Drying theory

Introduction

Drying of grain involves exposing grain to air with low relative humidity (RH) which will lead to evaporation of the moisture in the grain and then the moisture’s removal away from the grain. Since drying practices can have a big impact on grain or seed quality, it is important to understand some fundamentals of grain drying.

PURPOSE OF GRAIN DRYING

• Increase in shelf life/ Preservation
• Reduction n transportation
• Easy material handling
• Improve or maintain properties (flow ability or compressibility)

• In grain, moisture is present at two places: at the surface of the grain, ‘surface moisture’, and inside the kernel, ‘internal moisture’. Surface moisture will readily evaporate when grain is exposed to hot air. Internal moisture evaporates much slower because it first has to move from the kernel to the outside surface.
• As a result, surface moisture and internal moisture evaporate at a different rate. This difference results in a different ‘drying rates’ for different period of drying.
• The drying rate is defined as the rate at which grain moisture content declines during the drying process.
• It is normally expressed in percent moisture removed per hour [%/hr]. Typical drying rates of rice dryers are in the 0.5%/hr to 1%/hr range.

A drying curve, as illustrated in the figure below, shows how the grain moisture content (MC) and grain temperature change over time. As can be seen in the chart, the drying rate is not constant but changes over time. The temperature of the grain equally changes over time.

Heat transfer in grain drying

THIN LAYER DRYING

• The thin-layer drying shows the condition of nearly complete exposure of grains to heated air.

• The thickness of grains in thin-layer drying is normally upto 15cm.

The drying action can be represented on the basis of Newton's law by replacing moisture content in place of temperature

DEEP BED DRYING

• In deep bed dryers, the drying takes place in a drying zone and the layer of grains is more that 15cm.
• If drying air is 43 degree celcius, the thickness of grain layer is limited to 45cm.
• Deep bed drying system has been analysed by Hukil. The Hukil analysis is suitable for correlating the drying period, product moisture and product depth factors for the solution of drying system.

Factor's on Drying Process

• Air temperature
• Air velocity
• Air humididty
• Air exposure time

DRYING PERIODS

• Preheating period (drying rate is almost 0): When wet grain is exposed to hot air, initially only a very slight change in MC is observed. This happens because all the heat provided in the drying air is used to heat up the grain to the drying temperature.

• Constant-rate period (drying rate is constant in time): Once the grain is at the drying temperature, water starts to evaporate from the surface of the grain. During this period, all the heat from the drying air is used to evaporate surface moisture and the amount of moisture removed from the grain is constant in time. It is therefore called the constant-rate period. During this period, grain temperature is constant as well.

Falling-rate period (drying rate declines over time): As time passes, it takes more time for internal moisture to appear at the surface, and evaporation of water is no longer constant in time. As a result, drying rate will decline, and some of the heat from the drying air will heat up the grain. For paddy grain, the falling-rate period typically occurs at around 18% grain moisture content.

EFFECTES OF FACTORS ON DRYING

• Effect of air temperature :

The rate of drying increases with the rise of air temperature. But the equilibrium moisture content falls as the air temperature increases.

• Effect of air Velocity:

The rate of drying increases with the increase in air velocity as it increases the rate of evaporation from the surface of grains.

• Effect of air humidity:

The rate of drying increases with increase in humidity of the air

• Effect of exposure time:

The drying time increases as the total exposure time decreases.