The drying of materials is often the final operation in a manufacturing process, carried out
immediately prior to packaging or dispatch. Drying refers to the final removal of water,
or another solute, and the operation often follows evaporation, filtration, or crystallisation.
In some cases, drying is an essential part of the manufacturing process, as for instance
in paper making or in the seasoning of timber, although, in the majority of processing
industries, drying is carried out for one or more of the following reasons:
(a) To reduce the cost of transport.
(b) To make a material more suitable for handling as, for example, with soap powders,
dyestuffs and fertilisers.
(c) To provide definite properties, such as, for example, maintaining the free-flowing
nature of salt.
(d) To remove moisture which may otherwise lead to corrosion. One example is the
drying of gaseous fuels or benzene prior to chlorination.
With a crystalline product, it is essential that the crystals are not damaged during the
drying process, and, in the case of pharmaceutical products, care must be taken to avoid
contamination. Shrinkage, as with paper, cracking, as with wood, or loss of flavour, as
with fruit, must also be prevented. With the exception of the partial drying of a material by
squeezing in a press or the removal of water by adsorption, almost all drying processes
involve the removal of water by vaporisation, which requires the addition of heat. In
assessing the efficiency of a drying process, the effective utilisation of the heat supplied
is the major consideration.
The moisture content of a material is usually expressed in terms of its water content
as a percentage of the mass of the dry material, though moisture content is sometimes
expressed on a wet basis, as in Example 16.3. If a material is exposed to air at a given
temperature and humidity, the material will either lose or gain water until an equilibrium
condition is established. This equilibrium moisture content varies widely with the moisture
content and the temperature of the air, as shown in Figure 16.1. A non-porous insoluble
solid, such as sand or china clay, has an equilibrium moisture content approaching zero
for all humidities and temperatures, although many organic materials, such as wood,
textiles, and leather, show wide variations of equilibrium moisture content. Moisture may
be present in two forms: