Removal of small hydrocarbons (ethane, propane, butane) from natural gas streams using the ionic liquid 1-ethyl-3 -methylimidazolium tris(pentafluoroethyl)trifluorophosphate

• Absorption of light n-alkanes in an ionic liquid.
• Phase behavior as a function of carbon number.
• Selectivities relative to CO2.
• Separation of CO2 from natural gas.
• Comparison physical solvent with chemical solvent.

From our earlier work it was found that the ionic liquid (IL) 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([emim][FAP]) shows a very high carbon dioxide (CO2) solubility and a low methane (CH4) solubility, with the consequence that this ionic liquid has a high CO2/CH4 selectivity (M. Althuluth, M.C. Kroon, C.J. Peters, Ind. Eng. Chem. Res., 2012, (51), pp 16709–16712). In this paper we measure for the first time the absorption capacity in the same IL of other small hydrocarbons, e.g. ethane (C2H6), propane (C3H8) and butane (C4H10). The solubility of these hydrocarbons in [emim][FAP] has been determined as a function of temperature and concentration by measuring bubble points and cloud points in a temperature window of 290–365 K and at pressures up to 10 MPa, using a synthetic method. It was observed that at similar conditions the solubility decreases in the following order: C4H10 > CO2 > C3H8 > C2H6 > CH4. Maximum selectivities for CO2/hydrocarbon separations are established at the lowest temperatures. A comparison of the absorption enthalpies of the various hydrocarbons studied in this work showed that C4H10 has the highest value (Δhabs∞=−17.27 kJ/mol), which is consistent with its highest solubility in [emim][FAP].Furthermore, it was found that at higher hydrocarbon concentrations in the binary mixture with [emim][FAP] a liquid–liquid immiscibility occurred. In addition, it was observed that the second liquid phase can be dissolved in the IL at higher pressures. The chain length of the hydrocarbon had a significant influence on the liquid–liquid immiscibility which may have consequences for the recovery of small hydrocarbons from natural gas streams.

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