Page 149 - Pressure Swing Adsorption

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124 PRESSURE SWING ADSORPTION EQUILIBRIUM THEORY 125

The blowdown step ,s cnucal · to this cycle smce 1t 1s the source of the that the oower requ1Ted would be equivalent. Exoenmentallv. however. these
heavy component product. It follows the nnse step, so the bed is presumed to cycles performed differently, and the resulting recovehes and product ouri•
contam_ only the oure, heavy component. The most direct approach to ties are compared m Section 4.5. That sectwn also corhoares the expenmen-
determine the net effluent is simply to ctetermme the difference the initial tal results with the theoretical predictions presented here I see Figures 4.14(a)
and final states, according to Eo. 4.18 (since the contents are assumed to be and 4.J4(b)].
oure A). The result 1s:
4.4.6 Four-Step PSA Cycle: Columns with Dead Volume
1) l This sectmn covers a method for est1matmg the magnitude of effects caused
by dead volume, via an equilibnum model. The effects of dead voiume are
(4.41)
diverse-they vary deoending on which end of the adsorbent bed is affected,
Thus, as for pressurization, the rate of blowdown is 1mmateriaJ· only the and they depend on which step of the cycle is being 'considered. As shown
mittal and finai pressures matter. ' here, the effects are also typically more severe when adsorbent se1ect1vitv 1s
The light an_d heavy oroduct recoveries from the current five-step cycle~ poor.
based on complete purge and nonlinear isotherms are: Notwithstanding the last assumpt10n stated m Section 4. i, an inescapable
feature of adsorption columns is dead volume ·at both the feed end and
product end of the fixed bect. The pnmary reason ts that, if plug flow 1s to be
( 4.42A)
established in the adsorbent bed, the gas must be allowed uniform access m
the direcllon of flow. For an ordinary cylinctncal column, with axial feed and
discharge nozzles and bad retention plates, this coU:ld amount to a dead
volume (at each end) of 5 to 10% or more of the volume of the adsorbent
Pf '1'1 + (1 - 0)YF - 1] - '1'2
RA = ------~~-~-~ (4.43) bed.
Py.( ip 3 - 9) Breakthrough curves obtamed from most commerc1al columns exhibit
"rounding" ansmg from axial dispersmn, mass transfer resistance, or back
where 0 = 0(PH, y = I, YF), ip, = OiP,,, y = l, re)/9,(P , y = 1, y = 0),
11 mix.mg before or after the adsorbent bed. If dead volume is largely responsi-
'P2 = 0iP,,, y = 1, y.)f0iPL> y = 1, y = 0), and "'' = OiP , y = 1, Yr)/
11
0)P11, Yo, y = 0). ble for the rounding, it should be easy to diagnose., At a given pressure,
temperature, velocity, etc., one can simply compare the breakthrough curve
For linear isotherms, these equations simplify to:
of a conventional column with one in which dead volume has been mim~
m1zed. Examples of the iatter, shown in Figure 4.2, were obtained with
(4.44)
commercial adsorbents at ve1ocit1es and pressures typical of commercrnl
systems, but in a column m which dead volume was minimized. Of course, if
I
RA= 1- ----- ( 4.45) rounding eXIsts, it could also be an artifact of the ;slow response of the
(! - f3).f/YA, sampling mstrument.
Dead volume at the product end of a PSA column affects the steps
ln this cycle, Just as for the four-steo cycle ment10ned in Section 4.4.3, the
differently, as follows:
recovery of the light component can be improved by reducing the amount of
pure light product consumed in the purge step. The analysis of incomplete Blowdown. If, during the previous feed step, breakthrough had not
~urge here is identical to that presented earlier, since the steps are basically begun, retained pure product partially purges the bed dur-
the same. A mmor detail 1s that, at the outset of the purge step, more of the mg blowdown; if breakthrough had begun, the gas that
heavy component remains m the column than m the four-step cycle, when the l expands from this volume is mereiy additional (unneces-
hlowdown step follows the feed step. sary) waste.
The present cycle differs some~hat from the cycle suggested by Sircar 22 t Purge. If breakthrough had not begun dunng the feed step, less
which also includes a rinse step and ts described in Section 6.4. Differenc~s gas 1s consumed during purge because of the contribution
between the cycles are mamly 1n the details, such as the flow direction m J ctunng blowdown; if breakthrough had begun, excess gas IS
ccrtam steps. Both cycles were reduced to practice for the purpose of I necessary to cleanse this volume before the adsorbent can
splitting air, and the ranges of pressure ratios are nearly idcnt1c81, i~lPlymg bcgm to be purged.