Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes

Definitions

• This invention relates to a method and an apparatus for the on-site generation of a gas, particularly, but not exclusively, 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.
• relatively small chlorine gas generators can be used in rural communities to purity and render potable water drawn from small dams and rivers.
• gasses 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.
• 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.
• 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.
• thermosyphon effect 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;
• 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.
• 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.
• a metal halide preferably sodium chloride, alternatively potassium chloride
• 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.
• 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.
• 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.
• water preferably distilled, alternatively demineralized, water
• 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.
• 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.
• the invention also extends to an apparatus for the on-site generation of a gas
• 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.
• 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.
• the replenishment means prefferably 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.
• a salt separator preferably a strainer
• 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.
• a metal halide solution preferably sodium chloride, alternatively potassium chloride
• 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.
• 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.
• 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.
• 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.
• Figure 1 is a schematic diagram of one embodiment of a method for the on-site generation of chlorine gas according to the invention
• Figures 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 Figure 1 ;
• Figures 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 Figure
• a method for the on-site generation of chlorine gas comprises the steps of:
• thermosyphon effect 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;
• the method being characterised in that at no time is the electrolyte solution stored in a reservoir.
• 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.
• 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).
• 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).
• 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.
• the electrolyte solution is dissociatable into positively and negatively charged ions at least one of which is an ion of a gaseous element.
• 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.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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).
• FIG. 3 A, B and C details of a series of electrolysis cells (40) for use in the apparatus of Figure 2 are shown.
• electrolysis cells (40) 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.
• 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.
• the catholyte electrolysis solution ingresses the cell at the opposite bottom corner 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).

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Hybrid Cells (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)

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• 2001
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