1. ADSORPTION
The removal of dissolved substances from solution using adsorbents such as activated carbon

2. ADSORPTION
Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic.

3. d

4. Adsorption Process Classified as Physical and Chemical Ads.
1) Physical adsorption
The gas molecules adhere to the surface of the solid adsorbent as a result of intermolecular attractive forces (van der Waals forces) between them
The process is exothermic: the heat liberated is in the order of the the enthalpy of condensation of vapor (2-20 kJ/gmole)
The process is reversible (recovery of adsorbent material or adsorbed gas is possible) by increasing the temperature or lowering the adsorbate conc.
Physical adsorption usually directly proportional to the amount of solid surface area
Adsorbate can be adsorbed on a monolayer or a number of layers
The adsorption rate is generally quite rapid

5. Adsorption Process 2) Chemical adsorption
Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption
Heat liberated during chemisorption is in the range of kj/g mole
It is frequently irreversible. On desorption the chemical nature of the original adsorbate will have undergone a change.
Only a monomolecular layer of adsorbate appears on the adsorbing medium

6. Adsorption Mechanism 2) Chemical adsorption
Results from a chemical interaction between the adsorbate and adsorbent. Therefore formed bond is much stronger than that for physical adsorption
Heat liberated during chemisorption is in the range of kj/g mole

7. Properties of Activated Carbon
Adsorbent Material
Silica gel
Activated alumina
Activated carbon
Synthetic zeolite
Molecular sieve
Dehyrdating purposes
Properties of Activated Carbon
Bulk Density
22-34 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
15-25 Å
Regeneration Temperature
(Steaming) oC
Maximum Allowable
Temperature
150 oC

8. Properties of Silica Gel Properties of Activated Alumina
Adsorbent Material
Properties of Silica Gel
Bulk Density
44-56 lb/ft3
Heat Capacity
BTU/lboF
Pore Volume
0.37 cm3/g
Surface Area
750 m2/g
Average Pore Diameter
22 Å
Regeneration Temperature oC
Maximum Allowable Temperature
400 oC
Properties of Activated Alumina
Bulk Density
Granules
38-42 lb/ft3
Pellets
54-58 lb/ft3
Specific Heat
BTU/lboF
Pore Volume
cm3/g
Surface Area
m2/g
Average Pore Diameter
18-48 Å
Regeneration Temperature (Steaming) oC
Maximum Allowable Temperature
500 oC

9. Properties of Molecular Sieves
Adsorbent Materials
Properties of Molecular Sieves
Anhydrous Sodium Aluminosilicate
Anhydrous Calcium Aluminosilicate
Anhydrous Aluminosilicate
Type 4A 5A
13X
Density in bulk (lb/ft3) 44 38
Specific Heat (BTU/lboF)
0.19 -
Effective diameter of pores (Å) 4 5 13
Regeneration Temperature (oC)
Maximum Allowable Temperature (oC)
600
Crystalline zeolite
Uniform pores to selectively separate compounds by size & shape

10. Adsorption Isotherm
The amount of gas adsorbed per unit of adsorbent at equilibrium is measured against the partial pressure of the adsorbate in the gas phase gives equilibrium adsorption isotherm

11. Adsorption Isotherm
In general, an adsorption isotherm relates the volume or mass adsorbed to the partial pressure or concentration of the adsorbate in the main gas stream at a given temperature
The equilibrium concentration adsorbed is very sensitive to T
There are many equations proposed to fit analyticaly the various experimental istoherms

12. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb on to a surface is equal to the rate at which they desorb.
The capacity of an adsorbent for a particular adsorbate involves the interaction of three properties: the concentration C of the adsorbate in the fluid phase, the concentration Cs of the adsorbate in the solid phase and the temperatureT of the system.

13. If one of these properties is kept constant, the other two may be graphed to represent the equilibrium. The commonest practice is to keep the temperature constant and to plot C against Cs to give an adsorption isotherm. When Cs is kept constant, the plot of C against T is
known as an adsorption isostere.
In gas–solid systems, it is often convenient to express C as a pressure of adsorbate. Keeping the pressure constant and plotting Cs against T gives adsorption isobars. The three plots are shown for the ammonia-charcoal system in Figure 17.4 which is taken from the work of BRUNAUER.

15. At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other, that is:
Where ∆𝐻 the enthalpy change per mole of adsorbate as it transfers from gaseous to adsorbed phase.

16. Adsorption Isotherms, Langmuir
In Langmuir isotherm assuming a unimolecular layer can be obtained by a kinetic approach: at equilibium knowing that the rate of adsorption is equal to the rate of desorption:
ra=CaP(1-f)
rd=Cdf
ra: rate of adsorption
Ca, Cd: constant
P: partial pressure of the adsorbate
f: is the occupied fraction of the total solid surface 16

17. Adsorption Isotherms, Langmuir
Since we assume a monolayer coverage, the mass of adsorbate per unit mass of adsorbent (a) is also proportional to f:
Langmuir Isotherm 17

18. BET (Brunauer, Emmett and Teller) isotherm:
This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of adsorption for each layer except for the first layer.

20. CS =saturation (solubility limit) concentration of the solute
CS =saturation (solubility limit) concentration of the solute. (mg/liter)
qe = mass of material adsorbed (at equilibrium)
KB = a parameter related to the binding intensity for all layers.
Note: when Ce << CS and KB >> 1 and K = KB/Cs BET isotherm approaches Langmuir isotherm.