Chemistry Reference
In-Depth Information
10.3.1.2 Mixing Zone
The gap for the annular mixing zone is balanced between being so narrow that
the dispersion rises up and enters the lower collector ring, and so wide that mixing
becomes poor and stage efficiency suffers. In a 2-cm contactor in which the annular
gap was too wide, contactor operation exhibited a problem related to the absence of
phase inversion (Leonard et al., 1980a). Once the mixing zone had the organic phase
as the continuous phase, this condition remained even at low O/A low ratios, such
as 0.2 and 0.3, instead of the aqueous phase becoming the continuous phase. This
condition gave the dispersion a high structural viscosity that inhibited its flow into
the RI. Instead, the dispersion backed up into the annular mixing zone until it flowed
out the less-dense-phase exit port. This problem was corrected by making the gap
smaller (Leonard et al., 1997).
When contactors are developed for a new solvent-extraction process, typically a
single-stage contactor is built first. Its annular gap is sometimes made too wide so
that an insert can be added to reduce the gap. Various gap widths are then tested, and
the one that gives the best operation is specified for the new contactor stages. This
procedure allows one to maximize the performance of the mixing zone. Typically,
an annular gap that is about 9% of the rotor diameter will balance these competing
needs. As the annular gap is increased with other factors being held constant, the
liquid height in the mixing zone decreases. As the annular gap is decreased, the
shear rate increases. If shear rates are high enough, a stable or persistent emulsion
could form. In general, liquid-liquid pairs chosen for solvent extraction are tested
to check that the dispersions formed by the two immiscible liquids do not become
stable emulsions.
The bottom of the mixing zone contains stationary vanes attached to the bot-
tom of the contactor housing. These vanes stop the rotation of the dispersion that
is created in the annular gap and allows the dispersion to flow under the rotor. The
opening into the rotor is located at the center of the bottom surface of the rotor. If
the bottom vanes were not there, the dispersion would back up in the annular mixing
zone and flow out via the lower collector ring. If this happened, a breaking dispersion
would flow out the less-dense-phase exit port.
The height of the bottom vanes is not very important. Typically, it is 0.3 cm plus
6% of the rotor diameter. The gap between the top of the bottom vanes and the bot-
tom of the rotor should be small: typically, 0.15 cm plus 2% of the rotor diameter.
Larger gaps will give increased liquid height in the annular mixing zone. Usually,
there are eight vanes under the rotor. Use of four vanes would increase the average
liquid height in the annular mixing zone; however, the variation of liquid height with
time (pulsing) would also increase.
In addition to the number of vanes, the vane height, and the gap between the
vanes and the rotor, the vanes can be straight or curved. In almost every case,
practice at ANL has used straight vanes that extend to the inner wall of the contac-
tor housing, as shown in Figure 10.7. One type of commercial annular centrifugal
contactors, made by CINC Industries (Carson City, NV), has curved vanes that
extend halfway into the annular mixing zone (Meikrantz et al., 1998b). Macaluso
(2008), who designed these vanes, investigated the extension of eight curved vanes
