Process for desulphurizing olefinic gasolines, comprising at least two distinct hydrodesulphurization steps

The invention relates to a process for producing gasolines with a low sulphur and mercaptans content, which comprises at least two hydrodesulphurization steps operated in parallel on two distinct gasoline cuts. Said process optionally comprises a single hydrogen purification and recycling section. A hydrodesulphurization step corresponds to one or more hydrodesulphurization sections. A hydrodesulphurization section corresponds to one or more beds.

The production of fuel for gasoline or diesel engines satisfying new environmental standards requires in particular that their sulphur content be substantially reduced. Environmental specifications constrain refiners to reduce the amount of sulphur in the gasoline and gas oil pool to values of at most 50 ppm in 2005 and to at most 10 ppm on the first of January 2009 within the European community.

The feed to be treated is generally a gasoline cut containing sulphur, such as a gasoline cut from a coking unit, visbreaking unit, steam cracking or catalytic cracking unit (FCC). Said feed is preferably constituted by a gasoline feed from a catalytic cracking unit the distillation range of which is in the range 0° C. to 300° C., preferably in the range 0° C. to 250° C. In the remainder of the text, we shall speak in general of catalytically cracked gasoline, broadening this definition to gasolines which may, in addition to a catalytically cracked gasoline portion, contain gasoline fractions from other conversion units.

The catalytically cracked gasolines may constituted 30% to 50% by volume of the gasoline pool and generally have high mono-olefin and sulphur contents. However, almost 90% of the sulphur present in reformed gasoline is attributable to the gasoline from catalytic cracking. Gasoline desulphurization, and principally FCC gasolines, is thus of crucial importance in hitting current and future specifications. However, the mono-olefins contained in the gasoline make a major contribution to their octane number. To keep the octane number of gas from olefinic cracking high, it is necessary to limit the degree of hydrogenation of mono-olefins during hydrodesulphurization treatment of the gasolines. Selective hydrodesulphurization processes were developed for this purpose.

Further, desulphurized gasolines must also satisfy the specifications as regards corrosive power. The corrosive power of gasolines is essentially due to the presence of acidic sulphur-containing compounds such as mercaptans. The desulphurized gasolines must thus contain few mercaptans to limit their corrosivity. However, it is now known that in units for selective hydrodesulphurization of olefinic gasolines, H 2 S present in the reactor may react with the non-hydrogenated mono-olefins to form mercaptans. The fraction of mercaptans in the gasoline produced is generally higher when the sulphur content of the gasoline is lower. To minimize the mercaptans content, it is generally preferable to operate with a high hydrogen flow rate. However, this involves high costs related to the compressor, recycling and hydrogen purification. To overcome this problem, the present invention presents a solution which can limit the energy consumption of the compressor, while reducing the mercaptans content and increasing the octane number for a constant sulphur content in the desulphurized gasoline.

Further, in contrast, changes in the worldwide automotive market have forced refiners to investigate the maximum flexibility and thus the possibility of maximizing production of gas oil for diesel-engined vehicles or the production of gasoline, depending on the circumstances. Thus, it may be highly advantageous for a refiner to have at his disposal the cheapest possible option of sending the heavy gasoline fraction either to the gasoline pool or to the middle distillates pool, as required.

In summary, the present invention proposes a novel solution for economically solving the three-fold problem of reducing the sulphur content in fuels, limiting the mercaptans content in low sulphur gasolines and flexibility of orientation of the production of fuel towards gasoline cuts or middle distillates, depending on market requirements. Further, in the current context of reducing greenhose gas emissions, it is important to integrated into any novel idea the problem of controlling energy consumption. The process described in the context of the present invention is innovative as it allows simultaneous treatment of the three-fold problem described above while limiting the energy consumption due to the necessary compression of the hydrogen which is recycled to the hydrodesulphurization steps. Gasolines are obtained with a research octane number (RON) and sulphur content (S) which are such that RON≧90.70 and [S]≦50 ppm, preferably RON≧90.70 and [S]≦37 ppm, more preferably RON≧90.75 and [S]≦35 ppm and more preferably RON≧90.80 and [S]≦31 ppm. Preferably, each pair is associated with a motor octane number (MON) such that MON≧79.45, preferably MON≧79.50 and more preferably MON≧79.55.

Examination of the Prior Art

European patent application EP-A1-0 725 126 describes a method for desulphurizing catalytically cracked gasolines while limiting the octane number loss through mono-olefin hydrogenation. That method consists of distilling the gasoline into several fractions, including at least one fraction which is rich in compounds which are difficult to desulphurize selected from thiophene and alkylthiophenes, and a fraction which is rich in compounds which are easy to desulphurize, selected from thiacyclopentane, alkylthiacyclopentanes, benzothiophene and alkylbenzothiophenes. At least one of those two fractions is treated by a hydrodesulphurization process and then mixed with the untreated fraction.

That method has the disadvantage of necessitating an analysis of the different fractions before treatment, and does not describe how to select the fractions to limit the quantity of mercaptans in the final desulphurized product.

United States patent U.S. Pat. No.-B2-6,596,157 describes a process for desulphurizing gasoline cuts from cracking units, based on the treatment, in parallel, of the heavy fraction of the gasoline termed HCN (heavy cat naphtha) under hydrodesulphurization conditions which are non-selective and of the intermediate fraction of the gasoline termed ICN (intermediate cat naphtha) under selective hydrodesulphurization conditions, wherein the intermediate gasoline (ICN) is heated by the hydrotreated heavy fraction (HCN).

That patent does not describe how to treat the various cuts to limit the fraction of sulphur-containing compounds in the form of mercaptans in the desulphurized gasoline. Further, according to the disclosure of that patent, the light fraction of the gasoline, termed LCN (light cat naphtha), must generally undergo a complementary desulphurization treatment, for example extracting mercaptans by washing using a solution containing sodium hydroxide.

Regarding the. problem of extracting mercaptans in desulphurized cracking gasolines, the current solutions described in U.S. Pat. No. 6,960,291 consist of post-treating gasolines from selective hydrotreatment to deplete them in mercaptans. Many methods have been envisaged. As an example, we can cite WO-A-01/79391, which describes methods for treating partially desulphurized gasolines to reduce the mercaptans content using various methods such as adsorption, extraction with sodium hydroxide, heat treatments, etc. However, those methods have the disadvantage that they necessitate the use of a supplemental step for treating the gasoline and do not have the flexibility of sending certain cuts either to the gasoline pool or to the middle distillates pool.

US-A-2003/0042175 describes a method for desulphurizing cracking gasolines comprising various treatment steps to reduce the sulphur content. That process comprises a diolefins hydrogenation step, a step for the transformation of light sulphur-containing compounds by weighting, a step for distilling the gasoline into several cuts and at least one step for desulphurization of at least a portion of the heavy fraction of the gasoline produced. However, that patent does not disclose how to treat the gasolines to minimize the mercaptans content in the desulphurized gasoline, nor how to treat the hydrogen from the hydrodesulphurization steps.

The invention is based on the differential treatment of the various cuts constituting the gasoline cut.

It is known that in catalytically cracked gasolines, the light fractions are rich in mono-olefins and in saturated sulphur-containing compounds such as mercaptans and sulphides. The term “light fraction” means gasoline fractions with a boiling point of less than 100° C., preferably less than 80° C. and more preferably less than 65° C. The heavy fraction of the gasoline is rich in benzothiophene type sulphur-containing compounds such as benzothiophene and alkyl benzothiophenes and to a lesser degree, it is rich in alkyl thiophenes. Further, it is rich in aromatic compounds and poor in olefinic compounds. The heavy fraction of the gasoline is constituted by hydrocarbons with a boiling point of more than 160° C., preferably more than 180° C. and more preferably more than 207° C. That heavy fraction of the gasoline is generally that which contains the most sulphur. The heavy fraction of the gasoline may be incorporated either into the gasoline pool or into the middle distillates fraction to produce kerosenes or gas oils. The core fraction corresponds to the intermediate fraction between the light fraction and the heavy fraction. The core fraction of the gasoline is rich in mono-olefins and in sulphur-containing thiophene type compounds, including thiophene, methyl thiophenes and other alkyl thiophenes.

In general, the various fractions of the gasoline are obtained by distilling the effluent from the catalytic cracking unit.

The mixture constituted by the light fraction of the gasoline and the intermediate fraction or the intermediate fraction alone is treated in a first hydrodesulphurization step denoted HDS 1 . This step comprises bringing the gasoline to be treated into contact with hydrogen in one or more hydrodesulphurization reactors in series, containing one or more suitable catalysts for carrying out selective hydrodesuiphurization, i.e. with a degree of mono-olefins hydrogenation of less than 60%, preferably less than 50% and more preferably less than 40%.

The operating pressure in this step is generally in the range 0.5 MPa to 5 MPa, and preferably in the range 1 MPa to 3 MPa. The temperature is in the range 200° C. to 400° C. and preferably in the range 220° C. to 380° C. In the case in which the treatment is carried out in a plurality of reactors in series, the mean operating temperature of each reactor is greater by at least 5° C., preferably by at least 10° C. and more preferably by at least 15° C. than the operating temperature of the preceding reactor.

The quantity of catalyst used in each reactor is such that the ratio between the flow rate of the gasoline to be treated, expressed in m 3 per hour under standard conditions, per m 3 of catalyst (also termed the hourly space velocity) is in the range 0.5 h −1 to 20 h −1 , preferably in the range 1 h −1 to 15 h −1 . Most preferably, the first reactor is operated with an hourly space velocity in the range 2 h −1 to 8 h −1 .

The hydrogen flow rate is such that the ratio between the hydrogen flow rate, expressed in normal m 3 per hour (Nm3/h) and the flow rate of the feed to be treated, expressed in m 3 per hour under standard conditions, is in the range 50 Nm 3 /m 3 to 1000 Nm 3 /m 3 , preferably in the range 70 Nm 3 /m 3 to 800 Nm 3 /m 3 .

The degree of desulphurization achieved during the HDS 1 step is generally more than 80% and preferably more than 90%.

After the hydrodesulphurization step, the reaction mixture is cooled to a temperature of less than 60° C. to condense the hydrocarbons. The gas and liquid phases are separated in a separator. The liquid fraction which contains the desulphurized gasoline and a fraction of the dissolved H 2 S is sent to a stripping section; the gas fraction, which is principally constituted by hydrogen and which contains the majority of the H 2 S, is sent to a purification section.

The heavy fraction of the gasoline is treated in a distinct hydrodesulphurization step denoted HDS 2 . This step comprises bringing the gasoline to be treated in contact with hydrogen in one or more hydrodesulphurization reactors in series containing one or more suitable catalysts for carrying out hydrodesulphurization. Preferably, hydrodesulphurization of the heavy gasoline is carried out in a single step, on a single reactor. Hydrodesulphurization may be carried out selectively or non-selectively. In the first case, the degree of hydrogenation of the mono-olefins is less than 90%, preferably less than 80% and more preferably less than 60%.

The operating pressure in this step is generally in the range 0.5 MPa to 10 MPa, preferably in the range 1 MPa to 8 MPa. The temperature is in the range 220° C. to 450° C. and preferably in the range 250° C. to 380° C. In the case in which the treatment is carried out in a plurality of reactors in series, the mean operating temperature in each reactor is greater by at least 5° C., preferably by at least 10° C. and more preferably by at least 15° C. than the operating temperature of the preceding reactor.

The quantity of catalyst employed in each reactor is such that the ratio between the flow rate of the gasoline to be treated, expressed in m 3 per hour under standard conditions, per m 3 of catalyst (also termed the hourly space velocity) is in the range 0.3 h −1 to 20 h −1 , preferably in the range 0.5 h −1 to 15 h −1 . More preferably, the first reactor is operated with an hourly space velocity in the range 1 h −1 to 8 h −1 .

The hydrogen flow rate is such that the ratio between the flow rate of hydrogen, expressed in normal m 3 per hour (N m 3 /h) and the flow rate of the feed to be treated, expressed in m 3 per hour under standard conditions, is in the range 30 N m 3 /m 3 to 800 N m 3 /m 3 , preferably in the range 50 N m 3 /m 3 to 500 N m 3 /m 3 . Preferably, this ratio is less than 80% of the ratio of the flow rates employed for desulphurization in hydrodesulphurization step HDS 1 , preferably less than 60%, more preferably less than 50% and still more preferably less than 40% of the ratio of the flow rates employed for desulphurization in the hydrodesulphurization step HDS 1 .

After the hydrodesulphurization step, the reaction mixture is cooled to a temperature of less than 60° C. to condense the hydrocarbons. The gas and liquid phases are separated in a separator. The liquid fraction which contains the desulphurized gasoline as well as a fraction of dissolved H 2 S is sent to a stripping section; the gaseous fraction principally constituted by hydrogen and which contains the majority of H 2 S, is sent to a purification section.

Any catalyst with good selectivity as regards hydrodesulphurization reactions may be used in steps HDS 1 or HDS 2 . As an example, catalysts are used comprising an amorphous porous mineral support selected from the group constituted by aluminas, silicon carbide, silica, silica-aluminas or titanium or magnesium oxides, used alone or as a mixture with the alumina or the silica-alumina. It is preferably selected from the group constituted by silica, the family of transition aluminas and silica-aluminas. Highly preferably, the support is essentially constituted by at least one transition alumina, i.e. it comprises at least 51% by weight, preferably at least 60% by weight, more preferably at least 80% by weight, or even at least 90% by weight of transition alumina. It may optionally be solely constituted by a transition alumina.

The specific surface area of the support is generally below 200 m 2 /g and preferably less than 150 m 2 /g. The porosity of the catalyst before sulphurization is such that it has a mean pore diameter of more than 20 nm, preferably more than 25 nm or even 30 nm and usually between 20 and 140 nm, preferably between 20 nm and 100 nm, and highly preferably between 25 and 80 nm. The pore diameter is measured by mercury porosimetry in accordance with the ASTM D4284-92 standard with a wetting angle of 140°.

The hydrodesulphurization catalyst contains at least one group VI metal and/or at least one group VIII metal on a support. The group VI metal is generally molybdenum or tungsten; the group VIII metal is generally nickel or cobalt. In accordance with the invention, the surface density of the group VI metal is in the range 2×104 −4 to 4.0×10 −3 grams of oxide of said metal per m 2 of support, preferably in the range 4×10 −4 to 1.6×10 −3 g/m 2 .

Highly preferably, a catalyst or a concatenation of catalysts as described in patent application US2006000751A1 is used. They are catalysts comprising a support selected, for example, from refractory oxides such as aluminas, silicas, silica-aluminas or magnesia, used alone or as a mixture, and a group VI metal, preferably molybdenum or tungsten which may or may not be promoted by a group VIII metal, preferably cobalt or nickel. Said catalysts have a mean pore diameter of more than 22 nm. In the optional case of a concatenation of catalysts, the process comprises a succession of hydrodesulphurization steps, such that the activity of the catalyst in a step n+1 is in the range 1% to 90% of the activity of the catalyst in step n.

However, it is possible to use a non-selective catalyst in step HDS 2 . As an example, catalysts are used comprising a porous amorphous mineral support selected from the group constituted by aluminas, silicon carbide, silica, silica-aluminas or titanium or magnesium oxides, used alone or as a mixture with alumina or silica-alumina. It is preferably selected from the group constituted by silica, the transition alumina family and silica-aluminas. Highly preferably, the support is essentially constituted by at least one transition alumina, i.e. it comprises at least 51% by weight, preferably at least 60% by weight, more preferably at least 80% by weight, or even at least 90% by weight of transition alumina. It may optionally be solely constituted by a transition alumina. The hydrodesulphurization catalyst contains at least one group VI metal and/or at least one group VIII metal on a support. The group VI metal is generally molybdenum or tungsten and the group VIII metal is generally nickel or cobalt. The selective or non-selective nature of the hydrodesulphurization catalyst as defined above in the description of the invention generally depends on the composition and the mode of preparation of said catalyst. Simple ways of varying the selectivity consist, for example, of modifying the amounts of group VIII and group VI meals or optionally the mole ratio between the quantities of the group VIII and group VI metals for a given support or varying the specific surface area of the support for constant amounts of metals.

In a preferred implementation of the invention, the excess hydrogen from the hydrodesulphurization steps HDS 1 and HDS 2 may be combined and treated in a single purification section. The purified hydrogen is then recycled to at least one of the hydrodesulphurization steps HDS 1 and HDS 2 after a compression step to compensate for the pressure drops across the process. a makeup of fresh hydrogen is carried out either before or after the compression step to compensate for the consumption of hydrogen in the hydrodesulphurization reactors.

It is possible to envisage admitting at least all of the hydrogen necessary for the reactions occurring in the two hydrodesulphurization steps through a single one of the hydrodesulphurization steps, preferably HDS 2 . To this end, the whole supply of hydrogen necessary for steps HDS 1 and HDS 2 is thus sent to just one of these steps, preferably HDS 2 , and the gas purge from this step is sent to the purification treatment from which the purified gaseous product is recycled only to the other hydrodesulphurization step. That type of layout can minimize the flow rate of recycled hydrogen passing via the compressor in the case in which all of the hydrogen consumed in the two hydrodesulphurization steps is sent through the HDS 2 unit and the gas purge from this unit is sent to the purification treatment where the gas purified product is only recycled to the HDS 1 step. Since the HDS 1 unit necessitates a lower hydrogen flow rate compared with the HDS 2 unit, less hydrogen needs to be recycled via the compressor and hence the energy consumption of the compressor is reduced.

It is possible to envisage a combined treatment of the vapour fractions from the two stripping columns dedicated to each hydrodesulphurization step (cooling, recycling the condensed liquid to each of the stripping and purge columns combined with H 2 S rich gas sent to a purification step).

In the case in which the product from the hydrodesulphurization step HDS 2 is sent to the diesel pool, the fact of providing a HDS 2 step dedicated to the heavy gasoline means that this gasoline cannot be co-mixed with middle distillate cuts in another hydrotreatment step and thus releases the capacity in said hydrotreatment step, and as a result increases the capacity of the refinery production.

While remaining within the context of the invention, it is possible to carry out a pre-treatment of the feed the principal aim of which is:

selectively hydrogenating the diolefins to mono-olefins; transforming the light saturated sulphur-containing compounds, principally mercaptans, into heavier sulphides or mercaptans by reaction with the mono-olefins.

The hydrogenation of diolefins to mono-olefins is illustrated below by the transformation of 1,3-pentadiene, which is an unstable compound which polymerizes easily into pent-2-ene by addition of hydrogen. However, secondary mono-olefin hydrogenation reactions must be limited since, as shown in the example below, they would result in the formation of n-pentane.

The sulphur-containing compounds which are to be transformed are principally mercaptans and sulphides. The principal mercaptan transformation reaction consists of thioetherification of mono-olefins by mercaptans. This reaction is illustrated below by the addition of propane-2-thiol to pent-2-ene to form a propylpentylsulphide.

In the presence of hydrogen, sulphur-containing compound transformation may also be carried out by the intermediate formation of hydrogen sulphide which may then add to the unsaturated compounds present in the feed. However, this is a minor pathway under the preferred reaction conditions.

In addition to mercaptans, the compounds which may be transformed into heavier compounds are sulphides, principally dimethylsulphide, methylethylsulphide and diethylsulphide, CS 2 , COS, thiophane and methylthiophane.

In certain cases, it is also possible to observe reactions for transforming light nitrogen-containing compounds, principally nitrites, pyrrole and its derivatives, into heavier compounds.

This pre-treatment step consists of bringing the feed to be treated into contact with a stream of hydrogen and catalyst containing at least one metal from group VIB (group 6 in the new periodic table notation: Handbook of Chemistry and Physics, 76 th edition, 1995-1996) and at least one non-noble group VIII metal (groups 8 , 9 and 10 of said classification), deposited on a porous support.

The catalyst of the invention may be prepared using any technique which is known to the skilled person, in particular by impregnating elements from groups VIII and VIB onto the selected support. Said impregnation may, for example, be carried out using the preparation technique known to the skilled person as dry impregnation, in which exactly the desired quantity of elements is introduced in the form of salts which are soluble in the selected solvent, for example demineralized water, to fill the porosity of the support as exactly as possible. The support, which is by then filled with solution, is preferably dried. The preferred support is alumina, which may be prepared from any type of precursor and shaping tools which are known to the skilled person.

The catalyst is normally used in a sulphurized form obtained after treatment at temperature in contact with a decomposable sulphur-containing organic compound which can generate hydrogen sulphide (H 2 S), or directly in contact with a flow of gaseous H 2 S diluted in H 2 . This step may be carried out in situ or ex situ (i.e. inside or outside the hydrodesulphurization reactor) at temperatures in the range 200° C. to 600° C. and more preferably in the range 300° C. to 500° C.

The feed to be treated is mixed with hydrogen before being brought into contact with the catalyst. The quantity of hydrogen which is injected is such that the mole ratio between the hydrogen and the diolefins to be hydrogenated is more than 1 (stoichiometry) and less than 10, preferably in the range 1 to 5 mol/mol. Too large an excess of hydrogen may cause too much hydrogenation of mono-olefins and as a result, reduce the octane number of the gasoline. The whole feed is generally injected into the reactor inlet. However, it may be advantageous in some cases to inject a fraction or all of the feed between two consecutive catalytic beds placed in the reactor. This implementation can allow the reactor to continue operating if the inlet to the reactor is blocked with deposits of polymers, particles or gums present in the feed.

The mixture constituted by gasoline and hydrogen is brought into contact with the catalyst at a temperature in the range 80° C. to 220° C., preferably in the range 90° C. to 200° C. a liquid hourly space velocity (LHSV) in the range 1 h −1 to 10 h −1 , the units for the liquid hourly space velocity being a litre of feed per litre of catalyst per hour (l/l.h) The pressure is adjusted so that the reaction mixture is mainly in the liquid form in the reactor. The pressure is in the range 0.5 MPa to 5 MPa and is preferably in the range 1 to 4 MPa.

The gasoline treated under the conditions mentioned above has a reduced diolefins and mercaptans content. Generally, the gasoline produced contains less than 1% by weight of diolefins and preferably less than 0.5% by weight of diolefins. The amount of light sulphur-containing compounds with a boiling point lower than that of thiophene (84° C.) which is generally converted is more than 50%. Thus, it is possible to separate the light fraction of the gasoline by distillation and to send said fraction directly to the gasoline pool without complementary treatment.

The preferred implementations of the process of the present invention are illustrated in the flowsheets of FIG. 1 , FIG. 2 and FIG. 3 .

Description of FIG. 1

A core gasoline, gasoline A, moving in line 1 was mixed with hydrogen from a recycled compressor P 1 via line 20 . The mixture so formed is injected into the reaction section R 1 . The effluent moving via line 4 is cooled in the exchanger section E 1 to condense the hydrocarbons then the mixture is injected into the separation section S 1 via a line 6 . The separation section S 1 produces a gaseous fraction extracted via a line 8 which is essentially constituted by hydrogen, H 2 S and light hydrocarbons and a liquid fraction extracted via a line 9 . The liquid fraction is then injected into a stabilization section C 2 which extracts H 2 S dissolved in the hydrocarbons via an overhead line 15 . The gasoline recovered from the bottom of column C 2 via a line 16 may be sent directly to the gasoline pool.

A heavy gasoline, gasoline B, moving in line 3 is mixed with fresh hydrogen supplied via a line 2 . The mixture so constituted is injected into the reaction section R 2 . The effluent moving via line 5 is cooled in the exchanger section E 2 to condense the hydrocarbons, then the mixture is injected into the separation section S 2 via a line 7 . The separation section S 2 produces a gas fraction, extracted via line 10 , which is essentially constituted by hydrogen, H 2 S and light hydrocarbons, and a liquid fraction is extracted via a line 11 . The liquid fraction is then injected into a stabilization section C 3 which extracts the H 2 S dissolved in the hydrocarbons via an overhead line 17 . The heavy desulphurized gasoline recovered via a line 18 may be sent either to the gasoline pool or to the middle distillates pool.

The stabilization sections C 2 and C 3 each comprise a distillation column. To limit the operating costs and investment costs, it is advantageous to combine the distillates from said two columns before cooling them to condense them and to send them together to a reflux drum. The two columns may then be operated with a common reflux section.

The hydrogen from separators S 1 and S 2 respectively via lines 8 and 10 are mixed before being treated in a common purification section C 1 , which consists of washing with an aqueous amine solution using a well known technique. After purification, the hydrogen, free of H 2 S, moving in line 13 and compressed in a recycle compressor P 1 , is then mixed with gasoline A via line 20 .

In a further variation of the invention, makeup hydrogen is injected via a line 12 upstream of the purification section C 1 . The hydrogen necessary for treatment of gasoline B is then injected via a line 19 into the hydrodesulphurization step of the reaction section R 2 .

Description of FIG. 2

The numbers used in this Figure correspond to those used for FIG. 1 .

A further variation of the invention is presented in FIG. 2 . In this variation, a fraction of the hydrogen coming from the separation section SI via line 8 is injected, without a purification treatment, into the reaction section R 2 via a line 21 .

Description of FIG. 3

FIG. 3 illustrates the concatenations of the pre-treatment step principally consisting of hydrogenating diolefins, transforming the light sulphur-containing compounds into heavier compounds and the selective hydrodesulphurization step.

The pre-treatment step R 3 may be carried out either on the total gasoline injected via line 1 or on the gasoline recovered overhead via line 3 from distillation column C 4 . In the latter case, the gasoline A is sent directly to column C 4 without pre-treatment.

When the pre-treatment is applied to all of the gasoline, hydrogen is injected via a line 10 upstream of the pre-treatment step R 3 which corresponds to the step for selective hydrogenation and transforming the light saturated sulphur-containing compounds into heavier compounds. The gasoline produced is then distilled into two cuts in column C 4 —a heavy cut extracted via line 4 which corresponds to the heavy gasoline described in the text, and a lighter fraction recovered via line 3 which corresponds to the mixture of the core gasoline and the light gasoline described in the text. The light fraction is then distilled in a second column, C 5 , which can separate the core gasoline which leaves via line 6 from the light gasoline which exits via line 7 . The light gasoline recovered via line 7 is generally depleted in sulphur and may be sent directly to the gasoline pool without complementary treatment.

The core and heavy gasolines respectively recovered via lines 6 and 4 are treated in one or more hydrodesulphurization sections in accordance with the invention and can recover a gasoline H via line 8 and a gasoline J via line 9 , sent respectively to the gasoline pool and to the middle distillate pool.

It may be advantageous to produce the three gasoline cuts described in a single column provided with a side stream take-off from which the core gasoline is extracted. In a further implementation of the invention, the column for distillation of all of the gasoline injected via line 1 may be a single column with an internal wall.

When the pre-treatment step R 3 is applied to the gasoline from line 3 , hydrogen is injected via line 11 . This implementation has the advantage of sending to the pre-treatment step R 3 only a fraction of the gasoline corresponding to the fraction which is depleted in heavy gasoline, which reduces the quantities of gasoline to be treated, as well as the presence of potential contaminants for said catalysts, such as arsenic or silicon, which are generally concentrated in the heavy gasoline fractions.

Preparation of feeds

A gasoline a the boiling points of which were in the range 6° C. to 236° C., derived from a catalytic cracking unit, was distilled in a batch distillation column to produce four cuts:

A cut a 1 corresponding to the 6° C.-188° C. fraction;

A cut a 2 corresponding to the 188° C.-236° C. fraction;

A cut a 3 corresponding to the 6° C.-209° C. fraction;

A cut a 4 corresponding to the 209° C.-236° C. fraction.

The characteristics of the various cuts are shown in Table 1.