Page 59 - Pressure Swing Adsorption

34 PRESSURE SWING ADSORPTION FUNDAMENTALS OF ADSORPTION 35

Table 2.5. {Cm1111med) Table 2.6. Pore Structure of Typical Adsorbents
Temp. range Press. rnnge l lomogeneous-Un1mnd<1I Pore Size Comrosnc-Bim(1d<1l Pore Size
Sorbent Snrha!c (K) (atm) Reierem:c Distribution Distribution

SA Zeolite 144 1.0 Danner 25 Silica Gel Carbon Molecular Sieves
172-273 1.0 Noian-n Activated Alumina Pe!kted Zeolites
CMS(Takeda) 195-323 1-11.0 Kawozoe 34 Activated Carbon Macroreucutar ion exchange resins
CMS(BF) 303 0-0.9 Ruthven~~ Homogeneous 10n exchange resms
190-273 0-0.9 Ruthven 14
CMS(Takcda) 77 0-fl_{) llorvalh.n
77-323 0-(L(J Kawazoe- 14
CMS(BF) 273-33:\ 0-0.9 RuthvenH
comoosi!e (Table 2.6). These are illustrated m Figure 2. 13. In the "homoge-
See also Adsorption Equilibrium Data Hmulbook, D. P. Va\en:rncla and A. L Mvers, Prentice Hall.
Englewood Cliffs, N.J. (19K9), which provides .t useful summarv ol the av:1ilablc adsorpll!ln ClJuilibm1m neous" actsorhcnts the pore structure persists, on the same scale. ttiroughout
data for a wide rnnge ot svstems. the entire particle: so the distribut10n of pore size ·is unimodal. Bv contrast
the composite adsorbent oart1cles are formed by aggregation of small micro-
porous m1crooart1cles, sometimes with the aid of a hinder. As a result the
isotherm daia with literature references. The molecules of argon, oxygen, and ~ore size dist-ribui1011 has a well-defined bimodal chantctcr with m1cropores
nitrogen are of similar size and polanzabilitv so their van der Waals mterac-
~ithin the m1cropart1cles connected through the· rnacropores within the
t1ons are similar. As a result nonoolar adsorbents show very tittle selectivity oellet.
hetween these species, as exemplified tly the s1milanty m the 1sotllerms for In a composite adsnrheni there are three distmct res1stances to mass
nitrogen and oxygen on a carbon molecular sieve (Figure 2.11 ). By contrasi,
transfer, as illustra1cd in Figure 2.14. Under practic,i.1 conditmns of opcrntH1n
tt1e alurnrnum-nch zeolites show preferential adsorntwn of niirogen as a
the external film resistance 1s seldom, if ever, rate limiting; so that the
result of the field gradient Quadrupole 1nteract1on energy. 5A zeolite 1s the
sorpt1on/desorot1on rate 1s generally controlled bv either macrooorc or
most commonly used adsorbent for air separation (to produce oxygen) and m1crooore diffusion or by the combined effects of these resistances.
the separation factor (essentially the same as the ratio of Henry constants) A proper understanding of kinetic effects m PSA svstems therefore
for this adsorbent 1s at1out 3.3 at ambient conditions (sec Figure 2.12). This requires an understanding of-the mechanisms of both macropore anti m1cro-
value 1s almost independent of co111oos1t1on m conformity with the Langmuir po~e diffus10n. Only a bnef summary 1s given here; u more detaiied account
model. The separation factors for most other commerc1ai zeolites are similar has been given by Karger and Ruthven. 411
although very much higher seoaratton factors (8-10) have been reported by
Coe for well dehydrated Ca X or Li X as well as for Ca or Li chabaz1tes. 38 3
• 1l
The electric field gradient within a zeolite 1s enhanced by the presence of
2
divalent cation (Ca + ). However, anv traces of moisture can lead to cation
hydroiys1s, leading to tlle formation of two smgly charged ions:
Caz++ 2H 0=CaOH++ H,o+ (2.22)
2 \
with consequent loss of mtrogen select1vttv.
M1croporous J
-micro--particle
2.3 Adsorption Kinetics
lo> lb)
The raie of phys1cai adsorptmn 1s gcncrnlly controlled by diffusional lirmta-
Figure 2.13 Two common 1ypcs of microporous a<lsorhcnL (;1) lfomogcnc<H1<; parti-
t10ns rather t11an by the actual rate of equilibrat10n at a surface, which, for
cle with a wide range oi pore size (e.g., activated alumma or silica geL'(b) Composite
ohys1cal adsorption, 1s normally very rapid. From the oersoect1ve of sorptton pellet formed hv aggregat10n of small microporous microparticlcs (e.g., zeolite or
kinetics, adsorbents mav be divided into two broad classes: homogeneous and carbon molecular sieve adsorbents).