Absorption of SO2 by renewable ionic liquid/polyethylene glycol binary mixture and thermodynamic analysis

Abstract

Separation and removal of SO₂ before atmospheric release of the flue gas is essential to combat acid rain formation. Traditional chemical absorbents and conventional ionic liquids (ILs) suffer from several shortcomings including waste, second pollution, high viscosity, poor biodegradability and low absorption capacity. In this work, two renewable ILs where both cation and anion come from biodegradable materials, 2-hydroxyethyl-trimethylammoniume levulinate ([Choline][LA]) and 2-hydroxyethyl-trimethylammoniume lactate ([Choline]L), were synthesized and mixed with polyethylene glycol 200 (PEG200) to form 40 w% ILs binary mixture as SO₂ absorbents. Solubilities of SO₂ in the above two mixtures were measured under the temperature range of 303.15–333.15 K and the pressure up to 1.2 bar, with the result of 6.97–7.24 mol kg⁻¹ absorbents at 1.0 bar and 303.15 K. When the SO₂ pressure is reduced to 0.039 bar, the solubilities are still 1.77–1.84 mol kg⁻¹ absorbents. Reaction Equilibrium Thermodynamic Model (RETM) was used to analysis the thermodynamic behavior of SO₂ in present ILs mixtures. The obtained absorption enthalpies are as low as −39.81 and −51.69 kJ mol⁻¹ for the two binary mixtures, respectively. The two mixtures possess one or more improvements over commonly reported ILs in the aspects of biodegradability, absorption capacity, cost and energy consumption for regeneration of absorbents.