Drying principles and general considerations

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Drying Mechanisms

In the process of drying heat is necessary to evaporate moisture from the grain and a flow of air is needed to carry away the evaporated moisture. There are two basic mechanisms involved in the drying process; the migration of moisture from the interior of an individual grain to the surface, and the evaporation of moisture from the surface to the surrounding air. The rate of drying is determined by the moisture content and the temperature of the grain and the temperature, the (relative) humidity and the velocity of the air in contact with the grain.

Figure 5.1 (see Figure 5.1. Drying and Drying Rate Curves.) demonstrates the drying of a single layer of grain exposed to a constant flow of air. The moisture content falls rapidly at first but as the grain loses moisture the rate of drying slows. In general the drying rate decreases with moisture content, increases with increase in air temperature or decreases with increase in air humidity. At very low air flows increasing the velocity causes faster drying but at greater velocities the effect is minimal indicating that moisture diffusion within the grain is the controlling mechanism.

Grains are hydroscopic and will lose or gain moisture until equilibrium is reached with the surrounding air. The equilibrium moisture content (EMC) is dependent on the relative humidity and the temperature of the air; EMCs for a range of grains are shown in Table 5.1 (see Table 5.1 Grain Equilibrium Moisture Contents).

The relationship between EMC, relative humidity and temperature for many grains has been modelled by numerous researchers; the results of which have been summarized by Brooker et al. (1974).

It is very important to appreciate the practical significance of the EMC. Under no circumstances is it possible to dry to a moisture content lower than the EMC associated with the temperature and humidity of the drying air; for example, the data in Table 5.1 show that paddy can only dry to a moisture content of 16.7% when exposed to air at 25°C and 90% relative humidity. If paddy at a moisture content less than 16.7% is required then either the temperature of the drying air has to be increased or its humidity reduced.

The drying of grains in thin layers where each and every kernel is fully exposed to the drying air can be represented in the form:

MR = f(T, h, t); (1)

where (the moisture ratio);

MC is the moisture content of the grain at any level and at any time, % dry basis (%db);

MC_e is the equilibrium moisture content (%db);

MC_o is the initial moisture content of the wet grain (%db);

T is the air temperature (°C);

h is the air relative humidity; and

t is the drying time.

Empirical data have been used to determine mathematical approximations of the relationship between drying rate and air conditions. Relationships for many grains have been summarized by Brook & Foster (1981). For example, a thin layer equation for paddy (Teter 1987) is:

MR = exp.(-X * t^Y); (2)

where X = 0.026 - 0.0045h + 0.01215T; and

Y = 0.013362 + 0.194h - 0.000177h² + 0.009468T,

with h expressed as a percentage, and T in °C.

In the drying of grain in a deep bed, whilst individual kernels may all be losing moisture at different rates, the overall drying rate will remain constant for a long period. The air absorbs moisture as it moves through the bed until it becomes effectively saturated and moves through the remaining layers of grain without effecting further drying. Figure 5.2A (see Figure 5.2. Drying Zone in Fixed-bed Drying.) shows the three zones present within a thick drying bed at an intermediate time within the drying operation. Drying takes place within a discrete zone, the size of which depends on the moisture content of the grain and the temperature, humidity and velocity of the air. Below the drying zone is the dried zone where the grain is in equilibrium with the air. Above the drying zone is the un-dried zone wherein the grain remains unchanged from its initial condition. In a shallow bed as in Figure 5.2B the drying zone is thicker than the bed depth and drying would occur initially throughout the bed.

The change in temperature and humidity of air as it moves through a bed of grain depends on the rate at which moisture is being evaporated from each kernel as an individually exposed element. Knowledge of the effect of grain moisture content, other grain properties, the temperature, humidity and flow rate of the air upon fully exposed kernels is essential to an understanding of how drying would proceed within a bed.

Unfortunately no theory has been developed that accurately and practically describes the thin layer drying rate. As described above many empirical relationships have been established and these have to be used in prediction of drying time (see below). Accurate prediction of drying time is further inhibited by the variability of key factors encountered in practice, particularly so for the simple drying systems that are the most appropriate for use in developing countries. For example the moisture content of individual grains is likely to vary considerably within a batch and in the case of drying with a heater of constant heat output the temperature of the drying air will vary with changes in ambient air temperature.

Air Properties

The properties of the air flowing around the drying grain are a major factor in determining the rate of removal of moisture. The capacity of air to remove moisture is principally dependent upon its initial temperature and humidity; the greater the temperature and lower the humidity the greater the moisture removal capacity of the air.

The relationship between temperature, humidity and other thermodynamic properties is represented by a psychrometric chart as shown in Figure 5.3 (see Figure 5.3. CIBS Psychrometric Chart.). It is important to appreciate the difference between the absolute humidity and relative humidity of air. The absolute humidity is the moisture content of the air (mass of water per unit mass of air) whereas the relative humidity is the ratio, expressed as a percentage, of the moisture content of the air at a specified temperature to the moisture content of air if it were saturated at that temperature.

The changes in air conditions when air is heated and then passed through a bed of moist grain are shown in Figure 5.4 (see Figure 5.4. Representation of Drying process.). The heating of air from temperature TA to TB is represented by the line AB. During heating the absolute humidity remains constant at HA whereas the relative humidity falls from hA to hB. As air moves through the grain bed it absorbs moisture. Under (hypothetical) adiabatic drying sensible heat in the air is converted to latent head and the change in air conditions is represented along a line of constant enthalpy, BC. The air will have increased in both absolute humidity, Hc, and relative humidity, hc, but fallen in temperature, Tc. The absorption of moisture by the air would be the difference between the absolute humidities at C and B. (HC-HA)

If unheated air was passed through the bed the drying process would be represented along the line AD. Assuming that the air at D was at the same relative humidity, hc, as the heated air at C then the absorbed moisture would be (HD-HA), considerably less than that absorbed by the heated air (HC-HA)

Physical Properties of Grain

Comprehensive data on the numerous physical and thermal properties of grain are available in texts such as Brooker et al. (1974) and Brook & Foster (1981).

Moisture Content.

Convention dictates that moisture contents of grains are usually measured on a wet basis, ie the mass of moisture per unit mass of wet grain and written as X % (wb). The alternative measure refers to the measurement on a dry basis (X%(db)) which is the mass of moisture per unit mass of completely dry grain. Conversion between the two measurements is shown in Table 5.2. All moisture contents given in the text are on a wet weight basis, unless otherwise stated. Table 5.3 (see Table 5.3. Moisture loss during drying.) shows the mass of water lost from wet grain during drying for a range of initial and final moisture contents.

Table 5.2. Conversion of Moisture Contents.

Bulk Density.

The bulk density of grain is the weight per unit volume. Moisture content has an appreciable effect on the bulk density (see Chapter 3 for more detail).

Resistance to Air Flow.

The energy required to force air through a bed of grain is dependent on the air flow, the grain depth and physical properties of the grain such as surface and shape factors, the kernel size distribution, moisture content, and the quantity and nature of contamination, stones, straw, weeds etc. The relation between air flow and the pressure drop generated across the bed for selected grains is illustrated in Figure 5.5 (see Figure 5.5. Resistance of Grains and Seeds to Air Flow.). The data generally refer to clean and dry grain and correction factors of up to 1.4 are used for very wet and dirty grain (Teter 1987).

Latent Heat of Vaporization. Energy in the form of heat must be supplied to evaporate moisture from the grain. The latent heat of vaporization, Lh, for a grain depends on its moisture content and temperature and is appreciably greater than the latent heat of evaporation of water. The latent heat of vaporization for paddy at selected moisture contents and temperatures is shown in Table 5.4 (see Table 5.4. Latent Heat of Vaporization of Paddy.). Data for other grains have been reported by Brooker et al. (1974)

Estimation of Drying Time

A basic design procedure for the field worker is best illustrated for the design of a batch type dryer although the principles can be applied to a certain extent in the design of continuous multi-stage systems.

Assumed ambient air conditions are a dry bulb temperature of Ta and a relative humidity of RHa; from the psychrometric chart the wet bulb temperature, Twa, the enthalpy, Ha, and the absolute humidity ha can be derived. The air is heated to a selected safe drying temperature, Tb, thereby raising the enthalpy of the air to Hb.

The wet grain of equivalent bone-dry mass G has a moisture content of MCw%(db) and is to be dried to a moisture content of MCd%. A mass air flow of V is available.

The moisture, M, to be removed,

M = G * (MC_w - MC_d); (3)

It is assumed that throughout the drying period the air will exhaust from the bed at a constant wet bulb temperature and in equilibrium with the uppermost layers of grain. Initially the exhaust air will be in equilibrium with grain at MCw moisture and finally in equilibrium with grain at MCd moisture. By superimposing equilibrium moisture content data on to the psychrometric chart for the initial and final moisture contents the humidity of the exhaust air at the beginning and end of drying can be found. An average of the initial and final exhaust air relative humidities, hea is taken for calculation of drying time, td:

; (4)

An alternative, more complex but more accurate method for the estimation of drying performance is the technique based on dimensionless drying curves as initially developed by Hukill (1947). The methodology permits the estimation of the moisture content of grain at any level within the bed at any time after initiation of drying. It can be used for any grain for which EMC and thin layer drying data are available as is the case for most cereal grains.

The methodology involves the use of bulk drying curves as depicted in Figure 5.6 and calculation of three parameters, moisture ratio, time unit and depth factor.

The moisture ratio, MR is calculated from Equation 1:

The time unit, Y, is calculated using the equation:

where t0.5 is the half-response time, the time required for fully exposed grain to reach a moisture ratio of 0.5 under the drying air conditions employed. It can be calculated from the thin layer drying equations as in Equation 1 with MR assigned a value of 0.5.

The depth factor, D, is defined as the depth of the bed that contains the mass of grain, DM, that can be dried from the initial moisture ratio MR = 1 to a final moisture ratio MR = 0 with the sensible heat available over the period of one half-response time as the air cools to its wet bulb temperature. DM is calculated thus:

; (6)

where C_p is the specific heat of air. The number of depth factors within the bed is found from the expression:

; (7)

where d is the bed depth.

The curves in Figure 5.6 are represented by the equation:

; (8)

By transposing the drying conditions to these units and using either Figure 5.6 (see Figure 5.6. Dimensionless Drying Rate Curves.) or Equation 8 it is possible to predict when any layer within the bed reaches a desired moisture content.

More rigorous approaches to the design of drying systems have been developed. These include the methods based on thin layer drying equations described by Brook & Foster (1981) and Brooker et al. (1974). Many of these have been developed into sophisticated simulation techniques (Brooker et al. 1974).

The drying conditions for specific grains and situations are many and varied. Drying will take place under any conditions where grain is exposed to a flow of unsaturated air. Very fast drying can be accomplished using large volumes of high temperature air but, if carried through to completion, is likely to be inefficient in energy use and liable to damage the grain by over-heating and/or over-drying. Conversely slow drying, as in sun drying in inclement weather, provides conditions for continued respiration and deterioration of the grain leading to both quantitative and qualitative losses and the growth of moulds.

Drying Efficiency

The efficiency of the drying operation is an important factor in the assessment and selection of the optimum dryer for a particular task. There are three groups of factors affecting drying efficiency:

* those related to the environment, in particular, ambient air conditions;
* those specific to the crop;
* those specific to the design and operation of the dryer.

There are several different ways of expressing the efficiency of drying, of which the sensible heat utilization efficiency (SHUE), the fuel efficiency, and the drying efficiency are the most useful.

The SHUE takes into account the sensible heat attributable to the condition of the ambient air and any heat added to the air by the fan as well as the heat supplied by combustion of the fuel. It is defined as:

The fuel efficiency is based only on the heat available from the fuel:

It can be appreciated that the fuel efficiency would be significantly different for the operation of the same dryer at two locations with widely different ambient conditions. With low temperature drying, particularly in dry climates, the heat supplied from the fuel may be less than half of the total sensible heat and the fuel efficiency may exceed 100%. Direct comparison of the performance of dryers at separate locations is not possible using the fuel efficiency.

The drying efficiency, defined as:

is the expression to be used for evaluation of dryer designs or comparison between dryers, since it is a measurement of the degree of utilization of the sensible heat in the drying air.

Foster (1973) evaluated the fuel and drying efficiencies of several types of dryers used with maize. Over a wide range of conditions, continuous-flow dryers were found to have a fuel efficiency of 38% and a drying efficiency of 51%, batch dryers 42% and 58%, dryeration 61% and 78%, and two-stage drying, 60% and 79%, respectively.

Effect of Drying on Grain Quality

The drying operation must not be considered as merely the removal of moisture since there are many quality factors that can be adversely affected by incorrect selection of drying conditions and equipment. The desirable properties of high-quality grains include:

• low and uniform moisture content;
• minimal proportion of broken and damaged grains;
• low susceptibility to subsequent breakage;
• high viability;
• low mould counts;
• high nutritive value;
• consumer acceptability of appearance and organoleptic properties.

Moisture Content. It is essential that the grain after drying is at a moisture content suitable for storage. As discussed the desired moisture content will depend on the type of grain, duration of storage, and the storage conditions available. It is also important that the drying operation is carried out to minimize the range of moisture levels in a batch of dried grain. Portions of under-dried grain can lead to heating and deterioration.

Stress Cracking and Broken Grains. Drying with heated air or excessive exposure to sun can raise the internal kernel temperature to such a level that the endosperm cracks. The extent of stress cracking is related to the rate of drying. Rapid cooling of grain can also contribute to stress crack development.

Nutritive Value. Grain constituents such as proteins, sugars and gluten may be adversely affected when the grain attains excessive temperatures. The feeding value of grains can be lowered if inadequately dried.

Grain Viability. Seed grain requires a high proportion of individual grains with germination properties. The viability of grain is directly linked to the temperature attained by grains during drying (Kreyger 1972).

Mould Growth. Many changes in grain quality are linked to the growth of moulds and other microorganisms. The rate of development of microorganism is dependent on the grain moisture content, grain temperature, and the degree of physical damage to individual grains. Mould growth causes damage to individual grains resulting in a reduction in value. Under certain circumstances mycotoxin development can be a particular hazard.

Appearance and Organoleptic Properties. The colour and appearance as perceived by the customer and/or consumer. For example, the colour of milled rice can be adversely affected if the paddy is dried with direct heated dryers with poorly maintained or operated burners or furnaces.

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