FR2757084A1 - USE FOR THE DIRECT OXIDATION OF SULFUR COMPOUNDS IN SULFUR AND / OR LOW TEMPERATURE SULPHATES FROM A MOLYBDENE-BASED CATALYST - Google Patents

Abstract

The invention concerns the direct oxidation at low temperature of sulphurous compounds into sulphur and/or sulphates using catalysts comprising an active phase containing molybdenum and a support with base of titanium dioxide, of zirconia, of silica, of silica-aluminium or aluminium oxide, said silica, silica-aluminium and aluminium oxide having a specific surface area of at least 20 m<2>/g and a total porous volume of at least 0.3 cm<3>/g.

Description

USE FOR DIRECT OXIDATION OF SULFUR COMPOUNDS IN
LOW TEMPERATURE SULFUR AND / OR SULPHATES
OF A MOLYBDENE-BASED CATALYST
The present invention relates to catalysts for the treatment of gases, in particular industrial gaseous effluents, containing sulfur compounds, with a view to the catalytic transformation of these compounds into easily removable products.

It relates more particularly to catalysts for the direct oxidation of hydrogen sulphide to sulfur and / or to sulphates.

The desulphurization of gases containing low concentrations of hydrogen sulphide is experiencing a notable development. This treatment is applied in different areas:
- for constraints related to environmental protection, effluents
from Claus units, also called "tail gas", are increasingly
no longer subjected to additional treatment, called tail gas treatment.

The residual compounds of these tail gases (sulfur vapor, carbon dioxide
sulfur, organic sulfur compounds: CS2, COS, ...) can be eliminated by
hydrogenation into hydrogen sulfide, then this hydrogen sulfide is transformed into
elemental sulfur and / or sulphates.

It is also possible to envisage, at the output of the last Claus reactor, to put in
implements a deliberate shift in the H2S / S02 ratio then to pass the effluent through
a catalyst for converting the remaining SO2 into sulfur via the reaction of
Claus (2 H2S + S02 3 / x Sx + 2 H2O). Then, a final direct oxidation step
H2S in sulfur and / or sulphates completes the treatment.

- deposits are also treated (natural gas, geothermal sources, etc.)
containing gases naturally having a low concentration of
hydrogen sulfide. In some cases, process-based desulfurization
Claus turns out to be difficult if not impossible and a method of direct conversion of
hydrogen sulfide in sulfur and / or sulfates is preferable.

The present invention relates to the processes implementing the final reaction of conversion of hydrogen sulphide into elemental sulfur and / or sulphates in the presence of oxygen, and in particular at a temperature below the dew point temperature of the sulfur, ie less than 200 "C. The advantage of working at such a temperature is, on the one hand, to be able to recover the sulfur in liquid or solid form in the porosity of the catalyst, and on the other hand, to displace the thermodynamic equilibrium in the direction favorable to the sulfur formation reaction.

However, the drawback of this type of process is that simultaneously with the formation of elemental sulfur, it is common to produce sulfur dioxide in parallel, which is not retained on the catalyst, resulting in a loss in the yield of the gas purification.

Thus, the catalysts of the prior art do not make it possible to ensure the best possible conversion of H2S with maximum selectivities for sulfur and / or sulfates and minimum selectivities for SO2.

An aim of the present invention is to provide a catalyst making it possible, during the direct oxidation reaction of sulfur compounds at low temperature (T <200 OC), to increase the conversion of H2S while minimizing the conversion to SO2. .

In particular, an aim of the present invention is to provide a catalyst making it possible to obtain, during the reaction defined above, a degree of conversion of H2S close to 100% and a rate of yield of SO2 close to 0%. .

With this aim, the invention relates to the use, for the direct oxidation of sulfur-containing compounds to sulfur and / or sulphates at a temperature below 200 ° C., of a catalyst comprising an active phase comprising molybdenum and a support based. of titanium dioxide, zirconia, silica, silica-alumina or alumina, said silica, silica-alumina and alumina having a specific surface area of at least 20 m2 / g and a total pore volume of at least 0 , 3 cm3 / g.

Other details and advantages of the invention will emerge more clearly on reading the description which follows.

The invention relates firstly to the use for the direct oxidation of sulfur compounds to sulfur and / or sulphates at a temperature below 200 ° C. of a catalyst comprising an active phase comprising molybdenum and a support based on dioxide. of titanium, zirconia, silica, silica-alumina or alumina, said silica, silica-alumina and alumina having a specific surface area of at least 20 m2 / g and a total pore volume of at least 0.3 cm3 / g.

The catalyst according to the invention has an active phase based on molybdenum which may be in the form of at least one oxide or a salt of molybdenum, the latter possibly being chosen, for example, from nitrate, sulphate or chloride. molybdenum, an ammonium salt, ..

Generally, the molybdenum element content of the catalyst according to the invention is at least 1% by weight relative to the catalyst, preferably at most 50%, even more preferably between 2 and 30%.

The active phase of the catalyst according to the invention may further comprise at least one element other than molybdenum chosen from: iron, titanium, nickel, cobalt, tin, germanium, gallium, ruthenium, I antimony, niobium, manganese, vanadium, chromium, phosphorus, zinc, bismuth, alkali group, alkaline earth group, rare earth group and yttrium.

Like molybdenum, these elements can be present in the form of oxides or salts such as sulfates, nitrates, chlorides,
The content of element (s) other than molybdenum may be at most 40% by weight relative to the catalyst, preferably at most 30%, even more preferably at most 20%.

The support for the catalyst according to the invention may be based on alumina, zirconia, silica, silica-alumina or titanium dioxide.

The alumina, silica or silica-alumina of the catalyst according to the invention have a specific surface area (SS) of at least 20 m2 / g, preferably of at least 40 m2 / g.

This specific surface is a surface measured by the BET method.

By surface measured by the BET method is meant the specific surface determined by nitrogen adsorption in accordance with the ASTM D 3663-78 standard established from the BRUNAUER - EMMETT - TELLER method described in the periodical "The
Journal of the American Society ", 60, 309 (1938).

The alumina, silica or silica-alumina of the support of the catalyst according to the invention also have a total pore volume (TPV) of at least 0.3 cm3 / g, preferably at least 0.4 cm3 / g. This VPT is measured as follows: the value of the grain density and of the absolute density is determined: the grain (Dg) and absolute (Da) densities are measured by the picnometric method respectively with mercury and with helium, the VPT is given by the formula:
1 1
Dg - Da
Usually the catalysts according to the invention are in the form of balls, however any form can be envisaged, in particular extruded, crushed, monoliths, honeycombs,
These catalyst supports can be obtained by any technique known to those skilled in the art.

With regard to alumina, the alumina powder used as a starting material for the preparation of the support can be obtained by conventional methods such as the precipitation or gel method, and the method by rapid dehydration of a hydroxide d. alumina such as Bayer's hydrate (hydrargillite). This latter alumina is the preferred one of the invention.

If these are alumina beads, they may be obtained from shaping by coagulation into drops. This type of beads can for example be prepared according to the teaching of patents EP-A-0 015 801 or EP-A-0 097 539. The porosity control can be carried out in particular according to the process described in patent EP- A-0 097 539 by coagulation in drops of a suspension or of an aqueous dispersion of alumina or of a solution of a basic aluminum salt in the form of an emulsion consisting of an organic phase, an aqueous phase and a surfactant or an emulsifier. Said organic phase can in particular be a hydrocarbon, the surfactant or emulsifier is, for example, Galoryl EM 10.

The supports in the form of balls can also be obtained by agglomeration of an alumina powder. The agglomeration in the form of balls is carried out directly on the alumina powder by rotating technology. By rotating technology is meant any device in which the agglomeration is carried out by contacting and rotating the product to be granulated on itself. As an apparatus of this type, mention may be made of the rotating bezel, the rotating drum. This type of process makes it possible to obtain beads of controlled dimensions and pore distributions, these dimensions and these distributions being, in general, created during the agglomeration step. The control of the volumes of the pores of given diameter can also be carried out during this agglomeration step by an adequate adjustment of the rate of introduction of the alumina powder and optionally of water, of the speed of rotation of the apparatus or when introducing a shaping primer.

If the supports are extrudates of alumina, they can be obtained by mixing and then extruding an alumina-based material, said material possibly resulting from the rapid dehydration of hydrargillite or from the precipitation of boehmite or pseudi-boehmite. alumina. The porosity of the extrudates can be controlled by the operating conditions for mixing this alumina before extrusion. The alumina can also be mixed during kneading with porogens. By way of example, the extrudates can be prepared by the preparation process described in US-A3,856,708.

If the alumina supports are crushed, they can be obtained from the crushing of any type of alumina-based material such as, for example, balls obtained by all types of process (coagulation in drops, bezel or rotating drum) , extrudates, etc. The porosity of these crushed products is controlled by the choice of the alumina-based material which is crushed to obtain them.

Whatever the form of the alumina supports, the porosity can be created by various means such as the choice of the particle size of the alumina powder or the mixture of several alumina powders of different particle sizes. Another method consists in mixing with the alumina powder, before or during the agglomeration or extrusion steps, a compound, called a porogen, which disappears completely on heating and thus creating porosity in the supports.

As pore-forming compounds used, there may be mentioned, by way of example, wood flour, charcoal, sulfur, tars, plastics or emulsions of plastics such as polyvinyl chloride, polyvinyl alcohols, mothballs or the like. The amount of added pore-forming compounds is not critical and is determined by the desired pore volume.

The support may also include additives to facilitate shaping and additives to improve the final mechanical properties.

In the preparation of the catalyst of the invention, use may be made of the additives conventionally used in shaping techniques. These additives give the paste obtained by mixing the rheological properties suitable for shaping. By way of example, mention may be made in particular of: cellulose, carboxymethylcellulose, carboxyethylcellulose, tall oil, xanthan gums, surfactants, flocculants such as polyacrylamides, carbon black, starches, Stearic acid, polyacrylic alcohol, polyvinyl alcohol, biopolymers, glucose, polyethylene glycols, etc. The amount of these additives can vary between 0 and 15% by weight relative to the weight of the catalyst.

In addition, it is possible to use additional constituents capable of improving the mechanical qualities of the formulations. These constituents can be chosen from clays, silicates, alkaline earth sulphates, ceramic fibers, asbestos,. . These constituents can be used in amounts by weight relative to the support of up to 99.5% by weight, particularly up to 60%, preferably up to 30%.

Following their shaping, the alumina supports obtained can be subjected to various operations intended to improve their mechanical resistance such as curing by keeping in an atmosphere with a controlled humidity level, followed by calcination and then by an impregnation of the supports with a solution of one or more acids and a hydrothermal treatment in a confined atmosphere.

Finally, after these different treatments, the supports are dried and calcined.

When the catalyst support is based on titanium dioxide, it can be obtained by any method of preparation known to those skilled in the art, for example according to the teaching of documents EP-A-038741 and EP-A-060741.

The catalyst according to the invention can be in the form of beads with a diameter of between 1 and 10 mm, preferably between 1 and 6 mm, or in the form of extrudates with a cross section of between 0.7 and 5 mm, preferably between 1 and 3 mm.

The deposition of the catalytic phase on or in the support can be carried out by any method known to those skilled in the art. It can be carried out, for example, by impregnating the support already prepared with the catalytically active elements or precursors of these elements, or by mixing the catalytically active elements or the precursors with alumina or titanium dioxide during the setting. in the form of these materials. In the case of alumina, the deposition of the catalytic phase in the support can also be carried out by coprecipitation of the alumina and of the catalytically active elements or their precursors.

In the case of deposition by impregnation, this is done in a known manner by bringing the support into contact with a solution, a sol or a gel comprising at least one catalytically active element in the form of an oxide or a salt or 'one of their precursors.

The operation is generally carried out by soaking the support in a determined volume of solution of at least one precursor of a catalytically active element. By solution of a precursor of a catalytically active element is meant a solution of a salt or compound of the element or of at least one of the elements constituting the catalytic phase, these salts and compounds being thermally decomposable.

The salt concentration of the solution is chosen as a function of the quantity of active phase to be deposited on the support.

The active phase impregnation surface is determined by the volume of solution adsorbed. Thus, the adsorbed volume of the catalytic phase is equal to the total pore volume of the support to be impregnated. It is also possible to impregnate the support by soaking it in the solution of catalytically active element precursor and to remove the excess solution by draining.

According to a preferred embodiment, the active phase is deposited by dry impregnation.

The support can then be subjected to a drying and optionally calcination operation. For example, the catalyst can be calcined at a temperature between 200 and 1000 ° C, preferably between 300 and 800 ° C.

It is also possible to repeat these operations with the same support after having dried and calcined it and to successively deposit several elements on the support and on determined surfaces, which may vary.

Thus, according to one variant, it is possible to impregnate an alumina support with a molybdenum compound and to calcine it. This support can then be treated with another element as defined above.

When the deposition of the active phase is carried out during shaping, the catalytically active elements or their precursors are mixed with the raw material of alumina or titanium dioxide before its shaping.

The invention also relates to the use of the catalyst described above for the direct oxidation of sulfur compounds to elemental sulfur. This use makes it possible in particular to treat effluents containing quantities of H2S of less than 2% by volume.

The contact times of the reaction medium with the catalyst can range from 0.5 to 20 s, preferably from 1 to 10 s, or even from 2 to 8 s, these values being given under standard conditions of pressure and temperature, preferably, the direct oxidation is carried out at a contact time of at least 3 s.

Firstly, the gas to be treated with the addition of oxygen is passed over the catalyst according to the invention at a temperature of between 50 and 200 "C, preferably between 100 and 170" C.

Secondly, the sulfur-laden catalyst is swept away using an oxygen-free gas at a temperature between 200 and 500 "C.

This process is particularly applicable to the treatment of gas mixtures with a low H2S content, because the reaction is very exothermic and a significant rise in temperature would have the consequence, on the one hand, of the vaporization of at least part of the sulfur produced by the process. the reaction and on the other hand the oxidation of the H2S would be less selective: there would be larger quantities of S 2 formed.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLES
Preparation of catalysts according to the invention
The characteristics of the supports used are collated in Table 1.

Table 1

<tb><SEP> Surface <SEP> Volume <SEP> Granulometry <SEP> V <SEP> 100A <SEP> V <SEP> 0, îiim <SEP>
<tb><SEP> Specific <SEP> type <SEP> porous <SEP> total <SEP> (mm) <SEP> (cl3 / 9) <SEP> (cm3 / g) <SEP>
<tb><SEP> (mCg) <SEP> (cm) <SEP>
<tb> 1 <SEP> Alumina <SEP> balls <SEP> 192 <SEP> 0.71 <SEP> 2.0 <SEP> - <SEP> 3.2 <SEP> 0.45 <SEP> 0 , 29
<tb> 2 <SEP> Alumina <SEP> balls <SEP> 114 <SEP> 1.05 <SEP> 2.0 <SEP> - <SEP> 3.2 <SEP> 0.92 <SEP> 0 , 34
<tb> 3 <SEP> Extruded <SEP> 231 <SEP> 0.60 <SEP> 1.6 <SEP> 0.37 <SEP> 0.03
alumina <tb><SEP>
<tb> 4 <SEP> Extruded <SEP> from <SEP> 124 <SEP> 0.36 <SEP> 3.5 <SEP> 0.27 <SEP> 0.11
<tb><SEP> TiO2
<tb>
Different catalysts are produced by impregnating these supports with different salts.

Impregnation is carried out dry. The drying and calcining conditions depend on the nature of the impregnation salt and are indicated below:
- sulphate: drying: 15 h at 120 0C
calcination: 3 h at 350 "C
- nitrate: drying: 15 h at 85 "C
calcination: 3 h at 500 "C
- acetate: drying: 15 h at 120 0C
calcination: 3 h at 400 "C
- ammonium heptamolybdate: drying: 15 h at 120 "C
calcination: 3 h at 500 "C
The salts used for the different metals are as follows:
- molybdenum: ammonium heptamolybdate: (NH4) 6Mo7024
- iron: sulfate or nitrate
- cobalt sulfate
- nickel: sulphate or nitrate
- manganese: acetate
- calcium: sulphate
- phosphorus: phosphoric acid
The beads impregnated with both molybdenum and another element were first impregnated with this other element, then they were dried at 85 ° C for 15 h.

They were then impregnated with ammonium heptamolybdate, then dried at 120 ° C for 15 h and calcined at 500 ° C for 3 hours.

Catalytic test
The catalytic test protocol takes place in three stages.

The catalyst is first subjected to pre-aging in order to test it under normal conditions of use. This pre-aging is a presulfation at 140 "C by passage for 20 hours of an effluent whose average composition by volume is:
1.5% H2S
2.4% O2
30% H2O
66.1% of N2
The catalyst is then subjected to regeneration at 300 ° C. by passing an effluent for 6 hours, the average composition of which by volume is:
30% of H20
70% of N2
then passage for 8 hours of another effluent whose average composition by volume is:
2.6% H2S 30% H20
67.4% of N2
Finally, the direct oxidation reaction is carried out at 140 ° C. by passing an effluent comprising:
2500 ppm H2S
4000 ppm O2
30 9G H20
69.35% of N2.

During this last step, the conversion of H2S and the yield p of SO2 are measured for a reaction time of 3 hours and a contact time of 4 s or 2 s.

The results are collated in Table 2.

Table 2

<tb><SEP> Time <SEP> of <SEP> contact: <SEP> 4 <SEP> s <SEP> Time <SEP> of <SEP> contact: <SEP> 2 <SEP> s
<tb><SEP> Composition <SEP> of
<tb><SEP> catalyst <SEP> a <SEP>: <SEP> conversion <SEP>, 5: <SEP> yield <SEP> a <SEP>: <SEP> conversion <SEP>'3:<SEP> yield <SEP>
<tb><SEP> (nature <SEP> of <SEP> salt <SEP> of <SEP> H2S <SEP> in <SEP> SO2 <SEP> of <SEP> H2S <SEP> in <SEP> SO2
<tb><SEP> impregnation)
<tb><SEP> BALLS <SEP> 1 <SEP> (alumina)
<tb><SEP> 3.8 <SEP>% <SEP> Ni <SEP> (sulfate) <SEP> 98 <SEP> 22 <SEP> 96 <SEP> 26
<tb><SEP> 3.8 <SEP>% <SEP> Co <SEP> 99 <SEP> 34 <SEP> 99 <SEP> 41
<tb><SEP> 3,5 <SEP>% <SEP> Mn <SEP> 35 <SEP> 4 <SEP> 33 <SEP> 4
<tb> 1.9 <SEP>% <SEP> Fe <SEP> (sulfate) <SEP> 97 <SEP> 23 <SEP> 86 <SEP> 16
<tb> 6.9 <SEP>% <SEP> Fe <SEP> (sulfate) <SEP> 97 <SEP> 81 <SEP> 91 <SEP> 41
<tb><SEP> 5.9 <SEP>% <SEP> Mo <SEP> 100 <SEP> 0 <SEP> 64 <SEP> 0
<tb><SEP> 15.2 <SEP>% <SEP> Mo <SEP> 93 <SEP> 0 <SEP> 66 <SEP> 0
<tb><SEP> 5.9 <SEP>% <SEP> Mo <SEP> 100 <SEP> 25 <SEP> 99 <SEP> 27
<tb> 3.3 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> 10.7 <SEP>% <SEP> Mo <SEP> 100 <SEP> 4 <SEP> 100 <SEP> 13
<tb> 1.6 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> 18.7 <SEP>% <SEP> Mo <SEP> 100 <SEP> 8 <SEP> 100 <SEP> 12
<tb> 0.7 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> 15.0 <SEP>% <SEP> Mo <SEP> 100 <SEP> 12 <SEP> 92 <SEP> 13
<tb> 0.5 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> 14.8 <SEP>% <SEP> Mo <SEP> 83 <SEP> 4 <SEP> 59 <SEP> 1
<tb><SEP> 1.0 <SEP>% <SEP> P
<tb><SEP> BALLS <SEP> 2 <SEP> (alumina)
<tb><SEP> 8,2 <SEP>% <SEP> Mo <SEP> 99 <SEP> 9 <SEP> 100 <SEP> 4
<tb><SEP> 2.1 <SEP>% <SEP> Ni <SEP> (nitrate)
<tb><SEP> EXTRUDES <SEP> 3 <SEP> (alumina)
<tb><SEP> 15.1 <SEP>% <SEP> Mo <SEP> 100 <SEP> 8 <SEP> 96 <SEP> 11
<tb> 0.29 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> 15.0 <SEP>% <SEP> Mo <SEP> 100 <SEP> 5 <SEP> 100 <SEP> 15
<tb> 0.58 <SEP>% <SEP> Fe <SEP> (nitrate)
<tb><SEP> EXTRUDES <SEP> 4 <SEP> (titanium dioxide <SEP><SEP>)
<tb> 3.9 <SEP>% <SEP> Fe <SEP> (nitrate) <SEP> 100 <SEP> 50 <SEP> 97 <SEP> 43
<tb><SEP> 4.2 <SEP>% <SEP> Ca
<tb><SEP> 6.0 <SEP>% <SEP> Mo <SEP> 100 <SEP> 0 <SEP> 67 <SEP> 0
<tb><SEP> 4.1% <SEP> Ca
<tb>
It is found that practically all the catalysts based on molybdenum make it possible to obtain a good total conversion of H2S, while minimizing the formation of SO2.

On the contrary, the catalysts of the prior art exhibit either a good conversion of H2S but simultaneously a high yield of SO2 (such as those based on Co, Fe and Ni); or a low yield of SO2 but poor conversion of H2S (such as those based on Mn).

Claims (7)

1. Use for the direct oxidation of sulfur compounds to elemental sulfur and / or to sulphates at a temperature below 200 "C of a catalyst, characterized in that said catalyst comprises an active phase comprising molybdenum and a support based on titanium, zirconia, silica, silica-alumina or alumina dioxide, said silica, silica-alumina and alumina having a specific surface area of at least 20 m2 / g and a total pore volume of at least 0, 3 cm3 / g. 2. Use according to claim 1, characterized in that the active phase of the catalyst comprises at least one oxide or a salt of molybdenum. 3. Use according to any one of claims 1 or 2, characterized in that the catalyst is in the form of beads with a diameter of between 1 and 10 mm, preferably between 1 and 6 mm. 4. Use according to any one of claims 1 to 2, characterized in that the catalyst is in the form of extrudates with a cross section of between 0.7 and 5 mm, preferably between 1 and 3 mm. 5. Use according to any one of claims 1 to 4, characterized in that the molybdenum element content is at least 1% by weight, preferably at most 50% by weight relative to the catalyst. 6. Use according to any one of claims 1 to 5, characterized in that the active phase of the catalyst comprises at least one element other than molydene chosen from: iron, titanium, nickel, cobalt, tin. , germanium, gallium, ruthenium, antimony, niobium, manganese, vanadium, chromium, phosphorus, zinc, bismuth, the alkali group, the alkaline earth groups, the alkaline earth group. rare earths and yttrium. 7. Use according to claim 6, characterized in that the content of element (s) other than molybdenum is at most 40% by weight relative to the catalyst.