EP1746144B1 - New process for olefinic gasoline desulfurisation which limits mercaptan content - Google Patents

Abstract

The hydrodesulfurization of a gasoline containing less than 0.1 wt.% sulfur comprises hydrodesulfurizing the gasoline in contact with a bimetallic catalyst in a hydrodesulfuration reactor (12) operating at an hourly superficial velocity (HSV) of 0.1-20 h-1>, at 220-350[deg]C and 0.1-5 MPa; and recycling a first fraction of the desulfurized gasoline from an outlet to an inlet of the reactor with a recycle ratio of 0.1-3 times the flow rate of the gasoline to be desulfurized.

Description

Field of the invention

In particular, the production of reformulated species that meet the new environmental standards requires a much greater reduction in their sulfur content. Indeed, the environmental standards force refiners to lower the sulfur content in the gasoline pool to values of less than or equal to 50 ppm in 2005, which will have to be reduced to 10 ppm by January 1, 2009 within the community. European. In addition, the desulphurized species must also meet the specifications in terms of corrosive power. The corrosive power of gasolines is essentially due to the presence of acid sulfur compounds such as mercaptans.

Desulphurized species must therefore contain few mercaptans to limit their corrosivity.

The feedstock to be treated is generally a sulfur-containing gasoline cut such as, for example, a petrol cut from a coking unit, visbreaking, steam cracking or catalytic cracking unit (FCC). Said feedstock is preferably composed of a gasoline cutter from a catalytic cracking unit whose distillation range is typically between 70 ° C and about 250 ° C.

In the rest of the text, we will talk generally about catalytic cracking gasoline by extending this definition to species that may contain more of a majority of gasoline from a catalytic cracking unit, gasoline fractions from other conversion units.

Catalytic cracking gasolines can make up 30% to 50% by volume of the gasoline pool and generally have high olefin and sulfur contents. However, the sulfur present in the reformulated gasoline is attributable, to nearly 90%, to gasoline from catalytic cracking. The desulphurisation of gasolines, and mainly of FCC species, is therefore of crucial importance for the respect of current and future standards.

Examination of the prior art

Hydrotreating or hydrodesulfurization of catalytic cracking gasolines, when carried out under conventional conditions, makes it possible to reduce the sulfur content of the cut. However, these processes have the major disadvantage of causing a very large drop in the octane number of the cut, due to the hydrogenation of a large part, indeed of all the olefins under the usual conditions of hydrotreating.

There are methods for deep desulfurization of FCC gasolines while maintaining the octane number at an acceptable level.

These processes are, for the most part, based on the principle of selective hydrodesulphurisation aimed at converting the sulfur compounds into H 2 S, while limiting the hydrogenation of olefins to paraffins, a transformation which induces a significant loss of octane.

However, the efficiency of these processes is limited by the formation of so-called recombinant mercaptans resulting from the addition of the H 2 S formed in the reactor with the residual olefins. The formation of these recombinant mercaptans is in particular described in the patent US6,231,754 and the patent application WO01 / 40409 which teach various combinations of operating conditions and catalysts to limit the formation of recombinant mercaptans.

Other solutions to the problem of the formation of recombinant mercaptans are based on a treatment of partially desulfurized species to extracting said recombinant mercaptans. Some of these solutions are described in the patent applications WO 02/28988 or WO 01/79391 .

The method described in the present invention makes it possible to significantly reduce the formation of recombinant mercaptans, and to limit the loss of octane during the desulphurization step, without resorting to a complementary step of treating gasoline. Indeed, it has been found by the inventors that it was possible to improve the performance of selective desulphurisation processes of gasolines, by recycling a fraction of the desulfurized gasoline.

It has indeed been observed that by mixing the feedstock to be treated with a fraction of the desulfurized gasoline, it was possible to significantly reduce the fraction of sulfur compounds present as mercaptans in the desulfurized effluents, while maintaining the octane at high levels.

The implementation of appropriate recycling to desulphurize the species is described in some publications, but under conditions and to address problems different from the subject of the invention.

For example, it is taught in the patent US 2,431,920 that the desulfurization, hydrogenation and dehydrogenation reactions of gasolines are improved by recycling a fraction of the desulfurized effluent in order to limit the sulfur content of the feedstock to less than 0.1% by weight.

The patent US 2,431,920 relates to gasoline fractions which contain more than 0.1% by weight of sulfur (ie more than 1000 ppm by weight) in order to desulphurize these fractions and to saturate at least a part of the olefins.

The EP 1 369 466 , US 5,968,346 and US 2004/0226863 also describe hydrodesulfurization processes.

The present invention differs from the prior art in that it is intended to desulphurize in a very thorough manner gasolines which contain less than 0.1% by weight of sulfur, while precisely limiting the degree of hydrogenation of the olefins as well as the formation of mercaptans.

Brief description of the figure:

The figure 1 represents a diagram of the process according to the invention in which the optional elements of the process are shown in dotted lines.

Brief description of the invention:

The invention can be described as a process for the hydrodesulfurization of a gasoline containing less than 0.1% by weight of sulfur, from a catalytic cracking unit, or a gasoline from other conversion units, and containing preferably at least one part of catalytic cracking gasoline, comprising at least one hydrodesulfurization reactor using a bimetallic catalyst working at a VVH between 0.1 h ^-1 and 20 h ^-1 , a temperature of between 220 ° C. and 350 ° C, and a pressure of between 0.1 MPa and 5 MPa (1 MPa = 10 ⁶ Pascal = 10 bar), characterized by the recycling of a fraction of the desulfurized gasoline at the inlet of the hydrodesulfurization reactor the recycling rate being between 0.2 and 2 times the gasoline flow to be desulphurized.

The hydrodesulfurization reactor used in the process according to the invention will generally be a fixed bed reactor, the size of the catalyst grains being of the order of a few millimeters, and preferably between 1 and 4 mm.

The catalyst used in the process comprises at least one group VIII element and a group VIb element, deposited on a porous support, the group VIII element preferably being iron, cobalt or nickel, preferably cobalt and the group VIb element preferably being molybdenum or tungsten, preferably molybdenum.

The hydrodesulfurization catalyst consists of a porous support having a specific surface area of less than 200 m ² / gram.

The process according to the invention can in certain cases use a finishing reactor located downstream of the hydrodesulfurization reactor, the said finishing reactor using either a monometallic catalyst or a bimetallic catalyst of the same type as that used in the reactor. hydrodesulfurization.

In the case where the process comprises a finishing reactor, it is particularly advantageous to carry out the recycling of at least a portion of the desulfurized gasoline at a point located between the hydrodesulfurization reactor and the finishing reactor.

It is also particularly advantageous to carry out the recycling of at least a portion of the desulphurized gasoline between two catalytic beds of the hydrodesulfurization reactor (or one of the hydrodesulfurization reactors when they are several to operate in series or in parallel).

Detailed description of the invention

The invention relates to a process for the desulfurization of gasolines containing less than 0.1% by weight of sulfur in the form of any type of sulfur compounds (1000 ppm by weight), preferably less than 950 ppm by weight of sulfur, more preferably less than 900 ppm of sulfur and very preferably less than 850 ppm of sulfur, and including any type of chemical compounds including olefins. The present process finds particular application in the conversion of conversion gasolines, and in particular the species from catalytic cracking, fluid-bed catalytic cracking (FCC), a coking process, a visbreaking process, or a pyrolysis process.

The process according to the invention makes it possible to produce a gasoline with a very low sulfur content and improved octane number. The sulfur content of the gasoline obtained by means of the process according to the invention is thus generally less than 30 ppm by weight, preferably less than 28 ppm by weight, and very preferably less than 25 ppm by weight. The mercaptan content of said gasoline is preferably less than 25 ppm by weight, more preferably less than or equal to 22 ppm by weight and very preferably less than or equal to 20 ppm by weight.

The process according to the invention comprises at least one step of hydrodesulfurization of the gasoline to be treated possibly followed by a step of finishing the hydrodesulfurization.

The hydrodesulphurization is carried out in at least one fixed-bed reactor which may comprise a plurality of catalytic beds separated by an injection zone for a cold fluid called a cooling zone, making it possible to control the rise in temperature along the reactor.

The finishing step is also carried out in at least one fixed-bed reactor which may comprise several catalytic beds.

The desulfurized gasoline can be recycled at the inlet of the hydrodesulfurization reactor, or between two consecutive beds of catalyst at the cooling zone, or between the hydrodesulfurization reactor and the finishing reactor. .

Whatever the recycling point or points chosen, the total flow rate of recycled gasoline corresponds to a flow rate of between 0.2 and 2 times the flow rate of gasoline to be desulphurized, and very preferably between 0.2 and 1 time. the flow of gas to be desulphurized.

The recycled gasoline is characterized in that it has a sulfur content lower than the sulfur content of the gasoline to be desulphurized, and preferably, a sulfur content at least two times lower than the sulfur content of the essence to desulphurize.

The operating conditions of the hydrodesulphurization reactor are those typically used to selectively desulphurize olefinic species. The operation will be carried out, for example, at a temperature of between 220 ° C. and 350 ° C., under a general pressure of between 0.1 and 5 MPa, preferably between 1 MPa and 3 MPa.

The space velocity will generally be between about 0.1 h ^-1 and 20 h ^-1 (expressed as the volume of liquid gas to be desulfurized per volume of catalyst per hour), preferably between 0.1 h ^-1 and 10 h ^{- 1} , and very preferably between 0.5 h ^-1 and 8 h ^-1 .

The ratio of the hydrogen flow rate on the gasoline flow to be desulphurized will generally be between 50 liters / liter and 800 liters / liter, and preferably between 100 liters / liter and 400 liters / liter.

The hydrodesulfurization reactor contains at least one bed of hydrodesulfurization catalyst comprising at least one group VIII element, and a group VIb element, deposited on a porous support.

The group VIII element is preferably iron, cobalt or nickel.

The group VIb element is preferably molybdenum or tungsten.

The content of group VIII element expressed as oxide is generally between 0.5% by weight and 15% by weight, and preferably between 0.7% by weight and 10% by weight.

The metal content of the group VIb is generally between 1.5% by weight and 60% by weight, and preferably between 2% by weight and 50% by weight.

The porous support of the hydrodesulfurization catalyst is selected from the group consisting of silica, alumina, silicon carbide or any mixture of said elements of the group.

To minimize the hydrogenation of the olefins, it is advantageous to use an alumina-based support whose specific surface area is less than 200 m ² / g, preferably less than 150 m ² / g, and very preferably lower at 100 m ² / g.

The porosity of the hydrodesulfurization catalyst is such that the average pore diameter is generally greater than 20 nm, and preferably between 20 and 100 nm (1 nm = 1 nanometer = 10 ^-9 meter).

The surface density of the metal of group VIB is preferably between 2.10 ^-4 and 40.10 ^-4 gram of oxide of said metal per m ² of support, preferably between 4.10 ^-4 and 16.10 ^-4 g / m ² of support.

The Group VIb and VIII elements being active in hydrodesulfurization in their sulfurized form, the catalyst generally undergoes a sulphurization step before it comes into contact with the feedstock to be treated.

Generally, this sulphurization is obtained by a heat treatment of the solid during which it is brought into contact with a decomposable sulfur compound and hydrogen sulphide generator. The catalyst can also be directly contacted with a gas stream comprising hydrogen sulfide.

This sulphurization step may be carried out ex situ or in situ, ie inside or outside the hydrodesulfurization reactor.

Optionally, before contacting the feedstock, the sulfurized catalyst may also have been subjected to a carbon deposition step so as to deposit a certain carbon content, preferably less than or equal to 2.8% by weight.

This carbon deposition step aims at improving the selectivity of the catalyst by preferentially reducing the hydrogenating activity of the catalyst.

Preferably, the carbon content deposited is between 0.5% and 2.6% by weight. This carbon deposition step can be carried out before, after, or during the step of sulphurizing the catalyst.

The method may use a hydrodesulfurization finishing step using a catalyst comprising at least one element selected from group VIII elements, deposited on a porous support such as for example alumina or silica.

The element content of group VIII is between 1% and 60% by weight, and preferably between 2% and 20% by weight. The said group VIII element is introduced in the form of a metal oxide, then it is sulphurized before use.

This finishing step is mainly used to decompose saturated sulfur compounds such as mercaptans or sulphides contained in the hydrodesulphurization effluent.

When present, this finishing step is carried out at a temperature higher than the hydrodesulfurization step.

According to another particular embodiment of the invention, the finishing step will be carried out on a hydrodesulphurization catalyst comprising at least one group VIII element and a Group VIb element, deposited on a porous support.

The group VIII element is preferably iron, cobalt or nickel.

The group VIb element is preferably molybdenum or tungsten.

The content of group VIII element expressed as oxide is between 0.5% by weight and 10% by weight and preferably between 0.7% by weight and 5% by weight.

The metal content of group VIb is between 1.5% by weight and 50% by weight, and preferably between 2% by weight and 20% by weight.

The porous support is selected from the group consisting of silica, alumina, silicon carbide or any mixture of said constituent elements.

To minimize the hydrogenation of the olefins, it is advantageous to use an alumina-based support whose specific surface area is less than 200 m ² / g, preferably less than 150 m ² / g, and very preferably lower at 100 m ² / g.

The porosity of the catalyst used in the finishing step is such that the average pore diameter is greater than 20 nm, and preferably between 20 nm and 100 nm.

The surface density of the group VIb metal is preferably between 2.10 ^-4 and 40.10 ^-4 gram of oxide of said metal per m ² of support, preferably between 4.10 ^-4 and 16.10 ^-4 g / m ² .

The catalyst of the finishing step is characterized by a catalytic activity generally of between 1% and 90%, preferably between 1% and 70%, and very preferably between 1% and 50% of the catalytic activity of the catalyst. main catalyst of hydrodesulfurization.

The following description of the process will be better understood when reading the figure 1 attached which is not limited to the various configurations that can take the method according to the invention.

The figure 1 shows a hydrodesulfurization reactor divided into two catalytic beds, and a finishing reactor divided into two catalytic beds.

Several hydrodesulfurization reactors operating in parallel or in series, and several finishing reactors operating in parallel or in series are perfectly possible and remain within the scope of the invention.

Similarly, the division of each reactor in addition to two catalytic beds remains perfectly within the scope of the invention. The dotted frame around the finishing reactor means that this finishing step is optional.

The gasoline to be treated is introduced via line (1) and then mixed with hydrogen introduced via line (2) and heated by an exchanger train and / or an oven (11). The hydrogen of the line (2) consists of a mixture of the hydrogen recycled by the line (10) and the additional hydrogen introduced by the line (23).

The mixture brought to the temperature and the pressure necessary to reach the desired desulfurization rate, is generally in the vapor phase in the line (3).

It is sent to a reactor (12) containing at least one bed of hydrodesulfurization catalyst used in a fixed bed.

The effluent from the reactor (12) contains hydrocarbons and sulfur compounds that have not reacted, paraffins from the hydrogenation of olefins, H ₂ S from the decomposition of sulfur compounds, and recombinant mercaptans. from the addition reactions of H ₂ S on olefins.

The effluent from the reactor (12) is sent via the line (4) into an exchange train (13) in order to condense the hydrocarbon fraction (the part of the figure 1 in the dashed rectangle is then absent from the scheme according to this variant).

The mixture of liquid hydrocarbons and hydrogen is then separated in a separator tank (14) which makes it possible to recover a liquid fraction in bottom by the line (6) constituted mainly of the desulfurized gasoline, and at the top, a gaseous fraction. by line (5) consisting mainly of hydrogen and H ₂ S.

The gaseous effluent is directed by line (5) to a washing section (15) to separate H ₂ S from hydrogen.

The liquid effluent brought by the line (6) is expanded and injected into a stripping column (17) which makes it possible to extract at the top, via the line (9), the residual H ₂ S dissolved in the hydrocarbons.

The desulfurized gasoline is recovered at the bottom of the stripping column by the line (7). A fraction of this desulfurized gasoline is withdrawn via line (8) and mixed with the feedstock introduced via line (1).

According to another embodiment of the process, the hydrodesulfurization carried out in the reactor (12) is followed by a step of finishing the hydrodesulfurization carried out in the finishing reactor (19). In this case, the reaction mixture recovered at the line (4) can be reheated by an exchange train or an oven (18) and then sent into the finishing reactor (19) (implementation of the part of the figure 1 located in the rectangle in broken lines).

The recycling of a fraction of the desulphurized gasoline can be carried out either by the line (1), at the inlet of the hydrodesulphurization reactor, or by the line (20) between two catalyst beds of the hydrodesulfurization reactor ( 12), either by the line (22) between two catalyst beds of the finishing reactor (19), or by the line (21) between the hydrodesulfurization reactor (12) and the finishing reactor (19).

A combination of all these recycling is also possible and remains perfectly within the scope of the invention. The combination of recycling means that some of the desulfurized gasoline can be recycled at each of the various recycling points previously listed. In this case, the distribution of the recycling flow between the different recycling points can be absolutely arbitrary.

Example 1 (according to the prior art)

A continuously operating hydrodesulfurization reactor is charged with 100 ml (ml is the abbreviation of milliliter) of HR806 catalyst marketed by the company Axens. This catalyst based on cobalt and molybdenum oxides is sulfided with a mixture of H 2 and DMDS under conventional sulfurization conditions, in order to convert at least 80% of the metal oxides of molybdenum and cobalt into sulfides.

The characteristics of gasoline A from a catalytic cracking unit are summarized in Table 1. <b> Table 1 </ b> Gasoline Analysis A Sulfur, ppm 924 ppm weight Bromine index, mg / 100ml 50 Initial point simulated distillation 54 ° C Simulated distillation end point 210 ° C

• Température = 285 °C
• Pression = 2,5 MPa
• Débit de charge = 400 ml/h
• Débit d'hydrogène = 144 l/h

This gasoline is treated on the HR806 catalyst under the following conditions:

• Temperature = 285 ° C
• Pressure = 2.5 MPa
• Flow rate = 400 ml / h
• Hydrogen flow rate = 144 l / h

At the outlet of the hydrodesulfurization reactor, the reaction mixture is cooled and the gasoline is separated from the hydrogen in a gas / liquid separator.

Recovered gasoline is stripped by a nitrogen flow to remove residual H ₂ S and analyzed.

The gasoline produced contains 32 ppm of sulfur, of which 22 ppm in the form of mercaptans, and a bromine value of 30 mg / 100ml.

Example 2 (according to the invention).

A test is conducted on gasoline A under conditions similar to Example 1. A fraction of the liquid recipe from the stripper is returned to the charging pot using a pump. The recycle rate is calculated as the recycle rate divided by the fresh charge rate.

• Pression = 2,5 MPa
• Débit de charge à désulfurer = 400 ml/h
• Débit d'hydrogène = 144 l/h

The conditions of the test are as follows:

• Pressure = 2.5 MPa
• Charge rate to desulfurize = 400 ml / h
• Hydrogen flow rate = 144 l / h

The temperature is adjusted in increments of 1 ° C to obtain about 30 ppm of sulfur in the recipe.

The recycle rate is adjusted to obtain recycle rates in a range of 0.2 to 3. For each recycle rate, a sample of desulphurized gasoline is recovered and analyzed. Table 2 presents the analyzes performed on the different samples. Recycling rate 0.2 0.5 1 2 3 total flow l / h 0.48 0.6 0.8 1.2 1.6 Temperature, ° C 285 285 286 289 291

Analysis of the recipe

S, ppm 29 29 30 29 30 RSH, ppm 20 19 9 17 13 10 IBr mg / 100 ml 30.5 31.4 31.9 31.3 30.7

The operation of the reactor with recycling of a fraction of the recipe makes it possible, for the same sulfur content in the recipe, to produce a gasoline having a lower mercaptan content and a higher olefin content.

As the recycling rate increases, the mercaptan content of the effluent decreases, and the olefin content of the effluent measured by the bromine index (IBr) remains stationary, which preserves the value of the index of octane.

Claims (11)

1. A process for hydrodesulphurizing a gasoline containing less than 0.1 % by weight of sulphur derived from a catalytic cracking unit or from other conversion units, said process comprising at least one hydrodesulphurization reactor using a bimetallic catalyst operating at a HSV in the range 0.1 h^-1 to 20 h^-1, a temperature in the range 220°C to 350°C and a pressure in the range 0.1 MPa to 5 MPa, and comprising recycling a fraction of the desulphurized gasoline to the inlet to the hydrodesulphurization reactor with a recycle ratio in the range 0.2 to 2 times the flow rate of the gasoline to be desulphurized.
2. A hydrodesulphurization process according to claim 1, in which the hydrodesulphurization reactor functions at a HSV in the range 0.1 h^-1 to 10 h^-1, preferably in the range 2 h^-1 to 8 h^-1, and at a pressure in the range 1 MPa to 3 MPa with a hydrogen/feed volume ratio in the range 50 litres/litre to 800 litres/litre, preferably in the range 100 litres/litre to 400 litres/litre.
3. A hydrodesulphurization process according to claim 1 or claim 2, in which the catalyst employed comprises at least one element from group VIII and an element from group VIb, deposited on a porous support, the group VIII element preferably being iron, cobalt or nickel, and the group VIb element preferably being molybdenum or tungsten.
4. A hydrodesulphurization process according to any one of claims 1 to 3, in which the amount of group VIII element, expressed as the oxide, is in the range 0.5% by weight to 15% by weight, preferably in the range 0.7% by weight to 10% by weight, and the amount of group VIb metal is in the range 1.5% by weight to 60% by weight, preferably in the range 2% by weight to 50% by weight.
5. A hydrodesulphurization process according to any one of claims 1 to 4, in which the hydrodesulphurization catalyst comprises a porous support with a specific surface area of less than 200 m²/gram, preferably less than 150 m²/g, and more preferably less than 100 m²/g.
6. A hydrodesulphurization process according to any one of claims 1 to 5 in which, prior to bringing it into contact with the feed, the catalyst undergoes a step for depositing carbon such that said carbon deposit represents an amount of 2.8% by weight or less of said catalyst, preferably in the range 0.5% to 2.6% by weight.
7. A hydrodesulphurization process according to any one of claims 1 to 6, in which the hydrodesulphurization reactor is followed by a finishing reactor and in which the catalyst used in the finishing reactor has an amount of group VIII element, expressed as the oxide, in the range 0.5% to 10% by weight, preferably in the range 0.7% to 5% by weight, and has an amount of group VIb element in the range 1.5% to 50% by weight, preferably in the range 2% to 20% by weight.
8. A hydrodesulphurization process according to any one of claims 1 to 7, in which the catalyst used in the finishing reactor has a catalytic activity in the range 1% to 70%, preferably in the range 1% to 50% of the catalytic activity of the catalyst used in the hydrodesulphurization reactor.
9. A hydrodesulphurization process according to any one of claims 1 to 6, in which the hydrodesulphurization reactor is followed by a finishing reactor and in which the catalyst used in the finishing reactor is a monometallic catalyst containing a group VIII element the amount of which, expressed as the oxide, is in the range 1% to 60% by weight, preferably in the range 2% to 20% by weight.
10. A hydrodesulphurization process according to any one of claims 1 to 9, in which the recycle of a portion of the desulphurized gasoline is made to a point located between the hydrodesulphurization reactor and the finishing reactor.
11. A hydrodesulphurization process according to any one of claims 1 to 9, in which the recycle of a portion of the desulphurized gasoline is made between two catalytic beds located in the same hydrodesulphurization reactor.

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