Introduction
Fischer Tropsch (FT) catalysis is an essential chemical process that changes several compounds such as synthesis gas (syngas), methane, or biomass to petrochemical substituents and transports fuels (“Oxford Catalysts Group” par. 2). Catalyst is an essential component of the FT process. Syngas, which contain a mixture of hydrogen and carbon monoxide, is changed to hydrocarbons in the presence of catalysts such as iron, nickel, or cobalt. The products of the reaction include jet fuel, naphtha, diesel, and bases for synthetic lubricants (“Oxford Catalysts Group” par. 3).
Franz Fischer and Hans Tropsch initially developed the FT process in 1923 (Dlamini 31). The process was used in Germany during World War II to produce cheap fuel (“Oxford Catalysts Group” par. 1). In addition, the Apartheid period in South Africa saw immense use of the FT process to produce low-priced fuel. The early FT plant was established in 1936 and had an annual production capacity of 70,000 tonnes. Two years later there were nine FT plants in Germany whose output capacity was 660,000 tonnes annually. The abundance of coal in South Africa saw the development of a large-scale FT plant in Sasolburg in 1955. Two more FT plants were developed in South Africa 25 years later. The efficiency of the FT processes saw the establishment of numerous plants to fulfill the increasing demand for transport fuels.
Fundamentals
FT catalysis is a gas-to-liquid (GTL) process whose raw materials are a source of carbon and hydrogen, a catalyst, high temperatures, and high pressure. Franz Fischer and Hans Tropsch obtain Synthol, a combination of hydrocarbons and oxygen-containing compounds by reacting hydrogen and carbon monoxide in the presence of alkalized iron (Dlamini 31). They use a pressure of 50 atmospheres and temperatures between 400 and 450 oC. However, the FT plants in South Africa use “precipitated iron catalyst supported on silica and promoted by copper and an alkali” (Dlamini 32).
The actual process that takes place in FT plants is a polymerization reaction that produces various oligomers. This process can be categorized into three key reactions, which are main reactions, side reactions, and reactions that lead to catalysis alterations. The main reactions cause the formation of paraffin, olefin or can involve a water gas shift reaction. The chemical equations below depict the general equations of the main reactions (Dlamini 35).
Paraffin Formation
nCO + (2n + 1)H2 → CnH2n + 2 + nH2O
Olefin Formation
nCO + 2nH2 → CnH2n + nH2O
Water-Gas Shift (WGS) Reaction
CO + H2O ↔ CO2 + H2
The side reactions can produce alcohol or result in a Boudouard reaction, whereas the catalyst modification reactions can either oxidize or reduce the catalyst. The catalyst modification process can also form carbides.
The type of catalyst and the hydrogen to carbon monoxide ratio in the syngas determine the nature of the main reaction. For example, high H2/CO ratios and catalysts with robust hydrogenating capacities give preferentiality to the formation of paraffin. Low H2/CO ratios and catalysts with weak hydrogenating tendencies, on the other hand, support the formation of olefins.
FT reactions also produce water. However, water is an unwelcome component because it affects the conversion of syngas and the selectivity of the hydrocarbons. Water also influences the durability of the catalyst because it affects the extent of syngas adsorption on the catalyst, chain instigation, chain growth, and the process of methanation. Water, in addition, affects the process of hydrogenation to paraffin or dehydrogenation to olefins (Dlamini 35). It is, therefore, crucial to comprehend the process of water formation to enhance FT processes. WGS reactions help solve water problems by utilizing water and carbon monoxide from the FTS reaction. Potassium-promoted catalysts help improve WGS reactions, which also compensate for the low amounts of hydrogen in syngas.
The catalysts in FTS require activation by a reduction process using carbon monoxide, hydrogen, or syngas. Catalysis using cobalt needs activation by hydrogen and produces metallic cobalt, which is the active phase in the process. Studies show that reducing pre-treatments produce profound effects on the catalysis and selectivity for catalysts containing iron.
Current Status
Traditional FT plants characteristically show a low conversion efficacy of 50% and below because they are made to work well with high capacities (a minimum of 5,000 BPD). Therefore, their optimum and economic production is attained at high capacities of 30,000 BPD and above (“Oxford Catalysts Group” par. 3). It is, therefore, necessary to develop FT plants whose production capacities can be modified to suit different situations with minimum wastage. Oxford catalysts have FT microchannel reactors that give economic yields on smaller scales than the customary FT plants (“Oxford Catalysts Group” par. 6).
These microchannel reactors are small, have thin channels, and disperse heat faster than traditional reactors. Consequently, it is possible to use large volumes of active catalysts that make it possible to attain high rates of conversion.
Proposed Research
The reaction mechanisms in the FT process are still under scrutiny. There are several proposed mechanisms of the various steps involved in the FT process. However, the role of the chemisorbed carbon monoxide molecules in the process of chain growth remains a point of controversy. It is vital to understand all steps, their protagonists, and antagonists to be able to optimize the FT process. Therefore, further investigations on the function of the chemisorbed CO molecules are necessary for a better understanding of the FT process.
Works Cited
Dlamini, M. W. 2012, Literature Review: Fischer-Tropsch Synthesis. Web.
Oxford Catalysts Group. n.d. Web.