Page 294 - Pressure Swing Adsorption

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270 PRESSURE SWING ADSORPTION
EXTENSIONS OF THE PSA CONCEPT
271
these sm1Ple arguments; ootirnai performance is obtamed when the phase
angle difference is about 45° and the ratio of the piston strokes ts about 3.5:1. \
As a separation process the pressure swmg oarametnc pump has two
maJor advantages:
I. It can produce two oure products; so complete resolution of a hinary
hot space
nuxture may lie achieved without the complexity of the purge and nnse (wod~ing gas 1s
steps that are reauirect to accomplish this in the normai PSA mode (sec / found in here
while it expands)
Sect10n 6.3).
2. The system can be easily designed to provide efficient energy recovery, ,-regenerator
since, on the -exoans1on stroke, the 01stons are dnven by the pressure of
the gas. Conseivation of this energy for use in the next compression stroke displacer ~cooler
c3n be easily accomolished either usmg a flywheel or by coupling together
two umts operatmg-out of phase.
There 1s, however, one serious disadvantage: the pistons and cylinders
must be large enough to accommodate virtually all the gas desorbed from the cold space
bed at the lowest pressure of the cycle. For a bench-scale umt this 1s not a (working gas 1s
~ found in here
serious problem, but it does present a serious obstacle to scaleup. when compressed)
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7 .2 Thermally Coupled PSA

In the previous section we considered the molecular gate as a oressure-driven l Figure 7.4 The pnnc1ple of operation of the Stiriing engme (dispiacer type).
oarametnc oumo. This system is also closely related to the Stirling engine
and thus to a nOvei class of processes that utilize oenodic vanatiOns in both
pressure and temperature together with an oscillating gas flow to effect an working gas, and the eqmvaient amount of heat is transferred from the cold
energetically efficient separation. The basic elements of a Stirling engme are region to the hot region.
shown in Figure 7.4. As in the molecular gate there are two 01stons: a In order to reduce unnecessary heat losses, a regenerative heat exchanger
pressure piston and a displacer, in an arrangement that ts similar tn essence IS mclucted between the hot and cold regions. This is essentially a soace
to that shewn in Figure 7 .2. The working gas 1s transferred backWarcts and
oackect with high-heat-capacity matenal that 01cks up heat from the hot gas
forward between the ''hot space" and the "cold space" by the displacer
as It flows to the cold space, stores 1t. and transfers it to the cold gas
otston. There is very littie difference in pressure between the hot and cold
returning from the cold space on the next stroke of the displacer.
~paces: so the disoiacer does very little mechamcal work. However, the A thermally coupled PSA (TCPSA) system can be thought of as a Stirling
4
Pressure throughOll~ the system varies sinusoidally as a result of the move- engme in which the regenerator 1s packed with a selective adsorbent and the
ment of the pressure piston. When 'the system operates as an engme, gas
gas to be separated 1s the working fluid. Two possible arrangements are
expands m the hot space and flows mto the cold space, ctnvmg down the
shown m Figure 7.5. The arrangement shown m Figure 7.5(a) is directly
pressure mston. The displacer then moves down, transfernng the cold gas at
analogous to the displacer-type Stiriing engme (Figure 7.4). The disoiacer
low pressure back to the hot space. The pressure piston is then raised,
transfers the cold gas, at high pressure, from the cold space. through the
mcreasing the pressure m the system, while the gas m the ·hot space ts
adsorbent bed, where the preferentially adsorbed component 1s retained. The
heated, causmg a further nse m pressure, and the cycle is repeated. In this heat of adsorption raises the temperature of the gas flowing through to the
mode of operation the net effect is that heat 1s transferred by the gas from
hot spa·ce, where Jt 1s heated further from an external heat source. The
the hot to -cold regions and an eqUivalent amount of work is delivered to the pressure in the system 1s then decreased and the hot g11s 1s passed back
pressure piston. The system can also he operated in reverse as a heat pumo through the adsorbent bed. The prcfcrcnt1a/ly adsorbed species is dcsorhed
or refrigeratm;. In that mode, work 1s dOne by the pressure piston on the
and earned down with the gas tlow, which 1s cooled by the heat of desorp-