Ionic liquids for CO2 separation. A gas solubility study

Background Unprocessed Natural Gas (NG) usually contains high amounts of the acid gases carbon dioxide (CO2) and hydrogen sulphide (H2S) and other heavier hydrocarbons such as ethane and ethylene (C2), propane (C3), butane (C4). Removal of acid gas from NG (gas sweetening process) is commonly done by chemical or physical absorption. Physical solvents are usually preferred when high concentration of acid gas is present as it is the case in the Arab Gulf reservoirs. Aim Ionic liquids have been proposed as alternative solvents for numerous chemical processes. In order to study the feasibility of using ionic liquids as solvents for gas sweetening, the solubility of CO2, CH4 and other hydrocarbons present in the NG should be studied. The aim of this work is to determine the selectivity of CO2 over CH4 using the low-viscosity ionic liquid 1-hexyl-3-methylimidazolium tetracyanoborate ([hmim][TCB]) as absorbent. Also, the absorption of other hydrocarbons in this solvent is studied. Methods The solubility of several gases in the ionic liquid [hmim][TCB] has been experimentally determined using a Cailletet equipment at temperatures up to 400 K and pressures up to 15 MPa. Results The solubility of CO2 in [hmim][TCB] has been found to be up to 10 times higher than the solubility of CH4.Interestingly, the solubility of CO2 is highly dependent on the temperature while the solubility of CH4 is practically not influenced by the changes in temperature. Therefore, it is possible to find the optimum conditions to maximize the absorption of CO2 while minimizing the content of CH4. In the same way, the recovery of the solvent is designed to maximize the recovery of CH4. C2 and C3 have lower solubility than CO2 in [hmim][TCB], but as the alkyl chain increases, they become more soluble in the ionic liquid. At certain conditions, the mixture reaches a liquid immiscibility area and a second liquid (almost pure hydrocarbon) is formed. Conclusions A high selectivity of CO2 over CH4 has been found for the ionic liquid [hmim][TCB]. Additionally, solubility of ethane and propane in the ionic liquid has been found to be lower than the solubility of CO2. In other words, ionic liquids may be an alternative as solvents for gas sweetening