February 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
usage ratio, based on feed gas, to increase with increasing bed length or conversion. Thus, (1) predicts the high usage ratios a t low fractional conversions; (2) predicts the decrease in usage ratio with increasing fractional conversion; and (2) and (3) predict the minimum and the increase of the usage ratio at higher fractional conversions. Koelbel and Engelhardt (IS)found the rate of the water-gas reaction with a carbon monoxidewater mixture at atmospheric pressure to be strongly a function of temperature and suggested that on this basis the usage ratio in the synthesis should increase with decreasing temperature. However, the results of Table I11 and those of many other tests in this laboratory indicate that the usage ratio is virtually independent of temperature from 215" to 280" C . when the conversion is maintained a t about 65% with a gas eontaining equal parts of hydrogen and carbon monoxide. Nevertheless, these results and those of Koelbel and Engelhardt are not necessarily contradictory; they may be explained by postulating that the rate of the water-gas reaction is proportional to the rate of synthesis-Le., the temperature dependencies of both processes are the same. With fused catalyst D-3001, the rate of synthesis is strongly dependent on particle size, the activity increasing with increasing external area (per gram) of the particles. The data in Figure 7 may be approximated by assuming that the active portion of the catalyst is oonfined to a zone of constant depth from the external surface. Specifically, the particles were assumed to be spherical and the rate was assumed to be proportional to the volume of catalyst (per gram of catalyst) in this zone. On this basis, the depth of the active zone waa calculated to be about 0.1 Obviously, this is an oversimplification of the problem, but it demonstrates in a striking manner the fact that only a small fraction of the total surface area of the catalyst may be accessible in the synthesis. On the basis of the equations of Thiele (21) for simple types of reactions, it may be expected that in the range of Thiele's modulus in which the rate decreases with increasing particle size, the rate per unit total surface area will also increase with increasing pore diameter of the catalyst; this increase has been observed in tests with a fused catalyst in which the pore diameter has been changed by varying the temperature of reduction. Surface areas and pore diameters of catalysts D-3001 and P-3003.24 are presented in (1.2). These results for the Fischer-Tropsch synthesis are very different from those observed with similar fused iron catalysts in the decomposition of ammonia, where the rate was independent of particle size (9, 10). With iron Fischer-Tropsch catalysts, the alkali content is the
39P
most important composition variable. In early tests a t atmospheric pressure, Fischer ( 11) observed that iron-copper decomposition catalysts containing 0.3% potassium carbonate showed the maximum activity and life. Other alkali and alkaline-earth carbonates, such as rubidium carbonate, sodium carbonate, and barium carbonate, were somewhat effective aspromoters. Scheuermann (19) studied the effect of alkali on a fused magnetite-magnesium oxide catalyst and found maximum activity a t 0.76 part potassium oxide per 100 parts by weight of iron. The yield of wax increased to a maximum at 1.05 parts potassium oxide per 100 parts of iron and then decreased. Scheuermann considered the ratio of alkali to the amount of structural promoter as well as the ratio of alkali to iron to be important in determining the amount of alkali for optimum activity. On the other hand, Pichler (17') observed that the rate of synthesis on precipitated iron catalyst, which did not contain copper and which had been pretreated in carbon monoxide or synthesis gas, was essentially independent of alkali content; however, the average molecular weight, the fraction of oxygenated organic molecules produced, and the rate of carbon deposition increased with alkali content. The data of the present paper indicate that the effect of alkali on activity may be different on precipitated catalysts from that on fused, sintered, or cemented catalysts. These tests with precipitated catalysts indicate that the activity increased only gradually with alkali content. With the other types of catalysts, the activity increased sharply to a maximum a t 0.3 to 0.6 part of potassium oxide per 100 parts of iron; it then decreased. I n all cases the rate of deposition of carbon in the catalysts increased with alkali content as shown in Table IV, and the average molecular weight and the fraction of oxygenated molecules dissolved in the hydrocarbon fractions increased with increasing alkali content, a t least to the concentration corresponding to highest activity. The usage ratio decreased with increasing alkali content in the range 0 to 0.8 part of potassium oxide per 100 parts of iron. Thus, the selectivity and activity of iron catalysts are dependent on the alkali concentration. The reason for this effect of alkali on activity and selectivity is not understood. On the basis' of other studies in this laboratory and elsewhere (I&?), the product distribution data of Table I11 and Figure 8 are considered representative of the effect of temperature on the nature of the products. With increasing temperature, the average molecular weight and the fraction of oxygenated organic molecules decreased. The olefin content remained essentially constant; however, there was a shift from alpha to internal doublebond olefins as the temperature was increased.
(Fischer-Tropsch Synthesis)
EFFECT OF NITRIDING ON THREE TYPES OF IRON CATALYSTS J. F. SCHULTZ, B. SELIGMAN, L. SHAW,
A
RECENT paper (6) reported the changes in the activity and selectivity of a fused magnetite-magnesium oxidepotassium oxide catalyst in the Fischer-Tropsch synthesis produced by converting the iron of the catalyst to an
