Slide Kuliah PAP (Destilasi)

8/21/2019 Slide Kuliah PAP (Destilasi)

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PENDAHULUANDistillation didefinisikan sebagai :

suatu proses dimana campuran cairan atau uapdari dua zat atau lebih dipisahkan menjadi fraksikomponen-komponennya sesuai dengankemurnian yang diinginkan, denganmenggunakan atau menghilangkan panas.

Distilasi berdasarkan fakta bahwa uap yang

dihasilkan dari campuran yang mendidih akanlebih kaya komponen yang titik didihnya lebihrendah dibanding dengan komponen yangmempunyai titik didih yang lebih tinggi.

Oleh karena itu, ketika uap tersebut didinginkan

dan dikondensasikan, hasil kondensatnya akanmengandung lebih banyak komponen voletile.Pada keadaan yang sama, komponen yang kurangvoletile jumlahnya lebih sedikit.

Kolom Distilasi didisain untuk mendapatkan

pemisahan ini secara efisien.


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Introduction

Although many people have a fair idea whatdistillation means, the important aspects thatseem to be missed from the manufacturing pointof view are that:

distillation is the most common separation technique

it consumes enormous amounts of energy, both interms of cooling and heating requirements

it can contribute to more than 50% of plant operatingcosts

The best way to reduce operating costs of existingunits, is to improve their efficiency and operationvia process optimisation and control. To achievethis improvement, a thorough understanding ofdistillation principles and how distillation systemsare designed is essential.


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Introduction

The purpose of this set of notes is to expose youto the terminology used in distillation practice andto give a very basic introduction to:

types of columns

basic distillation equipment and operation column internals

reboilers

distillation principles

vapour liquid equilibria

distillation column design and the factors that affect distillation column operation


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TYPES OF DISTILLATION

COLUMNS

There are many types of

distillation columns, eachdesigned to perform specifictypes of separations, and each

design differs in terms ofcomplexity.


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COLUMNSBatch and Continuous Columns

One way of classifying distillation column type is to look at howthey are operated. Thus we have: batch and continuouscolumns.

Batch Columns

In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a 'batch' and then thedistillation process is carried out. When the desired task isachieved, a next batch of feed is introduced.

Continuous Columns

In contrast, continuous columns process a continuous feedstream. No interruptions occur unless there is a problem withthe column or surrounding process units. They are capable ofhandling high throughputs and are the most common of the twotypes. We shall concentrate only on this class of columns.


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COLUMNSTypes of Continuous ColumnsContinuous columns can be further classified according to:

the nature of the feed that they are processing,

binarycolumn - feed contains only two components

multi-componentcolumn - feed contains more thantwo components

the number of product streams they have

multi-productcolumn - column has more than twoproduct streams

where the extra feed exits when it is used to helpwith the separation,

extractivedistillation - where the extra feedappears in the bottom product stream

azeotropicdistillation - where the extra feed

appears at the top product stream


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COLUMNSTypes of Continuous Columns

the type of column internals

traycolumn - where trays of various designs

are used to hold up the liquid to provide bettercontact between vapour and liquid, hence betterseparation packed column - where instead of trays,'packings' are used to enhance contact betweenvapour and liquid


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BASIC DISTILLATION

EQUIPMENT AND OPERATIONMain Components of Distillation Columns

Distillation columns are made up of several components,each of which is used either to tranfer heat energy orenhance materail transfer. A typical distillation containsseveral major components:

a vertical shellwhere the separation of liquid componentsis carried out

column internals such as trays/platesand/or packingswhich are used to enhance component separations

a reboilerto provide the necessary vaporisation for the

distillation process a condenserto cool and condense the vapour leaving the

top of the column

a reflux drumto hold the condensed vapour from the topof the column so that liquid (reflux) can be recycled back

to the column


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BASIC DISTILLATION

EQUIPMENT AND OPERATION

The vertical shellhouses the columninternals and

together with thecondenser andreboiler, constitute adistillation column. Aschematic of a typical

distillation unit witha single feed and twoproduct streams isshown below:


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BASIC DISTILLATION


Basic Operation and Terminology

The liquid mixture that is to be processed isknown as the feedand this is introduced

usually somewhere near the middle of thecolumn to a trayknown as the feed tray. Thefeed tray divides the column into a top(enrichingor rectification) section and abottom (stripping) section. The feed flows

down the column where it is collected at thebottom in the reboiler.


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BASIC DISTILLATION

EQUIPMENT AND OPERATIONBasic Operation and Terminology

Heat is supplied to the reboiler to

generate vapour. The source of

heat input can be any suitablefluid, although in most chemicalplants this is normally steam. Inrefineries, the heating source maybe the output streams of other

columns. The vapour raised in thereboiler is re-introduced into theunit at the bottom of the column.The liquid removed from thereboiler is known as the bottomsproductor simply, bottoms.


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BASIC DISTILLATION


Basic Operation and Terminology

The vapour 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 aholding vessel known as the refluxdrum. Some of this liquid isrecycled back to the top of thecolumn and this is called the

reflux. The condensed liquid thatis removed from the system isknown as the distillateor topproduct.

Thus, there are internal flowsofvapour and liquid within the columnas well as external flows of feedsand product streams, into and out

of the column.


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COLUMN INTERNALS

Trays and Plates

The terms "trays" and "plates" are used interchangeably.There are many types of tray designs, but the mostcommon ones are :

Bubble cap trays

A bubble cap tray has riser orchimney fitted over each hole, anda cap that covers the riser. Thecap is mounted so that there is aspace between riser and cap toallow the passage of vapour.Vapour rises through the chimneyand is directed downward by thecap, finally discharging throughslots in the cap, and finallybubbling through the liquid on the

tray.


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COLUMN INTERNALS

Valve trays

In valve trays, perforations are covered by liftablecaps. Vapour flows lifts the caps, thus self creating a

flow area for the passage of vapour. The lifting capdirects the vapour to flow horizontally into the liquid,thus providing better mixing than is possible in sievetrays.


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COLUMN INTERNALS

Sieve traysSieve trays are simply metalplates with holes in them.Vapour passes straightupward through the liquid on

the plate. The arrangement,number and size of the holesare design parameters.

Because of their efficiency, wideoperating range, ease of

maintenance and cost factors,sieve and valve trays havereplaced the once highly thoughtof bubble cap trays in manyapplications.


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COLUMN INTERNALS

Liquid and Vapour Flows in a Tray ColumnThe next few figures show the direction of vapour and

liquid flow across a tray, and across a column.


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COLUMN INTERNALS

Each tray has 2 conduits, one on each side, called

downcomers. Liquid falls through the downcomers bygravity from one tray to the one below it. The flow acrosseach plate is shown in the above diagram on the right.

A weiron the tray ensures thatthere is always some liquid

(holdup) on the tray and isdesigned such that the the holdupis at a suitable height, e.g. suchthat the bubble caps are coveredby liquid.

Being lighter, vapour flows up thecolumn and is forced to passthrough the liquid, via theopenings on each tray. The areaallowed for the passage of vapour

on each tray is called the activetray area.


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COLUMN INTERNALS

The picture on the left is aphotograph of a section of apilot scale column equipedwith bubble capped trays.

The tops of the 4 bubblecaps on the tray can just beseen. The downcomer inthis case is a pipe, and isshown on the right. Thefrothing of the liquid on the

active tray area is due toboth passage of vapourfrom the tray below as wellas boiling.


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COLUMN INTERNALS

As the hotter vapour passes through the liquid on thetray above, it transfers heat to the liquid. In doing so,some of the vapour condenses adding to the liquid on

the tray. The condensate, however, is richer in the lessvolatile components than is in the vapour. Additionally,because of the heat input from the vapour, the liquid onthe tray boils, generating more vapour. This vapour,which moves up to the next tray in the column, is richerin the more volatile components. This continuous

contacting between vapour and liquid occurs on eachtray in the column and brings about the separationbetween low boiling point components and those withhigher boiling


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COLUMN INTERNALSTray Designs

A tray essentially acts as a mini-column, each accomplishinga fraction of the separation task. From this we can deducethat the more trays there are, the better the degree ofseparation and that overall separation efficiency will dependsignificantly on the design of the tray. Trays are designed tomaximise vapour-liquid contact by considering

the liquid distribution and

vapour distribution

on the tray. This is because better vapour-liquid contactmeans better separation at each tray, translating to bettercolumn performance. Less trays will be required to achievethe same degree of separation. Attendant benefits includeless energy usage and lower construction costs.

There is a clear trend to improve separations bysupplementing the use of trays by additions of packings.


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COLUMN INTERNALS

Packings

Packings are passive devices that are designed toincrease the interfacial area for vapour-liquid contact.The following pictures show 3 different types ofpackings.

These strangely shaped pieces are supposed to impart goodvapour-liquid contact when a particular type is placed togetherin numbers, without causing excessive pressure-drop across apacked section. This is important because a high pressure dropwould mean that more energy is required to drive the vapour up

the distillation column.


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COLUMN INTERNALS

Packings versus Trays

A tray column that is facing throughput problems maybe de-bottlenecked by replacing a section of trays withpackings. This is because:

packings provide extra inter-facial area for liquid-vapourcontact

efficiency of separation is increased for the samecolumn height

packed columns are shorter than trayed columns

Packed columns are called continuous-contact columnswhile trayed columns are called staged-contact columnsbecause of the manner in which vapour and liquid arecontacted.


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COLUMN REBOILERS

There are a number of designs of reboilers. It is beyond the scope

of this set of introductory notes to delve into their design principles.However, they can be regarded as heat-exchangers that arerequired to transfer enough energy to bring the liquid at thebottom of the column to boiling boint. The following are examplesof typical reboiler types.


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COLUMN REBOILERS


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COLUMN REBOILERS
http://www.rccostello.com/Klarex.html


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DISTILLATION PRINCIPLES

Separation of components from a liquid mixture viadistillation depends on the differences in boiling pointsof the individual components. Also, depending on theconcentrations of the components present, the liquidmixture will have different boiling point characteristics.Therefore, distillation processes depends on the vapourpressurecharacteristics of liquid mixtures.


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Vapour Pressure and Boiling

The vapour pressure of a liquid at a particulartemperature is the equilibrium pressureexerted bymolecules leaving and entering the liquid surface. Hereare some important points regarding vapour pressure:

energy input raises vapour pressure

vapour pressure is related to boiling a liquid is said to boil when its vapour pressure equalsthe surrounding pressure

the ease with which a liquid boils depends on itsvolatility

liquids with high vapour pressures (volatile liquids) willboil at lower temperatures

the vapour pressure and hence the boiling point of aliquid mixture depends on the relative amounts of thecomponents in the mixture

distillation occurs because of the differences in the

volatility of the components in the liquid mixture


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The Boiling Point Diagram

The boiling point diagramshows how the equilibriumcompositions of the components in a liquid mixture vary withtemperature at a fixed pressure. Consider an example of a liquidmixture containing 2 components (A and B) - a binarymixture.This has the following boiling point diagram.

The boiling point of A is that atwhich the mole fraction of A is 1.The boiling point of B is that atwhich the mole fraction of A is 0.In this example, A is the morevolatile component andtherefore has a lower boilingpoint than B. The upper curve inthe diagram is called the dew-point curvewhile the lower one

is called the bubble-point curve.


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The dew-point is the temperature at whichthe saturated vapour starts to condense.

The bubble-point is the temperature atwhich the liquid starts to boil.

The region above the dew-point curve shows theequilibrium composition of the superheatedvapour while the region below the bubble-

point curve shows the equilibrium compositionof the subcooledliquid.


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For example, when a subcooled liquid with molefraction of A=0.4 (point A) is heated, itsconcentration remains constant until it reaches

the bubble-point (point B), when it starts toboil. The vapours evolved during the boilinghas the equilibrium composition given by pointC, approximately 0.8 mole fraction A. This isapproximately 50% richer in A than the

original liquid.This difference between liquid and vapour

compositions is the basis for distillationoperations.


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DISTILLATION PRINCIPLESRelative Volatility

Relative volatilityis a measure of the differences in volatilitybetween 2 components, and hence their boiling points. Itindicates how easy or difficult a particular separation will be.The relative volatility of component i with respect tocomponent j is defined as

yi= mole fraction of component i in thevapour

xi= mole fraction of component i in theliquid

Thus if the relative volatility between 2components is very close to one, it is anindication that they have very similar vapourpressure characteristics. This means thatthey have very similar boiling points andtherefore, it will be difficult to separate the

two components via distillation.


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VAPOUR LIQUID EQUILIBRIA

Distillation columns are designed based on theboiling point properties of the components inthe mixtures being separated. Thus the sizes,

particularly the height, of distillation columnsare determined by the vapour liquidequilibrium (VLE) data for the mixtures.


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Vapour-Liquid-Equilibrium (VLE)Curves

Constant pressure VLE data isobtained from boiling pointdiagrams. VLE data of binarymixtures is often presented asa plot, as shown in the figureon the right. The VLE plotexpresses the bubble-pointand the dew-point of a binarymixture at constant pressure.The curved line is called theequilibrium lineand describesthe compositions of the liquidand vapour in equilibrium atsome fixed pressure.
http://lorien.ncl.ac.uk/ming/distil/distilpri.htmhttp://lorien.ncl.ac.uk/ming/distil/distilpri.htmhttp://lorien.ncl.ac.uk/ming/distil/distilpri.htmhttp://lorien.ncl.ac.uk/ming/distil/distilpri.htm


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This particular VLE plot shows a binary mixture that has a uniform

vapour-liquid equilibrium that is relatively easy to separate. The nexttwo VLE plots below on the other hand, shows non-ideal systems

which will present more difficult separations. We can tell from the

shapes of the curves and this will be explained further later on.


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The most intriguing VLE curves are generated byazeotropic systems. An azeotrope is a liquid mixturewhich when vaporised, produces the same compositionas the liquid. The two VLE plots below, show two

different azeotropic systems, one with a minimumboiling point and one with a maximum boiling point. Inboth plots, the equilibrium curves cross the diagonallines, and this are azeotropic pointswhere theazeotropes occur. In other words azeotropic systemsgive rise to VLE plots where the equilibrium curvescrosses the diagonals.


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Note the shapes of the respective equilibrium lines in relation tothe diagonal lines that bisect the VLE plots.


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Both plots are however, obtained from homogenousazeotropic systems. An azeotrope that contains oneliquid phase in contact with vapour is called ahomogenous azeotrope. A homogenous azeotrope

cannot be separated by conventional distillation.However, vacumn distillationmay be used as the lowerpressures can shift the azeotropic point.Alternatively, anadditional substance may added to shift the azeotropicpoint to a more favourable position.

When this additional component appears in appreciableamounts at the top of the column, the operation iscalled azeotropic distillation.

When the additional component appears mostly at thebottom of the column, the operation is called extractivedistillation


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The most intriguing VLE curves are generated byazeotropic systems. An azeotrope is a liquid mixturewhich when vaporised, produces the same compositionas the liquid. The two VLE plots below, show two

different azeotropic systems, one with a minimumboiling point and one with a maximum boiling point. Inboth plots, the equilibrium curves cross the diagonallines, and this are azeotropic pointswhere theazeotropes occur. In other words azeotropic systemsgive rise to VLE plots where the equilibrium curvescrosses the diagonals.


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Note the shapes of the respective equilibrium lines in relation tothe diagonal lines that bisect the VLE plots.


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The VLE curve on the left isalso generated by anazeotropic system, in this casea heterogenous azeotrope.

Heterogenous azeotropes canbe identified by the flatportion on the equilibriumdiagram. They may beseparated in 2 distillationcolumns since these

substances usually form twoliquid phases with widelydiffering compositions. Thephases may be separatedusing settling tanks underappropriate conditions.


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DISTILLATION COLUMN

DESIGNAs mentioned, distillation columns are designed

using VLE datafor the mixtures to beseparated. The vapour-liquid equilibrium

characteristics (indicated by the shape of theequilibrium curve) of the mixture willdetermine the number of stages, and hencethe number of trays, required for the

separation. This is illustrated clearly byapplying the McCabe-Thielemethod to designa binary column.
http://lorien.ncl.ac.uk/ming/distil/distilvle.htmhttp://lorien.ncl.ac.uk/ming/distil/distilvle.htm


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DISTILLATION COLUMN

DESIGNMcCABE-THIELE DESIGN METHOD

The McCabe-Thiele approach is a graphical one, and usesthe VLE plot to determine the theoretical number of

stages required to effect the separation of a binarymixture. It assumes constant molar overflowand thisimplies that:

molal heats of vaporization of the components areroughly the same

heat effects (heats of solution, heat losses to and fromcolumn, etc.) are negligible

for every mole of vapor condensed, 1 mole of liquid isvaporized


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DISTILLATION COLUMN

DESIGNThe design procedure is simple. Given the VLE diagramof the binary mixture, operating lines are drawn first.

Operating lines define the mass balance relationships

between the liquid and vapour phases in the column. There is one operating line for the bottom (stripping)

section of the column, and on for the top (rectificationor enriching) section of the column.

Use of the constant molar overflow assumption also

ensures the the operating lines are straight lines.


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DISTILLATION COLUMN

DESIGN

Operating Line for theRectification Section

The operating line forthe rectificationsection is constructed

as follows. First thedesired top productcomposition is locatedon the VLE diagram,and a vertical lineproduced until itintersects the diagonal

line that splits the VLEplot in half. A line withslope R/(R+1)is thendrawn from thisinstersection point asshown in the diagram

below.

Ris the ratio of reflux flow (L) to distillateflow (D) and is called the reflux ratioand is ameasure of how much of the material going

up the top of the column is returned back tothe column as reflux.


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DISTILLATION COLUMN

DESIGNOperating Line for the Stripping Section

The operating line for the stripping section is constructed in a similarmanner. However, the starting point is the desired bottom productcomposition. A vertical line is drawn from this point to the diagonal line,and a line of slope Ls/Vsis drawn as illustrated in the diagram below.

Lsis the liquid rate down the stripping section of the column, while Vsisthe vapour rate up the stripping section of the column. Thus the slopeof the operating line for the stripping section is a ratio between theliquid and vapour flows in that part of the column.


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DISTILLATION COLUMN

DESIGN


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DISTILLATION COLUMN

DESIGNEquilibrium and Operating Lines

The McCabe-Thiele method assumes that theliquid on a tray and the vapour above it are in

equilibrium. How this is related to the VLE plotand the operating lines is depicted graphicallyin the diagram on the right.

S O CO


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DISTILLATION COLUMN

DESIGN


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DISTILLATION COLUMN

DESIGNNumber of Stages and Trays

Doing the graphical construction

repeatedly will give rise to a number of'corner' sections, and each section willbe equivalent to a stage of thedistillation. This is the basis of sizing

distillation columns using the McCabe-Thiele graphical design methodology asshown in the following example.


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DISTILLATION COLUMN

DESIGNGiven the operating lines forboth stripping andrectification sections, thegraphical constructiondescribed above was applied.This particular exampleshows that 7 theoreticalstages are required toachieve the desiredseparation. The requirednumber of trays (as opposed

to stages) is one less thanthe number of stages sincethe graphical constructionincludes the contribution ofthe reboiler in carrying outthe separation.


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DISTILLATION COLUMN

DESIGNThe actual number of trays required is given by

the formula:

(number of theoretical trays)/(tray

efficiency)Typical values for tray efficiency ranges from 0.5

to 0.7 and depends on a number of factors,such as the type of traysbeing used, andinternal liquid and vapour flow conditions.

Sometimes, additional trays are added (up to10%) to accomodate the possibility that thecolumn may be under-designed.


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DISTILLATION COLUMN

DESIGNAs mentioned, distillation columns are designed

using VLE datafor the mixtures to beseparated. The vapour-liquid equilibrium

characteristics (indicated by the shape of theequilibrium curve) of the mixture willdetermine the number of stages, and hencethe number of trays, required for the

separation. This is illustrated clearly byapplying the McCabe-Thielemethod to designa binary column.
http://lorien.ncl.ac.uk/ming/distil/distilvle.htmhttp://lorien.ncl.ac.uk/ming/distil/distilvle.htm


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DISTILLATION COLUMN

DESIGNMcCABE-THIELE DESIGN METHOD

The McCabe-Thiele approach is a graphical one, and usesthe VLE plot to determine the theoretical number ofstages required to effect the separation of a binarymixture. It assumes constant molar overflowand thisimplies that:

molal heats of vaporisation of the components areroughly the same

heat effects (heats of solution, heat losses to and fromcolumn, etc.) are negligible

for every mole of vapour condensed, 1 mole of liquid isvaporised


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DISTILLATION COLUMN

DESIGNThe design procedure is simple. Given the VLE diagramof the binary mixture, operating lines are drawn first.

Operating lines define the mass balance relationshipsbetween the liquid and vapour phases in the column.

There is one operating line for the bottom (stripping)section of the column, and on for the top (rectificationor enriching) section of the column.

Use of the constant molar overflow assumption also

ensures the the operating lines are straight lines.

DISTILLATION COLUMN


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DISTILLATION COLUMN

DESIGN

Operating Line for theRectification Section

The operating line forthe rectificationsection is constructed

as follows. First thedesired top productcomposition is locatedon the VLE diagram,and a vertical lineproduced until itintersects the diagonal

line that splits the VLEplot in half. A line withslope R/(R+1)is thendrawn from thisinstersection point asshown in the diagram

below.

Ris the ratio of reflux flow (L) to distillateflow (D) and is called the reflux ratioand is ameasure of how much of the material going

up the top of the column is returned back tothe column as reflux.

DISTILLATION COLUMN


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DISTILLATION COLUMN

DESIGNOperating Line for the Stripping Section

The operating line for the stripping section is constructed in a similarmanner. However, the starting point is the desired bottom productcomposition. A vertical line is drawn from this point to the diagonal line,and a line of slope Ls/Vsis drawn as illustrated in the diagram below.

Lsis the liquid rate down the stripping section of the column, while Vsisthe vapour rate up the stripping section of the column. Thus the slopeof the operating line for the stripping section is a ratio between theliquid and vapour flows in that part of the column.


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DISTILLATION COLUMN

DESIGN


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DISTILLATION COLUMN

DESIGNEquilibrium and Operating Lines

The McCabe-Thiele method assumes that theliquid on a tray and the vapour above it are in

equilibrium. How this is related to the VLE plotand the operating lines is depicted graphicallyin the diagram on the right.

DISTILLATION COLUMN


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DISTILLATION COLUMN

DESIGN


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DISTILLATION COLUMN

DESIGNA magnified section of the operating line for the strippingsection is shown in relation to the corresponding n'th stage inthe column. L's are the liquid flows while V's are the vapourflows. x and y denote liquid and vapour compositions and thesubscripts denote the originof the flows or compositions. Thatis 'n-1' will mean from the stage below stage 'n' while 'n+1'will mean from the stage above stage 'n'. The liquid in stage'n' and the vapour above it are in equilibrium, therefore, xnandynlie on the equilibrium line. Since the vapour is carried to thetray above without changing composition, this is depicted as a

horizontal line on the VLE plot. Its intersection withthe operating line will give the composition of the liquid on tray'n+1' as the operating line defines the material balance on thetrays. The composition of the vapour above the 'n+1' tray isobtained from the intersection of the vertical line from this pointto the equilibrium line.


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DISTILLATION COLUMN

DESIGNNumber of Stages and Trays

Doing the graphical construction

repeatedly will give rise to a number of'corner' sections, and each section willbe equivalent to a stage of thedistillation. This is the basis of sizing

distillation columns using the McCabe-Thiele graphical design methodology asshown in the following example.


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DISTILLATION COLUMN

DESIGNGiven the operating lines forboth stripping andrectification sections, thegraphical constructiondescribed above was applied.

This particular exampleshows that 7 theoreticalstages are required toachieve the desiredseparation. The requirednumber of trays (as opposed

to stages) is one less thanthe number of stages sincethe graphical constructionincludes the contribution ofthe reboiler in carrying outthe separation.


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DISTILLATION COLUMN

DESIGNThe Feed Line (q-line)

The diagram above also shows that the binaryfeed should be introduced at the 4'th stage.

However, if the feed composition is such thatit does not coincide with the intersection of theoperating lines, this means that the feed is nota saturated liquid. The condition of the feedcan be deduced by the slope of the feed line

or q-line. The q-line is that drawn between theintersection of the operating lines, and wherethe feed composition lies on the diagonal line


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DISTILLATION COLUMN

DESIGNDepending on the state of the

feed, the feed lines will havedifferent slopes. For example,

q = 0 (saturated vapour)

q = 1 (saturated liquid)

0 < q < 1 (mix of liquid andvapour)

q > 1 (subcooled liquid)

q < 0 (superheated vapour)The q-lines for the various feed

conditions are shown in thediagram on the left.


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DISTILLATION COLUMN

DESIGNUsing Operating Lines and the Feed Line in McCabe-

Thiele Design

If we have information about the condition of the feedmixture, then we can construct the q-line and use it inthe McCabe-Thiele design. However, excluding theequilibrium line, only two other pairs of lines can beused in the McCabe-Thiele procedure. These are:

feed-line and rectification section operating line

feed-line and stripping section operating line stripping and rectification operating lines

This is because these pairs of lines determine the third.


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DISTILLATION COLUMN

DESIGNOVERALL COLUMN DESIGN

Determining the number of stages required for thedesired degree of separation and the location of thefeed tray is merely the first steps in producing an

overall distillation column design. Other things thatneed to be considered are tray spacings; columndiameter; internal configurations; heating andcooling duties. All of these can lead to conflictingdesign parameters.

Thus, distillation column design is often an iterativeprocedure. If the conflicts are not resolved at the designstage, then the column will not perform well in practice.The next set of notes will discuss the factors that canaffect distillation column performance.


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EFFECTS OF THE NUMBER OF

TRAYS OR STAGESHere we will expand on the design of columns by

looking briefly at the effects of

the number of trays, and

the position of the feed tray, and

on the performances of distillation columns



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TRAYS OR STAGES

Effects of the Number of Trays

It can be deduced from the previous section on distillationcolumn designthat the number of trays will influencethe degree of separation. This is illustrated by thefollowing example.

Consider as a base case, a 10 stage column. The feed is abinary mixture that has a composition of 0.5 molefraction in terms of the more volatile component, andintroduced at stage 5. The steady-state terminalcompositions of about 0.65 at the top (stage 1) and 0.1at the bottom (stage 10) are shown below:
http://lorien.ncl.ac.uk/ming/distil/distildes.htmhttp://lorien.ncl.ac.uk/ming/distil/distildes.htmhttp://lorien.ncl.ac.uk/ming/distil/distildes.htmhttp://lorien.ncl.ac.uk/ming/distil/distildes.htm


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TRAYS OR STAGES

Composition Profile: 10 stages, feed at stage 5



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TRAYS OR STAGESSuppose we decreasethe number of stages to 8, and keep the

feed at the middle stage, i.e. stage 4. The resulting compositionprofile is:


We can see that the top composition has decreased while the bottom

composition has increased. That is, the separation is poorer.



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TRAYS OR STAGES

Now, if we increasethe number of stages to 12, and againintroduce the feed at mid-column, i.e. stage 6, the compositionprofile we get is:



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TRAYS OR STAGESAgain, the composition has changed. This time

the distillate is much richer in the morevolatile component, while the bottoms has less,

indicating better separation.Thus, increasing the number of stages will

improve separation.


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TRAYS OR STAGESEffect of Feed Tray Position

Here we look at how the position of the feed trayaffects separation efficiency. Suppose we have

a 20 stage column, again separating a binarymixture that has a composition of 0.5 molefraction in terms of the more volatilecomponent. The terminal compositions

obtained when the feed is introduced at stages5, 10 and 15 (at fixed reflux and re-boil rates)are shown in the following plots.


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TRAYS OR STAGES

Composition profile: 20 stages, feed at stage 5


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TRAYS OR STAGES



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TRAYS OR STAGES



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TRAYS OR STAGESAs the feed stage is moved lower down thecolumn, the top composition becomes less richin the more volatile component while thebottoms contains more of the more volatilecomponent. However, the changes in topcomposition is not as marked as the bottomscomposition.

The preceding examples illustrate what can

happen if the position of the feed tray isshifted for this particular system. They shouldnot be used to generalise to other distillationsystems, as the effects are not straightforward.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATION

The performance of a distillation column isdetermined by many factors, for example:

feed conditions: state of feed

composition of feed

trace elements that canseverely affect the VLEof liquidmixtures

internal liquid and fluid flow conditions

state of trays (packings)

weather conditions

FACTORS AFFECTING
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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATION

Feed Conditions

The state of the feed mixture and feed compositionaffects the operating lines and hence the number of

stages required for separation. It also affects thelocation of feed tray. During operation, if the deviationsfrom design specifications are excessive, then thecolumn may no longer be able handle the separationtask. To overcome the problems associated with the

feed, some column are designed to have multiple feedpoints when the feed is expected to containing varyingamounts of components.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONReflux Conditions

As the reflux ratio is increased, the gradient ofoperating line for the rectification sectionmoves towards a maximum value of 1.Physically, what this means is that more andmore liquid that is rich in the more volatilecomponents are being recycled back into thecolumn. Separation then becomes better and

thus less trays are needed to achieve thesame degree of separation. Minimum trays arerequired under total refluxconditions, i.e.there is no withdrawal of distillate.

FACTORS AFFECTING

DISTILLATION COLUMN


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DISTILLATION COLUMN

OPERATION

Reflux Conditions

On the other hand, as reflux isdecreased, the operating line for therectification section moves towards theequilibrium line. The pinch between

operating and equilibrium lines becomesmore pronounced and more and moretrays are required.This is easy to verifyusing the McCabe-Thiele method.

The limiting condition occurs at minimumreflux ratio, when an infinite number of

trays will be required to effect separation.Most columns are designed to operatebetween 1.2 to 1.5 times the minimumreflux ratiobecause this is approximatelythe region of minimum operating costs(more reflux means higher reboiler duty).

FACTORS AFFECTING
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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATION

Vapour Flow Conditions

Adverse vapour flow conditions can cause

Foaming

Entrainment

Weeping/dumping

Flooding

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONFoaming

Foaming refers to the expansion of liquiddue to passage of vapour or gas.Although

it provides high interfacial liquid-vapourcontact, excessive foaming often leads toliquid buildup on trays. In some cases,foaming may be so bad that the foam mixeswith liquid on the tray above. Whether

foaming will occur depends primarily onphysical properties of the liquid mixtures,but is sometimes due to tray designs andcondition. Whatever the cause, separationefficiency is always reduced.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONEntrainment

Entrainment refers to the liquid carried byvapour up to the tray above and is again

caused by high vapour flow rates. It isdetrimental because tray efficiency isreduced: lower volatile material is carried toa plate holding liquid of higher volatility. It

could also contaminate high purity distillate.Excessive entrainment can lead to flooding.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONWeeping/Dumping

This phenomenon is caused by low vapourflow. The pressure exerted by the vapour is

insufficient to hold up the liquid on the tray.Therefore, liquid starts to leak throughperforations. Excessive weeping will lead todumping. That is the liquid on all trays willcrash (dump) through to the base of the

column (via a domino effect) and thecolumn will have to be re-started. Weepingis indicated by a sharp pressure drop in thecolumn and reduced separation efficiency.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONFlooding

Flooding is brought about by excessivevapour flow, causing liquid to be entrained

in the vapour up the column. The increasedpressure from excessive vapour also backsup the liquid in the downcomer, causing anincrease in liquid holdup on the plateabove. Depending on the degree of

flooding, the maximum capacity of thecolumn may be severely reduced. Floodingis detected by sharp increases in columndifferential pressureand significantdecrease in separation efficiency.

FACTORS AFFECTING


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DISTILLATION COLUMN

OPERATION

Column DiameterMost of the above factors that affect columnoperation is due to vapour flow conditions:

either excessive or too low. Vapour flowvelocity is dependent on column diameter.Weeping determines the minimum vapour flowrequiredwhile flooding determines themaximum vapour flow allowed, hence column

capacity.Thus, if the column diameter is notsized properly, the column will not performwell. Not only will operational problems occur,the desired separation duties may not beachieved.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONState of Trays and Packings

Remember that the actual number of traysrequired for a particular separation duty is

determined by the efficiency of the plate, andthe packings if packings are used.Thus, anyfactors that cause a decrease in tray efficiencywill also change the performance of thecolumn. Tray efficiencies are affected by

fouling, wear and tear and corrosion, and therates at which these occur depends on theproperties of the liquids being processed. Thusappropriate materials should be specified fortray construction.

FACTORS AFFECTING


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FACTORS AFFECTING

DISTILLATION COLUMN

OPERATIONWeather Conditions

Most distillation columns are open to theatmosphere. Although many of the columns

are insulated, changing weather conditionscan still affect column operation. Thus thereboiler must be appropriately sized to ensurethat enough vapour can be generated during

cold and windy spells and that it can be turneddown sufficiently during hot seasons. Thesame applies to condensors.

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