CAPE PENISULA UNIVERSITY OF TECHNOLOGYBELLVILLE CAMPUS

DEPARTMENT OF CHEMICAL ENGINEERINGND : CHEMICAL ENGINEERING

SUBJECT : ChemTech III B

LECTURER : G Hangone

STUDENT : Richardt Johan Loots

STUDENT NO. : 214196585

SectionPossible Mark

Student's Mark

Title Page 1

Table of Contents 3

List Symbols 2

Structure 2

Executive Summary 5

Introduction 5

Theory 15

Procedure 10

Results 25

Discussion 25

Conclusion 5

References 2

Total 100

I certify that this report is my own unaided work, except for the assistance received from the teaching staff. I undertake not to pass this report onto any other stu

Contents

I.List of Symbols........................................................................................................................................................................ ii

II.Executive Summary............................................................................................................................................................... iii

1.Introduction...........................................................................................................................................................................1

2.Theory................................................................................................................................................................................... 2

2.1 Basic distillation Principles.............................................................................................................................................2

2.1.1 Distillation theory...................................................................................................................................................2

2.1.2 Vapour Pressure.....................................................................................................................................................2

2.1.3 Boiling Point Diagrams...........................................................................................................................................2

2.1.4 Relative Volatility...................................................................................................................................................3

2.1.5 Efficiency................................................................................................................................................................3

2.2 Equilibrium Stage Concept............................................................................................................................................4

2.3 McGabe Thiele Theory...................................................................................................................................................5

2.4 Distillation Methods.......................................................................................................................................................6

2.4.1 Flash Distillation/Equilibrium.................................................................................................................................6

2.4.2 Batch Distillation...................................................................................................................................................6

2.4.3 Fractional Distillation..............................................................................................................................................6

2.4.4 Steam Distillation...................................................................................................................................................6

2.4.5 Vacuum Distillation...............................................................................................................................................6

2.4.5 Azeotropic Distillation............................................................................................................................................6

2.5 Distillation Column Types...................................................................................................................................................7

2.5.1 Batch Column..............................................................................................................................................................7

2.5.2 Continuous Column.....................................................................................................................................................7

2.6 Distillation Tray Types.........................................................................................................................................................7

2.6.1 Bubble Cap Trays.........................................................................................................................................................7

2.6.2 Valve Trays..................................................................................................................................................................7

2.6.3 Sieve Trays..................................................................................................................................................................7

3. Methodology........................................................................................................................................................................8

3.1 Apparatus.......................................................................................................................................................................8

3.2 Procedure......................................................................................................................................................................9

4. Results................................................................................................................................................................................10

5. Discussion...........................................................................................................................................................................12

6. Conclusion..........................................................................................................................................................................12

7. References..........................................................................................................................................................................13

Appendix...................................................................................................................................................................................a

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I.List of Symbols

II.Executive Summary

The main objective of the practical was to understand the principles of distillation, The calculation methods around a distillation unit using the McGabe-Thiele method to assist with theoretical plate calculation and determine the efficiency of the column. Comparing Theoretical values to the actual

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values and finally completing a energy balace around the condenser of the distillation unit to determine energy displacement.

The distillation column was run and steady state was achieved the reboiler valve was closed which practically rendering it a batch distillation.

One objective was to calculate the number of theoretical plated needed for this particular distillation and was calculated to 7 which is a relevant since the column has 8 actual plates and efficiency loss was accounted for thus the lower value for theoretical plates makes sense

The values for theoretical plates was obtained by using the McGabe-Thiele method by calculating the molar flowrate of the distillate and using this values to calculate the operating line. This line was then plotted and the steps were done.

Another objective was to do a energy Balance around the condenser, this task was carried out and it was calculated the a total of 481.3 J/s was lost to the surroundings and was determined that the reaction was strongly exothermic. The total amount of heat was gained by the water was 239.1 J/s and the total amout of heat lost by the distillate was 726.4 J/s

It was concluded the experiment was carried out successfully and all objectives was completed.

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1.Introduction

Distillation is one of the oldest methods to separate liquids based on thei volatilites and has been used for centuries. In the modern day industria it is mainly used for oil refining and the manufacture of alcoholic beverages.

Distillation is a method of separating the components of a solution which depends upon the distribution of the substances between a gas and a liquid phase, applied to cases where all components are present in both phases. Instead of introducing a new substance into the mixture in order to provide the second phase, as is done in gas absorption or desorption, the new phase is created from the original solution by vaporization or condensation.

In this experiment, the concern is over the separation of the components of a liquid solution of ethanol and water. By the application of heat, the solution of ethanol-water is partially vaporized and thereby creates a gas phase consisting of nothing but ethanol and water. Since the gas is richer in ethanol than the residual liquid, a certain amount of separation results. By repeated vaporizations and condensations, it is possible to make as complete a separation as desired, recovering both components of the mixture in as pure a state as needed.

In the following report, the determination of the number of trays of the distillation column is the main focus of the experiment. The number of trays will be determined theoretically. The use of flow rates and temperatures are needed to determine the theoretical number of trays which will be calculated

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2.Theory

2.1 Basic distillation Principles

2.1.1 Distillation theory

Distillation is the separation of components from a liquid mixture depending on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Distillation processes depends on the vapor pressure characteristics of liquid mixtures.

2.1.2 Vapour Pressure

The vapor pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface interface

2.1.3 Boiling Point Diagrams

The boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containing 2 components (A and B) - a binary mixture. This has the following boiling point diagram

The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at which the mole fraction of A is 0. In this example, A is the more volatile component and therefore has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble-point curve.

The region above the dew-point curve shows the equilibrium composition of the superheated vapor while the region below the bubble-point curve shows the equilibrium composition of the subcooled liquid.

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Figure 1 : Boiling Point Diagram

Dew-point: the temperature at which the saturated vapor starts to condense. Bubble-point: the temperature at which the liquid starts to boil.

2.1.4 Relative Volatility

A measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. Thus if the relative volatility between 2 components is very close to one, it is an indication that they have very similar vapor pressure characteristics. This means that they have very similar boiling points and therefore, it will be difficult to separate the two components via distillation.

2.1.5 Efficiency

In order to obtain the column efficiency for both the batch and continuous distillation systems

should be performed at total reflux. The overall column efficiency is defined as:

𝐸𝑜𝑣𝑒𝑟𝑎𝑙𝑙= NtheoreticalNactual

Where Nactual is the actual number of trays in the column and Ntheoretical is the number of theoretical stages, i.e. the number of trays that would be required to achieve the experimentally observed separation

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2.2 Equilibrium Stage Concept

The equilibrium stage concept is the most important part of distillation since this is where the most important assumptions are made. In the equilibrium stage the vapour-liquid is assumed in every stage, the liquid is transported to the stages at the bottom and the vapour to the top stages. The columns that have trays it is reasonable to describe the actual physics behind the columns, but it is not used in packed columns. It is said that the calculations from the equilibrium stage concept it works in all columns very well and even in packed columns.

Consider a vapour and a liquid that are in contact with each other as shown in Figure 2. Liquid molecules are continually vaporizing, while vapour molecules are continually condensing. If two chemical species are present, they will generally condense and vaporize at different rates. When not at equilibrium, the liquid and the vapour can be at different pressures and temperatures and be present in different mole fractions. At equilibrium the temperatures, pressures, and fractions of the two phases cease to change. Although molecules continue to evaporate and condense, the rate at which each species condenses is equal to the rate at which it evaporates. Although on a molecular scale nothing has stopped, on the macroscopic scale, where we usually observe processes, there are no further changes in temperature, pressure, or composition.

Dalton’s law of partial pressures : P=∑ P A

that is the total pressure is equal to the summation of the partial pressures. Since in an ideal gas or vapour the partial pressure is proportional to the mole fraction of the constituent, then PA= y A .P For an ideal mixture, the partial pressure is related to the concentration in the liquid phase by Raoult’s law which may be written as: PA=PA

0 . xA

where PA0 is the vapour pressure of pure A at the same temperature. This relation is usually found to

be true only for high values of x A, or correspondingly low values of xB, although mixtures of organic isomers and some hydrocarbons follow the law closely. For low values of x A, a linear relation betweenPA and x A again exists, although the proportionality factor is Henry’s constant H , and not

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Figure 2 : Equilibrium Liquid-Vapour interaction

the vapour pressure PA0 of the pure material. For a liquid solute A in a solvent liquid B,

Henry’s law takes the form :PA=H x A (Backhurst et al., 2002)

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2.3 McGabe Thiele Theory

For a binary system, the classic McCabe-Thiele graphical technique can be used to design the distillation column and to explore the effect of process variables. Although it is quite simple to use, this simplicity comes with a price. The technique uses a number of assumptions concerning the operation of the distillation column. It is important to analyze whether these assumptions are reasonable for the batch distillation unit being used. To use the McCabe-Thiele analysis, one needs the vapor-liquid equilibrium for the system plotted as a y versus x diagram. The second part is the operating line for the column which is a formulation of the mass balance from stage to stage; this requires the overall gas and liquid flows and the selected reflux ratio. When the distillation is continuous and feed enters the middle of the column, there are separate operating lines (mass balances) for the column above and below the point of feed introduction. For batch distillation, there is only one operating line since the molar flows of gas and liquid are constant throughout the column. The number of stages is determined by moving from equilibrium to operating curve such as occurs in the column. This generates a step structure which gives the number of stages. While the method is simple to do in practice, it takes some analysis to understand the concepts.

Upon inspection of Figure 2 Above the number of trays and the feed tray can easily be determined by using this method if all compositions are known around the distillation column. Once All the required values are known the stages can be determined

Values needed: xF, q, xD, xB, and RD (calculated with balances around column)

1. Plot the equilibrium curve.2. Calculate the slope q/(q-1) of the feed line.3. Plot the feed line using x=xF and the slope.4. Calculate the y-intercept of the rectifying line xD/(RD+1).5. Plot the rectifying line using (xd,xd) and the intercept.6. Draw the stripping line connecting the intersection of the rectifying and feed lines and the

point (xb,xb).

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Figure 3 : Typical McGabe Thiele Diagram

7. Correct for stage efficiency by drawing the "effective" equilibrium curve between the equilibrium curve and the operating lines.

8. "Step off" equilbrium stages. (McCabe & Smith, 1976)

2.4 Distillation Methods

2.4.1 Flash Distillation/Equilibrium

This is type of distillation that occurs in a single-stage in which a liquid mixture is partially vaporized. The vapour is allowed to come to equilibrium with the residual liquid and the resulting vapour and liquid phases are separated and removed from the apparatus.

2.4.2 Batch Distillation

A Feed is charged to a heated kettle .The liquid charge is boiled slowly and the vapour s are withdrawn as rapidly as possible to a condenser, where the condensed vapour (distillate) is collected . The first portion of vapour condensed is richest in the more volatile component . As vaporization proceeds, the vaporized product becomes leaner in the more volatile component

2.4.3 Fractional Distillation

Fractional distillation is essentially the same as simple batch distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with packing or on larger scale like the petroleum industry it will have trays and bubble caps. fractional distillation results in better separation between different compounds due to packing or "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compounds over and over. The more volatile liquids will tend to push towards the top of the fractionating column, while lower boiling liquids will stay towards the bottom, giving a better separation between the liquids.

2.4.4 Steam Distillation

When a mixture of two practically immiscible liquids is heated while being agitated to expose the surface of one liquid to the vapour phase, each constituent independently exerts its own vapour pressure as a function of temperature as if the other constituent were not present. Consequently, the vapour pressure of the whole system increases. Boiling begins when the sum of the partial pressures of the two immiscible liquids just exceeds the atmospheric pressure. In this way, many organic compounds insoluble in water can be purified at a temperature well below the point at which decomposition occurs

2.4.5 Vacuum Distillation

Vacuum distillation is distillation at a reduced pressure. Since the boiling point of a compound is lower at a lower external pressure, the compound will not have to be heated to as high a temperature in order for it to boil. Vacuum distillation is used to distill compounds that have a high boiling point or any compound which might undergo decomposition on heating at atmospheric pressure. The vacuum is provided either by a water aspirator or by a mechanical pump.

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2.4.5 Azeotropic DistillationA process by which a liquid mixture is separated into pure components with the help of an additional substance or solvent.

2.5 Distillation Column Types

2.5.1 Batch Column

In batch operation, the feed to the column is introduced in batches. That is, the column is charged with a 'batch' and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced with the remainder of feed removed from the column.

2.5.2 Continuous Column

Continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types.

2.6 Distillation Tray Types

2.6.1 Bubble Cap Trays

A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray

2.6.2 Valve Trays

In valve trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays

2.6.3 Sieve Trays

Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters

‘’Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications’’(McCabe & Smith, 1976)

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3. Methodology

3.1 Apparatus

Refractometer Stopwatch Measuring cylinder Continuous Distillation Unit ( Figure 4)

Figure 4 : Computer Controlled Continuous Distillation Unit

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3.2 Procedure

The mixture of 50/50% ethanol and water was already present in the reboiler at the bottom of the distillation column. The volume was checked and ensured to be enough

The tap of the cooling water was then opened.

The heating element was then switched on and it was set to the value higher than the boiling point temperature of the most volatile component and lower than the temperature of the least volatile component as the set point.

The system was left to reach steady state and produce a steady flow of distillate.

The temperatures of the cooling water in the inlet and outlet aswell as the flow rate and reflux timing was then recorded.

The valve that allows the flow of the distillate back to the column was then closed and the time it takes to fill 50ml of the tank was then recorded.

the distillate and bottoms was collected and measured with the refractometer to determine it’s composition.

The system was shut down after the necessary values and measurements were recorded.

The recorded values was processed and calculation was performed for the distillation column, from the results conclusions were drawn.

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4. Results

Table 1: Recorded Values

Variable Unit ValueCoolant in °C 12.6Coolant out °C 14.2Coolant flowrate l/min 2.2

Volume Distillate ml 50Distillate time s 412

Volumetric Fraction Bottoms % 47.2Volumetric Fraction Distillate % 89.2

Table 2 : Calculated Values

Calculated variables Value unitsComposition of ethanol in distillate (XD) 0.725 /

Composition of ethanol ii bottoms (Xw) 0.22 /

Moles of ethanol in distillate 1.52 mol

Mole s of ethanol in bottoms 0.81 mol

Composition of water in the distillate 0275 /

Composition of water in the bottom 0.78 /

Moles of water in distillate 0.577 mol

Moles of water in bottom 2.93 mol

Reflux ratio 1 Mol/mol

Heat gained by coolant 239.1 J/s

Heat lost by distillate 726.4 J/s

Heat lost from system 487.3 J/s

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Figure 5: Vapour-liquid equilibrium curve of ethanol and water

5. Discussion

The objective of the practical was to determine the amount of theoretical plates needed and complete a energy balance around the condenser.

Firstly looking at the number of theoretical plates that was determined shown in figure 5 that a value of 7 theoretical plates were calculated this value corresponds to the number of actual plates the column has, which is 8 plates.By standard more actual plates are always required than that of the theoretical plates. This is due to physical plates not being 100% efficient at equilibrium states and this should always be kept in mind when designing a distillation column or unit

Regarding the energy balance calculation it was calculated that the total energy lost was 487.3 J/s which makes sense since the condenser is not insulated as insulation would defeat the purpose of the condenser, which is to remove energy from the distillate to condense it.

It can also be seen that more heat was given off by the distillate, 726.4 J/s lost, than the amount of heat gained by the coolant 239.1 J/s . This indicates that heat was lost to the environment which is beneficial to the condenser as it helps with cooling, but not the column as this could contribute to a decrease in column efficiency and heat losses which works against the distillation process

When looking at column efficiency it was calculated the this distillation column had an efficiency of around 87.5% wich is considerably high and a overall good efficiency for a ethanol distillation

Furthermore The considering the mole fraction that were obtained it can clearly be seen that the fraction of the lighter key which is ethanol is dominant in the distillate which indicate that the distillation was in fact successful in separating the ethanol from the water.

6. Conclusion

The experiment was carried out successfully and the concept of distillation, theretical plates and the transfer of heat or the flow of energy was understood.

The number of theoretical plates determined and to have value of 7, compared to the amount of actual plates the value obtained makes sense taking into consideration the column efficiency

The energy balance was carried out and it was noticed that energy was in fact gained by the water almost double the amount was lost by the distillate which means that the process is exothermic and heat was given off by the distillate to the coolant and the surroundings.

Reccomendations: although time is of the essence in practicals it would be easier if the whole process ,from start to finish, was carried out by the students to eliminate misconceptions and the

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need for assumptions after the practical. Especially have the students prepare the ethanol water mixture instead having it in the column on arrival.

7. References

[1]Backhurst, J., Harker, J., Richardson, J. & Coulson, J. 2002. Coulson & Richardson's chemical engineering, J.M. Coulson and J.F. Richardson. Oxford: Butterworth-Heinemann.

[2]Cheremisinoff, N. 2000. Handbook of chemical processing equipment. Boston: Butterworth-Heinemann.

[3]McCabe, W. & Smith, J. 1976. Unit operations of chemical engineering. New York: McGraw-Hill.

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Appendix*Assuming 100 ml basis

Distillate mole fractions(D)

Ethanol89.6 % ethanol

89 .6ml 1m3

(100 )3ml=8.96×10−5m3

*density ethanol = 789kg/m^3

m=ρV ¿789×9.06×10−5¿0.07kg¿Mw

ethanol = 46.07 g/mol

n= mMw

¿ 0.0746.07¿1.52mol

Water

10. 4ml 1m3

(100 )3ml=1 .04×10−5m3

*density Water = 1000 kg/m^3

m=ρV ¿1000×1.04×10−5¿0.0104k g¿Mw Water 1 = 18.015g/mol

n= mMw

¿ 0.010418.015 ¿0. 577mol

xD=n

n+n2= 1. 521.52+0.577

=0.725

∴ xH2O=0.275

Bottoms mole fractions

Ethanol47.2 % ethanol

47.2ml 1m3

(100 )3ml=4.72×10−5m3

*density ethanol = 789kg/m^3

m=ρV ¿789×4.72×10−5¿0.0372kg¿Mw

ethanol = 46.07 g/mol

n= mMw

¿ 0.037246.07 ¿0.81mol

Water

52.8ml 1m3

(100 )3ml=5.28×10−5m3

*density Water = 1000 kg/m^3

m=ρV ¿1000×5.28×10−5¿0.0528 kg¿Mw

Water 1 = 18.015g/mol

n= mMw

¿ 0.052818.015¿2.93mol

xw=n

n+n2= 0.810.81+2.93

=0.22

∴ xH2O=0.78

Distillate molar flow rate

*50 ml in 412s = 1.214×10−7m3

s

Density of distillate

ρmix=∑ m

❑

∑V¿ 0.07+0.0104

0.0001¿804 kg

m3

Mass flow rate of distillate

m=ρmix×V¿ 804kgm3 × 1.214×10

−7m3

s¿kg /s

a

Molelcular mass of distillate

Mw ,mix=¿¿ xD×Mw ,eth+xH 2O×Mw ,H2O

Mw ,mix=¿0.725× 46.07+0.275×18.015¿

Mw ,mix=¿38 .36 kg / kmol ¿

Molar flowrate of distillate

n= mMw ,mix

¿ 9.76×10−5

38.36

¿2. 54×10−6 kmol/s

∴D=2.5 4×10−6 kmol/s

∴Ln=D

Rectifier Balance

¿Reflux ratio=Ln

D=44=1

V n=Ln+D

V n=2(2.5 4×10−6)

V n=5.08×10−6kmol/s

Component Balance (TOL)ynV n=lnx+1+xdD

yn=lnVn

xn+1+DVn

xd

yn=2.54×10−6

5.08×10−6 xn+1+2.5 4×10−6

5.08×10−6×0.725

yn=0.5xn+1+0.375

∴ y−intercept=0.36 for TOL

Energy Balance around Condenser

m=0.0366 kg/ s

C p ,water@12.6℃=4.085 kJkg . K

C p , ETH@81.1℃=3.065 kJkg .K

T water∈¿=12.6℃ ¿

T waterout=14.2℃

∆H vapwater=2260kJkg

∆ H vapETH=838kJkg

Qwater=mC p , water∆T

Q=0.0366×4.085×1.6

Q=239.1 Js

*assuming the ethanol was cooled to just below its boiling point of 78.37℃ and entered at 81.1℃

QETH=mCp , ETH∆T

Q=3.065×0.1×(−2.37)

Q=−726.4 Js

Qlost (Q¿¿ETH )+(Qwater)¿

Qlost=−726.4+239.1

Qlost=−487.3 Js

b