Flash distillation (also known as flash evaporation) is steadystate, singlestage distillation. In this Demonstration, the liquid feed contains two components. An adiabatic throttle reduces the pressure and a fraction of the liquid feed evaporates to maintain constant enthalpy at the lower pressure. Phase equilibrium data at the outlet pressure helps determine the composition and flow rates of the outlet streams.
The

equilibrium curve is generated using the modified Raoult's law:
(1)
,
(2)
,
where
and
are the liquid and vapor mole fractions (
for the more volatile component),
,
,
is total pressure and
is the saturation pressure, which is calculated using the Antoine equation:
(3)
.
The activity coefficients
in this simulation are pseudorandom.
Once the equilibrium curve has been determined, a system can be fully defined by mass balances:
(6)
,
(7)
,
where
,
and
are the mole fractions of the more volatile component in the feed, bottoms (liquid) and distillate (vapor) streams, respectively. The flow rates (
,
,
) are for the feed, bottoms and distillate, respectively.
In the case of this Demonstration,
and
are determined graphically. This is done by reading the
value and
value at the intersection of the
line with the equilibrium curve. The
line equation is a function of the vapor and liquid flow rates and feed conditions:
(8)
,
(9)
.