University of Bahrain

College of Engineering

Department of Chemical Engineering

(CHEG 413)

Unit operation laboratory

Experiment 6

Investigation the effects of a constant reflux ratio on product composition in batch distillation between ethanol and water

name Id. number task

Ahmed Sameer 20073435 leaderFaisal AlShowaikh 20070590 secretary

Saeed sawar 20001791 Safety officerFaisal AlShowaikh 20070590 member

Supervised by: Dr. Fatma Marhoon

expirement Date: 21/05/2012

Submission Date: 04/06/2012

Semester 2, 2011-2012

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Abstract:

Ethanol and water have different boiling points so they separated from each other by using batch distillation. The mole fraction of the components at the top and the bottom of the column was determined by using refractive indices calibration curve which is constructed at different ethanol and water volume percent. The column was operated at total reflux at the beginning of experiment until the ethanol evaporated then a 0.8 reflux ratio is specified in order to determine the effect of constant reflux ratio on product mole fraction. The refractive indices and the system temperatures were noted at every 10 minute interval for 1 hour. Then the system was simulated by using ASPEN PLUS software and the simulation results were compared with the experimental results. We obtain an error between 0.5% and 35%. We found that the mole fraction of light component increases with the time at the top of the column and decreases at the bottom. We recommended taking more points in the calibration curve in order to decrease the percentage of error.

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Table of Contents

Abstract....................................................................................................................................2

I.Introduction and Background.................................................................................................4

II.Theory....................................................................................................................................6

III.Industrial Applications..........................................................................................................8

IV.Apparatus and Procedures...................................................................................................9

IV.1 Apparatus......................................................................................................................9

IV.2 Material....................................................................................................................10

IV.3 Procedure....................................................................................................................10

V.Results and disscusion.........................................................................................................13

VI.Conclusions and Recommendations...................................................................................15

VII.References.........................................................................................................................16

VIII.Appendices.......................................................................................................................17

A. Raw Data........................................................................................................................17

B. computer simulation......................................................................................................20

C. Nomenclature.................................................................................................................24

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I. Introduction and Background

Distillation is one of the oldest and still most common methods for both the purification and the identification of organic liquids. It is a physical process used to separate chemicals from a mixture by the difference in how easily they vaporize. As the mixture is heated, the temperature rises until it reaches the temperature of the lowest boiling substance in the mixture, while the other components of the mixture remain in their original phase in the mixture. The resultant hot vapor passes into a condenser and is converted to the liquid, which is then collected in a receiver flask. The other components of the mixture remain in their original phase until the most volatile substance has all boiled off. Only then does the temperature of the gas phase rises again until it reaches the boiling point of a second component in the mixture, and so on.

Batch distillation is a type of distillation process. In batch distillation, a still is charged with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process.

Distillation is a fundamentally important process in both chemistry and industry. Distillation probably accounts for 90% of all separation processes in the chemical industry and is also a significant user of energy due to the necessary heating involved. Batch distillation is preferable rather than other types in a case of small scale and high purity production and in laboratory experiments. Batch distillation is usually used in the pharmaceutical industry and in wastewater treatment units.

The first clear evidence of distillation comes from Greek alchemists working in Alexandria in the first century AD. Distilled water has been known since at least ca. 200 AD, when Alexander of Aphrodisias described the process. Arabs learned the process from the Egyptians and used it extensively in their chemical experiments.

Early forms of distillation were batch processes using one vaporization and one condensation. Purity was improved by further distillation of the condensate. Greater volumes were processed by simply repeating the distillation. Chemists were reported to carry out as many as 500 to 600 distillations in order to obtain a pure compound

In the early 19th century the basics of modern techniques including pre-heating and reflux were developed, particularly by the French, then in 1830 a British Patent was issued to Aeneas Coffey for a whiskey distillation column, which worked continuously and may be regarded as the archetype of modern petrochemical units. In 1877, Ernest Solvay was granted a U.S. Patent for a tray column for ammonia distillation and the same and subsequent years saw developments of this theme for oil and spirits.

[1]

In this experiment ethanol and water are separated from each other in batch distillation to accomplish the following Objectives:

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1. To calibrate a refractive index graph for distilled water-ethanol .2. To perform batch distillation on a binary mixture using a sieve plate column.3. To investigate the effects of a constant reflux ratio on product composition at top and bottom.4. Compare your results with the theoretical ones from computer simulation. [2]

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II. Theory

Figure II-1: typical batch distillation configuration.

In batch distillation a mixture of binary or multicomponents is charged to the column pot as shown in figure II-1. The feed is heated to a boiling point temperature of the lowest volatile component in order to separate it from other components. Vapor flows upwards in the rectifying column and condenses at the top. Usually, the entire condensate is initially returned to the column as reflux. In batch distillation the time, reflux ratio and the concentration of the products is the main factor that’s control the efficiency of separation.

Reflux ratio is the ratio of the returned liquid to the column to the distillate. When reflux ratio increases more liquid will contact with the vapor and therefore better separation is achived.

There are three cases in batch distillation:

Figure II-2: configurations of Differential distillation at the left and batch rectification at the right.

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1- Differential distillation: in this case there is no reflux as shown in figure II-2 and all condensate is drawn from the condenser.

2- Binary batch rectification with constant distillate composition and variable reflux: in this case reflux ratio is varied to achieve a constant distillate composition that meets the specified purity. A complex control system is required, including a composition sensor on the distillate.

3- Binary batch rectification with constant reflux and variable distillate composition: this method is easier than pervious method because only flow seniors are required to control the reflux ratio. [3]

For the third method instantaneous distillate and still bottoms compositions vary with time.

The concentration of light component at the top of the column increases with time and the concentration of heavy component at the bottom decreases.

Batch distillation is usually carried out in two steps:

1- Operating at total reflux ratio until the system reach steady state.2- Operating at a desired reflux ratio.

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III. Industrial Applications

The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinal (herbal distillate), and food processing. The latter two are distinctively different from the former two in that in the processing of beverages, the distillation is not used as a true purification method but more to transfer all volatiles from the source materials to the distillate.

Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components—notably oxygen, nitrogen, and argon— for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with higher alcohol content. The premises where distillation is carried out, especially distillation of alcohol are known as a distillery.

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IV. Apparatus and Procedures

IV.1 Apparatus:

1. LS-32203 continous distillation column2. Refract meter & refractive index data 3. Beakers

Figure IV.1-1: continues distillation column

LS-32203 continuous distillation column that’s shown in Figure IV.1-1 provides tools to investigate the effects of temperature and reflux ratio on separation efficiency in a batch distiller.This unit is formed by a boiler, on which two types of columns can be adapted (plate columns and Raschig columns), a reflux system, a tank for distillation, a vacuum pump and a feeding pump (for continuous feeding). The steam which is heated by the boiler and reaching the head of the column is sent to a total condenser. The cooling water flow going through the condenser can be adjusted and is indicated in a flow meter. Distillation can be carried out at low pressures with the help of an adjustable vacuum pump. The load loss in the column can be measured with a pressure meter. The column can work both continuously and discontinuously.

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For proceeding to continuous feeding, a pump that can inject the substance directly into the boiler or in any of the plates is available. The temperatures of the system are measured through sensors placed in strategic positions.

Figure IV.1-2: refract meter

Refract meter is that device which measures the refractive index of the material the Operating the device as simple as placing a sample on the artificial sapphire prism, closing the presser and reading the answer on the Digital Display ,all in just a few seconds.

IV.2 materials:

The materials used in this experiment is ethanol and water

IV.3 procedure:

Calibration procedure:

1- Prepare 10 ml of ethanol and measure the refractive index.2- Add 10 ml of water to the prepared ethanol and measure the refractive index.3- Repeat the second step until reach 90 ml of water.4- Prepare 10 ml of water and measure the refractive index.5- Add 10 ml of ethanol to the prepared water and measure the refractive index.6- Repeat the fifth step until reach 90 ml of ethanol.

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Figure IV.3-1: batch distillation unit configuration

Experiment procedure:

1. Prepare a 10 L batch of binary mixture (ethanol and water) of known composition.

Recommended for 50% volume ethanol in water mixture.

2. Collect a 10 mL of the prepared sample using a syringe and test the refractive index.

3. Pour the mixture into the reboiler.

4. Switch ON the main power switches on the unit.

5. Close all the valves on this unit.

6. Open valves V8 and V11 if initial vacuum suction is required. Keep an eye on the pressure

gauge, close V8 and V11 when the vacuum pressure reach 0.2 bar.

7. Open valve V9. Start the cooling water supply. NOTE: Do not open valves V8 and V11 when

the experiment is running.

8. Turn on the Reboiler heater ON/OFF switch located on the Control Panel.

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9. Set the power of the boiler to maximum using the power regulator.

10. Set the temperature of reboiler, T15 to 95ºC by pressing UP or DOWN button. Note: high

power setting is required in order to supply sufficient energy to warm the system up in the

initial startup.

11. Turn on the reflux switch and set the reflux to 100%. The reflux controller indicates

percentage of condensate returning to the column. Open valve V12 on the Top product tank.

12. Wait for the vapor to condense in the glass condenser. Note: please take note of the

temperature change on T11, when T11 reaches about 80ºC, it will begin to show condensate

collection.

13. Once the top product appears in the divider vessel, leave the column at total reflux (100%)

for another 10 minutes to allow the system to stabilize.

14. After that, record down all the temperatures as the initial experiment point.

15. Change the reflux control to a desired value i.e. 80%.

16. At every intervals of 10 minutes, record down all temperatures and draw a small sample

(approximately 10ml) from the top product tank (DV3) and reboiler tank (DV1). Measure and

note down the refractive index of each sample at constant room temperatures. To quickly cool

down the reboiler’s sample, place the sample tube in a beaker filled with cold water. Note: you

are recommended to wear protective gloves, glasses, laboratory cloth, long pants and covered

shoes when taking samples from top product (DV3) and reboiler tank (DV1).

17. Take 6 set of readings. Then, empty the top product vessel and measure its volume. This will

give us the product output rate of the system. [2]

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V. Results and discussion

The results of light volatile component (ethanol) fraction at the top and the bottom of the column were obtained from the experiment and the computer simulation by using ASPEN PLUS at every 10 minute interval. The percentage error between the experimental and simulation results was calculated as shown in table v-1.

Table v-1: experimental and simulation results

Time (min)

experiment Simulation Error%

product X feed X product X feed X product X feed X0 0.7390 0.1900 0.5950 0.2271 24.201 16.336

10 0.5400 0.2220 0.5960 0.2207 9.3959 0.589020 0.5420 0.1390 0.5975 0.2143 9.2887 35.13730 0.5500 0.1970 0.6022 0.2077 8.6682 5.151640 0.5420 0.1830 0.6107 0.2007 11.249 8.819150 0.5460 0.1780 0.6203 0.1935 11.978 8.010360 0.5700 0.1800 0.6288 0.1858 9.3511 3.1216

0 10 20 30 40 50 60 700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

X vs Time

X product (exp values)X feed (exp values)X product (sim values)X feed (sim values)

T (min)

X

Figure V-1: time profile of ethanol mole fraction at the top and the bottom of the column obtained from experimental and simulation results.

From table V-1 and figure V-1, it’s clear from the simulation results that the mole fraction of ethanol at the top of the column increases with time and for the bottom decreases with time.

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The experimental result doesn’t follow this path and it’s fluctuating with time. The simulation results represent the normal case of batch distillation because with time more ethanol will be evaporated and the mole fraction should be increased at the top and decreased at the bottom. The error between simulation results and experimental results at the bottom is less than at the top. The error is changing from 0.5890% and 35.137%. The sources of error may come from:

1- The accuracy of timer (± 0.1%)2- The accuracy of Refract meter (± 0.00001%)3- The refractive indices do not reflect the fraction of the components significantly. For

example, the refractive index is changing only 0.03 when the fraction of ethanol changed from 0 to 100%. The calibration curve is not linear so the curve fitting cannot be used. The fraction of ethanol was obtained from the curve.

4- The system was simulated at different temperatures.

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VI. Conclusions and Recommendations

To conclude, the batch distillation system was studied experimentally and by using computer simulation. The experimental results were conflicted with simulation values at the first half of the experiment when the amount of light component at the top of the column decreases with time. The error range was obtained to be between 0.5% and 35%. There were some errors that are affecting the results. However, to decrease the percentage of error and to improve the future experiments the following points are recommended:

1- Since there are no other common methods to indicate the fraction of the distillation products we recommended to take more points in refractive calibration curve.

2- Using more than one reflux ratio in order to compare between the effects of high and low reflux ratio on the product composition.

3- Study the effect of boiling temperature on the composition of the products by changing the temperature more than 1 time at constant reflux ratio.

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VII. References

[1] wikipedia. http://en.wikipedia.org/wiki/Batch_Distillation[2] The laboratory manual which is handled by the instructure.[3] J. D. Seader, Ernest J. Henley, "Separation Process Principles 2nd Edition" W iley | 2005

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VIII. Appendices

A. Raw Data

Calibration curve data:

Table VIIIA-1: Calibration curve data

Ethanol

volume V, (ml)

water volum

e V, (ml)

Refractive index

0 10 1.3329710 10 1.3588720 10 1.3626630 10 1.3629340 10 1.364850 10 1.364960 10 1.3649270 10 1.3648580 10 1.3647790 10 1.3696310 0 1.3630410 10 1.3601110 20 1.3517710 30 1.347110 40 1.3438710 50 1.3417410 60 1.3403610 70 1.3393510 80 1.3384910 90 1.33799

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The calibration curve is constructed by converting the volume of ethanol to mole fraction by the following relation:

n= VM .W

( ρ )

Where M .W is the molecular weight and ρ is the density.

The properties of ethanol and water was determined by using ASPEN HYSYS software and listed in table VIIIA-2.

Table VIIIA-2: density and molecular weight of ethanol and water at ambient conditions.

Density, ρ(kg/L) Molecular weight, M .W(kg/mol)

Ethanol Water Ethanol Water

0.79600 0.99704 0.04607 0.01801

Table VIIIA-3: obtained mole fraction of ethanol.

Ethanol moles

Water moles

Ethanol mole

fraction0 0.55360

80

0.172781

4.982471

0.033515

0.172781

4.428864

0.037548

0.172781

3.875256

0.042683

0.172781

3.321648

0.049445

0.172781

2.76804 0.058753

0.172781

2.214432

0.072378

0.172781

1.660824

0.09423

0.172781

1.107216

0.134985

0.17278 0.55360 0.23786

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1 8 20.34556

10.55360

80.38431

20.51834

20.55360

80.48355

0.691122

0.553608

0.555239

0.863903

0.553608

0.609451

1.036683

0.553608

0.651883

1.209464

0.553608

0.685998

1.382244

0.553608

0.714024

1.555025

0.553608

0.737456

0.172781

0 1

1.33 1.335 1.34 1.345 1.35 1.355 1.36 1.365 1.37 1.3750

0.050.1

0.150.2

0.250.3

0.350.4

0.450.5

0.550.6

0.650.7

0.750.8

0.850.9

0.951

1.05

X vs refractive index

refractive index

Etha

nol m

olef

racti

on (X

)

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Figure VIIIA-1: calibration curve between Ethanol mole fractions and refractive indices.

The calibration curve is constructed by plotting the mole fraction of ethanol which is listed in Table VIIIA-3 verses the refractive indices from Table VIIIA-1

Experiment data:

Table VIIIA-4: experiment data

Time (min)

Product refractive index

feed refractive index

0 1.36981 1.3571410 1.36474 1.3587820 1.36475 1.3569430 1.36484 1.3575440 1.36475 1.3567450 1.36479 1.356160 1.36486 1.35642

Simulation data:

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Figure VIIIA-2: simulation results for the top section of the column.

Figure VIIIA-3: simulation results for the bottom section of the column.

B. computer simulation

The system was simulated by using ASPEN PLUS software. ASPEN provide strong tools to model, simulate chemical processes and to find optimum operating conditions and optimum design.

We try to simulate the system at the experiment reboiler temperature but we got an error message that is said the column become dry during the operation so we choose to specify the feed temperature only and the software will determine the appropriate boiling temperature.

The following is a summary of simulation steps:

1- First, the batch distillation column is added to the process flow sheet with the material streams.

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2- The components (ethanol and water) are added.

3- An appropriate equation of state is added to calculate the properties of the components. Peng Robinson is chosen in this case.

4- The specification of feed stream including the temperature, pressure and the volumetric flow rate is entered.

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5- The specification of the column is entered. The feed time is chosen to be 1 minute to give 10 liter total feed since the feed flow rate is specified in the previous step to be 10L/min.

6- Two operating steps are chosen. The first step for the column startup at total reflux and the second for the column at the specified reflux ratio.

7- In the second operating step, the Stop criterion is chosen to be based on the operation time. the time is changed from 0 to 1 hour at every 10 minutes interval and the results is obtained

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c. nomenclature

X : mole fraction of ethanolV : volume

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M.W : molecular wieghtρ : density

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