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Ashley L. Marbella January 7,
2011
A-2L
Exercise No. 5
Drying of Palay Grains
Introduction
Drying is a technology used in the preservation of grains. The grain is dried to
the level in which the level of moisture is safe for storage. Solar drying was being
used since ancient times. This process involves the transient transfer of heat and
mass in the product. Drying occurs by the evaporation of the liquid in the grain due
to heat (Chakraverty et al 2003).
In convective type dryers, hot air is used as a source of heat in evaporation,and as a medium for transport of evaporated moisture. There is a direct contact
between the material and the hot air in convection drying. In this drying system,
blower, duct, perforated floor, and exhaust are required (Chakraverty et al 2003).
The moisture content of the grain tends to balance with that of the
surrounding. The dry air absorbs the moisture from the grain. The humidity and
temperature of the air dictate the amount of water that can be absorbed by the
volume of air. When water from the surface of the grain evaporates, the water
inside the grain migrates to the surface. Water moves from moist area to the area
of less moist ( Dendy et al 2001).
Methodology
The initial moisture content of the palay grains was determined using the
Dole moisture meter. Three measurements were taken, and the average value was
recorded.
Two 250 g of the grain samples were weighed and placed in two drying
containers. The samples were placed at the laboratory dryer. The fan was activated,
and the velocity of the air was measured. The relative humidity of the air was
determined using the psychrometric chart. The heater switch was turned on, and
the temperature at the chamber was observed to be at 50 degrees Celsius. The
weight of the sample was measured for every thirty minute interval. The operation
was done until four data points were obtained. The final moisture content was
determined.
Results and Discussion
The total amount of moisture in kg removed from the sample was 30.575.
The drying rate was highest at the beginning of drying. The average drying rate was
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0.54 kg water per second. After 120 minutes, the drying rate calculated was 0.1 kg
water per second. The average amount of heat required to raise the air temperature
from ambient to drying temperature was 6.39 kJ/s. The drying constant is 0.0129.
The heat utilization factor is 0.230769.
Table 1. Drying characteristics of palay grain
Period of Drying
Amount ofmoistureremoved (kg)
Drying rate (kgwater/s) Q (kJ/s)
Initial (time = 0)
After 30 minutes 16.115 -0.425 6.359012
After 60 minutes 6.67 0.083333 8.395534
After 90 minutes 4.81 0 5.637179
After 120 minutes 2.98 -0.33333 5.173006
Average 6.391183
Total 30.575
Figure 1 shows the weight of the sample versus drying time in minutes. The
weight of the sample decreases over time. Based on the graph, the change in
weight over time was highest after thirty minutes. The smallest slope was at after
120 minutes.
Figure 1. Sample weight vs time
The moisture content wet basis versus time was shown in figure 2. The
moisture content of the grain is indirectly proportional to time. The final moisture
content of the sample was 12.65 %.
Figure 2. Percent Moisture Content versus time
If the grain sample was allowed to be dried for an extended period of time,
the moisture content in the dry basis would be 14 %, and the final weight would be
219 g.
Figure 1 and figure 2 were both quadratic functions. The R-values were close
to 1. The r value equal to 1 implies that the equation perfectly fits the data. The
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behavior of the graphs if time was extended was shown in figure 3 and figure 4. The
graphs were increasing after 120 minutes, based on the quadratic equation. But in
reality, the graph should be constant after the time was extended.
Conclusion
The amount of moisture content of the grain is indirectly proportional to time
in drying. The highest rate of moisture reduction was observed at the beginning of
the drying process. This rate reaches a constant value as time increases. The
amount of heat required to raise the temperature of air from ambient to drying
temperature was 6.39 kJ/s.
References
Chakraverty, A., Mujumdar, A., Raghavan, G. S., and Ramaswamy, H. 2003.Handbook of Postharvest Technology Cereals, Fruits, Vegetables, Tea, andSpices. New York: Marcel Dekker Inc. 119-120 p.
Dendy, D. A. V., and Dobraszczyk, B. 2001. Cereals and cereal products: chemistryand technology. USA: Aspen Publishers Inc. 29 p.
Appendix
Figure 3. Sample weight vs time
Figure 4. Percent Moisture Content versus time
Table 2. Raw Data for Drying of the grain
Period of DryingWeight of Container+ Sample (g) Weight of Sample (g)
Sample 1 Sample 2Sample1
Sample2 MCWB (%)
Initial (time = 0) 855.36 871.8 250 250 23.33
After 30 minutes 846.76 848.09 233.51 234.26 18.05
After 60 minutes 840.05 841.46 226.8 227.63 15.64
After 90 minutes 835.29 836.6 222.04 222.77 13.82
After 120 minutes 787.57 797.03 219.07 219.78 12.65
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Sample Calculation:
MCWB %= 100-(Wi*(100-MCi)/Wf)= 100-(250*(100-23.33)/((233.51+234.26)/2))
Table 3. Drying characteristics of palay grain
PeriodofDrying
Velocity ofair(ft/min)
Dryingrate (kgwater/s)
Amount ofmoistureremoved (kg)
Volumetric flowratem3/s
Specificvolumem3/kg
Massflowratekg/s Q (kJ/s)
Sample 1
Sample 2
Initial (time =
0)After30min. 1400 0.537 16.49 15.74
0.639018
0.9225
0.692703
6.359012
After60min. 1500 0.222 6.71 6.63
0.684663 0.915
0.748265
8.395534
After90min. 1600 0.16 4.76 4.86
0.730307 0.925
0.789521
5.637179
After120
min. 1500 0.099 2.97 2.99
0.68466
3 0.945
0.7245
11
5.1730
06
Total 30.93 30.22Average
30.575
6.391183
Sample calculation:
Drying rate= W/30= (250-233.51)/30= 0.537 kg/s
Amount of moisture removed= (250-233.51)= 16.49 kg
Amount of heat reqd to raise air temp= m*Cp *T= 0.692703 *1.02*(50-41)=
6.359012334 kJ/s
Drying constant=(y/x)/ (0.4343*f) = (7/19)/ (0.4343* 66) =0.0129
(M-Me)/( Mo-Me)= e^(-0.012t)
Heat Utilization factor= (38-41)/ (28-41)= 0.230769
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Time Scale factor was determined from figure 7
Figure 5. Change of moisture over time versus drying time
Figure 6. Change of moisture over time versus percent moisture content