Chemistry Reference
In-Depth Information
slow down. For this same reason, the interstage line should have a constant diameter.
In the SRL and ANL designs, the interstage lines curve gently around to the next
stage. In this way, liquid momentum is conserved and the liquid flows smoothly into
the next stage. The interstage lines should be large enough that the flowing liquid
fills no more than half of the cross-sectional area available for flow. The same rule
should hold for the exit lines to an effluent tank. In both cases, the liquid flow is gov-
erned by liquid flow in an open channel with a free surface. By keeping the interstage
lines open, the air pressure around the rotor will be the same in all stages. Lines
for external feed into a contactor stage, since, typically, they are driven by a pump,
can be much smaller. This condition prevents other-phase flow by gravity into these
lines, at least during contactor operation.
So far, we have discussed the optimum contactor design. Some designs are less
than optimal, either because of an operating error or the need to address some addi-
tional design constraint. The contactor still works, but the maximum achievable
throughput will be less. One such case occurs when the direction of rotor rotation is
reversed. Typically, this reversal is due to an installation or operating error. We have
tested such operation at Argonne, and the contactor still worked, although the maxi-
mum throughput was reduced. In a hypothetical contactor design, a contactor hous-
ing could be machined out of a single steel block. In this design, the collector ring
might not be a ring at all. Instead, it could a flat square trough that catches the liquid
exiting from the rotor. There would have to be a circular opening in the center of the
trough so that the contactor rotor could be inserted. For such a square collector, the
interstage line might be a vertical hole drilled in the housing block in one corner of
the trough. At the bottom of this vertical interstage passage, the line could make a
sharp 90° turn into the next stage. While such a design has not been tested relative to
a circular collector ring with a tangential exit port, its maximum throughput would
most likely be less. However, because of its compactness, such a design might still be
useful. In fact, it could eliminate the use of external interstage lines.
10.3.1.5 Motor
The motor above the contactor serves four functions by driving the rotor. First, it
mixes the two immiscible liquids. Second, it creates the centrifugal force that rapidly
separates the liquid-liquid dispersion. Third, it lifts the less-dense phase so that it can
flow by gravity to the next stage or to an effluent tank. Finally, it lifts the more-dense
phase so that it can also flow by gravity to the next stage or to an effluent tank. At
the same time, all the bearings are kept out of the liquid so that they have a normal
long life. All of the contactors built at SRL and ANL use the bearings in the motor
as the bearings for the rotor. As long as a well-balanced motor/rotor assembly spins
at less than 70-80% of its first critical speed, the contactor will not have excessive
vibrations because of the spinning rotor. Leonard et al. (1993) developed a method
for calculating the first critical speed of the motor/rotor assembly using an electronic
spreadsheet. As noted in this reference, the first critical speed must be calculated and
measured with the liquids in the contactor rotor.
The initial SRL and ANL contactors used motorized spindles to drive the con-
tactor rotors. These spindles have a large-diameter shaft that gives good stiffness
to the motor/rotor assembly. The shaft is also hollow so that it can supply air to the
