COS-Shift to Assist Gas Purification

Like almost all gasification processes, coal gasification produces not only H2S but also COS from the sulfur contained in the raw matmal. For instance, coal 

Wherever the absorption or adsorption processes used to clean the gases have a high absorptivity for H2S but can eliminate COS only at high cost or not at all, the processes are designed in such a way that the sulfur contained in the gas reaches the gas purification unit in the form of H2S. Since, however, most modem gas purification units are capable of removing not only H2S but also COS and other organic sulfur components efficiently enough to meet the requirements for methanol production, COS hydrolysis is today used only in special cases. 

A distinction is made between COS hydrogenation converting the COS to H2S at temperatures between 350 and 450 °C together with CO shift conversion, and COS hydrolysis where the COS is converted in a separate conditioning stage at temperatures between 120 and 300 °C. The former produces an equilibrium according to the formula 

Since in almost all methanol production plants only some part of the raw gas is carried through a CO shift conversion unit, COS hydrolysis is more important for coal gases than COS hydrogenation which takes place parallel with the water gas reaction. The equilibrium equation for COS hydrolysis is 

Some equilibrium figures (kp2) for this reaction are also listed in Table 2.5. 

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