Environmental Engineering Reference
In-Depth Information
PURE SOLUTE (
A
)
One-phase
region
Plait point
Tie-line
PURE
SOLVENT (
S
)
PURE
DILUENT (
B
)
Two-phase region
0
100%
Solvent
0
Diluent
100%
Figure 3.10
Three-component phase equilibrium diagram.
The three apexes of the equilateral triangle (Figure 3.10) represent each of the three
pure components. In this figure, pure solvent (
S
) is the lower left corner, pure solute
(
A
) is the top apex, and pure diluent (
B
) is the lower right corner. The curved saturation
line in the interior of the triangle forms a solubility envelope, under which lies the two-
phase region of immiscible solvent and diluent. At any point above this line the system
is completely miscible and forms only one phase. Extraction is not an effective means of
separation in this one-phase region, because the solvent and diluent must be immiscible
in order for partitioning of the solute from diluent to solvent and subsequent separation to
occur. If a system is operating in this one-phase regime, a simple addition of solvent can
dilute the mixture into the two-phase region. The plait point is similar to an azeotrope for
vapor-liquid equilibrium because it is the point at which the two phases have identical
compositions. The saturation line to the left of the plait point is the extract (solvent-rich)
phase, and the equilibrium compositions of the extract phase are given by points on this
line. The saturation line to the right of the plait point is the raffinate (diluent-rich) phase,
and the equilibrium compositions of the raffinate phase are given along this line. It should
be noted that one corner could represent any of the pure components, and some triangle
diagrams will be plotted differently than others.
Any point inside the two-phase region represents a mixture which will separate into
two phases along a tie-line. The compositions of these two phases lie on the saturation line
at the ends of the tie-line. The most important thing to remember about tie-lines is that
they show the compositions of two phases
which are in equilibrium with each other
. The
data used to plot the saturation line and the tie-lines are usually obtained experimentally;
there are no simple equilibrium relationships or equations as in Henry's Law used in
distillation analyses. A tie-line that slopes down from the extract line to the raffinate line
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