Lab10 Complete

8/11/2019 Lab10 Complete

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EPF 3105 Food Process Engineering

Laboratory 2

LECTURER NAME : Dr. Roseliza Binti Kadir Basha

SESSION TIME : Wednesday (2.00P.M. - 5.00PM)

GROUP : 4

GROUP MEMBERS:

Lee ZiQing 168587

Nur Leha Binti Mansor 170016

Norhafiza Binti Kamal 170078

NurAin Binti Mohd Jaafar 167219

Siti Nur Adibah Binti Hamzah 167259

Faculty of Engineering


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Experiment 10: Naphthalene sublimation by convection / convective mass transfer

Introduction:

The naphthalene sublimation technique is an experimental technique employed to

determined heat transfer coefficients in convection flows. The basic characteristic of thetechniques is that the heat transfer problem to be investigated is replaced by analogous mass

transfer problem. In the laboratory, only mass transfer experiments are performed, and then heat

transfer results are obtained by exploring the concept of analogy between heat and mass transfer.

Naphthalene is employed in the mass transfer experiment because of some of its properties, such

as the fact that it sublimes at room temperature, its low toxicity and its good casting and

machining properties.

A typical apparatus employed in naphthalene sublimation forced-convection experiments

consists basically of an open loop flow circuit. The circuit includes a test section, where the

naphthalene pieces are exposed to the airstream, a flow or velocity-measurement section and ablower.

The flow circuit is normally operated in the suction mode, and the test section is located

at the upstream end of the circuit. The choice of such an arrangement guarantees that the flowing

air will not be heated or contaminated with lubricating oil in the blower before it reaches the test

section.

In typical applications of the technique, it is desirable to make certain that the air entering

the flow circuit is free of naphthalene vapour. In other words, it is convenient that the air

entering the test section has its bulk mass fraction of naphthalene vapour equal to zero. The main

reason for such practice is to avoid complicated mass-fraction measurements in determining the

air inlet conditions. To achieve this zero mass-fraction inlet condition, it is crucial that the

environment from where fresh air is drawn into the flow circuit has no connections with the

environment to where air containing naphthalene vapour is exhausted.

In natural convection experiments, the experimental apparatus acquires different

characteristics. The test section with the naphthalene pieces is normally held by a frame that

often allows change of orientation of the active surfaces with respect to the driving body force

field. In situations of external or open-cavity flows, protection against stray air currents must be

provided, such as a protecting enclosure or a surrounding channel. This protection must be

designed such that possible changes in freestream concentration of naphthalene vapour are kept

to a minimum; otherwise the mass transfer rates may be seriously affected. Due to the typically

low mass transfer coefficients, enough sublimation is achieved only with data runs that last

several hours. Hence, to prevent significant changes in the mass-fraction boundary condition

during the experiments, the air temperature should be controlled very tightly.


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Result:

Table 1: Results obtained from the experiment of sublimation of naphthalene in 30% of

Area of Aerodynamic Unit

Time

(minutes)

Temperature

(C)

Velocity of air

(m/s)

Weight (g)

10 25.9 9.4 3.179

20 25.2 9.1 3.170

30 24.6 8.8 3.154

Temperature = 25.2 C (298.2 K)

Velocity of air = 9.1 m/s

Initial mass of naphthalene = 3.193 g

Final mass of naphthalene = 3.154 g

Initial diameter of naphthalene = 1.8 cm

CALCULATION

Constant properties:

Density, = 1.170 kg/m3

Diffusivity, Dnaphthalene vapor + [email protected] = 6.1346 x 10-6

m2/s

Viscosity of air, = 1.864 x 10

-5

Pa.s

Gas constant, R = 8314 J/K.kg mol


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=

kgmol/s.m2.Pa

Since the gas is dilute, and .

Therefore, kg mol/s.m

2.Pa

From the naphthalene vapor pressure versus temperature graph, the pressure is 8.8 Pa when the

temperature is 25.2 C.

Note that pA1= 8.8 Pa and pA2=0 Pa (pure air)

= (8.8- 0)

= 1.66872 10-7

kg mol/s.m2

= 1.6682 10-7

where M is the molecular weight of naphthalene= 128kg/kg mol

= 2.1353 10-5

kg/m2.s

M sublimated (calculated ) =

=

=

= 3.91 10-5

kg

= 0.03912 g


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The area of the sphere, A =

=

= 1.018 10-3

m

Total amount sublimated (calculated) =

= 1.6687 10-7

1.018 10-3

= 1.6987 10-10

kg mol/s

M sublimated (experimentally) = 0.039 g

M sublimated (calculated) = 0.03912 g

Total amount sublimated (experimentally) = 1.693 x 10-10

kg mol/s

Total amount sublimated (calculated) = 1.6987 x 10-10

kg mol/s

Percentage error of mass sublimated

% error M sublimated = -

100%

=

= 0.31 %


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Time

(minutes)

Temperature

(C)

Velocity of air

(m/s)

Weight (g)

10 26.1 22.7 3.097

20 26.0 22.1 3.092

30 26.5 24.9 3.076






CALCULATION




m2/s

Viscosity of air, = 1.864 x 10-5

Pa.s



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Experimentally

M sublimated (experimentally)= initial weight of the naphthalenefinal weight of the naphthalene

= 3.112g -3.076g

= 0.036 g

In 30 minutes, the mass of naphthalene sublimated is 0.036g in 60% area open of aerodynamic

unit.

The molecular weight of naphthalene, C10H8 = [10(12) + 8] = 128 kg/kg mol

The number of mole, n =

=

= 0.02431 mol

Total amount sublimated (experimentally) =

x

x

x

= 1.5625 x 10-10

kg mol/s

Theoretically:

The Schmidt number is

NSc =

=

= 2.597


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The Reynolds number is

NRe =

vD

=

= 1.8077 104

For gases, for a Schmidt number range of 0.6-2.7 and a Reynolds number range of 1-48000,

=

= 138.89

The Sherwood number is

138.89 =

= 0.047 m/s

From the table 7.2-1 (Textbook of Transport processes and Separation Process Principle)

Hence, for T = 26.2 + 273 = 299.2 K


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=

kgmol/s.m2.Pa

Since the gas is dilute, and .

Therefore, kg mol/s.m

2.Pa

From the naphthalene vapor pressure versus temperature graph, the pressure is 9.7 Pa when the

temperature is 26.2 C.

Note that pA1= 9.7 Pa and pA2=0 Pa (pure air)

= (9.7- 0)

= 1.8327 10-7

kg mol/s.m2

= 1.8327 10-7

where M is the molecular weight of naphthalene= 128kg/kg mol

= 2.3459 10-5

kg/m2.s

M sublimated (calculated ) =

=

=

= 4.30 10-5

kg

= 0.04298 g


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The area of the sphere, A =

=

= 1.018 10-3

m

Total amount sublimated (calculated) =

= 1.8327 10-7

1.018 10-3

= 1.8657 10-10

kg mol/s

M sublimated (experimentally) = 0.036 g

M sublimated (calculated) = 0.04298 g

Total amount sublimated (experimentally) = 1.5625 x 10-10

kg mol/s

Total amount sublimated (calculated) = 1.8657 10-10

kg mol/s

Percentage error of mass sublimated

% error M sublimated = -

100%

=

= 16.24 %


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Time

(minutes)

Temperature

(C)

Velocity of air

(m/s)

Weight (g)

10 27.6 28.0 3.110

20 27.7 28.2 3.082

30 26.8 27.8 3.053






CALCULATION




m2/s

Viscosity of air, = 1.864 x 10-5

Pa.s



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Experimentally

M sublimated (experimentally)= initial weight of the naphthalenefinal weight of the naphthalene

= 3.111g -3.053g

= 0.058 g

In 30 minutes, the mass of naphthalene sublimated is 0.058 in 90% area open of aerodynamic

unit.

The molecular weight of naphthalene, C10H8 = [10(12) + 8] = 128 kg/kg mol

The number of mole, n =

=

= 0.02430 mol

Total amount sublimated (experimentally) =

x

x

x

= 2.5174 x 10-10

kg mol/s

Theoretically:

The Schmidt number is

NSc =

=

= 2.597


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The Reynolds number is

NRe =

vD

=

= 1.8077 104

For gases, for a Schmidt number range of 0.6-2.7 and a Reynolds number range of 1-48000,

=

= 138.89

The Sherwood number is

138.89 =

= 0.047 m/s

From the table 7.2-1 (Textbook of Transport processes and Separation Process Principle)

Hence, for T = 27.4 + 273 = 300.4 K


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Discussion:

From the result in this experiment, area of the aerodynamic unit will affect the flow rate

and temperature of the air. We choose 3 area of aerodynamic unit that will give 3 different

results from the experiment. 3 different areas of aerodynamic unit are 30%, 60% and 90%.

Firstly, we are using 30% area of aerodynamic unit. We can see that the mass of the

naphthalene reduced from 3.193g to 3.154g with difference of 0.039g. The value of flow rate and

temperature in first 10 minutes is 9.4 m/s and 25.9oC respectively. For the second and third 10

minutes, the flow rate and temperature are 9.1m/s,25.2oC and 8.8m/s,24.6

oC respectively. From

the results, mass of naphthalene reduced slowly with decreasing in temperature and flow rate of

the air. The experimental value of sublimation rate is 1.693 x 10-10

kg mol/s while theoretical

value is 1.6987 x 10-10

kg mol/s. Percentage error for sublimation rate is 0.31 %.

Second part, by using 60% area of aerodynamic unit, the mass of the naphthalene reduced

faster than using a 30% area of aerodynamic unit. Mass of naphthalene before experiment is

3.112g and final value after 30 minutes is 3.076g with difference of 0.036 g. Theoretically, the

mass difference should be larger than using 30% area of aerodynamic unit. It is because the flow

rate tend to be faster and the convection diffusion must be faster as well. However, some of the

experimental error occurred during experiment, which never cause the mass difference to be

larger than that. The factor that cause it to happen might not proper handling of naphthalene and

inconsistent shape of naphthalene compare to the previous one. The value of flow rate and

temperature of 60% area of aerodynamic unit in first 10 minutes is 22.7m/s and 26.1oC

respectively. For second and third 10 minutes, flow rate and temperature value are 22.1m/s ,26.0

oC and 24.9m/s, 26.5

oC respectively. The experimental value of sublimation rate is 1.5625 x 10

-

10kg mol/s while theoretical value is 1.8657 10

-10kg mol/s. Percentage of error in this

experiment is quite big which is 16.24 %. There might be some error during experiment. For

example, disturbance during the air flow when there has blocker at inlet of the equipment. This

might affect the result in experiment. There has also zero error during weighing naphthalene

before and after experiment.

Lastly, using 90% area of aerodynamic unit, the mass of naphthalene decreased fastest by

time. The mass of naphthalene before experiment is 3.131g and final value is 3.053g with

difference of 0.058 g. By this result, we can determine that the area of the aerodynamic unit


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gives big impact on flow rate and temperature of the air. The flow rate and temperature for the

first 10 minutes is 28.0m/s and 27.6oC respectively. For the second and third 10 minutes, flow

rate and temperature of the air flow slightly decreasing as you can determine it in the result data.

The experimental value of sublimation rate is 2.5174 x 10-10

kg mol/s while theoretical value is

2.0881 10-10

kg mol/s. Percentage error for sublimation rate is 20.58 %. The percentage error is

high might be because of the air surrounding that already reduced the mass of naphthalene before

experiment. Furthermore, we used hand to touch the naphthalene when measured the weight

might cause the reduction of size as well.

From the result mention before, we can determine that mass of naphthalene decreased

when high percentage of area of aerodynamic unit is applied. This is due the faster flow rate and

thus faster mass convection. Reducing of mass is due to the sublimation of the naphthalene by

force air. The area of the aerodynamic unit will affect the flow rate and temperature of the air.

The increasing temperature and air flow causes a marked reduction of the sample radius as the

mass of naphthalene reduced by time. This is because the flow rate and temperature is high and

causing the rate of sublimation increased.

In the first stage, the sublimation rate increases in time because the temperature of the

surface sample rises and affects both desorption of molecules from the solid and movement of

the vapour formed by the diffusion layer which is the sublimation heat, the naphthalene vapour

pressure at the interface gassolid, and the diffusion coefficient of the naphthalene vapours in air

are functions of temperature.

Dimensionless parameters are often used to correlate convective transfer data. In

momentum transfer Reynolds number and friction factor play a major role. In the correlation of

convective heat transfer data, Prandtl and Nusselt numbers are important. Some of the same

parameters, along with some newly defined dimensionless numbers, will be useful in the

correlation of convective mass-transfer data.


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Appendix:

Figure 1: Naphthalene sphere being weighed

Figure 2: Aerodynamic Unit

Figure 3: Placement of specimen in the machine

Lab10 Complete

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