(94c) Mass Transfer Studies for Air Separation By Frequency Response Method

The frequency response (FR) method, a pseudo-steady state relaxation technique employing perturbation frequency, plays an essential role in discriminating between multi-kinetic mechanisms in microporous materials for separation and catalytic processes. The powerful technique couples with mathematical models, enabling consideration of various resistances, such as micropore diffusion, macropore diffusion, surface barriers, and external film resistance. Through judicious application of FR methods, it is not only possible to identify dominating mass transfer resistances but also to extract reliable mass transfer coefficients based on corresponding mathematical models.

Examples will be discussed for microporous materials for air separation. As an example, carbon molecular sieve (CMS) is the kinetics adsorbent that adsorbs O₂ faster than N₂ and Ar, and employed in commercial pressure swing adsorption processes for N2 production, but its potential for kinetic separation of O₂/N₂ or O₂/Ar at lower temperatures remains largely unexplored. The study evaluated equilibrium and kinetics properties for pure O₂, N₂, and Ar in commercial Shirasagi MSC-3K 172 at sub-ambient temperatures. The kinetic selectivity for O₂/N₂ near doubles (from 17 to 33) and more than triples (from 24 to 88) for O₂/Ar, when temperature is reduced from 25 to -25 °C, the isotherm capacities for three gases increase similarly. Another example will be on novel zeolite ITQ-55 synthesized by Avelio Corma’s group, controlled by micropore diffusion in contrast to CMS which is mostly controlled by surface barrier. More surprisingly, we found that the kinetic selectivity of O₂/N₂ increases with decreasing temperature, exhibiting exceptional performance at sub-ambient temperatures.