0/6/99 Equilibrium Staged Separations 1
EQUILIBRIUM STAGED
SEPARATIONS
u Single Stage Flash [Flash Drum]
u Distillation
eSimple Distillation
eComplex Distillation
eNon-Ideal Distillation
u Gas Absorption and Stripping
u Liquid-Liquid Extraction
u Multi-Effect Evaporation
0/6/99 Equilibrium Staged Separations 2
PHASE EQUILIBRIUM
uConditions for thermodynamic equilibrium between two
phases
fi *(1) = fi *(2),i = 1,…,nc
T(1) = T(2) and
P(1) = P(2)
u For an ideal system
fi*(V) = yi P and fi*(L) = xi PiS(T)
0/6/99 Equilibrium Staged Separations 3
PHASE EQUILIBRIUM DEFINITIONS
u K-value Ki
Ki = yi/xi
For an ideal system Ki = PiS(T)/ P
u Relative Volatility ajk of component j with respect to
component k
ajk = Kj / Kk
0/6/99 Equilibrium Staged Separations 4
VAPOR
V
Yi
LIQUID
L
Xi
Heating
Q or
Cooling
FEED
F
Zi
FLASH DRUM
0/6/99 Equilibrium Staged Separations 5
FLASH DRUM PERFORMANCE
u Separation Factor SFjk between components j and k
SF jk = SRj / SRk
But SRj = V Yj / L Xj and SRk = V Yk / L Xk
So SF jk = [V Yj / L Xj ] / [V Yk / L Xk ] = Kj / Kk = ajk
u Conclusion
The maximum separation is limited by the relative
volatility ajk which is set by nature.
u Fractional Recovery of component j
FRj = Vyj / F zj = Vyj / [Vyj + L xj ] = [ Sj / Sj + 1]
where Sj = VKj / L
0/6/99 Equilibrium Staged Separations 6
F
Zi
L1
X1i
V1
Y1i
V2
Y2i
L2
X2i
TWO-STAGE FLASH
0/6/99 Equilibrium Staged Separations 7
TWO-STAGE FLASH PERFORMANCE
u How can chemical engineers improve on the single -
stage flash? Answer,Try two stages.
u Overall Separation Factor
SF1jk = SF2jk = a jk
So SFOVERALLij = a ik2
Since a ij > 1,this is an improvement in the separation factor.
u Overall Fractional Recovery of component j
If both stages are operated at the same Sj,then
FROVERALLj = FR1j FR2j = FRj2 < FRj
So the overall recovery factor is lower for two stages than for one.
0/6/99 Equilibrium Staged Separations 8
THE SOLUTION
COUNTERCURRENT
MULTI-STAGING
OF
FLASH UNITS
0/6/99 Equilibrium Staged Separations 9
F
COUNTERCURRENT
MULTI-STAGE
FLASH SYSTEM
V
L
F
0/6/99 Equilibrium Staged Separations 10
D
F
B
F1
F2
D
S1
S2
B
SIMPLE
DISTILLATION
COMPLEX
DISTILLATION
0/6/99 Equilibrium Staged Separations 11
CONDITIONS FOR SIMPLE DISTILLATION
u Single Feed
u No Sidestreams
u Ideal or relatively ideal VLE
e No azeotropes
e No tangent pinches
0/6/99 Equilibrium Staged Separations 12
XY Diagram for Alpha = 4
0.0
0.2
0.4
0.6
0.8
1.0
0 0.2 0.4 0.6 0.8 1
X
Y
Ideal
VLE
0/6/99 Equilibrium Staged Separations 13
A
B
A B
B
E E
EXTRACTIVE DISTILLATION
0/6/99 Equilibrium Staged Separations 14
EXTRACTIVE DISTILLATION
u Use when relative volatility is poor (a AB < 1.1)
u Use only for a binary feed if possible
u Requires a mass separating agent (MSA) to enhance the
relative volatility (aka entrainer)
e Entrainer must increase a AB significantly
e Entrainer must be less volatile than A
e Entrainer should be less volatile than B
e Entrainer should be non toxic,non corrosive,etc.
u A sccond distillation column is required to recover the
entrainer E