Page 62 - Separation process engineering

Temperature, T 1

Pressure, p 1

Of the remaining, the designer will usually select first:
Drum pressure, p drum

The drum pressure must be below the critical pressure for the mixture so that a liquid phase can exist. An
approximate value of the critical pressure can be calculated from

(2-1)

where x are the liquid mole fractions and p and p are the critical pressures of the pure
i C,i C,mixture,approx
components and of the mixture (Biegler et al., 1997). In addition, as the pressure increases, the pressure
and temperature of the feed and the condensation temperature of the vapor increase. We prefer a feed
temperature that can be readily obtained with the available steam (T < T steam + 5°C), and if the vapor
F
product will be condensed, we prefer a condensation temperature that is at least 5°C above the available
cooling water temperature.
A number of other variables are available to fulfill the last degree of freedom.

As is true in the design of many separation techniques, the choice of specified design variables controls
the choice of the design method. For the flash chamber, we can use either a sequential solution method or
a simultaneous solution method. In the sequential procedure, we solve the mass balances and equilibrium
relationships first and then solve the energy balances and enthalpy equations. In the simultaneous solution
method, all equations must be solved at the same time. In both cases, we solve for flow rates,
compositions, and temperatures before we size the flash drum.

We will assume that the flash drum shown in Figure 2-1 acts as an equilibrium stage. Then vapor and
liquid are in equilibrium. For a binary system the mole fraction of the more volatile component in the
vapor y and its mole fraction in the liquid x and T drum can be determined from the equilibrium
expressions:

(2-2a)

(2-2b)

To use Eq. (2-2) in the design of binary flash distillation systems, we must take a short tangent and first

discuss binary vapor-liquid equilibrium (VLE).

2.2 Form and Sources of Equilibrium Data

Equilibrium data are required to understand and design the separations in Chapters 1 to 16 and 18. In
principle, we can always experimentally determine the VLE data we require. For a simple experiment,
we could take a chamber similar to Figure 1-2, fill it with the chemicals of interest, and at different
pressures and temperatures, allow the liquid and vapor sufficient time to come to equilibrium and then
take samples of liquid and vapor and analyze them. If we are very careful, we can obtain reliable