Effect of polyhydroxyalcohols on COS absorption in aqueous methyldiethanolamine

The removal of COS from natural gas is an area of increasing interest in natural gas sweetening, in view of tightening sulfur specifications. In an attempt to improve the absorption of COS in aqueous methyldiethanolamine (MDEA) solutions, the addition of polyhydroxyalcohols (PHA) was studied in small-scale laboratory tests. The addition of 25-30 wt.% ethylene glycol or glycerol increased the reaction rate by a factor up to 5-10, depending on the solvent loading and MDEA concentration. However, pilot plant studies revealed that the COS absorption could not be improved, or even worsened, under practical absorbing conditions. This is attributed to increased solvent viscosity which counteracts the increased reaction rate. The selectivity with respect to CO2 absorption improved, however, in some cases by up to a factor of two.