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Introduction
Activated carbon is a black solid substance resembling granular or powdered charcoal. It is extremely porous with a large surface area, and typically produced from organic precursors such as Charcoal, coconut shells,  wood chips, sawdust, lignite etc.

The raw material is first heated in an inert environment to obtain the carbonaceous material, which is activated further to derive a highly porous final product. For the activated material, surface areas typically range from 500-1400 m2/g. The activated carbon particle has mainly two types of pores existing in it, viz. macropores and micropores, through which adsorption takes place.

The macropores provide a passageway to the interior of the particle into the micropores but do not contribute substantially to the particle surface area. The micropores, on the other hand, are responsible for the large surface area of activated carbon particles created during the activation process. It is the micropores where adsorption largely takes place. Thus, two main parameters are relevant to the performance of the activated carbon namely, the surface area and the pore volume or structure. The pore volume limits the size of the molecules that can be adsorbed whilst the surface area limits the amount of material, which can be adsorbed. The mechanical strength of the activated carbon is also an important factor for prevention of damage due to regeneration, recycling etc.

Activated carbon has several important uses including solution purification (as in the clean-up of cane, beet and corn-sugar solutions), removal of tastes and odours from domestic and industrial water supplies, vegetable and animal fats and oils, alcoholic beverages, chemicals and pharmaceuticals and in the waste water treatment. It also finds use in purification of gases, liquid phase recovery, separation processes and as catalyst and catalyst supports. Many organic compounds such as chlorinated & non-chlorinated solvents, gasoline, pesticides and trihalomethanes can be adsorbed by activated carbon. It is also effective for removal of chlorine and moderately effective for removal of some heavy metals. It is used for liquid phase adsorption or de-colorizations which are usually light, fluffy powders produced from low-density material such as sawdust or peat. However, for gas phase adsorption, there is a need for hard, dense granular materials produced from high-density raw materials such as coconut shells, palm kernel shells, coal or coke.

High-grade activated carbon can be obtained from woody material like wood charcoal with inherent mechanical strength, high carbon and low ash content.

Carbonization or Pyrolysis of Wood
Pyrolysis is the process in which wood is heated, decomposed and eventually converted into desired product in absence of air in the fixed bed reactor. The pyrolysis includes carbonization (destructive/dry distillation of wood), charcoal processing, gasification, activated carbon processing. The pyrolysis products is wood charcoal. The activation of the raw materials is done normally in the presence of chemical agents such as zinc chloride, magnesium chloride, calcium chloride or phosphoric acid. The carbonized material is treated with oxidizing gas in a furnace at 800-10000C under the conditions that permit removal of nearly all the adsorbed hydrocarbons and some of the carbon to increase the surface area.
  • Steam Activation
    The steam activation process is introduced in India by Dr. R. G. W. Farnell of U.K. First steam activation plant was installed in India in 1958 in Rajpipla and second plant came in operation in 1962 in Najibabad, Distt. Bijnore.in able guidance of  Mr. S. B. Dhungat.
     

  • Chemical Activation

  • Chemical activation is generally used for the production of activated carbon from sawdust, wood or peat. The raw material is impregnated with a strong dehydrating agent, typically phosphoric acid (H3PO4) or zinc chloride (ZnCl2) mixed into a paste and then heated to temperatures of 500 - 800°C to activate the carbon. The resultant activated carbon is washed, dried and ground to powder. Activated carbons produced by chemical activation generally exhibit a very 'open' pore structure, ideal for the adsorption of large molecules.
This technique is generally used for the activation of coconut shell and charcoal subsequent to carbonization. The charcoal is activated by reaction with steam at a temperature of 800-1000oC under controlled atmosphere in a Rotary Shell to facilitate uniform heat distribution and improved gas–solid contact. The reaction between steam and charcoal takes place at the internal surface area.

Initially, gasification of the carbonized material with steam occurs and the following reaction, known as the Water-Gas reaction, takes place :
C + H20 -> CO + H2 -175,440 kJ/(kg mol)
This reaction being exothermic, temperature is maintained by partial burning of the CO and H2 formed, as follows :
2CO + O2 -> 2CO2 +393,790 kJ/(kg mol)
2H2 + O2 ->2H2O +396,650 kJ/(kg mol)

Flow Diagram for Activated Carbon Production
Flow Diagram for Activated Carbon Production

The process described above produces a carbonaceous substance with many small pores and thus a very large surface area, which is then crushed to yield a granular or pulverized product. Granular activated carbon (GAC) is more commonly used in water systems than pulverized activated carbon (PAC). Depending on the characteristics (particle size, pore size, surface area) of the granular carbon used, these are effective for removing organic chemicals in industrial waste and trace organics, lead, taste and odours in drinking water.

The charcoal based activated carbon is a natural product like the one derived from coconut shell. But the micropore of charcoal based activated carbon is more developed than coconut shell activated carbon. For example, adsorption of Methyl Blue on coconut shell based activated carbon usually ranges at 150-300mg/gm., but Charcoal activated carbon shows adsorption up to 200mg/g, the highest could reach to 400mg/gm. Thus Charcoal based activated carbon could be appropriate for liquid phase absorption (purification of drinking water, food and purification of medicine). The ability of Charcoal activated carbon to remove waste nitrogen from water is also very good.

Adsorption and absorption - two different phenomena
Adsorption is therefore a surface phenomenon. It should not be confused with absorption, which is defined as the filling of a porous body by a liquid, such liquid being retained only by capillarity. A sponge filling with water is a typical example of absorption.

One important point to remember is that, although adsorption occurs irrespective of the initial concentration of the pollutant, it is particularly effective when there are only traces of adsorbate present.

Chemical Properties
The filter surface may actually interact chemically with organic molecules. Also electrical forces between the activated carbon surface and some contaminants may result in adsorption or ion exchange. Adsorption is also affected by the chemical nature of the adsorbing surface. The chemical properties of the adsorbing surface are determined to a large extent by the activation process. Activated carbon materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants. For example, chloroform is adsorbed best by activated carbon that has the least amount of oxygen associated with the pore surfaces. Chemical Properties are:-
  1. Methylene Blue
    • measure of mesopore structure
  2. Caramel dp (Molasses No.)
    • measure of macropore structure
    • important for decolorizing performance
  3. Ash Content
    • reduces overall activity of activated carbon
    • reduces efficiency of reactivation
    • metals can leach out of activated carbon resulting in discoloration
    • acid/water soluble ash content is more significant than total ash content
  4. pH
    • Adsorption capacity increases under pH conditions, which decrease the solubility of the adsorbate (normally lower pH).
    • covers determination of the pH of a water extract of activated carbon
  5. Acid Solubles
    • Covers the determination of the acid soluble impurities of powdered activated carbons, using an atomic absorption spectroscopy method by direct aspiration.
  6. Water Solubles
    • Covers the determination of the water soluble content of (unused) powdered activated carbons. Water solubles are materials that can be extracted by distilled water under reflux conditions and are expressed as a percentage of dry carbon weight.

Physical Properties
The amount and distribution of pores play key roles in determining how well contaminants are filtered. The best filtration occurs when pores are barely large enough to admit the contaminant molecule. As contaminants come in different sizes, they are attracted differently depending on pore size of the filter.  Physical properties are:-

  1. Surface Area
    • measure of adsorption capacity (Note: pore size distribution/pore volume is also important to determine ultimate performance)
  2. Apparent Density
    • higher density provides greater volume activity and normally indicates better quality activated carbon
  3. Particle Size
    • smaller size provides quicker rate of adsorption which reduces the amount of contact time required
    • smaller size results in greater pressure drop
Contaminant Properties
A general rule of thumb is that similar materials tend to associate. Organic molecules and activated carbon are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the activated carbon rather than staying dissolved in a dissimilar material like water. Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores.

Water Temperature and pH
Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered, many organic chemicals assume more adsorbable form.

Exposure Time
The process of adsorption is also influenced by the length of time that the activated carbon is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of activated carbon in the filter and reducing the flow rate of water through the filter.
 


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  • Activated carbon materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants.

  • The amount and distribution of pores play key roles in determining how well contaminants are filtered.

  • Organic molecules & activated carbon are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the activated carbon rather than staying dissolved in a dissimilar material like water.