Activated carbon is a very
useful and
versatile compound used for cleaning water and gas streams of
harmful or
unwanted compounds. Activated carbon is used in everything from household
water filters to
industrial waste cleaning.
What is Activated Carbon ? (AC)
Activated carbon (AC) is a
crude form of
graphite, (also used in lead pencils). The carbon has a random imperfect structure that is highly
porous. The graphite structure gives the carbon a very large
surface area, which allows the
carbon to
adsorb a wide range of compounds.
The process of adsorption is made possible due to carbon’s surface chemistry. The atoms forming the surface area are not fully bonded (usually 4 bonds present with surrounding carbon atoms), as such they have non-bonded electrons (also the reason why graphite is conductive), which can readily bond to other reactive compounds.
AC can be made from several carbon sources, mainly from non-renewable sources such as wood, coal and bitumen, however other sources such as coconut shells are often used. Each carbon source leads to differing final properties of the end product and as such the source is not always interchangeable. The main use of activated carbon is in aqueous or liquid systems such as water treatment plants and home aquarium filtering. Activated carbon will remove dissolved organic compounds from water. As a result AC will remove colour, odour, and contaminants from water.
What types of Activated Carbon are there ?
There are several sizes of activated carbon, each manufactured to suit a specific use. The two main ways of using AC is in liquid (remove dissolved organic contaminants) or gaseous
phase (remove
volatile organic compounds from the air).
- Firstly, Granular Activated Carbon (GAC), this form can be used in both liquid with irregular shaped particles, sizes range from 0.2 to 5 mm.
- Secondly, Powder Activated Carbon (PAC), this form is usually used in liquid phases or in gas flues; PAC is made up of pulverized carbon with an average size less than 0.18mm.
- Thirdly activated carbon pellets, these are generally used for gas phases, made up of extruded and cylinders with diameters from 0.8 to 5 mm. This type is used for gas phases due to their ability to be used in varying pressures, with high mechanical strength and low dust content. Activated carbon is also available in specialised forms such as a cloth and fibres.
Applications and Common Uses
Groundwater treatment; AC can be used as a single treatment to remove compounds such as
chlorinated hydrocarbons and
aromatic compounds including
benzene,
toluene,
ethylbenzene and
xylene (BTEX).
Volatile Organic Compounds (VOCs) can be removed from the air. Activated carbon is used for VOC abatement in the following applications:
- Groundwater remediation
- Soil vapour extraction
- Chemical and Pharmaceutical Industries
- Coating and Printing Industries
- Degreasing and cleaning
- Solvent Recovery
- Tank venting
- Ventilation and air conditioning systems .
Cleaning of industrial wastewater
"Landfill Leachate (off drained water) Treatment. Granular activated carbon in combination with biological pretreatment is the leading technology for the treatment of landfill leachate for the removal of Chemical Oxygen Demand (COD), Adsorbable Organic Halogens (AOX) and other toxic substances. Granular activated carbon is now used at over 50 sites in Europe for this application".
Pharmaceutical and chemical purification. “Activated carbon is suitable for the decolourisation and purification of a wide range of organic and inorganic compounds including:
- Amines
- Hydrochloric and other mineral acids
- Amino acids
- Glycols
- Hydrocarbons
- Pharmaceuticals
- Activated carbon is used to purify a wide range of pharmaceuticals and intermediates”.
What happens when the carbon is saturated?
The three main options after use are disposal in
landfill, or if used in toxic situations can be rendered safe by
incineration. The third option is
regeneration; this involves removing the adsorbed compounds from the carbon and re-activating the carbon to allow re-use.
Reactivation can be done using a special
furnace operating at over 800°C with high temperature steam. The same process can also be carried out
chemically or by high pressure.
"At Feluy, approximately 50 km south of Brussels in Belgium, Chemviron Carbon operates the World's largest Reactivation Centre. A Reactivation Centre is also located in Grays, Essex serving Customers in the United Kingdom".
Bibliography
http://www.arcesystems.com/products/carbon/carbequip.htm
http://www.chemvironcarbon.com/activity/index.htm
http://www.ext.nodak.edu/extpubs/h2oqual/watsys/ae1029w.htm