Page 317 - Pressure Swing Adsorption

294 PRESSURE SWING ADSORPTION MEMBRANE PROCESSES 295
Table 8.2. Potential of Zeolite Membranes"'
100
CO ( A) CH (fl)
o- 2 4 KD=rr .~ .,,.,,
C0 2 /CH 4 separation K D KD = 1r K D
0~ "•
2
~ • 4A zeoliteb 35 X JO' 8.6 X to~ IO 3 X 10- 5 30 s·x 10-n 1.5 X 10- 9 2 X 10 4
---------0 SA zeoliteb 4500 2 X 10- 4 9 X 10- 3 50 1.4 X 10- 6 7 X 10- 5 130
..-o Rubber 10-I, 2.3x 10- 7 4.3
L.D. 0.96 X 10- 7 2.2 X 10- 11 4.4
10 ~ i Polvthene -
•
I
0 ·l N (A) CH, (B)
2
-01----- □----- □----- I
""·'
0- N 2 /CH 4 separation D KD = r. a=-
I K KD = 1T K D
H /CO i ""•
2 2 w-9 11 9
i 4A zeolite" 20 9 X 10- 7 2 X 10-s 30 5X 10- 6 l.,S: X 10-: 0.31
13
5
7:x 10-s
50 1.4 X 10-
SA zeolite
24
2.2 X 10
I Rubber 6.2 X 10- 11 2.3°" X 10- 7 0.27
L.D.
I Polythene 7.4 X 10- 9 2.2 X 10- 11 0.34
2
3
Ki:; m ccSTP/cm atm; D ism cm s-i; rr 1s m cm'STP/cmatmsec.
b .Zeolite has higher permeability and seieccjvily.
2.8 2. 7 2.8 2.9 3 3.i 3.2 3.3 4A zeolite membrane would allow removal of N 2 (mmor component) as permeate with permeal:>ili1v
comparable with polymeric membrane.
(b)
8. 1.4 Permeability versus Selectivity
Figure 8.3(b) , Variation of se\ecuv1ty with temperature for a polyamide asymmetnc
hollow fiber membrane as in Figure 8.3(a). The ideal membrane would have both high seJectiVity and high absolute
permeability (to allow a high throughput per unit area). Unfortunately there
1s often a high degree of c;:ompensation between oenneability and selectivity;
10~--------, I materials with a high selectivity generally have low permeability and vice
I versa. Some examples are shown m Figure 8.4 and Table 8.2. The selection of
N/CH 4
O/N 2
• the best material therefore gene'rally involves finding the oot1mal comorom1se
•
based on an economic evaluation .
• . \
•••
,
8.2 Membrane Modules
Since the flux varies inversely with the membrane thickness, It 1s desirable
that the active membrane should be as thin as oossible, The· limitation 1s of
course the physical strength, smce the membrane must be strong enough not
to rupture under the applied pressure;, which 1s often :quite large. For a g1ven
oL........._~ ........ -~ ....... ~~ pressure difference the throughout is directly proportionaJ to the ·membrane
.01 .1 1 10 100 1000 area. The challenge for the deSigner 1s therefore to tillnimize the membrane
N permeabUlty (Ba.rrers) thickness and ·maximize the membrane area per unit of moduie volume.
2
The active membrane 1s generally a thin polymer film supported on a
Figure 8.4 Variation of seiectavity with per?1eability for O2-N2 separatmn on macrooorous support that provides physical strength but makes no contribu-
poiymenc membranes.* (From Koros et al./ with perm1ssmn.)
· cm 1 (STP)·cm m·mole
*l Barrer=lo- 10 ~ H o. 335 m·s·TPa
~m.,·s•_rm g_