Page 72 - Separation process engineering

Two other points often used on the McCabe-Thiele diagram are the x intercept (y = 0) of the operating
line and its intersection with the y = x line. Either of these points can also be located algebraically and
then used to plot the operating line.

The intersection of the operating line and the y = x line is often used because it is simple to plot. This
point can be determined by simultaneously solving Eq. (2-11) and the equation y = x. Substituting y = x
into Eq. (2-11), we have

or

or

since V + L = F, the result is y = z and therefore

(2-18)

The intersection is at the feed composition.
It is important to realize that the y = x line has no fundamental significance. It is often used in graphical
solution methods because it simplifies the calculation. However, do not use it blindly.

Obviously, the graphical technique can be used if y, x, or T drum is specified. The order in which you find
points on the diagram will depend on what information you have to begin with.

Example 2-1. Flash separator for ethanol and water

A flash distillation chamber operating at 101.3 kPa is separating an ethanol-water mixture. The feed
mixture is 40 mol% ethanol and F = 100 kmol/h. (a) What is the maximum vapor composition and (b)
what is the minimum liquid composition that can be obtained if V/F is allowed to vary? (c) If V/F =