ABSTRACT

In this section, we have done two experiments; the first one was the experiment of

determining the column pressure drops and second was the experiment of determining the

composition of mixture of toluene-methylcyclohexane using refractometer. In determining the

column pressure drops, we also managed to determine the boil-up rate of the mixture and

observed the foaming conditions over the trays. In the second experiment, we did also find the

refractive index, mass, mole fraction and the volume of components in the mixture of toluene-

methylcyclohexane. The equipments and apparatus needed in the first experiment were a

continuous distillation column, measuring cylinder, power supply, and stopwatch. While the

experiment of determining the mixture compositions required a refractometer.

Method applied for the first experiment was no different from batch distillation in

industry. The distillation operated at total reflux which means there was no feed or top product or

bottom product, since the entire formed vapor will after condensation, return to the column.

Setting the power at several different intensities, we were able to make comparison of the boil-up

rate, the pressure drops and the degree of foaming for every different power supplied. The

experiment of determining the mixture compositions was rather simple; the sample was collected

and inserted into the refractometer and the refractive index was recorded. Using the data given,

we were able to determine the mole fraction of each component, mass and volume as well. After

all the calculations have been made, the graph was plotted.

Based on the graph plotted, we were able to observe the relationship for the boil-up rate

and the pressure drops in the column for the first experiment and the relationship between the

refractive index and the compositions of components.

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INTRODUCTION

Distillation is defined as a process in which a liquid or vapour mixture of two or more

substances is separated into its component fractions of desired purity, by the application and

removal of heat. Distillation is based on the fact that the vapor of a boiling mixture will be richer

in the components that have lower boiling points, therefore when this vapor is cooled and

condensed, the condensate will contain more volatile components. At the same time, the original

mixture will contain more of the less volatile material. There are many types of distillation

column, each designed to perform specific types of separation, and each differs in terms of

complexity.

1.) Batch column: in batch operation, the feed to the column is introduced batch-wise. That is,

the column is charge with a `batch` and then the distillation process is carried out. When

the desired task is achieved, a next batch of feed is introduced.

2.) Continuous column: in contrast, continuous columns process a continuous feed stream. No

interruptions occur unless there is a problem with the column or surrounding process unit.

They are capable of handling high throughputs and are the more common of the two types,

trays column and packed column.

Binary column – feed contains only two components 

Multi-component column – feed contains more than two components

Tray column (internal) – where trays of various design are used to hold up the

liquid to provide better contract between vapor and liquid

Packed column – where instead of trays `pickings` are used to enhance contact

between vapor and liquid.

Distillation columns are made up of several components, each of which is used either to

transfer heat energy or enhance material transfer. A typical distillation contains several

major components:

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1.) A vertical shell where the separation of fluid components is carried out

2.) Column internals such as trays/plates or pickings which are used to

enhance components separations

3.) A reboiler to provide the necessary vaporization for the distillation process

4.) A condenser to cool and condensed the vapor leaving the top of the column

5.) A reflux drums to hold the condensed vapor from the top of the column so that liquid

(reflux) can be recycle back to the column.

Operation and terminology; the liquid mixture that is to be processed is known as the

feed and this is introduced usually somewhere near the middle of the column to a tray known as

the feed tray. The feed tray divides the column into a top (enriching) section and bottom

(stripping) section. The feed flows down the column where it is collected at the bottom in there

boiler. Heat is supplied to the reboiler to generate vapor. The source of heat input can be any

suitable fluid, although in most chemical plants this is normally stream. In vapor raised in there

boiler is re-introduced into the unit at the bottom of the column. The liquid removed from their

boiler is known as the bottom product.

The vapor moves up the column, and as it exits the top of the unit, it is cooled by a

condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some

of this liquid is recycled back to the top of the column and this is called the reflux. The

condensed liquid that is removed from the system is known as the distillate or top product. Thus,

there are internal flows of vapor and liquid within the column as well as external flows of feeds

and product streams, into and out of the column.

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OBJECTIVES

The objectives of this experiment are:

1. To determine the pressure drop of the distillation column for various boil-up rates, to

observe the degree of forming on trays for each power increment,

2. To plot the curve relating pressure drop and boil-up rates,

3. To determine the refractive index for unknown concentration of methylcyclohexane and

toluene from the distillation column for each power increment

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THEORY

Distillation is a process of separating two or more miscible liquids by taking advantage

of the boiling point differences between the liquids. To understand how distillation works

consider the mixture of toluene and methylcyclohexane for this distillation experiment. Heat is

added to the mixture of toluene and methylcyclohexane and eventually the most volatile

component (in this case methylcyclohexane) begins to vaporize.

As the methylcyclohexane vaporizes it takes with it molecules of toluene. The

methylcyclohexane-toluene vapor mixture is then condensed and evaporated again, giving a

higher mole fraction of methylcyclohexane in the vapor phase and a higher mole fraction

of toluene in the liquid phase.

This process of condensation and evaporation continues in stages up the column until the

methylcyclohexane rich vapor component is condensed and collected as tops product and the

water rich liquid is collected as bottoms product.

To understand distillation, first consider what happens upon heating a liquid. At any

temperature, some molecules of a liquid possess enough kinetic energy to escape into the

vapor phase (evaporation) and some of the molecules in the vapor phase return to the liquid

(condensation).

Equilibrium is set up, with molecules going back and forth between liquid and vapor. At

higher temperatures, more molecules possess enough kinetic energy to escape, which results in a

greater number of molecules being present in the vapor phase. If the liquid is placed into a closed

container with a pressure gauge attached, one can obtain a quantitative measure of the degree

of vaporization. This pressure is defined as the vapor pressure of the compound, and can be

measured at different temperatures.

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MATERIAL & APPARATUS

MATERIALS

Methylcyclohexane-toluene

25 mol percent, 50 mol percent and 75 mol percent methylcyclohexane

APPARATUS

Continuous distillation column (UOP3CC)

100mL measuring cylinder

Stopwatch

Refractometer

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PROCEDURES

General Start-Up Procedure

1. Electrical power to the control console is switched on. This is achieved by moving

the ELCB switch to the UP position. The LOW LEVEL lamp in the reboiler heater

section of the console is illuminated. The reboiler power, reflux timer, column

temperature and process temperature digital displays are also illuminated.

2. All the valves (V1 to V5) are ensured to be in the closed position.

3. Laboratory cold water supply is turned on. Flow control valve (V5) is opened to

give maximum flow into the condenser via flowmeter (F11). Any leakages are

checked.

4. The manometer tube is filled until an equal level is visible about halfway up the

scale. Valves (V6 and V7) are closed.

5. Reboiler heater power is switched on at the console and power to the heater is

adjusted to 1.50 kW. Water in the reboiler began to heat up and this is observed by

selecting (T9) on the process temperature digital display..

6. V6 and V7 are opened and the pressure difference in the manometer is observed.

V6 and V7 are closed.

7. 90 mL of the sample is collected at valve V3 and the time taken for collecting the

sample is recorded.

Procedure For Experiment A:

1. Reboiler heater power is switched on at the console and power to the heater is

adjusted to 0.50 kW. Water in the reboiler began to heat up and this is observed by

selecting (T9) on the process temperature digital display.

2. The system is let stabilize first for 10 minutes.

3. V6 and V7 are opened and the pressure difference in the manometer is observed.

V6 and V7 are closed.

4. 90 mL of the sample is collected at valve V3 and the time taken for collecting the

sample is recorded.

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5. Few drops of the sample are taken and the refractive index for the sample is

checked by using the refractometer.

6. Steps 1 to 4 are repeated by varying the heater power.

Procedure For Experiment B: Determining the Mixture Composition

1. The refractive index (R.I) of pure methylcyclohexane and pure toluene are

measured.

2. Small quantities of 25 mol percent, 50 mol percent and 75 mol percent

methylcyclohexane are made up and their R.I are measured. The volume of

constituents are calculated as shown in the calculation

General Shut-Down Procedure

1. The heater power, feed pump motor and reflux valve are switched off at the

console.

2. The equipment is disconnected from the electrical supply when the equipment is

not in used.

3. All of the water are drained from the system using the drain valves V2, V3, V4,

V11 and by breaking the pipe connections of the bottom product cooler and the

feed pump.

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RESULTS

A. Using distillation column

Power (kW) Collection time (s)

Boil-up rate (L/s)

Pressure drop (cm H2O)

Refractive index

0.5 120 0.00075 69 1.441720.75 37 0.0024 80 1.453941.0 22 0.0041 161 1.445841.25 5 0.018 170 1.458761.50 9 0.01 155 1.46849

Power (kW) Degree of foaming on trays

Concentration of unknown (toluene mol %)

0.5 Gentle localized0.75 Violently localized1.0 Super violently localized1.25 Super violently localized1.50 Over whole tray

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B. Without using distillation column (finding refractive index from mixing of solution )

Sample Mixture percentage (mol %) Refractive indexMethylcyclohexane Toluene

A 0 100 1.49612B 25 80 1.47402C 50 60 1.45526D 75 40 1.43716E 100 20 1.42383

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DISCUSSION

In the experiment of determining the pressure drop of the mixture in the column, we were not only determining the pressure drop alone, but also the boil-up rate and the observation of the degree of foaming on each tray were recorded. To obtain the data, first thing we did was controlling the power intensity of the boiler. The power controller was turned until a reading we decided was obtained on the digital wattmeter. After that, 90 mL of the mixture were collected and the times of collection were recorded. This procedure enables us to calculate its boil-up rate. Then, we may take the reading of pressure drops by observing the manometer.

As the experiment went to the end, the pressure drops seem to increase as the power and boil-up rate increased. However the last reading of pressure drop did not show the similar pattern in contrast. This was due to the technical error; the sample of the mixture from the column has overflowed into the manometer arms, caused the formation of two layers of fluid in the manometer. This situation brought to decreasing pressure difference which violates the theory of pressure drop increase with increasing boil-up rate. Note that the distillation practice was operated at total reflux, meaning the entire formed vapor returned to the column after condensation.

Meanwhile for the experiment of determining mixture compositions using refractometer, we managed to obtain not only the compositions of the methylcyclohexane and toluene, but also their refractive index. By only calculating the composition of methylcyclohexane, we would also get the composition of toluene, since their composition made a total of 100 percent mixture. As stated in the result, the refractive index recorded decreased with increasing mole fraction of methylcyclohexane. This statement refers that as the mole fraction of toluene increased, the refractive index increased as well in almost proportional manner.

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CONCLUSION

The experiment of determining column pressure drop was done in order to achieve the objectives of performing batch distillation and to determine the overall pressure drop. Besides, we were able to observe the degree of foaming of mixture of toluene and methylcyclohexane on each tray. It was seen that as we adjusted the power of boiler heater to be higher, the corresponding boil-up rates were also increased. In spite of that, the pressure drops recorded for various boil-up rates were also increased as the boil-up rates increased. This relationship was also shown diagrammatically by plotting the graph of pressure drop against the boil-up rates. From the graph, it can be seen clearly that boil-up rate increased as the pressure drop increased, but not in proportional relation. The degree of foaming on trays also showed a trend of gentle to a more violent condition as the boil-up rates increased. At the beginning, the boiled mixture was gently localized. After some times, it was seen that the condition has altered to violently localize. It was then followed by the condition of foaming gently over whole tray, foaming violently over whole tray, and finally, we observed that there was an amount of liquid flooding in the column.

The second experiment, the experiment of determining mixture compositions was generally done to determine the relationship between the mixture composition or concentration or mole fraction with their refractive index. The refractive index can be read using equipment called refractometer. Based on the result, it can be seen that as the concentration of methylcyclohexane decreased, the refractive index increased. This relationship was also shown diagrammatically by plotting the graph of refractive index at room temperature against the mole fraction of methylcyclohexane. The graph clearly showed that the refractive index decreased as increasing mole fraction of methylcyclohexane.

In short, the objectives of these experiments were achieved though there were errors occurred during readings of pressure drop in experiment of determining the column pressure drops were taken.

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RECOMMENDATIONS

Several recommendations to be suggested to improve the technique applied in these experiments other than obtaining accurate data and following the theory are by repeating some important procedures twice or three times. We may then calculate or obtain the average data. Besides, the errors should be avoided such as the overflowed of the sample from the column into the manometer arms which affected the values of pressure drops significantly and totally violates the theory. Therefore, the sample that entered the manometer arms should be taken out as manometer contains only one type of fluid that is continuously fill the arms to measure the pressure drops. One more important thing should be bare in mind was to wear gloves during the collection of the sample as the temperature of the boiling sample was relatively high. In spite of that, it is necessary to ensure that the distillation column can be operated properly with no leakage and functional valves so that there would be no errors during collecting the samples. Whereas for experiment of determining the mixture compositions, the refractometer needs to be handled carefully since it is very sensitive equipment. The sample should be gently inserted into the refractometer and the reading may be taken after stabilized.

REFERENCES

Geankopolis, Christie John. Transport Processes and Separation Process Principles, 4th ed.

New Jersey: Prentice Hall, 2003

Laboratory Manual

http://en.wikipedia.org

http://www.engineeringtoolbox.com

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APPENDICES

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