equilibrium stage separations

8/11/2019 Equilibrium Stage Separations

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0/6/99 Equilibrium Staged Separations 1

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




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)




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




VAPOR V Yi

LIQUID LXi

Heating Q or

Cooling

FEED F Zi

FLASH DRUM




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




F Zi

L 1 X1i

V1 Y1i

V2 Y2i

L 2 X2i

TWO-STAGE FLASH




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.




THE SOLUTION

COUNTERCURRENT MULTI-STAGING

OF

FL ASH UNI TS




F

COUNTERCURRENT

MULTI-STAGE

FLASH SYSTEM

V

L

F




D

F

B

F 1

F 2

D

S1

S2

B

SIMPLE

DISTILLATION

COMPLEX

DISTILLATION




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




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




AB

A B

BE

E

EXTRACTIVE DISTILLATION




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

equilibrium stage separations

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