Material balance for methanol production from synthesis gas
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Answer:
Manufacture of methanol
(a) Production of synthesis gas
(i) Traditional methods
Methanol is manufactured from synthesis gas which is a mixture of carbon monoxide and hydrogen.
The feedstock, over the last 40 or more years, has been oil or natural gas. Coal, particularly in China, coal, rather than natural gas or oil, is being used as the feedstock.
(ii) ‘Green’ methanol
There have been major developments to produce methanol which is largely ‘green’.
Any solid biomass including for example agricultural, city and industrial waste can be used to make synthesis gas using techniques similar to its production from coal.
More recent developments include a plant in the Netherlands, which is using liquid propane-1,2,3-triol (glycerol), a by-product from the production of biodiesel, from animal fats and vegetable oils, to produce the gas.
Another ‘green’ route is to use waste carbon dioxide. Although the first such plant is linked to geothermal energy, it could be used to convert carbon dioxide waste from, for example, lime kilns and steel manufacture, to methanol.
(b) Synthesis of methanol
Synthesis gas is catalytically converted to methanol at elevated temperatures and pressures in a fixed bed reactor. The catalyst is an alumina pellet coated with copper and zinc oxides.
The main methanol synthesis reaction may be written:
From considering the energetics of the reactions, it can be seen that the yield of methanol is favoured by high pressures and low temperatures. A low pressure process came about by the discovery of a copper-based catalyst which was active at 475-575 K, thus allowing economical conversions to occur at 40-100 atm. One plant, for example, operates at 525- 575 K and 100 atmospheres. It eventually achieves a 97% conversion of the reactants.
The actual mechanism for the formation of methanol has been an active area of research. By using radioactive 14CO2 it is believed that the majority, if not all, of the methanol is derived via CO2.