PM3125 Lectures 16to17 Evaporation

7/25/2019 PM3125 Lectures 16to17 Evaporation

1/45

Prof. R. Shanthini05 J

Content of Lectures 13 to 18:

Evaporation:- Factors affecting evaporation

- Evaporators

- Film evaporators

- Single effect and multiple effect evaporators

- Mathematical problems on evaporation

PM3125: Lectures 16 to 18


2/45


Example 4:

Estimate the requirements of steam and heat transfer surface and the

e!aporatin" temperatures in each effect for a triple effect e!aporatore!aporatin" 5## $" h%1of a 1#& solution up to a 3#& solution'

(team is a!aila)le at 2## $Pa "au"e and the pressure in the e!aporation

space in the final effect is 6# $Pa a)solute' *ssume that the o!erall heat

transfer coefficients are 22+# 2### and 142# , m%2

s%1

-.%1

in the firstsecond and third effects respecti!el/'

0e"lect sensi)le heat effects and assume no )oilin"%point ele!ation and

assume equal heat transfer in each effect'

(ource: http:'nifst'or"'nunitoperationse!aporation2'htm


3/45


!erall heat transfer coefficients are 22+# 2### and 142# , m%2s%1-.%1in the

first second and third effects respecti!el/'

10% solution

30% solution

500 kg h-1

200 kPa (g)

60 kPa (abs)

Estimate the requirements of steam and heat transfer surface and the

e!aporatin" temperatures in each effect'

0e"lect sensi)le heat effects and assume no )oilin"%point ele!ation and

assume equal heat transfer in each effect'


4/45


Overall mass balance:

ata: * triple effect e!aporator is e!aporatin" 5## $"h of a 1#& solution

up to a 3#& solution'

oli!s olvent ("ater) olution (total)

#ee! 1#& of total

5# $"h

5## $"h 7 5# $"h

45# $"h

5## $"h

$oncentrate!ro!uct

5# $"h 16+ $"h 7 5# $"h

11+ $"h

5#3#91##

16+ $"h

&aour 'romall e''ects

# 333 $"h 5## $"h % 16+ $"h

333 $"h


5/45


team roerties:

ata: (team is a!aila)le at 2## $Pa "au"e and the pressure in the

e!aporation space in the final effect is 6# $Pa a)solute' 0e"lect sensi)le

heat effects and assume no )oilin"%point ele!ation9

teamressure

aturationtemerature

atent heat o'vaouriation

2## $Pa "9 2 )ar "9

3 )ar a)s9

133'5o

. 2164 $,$"

6# $Pa a)s9

#'6 )ar a)s9

86'#o. 22;3 $,$"


6/45


#irst e''ect econ! e''ect *hir! e''ect

(teamtemperature

133'5o.


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eat balance:

ata:*ssume that the o!erall heat transfer coefficients are 22+# 2###

and 142# , m%2s%1-.%1in the first second and third effects respecti!el/'

*ssume equal heat transfer in each effect'

q1 q2 q3 hich "i!es >1*1=2*2=3*3=1 >2and >3are "i!en'

*1 *2and *3can )e found if =


8/45


#irst e''ect econ! e''ect *hir! e''ect

(team temperature 133'5o. 1 22+# , m%2

s%1-.%1

>2 2### , m%2

s%1-.%1

>3 142# , m%2

s%1-.%1

Latent heat of

!apouriation ofsteam

@1 2164 $,$" @2 22## $,$" @3 224# $,$"

Latent heat of!apouriation ofsolution

22## $,$" 224# $,$" 22;3 $,$"

Proerties in all e''ects:


9/45


#irst e''ect

(team

temperature

133'5o.

(olutiontemperature

1 22+# , m%2

s%1-.%1

Latent heat of!apouriation

of steam

@1 2164 $,$"

Latent heat of!apouriationof solution

22## $,$"

$onsi!er the 'irst e''ect:

team use! . /

*ssumin" feed enters at the )oilin" point

(1@19

A1Latent heat of !apouriation of solution9

here

(1is the flo rate of steam used in the

first effect and

A1is the flo rate of !apour lea!in" the

first effect'


10/45


econ! e''ect

(team

temperature


11/45


*hir! e''ect

(team

temperature


12/45


team econom,:

(121649 A122##9 A2224#9 A322;39

Aapour lea!in" the s/stem A1C A2C A3 333 $"h from the mass)alance9


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eat trans'er area:

#irst e''ect

(team

temperature

133'5o.

(olutiontemperature

1 22+# , m%2

s%1-.%1

Latent heat of!apouriation

of steam

@1 2164 $,$"

Latent heat of!apouriationof solution

22## $,$"

*1 (1@1 >1=


14/45


Otimum boiling time:

In evaporation, solids may come out of solution and form a deposit

or scale on te eat transfer surfaces!

"is causes a gradual increase in te resistance to eat transfer!

If te same temperature difference is maintained, te rate of

evaporation decreases #it time and it is necessary to sut do#n te

unit for cleaning at periodic intervals!"e longer te $oiling time, te lo#er is te num$er of sutdo#ns

#ic are re%uired in a given period altoug te rate of evaporation

#ould fall to very lo# levels and te cost per unit mass of material

andled #ould $ecome very ig!& far $etter approac is to ma'e a $alance #ic gives a minimum

num$er of sutdo#ns #ilst maintaining an accepta$le trougput!


15/45


Otimum boiling time:

It as long $een esta$lised tat, #it scale formation, te overall

coeffcient of eat transfer ()* may $e e+pressed as a function of te

$oiling time (t* $y an e%uation of te form:

1). a t / $ (#ere a and $ are to $e estimated*

"e eat transfer rate is given $y d0

dt. ) & "

Com$ining te a$ove t#o e+pressions, #e get d0

dt. & "

(a t / $*2!

Integration of te a$ove $et#een 2 and 0$and 2 and t$gives

0$. ( & "a* 4(at$/$*2!5 $2!6

#ere 0$is te total eat transferred

in te $oiling time t$!


16/45


Otimum boiling time to ma4imie heat trans'er:

Let us optimi7e te $oiling time so as to ma+imi7e te eat

transferred and ence to ma+imi7e te solvent evaporated:

If te time ta'en to empty, clean and rell te unit is tc, ten te total

time for one cycle is t . (t$/ tc* and te num$er of cycles in a period

t9is t9(t$/ tc*!

"e total eat transferred during tis period is te product of te eat

transferred per cycle and te num$er of cycles in te period or:

09. 0$t9(t$/ tc* . ( & "a* 4(at$/$*2!5 $2!6 t9(t$/ tc*

"e optimum value of te $oiling time #ic gives te ma+imum

eat transferred during tis period is o$tained $y differentiating te

a$ove and e%uating to 7ero #ic gives:

t$,optimum. tc/ (a* (a$tc*2!


17/45


Otimum boiling time to minimie cost:

"a'e Ccas te cost of a sutdo#n and te varia$le cost during

operation as C$

, ten te total cost during period t9

is:

09. ( & "a* 4 (at$/$*2!5 $2!6 t9(t$/ tc*

"e optimum value of te $oiling time #ic gives te minimum

cost is o$tained $y differentiating te a$ove and e%uating to 7ero#ic gives:

t$,optimum. (Cc C$* / (a$CcC$*2!(aC$*

C". (Cc/ t$ C$* t9(t$/ tc*

)sing , #e can #rite

C". (Cc/ t$ C$* a 09 ( & " 4(at$/$*2!5 $2!6 ;


18/45


+4amle

In an evaporator andling an a%ueous salt solution, overall eat

transfer coefficient ) ('


19/45


@ata provided:

Since ) is in '2 m

B" . >2oCB

Latent eat of vapouri7ation of #ater . 322 '?'gB

tc. >!1= . >!1= + 3D22 s . 121 sB

Cc. As 12,222B

C$. As 1,222 per our . As 3!33 per s


20/45


For the case of maximum throughput:

t$,optimum. tc/ (a* (a$tc*2!

. (121* / ( 2!2222=* (2!2222=

+ 2! + 121*2!

. 8112 s . 7.81 h

eat transferred during $oiling:

0$. ( & "a* 4(at$/$*2!5 $2!6

. ( + >2 + >2 2!2222=*4(2!2222= + 8112 / 2!*2!5 2!2!6

. 46.9 x 106!


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!8 'g per cycle . %s 10.& per g


22/45


Aate of evaporation during $oiling

. 23=>!8 'g 8112 s . 2!= 'gs

Gean rate of evaporation during te cycle

. 23=>!8 'g (8112 s / 121 s* . 2!>=3 'gs


23/45


For the case of minimum cost:

eat transferred during $oiling:

0$. ( & "a* 4(at$/$*2!5 $2!6

. ( + >2 + >2 2!2222=*4(2!2222= + D8> / 2!*2!5 2!2!6

. 7#.6 x 106!

t$,optimum. (Cc C$* / (a$CcC$*2!(aC$*

. (12,2223!33*

/ (2!2222= + 2! + 12,222 + 3!33*2!(2!2222= + 3!33* . D8> s . 1&.6$ h


24/45


s + As 3!33 per s *

. As 32=D1 per cycle

. As 32=D1 per cycle #ater evaporated per cycle

. As 32=D1 per cycle 311!8 'g per cycle . As != per 'g


25/45


Aate of evaporation during $oiling

. 311!8 'g D8> s . 2!D1 'gs

Gean rate of evaporation during te cycle

. 311!8 'g (D8> s / 121 s* . 2!>> 'gs


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ma4imumthroughut

minimumcost

ptimum )oilin" time 1 h 1563 h

?eat transferred durin")oilin"

6 4 106k 26 4 106k

Mean rate of e!aporationper c/cle

03 kg7s 02 kg7s

.ost of operation per c/cle 8s 21301 erc,cle

8s 30612 erc,cle

.ost of operation per $" ofater e!aporated

8s 105 er kg 8s 5 er kg

'ummar(:


27/45

Prof. R. Shanthini

05 J

#alling 'ilm evaorators#alling 'ilm evaorators

In falling film evaporators te

li%uid feed usually enters te

evaporator at te ead of te

evaporator!

In te ead, te feed is evenly

distri$uted into te eating

tu$es!

& tin film enters te eating

tu$e and it flo#s do#n#ards at

$oiling temperature and is

partially evaporated!


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Prof. R. Shanthini

05 J http:!ideo'"eap'com!ideo822+43"ea%ie"and%animation%fallin"


29/45

Prof. R. Shanthini

05 J

"e li%uid and te vapor $ot

flo# do#n#ards in a parallel

flo#!

"is gravity-induced do#n#ard

movement is increasingly

augmented $y te co-current

vapor flo#!

"e separation of te

concentrated product from its

vapor ta'es place in te lo#er

part of te eat e+canger and

te separator!

In most cases steam is used for

eating!


30/45

Prof. R. Shanthini

05 J

Falling film evaporators can $e operated #it very lo#

temperature differences $et#een te eating media and te $oiling

li%uid!

"ey also ave very sort product contact times, typically Hust a

fe# seconds per pass!

"ese caracteristics ma'e te falling film evaporator particularlysuita$le for eat-sensitive products, and it is today te most

fre%uently used type of evaporator!


31/45

Prof. R. Shanthini

05 J

o#ever, falling film evaporators must $e designed very carefully

for eac operating conditionB

sufficient #etting (film tic'ness* of te eating surface $y li%uid

is e+tremely important for trou$le-free operation of te plant!

If te eating surfaces are not #etted sufficiently, dry patces and

#ill occur!

"e proper design of te feed distri$ution system in te ead of

te evaporator is critical to acieve full and even product #etting

of te tu$es!


32/45

Prof. R. Shanthini

05 J

ecause of te lo# li%uid olding volume in tis type of unit, te

falling film evaporator can $e started up %uic'ly and canged to

cleaning mode or anoter product easily!

Falling film evaporators are igly responsive to alterations of

parameters suc as energy supply, vacuum, feed rate,

concentrations, etc!


33/45

Prof. R. Shanthini

05 J

*,es o' evaorators

&ertical #alling #ilm +vaorators:

"e tu$e lengt is

typically D m to 11 m,

$ut can $e as sort as

1! m to 3 m (for

e+ample, in deep

vacuum applications*!

@iameters are

typically 2 mm to D>

mm!

* ' t


34/45

Prof. R. Shanthini

05 J

*,es o' evaorators

&ertical #alling #ilm +vaorators:

* ' t


35/45

Prof. R. Shanthini

05 J

*,es o' evaorators

oriontal #alling #ilm +vaorators:

"e li%uid is evaporated at te outside of te tu$es! It flo#s from one

tu$e to te oter in form of droplets, Hets or as a continuous seet!

Beed

istri)utor

(team.ondensate

.oncentrate.oncentrate

Aapour

* es o' e aorators


36/45

Prof. R. Shanthini

05 J

*,es o' evaorators

oriontal #alling #ilm +vaorators:

"e li%uid is evaporated at te outside of te tu$es! It flo#s from one

tu$e to te oter in form of droplets, Hets or as a continuous seet!

*,es o' evaorators


37/45

Prof. R. Shanthini

05 J

*,es o' evaorators

oriontal #alling #ilm +vaorators:@ue to te impinging effect #en flo#ing from one tu$e to te oter

te eat transfer is iger compared to vertical falling filmevaporators!

In addition tis unit type can $e operated #it even lo#er pressure

drops compared to te vertical design!

It is also possi$le to design a iger eat transfer area for a givensell compared to te vertical units! 9erforated plates or specially

designed spray no77les can $e used in order to guarantee a even

li%uid distri$ution for eac tu$e!

Cleaning of te outside tu$es can $e difficult, terefore tis type ofevaporators is not used for processes #it tendency to foul!

"u$e dimensions are typically 2!= to 1JJ!

#lo" characteristics in vertical 'ilm 'lo"


38/45

Prof. R. Shanthini

05 J


"e li%uid film can $e o$served in different ydrodynamic

conditions!

"is conditions are caracterised $y film %e(nol)s number,

defined as follo#s:

m 5 total mass flo# rate of condensate ('gs*

@ 5 tu$e diameter (m*

KL 5 li%uid viscosity (9a!s*

Aefilm. . .>m

@KL

> (m@*

KL

> + Gass flo# rate circumference

Li%uid viscosity



39/45

Prof. R. Shanthini

05 J


*ure laminar flo+

Aefilm M 32

"is flo# condition can ardly

ever $e encountered in tecnical

processes!nly in very viscous flo#s tis

flo# condition can $e

encountered! ut even tan in

literature it is mentioned tat#avy $eaviour #as o$servedN!

Aefilm . 12 (already #avy*



40/45

Prof. R. Shanthini

05 J


,av( laminar flo+

Aefilm M 1822

"e tic'ness of a #avy laminar

fluid film is reduced compared to

a pure laminar film!

Smaller average film tic'ness

and increased partial tur$ulence

yield a iger eat transfer

compared to pure laminar flo#conditions!

Aefilm . 22



41/45

Prof. R. Shanthini

05 J


-urbulent flo+

Aefilm O 1822

&part from te near to te #all

laminar su$ layer te flo# is

fully tur$ulent!In tis region eat transfer

increases #it increased

tur$ulence #ic means #it

increased Aeynolds num$er

Aefilm . 222

eat *rans'er


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Prof. R. Shanthini

05 J

eat *rans'er

q > * =< > *


43/45

Prof. R. Shanthini

05 J

%*ll ph/sical properties of the liquid are

e!aluated at the film temperature


44/45

Prof. R. Shanthini

05 J

eat *rans'erBor tur)ulent flo Hefilm 18##9 the steam%side condensation coefficient

for !ertical surfaces can )e calculated )/ the folloin" equation:

0u #'##++h L

$L

JL2" L3

KL2

13He

#'4

eat *rans'er


45/45

Prof. R. Shanthini

eat *rans'erBor laminar flo Hefilm I 18##9 the steam%side condensation coefficient

for horiontal surfaces can )e calculated )/ the folloin" equation:

0u #'+25h $L

JLJL % JA9 " @ 3

0KL $L=

Documents

PM3125 Lectures 16to17 Evaporation