The drying of any product (including biological products) is often the last stage of a manufacturing process (McCabe et al., 1984; Coulson and Richardson, 1991). It involves the final removal of water from a heat-sensitive material ensuring that there is minimum loss in viability, activity or nutritional value. Drying is undertaken because:
(a) The cost of transport can be reduced.
(b) The material is easier to handle and package.
(c) The material can be stored more conveniently in the dry state.
A detailed review of the theory and practice of drying can be found in Perry and Green (1984). It is important that as much water as possible is removed initially by centrifugation or in a filter press to minimize heating costs in the drying process. Driers can be classified by the method of heat transfer to the product and the degree of agitation of the product. In contact driers the product is contacted with a heated surface. An example of this type is the drum drier (Fig. 10.31), which may be used for more temperature stable bio-products. A slurry is run onto a slowly rotating steam heated drum, evaporation takes place and the dry product is removed by a scraper blade in a similar manner as for rotary vacuum filtration. The solid is in contact with the heating surface for 6-15 seconds and heat transfer coefficients are generally between 1 and 2 kW m~2 KT1. Vacuum drum driers can be used to lower the temperature of drying.
A spray drier (Fig. 10.32) is most widely used for drying of biological materials when the starting material is in the form of a liquid or paste. The material to be dried does not come into contact with the heating surfaces, instead, it is atomized into small droplets through for example a nozzle or by contact with a rotating disc. The wide range of atomizers available is described in Coulson and Richardson (1991). The droplets then fall into a spiral stream of hot gas at 150° to 250°. The high surface area:volume ratio of the droplets results in a rapid rate of evaporation and complete drying in a few seconds, with drying rate and product size being directly related to droplet size produced by the atomizer. The evaporative cooling effect prevents the material from becoming overheated and damaged. The gas-flow rate must be carefully regulated
so that the gas has the capacity to contain the required moisture content at the cool-air exhaust temperatu: (75° to 100°). In most processes the recovery of very small particles from the exit gas must be conducted using cyclones or filters. This is especially important for containment of biologically active compounds. The jet spray drier is particularly suited to handling heat sen»i-tive materials. Operating at a temperature of around 350°, residence times are approximately 0.01 second', because of the very fine droplets produced in the atomizing nozzle.
Spray driers are the most economical available for handling large volumes, and it is only at feed rates below 6 kg min 1 that drum driers become more economic.
Freeze drying is an important operation in the production of many biologicals and pharmaceuticals. The material is first frozen and then dried by sublimation in a high vacuum. The great benefit of this technique is that it does not harm heat sensitive materials. The process is often termed lyophilization when the solvent being evaporated is water.
Fluidized bed driers are used increasingly in the
rmaceutical industry. Heated air is fed into a champs f fluidized solids, to which wet material is continuously added and dry material continuously removed. Very high mass-transfer rates are achieved, giving rapid evaporation and allowing the whole bed to be maintained in a dry condition.
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