AIM: Differential distillation, distillation process, distillation calculation, obtain distillation data for binary mixture.
OBJECTIVES: To compare experimentally (F/W) values to that of calculated from Rayleigh’s equation.
PRINCIPLE/THEORY:
If an infinite number of successive flash vaporization of liquid is carried out with a number of decimally small portions of liquid each time, then the net result would be equivalent to a simple distillation of differential distillation. In practice, the method can be approximated to a batch of liquid containing charged to a kettle or still fitted with some of the heating devices such as a steam jacket. The charge is boiled slowly and the vapours are withdrawn rapidly as they form through a condenser where they are liquefied and the condensate is collected in the reservoir. The first portion of the distillate will be richest in more volatile substance and as distillation proceeds, the vaporized product becomes lean.
Rayleigh’s equation which can explain this phenomenon by EQUATION
Modified Rayleigh’s equation
Log (Fx_{F} /Wx_{w}) = a log [F (1x_{F}) /W (1x_{w} )]
a = Öa_{W} a_{F}
Where F is total number of moles of the feed, W is the numbers moles of residue XF and XW are mole fractions of more volatile substance in feed, and residue, which relates the number of moles of ‘A’ remaining in the residue Wxw to that of ‘B’ remaining W (1 Xw). These expressions are most likely to be valid for an ideal mixture, for which ‘a’ is most nearly constant.
Log (Fx_{F} /Wx_{w}) = a log [F (1x_{F}) /W (1x_{w} )]
a = Öa_{W} a_{F}
Where F is total number of moles of the feed, W is the numbers moles of residue XF and XW are mole fractions of more volatile substance in feed, and residue, which relates the number of moles of ‘A’ remaining in the residue Wxw to that of ‘B’ remaining W (1 Xw). These expressions are most likely to be valid for an ideal mixture, for which ‘a’ is most nearly constant.
EQUIPMENT USED:
 Simple distillation column
 Mantle heater
 250 ml beakers2
 50 ml beaker1
 Specific gravity bottle (25 ml)
 Thermometer
MATERIAL USED: Methanol –water.
EXPERIMENTAL PROCEDURE:
1. Measure exactly 300 ml using a measuring jar and transfer this 300 ml of feed into a beaker. In case of feed in the bottle is less than 300 ml make up with pure methanol.
2. Set aside 50 ml of the feed for measurement of specific gravity. Find specific gravity.
3. The balance 250 ml is transferred into round bottom flask of the setup.
4. Assemble the glass setup carefully. i.e. connect the condenser, thermal well etc. Place a thermometer in the thermal well of the still.
5. Start the condenser water supply. Ensure a rich flow of water through the condenser.
6. Check all the connections and then slowly heat the feed by using mantle heater.
7. When vapour is produced, it will condense in the condenser and collected in a 250 ml beaker.
8. This procedure is continued up to half the volume of feed is vaporized and condensed.
9. The volume of feed in the beaker is reduced to half the volume stop the heating process while supplying water to the condenser about 15 minutes.
5. Start the condenser water supply. Ensure a rich flow of water through the condenser.
6. Check all the connections and then slowly heat the feed by using mantle heater.
7. When vapour is produced, it will condense in the condenser and collected in a 250 ml beaker.
8. This procedure is continued up to half the volume of feed is vaporized and condensed.
9. The volume of feed in the beaker is reduced to half the volume stop the heating process while supplying water to the condenser about 15 minutes.
10. Make sure that no vapours are produced to shut the water supply.
11. Cool the residue, the specific gravity, or R.I for the residue and distillate are found.
12. The volume of the distillate and residue are also measured accurately using a measuring jar.
13. A calibration chart is prepared for (specific gravity or R .I) to the mole fraction of methanol in the solution.
11. Cool the residue, the specific gravity, or R.I for the residue and distillate are found.
12. The volume of the distillate and residue are also measured accurately using a measuring jar.
13. A calibration chart is prepared for (specific gravity or R .I) to the mole fraction of methanol in the solution.
OBSERVATIONS:
Volume of feed = _____________ ml .
Room temperature = _____________ `C .
Distillate temperature =_______________`C.
Barometric pressure =728 mmHg .
Volume of distillate = _____________ml.
Volume of residue =______________ml.
Temperature of the first bubble starts at =_________`C.
Temperature at the end of distillation = ______________`C.
Specific gravity of the feed =_________
Specific gravity of the distillate =________
Specific gravity of the residue =_________
TemperatureVapor pressure data:
Get the temperature –vapor pressure data for both methanol and water from Perry’s handbook.
Selection of upper and lower limits of temperatures.
First temperature Boiling point of more volatile component (here it is methanol ) corresponding to site barometric pressure (728 mmHg).
Last temperature boiling point of less volatile component (here it is water ) corresponding to site barometric pressure (728 mmHg).
Any number of temperature between the above two may be selected for filling up table 1
DATA ANALYSIS:
plot the graph between S.G Vs mole fraction X.
Get the temperature – vapor pressure data of water, methanol from Perry’s handbook.
Then plot the graph between vapor pressure of methanol Vs T and Vapor pressure of water Vs Tin the same graph.
Calculation X, Y* data for the methanolwater system at Pt =728 mmHg using Raoult’s law.( α are also computed).
NOTE: refer example problem (1) in distillation chapter of R.E Treybal (mass transfer book ).
Plot X Vs Y* (Take equal scale on both axes).
from graph 1, find X f ,X d ,Xw .
Calculate moles of feed, distillate & residue using the values of Xf, Xd and Xw obtained from step6.
compute ln (F/W) ( i.e. experimental value ).
Plot the graph between 1/(Y*X) Vs X. This graph was plotted using data available in step 4. The area under this curve between the limits Xf & Xw gives the RHS of Rayleigh ‘s equation. This gives ln (F/W) (theoretical value).
Graphs to be plotted:
For a methanolwater system,
Specific gravity Vs mole fraction X.
Plot Y* Vs X.
Vapor pressure Vs Temperature of both methanol and water.
1/(Y*X) Vs X (report area under the curve between the limits Xf and Xw ).
RESULTS:
ln(F/W) experiment =
ln(F/W) rayleigh’s equation =
DATA TABLES:
Table 2:
Volume of feed = _____________ ml .
Room temperature = _____________ `C .
Distillate temperature =_______________`C.
Barometric pressure =728 mmHg .
Volume of distillate = _____________ml.
Volume of residue =______________ml.
Temperature of the first bubble starts at =_________`C.
Temperature at the end of distillation = ______________`C.
Specific gravity of the feed =_________
Specific gravity of the distillate =________
Specific gravity of the residue =_________
S.G.sample= (Wt of S.G. Bottle + Sample ) – (Wt of the empty S.G Bottle)/{(Wt of the S.G bottle + Distillate water )(Wt of empty S.G Bottle )}
TemperatureVapor pressure data:
Get the temperature –vapor pressure data for both methanol and water from Perry’s handbook.
Selection of upper and lower limits of temperatures.
First temperature Boiling point of more volatile component (here it is methanol ) corresponding to site barometric pressure (728 mmHg).
Last temperature boiling point of less volatile component (here it is water ) corresponding to site barometric pressure (728 mmHg).
Any number of temperature between the above two may be selected for filling up table 1
DATA ANALYSIS:
plot the graph between S.G Vs mole fraction X.
Get the temperature – vapor pressure data of water, methanol from Perry’s handbook.
Then plot the graph between vapor pressure of methanol Vs T and Vapor pressure of water Vs Tin the same graph.
Calculation X, Y* data for the methanolwater system at Pt =728 mmHg using Raoult’s law.( α are also computed).
NOTE: refer example problem (1) in distillation chapter of R.E Treybal (mass transfer book ).
Plot X Vs Y* (Take equal scale on both axes).
from graph 1, find X f ,X d ,Xw .
Calculate moles of feed, distillate & residue using the values of Xf, Xd and Xw obtained from step6.
compute ln (F/W) ( i.e. experimental value ).
Plot the graph between 1/(Y*X) Vs X. This graph was plotted using data available in step 4. The area under this curve between the limits Xf & Xw gives the RHS of Rayleigh ‘s equation. This gives ln (F/W) (theoretical value).
Graphs to be plotted:
For a methanolwater system,
Specific gravity Vs mole fraction X.
Plot Y* Vs X.
Vapor pressure Vs Temperature of both methanol and water.
1/(Y*X) Vs X (report area under the curve between the limits Xf and Xw ).
RESULTS:
ln(F/W) experiment =
ln(F/W) rayleigh’s equation =
DATA TABLES:
Preparation of calibration chart:
S.No

A volume of
methanol

Volume of
water

Specific
gravity

Mole fraction
of methanol

1

30

0
 
2

25

5
 
3

20

10
 
4

15

15
 
5

10

20
 
6

5

25
 
7

0
 30 
Table 1:
S.No

Temperature

PA(mmHg) (methanol)

PB (mmHg)
(water)

XA = (PtPA) /
(PA PB)

Y A= PA.XA/Pt

Table 2:
S.No

X

Y*

1/(Y *X)

α =(PA/ PB)
