GB2225340A

GB2225340A - Circulation of electrolyte in an electrochemical cell, using Taylor vortices

Info

Publication number: GB2225340A
Application number: GB8827289A
Authority: GB
Inventors: John Desmond Thornton
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-11-22
Filing date: 1988-11-22
Publication date: 1990-05-30
Also published as: GB8827289D0

Abstract

In an electrochemical cell for decomposition of organic waste liquids having an anode compartment 16 and a cathode compartment 18 separated by a porous pot 20, the anode 26 is driven by a shaft 28 having an axial passage 32 extending from an upper inlet 34 in the vicinity of the liquid level to a lower outlet adjacent a turbine 36. The rotating anode 26 produces Taylor vortices in annular space 29 and liquid is drawn from layer 50 through passage 32 and emerges to contact the anode 26. In one use, organic solvent such as tributyl phosphate/odourless kerosene is destroyed. Fresh solvent is added through an inlet. A helical cooler 44 may also be provided. <IMAGE>

Description

An electrochemical cell This invention relates to an electrochemical cell.

The invention has particular, but not exclusive, application to an electrochemical cell for use in electrochemical decomposition of organic waste liquids such as spent tributyl phosphate/odourless kerosene as used in the nuclear industry in the course of reprocessing nuclear fuel materials.

According to the invention there is provided an electrochemical cell in which the contents of the cell are circulated by a generally vertically disposed, rotatably driven conduit having an inlet in the vicinity of the liquid level within the cell and an outlet disposed below the inlet, the conduit being so arranged that, in response to rotation of the conduit, liquid is induced to flow through the conduit in a direction from the inlet to the outlet.

Preferably the conduit carries, or forms part of, one of the electrodes of the cell. The conduit for example may mount a cylindrical electrode which is rotatably fast with the conduit.

To induce the flow of liquid the rotatable conduit is conveniently provided with means, eg a turbine, which as a result of rotation of the conduit induces liquid flow in a direction from the conduit inlet to its outlet.

To promote further understanding of the invention one embodiment will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a vertical cross-sectional view through an electrochemical cell in accordance with the invention; and Figure 2 is an enlarged fragmentary view of the turbine mixer.

Referring to the drawings,the cell shown is primarily intended for the electrochemical destruction of organic solvent such as spent tributyl phosphate/ odourless kerosene (TBP/OK) derived from a nuclear reprocessing plant - however the cell may have applications in other forms electrochemical processes.

The cell comprises an outer vessel 10 provided with a lid 12 which is sealingly clamped, via seal 14, by means of bolts or other fasteners (not shown). The interior of the vessel 10 is divided into an anode compartment 16 and a cathode compartment 18 by a porous pot 20 having a flange 24 by means of which it is sealingly clamped, via seals 22, between the lid 12 and an internal shoulder on the vessel 10. It will be seen that the sealing arrangement between the lid 12, the vessel 10 and the pot 20 provides a double seal between the environment and the anode and cathode compartments 16, 18. In addition, the edge of the pot flange 24, which will become wet by capillary action, is insulated from the environment.

The anode compartment incorporates an anode 26 in the form of a drum (which may be of polished platinised titanium) carried by a generally vertical shaft 28 which is journalled in the lid 12 via a gas tight mechanical seal 30. The shaft 28 at its upper end is coupled to a drive motor (not shown) for rotating the shaft and drum.

The shaft 28 is provided with an axial passage 32 extending from an inlet 34 to the lower end of the shaft and the shaft, at its lower end, terminates in a turbine mixer 36 which is shown in greater detail in Figure 2.

The mixer 36 comprises upper and lower discs 38 and a number of blades 40 arranged, in the illustrated emmbodiment, in a cruciform configuration but with their radially inner extremities spaced from one another. If desired, a lesser or greater number of blades may be employed.

The vessel 10 is enclosed in a cooling jacket 42 and the anode compartment is provided with a double helical cooling coil 44 to keep the contents of the cells within an optimum temperature range. If desired the cooling coil 44 can pass through electrically insulated couplings and be connected electrically to the anode. The coolant supply can be fed from a header tank. The cathode compartment 18 incorporates a cathode 48, of for example polished platinised titanium, which may be part-annular and is connected to the electrical supply via a lead passing through an insulating sleeve or bush 48a provided in the vessel 10, or alternatively, in the lid 12. The anode 26 is connected to the electrical supply via the shaft 28 which is electrically insulated from the lid 12 by the seals 30.

The anode compartment 16 is filled with an anolyte which may comprise nitric acid and silver nitrate (which provides silver for use as an electrochemically regenerable oxidising species, ie divalent silver ions) and the solvent (TBP/OK) is added together with any water necessary to replenish the anolyte, via suitable inlets, to maintain a liquid level within the anode compartment above the inlet 34 of the shaft 28 such that the inlet 34 is, at all times, immersed in the layer 50 of supernatant organic solvent which tends to form above the aqueous anolyte. The inlet 34 is in the vicinity of the liquid level in the cell.

As the shaft 28 rotates, organic solvent is drawn from the layer 50 through the inlet 34 and passes along the shaft 32 to the turbine mixer 36 where it is discharged in the form of small drops by the turbine blades 40. The droplets then pass upwardly through the anode compartment 16 to the top of the anolyte and coalesce again with the organic layer 50. The rotating anode 26 sets up Taylor effect vortices in the annular space 29 between the anode 26 and the porous pot 20.

This produces a controlled flow pattern. Taylor vortices are discussed in Transactions of Institute of Chemical Engineers 1953 Volume 31 p.289 and Philosophical Transactions 1923, 223A, 289. Thus, the continuous anolyte containing silver ions is repeatedly brought into contact with the anode surface because of the vortices and this ensures rapid oxidation of monovalent silver ions to the divalent form thereby ensuring a high concentration of the latter ion which is essential for the continued oxidation of the solvent. During operation a substantially continuous dispersion is present in the anode compartment.

The cathode compartment 18 is filled, via inlet 52 with catholyte comprising nitric acid. Off-gases generated in the course of reaction, eg CO2, Ru04, NO and N02 are removed via suitable nozzles (not shown) for subsequent treatment. The cathode compartment is provided with a drain outlet 54 (normally closed) and the anode compartment is provided with a suction tube 56 for drainage purposes. In operation, the gas space above the anolyte may be maintained below atmospheric pressure so that any imperfections in the sealing arrangements merely result in a small ingress of air into the anode compartment and which may be subsequently vented from an absorber.

In the above process, two quite separate sets of reactions occur; one set at the anode and the other at the cathode. For this reason, it is necessary to keep the anolyte and the catholyte physically separate but yet, at the same time, to maintain an electrically conducting path between the two solutions. This is accomplished by separating the two liquors by a porous diaphragm through which ions can pass but gross mixing of anolyte and catholyte is avoided. The process is catalytic in the sense that Ag+ ions have first to be converted to Ag2+ ions at the anode before organic matter can be oxidised. The ionic conversion is achieved by rapid and repeated contact of the solution with the surface of the anode where this change takes place. The fluid is brought into repeated contact with the anode through the agency of Taylor vortices.

Thus whilst the oxidising agent, Ag2+, is produced only at the surface of the anode, oxidation of organic matter takes place in the bulk solution after the Ag2+ ions have migrated away from the anode. This oxidative process is accompanied by a parallel reduction of these ions to Ag+ after which no further reaction can take place until they have returned to the anode surface and been themselves reoxidised to Ag2+. A positive direction of flow is maintained as well as frequent fluid contact with the anode.

The arrangement provides a smooth cylindrical rotating anode and makes use of Taylor vortices to bring about rapid and repeated contact of the anolyte with the anode surface. Contact will be established between an element of fluid and the surface of the anode once for each rotation of the fluid vortex. This time is also governed inter alia by the diameter of a vortex and hence by the annular gap between the anode and the surrounding porous pot. Thus by making the annular gap smaller, the vortex diameter will become smaller and the frequency of fluid contact with the anode that much greater. Absence of fixed vanes makes resistance to fluid motion small.

Claims

1. An electrochemical cell in which the contents of the cell are circulated by a generally vertically disposed, rotatably driven conduit having an inlet in the vicinity of the liquid level within the cell and an outlet disposed below the inlet, the conduit being so arranged that, in response to rotation of the conduit, liquid is induced to flow through the conduit in a direction from the inlet to the outlet.

2. An electrochemical cell as claimed in claim 1, in which the conduit carries one of the electrodes of the cell.

3. An electrochemical cell as claimed in claim 1, in which the conduit forms part of one of the electrodes of the cell.

4. An electrochemical cell as claimed in claim 2, in which the conduit mounts a cylindrical electrode which is rotatably fast with the conduit.

5. An electrochemical cell as claimed in any preceding claim, in which the conduit is provided with means which as a result of rotation of the conduit induces liquid flow in a direction from the conduit inlet to its outlet.

6. An electrochemical cell as claimed in claim 6, in which the means comprises a turbine.

7. An electrochemical cell as claimed in claim 6, in which the turbine is at the lower end of the conduit.

8. An electrochemical cell as claimed in any preceding claim, in which the conduit is associated with one of the electrodes and is separately a porous element from the other electrode.

9. An electrochemical cell as claimed in any preceding claim, in which rotation of the conduit produces vortices in the electrolyte.

10. An electrochemical cell substantially as hereinbefore described with reference to and as shown in the accompanying drawings.