Adsorption is one of the physical processes used in trapping substances on the surface of a solid material. It is a surface phenomenon. The solid is called absorbent. The absorbent is placed in a fixed bed and continuously passing of fluid is done through he bed till the solid becomes saturated.
Adsorption is similar to ion exchange resin because, they are carried out in semi batch and fixed bed.
In gaseous state, adsorption is similar to chromatography but the feed is not continuous. The individual components moves at different rates and collected at exist.
Adsorption can be physiosorption or chemisorptions.
Physiosorption: this considers the van der Waals forces of attraction.
CHEMISORPTION: chemical bonds are considered. The predominant bond is covalent. The bond is responsible for trapping adsorbate to the absorbent. in relation to surface energy, adsorption can be similar to surface tension.
Adsorption takes place in porous material and on the walls of pores of specific sites inside a particle. The recovery organic solvent such as paints can be achieved by adsorption process. Using carbon, pollutants can be removed and also better ventilation can be achieved.
Gases are dried using adsorption process. Example is the entrapment of water molecules by silica gel. It can used to separate organic and=] petroleum products.
- Fixed bed apparatus: it has the set up below (diagram)
- Gas dryer: it has a similar set up above. In fixed bed apparatus, regeneration can be carried out using hot inert gas but in gas dryer, hot gas can be used. Notice; at low pressure, desorption takes place, therefore, after regeneration the pressure should be maintained.
- Adsorption from liquid: the major application is the removal of impurities from body of substances. This purification process is done at a high concentration of the adsorbate. Example, using activated charcoal to remove pollutant from waste solution.
- Pressure swing adsorption: this helps to supply hydrogen to a process stream.
Adsorption isotherm is the equilibrium relationship between the concentration of a liquid and partial pressure of a gas with the absorbent medium at constant temperature.
Types of isotherm
- Freundlish: it has a relationship of
m= mass P=pressure x= mass of adsorbate and Kc and n are constant.
- Linear: the amount of substance adsorbed is proportional to the concentration of the fluid.
- Langmuir: this gives when the adsorption process is reversible.
A=gas and B = adsorbate site
Θ= surface coverage
θ is difficult to determine experimentally. At STP, the adsorbent gas forms a single or mono layer and thus represented as:
Plotting the reciprocal of V and P to obtain K and Vsing and a straight line could be obtain as
The adsorption enthalpy can be determined using
Hads change =change Hliq – RT(InC)
Adsorption is mostly exothermic process.