The addition of a third substance,
usually toluene, to the water-ethanol mixture
increases the relative volatility of water and
ethanol and makes the separation possible.
Water, ethanol and toluene from a ternary azeotrope,
which boil at approximately 4 °C below pure ethanol.
This azeotrope forms two phases, one rich in water
and ethanol, the other in toluene.
By using two distillation columns, water-ethanol
mixture may be separated to give substantially
pure components. Figure 1 shows
a basic flow diagram for the process.
The feed stream (1) is mixed (3) with the ternary
azeotrope (2) leaving the top of the primary column.
The two-phase mixture is then divided in the separator
into a light, toluene rich phase (5) and a heavy
, ethanol-water phase (4). The ethanol-water phase
(4) is fed to the secondary column which produces
the ethanol-water azeotrope, with traces of toluene,
as tops product (6), and water as the bottoms
The distillate (6) from the secondary column
is then mixed with the toluene phase (5) leaving
the separator and is fed to the primary column
(8). The primary column produces ternary azeotrope
as the tops product (2) and absolute alcohol as
the bottoms product (9).
The absolute alcohol has a water content not
greater than 0.1% and toluene levels of 2% or