CALCULATION OF MINIMAL ENERGY-REQUIREMENTS FOR NONIDEAL RECTIFICATION

Minimum Reflux Calculation for Nonideal Distillations. The minimum reflux ratio is the lower limit of operation for a distillative separation. Due to rising energy costs one is increasingly forced to approach this thermodynamically defined minimum energy consumption. This paper presents an over-view of the important methods of calculation published to date. First, a concise physical and mathematical description of the problem is given to introduce the concept of pinch points. These are characteristic regions in separation profiles of distillation columns where an infinite number of stages would be required under minimum reflux conditions. Then, shortcut and rigorous problem solutions for both ideal and nonideal mixtures are reported. Ideal mixtures can be computed using the well-known Underwood equations, which also cover complex column designs (multiple feeds/products, sidestream strippers and enrichers). However, pronouncedly nonideal systems still require time-consuming simulations of columns with a large number of stages. Hence, recent developments attempt to avoid column simulations and to compute pinch points directly.