Chemistry: fischer-tropsch processes; or purification or recover – Group viii metal containing catalyst utilized for the...
Reexamination Certificate
1987-11-06
2001-11-20
Mulcahy, Peter D. (Department: 1713)
Chemistry: fischer-tropsch processes; or purification or recover
Group viii metal containing catalyst utilized for the...
Reexamination Certificate
active
06319960
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
This invention relates to a method for converting hydrogen and carbon monoixde to heavy hydrocarbons in a fixed bed reactor by a catalytic reaction where the catalyst pellet is designed so as to optimize the CO conversion and methane selectivity. Selectivity to methane is the percentage of the total CO moles converted.
A metal catalyst (e.g. cobalt or ruthenium) on a support (e.g., titania or silica) which may be promoted by different metals (e.g., rhenium, hafnium and others) are used for synthesis of heavy hydrocarbons from a mixture of carbon monoxide and hydrogen. The principal reaction may be expressed as:
where the distribution of the hydrocarbon products can be approximated by the Flory-Schultz expression. The fraction of oxygenates and olefins in the product is small.
An important consideration in the development of the hydrocarbon synthesis process is to minimize the production of light hydrocarbons (C
1
-C
4
), especially of methane. The fraction of methane in the product exceeds that predicted by the Flory-Schultz distribution.
Another important consideration is to maximize the productivity, defined as the number of CO moles converted per unit time and reactor volume, so as to minimize the volume of the reactor in which the reaction is carried out.
Both considerations have been met with available catalyst powder of the size 80-140 mesh (approximately 0.15 mm in diameter). However, additional factors should be considered in the design of a fixed bed reactor; namely, the pressure drop in the reactor, and the removal of the heat generated by the reaction.
These require the design of catalyst pellets which retain the properties of the powder catalyst (80-140 mesh) but are larger in size (>1.0 mm). However, since the reactants have to diffuse through liquid-filled pores, the longer diffusion path may create concentration gradients within the pellet. Such gradients alter the hydrogen to carbon monoxide ratio in the pellet due to the lower diffusivity of the latter. As a result the selectivity to methane, which depends on this ratio, increases considerably. Furthermore, since the rate of reaction depends on the concentration of the two reactants, the productivity is smaller in a pellet than in powder.
Because the pellets have to be used in a fixed bed reactor, the design of the catalyst pellet has to be directed toward minimizing the methane selectivity and maximizing the productivity. The catalyst of the present invention is designed to achieve this purpose.
SUMMARY OF THE PRESENT INVENTION
The present invention is a method for converting hyudrogen and carbon monoxide to heavy hydrocarbons in a fixed bed reactor by a catalytic reaction. The method includes contacting the hydrogen and carbon monoxide in the reactor at reaction conditions with a supported catalyst pellet. The support includes an inert or hollow core and an outer layer of porous inorganic refractory oxide. The outer layer has a thickness determined so as to optimize CO conversion to heavy hydrocarbons so that conversion to methane is maintained at a predetermined level. The thickness is determined by relating the rate of diffusion of the CO and the hydrogen to a rate of reaction in the porous inorganic oxide for a predetermined support geometry, partial pressures, and temperatures. The support may take on many shapes e.g., sphere, ring or semi-circle.
A metal catalyst (e.g. cobalt or ruthenium) on a support (e.g., titania or silica) promoted by different metals (e.g., rhenium, hafnium and others) are used for synthesis of heavy hydrocarbons from a mixture of carbon monoxide and hydrogen.
The concentrations of both hydrogen and carbon monoxide decrease as they diffuse into the pellet due to significant mass transfer resistance inside the pores. The global rate of CO conversion in the pellet decreases. Furthermore, the methane production rate increases which is a result of its dependency on the ratio between the hydrogen to CO concentration which increases in the pellet. This ration increases if the parameter &ggr; (see equation 11 below) is less than unit. This behaviour was observed in Co or Ru catalysts supported on titania, silica or alumina.
At a certain depth in the pellet, the hydrogen to CO ratio reaches values which cause most of the CO to be converted to methane which is detrimental to the process. This depth which is called the optimum rim thickness can be determined from the pellet model. Increasing the rim thickness diverts most of the marginal CO conversion to methane while decreasing the rim thickness decreases the CO conversion significantly. Therefore, for an optimal operation, the optimal rim thickness should be determined.
As will be discussed below with respect to
FIGS. 2 and 3
, it is not possible to simultaneously maximize CO conversion and minimize methane conversion. However, it is possible to choose a rim thickness so as to optimize CO conversion to heavy hydrocarbons.
The process of the present invention is carried out at a rate of CO conversion to hydrocarbons such that the percentage of methane production is maintained at a predetermined low level so as to make the entire process useful and practical.
As the rim thickness increases above the optimal rim thickness, the marginal increase in CO conversion is accompanied by an increase in the percentage of methane production in the converted carbon monoxide. This is observed in
FIGS. 2 and 3
discussed below. At the optimal rim thickness, most of the increase in CO conversion goes into methane production. Since an object of the present invention is to limit methane production in the converted CO, a rim thickness must be chosen at about this value.
The supported catalyst may be fabricated by a number of different methods known in the art, see e.g., Scientific Basis For the Preparation of Heterogenous Catalyst, Preprints of the Fourth International Symposium, Sep. 1-4, 1986, Louvain-La-Nueve, Belgium.
REFERENCES:
patent: 4413064 (1983-11-01), Beuther et al.
patent: 4599481 (1986-07-01), Post
patent: 855317 (1985-08-01), None
Arcuri Kym B.
Behrmann William C.
Herskowitz Mordechay
Mauldin Charles H.
Exxon Research & Engineering Company
Lerner David Littenberg Krumholz & Mentlik LLP
Mulcahy Peter D.
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