Lect - 21 Evoporation Lecture 1 of 3.pptx

Embed Size (px)

Citation preview

  • 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