Chemistry Reference
In-Depth Information
zone (Chow and Leonard, 1993). The special two-diameter rotor, discussed above for
large contactors, can also control the mixing when the annular mixing is still more
than is needed. In some cases, reduced mixing might prevent emulsion formation. In
all cases, the reduced operating costs are a benefit.
Mixing can also be controlled by designing the inside of the contactor housing
with two diameters. In the upper region of the housing, the diameter would be large
so that mixing is low. In the lower region, the diameter would be smaller so that the
annular gap is small and the mixing is high. In addition, with the smaller annular gap
at the bottom, the ALH would be higher than with the larger annular gap throughout
the mixing zone for the same throughput. Such a configuration should be easier to
build than the two-diameter rotor design and would allow a separating zone with
a constant cross-sectional area. If an insert is used to create the smaller bottom
annular gap, various mixing-zone configurations could easily be tested by making a
number of inserts. In fact, this approach is reasonable to use with all contactors, at
least while a process is being developed.
10.3.8 a
Q u e o u S
a n d
o
r g a n i C
P
r o P e r t i e S
When evaluating whether or not an aqueous and organic (solvent) pair is suitable for
carrying out a solvent extraction, the most important characteristic is the distribu-
tion ratios of the components to be extracted and of those to be left in the fluid. Once
the distribution ratios are found to be favorable, the immiscible liquid-liquid pair
must be characterized to determine if the pair can be used in commercial solvent-
extraction equipment. This characterization is best done by the batch dispersion-
number test (Leonard, 1995). This test can be performed easily and quickly with no
special equipment. If the results are favorable, the densities of the two phases need
to be considered. If the difference is less than 10%, plant operation could be difficult.
As a rule of thumb, the density difference should be 15% or greater. The liquid vis-
cosity is important in that more power will be required to turn the rotor if the viscos-
ity is higher. The liquids also need to be able to flow easily from stage to stage.
10.3.9 z
e r o
-P
of i n t
a
n a l y S i S
When rotors are made, they should have the desired hydraulic performance. Also,
contactor fabricators may want to compare the hydraulic performance of a set of
rotors with one or more previous sets of rotors. To do this, the zero-point test has been
devised (Leonard et al., 2002b). In this test, a rotor is operated in a single-stage unit
or as a single stage in a multistage unit with a single liquid phase, usually water. The
water is pumped to the unit at a fixed flow rate, and the output flow from the more-
dense-phase exit is measured. Then, the flow rate is increased, and the test repeated
several times. At some point, the liquid will not only flow out of the more-dense-
phase exit, but also the less-dense-phase exit. The average of the highest flow rate
with no flow out of the less-dense-phase exit and the lowest flow rate with flow out
of the less-dense-phase exit is called the “zero-point flow rate,” or simply, “the zero
point.” Beyond the zero point, the flow rate increase is continued, and the liquid flow
out of both exit ports is measured. Then, one plots the total flow rate on the
x
-axis of a
