Sultan Qaboos University

College of Engineering

Petroleum and Chemical Engineering Department

Chemical Engineering Lab.II ( CHPE4712 ) - Section # ( 20 )

Exp. 4 (Batch Distillation)

By :

Malik Mohamed Khalifa Al Malki 89221 Ahmed Hamed Mohamed Al Qasmi 88805 Salim Sulaiman Ali Al Rubkhi 89704 Salah Rashid Said Al Jaafari 90637 To :

Dr. Mohammed Al abri

I

Abstract

The pressure drop of the batch distillation was calculated for different boil up

rates by using binary mixture of (50% vol.) methylcyclohexane and (50% vol.)

toluene. It was observed that increasing boil up rate will increase the pressure drop.

The refractometer was used to obtain the composition in the distillate and

bottom. Refractometer measure the angle of reflection based on the composition of

the methylcyclohexane and toluene.

The efficiency was calculated for four trails by using Fenske's method. The efficiency

was reached 65% as maximum and 43% as minimum.

II

Nomenclature

Symbols Description

푛 Number of theoretical stages

푋 The mole fraction of the methylcyclohexane

푋 The mole fraction of the Toluene

훼 The volatility of methylcyclohexane

훼 The volatility of toluene

훼 The average volatility of methylcyclohexane and toluene

퐸 The efficiency of the column

III

Table of Contents

Abstract ...................................................................................................................... I

Nomenclature ............................................................................................................ II

Table of Contents .................................................................................................... III

List of Figures ......................................................................................................... IV

List of Tables ............................................................................................................V

Equipments Set-up .................................................................................................... 2

Procedure .................................................................................................................. 4

Result and Discussion................................................................................................ 5

1. Experiment 1 ................................................................................. 5

1.1 Power and Boil-up Rate ....................................... 5

1.2 Column efficiency ................................................ 6

2. Experiment 2 ................................................................................. 7

Conclusion ................................................................................................................ 9

References ............................................................................................................... 10

Appendices.............................................................................................................. 11

IV

List of Figures

Figure 1: Experiment Parts and Set-up ....................................................................... 3 Figure 2: Control Panel and Refractometer ................................................................ 3 Figure 3: Pressure Drop and Boil-up Rate Relation .................................................... 5 Figure 4: Toluene Composition at Different Boil-up Rates ........................................ 6 Figure 5: Column Efficiency and Boil-up Rate .......................................................... 7 Figure 6: Methylcyclohexane Composition with Time at Different Reflux ................ 8 Figure 7: Macabe-Theile diagram .............................................................................. 8

V

List of Tables

Table 1: Pressure Drop and Boil-up Rate at Different Power Values .......................... 5 Table 2: Temperatures and Compositions at Various Boil-up Rates ........................... 6

1

Introduction

One of the major chemical processes used in industry is the separation of liquid mixtures into their original component using distillation. Mainly there are two types of distillation either continuous distillation or batch distillation. Batch distillation is usually more preferable in some condition for example when small amount of material are to be handled. In industry batch distillation is used for purifying products or recovering solvents or valuable reactants from waste stream.[1] Furthermore, there are environmental applications of distillation such as the separation of organic solvent/water mixtures and water removal for volume reduction prior to disposal of hazardous waste mixtures. [2]

In general, batch distillation has more flexibility than continuous distillation in terms of dealing with flow rate, temperature and pressure. In addition batch distillation often means simpler operation and lower capital cost than continuous distillation. The disadvantage of batch distillation is the high cost of energy, as it most often requires more energy than continuous distillation. In this experiment it is desired to:

Determine the pressure drop over the distillation column for various boil up

rates.

Determine the overall efficiency at varying boil up rates.

Determine the effect of varying the reflux ratio on top and bottom composition

with time.

Use the refractormeter to determine the mixture compositions.

These objectives will give a better understanding in knowing the affect of different parameters on the whole distillation process. Usually chemical engineers interfere some conditions while operating distillation processes that for sure they should know the affect of different parameters such as boil up rate, reflux ratio and efficiency of the column on the process itself. Knowing these parameters can be understood firstly in a small scale level such as lab instruments that we are dealing with in this experiment, so as to feel and understand the full process.

The distillation column consists of a certain number of trays these trays are usually called stages. The mixture vapor passes through the sieves across each tray. As the vapor passes up to the top of column it is cooled and condensed in the condenser. The condensed liquid either recycled back to the top of the column and called the reflux or removed out of the column and called as the distillate. The theoretical number of stages is calculated using Fenske equation:

2

푛 + 1 =[ ×( ) ]

( ) (1)

Where (푛) is the number of theoretical stage, (푥 ,푥 ) are the mole fractions of the more and least volatile component respectively and (훼 ) is the relative volatility which calculated by equation (2):

(훼 ) = √훼 . 훼 (2)

The efficiency of the column is calculated as:

퐸 =

× 100% (3)

Equipments Set-up

The batch distillation column unit which was used in this experiment was setup as shown in figure (1). This unit consists of a reboiler, condenser, column, manometer, receiver and valves. The reboiler contain heater to reboil-up liquid. Also, water was used in the condenser to condense the vapor. The column contains 8 sieve trays and it is the main part of this unit where the separation is occurs. Pressure drop is measured using the manometer. The product is collect in the receiver. The valves are used to control flow rates.

3

Figure 1: Experiment Parts and Set-up

Heater use electric power from control panel which is shown in figure (2). Refractometer device is used to measure the compositions of the top and bottom products. It is used to measure the reflection angle based on the mixture composition. The composition is calculated using correlation between angle and compositions. The Refractometer device is shown in figure (2).

Figure 2: Control Panel and Refractometer

4

Procedure

The experiment went throw many steps. The distillation column was stetted up as shown in figure (1). Before the experiment is started all the valves was closed. First, valve 10 on the reflux pipe is opened and the equipments are set up at total reflux. The power supply needed to boil up the liquid was started at fully situation. Then, the system waited for 20 minutes to be in equilibrium at all stages. When the vapor start rising up, the power was reduced to 0.5 KW in the same time the water supplier was opened at 3 L/min to cool down the mixture of methylcyclohexane and toluene by opening valve 5. A er that, the temperatures T1 and T8 were recorded by means of temperature sensors. In addition of that, two samples were taken, one from bottom stage (V2) and one from the dis llate stage (V3). The samples were used to determine the fractions of components using the refractometer.

In refractometer, small amount from both top and bottom was taken and spread over the glass piece in the device separately then the sample is covered by the covering part in the refractometer. Then, the sample was analyzed. The refractometer measure the reflaction angel passed on the mixture composition and the composition is calculated using next equation (4):

푇표푙푢푒푛푒푉표푙% = (퐶24 − 1584) 0.0797⁄ (4)

The pressure drop is calculated using the manometer by opening two valves (V6, V7). In addition, the boil up rate is calculated by operating V3 and the condensate mixture was collected in a measuring cylinder at that time the duration taken to fill the measuring cylinder to a certain level is measured. To be accurate and being sure that the flow from the V3 does not contain vapors, the level of the liquid in the feeding pipe was observed. Main while the degree of foaming was observed based on how violence it is in the trays. Then, the power was increased by 0.15 KW increment and the same steps was repeated for all trials.

In the second part of the experiment, the reflux ratio was changed by using reflux controller. The reflux ratio was set to return the condensate to the column for one seconds and one second to the product receiver. Then the same measurements were taken every 10 minutes for 4 trials. Finally, the reflux ratio was changed to 5:1 seconds and the same procedure was repeated.

5

Result and Discussion

1. Experiment 1

1.1 Power and Boil-up Rate

Table (1) shows the boil-up rate and pressure drop over the column at different power values. The boil-up rate is 0.655 L/hr at a power of 0.5 KW and the pressure drop was measured to be 58 mmH2O. For boil-up rates of 0.818 and 0.988 L/hr the measured values of pressure drop was 67 and 82 mmH2O respectively. As the power increase the boil-up rate will be higher and as a result the pressure drop will increase. Increasing the power input to the reboiler causes more formation of vapor with higher velocity. If the velocity increased, the pressure drop increases because it is proportional to the square of velocity. Figure (3) shows the effect of boil-up rate in pressure drop.

Table 1: Pressure Drop and Boil-up Rate at Different Power Values

Power, KW Boil-up Rate, L/hr Pressure Drop, mm H2O Top Bottom Overall 0.5 0.655 58

0.65 0.770 63 0.8 0.818 67

0.95 0.988 82

Figure 3: Pressure Drop and Boil-up Rate Relation

1.75

1.8

1.85

1.9

1.95

1 1.05 1.1 1.15 1.2 1.25Log

Pres

sure

Dro

p (m

m H

2O)

Log Boil up Rate(L/hr)

Pressure Drop Vs Boil-up Rate

6

0

10

20

30

40

50

60

0.6 0.7 0.8 0.9 1

Tolu

ene

Com

posi

tion

Boil-up Rate L/hr

"Top"

"Bottom"

1.2 Column efficiency

After running the experiment, temperatures and compositions were calculated at top and bottom. In this part of experiment, the batch distillation was running at a total reflux. The top and bottom compositions were analyzed using refractometer.

Since the boiling point of toluene is 110.6°C and that of methylcyclohexane is 100.9oC, methylcyclohexane is the more volatile component or in other word the light key. As a common sense, the composition of toluene at the bottom should be greater than that at top and that was noticed in the result obtained and shown in figure (4). The top and bottom temperatures were measured at different boil-up rates. Boil-up rates, compositions and temperatures are tabulated in table (2).

Table 2: Temperatures and Compositions at Various Boil-up Rates

Boil-up rate (L/hr)

T1 (oC)

T8 (oC)

Toluene top mole %

Toluene bottom mole %

0.655 102.9 104.4 29.31 56.16 0.771 102.6 104.3 25.11 50.24 0.818 102.9 104.6 22.53 49.03 0.988 103.1 105 19.21 48.22

Figure 4: Toluene Composition at Different Boil-up Rates

7

At a boil-up rate of 0.655 L/hr, the top and bottom compositions of methylcyclohexane are 0.7069 and 0.4384 respectively. Vapor-liquid equilibrium data was used to calculate the relative volatility at top and bottom and the average relative volatility was obtained. The efficiency of the column was determined using Fenske's method. The real number of stages is eight trays. The efficiency of the column is 57.3% when the boil-up rate was 0.818 L/hr. Increasing the boil-up rate cause an increase in the column efficiency as obtained from the experiment and the relation is shown in figure (5). Increasing the boil-up rate may cause more of light key component to evaporate and as a result more separation was achieved. There is a high increase at some points in the efficiency relation and that is happen due to the differences in composition determination. There is a large human error in reading from the refractometer. Also the refracted angle is hard to read because the sensitivity of the calibration screw and the difficulty in adjusting the graduation.

Figure 5: Column Efficiency and Boil-up Rate

2. Experiment 2

In this part, the reflux ratio was changed. The product composition was measured with time at different reflux and constant power and boil-up rate. 1:1 and 5:1 reflux ratios were used. 1:1 reflux ratio means that the liquid leaves the condenser is diverted one second to the column and one second to the receiver as a distillate. For each reflux ratio, the compositions of methylcyclohexane at top and bottom were measured with time. It can be seen from figure (6) that the separation is increasing with time at both reflux ratios.

40

45

50

55

60

65

10 11 12 13 14 15 16 17

Effic

ienc

y

Boil-up Rate (L/hr)

8

Figure 6: Methylcyclohexane Composition with Time at Different Reflux

Better separation can be achieved at higher reflux ratio. The reason can be explained from Macabe-Theile diagram that is shown in figure (7). If the reflux ratio increases, the slope of the operating line will increase and that cause fewer trays were required to achieve the same separation. Since the number of stages is fixed in this experiment, the separation will be better.

Figure 7: Macabe-Theile diagram

0.66

0.68

0.7

0.72

0.74

0.76

0.78

0.8

0.82

0.84

0 10 20 30 40 50

met

hylc

yclo

hexe

ne T

op co

mpo

sitio

n

Time (min)

١:١

٥:١

9

Conclusion

The first part of the experiment, the pressure drop was measured at different boil-up rates. It was noticed that the pressure drop increase as boil-up rate increase due to the increase of vapor velocity. The column efficiency was calculated at various boil-up rates. The maximum efficiency was found to be 64.5%.

In the second part, methylcyclohexane top and bottom mole fractions was measured for two reflux ratios 1:1 and 5:1. At higher reflux, better separation was obtained.

There are some results that seem not consistence due to errors. The larger errors are made by human mistakes. Human errors are found in measuring the boil-up rates and reading from the refractometer. Other errors are referred to devices such as the refractometer. There are some difficulties at adjusting the calibration screw and reading the refracted angle.

10

References [1] Study the Performance of Batch Distillation Using GPROM. roukela: National Institute of Technology. (2010-2011). [2] Richard D. Noble, P. A. (2004). Principles of Chemical Separations with Environmental Applications. Cambridge University Press.

11

Appendices

Efficiency was calculated using Fenske Equation:

avAB

bA

B

dB

A

xx

xx

n)log(

log

1

Where n = number of theoretical plates xA = mole fraction of more volatile component xB = mole fraction of least volatile component

av = average relative volatility Subscripts D, B indicate distillate and bottom respectively.

The molecular mass and density methylcyclohexane and toluene are in Table 6.

Component Molecular Mass (kg/kmol)

Density (kg/L)

Toulene 92.15 0.867 Methylcyclohexane 98.19 0.774