A study of kinetic air separation at low temperatures: Oxygen, nitrogen, and argon in carbon molecular sieve

Abstract

Carbon molecular sieves (CMS) are among the most promising materials for industrial gas separations, including their utilization for the kinetical removal of O2 from N2 and Ar in commercial pressure swing adsorption processes. However, little is known about the characteristics of these kinetics processes at lower temperatures. In this article, a kinetic study was carried out at three temperatures (-25, 0, and 25 °C) for pure O2, N2, and Ar using a commercially available Shirasagi MSC-3K-172. A pressure-swing frequency response technique was applied to determine the dominating mass transfer resistances, as well as corresponding temperature dependences, revealing that a combined resistance model of surface barrier and micropore diffusion best describes the behavior of all three gases. In addition, this MSC-3K-172 exhibited higher kinetic rates but lower kinetic selectivities as compared to Shirasagi MSC-3R-162 and −170, which showed only surface barrier resistances for N2 and Ar. When the temperature was reduced from 25 °C to −25 °C, the equilibrium capacities for three gases increase similarly, but the kinetic rates decrease to varying extents, resulting in an unexpected increase in kinetic selectivity – the value near doubled (from 17 to 33) for O2/N2 and more than tripled (from 24 to 88) for O2/Ar. The observed kinetic selectivities were found to correlate well with the difference of the activation energies between the gas pairs. These activation energies are 36, 29, and 21 kJ/mol for Ar, N2, and O2, respectively, greater than the isosteric heats of adsorption, which follow the reverse order. Together, these results indicate a delicate balance among kinetic selectivities, controlling mass transfer resistances and rate parameters, and equilibrium capacities, and can be utilized for the design of kinetic separations with CMSs, especially when extended to sub-ambient temperatures.