Adsorption By Usman Malick

1. Adsorption

2. Basics:
A mass transfer process which involves
the accumulation of substance at the
interface of the two phases, such as,
liquid-liquid, Gas-liquid , Gas-solid or
Liquid-solid interface.
OR
Adsorption is adhesion of atoms, ions
or molecules from a gas, liquid or
dissolved solid to a surface.

3. Adsorbate &Adsorbent:
• Adsorbate:
• The molecular species or substance, which
concentrates or accumulates at the surface
is termed Adsorbate.
• Adsorbent:
• The material on the surface of which the
adsorption takes place is called Adsorbent.

6. Adsorbents:
• Desirable traits
High selectivity and adsorbate capacity (high surface Area/Weight).
Favorable kinetic and transport properties.
High chemical, mechanical, and thermal stability.
Low fouling and easy regeneration.
No side reactions.
Cheap.

7. Types Of Adsorption:
Physical Adsorption or physisorption:
A kind of adsorption in which the accumulation of adsorbate
on the surface of a solid occurs on account of weak van der
Waals’ forces, is termed as physical adsorption or
physisorption.
Chemical adsorption or chemisorption.
A kind of adsorption in which the molecules or atoms are held
to the solid surface by chemical bonds, is termed as chemical
adsorption.

8. Comparison:
Chemical Adsorption
Occurs due to
the chemical
bond formation.
Highly specific in
the nature.
Irreversible
reactions takes
place.
High activation
energy required.
Possible over a
wide range of
temperatures
Physical Adsorption
• Occurs due to the wander-Waal
forces.
• Not specific in the nature.
• Reversible in nature.
• No appreciable energy is
required.
• Only significant at relatively low
temperatures.

9. Differences between gas and liquid adsorption:
Gas adsorption:
Fast mass transfer.
Concentration (and adsorption) determined by P and T.
Removing adsorbed gases (regeneration) through P & T.
Liquid adsorption:
Slower mass transfer.
Adsorbent pores already filled with liquid.
Adsorbents can be less selective and competitive adsorption between components is critical.

10. Factors
Affecting
Adsorption:
• TEMPERATURE:
• An exothermic process. Chemical adsorption first
increases with rise in the temperature and then starts
decreasing.
• PRESSURE:
• With increase of pressure, adsorption increases up to
certain extent till saturation level is achieved.
• SURFACEAREA:
• Adsorption capacity of adsorbent increases with
increase in its surface area.

11. Adsorbents Mostly Used.
• Silica gel:
• Chemically inert, nontoxic, polar and
dimensionally stable (< 400 °C ).
• It is prepared by the reaction between
sodium silicate and acetic acid.
• Activated carbon:
• Effective adsorbent because it is a highly
porous material and provides a large surface
area.

12. Different Sizes Used:
• Microporous Adsorbents:
Pore size range 2Ao to 20Ao
• Mesoporous Adsorbents:
Pore size range 20Ao to 500Ao
• Macro porous Adsorbents:
Pore size range > 500Ao

14. ADSORPTION ISOTHERMS:
• The variation in the amount of adsorbate adsorbed by the adsorbent
with pressure at constant.
• Temperature is expressed by a curve termed as adsorption isotherm.
• FREUNDLICH ADSORPTION ISOTHERM:
x/m =Kp 1/n
x= Amount of adsorbate.
m= Amount of Adsorbent.
P= Pressure.

15. Other types of Isotherms:
• Langmuir
• model applies to gases adsorbed on solid surfaces.
• a semi-empirical isotherm with a kinetic basis and was derived
based on thermodynamics.
• BET
• Often molecules do form multilayers, that is, some are adsorbed on
already adsorbed molecules, and the Langmuir isotherm is not valid.

16. Applications of Adsorption:
Air pollution masks:
These consists of silica gel or activated charcoal powder, when dust or smoke are
paused through them, those particles get adsorbed on the surface of these materials.
Purification of water:
By the addition of alum stone to the water, impurities get adsorbed on the alum and
water gets purified.
Removal of moisture and humidity:
Moisture in the air is removed by placing silica gel on which water molecular gets
adsorbed.
Drying gases and liquids before loading them in industrial systems.

17. Carbon capture and storage
Typical adsorbents proposed for carbon capture and
storage are zeolites and MOFs.
Adsorbents can be regenerated by temperature or pressure.
• MOFs:
• Metal Organic Frame works consisting of the organic porous materials.