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Abstract: The extensive kinetic data accumulated, over many years, from a series of experimental studies of the kinetics of adsorption/desorption of a wide range of different sorbates in type A zeolites are reviewed and analyzed in an attempt to develop a coherent understanding of the behavior of these systems. Kinetic data for large laboratory synthesized crystals and small commercial crystals, measured under similar conditions, have been studied in detail. In well dehydrated crystals the sorption rates are generally controlled by intracrystalline diffusion but exposure to traces of water leads to the development of surface resistance and a pronounced reduction in the sorption rate. Zeolite samples of different origin show widely different sorption rates but the diffusional activation energies (for a given sorbate) are essentially constant. The differences between the different samples appear to be due mainly to differences in the cation distribution caused by differences in the initial dehydration procedure. The ideal cation distribution (in 5A) in which all window sites are unoccupied is realized only in very carefully dehydrated samples in which the water was removed slowly at gradually increasing temperature under a high vacuum. In the small commercial 5A crystals many of the windows are blocked, even though all the cations could theoretically be accommodated in the 6-ring sites. This is probably due to cation hydration reactions which are likely to occur when the dehydration conditions are not carefully controlled. [Copyright &y& Elsevier]

The kinetics of adsorption and desorption for an adsorbent particle, subjected to a step change in surface concentration, have been analyzed in detail for isothermal diffusion-controlled systems in which the diffusivity increases strongly with loading. The transient adsorption curves retain the same shape as for a constant diffusivity system, even when the diffusivity varies by more than an order of magnitude. The effective diffusivity increases with the size of the concentration step and depends on both the diffusivity ratio (between final and initial values) and the form of the concentration dependence. When the diffusivity ratio is high the desorption rate also increases with step size but by a much smaller factor. For very high diffusivity ratios (more than an order of magnitude) the shape of the desorption curves changes showing a much slower final approach to equilibrium than for a constant diffusivity system. A simple model is developed for such systems leading to a simple analytic expression for the desorption curve. The theory is validated by comparison with representative experimental data and used to explain some of the apparent anomalies in previously published studies. [Copyright &y& Elsevier]

The performance of an endless belt counter-current liquid phase adsorption contactor has been studied experimentally for the removal of Cu2+ from aqueous solution by a cation exchange resin. Although, such a device appears to offer the possibility of a highly efficient contactor, the performance is in fact severely constrained by the effect of solid side mass transfer resistance (diffusion of Cu2+ within the resin) and by internal recirculation of the fluid, which becomes severe at high belt speeds. As a result of these constraints the system behaves effectively as a single theoretical stage and both the steady state and transient behaviour are adequately modeled on that basis. [Copyright &y& Elsevier]

A preface for the October 2007 issue of the "Adsorption Science & Technology" is presented.

Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements. [ABSTRACT FROM AUTHOR]