Patent Number: 050229737
Section: summary

This invention relates to enhanced solvent extraction, and in particular to a method and an apparatus for electrostatically enhanced solvent extraction. In known electrostatic solvent extraction cells, relatively high extraction rates can be obtained if the dispersed phase is charged electrically by passing it through charged nozzles, and then subsequently accelerating the charged dispersed phase droplets in an electrical field. This procedure not only gives rise to very small drops (and hence a large interfacial area for mass transfer) but also involves very short contact times by virtue of the high drop velocity. The process is thus ideally suited to the extraction of labile and unstable materials, frequently of biological origin. UK Patent 1205562 envisaged a column comprising a number of stacked sieve plates and a bank of nozzles which are in communication with an inlet for the dispersed phase. The nozzles are arranged to discharge longitudinally along the column, and when a potential is applied to the nozzles, charged droplets of the dispersed phase are discharged longitudinally towards the plates. This arrangement works well on the small scale but becomes progressively less efficient with scale-up due to the fact that the inner nozzles in the bank tend to be shielded electrostatically by the outer ones and therefore carry a smaller charge. As a consequence, only the outer nozzles produce the very small droplets required whilst the inner nozzles give rise to larger droplets having a lower specific area for mass transfer. Furthermore, since the larger droplets are not so highly charged, they are incapable of being accelerated to high velocities and so the advantage of low contact times is lost. According to one aspect of the present invention, there is provided a method of electrostatically enhanced solvent extraction in which a dispersed phase interacts with a continuous liquid phase flowing through a column in counter-current relationship, wherein the improvement comprises locating interacting means in the column so as to catch the dispersed phase flowing in the column and so as to define a space at a side of the interacting means between the side and the side of the column and through which space the continuous phase flows, and providing an electric field to cause the discharge of charged droplets of the dispersed phase sideways from the interacting means through the space toward the side of the column. According to another aspect, the invention provides apparatus for electrostatically enhanced solvent extraction comprising a column in which a continuous phase is arranged to flow therethrough in counter-current relationship with a dispersed phase, wherein the improvement comprises, means for catching a dispersed phase flowing through the column and for interacting the dispersed phase with the continuous phase, the interacting means being located in the column to define a space at a side thereof between the side and the side of the column and through which space the continuous phase is arranged to flow, and means for providing an electric field to cause the discharge of charged droplets of the dispersed phase sideways from the interacting means through the space toward the side of the column. In a further aspect, the invention provides apparatus for electrostatically enhanced solvent extraction, the apparatus comprising a column for the counter-current flow of a continuous phase and a dispersed phase therethrough, one or a plurality of inlets for the introduction of the dispersed phase into the column to contact the continuous phase, at least one receptacle at an intermediate location in the column for catching the dispersed phase flowing through the column, the receptacle having side walls, an open top, and a closed base, the receptacle being located to define a space at the side thereof between the side of the receptacle and the column and through which space the continuous phase is arranged to flow, the receptacle having a plurality of discharge ports extending through the side walls and arranged to discharge the dispersed phase through the side walls and through the space toward the side of the column, and means for applying an electric potential to the receptacle to cause the discharge of charged droplets of the dispersed phase through the discharge ports. Preferably, the cell comprises a column with the discharge ports being arranged to discharge droplets towards the side of the column. Desirably, the flow of the continuous phase is biased away from the centre of the column so as to enhance the interaction of the continuous phase with the charged droplets. Advantageously, the electric field strength applied to the receptacle is at least 1.5 kV/cm. Preferably a collector is disposed below the receptacle, the collector having radially disposed riser ports for the upward flow of the continuous phase therethrough, and a central outlet for the downward flow of dispersed phase therethrough. Conveniently the collector has a side wall that extends to at least the same height as the discharge ports in the cell, and the electric potential is applied between the discharge ports and the side wall. Advantageously, the base of the collector is of frusto-conical form to bias the flow of the dispersed phase towards the central outlet, and bias the flow of the continuous phase towards the ports. Preferably, a plurality of receptacles and collectors are located in series in the cell, and alternate receptacles may be oppositely charged. Conveniently, the collectors may be at earth potential. Use of the invention overcomes the aforementioned problem by arranging nozzles in such a manner that no matter how many nozzles are employed, electrostatic shielding is avoided and all the nozzles can carry the same charge. This arrangement enables large diameter electrostatic solvent extraction columns to be envisaged and so opens the way to the exploitation of electrostatically enhanced solvent extraction on an industrial scale.