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Adsorption (Chemical Engineering) 263

2
σ L A 1 2
HETP ≡ = + A + A 3 v (23)
¯ t 2 v
can be easily derived by substituting the approximate
relationship D L ≈ 0.7D m + vR p in Eq. (22), whence
it follows that coefficient A 1 1.4D m , A 2 = 2R p , and
A 3 = 2ε/(1 − ε)kK.
In the low-velocity region the axial dispersion coeffi-
cient D L is approximately independent of gas velocity and
Eq. (22) can be rearranged to give:
2
σ L D L ε 1
= + (24)
2
2¯ t v v 2 1 − ε kK
2
2
from which it is evident that a plot of (σ L/2¯ t v) ver-
2
sus 1/v should be linear with slope D L and intercept
ε/(1 − ε)kK. This provides a simple means of separating
the effects of axial dispersion and mass transfer resistance.
The shape of the response peak is rather insensitive to the
nature of the mass transfer resistance, however, so even
by more sophisticated methods of analysis it is generally
not possible to establish the relative importance of the in-
dividual mass transfer resistances except by varying the FIGURE 8 Schematic diagram showing the two basic modes of
adsorbent particle size and/or crystal size. operating an adsorption separation process: (a) cyclic batch two-
bed system; (b) continuous countercurrent system with adsorbent
recirculation. Concentration profiles through the adsorbent bed
VII. CYCLIC BATCH are indicated. Component A is more strongly adsorbed than B.
(Reprinted with permission from Ruthven, D. M. (1984). “Prin-
ADSORPTION PROCESSES
ciples of Adsorption and Adsorption Processes,” copyright John
Wiley & Sons, New York.)
The general mode of operation of a cyclic batch adsorp-
tion process is illustrated in Fig. 8. In its simplest form
such a process employs two adsorbent beds, each of which the regeneration desorption step. Some examples of such
is alternately saturated and regenerated. During the satu- processes are given in Table II.
ration or adsorption cycle, adsorption is continued until Processesofthistypecanbefurtherclassifiedaccording
the mass transfer zone has almost reached the bed outlet. to the method used to regenerate the spent bed: thermal
At this point the beds are switched so that the spent bed swing, pressure swing, purge gas stripping, or displace-
is replaced by a freshly regenerated bed, while the more ment desorption. In a thermal swing process desorption is
strongly absorbed species is removed from the spent bed in accomplished by raising the temperature of the bed, either

TABLE II Examples of Cyclic Adsorption Separation Processes
Liquid or Regeneration
Process gas phase a Adsorbent Selectivity method
Drying of gas streams G 13X, 4A, or 3A molecular Equilibrium Thermal swing or
sieve pressure swing
Drying of solvents L 4A sieve Equilibrium Thermal swing
Solvent recovery G Activated carbon Equilibrium Steam stripping
H 2 recovery G Molecular sieve Equilibrium Pressure swing
Air separation G Carbon molecular sieve Kinetic Pressure swing
Zeolite Equilibrium
Linear paraffins separation G 5A molecular sieve Shape-selective Displacement or
sieving vacuum desorption
Wastewater purification L Activated carbon Equilibrium Steam stripping
a Liquid; G, gas.