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    Solid Liquid

    ExtractionPrepared by Dr.Nagwa El-Mansy

    Chemical Engineering Department

    Cairo University

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    Solid-Liquid Extraction

    Solid liqid e!traction"leaching# means the removalo$ a

    constitent $rom a mi!tre o$ solids by bringing the

    solid material into contact with a liqid solvent thatdissolves this particlar constitent.

    Applications:-

    %- leaching o$ soybean oil $rom &a'ed soybeans with

    he!ane.(- )eaching o$ sgar $rom sgar beets with hot

    water.

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    *- Prodction o$ vegetable oils with organic solvents

    sch as he!ane + acetone and ether by e!traction

    the oilpeants + soybeans+ sn&ower seeds+ cotton seeds

    and

    halibt livers.

    ,- Solble tea is prodced by water leaching o$ tea)eaves.

    - old is leached $rom its ore sing an aqeos

    sodim cyanide soltion.

    /- E!traction o$ copper o!ide $rom low grade oreswith

    dilte 0(S1,acid.

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    Mechanism of leaching:-

    E!traction involves two steps which are2-

    %- Contacting step2- o$ solvent and the material to be3reated+ so as to trans$er solble constitent to the

    Solvent.

    (- Separation step2- o$ the soltion $ormed $rom the

    relatively e!hasted solids.3he above two steps may be condcted in separate

    Eqipment or in one and the same eqipment.

    Soltion reslting $rom separation step is termed

    1ver&ow+ Solids le$t over are termed Under&ow.

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    E!traction 3erminology2-

    %-Contacting Step2-

    4asically it5s a mass trans$er step + it aims attrans$erring

    3he solble constitent $rom the solid phase into the

    liqid phase by di6sion and dissoltion.

    3he solte is 7rst dissolved $rom the sr$ace o$ the

    solid+ then passes into the main body o$ the soltion by

    di6sion.

    3his process may reslt in the $ormation o$ pores in the

    solid material which e!poses $resh"new# sr$aces tosbseqent solvent penetration to sch sr$aces.

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    8n ideal contacting "mi!ing# stage yields a prodct in

    3hermodynamic Eqilibrim9 No mass trans$er.

    No heat trans$er. No momentm

    trans$er.

    :hy its di;clt to reach an ideal stage behavior+

    thermodynamic eqilibrim than the solbility in the solvent+

    so the reali?ation o$ an ideal stage needs rather longtimes i$ the operations is carried ot batch wise or an

    E!cessively large apparats i$ the operation is continos.

    How to increase the rate of mass transfer?

    %- @ncrease the agitation speed+ thic'ness o$ the bondary

    )ayer decreases.

    (- 8s the temperatre increase+ the solbility increase+

    0ence+ the di6sion increases + the viscosity decreases

    leading to decrease in the 7lm thic'ness + hence the pore

    resistance decreases.

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    *-Si?e redction o$ solid to increase the e!posed mass

    trans$er area.

    (-Separation step2-

    3his is a momentm trans$er step that can be carried

    ot by "settling or 7ltration#.

    Calclations2-

    3he design problem o$ calclating nmber o$theoretical stages reqire simltaneos soltion o$

    material balance and eqilibrim relations.

    :e have three component system2-

    %- solte "8#(- @nert solid"4#

    *- Solvent "S#

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    Aepresenting the three component systemon right

    angle triangle2-

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    Addition of two streams:-

    AP AQ AR

    BP BQ BR

    SP SQ SR

    P + Q = R

    P x + Q x = R x

    P x + Q x = R x

    P x + Q x = R xBy using lever arm principle,

    the length and amounts

    are calculated as follows!PR Q a

    = =RQ P "

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    Sutraction of two streams:-

    A# A$ A%

    B# B$ B%

    S# S$ S%

    # ! $ = %

    # x ! $ x = % x

    # x ! $ x = % x

    # x ! $ x = % xBy using lever arm principle,

    the length and amounts

    are calculated as follows!

    #% $ "&= =

    $% # a& + "&

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    Equilirium !elations:-

    8-)ocs o$ nder &ow2-

    %-3he mass o$ soltion retainedBnit masso$ inert

    insolble solids is obtained e!perimentally

    as a $nctiono$ soltion composition 8.3he data is

    sally available

    in tablar $orm.

    "#g solution $ #g solids % &"A'S%$(

    )A & " A$ A' S%

    .* .%

    .*( .(

    .* .*

    ., .,

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    (- @n some cases the soltion retainedB'gsolids is

    appro!imated constant.i.e independent o$

    composition.

    3his means that thelocs o$ nder&ow

    is a line parallel

    to the hypotense.

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    *-)ocs is a straight line2-

    )ocs between two points " + . # F " .,+

    #

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    ,- GG @b solvent retainedB @b solte-$reesolid2-

    ( )

    ( )

    S

    B

    S

    A S

    S A S A S

    S A

    S A

    S A

    A

    S= r = '

    BS

    xA+B+S rB x

    A+B+S

    x = r' ! ( x + x )

    x = r ! r (x + x ) = r ! r x + r x

    ('+ r) x = r ! r x

    r rx = ! x ( *t&s a striaght line euation)

    '+ r '+ r

    *f x = x ='

    *f x =

    = =

    S

    x = '-.

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    4- )ocs o$ over&ow2-

    8lso the over&ow consists generally o$

    soltion only +i.e"solid-$ree or inert-$ree# its locs is2-

    %-3he hypotense.

    (- @n some cases the over&ow contains %Hsolids and

    IH soltion.

    4 2 8 J S

    % 2 I

    !4K 4 B " 8 J 4 J S # K .%

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    *- GGG..and the over&ow $rom the 7rststage contains

    %H $rom the solids in the $eed.

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    @deal E!traction Stage2-

    @deal e!traction stage indicates

    whether or not we are close

    to thermodynamic eqilibrim.

    3he ideal stage in leaching is represented as astraight

    line going throgh point "4# and ctting the two locs

    in !%and y%.

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    Methods of *peration "+,pes of contact%:-

    -Simple single stage:-

    Considering a theoretical e!traction stage where )

    with composition !is broght into contact with a

    solvent Lhaving composition y . 3he amont )%and

    Composition !%o$ the

    prodct nder&ow aswell as

    the amont L%and

    comp. y%

    o$ the over&ow are to be

    determined.

    determ

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    ' '

    ' ' ' ' $

    $

    ' ' $

    $

    $

    $

    /$B! 0 + 1 = 0 + 1 = $

    2$B! 0 x + 1 y = 0 x + 1 y = $ x

    x , y , x lies on straight line and x , y , x lies on straight line

    y x 0 a = =

    x x 1 "

    0 0 y x = =0 +1 $ y x

    ' $ '

    ' $ '

    ' ' ' $' '

    ' ' ' $

    a= get a 3 "

    a+"

    Also!

    y x 0 d= =x x 1 c

    0 0 y x d= = get 0 31

    0 +1 $ x x d+c

    =

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    $

    $

    ' '

    ' '

    x is the point of intersection

    "etween the two straight lines,

    2onnect x with (B) to cut thetwo loci in x and y 45o o"tain

    the flow rates (amounts 1 , 0 )

    apply lever arm principle4

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    .- Multi-stage cross current contact:-

    '

    '

    '

    ' '

    ' ' ' ' $

    $

    ' ' $

    $aterial Balance on first Stage!

    /$B! 1 + 0 = 1 + 0 = $

    2$B! 1 y + 0 x = 1 y + 0 x = $ x

    y , x , x lies on a straight line4

    3 y , x , x lies on a s

    traight line4

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    '

    '

    $

    $ ' '

    ' '

    x is the point of intersection "etween the two straight lines,

    connect x with (B) to cut the two loci in x and y 45o o"tain

    the flow rates (amounts 1 , 0 ) apply lever arm principle4

    $aterial Bal

    .

    .

    .

    . .

    . . . . $

    $

    ' ' $

    ance on second Stage!

    /$B! 1 + 0 = 1 + 0 = $

    2$B! 1 y + 0 x = 1 y + 0 x = $ x

    y , x , x lies on a straight line4

    3 y , x , x lies on a straight lin

    .

    .

    $

    $ . .

    . .

    e4

    x is the point of intersection "etween the two straight lines4

    Again connect x with (B) to cut the two loci in x and y 4

    5o o"tain the flow rates (amounts 1 , 0 ) apply lever arm principle

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    /- Multi-stage counter current contact:-

    Considering a $eed with rate )Dand

    composition !D

    which is to be treated with a solvent having a

    composition ynJ%at a rate o$ LnJ%. 3he weight

    $ractionin the U. sholdn5t e!ceed a given vale !n8.

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    3o determine the nmber o$ ideal stages reqired to

    achieve this e!traction dty se the $ollowing steps2-

    3he overall M.4 on the n stages may be written as2-

    = ! + ynJ%+ and !nare located $rom the speci7cationso$ the problem.

    = !M + !and ynJ%lies on the same straight line where

    !M

    Divides the distance between !and ynJ%in the ratioo$ LnJ% B ).

    n+' ' n

    n+' n+' ' ' n n $

    /$B! 1 + 0 = 1 + 0 = $

    2$B! 1 y + 0 x = 1 y + 0 x = $ x

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    M being the sm o$ )nand L%+ the points

    !n+!M+y%lies

    on one straight line. 0ence y% may belocated by

    E!trapolating !n !M to meet the locs o$ the

    over&ow.n+' n '

    n+' n+' ' '

    n+' n

    '

    5he $4B on each stage can "e written as!

    1 ! 0 1 ! 0 = R

    Also 1 y ! 0 x = 1 y ! 0 x = R

    y , x , R lies on a straight line4

    3 y , x , R lies on a straight line4

    R is lo

    n n

    =

    ' n n+'

    ' ' . . n n

    ' ' . .

    cated "y extrapolating x y and x y 4

    5ie line is drawn "etween y x , y x ,4444444444, y x

    "y 6oining ( , x ,y ) , ( , x ,y ) and so on4

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    0ondition of in1nite numer of stages:-

    3o increase over&ow composition the nmber o$ stages

    mst be increased.

    the nmber o$ stages

    increases till reaching

    the ma!imm over&ow

    composition "y%ma!#.Steps:-

    Connect ynJ%with "4#

    3o ct U. locs in ! n min

    connect it with !M thene!tend the line to ct 1.

    in y %ma!.

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    Stage E2cienc,:-

    8 deviation between the behavior o$ actal and

    3heoretical stages is e!pected.@n theoretical stage2-

    "SolventB Solte# 1. K "SolventB Solte# U.

    8ctal stage is less e;cient becase2-

    %- Slow rates o$ mass trans$er reslting in

    nattainment o$ 3.D.E "complete dissoltion# inthe

    contact time provided $or the stage.(-3he solte is adsorbed by the solid to a higher

    content than the solvent.

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    *- 3he pore strctre o$ the solid-solte mi!tre issch

    that only a portion o$ the solte is accessible to the

    solvent.

    0alculation of actual numer of stages:-

    8- Constrction when over&ow e;ciency is 'nown2-

    3he tie line representingan actal stage passes no

    )onger throgh the origin "4#.

    n+' n act/47 8

    n+' n

    y ! y9 = 4:

    y ! y

    =

    Actual change in concentration95heoretical change in concentration

    =

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    4- Constrction when nder&ow e;ciency is'nown2-

    3he tie line representing

    an actal stage passes no

    )onger throgh the origin "4#.

    n!' n act;47 8

    n!' n

    ! x9 = 4:

    ! x

    x

    x

    =

    Actual change in concentration

    9 5heoretical change in concentration=

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    Solids-free coordinates:-

    3he constrction is in general less crowded

    than inthe case o$ trianglar diagram. No calclations

    are

    needed be$ore plotting the eqilibrim

    relations +btthe same procedres described be$ore are

    $ollowed.

    3he amonts o$ varios streams shold bee!pressed

    on solid-$ree basis.

    Some notes on Aectanglar Diagram System2-

    3he abscissa O "wei ht $raction o$ solte on a

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    Equilirium !elations:-

    5he concentration of inert (B) in the solution mixture

    is expressed in %g units!

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    Some notes on the diagram!

    Pure A = ' , B = , S =

    A A B x or y = = , # = A + S A + A + S '

    Pure S = ' , A = , B = A B

    x or y = = = , # = A + S + ' A + S '

    P

    = =

    +

    = =

    +

    ure B = ' , A = , S =

    A B 'x or y = = = , # =

    A + S + A + S = =

    +

    i lid

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    8 typical eqilibrim diagram is shown in the$ollowing

    igre.

    3he lower crve o$ 3vs ,A+ where N K on thea!is+ represents the locs o$ over&ow"where all the

    solids has been removed#.

    3he pper crve 3vs xA + Aepresents the locs o$

    nder&ow.3he tie lines are vertical+ and on !-y diagram+ the

    Eqilibrim line is y8K !8

    n+' n+'

    %g inert solid7or the entering solid feed to "e leached, # =

    %g solute A

    and x = '4 4 And for pure solvent # = , y = 4

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    +,pes of contact:-

    "%#Simple Single Stage2-3he application o$ material

    balance eqations in connction with sch diagram +

    the qantities o$ varios streams shold bee!pressed

    on solid-$ree basis.

    )K mass o$ $eed.

    K "8J4# or "8J4JS#.

    )5K mass o$ $eed on solid

    ree-basis K "8# or "8JS#.

    LK mass o$ solvent K "SJ8#

    L5K mass o$ solvent on solid

    ree-basis K "SJ8#.

    /$B 0 &+ 1 & 0 &+ 1 & $&

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    ' '

    ' ' ' ' $

    $

    ' ' $

    $

    $

    /$B! 0 &+ 1 & = 0 &+ 1 & = $&

    2$B! 0 &x + 1 & y = 0 &x + 1& y = $& x

    x , y , x lies on straight line

    and x , y , x lies on straight line

    y x 0 & a

    = =x x 1 & "

    0 & =0 &+1 &

    $

    ' $ '

    ' $ '

    ' ' ' $' '

    ' ' ' $

    0 & y x a= = get a 3 "

    $ y x a+"

    Also!

    y x 0 & d= =

    x x 1 & c

    0 & 0 & y x d= = get 0 &31 &

    0 &+1 & $& x x d+c

    =

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    ' '' '

    ' '

    0 A + S B

    = (A+S) + B

    = 0 & + B

    0 A + S B = +

    A + S A + S A + S

    0 = ' + #0 &

    00 & =

    ' + #1 0

    also 1 & = 3 0 &'+# '+ #

    = +

    =

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    Single stage $or the other type o$eqilibrim2-

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    *- Mlti-stage conter crrent contact2-

    n+' ' n

    n+' ' n

    n+' n+' ' ' n n $

    n+' n ' . '

    n+' n+

    /$B! 1 + 0 = 1 + 0 = $

    1 & + 0 & = 1 & + 0 & = $&

    2$B! 1 & y + 0 & x = 1 & y + 0 & x = $& x/perating relations!

    1 & ! 0 & 1 & 0 & =1 & ! 0 & R

    1 & y

    = = =

    ' n n ' ' . . ' ' R ! 0 &x =1 &y !0 &x = 1 &y ! 0 &x = R x

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    Equipments in leaching:-

    %- i!ed-bed leaching2-

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    i!ed bed leaching is sed in the beet sgar and is also

    $or e!traction o$ pharmaceticals $rom bar's and seeds+

    and $or other processes.

    @n the igre+ the cover is removable so sgar beet

    slices can be dmped into the 4ed. 0ot water at Q% to

    QQRC &ows into the bed to leach ot the sgar.

    3he leached sgar soltion &ows ot the bottom onto

    the ne!t tan' in series. 3he top and bottom covers are

    removable so that the leached beets can be removed

    and a $resh charge added. 8bot IH 1$ the sgar in

    the beets is leached.

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    (-4ollman bc'et-type e!tractor2-

    D lid dd d h i h id

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    Dry solids are added at the pper right side to a

    Per$orated bas'et or bc'et. 3he solid leached by a

    dilte soltion called hal$ miscella.

    3he liqid downward throgh the moving bc'etsand

    is collected at the bottom as the strong soltion or$ll

    miscella.

    3he bc'ets moving pward on the le$t are leached

    conter crrently by $resh solvent sprayed on thetop

    bc'et. 3he wet &as's are dmped as shown in the

    igre and removed continosly.

    * ild b d

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    *-0ildebrandt screw-conveyor e!tractor2-

    C i $ h d

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    Consists o$ three screw conveyers arrangedin a U

    shape.

    3he solids are charged at the top rightconveyed

    downward+ across the bottom + and then p

    the other)eg . 3he solvent &ows conter crrently to

    the solid.

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