Temperature and pressure dependencies of diffusion coefficient of carbon dioxide in ionic liquids

In this study, the diffusion coefficients of carbon dioxide (CO2) in ionic liquids were experimentally measured using the phase-shifting interferometer. The temperature dependence of the diffusion coefficient from 298 K to 323 K and the pressure dependence in the range of 0.2 MPa-CO2 to 1.0 MPa-CO2 were evaluated. Ionic liquids are expected to replace conventional amine solutions as absorbents in CO2 separation and capture processes because of their non-volatility and excellent gas absorption performance. Understanding the diffusivity of CO2 into ionic liquids at the gas-liquid interface is essential to realize a novel and efficient CO2 separation and capture process. In this study, the phase-shifting interferometer was applied to precisely observe the CO2 diffusion process in the vicinity of the gas–liquid interface in a short duration. As absorbents, l-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) were used to evaluate the effect of anions. The diffusion coefficient of CO2 into ionic liquids was experimentally evaluated from transient concentration distributions. As results, the measured diffusion coefficients were on the order of 10−10 m2/s for both the temperature and pressure dependency experiments. The diffusion coefficient of CO2 in both the ionic liquids was largely constant within the pressure range applied in the experiment, and no strong pressure dependence was observed. In contrast, the diffusion coefficient of [bmim][Tf2N] showed a strong temperature dependence, increasing monotonically with increasing temperature, in the applied temperature range, which is typically employed in absorption towers.