Using CFD to Assist in the Design of Solvent
Extraction Pumper-Mixers
Added to Post Mixing on Nov 1, 2003. Presented in part at
Hydro 2003, Vancouver on Aug 27, 2003.
Introduction
A solvent extraction pumper mixer is a special class of
mixing impeller. The impeller must create a suction when rotating,
thus creating a change in pressure or head. Through this action,
flow is generated much like a pump. This head and flow
requires power. The solvent extraction (SX) pumpers is the heart
of an SX circuit. All of these parameters are required in order
to properly design an SX plant. In the past, very few companies
took the time to study this experimentally and use the data
to scale-up. It all started about 30 years ago with General Mills.
Then came Davy (now Kvaerner), Krebs, Avendeño, Lightnin
and Outokumpu. The approach for scale-up is as different as
the companies that have done the lab studies. So are the
pumper designs.
Experimentation requires tanks, false bottoms, orifice
plates, entrance and exit pipes, piping, a settler, weirs, a
torque cell, and a flow meter. When using the organic and
aqueous phases, it can get rather messy. Because of the continuous
flow conditions, large inventories of reagents are required for
these studies. No wonder so few studies have been done.
A more elegant method is using CFD, computational fluid dynamics,
to study the SX-pumper. This has not been done to our knowledge
for a systematic and fundamental study because
- An SX circuit is asymmetric and therefore a complicated
360 degree model must be made from scratch, which includes
the inlet and outlet pipes, the false bottom, the orifice,
the pumper, the baffles, and the overflow-underflow connection
to the auxiliary tank or the settler.
- The close proximity of the pumper to the stationary
false bottom requires a very tight rotating reference frame.
- Most commercially available CFDs take notoriously long times
to converge to a solution.
- Furthermore, most commercially available CFD programs are
too diffusive in order to assist in the convergence, and
thus violates some rules of physics.
- For this reason, most CFD results need to be validated
by comparing it and tweaking the code to an experiment,
otherwise the results will be looked upon with skepticism.
This article will discuss how Post Mixing
uses CFD to understand, study, and design SX circuits. You
will probably be amazed at how accurate and quick CFD can be when
using the right program, Acusolve, and the right resources,
Keith Johnson.
Background: Solvent Extraction Systems
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