A useful method for a binary mixture employs an analysis based on the McCabe-Thiele graphical method. In addition to the usual assumptions of adiabatic column and equimolal overflow on the trays, the following procedure assumes negligible holdup of liquid on the trays, in the column, and in the condenser.

As a first step in the calculation, the minimum-reflux ratio should be determined. In Fig. 13-100, point D, representing the distillate, is on the diagonal since a total condenser is assumed and xD = yD. Point F represents the initial condition in the still pot with coordinates xpi, ypi. Minimum internal reflux is represented by the slope of the line DF,

where L is the liquid flow rate and V is the vapor rate, both in moles per hour. Since V = L + D (where D is distillate rate) and the external-reflux rate R is defined as R = L/D, then

or R-=i^fc; ,13-141' The condition of minimum reflux for an equilibrium curve with an inflection point P is shown in Fig. 13-102. In this case the minimum internal reflux is

The operating reflux ratio is usually 1.5 to 10 times the minimum. By using the ethanol-water equilibrium curve for 101.3 kPa (1 atm) pressure shown in Fig. 13-101 but extending the line to a convenient point for readability, (L/V)min = (0.800 - 0.695)/(0.800 - 0.600) = 0.52 and Rmin = 1.083.

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