Page 361 - Pressure Swing Adsorption

(" i I ;I I

338 PRESSURE SWING ADSORPTION APPENDIX C 339

C.2.22 Kumar et al. not perfectly understood. In addition, 1t has been: pos&ible to secure patents
with equivalent ftowsheets even m the 1980s (cf Figure C.3). Those facts
One -of the more difficult separations among atmospheric gases 1s to split speak volumes about the subtleties mvolved m PSA.
argon from oxygen. This is due to nearly identical adsorption isotherms that In one of the very first disclosures of PSA concepts, Finlayson and Sharo 2
these components exhibit with most zeolites. Kumar et al., .who filed in 1982 made some suggest10ns that now might be thought of as mundane, but were
19
and received a patent in 1984, approached the problem of purifying argon truly farsighted at the time. For example, they gave a detailed example of a
containing nimor amounts (e.g .• less than 10% each) of nitrogen and oxygen smgle PSA system bemg used to alter the ratio of hydrogen and carhon
by first removing the nitrogen vm zeolite, then removing oxygen usmg carbon monoxide in water gas from about 1 : 1 to 9: 1, 2: 1 and J : 8, respect1vciy, for
molecular sieve. Essentially, they used looseiy coupled PSA systems m which subseauent synthesis of ammonia, methanol, and aliphatic acids. They also
the product of the first was the feed to the second. Followmg similar recognized activated carbon and silica gel as promising adsorbents. Finally,
reasoning, HayBshi et al., who filed in 1983 and were awarded a patent in they suggested that, "nitrogen, hydrogen, oxyge·n, carbon monoxide, and
I 985, 211 were able to split air to ob tam both high-purity oxygen and argon. methane" were candidates for purification by PSA. Even though these
applications were suggested over 60 years ago, the :PSA hydrogen purification
(the easiest of the group) did not become oractICal on a commercial scale
C.2.23 Wiessner and Bolkart
until the late 1960s. Similarly, separation· of either relatively pure oxygen or
A subtle variat1on,on the technique for recovering the most strongly adsorbed mtrogen from air did not become commercially Viable until the late 1970s,
components from a mixture was suggested by Wiessner and Bolkart in 1988. 65 and splitting carbon monoxide and methane from contammants oniv hecame
They allowed blowctown to an intermediate pressure, then purged the resid- moderateiy successful in the late 1980s. Despite the achievements of modern
ual heavy component. Subseauently, they completed blowdown, and as an technology, improvements in all of the applications are still bemg sought.
oot10n purged agam. This allows a valuable component to be recovered at a One wonders what, if Finiayson and Sharp were with us today, they would
a moderately adsorbed component to be removed prior to dropping the l In view of the incremental nature of Batta's oa:tent in 1971, 51 it could be
moderate oressure, above the mm1mum pressure m the PSA cycle, allowmg expect to be possible via PSA 60 years from now.
higher overall recovery with minimal recompression cost. It could also oermit
though of as a sort of turning pmnt, at least for hydrogen production. It
pressure sufficiently to desorb a more strongly adsorbed comoonent. ! seemed to mark the passage from dramatic mnovat1ons m which substantial
!
improvements over pnor art were claimed, to an onslaught of mmor refine-
ments of existmg cycles. A similar turning oomt :occurred ·for PSA oxygen
C.2.24 Tagawa et al.
production in the eariy 1980s. That 1s not to say that mnovauons ceased. lt
An exampie of the refinement that 1s prevalent m recent PSA oatents was was Just that most of the "obvious" modes of PSA operatwn had aireadv
given by Tagawa et al. m 1988. 66 They compared three different methods for been discovered, if ·not perfected.
pr~ssure eoualizatmn, m a process to split oxygen from air using SA zeolite. Most of the topics covered in this book are based on mathematicai models,
Thelf opuons included; (1) from the product end of one column to the feed although in some instances exoenmental verification is mentioned. Con-
end of a parallel column, (2) from the product end of one column to the versely, this appendix: covers the main repository of ongmal id.eas related to
oroduct end of a parallel column, and (3) from the feed end of one column to pressure swing adsorption cycles, and apparently none of them depended on
the feed end of a parallel column. They state that the first ts m the onor art. mathematical models to reach fruition. This is not to say tl1at some may not
They aiso claim that (2) and (3) can be conducted simultaneously. have achieved more resounding success if an adequate mathematical model
had been available. It may be worthwhile to consider, for a moment, the
remforcing aspects of the patent literature and the conclusions obtamed
I For example, there are significant distinct10ns (n the ways m which even
C.3 Concluding Remarks independently, though much later, via mathemat1c-a1 models.
To close this appendix, some comments are approoriate. First, a few overall l i the simplest steps can be accomplished. From a theoret1ca1 pomt of view,
1moress10ns are given about specific patents and phases that they seem to fall i Section 4.6 shows that pressunzation with product is v1rtually always suoenor
into. Then a few comments are made about the relation of this appendix to , I to pressurization with .feed. That potnt 1s made as strongly, if not as
other parts of this book. succinctly, in patents by different methods of pressure cquaiizi1t1on, as
First, 11 1s remarkable that the basic ftowsheet for PSA has been in tile ! illustrated by the references listed m Sections C.2:8 and C.2.24. Soecifieally,
! when the purified product from one column 1s use'd partially to pressurize a
public domam for over sixty years (cf. Figure C.l), yet some aspects are still
l