Page 111 - Pressure Swing Adsorption

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86 PRESSURE SWING ADSORPTION PSA CYCLES: BASIC PRINCIPLES 87

(o l {bl N,
- 1.l .tl.O ~
' . 0 a. 02 tThmnle
~
0 0.8 :,
E ;
E -a '7
C
;..0.1. .e Nl
u u
0
Lt 00
0 4 8 12 16 JO 60 90
p (otm) [mm)
Figure 3.15 (a) Equilihnum isotherms and (b) expenmental uptake curves for Adsorber A Adsorber 8
sorptmn of 0 and N on Bergbau-Forschung carbon molecular sieve. (From Ref. 21;
2 2
reprmtcd with pcrm1ss1on.)
'
nitrogen processes avoided this difficulty by usmg a vacuum to clean the bed 3w•v ~r----------.-----------107' 3-way
valve ~
I
..,,.~ valve
rather than a purge, as illustrated m Figure 3.12. The general scheme for a '---------!-~-,-----~
vacuum swmg carbon sieve process to produce nitrogen IS shown m Figure I
V,cuum pump .
3.16. A better option ts, however, available. At the end of the blowdown step ~Air
the adsorbent contains both oxygen (fast diffusing) and mtrogen (slow diffus- i Compre=r
ing), Thus. if the bed is simply dosed at the product end and left for a period
of lime, the oxygen will diffuse out first, followed bv nitrogen, so the system
Figure 3.16 Schcmauc diagram of the Bergbau-Forschung PSA air separation
is, m effect, self-purging. The product ounty is directly controlled by increas- process. (From Ref. 21; reprinted with ocrm1ss1on.)
ing feed pressure, and pressure equalization 1s incorporated to reduce the
blowdown loss. A dual-ended pressure eoualizatlon ts used in which the feed
and prodllct ends of the high-pressure bed are connected to the respective
equalizatmn and a self-purging desorption step. Connectmg both ends of the
ends of the low-pressure bed. Most modern nitrogen PSA units therefore beds allows rapid pressure equalization and improves product punty smce
operate on the cycle shown m Figure 3.17, which mcoroorates both pressure
the oxygen-rich gas remams at the feed end of thei oressunzed bed. The
higher recovery advantage of dual-ended pressure eoua1izat1on, however.
decreases with increasing product purity.
Table 3.2. Equilibnum and Kinetic Data of Oxygen, Nitrogen, Methane, and
CarbOn Dioxide on Bergbau - Forschung Carbon Molecular Sieve and Kinetic selecliv1iy may he increased hy increasing the diffusivity of the
4A Zeolite at 25"C faster component, oxygen, or further decreasing the diffus1v1ty of the slower
component, nitrogen. An mcreasc m oxygen diffusivity will mcrease the
Diffus1onai Henrv·s Saturation nitrogen product punty without sermusiy affectmg the rccoverv (see Figure
1
time constant (s- ) constant" constant (g moles/cm·')
5.9). Intuitively ,t may appear that nttrogen recovery will mcreasc with
Oi-CMS" 2.70X IO-; 9.25 2.64 x IO"' 1 decreasing nitrogen diffusivity, and the hcst s1tuat1on Would be reached when
N 2 -CMS" 5.90 X 10- 5 8.90 2.64 X 10--_; there IS practically no ocnetrat1on of nitrogen. What 1s overlooked m this
Oi-4N 8.51 X 10-' 2.10 L72x 10- 2 mtutttve argument 1s tlle role of the desorbing nitrogen m a self-purging
N -4N 8.99 X 10-~ 4.26 i.20X 10- 2 cycle. The desorbing nitrogen cleans the void volume of the adsorot,on
2
COi-CMSd 9.00'x 10-~ 135.83 2.85 X 10- 3
CHcCMSd 5.00 X 10-t. 25.83 l.74 X 10-_; column, and, unless there ts significant uotake of rntrogen during tile high-
pressure adsorption step, inadequate self-purging W:ill result 10 increased
Dimen~mnless bas1~. oxygen contammat1on in the nitrogen product. There 1s therefore a lower
Source; Kinetic data from Ref. 24 and equilibrium data trom ReL 22. limit of nitrogen diffus1vt1y below which the self-purging cycle becomes
Source: Ref. 23.
d Sourre; Ref. 20. meffective.