Diffusional limitations in adsorption processes

1
Diffusional limitations in adsorption processes

• Adsorption can be mass transport limited
• Morphology, size and geometry of adsorbent
  influences mass transfer
• Mechanisms
• External mass transport (controlled by diffusion
  from the bulk solution to the surface of the
  adsorbent)
• Internal mass transport (controlled by diffusion
  within a porous adsorbent)
• With porous adsorbents commonly used in
  bioseparation, the second mechanism predominates.

Adsorbent
Porous adsorbent
Film
2
Batch adsorption
Equilibrium line or isotherm
Operating line or material balance
CB0 (S/A)CU0
Equilibrium line
CB
Slope -S/A
Operating line
CB
CU
CU
3
Packed bed adsorption
Feed
Eluent
Wash liquid
Product
Spent feed
Wash
Adsorption
Washing
Elution
Elution
Washing
Adsorption
Concentration
Impurities
Target molecule
Time
4
Breakthrough curve
5
Zones within packed bed
6
Analysis of packed bed adsorption
At t 0 and z 0, CU CF t t and z l,
CU CF t t and z l, CU B CF where B
(C/CF)
For RHS 0 and CB KCU
For RHS ? 0
7
Approximate analysis of packed bed adsorption
CF
t
CU
t
0
Equilibrium zone
l
Adsorption zone
0
z
8
Other adsorption devices

• Limitations of packed beds
• High pressure drop
• Inability to handle feed with particulate
  material
• Need to use low flow rates
• Radial dispersion problems

Stack of porous membranes
Feed
Feed
Membrane adsorber
Fluidised bed adsorber