Modelling & Simulation of Binary Distillation Column

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“MODELLING & SIMULATION OF BINARY

DISTILLATION COLUMN”

CONTE

NTS

• •

Introduction

•

Vapour Liquid Equilibrium

•

Types of Distillation

1. 1. Ba

Batch

tch Dis

Distil

tillat

lation

ion

2. 2. Con

Contin

tinuou

uous Disti

s Distilla

llatio

tion

n

•

Simple Distillation

•

Flash Evaporation

•

Fractional Distillation

•

T

Types o

ypes of Azeotropes

f Azeotropes

•

Separation of Azeotropes

•

Steam Distillation

•

V

Vacuum

acuum Distillati

Distillation

on

•

Extractive Distillation

•

Theoretical plates

•

Methods of calculating no. of

Methods of calculating no. of stages

stages

1. 1. Fe

Fens

nske E

ke Equ

quat

atio

ion

n

2. 2. McC

McCabe

abe-Thi

-Thiele

ele Met

Method

hod

•

Modelling of McCabe-Thiele Method

•

Assumptions of McCabe-Thiele Method

•

Introduction to Simulation

•

Flow Chart of Simulation Program

•

Problem

•

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• •

Appendix

• •

Bibliography

Introduction

A process in which a liquid or vapour mixture of two or more substances is A process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application separated into its component fractions of desired purity, by the application and removal of heat.

and removal of heat. Or in other words:

Or in other words: DistillationDistillation is a widely used method for separatingis a widely used method for separating mixtures based on differences in the conditions required to change the phase mixtures based on differences in the conditions required to change the phase of components of the mixture. To separate a mixture of liquids, the liquid of components of the mixture. To separate a mixture of liquids, the liquid can be heated to force components, which have different boiling points, into can be heated to force components, which have different boiling points, into the gas phase. The gas is then condensed back

the gas phase. The gas is then condensed back into liquid form and collected.into liquid form and collected. Repeating the process on the collected liquid to improve the purity of the Repeating the process on the collected liquid to improve the purity of the product is called double distillation. Although the term is most commonly product is called double distillation. Although the term is most commonly applied to liquids, the reverse process can be used to separate gases by applied to liquids, the reverse process can be used to separate gases by liquefying components using changes in temperature and/or pressure.

liquefying components using changes in temperature and/or pressure.

Distillation is used for many commercial processes, such as production of Distillation is used for many commercial processes, such as production of gasoline, distilled water, xylene, alcohol, paraffin, kerosene, and many other gasoline, distilled water, xylene, alcohol, paraffin, kerosene, and many other liqu

liquids. ids. TTypes ypes of of distildistillatilation on incluinclude de simplsimple e distidistillatllation ion (desc(described ribed here),here), fractional distillation (different volatile 'fractions' are collected as they are fractional distillation (different volatile 'fractions' are collected as they are produced), and destructive distillation (usually, a material is heated so that it produced), and destructive distillation (usually, a material is heated so that it

decomposes into compounds for

decomposes into compounds for collection).collection).

Distillation is based on the fact that the vapour of a boiling mixture will be Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points.

richer in the components that have lower boiling points.

Therefore, when this vapour is cooled and condensed, the condensate will Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture contain more volatile components. At the same time, the original mixture wil

will l concontaitain n mormore e of of the the lesless s volvolatiatile le matmaterierial. al. DisDistiltillalatiotion n cocolumlumns ns areare designed to achieve

designed to achieve this separation efficientlythis separation efficiently.. Al

Alththouough gh mamany ny pepeopople le hahave ve a a faifair r ididea ea whwhat at “d“disistitillllatatioion” n” memeanans, s, ththee important aspects that seem to be missed from the manufacturing point of important aspects that seem to be missed from the manufacturing point of view are that:

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•

• Distillation is the most Distillation is the most common separation technique.common separation technique. •

• It consumes enormous amounts of energy, both in terms of coolingIt consumes enormous amounts of energy, both in terms of cooling

and heating requirements. and heating requirements.

•

• It can contribute to more than 50% It can contribute to more than 50% of plant operating costs.of plant operating costs.

The best way to reduce operating costs of existing units, is to improve their The best way to reduce operating costs of existing units, is to improve their efficiency and operation via process optimization and control. To achieve efficiency and operation via process optimization and control. To achieve this improvement, a thorough understanding of distillation principles and this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.

how distillation systems are designed is essential. The Distillation of the Crude Oil in

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Vapor-Liquid Equilibrium

V

Vapor-liquid equilibrium, abbreviated as VLE by apor-liquid equilibrium, abbreviated as VLE by some, is some, is a condition wherea condition where aa liquidliquid anand d ititss vapor vapor (g(gaas s phphasasee) ) aare re inin equilibriumequilibrium wiwith th eaeach ch ototheherr, , aa condition or state where the rate of

condition or state where the rate of evaporationevaporation (liquid changing to vapor)(liquid changing to vapor) equals the rate of

equals the rate of condensationcondensation (vapor changing to liquid) on a molecular (vapor changing to liquid) on a molecular lev

level el susuch ch ththat at ththerere e is is no no nenet t (o(oveverarallll) ) vavapopor-r-liliquiquid d ininteter r coconvnverersisionon.. Although in theory equilibrium takes forever to reach, such an equilibrium is Although in theory equilibrium takes forever to reach, such an equilibrium is practically reached in a relatively closed location if a liquid and its vapor are practically reached in a relatively closed location if a liquid and its vapor are

allowed to stand in contact with

allowed to stand in contact with each other long enough with no interferenceeach other long enough with no interference or only gradual interference from the

or only gradual interference from the outside.outside.

VLE Data Introduction

Th

The e coconcncenentrtratatioion n of of a a vavapopor r in in cocontntacact t wiwith th itits s liliququidid, , esespepecicialally ly atat eq

equiuililibrbriuium, m, is is ofofteten n gigiveven n in in teterms rms of of vapovapor r presspressureure, , whwhicich h cocoululd d bebe aa partial pressure partial pressure (part of the total gas(part of the total gas pressure pressure) if any other gas(es) are) if any other gas(es) are present with the vapor. The equilibrium vapor pressure of a liquid is usually present with the vapor. The equilibrium vapor pressure of a liquid is usually

very dependent on

very dependent on temperaturetemperature. At vapor-liquid equilibrium, a liquid with. At vapor-liquid equilibrium, a liquid with individual components (compounds) in certain

individual components (compounds) in certain concentrationsconcentrations will have anwill have an equilibrium vapor in which the concentrations or partial pressures of the equilibrium vapor in which the concentrations or partial pressures of the vapor components will have certain set values depending on all of the liquid vapor components will have certain set values depending on all of the liquid component concentrations and the temperature. This fact is true in reverse component concentrations and the temperature. This fact is true in reverse al

alsoso; ; if if a a vavapopor r wiwith th cocompmpononenents ts at at cecertrtaiain n coconcncenentrtratatioions ns or or papartirtialal pressures is in vapor-liquid equilibrium with its liquid, then the component pressures is in vapor-liquid equilibrium with its liquid, then the component ccoonncceennttrraatitioonns s iin n tthhe e lliiqquuiid d wiwilll l bbe e sseet t ddeeppeennddeennt t oon n tthhe e vvaappoor r concentrations, again also depending on the temperature. The equilibrium concentrations, again also depending on the temperature. The equilibrium concentration of each component in the liquid phase is often different from concentration of each component in the liquid phase is often different from it

its s coconcncenentrtratatioion n (o(or r vavapopor r prpresessusurere) ) in in ththe e vavapopor r phphasase, e, bubut t ththere ere is is aa co

corrrrelelatatioion. n. SuSuch ch VLVLE E coconcncenentrtratatioion n dadata ta is is ofofteten n knknowown n or or cacan n bebe de

detetermrminined ed exexpeperirimementntalally ly fofor r vavapoporr-l-liqiquiuid d mimixtxturures es wiwith th vavaririououss co

compmpononenentsts. . In In cecertrtaiain n cacaseses s susuch ch VLVLE E dadata ta cacan n be be dedetetermrminined ed or or aapppprrooxxiimmaatteed d wwiitth h tthhe e hheellp p oof f cceerrttaiain n tthheeooririees s ssuucch h aass Raoult'sRaoult's Law

Law,, Dalton's LawDalton's Law, and/or , and/or Henry's LawHenry's Law.. Su

Such ch VLVLE E ininfoformrmatatioion n is is ususefeful ul in in dedesisiggniningng columnscolumns for for distillationdistillation,, especially

especially fractional distillationfractional distillation, which is a particular specialty of , which is a particular specialty of chemicalchemical engineers

engineers. . DisDistiltillatlation ion is is a a proprocescess s useused d to to sepseparaarate te or or parpartiatially lly sepseparaaratete ccoommppoonneenntts s iin n a a mmiixxttuurre e bbyy boiling boiling (v(vapapororizizatiationon) ) fofollllowoweded

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by

by condensationcondensation. . DDiissttiillllaattiioon n ttaakkees s aaddvvaannttaagge e oof f ddiiffffeerreennccees s iinn concentrations of components in the liquid and vapor phases.

concentrations of components in the liquid and vapor phases.

In mixtures containing two or more components where their concentrations In mixtures containing two or more components where their concentrations are

are cocompmparared ed in in ththe e vavapopor r anand d liliququid id phphasaseses, , coconcncenentrtratatioions ns of of eaeachch component are often expressed as

component are often expressed as mole fractionsmole fractions. A mole fraction is number . A mole fraction is number of

of molesmoles of a given component in an amount of mixture in aof a given component in an amount of mixture in a phase phase (either (either va

vapopor r or or liliququid id phphasase) e) didivivideded d by by ththe e tototatal l nunumbmber er of of momoleles s of of alalll components in that amount of mixture in that p

components in that amount of mixture in that phase.hase. Bi

Binanary ry mimixtxturures es arare e ththosose e hahaviving ng twtwo o cocompmpononenentsts. . ThThreree-e-cocompmpononenentt mi

mixtxturures es cocoululd d be be cacalllled ed teternrnarary y mimixtxturureses. . ThTherere e cacan n be be VLVLE E dadata ta fofor r mixtures with even more components, but such data becomes copious and is mixtures with even more components, but such data becomes copious and is often hard to show graphically. VLE data is often shown at a certain overall often hard to show graphically. VLE data is often shown at a certain overall p

preressssurure, e, susuch ch as as 11 atmatm or or whwhatatevever er prpresessusure re a a prprococesess s of of inintetererest st isis conducted at. When at a certain temperature, the total of partial pressures of conducted at. When at a certain temperature, the total of partial pressures of all the components becomes equal to the overall pressure of the system such all the components becomes equal to the overall pressure of the system such that vapors generated from the liquid displace any air or other gas which that vapors generated from the liquid displace any air or other gas which ma

mainintatainined ed ththe e ovovereralall l prpresessusurere, , ththe e mimixtxturure e is is sasaid id toto boil boil aand nd ththee co

corrrresespopondndining g tetempmpererataturure e is is ththee bo boiliiling ng poipointnt(T(Thihis s asassusumemes s exexcecessss pressure is relieved by letting out gases to maintain a desired total pressure). pressure is relieved by letting out gases to maintain a desired total pressure).

A boiling point at an overall pressure of 1 atm is called the

A boiling point at an overall pressure of 1 atm is called the normal boilingnormal boiling point

point..

Thermodynamic Description of Vapor-Liquid Equilibrium

Th

The e fifieleld d of of thermodynamicsthermodynamics descdescribes ribes when when vapovapor-lir-liquid quid equiequilibriulibrium m isis possible, and its properties. Much of the analysis depends on whether the possible, and its properties. Much of the analysis depends on whether the

vapor and liquid consist of a s

vapor and liquid consist of a single component, or if they are mixturesingle component, or if they are mixtures..

Pure (single-component) systems

If the liquid and vapor are pure, in that they consist of only one molecular If the liquid and vapor are pure, in that they consist of only one molecular component and no impurities, then the equilibrium state between the two component and no impurities, then the equilibrium state between the two phases is described by the following equations:

phases is described by the following equations:

And And

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where

where and and are are thethe pressures pressures witwithin hin the the liqliquid uid and and vapvaporor,, an

and d arare e ththee temperaturestemperatures wiwiththiin n ththe e lliqiquuid id aand nd vavapoporr, , anandd and

and are are the the molar molar Gibbs free energiesGibbs free energies (units of energy per (units of energy per amount of amount of substance

substance) within the liquid and vapor, respectively.) within the liquid and vapor, respectively. [4][4] In other words, theIn other words, the temperature, pressure and molar Gibbs free energy are the same between the temperature, pressure and molar Gibbs free energy are the same between the two phases when they are at

two phases when they are at equilibrium.equilibrium.

An equivalent, more common way to express the vapor-liquid equilibrium An equivalent, more common way to express the vapor-liquid equilibrium condition in a pure system is by using the concept of

condition in a pure system is by using the concept of fugacityfugacity. Under this. Under this view

view, equilibrium is , equilibrium is described by the following equation:described by the following equation:

whe

where re and and are are thethe fugacitiesfugacities oof f tthhe e lliiqquiuid d aanndd vapor,

vapor, respectivelyrespectively, , at at the the system system temperature temperature and and pressure pressure ..[5][5] UsingUsing fugacity is often more convenient for calculation, given that the fugacity of fugacity is often more convenient for calculation, given that the fugacity of the liquid is, to a good approximation, pressure-independent,

the liquid is, to a good approximation, pressure-independent, [6][6] and it isand it is often

often convconvenieenient nt to to use use the the quanquantity tity , , the the dimendimensionlsionless ess fugacifugacityty coefficient, which is 1 for an

coefficient, which is 1 for an ideal gasideal gas..

Multicomponent systems

In a multicomponent system, where the vapor and liquid consist of more In a multicomponent system, where the vapor and liquid consist of more th

than an onone e tytype pe of of momolelecuculele, , dedescscriribibing ng ththe e eqequiuililibrbriuium m ststatate e is is momorere com

compliplicatcated. ed. For For all compoall componennents ts in in the systethe system, m, the equilthe equilibribrium stateium state between the two phases is described by

between the two phases is described by the following equations:the following equations:

where

where and and are are the the temptemperaturerature e and and presspressure ure for for each each phasephase, , andand an

and d arare e ththee p parartitial al momolalar r GiGibbbbs s frfree ee enenerergygyalsalso o calcalledled chemicalchemical potential

potential (units of energy per (units of energy per amount of substanceamount of substance) within the liquid and) within the liquid and vapor, respectively, for each phase. The partial molar Gibbs free energy is vapor, respectively, for each phase. The partial molar Gibbs free energy is defined by:

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wh

wherere e is is ththe e ((extensiveextensive) Gi) Gibbbbs frs free eee enenergrgyy, an, and d is tis thehe amamouount nt of of substance

substance of of component component ..

T

Types Of

ypes Of Distillation Columns

Distillation Columns

There are many types of distillation columns, each designed to perform There are many types of distillation columns, each designed to perform specific types of separations, and each design differs in

specific types of separations, and each design differs in terms of complexity.terms of complexity. One way of classifying distillation column type is to look at how they are One way of classifying distillation column type is to look at how they are operated. Thus we have:

operated. Thus we have:

1.

1. Batch andBatch and

2.

2. Continuous columns.Continuous columns.

Batch Columns

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

introduced.

Heating an ideal mixture of two volatile substances A and B (with A having Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility

the higher volatility, or , or lower boiling point) in a lower boiling point) in a batch distillation setup (suchbatch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid which contains a mixture of A and B. The results in a vapor above the liquid which contains a mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the ratio between A and B in the vapor will be different from the ratio in the liquid: the ratio in the liquid will be determined by how the original mixture liquid: the ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This in turn means through the condenser and is removed from the system. This in turn means

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that the ratio of compounds in the remaining liquid is now different from the that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e. more enriched in B

initial ratio (i.e. more enriched in B than the starting liquid).than the starting liquid).

The result is that the ratio in the liquid mixture is changing, becoming richer The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which in component B. This causes the boiling point of the mixture to rise, which in turn results in a rise in the temperature in the vapor, which results in a in turn results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an changing ratio of A : B in the gas phase (as distillation continues, there is an in

incrcreaeasising ng prpropoporortition on of of B B in in ththe e gagas s phphasase)e). . ThThis is reresusultlts s in in a a slslowowlyly changing ratio A : B in

changing ratio A : B in the distillate.the distillate. If

If the differthe difference in ence in vapovapor r pressupressure between the re between the two componetwo components A and B nts A and B isis large (generally expressed as the difference in boiling points), the mixture in large (generally expressed as the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and the beginning of the distillation is highly enriched in component A, and wh

when en cocompmpononenent t A A hahas s didiststililleled d ofoff, f, ththe e boboililining g liliquiquid d is is enenricrichehed d inin component B.

component B.

Continuous Columns

In

In cocontrntrastast, , concontitinuonuous us colcolumnumns s proprocescess s a a concontintinuouuous s feefeed d strstreameam. . NoNo interruptions occur unless there is a problem with the column or surrounding interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the process units. They are capable of handling high throughputs and are the most common of the two types. We shall concentrate only on this class of most common of the two types. We shall concentrate only on this class of columns.

columns.

Continuous distillation is an ongoing distillation in which a liquid mixture is Continuous distillation is an ongoing distillation in which a liquid mixture is con

contintinuouuously sly (wi(withthout out intinterruerruptiption) on) fed fed intinto o the the proprocescess s and and sepseparaaratedted fractions are removed continuously as output streams as time passes during fractions are removed continuously as output streams as time passes during the operation. Continuous distillation produces at least two output fractions, the operation. Continuous distillation produces at least two output fractions, including at least one

including at least one volatilevolatile distildistillate fractionlate fraction, , whicwhich h has has boilboiled ed and beenand been separately captured as a vapor condensed to a liquid. There is always a separately captured as a vapor condensed to a liquid. There is always a bottoms (or residue) fraction, which is the least volatile residue that has not bottoms (or residue) fraction, which is the least volatile residue that has not been separately captured as

been separately captured as a condensed vapor.a condensed vapor. Con

Contintinuouuous s disdistiltillatlation ion difdifferfers s frofrom m batbatch ch distdistillillatiation on in in the the resprespect ect thathatt conce

concentratintrations ons shoulshould d not not chanchange ge over time. over time. ContContinuouinuous s distidistillatllation ion can can bebe run at a

run at a steady statesteady state for an arbitrary amount of time. Given a feed of in afor an arbitrary amount of time. Given a feed of in a specified composition, the main variables that affect the purity

specified composition, the main variables that affect the purity of products inof products in continuous distillation are the reflux ratio and the number of theoretical continuous distillation are the reflux ratio and the number of theoretical equilibrium stages (practically

equilibrium stages (practically, the number of , the number of trays or the trays or the height of packing).height of packing). Reflux is a flow from the condenser back to the column, which generates a Reflux is a flow from the condenser back to the column, which generates a rec

recycycle le ththat at alallolows ws a a bebetttter er sesepapararatition on wiwith th a a gigiveven n nunumbmber er of of trtrayays.s. Equilibrium stages are ideal steps where compositions achieve vapor-liquid Equilibrium stages are ideal steps where compositions achieve vapor-liquid

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equilibrium, repeating the separation process and allowing better separation equilibrium, repeating the separation process and allowing better separation given a reflux ratio. A column with a high reflux ratio may have fewer given a reflux ratio. A column with a high reflux ratio may have fewer stages, but it refluxes a large amount of liquid, giving a wide column with a stages, but it refluxes a large amount of liquid, giving a wide column with a large holdup. Conversely

large holdup. Conversely, a column , a column with a low reflux ratwith a low reflux ratio must have a io must have a largelarge number of stages, thus requiring a

number of stages, thus requiring a taller column.taller column. Co

Contntininuouous us didiststilillalatition on rereququirires es bubuilildiding ng anand d coconfnfigigururining g dededidicacatetedd equipment. The resulting high investment cost restricts its use to the large equipment. The resulting high investment cost restricts its use to the large scale.

scale.

Types of Continuous Columns

Continuous columns can be further classified according to: Continuous columns can be further classified according to:

1.

1. The nature of the feed that they are processingThe nature of the feed that they are processing

•

• Binary column - feed contains only two components.Binary column - feed contains only two components. •

• MuMultlti-i-cocompmpononenent t cocolulumn mn - - fefeed ed cocontntaiains ns momore re ththan an twtwoo

components. components.

2.

2. The number of product streams they haveThe number of product streams they have •

• MulMulti-ti-proproducduct t colcolumn umn - - cocolumlumn n has has mormore e ththan an two produtwo productct

streams. streams. 3

3.. WhWherere e tthe he eextxtra ra fefeeed d exexitits s wwhehen n it it iis s uusesed d to to hhelelp p wwitith h ththee separation

separation

•

• ExtExtracractivtive e disdistiltillalatiotion n - - whewhere re the the extextra ra feefeed d appappearears s in in thethe

bottom product stream. bottom product stream.

•

• Azeotropic distillation - where the extra feed appears at the topAzeotropic distillation - where the extra feed appears at the top

product stream. product stream.

4.

4. The type of column internalsThe type of column internals •

• Tray column - where trays of various designs are used to holdTray column - where trays of various designs are used to hold

up

up the the liliquiquid d to to proprovidvide e betbetter ter concontactact t betbetweeween n vapvapour our andand liquid, hence better separation.

liquid, hence better separation.

•

• Packed column - where instead of trays,Packed column - where instead of trays, 'Packing''Packing' are used toare used to

enhance contact between vapour and liquid. enhance contact between vapour and liquid.

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Simple Distillation

In simple distillation, all the hot

In simple distillation, all the hot vapors produced are vapors produced are immediately channeledimmediately channeled int

into o a a concondendenser ser whiwhich ch coocools ls and and cocondendensenses s the the vapvapors. Thereors. Thereforfore, e, thethe distillate will not be pure

distillate will not be pure its composition will be identical to the its composition will be identical to the compositioncomposition of the vapors at the given temperature and pressure, and can be computed of the vapors at the given temperature and pressure, and can be computed from

from Raoult's lawRaoult's law..

As a result, simple distillation is usually used only to separate liquids whose As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C)

boiling points differ greatly (rule of thumb is 25 °C) [25][25] or to separateor to separate liquids from in volatile solids or oils. For these cases, the vapor pressures of liquids from in volatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be the components are usually sufficiently different that Raoult's law may be ne

neglglececteted d dudue e to to ththe e ininsisiggninifificacant nt cocontntriribubutition on of of ththe e leless ss vovolalatitilele co

compmpononenent. t. In In ththis is cacasese, , ththe e didiststilillalate te mamay y be be susufffficicieientntly ly pupure re fofor r ititss intended purpose.

intended purpose.

The liquid mixture that is to be processed is known as the feed and this is The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray introduced usually somewhere near the middle of the column to a tray

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kn

knowown n as as ththe e fefeed ed trtrayay. . ThThe e fefeed ed trtray ay didivividedes s ththe e cocolulumn mn ininto to a a totopp (enriching or rectification) section and a bottom (str

(enriching or rectification) section and a bottom (stripping) section. The feedipping) section. The feed flows down the column where it is

flows down the column where it is collected at the bottom in the reboiler.collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapour. The source of heat input Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable fluid, although in most chemical plants this is normally can be any suitable fluid, although in most chemical plants this is normally steam. In

steam. In refinerefineries,ries, the heating source may be the output streams of other the heating source may be the output streams of other colum

columns. ns. The vapouThe vapour raised in the reboiler is re-intror raised in the reboiler is re-introduced intduced into the unit ato the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or

the bottoms product or simplysimply, bottoms., bottoms.

The vapour moves up the column, and as it exits the top of the unit, it is 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 a holding vessel cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of known as the reflux drum. Some of this liquid is recycled back to the top of th

the e cocolulumn mn anand d ththis is is is cacalllled ed ththe e rereflfluxux. . ThThe e cocondndenensesed d liliququid id ththat at isis removed from the system is

removed from the system is known as the distillate or top product.known as the distillate or top product. Thus, there are internal flows

Thus, there are internal flows of vapour and liquid within the column as wellof vapour and liquid within the column as well as external flows of feeds

as external flows of feeds and product streams, into and out of and product streams, into and out of the column.the column.

Flash Evaporation

Fla

Flash sh (or (or parpartiatial)l) evaporationevaporation is is the the parpartiatiall vaporizationvaporization thathat t occoccurs urs whewhenn aa satursaturated ated liquliquidid ststreream am unundedergrgoeoes s a a redreducuctition on in in prpresessusure re by by papassssiningg through a

through a throttling valvethrottling valve or other throttling device. This process is one of or other throttling device. This process is one of the simplest

the simplest unit operationsunit operations. If the throttling valve or device is located at the. If the throttling valve or device is located at the entry into a

entry into a press pressure ure vesselvessel so that the flash evaporation occurs within theso that the flash evaporation occurs within the vessel, then the vessel is often referred to as a

vessel, then the vessel is often referred to as a flash drumflash drum.. If

If ththe e sasatuturarateted d liliququid id is is a a sisingnglele-c-comompoponenent nt liliququid id (f(for or exexamamplple,e, liquid

liquid propane propane or liquidor liquid ammoniaammonia), a part of ), a part of the liquid immediately "flashes"the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the

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temperature

temperature of the liquid at the reduced pressure. This is often referred to asof the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional

"auto-refrigeration" and is the basis of most conventional vapor compressionvapor compression refrigeration

refrigeration systems.systems.

If the saturated liquid is a multi-component liquid (for example, a mixture If the saturated liquid is a multi-component liquid (for example, a mixture of

of propane propane,, isobutaneisobutane and normaland normal butane butane), the flashed vapor is richer in the), the flashed vapor is richer in the more

more volatilevolatile components than is the remaining liquid.components than is the remaining liquid.

Flash Evaporation of a

Flash Evaporation of a Single-Component Liquid

Single-Component Liquid

The

The flaflash sh evaevaporaporatition on of of a a sinsinglegle-com-componponent ent liqliquid uid is is an an iseisentrntropiopic c i.ei.e.,., constant

constant entropyentropy) process and is often referred to as an) process and is often referred to as an adiabaticadiabatic flashflash. . TheThe following equation, derived from a simple heat balance around the throttling following equation, derived from a simple heat balance around the throttling valve or device, is used to predict how much of a single-component liquid is valve or device, is used to predict how much of a single-component liquid is vaporized.

vaporized.

X = 100 (H

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where: where:

X

X = = wweeiigghht t ppeerrcceennt t vvaappoorriizzeedd H

HuLuL = = upupststreaream m liliququid id enenththalalpy py at at upupststreream am tetempmpererataturure e anandd

pressure, J/kg pressure, J/kg H

HdVdV = = flflasashhed ed vavapopor r eentnthhalalppy y at at dodownwnststreream am prpresessusure re anandd

corresponding saturation temperature, J/kg corresponding saturation temperature, J/kg H

HdLdL = = reresisidudual al liliququid id enenththalalpy py at at dodownwnststreream am prpresessusure re anandd

corresponding saturation temperature, J/kg corresponding saturation temperature, J/kg

If the enthalpy data required for the above equation is unavailable, If the enthalpy data required for the above equation is unavailable, then the following equation may be used.

then the following equation may be used.

X = 100 · c

pp

(Tu – T

dd

) ÷ H

vv

where: where:

X

X = = wweeiigghht t ppeercrceennt t vvaappoorriizzeedd

cc p p = liquid= liquid specific heatspecific heat at upstream temperature and pressure,at upstream temperature and pressure,

J/(kg °C) J/(kg °C) T

Tuu = = upstream upstream liquid liquid temperature, temperature, °C°C

T

Tdd = liquid= liquid saturation temperaturesaturation temperature corresponding to thecorresponding to the

downstream pressure, °C downstream pressure, °C H

Hvv = = liliququidid heaheat t of of vapvaporiorizazatiotionn at at dodownwnststreream am prpresessusure re anandd

corresponding saturation temperature, J/kg corresponding saturation temperature, J/kg

(Note: The words "upstream" and "downstream" refer to before and after the (Note: The words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.)

liquid passes through the throttling valve or device.) This type of flash e

This type of flash evaporation is used in thevaporation is used in the desalinationdesalination of brackish water or of brackish water or oc

oceaean n wawateter r by by ""Multi-Stage FlashMulti-Stage Flash Distillation." The water is heated andDistillation." The water is heated and then routed into a reduced-pressure flash evaporation "stage" where some of then routed into a reduced-pressure flash evaporation "stage" where some of the water flashes into steam. This steam is subsequently condensed into the water flashes into steam. This steam is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a free water. The residual salty liquid from that first stage is introduced into a se

secocond nd flflasash h evevapapororatiation on ststagage e at at a a prpresessusure re lolowewer r ththan an ththe e fifirsrst t ststagagee pr

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condensed into more salt-free water. This sequential use of multiple flash condensed into more salt-free water. This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are evaporation stages is continued until the design objectives of the system are met. A large part of the world's installed desalination capacity uses met. A large part of the world's installed desalination capacity uses multi-stage flash distillation. Typically such plants have 24 or more sequential stage flash distillation. Typically such plants have 24 or more sequential stages of flash evaporation.

stages of flash evaporation.

Equilibrium Flash of a Multi-Component Liquid

The equilibrium flash of a multi-component liquid may be visualized as a The equilibrium flash of a multi-component liquid may be visualized as a simple

simple distillationdistillation prprococesess s ususiing ng a a sisingnglele equilequilibriuibrium m stagstagee. . IIt t iis s vveerryy different and more complex than the flash evaporation of single-component different and more complex than the flash evaporation of single-component liqu

liquid. id. For a For a multimulti-com-componenponent t liqliquid, calculatiuid, calculating ng the amounts the amounts of of flashflasheded va

vapopor r anand d reresisidudual al liliququid id in in eqequiuililibrbriuium m wiwith th eaeach ch ototheher r at at a a gigivevenn temperature and pressure requires a trial-and-error

temperature and pressure requires a trial-and-error iterativeiterative solution. Such asolution. Such a calculation is commonly referred to as an equilibrium flash calculation. It calculation is commonly referred to as an equilibrium flash calculation. It involves solving the Rachford-Rice equation:

involves solving the Rachford-Rice equation:

Where: Where:

zzii is the mole fraction of componentis the mole fraction of component ii in the feed liquid (assumed to bein the feed liquid (assumed to be

known); known); β

β is the fraction of feed is the fraction of feed that is vaporised;that is vaporised; K

K ii is the equilibrium constant of componentis the equilibrium constant of component ii..

The equilibrium constants

The equilibrium constants K K ii are in general functions of many parameters,are in general functions of many parameters,

though the most important is arguably temperature; they are defined as: though the most important is arguably temperature; they are defined as:

Where: Where: x

xii is the mole fraction of is the mole fraction of componentcomponent ii in liquid phase;in liquid phase;

y

yii is the mole fraction of is the mole fraction of componentcomponent ii in gas phase.in gas phase.

O

Onncce e tthhe e RRaacchfhfoorrdd--RRiicce e eeqquuaattiioon n hhaas s bbeeeen n ssoollvveed d ffoor r ββ, , tthhee compositions xi and

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The Rachford-Rice equation can have multiple solutions for

The Rachford-Rice equation can have multiple solutions for ββ, at most one, at most one of which guarantees that all

of which guarantees that all xxii andand yyii will be positive. In will be positive. In particularparticular, if , if there isthere is

only one

only one ββ for which:for which:

The

Then n thathatt ββ is the solution; if there are multiple suchis the solution; if there are multiple such β'sβ's, it means that, it means that either

either K K maxmax<1 or <1 or K K minmin>1, indicating respectively that no gas phase can be>1, indicating respectively that no gas phase can be

sustained (and therefore

sustained (and therefore β=0β=0) or conversely that no liquid phase can exist) or conversely that no liquid phase can exist (and therefore

(and therefore β=1β=1).). It is possible to use

It is possible to use Newton's method Newton's method for solving the above Rachford-Ricefor solving the above Rachford-Rice equation, but there is a risk of converging to the wrong value of

equation, but there is a risk of converging to the wrong value of ββ; it is; it is im

impoportrtanant t to to iininititialalizize e ththe e sosolvlver er to to a a sesensnsibible le ininititiaial l vavalulue, e, susuch ch asas (β

(βmaxmax+β+βminmin)/2)/2 (which is however not sufficient: Newton's method makes no(which is however not sufficient: Newton's method makes no

guarantees on stability), or, alternatively, use a bracketing solver such as the guarantees on stability), or, alternatively, use a bracketing solver such as the bisection or the

bisection or the Brent methodBrent method, which are guaranteed to converge but can be, which are guaranteed to converge but can be slower.

slower.

The equilibrium flash of multi-component liquids is very widely utilized The equilibrium flash of multi-component liquids is very widely utilized in

in pet petroleuroleum m refinrefinerieseries,, petrochemical petrochemical andand chemichemical cal planplantsts andand natnaturaural l gasgas processing

processing plants.plants.

Fractional Distillation

Fractional distillation is the separation of a mixture into its component parts, Fractional distillation is the separation of a mixture into its component parts, or

or frafractctioionsns, , susuch ch as as in in sesepaparatratiningg chemichemical cal compocompoundsunds by by ththeieir r boiling boiling point

point by heating them to a temperature at which several fractions of theby heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of

compound will evaporate. It is a special type of distillationdistillation. Generally the. Generally the component parts boil at less than 25 °C from each other under a pressure of component parts boil at less than 25 °C from each other under a pressure of

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one atmosphere (

one atmosphere (atmatm). If the difference in boiling points is greater than). If the difference in boiling points is greater than 25 °C, a

25 °C, a simple distillationsimple distillation is used.is used.

Using the Phase Diagram

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If you boil a liquid mixture C1, you will get a vapour with composition C2, If you boil a liquid mixture C1, you will get a vapour with composition C2, which you can condense to give a liquid of that same composition (the pale which you can condense to give a liquid of that same composition (the pale b

blulue e lilinenes)s). . If If yoyou u rereboboil il ththat at liliququid id C2C2, , it it wiwill ll gigive ve a a vavapopour ur wiwithth composition C3. Again you can condense that to give a liquid of the same composition C3. Again you can condense that to give a liquid of the same new composition (the red lines).

new composition (the red lines).

Reboiling the liquid C3 will give a vapour still richer in the more volatile Reboiling the liquid C3 will give a vapour still richer in the more volatile component B (the green lines). You can see that if you were to do this once component B (the green lines). You can see that if you were to do this once or twice more, you

or twice more, you would be able to collect a liquid would be able to collect a liquid which was virtually purewhich was virtually pure B. The secret of getting the more volatile component from a mixture of B. The secret of getting the more volatile component from a mixture of liq

liquiduids s is is obvobviouiously sly to to do do a a sucsuccescessiosion n of of boboiliiling-ng-concondendensinsing-rg-reboeboiliilingng operations. It isn't quite so obvious how you get a sample of pure A out of operations. It isn't quite so obvious how you get a sample of pure A out of this. That will become clearer in a while.

this. That will become clearer in a while.

The Vapour

This new vapour will again move further up the fractionating column until it This new vapour will again move further up the fractionating column until it gets to a temperature where it can condense. Then the whole process repeats gets to a temperature where it can condense. Then the whole process repeats itself.

itself.

Each time the vapour condenses to a liquid, this liquid will start to trickle Each time the vapour condenses to a liquid, this liquid will start to trickle back down the column where it will be reboiled by up-coming hot vapour. back down the column where it will be reboiled by up-coming hot vapour. Each time this happens the new vapour will be richer in the more volatile Each time this happens the new vapour will be richer in the more volatile component.

component.

The aim is to balance the temperature of the column so that by the time The aim is to balance the temperature of the column so that by the time vapour reaches the top after huge numbers of condensing and reboiling vapour reaches the top after huge numbers of condensing and reboiling operations, it consists only of the more

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Wh

Whetetheher r or or nonot t ththis is is is popossssibible le dedepependnds s on on ththe e didiffferferenence ce bebetwtweeeen n ththee boiling points of the two liquids. The closer they are together, the longer the boiling points of the two liquids. The closer they are together, the longer the

column has to be. column has to be.

The Liquid

So what about the liquid left behind at each reboiling? Obviously, if the So what about the liquid left behind at each reboiling? Obviously, if the vapour is richer in the more volatile component, the liquid left behind must vapour is richer in the more volatile component, the liquid left behind must be getting richer in the other one.

be getting richer in the other one.

As the condensed liquid trickles down the column constantly being reboiled As the condensed liquid trickles down the column constantly being reboiled by up-coming vapour, each reboiling makes it richer and richer in the less by up-coming vapour, each reboiling makes it richer and richer in the less volatile component - in this case, A. By the time the liquid drips back into volatile component - in this case, A. By the time the liquid drips back into the flask, it

the flask, it will be very will be very rich in A indrich in A indeed.eed.

So, over time, as B passes out of the top of the column into the condenser, So, over time, as B passes out of the top of the column into the condenser, the liquid in the flask will become richer in A. If you are very, very careful the liquid in the flask will become richer in A. If you are very, very careful over temperature control, eventually you will have separated the mixture over temperature control, eventually you will have separated the mixture into B in

into B in the collecting flask and A in the original flask.the collecting flask and A in the original flask. Finally

Finally, what is , what is the point of the point of the packing in the the packing in the column?column?

To make the boiling-condensing-reboiling process as effective as possible, it To make the boiling-condensing-reboiling process as effective as possible, it has to happen over and over again. By having a lot of surface area inside the has to happen over and over again. By having a lot of surface area inside the column, you aim to have the maximum possible contact between the liquid column, you aim to have the maximum possible contact between the liquid trickling down and the hot vapour rising.

trickling down and the hot vapour rising.

If you didn't have the packing, the liquid would all be on the sides of the If you didn't have the packing, the liquid would all be on the sides of the condenser, while most of the vapour would be going up the middle and condenser, while most of the vapour would be going up the middle and never come into contact with it.

never come into contact with it.

Azeotrope

An Azeotrope (pronounced /ay-ZEE-ə-trope/) is a mixture of two or more An Azeotrope (pronounced /ay-ZEE-ə-trope/) is a mixture of two or more liquids (chemicals) in such a ratio that its composition cannot be changed by liquids (chemicals) in such a ratio that its composition cannot be changed by simple

simple distillationdistillation. This occurs because, when an azeotrope is boiled, the. This occurs because, when an azeotrope is boiled, the resulting vapor has the same rat

resulting vapor has the same ratio of constituents as the original mixture.io of constituents as the original mixture. Because their composition is unchanged by distillation, azeotropes are also Because their composition is unchanged by distillation, azeotropes are also ca

callleled d (e(espspececiiaalllly y in in ololdeder r ttexextts) s) ccononststanant t bbooililining g mimixtxtuureres. s. ThThee word azeotrope is derived from the Greek words ζέειν (boil) and τρόπος word azeotrope is derived from the Greek words ζέειν (boil) and τρόπος (change) combined with the prefix α- (no) to give the overall meaning, “no (change) combined with the prefix α- (no) to give the overall meaning, “no change on boiling.”

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T

Types

ypes of Azeotropes

of Azeotropes

Each azeotrope has a characteristic

Each azeotrope has a characteristic boiling point boiling point. The boiling point of an. The boiling point of an azeotrope is either less than the boiling points of any of its constituents (a azeotrope is either less than the boiling points of any of its constituents (a p

pososititivive e azazeoeotrtropope)e), , or or grgreaeateter r ththan an ththe e boboililining g popoinint t of of anany y of of ititss constituents (a negative azeotrope).

constituents (a negative azeotrope).

A well known example of a positive azeotrope is 95.6%

A well known example of a positive azeotrope is 95.6% ethanolethanol and 4.4%and 4.4% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.1°C, which is lower than either of its constituents. azeotrope boils at 78.1°C, which is lower than either of its constituents. Ind

Indeed eed 78.78.1°C 1°C is is the the minminimuimum m temtemperperatuature re at at whiwhich ch any any ethethanoanol/wl/wateater r so

solulutition on cacan n boboilil. . In In gegeneneraral, l, a a poposisititive ve azazeoeotrtropope e bboioils ls at at a a lolowewer r temperature than any other ratio of its constituents. Positive azeotropes are temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures.

also called minimum boiling mixtures. An example of a negative azeotrope is

An example of a negative azeotrope is hydrochloric acidhydrochloric acid at a concentrationat a concentration of

of 2020.2.2%% hydrhydrogen ogen chlochlorideride anand d 7979.8.8% % wawateter r (b(by y weweigightht). ). HyHydrdrogogenen chloride boils at –84°C and water

chloride boils at –84°C and water at 100°C, but the at 100°C, but the azeotrope boils at 110°C,azeotrope boils at 110°C, which is higher than either of its constituents. The maximum temperature at which is higher than either of its constituents. The maximum temperature at wh

whicich h anany y hyhydrdrocochlhlorioric c acacid id sosolulutition on cacan n boboil il is is 111010°C°C. . In In gegeneneraral, l, aa negative azeotrope boils at a higher temperature than any other ratio of its negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called maximum boiling mixtures. constituents. Negative azeotropes are also called maximum boiling mixtures. Azeotropes consisting of two constituents, such as the two examples above, Azeotropes consisting of two constituents, such as the two examples above, are

are calcalled led bibinarnary y azeazeotrotropeopes. s. ThoThose se conconsissistinting g of of thrthree ee conconstistituetuents nts areare calle

called d ternarternary y azeoazeotropetropes. s. AzeoAzeotropetropes s of of more more than than three three consticonstituentuents ts areare also known.

also known.

More than 18,000 azeotropic mixtures have been documented. More than 18,000 azeotropic mixtures have been documented.

Combinations of solvents that do not form an azeotrope when mixed in any Combinations of solvents that do not form an azeotrope when mixed in any proportion are said to be

proportion are said to be zeotropiczeotropic..

When running a binary distillation it is often helpful to know the azeotropic When running a binary distillation it is often helpful to know the azeotropic composition of the mixture.

composition of the mixture.

Separation of Azeotrope Constituents

Distillation is one of the primary tools that chemists and chemical engineers Distillation is one of the primary tools that chemists and chemical engineers use to separate mixtures into their constituents. Because distillation cannot use to separate mixtures into their constituents. Because distillation cannot sep

separaarate te the the conconstistituetuents nts of of an an azeazeotrotropeope, , the the sepseparaaratiotion n of of azeazeotrotropiopicc mi

mixtxturures es (a(alslso o cacalllled ed azazeoeotrtropope e brbreaeakikingng) ) is is a a totopipic c of of coconsnsidideraerablblee interest. Indeed this difficulty led some early investigators to believe that interest. Indeed this difficulty led some early investigators to believe that azeotropes were actually compounds of their constituents. But there are two azeotropes were actually compounds of their constituents. But there are two

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reasons for believing that this is not the case. One is that the

reasons for believing that this is not the case. One is that the molar molar ratio of ratio of the constituents of an azeotrope is not generally the ratio of small integers. the constituents of an azeotrope is not generally the ratio of small integers. For example, the

For example, the azeoazeotrope formed by trope formed by water andwater and acetonitrileacetonitrile contains 2.253contains 2.253 moles of acetonitrile for each mole of water. A more compelling reason for moles of acetonitrile for each mole of water. A more compelling reason for believing that azeotropes are not compounds is, as discussed in the last believing that azeotropes are not compounds is, as discussed in the last section, that the composition of an azeotrope can be affected by pressure. section, that the composition of an azeotrope can be affected by pressure. Contrast that with a true compound, carbon dioxide for example, which is Contrast that with a true compound, carbon dioxide for example, which is two moles of oxygen for each mole of carbon no matter what pressure the two moles of oxygen for each mole of carbon no matter what pressure the gas is observed at. That azeotropic composition can be affected by pressure gas is observed at. That azeotropic composition can be affected by pressure suggests a means by which such

suggests a means by which such a mixture can be separated.a mixture can be separated.

Azeo

Azeotropi

tropic D

c D ist

istilla

illation

tion

Other methods of separation involve introducing an additional agent, called Other methods of separation involve introducing an additional agent, called an

an EnEntrtraiainenerr, , ththat at wiwill ll afaffefect ct ththee volatilityvolatility oof f oonne e oof f tthhe e aazzeeoottrrooppee constituents more than another. When an entrainer is added to a binary constituents more than another. When an entrainer is added to a binary azeotrope to form a

azeotrope to form a ternary azeotrope, and the resulting mixture distilled, theternary azeotrope, and the resulting mixture distilled, the method is called azeotropic distillation. The best known example is adding method is called azeotropic distillation. The best known example is adding benzene

benzene or or cyclohexanecyclohexane to the to the water/ethanol azeotrope. Wwater/ethanol azeotrope. With cyclohexane asith cyclohexane as the entrainer, the ternary azeotrope is 7% water, 17% ethanol, and 76% the entrainer, the ternary azeotrope is 7% water, 17% ethanol, and 76% cyclohexane, and boils at 62.1°C. Just enough cyclohexane is added to the cyclohexane, and boils at 62.1°C. Just enough cyclohexane is added to the water/ethanol azeotrope to engage all of the water

water/ethanol azeotrope to engage all of the water into the ternary azeotrope.into the ternary azeotrope. When the mixture is then boiled, the azeotrope vaporizes leaving a residue When the mixture is then boiled, the azeotrope vaporizes leaving a residue composed almost entirely of the excess ethanol.

composed almost entirely of the excess ethanol.

Chemical Action Separation

Another type of entrainer is one that has

Another type of entrainer is one that has a strong chemical affinity for one of a strong chemical affinity for one of the constituents. Using again the example

the constituents. Using again the example of the water/ethanol azeotrope, theof the water/ethanol azeotrope, the liquid can be shaken with

liquid can be shaken with calcium oxidecalcium oxide, which reacts strongly with water to, which reacts strongly with water to fo

form rm ththee nonvolatilenonvolatile compound,compound, calccalcium ium hydrohydroxidexide. . NNeeaarrlly y aalll l oof f tthhee calcium hydroxide can be separated by

calcium hydroxide can be separated by filtrationfiltration and theand the filtratefiltratere distilled tore distilled to obtain nearly pure ethanol.

obtain nearly pure ethanol.

A more extreme example is the azeotrope of 1.2% water with 98.8%

A more extreme example is the azeotrope of 1.2% water with 98.8% diethyldiethyl ether

ether . Ether holds the last bit of water so tenaciously that only a very. Ether holds the last bit of water so tenaciously that only a very powerful

powerful desiccantdesiccant such assuch as sodiumsodium metal added to the liquid phase can metal added to the liquid phase can resultresult in completely dry

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Anhydrous

Anhydrous calcium chloridecalcium chloride is used as is used as a desiccana desiccant for t for dryindrying a g a wide variewide varietyty of

of sosolvlvenents ts sisincnce e it it is is ininexexpepensnsivive e anand d dodoes es nonot t rereacact t wiwith th momostst nonnon aqueous

aqueous solvents.solvents. ChloroformChloroform iis s aan n eexxaammpplle e oof f a a ssoollvveennt t tthhaat t ccaan n bbee effectively dried using calcium chloride.

effectively dried using calcium chloride.

Distillation using a Dissolved Salt

When a

When a saltsalt is dissolved in a solvent, it always has the effect of raising theis dissolved in a solvent, it always has the effect of raising the boiling point of that solvent - that is it decreases the

boiling point of that solvent - that is it decreases the volatilityvolatility of the solvent.of the solvent. When the salt is readily soluble in one constituent of a mixture but not in When the salt is readily soluble in one constituent of a mixture but not in another, the volatility of the constituent in which it is soluble is decreased another, the volatility of the constituent in which it is soluble is decreased an

and d ththe e ototheher r coconstnstititueuent nt is is ununafaffefectcteded. . In In ththis is wawayy, , fofor r exexampamplele, , it it isis p

posossisiblble e to to brbreaeak k ththe e wawateter/r/etethahanonol l azazeoeotrotrope pe by by didissssololvivingng potassium potassium acetate

acetate in it and distilling the result.in it and distilling the result.

Examples of

Examples of azeotrope

azeotropess

Proportions are by weight Proportions are by weight::

•

• nitnitric ric aciacidd (6(688%) %) // water water , , boboilils s aat t 12120.0.55°C °C at at 1 1 atatm m (n(neegagattivivee

azeotrope) azeotrope)

•

• perchloric acid perchloric acid (28.4%) / water, boils at 203°C (28.4%) / water, boils at 203°C (negative azeotrope)(negative azeotrope) •

• hydrhydrofluoriofluoric c acidacid (3(35.5.6%6%) ) / / wwataterer, , boboilils s at at 11111.1.3535°C °C (n(negegatativivee

azeotrope) azeotrope)

•

• ethanolethanol (96%) / water, boils at 78.1°C(96%) / water, boils at 78.1°C •

• sulfuric acidsulfuric acid (98.3%) / water, boils at 338°C(98.3%) / water, boils at 338°C •

• acetoneacetone // methanolmethanol // chloroformchloroform form an intermediate boiling (saddle)form an intermediate boiling (saddle)

azeotrope azeotrope

•

• diethyl ether diethyl ether (33%) /(33%) / halothanehalothane (66%) a mixture once commonly used(66%) a mixture once commonly used

in

in anaesthesiaanaesthesia..

•

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Complex Azeotr

Complex Azeotrope

ope Systems

Systems

The rules for positive and negative azeotropes apply to all the examples The rules for positive and negative azeotropes apply to all the examples di

discscusussesed d so so fafarr. . BuBut t ththerere e are are sosome me exexampampleles s ththat at dodon'n't t fifit t ininto to ththee categories of positive or negative azeotropes. The best known of these is the categories of positive or negative azeotropes. The best known of these is the tteerrnnaarry y aazzeeoottrrooppe e ffoorrmmeed d bby y 3300%% acetoneacetone, , 4477%%chloroformchloroform, , aanndd 23%

23% methanolmethanol, which boils at 57.5°C. Each pair of these constituents forms, which boils at 57.5°C. Each pair of these constituents forms a

a binabinary ry azeotazeotrope, rope, but but chlorchloroform/oform/methamethanol nol and and acetoacetone/mene/methanothanol l bothboth fo

form rm poposisititive ve azazeoeotrtropopes es whwhilile e chchlolororofoform/rm/acacetetonone e foforms rms a a nenegagatitiveve azeotrope. The resulting ternary azeotrope is neither positive nor negative. azeotrope. The resulting ternary azeotrope is neither positive nor negative. Its boiling point falls between the boiling points of acetone and chloroform, Its boiling point falls between the boiling points of acetone and chloroform, so it is neither a maximum nor a

so it is neither a maximum nor a minimum boiling point. This type of systemminimum boiling point. This type of system is called a

is called a Saddle AzeotropeSaddle Azeotrope. Only systems of three or more constituents. Only systems of three or more constituents can form saddle azeotropes.

can form saddle azeotropes.

A rare type of complex binary azeotrope is one where the boiling point and A rare type of complex binary azeotrope is one where the boiling point and condensation point curves touch at two points in the phase diagram. Such a condensation point curves touch at two points in the phase diagram. Such a sy

syststem em is is cacalllled ed a a dodoububle le azazeoeotrtropope, e, anand d wiwill ll hahave ve twtwo o azazeoeotrtropopicic ccoommppoossiittiioonns s aannd d bbooiilliinng g ppooiinnttss. . AAn n eexxaampmplle e iis s wwaatteer r aannd d N N--methylethylenediamine.

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Steam Distillation

Ste

Steam am didiststilillalatition on is is a a spspececiaial l tytype pe of of distillationdistillation (a(a separaseparation tion procprocessess)) for temperature sensitive materials like natural

for temperature sensitive materials like natural aromaticaromatic compounds.compounds. Many

Many organic compoundsorganic compounds tend totend to decomposedecompose at high sustained temperatures.at high sustained temperatures. Separation by normal distillation would then not be an option, so water Separation by normal distillation would then not be an option, so water or

or steamsteam is introduced into the distillation apparatus. By adding water or is introduced into the distillation apparatus. By adding water or steam, the

steam, the boiling points boiling points of the compounds are depressed, allowing them toof the compounds are depressed, allowing them to evaporate at lower temperatures, preferably below the temperatures at which evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable. If the substances to be the deterioration of the material becomes appreciable. If the substances to be distilled are very sensitive to heat, steam distillation can also be combined distilled are very sensitive to heat, steam distillation can also be combined with

with vacuum distillationvacuum distillation. After distillati. After distillation the vapors are on the vapors are condensed as usual,condensed as usual, usually yielding a

two-usually yielding a two- phase system phase system of water and the organic compounds,of water and the organic compounds, allowing for simple separation.

allowing for simple separation.

Principle

When a mixture of two practically

When a mixture of two practically immiscibleimmiscible liquids is heated while beingliquids is heated while being agitated to expose the surfaces of both the liquids to the vapor phase, each agitated to expose the surfaces of both the liquids to the vapor phase, each const

constitueituent nt indeindependependently ntly exerexerts ts its its ownown vapovapor r pressupressurere as a function of as a function of temperature as if the other constituent were not present. Consequently, the temperature as if the other constituent were not present. Consequently, the vapor pressure of the whole system increases. Boiling begins when the sum vapor pressure of the whole system increases. Boiling begins when the sum o

of f tthhee pa partirtial al prepressussuresresof of ththe e twtwo o imimmimiscsciiblble e lliiqquiuids ds jjuust st exexceceededss the

the atmospheric pressureatmospheric pressure (approximately 101 kPa at sea level). In this way,(approximately 101 kPa at sea level). In this way, many organic compounds insoluble in water can be

many organic compounds insoluble in water can be purified at a temperaturepurified at a temperature wel

well l belbelow ow the the poipoint nt at at whiwhich ch decdecompomposiositiotion n occoccursurs. . For For exaexamplmple, e, thethe boiling point of

boiling point of bromobenzene bromobenzene is 156 °C and the boiling point of water isis 156 °C and the boiling point of water is 100 °C, but a mixture of the two boils at 95 °C. Thus, bromobenzene can be 100 °C, but a mixture of the two boils at 95 °C. Thus, bromobenzene can be easily distilled at a temperature 61 C°