Activated Carbon from CPL Carbon Link:Activated Carbon Properties

1.3 Properties of Activated Carbon

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 International Union of Pure and Applied Chemistry (IUPAC) defines the pore size distribution as:

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

The macropores are used as the entrance to the activated carbon, the mesopores for transportation and the micropores for adsorption.

View of pore structure - scanning
electron microscope

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.

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.

Comparative pore
size distribution 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.

The important properties of activated carbon relevant to specific applications are considered further in the text.