drying of solids(sand)
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Drying of solids(sand)TRANSCRIPT
Course Number: ChE-302
Course Title: Chemical Engineering Laboratory-II
Experiment Number: 04
Name of the Experiment: Drying of solids
Submitted by:
Mahe Rukh
Student Number: 1202036
Section: A2 Group Number: 02
Department of Chemical Engineering
BUET
Partners’ Student Numbers: 1202037
1202038
1202039
1202040
Date of
Performance:10
/10/15
Date of Submission:
31/10/15
Submitted to:
Dr. Md. Tanvir Sowgath
Assistant Professor
Department of Chemical Engineering
BUET
Summary Drying of solid is of great importance in industrial production and in chemical engineering application.
Various kind of process for drying are of day to day use in various fields. That is why conducting this
experiment is very effective as it gave us vivid idea of the phenomena- drying of solids e.g. sand. For this
purpose, two trays were used. Sand was first poured on them and then water was mixed. In a vacuum
dryer the trays were heated and moisture content from sand was removed by steam supplied from
steam generator where the flow rate of steam was maintained at 30psi. After every five minute trays
were drawn out and moisture content in them was calculated by weighing their weight and comparing it
to initial weight of tray and sand. From the collected data drying rate, moisture content was measured
and graphs were drawn focusing drying characteristics of sand.
Introduction The removal of relatively small amounts of water or other liquid to reduce the content of
remaining liquid to acceptably low amount in a solid matter is known as drying of solids. this is a
mass transfer process were liquid is removed by means of evaporation. So, we can see drying of
solids need both mass transfer and heat transfer simultaneously. A source of heat and an agent is
involved in this process. Most commonly, a gas stream such as air applies the heat by convection
and carries away the liquid in solid as vapor.
There are various methods of drying e.g. direct drying, indirect drying, dielectric drying, freeze
drying, supercritical drying and natural drying. During thermal drying two things occur at a time.
Transfer of moisture content from internal space to solid surface with subsequent evaporation
and Transfer of heat to raise the wet solid temperature and to evaporate the moisture content. The
drying rate depends on how these two processes proceed. Heat transfer process occurs due to
conduction, convection or radiation mode of heat flow or in some cases as a result of a
combination of any of these effects. In our experiment we have used indirect method of drying
where it is important to have the maximum contact between heated dryer metal surfaces and the
solid undergoing drying and maximum movement at areas of contact. Here, the drying operation
does not depend upon the effect of convection from heated air or gases. Air flow through such
dryer is only required to remove moisture as vapor from the system as it is liberated in the course
of drying.
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External conditions such as temperature, humidity, water velocity relates with drying of solids.
Theoretical models often consider not only external conditions but also the mechanisms of
internal moisture movement and its consequent effect. The solids fed to dryer can be in the form
of flakes, granular, crystals and may have widely different properties. There are over 100 distinct
types of dryers. Of them tray dryer is most common. Tray dryer is one kind of batch dryer which
can handle both direct drying and indirect drying. The dryer operates under a vacuum, and heat
transfer is largely by conduction or by radiation. The trays are enclosed in a big cabinet, which is
evacuated. Water vapor produced is usually condensed, so that the vacuum pumps need only to
deal with non-condensable gases.
Experimental work The following things were used in this experiment-
1. Two trays
2. Sand
3. Water
4. Vacuum pump
5. Vacuum dryer
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Figure 01: A schematic diagram of vacuum dryer
Figure -02: Schematic diagram of vacuum dryer.
Procedure 1. At first the weight of the two empty trays were taken.
2. Tray weight with sand was taken. Then water was added to moisten the sand and trays were reweighted.
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3. Trays were vacuum dried by putting inside the drier.
4. At an interval of five minutes’ vacuum drying was stopped by releasing vacuum drying to
take the reading of weight of tray containing sand and water
5. Reading was taken until constant weight readings were achieved.
6. Data for calculation were noted.
Observed Data Weight of each empty tray = 6.5 lbm
Tray 1
Weight of Tray + sand = 14 lbm
Weight of Tray + sand + water = 15 lbm
Tray 2
Weight of Tray + sand = 15.0 lbm
Weight of Tray + sand + water = 16.5 lbm
Table 1: Representing weight of Tray +Sand + Water at different time
No of Obs
Time
(min)
Weight ( lbm) of
Tray 1+Sand+Water
Weight(lbm) of
Tray 2 +Sand+ Water
01. 5 14.88 16.31
02. 10 14.75 16.07
03. 15 14.50 15.81
04. 20 14.25 15.68
05. 25 14.19 15.50
06. 30 14.13 15.25
07. 35 14.00 15.13
08. 40 14.00 15.00
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09. 45 14.00 15.00
Calculated data
Table 2: Drying Rate and Moisture Content of Tray 1 at Different Time
Observation No. Time
(Min)
Moisture decrease rate
C =(W0-Wt)/t
lb/min
Drying rate, R
lb/ft2.min
Moisture content
lb water/ lb solid
1 5 0.024 .0090 0.117
2 10 0.026 0.010 0.10
3 15 0.05 0.0183 0.067
4 20 0.05 0.0183 0.033
5 25 0.012 0.0044 0.025
6 30 0.012 0.0044 0.0173
7 35 0.026 0.0096 0.0
8 40 0.0 0.0 0.0
9 45 0.0 0.0 0.0
Table 3: Drying Rate and Moisture Content of Tray 2 at Different Time
Observation No. Time
(Min)
Moisture decrease rate
C =(W0-Wt)/t
lb/min
Drying rate, R
lb/ft2.min
Moisture content
lb water/ lb solid
1 5 0.038 0.0140 0.154
2 10 0.048 0.0177 0.125
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3 15 0.052 0.0191 0.095
4 20 0.026 0.0096 0.080
5 25 0.036 0.0132 0.059
6 30 0.05 0.0183 0.029
7 35 0.024 0.0088 0.015
8 40 0.026 0.0096 0.0
9 45 0.0 0.0 0.0
Sample calculation Calculation for observation no 1 of tray 1:
Initial weight of sand+ tray 1+ water after 5 min, W0 = 14.88 lb
Weight of sand+ tray+ water after 10 min, Wt = 14.75 lb
Moisture decrease rate , C= (Wo-Wt)/time= (14.88-14.75)/5
= 0.026 lb/min
Area of tray 1, A = 29x 13.5 inch2
= 391.5 inch2
= 2.71875 ft2
Drying rate R= C/A
=
0.01252.71875 lb/min ft2
= 0.0096 lb/min ft2
Moisture content =
lb of waterlb of dry solid
= (weight of sand+ water+ tray at 5 minutes)-(weight of sand+ tray)
(initial weight of sand + tray)- (weight of tray)
= (14.88—14)/ (14-6.5)
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=0.1173
Graphical representation
Graph 01: Drying rate versus time
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Graph 02: Moisture content vs time
Graph-03 : Drying rate versus moisture content
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Results In case of both Tray 1 & Tray 2 100% removal of the moisture was observed. For tray-1 moisture
content reduced to zero after 35 minutes and for tray-2 it took 40 minutes.
Discussion
Discussion on result
Moisture decrease rate should have been same for two tray-1 and tray-2 for same number of
observations as they were dried in same dryer. But from data it is seen that they are not same.
Discussion on graphs
By using the calculated data, following three graphs are drawn for both trays:
1) Plot of Drying rate vs. Time
2) Plot of Moisture Content vs. Time
3) Plot of Drying Rate vs. Moisture Content
Plot of Drying Rate vs. Time
the drying rate increased for some times at first, then started decreasing and finally became
constant in both cases. But drying rate does not decrease with decreasing of moisture content,
nevertheless drying rate fluctuate too much
Plot of Moisture Content vs. Time
From graphs 1 and 2, we see that moisture content decreases with time as usual and
consequently it becomes zero which was supposed to be obtained.
Plot of Drying Rate vs. Moisture Content
There should have been constant drying period but from graphs we see there was not any.
Errors that may have occurred in the experiment-
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1. the pressure gauge that measured steam pressure showed fluctuations during the
experiment but the flow rate should have been constant.
2. The measuring device was operated manually and weight measured using this device may
have caused inaccuracy.
3. The trays were taken out in every 5 minutes. So there is chances that some moisture was
lost during weight measurement.
4. In the vacuum dryer maintaining constant pressure was a struggle. So it affected heat
transfer between the wet solid and steam. Also vacuum could not be maintained perfectly
as the door of the dryer had to be opened and thus air entered the dryer every time and it
was difficult to evacuate it by the vacuum pump so fast, so the drying conditions might
have changed.
5. During the mixing some weight loss may have occurred and even a small loss of sand
particle may cause error in the experiment.
Ways to improve the experiment-
1. Mixing should be done with care
2. Using digital weight measuring device instead of manual one.
3. Constant pressure at both steam inlet to dryer and in the vacuum dryer should be
maintained.
Nomenclature
Symbol Name Unit(If Applicable)
A Area of the tray ft2
C Moisture Decreasing rate lbm/minFt Feet - lbm Pound mass -
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T Time minuteW Moisture Content lbm/ lbm
References 1. Foust, Alan S.; Wenzel, Leonard A.; Clump, Curtis W.; Maus, Louis; and
Andersen, L. Brice: Principles of Unit Operations, 2nd ed., John Wiley & Sons,
New York, 1994.
2. McCabe, Warren L.; Smith, Julian C.; and Harriott, Peter: Unit Operations of
Chemical Engineering, 6th ed., McGraw-Hill, New York, 2001.
3. Genskow, L.R.; Beimesch,W.E.; Hecht, J.P.; Kemp,I.C.; Langrish,T.;
Schwartzbach, C.; Smith: Chemical Engineers' Handbook. Mc Graw Hill
Professional,2007.
4. Drying, Wikipedia, Retrieved from https://en.wikipedia.org/wiki/Drying
5. Industrial and engineering chemistry, ACS publications ,Retrieved from
http://pubs.acs.org/doi/abs/10.1021/ie50238a021
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Marking Scheme: Normal Report
Name: Mahe Rukh
Student number: 1202036
Section and marks allocated Marks
Summary(1)
Introduction (1)
Experimental Work (1.5)
Observed Data (1)
Calculated Data (1)
Sample Calculation (1)
Graphs (1)
Results and Discussion (1)
References and Nomenclature (0.5)
Writing Quality and Style (1)
Total (10)
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