Slide 1

Jangam, Mujumdar - Basic Concepts and Definition

depression below the equilibrium vapor pressure of water at the same temperature is
due to its confinement in smaller capillaries. Water in region C is even more loosely held

in larger capillaries. It is available for reactions and as a solvent.

Figure 1.4. Typical sorption isotherm
Figure 1. 5 shows the various types of moisture defined in Error! Reference source
not found.. Desorption isotherms are also dependent on external pressure. However, in
all practical cases of interest, this effect may be neglected.

According to Keey (1978), the temperature dependence of the equilibrium moisture
content can be correlated by:

*

tan

*

X

T

X

cons t

 =−α











∆

∆

Ψ=

(1.4)

where X is the dry-basis equilibrium moisture content, T is the temperature and (^) Ψ is
the relative humidity of air. The parameter ranges from 0.005 to 0.01 K-1. This correlation
may be used to estimate the temperature dependence of X if no data are available.
For hygroscopic solids, the enthalpy of the attached moisture is less than that of
pure liquid by an amount equal to the binding energy, which is also termed the enthalpy
of wetting, Hw (Keey, 1978). It includes the heat of sorption, hydration and solution and

may be estimated from the following equation:

d

d T

H

Xcons t R T

w

g

(ln )

( / ) tan

Ψ ∆

(^1) =

= − (1.5)

Adsorption

Desorption

B

C

A

% Relative Humidity

Moisture content

20 40 60 80 100