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Chemical Engineering Operations
Solvent extraction (liquid-liquid extraction)
Dr. Anand V. Patwardhan
Professor of Chemical EngineeringInstitute of Chemical Technology
Nathalal M. Parikh Road
Matunga (East), Mumbai-400019
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Liquid-Liquid Extraction / Solvent Extraction
Separation of constituents of liquid solution by contact
,
original solution distribute differently between the two
.
EXTRACTION:
1. Carrier phase is a liquid, not a solid, so the physical
se aration techni ues chan e
2. Two distinct li uid hases develo hence non-
2uniformity of resulting solutions.
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Driving force: chemical differences, not the vapor
pressure erences, an ence can e use w endistillation is impractical.
For example: to separate materials with similar boiling
,
compounds.
Common applications: separation and purification of
broth, etc.
3
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Distillation and evaporation generally produce
n s e pro uc s; qu ex rac on genera y oes no .
products.
Secondary separation: by distillation or evaporation.
The overall process cost thus must be considered
Extraction may become economical for dilute aqueous
solutions when eva oration would re uirevaporisation of very large amounts of water.
4
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Terminology
Feed: solution to be treated.
Solvent: liquid used in contacting.
Extract: enriched solvent product.
Raffinate: depleted feed.
Process: single stage, multistage crosscurrent, or
countercurrent.
5
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A typical ternary diagram
6
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Plait point:
Located near the top of the
two- hase envelo e at the
inflection point.
represen s a con on
where the 3-component
phases, but the phases have
.
(analogous to azeotropicmixture of liquid and
vapor.)
7
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2 main classes of liquid-liquid equilibrium:
Class I system: 1 immiscible pair of compounds.
,
triangular diagram.
Solvent Selection:
.
Distribution coefficients: y/x at equilibrium; large
values pre erable.
Insolubilit : should not be soluble in carrier feedliquid.
8
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Recoverability: constraints such as azeotropes.
Density: must be different phases can be separated by
settling.
Interfacial tension: if too high, liquids will be difficult
.
Chemical reactivity: solvent should be inert and stable.
Viscosity, Vapor pressure, Freezing point: low values
Safety: toxicity, flammability.
Cost:
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Calculations:
1. Number of stages needed to make a separation.
2. Amount of solvent needed to make a separation.
3. Liquid-Liquid equilibrium is not available as
, .
Choice of graphical approaches:
i. McCabe-Thiele approach: if y versus x data is
available mass fraction of solute in E- hase versusmass fraction in R-phase). The curve begins at the
ori in and ends with the lait oint com osition.
10
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ii. Hunter-Nash method: Equilateral triangle
agram: cons ruc on can e one rec y on etriangle.
iii. Rectangular equilibrium diagrams.
11
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Solvent-to-Feed Ratio:
For a given feed mixture, required degree of
, ,
choice of solvent, there exists a minimum solvent-to-
.
,
extract phase in equilibrium with the entering feed.
A theoretical upper limit or maximum solvent-to-feed
ratio also can be determined. The maximum solvent-to-feed ratio is thus that which puts the mixture on the
hase boundar or sin le hase.
12
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Extraction equipment selection
Design constraints:
(1) Maximise area of mass transfer,
(2) Adjust flow feeds for maximum solute recovery.
3 main types of extractors:
Mixer-settlers: when only one equilibrium stage is
needed. The two hases are added and mixed.Separation based on density differences.
Disadvanta e: re uires lar e-volume vessel and a hi h
13solvent demand.
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Contacting columns: packed / tray / spray column. In
case o pac e co umn, e pac ng ma er a s ou ewetted by the continuous phase. The flow in a column
.
less than 4%. Also, it can offer multiple stages.
14
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Advantages and disadvantages of the various extractor types
Unit Advantages Disadvantages Efficient
Low head room
Mixer-
Settler
Induces good
contacting
Can handle an
arge oor
High setup costs
High operation costs
number of stages
Columns Small investment costs
High head room
w ou
Agitation) Low operating costs
cu o sca e up rom a
Less efficient than mixer-settler
Good dispersion
(with
Agitation)
Low investment costs
Can handle any
number of stages
differences
Does not tolerate high flow ratios
Centrifugal
Can separate small
density differences
High setup cost
High operating and maintenance
15
Small liquid inventory
Cannot handle many stages
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Liquid-Liquid Equilibrium
Chemical potential of both liquid phases must be equal,
L L L L L L1 2 1 1 2 2
x xi i i i i i
For a multi-component system, the UNIQUAC equation
combinatorialln ln l residualn
17
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Activity coefficient models for finding mole fractions:
UNIFAC (UNIquac Functional group Activity
-
Combinatorial and residual activities: based on
statistical mechanical theory and compositions are
computed from the size and energy differencesbetween the molecules in the mixture. The
relationships for these two activities are available.
18
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Theory Ternary Phase Diagram
General principles:
within the triangle to the three sides equals the altitude
.
Each apex of triangle represents one pure component.
Any point of a side of the triangle represents a binary
Lines may be drawn parallel to the sides of the
equ a era r ang e or e p o ng o e
compositions.
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Phase diagram for a three component system
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Ternary phase diagram: directly from experimental
a a.
,
experimentally by a cloud point titration.
For example, a solution containing A and C with
,
is added until the onset of cloudiness (haze) due to
Then the composition can be plotted onto the
ternar hase dia ram.
21
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Tie lines: Join points (equilibrium) on miscibility
oun ary.
from an experiment
For example, A mixture may be prepared with
, ,
B). If we allow it to equilibrate, then we can
raffinate (R) phase.
22
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Point F: feed; Point S: solvent.
Point H: composition of feed and solvent at
.
(S) compositions for each component
Points Rand E: compositions of Raffinate and
Point P: lait oint: At this oint onl one li uidphase exists and the compositions of two effluents are
e ual.
23
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Curve JRDPEK: equilibrium between all three
componen s.
will exist.
Area above JRDPEK: only one liquid phase.
Operating line: FE and SR.
Equilibrium constraint: chemical potential
x xA A A A A A
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Separation factory
E
R Solute fluxes in Raffinate and Extract:
Extract:N K A x xE E E
Raffinate:N K A x xR R R
,
KE = overall mass transfer coefficient on Extract side
R
A = total available mass transfer areai =E
xiR= solute concentration at interface on Raffinate side
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A = carrier solvent B = solvent used for extraction C = component to be extracted
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The figure gives a general ternary diagram for a
es re so u e , an ex rac ng so ven an acarrier solvent (A) from which the solute is to be
.
.
extract C from feed.
The Raffinate composition (R) is specified with respect
to the recover ofC.
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Composition Flow rate
g ven g venF A and C YES YES
S B
YES, usually pure
OR
NO, obtained
fromre a ve y pure ca cu a on
NO, obtainedNO, obtained
E
large C fromcalculation
component
balance
A with
YES, recovery
amount
NO, obtained
fromsmall C of solute C
needed
component
balance
28
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,determination of the
minimum solvent-to-feedra o min
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Step 1: Determine the minimum solvent-to-feed ratio
min. s s nee e o n ou ex raccomposition)
Procedure:
1. Draw an operating line from S to Rthat extends
beyond the boundaries of triangular diagram.
2. Each tie line is considered to be a pinch point, and line
drawn rom each t e l ne to operat ng l ne s named P ,
, Pn.
3. The Pinch Point farthest away from Ris called Pmin.
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4. After Pmin is established, a line is drawn from Pmin,
roug ee compos on , o e o er s e oequilibrium curve. This point will represent E1.
5. After E1 is known, a mass balance around the system
= =.
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Solving for (Smin/F), we get,
x xS A Amin F M
F x xA AM S
Let (S/F)actual = z (S/F)min (1 < z < 2, generally).
The new mixing point (M) is determined by movingalong the FS line until the new ratio point is reached.
(FM )
M S
l
l
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Step 2: Determine E (extract composition)
Procedure:
1. A line is drawn from the raffinate composition (R),
side of the equilibrium line.
2. This is the extract (solute-rich solvent) composition
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Step 3: Determine Operating Point P
Procedure:
1. Operating point is a graphical point that represents
.
.
(R) points on diagram (operating line)
3. Draw a line connecting extract (E) and feed (F). The
oint at which these two lines intersect P is o eratinpoint P.
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extract (E) composition
36
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Step 4: Number of stages
Procedure:
1. Follow tie line from E to the other side of equilibrium
stage.
2. Another operating line drawn from operating point P,
is an equilibrium stage of the system.
3. This procedure is repeated until stages have been
constructed to R the raffinate com osition.
37
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Step 5: Calculation of unknown flow rates
Procedure:
. .
x F x S x R x EAF AS AR AEx F x S x R x E
BF BS BR BE
xAs and xBs = fractions ofA and B for the specifiedstreams.
F, S, R, and E = flow rates of feed, solvent, raffinate,
.
Rand E = unknowns.
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Step 6: Determination of Extraction Column Diameter
Procedure:
1. Diameter of extraction column must be enough to
flooding.
2. Estimation of column diameter for L-L contacting-
contactors due to larger number of important
variables.
39
3 V i bl f l l i l di
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3. Variables necessary for calculating column diameter
nc u e:
Viscosit and densit of continuous hase
Geometr of internals
40
4 C l di t b b t d t i d th h
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4. Column diameter may be best determined through
sca e up o a ora ory es runs:
. P .
. .
.
constant for larger scaled up commercial units.
iv. The superficial velocity data will be used to
calculate the column diameter throu h thefollowing correlation derivation:
41
5 A t l l it f di d (d l t)
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5. uD = Actual average velocity of dispersed (droplet)
p ase.
uC = Actual average velocity of the continuous phase.
UD = Superficial velocity of dispersed phase.
U = Su erficial velocit of continuous hase.
D= Volume fraction of dispersed phase in column
ur = verage rop e r se ve oc y re a ve o con nuous
phase
= apac ty Parameter or the extract on column
CD = Drag CoefficientD, C = Densities of dispersed and continuous phases
=
42
{1 } F t hi h t f hi d d i i
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{1D} = Factor which accounts for hindered rising
e ec o o er rop e s
u0 = Characteristic rise velocity for a single droplet
= Viscosity (subscript will determine component)
= Interfacial tension subscri t will determinecomponent)
=
DT = Column diameter
g = ccelerat on due to grav ty
MD, MC = Mass flow rates of dispersed and continuousphases
43
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Countercurrent flows ofdispersed and continuous
liquid phases in a column
Diameter Calculation Procedure:
Step A: Determination of Column Total Capacity
relative to the column wall are:
UU CDu ; u1D C
44
The average droplet rise velocity relative to the
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The average droplet rise velocity relative to the
con nuous p ase s e sum o ese ve oc es or
counter-current operation):
CDu1
D D
This relative velocity is also expressed in terms of
forces acting upon droplet including drag forces,gravitational forces, and buoyancy forces. These
variables are combined into one parameter called C:
dCapacity Paramet P
3 Cer C
45
If the droplet diameter dp is not known C may be
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If the droplet diameter dp is not known C may be
o a ne roug a corre a on ea er, . .; en ey,
E.J.Separation Process Principles, John Wiley and
, , . ,
experimental data from operating equipment.
2 1C Du C 1 12 fr
C From ex erimental data Ga ler et al. found that the
right-hand-side of the above equation may be
ex ressed as:0 D
46
Eliminating the relative velocity (u ) by combining the
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Eliminating the relative velocity (ur) by combining the
a ove equa ons:UU
CD1 0 D
D D
...
D.
value ofUC/uo.
This graph represents the holdup curve for the liquid-
li uid extraction column.
A typical value ofU /u may be assumed 0.1.
47
Typical holdup curve for liquid-liquid extraction
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Typical holdup curve for liquid-liquid extraction
Floodin oint
UD
UC 0.1
u0
u
0
48
At fixed UC an increase in UD results in a increased
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At fixed UC, an increase in UD results in a increased
va ue o o upD
un e oo ng po n s reac e
at the maximum:
D D U
C On the other hand, with UD fixed, UC may be increased
until the floodin oint is achieved at:
U 0C D U
D
49
Inserting these derivatives into equation (1) results in
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Inserting these derivatives into equation (1) results in
e o ow ng express on orD
a oo ng con ons.
The subscript fdenotes flooding:
1 2U U
C CU UD fD D
50
Apply derivatives of Equation (1) into Equation (2), the
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Apply derivatives of Equation (1) into Equation (2), the
express on so ve s mu aneous y resu ng n e
following Figure for the variation of total capacity as a
Total Capacity versus Phase flow ratio
U U
u
Asymptotic limit = 0.25
51
U U
D C
Step B: Characteristic Rise Velocity calculation
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Step B: Characteristic Rise Velocity calculation
Dimensionless quantity [(u0 C C)/( )] may be
. , ,
experimentally.
Hence, the characteristic rise velocity for a single
0.01u
0C C
52
Step C: Calculation of the superficial velocities at
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p p
velocity for best performance.
The sum of superficial velocities is found by reading the
ratio graph., and multiplying by the characteristic risevelocit then dividin the uantit b 2.
53
Step D: Determination of the Total Volumetric Flow rate
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p
e o a vo ume r c ow ra e s a unc on o e mass
flow rates:
UUCDQ
D C
-
The cross-sectional area is the total volumetric flowrate divided b the sum of the su erficial velocities at
50% of flooding: Q
TotalC U UC D 50% floodin
54
Step F: Determination of Column Diameter
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p
The column diameter may be found from the cross-
1 2
D 4A
55
Step 7: Determine the Height of the Column
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Procedure:
HETS (Height Equivalent to a Theoretical Stage) gives
.
,
experimental data suggest that the dominant physical
a. Interfacial tension
b. Phase viscosities
56
c. ens y erence e ween p ases
HETS is estimated by conducting laboratory
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exper men s o e erm ne e co umn ame er as
discussed in step 6.
These values are scaled to commercial size column by
diameter raised to an exponent, which may vary from
. . , .
following figure shows the HETS for columns and
rotar contactor.
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58
Find Value of (HETS/DT1/3)
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From Figure, determine the value of (HETS/DT1/3) at a
.
the column diameter:
HETS = (HETS/DT1/3) . DT1/3
Total height of the column is derived from the number
of e uilibrium sta es Ste 4 :
Total Hei ht = HETS Number of E uilibrium Sta es
59
Performance of Several Types of Column Extractors
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Extractor Type 1/HETS,(m1)
UD + UC,(m/hr)
Packed Column 1.5 - 2.5 12 - 30
. .
Rotating Disk2.5 - 3.5 15 - 30
Karr Column 3.5 - 7.0 30 - 40
60
Important Properties in Liquid-Liquid Extraction:
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1. Temperature: smaller role in extraction than in other
.
streams fed into the column.
There is no heating requirement for the process and
For these reasons extraction can be re arded as anisothermal process.
61
2. Pressure: small role in extraction.
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When combined with the temperature considerations,
-
phase liquid region.
Isothermal and isobaric condition is beneficial to
.
- -
dependent.
62
3. Activity coefficients: most important; determine the
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m sc y an ence e par on ac or o e so u e.
-
are the most accurate in predicting the activities of the
,equations.
Once a predictive model has been plotted on a
equilibrium line experimentally for the most accurate
data.
63
4. Viscosity: affects flooding and choice of equipment.
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Flooding of extraction columns is analogous to
.
spray or packed columns.
64
COSTS INVOLVED
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Economic tradeoff exists for the design:
1. At fixed solvent feed ratio, amount of solute extracted
. ,value of the unextracted solute is balanced against the
.
required decreases as the solvent rate increases. The
ca acit of the e ui ment necessar for handlin thelarger solvent flow increases with higher solvent rate.
Thus the cost of the e ui ment asses throu h a
65minimum.
3. As solvent rate increases the extract solutions become
more u e ere ore e cos o so ven remova s
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more u e. ere ore, e cos o so ven remova s
increased as well as the operating cost for increased
.
.annualized cost (investment and operating costs) must
reflux rate.
5. Cost models have been developed for the various types
of extractor desi n such as column t e extractormixer-settler, and continuous centrifugal extractor.
66
REFERENCES
1 S d J D H l E J S ti P P i i l J h Wil
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1. Seader, J.D.; Henley, E.J. Separation Process Principles. John Wiley
and sons, New York, 1999.
. , . . .
Company, Houston, 1994.
3. Sandler, S.I. Chemical and Engineering Thermodynamics. John Wileyand sons, New York, 1998.
4. Treybal, R.E. Mass Transfer Operations. McGraw-Hill, New York,
.
5. Douglas, J.M. Conceptual Design of Chemical Processes. McGraw-Hill, New York, 1988.
6. Hanson, C.; Baird, M.H.I.; Lo, T.C. Handbook of Solvent Extraction.
John Wiley and sons, New York, 1983.
7. Reid, R.C.; Prausnitz, J.; Poling, B. The Properties of Gases and
Liquids, 4th edition. McGraw-Hill, New York, 1987.
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