Separation selectivity of various gases in the ionic liquid 1-ethyl-3-methylimidazolium tris (pentafluoroethyl) trifluorophosphate

The separation of carbon dioxide (CO2) from methane (CH4) is an important process in many industrial areas such as natural gas processing and biogas purification [1]. Natural gas also contains significant amounts of ethane, some propane, butane, and other higher hydrocarbons [2]. In natural gas treating, loss of heavy hydrocarbons is a concern. It is desirable to recover these compounds due to practical problems. There are many methods available for the removal of acid gases from gas streams. The most commonly used are chemical solvents, physical solvents, membranes and cryogenic fractionation [3]. Physical solvents tend to be favoured over chemical solvents when the concentration of acid gases is very high. However, if the concentration of heavy hydrocarbons is high, a physical solvent may not be the best option due to higher co-absorption of hydrocarbons. An acceptable solvent should have a high capacity for acid gas and a low capacity for hydrocarbons [4]. Therefore, ILs have been proposed to be used as a solvent for CO2 capture, because of their advantageous properties over conventional ones [5]. We found that the ionic liquid (IL) 1-ethyl-3 methylimidazoliumtris(pentafluoroethyl) trifluorophosphate ([emim][FAP]) shows the highest carbon dioxide (CO2) solubility of all ILs studied so far and shows higher selectivities for CO2/CH4 separations than any other IL, indicating the promising potential of using this IL for the separation of CO2 from natural gas. Also, the solubilities of C2H6, C3H8 and C4H10 in ([emim][FAP]) were measured and compared to the CO2 solubility in the same IL. The separation ratio between CO2 and hydrocarbons decreases as the hydrocarbon chain becomes longer. The selectivity increase accordingly to the order: SCO2/C4H10 <SCO2/C3H8 <SCO2/C2H6 <SCO2/CH4. A maximum selectivity is achieved at lower temperatures. Therefore, the CO2 removal from a natural gas stream is recommended to be performed at low temperatures in order to achieve the best separation. The obtained solubility data are modelled with the Peng-Robinson equation of state combined with quadratic mixing rule. The calculated data have been found to be in a good agreement with the experimental results.