Page 1
26.02.2012
1
SEPARATION PROCESSES
LIQUID-LIQUID EXTRACTION PROCESSES
Equilibrium Relations in LL Extraction
Transport Processes and Separation Process Principles CHRISTIE J.
GEANKOPLIS
Unit operations in chemical engineering by Mccabe, Smith and Harriot
PRINCIPLES AND MODERN APPLICATIONS OF MASS TRANSFER
OPERATIONS by Jaime Benitez
Separation process principles by Seader and Henley
Separation processes by King
Chemical Engineering by Coulson and Richardson
LIQUID-LIQUID EXTRACTION PROCESSES
removing one constituent from a liquid by means of an other liquid
solvent
When separation by distillation is ineffective or very difficult (close-boiling
mixtures or substances / or heat sensitive mixtures), liquid extraction is one
of the main alternatives to consider
recovery of penicillin from the fermentation broth by extraction with a
solvent such as butyl acetate
recovery acetic acid from dilute aqueous solutions
Page 2
26.02.2012
2
Equilibrium Relations in LL Extraction
Equilateral triangular coordinates are often used to represent the equilibrium
data of a three-component system, since there are three axes
A, B, or C : a pure component
point M a mixture of A, B, and C
• the perpendicular distance from the
point M to
• the base AB : the mass fraction xc of
C in the mixture at M,
• to base CB the mass fraction xA of A,
• to base AC the mass fraction xB of B
xA + xB + xc = 0.40 + 0.20 + 0.40 = 1.0
Liquid-liquid phase diagram where components A and B are partially
miscible
• C dissolves completely in A or in B.
• A is only slightly soluble in B and B slightly soluble in A.
• The two-phase region is included inside below the curved envelope.
• An original mixture of composition M will separate into two phases a and b
which are on the equilibrium tie line through point M.
• Other tie lines are also shown. The two phases are identical at point P, the Plait
point.
Page 3
26.02.2012
3
Phase diagram where the solvent pairs b—c and a-c are partially miscible.
Page 4
26.02.2012
4
A: acetic acid, B: water, C: isopropyl ether solvent
The solvent pair B and C are partially miscible.
The concentration of the component C is plotted on the vertical axis and that
of A on the horizontal axis.
The concentration of component B is obtained by difference from
inside the envelope: two phase
Outside: one-phase region
A tie line g-i connecting the raffinate and extract layers
raffinate layer : water-rich layer
extract layer : ether-rich solvent layer g,
The raffinate composition is designated by x and the extract by y
Page 5
26.02.2012
5
the mass fraction of C in the extract layer : yc
the mass fraction of C in the raffinate layer and as xc
To construct the tie line gi using the equilibrium yA — xA plot below the phase
diagram, vertical lines to g and i are drawn.
Page 6
26.02.2012
6
Equilibrium: System Water-Chloroform-Acetone
Equilibrium extraction data for the system
water-chloroform-acetone at 298 K and 1 atm are given in Table. Plot these
data on a right-triangular diagram, including in the diagram a conjugate or
auxiliary line that will allow graphical construction of tie lines.
Page 7
26.02.2012
7
Chloroform as solvent (B), water as diluent (A), and
acetone as solute (C).
This system exhibits a type I equilibrium behavior. The
graphical construction to generate the conjugate line
from the tie-line data given (e.g., line RE) is shown, as
well as the use of the conjugate line to generate other
tie lines not included in the original data. Line LRP is
the saturated raffinate line, while line PEK is the
saturated extract line.
Single-Stage Equilibrium Extraction
Derivation of lever-arm rule for graphical addition.
two streams, L kg and V kg, containing components A, B, and C, are mixed
(added) to give a resulting mixture stream M kg total mass.
overall mass balance and a balance on A,
Graphical addition and lever-arm rule : (a) process flow, (b) graphical addition
Page 8
26.02.2012
8
An original mixture weighing 100 kg and containing isopropyl ether (C), acetic
acid (A), and water (B) is equilibrated and the equilibrium phases separated The
compositions of the two equilibrium layers are for the extract layer (V) yA = 0.04,
yB = 0.02, and yc = 0.94, and for the raffinate layer (L) xA =0.12, xB = 0.86, and
xc = 0.02. The original mixture contained 100 kg and xAM = 0.10. Determine the
amounts of V and L.
Page 9
26.02.2012
9
M = 100 kg and xAM= 0.10
L = 75.0 and V = 25.0
L = 72.5 kg and V = 27.5 kg
Single-stage equilibrium extraction
separation of A from a mixture of A and B by a solvent C in a single equilibrium
stage.
the solvent, as stream V2 and the stream L0 enter
The streams are mixed and equilibrated and
the exit streams L1 and V1 leave in equilibrium with each other.
Page 10
26.02.2012
10
Single-Stage Extraction
Pure chloroform is used to extract acetone from a feed containing 60 wt%
acetone and 40 wt% water in a co-current extractor. The feed rate is 50 kglh,
and the solvent rate is also 50 kg/h. Operation is at 298 K and 1 atm. Find the
extract and raffinate flow rates and compositions when one equilibrium stage
is used for the separation. (assume water and chloroform do not mix)
Page 11
26.02.2012
11
V= C, Solvent, chloroform, ya1=0, yC1=1.0
L= Liquid mixture, A: acetone, B=water
xA1=0.60, xB1=0.40
A: acetone
C: chloroform B: water
V1
L1
V2
L2
V1+L1=V2+L2=M
V1yA1+L1xA1=V2yA2+L2xA2=MxAM
V1yB1+L1xB1=V2yB2+L2xB2=MxBM
V1yC1+L1xC1=V2yC2+L2xC2=MxCM
L1
V1
yA2
xA2
V1+L1=V2+L2=M
V1yA1+L1xA1=V2yA2+L2xA2=MxAM
V= C, Solvent, chloroform, ya1=0, yC1=1.0
L= Liquid mixture, A: acetone, B=water
xA1=0.60, xB1=0.40
L1
V1
yA2
xA2
Tie line is constructed through by trial and error, using conjugate line extract
(V2, yA2) and rafinate (L2, xA2) locations are obtained. The concentrations
yA2=0.334, xA2=0.189
Page 12
26.02.2012
12
L1
V1
yA2
xA2
Continuous Multistage Countercurrent Extraction
Page 13
26.02.2012
13
Material Balance for Countercurrent Stage Process Pure solvent isopropyl
ether at the rate of VN+1= 600 kg/h is being used to extract an aqueous
solution of L0 = 200 kg/h containing 30 wt % acetic acid (A) by countercurrent
multistage extraction. The desired exit acetic acid concentration in the
aqueous phase is 4%. Calculate the compositions and amounts of the ether
extract V1 and the aqueous raffinate LN.
Page 14
26.02.2012
14
Pure solvent isopropyl ether (C)
VN+1= 600 kg/h yAN+1=0 yCN+1=1.0
B: aqueous solution of L0 = 200 kg/h containing 30 wt % acetic acid (A) exit
acetic acid concentration in the aqueous phase is 4%.
xA0= 0.30, xB0=0.70, xC0=0, xAN=0.04
L0xA0+VN+1yAN+1=LNxAN+V1yA1=MxAM
VN+1= 600 kg/h yAN+1=0 yCN+1=1.0
L0 = 200 kg/h xA0= 0.30, xB0=0.70, xC0=0, xAN=0.04
Page 15
26.02.2012
15
VN+1= 600 kg/h yAN+1=0 yCN+1=1.0
L0 = 200 kg/h xA0= 0.30
xB0=0.70, xC0=0, xAN=0.04
xCM=0.75, xAM= 0.075
L0xA0+VN+1yAN+1=LNxAN+V1yA1=MxAM
VN+1 and L0 are plotted
LN is on phase boundary
It can be plotted at xAN=0.04
For the mixture point M
xCM=0.075, xAM=0.75
Draw a line from the point of
LN and M at equilibrium of
extract phase gives V1,
yA1=0.08 and
yC1=0.90
, yCN+1, yAN+1
yC1, yA1
xCN, xAN
xC0, xA0
VN+1= 600 kg/h yAN+1=0 yCN+1=1.0
L0 = 200 kg/h xA0= 0.30
xB0=0.70, xC0=0, xAN=0.04
xCM=0.75, xAM= 0.075
yA1=0.08 and
yC1=0.90
LN= 136 kg/h
V1= 664 kg/h
Page 16
26.02.2012
16
Stage-to-stage calculations for countercurrent extraction
stage by stage to determine the concentrations at each stage and the total
number of stages N needed to reach LN
total balance on stage 1
total balance on stage n
Assumption: D (kg/h) is constant for all stages
Page 17
26.02.2012
17
balance on component A, B, or C.
Page 18
26.02.2012
18
Number of Stages in Countercurrent Extraction
Pure isopropyl ether (C) of 450 kg/h is being used to extract an aqueous solution of
150 kg/h with 30 wt % acetic acid (A) by countercurrent multistage extraction. The
exit acid concentration in the aqueous phase (B) is 10 wt %. Calculate the number
of stages required.
VN+1=450, yAN+1=0, yCN+1=1.0
L0=150, xA0=0.30, xB0=0.70, xC0=0, xAN=0.10
(150 0) (450 1.0)0.75
150 450
x x
(150 0.30) (450 0)0.075
150 450
VN+1=450, yAN+1=0, yCN+1=1.0
L0=150, xA0=0.30, xB0=0.70, xC0=0, xAN=0.10
Page 19
26.02.2012
19
Countercurrent-Stage Extraction with Immiscible Liquids
If the solvent stream VN+l contains components A(solute) and C(solvent) and
the feed stream L0 contains A (solute) and B(feed solvent) and components
B and C are relatively immiscible in each other, the stage calculations are
made more easily.
The solute A is relatively dilute and is being transferred from L0 to VN+1
where L’ = kg inert B/h, V’ = kg inert C/h, y = mass fraction A in V stream, and x = mass fraction A in L
stream
Slope of the operating line is L’ / V’ (if y and x are dilute)
Page 20
26.02.2012
20
EXAMPLE Extraction of Nicotine with Immiscible Liquids. An inlet water solution of
100 kg/h containing 0.010 wt fraction nicotine (A) in water is stripped with a
kerosene stream of 200 kg/h containing 0.0005 wt fraction nicotine in a
countercurrent stage tower. The water and kerosene are essentially immiscible in
each other. It is desired to reduce the concentration of the exit water to 0.0010 wt
fraction nicotine. Determine the theoretical number of stages needed. The
equilibrium data are as follows (C5), with x the weight fraction of nicotine in the
water solution and y in the kerosene.
Page 21
26.02.2012
21
L0=100 kg/h, x0=0.010
VN+1= 200 kg/h, yN+1= 0.0005, xN=0,0010
L’=L(1-x)=L0(1-x0)=100(1-0.010)=99.0 kg water/h
V’=V(1-y)=VN+1(1-yN+1)=200(1-0.0005)=199.9 kg kerosen/h
1
1
0.010 0.0005 0.001099.0 199.9 99.0 199.9
1 0.010 1 0.0005 1 0.0010 1
y
y
y1=0.00497
N=3.8 theoretical plate
L0=100 kg/h, x0=0.010
VN+1= 200 kg/h, yN+1= 0.0005, xN=0,0010 y1=0.00497
Page 22
26.02.2012
22
Minimum Solvent and Countercurrent Extraction of Acetone. An aqueous
feed solution of 1000 kg/h containing 23.5 wt % acetone and 76.5 wt % water is
being extracted in a countercurrent multistage extraction system using pure
methyl isobutyl ketone solvent at 298-299 K. The outlet water raffinate will
contain 2.5 wt % acetone. Use equilibrium data from Appendix A.3.
a) Calculate the minimum solvent that can be used. [Hint: In this case the tie
line through the feed L0 represents the condition for minimum solvent flow rate.
This gives V1 min . Then draw lines LN V1 min and L0 VN+1 to give the mixture point
Mmin and the coordinate xAMmin, find VN+ 1 min the minimum value of the solvent
flow rate VN+ 1]
b) Using a solvent flow rate of 1.5 times the minimum, calculate the number of
theoretical stages.
Page 23
26.02.2012
23
L0=1000 kg/h
xA0=0.235
xC0=0
VN+1=? kg/h
yAN+1=0
yCN+1=1.0
xAN=0.025
A:Acetone
C:MIK
Tie line on L0(xA0), V1(y1)min
xA
yA
xA0
y1min
L0, xA0
VN+1
A
C
LN,
xAN=0.025
V1 min y1 min
Dmin
Mmin
Tie line
xAmin=0.175
0 0 1min 1 1minmin
0 1min 1min
1min
1000(0.235) (0)0.175
1000
340 /
A N AN NAM
N N
N
L x V y Vx
L V V
V kg h
VN+1=1.5(VN+1 min)=1.5(340)=510kg/k L0=1000 kg/h
xA0=0.235
xC0=0
yAN+1=0
yCN+1=1.0
0 0 1 1
0 1
1000(0.235) 510(0)
1000 510
0.156
A N ANAM
N
AM
L x V yx
L V
x
0 0 1 1
0 1
1000(0) 510(1.0)
1000 510
0.337
C N CNCM
N
CM
L x V yx
L V
x
Page 24
26.02.2012
24
M(xAM and xCM)
M(0.156, 0.337)
xAN=0.025 given
LN-M line gives V1, y1
yA1=0.290, yC1=0.645
xCN=0.020 from graph
Lo+VN+1=LN+V1=M
1000+510=LN+V1
V1=1510-LN
1 1
1
(0.025) (1510 )(0.290)0.156
1510
N AN AAM
N
N N
L x V yx
L V
L L
LN=763, V1=747
To calculate D point
0 0 1 1
0 1
0 0 1 1
0 1
1000(0) 747(0.645)
1000 747
1.91
1000(0.235) 747(0.290)
1000 747
0.073
C CC
C
A AA
C
L x V yx
L V
x
L x V yx
L V
x
D
D
D
D
Page 25
26.02.2012
25
L0, xA0 A
LN, xAN
V1 , y1
M
VN+1
C
xA
yA
x1
y1
Draw tie line y1 (V1), x1(L1)
Draw L1D,
Draw tie line y2 (V2), x2(L2)
Draw L2D
Draw tie line y3 (V3), x3(L3)
Repeat steps to LN
N= 5 stage
V2 , y2 y2
x2
L1
L2
V3, y3
Countercurrent Extraction of Acetic Acid and Minimum Solvent. An
aqueous feed solution of 1000 kg/h of acetic acid-water solution contains 30.0 wt
% acetic acid and is to be extracted in a countercurrent multistage process with
pure isopropyl ether to reduce the acid concentration to 2.0 wt % acid in the final
raffinate. (a) Calculate the minimum solvent flow rate that can be used.
(b) If 2500 kg/h of ether solvent is used, determine the number of theoretical
stages required. (Note: It may be necessary to replot on an expanded scale the
concentrations at the dilute end.)
Page 26
26.02.2012
26
Extraction with Immiscible Solvents. A water solution of 1000 kg/h containing
1.5 wt % nicotine in water is stripped with a kerosene stream of 2000 kg/h
containing 0.05 wt % nicotine in a countercurrent stage tower. The exit water is to
contain only 10% of the original nicotine, i.e., 90% is removed. Calculate the
number of theoretical stages needed.