VLE-UNIT

CHEMICAL ENGINEERING LABORATORY

CHE 465

VAPOR-LIQUID EQUILIBRIUM UNIT

EXPERIMENT

NAME MATRIX NO.AHMAD IFWAT AHMAD KAMAL 2011144359NUR SYAHIDAH MOHD SUBARI 2011785691MUHAMMAD SHAMIL AZHA IBRAHIM 2011195429

LECTURER : MISS ADIEB

DATE OF SUBMISSION : 22ND OCTOBER 20121

2

ABSTRACT

The experiment was carried out to investigate the relationship between vapour and liquid at

equilibrium and at atmospheric pressure. The experiment was also conducted to construct the equilibrium

curves at atmospheric pressure for binary system namely methanol and water. The experiment was carried

out using the Vapour Liquid Equilibrium (VLE) unit. A mixture of methanol-water with known composition

is initially fed into the evaporator. When the heater is switched on, the mixture will start to boil. The mixture

vapour will rise up and will be cooled down by the condenser at the top of the evaporator. As the vapour

starts to condense, the liquid falls back into the evaporator. The system will stabilize and finally reach an

equilibrium state when temperature remains constant. Samples of vapour and liquid are taken to determine

their compositions. At the end of the experiment, a graph of mole fraction of vapour against mole fraction of

liquid and a graph of temperature against mole fraction of liquid and vapour were plotted. This equilibrium

curves at atmospheric pressure for binary system namely methanol and water clearly shows the relationship

between vapour and liquid at equilibrium and at atmospheric pressure. It can be said that from the graphs,

the relationship between vapour and liquid at equilibrium and at atmospheric pressure is that they exist in

linear. The experiment was considered a success as all the objectives were achieved.

3

INTRODUCTION

Vapour-liquid equilibrium data are the basic information of the system required for the design of

equilibrium stages of vapour-liquid separation equipment like distillation. The unit can be used to study any

binary system as well as multi component system.

Vapour-liquid equilibrium unit is a condition where liquid and its vapour (gas phase) are

in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapour)

equals the rate of condensation (vapour changing to liquid).

Several kinds of equilibria are important in mass transfer. In all situations, two phases are involved,

and all combinations are found except two gas phases or two solid phases. The controlling variables are

temperature, pressure and concentrations. To classify equilibria and to establish the number of independent

variables, phase rule is used.

Equation 1

∅=number of degreesof freedom

C=number of components

P=numberof phases

In this case, for VLE (Vapour Liquid Equilibrium) unit, two components are used and found in both

phases. Thus, its degree of freedom is:

∅=2−2+2=2

When the pressure is fixed (isobaric), only one variable can be change independently such as the

liquid-phase concentration and both temperature and vapour phase concentration then follow.

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∅=C−P+2

OBJECTIVES

The objectives of the experiment are:

1. To investigate the relationship between vapour and liquid at equilibrium and at atmospheric pressure.

2. To construct the equilibrium curves at atmospheric pressure for binary system namely methanol and

water.

5

THEORY

Vapour-liquid equilibrium unit is suitable for investigating the relationship between vapour and

liquid at equilibrium at normal pressure and at high pressure up to 20.0 bars. Equilibrium data represent the

composition of the mixture in the vapour phase (Y) and that in the corresponding equilibrium liquid phase

(X) at equilibrium. In order to separate a binary mixture using distillation process, there must be differences

in volatilities of the components. The greater the difference, the easier it is to do so. Volatility is the measure

of an element to evaporate easily by means element with lower boiling point.

A vapour-liquid equilibrium unit is carried out by manipulating its mixture composition such as the

volume of methanol and water. It is more convenient to express compositions using mole fraction. Mole

fraction is the number of moles of one component to the total number of moles in the mixture.

The compositions are presented in mole fractions of the more volatile component. Equilibrium

compositions are functions of temperature and pressure. Therefore the data are reported under isothermal or

isobaric conditions.

In order to be able to predict the phase behaviour of a mixture, limits of phase changes are to be determined.

The limits in the case of gas-liquid phase changes are called the bubble point and the dew point.

The bubble point is the point at which the first drop of a liquid mixture begins to vaporize.

The dew point is the point at which the first drop of a gaseous mixture begins to condense.

Plotting both the bubble and the dew points on the same graph could come up with what is called a P-xy or

a T-xy diagram, depending on whether it is graphed at constant temperature or constant pressure. The "xy"

implies that the curve is able to provide information on both liquid and vapour compositions.

o Example: Binary system of benzene-toluene mixture.

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mole fraction= molesof componenttotalnumber of moles

Isochoric condition (constant temperature)

Graph of pressure versus benzene composition

P-xy diagram

Benzenecomposition (mole fraction)

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Isobaric conditions (constant temperature)

Graph of temperature versus benzene composition

T-xy diagram

Benzenecomposition (mole fraction)

8

Temperature(¿0C)¿

Graph of benzene composition in vapour (Y) and liquid (X) phase.

XY diagram

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APPARATUS / MATERIALS

Beaker

Thermometer

Refractometer

Water

Methanol

Tissue paper

Goggles

Gloves

Measuring Cylinder

Sample bottles

VLE Equipment

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PROCEDURE

General Start-up procedures

1. A quick operation was prepared to ensure that the unit was in proper operating condition.

2. The unit was connected to the nearest power supply.

3. The valves were opened at the feed port and the level sight tube ( V1, V2, and V3)

4. The boiler was filled with distilled water through the feed port and make sure that the water level is

at about half of the boiler’s height. Then, the valves, V1 and V2 and the level sight tube were closed.

5. The power supply switch was turned on.

6. The experiment was then carried out.

General Shut-down procedures

1. The heater was switched off and the boiler temperature was allowed to drop.

Note: The valve at the water inlet port was made sure not to be opened as it is highly pressurized at

high temperature.

2. The main switch and the main power supply was switched off.

3. The water next use was retained.

4. The upper part of the level sight tube, V3 was opened to drain off the water. V1 and V2 was then

opened to drain off the water.

Sampling procedures

1. Vapour sampling from the condenser.

i) Vent valve V6 was ensured to be opened and drain valve V7 was closed.

ii) Valve V5 was slowly opened to allow some condensed vapour from the condenser to flow

into the top sample collector. Valve V5 is closed.

iii) Valve V7 was opened to collect the sample in a sampling vial.

iv) The cap on the vial was immediately closed and the sample was immersed in cold water.

v) The sample is used on different litres of water and methanol as shown on result.

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2. Liquid sampling from the evaporator

i) Valve V4 was ensured to be opened and drain valve V3 is closed.

ii) Valve V12 was opened to allow cooling water to flow through the bottom sample collector.

iii) Then, valve V2 was slowly opened to allow some liquid from the evaporator to flow into the

sample collector. Valve V2 was closed back.

iv) Valve V3 was opened to let the sample in a sampling vial to be collected.

v) The cap on the vial was immediately closed and the sample was immersed in cold water.

vi) The sample is used on different water and methanol as shown on results.

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RESULTS

Volume of Water

(L)

Volume of Methanol

(L)

Temperature(¿0C)¿

Vapour Liquid

Refractive Index

Vapour Liquid

3.0 0.1 89.4 97.9 1.3386 1.3328

3.0 0.3 86.4 94.7 1.3381 1.3327

3.0 0.5 84.6 91.7 1.3389 1.3329

3.0 1.0 81.1 86.3 1.3401 1.3357

3.0 2.0 76.2 80.2 1.3404 1.3373

3.0 3.0 73.9 77.3 1.3393 1.3398

Table 1

Volume of Water

(L)

Volume of Methanol

(L)

Temperature(¿0C)¿

Vapour Liquid

Refractive Index

Vapour Liquid

1.0 2.0 70.6 73.2 1.3360 1.3386

1.0 3.0 69.0 71.6 1.3403 1.3407

1.0 5.0 67.1 69.3 1.3350 1.3388

Table 2

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CALCULATIONS

Density of water (H ¿¿2O)=1g /cm3 ¿

Density of methanol (CH 3OH ¿=0.7918 g /cm3

Molecular weight H 2O=18g /mol

Molecular weight CH 3OH=32 g/mol

Number of moles= Mass of substanceMolecularweight of substance

Therefore,

i) Number of moles of H 2O

Volume H 2O=3 L

Mass of H 2O=1g H 2O

cm3H 2O×

106 cm3

1000 L×3 LH 2O=3000 g H 2O

Molesof H2O=3000 g H 2O×1mol H 2O

18 g H 2O=166.667mol

Volume H 2O=1L

Mass of H 2O=1g H 2O

cm3H 2O×

106 cm3

1000 L×1L H 2O=1000g H 2O

Molesof H2O=1000 gH 2O×1mol H 2O

18 g H 2O=55.556mol

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ii) Number of moles CH 3OH

Volume CH 3OH=0.1 L

Mass of CH 3OH=0.7918gCH 3OH

1cm3CH3OH×

106cm3

1000 L×0.1 LCH 3OH=79.18 gCH 3OH

Molesof CH 3OH=79.18 gCH 3OH×1molCH 3OH

32gCH 3OH=2.4744molCH3OH

Volume CH 3OH=0.3 L


1cm3CH3OH×

106cm3

1000 L×0.3 LCH 3OH=237.54 gCH3OH

Molesof CH 3OH=237.54 gCH 3OH×1molCH3OH

32gCH 3OH=7.4231molCH 3OH

Volume CH 3OH=0.5 L


1cm3CH3OH×

106cm3

1000 L×0.5 LCH 3OH=395.9gCH 3OH

Molesof CH 3OH=3.95 .9 gCH3OH×1molCH 3OH


Volume CH 3OH=1.0 L


1cm3CH3OH×

106cm3

1000 L×1.0 LCH 3OH=791.8 gCH 3OH

15



Volume CH 3OH=2.0 L


1cm3CH3OH×

106cm3

1000 L×2.0 LCH 3OH=1583.6 gCH 3OH

Molesof CH 3OH=1583.6 gCH 3OH ×1molCH 3OH

32 gCH3OH=49.4875molCH 3OH

Volume CH 3OH=3.0 L


1cm3CH3OH×

106cm3

1000 L×3.0 LCH 3OH=2375.4 gCH 3OH



Volume CH 3OH=2 L


1cm3CH3OH×

106cm3

1000 L×2LCH 3OH=1583.6 gCH 3OH

Molesof CH 3OH=1583.6 gCH 3OH ×1molCH 3OH

32 gCH3OH=49.4875molCH 3OH

Volume CH 3OH=3 L


1cm3CH3OH×

106cm3

1000 L×3 LCH 3OH=2375.4 gCH 3OH

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Volume CH 3OH=5 L


1cm3CH3OH×

106cm3

1000 L×5 LCH 3OH=3959 gCH 3OH



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Volume

H 2 0 used

(L)

Volume

CH 3OH

used (L)

Mole

H 2 0

(mol)

Mole

CH 3OH

(mol)

Moltotal

mol (H 20+CH 3OH )

Mole

fraction

H 2 0

(mol H 2 0

mol total )

Mole fraction

CH 3OH

(molCH 3OH

mol total )

3.0 0.1 166.667 2.4744 169.1414 0.99 0.01

3.0 0.3 166.667 7.4231 174.0901 0.96 0.04

3.0 0.5 166.667 12.3719 179.0389 0.93 0.07

3.0 1.0 166.667 24.7438 191.4108 0.87 0.13

3.0 2.0 166.667 49.4875 216.1545 0.77 0.23

3.0 3.0 166.667 74.2313 240.8983 0.69 0.31

1.0 2.0 55.556 49.4875 105.0435 0.53 0.47

1.0 3.0 55.556 74.2313 129.7873 0.42 0.58

1.0 5.0 55.556 123.7188 179.2748 0.31 0.69

Mole fraction of Methanol (CH 3OH ).

Table 3

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Composition of Methanol (CH 3OH ) in mole fraction

Temperature(¿oC )¿ Mole fraction Refractive Index, RI Mole fraction

Vapour Liquid H 2 0

(Mol H 20

Moltotal )

CH 3OH

(MolCH3OH

Moltotal )

Vapour Liquid CH 3OH

(vapour)

Y

CH 3OH

(liquid)

X

89.4 97.9 0.99 0.01 1.3386 1.3328 0.0134 0.0133

86.4 94.7 0.96 0.04 1.3381 1.3327 0.0535 0.0533

84.6 91.7 0.93 0.07 1.3389 1.3329 0.0937 0.0933

81.1 86.3 0.87 0.13 1.3401 1.3357 0.1742 0.1736

76.2 80.2 0.77 0.23 1.3404 1.3373 0.3083 0.3076

73.9 77.3 0.69 0.31 1.3393 1.3398 0.4152 0.4153

70.6 73.2 0.53 0.47 1.3360 1.3386 0.6279 0.6291

69.0 71.6 0.42 0.58 1.3403 1.3407 0.7774 0.7776

67.1 69.3 0.31 0.69 1.3350 1.3388 0.9212 0.9238

Table 4

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Temperature and mole fraction of Methanol (CH 3OH )

Temperature(¿0C)¿ Mole fraction

Y

(Vapour)

X

(Liquid)

Y

(Vapour)

X

(Liquid)

96.8 94.7 0.0134 0.0133

93.4 90.7 0.0535 0.0533

91.9 87.6 0.0937 0.0933

88.7 84.9 0.1742 0.1736

85.1 79.3 0.3083 0.3076

80 76.8 0.4152 0.4153

77.4 73.1 0.6297 0.6291

73.2 70.4 0.7774 0.7776

69.6 66 0.9212 0.9238

Table 5

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Graph of Temperature versus mole fraction (x,y)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

20

40

60

80

100

120

0

20

40

60

80

100

120

T-xy diagram

liquidvapour

Composition of methanol (mole fraction)

Tem

pera

ture

(C

)⁰

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Graph of mole fraction in liquid versus mole fraction in vapour

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

XY diagram

X =

mol

e fr

acti

on o

f m

etha

nol i

n liq

uid

phas

e

Y = mole fraction of methanol in vapour phase

22

DISCUSSION

The experiment was carried out to investigate the relationship between vapour and liquid

at equilibrium and at atmospheric pressure. The experiment was also conducted to construct the

equilibrium curves at atmospheric pressure for binary system namely methanol and water. The

experiment was carried out using the Vapour Liquid Equilibrium (VLE) unit. A mixture of

methanol-water with known composition is initially fed into the evaporator. When the heater is

switched on, the mixture will start to boil. The mixture vapour will rise up and will be cooled

down by the condenser at the top of the evaporator. As the vapour starts to condense, the liquid

falls back into the evaporator. The system will stabilize and finally reach an equilibrium state

when temperature remains constant. Samples of vapour and liquid are taken to determine their

compositions. At the end of the experiment, a graph of mole fraction of vapour against mole

fraction of liquid and a graph of temperature against mole fraction of liquid and vapour were

plotted. This equilibrium curves at atmospheric pressure for binary system namely methanol and

water clearly shows the relationship between vapour and liquid at equilibrium and at atmospheric

pressure. It can be said that from the graphs, the relationship between vapour and liquid at

equilibrium and at atmospheric pressure is that they exist in linear.

23

CONCLUSION

At the end of the experiment, a graph of mole fraction of vapour against mole fraction of

liquid and a graph of temperature against mole fraction of liquid and vapour were plotted. This

equilibrium curves at atmospheric pressure for binary system namely methanol and water clearly

shows the relationship between vapour and liquid at equilibrium and at atmospheric pressure. It

can be said that from the graphs, the relationship between vapour and liquid at equilibrium and at

atmospheric pressure is that they exist in linear. The experiment was considered a success as all

the objectives were achieved.

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RECOMMENDATION

To improve the experiment and obtain best results, the experiment should be repeated

three times in order to get average readings. This will reduce the deviation from theoretical

results. The experiment itself took a mere four hours to be done once, so with insufficient time,

the experiment could only be done once. To get better results, the experiment should have been

repeated twice.

Besides that, the sample bottles should be cleaned and washed thoroughly so that there

are no impurities inside the bottle. Any impurities in the sample bottles would effect the results

of the experiment.

Next, readings taken from the measuring cylinder should be taken at eye level to avoid

parallax error. Also, while pouring methanol into the measuring cylinder, goggles should be

worn to avoid the methanol from splashing into the eyes. If this accident does occur, wash

thoroughly with water.

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REFERENCES

1. http://en.wikipedia.org/wiki/Vapor%E2%80%93liquid_equilibrium RETRIEVED 21

OCTOBER 2012

2. http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Vapor-Liquid-

Equilibrium-843.html RETRIEVED 21 OCTOBER 2012

3. http://en.wikipedia.org/wiki/Methanol_(data_page) RETRIEVED 21 OCTOBER 2012

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