Conditioning of Methanol Synthesis Gas

Almost without exception, the raw gases produced from coal contain more carbon monoxide than required for methanol production. This is why most coal-to-methanol plants include facilities to shift some of the carbon monoxide contained in the raw gas. 

A distinction is normally made between clean gas conversion where the CO shift conversion unit is preceded by a gas purification stage to remove the major part of the sulfur components and any higher hydrocarbons that may be present in the gas, and sour gas conversion which accepts the untreated coal gas as a feed. A special type of the latter is what has been termed raw gas conversion, where considerable quantities of high boiling hydrocarbons are not eliminated from the raw gas but are left to pass the CO shift conversion stage. 

Depending on the temperatures at which the carbon monoxide is shifted, another distinction is made between high-temperature shift conversion (300-500 °C) and low-temperature shift conversion (180-280°C). Low- temperature shift conversion is, however, normally used only if the residual CO content in the converted gas has to be very low. As this is not the case fcx methanol production, and as there is no reason to put up with the high vulnerability to sulfur of the copper catalysts used for low-temperature conversion nor their considerable cost, the following description will be limited to high-temperature conversion. 

Independently of the overall pressure, but depending upon the temperature of the gas mix, thoe is a firm relationship - defined by the water gas equilibrium -between carbon monoxide, carbon dioxide, hydrogen and steam. 

The literature contains numerous formulations for this dependence on temperature one frequently used equation [2.23] is 

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