Page 147 - Pressure Swing Adsorption

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PRESSURE SWING ADSORPTION EQUILIBRIUM THEORY 123
inherent costs of increased cornolex1ty and additional power for recomoress~ 5-STEP PSA CYCLE
mg the product, if that 1s necessary.

4.4.5 Five-Step PSA Cycle: Incorporating Rinse PRESSURIZATION FEED RINSE SLOWDOWN PURGE
(WITH PRODUCT)
and Incomplete Purge
y.o
In this sec_tion a rinse step is added to the four.step PSA cycie discussed in v=O
Section 4.4.3. The added rmse step follows the feed step, and 11 begms by
admitting the pure heavy component to the bed. This displaces the residual
feed, which is recycled. In so doing, the adsorbent bed becomes saturated
with the heavy component. Therefore, durmg the blowctown step, the heavy
component 1s recovered as the pressure droos from PH to PL· At least oart
of the heavy component must be recomoressed (to PH) for use m the I
subseauent nnse step. The cycle is shown schemat1cally m Figure 4.11. , I
Thoug11 called rinse, the action of this step could also be thought of as a
simple wave. Arguments can be made for directing the flow dunng the nnse I ~ t
high-pressure purge. A maJor formal distinction between rinse and purge is
that rmsing involves a composition shock wave, while ourgmg mvotves a

step either cocutrent or countetcurrent to the feed. Factors such as mechani- I YF YF y .. i Y>O
cal compiexity and product ounty affect the choice. For now, since the
mathematical modd to be disc~ssed assumes local eauilibrium, 1mpJymg that
1
ideal shock fronts exist durmg the feed and rmse steps, the direction does not I 0
affect performance. To be definite (and to favor the mechanically simoler I
version), the rinse flow is taken to be counter to that of the feed. _J
.......
The present PSA cycle also mcludes mcomplete ourgmg. The eauat1ons I N 0.2
that govern the purge, pressunzat1on, and feed steps m this cycle are
identical to those that apply in the four-step cycle covered m Sections 4.4.1 I C
0 0.4
and 4.4.3. Similarly, the equations that govern the rmse step m this cycle are ·.;:::
analogous to those for the feed step. That 1s, the relation between the 'ijj
0
mterstitial veioc1ty, the length of the bed, the step time, and the column I 0... 0.6
isotherm 1s obtained from EQs. 4.5, 4.7, 4.9, 4.10, and follows along the same cu
lines as Ea. 4.20: ·x 0.8
I
( 4.38) <l'.
Similarly, the corresoonding expression mvolvmg the rmse steo molar ef- 1.0
fluent rate, step time, pressure ratio, and coefficients that represent geometrv
and adsorbent-adsorbate mteractions 1s similar to Eq. 4.22. As a result, the Pressun- Feed Rinse Slowdown Purge
molar quantities m the effluent and influent are: I zation
Figure 4.11 Five step cycle-mcluding a nnse steo. (Too) Flows and composition
Q0"'1IR = <f,Pf3A/0a ( 4.39) associated with each step. (Bottom) Position-verus-t1me representation of each step.
I
Q,/IR = </>?1[1 + (0 - 1)YA,]f3A.,/0A (4.40) Shaded region depicts penetration heavy component. X denotes fraction of ·comple-
tion of purge steo.
where O = O(Ptt, Ya, y = !). Note that the form of Ea. 4.40 ts identical to
that of Eq. 4.24. For nonlinear isotherms, smce the parameters are evaluated
under different conditions, the results may be different.