Page 219 - Pressure Swing Adsorption

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196 PRESSURE SWING ADSORPTION DYNAMIC MODELING OF A PSA SYSTEM 197
Ta.hie 5.6. (Conttrmed) Ta.hie 5.8. Kinetic and Equilibnum Data and Other Common
Parameter values Used in the Simulations of PSA Air
Except 'for the following changes all other eQuations m Table 5.2 appiy. The subscnpt i has Separation for Nitrogen Production"
the same meaning as m Table 5.2.
The appropr,iate forms for the diffusion equations are obtamed bv substltutmg Eqs. 9 and 10 in I Adsorbent Carbon molecular sieve
the particle balance equations for component A (i = A m Eq. 3 and componem B (i = B m (Bergbau-Forscnung)
Eq. 3): Bed length (cm) 35.0
D,rn [<1-0 )(·a2qA +3. iJqA·)+B ("2"a +~ a"")] 1 Bed radius (cm) 1.75
aq, .. = - ! Particle size (cm) 0.3175 (pellet)
at •-OA-8 B ar1 r Jr A ar1 r rr 3
8 Particle density (g/cm ) 0.9877
DAO ( JOA iJ8B)(i)qA ilqB) (11) i Bed voidage 0.40
+ (1-0A-8»)2 (1-8B)Jr+OATr ¥+Jr Ambient temperatuTe (°C) 25.0
PecJet numbeT 1000.0
iiqs = DBo [<i-o )(a~qn +I iJq")+o (<iqA +~ iJqA)] Eauilibnum constam
foroxygen(KA)
at · 1- 8A - 8 8 A . ar2 r Jr 8 Jr2 r Jr Equilibrmm constant
for nitrogen (K )
D80 [ ) iJ.88 JOA j ( JqB iJqA) (12) 8
+ (1-6,,t-8B)2 (l-OA ar+8BJr 7rr+7ir Saturation constant for
oxygen (qA )(moljcm·') 2.64 X JO-lb
5
Similarly the appropnate boundary conditions for the two components at the particle surface are Samration constant for
3
obtamed by subs~ituung Eqs. 9 and to in Eq. 5 written for components A and B and solving nitrogen (q 85 ) (moljcm ) 2.64 X l0-Jb
simultaneously for (aqA /Jr)lr-Rp and (iJq 8 I Jr)l,-RP; Limiting diffusional time
constant for oxygen (DAn/r}) (s- l) 2.7 X 10- 3 ,
(13) Limiting diffusional time
constant for nitrogen (D» /r}) (s- 1 ) 5.9 X 10-s,
0
(14)
fl values of 14 and 85 for oxygen and mtrogen, respecnvely, taken from the
correlacion of Nakao and Suzuki have been used m computl'ng the LDF model
Equauom, 9 and 10 ure 1rue for qAS"" q 8 s; if this 1s not true, the expressions will conwm additional predictions.
terms, b Ruthven e1 111.·\"'
~ FarooQ and Ruihven. u,
Table 5.7. PSAAir Separation for Nitrogen Production on a Carbon Molecular Sieve:
Summary of Experimental Conditions, Product Purity, pressures (3-7 atm) and therefore provide a suitable database. The· exoen-
Recovery, and Productivity" mental conditions and the observed product ounty, recovery, and oroduct1v-
1ty are summarized in Table 5.7. The equilibnum and kinetic parameters
Adsorption Moie% % Recovery Produclivity taken from independent, smgle-component measurements and used 1n s1mu-
L/ 11 011 pressure oxygen of ( cm'N,) Jating these experimental runs are given m Table 5.8, The diffusion model
Run No. ratio(s) (atm) in product nitrogenh hr. cm·; adsorbent
(with constant and vanable diffus1v1ty) and the LDF model predictions are
compared with the experimental results in Figure 5.8. The oredicttons of the
25 3.0 10.5 56.4 81
2 25 4.4 7.5 53.7 106.2 (: constant-diffusivity pore diffusion model and the LDF model, with D vaiues
3 25 5.8 6.0 49,2 137.4 adjusted for cycle time according to the correlation of Nakao and Suzuki, 35
4 25 6.8 4.4 42.1 135.75 are very close. (This agreement provides additional confirmation of the
5 37 3.0 4.0 29.2 27.4 results shown m Figure 5.3.) It 1s clear (from Figure 5;8) that the concentra-
0 37 4.4 1.8 21.6 30,76 tion-dependent diffus1v1ty model predicts the correct qualitative trends for
7 37 5.8 0.7,0.75 II.I 21.4
8 37 6.8 0.6,0.7 7.7 17.3 purity and recovery over· the range of experimental variables exammed. The
constant-diffus1v1ty model (and the LDF model), on the other hand, cannot
Feed composilion - 21% oxygen, 79% nitrogen, blowdown/desorptmn pressurn = 1 atm, predict the correct trend of recovery with operating pressure, and, even
rres~un:rntion/h\owdown time - 2 s, adsorpt1on/dcsorption time - 60 s, pressure equalizution - 2 s. though this model predicts the correct trend for the variation of product
Corrected for pressunz11tion gas quantity.
Sour.·e: 1-lass,111 et al. 111 punty, the qualitative disagreement at the higher pressures 1s too large.