experimental modeling of a sustainable natural vacuum desalination unit for domestic water...

7/28/2019 Experimental Modeling of a Sustainable Natural Vacuum Desalination Unit for Domestic Water Production

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University of Bahrain

College of Engineering

Chemical Engineering Department

CHEG 431Senior Project

Semester I - Academic Year 2012-2013

Experimental Modeling of a

Sustainable Natural Vacuum

Desalination unit for Domestic water

production

A report submitted to University of Bahrain in partial fulfillment of the

requirements for the degree of Bachelor of Science in Chemical Engineering.

Submitted on: 7th of January 2013

Done by: I.D. No.Ahmed Sameer 20073435

Abdulaziz Shami 20073521

Under Supervision:Dr. Raed Al-Jawdar AdvisorProf. Teomen Ayhan Co-Advisor


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1. Introduction:

Figure 1.1 global water resources

Sweet water or drinking water has become one the most important challenges faces the

world due to the high growth of population and the limited resources. Earth containsaround 1.4(109) km of water which covers 70% of plant surface area. The amount of

sweet water is approximately 2.5% and 80% of it is frozen. Only 0.5% of water around

the world is supporting the life [1]. According to United Nations Population Fund the

world population in 2012 is around 7 billion and expected to be 10 billion in 2100. The

recommended amount of water usage for person is 1.8 Liter per day [2] which mean the

world is using 5.99(1012) Liter of water per year. In additional to human drinking usage

sweet water is used in cultivation and house working usage.

Figure 1.2 forecast water distribution at 2025
http://en.wikipedia.org/wiki/United_Nations_Population_Fundhttp://en.wikipedia.org/wiki/United_Nations_Population_Fund


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In additional to limited water resources the sweet water is not distributed though the

world (see figure 1.2).

Figure 1.3 typical water salinity

The world is trend to use desalination process to produce sweet water from salinity water.

The process is to remove salts from the water which consists mainly from 55% by mass

of chloride and 30% of sodium. The first water desalination plant where installed in

Kuwait in 1957 by Westinghouse. In 1960, the First MSF plants commissioned in

Shuwaikh, Kuwait and in Guernsey, Channel Island. The MSF unit in Shuwaikh had 19

stages, a 4550 m3/d Capacity. In 1966, the co-generation method was reduced the cost of

desalination by 50%. The idea of low temperature mechanical vapor compression with

low temperature was introduced in 1980. The largest multistage flashing unit was

constructed united Arab emirates in 1996 with capacity 57,735 m3/d.

According to the International Desalination Association, in 2009, 14,451 desalination

plants operated worldwide, producing 59.9 million cubic meters per day, a year-on-year

increase of 12.3%. The production was 68 million m3 in 2010, and expected to reach 120

million m3 by 2020.

Figure 1.4 industrial desalination techniques


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There are two main techniques used in industrial water desalination, evaporation by using

thermal energy and reverse osmosis by using mechanical energy (see figure 1.4).

Figure 1.5 evaporation process

In flash evaporation process water is evaporated by using steam and separated from salts.

Then water condensed by using cooling water and the fresh water (see figure 1.5).

Figure 1.6 evaporation with vapor compression process


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A modified evaporation process is including vapor compressor thats compressed the

vapor from the condenser to generate steam and using it to evaporate the feed (see figure

1.6).

Figure 1.7 osmosis and reverse osmosis processes

Reverse osmosis: Osmosis is the net movement ofsolvent molecules through a

partially permeable membrane into a region of highersolute concentration, in order to

equalize the solute concentrations on the two sides. Osmosis pressure is depending on

the concentration in both sides and its the driver of the solvent movement. by Appling a

pressure over than osmosis difference the solvent will transfer in reverse direction andthis is what called reverse osmosis [1] (see figure 1.7).

The cost of desalination process depends mainly on the energy used to remove salts from

water. Desalination process consume huge amount of energy. Desalination consume

around 0.4% of global electricity production and the cost of desalination process is equal

to 0.5 $ per cubic meter[3].

In the recent years, many researches were conducted to reduce the cost of desalination.

One of the suggested techniques is to operate the system at natural vacuum. The benefit

from this is to reduce the amount of required heat. In additional to that, desalinationprocess under vacuum pressure can work without external heat source by using solar

energy. This suggestion is still under study to enhance the amount of water produced

which is considered as small in current time. In this project natural vacuum desalination

system based on solar energy is used. The process contains evaporation and condensation

sides. The fresh water is evaporated in the evaporation side and the vapor condensed in

the condenser.
http://en.wikipedia.org/wiki/Solventhttp://en.wikipedia.org/wiki/Semipermeable_membranehttp://en.wikipedia.org/wiki/Solutehttp://en.wikipedia.org/wiki/Solutehttp://en.wikipedia.org/wiki/Semipermeable_membranehttp://en.wikipedia.org/wiki/Solvent


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The aim of this project is to:

1- Studying the water vacuum desalination system and the effect of temperature and thepressure in the flow rate of sweet water.

2- Setting up a procedure to run the system.3- Finding a correlation between the pressure and the temperature by using

experimental model.

One of the experiment aims is to compare between the experimental results and a

theoretical model was constructed as a senior project by past students but due to the time

constrain and the complexity of the system since it run at the first time so decide to

construct an experimental model only.


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2. Theory

2.1 Vacuum pressure and evaporation process

Figure 2.1.1 Natural Vacuum Desalination system[5]

For approval the formation of natural vacuum pressure generated in the system applies

Bernoullis equation for incompressible flows to obtain the pressure at point 3 and 5:

Equation (2.1.1)

Where:

is the fluid flow at the chosen point is the acceleration due to gravity

is height at the chosen point is density of the fluid is the pressure at chosen point


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Assumptions:

At initial condition for the vaporization side (1 and 3) :

and (As reference point)

By using the assumptions in equation (1), the amount of can simplified as:

Equation (2.1.2)

In equation (2), if the is Temperature is ambient ( ): and

, then is obtained:

Similarly, if apply the Bernoullis equation and condition between point 7 and 5, the

value of pressure at 4:

That mean when operation of water at height 10.33 m, the value of pressure at point 3

and 5 ( and) is found zero. This also means that the natural vacuum is createdbetween the evaporation and condensation tanks.


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Figure 2.1.2 Molecular motions at different temperatures

Figure 2.1.3 Molecular collusion at different pressure from low to high

The kinetic energy of an object is the energy which it possesses due to its motion or the

required Work to accelerate a body of a given mass from rest to its stated velocity. The

kinetic energy is expressed by:

Equation (2.1.3)Where:

is the kinetic energy is the mass is the velocity

Low temperature High temperature
http://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Motion_(physics)http://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Motion_(physics)http://en.wikipedia.org/wiki/Energy


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According to The kinetic theory of gases, the kinetic energy can be related to the

temperature by the Maxwell-Boltzmann distribution:

Equation (2.1.4)

Where:

is the kinetic energy per degree of freedomK is the Boltzmann constant

T is the temperature.

From the second equation its clear that the kinetic energy increase with the temperature

(see figure 2.1.2).

Intermolecular forces are forces of attraction or repulsion which act between neighboring

particles (atoms, molecules or ions).

As the pressure increase, repulsive forces from molecules oppose the decrease in volume.

The frequency of collisions also increases at higher pressure, thereby increasing the

contribution of these intermolecular forces (see figure 2.1.3).

Figure 2.1.4 Water Saturation curve
http://en.wikipedia.org/wiki/Kinetic_theoryhttp://en.wikipedia.org/wiki/Maxwell-Boltzmann_distributionhttp://en.wikipedia.org/wiki/Maxwell-Boltzmann_distributionhttp://en.wikipedia.org/wiki/Kinetic_theory


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Evaporation process occurs when the kinetic energy of liquid molecules overcome and

break liquid-phase intermolecular forces, this happen when the temperature reaches the

saturation temperature. The saturation temperature is defined as the temperature for a

corresponding saturation pressure at which a liquid boils into its vapor phase (see Figure

2.1.4).

The relationship between the boiling temperature and the pressure can be described by

Clausius-Clapeyron equation:

( )

Equation (2.1.5)

TB is the normal boiling point,

R is the ideal gas constant, 8.314 J K mol

P0 is the vapor pressure at a given temperature, atm

Hvap is the heat of vaporization of the liquid, J/mol

T0 isthe given temperature, K

Based on previous results, the evaporation rate will increase with the increase of

temperature and the decrease of the pressure. When the molecules liberate from the

liquid the average kinetic energy of the remaining molecules will decrease and therefore

the temperature will decrease and this is called evaporative cooling. [4]
http://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Clausius-Clapeyron_equationhttp://en.wikipedia.org/wiki/Ideal_gas_constanthttp://en.wikipedia.org/wiki/Ideal_gas_constanthttp://en.wikipedia.org/wiki/Clausius-Clapeyron_equationhttp://en.wikipedia.org/wiki/Kinetic_energy


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2.2 Experimental model:

Models are used to relate the input and the output or the results of the process. Models

are used to obtain the optimum operating conditions, controlling the system or to design

a process. There are two common types of models, the first type is the theoretical model

and its obtained from the system balance equation (energy and mass). The second type

is the experimental models which is constructed by conducting experiments at different

input variable values and obtains the results then correlate between the inputs and

outputs mathematically. The theoretical model is more complicated than the

experimental model since its detailed. However the results obtained from the theoretical

model is more realistic and determining the model is less expensive than the

experimental model because the second one needs a lot of experiments.

To relate three variable two independent and one dependent multiply regression method

is used. The following equation for three variables linear regression:

Equation (2.2.1)Where:

Y is the output

and are the inputs variable, and are the regression coefficientBy determining the regression coefficient, a correlation between the inputs and the

output can be determined.

To check how a regression line fits a set of data, the coefficient of multiple correlation

(R2) is used and Its a number between zero and one. For example if (R2) is equal to 0.8

that mean 80% of the variation of y around its mean is explained by the regressors x1 and

x2.


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3. Apparatus and Procedure:

3.1 Experiment strategy

As mentioned before in the introduction, the main purpose of the experiment is to study

the effect of temperature and pressure on the condensing liquid flow rate. Due to thecomplexity of the system and time constrain three different pressures only and three

temperatures for each one were choosing to run the experiment.

Figure 3.1.1 experiment strategy

The highest vacuum value was choose according to lowest possible pressure obtained

from the system. We reach 27 torr one time only so we set the minimum pressure to be

60 torr. The idea of the system is to operate at ambient temperature only without external

heaters but due to the time limitation and the aim of the project we use heaters the

temperature values were set starting from 40 Celsius to 60 Celsius.

The main defect in the system is the leakage and the vacuum was loosed rapidly at the

first runs so we didnt get constant pressure values. The main purpose of the experiment

is to use natural vacuum but to fixing the leakage problem will consume the time so we

control the pressure manually by using vacuum pump.

The idea of the process is to condense the vapor without using cooling system. However,

we use cooling jacket at constant temperature.

Additional sets were used to obtain the experimental model. Some failed runs were

occurred and rejected. We use tab water instead of sea water because its not available.

With the current configuration the system required at least 4 persons to run the

experiments. Prof. Teomen Ayhan from the department of mechanical engineering and

engineer Juma were worked with us to complete the task.

We take the first month to study the system thermally and to prepare a proper procedure.

60 torr

40 C0 50 C0 60 C0

70 torr

40 C0 50 C0 60 C0

80 torr

40 C0 50 C0 60 C0


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3.2 Apertures:

Figure 3.2.1 upper side of natural vacuum system

The apparatus was installed by Prof. Teomen Ayhan in building number 14 in isa town

campus. The system is U shape from the ground to the roof and it is contains:

1- Two tanks for feed and discharge at the ground.2- U shape pipe with 25 cm diameter at the roof of the building. The right side

represents the evaporation part and the left side for the condensation.

3- Two pipes connected the ground tanks to the U shape pipe.4- 3 heaters in evaporation part.5- Sheller support water to the cooling jacket which located around the central and

condensation pipe.

6- Vacuum pump connected to central part.7- Two manometers at right and left sides.8- Two glasses tubes for each side to observe levels of water.9- 12 Temperature sensors distributed around the U shape pipe.10-2 pressure transducer in the condensation and evaporation parts.

There is also a control room contain two computers connected with sensors, transducers

and ambient temperature sensor.


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Figure 3.2.2 (a) typical view of the vapor tank connected with the evaporation column (Part 1) and the

condensation column (Part2). (b) The view of the evaporation column.[5]

Figure 3.2.3 small scale model on the unit

There are also small scale model for the system as shown in figure (3.2.2) under construction.

This model is easy to study and will reduce the experimental error.


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3.3 Procedure:

Figure 3.3.1 Schematic diagram of the experimental set-up.[5]

1- Open air release valve.2- Full the system by water.3- Close air release valve.4- Turn on heater and Sheller to specific temperatures,5- Reduce specific and equal amount of water from two tanks to have length of

10.33 m.

6- Start the experiment after reaches the specific temperatures for the vaporizationand condensation sides and record the time and temperature readings.7- Record the level of water from the condensation side every 15 minutes.

8- Check the pressure, if exceeds the set point operate the vacuum pump then turnoff the pumps when return to set point.

9- Check temperature, if changes control it by adjust heaters.10-Repeat steps with different temperatures and pressures according to the

experiment strategy.


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4. Results and Discussion:

4.1 Investigation of the effect of pressure and Temperature

Table 4.1.1 Expermintal results

No. of run Temperature

(degC)

Pressure

(torr)

Amount of water

(L/day)

1 40 80 0.00

2 50 80 10.47

3 60 80 39.04

4 40 70 3.88

5 50 70 21.20

6 60 70 46.36

7 40 27 23.558 40 60 8.99

9 50 60 26.69

10 60 60 50.63

Figure 4.1.1 Postion of expermital runs in water staurtion

1

10

100

1000

0 20 40 60 80 100

SaturaionPressure(Torr)

Temperature (degC)

Water staurtaion pressure

P= 60 torr

P= 70 torr

P= 27 torr

P= 80 torr

Liquid

Steam


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As shown from Figure (4.1.1), each point is a run of experiment with its pressure and

temperature conditions. Also, most of the points located in steam side where they are in

super-heated steam situation. In that situation the temperature decreases in evaporation

due to evaporative cooling as mention in Chapter 2. Some of molecules skip from liquid

and the average liquid kinetic energy decrease which lead to decrease the temperature.

Figure 4.1.2 Effect of pressure in the condensed water flow rate at different temperature

From figure (4.1.2), as shown the flow rate decreases with increase of pressure (which

means decreases of Vacuum) for each temperature. This happen because increasing of

pressure leads to increase the collision between molecules and to increase intermolecular

force. The kinetic energy thats overcome the intermolecular force is related to the

temperature and the temperature is constant therefore, the amount of bonds thats break

will be less and less amount of vapor will liberate from liquid.

0

10

20

30

40

50

60

60 65 70 75 80 85

Waterflowrate

(L/day)

Pressure (Torr)

Water flow rate vs. Pressure

T= 40 C

T= 50 C

T= 60 C


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Figure 4.1.3 Effect of Temperature in the condensed water flow rate at different pressures

From figure (4.1.3), as shown the flow rate increases with increase of for each pressure. This

happen because increasing the temperature will increase the kinetic energy where the

intermolecular force is related to the pressure and the pressure is constant so more amount of

bonds will break and more amount of vapor will liberate from liquid.

Based on the previous results, the best condition is to run the evaporator at highest possible

temperature and lowest pressure to increase the evaporation rate.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

30 35 40 45 50 55 60 65

Waterflowrate(L/day)

Temprature ( C )

Water flow rate vs. Temprature

P= 80 torr

P= 70 torr

P= 60 torr


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4.2 Experimental model:

Figure 4.2.1 Model scheme Diagram

The following correlation between the output liquid volumetric flow rate and the inputs

(boiling temperature and the pressure) was obtained by multiple linear regression method.

The regression coefficient was determined by using regression tool in Microsoft EXCEL

software. (For more information see Appendix B)

The coefficient of multiple correlations (R2

) was determined to be 0.96 which mean 96%of the variation of y around its mean is explained by the regressors P and T.

Table 4.2.1 Compression between Actual and Estimated flow rates

Run

No.

Boiling temperature

(T) , C0

Pressure (P),

torr

Actual flow rate

L/day

Estimated flow

rate, L/day

Error

percentage %

1 40.00 80.00 0.00 -5.16 -

2 50.00 80.00 10.47 16.52 57.80

3 60.00 80.00 39.04 38.19 2.17

4 40.00 70.00 3.88 0.58 84.96

5 50.00 70.00 21.20 22.26 5.04

6 60.00 70.00 46.36 43.94 5.23

7 40.00 27.00 23.55 25.29 7.40

8 40.00 60.00 8.99 6.33 29.60

9 50.00 60.00 26.69 28.01 4.94

10 60.00 60.00 50.63 49.69 1.87


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Figure 4.2.2 Normal Probability Plot

From table (4.2.1), the error percentage between the estimated and actual liquid flow rate

is less than 8% for 6 points and its increased to 29.6% at point 8 and exceed 50% inpoints 2 and 4 respectively. The error percentage cannot be calculated at the first point

because infinity value will be obtained since dominator is equal to zero. The results of the

points 4 and 8 were affected due to the value of the first point. From the normal

probability plot the non-linearity occurs at the beginning of curve. To solve this problem,

more points of temperature and pressure that gives zero liquid flow rates should be

plotted. However this correlation can be used as preliminary estimation of the flow rate at

certain pressure and temperature.

0

20

40

60

0 20 40 60 80 100Orderedr

esponse

Sample Percentile

Normal Probability Plot


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5. Conclusion and Recommendations:

5.1 Conclusion

To conclude, the water vacuum pressure desalination unit was operated for the first time

and experiments were conducted to investigate the effect of temperature and pressure in

the flow rate of water. The results were equivalent with the theory thats stat e the

evaporation rate increases with increase the temperature and vice versa for the pressure.

An experimental model was constructed to describe the relationship between the

temperature, pressure and the flow rate. Only 11 results were used to construct the model

due to the complexity of the system and the time limitation. A procedure to run the

system was set and some recommendations were suggested to simplify the operation and

to enhance the heat transfer.


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5.2 Recommendations:

1- Using pressure control system to adjust the pressure automatically. The systemmeasures the pressure inside the column and compares it with the set point. Ifthere is an action need to be taken the system adjust the vacuum pump to

eliminate the difference between the measurement value and the set point.

2- Using solar power to generate electricity in order to operate the heater and waterpump. Photovoltaic is a method used to transfer solar radiation directly to

electricity by using semi-conductors.

3- Using fan to increase the speed of the vapor from the evaporation section to thecondensation section and therefore decrease the time of vapor movement and

increase the condensation rate.

4- Using level measurement connected to computer in order to decrease the humanerror and to monitor the level easily.

5- Manufacturing all connection in the system in high quality to reduce property ofleakage.

6- Removing the central section of the pipe to reduce the leakage and watermovement time to the condenser.

7- Using sea water instead of tab water to be more realistic and to achieve the mainobjective of the system.

8- Adding water treatment section by add chemicals such as oxygen Scavenger tofeed to prevent non-condensable gases from entering the system.
http://en.wikipedia.org/wiki/Scavenger_(chemistry)http://en.wikipedia.org/wiki/Scavenger_(chemistry)


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References:

1- El-Dessouky and Ettouney.Fundamentals of Salt Water Desalination, 1st Edition,Elsevier Science, 2002

2- Greenhalgh, Alison (March 2001). "Healthy living - Water". BBC Health.http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.sht

ml. Retrieved 2007-02-19.

3- "Black & Veatch-Designed Desalination Plant Wins Global Water Distinction,"Black & Veatch Ltd., via edie.net, 2006-05-04. Retrieved on 2007-08-20.

4- Atkins.Physical Chemistry,8th Edition5- T.Ayhan,H. Al Madani. Feasibilty study of renewable energy powered seawater

desalination technology using natural vacuum technique,Sience Direct, 2010.
http://store.elsevier.com/authorDetails.jsp?authorId=ELS_1023379http://store.elsevier.com/authorDetails.jsp?authorId=ELS_1023346http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtmlhttp://en.wikipedia.org/wiki/BBChttp://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtml.%20Retrieved%202007-02-19http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtml.%20Retrieved%202007-02-19http://www.edie.net/news/news_story.asp?id=11402&channel=0http://www.sciencedirect.com/science/article/pii/S0960148109002833http://www.sciencedirect.com/science/article/pii/S0960148109002833http://www.sciencedirect.com/science/article/pii/S0960148109002833http://www.sciencedirect.com/science/article/pii/S0960148109002833http://www.edie.net/news/news_story.asp?id=11402&channel=0http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtml.%20Retrieved%202007-02-19http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtml.%20Retrieved%202007-02-19http://en.wikipedia.org/wiki/BBChttp://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtmlhttp://store.elsevier.com/authorDetails.jsp?authorId=ELS_1023346http://store.elsevier.com/authorDetails.jsp?authorId=ELS_1023379


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Appendices

Appendix A: Raw Data

Appendix B: Experimental Model Calculation


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Appendix A: Raw Data

No 1 No 2

Data& Time Minutes Level(cm) Data& Time Minutes Level(cm)15-Oct 15-Oct

03:30 0 0 03:30 0 0

03:45 15 1.5 03:45 15 1.5

04:05 35 3.1 04:05 35 3.1

04:25 55 0.7 04:25 55 0.7

Temp. (C) 40 Temp. (C) 50

Pressure (torr) 60 Pressure (torr) 60

No 3 No 4

Data& Time Minutes Level(cm)

Data& Time Minutes Level(cm)18-Oct 14-Nov

02:00 0 0 02:39 0 0

02:19 19 1.5 02:57 18 0.3

02:40 40 3.1 03:39 60 0.4

03:00 60 4.3 04:10 91 0.5



No 5 No 6


Data& Time Minutes Level(cm)14-Nov 18-Nov

05:51 0 0 04:30 0 0

06:08 17 1.1 04:45 15 0.75

06:23 32 2.1 05:00 30 0.9




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No 7 No 8


Data& Time Minutes Level(cm)13-Dec 23-Dec

06:31 0 0 05:11 0 0

06:55 24 0.8 05:26 15 0

05:41 30 0

05:56 45 0



No 9 No 10



29-Dec 29-Dec04:42 0 0 06:20 0 0

04:57 15 0.2 06:38 18 0.9

05:17 35 0.5 06:58 38 2.1

05:36 54 0.8




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Appendix B: Experimental Model Calculation

Output from Excel:

Regression Statistics

Multiple R 0.98471247

R Square 0.96965864

Adjusted R Square 0.95954485

Standard Error 3.36547618

Observations 9

ANOVA

df SS MS F

Significance

F

Regression 2 2171.842 1085.921 95.87494 2.79E-05

Residual 6 67.95858 11.32643

Total 8 2239.8

Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Lower 95.0% Upper 95.0

ntercept -45.907322 7.164077 -6.40799 0.000681 -63.4372 -28.37745783 -63.4372 -28.377

40 2.16783642 0.15656 13.84666 8.83E-06 1.784747 2.550925582 1.784747 2.55092

80 -0.5746017 0.084972 -6.76228 0.00051 -0.78252 -0.366683788 -0.78252 -0.3666


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Nomenclature:

is the fluid flow at the chosen point is the acceleration due to gravity

is height at the chosen point is density of the fluid is the pressure at chosen point is the kinetic energy is the mass is the velocity is the kinetic energy per degree of freedomK is the Boltzmann constant

T is the temperature.

TB is the normal boiling point,

R is the ideal gas constant, 8.314 J K1

mol1

P0 is the vapor pressure at a given temperature, atm

Hvap is the heat of vaporization of the liquid, J/molT0 isthe given temperature, K

Y is the output from regression function

and are the inputs variable, and are the regression coefficient
http://en.wikipedia.org/wiki/Ideal_gas_constanthttp://en.wikipedia.org/wiki/Ideal_gas_constant

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