FR2993570A1 - PROCESS FOR THE PRODUCTION OF A LIGHT LOW SULFUR CONTENT - Google Patents

Abstract

The present invention relates to a process for treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans. The process comprises the following steps: a) a demercaptation step is carried out by adding at least a portion of the mercaptans to the olefins by contacting the gasoline with at least a first catalyst; b) a step of treating the gasoline from step a) with hydrogen in a distillation column comprising at least one reaction zone including at least a second catalyst. The operating conditions and the second catalyst of step b) are chosen so that in said distillation column a separation of the gasoline resulting from step a) is carried out simultaneously into a light gasoline fraction and a hydrogen fraction. heavy gasoline and a reaction of thioetherification and selective hydrogenation of diolefins of a fraction of the gasoline from step a) by contacting with the second catalyst.

Description

The present invention relates to a method of treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans to provide a light fraction of this gasoline with very low sulfur content while preserving the octane number .

State of the art The production of reformulated species meeting the new environmental standards requires in particular that their concentration in olefins be reduced slightly but significantly their concentration of aromatics (especially benzene) and sulfur. Catalytic cracking gasolines, which can represent 30 to 50% of the gasoline pool, have high olefin and sulfur contents. The sulfur present in the reformulated gasolines is attributable, to nearly 90%, to the catalytic cracking gasoline (FCC, "Fluid Catalytic Cracking" or catalytic cracking in fluidized bed). The desulphurisation (hydrodesulphurisation) of gasolines and mainly FCC species is therefore of obvious importance for the achievement of specifications. Pretreatment by hydrotreatment (hydrodesulfurization) of feeds sent to catalytic cracking leads to FCC gasolines typically containing less than 100 ppm of sulfur. These hydrotreatment units, however, operate in severe conditions of temperature and pressure, which implies a high consumption of hydrogen and a high investment. In addition, the entire charge must be desulfurized, resulting in the processing of very large load volumes.

It is therefore necessary, in order to meet the sulfur specifications, to post-treat by hydrotreating (or hydrodesulfurization) the catalytic cracking gasolines. When this post-treatment is carried out under standard conditions known to those skilled in the art, it is possible to further reduce the sulfur content of the gasoline. However, this process has the major disadvantage of causing a very significant drop in the octane number of the cut, due to the saturation of olefins during the hydrotreatment. US Pat. No. 4,131,537 teaches the advantage of splitting the gasoline into several cuts, preferably three, depending on their boiling point, and of desulfurizing them under conditions which may be different and in the presence of a catalyst. comprising at least one Group VIB and / or Group VIII metal. It is stated in this patent that the greatest benefit is obtained when the gasoline is split into three cuts, and when the cut having intermediate boiling points is treated under mild conditions. French Patent FR 2,785,908 teaches the advantage of splitting the gasoline into a light fraction and a heavy fraction and then performing a specific hydrotreatment of light gasoline over a nickel-based catalyst, and a hydrotreating heavy gasoline on a catalyst comprising at least one Group VIII metal and / or at least one Group VIb metal. US Pat. No. 6,440,299 describes a process for removing mercaptans from a hydrocarbon feedstock using a catalytic distillation column. The catalytic bed of the column is located above the feed to treat only the light fraction of the load. The catalyst used is a supported catalyst based on nickel sulphide on which the mercaptans are removed by an addition thioetherification reaction on the diolefins. As illustrated in the example of this patent, the process makes it difficult to simultaneously obtain very low levels of sulfur on the light fraction of the treated gasoline. Indeed, when the amount of diolefins in the feed is low and / or the amount of mercaptans is important, the conversion kinetics of mercaptans on the catalyst is disadvantaged. To maintain a high conversion, it is necessary either to increase the temperature or to limit internal traffic to the column. Operating at higher temperature at iso-point cutting of light gasoline can only be done by increasing the pressure in the column however this increase is limited by the design of the column. Limiting internal traffic (for example by lowering the internal reflux ratio) has the disadvantage of degrading the separating power of the column, which would favor the recovery of light mercaptans not converted into the light fraction.

Also known is US Pat. No. 5,807,477, which discloses a process for treating a catalytic cracking gasoline which uses a first thioetherification step in which the mercaptans are reacted with the diolefins of the feedstock in the presence of a catalyst. Group VIII metal oxide base. The gasoline having undergone this first stage is then sent to a distillation column comprising a catalytic hydrogenation section containing another Group VIII metal oxide catalyst. The catalytic column is configured to function as a depentanizer. Thus compounds having a carbon number greater than or equal to 6 and including the thioethers formed during the first step are extracted at the bottom of the column while the compounds having a carbon number less than or equal to 5 are distilled towards the head of the column and contacted with the hydrogenation catalyst where a selective hydrogenation of unreacted diolefins is carried out during the first step.

However, the process described above makes it difficult to obtain a light fraction of the gasoline that meets the future sulfur specifications, ie whose upper limit in total sulfur in gasoline is 50 ppm by weight, or even 30 or 10 ppm by weight in some countries. As indicated in the example of US Pat. No. 5,807,477, the first step of the process does not convert light mercaptans or little. Indeed, the elimination of mercaptans is by addition to the diolefins of the charge. However, it is stated in the patent that such a catalyst based on Group VIII metal oxide also catalyzes the selective hydrogenation of diolefins. The two reactions are therefore competing with diolefins and this results in a limited conversion to thioetherification. Once injected into the column (second stage), the light mercaptans, especially those which are lighter than the cut point of the column, will end up favorably in the vapor phase in the rectification zone and will then be difficult to convert to the catalytic bed. The light gasoline produced at the top of the catalytic column therefore contains a large fraction of the light mercaptans present in the initial charge.

Similarly, it is found in the example mentioned in the patent that the H2S is not converted at all in this first step, which means that if H2S is present in the feedstock, it will inevitably end up in the catalytic column located downstream. However, it is known to one skilled in the art that the Group VIII metal hydrogenation catalysts also catalyze the addition reactions of the H 2 S on the diolefins and / or the olefins, which can induce the formation of additional mercaptans near the top of the column, which are found mainly in the light fraction of gasoline. H2S and mercaptans are also known to be inhibitors of this type of catalyst, especially when the Group VIII metal is palladium.

The invention as presented in US Pat. No. 5,807,477 thus makes it difficult to deplete the light fraction of gasoline to very low levels of sulfur, which generally requires the use of a post-treatment of this fraction in order to meet the specifications. An object of the invention is therefore to provide a process for producing a light gasoline with a very low sulfur content, that is to say having a sulfur content of less than 50 ppm by weight and preferably less than 30 ppm or ppm, while limiting octane loss, which is also relatively simple and requires the lowest possible investment. SUMMARY OF THE INVENTION For this purpose, there is provided a method of treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans, said process comprising the following steps; a) a step of demercaptation is carried out by adding at least a portion of the mercaptans to the olefins by contacting the gasoline with at least a first catalyst, at a temperature of between 50 and 250 ° C, at a temperature of pressure between 0.4 and 5 MPa and a liquid space velocity (LHSV) between 0.5 and 10 h -1, the first catalyst being in sulphide form and comprising a first support, at least one metal selected from group VIII and at least one metal selected from the group VIb of the Periodic Table of the elements, the weight, expressed as the oxide equivalent of the metal selected from group VIII relative to the total weight of catalyst, is between 1 and 30% and the weight , expressed in oxide equivalent of the metal selected from the group VIb, is between 1 and 30% relative to the total weight of catalyst; b) a step of treating the gasoline resulting from step a) with hydrogen in a distillation column comprising at least one reaction zone comprising at least a second catalyst comprising a second support and at least one group VIII metal, the conditions of step b) being chosen so that in said distillation column the following operations are carried out simultaneously: I) a distillation so as to separate the gasoline resulting from step a) into a fraction of light gasoline depleted of sulfur compounds and a heavy gasoline fraction having a boiling point higher than that of light gasoline and comprising the majority of sulfur compounds, the light gasoline fraction being evacuated to a point located above the reaction zone and the heavy gasoline fraction being discharged at a point below the reaction zone; II) contacting a fraction of the gasoline from step a) with the second catalyst so as to carry out the following reactions: (i) a thioetherification by adding a portion of the mercaptans with a portion of the diolefins to form thioethers, (ii) selective hydrogenation of a portion of the diolefins to olefins, and optionally (iii) isomerization of the olefins.

The process according to the invention implements a first step a) in which the sulfur compounds of the mercaptan type (R-SH) are converted into heavier sulfur compounds by reaction with the olefins present in the gasoline to be treated. The demercaptation reactions according to the invention are characterized by a reaction of the mercaptans with olefins: either by direct addition to the double bond to produce sulphides with a higher boiling point, or by a hydrogenolysing route: the hydrogen present in the the reactor will produce H2S by contact with a mercaptan which will add directly to the double bond of an olefin to form a heavier mercaptan, that is to say at the higher boiling point. This route is however a minority in the preferred conditions of the reaction. This first step of increasing the weight of the mercaptans reaches very high conversions (> 90% and very often> 95%) because the demercaptation reactions are carried out selectively on the olefins which are generally present at very high levels. The lightest mercaptans are the most reactive in this first step a). Similarly, in case of presence of H2S in the feed, it is converted into mercaptan (which itself can be converted) by addition to the olefins with the catalyst at the selected conditions. The removal of the H 2 S from this first stage is advantageous insofar as it makes it possible to prevent the inhibition of the catalyst of the catalytic column of the second stage (the H 2 S being an inhibitor of the catalyst used) and to avoid the H2S training leading with the light fraction. The conversion of the H 2 S upstream of the distillation column to heavy mercaptans or sulphides which will exit in the heavy fraction of the catalytic distillation column ensures a very low sulfur content on the light fraction. Thus the H2S present up to a content of the order of 10 ppm by weight in the feed can be converted to 100%. The demercaptation and elimination reactions of H2S are preferably carried out on a catalyst comprising at least one element of group VIII (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995- 1996), at least one element from the group Vlb (group 6 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and a support. The group VIII element is preferably chosen from nickel and cobalt and in particular nickel. The group VIb element is preferably selected from molybdenum and tungsten and very preferably molybdenum. Before contacting with the charge to be treated, the catalyst undergoes a sulphurization step. The catalyst performs the desired demercaptation reactions only in its sulfide form. Sulfurization is preferably carried out in a sulforeductive medium, that is to say in the presence of H 2 S and hydrogen, in order to convert the metal oxides to sulphides such as, for example, MoS 2 and Ni 3 S 2. The process according to the invention comprises a second step b) of treating the gasoline produced in step a) in a distillation column provided with a reaction section. The distillation column is configured to work under operating conditions which simultaneously make it possible: - to convert the light mercaptans which would not have reacted during step a) by thioetherification with the diolefins so that the thioethers formed at 30 during the second stage are also trained in the heavy gasoline fraction. The catalyst used in the distillation column also performs a selective hydrogenation of the diolefins and optionally allows isomerization of olefins whose double bond is in the outer position in the internal position. - To separate the pretreated gasoline in step a) in at least two fractions, namely a fraction of gasoline called "light" depleted of sulfur compounds and which contains most of the olefins and a gasoline fraction called " which comprises the majority of sulfur compounds, in particular thioethers formed in step a) and b). The reaction of selective hydrogenation of diolefins to olefins is especially important when the light cut gasoline is used as a charge of an etherification unit (type TAME for example) because these highly unsaturated compounds easily form gums in this case. type of processes. When this light cut is sent directly to the gasoline pool, it is also advantageous to hydrogenate diolefins because they tend to produce gums when oxygen enters the storage tanks. In addition, this second step also makes it possible to weigh down the residual mercaptans that would not have been transformed during step a) by reacting the residual mercaptans with the diolefins. Finally, depending on the Group VIII metal selected for one of the supported catalysts of the column, preferably palladium, the invention may provide isomerization of olefins from the external position to the internal position. This isomerization is advantageous because it makes it possible to maintain the octane number of the cut, or even to improve it, despite the hydrogenation of the olefins which can take place in each of the steps. An advantage of the process according to the invention lies in the fact that step a) does not generate hydrogen sulphide (I-12S) but on the contrary makes it possible to convert the H2S that may be present in the feed to be treated. Indeed, the presence of H2S at the inlet of the catalytic column could, as indicated above, induce the formation of additional mercaptans on the catalytic zone of the column which could then exit in the light fraction. The absence of H2S which is a catalyst inhibitor used in the present invention also makes it possible to improve the efficiency of the catalyst of the catalytic column.

Another advantage of the process is that the high conversion of the mercaptans during step a) induces a significant increase in the molar ratio between mercaptans and diolefins at the column inlet. The effectiveness of the thioetherification of the residual mercaptans in step b) is then improved because the mercaptans / diolefins ratio is favorable. Another advantage of the sequence of steps a) and b) is related to the presence of a synergy of these two steps for the conversion of mercaptans due to a differentiated reactivity of the mercaptans as a function of their chain length in the two step. Indeed, the lighter mercaptans (methanethiol, ethanethiol in particular) are the most reactive in step a). Conversely, the heavier mercaptans that are likely to be entrained in the light cut (eg 1- and 2-propanethiol), ie those whose boiling point is close to the cutting point of the column, are the least reactive in step a) while they are more easily converted into the catalytic column since favorably present in the liquid phase at the catalytic zone. The synergy of the two reactions thus makes it possible to obtain a light cut of the gasoline at a very low sulfur content whatever the nature of the mercaptans initially present in the feed. Another advantage of the process according to the invention lies in the fact that it is not necessary to desulfurize the light fraction of the gasoline coming from the distillation column because the majority of the sulfur compounds have been converted into weight compounds. higher molecular in steps a) and b) so that they are trained in the heavy gasoline fraction. Catalytic hydrogenation reactions are not required in step a), but if hydrogen is added it serves essentially to maintain a hydrogenating surface state of the catalyst to ensure a high yield on the demercaptation reactions. The method according to the invention is therefore not penalized by the low pressures. Another advantage of the process is therefore that the two steps can be performed at the same pressure (with the pressure drop close) because step a) requires only little dissolved hydrogen, or not at all and step b ) is generally performed at relatively low pressures (less than 1.5 MPa) in order to minimize utility consumption while providing a temperature necessary for the thioetherification reaction. This step b) also requires a relatively small amount of makeup hydrogen to selectively hydrogenate only the diolefins.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing a light fraction of a gasoline having a low sulfur content from a gasoline, preferably originating from a catalytic cracking unit, from coking or visbreaking. According to the invention, the gasoline first undergoes a step a) of transforming the sulfur compounds, essentially the lighter mercaptans of the gasoline, on the olefins in order to increase their molecular weight. The process further comprises a second step b) which comprises passing all or part of the gasoline from step a) into a distillation column comprising a catalytic reaction zone. In step b) where a fractionation of the gasoline is carried out in at least two fractions (a light fraction and a heavy fraction), the light fraction is treated under conditions and on a catalyst which make it possible (i) to form thioethers by adding a portion of the mercaptans which have not reacted in step a) with a portion of the diolefins to form sulfur compounds of higher boiling point which are subsequently in the gasoline heavy, (ii) selectively hydrogenating at least a portion of the diolefins to olefins and optionally (iii) isomerizing the double-bonded internal double-bonded olefins. This sequence makes it possible to obtain in fine a light fraction whose sulfur content has been lowered without any significant decrease in the olefin content, even at high conversion rates, without it being necessary to treat this. light gasoline by means of a hydrodesulphurization section or using methods to restore the octane number of gasoline. The process according to the invention thus makes it possible to provide a light gasoline fraction whose total sulfur content is less than 50 ppm by weight, preferably less than 30 ppm, or even less than 10 ppm by weight. As used herein, the term "catalytic column" refers to an apparatus in which the catalytic reaction and product separation takes place at least simultaneously. The equipment employed may comprise a distillation column equipped with a catalytic section, in which the catalytic reaction and distillation take place simultaneously at the specifically chosen cutting point. It may also be a distillation column in association with at least one reactor disposed inside said column and on a wall thereof. The internal reactor may be operated as a vapor phase reactor or as a liquid phase reactor with a co-current or countercurrent liquid / vapor circulation. The implementation of a catalytic distillation column has the advantages, compared with the implementation of a system comprising a reactor and a distillation column, of reducing the number of unit elements, hence a higher investment cost. reduced. The use of a catalytic column allows control of the reaction while promoting an exchange of the heat released; the heat of reaction can be absorbed by the heat of vaporization of the mixture. The gasoline to be treated The process according to the invention makes it possible to treat any type of sulfur-containing gasoline cut, preferably a petrol cut from a catalytic cracking unit, whose range of boiling points typically extends from approximately the boiling points of the hydrocarbons with 2 or 3 carbon atoms (C2 or C3) up to about 250 ° C., preferably since about the boiling points of the hydrocarbons with 2 or 3 carbon atoms (C2 or C3) ) up to about 220 ° C, more preferably from about the boiling points of the 5-carbon hydrocarbons up to about 220 ° C. The process according to the invention can also treat feeds having end points lower than those mentioned previously, such as for example a C5-180 ° C cut. The sulfur content of catalytic cracked gasoline (FCC) gasoline cuts depends on the sulfur content of the FCC-treated feedstock, the presence or absence of FCC feedstock pretreatment, and the endpoint of the FCC. chopped off. Generally, the sulfur contents of the entirety of a petrol cut, in particular those coming from the FCC, are greater than 100 ppm by weight and most of the time greater than 500 ppm by weight. For gasolines with end points higher than 200 ° C., the sulfur contents are often greater than 1000 ppm by weight, and in some cases they may even reach values of the order of 4000 to 5000 ppm by weight. In addition, gasolines from catalytic cracking units (FCC) contain, on average, between 0.5% and 5% by weight of diolefins, between 20% and 50% by weight of olefins, between 10 ppm and 0.5%. sulfur weight of which generally less than 300 ppm of mercaptans. Mercaptans are generally concentrated in the light ends of gasoline and more precisely in the fraction whose boiling point is below 120 ° C. It should be noted that the sulfur compounds present in the gasoline may also comprise heterocyclic sulfur compounds, such as, for example, thiophenes, alkylthiophenes or benzothiophenes. Step a) Increasing the mercaptans with the olefins This step consists of converting the light sulfur compounds of the mercaptan family, that is to say the compounds which at the end of step b) of distillation would be in the light gasoline, heavier sulfur compounds that will be drawn into the heavy gasoline during step b) distillation. During this step a) a demercaptation reaction takes place which sees the addition of the mercaptans to the olefins of the feed in the presence of a catalyst. Typically the mercaptans that can react during step a) are the following (non-exhaustive list): methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, isopropyl mercaptan, iso-butyl mercaptan, tert-butyl mercaptan, n-butyl mercaptan, sec-butyl mercaptan, iso-amyl mercaptan, n-amyl mercaptan, α-methylbutyl mercaptan, α-ethylpropyl mercaptan, n-hexyl mercaptan, 2- mercapto-hexane. The demercaptation reaction is preferably carried out on a catalyst comprising at least one element of group VIII (groups 8, 9 and 10 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996), at least one element of the Vlb group (group 6 of the new periodic classification Handbook of Chemistry and Physics, 76th edition, 1995-1996) and a support. The group VIII element is preferably chosen from nickel and cobalt and in particular nickel. The group VIb element is preferably selected from molybdenum and tungsten and most preferably molybdenum. The catalyst support is preferably selected from alumina, nickel aluminate, silica, silicon carbide, or a mixture of these oxides. Alumina is preferably used, and even more preferably pure alumina. Preferably, a support having a total pore volume measured by mercury porosimetry of between 0.4 and 1.4 cm 3 / g and preferably between 0.5 and 1.3 cm 3 / g is used. The specific surface of the support is preferably between 70 m 2 / g and 350 m 2 / g.

According to a preferred variant, the support is a cubic gamma alumina or delta alumina. The catalyst used in step a) generally comprises: a content by weight of oxide of the group VIb element between 1 and 30% by weight relative to the total weight of the catalyst, a content by weight of oxide of the group VIII element between 1 and 30% by weight relative to the total weight of the catalyst, a sulphidation rate of the metals constituting said catalyst at least equal to 60%, a molar ratio between the metal of the group VIII and the metal of the group VIb between 0.6 and 3 mol / mol, - a support consisting of gamma alumina or surface delta between 70 m2 / 9 and 350 m2 / g In particular, it has been found that the performances are improved when the catalyst has the following characteristics: the content by weight of oxide of the group VIb element in oxide form is between 4 and 20% by weight relative to the total weight of catalyst, preferably between 6 and and 18% by weight; the content of Group VIII metal expressed in oxide form is between 3 and 15% by weight and preferably between 4% by weight and 12% by weight relative to the total weight of catalyst; the molar ratio between the non-noble metal of group VIII and the metal of group VIb is between 0.6 and 3 mol / mol and preferably between 1 and 2.5 mol / mol, a support consisting of surface gamma alumina of between 180 m2 / g and 270 m2 / g A preferred embodiment of the invention corresponds to the use of a catalyst containing a nickel oxide content by weight (in NiO form) included between 4 and 12%, a content by weight of molybdenum oxide (in MoO3 form) of between 6% and 18% and a nickel / molybdenum molar ratio of between 1 and 2.5, the metals being deposited on a support consisting of only alumina and the sulphidation rate of the metals constituting the catalyst being greater than 80%. The catalyst according to the invention can be prepared using any technique known to those skilled in the art, and in particular by impregnation of the elements of Groups VIII and Vlb on the selected support. After introduction of the elements of groups VIII and VIb, and possibly shaping of the catalyst, it underwent an activation treatment. This treatment generally aims to transform the molecular precursors of the elements in the oxide phase. In this case it is an oxidizing treatment but a simple drying of the catalyst can also be carried out. In the case of an oxidizing treatment, also known as calcination, this is generally carried out under air or under dilute oxygen, and the treatment temperature is generally between 200 ° C. and 550 ° C., preferably between 300 ° C. C and 500 ° C. After calcination, the metals deposited on the support are in oxide form. In the case of nickel and molybdenum, the metals are mainly in the form of MoO 3 and NiO. Before contacting with the feedstock to be treated, the catalysts undergo a sulphurization step. Sulfurization is preferably carried out in a sulforeductive medium, that is to say in the presence of H 2 S and hydrogen, in order to convert the metal oxides to sulphides such as, for example, MoS 2 and Ni 3 S 2. Sulfurization is carried out by injecting onto the catalyst a stream containing H2S and hydrogen, or a sulfur compound capable of decomposing into H 2 S in the presence of the catalyst and hydrogen. Polysulfides such as dimethyl disulphide (DMDS) are H 2 S precursors commonly used to sulphide catalysts. The temperature is adjusted so that the H2S reacts with the metal oxides to form metal sulfides. This sulphurization may be carried out in situ or ex situ (inside or outside the reactor) of the demercaptation reactor at temperatures between 200 and 600 ° C. and more preferably between 300 and 500 ° C.

In step a) of adding the mercaptans to the olefins, the feedstock to be treated is contacted with the catalyst in sulphide form. The demercaptation reactions according to the invention are characterized by elimination of mercaptans on olefins: either by direct addition to the double bond to produce sulphides with a higher boiling point or by a hydrogenolysing route: hydrogen (s') it is present) in the reactor will produce H2S by contact with a mercaptan which will add directly to the double bond of an olefin to form a heavier mercaptan, ie to the boiling point more Student. This route is however a minority in the preferred conditions of the reaction. Similarly, in case of presence of H2S in the feed, it is converted into mercaptan (which itself can be converted) by addition to the olefins with the catalyst at the selected conditions. This H2S may come from recycle gas or make-up hydrogen that contains some impurities. This H2S can also sometimes be solubilized in the liquid charge. This step can be carried out without adding hydrogen to the reactor, but preferably it is injected with the feed so as to maintain a hydrogenating surface state of the catalyst suitable for high conversions in demercaptation. Typically, step a) operates with an H 2 / HC ratio of between 0 and 25 Nm 3 of hydrogen per m 3 of filler, preferably between 0 and 10 Nm 3 of hydrogen per m 3 of filler, very preferably between 0 and 5 Nm3 of hydrogen per m 3 of feed, and even more preferably between 0.5 and 2 Nm 3 of hydrogen per m 3 of feed.

The entire charge is usually injected at the reactor inlet. However, it may be advantageous in some cases to inject a fraction or the entire charge between two consecutive catalytic beds placed in the reactor. This embodiment makes it possible in particular to continue operating the reactor if the inlet of the reactor is clogged by deposits of polymers, particles, or gums present in the load. The gasoline to be treated is brought into contact with the catalyst at a temperature of between 50 ° C. and 250 ° C., and preferably between 80 ° C. and 220 ° C., and even more preferably between 90 ° C. and 200 ° C. C, with a liquid space velocity (LHSV) between 0.5 11-1 and 10 h '', the unit of the liquid space velocity being the liter of feed per liter of catalyst per hour (1 / 1h). The pressure is between 0.4 MPa and 5 MPa and preferably between 0.6 and 2 MPa and even more preferably between 0.6 and 1 MPa.

At the end of step a) the gasoline treated under the conditions set out above, has a reduced mercaptans content. Generally, the gasoline produced contains less than 50 ppm by weight of mercaptans, and preferably less than 10 ppm by weight. Light sulfur compounds whose boiling point is lower than that of thiophene (84 ° C) are generally converted to more than 80% or even more than 90%. The olefins are not or very little hydrogenated, which allows to maintain a good octane output of step a). The hydrogenation rate of the olefins is generally less than 2%. Stage b) of treatment of the gasoline resulting from stage a) The second stage of the process according to the invention involves a distillation column incorporating a catalytic reaction section. In said column a distillation of the gasoline from step a) is carried out in at least two sections, namely: a "light" section which contains a gasoline softened with sulfur, whose range of boiling points typically extends from about the initial point of the input charge of the catalytic column to an end point generally between 60 ° C and 100 ° C and - a so-called "heavy" cut whose range of boiling points typically extends from about the end point of the light gasoline cut to about the end point of the charge of the catalytic column and which contains the majority of the heavy sulfur compounds, that is to say sulfur compounds whose boiling temperature is greater than the cutting point chosen for the operation of the column. The catalytic distillation column comprises a reaction section comprising a catalytic bed intended to carry out at least one thioetherification reaction of the mercaptans which would not have reacted with the olefins during step a), a selective hydrogenation of the diolefins of the gasoline, and possibly isomerization of olefins. For this purpose, the reaction section is arranged in the distillation column such that the "light" gasoline which distills up the column is brought into contact with the catalyst bed. The thioetherification reaction of step b) consists in reacting the residual mercaptans with the diolefins contained in the gasoline resulting from stage a) to form thioethers whose boiling point is higher than that of the residual mercaptans. so that they are trained in the "heavy" section at the bottom of the catalytic column. The operation of the catalytic column involves the simultaneous presence of two phases in the reaction zone, namely a liquid phase and a vapor phase which comprises most of the hydrogen and light hydrocarbons. As with any distillation, there is a temperature gradient in the system such that the lower end of the column comprises the compounds whose boiling point is higher than that of the upper end of the column. The distillation makes it possible to separate the compounds present in the feed by difference in boiling temperature. The heat of reaction, if any, generated in the catalytic column is evacuated by vaporization of the mixture on the distillation plate concerned. Consequently, the thermal profile of the column is very stable and the catalytic reactions which take place on the bed present at the top of the column do not disturb its operation. Likewise, this stability of the thermal profile makes it possible to have stable reaction kinetics since they are isothermal on each separation stage. Preferably, the catalyst used in the reaction section is also capable of selectively hydrogenating the diolefins of the "light" fraction to the corresponding olefins by means of a hydrogen feed. Optionally, the catalyst of the reaction section also makes it possible to isomerize the olefins whose double bond is in an external position to an isomer whose double bond is in the internal position. According to the invention, the catalyst employed in the reaction section comprises at least one Group VIII metal deposited on a porous support and can be originally in the form of small-diameter extrudates or spheres. The catalyst has a structural form suitable for catalytic distillation in order to act both as a catalytic agent for carrying out the reactions but also as a material transfer agent in order to have separation stages available along the bed.

According to a preferred embodiment, the metal may be chosen from nickel and palladium. If the metal is palladium it is preferably the only active metal in the catalyst. When the metal is nickel, the content by weight of the Group VIII metal relative to the total weight of catalyst, expressed as oxide, is generally between 10 and 60%. When the metal is palladium, the content by weight of the Group VIII metal relative to the total weight of catalyst (% Pd metal) is generally between 0.1 and 2%.

The porous support of the catalyst of step b) can be selected from alumina, nickel aluminate, silica, silicon carbide, or a mixture of these oxides. Alumina is preferably used, and even more preferably pure alumina.

A catalyst which is particularly suitable for carrying out the addition of residual mercaptans to diolefins and a selective hydrogenation of diolefins comprises 40 to 60% by weight of nickel, deposited on an alumina support. The catalytic reaction involves hydrogen which is mixed with the light gasoline which distills toward the top of the column and the mixture is brought into contact with the catalyst as defined. In the reaction section, the hydrogen / diolefin molar ratio is generally between 1 and 10 mol / mol. However, it is preferable to operate in the presence of a small excess of hydrogen relative to the diolefins in order to avoid excessive hydrogenation of the olefins and ensure a good octane number.

The operating pressure of the catalytic distillation column is generally between 0.4 and 5 MPa, preferably between 0.6 and 2 MPa and preferably between 0.6 and 1 MPa. The temperature in the reaction zone is generally between 50 and 150 ° C, preferably between 80 and 130 ° C.

It should be noted that this pressure can be substantially the same (at the pressure drop by) as that prevailing in the reactor of step a). The hydrogen required in step b) is relatively low since it serves essentially to maintain the catalyst in a hydrogenating state and to hydrogenate only the diolefins to olefins. The pressure control of the column can impose this one on both stages. Thus, a recycling of the hydrogen recovered in the head of the column can optionally be implemented. The recycle hydrogen can be injected, for example, directly into the catalytic distillation column of step b) or can be injected with the makeup hydrogen of the distillation column or reactor 2 and can also be injected directly. in the reactor of step a). According to a particular embodiment, the distillation column is configured to function as a depentanizer. According to another particular embodiment, the distillation column is configured to function as a dehexanizer. In the context of the invention, it is also possible to use more than one catalytic bed in the reaction zone, for example two separate catalytic beds separated from one another by a space. BRIEF DESCRIPTION OF THE FIGURES These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, with reference to the drawings of the figures, in which: FIG. first diagram of the method according to the invention; FIG. 2 shows a second diagram of the method according to the invention. The figures are not drawn to scale. Generally, similar elements are denoted by identical references in the figures. FIG. 1 shows a first diagram of the process according to the invention for treating a gasoline feedstock comprising, in particular, olefins, diolefins and sulfur compounds of the mercaptan type and of the thiophenic family in order to provide a light fraction of gasoline having a total sulfur content of less than 30 ppm (weight), preferably less than 20 ppm (weight), or even less than 10 ppm (weight). According to the process, the gasoline feedstock to be treated is optionally sent with a supplement of hydrogen in a demercaptation reactor 2 through a feed line 1.

The reactor 2 comprises a catalytic section provided with a catalytic bed specifically chosen in order to carry out the selective addition of the mercaptans to the olefins in order to increase their molecular weight. . The reactor is preferably a fixed catalytic bed reactor which operates in a three-phase or two-phase system and one of the phases (the catalyst) is solid.

The demercaptation catalyst according to the invention comprises a support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic table of elements. The weight, expressed as oxide equivalent of the metal selected from group VIII relative to the total weight of catalyst, is generally between 1 and 30%. The weight, expressed as the oxide equivalent of the metal selected from the group VIb, is between 1 and 30% relative to the total weight of catalyst. Preferably the group VIII metal is selected from nickel and cobalt and the group VIb metal is selected from molybdenum and tungsten. The demercaptation reactions are generally conducted at a temperature between 50 and 250 ° C, at a pressure of between 0.4 and 5 MPa and a VVH of between 0.5 hr and 10 hr-1. All or part of the feedstock treated in the mercaptan weighing reactor 2 is then sent via line 3 to a distillation column 4 comprising a reaction zone 5 equipped with a catalytic bed, the assembly being also designated by the term "catalytic column". Although FIG. 1 shows a reaction zone 5 having a single catalytic bed, it is quite possible to use more than one catalytic bed, the beds being arranged successively in the column. The catalytic column 4 is configured and operated to split the treated gasoline charge in the reactor 2 into two slices (or fractions), namely a heavy cut and a light cut. Furthermore, the catalytic bed is disposed in the upper part of said column so that the light section meets the catalytic bed during the fractionation. In the context of the invention, use is made of the upper part of the column which includes the reaction zone 5 to achieve not only an increase reaction of the light mercaptans which have not reacted in reactor 2, but also a selective hydrogenation. diolefins present in the light cut or even isomerization of olefins. For this purpose, the light cut is brought into contact in the presence of hydrogen with at least one catalyst of the reaction zone 5 which comprises a support and at least one metal of group VIII of the periodic table of the elements. As shown in FIG. 1, the hydrogen is directly sent to the catalytic column 5 via a pipe 6 whose injection point is situated upstream of the reaction zone. It should be noted, however, that the hydrogen can be mixed directly with the effluent leaving reactor 2 at line 3, before the effluent is transferred to the catalytic column, as shown in dashed lines in FIG. Referring to FIG. 1, the light cut containing gasoline softened to sulfur, the unreacted hydrogen and possibly some hydrogen sulphide is withdrawn through line 8. The cut is then partially condensed in order to separate the non-condensable compounds by passing through a heat exchanger 9 before being sent to a separator 11. The latter makes it possible to recover, via the line 14, a gaseous fraction containing essentially hydrogen and a treated gasoline part of which is sent through the line 13 to the gasoline pool and the other part is recycled through line 12 in the catalytic column 4 to ensure a reflux therein. The hydrogen recovered in the overhead gases can be recycled using a compressor. This recycle can be sent to the makeup hydrogen of the column or the makeup hydrogen of the reactor.

Preferably, the light section is withdrawn a few trays below the head of the catalytic distillation column 5. FIG. 2 illustrates a second embodiment which differs from that shown in FIG. 1 by the additional presence of a withdrawal. intermediate side 15 which allows for example to specifically recover a petrol cut containing a good part of the thiophene compounds which can serve as a load in a secondary hydrodesulfurization type treatment unit. This withdrawal can be performed below the catalytic zone as shown in Figure 2 or at an intermediate point in the middle of the catalyst bed. It is also pointed out that the heavy fraction which is withdrawn at the bottom of the catalytic column by line 7 can be treated in a hydrodesulfurization unit in order to convert the sulfur compounds into H 2 S and then combined with the light gasoline extracted into head of the catalytic column. Example 10 In a fixed flow-down bed reactor are charged 50 cc of NiMo catalyst 8/8 on nickel aluminate support in the form of 2-4 mm spheres. This is firstly sulphurated by injection for 4 h at VVH = 211-1, at 350 ° C. and 2.5 MPa of a heptane feed containing 4% DMDS under a hydrogen flow rate of 500 NL. / L. Under these conditions, the DMDS decomposes to H2S and permits the sulfuration of the catalyst. The feedstock used for the test is an FCC gasoline with an initial boiling point PI = 2 ° C and a boiling point PF = 208 ° C. The operating conditions are as follows: P = 1.0 MPa T = 100 ° C VVH = 3h-1 H 2 / HC = 2 NL / L A speciation analysis of the sulfur compounds gives us: Compounds Charge (ppm) Effluent ( ppm) H2S 5 <0.1 RSH C1-C6 213 8 THIOPHENIC 336,365 THIOPHANES 11 18 BENZOTHIOPHENIC 333,322 SULFIDES AND RSH C6 + 50 268 Total 943,981 The breakdown of mercaptans by carbon number is as follows: Compounds Charge (ppm) Effluent (ppm) Sum: C1SH-C3SH ppm 211 7 Sum RSH ppm 213 8 It is found that under these conditions a conversion of 96.7% is achieved on mercaptans from Cl to C3. These mercaptans are the sulfur compounds most likely to be in the light fraction of the gasoline after distillation, which would lead to more than 25 ppm of mercaptans in the light fraction. Analysis by chromatography of the feedstock and effluent gives the following results for the hydrocarbon families: Compounds Charge (%) Effluent (%) Parrafines 29.0 28.9 Olefins 50.0 49.8 Naphthenes 8.8 8.9 Aromatic 12.2 12.2 Diolefins It is found that the olefins have hardly been hydrogenated between the inlet and the outlet of the reactor 2. The octane number has therefore not been degraded. In addition, the chromatographic method used makes it possible to identify the C5 diolefins, which come out of the olefin family, and which give us an idea of the hydrogenation of these highly unsaturated compounds most likely to be in the top cut. . These diolefins are: isoprene, 1,3-cispentadiene and 1,3-transpentadiene. Their conversion is 17% in the reactor. The reactor effluent is then sent into a catalytic column 5 cm in diameter 10 and 12 m in height. This column is loaded at the top with 3 m of a catalyst which contains 0.3% by weight of Pd supported on alumina base. The operating conditions are as follows: Head pressure: 0.9 MPa Average catalyst bed temperature: 130 ° C. Head product yield: 25% weight H2 flow / feed rate: 2 N liter / liter The following results are obtained in the light fraction recovered in the upper part of the catalytic column: Total RSH content: 1 ppm weight Total S content: 8 ppm weight Diolefin content: 100 ppm weight This 2nd step makes it possible to finalize the conversion of the mercaptans by addition on diolefins and allows recovering less than 10 ppm by weight of sulfur in the light fraction recovered by lateral withdrawal above the catalytic zone.

Claims (3)

REVENDICATIONS1. A process for treating a gasoline comprising diolefins, olefins and sulfur compounds including mercaptans, said process comprising the following steps: a) performing a demercaptation step by adding at least a portion of the mercaptans to the olefins by contacting the gasoline with at least a first catalyst, at a temperature between 50 and 250 ° C, at a pressure of between 0.4 and 5 MPa, with a liquid space velocity (LHSV) of between 0.5 and 11-1, the first catalyst being in sulphide form and comprising a first support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic table of elements, the weight, expressed in oxide equivalent of the metal selected from group VIII relative to the total weight of catalyst, is between 1 and 30% and the weight%, expressed as oxide equivalent of the metal selected from the group VIb is between 1 and 30% relative to the total weight of catalyst; b) a step is carried out for treating the gasoline resulting from step a) with hydrogen in a distillation column comprising at least one reaction zone including at least one second catalyst comprising a second support and at least one metal group VIII, the conditions of step b) being chosen so that in said distillation column is carried out simultaneously the following operations: I) a distillation so as to separate the gasoline from step a) into a fraction light gasoline depleted in sulfur compounds and a heavy gasoline fraction having a boiling temperature higher than that of light gasoline and comprising the majority of sulfur compounds, the light gasoline fraction being evacuated to a point located above the reaction zone and the heavy gasoline fraction being discharged at a point below the reaction zone; II) contacting a fraction of the gasoline from step a) with the second catalyst so as to carry out the following reactions: (i) a thioetherification by adding a portion of the mercaptans with a portion of the diolefins to form thioethers; (ii) selective hydrogenation of a portion of the diolefins to olefins; and optionally (iii) isomerization of the olefins. 2. Method according to claim 1, wherein step b) is carried out at a pressure of between 0.4 and 5 MPa, preferably between 0.6 and 2 MPa, and at a temperature of between 50 and 150 ° C. preferably between 80 and 130 ° C in the reaction zone. 3. Method according to claims 1 or 2, wherein the first catalyst comprises nickel and molybdenum, with a weight of nickel relative to the total weight of catalyst, expressed as oxide, between 1 and 30%, preferably between 4 and 12% and a weight% of molybdenum relative to the total weight of catalyst, expressed as oxide, of between 1 and 30%, preferably between 6 and 18%. Process according to one of the preceding claims, in which the second catalyst comprises nickel with a weight of nickel relative to the total weight of catalyst, expressed as oxide, of between 10 and 60%. Process according to one of Claims 1 to 3, in which the second catalyst comprises palladium with a weight of palladium metal relative to the total weight of catalyst of between 0.1 and 2%. A process as claimed in any one of the preceding claims wherein the H 2 / HC ratio in step a) is 0 to 25 Nm 3 of hydrogen per m 3 of filler and more preferably 0.5 to 2 Nm 3 of hydrogen per m3 of charge. 7. Method according to one of the preceding claims wherein the gasoline is a catalytic cracking gasoline. 154. 5. 2 0 6.8. Method according to one of the preceding claims wherein the distillation column is configured to function as a depentanizer or dehexanizer. 9. Method according to one of the preceding claims wherein is carried out a lateral withdrawal of an intermediate gasoline cut. 10. Method according to one of the preceding claims wherein the pressure used in the distillation column of step b) is equal to the pressure used in the reactor of step a) at the pressure drop. of the hydraulic circuit near. 11. Method according to one of the preceding claims wherein there is carried out on the light gasoline a condensation and separation step in order to recover unconsumed hydrogen. 12. The method of claim 11 wherein the unconsumed hydrogen is recycled in step a) or b). 13. Method according to one of the preceding claims wherein in step a) is carried out concomitantly an addition of the H2S present in the gasoline to be treated on olefins. 14. Method according to one of the preceding claims wherein the heavy gasoline fraction from step b) is treated in a hydrodesulfurization unit.