Page 286 - Separation process engineering

number below the feed stage, N − N , can be estimated as,
f

(7-41)

Since neither procedure is likely to be very accurate, they should only be used as first guesses of the feed
location for simulations.
The Gilliland correlation can also be fit to equations. Liddle (1968) fit the Gilliland correlation to three

equations. Let x = [L/D − (L/D) min ]/(L/D + 1). Then

(7-42a)

while for 0.01 < x < 0.90

(7-42b)

and for 0.90 ≤ x ≤ 1.0

(7-42c)

For most situations Eq. (7-42b) is appropriate. The fit to the data is shown in Figure 7-3. Naturally, the
equations are useful for computer calculations. Erbar and Maddox (1961) (see King, 1980, or Hines and
Maddox, 1985) developed a somewhat more accurate correlation that uses more than one curve.

As a rough rule of thumb we can estimate N = 2.5 N min . This estimate then requires only a calculation of
N min and will be useful for very preliminary estimates.

Example 7-3. Gilliland correlation

Estimate the total number of equilibrium stages and the optimum feed plate location required for the
distillation problem presented in Examples 7-1 and 7-2 if the actual reflux ratio is set at L/D = 2.

Solution

A. Define. The problem was sketched in Examples 7-1 and 7-2. F = 100, L/D = 2, and we wish to

estimate N and N .
F
B. Explore. An estimate can be obtained from the Gilliland correlation, while a more exact
calculation could be done with a process simulator. We will use the Gilliland correlation.

C. Plan. Calculate the abscissa

determine the ordinate