AENG 252 :: Lecture 05

AENG 252 :: Lecture 05 :: DRYING – GRAIN DRYING, TYPES OF DRYING, TYPES OF DRYERS, IMPORTANCE OF DRYING

Permits long time storage of grain without deterioration
Permits continuous supply of product thro’ out the year
Permits early harvest which reduces field damage and shattering loss
Permits the farmers to have better quality product
Makes products available during off season

Drying theory

Convection process in which moisture from a product is removed
The water content of agricultural product is given in terms of moisture content
They gain or loose moisture as per the atmospheric conditions
Moisture migration into or from a product is dependent on the difference of vapour pressure between atmosphere and product
If the vapour pressure of grain is greater than atmospheric vapour pressure, transfer of moisture from grain to atmosphere takes place
If the atmospheric vapour pressure is greater than grain vapour pressure, grain absorbs moisture from atmosphere

Drying rate periods
Divided into 3 periods

Constant rate period
First Falling rate period
Second falling rate period

Constant rate period

Moisture migration rate from inside of product to its surface is equal to the rate of evaporation of water from surface
This period continues till critical moisture content is reached
Critical moisture content: Moisture content of a product where constant rate drying ceases and falling rate starts
This period is very short for agricultural products
Drying of sand and washed seeds takes place in constant rate period

Falling rate period

Most of the agricultural products are dried in falling rate drying period
Movement and diffusion of moisture in interior of grains controls the entire drying process

Controlled by

Migration of moisture from interior of grains to upper surface due to water vapour diffusion

Removal of moisture from the surface

Divided into two periods

First falling rate period
Second falling rate period

First falling rate

Unsaturated surface drying
Drying rate decreases because of the decrease in wet surface area
Fraction of wet surface decreases to zero, where first falling rate ends

Second falling rate

Sub surface evaporation takes place & it continues until the equilibrium moisture content is reached

Mechanism of drying process

Movement of moisture takes place due to

Capillary flow – Liquid movement due to surface forces
Liquid diffusion – Liquid movement due to difference in moisture concentration
Surface diffusion - Liquid movement due to moisture diffusion of the pore spaces
Vapour diffusion – vapour movement due to moisture concentration difference
Thermal diffusion - vapour movement due to temperature difference
Hydro dynamic flow – water and vapour movement due to total pressure difference

Thin layer drying

Process in which all grains are fully exposed to the drying air under constant drying conditions i.e. at constant air temp. & humidity.
Up to 20 cm thickness of grain bed is taken as thin layer
All commercial dryers are designed based on thin layer drying principles
Represented by Newton’s law by replacing moisture content in place of temperature

M-Me/Mo-Me = e -Kq
M – Moisture content at any time q, % db
Me- EMC, %db
Mo – Initial moisture content, %db
K – drying constant
q - time, hour

Deep bed drying

All grains are not fully exposed to the same condition of drying air
Condition of drying air changes with time and depth of grain bed
Rate of airflow per unit mass of grain is small
Drying of grain in deep bin can be taken as sum of several thin layers
Humidity & temperature of air entering & leaving each layer vary with time
Volume of drying zone varies with temp & humidity of entering air, moisture content of grain & velocity of air

Deep bed drying characteristics at different depths
Continuous flow dryer

Columnar type dryer in which wet grains flow from top to the bottom of the dryer
Two types

Mixing
Non-mixing

Mixing

Grains are diverted in the dryer by providing baffles
Use low air flow rates of 50-95 m3/min/tonne
Zig-zag columns enclosed by screens are used to achieve mixing
High drying air temperature of 65°C is used

Baffle dryer

Continuous flow mixing type dryer
Consists of receiving bin, drying chamber fitted with baffles, plenum fitted with hot air inlet
Baffles are fitted to divert the flow & also for mixing
Grain fed at the top & move downward in a zig-zag path where it encounters a cross flow of hot air
Bucket elevator is used to recirculate the grain till the grain is dried to desired moisture level
Uniformly dried product is obtained

Mixing type baffle dryer
Non-mixing

Grains flow in a straight path
Baffles are not provided and drying takes place between two parallel screens
High airflow rates can be used
Drying air temp. of 54°C is used

Feed hopper
Plenum chamber
Exit air
Dry grain outlet
Screened grain column

Continuous flow dryer (Non-mixing)
Recirculatory Batch dryer

Continuous flow non mixing type
Consists of 2 concentric circular cylinders, set 15-20 cm apart

Bucket elevator is used to feed & recirculated the grain
Centrifugal blower blows the hot air into the inner cylinder, acts as a plenum
Grain is fed at the top of the inside cylinder; comes in contact with a cross flow of hot air
The exhaust air comes out through perforations of the outer cylinder
Grain is recirculated till it is dried to desired moisture content
Drying is not uniform as compared to mixing type

Recirculating batch dryer

LSU dryer

Developed at Louisiana state university (LSU)
Continuous mixing type dryer
Developed specifically for rice to ensure gentle treatment, good mixing & good air to grain contact
Consists of rectangular chamber, holding bin, blower with duct, grain discharging mechanism and air heating system
Layers of inverted V shaped channels are installed in the drying chamber; heated air is introduced through these channels at many points
Alternate layers are air inlet & outlet channels; arranged one below the other in an offset pattern
Inlet port consists of few full size ports & two half size ports; all ports are of same size arranged in equal spacing
Ribbed rollers are provided at the bottom of drying chamber for the discharge of grain
Capacity varies from 2-12 tonnes
Recommended air flow rate is 60-70 m3/min/tonne
Air temp. are 60 &85°C for raw & parboiled paddy
Uniformly dried product can be obtained
Can be used for different types of grain
High capital investment

LSU Dryer

Garner
Duct
Dry material outlet
Hopper
Continuous flow
Door
Roof

LSU Dryer
Tray driers

In tray dryers, the food is spread out, generally quite thinly, on trays in which the drying takes place.
Heating may be by an air current sweeping across the trays, or heated shelves on which the trays lie, or by radiation from heated surfaces.
Most tray dryers are heated by air, which also removes the moist vapours.

Fluidized Bed Dryers
In a fluidized bed dryer, the food material is maintained suspended against gravity in an upward-flowing air stream.
Heat is transferred from the air to the food material, mostly by convection

Pneumatic Dryers

In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension.
Heated air accomplishes the drying and often some form of classifying device is included in the equipment.
In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying

Rotary Dryers

The foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls.
In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.

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