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.
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