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• Srinyanavel ஸ்ரீஞானவேல்

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Title Experiment 3: Tray Drying Objectives To perform drying test on solids To investigate the effects of air velocity on drying rate To perform heat and mass transfer analysis of a drying process Introduction Drying is one of the most common methods used in the food processing industry. Drying was traditionally carried under the hot sun. However, drying foodstuff nowadays can easily be carried out in an oven or a dehydrator at any time of the day. The purpose of drying is to remove moisture from food so that microbial and bacteria growth is hindered. The drying method has its advantages. Packaging becomes much easier as well as economical. Foods that are dried also last longer; hence their shelf life is prolonged. Some of the more sophisticated methods of drying include freeze-drying and spray drying. Freeze drying is a dehydration process whereby the food product is frozen and the surrounding pressure is decreased to allow the frozen water to sublime off the food material. Spray drying involves drying out a liquid or a slurry compound by rapidly drying with a burst of hot gas. This method is commonly used in dehydrating temperature-sensitive substances such as pharmaceutical products and milk powder. Our tray drying experiment investigates the drying property of sand. We also study the effects of variable wind velocity unto the drying rate of sand. For the first experiment, we expect to see a limit in the drying of sand. The sand will dry up to a certain point where the resulting mass will remain a constant as there is no water let to evaporate. For the second part of the experiment, we expect the higher wind velocity to increase the rate of evaporation. We hypothesise so because as the wind increases in speed, the rate of removal of water will increase.

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Materials and Equipment   

Tray dryer Sand Tray

Figure 1 Tray Dryer Unit

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Data Analysis:Experiment 1 Initial Mass of sand (g) Water added (g) Total Mass (g)

0.787 0.065 0.99 7.62910

Moisture content (%)

8

Cross Section Area of tray (m2) Fan frequency (Hz) Heater temperature (oC)

0.06493 8 65

Mass of

Product Moisture Content

Time

Mass

T1DB

T2WB

T3DB

T4WB

water Evaporated

(min)

(kg) 0.99 0.98 0.972 0.964 0.956 0.949 0.942 0.936 0.93 0.925

(oC)

(oC)

(oC)

(oC)

(kg)

0 10 20 30 40 50 60 70 80 90

47.3 47 45.3 47.2 44.3 47.1 46.7 46.7 46.4

37 37.8 37.9 38.6 38.6 38.7 38.6 38.5 38.6

43.8 44.3 44.3 45.3 46.8 45.8 45.6 48.3 44

30.8 30.8 31.4 30.9 31.2 31.8 32.5 31.3 31.7

0 0.01 0.018 0.026 0.034 0.041 0.048 0.054 0.06 0.065

(%) 6.566 5.612 4.835 4.046 3.243 2.529 1.805 1.175 0.538 0.000

Time

Drying rate

Humidity

Humidity

Efficiency

Rate of heat

(min)

(kg/min)

relative (%)

relative (%)

(%)

transfer by

(before tray)

(after tray) 0 15.69507 12.27273 4.926108 8.558559 -12.9534 5.882353 5.069124 -7.37327

convection (J) 0 16.06589 14.35012 11.54248 13.41424 8.890833 13.10228 12.63434 12.79032

0 10 20 30 40 50 60 70 80

0 0.001 0.0008 0.0008 0.0008 0.0007 0.0007 0.0006 0.0006

51.4 55.6 62.3 58.1 69.6 58.9 59.9 59.5

39.2 37.8 39.9 35.6 32.9 37.3 40.2 29.8 3

90 0.0005 Experiment 2

Fan Frequenc

Mass

61.1

T1DB

T2WB

41.9

T3DB

11.21495

Mass of

Product

water

Moisture

T4WB

y

12.1664

evaporated(g

Fan

(kg) (oC) (oC) (oC) 0.961 49 38.3 46.4 0.952 47.9 37.8 46.7 0.946 45.6 38.1 45.4 0.94 45.1 38 45.7 0.934 45.9 37.7 45.7 0.929 45.6 37.8 45.5 Drying Drying Air

frequency

time (min)

(Hz) 8 9 10 11 12

(Hz)

(oC) 32.4 31.8 31.4 32.2 31.8 31.3 Air

)

Content (%) 0 3.330 0.009 2.416 0.015 1.797 0.021 1.170 0.027 0.535 0.032 0.000 Efficiency Rate of

rate

humidity

humidity

(%)

(kg/min)

(%)

(%) (after

transfer by

(before

tray)

convection

tray) 0 0 50.9 37.7 10.83333 8 10 0.0009 52.5 35 5.240175 9 10 0.0006 62 37 0.970874 10 10 0.0006 63.5 38.9 -2.98507 11 10 0.0006 59.1 37.5 0.956938 12 10 0.0005 60.7 36.4 0.485437 Note: Calculation was done by assuming 0.929kg as the mass of dry sand

heat

(J) 16.68981 15.75393 11.69846 11.07455 12.79032 12.1664

Mass balance Experiment 1 Mass of moisture sand = Mass of dry sand + Mass of water evaporated (assume no generation and consumption in this process) Mass of moisture sand = 0.99kg Mass of dry sand = 0.925kg (Assume it is dry) Mass of water evaporated = 0.065kg 0.99kg = 0.925kg + 0.065kg

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Experiment 2 Mass of moisture sand = 0.961kg Mass of dry sand = 0.929kg (Assume it is dry) Mass of water evaporated = 0.032kg 0.961kg=0.929kg+0.032kg Energy balance Experiment 1 Average T1=46.444oC (assume to be the average air temperature) Average T2=38.256oC (assume to be the average sand's surface temperature) Heat transfer coefficient of air, h =24.023 J/m2oC Cross section area, A = 0.065m2 Formula for calculation= q=h A (T1-25oC) Rate of heat transfer by convection, q=33.449J Cp for sand= 0.83kJ/kgoC Sensible heat for the sand, q = 0.852*0.83*(38.256-25) = 9.374J [q= m Cp (T2 -25oC)] Heat of vaporization, q= 24.075J [mass of water x Hvap=0.01kg x 2407.5144kJ/kg] Heat requirement= 33.449J Experiment 2 Average T1= 46.517oC (assume to be the average air temperature) Average T2= 37.95oC (assume to be the average sand's surface temperature) Heat transfer coefficient of air, h =24.023 J/m2oC Cross section area, A = 0.065m2 Rate of heat transfer by convection, q=33.562J 5

Cp for sand= 0.83kJ/kgoC Sensible heat for the sand, q = 0.823*0.83*(37.95-25) = 8.846J [q= m Cp (T2 -25oC)] Heat of vaporization, q= 21.747J [mass of water x Hvap=0.01kg x 2407.5144kJ/kg] Heat requirement= 36.023J

Experiment 1: Average T1 Average T3 Overall efficiency Experiment 2: Average T1 Average T3 Overall efficiency

46.4444444 45.3555556 5.07772021 46.5166667 45.9 2.86599535

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Graph for Experiment 1

Moisture content vs. Drying time

Moisture content(%)

7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00

E.1

0

10 30 50 70 90 20 40 60 80

Drying time(min)

Drying rate vs. Moisture content 0 0 0 0

E.1

Drying rate(1/min) 0 0 0 1.00 3.00 5.00 7.00 0.00 2.00 4.00 6.00

Moisture content(%)

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Graph for experiment 2

Moisture content vs. Drying time

Moisture content(%)

3.500 3.000 2.500 2.000 1.500 1.000 0.500 0.000

E.2

0

10 20 30 40 50

Drying time(min)

Drying rate vs. Moisture content 0 0 0 E.2

Drying rate(1/min) 0 0 0 0.00

1.00

2.00

3.00

4.00

Moisture content(%)

8

Drying rate vs. Air velocity 0 0 0 E.2

Drying rate(1/min) 0 0 0 8

9

10

11

12

Air Velocity (Hz)

Efficiency vs. Air velocity

E.2

Efficiency(%) 8

9

10

11

Air velocity(Hz)

9

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Discussion The two experiments were conducted to determine a better condition for drying process in term of time consumption and cost. The results of these two experiments was compared and discussed. Experiment 1 was carried out as a control test to observe the drying process of sand under a condition which we assume to have constant temperature (65 ℃ ) and constant air velocity (8Hz). The results show that, moisture content of the sand decreases as drying time increases. As the drying time increase, the amount of water loss through evaporation will increase, and this contribute to the drop of moisture content in sand. Moisture content’s effect. Based on the data obtained, drying rate is proportional to the moisture rate. This also means that the drying rate decreases as time proceeds. This is because drying will occur through the surface first and only then proceed to the next layer below. The fastest rate will occur at the initial period of the process since more water is located at the surface. After some time, the surface water will decrease and water at a lower level will have to be evaporated. At this level, some water might still be trapped inside the sand and it becomes more difficult to escape from the sand. Thus, more time is required for this water to escape and vaporize. As the level of drying become deeper, the drying rate will decrease. Since no reaction occurs in any drying process, thus, the mass is conserved. Mass balance was performed and the results satisfy what the theory says. Besides, heat balance is also done to determine the performance of the drying. However, the balance does not give a desired result. This indicates that heat does not completely transfer to the sand. Besides, performing heat transfer calculations are very difficult as we only consider heat convection, other heat transfer mechanisms were not considered. This explains the 5% overall efficiency for the dryer in experiment 1. The energy from the hot air cannot be fully transferred to the sand, some heat might be lost to the surroundings; showing that this system is not adiabatic.. In experiment 2, the air velocity was manipulated and the effect of air velocity to the drying rate was observed. The moisture content and drying rate follows the same trend as in experiment 1 as the drying time increases. The moisture content decreases 10

with the drying time and drying rate will decrease with moisture content. It is important to compare the drying rate on different air velocities. There is a big error in this part as the results obtained are totally different with the theory. The drying rate is supposed to increase with the air velocity, but the graph shows an opposite result. After some analysis, this result is now explainable. Drying rate is influenced by air velocity, but it also depends on the moisture content. If were to find out the influence of air velocity on drying rate, other drying rate dependent variables need to be kept constant, and experiment 2 does not satisfy that. Moisture content will have a negative effect to drying rate, whereas air velocity will have positive effect to drying rate. When these two variables occur, the effect will superimpose and give a new effect on the drying rate. Moisture content’s effect is dominant when the rate decreases from 0.0009 to 0.0006. Then, the rate becomes constant as the air velocity’s effect and moisture content’s effect cancelled out each other. Finally, the rate decreases again, this time caused by the moisture content’s effect. The result also shows that air velocity will affect the efficiency of the dryer. The efficiency measures the effectiveness of heat transfer to the sand’s moisture for vaporization. If the efficiency is decreased, it indicates that less heat is absorbed by the sand to vaporize the water. This is true when air moves with a fast speed, it carries the water at the sand’s surface at a fast rate before it evaporate, thus, less heat is needed to remove the water from sand. The experiment gives us many unexpected results and is probably caused by the errors listed below. Firstly, we think that the uneven moisture on the sand’s surface might affect the drying rate. If most of the water is located at the lower layer of the sand, then the drying rate will not follow the original trend. Besides, the temperature that was assumed to be constant was in reality fluctuating. This may be caused by the instrument being not well insulated, and heat will be lost to the surroundings easily. It is impossible to maintain a constant temperature. As we know that, the drying rate is highly dependent on temperature, the temperature fluctuations cause the water to escape at a different rate. Furthermore, errors might come from the data collection as the temperature values were not recorded at the instant time. All the four temperature displays are mounted together in a single monitor. It takes time to record, change and record the four temperature readings. And some times, the monitor faces technical glitches and we assume it to be the right reading. Since temperature will never be 11

constant, the reading must be recorded fast to minimize the error in the temperature readings. Wrong readings recorded will contribute to calculation errors. Moreover, the sand has to be reweighed at every 10 minute intervals. And at each of the time, it takes us about 3 minutes to reheat the sand again. As this period of time, some water might vaporized to the surroundings and the drying rate as this moment had not been taken into account. It is better to have four monitors to display all the dry bulb and wet bulb temperatures. If this change can be made to the instrument, the all the temperatures reading can be recorded more effectively and gives less room for error. A better insulation can minimize heat loss to the surroundings and help achieve the constant temperature assumption. Installation of insulation to the instrument might improve the result’s reliability. It is advisable to dry the sand continuously without taking out to reweigh the sand at every time interval. If possible, weigh the sand in dryer without taking it out. The moisture content will affect the drying rate, thus experiment 2 cannot clearly show the relationship between air velocity and drying rate. It is better to design a new experiment which keeps the moisture content constant for every air velocity used. But to prepare a same mass of sand with equal moisture content is quite difficult. For our experiment, we cannot determine the influence of air velocity to the drying rate, and thus, we cannot determine the optimization condition to dry the sand. A complete research (in appendix) shows that temperature influence is higher on the drying rate. The optimum condition can be found out from experiment, but high temperature and air velocity is rather costly. As a reminder, the drying process cannot be conducted under extremely high temperatures as the surface of the sand might become hard or even get burnt. The best conditions for the drying process with the consideration of the cost and time consumption only can be determined after complete analyses on the cost we have to pay for every Celsius and Hertz. Conclusion The effect of air velocity on drying rate was investigated in this experiment. Theory says high air velocity increases the drying rate. But, our experiment shows opposite 12

result due the moisture content’s effect. Drying is costly and time consuming, thus, a optimum condition is needed to be found to reduce the cost and time consumed. The experiment should be redesigned to correctly examine the influence of air velocity on drying rate.

Reference Principles of mass transfer and separation processes by Binay.K.Butta 2007 by Prentice-Hall of India

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