EXERGY ANALYSIS OF DISTILLATION PROCESSES

Distillation is a unit operation in which two main processes are invol ved: heat transfer for vaporizing and condensing and mass transfer for the separation of the mixture. A distillation unit can be described a s an exergy converter: that is a unit which converts thermal exergy in to chemical exergy. To obtain a complete graphical representation of m ass, enthalpy and exergy balances in a fractional distillation, we pro pose the use of a diagram: the specific enthalpy (in kJ/kg of mixture) is plotted against the Carnot factor theta to show the heat and mass transfer effects simultaneously. The application of the methodology to the distillation of an ammonia-water mixture is presented. Convention al fractional distillation consumes a lot of exergy. We propose a new type of distillation in which the reboiler and the condenser, normally located at the bottom and at the top of the distillation column, are replaced by two heat exchangers integrated in the column itself. The e xchanger integrated in the bottom of the column inputs heat to the col umn, and the other exchanger located in the upper part of the column r emoves heat. This arrangement minimizes the creation of entropy in the column and therefore maximizes the exergetic effectiveness. This new process, called ''diabatic'' or ''quasi-reversible'' distillation shou ld bring important improvements to conventional distillation, not only from the point of view of energy use (large reduction in the consumpt ion of heat carrier fluids for heating and cooling), but also in terms of capital investment as the cost of fitting a heat exchanger in the distillation column will probably be less than the cost of a reboiler or condenser and reflux head.