Tray Drying Report.pdf

UEMK2411 CHEMICAL ENGINEERING LABORATORY I GROUP 09

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.


2

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

Mo

istu

re C

on

ten

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

Dry

ing

Rat

e(g

/min

)

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.


8

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

Dry

ing

rate

(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.

Online version available at:

http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPL

AY_bookid=213&VerticalID=0

Documents

Tray Drying Report.pdf