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.