In this essay we will discuss about:- 1. Classification of Adsorbent 2. Characterisation of Adsorbents 3. Mechanism 4. Equipment 5. Regeneration.
Essay # 1. Classification of Adsorbent:
Potential adsorbent can be classified into three groups:
(a) Nonpolar solids:
Where the adsorption is mainly physical e.g., activated carbon.
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(b) Polar solids:
Where the adsorption is chemical and no change in the chemical structure of the molecules or the surface occurs e.g. silica and aluminium oxides. These materials adsorb both polar and nonpolar molecules, but they exhibit preference or polar molecules.
(c) Chemical Adsorbing surfaces which adsorb the molecules and then release them after reaction, which may be either catalytic, leaving the surface unchanged or non-catalytic requiring replacement of the surface atoms.
Adsorbent can be classified as conventional or non-conventional adsorbents convention adsorbents include activated carbon, polymeric carbon, aluminous, silica gel, bauxite etc. Non-conventional adsorbents includes, fly-ash, hardwood, lignite, peat, sawdust etc.
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Activated Carbon:
Among the various adsorbent mentioned above activated carbon is most commonly used adsorbent because it has large surface area per unit weight or volume of solid. Activated carbon is the residue obtained from various carbonaceous materials like coal, nut shells, wood, paper mill sludge, petroleum sludge, which are subjected to various steps like drying, carbonising and activating.
Activated carbon is effective in adsorbing molecules of organic substances, with less selectivity than is exhibited by other more polar adsorbents. Activated carbon is effective in adsorbing organic molecules even from humid gas stream also.
Essay # 2. Characterisation of Adsorbents:
Some of the important parameters for characterisation of adsorbents are-particle size, surface area, pore size, pore volume and pore volume distribution, density, adsorption capacity, iodine number and molasses number, hardness and abrasion resistance.
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Characteristics of some commonly used adsorbents are given in Table 10.3. The total adsorptive capacity of a sample of activated carbon or any adsorbent may be measured by its activity or relativity for a standard vapour.
Adsorbent Impregnations:
The effectiveness of adsorbent can be increased by impregnations.
The effectiveness of adsorbent impregnation may be related with any of the following modes of action:
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(a) The impregnant may be a reagent that chemically converts a pollutant of a harmless or adsorbable product.
(b) The impregnant may be a catalyst that acts continuously.
(c) The impregnant may be a catalyst that acts intermittently.
Table 10.4 lists some adsorbent impregnations and summarises their modes of action.
Essay # 3. Mechanism of Adsorption:
The overall kinetics of the adsorption system used are affected by both type of gas solid contacting method and transport processes of the adsorbate from the bulk gas to the internal adsorbate surface.
The following transport process can affect the overall rate of the adsorption process and should be kept in view:
(a) Transfer of adsorbate from the bulk gas stream to the external surface of the adsorbent.
(b) Diffusion through the passage within the particles.
i. Through Macropores,
ii. Through Micropores,
iii. Adsorbs at an appropriate site is adsorption on internal particle surface.
(c) Time for adsorption to take place is sum of the times for each step.
Bach of the process depends on the system operating conditions and physical and chemical characteristics of the gas stream and solid adsorbent. The slowest of these steps will determine the rate for the whole process and is called rate determining step.
Effect of Process Variables:
Some of the important variable affecting adsorption are air flow rate, temperature, vapour concentration, humidity, degree of regeneration, contact time, mode of operation method adopted etc.
Equilibrium Isotherms:
The equilibrium characteristics of a solid gas system are described by a curve of the concentration of adsorbed gas on the solid as a function of the equilibrium partial pressure of the gas at constant temperature. Such cure is called adsorption isotherms. The most well-known isotherms are shown in Table 10.5 which indicates the dependence of the heat of adsorption on surface coverage which each isotherm implies.
1. Langmuir Isotherms:
V = vol. of adsorbate adsorbed per unit mass
Vm = volume of adsorbate for monolayer coverage
p = partial pressure
Assumptions:
1. Heat of adsorption constant
2. Adsorption limited to single layer
3. Adsorbed molecules are not free to move on the surface
2. Freundlich Isotherm:
V = a pl/n n > I
a = Freundlic constant.
p = partial pressure
3. Branauer, Emmett, Teller (BET) Isotherm:
p = partial pressure
C = BET constant
v = volume of adsorbate adsorbed
vm = volume of adsorbate adsorbed for monolayer coverage
Assumptions:
Adsorbent surface is composed of uniform energies of adsorption sites and the energy localised sites do not interfere with neighbouring site.
1. Adsorbent molecule do not migrate on the surface of adsorbent.
2. The energy of adsorption holds the first monolayer whereas the condensation of energy of adsorbate is responsible for adsorption of subsequent layers.
3. A given layer need not complete the formation to the initiation of successive layers.
Break through Curve:
The design of a fixed-bed adsorption column would normally require a model that one predict the break through curve. The curve of effluent concentration as a function of time thus has an S-shaped appearance that may be steep or relatively Hat depending on the rate of adsorption, the nature of adsorption equilibrium. The fluid velocity, the inlet concentration and the length of the bed.
The time at which the break through curve first begins to rise appreciably is called the break point. The passage of an adsorption wave through a stationary bed during an adsorption cycle is given in Figure 10.2.
Design Principles of Adsorption Equipment:
The general requirements that adsorption phenomenon impose an equipment design are as follows:
(a) Long enough duration of contact between air stream and sorbent bed for adequate sorption efficiency.
(b) Sufficient sorption capacity to provide desired service life.
(c) Small enough resistance to air flow to allow adequate operation of the air moving devices being used.
(d) Uniformity of distribution of airflow over the sorbent bed to ensure full utilisation of the sorbent.
(e) Adequate pre-treatment of air to remove non adsorbable particles which would impair the action of sorbent bed.
(f) Provision for removing the sorbent after it has reached saturation.
Essay # 4. Equipment of Adsorption:
Various types of adsorption equipment commonly used are given in Table 10.6. Most adsorption systems are of the cycle fixed bed type. Other type of adsorbes are moving bed or fluidized bed, rotating bed, falling bed. Some of the important adsorption equipment’s commonly used are given in table 10.6 and Fig. 10.3, 10.4, 10.5, 10.6.
Essay # 5. Regeneration of Adsorbents:
Regeneration of adsorbents is very important operation in adsorption and major part of running cost of any adsorption process lies in this regeneration Step, the various commonly used regeneration-adsorption cycles are thermal swing cycle, pressure swing cycle, and purge gas stripping cycle. Displacement cycles and combination cycles (6) Fig. 10.7.
