Activated carbon is pure carbon (carbon content > 90%), either in form of microcrystalline or amorphous graphite. The material is highly porous with a specific surface area of about 500 g/m2 – 1,500 g/m2. This means that 3g of activated carbon can have the surface area of a football field. The material is able to adsorb a large number of (mainly organic) molecules. It is therefore used as a filter material for many applications, for example in water purifiers, in tobacco smoke filters or in the automobile industry to prevent the evaporation of gasoline fumes.
The size distribution and the shape of the carbon particles determines the efficiency and throughput of the filter. The amount of fine particles (< 1 mm) is an especially critical parameter as these might block the filter. Sieve analysis is the most common method used for particle sizing of activated carbon, however, it does have some disadvantages. Firstly, sieving raises dust through abrasion so that the measurement process modifies the original size distribution of the sample. Secondly, sieving does not allow for shape analysis. If, however, products like carbon rods are to be analysed, it is essential to determine the length and diameter of the rods
precisely and with a high statistical accuracy.
The CAMSIZER digital image processor uses two high resolution CCD cameras for simultaneous particle size and shape analysis. It is suitable for all dry, pourable bulk goods. All particles fall freely through the measurement field where they are irradiated by a stroboscope light source from one side. The system then evaluates the particles’ projections. Due to the short exposure time of only a few microseconds, the digital images are extremely sharp. Features such as the great accuracy of the analysis, the wide measuring range from 30 µm to 30 mm, the high sample throughput and the fact that hardly any cleaning is necessary make this instrument ideal for the characterisation of activated carbon.
In contrast to sieving, a measurement with the digital image processor only takes a few minutes. Due to the high throughput, the analysis of larger sample volumes, for example the complete activated carbon filling of a filter, can be carried out without any problem. Analysing a large volume makes the single measurement more representative
and saves the additional working step of sample division.
Classic sieve analysis is usually carried out with a maximum of eight sieves, i.e. the obtained size distribution curve is limited to a few data points. The digital image processor obtains and stores the results for at least 1,000 size classes, which corresponds to a sieve stack with a height of 50 m. During sieving, the activated carbon particles rub against each other, which leads to an increase of the fine particles and a high dust exposure in the laboratory.
The digital image processor measures the length and width of the particles independently thus providing important information about particle shape, which is especially interesting for extruded carbon rods. If sieving is to be replaced by digital image analysis, it is important that the results correspond to results obtained with sieving, so that existing product specifications can be easily adopted.
As the digital image processor measures particles in free fall, projections can lead to deviations from the sieve analysis results. The type of these deviations and their extent are determined by particle shape. The digital image processor software allows for the creation of material-specific fitting algorithms, which compensate for these differences entirely to allow 100% compatibility to sieve analysis. These fitting functions can be generated by the user in three
simple steps.
