Batch and continuous drying
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by Maitri Naewbanij and Viboon Thepent
Drying is one of the crucial steps in food processing and preservation. It is the means of providing grain at optimum moisture content for processing, such as rice milling and wheat milling processes, as well as rendering grain moisture content at stage where moisture is unavailable for mould growth. In preserving grain without deterioration, drying is the cheapest among other methods, ie, chemical application and controlled atmosphere storage. Drying methods and operation are rather specific for each kind of grain and its use. Skilled operators are often required for the drying operation.
PRINCIPLE OF DRYING
Drying is the process of moisture removal from the product, or grain in this case. Since grain is a hydroscopic material which can either absorb or desorb moisture from the air or its surroundings depending on the difference in vapour pressure, moisture transferred from a higher vapour pressure, to the lower one. In the sundrying process, grain is heated by solar radiation thus creating a higher vapour pressure in grain than the surrounding air. In the same manner, the heated air drying process starts when the grain is heated (by conduction) when it comes in contact with the air. Higher velocity air flow in heated air drying has the advantage of reducing the boundary layer of the grain, thereby increasing the heat transfer coefficient of the grain, as well as increasing the rate of moisture movement from grain to the surrounding air. Therefore, the drying rate of a specific kind of grain is dependent on both air temperature and air flow rate.
When the heated air is forced through the grain mass, the vapour pressure of the grain increases as the temperature of the grain increases. The vapour pressure at the grain surface increase rapidly, since the surface of grain is heated first during the initial stage of drying. Center grain temperatures rise slowly as the moisture at the center of grain slowly migrates to the drying air. Drying rates of grain, therefore, can be characterised into four stages (Fig. 1 Illustration of constant rate and falling-rate drying periods.), the heating period (AB), the constant rate period (BC), the first falling rate period (CD), and the second falling rate period (DE).
The constant rate period occurs during the drying of the surface moisture. In this period, the grain temperature does not increase appreciably due to the effect of evapourative cooling. Removal of moisture from the grain is fastest during this period.
The falling rate periods occur after the removal of the surface moisture or during the removal of moisture from within the grain. During this period of drying, grain temperature is slowly by the incoming hot air, thus, the vapour pressure within the kernel increases and slowly moves out to the drying air. Drying at this stage is dependent on grain characteristics, temperature of the kernel, and the vapour pressure of the hot air.
CALCULATION OF MOISTURE CONTENT
Grain moisture content may be expressed as percentage moisture on dry matter and percentage moisture of wet grain depending on the purpose for its use. In research or scientific purpose, the moisture content of grain is usually expressed as percentage of moisture on dry matter basis, which is expressed as in equation (1)
M(db) = moisture content, % dry basis.
For commercial purposes, the grain moisture content is expressed as percentage moisture on wet grain basis. This is so because most of the moisture meters can only measure the grain moisture content accurately in percentage moisture on wet grain basis. Mathematically, A can be expressed as :
Calculation of moisture reduction or weight of water removed from grain is one of the many important requirements in designing a grain dryer. It gives the rough idea of how much air, fuel and power are needed to dry a certain kind of grain.
If initial moisture content of grain (M1), initial weight (W1) and desired moisture content (M2) are determined, then final grain weight (W2) is calculated by the following equation:
Determine the heat requirement to dry 2 tonnes of corn if the initial moisture content is 25% and is to be dried down to 15%. Assuming the heat utilisation factor of the drying system is 80% with the air at temperature remaining constant at 90°C. Ambient air conditions may be taken as 30°C, 7.0% R.H, and air is supplied at the rate of 0.33 cu.m/s/cu.m of grain.
Amount of water needed to evaporate from corn can be calculated as;
Wt. of water at 25% MC = 0.25 x 2000 kg = 500 kg
Wt of dry matter = 200 - 500 = 1500 kg
0.15 (Wt. of water + 1500) = Wt. of water
Wt. of water 15% = = 264.71 kg
Thus, amount of water to be removed from corn is,
500 - 264.71 = 235.29 kg
Method B: Using equation (3) to determine final weight
Amount of water to be removed = W1 - W2 = 2000 -1764.71 = 235.29 kg
Calculation for the drying air requirement to evapourate 235.29 kg of water from corn requires the knowledge of psychrometrics.
The psychrometric chart tells the property of air at any given air condition. It contains five types of lines having the vertical axis of absolute humidity (ratio of moisture contain in a unit weight of air), the horizontal axis of air temperature. The curved line upward from left to right is the relative humidity line (percentage of moisture to the potential moisture holding capacity of a specific moisture holding capacity of a specific volume of air at given condition), the slanted line is the wet bulb temperature which is consiged with the constant enthalpy (constant heat energy) line, and the incline upward from left to right line is the specific volume line. To be able to use this chart, we must know the characteristic of processes described on the chart and two properties of air at a given condition. That is when a point is located, then a line can be drawn according to the process specified. The processes discribed on the psychrometric chart are shown in Figure (see Fig.2 A skeleton psychrometric chart) (2).
Thus given the ambient air condition, 30°C or 86°F and 70% R.H., the initial point (1) can be located on the psychometric chart (Fig 3 Psychromatric chart: high temperature). Point (2) is determined by the heating process or air temperature raised to 90°C (194°F). Point (3) is determined by following the wet bulb temperature line up to the temperature of the exhaust air. Temperature of the exhaust air (air temperature leaving the grain mass) can be determined from the given heat utilisation factor (HUF) and the air temperature at point (1) and point (2) as follows:
The amount of air required can, therefore, be calculated by dividing the amount of water to be removed form corn by the difference in reading of the absolute humidity values between point (3) and point (2), that is:
(sp.vol.of air = 0.88 cu. M/kg)
The heat requirement can be calculated by multiplying the total weight of air requirement by the different in readings of enthalpy of point (2) and point (1), or calculated
Total heat required 12,383.68 (143.5 78) = 811,131.04 kj
In the case where air is supplied at the rate of 0.33 cu.m/sec per cu.m of grain then the burner capacity can be computed as;
for shelled corn, the specific volume = 720 kg/cu.m
volume of 2 tonnes corn = = 2.78 cu.m
Total air folw = 0.33 x 2.78 = 0.9174 cu.m/s = 3300 cu. m/hr
Drying time requited = = 3.3 hr
and burner capacity
Batch drying is a system at which a certain volume of grain is being dried at a time. The volume is fixed by the holding capacity of a dryer, and dried to required moisture. After unloading the dried grain from the dryer, then drying for the next batch of grain can be performed. Batch drying, therefore, requires loading and unloading time for each batch of grain needed to be dried.
CONFIGURATION OF BATCH DRYING BINS
Several configurations and designs of batch drying bin have been manufactured and are widely used in many countries around the world. They may be stationary bins or portable bins. However, the batch drying bins can be classified into two groups; (a) static grain dryer, and (b) mixing - grain dryer.
The static - grain dryer has many configurations as shown in Figure (see Fig. 4 Principle of static - batch grain dryers)(4). They may be a thin layer bed, thick layer bed, vertical column thin layer, and round structure radial dryers. The static - grain dryer has a disadvantage of over-drying the grain nearest to the wall of the incoming hot air. Grain nearest to the wall of the incoming hot air loses its moisture first to the hot air, while temperature of the air dropped, at the same time the air absorbs moisture given off from the grain. Thus, the hot air reduces its temperature and capacity while moving through the grain bed. The difference in grain moistures between the layer nearest to the hot air and the outer layer varies with grain thickness, temperature of hot air, and airflow rate.
The mixing - grain batch dryer is designed to come over this drying problem of the static grain dryer. Design of the dryers usually is of a tall vertical drying column to facilitate the mixing operation. Mixing is achieved by recycling the discharged grain back to the dryer 3-4 cycles before its moisture reduced to the desire level. A fairly uniform grain moisture content may be obtained by this dryer if the recycling time is short. Figure (5) shows various designs of the mixed grain batch dryer.
The term continuous drying is where grain is continuously flowing though a dryer without stopping. The dryer itself has the same features as that of the mixed grain dryer. However, it requires several buffer bins for holding the discharged gram. The operator for the continuous flow dryer must have knowledge of grain drying management to programme the dryer to operate at its maximum efficiency. The continuous drying system offers the lowest operating costs as compared to batch drying systens Furthermore, a uniform grain moisture content after drying is obtained, and the drying capacity of the continuous flow dryer is higher than that of the mixing grain batch dyer if the same dryer and drying conditions are used.
Continuous flow drying is usually employed in relatively large grain complexes. The system can handle a large quantity of grain, and offer greater flexibility for drying operations. Figure 6 shows the typical operation pattern of the continuous drying system. Wet grain after being cleaned is put into the continuous dryer which continuously discharges grain into the buffer bins. Moisture content of the grain after passing the dryer may be reduced 2 to 4% depending on the grain moisture content at intake, drying air temperature and the air flow rate. The grain is held in the buffer bins 6 hours or more allowing for the equalisation of moisture within the kernel. For rice drying, the reduction of grain moisture should not go beyond 2% for each pass. Higher rates of moisture reduction causes crack or fissure development within the kernel. The partially dried grain after being tempered in the buffer bin is recycled into the dryer until the desired moisture is reached. The final pass before moving grain into storage is the cooling phase. This is to bring down the grain temperature to that of the surrounding ambient air temperature. Grain continues giving off moisture until the surrounding air reaches equilibrium conditions with the vapour pressure inside the grain. Cooling down of the inter stitial air causes condensation to occur on the grain surface, thus, a favourable condition for mould growth.
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