文档介绍：EQUILIBRIUM STAGED
SEPARATIONS
u Single Stage Flash [Flash Drum]
u Distillation
ð Simple Distillation
plex Distillation
ð Non-Ideal Distillation
u Gas Absorption and Stripping
u Liquid-Liquid Extraction
u Multi-Effect Evaporation
0/6/99 Equilibrium Staged Separations 1
PHASE EQUILIBRIUM
uConditions for thermodynamic equilibrium between two
phases
*(1) *(2)
fi = fi , i = 1,…,nc
(1) (2)
T = T and
P(1) = P(2)
u For an ideal system
*(V) *(L) S
fi = yi P and fi = xi Pi (T)
0/6/99 Equilibrium Staged Separations 2
PHASE EQUILIBRIUM DEFINITIONS
u K-value Ki
Ki = yi/xi
S
For an ideal system Ki = Pi (T)/ P
u Relative Volatility αjk ponent j with respect to
component k
αjk = Kj / Kk
0/6/99 Equilibrium Staged Separations 3
FLASH DRUM
VAPOR
V
Yi
FEED
F
Zi
Heating
Q or
Cooling
LIQUID
L
Xi
0/6/99 Equilibrium Staged Separations 4
FLASH DRUM PERFORMANCE
u Separation Factor SFjk ponents 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 = αjk
u Conclusion
The maximum separation is limited by the relative
volatility αjk which is set by nature.
u Fractional Recovery ponent j
FRj = Vyj / F zj = Vyj / [Vyj + L xj ] = [ Sj / Sj + 1]
where Sj = VKj / L
0/6/99 Equilibrium Staged Separations 5
V2
Y2i
TWO-STAGE FLASH
V1
Y1i
F L2
Zi X2i
L1
X1i
0/6/99 Equilibrium Staged Separations 6
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 = α jk
2
So SFOVERALLij = α ik
Since α ij > 1, this is an improvement in the separation factor.
u Overall Fractional Recovery ponent j
If both stages are operated at the same Sj, then
2
FROVERALLj = FR1j FR2j = FRj < FRj
So the overall recovery factor is lower for two stages than for one.
0/6/99 Equilibrium Staged Separations 7
THE SOLUTION
COUNTERCURR