Separation of Molecules, Macromolecules and Particles Principles, Phenomena and Processes

Separations in which the feed phase or feed-containing phase has a bulk motion parallel to the direction of the force causing separation have been studied in Chapter 6. In many separation techniques/processes/operations, the feed phase or feed-containing phase has a bulk motion perpendicular to the direction of the driving force. In a number of situations, the feed is introduced in small amounts in a carrier fluid whose bulk motion is perpendicular to the force direction. Such separations will be studied in this chapter.

For separations based on distribution of species between two phases in equilibrium, we study in Section 7.1 primarily those two-phase systems where the second phase (e.g. adsorbent particles in a packed adsorbent bed) is stationary; the first phase, which is more often the feed solution/mixture, moves perpendicular to the direction of chemical potential driving force between the two immiscible phases (Figure 7.0.1(a)). This first phase (the mobile phase) bulk motion is generally in one direction. The benefits of such bulk motion in terms of extreme purification achievable in two-component systems and multicomponent separation capability will be illustrated. In the cyclic processes studied next, the direction of motion of the mobile phase is periodically reversed; the direction of force is also reversed, except it remains perpendicular to the bulk-phase flow direction (Figures 7.0.1(b) and (c)). The mobile phase is sometimes generated by the separation operation, as in the case of the blowdown phase of pressure swing adsorption (PSA). The elution chromatographic process considered next involves injection of a sample to be separated into a carrier fluid flowing perpendicular to the direction of the force between the fluid and the stationary adsorbent phase (Figure 7.0.1(d)); this and other related chromatographic processes are also studied in Section 7.1.5. The imposition of an electrical force parallel to the direction of flow in a packed chromatographic column is studied in Section 7.1.6 in what is called counteracting chromatographic processes.