Equilibrium Stage Separations
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EQUILIBRIUM STAGEDSEPARATIONS
u Single Stage F lash [Flash Drum]
u Distillation
ð Simple Di stil lation
ð Complex Distil lation
ð Non-I deal Distil lation
u Gas Absorption and Stripping
u L iquid-L iquid Extraction
u M ul ti-Ef fect Evaporation
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PHASE EQUILIBRIUM
u Conditi ons for thermodynamic equi l ibri um between two phases
f i *(1) = f i *(2) , i = 1,…,nc T (1)
= T (2) and
P (1) = P (2)
u F or an i deal system
f i *(V) = y i P and f i * ( L ) = x i P i
S (T)
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PHASE EQUILIBRIUM DEFINITIONS
u K-value K i
K i = y i /x i
F or an ideal system K i = P i S (T)/ P
u Relative Volatil i ty jk of component j with respect to component k
j k = K j / K k
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VAPOR V Yi
LIQUID LXi
Heating Q or
Cooling
FEED F Zi
FLASH DRUM
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FLASH DRUM PERFORMANCE
u Separation F actor SF jk between components j and k SF jk = SR j / SR k
But SR j = V Y j / L X j and SR k = V Y k / L X k
So SF j k = [V Y j / L X j ] / [V Y k / L X k ] = K j / K k = jk
u Conclusion The maximum separation is l imited by the relative volatility j k whi ch is set by nature.u F ractional Recovery of component j
F R j = Vy j / F z j = Vy j / [Vy j + L x j ] = [ S j / S j + 1]
where S j = VK j / L
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F Zi
L 1 X1i
V1 Y1i
V2 Y2i
L 2 X2i
TWO-STAGE FLASH
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TWO-STAGE FLASH PERFORMANCE
u H ow can chemical engineers improve on the single - stage f lash? Answer: Try two stages.
u Overall Separation F actor
SF 1jk = SF 2jk = jk
So SF OVERALLij = ik 2
Since i j > 1, thi s is an improvement in the separation factor.
u Overall F ractional Recovery of component j
I f both stages are operated at the same S j , then
F R OVERALLj = F R 1j F R 2j = F R j 2 < F R j
So the overall recovery factor is lower for two stages than for one.
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THE SOLUTION
COUNTERCURRENT MULTI-STAGING
OF
FL ASH UNI TS
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F
COUNTERCURRENT
MULTI-STAGE
FLASH SYSTEM
V
L
F
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D
F
B
F 1
F 2
D
S1
S2
B
SIMPLE
DISTILLATION
COMPLEX
DISTILLATION
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CONDITIONS FOR SIMPLE DISTILLATION
u Single F eed
u No Sidestr eams
u I deal or relatively ideal VL E
ð No azeotr opes
ð No tangent pinches
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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 1X
Y
IdealVLE
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AB
A B
BE
E
EXTRACTIVE DISTILLATION
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EXTRACTIVE DISTILLATION
u Use when relative volatil i ty is poor ( AB < 1.1)
u Use only for a binary feed if possible
u Requi res a mass separating agent (M SA) to enhance the relative volatil i ty (aka entr ainer)
ð Entr ainer must increase AB signifi cantly ð Entr ainer must be less volatil e than Að Entr ainer should be less volatil e than B
ð Entr ainer should be non toxi c, non corr osive, etc.
u A sccond disti l lation column is required to recover the entr ainer E