experiment b group 24

1

Experiment B: Batch Distillation (small scale)

11/03/11

Group 24

James Blair

Anouluwatomi Are

Andres Palacios

Rafaila Paspati

Muhammad Usman

Table of Contents: 1. Introduction…………………………………………………………………………….…2

2. Experimental Method……………………………………………………………….6

3. Results ………………………………………………………………………………7

4. Discussion of results ……………………………………………………………….16

5. Conclusion………………………………………………………………………………19

6. References ………………………………………………………………………………20

7.Nomenclature………………………………………………………………………………20

2

ABSTRACT

This report aims to discuss our results of our attempt to achieve the required separation by distillation set out

in the problem statement. This will be achieved by analysing the effects different variables had on the

product quality and speed of distillation, comparing our findings with our researched theory. A

comprehensive analysis of the results will be given by comparing three different methods of measuring the

methanol v/v% of the distillate, concluding what the optimum operating conditions are for the distillation

column, and which method of measuring methanol v/v% is the most reliable.

1-INTRODUCTION

Problem Statement

The requirement of this experimental problem is the recovery of methanol from methanol/water mixture

through a batch distillation process, since this requirement occurs infrequently. The problem states that a

strategy has to be devised in order to achieve the required separation by characterising the performance of

the available system. The experimental plan should be cost effective, achievable in the allotted time and

conform to safety, health and environmental requirements. The above should occur by using a feed which

contains 10% v/v methanol, recovering 85 ml of methanol as a composition ≥ 97% vol% methanol of the

collected distillate.

Objectives:

Gain a greater understanding on the operability of distillation columns in terms of reflux ratios and

how different ratios affect the distillate composition and time efficiency.

Determining which of the experiments done is more cost effective by understanding trade-off

between cycle time and value of recovered material to maximize profit.

Determining which method of measuring the methanol v/v% is the most reliable

Appreciating the importance of environmental, health and safety issues when dealing with hazardous

chemicals.

Literature Review:

Distillation is a process in which a liquid or vapour mixture of two substances, in this case methanol and

water are separated into its component fractions of desired purity by the application or the removal of heat.

Distillation may be carried out either as a batch operation or as a continuous operation. Usually the

operation is effected in continuous-contact equipment or in stage–wise contacts towers. Batch distillation is

more preferable to continuous distillation when quantities being handled are small, as the requirement to

recover methanol from a methanol/water mixture only occurs infrequently.

During the batch distillation process the feed is not supplied continuously to the column but it is supplied in

batches which results unsteady state since the conditions that occur in the system change with time. The

basic concept of a distillation process is that pure liquids exhibit different volatilities at a given temperature.

More specifically a very low temperature means that not enough distillate is collected whether a high

temperature results in contamination of distillate with the lower volatile component and thus if heat is

applied to a liquid mixture of these substances those having higher vapour pressures. If this vapour is

condensed it should be clear that a certain amount of purification will be achieved. The advantage of using

batch distillation is that it allows a better product integrity i.e. each product of the process can clearly be

identified in terms of the feed involved and the conditions processing especially in pharmaceutical and food

industries. Another advantage is that it is flexible in accommodating changes in feed composition, changes

in production rate, and changes in product formulation. However a disadvantage is that energy requirement

in batch distillation is higher than in continuous distillation which has an effect by increasing the risk of the

decomposition of the substances. (Distillation and Absorption notes, Webb 2010)

As it is mentioned above the batch distillation apparatus consists of a vertical column with a 30 equally

spaced trays mounted inside of it in which the separation of the vapour and liquid occurs. As the process is

3

taking place, the vapour passes up through the column and into the condenser where it is condensed into

liquid. The liquid from tray 1 then flows into the base of the column and then into the Reboiler where it is

partially vaporised. In addition the reflux divider acts as a vacuum jacket in which vapour can be trapped

without disturbing the distillation process for sampling purposes. (Walden L.S. Laukhuf 1985)

Background Theory

Batch distillation is often used for preliminary separation for multi-component systems or in processes

where quantities being handled are small, also when the material to be distilled is heavily fouling or is high

in solids. In batch distillation the assumptions that are stated are perfect insulation and minimal hold up in

vapour space.

http://en.wikipedia.org/wiki/File:BatchRectifier.png

Figure 1-Batch distillation column

The basic concept of distillation is the fact that components of a mixture do not have the same volatilities

and so the compositions of vapour and liquid are often different. However there are relationships between

the vapour and the liquid at equilibrium in order to predict the equilibrium composition of one stream if the

composition of the other is known.

Dalton’s Law: PA = yA P (1)

Raoult’s Law: PA = xA PθA (2)

Relative volatility:

αΑΒ = PθΑ / P

θB

From (1) and (2) αΑΒ =

(3)

During the experiment the relative volatile is assumed to remain constant since it does not affect the

efficiency of the distillation process. The compositions in equation (3) A, B are those which are in

equilibrium. The value of a will vary over the range from x=0 to x=1. If yA = xA then α = 1.0 and no

separation is possible by simple distillation. (Charles A. Plank 1985)

The first vapours are in equilibrium with the initial composition of the liquid in the still. As time progresses

the liquid becomes leaner in the more volatile component (MVC) and so the boiling point rises inevitably

the distillate also becomes leaner in the MVC. If Si moles of liquid are initially charged to the still then after

a time there will be S moles remaining. During this time the composition of the liquor will have fallen from

xSi initially to xs. However there will be Sf moles left in the still and the amount of distillate will be given by:

D xd = Si xSi – Sf xSf (4)

http://en.wikipedia.org/wiki/File:BatchRectifier.png

4

During any point through the distillation process the vaporisation of δS moles of liquid will result in the

production of δS moles of vapour. The change in composition of the liquid will be a loss of δ (SxS) moles of

MVC, where in the vapour phase it will be yδS. For the whole process according to Rayleigh’s Equation:

Ln

=

(5)

In addition another variable that is really significant during a distillation process is the optimum reflux ratio.

The reflux ratio is the ratio of the amount of the liquid which has returned to the column as reflux over the

amount of overhead product produced.

(in this case)

As time proceeds, the composition of the top product xd falls which means that the amount of the distillate is

reduced, hence, from the equation we have a higher value of the reflux ratio. Where on the other hand if the

amount of distillate increases then we obtain a lower value of the reflux ratio. (McCabe WL, Smith JC 1993,

Coulson JM & Richardson JF 2002)

However the value of the reflux ratio is relevant to the cost of operation. The costs of unit is roughly

proportional to the total plat area which in other words is the number of plates times the cross sectional area

of the column, and because the reflux ratio is relevant to the number of plates, the fixed costs for the column

first decrease and then increase with reflux ratio. The total cost of the operation is equal to the sum of the

fixed costs and the cost for heating and cooling. (Distillation and Absorption notes, Webb 2010)

Figure 2- TAC vs. Reflux ratio diagram

http://www.separationprocesses.com/Distillation/Fig064.htm

However in order for the plan to be cost effective from the diagram it is obvious that in order to obtain the

minimum costs for the operation we have to obtain the optimum reflux ratio i.e. a minimum at a definite

reflux ratio not much greater than the minimum reflux.

Refractometer:

A Refractometer is a laboratory or field device which is used to accurately measure the composition of

distillate mixture by measuring its refraction index. Light is passed through the mixture, causing the

5

refraction of light. This is then measured on a scale to an accuracy of five decimal places and the values are

then compared to literature values to determine the characteristics of the mixture.

Figure 3 shows that change of angle of light as it passes through from one medium to another. The change in

angle is due to a change in phase velocity where the wavelength of light changes at constant frequency.

Figure 3- Refractive index measurement

Figure 4- Refractometer

Gas Chromatography: Gas chromatography is the second method used to analyse the distillate sample’s composition. The way it works

is that the sample is transferred by an inert gas into a column filled with inert liquid, which adsorbs by an inert

solid. After the activation of heater, the sample data is sent to a detector and then on a recorder. The inert gas is

usually argon, nitrogen, carbon dioxide or helium. The chosen inert gas is dependent on the type of detector and

often the carrier gas system has a molecular sieve that removes water and other impurities from the sample.

Figure 5- Gas chromatography machine http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm

The gas chromatogram produces a series of peaks which represent the compounds in the mixture. The area under

these peaks is relative to the amount of a particular compound present in the sample, so a larger peak will

represent higher percentage.

The equation below can be used to determine refractive index.

=

= refractive index

http://www.google.co.uk/imgres?imgurl=http://www.bikudo.com/photo_stock/691102.jpg&imgrefurl=http://www.bikud

o.com/product_search/details/100744/digital_refractometer.

html&usg=___MCwowfqt4Fwxnu0uMWkEx8C8oo=&h=320&w=320&sz=21&hl=en&start=77&zoom=1&tbnid=EJr2

AWQIzYFOEM:&tbnh=113&tbnw=118&ei=4Z55TaTJL4u

I4gavltHYBQ&prev=/images%3Fq%3Drefractometer%26u

m%3D1%26hl%3Den%26sa%3DN%26biw%3D1366%26b

ih%3D519%26tbs%3Disch:10%2C1350&um=1&itbs=1&ia

ct=hc&vpx=1100&vpy=191&dur=21&hovh=225&hovw=225&tx=179&ty=124&oei=yZ55TYLyFIiAhQeEtZTyBg&p

age=4&ndsp=27&ved=1t:429,r:26,s:77&biw=1366&bih=51

9

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm

6

Section 2: Experimental Method 2.1 Systems Used

In order to carry out the batch distillation experiment, solution of methanol and de-ionised water is used.

2.2 Apparatus

Pipette – 5 ml (± 0.05ml)

Pipette Bulb

Measuring Cylinders – 25ml (±0.5ml) – 10 ml (±0.1ml)

Volumetric flask - 1000 ml (± 0.4m)

Sampling Vessels - 3ml

Distillation Column ( 30 trays)

Isomantle & Reboiler

Reflux divider

Condenser

Iludest distillation controller

Timer/Stop watch

Gas Chromatogram

Refractometer

Digital density meter

(Blackboard, 2011)

Figure 6- Distillation column operation

Condenser

Reflux Divider

30 tray column vacuum jacketed

Reboiler and Isomantle

Refractometer

Digital density meter

7

2.3 Experiment & Procedure

In order to analyse the composition samples collected as distillate from the distillation process, a calibration

curve was plotted. The calibration was created by plotting refractive indices against methanol compositions.

Eleven samples were made of de-ionised solutions of water and methanol (0-100%) with different

increments. In order to make the curve as accurate as possible it was particularly important to get accurate

results for the higher concentration solutions, as this was the area of the curve we would be most interested

in.. These samples were tested using the Refractometer which gave the refractive index value of the given

sample. This refractive index value was then compared with literature value to get the composition of

methanol in wt %. A digital density meter was used to find the densities of water and methanol and then

used to get the actual methanol v/v %. The composition of the distillate was also tested by the lab technician

using the gas chromatogram.

Following is a table indicating different solution samples:

Volume of

Methanol (ml)

Volume of

Water (ml)

Methanol %

v/v

0 100 0

10 90 10

20 80 20

40 60 40

50 50 50

60 40 60

80 20 80

85 15 85

95 5 95

100 0 100

Table 1 – Compositions of solutions used to create the calibration curve

The methanol feed solution (10% v/v) was prepared using pipette of 50ml and 100ml of 10% methanol and

then transferred to a conical flask of 1000ml with 900ml of de-ionised water.

In order to make sure that suitable amount of data is collected, the group was split into sub groups. Two

people went off to fume cupboard and made samples of water/methanol solution and were measuring the

densities. Two people were operating the distillation column to get different samples of distillates and the

remaining individual was operating the Refractometer to measure the composition of standards and the

samples of still, feed and distillate.

Refractometer Operation:

Place a few drops of the sample using pipette on the prism of the Refractometer and then close the

hinged prism.

Dispose the pipette and wipe the Refractometer using tissue.

Press the read button to activate the Refractometer and only note down the results when the machine

reads 20.0˚C. At times the temperature went above 20.0˚C; therefore ice was poured into the water

bath which was connected to Refractometer to adjust the temperature.

Convert the refractive index value to methanol % w/w using literature.

When using the Refractometer, the methanol percentage value observed from literature is by weight. This

value is then converted to get the actual methanol percentage by volume using the densities.

Distillation Column Operation:

Turn on cooling water, thermometer and distillation controller

Turn on the Isomantle using Iludest Distillation controller

8

Charge the Reboiler with 1000 ml of feed

Set distillation controller to ‘start’

Temperatures observed on sensors, until steady state is reached (constant temperature at the top of

the column, about 64 °C)

Set reflux ratio to desired ratio by varying R (return to column) using the dial on the right hand side

of distillation controller, then set system to ‘distil’

Allow to distil and collect samples at set time intervals and record the volume collected

Repeat process using different reflux ratios

This method was followed to find the optimum reflux ratio in the first lab session. The next lab

session followed the same procedure with a few additional steps :

Set function back to ‘start’

Allow for batch to reheat for 46 minutes until steady-state is again achieved (no more flooding seen

in column)

Set system back to ‘distil’ on the chosen reflux ratio

Keep repeating process until roughly 85cm3 of methanol is collected

Use Refractometer to measure the composition of methanol

A digital density meter was used to measure the density of the water and methanol to convert wt% to v/v.

Digital Density Meter Operation:

Inject a sample of liquid into the sample chamber on the right hand side of the meter using a syringe

Ensure that no bubbles are in the sample chamber as this would affect the density

Note down the density which appears on the display of the meter

Press the vacuum button to clean sample chamber of liquid and remove the syringe

Repeat the process to take the average density

The samples collected at the end of the experiment are given to the lab technician so the composition can be

measured using gas chromatogram.

2.4 Health & Safety Precautions

Methanol is a toxic and highly flammable therefore care must be taken when handling methanol. Exposure

to methanol can occur via absorption, contact with the eyes, inhalation or ingestion and high vapour

concentration of methanol can cause burning and irritation in eyes the short term exposure limit (STEL) of

methanol is 250 ppm. The STEL sets limits on excursions for periods of up to 15 minutes, four times per

day with at least 60 minutes between exposure periods, so long as individuals are suffering no irritation or

discomfort. Considering the factors mentioned above, extra care was taken and all the standards were made in a fume

cupboard. All the individuals made use of protective personal equipment such as goggles, gloves and lab

coats while operating the distillation column in order to avoid contact of methanol with skin and eyes. Extra

care was taken when transferring methanol to conical flasks and taking samples of the methanol/water

mixtures to avoid any spillages of solution and breakage of glass as both can harm the individual. In case of

this occurring, this should be cleared and disposed of appropriately.

Since methanol is highly flammable, within the approximate temperature range of 12°C to 41°C, methanol

will produce a concentration of vapour that is explosive upon contact with an ignition source. Therefore

individuals must ensure that there is no ignition source near methanol. Mixtures of methanol should be kept

away from any heat or hot surfaces as it has a flash point of 12°C.

2.5 Environmental Factors

Due to the nature of methanol, environmental factors need to be considered too. When disposing of

methanol, depending on the quantity, different methods should be used. If large quantity of waste methanol

9

present, it should be disposed of at a licensed waste solvent company which lab technician or the university

department would do. In the case of this experiment, if quantities of methanol were very small, it could be

flushed down the sink with about 3-4 times water with it or a disposing bottle could be used (Methanex,

2006).

3-Results

Observed Results

Plotting the calibration curves

The refractive indices at different volume percentage composition were recorded on Tuesday 1st of March and are

displayed on Table 2. In order to obtain an accurate curve, various intervals from 0-100 v/v % were taken.

% Volume methanol Refractive index Gas Chromatography Reading

0 - 20123

10% 1.33450 3311318

20% 1.33729 6464654

40% 1.34116 11905162

50% 1.34237 16035269

60% 1.34255 19528680

80% 1.33931 27276800

85% 1.33773 29296368

95% 1.33248 31119534

100% 1.32871 30925116

Table 2-Refractive Index and Gas Chromatography readings for control solutions.

A calibration curve of refractive index against volume percentage of methanol was plotted so that we could use the

curve to estimate the methanol percentage of sample solutions taken during distillation.

Figure 7-Calibration Curve for refractive index vs. methanol v/v%

1.328

1.33

1.332

1.334

1.336

1.338

1.34

1.342

1.344

0 20 40 60 80 100

Re

frac

tive

Ind

ex

% v/v of Methanol

Refractive index

Refractive index

10

Figure 8- Methanol % in control solutions versus Gas chromatography area

Experiment 1

On Tuesday 1st of March the first distillation experiment was carried out in order to study the effects of reflux ratios

and volume collected. The reflux ratios were changed gradually from values of 1-7 and the distillate was recovered

after 5 minute intervals. This was repeated for three runs until the total volume collected was as close as possible to 85

ml; thus, assuming that mostly methanol is collected in the distillate.

The refractive indices for each distillate were measured in order to then be able to obtain from literature values the

weight percentages of the solutions collected. The chromatography data for each distillate was also collected. These

results are represented in the following tables.

Time (min)

Reflux

Ratio

Distillate

Collected (ml)

Refractive

index

Weight fraction of

methanol/water

(w/w %)

Gas Chromatography

– Integrated Area

5 1 6.0 1.33001 99.00 32490900

10 2 9.5 1.33134 97.00 31365402

15 3 5.5 1.33160 93.00 31365402

20 4 4.0 1.33275 96.00 32609414

25 5 3.5 1.33070 95.00 32162062

30 6 2.5 1.32906 96.94 33397704

35 7 3.0 1.33299 98.80 30822512

Table 3- Observed results from 1st run

Integrated area = (340276 × V/V%) - 476090

-5000000

0

5000000

10000000

15000000

20000000

25000000

30000000

35000000

40000000

0 20 40 60 80 100 120

Inte

rgra

ted

are

a

V/V % of methanol

0

2

4

6

8

10

0 2 4 6 8

Dis

tilla

te C

olle

cte

d (

ml)

Reflux Ratio

11

Figure 8 – Amount of distillate collected vs. reflux ratio for 1st run

Time (min)

Reflux

Ratio

Distillate

Collected Refractive index

Weight fraction of

methanol/water

(w/w %)

Gas Chromatography

– Integrated Area

40 1 6.7 1.32905 99.93 32792524

45 2 4.8 1.33676 85.30 33423538

50 3 3.3 1.32911 99.70 32847786

55 4 3.2 1.33038 98.03 32365770

60 5 3.2 1.32941 100.00 32047204

65 6 1.8 1.32941 99.42 33456402

70 7 1.8 1.32908 100.00 33715829

Table 4 – Observed results from 2nd

run

Figure 9- Amount of distillate collected vs. reflux ratio for 2

nd run

Time (min)

Reflux

Ratio

Distillate

Collected

(ml) Refractive index

Weight fraction of

methanol/water

(w/w %)

Gas

Chromatography

– Integrated Area

80 1 12.0 1.32955 99.50 33300782

90 2 6.0 1.33214 95.00 31639912

Liquid left in still - - 1.33336 1.5 935353.9

Table 5 – Observed results from 3rd

run

It would not have been sensible to plot a graph for the third run as there were only two values.

Experiment 2

On Friday 4th March the distillation was carried out again using reflux ratios of 2 and 3. This was done as from figures

8 and 9 it can be seen that in general, smaller reflux ratios gave larger volumes of distillate within the same interval,

whilst still being within the desired final distillate composition. This conclusion was arrived at by observing the

volume percentages using the density calculation method, as some of our values for methanol v/v% using the

calibration curve method were clearly anomalous (85% v/v for reflux ratio 2).

0

1

2

3

4

5

6

7

8

0 2 4 6 8

Dis

tilla

te C

olle

cte

(m

l)

Reflux Ratio

12

Table 6- Observed results from experiment two (I.S.S= insufficient sample size to obtain value)

The density of pure water and methanol were recorded using digital density meter in order to convert weight

percentage into volume percentage.

Pure component Density (kg m-3

)

Water 999.6

Methanol 792.3

Table 7- Measured densities of pure water and methanol.

Derived Results Using our observed results, it was possible to calculate the final methanol v/v% for each sample and therefore overall

using three different methods. Firstly by using a graph obtained from the gas chromatography readings, secondly

using the recorded densities of the two components, and finally using our plotted calibration curve.

1) Gas Chromatography method

Using figure 2 and equation 2.1:

Experiment 1

Reflux Ratio Volume percentage of

methanol (v/v %)

Actual volume of

methanol collected (ml)

1 96.88 5.81

2 93.58 8.89

3 93.58 5.15

4 97.23 3.89

5 95.92 3.36

6 99.55 2.49

7 91.98 2.76

Table 8- Volume percentage of methanol for 1st run using gas chromatography readings

Reflux Ratio Volume percentage of methanol (v/v

%)

Actual volume of


1 97.77 6.55

2 99.62 4.78

3 97.93 3.23

4 96.52 3.09

5 95.58 3.06

6 99.72 1.79

7 100.48 1.81

Time (min) Reflux Ratio

Distillate Collected

(ml)

Refractive

index

Weight fraction

of

methanol/water

(w/w %)

Gas

Chromatograp

hy – Integrated

Area

10 2 6.7 1.33043 98.2 586491.1

20 2 4.8 1.32916 99 601219.6

30 2 3.3 1.33029 99 512933.8

40 2 3.2 1.33485 90 578444.2

50 2 3.2 1.33597 88 I.S.S

60 3 1.8 1.33392 92 I.S.S

70 3 1.8 1.33194 95 558869.8

80 3 1.8 1.33180 96 I.S.S

90 3 1.8 1.33360 93 544084.7

Liquid left in

still - - 1.33334

2 9463.2

13

Table 9- Volume percentage of methanol for 2nd

run using gas chromatography readings


%)

Actual volume of


1 99.26 11.91

2 94.38 5.66

Table 10- Volume percentage of methanol for 3rd

run using gas chromatography readings

Experiment Total Amount of distil

collected (ml)

Total amount of


%v/v methanol of

total distillate

1 76.8 74.23 96.7

Table 11- Total amount of methanol collected and overall v/v% of methanol

As a result of the fact that several of our samples for experiment 2 were insufficient to obtain a gas chromatography

reading, we decided not to perform the calculations using this method for experiment 2, as the result would have been

incomplete and therefore invalid.

2) Densities method

Another way to analyse the composition of methanol in the solution is to use the measured densities of water and

methanol with the recorded w/w% for each sample to find the total volume percentage of methanol in the distillate

using equations 3.1-3.6.

Experiment 1

Reflux Ratio

Volume percentage of methanol (v/v %) Actual volume of methanol

collected (ml)

1 99.21 5.95

2 97.61 9.27

3 94.37 5.19

4 96.80 3.87

5 96.00 3.36

6 97.56 2.44

7 99.05 2.97

Table 12-Volume of methanol collected from the 1st run using measured densities

Reflux Ratio

Volume percentage of methanol (v/v %) Actual volume of methanol

collected (ml)

1 99.94 6.70

2 87.98 4.22

3 99.76 3.29

4 98.43 3.15

5 100.00 3.20

6 99.54 1.79

7 100.00 1.80

Table 13-Volume of methanol collected from the 2nd

run using measured densities

Reflux Ratio

Volume percentage of methanol

(v/v %)

Actual volume of methanol

collected (ml)

1 99.60 11.95

2 96.00 5.76

Table 14-Volume of methanol collected from the 3rd

run using measured densities

14

Experiment 2

Reflux Ratio


(v/v %)


collected (ml)

2 99.21 20.70

2 99.21 19.84

2 91.91 17.86

2 90.25 4.60

2 93.55 2.71

3 96.00 1.87

3 96.80 3.84

3 94.37 3.87

3 99.21 0.94

Table 15-Volume of methanol collected from the 1st run using measured densities


collected (ml)

Total amount of


%v/v methanol of

total distillate

1 76.8 74.92 97.5

2 78 76.23 97.7

Table 16- Total amount of methanol collected and overall v/v% for each experiment

3) Calibration Curve method

Using figure 1 we were able to read off the volume percentage of methanol for each sample and from that calculate

the overall volume percentage of the total distillate collected using equation 3.6.

Experiment 1

Reflux Ratio Volume percentage of methanol (v/v %) Actual volume of methanol

collected (ml)

1 96 5.76

2 95 9.03

3 93 5.12

4 97 3.88

5 99 3.47

6 93 2.33

7 96 2.88

Table 17- Volumetric composition of the 1st run using calibration curve


%)

Actual volume of


1 98 6.57

2 85 4.08

3 98 3.23

4 98 3.14

5 98 3.14

6 98 1.76

7 98 1.76

Table 18- Volumetric composition of the 2nd

run using calibration curve

Reflux Ratio

Volume percentage of methanol (v/v %) Actual volume of


1 99.60 11.95

2 96.00 5.76

Table 19- Volumetric composition of the 3rd

run using calibration curve

15

Experiment 2

Reflux Ratio


(v/v %)


collected (ml)

2 98 20.58

2 99 19.80

2 91 16.38

2 94 4.70

2 92 2.76

3 25 or 85 0.50 or 1.70

3 98 3.92

3 97 3.88

3 99 0.99

Table 20- Volumetric composition of the 2nd

experiment using Calibration Curve


collected (ml)

Total amount of


%v/v methanol of

total distillate

1 76.8 73.84 96.1

2 78 73.51 or 74.71 94.2 or 95.8

Table 21- Total amount of methanol collected and overall v/v% for each experiment

Calculations

1) Feed Preparation

Volume of solution= Volume of methanol + Volume of water (ml) equation (1.1)

equation (1.2)

equation (1.3)

E.g. For an 85 %v/v methanol for 1 litre solution

2) Composition Calculation

%wt of water = 100 - %wt of methanol equation (2.1)

E.g. %wt of water=100-98.80=1.20

equation (2.2)

E.g. Density of water = 999.6kg m-3

m

3 kg

-1

equation (2.3)

E.g. Density of methanol = 792.3kg m-3

m

3 kg

-1

Vs= Vws+ Vms equation (2.4)

Vs= 0.0012 + 0.125 =0.1262m3 kg

-1

equation (2.5)

%

16

3) Calculating total amount of methanol collected

Volume of methanol collected= %v/v of methanol* distillate collected equation (3.1)

E.g. Volume of methanol collected= 99.05* 3ml=2.97ml

To find the %v/v methanol of total distillate collected, the sum of all methanol collected is divided by the sum of

distillate collected.

4) Calculating volumetric composition of distillate using Chromatography curve

Using the equation obtained from the Chromatography curve to find the volumetric composition of distillate.

equation (4.1)

E.g.

91.98

4.0-Discussion of Results Analysis

Once Experiment 1 was done, an analysis was made using the correlation of refractive index and weight percentage to

have an idea of which reflux ratios obtained gave the highest volume percentages and had compositions of 97 v/v % or

above. From Table 12, it can be observed that a reflux ratio of 2 gave around 98 v/v % and the largest collected

volume. This reflux ratio was adopted for Experiment 2 and it was then decided due to the theory that as the

experiment proceeds, an increase in reflux ratio will increase the amount of methanol collected. The results from

Table 13 confirm this as in the second run the composition of methanol decreased dramatically for a reflux ratio of 2.

At this instance a reflux ratio of 3 was chosen and by observing Table 18 it can be seen that this value gave the desired

composition and a volume of distillate which was larger than those of greater reflux ratios.

The results from Figure 2 and 3 demonstrate that as the reflux ratio increases, the respective amount of distillate

collected decreases as more is returned to the column. The volume fraction of methanol; however, generally increases

in the distillate over time. This is demonstrated by the results obtained from the Refractive Index analysis in Tables 8-

10. There is therefore a trade off between purity and collected volume over time.

In theory the Gas Chromatography composition analysis is should be more accurate as it finds the composition of

methanol in the mixture by knowing the number of components and what they are. On the other hand, the refractive

index gives the proportion of the components once it is known how many components there are. In terms of

practicality the calibration curve is the fastest way of obtaining results; yet, it is less accurate than using literature

values correlating refractive index and weight percentages. The downside with the refractive index method is that one

value of refractive index can sometimes give two values of composition and thus confusion can arise as it is shown in

Tables 20 and 21. The method of choosing the correct method to measure compositions demonstrates once again a

trade-off between time required to obtain composition values and accuracy.

The final compositions obtained for Experiment 1 can be compared for the three methods and the refractive index

analysis using literature values is the closest to the GC analysis by a difference of 0.8%. This implies that the most

efficient way to measure composition for this experiment would be to use the weight fractions from the literature

values as it is the most time efficient. The solution can also be assumed to contain purely water and methanol and is

something that can be obtained from the feed provider. In this experiment it is also important to state that since the

values of composition are either very high or very low, when obtaining composition from the refractive index

literature values, if there are two possible values, it will be obvious which composition is the correct one. For

example, when we obtained the result of 25 or 85%, as we know the composition of the distillate should be high,

clearly the correct value is 85% not 25%.

Each experiment reached an end after 90 minutes. This was due to the fact that the amount of distillate that was being

collected after this time was insignificant and carrying on collecting amounts of this size it would have taken too long

to collect the desired 85ml.

17

Experiment 2 showed to be successful as it approached the target compositions and desired volume. From Table 16,

the final composition of methanol in the total amount of distillate collected was of 97.7 v/v % methanol and a total

volume of 76.2 ml was collected, close to the 85 ml required. Experiment 1 was also carried out in 90 minutes and the

target composition was not achieved. The total volume of methanol recuperated was also lower than Experiment 2 as

shown in Table 16. This demonstrates that there is a trade-off between the cycle time of a batch distillation and the

purity of the methanol. In industry this trade-off is the basis of the cost-benefit analysis where the amount of

methanol has to be collected in the most time efficient way while producing large volumes and high methanol

compositions. In order to achieve this, an objective function has to be satisfied where the total annualised cost has to

be at its minimum. According to Sinnott and Towler (2009), the total annualised costs for the operation of a

distillation column would be:

Total Annualised Cost (TAC) = Fixed costs + Variable costs

The fixed costs consist of the cost of the column and installation. The variable costs refer to the cost of the feed and

the utilities such as cooling water and the power requirements of the Reboiler. According to the theory, the heating

duty of the Reboiler is also dependent on the reflux ratio, as a higher reflux ratio requires higher operating costs due

to the fact that the liquid returning has to be heated once again. In Experiment 1, higher reflux ratios were used for

and thus had larger operating costs than Experiment 2. The total profits would be the difference on the revenue made

from selling the methanol and the total annualised cost:

Total Profits = Revenue - TAC Experiment 2 would obtain larger total profits than Experiment 1 as it produces larger revenues and lower total

annualised costs. This means that Experiment 2 is more cost-effective than Experiment 1.

In the experiment, the heating duty of the Reboiler played a major role in the experiment as when the temperature of

the vapour on the top of the experiment went over the boiling point of pure methanol, larger amounts of water vapour

were being collected. This could explain the low compositions of methanol during minutes 40-50 in Table 6 for

Experiment 2. In order to avoid this happening, a negative feedback has to be put into place by the use of a sensor and

an actuator in order to maintain that temperature at around 64°C and thus collect only methanol. This way an optimum

reflux ratio could be achieved and the desired composition could be obtained in a shorter space of time which is

favourable, as the faster the product can be produced, the more profit will be made.

The experiment reached an end as the distillate volume collected after 90 minutes was insignificant and would have

taken too long to collect 85 ml.

Sinnott, R. Towler, S., 2009. Chemical Engineering Design. 5th ed. Oxford: Elsevier Ltd.

Errors

During the experiment there were many areas where errors could have occurred and therefore affected the accuracy of

our observed and derived results. These errors can be classed into two categories, systematic and random errors.

Systematic errors

Due to the fact that there were many different variables that we were required to measure in this experiment, there

were many different possible sources of systematic error. In the preparation of the standard solutions for the

calibration curve and all subsequent distillation feeds, we used a pipette to measure the volumes that had an

uncertainty in measurement of ±0.05ml. As the total volume of each solution prepared using the pipettes was a litre

(1000ml) then in theory the maximum possible error in the total amount is only ±0.1ml (2×±0.05ml) which is a mere

±0.01% percentage error:

.

In fact the maximum possible influence this systematic error could have is on the 10% v/v methanol solution for the

calibration curve. For this 10ml was measured and added to 90ml3 of water using the pipette. If the worst possible

errors occurred, then the solution could only be as much as 10.05% methanol and as little as 9.95%.

In a similar fashion, the maximum possible systematic error in measuring the refractive index is equally as small.

Measuring the refractive index is important as it determines the accuracy of the calibration curve and the refractive

index machine has a very small uncertainty of ±0.00005. As our smallest refractive index measurement (100%

methanol solution) of 1.32871, this uncertainty is so small in comparison that it is not worth quantifying the possible

errors measurement.

The density meter used to measure the density of pure methanol and water solutions had an uncertainty of ±0.05kg m-

3. Like the refractive index measurements, when comparing this uncertainty to the measured values we obtained, the

smallest of which was 792.3kg m-3

, any possible error is too negligible to be worth quantifying.

18

There will also be an uncertainty in measurement of the gas chromatography machine but this is so small compared to

the magnitude of the values produced by the machine that it is simply too negligible to mention.

The most influential source of systematic errors most likely came from measuring the volume of distillate collected in

the samples. For the 1st run of experiment 1 the measuring cylinder used had an uncertainty of ±0.5ml. Considering

the volume measured in that run was only 9.5ml and the smallest was 2.5ml, this is a very significant potential source

of error. For the 9.5ml measurement this equates to a possible percentage error of ±5.3%, but with the smallest

measurement the potential error is ±20%. Anything over 5% is quite significant, but to have a possible percentage

error of ±20% is really unacceptable. This is why for all consequent measurements we changed to a more accurate

measuring cylinder which only had an uncertainty of ±0.1ml. This should clearly lead to more accurate results,

however, considering the smallest amount of distillate this cylinder was used the measure was 1.8ml, this still creates

a maximum possible percentage error in measurement of ±5.6%, which is still quite large.

There will also be an uncertainty in measurement of the gas chromatography machine but again this is so small

compared to the magnitude of the values produced by the machine that it is simply too negligible to mention.

Random errors

Random error could have easily occurred throughout the experiment in a number of different ways. The most obvious

is when using the calibration curves to calculate the methanol v/v% of the total distillate collected. In order to obtain

the v/v% of methanol for each sample we had to read off a value from the calibration curve given the measured

refractive index. Although the refractive indices were measured very accurately, error in human judgement in where

the values exactly intersected the curve giving a corresponding v/v% reading will not have been exact and could have

lead to errors. Any errors in values calculated using the gas chromatography data are also most likely due to

uncertainties from its calibration curve.

Errors in human judgement will also have been a factor in possible errors in the time measurements. Although it was

intended that samples of distillate were taken at exact time intervals, it was down to human judgement when precisely

to take the samples, so it is possible that an error of a few seconds could have occurred. It was important that the

samples were taken over defined time intervals so that it was possible to analyses which reflux ratio was the most

efficient at producing the desired distillate. Seeing as all the times that we measured were in the scale of minutes and

any possible error was only a few seconds, possibly less, any error in the time measurements should not have had a

great effect on the accuracy of our results.

Using the calibration curve method could have led to an error in the methanol v/v% measurement due to the fact that

the reading assumes that there are only two components in the analysed sample. Although in theory we used 100%

pure methanol and water, even a small presence of an impurity could affect the refractive index readings and give an

incorrect value for the volume composition.

When collecting the samples from the column between collecting the sample in the measuring cylinder and

transferring the sample to a container to be analysed, as the sample is often at a higher temperature than 65oC when it

is collected, it is inevitable that some of the methanol in the mixture will vaporise. Although this amount should be

very small each time, it does mean that our recorded value of total methanol recovered should be less than the amount

that was actually collected from the distillation and therefore give an untrue value for our total methanol recovered

and a smaller v/v% value than is actually true.

Although random errors cannot be quantified, they are clearly significant enough to affect the outcome of our

experiment, as shown by the fact that we did encounter a small number of anomalous errors, for example, the

methanol composition going over 100 v/v % in Table 8 and being as low as 85%v/v in table 17.

Propagation of errors

As the errors due to measurement are negligible for most of our measurements it seems redundant to calculate the

propagation of those errors as for the most part, they shouldn’t have had a great effect on subsequent calculations.

However, the uncertainty in measuring the amount of distillate collected each time is significant enough to have an

effect on the calculations, and as the most important aspect of the experiment was to achieve the desired overall

methanol v/v%, it is important to know the possible error in this measurement.

19

Analysing the error can be done in one of two ways. The simplest method of calculating this error is to assume that the

error in each individual measurement is the maximum and that they all add up. Using the equation:

Where ∆Z is the possible error and ∆x and ∆y are errors in the variables. So for experiment 1if we take ∆Z to be the

error in total distillate collected, and consider that all the collected samples in the 1st run had an uncertainty of±0.5ml

and all subsequent distillate measurements had an uncertainty of ±0.1ml:

This gives the maximum possible errors that could have occurred in the calculation of total volume of distillate

collected for both experiments. However, it is unlikely that the errors in the sample measurements combined in the

worst possible way as suggested by this method. Therefore, it is better to describe the likely error that occurred in

these calculations by using the following equation for error calculation where Z= x+y:

Given the two total volumes collected for experiment 1 and 2 were 76.8ml and 78ml respectively, these error values

give the following percentage error values for each experiment:

Experiment Maximum percentage error Likely percentage error

1 ±5.73% ±1.77%

2 ±1.15% ±0.38%

5-Conclusion

From the results, it can be observed that there is a correlation between the volume of distillate collected and reflux

ratio as the larger the reflux ratio, the lower the volume of distillate collected. In theory the GC analysis is the most

accurate; yet, for this experiment in terms of practicality the refractive index method using literature values is more

adequate.

It can be concluded that in Experiment 2 the desired composition and target volumes are achieved by using reflux

ratios of 2 and 3. The final collected composition had a value of 97 v/v % and a total volume of 76 ml. It is also observed that an industrial level there is a trade-off between operating costs and cycle time to maximize

methanol purity and thus profits. Experiment 2 showed to be the most cost effective experiment as it satisfied these

conditions. Further investigation would be required to determine whether one single optimum reflux ratio and the

target volume could be achieved by applying the improvements mentioned in the analysis.

The experiment also emphasized the importance of health, safety and environmental requirements when dealing with

methanol. These implications apply at an industrial level when it comes to disposing of large quantities of methanol

appropriately.

20

6-References Sinnott, R. Towler, S., 2009. Chemical Engineering Design. 5

th ed. Oxford: Elsevier Ltd.

Methanex. (2006). Technical Information & Safe Handling Guide for Methanol. Available:

http://www.methanex.com/products/documents/TISH_english.pdf. Last accessed 9th mar 2011

Department of Technical Vocational Education. (2006). Mass Transfer. Available:

www.most.gov.mm/techuni/media/ChT03032MassTransfer.doc+relevance+between+cost+of+operation+an

d+reflux+ratio&hl=en&gl=uk&pid=bl&srcid=ADGEESiM9glw.Last accessed 9th mar 2011.

Scully, P. 2011. System Measurements Notes.

Webb, C. 2011. Distillation and Absorption Notes

Mbeychok. (2006). BatchRectifier. Available: http://en.wikipedia.org/wiki/File:BatchRectifier.png. Last accessed 9th mar 2011. Unknown. (2003).Distillation Diagram. Available: http://www.separationprocesses.com/Distillation/Fig064.htm. Last accessed 9th mar 2011. Mars Traders. (). Digital Refractometer. Available: http://www.bikudo.com/product_search/details/100744/digital_refractometer.html. Last accessed 9th mar 2011.

Unknown. (). Gas Chromatography. Available: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm. Last accessed 9th mar

2011.

7-Nomenclature

Symbol Definiton Units

Vws Volume of water per kg of solution m3 kg

-1

Vms Volume of methanol per kg of solution m3 kg

-1

Vs Volume of solution per kg of solution m3 kg

-1

ρm Density of water kg m-3

ρw Density of methanol kg m-3

P Total Pressure Pa

PA Partial Pressure of component A Pa

PθΑ Vapour pressure at standard conditions of component A Pa

PθB Vapour pressure at standard conditions of component B Pa

yA Mole fraction of A in vapour phase mol %

xA Molel fraction of A in liquid phase mol %

xb Molel fraction of B in liquid phase mol %

α Volatility -

L Liquid returning to column mol

D Amount of distillate mol

R Reflux ratio -

r Liquid returning to column mol

W Liquid returning to column mol

Sf Final numbers of moles in still mol

Si Initial number of moles in still mol

xsf Final composition in still mol %

xsi Initial composition in still mol %

experiment b group 24

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