Phase equilibria and diffusion behavior of high pressure CO2 in tetra-n-heptyl ammonium bromide

Abstract

The solid–liquid–gas equilibrium data of the tetra-n-heptyl ammonium bromide ([thepAm][Br])–CO2 system, and then the solubility and absorption kinetic data of CO2 in different phases of [thepAm][Br] were measured by a high pressure quartz spring method. Results showed high molar fractions of CO2 in solid [thepAm][Br] (0.921 at 15.0MPa and 313.2K; 0.567 at 5.0MPa and 313.2K) which were even higher than those in liquid [thepAm][Br] at same pressures and those in other ionic liquids (ILs) reported in the literature at relatively high pressures. The study also revealed that CO2 absorption rate was very pressure dependent. The Peng–Robinson equation of state (PR-EoS) with the van der Waals one-fluid mixing rules and the NRTL model were employed to calculate the phase equilibrium data by looking the solid [thepAm][Br] as a special liquid. A 1-D diffusion model with a concentration dependent diffusion coefficient equation combined with the NRTL model was developed to calculate the absorption kinetic data of CO2 in [thepAm][Br], suggesting that the diffusion coefficient was not a constant at high pressures.