Diesel oil is being made by chemically treating natural gas.
.The advantages to making diesel from methane are clear. Diesel can be used in car and train engines whereas methane cannot, and diesel is worth more. There are a few methods of converting methane, the simplest alkane, into diesel, a complex mixture of hydrocarbons made up of carbon chains typically ten to twenty atoms long. The two main industrial processes are the Fischer-Tropsch Process and the Mobil Process.
The Fischer-Tropsch Process can be used to synthesise diesel but it can also be used to synthesise other hydrocarbons, such as petrol (gasoline), which has shorter carbon chains. The methane is filtered so that any sulphur-based impurities are totally removed which would stop the catalysts from doing their job. The methane is then mixed with steam and subjected to temperatures and pressures, such that carbon monoxide and hydrogen gas result from this. The working temperature of the reaction is typically 300 degrees Celsius. The reaction could be sped up by increasing the temperature but that would lower the yield, so a compromise is met. The pressure can vary up to about one megapascal. Higher pressures would give a greater yield but the processing plant would be more expensive to build and maintain.
The catalyst used can be either iron or cobalt. The catalyst ensures that the hydrogen and carbon monoxide react together in such a way that their molecular orientations are more energetically favourable. The hydrogen molecules and the carbon monoxide molecules break up and carbon-carbon bonds form creating long hydrocarbon chains. The stoichiometric proportions of hydrogen and carbon monoxide determine the dominant product of the reaction, with cetane (sixteen carbon atoms long) desired if making diesel.
The Mobil Process differs in that it makes higher alkanes by making methanol as an intermediate compound. Methanol is made from methane by steam reformation followed by the water-shift reaction (as is the case with the Fischer-Tropsch Process) and a direct combination of hydrogen and carbon monoxide. Two molecules of methanol can react together to to make methoxymethane and water. A specific zeolite catalyst can cause further chemical dehydration of the methoxymethane to make a mixture of hydrocarbons, about four-fifths of which are larger than butane molecules in terms of relative molecular mass, and can therefore be used in car engines. The major limitation of this reaction is the reduced working life of the catalyst which turns the methanol into fuel.
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