Equilibrium theory revisited. Isothermal fixed-bed sorption of binary systems-III. Solutes with type I, II and IV parent isotherms: phase separation phenomena

Abstract

In the third part of this series, we analyse the isothermal equilibrium sorption of two solutes with Type I, II and IV parent isotherms, with particular reference to conditions which may lead to phase separation (condensation/precipitation). The study begins with a general survey of displacement operations based on concepts developed in Parts I and II. A displacement effectiveness scale is developed in terms of single solute isotherm shape and location of the watershed point, determined by drawing a tangent to the origin of the light component isotherm. Effectiveness is judged by the concentration range over which the displacement agent is active, and the degree of desorbate enrichment (roll-up) obtained. The procedure allows, by simple inspection of the parent isotherms, to determine conditions which favour rapid displacement and high levels of enrichment. A second geometrical construction, that of a tangent from the q- Y origin to an inflecting Type II or IV isotherm, emerges as a critical dividing line beyond which displacement leads to a “catastrophic” jump discontinuity in plateau concentrations, with potential for producing high enrichment levels. Phase separation phenomena are addressed in order of increasing complexity. Single component separation of either the light or heavy solute may take place depending on the initial and feed concentrations. Some simple criteria for its occurrence are given, and attention drawn to the possibility of isolating a solute by displacement precipitation. Simultaneous separation of both solutes occurs when the hodograph trajectories cross a binary phase boundary. Hodographs are presented of two real solute pairs of Type II isotherm parentage with potential for simultaneous condensation. The effect of isotherm shape and phase separation on solute trajectories is discussed. An example is given of a real solute with a Type II isotherm which would experience a positive jump discontinuity in roll-up values upon displacement with a lighter component.