Adsorption inert-purge

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. 

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 ... 

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 ... 

Compared to adsorption s use in bulk-gas separations, its use in gas purifications is much more frequent (see Table I and references 13, 36 and 37), and the technology is, for the most part, more conventional Temperature-swing adsorption, often combined with inert-purge stripping, is by far the most common process used Two or more fixed beds operated in parallel, typically with one adsorbing and one or more regenerating, constitute the standard flowsheet ... 

The simplest cycle is the so-called inert-purge cycle, shown in Figure 14.19. It is basically a two-step process of a few minutes or hours duration. First is adsorption (often with heat evolution). 

FIGURE 14.19 Simple two-column inert-purge adsorption system. 

Liquid Adsorption. Adsorption takes place selectively because of (1) polarity differences between adsorbate and unadsorbed liquid, or (2) differences in size and structural characteristics of molecules. In the first category we may mention activated carbon, in the second one the molecular sieves. Liquid adsorption is suitable for both recovery of small amounts from dilute solutions, and for bulk separations. Purification is feasible with very selective adsorbent. Recovery of adsorbate and regeneration of the adsorbent may be performed by (1) Thermal Swing Adsorption, (2) Pressure Swing Adsorption, (3) inert-purge swing, or (4) displacement desorption. 

As adsorption occurs, the adsorbent increases in temperature as the heat of adsorption is released. Conversely, cooling occurs during regeneration. Because adsorption capacity is reduced as adsorbent temperature rises, inert-purge processes are usually limited to from 1 to only a very few kg adsorbate per 100 kg adsorbent. Cycle times, in ctMitrast to temperature-swing processes, are only a few minutes and almost always less than 10 min. 

Recently, a quite different use for an inert-purge cycle has emerged the removal of large amounts (up to and exceeding 20 wt. %) of water from organic streams. Thus, inert-purge adsorption can now compete directly with azeotropic and extractive distillation and other means for a number of azeotrope-breaking separations, the most common of which is the production of dry ethanol. The process, as depicted in Fig. 

---