Secondary drying surface area

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

Current legislation in most developed countries classifies as hazardous most fine and all ultrafine particulate solids separated and collected from fluids with environmental control devices of any kind. In many cases, the mere fact that solids are micron or submicron sized, constitutes a reason for this classification. Owing to their fineness, dusts and slurries or moist residues from pollution abatement that are or become dry particulate matter easily become airborne causing a renewed threat to the environment and to human and animal or plant life (secondary pollution). Also, the large surface area of fine particulate solids results in high solubility so that toxic or otherwise undesirable substances may leach out and spill into surface water or end-up in aquifers if such fines are stored outside or deposited in unprotected landfills. 

Pikal et al studied a 5%-povidone (polyvinyl pyrrolidone, PVP) solution, dried from an 8-ml fill volume, of 2-cm depth, and from a 4-ml fill volume at a 1-cm fill depth. In this instance, the fill depth had little effect on the specific surface area of the dried product (2.5 m g compared to 2.3 m g )- It was found that the rates of secondary drying (as measured by the mean ratio of the (1 - F) values), the normalised water contents of the two products were essentially identical. A mean ratio of (1 - F) of 1.19 0.17 was found, indicating that cake thickness did not significantly affect secondary drying kinetics. It can thus be concluded that material at the top of the cake dries at a similar rate to material at the bottom, provided overheating is avoided. Similar experiments were carried out on an amorphous, formulated moxalactam product, as shown below. 

Although the sample with the lowest specific surface area has double the fill depth, the correlation between drying rate and specific surface area is good. It also demonstrates that cake depth does not significantly affect secondary drying kinetics. 

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