Document Type


Date of Award


Degree Name

Master of Science in Chemical Engineering - (M.S.)


Chemical Engineering and Chemistry

First Advisor

Ralph Ceccheti

Second Advisor

John E. McCormick

Third Advisor

Edward Charles Roche, Jr.


A short-cut method is presented for calculating the minimum, reflux rate for multicomponent systems. This new method is best suited for use on the computer and gives a very accurate estimate of this important factor in the design of fractionating columns. It does so without going to the tedious and time-consuming calculations of the rigorous methods.

The calculation approach is based on a simplification of the Thiele and Geddes method for fractionating columns under minimum reflux conditions.

The program is written in Fortran IV language and was used on an IBM 370 computer. Input specifications include the split of the light and heavy key components and feed conditions and composition. The liquid rate at the rectifying pinch zone, which represents the constant internal minimum reflux, is then calculated.

This method is an iterative procedure and the program assumes constant molal overflow between the rectifying and stripping pinch zones. The overall material balance for every trial is based on temperatures calculated from the composition of the previous trial. However, the liquid rate at the rectifying pinch zone temporarily remains constant until convergence is obtained on the distillate com-position and pinch zone temperatures. The liquid rate is then changed in the direction of convergence on the key component specifications and the calculations are repeated until converged.

The true minimum reflux was calculated on a rigorous plate-to-plate program by a parameter study of the actual required reflux for a series of column designs with increasing stages and with each design making the same separation for the key components. These values were asymtotically extrapolated to the case for infinite stages, and the minimum reflux value obtained was compared with the results given by the new procedure. The results demonstrated the reliability and usefulness of this new short-cut method.



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