Fischer-Tropsch normal pressure synthesis

Medium Pressure Synthesis. Pressures of 500—2000 kPa (5—20 atm) were typical for the medium pressure Fischer-Tropsch process. Cobalt catalysts similar to those used for the normal pressure synthesis were typically used at temperatures ranging from 170 to 200°C ia tubular "heat exchanger" type reactors. 

Fischer-Tropsch synthesis. (Synthol process Oxo synthesis). Synthesis of hydrocarbons, aliphatic alcohols, aldehydes, and ketones by the catalytic hydrogenation of carbon monoxide using enriched synthesis gas from passage of steam over heated coke. The ratio of products varies with conditions. The high-pressure Synthol process gives mainly oxygenated products and addition of olefins in the presence of cobalt catalyst (Oxo synthesis) produces aldehydes. Normal-pressure synthesis leads mainly to petroleum-like hydrocarbons. 

Fischer and Tropsch used cheap catalysts (alkalized iron turnings) and high temperature and pressures in their initial development work. The products of the reaction were oxygenated compounds (Synthol). Later they used catalysts of high activity, low temperature, and low pressure. As a result of these experiments the opinion was established that the optimum conditions for production of hydrocarbons are atmospheric pressure and lowest possible temperatures (normal-pressure-synthesis). 

The concentrations of hydrocarbons adsorbed in catalyst pores, especially of the heavier products, depend on their partial pressure in the catalyst bed. The partial pressure of the products at the same conversion and selectivity levels depends on the total pressure in the reactor. Therefore, Fischer-Tropsch synthesis at total pressure of 20 bar (tt CO=2) would result in a higher partial pressure of hydrocarbons. The higher partial pressure would lead to the condensation of reaction products which are normally in gaseous phase... 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

The normal synthesis gas required for the Fischer-Tropsch process is a mixture of 2 volumes of hydrogen and 1 volume of carbon monoxide. Recent practice in the Ruhr when using cobalt catalyst showed ratios of from 1.8 to 2.0 for the atmospheric pressure process and ratios as low as 1.5 for the medium-pressure process. In the Ruhrehemie plant at Sterkrade, the medium-pressure process used different ratios for each of the three stages of the synthesis, 1.4, 1.6, and 1.8, respectively, by introducing the requisite amount of "converted water gas before each stage. In all cases, the synthesis gas contained inert constituents which seldom exceeded 20% by volume. 

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