, 2,3,4 trimethylthiophene, 2,3,5 trimethylthiophene, benzothiophene.

  Depending on the cutting point of the gasoline and the operating conditions of the catalytic cracking unit, certain compounds may of course be absent from this gasoline. In addition, when treating high boiling point feeds, the presence of alkyl compounds of benzothiophene is also conceivable,

  even compounds derived from dibenzothiophene.

  The process according to the invention generally comprises at least a first step (step a), carried out by passing the filler, preferably consisting of the entire gasoline cut, over a catalyst which makes it possible to hydrogenate, at least in part, the compounds unsaturated sulfur present in said charge, such as for example thiophenic compounds, in saturated compounds, such as for example thiophanes (or thiacyclopentane), or mercaptans according to a succession of reactions described below

X X SHSH

  Thiophene Thiophane Mercaptan 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 partially under the form of sulphides. When such a catalyst is used, the operating conditions are adjusted in order to be able to hydrogenate at least partly the saturated compounds, such as the thiophene compounds, while limiting the hydrogenation of the olefins. The process according to the invention comprises a second step (step b) in which the sulfur-containing saturated compounds are converted into H2S according to the reactions: S H2S SH This treatment can be carried out on any catalyst that makes it possible to convert the saturated sulfur compounds (mainly the thiophanes or mercaptans). It may for example be carried out on a nickel-based catalyst,

molybdenum, or cobalt.

  The thus desulphurized gasoline is then optionally stripped (that is to say that a gaseous stream, preferably containing one or more inert gases, is passed through.

  through this gasoline), in order to eliminate the H2S produced during hydrodesulfurization.

  The terms first step (step a) and second step (step b) do not exclude the possible presence of another step, in particular a step of pretreatment of the charge, consisting for example in the selective hydrogenation of the dienes present in load. Such an optional pretreatment step is to

  preferably located before step a of the method according to the invention.

  Detailed Description the invention:

  It has been unexpectedly discovered that the combination of two suitable catalysts, a first catalyst promoting the conversion of sulfur unsaturated compounds present in gasoline, such as for example the thiophene compounds, and at least one second catalyst promoting the transformation selectively saturated sulfur compounds, linear or cyclic already present in gasoline or produced in the first stage, allows to ultimately obtain a desulphurized gasoline does not have a significant decrease in olefin content or the index d 'octane; this without the need to split gasoline, or to resort to processes to restore the octane number of gasoline. Hydrodesulfurization rates

  are reached, under reasonable operating conditions specified below.

  after, including when operating with a recycling of hydrogen not consumed

  and optionally containing hydrogen sulfide (H2S).

  The sulfur species contained in the fillers treated by the process of the invention may be mercaptans or heterocyclic compounds, such as, for example, thiophenes or alkylthiophenes, or heavier compounds,

  such as benzothiophene or dibenzothiophene. These hetero compounds

  cyclic, unlike mercaptans, can not be removed by extractive processes. These sulfur compounds are, on the other hand, removed by the process according to the invention which leads to their at least partial decomposition into hydrocarbons.

and H2S.

  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 greater than 200 ° C., the sulfur contents are often greater than 1000 ppm by weight, and in some cases they may even reach

  the order of 4000 to 5000 ppm by weight.

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

  Hydrogenation of the Dienes: The hydrogenation of the dienes is an optional but advantageous step which makes it possible to eliminate, before hydrodesulphurization, almost all the dienes present in the gasoline cutter containing sulfur to be treated. It is preferably carried out before 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 formed by platinum, palladium and nickel, and a support. For example, use will be made of a nickel-based catalyst deposited on an inert support, such as, for example, alumina, silica or a support containing at least 50% alumina. This catalyst operates at a pressure of 0.4 to 5 MPa, at a temperature of 50 to 250 C, with a liquid hourly space velocity of 1 to 10 h -1. Another metal may be combined to form a bimetallic catalyst, such as, for example,

molybdenum or tungsten.

  It may be particularly advantageous, especially when processing cuts having a boiling point below 160 ° C, to operate under conditions such that at least partial softening of the gasoline is achieved, that is, that is, some reduction in the mercaptan content. To do this, it is possible to use the diene hydrogenation procedure described in patent application EP-A-0 832 958, which

  advantageously uses a palladium-based catalyst.

  The choice of operating conditions is particularly important. The operation will generally be carried out under pressure in the presence of a quantity of hydrogen in small excess relative to the stoichiometric value necessary for hydrogenating 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. The temperature is most generally between about 50 and about 250 C, and

  preferably between 80 and 200 C, and more preferably between 160 and 190 C.

  The pressure employed is sufficient to maintain more than 80%, and preferably more than 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, and more preferably between 1 and 4 MPa. The space velocity is from about 1 to about 10 h -1,

preferably between 4 and 10 h-1.

  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 one embodiment of the invention, the step of hydrogenation of the dienes takes place in a catalytic hydrogenation reactor which comprises a catalytic reaction zone traversed by the totality of the charge and the quantity

  of hydrogen necessary to effect the desired reactions.

  Hydrogenation of Unsaturated Compounds of Sulfur (Step a): This step consists of converting at least a portion of the unsaturated sulfur compounds such as the thiophene compounds into saturated compounds, for example into

  thiophanes (or thiacyclopentanes) or in mercaptans.

  This step may for example be carried out by passing the feedstock, 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 partly in sulphide form, at a a temperature of between about 210 ° C. and about 320 ° C., preferably between 220 ° C. and 290 ° C., under a pressure generally of 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 10 hours (expressed as volume of liquid per volume of catalyst per hour), preferably between 3 and 8 hours -1. H2 / HC ratio is included

  between 100 to 600 liters per liter and preferably between 300 and 600 liters per liter.

  In order to achieve, at least partially, the hydrogenation of unsaturated sulfur compounds of the gasoline according to the process of the invention, at least one catalyst, comprising at least one element of group VIII (Group 8 metals), is generally used. 9 and 10 of the new classification, ie iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium or platinum) and / or at least one element of the group VIb (metals of Group 6 of the new classification, ie chromium, molybdenum or tungsten), on a suitable support. The element of group VIII, when present, is usually nickel or cobalt, and the element of the Vib group, when present, is usually molybdenum or tungsten. Combinations such as nickel-molybdenum or 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.

  After introduction of the element or elements and possibly shaping of the catalyst (when this step is carried out on a mixture already containing the basic elements). the catalyst is in a first activated step. This activation may correspond to either an oxidation, then a reduction, or a direct reduction, or a calcination only. The calcination step is generally carried out at temperatures ranging from about 100 to about 600 ° C. and preferably between 200 ° C. and 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 stream of hydrogen at a temperature preferably of at least 300 C.

  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 when the sulfur or a sulfur compound has been introduced on the catalyst. The sulfur or a sulfur compound may be introduced ex situ, that is to say outside the reactor where the process according to the invention is carried out, or in situ, ie in the reactor used for the process according to the invention. 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 H2S, 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.

  According to the process of the invention, the conversion of 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% and so

  preferred less than 40% during this step.

  The effluent resulting from this first hydrogenation step is then sent to

  the level of the catalyst which makes it possible to decompose saturated sulfur compounds into H2S.

  -Decomposition of sulfur saturated compounds (step b): In this step, the saturated sulfur compounds are converted, in the presence of hydrogen on a suitable catalyst. This transformation is carried out without substantial hydrogenation of the olefins, ie during this step the hydrogenation of the olefins is generally limited to 20% by volume relative to the olefin content of the starting gasoline, and preferably, limited to 10% in

  volume relative to the olefin content of gasoline.

  The catalysts which may be suitable for this stage of the process according to the invention, without this list being limiting, are catalysts comprising at least one base metal chosen from the group formed by nickel, cobalt, iron, molybdenum, tungsten. These metals can be used alone or in combination, they are of

  preferably supported and used in their sulfurized form.

  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.

  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 is suitable for the invention. The supports of the catalysts used in this step 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, the latter oxides may be used alone or in admixture with alumina or silica-alumina. Preferably, the supports are transition aluminas or silicas whose specific surface area is between 25 and 350 m 2 / g. Natural compounds, such as for example kieselguhr or kaolin, may also be suitable as catalysts used

in this step of the process.

  After introducing the base metal and possibly forming the catalyst (when this step is carried out from a mixture already containing the base metal), the catalyst is in a first activated step. This activation may correspond to either an oxidation, then a reduction or a direct reduction, or only to a calcination. The calcination step is generally carried out at temperatures ranging from about 100 to about 600 ° C. and preferably between 200 ° C. and 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

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  flow of hydrogen at a temperature of at least 300 C. The reduction can also

  to be carried out partly by means of chemical reducers.

  The catalyst is preferably used at least in part in its sulfurized form. This has the advantage of minimizing the risks of hydrogenation of unsaturated compounds such as olefins or aromatic compounds during the start-up phase. The introduction of sulfur can occur between different activation steps. Preferably, no oxidation step is performed when the sulfur or a sulfur compound is introduced on the catalyst. The sulfur or a sulfur compound may 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, and 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 H2S, 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 of 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.

  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

preferred between 4 and 10% by weight.

  The hydrodesulphurization carried out during this step is intended to convert the saturated sulfur compounds of gasoline to H2S which has already undergone at least one prior hydrogenation of unsaturated sulfur compounds, so as to obtain an effluent, which will respond the desired specifications in terms of compound content

  sulfur. The gasoline thus obtained has only a low octane loss.

  It has been found that the use of this second catalyst in this step, under particular operating conditions, makes it possible to decompose the saturated compounds, contained in the effluent resulting from the preceding step, into H 2 S. 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 b is generally limited to at most 20% by volume of olefins,

  preferably at most 10% by volume.

  The treatment for decomposing the saturated sulfur compounds from the first step of the process is carried out in the presence of hydrogen, with the catalyst comprising at least one base metal selected from the group formed by nickel, cobalt, iron, molybdenum, tungsten, at a temperature of between about 250 ° C. and about 350 ° C., preferably between about 260 ° C. and about 350 ° C., more preferably between about 260 ° C. and about 320 ° C., under low to moderate pressure, generally between about 0.5 and about 5 MPa, preferably between 0.5 and 3MPa, more preferably between 1 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 hr. The H2 / HC ratio is adjusted according to the desired hydrodesulfurization rates in the range of from about 100 to about 600 liters per liter, preferably from 300 liters per liter. All or part of this hydrogen can come from step a or a recycling of the unconsumed hydrogen from step b. This hydrogen from

  steps a or b may optionally contain non-separated H2S.

  Preferred Embodiments of the Process According to the Invention One of the possibilities of implementing the process according to the invention consists, for example, in passing the gasoline to hydrodesulfurize through two separate reactors containing respectively for the first reactor: or part, preferably all, of a catalyst allowing, at least in part, the hydrogenation

2790000

  unsaturated sulfur compounds (step a), such as for example thiophenic compounds, sulfur saturated compounds (such for example thiacyclopentanes or mercaptans) and for the second reactor: a catalyst for decomposing saturated compounds of sulfur into H 2 S ( step b), and possibly another part of the catalyst required for step a, preferably at the head of bed. Between the two reactors, systems can be implemented to dissociate possibly

  the operating conditions of the two reaction zones.

  In another configuration of the process according to the invention, the two catalysts

  can be placed in series in the same reactor.

  In both cases, the two catalytic zones can operate under different or identical conditions of pressure, temperature, VVH, and H2 / filler ratio. In summary, the process according to the invention consists of a gasoline production process. low sulfur content characterized in that it comprises at least two stages:

  a) a hydrogenation step of the unsaturated sulfur compounds.

  b) a decomposition step of the saturated sulfur compounds.

  wherein: a pretreatment step for hydrogenating diolefins in the feed is

  possibly performed before step a.

  M the load is preferably constituted by the whole of a section esence, of

  preferably a catalytic cracking gasoline.

  Step a is carried out by passing the feedstock, in the presence of hydrogen, over a catalyst that makes it possible to hydrogenate the unsaturated sulfur compounds, and preferably comprising at least one element from group VIII and / or at least one element at least partially in the sulfide form of group VIb, and wherein the group VIII element, when present, is preferably nickel or cobalt, and the element of group VIb, when present, is preferably the

molybdenum or tungsten.

  Step a is carried out at a temperature of between about 210 ° C. and about 320 ° C., under a pressure generally of between about 1 and about 4 MPa, with a liquid space velocity of between about 1 and about 10 hours, and H2 / HC ratio of between about 100 and about 600 liters. Step b is carried out in the presence of a catalyst for decomposing the saturated sulfur compounds, preferably comprising at least one base metal chosen from the group formed by nickel, cobalt, iron, molybdenum, tungsten, the base metal content being between 1 and 60% by weight, preferably between 5 and 20% by weight, and said metal preferably being sulfurized. Step b is carried out at a temperature between about 250 ° C. and about 350 ° C., a pressure of between about 0.5 and about 5 MPa, a liquid space velocity between about 0.5 and about 10 hours and a

  H2 / HC ratio between about 100 and about 600 liters per liter.

  the process can optionally be implemented by means of a single reactor containing the catalysts required for steps a and b, pretreatment reactor of the feedstock (such as, for example, a diene hydrogenation reactor) not included. It can also be optionally implemented using at least two separate reactors, excluding pretreatment reactor of the feedstock, the first reactor containing the catalyst required for step a

  and the second at least that required in step b.

  With the process according to the invention as described, it is possible to achieve high levels of hydrodesulphurization while limiting the loss of olefins and by

  consequently the decrease of the octane number.

  The examples below illustrate the invention.

  Table 1 shows the characteristics of the feedstock (catalytic cracking gasolines) treated by the process according to the invention. The analytical methods used to characterize the charges and effluents are as follows: gas phase chromatography (GPC) for the hydrocarbon constituents;

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  - NF M 07052 method for total sulfur; - method NF EN 25164 / M 070262 / ISO 5164 / ASTM D 2699 for the research octane number; - Method NF EN 25163 / M 07026-1 / ISO 5163 / ASTM D 2700 for the motor octane number.

  Table 1: Characteristics of the load used.

  Load Density 0.75 Starting Point (C) 40 C End Point (C) 2000C Olefins Content (% vol.) 32 Total S (ppm) 1200

RON 90

MY 78

(RON + MON) / 2

  The analysis of sulfur compounds in the feed by gas chromatography coupled with a specific detector of PFDP (Pulse Flame Photometry) type

  Detector) leads to the results presented in Table 2.

  Table 2. Nature and concentration of sulfur compounds present

in charge.

  Concentrated sulfur compounds identified (ppm)

THIOPHENE 235

  Mercaptans 0 Methylthiophenes 487 Thiacyclopentane 82 Methylthiacyclopentane 40 C2 Thiophenes 227 diethylsulfide 11 C3 Thiophenes 26 C2 Thiacyclopentanes 46 C3 Thiacyclopentanes 46 Example 1 (comparative): hydrodesulfurization of gasoline over a catalyst allowing

  the conversion of unsaturated sulfur products.

  ml of HR306C catalyst, sold by the company Procatalyse, are placed in an isothermal tubular reactor, with a fixed bed of catalyst. The catalyst is first sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock consisting of 2% by weight of sulfur in the form of

  dimethyldisulphide in n-heptane.

  The hydrodesulfurization operating conditions are as follows: VVH = 4 h -1, H 2 / HC = 400 I / l, P = 2.7 MPa. Under these conditions, the effluent after desulphurization

  220 C, 230 C and 250 C, the characteristics described in Table 3.

  Table 3 Comparison of load characteristics and

desulfurized effluent.

  Load Effluent Effluent Effluent Temperature (C) 220 C 230 C 250 C S total (ppm) 1200 587 305 96 Olefins (% vol.) 32 27 24 16

MON 78 77.5 77 75

RON 90 89.1 87.7 83

(RON + MON) / 2 84 83.3 82.4 79

Loss in octane - 0.7 1.6 5

% HDS * 51 75 92

% HDO ** 16 25 50

  * HDS refers to the rate of hydrodesulfurization **% HDO refers to the hydrogenation rate of olefins These results show that on the cobalt and molybdenum catalyst, the high levels of desulphurization are accompanied by a significant loss in

  olefins and therefore a significant loss in octane.

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  On the other hand, the analysis of the nature of the sulfur compounds present in

  effluents leads to the results presented in Table 4.

Table 4

  Sulfur compounds Concentration Concentration Concentration Effluent effluent effluent effluent concentration identified (ppm) 51% HDS 75% HDS 92% HDS (ppm) -rn __ m_9ppm_ Thiophene 235 45 19 0 Mercaptans 0 161 125 68 Methylthiophenes 487 53 25 Thiacyclopentane 82 71 16 8 Methylthiacyclopentane 40 105 55 15 C2 Thiophenes 227 68 16 0 Diethylsulfide 11 0 0 0 C3 Thiophenes 26 8 4 0 C2 Thiacyclopentanes 46 65 35 5 C3 Thiacyclopentanes 46 11 10 5 this catalyst the unsaturated compounds of sulfur are

  converted significantly even if the desulphurisation rate is less than 75%.

  Example 2 (comparative): hydrodesulphurization of gasoline on a catalyst allowing

  the conversion of compounds saturated with sulfur.

  The gasoline whose characteristics are described in Table 1 is subjected to a hydrotreatment on a nickel-based catalyst in a tubular reactor

  isothermal, fixed catalyst bed. The catalyst is prepared as follows.

  It is prepared from a transition alumina of 140 m2 / g which is

  ball shape 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. The catalyst (100 ml) is then 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 sulfur in the form of

  dimethyldisulphide in n-heptane.

  The gasoline is then subjected to hydrotreatment under the following conditions:

  VVH = 2 h -1, P = 2.7 MPa, H2 / HC = 400 expressed in liter of hydrogen per liter of feedstock.

  The temperature of the tests is 300 C and 350 C. The characteristics of the effluents

  thus obtained are presented in Table 5.

  Table 5: Characteristics of gasolines after hydrodesulphurization (HDS) on nickel catalyst Effluent load obtained Effluent obtained after HDS at 300 ° C. after HDS at 350 ° C. Total (ppm) 1200 660 300 Olefins (% vol.) 32 31 29

MON 78 78 78

RON 90 90 89

(RON + MON) / 2 84 84 83.5

  Loss of octane 0 0.5% of HDS 45 75% of HDO 3 9 - The nickel-based catalyst thus makes it possible to desulphurize gasoline without consuming olefins. However, with this catalyst, it is difficult to achieve high levels of hydrodesulfurization, except by working at temperatures significantly higher than 300 C, which generates a loss of octane more

  important and imposes constraints on the process.

  The results of analysis of the nature and concentration of sulfur compounds

  obtained after hydrodesulfurization (HDS) are reported in Table 6.

Table 6

  Sulfur compounds Concentration Concentration Identified concentration of effluent effluent load (ppm) 45% HDS 75% HDS (ppm) (ppm) Thiophene 235 132 147 Mercaptans 0 30 3 Methylthiophenes 487 271 92 Thiacyclopentane 82 12 2 Methylthiacyclopentane 40 5 1 C2 Thiophenes 227 202 47 diethylsulfide 11 0 0 C3 Thiophenes 26 8 8 C2 Thiacyclopentanes 46 0 0 C3 Thiacyclopentanes 46 0 0 It can thus be noted that on this type of catalyst, the saturated sulfur compounds are converted significantly. Example 3 (comparative): hydrodesulfurization with a cobalt-molybdenum catalyst and a

recycles hydrogen.

  The gasoline whose characteristics are described in Table 1 is subjected to hydrodesulfurization on a conventional hydrotreating catalyst in an isothermal tubular reactor. 25 ml of HR306C D catalyst, sold by the company Procatalyse, are placed in the hydrodesulfurization reactor. The catalyst is first sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a feedstock consisting of 2% by weight of sulfur in the form of

  dimethyldisulphide in n-heptane.

  The operating conditions of the hydrodesulphurization are as follows: VVH = 4 hours, H2 / HC = 400 I / 1, P = 2.7 MPa. The partial pressure of H2S at the reactor inlet of 0.023 MPa to simulate the H2S bring by the recycle of hydrogen to

22; 2.79 million

  level of the unit. The temperature was raised to 250 C and 270 C. The characteristics

  effluents thus obtained are shown in Table 7.

  Table 7: Characteristics of gasolines after hydrodesulphurization with the catalyst based on cobalt and molybdenum Charge Effluent obtained Effluent obtained after HDS after HDS

at 250 C to 270 C

  S total (ppm) 1200 1100 953 Olefins (% vol.) 32 19 12

MON 78 76 75

RON 90 84 81

(RON + MON) / 2 84 80 78

  Loss of octane 46% of HDS 8 21% of HDO 41 63 These results show that in the presence of H2S, it is difficult to obtain on conventional sulphide catalysts a high level of HDS with a loss in olefins limited. This type of catalyst therefore requires the use of hydrogen without H2S to lead to good performance, which can in some cases

increase the cost of the process.

  Table 8 presents the results of analysis of the nature and concentration

  sulfur compounds obtained after hydrodesulfurization.

23; 2.79 million

Table 8

  Identified sulfur compounds Concentration Concentration Concentration of effluent effluent loading (ppm) 8% HDS 20% HDS Thiophene 235 10 0 Mercaptans 0 699 631 Methylthiophenes 487 5 0 Thiacyclopentane 82 150 173 Methylthiacyclopentane 40 85 60 C2 Thiophenes 227 18 0 diethylsulfide 11 0 0 C3 Thiophenes 26 0 0 C2 Thiacyclopentanes 46 98 53 C3 Thiacyclopentanes 46 35 36 Example 4 (according to the invention): Hydrodesulfurization with a chain of hydrogenation catalysts of unsaturated compounds and decomposition of

  compounds saturated with sulfur, and with recycle of hydrogen.

  The species whose characteristics are described in Table 1 is subject to

  hydrodesulphurization on a series of catalysts in isothermal tubular reactor.

  ml of HR306C catalyst, sold by the company Procatalyse, and 50 ml of the catalyst obtained according to the same protocol as that described in Example 2 are placed in the hydrodesulfurization reactor. The catalysts are first sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a

  filler consisting of 2% sulfur in the form of 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 = 400 1/1, P = 2.7 MPa.

  The temperature of the catalytic zone comprising the catalyst HR306C) is 250.degree. C., the temperature of the catalytic zone containing the catalyst of example 2 is 24 t2790000 of 290.degree. C. in order to simulate the H.sub.2 S provided by the recycle of the hydrogen, a quantity of H2S corresponding to a partial pressure of 0.023 MPa is injected at the reactor inlet.

  The characteristics of the effluent thus obtained are presented in Table 9.

  Table 9: Characteristics of gasolines after hydrodesulphurization with catalyst chain Total Effluent S (ppm) 1200 96 Olefins (% vol.) 32 23

MON 78 77

RON 90 87

(RON + MON) / 2 84 82

  Loss of octane 2.0% of HDS 92% of HDO 28 Thus with the chain of catalysts, it is possible to reach high hydrodesulfurization rates, with a limited consumption of olefins and a catalyst operating temperature allowing to convert the sulfur saturated compounds lower than in the case where it is used alone to treat gasoline

departure.

  Example 5 (according to the invention): hydrodesulfurization with a chain of hydrogenation catalysts for unsaturated compounds and decomposition of

  compounds saturated with sulfur, with recycle of hydrogen.

  The species whose characteristics are described in Table 1 is subject to

  hydrodesulphurization on a series of catalysts in isothermal tubular reactor.

  ml of HR306C catalyst, sold by the company Procatalyse, and 50 ml of the catalyst obtained according to the same protocol as that described in Example 2 are placed in the hydrodesulfurization reactor. The catalysts are first sulphurated by t2790000 treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a

  filler consisting of 2% sulfur in the form of 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 = 400 1/1, P = 2.7 MPa. The temperature of the catalytic zone comprising the catalyst HR306C E) is 230 ° C., the temperature of the catalytic zone containing the catalyst of Example 2 is 270 ° C. In order to simulate the H2S provided by the hydrogen recycle , a quantity of H2S corresponding to a partial pressure of 0.023 MPa is injected at the inlet of the

reactor.

* The characteristics of the effluent thus obtained are presented in

table 10.

  Table 10: Characteristics of gasolines after hydrodesulphurization with the catalyst chain Total Effluent S (ppm) 1200 240 Olefins (% vol.) 32 26

MON 78 77.8

RON 90 88.6

(RON + MON) / 2 84 83.2

  Loss of octane 0.8% of HDS 80% of HDO 19 Thus with the linking of catalysts, it is possible to reach high hydrodesulphurization rates, with a limited consumption of olefins and an operating temperature of catalyst for converting compounds

  saturated sulfur lower than in the case where it is used alone.

  Example 6 (comparative): hydrodesulphurization with a chain of hydrogenation catalysts for unsaturated compounds and decomposition of saturated compounds of

  sulfur operating at low temperature, with recycling of hydrogen.

  The species whose characteristics are described in Table 1 is subject to

  hydrodesulphurization on a series of catalysts in isothermal tubular reactor.

  ml of HR306C catalyst, sold by the company Procatalyse, and 50 ml of the catalyst obtained according to the same protocol as that described in Example 2 are placed in the hydrodesulfurization reactor. The catalysts are first sulphurized by treatment for 4 hours under a pressure of 3.4 MPa at 350 ° C., in contact with a

  filler consisting of 2% sulfur in the form of dimethyl disulphide in n-heptane.

  The operating conditions of hydrodesulfurization are as follows

  VVH = 1.33 h -1 relative to the entire catalytic bed H2 / HC = 400 I / 1, P = 2.7 MPa.

  The temperature of the catalytic zone comprising the catalyst HR306C is 230 ° C., the temperature of the catalytic zone containing the catalyst of Example 2 is 200 ° C. In order to simulate the H2S provided by the recycle of hydrogen, a amount of H2S corresponding to a partial pressure of 0.023 MPa is injected at the inlet of the reactor.

  The characteristics of the effluent thus obtained are presented in Table 11.

  Table 11: Characteristics of gasolines after hydrodesulphurization with the catalyst chain, the catalyst used to decompose saturated sulfur compounds operating at low temperature Total Effluent S Charge (ppm) 1200 900 Olefins (% vol.) 32 25

MON 78 77.5

RON 90 88

(RON + MON) / 2 84 82.8

  Loss of octane 1.2% of HDS 25% of HDO 22 Thus with the chain of catalysts, but with an operating temperature of the catalyst decomposing the compounds saturated with sulfur of 200 C, it is not

  possible to achieve high hydrodesulfurization rates.

Claims (12)

  1. Process for producing gasoline with a low sulfur content, characterized in that it comprises at least two stages: a) a step of hydrogenation of the unsaturated sulfur compounds,   b) a decomposition step of the saturated sulfur compounds.   The method of claim 1, wherein a pretreatment step for   hydrogenating the diolefins of the charge is carried out before step a.   3. Method according to one of claims 1 or 2 wherein the load is constituted by the whole of the petrol cut.   The method of any one of claims 1 to 3 wherein the charge is   a catalytic cracking gasoline.   The method of any one of claims 1 to 4 wherein step a is   carried out by passing the charge, in the presence of hydrogen, over a catalyst comprising at least one group VIII element and / or at least one element of the   group VIb, at least partly in sulphide form.   The method of claim 5 wherein the group VIII element, when present, is nickel or cobalt, and the group VIb element, when present, is molybdenum or tungsten.   7. The method of claim 5 wherein step a is carried out at a temperature between about 210 C and about 320 C, at a pressure generally between about 1 and about 4 MPa, with a space velocity of the liquid between about 1 and about 10 h -1, and an H2 / HC ratio included   between about 100 and about 600 liters.   8. Method according to one of claims 1 to 7 wherein step b is performed in   the presence of a catalyst comprising at least one base metal chosen from   group formed by nickel, cobalt, iron, molybdenum, tungsten.   The method of claim 8 wherein the base metal content is   between 1 and 60% by weight and said metal is sulphurized.   10. Method according to one of claims 1 to 10 wherein step b is performed at   a temperature of from about 250 ° C. to about 350 ° C., a pressure of from about 0.5 to about 5 MPa, a space velocity of the liquid of from about 0.5 to about 10 h -1, and an H2 / HC ratio of from about 0.5 to about 5 MPa; 100 and about 600 liters per liter.   11. Method according to any one of claims 1 to 11 implemented at   a single reactor containing the necessary catalysts in steps a and b,   pre-treatment reactor not included.   12. Method according to any one of claims 1 to 11 implemented at   means of at least two separate reactors, excluding charge pretreatment reactor, the first reactor containing the catalyst required in step a and the   second at least the one needed in step b.