Investigation of Pathways of CO and CO2 in a Fischer-Tropsch System Using Tracer Studies, Development of Reaction Mechanism and Kinetic Expressions PDF Download

Author: Debanjan Chakrabarti
Publisher:
ISBN:
Category : Pretroleum products
Languages : en
Pages : 228

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Book Description
The Fischer-Tropsch (FT) synthesis is an indirect feeds-to-liquids process to produce synthetic crude oil from any carbonaceous source such as coal, natural gas or biomass. The carbonaceous source is converted to synthesis gas by gasification or reforming, which then undergoes simultaneous polymerisation and hydrogenation steps to form the hydrocarbon and oxygenate rich synthetic crude oil or syncrude, which can be refined to obtain gasoline, diesel, jet fuels and petrochemicals just as obtained from conventional crude oil. This provides an alternate source of hydrocarbon rich transportation fuels at a time when conventional crude oil reserves are getting depleted and oil demand is increasing. The product formed from the FT reaction contains hydrocarbons and oxygenates ranging from C1 to over C80 or so, along with CO2. An ideal operation would lead to maximization of the naphtha (C5-C11) and distillate (C11-C22) fractions in the product, while decreasing the selectivity of methane, C2-C4 gases, CO2 and heavy waxes. This can be achieved either by improvements in reactor design, manipulating operating parameters, or by developments in catalyst design. However, an understanding of the reaction mechanism of the process is essential to properly exploit these techniques. In the nearly 90 years since its discovery, the process has been studied extensively and been commercialized successfully. However, there still exists a lack of clarity with respect to the reaction pathways and surface intermediates involved in the system of reactions. Thus, there exists no consensus on the reaction mechanism of the FT system. In this thesis, the mechanisms of the reactions in the cobalt as well as iron catalyst-based FT systems have been investigated by conducting experiments and correlating the interpretation of the results with experimental observations in the literature. Based on the derived mechanisms, kinetic expressions have also been derived to represent each FT system. A study of CO2 in the cobalt-alumina based FT system was conducted by means of periodic feeding studies and investigations involving 14CO2 co-feeding. It was found that the CO2 in the cobalt catalyst system was capable of forming an oxygen free carbon intermediate and short chain hydrocarbons directly, without first undergoing a reverse water gas shift reaction to form CO. This was found to be a secondary methane formation pathway on cobalt catalysts. Investigations with 13C 18O indicated the existence of two carbon pools on the cobalt catalyst, one a CHx surface species and the other an adsorbed CO species. The insertion of the adsorbed CO species onto the CHx species resulted in the formation of a C2 oxygenate intermediate, which could either be hydrogenated to terminate as alcohol, or undergo hydrogen assisted C-O dissociation to form the C2 hydrocarbon intermediate. The C2 hydrocarbon intermediate could be desorbed as ethylene or hydrogenated to ethane. This indicated that the chain growth step took place by the CO insertion mechanism. The alcohols and hydrocarbons were found to originate from a common parent chain. A main hydrocarbon formation reaction was found to be the same on cobalt as well as iron catalysts. However, there were differences in the secondary reactions involved in each catalyst system. The methane as well as methanol formation was found to be the result of parallel pathways on cobalt catalysts - one via the FT reaction pathway, and the second via a rapid hydrogenation of adsorbed CO and CO2. The second pathway was negligible on iron catalysts. However, iron catalysts are known to be water gas shift active, which leads to the formation of CO2. However, on cobalt catalysts, any CO2 formed is either the result of a disproportionation reaction of CO to form C surface species and CO2, or by dissociation of CO on the catalyst surface followed by recombination of the C and O species. It was also inferred that the C2+ intermediates were attached to the catalyst via the terminal and adjacent-to-terminal carbon atoms, with both these atoms being available for chain growth. This explained the negative deviations of the C2 species from the ASF trend as well as the branching behaviour observed in the hydrocarbon product. Based on the detailed mechanism, kinetic expressions were derived for fitting to experimental data.