Activated carbon is a carbonaceous material characterized by a well developed pore structure and a very large internal surface area.
These characteristics provide activated carbon with a very strong adsorptive property.
a – micropores
b , f – mesopores (spherical and cilindrical)
c , e , d, – macropores (passing, internal, superficial)
Activated carbon is mainly produced from
– coconut shell
The yield of activated carbon is in the range between 15-35 % depending upon the raw material
Generally coal gives high yield (30-35%) while coconut shell gives a lower yield (15-20%)
The approximate carbon content for different raw materials is shown in table 1
|Tab. 1 – Approximate carbon content of raw materials|
|Material||Carbon content (%)|
Activated carbon is produced in two different stage carbonization of raw material activation of the carbonized product at high temperature by means of steam process or chemical process.
Is the first stage during which, in absence of air and at medium temperature (roughly 600°C) the raw material is partially carbonized so that dehydration and devolatilisation occur; in this stage an intermediate product is obtained which has small pore dimension;
The volatile content is reduced below the 15%
Is the stage during which the carbonized product is activated at a temperature of 900-1100°C and carbon is removed from the pore so that very high porous material is obtained.
Is generally used to produce both coconut and coal based activated carbon at temperature of 800-1000°C.
A subsequent step of the steam process could be the acid washing of activated carbon in order to reduce the ash content of the final product.
Is generally used to produce activated carbon from wood or peat.
It consists in mixing an activating agent (phosphoric acid, zinc chloride) to open up the structure of the cellulose.
This type of activation allows to obtain good pore distribution without any further treatment.
GENERAL SCHEME OF ACTIVATION
BLOCK DIAGRAM SINGLE STAGE
G R A N U L A R
E X T R U D E D ( PELLET )
P O W D E R
The adsorption capacity of activated carbon is determined mostly by two factors
INTERNAL SURFACE AREA
Its determined by mean of N2 adsorption and represent the surface area of the activated carbon and it is expressed in m2/g
Internal surface area varies in the range 500-1800
PORE SIZE DISTRIBUTION
Micropore: 0-20 A°
Macropore: > 50 A°
Micropores and mesopores are supposed the be the most effective for the adsorption phenomenon and represent 80-90 % of the pore distribution.
Distribution of pores depends upon the starting material
Coconut shell a.c. have a predominance of micropore (roughly 90%)
Coal based a.c. have a wide pore distribution (micropore/mesopore distribution is 60/30%)
Wood – peat based a.c. have high proportion of macropore (40-50%)
Test Method AWWA B 600-78
Is the capacity of adsorption towards the Iodine; since iodine is a small molecule this parameters gives indication about the micropore percentage and the total surface area.
Is the quantity of activated carbon used to reduce the concentration of a solution of phenol from 200 mg/l down to 20 mg/l; It gives indication of mesopore and capacity of adsorption of medium molecule
Is the quantity of M.B. adsorbed by 100 g activated carbon;
It gives indication about macropore quantity of the activated carbon and the decolourization capacity.
This parameter may give indication of the activation of activated carbon of same raw material; it’s important mainly for designing the plants and the filters.
Represents the quantity of inert material of an a. c.
Particle size distribution
Test method ASTM D 2862
Is generally expressed in US mesh and represent the particle size of an activated Carbon
Correspondence between U.S. mesh (A.S.T.M.) ad Millimeters
|U.S. mesh (A.S.T.M.)||Milimeters|
Type of activated carbon (raw material, pore size distribution, mesh size)
Type of contaminant (molecular weight, chemical structure, presence of other compounds)
Concentration of contaminant
Temperature of fluid phase
Humidity of fluid phase (gas stream)
“Each type of activated carbon as a capacity of adsorption towards each contaminants”
This capacity is determined or expressed by a “specific adsorption isotherm” and represent the capacity of an activated carbon to remove a contaminant from a stream (liquid or gas)
This capacity is called “loading of activated carbon” and it is expressed as X/M where X is the weight of contaminants adsorbed (g)
M is the weight of activated carbon (g) The definition is “quantity of contaminant adsorbed per unit of weight of activated carbon”
DIAGRAM OF AN ADSORPTION TEST