mass transfer - distillation final 20.1.2016.pptx

7/24/2019 Mass transfer - Distillation final 20.1.2016.pptx

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Distillation


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Boiling Point - Composition

Curve for Cyclohexane-Toluene

mixture


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11/66

VLE An Azeotrope

Ethyl acetate Ethanol VLE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa (EA liquid)

Ya(EAvapo

ur)

EA and Eth form an Azeotrope at55% EAAn azeotrope is formed when

the liquid and vapourcompositions are the same

Separation b conventionaldistillation is not possible

Dewpoint and bubble point

are the same at theazeotrope

!suall occurs at a particularmole fraction" #utside thispoint separation is possible

$an have a minimum ormaimum boilin& pointazeotrope

$an limit the separation andpurit of the product

$han&in& the pressure can bethe solution Etractive


12/66

Ethyl acetate Ethanol VLE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa (EA liquid)

Ya(EAvapour)

Ethyl acetate Ethanol VLE (T-x-y)

70

71

72

73

74

75

76

77

78

79

80

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa, Ya (EA)

TempC

Bubble

Dew

'in () Azeotrope EA Ethanol


13/66

ater !ormic Acid VLE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa ("#$ liquid)

Ya("#$

vapour)

ater !ormic Acid VLE (T-x-y)

99

100

101

102

103

104

105

106

107

108

109

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa, Ya ("#$)

TempC

Bubble

Dew

#ther azeotropic mitures include *water + nitric acid, *water +hdrochloric acid, and man *water alcohols,

'a () Azeotrope + -ater.ormic Acid


14/66

/f equilibrium data are not available )artial )ressure0 Dalton and 1aoult

Dalton2s Law

)a3 a)

)ais the partial pressure0 ais the vapour mole fraction and ) is the totalpressure

1aoult2s law applies to an ideal miture

)a3 )oaa )

oa is the vapour pressure0 a is the liquid mole fraction

-e assume we are dealin& with ideal mitures"

$ombinin& Dalton and1aoult4

.rom 1aoult2s law and Dalton2s law0 we have

6herefore0 if we 7now the vapour pressure we can calculate the molefractions of the liquid and vapour phases

8ou can then plot an + dia&ram or a 6++ dia&ram" 1emember0 this isfor constant pressure onl"

T

a

o

aa

P

xPy =o

b

o

a

o

bT

aPP

PPx

=


15/66

Temperatu

re

(%C)

Vapor

pre&&ure

(mm"')

0 4.5

5 6.8

10 9.0

20 17.3

30 31.5

40 55.5

50 92.3

60 149.3

70 234.0

80 354.9

90 525.8

100 760.0

Water 'ethanol


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17/66

9" .rom 1elative Volatilit

o

b

o

aba

P

P=,

6his is another wa to &et mole fractions" -e can determine therelative volatilit from the Vapour )ressures" !se the de:nition of

relative volatilit and 1aoult2s law to &et the followin&

is a function of V) which is a function of 6" ;/f ou need


18/66

Activit Eth ?9#Ethanol and -ater are separated b distillation" 6he 6++ dataare &iven as follows" )err is a source for data ;limited=

Temp Xa Ya

100.0 0.000 0.000

89.0 0.072 0.38985.3 0.124 0.470

82.7 0.234 0.545

81.5 0.327 0.583

79.8 0.508 0.656

79.3 0.573 0.684

Ethanol Wate

75

80

85

90

95

100

105

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Xa, Ya (Eth)

TempC

.ermentation &ives an ethanol conc of about @5%" -hathappens when the miture is boiled -hat is the hi&hest conc ofeth that can be achieved in this wa


19/66

S/')LE D/S6/LLA6/#>

6otal mole present when time t3 oBL

*

*

( ) ( )Lx y dL L dL x dx

Lx y dL Lx Ldx xdL dxdL

= +

= +

*

*

( )

( )

f

W

dL y x Ldx

dL dx

L y x

x x L F

x x L W

=

=

= =

= =


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.LAS? D/S6/LLA6/#>


22/66


23/66

Steam Distillation

Steam Distillation :

)ossible to distill an or&anic compound at much lower temp" At constant sstem pr")60steam lowers the partial C vapor pressure

of or&anic compound C its correspondin& boilin& pt" Due to immiscibilit of water 0it can be separated from product b

simple condensation C followed b decantin&"Application )uri:cation of heat sensitive material as an alternativeto


24/66

Azeotropic & Extractive Distillation.

Ver close boilin& mi can be separated economicall b this technique"

Solvent when added will increase the dierence between volatilities ofli&ht C heav component" 6he attraction of solvent to one of thecomponent reduces the volatilit of solvent C the component to whichitis attracted"


25/66

Solvent for Distillation should be>on+corrosive"Should not react with feed to form undesirable product">on+toic"Azeotropic solvent should have volatilit near the ma


26/66

Overheadvapor

Top stage

Totalcondenser

Rectifying section(Enriching section)

Stripping section(Exhausting section)

Feed

Bottomof column

Partial reboiler

Bottom stage

eed stage

Distillate

Bottomproduct

Topof column

Boilup


27/66

Applications: the most widely used large-scale method for separating homogeneous fluid mixtures

in the chemical and petrochemical industry

if no azeotropes are encountered, overhead and bottom products may be obtained in

any desired purity

suitable for the separation of liquid mixtures of components having similar boilingpoints into their individual components (at low relative volatility, but >,!"#

$quipment: %ray &olumns (stagewise contact

between the phases on individual

trays#

'aced &olumns (continuous contact bet-

ween the phases on the surface of a pac-

ing material#


28/66

Types of Binary Distillation Calculation

$nergy requirements and heat exchanger design for a given adiabatic separation process (calculate the

heat duties of the condenser and reboiler, specify the heating steam consumption and coolant

requirements, thermal-hydraulic design of the condenser and reboiler#

)etermination of main dimensions of the distillation column: estimating the number of equilibrium

stages required for a given separation, the column height and the column diameter for a desired

pressure drop (H* f(N#, D#

Historical Review of Calculation Methods

until +!s: simplified, partially graphical design procedures for tray columns separating binary

mixtures: Ponchon-Savarit(+.#, McCabe-Thiele(+"#

approximate calculation methods for the solution of multicomponent, multistage sepa-

ration problems (/hortcut methods#: Fenske(+0#, Gilliland(+1!#, Underwood(+12#

design of paced columns based on 3%4.5%4 concepts: Chilton, Colburn(+0"#

in the present: complex mathematical matrix methods allow to find exact solutions of nonlinear equation

systems: an!-Henke(+22#, Na"htali-Sandhol#(+#

commercial process simulation software allowing design and rating calculations of tray and

paced columns operating at steady or unsteady state conditions (A/'$3, &5$6&A)#


29/66

.undamentals of (inar DistillationAssumptions for an Approimate $alculation of the (inar

DistillationA&&umption& and impliication&*

1! The two "omponent# ha$e e%ual an& "on#tant mola enthalp'e# o( $apo')at'on *latent heat#!.

2! The "omponent heat "apa"'t+ "han,e# an& the heat o( m'-'n, ae ne,l','ble "ompae& to the heat o($apo')at'on *"on#'&e'n, '&eal beha$'ou o( b'na+ m'-tue#!.

3! The &'#t'llat'on "olumn the "on&en#e an& the ebo'le ae well 'n#ulate& #o that heat lo##e# to en$'onmentae ne,l','ble.

4! The pe##ue '# "on#tant thou,hout the "olumn no pe##ue &op o""u#.

The abo$e a##umpt'on# lea& to the concept o con&tant molar overlo+. Th'# appoa"h a##ume# that the

amount o( mole"ule# wh'"h e$apoate an& wh'"h "on&en#ate 'n ea"h #ta,e ae the #ame o neal+ the #ame.

That mean# that all l'%u'& an& $apo mola (low ate# 'n the e"t'(+'n, #e"t'on ae "on#tant an& that all l'%u'& an&

$apo mola (low ate# 'n the #t'pp'n, #e"t'on ae "on#tant but not the #ame a# tho#e 'n the e"t'(+'n, #e"t'on.

/uthe e%u'ement# ae

5! 'net'" an& potent'al ene,'e# ae ne,l','ble.

6! The &'#t'llat'on "olumn '# opeate& at "ont'nuou# #tea&+ #tate "on&'t'on#.

7! The #team# lea$'n, ea"h #ta,e ae a##ume& to be 'n $apol'%u'& e%u'l'b'um. The l'%u' an& $apo# ae

alwa+# at the' bubble po'nt# an& &ew po'nt# e#pe"t'$el+.


30/66


31/66

Basic Operation and Terminology

The $apou mo$e# up the "olumn an& a# 't

e-'t# the top o( the un't 't '# "oole& b+ a

"on&en#e. The "on&en#e& l'%u'& '# #toe& 'n a

hol&'n, $e##el nown a# the e(lu- &um.ome o( th'# l'%u'& '# e"+"le& ba" to the top

o( the "olumn an& th'# '# "alle& the e(lu-. The

"on&en#e& l'%u'& that '# emo$e& (om the

#+#tem '# nown a# the &'#t'llate o top

po&u"t.

"eat i& &upplied to the reoiler to 'enerate

vapour The &ource o heat input can e any

&uitale luid, althou'h in mo&t chemical

plant& thi& i& normally &team .n reinerie&, the

heatin' &ource may e the output &tream& o

other column& The vapour rai&ed in the

reoiler i& re-introduced into the unit at the

ottom o the column The liquid removed rom

the reoiler i& /no+n a& the ottom& product

or &imply, ottom&


32/66

%he mathematical-graphical McCabe-Thiele Methodcan be used to determine the number of ideal stages N needed for a givenseparation of a binary mixture (to produce a distillate and a bottom product # and column operating pressure7 %he methoduses material balances around certain parts of the column and the equilibrium curve7

Rectifying section including the total condenser:

V 3 L D0 L 3 V D0 D 3 V + L

Stripping section including the partial reoiler:

@

'c$abe+6hiele 'ethod for 6raed 6owers

n

n

x

L

1

1

+

+

n

n

y

Vplaten th

DxD,

V

LDLV

balancematerialOverall

nn +=

+1

11

1

11

1

11

++

+=

=+++=

+=

+=

+

++

+

++

R

xx

R

Ry

D

LRDL

DxxDL

Ly

V

DxxV

Ly

DxxLyV

AbalanceComponent

D

n

n

Dn

n

nn

n

D

n

n

n

n

Dnnnn

FxF,

1

1

+

+

m

m

y

V

1

1

+

+

m

m

x

L

platem

th

WxW,

WVL

balancematerialOverall

mm +=

+1

WL

WxxWL

Ly

WL

WxxWL

Ly

V

WxxV

Ly

WxyVxL

AbalanceComponent

m

W

m

m

m

W

n

m

m

n

m

W

n

m

m

m

Wmmmm

+

=

+

=

+=

+=

+

++

+

++

1

11

1

11


33/66

%he quantities of the liquid and the vapor streams change abruptly at the feed tray $since the feed may consist ofliquid, vapor, or a mixture of both7 %he feed-stage condition influences the change in slope of the operating lines:

a# 8eed is a saturated liquid b# 8eed is a saturated vapor

1S

SS

f @

f !@

f

f @

f + @

f

!"""""#"""""

sec

$"""""

%""""""

sec

&"""""")1(

'""""")1(

""""""

1"""""

W

D

WxxLyVWLV

tionlowerbalancematerialOver

DxLxVy

balanceAComponent

DLV

tionupbalancematerialOver

FVV

FVV

FLL

FLL

+=

=

+=

+=

=

+=

=

+=

( ) ( )

( )

xx

y

FxFxFy

invalue!LLVV"ub

WxDxLLxVVy

n#!ub!tracti

F

F

WD

+

=

+=

++=

11

1*""""")1(

+,

+"""""

!$

FxF,FxF,

L L

L2 L2

V V

V2

V2


34/66

'ossible feed-stage conditions:

a# /aturated liquid

b# /aturated vapor

c# /ubcooled liquid

d# /uperheated vapor

e# 'artially vaporized (liquid 9 vapor#

$ffect of thermal condition of feed

%

&'

FF&'

('

)'F @

@

"G F

"3 F

F G "G @"3 @

"H @

Eq" ;I=

!

!

!

F

F

F

F

F

h h % * * F + +

h h % * * + + F

h h % * * F + +

h h % * * + + F

h h h % * * * F + + + F

= = = + =

= = = = +

< > > + < < > +

< < < < < < + < < +

% % C C C C C

% % C C C C C

% % C C C C C

% % C C C C C

% % % C C C C C C C C

"lope

=

1

+=

=

)(

1

fbp TTC

ionvapouri$atofheatlatent%olal

vapour!aturatedtofeedofmoleconvertto&ner#yliuidcoldFor


35/66

;n +", 6c&abe and %hiele published an approximate graphical method for combining the equilibrium curve for a bi-nary system with operating lines to estimate the number of equilibrium stages required for a desired degree of separa-tion of the feed7

# )raw the equilibrium curve

# )raw the rectifying section operating line

a#


36/66


37/66

! Minimum Reflu% Ratio rmin

As the reflux ratio decreases from the limiting case of total reflux, the intersection of the two operating lines andthe %-line moves from the diagonal toward the equilibrium curve7 %he number of equilibrium stages requiredincreases because the operation lines move closer and closer to the equilibrium curve, thus requiring more andmore stairs to move from the top of the column to the bottom7 8inally, a limiting condition is reached when thepoint of intersection / is on the equilibrium curve and therefore an infinite number of stages is required7

%he minimum reflux ratio rmincan be determined graphically

from the ordinate intercept of the rectifying operating lineor by the Underwood equation (that can be applied to saturated-liquid feed, %* #:

The minimum reflu% ratio corresponds to the need for an infinite numer of stages at a minimum

oilup ratio& thus at minimum operating costs necessary for the separation(

min

N

r

(

)F @

@

,)% D)%

!min(% !minmin

D)(r

=+

%%

S

F&%

)inch)oint

J )rof" Dr" '" 1eppich J $onceptual Desi&n of Distillation0 Absorption and Strippin& Sstems J KM J


38/66

An distillation column must be operated between the twolimiting conditions of minimum reflux rminand total reflux,

with the corresponding number of equilibrium stages re-quired varying from infinity to the minimum number Nmin7

%he reflux ratio to be used for a new design should be theoptimum, the one for which the total annual cost Ctotof the

distillation, which is the sum of the installed capital andoperating costs, will be the least7

%he reflux ratio influences both the number of stages

required (and thus the installed capital cost Cca".r# andthe energy requirements (and thus the operating costs

Co"r#7

%he total cost must pass through a minimum at theoptimum reflux ratio, that frequently occurs in the range of

,!" ? rmin@ ropt@ ," ? rmin

at high energy costs at high costs of construction materials

A first estimate of the optimum reflux ratio can be obtainedfrom ropt* , ? rmin7

Cca"

Co"

Ctot

total installed annual capital cost

annual operating and maintenance cost

(heating and cooling costs#

total annual cost

rmin ropt

r

r=

&op ;r=

&tot;r=

&*NOa,


39/66

.or a non+ideal sstem0 where the molarlatent heat is no lon&er constant and where

there is a substantial heat of miin&0 thecalculations become much more tedious"

.or binar mitures of this 7ind a &raphical

model has been developed b 1!?E'A>>0)#>$?#>0 and SAVA1/60 based on the use ofan enthalp+composition chart"

/t is necessar to construct an enthalp+composition dia&ram for particular binarsstem over a temperature ran&e coverin&the two+phase vapor+liquid re&ion at the

pressure of the distillation"


40/66

6he followin& data are needed

@" ?eat capacit as a function oftemperature0 composition and pressure"

9" ?eat of miin& and dilution as a function of

temperature and composition"K" Latent heats of vaporization as a function

of composition and pressure ortemperature"

P" (ubble+point temperature as a function ofcomposition and pressure"

Enthalp of liquid


41/66

Enthalp+composition dia&ram

n

iim hh

/n Qre&ularR O ideal mitures

oiii hxh =

.or &aseous O vapor mitures atnormal 6 and )

n

iii

n

iim yhH

Enthalp of liquid

soBBAAmix

Hhxhxh ;K=

sorefB!BrefA!Amix H""#x""#xh

mimixmix hH

BBAAmix xx

;P=

;5=

;=


42/66

q.

V

L

#ver+all material balance

. 3 V L

. .3 V L

. h.3 V ? L h

;M=

;I=

;=

6he enthalp+concentration dia&ram ma be used to evaluate &raphicall theenthalp and composition of streams added or separated"

Stead+state Towsstem with phase

separation and heatadded

$omponent material balance

Enthalp balance

.or adiabatic process0 q 3 F

V ;? h.= 3 L ;h. h=

V ; .= 3 L ;. =

hh

hH

$

%

&

&

=

xx

xy

$

%

&

&

Substitutin& eq" ;M= to ;= &ives

Substitutin& eq" ;M= to ;I= &ives

hLHVhLV F

;@F=

;@@=

;@9=

;@K=

$ i ;@@= t ;@K= i


43/66

$omparin& eq" ;@@= to ;@K= &ives

xx

xy

hh

hH

&

&

&

&

=

Eq" ;@P= can be rearran&ed

xx

hh

xy

hH

&

&

&

&

=

;@P=

;@5=

h

h.

?

L

.

V

.

Enthalp+concentration lines adiabatic0 q 3 F

:is$&lineofslope"he'''

&

&

xy

hH

:'''

isFLlineofslopeThexx

hh

&

&

""o&'n, to e%. *15! the #lope# o( both

l'ne# ae the #ame.

'n"e both l'ne# ,o thou,h the #ame po'nt

*/! the l'ne# l'e on the #ame #ta',ht l'ne.


44/66

L

.

V?

h.

h

.

A

(

LEVE1+A1' 1!LE )1/>$/)LE

hh

hH

$

%

=

$onsider trian&le L(V

$

%

hh

hH

BA

A$

%

$

'''

'''

'''

'''

==

'''

'''

%

$

%=

Similarl '''

'''

%

%

$= '''

'''

%

%=


45/66

D0 D0 ?LD

(0 (0 ?L(

..?.

qD

q(

V@

LFF?LF

#ver+all material balance

. 3 D ( ;@=


. .3 D D ( ( ;@

. .3 D D ;. D= ( ;@

BD

B

xx

xxD

=

;@=

Enthalp balance

;9F=BDDB

hB(hD(h


46/66

TOTAL CONDENSER

V@3 LF D ;9@=

V@@3 LFF D F ;99=

'aterial balance around condenser


Enthalp balance

A

V@

L@

LF

DD

qD

qD V@?@3 LFhF D hD;9K=

$ombinin& eqs" ;9@= and ;9P=

D)

)DD*

hH

H+h

D

%

=

/nternal reTu is shown as

*DD

)DD

)

*

h+h

H+h

$

%

=

;95=

;9=

Desi&natin& D(+ DD=

V@?@3 LFhF D ;hD UD= ;9P=


47/66

/nternal reTu between each plate0until a point in the column isreached where a stream is addedor removed0 can be shown as

mDD

mDD

m

m

hQh

HQh

V

L

=

)

)

;9M=

A

Vm@

Lm

LF

DD

qD

m

LF0 DhF0 hD

V@

?@

;hD UD=0 D

hD UD ?@

?@ hD

@0 F0 D

?o

rh


48/66

hF0 F

V@

?D0 D

;?D UD=0 D

?D UD ?@

?@ hF

@0

F0

D

?o

rh

D

6he material balance equation mabeEnthalp balanceof total condensor


49/66

RECTIFYING SECTION

V@

VK

Vn@

L@

L9

Ln

LFDD

!L

qD

rearran&ed in the from of dierence

LF V@3 L@ V93 L9 VK

3 " " " " 3 Lm Vm@

3 D 3

;9I=LF V@3 D 3

n

V9

$ombinin& eqs" ;K5= and ;K=

D3 ;KF=

.or the component material balance

LFF V@@3 L@@ V99

3 L99 VKK3 " " " "

3 Lmm Vm@m@

3 D D3

;9=LFF V@@3 D D3

.or the enthalp balanceLFhF V@?@3 L@h@V9?93 L9h9VK?K3 " " " "

3 Lmhm Vm@?m@3 D ;hD UD= 3 h;K@=

$ombinin& eqs" ;9K= and ;K@=

h3 hD UD ;K9=

a p ba a ceo o a co de so

qD V@?@3 LFhF D hD;9K=


50/66

6hese K independent equations *eqs" ;9I=0 ;9=0 and ;KF=, can be written forrectifin& section of the column between each plate"

#n the enthalp scale and on the composition scale0 the dierences inenthalp and in composition alwas pass throu&h the same point0 ;*D0 ;hD

UD=,

6his is desi&nated as point 0 the dierence point0 and all lines correspondin&

to the combined material and enthalp balance equations ;operatin& lineequations= for the rectifin& section of the column pass throu&h thisintersection"

)LA6E+6#+)LA6E 1A)?/$AL )1#$ED!1E .#1 DE6E1'/>/> 6?E


51/66

)LA6E 6# )LA6E 1A)?/$AL )1#$ED!1E .#1 DE6E1'/>/> 6?E>!'(E1 #. EU!/L/(1/!' S6AES

@" !se 10 D0 ?Dor hDto establish the location of point with 3 Dand

h3 hD UDor h3 ?D UD

9" !se Equilibrium data alone to establish the point L@at ;@0 h@=" Since

L@is assumed to be a saturated liquid0 @must lie on the saturated+

liquid line"

K" Draw the operatin& line between L@and " 6his line intersects the

saturated+vapor line at V9;

90 ?

9="

P" 1epeat steps 9 and K until the feed plate is reached"

or

?o

rh V@V9VKVP

DL@L9LK

;0 h=

6he material balance equation mabe rearran&ed inthe from of dierence


52/66

61/))/> SE$6/#>

(

(q(

m

>

L)mV

)MV

MV )ML

ML

the from of dierence

MMMM VLBVL ))

...), =MM LL

)mm VL ;KK=

.or the component material balance

MMMMBMMMM yVxLxByVxL + ))))

...)),, =MMMM yVxL

xyVxL mmmm ));KP=

$ombinin& eqs" ;PF= and ;P@=

Bxx=;K5=

.or the enthalp balance

;K=

MMMBBMMMM HVhLQhBHVhL ))))

...)),, =MMMM HVhL

hHVhL mmmm ))

$ombinin& eqs" ;@= and ;P=

BB Qhh ;KM=


53/66

6hese K independent equations *eqs" ;KK=0 ;KP=0 and ;K5=, can be written forstrippin& section of the column between each plate"

#n the enthalp scale and on the composition scale0 the dierences in enthalpand in composition alwas pass throu&h the same point0 *(0 ;h( U(=,"

6his is desi&nated as point 0 the dierence point0 and all lines correspondin&to the combined material and enthalp balance equations ;operatin& lineequations= for the strippin& section of the column pass throu&h thisintersection"

3$B

;KI=

$ombinin& eq" ;KI= with eqs" ;9I= and;KK= &ives

;K=

Equation ;K= implies that lies on the etension of the strai&ht line passin& throu&h and "

U(is usuall not 7nown" /t can be derived from over+all material balance

)LA6E+6#+)LA6E 1A)?/$AL )1#$ED!1E .#1 DE6E1'/>/> 6?E


54/66

>!'(E1 #. EU!/L/(1/!' S6AES

@" Draw a strai&ht line passin& throu&h . and "

9" Draw a vertical strai&ht line at (all the wa down until it intersects the

etension of line .inK" Assumin& the reboiler to be an equilibrium sta&e0 the vapor V'@is in

equilibrium with the bottom stream"

P" !se equilibrium data alone to establish the value of m@on the

saturated+vapor line"

5" Draw the operatin& line between Lm;m0 hm= and V'@" 6his line intersectsthe saturated+liquid line at

" 1epeat steps P and 5 until the feed plate is reached"

or

?o

rh h(

)-$ -$ )-$

0x,

L' L'+

@

TOTAL COLUMN


55/66

TOTAL COLUMN

D

(

.

qD

q(

V@

LF

6he construction ma start

from either side of thedia&ram0 indicatin& eitherthe condition at the top orthe bottom of the column"

)roceed as eplained inprevious slides"

/n either case0 when anequilibrium tie line crosses

the line connectin& thedierence points throu&h thefeed condition0 the otherdierence point is used to

complete the construction"


56/66

?o

rh

.

@9KP5MI

. D(

?o

rh

V@

@

@

L@

EWA')LE K


57/66

EWA')LE K

!sin& the enthalp+concentration dia&ram from Eample 90 determinethe followin& for the conditions in Eample @0 assumin& a saturatedliquid feed"

a" 6he number of theoretical sta&es for an operatin& reTu ratio of 13 LFOD 3 9"5

b" 'inimum reTu ratio LFOD"

c" 'inimum equilibrium sta&es at total reTu"d" $ondenser dut feedin& @F0FFF lb of feedOhr0 (tuOhr"

e" 1eboiler dut0 (tuOhr"

S#L!6/#>

;a= .rom the &raph hD3 hF3 50@@M calOmole

?@3 @90M9K calOmole

12

2113

h"

"4h

1

L

=

225,65#7,2#

5#7,2#4225,66# 1

=

UD3 909@ calOmole


58/66

h

3 hD

UD

3 50@@M ; 909@= 3 K@0MKI calOmole

6he coordinate of point is

3 D3 F"M

Draw a strai&ht line passin& throu&h and ."

Etend the line until it intersects a vertical line passin& throu&h(0 at

Draw operatin& lines and equilibrium lines in the whole columnusin& the method eplained in the previous slides"

>umber of sta&es 3 @@


59/66

.

3 9@0MFF calOmole

;b=


60/66

.

12

2113

h"

"4h

1

L

=

12

2

h"

"h

=

225,65#7,2#

5#7,2#533,#2

1

L

min

3

=

3 @"@I

;c=


61/66

.

@9KP5M

> 3 M

;c=

;d= h U 3 h 3 K@ MKI calOmole


62/66

;d= hD UD3 h3 K@0MKI calOmole

hD3 50@@M calOmole

UD3 909@ calOmole

!molel237

hr!l333,23

!mole

1mole8#93

molecal

molel0tu:2

mole

cal9#2,#941

3 @0I@0IPK (tuOhr

;e= h( U(3 @P0K5F calOmole

h(3 50II calOmole

U(3 @P0K5F 50II 3 9F09K calOmole

!molel237

hr!l333,23

!mole

0mole6583

molecal

molel0tu:2

mole

cal#79,#941

3 90K@0M5@ calOmole


63/66

A#"/S AE#"012

D0S"0%%A"01 #%3-1!E/A"01)apour *low Conditions

+dverse vapour flow conditions can cause

*oaming

#ntrainment

,eeping-dumping

*looding

*oaming


64/66

*oaming8oaming refers to the expansion of liquid due to passage of vapour

or gas7 Although it provides high interfacial liquid-vapour contact,excessive foaming often leads to liquid buildup on trays7 ;n some cases,foaming may be so bad that the foam mixes with liquid on the trayabove7 hether foaming will occur depends primarily on physicalproperties of the liquid mixtures, but is sometimes due to tray designsand condition7 hatever the cause, separation efficiency is alwaysreduced7

#ntrainment$ntrainment refers to the liquid carried by vapour up to the tray

above and is again caused by high vapour flow rates7 ;t is detrimental

because tray efficiency is reduced: lower volatile material is carried to aplate holding liquid of higher volatility7 ;t could also contaminate highpurity distillate7 $xcessive entrainment can lead to flooding7

,eeping-Dumping%his phenomenon is caused by low vapour flow7 %he pressure exerted bythe vapour is insufficient to hold up the liquid on the tray7 %herefore,liquid starts to lea through perforations7 $xcessive weeping will lead todumping7 %hat is the liquid on all trays will crash (dump# through to thebase of the column (via a domino effect# and the column will have to bere-started7 eeping is indicated by a sharp pressure drop in the columnand reduced separation efficiency7

*looding8looding is brought about by excessive vapour flow causing liquid to be


65/66

8looding is brought about by excessive vapour flow, causing liquid to beentrained in the vapour up the column7 %he increased pressure fromexcessive vapour also bacs up the liquid in the downcomer, causing anincrease in liquid holdup on the plate above7 )epending on the degree offlooding, the maximum capacity of the column may be severely reduced7

8looding is detected by sharp increases in column differential pressureandsignificant decrease in separation efficiency7

Column Diameter6ost of the above factors that affect column operation is due to vapour

flow conditions: either excessive or too low7 Bapour flow velocity is dependenton column diameter7 eeping determines the minimum vapour flow required

while flooding determines the maximum vapour flow allowed, hence columncapacity7%hus, if the column diameter is not sized properly, the column willnot perform well7 3ot only will operational problems occur, the desiredseparation duties may not be achieved7

State of Trays and .ac/ingsCemember that the actual number of trays required for a particular

separation duty is determined by the efficiency of the plate, and the pacings ifpacings are used7%hus, any factors that cause a decrease in tray efficiency willalso change the performance of the column7 %ray efficiencies are affected byfouling, wear and tear and corrosion, and the rates at which these occurdepends on the properties of the liquids being processed7 %hus appropriatematerials should be specified for tray construction7


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,eather Conditions

6ost distillation columns are open to the atmosphere7 Although many of the

columns are insulated, changing weather conditions can still affect column

operation7 %hus the reboiler must be appropriately sized to ensure that enoughvapour can be generated during cold and windy spells and that it can beturned down sufficiently during hot seasons7 %he same applies to condensors7

mass transfer - distillation final 20.1.2016.pptx

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