Page 69 - Separation process engineering

2.4 Binary Flash Distillation

We will now use the binary equilibrium data to develop graphical and analytical procedures to solve the
combined equilibrium, mass balance and energy balance equations. Mass and energy balances are written

for the balance envelope shown as a dashed line in Figure 2-1. For a binary system there are two
independent mass balances. The standard procedure is to use the overall mass balance,

(2-5)

and the component balance for the more volatile component,

(2-6)

The energy balance is

(2-7)

where h , H , and h are the enthalpies of the feed, vapor, and liquid streams. Usually Q flash = 0, since the
F
L
v
flash drum is insulated and the flash is considered to be adiabatic.
To use the energy balance equations, we need to know the enthalpies. Their general form is

(2-8)

For binary systems it is often convenient to represent the enthalpy functions graphically on an enthalpy-
composition diagram such as Figure 2-4. For ideal mixtures the enthalpies can be calculated from heat
capacities and latent heats. Then,

(2-9a)

(2-9b)