PROCESS FOR PRODUCING SYNTHETIC LIQUID HYDROCARBONS FROM NATURAL GAS

Abstract

A process synthesizes C.sub.5 and higher hydrocarbons from natural gas through intermediate conversion of natural gas to synthesis gas and subsequent conversion of CO and H.sub.2 by Fischer-Tropsch synthesis. The process includes steam reforming of natural gas in a steam reforming reactor to form synthesis gas, separating carbon dioxide from the synthesis gas by a liquid absorption method to a residual carbon dioxide content in the synthesis gas no more than 5 vol. %, separating an excess of hydrogen from the synthesis gas by a hydrogen-permeable membrane apparatus to a H.sub.2:CO molar ratio in the range of 1.9 to 2.3 and synthesizing liquid hydrocarbon from the synthesis gas by Fischer-Tropsch synthesis.

Claims

1. A process for producing synthetic liquid hydrocarbons from natural gas, the process comprising subsequent steps of: steam reforming of natural gas in a steam reforming reactor to form synthesis gas; separating carbon dioxide from the synthesis gas by a liquid absorption method to provide a remaining carbon dioxide content in the synthesis gas no more than 5 vol. %; separating an excess of hydrogen from the synthesis gas by using a hydrogen-permeable membrane apparatus to provide a H2:CO molar ratio in the range of 1.9 to 2.3; and synthesizing liquid hydrocarbon from the synthesis gas by Fischer-Tropsch synthesis.

2. The process of claim 1 wherein the excess of hydrogen separated from the synthesis gas is used as fuel in the step of steam reforming.

3. The process of claim 1 wherein carbon dioxide separated from the synthesis gas is mixed with natural gas and fed at an inlet of the steam reforming reactor.

4. The process of claim 1, wherein the steam reforming is carrying out at a pressure of a natural gas-steam mixture in the range of 22 to 35 bars.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows a flow chart of the process for producing liquid hydrocarbons from natural gas according to the above-mentioned article by I. S. Ermolaev et al.

[0021] FIG. 2 shows a flow chart of the process for producing liquid hydrocarbons from natural gas according to the above-mentioned U.S. Pat. No. 6,881,394 B2;

[0022] FIG. 3 shows a flow chart of the process for producing liquid hydrocarbons from natural gas according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The process of the present invention is carried out in the following way.

[0024] Steam reforming of natural gas is carried out in a reformer 1 (steam reforming reactor) having burners 2 so that synthesis gas (herein also referred as to “syngas”) with a H.sub.2:CO molar ratio of about 2.4 to 2.8 and CO.sub.2 content of about 18% is produced. This syngas is fed to an absorption unit 3 (referred to as “absorber” in Figures) for separation of CO.sub.2 from syngas to residual CO.sub.2 content no more than 5 vol. % by a liquid absorption method. Present-day technologies of CO.sub.2 separation are based on liquid absorption by amine solutions (such as methyldiethanolamine (MDEA), diethanolamine or complex amines) or potash solution. All these solutions enable achieving an equal result and MDEA is the most common due to commercial successes of BASF that offered MDEA. CO.sub.2 separated from syngas is mixed with natural gas supplied in a tube side of the reformer 1. Syngas purified of CO.sub.2 is passed over hydrogen-permeable membranes of a membrane unit 4 to separate excess hydrogen whereby syngas having H.sub.2:CO molar ratio of 1.9 to 2.3 is produced. Syngas is then fed to a Fischer-Tropsch reactor 5 (referred to as “FT reactor” in the Figures and the following Examples) in which synthetic liquid hydrocarbons are produced from syngas. Fischer-Tropsch synthesis off-gases separated in a separator 6 are supplied as fuel gas to the burners 2 of the reformer 1 where the off-gases partially replace natural gas intended to burn in the burners 2. Hydrogen separated by the membrane unit 4 is also supplied to the burners 2 for partial substitution of natural gas as fuel gas. Supply of CO.sub.2 separated from syngas in the tube side of the reformer 1 enables to shift reforming reaction equilibrium so that the H.sub.2:CO molar ratio in the range of 2.4 to 2.8 is achieved.

[0025] In the process of the present invention, although the amount of CO.sub.2 separated from syngas is insufficient to provide the required H.sub.2:CO molar ratio of 2 directly in the reformer 1, the absorption unit 3 according to the present invention is more than twice as small and cheap as an amine treatment unit in the process by I. S. Ermolaev et al. Therefore, at the outlet of the reformer 1 according to the present invention, there is a syngas with a H.sub.2:CO molar ratio of 2.4 to 2.8 and moderate CO.sub.2 amount that is then separated in the absorber 3. The content of excess hydrogen in the syngas after steam reforming is also substantially lower than in the process of U.S. Pat. No. 6,881,394 and so the membrane unit 4 is accordingly smaller in sizes and cheaper. Moreover, the combination of the absorption unit 3 and the membrane unit 4 enables producing low-CO.sub.2 syngas without the use of expensive separation of CO.sub.2 from combustion gas. Moreover, the process of the present invention enables use of more structurally and technologically simple and cheap amine treatment variants in which CO.sub.2 is separated from syngas to a residual content no more than 5 vol. % with no substantial loss of Fischer-Tropsch synthesis efficiency.

[0026] It should be noted that for the purposes of the present invention it is important that the hydrogen separation by membranes should be carried out not until the syngas has been purified of carbon dioxide but not vice versa. According to the invention, the combination of steam reforming and liquid separation of carbon dioxide yields syngas having a minor hydrogen excess. This excess can be separated and removed easily by the membrane unit 4 of small capacity when the separated hydrogen amount is just enough to cover fuel gas needs of the reformer 1 (hydrogen covers a part of the needs whereas the rest needs are covered by off-gases from the Fischer-Tropsch reactor 5). If membrane treatment in the unit 4 is carried out before absorption in the unit 3, the membranes of the unit 4 will be forced to process carbon dioxide rich syngas characterized by lower hydrogen partial pressure. It was found that in such case the required hydrogen recovery rate either cannot be achieved or is achieved at lower selectivity, i.e. hydrogen is separated together with carbon dioxide and no longer utilizable as fuel gas in a steam reforming reactor.

[0027] As provided by the present invention, the combination of the reformer 1, the undersized absorption unit 3 and the undersized membrane unit 4 placed thereafter creates an unexpected and superadditive effect enabling achievement of the main technical result of the present invention, namely achievement of sufficiently high carbon efficiency without the use of expensive equipment for CO.sub.2 separation from combustion gas. Moreover, the process of the present invention enables to eliminate the problem of oxygen contained in the amine solution (because CO.sub.2 is not separated from combustion gas of the reformer 1), eliminate necessity of consumption of an additional natural gas as fuel gas for reforming burners, substantially reduce of hydrogen amount used as fuel gas for reforming burners (hydrogen combustion is energy-wise disadvantageous versus combustion of natural gas or off-gases from Fischer-Tropsch synthesis) and use of the more simplified and inexpensive absorption unit 3 for separating CO.sub.2 from syngas to residual CO.sub.2 content no more than 5 vol. %.

[0028] According to the present invention, steam reforming reaction is preferably carried out under a pressure in the range of 22 to 35 bars. At a pressure below 22 bars, carrying out effective Fischer-Tropsch synthesis becomes impossible because syngas pressure at an inlet of the Fischer-Tropsch reactor 5 can be provided at a level only below 18 bars resulting in sharp falloff in productivity of the catalyst used in Fischer-Tropsch synthesis. At a pressure above 35 bars, weight and cost of the equipment for steam reforming and liquid absorption substantially increase.

[0029] Hereinafter, embodiments of processes for producing synthetic liquid hydrocarbons from natural gas are provided wherein Examples 1 to 4 illustrate implementation of the present invention process whereas Examples 5 to 9 are given as comparisons with the present invention process.

EXAMPLE 1

[0030] Natural gas containing 96% of methane was fed under a pressure of 25 bars for mixing with water steam at a steam:gas volume ratio of 2.55. The resulting steam-gas mixture was fed in the tube side of the reformer 1 where, over a nickel catalyst, the steam-gas mixture was converted to syngas. After separation of unreacted water from syngas, H.sub.2:CO molar ratio in the produced syngas was 2.8 and CO.sub.2 content was 12%. This syngas was fed to the absorption unit 3 (amine treatment absorber) where CO.sub.2 was separated by means of MDEA solution to a residual content of 0.5%. Rich amine (MDEA) solution was fed to a regenerator (not shown in Figs.) where CO.sub.2 was released at a temperature above 115° C. The released CO.sub.2 gas was compressed to a pressure of 24 bars and fed for mixing with the steam-gas mixture at an inlet of the reformer 1. Syngas purified of CO.sub.2 was passed over polymer hydrogen-permeable membranes of the membrane unit 4 thereby excess hydrogen was separated and syngas with a H.sub.2:CO molar ratio of 2.2 was produced. This syngas was fed to the Fischer-Tropsch reactor 5 where synthetic liquid hydrocarbons (SLH) were produced over a cobalt catalyst. Products of Fischer-Tropsch synthesis are SLH, water and off-gases. Off-gases were mixed with hydrogen separated by the membranes and supplied for burning in the burners 2 of the reformer 1 to generate heat required for maintaining endothermic steam reforming reaction. An integral carbon efficiency of the process was 50%.

[0031] Processes of Examples 2 to 4 according to the present invention and comparative Examples 5, 6 were carried out similar to Example 1. Comparative Example 7 show results of carrying out the process for producing liquid hydrocarbons according to I. S. Ermolaev at al. and comparative Examples 8, 9 show results of carrying out the process according to U.S. Pat. No. 6,881,394. Quantitative data for all Examples 1 to 9 are given in Table. A temperature of syngas at an outlet of the reformer tubes was 880° C. in all Examples except for Examples 4 and 9.

TABLE-US-00001 TABLE H.sub.2:CO Steam H.sub.2:CO at the CO.sub.2 at reforming at the FT the FT Carbon Example pressure, reformer reactor reactor efficiency, number bar outlet inlet inlet % % Note 1 25 2.8 2.2 0.2 50 The present invention 2 35 2.7 2.2 0.2 46 The present invention 3 25 2.7 2.2 5.0 46 The present invention 4 25 2.6 2.2 0.2 57 The present invention * 5 18 2.7 2.2 0.2 39 Out of the scope of the present invention 6 40 2.4 2.0 0.5 38 Out of the scope of the present invention 7 25 2.2 2.2 0.3 50 According to I.S. Ermolaev at al. 8 25 4.3 2.2 11 29 According to U.S. Pat. No. 6,881,394 9 25 3.6 2.2 6 39 According to U.S. Pat. No. 6,881,394 * * With use of an expensive reformer having improvement characteristics (a temperature of syngas at the outlet of the reformer tubes was 1,000° C., a steam:gas ratio was 2.1).