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

抄録

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.