Displacement purge gas

A major requirement for a successful displacement purge gas/vapor, the displacer, however, is that it should not be too strongly adsorbed compared to the feed species to be desorbed. If it is, then it would be difficult to desorb it during the adsorption part of the next cycle. In fact, ideally, it should have an almost identical affinity for adsorption on... 

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). 

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. 

Air purging This refers to the displacement of gas or gas/air mixture with air. The purge is complete with ... 

If, as illustrated in figure 12.6, the isothermal starting lines of the various curves do not coincide, then A< >o, A< cai, and Aheat transfer change between runs, for example, due to a variation in the purge gas flow or the fact that it is virtually impossible to relocate the crucible containing the sample exactly in the position used for the calibrant run (normally the reference crucible remains in place throughout a series of runs). Note that a similar correction should have been used in the computation of heat flow or area quantities if, in the example of figure 12.4, the isothermal baselines of the main experiment and the zero line were not coincident. 

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. 

In contrast to perfect displacement, Eq. (4) demonstrates that perfect mixing leads to a reduction in the air content by a factor of e (0.37) after one chamber volume of purge gas is consumed. Similarly, it may be shown that 8.8 chamber volumes are required to reduce the air content to 150 ppm, and about 10 are necessary to attain 45 ppm. Unless a short circuit occurs because of the inappropriate placement of entrance and exit ports (Fig. 2.2c), the actual situation can be expected to fall between perfect displacement and perfect mixing. Experi-... 

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 ... 

In pressure-swing processes, it Is quite common to use a fraction of the less-adsorbed gas product as a low-pressure purge gas, as shown In Figure 2. Often the purge flow is in the opposite direction from the feed flow. Rules for the minimum fraction of less-adsorbed gas product for displacing... 

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 ... 

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. 

Displacement desorption. The method is essentially the same as for purge gas stripping but the regeneration stream is adsorbed and displaces the adsorbed species... 

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