Fischer-Tropsch medium 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. 

The two important discoveries in the search for iron-based Fischer-Tropsch catalysts were (a) the finding that the addition of alkali yielded significant improvements in the activity and selectivity (to liquid products) of iron catalysts (15), and (b) the development of the medium-pressure synthesis (16). In 1943 a pilot plant was constructed at Schwarz-Leide in Germany for the comparative testing of iron-based catalysts. However, the outcome of World War II curtailed its activities. After 1945 many of the plants were destroyed and, for those remaining, recommencement of operation was forbidden for several years. Of the three plants restarted, the last at Bergkamen was closed in 1962. 

Iron catalysts used in Fischer-Tropsch synthesis are very sensitive to conditions of their preparation and pretreatment. Metallic iron exhibits very low activity. Under Fischer-Tropsch reaction conditions, however, it is slowly transformed into an active catalyst. For example, iron used in medium-pressure synthesis required an activation process of several weeks at atmospheric pressure to obtain optimum activity and stability.188 During this activation period, called carburization, phase... 

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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