distillation process prensation

7/23/2019 Distillation Process Prensation

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A Lecture on DistillationA Lecture on Distillation



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#



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#



&(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#



'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



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



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%



'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#



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%



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#



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#



'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



'OL5;N

INT(&NALS



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!#



BUBBLE CAP TRAY



'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%



SIEVE TRAY



VALVE TRAY



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#



%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#



T+%(S O8 DISTILLATION There are di)erent t!pes odistillation#

'onventional distillationSteam Distillation

*acuum Distillation

8ractional DistillationA?eotropic Distillation

(xtractive Distillation



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#



*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#



'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



%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



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#

http://lorien.ncl.ac.uk/ming/distil/distilvle.htm

http://lorien.ncl.ac.uk/ming/distil/distilvle.htm



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#



'ont222#

This condition however leads tonumer o di-culties#

oodin$%rimin$Boamin$C

entrainment

weepin$Gdumpin$'onin$



(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!#



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#



%&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 $as pressure drop to leadto oodin$#



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

distillation process prensation

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