Page 94 - Separation process engineering

2.8 Three-Phase Flash Calculations

Many systems, particularly mixtures of nonpolar organics and polar compounds such as water, will form
two liquid phases and one vapor phase. A binary example, n-butanol and water, is discussed later (see
Figure 8-2 and Problem 8.D3). In this section we consider calculations for multicomponent liquid-liquid-

vapor systems. For example, if a vapor mixture of gasoline and water is partially condensed, the result
will be an aqueous layer with a high mole fraction of water, an organic phase containing very little water,
and a vapor phase. The different components of gasoline will distribute between the three phases
differently.
With three phases, the component mass balance for a flash distillation system is

(2-59)

which is equivalent to Eq. (2-36). There are i independent component mass balances, but the overall
balance is not independent, since it is obtained by summing all of the component balances.

When there are three phases, we can write three equilibrium distribution relationships for each
component i,

(2-60a,b,c)

Solving Eq. (2-60a) for y, and substituting this into Eq. (2-60b), rearranging and comparing to Eq. (2-
i
60c) we obtain

(2-61)

Thus, only two of the three K values for each component are independent. Of course, all the K values are,
in general, functions of temperature, pressure, and composition.
We can now follow exactly the same steps as were used to derive the Rachford-Rice equation [Eqs. (2-
37) to (2-42)] to derive two equations for the three-phase flash.

(2-62)

(2-63)

If temperature and pressure are specified and correlations for the equilibrium parameters are available,
these two forms of the Rachford-Rice equation can be solved simultaneously for L /F and V/F. Then
liquid_1
y, x i,liquid_1 , and x i,liquid_2 can be calculated from the three-phase equations equivalent to Eqs. (2-37) and
i
(2-38) (see Problem 2.C9). If only liquid 1 and vapor are present, then an equation equivalent to Eq. (2-