School of Chemical & Biomedical Engineering

Nanyang TechnologicalUniversity

AY2011/2012 SEMESTER 1CH3702

Experiment CE4

Venue: N1.2-B5-01

Distillation

Lab Group : GP 10

Date of Experiment : 22nd September 2011

Name : LOW LIANG YUAN U0921874C

TAN JUN HONG ADRIEL U0921800G

PHUA JIAN XUN U0921440B

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Table of Contents

1. INTRODUCTION.............................................................................................................................3

2. RESULTS AND CALCUATION..........................................................................................................3

2.1 Calculation of individual components’ mole fraction............................................................3

2.2 Temperature distribution along the column..........................................................................7

2.3 Calculation of the overall efficiency of the column................................................................7

2.3.1 Fenske equation.............................................................................................................7

2.3.1.1 Calculation of the vapour pressure using Antoine Equation.....................................8

2.3.1.2 Calculation of the average relative volatility.............................................................9

2.3.1.3 Calculation of the theoretical number of plates required in total reflux...................9

2.3.1.4 Calculation of the overall column efficiency.............................................................9

2.3.2 McCabe-Thiele Method...............................................................................................10

2.3.2.1 Calculation of the theoretical number of plates required in total reflux..............11

2.3.2.2 Calculation of the overall column efficiency........................................................11

3. DISCUSSION.................................................................................................................................12

3.1 General observations of the distillation column..................................................................12

3.2 Observation of the distillation column at different heat load (0.7 kW and 0.3 kW)............14

3.3 Evaluation of overall column efficiency calculated using different approach......................15

3.4 Other Source of error...........................................................................................................17

4. CONCLUSION...............................................................................................................................17

5. REFERENCE..................................................................................................................................17

APPENDIX A– H–xy Diagram of Ethanol- Water system.....................................................................18

APPENDIX B– Signed Logsheet............................................................................................................19

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1. INTRODUCTION

Distillation is an important industrial separation technology used to separate a liquid mixture into its different components. The basis of the separation lies in the differences in the components’ volatilities (how readily each component vapourises) in the boiling liquid mixture. From the distillation process, at least two output components of a volatile distillate component and a bottom component will be produced.

In this experiment, we will elucidate the effect of changing power input (to the reboiler) on the output components’ purity. This distillation process involves a binary mixture of ethanol and water. We aim to determine and investigate the following:

Column efficiency by applying Fenske Equation

Number of theoretical plates for a binary mixture by McCabe – Thiele’s method

The steady state distillation of a binary mixture under continuous operation

2. RESULTS AND CALCUATION

In the experimental run, the distillation column was allowed to reach steady state before the readings were taken. Stability of the system was observed when there was no appreciable fluctuation of the temperature readings. Once the system reached steady state, the overhead and bottom products were collected at 10 minute interval. In order to determine the mole fraction of the individual components for the overhead and bottom products, Gas Chromatography (GC) was employed. The experimental values for both overhead and bottom products are recorded in Tables 1 and 2 below.

2.1 Calculation of individual components’ mole fraction

Table 1 : Data of overhead product for total reflux with power of 0.7kW supplied to reboiler

Trial Number 1 2 3 4

Area under graph for ethanol (25 uV.s)2950.18

43072.146 2881.312 2866.215

Area under graph for water (25 uV.s)245.562

4234.1015 231.8322 230.7645

Amount of ethanol (mol ) 2.13E-06 2.22E-06 2.08E-06 2.07E-06Amount of water (mol ) 5.74E-07 5.52E-07 5.47E-07 5.45E-07Concentration of ethanol (mol/m3 ) 21.3078 22.1969 20.8057 20.6956Concentration of water (mol/m3 ) 5.7420 5.5188 5.4746 5.4538Mole fraction of ethanol 0.7877 0.8009 0.7917 0.7914

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Mole fraction of water 0.2123 0.1991 0.2083 0.2086

Table 2: Data of bottom product for total reflux with power of 0.7kW supplied to reboiler

Trial Number 1 2 3 4Area under graph for ethanol (25 uV.s) 2221.59 1968.836 1607.087 1400.381Area under graph for water (25 uV.s) 2432.052 2231.402 1896.97 1668.475Amount of ethanol (mol ) 1.60E-06 1.42E-06 1.15E-06 1.00E-06Amount of water (mol ) 4.83E-06 4.44E-06 3.79E-06 3.35E-06Concentration of ethanol (mol/m3 ) 15.9960 14.1532 11.5159 10.0089Concentration of water (mol/m3 ) 48.3293 44.4211 37.9072 33.4567Mole fraction of ethanol 0.2487 0.2416 0.2330 0.2303Mole fraction of water 0.7513 0.7584 0.7670 0.7697

Table 3: Data of total reflux with power of 0.7kW supplied to reboiler

Mole fraction of overhead product

Mole fraction of bottom product

Trial Number

Pressure Drop(cm H2O)

Ethanol Water Ethanol Water

1 7.9 0.7877 0.2123 0.2487 0.75132 7.9 0.8009 0.1991 0.2416 0.75843 7.9 0.7917 0.2083 0.2330 0.76704 7.9 0.7914 0.2086 0.2303 0.7697

Average 7.9 0.7929 0.2071 0.2384 0.7616

Sample Calculation

Taking values from trial 1 for the sample calculation.

The relationship between the area under the curve obtained from the Gas Chromatography and the amount of ethanol in the product streams is given by the following equation:

Area=1371648799× Amount+27.507349

Using the above equation, the amount of ethanol present in the overhead product and the bottom product was calculated. With the resultant values, the concentration of ethanol and water were then calculated in their respective streams.

Amount of ethanol in the overhead product = Area−27.507349

1371648799

= 2950.18359−27.507349

1371648799

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= 2.13×10−6 mol

Amount of ethanol in the bottom product = Area−27.507349

1371648799

= 2221.59009−27.507349

1371648799

= 1.60×10−6 mol

Concentration of ethanol in the overhead product = Amount of ethanolVolume of sample

= 2.13×10−6mol

0.1×10−6m3

= 21.3078 mol/m3

Concentration of ethanol in the bottom product = Amount of ethanolVolume of sample

= 1.60×10−6mol

0.1×10−6m3

= 15.9960 mol/m3

Whereas, the relationship between the area under the curve obtained from the Gas Chromatography and the amount of water in the product streams is given by the following equation:

Area=513413665× Amount−49.239188

Amount of water in the overhead product = Area+49.239188

513413665

= 2950.18359+49.239188

513413665

= 5.74×10−7mol

Amount of water in the bottom product = Area+49.239188

513413665

= 2432.05249+49.2391889

513413665

= 4.83×10−6 mol

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Concentration of water in the overhead product = Amount of ethanolVolume of sample

= 5.74×10−7mol

0.1×10−6m3

= 5.7420 mol/m3

Concentration of water in the bottom product = Amount of ethanolVolume of sample

= 4.83×10−6mol

0.1×10−6m3

= 48.3293 mol/m3

Using the concentration of the water and ethanol, the mole fraction of water and ethanol were then calculated in their respective overhead and bottom stream.

Mole fraction of ethanol in the overhead stream

= Concentrationof ethanol

Concentrationof ethanol+Concentrationof water

= 21.3078mol /m3

21.3078mol/m3+5.7420mol /m3

= 0.7877

Mole fraction of water in the overhead stream

= Concentrationof water

Concentrationof et hanol+Concentrationof water

= 5.7420mol /m3

21.3078mol/m3+5.7420mol /m3

= 0.2123

Mole fraction of ethanol in the bottom stream

= Concentrationof et hanol

Concentrationof et hanol+Concentrationof water

= 15.9960mol /m3

15.9960mol/m3+48.3293mol /m3

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= 0.2487

Mole fraction of water in the bottom stream

= Concentration of water

Concentrationof ethanol+Concentrationof water

= 48.3293mol/m3

15.9960mol/m3+48.3293mol /m3

= 0.7513

2.2 Temperature distribution along the column

The table 4 below records the average temperature along the column at steady state with operating conditions of total reflux and 0.7 kW of power supplied to the reboiler.

Table 4: Temperature distribution along the column

T1 (℃) T2(℃) T3(℃) T4(℃) T5(℃) T6(℃) T7(℃) T8(℃) T9(℃)75.0 75.7 76.2 77.3 77.5 77.8 78.4 79.1 83.4

2.3 Calculation of the overall efficiency of the column

The overall efficiency of the column was calculated by obtaining the number of theoretical plates required for a total reflux system. The 2 methods used for obtaining the number of theoretical plates were:

1. Fenske Equation

2. McCabe-Thiele Method

The following sections below will illustrate how the overall efficiency of the column using Fenske Equation and McCabe-Thiele Method was calculated.

2.3.1 Fenske equation

In 1932, Fenske developed an equation to calculate the minimum number of plates required for binary distillation at total reflux. This equation is as follow:

n+1=log [(

xA

xB

)d

(xB

x A

)b

]

log [(α AB)av ]

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Where n = number of theoretical plates XA = mole fraction of the most volatile component XB = mole fraction of the least volatile component

(α AB)av = average relative volatility

The average relative volatility was calculated using the following equation:

(α AB)av=√(α AB)d (α AB)b

Where (α AB)d = ratio of vapour pressure of more volatile component to the less volatile component for the overhead stream.

(α AB)b = ratio of vapour pressure of more volatile component to the less volatile component for the bottom stream.

The vapour pressure at a given temperature was calculated from Antoine equation which is shown below:

log10 PA¿=A− B

T +C

Where PA* = Vapour pressure of a substance A at a given temperature (mm Hg)

A ,B ,C = Constants that are unique for different substanceT = Temperature (℃)

2.3.1.1 Calculation of the vapour pressure using Antoine Equation

The following constants were used for ethanol and water in the Antoine Equation:

Table 5: Constants for ethanol and water in Antoine Equation

A B C

Ethanol 8.11220 1592.864 226.184

Water 7.96681 1668.210 228.000

Overhead stream temperature T1 = 75.0℃log10 Pethanol

¿=8.11220− 1592.86475.0+226.184

Pet hanol¿=¿ 666 mm Hg

log10 Pwater¿=7.96681− 1668.210

75.0+228.000

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Pwater¿=¿ 289 mm Hg

Reboiler temperature T9 = 83.4℃log10 Pethanol

¿=8.11220− 1592.86483.4+226.184

Pet hanol¿=¿ 927 mm Hg

log10 Pwater¿=7.96681− 1668.210

83.4+228.000

Pwater¿=¿ 407 mm Hg

2.3.1.2 Calculation of the average relative volatility

Using the values of vapour pressure calculated from Antoine Equation, (α AB)d and (α AB)b were then calculated. In this experiment, the more volatile component is ethanol. Therefore, A was taken to be ethanol component and B was taken to be the water component.

(α AB)d=Pet hanol

¿

Pwater¿ =

666mmHg289mmHg

=2.304

(α AB)b=Pethanol

¿

Pwater¿ =

927mmHg407mmHg

=2.278

(α AB)av=√(α AB)d (α AB)b=√2.304 ×2.278=2.291

2.3.1.3 Calculation of the theoretical number of plates required in total reflux

n+1=log [(

xA

xB

)d

(xB

x A

)b

]

log [(α AB)av ]

n+1=log [( 0.7929

0.2071)( 0.7616

0.2384)]

log [2.291]

n+1=3.02

n=2.02

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2.3.1.4 Calculation of the overall column efficiency

In this experiment, 8 plates plus 1 reboiler were used for the distillation process. Therefore, the actual number of plates used was 8.

Column efficiency= Number of theoretical PlatesNumber of actual Plates

×100 %

¿2.02

8×100 %

¿25.3 %

2.3.2 McCabe-Thiele Method

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12

3

4

R

Fig. 1: McCabe-Thiele’s diagram for Ethanol-Water system

2.3.2.1 Calculation of the theoretical number of plates required in total reflux

From Fig. 1, there are 4 plates +1 reboiler

Therefore, the number of theoretical plates = 4

2.3.2.2 Calculation of the overall column efficiency

In this experiment, 8 plates plus 1 reboiler were used for the distillation process. Therefore, the actual number of plates used was 8.

Column efficiency= Number of theoretical PlatesNumber of actual Plates

×100 %

¿48×100 %

¿50.0 %

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XD = 0.7929XB = 0.2384

3. DISCUSSION

3.1 General observations of the distillation column In this experiment, 8 litres of 30 mol% of ethanol/water mixture with a heat load of 0.7 kW provided to the reboiler undergoes distillation under a total reflux condition. Fig.2 below shows the temperature distribution along the distillation column (from T1 = temperature at top plate to T8 = temperature at bottom plate).

Fig. 2: Temperature distribution along the distillation column (from T 1 to T8) with reboiler heating load of 0.7kW under total reflux condition

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Temperature distribution along distillation column

Temperature at reboiler, T9

From Fig. 2, it was observed that the temperature in the reboiler, T9 increases steadily on the

temperature distribution profile and eventually stabilise at a temperature of 83.4℃. At this point, the liquid mixture reaches its boiling point and starts to form vapour. As the system was conducted under total reflux condition, all the vapour that rose through the column was eventually condensed back to the liquid mixture. It was then returned to top plate and flowed down the column. As seen in Fig.2, the system reaches an equilibrium condition when the temperature along the column (T1 to T8) reached to an average steady temperature. At this point, it can be assumed that the amount of moles of liquid vapourise is equal to the amount of moles of vapour that is condensed.

1 2 3 4 5 6 7 8 970

72

74

76

78

80

82

84

86

Temperature distribution along the column and reboiler

Plate number

Tem

pera

ture

()

℃

Fig 3: Temperature distribution along the column

It was also observed that the temperature in the plates decreases from the bottom plate to the top plate as seen in Fig. 3. This was probably due to the interaction between the hot vapour flowing up the column and the downward flowing cooler liquid that was condensed. As a result, heat was transferred from the hot vapour to the cooler liquid. This created a temperature gradient in the column which resembles a countercurrent cascade of flash chamber with intermediate heat exchangers. Therefore, the temperature at the top plate will be lower than the bottom plate due to the higher heat transfer at the top plate.

In the experiment, it was observed that the bubbling action at the bottom plate was more significantly visible than that at the top plate. This is because at the bottom plate, the vapour flowing through the sleeve on the plate experienced less resistance from the downward flowing liquid as compared to the top plate. Thus at the top plate, less bubbling action was seen.

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Reboiler

Top plate

Bottom plate

3.2 Observation of the distillation column at different heat load (0.7 kW and 0.3 kW)

In the comparison between the different heat load used (0.7 kW and 0.3 kW), the following observations were noted:

The bubbling action at the plates in the distillation column with heat load of 0.7 kW was observed to be greater than that of 0.3 kW. This was probably due to the higher vapour flow rate present in the column with the heat load of 0.7 kW as compared to 0.3 kW. When a higher power rating is supplied to the reboiler, more heat will be transferred to the liquid. This resulted in a greater amount of liquid being vapourise which then leads to a higher vapour flow rate in the column.

A greater amount of liquid was observed to be flowing down the column for the 0.7 kW heat load as compared to the 0.3 kW heat load. This higher flow rate of liquid can be further seen at the downcomer (in which at heat load of 0.7 kW), a higher liquid level was observed as more liquid was accumulated than at a heat load of 0.3 kW. This is because the amount of liquid being vapourises increases with increasing reboiler heat load. Hence under total reflux condition, a greater amount of vapour was condensed back into liquid for the 0.7 kW heat load as compared to the 0.3 kW. As a result, more liquid flowed down the column at higher heat load.

The pressure drop across the column at 0.7kW heat load was observed to be higher than that of 0.3kW. At heating load of 0.7kW, more liquid were vapourised than that at 0.3 kW heat load. Since the liquid flow rate also becomes greater at higher heating load, a greater resistance would be experience by the vapour flowing through the sieve as it comes into contract with the downward flowing liquid on the tray. As a result, the pressure drop across the entire column will be greater at higher heating load.

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3.3 Evaluation of overall column efficiency calculated using different approach

In this experiment, the actual number of plates used was 8 plates plus 1 reboiler. In the calculation of the overall efficiency, two methods, namely the McCabe-Thiele diagram and the Fenske Equation were used.

Table 6: Overall column efficiency for Fenske equation and McCabe-Thiele method

MethodActual number

of platesTheoretical number

of platesOverall Column Efficiency (%)

Fenske Equation 8 2.02 25.30McCabe-Thiele

Diagram8 4 50.00

From Table 6, it can be seen that both methods yield a low overall column efficiency of 25.30% for Fenske Equation and 50% for McCabe-Thiele method.

During the analysis of the mixture using gas chromatography, 3 peaks were observed. This implied that the mixture contains impurity. The impurity was identified as methanol. As methanol is more volatile than ethanol, under normal circumstances methanol will evaporate preferentially over ethanol. Thus in the mixture, the most volatile component is not ethanol. As a result, the mole fraction of ethanol in the distillate and condensate would be different from the actual scenario of only ethanol and water mixture. Since the calculation done using Fenske equation and McCabe-Thiele method in this experiment was based on the assumption that the system that we were working with was a binary system, present of methanol impurity would render inaccuracy in the calculation of the theoretical number of plates.

The reasons for the low overall column efficiency for while using the Fenske equation could be due to the following:

Firstly, calculation using Fenske equation is based on the relative volatility of the components. However, as the mixture contains methanol impurity, this would affect the accuracy of the relative volatility of the components. This in turn affects the accuracy of the theoretical number of plates calculated.

Secondly, the Fenske equation used is only valid for a system with constant relative volatility. However, an increased in pressure or temperature in the plates would have a significant effect on the relative volatility of the components in the liquid mixture (as seen in Fig.4 below). Although ideally, it was assumed that the pressure within the column is constant throughout the plates, in actual fact, the pressure does change along the column. Therefore, this would confer inaccuracy in the calculation of the theoretical number of plates.

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Fig.4: Diagram of the effects of temperature or pressure on the relative volatility (Beychok, 2008)

As for the McCabe-Thiele method, the reasons for the low overall column efficiency could be due to the following:

In the calculation, it was assumed that the vapour-liquid equilibrium was established at each plate, in which a saturated vapour and saturated liquid at equilibrium will leave the plate. However, in the actual operation, the vapour-liquid equilibrium may not be achieved as perfect mixing may not occur. Thus the number of plates calculated would defer from the actual operations.

McCabe- Thiele Method is dependent on the constant molal overflow (CMO) which requires the molar heat of vapourization between the components (ethanol and water) in the binary system to be as close as possible. However, in this experiment, the difference between the latent heat of vapourization of ethanol (838kJ/kg) and water (2258kJ/kg) differs significantly. Thus conferring to the inaccuracy in the calculation of the number of plates calculated.

In the McCabe-Thiele method, it was also assumed that the saturated liquid line and the saturated vapour lines on the enthalpy-composition, H-xy diagram are parallel. However, as seen in Appendix B, the saturated liquid line and saturated vapour line are not parallel. Therefore, the assumption that heat of vapourisation is linear dependent on the mass fraction is only an approximation. This would in turn result in inaccuracy in the number of plates calculated.

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3.4 Other Source of error

When the heater was providing a heat load of 0.7kW, excessive entrainment was observed due to the high vapour velocity. Entrainment refers to the liquid being carried by the high vapour flow rate up to the plates. This excessive entrainment will reduce the efficiency of the column as flooding occurs. Hence, both the values of overall column efficiency calculated using Fenske equation and McCabe-Thiele method will not accurately represent the system as both methods does not take into account the decrease in efficiency due to entrainment. Therefore, plates with more efficient downcomer design can be used to minimize the effect of entrainment.

4. CONCLUSION

This experiment involved the operation of a distillation column under different heating load of 0.7 kW and 0.3 kW. At 0.7 kW reboiler heating load, it was observed that there were greater bubbling action and higher liquid flow rate within the column as compare to that of 0.3 kW. This was due to the greater amount of liquid being vapourise at 0.7 kW heating load as compared to 0.3 kW.

For the calculation of the overall column efficiency, two methods were used, namely the Fenske equation and the McCabe-Thiele method. As seen in Table 6, both methods yielded a low efficiency of ~25.30% (Fenske equation) and ~50% (McCabe-Thiele method). The reason for this low efficiency was due to the presence of methanol impurity in the Ethanol-water mixture. Since both methods operate under the assumption of a binary system, introduction of methanol impurity would adversely affect the accuracy of the calculation for the theoretical number of plates.

5. REFERENCE

1. Milton Beychok (2008). “Diagram of the effect of temperature or pressure on relative

volatility”.

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APPENDIX A– H–xy Diagram of Ethanol- Water system

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APPENDIX B– Signed Logsheet

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