Page 186 - Separation process principles 2

Summary 151

Start EXAMPLE 4.19
F, z fixed
' p, T of equilibrium In a process for producing styrene from toluene and methanol, the
phases fixed
gaseous reactor effluent is as follows:
Component kmoyh
three-phase Hydrogen 350
Methanol 107
Solution Water 49 1
not found Toluene 107
Ethylbenzene 141
Styrene 350
Y = VIF If this stream is brought to equilibrium at 38°C and 300 @a, com-
Solution
not found pute the amounts and compositions of the phases present.

SOLUTION
- Because water, hydrocarbons, and a light gas are present in the mix-

not found ture, the possibility exists that a vapor and two liquid phases may be

Single-phase present, with the methanol being distributed among all three
solution phases. The isothermal three-phase flash module of the ChemCAD
simulation program was used with Henry's law for hydrogen and
the UNIFAC method for estimating liquid-phase activity coeffi-
I '" cients for the other components, to obtain the following results:
liquid
Figure 4.34 Algorithm for an isothermal three-phase flash.
kmoyh
Component V 1 ) ~(2)
Calculations for an isothermal three-phase flash are
Hydrogen 349.96 0.02 0.02
difficult and tedious because of the strong dependency of
Methanol 9.54 14.28 83.18
K-values on liquid-phase compositions when two immiscible Water 7.25 8.12 475.63
liquid phases are present. In addition, it is usually not obvious Toluene 1.50 105.44 0.06
that three phases will be present, and calculations may be nec- Ethylbenzene 0.76 140.20 0.04
348.64
0.14
1.22
essary for other combinations of phases. A typical algorithm Styrene ---
for determining the phase conditions is shown in Figure 4.34. Totals 370.23 616.70 559.07
Because of the complexity of the isothermal three-phase flash
As would be expected, little of the hydrogen is dissolved in either of
algorithm, calculations are best made with a steady-state,
the two liquid phases. Little of the other components is left uncon-
process-simulation computer program. Such programs can
densed. The water-rich liquid phase contains little of the hydrocar-
also perform adiabatic or nonadiabatic three-phase flashes by
bons, but much of the methanol. The organic-rich phase contains
iterating on temperature until the enthalpy balance, most of the hydrocarbons and small amounts of water and methanol.
Additional calculations at 300 kPa indicate that the organic
phase condenses first with dew point = 143°C and secondary dew
point = 106°C.
is satisfied.

SUMMARY

1. The phase rule of Gibbs, which applies to intensive variables 3. Minimum- or maximum-boiling azeotropes, which are formed
at equilibrium, determines the number of independent variables by close-boiling, nonideal liquid mixtures, are conveniently repre-
that can be specified. This rule can be extended to the more general sented by the same types of diagrams used for nonazeotropic
determination of the degrees of freedom (number of allowable (zeotropic) binary mixtures. Highly nonideal liquid mixtures can
specifications) for a flow system, including consideration of exten- form heterogeneous azeotropes involving two liquid phases.
sive variables. The intensive and extensive variables are related by 4. For multicomponent mixtures, vapor-liquid equilibrium-phase
material and energy balance equations together with phase equilib-
compositions and amounts can be determined by isothermal-flash,
rium data in the form of equations, tables, andlor graphs.
adiabatic-flash, and bubble- and dew-point calculations. When the
2. Vapor-liquid equilibrium conditions for binary systems are mixtures are nonideal, the computations are best done with process-
conveniently represented and determined with T-y-x, y-x, and simulation computer programs.
P-n diagrams. The relative volatility for a binary system tends to
5. Liquid-liquid equilibrium conditions for ternary mixtures are I
1.0 as the critical point is approached. best determined graphically from triangular and other equilibrium