(94d) How CMS Structure Affects the PSA Performance

The structural characterization of Carbon Molecular Sieves (CMS) remains challenging due to a complex network of partially restricted pores exhibiting substantial diffusion limitations. Yet, a unique arrangement of the deposited pyrocarbon layers within the porous carbon particle dictates the adsorption selectivity of CMS and therefore the overall process performance of kinetically-controlled separations such as N₂/O₂ or CH₄/CO₂. For this reason, the process-based evaluation of CMS quality is still considered the most foolproof method with respect to a particular application, as it directly provides data about the separation effectiveness in authentic operating conditions.

Within the last decades, many research groups studied CMS materials focusing strongly on the determination of governing mass transfer mechanism, which comprehension is essential for establishing the predictive mathematical model and accurate process simulations. Publications vastly report mechanisms of surface barrier, diffusion, or superposition of both. However, no straightforward interpretation of differences among found mechanisms is attainable due to the lack of an insight into the unique material texture as well as surface chemistry, which are at the bottom of specific interactions with various adsorbates at the micropore entrances, playing a key role to adsorber dynamics and consequently PSA system performance.

Combining the expertise on manufacturing strategies of CMS with structural characterization, kinetic studies, and process performance analysis, allows for a holistic understanding of gas separations on CMS materials. Moreover, this enables the fabrication of improved adsorbents in favour of PSA/VSA systems exhibiting reduced CAPEX and/or OPEX as well as the design of novel materials.

The background of the CMS production method via Chemical Vapour Deposition (CVD) and the effect of reaction conditions on material properties are presented. The discussion is supported by experimental results of advanced textural characterization along with N₂-PSA/VSA process data and mass transfer kinetics considerations.