Page 43 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001-13 May 7, 2001 12:29

Adsorption (Chemical Engineering) 265

allow detailed mathematical modelingalongthe lines indi- no information on the regeneration conditions needed. In
cated in the previous sections, one can in principle predict practice, in most two-bed purification processes the de-
the dynamic capacity for any defined feed and regenera- sorption step in fact controls the cycle, either directly or
tion conditions. An a priori design of the bed is therefore through the heat balance. Initial design of the regeneration
feasible. Such an approach has been adopted only rather cycle is commonly based on the assumption that during
infrequently, however, probably because the capability of desorption the column approaches equilibrium. However,
solving the governing equations for the more complex at the low concentrations prevailing during the later steps
systems typical of industrial operations has been achieved of desorption, kinetic effects may be important, so a more
only recently. A more common approach is to base the de- detailed analysis is desirable.
sign on experimental measurements of dynamic capacity Another factor that is particularly important in the re-
using the LUB concept. A breakthrough curve is measured generation of molecular sieve driers is the rate at which
using the same adsorbent under the same hydrodynamic the temperature is raised during regeneration. If this is too
conditions but in a laboratory-scale column. The LUB, rapid relative to the rate of moisture removal, one may get
which is essentially a measure of the width of the mass rapid desorption of moisture from the initial section of the
transfer zone, is given by bed, which is in contact with the hot desorbent gas, fol-
lowedbycondensationofliquidwaterinthecoolerregions

LUB = (1 − ¯ q /q 0 )L = (1 − t /¯ t)L (25)
some distance from the inlet, with serious consequences
where q 0 is the adsorbed-phase concentration in equilib- for adsorbent life.

rium with the feed, t the break time, and ¯ t the mean in- To avoid the possibility of fluidizing the bed the system
tention time. These quantities can be calculated directly is normally operated in the downflow mode with upflow
by integration from an experimental breakthrough curve desorption since the gas velocity during desorption is nor-
(Fig. 9), mally lower than that during adsorption. The maximum
upflow velocity is normally limited to 80% of the mini-
∝

(1 − c/c 0 ) dt mum fluidization velocity, while velocities as high as 1.8
¯ t =
0 times minimum fluidization can be tolerated in downflow.
(striped area in Fig. 9)
t

B. Pressure Swing Processes
t = (c − c/c 0 ) dt
0 The general features of a simple two-bed pressure swing
(hatched area in Fig. 9) adsorption (PSA) system are shown in Fig. 10, and de-
tails of two simple cycles are shown in Fig. 11. One of
where c 0 is the feed concentration of sorbate. The effective
the important features of such processes is that the less
capacity of a column length L will be the equilibrium ca-
strongly adsorbed species (the raffinate product) can be

pacity of a column of length L , where L = L − LUB, and
recovered at high purity but at relatively low fractional
on this basis the size of a column required for a given duty
recovery, while the more strongly adsorbed species (the
can be readily estimated. It is important that the experi-
extract product) is always recovered in impure form dur-
mental LUB be measured under conditions that are pre-
ing the blowdown and purge steps. This type of process, is
cisely analogous to the large-scale process. For example,
therefore especially suitable for gaseous separations when
if the small laboratory column operates isothermally while
the feed is inexpensive and the less strongly adsorbed
the full-scale unit is adiabatic, the LUB may be seriously
species is the required product. All three major indus-
underestimated, leading to an inadequate design. Further-
trial applications of PSA (air drying, air separation, and
more, the method is valid only for a constant-pattern sys-
hydrogen purification) fulfill these requirements.
tem (adsorption with a favorable isotherm) and provides
PSA systems are well suited to rapid cycling, making it
possible to obtain relatively large througput with relatively
small adsorbent beds. However, the energy efficiency of
such processes is not high, and since mechanical energy
is generally more expensive than heat, PSA systems are
generally not economic for large-scale operations. Their
advantage lies in their compactness and simplicity, mak-
ing them ideal for applicatins such as the production of
medical oxygen in the home or in hospitals in remote ar-
FIGURE 9 Sketch of a typical breakthrough curve showing rela- eas. However, with recent improvements in process effi-
--

tionship between break time t and mean retention time t. ciency PSA processes are economically competitive with