Displacement purge

Stea.ming Retjuirements. The steaming of fixed beds of activated carbon is a combination of thermal swing and displacement purge swing. The exothermic heat released when the water adsorbs from the vapor phase is much higher than is possible with heated gas purging. This cycle has been successhiUy modeled by equiUbrium theory (128). 

Regenerative adsorption units can be operated by a thermal-swing cycle, pressure-swing cycle, displacement-purge cycle, or inert-purge cycle. Combinations of these are frequently employed. 

Displacement-purge forms the basis for most simulated continuous countercurrent systems (see hereafter) such as the UOP Sorbex processes. UOP has licensed close to one hundred Sorbex units for its family of processes Parex to separate p-xylene from C3 aromatics, Molex tor /i-paraffin from branched and cyclic hydrocarbons, Olex for olefins from paraffin, Sarex for fruc tose from dextrose plus polysaccharides, Cymex forp- or m-cymene from cymene isomers, and Cresex for p- or m-cresol from cresol isomers. Toray Industries Aromax process is another for the production of p-xylene [Otani, Chem. Eng., 80(9), 106-107, (1973)]. Illinois Water Treatment [Making Wave.s in Liquid Processing, Illinois Water Treatment Company, IWT Adsep System, Rockford, IL, 6(1), (1984)] and Mitsubishi [Ishikawa, Tanabe, and Usui, U.S. Patent 4,182,633 (1980)] have also commercialized displacement-purge processes for the separation of fructose from dextrose. 

Chromatography Chromatography is a sorptive separation technique that allows multicomponent separations in both gas and Hquid phase. As a preparative tool, it is often used as a displacement-purge process, although many applications employ an inert-displacement mode, especially for use in analysis. General characteristics and operating modes are discussed in a separate part of this section. 

FIG. 16"41 Ideal displacement-purge swing cycle. Repiinted with peimission of UOP.)... 

While inert and displacement purge regeneration is widely used in liquid phase separations, there are few industrially relevant inert purge systems employed in gas phase separations. It is sufficient to note that an inert purge regeneration can be done and it will generally be most effective at relatively high adsorption temperatures. 

Displacement-Purge Cycle This cycle, which is somewhat similar to the previous one, differs from it in that a gas or vapor which adsorbs about as strongly as the adsorbate is used to remove the adsorbate (see Figure 1) Removal is thus facilitated both by adsorbate partial-pressure reduction in the fluid around the particles and by competitive adsorption of the displacement medium. As with the inert-purge cycle, the maximum delta loading is the equilibrium loading ... 

Figure 1. Displacement-purge cycle. A, adsorbate B, less-adsorbed component and D, displacement agent.

Gas bulk-separation processes consist mostly of pressureswing adsorption (PSA) variations and displacement-purge and Inert-purge processes Recently, however, a chromatographic cycle has been commercialized Below, these cycles will be discussed ... 

Displacement-Purge and Inert-Purge Cycles By far the most widespread embodiment of these cycles Is the separation of normal and lso-parafflns In a variety of petroleum fractions. These fractions In general contain several carbon numbers and can Include molecules from about C5 to about C g All of these separations use 5A molecular sieve as the adsorbent Its 0.5 nm pore diameter Is such that normal paraffins can enter but lso-parafflns are excluded this constitutes the basis for separation. 

As mentioned earlier, the use of a non-condensable, non-adsorbing carrier gas simplifies the task of recovery of products compared to that task in displacement-purge adsorption. At the same time, the scale-up of this process means that the effects of separation-inhibiting thermal gradients accompanying adsorption and desorption have been dealt with successfully. 

---