Page 91 - Pressure Swing Adsorption

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CHAPTER
3

PSA Cycles: Basic Principles

The mode of operation of a PSA separation process 'was explained in general
terms m Chapter 1 without any discussion of the ;details of the ooeratmg
cycle. The choice of a stutable ooeratmg cycle 1s in- fact cnt1ca1, and a wide
range of different cycles have been oroposed to oot1rn1ze different aspects of
the overall process. The underlying pnnc1ples govern mg the choice of ooerat-
mg cycle are reviewed m this chapter.
PSA processes may be categorized according to the nature of the adsorp-
tion selectivity (equilibnum or kinetic) and whether· the less strongly (or less
rapidly) adsorbed species (the raffinate product) or the more strongly (or
more raoidly) adsorbed soec1es (the extract oroduct) 1s recovered at high
ounty. The vanous possible combinations of these criteria v1eld four different
classes of process. Since the factors that dommate_ the ch01ce of operating
cycle and the mode of operation are somev,'hat different, these four cate•
gones are discussed sequentrnlly.

3.1 Elementary Steps

Any PSA cycle can be considered as a sequence of elementary steps, the
most common of which are:
1. Pressunzatton (with feed or raffinate product);
2. High~pressure feed with raffinate withdrawal;
3. Depressunzat1on or "blowdown" (cocurrcm ot countcrcurrem rn the
feed);

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