As processes for synthesizing hydrocarbons from a syngas, the Fischer-Tropsch reaction, methanol synthesizing reaction, and the like are well known. It is known that the Fischer-Tropsch reaction proceeds with a catalyst of an iron group such as iron or cobalt or a platinum group such as ruthenium, methanol synthesizing reaction with a copper-based catalyst, and oxygen-containing C2 (ethanol, acetaldehyde, and the like) synthesis with a rhodium-based catalyst. In addition, the catalytic ability of the catalyst for use in the synthesis of these hydrocarbons is strongly linked to dissociative adsorption ability for carbon monoxide [for example, see Non-Patent Reference 1 (“Kin-itsu Shokubai to Fukin-itsu Shokubai”, collaboration of Hidai and Ichikawa, Maruzene, published in 1983)].
Incidentally, in recent years, a gas oil of a low sulfur content has been desired from the viewpoint of air environmental conservation, and this trend may still more increase hereafter. Moreover, from the viewpoint that crude oil resources are limited, it is desired to develop an alternative energy source and this development may be increasingly desired hereafter. As a technology responding to these requests, GTL (gas to liquids) is present, which is a technology synthesizing liquid fuels such as kerosene and gas oil from natural gas (main component: methane gas) whose minable reserves are said to be comparative to crude oil in terms of energy. Since natural gas contains no sulfur or, even if contains hydrogen sulfide (H2S) and the like which are easy to desulfurize, the resulting liquid fuels such as kerosene and gas oil hardly contain sulfur and possess an advantage that they can be utilized as high-performance diesel fuels having a high cetane number, so that GTL has recently attracted increasing attention.
As a part of the above GTL, a process for producing hydrocarbons from a syngas by the Fischer-Tropsch reaction (hereinafter referred to as “FT reaction”) has been actively investigated. At the production of hydrocarbons by the FT reaction, it is important to efficiently synthesize hydrocarbons corresponding to C10 to C16 in order to enhance a yield of a kerosene and gas oil fraction. In general, a hydrocarbon distribution of the hydrocarbon products in the FT reaction follows the Shultz-Flory rule. In the Shultz-Flory rule, a chain growth probability, α value, tends to decrease remarkably with elevation of the reaction temperature, that is, when the reaction temperature elevates, there is a tendency that carbon numbers of formed hydrocarbons remarkably decrease. In the past, technological development such as catalyst development has been actively investigated for the purpose of solving a problem how to disengage the restriction of the Shultz-Flory rule and how to synthesize hydrocarbons having specific carbon numbers selectively. However, a technology capable of sufficiently solving the problem has not yet been proposed. Recently, there becomes common an idea of enhancing yields of fractions capable of being easily convertible into a kerosene and gas oil fraction by hydrogenolysis, such as waxes and, as a result, enhancing a yield of a kerosene and gas oil fraction by hydrogenolysis of the waxes, without persisting in disengaging the restriction of the Shultz-Flory rule. However, the chain growth probability is currently about 0.85 and it is one recent technical problem how to enhance the probability. On the other hand, when the chain growth probability is enhanced too much, almost all of formed hydrocarbons are waxes, so that there arises a handling problem because the waxes are in turn apt to solidify during process operations. Also, in view of the general performance of a catalyst, the chain growth probability of about 0.95 is considered to be virtual upper limit.
As another method for further enhancing the yield of the kerosene and gas oil fraction, it is considered to be effective to use a catalyst having an excellent performance whose hydrocarbon-producing ability, i.e., activity is high, yields of gaseous components are low, liquid yields and chain growth probability are high, and activity is stably exhibited for a long period of time.
Hitherto, various catalysts for the FT reaction have been proposed and, as catalysts for the purpose of high selectivity to olefins, ruthenium-based catalysts, such as a catalyst wherein ruthenium is supported on a manganese oxide support and a catalyst wherein a third component is further added to the ruthenium-supported catalyst have been proposed [see Patent Document 1 (JP-B-3-70691) and Patent Document 2 (JP-B-3-70692)].
However, in the FT method using these ruthenium-based catalysts, enhancement of the yield of the above kerosene and gas oil fraction cannot be sufficiently achieved. Namely, the above ruthenium-based catalysts are excellent in selectivity to olefins but their catalytic activity is low and a yield of a liquid hydrocarbon fraction having 5 or more carbon atoms (hereinafter abbreviated as “C5+”) itself is low.
The present inventors invented a process for producing hydrocarbons, which comprises subjecting a catalyst to reduction treatment beforehand, dispersing the treated catalyst in liquid hydrocarbons in an concentration of 1 to 50% by weight, and bringing a mixed gas containing hydrogen and carbon monoxide as main components into contact with the dispersed catalyst at a pressure of 1 to 10 MPa at a reaction temperature of 170 to 300° C., the catalyst being obtained by supporting a sodium compound on a support composed of an aluminum oxide and a manganese oxide in an amount of 0.1 to 10% by weight on the basis of the catalyst and by further supporting ruthenium in an amount of 1 to 30% by weight on the basis of the catalyst and exhibiting a specific surface area of 60 to 350 m2/g and a bulk density of 0.8 to 1.8 g/ml, and they filed a patent application thereon [see Patent Document 3 (JP-A-2003-3174)].
The process for producing hydrocarbons according to the above invention is an excellent process in view of a high chain growth probability, an excellent selectivity to olefins, and capability of carrying out the reaction stably and smoothly with a high catalytic activity but, from the viewpoint of productivity of C5+, further enhancement thereof is desired. Namely, in general, when a catalyst having a high productivity of objective products per catalyst weight is used, it can be expected to reduce a catalyst cost and a facility cost, for example, catalyst weight to be used for obtaining the same amount of objective products can be reduced and, as a result, a reactor can be small-sized. Therefore, even in a process for producing hydrocarbons such as the process for producing hydrocarbons according to the above previous invention, further enhancement of C5+ productivity of a catalyst to be used is desired.    [Patent Document 1]
JP-B-2-70691    [Patent Document 2]
JP-B-2-70692    [Patent Document 3]
JP-A-2003-3174    [Non-Patent Reference 1]
“Kin-itsu Shokubai to Fukin-itsu Shokubai”, collaboration of Hidai and Ichikawa, Maruzene, published in 1983