Some Experiments on Packed-Bed Drying

Abstract

Drying equipment is described in which streams of air of accurately-controlled temperature and humidity may be passed over the surface of a packed bed. The materials used in the beds are sieved spherical particles of copper, glass and polystyrene whilst the liquids are water, methanol and a series of organic solvents. Experiments are described in which the effect of physical variables on drying rate is measured. Surface tension is shown to have a major effect on the shape of drying rate curves; under the experimental conditions water is the only liquid for which the rate of drying during the constant-rate period is equal to that from a free liquid surface. The equivalent period for liquids of lower surface tension corresponds to a distinctly lower drying rate. Possible reasons for this behaviour are discussed. An increase in liquid viscosity causes reduction in drying rate, but only when the rate of drying is itself high. The effect of using solids of differing thermal conductivities is investigated. At high drying rates, increasing thermal conductivity produces increasing drying rate. Surprisingly a mixed bed (e.g. of copper and glass) often dries faster than beds of the constituent materials alone. Split beds are also used (beds of which the halves are of different materials). For these beds it is possible to observe the growth of dry patches on the surface of the beds and the manner in which liquid appears to migrate across the interface between the particles of the two halves. The temperature distribution across the interface is shown to be abnormal in part of the drying cycle. A conclusion which may be drawn from the work is that it is often an advantage, from the drying standpoint, to have particles that have either a wide range of thermal conductivities or a wide range of particle sizes.