Progress in Parametric Pumping

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Abstract
Parametric pumping is the name given to a novel separation device developed by Richard Wilhelm and co-workers(1,2,3). The principle of operation depends on an immobile phase (e.g. adsorbent) to alternately retard and release selected species. This action coupled with a synchronous periodic fluid flow causes the selectively adsorbed species to be literally pumped from one region to another. In the successful experiments(2,3) a single packed column was used with reservoirs attached to each end. A water jacket surrounding the bed was heated or cooled synchronously with periodic fluid motion. Figure 1 outlines the principle of operation. Essentially, the process operates in a “bucket-brigade” fashion. If one follows the travels of a single solute (see Figure 1) in a non-adsorbable solvent we see that during downflow the column is cooled thus retarding (adsorption step) the solute, allowing the solvent to move unimpeded. When the flow direction changes to an upward motion, the column is heated, releasing (desorption step) the solute and sweeping it upward. Repeating this cycle, one sees that the solute hops in bucket-brigade fashion toward the upper reservoir until it is eventually captured. Separation factors (i.e., ratio of rich to lean reservoir composition) as large as 105:1 have been reported(2,3). The fundamental driving force for separation is the adsorbed phase concentration difference at the two operating temperatures. This difference depends on the shape of the isotherms as shown in Figure 2, The method of operation thus outlined has been called the direct thermal-mode(3). The principle of operation is not limited to a thermally induced retardation-release step, and in fact Thompson and Bass(4,5) recently used electrode potential to induce retardation. Furthermore, in a short note Lee and Kirwan(6) outlined results whereby a porous carbon electrode sustained markedly different adsorption levels of biosubstances (glucoamylase) with Increases in applied voltage. In one of his earlier papers(3), Wilhelm suggests a variety of driving potentials may be used, including electrical, pressure, magnetic, and chemical (e.g., pH). Obviously, any potential which can be effectively tuned so as to induce a retard-re lease mechanism could be used in designing a parametric pump. The trick is to discover a retardation-release mechanism which is selective, that is, only the desired species is pumped, while other species remain stagnant or move in the opposite direction (e.g., abnormal isotherm). By way of introduction, it should be noted that the parametric pumping technique is not limited to closed (batch) systems, but can be operated in a continuous and semi-continuous mode. Recent work on such open systems will also be treated in another section.

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