10 Introduction to Complex Distillation Methods

8/11/2019 10 Introduction to Complex Distillation Methods

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Chapter 10

Introduction to Complex

Distillation Methods

In the preceding chapters concerning distilla-

tion (chapters 2-7), we have learned how to sepa-

rate binaryand multi-componentmixtures by the

rather simpledistillation systems (either continu-

ousor batch)

Unfortunately, however, the knowledge of such

simple distillation systemsonly is normally not

enoughfor the industrial-scaledistillation, as

the simpledistillation systems are notcapableof,

e.g.,


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completelyseparating azeotropic mixtures

separatingthe mixtureswhen the relative

volatilityis close to unity (1)[in other

words, when the boiling temperaturesof

the componentsin the mixturesare

relatively closeto each other/one another];

note that if we try to do so, the distillation

costcould be very expensive

To overcomesuch problems/complications,

the complexdistillation systems/techniques must

be employed


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Such complexdistillation methods include

azeotropicdistillation

pressure-swing or two-pressuredistillation

extractivedistillation

reactivedistillation

The details of these complexdistillation tech-

niques are as follows

10.1Breaking Azeotropes with Other Separators

When the mixture becomes azeotropic(i.e.

the mixture behaves as it is the pure substance),

it limitsthe separation by distillation(i.e.it

cannot be separated from each other/one

another by distillation)


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For example, when the mixture of ethanol

(EtOH) and waterreaches the concentrationof

0.8943mole fraction of EtOH, it can no longer

be separatedby the simple/ordinarydistillation

To breakthe azeotrope, or to getthe mixture

with the mole fractionof EtOHhigher than

0.8943, how would we do?

Lets consider the simplest, but less likelyto

be used, technique, illustrated in Figure 10.1


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By employing this technique, the distillate(or

the mixture of A and B) whose concentrationis

close/nearto the azeotropic point from thefirst

distillation columnis sent to another separation

device, which is able to yield pureA

Figure 10.1: The distillation column coupled

with another separating device for braking the

azeotrope

(from Separation Process Engineering by Wankat, 2007)


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Note that the waste streamin Figure 10.1 is

the streamthat contains the mixtureof the sepa-

ratingcomponents (e.g., the mixture of A + B)

However, some questions arise:

If the other techniqueor the other separa-

tion deviceis more effectivethan the dis-

tillationin separating the mixture, why do

we use the distillation in the first place?

or why dont we use the other technique

in the first place?

What/how would we do with the wastestream (i.e.the mixture of separating

components)?


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Lets consider another technique, which is

more likelyto be employed, shown in Figure 10.2

Figure 10.2: The distillation column coupled

with another separating device in which the

bottom product of the other separating deviceis recycled back to the distillation column



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In this separating scheme, the distillateat or

near the azeotropicconcentration is sent to the

other separation device(s), in which the waste

stream from the other separation device(s) is sent

back to the distillation column

By employing this technique, thefirstdistilla-

tion column operates as a two-feedcolumn (i.e.

thefeedand the recycle stream)

This arrangementis commonly usedin indus-

try

A more complexsystem used to breakan

azeotropeor azeotropesis as illustrated in Figure

10.3


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Figure 10.3: The complex separation system, com-

prising the distillation column and another sepa-

rating device, used for breaking azeotrope(s)(from Separation Process Engineering by Wankat, 2007)

The product from the additional separating

device(s) is returned to the distillation column as

a reflux, in which1o

x y>


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When the refluxhas higher concentration( )ox

thanthe azeotropic distillate( )1y , as depicted in

the McCabe-Thiele diagram in Figure 10.3, the

distillationcan continueuntil the desired concen-

trationof A(or even pureA) is obtained

10.2Binary Heterogeneous Azeotropic

Distillation Processes

Surprisingly, the presenceof anotherazeotro-

picmixture can be used to separate the existing

azeotropicsystem, especially when the two azeo-

tropic systems are heterogeneousto each other

Such technique is called the azeotropicdis-

tillation


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Azeotropicdistillation is performed by adding

a solventor an entrainerto the system; this

entrainerformsan azeotropic mixturewith one

or both of the components

An example of the rather simple(although

not common) azeotropicdistillation is the distil-

lation of n-butanoland water

When the vapour that comes out of the top

of the distillation column with the concentration

of azy (this is an azeotropicconcentration) is con-

densed by a condenser, it is separatedinto two

separate liquidphases:


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one is the water-richphase (the aphase)

another one is the organic(i.e.n-butanol)

phase (the bphase)

as shown in Figure 10.4; note that each phase has

the samevapour phaseconcentration of n-buta-

nol,B

y , but different liquid phaseconcentrations

( )0.01 0.02 and 0.41 0.42B Bx xa b

- -

The distillation system used for this binaryheterogeneous azeotropic system is as depicted in

Figure 10.5

In this distillation system, the column 1is

the strippingcolumn receiving the a(water-rich)

phase with the concentration ofB

xa

from the

separator


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Figure 10.4: The heterogeneous azeotropic system

of n-butanol and water


The McCabe-Thiele diagram for the 1stcolumn

is on the left hand side(LHS) of Figure 10.6; note

that, for the 1stcolumn, n-butanol is the more

volatilethan water; thus, the bottom productcan

be pure water(i.e., bot

0B

x )


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Figure 10.5: The distillation system for the

heterogeneous binary azeotrope


Another liquid phase, or theb

(organic) phaseis sent to the column 2 (the 2ndcolumn), which is

also a strippingcolumn


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Figure 10.6: The McCabe-Thile diagram for the

two-column distillation system for the heteroge-

neous binary azeotrope


On the contrary to the 1stcolumn, in the 2nd

column, n-butanol is the less volatilecomponent,

as it is evident that the equilibriumline is under

the operatingline


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This phenomenoncan occursince a solvent

or an entrainerwith a highboiling temperature

that can dissolveonly n-butanol is addedto the

distillationsystem

As a result, the bottomstream of the 2ndco-

lumn can have a very high concentration(or even

pure) of n-butanol

Another commercialexample of breaking aze-

otrope using the entraineris the addition of

benzene(i.e.the entrainer) into the mixture ofethanol(EtOH) and water


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10.3Pressure-swing or Two-pressure Distillation

As we have learned previously, the changes in

both temperature and pressureaffect the vapour-

liquid equilibrium(VLE)

The changes in temperature and pressurecan

also alter() the compositionof the azeo-

trope

For example, at 1 atm (760 mm Hg), the aze-

otropicconcentration of the ethanol-watermix-ture is at 0.8943mole fraction of ethanol, as

mentioned previously


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However, at the pressure below70 mm Hg,

the azeotropedisappearscompletely

Note that, since, the distillationis commonly

operatedat a constant pressure, it is more con-

venientto alterthe systems pressurethan the

temperature

To appreciate () how the pressure-swing

or the two-pressuredistillation works, lets con-

sider the pressure-swingdistillation system, which

is used to separate the mixture of methyl ethylketone (MEK) and water, shown in Figure 10.7


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Figure 10.7: The pressure-swing distillation

system for breaking the azeotrope


The column 1 (or the 1

st

column) operates at1 atm, and the azeotropic concentrationof the

mixture is 35 wt% waterand 65% MEK; note

that, at 1 atm, MEKis more volatilethan water


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If thefeedto the 1stcolumnhas a concentra-

tion of MEK lower than65%, which is lower than

the azeotropicconcentration,

the distillatecan have as high concentra-

tionof MEKas 65%(i.e.at the azeotropic

concentration)

the bottomcan be the mixturewith a very

high concentrationof water(or even pure

water)

The relative volatilityof MEKand water

changesafter the pressurereaches ~6.8 atm(or

~100 psia)


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At the pressurehigher than~6.8 atm, water

becomes more volatilethan MEK

Additionally, the azeotropicconcentration at

the pressureof ~6.8 atmchanges from 35% water

(at 1 atm) to 50% water

To obtain pureMEK, the distillate from the

1stcolumn, which contains the mixture of 35%

water and 65% MEK, is sent to the column 2(or

the 2ndcolumn) operating at ~6.8 atm

Since, at this pressure (~6.8 atm), wateris

more volatilethan MEK, and the azeotropic con-

centration is at 50% water and 50% MEK,


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the distillateof the 2ndcolumn can be the

mixture with the concentrationof waterof

as high as 50%

the bottomof the 2ndcolumn can be the

mixture with a very high concentrationof

MEK(or even pureMEK)

10.4Extractive Distillation

Extractive distillationis another complexdis-

tillation system used for separatingthe azeotropic

mixture(s)


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In the extractivedistillation, a solventis added

to the distillation column such that only one com-

ponent (e.g., B from the mixture of A + B) is

attracted to it

If the solventhas a highboiling point, the

volatilityof the mixtureof the solventand species

Bis reduced, until it is lowerthan that of species

A

Accordingly, species Abecomes the more vo-

latilecomponent (than the mixture of species B+ solvent); thus, it can easily be removed from

the distillation system as the distillate


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An exampleof extractive distillation is as illu-

strated in Figure 10.8

Figure 10.8: An extractive distillation system


In this system, the solventwith a high boiling

pointis added to the 1stcolumn several stages

abovethefeed stageand a few stages belowthe

topof the column


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In the topsection (i.e.the section abovethe

solvent feeding point), species Ais removedfrom

the column as the distillate; this is because the

solvent, which is a high boiling temperaturesub-

stance, is mixedwith species B, and the resulting

mixturehas a very high boiling temperaturerela-

tive to that of species A

In the middle section(i.e.the section from

thefeeding pointof the solventto thefeed stage),

the solventseparatesspecies Bfromthe mixture

of A + B


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It is important to note that the amountof

the solventmust be large enoughto completely

separatespecies Bfromspecies A, otherwise, the

distillatewill notbe pureA, as species Bcan

forman azeotropic mixture withspecies A

In the bottom section(i.e.the section below

thefeed stage), since the mixtureof the solvent

and species Bis less volatilethan species A, the

mixtureof species Band the solventis removed

from the 1stcolumn as the bottom

The solventcan then be separatedfrom spe-

cies Bin the 2nddistillation column


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If the solventis chosenproperly, it should

easilybe removedfrom species Bby the simple

distillation column with onlyafewequilibrium

stages

The solventobtained from the 2ndcolumn can

be recycledto the 1stcolumn to be usedas the

solventagain

Note that, generally, the extractivedistillation

is notapplicablefor separating the mixtureof

isomers


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10.5Azeotropic Distillation with Added Solvent

When a homogeneous azeotropeis formed,

the proceduresused for heterogeneousazeotropic

distillation cannotbe employed

In order to solvethis problem, the solvent(or

an entrainer) thatformsa binaryor ternary

azeotropeis addedto the systemto enablethe

separationof the mixture

An example of the azeotropic distillationwithadded solventis as illustrated in Figure 10.9


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Figure 10.9: The separation of butadiene from

butylenes using ammonia as an entrainer


At the temperature of 40 oC, the azeotropeof

the mixtureof butadieneand butyleneexiting the

1stcolumn as the top product is homogeneous,

and, as a result, very difficultto be separated

from each other; note that, as butadieneis the

less volatilecomponent, it leavesthe 1stcolumn

purely(or nearly purely) as the bottom


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Ammoniais addedto the distillation column

either asa refluxor asafeed

The mixtureof butadiene + butylenes + am-

monia is condensedat the condenser

At a temperature below20 oC(note that, the

colderthe temperatureof the settler, the purer

the twoliquid phases), twoliquidphasesare

formed atthe liquid-liquid settler:

the upperphase: contains mainlybutylene

and ammonia(with a small amount of

butadiene)

the lowerphase: contains mainlyammonia,

with small amounts of butadiene and buty-

lene


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The lowerphase is returnedto the distillation

column (to be usedas the recycle entrainer),

while the upperphase is sent to the subsequent

strippingcolumn

In the strippingcolumn, the mixtureof buty-

leneand ammoniacan be distilled and thus sepa-

rated into

the topproduct (the distillate), which isthe azeotropicmixture of butyleneand

ammonia

the bottomproduct (the bottom), which

can be purebutylene

The distillateof the stripping column is recy-

cledto the liquid-liquid settler


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Another example of the azeotropic distillation

with the added solventis as shown in Figure 10.10

In Figure 10.10, species Aand the solvent(or

the entrainer) form the azeotropewith a low

boiling point; thus leavingthe distillationcolumn

as the distillate

Species B, whose boiling temperatureis higher

than the mixture of species A and the solvent (in

other words, species B is a less volatilecompo-

nent) can be removedfrom the column as pureBas the bottom


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Figure 10.10: An azeotropic distillation with a

low boiling temperature azeotrope


Species Acan be removedfrom the solventby a water-wash extraction


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The additionaldistillation column(on the

RHS of Figure 10.10) is used to separatethe sol-

ventfrom water

The thirdexample is commonly used for sepa-

rating ethanol(EtOH) from water, using a hydro-

carbonas an entrainer, as depicted in Figure

10.11

Previously, benzeneis used as the solvent

(entrainer); however, because of its toxicity (as

it is a carcinogenicagent), it has been replacedby some other hydrocarbons, e.g., diethyl ether,

n-pentane, or n-hexane


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Figure 10.11: A ternary azeotropic distillation

for separating ethanol from water


The mixture of EtOH and water at nearly

azeotropic concentration(~70-90% EtOH) is fed

into the azeotropicdistillation column, in which

the hydrocarbon solvent/entraineris added to

the column as a reflux


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The vapour distillateis condensedand then

separatedinto twoliquid phases:

the upperlayer or the solventlayer

the waterlayer

The compositionof each layerdepends onthesolventused

For example, if n-hexaneis used,

the upperlayer contains 96.9 wt% hexane,

2.9% EtOH, and 0.5% water

the loweror waterlayer contains 6.2%

hexane, 73.7% EtOH, and 20.1% water

The upperlayer is returnedto the azeotropic

columnas a refluxto be used as the recycle en-

trainer


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The lowerlayer (with a small amount of

n-hexane, which is used to break the azeotrope)

is sentto the stripping column, to obtain pure

wateras the bottom

The additionof solvent(e.g., n-hexane) into

the azeotropiccolumn (the LHScolumn) makes

the azeotropicmix-ture of EtOH + water and

the solvent becomemore volatilethan EtOH

Thus, EtOH(which is now less volatile) can

be removed purelyas the bottom

10.6Reactive Distillation

The advantageof having the reactivedistilla-

tion system or the distillation witha chemical


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reactionis that the product(s)of the reactioncan

be removedcontinuously bythe distillation, which

drives the reversiblereaction to completion

The arrangement of the distillation system for

the reversiblereactions:

A C

or A C + D

is as depicted in Figure 10.12

If the reactantis less volatilethan the pro-

duct(s), the arrangementon the LHS(A) is ap-propriate; note that the bottom bleedis required

to preventthe build-upof non-volatileimpurities


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Figure 10.12: The reactive distillation system for

a single-reactant reaction


If the reactantis more volatilethan the pro-

duct(s), the arrangementon the RHS(B) is ap-

propriate; note that the top bleedis required to

re-move volatileor gaseouscomponents

The arrangement for the reaction:

A + B C + D


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in which the reactants are of intermediate volati-

lity between the two products (i.e.C and D), is

as shown in Figure 10.13

Figure 10.13: The reactive distillation system

with two reactants


Note also that the arrangements Cand Dare

widely usedfor the esterificationreaction:

Fatty acid + Alcohol Ester+ Water

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10 Introduction to Complex Distillation Methods

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