Volatile organic compounds absorption in non-aqueous solvents: a critical review based on hydrodynamics and mass transfer considerations

The search for alternative solvents to water for the absorption of hydrophobic volatile organic compounds (VOCs) has led to the publication of numerous scientific papers over the past decades. The solvents considered must meet stringent constraints, especially a very low volatility. Such solvents are typically one to two orders of magnitude more viscous than water. In particular, many authors synthesized and screened for this particular application a multitude of solvents claimed to be green, such as ionic liquids or deep eutectic solvents. This critical review summarizes the main physico-chemical properties of the solvents studied, together with the measurement techniques used and the type of gas–liquid contactor implemented. Above all, it aims to provide a practical guide to gas–liquid transfer with a focus on the main criteria to be evaluated to confirm the potential of any non-aqueous solvent for VOC absorption. To date, the use of non-aqueous solvents would require to implement packed columns at industrial scale. For this purpose, hydrodynamics and mass transfer simulations are proposed. They assess the influence of multiple parameters, such as viscosity, Henry’s law constant, liquid-to-gas mass flow rates ratio, etc. considering different non-aqueous solvents (transformer oil, two deep eutectic solvents, one ionic liquid, di-2-ethylhexyl adipate, and a silicone oil). These simulations enabled to draw crucial conclusions about the hydrodynamics of non-aqueous solvents and their performances to remove VOCs. Finally, this critical review underlines the main criteria to respect when evaluating alternative solvents to water for the absorption of hydrophobic VOCs.