EP1002853B1 - Process for the production of low sulfur gasolines - Google Patents

Description

The present invention relates to a process for the production of gasolines with a low sulfur content, which makes it possible to recover the totality of a petrol fraction containing sulfur, to reduce the total sulfur and mercaptan contents of said petrol fraction to very low levels. levels, with no significant decrease in fuel efficiency, and minimizing the decrease in octane number.

Prior art:

The production of reformulated species that meet the new environmental standards requires, in particular, that their concentration of olefins and / or aromatics (especially benzene) and sulfur (including mercaptans) be reduced. Thus, the catalytic cracking gasolines have high olefin contents, and the sulfur present in the reformulated gasoline is attributable, to nearly 90%, to catalytic cracking gasoline (FCC). in a fluidized bed). The desulphurisation (hydrodesulphurisation) of gasolines and mainly of FCC species is therefore of obvious importance.

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, resulting in the processing of very large load volumes.

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

The separation of light gasoline and heavy gasoline before hydrotreatment has already been claimed in US-A-4 397 739. In this patent, there is claimed a process for the hydrodesulphurisation of gasolines comprising a fractionation of gasoline in a light fraction and a heavy fraction and the 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 that may be different. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three slices and when the cut having intermediate boiling points is treated under mild conditions.

EP-A-0 725 126 discloses a method for hydrodesulphurizing a cracking gasoline in which the gasoline is separated into a plurality of fractions comprising at least a first fraction rich in compounds 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.

It is also interesting to note that the so-called "easy" desulphurizing compounds are, according to the indications of the patent application EP-A-0 725 126, benzothiophene and methylbenzothiophene whose boiling points are respectively 220 ° C and 244 ° C. These compounds are therefore found in the so-called "high boiling point" section of US Pat. No. 4,131,537, which according to this patent requires the most severe treatments to be desulfurized.

It has also been proposed in US Pat. No. 5,290,427 to hydrotreat the species by fractionating the gasoline, then to desulphurize the fractions and to convert the desulfurized fractions to a ZSM-5 zeolite, in order to offset by isomerization the recorded octane loss.

US-A-5,318,690 discloses a process with gasoline fractionation and softening of the light fraction, while the heavy fraction is desulfurized, then converted to ZSM-5 and desulfurized 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 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 production of gasolines with a low sulfur content, which makes it possible to recover the totality of a petrol fraction containing sulfur, to reduce the total sulfur and mercaptan contents of said petrol fraction to very low levels. levels, with no significant decrease in fuel efficiency, and minimizing the decrease in octane number.

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. The method according to the invention comprises a separation of said essence into a light fraction and a heavy fraction, hydrodesulfurization of the light gasoline on a nickel-based catalyst, hydrodesulfurization of the heavy fraction on a catalyst comprising at least cobalt and / or at least one Group VIb metal, and the mixture of the desulphurized fractions.

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 having 5 carbon atoms (C5) up to about 220 ° C. The end point of the gasoline cut depends on the refinery from which it comes and the constraints of the market, but generally remains within the limits indicated above.

The process according to the invention comprises a separation of the essence into two fractions: a light fraction (also hereinafter referred to as light cut or light gasoline), the end point of which is generally less than or equal to approximately 160 ° C., preferably lower at 140 ° C and more preferably below 120 ° C, a heavy fraction (also hereinafter referred to as heavy cut or heavy gasoline) which is constituted by the heavy fraction complementary to the light gasoline.

In general, the cutting point is chosen so as to maximize the olefin content in the light cut. This content can be easily determined, for example by means of the determination of the bromine number, generally available on the site.

The hydrodesulphurization (also called hydrotreatment) of the light gasoline is carried out on a nickel-based catalyst described in a patent application filed simultaneously, and the hydrodesulphurization of the heavy fraction on a conventional hydrotreatment (hydrodesulfurization) catalyst comprising a Group VIII metal and a Group VIb metal.

The light and heavy cuts thus desulfurized are then mixed. The effluent obtained may optionally be stripped in order to remove the H ₂ S produced during the hydrodesulfurization.

It is also possible, and particularly preferred when the gas to be desulphurized contains polyolefins (dienes), to perform a selective hydrogenation of the gasoline before fractionation.

Detailed Description the invention:

It has been unexpectedly observed that the combination of this simple fractionation of a gasoline with a hydrodesulfurization on a catalyst consisting of nickel supported by the light fraction and a hydrodesulphurization on a conventional catalyst of the heavy fraction makes it possible to obtain, after the desulphurized fractions have been mixed, a desulfurized gasoline does not show a significant decrease in the olefin content or the octane number.

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. These heterocyclic compounds, unlike mercaptans, can not be removed by the extractive processes. These sulfur compounds are consequently eliminated by hydrotreatment, which leads to their decomposition into hydrocarbons and H ₂ S.

In the light fraction can be found sulfur compounds whose boiling points are below 160 ° C or even below 140 ° C and preferably below 120 ° C. Among these, mention may be made of methanethiol (Peb = 6 ° C), ethanethiol (Peb = 35 ° C), propanethiol (Pteb = 68 ° C), thiophene (Peb = 84 ° C), thiacyclobutane (Peb = 95 ° C), pentanethiol (Peb = 99 ° C), 2-methylthiophene (Peb = 113 ° C), 3-methylthiophene (Peb = 115 ° C), thiacyclopentane (Peb = 121 °) C), 2-methylthiacyclopentane (bp = 133 ° C), 2-ethylthiophene (bp = 134 ° C), 3-ethylthiophene (bp = 136 ° C), 2-5 dimethylthiophene (bp = 137 ° C) ), 3-methylthiacyclopentane (Peb = 139 ° C), 2,4-dimethylthiophene (Peb = 141 ° C), 2,3-dimethylthiophene (Peb = 142 ° C), 2,5-dimethylthiacyclopentane (Peb = 142 ° C), 3,3-dimethylthiacyclopentane (Peb = 145 ° C), 3,4-dimethylthiophene (Peb = 145 ° C), 2,3-dimethylthicyclopentane (Peb = 148 ° C), 2-isopropylthiophene (bp = 153 ° C), 3-isopropylthiophene (bp = 157 ° C) and 3-ethyl-2-methylthiophene (bp = 157 ° C).

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. Light fractions naturally have a lower sulfur content than heavier cuts.

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, they can even in certain cases reach values of 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 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 petrol fraction containing sulfur to be treated. It generally takes place in the presence of a catalyst comprising at least one Group VIII metal, preferably selected from the group consisting of platinum, palladium and nickel, and a support. For example, a catalyst containing 1 to 20% by weight of nickel deposited on an inert support, such as, for example, alumina, silica, silica-alumina or a support containing at least 50% alumina, will be used. . 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 treating cuts having a boiling point below 160 ° C to operate under conditions such that at least partial softening of the gasoline is obtained, that is, that is, some reduction in the mercaptan content. To do this, we can use the procedure described in the patent application FR-A-2,753,717, which 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 with 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 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 0.4 and 5 MPa and preferably greater than 1 MPa. The pressure is advantageously between 1 and 4 MPa. The space velocity is from about 1 to about 10 h ^-1 , preferably from 4 to 10 h ^-1 .

The light fraction of the catalytic cracking gasoline fraction can contain up to a few% by weight 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 entire charge and the amount of hydrogen necessary to effect the desired reactions. .

- Separation of light gasoline and heavy gasoline:

This step consists of splitting the gasoline into two fractions, a light fraction, also called light gasoline, and a heavy fraction also called heavy gasoline. The cutting point between these two species corresponds to the point final boiling point (also known as the end point) of light gasoline, and the initial boiling point (also known as the initial point) of heavy gasoline. It is at a temperature generally below 160 ° C in terms of boiling point, preferably below 140 ° C, and more preferably below 120 ° C.

The light gasoline thus has an end point (cutting point between the light fraction and the heavy fraction) generally greater than or equal to about 160 ° C, preferably greater than 140 ° C and more preferably greater than 120 ° C.

Heavy gasoline is the heavy fraction complementary to light gasoline. It has an initial point generally greater than or equal to approximately 160 ° C, preferably greater than 140 ° C and more preferably greater than 120 ° C.

This separation can be carried out using any techniques known to those skilled in the art, such as, for example, distillation or adsorption.

- Hydrodesulfurization of the light fraction:

The end point of the light gasoline cut depends of course on the refinery, but remains within the limits indicated above. The filler is preferably a light gasoline derived from the separation of a catalytic cracking gasoline.

Suitable catalysts are catalysts consisting of supported nickel.

The nickel content of the catalyst used according to the invention is generally between about 1 and about 80% by weight, preferably between 5 and 70% by weight and even more preferably between 10 and 50% by weight. Preferably, the catalyst is generally shaped, preferably in the form of beads, extrudates, pellets, or trilobes. The nickel may be incorporated in the catalyst on the preformed support, it may also be mixed with the support before the shaping step. Nickel is generally introduced in the form of a precursor salt, generally soluble in water, such as, for example, nickel nitrate. 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 the process of the invention are generally porous solids chosen from refractory oxides, such as, for example, aluminas, silicas and silica-aluminas, magnesia, as well as titanium oxide 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. The supports chosen from natural compounds (for example kieselguhr or kaolin) may also be suitable as supports for the catalysts of the process according to the invention.

After introducing the nickel and possibly forming the catalyst (when this step is carried out on a mixture already containing nickel), 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 of from about 100 to about 600 ° C and preferably from 200 to 450 ° C under an air flow rate. The reduction step is performed under conditions to convert at least a portion of the oxidized forms of nickel to metal. 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 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 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 sulphurized 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 of the invention. In this reactor, the catalyst is then treated in hydrogen in order to convert at least a portion of the nickel into 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.

After sulfurization, the sulfur content of the catalyst is generally between 0.5 and 25% by weight, preferably between 4 and 20% by weight.

The hydrodesulphurization of the light fraction of gasoline is intended, by using the catalyst described above, to convert the sulfur-containing compounds of the cut into H ₂ S, so as to obtain an effluent, which after mixing with the Desulfurized heavy gasoline will meet the desired specifications in terms of sulfur compound content. The light cut produced has the same distillation range and a slightly lower octane number, due to the partial but inevitable saturation of the olefins.

The operating conditions of the hydrotreatment reactor according to the present invention must be adjusted so as to reach the desired level of hydrodesulfurization, and in order to minimize the octane loss resulting from the saturation of the olefins. The catalyst used in the process according to the invention generally makes it possible to convert at most 70% of the olefins, preferably at most 60-65% of the olefins, and more preferably less than 20% of the olefins (the diolefins being totally or almost totally hydrogenated). ). With the catalyst of the process according to the invention, it is thus possible to achieve high levels of hydrodesulphurization while limiting the loss of olefins and therefore the reduction of the octane number.

The hydrodesulfurization of the light fraction is carried out in the presence of hydrogen, with the nickel-based catalyst having a temperature of between about 160 ° C and about 420 ° C, at low to moderate pressure, generally between about 0.5 and about 8 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 the temperature is between 200 ° C and 400 ° C, and very preferably between 290 ° C and 350 ° C. Preferably the pressure is between 1 and 3 MPa.

- Hydrodesulfurization of the heavy fraction:

The fraction corresponding to heavy gasoline is subjected to conventional hydrotreatment (hydrodesulphurization) carried out on a conventional hydrotreating catalyst in order to convert the sulfur-containing compounds of the cut into H ₂ S, and so as to obtain an effluent, after mixing with the light desulfurized gasoline, which meets the desired specifications in terms of content of sulfur compounds.

The heavy fraction thus desulphurized has the same distillation range and a slightly lower octane number than before hydrotreatment, because of the total saturation of the olefins. This loss of octane is limited because the heavy fraction (heavy gasoline) has an olefin content generally less than 20% by weight and preferably less than 10% by weight.

The operating conditions of the hydrotreating reactor according to the present invention must be adjusted to await the desired level of desulfurization. At least 90% of the sulfur compounds present in heavy gasoline are generally converted to H ₂ S.

The heavy fraction is hydrotreated in the presence of hydrogen with a catalyst consisting of cobalt and at least one Group VIb metal supported at a temperature of from about 160 ° C to about 420 ° C under generally from about 0.5 to about 8 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 6 h ^-1 . The H ₂ / HC ratio is adjusted according to the desired desulfurization rates in the range of 100 to 600 liters per liter and preferably 300 to 600 liters per liter.

Preferably the temperature is between 200 ° C and 300 ° C. Preferably the pressure is between 2 and 4 MPa.

In order to carry out the hydrotreatment reaction of heavy gasoline according to the process of the invention, at least one conventional hydrodesulphurization catalyst, comprising cobalt and at least one metal from group VIb (Group 6 metals), is generally used. the new classification, ie chromium, molybdenum or tungsten), on a suitable support. The metal of group Vlb, 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.

- Implementation of the method according to the invention

• une fraction légère, dont les points intitial et final sont par exemple de 20 °C et 160 °C respectivement, et qui renferme la plus grande partie des oléfines et une partie des composés soufrés.
• une fraction lourde, dont le point initial est par exemple supérieur à 160 °C, et qui renferme les composés soufrés les plus lourds et, comme composés insaturés, peu d'oléfines mais principalement des composes aromatiques.

The process according to the invention as described above may for example be carried out in a configuration which comprises, in a first step, the separation, for example a distillation, of the essence into two fractions:

• a light fraction, whose initial and final points are for example 20 ° C and 160 ° C respectively, and which contains most of the olefins and a part of the sulfur compounds.
• a heavy fraction, whose initial point is for example greater than 160 ° C, which contains the heavier sulfur compounds and, as unsaturated compounds, few olefins but mainly aromatic compounds.

Each of the two fractions is then subjected to hydrodesulfurization, under the conditions described above, in order to eliminate almost completely the sulfur of the heavy fraction and to eliminate a part of the sulfur present in the light fraction, preferably limiting to to reach the sulfur content necessary for the product obtained by mixing the two hydrodesulphurized sections has a sulfur content corresponding to the desired specifications.

Another possibility is to place the reaction zones where the hydrodesulphurization reactions of the light and heavy gasoline fractions are carried out outside the distillation zone, but to use reaction zones as feedstock. hydrodesulphurizing the liquid fractions taken from trays in the distillation zone, with recycling of the desulphurized effluents to said distillation zone, at one or more levels above or below, preferably in the vicinity, of the sampling levels.

It is also possible to implement another configuration, in which the hydrotreatment catalysts for treating the light and heavy fractions of the gasoline are placed directly in the distillation zone allowing the separation of the light fraction from the heavy fraction. .

The examples below illustrate the invention without limiting its scope.

• chromatographie en phase gaz (CPG) pour les constituants hydrocarbonés ;
• méthode NF M 07022/ASTM D 3227 pour les mercaptans ;
• méthode NF M 07052 pour le soufre total :
• méthode NF EN 25164/M 07026-2/ISO 5164/ASTM D 2699 pour l'indice d'octane recherche ;
• méthode NF EN 25163/M 07026-1 ISO 5163/ASTM D 2700 pour l'indice d'octane moteur.

Tableau 1 : Caractéristiques de la charge utilisée. Charge Densité 0,75 Point initial (°C) 80°C Point final (°C) 240°C teneur en oléfines (% vol.) 25 S total (ppm) 4500 S ex mercaptans (ppm) 0 RON 95 MON 82 (RON + MON)/2 88.5 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 loads and effluents are as follows:

• gas phase chromatography (GC) for hydrocarbon constituents;
• method NF M 07022 / ASTM D 3227 for mercaptans;
• method NF M 07052 for total sulfur:
• method NF EN 25164 / M 07026-2 / 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. Charge Density 0.75 Initial point (° C) 80 ° C End point (° C) 240 ° C olefin content (% vol.) 25 S total (ppm) 4500 S ex mercaptans (ppm) 0 RON 95 MY 82 (RON + MON) / 2 88.5

Example 1 (comparative): hydrodesulphurization of unfractionated gasoline.

25 ml of HR306C® catalyst, marketed 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% of sulfur in the form of dimethyl disulphide in n-heptane.

The hydrodesulfurization operating conditions are as follows: T = 270 ° C., VVH = 4 h ^-1 , H ₂ / HCl = 125 l / l, P = 2.7 MPa. Under these conditions, the effluent after desulfurization has the characteristics described in Table 2. Table 2: Comparison of the characteristics of the feedstock and desulphurized effluent. Charge Effluent S total (ppm) 4500 315 S ex mercaptans (ppm) 0 150 Olefins (% vol.) 25 8 MY 82 76 RON 95 85 (RON + MON) / 2 88.5 80.5 Loss in octane - 8 % HDS * 93.1 % HDO ** 68 *% HDS is the rate of hydrodesulfurization **% HDO refers to the hydrogenation rate of olefins

Example 2 (according to the invention): hydrodesulphurization of fractionated gasoline.

The species whose characteristics are described in Table 1 is divided into two sections, one having an end point of 110 ° C (light cut) and the other an initial point of 110 ° C (heavy cut). The characteristics of the distillate species and the yield of each cut is described in Table 3. Table 3: Characteristics of distilled species and yield of each cut Charge Light petrol Heavy gas Volume (%) 45 55 S total (ppm) 4500 1600 6900 S ex mercaptans (ppm) 0 0 0 Olefins (% vol.) 25 46 7.5 Initial point (° C) 80 80 110 End point (° C) 240 110 240

The heavy fraction of the gasoline is subjected to hydrodesulfurization on a conventional hydrotreating catalyst in an isothermal tubular reactor. 25 ml of HR306C® catalyst, marketed 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% of sulfur in the form of dimethyl disulphide in n-heptane.

The operating conditions of the hydrodesulfurization are as follows: T = 280 ° C., VVH = 4 h ^-1 , H ₂ / HCl = 125 l / l, P = 2.7 MPa. Under these conditions, the effluent after hydrodesulfurization has a sulfur content of less than 1 ppm and an olefin content of less than 1% by volume.

The light fraction of the gasoline is subjected to hydrotreatment on a supported nickel catalyst in an isothermal tubular reactor. The catalyst is prepared as follows.

It 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. 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 filler containing 4% sulfur as dimethyl disulphide in n-heptane.

The hydrodesulfurization of the light gasoline is then carried out. The temperature is 280 ° C, the charge rate is 200 ml / hour. The H ₂ / charge ratio expressed in liters of hydrogen per liter of filler is 400, the operating pressure is 2.7 MPa.

Under these conditions, the analysis of the liquid effluent leads to the results presented in Table 5. Table 5: Hydrodesulfurization of Light Gasoline on Nickel Catalyst Light petrol Lightly desulphurated gasoline S total (ppm) 1600 700 S ex mercaptans (ppm) 0 20 Olefins (% vol.) 46 43 Initial point (° C) 80 80 End point (° C) 110 110

Light gasoline and heavy gasoline desulphurized separately are then mixed. The product obtained has the following characteristics: Table 6: Characteristics of light gasoline - heavy gasoline mixture after hydrodesulfurations Charge Desulfurized gasoline S total (ppm) 4500 315 S ex mercaptans (ppm) 0 9 Olefins (% vol.) 25 19.5 MY 82 81.2 RON 95 92 (RON + MON) / 2 88.5 86.6 Loss in octane - 1.9 % HDS * 93.1 % HDO ** 22 *% HDS is the rate of hydrodesulphurization **% HDO refers to the hydrogenation rate of olefins

Example 3 (comparative): hydrodesulphurization of fractionated gasoline using a cobalt-molybdenum catalyst.

The species whose characteristics are described in Table 1 is divided into two sections, one having an end point of 110 ° C (light cut) and the other an initial point of 110 ° C (heavy cut). The characteristics of the distilled gasolines and the yield of each cut are described in Table 3 of Example 2.

The heavy fraction of the gasoline is subjected to hydrodesulfurization on a conventional hydrotreating catalyst in an isothermal tubular reactor. 25 ml of HR306C® catalyst, marketed 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% of sulfur in the form of dimethyl disulphide in n-heptane.

The operating conditions of the hydrodesulfurization are as follows: T = 280 ° C., VVH = 4 h ^-1 , H ₂ / HCl = 125 l / l, P = 2.7 MPa. Under these conditions, the effluent after hydrodesulfurization has a sulfur content of less than 1 ppm and an olefin content of less than 1% by volume.

The light fraction of the gasoline is subjected to hydrodesulfurization on the HR306C® catalyst in an isothermal tubular 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% of sulfur in the form of dimethyl disulphide in n-heptane.

The hydrodesulfurization of the light gasoline is then carried out under the following conditions: T = 220 ° C, VVH = 4 h ^-1 , H ₂ / HC = 400 l / l, P = 2.7 MPa.

Under these conditions, the analysis of the liquid effluent leads to the results presented in Table 7. Table 7: Hydrodesulfurization of light gasoline over HR 306CⓅ catalyst Light petrol Lightly desulphurated gasoline S total (ppm) 1600 700 S ex mercaptans (ppm) 0 250 Olefins (% vol.) 46 36 Initial point (° C) 80 80 End point (° C) 110 110

Light gasoline and heavy gasoline desulphurized separately are then mixed. The product obtained has the following characteristics: Table 8: Characteristics of light gasoline - heavy gasoline mixture after hydrodesulphurations Charge Desulfurized gasoline S total (ppm) 4500 315 S ex mercaptans (ppm) 0 113 Olefins (% vol.) 25 16 MY 82 78.6 RON 95 88.6 (RON + MON) / 2 88.5 83.6 Loss in octane - 4.9 % HDS * 93.1 % HDO ** 36 *% HDS is the rate of hydrodesulfurization **% HDO refers to the hydrogenation rate of olefins

Claims (8)

1. A process for producing a gasoline with a low sulphur content, wherein said process comprises:

   • a separation of a gasoline containing sulphur into a light fraction and a heavy fraction, the cut point being selected in such a way that the light cut contains the main part of the olefins from the feedstock and the heavy fraction shows an olefin content which is less than 20% by weight.

   • a hydrodesulphurisation of the light fraction on a catalyst constituted by supported nickel,

   • a hydrodesulphurisation of the heavy fraction on a catalyst constituted by supported cobalt and at least one group VIB metal, and

   • the mixing of the desulphurised fractions.
2. A process according to claim 1, wherein the sulphur-containing gasoline originates from a catalytic cracking process.
3. A process according to claim 1 or 2 wherein the group VIB metal is molybdenum or tungsten.
4. A process according to any one of the preceding claims, wherein prior to separation, the dienes present in the sulphur-containing gasoline cut are hydrogenated.
5. A process according to any one of the preceding claims, wherein the cut point between the light fraction and the heavy fraction is at a temperature of less than 160°C.
6. A process according to any one of the preceding claims, wherein the light fraction and the heavy fraction are hydrodesulphurised in the presence of hydrogen, at a temperature in the range 160°C to 420°C, at a pressure in the range from about 0.5 to about 8 MPa, at a liquid space velocity in the range from about 0.5 to about 10 h^-1 and an H₂/HC ratio in the range from about 100 to about 600 litres per litre.
7. A process according to any one of the preceding claims, wherein separation is carried out in a distillation column and wherein the feeds from the hydrodesulphurisation reactors are withdrawn from different levels in said column and the effluents from said reactors are returned to said column.
8. A process according to any one of the preceding claims, wherein separation is carried out in a distillation column and wherein the hydrodesulphurisation catalysts are placed inside said column.