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Figure 1.1:Simple representation of distillation in a plate column [3].

While conducting the experiment total reflux and finite reflux is the main parameters. As an

information, total reflux condition is valid when the nearly no product removal occurs. All the

overhead vapor is condensed and returned as reflux. Therefore; L/D ratio (reflux ratio) is

infinite. Thus, it makes the operating lines the 450 line. In addition, in this situation, the

operating lines are as far as they can get from the equilibrium curve, so the number of

theoretical stages can be obtained as minimum number [4]. Moreover, in the finite reflux ratio

a higher number of theoretical stages could be obtained than total flux situation.


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2. EXPERIMENTAL PROCEDURES

In this experiment continuous distillation column used to distillate ethyl alcohol -water

binary mixture at total reflux and finite reflux.

The setup consists of a sieve tray column with seven trays, reboiler, cooling system,

pump and flowmeter and there are 6 thermometers which read temperature of different parts

of the system. Sketch of the distillation unit is drawn using Edraw Max with measurements of

important parts shown below Figure 2.1.

Figure 2.1:Experimental setup

In the first part of the experiment distillation with total reflux condition is examined.

After the power supplied, valves V1, V2, V4 and V6 are closed and V3 and V5 opened in

order to reboiler E1 to be filled with mixture. Mixture is pumped until starts to passing to tank


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D3. E2 the cooling unit fills totally and flow rate adjust as 200 l/h. then heater is switched of.

When vapor condensed in cooling unit start refluxed into the column. Reflux ratio adjusted

5:1 on switchboard. Every 5 minutes temperatures are recorded and compared previous values

of each thermometer. After observing the no-change of previous temperature values and

initial ones it is understood that system reach steady state and samples are taken from bottom

and distillate.

In the second part of the experiment the same procedure is followed until reaching the

steady state. System reached steady state solenoid valve S1 opened on switchboard and pump

flow rate settled at 50%.Distillation ended with switch off J1 heater, S1selonoid valve and

pump G1. Then again samples are taken from bottom and distillate.

Samples are analyzed by refractometer by using refractive index of mixtures ethyl

alcohol and water. Therefore weight fraction of bottom and distillate composition is

determined by using refractive indexes.


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3. RESULTS

3.1. Number of Ideal Stages at Total Reflux

To fix the operating lines, reflux ratio (L/D) must be set. One of the limiting values of

reflux ratio is that of total reflux, which means reflux ratio is infinity. At total reflux, L/V

becomes 1 and operating line of enriching and stripping section is 450 line [5]. Table 3.1.1

shows mole fractions of bottoms and distillate which are found from refractive indices.

Number of stages is determined from McCabe-Thiele Diagram as in Figure 3.1.1.

Table 3.1.1: Refractive indices and compositions of ethanol-water in total reflux [6].

Refractive index Mass fraction of

water

Mass fraction of

ethanol

Mole fraction of

ethanol

Bottoms 1.3370 0.9291 0.0709 0.029

Distillate 1.3640 0.15 0.85 0.69

Figure 3.1.1:Number of stages at total reflux


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3.2. Plate Efficiency at Total Reflux

From Figure 3.1.1, it is seen that there is 2.2 ideal stages and 1 reboiler at total reflux

and efficiency is found as,

3.3. Number of Ideal Stages at Finite Reflux

At finite reflux, reflux ratio is set to 5 and compositions of bottoms, distillate and feed

are determined by using refractive indices as done previously.

Table 3.3.1: Refractive indices and compositions of ethanol-water in finite reflux [6].


water

Mass fraction of

ethanol

Mole fraction of

ethanol

Bottoms 1.3370 0.9291 0.0709 0.029

Distillate 1.3640 0.15 0.85 0.69

Feed 1.3630 0.2266 0.7734 0.57

To obtain number of ideal stages, operating lines of stripping and enriching section

and q-line must be determined. Calculations of slopes q-line and enriching section are shown

in Appendix part of the report. q is calculated as 1.14 and slope of q-line is found as 8.14.

Also, slope of enriching section is found as 0.83. When these operating lines are drawn into

equilibrium curve and 450line, stripping section operating line is obtained by drawing a line

from mole fraction of bottoms on 450line to cross section of q-line and enriching section line.

After that, number of ideal stages is determined by using McCabe-Thiele Method and it is

shown in Figure 3.3.1.


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Figure 3.3.1:Number of stages at finite reflux

3.4. Plate Efficiency at Finite Reflux

From Figure 3.3.1, it is obvious that there are 2.7 ideal stages and 1 partial reboiler;

therefore, efficiency is,


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4. DISCUSSIONS

The main aim of the experiment is to determine the efficiency of the plate column

separating water and ethanol by using McCabe-Thiele method.

Some assumptions are made to use McCabe-Thiele method. One of the assumption is

adiabatic column condition and thermal capacitance of column material is negligible. The

column was not isolated; therefore, there is heat loss between the column and surroundings. It

is also assumed that total condensation at the top of column. This problem is most likely

overcome by both cooling jacket and cooling coil. Constant pressure is assumed throughout

the column although there is certain pressure drop throughout the column.

One of the important assumption is that latent heats are equal. This assumption is valid

for water and ethanol mixture. The heat of vaporization of water is 40,626 kj/kmol while the

heat of vaporization of the ethanol is 39,422 kj/kmol. Therefore, the heat of vaporizations are

close enough to assume that they are equal. Besides, sensible heat differences between the

stages are also assumed negligible throughout the calculations. Proven of equal latent heat

assumption also proves equimolar counter-diffusion. The heat released by one mole of vapor

condensation is approximately equal to the heat required to vaporize one mole of the liquid.

This means the number of molecules passing from the vapor phase to the liquid phase and

vice versa holds.

Plate efficiency at total reflux is found as 0.31 whereas it is found 0.39 when reflux

ratio is equal to 5. This result show that efficiency increase with increasing reflux ratio.

However, if the reflux ratio is too high then condenser duty is very large. There is an optimum

value for each distillation column between reflux ratio, condenser duty and efficiency.

The efficiency values are not reliable. Although the reflux ratio is set to 5 by using a

mechanism closed for 5 seconds and open for 1 seconds, the real reflux ratio is different. Thecertain amount of liquid falls to the distillate storage instead of falling into the column when

the mechanism is open.

The heat balance for the system was not made in this report. Since, neither heat given

by the heater of the reboiler nor the vapor flow rate are known.


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5. CONCLUSIONS

In this experiment main aim is to examine continuous sieve tray distillation column at total

reflux condition and finite reflux condition. Several assumptions made for using McCabe-

Thiele method; adiabatic column, total condensation at the top column, constant pressurethroughout the column. Also it is assumed that latent heats are equal so that equimolar

counter-diffusion is proved. When results analyzed it is observed efficiency increase with

increasing reflux ratio. Although reflux ratio settled, real reflux ratio is different. it is

supposed to falling into column when mechanism open but some amount of liquid falls

distillate storage.


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6.REFERENCES

[1]-Experiment 25 Distillation In A Plate Column laboratory manual. Retrieved May 4th

2013 fromhttp://www.metu.edu.tr/~tezsevin/ChE420/Manuals.htm

[2]- What is distillation? Retrieved May 4th2013 from

http://chemistry.about.com/cs/5/f/bldistillation.htm

[3]- Distillation an introduction column internals. Retrieved May 4th2013 from

http://lorien.ncl.ac.uk/ming/distil/distilint.htm

[4]- Distillation IV: Calculations. Retrieved May 4th2013 from

http://facstaff.cbu.edu/rprice/lectures/distill4.html

[5]- Geankoplis C.J. (2003) Transport Processes and Separation Process Principles. 4thed.

Massachusetts: Pearson Education Inc.

[6]- Troy A.S., Refractive Index of Ethanol-Water Mixtures and Density and Refractive

Index of Ethanol-Water-Ethyl Ether Mixtures, Northern Regional Research Laboratory,

Peoria, Illinois,1946.

[7]-McCabe, W.L., Smith J.C., Harriot P., (2005). Unit Operations of Chemical Engineering.

7thed. New York: McGraw Hill

[8]-National Institute of Standards and Technology. Retrieved May 4th

2013 from

http://www.nist.gov/chemistry
http://www.metu.edu.tr/~tezsevin/ChE420/Manuals.htmhttp://www.metu.edu.tr/~tezsevin/ChE420/Manuals.htmhttp://www.metu.edu.tr/~tezsevin/ChE420/Manuals.htmhttp://chemistry.about.com/cs/5/f/bldistillation.htmhttp://chemistry.about.com/cs/5/f/bldistillation.htmhttp://lorien.ncl.ac.uk/ming/distil/distilint.htmhttp://lorien.ncl.ac.uk/ming/distil/distilint.htmhttp://facstaff.cbu.edu/rprice/lectures/distill4.htmlhttp://facstaff.cbu.edu/rprice/lectures/distill4.htmlhttp://www.nist.gov/chemistryhttp://www.nist.gov/chemistryhttp://www.nist.gov/chemistryhttp://facstaff.cbu.edu/rprice/lectures/distill4.htmlhttp://lorien.ncl.ac.uk/ming/distil/distilint.htmhttp://chemistry.about.com/cs/5/f/bldistillation.htmhttp://www.metu.edu.tr/~tezsevin/ChE420/Manuals.htm


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7. APPENDIX

A. Equilibrium Data for Ethanol-Water System

Equilibrium data for ethanol-water system is shown in Table A.1. Data is available for

mass fraction of ethanol; however, mole fractions are needed for diagrams. Therefore, mass

fractions are converted to mole fractions by using the following equation.

Where,

wA is weight of ethanol for 100 kg mixture,

wB is weight of water for 100 kg mixture,

mwAis molecular weight of ethanol,

mwBis molecular weight of water.

Table A.1: Equilibrium data for Ethanol-Water Mixture [5].

T x(mass fraction) y(mass fraction) x(mole fraction) y(mole fraction)

100 0 0 0 0

98.1 0.02 0.192 0.008 0.085

95.2 0.05 0.377 0.02 0.19

91.8 0.1 0.527 0.04 0.3

87.3 0.2 0.656 0.09 0.43

84.7 0.3 0.713 0.14 0.49

83.2 0.4 0.746 0.2 0.53

82 0.5 0.771 0.28 0.57

81 0.6 0.794 0.37 0.6

80.1 0.7 0.822 0.48 0.64

79.1 0.8 0.858 0.61 0.7

78.3 0.9 0.912 0.78 0.8

78.2 0.94 0.942 0.86 0.86

78.1 0.96 0.959 0.9 0.9

78.2 0.98 0.978 0.95 0.95

78.3 1 1 1 1


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By using data in Table A.1, equilibrium curve for ethanol-water is drawn as shown in

Figure A.1 and boiling point diagram is plotted as in Figure A.2.

Figure A.1: Equilibrium curve for ethanol-water

Figure A.2: Boiling Point diagram for ethanol-water

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1y,

molefraction

ofethanolinvaporphase

x,mole fraction of ethanol in liquid phase

Ethanol-Water Equilibrium

0

10

20

30

40

5060

70

80

90

100

110

120

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Temperat

ure,T

(C)

mole fraction of ethanol in liquid,x or vapor,y

Boiling Point Diagram for Ethanol-Water


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B. Refractive Index of Ethanol-Water Mixture

To find compositions of bottom product, distillate and feed, refractive indices are used

[6]. Compositions are found as mass fraction, then they are converted to mole fraction by

using Equation A.1.

Table B.1: Refractive indices and compositions of ethanol-water in total reflux [6].


water

Mass fraction of

ethanol

Mole fraction of

ethanol

Bottoms 1.3370 0.9291 0.0709 0.029

Distillate 1.3640 0.15 0.85 0.69

Table B.2: Refractive indices and compositions of ethanol-water in finite reflux [6].


water

Mass fraction of

ethanol

Mole fraction of

ethanol

Bottoms 1.3370 0.9291 0.0709 0.029

Distillate 1.3640 0.15 0.85 0.69

Feed 1.3630 0.2266 0.7734 0.57

C. Calculation of Slope of q-line

Condition of feed is represented by the quantity q, which is defined as [5],

Equation C.1 is also written in terms of enthalpies,


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Substituting these values into Equation C.3,

q-line equation is,

Therefore, slope of q-line is,

D. Calculation of Slope of Enriching Section

Operation line of enriching section is defined as [5],

During the experiment, reflux is set to 5:1. Therefore, slope of enriching section is,

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