GB2257157A - Process for the production of liquid hydrocarbons from natural gas in the presence of a catalyst based on zeolite and noble metals - Google Patents

Description

2 2, 71 5 7 C- 1 The present invention concerns a process for the

production of liquid hydrocarbons fran natural gas. More specifically this invention concerns the conversion of natural gas of which the major constituent is methane into liquid products which are easier to transport. Still more specifically this invention concerns a process which is characterised by:

(a) separating the natural gas into at least two fractions, a first fraction of the gas which is enriched with methane and a second fraction which is enriched with C 2+ alkanes (ethane, propane and higher alkanes), (b) at least partially selectively oxidising methane by means of molecular oxygen in the presence of a methane oxidising coupling catalyst, (c) at least partially mixing the fraction which is enriched with C 2+ alkanes with the effluent frcm the selective oxidation step when at least 80% of the molecular oxygen introduced in step (b) has already been consumed in step (b), (d) pyrolysing the mixture resulting from step (c), (e) after having adjusted the temperature of the mixture from step (d) to a temperature of between 30CC and 7500C and more particularly between 43CC and 54CC, at least partially converting the olef ins and possibly in part the C 2+ alkanes into aromatics in the presence of a particular catalyst containing a zeolite, one or more metals of group VIII and/or rhenium, one or more additional metals selected from tin, lead, indium and germanium, possibly an alkali or alkaline-earth metal and possibly a matrix. The composition of the catalyst is indicated hereinafter.

The basic principles of the process for the oxypyrolysis of natural gas which combines selective oxidation of methane and pyrolysis 3 0 of the C 2+ alkanes formed and added to the oxidation effluent are 2 described in three French patents (Nos 2 629 451, 2 636 627 and 2 641 531) ary-1 in an article (Appl. Catal., Vol 58 (1990) 269).

In a first step, step (a), of that process, the natural gas is separated into at least two fractions, the first being methane with a reduced content of C 2+. alkanes (ethane, propane and higher alkanes) and the second fraction being composed of higher C 2+ alkanes with a reduced methane content. A part at least of the first fraction is then mixed with oxygen (pure or enriched) to give 0.4 mole of molecular oxygen per mole of carbon. The mixture may advantageously contain substantial amounts of water vapour. The molar amount of water vapour with respect to the methane can be betwe-en 0 and 4, preferably between 0 and 2, still more preferably between 0 and 1 and more particularly between 0.05 and 0. 50. The water vapour permits handling of the oxygen/hydrocarbons mixtures with a higher degree of safety and permits a simultaneous increase in the degree of selectivity in respect of coupling products and conversion of the methane in the step for selective oxidation of the methane.

The second step in the process, step (b), comprises consuming the molecular oxygen by a selective catalytic oxidation reaction at temperatures which are at least higher than 6500C, preferably higher than 7500C and still more preferably higher than 8000C. Generally, the pressure is between 1 and 15 bars, preferably between 1 and 10 bars and more particularly between 1 and 4 bars. That reaction which is generally referred to as methane oxidising coupling is carried out in the presence of a catalyst which is stable at high temperature. Catalysts which are stable at high temperature are generally those which contain at least one refractory oxide such as magnesia, calcium oxide, strontium oxide and the other oxides which are to be found in Table 3 of the Article which appeared in Appl. Catal. Vol 67 (1990) 47.

The catalysts which are of particular interest in regard to methane 3 oxidising coupling are inter alia those described in French patent No 2 629 451 and in the articles which appeared in Appl. Catal. Vol 67 (1990) 47 and Chem. Soc. Rev., Vol 18 (1989) 251.

The selective oxidation step can be carried out in a fixed bed reactor, a moving bed reactor or a transported bed reactor. The use of a fixed bed reactor is particularly advantageous when the catalyst does not have good properties in regard to mechanical strength and for relatively low methane conversion rates per pass, for example of less than 20%. Moving bed reactors such as boiling bed reactors or transported bed reactors are highly attractive when the conversion rate per pass is for example higher than 20%. Circulation of the catalyst permits better control of the temperature involved by heat exchange between the charge, the catalyst and the effluents.

Irrespective of the reactor used for the step for the selective oxidation of methane, the selective oxidation step is highly exothermic. It is therefore very important to lower the tenperature of the effluent to limit the formation of acetylene and coke which way be formed, with the high contact times at elevated temperature. For that reason, it is often advantageous to inject into the hot effluent at least a part of the second fraction which is composed of higher C 2- falkanes with -a reduced methane content, which come from the first step. The generally paraffinic C 2+ hydrocarbons serve to lower the temperature by effecting a thermochemical quenching operation, in other words: they absorb the heat given off by the oxidation step and are transformed into olefins and hydrogen.

The third step, step (c), comprises at least partially adding the C 2+ paraffins obtained in step (a) to the effluent from the step involving selective oxidation of the methane, when at least 80% and preferably at least 95% of the molecular oxygen introduced in step (b) has already been consumed in the second step. That mode of operation has three advantages:

4 1. there is no need to operate in step (b) for the selective oxidation of methane in the presence of C2_,.hydrocarbons which are more oxidisable than methane; 2. the addition of C 2_,_ hydrocarbons to the effluent from the selective oxidation of the methane, after at least 80% and preferably at least 95% of the molecular oxygen introduced in step (b) has already been consumed, permits the use of the majority of the molecular oxygen for activation of the methane and not for dehydrogenation of the C 2 alkanes; and 3. the addition of the C2.hydrocarbons to the effluent fram the S. elective oxidation of the methane, after at least 80% and preferably at least 95% of the molecular oxygen introduced in step (b) has already been consumed, permits the transformation by absorption of heat of the C 2.t. alkanes into olefins and hydrogen, which latter can be used for the recovery of carbon, for example by hydrogenating the CO into methane.

The fourth step, step (d) comprises maintaining the temperature of the combined effluents for a sufficiently long period of time to permit preferably the production of an ethylene/ethane molar ratio of at least 1. 2 and preferably higher than 1.4. In the course of this step, the residence time is generally between 50 milliseconds and 10 seconds, preferably between 100 milliseconds and 2 seconds.

In accordance with a method for the production of liquid hydrocarbons from natural gas, which is not part of this invention, the gaseous effluents issuing from the fourth step, step (d), are adjusted to a temperature of lower than 100% ccmpressed and introduced into the separation system. The compressed gas then has water and CO 2 removed therefrorn before an operation is effected for the separation of higher hydrocarbons (C 2 C 2 =, C 3' C3 = and higher hydrocarbons) and light gases (CO, H 2' CH 4). The higher hydrdocarbons are then:

1. separated to produce olefins (ethylene, propylene and higher olefins) and alkanes which can be recycled, 2. oligomerised and/or aramatised for the production of liquid hydrocarbons and alkanes which can be recycled, or 3. dimerised to produce liquid C 4+ olefins and alkanes which can be recycled.

In accordance with the method of the present invention, the effluent coming fran the oxypyrolysis operation in the gaseous state and containing gases including hydrocarbons such as ethylene, ethane and propylene is advantageously treated over an arcmatisation catalyst.

It is in f act a matter of great attraction to impart a value to such C 2-C3 hydrocarbons as liquid products, being for example major petrochemical intermediates.

The effluents from step (d) which are then used for the charge for step (e) comprise:

1. at least 40% and at most 95% by weight of the canbination of methane plus water, 2. at least 5%, preferably at least 10%, and still more preferably at least 15% by weight of non-paraffinic hydrocarbon ccmpounds, 3. less than 1%, preferably less than 0.5% and still more preferably less than 0.1% by weight of molecular oxygen, and 4. other compounds (in variable quantities depending on the initial charge of step (a) and the operating conditions in steps (a) to (d)), such as: N 2 CO, CO 2, H 2 and C 2+ alkanes.

The fifth step, step (e), therefore comprises adjusting the effluents of step (d) to a temperature of between 30CC and 75CC and more particularly between 43CC and 540% then the effluent is brought into contact with a particular arematisation catalyst which permits at least partial transformation of the olefins into liquid hydrocarbons. Tr ansformation of such olefins and possibly at least in part of the C 2 alkanes into liquid hydrocarbons may have important consequences on the dimensions of the separation system. In addition, the grouping of a hot 6 unit such as the aromatisation unit with other hot units prior to the cold separation steps has positive consequences in terms of the capital investment required for the operational assembly for putting natural gas into a useful form. 5 The invention has two major advantages. It least partial transformation of the olefins and possibly the C 2-t- alkanes into arcoatics before passing into the separation system permits: 1. a reduction in the dimensions of the separation system, and 2. a reduction in the utilities or services required by virtue of combining a hot unit (aromatisation) with other hot units (oxidation, pyrolysis).

The following non-limiting examples illustrate different embodiments which may be employed to obtain liquid hydrocarbons from natural gas in accordance with the invention. These examples are illustrated by Figures 1 and 2.

A preferred embodiment of the present invention comprises using a boiling bed reactor for the selective oxidation and pyrolysis steps (oxypyrolysis) and a reactor of the moving bed type for the arcmatisation operation. In this embodiment, the methane-enriched gas (3) coming fran separation of the natural gas (1) in the separator (2) is mixed with the molecular oxygen (5). When it is advantageous to add water vapour to the charge, it is generally added to the methane or to the oxygen, or to the methane and the oxygen before mixing the methane with the oxygen. It is highly important to control the temperature of the gaseous mixture before it passes into the reactor. For that purpose, it is possible to heat independently the gases containing the methane (3) and the oxygen (5) or the whole of the gases (6). The gases which are preheated to a temperature which is generally lower than 750'C and preferably lower than 600"C are brought into contact with the methane oxidising coupling catalyst in the laiaer part (7a) of the 7 reactor (7) in Figure 1 or in the upstream part (7a) of the reactor (7) in Figure 2.

The catalyst for this type of procedure must enjoy good mechanical strength. The size of the particles is generally between 20 microns and 4 m in diameter. The size of the catalyst particles is variable in dependence on the operating conditions of the unit and in dependence on the density of the catalyst. Although it is not possible to mention all the catalysts for the oxidising coupling of methane, which are of potential attraction in regard to an application in a boiling bed reactor, mention way be made of the catalysts which are described in French patent No 2 629 451, in the article in Appl. Catal., Vol 76 (1990) 47, in the article in Chem. Soc. Rev., Vol 18 (1989) 251, and for example catalysts such as: BaCO 3 /Al 2 0 3 (generally but not necessarily in the presence of a few ppm of a source of chlorine in the charge); Pb/Al 2 0 3 (often in the presence of an alkali metal and/or an anion containing sulphur or phosphorus); mixed oxides containing Na, B, Mg and Mn (such as NaB2Mg4Mn 20X); La/MgO (often in the presence of one or more oxides of alkaline-earth or alkali metals and/or other oxides of lanthanides and possibly boron); the combinations Na or K, one or more oxides of alkaline-earth metals and possibly boron; perovskites MCeO 3 ( M=Sr or Ba) and mixed oxides containing thori-Lun or yttrium.

After having been subjected to selective oxidation in the reactor (7). and at a location at which at least 80% of the molecular oxygen introduced in step (b) has been consumed, a part at least of the fraction of the gas which is enriched with C 2+ alkanes (4) is added to the reactor. The addition of that fraction can be effected to the expanded bed of the catalyst (Figure 1) or at a higher level than the catalyst (such as in the disengagement zone).

After a sufficiently long residence time in the reactor (step (d) of the process) to obtain the desired propor-tion of olefins, the gases 8 are passed by way of the line (8) to an exchanger (9) in which the temperature of those gases is lowered to a temperature of between 30CC and 75CC and more particularly between 4300C and 5400C. Those effluents are then passed in Figure 1 by way of the line (10) to the aramatisation reactor (11c). If the operating pressure of the aromatisation reactor (11c) is higher than the pressure in the line (10), the effluents may advantageously be ccmpressed at that moment.

In Figure 1, the aramatisation reactor (11c), in one of the preferred embodiments, receives the charge to be arcmatised, with fresh catalyst, by way of the upper part of the reactor. In that way he catalyst which has been deactivated by coking is removed at the bottom of the reactor. The deactivated catalyst is then subjected to regeneration at Uld) before being re-introduced by way of the line (13) into the upper part of the reactor (11c). The charge which passes into the reactor is brought into contact with an arcmatisation catalyst which, in the present invention, is defined as indicated hereinafter.

The catalyst is a particular catalyst containing:

1. a zeolite, group formed by the 2. at least one metal selected from the/rietals of group VIII of the family of platinum (Ru, Rh, Pd, Os, Ir and Pt) and rhenium, 3. at least one additional metal, 4. possibly at least one alkali or alkaline-earth metal, and 5. possibly at least one matrix.

It is thus possible to use the catalyst described in French patent application No 90/06557.

That catalyst comprises a zeolite of structure MFI on the one hand, and on the other hand a generally amorphous matrix or support on which there is deposited at least one metal selected from the group formed by metals of group VIII (Ru, Rh, Pd, Os, Ir and Pt) and rhenium and at least one additional metal selected from the group formed by 9 tin, germanium, lead and indium, said support possibly containing at least one alkali metal or at least one alkaline-earth metal selected from the group formed by lithium, sodium, potassium, rubidium, caesium, barium, calcium, baryllium, magnesium and strontium.

The zeolite of structure MFI constituting a part of the catalyst of the present invention can be prepared by any procedures which are known in the prior art. Synthesis of the zeolite MFI can be effected in a conventional medium OH in the presence or absence of organic structuring agents and/or alcohol. Synthesis of zeolite MFI in an CJ medium in accordance with the procedures known in the prior art is broadly described in the following document: Synthesis of High Silica Zeolites, P Jacobs and J Martens, Studies in Surface Science and Catalysis, Volume 33, Elsevier editor, 1987. Zeolite MFI can also be synthesised in less conventional media, such as for example a fluoride medium (EP-A- 172 068 of C.F.Rj.

After synthesis, the zeolite MFI is transformed into a hydrogen form by total or partial elirnination of the organic compounds and/or the alkali or alkaline-earth cations that it possibly contains after the synthesis operation. All the procedures known in the prior art can be used for going to the hydrogen form, such as for example calcination operations with or without an oxidising atmosphere, ion exchange operations which nay or may not be followed by calcination, various chemical treatments, etc.

All the zeolites MFI which are synthesised in the Si-Al system are suitable for the present invention. However their Si/Al ratio will be higher than 7, preferably higher than 25 and still more preferably between 40 and 200.

The supports for the metals which are added to the zeolite are generally selected from oxides of the metals of groups II, III and/or IV of the periodic system of elements such as for example the oxides titanium,, taken alone or of magnesium, aluminium,/ zirconium, thorium or silicon as a mixture with each other or with oxides of other elements in the period system such as for example boron. It is also possible to use charcoal.

The preferred support is alumina; the specific surface area of the alumina may advantageously be between 10 and 600 m 2 per gram, preferably between 150 and 400 m 2 /g.

The composite catalyst (matrix-zeolite mixture) of the present invention can be prepared in two ways, the principle of which is set out below.

First way: mixture of the zeolite MFI with the support; this mixture can be produced as between two powders, the two shaped solids, a powder and one of the shaped solids. It is also possible for the two solids to be shaped in combination by all the procedures which are known in the prior art; pelletisation, extrusion, formation as tablets, coagulation as drops, or drying by atomisation. In those shaping operations, it is possible if necessary to add a shaping additive (silica etc). After mixing and/or shaping, the various active agents are then deposited on the support (in the presence therefore of the zeolite).

Second way: the active agents are first deposited on the support -which is mixed and shaped with the zeolite MFI under the same conditions as above. In an alternative procedure the zeolite may be introduced into the composite catalyst at any one of the steps involving deposit of the active agents on the support.

The preferred method of preparation comprises depositing the active agents on the support and then introducing the zeolite into the final catalyst by shaping of the two powders. The shaping operation will preferably be carried out after a micronic crushing operation which can be performed using the moist crushing procedure.

11 The composite catalyst contains between 1 and 99% by weight of zeolite, the amount to make up 100% being formed by the support (matrix) which is charged with different active agents. The respective proportion of zeolite and support varies over a wide range at it depends on the one hand on the Si/Al ratio of the zeolite and on the other hand the content of active agents in the support.

The part of the composite catalyst comprising noble metal is generally prepared using conventional methods comprising impregnating the support by mans of solutions of compounds of the metals which are to be introduced. The procedure uses either a common solution of those metals or separate solutions for the metal or metals of group VIII (Ru, Rh, Pd, Ir and Pt) and/or rhenium and for the additional metal or metals. When a plurality of solutions are used, it is possible to carry out intermediate drying and/or calcination operations. The procedure is usually terminated by a calcination operation for example at between about 500 and 1000'C, preferably in the presence of free oxygen, for example by providing an air sweep effect.

The platinum (and possibly another noble metal in the platinum group) can be incorporated into the support by impregnation of that support by means of a suitable aqueous or non-aqueous solution containing a salt or a compound of the noble metal. The platinum is generally introduced into the support in the form of chloroplatinic acid but it is also possible to use compounds such as ammonium chloroplatinate, platinum dicarbonyl dichloride, hexahydroxyplatinic acid, palladium chloride and palladium nitrate.

The elements selected from the group consisting of tin, germanium, lead and indium. may be introduced by way of compounds such as for example chlorides, bromides and nitrate of tin, halides, nitrate, acetate and carbonate of lead, chloride and oxalate of germanium, and nitrate and chloride of indium.

12 The element selected from the group consisting of alkali and alkalineearth metals can be introduced by way of compounds such as halides, nitrates, carbonates, cyanides and oxalates.

A process for the production of the matrix (support) which is charged with active metals comprises the following steps:

(a) introducing on to the support at least one element selected from the group consisting of alkali and alkaline-earth metals, (b) calcination of the product obtained in step (a), (c) introducing on to the support at least one or more metals of group VIII (Ru, Rh, Pd, Ir and Pd) and/or rhenium, in the form of at least one halogenated compound of said metal, (d) calcination of the product obtained in step (c), and (e) introducing on to the product obtained in step (b) at least one additional metal in the form of at least one organometallic compound of said metal M.

Amng the ccinpounds of the metal or metals of group VIII (Ru, Rh, Pd, Os, Ir and Pt) and/or rhenium. which are used in the present invention, mention my be made by way of example of ammoniated ccuplexes.

Mention will be made in particular in the case of platinum of the salts of platinum IV hexamnines of the formula (Pt(NH 3)6 X 4 in which X is a halogen atom selected frcm the group formed by fluorine, bramine, iodine, and preferably chlorine, the salts of platinum IV halogenopentammines of the formula (PtX(NH 3)53' and platinum IV tetrahalogenodianTnines of the formula PtX 4 M 3)2 in which X is as specified above. The complexes of platinum with halogens-polyketones and halogenated polyketonic compounds of the formula H(Pt(acac)2) in which X is as specified above and 'acac' represents the residue of the formula C 5702 derived fran acetylacetone.

The introduction of one or more metals of group VIII (Ru, Rh, Pd,

Ir and Pt) and/or rhenium. is preferably effected by impregnation by I 13 mans of an aqueous or organic solution of one of the above-mentioned organometallic compounds. Among the organic solvents which can be used:mention can be made of paraffinic, naphthenic or aromatic hydrocarbons and halogenated organic compounds having for example frcm 1 to 12 carbon atoms in their molecule. Mention will be made in particular of n-heptane, methylcyclohexane, toluene and chloroforin. It is also possible to use mixtures of solvents.

The supports are conventional supports such as those defined above and already containing for example an alkali or alkaline-earth metal.

After the introduction of one or more mtals of group VIII (Ru, Rh, Pd, Ir and Pt) and/or rhenium, the product obtained is possibly dried then calcined, preferably at a temperature of from about 400 to 10000C.

After that calcination operation, the additional metal or metals is or are then introduced, possibly prior to the introduction of said metal M, the procedure includes a reduction operation using hydrogen at high temperature, for example from. 300 to 500C. The reduction operation nay comprise for example a slow rise in temperature in a flow of hydrogen to the maximum reduction temperature which is for example between 300 and 500'C and preferably between 350 and 450'C, followed by maintaining the hydrogen atmosphere for fran 1 to 6 hours at that temperature.

The additional metal M. can be introduced before or after the introduction of the noble mtal. If it is introduced before the noble metal, the compound used will be selected from the group formed by halides, nitrates, acetates, carbonates and oxalates of the additional metal. The introduction operation will advantageously be effected in aqueous solution. In that case, before dealing with introduction of the noble mtal, a calcination operation is effected in air at a temperature of between 400 and 1000'C.

The additional metal M can be introduced after the introduction of the noble metal in the form of at least one organic ccmpound selected fran the group formed by complexes, in particular polyketonic ccmplexes, of the metals M and the hydrocarbyl metals such as alkyls, cycloalkyls, aryls, alkylaryls and arylalkyls metals.

The introduction of the metal M is advantageously effected by mans of a solution in an organic solvent of the organcmetallic compound of said metal M. It is also possible to use organchalogenated ccmpounds of the metals M. As ccmpounds; of metals M, mention will be made in particular of tetrabutyl-tin, tetramethyl-tin, tetrapropylgermanium, tetraethyl-lead, indiumacetylacetonate and triphenylindium.

The impregnation solvent is selected fran the group formed by paraffinic, naphthenic or aromatic hydrocarbons containing from 6 to 12 carbon atans per molecule and halogenated organic compounds containing frcm 1 to 12 carbon atcms per molecule. Mention nay be made of n-heptane, mthylcyclohexane, toluene and chloroform. It is possible to use mixtures of the solvents defined above.

This method of introducing the metal M has already been described in US-A-4 548 918. However the ccmbination of the method of introducing one or more metals of group VIII (Ru, Rh, Pd, Ir, or Pt) and/or Re and the method of introducing the metal M gives a particular synergy.

Preferably the part of the canposite catalyst (zeolite and matrix) containing the noble metal contains by w-eight with respect to the support: (a) about 0. 01 to 2% and more particularly about 0. 1 to 0. 5% of at least one or more metals of group VIII (Ru, Rh, Pd, Os, Ir or Pt) and/or Re, 30 (b) about 0.005 to 2%, preferably 0.01 to 0.50% of additional metal when it is tin or 0.005 to 0.70% and preferably about 0. 01 to 0.6% and more particularly 0.02 to 0.50% when the additional metal is selected from the group formed by germanium, lead and indium, (c) about 0. 01 to 4% and more particularly about 0. 1 to 0. 6% of at least one metal selected from the group formed by alkali and alkaline- earth metals and preferably lithium and potassium and mixtures thereof.

When there are at least two metals of the family tin, germanium, lead and indium, the overall content of metals of that family is about 0.02 to 1. 20% and preferably 0.02 to 1.0% and more particularly 0.03 to 0.80%.

On issuing from the operation for preparation of the part of the composite catalyst containing the noble metal, it is generally calcined at between 450 and 10000C but the catalyst, on issuing from the calcination operation, may advantageously be subjected to an activation treatment in a hydrogen atmosphere at high temperature, for example 300 to 5000C, in order to produce a more active metallic phase. The procedure involved in that treatment in hydrogen comprises for example a slow rise in temperature in a f low of hydrogen to the maximum reduction temperature, which is for example between 300 and 20 500'C and preferably between 350 and 450'C, followed by holding that temperature for a period of f rcin 1 to 6 hours. This type of preparation of the catalyst results in a solid in which the metals are distributed homogeneously throughout the entire volume of the catalyst grain and are in a metallic state after the 25 reduction treatment in a flow of hydrogen at between 300 and 500'C followed by being held at the selected final temperature in a hydrogen atmosphere for a period of from 1 to 6 hours. By way of example, a moreparticular method for preparation of the supports comprises effecting the following steps: 30 (a) impregnating an alumina support with an aqueous solution of lithium nitrate, 16 (b) drying the product obtained in step (a), (c) calcining the product obtained in step (b), (d) impregnating the product obtained in step (c) by an aqueous solution of a compound of a metal selected from the group formed by 5 tin, germanium, indium and lead, (e) drying the product obtained in step (d), (f) calcining the product obtained in step (e), (g) impregnating the product obtained in step (f) by a solution of platinum acetylacetonate in toluene, (i) - (h) drying the product obtained in step (g), (i) calcining the product obtained in step (h), and (j) reducing in a flow of hydrogen the product obtained in step Another advantageous method for preparation of the catalysts can be carried out with the following steps:

(a) impregnating an alumina support with an aqueous solution of lithium nitrate, (b) drying the product obtained in step (a), (c) calcining the product obtained in step (b), (d) impregnating the product obtained in step (c), with an armoniacal solution of platinum tetranmine chloride, (e) drying the product obtained in step (d), (f) calcining the dry product obtained in step (e), (g) reducing the product obtained in step (f) in a f low of hydrogen, (h) bringing the product obtained in step (g) into contact with a hydrocarbon solvent and with said organic compound of said metal M, for example by immersing the mass in a hydrocarbon solvent already containing the organic compound or inimrsing the mass in a hydrocarbon solvent and then injecting into the resulting mixture a solution of the organic compound of said metal M in a hydrocarbon solvent and for 17 example that in which said mass was imnersed, and W reducing the product obtained in step (h) in a f low of hydrogen.

The catalyst which is obtained by the foregoing procedures and which can possibly be subjected to a calcination treatment in air at a temperature which is generally between 3500C and 6900C is used for the aromatisation reaction of the oxypyrolysis gases. That reaction is of par-ticular attraction as it makes it possible to impart value to gaseous hydrocarbons as liquid products involving a higher added value (primarily benzene, toluene and xylenes).

The oxypyrolysis effluent charge containing, amongst other compounds, ethylene and/or ethane and/or propylene, is brought into contact with the above-described arcmatisation catalyst at a temperature of between 3000C and 7500C and more particularly between 430'C and 540'C, and with an hourly flow rate by mass of charge with respect to the weight of catalyst (PPH) of between 0.5 and 150 h-lF preferably between 1 and 80 h-l. The operating pressure will advantageously be between 1 and 18 bars, preferably between 1 and 12 bars.

The effluent fran the aron-atisation reactor Ulc) which is enriched with araTatic products is transferred by way of the conduit (12) to the separation system.

Another preferred embodiment of the invention involves using a fixed bed reactor (7) for the selective oxidation and pyrolysis steps, preferably but not necessarily in the same enclosure (here therefore there are two separate zones). In accordance with a particular procedure (see Figure 2), the arcmatisation operation can be carried out in at least one reactor which is defined by Ula) and (11b), which c an be taken 'out of circuit' to regenerate the catalyst that it contains (referred to in the English language as a 'swing reactor,). In this embodiment of the present invention, the catalysts used for the selective oxidation of methane and at least partial arcmatisation 18 of the olef ins and possibly in part of the C 2± alkanes do not necessarily have to be highly resistant to attrition. It is appropriate therefore for the catalysts to be used in the form of extrudates, grains or crushed materials. All the catalysts referred to hereinbef ore for application in a boiling bed reactor can be used for an application in a fixed bed reactor. The catalyst for this type of embodiment does not need to afford a particularly high level of mechanical strength.

When it is advantageous to add water vapour to the charge, it is generally added to the methane or to the oxygen, or to the methane and to the oxygen before the methane is mixed with the oxygen. It is highly important to control the temperature of the gaseous mixture before it passes into the reactor. For that purpose, in the case of a fixed bed reactor, the gases containing the methane (3) and the oxygen (5) are heated independently before the methane is mixed with the oxygen. Mixing as between the methane and the oxygen is effected in the chamber of the reactor (7) prior to contact with the catalyst. The mixing operation being carried out in the reactor, the conduit (6) is possibly not required for that embodiment. Nonetheless it is often advantageous for a part of the operation of preheating the gases to be carried out in the reactor. The water present in the charge is capable of receiving, by thermal radiation, the heat which is given off by the catalyst. Therefore, by virtue of that method, a part of the preheating operation can be carried out in the oxidation reactor. The gases which are preheated to a temperature which is generally lower than 750'C and preferably lower than 600'C are then brought into contact with the catalyst for oxidising coupling of the methane, in the reactor (7).

After having been subjected to selective oxidation in the reactor (7) and at a location at which at least 80% of the molecular oxygen introduced in step (b) has been consumed, at least a part of the 19 fraction of the gas which is enriched with C 2.alkanes (4) is added in the reactor. The location at which that fraction is added is generally after the catalytic bed, for reasons of simplicity of operation.

After a sufficiently long residence time (pyrolysis) to give the desired proportion of olefins (ethylene/molar ratio of at least 1.2), the gas is passed towards the arematisation reactor by way of the lines (8) and (10), through the unit (9). The assembly of the aromatisation reactor which is defined by (11a) and (11b) operates in a discontinuous mode. The part Ula) of the assembly of the reactor, for example, receives the gas to be arcmatised at a temperature and for a suf f iciently long time to transform a part at least of the olefins and possibly a part of the C2.,.alkanes into arcmatics. During a period at least of that transformation operation in the part (11a) of the reactor assembly, the part (11b) is put into the regeneration mode to remove the carbon which has been deposited on the catalyst during the preceding cycle. After that regeneration step the part (11b) is put back into service again and so forth.

The effluent which is arcmatised in the aromatisation reactor (11a and llb) is then passed to the separation system by way of the line (12).

The following examples set out the process in greater detail without however limiting the scope thereof.

EXAMPLE 1: Preparation of a zeolite MFI:

Zeolite MFI is synthesised in the presence of an organic structuring agent using one of the csitions known in the prior art (US-A-3 702 886). That zeolite is transformed into an H form by the following treatments: calcination in an air-nitrogen mixture (10% oxygen in the mixture) at 55CC for 4 hours, 30 - three exchanges in NH 4 NO 3 5N at 1000C, and calcination in air at 5300C for 5 hours at a flow rate of 6 1/h/g.

The Si/Al ratio of zeolite HMFI is equal to 49, its pore volume as measured by nitrogen adsorption at 77K is higher than 0.160 cm 3 /g.

EXAMPLE 2: Preparation of an alumina C containing by weight 0.3% of platinum, 0.3% of tin and 0.5% of lithium:

Preparation of an alumina A containing 0.50% of lithium by weight.

Alumina A is prepared by adding to 100 grams of alumina (with a specific surface area of 240 m 2 /g and a pore volume of 0. 58 an 3 /g) an 3 of an aqueous solution of lithium nitrate.

The constituents are left in contact for 6 hours, followed by drying without heating and then drying for 1 hour at a temperature of 100 to 120'C, followeed by calcination for 2 hours at 530'C.

Preparation of an alumina B containing by weight 0. 5% of lithium and 0.3% of tin.

An aqueous solution of tin acetate is brought into contact with the foregoing alumina A in a proportion of 100 cm 3 of solution for 100 grams of alumina A for a period of 6 hours.

The solid obtained is then dried without heating and then dried for 1 hour at a temperature of 100 to 1200C, then calcined at 5300C.

Preparation of an alumina C containing by weight 0. 5% of lithium, 0.3% of tin and 0.3% of platinum.

Taking the calcined solid B containing lithium and tin, this procedure then involves impregnating the platinum.

The procedure involves adding to the preceding solid B, 100 cm 3 of a solution of platinum acetylacetonate in toluene. The concentration of platinum in that solution is equal to 3 grams per litre.

The constituents are left in contact for 6 hours, followed by drying without heating, drying for 1 hour at a temperature of 100 to 1 200C and then calcination for 2 hours at 5300C. The material is then reduced in a flow of dry hydrogen for a period of 2 hours at 450'C.

21 EXAMPLE 3: Preparation of the arematisation catalyst:

The aramatisation catalyst is obtained by pelletisation of an equalweight mixture of zeolite MFI of Example 1 and solid C of Example 2.

EXAMPLE 4: Transformation of an oxypyrolysis effluent:

The catalyst of Example 3 was used for arcimatisation of an oxypyrolysis effluent (10), the ccimposition by weight of which is set out in Table 1. Hereinafter the oxypyrolysis effluent is referred to as the 'charge' and its products of transformation on the aromatisation catalyst are referred to as 'products'.

TAB LE 1 Charge % by weight C 1 54.5 C 13.7 2 C 2 5.22 C- 0.70 3 H 2 1.18 CO 2.00 CO 2 7.60 H 2 0 15.1 The aranatisation catalyst was charged into a fixed bed reactor of stainless steel in which the operating conditions were as follows: temperature: 49CC pressure: atmospheric - hourly f low rate of liquid charge equal to 35 times the weight of the catalyst.

The results in terms of percentage by weight of the products found in line (12) are set out in Table 2.

22 T A B L E 2 Products % by weight c 1 c 2 c 2 c 3 c 3 c 4 c 4 nonarcrnatic C 5+ Arcinatics H 2 CO CO 2 H,0 55.2 2.52 5.84 0.37 0.89 0.23 0.39 0.26 8.31 1.32 1.55 8.30 14.8 23

Claims (15)

1. A process for the production of liquid hydrocarbons from natural gas characterised by:

(a) separating the natural gas into at least two fractions, a first fraction of the gas which is enriched with methane and a second fraction which is enriched with C2. alkanes (ethane, propane and higher alkanes), (b) selectively oxidising at least a part of the methane by means of 10 molecular oxygen in the presence of a methane oxidising coupling catalyst, (c) mixing at least in part the fraction which is enriched with C2. alkanes with the effluent from the selective oxidation step when at least 80% of the molecular oxygen introduced in step (b) has already been consumed in step (b), (d) pyrolysing the mixture resulting from step (c), (e) after adjusting the temperature of the mixture from step (d) to a temperature of between 3OTC and 75TC, at least partially transforming the olefins and possibly in part the C2. alkanes into aromatics in the presence of an aromatisation catalyst containing: a zeolite, at least one group VIII metal of the platinum family and/or rhenium, and at least one of the following additional metals:

tin, germanium, lead and indium.

2. A process according to Claim 1 in which in the course of step (e), the temperature of the mixture from step (d) is adjusted to between 43TC and 54TC.

3. A process according to Claim 1 or 2 in which, at the discharge from step (a), a part at least of the first fraction (methane fraction) is mixed with oxygen, the content of which can be up to 0.

4 mole of molecular oxygen per mole of carbon.

24 4. A process according to Claim 3 in which the mixture contains 0 to 4 moles of water vapour per mole of methane.

5. A process according to any one of Claims 1 to 4 in which at least a part of the second fraction (C2+) obtained in step (a) is injected into the zone for selective oxidation of the methane (step (b)).

6. A process according to any one of Claims 1 to 5 in which, in the course of step (d), the temperature of the combined effluents obtained in step (c) is maintained until the production by pyrolysis of an ethylene/ethane molar ratio of at least 1.2:1, the temperature being between 800'C and 950'C, with a residence time of between 50 milliseconds and 10 seconds.

7. A process according to any one of Claims 1 to 6 in which the aromatisation catalyst further contains at least one alkali or alkalineearth metal.

8. A process according to any one of Claims 1 to 7 in which the aromatisation catalyst further contains a matrix.

9. A process according to any one of Claims 1 to 8 in which the zeolite is an MFI.

10. A process according to Claim 8 in which the catalyst contains from 1 to 99% by weight of zeolite, the amount to make up 100% being formed by the support (matrix) that is charged with active agents (metals).

11. A process according to Claim 10 in which the zeolite is an MFI.

12. A process according to any one of Claims 8 to 10 in which, by weight with respect to the matrix, the catalyst contains from 0.01 to 2% of at least one metal from the group formed by noble metals of the 35 platinum family and rhenium, from 0.005 to 2% of the said additional metal when it is tin or from 0.005 to 0.70% of the said additional metal when it is germanium, lead or indium, and from 0.01 to 4% of at least one alkali or alkaline-earth metal.

13. A process according to Claim 1 carried out in apparatus substantially as hereinbefore described in Figure 1 or Figure 2 of the accompanying drawings.

14. A process according to Claim 1, in which the catalyst in step (e) is substantially as hereinbefore described in Example 2 or 3.

15. Liquid hydrocarbons obtained by a process according to any one of Claims 1 to 14.