Adsorption - Principles of Chemical Separations with Environmental Applications - page 194

FLUID PHASE SORBENT

1 2 PORE

3

4

Figure 7.2 Adsorption mechanisms.

3 Transfer of the adsorbate from the particle surface to the interior of the adsorbent via
the pore network. This step can be accomplished in two ways: pore diffusion (diffusion
through the fluid in the pore); and surface diffusion (the particle travels along the pore
surface).
4 Physical or chemical binding of the adsorbate to the internal surface of the adsorbent.
This step is controlled by the molecular interactions described previously for adsorption.
Steps 1 and 4 are usually the fastest steps, and therefore are not considered to contribute
to the overall rate of adsorption. The rate-determining step is typically Step 3, although
changes in fluid flowrates can affect mass transfer across the fluid-particle boundary layer
(Step 2).
For fixed beds, Step 2 can be described by:

17 Re − 0 . 415

j = 1 . 10 < Re < 2500 (7.1)

k
v s Sc 0 . 067

= = Chilton-Colburn j factor (7.2)

ρ f ν s d p
µ

where Re = = Reynolds number

µ

Sc =

D AB =

Schmidt number

ρ

ρ f = fluid density

µ = fluid viscosity

v s = fluid superficial velocity

d p = particle diameter

k = mass transfer coefficient

D AB = diffusion coefficient of sorbate in fluid.

High values of k correspond to less mass transfer resistance for this step. One way to in-
crease k is to increase

v s . This can cause problems, though, since the contact time in the bed
would be reduced. This can lead to breakthrough with a larger portion of the bed
unused.