UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
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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UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
Digital scale
Axial Fan
Control Panel Drying Chamber 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 cross sectional area of trays
7.467057101 % 0.0752 m^2
Axial fan Frequency
8 Hz
Heater temperature
Time
Mass, m
(min)
(kg)
0
0.936
65 deg C
T1 Dry
T2 Wet
T3 Dry
Bulb Inlet
Bulb Inlet
Bulb Outlet
( )
( )
( )
41.7
30.5
37.2
T4 Wet Bulb Outlet ( ) 27.5 2
UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
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
Mass of
Product
Air
Air humidity
drying rate
(min)
evaporated
Moisture
Humidity
after tray (%)
(kg/min)
water (kg)
Content (%)
before tray (%)
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: 3
UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
( )
(
)
(
)
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.
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UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
Graph of Moisture Content(%) vs Time(min) Moisture Content(%)
8 6 4 2 0 -2 -4
0
20
40
60
80
100
Time(min)
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UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
Graph of Drying Rate(g/min) vs Moisture Content(%) Drying Rate(g/min)
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
-4
-2
0
2 4 Moisture Content(%)
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8
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 cross sectional area of trays
7.250341997 % 0.0752 m^2
Axial fan Frequency
10 min
Heater temperature
65 ( )
Time (min)
Mass, m
T1 Dry
T2 Wet
T3 Dry Bulb
T4 Wet Bulb
(kg)
Bulb Inlet
Bulb Inlet
Outlet ( )
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
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UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
Fan
Mass of
Product
Air Humidity
Air
Drying
Frequency
evaporated
Moisture
before tray
humidity
rate
(Hz)
water (kg)
Content
(%)
after tray
(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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UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
Drying rate(g/min)
Graph of Drying rate(g/min) vs Fan frequency(Hz) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
14
Fan Frequency(Hz)
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 an interval of
. Results were recorded in
. 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 8
UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
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 9
UEMK2411 CHEMICAL ENGINEERING LABORATORY I
GROUP 09
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. Online version available at: http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPL AY_bookid=213&VerticalID=0
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