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 Drying was tradit tradition ionally ally carried carried under under the hot sun. sun. Howeve However, r, drying drying foodst foodstuff uff 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. ac!aging becomes much easier as well as economical. "oods that are dried also last longer# hence their shelf life is prolonged. $ome of the more sophisticated methods of drying include free%e&drying and spray drying. "ree%e drying is a dehydration process whereby the food product is fro%en and the surrounding pressure is decreased to allow the fro%en water to sublime off the food material. $pray drying involves drying out a li'uid 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 mil! powder. (ur tray drying drying experiment investigates investigates the drying drying property property of sand. )e also study the effects of variable wind velocity unto the drying rate of sand. "or 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. "or the second part of the experiment, we expect the higher wind velocity to increase the rate of evaporation. )e hypothesise so because as the wind increases in speed, the rate of removal of water will increase.
1
Materials and Equipment • • •
Tray dryer $and Tray
Figure 1 Tray Dryer Unit
2
Data Analysis!
Experiment * +nitial ass of sand -g )ater added -g Total ass -g
/.010 /./2 /.44 0.264*/
oisture content -5
1
7ross $ection 8rea of tray -m6 "an fre'uency -H% Heater temperature -o7
/./2943 1 2
ass of
roduct oisture 7ontent
Time
ass
T*D
T6)
T3D
T9)
water Evaporated
-min
-!g /.44 /.41 /.406 /.429 /.42 /.494 /.496 /.432 /.43 /.46
-o7
-o7
-o7
-o7
-!g
/ */ 6/ 3/ 9/ / 2/ 0/ 1/ 4/
90.3 90 9.3 90.6 99.3 90.* 92.0 92.0 92.9
30 30.1 30.4 31.2 31.2 31.0 31.2 31. 31.2
93.1 99.3 99.3 9.3 92.1 9.1 9.2 91.3 99
3/.1 3/.1 3*.9 3/.4 3*.6 3*.1 36. 3*.3 3*.0
/ /./* /./*1 /./62 /./39 /./9* /./91 /./9 /./2 /./2
-5 2.22 .2*6 9.13 9./92 3.693 6.64 *.1/ *.*0 /.31 /.///
Time
Drying rate
Humidity
Humidity
Efficiency
-min
-!g;min
relative -5
relative -5
-5
transfer by
-before tray
-after tray / *.24/0 *6.60603 9.462*/1 1.14 &*6.439 .11633 ./24*69 &0.30360
convection -= / *2./214 *9.3/*6 **.9691 *3.9*969 1.14/133 *3.*/661 *6.23939 *6.04/36
/ */ 6/ 3/ 9/ / 2/ 0/ 1/
/ /.//* /.///1 /.///1 /.///1 /.///0 /.///0 /.///2 /.///2
*.9 .2 26.3 1.* 24.2 1.4 4.4 4.
34.6 30.1 34.4 3.2 36.4 30.3 9/.6 64.1 3
4/ /./// Experiment 6
"an "re'uenc
ass
2*.*
T*D
T6)
9*.4
T3D
**.6*94
ass of
roduct
water
oisture
T9)
y
*6.*229
evaporated-g
-H%
-o7 -o7 -o7 94 31.3 92.9 90.4 30.1 92.0 9.2 31.* 9.9 9.* 31 9.0 9.4 30.0 9.0 9.2 30.1 9. Drying 8ir
"an
-!g /.42* /.46 /.492 /.49 /.439 /.464 Drying
fre'uency
time -min
1 4 */ ** *6
-H%
-o7 36.9 3*.1 3*.9 36.6 3*.1 3*.3 8ir
7ontent -5 / 3.33/ /.//4 6.9*2 /./* *.040 /./6* *.*0/ /./60 /.3 /./36 /./// Efficiency
rate
humidity
humidity
-5
heat
-!g;min
-5
-5 -after
transfer by
-before
tray
convection
tray
-= / / /.4 30.0 */.13333 *2.2141* 1 */ /.///4 6. 3 .69/*0 *.0343 4 */ /.///2 26 30 /.40/109 **.24192 */ */ /.///2 23. 31.4 &6.41/0 **./09 ** */ /.///2 4.* 30. /.42431 *6.04/36 *6 */ /./// 2/.0 32.9 /.91930 *6.*229 >ote: 7alculation was done by assuming /.464!g as the mass of dry sand Mass balance
Experiment * ass of moisture sand ? ass of dry sand @ ass of water evaporated -assume no generation and consumption in this process ass of moisture sand ? /.44!g ass of dry sand ? /.46!g -8ssume it is dry ass of water evaporated ? /./2!g /.44!g ? /.46!g @ /./2!g
4
Experiment 6 ass of moisture sand ? /.42*!g ass of dry sand ? /.464!g -8ssume it is dry ass of water evaporated ? /./36!g /.42*!g?/.464!g@/./36!g Energy balance
Experiment * 8verage T*?92.999o7 -assume to be the average air temperature 8verage T6?31.62o7 -assume to be the average sandAs surface temperature Heat transfer coefficient of air, h ?69./63 =;m6o7 7ross section area, 8 ? /./2m6 "ormula for calculation? '?h 8 -T*&6o7
7 p for sand? /.13!=;!g o7 $ensible heat for the sand, ' ? /.163B/.13B-30.4&6 ? 1.192= C'? m 7 p -T6 &6o7 Heat of vapori%ation, '? 6*.090= Cmass of water x H vap?/./*!g x 69/0.*99!=;!g Heat re'uirement? 32./63=
E"periment 1 8verage T* 8verage T3 (verall efficiency E"periment # 8verage T* 8verage T3 (verall efficiency
92.9999999 9.32 ./0006/6* 92.*22220 9.4 6.12443
6
$rap% &or E"periment 1
Moisture content vs. Drying time 7.00 6.00 5.00 4.00
Moisture content(%)
3.00
E.1
2.00 1.00 0.00 10 0
30
20
50
40
70
60
90
80
Drying time(min)
Drying rate vs. Moisture content 0 0 0 0
Drying rate(1/min)
E.1
0 0 0 1.00 0.00
3.00
2.00
5.00
4.00
7.00
6.00
Moisture content(%)
7
$rap% &or e"periment #
Moisture content vs. Drying time 3.500 3.000 2.500 2.000
Moisture content(%)
E.2
1.500 1.000 0.500 0.000 0
10 20 30 40 50
Drying time(min)
Drying rate vs. Moisture content 0 0 0
Drying rate(1/min)
E.2
0 0 0 0.00
1.00
2.00
3.00
4.00
Moisture content(%)
8
Drying rate vs. Air velocity 0 0 0
Drying rate(1/min)
E.2
0 0 0 8
9
10
11
12
Air Velocity (Hz)
Efciency vs. Air velocity
E.2
Efciency(%) 8
9
10
11
Air velocity(Hz)
9
12
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 * was carried out as a control test to observe the drying process of sand under a condition which we assume to have constant temperature -2 ℃ and constant air velocity -1H%. The results show that, moisture content of the sand decreases as drying time increases. 8s 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 e&&ect . ased 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. 8fter some time, the surface water will decrease and water at a lower level will have to be evaporated. 8t 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 re'uired for this water to escape and vapori%e. 8s the level of drying become deeper, the drying rate will decrease. $ince no reaction occurs in any drying process, thus, the mass is conserved. ass balance was performed and the results satisfy what the theory says. esides, 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. esides, 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 *. 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.. +n experiment 6, 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 * as the drying time increases. The moisture content decreases 10
with the drying time and drying rate will decrease with moisture content. +t 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. 8fter some analysis, this result is now explainable. Drying rate is influenced by air velocity, but it also depends on the moisture content. +f were to find out the influence of air velocity on drying rate, other drying rate dependent variables need to be !ept constant, and experiment 6 does not satisfy that. oisture content will have a negative effect to drying rate, whereas air velocity will have positive effect to drying rate. )hen these two variables occur, the effect will superimpose and give a new effect on the drying rate. oisture contents effect is dominant when the rate decreases from /.///4 to /.///2. Then, the rate becomes constant as the air velocitys effect and moisture contents effect cancelled out each other. "inally, the rate decreases again, this time caused by the moisture contents 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 sands moisture for vapori%ation. +f the efficiency is decreased, it indicates that less heat is absorbed by the sand to vapori%e the water. This is true when air moves with a fast speed, it carries the water at the sands 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. "irstly, we thin! that the uneven moisture on the sands surface might affect the drying rate. +f most of the water is located at the lower layer of the sand, then the drying rate will not follow the original trend. esides, 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. +t is impossible to maintain a constant temperature. 8s we !now that, the drying rate is highly dependent on temperature, the temperature fluctuations cause the water to escape at a different rate. "urthermore, errors might come from the data collection as the temperature values were not recorded at the instant time. 8ll the four temperature displays are mounted together in a single monitor. +t ta!es time to record, change and record the four temperature readings. 8nd some times, the monitor faces technical glitches and we assume it to be the right reading. $ince temperature will never be 11
constant, the reading must be recorded fast to minimi%e the error in the temperature readings. )rong readings recorded will contribute to calculation errors. oreover, the sand has to be reweighed at every */ minute intervals. 8nd at each of the time, it ta!es us about 3 minutes to reheat the sand again. 8s this period of time, some water might vapori%ed to the surroundings and the drying rate as this moment had not been ta!en into account. +t is better to have four monitors to display all the dry bulb and wet bulb temperatures. +f this change can be made to the instrument, the all the temperatures reading can be recorded more effectively and gives less room for error. 8 better insulation can minimi%e heat loss to the surroundings and help achieve the constant temperature assumption. +nstallation of insulation to the instrument might improve the results reliability. +t is advisable to dry the sand continuously without ta!ing out to reweigh the sand at every time interval. +f possible, weigh the sand in dryer without ta!ing it out. The moisture content will affect the drying rate, thus experiment 6 cannot clearly show the relationship between air velocity and drying rate. +t is better to design a new experiment which !eeps the moisture content constant for every air velocity used. ut to prepare a same mass of sand with e'ual moisture content is 'uite difficult. "or our experiment, we cannot determine the influence of air velocity to the drying rate, and thus, we cannot determine the optimi%ation condition to dry the sand. 8 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. 8s 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 7elsius and Hert%. (onclusion
The effect of air velocity on drying rate was investigated in this experiment. Theory says high air velocity increases the drying rate. ut, our experiment shows opposite 12
result due the moisture contents 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.
)e&erence
rinciples of mass transfer and separation processes by inay.F.utta 6//0 by rentice&Hall of +ndia
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