EP1849850B1 - Method of desulphurating olefin gasolines comprising at least two distinct hydrodesulphuration steps - Google Patents

Description

Field of invention

The invention relates to a process for the production of gasolines with a low sulfur and mercaptan content which comprises at least two hydrodesulphurization stages carried out in parallel on two distinct cuts of the gasoline. This process optionally includes a single hydrogen purification and recycling section. A hydrodesulfurization step corresponds to one or more hydrodesulfurization sections. A hydrodesulfurization section corresponds to one or more beds.

Description of the problem

The production of fuels for gasoline or diesel engines that meet new environmental standards requires in particular that their sulfur content be significantly reduced. Indeed, environmental standards force refiners to lower the sulfur content in the gasoline and diesel pool to values below or at most equal to 50 ppm in 2005, and which will have to be reduced to 10 ppm on January 1, 2009 within the European community.

The feedstock to be treated is generally a sulfur-containing gasoline cut such as, for example, a gasoline cut from a coking, visbreaking, steam cracking or cracking unit. catalyst (FCC). Said feed preferably consists of a gasoline cut from a catalytic cracking unit, the distillation range of which is between 0°C and 300°C and preferably between 0°C and 250°C. In the rest of the text, we will speak generally of catalytic cracked gasoline by broadening this definition to gasolines which may contain, in addition to a part of catalytic cracked gasoline, gasoline fractions from other conversion units. .

Catalytic cracked gasolines can constitute 30% to 50% by volume of the gasoline pool and generally have high mono-olefin and sulfur contents. However, the sulfur present in reformulated gasoline is attributable, at nearly 90%, to gasoline resulting from catalytic cracking. The desulphurization of gasolines, and mainly FCC gasolines, is therefore of crucial importance for compliance with current and future standards. However, the mono-olefins contained in gasoline contribute significantly to their octane rating. In order to maintain the octane number of olefinic cracked gasolines at high values, it is necessary to limit the rate of hydrogenation of the mono-olefins during the treatment of the gasolines by hydrodesulphurization. For this, so-called selective hydrodesulfurization processes have been developed.

In addition, the desulphurized gasolines must also meet the specifications in terms of corrosiveness. The corrosive power of gasoline is essentially due to the presence of acid sulfur compounds such as mercaptans. Desulphurized gasolines must therefore contain few mercaptans to limit their corrosiveness. However, it is now known that in units for the selective hydrodesulphurization of olefinic gasolines, the H ₂ S present in the reactor can react with non-hydrogenated mono-olefins to form mercaptans. The fraction of mercaptans in the gasoline produced is generally higher the lower the sulfur content of the gasoline. To minimize the mercaptan content, it is generally preferable to work with a high flow rate of hydrogen. However, this induces significant costs in terms of the compressor, recycling and purification of hydrogen. In order to respond to this problem, the present invention presents a solution making it possible to limit the energy consumption of the compressor, while reducing the mercaptan content and increasing the octane number for a constant sulfur content of the desulfurized gasoline.

Moreover, the contrasting evolution of automotive markets around the world is pushing refiners to seek maximum flexibility and therefore the possibility of maximizing diesel fuel cut production for diesel-powered vehicles or gasoline production depending on the circumstances. Thus, it can be very advantageous, for a refiner, to have the most economical possibility possible of sending the heavy fraction of the gasoline, either to the gasoline pool, or to the middle distillates pool according to its needs.

In summary, the present invention proposes a new solution to respond economically to the triple problem of reducing the sulfur content in fuels, limiting the mercaptan content in gasolines with a low sulfur content, and flexibility in orienting fuel production towards gasoline or middle distillate cuts according to market needs. Furthermore, in the current context of reducing greenhouse gas emissions, it is important to integrate the problem of controlling energy consumption into any new idea. The process diagram described in the context of this invention is innovative because it makes it possible to simultaneously deal with the triple problem described above, while limiting the energy consumption due to the necessary compression of the hydrogen which is recycled in the steps of hydrodesulfurization. The aim is to obtain gasoline whose research octane number (RON) and sulfur content [S] couples are such that RON• 90.70 and [S]• 50ppm, preferably RON• 90.70 and [S] • 37ppm, more preferably RON• 90.75 and [S]• 35ppm and very preferably RON• 90.80 and [S]<31ppm. Preferably, each torque will be associated with an engine octane number (MON) such as MON• 79.45, preferably such as MON• 79.50, even more preferably such as MON• 79.55.

Review of prior art.

The patent application EP0725126-A1 describes a method for desulfurizing gasolines from catalytic cracking while limiting the loss of octane by hydrogenation of mono-olefins. This method consists in distilling the gasoline into several fractions, including at least a fraction rich in compounds which are difficult to desulphurize chosen from thiophene and alkylthiophenes, and a fraction rich in compounds which are easy to desulphurize chosen from thiacyclopentane, alkylthiacyclopentanes, benzothiophene and alkylbenzothiophenes. At least one of these two fractions is treated by a hydrodesulphurization process and is then mixed with the untreated fraction. This method has the disadvantage of requiring an analysis of the various fractions before treatment, and does not describe how to choose the fractions to limit the quantity of mercaptans in the final desulfurized product.

The patent US 6596157 B2 describes a process for the desulphurization of gasoline cuts from cracking units based on the treatment, in parallel, of the heavy fraction of gasoline called HCN (Heavy Cat Naphtha according to the Anglo-Saxon terminology), under non-selective hydrodesulphurization conditions and of the intermediate gasoline fraction called ICN (Intermediate Cat Naphtha according to the Anglo-Saxon terminology) under selective hydrodesulphurization conditions, for which the intermediate gasoline (ICN) is heated by the hydrotreated heavy fraction (HCN) stream.

This patent does not describe how to treat the different cuts to limit the fraction of sulfur compounds in the form of mercaptans in the desulfurized gasoline. Moreover, according to the teaching of this patent, the light fraction of the gasoline called LCN (Light Cat Naphtha according to the Anglo-Saxon terminology), must generally undergo an additional desulphurization treatment, for example an extraction of the mercaptans by washing with means of a solution containing soda.

Regarding the problem of extracting mercaptans from desulphurized cracking gasolines, the solutions currently envisaged described in the patent US 6960291 consist of post-treating the essences resulting from selective hydrotreating in order to deplete them in mercaptans. The methods considered are multiple. For example, the patent WO 01/79391 which describes methods for treating partially desulfurized gasolines to reduce their mercaptan content on the basis of different methods such as adsorption, extraction with soda, heat treatments, etc. However, these methods present the the disadvantage that they require the implementation of an additional gasoline treatment step and do not offer the flexibility of sending certain cuts either to the gasoline pool or to the middle distillates pool.

The patent application US 2003/0042175 describes a method for desulphurizing cracked gasolines comprising different treatment steps to reduce the sulfur content. This process comprises a stage of hydrogenation of the diolefins, a stage of transformation of the light sulfur compounds by weighting, a stage of distillation of the gasoline into several cuts and at least one stage of desulphurization of at least part of the heavy fraction gasoline produced. However, this patent does not teach how to treat the gasolines to minimize the mercaptan content of the desulfurized gasoline, nor how to treat the hydrogen resulting from the hydrodesulfurization stages.

Description of the invention :

The invention is based on the differentiated treatment of different cuts constituting the gasoline cut.

The invention relates to a process for the production of gasolines with a low sulfur and mercaptan content according to the subject of claim 1.

It is known that in gasolines from catalytic cracking, the light ends are rich in mono-olefins and in saturated sulfur compounds such as mercaptans and sulphides. By light fraction is meant the gasoline fractions whose boiling point is below 100°C, preferably below 80°C and very preferably below 65°C. The heavy fraction of gasoline is rich in sulfur compounds of the benzothiophene type such as benzothiophene and alkylbenzothiophenes and to a lesser degree is rich in alkylthiophenes. In addition, it is rich in aromatic compounds and low in olefinic compounds. The heavy fraction of gasoline consists of hydrocarbons whose boiling point is above 160°C, preferably above 180°C and very preferably above 207°C. This heavy fraction of gasoline is generally the one that contains the most sulphur. The heavy gasoline fraction can be incorporated either into the gasoline pool or into the middle distillate fraction to produce kerosene or gas oil. The heart 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 sulfur compounds of thiophene types, including thiophene, methyl-thiophenes and other alkylthiophenes.

Generally, the various gasoline fractions are obtained by distillation of the effluent from the catalytic cracking unit.

The mixture consisting of the light gasoline fraction and the intermediate fraction or the intermediate fraction alone is treated in a first hydrodesulfurization stage called HDS1. This step consists of bringing the gasoline to be treated into contact with hydrogen, in one or more hydrodesulphurization reactors in series, containing one or more catalysts suitable for carrying out the hydrodesulphurization selectively, i.e. say with a degree of hydrogenation of the mono-olefins of less than 60%, preferably less than 50% and very preferably less than 40%.

The operating pressure for this step is between 0.5 MPa and 5 MPa, and preferably between 1 MPa and 3 MPa. The temperature is between 200°C and 400°C and preferably between 220°C and 380°C. In the case where the treatment is carried out in several reactors in series, the average operating temperature of each reactor will be higher by at least 5°C, preferably by at least 10°C and very preferably by at least 15 °C at the operating temperature of the reactor which precedes it.

The quantity of catalyst used in each reactor is such that the ratio between the flow rate of gasoline to be treated, expressed in m ³ per hour at standard conditions, per m ³ of catalyst (also called space velocity) is between 0.5 h ^-1 and 20 h ^-1 and preferably between 1 h ^-1 and 15 h ^-1 . Very preferably, the first reactor will be operated with a space velocity of between 2 h ^-1 and 8 h ^-1 .

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

The degree of desulphurization reached during stage HDS1 is greater than 80% and preferably greater than 90%.

After the hydrodesulfurization step, the reaction mixture is cooled to a temperature below 60° C. in order to condense the hydrocarbons. The gas and liquid phases are separated in a separator. The liquid fraction which contains the desulfurized gasoline as well as a fraction of the dissolved H ₂ S is sent to a stripping section, the gaseous fraction consisting mainly of hydrogen and which contains the majority of the H ₂ S is sent to a purification section.

The heavy gasoline fraction is treated in a separate hydrodesulfurization step called HDS2. This step consists of bringing the gasoline to be treated into contact with hydrogen, in one or more series hydrodesulphurization reactors containing one or more catalysts suitable for carry out hydrodesulphurization. Preferably, the hydrodesulfurization of the heavy gasoline will be carried out in a single step, in a single reactor. The hydrodesulphurization can 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 very preferably less than 60%.

The operating pressure for this step is between 0.5 MPa and 10 MPa, and preferably between 1 MPa and 8 MPa. The temperature is between 220°C and 450°C and preferably between 250°C and 380°C. In the case where the treatment is carried out in several reactors in series, the average operating temperature of each reactor will be higher by at least 5° C., preferably by at least 10° C. and very preferably by at least minus 15°C at the operating temperature of the reactor which precedes it.

The quantity of catalyst used in each reactor is such that the ratio between the flow rate of gasoline to be treated, expressed in m3 per hour at standard conditions, per m3 of catalyst (also called space velocity) is between 0.3 h ^{- 1} and 8:00 ^p.m. and preferably between 0.5: ⁰⁰ a.m. and ^3:00 p.m. Very preferably, the first reactor will be operated with a space velocity of between 1 h ^-1 and 8 h ^-1 .

The hydrogen flow rate is such that the ratio between the hydrogen flow rate expressed in normal m ³ per hour (Nm ³ /h) and the feed rate to be treated expressed in m ³ per hour at standard conditions is between 30 Nm ³ /m ³ and 800 Nm ³ /m ³ , preferably between 50 Nm ³ /m ³ and 500 Nm ³ /m ³ . This ratio will be less than 80% of the ratio of the flow rates used to desulfurize in the HDS1 hydrodesulfurization step, preferably less than 60%, very preferably less than 50% and even more preferably less than 40 % of the flow rate ratio used to desulphurize in the HDS1 hydrodesulphurization step.

After the hydrodesulfurization step, the reaction mixture is cooled to a temperature below 60° C. in order to condense the hydrocarbons. The gas and liquid phases are separated in a separator. The liquid fraction which contains the desulfurized gasoline as well as a fraction of the dissolved H ₂ S is sent to a stripping section, the gaseous fraction consisting mainly of hydrogen and which contains the majority of the H ₂ S is sent to a purification section.

Any catalyst exhibiting good selectivity with respect to hydrodesulphurization reactions can be used in the HDS1 or HDS2 stages. By way of example, catalysts will be used comprising an amorphous and porous mineral support chosen from the group consisting of aluminas, silicon carbide, silica, silica-aluminas or even titanium or magnesium oxides used alone or mixed with alumina or silica-alumina. It is preferably chosen from the group consisting of silica, the family of transition aluminas and silica-aluminas. Very preferably, the support consists essentially of at least one transition alumina, that is to say it comprises at least 51% by weight, preferably at least 60% by weight, very preferably at least 80 % by weight, or even at least 90% by weight of transition alumina. It may optionally consist solely of a transition alumina.

The specific surface of the support is generally less than 200 m2/g and preferably less than 150 m2/g. The porosity of the catalyst before sulfurization is such that it has an average pore diameter greater than 20 nm, preferably greater than 25 nm or even 30 nm and often between 20 and 140 nm, preferably between 20 and 100 nm, and very preferably between 25 and 80 nm. The pore diameter was measured by mercury porosimetry according to the ASTM D4284-92 standard with a wetting angle of 140°.

The hydrodesulfurization catalyst contains at least one Group VI metal and/or at least one Group VIII metal on a support. Group VI metal is usually molybdenum or tungsten the Group VIII metal is usually nickel or cobalt. The surface density of the group VI metal is comprised according to the invention between 2.10 ^-4 and 4.0.10 ^-3 grams of oxide of said metal per m2 of support, preferably between 4.10 ^-4 and 1.6.10 ^-3 g/m ² .

In the possible case of a catalyst sequence, the process comprises a succession of hydrodesulphurization stages, such that the activity of the catalyst of a stage n+1 is between 1% and 90% of the activity of the catalyst of step n.

It is however possible to use a non-selective catalyst in the HDS2 stage. By way of example, catalysts will be used comprising an amorphous and porous mineral support chosen from the group consisting of aluminas, silicon carbide, silica, silica-aluminas or even titanium or magnesium oxides used alone or mixed with alumina or silica-alumina. It is preferably chosen from the group consisting of silica, the family of transition aluminas and silica-aluminas. Very preferably, the support consists essentially of at least one transition alumina, that is to say it comprises at least 51% by weight, preferably at least 60% by weight, very preferably at least 80 % by weight, or even at least 90% by weight of transition alumina. It may optionally consist solely of a transition alumina. The hydrodesulfurization catalyst contains at least one Group VI metal and/or at least one Group VIII metal on a support. Group VI metal is usually molybdenum or tungsten and Group VIII metal is usually nickel or cobalt. The selective nature or not of the hydrodesulfurization catalyst, in the sense defined above in the description of the invention, generally depends on the composition and the method of preparation of said catalyst. Simple ways to vary the selectivity consist, for example, in modifying the contents of metals of group VIII and of group VI or possibly the molar ratio between the quantities of metals of group VIII and of group VI for a given support or to make vary the specific surface of the support for constant metal contents.

According to a preferred embodiment of the invention, the excess hydrogen from the HDS1 and HDS2 hydrodesulfurization steps can be collected and treated in a single purification section. The hydrogen thus purified is then recycled to at least one of the hydrodesulphurization stages HDS1 and HDS2 after a compression stage to compensate for pressure drops through the process. A make-up of fresh hydrogen is carried out, either before or after the compression stage in order to compensate for the consumption of hydrogen in the hydrodesulphurization reactors.

It is possible to envisage admitting at least all the hydrogen necessary for the reactions occurring in the two hydrodesulphurization stages through only one of the hydrodesulphurization stages, preferably HDS2. For these purposes, all of the hydrogen supply necessary for stages HDS1 and HDS2 is then sent to only one of these stages, preferably HDS2, and the gas purge from this stage is sent to the purification treatment, the purified gaseous product of which n is only recycled to the other hydrodesulfurization step. This type of scheme makes it possible to minimize the flow of recycled hydrogen passing through the compressor in the event that all the hydrogen consumed in the two hydrodesulphurization stages is sent through the HDS2 unit and the gas purge from this unit is sent to the purification treatment, the purified gas product of which is only recycled to stage HDS1. Indeed, the HDS1 unit requiring a lower hydrogen flow compared to the HDS2 unit, less hydrogen has need to be recycled by the compressor and thus the energy consumption of the compressor is lower.

It is possible to envisage a combined treatment on the vapor fractions from the two stripping columns dedicated to each hydrodesulphurization step (cooling, recycling of the condensed liquid to each of the stripping columns and combined purging of gas rich in H2S sent to a purification step.

In the case where the product of the HDS2 hydrodesulphurization stage is sent to the diesel pool, the fact of having an HDS2 stage dedicated to heavy gasoline means that this gasoline is not co-mixed with middle distillate cuts in another hydrotreatment therefore to free up capacity in said hydrotreatment and consequently to increase the production capacity of the refinery.

• d'hydrogéner sélectivement les dioléfines en mono-oléfines
• de transformer les composés soufrés légers saturés et principalement les mercaptans, en sulfures ou mercaptans plus lourds par réaction avec les mono-oléfines

It is possible, while remaining within the scope of the invention, to implement a pre-treatment of the load, the purpose of which is mainly:

• to selectively hydrogenate diolefins to mono-olefins
• to transform saturated light sulfur compounds and mainly mercaptans, into heavier sulfides or mercaptans by reaction with mono-olefins

The hydrogenation reactions of diolefins to mono-olefins are illustrated below by the transformation of 1,3 pentadiene, an unstable compound which can easily polymerize, into pent-2-ene by hydrogen addition reaction. However, it is sought to limit the secondary hydrogenation reactions of the mono-olefins which, in the example below, would lead to the formation of n-pentane.

The sulfur compounds which it is sought to convert are mainly mercaptans and sulphides. The main transformation reaction of mercaptans consists of a 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 propyl pentyl sulfide.

In the presence of hydrogen, the transformation of the sulfur compounds can also go through the intermediate formation of hydrogen sulphide which can then add to the unsaturated compounds present in the charge. However, this route is in the minority under the preferred reaction conditions. In addition to the mercaptans, the compounds capable of being thus transformed and made heavier are the sulphides and mainly dimethyl sulphide, methyl ethyl sulphide, diethyl sulphide, CS ₂ , COS, thiophane and methyl thiophane.

In some cases, we can also observe reactions of weighting of light nitrogenous compounds, and mainly nitriles, pyrrole and its derivatives.

This pretreatment step consists of bringing the charge to be treated into contact with a flow of hydrogen and with a catalyst containing at least one metal from group VIb (group 6 according to the new notation of the periodic table of the elements: Handbook of Chemistry and Physics, 76th edition, 1995-1996 ) and at least one metal from group VIII (groups 8, 9 and 10) of said classification, deposited on a porous support.

The catalyst according to the invention can be prepared by means of any technique known to those skilled in the art, and in particular by impregnation of the elements of groups VIII and VIb on the selected support. This impregnation can for example be carried out according to the method of preparation known to those skilled in the art under the terminology of dry impregnation, in which just the quantity of desired elements is introduced in the form of soluble salts in a chosen solvent, by example of demineralized water, so as to fill as exactly as possible the porosity of the support. The support thus filled with the solution is preferably dried. The preferred support is alumina, which can be prepared from any type of precursors and shaping tools known to those skilled in the art.

The catalyst is usually used in a sulphide form obtained after temperature treatment in contact with a decomposable organic sulfur compound and generator of hydrogen sulphide (H ₂ S) or directly in contact with a gas stream of dilute H ₂ S in _H2 . This step can be carried out in situ or ex situ (i.e. inside or outside the hydrodesulphurization reactor) at temperatures between 200 and 600° C. and more preferably between 300 and 500° C. .

The charge to be treated is mixed with hydrogen before being brought into contact with the catalyst. The quantity of hydrogen injected is such that the molar ratio between the hydrogen and the diolefins to be hydrogenated is greater than 1 (stoichiometry) and less than 10, and preferably between 1 and 5 mole/mole. Too much excess hydrogen can lead to strong hydrogenation of mono-olefins and consequently a decrease in the octane number of gasoline. The entire feed is generally injected at the reactor inlet. However, it may be advantageous, in certain cases, to inject a fraction or all of the charge between two consecutive catalytic beds placed in the reactor. This embodiment makes it possible in particular to continue to operate the reactor if the inlet of the reactor is clogged by deposits of polymers, particles, or gums present in the charge.

The mixture consisting of gasoline and hydrogen is brought into contact with the catalyst at a temperature between 80°C and 250°C, and preferably between 90°C and 220°C, with a liquid space velocity ( LHSV) between 1 h ^-1 and 10 h ^-1 , the unit of the liquid space velocity being the liter of charge per liter of catalyst and per hour (II ^-1 .h ^-1 ). The pressure is adjusted so that the reaction mixture is mainly in liquid form in the reactor. The pressure is between 0.5 MPa and 5 MPa and preferably between 1 and 4 MPa.

The gasoline treated under the conditions stated above has a reduced content of diolefins and mercaptans. Generally, the gasoline produced contains less than 1% by weight of diolefins, and preferably less than 0.5% by weight of diolefins. Light sulfur compounds whose boiling point is lower than that of thiophene (84°C) are generally converted at more than 50%. It is therefore possible to separate the light fraction from the gasoline by distillation and to send this fraction directly to the gasoline pool without additional treatment.

Preferred embodiments of the process of the present invention are illustrated in figure 1 , the figure 2 and the picture 3 .

Description of Figure 1:

A core gasoline, gasoline A circulating via line 1 is mixed with hydrogen from the recycle compressor P1, via line 20. The mixture thus formed is injected into the reaction section R1. The effluent circulating via line 4 is cooled in the exchanger section E1 in order to condense the hydrocarbons then the mixture is injected into the separation section S1 via line 6. The separation section S1 produces a fraction gas extracted by line 8, which essentially consists of hydrogen, of H ₂ S and light hydrocarbons and a liquid fraction extracted via line 9. The liquid fraction is then injected into a stabilization section C2 which extracts via line 15, at the top, the H ₂ S dissolved in the hydrocarbons. The gasoline recovered at the bottom of column C2 via line 16 can be sent directly to the gasoline pool.

A heavy gasoline, gasoline B circulating via line 3 is mixed with fresh hydrogen supplied via line 2. The mixture thus formed is injected into the reaction section R2. The effluent circulating via line 5 is cooled in the exchanger section E2 in order to condense the hydrocarbons then the mixture is injected into the separation section S2 via line 7. The separation section S2 produces a fraction gas extracted by line 10, which essentially consists of hydrogen, H ₂ S and light hydrocarbons and a liquid fraction extracted by line 11. The liquid fraction is then injected into a stabilization section C3 which extracts by line 17, at the top, the H ₂ S dissolved in the hydrocarbons. The desulfurized heavy gasoline recovered via line 18 can be sent either to the gasoline pool or to a middle distillate pool.

The stabilization sections C2 and C3 each comprise a distillation column. It is advantageous, in order to limit the operating and investment costs, to combine the distillates from these two columns before cooling them in order to condense them, and to send them jointly to the reflux drum. Both columns can thus be operated with a common reflux section.

The hydrogens from separators S1 and S2 via lines 8 and 10 respectively are mixed before being treated in a common purification section C1 which consists of washing with an aqueous amine solution according to a technique well known to man. of career. After purification, the hydrogen stripped of I'H ₂ S and circulating through line 13 and compressed in a recycle compressor P1 and is then mixed with gasoline A through line 20.

According to another variant of the invention, the make-up hydrogen is injected via line 12 upstream of purification section C1. The hydrogen needed for treatment of gasoline B is then injected via line 19 into the hydrodesulphurization stage of reaction section R2.

Description of Figure 2

The figures used in this figure correspond to those used for the figure 1 .

Another variant of the invention is presented on the figure 2 . According to this variant, a fraction of the hydrogen from the separation section S1 via line 8 is injected, without purification treatment, into the reaction section R2 via line 21.

Description of Figure 3

The picture 3 illustrates the sequences of the pretreatment stage consisting mainly of hydrogenating the diolefins and making the light sulfur compounds heavier and of the selective hydrodesulphurization stage.

The pretreatment stage R3 can be implemented either on the total gasoline injected via line 1, or on the gasoline recovered at the distillation head in column C4 via line 3. In the latter case, the gasoline A is sent directly to column C4 without pretreatment.

When the pretreatment is applied to the total gasoline, the hydrogen is injected via line 10, upstream of the pretreatment stage R3 which corresponds to the stage of selective hydrogenation and weighting of the saturated light sulfur compounds. The gasoline produced is then distilled in two cuts in column C4, a heavy cut extracted by line 4 which corresponds to the heavy gasoline described in the text, and a lighter fraction recovered by line 3 which corresponds to the mixture of heart essence and light essence described in the text. The light fraction is then distilled in a second column, C5 which makes it possible to separate the core gasoline which leaves via line 6 from the light gasoline which leaves via line 7. The light gasoline recovered by line 7 is generally low in sulfur and can be sent directly to the gasoline pool without additional treatment.

The core and heavy gasolines recovered by lines 6 and 4 respectively are treated in one or more hydrodesulphurization sections in accordance with the invention and making it possible to 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 equipped with a lateral withdrawal from which the core gasoline is extracted. According to another embodiment of the invention, the distillation column for the total gasoline injected via line 1 can be a single column with an internal wall.

When the pretreatment stage R3 is applied to the gasoline from line 3, the hydrogen is injected via line 11. This embodiment has the advantage of only sending into the pretreatment stage R3 a fraction of gasoline corresponding to the fraction freed from heavy gasoline, which reduces the quantities of gasoline to be treated, as well as the presence of potential contaminants from catalysts such as arsenic or silicon which are generally concentrated in the heavy fractions of gasoline.

Examples: Preparation of loads:

• Une coupe a1 correspondant à la fraction 6°C - 188°C
• Une coupe a2 correspondant à la fraction 188°C - 236°C
• Une coupe a3 correspondant à la fraction 6°C - 209°C
• Une coupe a4 correspondant à la fraction 209°C - 236°C

A gasoline a , whose boiling points are between 6°C and 236°C, from a catalytic cracking unit is distilled in a discontinuous distillation column in order to produce four cuts:

• A section a1 corresponding to the fraction 6°C - 188°C
• A section a2 corresponding to the fraction 188°C - 236°C
• A section a3 corresponding to the fraction 6°C - 209°C
• A section a4 corresponding to the fraction 209°C - 236°C

The characteristics of the different cuts are gathered in Table 1. Table 1: characteristics of distilled cuts at a1 a2 a3 a4 100% 85% 15% 92% 8% S ppm weight 390 253 1173 250 1875 BrN g/100g 41 46.5 11 43 6 RON 91.7 91.5 94.5 92.1 94.3 MY 80 81.3 82.8 81.9 82.9 Chopped off °C 6-236 6-188 188-235 6-209 209-235 -ppm weight is the weight content of sulfur in parts per million measured according to the ASTM method ASTM D-5453.
-BrN (Bromine Number according to the Anglo-Saxon terminology) is the bromine index measured according to the ASTM D-1159 method.

Example 1 (comparative):

A volume of 100 ml of HR806S catalyst (sulfur catalyst based on cobalt and molybdenum) marketed by the company Axens is loaded into the reactor of a pilot unit. This catalyst has the particularity of being presulfided and preactivated ex situ. It therefore does not require an additional sulfurization step.

Gasoline a is mixed with hydrogen before being injected into the reactor. The fuel flow is 400 ml/h and the hydrogen flow is 116 normal liters per hour. The hydrogen flow is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I. The temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called c1 , is cooled and stripped by a flow of hydrogen in order to eliminate dissolved H ₂ S.

After analysis, this gasoline contains 38 ppm of sulfur of which 14.0 ppm are in the form of mercaptans. Its Research Octane Number (RON) is 90.60 and its Motor Octane Number (MON) is 79.40.

Example 2 (comparative ):

340 ml/h of a1 gasoline are mixed with 98 normal liters per hour of hydrogen and injected into a volume of 85 ml of HR806S catalyst. The hydrogen flow is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 300 NI/I. The reactor temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called b1 , contains 19 ppm of sulfur, including 8 ppm in the form of mercaptans.

60 ml/h of a2 gasoline are mixed with 14.4 normal liters per hour of hydrogen and injected into a volume of 15 ml of HR806S catalyst. The hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 240 NI/I. The reactor temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called b2 , contains 90 ppm of sulfur, including 4 ppm in the form of mercaptans.

Overall, for the processing of cuts a1 and a2, the hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I.

The essences b1 and b2 are mixed up to 85% weight of essence b1 and 15% weight of essence b2. The mixture thus formed, called c2 , is analyzed. It contains 30 ppm of sulfur of which 8.0 ppm in the form of mercaptans. His its research octane number (RON) is 90.80 and its engine octane number (MON) is 79.50. Fraction b2 can also be sent to the very low sulfur middle distillate pool.

By comparing the gasolines c1 and c2 produced in examples 1 and 2, it appears that the parallel treatment of the gasolines separated into two distinct cuts while maintaining, overall, the same hydrogen flow rate makes it possible to improve the index of octane of desulfurized gasoline, but above all to significantly reduce the mercaptan content.

Example 3 (according to the invention):

Gasoline b1 is obtained according to the preparatory mode described in example 2.

60 ml/h of a2 gasoline are mixed with 6.3 normal liters per hour of hydrogen and injected into a volume of 15 ml of HR806S catalyst. The hydrogen flow is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 105 NI/I. The reactor temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called b5 , contains 135 ppm of sulfur, including 6 ppm in the form of mercaptans.

The essences b1 and b5 are mixed up to 85% weight of essence b1 and 15% weight of essence b5. The mixture thus formed, called c4, is analyzed. It contains 36 ppm of sulfur including 8.0 ppm in the form of mercaptans. Its Research Octane Number (RON) is 90.90 and its Motor Octane Number (MON) is 79.60. Fraction b5 can also be sent to the very low sulfur middle distillate pool.

Overall, for the processing of cuts a1 and a2, the hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 270 NI/I.

Example 4 (comparative ):

368 ml/h of a3 gasoline are mixed with 108.2 normal liters per hour of hydrogen and injected into a volume of 92 ml of HR806S catalyst. The hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 294 NI/I. The reactor temperature is adjusted to 260°C. The gasoline produced, called b3 , contains 20 ppm of sulfur, including 7 ppm in the form of mercaptans.

32 ml/h of a4 gasoline are mixed with 7.5 normal liters per hour of hydrogen and injected into a volume of 8 ml of HR806S catalyst. The hydrogen flow is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 234 NI/I.

The reactor temperature is adjusted to 260°C. The gasoline produced, called b4 , contains 140 ppm of sulfur, including 3 ppm in the form of mercaptans.

The b3 and b4 gasolines are mixed up to 92% by weight of b3 gasoline and 8% by weight of b4 gasoline. The mixture thus formed, called c3 , is analyzed. It contains 30 ppm of sulfur including 7.0 ppm in the form of mercaptans. Its Research Octane Number (RON) is 91.00 and its Motor Octane Number (MON) is 79.70. Fraction b4 can also be sent to the very low sulfur middle distillate pool.

Overall, for the processing of the a3 and a4 cuts, the hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I.

By comparing examples 2 and 4, it is clear that it is advantageous to separate gasoline from a heavy fraction whose boiling point is higher than 209°C in order to treat it in an independent hydrodesulphurization section, because this improves the octane number of desulfurized gasoline and reduces the mercaptan content.

Example 5 (comparative):

340 ml/h of gasoline a1 are mixed with 98.6 normal liters per hour of hydrogen and injected into a volume of 85 ml of HR806S catalyst. The hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I. The reactor temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called b6 , contains 22 ppm of sulfur, including 9 ppm in the form of mercaptans.

60 ml/h of a2 gasoline are mixed with 17.4 normal liters per hour of hydrogen and injected into a volume of 15 ml of HR806S catalyst. The hydrogen flow is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I. The reactor temperature is adjusted to 260° C. and the pressure to 2 MPa. The gasoline produced, called b7 , contains 80 ppm of sulfur, including 4 ppm in the form of mercaptans.

Overall, for the processing of cuts a1 and a2, the hydrogen flow rate is such that the H2/HC ratio in normal liters of hydrogen per liter of charge is equal to 290 NI/I.

The b6 and b7 gasolines are mixed up to 85% weight of b6 gasoline and 15% weight of b7 gasoline. The mixture thus formed, called c5 , is analyzed. It contains 31 ppm of sulfur including 8.3 ppm in the form of mercaptans. Its Research Octane Number (RON) is 90.65 and its Motor Octane Number (MON) is 79.40. Table 2: comparison of the performances obtained Example 1 comparison 2 comparative 3 invention 4 comparative 5 comparative Cutting point (°C) No 188 188 209 188 H2/HC (HDS2) compared to H2/HC (HDS 1) in % No 80 35 80 100 sulfur content (in ppm) 38 30 36 30 31 mercaptan content (in ppm) 14.0 8.0 8.0 7.0 8.3 RON 90.60 90.80 90.90 91.00 90.65 MY 79.40 79.50 79.60 79.70 79.40

H2/HC (HDS2) relative to H2/HC (HDS1) in %: ratio between the hydrogen flow rate expressed in normal m ³ per hour and the feed flow rate to be treated expressed in m ³ per hour at standard conditions as a percentage of ratio of flow rates used to desulphurize in the HDS1 hydrodesulphurization step

The comparison of examples 1, 2, 4 and 5 shows that the parallel treatment of the gasolines separated into two distinct cuts while maintaining, overall, the same hydrogen flow rate makes it possible to improve the octane number of the desulfurized gasoline, and above all to significantly reduce the sulfur and mercaptan contents.

Furthermore, when the hydrogen flow rate in the HDS2 hydrodesulfurization step is such that the ratio between the hydrogen flow rate expressed in normal m ³ per hour and the feed rate to be treated expressed in m ³ per hour at the conditions standards is less than 80% of the ratio of the flow rates used to desulphurize in the HDS1 hydrodesulphurization step (examples 2 and 4 a significant reduction in the sulfur and mercaptans 1.

Claims (6)

1. Process for producing gasolines with a low content of sulfur and mercaptans, the feedstock corresponding to a gasoline from a catalytic cracking unit, the distillation range of which is between 0°C and 300°C, said gasoline being distilled into three fractions:

   - a light fraction corresponding to a gasoline fraction with a boiling point below 100°C, said light fraction being rich in monoolefins and in saturated sulfur-containing compounds;

   - a heavy gasoline fraction corresponding to a gasoline fraction with a boiling point above 180°C, said heavy fraction being rich in sulfur-containing compounds of benzothiopene types, rich in aromatic compounds, to a lesser degree rich in alkylthiophenes, and poor in olefinic compounds;

   - a core fraction corresponding to the intermediate fraction between the light fraction and the heavy fraction, said core fraction being rich in monoolefins and in sulfur-containing compounds of benzothiopene types,

   said process comprising at least two hydrodesulfurization steps HDS1 and HDS2 carried out at the same time on two different cuts of the gasoline constituting the feedstock;

   the HDS1 step treating the mixture constituted of the light gasoline fraction and said intermediate fraction or the intermediate fraction alone consisting in bringing the gasoline to be treated into contact with hydrogen at a temperature between 200°C and 400°C, a pressure between 0.5 and 5 MPa, and at a space velocity, defined as being the ratio of the flow rate of gasoline to be treated, expressed in m³ per hour, under standard conditions, per m³ of catalyst, of between 0.5 h^-1 and 20 h^-1, in one or more reactors in series containing one or more catalysts suitable for carrying out the hydrodesulfurization selectively with a degree of hydrogenation of the monoolefins of less than 60%, said catalyst containing at least one group VI metal and/or at least one group VIII metal on a support, the porosity of the catalyst before sulfurization having a mean pore diameter of greater than 20 nm measured by mercury porosimetry according to the ASTM D4284-92 standard with a wetting angle of 140°, the surface density of the group VI metal is, according to the invention, between 2 × 10^-4 and 4.0 × 10^-3 grams of oxide of said metal per m² of support, it being understood that during a catalyst concatenation, the process comprises a series of hydrodesulfurization steps such that the activity of the catalyst of a step n+1 is between 1% and 90% of the activity of the catalyst of step n,

   and in which the flow rate of hydrogen in the hydrodesulfurization unit HDS1 is such that the ratio of the hydrogen flow rate, expressed in normal m³ per hour to the flow rate of feedstock to be treated, expressed in m³ per hour, under standard conditions, is between 50 Nm³/m³ and 1000 Nm³/m³, the degree of desulfurization achieved during the HDS1 step being greater than 80%,

   then, after the hydrodesulfurization HDS1, the reaction mixture is cooled to a temperature below 60°C in order to condense the hydrocarbons, the gas and liquid phases being separated in a separator, said liquid fraction containing the desulfurized gasoline and also a fraction of the dissolved H₂S is sent to a stripping section, said gas fraction, constituted mainly of hydrogen and which contains most of the H₂S, is sent to a purification section, and the step HDS2 treating the heavy fraction of the gasoline by bringing the gasoline to be treated into contact with hydrogen at a temperature between 220°C and 450°C, a pressure between 0.5 and 10 MPa, and at a space velocity, defined as being the ratio of the flow rate of gasoline to be treated, expressed in m³ per hour, under standard conditions, per m³ of catalyst, of between 0.3 h^-1 and 20 h^-1, in one or more reactors in series containing one or more catalysts suitable for carrying out the hydrodesulfurization selectively or nonselectively, the degree of hydrogenation of the monoolefins being less than 90% in the case of selective hydrodesulfurization, said catalyst containing at least one group VI metal and/or at least one group VIII metal on a support, the porosity of the catalyst before sulfurization having a mean pore diameter of greater than 20 nm measured by mercury porosimetry according to the ASTM D4284-92 standard with a wetting angle of 140°, the surface density of the group VI metal is, according to the invention, between 2 × 10^-4 and 4.0 × 10^-3 grams of oxide of said metal per m² of support, it being understood that during a catalyst concatenation, the process comprises a series of hydrodesulfurization steps such that the activity of the catalyst of a step n+1 is between 1% and 90% of the activity of the catalyst of step n,

   and in which the flow rate of hydrogen in the hydrodesulfurization unit HDS2 is such that the ratio of the hydrogen flow rate, expressed in normal m³ per hour to the flow rate of feedstock to be treated, expressed in m³ per hour, under standard conditions, is between 30 Nm³/m³ and 800 Nm³/m³, then, after the hydrodesulfurization HDS2, the reaction mixture is cooled to a temperature below 60°C in order to condense the hydrocarbons, the gas and liquid phases being separated in a separator, said liquid fraction containing the desulfurized gasoline and also a fraction of the dissolved H₂S is sent to a stripping section, said gas fraction, constituted mainly of hydrogen and which contains most of the H₂S, is sent to a purification section, and in which the flow rate of hydrogen in the hydrodesulfurization step HDS2 is such that the ratio of the hydrogen flow rate, expressed in normal m³ per hour to the flow rate of feedstock to be treated, expressed in m³ per hour, under standard conditions, is less than 80% of the ratio of the flow rates used to desulfurize in the hydrodesulfurization step HDS1.
2. Process according to Claim 1, comprising a single section for purifying and recycling excess hydrogen from the hydrodesulfurization steps HDS1 and HDS2.
3. Process according to Claim 1 or Claim 2, in which at least all of the hydrogen necessary for the reactions occurring in the two hydrodesulfurization steps is admitted through only one of the hydrodesulfurization steps and in which the gas purge from this hydrodesulfurization step is sent to the purification treatment, the purified gaseous product from which is recycled only to the other hydrodesulfurization unit.
4. Process according to Claim 3, in which at least all of the hydrogen necessary for the reactions occurring in the two hydrodesulfurization steps is admitted through the hydrodesulfurization step HDS2.
5. Process according to one of Claims 1 to 4, in which the feedstock is pretreated so as to:

   - selectively hydrogenate the diolefins to monoolefins;

   - convert the saturated light sulfur-containing compounds into heavier sulfides or mercaptans by reaction with the monoolefins.
6. Process according to one of Claims 1 to 5, in which the group VI metal is molybdenum or tungsten and the group VIII metal is nickel or cobalt.

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