A REVIEW ON MINIMUM ENERGY CALCULATIONS FOR IDEAL AND NONIDEAL DISTILLATIONS

The minimum energy requirement of a distillation sets a lower, thermod ynamically defined operating limit, which is increasingly important in practice due to growing interest in saving energy. During the concept ual design phase this energy information can also be used to quickly c ompare distillation configurations. This paper gives a summary of the most important methods published to date for the calculation of the mi nimum energy requirement. Firstly, the occurrence of so-called pinch z ones will be systematically described. These are sections of the colum n in which at minimum reflux an infinite number of separation stages w ould be necessary. Then exact and approximating solutions of the probl ems both for ideal and for nonideal mixtures will be discussed. For id eal mixtures a rapid calculation is possible using the well-known Unde rwood equations, which can also be applied to complex columns (e.g., s everal feeds and side products, side stream strippers and enrichers). However, strongly nonideal multicomponent mixtures still require time- consuming simulations of columns having large numbers of plates. In su ch cases serious convergence problems must often be reckoned with. Rec ent developments aim at avoiding column simulations and at calculating pinch points directly.