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A generalized parallel pore and surface diffusion model and associated dynamic simulation program have been developed for multicomponent fixed-bed ion-exchange processes. Both equilibrium and nonequilibrium mass action laws are used to describe stoichiometric ion exchange. Model equations are solved numerically for frontal, pulse, or sequential loading processes. Analytical solutions obtained from a local equilibrium theory for binary systems and experimental data of two multicomponent systems served as benchmarks for the numerical solutions. The results indicate that the parallel pore and surface diffusion model should be considered for nonlinear large-particle systems. A parametric study shows that a major difference in fixed-bed dynamics between mass action and Langmuir systems lies in the propagation of diffuse waves of multivalent ions. Generally, the higher the valence or mass action equilibrium constant, the more pronounced the tailing of diffuse waves, which results in apparent adsorption hysteresis in a loading and washing cycle. The apparently irreversibly adsorbed multivalent ions can be eluted by concentrated solutions of lower valence ions, as a result of the relative selectivities of the higher valence against lower valence ions decreasing with increasing total solution phase concentration. This can lead to changes from favorable to unfavorable isotherms and self-sharpening waves to diffuse waves, or vice versa. Other results show that elution order can be reversed for heterovalent ions in elution and displacement chromatography.

Oxygen concentrations above 99.5% are required for several applications, mainly in the medical and aerospace fields. Two-stage pressure swing adsorption (PSA) processes, combining kinetic separation with equilibrium separation, have been developed for producing 99+% oxygen from air. Argon and nitrogen are kinetically removed from the air feed using a carbon molecular sieve adsorbent and the remaining nitrogen is removed using a N 2 /O 2 selective zeolite. Despite that, two-stage processes are often unattractive, complex, and energy consuming, requiring two or more compressors/vacuum pumps. Moreover, most of the two-stage units described in literature are unable to reach the required oxygen purity of 99.5%. This work studies three energy-efficient two-stage vaccuum PSA (VPSA) processes, combining an equilibrium based PSA (EPSA) or a kinetic based PSA (KPSA) for the first stage, with a VPSA unit packed with the Ar/O 2 selective zeolite AgLiLSX for the second stage, aiming to produce 99.5+% oxygen; the use of zeolite AgLiLSX allows removing argon besides nitrogen. The best two-stage VPSA configuration allowed obtaining a 99.8% oxygen stream at 6% of recovery and a 99.5+% oxygen stream at 14+% of recovery.

Carbon molecular sieve membranes (CMSM) were prepared on α-alumina supports by carbonization of a resorcinol-formaldehyde resin loaded with boehmite. Two series of carbon membranes produced at 500 ºC and 550 ºC carbonization end temperatures were prepared. The influence of the carbonization end temperature on the structure, morphology and performance of the membranes was examined by scanning electron microscopy, thermogravimetric analysis, CO2 adsorption and permeation to N2, O2, He, H2 and CO2 at temperatures from 25 ºC to 120 ºC. SEM photographs showed carbon membranes with a thin and very uniform layer and a thickness of ca. 3 m. Carbon dioxide adsorption isotherms revealed that all the produced carbon membranes have a welldeveloped microporous structure. Nevertheless, the membranes carbonized at 550 ºC have more ultramicropores and a narrower pore size distribution. The permselectivity of CMSM prepared at this temperature surpasses the Robeson upper bound for polymeric membranes, especially regarding ideal selectivities of pairs O2/N2 (O2 permeation rate: 9.85 x10 -10 mol m -2 s -1 Pa -1 and ideal selectivity: >11.5), H2/N2 (H2 permeation rate: 5.04 x10 -8 mol m -2 s -1 Pa -1 and ideal selectivity: >586) and He/N2 (He permeation rate: 4.68x10 -8 mol m -2 s -1 Pa -1 and ideal selectivity: >544).

Commercial adsorbents do not exhibit argon/oxygen equilibrium selectivity above 1. However, in the past decade, Air Products and Chemicals developed an argon/oxygen selective silver-based zeolite, AgLiLSX. In this contribution, the authors studied and characterized the AgLiLSX adsorbent to provide fundamental data to evaluate its potential for high-purity oxygen production in a single-stage PSA unit. Oxygen, nitrogen, and argon adsorption isotherms and breakthroughs curves were obtained, and moderate equilibrium selectivity (αN2/O2 = 4.98 and αAr/O2 = 1.14 at 1 bar and 25 °C), high working capacity (0.45 mol·kg–1 for nitrogen, between 1.4 and 0.2 bar at 25 °C), and superior performance were observed. The authors concluded that this adsorbent can allow the production of a 95+% oxygen stream in a single-stage PSA operation.

The maximum oxygen concentration obtained using a conventional single-stage pressure swing adsorption unit is ca. 95% balanced mostly with argon. However, there are several applications requiring simple and compact units for producing high-purity oxygen (≥99%), such as medical, military, or aerospace. This article studies a single-stage vacuum pressure swing adsorption (VPSA) unit, loaded with silver-based zeolite AgLiLSX, for producing ca. 1 L STP ·min −1 of high-purity oxygen. The unit was designed on the basis of the experimental and simulation results obtained with a lab-scale unit. For a product concentration of 99.0%, the recovery obtained was ca. 8.0% with a productivity of 9.0 m 3 ·h −1 ·ton −1 .