Page 167 - Separation process principles 2

132 Chapter 4 Single Equilibrium Stages and Flash Calculations

When values of xi are small, Kb approaches KD. As dis- devised to determine the equilibrium compositions. Exam-
cussed in Chapter 2, the distribution coefficient, KD,, which ples of phase diagrams are shown in Figure 4.14 for the
can be determined from activity coefficients using the ex- water (A)-ethylene glycol (B)-furfural (C) system at 25OC
pression KDB = yz)/yA1) when mole fractions are used, is a and a pressure of 101 kPa, which is above the bubble-point
strong function of equilibrium-phase compositions and tem- pressure, so no vapor phase exists. Experimental data for this
perature. However, when the raffinate and extract are both system were obtained by Conway and Norton [18]. The pairs
dilute in the solute, aclivity coefficients of the solute can be water-ethylene glycol and furfural-ethylene glycol are each
approximated by the values at infinite dilution so that KDB completely miscible. The only partially miscible pair is
can be taken as a constant at a given temperature. An exten- furfural-water. Furfural might be used as a solvent to remove
sive listing of such KDB values in mass fraction units for var- the solute, ethylene glycol, from water; the furfural-rich
ious ternary systems is given in Perry's Handbook [9]. If phase is the extract, and the water-rich phase is the raffinate.
values for FB, xr), St and KD, are given, (4-25) can be Figure 4.14a, an equilateral-triangular diagram, is the
solved for xF). most common display of ternary liquid-liquid equilibrium
data in the chemical literature. Any point located within or
on an edge of the triangle represents a mixture composition.
Such a diagram has the property that the sum of the lengths
of the perpendicular lines drawn from any interior point to
A feed of 13,500 kglh consists of 8 wt% acetic acid (B) in water
the sides equals the altitude of the triangle. Thus, if each of
(A). The removal of the acetic acid is to be accomplished by
liquid-liquid extraction at 25OC with methyl isobutyl ketone sol- the three altitudes is scaled from 0 to 100, the percent of, say,
vent (C), because distillation of the feed would require vaporization furfural, at any point such as M, is simply the length of the
of large amounts of water. If the raffinate is to contain only 1 wt% line perpendicular to the base opposite the pure furfural
acetic acid, estimate the kilograms per hour of solvent required if a apex, which represents 100% furfural. Figure 4.14a is con-
single equilibrium stage is used. structed for compositions based on mass fractions (mole frac-
tions and volume fractions are also sometimes used). Thus,
SOLUTION the point M in Figure 4.14a represents a mixture of feed
and solvent (before phase separation) containing 18.9 wt%
Assume that the camer (water) and the solvent are immiscible.
water, 20 wt% ethylene glycol, and 61.1 wt% furfural.
From Perry S Handbook, take KD = 0.657 in mass-fraction units for
this system. For the relatively low concentrations of acetic acid in The miscibility limits for the furfural-water binary system
this problem, assume that Kb = KD. are at D and G. The miscibility boundary (saturation or binodal
curve) DEPRG can be obtained experimentally by a cloud-
point titration; water, for example, is added to a (clear) 50 wt%
solution of furfural and glycol, and it is noted that the onset of
cloudiness due to the formation of a second phase occurs when
The raffinate is to contain 1 wt% B. Therefore,
the mixture is 11 % water, 44.5% furfural, and 44.5% glycol by
xp = 0.01/(1 - 0.01) = 0.0101 weight. Other miscibility data are given in Table 4.5, from
which the miscibility curve in Figure 4.14a was drawn.
From (4-25), solving for EB,
xf) Table 4.5 Equilibrium Miscibility
1
EB = 3 = (0.087/0.0101) - 1 = 7.61 Data in Weight Percent for the
-
XB Furfural-Ethylene Glycol-Water
From (4-24), the definition of the extraction factor, System at 2S°C and 101 kPa
Ethylene
Furfural Glycol Water
This is a very large solvent flow rate compared to the feed rate-
more than a factor of lo! Multiple stages should be used to reduce
the solvent rate or a solvent with a larger distribution coefficient
should be sought. For 1-butanol as the solvent, KD = 1.613.

In the ternary liquid-liquid system, shown in Figure 4.13b,
components A and C are partially soluble in each other and
component B again distributes between the extract and raffi-
nate phases. Both of these exiting phases contain all compo-
nents present in the feed and solvent. This case is by far the
most commonly encountered, and a number of different
phase diagrams and computational techniques have been