Total reflux, minimum plates

Fig. 2.3.2-9 Balance lines and staircase construction, a) Minimum reflux ratio, infinite plate number, b) Finite reflux ratio, resp. finite plate number, c) Total reflux, minimum plate number.

In general the same type of information given by the constant 0/V method can be obtained by the use of the Ponchon and Savarit method. For example, the cases of total reflux, minimum reflux ratio, and optimum feed-plate location can be easily solved. 

Example 8-12 Minimum Theoretical Trays/Plates/Stages at Total Reflux... 

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

The program starts up the column at total reflux (R very high). After steady state is reached on all plates, vary the reflux ratio interactively and attempt to carry out the distillation in minimum time, while attempting to... 

Giluland(30) has given an empirical relation between the reflux ratio R and the number of plates n, in which only the minimum reflux ratio Rm and the number of plates at total reflux nm are required. This is shown in Figure 11.42, where the group [in + 1) — (nm + l)]/(n + 2) is plotted against R — Rm)/(R + 1). 

The minimum reflux ratio, Rm is calculated using Underwood s method (Example 11.16) as 0.83 and, using Fenske s method, Example 11.17, the number of plates at total reflux is nm = 8. The following data have been taken from Figure 11.42, attributable to Gilliland(30) ... 

This measure was based upon the ratio of the minimum necessary number of plates, A min (averaged over the reboiler composition) in a column to the actual number of plates in the given column, Nj. Christensen and Jorgensen assumed that the mixture has a constant relative volatility a and the column operates at total reflux using constant distillate composition (x o) strategy (section 3.3.2) and evaluated Nmin using the Fenske equation ... 

In this approach, Fenske s equation [Ind. Eng. Chem., 24, 482 (1932)] is used to calculate which is the number of plates required to make a specified separation at total reflux, i.e., the minimum value of N. Underwood s equations [/. Inst. Pet., 31, 111 (1945) 32,598 (1946) ... 

K = equilibrium constant for a component in the feed n = number of components N = number of theoretical plates = minimum number of theoretical plates at total reflux q = pseudo-ratio of liquid to vapor for the feed R = reflux ratio Rn,in = minimum reflux ratio... 

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