Distribution Between C2 and C3 in the Low Temperature Fischer-Tropsch Synthesis over Cobalt and Iron Catalysts

Distribution between C₂ and C₃ in the low temperature Fischer-Tropsch Synthesis over cobalt and iron catalysts

Xiaojun Lu, Xinying Liu, Diane Hildebrandt and David Glasser

 Material and Process Synthesis Engineering, College of Science, engineering and Technology, University of South Africa, Johannesburg, South Africa. Email: lux@unisa.ac.za. Tel: +27 11 6709041

Keywords: Fischer-Tropsch Synthesis, cobalt catalyst, iron catalyst, product distribution.

Abstract

Deviation of C₂ (including olefin and paraffin) in the ASF distribution in the Fischer-Tropsch Synthesis (FTS) has been observed consistently but relatively few studies have focused on this. Explanations for C₂ deviation have been proposed by researchers and secondary reaction of ethene is suggested as the cause for the deviation.[1] However, the distribution between C₂ and C₃ has not been investigated as comprehensively as the C₂+ products in the FTS reaction. This study mainly focuses on the distribution between C₂ and C₃ products in FTS over both a supported cobalt catalyst (10% Co/90% TiO₂) and a precipitated iron catalyst (Fe/SiO₂/K/Cu). The FTS experiments over the cobalt catalyst were conducted in a fixed bed reactor, a CSTR, and a batch reactor under wide range of operating conditions including varying the H₂/CO ratio in the feed, the reaction temperature, and space velocity (SV) of the feed gas. The FTS experiments over the Fe catalyst were conducted in fixed bed reactors with H₂, CO, and syngas as activation reagents respectively. The distributions between C₂ and C₃ were investigated both for olefins and paraffins. C₃H₆/C₂H₄ ratio was found to be a function of CO conversion only (as shown in Figure 1 below) regardless the reasons (such as temperature, SV, and H₂/CO ratio in the feed) causing the change of conversion. The overall C₃/C₂ was found to be decided by reaction temperature mainly and space velocity has no effect on it. The unique distribution between C₂ and C₃ olefinic products suggests it is related to mechanism of FT reaction, as this phenomenon could not be explained by kinetics because such distribution is not affected by partial pressures of reactants and products. A model, based on competitive reactions between CO and olefins is being developed to describe the product distributions between C₂ and C₃ hydrocarbons.

Figure 1 distribution between C₂ and C₃ olefins as a function of CO conversion for Co/TiO₂ (left) and precipitated Fe (right) catalysts

Reference:

[1]   Van der Laan, G P and Beenackers, A A C M. Catal. Rev.-Sci. Eng., 1999, 41, 255¨C318.