What is Activated Carbon

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DEFINITION

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.

 

PORE STRUCTURE OF ACTIVATED CARBON

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a   –  micropores

b , f  –  mesopores (spherical and cilindrical)

c , e , d,  – macropores (passing, internal, superficial)

 

RAW MATERIAL

Activated carbon is mainly produced from

           –  coal

            – coconut shell

            – wood

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 (%)
Coal 65-70%
Coconut shell 40-45%
Wood 35-40%

 

 PRODUCTION OF ACTIVATED CARBON

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.

CARBONIZATION

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%

 

ACTIVATION

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.

 

STEAM PROCESS

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.

 

CHEMICAL ACTIVATION

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 

(SINGLE KILN)

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BLOCK DIAGRAM SINGLE STAGE 

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CLASSIFICATION OF ACTIVATED CARBON

G R A N U L A R

  • MAINLY USED FOR WATER TREATMENT (POTABLE, URBAN AND INDUSTRIAL WASTE WATER)
  • DIFFERENT MESH SIZE
  • DIFFERENT STARTING MATERIAL ( COCONUT, MINERAL, VEGETAL )

 

E X T R U D E D ( PELLET )

  • MAINLY USED FOR AIR TREATMENT
  • DIFFERENT DIAMETER 2 – 3 – 4 mm (most common)
  • GENERALLY MINERAL BASED

P O W D E R

  • USED FOR
  • WATER TREATMENT (DRINKING AND WASTE)
  • INDUSTRIAL PROCESS (WINE INDUSTRY, PHARMACEUTICAL, FOOD INDUSTRY)
  • AIR TREATMENT (INCINERATORS)
  • SIZE: GENERALLY UNDER 325 mesh
  • DIFFERENT RAW MATERIAL (COAL, COCONUT, VEGETAL)

 

CHARACTERIZATION OF  ADSORPTION CAPACITY

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°

Mesopore:        20-50A°

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%)

 

MAIN PARAMETERS THAT CHARACTERIZE ACTIVATED CARBON

  • Iodine number (mg/g)
  • Surface area B.E.T. (m2/g)
  • Methylene blue adsorption (g/100g)
  • Apparent density (kg/dm3)
  • Moisture (%)
  • Ash content (%)
  • Hardness (%)
  • Phenol adsorption (g/l)
  • Particle size distribution (% – mesh)

 

  • Iodine number (mg/g)

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.

  • Phenol number (g/l) Test Method AWWA B 600-78

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

  • Methylene blue adsorption (g/100g) Test Method ASTM Spectrofotometric

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.

  • Bulk Density (g/l) Test Method ASTM D 2854)

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.

  • Ash content (%) Test Method ASTM D 2866

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
4 4.76
6 3.36
8 2.38
12 1.68
16 1.19
20 0.84
40 0.42
200 0.074
325 0.044

PARAMETERS AFFECTING THE ADSORPTION PROCESS ON ACTIVATED CARBON

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)

 

ADSORPTION ISOTHERM

“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

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