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A Lecture on DistillationA Lecture on Distillation
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DISTILLATIONDistillation is defned as:a process in which a liquid or vapourmixture o two or more sustances isseparated into its componentractions o desired purit!" ! theapplication and removal o heat#
Distillation is ased on the actthat the vapour o a oilin$ mixturewill e richer in the componentsthat have lower oilin$ points#
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DISTILLATION %&IN'I%L(Separation o components rom a liquidmixture via distillation depends on thedi)erences in oilin$ points o theindividual components# Also" dependin$on the concentrations o the componentspresent" the liquid mixture will have
di)erent oilin$ point characteristics# Thereore" distillation processes dependson the vapour pressure characteristics oliquid mixtures#
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&(LATI*( *OLATILIT+&elative volatilit! is a measure o the di)erences in
volatilit! etween , components" and hence theiroilin$ points# It indicates how eas! or di-cult aparticular separation will e# The relative volatilit!o component .i/ with respect to component .0/ isdefned as
!i 1 mole raction o component .i/ in the vapourxi 1 mole raction o component .i/ in the liquid
Thus i the relative volatilit! etween ,components is ver! close to one" it is an indicationthat the! have ver! similar vapour pressurecharacteristics# This means that the! have ver!similar oilin$ points and thereore" it will edi-cult to separate the two components viadistillation#
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'ONT222#It is the ratio o the concentrationratio o A 3 4 in $as phase to that
o liquid phase#
*)1(
)1(*
)1/(
*)1/(*
y x
x y
x x
y y
−
−
=
−
−
=α
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ID(AL SOL5TION 6ID(AL SOL5TION 6
&AO5LT/S LA7&AO5LT/S LA7
8or an ideal solution "the equiliriumpartial pressure p9 o a constituent
at a fxed temperature equals theproduct o its vapor pressure pwhen pure at this temperature and
its mole raction in liquid# %9A1 %A x
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4ASI' DISTILLATION4ASI' DISTILLATION
(5I%;(NT(5I%;(NTMAIN COMPONENT OF DISTILLATION COLUMN
Distillation columns are made up of severalcomponents eac! of "!ic! is used eit!er totransfer !eat ener#$ or en!ance material
transfer% A t$pical distillation contains severalma&or components'
a vertical s!ell "!ere t!e separation ofli(uid components is carried out%
column internals suc! as tra$s)plates and)or
pac*in# "!ic! are used to en!ancecomponent separations%
a re+oiler to provide t!e necessar$vapori,ation for t!e distillation process%
a condenser to cool and condense t!e
vapour leavin# t!e top of t!e column%
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'OL5;N &(4OIL(&S There are a numer o desi$ns o reoiler#It is e!ond the scope o this set o
introductor! notes to delve into theirdesi$n principles# <owever" the! can ere$arded as heat6exchan$ers that arerequired to transer enou$h ener$! to
rin$ the liquid at the ottom o thecolumn to oilin$ point# The ollowin$ areexamples o t!pical reoiler t!pes#
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T-PE OF DISTILLATIONT-PE OF DISTILLATION
COLUMNCOLUMN
T!ere are man$ t$pes ofdistillation columns eac!desi#ned to perform speci.c
t$pes of separations and eac!desi#n di/ers in terms ofcomple0it$%
One "a$ of classif$in# distillation
column t$pe is to loo* at !o" t!e$are operated% T!us "e !ave'
+atc! and
continuous columns%
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DIS'&I%TION1atc! Columns In atch operation" the eed
to the column is introduced atch6wise# Thatis" the column is char$ed with a =atch= andthen the distillation process is carried out#7hen the desired tas> is achieved" a nextatch o eed is introduced#
Continuous Columns In contrast"continuous columns process a continuouseed stream# No interruptions occur unlessthere is a prolem with the column or
surroundin$ process units# The! are capaleo handlin$ hi$h throu$hputs and are themost common o the two t!pes# 7e shallconcentrate onl! on this class o columns#
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T+%(S O8 'ONTIN5O5S
'OL5;N'ontinuous columns can e urtherclassifed accordin$ to:
T!e nature of t!e feed t!at t!e$ areprocessin#"
4inar! column 6 eed contains onl! two components
;ulti6component column 6 eed contains more thantwo components
T!e num+er of product streams t!e$!ave
;ulti6product column 6 column has more than twoproduct streams#
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'ONT2222##2!ere t!e e0tra feed e0its "!en it isused to !elp "it! t!e separation"
(xtractive distillation 6 where the extra eedappears in the ottom product stream
A?eotropic distillation 6 where the extra eedappears at the top product stream
T!e t$pe of column internals
Tra! column 6 where tra!s o various desi$ns areused to hold up the liquid to provide etter
contact etween vapour and liquid" hence etterseparation
%ac>ed column 6 where instead o tra!s"=pac>in$s= are used to enhance contact etweenvapour and liquid
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'OL5;N
INT(&NALS
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T&A+S AND %LAT(S
The terms @tra!s@ and @plates@ are usedinterchan$eal!# There are man! t!pes otra! desi$ns" ut the most common onesare :
1u++le cap tra$sA ule cap tra! has riser or chimne! fttedover each hole" and a cap that covers theriser# The cap is mounted so that there is a
space etween riser and cap to allow thepassa$e o vapour# *apour rises throu$h thechimne! and is directed downward ! the cap"fnall! dischar$in$ throu$h slots in the cap"and fnall! ulin$ throu$h the liquid on the
tra!#
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BUBBLE CAP TRAY
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'ONT22223alve tra$s
In valve tra$s perforations are covered +$lifta+le caps% 3apour 4o"s lifts t!e caps t!usself creatin# a 4o" area for t!e passa#e ofvapour% T!e liftin# cap directs t!e vapour to4o" !ori,ontall$ into t!e li(uid t!usprovidin# +etter mi0in# t!an is possi+le insieve tra$s%
Sieve tra$sSieve tra$s are simpl$ metal plates "it! !olesin t!em% 3apour passes strai#!t up"ardt!rou#! t!e li(uid on t!e plate% T!earran#ement num+er and si,e of t!e !olesare desi#n parameters%
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SIEVE TRAY
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VALVE TRAY
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LI5ID6*A%O5& 8LO7
The next ew f$ures show the direction ovapour and liquid ow across a tra!" andacross a column#
(ach tra! has , conduits" one on each
side" called .downcomers/# Liquid allsthrou$h the downcomers ! $ravit! romone tra! to the one elow it# The owacross each plate is shown in the dia$ram
A weir on the tra! ensures that there isalwa!s some liquid BholdupC on the tra!and is desi$ned such that the the holdupis at a suitale hei$ht" e#$# such that theule caps are covered ! liquid#
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%A'INES
%ac>in$s are passive devices that aredesi$ned to increase the interacial area orvapour6liquid contact# The ollowin$ picturesshow F di)erent
These stran$el! shaped pieces are supposedto impart $ood vapour6liquid contact when aparticular t!pe is placed to$ether innumers" without causin$ excessive
pressure6drop across a pac>ed section# Thisis important ecause a hi$h pressure dropwould mean that more ener$! is required todrive the vapour up the distillation column#
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T+%(S O8 DISTILLATION There are di)erent t!pes odistillation#
'onventional distillationSteam Distillation
*acuum Distillation
8ractional DistillationA?eotropic Distillation
(xtractive Distillation
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ST(A; DISTILLATION
It is convenient technique or distillin$certain di-cult6to6handle ;aterials# Asollow6
'hemicals havin$ relativel! hi$h oilin$ pointsat atmospheric pressure#
'hemicals that are li>el! to decompose evenat their atmospheric oilin$ points# e#$# att!acids#
'hemicals that pose ha?ards when distilled !
the application o direct heat# e#$ Turpentine#Steam distillation is practiced in coal –
chemicals" ats and oils" petroleum"petrochemicals" paper industries etc#
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*A'55; DISTILLATIONIt is normall! reerred to those distillationwhich are carried out under vacuum#*acuum distillation has some enefts are asunder#
The volatilit! o the components $enerall! riseswith the lowerin$ o the oilin$ points# So thatrequired column hei$ht will e reduced# &equiredlower reux ratio#
%ol!meri?ation and thermal decomposition ocomponents can e prevented# The columntemperature is at its hi$hest at the ottomsection where product de$radation"pol!meri?ation can happen# So lower ottomtemperatures can $enerate less unwantedchemical chan$es#
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'ONT222## There are some or$anic compounds thatare thermall! unstale even attemperature near their atmosphericoilin$ points#
Separation under vacuum u!s aneconom! in reoiler ener$!
requirement#So less costl! ener$!sources li>e low pressure steam or wasteheat steam or hot water can e used
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%A'INES *s T&A+S%A'INES *s T&A+SA tra! column that is acin$ throu$hputprolems ma! e de6ottlenec>ed !replacin$ a section o tra!s with pac>in$s#
This is ecause:pac>in$s provide extra inter6acial area or liquid6
vapour contacte-cienc! o separation is increased or the samecolumn hei$ht
pac>ed columns are shorter than tra!ed columns
%ac>ed columns are called continuous6contact columns while tra!ed columns arecalled sta$ed6contact columns ecause othe manner in which vapour and liquid arecontacted#
8A'TO& A88('TINE
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8A'TO& A88('TINEDISTILLATION O%(&ATION
The perormance o a distillation column is determined! man! actors" or example:
eed conditions
state o eed
composition o eed
trace elements that can severel! a)ect the *L( oliquid mixtures
internal liquid and uid ow conditions
state o tra!s Bpac>in$sC
weather conditions
Some o these will e discussed elow to $ive an ideao the complexit! o the distillation process#
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DI88I'5LTI(S INDISTILLATION 'OL5;N
8or sta$e or tra! e-cienc! to e hi$h the time ocontact should e lon$ to permit the di)usion# Inorder to provide lon$ contact time the liquid pool
on each tra! should e deep#so that ules o$as will required a lon$ time to rise throu$h theliquid#
4ut when $as ule onl! slowl! throu$h theopenin$ on the tra! the ule are lar$e 3 the
interacial surace is small 3 much o it ma! evenpass over the tra! without havin$ contact the $as#
So" or hi$h tra! e-cienc! we required deep poolo liquid 3 hi$h $as velocities#
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'ont222#
This condition however leads tonumer o di-culties#
oodin$%rimin$Boamin$C
entrainment
weepin$Gdumpin$'onin$
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(NT&AIN;(NT
7hen the $as velocities is relativel! hi$h" itdispersed ver! thorou$hl! into liquid whichin turn is a$itated into roth#this provide
lar$e interacial surace area# 4ut at hi$h$as velocities" when the $as is disen$a$edrom the roth small droplets o liquid wille carried ! the $as to the tra! aove#
This is called entrainment# This will reducethe concentration chan$e rou$ht aout !the mass transer which lead to decreasethe tra! e-cienc!#
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8LOODINEEreat liquid depth 3 hi$h $as velocities othresult in hi$h pressure drop or $as " owin$throu$h the tra!#
7ith the lar$e pressure di)# In the space
etween the tra!" the level o liquid leavin$ thetra! at relativel! low pressure#
As the pressure di)# Increase due to increaserate o ow o either $as or liquid the level indown spot will rise urther to permit the liquid to
enter the lower tra!# 8urther increase in ow ratethe liquid will fll the entire space etween thetra!s#the tower is then ooded#
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%&I;INE8or liquid $as comination which tend tooam excessivel!" hi$h $as velocities ma!lead to primin$#
<ere the oam persists throu$hout thespace etween the tra! and the liquid iscarried ! the $as rom one tra! to anothertra!#the liquid so carried recirculatesetween tra!s 3 the added liquid6handlin$load increase the $as pressure drop to leadto oodin$#
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CONIN5 2EEPIN5 6CONIN5 2EEPIN5 6
DUMPIN5DUMPIN5
I liquid rates are to low" the $as risin$throu$h the openin$s o the tra! ma!push the liquid awa! B'onin$C and contact
o $as 3 liquid is poor#I the $as rate is to low" much o the liquidma! rain down throu$h the openin$s othe tra!B7eepin$C thus to ailin$ to otain
o complete ow over the tra!#At ver! low $as ow rate none o liquidreaches the downspout#BDumpin$C
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