Equilibrium theory revisited. Isothermal fixed-bed sorption of binary systems-II. Non-Langmuir solutes with type I parent isotherms: Azeotropic systems

Abstract

The preceding analysis of the equilibrium percolation of binary Langmuir solutes (Part I) is extended to systems with arbitrary equilibrium relations and parent isotherms of Type I. The use of “extended” binary Langmuir isotherms is proposed as the best means for providing some analytical insight while satisfying important thermodynamic and mathematical requirements. We show that the simple method for locating the watershed point given in Part I can be extended to broad classes of parent solute isotherms. It consists of determining, analytically or graphically, the intersection of the light component Henry's line with the heavy component isotherms. This opens up a rapid path for assessing the effectiveness of displacement agents in purging a loaded bed and achieving high levels of desorbate enrichment (see Part III). The behaviour of binary non-Langmuir solutes, both with and without selectivity reversal is addressed. The latter case yields solute trajectories and other properties which resemble, in all major aspects, those established for Langmuir solutes (Part I). It is shown that under certain conditions minor changes in the equilibrium parameters can lead to serious aberrations in the predicted solute trajectories. The use, in particular, of single component isotherms to represent strongly sorbed solutes yields highly erroneous predictions of binary desorption and displacement processes. The effect of selectivity reversal is expressed through azeotropic loci, incorporated in the hodograph diagrams. These curves constitute deflecting barriers in some cases and can be traversed by the solute trajectories in others. The distinguishing feature of the hodographs is the shape of the negative characteristics in q- Y space which inflect across a diagonal in much the same way as binary vapour-liquid azeotropes. The resulting solute fronts still follow the binary Langmuir pattern in most instances, except in certain displacement operations and in cases of simultaneous azeotropic propagation. The inflecting form of the Γ − characteristics also gives rise to the interesting possibility of isolating pure gaseous solute fractions from a binary feed through the use of dual column systems operated at different total pressures. Various solute pathways, created by the presence of the azeotropic curves are analyzed, and compared with experimental column breakthrough curves. The extreme sensitivity of certain azeotropic pathways to the precise form of the equilibrium relation is discussed.