In this article we will discuss about:- 1. Introduction to Grain Drying 2. Process of Grain Drying 3. Principle 4. Methods.
Introduction to Grain Drying:
Grain drying is the process for conditioning the grains for safe storage. Grain is a living organism and during the drying process, its life must be fully safe guarded. Correct drying method preserves the quality, nutritive value and viability of grain.
The principal parameters which govern the condition of drying are:
(a) Maximum temperature to which the grain is heated during drying.
(b) The duration for which the grain is maintained in the heated state.
(c) Temperature of the drying agent supplied to the drier.
(d) Velocity of the drying agent.
(e) Relative humidity of air.
The principal requirement of the drying process is that dried grain must fully conserve its seed or food grain quality. The quality of seed grain is determined by germination capacity and the energy required for it. Food grains are valued according to the quality (density) of their gluten content.
Process of Grain Drying:
At a given temperature and relative humidity, air has the capacity to hold a specific amount of moisture. As the air moves through grains, there is transfer of moisture from one to another grain unless moisture equilibrium exists between the grain and the air.
In forced air drying method, the moisture moves from the grain to the air. The air is used as medium to supply heat, needed for evaporation of moisture from grains and as a carrier to remove the evaporated moisture from the grain. Thus the process of drying is heat and mass transfer process.
Evaporation of moisture needs about 600 kilo calories of heat per kg of water. This amount of heat must be supplied by reducing the heat of drying air. The heat lost by drying air is utilized for moisture evaporation and is converted into the latent heat of water vapour in the air.
Thus the total heat content of the drying air remains almost constant. If the grain and the air in contact is allowed to remain together for longer duration of time, moisture transfer takes place between the grain and air, until equilibrium condition is established.
Partial pressure of water vapour in the grain is equal to the partial pressure of water vapour in the air. The moisture content of the grain at which this stage is reached is called Equilibrium moisture content (EMC) of the grain. The relative humidity of air at this stage is called Equilibrium relative humidity (ERH) of the air.
Principle of Grain Drying:
Grain drying is based on two principles:
1. Thin layer drying
2. Deep bed drying
Commercial driers normally use the method of thin layer drying. Farm driers follow method of deep bed drying.
1. Thin Layer Drying:
Thin layer drying refers to the drying of grains which are entirely exposed to the air, moving through the grains.
Special features of thin layer drying:
i. Grain depth is not more than 20 cm.
ii. At a given relative humidity, the drying rate is proportional to the difference between grain moisture content (MC) and equilibrium moisture content (dry basis).
iii. Rate of drying is proportional to the difference between vapour pressure of grain and vapour pressure of drying air.
iv. At a given moisture content, the drying rate is proportional to the difference between the dry bulb temperatures of air in equilibrium with the grain.
The drier which uses the principle of thin layer drying is known as Continuous flow drier.
Continuous flow drier is of two types:
(a) Non-mixing columnar drier
(b) Mixing drier.
(a) Non-Mixing Columnar Drier:
The wet grain descends between two parallel screens usually set about 15 cm apart while heated air blows through the screen. No appreciable mixing occurs during the process.
(b) Mixing Drier:
Mixing driers are available in several designs.
The popular designs are:
(i) Baffle type
(ii) Louisiana state university (LSU) type
2. Deep Bed Drying Process:
Deep bed drying process includes bin or batch type driers. When the drying air has to pass through a layer of more than 20 cm thickness of grain, it is called Deep bed drying process. In this process, grain does not move. Grain is contained in a bin.
The natural or heated air is forced through the bottom of the bin upward through the wet grain. The temperature, humidity, saturated vapour pressure and specific volume of drying air change as the air passes through the grain. Consequently the drying potential of air decreases as it moves upwards.
The bottom layers dry first and the drying zone gradually moves upward. Deep bed drying method may be thought as a process of drying grain in several thin layers, in which the temperature and humidity of air, entering and leaving each layer vary with time depending upon the stage of drying.
This type of drying system consists of the following:
(а) Structure for holding grain
(b) Suitable fan or blower for supplying air
(c) Air distribution system
(d) Heating unit (if heated air is to be passed).
Circular bins made of plain or corrugated steel sheets are commonly used in this system. The main consideration is to provide a tight structure, which prevents leakage of air and moisture through floors and walls.
Two types of air distribution system may be used for this drier:
i. Perforated or bottom open ducts placed on a solid floor.
ii. Perforated false floor with air introduced below it.
The second system distributes air more uniformly and offers less resistance to air flow than the first one. The net area of perforation is about 15% of the gross floor area. The air flow rate is approximately 0.40 to 0.60 cubic metre/min/100 kg of wet grain, depending upon weather conditions and initial moisture content of grain. The air flow rate, limits the depth of grain to a maximum of about 2.40 metres for grain having 20% moisture. Maximum safe temperature of drying air is considered to be about 44°C for paddy.
Methods of Grain Drying:
There are three general methods of drying grains:
1. Unheated air drying
2. Unheated air drying with some additional heat and
3. Heated air drying
1. Unheated Air Drying:
In this type of drying, atmospheric air is forced through the grains. It is an economical and slow process. This process is connected with a method called Batch or Bin drying. The seeds are kept in a bin. The drier consists of a structure for storing the seed, a blower, a motor and an air distribution system.
A false floor with small perforations is used to hold the seed. The area beneath this floor consists of a chamber containing air called Plenum chamber or Air chamber. The air is blown through a blower driven by a motor. This is a simple method with low investment requiring very less supervision. The main drawback is that if the weather condition is not favourable, this method becomes ineffective. The rate of drying is very slow and there are chances of damage of grains by mold growth due to prolonged drying.
2. Unheated Air Drying with Additional Heat:
The method engaged additional equipment for heating purpose. This process of drying does not depend much upon weather conditions. The drying time is also reduced to a great extent. The equipments required are same as that of unheated air drying with an addition of heating unit. It is recommended that this method be employed during humid and cold weathers, or during prolonged period of high humidity (above 75%).
3. Heated Air Drying:
Heated air drying is the process of drying grain by heated air, forced through the grains. It is quick and efficient process of drying. It is independent of weather conditions but the initial cost is high.
Types of Drier:
1. Louisiana State University (LSU) Type:
This type of drier consists of bin in which layers of ‘A’ shaped air channels are provided. Each layer of channel is offset from one above so that the top of the ‘A’ shaped channel split the stream of grain causing thorough mixing. Alternate layers are air inlet channels and air outlet channels. In the inlet channels, the end openings on one side face the air plenum chamber, while the end openings on opposite side are closed. In the air outlet channels, the end openings on the plenum side are closed and on the opposite side they are opened to air exhaust system.
Air temperature and drying time of grain are kept within certain limits. The drying air temperature varies from 55°C to 70°C. Drying is phased in 3 to 4 stages, and each stage is called a pass.
In case of paddy the grain is brought to the drier at about 22% moisture (wet basis), passed through a scalper and fed to the drier. As it flows beneath the drier for about 30 minutes, it loses about 4% moisture.
This is the first pass. It is transferred to a bin for tempering for about 8 hours. It is again fed through the drier for 30 minutes (second pass), when it loses about 3% moisture i.e. the moisture content is about 15%. It is conveyed to the tempering bin for 8 to 12 hours.
When it is passed through the drier for the third time for 30 minutes, it loses 2% moisture. The paddy is now dried to 13% moisture (wet basis), which is considered safe for storage. The rate of drying decreases as the moisture content goes down gradually.
2. Baffle Type:
In Baffle type drier, moist grain flows downward in a zigzag path by means of baffles provided in the way. The heated air is forced through the grains uniformly.
Depending upon the methods of construction, a few common driers are:
(a) Sack drier
(b) Revolving drum drier
(c) Closed circuit drier with dehumidifier.
(a) Sack Drier:
Sack drier is useful when small quantity of grains is to be handled. In sack drying, it is desirable to use maximum air temperature of 110°F only. The bags should be turned once during the drying operation. There is a blower connected to the air chamber beneath the floor to supply air. Suitable building with perforated floor should be used for this purpose.
(b) Revolving Drum Type Drier:
This method consists of a large drum 1 to 2 m in diameter, 3 to 6 m in length placed on a slight inclined position. At the higher end, the grain is fed and as the unit rotates the grain is dropped through the air, flowing in the opposite direction to grain flow. In this process, continuous mixing of the seed with the drying air takes place.
(c) Closed Circuit Drier with Dehumidifier:
A dehumidifier is used to dry the air before it passes through the seed. In this process the seed does not move during drying period. A closed circuit is needed in this process.