A Novel Method and Correlation for Measuring and Evaluating the Diffusion Coefficients of Gases in Liquids

Abstract

Knowledge of the diffusion coefficient of fluid systems is essential for designing and optimizing enhanced oil recovery, fluid separation, and storage processes. In this study, the diffusion coefficients of the gases (CO2 and CH4) in liquids, n-hexadecane, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], and tri-ethylene glycol (TEG) were determined using a newly developed correlation. The correlation was derived by adjusting Fick's equation to the experimental data on liquid droplet diameter obtained from pendant drop tensiometry. It also takes into account the vapour-liquid equilibrium of the liquid-gas system. The experiments were conducted at temperatures ranging from 20°C to 50°C and pressures up to 200 bar in a high-pressure, high-temperature view cell. The diffusion coefficient of the fluid systems increases at higher temperatures and pressures. However, a higher diffusion coefficient, greater than 10-9 m2/s, is determined under relatively lower temperature 30°C, and high pressure (P > 70 bar) conditions in a miscible system, as in the case of the n-hexadecane-CO2 system. The diffusion coefficient values obtained in this work agree with the published data for comparable fluid systems, taking values in the order of magnitude of 10-10 to 10-9 m2/s. However, the results from the Cui et al. (2023) correlation were found to be lower, especially at moderate to low pressures (P ≤ 50 bar), with this work addressing this discrepancy. Additionally, CO2 miscibility with n-hexadecane and the resulting 5.3% greater swelling of the liquid droplets at 30°C and 50 bar compared to CH4 at the same conditions make application of CO2 for enhanced oil recovery favorable. The greater swelling of the ionic liquid [BMIM][PF6] compared to TEG by CO2 under the same temperature and pressure conditions demonstrates increased solubility of CO2 in [BMIM][PF6]. Thus, [BMIM][PF6] has a greater capacity to absorb CO2, making it more effective than TEG in CO2 capture.