LIQUID-LIQUID CONTACTOR COLUMNS

Abstract: A liquid-liquid contactor column 10 has a solvent input 16 and an aqueous phase input 22. Lighter phase liquid is discharged at 31 and heavier phase is moved by an air-lift 26 to a receiver. The air supply line 28 to the air-lift 26 includes valve 32 and a pressure sensing transducer- 34 senses pressure in line 28 and sends electric signals to signal processing unit 52 which may for example close down operation of the column, or effect other changes to maintain operating characteristics of the column. The column could be pulsed by means which also send signals to unit 52, and could deliver to an external settler.

Description: This invention relates to liquid-liquid contactor columns. One form of contactor is the pulsed column type in which the rate of mass transfer is enhanced by hydraulic pulsation of the liquids in the column through packing within the column, one liquid forming a continuous phase and the other a dispersed phase. In one known design of pulsed column for use in situations where a solute-containing aqueous liquid (such as uranium/plutonium-bearing nitric acid) constitutes the dispersed phase and flows downwardly through the column for contact with a continuous phase constituted by a solvent extractant such as tri-n-butyl phosphate diluted with odourless kerosene, a phase-disengaging section, known as the settler, is located at the bottom of the column. To secure stable operation of the column and satisfactory mass transfer between the two phases, it is necessary to regulate the position of the liquid-liquid interface within the settler and consequently, to enable such regulation to be effected, the position of the interface has to be monitored. One method of monitoring for this purpose involves the use of pneumercators inserted into each phase for measuring differential pressure in the settler, and signal processing circuitry connected to the pneumercators. However, because the bottom settler is subject to the pulsating pressure head exerted by the column, accurate sensing of the interface is difficult and involves the use of complex electronics for performing the necessary signal processing. Moreover, the interface is not, in practice, a well-defined boundary - instead it comprises a dispersion band formed by an emulsion of the two phases. It has previously been proposed to employ a separate or external settler instead of using a section of the pulsed column as the settler. In this instance, a mixture of the two liquid phases is pumped by an air-lift from the bottom of the column into a separate settler where they are separated, the separated lower density phase then being available for recycle to the column and the separated higher density phase being passed, via a lute, to subsequent process operations. With an external settler, the position of the liquid-liquid interface is essentially determined by substantially non-variable parameters, such as: the relative heights of the lower density phase off-take and the lute for the higher density phase; the densities of the two phases; their relative proportions in the dispersion band; and the flow resistances present. In practice, the parameters may be selected so that the interface lies at a substantially well-defined level and the interface remains in this position (in the absence of fault conditions) because any fluctuations in the flowrates of the liquids to the settler result in only slight changes in the relative heights of the separated liquids at the lute for the higher density phase and the off-take for the lower density phase. In pulsed columns using an integral settler in situations where radioactive liquors are being handled, various parameters such as interface movement, total column height (and hence dispersed phase hold-up), pulsing amplitude and pulsing frequency can be monitored by means of a system of pneumercators. According to this invention there is provided a liquid-liquid contactor column including an air lift for lifting liquid from a lower part of the liquid-liquid contactor column and means for indicating the air pressure applied to the air lift, characterised in that the means (34) for indicating the air pressure applied to the air lift (26, 28) comprises a pressure transducer (34) adapted to produce electrical signals related to the magnitude of the air pressure applied to the air lift (26, 28) and that there is included a signal processor (52) adapted to produce control signals related to the electrical signals produced by the transducer (34) and means (32) responsive to the control signals to vary the pressure of the air in the air lift (26, 28). German Patent No. 598,595 discloses a liquid treatment plant in which a dense phase is passed from one treatment chamber to another via a pressurised pneumatic transfer system and means are provided for indicating the air pressure applied to the pneumatic transfer system, but no more is done than that and no use is made actively to derive control signals related to the indications of the air pressure applied to the transfer system and using these signals to control the action of a liquid-liquid contactor column. The air-lift need not be pulsed. When the column is pulsed, electrical means for processing said signals may discriminate between first signal components attributable to pulsing of the column and second signal components attributable to pulsing of the air-lift means. The air-lift means may deliver liquid from below a continuous phase inlet to a separate settler vessel. As used herein, "air-lift means" is to be construed as covering the use of air or other gases to feed contents of the column to a receiver. The electrical signal processing means may also be arranged to effect averaging of the electrical signals produced by the pressure sensing means. In practice, the average pressure prevailing in the air-supply line to the air-lift means is related to the dispersed phase hold-up prevailing within the column, i.e. average pressure increases generally linearly with column weight (which is a measure of dispersed phase hold-up). The electronic signal processing means may comprise means for detecting variation of the first and/or second signal components, and means responsive to the detecting means for modifying operation of the column and/or the air-lift means in dependence upon such variations. For example, if the detecting means detects a change in the amplitude of the first signal component (which change may be indicative of a change in the operational state of means for pulsing the column in the case where the column is a pulsed column), the modifying means may act on the column-pulsing means in such a way as to restore the amplitude of the first signal component to a pre-determined value or a value lying within a pre-determined range. Similarly, if the detecting means detects a change in the frequency of the first signal component, then the modifying means may act on timing means associated with the column-pulsing means to restore the column pulsing frequency to a pre-determined value or a value within a pre-determined range. In another instance, if the detecting means detects a change in the averaged output of the electrical processing means (indicating a change in the dispersed phase hold up), the modifying means may act to change the air supply to the air-lift means and thereby counteract the change in dispersed phase hold up. In other circumstances, the modifying means may act to shut down operation of the column;, for instance, if the first signal component is absent this is indicative of malfunction of the column-pulsing means and column operation may be shut down while the malfunction is rectified. The invention may be performed in various ways and some specific embodiments with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows an extractor column; Figure 2 shows another column; Figure 3 shows a column with external settler; Figure 4 shows a pulsed column; and Figure 5 shows a graph. The invention is applicable to any form of liquid-liquid contactor column. In the arrangement, operation of the column is monitored using the pressure in an air-lift riser of an air-lift, the air-lift being used to remove the heavier phase. Referring to Figure 1, the plant comprises a liquid-liquid contactor column 10 fitted in conventional manner with a large number of horizontally disposed perforated plates (or sieve plates) 11. The column 10 is supplied with an organic solvent phase via line 16. An aqueous phase is supplied to the column via line 22. Where the column forms part of a nuclear fuel reprocessing plant, the aqueous phase typically comprises nitric acid containing dissolved nuclear fuel and the solvent phase typically comprises tri-n-butyl phosphate in an odourless kerosene diluent. The column may be operated with the aqueous liquor as the dispersed phase. Following thorough dispersion and mixing, with accompanying mass transfer of solute between the phases, the phases are withdrawn continuously from the column 10, the heavier phase by means of air-lift limb 26 supplied with air via air supply line 28, and the lighter phase overflowing through line 31. The plant, if employed for nuclear fuel reprocessing, is contained within shielding, part of which is depicted by reference numeral 30. The air supply line 28 passes through the shielding 30. Flow through the line 28 is controllable by a flow-controlling valve 32 and the pressure in the line 28 is sensed by a pressure transducer 34, both the valve 32 and the transducer 34 being located externally of the shielded containment cell. The liquid-liquid interface for light phase dispersal is at 29 and the lighter phase (the solvent) is removed at 31. The heavier phase is removed by the air lift 26. In operation,t he air lift action creates a series of air pockets within the air-lift limb 26 which transport slugs of the heavier phase to an external receiver, the slugs being delivered at regular intervals. Figure 5 illustrates the variation of the air pressure in air-supply line 28 as sensed by the transducer. It will be seen that the transducer output consists of a series of pulses 46 corresponding to build-up of the air pockets in the air lift limb 26 followed by troughs 48 corresponding to the air pocket and slug travel up the limb. For a given air flow rate, set by valve 32, the troughs 48 occur at regular intervals. The pulses 46 as shown have a higher frequency oscillation superimposed thereon comprising a series of regularly-spaced peaks 50 referred to below. The peaks 50 are absent in operation of the Figure 1 arrangement. The output of the transducer 34 is fed to electronic processing instrumentation 52 which may be computer-based, for producing a record of the pressure variations occurring during operation of the plant. The instrumentation 52 serves to process the signals derived from the transducer 34 so as to determine the signal content attributable to the slugging action of the air-lift limb 26. In this way, malfunctions of the plant can be detected from a single signal obtained from a sensor 34 located externally of the shielding cell. The instrumentation 52 may function as a controller for adjusting various control components of the plant in dependence upon the signal components present in the output of the pressure transducer. For instance, if the value of the transducer signal, after averaging by the instrumentation 52, deviates from a pre-set value, the latter may adjust the flow valve 32 via control line 58 to restore the averaged value of the transducer signal to a desired value or within a desired range. The plates 11 can be replaced by other forms of packing, for example Raschig rings, Lessing rings, wire mesh, and can take the form of fixed material with sufficient voidage to allow flow of the liquids. In Figure 2, the liquid-liquid interface for heavy phase dispersal is at 41, the level of lighter phase (solvent) being at 43. In this case also slugs of heavier phase liquid are delivered at 27. In Figure 3, the lift 26 feeds an external settler 24 which comprises a settler vessel 36 provided with an air-disentrainment chamber 38 via which the mixed phases are fed to the vessel 36. Phase disengagement takes place in the settler, the interface between the two phases being indicated at 40. The lighter phase may be taken off via a recycle line 20 connecting with a main supply line 18 and with line 16, and the heavier phase e.g. aqueous raffinate being taken of via a lute arrangement 42 and line 44 for subsequent treatment. Figure 4 shows a pulsed column arrangement having similarity to Figure 3. In this case the column is pulsed in known manner by a pulsing device 14 (including timer means 14a) which applies pulses of well-defined amplitude and frequency to the contents of the column through a pulsing limb 12. In this case the peaks 50, Figure 5, are present and correspond to pulsing of the column i.e. the superimposed perturbation has a frequency and amplitude governed by the frequency and amplitude imparted to the column by the pulsing device 14. The signals on line 58 will include a component attributable to column pulsing. If, for example, a blockage occurs between the column 10 and the air inlet point of the limb 26, the higher frequency component (i.e. peaks 50) will be absent and, in this event, automatic shut-down of the plant may be initiated. Also, the instrumentation 52 may include circuitry for monitoring the frequency and amplitude of the signal component corresponding to column pulsing so that, if the frequency and/or amplitude deviates from certain values or ranges the controller 52 produces corrective signals on line 56 to effect adjustments of the pulsing device 14. It will be understood that the columns of Figures 1 to 3 could be pulsed. Thus, in any extraction or liquid-liquid contactor column an air-lift can be used to lift the more dense (heavy) phase from the base of the column to a product receiver and/or another part of the process. The pressure of the air in the air-lift riser can be used to monitor the total weight of the liquids in the column and hence the dispersed phase hold-up (DPHU). This in turn can be used to monitor the approach to flooding in the column since, as the flow-rates of the liquids fed to the column are increased, the DPHU increases. At some point the limiting flow-rates are reached and the column will flood. The column can be monitored in this way regardless of whether the phase separating interface is at the top of the column, the base of the column or in an external settler, see Figures 1, 2, 3 and 4 respectively. The instrumentation 52 may include electronic signal processing means arranged to average the electrical signals from the sensing means. The processing means may comprise means for detecting variation of the first and/or second signal components, and means responsive to the detecting means for modifying operation of the column and/or the air-lift means in dependence upon such variations. The detecting means and modifying means may act as indicated above.

Claims: A liquid-liquid contactor column including an air lift for lifting liquid from a lower part of the liquid-liquid contactor column and means for indicating the air pressure applied to the air lift, characterised in that the means (34) for indicating the air pressure applied to the air lift (26, 28) comprises a pressure transducer (34) adapted to produce electrical signals related to the magnitude of the air pressure applied to the air lift (26, 28) and that there is included a signal processor (52) adapted to produce control signals related to the electrical signals produced by the transducer (34) and means (32) responsive to the control signals to vary the pressure of the air in the air lift (26, 28). A liquid-liquid contactor column according to Claim 1 wherein there is included means (14) for pulsing the action of the liquid-liquid contactor column (10) and the signal processor (52) is adapted to distinguish between a first component of the electrical signals from the transducer (34) attributable to the pulsing of the liquid-liquid contactor column (10) and a second component of the electrical signals from the transducer (34) attributable to fluctuations in the pressure of the air in the air lift (26, 28). A liquid-liquid contactor column according to Claim 2 wherein the signal processor (52) and control means (32) are adapted to respond to variations in the first component of the electrical signals from the transducer (34) to control the action of the means (14) for pulsing the liquid-liquid contactor column (10). A liquid-liquid contactor column according to Claim 3 wherein the signal processor (52) and control means (32) are adapted to respond to changes in the amplitude of the first signal component and vary the amplitude of the action of the means (14) for pulsing the liquid-liquid contactor column (10) to restore the amplitude of the first signal component to a value within a pre-determined range of values. A liquid-liquid contactor column according to Claim 3 wherein the signal processor (52) and control means (32) are adapted to respond to changes in the frequency of the first signal component and vary the frequency of the action of the means (14) for pulsing the liquid-liquid column (10) to restore the frequency of the first signal component to a value within a pre-determined range of values. A liquid-liquid contactor column according to any of Claims 2 to 5 wherein the signal processor (52) and control means (32) are adapted to respond to changes in the average value of the electrical signals from the transducer 34 to vary the air pressure supplied to the airlift (26, 28) so as to counteract changes in the hold-up in the liquid-liquid contactor column (10) of the dispersed phase. A liquid-liquid contactor column according to Claim 3 wherein the signal processor (52) and control means (32) are adapted to shut down the operation of the liquid-liquid contactor column (10).

Inventors: LOGSDAIL, DENNIS HAROLD, PARKER, DOUGLAS EDWARD
Applicants: UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1989
Main classification: B01D11
IPC codes: B01D11