FR2836064A1 - Removal of hydrogen sulfide and hydrocarbons from hydrogen using a pressure swing adsorption unit with an integral compressor - Google Patents

Abstract

Treatment of gas mixture comprising hydrogen sulfide (H2S), hydrogen and hydrocarbons using a pressure swing adsorption (PSA) unit with an integral compressor comprises using PSA unit comprising H2S adsorption bed (B1) operating at lower pressure than hydrocarbon adsorption bed (B2). During decompression, B2 is charged with recycle gas from compressor and B1 is charged with purge gas from B1. Treatment of a gas mixture comprising hydrogen sulfide (H2S), hydrogen and hydrocarbons using a pressure swing adsorption (PSA) unit with an integral compressor comprises using a PSA unit comprising an H2S adsorption bed (B1) operating at a lower pressure than a hydrocarbon adsorption bed (B2). During decompression, B2 is charged with recycle gas from the compressor and B1 is charged with purge gas from B1. The recycle gas is a mixture of purge gas from B2 and overhead gas from B1 during decompression.

Description

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 The present invention relates to a method for treating a gaseous mixture comprising hydrogen, H 2 S and hydrocarbons, aimed at obtaining three gas streams: one comprising mainly hydrogen, the second comprising mainly hydrogen, H2S and the third mainly comprising hydrocarbons, while maintaining the gaseous flow of hydrogen at the pressure of the initial gas mixture.

 The present invention relates to the treatment of products derived from hydrotreatment processes (HDT), widely used in the refining industry. Various types of hydrotreatment processes coexist in most refineries and process a large number of refinery products, particularly the following cuts: gasoline, kerosene, diesel, vacuum distillates, oil bases. These hydrotreatment processes make it possible to adjust certain properties of the refining products such as the contents of sulfur, nitrogen, aromatic compounds or the number of cetane. Sulfur is often the key property (units are often called HDS for hydrodesulfurization) and is subject to increasingly stringent specifications leading refiners to look for ways to improve these units.

 The hydrogenation chemical reactions take place in a reactor where the hydrocarbon feedstock is mixed with a hydrogen stream (in large excess) and passes over a catalyst bed. Part of the hydrogen reacts with sulfur, nitrogen and unsaturated organic compounds producing hydrogen sulphide (H2S), ammonia (NH3), light hydrocarbons (HC) C1-C6 and heavier saturated compounds . At the outlet of the reactor, a liquid / vapor separation tank allows the recovery of the hydrogen-rich gas phase which is recycled to create this excess of hydrogen (hereinafter referred to as recycle gas). This gas contains, in addition to hydrogen, most of the volatile compounds created in the reactor and which tend to concentrate there. The hydrogen consumed chemically as well as the hydrogen lost by mechanical losses, dissolution or purge is compensated by a hydrogen-rich makeup gas whose composition varies according to its mode of production. Typically the hydrogen content of this gas (hereinafter referred to as make-up gas) varies between 70 mol% and 99.9 mol%, the balance being generally methane or a mixture of light hydrocarbons.

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 An essential parameter of the hydrotreatment reaction is the hydrogen partial pressure at the outlet of the reactor. This partial pressure depends on the total pressure of the unit (fixed during the "sizing" of the unit), the degree of vaporization of the hydrocarbon feed (fixed by the total pressure and the operating temperature) and especially the hydrogen concentration of the two makeup and recycle gases used. The H2S partial pressure is a second important parameter which depends mainly on the H2S content of the recycle gas, therefore the sulfur of the feedstock and the desulfurization rate applied during the hydrotreatment. It is therefore desirable in the hydrodesulfurization units to increase the hydrogen partial pressure and to reduce the H 2 S partial pressure by purifying one or both make-up and recycle gases. The objective is to minimize the H2S and hydrocarbon contents.

 The H2S removed from the recycle gas must then be treated, this treatment being generally carried out in a sulfur production Claus unit.

However, such a unit can process only gases without hydrocarbons.

 The prior art already offers various solutions to achieve the goal of minimizing the levels of H2S and hydrocarbons in the recycle gas. Thus, a first solution is to purge the recycle gas to limit the concentration of H2S and hydrocarbon: a fraction of the recycle gas is taken to evacuate incondensable gases accumulated in the recycling loop. These gases are evacuated to the so-called fuel gas network; this network is present in all refineries and collects all gaseous effluents that can be recovered in the form of energy. However, this high-pressure purge has several drawbacks: the impact on the partial pressures of hydrogen and H 2 S is low, the recycle gas being rich in hydrogen, the primary consequence of purging is the loss of hydrogen to the hydrogen. fuel gas network of the refinery. This hydrogen is then poorly valued since it is used as fuel. because of this loss, a larger amount of make-up gas must be introduced. But the make-up gas is compressed to go from the pressure of the hydrogen network to the operating pressure of the unit. The high pressure purge is therefore limited by the capacity of the makeup gas compressor.

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 A second solution consists in carrying out a step of washing the recycle gas with an amine solution. During this wash, H2S is completely absorbed and desorbed by regeneration of the amine and finally converted to liquid sulfur in a downstream Claus unit, for example. However, the washing only concerns H2S and does not remove any hydrocarbon from the recycle gas. The impact on H2S partial pressure is important, but the impact on hydrogen partial pressure is negligible. The performance gain in hydrodesulfurization achieved by this washing therefore remains modest. In addition, amine solutions pose problems of corrosion and foaming.

 A third solution of the prior art, which is widespread for H2 / CO / CH4 mixtures, consists in purifying the hydrogen of the adsorption recycling gas. This adsorption makes it possible to reach purity levels higher than 99.5%. The application of the adsorption to a hydrodesulfurization recycle gas is for example described in JP 57055992. The adsorption treatment of the recycle gas or the mixture of the recycle gas or the make-up gas influences the performance of the recycle gas. hydrodesulphurization significantly. However, this solution is never applied industrially to the treatment of these gases because of low yields. Indeed, the hydrogen yield of adsorption units is limited to 70 to 90%. The loss of hydrogen must therefore be offset by the use of a larger amount of makeup gas.

The increase of the make-up gas flow can reach 100% in the case of the treatment of the entire recycle gas by a PSA (Pressure Swing Adsorption) method.

The use of a PSA therefore implies a high cost of hydrogen; this use is also greatly limited by the capacity of the make-up gas compressor and is in practice impossible without heavy investment.

 A fourth solution of the prior art is the recovery of the hydrogen contained in the recycle gas by treatment of this gas with a membrane permeable to hydrogen. This type of membrane makes it possible to obtain good hydrogen purities (90 to 98%) and acceptable yields (80 to 98% depending on the desired purity). The cost is moderate compared to previous solutions.

This solution in the field of hydrodesulfurization units has been described in EP-A-061 259. This solution is industrially applied but the problem remains the need for recompression of the recycle gas after its passage.

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 in the membrane. Indeed, the purified hydrogen is produced at reduced pressure and the performance of the membrane is better than the production pressure is low. The integral treatment of the recycle gas is in practice impossible. The membrane is thus generally placed on a bypass of the recycle gas) and the hydrogen produced is returned to the suction of the make-up gas compressor to return to the pressure of the unit. The flow rate treated by the membrane, and consequently its efficiency, is once again limited by the capacity of the makeup gas compressor.

 A fifth solution is the use of reverse-selectivity membranes, which keep the hydrogen under pressure. However, these membranes have low hydrogen / hydrocarbon selectivities (particularly for hydrogen / methane separation). They can therefore allow the integral treatment of the recycle gas (as described in US Pat. Nos. 6,190,540 and 6,189,996) to achieve a more selective hydrocarbon purge, but a compromise between the loss of hydrogen and the degree of purification. Hydrogen must be found. If the objective is the thorough purification of the recycling gas (hydrogen purity of 90% or even 95%) then the hydrogen losses are very important (30%, 50% or even much more) and the limitations are identical to those a simple purge (corresponding to the first solution mentioned above). If the objective is to reduce hydrogen losses as compared to a conventional high-pressure purge (as in the first solution above), then purification and impact on hydrodesulfurization performance are very moderate. The last disadvantage is that the loss of hydrogen varies with the composition of the recycle gas to be treated: it is all the stronger because the recycling gas to be treated is rich in hydrogen.

 Only one solution describes a ternary separation of HzS, H2 and hydrocarbons. It consists in using the combination of a PSA and a membrane. The membrane processes the waste gas from a PSA in a membrane: there is an improvement in hydrogen yield and a coarse separation of the two species of waste gas. However, due to H2S / hydrocarbon selectivities always less than 50, no membrane can today produce a sufficiently pure H2S flux for

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 feed a Claus unit. Similarly, no membrane allows the production of hydrocarbons completely free of H2S.

 Finally, a last way of purifying the recycling gas of the HDS unit is to isolate H2S by adsorption process TSA (Temperature Swing Adsorption) with regeneration of the adsorbent by temperature rise. However, this solution has the disadvantage of being expensive and difficult to implement due to the heating of the adsorbent by the gas.

 There is therefore a need to improve the hydrodesulfurization units of the prior art and in particular the treatment of the recycle gas and the use of this gas and make-up gas. The object of the invention is to propose a process for treating gas from a hydrodesulphurization unit so as to obtain a recycle gas having a high purity of hydrogen without reducing the pressure of the gas and without loss of hydrogen during this treatment.

 Another object of the invention is to propose a process for treating the gas resulting from a hydrodesulfurization unit in order to eliminate the H2S contained therein and to obtain a gas essentially comprising H2S so as to be able to treat it directly into a Claus unit.

termes"entrée"et"sortie"désignent tes extrémités d'entrée et de sortie d'un adsorbeur en phase d'adsorption ; l'expression"co-courant"désigne te sens de circulation du gaz dans l'adsorbeur pendant cette phase d'adsorption, et l'expression "contre-courant" désigne le sens inverse de circulation. The invention relates to a pressure-swing adsorption separation process of a gas mixture, in which a pressure modulation cycle is implemented for the or each adsorber, comprising a succession of stages which define phases of adsorption, decompression, purge and pressure rise. The invention can be implemented with so-called PSA cycles, in which the adsorption is carried out at a pressure that is clearly greater than atmospheric pressure, typically of the order of 10 to 50 bar, while the minimum pressure of the cycle is substantially equal to either the atmospheric pressure or at a pressure of a few bars. These processes comprise different combinations of adsorption, decompression, purge and recompression steps of the adsorbers. In addition, in what follows, the

"inlet" and "outlet" terms refer to the inlet and outlet ends of an adsorber in the adsorption phase; the term "co-current" designates the flow direction of the gas in the adsorber during this adsorption phase, and the expression "countercurrent" designates the opposite direction of circulation.

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 The invention relates exactly to a method of treating a gaseous mixture comprising at least H 2 S, H 2 and hydrocarbons with the aid of a treatment device comprising a pressure swing adsorption unit (PSA) associated with an integrated compressor , in which is carried out, for each adsorber of the unit, a pressure modulation cycle comprising a succession of phases which define adsorption, decompression, purge and pressure rise phases, in which: each adsorber comprises two different adsorbent beds, one permitting the adsorption of H2S, and the other the adsorption of the hydrocarbons; during the adsorption phases of the two beds, the gaseous mixture to be treated is brought into contact with the PSA beds so as to adsorb H2S and hydrocarbons and to produce at the head of the PSA beds a gas comprising essentially hydrogen, - during the decompression phase of the adsorbent bed allowing the adsorption tion of the hydrocarbons, at least part of a gas coming from the compressor integrated in the treatment device, said recycle gas, is introduced into said bed during the decompression phase of the adsorbent bed allowing the adsorption of H2S is introduced into said bed purge gas from the purge phase of said adsorbent bed, - the recycle gas is derived from the compression of a gas mixture comprising at least :. the purge gas produced during the purge phase of the adsorbent bed for the adsorption of hydrocarbons, and. the gas produced at the head of the adsorbent bed for the adsorption of H2S during the decompression phase of said bed.

 Other characteristics and advantages of the invention will appear on reading the description which follows. In the remainder of the description, the CPSA designates the treatment device comprising a pressure swing adsorption unit (PSA) associated with an integrated compressor used in the method according to the invention.

 The method according to the invention aims to treat a gaseous mixture comprising at least H2S, H2 and hydrocarbons which may be a stream resulting from a hydrodesulfurization process. Generally, the gaseous mixture to be treated has a temperature of between 20 and 80 ° C., preferably between 30 and 50 ° C.

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 generally has a pressure of between 15 and 80 bar absolute, preferably between 20 and 50 bar absolute. The composition of this type of mixture is usually the following: at least 0.2% by mole, preferably at least 0.5% by mole, generally at most 10% mole, even more preferably between 1 and 2, 5% by mole of H2S, between 60 and 98%, preferably between 85 and 95% by mole of hydrogen, at least 1% by mole of hydrocarbons, and more precisely: at most 20%, preferably between 0.5 and 5% by mole of CH4, at most 10%, preferably between 0.1 and 3% by mole of C2 hydrocarbons, at most 5%, preferably between 0.05 and 1 mol% of C3 hydrocarbons. at most 2%, preferably between 0.02 and 0.5 mol% of C4 hydrocarbons. at most 0.5%, preferably at most 0.06% by mole of hydrocarbons having a carbon number greater than or equal to 5.

 As indicated above, each adsorber of the PSA comprises two different adsorbent beds, one permitting the adsorption of H2S, and the other the adsorption of the hydrocarbons. This combination of several families of adsorbents allows a selective adsorption then an easy desorption of H2S and hydrocarbons, in particular heavy hydrocarbons (C4, Cs and C6) without a regeneration at a temperature higher than 100 C is necessary. The PSA may comprise a variable number of adsorber pairs, generally greater than 3.

Each adsorber is composed of two different adsorbent beds, said adsorbents being chosen from: activated carbons, silica gels, aluminas or molecular sieves. Preferably, the silica gels must have a pore volume of between 0.4 and 0.8 cm 3 / g and a mass area greater than 600 m 2 / g. Preferably, the aluminas have a pore volume greater than 0.2 cm 3 / g and a mass area greater than 220 m 2 / g. The zeolites preferably have a pore size of less than 4.2 Å, an Si / Al molar ratio of less than 5 and contain Na and K. The activated carbons preferably have a mass area greater than 800 m 2 / g and a micropores between 8 and 20 A. According to a preferred embodiment, each PSA adsorber is

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 composed of a double bed comprising: in the lower part a protective bed composed of alumina or silica gel surmounted by a bed of zeolite having a pore size of less than 4.2 A and adsorbing exclusively H2S. The proportions vary according to the nature of the gaseous mixture to be treated (in particular according to its percentages of H2O and H2S). The upper bed is preferably in a separate adsorber; it may consist of a combination of activated carbon and silica gel and preferentially adsorb hydrocarbons.

 According to the process of the invention, during the adsorption phase, the gaseous mixture to be treated is brought into contact with the first adsorber of the PSA: the gaseous mixture is introduced at the bottom of the bed in the direction said co-current . During this contacting step, H2S is essentially adsorbed on the adsorbent bed situated in the lower part of the adsorber, while the hydrocarbons adsorb on the adsorbent bed located in the upper part of the adsorber. A gas comprising essentially hydrogen is produced at the pressure of the gas mixture initially introduced into the treatment device decreased by about one bar of pressure drop. During this step, the hydrogen produced is generally of a purity greater than at least 97 mol%, preferably greater than at least 99%, or even greater than at least 99.5%. In the case of the treatment of a gas from a hydrodesulphurization unit, the gas comprising essentially hydrogen obtained can be recycled in the hydrodesulphurization process because it has the hydrogen purity and the pressure necessary for this type. hydrodesulfurization process.

 During the decompression and purge phases of the process according to the invention, the adsorbent beds do not undergo the same cycle of pressure variations.

 Thus, with regard to the bed of the adsorbent for the adsorption of hydrocarbons (that is to say the bed of the upper part of the adsorber, hereinafter referred to as "high bed" of the adsorber ), during the decompression phase, several steps are implemented, including at least a first step in which is introduced in co-current in the high bed at least a portion of the recycle gas from the compressor integrated in the treatment device. This recycle gas has a pressure P 'lower than the pressure P of the gaseous mixture treated by the treatment device. This step of decompression with

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 current of the decompression phase produces a gaseous flow essentially comprising hydrogen, thus of a H 2 purity equivalent to that of the gas resulting from the adsorption step described above, that is to say from hydrogen purity of at least greater than 97 mol%. This gaseous flow essentially comprising hydrogen is generally used during the purge phases of the two beds of adsorbent: it is introduced against the current of the high bed and the low bed during their respective purge phases. In a final step of the high bed decompression stage, a waste gas is produced. This stage of production of the high bed waste gas can be obtained by countercurrent decompression initiated at the pressure P "(<P ') of the PSA, this waste gas of the high bed is a gas comprising essentially hydrocarbons, it is that is, a hydrocarbon content of at least 70 mol% and a content of H2S generally of not more than 0.05 mol% This waste gas is removed from the process. high bed, a gas is introduced against the current of this bed.As indicated above, this gas is generally a gas stream from the decompression step co-current of the high bed. This purge gas generally comprises between 40 and 75 mol% H 2 and has a low H 2 S content of not more than 0.05 mol% .The flushing gas of the high bed is used as recycle gas. after recompression.

 As regards the bed of the adsorbent for the adsorption of H2S (that is to say the bed of the lower part of the adsorber, hereinafter referred to as the "low bed" of the adsorber), during the decompression phase, several steps are carried out, including at least a first step during which the purge gas from the purge phase of said adsorbent bed, optionally mixed with a gas, is introduced into said bed; rich in H2S. This rich gas comes from a source outside the CPSA; it may be for example gas from the fuel-gas acid network (that is to say containing H2S) of a refinery. The introduction is co-current. This step of co-current decompression of the low bed decompression stage produces a gas stream comprising hydrogen, hydrocarbons and a low H2S content, that is to say at most 0.05% in mole. As indicated above, this gas stream is used as recycle gas after recompression. In a final stage of the low bed decompression stage, a waste gas is produced. This production of

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 waste gas from the low bed can be obtained by countercurrent decompression; this waste gas of the low bed is a gas essentially comprising H 2 S, that is to say comprising an H 2 S content of at least 75 mol%. This waste gas generally comprises a hydrocarbon content of at most 2 mol%. This waste gas is removed from the process. Because of its low hydrocarbon content, it can be processed directly in a Claus unit. During the purge phase of the low bed, a gas is introduced against the current of this bed.

 As indicated above, this gas is generally a gas stream from the decompression stage co-current of the high bed. A purge gas is produced containing between 40 and 80% of H2S and at most 5% of hydrocarbons. As indicated above, this purge gas is used in the co-current decompression step of the low bed decompression stage.

 According to an essential characteristic of the invention, the recycle gas is derived from the compression of a gas mixture comprising at least the purge gas produced during the purge phase of the high bed, and the gas produced at the head of the bed. down during the decompression phase of said bed. It is also possible to add a third gas to this mixture to obtain the recycle gas: thus, one can mix a gas rich in hydrocarbons devoid of H2S (ie containing less than 0.01% of H2S ) the purge gas produced during the purge phase of the high bed and the gas produced at the head of the low bed during the decompression phase of said bed. This gas rich in hydrocarbons can for example be derived from the non-acid fuel network (or "softened") of a refinery. Before they are introduced into the compressor, these two or three gases are usually mixed in a mixer. According to a variant of the invention, at least a portion of the recycle gas can be brought into contact with the PSA beds during the adsorption phases of the two beds.

 According to one particular embodiment of the invention, the treatment device may comprise a hydrogen-permeable membrane placed so as to treat at least a portion of the gas leaving the compressor. Thus, the mixing of the purge gas produced during the purge phase of the high bed, and the gas produced at the head of the low bed during the decompression phase of said bed is compressed by the compressor can be treated in whole or in part in a membrane permeable to hydrogen. A hydrogen selective membrane is generally used which produces a hydrogen-rich permeate and a retentate

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contre-courant dans le lit de l'adsorbant haut lors de la phase de purge dudit lit. Le rétentat de la membrane perméable à l'hydrogène peut être utilisé en tant que gaz de recycle ou en mélange avec une partie du gaz de recycle si ce dernier n'a pas été totalement traité par la membrane. rich in hydrocarbons depending on the nature of the gaseous mixture treated. It may be a polymer membrane having a resistance to H2S, for example based on polyimide or polyaramid, preferably polyaramid (polyparaphenylene-therephthalamide). The permeate gas issuing from the hydrogen-permeable membrane may be mixed with the gaseous flow essentially comprising hydrogen resulting from a high-bed co-current depressurization before being used for the purge phase, or for purge the bed high, either for the purge of the low bed. Thus, at least a portion of the permeate of the hydrogen permeable membrane can be introduced countercurrent into the bed of the low adsorbent during the purge phase of said bed. Similarly, at least a portion of the permeate of the hydrogen permeable membrane can be introduced at

against the current in the bed of the high adsorbent during the purge phase of said bed. The retentate of the hydrogen permeable membrane may be used as a recycle gas or in admixture with a portion of the recycle gas if it has not been fully treated by the membrane.

 Preferably, the process uses at least three pairs of adsorbent beds cyclically under pressure one after the other. Figure 1 illustrates a cycle of operation of a CPSA of 8 adsorbers (R01 to R08). In this figure, where the times t are plotted on the abscissa and the absolute pressures P on the ordinate, the lines oriented by arrows indicate the movements and destinations of the gas flows, and, in addition, the flow direction in the adsorbers. When an arrow is in the direction of increasing ordinates (towards the top of the diagram), the current is said to cocurrent in the adsorber. If the upward arrow is below the line indicating the pressure in the adsorber, the current enters the adsorber through the inlet end of this adsorber. If the arrow pointing upwards is situated above the line indicating the pressure, the current leaves the adsorber through the outlet end of this adsorber, the inlet and outlet ends being respectively those of the gas to be treated and gas withdrawn during the production / adsorption phase. When an arrow is in the direction of decreasing ordinates (towards the bottom of the diagram), the current is said against the current in the adsorber. If the downward arrow is located above the line indicating the pressure of the adsorber, the gas flow enters the adsorber through the outlet end of the adsorber, the inlet and outlet ends being always those of gas to

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treat and gas withdrawn during the production / adsorption phase. Each adsorber
R01 to R08 follows the cycle of FIG. 1, being offset with respect to the adsorber preceding it by a duration called phase time and equal to the duration T of the cycle divided by eight, ie divided by the number of adsorbers in function.

 The cycle of FIG. 1 thus comprises eight phase times and illustrates the phase time / adsorber duality, namely that at any instant of the operation of the CPSA unit, each adsorber is in a different phase time.

 The process according to the invention can be used to treat a gaseous mixture resulting from a hydrodesulfurization unit. It can be implemented at different locations of a conventional hydrodesulfurization unit. FIG. 2 represents the diagram of a conventional hydrodesulphurization unit (HDS): a charge of liquid hydrocarbons to be treated (1) and containing sulfur-containing molecules is introduced into the hydrodesulfurization reactor (HDS reactor) in a mixture with a gas stream essentially comprising hydrogen (7). In the reactor, in the presence of a solid catalyst, numerous hydrogenation reactions take place and in particular transform the organic sulfur molecules into H2S hydrogen sulfide. It leaves the reactor a biphasic flow (2) which is led to a high pressure separation unit producing two streams: a stream (3) comprising essentially hydrogen and H2S and a stream (21) comprising mainly hydrocarbons and H2S. The stream (21) is then generally expanded and treated in a low pressure separation unit producing two streams: a stream (22) comprising the hydrocarbons produced by the HDS unit and a stream (35) comprising substantially H2S. This last stream (35) is injected into the fuel-gas acid network of the refinery. The flow (3) can be divided into two parts: - one (5) is compressed by the recycle compressor (C2) to be reused in the HDS reactor: it is the recycling gas (6 the other (33) is conveyed to one of the acid gas networks of the refinery.

The line (33) comprises a purge valve which can be opened, for example to regulate the pressure of the unit.

 The HDS unit is also supplied with a fresh hydrogen stream (11) (this is the makeup gas) which is taken from the refinery's hydrogen network and then compressed by the auxiliary compressor ( C1), if necessary, up to the pressure of the HDS reactor to generate the flow (12). Recycling gas

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 compressed (6) and the compressed makeup gas (12) form the hydrogen-rich gas stream (7) introduced into the HDS reactor.

 FIG. 3 illustrates the possible locations of the CPSA in a hydrodesulfurization unit defined according to FIG. 2: in position 1, the CPSA processes the gaseous mixture comprising hydrogen, hydrocarbons and H 2 S at high pressure (FIG. ) from the high pressure separation unit of the hydrodesulfurization unit, in position 2, the CPSA processes the gaseous mixture comprising hydrogen, hydrocarbons and high pressure H2S (3) from the high-pressure separation unit of the hydrodesulphurization unit when this mixture is at the intake of the recycle compressor (C2); in position 3, the CPSA processes the recycle gas (6) from the compressor C2 - in position 4, the CPSA processes the mixture (7) of the recycle gas (6) and the makeup gas (12) (from the hydrogen network), - in position 5, the CPSA processes the make-up gas (12) from the compressor C2, - in position 6, the CPSA processes the fresh hydrogen (11) taken from the network hydrogen from the refinery.

 According to a first preferred variant of the invention, the process according to the invention can be used to treat the gaseous mixture comprising hydrogen, hydrocarbons and high-pressure H2S (3) originating from the separation unit. high pressure of the hydrodesulfurization unit. This variant is illustrated in FIG. 4. The gaseous mixture comprising hydrogen, hydrocarbons and high pressure H2S (3) is introduced into the treatment device and treated according to the method of the invention. During the adsorption phase of the PSA, a hydrogen-rich gas (4) is recovered at the outlet of the adsorber, with a pressure drop of less than 1 bar, which can be sent to the recycle compressor (C2). then to the HDS reactor. During the decompression phase of the low bed of the PSA, a concentrated waste gas (32) H2S is recovered which can feed the acid gas network of the refinery, to be treated by Claus process. During the decompression phase of the high bed of the PSA, it recovers a waste gas (33) concentrating hydrocarbons that can feed the fuel-gas network of the refinery. According to this first

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 Alternatively, it is possible to treat only a portion of the gaseous mixture comprising hydrogen, hydrocarbons and H2S at high pressure (3) through the diversion line ("by-pass" in English) ( 41) of the CPSA. The line (33) can be opened to purge a fraction of the gas produced by the process according to the invention. Two optional modes can be implemented according to this first variant. According to a first optional mode, the treatment device can be treated in addition to the gaseous mixture (3), all or part of the gaseous effluent of the low pressure separation section (35) as well as the acid or non-acidic fuel gas, The non-acidic fuel gas is mixed with the purge gas produced during the purge phase of the high bed and with the gas produced at the head of the bed of the low adsorbent during the cooling phase. decompression of said bed, before being compressed by the compressor of the treatment device, then optionally treated by the membrane. The acid fuel gas and the gas (35) are mixed with the purge gas of the low bed from the purge phase of said bed before being introduced into the low bed for the co-current decompression phase.

According to a second optional mode, all or part of the hydrogen-rich gas from the device according to the invention (34) can be sent to the existing multi-stage makeup compressor (C1) (for example between two compression stages). This relieves the recycle compressor (C2) which can sometimes be limited (for example due to the low molecular weight of the gas to be compressed due to purification).

 According to a second preferred variant of the invention, the process can be used to treat the mixture (7) of the recycle gas (6) and the makeup gas (12) (from the hydrogen network). This variant is illustrated in FIG. 5. The process produces: a gas comprising essentially hydrogen (8), with a pressure drop of less than 1 bar, which is sent to the HDS reactor, -a waste gas (32) concentrating the hydrocarbons and supplying one of the refinery's low-pressure acid gas networks; - a residual gas (33) concentrating H2S and supplying one of the non-acid fuel gas networks of the refinery.

 This configuration is useful when the makeup gas purity is low (less than 95 mol%). It is possible to treat only a portion of the recycle gas (6) and the make-up gas (12) due to the presence of a line of

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derivation (41). Two optional modes can be implemented according to this first variant. According to a first optional mode, the treatment device can treat, in addition to the mixture (7), all or part of the gaseous effluent of the low-pressure separation section (35) as well as the non-acidic or acidic fuel gas represented by 31 in FIG. 5. The non-acidic fuel gas is mixed with the purge gas produced during the purge phase of the high bed and with the gas produced at the head of the bed of the low adsorbent during the decompression phase of said bed, before being compressed by the compressor of the treatment device, then optionally treated by the membrane. The acid fuel gas and the gas (35) are mixed with the purge gas of the low bed from the purge phase of said bed before being introduced into the low bed for the co-current decompression phase.
Fig. 6 shows an alternative scheme of a conventional hydrodesulfurization unit (HDS) in which the compressed makeup gas is mixed with the recycle gas before it has been recompressed. The compressor C2 thus allows the compression of the makeup gas and the recycle gas. In this configuration of the hydrodesulfurization unit (HDS), the CPSA can treat different gaseous mixtures of the unit: - in position 1 ', the CPSA processes the gaseous mixture comprising hydrogen, hydrocarbons and H2S at high pressure (3) from the high pressure separation unit of the hydrodesulphurization unit, in the 2 'position, the CPSA processes, as in the above position, the gaseous mixture comprising hydrogen, hydrocarbons and high-pressure H2S (3) from the high-pressure separation unit of the hydrodesulphurization unit but after the eventual removal of a purge gas (which can supply another refining unit with example), in the 3 'position, the CPSA processes the mixture of the compressed makeup gas and the uncompressed recycle gas at the intake of the recirculation compressor C2. At the 4' position, the CPSA processes the gas mixture. recycling and make-up gas at the discharge of the recycling compressor C2, - in position 5 ', the CPSA processes the make-up gas at the boost of the auxiliary compressor C1, - at position 6', the CPSA processes the fresh hydrogen taken from the hydrogen network of the refinery at the compressor suction. C1.

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 The process of the invention makes it possible to achieve hydrogen yields on the recycle gas greater than 95%. The hydrogen-selective membrane makes it possible to reduce the energy required to compress the purge effluent and to increase the productivity of the adsorbent or to reduce its size.

 The invention thus makes it possible to purify the recycling gas of a hydrodesulfurization unit combining both high purity (at least equal to 97 mol%), high efficiency (greater than 95%) and obtaining hydrogen under pressure (pressure difference between the gaseous mixture to be treated and the purified hydrogen obtained less than 1 bar). The method according to the invention therefore allows the integral treatment of the recycle gas, and optionally make-up gas. The impact induced on the refining unit is thus much greater than that obtained with any other solution of the prior art. The invention makes it possible to obtain in the hydrodesulfurization unit a hydrogen partial pressure of between + 10% and + 60% and a partial pressure of Hz S reactor output of between -50% and -80%. %. This impact on partial pressures results in an improvement in performance that can be expressed in different ways: - reduction of the sulfur content in the hydrocarbons produced in the hydrodesulphurisation unit from -30% to -80%, - temperature equivalent to base temperature + 100C at 25 ° C, - catalyst amount consumption decreased by 35% to 50%.

 The invention makes it possible to treat both the high-pressure gas stream rich in hydrogen and the low-pressure streams still containing a large amount of hydrogen (25 to 70 mol%) usually intended for fuel oil. Thanks to these low pressure "make-up" streams, the high-pressure hydrogen yield, intended for recycling to the HDS reactor, of the process can exceed 100%. The device according to the invention is not only a means for purifying the various streams comprising hydrogen but also a means for recovering hydrogen.

 Finally, the invention makes it possible to separate the hydrocarbons from the H 2 S in the waste gases, which makes it possible to treat the gas comprising essentially H 2 S directly in a Claus unit.

<Desc / Clms Page number 17>

EXAMPLE
Table 1 below gives a comparison of the results obtained during the use of a conventional PSA, the implementation of the method according to the invention without a hydrogen selective membrane and with a hydrogen-selective membrane. with identical adsorbent volume. Table 2 gives the material balance obtained during the implementation of the process according to the invention without a membrane that is selective for hydrogen.

 The treated gaseous mixture is composed of: 90 mol% of H 2, 2 mol% of H 2 S and 8 mol% of hydrocarbons (of which 4 mol% of CH 4). The adsorbent is a combination of at least two adsorbents taken from the following families: activated carbon, activated aluminas, silica gels, zeolites. The pressure of the feed gas is 35 bar. The pressure of the waste gas is 6 bar. The average temperature of the adsorbent beds is 400C.

Table 1

<Tb>
<tb> Purity <SEP> of <SEP> Yield <SEP> Capacity <SEP> of
<tb> the H2 <SEP> product <SEP> in <SEP> H2 <SEP> production <SEP> in <SEP> Nm3 / h
<tb> PSA
<tb> 98, <SEP> 5% <SEP> 84, <SEP> 3% <SEP> 30 <SEP> 000
<tb> Process <SEP> according to <SEP> the invention
<tb> without <SEP> membrane <SEP> selective <SEP> 98, <SEP> 5% <SEP> 97, <SEP> 8% <SEP> 54000
<tb> to <SEP> hydrogen
<tb> Process <SEP> according to <SEP> the invention
<tb> with <SEP> membrane <SEP> selective <SEP> 98, <SEP> 5% <SEP> 98.1 <SEP>% <SEP> 73 <SEP> 500
<tb> to <SEP> hydrogen
<Tb>

<Desc / Clms Page number 18>

Table 2

<Tb>
<tb> Compositions <SEP> Load <SEP> Product <SEP> 1 <SEP>: <SEP> gas <SEP> Product <SEP> 2 <SEP>: <SEP> gas <SEP> Product <SEP> 3 <SEP >: <SEP> gas
<tb> Molars <SEP> comprising <SEP> comprising <SEP> comprising
<tb> essentially <SEP> H2 <SEP> essentially <SEP> essentially
<tb> directed <SEP> to <SEP> reactor <SEP> of <SEP> H2S
<tb> HDS <SEP> hydrocarbons <SEP> directed <SEP> to <SEP> unit
<tb> directed <SEP> to <SEP> fuel-Claus
<tb> gas <SEP> no <SEP> acid
<tb> Hydrogen <SEP> 91, <SEP> 1 <SEP>% <SEP> 98, <SEP> 5% <SEP> 22, <SEP> 6 <SEP>% <SEP> 7, <SEP> 6%
<tb> Hydrocarbons <SEP> 7, <SEP> 1 <SEP>% <SEP> 1, <SEP> 5% <SEP> 77, <SEP> 4 <SEP>% <SEP> 1%
<tb> HzS1, <SEP> 8% <SEP><<SEP> 50 <SEP> ppm <SEP><100<SEP> ppm <SEP> 91, <SEP> 4 <SEP>%
<tb> Flow rate, <SEP> Nm3 / h <SEP> 1 <SEP> 0 <SEP> 000 <SEP> 9064 <SE> 739 <SEP> 197
<Tb>

Claims (16)

Process for the treatment of a gaseous mixture comprising at least H 2 S, H 2 and hydrocarbons with the aid of a treatment device comprising a pressure swing adsorption unit (PSA) associated with an integrated compressor, in which one employs, for each adsorber of the unit, a pressure modulation cycle comprising a succession of phases which define adsorption, decompression, purge and pressure rise phases, characterized in that: - the PSA comprises two different beds of adsorbents, one allowing the adsorption of H2S, and the other the adsorption of the hydrocarbons, the bed allowing the adsorption of H2S operating at a pressure lower than the pressure at which the bed operates allowing the adsorption of the hydrocarbons, during the adsorption phases of the two beds, the gaseous mixture to be treated is brought into contact with the PSA beds so as to adsorb H2S and hydrocarbons and to produce at the head beds of the PSA a gas comprising essentially hydrogen, - during the decompression phase of the adsorbent bed for the adsorption of hydrocarbons, is introduced into said bed at least a portion of a gas from the compressor integrated in the device treatment, said recycle gas, during the decompression phase of the adsorbent bed for the adsorption of H2S, is introduced into said bed purge gas from the purge phase of said adsorbent bed, the recycle gas is derived from the compression of a gas mixture comprising at least: the purge gas produced during the purge phase of the adsorbent bed for the adsorption of hydrocarbons, and. the gas produced at the head of the adsorbent bed for the adsorption of H2S during the decompression phase of said bed. 2. Method according to claim 1, characterized in that the treatment device comprises a hydrogen permeable membrane placed so as to treat at least a portion of the gas leaving the compressor. <Desc / Clms Page number 20>

3. Method according to claim 1 or 2, characterized in that during the decompression phase of the adsorbent bed for the adsorption of H2S, is introduced into said bed purge gas from the purge phase said adsorbent bed mixed with a gas rich in H2S. 4. Method according to claim 2, characterized in that at least a portion of the permeate of the hydrogen permeable membrane is introduced into the adsorbent bed for the adsorption of H2S during the purge phase of said bed . 5. Method according to claim 2, characterized in that at least a portion of the permeate of the hydrogen permeable membrane is introduced into the adsorbent bed for the adsorption of hydrocarbons during the purge phase of said bed . 6. Method according to claim 2, characterized in that the retentate of the membrane permeable to hydrogen is introduced into the adsorbent bed for the adsorption of hydrocarbons during the decompression phase of said bed. 7. Method according to one of the preceding claims, characterized in that at least a portion of the recycle gas is contacted with the PSA beds during the adsorption phases of the two beds. 8. Method according to one of the preceding claims, characterized in that the recycle gas is derived from the compression of a gas mixture comprising a gas rich in hydrocarbons. 9. Method according to one of the preceding claims, characterized in that during the adsorption phase, a gas is produced comprising substantially hydrogen having a pressure P decreased by about one bar of pressure drop. <Desc / Clms Page number 21> 10. Method according to one of the preceding claims, characterized in that during the adsorption phase, a gas is produced comprising substantially hydrogen having a purity greater than at least 97 mol%. 11. Method according to one of the preceding claims, characterized in that the gaseous mixture comprising at least H2S, H2 and hydrocarbons is a stream from a hydrodesulfurization process. 12. The method of claim 11, characterized in that during the adsorption phase, the gas comprising substantially hydrogen produced is recycled in the hydrodesulfurization process. 13. The method of claim 11 or 12, characterized in that the gaseous mixture comprising at least H2S, H2 and hydrocarbons to be treated is the gaseous mixture at high pressure comprising essentially hydrogen, hydrocarbons and HsS ( 3) from the high pressure separation unit of a hydrodesulfurization unit. 14. The method of claim 11 or 12, characterized in that the gaseous mixture comprising at least H2S, H2 and hydrocarbons to be treated is the mixture (7) of the recycle gas (6) and the makeup gas (12). a hydrodesulfurization unit. 15. Method according to one of claims 13 to 14, characterized in that the waste gas from the decompression phase of the adsorbent bed for the adsorption of hydrocarbons is sent to the non-acid fuel gas network. 16. Method according to one of claims 13 to 15, characterized in that the waste gas from the decompression phase of the adsorbent bed for the adsorption of HzS is sent to the acid gas fuel network.