Introduction

Adsorption on porous carbons was described as early as 1550 B.C. in an ancient Egyptian papyrus and later by Hippocrates and Pliny the Elder, mainly for medicinal purposes. In the 18th century, carbons made from blood, wood and animals were used for the purification of liquids. All of these materials, which can be considered as precursors of activated carbons, were only available as powders. The typical technology of application was the so-called batch contact treatment, where a measured quantity of carbon and the liquid to be treated were mixed and, after a certain contact time, separated by filtration or sedimentation.

At the beginning of the 19th century the decolourisation power of bone char was detected and used in the sugar industry in England. Bone char was available as a granular material which allowed the use of percolation technology, where the liquid to be treated was continuously passed through a column. Bone char, however, consists mainly of calcium phosphate and a small percentage of carbon; this material, therefore was only used for sugar purification.

At the beginning of this century the first processes were developed to produce activated carbons with defined properties on an industrial scale. However, the steam activation (V. Ostreijko, 1900 and 1901) and chemical activation (Bayer, 1915) processes could only produce powder activated carbon. During the First World War, steam activation of coconut char was developed in the United States for use in gas masks. This activated carbon type contains mainly fine adsorption pore structures suited for gas phase applications.

Today many users are switching from the traditional use of powdered activated carbon as a disposable chemical to continuous adsorption processes using granular activated carbon combined with reactivation. By this change they are following the modern tendency towards recycling and waste minimization, thereby reducing the use of the world's resources.

Activated carbon is the trade name for a carbonaceous adsorbent which is defined as follows: activated carbons are non-hazardous, processed, carbonaceous products, having a porous structure and a large internal surface area. 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. The volume of pores of the activated carbons is generally greater than 0.2 mlg^-1. The internal surface area is generally greater than 400 m²g^-1. The width of the pores ranges from 0.3 to several thousand nm.

All activated carbons are characterized by their ramified pore system within which various mesopores (r = 1 - 25 nm), micropores (r = 0.4 - 1.0 nm) and sub micropores (r < 0.4 nm) branch off from what we call macropores (r > 25 nm). Activated carbons have been used for many years quite successfully for adsorptive removal of impurities from exhaust gas and waste water streams. However, for cost-effective removal of certain impurities contained in gases (such as hydrogen sulphide, 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.

General properties

Activated carbon is a crude form of graphite, the substance used for pencil leads. It differs from graphite by having a random imperfect structure which is highly porous over a broad range of pore sizes from visible cracks and crevices to molecular dimensions. The graphite structure gives the carbon it's very large surface area which allows the carbon to adsorb a wide range of compounds. Activated carbon has the strongest physical adsorption forces or the highest volume of adsorbing porosity of any material known to mankind. Activated carbon can have a surface of greater than 1000m²/g. This means 5 grams of activated carbon can have the surface area of a football field.

Adsorption is the process by which liquid or gaseous molecules are concentrated on a solid surface, in this case activated carbon. This is different from absorption, where molecules are taken up by a liquid or gas.

Activated carbon can made from many substances containing a high carbon content such as coal, wood and coconut shells. The raw material has a very large influence on the characteristics and performance activated carbon.

Properties

Activated carbon is the generic term used to describe a family of carbonaceous adsorbents with a highly crystalline form and extensively developed internal pore structure. A wide variety of activated carbon products is available exhibiting markedly different characteristics depending upon the raw material and activation technique used in their production. In selecting an activated carbon, it is important to have a clear understanding of both the adsorptive and physical characteristics of the material in order to optimise the performance capabilities.

Adsorptive characteristics surface area (BET N₂) - measurement, using nitrogen (N₂), of the extent of the pore surface developed within the matrix of the activated carbon. Used as a primary indicator of the activity level, based on the principle that the greater the surface area, the higher the number of adsorptive sites available.

Pore size distribution - determination of the pore size distribution of an activated carbon is an extremely useful way of understanding the performance characteristics of the material. The macropores are used as the entrance to the activated carbon, the mesopores for transportation and the micropores for adsorption. The International Union of Pure and Applied Chemistry (IUPAC) defines the pore size distribution as:

• Micropores r < 1 nm
• Mesopores 1 < r < 25 nm
• Macropores r > 25 nm

Physical characteristics

Hardness - an important factor in system design, filter life and product handling. There are large differences in the hardness of activated carbons, depending on the raw material and activity level.
Bulk Density - should be carefully considered when filling fixed volumes as it can have considerable commercial implications. The backwashed and drained density will show a lower value due to the water film between the particles of activated carbon.
Iodine number - measurement of the porosity of an activated carbon by adsorption of iodine from solution.
Carbon tetrachloride activity - measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapour.
Particle size distribution - the finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapour phase systems this needs to be considered against pressure drop, which will affect energy cost. Careful consideration of particle size distribution can provide significant operating benefits.

Forms of activated carbon

Granular Activated Carbon (GAC) - irregular shaped particles with sizes ranging from 0.2 to 5 mm. This type is used in both liquid and gas phase applications.

Powder Activated Carbon (PAC) - pulverized carbon with a size predominantly less than 0.18 mm (US Mesh 80). These are mainly used in liquid phase applications and for flue gas treatment.

Pelleted Activated Carbon - extruded and cylindrical shaped with diameters from 0.8 to 5 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content. Activated carbon is also available in special forms such as a cloth and fibres.

Chemisorption

The technique of chemical impregnation is used to increase significantly the performance efficiency and adsorptive capacity of activated carbon for many gases which are difficult to adsorb onto base products. We manufacture a wide range of chemically impregnated activated carbons, using a variety of production methods that have been developed during many years of operation. The technique of impregnation is critical to ensure a homogeneous distribution of the chemicals on the surface of the activated carbon, thus maintaining maximum access to the pore structure to provide the target contaminants with the maximum contact efficiency to the impregnant.

Zeolite

Natural zeolite is mined from the earth. It is a cream-colored mineral. Zeolite has exceptional abilities for capturing small molecules such as ammonia, formaldehyde and sulphides. The ability of activated zeolites to adsorb many gases on a selective basis is in part determined by the size of the channels ranging from 2.5 to 4.3 Å in diameter (according to zeolite type). Specific channel size enables zeolites to act as molecular gas sieves and selectively adsorb such gases as ammonia, hydrogen sulphide, carbon monoxide, carbon dioxide, sulphur dioxide, water vapour, oxygen, nitrogen, formaldehyde and others. The ability of zeolites to adsorb certain gases readily lends them to odour control applications.

Public toilets, horse stables, chicken houses and feed lots all release offensive ammonia fumes causing distress to the animals as well as human beings. These problems can be arrested by the application of zeolite products. Homeowners with pets often have pet litter trays which cause to unpleasant and offensive odours. Zeolite placed in the litter tray with the normal litter can eliminate these odours by adsorbing ammonia and volatile organic compounds. The potential use of zeolites in many industries for select removal of gases from composite gaseous mixtures is seemingly endless.