ELECTROLYTE REGENERATION

Abstract

A system for regenerating electrolyte from an electrochemical cell comprises: a directing means for receiving electrolyte from an electrochemical cell, the directing means having a first outlet and a second outlet; a sensor or sample port positioned between the electrochemical cell and the first and second outlets via which the presence of a contaminant in the electrolyte may be measured; and a purification stage for receiving electrolyte from the second outlet and producing regenerated electrolyte, the purification stage comprising an ion exchange chamber. When a contaminant is detected, the directing means directs at least a portion of the electrolyte through the second outlet to the purification stage. The system is particularly suitable for use with an AEM electrolyser.

Claims

1. A system for regenerating an electrolyte from an electrochemical cell, said system comprising: a directing means for receiving the electrolyte from an electrochemical cell, the directing means having a first outlet and a second outlet; a sensor or a sample port positioned between the electrochemical cell and the first outlet and the second outlet via which the presence of a contaminant in the electrolyte may be measured; and a purification stage for receiving electrolyte from the second outlet and producing regenerated electrolyte, the purification stage comprising an ion exchange chamber; wherein, when a contaminant is detected, the directing means directs at least a portion of the electrolyte through the second outlet to the purification stage.

2. The system according to claim 1, wherein the system comprises the electrochemical cell from which the electrolyte is regenerated.

3. The system according to claim 1, wherein the electrochemical cell comprises an electrolyser or a stack of electrolysers, wherein, when the stack of electrolysers is used, the system comprises means for combining the electrolyte streams from each of the electrolysers into a single stream upstream of the first outlet and the second outlet.

4. The system according to claim 3, wherein the electrolyser or each electrolyser of the stack of electrolysers is an anion exchange membrane (AEM) electrolyser and more preferably an APM electrolyser operating with a substantially dry cathode.

5. The system according to claim 1, wherein the sensor comprises a pH sensor, a conductivity sensor, or both.

6. The system according to claim 1, wherein the sensor is immersed in the electrolyte, or the electrolyte is intermittently sampled via the sample port.

7. The system according to claim 1, wherein the sensor or the sample port is positioned on the directing means.

8. The system according to claim 1, wherein the ion exchange chamber comprises a cation exchange resin or an anion exchange resin, wherein: when the cation exchange resin is used, the purification stage comprises means for maintaining the temperature in the ion exchange chamber at less than 150 C.; and when the anion exchange resin is used, the purification stage comprises means for maintaining the temperature in the ion exchange chamber at less than 80 C.; and the ion exchange chamber optionally comprises a temperature sensor.

9. The system according to claim 1, wherein the ion exchange chamber comprises two or more sub-chambers, wherein, optionally, the two or more sub-chambers are arranged in parallel.

10. The system according to claim 1, wherein the ion exchange chamber is adapted to process each of: the electrolyte, and at least one of water and a liquid for regenerating the ion exchange chamber, wherein the ion exchange chamber preferably comprises: a separate inlet and outlet for each of: the electrolyte, and the at least one of water and a liquid for regenerating the ion exchange chamber; or a single inlet and outlet for each of: the electrolyte, and the at least one of water and a liquid for regenerating the ion exchange chamber; and wherein, optionally, the ion exchange chamber comprises means that allows the flow of only one of the water or the liquid through the ion exchange chamber at any given time.

11. The system according to claim 10, wherein the system comprises at least one of a water softening stage and a reverse osmosis stage for further treating the water that passes through the ion exchange chamber.

12. The system according to claim 1, wherein the purification stage comprises a first ion exchange chamber and a second ion exchange chamber, the first ion exchange chamber comprising a cation exchange resin and the second ion exchange chamber comprising an anion exchange resin.

13. The system according to claim 12, wherein the first ion exchange chamber and the second ion exchange chamber are arranged in series such that the electrolyte passes through the first ion exchange chamber and then the second ion exchange chamber, optionally wherein a bypass mechanism allows the electrolyte to pass through only one of the first ion exchange chamber and the second ion exchange chamber.

14. The system according to claim 1, wherein the purification stage comprises a filter downstream of any ion exchange chamber.

15. The system according to claim 1, wherein at least one of the electrolyte from the at least one first outlet and the regenerated electrolyte is recycled to an electrochemical cell, and, optionally, electrochemical cell is the electrochemical cell from which the electrolyte originated.

16. The system according to claim 1, wherein the electrolyte is an aqueous solution comprising an ionic compound.

17. A method for regenerating an electrolyte from an electrochemical cell, said method comprising: passing the electrolyte from an electrochemical cell to a directing means, the directing means having a first outlet and a second outlet; measuring the presence of a contaminant in the electrolyte, via a sensor or sample port, between the electrochemical cell and the first outlet and the second outlet; when a contaminant is detected, directing at least a portion of the electrolyte through the second outlet of the directing means to a purification stage; and purifying the electrolyte from the second outlet in the purification stage to produce a regenerated electrolyte, the purification stage comprising an ion exchange chamber.

18. The system according to claim 16, wherein the ionic compound is a hydroxide salt.

19. The system according to claim 18, wherein the hydroxide salt is potassium hydroxide.

Description

[0099] To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

[0100] FIG. 1 shows graphs of pH against time and conductivity against time respectively;

[0101] FIGS. 2A and 2B show a schematic of a system in accordance with the present invention, without and with water treatment respectively;

[0102] FIGS. 3A and 3B show a schematic of an alternative system in accordance with the present invention, without and with water treatment respectively; and

[0103] FIGS. 4A and 4B show a purification stage with multiple sub-chambers, without and with water deionisation functionality respectively.

[0104] As has been shown in FIG. 1, direct measurement of the electrolyte pH is not as useful a method for monitoring electrolyte degradation and acidic contaminants because a significant drop in pH is only observed once the electrolyte degradation is approaching 100%. On the conductivity scale instead, the change in slope from 0% to 100% electrolyte degradation is very clear, and it is a very helpful parameter to keep track of the electrolyte status. Moreover, the conductivity is strongly dependent on the total alkalinity of the electrolyte solution which means that a further matching of total alkalinity and conductivity could be obtained if the requirements on any deionised input water are respected.

[0105] Referring to FIG. 2A, a schematic of a system in accordance with the present invention without water treatment functionality, there can be seen an electrolyser 1 with an outlet for electrolyte to go to tank 2. A conductivity sensor 3 in communication with a computing means (not shown) directs the flow of the electrolyte at the junction 17. If the conductivity has dropped below a predetermined threshold, the electrolyte is directed towards the purification system. If adequate conductivity is present, then the electrolyte flows from the tank 2 back to the electrolyser 1.

[0106] When the sensor 3 directs the electrolyte to the purification stage, it enters a first ion exchange chamber 5, which in this embodiment is a cation exchange chamber. The electrolyte flows through the first ion exchange chamber 5 to a second ion exchange chamber 6, which in this embodiment is an anion exchange chamber. The regenerated electrolyte is reintroduced to the electrolyser loop via the second ion exchange chamber's outlet stream 16.

[0107] The valves to control the flow of liquids, and computing means are not shown.

[0108] FIG. 2B differs from FIG. 2A in that it has the addition of water in its own treatment loop to create deionised water. Water is introduced via inlet 7 to softener 8 and onwards for further processing by reverse osmosis unit 9. The water then passes through a first ion exchange unit 10 and a second ion exchange unit 11. The treated water can be added to the electrolyser loop by pipe 15 and is not limited to where in the loop it is added.

[0109] FIG. 3A depicts an electrolyser loop similar to that depicted in FIGS. 2A and 2B. The difference can be seen in the purification stage 4, wherein the first ion exchange chamber 5 is provided with an inlet for the communication of a regenerating liquid 12, in this instance KCl for the cation exchange resin. Once regeneration has occurred, the regenerating liquid, the composition of which now altered, exits to waste via outlet 13a. The second ion exchange chamber 6 is provided a similar secondary inlet for the introduction of KOH 14 for the regeneration of the anion exchange resins, and disposed of via outlet 13b.

[0110] FIG. 3B differs from FIG. 3A in that the dual functionality of regenerating electrolyte and deionising water can be done by shared equipment. In this figure the earlier optional steps of softening and reverse osmosis are not shown. The water has its own inlet 7 to the cation exchange chamber 5, with a dedicated outlet for the communication of water to the second chamber 6 filled with an anion exchange resin.

[0111] Not shown are the valves and other control means adapted to ensure only water, or the electrolyte is in the ion exchange chamber 5, 6 at any given time. To allow for the dual treatment of water and electrolyte, each exchange chamber may be a stage, comprising a plurality of sub-chambers as depicted in FIG. 4A and 4B.

[0112] Not shown, but an optional feature, is a separate inlet and outlet to the cation exchange resin for the introduction and removal of an acid, notably H.sup.+ ions, such that the cation exchange resin can be switched between K.sup.+ and H.sup.+ for the regeneration of electrolyte and the deionisation of water. The newly conditioned resin may also be flushed once the resin has been conditioned before the regenerated electrolyte, or deionised water is used.

[0113] FIG. 4A is a closer look at the purification stage 4, including the regeneration lines 11, 12, 13a, 13b. The electrolyte is directed via inlet 20a,b,c to one of the ion exchange chambers 5a,b,c respectively. Computing means (not shown) are provided to control valves (not shown) to direct electrolyte to one ion exchange chamber 5a,b,c at a time. Regeneration liquid may be introduced to each exchange chamber by inlet 11a,b,c and removed via outlet 13a. The control system ensures regeneration is not conducted whilst electrolyte is flowing to avoid polluting the electrolyte.

[0114] The flow of electrolyte from the first stage to the second is similar to previous embodiments. A cross pipe means the outlet of exchange unit 5a does not necessarily have to go to anion exchange unit 6a.

[0115] Whilst FIG. 4B does not show the piping for the regeneration fluid, it may be present as depicted in earlier figures or in accordance with the description. This embodiment includes the water piping including inlet line 7. Much like the electrolyte shown in FIG. 4A the water can be deionised in a similar manner.

[0116] In this embodiment of the present invention the flow of water may be used to flush the regeneration liquid from each ion exchange chamber between use, with the flush water being disposed of through waste outlets 13a and b.

[0117] FIGS. 4A and 4B show the preferred embodiment with at least three ion exchange sub-chambers per stage as this would allow for deionisation of water, regeneration of electrolyte and regeneration of ion exchange resins without the need for any down time.

[0118] The present invention will now be described by reference to the following, non-limiting, examples:

Example 1

[0119] A 1 M KOH solution is used in an AEM electrolyser. This electrolyte, like others can suffer due to carbonation when exposed to air, i.e. the solution is gradually converted to potassium carbonate and then to potassium hydrogencarbonate.

[0120] A mixed bed containing both strong cationic and strong anionic exchange resins is flushed with a 30% KOH solution, preferably having a volume of at least 2 or 3 times the resin volume. The excess KOH is washed by rinsing with deionised water. The carbonated electrolyte is regenerated by flowing it into the charged mixed bed. This results in an isotonic regeneration obtaining a KOH solution having exactly the same osmotic pressure (in this case 1 M).

Example 2

[0121] In order to avoid handling caustic solutions, a 0.2 M sodium sulfate solution in water is used to fill an electrolyte tank. To obtain a potassium hydroxide solution in situ for use in electrolysis, a hydraulic circuit was connected to a mixed-bed resin that had been prepared similarly to that in the previous example. The sodium sulfate solution was passed through the mixed bed to obtain a 0.4 M KOH solution (i.e. an isotonic regeneration of the electrolyte). When the ion exchange is complete (e.g. as monitored by pH and/or conductivity), the electrolyte is no longer passed through the ion exchange resin and is instead diverted back to the electrolyser. When monitoring systems detect a consistent depletion of the electrolyte composition (e.g. pH decreases below 11.5 or conductivity falls below 60% of the starting value), the electrolyte is diverted back through the ion exchange resin. After regeneration of the electrolyte, the ion exchange resins are restored by flushing again with concentrated KOH solution.

[0122] The invention is not intended to be restricted to the details of the embodiments described above. For instance, any suitable ion exchange resin or equivalent may be used.

[0123] Furthermore, and as mentioned above, the present invention is not restricted to use with an electrochemical cell that is an electrolyser, but can also be used with a battery such as but not necessarily limited to a zinc-air, silver oxide or lead-acid battery.