Page 94 - Pressure Swing Adsorption

68 PRESS II RE SWING ADSORPTION
PSA CYCLES: BA51C PRINCIPLES 69
Table 3.1. Summary of the Elementary Steps Used in PSA Cycles
Table 3.1. (Conmwed)
Elemenrnry step Mo<le ot operatmn Principal features Elementary steri Mock of orierat1on Princira! feature~
Pressunzat1on i. Press11nzu11on with Enrichment or the less Pressure The high- and low- Comerves energy ;lnd
fee<l from the foed selectively adsorbed egu&lization pres,,ure bc<ls are separative work
end species m the gas phase
either connected
at the product end
through their product
2. Pressurization with Sharpens the concentration
ends or the teed and
raffinate product from front, which improves the
product ends of the
the product end pnor purity and recoverv of
high-pressure hed are
to feed pressunzat1on raffinate product connected lo the
respecuve ends ot the
High-pressure 1. Product (raffinate) Raffinate produc1 1s
low-pressure bed
adsorption withdrawal at constant delivered at high pressure
coiumn pressure Rinse The bed is purged with i'mprove~ extract product
2. The column r,ressure ·1s Verv high recovery of the the preferentrnllv purny when !he liRhler
;1llowed to decrease less sciectivliv adrnrhed
atl.'>orhcd specie., ;1ftcr spcoc~ <m.: coachorhed in
while the rullinuie species rnav be achieved.
high-pressurt: large .imount with heavier
product 1s drawn from hut lhc product 1s adsorption at fccU components
the product end delivered at low pressure
pressure m I he
direc:tmn of the feed
Blowdown I. Countercurrent Usetl when onlv raltinaie
blowdown to a low product 1s reomred at high
pressure punty; prevents contammat1on
of the product end with more 4. Desorption at the iower operating pressure; this mav be accomplished by
strongly adsorbed species
evacuation, purgmg the bed with the rnffinate product or, m a kinetically
2. Cocurrent blowdown Used when extract product
to an mtermediate ,s also reamred in high controlled process, by slow equilibration with consequent evoluuon of the
pressure pnor to punty; improves exlract slower-diffusing sorbate;
countercurrent product purltV and mav also 5. Pressure equalization (which JS used in many cycles. pnor to the blowdown
blowdown mcrease raffinate recovery step, to conserve energy and seoarative work);
6. Rinse (purging with the oreferentially adsorbect-soecies at high pressure,
Desorption I. Couniercurre111 Improves raffinate product
at low desorption with at the expense ot decrease following the adsorotmn step).
pressure product purge m recover,,: purge at The processes differ from one another 111 the sequence of the elementary,
subatmmphcnc pressure
steps and m the way in which these steps arc earned ouL Some of the mon;
reduces raflinate product
loss bm increases energy important vanants and the benefits derived from them are summarized 10
COSI Table 3.1.
2. Countercurreni Recovery enhancement while
To understand a PSA cycle properly It 1s necessary to know the way m
desorption without mamrnmmg high product
external purge purity 1s possible oniy m which the concentration profile moves and changes 'shape during each of the
elementary steos. Gas-phase concentratrnn profiles m an adsorotion column
certain kinetic separation
3. Evacuation High punty of both that undergoes m sequence oressunzatton, high-ptessure adsorot1on. blow-
extract and raffinate down, and low-pressure desorption are shown m Figure 3. i for both equilib-
products; advantageous over rium and kinetic separations. The profiles were calculated for pressurization
producl purge when the
allsorbei.l phase 1s very with feed gas (from the feed end with the product 'end closed), reverse flow
strongly hdd biowdown, and desorption with product purge, aiso in the reverse flow
direction. It 1s clear that the movements of the concentration wave are
(Cml!mued)
similar in both cases. During pressurization the initial gas in the hed 1s
pushed toward the closed product end, where it forms a plateau that 1s
significantly enriched in the less strongiy adsorbed species. The region before
the plateau shows the penetration of the feed gas. This behavior 1s m