Download - Lect - 21 Evoporation Lecture 1 of 3.pptx
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
1/56
EvaporationDr. Senthilmurugan S. Department of Chemical Engineering IIT Guwahati - CL204 - Part 2
Evaporators and its design concepts
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
2/56
5/12/16 | Slide 2
Content of Lectures
Evaporation
Types of evaporators
Factors affecting evaporation
Energy Balance for Single effect and ultiple effect
evaporators
!at"eatical pro#les on evaporation
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
3/56
5/12/16 | Slide $
Terinology
Evaporation % process #y &"ic"
li'uid &ater passes directly to t"e
vapor p"ase
Transpiration ( process #y &"ic"
li'uid &ater passes fro li'uid to
vapor t"roug" plant eta#olis
Su#liation ( process #y &"ic"&ater passes directly fro t"e
solid p"ase to t"e vapor p"ase
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
4/56
5/12/16 | Slide )
Evaporation
Suppose t"at &e "ave a dilute solution of a
solute *say+ sugar, dissolved in a solvent
*say+ &ater,
-e need to reove part of t"e solvent
*&ater, to produce a concentrated solution
T"is can #e ac"ieved #y "eating t"e
solution so as to evaporate t"e solvent
*&ater,
T"is process is .no&n as evaporation
T"e industrial e'uipent used for t"is
purpose in .no&n as an evaporator
0n process industry T"e t"ic. li'uor is t"evalua#le product and t"e vapour is
condensed and discarded
-ater industry concentrated sea &ater
discarded and vapour is condensed used
as product &ater
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
5/56
5/12/16 | Slide 5
Types of Evaporator
pen .ettle or pan evaporator
3ori4ontal(tu#e evaporator
ertical(type s"ort(tu#e evaporator
Long(tu#e vertical evaporators
p&ard flo& *cli#ing(fil,
7o&n&ard flo& *falling(fil,
Forced circulation
8gitated(fil evaporators
!ec"anical oveent
ltrasonic agitation
Single effect evaporators
!ultiple effect evaporators
For&ard feed
Bac.&ard feed
!i9ed feed
:arallel feed
T"eral distillation evaporators
!ulti Stage Flas" *!SF,/ Single
Effect 7istillation *SE7,
!ulti Effect 7istillation *!E7,
apour Copression *C, T"eral
!ec"anical
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
6/56
5/12/16 | Slide 6
Concentrate
Condensate
:an
Boiler
;ac.et
:ressure gauge
Stea
- simplest form of evaporators
- inexpensive
- simple to operate
- very poor heat economy
- in some cases paddles and scrapers
for agitation are used
Types of evaporatorspen .ettle or pan evaporator
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
7/56
5/12/16 | Slide <
Types of evaporators
relatively c"eap
used for non(
viscous li'uids
"aving "ig" "eat(
transfer coefficients
and li'uids t"at do
not deposit scales
poor li'uid
circulation *and
t"erefore unsuita#le
for viscous li'uids,
3ori4ontal(tu#e evaporator
7ilute feed
Condensate
Concentrated
product
apor
Stea
inlet
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
8/56
5/12/16 | Slide =
Types of evaporators
Li'uid is inside t"e tu#es
Stea condenses outside t"e tu#es
sed for non(viscous li'uids "aving "ig"
"eat(transfer coefficients and li'uids t"at
do not deposit scales
ertical ( tu#e evaporator
7ilute feed
Condensate
Concentrated
product
apor
Stea
inlet
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
9/56
5/12/16 | Slide >
Types of evaporators
>
Cli#ing(fil long(tu#e vertical evaporator Falling fil long(tu#e vertical evaporator
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
10/56
5/12/16 | Slide 1?
Types of evaporators
Forced(circulation evaporator &it" separate t&o(
pass "ori4ontal +#eating eleent
8gitated fil evaporator
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
11/56
5/12/16 | Slide 11
Types of evaporator Triple(effect evaporator
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
12/56
5/12/16 | Slide 12
Types of evaporator -it" respect to feed flo& direction
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
13/56
5/12/16 | Slide 1$
!ulti Stage Flas" *!SF,/Single Effect 7istillation *SE7,
Stea is used to "eat tu#es of saline &ater
3eated &ater flo&s into @stagesA t"at are at lo&er pressure
-ater #oils rapidly and @flas"esA into stea
7ifferent types of !SF tec"nology availa#le &it" respect to internal
configuration and flo& direction
Flo& diagra
C " a l l e
n g e 1 E n e
r g y e f f i c i
e n t e
v a p o r a
t i o n
t e c " n
o l o g y
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
14/56
5/12/16 | Slide 1)
!SF desalting
Brine "eater
To #oil incoing sea&ater to its #oiling point
Flas"ing c"a#er
-ater and vapor separation
Condensing c"a#er
Condensation of vapor to &ater
Eector
eove non(condensa#le gases
Sea &ater pup
To pup pretreated sea &ater to flas"
c"apers
Brine pup
To pup #rine fro flas"ing c"a#er Condensate pup
To pup condensate fro #rine "eater
:roduct pup
To pup product &ater fro flas"ing unit
!aor units
11%
19%
54%
8%
5% 1% 2%
Brine Heater Desuperheater !" !tagesBrine Blo#do#n Distillate Brine Blo#do#n $ump
Brine ecirculation &alve
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
15/56
5/12/16 | Slide 15
!ulti Effect 7istillation
T"e Brine is "eated in ultiple effect and ultiple
stages
Based on operating tep % "ig" and Lo&
Based on "eating arrangeent
ertical tu#e evaporator
3ori4ontal tu#e evaporator
Based on copression
!E7 &it" !ec"anical apor Copression *!C, !E7 &it" T"eral apor Copression *!E7(
TC,
vervie&
MED-TVC
MED-Horizontal tubes
MED-MVC
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
16/56
5/12/16 | Slide 16
!ulti Effect 7istillation3ori4ontal evaporators
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
17/56
5/12/16 | Slide 1<
!ulti Effect 7istillation-it" vertical evaporators
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
18/56
5/12/16 | Slide 1=
!ulti Effect 7istillation-it" t"eral vapor copression
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
19/56
5/12/16 | Slide 1>
apor copression
Tec"nology is siilar to !SF and !E7
C processes e9pose influent &ater in a tu#e to a vacuu t"at causes t"e &ater to
#oil T"e resulting vapor is transferred to anot"er tu#e+ &"ere it is copressed #ac.
into li'uid &ater
Tec"nology overvie&
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
20/56
5/12/16 | Slide 2?
T"eral desalinationCoparison
!ource ' 2((9 )ux esearch report to *BB
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
21/56
5/12/16 | Slide 21
Factors effecting evaporation *1/1,
Concentration in t"e li'uid
Li'uid feed to an evaporator is relatively dilute
So its viscosity is lo&+ and "eat(transfer coefficient "ig"
8s evaporation proceeds+ t"e solution #ecoes concentrated
So viscosity increases and "eat(transfer coefficient drops
7ensity and t"e #oiling point of solution also increase
Solu#ility
8s solution is "eated+ concentration of t"e solute in t"e solution increases
0n case t"e solu#ility liit of t"e solute in solution is e9ceeded+ t"en crystals
ay for
Solu#ility of t"e solute t"erefore deterines t"e a9iu concentration of t"esolute in t"e product strea
0n ost cases+ t"e solu#ility of t"e solute increases &it" teperature T"is
eans &"en a "ot concentrated solution fro an evaporator is cooled to roo
teperature+ crystalli4ation ay occur
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
22/56
5/12/16 | Slide 22
Factors effecting evaporation *2/),
T"e vapor pressure of a'ueous
solutions is less t"an t"at of &ater at
t"e sae teperature Conse'uently+
for a given pressure t"e #oiling point of
t"e solutions is "ig"er t"an t"at of pure
&ater T"e increase in #oiling point
over t"at of &ater is .no&n as t"e
#oiling(point elevation *B:E, of t"e
solution
7u"ringDs rule T"e #oiling point of a
given solution is a linear function of t"e
#oiling point of pure &ater at t"e saepressure
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
23/56
5/12/16 | Slide 2$
Factors effecting evaporation *$/),
:ressure and teperature
T"e #oiling point of t"e solution is
related to t"e pressure of t"e syste
T"e "ig"er t"e operating pressure of
t"e evaporator+ t"e "ig"er t"e
teperature at #oiling
8lso+ as t"e concentration of t"e
dissolved aterial in solution increases
#y evaporation+ t"e teperature of
#oiling ay rise *a p"enoenon .no&n
as #oiling point rise/elevation,
To .eep t"e teperatures lo& in "eat(
sensitive aterials+ it is often necessaryto operate under atosp"eric pressure
*t"at is+ under vacuu,
Effect of li'uid "ead and friction
High velocity due to
+oiling and +u++lerise
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
24/56
5/12/16 | Slide 2)
Factors effecting evaporation *)/),
Teperature sensitivity of aterials
:"araceuticals products+ fine c"eicals and foods are daaged &"en "eated to
oderate teperatures for relatively s"ort ties
So special tec"ni'ues are eployed to reduce teperature of t"e li'uid and tie of "eating
during evaporation
Scale deposition
Soe solutions deposit solid aterials *called scale, on t"e "eating surfaces T"e result is t"at t"e overall "eat(transfer coefficient *, ay drastically decrease+ leading to
s"ut do&n of t"e evaporators for cleaning purposes
!aterials of construction
Evaporators are ade of soe .ind of steel
3o&ever any solutions attac. ferrous etals and are containated #y t"e
Copper+ nic.el+ stainless steels can also #e used
Foaing and frot"ing
Solutions li.e organic copounds tend to foa and frot" during vapori4ation
T"e foa is carried a&ay along &it" vapour leaving t"e evaporator
Entrainent losses occur
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
25/56
5/12/16 | Slide 25
!et"od of operation of evaporators
Single(effect evaporation
-"en a single evaporator is used +t"e vapor fro t"e #oiling li'uid is
condensed and discarded T"is is called single effect evaporation
0t is siple #ut utili4es stea ineffectively
To evaporate 1 .g of &ater fro t"e solution &e re'uire 1(1$ .g of stea
!ultiple(effect evaporation 0ncreasing t"e evaporation per .g of stea #y using a series of evaporators
#et&een t"e stea supply and condenser is called ultiple effect evaporation
Econoy Econoy is t"e nu#er of .ilogras vapori4ed per .ilogra of stea fed
to t"e evaporator unit
Capacity Capacity is defined as t"e nu#er of .ilogras of &ater vapori4ed per"our #y evaporator unit
0n a single(effect evaporator t"e econoy is nearly al&ays less t"an 1+ #ut in
ultiple(effect e'uipent it ay #e considera#ly greater
T"e stea consuption+ in .ilogras per "our capacity / econoy
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
26/56
5/12/16 | Slide 26
3eat transfer coefficient
T"e overall resistance to "eat transfer #et&een t"e stea and t"e #oiling li'uid is
t"e su of five individual resistances *i, t"e stea(fil resistance t"e t&o scale
resistances+ *ii, inside and *iii, outside t"e tu#es *iv, t"e tu#e(&all resistance and
*v, t"e resistance fro t"e #oiling li'uid
0n ost evaporators t"e fouling factor of t"e condensing stea and t"e resistance of
t"e tu#e &all are very sall+ and t"ey are usually neglected in evaporatorcalculations 0n an agitated(fil evaporator t"e tu#e &all is fairly t"ic.+ so t"at its
resistance ay #e a significant part of t"e to total
For evaporators
( (
1 1 1 1 1i i i w
i o fo L w i f i
d d d x
U d h d h d k h h= + + + +
1 1 1 1 1o w o o
o o fo L i i i fi
d x d d
U h h d k d h d h
= + + + +
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
27/56
5/12/16 | Slide 2<
3eat transfer coefficient
Stea side coefficients
Fil &ise *refer condensation c"apter,
7rop &ise *refer condensation c"apter,
Li'uid side coefficient
T"e li'uid(side coefficient depends to a large e9tent on t"e velocity of t"e li'uid
over t"e "eated surface 0n natural(circulation evaporators t"e li'uid(side coefficient for dilute a'ueous
solutions is #et&een 15?? and $??? -/2 °C
Forced circulation one "as use appropriate correlation discussed pervious c"apters
T"e Col#urn e'uation for forced circulation viscus flo&+
7ittus % Boelter e'uation
verall coefficient
Because of t"e difficulty of easuring t"e "ig" individual fil coefficients in an
evaporator+ e9periental results are usually e9pressed in ters of overall
coefficients
For evaporators
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
28/56
5/12/16 | Slide 2=
Typical overall coefficients in evaporators
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
29/56
5/12/16 | Slide 2>
Calculation et"ods for single(effect evaporators
!team, !
$!, -!, H!
"eed, "
x", -", h"
.ondensate, !$!, -!, h!
&apor, &
y&, -1, H&
.oncentrate, )
x), -1, h)
$
-1
"eed/
" 0 mass flo# rate
x" 0 mass fraction of solute in feed-" 0 temperature of feed
h" 0 enthalpy of feed
&apor leaving the evaporator/
& 0 mass flo# rate
y& 0 mass fraction of solute in vapor -1 0 temperature of vapor
H& 0 enthalpy of vapor
.oncentrate leaving the evaporator/
) 0 mass flo# rate
x) 0 mass fraction of solute in concentrate-1 0 temperature of concentrate
h) 0 enthalpy of concentrate
!team/
! 0 mass flo# rate
$! 0 steam pressure
-! 0 steam temperature
H! 0 enthalpy of steamh! 0 enthalpy of condensate
$ 0 pressure in the evaporator
-1 0 temperature in the evaporator
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
30/56
5/12/16 | Slide $?
Calculation et"ods for single(effect evaporatorsverall material +alance/
" ) 3 &
!olute +alance/
" x" ) x) 3 & y&
f the vapor is free of solute/
" x" ) x)
Heat +alance/
" h" 3 ! H! ) h) 3 & H& 3 ! h!
e#riting/" h" 3 ! H! 6 h!7 ) h) 3 & H&
" h" 3 ! ) h) 3 & H&
#here H! 6 h!
!team, !
$!, -!, H!
"eed, "
x"
, -"
, h"
.ondensate, !
$!, -!, h!
&apor, &
y&, -1, H&
.oncentrate, )
x), -1, h)
$
-1
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
31/56
5/12/16 | Slide $1
Calculation et"ods for single(effect evaporators
nergy lost +y the steam
: ! ! H! 0 h!7
n case of no energy loss to the
environment, : amount of energy
gets transferred from steam to the
solution through the tu+e #all of
area * and overall heat transfer
coefficient ;<
-herefore,
: ; * =- ; * -! 0 -17
!team, !
$!, -!, H!
"eed, "
x", -", h"
.ondensate, !
$!, -!, h!
&apor, &
y&, -1, H&
.oncentrate, )
x), -1, h)
$
-1
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
32/56
5/12/16 | Slide $2
Ent"alpy s concentration diagra+Syste sodiu "ydro9ide(&ater
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
33/56
5/12/16 | Slide $$
Single Effect Evaporator
8 continuous single(effect evaporatorconcentrates >?
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
34/56
5/12/16 | Slide $)
Calculation et"ods for single(effect evaporators
Data provided:
" 9(>2 ?g@h
x" 1 #t % (
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
35/56
5/12/16 | Slide $5
Calculation et"ods for single(effect evaporators
verall material +alance/" ) 3 &
!olute +alance/
" x" ) x) no solute in the vapor7
Heat +alance/" h" 3 ! ) h) 3 & H&
#here H! 0 h!
Data provided/
" 9(>2 ?g@h
x" (
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
36/56
5/12/16 | Slide $6
Calculation et"ods for single(effect evaporators
Heat +alance/" h" 3 ! ) h) 3 & H&
#here H! 0 h!
Data ?no#n/
" 9(>2 ?g@hE ) F(48 ?g@h, & A(24 ?g@h-" A8.
$ 1(1
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
37/56
5/12/16 | Slide $<
Calculation et"ods for single(effect evaporatorsData ?no#n/
" 9(>2 ?g@hE ) F(48 ?g@h, & A(24 ?g@h
-" A8.
$ 1(1
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
38/56
5/12/16 | Slide $=
Calculation et"ods for single(effect evaporatorsData ?no#n/
" 9(>2 ?g@hE ) F(48 ?g@h, & A(24 ?g@h
-" A8.
$ 1(1
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
39/56
5/12/16 | Slide $>
Effects of processing varia#les on evaporator operation
Effect of feed teperature T"e inlet teperature of t"e feed "as a large effect on t"e evaporator operation
-"en feed is not at its #oiling point+ stea is needed first to "eat t"e feed to its #oiling post and t"en
to evaporate it
:re"eating t"e feed can reduce t"e si4e of evaporator "eat(transfer area
Effect of stea pressure
3ig" pressure provides "ig" Ts values+ and "ence TS % T1 &ill increase 3ig" pressure stea is "o&ever ore costly
T"erefore+ overall econoic #alances ust #e considered to deterine t"e optiu stea pressure
Effect of feed pressure
:ressure in t"e evaporator sets t"e #oiling point of t"e solution *T1,
Stea pressure deterines t"e stea teperature *Ts,
Since ' 8 *TS % T1,+ larger values of *TS % T1, &ill "elp reduce t"e "eat(transfer area needed and
"ence t"e cost of evaporator acuu can #e aintained in t"e solution side using a vacuu pup
For e9aple+ if t"e pressure in E9aple 1 is reduced to )1) .:a+ #oiling point of &ater reduces to
$)>> I and t"at &ould increase t"e *TS % T1, fro 1? I to $$$ I 8 large decrease in "eat(transfer
area &ould #e o#tained
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
40/56
5/12/16 | Slide )?
xample 2/
*n evaporator is used to concentrate 45AF ?g@h of a 2(% MaHsolution entering at F(. to a product of 5(% solids< -he pressure
of the saturated steam used is 1>( ?$a and the vapor space pressure
of the evaporator is at 12 ?$a< -he overall coefficient ; is 15F(
C@m2
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
41/56
5/12/16 | Slide )1
Calculation metho!" for "ingle-effect e#aporator"
Data provided:
" 45AF ?g@h
x" 2( #t % (
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
42/56
5/12/16 | Slide )2
Calculation metho!" for "ingle-effect e#aporator"
Amounts of vapor and liquid products =
", x" and x) are ?no#n, and therefore ) 1814 ?g@h and & 2>22 ?g@h
!team used =
! ) h) 3 & H& 6 " h"
h" enthalpy of 2(% solution at F(o. 214 ?I@?g
using the enthalpy0concentration chart7
h) enthalpy of 5(% solution at -1 G
using the enthalpy0concentration and +oiling6point rise charts7
!aturated temperature at $ (
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
43/56
5/12/16 | Slide )$
Calculation metho!" for "ingle-effect e#aporator"
Amounts of vapor and liquid products =
", x" and x) are ?no#n, and therefore ) 1814 ?g@h and & 2>22 ?g@h
!team used = ! =
! 1814 x 5(5 3 2>22 x 2FF> 0 45AF x 214 >2(494( ?I@h
latent heat of vapouriation of #ater at 1 +ar and 1152(494( @ 221F ?g@h A251 ?g@h
!team econom" = ?g vapouried @ ?g steam used = V # ! =
!team economy 2>22 @ A251 (
* ! @ ; -! 0 -17 K>2(494( J 1((( @ AF((L @ K15F( J 115
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
44/56
5/12/16 | Slide ))
xample A (Repeat Example 2 assuming that the thermal properties of the
liquid in the evaporator can be approximated by those of water/
*n evaporator is used to concentrate 45AF ?g@h of a 2(% MaH
solution entering at F(. to a product of 5(% solids< -he pressure
of the saturated steam used is 1>( ?$a and the vapor space pressure
of the evaporator is at 12 ?$a< -he overall coefficient ; is 15F(
C@m2
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
45/56
5/12/16 | Slide )5
Calculation metho!" for "ingle-effect e#aporator"
Data provided:
" 45AF ?g@h
x" 2( #t % (
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
46/56
Calculation metho!" for "ingle-effect e#aporator"
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
47/56
5/12/16 | Slide )<
Calculation metho!" for "ingle-effect e#aporator"
.ompare the solutions of xamples 2 and A and discuss the
importance of considering the +oiling pint rise and enthalpy
change of concentrated solution<
Dou$le-effect e#aporator"
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
48/56
5/12/16 | Slide )=
Dou$le-effect e#aporator"
Calculation metho!" for !ou$le-effect e#aporator"
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
49/56
5/12/16 | Slide )>
Calculation metho!" for !ou$le-effect e#aporator"
f li:uid is to +e evaporated in each effect,
and if the +oiling point of this li:uid is unaffected +y the solute concentration,
then #riting a heat +alance for the first evaporator/:1 ;1 *1 =-1 ;1 *1 -! 0 -17
!imilarly, in the second evaporator,
remem+ering that the OsteamO in the second is the vapor from the firstevaporator
and that this #ill condense at approximately the same temperature as it +oiled,
since pressure changes are small,
:2 ;2 *2 =-2 ;2 *2 -1 0 -27
f the evaporators are #or?ing in +alance, then all of the vapours from the
first effect are condensing and in their turn evaporating vapours in the
second effect< *lso assuming that heat losses can +e neglected, there is no
apprecia+le +oiling6point elevation of the more concentrated solution, and
the feed is supplied at its +oiling point,
q1 q2
Calculation metho!" for !ou$le-effect e#aporator"
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
50/56
5/12/16 | Slide 5?
Calculation metho!" for !ou$le-effect e#aporator"
f the evaporators are #or?ing in +alance,
then all of the vapours from the first effect are condensing
and in their turn evaporating vapours in the second effect<
*lso assuming that heat losses can +e neglected,
there is no apprecia+le +oiling6point elevation of the more concentrated solution,
and the feed is supplied at its +oiling point,
q1 q2
-hat is, ;1 *1 =-1 ;2 *2 =-2
"urther, if the evaporators are so constructed that !1 !2,
the foregoing e:uations can +e com+ined<
;2 @ ;1 =-1 @ =-2
-hat is, the temperature differences are inversely proportional to the overall heat
transfer coefficients in the t#o effects<
-his analysis may +e extended to any num+er of effects operated in series, in the same #ay
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
51/56
5/12/16 | Slide 51
Effect of #oiling(point elevationFor triple effect evaporators
281°.
252°.
2F1°.
22F°.
2(9°.
1>F°.
>9851(5P-total
;t*totalP-total
8 t i l ff t t
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
52/56
5/12/16 | Slide 52
8 triple(effect evaporator
Energy Balance e'uations for tripleeffect evaporator
*1, t"e rate of stea flo& to t"e first
effect+
*2, to *), t"e rate of flo& fro eac"
effect+ *5, t"e #oiling teperature in t"e first
effect+
*6, t"e #oiling teperature in t"e second
effect+ and
*
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
53/56
5/12/16 | Slide 5$
xample 4/
stimate the re:uirements of steam and heat transfer surface, and the evaporating
temperatures in each effect, for a triple effect evaporator evaporating 5(( ?g h61
of a 1(% solution up to a A(% solution<
!team is availa+le at 2(( ?$a gauge and the pressure in the evaporation space in
the final effect is F( ?$a a+solute< *ssume that the overall heat transfer
coefficients are 22>(, 2((( and 142( I m62 s61 Q.61 in the first, second and third
effects, respectively<
Meglect sensi+le heat effects and assume no +oiling6point elevation, and assume
e:ual heat transfer in each effect<
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
54/56
5/12/16 | Slide 5)
%a"" $alance *.g "(1,
Solids Li'uids Total
Feed 5? )5? 5??
:roduct 5? 11< 16<
Evaporation $$$
Fro stea ta#les+ t"e condensing teperature of
stea at 2?? .:a *g, is 1$)JC and t"e latent "eat is
216) .; .g (1 Evaporating teperature in final effect
under pressure of 6? .:a *a#s, is =6JC+ as t"ere is no
#oiling(point rise and latent "eat is 22>) .; .g(1E'uating t"e "eat transfer in eac" effect
q1 q2 q$
U 1 A1∆T 1 U 2 A2 ∆T 2 U $ A$∆T $
8nd ∆ T 1 K ∆T 2 K ∆T $ *1$) ( =6, )=JC
o&+ if A1 A2 A$
t"en ∆T 2 U 1∆T 1 /U 2 and ∆T $ U 1∆T 1 /U $
so t"at ∆T 1*1 K U 1/U 2 K U 1/U $, )=+
∆T 1 9 M1 K *22
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
55/56
5/12/16 | Slide 55
* , * , g
in second effect is *121 ( 1)6, 1?65JC latent "eat 22)? .; .g(1
in t"e t"ird effect is *1?65 ( 2?6, =6JC+ latent "eat 22>) .; .g(1
E'uating t"e 'uantities evaporated in eac" effect and neglecting t"e sensi#le "eat c"anges+
if &1
+ &2
+ &$
are t"e respective 'uantities evaporated in effects 1+2 and $+ and w s
is t"e
'uantity of stea condensed per "our in effect 1+ t"en
w 1 9 22?? 9 1?$ w
2 9 22)? 9 1?$
w $ 9 22>) 9 1?$
w s 9 216) 9 1?$
T"e su of t"e 'uantities evaporated in eac" effect ust e'ual t"e total evaporated in all
t"ree effects so t"at w 1 K w
2 K w
$ $$$ and solving as a#ove+
w 1 11$ .g "(1 w
2 111.g "(1 w
$ 1?=.g "(1
w s 115 .g "(1
Steam con"umption
0t re'uired 115 .g stea *w s
, to evaporate a total of $$$ .g &ater+ t"at is
?$5.g stea/.g &ater evaporated
&eat e'changer "urface.
-riting a "eat #alance on t"e first effect
*11$ 9 22?? 9 1???,/$6?? 22
-
8/17/2019 Lect - 21 Evoporation Lecture 1 of 3.pptx
56/56