Slide 1

Jangam, Mujumdar - Basic Concepts and Definition

bulb temperature is independent of the geometry of the drying object due to the analogy

between heat and mass transfer.

Figure 1.7. Typical textbook batch drying rate curve under constant drying conditions

The drying rate in the falling rate period(s) is a function of X (or Xf) and must be de-
termined experimentally for a given material being dried in a given type of dryer. If the
drying rate curve (N versus X) is known, the total drying time required to reduce the sol-
id moisture content from X 1 to X 2 can be simply calculated by:

t

M

A

dX

d N

s

X

X

= −∫

1

2

(1. 9)

Table 1.5 lists expressions for the drying time for constant rate, linear falling rates
and a falling rate controlled by liquid diffusion of water in a thin slab. The subscripts c
and f refer to the constant and falling rate periods, respectively. The total drying time is,
of course, a sum of drying times in two succeeding periods. Different analytical expres-
sions are obtained for the drying times tf depending on the functional form of N or the
model used to describe the falling rate, e.g., liquid diffusion, capillarity, evaporation-
condensation. For some solids, a receding front model (wherein the evaporating surface
recedes into the drying solid) yields a good agreement with experimental observations.
The principal goal of all falling rate drying models is to allow reliable extrapolation of
drying kinetic data over various operating conditions and product geometries.

The expression for tf in Table 1.5 using the liquid diffusion model (Fick's second law
of diffusion form applied to diffusion in solids with no real fundamental basis) is ob-
tained by solving analytically the following partial differential equation:

Moisture content (dry basis)

Drying Rate (kg m

-2 s

-1)

D C B^ A^

Falling Rate

Period

Constant Rate

Period

Initial

Transient

Period

Xe XC

(Internal heat/mass

transfer rate controlling)

(External heat/mass

transfer rate controlling)