Page 273 - Pressure Swing Adsorption

l interstitial gas 1s transferred through the feed end to the A bed (counter-
250 PRESSURE SWING ADSORPTION ' PSA PROCESSES 251

current), thus rectucmg the pressure from PH to Pi-
J 83. Countercurrent Deoressunzation II. The bed is connected to another B
bed, which 1s undergoing step 86 and more of the desorbate and
mterstitial gas 1s removed (countercurrently) through the feed end,
reducing the pressure to P •
2
B4. Countercurrent Deoressurizat10n III. The bed is blown down from P to
2
atmosohenc pressure, and the effluent gas, which contains a orooort1on
of the feed imourities together with some hydrogen, 1s reJectect.
B5. Countercurrent Purge. The bed is ourged at atmospheric oressure with
high-purity hydrogen product to desorb any impurities further. The
effluent is rejected.
B6. Cocurrent Pressunzat1on. The pressure m the bed is raised to P 2 by
+ 888 connecting with another B bed undergomg step BS.
I o-t-0, ..................... 0,00 B7. Countercurrent Pressurizat10n. Final pressurization of the B bed to PH
0
0-000-0\
10
is accomplished with hydrogen product, mtroduced from the product
end. During the later part of this step the B bed is connected with
+ + + another A bed undergoing step A6 and both beds are then pressurized
g::gg to PH.
"' 0- 0-
g: ~ g:
The B beds pass through two complete cycies (steps Bl to B7) while the A
beds go through one cycle so that each B bed handles the gas from two A
beds during the complete cycle. This approach reduces significantly the size
of the B beds. A key feature of this cycle 1s that =the A and B beds are
connected m senes dunng the adsorption step but thev are regenerated by
00 - 0
!'-- IO in two entirely different sequences. The overall performance 1s sumrnanzect m
00 0
00 0
Table 6.5. It 1s evident that the hydrogen product has a puntv greater than
99.999% and the fractional recovery 1s about 86-87%, while the CO prod-
2
uct is produced at a purity of about 99.4% with about 90% recovery. The
Gemini-8 process gives slightly lower ounty and recovery of CO , but there is
2
a significant reduction in the size of the compressors and the oower con-
sumption.
6.10 PSA Process for Concentrating a Trace Component

In the processes described so far in this chapter the objective has generally
been to produce a oure raffinate product, although in some cases the more
strongly adsorbed species (the extract product) 1s also recovered m concen-
trated form. However, particularly m environmental applications, it is often
necessary to concentrate a trace component for disoosal or further process-
ing_._ Provided that a sufficiently selective adsorbent 1s available, PSA appears
to be well smtect to such applications, although to date few, if any, processes
of this kind have been commercialized. Examoles of :two such processes that
have been developed to the pilot plant scale are described m this section.