NAME: Machell Lee

ID #: 05714408

LABORATORY EXERCISE: BD

LAB TITLE: Batch Distillation

GROUP: B2

DEPARTMENT: Chemical Engineering

DATE PERFORMED: February 7, 2006

DATE SUBMITTED: February 14, 2006

Abstract

The aim of this experiment was to (1) chart the operation of a simple distillation

operation and compare it with relevant theory and (2) compare a batch rectification

operation with a simple distillation.

A 1:1 ethanol-water mixture was distilled via simple and fractional distillation.

The time to collect successive 30cm3 portions of distillate was measured until 150 cm3

was collected. The temperature and density (using pycnometers) were measured on each

occasion and the mass fraction of the more volatile component (ethanol) in each was

determined. A graph of mass fraction against time was plot for both distillation processes

and their results compared.

Using the presented theory and the relevant experimental results the mole fraction

of ethanol in the distillate and in the waste were calculated for both processes. Fractional

distillation delivered a distillate of greater ethanol concentration. Fractional distillation

resulted in a fairly constant high mole fraction of ethanol in the distillate, which

emphasizes its efficacy as a separation process; while the less effective simple distillation

resulted in the smaller ethanol mole fraction decreasing with time.

Introduction

Distillation is a technique used to separate a mixture of two or more liquids. The

mixture is boiled and the vapour condensed and collected as distillate. Since the vapour is

richer than the liquid in the more volatile component the distillate will also richer in the

more volatile component since it has the same composition as the vapour being

condensed.

In batch distillation (differing from continuous distillation) the liquid is held

within the distilling flask while the distillate is continuously taken off. The original

distillate will have the highest concentration of the more volatile component (ethanol) but

this gradually decreases as its concentration in the liquid decreases. There are two types

of batch distillation: Simple and Fractional distillation.

In simple or differential distillation the vapour is directly condensed as it leaves

the flask and at any given time the vapour is in equilibrium with the liquid in the flask.

Since the concentration of both the liquid and vapour is changing, the process is a time

dependent one. However, if a rectifying column is installed (fractional distillation) a

continuous distillation of the liquid mixture occurs. As the vapour rises in the

fractionating column it is repeatedly condensed and vapourised. Since the vapour

becomes richer in the more volatile component at each instant, this results in a greater

concentration of product being formed. Also the distillate composition does not vary with

time. Thus, the latter is the more effective in separating mixtures.

Experimental

Apparatus and Chemicals

Electrical muffle heater

2 Round bottom flasks

Thermometer

Condenser

Measuring Cylinders

6 Pycnometers (density bottles)

Fractionating Column

Beaker

Ethanol

Distilled water

Procedure

1. The apparatus for simple distillation was set up.

2. The mass of the six pycnometer bottles, with their corresponding covers labelled

1-6 were measured.

3. A 1:1 mixture of water and ethanol was prepared in a round bottom flask using

125 ml of each solution.

4. The density of this mixture was determined by measuring the combined mass of

the mixture in the pycnometer, the vessel and the cover.

5. This known mass of feed was charged to the system.

6. The heater was put on maximum rate and cooling water put on the condenser.

7. The condensate was collected in a measuring cylinder and the timer started when

the first drop of condensate fell into the cylinder.

8. The time was noted at every 30 ml of condensate collected.

9. Each 30 ml sample was collected separately in a measuring cylinder and its

density measured using the pycnometer.

10. The temperature of each sample of condensate was also noted.

11. The experiment was stopped when 150 ml of distillate was collected.

12. The temperature and density of the bulk distillate was noted.

13. The residue was left to cool to a suitable temperature when its density was

determined.

14. A fractionating column was attached to the equipment and steps 3-13 repeated

for fractional distillation.

Analysis and Presentation of Results

m1 - mass of pycnometer and coverm2 - mass of sample with pycnometer and coverm3 - mass of sample onlyVolume used = 25 ml

Sample Calculation(Using initial reading from simple distillation)m3 = m2 - m1

= 50.4 – 28.2 = 22.2 g

Density =

= 0.888 gcm-3

Mass of feed, F = density x volume = 0.888 x 250 = 222 g

Mass fraction ethanol = 58.48% (using composition table)

Mass of ethanol = 222 x 0.5848 = 129.8256 g

Moles of ethanol =

Mass of water = 222 – 129.8256 = 92.1744 g

Moles of water =

Total moles = + = 7.936186

Mole fraction ethanol, xf = = 0.355469

Mass of distillate, D = 127.8 g Mass fraction ethanol = 73.65% (using composition table)

Mass of ethanol = 127.8 x 0. 7365= 94.1247g

Moles of ethanol =

Mass of water = 127.8 – 94.1247 = 33.6753g

Moles of water =

Total moles = + = 3.914073

Mole fraction ethanol, yD,av = = 0.52255 g

W = F – D = 222 - 127.8 = 94.2 g

Mass fraction ethanol = 39.48% (using composition table)

Mass of ethanol = 94.20 x 0.3948 = 37.19016 g

Moles of ethanol =

Mass of water = 94.2 – 37.1906 = 57.00984 g

Moles of water =

Total moles = + = 3.971828

Mole fraction ethanol, xw = = 0.203466

Table of results for simple distillation

  Time (s) m1 (g) m2 (g) m3 (g) density (gcm-3) Temperature (oC) mass fraction

Feed 0 28.2 50.4 22.2 0.888 33 58.48

  184.84 29.3 49 19.7 0.788 28 98.19

  182.03 28 48.1 20.1 0.804 28 92.59

  192.39 26.5 47.3 20.8 0.832 28 81.81

  537.17 29 49.8 20.8 0.832 28 81.81

  484.19 27.9 49.4 21.5 0.86 28 70.33

Distillate   28.2 49.5 21.3 0.852 28 73.65

Waste   29.3 52.5 23.2 0.928 31 39.48

mass feed (g) 222mass ethanol (g) 129.8256moles ethanol 2.821069mass water (g) 92.1744moles water 5.115117total moles 7.936186mole fraction ethanol 0.355469

mass distillate (g) 127.8mass ethanol (g) 94.1247moles ethanol 2.0453mass water (g) 33.6753moles water 1.868774total moles 3.914073mole fraction ethanol 0.52255

mass waste (g) 94.2mass ethanol (g) 37.19016moles ethanol 0.80813mass water (g) 57.00984moles water 3.163698total moles 3.971828mole fraction ethanol 0.203466

Graph of Distillation composition against time for Simple Distillation

0

20

40

60

80

100

120

0 184.84 182.03 192.39 537.17 484.19

Time (s)

Dis

tilla

tio

n C

om

po

sitio

n (

mas

s fr

acti

on

)

Simple Distillation

Table of results for fractional distillation

Time (s) m1 (g) m2 (g) m3 (g) density (gcm-3) Temperature (oC) mass fractionFeed 0 29.3 50.7 21.4 0.856 33 70.18

207.13 28 47.8 19.8 0.792 28 96.84202.6 26.5 47 20.5 0.82 28 86.55215.93 29 49.2 20.2 0.808 28 91.12252.65 27.9 48.3 20.4 0.816 28 88.1377.57 28.2 49.8 21.6 0.864 28 69.02

Distillate 28 48.3 20.3 0.812 27.5 89.78Waste 26.5 50.8 24.3 0.972 33 13.68

mass feed (g) 214mass ethanol (g) 150.1852moles ethanol 3.263477mass water (g) 63.8148moles water 3.541332total moles 6.804809mole fraction ethanol 0.479584

mass distillate (g) 121.8

mass ethanol (g) 109.352moles ethanol 2.376185

mass water (g) 12.44796moles water 0.690786total moles 3.066971mole fraction ethanol 0.774766

mass waste (g) 92.2

mass ethanol (g) 12.61296moles ethanol 0.274076

mass water (g) 79.58704moles water 4.416595total moles 4.690671mole fraction ethanol 0.05843

Graph of Distillation composition against time for Fractional Distillation

0

20

40

60

80

100

120

0 207.13 202.6 215.93 252.65 377.57

Time (s)

Dis

tilla

tion

Com

posi

tion

(mas

s fr

actio

n)

Fractional Distillation

Representation of results

Mole FractionEthanol In Feed

xf

Mole FractionEthanol In Distillate

yDtavg

Mole FractionEthanol In Residue

xw

SimpleDistillation

0.355469 0.52255 0.203466

FractionalDistillation

0.479584 0.774766 0.05843

Comparison with Theoretical Data

Simple Distillation

Upon integration this equation gives which is equal to the area under the curve x

vs. 1/(y*-x) curve, between the limits xw and xf,

where F is the total number of moles in the feed, and W is the total number of moles in the waste

= ln 1.9981

= 0.6922 Comparing with:Area of graph between xf and xw = 10.5 blocks x area of block

= 10.5 x 0.05 = 0.525 units2

Discussion

The first distillate from the simple distillation was found to be the highest in

concentration of ethanol (98.19 % by mass) and higher than that of the starting mixture

(58.48%) - as would be expected from the theory. As time increases however, there is a

gradual decrease in the concentration of ethanol in the distillate, that is, the composition

of the vapour was continuously changing with time. This proves that batch distillation is

a time dependant or unsteady process.

The graph for fractional distillation shows that the initial distillate was higher in

ethanol concentration than the original mixture (96.84%), as expected, but thereafter the

value remained fairly constant with some fluctuations. There was thus no obvious

relationship between the composition of the more volatile component (ethanol) in the

distillate and time; hence the composition did not vary with time.

The mole fraction of ethanol in the bulk distillate was higher for fractional

distillation (0.774766) than for simple distillation (0.552255) (similarly, the mass %)

therefore proving that fractional distillation provides a more effective separation of the

components in the mixture.

Since the aim of the experiment was simply to compare the two distillation

techniques, it was not necessary to completely separate the two liquids. Nevertheless,

neither of these two distillation methods would be able to fully carry out such a

separation because the ethanol-water mixture is a homogeneous azeotropic one, which

can only be separated by azeotropic distillation.

The change in the number of moles calculated from the experiment is similar to

that from the relevant theory with only a difference of (0.1672). Precautions were taken

throughout the experiment to ensure accuracy of results. These include ensuring that each

pycnometer was covered with the same cover each time to prevent any variations in mass

and hence the values calculated. Another precaution taken was to ensure that there was a

continuous flow of water in the condenser so that there were no bubble spaces to hamper

the condensation process.

However, this difference may have resulted from errors in the readings, and/or

experiment from assumptions or approximations. Possible sources of errors are:

1) The assumption that there was no loss in volume of the distillate when losses

occurred when switching measuring cylinders and measuring the densities

using the pycnometer.

2) Reaction time in starting and stopping the stopwatch.

3) The approximation involved in the counting squares method of determining

the area underneath the curve.

4) Improper securing of the connections in the apparatus which could have

caused vapour to escape into the surroundings.

5) Incomplete stirring or mixing of the original mixture which may have resulted

in a greater concentration of ethanol being on the surface or bottom of the

flask.

The experiment could have been improved by insulating the columns so

preventing heat loss and by placing petroleum jelly over the joints to ensure that

no vapour escapes the apparatus.

Conclusion

The mole fraction and (mass fraction) of ethanol in the distillate was higher

for fractional distillation than simple distillation.

The distillate produced from simple distillation was highest in concentration

of the more volatile component initially, and then decreased with time. Thus

simple distillation is a time dependent process.

Ethanol composition in the distillate produced from fractional distillation is

constant with time (independent of time).

Fractional distillation is a better separator of an ethanol-water mixture as it

results in a purer distillate being formed.

For simple distillation the change in the number of moles in the experiment

was approximately the same as that from the theory.

Refrerences

THE UNIVERSITY OF THE WEST INDIES Laboraory Manual CH11A –

Applied Chemistry I and CH17B – Chemical Engineering Laboratory I (2005-

2006)

McGAW, D.R.; MELLOWES, W.A.; YOUNG HOON, A. and FARABI, H.

Introduction to Chemical and Process Engineering

CHOPEY, M.P. Handbook of Chemical Engineering Calculations. Td Edition

1994, Mcgraw Hill