Method and apparatus for the on-site generation of a gas

Abstract

This invention relates to an apparatus and a method for the on-site generation of gas of relatively small quantities of a halogen gas, preferably chlorine gas. The apparatus has at least one electrolytic cell having an anolyte section and a catholyte section. At least one of these sections is connected, by fluid conduits, to a fluid heater which heats an electrolyte solution prior to its ingress into the section. Heating also facilitates circulation of the electrolyte solution through the apparatus by means of a thermosyphon effect. The heating element is, in turn, connected to an electrolyte replenishment means. The apparatus includes at least one gas separator which separates gas produced in the electrolytic cell from electrolyte solution. The apparatus lacks a reservoir for the storage of electrolyte solution.

Description

FIELD OF THE INVENTION

[0001] This invention relates to a method and an apparatus for the on-site generation of a gas, particularly, but not exclusively, chlorine gas.

BACKGROUND TO THE INVENTION

[0002] There are numerous advantages of having a facility for the on-site generation of chlorine gas. Chlorine gas is considered to be a hazardous substance and strict controls govern its storage and transport. In addition and because of its hazardous status, it is expensive to transport pressurised vessels containing liquid chlorine. This increases the costs of production facilities using the gas.

[0003] There is also a market, for on-site generators of relatively small volumes of chlorine gas, in facilities that use small quantities of chlorine gas. These facilities include water purification and sewage treatment plants and cooling towers where water used in these towers is chlorinated. To avoid having to store large quantities of liquid chlorine or chlorine in a solid granulated or pelletized form, these facilities could make use of an on-site and on demand apparatus for generating chlorine. Chlorine gas and sodium hypochlorite are also used as a disinfectant.

[0004] In addition, relatively small chlorine gas generators can be used in rural communities to purity and render potable water drawn from small dams and rivers.

[0005] In addition to chlorine generation apparatuses there is also a market for apparatuses which generate other gasses on an on-demand and on-site basis. Such gasses would include the halogen bromine which is used as an agricultural soil sterilizing agent and which is particularly effective in combating nematode infestations of the soil.

[0006] Apparatuses which generate chlorine gas by means of electrolysis are well known. These apparatuses generate chlorine gas from the anode of an electrolytic cell through which a solution of sodium chloride is passed. At the cathode hydrogen gas and sodium hydroxide are produced.

[0007] Many of the above-mentioned apparata are suitable for and have been used for the on-site generation of chlorine gas. One example is disclosed in U.S. Pat. No. 4,308,123. In this example an electrolytic cell having an anode and a cathode separated from one another by a chemically resistant ion exchange membrane permeable only to positively charged ions is used. The anode chamber is charged With an acidic sodium chloride solution while the cathode chamber is charged with a basic aqueous solution. When an electric current is passed through the chamber chlorine gas is produced at the anode and hydrogen and sodium hydroxide are produced at the cathode. Chlorine and sodium hydroxide generated may be combined to form sodium hypochlorite.

[0008] The above-described apparatus has a disadvantage in that anolyte and catholyte feed tanks or reservoirs as well as anolyte and catholyte surge tanks are necessary. These tanks represent a potential hazard particularly in a semi-industrial environment where strict safety controls may not be diligently enforced.

[0009] Furthermore, many known gas generators require pumps to circulate the electrolyte solutions. There pumps require a source of energy and, often sophisticated, control systems. In addition they also need regular maintenance which, in remote areas, is a disadvantage particularly given the potentially hazardous nature of the gases produced.

OBJECT OF THE INVENTION

[0010] It is an object of the present invention to provide a method and an apparatus for the on-site generation of gasses, particularly chlorine gas, which at least partly alleviates the above-mentioned disadvantages.

SUMMARY OF THE INVENTION

[0011] In accordance with this invention there is provided a method for the on-site generation of a gas said method comprising the steps of:

[0012] a) forming a dissociatable electrolyte solution which, in use, dissociates into positively charged and negatively charged ions at least one of which is an ion of a gaseous element;

[0013] b) heating the electrolyte solution upstream of at least one electrolytic cell and thereby causing it to circulate and re-circulate through conduits and through the or each electrolytic cell by means of a thermosyphon effect;

[0014] c) liberating, from the solution, at one electrode of the or each electrolytic cell, a gas;

[0015] d) passing the liberated gas and electrolyte solution through at least one gas separator to separate the gas from the electrolyte solution prior to re-circulating the electrolyte solution through the electrolysis cell; and,

[0016] e) at no time storing the electrolyte in a reservoir.

[0017] There is further provided for facilitating circulation of the electrolyte solution by entraining gas bubbles produced in the or each electrolysis cell and orientating conduits leading from the or each electrolysis cell to the or each gas separator substantially vertically thereby providing a gas lift effect.

[0018] The invention also provides for the electrolyte in the solution to be strengthened, if necessary, and for any make up water to be saturated by passing it through an electrolyte salt dissolving tube which is preferably mounted substantially horizontally, and for electrolyte salt in the tube to be replaced with fresh salt, preferably from a hopper.

[0019] The invention provides further for the electrolyte solution to be a metal halide, preferably sodium chloride, alternatively potassium chloride, for the gas generated at the anolyte side of the electrolysis cell to be a halogen, preferably chlorine, and for hydrogen gas and sodium, alternatively potassium hydroxide to be generated at the catholyte side of the electrolysis cell.

[0020] There is further provided for the anolyte and catholyte sections of the or each electrolytic cell to be separated from one another by an ion selective membrane, preferably a perfluoropolymer membrane, which allows the passage of sodium, alternatively potassium, ions therethrough but which is impermeable to a halogen, preferably chlorine, hydrogen gas and hydroxyl ions.

[0021] There is further provided for the addition of water, preferably distilled, alternatively demineralized, water to the sodium, alternatively potassium hydroxide solution at the catholyte side of the electrolyte cell to maintain the pre-determined concentration of sodium, alternatively potassium, in the catholyte solution.

[0022] There is also provided for the method to include the production of sodium hypochlorite, alternatively potassium hypochlorite by mixing chlorine and sodium hydroxide, alternatively chlorine and potassium hydroxide, produced by the method of the invention.

[0023] The invention also extends to an apparatus for the on-site generation of a gas comprising at least one electrolytic cell having an anolyte section and a catholyte section, at least one section being connected, by fluid conduits, to a fluid heater which, in use, heats an electrolyte solution prior to its ingress into said section and facilitates circulation of the electrolyte solution through the apparatus by means of a thermosyphon effect, the electrolytic solution being dissociatable into positively charged and negatively charged ions at least one of which is an ion of a gaseous element, the heating element in turn being connectable by fluid conduits to an electrolyte replenishment means, at least one gas separator which, in use, separates gas produced in the electrolytic cell from electrolyte solution, the apparatus lacking a reservoir for the storage of electrolyte solution.

[0024] There is further provided for the or each gas separator to be positioned operatively above the or each electrolysis cell and for conduits linking them to be orientated operatively and substantially vertically thereby facilitating circulation of the electrolytic solution by means of a gas lift effect.

[0025] There is also provided for the replenishment means to be a substantially horizontally orientated electrolyte salt dissolving tube through which electrolyte solution from the or each gas separator flows prior to flowing through the heating element, for the salt dissolving tube to be connected to an electrolyte salt replenishment hopper which contains a desired salt, and for the salt dissolving tube to be connected to a salt separator, preferably a strainer, which is connected to the heating element and which, in use, removes particulate salt from the electrolyte prior to its introduction into the heating element.

[0026] The invention provides further for the electrolyte to be a metal halide solution, preferably sodium chloride, alternatively potassium chloride, for the gas generated at the anolyte side of the electrolysis cell to be a halogen, preferably chlorine, and for hydrogen gas and sodium, alternatively potassium hydroxide to be generated at the catholyte side of the electrolysis cell.

[0027] There is further provided for the anolyte and catholyte sections of the or each electrolytic cell to be separated from one another by an ion selective membrane, preferably a perfluoropolymer membrane, which allows the passage of sodium, alternatively potassium, ions therethrough but which is impermeable to chlorine and hydrogen gas.

[0028] There is further provided for the addition of water, preferably distilled, alternatively demineralized, water to the sodium, alternatively, potassium hydroxide solution at the catholyte side of the electrolyte cell to maintain the concentration of sodium, alternatively, potassium hydroxide in the catholyte solution.

[0029] There is also provided for the apparatus to produce sodium hypochlorite, alternatively potassium hypochlorite, by mixing chlorine and sodium hydroxide, alternatively potassium hydroxide, produced by the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] One embodiment of the invention will be described below by way of example only and with reference to the accompanying drawings in which:

[0031] FIG. 1 is a schematic diagram of one embodiment of a method for the on-site generation of chlorine gas according to the invention;

[0032] FIGS. 2A to C are, respectively, a schematic first side view, a schematic second side view and a schematic plan view of an apparatus for the on-site generation of chlorine gas according to the method of FIG. 1; and

[0033] FIGS. 3A to C are, respectively, a front elevation, a plan view and a sectional part side view of an array of electrolysis cells used in the apparatus of FIG. 2.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0034] Referring to FIG. 1, a method for the on-site generation of chlorine gas comprises the steps of:

[0035] a) forming a first dissociatable, sodium chloride or brine, electrolyte solution (1) which is conveyed, through a conduit (2), to an anolyte section (3) of an electrolytic cell (7), and a second dissociatable, basic aqueous, solution (4) which is conveyed, through a conduit (5) to a catholyte section (6) of an electrolytic cell (7) which is divided into its sections by a perfluoropolymer membrane (8) which, while allowing the passage of sodium ions therethrough, is impermeable to chlorine, hydrogen and hydroxyl ions;

[0036] b) heating the first dissociatable electrolyte solution (1) prior to conveying it to the anolyte section (3) of the electrolytic cell (7) thereby causing it to circulate and re-circulate through the conduits and through the electrolytic cell (7) by means of a thermosyphon effect;

[0037] c) liberating, from the electrolyte solutions chlorine gas, in the anolyte section (3) and hydrogen gas, in the catholyte section (6);

[0038] d) entraining chlorine and hydrogen gas bubbles in the electrolyte solutions thereby facilitating circulation of the electrolyte solutions by means of gas lift, and collecting chlorine gas (9) and hydrogen gas (10);

[0039] e) the method being characterised in that at no time is the electrolyte solution stored in a reservoir.

[0040] In the above-described embodiment, the electrolysis cell (7) has an anode in its anolyte section (3) and a cathode in its catholyte section (6). The anode and cathode are connected to the positive and negative poles respectively of a direct current supply (11) which, in this embodiment, is a direct current power convertor. The direct current power convertor (11) receives alternating current (12) from a suitable alternating current source.

[0041] As the salt in the first dissociatable electrolyte solution (1) becomes depleted it is refreshed by adding salt from a salt supply hopper (14) to the substantially horizontally orientated electrolyte salt dissolving tube through which the first chlorinated electrolyte solution circulates before being strained, heated and recirculated to the electrolysis cell (7).

[0042] As the solution on the anode side becomes volumetrically depleted, it is compensated by the addition of pure fresh brine (200) into the system. This pure fresh brine is made up in item 201 where mains water is passed through a separate salt dissolving tube similar to that described in (1) above including a separate salt separator/strainer but the saturated solution so formed is then passed through a column containing a proprietary type resin which removes heavy metal anion impurities prior to transferring the purified solution via a conduit (200) to the anolyte system (1). The second dissociatable electrolyte solution (4) is refreshed by the addition of water from the water supply (13) after it has passed though a demineralizing unit (15).

[0043] In addition to chlorine and hydrogen gas, the method includes producing sodium hypochlorite in a reactor ((16). Sodium hypochlorite is formed by combining chlorine and sodium hydroxide produced in the anolyte and catholyte sections (3 & 6) of the electrolysis cell (7) respectively. Once produced the sodium hypochlorite is stored in a storage facility (17). Sodium hydroxide produced in the catholyte section (6) of the electrolysis cell (7) can also be drawn off and stored in a storage facility (18).

[0044] Referring to FIGS. 2A, B and C, an apparatus (20) for the on-site generation of chlorine gas comprises at least one electrolysis cell (21) having an anolyte section and a catholyte section. At least one section which, in this embodiment, is the anolyte section, is connected by a conduit (22) to a fluid heater (23) which, in use, heats an electrolyte solution prior to its ingress into said section of the electrolysis cell (21) and facilitates circulation of the electrolyte solution through the apparatus by means of a thermosyphon effect.

[0045] The electrolyte solution is dissociatable into positively and negatively charged ions at least one of which is an ion of a gaseous element. In this embodiment chlorine gas is generated and the electrolyte solution in the anolyte section of the apparatus becomes an acidic sodium chloride solution when chlorine meets with water to form hypochlorous acid which dissociates into positively charged sodium and hydrogen ions and negatively charged chlorine and hydroxyl ions. The chlorine and hydrogen ions combine with like ions to form chlorine and hydrogen gas each of which is circulated together with the electrolyte solution through gas separators (24 & 25) which separate the chlorine gas and hydrogen gas respectively from the electrolyte solutions. In this embodiment the hydrogen gas is a waste product and is vented to atmosphere while the chlorine gas is used or processed further to produce sodium hypochlorite by combining chlorine with sodium hydroxide, both of which are produced by the apparatus.

[0046] Each electrolysis cell (21) is divided into an anolyte section and a catholyte section by a perfluoropolymer membrane which allows sodium ions to pass therethrough but does not allow chlorine, hydrogen or hydroxyl ions to pass through it. This membrane effectively divides the apparatus as well as the electrolysis cell into an anolyte section and a catholyte section.

[0047] As electrolyte passes through the anolyte section of the electrolysis cell (21), chlorine gas is formed at the anode and becomes entrained in the electrolyte solution which is depleted. The entrained gas bubbles facilitate circulation of the electrolyte and entrained gas bubbles to a chlorine gas separator (24) by means of a gas lift. After passing through the chlorine gas separator (24), the depleted electrolyte flows through a conduit (26) and enters a substantially horizontally orientated electrolyte salt dissolving tube (27) which is supplied with sodium chloride salt from a salt hopper (28) through a chute (29). In the salt dissolving tube (27) the electrolyte solution is refreshed. Salt crystals in the electrolyte solution are removed by passing the refreshed electrolyte solution through a salt separator and strainer (30) from which it is returned to the fluid heater (23) for the process to be repeated.

[0048] As electrolyte passes through the catholyte section of the electrolysis cell (21), hydrogen gas is produced at the cathode. The bubbles of hydrogen gas, like the chlorine gas, become entrained in the electrolyte solution and facilitate circulation thereof through a hydrogen gas separator (25). After removal of the hydrogen and sodium hydroxide, water which has passed through a demineralisation column (31) is added to refresh the catholyte electrolyte solution which passes through the catholyte section of the electrolysis cell (21).

[0049] The catholyte electrolytic solution is not heated directly as is the anolyte electrolysis solution. It is, however, heated in the electrolysis cell (21) as a result of it being in contact with the heated anolyte electrolyte solution. It is envisaged that heating of the anolyte electrolysis solution prior to its introduction into the electrolysis cell improves the efficiency of the gas generation process for the electrolyte is at its optimum temperature. Electric current for the anode, cathode and heater is supplied by a mains alternating current supply. In the case of the supply to the anode and cathode it passes through a direct current convertor (not shown).

[0050] Referring to FIGS. 3A, B and C, details of a series of electrolysis cells (40) for use in the apparatus of FIG. 2 are shown. In this embodiment there are two electrolysis cells (40) each separated by a perfluoropolymer membrane (41) which is permeable to sodium ions but impermeable to chlorine, hydrogen and hydroxyl ions.

[0051] Each cell (40) has an anode (42) at which chlorine gas is generated and a cathode (43) at which hydrogen gas is generated. The cells (40) are formed by bolting together a series of plates, two of which are end plates (44) which have anolyte electrolyte solution inlets (45) and outlets (46) and catholyte electrolyte solution inlets (47) and outlets (48). The inner spacer plates (100) form the counter through which anolyte electrolysis solution flows in at a bottom corner of the plate and consequently the cell and egresses at the opposite top corner. In a similar fashion, the catholyte electrolysis solution ingresses the cell at the opposite bottom comer to the anolyte electrolysis solution, and egresses at the opposite top. The anolyte and catholyte thus flow in a countercurrent which, in use, maximises efficiency. The complete assembly is bolted together using backing plates (101) and the bolts (102).

[0052] It will be appreciated that numerous variations can be made to the above described embodiment of the invention without departing from the scope thereof. In particular, the embodiments describe a method and an apparatus for the generation of chlorine gas and hydrogen gas. The same apparatus can be used for the generation of bromine gas or, indeed, any gas which can be produced by an electrolytic reaction.

Claims

1. A method for the on-site generation of a gas comprising the steps of:

a) forming a dissociatable electrolyte solution which, in use, dissociates into positively charged and negatively charged ions at least one of which is an ion of a gaseous element;

b) heating the electrolyte solution upstream of at least one electrolytic cell and thereby causing it to circulate and re-circulate through conduits and through the or each electrolytic cell by means of a thermosyphon effect;

c) liberating, from the solution, at one electrode of the or each electrolytic cell, a gas;

d) passing the liberated gas and electrolyte solution through at least one gas separator to separate the gas from the electrolyte solution prior to re-circulating the electrolyte solution through the electrolysis cell; and,

e) at no time storing the electrolyte in a reservoir.

2. A method for the on-site generation of a gas as claimed in claim 1 in which circulation of the electrolyte solution is facilitated by entraining gas bubbles produced in the or each electrolysis cell and orientating conduits leading from the or each electrolysis cell to the or each gas separator substantially vertically thereby providing a gas lift effect.

3. A method for the on-site generation of a gas as claimed in claim 1 or in claim 2 in which the electrolyte in the solution is strengthened, if necessary, and any make up water is saturated by passing the electrolyte and make up water through an electrolyte salt dissolving tube.

4. A method for the on-site generation of a gas as claimed in claim 3 in which the electrolyte salt dissolving tube is mounted substantially horizontally.

5. A method for the on-site generation of a gas as claimed in claim 4 in which salt in electrolyte salt dissolving tube is replaced with fresh salt from a hopper.

6. A method for the on-site generation of a gas as claimed in any one of the preceding claims in which the electrolyte solution is a metal halide and gas generated at the anolyte side of the electrolysis cell is a halogen.

7. A method for the on-site generation of a gas as claimed in claim 6 in which the metal halide salt is sodium chloride and the and gas generated at the anolyte side of the electrolysis cell is chlorine.

8. A method for the on-site generation of a gas as claimed in claim 6 in which the metal halide salt is potassium chloride and and gas generated at the anolyte side of the electrolysis cell is chlorine.

9. A method for the on-site generation of a gas as claimed in any one of claims 6 to 8 in which hydrogen gas and sodium hydroxide or potassium hydroxide to be generated at the catholyte side of the electrolysis cell.

10. A method for the on-site generation of a gas as claimed in any one of the preceding claims in which the anolyte and catholyte sections of the or each electrolytic cell is or are separated from one another by an ion selective membrane which allows the passage of sodium or potassium, ions therethrough but which is impermeable to a halogen, to hydrogen gas and to hydroxyl ions.

11. A method for the on-site generation of a gas as claimed in claim 10 in which the ion selective membrane is a perfluoropolymer membrane.

12. A method for the on-site generation of a gas as claimed in any one of the preceding claims in which water is added to the sodium hydroxide or potassium hydroxide solution at the catholyte side of the electrolyte cell to maintain a predetermined concentration of sodium hydroxide or potassium hydroxide in the catholyte solution.

13. A method for the on-site generation of a gas as claimed in claim 12 in which the water is distilled or demineralized, water.

14. A method for the on-site generation of a gas as claimed in any one of claims 6 to 13 in which sodium hypochlorite or potassium hypochlorite is also produced by mixing chlorine and sodium hydroxide, or chlorine and potassium hydroxide.

15. A method for the on-site generation of a gas substantially as herein described with reference to and as illustrated in the accompanying drawings.

16. An apparatus for the on-site generation of a gas comprising at least one electrolytic cell having an anolyte section and a catholyte section, at least one section being connected, by fluid conduits, to a fluid heater which, in use, heats an electrolyte solution prior to its ingress into said section and facilitates circulation of the electrolyte solution through the apparatus by means of a thermosyphon effect, the electrolytic solution being dissociatable into positively charged and negatively charged ions at least one of which is an ion of a gaseous element, the heating element in turn being connectable by fluid conduits to an electrolyte replenishment means, at least one gas separator which, in use, separates gas produced in the electrolytic cell from electrolyte solution, the apparatus lacking a reservoir for the storage of electrolyte solution.

17. An apparatus for the on-site generation of a gas as claimed in claim 16 in which the or each gas separator is positioned operatively above the or each electrolysis cell and in which conduits linking the or each electrolysis cell is orientated operatively substantially vertically thereby facilitating circulation of the electrolytic solution by means of a gas lift effect.

18. An apparatus for the on-site generation of a gas as claimed in claim 16 or in claim 17 in which the replenishment means is a substantially horizontally orientated electrolyte salt dissolving tube through which electrolyte solution from the or each gas separator flows prior to flowing through the heating element.

19. An apparatus for the on-site generation of a gas as claimed in claim 18 in which the salt dissolving tube is connected to an electrolyte salt replenishment hopper which contains a desired salt, and is also connected to a salt separator, which is connected to the heating element, the salt separator removing particulate salt from the electrolyte prior to its introduction into the heating element.

20. An apparatus for the on-site generation of a gas as claimed in claim 19 in which the salt separator is a strainer.

21. An apparatus for the on-site generation of a gas as claimed in any one of claims 16 to 20 in which the electrolyte is a metal halide solution, the gas generated at the anolyte side of the electrolysis cell to be a halogen and hydrogen gas and a metal halide hydroxide are generated at the catholyte side of the electrolysis cell.

22. An apparatus for the on-site generation of a gas as claimed in claim 21 in which the metal halide is sodium chloride, the gas generated at the anolyte side of the electrolysis cell is chlorine and hydrogen gas and sodium hydroxide are generated at the catholyte side of the electrolysis cell.

23. An apparatus for the on-site generation of a gas as claimed in claim 21 in which the metal halide is potassium chloride, the gas generated at the anolyte side of the electrolysis cell is chlorine and hydrogen gas and potassium hydroxide are generated at the catholyte side of the electrolysis cell.

24. An apparatus for the on-site generation of a gas as claimed in any one of claims 16 to 24 in which the anolyte and catholyte sections of the or each electrolytic cell are separated from one another by an ion selective membrane which allows the passage of sodium or potassium, ions therethrough but which is impermeable to chlorine and hydrogen gas.

25. An apparatus for the on-site generation of a gas as claimed in claim 24 in which the ion selective membrane is a perfluoropolymer membrane.

26. An apparatus for the on-site generation of a gas as claimed in any one of claims 16 to 25 in which water is added to the sodium hydroxide or potassium hydroxide solution at the catholyte side of the electrolyte cell to maintain the concentration of sodium hydroxide or potassium hydroxide in the catholyte solution.

27. An apparatus for the on-site generation of a gas as claimed in claim 26 in which the water is distilled or demineralized water.

28. An apparatus for the on-site generation of a gas as claimed in any one of claims 22 to 27 in which the apparatus also produces sodium hypochlorite, or potassium hypochlorite, by mixing chlorine and sodium hydroxide, or by mixing chlorine and potassium hydroxide, produced by the apparatus.

29. An apparatus for the on-site generation of a gas substantially as herein described with reference to and as illustrated in the accompanying drawings.