A METHOD OF CONVERTING NATURAL GAS INTO FUELS

FIELD OF APPLICATION

The method of converting natural gas into fuels is applicable to the catalytic conversion of natural gas in the presence of a sorbent-catalyst into a mixture of hydrogen and carbon oxide (synthesis gas), which is a raw material for producing synthetic fuels and other chemical products.

PRIOR STATE OF THE ART

It is known that the mixture ofrom hydrogen and carbon oxide (synthesis gas) is widely used in chemical processes such as the synthesis of methanol, higher alcohols and aldehydes, in the production of synthetic motor fuels (Fischer- Tropsch process). A synthesis gas with determinate ratio of hydrogen and carbon oxide (H ₂ /CO) is needed for each of these processes. Reactions of catalytic and thermal conversion of the paraffins are used for obtaining a mixture of hydrogen and carbon oxide with one or another ratio of H ₂ /CO. (Gas Chemistry in the 21 century: Problems and Prospects, Proceedings of the Moscow Seminar on Gas Chemistry 2000 - 2002, pp. 138-141, Moscow, 2003). The vapor conversion of natural gas (methane) is most widely applied in both its catalytic and non-catalytic variants, where a synthesis gas with a ratio of H ₂ /CO > 3 is formed, which is suitable only for processes of ammonia synthesis. In addition, disadvantages of this process are the high original cost of the necessary overheated vapor and the formation of excessive quantities of carbon dioxide.

The conditions, at which the process of vapor conversion takes place, are as follows:

- for catalytic conversion - 1 ~ 850 ⁰ C, p ~ 1 - 4 MPa and a catalyst of Ni;

- for non-catalytic conversion - 1 ~ 1,000 - 1,600 ⁰ C, p < 0.5 MPa;

CH ₄ + H ₂ O + 206 KJ → CO + 3H ₂

In the carbon-acid conversion of methane, a mixture of hydrogen and carbon oxide in the ratio of H ₂ /CO ~ 1 is obtained, which is required in the reaction of formaldehyde production.

CH ₄ + CO ₂ + 247 KJ2 → 2CO + 2H ₂

The conditions for realizing the process of carbon-acid conversion are as follows: t ~ 750 - 850 ⁰ C, p ~ 2 MPa, a catalyst of Ni and Ni-containing compounds.

3CH ₄ + 2H ₂ O + CO ₂ + 659 KJ4 → 2CO + 8H ₂

The conditions for realizing the process of vapor-oxygen conversion are as follows: t K 900 - 950 ⁰ C, p ~ 2 - 4 MPa and a catalyst of Ni.

2CH ₄ + 1/2O ₂ + H ₂ O + 169 KJ2 → 2CO + 5H ₂

A synthesis gas in the ratio of H ₂ /CO = 2.5 is obtained in the vapor-oxygen conversion.

The reactions of the vapor, vapor-oxygen and carbon-acid conversions of methane are endothermal and accompanied by a process of coke formation, because of which they require high consumption of energy. Significant capital investments are also required for the equipment necessary for producing synthesis gas and they represent from 30 % to 70 % of the production costs, e. g. the costs of such a production as that of methanol and synthetic motor fuels (Oil and GazEurasia, p. 85, No. 9, 2003).

A method of producing synthesis gas at the ratio of H ₂ /CO ~ 2 by selective catalytic oxidation of hydrocarbons with oxygen is known as well (S. C. Tsang, J. B. Claridge and M. L. H. Green, Recent Advances in the Conversion of Methane to Synthesis Gas, Catalysis Today, 1995, V. 23, pp. 3-15). In contrast to the vapor and carbon-acid conversions, the selective catalytic oxidation (SCO) of hydrocarbons is conducted with greater selectivity; i. e. a smaller quantity of byproducts is obtained. In addition, the process is exothermal and runs efficiently for short time periods of contact, which permits its realization in autpthermal regime and allows reducing the size of the reactor, and this, in its turn, diminishes both the

energy costs and the capital expenses (D. A. Hickman, L. D. Schmidt, Synthesis Gas Formation by Direct Oxidation of Methane in Catalytic Selective Oxidation, ACS Symposium Series, 1993, pp. 416-426; P. M. Torniainen, X. Chu and L. D. Schmidt, Comparison of Monolith-Supported Metals for the Direct Oxidation of Methane to Syngas. J. Catal., 1994, V. 146, pp. 1-10). Carrying out concurrently the exothermal reaction of SCO and the endothermal vapor conversion of natural gas with the help of the same catalyst permits realizing a process of obtaining a mixture of hydrogen and carbon oxide enriched with hydrogen in an autothermal regime. (J. W. Jenkins and E. Shutt, The Hot Spot ™ Reactor, Platinum Metals Review, 1989, V. 33, 3, pp. 118-127).

Studying the process of SCO of methane in a pilot installation with block catalyst containing Pt-Pd (L. K. Hoshmuth, Catalytic Partial Oxidation of Methane over Monolith-Supported Catalyst, Appl. Catal., B: Environmental, V. 1, 1992, pp. 89) indicates that for contact time ~ 0.02 s complete oxidation of methane is effected in the front layer of the block, and vapor and carbon-acid conversions of methane in the next layers. For this reason, to obtain maximum quantity of synthesis gas the catalyst should be concurrently active in these three reactions. Accordingly, a catalyst with large total surface area is required for efficient performance of the slow reactions of methane conversion. At the same time, owing to the high gradient of the temperature along the length of the block, the catalyst should be characterized by high thermal resistance.

To perform the process of SCO for short contact times - 10 ^"2 S, grids of Pt- Rh or a block carrier containing 10 % of Rh are used, which is very expensive and economically ineffective. (Synthesis Gas Formation by Direct Oxidation of Methane in Catalytic Selective Oxidation, ACS Symposium Series, 1993, pp. 416- 426; P. M. Torniainen, X. Chu and L. D. Schmidt, Comparison of Monolith- Supported Metals for the Direct Oxidation of Methane to Syngas. J. Catal, 1994, V. 146, pp. 1-10).

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It is also known a method of SCO of methane for obtaining hydrogen and carbon oxide (US5149464) at temperature 650 - 900 ⁰ C and volumetric speed 40,000 - 80,000 h ^"1 (0.05 - 0.09 s) in the presence of a catalyst representing a transition metal or its oxide, deposited on a thermostable oxide of one of the following elements (M): Md, B, Al, Ln, Ga, Si, Ti, Zr, Hf, or on a perovskite-like mixed oxide of general formula MxM ¹ VOz with pyrochlorine structure, where M ¹ is a transition metal, including also on elements of group VII. The atomic ratio of the elements of group VIII to the non-noble elements in these compounds is 1:1 or 3:1, and the content of noble metals is 32.9 - 48 % of the mass. The conversion of methane in the presence of mixed oxides Pr ₂ Ru ₂ O ₇ , Eu ₂ Ir ₂ O ₇ or La ₂ MgPtO ₆ at volumetric speed 40,000 h ^"1 and t = 777 ⁰ C does nor exceed 94 %, and raising the volumetric speed to 80,000 h ^"1 leads to reduction of the methane conversion to 73 % and of the selectivity for CO and H ₂ to 82 % and 90 %, respectively.

The method of SCO of hydrocarbons for obtaining synthesis gas in the presence of a catalyst, based on mixed oxides with the structure of perovskites, is the closest in its technical essence and achievable effect to the method being applied for (RU2204434).

A major disadvantage of all described methods of producing synthesis gas by oxidation of methane with air is the presence of a considerable quantity of nitrogen in the synthesis gas. For instance, in the patent chosen by us as a prototype, when using air as an oxydizer in the presence of 27 volume % of methane in the reacting mixture, the concentration of synthesis gas is 50 volume % (the rest is nitrogen).

Using oxygen as an oxydizer requires expensive installations for separating the air, which raises significantly the costs of the synthesis-gas production.

The task of the invention consists in the creation of a method of converting natural gas into fuels by creating a thermostable catalyst for obtaining a mixture of hydrogen and CO that is efficient for short contact times for both the reactions of SCO of hydrocarbons with oxygen and the vapor and carbon-acid conversions of

hydrocarbons, and at that in the presence of sulphur-containing compounds, and also for the realization of the process of obtaining the mixture of hydrogen and carbon oxide by using this catalyst.

TECHNICAL ESSENCE OF THE INVENTION

This task is solved by creating a method of converting natural gas into fuels, which includes a phase of converting natural gas to a mixture of carbon monoxide and hydrogen (synthesis gas) and a phase of catalytic conversion of the synthesis gas into motor fuels. The process of obtaining the mixture of hydrogen and carbon oxide takes place at temperature 800 - 900 ⁰ C and pressure 0.1 - 1 MPa in the presence of a sorbent-catalyst, preliminarily saturated with oxygen as a result of its treatment with oxygen-containing gas. The sorbent-catalyst is based on aluminium oxides and mixed oxides, including oxides of nickel, cerium, zirconium and iron, and has the following composition in mass percentage: mixed oxide — no more than 20 mass %; aluminium oxide - the remaining part of the composition. The sorbent-catalyst is in the form of granules with surface-to-weight ratio 100 - 200 m ² /g. The regeneration of the sorbent-catalyst is performed at temperature 500 - 600 ⁰ C in a flow of gases, containing 1 - 5 volume % of oxygen, up to its saturation with oxygen, which ceases with terminating the absorption of oxygen by the catalyst.

An advantage of the method of converting natural gas into fuels consists in the fact that the catalyst is thermostable for obtaining a mixture of hydrogen and CO and efficient for short contact times for both the reactions of SCO of hydrocarbons with oxygen and the vapor and carbon-acid conversions of hydrocarbons, and at that in the presence of sulphur-containing compounds.

EXEMPLARY EMBODIMENT OF THE INVENTION

Using a catalyst, which is a complex composite containing mixed oxides with structure of perovskite or fluoride and transition and (or) noble metals and additional components with low coefficient of thermal expansion, determines that

the catalytic conversion of the mixture, containing hydrocarbon or a mixture of hydrocarbons and (or) air, or CO ₂ , or vapor or their mixtures, and also - but not obligatorily - sulphur compounds, is realized in the presence of such a catalyst. In this method it is observed high conversion of methane and high selectivity, thermostability of the catalyst, and at that it is not observed coke formation and contamination with sulphur-containing compounds.

This technical result is attained by using a catalyst that has the following composition in % of the mass:

A transition or noble element - no more than 10 % of the mass;

Mixed oxide - no more than 1 %;

Material with ultra-low coefficient of thermal expansion (no more than 8 * 10 ^~ 6 ⁰ C ^" 0 - no more than 95 %;

AL ₂ O ₃ - the remaining part of the mass;

The mixed oxide includes in itself an oxide with the structure of perovskite Mεl-yMyOz and (or) an oxide with the structure of fluoride M ¹ XM ² I-XOz, where:

M - an element ofgroup VIII (Pt, Rh, Ir);

M ¹ - rare earth or alkali earth elements;

M ² - an element of group IVb from the Periodic Table of Chemical Elements;

B - a transition element - elements of period IV from the Periodic Table with a 3d electron shell.

Values of x and y are in the following intervals: 0.01 < x < l; 0 <y < l.

Values of x, y and z are determined from the oxidation rate of the captions and their stoichiometric ratio.

The term ,,rare-earth elements" means elements belonging to the group of the rare-earth elements that includes elements of group III b and elements with 4f electron shell, for instance, La, Ce, Nd.

The term ,,alkali-earth elements" means elements belonging to the group Ha from the Periodic Table of Chemical Elements, for instance, Sr, Ca.

Including components with low or negative coefficient of thermal expansion (CTE) into the composition of the high-temparature catalysts permits regulating their coefficient of thermal expansion and in such a way catalysts with high thermoresistance are obtained. Cordierite, mullite, complex phosphates of zirconium with NZP structure, wolframates (M ₂ W ₃ O ₁₂ , MW ₂ Os), molibdates, vanadates (MV ₂ O ₇ ), and aluminium titanate are used as such components. (I. Naroi-Szabo, «Inorganic Crystallography)), Mir Publishers, Moscow, 1971).

The obtained complex composite of the catalyst has surface-to-weight ratio 2 — 200 m ² /g and may be in the form of tablets, rings, spheres or small blocks with cell structure.

The processes are effected by consecutive passing of a gas mixture containing hydrocarbon or a mixture of hydrocarbons and (or) air, or vapor, or their mixture with temperature 200 - 500 ⁰ C through a fixed layer of the catalyst.

To obtain the necessary composition of the mixture of hydrogen and carbon oxide it is necessary to modify the composition of the initial mixture. The initial mixture contains hydrocarbon or mixture of hydrocarbons and (or) air, or vapor, or CO ₂ , or their mixture, as well as, non-obligatorily, sulphur-containing compounds, the process being performed at temperatures 500 ⁰ C - 1,000 ⁰ C. Natural gas, methane, propane-butane mixture, mixture of heavier hydrocarbons, kerosene, etc. are used as hydrocarbon raw material. Oxygen, air, CO ₂ or water vapor is used as an oxygen-containing gas.

The catalysts proposed are prepared by applying the methods of mixing and impregnation with subsequent drying and heating up. The process of obtaining the mixture of hydrogen and carbon oxide takes place in a counter-flow reactor at temperatures 550 ⁰ C - 1,000 ⁰ C for different contact time and composition of the reacting mixture. The composition of the initial reacting mixture and the reaction products are analyzed chromatographically. The efficiency of the catalyst work is

determined from the methane conversion rate, the selectivity for CO and hydrogen, and from the quantity of obtained mixture of hydrogen and carbon oxide and their proportion.

As a result of the present invention a synthesis gas is obtained, which does not contain any ballast impurities of nitrogen, whereupon air is used as an oxydizer, and subsequently motor fuels will be produced from the synthesis gas. Using a synthesis gas, which does not contain any ballast impurities of nitrogen, for the production of motor fuels according to the Fischer-Tropsch reaction increases considerably the efficiency of the process of producing fuels and simultaneously permits diminishing the size of the technological equipment and reducing the investment expenses. The preset objective is attained by using solid sorbent-catalyst containing oxygen introduced during its preliminary treatment with air. This approach allows obtaining synthesis gas, not containing ballast nitrogen, from natural gas by using air as an oxydizer. In addition, the realization of the method proposed does not require feeding into the reaction apparatus, besides the methane, any other additional components as air, water vapor, CO2 or their mixture, which permits simplifying significantly the process, diminishing the energy costs and investment expenses.

Example 1. Ni, Ce, Zr and Fe oxides in powdered state are treated with a 10-percent solution Of HNO ₃ at temperatures 40 ⁰ C - 60 ⁰ C. The mass obtained is mixed with powder of γ-Al ₂ θ ₃ and kept for 12 h at temperature 50 ⁰ C, after which the temperature is increased to 100 ⁰ C, and at that the water bound physically is released and the weight of mixed oxides attains a constant value. The mixture obtained is pressed in the form of tablets with size 3 x 5 mm. Then the tablets are heated at temperature 800 ⁰ C for 5 h. The bulk weight of the catalyst is within 0.7 - 0.9 g/cm ³ . Then air feeding is terminated and the catalyst is purged with nitrogen for 2 h at 800 ⁰ C, methane being introduced subsequently. The conversion of methane at volumetric ratio of CEU/catalyst = 150 is 94 %, and the composition of the mixture obtained in volumetric percentage is as follows:

H ₂ - 60.0 volume %; CO - 30.0 volume %; CH ₄ - 2.0 volume %; CO ₂ - 5.0 volume %; H ₂ O - 3.0 volume %.

The synthesis gas produced is cooled, compressed and directed into a reactor for synthesis of hydrocarbons at temperature 300 ⁰ C and pressure 30 atm. The synthesis of liquid hydrocarbons takes place in the presence of a polyfunctional catalyst containing oxides of iron, zinc and boron in combination with a carrier, namely aluminium and its oxides. The motor fuels obtained are 19O g per 1 nm ³ of synthesis gas for conversion of the carbon oxides equal to 98 %. Under the same other conditions for the synthesis of liquid hydrocarbons, using a synthesis gas containing 50 volume % of nitrogen leads to reduction of produced motor fuels to 14O g per 1 nm ³ of synthesis gas.

Example 2. The sorbent-catalyst is prepared in the way presented in Example 1. The conversion of methane takes place at volumetric ratio CHφ/catalyst = 150 and temperature 700 ⁰ C. In this case the methane conversion rate is 60 %, and the composition of the synthesis gas produced in volumetric percentage is as follows:

H ₂ - 22.8 volume %; CO - 13.6 volume %; CH ₄ - 18.2 volume %; CO ₂ - 13.6 volume %; H ₂ O - 31.8 volume %.

The synthesis gas produced is cooled, compressed and directed into a reactor for synthesis of hydrocarbons at temperature 300 ⁰ C and pressure 30 atm. The synthesis of liquid hydrocarbons takes place in the presence of a polyfunctional catalyst containing oxides of iron, zinc and molybdenum in combination with a carrier, namely aluminium and its oxides and phosphates. The motor fuels obtained are 55 g per 1 nm ³ of synthesis gas for conversion of the carbon oxides equal to 90 %.

Example 3. The sorbent-catalyst is prepared and activated in the way presented in Example 1. The conversion of methane takes place at volumetric ratio CIVcatalyst = 150 and temperature 900 ⁰ C. In this case the methane conversion

rate is 96 %, and the composition of the synthesis gas obtained in volumetric percentage is as follows:

H ₂ - 62.0 volume %; CO - 30.5 volume %; CH ₄ - 1.5 volume %;

CO ₂ - 4.0 volume %; H ₂ O - 2.0 volume %.

The synthesis gas produced is cooled, compressed and directed into a reactor for synthesis of hydrocarbons at temperature 300 ⁰ C and pressure 30 atm. The synthesis of liquid hydrocarbons takes place in the presence of a polyfunctional catalyst containing oxides of iron, zinc and molybdenum in combination with a carrier, namely aluminium and its oxides and phosphates. The motor fuels obtained are 55 g per 1 nm ³ of synthesis gas for conversion of carbon oxides equal to 90 %.

Example 4. The process of producing the synthesis gas takes place in the way described in Example 1. Then the reactor temperature is diminished to 500 ⁰ C - 600 ⁰ C and the reactor is purged with nitrogen for 2 h, after which the sorbent- catalyst is subject to oxidizing regeneration in a flow of gases containing 1 - 5 volume % of oxygen. The process of regeneration ceases with terminating the absorption of oxygen by the sorbent-catalyst.