Page 354 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

356 Chapter 12 Adsorption

where
G s
H tOG ¼ (12.10b)
K Y a P

Y is the equilibrium composition in the gas corresponding to the adsorbate composition X and
K Y a P is the overall gas-phase mass transfer coefficient based on the specific surface area of the solid
particles ða P Þ.
Rotary beds are employed to combine the advantages of both moving and fixed bed. Several beds
are on a rotating drum (or wheel) that makes each bed to go through cycles of
adsorption and desorption. At specific locations, each bed aligns with the
appropriate inlet and outlet piping connections for adsorption and desorption.
Rotary beds
Most often these beds are partitioned sectors. Recovery of volatile solvent
from air is one of the common applications of rotary bed adsorber.
Fluidised beds are extensively used for recovery of vapours, drying of air with silica gel,
fractionation of light hydrocarbon vapours with carbon, etc. Fluidised beds are
attractive because (i) interparticle heat and mass transfer is high and (ii) when
fully fluidised, its net weight is supported by drag forces due to fluid flow that
Fluidized beds
results in the pressure drop (DP) to be almost independent of flow rate
(Eq. 12.11).
DP
¼ð1 aÞðr r Þg (12.11)
s
f
L bed
where r and r are the density of the solid and fluid, respectively, and a is the bed voidage at the onset
s
f
of fluidisation.
12.1.2 Adsorption mechanisms
Bonding of a solute on a solid surface can be physical bonding (Physisorption) or chemical bonding
(Chemisorption). Table 12.2 lists the major differences between the two processes. There may be
situations when bonds of both types may be present. A third mechanism e capillary condensation,
happens in the case of gas adsorption in porous media. This involves localised condensation inside
pores at temperatures above the dew point of the bulk fluid allowing the formation of multiple solute
layers on the surface.
Porous adsorbents with capillaries not too narrow on the molecular scale adsorb by the same
mechanism as nonporous adsorbents for a low relative partial
pressure (partial pressure/vapour pressure). With increasing pres-
sure, multilayer adsorption takes place with adsorbate condensing
Adsorption in Porous media
inside pores and the heat of adsorption similar to the heat of
condensation. This phenomenon termed “capillary condensation”
can be explained by considering the vapour pressure over a curved
surface. At the same temperature, the vapour pressure is lower over a concave surface as compared to a
flat or convex surface. In the narrow capillaries, the adsorbed liquid meniscus is concave. This results
in the vapour condensing at a lower pressure than the vapour pressure over a flat surface. Capillary
condensation causes hysteresis of adsorption equilibrium (Fig. 12.2) in porous adsorbents.