Adsorption

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 , i ons  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.

4. Adsorbents:

5. 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.

6. Comparison: Chemical Adsorption 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.

7. 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.

8. 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.

9. 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.

10. Different Sizes Used: Microporous Adsorbents: Pore size range 2A o to 20A o Mesoporous Adsorbents: Pore size range 20A o to 500A o Macro porous Adsorbents: Pore size range > 500A o

11. 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.

12. 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.

13. 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 .

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