Particle degradation can cause problems in a number of areas on account of the changes in particle shape and particle size distribution that can result. It is a particular problem with chemical materials that are coated, for it is the coating that is generally the friable element of the resulting material. Plant operating difficulties are often experienced because of the fines produced, and problems in handling operations can also result after the material has been conveyed.
Apart from the obvious problems of quality control with friable materials, changes in particle shape can also lead to subsequent process difficulties with certain materials. The appearance of the material may also change so that it is not so readily sold. Changes in particle size distribution can affect flow characteristics, which in the extreme can change a free-flowing material into one which will only handle with great difficulty, and with materials for subsequent sale this can lead to customer problems.
In pneumatic conveying systems plant operating difficulties can result if degradation causes a large percentage of fines to be produced, particularly if the filtration equipment is not capable of handling the fines satisfactorily. Filter cloths and screens will rapidly block if they have to cope with unexpectedly high flow rates of fine powder. The net result is that there is usually an increase in pressure drop across the filter, and this could be a significant proportion of the total pressure available in a low pressure system.
This means that the pressure drop available for conveying the material will be reduced, which in turn means that the mass flow rate of the material will probably have to be reduced in order to compensate. If this is not done there will be the risk of blocking the pipeline. Alternatively, if the filtration plant is correctly specified, with material degradation taken into account, it is likely to cost very much more as a result. This, therefore, provides a direct financial incentive to ensure that particle degradation is minimized, even if it does not represent a problem with respect to the material itself .
In many systems there is a need to store the conveyed material in a hopper or silo. Flow functions can be determined for bulk particulate materials, from which hopper wall angles and opening sizes can be evaluated, to ensure that the material flows reliably at the rate required. A change in particle size distribution of a material, as a result of conveying operations, however, can result in a significant change in flow properties. Thus a hopper designed for a material in the ‘as received’ condition may be totally unsuitable for the material after it has been conveyed. As a result it may be necessary to fit an expensive flow aid to the hopper to recover the situation.
Potential explosion problems
Many materials, in a dust cloud, can ignite and cause an explosion. Dust clouds are clearly quite impossible to avoid somewhere in a pneumatic conveying system, and so this poses a problem with regard to the safe operation of such systems. Of those materials that are potentially explosive, research has shown that it is only the fraction with a particle size less than about 200 µm that poses the problem. Degradation and attrition caused by pneumatic conveying, however, can result in the generation of a considerable number of fines, particularly if the material is friable. Even if the material did not represent a problem with respect to explosions in the ‘as received’ condition, the situation could be very different after the material has been conveyed.