Solar Drying: Fundamentals,Applications and Innovations - National ...

Visavale - Principles, Classification and Selection of Solar Dryers
moisture content at the corresponding water activity α w, over a range of values at constant
temperature. Figure 1.5 presents the absorption and desorption isotherms. It is
obvious that it has a slight hysteresis in re-absorbing water when the product has been
dried. There exist numerous mathematical models, theoretical, empirical and semiempirical
relationships, developed for various agricultural products, crops, grains, etc.
The three models most widely used in food drying are described here.
Figure 1.5. Sorption Isotherm (A) Absorption curve (B) Desorption curve (Belessiotis
and Delyannis, 2011)
1.6.1. Sorption Equations
A number of mathematical equations have been proposed to describe the sorption phenomena
but only the BET and GAB equations are widely accepted especially for crops.
These are based on the theory of water molecular layers absorbed onto the materials
surface, a modification for multi-layer absorption, of the Langmuir’s (1918) water molecular
mono-layer equation:
α w 1 − 1
= 1
(1.11)
X X mon C∙X
where are: X, the moisture content at the observation time period, X mon , the mono-layer
moisture content and is the Langmuir’s constant. As shown in the schematic model of
Kei on Figure 1.6 (Toei, 1996), it is assumed that, with respect to the externally exposed
molecules on each layer within the multi-layer, dynamic equilibrium between Langmuir’s
adsorption– desorption rates still holds for the exposed molecules on each layer.
The isotherm models used to fit the data were BET (Brunauer et al., 1938) (Eq. 1) and
GAB models (Van den Berg, 1984) (Eq. 1.12):
X
e
X
mCGAB
K
GABaw
=
[( 1 − K
GABaw
)(1 − K
GABaw
+ CGAB
K
GABaw
)]
(1.12)
where X e is the equilibrium moisture content (g water/g solids); X m is the monolayer
moisture content (g water/g solids); n is the number of layers; C BET, C GAB and K GAB are the
constants.
Drying of Foods, Vegetables and Fruits 11