Flash Distillation Cascade in a Constant Relative Volatility Mixture

feed quality q

0.5

feed composition z

0.5

relative volatility α

2.56

number of flash drums

5

Consider a binary mixture with a constant relative volatility,

α

. Assume that adequate heating of the liquid phases allows constant thermal quality,

q

, at the entrance of each unit of the cascade of flash drums. The feed composition (i.e. mole fraction of the light component) is

z

.

The liquid and vapor mole fractions of the two product streams (i.e. one vapor and one liquid stream leaving each flash vessel) are solutions of the mass balance and equilibrium relationship given by:

y=

q

q-1

x+

z

1-q

,

y=

αx

1+(α-1)x

.

A simple algebraic manipulation using these equations gives the following quadratic equation, which can be solved for

x

:

1-f

f

(α-1)

2

x

+α+

1-f

f

-

(α-1)

f

zx-

z

f

=0

;

here

f=1-q

is the vapor fraction of the feed.

This Demonstration shows the graphical solution of the cascade of flash drums. As you increase the number of flash vessels, the exit composition approaches that of the pure heavy component. This component is obtained more rapidly for larger values of the relative volatility.