Page 355 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

12.1 Introduction 357

Table 12.2 Physisorption versus Chemisorption.
Physisorption Chemisorption
Low enthalpy of adsorption (5e50 kJ/mol) High enthalpy of adsorption (200e400 kJ/mol)
Reversible Irreversible
Weak forces of attraction like van der Waals Chemical bonding involving orbital overlap and
forces, hydrogen bonding, etc. charge transfer
Multilayer adsorption. Generally, monolayer adsorption
BET isotherm used to model equilibrium. Langmuir isotherm used to model equilibrium
Observed at low temperature (higher temperature Observed at higher temperature
reduces surface coverage)
Not specific Highly specific adsorbate-adsorbent pairs
No surface reactions Surface reactions e dissociation, reconstruction,
catalysis possible

p*, Equilibrium partial pressure Adsorption

Desorption

q, kg adsorbate/kg adsorbent
FIGURE 12.2
Adsorption isotherm exhibiting hysteresis.

12.1.3 Adsorption equilibrium
Isotherms relating adsorbate loading capacity to adsorbate concentration in the process stream at a
given temperature are the most common form of expressing the equilibrium data. Equilibrium data
governs capital cost to a large extent since adsorption capacity decides the amount of adsorbent
required.