Dynamics of physical adsorption for porous grains with a complex internal structure

Abstract

The dynamics of physical adsorption for a porous medium consisting of porous grains having a complex internal structure is analysed. The analysis is based on a combined theoretical model describing the mass transfer of an adsorbate inside a porous adsorbent grain with a complex internal structure. The model takes into account the effect of the dead-end transport channels (pores) inside the porous adsorbent grain on the nature of the mass transfer of a substance. For frontal adsorption dynamics with convex isotherms equations have been developed for calculating the width of the zone corresponding to intensive mass exchange in the adsorber (the width of the stationary front). Equations were also obtained for calculating the times of relaxations due to different mechanisms of interphase mass transfer inside a porous grain with a complex internal structure and to mass transfer in the intergrain space of the porous medium. It is shown that a strong interaction can take place between the mass transfer within the zones of a porous grain and the dead-end transport channels. When this occurs the width of the stationary front and the relaxation times characteristic of mass transfer inside the porous grain are in quasilinear dependence on the geometrical dimension of the porous grain and the geometrical dimension of the microporous zone inside the porous grain. These dimensions are: radius R0 in the case of a porous grain of an adsorbent having a spherical form and radius R0 in the case of a microporous spherical zone. An analysis of the frontal dynamic curves and the measurement of the coefficients of diffusion inside the microporous zone by other physical methods (e.g., n.m.r. measurements) show a lack of coincidence of the values of the diffusion coefficients. The possible cause of this non-coincidence is indicated.