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Activated Carbon

Activated carbon


Activated Carbon is a form of carbon that has been processed to give it a large surface area. Activated carbons are non-hazardous, processed, carbonaceous products, having a porous structure and a large internal surface area.

 

Activated carbon is usually derived from charcoal. Due to its high degree of microporosity, just 1 gram of activated carbon has a surface area in excess of 500 m2 (about one tenth the size of an American football field). Sufficient activation for useful applications may come solely from the high surface area, though further chemical treatment often enhances the adsorbing properties of the material.

 

The large surface area is capable of adsorbing organic molecules from liquids and gases. These materials can adsorb a wide variety of substances, i.e. they are able to attract molecules to their internal surface, and are therefore called adsorbents.

 

Activated carbons have been used for many years quite successfully for adsorptive removal of impurities from exhaust gas and waste water streams. It has been historically used for removal of odor, removal of color pigments and various catalytic functions. However, applications of activated carbon have increased significantly in the recent years with the advancement of activated carbon process capability.

 

Removal of certain impurities contained in gases (such as hydrogen sulfide, mercury and ammonia), the adsorption capacities and the feasible removal rates must be substantially boosted by impregnation of the activated carbon by suitable chemicals. When these chemicals are deposited on the internal surface of the activated carbon, the removal mechanism also changes. The impurities are no longer removed by adsorption but by chemisorption.

 


Classification and Applications

Activated carbons are broadly classified here for general purpose based on their physical characteristics:

 

Powdered Activated Carbon (PAC)

PAC is made up of crushed or ground carbon particles, 95–100% of which will pass through a designated mesh sieve or sieve. They are activate carbons which are made in particulate form as powders or fine granules less than 1.0 mm in size with an average diameter between .15 and .25 mm. They present a large surface to volume ratio with a small diffusion distance.

 

 PAC is not commonly used in a dedicated vessel, owing to the high head loss that would occur. PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.

 

Granular Activated Carbon (GAC)

GAC is defined as the activated carbon being retained on a 50-mesh sieve (0.297 mm). Compared to PAC, GAC has a relatively larger particle size and consequently, presents a smaller external surface. Diffusion of the adsorbate is an important factor which makes it a preferred option for all adsoprtion of gases and vapors as their rate of diffusion are faster.

 

Granulated carbons are also used for water treatment, deodorization and separation of components of flow system. GAC can be either in the granular form or extruded (see below). GAC is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications. A 20×40 carbon is made of particles that will pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40 sieve (0.42 mm) (generally specified as 95% retained). The most popular aqueous phase carbons are the 12×40 and 8×30 sizes because they have a good balance of size, surface area, and head loss characteristics.

 

Extruded Activated Carbon (EAC)

EAC combines powdered activated carbon with a binder, which are fused together and extruded into a cylindrical shaped activated carbon block with diameters from 0.8 to 130 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content.

 

Impregnated Carbon

Impregnated activated carbons are carbonaceous adsorbents which have chemicals finely distributed on their internal surface. The impregnation optimizes the existing properties of the activated carbon giving a synergism between the chemicals and the carbon. Porous carbons containing several types of inorganic impregnant such as iodine, silver, cations such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for specific application in air pollution control.

 

For environmental protection, various qualities of impregnated activated carbon are available and have been used for many years in the fields of gas purification. Due to antimicrobial/antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. Drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and Al(OH)3 a floculating agent. Impregnated carbons are also used for the adsorption of H2S and thiols. Adsorption rates for H2S as high as 50% by weight have been reported.

 

Environmental Application

Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:
  • spill cleanup
  • groundwater remediation
  • drinking water filteration
  • air purification
  • volatile organic compounds capture
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