Page 313 - Pressure Swing Adsorption

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PRESSURE SWING ADSORPTION MEMBRANE PROCF-5SES 291
290
Membrane
MEMBRANE 1 t
FEED--+1 i-- RESIDUE
Nitrogen
Nitrogen
Hvdrogen High Pressure
I
High Pressure
PERMEATE A B A B
Low Pressure Hydrogen

Figure 8.1 Membrane separation process. X l - X y l - y

component can penetrate the membrane, 1s defined by:
A (8.1)
N; = ,,,-,5(Pm -P;L)
a' = y/(l-y}, r = P./Pl
wi1crc ( /Jill - P;t) represents the difference in the partial pressure of compo- x/(l-x)
nent ! across the membrane and O 1s the thickness. Figure 8.2 Definition of "local'' scparauon factor.
One may usefully define a local separat10n factor (a') by reference to the
s1tuat1on sketched in Figure 8.2.:
a'=y/(1-y) (8.2) eouilibrium constant) and the diffwav1ty of the adsorbed or dissolved species;
x/(1-x) so, to a first approxnnation we may write:
The flux ratio 1s given by: - 1TA K.4DA
a=-=--···· ( 8.5)
1Tn KsDa
NA = ,,,.A ( &JxA - YA\ YA YA (8.3)
NB 1rn &JXn-Yn) = Yn = 1-yA
This .e~?ress10n 1s not _exact, since 1t assumes syst_ern lineanty and ignore& the
where P = P / PL 1s the oressure ratio across the membrane. If the back ooss_1 b1hty of oarallei _contributions from other nonseiect1ve transparent
11
oressure 1s negligible, P- oo; and the separation factor approaches the mechanism such as Po1seuille flow. Nevertheless it serves to delineate the
permeability ratio (a): mam fa.ctors controlling membrane select1v1ty. Clearly a high selectivity mav
be achieved from a large difference m either diffl1s1v1ty or eauilibnum
, YA/Yn 7TA (8.4)
a=---4--=a constant or from a combination of hoth these factors. l.Jnfortunatcl-y there is
XA/X11 11"11
This 1s the most favorable situation. Any back pressure will reduce the
separation factor to a iower value, and the permeability ratio 1s therefore Table 8.1. Compensation of Diffusivity and Solubility for H S
2
sometimes referred to as the "intrms1c seoaration factor/' or the "selectivity." In Polymer Membranes
For an mert (nonadsorbing) m1croporous solid the permeability ratio 1s ,,. D K
essentially the ratio of Knudsen diffusivities, which is simply the inverse ratio Membrane T (°C) P (Torr) (cm 3 STPcm - i s- atm- 1 ) lcm ·s- l 1cm; · cm-·']
1
1
2
of the souare of the molecular weights. Such selectivities are therefore
modest'and too small to be of much practical interest. If, however, the pores Nylon 30 110 2.4 X 10-<J 3 X 10- IO 7.9
are small enough to offer significant steric hindrance to diffusion or if one or 30 621 2.6 X 10-\i 4.9 X JO- IO 5.3
Polwmvi 30 244 2.2 X If)·· ' 1 5.5 X ]fl -'J 04
b,)th or the components ate adsorbed on the pore wall or dissolved within the
tritluoro 45] 2.0 X Ill'" 4.9 X 10- 'I 0.4
solid,matrix, much Iarger separation factors are possible. For such a system acetate 751 2.0 X IC)-'J 6.8 X 10·-\l 0.3
the permeability depends on the product of the solubility (or the adsorption