Page 225 - Pressure Swing Adsorption

202 PRESSURE SWING ADSORPTION DYNAMIC MODELING OF A PSA SYSTEM 203

in a basic Skarstrom PSA L-ycle are oressurizatlon, high-pressure adsorption,
countercurrent blowdown, and countercurrent purge, as represented in Fig-
ure 5.l0(a). If mass transfer between solid and gas phases during the
pressunzallon and blowdown steos 1s assumed to be negligible, then at cyclic
steady state the amount adsorbed during the adsorption step should be equai
to tlle amount desorbed during the purge step. Transient PSA s1muiatt0n
with a frozen solid approximation during oressunzat10n and blowdown has
4 7 10 25
been validated for both purification orocesses - • • and kinetically con-
• Therefore, for these processes, the operatton of a
trolled bulk seoar~tion. 13 57
Skarstrom cycle at steady state can be viewed as a continuous countercurrent
flow (CCF) system m which the immobile solid phase adsorbs from the
high-pressure stream and desorbs to the purge stream with zero net accumu-
lation in the solid phase. This reoresentation is shown schematically m Figure
;,;.
5.!0(b). :/l
The idea of representmg the PSA system as a contmuous countercurrent ,;; ,~,
flow ooerat1on was first proposed by Suzuki. 58 He developed the CCF model 'l· f
for a trace component system and compared the steady-state concentration
profiles from this model with those from the transient simulation. In addition
to usmg the frozen solid approx1mat10n during pressunzat1on and blowctown
for the transient simulat10n, Suzuki also adopted rapid cycling to attain a

small throughput ratio so that solid- and gas-phase profiles also rernamed
nearly frozen during the adsorption and the desorpt1bn steps. Under these
conditions, the steady-state profiles from the CCF model were found to be m
good agreement with those from the transient s1mulat1on, as may be seen
from Figure 5.11. While these extreme assumohons may be realistic for a
purification process, in a bulk separation process there will generally be
significant excursions of the concentration orofiles during the adsorption and
the desorption steos. Farooq and Ruthven 59 extended the CCF model for a
bulk separation process (the model equations are giVen m Table 5.9) and
showed that, even without a very small throughput ratio. the CCF model still
correctly predicts the qualitative trends of experimental purity and recoveIY
data for a PSA nitrogen umt using carbon molecular sieve. The results,
mcluding transient model oredict1ons, are shown m Figure 5.12. This study
also showed that mass transfer during pressunzat1on and blowdown steos will
not impair the predict10ns of the CCF model. Although the transient model
10 '----'----'---'---'----'--~-~~
0.0 0.25 0.5 0.75 is quantitatively superior, the simplicity and computatmnal efficiency make
LENC.TH (mi the CCF moctei useful at least for mitial selection of the range of conditions
within which more detailed studies should be concentrated. There are,
Figure S.ll (a) Solid-phase and (b) gas-phase concentration Profiles from the CCF
model (-- adsorption, --- desorption) and the transient simulation model (0 end however, some limitations of this approach. The CCF model .solution 1s
of adsorption, • end of desorption) are compared for air drying on activated aiumina. invariant to changes in cycle time as iong as the· durations of the adsorptmn
(From Ref. 58; reprmted with perrmssion.) arid ctescnptmn steos are keot equal. The CCF model 1s applicable in