MASS TRANSFER.2 LIQUID-LIQUID EXTRACTION SECTION 1
May 24, 2015
MASS TRANSFER.2LIQUID-LIQUID EXTRACTION
SECTION 1
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Separation Technique
s
Distillation
Crystallization
Salting Out
Solvent Extraction
WHY LIQUID-LIQUID EXTRACTION
When 𝜶 nearly equal 1. For azeotropic mixtures. To avoid thermal decomposition. Some notes: 1- Separation is done according to different in solubility (not volatility).2- Problem with L-L extraction that we mix then we separate.
WHAT IS LIQUID-LIQUID EXTRACTION
Species involved: Solute + Inert Solvent
Each stage contain: Contact Separation
Solvent
Feed Raffinate
Extract
Separation
Vo, yo
Lo, xo
V1, y1
L1, x1
Contact
Vo: Mass flow rate of fresh solvent.
yo: Mass fraction of solute in solvent
Lo: Mass flow rate of liquid feed mixture
xo: Mass fraction of solute in meal
V1: Mass flow rate of Extract layer
y1: Mass fraction of solute in Extract
L1: Mass flow rate of Raffinate layer
x1: Mass fraction of solute in Raffinate
Extract: A B So Raffinate: A B S
TYPES OF TERNARY SYSTEMS
Formation of one pair of partially miscible liquids.
[closed ternary diagram]
Formation of two pair of partially miscible liquids.
[open ternary diagram]
FORMATION OF ONE PAIR OF PARTIALLY MISCIBLE LIQUIDS AS&BS are completely soluble.
AB is partially soluble.
M will be separated to two compositions; N & L
If S is solvent for A so as S increases, L goes up till P(Plait Point) at which A,B & S become one phase. This is done due to the increase in mutual solubility between A&B.
S
BA
P
II
I
MN
L
FORMATION OF ONE PAIR OF PARTIALLY MISCIBLE LIQUIDS
Equilibrium relation.
x
y
P
FORMATION OF TWO PAIR OF PARTIALLY MISCIBLE LIQUIDS
AS&AB are partially soluble.
B is completely soluble in S.
S
BAI
I
II
x
y
STAGE DEFINITION
It is a mechanical device or series that allow the solvent & solution to contact and separate. So; stage is:
Contact
Separation
Stage
1
V0, y0
L1, x1L0, x0
V1, y1
V0+L0=V1+L1=M
V0y0+L0x0=V1y1+L1x1=MxM
are on the same straight line y0, x0,
xM
are on the same straight line y1, x1, xM
SINGLE STAGE
AB
S
x
y
M
x0
x1
y1
y0
MULTI STAGE CROSS CURRENT
1 2L0, x0 L1, x1 L2, x2
V0, y0 V0, y0
V2, y2V1, y1
y0, x0, xM1 are on the same straight
line are on the same straight line y1,
x1, xM1
AB
S
x0
y0
x
y
y0, x1, xM2 are on the same straight
line are on the same straight line y2,
x2, xM2
M1
x1
y1
M2
y2
x2
MULTI STAGE COUNTER CURRENT
AB
S
x0
x
y
1 n
L0, x0 L1, x1 Ln, xn
V1, y1 Vn, yn Vn+1, yn+1
Vn+1+L0=V1+Ln=M
Vn+1yn+1+L0x0=V1y1+Lnxn=MxM
L0-V1=Ln-Vn+1=R
yn+1
M
xn
y1
R
x1
y2
x2
y3
x3
xn+1 xn-1
xn
b a
No. of stages= n+(a/a+b)But get a & b from AB line
yn+1
AB
S
x0
x
y
M
xn
y1
R
PROJECTION OF OPERATING LINE
yn+1
EquilibriumOperating
EXTRACT & RAFFINATE EFFICIENCY
x
y
x
y
Equilibrium
Operating
Equilibrium
Operating
90%=Extract efficiency 80%=Raffinate efficiency
0.9HH
0.8L
L
New Equilibrium
New Equilibrium
PROBLEM (1) Givens: A counter current-multi stage system. We need to recover Pyridine from aqueous solution by
Chloro-benzene. xn=0.01 x0=0.25 Vn+1/L0=10
Required:1. Maximum pyridine content that may be obtained in the
extract phase leaving the system. For ratio of solvent to feed of 10 determine:1. Composition and quantity (expressed as pounds per pound
of feed) of extract phase leaving the system.2. The number of equilibrium stages required.
PROBLEM (1)
Extract layer Raffinate layer
Chloro-benzene Water Pyridine Chloro-
benzene Water Pyridine
yS yB yA xS xB xA
0.69 0.03 0.28 0.02 0.73 0.25
0.8 0.02 0.18 0 0.88 0.12
0.91 0.02 0.07 0 0.97 0.03
0.96 0.01 0.03 0 0.99 0.01
PROBLEM (1)
xA
Extract layer Raffinate layerChloro-
benzene Water PyridineChloro-
benzene Water Pyridine
yS yB yA xS xB xA
0.69 0.03 0.28 0.02 0.73 0.25
0.8 0.02 0.18 0 0.88 0.12
0.91 0.02 0.07 0 0.97 0.03
0.96 0.01 0.03 0 0.99 0.01
Equilibrium
yA
AB
S
ExtractRaffinate
x0
yn+1
xn
PROBLEM (1)
To get maximum extract concentration, it will be at equilibrium with feed concentration (x0).
y1max=0.27
Note Here we make an approx.
as equilibrium not complete (otherwise we have to work like the next problem)
xA
yA
AB
S
x0
yn+1
xn
y1 max
PROBLEM (1)
As S/F=Vn+1/L0=10
Vn+1/(Vn+1+L0)=a/(a+b) 10/11=a/(a+b) a+b=10.1, a=9.18 Put Point(M).
xA
yA
AB
S
x0
yn+1
xn
x0
yn+1
b
a
y1=0.05
M
PROBLEM (1)
xA
yA
AB x0
yn+1
xn
y1
R
PROBLEM (1)
N.T.S=1.
xA
yA
AB x0
yn+1
xn
y1
R
Equilibrium
Operating
PROBLEM (1)
As S/F=Vn+1/L0=10
V1/(V1+Ln)=a/(a+b) 9/9.6=a/(a+b) V1/(V1+Ln)=9/9.6
L0/(Vn+1+L0)=1/11……..*
V1+Ln= Vn+1+L0
V1 /(Vn+1+L0)=9/9.6……**
By dividing *&** V1/ L0=(9*11)/(9.6*1)
V1/ L0=10.3125 lb/lb xA
yA
AB
S
x0
yn+1
xn
xn
y1
b
a
y1
M
PROBLEM (2)
Givens: A:Diphenyl hexane. B:Decosane. S:Furfural. Feed contains 0.2 A & 0.8 B. Multistage counter current system. Solvent contains 0.005 A. Raffinate product contains 0.01 A. Required: N.T.S if mass ratio of solvent to feed is 1.65. Ratio of solvent to feed when N.T.S equals 3. Maximum concentration of A that will be obtained.
PROBLEM (2)
Here the solubility saturation curve is given between xS , xA &xB so, this means that the solubility curve is closed.
Also the equilibrium curve is given:
xB xA xS
yS yB yA xS xB xA
Equilibrium
PROBLEM (2)
xn= 0.01
x0= 0.2
yn+1 = 0.005
Vn+1/L0=1.65
Vn+1/(Vn+1+L0)=1.65/2.65=0.62
Vn+1/(Vn+1+L0)=a/(a+b) 0.62=a/9.5 a= 5.89 Put Point M y1=0.15 N.T.S=4
AB
Sx
y
yn+1
x0
b
a
yn+1
X0=0.2 xn
M
y1
R
PROBLEM (2)
To get S/F that makes N.T.S=3.
We will assume y1. As N.T.S decreases, y1 decreases too so; assume it less than y1 of N.T.S=4
y1 assumed =0.1
What a luck!! Vn+1/L0=1.8
AB
S
x
y
yn+1
X0=0.2 xn
M
x0
y1 assumed
R
PROBLEM (2)
To get y1max;
a) Assume y1max
b) From the equilibrium, get x1 on the raffinate locus
c) If the line connecting y1max and x1 passes by xo, then the assumption is true. If not, repeat the steps
AB
Sx
y
yn+1
X0=0.2 xn
M
y1
R
y 1max assumed
X1
Thank you for your attention!Any Questions?