Drying

Definition

After threshing, the moisture content of most grain is too high for good conservation (13-14 percent).

"Drying" is the phase of the post-harvest system during which the product is rapidly dried until it reaches the "safe-moisture" level.

The aim of this dessication is to lower the moisture content in order to guarantee conditions favourable for storage or for further processing of the product.

Drying permits a reduction of losses during storage from causes such as:

• premature and unseasonable germination of the grain;
• development of moulds;
• proliferation of insects.

Moisture content

The moisture content of a product is a numerical value expressed in percentage. This is determined by the relationship between the weight of the water contained in a given sample of grain and the total weight of that sample:

when:

H% = the moisture content of the sample (in %);
Wwater = weight of the sample's water (in kg);
Wdm = weight of the sample's dry matter (in kg).

Therefore, to say that paddy has a 25 percent moisture content means that in a sample of 100 g of raw product there are 25 g of water and 75 g of dry matter.

Relative humidity

Grains are "hydroscopic", meaning that in ambient air they can either give off or absorb water in the form of vapour.

At a given temperature, however, the air cannot absorb unlimited quantities of water vapour.

The air is said to be "saturated" when, unable to absorb water vapour at a given temperature, it has a relative humidity of 100 percent.

The relative humidity of the air, expressed in percentage, is defined as the relationship between the weight of the water vapour contained in 1 kg of air and the weight of the water vapour contained in 1 kg of saturated air, at a given temperature:

when:

RH% = relative humidity of the air (in %).

The following table shows the maximal weights of water vapour contained in 1 kg of air:

 Air temperature 0ºC 10ºC 20ºC 30ºC 40ºC Maximal water vapour weight (in g) 3.9 7.9 15.2 28.1 50.6 Air temperature 50°C 60°C 70°C 80°C 90°C Maximal water vapour weight (in g) 89.5 158.5 289.7 580.0 1559

Air containing a given amount of water vapour tends to become saturated if its temperature is lowered.

On the contrary, if one wants to increase the "drying power" of this air (meaning its capacity for absorbing more water vapour), it is necessary to heat it.

For example, air containing 15.2 g of water vapour per kg has a relative humidity of:

Air-grain equilibrium

For a certain category of products and for a given temperature, an equilibrium in the exchange of water vapour between the grain and the air is represented by the curve called "hydroscopic equilibrium curve".

This curve shows, at a given temperature, the equilibrium between the moisture content of the grain (H%) and the relative humidity of the air (RH%).

The following graph gives the air-maize equilibrium curves at three temperatures (15°C, 20°C, and 35°C).

Air-maize equilibrium curve

As can be easily observed:

A) at 20°C, for a relative air humidity of 70 percent, equilibrium is reached when the moisture content of the grains of maize is 14 percent (point A);

B) ventilated by air at 55 percent relative humidity, the grains of maize lose moisture, and reach equilibrium when their moisture content is 12 percent (point B);

C) ventilated by air at 80 percent relative humidity, the grains of maize are rehumidified, and reach equilibrium when their moisture content is 16 percent (point C).

Drying process

Drying of products can thus be obtained by circulating air at varying degrees of heat through a mass of gram.

As it moves, the air imparts heat to the grain, while absorbing the humidity of the outermost layers.

Diagram of humidity exchanges between air and grain

In terms of physics, the exchange of heat and humidity between the air and the product to be dried is seen in the following phenomena:

• heating of the grain, accompanied by a cooling of the drying air;
• reduction in the moisture content of the grain, accompanied by an increase in the relative humidity of the drying air. But this process does not take place uniformly.

Indeed, the water present in the outer layers of grain evaporates much faster and more easily than that of the internal layers.

Thus it is much harder to lower the moisture content of a product from 25 to 15 percent from 35 to 25 percent.

It would be a mistake to think that this difficulty be overcome by rapid drying at high temperature. In fact, such drying conditions create internal tensions, producing tiny cracks that can lead to rupture of the grains during subsequent treatments.

For drying grain, essentially two methods are used:

• natural drying,
• artificial drying.

Both of them have advantages and disadvantages, and no ideal method exists that permits all needs to be met.

Natural drying

The natural drying method, which uses the techniques illustrated in the chapter on predrying, consists essentially of exposing the threshed products to the air (in sun or shade).

To obtain the desired moisture content, the grain is spread in thin layers on a drying-floor, where it is exposed to the air (in sun or shade) for a maximum of 10-15 days.

To encourage uniform drying, the grain must be stirred frequently, especially if it is in direct sunlight.

Furthermore, for drying to be effective, the relative humidity of the ambient air must not be higher than 70 percent.

For that reason, grain must not be exposed at night.

In fact, by bringing about an increase in the relative humidity of the air, the cold of the night fosters rehumidification of the grain.

For the same reasons, this method should not be used in humid regions or during the rainy season.

It must be remembered that insufficient or excessively slow drying can bring about severe losses of product during storage from the self-generated heat of "green" grain.

Finally, prolonged exposure of grain to atmospheric factors, and thus to pest attack (insects, rodents, birds) and micro-organisms (moulds), can also cause losses of product.

Despite these drawbacks, natural drying is advantageous in the following situations:

• when atmospheric conditions favour a reduction in moisture consent over a reasonably short time-span;
• when the quantities of grain to be dried are modest;
• when production organization and socio-economic conditions do not justify the cost of installing artifical drying equipment.

Artificial drying

The introduction of high-yielding crop varieties and the progressive mechanization of agriculture now make it possible to harvest large quantities of grain with a high moisture content in a short time.

In humid tropical and subtropical zones, given unfavourable weather conditions at harvest time, it is often difficult to safeguard the quality of products.

In order to satisfy the need for increasing agricultural production, it is therefore necessary to dry the products in relatively brief times, whatever the ambient conditions. Consequently, it is necessary to resort to artificial drying.

This method consists of exposing the grain to a forced ventilation of air that is heated to a certain degree in special appliances called "dryers".

Diagram of a static dryer: 1 Grain to be dried; 2 Hot, dry air; 3 Humid air.

Artificial drying and dryers

In its construction, the basic elements of a dryer are:

• the body of the dryer, which contains the grain to be dried;
• the hot air generator, which permits heating of the drying air;
• the ventilator, which permits circulation of the drying air through the mass of grain.

For artificial drying of grain, two types of dryer are used:

• static or discontinuous dryers;
• continuous dryers.

The former are inexpensive and can treat only modest quantities of grain; thus they are better adapted to the needs of small- and medium-scale centres for the collection and processing of products.

As for the latter, these are high-flow dryers that need a more complex infrastructure, complementary equipment and, above all, special planning and organizational They are therefore more appropriate for big centres, silos or warehouses, where very large quantities of product are treated.

Drying and static dryers

A current of hot air moves from bottom to top through a thick layer of grain.

Drying of the mass of grain does not take place in a uniform fashion: as it moves from the bottom to the top, the drying air imparts heat to the grain and absorbs moisture, losing its "drying power" in the process. The lower layers will therefore dry more rapidly than the upper ones.

During the drying process, the mass of grain is thus found to be divided into three areas:

• area of dry grain,
• drying area,
• area of humid grain.

The imaginary line separating the area of dry grain from the area undergoing drying is called the "drying front".

Diagram of the drying process: 1 Humid grain; 2 Drying area; 3 Drying front: 4 Dry grain.

In its slow movement from bottom to top, the drying front separates the already-dry grain from that which is undergoing or awaiting drying.

The speed with which the drying front moves depends on the characteristics of the drying air (temperature, relative humidity) and of the grain mass to be dried (type, moisture content, thickness of the layer).

When the drying is finished, the grain in the lower layers is in any case dryer than that in the upper layers.

To keep this difference in moisture content within acceptable bounds, it is important that the hot air used be of suitable temperatures and flows.

As an indication, let us say that a high temperature should entail a greater flow of air.

To reduce the risks of heterogeneity of moisture content, but also to limit the costs of the operation, prolonged drying of too-thick layers of grain should be avoided.

To obtain a more nearly homogeneous moisture content of the grain, it is possible to make provision for ventilation with air at ambient temperature, after the hot-air drying.

The construction and use of the various types of cabin dryers and radially ventilated dryers are based on these principles of operation and on these precautions.

In order to reduce still further the difference in moisture content of the grain, some dryers are equipped with special devices for stirring the grain during drying.

This is the principle behind circular dryers, for example, as well as "in-bin drying" and successive-lot dryers.

Drying and continuous dryers

A continuous flow of grain is passed in a thin layer through a shaft traversed by a current of very hot air. In its movement, the mass of grain is constantly stirred.

In this case, the mass of dried grain has a fairly uniform moisture content.

The temperature of the drying air must be kept within certain limits so as not to alter the food characteristics and germinative properties of the grain (temperature refers to the air, not to the grain). In fact, the grain is not in the very hot air current long enough to reach very high temperatures.

Diagram of a continuous dryer: 1 Grain to be dried; 2 Hot, dry air; 3 Humid air; 4 Dried grain.

During the final phase of drying, however, when the innermost moisture needs to be extracted, there is a risk of briefly overheating the grain.

It must also be remembered that rapid drying at a high temperature, as well as sudden cooling, can provoke cracking and breaking of the grains during milling.

To minimize this disadvantage, drying is sometimes done by swiftly putting the grain several times into the current of very hot air.

Between two of these passages, the grain is left at rest for several hours in order to homogenize the moisture content throughout the mass of grain.

Another system used to reduce the risks of breakage consists of exposing the grain to the hot drying air until the grain moisture content is slightly (2-3 percent) higher than that desired, then letting it rest for a few hours before submitting it to ventilation by ambient air.

The construction and use of various types of continuous dryers are based on these operating principles and precautions. There are vertical dryers (column, louvred, or baffle dryers) and horizontal or inclined dryers (in which the grain is moved by paddles or by conveyor belt, or on the cascade or mobile principle).

Main technical features of dryers

The most suitable choice and optimal use of a dryer depend on the relationship between certain technical features of the appliances and local production needs.

The main technical features of dryers are:

• evaporating power,
• air renewal, or specific flow,
• specific thermal consumption.

Evaporating power

In order to define the type of dryer needed, it must first be determined how much water per hour is to be eliminated during drying.

This figure should completely reflect local needs for drying the products. It can be deduced through analysis of the data on annual and seasonal production.

The technical feature that demonstrates a dryer's performance is its "evaporating power".

Evaporating power indicates the quantity of water a dryer can extract from the mass of product to be dried in an hour. Its unit of measurement is the kilogram of water evaporated per hour (kg water/in).

Although in their marketing literature manufacturers do not all explicitly show the evaporating power, it is the only feature by which the real performance of a dryer can be judged.

Indeed, if the evaporating power and the quantity of water per quintal of product to be eliminated are known, the "dryer output" (the quantity of product dried in an hour) can be calculated.

The unit of measurement of the dryer output is the quintal of product (wet or dry) per hour (q product/in).

Instead of indicating the evaporating power, many manufacturers usually indicate "dryer output" as the appliance's feature.

This indication is accurate only if it is specified for what initial and final degrees of humidity of the product this figure has been calculated, and if it stands in relation to the wet or dry product.

Other manufacturers often give the evaporating power, or rather the "evaporation capacity" of a dryer, in "points/hour": "point" means the quantity of water eliminated when the humidity of a quintal of the product is lowered by 1 percent.

Based on this notion of "point" of humidity, they claim to reduce the calculation of "dryer output" to a simple relationship between the evaporation capacity and the difference between the initial humidity of the product and its final humidity, after drying.

It can be seen, however, that for the same unit quantity of wet product, varying quantities of water to be eliminated can correspond to a "point" of humidity, depending on the final humidity desired.

Therefore, in order to show the true performance of the appliance, it is very important that manufacturers specify to what final product-moisture content the figure of the evaporation capacity of the dryer, expressed in points/hour, refers.

Air-renewal output

The "air-renewal" of a dryer, or its "specific flow", indicates the quantity of air per hour that passes through a cubic metre of product. Its unit of measurement is the cubic metre of air per cubic metre of product (m³/h/m³).

This feature, which is closely linked to evaporating power, basically depends on the power of the ventilation device and the thickness of the layer of grain to be dried.

High air-renewal (6 000 to 8 000 m³/h/m³) permits a shorter drying-time.

In these conditions, however, higher dryer flow can increase energy consumption and, above all, can increase the risks of cracking and breakage of the grains by too rapid drying.

On the other hand, more modest rates (2 000 to 4 000 m³/h/m³) permit better drying of the product but reduce the dryer flow rate.

The air-renewal of a dryer must thus be selected by seeking the best possible compromise between these factors and the local production conditions.

Specific thermal consumption

The "specific thermal consumption" of a dryer indicates the quantity of heat necessary to eliminate a kilogram of water from the mass of product to be dried.

This quantity of heat obviously includes both the heat used for heating (dryer, product) and evaporating the water, and the heat partly dispersed into the ambient air.

The unit of measurement of specific thermal consumption is the millitherm per kilogram of water evaporated (mth/kg water evaporated).

Discontinuous dryers generally have thermal consumption rates higher than 1 500 mth/kg water evaporated.

On the other hand, continuous dryers have a thermal consumption rate of between 850 and 1 200 mth/kg.

Manufacturers of dryers should always be explicitly asked to give the rate of thermal consumption.

Other drying methods

We have seen that natural drying is slow and entails great risks of loss of product.

On the other hand, artificial drying is fairly costly (purchase of dryers, use of fuel often derived from petrol, etc.).

To find a compromise solution to fulfil the needs of small rural communities, especially in humid tropical regions, a series of experiments was undertaken in the early 1970s aimed both at improving traditional methods of natural drying and at checking the validity of technological solutions that favoured the sun as a source of alternative energy.

These experiments not only added to the existing knowledge about drying procedures but also led to the construction of many prototypes of solar dryers.

With these appliances, drying of products is obtained:

• by a greenhouse effect produced within a drying-box exposed directly to the sun;
• by the circulation of heated air from solar captors;
• by the combined action of these two phenomena.

However, the relatively high costs of manufacture, a short life-span and variable performances have so far limited the adoption of solar dryers, particularly for drying grain.

Diagram of a solar dryer: 1 Solar panel; 2 Hot, dry air; 3 Grain to be dried.