The direct conversion of synthesis gas to chemicals
Du Toit, Ernest
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The catalytic conversion of synthesis gas, obtainable from the processing of coal, biomass or natural gas, to a complex hydrocarbon product stream can be achieved via the Fischer-Tropsch process. The Fischer-Tropsch synthesis process has evolved from being mainly a fuel producing process in the early 1950's to that of a solvent and speciality wax production process towards the end of the 1970's. From the early 1980's there has been a clear shift towards the production of commodity chemicals in addition to fuel. Advances in reactor technology, volatile crude oil markets and a world trend towards "clean" fuels may cause a shift towards coal and natural gas as the feedstock of choice for the chemical industry. Fischer-Tropsch plants are capital intensive ventures due to the complexity of the process. Viable returns on such projects can only be realised by adding value to the products obtained from such processes. The chemical industry places a high premium on certain chemicals such as olefins and higher alcohols. More selective production of such chemicals can contribute to increased 'profitability and thus more economically viable processes. The C8+alcohol and C6+ olefin product range can be labelled as valuable chemicals. A major limitation in the traditional Fischer-Tropsch process is the low selectivity towards these valuable chemicals. The product distribution observed for a Fischer-Tropsch catalyst system conforms to the SchulzFlory polymerisation mechanism, which is inherently non-selective. This investigation deals with an iron-based catalyst that can best be described as a chemically selective Fischer-Tropsch catalyst. The product spectrum achieved with this so-called "ChemFT" catalyst can be seen as a breakthrough in terms of producing chemicals directly from syngas. The investigation covers the following aspects: a review of the development of the ChemFT catalyst used in this investigation, the characterisation of the ChemFT catalyst, an experimental verification of the catalyst product spectrum with respect to alcohols and olefins, on both laboratory and pilot plant scale, the development of rate equations for'Fischer-Tropsch and Water-Gas-Shift activity. Experimental performance results of the ChemFT catalyst show high selectivity towards the desired alcohol product compared to traditional low temperature iron catalysts (8- 14 C atom% vs. 2 - 4 C atom %). Similar olefin selectivity is obtainable with lower long chain paraffin selectivity (little or no wax formation). It is concluded that the ChemFT catalyst differs from conventional Fischer-Tropsch iron catalysts as far as selectivity and typical process conditions are concerned. Published reaction rate equations were evaluated for applicability to such a scenario. Known Fischer-Tropsch reaction rate equations described the catalyst kinetics fairly well. The theoretical base thereof was further improved by modifying the equations to include the effect of catalyst vacant sites. Published Water-Gas-Shift rate equations did not adequately describe the catalyst. It was shown that the accuracy of the Water-Gas-Shift equation could be improved by modifying it to account for C02 adsorption. Reaction rate equations for both the Fischer-Tropsch and Water-Gas-Shift reaction rates that are valid in the typical operating conditions are proposed.
- Engineering