CH 3520 HEAT AND MASS TRANSFER LABORATORY

Title of the experiment: Plate column distillation

Date of experiment: 5/02/2013

Date of Report: 13/02//2013

For Instructors use only:

Item number Division Maximum Marks Actual Marks

1 Experimental Diagram 5

2 Experimental Procedure 5

3 Precautions 5

4 Experimental data 20

5 Sample calculations 15

6 Error analysis 10

7 Results and discussion 25

8 Suggestions for improvement 5

9 Conclusions 5

10 References 5

Total 100

Batch and group number: B8

Name Roll Number 1. Gokul Krishna CH10B017

2. Prabal Sabarawal CH10B087

3. T G Radhakrishna CH10B089

3.

Marks:

100

Plate Column Distillation Aim: To calculate the number of ideal stages required and the overall efficiency of plate distillation

column.

Apparatus: Measuring cylinders, beakers, refractometer, burette, chemicals: Toluene, CCl4, and

acetone.

Experimental Diagram:

Theory:

Any physical device that provides good contact between the vapour and liquid phases present in

industrial-scale distillation columns or laboratory-scale glassware distillation columns constitutes a

"plate" or "tray". Since an actual, physical plate is rarely a 100% efficient equilibrium stage, the number

of actual plates is more than the required theoretical plates.

We can use the following formula to calculate the efficiency of the plate column if we know both

number of theoretical stages and the number of ideal stages required for a particular operation. Number

of ideal stages required can be calculated from McCabe-Thiele method.

Where:

Na = the number of actual, physical plates or trays, Nt = the number of theoretical plates or trays and E =

the overall efficiency

Plate

For calculations we follow the McCabe-Thiele method. It is a graphical approach, considered to be the

simplest and most instructive method for the analysis of binary distillation. This method uses the fact

that the composition at each theoretical plate (or equilibrium stage) is completly determined by the mole

fraction of one of the two components. The McCabe-Thiele method is based on the assumption of

constant molar overflow which requires that:

The molal heats of vaporisation of the feed components are equal.

For every mole of liquid vaporized, a mole of vapour is condensed.

Heat effects such as heat of solution and heat transfer to and from the distillation column are

negligible.

Procedure:

Calibration:

1. Take 11 beakers with 10ml solution of CCl4 and Toluene such that volume of CCl4 varying from

0ml to 10ml.

2. Refractive index of each sample is measured using refractometer.

3. A graph between refractive index and mole fraction is plotted.

Distillation column experiment:

1. Wash all the vessels and round bottle flask with acetone.

2. A 1000 ml solution of toluene and CCl4 is made with 55% (v/v) toluene and 45% (v/v) CCl4

and fed into the round bottom flask.

3. Boiler is turned on and temperature is observed at the plates and flask.

4. Vapours are condensed using cold water supply from outside.

5. After sometime the temperatures at all the levels becomes constant. This indicates that the

system has reached steady state. Then Samples from the five taps are collected.

6. The samples should be cooled down to room temperature.

7. Refractive index of the samples is measured with the help of refractometer.

Precautions:

1. Clean all the vessels and round bottom flask using acetone

2. Avoid liquid spillage over heater coils at the bottom of distillation column.

3. Ensure proper supply of cold water in the condenser pipe and make sure there is no leakage.

4. Shake the samples well before measuring the refractive index.

5. Ensure that steady state is achieved before collecting samples.

6. Wear gloves while handling hot materials.

7. Wear safety goggles while making binary solutions.

8. Store the collected samples in tightly closed containers.

9. Measure the refractive index of the collected samples from plate column only after reaching

room temperature.

Experimental Data:

Molecular Mass of CCl4: 153.82 g mol−1

Molecular mass of Toluene: 92.14 g mol-1

Density of CCl4: 1.595 g/ml

Density of Toluene: 0.866 g/ml

Calibration:

Volume of CCl4

(ml) Moles of

CCl4

Volume of Toluene

(ml) Moles of

Toluene

Mole

fraction of

CCl4

Refractive

Index

0 0 10 0.09398741 0 1.49404

1 0.010369263 9 0.084588669 0.10919849 1.49177

2 0.020738526 8 0.075189928 0.216187426 1.48898

3 0.031107788 7 0.065791187 0.321033201 1.48654

4 0.041477051 6 0.056392446 0.423799572 1.48191

5 0.051846314 5 0.046993705 0.524547793 1.47998

6 0.062215577 4 0.037594964 0.623336735 1.47502

7 0.072584839 3 0.028196223 0.720223002 1.47141

8 0.082954102 2 0.018797482 0.815261038 1.47021

9 0.093323365 1 0.009398741 0.908503229 1.46499

10 0.103692628 0 0 1 1.45812

Graph of Refractive index vs. Mole fraction of CCl4

Linear fit for the data: y = -0.0345x+1.4962

y = -0.0345x + 1.4962

1.455

1.46

1.465

1.47

1.475

1.48

1.485

1.49

1.495

1.5

0 0.2 0.4 0.6 0.8 1 1.2

Ref

ract

ive

Ind

ex

Mole Fraction of CCl4

Refractive index of solution vs Mole fraction of CCl4

Distillation column experiment:

Tray spacing = 20 cm

No. Of trays = 5

Length of column = 100 cm

Total Volume of the mixture = 1000 ml

Volume of Toluene = 550 ml

Volume of CCl4 = 450 ml

Plate number

(from bottom)

Refractive index Temperature (oC) Mole fraction of CCl4

from :

x= (y-1.4962)/(-0.0345)

1(bottom) 1.4705 86 0.744927536

2 1.46937 83 0.777681159

3 1.46777 Not working 0.824057971

4 1.46577 Not working 0.882028986

5 (top) 1.46493 78 0.906376812

Calculations:

In order to calculate the number of ideal trays required for the operations, we are going to use McCabe

Thiele method.

According to this method, slope of operating line of absorption or rectifying section is

where, R is

reflux ratio.

Reflux ratio is defined as the ratio of reflux rate to distillate rate (R = L/D). In our case, distillate rate=0.

So, Reflux ratio, R = . Therefore, slope of absorption curve =

= 1. Therefore, y = x is our

operating line.

Plot CCl4 and toluene vapour liquid equilibrium curve and operating line y = x on the same graph. Then

drop the stages from the first point (concentration of CCl4 on the top plate) to the last point

(concentration of CCl4 on the bottom plate). Then count the number of steps. This number is the

required number of ideal stages to do the same separation.

From the graph we can see that, the number of ideal stages required to do the same separation is around

2.5. Since, the number of stages cannot be a fraction so, the number of ideal stages required = 3.

According to the formula, Efficiency of the plate column =

Therefore, efficiency of the plate column = 60 %

Vapour-Liquid Equilibrium Data for CCl4-Toluene system:

Mole fraction of CCl4 in liquid phase (x) Mole fraction of CCl4 in the vapour phase (y*)

0.025 0.061

0.05 0.1181

0.075 0.1715

0.1 0.2217

0.125 0.2688

0.15 0.313

0.175 0.3547

0.2 0.394

0.225 0.431

0.25 0.466

0.275 0.499

0.3 0.53

0.325 0.559

0.35 0.5879

0.375 0.614

0.4 0.639

0.425 0.6638

0.45 0.6867

0.475 0.7084

0.5 0.7292

0.525 0.748

0.55 0.767

0.575 0.7858

0.6 0.803

0.625 0.8195

0.65 0.8353

0.675 0.8504

0.7 0.8648

0.725 0.8787

0.75 0.892

0.775 0.9048

0.8 0.9171

0.825 0.9289

0.85 0.9402

0.875 0.9511

0.9 0.9616

0.925 0.9717

0.95 0.9815

0.975 0.9909

1 1

Graph showing number of ideal trays, VLE curve, and Operating line

*Dots are mole fraction of CCl4 on each plate (if we join these points we will get our operating line)

Errors and discussion:

Actual Efficiency of the plate column would be less than 60% because the actual number of trays

required to do the separation is approximately 2.5, but we have taken the number to be 3 in our

calculations. So, actual efficiency of the plate column would be around 50% which is much less than the

typical efficiency of a plate column which is around 65%. The efficiency of the plate column can be

improved by improving the contact between vapour and liquid by using bubble cap trays.

Least count of the burette used is 0.1 ml. So, there could be a maximum error of 0.05 ml in our readings.

Value of density is collected from “Chemical Engineer’s Handbook” by Robert Perry. This value need

not be the same as that of the sample we used.

Suggestion:

1. The inefficiency of plates occurs mainly because of two reasons – insufficient time of contact and

insufficient degree of mixing.

2. Better contacting is achieved by installing bubble-caps or valve caps at each perforation to

promote the formation of vapour bubbles flowing through a thin layer of liquid maintained by a

weir on each tray.

3. Insulation of the round bottomed flask can be improved in order to prevent heat loss to

atmosphere.

4. Few thermocouples were not working. They have to be replaced.

References:

“Mass Transfer Operation” by Robert Treybal, third edition

“Chemical Engineer’s Handbook” by Robert Perry and Don Green

“Separation Process Principles” by Seader and Henley