Page 60 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 49

thus a minimum reflux requirement. Minimum reflux is
calculated [ 1331 :

(L/Vmin) = (YD - Y~)/(xD - xi); (see Figure 8-33) (8-92)
Note that YD = XD on the diagonal of the equilibrium
plot, and yi and xi are points of intersection with the equi-
librium curve. For an abnormal equilibrium curve (as
compared to regular or normal shape) see Figure 8-34.
Once a minimum reflux has been established (which is
not an operating condition), then a realistic reflux ratio of
from 1.5 to as much as 10 times the minimum can be
selected. Of course, the larger the reflux value down the
column the more vapor has to be boiled up, and the
greater will be the required column diameter. So, some
XO x3
Mol fraction of volatiles in liquid, x economic balance must be determined.
Solving the typical Raleigh equation:
Figure 8-33. Variable reflux ratio at various theoretical plates to
achieve a specified separation from x, kettle to ~3 distillate over-
head. Note, all reflux ratios shown yield same separation, but with (8-93)
different numbers of theoretical platedstages; D = Distillate; F = Ket-
tle Conditions; x,, yo at equilibrium. Used by permission, Ellerbee,
R. W. Chem. Eng. May 28 (1 973), p. 110. The average distillate composition will be:

vapor generated by the kettle. The heat duty of the kettle
can be calculated using the appropriate latent and sensi-
ble heat of the mixture components.
For a constant reflux ratio, the value can be almost any
ratio; however, this ratio affects the number of theoretical
plates and, consequently, actual trays installed in the recti-
fication section to achieve the desired separation. Control
of batch distillation is examined in Reference 134.
The internal reflux ratio is L/V, and is the slope of the
operating line. The external reflux is [ 1331 :

R = L/D
andV = L + D

V/L=L/L+D/L=l+l/R= (R+l)/R
Then L/V = K/(R + I)

and, R= (L/V)/[I - (L/V)], see Figure 8-33.
Point F on the figure represents conditions in the kettle
or still with xi, yi, or x,, yo. Line DF represents slope of the
operating line at minimum reflux. The step-wise develop-
ment from point D cannot cross the intersection, F, where
the slope intersects the equilibrium line, and leads to an Mol Fraction Component C1 in Liquid
infinite condition, as point F is approached. Thus, an infi- Figure 8-34. Minimum reflux for abnormal equilibrium curve for
nite number of theoretical traydstages is required, and Batch Operation, constant reflux ratio.