How to Decide Activated Carbon Quality ?

In the environmental management field, using activated granular carbon to remove dissolved organic compounds and volatile organic emissions is a widely accepted procedure. However, there are significant differences in the abilities of various carbon products to remove these unwanted substances.

beadactivatedcarbonThe ability of activated carbon to remove contaminants is determined not by its weight or volume, but its Adsorption capacity, i.e., the amount of impurity removed by a given amount of activated carbon.

The higher this capacity, the more contaminants removed per, let’s say, cubic foot, the less carbon needed to perform a particular job. In the manufacture of activated carbons, a wide variety of raw materials and widely varying quality specifications are used.

While the raw material itself determines many of a carbons physical properties, its adsorption capacity is dependent on a precise and carefully controlled steam activation process

Quality is also an issue with reactivated carbons. The ability to decontaminate and reactive spent carbon to near virgin capacity is dependent not only on proper operation of the reactivation furnace but its close and careful monitoring (e.g., thermal degradation and the build up of inorganic ash constituents are common problems with carbons that are repetitively recycled).

The carbon manufacturing industry and ASTM have developed two critical tests that not only measure the quality of both virgin and reactivated carbon products but also predict its cost effectiveness:

The IODINE A D S O R P T I O N T E S T ( A S T M D4607) for measuring the LIQUID PHASE of activated carbons produce IODINE A D S O R P T I O N NUMBERS from 800 to 1250 mg/gr. (the higher the number the greater the capacity).

The CARBON T E T R A C H L O R I D E A D S O R P T I O N T E S T ( A S T M D3467) for the VA P O R P H A S E o f activated carbons produces CARBON T E T R A C H L O R I D E A D S O R PTION NUMBERS ranging from 45 to 70 percent by weight

When buying either virgin or reactivated carbon products, make sure that these adsorption numbers are specified. Then compare these to CARBTROL’s to insure the best activated and reactivated carbon value for your money.

CARBTROL’S NUMBERS:

Liquid Phase Virgin Carbon – CSL – Iodine Number

– 1100 mg/gr. (average) !

Vapor Phase Virgin Carbon – CSV – Carbon Tetrachloride Number

– 60-65% (average) !

Typical Properties of Granular Activated Carbon

Typical Properties of Granular Activated Carbon

Bituminous Sub-bituminous Lignite Nut Shell
Iodine Number 1,000-1,100 800-900 600 1,000
Molasses Number 235 230 300 0
Abrasion Number 80-90 75 60 97
Bulk Density as packed LB/CF 26-28 25-26 23 29-30
Volume Activity 26,000 25,000 13,800 0

Activated Carbon Iodine and molasses numbers measure pore size distribution.  Iodine number is a relative measure of pores at sizes of 10 to 2 Angstroms. It is reported in milligrams of elemental iodine adsorbed per gram of GAC and determines the area available on the GAC to adsorb low molecular weight organics.

Molasses number measures the degree a GAC removes color from a stock solution. It measures pores greater than 28 Angstroms. These are the pores responsible for removing larger molecular weight organics such as tannins.

Abrasion numbers represent the relative degree of particle size reduction after tumbling with a harder material. No reduction is rated 100, complete pulverization is zero.

Factors that affect the performance of Activated Carbons

Factors that affect the performance of Activated Carbons

Molecular weight:
As the molecular weight increases, the activated carbon adsorbs more effectively because the molecules are lea soluble in water. However, the pore structure of the carbon must be large enough to allow the molecules to migrate within. A mixture of high and low molecular weight molecules should be designed for the removal of the more difficult species.

pH:
Most organics are less soluble and more readily adsorbed at a lower pH. As the pH increases, removal decreases. A rule of thumb is to increase the size of the carbon bed by twenty percent for every pH unit above neutral (7.0).

Contaminant concentration:  The higher the contaminant concentration, the greater the removal capacity of activated carbon. The contaminant molecule is more likely to diffuse into a pore and become adsorbed. As concentrations increase, however, so do effluent leakages. The upper limit for contaminants is a few hundred parts per million. Higher contaminant concentration may require more contact time with the activated carbon. Also, the removal of organics is enhanced by the presence of hardness in the water, so whenever possible, place activated carbon units upstream of the ion removal units. This is usually the case anyway since activated carbon is often used upstream of ion exchange or membranes to remove chlorine.

Particle size:
Activated carbon is commonly available in 8 by 30 mesh (largest), 12 by 40 mesh (most common), and 20 by 50 mesh (finest). The finer mesh gives the best contact and better removal, but at the expense of higher pressure drop. A rule of thumb here is that the 8 by 30 mesh gives two to three times better removal than the 12 by 40, and 10 to 20 times better kinetic removal than the 8 by 30 mesh.

Flow rate:
Generally, the lower the flow rate, the more time the contaminant will have to diffuse into a pore and be adsorbed. Adsorption by activated carbon is almost always improved by a longer contact time. Again, in general terms, a carbon bed of 20 by 50 mesh can be run at twice the flow rate of a bed of 12 by 40 mesh, and a carbon bed of 12 by 40 mesh can be run at twice the flow rate of a bed of 8 by 30 mesh.  Whenever considering higher flow rates with finer mesh carbons, watch for an increased pressure drop!


Temperature:
Higher water temperatures decrease the solution viscosity and can increase die diffusion rate, thereby increasing adsorption. Higher temperatures can also disrupt the adsorptive bond and slightly decrease adsorption. It depends on the organic compound being removed, but generally, lower temperatures seem to favor adsorption.

Healing properties of activated charcoal

To understand how activated charcoal works, it is important to know the difference between “adsorb” and “absorb”. Sponges absorb liquids, and they can be squeezed out. Charcoal adsorbs liquids, and binds to toxic chemicals so that they cannot escape.

Healing properties of Activated Carbons:

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  • Detox: By binding with organic chemicals from pesticides, plastics, and other pollutants.
  • Detox: By binding with viruses and bacteria.
  • Gentle on the colon and does not damage the mucus lining of the intestines.
  • De- bloats by binding to gases.
  • Facilitates digestion.
  • May help lower cholesterol, triglycerides and lipids found in the blood.
  • Helps relieve constipation.
  • Helps with acne.
  • Kills parasites like Candida.
  • Removes the toxins in the human body. (The toxins are eliminated through feces).
  • Helps with food poisoning

Two Types of Absorption of Activated Carbons

There are two types of absorption of activated carbons:

Physical Adsorption – During this process, the adsorbates are held on the surface of the pore walls by weak forces of attraction known as Van Der Waals Forces or London dispersion forces.

Chemisorption – This involves relatively strong forces of attraction, actual chemical bonds between adsorbates and chemical complexes on the pore wall of the activated carbon.

Key Properties of Activated Carbon

Activated CarbonSurface Area – Generally, higher the internal surface area, higher the effectiveness of the carbon. The surface area of activated carbon is impressive, 500 to 1500 m2/g or even more; a spoonful of activated carbon easily equates the surface area of a soccer field.

It is in the activation process that this vast surface area is created. The most common process is steam activation; at around 1000°C steam molecules selectively burn holes into the carbonized raw material, thus creating a multitude of pores inside the carbonaceous matrix. In chemical activation, phosphoric acid is used to build up such a porous system at a lower temperature.

Total Pore Volume – Refers to all pore spaces inside a particle of activated carbon. It is expressed in milliliters per gram (ml/g), volume in relation to weight. In general, the higher the pore volume, the higher the effectiveness. However, if the sizes of the molecules to be adsorbed are not a good match to the pore size, some of the pore volume will not be utilized. Total pore volume (T.P.V.) differs by raw material source and type of activation method.

Pore Radius – The mean (average) pore radius, often measured in angstroms, differs by activated carbon type.

Pore Volume Distribution – Each type of carbon has its own unique distribution of pore sizes. They’re referred to as micropores (small), mesopores (medium) and macropores (large). Carbons for adsorbing many types of gas molecules are microporous. The best carbons for decolorization have a higher distribution of mesopores.

  • Micropores r < 1nm
  • Mesopores r 1-25nm
  • Macropores r > 25nm
    nm = nanometer  

Activated Carbon: Mesh Size and Ash Content

Mesh Size

The physical size, or mesh size, of a carbon must be considered in relation to the flow rate in the system it is to be used. Naturally, the smaller the carbon’s mesh size, the greater its resistance to flow. Thus, it is usual to select the smallest mesh size carbon that will satisfy the pressure drop limitations of the system.

Mesh Size
Mesh Size

 

Ash Content

Ash content is less important except where the carbon is used as a catalyst support since certain constituents of the ash may interfere or destroy the action of precious metal catalysts. Ash content also influences the ignition point of the carbon—this may be a major consideration where adsorption of certain solvents is concerned.

Density

The density of carbon is, of course, of great importance to many users in estimating the weight required to fill a vessel.

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