Patent Application
20170349838
The invention relates to the field of gas chemistry, particularly to a process for synthesizing C5 and higher hydrocarbons from natural gas through intermediate conversion of natural gas to synthesis gas (CO/H2 mixture) and subsequent conversion of CO and H2 by Fischer-Tropsch synthesis.
The Fischer-Tropsch process developed in the last century and immediately applied in industry was at first realized by fixed-bed catalytic reactors and then by reactors of ever-more complicated design. It was caused by requirements of increase in catalyst productivity and by needs of solution of rising problems in heat removing.
The Fischer-Tropsch synthesis is an exothermic process and occurs at elevated pressure in the presence of catalysts based on metals of group VIII of the Mendeleev periodic table.
High-pressure synthesis gas for carrying out the Fischer-Tropsch synthesis is produced by oxidation conversion of natural gas hydrocarbons including steam reforming methods, autothermal reforming methods and partial oxidation methods. Herewith, the steam reforming method is a preferred one as it needs no oxygen and is only based on reaction of natural gas with water steam at elevated temperature. It is important that steam reforming is an endothermic process that occurs inside reaction tubes over catalyst, and heat required for carrying out the reaction is provided by combustion of fuel gas in a shell side of a fuel gas reactor wherein fuel gas can be natural gas or any combustible gas.
However, none of the oxidation conversion methods yields synthesis gas fully meeting requirements of Fischer-Tropsch synthesis. Particularly, the H2:CO molar ratio should be adjusted (it should be near to 2) and CO2 consent should be corrected (it should be as low as possible for improvement of efficiency of a Fischer-Tropsch reactor). Coordination of synthesis gas composition with the requirements of Fischer-Tropsch synthesis (synthesis gas conditioning) can be implemented by various methods.
In the course of occurring the main process stream “natural gas—conditioned synthesis gas—Fischer-Tropsch synthesis product”, there are involved processes producing additional gas streams such as off gases from Fischer-Tropsch process. Effectiveness of the integrated technique of C5 and higher hydrocarbon synthesis from natural gas (in terms of kilograms of produced liquid hydrocarbons per 1,000 cubic meters of natural gas or in percentage of carbon efficiency) depends on how these additional streams utilized as process gases or fuel gases. Unfortunately, any methods that are being in use to improve of carbon efficiency result in increase of equipment costs and extra expenses for gas compression.
Improvement of carbon efficiency of the integral technique of C5 and higher hydrocarbon synthesis with no considerable increase of equipment costs is an important problem.
Known in the art is a process for producing liquid hydrocarbons from natural gas (I. S. Ermolaev, V. S. Ermolaev, V. Z. Mordkovich. Justification of Selection of Circulating Schemes in Synthesis of Liquid Hydrocarbons from Natural Gas. Theoretical Foundations of Chemical Technology, 2013, Vol. 47, No. 2, p.p. 201-201), comprising steam reforming natural gas, separating CO2 from synthesis gas by an absorption method with amine solution, separating additional CO2 from combustion gas of steam reforming, combining two streams of the separated CO2, compressing all the combined CO2 to a steam reforming pressure and feeding the compressed CO2 in a tube side of a steam reforming reactor. Such feeding CO2 in the tube side of the steam reforming reactor enables to shift reforming reaction equilibrium to achieve the required H2:CO molar ratio. Therefore, the steam reforming of the known process yields synthesis gas having the required H2:CO molar ratio of 2 and high content of CO2 that is then removed by amine treatment. After amine treatment, synthesis gas is directly fed to a Fischer-Tropsch reactor where synthetic liquid hydrocarbons are produced from synthesis gas and off-gases of Fischer-Tropsch synthesis is fed to steam reforming burners as fuel gas to partially replace natural gas. High carbon efficiency is an advantage of this known process. A drawback of this process is substantially increased equipment costs because, for providing such high carbon efficiency, separation of the additional CO2 from combustion gas is necessary to achieve the required H2:CO molar ratio. Separation of the additional CO2 from combustion gas involves use of large, unwieldy and expensive apparatus because combustion gas should previously be cooled, CO2 separation is carried out under adverse conditions of low pressure and low CO2 content, and oxygen should be removed from rich amine solution and separated CO2.
The prior art closest to the present invention is a process disclosed in U.S. Pat. No. 6,881,394 B2, 2005 (the process is schematically depicted in
On the other hand, the main disadvantage of this known process is that its use results in high loss in carbon efficiency of the integrated technique. Substantial decrease of carbon efficiency is due to the fact that large amount of hydrogen, which is the most valuable and hardly separable product of steam reforming (very much energy previously consumed to compress hydrogen has to be sacrificed for hydrogen separation), is inefficiently used as low-pressure fuel gas. Other disadvantage of the know process is that it is impossible to produce synthesis gas having CO2 content substantially less than 5% in order to use the synthesis gas in Fischer-Tropsch synthesis when the presence of CO in an amount of 5% and especially more substantially inhibits Fischer-Tropsch reaction and decreases process efficiency in whole.
A technical object of the present invention is providing a synthesis gas production process free from the disadvantages of the above-mentioned known processes, i.e. producing synthesis gas of optimal composition without the use of CO2 separation from combustion gas of steam reforming and by ensuring possibility of decreasing CO2 content in synthesis gas fed to a Fischer-Tropsch reactor substantially lower than 5%.
Said object is accomplished in the present invention by that a process for producing of synthetic liquid hydrocarbons from natural gas comprises 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 residual 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.
Moreover, according to the present invention, the excess of hydrogen separated from the synthesis gas can be used as fuel in the step of steam reforming, and carbon dioxide separated from the synthesis gas can be mixed with natural gas and fed at an inlet of the steam reforming reactor.
According to the present invention, the steam reforming is carrying out preferably at a pressure of a natural gas-steam mixture in the range of 22 to 35 bars.
The main technical result provided by the present invention is ensuring sufficiently high carbon efficiency without the use of expensive and power-consuming equipment for separation of CO2 from combustion gas. Moreover, the present invention substantially excels the known process of the above-mentioned U.S. Pat. No. 6,881,394 in carbon efficiency.
The process of the present invention is carried out in the following way.
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 H2:CO molar ratio of about 2.4 to 2.8 and CO2 content of about 18% is produced. This syngas is fed to an absorption unit 3 (referred to as “absorber” in Figures) for separation of CO2 from syngas to residual CO2 content no more than 5 vol. % by a liquid absorption method. Present-day technologies of CO2 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. CO2 separated from syngas is mixed with natural gas supplied in a tube side of the reformer 1. Syngas purified of CO2 is passed over hydrogen-permeable membranes of a membrane unit 4 to separate excess hydrogen whereby syngas having H2: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 CO2 separated from syngas in the tube side of the reformer 1 enables to shift reforming reaction equilibrium so that the H2:CO molar ratio in the range of 2.4 to 2.8 is achieved.
In the process of the present invention, although the amount of CO2 separated from syngas is insufficient to provide the required H2: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 H2:CO molar ratio of 2.4 to 2.8 and moderate CO2 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-CO2 syngas without the use of expensive separation of CO2 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 CO2 is separated from syngas to a residual content no more than 5 vol. % with no substantial loss of Fischer-Tropsch synthesis efficiency.
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.
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 CO2 separation from combustion gas. Moreover, the process of the present invention enables to eliminate the problem of oxygen contained in the amine solution (because CO2 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 CO2 from syngas to residual CO2 content no more than 5 vol. %.
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.
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.
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, H2:CO molar ratio in the produced syngas was 2.8 and CO2 content was 12%. This syngas was fed to the absorption unit 3 (amine treatment absorber) where CO2 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 CO2 was released at a temperature above 115° C. The released CO2 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 CO2 was passed over polymer hydrogen-permeable membranes of the membrane unit 4 thereby excess hydrogen was separated and syngas with a H2: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%.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014153237 | Dec 2014 | RU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/RU2015/000927 | 12/25/2015 | WO | 00 |
