Are ionic liquids an alternative for gas sweetening?

Natural Gas (NG) undergoes several treatment steps aiming to remove undesirable components such as carbon dioxide (CO2) and hydrogen sulfide (H2S), which are known as acid gases. A gas sweetening process is commonly performed by chemical or physical absorption. The latter is usually preferred when large amounts of acid gas should be removed because of the less energy demanding regeneration step. Ionic liquids (ILs) have been proposed as alternative solvents for numerous chemical processes due to properties such as chemical stability and negligible vapour pressure. Extensive research has been executed on the solubility of CO2 in ILs. It was found that CO2 is highly soluble in certain ILs. This fact encouraged us to propose ILs as an alternative solvent for CO2 capture. The aim of this work is to study the thermodynamic aspects of the feasibility of using ILs for CO2 separation from natural gas. The solubility of CO2, CH4 and other light hydrocarbons in the low-viscosity 1-hexyl-3-methylimidazolium tetracyanoborate ionic liquid ([hmim][TCB]) has been experimentally determined using a synthetic method (a Cailletet equipment) at temperatures up to 400 K and pressures up to 15 MPa. The solubility of CO2 in [hmim][TCB] has been found to be up to 20 times higher than the solubility of CH4. Interestingly, while the solubility of CO2 is highly dependent on the temperature, the solubility of CH4 in the IL is almost not influenced by temperature. Also it will be discussed why Henry’s law underestimates selectivities at elevated pressures. The binary P,T phase diagram of ethane and propane in [hmim][TCB] shows maximum solubility near the respective critical points of the two gases. It was observed that both hydrocarbons have a lower solubility than CO2 in [hmim][TCB]. However, there is a clear tendency that the higher the alkyl chain of the hydrocarbon, the higher is the solubility in [hmim][TCB]. Therefore, it is possible to find the optimum conditions to maximize the absorption of CO2 and minimizing the absorption of CH4 and other hydrocarbons.