The Theory of Drying

THE THEORY OF DRYING

Theories of drying are limited in application in that drying times are normally experimentally determined. Nevertheless, an appreciation of the scope and limitations of the different drying methods is given. The following terms are employed in discussing drying: humidity and humidity of saturated air, relative humidity, wet bulb temperature, and adiabatic cooling line. Other terms may be defined as follows:

Moisture content: This is usually expressed as a weight per unit weight of dry solids. Equilibrium moisture content: If a material is exposed to air at a given temperature and humidity, it will gain or lose moisture until equilibrium is reached. The moisture present at this point is defined as the equilibrium moisture content for the given exposure conditions. At a given temperature, it will vary with the partial pressure of the water vapor in the surrounding atmosphere. This is shown for a hypothetical hygroscopic material in Figure 7.1 in which the equilibrium moisture content is plotted against the relative humidity. Any moisture present in excess of the equilibrium moisture content is called “free water.”

FIGURE 7.1 The relation between equilibrium moisture content and relative humidity for a hygroscopic solid.

Equilibrium moisture content curves vary greatly with the type of material examined. Insoluble, nonporous materials, such as talc or zinc oxide, give equilibrium moisture contents of almost zero over a wide humidity range. A moisture content of between 10% and 15% may be expected for cotton fabrics under normal atmospheric conditions. Drying below the equilibrium moisture content for room conditions may be deliberately undertaken, particularly if the material is unstable in the presence of moisture. Subsequent storage conditions then become important for product stability.

The equilibrium moisture content at 100% relative humidity represents the minimum amount of water associated with the solid that still exerts a vapor pressure equal to a separate water surface. If the humidity is reduced, only part of the water evaporates before a new equilibrium is established. The water retained at less than 100% relative humidity must, therefore, exert a vapor pressure below that of a dissociated water surface. Such water is called “bound water.” Unlike the equilibrium moisture content, bound water is a function of the solid only and not of the surroundings. Such water is usually held in small pores bound with highly curved menisci, is present as a solution, or is adsorbed on the surface of the solid.

The value of equilibrium moisture content curves is illustrated by the examples given in Figure 7.2. The equilibrium moisture content of the antacid granules, composed of magnesium trisilicate granulated with syrup, is a sen-sitive function of relative humidity. If it is to be dried to a moisture content of 3%, air at a relative humidity of less than 35% must be used. 

FIGURE 7.2 Equilibrium moisture content curves for two tablet granulations.

With knowledge of the humidity of the circulating air, psychrometric charts may be used to determine the minimum air temperature that will dry the material to the required standard. (In fact, the temperature has an effect on the equilibrium moisture content that is independent of the humidity, but this can be neglected to a first approximation.)

The lactose granulation, on the other hand, has a low sensitivity to relative humidity. Drying at low relative humidities derived from high air temperatures causes only a marginal decrease in the final moisture content, and the stability of the active ingredients associated with the lactose filler could be impaired. This argument may only be applied to the final moisture content. It is not related to the rate of drying that would, of course, be greater at higher temperatures and lower humidities.

The effects of storage after drying may also be assessed from the equi-librium moisture content curves. Storage conditions are not critical for the lac-tose granulation. If the antacid formulation was stored at a relative humidity of only 65%, it would, given sufficient time, absorb moisture until the content was 9%. This could be associated with poor flow characteristics and its attendant difficulties during compression.

Dynamic vapor sorption techniques now exist, which allow thorough studies of moisture association with solids under a wide range of relative humidity conditions based on microbalance technology.