Pulsed perforated plate columns are differential type liquid-liquid extrac­tion contactors because they manifest continuous, rather than step, concen­tration profiles. These devices have been used for recycling used nuclear fuel since the Hanford PUREX plant was started up in 1955. All current commercial UNF recycling plants (THORP, UP3 and RRP) employing advanced PUREX separations technology use pulsed columns in three functions:

• Separation of uranium and plutonium from UNF.

• Separation of plutonium from uranium.

• Purification of the plutonium product.

A schematic of a typical industrial pulsed column used for processing used nuclear fuel is illustrated in Fig. 3.13. THORP pulsed columns are cylindrical and are filled with a series of horizontal perforated plates (Fig. 3.14), spaced at 2-inch intervals, and held together by a series of vertical tie-rods (Phillips, 1993a). In contrast, the La Hague plants use annular pulsed columns with inner and outer columns arranged concentrically. The plates, which in this case are rings, are spaced similarly to the ones in the cylindrical columns but are not perforated and alternately attached to the external and internal column walls. Some smaller cylindrical columns are also used and these are filled with alternating rings and discs, with the rings attached to the outer column wall and the disks carried on a central vertical tie-rod (Drain, 1997).

In all column contactors the more dense aqueous phase enters the column near the top while the less dense solvent phase enters through a pulse limb close to the bottom. A liquid-liquid dispersion is created as the phases flow counter-current through the plate perforations under the action of the periodic pulse applied to the solvent. This can be arranged to be a disper­sion of aqueous drops falling through a continuous solvent layer, or it can be a dispersion of solvent drops rising through a continuous aqueous phase. In the former case, the phase separation interface is formed and held in the “bottom settler” at the base of the column while in the latter case this interface is in the top settler. Any solids in the aqueous phase feed will typically congregate at the liquid-liquid interface and be directed with the dispersed phase. Therefore, when extracting uranium and plutonium into solvent from an aqueous UNF feed, the aqueous phase is dispersed so that any solids will be directed to the bottom settler where the aqueous drops coalesce at the solvent/aqueous interface. Thus, contamination of the solvent containing the separated uranium and plutonium is avoided. The top settler provides sufficient freeboard for the solvent to exit the column free of aqueous phase entrainment.

From compressed air reservoir

Pneumacators for density measurement

Top settler

There are two stable hydrodynamic operating regimes for pulsed columns. The mixer-settler operating regime manifests at low pulse frequencies and/ or amplitudes and flow rates when dispersion and coalescence entirely occurs between plates. At higher pulse frequencies and/or amplitudes and flow rates the mixer-settler regime transitions to the dispersive or emulsion regime where a continuous dispersion is established throughout the col­umn’s length. Drop sizes in the dispersion regime are typically 1-3 mm in a nitric acid — 30% tributyl phosphate systems with uranium and plutonium (Phillips, 1993b). At still higher pulse frequencies and/or amplitudes and flow rates, the dispersion becomes unstable, leading to gross wrong-phase entrainment in the products, a condition known as flooding. Pulsed columns are usually operated in the dispersive regime because the higher dispersed phase hold-up and higher interfacial area maximizes mass transfer effi­ciency and minimizes wrong-phase entrainment. However, they also provide good mass transfer performance when operating in the mixer-settler regime because the lower interfacial area is at least partially mitigated by the con­tinuous coalescence and re-dispersion of the drops, which continually exposes a fresh interface for mass transfer.

Detection and control of the position of the bulk interface in the top settler for a continuous aqueous phase, or bottom settler for a continuous organic phase, is critical for stable pulsed column operation and hence good mass transfer performance. Interface detection and control is achieved in the THORP pulsed columns using equipment with no in-cell moving parts so that no in-cell maintenance is required through the life of the facility. Interface detection is achieved using air bubbler tubes (“pneumercators”) to measure the relative densities of the two liquid phases above and below the interface and thus infer the interface position between the bubbler tubes (Phillips, 1993a). In solvent continuous columns, with the interface in the bottom settler, the pneumercator pressure signals are interfered with

by the large pulsing pressure applied. Rapid response pressure transducers and sophisticated signal processing are used to extract a reliable interface position signal from the pulse pressure wave “noise”. Interface position control is accomplished using an air lift system (see below) to control the rate of aqueous phase removal from the bottom settler. The airlift is coupled with a fluidic pump to provide a constant submergence for the air lift air input. When such a constant submergence is provided, liquid flows from airlifts are stable and increase reproducibly with increasing air flow, making them excellent control devices.

Some important chemical engineering attributes of pulsed columns are provided in Table 3.3.