8/6/2019 AE 31 exer 5

1/5

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

8/6/2019 AE 31 exer 5

2/5

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

8/6/2019 AE 31 exer 5

3/5

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

8/6/2019 AE 31 exer 5

4/5

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

8/6/2019 AE 31 exer 5

5/5

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