(612d) Transport of Carbon Dioxide in a Carbon Molecular Sieve

Transport in carbon molecular sieves (CMSs) is of practical importance in air separation for the production of N₂, but the literature includes models that typically include a combination of surface resistances and micropore diffusion.

We present a careful analysis of volumetric experiments and show that adsorption kinetics of CO₂ is due to a distribution of time constants, ie it cannot be described by a single linear driving force (LDF) constant or a combination of LDF and micropore diffusion. This should not be a surprise, given the way in which CMSs are produced, which is likely to produce a distribution of micropores that are then narrowed at the neck by repeated coke deposition.

Taking this into account we present a simple model that introduces a distribution of time constants (inverse of the LDF constant), which can be represented using a log-normal distribution with μ = 0. This results in an integral model that has only 2 parameters, the characteristic time constant and the standard deviation of the distribution. The linear model for the dynamic response of a volumetric system that includes the flow resistance through the valve can be solved analytically in the Laplace domain, but direct inversion is not possible via the method of residues given the continuous distribution of time constants. The analytical solution is therefore inverted numerically to obtain the time domain solution. The comparison with the kinetic experiments shows that the transport in the CMS can be described with very high accuracy using a surface barrier model with a simple distribution of time constants characterized by a single additional parameter.