Drying can be defined as the vaporization and removal of liquids from a solution, suspension or other solid-liquid mixture to form a dry solid. Drying occurs as a result of the vaporization of liquid by supplying heat to wet solid materials. In a drying process, heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor into the surrounding gas. It should be thought of not only as a thermal separation; but also as a means to control the quality of manufactured solid products.
Solids drying is complicated. It involves both heat and mass transfer occurring simultaneously, as well as physicochemical transformation. One tool for describing drying progress is known as a drying rate curve. This one-page reference provides information on how the components of a drying rate curve are used to describe a drying process.
Drying rate curve
The mass flux of the vapor leaving the surface of the solid per unit time is called the drying rate, and is denoted by the symbol ṁ (in kg/m2∙s). The drying rate is usually determined by measuring the change of moisture content with time dX/dt, as shown in Equation (1) [1].
ṁ = Ms/A dX/dt (1)
Where Ms is the mass of the solid and A is the surface area of the drying solid that is in contact with the drying agent. The drying rate ṁ depends on the conditions of drying and on the moisture content of the solid X (measured in kg liquid per kg of dry solid). For the humidity of a gaseous drying agent (usually air), the symbol Y is used (in kg vapor per kg of dry gas). The saturation humidity is denoted by Y*.
The relationship of the drying rate ṁ and the moisture content X under constant drying conditions can be represented by the function ṁ(X), also known as the drying rate curve.
Two drying regimes
Solids drying is understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two are demarcated by a break point known as the critical moisture content [1].
Constant-rate zone. In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1 [2]), section AB represents the constant-rate period. In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it [2].
FIGURE 1. Segment AB of the graph represents the constant-rate drying period, while segment BC is the falling-rate period
Within the constant-rate regime, the solid surface temperature remains constant and is equal to the wet-bulb temperature of the surrounding air (lowest possible temperature a surface can reach via evaporative cooling). During this period, liquid must be transported to the surface at a rate sufficient to maintain saturation. This zone ends when the superficial moisture has evaporated, and the solid’s surface can no longer sustain a continuous liquid film.
Falling-rate zone. A break in the drying curve (critical moisture content) occurs at Point B, and a linear fall in the drying rate is observed with further drying. This section (segment BC), is called the first falling-rate period. As drying proceeds, moisture reaches the solids surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the solid surface is no longer saturated, the surface temperatures will tend to rise above the wet bulb temperature.
This section (segment CD in figure), is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, osmotic pressure, capillary forces or through a vaporization and condensation cycle.
Thermodynamics and kinetics
The capacity of the air stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle. The two elements essential to this process are inlet-air temperature and air flowrate. Higher-temperature drying air means a greater vapor-holding capacity.
Surface-bound water is governed by the thermodynamic equilibrium, described by sorption isotherms. A material’s unique sorption isotherm provides a picture of how much moisture it will hold at a humidity level [1].
In the falling-rate regime, evaporation is governed by the kinetics of liquid migration in the interior of the solid (capillarity, diffusion).
Editor’s note: The content in this column was adapted from the following sources: 1. Tsotsas, E., Gnielinski, V. and Schundler, E., Drying of Solids Materials, Chapter in “Ullman’s Chemical Engineering and Plant Design,” Wiley-VCH, 2005; and 2. Parikh, D., Solids Drying: Basics and Applications, Chem. Eng., April 2014, pp. 42–45.