Grain Drying Fundamentals

• Drying refers to removing a small amount of moisture from a solid material by evaporation.
• Drying involves both heat and mass transfer operations.
• Thin layer drying is a grain drying process where all grains are fully exposed to drying air under constant temperature and humidity.
• Drying removes excess moisture for safe storage.
• Delaying drying can cause damage to the product.
• Drying too rapidly can cause stress cracks.
• Over-drying results in economic loss.
• Drying is essential for successful storability.
• Drying depends on air-water vapor mixtures and the moisture content of the product.
• The rate of drying depends on the vapor pressure difference between the grain and the ambient air.
• Dryers are evaluated based on capacity, efficiency, and product quality.
• The drying process includes four major stages:
  • Initial induction period: heat transfer raises material temperature.
  • Constant rate period: drying occurs on the material surface, similar to free water evaporation.
  • First falling rate period: evaporation from a saturated, gradually decreasing surface.
  • Second falling rate period: evaporation of moisture occurs below the material surface.
• The critical moisture content is when the drying rate starts to decline.
• Drying can increase farmers' income by reducing quality and quantity losses.
• Drying can expand market opportunities for farmers, allowing temporary storage without quality deterioration.
• Timely drying can prevent potential losses in quality and quantity of grain.
• Delay or improper drying can lead to losses, including:
  • Discoloration
  • Non-uniformity
  • Over-drying
  • Fissuring
  • Grain deterioration
  • Loss of seed viability
  • Reduced milling recovery
  • Altered texture, appearance, and taste.
  • Spillages/physical losses
  • Price reduction.
• Drying methods include:
  • Conduction
  • Convection
  • Radiation
  • Solar
  • Dielectric and Microwave
  • Chemical
  • Sack Drying

Conduction Drying

• Heat for drying is transferred to the wet solid mainly through conduction via a solid, typically metallic, surface.
• Some dryers are equipped with an agitator to move and mix the material bed, eliminating moisture gradients and increasing the drying rate.
• Conduction drying is characterized by:
  • Heat transfer through a solid, usually metallic, surface.
  • Surface temperatures may vary widely.
  • Can be operated under low pressure and in an inert atmosphere.
  • Effective removal of dust and dusty materials.
  • Agitation leads to more uniform drying and increased rate.
  • Can be carried out continuously or batch-wise.

Convection Drying

• Hot gases supply heat, vaporize moisture, and transfer heat to the wet solid convection.
• Characteristics of Convection Drying
  • It's dependent on heat transfer from the drying agent to the wet material.
  • Drying agents such as steam heated air, direct flue gases of agricultural waste, etc. can be used.
  • Drying temperatures varies widely.
  • Vapor content affects the drying rate and final moisture content at gas temperatures below boiling.
  • Dehumidification is needed for natural air drying, if atmospheric humidities are high.
  • Fuel consumption per kilogram of moisture evaporated is always higher than conduction drying.

Natural Air Drying

• The unheated air supplied by nature is utilized.

Advantages of Convection Drying

• Low initial investment and maintenance cost
• No fuel cost
• No fire hazard
• Least supervision
• Least mold growth

Disadvantages of Convection Drying

• Drying rate is very slow
• The drying period may extend to weeks.
• Weather-dependent
• It requires more drying space
• Useful for short-term storage in the farm
• Not suitable for humid tropics

Supplemental Heat Drying

• A amount of heat (temperature rises within 5-10°C) is required to reduce drying air relative humidity.

Heated Air Drying

• A large quantity of grain can be dried within a short time and marketed at once
• Used for both short- and long-term storage.

Types of Convection Dryers

• Flat bed dryers are cheap and simple but labor intensive.
• Inclined bed dryers are easy to unload but more expensive.
• Circular bin dryers are compact and cheap but airflow is uneven inside and outside.
• Convection drying is independent of weather and can be faster with use of higher drying capacity per fan horsepower.
• Convection can be used for both long term and short term grain storage.
• Convection drying disadvantages include skilled manpower requirements for condition control, and can lead to liquid fuel contamination.

Radiation Drying

• Moisture movement and evaporation occurs due to the difference in temperature and partial pressure of water vapor.
• Relies on absorption of radiant energy from the sun, transforming it into heat within the grain.
• It is aided using special infrared radiation generators, called infrared lamps.

Sun Drying

• The use of solar energy as the most popular drying method.
• In developing countries, a major quantity of grain is still dried in the sun.

Advatanges of Radiation Drying

• No fuel or mechanical energy required
• Operation is simple
• Viability, germination, and baking qualities are fully preserved
• Labor-oriented
• No pollution

Disadvantages of Radiation Drying

• Is completely dependent on weather
• Cant operate round the clock or round the year
• Excessive losses from animals
• The entire process is unhygienic
• Handling of large quantities of grain is unsuitable within a short harvest period

Infrared Drying

• Infrared rays penetrate materials and converted into heat energy.
• It uses Metallic and ceramic infrared lamps/surfaces heated to a specific temperature via open flame as generators for infrared radiation.

Advantages of using lamp radiation dryers:

• Small thermal inertia
• Simplicity and safety in operation

Using of lamp radiation dryers:

• High expenditure of electric power
• Low utilization factor

Solar Drying

• It effectively used in many tropical and subtropical countries where solar energy is abundant.
• It has an absorber to receive and absorb solar radiation.
• The energy absorber transforms the radiation energy to thermal energy for heating air flowing over it, which is used as a drying medium.

Dielectric and Microwave Drying

• The substance is heated by the dielectric loss.
• The molecules of the substance are placed in a field of high frequency current.
• Electrical energy is transformed into heat, when the molecules of the substance are polarized and oscillates with the frequency.

Chemical Drying

• Common salt is effective and convenient in preventing changes while in storage.
• Water is removed when wet paddy is treated with common salt, from kernel by osmosis.
• The common salt cannot penetrate into the endosperm through husk layer instead absorbs moisture from paddy.

Chemical Drying Advantages

• Chemical reduces damage to fungal, microbial, enzymatic activity
• It Does not affect the grain viability
• Chemical is satisfactory in quality of paddy
• Chemical has negligible dry matter loss
• Chemical does not affect quality of rice bran

Chemical Drying Disadvantages

• It may retain moisture in the husk due to sodium chloride content
• It has a shortened useful life of gunny (burlap sack)
• It changes to a dark yellow color husk
• It requires an additional drying

Sack Drying

• Suitable for drying quantities of seed to preserve varietal purity, strain and viability
• Grain bags are laid out over a tunnel system

Grain Dryers

• Devices remove the moisture of the grain
• Generally by forced or natural convection with or without addition of heat
• Reduce the grain moisture for storage (IRRI)

Grain Drying Options

• Field drying
• Natural Air/Low Temperature Grain Drying
• High-Temperature Grain Drying

Field Drying

• Low Cost
• Ease of Management
• Mat Drying and Pavement Drying

Multi-Purpose Drying Pavement (MPDP)

• Caused Fissures during grain rewetting
• The grains should be mixed
• Grain thickness is recommended to be set 2-4cm

Natural Air/Low Temperature Drying

• Controllers the humidity rather than temperature
• Drying allows grain layers reach equilibrium moisture content

High Temperature Grain Drying

• It employs elevated temperatures for rapid drying
• High temperature dries until content meets the desired level
• To reduces grains moisture, device are needed to force heated air through grain mass

Batch Type High Temperature Grain Drying

• Dryer of mechanical style
• Grain in the machine is contained and held to until moisture has been reached

Flat Bed Type

• Shallow bed batch
• Volume has a fixed volume; it stays stationary in horizontal holding bin

Recirculating Type

• Machine with ability to mix contents
• Operation is conducted while drying

Vertical Bin Type

• Drying machine with Batch
• The volume remains at a fixed state and is held in a vertical Bin

Flat Bed Dryer

• 6 ton per batch
• Time of Dryer is 6-8 hrs
• About 12 hp engine
• Use 35 kg rice hull/hr as fuel
• Measurements of 3x6 metres floor area
• 76cm blower
• All are part of a Flat Bed Dryer

Parts of Flat Bed Dryer

• Burner
• Blower
• Bin or Bed

Continuous Flow Dryer

• The dryer is constantly going and material is constantly pushed through
• The Dryer discharges

Concurrant Flow Rate

• Continuous
• Being the product is moved when the dryer is

Counter-Flow Type

• Dryer is continuous
• Drying and moves in separate courses

Cross-flow Type

• An ongoing Dryer
• Air is transverse

Mixing Type

• Uses column styled drying bin
• This Dryer has mixing ability

Non-Mixing Type

• Dryer for constant flowing
• Grains go and pass the column
• The system contains screens in its structure

Tray Dryers

• For tray drying, spread the food generally the trays are arranged for the drying, as a result of the equipment
• You could warm up by leading an amount if air through the trays

Tunnel Dryer

• Trays move along on a rolling system while the heat it is applied and vapors are removed
• When applying material an air is uses

Fluidized Bed Dryer

• Dryer uses fluid
• The components are then pushed against gravity in flow
• There must be horizontal components to help with convection

Bin Dryer

• The product is put in bin with a floor that has hole
• The device shoots streams of air blowing through the product
• It is then dried