Drying theory and drying rating curve

1. DRYING THEORY AND
DRYING RATING CURVE
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2. INTRODUCTION
 Drying of agricultural products is an important unit operation
under post harvest phase. It refers to removal of moisture from
grains and other products to a predetermined level, whereas
dehydration means removal of moisture to very low levels
usually to bone dry condition.
 Drying makes the food grains and other products suitable for safe
storage and protects them against attack of insects, molds and
other micro-organisms during storage.
Moisture and its removal
 During drying, the moisture from solids gets vaporised and
diffused in dilute environment.
 Removal of moisture takes place by high pressure or high
temperature conditions.
 Drying removes the volatile materials present inside solid
matters. In some products, drying is carried to bring down the
moisture content to a predetermined limit.

3. Utilities of drying
 Drying is thermo-physical and physico-chemical action and its dynamic
principles are governed by heat and mass transfer laws inside and
outside of the products.
 Drying is the most effective unit operation for protection of food grains
and safe storage of agricultural products. Proper drying makes the
material stable and the attack of insects and micro-organisms during
storage period can be kept at negligible level.
Various other advantages of drying are given below-
 Drying permits early harvest of crops. This leads to reduction of losses
by shattering. This also leads to permit time for preparation of land for
sowing of following crop.
 Drying helps in proper planning of harvesting season. If adequate
drying facilities are available then crop can be harvested as per the
availability of labourers.

4.  Drying of agricultural products to optimum moisture content results in
safe storage of products over a longer period. Grains are stored in large
quantities for long period in Government warehouses and industries
for their subsequent use as and when required.
 Storage of products after drying makes products available during off
seasons. Selling of grains in off season would fetch additional income
to growers as prices remain higher during these periods.
 When grains are stored, due to increase in the temperature and
moisture content, heat of respiration also increases. The heat of
respiration can be removed by aeration and drying. Thus the
temperature and moisture content of the products can be brought down
and the viability of seeds could be maintained.
 Some agricultural byproducts and waste products can also be
converted into useful products by proper drying.

5. THERMAL PROPERTIES
 Information of thermal properties of products leads
towards prediction of heat transfer rate for the product.
 For design of heating and refrigeration system for food
materials, information on following thermal properties are
necessary.
1. Specific heat
2. Thermal conductivity
3. Enthalpy
4. Thermal diffusion

6. Specific heat
 The specific heat of a product tells us about the amount of
heat required to change the temperature of the product.
 The specific heat of a product is not dependent on its mass
density.
 To predict the specific heat of a mixture, we should know
the values of specific heats of separate materials of the
mixture.
 Specific heat of any product can be determined by the
following expression.
𝑄 = 𝑚𝐶𝑝(𝑇2 − 𝑇1)
Where
Q = amount of heat, cal
m = mass of the product, g
𝐶𝑝= Specific heat, cal/g-oC
T1 and T2 = initial and final temperature of the product, oC

7. Thermal Conductivity
 The ability of heat conductance of a material can be judged by
its thermal conductivity value.
 The thermal conductivity of a material can be defined as the
amount of heat conducted across unit area and unit thickness of
material in unit time for unit change in temperature. Therefore,
thermal conductivity depends on the mass density of material.
 In agricultural products, thermal conductivity depends on their
composition. But the value of thermal conductivity is also
affected by percent void spaces, shape, size and arrangement of
void spaces and other factors which restrict the heat flow.
 Thermal conductivity of single grain is 03 to 0.6 kcal/m-h-°C
whereas thermal conductivity of a heap of grains ranges
between 0.1 to 0.15 kcal/m-h-°C. The reason for this is the pore
space between the grains. These pore spaces are occupied by air
whose thermal conductivity is quite lower (0.02 kcal/m-h-°C )
as compared to grains.

8.  Thermal conductivity can be determined by the following
equation:
𝑄 =
𝐴𝐾(𝑇2 − 𝑇1)
𝑡
Where
Q = amount of heat
K = thermal conductivity
t = thickness of material layer
A = Area of material perpendicular to direction of
heat flow
T1 and T2 = temperature at the surfaces of the
material

9. Enthalpy
 The heat content of a material is its enthalpy.
 It can also be said that the energy level of a material is enthalpy.
 The absolute value of enthalpy is very difficult to define but its
zero value is usually defined at oC or some other convenient
temperature.
 Enthalpy is generally used to quantify energy in steam.
𝑄 = 𝑚 (ℎ2 − ℎ1)
Where
Q = amount of heat, cal
m = mass, g
h2 and h1 = enthalpy at temperature T2 and T1 respectively,
cal/g
Thus, it can be said that the amount of heat energy required to heat
a material from temperature T1 to temperature T2 is equal to m (h2 -
h1).

10. Thermal diffusivity
 Thermal diffusivity is used to determine the heat transfer
rates in solid agricultural products of any shape.
 This property of material is related to the ability of material
to conduct heat and its ability to store heat energy.
 It can be determined by the following expression:
Thermal diffusivity =
Thermal conductivity
specific heat × mass density

11. DRYING RATE CURVE
 The plots of
moisture content
versus drying time
or drying rate versus
drying time or drying
rate versus moisture
content are known
as drying curves.

12. Constant Rate Period
 Some crops including cereal grains at high moisture content are
dried under constant rate period at the initial period of drying.
Falling rate period follows subsequently.
 For example, wheat is dried under constant rate period when its
moisture content exceeds 72%.
 In the constant rate period, the rate of evaporation under any
given set of air conditions is independent of the solid and is
essentially the same as the rate of evaporation from a free liquid
surface under the same condition.
 The rate of drying during this period is dependent upon
 (a) difference between the temperature of air and temperature of
the wetted surface at constant air velocity and relative humidity;
 (b) difference in humidity between air stream and wet surface at
constant air velocity and temperature; and
 (c) air velocity at constant air temperature and humidity.

13.  In the constant rate period, drying takes place by surface
evaporation and moisture moves by vapor pressure
difference.
 The moisture content at which the drying rate ceases to be
constant is known as the critical moisture content of the
solid.
 The average critical moisture content for a given type of
material depends upon the surface moisture concentration,
bed thickness of the material, and rate of drying.
 The critical moisture content of a product also depends
upon the characteristics of the solid such as shape, size,
and drying conditions.

14. Falling Rate Period
 Cereal grains are usually dried entirely under falling rate
period.
 The falling rate period enters after the constant drying rate
period and corresponds to the drying cycle where all
surface is no longer wetted and the wetted surface
continually decreases, until at the end of this period, the
surface is dry.
 The cause of falling off in the rate of drying is due to the
inability of the moisture to be conveyed from the center of
the body to the surface at a rate comparable with the
moisture evaporation from its surface to the surroundings.
 The falling rate period is characterized by increasing
temperatures both at the surface and within the solid.
Furthermore, changes in air velocity have a much smaller
effect than during the constant rate period.

15.  The falling rate period of drying is controlled largely by the
product and is dependent upon the movement of moisture within
the material from the center to the surface by liquid diffusion and
the removal of moisture from the surface of the product.
 The falling rate period of drying often can be divided into two
stages:
(1) unsaturated surface drying and
(2) drying where the rate of water diffusion within the
product is slow and also the controlling factor.
 Practically, all cereal grains are dried under falling rate period if
their moisture contents are not very high.

16. Principles of Grain Drying
Grain drying is based on two principles:
1. Thin layer Drying
2. Deep bed drying
 Because of the basic differences in drying characteristics
of grains in thin layer and deep bed, the whole grain drying
process is divided into thin layer drying and deep bed
drying.
Thin layer drying
 Thin layer drying refers to the grain drying process in
which all grains are fully exposed to the drying air under
constant drying conditions, i.e, at constant air temperature,
and humidity. Generally, up to 20cm thickness of grain bed
(with a recommended air-grain ratio) is taken as thin layer.
All commercial flow dryers are designed on thin layer
drying principles.

17. Deep bed drying
 In deep bed drying all the grains in the dryer are not fully
exposed to the same condition of drying air.
 The condition of drying air at any point in the grain mass
changes with time and at any times it also changes with
depth of the grain bed.
 Over and above the rate of air flow per unit mass of grain
is small compared to the thin layer drying of grain.
 All on farm static bed batch dryers are designed on deep
bed drying principle.
 The drying of grain in a deep bin can be taken as the sum
of several thin layers.
 The humidity and temperature of air entering and leaving
each layer vary with time depending upon the stage of
drying, moisture removed from the dry layer until the
equilibrium moisture content is reached.

18.  Little moisture is removed; rather a small amount may be
added to the wet zone until the drying zone reaches it.
 The volume of drying zone varies with the temperature and
humidity of entering air, the moisture content of grain and
velocity of air movement.
 Drying will cease as soon as the product comes in
equilibrium with the air.
 The condition of drying in deep bed is shown in fig.

19. Thank You