Pressure swing adsorption vacuum desorption

In the TIP process the Hysomer process is combined with the ISOSIV process which separates normal paraffins from branched ones by selectively adsorbing the normal fraction into zeolite CaA (pressure swing adsorption). Ajfter desorption (by applying vacuum) the normal paraffins are recycled. A schematic view... 

These differences become more pronounced with increasing deviations of the adsorption isotherm from a hnear relationship. As a rule the decision on the separation process as temperature swing adsorption (TSA), pressure swing adsorption (PSA), or vacuum swing adsorption (VSA) is a compromise between the steps of adsorption and desorption. Applying the PSA technique on approximately linear adsorption isotherms is advantageous. 

The adsorption literature often classifies the adsorptive processes in terms of (a) Pressure Swing Adsorption (PSA) when the adsorption step is carried out at a super-atmospheric pressure level and the lowest desorption pressure is ambient (Processes A, D, E, F, G), (b) Vacuum Swing Adsorption (VSA) when the adsorption step is carried out at near-ambient pressure level and the lowest desorption pressure is sub-ambient (Processes H, I) and (c) Pressure Vacuum Swing Adsorption (PVSA) when the adsorption step is carried out at a super-atmospheric pressure level and the lowest desorption pressure is sub-atmospheric (Processes B, C). 

Prior to ca. 1980, PSA systems were used with both adsorption and desorption pressures well above atmospheric. These systems were low in capital (due to simplicity) but high in power consumption (since both N2 and O2 in the feed are compressed compared with vacuum swing adsorption (VSA) where only the waste gas is evacuated). The availability of improved sorbents and lower cost vacuum equipment led to the development of VS A, which is typically operated with adsorption pressure slightly above atmospheric and desorption pressure of typically 0.2 atm. A further breakthrough occurred in 1989, with the invention of LiLSX zeolite (low silica X, with Si/Al = 1) (Chao, 1989). The LiLSX is currently the best commercial sorbent for air separation and will be discussed... 

Pressure range. The process performance is often enhanced for a PSA cycle with desorption at sub-atmospheric pressures, usually called vacuum swing adsorption (VPSA). 

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