Supercritical and near-critical Fischer-Tropsch synthesis: Effects of solvents

Abstract

The effect of solvents on the reaction performance of Fischer-Tropsch synthesis under supercritical and near-critical conditions was studied in a fixed bed reactor. Two kinds of solvents, pure solvents ( n-hexane and n-pentane) and mixed solvents (the solvent consists mainly of n-hexane and some contents of C 5 to C 10 normal paraffins and normal olefins), were chosen as the solvent. The catalyst used was a SiO 2 supported cobalt catalyst. The results showed that the partial pressure of n-hexane had slight influence on the CO conversion when the total pressure and the syngas partial pressure were kept constant. 1-olefins content in the products, however, increased with increasing partial pressure of n-hexane. 1-olefins content in the products under supercritical condition was obviously higher than that under non-supercritical conditions. Similar CO conversion, CH 4 and CO 2 selectivities, and hydrocarbon distribution were obtained in different pure media: n-pentane and n-hexane. 1-olefins content in n-pentane was slightly lower than that in n-hexane. When compared with pure n-hexane solvent, the solvent with mainly n-hexane and some C 5–10 components have similar effects on CO conversion, CH 4 and CO 2 selectivities, and the hydrocarbon selectivity of the products. This result suggested that the recycle of the supercritical solvent containing some content of lighter fractions (such as C 5–10) produced in the FT process is possible to reduce the amount of supercritical solvent during the supercritical FT process. The binary mixed solvent (25% n-hexane/75% n-decane) showed considerably higher 1-olefins selectivity than pure n-hexane probably due to its higher solubility to heavier products.