Jangam, Mujumdar - Basic Concepts and DefinitionFor certain materials and under conditions such as those encountered in freeze dry-
ing, a “receding-front” model involving a moving boundary between “dry” and “wet”
zones often describes the mechanism of drying much more realistically than does the
simple liquid diffusion or capillarity model. Examination of the freeze drying of a thin
slab indicates that the rate of drying is dependent on the rate of heat transfer to the
“dry-wet” interface and the mass transfer resistance offered by the porous dry layer to
permeation of the vapor which sublimes from the interface. Because of the low pres-
sures encountered in freeze drying, Knudsen diffusion may be significant. Liapis and
Marchello (1984) have discussed models of freeze drying involving both unbound and
bound moisture.
When drying materials under intense drying conditions, diffusion or capillarity
models generally do not apply. If evaporation can occur within the material there is a
danger of the so-called “vapor-lock” that occurring within the capillary structure causing
breaks in liquid-filled capillaries. This phenomenon can cause departure from the clas-
sical drying curve, e.g., no constant rate drying may appear under intense drying condi-
tions but may do so under milder drying conditions (Zaharchuk, 1993).
1. 4. ADVANCES IN FOOD DRYING
1.4.1. Use of advanced computational tools
Drying is a highly energy intensive unit operation. In food sectors, it is necessary to
maintain the product quality during drying in addition to the reduced drying cost. Some
products can be very high valued such as mango, strawberry, some marine products;
even highly expensive freeze drying can also be used for these products with enough
profit margins. However, products such as banana, sapota, green peas, corn, roots, vege-
tables cannot be dried using such expensive drying route. Hence it is very necessary to
develop cost-effective and innovative drying methods in food sectors. It is accepted fact
that mathematical modeling can be very helpful to achieve these expectations. Further,
when a drying method is to be developed for certain product, it is difficult to test all the
methods experimentally which is time consuming in addition to cost involvement. How-
ever, recently developed advanced computational tools can help compare different dry-
ing methods numerically.
Computational fluid dynamics is one of the well known tools used in various indus-
trial sectors. It has been used in chemical industries to study numerous unit operations
and can also be easily applied in drying. Jamaleddine and Ray (2010) have carried out a
comprehensive review of the CFD techniques applied to diverse problems in industrial
drying. They have reported that CFD solutions have been used in drying to optimize, to
retrofit, to develop equipment and processing strategies and replacing expensive and
time consuming experimentation. Mujumdar and Wu (2008) have highlighted the need
for cost-effective solution that can push innovation and creativity in drying. This can be
easily possible using highly efficient tools such as CFD. Some of the key advantages of
CFD in the drying sector are its ability to give information on comparison of different
geometries, its use as a powerful tool for troubleshooting purposes including the evalua-
tion of the effect of various parameters even in complex geometries (Mujumdar and Wu,