Patent Number: 050229737
Section: description

Referring now to FIG. 1 and FIG. 2, part of a solvent extraction column 10 of hollow cylindrical form is shown, and it is assumed that a heavier phase (usually aqueous) is dispersed and therefore moves down the column 10. In the column 10 a shallow receptacle 12 is surrounded by a funnel-shaped collector 14. In a typical operation, the receptacle 12 is charged to a high potential by means of a DC generator (not shown) and the collector 14 is at earth potential. The receptacle 12 dimensions are not critical but the receptacle 12 is made as small as possible in order to reduce the static hold-up of dispersed phase in the column 10. To this end, the underside 16 of the receptacle 12 may be inwardly dished as shown in order to reduce static hold-up still further. The receptacle 12 has an outer wall 13 supplied with six symmetrically disposed discharge ports in the form of nozzles 18 mounted as close to the bottom of the receptacle 12 as practicable. The receptacle 12 is supported in the column by means of one or more rods of non-conducting material (not shown) and is electrically insulated from all other components of the column 10. The collector 14 is similarly supported unless the column 10 is operated with the collector 14 at earth potential. Referring now to FIG. 1 and FIG. 3, the collector 14 has a vertical cylindrical wall 20 and a conical base 22 with a central outlet 24 through which the dispersed phase passes to the next receptacle 12 below. The base 22 has four symmetrically disposed radially displaced riser ports 26 through which the (lighter) continuous phase passes upwards through the column 10, the riser ports 26 having conical caps 28 defining gaps 29 in order to inhibit dispersed phase passing downwards through the riser ports 26 and thereby bypassing the receptacle 12 beneath. The diameter of the collector wall 20 is made such that it fits snugly within the column 10. As shown in FIG. 4, the column 10 may be constructed by stacking a series arrangement of receptacles 12 and collectors 14. In one method of operation receptacles 12 identified as (1), (3) and (5) are positively charged whilst receptacles 12 (2), (4) and (6) are negatively charged with respect to earth potential. All the collectors 14 identified as (1)-(6) are earthed. This arrangement has the advantage that since all the collectors 14 are earthed they need not be electrically insulated from the column 10. Furthermore the heavy phase leaving the bottom of the column 10 is electrically neutral. It is however possible to operate with alternative circuits and, as an example, all receptacles 12 may be positively charged whilst all collectors 14 are negatively charged. In FIG. 4, the conducting heavy phase (usually aqueous in practice) is fed to the top of the column 10 and enters receptacle 12 (1). The heavy phase flows through the charged nozzles 18 and issues as a charged spray of very small droplets of heavy phase. These droplets are attracted to the earthed collector 14 where they coalesce on the wall 20 and then flow by gravity down through the outlet 24 to the next receptacle 12 below where the entire process is repeated. Finally, the heavy coalesced phase leaves the collector 14 (6) and forms an interface 32 at the bottom of the column 10 from where it is withdrawn. The heavy coalesced phase could, in principle, be withdrawn directly from collector 14 (6) but by allowing it to flow to the bottom of the column 10 a small volume of column 10 is created for the introduction and distribution of the light continuous phase. The latter then rises up the column 10, passing through the riser ports 26 in each collector 14 on the way, and is withdrawn at the top of the column 10. Contact between the two phases is effected in two different ways. Thus the rising continuous phase encounters the fine spray of dispersed phase droplets leaving each nozzle 18 and at the same time contacts the falling film of dispersed phase on the wall 20 of each collector 14. The overall mechanism of mass transfer is therefore a combination of cross-flow and countercurrent transfer. By far the more important mode of transfer is that involving the droplet spray since in this case not only are the droplets oscillating because of the electrical charge that they carry but their interfacial area is also very high. The characteristics of the droplets, including their mean size, is controlled not by the nozzle 18 diameter but by the voltage applied to the nozzles 18. The important parameter is the electrical field strength between the tip of each nozzle 18 and the wall 20 of the collector 14. Referring again to FIG. 1, the distance between the tip of each nozzle 18 and the wall 20 is denoted by L. If V volts are applied to the nozzles 18, the field strength E is given by V/L. When E is less than 1 kV/cm, droplets form separately at the nozzles 18 although they are smaller than would be the case if no field were applied. When E is equal to 1.5 kV/cm or greater, myriads of very small droplets issue from the nozzles 18 and it is in this "spray" regime that the column 10 should operate. Although relatively high DC voltages may be called for, the current required is very small and only a few watts would be consumed by each receptacle 12. The invention is thus very economical in terms of energy utilisation. It should also be noted that the droplet size is independent of the nozzle 18 diameter in the spray regime. This is an important feature when extraction of liquors containing suspended solids is contemplated, such as biological broths which may contain cell fragments or other lysis products. In such situations, the nozzle 18 diameter may be made sufficiently large to avoid blocking without impairing the very small droplet size range needed to maintain a high interfacial area of contact. In principle the invention may be used in co-current as well as in counter-current flows through a solvent extraction column. The number of nozzles 18 may be selected to provide a required discharge of charged droplets towards the wall 20. There may be fewer or more than the six shown in FIG. 2. Although the nozzles 18 have been shown as aligned normal to the longitudinal axis of the column 10, if desired the nozzles 18 may be aligned at alternative angles. The number of riser ports 26 is selected to provide the required flow of the continuous phase, and may be fewer or more than the four shown in FIG. 3. The use of a relatively high wall 20 for the collector 14 has advantages, but if necessary the electric field could be applied between the nozzles 18 and the column 10.