Page 161 - Separation process principles 2

126 Chapter 4 Single Equilibrium Stages and Flash Calculations

In extractive distillation, as discussed in Chapter 1, a sol- Typically, the vapor phase is sent to a vapor separation sys-
vent is added, usually near the top of the column, to selec- tem, while the liquid phase is sent to a liquid separation sys-
tively alter the activity coefficients in order to increase the tem. Computational methods for a single-stage flash calcula-
relative volatility between the two species to be separated. tion are of fundamental importance. Such calculations are
The solvent is generally a relatively polar, high-boiling used not only for the operations in Figure 4.10, but also to
constituent, such as phenol, aniline, or furfural, which con- determine, anywhere in a process, the phase condition of a
centrates at the bottom of the column. stream or batch of known composition, temperature, and
pressure.
For the single-stage equilibrium operation with one feed
4.4 MULTICOMPONENT FLASH,
stream and two product streams, shown in Figure 4.10, the
BUBBLE-POINT, AND DEW-POINT
2C + 5 equations listed in Table 4.3 apply. (In Figure 4.10, T
CALCULATIONS
and P are given separately for the vapor and liquid products
AJEash is a single-equilibrium-stage distillation in which a to emphasize the subsequent need to assume mechanical and
feed is partially vaporized to give a vapor richer in the more- thermal equilibrium.) They relate the 3C + 10 variables
volatile components than the remaining liquid. In Fig- (F, V, L, z,, Yi, xi, TF, Tv, TL, PF, Pv, PL, Q) and leave C + 5
ure 4.10a, a liquid feed is heated under pressure and flashed degrees of freedom. Assuming that C + 3 feed variables F,
adiabatically across a valve to a lower pressure, resulting in TF, PF, and C values of zi are known, two additional variables
the creation of a vapor phase that is separated from the re- can be specified. The most common sets of specifications are
maining liquid in a flashhrum. If the vive is omitted, a low-
Isothermal flash
pressure liquid can be partially vaporized in the heater and
Bubble-point temperature
then separated into two phases in the flash drum. Alterna-
Dew-point temperat~~re
tively, a vapor feed can be cooled and partially condensed,
Bubble-point pressure
with phase separation in a flash drum, as in Figure 4.10b, to
Dew-point pressure
give a liquid that is richer in the less-volatile components. In
Adiabatic flash
both cases, if the equipment is properly designed, the vapor on adiabatic flash
and liquid leaving the drum are in equilibrium [4].
Percent vaporization flash
Unless the relative volatility is very large, the degree of
separation achievable between two components in a single Calculation procedures, described in the following for all
equilibrium stage is poor. Therefore, flashing (partial these cases, are well known and widely used. They all
vaporization) or partial condensation are usually auxiliary assume that specified values of feed mole fractions, zi, sum
operations used to prepare streams for further processing. to one.

Flash drum so thermal Flash
V, Y,. h,
Pv, Tv If the equilibrium temperature Tv (or TL) and the equilib-
Heater rium pressure Pv (or PL) of a multicomponent mixture are
specified, values of the remaining 2C + 5 variables are de-
termined from the same number of equations in Table 4.3.

Table 4.3 Equations for Single-Stage Flash Vaporization and
Partial Condensation Operations
Number of
Equation Equations
v. Y;, hv
A-*
Partial pVr TV (l)Pv= PL (mechanical equilibrium) 1
condenser 1 1 (2) Tv = TL (thermal equilibrium) 1
(3) ~i = Kix, (phase equilibrium) C
(4) Fzi = Vy, + Lx; (component material C
balance)
1
(5)F=VtL
I>
(total material balance)
PL.
TL
Flash drum L. xi, hL (6)hFF+Q=hvV+hLL (energybalance) 1
(7)C~i -Cxi =O (summations) 1
Figure 4.10 Continuous, single-stage equilibrium separation: I i %=2C+5
(a) flash vaporization (adiabatic flash with valve, isothermal flash
Ki = Ki{Tv, PV,Y? x) h~ = ~FITF, PF, Z]
without valve when Tv is specified); (b) partial condensation
hv = ~vP'v, Pv,rI = ~L~TL, PL,xl
(analogous to isothermal flash when TV is specified).