Derive and plot the operating equation for a binary flash distillation

Graphical Binary Flash Distillation

It is relatively common in separations (especially when we look at cascades of separation units) to examine graphical approaches to solutions.

The graphical solutions to be used always include an operating line and an equilibrium curve

DEFINITION

The equilibrium curve is a plot of the composition of a component in one phase versus the composition of the same component in the other phase.

A simple way to obtain the equilibrium curve is by using the relative volatility, which you may recall is defined as (for a binary system)

$\alpha_{AB} = \frac{K_A}{K_B} = \frac{y_A/x_A}{y_B/x_B} = \frac{y_A(1-x_A)}{(1-y_A)x_A}$

We can rearrange this to give:

$y = \frac{\alpha_{AB}x}{1+(\alpha_{AB}-1)x}$

DEFINITION

The operating line is a line representing the material balance equations.

There are several options of operating line format, but one form can be obtained, by rearranging the component mass balance for y:

y = -(L/V)x + (F/V)z

we then take the total material balance and we divide both sides by V to give:

(L/V) = (F-V)/V = (1-V/F)/(V/F)

combining these two equations gives:

y = [(V/F-1)/(V/F)]x + (F/V)z

If we denote the fraction of the feed that is vaporized (i.e., V/F) as f = V/F, we get the operating line as:

$y = \frac{f-1}{f}x+\frac{z}{f}$

Plotting these two lines yields the composition of both the exiting vapor (y) and liquid (x) as the intersection of the lines.

line intersections

NOTE:

The slope and intercept of the operating line are typically used to plot it in the first place. The dotted line along the diagonal is the y=x line and is sometimes used to facilitate plotting the operating line (i.e., set y=x in the feed line equation in order to find the location where the feed line crosses the dotted line). It has no physical significance in this application.

OUTCOMES:

Derive and plot the operating equation for a binary flash distillation on a y-x diagram