Factors in sulfur poisoning of iron catalysts in Fischer-Tropsch synthesis

Abstract

Poisoning of catalysts in the Fischer-Tropsch synthesis by sulfur compounds has been investigated at the U.S. Bureau of Mines as part of its program on the conversion of coal to liquid and gaseous fuels. The effects of alkali content, initial surface area, and particle size on the poisoning of fixed beds of reduced fused iron oxide catalysts are reported for synthesis tests with 1 H 2 + 1 CO gas containing 69 mg S as H 2S/m 3 at 21.4 atm. Chemisorption studies of CO at −195 °C and CO 2 at −78 °C on reduced catalysts on which H 2S was adsorbed indicate that sulfur chemisorbs on both metallic iron and on alkali promoter if present. However, the decrease in the moles of CO plus CO 2 chemisorbed was only 14 to 29% of the moles of H 2S chemisorbed on the catalyst. Alkali promoters increase both the activity of the catalyst and its resistance to poisoning. Addition of 0.51 g K 2O/100 g Fe to an alumina-promoted catalyst increased activity about sixfold and resistance to poisoning about fivefold. In the early and intermediate stages of poisoning, the selectivity was not significantly changed. The relative amounts of products in different boiling fractions and the concentrations of olefins and oxygenated molecules in these fractions remained essentially constant. Initial surface area of reduced catalysts was not an important factor in determining either activity or resistance to poisoning. Decreasing particle size from 6–8 mesh to 28–32 mesh (a fivefold increase in external area) increased the activity and the resistance to poisoning more than threefold. Apparently poisoning occurs in a thin active layer near the external surface of the particles. Previous work from this laboratory has shown the depth of this active layer to be about 0.1 mm. In three fixed-bed poisoning tests, the concentration of sulfur in the catalyst was determined as a function of bed length. Most of the sulfur was found in the inlet third of the catalyst.