Page 284 - Pressure Swing Adsorption

PRESSURE SWING ADSORPTION PSA PROCESSES 261
260
of expans10n of the product to atmospheric pressure. For a vacuum swi?g without pressure equalization there 1s a shallow rnm11num m the comoressor
process· m which the product 1s produced at subatmosphenc pr~ssure, the \ work curve at about 15 atm; while for the process with pressure equalization
latter quantity will be negative. Eauation 6.15 differs from the normal the compressor work 1s greatly reduced and there is a sharoer minimum at
definition of the First Law efficiency in that the seoarattve work associated l about 4 atm. The exerget1c efficiency reaches a maximum at aoout 5 atm for
with the waste product 1s excluded and the energy of compress10n (or I the process with pressure eQualization but increases monotomcally for the
rations were considered: the two-bed Skarstrom cycle with and without _a I grams for operation at their ootimal pressures (in terms of compressor work)
expansion) of the product to atmosohenc pressure is allowed for. process without oressure equalization. The correspdnding Grassman dia-
-A detailed exergy analysis of PSA air separation processes has been
are shown in Figure 6.26. The exerget1c efficiency is about 17% for the
40 41
For oxygen production tw? process ~onfigu-
reported by B·anerjee et al.
,
pressure equalization step. The variation of compressor work and exerget!c
efficiency with operating pressure 1s shown in Figure 6.25. For the cycle COtfflGURATlON
EXERGY
OF
WO«K lam
INPUT FEED
'• (a) 10•" I
(100¼)

PAOOl)Cl
OHGE~ 1014\
1)1J<] w, t~•J•.1
1100•1,1
l)OOO U•$'lol
••T~~c~l•O
sOSHS U\U C0NFIGURAI10N 1 EXO 0-0t (0.5°/o)
""·~•,.)
EX ERG¥
WORK OF
INPUT FEED
IHG 6•1J ( 70¼)
(a)
1100"/ol
(b)
t'wJTV
0,31 I l·6"/,)
COMPRES
f.XIT
7113
SlREAM rsc'-"=~, 111•1•,.,
)!178& 3•56
161"/ol ly 1-76150"/.J
y,:__.,~;.__E:X9 :0-0411•1"/ol
(c) O 0·01(0•3•1.1
AFTERCOOlER
l05S •lO ll•B'Vo) 0•1514•2•/•l
COMPRESSOR 8657 FEED
tOSSES I 21•5 •/•l EXERGY .. 0
53.!9
tu.,,.1 -0-09
i-2-s.,-. J
(b)
Figure 6.26, Grassman diagrams showmg losses of exergy for S~arstrom ~de (blo:,v-
down to atmosohenc pressure) for oxygen production (a) without .and ~b) with Figure 6.27 Grassman diagrams showing exergy losses for mtrogen production with
three different cycles. (a) Skarstrom cycle and (b) SkarstrO:m cvcJe with pressure
pressure eoualization. Ooeratmg pressures are, ,resoect1vely, 15.b and 3.9. b~r, t.he
Va1ues for Which the compressor work 1s m1mm1zed. (From Banenee et al., with equalization (PH)= 8 atm, PL= 1 atm); {c) vacuum swing cycle (PH= 1 atm, Pi.=
0.2 atm). (From Banef]ee et ai., 41 with pcrm1ss1on.)
perm1ss1on.)