Figure 2. Equilibrium moisture content as a function of relative humidity.
After a short heat-up period (not shown in Figure 1), the rate
of moisture removal is constant. During this constant-rate drying period, enough moisture is available to keep the surface of
the solid particles entirely wet. The temperature of the wet particle surface is the adiabatic saturation temperature, which in
the case of water and air is equal to the wet bulb temperature.
At a certain moisture content, dry regions begin to exist
on the surface, and the drying rate begins to decrease. This
moisture level is called the critical moisture content. The critical moisture content depends on the thickness of the bed of
material and the degree of mixing between the gas and solids.
The critical moisture content is therefore not a property of the
material itself and must be determined experimentally.
The gas used to dry the solids frequently contains moisture,
i.e., it has a relative humidity. The moisture content of the bulk
material leaving a dryer cannot be less than that which is in
equilibrium with the gas. This equilibrium moisture content is a
function of temperature and the relative humidity of the drying
gas. An example equilibrium curve is shown in Figure 2.
The rate of heat transfer during the constant-rate period of
drying is equal to the product of the total surface area of the
particles, the heat-transfer coefficient, and the temperature
The drying mechanisms are often complex as they involve
both transfer of heat either directly by contact with a hot gas
or indirectly from steam, combustion gas, or a heat-transfer
fluid and mass transfer of liquid from the bulk material to the
surrounding gas. Often, drying is conceptualized as a two-
step process, as illustrated by a typical drying curve shown in
Figure 1. A drying curve is obtained by placing a sample of bulk
material or suspending it on a balance inside a heated environ-
ment. The weight of the sample is then measured over time
under constant drying conditions. Ideally, the sample is dried
under conditions similar to those to be expected when the ma-
terial is processed in commercial equipment.
■ By Greg Mehos, Ph.D., P.E., Jenike & Johanson, Inc.
Improving energy efficiency and product quality through continuous drying
Feed Considerations for
■ 24 JULY/AUGUST 2014 | PHARMACEUTICAL PROCESSING
■ PHARMPRO. COM
■ DR YING
Figure 1. Typical drying curve illustrating constant and falling rate periods
and critical and equilibrium moisture contents.