1
TITLE OF EXPERIMENT
Tray Drying
OBJECTIVES
The objectives of this experiment were to perform drying test on solids, to investigate the effects of air velocity on drying rate and to perform heat and mass transfer analysis of a drying process.
INTRODUCTION
Like evaporation, drying is a mass-transfer process resulting in the removal of water or moisture from a process stream. While evaporation increases the concentration of non-volatile components in solution, in drying processes the final product is a solid. Drying processes reduce the solute or moisture level to improve the storage and handling characteristics of the product, maintain product quality during storage and transportation and reduce freight cost (less water to ship). Drying of solids in certain cases like wood, ceramics and soap has a remarkable fathom of the internal mechanism obtained that allows control of product standard. Surveys of drying of solids have been made from the so-called external viewpoint, wherein the effects of the external drying medium like air velocity, humidity, temperature and wet material shape and subdivision are studied with respect to their influence on the drying rate.
Tray dryer is used for drying solids by air or removes the moist vapours which must be supported by trays. Trays are designed to force the air to follow a longer zigzag route which increases the contact time between food and air, thus improve its efficiency. Heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces. It is most suitable in terms of cost and output when the production rate is small.
MATERIAL AND EQUIPMENT
The equipment used in this experiment was the tray dryer unit. This unit is designed to demonstrate the theoretical and practical aspects of solids drying.
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RESULTS AND CALCULATIONS
Part 1:
Initial mass of rice / dry sand 0.632 kg
mass of tray 0.253 kg
amount of water added 0.051 kg
total mass (wet sand + trays + holder) 0.936 kg Moisture content percentage 7.467057101 %
cross sectional area of trays 0.0752 m^2
Axial fan Frequency 8 Hz
Heater temperature 65 deg C
Time (min) Mass, m (kg) T1 Dry Bulb Inlet ( ) T2 Wet Bulb Inlet ( ) T3 Dry Bulb Outlet ( ) T4 Wet Bulb Outlet ( ) 0 0.936 41.7 30.5 37.2 27.5 Digital scale Axial Fan Control Panel Drying Chamber
3 10 0.928 45.9 36.8 40.7 29.5 20 0.92 45.8 38.2 40.9 30.5 30 0.904 40.0 37.0 39.0 31.3 40 0.898 38.7 36.8 38.7 30.8 50 0.891 47.1 38.8 42.6 31.3 60 0.884 47.5 37.5 42.9 32.0 70 0.879 48.2 36.8 46.7 32.0 80 0.874 48.4 35.0 45.9 35.6 90 0.868 47.9 39.2 45.3 31.0 Time (min) Mass of evaporated water (kg) Product Moisture Content (%) Air Humidity before tray (%) Air humidity after tray (%) drying rate (kg/min) 0 0.000 7.47 45.10 47.92 0 10 0.008 6.37 56.03 44.35 0.0008 20 0.016 5.25 62.21 47.71 0.0008 30 0.032 2.92 82.46 58.22 0.0016 40 0.038 2.02 88.44 57.11 0.0006 50 0.045 0.94 59.88 45.36 0.0007 60 0.052 -0.16 53.35 47.11 0.0007 70 0.057 -0.96 48.56 36.39 0.0005 80 0.062 -1.77 41.80 51.31 0.0005 90 0.068 -2.76 58.63 36.65 0.0006 Calculations:
By using the sample from , Mass balance:
4 ( ) ( ) ( )
Provided that is the total mass and is the tray mass.
( ) ( ) [ ( )]
for sand is obtained from Table A-14 from the Fundamentals of Heat and Mass Transfer 6th ed. by Frank P.Incropera.
5 -4 -2 0 2 4 6 8 0 20 40 60 80 100 M o is tu re C o n te n t(% ) Time(min)
Graph of Moisture Content(%) vs
Time(min)
6 Part 2:
Initial mass of rice / dry sand 0.678 kg
mass of tray 0.253 kg
amount of water added 0.053 kg
total mass (wet sand + trays + holder) 0.931 kg Moisture content percentage 7.250341997 %
cross sectional area of trays 0.0752 m^2
Axial fan Frequency 10 min
Heater temperature 65 ( )
Time (min) Mass, m (kg) T1 Dry Bulb Inlet ( ) T2 Wet Bulb Inlet ( ) T3 Dry Bulb Outlet ( ) T4 Wet Bulb Outlet ( ) 8 0.924 50.2 36.2 47.6 32.1 9 0.917 47.0 39.0 44.2 30.8 10 0.91 38.7 32.6 33.7 29.5 11 0.908 43.3 34.3 40.4 30.0 12 0.902 46.2 37.5 42.5 30.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 -4 -2 0 2 4 6 8 D ry in g R at e(g/m in ) Moisture Content(%)
Graph of Drying Rate(g/min) vs Moisture
Content(%)
7 Fan Frequency (Hz) Mass of evaporated water (kg) Product Moisture Content (%) Air Humidity before tray (%) Air humidity after tray (%) Drying rate (kg/min) g/min 8 0.007 6.29 41.00 34.55 0.0007 0.7 9 0.014 5.34 61.07 38.86 0.0007 0.7 10 0.021 4.38 65.75 73.60 0.0007 0.7 11 0.023 4.10 55.02 47.35 0.0002 0.2 12 0.029 3.28 57.81 42.63 0.0006 0.6
The calculation for the mass of evaporated water, moisture content and drying rate is the same and values will be provided in the tables.
( ) ( ) [ ( )]
for sand is obtained from Table A-14 from the Fundamentals of Heat and Mass Transfer 6th ed. by Frank P.Incropera.
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DISCUSSIONS
After conducting the experiment, the results obtained were tabulated and plotted into graphs. For the part 1 of the experiment, we were required to dry the sands in the tray drying equipment for about . Results were recorded in an interval of . When plotting the graph of Moisture Content VS Drying Time, the graph obtained found out to be having negative moisture content against drying time. Theoretically, the graph will be having a trend of linearly decreasing. However, the graph we obtained was decreasing and decreased further along the drying time. This might due the moisture of the sand initially contained some water. So when we dried the sand in the drying chamber for more than , we obtained a negative value for the moisture content. This is due to the initial rate of moisture that we took as a reference point.
Asides that, the graph of drying rate VS moisture content seems to be fluctuating along the -axis. Theoretically, the graph should be having a trend that will increase dramatically to a certain period. Then after that it will remain constant. However, the plots we got were like unstable drying rate. This might due to some reason that caused it to be. While conducting the experiment, the major problem that we encountered was the heater on the tray drier. According to the lab manual, the heater is supposed to heat up the air that being sucked into the chamber by the axial fan. Therefore, it has a sensor that will cause it to automatically control the power supply when the desired temperature reached and heat up against once the
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 2 4 6 8 10 12 14 D ry in g ra te (g/m in ) Fan Frequency(Hz)
Graph of Drying rate(g/min) vs Fan
frequency(Hz)
9 temperature drops. However, the malfunction of the heater might leads to the inaccuracy of the experimental results whereby the heater did not heat up when the temperatures of the air flow drops until it has a very big gap with the set value. This decreased the accuracy of the dry bulb temperature and the wet bulb temperature that we need for calculations.
Based on the results tabulated, we need to find the percentage of air humidity in the tray dryer. By having the values of wet bulb temperature and dry bulb temperature, with the help of psychrometric chart, the air humidity can be found. In our results, we used psychrometric calculator provided by Sugartech to find the air humidity. Based on the calculations, we calculated that the dryer has only an efficiency of which was extremely low for the efficiency.
For the Part 2 of the experiment, we manipulated the fan frequency to see the effect of fan frequency on the drying rate. The results were recorded down for an interval of for each set of fan frequency starting from to . From the results obtained, a graph of Drying Rate VS Fan Frequency was plotted. The graph plotted shows a constant drying rate across fan frequency. But when the fan frequency was set to , the drying rate decrease instantaneously to . This is because at that particular time, the tray dryer equipment became malfunction again. The heater did not heat up the air flow to the temperature that we set which was . This cause the air temperature to drop and thus the humidity inside the chamber at that moment increase. When the air humidity in the chamber increased, the evaporation of water from the sand hard to occur, thus the drying rate was low. However, the heater automatically turned on again when we change the frequency to . That was why the drying rate increase again after . Thus, after performing the calculations, we obtained that the efficiency of the same tray drying unit has a value of . It was much better compared to its efficiency of dryer in Part 1.
There were some precautions that need to be taken into account during the experiment. Firstly, the heater must be switched on and the fan must be turned on. Next, the wet bulb temperature sensor must always being make sure it was wet enough. In addition, when measuring the mass of the tray, sands and water, the weigh balance reading must be stable before the reading was recorded as the balance
10 is too sensitive until a single movement on it will affect the results. Safety measure had been taken when dealing with the tray dryer unit as it might be a very hot surface especially the heating element, thus gloves are prepared in order to prevent burn injuries.
CONCLUSION
As a conclusion, the objectives of the experiment were achieved. The drying test was successfully performed on the solids used – sands and the effect of air velocity on drying rate were studied. Asides that, the heat and mass transfer analyses of a drying process were performed by obtaining the values in the calculations.
REFERENCES
1. Drying of solids. (n.d.). Retrieved July 29, 2011, from Classof1:
http://classof1.com/homework_answers/chemical_engineering/drying_of_soli ds/
2. Henley, E. J., Seader, J., & Roper, D. (2011). Separation Process Principles 3rd Edition. Asia: John Wiley & Sons Pte Ltd.
3. Incropera, F. P., Dewitt, D. P., Bergman, T. L., & Lavine, A. S. (2005). Fundamentals of Heat and Mass Transfer. Asia: John Wiley & Son Inc. 4. Solids Drying. (n.d.). Retrieved July 29, 2011, from GEA Barr Rosin:
http://www.barr-rosin.com/applications/solids_drying.asp
5. Fellows, P.J. (2000). Food Processing Technology - Principles and Practice (2nd Edition). (pp: 309-340). Woodhead Publishing.
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