Adsorption - Principles of Chemical Separations with Environmental Applications - page 201

A quick estimate of T for gas-phase sorption can be made. H has a range of 1000 to

4000 kJ / kg (avg = 2500), C p g is approx. 1 kJ / kg · K and Y f is typically 0.01. Therefore,

0

kg carrier gas 2500 kg solute

kg solute

kJ

. 01

25 ◦ C) T = = 25K( ≡ .

kJ
kg carrier gas · K

1

So the maximum temperature rise in a gas-solid sorption can reach 25 ◦ C.
A few additional points are worth noting:
1 For constant partial pressure, Y f is inversely proportional to total pressure. Therefore, T

will decrease with an increase in total pressure. This translates to isothermal operation
for a total feed pressure of approximately 50 atm.
2 Carrier gases with high C p g (hydrogen, for example) will tend to reduce

T .

3 An increase in feed solute concentration will increase T .

7.8.6 Bed stability

Not all adsorption beds will develop stable MTZs. One requirement for stability (i.e., the
MTZ reaches a limiting size) is that the equilibrium line must be “favorable.” In the case of
a single adsorbate isothermally removed from a non-adsorbable component, the curve of
loading as a function of composition must be concave downward in the region of loading
below the stoichiometric point to be favorable. This effect is described in more detail
in Section 7.9. In non-isothermal adsorption it is possible for the temperature effects to
cause a favorable isotherm to become an unfavorable equilibrium line. This was discussed
previously in the context of the crossover ratio R .

7.8.7 Special considerations for liquids

Flow direction considerations for liquid systems are somewhat different than those for
vapor flow. In liquid or dense-phase flow the buoyancy force of the liquid must be con-
sidered as well as the pressure drop. During upflow adsorption, the flow velocity should
be low so as to not cause bed expansion (fluidization). As the flowrate exceeds this limit,
the pressure drop increase is small with increasing velocity. Sometimes liquid systems are
designed with some bed expansion (10% at the most) when it is desirable to limit pressure
drop. Upflow is preferred if the liquid contains any suspended solids, so that the bed will
not become plugged.
Prior to the introduction of a liquid into the adsorbent bed, there must be sufficient time
for any gas or vapor that may be trapped in the pores of the sorbent to outgas. Otherwise the
gas or vapor may contaminate a product during operation. In the case of upflow adsorption,
the effective bulk density may be lowered enough to cause excessive bed expansion or
flow channeling.