The Size and Size Distribution of the Solid Particles

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Chapter: Pharmaceutical Engineering: Solid-Liquid Extraction

The particle size of the solids determines the distance that solvent and solute must diffuse within the solid matrix.


THE SIZE AND SIZE DISTRIBUTION OF THE SOLID PARTICLES

The particle size of the solids determines the distance that solvent and solute must diffuse within the solid matrix. Since this offers the major diffusional resistance, reduction of the distance by comminution greatly increases the rate of leaching, the concentration gradient being effectively increased. In addition, the inverse relationship between particle size and surface area prescribes an increase in the area of contact between the matrix and the surrounding liquid. Transfer of solute at this boundary is therefore facilitated. In leaching by immersion, a further advantage conferred by size reduction is the ease with which finer particles are suspended. Finally, extensive cell rupture occurs during grinding, allowing more direct contact between solvent and solute and more rapid dissolution and diffusion.

Other factors, however, operate against size reduction. Leaching by per-colation demands the formation of a permeable bed. Low permeability will give low flow rates and low rates of extraction. Permeability is a complex function of both particle size and porosity, the former determining how a given void space is to be disposed within the bed. The disposition of the void space will consist of few channels of relatively large diameter, that is, a bed of high permeability, if the particle size is large. In leaching by immersion, the difficulties of separating solid and liquid increase as the particle size decreases.

The opposition of the factors suggests an optimum particle size for any particular extraction. This is determined to some extent by the physical nature of the solids. A dense, woody structure would be extracted as a fine powder. An example is given by the root of Ipecacuanha. A leafy structure, on the other hand, would be more satisfactorily leached as a coarse powder.

Both porosity and permeability are influenced by the particle size distri-bution. A high porosity is secured if the distribution is narrow. Small particles may otherwise fill the interstices created by the contact of larger particles. After grinding, therefore, it is often necessary to classify the product and remove undersize material. The undersize would then be bulked with the fines from other batches and separately extracted. A further advantage arising from a narrow size distribution is even packing and the creation of a regular system of pores and waists. This promotes even movement of solvent and solution through the bed.

In some cases, size reduction may take a particular form. Seeds and beans are often rolled or flaked to produce extensive cell rupture. In other processes, the cell wall, although depressing the rate of extraction, may make the extraction more selective by preventing the movement of unwanted materials of high molecular weight. Here the size reduction must leave most cells intact.

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