Syn-gas can also be produced by the reaction of CO2 with natural gas or other aliphatic hydrocarbons, generally termed CO2 or "dry" reforming, because it does not involve any steam. With a reaction enthalpy of AH = 70.7 kcal mol-1 , this reaction is more endothermic than steam reforming (49.1 kcal mol-1).
The syn-gas produced has also an H2/CO ratio of 1; much lower than the values of around 3 obtained with steam reforming. Whilst this is a disadvantage for methanol synthesis, it makes a suitable feed gas for other processes, especially iron ore reduction and Fischer-Tropsch synthesis. Several industrial processes, taking advantage of high CO and low water contents of synthesis gas produced by CO2 reforming, have thus been applied for the production of iron. For methanol production, however, hydrogen generated from other sources would have to be added to the obtained syn-gas. Combination of steam and CO2 reforming, which can be performed using the same catalysts, to reach adequate syn-gas composition is also possible.
218 | Chapter 12 Production of Methanol from Syn-Gas to Carbon Dioxide Syn-Gas from Petroleum and Higher Hydrocarbons
Natural gas is not the only hydrocarbon source used for syn-gas generation for methanol production. Although on a smaller scale, liquefied petroleum gas and the different fractions obtained during oil refining (and especially naphtha) are also employed to produce syn-gas for the manufacture of ammonia, methanol and higher alcohols (propanol, butanol, etc.). Crude oil, heavy oil, tar and asphalt can all be transformed to syn-gas. The methods used are similar to those for natural gas: steam reforming and partial oxidation or a combination of both.
The problem with higher carbon-rich feedstocks, is generally their higher content in impurities, and especially sulfur compounds or chlorine which can very rapidly poison the catalysts used for steam reforming and subsequent methanol synthesis. Therefore, considerably more capital must be invested in the purification steps, and other catalysts which are more resistant to poisoning may be employed. Heavy oils, tar sands and other hydrocarbon sources containing large and complex aromatic structures are also poor in hydrogen, resulting in a syn-gas that will be rich in CO and CO2 but deficient in hydrogen.
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