WO2014072600A1

WO2014072600A1 - Photocatalytic process for production of hydrogen and sulphur from hydrogen sulphide

Info

Publication number: WO2014072600A1
Application number: PCT/FR2013/052468
Authority: WO
Inventors: Jerome Majcher; Antoine Fecant; Denis Uzio
Original assignee: IFP Energies Nouvelles
Priority date: 2012-11-12
Filing date: 2013-10-16
Publication date: 2014-05-15
Also published as: FR2997940B1; FR2997940A1

Abstract

The invention relates to a photocatalytic process for production of hydrogen (28) and for recovery of solid sulphur (48) from hydrogen sulphide gas (6) in the presence of a light source which emits in the ultraviolet and/or visible spectrum (22). The process enables continuous operation and comprises the bringing into contact of a gas charge comprising hydrogen sulphide (6) with a basic aqueous solution (8) in a zone of heat exchangers comprising at least two exchangers (2) and (4) arranged in parallel and operating alternately, a gas/liquid separation (12), a photocatalytic step making it possible to form hydrogen and polysulphides (20), a gas/liquid separation (26) making it possible to recover the hydrogen (28), and the recycling of the polysulphides (30) in the operating heat exchanger (2) in order to cause said polysulphides to precipitate in the form of sulphur. The process is also aimed at recovering the sulphur in solid form (48).

Description

PHOTOCATALYTIC PROCESS FOR PRODUCING HYDROGEN AND SULFUR FROM HYDROGEN SULFIDE

The field of the invention is that of photocatalysis and more specifically photocatalytic reactions for the production of dihydrogen (H ₂ ) from hydrogen sulfide (H ₂ S).

The present invention relates to a photocatalytic process for producing hydrogen and recovering solid sulfur from hydrogen sulphide. The object of the invention is to provide a method for continuous operation. The process can advantageously be established on an existing refinery in addition to a Claus type process with daytime operation.

Hydrogen sulfide is a toxic chemical emitted in large quantities from natural sources and industrial processes. Even in trace amounts, H ₂ S has a poisonous effect on the Group VIII metal catalysts used in many industrial processes such as, for example, in the hydrogenation or synthesis of ammonia.

Most industrial processes for the removal of H ₂ S from gaseous effluents are based on a process for the absorption of H ₂ S in aqueous amine solutions, followed by a Claus process for decomposing H ₂ S in water and sulfur [H ₂ S + (1/2) 0 ₂ → H ₂ 0 + S].

Although the Claus process is the most used method for removing H ₂ S, it does not recover the hydrogen potentially stored in H ₂ S in the form of H ₂ dihydrogen. In addition, because of the high temperatures required, this process leads to high greenhouse gas emissions (C0 ₂ ).

Various strategies for converting H ₂ S to hydrogen and sulfur have been proposed to mitigate environmental problems and produce hydrogen. For example, the thermal decomposition of H ₂ S into sulfur and hydrogen is known.

Another strategy is the decomposition of H ₂ S into sulfur and hydrogen by the photocatalytic route. Indeed, part of the solar irradiation touching the surface of the earth can be converted into chemical energy by producing dihydrogen (H ₂ ) via the photocatalytic dissociation of compounds acting as a source of protons such as hydrogen sulfide (H ₂ S).

Since dihydrogen is a molecule considered as a dense and non-polluting energy carrier, the production of this molecule by photocatalysis could respond to the ever-increasing environmental constraints and the growing global energy needs. Indeed, the dihydrogen can be used, for example, directly as a fuel for internal combustion engines for transport, for the generation of electrical energy in fuel cells or even to provide a supplement of high purity hydrogen for the processes. hydrotreatment and / or hydroconversion processes that consume this gas in existing refineries.

The process according to the invention producing hydrogen from H ₂ S has many advantages over the new challenges facing the energy sector. The global trend to produce cleaner and therefore less sulphurized fuels leads to increasing H ₂ S production. In addition, the hydrotreating and hydrocracking units used to meet new specifications and increasing fuel demand have increased their capacity. hydrogen consumption.

In addition, the substitution by the process according to the invention, using as energy source light radiation, conventional H ₂ S treatment units or hydrogen production units (Claus process, steam reforming, etc.) makes it possible to reduce emissions. of greenhouse gases.

PRIOR ART

Photocatalytic processes for the production of dihydrogen from H ₂ S are known in the state of the art.

Thus EP 0,205,199 describes an implementation of this type of process. A Pyrex photocatalytic cell equipped with a stirring system is irradiated with visible radiation. The cell is filled with a NaOH-type base in which a stream of gaseous H ₂ S is bubbled in the presence of a dispersed catalyst (from the group ZnO, ZnO-Ru0 ₂ , ZnS, ZnSe and CuGaS ₂ ). Different performances expressed in flow rate of H ₂ are presented as a function of the photocatalyst used, the H ₂ S / NaOH ratio, the NaOH concentration and the wavelength. H ₂ S is injected through a first line into the photocatalytic cell while a second line discharges excess H ₂ S and H ₂ produced. This effluent is reinjected after compression by the first line, the whole being provided with a fresh H ₂ S inlet. This implementation therefore works until saturation of the H _{2 system} , the solid sulfur being deposited at the bottom of the cell. To restart a production cycle, the unit must be stopped.

US 6,572,829 discloses an implementation that connects three sections: a dissolution zone of the H ₂ S / insulation sulfur, a degassing zone of the H ₂ S and a photocatalytic reaction zone. H ₂ S bubbles through a liquid base in the first section. The solution containing HS ^" ions is sent into the degassing zone where excess H ₂ S is discharged Finally, the solution passes through the photoreactor where a dissociation of gaseous H ₂ and S ₂ ^2" ions on catalysts of the CdS group, Pt-CdS, ZnS or ZnFe ₂ O ₄ is carried out _. The latter recycled in the first section in the presence of H ₂ S will precipitate in solid sulfur. The bottom of the first section is filled with ceramic filters to isolate the sulfur. Thus this implementation works until saturation of the sulfur system. It is mentioned that in all cases the sulfur is solid and that it will have to be collected by opening the system and stopping the unit accordingly.

It is also described the production of hydrogen under visible radiation from an alkaline solution in which a flow of H ₂ S flows in the presence of photocatalysts Cdln ₂ S _{4 type} (Adv Funct Mater, 16, p 1349 (2006)), CuGa _x 1n _x 0 ₂ (Catalysis Communications, 9, p 395 (2008) and FeGa0 ₃ (Int J. Hydrogen Energy, 33, p 6586 (2008)) implemented under Finally, the work of Can Li et al (Chinese Journal of Catalysis, 29 (4), 313 (2008)) has shown that it is possible to produce hydrogen by photolysis of a flow of H ₂ S in gas phase under UV-Vis irradiation by the use of a photocatalyst based on ZnS doped in a fixed-bed photoreactor. Thus, the photocatalytic processes for the production of hydrogen from H ₂ S proposed in the state of the art require an intervention interrupting the process which strongly penalizes its productivity.

The object of the invention is to provide a photocatalytic process for producing hydrogen from H ₂ S allowing continuous operation.

Another object is to provide a method for replacing or supplementing at least partially the Claus type process in the daytime.

Another object of the invention is to provide a method for maintaining the polysulfides in the liquid phase in the photoreactor, to prevent them from being deposited on the photocatalyst. Indeed, the deposition of polysulfides on the photocatalysts leads to inactive photocatalysts.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a photocatalytic process for producing hydrogen and sulfur from H ₂ S for continuous operation.

More particularly, the present invention relates to a process for producing hydrogen and sulfur from a gaseous feedstock comprising hydrogen sulfide, comprising the steps of:

a) a gaseous feedstock comprising hydrogen sulfide H ₂ S is contacted with a basic aqueous solution and a liquid fraction comprising polysulfides produced in step d) in a contacting zone so as to dissolve at least one a part of hydrogen sulphide H ₂ S in ionic form, and so as to cause the precipitation of sulfur by decomposition of polysulfides,

b) a gas / liquid separation of the effluent obtained in step a) is carried out so as to recover a gaseous fraction comprising hydrogen sulphide and a liquid fraction comprising hydrogen sulphide in ionic form,

c) at least a portion of the liquid fraction comprising the hydrogen sulphide in ionic form resulting from step b) is brought into contact with a photocatalyst in the presence of a radiation source emitting at least in a range of lengths of wave greater than 280 nm, to produce a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulfides,

d) a gas / liquid separation of the effluent obtained in step c) is carried out so as to recover a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulfides, the liquid fraction being at least partially recycled to step a),

said method being characterized in that said contacting zone in step a) comprises at least a first and a second heat exchanger, step a) is carried out in the first heat exchanger, and then when the first exchanger of heat is charged with sulfur, the first exchanger is disconnected and step a) is carried out in the second heat exchanger, the first heat exchanger is emptied of its liquid content, a fluid is injected into the first heat exchanger at a temperature above the melting temperature of the sulfur, and the liquid sulfur is removed from the first heat exchanger.

The various stages of the process and the equipment that constitute it are detailed below.

Step a): Dissolution of the H 2 S in the basic aqueous solution

According to step a) of the process according to the invention, a gaseous feedstock comprising hydrogen sulfide H2S is brought into contact with a basic aqueous solution and a liquid fraction comprising polysulfides produced in step d) in a zone of contacting so as to dissolve at least a portion of the hydrogen sulfide H ₂ S in ionic form (H ₂ S + OH -> HS ^" + H ₂ O), and so as to cause the precipitation of sulfur by decomposition of polysulfide.

The gaseous feedstock comprising hydrogen sulfide H ₂ S is generally an industrial gaseous effluent from a refinery, such as an effluent from an amine wash or optionally a light hydrocarbon gas feed containing H ₂ S. In the In the case of the treatment of a light hydrocarbon feedstock of refinery containing hydrogen sulphide and coming from the desulfurization units, the composition of the gas to be treated is on average the following before washing with amines: 1% by mass H ₂ , 15% by weight C1, 18% by mass C2, 14% by mass C3, 13% by mass C4, 5% by mass C5 +, 34% by mass H ₂ S. When the gas to be treated has been subjected to an amine wash, the composition is on average 2.5% by weight H ₂ 0 and 97.5% by weight H ₂ S. Preferably, the feedstock comprising hydrogen sulphide H ₂ S is an effluent from an amine wash.

The basic aqueous solution is preferably selected from an aqueous solution of potassium hydroxide (KOH), sodium hydroxide (NaOH) or a mixture of both.

The contacting according to step a) can be carried out at a temperature between room temperature and 100 ° C. The contacting is generally carried out at a pressure between atmospheric pressure and 3 MPa (30 bar).

Preferably, the contacting is carried out at ambient temperature. Preferably, the contacting is carried out at atmospheric pressure.

When it is desired to valorize the hydrogen produced by photocatalysis in step c) by injection into the refinery network and then to use it in units such as hydrotreatment and / or hydroconversion, it is preferable to use a pressure between 2 MPa and 3 MPa (20 and 30 bar).

The pH in step a) is generally between 7 and 9, preferably between 7.6 and 8.5. In fact, the addition of hydrogen sulphide as the acid compound lowers the pH of the basic aqueous solution and the recycled liquid fraction and thus makes it possible to dissociate the H ₂ S in H ⁺ and HS ^" and to decompose the polysulfides to sulfur.

Any type of heat exchanger such as a plate heat exchanger or a shell-type heat exchanger can be used. Advantageously, a "shell-and-tube" type heat exchanger (standardized by the TEMA standard) is used which consists of a bundle of tubes arranged inside a shell called a shell. One of the fluids circulates inside the tubes (hot fluid, preferably water vapor) and the other inside the shell (mixture charge / solution basic aqueous / liquid fraction containing the polysulfides), around the tubes. At each end of the beam are attached distribution means which ensure the circulation of fluid within the beam in one or more passes. The calender is also provided with inlet and outlet pipes for the second fluid (which circulates outside the tubes). Preferably, the charge containing the hydrogen sulphide and the basic aqueous solution and the liquid fraction containing the polysulphides are injected countercurrently, that is to say that the charge containing the H ₂ S is injected into the portion lower the exchanger, while the basic aqueous solution and the liquid fraction containing the polysulfides are injected into the upper part of the exchanger. This improves the gas / liquid contact and consequently the dissolution of the hydrogen sulphide.

The method according to the invention allows continuous operation through a contacting zone in step a) comprising at least a first and a second heat exchanger. The first heat exchanger operates alternately with the second heat exchanger. Indeed, the contacting zone is composed of at least two exchangers arranged in parallel to process the load. These also act as decanters (recovery of sulfur). While one is running, the second is in regeneration and vice versa.

More particularly, said method is characterized in that said contacting zone in step a) comprises at least a first and a second heat exchanger, step a) is carried out in the first heat exchanger, and then when the first heat exchanger is charged with sulfur, the first exchanger is disconnected and step a) is carried out in the second heat exchanger, the first heat exchanger is emptied of its liquid content, a fluid is injected into the first exchanger of heat at a temperature above the melting temperature of the sulfur, and the liquid sulfur is removed from the first heat exchanger.

Preferably, the fluid passes through the heat exchanger in a first circuit, the contacting of step a) being performed in a second circuit, the first circuit being adapted to exchange heat with the second circuit. According to a preferred variant, the contacting of step a) is carried out in an area comprising at least two heat exchangers arranged in parallel, these two exchangers operating alternately, each heat exchanger enclosing a bundle of tubes inside the heat exchanger. a calender, a single exchanger being used for contacting step a) which progressively charges with sulfur by recycling the liquid fraction comprising polysulfides, the other exchanger being disconnected during this time, and when the exchanger used for the contacting of step a) is charged with sulfur, the exchangers are exchanged by putting into operation the other exchanger. The exchanger loaded with sulfur is disconnected, emptied of its liquid content and then subjected to an injection of a fluid at a temperature above the melting temperature of the sulfur in the tube bundle of the exchanger so as to melt and to recover sulfur. While an exchanger is used to effect the dissolution of H ₂ S and the precipitation of polysulfides in sulfur, the other is freed of sulfur, thus preparing it for tilting as soon as the first exchanger is charged with sulfur. As soon as the tilting is done, the sulfur is accumulated and the dissolution is carried out in the exchanger previously cleaned of sulfur and the other exchanger saturated with sulfur is cleaned and so on.

Step b): Separation of the gaseous H? S

According to step b) of the process according to the invention, a gas / liquid separation of the effluent obtained in step a) is carried out so as to recover a gaseous fraction comprising hydrogen sulphide and a liquid fraction comprising Hydrogen sulfide in ionic form.

This separation makes it possible to evacuate the gaseous fraction comprising undissolved hydrogen sulphide and thus makes it possible to send only a liquid fraction into the photocatalytic stage.

This separation is preferably carried out in a gas / liquid separator flask.

According to a preferred variant, the gaseous fraction comprising undissolved hydrogen sulfide from the gas / liquid separation is recycled to the exchanger in which the contacting of step a) is carried out. When operating with a pressure greater than atmospheric pressure in the exchanger in which the contacting of step a) is performed, the gaseous fraction comprising undissolved hydrogen sulphide from the gas / liquid separation is preferably recompressed before recycling in the exchanger.

Step c): Production of hydrogen by photocatalysis

According to step c) of the process according to the invention, at least a portion of the liquid fraction comprising hydrogen sulphide in ionic form resulting from step b) is brought into contact with a photocatalyst in the presence of a source of radiation emitting at least in a wavelength range greater than 280 nm, so as to produce a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulphides, and optionally sulphide ions and / or hydrogen sulphide dissolved having not been converted.

By photocatalysis, the sulfur contained in the liquid fraction comprising hydrogen sulfide in liquid form HS ^" is oxidized to polysulfides generally having the formula S _n ^{2 ~} with n = 2 to 6, and the protons generated by the reaction of oxidation of HS ^" are reduced to H ₂ dihydrogen _:

Oxidation: 2HS ^" + 2h + => S ^2" ₂ + 2H ⁺

Reduction: 2H ⁺ + 2 _e ^" => H ₂

By separating the gaseous fraction comprising undissolved hydrogen sulphide (which is an acidic compound) in step b), the liquid fraction sent in the photocatalytic step is less acidic than the charge mixture / basic aqueous solution. . The pH in step c) is generally between 10 and 13, preferably between 1, 5 and 12.5. Obtaining a pH in this range is important in order to maintain the polysulfides in solution and to prevent the polysulfides formed from precipitating (already) in the photoreactor on the photocatalyst causing its deactivation.

If necessary, an addition of basic aqueous solution may be injected with the liquid fraction comprising hydrogen sulfide in ionic form from step b) into the photocatalyst. This allows the regulation of the pH to a value between 10 and 13, thus avoiding any premature precipitation of polysulfides in the photocatalyst. The addition of the basic aqueous solution is preferably selected from an aqueous solution of potassium hydroxide (KOH), sodium hydroxide (NaOH) or a mixture of both. Preferably, the addition of the basic aqueous solution is identical to the basic aqueous solution of step a).

The implementation of the hydrogen production process is in a liquid medium. It is not necessary to have pure reagents. For example, the liquid medium may contain solvated ions (Na ⁺ , K ⁺ , S ^{2 "} , CO ₃ ^2" , Cl ^" , Br ^" , NO ₃ ^" , etc.).

The implementation of the process for producing dihydrogen can be carried out in all types of reactors adapted to a photocatalytic reaction. Thus, the implementation can be done in reactors totally glass or having non-absorbing optical windows to allow the radiation to reach the surface of the photocatalyst.

The type of reactor technology for implementing the photocatalyst is generally adapted to a thin film. Thus the thin-film fixed-bed reactors are advantageously employed, it is however also possible to use sloped or even cylindro-parabolic collector-type reactors. For this type of technology, the photocatalyst is advantageously deposited on a fabric-type support or deposited on a surface in contact with the light source in the form of a paint. Reactor technology can also be adapted to a suspension. Thus, fluidized bed type reactors are advantageously employed.

The radiation source emits at least a wavelength range greater than 280 nm, preferably 315 nm to 800 nm, which includes the UV spectrum and / or the visible spectrum. The radiation may be any UV-visible radiation such as natural sunlight, a type of lamp Hg, a lamp type Xe, a lamp type LED (Light Emitting Diode). Preferably, the radiation source is the natural sunlight. The luminous power is generally between 1 and 50 mW / cm ² , preferably between 1 and 35 mW / cm ² . The photocatalytic stage c) for producing hydrogen can be carried out at a temperature between room temperature and 100 ° C. The photocatalytic stage c) for producing hydrogen is generally carried out at a pressure comprised between atmospheric pressure and 3 MPa (30 bar).

The photocatalytic stage c) of hydrogen production has the advantage that it can be carried out under very mild operating conditions, especially at room temperature and / or at atmospheric pressure. Preferably, step c) is thus carried out at room temperature and / or at atmospheric pressure.

When it is desired to use the hydrogen produced by photocatalysis to inject it into the refinery's hydrogen network for use in refinery applications, such as hydrotreating and / or hydroconversion, one uses preferably a pressure between 2 MPa and 3 MPa (20 bar and 30 bar).

The photocatalyst employed may be any semiconductor material absorbing in the UV and / or visible and having a conduction band level strictly greater than 0V (versus NHE). The photocatalyst may for example be chosen from TiO ₂ , ZnS, ZnO, WO ₃ , MoSx, Fe ₂ O ₃ , CdS, used alone or as a mixture.

According to a particularly preferred variant, the photocatalyst is a composition comprising a mixture of zinc sulphide (ZnS) and molybdenum sulphide (MoSx), and in which the molar ratio Mo / Zn is between 0.01 and 1, 9, preferably between 0.5 and 1.5. The ratio is calculated on the basis of elements Mo and Zn. Such a photocatalyst is described in application FR12 / 01 .853.

The photocatalyst is preferably in the form of nanoparticles. The size of the nanoparticles is generally less than 1 μιτι (1000 nm), and preferably between 10 and 500 nm, more preferably between 10 and 200 nm.

In general, it is desired to obtain small particle sizes that make it possible to increase the surface / volume ratio in order to obtain a large surface area available for the photogenerated reactive species. The specific surface area of the particles is generally between 10 and 150 m ² / g. Alternatively, the photocatalyst may further comprise at least one Group VIII metal and / or Group IB in its metal, oxide or sulfide form. In the case where two metals are present, they can be in the form of an alloy. Preferably, the group VIII and / or IB group metal is chosen from platinum, palladium, gold, nickel, cobalt, ruthenium, rhodium or chromium in their metallic form or else from oxides. NiO _x , RuO _x , Cr ₂ 0 _3, CoO and Co ₃ 0 ₄ The content of the metal of group VIII and / or of group IB in its metallic, oxide or sulphide form is generally between 0.01 and 5% by mass metal on the photocatalyst, preferably between 0.1 and 2% metal.

The photocatalyst may be prepared according to any method of preparation known to those skilled in the art.

Step d): Separation of hydrogen produced

According to step d) of the process according to the invention, a gas / liquid separation of the effluent obtained in step c) is carried out so as to recover a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulfides. the liquid fraction being at least partly recycled in step a).

This separation is preferably carried out in a gas / liquid separator flask.

This separation makes it possible to extract the hydrogen produced during the photocatalysis reaction c).

This step makes it possible to produce very pure hydrogen. Generally, the hydrogen produced has a purity greater than 99%, preferably greater than 99.9%. Generally, hydrogen has a purity of 100%.

The hydrogen thus produced can be used, for example, as a fuel for internal combustion engines, for the generation of electric energy in fuel cells, or as a supplement for high purity hydrogen for hydrotreatment processes. highly efficient hydroconversion of this gas in existing refineries.

When it is desired to use hydrogen for applications in the refinery, and operates at atmospheric pressure in the photocatalytic stage, the hydrogen must be recompressed before its introduction into the hydrogen network of the refinery.

When it is desired to use hydrogen for applications in the refinery, and operates at a pressure above atmospheric pressure in the stage photocatalytic c), and especially at a pressure between 2 MPa and 3 MPa (20 and 30 bar), the gaseous fraction comprising hydrogen can be directly recycled into the hydrogen network of the refinery.

According to step d) of the process according to the invention, the liquid fraction comprising the polysulfides is at least partly, preferably entirely, recycled in step a). Contacting said liquid fraction with the gaseous feedstock comprising hydrogen sulfide in the contacting zone of step a) causes the precipitation of sulfur by decomposition of the polysulfides.

In fact, the hydrogen sulphide contained in the feedstock is an acid compound which precipitates the elemental sulfur by decomposing the polysulfides maintained in liquid form in the liquid fraction recycled at a higher pH (thanks to the degassing of the H ₂ S during step b).

If necessary, another acidic compound such as chloric acid, sulfuric acid or phosphoric acid may be added in addition to the hydrogen sulfide feed to precipitate the polysulfides contained in the recycled liquid fraction.

The exchanger in which the contacting of step a) is carried out will progressively be loaded into solid sulfur, affecting the operation, in particular a loss of charge.

When the exchanger used for the contacting of step a) is charged with sulfur, this exchanger is disconnected and step a) is carried out in the other heat exchanger.

The switching of the exchanger charged with sulfur to another exchanger is generally carried out by a set of valves at the inlet of the gaseous feed, the entry of the basic aqueous solution and the evacuation of the basic aqueous solution.

The exchanger charged with sulfur is then emptied of its liquid content, and then a fluid is injected into this heat exchanger at a temperature above the melting temperature of the sulfur (melt temperature between 12.8 ° C. and 19 ° C. C according to the nature of elemental sulfur according to CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief DR Lide, 81 st edition, 2000-2001); Then the liquid sulfur is removed from this heat exchanger.

Preferably, the injected fluid is water vapor.

According to a preferred variant according to which "shell-and-tube" heat exchangers are used, the fluid is injected into the tube bundle of the disconnected sulfur-charged exchanger for melting the sulfur, which is recovered in liquid form by an exit from the grille. The injection of the fluid is then operated in heat exchange tubes of the exchanger so that the sulfur becomes liquid and is in turn evacuable from the capacity of the exchanger. The cleaned exchanger regains its operability, the evacuated sulfur resolidifies at room temperature and is for example recoverable as such (fertilizer ...).

Figure 1 schematically shows a preferred variant of the method according to the invention.

The process is composed at the inlet of two heat exchangers (2) and (4) for treating the feedstock comprising hydrogen sulfide. These also act as decanters (precipitation of polysulfides). While one is running, the second is in regeneration and vice versa. The gaseous feedstock comprising hydrogen sulfide (6), the basic aqueous solution (8) and the liquid fraction comprising polysulfides (30) produced in step d) are introduced into the calender of a first exchanger (2). The liquid solutions fill the capacity of the exchanger in its shell part. An inlet of the charge is preferably positioned at the bottom of this calender to ensure maximum gas / liquid exchange. The contacting of the charge containing H ₂ S (6) and the basic aqueous solution (8) in the exchanger (2) has the objective of maximizing the dissolution of the H ₂ S in ionic form by adjusting pressure and flow rates. The effluent from the first exchanger (2) is fed via the line (10) into a gas / liquid separator tank (12) in which a gas / liquid separation of the effluent obtained in step a) is carried out so as to recovering a gaseous fraction comprising undissolved hydrogen sulphide (14) and a liquid fraction comprising hydrogen sulphide in ionic form (16). The gaseous fraction comprising hydrogen sulfide (14) is recycled to the exchanger (2), optionally after having been recompressed in the compressor (18). Fraction liquid comprising hydrogen sulphide in ionic form (16) is fed to the photocatalytic reactor (20) in which the liquid fraction comprising hydrogen sulphide in ionic form resulting from step b) is brought into contact with a photocatalyst in the presence of a radiation source emitting at least in a wavelength range greater than 280 nm, for example sunlight (22), so as to produce a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulfides. If necessary, an addition of basic aqueous solution (17) may be injected with the liquid fraction comprising hydrogen sulphide ion form from step b) in the photocatalytic reactor. This allows the regulation of the pH to a value between 10 and 13, thus avoiding any premature precipitation of the polysulfides in the photocatalytic reactor.

The effluent from the photocatalytic reactor containing the gaseous fraction comprising hydrogen and the liquid fraction comprising polysulfides is fed via line (24) into a gas / liquid separator tank (26) in which a gas / liquid separation is carried out. the effluent obtained in step c) so as to recover a gaseous fraction comprising hydrogen (28) and a liquid fraction comprising polysulfides (30). Depending on the use of the hydrogen produced, it can be recompressed in the compressor (32) in order to achieve, for example, the pressure necessary for application in a refinery, such as in hydrotreating and / or hydroconversion.

The liquid fraction comprising the polysulfides (30) from step d) is recycled via a pump (34) into the exchanger (2) of step a). The introduction of the feedstock comprising hydrogen sulphide (6) as an acidic compound precipitates the elemental sulfur by decomposition of the polysulfides contained in the recycled fraction which is deposited in the shell of the exchanger (2). The exchanger (2) is thus gradually loaded with sulfur. Once the exchanger (2) is charged with sulfur, the exchangers are exchanged by putting into service the other exchanger (4) for contacting step a), while the exchanger charged with sulfur ( 2) is offline. The tilting of the sulfur-filled exchanger (2) to another exchanger (4) is generally effected by a set of valves at the inlet of the gaseous feed (36), the entry of the basic aqueous solution ( 38) and the evacuation of the basic aqueous solution (40) (the exchanger (4) being identical to the exchanger (2), the reference signs are indicated by an apostrophe "'").

In order to recover the sulfur contained in the shell of the exchanger loaded with sulfur (2), it is emptied of its liquid content by the valve (42). Then, a fluid (44) is injected into the tube bundle of the disconnected sulfur-charged exchanger (2) at a temperature above the melting temperature of the sulfur to melt the sulfur, preferably steam. water. This fluid is evacuated by the line (46). The injection of the fluid (44) then melts the sulfur by heat exchange, which is removed by the valve (42). The evacuated sulfur (48) resolidifies at room temperature. The exchanger (2) is thus cleaned and can be reconnected for contacting step a) as soon as the other exchanger (4) is charged with sulfur.

Claims

Claims A process for producing hydrogen and sulfur from a gaseous feedstock comprising hydrogen sulfide, comprising the steps of:

c) at least a portion of the liquid fraction comprising the hydrogen sulphide in ionic form resulting from step b) is brought into contact with a photocatalyst in the presence of a radiation source emitting at least in a range of lengths of wave above 280 nm, so as to produce a gaseous fraction comprising hydrogen and a liquid fraction comprising polysulfides,

2. Method according to claim 1, wherein the first heat exchanger operates alternately with the second heat exchanger.

3. Method according to claims 1 or 2, wherein the fluid passes through the heat exchanger in a first circuit, the contacting of step a) being performed in a second circuit, the first circuit being adapted to exchange the heat with the second circuit.

4. Process according to claims 1 to 3, wherein the pH in step a) is between 7 and 9.

5. Process according to claims 1 to 4, wherein the pH in step c) is between 10 and 13.

The process according to claims 1 to 5, wherein the basic aqueous solution is selected from aqueous potassium hydroxide (KOH), sodium hydroxide (NaOH) or a mixture of both.

The process of claims 1 to 6, wherein the feedstock comprising hydrogen sulfide is an effluent from an amine wash.

The method of claims 1 to 7, wherein the radiation source in step c) is natural sunlight.

9. Process according to claims 1 to 8, wherein the gaseous fraction comprising hydrogen sulfide from the gas / liquid separation of step b) is recycled to the exchanger in which the contacting of the step a) is performed.

10. Process according to claims 1 to 9, wherein the injected fluid is water vapor.

1 1. Process according to claims 1 to 10, wherein the contacting according to step a) is carried out at a temperature comprised between the temperature ambient and 100 ° C and at a pressure between atmospheric pressure and 3 MPa (30 bar).

12. Process according to claims 1 to 1 1, wherein the photocatalytic step c) of hydrogen production is carried out at a temperature between room temperature and 100 ° C, and at a pressure between atmospheric pressure and 3 ° C. MPa (30 bar).

13. Process according to claims 1 to 12, in which the photocatalyst is a composition comprising a mixture of zinc sulphide (ZnS) and molybdenum sulphide (MoSx), and in which the molar ratio Mo / Zn is between 0, 01 and 1, 9.

14. The method of claims 1 to 13, wherein the photocatalyst is in the form of nanoparticles whose size is less than 1 μιτι.

The method of claims 1 to 14, wherein the photocatalyst further comprises at least one Group VIII and / or Group IB metal in its metal, oxide or sulfide form.