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Basic principles of grain drying
by Prof. F.W. Bakker-Arkema
ABSTRACT
A thorough understanding is required of the principles of drying and the fundamentals of grain deterioration in order for the agricultural engineer to adequately design a grain drying system. The topics of psychrometrics, equilibrium moisture content, grain quality, air movement, and drying theory are reviewed at the technical level required to fully comprehend subsequent papers to be presented at the grain drying seminar in Nanjing, China. This review closely follows six chapters from the standard book on grain drying by Brooker et al. (1974), which in the Chinese translation, is used as a text during the seminar. The author's five papers along with the Brooker textbook should aid in acquainting Chinese engineers with the latest designs and operations of grain dryers for small-scale and largescale installations, and in transferring up-to-date drying technology from abroad into China.
INTRODUCTION
Cereal grains and legumes are usually harvested at moisture contents too high for safe storage. Thus, drying is a necessity. A large amount of water (103.5 kg per tonne of wet material) has to be removed in drying wet grain from 22 to 13% (w.b.)*. Sufficient drying air has to be provided to the grain to assure that drying to safe-storage moisture contents is completed before microbial deterioration of the grain commences. This is the main objective of all sun and mechanical grain drying systems.
Although much grain in the world is still sun. dried, this review of drying principles will stress the mechanical drying of the crop in bulk. The paper can be considered a synopsis of the book "Drying Cereal Grains" by Brooker, Bakker-Arkema and Hall. Each section represents one Chapter in this text.
Proper understanding of the fundamentals of grain drying requires a basic understanding of the topics of psychrometrics, grain deterioration, grain moisture equilibrium, air movement, and drying theory. Each of these topics will be reviewed.
MOIST AIR PROPERTIES
This section is covered in detail in Chapter 2 of Brooker et al. (1974).
The air to be used in grain drying can be considered a mixture of dry air and water vapor. Psychrometrics refers to the thermodynamic relationships between dry air and water vapor.
There are three air humidity terms which need to be understood by grain dryer operators: vapor pressure, relative humidity, and absolute humidity. The water vapor pressure is the partial pressure exerted by the water vapor molecules in moist air; at saturation it is called the saturated vapor pressure. The relative humidity is the ratio of the actual water vapor pressure in the air to the water vapor pressure in saturated air; the relative humidity is expressed as a decimal or percentage. The absolute humidity is the weight of water vapor in the air per unit weight of dry air; absolute humidity values of drying air range from 0.005 to 0.1 kg/kg. Each of the three air humidity terms is used frequently in grain drying calculations.
Three air temperature terms require consideration in dryer design. dry-bulb, wet-bulb, and dew-point temperature. The dry-bulb temperature is the value registered by an ordinary thermometer. The wetbulb temperature is the temperature indicated by a wetwick covered thermometer with air passing over the wick at a speed of at least 5 m/s. The dewpoint temperature is the temperature at which condensation occurs if moist air is cooled at constant absolute humidity. The three temperature terms are of significance in understanding grain drying principles.
Two additional thermodynamic terms of the drying air are required in grain drying calculations: enthalpy and specific volume. The enthalpy of moist air is the heat content of the air per unit weight of dry air above a certain reference temperature. The specific volume of moist air is defined as the volume per unit weight of dry air. Note that both of these thermodynamic properties are defined in terms of dry air; the same is true for the absolute humidity.
In Chapter 2 of Brooker et al. (1974), equations are given for calculating each of the eight thermodynamic properties of moist air defined in the previous paragraphs. Because the thermodynamic properties of air are so frequently needed in analyzing grain-drying calculations, charts have been constructed, for various dry-bulb temperature ranges and atmospheric pressures, of the values calculated from the Brooker psychrometric equations. These charts are called psychrometric charts.
The vertical axis of the psychrometric chart usually represents the absolute humidity (and the vapor pressure), the horizontal axis the dry-bulb temperature. Diagonal lines represent constant enthalpy (and wetbulb) values. The relative humidity lines are curved. The specific volume lines are drawn obliquely to the horizontal axis. If two of the thermodynamic values of the air are known, the other properties can be read directly from the psychrometric chart.
Several processes in grain drying can be followed directly on the psychrometric chart. These include: heating, humidifying, condensing and drying.
GRAIN QUALITY
This material is covered in more detail in Chapter 3 of Brooker at al. (1974).
The objective of post-harvest drying is to maintain the desired qualities of the grain. It depends on the enduser of the grain which grain quality factor is most essential. Desirable properties of the dried grain to be considered by the grain dryer designer might include: (1) appropriately low and uniform final moisture content, (2) low moldcount of the dried kernels, (3) low percentage of broken and damaged kernels, (4) high viability (seed), (5) high head-yield (rice), (6) high bakingquality (wheat), (7) high oil-recovery (soybeans), (8) high starch-yield (sorghum), (9) high protein-content (wheat), and (10) high test-weight (maize).
Many countries have official grain standards under which grain is traded; unfortunately, no international standards have as yet been adopted. Few of the properties listed in the previous paragraph are contained in the grain standards. In the United States, the standards for the six grades of paddy includes maximum limits of heat damaged, chalky and physically damaged kernels, and specified color requirements; moisture content is not part of the grades. In the Philippines, the five standard grade requirements for paddy include maximum limits of nine factors, including foreign matter, moisture content, and cracked kernels. The United States standard for the six grades of maize includes the factors of moisture content, test weight, broken kernels and foreign material, and damaged kernels.
The drying-air temperature can have a significant effect on grain-quality although it should be emphasized that the kernel-temperature rather than the drying-air temperature should be considered in assessing kerneldamage. In many grain dryers, there is no significant difference between the air and kernel temperatures (e.g. in in-store dryers), but in some dryer designs the maximum kernel-temperature is far below the inlet airtemperature (e.g. in concurrent-flow dryers). The maximum allowable grain-temperature depends on (1) the use to be made of the grain, (2) the moisture content of the grain, and (3) the type of grain. For seed grain above 24% (w.b.) moisture content the safe dryingtemperature is 43°C, and below 24% (w.b.) is 49°C; for milling-wheat above 25% (w.b.) the maximum temperature is 60°C, while at moistures below that level 66°C drying air can be used. In feedgrain, kerneltemperatures in the 100-120°C range do not affect the nutritive value of the grain but may increase the susceptibility of the kernels to breakage.
Research is being conducted to quantify the grainquality deterioration during the drying process. First order reaction equations have been developed for the decrease during drying in seed-viability of wheat and the increase in breakage susceptibility of maize. A somewhat different set of equations has been developed for the dry-matter loss and mold development during lowtemperature in-store drying.
GRAIN EQUILIBRIUM MOISTURE CONTENT
Chapter 4 in Brooker et al. (1974) should be consulted for additional information on the topic
The concept of equilibrium moisture content (EMC) is important in grain drying because the EMC determines the minimum moisture content to which a grain is dried under a given set of drying conditions. The EMC is defined as the moisture content of a biological product after it has been exposed to a particular environment for an infinity long period of time. The EMC of grain is dependent upon the humidity and temperature of the air, and on the grain-type, grainvariety, grain-maturity, and grain-history.
As an example of the effect of grain-type, consider 16% (w.b.) moisture content wheat and oats stored at 30°C and 75% relative humidity. Because of the difference in the equilibrium moisture contents of the two crops at 30°C-75% RH, the wheat will absorb moisture while the oats will lose water.
EMC values of different grains have been determined over a wide range of temperatures and relative humidities. These values are available in the literature in table and in graph form. The graphs are known as EMC isotherms, and are plots at a particular temperature of the percent moisture content (on the coordinate) versus the percent relative humidity (on the abscissa).
To facilitate dryer design calculations, equations have been developed for the sigmoid-shaped EMC isotherms. International agreement appears to have been reached to recommend the theoretically-based Guggenhein-Anderson-de Boer (GAB) isotherm for the calculation of the EMC values of all food products including grains. The empirical isotherm equation developed specifically for grains by Chung-Pfost has been used extensively in the past for grain dryer design.
EMC data allow calculation of the heat of vaporization of moisture from the grain. This value is a measure of the energy requirement to dry grain at different moisture contents and temperatures. Knowledge of the heat of vaporisation of a grain is essential for the calculation of the fuel consumption of the crop during the drying process.
AIR MOVEMENT
The topic of air movement is discussed in more detail in Chapter 5 in Brooker et al. (1974).
Drying air fulfills two functions in a mechanical grain drying system: (1) to carry the necessary energy to the grain to evaporate the moisture, and (2) to carry the evaporated water out of the grain mass. When air is forced through a bed of grain, resistance to the flow develops because of friction and turbulence. The resistance, called the pressure drop, is overcome by providing an excess pressure on the air entrance side of the grain mass, or by providing a vacuum on the air exit side. The pressure drop through the layer of grain depends on the rate of airflow, the physical characteristics of the grain kernels, the bed porosity, the thickness of the layer, the percentage of impurities in the grain, and the method of filling the dryer.
Pressure drop data for grains and legumes have been determined experimentally over a wide range of airflow rate. The data is usually plotted on log-log paper in terms of mm of water column (or Pascal) per meter of bed depth versus airflow rate in m³ per sec per m² of bed area Equations are available in the literature expressing the pressure drop through a grain mass in terms of airflow rate, percentage of fines, moisture content, and bed depth.
In addition to the resistance in the grain, the drying air can encounter resistance in the air-ducting system of the dryer. The sum of the grain and ducting resistances represents the system resistance of the dryer.
The air-moving device used in grain drying systems is the fan; it should be able to deliver the specified volume of air at the correct pressure Two types of fans are in use in grain-drying installations, the axial-flow and the centrifugal. Axial-flow fans have one or more impellers (with radial blades) rotating within a cylindrical casing; air flows parallel to the axis of the fan shaft. Centrifugal fans have an impeller (with blades around its periphery) rotating within a scroll-shaped casing; the air enters parallel to the impeller shaft and is fumed 90° before discharge Axial fans are noisier than centrifugal fans, operate at lower pressures, require less space, and are in general less expensive in delivering a certain air volume.
The performance of geometrically similar fans is governed by a series of fan laws which express the effect of speed of rotation of a fan on the volume of air delivered, the pressure developed and the power required. Proper matching of the pressure drop versus airflow curve of a fan rotating at a certain speed with that of a drying system results in the operation of the fan at the desired characteristic pressure and airflow rate of the dryer. If the match between fan and drying system is correct, the fan will operate close to the optimum efficiency range of the fan, and near the rated horsepower.
GRAIN DRYING CALCULATIONS
This section parallels Chapter 8 in Brooker et al. (1974).
Drying is a process of simultaneous heat and moisture transfer. A number of biological products, when drying as single particles under constant external conditions, exhibits a constant-rate moisture loss during the initial drying period, followed by a falling-rate drying period. Grains and legumes, however, dry entirely within the falling-rate period.
In order to model the drying of a grain dryer, the drying-rate characteristics of the individual kernels have to be known in terms of the moisture and temperature changes occurring at different drying conditions. The drying rate equation fulfills this purpose.
Due to the complexity of the falling-rate graindrying process, engineers prefer to lump the effects of the different physical drying-transport mechanisms (ie. liquid diffusion, capillary flow thermal diffusion, vapor diffusion) into a simplified semi-theoretical diffusion equation with a concentration and temperature dependent diffusion-coefficient. Values for the effective diffusion-coefficient have been published for most grains including paddy and maize. A purely empirical dryingrate expression, the so-called "thin-layer" equation, is frequently used by grain dryer designers; values for the "drying constant" of cereals and legumes are available in the literature.
Thin-layer (or diffusion) equations describing the drying-rate of individual grains are an essential part of deep-bed grain-drying models of in-store and continuous-flow dryers. There are basically two types of drying models, the differential-equation type and the heat-mass balance type. Each can be divided into nonequilibrium and equilibrium models. The nonequilibrium/differential-equation grain-drying models are the most fundamental and general in nature, and give the most accurate predictions of the dryingrate, the moisture content distribution, and the energy consumption of a particular dryer-type, regardles of its configuration or grain-type.
REFERENCES
Brooker, D B., Bakker-Arkema, F.W., and Hall, C.W. 1974. Drying Cereal Grains. AVI Publishing Co., West port, CT. U.S.A. Available in Chinese Translation.