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

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 C5 to C10 normal paraffins and normal olefins), were chosen as the solvent. The catalyst used was a SiO2 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, CH4 and CO2 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 C5–10 components have similar effects on CO conversion, CH4 and CO2 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 C5–10) 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.