3242.     ---------------.  [SPENCER, W. D.]  Science of Coal-to-Oil Conversion.  IV.  Fischer-Tropsch Synthesis.  Petroleum (London), vol. 7, 1944, pp. 90-94; Chem. Abs., vol. 39, 1945, p. 2391.

        Other articles in this series deal with:  General Considerations, pp. 25-28; Low-Temperature Carbonization, p. 34-36, 39; Hydrogenation of Coal, p. 76-79; High-Temperature Carbonization, pp. 135-139; Summary and General Conclusions, pp. 184-186.  The development of the synthesis process began in 1902 with Sabatier and Senderens, who investigated the reactions CO+3 H₂=CH₄+H₂O and CO₂+4 H₂=CH₄+2 H₂O at 200°-250° over a reduced Ni catalyst.  In 1913, the Badische Anilin und Sodafabrik, by employing catalysts consisting of the oxides of Co, Os, or Zn at 400° and 100 atm., obtained a complex mixture of hydrocarbons and their O derivatives.  By increasing the pressure, the I. G. Farbenindustrie A.-G. later developed the process for synthesizing MeOH from water gas.  Fischer and Tropsch in 1922, using an Fe catalyst impregnated with K₂CO₃ and passing over it a mixture of CO and H₂ at 400°-450° and 100-150 atm., obtained a product, which they called Synthol, consisting of a mixture of alcohols, aldehydes, ketones, and fatty acids.  By reducing the pressure and the temperature, a less highly oxygenated product was obtained, but the rate of reaction likewise fell off, making it necessary to find a more active catalyst.  It was not until 1928 that Fischer, with Koch and Meyer, succeeded in producing catalysts that were active at atmospheric pressure and temperatures below 200° and capable of retaining their activity for several months.  To preserve the catalyst activity, it was found that S present in the synthesis gas, which consisted preferably of a water gas containing H₂ and CO in the ratio 2:1, must not exceed 0.1%.  The product obtained by this latest process was called Kogasin; it consisted of gaseous liquid and solid hydrocarbons composed mainly of straight-chain paraffins and olefins and could be broken down into a series of fractions consisting of low-boiling hydrocarbons (up to 30° C), gasoline (30°-200°), diesel oil (200°-350°), and solid paraffin wax.  The exact composition of the fractions is a function of the catalyst and of the operating conditions.  By 1934, the process had reached such a stage of development that it was taken over by the Ruhrchemie A.-G., and a plant was constructed and put into operation in 1936 at Oberhausen.  By 1940 German output of oil from the Fischer-Tropsch process was estimated at 1,000,000 tons per yr.  The theory of the process may be represented empirically by the equation:

CO+2H₂=(CH₂)+H₂O+48,000 cal.

It appears that the 1^st stage in the process is adsorption of CO on the catalyst, followed by reduction to carbide, which reacts with the H₂ to form CH₂ groups, which either combine with H₂ to form CH₄ or polymerize with formation of higher hydrocarbons.  The catalyst usually used industrially consists of Co:ThO₂:kieselguhr in the proportions 100:18:100.  Fe or Ni can be used.  The latter cannot be used at medium or high pressure, since Ni carbonyl is formed and removed from the system because of its volatility.  Under low pressure, it increases the degree of saturation of the product.  Fe gives good results.  At low pressures, the yield of hydrocarbons is small but increases with increase in pressure.  The operating temperature should be higher than with Co, and the ratio H₂:CO should be lower.  The products are more unsaturated and contain less wax.  A new catalyst is reported, but not named, with which a C₄ fraction is obtained containing 90% of isobutane.  The temperature is above 200° and the pressure above 10 atm.  With another catalyst, naphthenes and aromatics can be obtained.  With Ru as a catalyst, a temperature of 200°, and pressures of 100-20 atm., a mixture of solid waxes is obtained with a m. p. up to 134° C.  the Fischer-Tropsch synthesis was originally operated at atmospheric pressure to cut down the formation of alcohols and other oxygenated compounds, but it has been found that certain advantages exist in using pressures of 7-8 atm., among which is the reduction in size of the plant.  The theoretical yield of hydrocarbons from 1 cu. m. of ideal synthesis gas (66.7% H₂, 33.3% CO) is 208 gm., but, in practice, the yield does not normally exceed 140 gm. per m.³ for a 1-stage medium-pressure process or 160 gm. for a 2-stage process.  Several steps occur in the manufacture of oils:  (a) Manufacture of synthesis gas; (b) purification of gas; (c) hydrocarbon synthesis; (d) manufacture of catalyst; (e) distillation and refining of primary products; (f) further treatment of products, for example, cracking or isomerization of gasoline, conversion of wax to lubricants.  Each of these steps is discussed with a variety of data and a flow sheet.  The capital cost of a plant (100,000 tons per yr. of finished products) should not exceed ₤3,300,000 (based on 1938 prices) or ₤7 3s. per ton per yr. of coal consumed, calculating 4.5 tons per ton of finished products.  Such a plant, allowing for certain improvements in operation and design and 10% for depreciation and overhead, should produce gasoline and diesel oil at a cost not to exceed 9 d. per gal.  Whether the Fischer-Tropsch process is assessed on its own merits or regarded as an adjunct to carbonization, it has the advantage that all products are valuable materials.  Although emphasis is usually placed on the production of motor fuels, it is possible that in the future the real value will be considered to be its use in the manufacture of synthetic chemicals giving hydrocarbons, alcohols, acids, esters, organic solvents, plastics, and many other aliphatic compounds containing C, H, and O.  Coal tar would remain the chief source of aromatic compounds, and petroleum refinery byproducts would provide a source of aliphatic hydrocarbons and chemicals based on them.  This would provide a basis for a coordinated group of processes feeding the organic chemical industries.