Mccausland, Linda Jane (20 Shepherd Gardens, Abingdon, Oxfordshire OX14 5PR, GB)
Turner, Andrew Derek (17 Loyd Close, Abingdon, Oxfordshire OX14 1XR, GB)
Legg, Stuart Anton (11 Ashwood Drive, Shaw Newbury, Berkshire RG14 2PN, GB)
Mccausland, Linda Jane (20 Shepherd Gardens, Abingdon, Oxfordshire OX14 5PR, GB)
Turner, Andrew Derek (17 Loyd Close, Abingdon, Oxfordshire OX14 1XR, GB)
| 1. | A method of treating matter by mediated electrochemical redox reaction in which method an electrolyte containing ions of an electrochemically re generable primary oxidising or reducing species is subjected to an electric potential within an electrochemical cell and the matter is added to the electrolyte either continuously or periodically thereby to be subjected to an oxidation or reduction process in which the primary oxidising or reducing species is reduced or oxidised respectively and regenerated by the electric potential, characterised in that the electrolyte containing the said ions and the matter intermixed therewith is subjected to ultrasonic and/or sonic insonation directly to enhance the respective oxidation or reduction interaction between the ions and the matter in a vessel which is outside the electrochemical cell and the ultrasound and/or sound is concentrated within the electrolyte away from walls of the vessel. |
| 2. | A method as claimed in claim 1 wherein the insonation is sonic. |
| 3. | A method as claimed in claim 1 wherein the insonation is ultrasonic. |
| 4. | A method as claimed any of the preceding claims, wherein the matter is to be oxidised and the said ions are of silver in an acidic aqueous electrolyte. |
| 5. | A method as claimed in claim 4, wherein said acidic electrolyte comprises nitric acid. |
| 6. | A method as claimed any of the preceding claims, wherein the matter is solid matter. |
| 7. | A method as claimed in any of claims 4 to 6, wherein the matter comprises or includes organic matter. |
| 8. | A method as claimed in claim 7, wherein the electrolyte in the cell is separated into catholyte and anolyte regions by an ion permeable membrane and a proportion of the catholyte is extracted for feed into the anolyte to compensate for transfer of ions of the said oxidising species, water and organic molecules from anolyte to catholyte in the electrochemical cell. |
| 9. | A method as claimed in claim 8, wherein said extracted catholyte is subject to a solids concentration process, a high solids fraction being fed into the anolyte and a low solids fraction being returned to the catholyte. |
| 10. | A method as claimed in claim 9, wherein said extracted catholyte is cooled prior to being subjected to said solids concentration process, the cooling encouraging precipitation of dissolved organic matter thereby to enhance the return of organic matter to the anolyte. |
| 11. | A method as claimed in any of the preceding claims, wherein matter is supplied as a slurry of solids suspended in water and is subjected to a solids concentration process just prior to mixing with anolyte, the high solids fraction being fed into the anolyte and mixed therewith. |
| 12. | Apparatus for use in the treatment of matter, which apparatus comprises an electrochemical cell having a cathode, an anode, a permeable separator between the anode and cathode forming an anode region and a cathode region within the cell, an electrolyte containing ions of an electrochemically regenerable primary oxidising or reducing species, means for mixing the matter continuously or periodically with respectively anolyte for oxidising or catholyte for reducing, and one or more ultrasonic and/or sonic transducers mounted for insonating the said mixture in a vessel outside the electrochemical cell such that the ultrasound and/or sound is concentrated within the electrolyte and away from walls of the vessel to enhance the respective oxidation or reduction interaction between the said ions and the matter. |
| 13. | Apparatus according to claim 12 Wherein the transducers all operate at the same frequency. |
| 14. | Apparatus according to claim 12 wherein the transducers all operate at different frequencies. |
| 15. | Apparatus as claimed in claim 12, wherein the matter is to be oxidised and the said ions are of silver in an acidic aqueous electrolyte. |
| 16. | Apparatus as claimed in claim 15, wherein said acidic electrolyte comprises nitric acid. |
| 17. | Apparatus as claimed in claim 15 or claim 16, wherein an anolyte vessel is connected for circulation of anolyte between the anolyte vessel and the anolyte region of the electrochemical cell, a catholyte vessel is connected for circulation of catholyte between the catholyte vessel and the catholyte region of the electrochemical cell, and the ultrasonic and/or sonic transducer or transducers are mounted on the said anolyte vessel. |
| 18. | Apparatus as claimed in claim 17, wherein a connection is provided for extracting and feeding a proportion of catholyte from the catholyte vessel into the anolyte vessel to compensate for transfer of silver, water and organic molecules from anolyte to catholyte in the electrochemical cell. |
| 19. | Apparatus as claimed in claim 18, wherein the said connection between the catholyte vessel and the anolyte vessel includes means for effecting a solids concentration process, a high solids fraction being fed into the anolyte vessel and a low solids fraction being returned to the catholyte vessel. |
| 20. | A method of treating matter by mediated electrochemical redox reaction in which method an electrolyte containing ions of an electrochemically re generable primary oxidising or reducing species is subjected to an electric potential within an electrochemical cell and the matter is added to the electrolyte either continuously or periodically thereby to be subjected to an oxidation or reduction process in which the primary oxidising or reducing species is reduced or oxidised respectively and regenerated by the electric potential, characterised in that the electrolyte containing the said ions and the matter intermixed therewith is subjected to sonic insonation directly to enhance the respective oxidation or reduction interaction between the ions and the matter in the electrochemical cell. |
| 21. | Apparatus for use in the treatment of matter, which apparatus comprises an electrochemical cell having a cathode, an anode, a permeable separator between the anode and cathode forming an anode region and a cathode region within the cell, an electrolyte containing ions of an electrochemically regenerable primary oxidising or reducing species, means for mixing the matter continuously or periodically with respectively anolyte for oxidising or catholyte for reducing, and one or more sonic transducers mounted for insonating the said mixture in the electrochemical cell to enhance the respective oxidation or reduction interaction between the said ions and the matter. |
Patent specification EP 0 297 738 describes a method and apparatus for electrochemical treatment of organic waste matter using an aqueous electrolyte comprising nitric acid and containing silver ions as an electro- chemically re-generable primary oxidising species.
Operated at a temperature between 50°C and 90°C, the cell is particularly effective in decomposing organic waste matter.
Patent specification EP 0 771 222 describes developments of the apparatus of EP 0 297 738 for preventing or reducing the build-up of contamination of electrolyte by one or more of the elements sulphur, nitrogen, chlorine, bromine or iodine. Reference is made to organic waste which has assumed importance in recent years in the form of explosive material and chemical weapons required to be destroyed, for example, under International Treaty arrangements. Further examples of organic waste requiring destruction for which the method and apparatus has application are wastes containing agrochemicals (pesticides and herbicides) and toxic pharmaceuticals.
The method and apparatus described in these prior patent specifications provides a relatively safe and effective route for the disposal of such material and EP 0 771 222 addresses the problems of build-up of contamination in the electrolyte. Certain waste materials for disposal present additional hazards.
Patent application GB 01 02843.3 describes measures to protect against these, to improve the overall environmental acceptability of the apparatus and reduce the possibilities for fouling of the electrochemical cell by solids in the electrolyte.
US 5707508 mentions the use of ultrasound applied to emulsify a feed of liquid organic material for admixture with electrolyte to enhance the oxidation rate of the organic material in an electrochemical cell.
The invention is based upon the appreciation that particularly effective enhanced interaction between ions of the electrochemically re-generable species and matter undergoing treatment is achieved by applying ultrasound or sound directly to the reaction mixture including the matter to be treated and the said ions.
The invention provides, in one of its aspects, a method of treating matter by mediated electrochemical redox reaction in which method an electrolyte containing ions of an electrochemically re-generable primary oxidising or reducing species is subjected to an electric potential within an electrochemical cell and the matter is added to the electrolyte either continuously or periodically thereby to be subjected to an oxidation or reduction process in which the primary oxidising or reducing species is reduced or oxidised respectively and re-generated by the electric potential, characterised in that the electrolyte containing the said ions and the
matter intermixed therewith is subjected to ultrasonic and/or sonic insonation directly to enhance the respective oxidation or reduction interaction between the ions and the matter in a vessel which is outside the electrochemical cell and the ultrasound and/or sound is concentrated within the electrolyte away from walls of the vessel.
Either ultrasound or sound may be used to enhance the interaction between the ions and the matter to be treated. Ultrasound is typically sound at frequencies above 20kHz. Sound is typically at frequencies up to 20 kHz. In one embodiment ultrasound is used. In a preferred embodiment sound is used. Preferably the sound is at a frequency of from 7 to 20kHz, preferably from 10 to 20kHz. In a further embodiment, a mixture of ultrasound and sound may be used.
Where the matter is to be oxidised, the said ions are preferably of silver in an acidic aqueous electrolyte, preferably nitric acid electrolyte, although methanesulphonic acid can be used as electrolyte. The method is particularly suitable where the matter is solid matter which may be or may include organic matter. The organic matter, which includes biological organic matter, is any matter based on carbon atoms and may thus include biological waste. The matter may also be chemical weapons, explosive material or matter that is contaminated with nuclear waste.
The invention provides, in another of its aspects, apparatus for use in the treatment of matter, which apparatus comprises an electrochemical cell having a cathode, an anode, a permeable separator between the anode and cathode forming an anode region and a cathode region within the cell, an electrolyte containing ions of
an electrochemically re-generable primary oxidising or reducing species, means for mixing the matter continuously or periodically with respectively anolyte for oxidising or catholyte for reducing, and one or more ultrasonic and/or sonic transducers mounted for insonating the said mixture in a vessel outside the electrochemical cell such that the ultrasound and/or sound is concentrated within the electrolyte and away from walls of the vessel to enhance the respective oxidation or reduction interaction between the said ions and the matter.
The vessel where the electrolyte, typically anolyte, is exposed to sound and/or ultrasound forms part of the apparatus for recirculating the electrolyte and is outside the electrochemical cell. It may be a separate vessel or a pipe. Preferably it is a separate vessel such as a cylindrical container through which the electrolyte, preferably anolyte, flows.
The sound and/or ultrasound is provided by transducers which may be inside or outside the vessel.
For example, they may be bonded to or in contact with the outside of the vessel, sound may be introduced via a probe projecting downwardly through the upper surface of the liquid (s) or by a providing a downwardly directed sonic or ultrasonic horn on the top wall of the vessel.
The sound and/or ultrasound transducers, probe or horn are arranged so that the sound and/or ultrasound is concentrated within the electrolyte away from the vessel walls. Thus, the acoustic energy density of the sound and/or ultrasound is greater in the electrolyte than at the vessel walls. Preferably the acoustic energy density of the sound and/or ultrasound is less than 3 Watts/cm2, preferably less than 2.5 W/cm2, more preferably less than
2 W/cm2 at the walls of the vessel. This reduces cavitation at the vessel walls.
The sound and/or ultrasound transducers may be connected so as to operate in different combinations such that the energy is concentrated within the electrolyte.
For example, the transducers may be arranged in rows around the vessel. In that case, typically at least a row of transducers is on at the same time in order to concentrate the ultrasound or sound energy within the electrolyte. Using a number of transducers all at the same frequency allows the ultrasound or sound to be focussed within the vessel. Different transducers may be operated at different frequencies, preferably simultaneously. This also results in an increased concentration of ultrasound and/or sound towards the centre of the vessel and away from the walls.
Cavitation blocking occurs when the acoustic energy density of the sound and/or ultrasound a small distance into the electrolyte causes enough cavitation to shield the rest of the electrolyte from the sound waves and they fail to penetrate the liquid. The energy density at which cavitation blocking occurs varies with frequency. At a frequency of 20kHz, cavitation blocking can occur at 2. 5W/cm2 depending on the system and on temperature.
However, at high frequencies, cavitation blocking occurs at a higher energy density. Typically the power supplied to the ultrasound and/or sound transducers is chosen so as to avoid cavitation blocking The vessel where the sound and/or ultrasound is applied may comprise an inner removable vessel which contains the anolyte and waste matter. After a period of use it is convenient to be able to remove the inner removable vessel so as to be able to remove any solids
that have built up in the unit. The removable vessel may take the form of a basket, or preferably a solid vessel as this minimises the damage done to the unit by the sound and/or ultrasound waves. In a preferred embodiment the removable vessel is a vessel with solid sides and base, the base being provided with connections to inlet and/or outlet pipes in the outer vessel.
The present invention also provides a method of treating matter by mediated electrochemical redox reaction in which method an electrolyte containing ions of an electrochemically re-generable primary oxidising or reducing species is subjected to an electric potential within an electrochemical cell and the matter is added to the electrolyte either continuously or periodically thereby to be subjected to an oxidation or reduction process in which the primary oxidising or reducing species is reduced or oxidised respectively and re- generated by the electric potential, characterised in that the electrolyte containing the said ions and the matter intermixed therewith is subjected to sonic insonation directly to enhance the respective oxidation or reduction interaction between the ions and the matter in the electrochemical cell.
The invention provides, in another of its aspects, apparatus for use in the treatment of matter, which apparatus comprises an electrochemical cell having a cathode, an anode, a permeable separator between the anode and cathode forming an anode region and a cathode region within the cell, an electrolyte containing ions of an electrochemically re-generable primary oxidising or reducing species, means for mixing the matter continuously or periodically with respectively anolyte for oxidising or catholyte for reducing, and one or more sonic transducers mounted for insonating the said mixture
in the electrochemical cell to enhance the respective oxidation or reduction interaction between the said ions and the matter.
Sound is typically at frequencies up to 20 kHz.
Preferably the sound is at a frequency of from 7 to 20kHz, preferably from 10 to 20kHz.
The sound is provided by transducers which may be inside or outside the vessel. For example, they may be bonded to or in contact with the outside of the vessel, sound may be introduced via a probe projecting downwardly through the upper surface of the liquid (s) or by a providing a downwardly directed sonic horn on the top wall of the vessel.
The sound transducers, probe or horn are arranged so that the sound is concentrated within the electrolyte away from the vessel walls. Thus, the acoustic energy density of the sound is greater in the electrolyte than at the vessel walls. Preferably the intensity of the sound is less than 3 Watts/cm2, preferably less than 2.5 W/cm2, more preferably less than 2 W/cm2 at the walls of the vessel. This reduces cavitation at the vessel walls.
The sound transducers may be connected so as to operate in different combinations. Preferably they are arranged such that the energy is concentrated within the electrolyte. For example, the transducers may be arranged in rows around the vessel. In that case, typically at least a row of transducers is on at the same time in order to concentrate the sound energy within the electrolyte. Using a number of transducers all at the same frequency allows the sound to be focussed within the vessel. Different transducers may be operated at different frequencies, preferably simultaneously. This
also results in an increased concentration of sound towards the centre of the vessel and away from the walls.
Specific constructions of apparatus and methods embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which: Figure 1 is a schematic representation of a part of an apparatus for use in the decomposition of waste matter, and Figure 2 is a diagrammatic representation, of a part of an apparatus for use in the decomposition of waste matter.
The principle of operation of the apparatus of these examples, which is explained in GB 01 02843.3 and also in EP 0 297 738 is straightforward. In an electrochemical cell, an electrolyte of nitric acid containing silver ions is separated by a membrane (typically Nafion) into an anode region and a cathode region. Waste matter to be decomposed is mixed with the anolyte. Ag ions in the anolyte either directly themselves or via secondary oxidising species oxidise the waste matter. The reduced Ag ions produced in this process are electrochemically re-generated in the cell.
Figure 1 shows part of an apparatus designed for handling organic waste supplied at 11 as a slurry with excess water, such as may be required when the waste is explosive. A full description of this apparatus may be found in GB 01 02843.3.
The heart of the apparatus is electrochemical cell 12 having an anolyte region 13 and catholyte region 14 separated by membrane 10. A main reaction anolyte vessel 15 is supplied with anolyte (thus held separately from the anolyte region 13 of the electrochemical cell 12) and other process streams as will be described below. A catholyte vessel 17 provides a holding and management vessel for catholyte separate from the catholyte region 14 of the electrochemical cell 12.
Electrolyte supply for the anolyte vessel 15 and catholyte vessel 17 at startup and for any makeup required during processing is provided via 16,18 respectively from a supply of silver nitrate solution, a supply of nitric acid and a supply of process water.
Feed, in this example, of a slurry in water of solid organic waste at 11 passes first to hydrocyclone 28 from which a solids rich component passes via fluidic vortex mixer 29 to the anolyte vessel 15. The light fraction (mainly water) from the hydrocyclone 28 is returned via 21 to a slurrying plant (not shown) where the feed supply is prepared. The oxidation reactions driven by Ag ions take place in the anolyte vessel 15, with corresponding reduction of Ag to Ag. A flow of anolyte from the anolyte vessel 15 to the electrochemical cell 12, where re-generation of Ag to Ag takes place, is driven by a pump 31 via hydrocyclone 32. Solids in this flow are separated out in the hydrocyclone 32 and returned via fluidic vortex mixer 29 to the anolyte vessel 15, while the solution containing Ag ions for regeneration pass via a filter (not shown) to remove any particulates remaining and heat exchanger 33 to the anolyte region 13 of the electrochemical cell 12. The filter may be ceramic
or of PVDF (polyvinylidene fluoride) for nuclear applications. Anolyte containing re-generated Ag is returned from the anolyte region 13 to the anolyte vessel 15 via fluidic vortex mixer 29. In this way, the electrochemical cell 12 is protected from exposure to quantities of solids which would tend to foul the membrane 10.
A pump 23 provides a controlled bleed of catholyte from the catholyte vessel 17 to hydrocyclone 34, which separates the bleed stream into a solids rich component passed into the anolyte vessel 15 and a solids depleted component returned to the catholyte vessel 17. By applying cooling to this bleed stream from the catholyte vessel 17, sparingly soluble organic matter in solution is encouraged to precipitate out, thus further reducing concentration of organic matter in the catholyte. The flow rate is controlled so that the volumetric return to the anolyte vessel matches the volumetric transfer of water, silver and organic molecules across the membrane 10 from anolyte to catholyte.
Apparatus and techniques for the handling of off- gases from the catholyte vessel, removal of residual contaminants from the anolyte and the catholyte and regeneration of anolyte and catholyte are described in GB 01 02843. 3 In accordance with the present invention, the anolyte vessel 15 is provided with a plurality (eg up to 10) of, high intensity ultrasonic transducers attached around its walls and focused to concentrate ultrasonic energy within the anolyte away from the vessel walls. By microcavitation effects, and efficient coupling of energy from the ultrasound, this serves to enhance directly the action of the oxidising species in the anolyte upon the
matter, solid organic waste in the present example, therein. The ultrasound also enhances the mass transfer of redox reagent to the surface-increasing reaction rate/effectiveness. This andthe effective increase in interfacial area for reaction which this provides and consequent improvement in current efficiency means that a smaller plant and reduced energy consumption are achieved for an equivalent quantity of waste treated.
Figure 2 illustrates a modified form of cylindrical anolyte vessel 15a for the batch treatment of solid matter contained in a cylindrical basket 41 removably and replaceably received within the vessel 15a. The basket 41 in this example has a solid cylindrical wall and a mesh floor and mesh lid to allow passage therethrough of anolyte.
The cylindrical vessel 15a of titanium, preferably highly polished on the inside, has a removable lid 42, inlet pipelines 43,44 and corresponding outlet pipelines 45,46 for respectively anolyte flow from an anode region of an electrochemical cell (equivalent to anode region 13 in the example of Figure 1), and flow of fluid (e. g. gas) to encourage agitation within the vessel 15a. Anolyte flow is upwards through the basket 41 to reduce the possibility of a blockage forming by collection of solid matter on the mesh floor. In other respects the process flows for a mediated electrochemical oxidation reaction, eg with Ag, would be as described for Figure 1 and in GB 01 02843.3.
Bonded with adhesive to the outside of the wall of the vessel 15a is a plurality of ultrasonic transducers 47. In this example there are 10 such transducers 47, of which 4 can be seen in the Figure 2. The transducers 47
are designed to provide high intensity, up to 3 watts/cm2, of acoustic energy density, within the vessel focused at a radiation depth, depending upon the size of the vessel, but typically between 1 cm and 70 cm, to minimise exposure of the vessel wall to ultrasound and thereby minimise consequential damage to the vessel. The transducers will typically operate at between 20 kHz and 60 kHz, and typical power dissipation within the vessel 15a is in the range 10 to 200 watts/litre. The ultrasonic power supply can be fixed or swept frequency, or pulsed.
Here again, the effect of the intense ultrasonic radiation, through cavitation at the microscopic scale, is to achieve intimate contact between the ions of the oxidising or reducing (as the case may be) species and the dispersed phase of matter which is to react therewith.
The consequent increase in the effective interfacial area for reaction permits a reduction in size of anolyte vessel 15a for an equivalent throughput. This and the enhancement of current efficiency in turn permits a corresponding reduction in the overall plant size and energy consumption.
The invention is not restricted to the details of the foregoing examples. For example, sound may be used instead of ultrasound.
For instance, Referring to Figure 1, the three streams fed to the fluidic vortex mixer 29 need not necessarily be mixed in this way, but may be fed directly to the anolyte vessel 15. This may, indeed, be preferable for the concentrated feed slurry of organic
waste matter, where this is explosive, to minimise the path length before admixture with bulk anolyte.
Referring to Figure 2, the basket 41 may have solid sides and a solid base with appropriate connections to the inlet pipes 44 and 43.
The construction materials for the plant are chosen according to the nature/corrosiveness of the materials to be contained and taking into account the need for transparency to ultrasound. For example the feed and anolyte containment where organic fuel is to be treated is desirably titanium, with stainless-steel for the catholyte. Alternatively PTFE/PVDF either as construction material or lining can be used for both anolyte and catholyte containment. Where halogen containing warfare agents are to be treated, then PTFE/PVDF either as construction material or lining is required. A wall thickness of less than 3 mm is desirable. Alternative materials for the anolyte and catholyte containment are zirconium, possibly with tantalum or niobium alloy facings, or ceramics, such as alumina.
Heating for the anolyte/catholyte vessel may be provided by electrical band heaters or steam coils around the vessel between the transducers 47. Cooling for the transducers themselves may be provided by water cooling or heat pipes.
It will be appreciated that, where the reaction required is a reduction reaction, this will take place in the catholyte vessel, and the ultrasonic transducers would be mounted on the catholyte vessel. Where a combination of reduction and oxidation reactions can be provided simultaneously, it would be appropriate to provide for ultrasonic insonation of both the anolyte and
the catholyte vessels. Suitable ion species for mediated electrochemical reduction are V/V or Cr3+/Cr2+.
For nuclear applications the plant components should be made from metal, or ceramic. The most resistant plastics are PVDF (polyvinylidene fluoride), FEP (fluorinated ethylene-propylene), or PFA (perfluoroalkoxyethylene) in particular when provided with glass or mineral fillings.
The cell membrane (10 in Figure 1) can be of microporous ceramics, possibly with PVDF or Nafion binders. A thin fine pore surface layer can minimise transmembrane flow and assist accurate pressure balancing between anolyte and catholyte regions.
Where the matter is solid waste, which has been chopped or shredded for example to reduce its size, solution access to all surfaces can be further encouraged by applying pressure swings near the boiling point of the electrolyte. Thus, reduced pressure causes boiling which expels liquid. Restored pressure condenses vapour and fresh electrolyte is drawn in. The temperature and range of pressure swing are chosen as a compromise between excessive pressure reduction and too high an operating temperature. For Ag operation needs to be below about 90°C, preferably below 70°C to avoid excessive reaction with water. Pressure equalisation on either side of the cell membrane 10 is desirable during such pressure cycling, to avoid fatigue. Alternatively the electrochemical cell could be isolated from the reaction vessel during pressure cycling.
It can also be advantageous, particularly if the matter being treated is organic fluid, to provide a
transducer mounted on the vortex mixer 29 for ultrasonic insonation of the slurry as it passes therethrough.
Where the material being treated is organic fluid, and particularly if the density differentials render hydrocyclones ineffective, an alternative is to use a coalescer comprising, for example a fibrous structure of a material, such as PVDF, which is wetted by the minor organic phase.