Mass Transfer of Binary Mixtures of Oxygen and Argon in a Carbon Molecular Sieve

Abstract

Transport of binary O 2 /Ar gas mixtures in a carbon molecular sieve (CMS) has been studied using concentrationswing frequency response. The rate data for this binary gas system are described well by the transport models (surface barrier for Ar and surface barrier plus micropore diffusion for O 2 ) and parameters that were found to govern transport of the respective pure gases on the same CMS. These results suggest that transport of O 2 is not impeded by the presence of Ar. However, the experimental data do not rule out the possibility of some enhancement of the O 2 transport rate. Furthermore, O 2 and Ar isotherms on this CMS were observed to be linear in the region over which frequency response experiments were performed, suggesting that the micropore diffusion component of O 2 transport behavior is unaffected by the presence of Ar. Because transport of Ar in this CMS is much slower than O 2 , the dynamic response of this system owes the majority of its character to O 2 adsorption, and the effect of O 2 on Ar transport remains hidden. INTRODUCTIONCarbon molecular sieve (CMS) materials have received attention as potential adsorbents for the production of high-purity (>99%) O 2 from air by pressure swing adsorption (PSA). 1−14 In traditional zeolite-based PSA processes, the O 2 product is often limited to 94−95% purity, as the similar adsorption behavior of O 2 and Ar on many zeolites results in a substantial Ar impurity in the product. 1,2 In CMS, however, O 2 adsorbs much more rapidly than Ar, allowing separation of these two gases to be effected by the difference in their adsorption rates.To rigorously design a PSA process utilizing CMS or any other adsorbent, accurate knowledge of the adsorption rate behavior of the relevant adsorbent/gas system is desirable. Adsorption rates are generally limited by mass transfer, which can occur by one or more of a variety of possible mechanisms including micropore diffusion, macropore diffusion (ordinary diffusion, Knudsen diffusion, or Poiseuille flow), transport across a surface barrier, and external mass transfer. The existence of cross-coefficients introduced by multicomponent transport models can further complicate adsorption rate behavior. As a result, adsorption rates are an important and nontrivial subject worthy of thorough investigation.As CMS has long been recognized for its ability to produce purified N 2 from air, many of the studies of adsorption rates of atmospheric gases on CMS have been focused on O 2 and N 2 . 15−28 However, with the exception of a few studies, 24,25,29 adsorption rates of Ar have been largely ignored. Moreover, though adsorption rates of pure gases are generally presumed to differ from those of gases in multicomponent mixtures, studies of adsorption rates in such mixtures have been infrequent. 26 We thus conclude that there remains a need for further study, especially regarding adsorption rates of mixtures of O 2 and Ar.In a recent work, 30 we studied pure component adsorption rates of O 2 , N 2 , and Ar on two varieties of CMS using pr...

Identification of mass transfer resistances in microporous materials using frequency response methods

“…However, the effect of cross-coefficients introduced by multicomponent interaction is system-dependent. As a result of a binary study of O 2 /Ar mixture in CMS using CSFR [72], the experimental data was described well using the pure component transport models and coefficients with no additional cross-coefficients. The data suggested that the adsorbed gas could be treated as noninteracting, contrary to previous CO 2 and CH 4 in CMS system.…”

“…While many investigations demonstrate micropore diffusion in zeolites, a few examples show a surface barrier dominating or contributing to the overall mass transfer, especially in CMSs [15,63,64,72,73]. CMS is designed to separate targeted molecules kinetically by generating a restriction at the pore mouth of micropore opening.…”

“…Throughout the history of the application of FR techniques to adsorption systems, mathematical models have been developed for various physical systems with different rate-limiting mechanisms [31, 33, 38-40, 46, 47, 56, 67-69]. Systems have been identified with diffusion using the Fickian diffusion model [4,16,23,70], first-order reaction kinetics [71], a surface barrier with linear driving force model [3,15,63,64,72,73], parallel diffusion processes [52], and heat effects with non-isothermal models [40,42,47].…”

Evidence of Combined Heat-Transfer and Surface Barrier Resistances for Propane in Commercial ZIF-8 Crystals Probed by Pressure-Swing Frequency Response

“…38,39 Most investigations in zeolites were reported to have an intracrystalline diffusion (also called micropore/nanopore diffusion) by Fickian diffusion model. 36,40−42 In contrast, a few examples suggested the surface barrier dominating or contributing to the overall mass transfer for systems including nitrogen, oxygen, and methane in carbon molecular sieves, 27,29,32,43,44 methane diffusion in commercial zeolite LTA with small crystals, 45,46 CO 2 and propane in theta-1, 38 cyclohexane in silicate-1, 7 and water vapor in NaX system. 47 FR also allows a broad coverage of measurable diffusivities varying from 10 −17 to 10 −7 m 2 /s by incorporating a wider range of crystal size and perturbation frequencies, 38 facilitating direct comparison of diffusivities with other techniques, i.e., fast diffusivities from microscopic methods and slow diffusivities from other macroscopic methods as uptake.…”

“…The shape of response curves, which depends on controlling equations of kinetics, is distinct for surface barrier and diffusion mechanisms providing clear separation. Various frequency modulation methods have been developed for batch systems using a volume-swing FR − and continuous flow systems using a pressure-swing − or a concentration-swing FR − with reviews on these techniques detailed elsewhere. , Most investigations in zeolites were reported to have an intracrystalline diffusion (also called micropore/nanopore diffusion) by Fickian diffusion model. ,− In contrast, a few examples suggested the surface barrier dominating or contributing to the overall mass transfer for systems including nitrogen, oxygen, and methane in carbon molecular sieves, ,,,, methane diffusion in commercial zeolite LTA with small crystals, , CO 2 and propane in theta-1, cyclohexane in silicate-1, and water vapor in NaX system . FR also allows a broad coverage of measurable diffusivities varying from 10 –17 to 10 –7 m 2 /s by incorporating a wider range of crystal size and perturbation frequencies, facilitating direct comparison of diffusivities with other techniques, i.e., fast diffusivities from microscopic methods and slow diffusivities from other macroscopic methods as uptake.…”