Page 25 - Pressure Swing Adsorption

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CHAPTER
1

Introduction

Pressure swing adsorotion {PSA) 1s not a new orocess and, like mosr good
mventtons, with the advantage of hincts1ght the onnciole appears obvious. As
m all adsorption separation processes, the essential reoulfement 1s an adsorH
bent that preferentially adsorbs one comoonem (or one family of related
components) from a mixed feed. This select1v1ty may depend on a difference
m actsorotlon equilibrium or on a difference m sorpt1on rates (kinetic
seiectivity). ln certam cases the difference m rates may be so great that the
slower-diffusing species 1s m effect tota\lV excluded from the ad5orhent
{s1ze-select1ve s1evmg), and m this situation a verv efficient separation can
obv10usly be achieved.
All adsorot1on separation processes involve two ormc1oal steos: (1) adsom-
tton, durmg which the oreferentrnllv adsorbed species are picked up from the
feed; (2) regeneratton or desorvtton, ctunng which these species are removed
from the adsorbent, thus "regenerating" the adsorbent for use m the next
cycle. The gcncrai concept 1s shown m Figure 1 .-1. It is possible to ohtain
useful products from either the adsorption or regeneration steps or from
both steps. The effluent dunng the adsorption step is purified "raffinate"
product from which the preferent1ally adsorbed species have been removed.
The ctesorbate that 1s recovered durmg the regeneration step contams the
more strongly adsorbed species m concentrated form (relative to the feed)
and is sometimes called the "extract" product.
The essential feature of a PSA process 1s that, during the regeneratJOn
step, the preferentially adsorbed species are removed by rcducmg the total
oressure, rather than by ra1smg the temperature or ourgmg with a displacing