Process for the conversion of a synthesis gas to hydrocarbons in the presence of beta-SiC and effluent from this process

The present invention relates to a process for the conversion of synthesis gas to C 2 + hydrocarbons by Fischer-Tropsch synthesis. It also relates to a Fischer-Tropsch synthesis process which produces an effluent exhibiting a specific distribution of the liquid fraction, and to this effluent.

Numerous documents are known which describe the Fischer-Tropsch (FT) synthesis using metal catalysts supported on various catalytic supports.

The documents WO-A-01/12323 and WO-A-01/54812 (Battelle) mention carbides generically as support or as interfacial layer for catalytic systems for the FT synthesis. There is no mention of the type of carbide which may be used.

U.S. Pat. Nos. 5,648,312, 5,677,257 and 5,710,093 (Intevep) disclose a specific catalytic support suitable for the FT synthesis (capable of being employed in a fixed bed, an ebullating bed or as a slurry). The catalytic entity in these documents is a metal from Group IVB or VIII, or a mixture, in particular zirconium and cobalt.

This support is obtained in particular by the formation of a suspension of silica and of silicon carbide in a basic solution, the formation of drops and then their separation as spheres, and then the transfer of the spheres into an acidic solution to result in a catalytic support comprising a substantially homogeneous mixture of silica and of SiC. Processes for the synthesis of hydrocarbons from synthesis gas use this catalytic support, which is a homogeneous mixture of particles of silica and of SiC, in an amount of 10 to 50% of the support, the support having a specific surface of at least approximately 30 m 2 /g (preferably of greater than 40 m 2 /g), mean pore diameters of at least approximately 150 Å and a particle size of at least 0.1 mm. From the synthesis, the SiC necessarily has the alpha form in these documents, the binder of this SiC being the silica. The specific surface is conferred by the silica binder rather than by the form of the SiC; the latter is used in fact more as a heat sink rather than as a support as such.

The presence of silica and/or of alumina as binder can have a negative effect on the thermal conductivity of the support. In addition, this binder, as support, additionally exhibits the disadvantage of reacting with the catalytic entity, thus resulting in a loss in the activity over time.

The invention provides a novel FT synthesis process using a novel catalytic support comprising β-SiC. A support is thus obtained which, in comparison with the supports of the prior art, is particularly suitable for the exothermic reaction of the FT synthesis.

Thus, the invention provides a process for the conversion of carbon monoxide to C 2 + hydrocarbons in the presence of hydrogen and of a catalyst comprising a metal and a support comprising silicon carbide, characterized in that the support comprises more than 50% by weight of silicon carbide in the beta form.

A further subject-matter of the invention is a process for the conversion of carbon monoxide to C 2 + hydrocarbons in the presence of hydrogen and of a catalyst, characterized in that the hydrocarbon effluent comprises more than 70 mol % of a mixture comprising from 50 to 90 mol % of C 6 to C 12 hydrocarbons and from 10 to 50 mol % of C 13 to C 24 hydrocarbons.

A further subject-matter of the invention is a C 2 + hydrocarbon effluent comprising hydrocarbons, methane and CO 2 which comprises more than 70 mol % of a mixture comprising from 50 to 90 mol % of C 6 to C 12 hydrocarbons and from 10 to 50 mol % of C 13 to C 24 hydrocarbons.

The invention is described in more detail with reference to the appended drawings, in which:

FIG. 1A gives the results of the yields of the various fractions as a function of the time under flow for a first embodiment;

FIG. 1B gives the distribution of the hydrocarbons within the liquid fraction for this first embodiment;

FIG. 2A gives the results of the yields of the various fractions as a function of the time under flow for a second embodiment;

FIG. 2B gives the distribution of the hydrocarbons within the liquid fraction for the second embodiment;

FIGS. 3A and 3B give the results of the yields of the various fractions as a function of the time under flow for third and fourth embodiments;

FIG. 4A gives the results of the yields of the various fractions as a function of the time under flow for a fifth embodiment;

FIG. 4B gives the distribution of the hydrocarbons within the liquid fraction for this fifth embodiment.

The beta-SiC is prepared by a gas/solid reaction between intimately mixed (without liquid) SiO vapour and solid carbon. For further details with regard to the β-SiC, reference may be made to the following patent applications and patents, incorporated by reference in the present application: EP-A-0 313 480, EP-A-0 440 569, U.S. Pat. No. 5,217,930, EP-A-0 511 919, EP-A-0 543 751 and EP-A-0 543 752. In comparison with the alpha form, the β-SiC is characterized in particular in that it exists in the pure state without binder. The crystals are of face-centred cubic type. In general, the specific surface of the β-SiC is between 5 and 40 m 2 /g and preferably between 10 and 25 m 2 /g.

The β-SiC can be prepared in the form of a powder, grains, extrudates (without binder), foam, monolith, and the like. The size of the SiC can vary according to the type of process employed (fixed bed, ebullating bed, slurry bed). It is thus possible, according to one alternative form, to use a size of between 0.1 and 20 mm, preferably between 1 and 15 mm. According to another alternative form, it is possible to use a size of between 1 and 200 μm, preferably between 5 and 150 μm.

This β-SiC has very good mechanical properties. Because of its very good thermal conductivity, generally much greater than that of metal oxides, hot spots are limited to the surface of the catalyst. The selectivity is thus improved.

According to one embodiment, the support of the catalyst comprises from 50 to 100% by weight of beta silicon carbide in the particulate state and preferably 100% of the said silicon carbide.

Use may conventionally be made, as main metal catalyst, of metals from Group VIII, for example in particular cobalt, iron or ruthenium, cobalt being particularly preferred. Use may also be made conventionally, at the same time, of a promoter. Mention may be made, among the promoters, of another metal from Group VIII or also of metals chosen from the group consisting of Zr, K, Na, Mn, Sr, Cu, Cr, W, Re, Pt, Ir, Rh, Pd, Ru, Ta, V, Mo and their mixtures. Mo is preferred.

The content of main metal, in particular cobalt, is conventionally greater than 5%, typically between 10 and 50%, of the final weight of the catalyst, in particular between 20 and 35% by weight. The content of promoter, in particular molybdenum, is conventionally between 0.1 and 15% of the final weight of the catalyst, in particular between 2 and 10% by weight. A primary metal/promoter ratio by weight is conventionally from 10:1 to 3:1.

The catalytic metal is deposited conventionally. For example, use may be made of the impregnation of the pore volume by a salt of the metal, for example cobalt nitrate. Use may also be made of the evaporated drop (also known as egg shell) method, by dropwise addition of a metal salt solution at ambient temperature to a support at high temperature, resulting in deposition essentially at the surface, for example a cobalt nitrate solution under air to a support at 200° C.

The catalytic bed can be fixed, ebullating or as a slurry. A fixed bed will be preferred.

The Fischer-Tropsch synthesis reaction is generally carried out under the following operating conditions:

total pressure: 10 to 100, preferably 20 to 50, atmospheres; reaction temperature: 160 to 250° C., preferably 180 to 220° C.; GHSV varying from 150 to 5000 h −1 , preferably from 200 to 1000 h −1 ; H 2 /CO ratio of the starting synthesis gas of between 1.2 and 2.8, preferably between 1.7 and 2.3.

The present invention also relates to an FT synthesis process which produces a hydrocarbon effluent which comprises at least 70 mol % of C 5 to C 25 hydrocarbons and in which the distribution is centred around relatively light hydrocarbons, approximately from C 7 to C 10 . This type of distribution is not conventional, in particular for a process carried out in a fixed bed.

According to one embodiment, the hydrocarbon effluent comprises more than 70 mol % of a mixture comprising from 50 to 90 mol % of C 6 to C 12 hydrocarbons and from 10 to 50 mol % of C 13 to C 24 hydrocarbons.

According to one embodiment, the hydrocarbon effluent comprises at most 10 mol % of branched hydrocarbons and olefins and/or less than 2 mol % of C 1 to C 20 alcohol.

According to one embodiment, in the hydrocarbon effluent, the content of methane and of CO 2 is less than 20 mol %.

Another subject-matter of the invention is this specific FT synthesis effluent.

In the patent application, the ratios are molar, unless otherwise mentioned.

The following examples illustrate the invention without limiting it.