WO2009103895A2

WO2009103895A2 - Method for oxidising h2s into sulphur using a catalyst carried by a porous sic foam

Info

Publication number: WO2009103895A2
Application number: PCT/FR2008/001776
Authority: WO
Inventors: Cuong Pham-Huu; Charlotte Pham; Patrick Nguyen
Original assignee: Sicat; Centre National De La Recherche Scientifique; Universite De Strasbourg
Priority date: 2007-12-21
Filing date: 2008-12-18
Publication date: 2009-08-27
Also published as: WO2009103895A3; FR2925356A1

Abstract

The invention relates to a method for processing a gas containing H2S into sulphur, in which the gas is contacted with a catalyst that comprises a substrate made of an SiC porous foam having a specific surface BET higher than 1 m2/g, preferably higher than 5 m2/g, a density between 0.1 g/cm3 and 0.4 g/cm3, and an active phase containing at least one transition metal element.

Description

Process for oxidizing H ₂ S to sulfur using a catalyst supported by a porous SiC foam

Technical field of the invention

The present invention relates to the use of catalyst supports in the form of a porous SiC foam as a catalyst support for the mild oxidation of H ₂ S to elemental sulfur.

State of the art

Standards on the discharge of pollutants into the atmosphere are becoming more stringent; this applies in particular for releases of sulfur products. Thus, it becomes more and more necessary to purify the gaseous effluents before their emission into the atmosphere. For this purpose, there are numerous methods for removing H ₂ S from the gaseous effluent to be treated. One of the best known processes is the Claus process which converts H ₂ S into elemental sulfur. However, not all H ₂ S entering the unit is converted to sulfur due to thermodynamic limitations:

2H ₂ S + SO ₂ → 2H ₂ O + V s _n

^{2 2 2} Zn "(Equation 1)

Residual L ¹ H ₂ S and SO ₂ contained in the Claus process effluent (tail gas) must be reprocessed to minimize sulfur product concentrations in the gas tail before combustion and re-emission into the atmosphere. These tail gas treatments of the Claus process are sometimes referred to as "post-Claus processes". There are several post-Claus processing methods, including the Scot method (GB 1,461,070) where the tail gas added to hydrogen passes through a catalytic bed consisting of a group transition metal sulfide VI (especially Cr, Mo, W) and / or Group VIII (in particular Ni, Pd, Pt), supported on an inorganic oxide. The SO2 is reduced to H ₂ S which is separated by liquid adsorption of the rest gas. The disadvantage of this method is that it requires a complex installation to implement and expensive; moreover, the adsorption of H ₂ S requires a high energy consumption. In the so-called BSR-Selectox process (US 4,311,683 - Union OII Company of California), the SO2 contained in the tail gas ("off gases" or "tail gases" in English) is hydrogenated in H ₂ S to using a catalyst consisting of cobalt oxide (3-8%) and molybdenum oxide (8-20%) supported on alumina. Then the H ₂ S is oxidized to sulfur using a catalyst comprising an oxide and / or vanadium sulphide supported on a porous refractory oxide (typically alumina and / or zeolite). As for the Scot process, the BSR-Selectox process requires a step of cooling the gases before the catalytic oxidation step in order to condense the water vapors from the Claus process. In addition, the catalysts of these two processes are sensitive to water (absorption / deactivation).

To suppress the H ₂ S content in low concentrations, that is to say less than 25% by volume, more particularly between 0.001% and 20% and even more particularly between 0.005 and 10% by volume, contained in gases from different sources, H ₂ S can be catalytically oxidized to sulfur according to the reaction:

H ₂ S + V ₎ O ₂ → S + H ₂ O 2/2 ^{2 2} (Equation 2)

In such processes, the acid gas to be treated is mixed with a suitable amount of oxygen-containing gas, such as, for example, air, oxygen or an oxygen-enriched gas, and is contacted with the oxygen-containing gas. catalyst for oxidizing H ₂ S to sulfur. If this reaction takes place at a temperature below the dew point of sulfur, the sulfur formed is deposited on the catalyst, and more particularly in the porosity of the catalyst. Thus, the catalyst loaded with sulfur must cyclically undergo regeneration steps during which the sulfur is vaporized at a temperature typically between 200 and 500 ° C, and transported outside the catalyst bed by a non-oxidizing carrier gas. The reaction can also be carried out above the dew point of sulfur, more particularly above 180 ° C., and in this case, the sulfur formed is in vapor form and / or in liquid form, and it condenses then outside the catalytic bed in a cold zone at the end of the reactor. A very large number of works have been published to explore the catalysts, active phases and catalyst supports that can be used for this reaction. This reaction is carried out using catalysts based on titanium oxide (EP 0078 690 A - Mobil OiI Corporation), based on titanium oxide containing a sulphate of alkaline earth metal (WO 83/02068 - EIf Aquitaine), based on titanium oxide containing nickel oxide and optionally aluminum oxide (EP 0 140 045 A - BASF AG), or based on a mixed oxide of titanium oxide / zirconium oxide, or based on silica combined with a transition metal selected from the following list: Fe, Cu, Zn, Cd, Cr, Mo, W, Co and Ni and preferably with Fe which can be combined with one or more precious metals such as Pd, Pt, Ir and Rh, and more preferably Pd (FR 2 511 663 - EIf Aquitaine). The most widely used catalysts, which are the industry benchmarks, are catalysts based on a low surface area alpha alumina comprising iron (5%) stabilized with Cr (0.5%) (US 4,818,740). - VEG-Gasinstituut) or silica-based catalysts with a specific surface area of the order of 20 m ² / g comprising 4% iron stabilized with 1% Cr or P (US 5,286,697 - VEG-Gasinstituut). A catalyst comprising an active phase of vanadium deposited on silica has also been described (see the article by JS Chung et al., "Removal of H ₂ S and / or SO 2 by catalytic conversion technologies" published in Catalysis Today 35, pp. 27-43 (1997)). More recently, catalysts based on mixed iron-niobium oxides have been described for this reaction (see the article by DW Park et al., "Prduction of ammonium thiosulfate by the oxidation of hydrophil sulfide over Nb-Fe mixed oxide catalysts"). Published in Catalysis Today 93-95, pp. 235-240 (2004)). Catalysts comprising an active phase of iron and chromium deposited on a metal alloy support (Fe, Cr, Ni) or silica (US 5,352,422 - VEG Gasinstituut) are also known. Supports based on alumina or silica are carriers with a certain chemical reactivity; mention may be made, for example, of sulfation of alumina which leads to destruction of the support or to progressive deactivation of the active phase. Alumina and silica can also catalyze the reverse reaction of the Claus reaction from a certain temperature where hydrogen sulphide and sulfur dioxide are formed from sulfur and water. And finally, these supports have a very low thermal conductivity, which promotes the formation of hot spots on the catalyst surface; these hot spots can be responsible for thermal runaway within the reactor that is highly detrimental to the life of the catalyst. In addition, temperature jumps can result in an undesirable drop in selectivity or even the destruction of the catalyst itself. Silicon carbide, and in particular β-SiC with a high specific surface area, is known as a catalyst support, and is of interest especially for highly endothermic or highly exothermic reactions, or in the presence of reagents or corrosive products. It is known that SiC has excellent thermal conductivity compared to other catalyst supports, as well as good chemical resistance at high temperatures, which allows it to be used at elevated temperatures. Β-SiC can be obtained by the reaction between SiO vapors with reactive carbon at a temperature of between 1100 ° C. and 1400 ° C. (Ledoux process, see EP 0 313 480 B1), or by a process in which a mixing a liquid or pasty prepolymer and a silicon powder is extruded, crosslinked, carbonized and carburized at a temperature of between 1000 ° C. and 1400 ° C. (Dubots process, see EP 0440 569 B1 or EP 0952 889 B1) .

Furthermore, β-SiC foams, which can be obtained by a variant of the Dubots process, including the impregnation of a polyurethane foam with a suspension of a silicon powder in an organic resin (Prin process, see EP) are known. 0624 560 B1, EP 0 836 882 B1 or EP 1 007207 A1).

These different forms of SiC, and in particular of β-SiC, can serve as a catalyst or as a catalyst support. In the latter case, a catalytically active phase is deposited on the support, generally from a precursor which is deposited by liquid or gaseous phase, and which must most often be activated, for example by reduction of the metal compound which it contains. US 6,372,193 (EIf Exploration Production) describes the use of a catalyst supported on β-SiC granules for the direct oxidation reaction of H ₂ S elemental sulfur at high temperature (> 180 ° C) . According to the state of the art, as represented for example by the document US Pat. No. 6,372,193 cited above, the catalysts for the oxidation reaction of H ₂ S in elemental sulfur are in the form of extrudates, granules or or spheres. This type of stack generates significant losses of loads, thereby limiting flow rates and therefore productivity. The oxidation reaction of H ₂ S to sulfur is highly exothermic (.DELTA.h -222AJ.wo = / ^ \ _e continuous cooling of the catalyst support may be required. However, in the β-SiC-based catalysts according to the state of the technical, the evacuation of the heat of reaction is essentially by a heat exchange with the reaction gases because inter-granular contact occurs only slightly in the heat transfer.

The selective oxidation of H ₂ S to sulfur is also catalyzed by a catalyst comprising SiC nanotubes as support and nickel as the active phase, as briefly described in the article "Silicon Carbide, a novel catalyst support for heterogeneous catalysis. By M. Ledoux and C. Pham-Huu, published in Cat Tech, Vol. 5, p. 226 - 246 (2001). This process is conducted at a temperature of 333 K, i.e., below the melting point of sulfur.

The problem that the present invention seeks to solve is to propose a novel process for oxidizing H ₂ S to sulfur, which uses a catalyst support which has a low pressure drop and which is particularly resistant to the conditions that occur during heating. selective oxidation of H ₂ S to sulfur, ie a catalyst support having good thermal conductivity as well as good chemical stability at high temperature.

Objects of the invention According to the invention, this problem is solved by the use of a catalyst support in the form of a porous SiC foam with a BET specific surface area greater than 1 m ² / g, and preferentially greater than 5 m ² / g, and having a density of between 0.1 g / cm ³ and 0.4 g / cm ³ . The foam may be lined with nanotubes or nanofibers. An β-SiC foam is particularly preferred. The foam has an active phase, deposited on the foam and / or the nanotubes or nanofibers, if present. The active phase comprises at least one transition metal element.

An object of the invention is therefore a process for treating a gas containing H ₂ S in sulfur, in which said gas is brought into contact with a catalyst, said process being characterized in that said catalyst comprises a catalyst support. porous foam. Description of figures

FIG. 1 shows the result of a catalytic test carried out on a catalyst comprising a cellular foam of β-SiC according to Example 1, which represents the invention. FIG. 2 shows the result of a catalytic test carried out on a catalyst in the form of extrudates, according to example 2, which represents the state of the art. FIG. 3 compares the pressure drop in a honeycomb foam of β-SiC (curve 1) with that in extruded β-SiC (curve 2) for different gas velocities.

Description

The process according to the invention uses a catalyst for the selective oxidation reaction of hydrogen sulfide elemental sulfur; this catalyst is composed of an active phase and a support. The active phase comprises at least one metal. The support consists of a foam. Silicon carbide or carbon foam is preferred. In a particularly preferred embodiment, a cellular foam is used. In the context of the present invention, it is advantageous to minimize the microporosity of the catalyst support. Indeed, the access of the reaction gases to the micropores is typically by a diffusion mechanism, which slows the reactions, and which can, in some cases, promote parasitic reactions; there is often a decrease in selectivity when a diffusion mechanism occurs. For this reason, foams, and in particular carbon foams, having a high microporosity are not particularly suitable for carrying out the present invention. In the context of the present invention, a β-SiC cellular foam is more particularly preferred. Its macroscopic structure is generated directly during the synthesis of the material, without the need for any additional shaping as is generally the case with α-SiC-based supports where postsynthesis shaping is necessary to give the structure macroscopic final. Α-SiC supports may, however, be used within the scope of the present invention, even if they are not preferred. Metal foams are not suitable in the context of the present invention. In the context of the present invention, the term "specific surface" means the specific surface area determined according to the well-known method of Brunauer, Emmet and Teller (BET method).

The porosity of a material is usually defined by reference to three categories of pores which are distinguished by their size: the microporosity (diameter less than about 2 nm), the mesoporosity (diameter of between about 2 and about 50 nm) and the macroporosity (diameter greater than about 50 nm).

In some embodiments of the present invention, a β-SiC foam is preferred which is an open-pored foam. Here we mean by "foam foam" a foam that has both a very low density and a large pore volume. The β-SiC cellular foam used in the context of the present invention has a density of between 0.05 g / cm ³ and 0.5 g / cm ³ . In general, for its application as a catalyst or catalyst support, below a density of 0.05 g / cm ³ , there are problems with the mechanical strength of the foam, whereas above 0 , 5 g / cm ^3. , The pore volume alveolar will be reduced and the pressure drop will increase without providing a functional advantage. Advantageously, the density is between 0.1 and 0.4 g / cm ³ .

In a variant of the present invention, the porous foam is lined with nanotubes or nanofibers. These nanotubes or nanofibers may be carbon or silicon carbide.

In a preferred embodiment, an SiC foam is used, advantageously a β-SiC foam, lined with carbon nanotubes or nanofibers.

In another preferred embodiment, an SiC foam, advantageously a β-SiC cellular foam, filled with nanotubes or nanofibers, is used.

SiC.

In this way, a support particularly suitable for the exothermic reaction of the selective oxidation reaction of H ₂ S in elemental sulfur is obtained with respect to the supports of the previous part. This support offers numerous advantages, particularly in terms of operating parameters. Β-SiC is known in the state of the art in various forms, and β-SiC foams are commercially available, for example from ERG Materials and Aerospace Corporation, Oakland, USA. A β-SiC support that can be used in the context of the present invention is prepared by a gas / solid reaction between SiO in vapor form generated in situ and solid carbon intimately mixed, as described in patent documents EP 0 313 480, EP 0440 569, US 5,249,930, EP 0 511 919, EP 0543 751, US 5,449,654, US 6,251,819 and EP 0543,752. Compared to the α-SiC form, which is generally obtained in the form of a powder thus requiring the presence of binders for final macroscopic shaping, the β-SiC synthesized by the method described above is obtained in various macroscopic forms directly after synthesis without the need to add binders. The binders could significantly modify the thermal conductivity of the material but also its microstructure, which would be detrimental to the catalytic process envisaged. The binder could also have interactions with the active phase and alter its catalytic properties. Α-SiC foam, although not preferred, could also be used. This product can be obtained for example from an α-SiC powder, a suspension is infiltrated in a polyurethane foam which is then subjected to sintering at high temperature. Its specific surface is quite weak. In any case, a foam, to be used in the context of the present invention, must have a specific surface area of at least 1 m ² / g, but foams having a specific surface area of at least 5 m are preferred. ² / g and even more preferably at least 10 m ² / g. Porous foams, the porosity of which consists essentially of meso- and macro-pores, and in which less than 5% by volume of the porosity is due to the micropores which can give rise to problems of diffusion of the reagents and products which are harmful for selectivity in the intended process.

Regarding α-SiC foam, less preferred than carbon foam or β-SiC foam, its higher density makes it a heavier product, which must be taken into account when designing the reactor , and its specific surface is rather weak and the absence of mesopores decreases the catalytic activity. However, its thermal conductivity can be, even in the presence of a binder, higher than that of the β-SiC foam. In general, that is to say with or without nanotubes or nanofibers, it is preferred to use in the context of the present invention a β-SiC foam with a specific surface area of between about 5 m ² / g and about 40 m ² / g and preferably between 10 m ² / g and 25 m ² / g; these values do not take into account the additional specific surface of the nanofibers or nanotubes that may be present. Foams according to the state of the art whose specific surface area is above 40 m ² / g tend to have a high microporosity. A particularly preferred foam is a β-SiC foam, which has variable pore openings between 800 and 5000 microns, preferably between 1000 and 4000 microns. The open porosity (macroporosity) of this SiC foam can vary from 30 to 90%, especially from 50 to 85%.

After the deposition of nanofibers or nanotubes, the specific surface area can reach or even exceed 100 m ² / g. The additional surface area due to these nanofibers or nanotubes essentially corresponds to their external surface, excluding micropores, which has important advantages for the selectivity of catalyzed chemical reactions.

We describe here a process for preparing a cellular foam of β-SiC.

In a first step, a silicon powder is supplied, which is mixed with a thermosetting resin. The thermosetting resin may be pure or diluted in a suitable solvent, such as ethanol, acetone, or other suitable organic solvent. This makes it possible to reduce its viscosity, which favors a good mixture with the powder of the active phase, and possibly the infiltration at the next step of said process.

As the thermosetting resin, phenolic or furfuryl resins can advantageously be used.

In a second step, the mixture thus obtained is infiltrated onto a polymer foam, and the infiltrated foam is dried. As the polymer foam, a polyurethane foam is advantageously used, characterized in that it has an open macroscopic structure due to its cellular structure, the mean diameter of which can be selected between 600 and 4500 μm. When preparing foams for catalytic application using the process according to the invention, the preferred foams having a cell size of between 900 and 3000 μm, and preferably between 2000 and 3000 μm; a typical value is about 2500 μm.

After infiltration, particularly advantageously, the resulting foam is allowed to dry in ambient air, for example for 12 hours. This period can be reduced by increasing the temperature of the medium where the drying takes place.

Then the whole is heated to a first temperature, sufficient to cause the polymerization of the resin, and then the whole is heated to a second temperature, greater than the first, and sufficient to cause the carbonization of the resin and the foam infiltrated. In general, the polymerization temperature is between 130 ° C. and 200 ° C. and the carbonization temperature is between 500 ^° C. and 800 ° C. If a resin that can be cured with a curing agent is used, the polymerization temperature may be lower. Particularly advantageously, the polymerization of the phenolic resin is carried out at a temperature of between about 130 ^° C. and about 170 ^° C., typically at about 150 ^° C., for about two hours in air.

The solid thus obtained is carbonized by heating it to a temperature sufficient to carbonize the resin and, if present, the polymer foam and the powdered carbon precursors. This carbonization temperature is between 500 ° C. and 800 ^° C., preferably between 600 ° C. and 700 ° C., and even more preferably between 630 ° C. and 680 ^° C. By choosing a suitable heating rate, the steps of Polymerization and carbonization, which correspond to two different types of chemical reactions, can be carried out continuously, without the need for a temperature plateau at the carbonization stage. For example, this can be achieved by heating under helium flow (e.g., 100 mL min ^"1) from room temperature to about 650 ⁰ C for about 6 hours with a steady heating rate of 2 ⁰ C min ^"1 .

The solid after carbonization must then be carburized to form the structure of the β-SiC support. Carburizing can be carried out at a temperature of between 1200 ^° C. and 1450 ^° C. By way of example, under atmospheric pressure, the carburation requires a temperature of between 1300 ^° C. and 1400 ° C., and preferably between 1350 ° C. and 1400 ° C., whereas under vacuum a slightly lower temperature, for example 1200 ° C., may be suitable.

In another embodiment, under an argon stream, it is possible to typically carburize at a temperature of between 1250 ° C. and 1450 ° C.

According to an advantageous embodiment of the present invention, this β-SiC foam is used as a substrate for depositing nanotubes or nanofibers.

The advantage of a β-SiC foam is that its open structure allows to use a much higher flow rate without suffering a significant loss of load through the catalyst bed. This is known from the state of the art (see the article "Pressure drop measurements and modeling on SiC foams" by M. Lacroix et al., Published in Chemical Engineering Science (2007), doi 10.1016 / j.ces.2007.03 .027). This effect is shown in FIG. 3 which compares a β-SiC foam with β-SiC extrudates; the volume of the two materials was chosen for the same value of VSH (hourly space velocity, also called hourly volume velocity, WH). The pressure drop was calculated according to the model described in the publication by M. Lacroix et al. However, a low pressure drop is advantageous because it reduces the compression of the reaction gases, as well as the recycling of gaseous effluents in the process. Another advantage of the SiC foam, in particular the β-SiC foam, is that it makes it possible to achieve a reagent diffusion time which is very low, in particular less than the second. The cellular foam of β-SiC therefore makes it possible to obtain very high productivities, especially at high space velocity.

Moreover, β-SiC has interesting mechanical and thermal properties. Monolithic blocks, or more divided forms (granules, rods, etc.) can be made which can be used as a catalyst support in fixed bed or "fixed slurry" catalytic processes. Its very good thermal conductivity, generally much greater than that of metal oxides, makes it possible to limit hot spots on the surface of the catalyst, which are generally at the origin of undesirable side reactions. This improves the selectivity towards the intended useful products. The Applicant has found that the thermal conductivity remains very good even for foam foams synthesized according to the methods described above. Indeed, this foam is interconnected throughout the structure of the material, which thus promotes the transfer of heat throughout the material through the absence of insulating areas as could be the case in a shaped structure in the presence inorganic binder with low thermal conductivity or in a bed consisting of a stack of grains or extrudates. The absence of binders thus makes it possible to maintain the intrinsic thermal conductivity in all points of the support. In the context of the present invention, the catalyst support, if it is SiC, does not need to be pure SiC. It may contain impurities (for example an excess of carbon, or metal oxides) or additives (for example metallic elements having a catalytic effect, which may be present, for example in the metallic form, or in the form of an oxide or oxycarbide), as long as it does not interfere with the operation of the process. A β-SiC foam catalyst support may also contain a small proportion of α-SiC as an additive, for example to increase the thermal conductivity of the material. In general, a β-SiC foam that can be used in the context of the present invention must contain more than 50% by weight of silicon carbide in beta form. According to a preferred embodiment, the catalyst support contains from 50% (and preferably 70%) to substantially 100% by weight of beta-silicon carbide foam, and preferably substantially 100% of said silicon carbide, the remainder being be composed of impurities of very diverse origin: iron, aluminum, various inclusions contained in the raw materials, residual carbon linked to the excess of carbon used during the synthesis, SiO ₂ originating in particular from the surface oxide of silicon and SiC as well as the combustion under air of the excess carbon used during the synthesis, etc.

As the active phase, Group VIII metals or their compounds, for example iron or nickel, may be used conventionally, iron being preferred. The active phase content (especially in the case of iron) is generally between 1 and 10% of the final weight of the catalyst, advantageously between 2 and 5%, and more preferably between 3 and 4%. It is preferred to use iron, especially in oxide form, for an oxidation process conducted at high temperature (that is to say typically greater than 190 ^° C., and preferably at a temperature of between 230 ° C and 250 ° C), and nickel for an oxidation process conducted at a low temperature (that is to say typically between 20 ° C and 120 ° C). With nickel, the undesirable formation of SO2 is observed above 120 ° C. A temperature of between 40 ° C and 80 ° C is preferred; at 60 ° C we obtain very good results.

The deposition of the active phase can be done by known techniques as such. For example, the method of impregnating the pore volume with a salt of the metal, for example iron nitrate, may be used, the impregnating solvent possibly being water or a water / ethanol mixture or ethanol. It may be advantageous to add an agent increasing the viscosity of the solution, miscible with the impregnating solution, including glycerol. This additive will control the rate of evaporation and control the size of the precursor particles. It is also possible to use the said bi-phasic impregnation method, which is described in document EP 0208 635, in which the pore volume, essentially the hydrophilic mesopores, of the support is initially saturated with water, and then the precursor of the active phase (for example an iron salt as iron acetylacetonate) is deposited using a solvent immiscible with water (benzene or toluene). In another variant of the impregnation method, the SiC foam-based support is completely immersed in a solution containing the precursor salt of the desired active phase, then the impregnated foam is removed from the solution and dried at room temperature. the air. An optional calcination step may be added to convert the precursor salt into an insoluble species in the impregnating solution. The impregnation operation may be repeated several times until the solution containing the salt of the active phase is exhausted. The thus impregnated support is then dried. The binary mixtures of these active phases mentioned above could also be used. The catalytic activity could also be improved or stabilized by adding other promoters to better distribute the particles of the active phase over the entire surface of the support.

In a particular embodiment, the supports impregnated by the various methods described above are then treated in the following manner: drying in air at a temperature of between 90 and 120 ° C. (for example about 100 ° C.) for a period of time between 0.5 and 6 hours (for example about 2 hours), then calcination under air at a temperature of between 250 and 450 ° C. for a period of between 0.5 and 4 hours (for example about 2 hours at about 350 hours). ° C) to transform the salt into its corresponding oxide. With the catalyst according to the invention, the selective oxidation reaction of H ₂ S in elemental sulfur is generally carried out under the following operating conditions: the pressure is advantageously atmospheric pressure, the reaction temperature is between 180 ° C. C and 300 ° C (preferably between 230 ° C and 270 ° C), the ratio O ₂ / H ₂ S is between 1 and 10 (preferably between 1 and 5), the concentration of H ₂ S in the phase gas is between 0.1 and 10% by volume (and more particularly between 0.5 and 3% by volume), the concentration of water vapor is advantageously between 10% and 50% by volume, and advantageously that which typically encountered at the output of a Claus unit (that is to say typically of the order of 30% by volume), the hourly space velocity (NTP) is typically between 700 h ^-1 and 5000 h ^-1 (preferably between 1200 h ^-1 and 4000 h ^-1) . The catalyst according to the invention makes it possible to obtain oufre (expressed as% hydrogen sulfide converted to sulfur (extremely high. It is the same for the selectivity (ratio of a quantity of species formed on the amount of transformed reagent) which remains high, and especially at high temperature. Typically, a catalyst according to the invention makes it possible to achieve sulfur yields of between 80% and 99%, particularly between 90% and 95%. Such performance can be obtained according to the invention in particular thanks to the macroscopic structure of β-SiC porous alveolar foams having a large exchange surface and thanks to its porosity (meso-) specially adapted to the process. In this very preferred embodiment, the selectivity for its part is between 80% and 99%, more particularly between 91% and 97%. This excellent selectivity is attributed on the one hand to the absence of micropores in the material based on silicon carbide, which limits the diffusion phenomena, thus promoting better evacuation of the reaction product, in this case sulfur, out of the catalytic zone, and secondly to the intrinsic thermal conductivity of SiC, the latter allows a better homogenization of the temperature in the catalytic bed thus avoiding the formation of hot spots detrimental to the selectivity. This trend is all the more affirmed in the case of a related structure of foam type having a continuity of material. For the oxidation reaction of H ₂ S, this has a very significant advantage. Indeed, the oxidation reaction of H ₂ S is a strongly exothermic reaction (ΔH = - 222 kJ / mole). The heat released can be estimated by converting 1% H ₂ S into a typical flow to be treated using equation (3); this heat released is close to 60 ^° C. in a perfectly adiabatic reactor.

ATad = ^ω ° (-AH) MC

^PG (Equation 3)

In this equation, M represents the molecular weight, C _PG the heat capacity of H ₂ S, ΔH the reaction enthalpy, ω ₀ the mass fraction of H ₂ S in the incoming gas stream. However, the formation of sulfur dioxide is favored by a rise in temperature. One of the advantages of the catalyst support according to the invention is that the heat generated by the reaction is rapidly dissipated in the material due to the excellent intrinsic thermal conductivity of the SiC. This conduction is all the more favored by the related structure of cellular foams of SiC. This gives a good axial and radial dispersion of heat, which is less the case for a catalytic bed consisting of a stack of particles (spheres, extruded ...) where the inter-grain contacts are as many thermal barriers.

In other embodiments, for example using a carbon foam support, good performance is also obtained in terms of conversion efficiency and selectivity.

The catalyst support according to the invention therefore leads to a better homogeneity of the temperature over the entire catalyst. This avoids hot spots and thermal runaway. It is also important to underline that the selective oxidation reaction of H ₂ S is normally carried out in adiabatic mode and that the heat formed is evacuated exclusively by gas-solid exchanges and not by solid-solid exchanges (catalyst-wall). . The consequences of this physical property are multiple: It opens the possibility of working at higher space velocities, which leads to an increase in the number of moles converted per unit of time and, consequently, to a higher clearance heat. It opens the possibility of converting higher H ₂ S concentrations, which leads to additional heat release.

It leads to the extension of the life of the catalyst, in particular by limiting the fiïttage. The catalyst supports according to the state of the art, in particular alumina and silica, do not normally make it possible to work beyond 1.5% H ₂ S by volume, otherwise there is a risk of a rapid deactivation of the catalyst. catalyst, as well as the formation of hot spots detrimental to the selectivity.

As indicated above, in a preferred embodiment of the present invention, the foam is packed with nanotubes or nanofibers. Carbon nanotubes or nanofibers may be deposited by any known technique. We describe here a suitable process:

In a first step, a growth catalyst of nanotubes or nanofibers is incorporated in the porous SiC support. This catalyst is intended to promote the growth of carbon nanotubes or nanofibers. Advantageously, nickel is used, in particular to manufacture carbon nanofibers, or iron, cobalt or a mixture of iron and cobalt to make carbon nanotubes. Any other binary or ternary mixture of these three elements may also be used. We describe here a typical embodiment for this step. The porous SiC support is impregnated with a solution of an active phase precursor. An aqueous or alcoholic solution is suitable. The precursor may be a salt of a transition metal, for example Ni (NO ₃ ) ₂ . The metal filler is advantageously between 0.4% by mass and 3% by mass, and preferably between 0.5% and 2%. After the impregnation, it is dried in an oven, preferably at a temperature between 80 ° C and 120 ° C for 1 to 10 hours, and then calcined under air or in an inert atmosphere at a temperature between 250 ° C and 500 ° C. The active phase precursor is then converted into the active phase, preferably by reduction under reducing gas to a suitable temperature, for example between 25O ⁰ C and 500 ° C under hydrogen. The duration of this reduction is typically between 0.2 hours and 3 hours. In a second step, carbon nanotubes or nanofibers are grown from a mixture comprising at least one hydrocarbon and hydrogen. The temperature of the reaction must be between 300 ^° C. and 1000 ^° C., and is preferably between 600 ^° C. and 800 ^° C. The hydrocarbon is advantageously an aliphatic, olefinic, acetylenic or aromatic C1 to C10 hydrocarbon. The aliphatic, olefinic or acetylenic hydrocarbons may be linear or branched. C 1 to C 4 aliphatic or olefinic hydrocarbons, especially those of C 2 or C 3, are preferred. Acetylene is also suitable. Among the aromatic hydrocarbons that can be used is toluene which, mixed with ferrocene, leads, according to the findings of the present inventors, to the formation of aligned carbon nanotubes.

Thus we obtain nanofibers or carbon nanotubes. To obtain SiC nanofibers, a third step is necessary.

In this third optional step, the carbon nanotubes or nanofibers are transformed into SiC nanotubes and nanofibers by reacting them with an SiO vapor in a heat treatment chamber. The SiO vapor can be produced in the heat treatment chamber, as close as possible to the carbon structures to be converted into SiC. In one embodiment, the generation of SiO can be ensured by heating a mixture of Si and SiO ₂ placed in the vicinity of carbon nanotubes or nanofibers. In another embodiment, the carbon nanotubes or nanofibers may be embedded in an SiC precursor matrix (this term is explained below) containing, for example, a mixture of Si and phenolic resin. To obtain β-SiC, the reaction temperature is advantageously between 1000 ° C. and 1500 ^° C., preferably between 1050 ^° C. and 110 ^° C., and even more preferably between 1150 ^° C. and 1350 ^° C.

Depending on the duration of the reaction, a partial or complete conversion of carbon nanotubes or nanofibers into SiC nanofibers, and in particular β-SiC, can be obtained. The length of these nanotubes or nanofibers can reach about 1 μm or more.

The advantage of this type of cellular structure filled with nanofibres / nanotubes of SiC is on the one hand to keep the advantages of an alveolar foam of SiC that is to say: - Total control of the shape / size macroscopic;

- Control of the size of the cells (from 500 μm to 5000 μm);

- low pressure drop;

related structure promoting thermal transfer in the catalytic bed. To these advantages are added those of nanoscopic fibrous structures (from nanometer to micrometer) namely the large geometrical exchange surface (from 10 mVg to several hundred m ² / g) as well as the surface properties of these fibers which allow to facilitate the control of the particle size during the deposition of the active phase. Finally, we obtain a honeycomb structure of silicon carbide which has a low pressure drop and which develops a large specific surface area of between 10 m ² / g and 100 m ² / g, preferably between 20 m ² / g and 60 m ² / g.

The catalyst is prepared in the same manner as described for naked SiC foams. Such a support therefore makes it possible to perfectly control the size of the particles of the active phase and prevents their coalescence (filtration) under a reaction flow over time. A more active and stable catalyst is thus obtained as a function of time, without appreciable degradation of the selectivity.

Such a catalyst is operational under the same process conditions as those written for naked SiC foams. Just as for a catalyst prepared from bare SiC foam, a catalyst prepared from SiC foam filled with nanofibers / SiC tubes can work at very high space velocities (over 4000 h ^"1 ). that the output streams of a Claus unit have a space velocity of the order of 1000 to 1500 h ^-1 , such a catalyst prepared according to the invention would significantly reduce the volume of the reactor.

The following examples represent various embodiments of the present invention and thus illustrate the invention, but they do not limit it. In these examples, reference is made to the parameters VSH (hourly space velocity) and WHSV (hourly space velocity per mass of catalyst), which are known to those skilled in the art and defined as follows: flux passing over the catalyst per hour {mL I h)

Apparent volume of catalyst (mL) amount of H ₂ S per unit of time (g / h)

WHSV {h ^{~ λ} ) = mass of the catalyst (g)

EXAMPLES Example 1 In this example according to the invention, a support based on β-SiC, in the form of a monolithic foam with an average cell opening size of approximately 1100 μm and a specific surface area of 11 m ² / g, was impregnated by the porous volume method with an aqueous glycerol solution in which iron nitrate was dissolved. The mass of the precursor salt was calculated to have an iron load of 3% by weight relative to the support. The impregnated product was then dried under air at 100 ° C for 2 h and then calcined under air at 350 ° C for 2 h to convert the precursor salt to its corresponding oxide.

The selective oxidation reaction of H ₂ S in elemental sulfur was carried out under the following conditions: atmospheric pressure, reaction temperature 230 and 25O ⁰ C, apparent volume of the catalyst 3 cm ^3. , Mass of the catalyst (support + active phase) 1 g, flow rate of H ₂ S 12 to 48.5 cm ³ ./min diluted to 4% in helium, flow rate of O ₂ from 6.2 to 24.7 cm ³ ./min diluted at 20% in helium, helium diluent flow rate of 31.2 to 124.2 cm ³ .min, hourly space velocity VSH from 1000 to 4000 h ^-1 , hourly space velocity WHSV of 0.042 to 0.168 If ¹ . Table 1 illustrates the results of the catalytic test at 230 ° C.

Table 1

Table 2 illustrates the results of the catalyst at 250 ° C.

Table 2

In order to test the stability of the catalyst, the test was conducted over 265 hours and no deactivation was observed. Figure 1 summarizes the catalytic test in all the different conditions.

Example 2

In comparison, Figure 2 shows a catalytic test performed on silicon carbide extrusions according to the state of the art impregnated in the same manner as in Example 1 with the same iron load. The operating conditions were as follows: - atmospheric pressure;

reaction temperature: 230 and 250 ° C .;

apparent volume of the catalyst: 2.5 cc;

catalyst mass (support + active phase): 1.7 g;

from 30.7 to 40.9 cc / min of H ₂ S diluted to 4% in helium; from 15.6 to 28.3 cc / min of O ₂ diluted to 20% in helium;

78.7 to 104.9 cc / min of helium (diluent);

VSH ranging from 3000 to 4000 h ^-1 ;

WHSV ranging from 0.067 to 0.09 h ^-1 ;

At 25O ⁰ C we find:

With an equal mass of catalyst, the foam structure catalyst manufactured according to Example 1 makes it possible to work at higher flow rates, which is practically twice as much with respect to extrudates and with a lower pressure drop.

Claims

A process for treating a gas containing H ₂ S in sulfur, wherein said gas is contacted with a catalyst, said method being characterized in that said catalyst comprises a porous SiC cellular foam support having a surface BET specific greater than 1 m / g, and preferably greater than 5 m / g, and a density between 0.1 g / cm ³ and 0.4 g / cm ³ , and an active phase comprising at least one metal element of transition.

2. The method of claim 1, wherein said SiC foam is a β-SiC foam.

3. Method according to any one of claims 1 to 2, wherein said support has a BET specific surface area of between 5 m ² / g and 40 m ² / g, and preferably between 10 m ² / g and 25 m ² / boy Wut.

The method of any one of claims 1 to 3, wherein less than 5% by volume of the specific surface area of said support is due to micropores.

5. Process according to any one of claims 1 to 4, wherein said foam is lined with nanotubes or nanofibers.

The method of claim 5, wherein said nanotubes or nanofibers are carbon or SiC.

7. The method of claim 5 or 6, wherein the specific surface area of said support is greater than 100 m ² / g.

8. Process according to any one of claims 1 to 7, in which the active phase is nickel, if the process is carried out at a temperature not exceeding 120 ^° C., and preferably at a temperature of between 40 ° C. and 80 ° C. vs.

9. Process according to any one of claims 1 to 7, characterized in that the reaction temperature is between 180 ° C and 300 ° C and preferably between 230 ^° C and 270 ° C.

10. Process according to any one of claims 1 to 9, characterized in that the ratio O ₂ / H ₂ S is between 1 and 10, preferably between 1 and 5.

11. Method according to any one of claims 1 to 10, characterized in that the concentration of H ₂ S in the gas phase is between 0.1 and 10% by volume, and preferably between 0.5 and 3% by volume. volume.

12. The method of claim 11, characterized in that the hourly space velocity (VSH) is between 700 h ^-1 and 5000 h -1, and preferably between 1200 h ^-1 and 4000 h ^-1 .