Adsorption isotherms and kinetics of carbon dioxide on Chinese dry coal over a wide pressure range

A gravimetric method with in situ density measurement is used to determine the adsorption isotherms and kinetic characteristics of CO₂ on Chinese dry coal plug at 293.29, 311.11, 332.79 and 352.57 K and pressures up to 19 MPa. The adsorption and desorption process is reversible, which shows that it is a process of physical adsorption for CO₂ on coal. The excess adsorption increases with the increasing pressure at low pressures until the CO₂ phase transitions pressure is reached. Above this pressure, the excess adsorption decreases with the increasing pressure. The adsorption behaviour is described using the CO₂ density instead of pressure in four thermodynamic models such as modified Langmuir, Langmuir + k, DR and DR + k. It is found that the modified Langmuir + k and DR + k models are more suitable for liquid and supercritical CO₂ adsorption, respectively. The adsorption kinetics data are also obtained during the measurement of adsorption isotherms. The experimental data are fluctuant at the initial time range due to the temperature variation in the adsorption cell after high-pressure CO₂ is injected. The diffusivity is estimated using a modified unipore model. It is observed that the kinetic parameter C, accounting for the effect of gas diffusivity, increases with the increasing pressure at low pressures and has no obvious relations with pressure at high pressures. In this study, C value has no dependencies with temperature for CO₂, and the order of magnitude of the effective diffusivity is approximately 10⁻⁵ to 10⁻⁴ s⁻¹.