An effective method for resolving spatial distribution of adsorption kinetics in heterogeneous porous media: application for carbon dioxide sequestration in coal

In adsorption studies, equilibrium adsorption properties are relatively easy to determine compared to kinetic parameters. Especially, it is not possible to study the spatial distribution of adsorption kinetics in structurally heterogeneous adsorbents by the available and almost standardized macroscopic methods. This paper presents application of quantitative X-ray computerized tomography (CT) imaging in investigating the spatial distribution of adsorption kinetics. The approach presented in the paper is demonstrated for mapping the adsorption kinetics of carbon dioxide (CO2) on a Pittsburgh coal sample to demonstrate the applicability of the method for an extreme case and also for the environmental importance of sequestration studies. From the experimental point of view, and thus generating representative data for simulators, there are usually two main problems related to the determination of the kinetics of adsorption in sequestration. First, it is necessary to study the kinetics on consolidated samples kept under representative pressure conditions. Second, there should be a suitable experimental and analysis technique that enables determination of local mass transfer parameters within individual lithotypes in an extremely complex system, like coal. During the experiment that was designed to address these problems, CO2 pressure was increased incrementally (1.7, 3.06, 4.42MPa) as adsorption equilibrium was attained in the coal that was kept under 1.36-MPa constant effective stress. The sample was frequently scanned during the transient period of uptake. Local CO2 adsorption was quantified by processing the images taken at transient period of uptake to study the equilibrium and dynamics of adsorption process in different coal microlithotypes. In paper, it is presented that the application of X-ray CT to adsorption studies helps to evaluate the kinetics of the process, especially in composite materials within which the kinetics behavior of a local region or of a point is needed. This can be especially useful in understanding the diffusion process at different locations in a heterogeneous adsorbent, where this understanding can enhance designing/modeling the adsorption systems or in increasing their efficiency. Application of the technique to coal-CO2 case helps to resolve mass transport properties of different microlithotypes.