Page 301 - Pressure Swing Adsorption

,I 1
278 PRESSURE SWING ADSORPTION EXTENSIONS OF THE PSA CONCEPT 279

This 1s essentially the cycle that would be used. in a conventional PSA system utilizes a single adsorotwn column oacked with smaller adsorbent
process, in order to recover both products. Of course, m a conventional PSA particles 000-500 µm). As a consequence of-the small oart1cle s·ize, pressure
process these steps would be distinct, whereas, in a TCPSA cycie, they are droo through the bed 1s high 0-2 atm), and the cycle time (typ1callv 3-10
merged, but this does not represent an essen!Ial difference. sec) ts much shorter than m conventionai PSA system; hence the name
One may also choose to regard a TCPSA process as analogous to a "rapid pressure swmg."
distillat1on or countercurrent extraction orocess m which the light and heavy The cycle (Figure 7.10) 1s very s1mole, mvolvmg m its ongmal conceot10n
products are refluxed at each end of the column. The reflux ratio reqmred to only two steps of equal duration: a combined pressunzat,on-oroduct with-
produce pure products depends on the separat10n factor (or relative volatil- I drawal step and the exhaust steo. The RPSA cycle may thus be regarded as a
ity). Just as m a distillation process 1t 1s possible to obtam pure products even PSA cycle in which the oressunzat10n and feed stf!ips are merged and the
I
when the relative volatility is small, by using a high reflux ratio; so, m a ' purge step ts elimmatcd. Regeneration of the adsorbent occurs only durmg
,1
TCPSA process increased reflux may be used to compensate for a low j the countercurrent depressunzat1on step (normally to atmosphenc pressure).
pressure ratio. Whereas convent10nal PSA processes generally operate at A large pressure droo m the direction of flow, durmg the combined oressur-
relatively high pressure ratios but with low ·reflux (in the form of countercur- I izat1on-oroduct withdrawal step, 1s needed to mamtam the reouired purity of
rent purge), TCPSA systems generally operate at much iower pressure ratios the raffinate oroctuct. The pressure gradient between the feed and -the
but with higher reflux. The tradeoff in terms of power consumption obviously oroduct end also allows continuous withdrawal of the raffinate product even
depends on many factors, some of which are system dependent. during the oenod m which the bed 1s bemg regenerated by ctepressunzat1on
from the feed end.
7 .2.3 Scaleup Considerations Dunng the vressunzation-product step the more strongly adsorbed soec1es
I travels less rapidly through the column; so, provided that the duration of the
To date only small laboratory-scale versions of a TCPSA system have been feed step is not too long, the less strongly adsorbed component may be
built, and, although the viability of the concept has been amply demon-
removed at the outlet as a pure raffinate product, Just as in a conventional
strated, important Questions concerning the prospects for scaleuo remam to
PSA process. However, during the countercurrent- deoressurization step,
be resoived. The mam difficulty is the size of the pistons (and cylinders) (see withdrawal of the raffinate product continues at the bed outlet while the flow
Section 7.1). In a typical laboratory unit the ratio of cylinder voiume to m the inlet region 1s reversed. The more strongly adsorbed species 1s thus
adsorbent volume 1s about 10, although this figure varies widely depending on desorbect and removed as a waste product from the feed end of the column.
the adsorbent and the gas composition. Direct scateuo to a production umt, The result of this pattern of pressure and flow vanation rs that. m the inlet
maintammg this ratio, 1s obviously unattractive, since the pistons and cylin- regmn of the bed, the concentrat10n front moves alternatively forwards and
ders become too large and eXPensive. The most obvious way to avoid this backwards, but with a net forward bias, as m a oaratnetric oumo. Since the
difficulty 1s to decrease the cycle time. This would give a proportionate wave velocity is higher for the less strongly adsorbed species, the mole
mcrease m throughput for a given size of system. However, mass transfer fraction of this species increases continuously as the sample of gas progresses
resistance and pressure drop consideratmns impose severe restrictions on the towards the outlet of the bed. This mechanism by which the progressive
cyCte time (and the associated gas flow rates). As a result, with a packed ;' I ennchrnent occurs has been likened to a ratchet. 7
actsorbent bed as the mass transfer device the cycle time cannot be reduced
As a result of the short cycle time the oroduct1v1ty ;m the type of system JS
beiow about 2-3 sec (20-30 rpm). This limitation might, however, be over- generally much greater than for a conventwnal PSA process, operatmg at
come by an improved adsorbent configuration. A monolithic adsorbent or a
comparable product ounty and recovery. This advantage 1s, however, offset
parallel plate contactor with sufficiently small plate spacing anct sufficiently by the much higher energy requirement. A detailed summary of the earlier
uniform gas channels offers the potential for a much faster cycle-up to expenrnental studies has been given by Yang. 8
perhaos 200-300 rpm. At such speeds the cylinder volume per unit through-
put becomes muc11 more reasonable; so that intermediate- and large-scale
7.3. I Modeling and Simulation
applications appear cost effective relative to conventional PSA systems._
The modeling of an RPSA process is similar in orincmle to that of a
7.3 Single-Column Rapid PSA System conventional PSA system (as discussed in Chapters 4 and 5) except that the
assumption of negligible pressure drop, which 1s generally a valid approxima-
A-third PSA variant that may also be regarded as a variant of the parametric tio~ durmg the feed an~ purge steps of a_ conventio:nai cycle, 1s no longer
5 6
pumo was suggested by Kadlec and co-workers m the early 1970s. • This vahd. The pressure gradient plays a key role m an RPSA process and must