# Fischer–Tropsch synthesis using H2/CO/CO2 syngas mixtures: A comparison of paraffin to olefin ratios for iron and cobalt based catalysts

Fischer–Tropsch synthesis (FTS) using H2/CO/CO2 syngas mixtures over cobalt and iron based catalysts was carried out in a fixed-bed reactor. CO2 rich feeds produce products that are mostly light hydrocarbons with higher molar paraffin to olefin (P/O) ratio, whereas CO rich feeds shift the product composition to an FT type product with higher olefin product selectivity over both iron and cobalt based catalysts. Although the P/O ratio for FTS is strongly dependent on the operating conditions, the experimental evidence shows that the linear relationship between P(n+1)/O(n+1) and P(n)/O(n) holds for a large number of experiments. It is also shown to be independent of the type of the reactor, the composition of the syngas, reaction conditions and the kind of catalyst. Two features about the ratio of ξ = [P(n+1)/O(n+1)]/[P(n)/O(n)] for the FT products have been identified: (1) with n > 2, the experimental values of ξn>2 are higher than 1, fairly constant and independent of chain length n; (2) with chain length n = 2, the ratio of P3/O3 to P2/O2 (ξn=2) is significantly different, and shows that ξn=2 ≪ ξn>2. An equilibrium hypothesis is considered in an attempt to explain this experimental phenomenon.A simple vapor–liquid equilibrium (VLE) model indicates that the ratio of P(n+1)/O(n+1) to P(n)/O(n) changes in a range of (1, 1/β), where β is the variation of the vapor pressure coefficient, which is related to the incremental energy of vaporization per CH2 unit of the hydrocarbon chain. Our experimental results support the expression when the chain length n > 2. But with chain length n = 2, this expression is unable to explain the relationship between P3/O3 and P2/O2. Another model, based on quasi reaction equilibrium, is developed to explain the linear relationship between P(n+1)/O(n+1) and P(n)/O(n). We assume that the reaction of Cn+1H2n+2 + CnH2n+2 = Cn+1H2n+4 + CnH2n reaches quasi-equilibrium. Because the experimental results are quite close to the equilibrium calculations, we postulate that the product distribution might be determined by considering reaction equilibrium.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (169 K)Download as PowerPoint slideHighlights► The product selectivities between the feeds of CO2-rich syngas and CO-rich syngas are far different. ► P/O ratio for FTS is strongly dependent on the operating conditions. ► Linear relationship between P(n+1)/O(n+1) and P(n)/O(n) holds for many experiments. ► Vapor–liquid equilibrium and quasi reaction equilibrium have been used to explain the phenomenon.

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Journal: Applied Catalysis A: General - Volumes 433–434, 8 August 2012, Pages 58–68

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