Page 114 - Pressure Swing Adsorption

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l'HESSUHE SWING AIJSOHl'TION PSA CYCLES: BASIC l'HINCIPLES 89
t BYPACOUCT
~

COL. I FEED r--'-~ BYPRODUCT
TANK TANK

FEED
AIR
i
COL.2
G
u
A L
R
0 FEED u
Feed COMPRESSOR
B M
Pressurization Adsorption Pressure Slowdown Desorption Pressure E N
Equalization Equalization 0
Figure 3.17 The sequence of operations 1n a two-hcd PSA cycic 1nclut.ling dtrnl-cndcd
pressure cqunliz,1tmn und no purge.

3.4.2 Kinetic and Equilibrmm Effects in Opposition
When equilibrium select1v1ty favors the slower-diffusing component, as m air
separation for mtrogen production on modified 4A zeolite (see Table 3.2 for
quantitative values), the system cannot be made self-purging smce the amount
of desorbing nitrogen 1s not sufficient to eliminate the residuai oxygen
concentration in the bed. For such a system convent1onal purge or vacuum
regeneration is necessary. Shin and Knaebel 19 have reoortcd an extensive
expcnmental study of this system. They adopted the Skarstrom cycle 111 a
LEGEND
single-bed apparatus and therefore enJoyed the additional freedom to opt1-
m1ze the cycle by vmymg independently the duration of the individual steps. PC Pressure Controller
Their expenmental system 1s shown m Figure 3.18. In such an arrangement PT Pressure Transducer
an additional tank was used for product storage On order to supply the FC Flow Controller
FM Flow Meter
purge) and pressure ·equalization cannot be performed. However, the con~
SV Solenoid valve
te·nts of the auxiliary -tank may be used to oarhally reoressurize the bed in a NO Normally Open
manner essentially eomvalent to a pressure equalization step. NC Normally Clos&d
Blowdown and purge under vacuum were also investigated in this study for COM Common
comparison. Some of the more important findings are summarized m Tables Figure 3.18 The smgie-bed experimental PSA svsiem used by Shin and Knaebel to
3.3 and 3.4. As the pressure ratio increases, the punty increases, but the study air separatmn on a modified 4A zeolite. (From Ref. 19}