Exergy analysis of distillation processes

Distillation is a unit operation in which two main processes are involved: heat transfer for vaporizing and condensing and mass transfer for the separation of the mixture. A distillation unit can be described as an exergy converter: that is a unit which converts thermal exergy into chemical exergy. To obtain a complete graphical representation of mass, enthalpy and exergy balances in a fractional distillation, we propose 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. Conventional 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 exchanger integrated in the bottom of the column inputs heat to the column, and the other exchanger located in the upper part of the column removes 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 should bring important improvements to conventional distillation, not only from the point of view of energy use (large reduction in the consumption 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.