Overview of F d Separations

The F( + )d methods, like techniques in the F(+)cd class, require differential flow for separation. However the nature of the differential flow differs in the two cases. Since the F(+)d methods induce enrichment only at an interface between two phases, the sole requirement of differential flow is that one phase assumes motion relative to another phase. It is usually an easy matter to instigate the relative motion of phases. 

A few simple differences in the properties of immiscible phases make possible their relative displacement. Most simply, if the phases have different densities they will automatically acquire a relative motion in a gravitational field. Thus in adsorptive bubble separation methods, bubbles injected into a column of liquid rise toward the upper surface. Separation occurs by combining the relative enrichment of components at the bubble interface with the continuous displacement of bubbles through the liquid [33-35]. 

Fractional distillation works similarly. There is a division into vapor and liquid phases, the first enriched in solute(s) of relatively high vapor pressure and the second enriched in solutes of low vapor pressure. This enrichment is amplified by the relative displacement of phases. Again the relative motion occurs by virtue of the differences in densities of the phases coupled with the action of gravity. The downward flow (reflux) of the liquid stream is a direct consequence of gravity and is responsible for the accumulation of high-boiling components at the bottom of a distillation column. The motion of the vapor, countercurrent to the liquid stream, sweeps low boilers to the top [36]. 

The phase left in the large interstices between support particles is unanchored and can be driven by an applied pressure gradient. It is necessary, of course, that the viscosity of this mobile phase be relatively low so that flow can occur without excessive pressure drops. 

We note that electroosmotic flow, originating at any charged interface (i.e., almost any real interface), is an effective mechanism for relative phase displacement. Its use in chromatography has been championed by Knox and co-workers [38]. 

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