experiment b group 24
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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
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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
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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)
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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
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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
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o.com/product_search/details/100744/digital_refractometer.
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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
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age=4&ndsp=27&ved=1t:429,r:26,s:77&biw=1366&bih=51
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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
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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
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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
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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
methanol collected (ml)
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
Reflux Ratio Volume percentage of methanol (v/v
%)
Actual volume of
methanol collected (ml)
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
methanol collected (ml)
%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
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Experiment 2
Reflux Ratio
Volume percentage of methanol
(v/v %)
Actual volume of methanol
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
Experiment Total Amount of distil
collected (ml)
Total amount of
methanol collected (ml)
%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
Reflux Ratio Volume percentage of methanol (v/v
%)
Actual volume of
methanol collected (ml)
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
methanol collected (ml)
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
Volume percentage of methanol
(v/v %)
Actual volume of methanol
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
Experiment Total Amount of distil
collected (ml)
Total amount of
methanol collected (ml)
%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 %
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