Electrochemical cell comprising electrically conductive diamond electrodes

Abstract

An electrochemical cell for treating a fluid, the electrochemical cell comprising: at least two opposing electrodes defining a flow path for the fluid between the electrodes, where at least one of the electrodes is formed of electrically conductive diamond material; drive circuitry configured to apply a potential across the electrodes such that a current flows between the electrodes when the fluid is flowed through the flow path between the electrodes; and a housing in which the electrodes are disposed, the housing comprising pressure seals configured to containing the fluid within the fluid path and a support structure for supporting the electrodes, wherein the support structure and the pressure seals are configured such that the electrochemical cell has an operating pressure in a range 2 to 10 bar within which the electrodes are supported without fracturing and within which the fluid is contained within the flow path, wherein the electrodes are spaced apart by a distance in a range 0.5 mm to 4 mm, and wherein the drive circuitry is configured to apply a potential across the electrodes giving a current density ≥15,000 Amp/m.sup.2 over an electrode area of at least 20 cm.sup.2 for an operating voltage of no more than 20 V.

Claims

1. An electrochemical cell for treating a fluid, the electrochemical cell comprising: at least two opposing electrodes defining a flow path for the fluid between the electrodes, where at least one of the electrodes is formed of electrically conductive diamond material; drive circuitry configured to apply a potential across the electrodes such that a current flows between the electrodes when the fluid is flowed through the flow path between the electrodes; and a housing in which the electrodes are disposed, the housing comprising pressure seals configured to containing the fluid within the flow path and a support structure for supporting the electrodes, the support structure being disposed in the flow path between the opposing electrodes, wherein the support structure and the pressure seals are configured such that the electrochemical cell has an operating pressure in a range 3 to 10 bar within which the electrodes are supported without fracturing and within which the fluid is contained within the flow path, wherein the electrodes are spaced apart by a distance in a range 0.5 mm to 3.5 mm, and wherein the drive circuitry is configured to apply a potential across the electrodes giving a current density ≥15,000 Amp/m.sup.2 over an electrode area of at least 20 cm.sup.2 for an operating voltage of no more than 20 V.

2. An electrochemical cell according to claim 1, wherein the electrodes include two end electrodes and one or more intermediate electrodes disposed between the end electrodes forming a bipolar cell configuration.

3. An electrochemical cell according to claim 1, wherein the support structure and the pressure seals are configured such that the operating pressure of the electrochemical cell is at least 4 bar.

4. An electrochemical cell according to claim 1, wherein the pressure seals are configured such that the operating pressure of the electrochemical cell is no more than 8 bar.

5. An electrochemical cell according to claim 1, wherein the pressure seals are configured such that the operating pressure of the electrochemical cell is no more than 5 bar.

6. An electrochemical cell according to claim 1, wherein the electrode spacing is no more than 3.0 mm, 2.5 mm, or 2 mm.

7. An electrochemical cell according to claim 1, wherein the electrode spacing is no less than 0.75 mm, 1 mm, 1.25 mm, or 1.5 mm.

8. An electrochemical cell according to claim 1, wherein the electrode area is at least 40 cm.sup.2, 60 cm.sup.2, 80 cm.sup.2, 100 cm.sup.2, or 120 cm.sup.2.

9. An electrochemical cell according to claim 1, wherein the electrode area is no more than 200 cm.sup.2, 150 cm.sup.2, or 130 cm.sup.2.

10. An electrochemical cell according to claim 1, wherein the drive circuitry is configured to apply a current density ≥20,000 Amp/m.sup.2, ≥25,000 Amp/m.sup.2, ≥28,000 Amp/m.sup.2, or ≥29,000 Amp/m.sup.2 over the electrode area.

11. An electrochemical cell according to claim 1, wherein the drive circuitry is configured to apply a current density no more than 40,000 Amp/m.sup.2, 35,000 Amp/m.sup.2, or 32,000 Amp/m.sup.2 over the electrode area.

12. An electrochemical cell according to claim 1, further comprising a pump system for pumping fluid through the cell with a flow velocity of at least 1 ms.sup.−1, 1.3 ms.sup.−1, 1.6 ms.sup.−1, or 2 ms.sup.−1.

13. An electrochemical cell according to claim 12, wherein the pump system is configured such that the flow velocity is no more than 10 ms.sup.−1, 5 ms.sup.−1, or 3 ms.sup.−1.

14. An electrochemical cell according to claim 12, wherein the electrochemical cell has an operating pressure in a range 3 to 8 bar within which the electrodes are supported without fracturing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a bipolar electrochemical cell with a parallel flow configuration according to WO2008/029258;

(3) FIG. 2 illustrates a bipolar electrochemical cell with a serpentine flow configuration according to WO2008/029258;

(4) FIG. 3 illustrates a bipolar electrochemical cell with a parallel flow configuration and a plurality of support structures disposed between the electrodes according to WO2012/049512;

(5) FIG. 4 illustrates an example of a support structure according to WO2012/049512;

(6) FIG. 5 illustrates an example of an electrochemical cell according to WO2012049512; and

(7) FIG. 6 illustrates a modified electrochemical cell according to an embodiment of the present invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

(8) An embodiment of the present invention is based on a modified version of the proprietor's own earlier diamond-based bipolar electrochemical cell configuration as described in WO2012/049512. The configuration described in WO2012/049512 comprises a support structure, electrode spacing (5 mm), and seals configured to provide an operating pressure of approximately 1 bar and a current density of 5,000 Amp/m.sup.2 over an electrode area of at least 20 cm.sup.2 for an operating voltage of 8 V or a current density of 10,000 Amp/m.sup.2 over an electrode area of at least 20 cm.sup.2 for an operating voltage of 12 V. In practice, it has been found that it would be advantageous to increase the current density for a given applied potential while retaining a large electrode area. While larger current densities have been achieved for diamond-based electrochemical cell configurations with smaller electrodes, it is the combination of a high current density and a large electrode area which results in a more efficient electrochemical processing cell.

(9) FIG. 5 illustrates an example of an electrochemical cell according to WO2012049512. The configuration comprises the following components: main body assembly 20; spring loaded clamp 22; manifold 24; cassette assembly 26; end electrode 28; outer clamp ring 30; adapter 32; o-rings 34, 36, 38; studding 40; washer 42; nut 44; and studs 46, 48.

(10) FIG. 6 illustrates a modified electrochemical cell according to an embodiment of the present invention. The configuration comprises the following components: o-ring 60; outer clamp ring 62; disk spring 64; trunnion 66; threaded studs 68, 70; washer 72; nut 74; main body 76; spring clamp 78; electrode assembly 80; cassette assembly 82; threaded stud 84; o-ring 86; and manifold 88.

(11) The present inventors have reduced the electrical resistivity of the diamond-based electrochemical cell in two major ways: (i) reducing the diamond electrode spacing; and (ii) redesigning the housing including the diamond electrode support structure and the pressure seals to increase the operating pressure and flow velocity. In this regard, reducing the electrode spacing reduces the distance through which current is required to pass through the fluid between the electrodes thus reducing electrical resistance. Increasing the operating pressure reduces the size of gas bubbles in the fluid thus reducing the resistivity of the fluid. Furthermore, fluid flow velocity can be increased without exceeding operating pressure limits of such a modified electrochemical cell. Increasing fluid flow velocity aids in removal of bubbles from the surface of the diamond electrodes which again leads to a reduction in the resistivity of the fluid between the diamond electrodes.

(12) At the same time, desired operating conditions must be balanced against tolerances of the components of the cell. For example, operating at too high a pressure can lead to fracture of the diamond electrodes, especially relatively thin, large area diamond electrodes. While the electrode area can be reduced to increase robustness to cracking, a reduction in electrode area reduces the oxidative capacity of the cell. As such, a balance must be struck between increased operating pressure, retention of large area electrodes, and robustness to leakage and cracking. Furthermore, reducing electrode spacing results in a reduced size of flow path for the fluid, which can cause problems of clogging of the flow path with solid particulates in the fluid. Furthermore, reducing the size of the flow path between the electrodes increases fluid flow velocity for a given flow rate. While an increase in flow velocity is advantageous for the aforementioned reason of bubble removal, if the increased flow velocity results in too much of a pressure increase then this can lead to leakage or electrode fracture. As such, the present inventors have defined upper and lower limits for operating pressure and electrode spacing and redesigned the electrode support structure and pressure seals to achieve the desired electrode spacing and operating pressure while retaining a robust configuration.

(13) In light of the above, an electrochemical cell configuration is provided which comprises: at least two opposing electrodes defining a flow path for the fluid between the electrodes, where at least one of the electrodes is formed of electrically conductive diamond material; drive circuitry configured to apply a potential across the electrodes such that a current flows between the electrodes when the fluid is flowed through the flow path between the electrodes; and a housing in which the electrodes are disposed, the housing comprising pressure seals configured to containing the fluid within the fluid path and a support structure for supporting the electrodes, wherein the support structure and the pressure seals are configured such that the electrochemical cell has an operating pressure in a range 2 to 10 bar within which the electrodes are supported without fracturing and within which the fluid is contained within the the flow path, wherein the electrodes are spaced apart by a distance in a range 0.5 mm to 4 mm, and wherein the drive circuitry is configured to apply a potential across the electrodes giving a current density ≥15,000 Amp/m.sup.2 over an electrode area of at least 20 cm.sup.2 for an operating voltage of no more than 20 V.

(14) The electrodes may include two end electrodes and one or more intermediate electrodes disposed between the end electrodes forming a bipolar cell configuration. However, it should be noted that the electrochemical cell configuration is not limited to a bipolar cell configuration and the teachings of this specification can also be applied to alternative configurations as described, for example, in WO2014/090663.

(15) The support structure and the pressure seals may be configured such that the operating pressure of the electrochemical cell is at least 3 or 4 bar and optionally no more than 8 or 5 Bar. The lower limit to the pressure configuration is defined by the desire to increase pressure and thus reduce the size of bubbles and thus the resistivity of the fluid. The upper limit to the pressure configuration is defined by the pressure at which leakage and/or electrode fracture occurs. This is dependent on the strength of the diamond electrodes, the support structure for the electrodes, and the pressure rating of the pressure seals. The pressure seals may be selected to fail at a pressure which is below that where the electrodes fracture. Furthermore, a leakage detection system may be provided to shut down the cell on detection of a leak. Such a combination of features limits cell damage if, for example, solid particulates block the flow path leading to an increase in pressure which could damage the cell.

(16) The electrode spacing may be no more than 3.5 mm, 3.0 mm, 2.5 mm, or 2 mm and optionally no less than 0.75 mm, 1 mm, 1.25 mm, or 1.5 mm. The upper limit for the electrode spacing is set by the desire to reduce the distance between the electrodes thus reducing the resistance of the cell and also providing an increased fluid flow velocity and pressure for a given volume flow rate which reduces the size and number of bubbles thus reducing the sensitivity of the fluid between the electrodes. The lower limit for the electrode spacing is set by the desire to avoid blockages caused by solid particulates and to ensure that the flow velocity and pressure is not so high that leakage or electrode fracture becomes problematic.

(17) The electrode area may be at least 40 cm.sup.2, 60 cm.sup.2, 80 cm.sup.2, 100 cm.sup.2, or 120 cm.sup.2 and optionally no more than 200 cm.sup.2, 150 cm.sup.2, or 130 cm.sup.2. The electrode area is the surface area of one side of an electrode exposed to the fluid. The lower limit is set by the desire to have a large electrode area which increases oxidative capacity. The upper limit is set by the cost of the electrodes and the difficulty in supporting larger area diamond electrodes while operating at high pressures without the electrodes fracturing.

(18) The drive circuitry can be configured to apply a current density ≥20,000 Amp/m.sup.2, ≥25,000 Amp/m.sup.2, ≥28,000 Amp/m.sup.2, or ≥29,000 Amp/m.sup.2 over the electrode area and optionally a current density no more than 40,000 Amp/m.sup.2, 35,000 Amp/m.sup.2, or 32,000 Amp/m.sup.2 over the electrode area. These current densities are achieved for an operating voltage of no more than 20 V. It is desired to provide an extremely high current density in order to provide a high oxidative capacity. It is further desired that this should be provided at a relatively low voltage as available power may be limited at point of use and higher powers will result in large operating costs. The upper limit will be set by the desire to keep power requirements and cost down.

(19) The electrochemical cell may be provided with a pump system for pumping fluid through the cell with a flow velocity of at least 1 ms.sup.−1, 1.3 ms.sup.−1, 1.6 ms.sup.−1, or 2 ms.sup.−1 and optionally no more than 10 ms.sup.−1, 5 ms.sup.−1, or 3 ms.sup.−1. As previously described, increased flow velocity can remove bubbles from the electrode surfaces and increase pressure thus reducing bubble size. Both of these effects result in a reduced resistivity and thus an increased current density for a given applied voltage. However, if the flow velocity is too high then the pressure may increase to such an extent that leakage of diamond electrode fracture is problematic.

(20) As compared to the previous cell configuration as illustrated in FIG. 5, the modified cell configuration as illustrated in FIG. 6 now has a linear configuration with a flanged inlet and outlet. The electrode clamping force has been increased and the material of the O-ring seals changed taking into account chemical resistance, loading, and thermal effects. Finite Element Analysis (FEA) has been utilized to optimize seal loading and materials selection based on loading/thermal effects. In this regard, it should be noted that electrochemical cells designed to operate at higher current densities and water pressures increase the demand on the components used to seal and package the diamond electrodes. Polymer components exposed to the electrolyte are advantageously fluoropolymers or related materials to resist the increased chemical corrosion found in the local vicinity of electrolysis. Increasing the operating pressure requires the compressive force used for the sealing to be increased and the use of coned-disk washers on all fasteners to apply a pre-loading force is advantageous. The support interface between the diamond electrode and the backing clamp must be sufficiently stiff so as to prevent tensile stresses from fracturing the diamond electrodes while a compressive clamping force is applied to the sealing surface.

(21) The modified bipolar cell configuration comprises 21 boron doped diamond electrodes having a 1.6 mm spacing. Each electrode has a diameter of 130 mm and a thickness of 0.6 mm. The cell is configured to operate at a pressure of around 3 bar, a flow rate of around 25,000 litres/hour, a flow velocity >1.0 m sec.sup.−1, and a current density of 28,000 Am.sup.−2 or more. Power requirements are 120 to 250 kW and the Maximum Oxidation Capacity is approximately 2 kg h.sup.−1 compared with a previous value of 0.5 kg h.sup.−1.

(22) While the embodiment described above is in a bipolar configuration, it is envisaged that the present invention can be applied to other electrochemical cell configurations. Furthermore, it may be noted that in operation the anode electrodes function to generate oxidizing species. As such, in one configuration the plurality of electrodes comprise a plurality of opposing pairs of electrodes, each opposing pair of electrodes comprising one electrode which is formed of a solid sheet of electrically conductive diamond material and another electrode which is formed of non-diamond material such as a metal or metal composite material. However, in preferred configurations the plurality of electrodes comprise a plurality of opposing pairs of electrodes where both electrodes in each opposing pair of electrodes are formed of a solid sheet of electrically conductive diamond material.

(23) In addition to water treatment applications as described herein, embodiments of the present invention can also be used to generate bleach (OCl.sup.−) via the electrolysis of brine solutions.

(24) While this invention has been particularly shown and described with reference to embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention which is defined by the appending claims.