Patent Application
20100044242
The present invention relates in general to electrochemical processes for the production of a chlorine-based oxidant product. More particularly, the present invention provides an electrochemical method for producing one or more chlorine-based oxidants, such as sodium hypochlorite and hypochlorous acid, through the use of a multi-compartment, electrolytic cell that includes an anion-conductive membrane, an alkali cation-conductive membrane, and an anolyte having an acidic pH.
Some chlorine-based oxidants, such as sodium hypochlorite, are commonly used as disinfecting and bleaching agents. In one example, sodium hypochlorite (NaOCl) is often used to bleach and launder cloth fabrics (e.g., clothing); to disinfect surfaces, such as floors and medical equipment in hospitals; to sanitize water in wells, waste-water treatment plants, and other water systems; and for a wide variety of other applications. In some instances, sodium hypochlorite is marketed as a 3-6 weight % (wt %) solution for use as household bleach. In other instances, stronger solutions are marketed for use in the chlorination of water (e.g., swimming pools) and for use in medical applications. The exact amount of sodium hypochlorite required for a particular application, however, depends on the quantity of water used, the water's chemistry, the water's temperature, the presence or absence of sediment in the water, contact time, and other similar factors.
Sodium hypochlorite can be produced in a variety of manners. In one example of a conventional method, sodium hypochlorite is produced as chlorine is passed into a cold and dilute solution of sodium hydroxide. In another example, sodium hypochlorite is produced through the electrolysis of brine in a double compartment electrolytic cell. In this example, the hydrolysis process produces caustic soda (sodium hydroxide) and chlorine gas, which are mixed together to form sodium hypochlorite.
While the above-mentioned production methods are used to create large amounts of sodium hypochlorite, such methods are not without their shortcomings. In one example, some methods for producing sodium hypochlorite are inefficient and expensive. For instance, some methods require relatively large amounts of electricity to be spent for each unit of sodium hypochlorite that is produced. In another example, certain conventional production processes are essentially immobile and thus prevent sodium hypochlorite, which has a limited shelf life, from being produced at the site where it is to be used. In another example, some conventional methods expose components of an electrolytic cell, namely the cathode and anode, to relatively harsh conditions (e.g., scaling and degradation), which tend to shorten the components' operational lifespan. In still another example, some methods tend to produce little hypochlorous acid (HOCl, including its ion ClO−), which acts as even a better disinfectant than sodium hypochlorite. In a final example, some methods are not able to produce sodium hypochlorite at a near-neutral pH, which may easier to handle than extremely acidic or basic forms of the oxidant.
The present invention provides methods and apparatus for the electrochemical production of solutions of one or more chlorine-based oxidants, such as alkali hypochlorite and/or hypochlorous acid and chlorine, from a feed stream comprising an alkali-chloride salt and through the use of a multi-compartment, electrolytic cell comprising alkali cation-conductive membrane technologies. More generally, the present invention provides methods and apparatus for the electrochemical production of solutions of one or more halogen-based oxidants such as alkali hypohalite and/or hypohalous acid and halogen gas from a feed stream comprising an alkali-halide salt and through the use of a multi-compartment, electrolytic cell comprising alkali cation-conductive membrane technologies. The methods and apparatus of the present invention may provide the capability of continually generating alkali hypohalite and/or hypohalous acid from seawater, salt brine, reverse osmosis (R.O.) brine, aqueous salt solutions, tap water, and other alkali halide-based solutions such as sodium chloride and the like. The alkali metal source of the solutions could be any alkali metal including without limitation, lithium, sodium and potassium. The halogen source of the solution could be fluorine, chlorine, bromine and iodine. Accordingly, it will be appreciated by those of skill in the art that references throughout the specification and claims to sodium could be substituted with lithium, potassium, or other suitable alkali metals. Furthermore, references throughout the specification and claims to chlorine could be substituted with fluorine, bromine, iodine, or other suitable halides.
Generally, the multi-compartment, electrolytic cell comprises a first compartment, a second compartment, and a third compartment, which are each configured to hold an amount of fluid. The first, or anolyte, compartment includes an anode electrode that is positioned to contact an acidic anolyte fluid within that compartment. Similarly, the third, or catholyte, compartment includes a cathode electrode that is positioned to contact a catholyte fluid within that compartment. The second, or middle compartment is positioned intermediate to, and is in operable communication with, both the first (anolyte) and the third (catholyte) compartments. Indeed, in some implementations, the first and the second compartments are separated by an anion-conductive membrane (e.g., an ACS® membrane from Astom Corp.), while the second and third compartments are separated by a cation-conductive membrane (e.g., a NaSICON-type membrane) that is selective to one type of material (e.g., sodium ions).
The electrolytic cell can be used in any suitable manner that allows sodium hypochlorite, hypochlorous acid (including hypochlorite (ClO−), chlorine or another chlorine-based oxidant product to be produced through the cell's use. For example, one or more feed streams can be added to the electrolytic cell, charge can be passed between the electrodes, and the fluids from the various compartments can be mixed in a variety of ways to form solutions comprising different concentrations of chlorine-based oxidant products (e.g., sodium hypochlorite).
The feed streams added to the cell can comprise any fluid or fluids that allow the cell to function properly and to produce sodium hypochlorite, hypochlorous acid, or another chlorine-based oxidant product. For example, water, an aqueous solution of a soluble alkali-chloride salt (e.g., sodium chloride), and/or an acid containing chlorine (e.g., hydrochloric acid (HCl)) can be added (e.g., as an anolyte) to the first compartment. In another example, an aqueous solution of an alkali-chloride salt or alkali hydroxide (e.g., sodium chloride or sodium hydroxide) can be added (as an electrolyte) to the second compartment. In still another example, water, an aqueous solution containing an alkali-chloride salt (e.g., brine, seawater, tap water containing sodium chloride, etc.), and/or an aqueous solution of an alkali-containing base (e.g., sodium hydroxide) can be added (as a catholyte) to the third compartment.
The pH and concentration of the feed stream or streams added to the compartments can be controlled so the fluid in each compartment has a pH that allows the cell to function as intended. In other words, the fluids in the 3 compartments can be tailored to have any suitable pH. That said, the anolyte in the first compartment may have a pH that is less than about 7, and in some instances, less than about 4. This acidic pH of the anolyte may increase the amount of chlorine gas and hypochlorous acid and other chlorine-based oxidants that are generated at the anode during cell use.
With reference to the electrolyte in the second or middle compartment, the electrolyte may have a pH greater than about 5.5. In one embodiment, the middle compartment has a pH of between about 6 and about 14. Additionally, the catholyte in the third compartment may have a pH between about 7 and about 14.
The acidic pH will also prevent precipitation of water insoluble salts of calcium, magnesium on the anionic and cationic membranes. This will increase the longevity of electrolytic cell operation when used with salt water sources such as sea water, R.O. brine water and tap water.
In some instances, the cell is configured to direct and mix fluid from one or more compartments into one or more other compartments of the cell. Thus, the cell may mix the fluids and cause chemical reactions to occur in desired compartments, or even outside of the cell.
In one non-limiting example of a suitable method for using the cell to produce sodium hypochlorite and hypochlorous acid as the chlorine-based oxidant products, a feed stream comprising an aqueous sodium chloride solution is added into the first and second compartments and water or aqueous sodium hydroxide is fed into the third compartment. As current is passed between the anode and the cathode, sodium chloride in the second compartment is split, and its anions (e.g., Cl−) and cations (e.g., Na+) are transported through their respective anion- and cation-conducting membranes. Additionally, as current passes between the electrodes, hypochlorous acid, chlorine gas and hydrochloric acid (including ions thereof) accumulate in the first compartment and sodium hydroxide accumulates in the third compartment. To control the pH of the second compartment, an effluent from the third compartment (containing sodium hydroxide) is added to the second compartment. Furthermore, to form sodium hypochlorite, an effluent from the first compartment is mixed with an effluent from the second compartment, which contains sodium hydroxide from the effluent from the third compartment. While this mixing may occur in any suitable location, in some instances, the effluent from the first compartment and the effluent from the second compartment are mixed in a vessel disposed outside of the electrolytic cell.
In a second non-limiting example, a feed stream comprising water, an aqueous sodium chloride solution, and/or hydrochloric acid is added to the first compartment for the production of hypochlorous acid and chlorine. Another feed stream comprising a sodium chloride solution is fed as an electrolyte into the second compartment. In this second compartment, sodium chloride in the electrolyte is split and its anions and cations are transported through the anion-conductive and cation-conductive membranes, respectively, as charge is passed between the electrodes. In some instances, the electrolyte is continuously re-circulated through the second compartment, causing the ion content of the electrolyte to become depleted.
In this second example, a final feed stream comprising water, an aqueous sodium chloride solution sodium chloride, and/or an aqueous solution of sodium hydroxide is added as a catholyte to the third compartment, where sodium hydroxide is produced. In some cases, the catholyte is then continuously re-circulated through the third compartment, causing the concentration of sodium hydroxide in that compartment to increase.
In this second example, an effluent containing hypochlorous acid, hydrochloric acid, and unreacted chlorine can be obtained in an effluent from the first compartment. Then, to form sodium hypochlorite, this effluent from the first compartment is mixed and reacted with a source of sodium hydroxide (e.g., effluent from the third compartment or catholyte). While this mixing can occur in any suitable location, in some implementations or embodiments, the mixing of the effluent from the first compartment and the sodium hydroxide source occurs in a vessel separate from the cell. In this way, sometimes undesirable HOCl and Cl2 is converted into NaOCl to make a more concentrated end product of NaOCl.
In a third non-limiting example, a feed stream comprising water, an aqueous sodium chloride solution, and/or hydrochloric acid is added to the first compartment, for the production of hypochlorous acid and chlorine. A second feed stream comprising a sodium chloride solution is also added as an electrolyte and catholyte to the second and third compartments, respectively. In some cases, this catholyte/electrolyte is re-circulated through both the second compartment, in which sodium chloride is split, and the third compartment, in which sodium hydroxide and hydrogen are formed.
In this third example, an effluent containing hypochlorous acid, hydrochloric acid, and unreacted chlorine can be obtained from the first compartment. To form sodium hypochlorite, this effluent from the first compartment is mixed with a source of sodium hydroxide (e.g., effluent from the second and/or the third compartment). While this mixing can occur in any suitable location, in some implementations or embodiments, the mixing of the effluent from the first compartment and sodium hydroxide source occurs in a vessel disposed outside of the cell.
In a fourth non-limiting example, a feed stream containing water, an aqueous sodium chloride solution, and/or hydrochloric acid is added as an anolyte to the first compartment where hypochlorous acid and chlorine are formed. Another feed stream comprising a sodium chloride or alkaline sodium chloride solution is added as an electrolyte to the second compartment, where sodium chloride splitting occurs. In some cases, this electrolyte is continuously re-circulated through the second compartment, causing the ion concentration of the electrolyte to be depleted. A final feed stream containing water, an aqueous sodium chloride solution, and/or sodium hydroxide is added as a catholyte to the third compartment, where sodium hydroxide and hydrogen are formed as current passes between the electrodes.
In this forth example, an effluent containing hypochlorous acid, hydrochloric acid, and unreacted chlorine can be obtained in an effluent from the first compartment. To form sodium hypochlorite, this effluent can be mixed directly with an effluent from the third compartment containing sodium hydroxide. While this mixing can occur in any suitable location, in some implementations, the mixing of the effluent from the first compartment and the effluent from the third compartment occurs in a vessel disposed outside of the electrolytic cell.
In a fifth non-limiting example, a feed stream containing water, an aqueous sodium chloride solution, and/or hydrochloric acid is added as an anolyte to the first compartment, where hypochlorous acid and chlorine are formed. Another feed stream comprising a sodium chloride solution is added as an electrolyte to the second compartment, where sodium chloride splitting occurs. In some cases, this electrolyte is continuously re-circulated through the second compartment, causing the ion concentration of the electrolyte to be depleted. A final feed stream containing water, an aqueous sodium chloride solution, and/or sodium hydroxide is added as a catholyte to the third compartment, where sodium hydroxide and hydrogen are formed during cell operation.
In this fifth example, to form sodium hypochlorite an effluent containing sodium hydroxide is channeled from the third compartment, into the second compartment, and then into the first compartment where the sodium hydroxide reacts with hypochlorous acid, hydrochloric acid, and/or unreacted chlorine that are formed in the first compartment.
In a final non-limiting example, a feed stream containing water, an aqueous sodium chloride solution, and/or hydrochloric acid is added as an anolyte to the first compartment for the formation of hypochlorous acid and chlorine at the anode. Another feed stream comprising a sodium chloride solution is added as an electrolyte to the second compartment, where sodium chloride splitting occurs. A final feed stream containing water, sodium chloride, and/or sodium hydroxide is added as a catholyte to the third compartment for the formation of sodium hydroxide.
In this final example, chlorine-based oxidant stream is formed as an effluent from the first compartment (comprising hypochlorous acid, hydrochloric acid, and chlorine) is fed directly into the third compartment (comprising sodium hydroxide).
While the described systems and methods have proven particularly useful for the production of sodium hypochlorite, the skilled artisan will recognize that the described methods may be modified to produce one or more other chlorine-based oxidant products, such as lithium hypochlorite and/or potassium hypochlorite. For example, instead of using sodium chloride in the electrolyte and a NaSICON membrane, the described methods may use another alkali-chloride salt (e.g., lithium chloride (LiCl), potassium chloride (KCl), etc.) solution as the electrolyte with a membrane (e.g., a LiSICON membrane, a KSICON membrane, etc.) that is capable of transporting selected cations from the salt solution into the third compartment.
These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
In order that the manner in which the above-recited and other features and advantages of the invention are obtained and will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that the drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of suitable ion-conducting membranes, feed streams, methods for mixing fluids inside and/or outside an electrolytic cell, etc., to provide a thorough understanding of embodiments of the invention. One having ordinary skill in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
The present invention relates to systems and methods for producing a one or more halide-based oxidants, such as sodium hypochlorite, chlorine and hypochlorous acid, through the use of a multi-compartment electrolytic cell comprising alkali cation-conductive and anion-conductive membrane technologies. In one embodiment, the use of the electrolytic cell, both pure and impure aqueous sodium chloride solutions, such as seawater, brine, tap water including sodium chloride, and mixtures thereof, can be used to produce sodium hypochlorite, chlorine and hypochlorous acid. To provide a better understanding of the describe systems and methods, the electrolytic cell is first described, followed by a description of a variety of methods for using the cell.
The anode electrode 108 can comprise one or more of a variety of materials that allow it to evolve halogen such as chlorine in preference over oxygen when it is contacted with an acidic anolyte comprising chlorine ions and when current is running between the electrodes. Some non-limiting examples of suitable anode materials comprise dimensionally stabilized anode-platinum on titanium (DSA), platinized titanium, ruthenium IV dioxide (RuO2) on titanium, and other suitable anode materials that are well known in the art.
The cathode electrode 110 can comprise one or more of a variety of suitable materials that allow it to evolve hydrogen when the current is run between the electrodes, and when the cathode is disposed in a catholyte. Some non-limiting examples of suitable cathode materials include nickel, stainless steel, and other conventional materials that are stable in a caustic pH.
In some specific embodiments, the alkali-ion conducting ceramic membrane compositions comprising NASICON materials may include at least one of the following: materials of general formula M1+xMI2SixP3−xO12 where 0≦x≦3, where M is selected from the group consisting of Li, Na, K, or Ag, or mixture thereof, and where MI is selected from the group consisting of Zr, Ge, Ti, Sn, or Hf, or mixtures thereof; materials of general formula Na1+zLzZr2−zP3O12 where 0≦z≦2.0, and where L is selected from the group consisting of Cr, Yb, Er, Dy, Sc, Fe, In, or Y, or mixtures or combinations thereof; materials of general formula MII5RESi4O12, where MII may be Li, Na, or Ag, or any mixture or combination thereof, and where RE is Y or any rare earth element. In some specific embodiments, the NASICON materials may include at least one of the following: non-stoichiometric materials, zirconium-deficient (or sodium rich) materials of general formula Na1+xZr2−x/3SixP3−xO12−2x/3 where 1.55≦x≦3. In some specific embodiments, the alkali-ion conducting ceramic membrane compositions comprising NASICON materials may include at least one of the following: non-stoichiometric materials, sodium-deficient materials of general formula Na1+x(AyZr2−y)(SizP3−z)O12−δ where A is selected from the group consisting of Yb, Er, Dy, Sc, In, or Y, or mixtures or combinations thereof, 1.8≦x≦2.6, 0≦y≦0.2, x<z, and δ is selected to maintain charge neutrality.
In some embodiments of the present invention it may be advantageous to employ polymeric anion-conductive membranes that are substantially impermeable to at least the solvent components of the anolyte solution in the first. A variety of polymeric anion-conductive membrane materials are known in the art and would be suitable for constructing the polymeric anion-conductive membrane of the present invention, as would be understood by one of ordinary skill in the art. In some specific embodiments, the polymeric anion-conductive membranes may include at least one of the following: NEOSEPTA® anion exchange membranes (Astom Corp.) such as grades NEOSEPTA® AM-1, NEOSEPTA® AM-3, NEOSEPTA® AMX, NEOSEPTA® AHA, NEOSEPTA® ACM, NEOSEPTA® ACS, NEOSEPTA® AFN, or NEOSEPTA® AFX; Ionac® MA-3475 or MA-7500 anion membranes (Sybron Chemicals Inc, NJ); ULTREX™ AMI-7001 anion membrane (Socada LLC, NJ); and PC-SA, PC-SA/HD, PC 100 D, PC 200 D, PC Acid 60, or PC Acid 100 anion membranes (PCA GmbH, Germany).
While not shown in
The cell may be used with any suitable feed stream or streams that allow a chlorine-based oxidant product (e.g., sodium hypochlorite, chlorine and/or hypochlorous acid) to be produced when the cell is operated. In one example, the feed stream that is initially added to the first compartment as the anolyte is selected from water and a chlorine-based solution, such as an aqueous solution comprising an alkali-chloride salt (e.g., sodium chloride, lithium chloride, potassium chloride, etc.) and/or an aqueous solution of hydrochloric acid. In another example, the feed stream that is initially introduced into the second compartment as the electrolyte comprises an aqueous solution containing an alkali-chloride salt (e.g., sodium chloride, lithium chloride, potassium chloride, etc.). In still another example, the feed stream that is initially fed into the third compartment as the catholyte comprises water, an aqueous solution including an alkali-chloride salt (e.g., sodium chloride, lithium chloride, potassium chloride, etc.), and/or an aqueous solution containing an alkali-base (e.g., sodium hydroxide, lithium hydroxide, potassium hydroxide, etc.). In one embodiment, the chlorine-based oxidant product comprises sodium hypochlorite present at a concentration between about 0.1 wt % and about 30 wt %. In another embodiment, the chlorine-based oxidant product comprises sodium hypochlorite present at a concentration between about 0.1 wt % and about 15 wt %. In yet another embodiment, the chlorine-based oxidant product comprises sodium hypochlorite present at a concentration between about 0.1 wt % and about 5 wt %.
The described systems and methods may be used to form one of a variety of chlorine-based oxidants, such as sodium hypochlorite, lithium hypochlorite, and potassium hypochlorite. In one example in which the electrolytic cell is used to produce lithium hypochlorite, the cell may comprise a LiSICON-type membrane, the alkali-chloride salt in the feed stream may comprise an aqueous lithium chloride solution, and the alkali-base may comprise lithium hydroxide. In another example, where the cell is used to produce potassium hypochlorite, the cell may comprise a KSICON-type membrane, the alkali-chloride salt may comprise potassium chloride, and the alkali-base may comprise potassium hydroxide. In a final example, where the cell is used to produce sodium hypochlorite, the cell may comprise a NaSICON-type membrane, the alkali-chloride salt may comprise sodium hypochlorite, and the alkali-base may comprise sodium hydroxide.
Where a feed stream comprises a chemical in addition to water (e.g., hydrochloric acid, an alkali-chloride salt, and/or an alkali-base, the feed stream may have any suitable amount of the additional chemical that allows the cell to produce a chlorine-based oxidant product. in one embodiment, where a feed stream added to the first compartment comprises hydrochloric acid, the feed stream can comprise any suitable concentration of hydrochloric acid. Indeed in some embodiments, the feed stream comprises between about 0.1 wt % and about 26 wt % hydrochloric acid. In other embodiments, the feed stream comprises between about 0.1 wt % and about 10 wt % hydrochloric acid. In still other embodiments, the feed stream added to the first compartment comprises between about 0.1 wt % and about 8 wt % hydrochloric acid
Where a feed stream added to the first, second, and/or third compartment comprises an aqueous solution of sodium chloride (or another alkali-chloride salt), the stream may comprise any suitable concentration of the alkali-chloride salt (e.g., sodium chloride). In some embodiments, the concentration of sodium chloride in a feed stream is between about 0.2 wt % and about 26 wt %. In other embodiments, the concentration of sodium chloride in a feed stream is between about 2 wt % and about 20 wt %. In still other embodiments, the sodium chloride concentration in the feed stream is between about 3 wt % and about 13 wt % (e.g., about 10 wt %±2 wt %). For example where a feed stream comprising sodium chloride is added to the first compartment as an anolyte, the feed stream can comprise between about 2.5 wt % and about 4.5 wt % sodium chloride. In another example where the feed stream comprises sodium chloride and is initially added to the second compartment as an electrolyte, the feed stream can comprise between about 8 wt % and about 12 wt % sodium chloride.
Where a feed stream added to the second and third compartment as a catholyte comprises an alkali-base, such as sodium hydroxide, the feed stream can comprise any suitable amount of the alkali-base (e.g., sodium hydroxide) that allows the cell to function as intended. For example, in some embodiments, the feed stream comprises between about 1 wt % and about 26 wt % sodium hydroxide. In other embodiments, the feed stream comprises between about 5 wt % and about 20 wt % sodium hydroxide. In still other embodiments, the feed stream comprises between about 8 wt % and about 12 wt % sodium hydroxide. In still other embodiments, the feed stream comprises a mixture of sodium hydroxide and sodium chloride.
To better explain how the electrolytic cell can be used to produce one or more chlorine-based oxidants, several representative embodiments of suitable methods and systems are described with reference to
It should be noted that throughout this disclosure, the reaction names (e.g., R-1, R-2, . . . Rn) are used for the sake of simplicity and not to identify any particular order in which specific chemical reactions occur. Additionally, while examples of suitable locations for the reactions are discussed, the discussed locations are provided for example only and are not intended to limit the scope of the invention.
According to some embodiments,
To form sodium hypochlorite,
A second non-limiting embodiment (Scheme B) of a method and system for producing sodium hypochlorite and/or hypochlorous acid is illustrated in
After the various feed streams (26, 28, and 30) are added to their respective compartments,
As the power source (not shown in
At reaction R-2,
As shown by reactions R-2 and R-3 in
In a variation of the system and method shown in
To form sodium hypochlorite, the effluent 42 from the first compartment 102 can be reacted, according to reactions R-5 and R-7 (shown in Table 2), with a source of sodium hydroxide in a vessel that is separate from the cell. While the sodium hydroxide source may comprise any suitable source of sodium hydroxide, in some embodiments, the sodium hydroxide source comprises the effluent 41 from the second and third compartments which comprises sodium hydroxide.
In the described methods, the pHs of the various compartments can be controlled in any suitable manner, including, but not limited to: controlling the pH of the various feed streams and/or effluents (e.g., by controlling the feed streams' and effluents' sodium-chloride concentrations, hydroxide concentrations, hydrochloric acid concentrations, etc.), changing effluent mixing schemes, changing the current passing between the anode and the cathode, and/or through other known or novel methods for controlling pH.
The fluids (e.g., the anolyte, electrolyte, and catholyte) in each compartment can have any suitable pH. In some embodiments, the pH of the anolyte in the first compartment is generally maintained at an acidic pH, or at a pH less than about 7 (e.g., between about 7 and about 1.5). Indeed, in some embodiments, the pH of the anolyte is maintained at a pH below about 5.5. In still other embodiments, the pH of the anolyte in the first compartment is maintained below a pH of about 4.5 (e.g., between about 2 and about 4.2).
The acidic pH of the anolyte may serve several purposes, including increasing the amount of oxidants (e.g., chlorine gas) produced at the anode. Indeed, at low pHs, more of the current passing between the anode and the cathode may generate chlorine than would occur at higher pHs, which would tend to cause the current to generate more oxygen. A portion of this increased chlorine may then be reacted with water to form hypochlorous acid and hydrochloric acid, which can be reacted with sodium hydroxide to form sodium hypochlorite. Another portion of the chlorine can be reacted according to reaction R-7 to produce sodium hypochlorite. By capturing the additional chlorine, using sodium hydroxide, the current efficiency for the production of sodium hypochlorite can be increased or maximized.
The usage of anolyte of pH less than 4 may prevent precipitation of scale forming salts present in the first feed stream onto the anionic membrane. The usage of anolyte of pH above about 5.5 may also prevent precipitation of scale forming salts present in the electrolyte onto the cationic membrane.
In some embodiments, the pH of the fluid (electrolyte) in the second compartment is maintained at a pH between about 7 and about 13. Accordingly, the anolyte in the first compartment can have an acidic pH, while one or both of the ion-conductive membranes (e.g., a NaSICON membrane) can be protected, to some extent, from the degradation and damage which may occur at low pHs.
In some embodiments, the pH of the catholyte in the third compartment is maintained at a pH between about 7 and about 14. In one embodiment, to control the pH of the second and third compartments, voltage is applied to the electrodes to allow a plurality of alkali ions in the second compartment to pass through the cation-conductive membrane into the third compartment. In another embodiment, the concentration of sodium hydroxide in the third compartment is controlled.
In the described methods, the power supply can provide any suitable voltage that allows the cell to produce a chlorine-based oxidant product, such as sodium hypochlorite. In some embodiments, the power supply provides the cell with between about 1 and about 15 volts. In other embodiments, the power supply causes between about 2 and about 10 volts to pass between the anode and cathode. In still other non-limiting embodiments, the power supply causes between about 4 and about 9 volts of electricity to pass between the electrodes.
The power supply can also provide any suitable current density to the cell. Indeed, in some embodiments, the power supply provides between about 20 and about 100 mA/cm2. In other embodiments, the power supply is used to provide a current density between about 30 and about 75 mA/cm2. In still other embodiments, the power supply provides a current density between about 32 and about 60 mA/cm2.
A final effluent comprising sodium hypochlorite, hypochlorous acid, and/or another chlorine-based oxidant product that is produced by the cell may flow at any suitable rate that provides the final effluent with a suitable concentration of the chlorine-based oxidant product. Different flow rates can alter the pH and/or concentration of the final effluent. The skilled artisan will recognize that the actual flow rate of the final effluent from the cell can depend on several factors, including, but not limited cell size, temperature, ambient pressure, etc. In one embodiment, the flow rate of the final effluent is between about 2 and about 30 ml/min. In another embodiment, the flow rate of the final effluent is between about 3 and about 20 ml/min. In still another non-limiting embodiment, the flow rate of the final effluent is between about 5 and about 15 ml/min.
Where the described systems and methods are used to produce sodium hypochlorite, the electrolytic cell may produce any suitable concentration of sodium hypochlorite. In one example, the described systems and methods produce solutions comprising between about 0.5 and about 30 wt % sodium hypochlorite. In another example, the described systems and methods produce solutions comprising between about 4 wt % and about 13 wt % sodium hypochlorite. In still another example, the described systems and methods produce solutions comprising between about 8 and about 12 wt % sodium hypochlorite.
The described systems and methods can be varied in any suitable manner. For instance, in addition to the described components, the electrochemical cell may comprise any other suitable component, such as a coolant system, a conventional pH controlling system, etc. Indeed, because the described systems and methods may function best between about 5° and about 30° Celsius, in some embodiments, the described cell is used with a coolant system. In another example, additional chemical ingredients are added to the cell for any suitable purpose (e.g., to modify fluid pH, to combat scaling on the electrodes, etc.). In still another example, effluents from one or more compartments are fed into a desired compartment or compartments at any suitable time (e.g., any suitable time after the introduction of a feed stream into the cell) and in any suitable amount.
The described systems and methods may have several beneficial characteristics. In one example, because the described methods can use an acidic anolyte, the described methods may be able to produce higher concentrations of a chlorine-based oxidant product, such as sodium hypochlorite, than may some similar methods involving an anolyte with a basic pH. In another example, the described methods are able to produce relatively high concentrations of sodium hypochlorite with relatively small amounts of energy (e.g., a current density between about 20 and about 50 mA/cm2). In still another example, the described methods may use inexpensive ingredients, such as seawater, brine, tap water with sodium chloride, etc. In still another example, the described methods may be used to produce chlorine-based oxidants, such as sodium hypochlorite, on demand and continuously, as desired. In still another example, some embodiments of the electrolytic cell may be portable and, thereby, allow sodium hypochlorite or another chlorine-based oxidant product to be produced at the site where it will be used.
The following non-limiting example is given to illustrate various embodiments within the scope of the present invention. This example is given by way of demonstration only, and it is understood that the following example is not comprehensive or exhaustive of the many types of embodiments of the present invention that can be prepared in accordance with the present invention.
In one example of how the electrolytic cell functions, a cell was prepared and operated according to the systems and methods shown in
Additionally, in this example, chlorine gas produced in the first compartment was reacted with an external source of sodium hydroxide in a vessel separate from the electrolytic cell to produce sodium hypochlorite. The concentration of the sodium hypochlorite produced by this process was about 10.23 g/L. The current efficiency of this process was measured to be 100% and the power consumption was 2.23 kWh/lb of sodium hypochlorite produced.
While specific embodiments and examples of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.
This application claims the benefit of U.S. Provisional Application No. 61/091,627, filed Aug. 25, 2008, entitled “Three Compartment Apparatus and Method for Producing Sodium Hypochlorite” and U.S. Provisional Application No. 61/120,737, filed Dec. 5, 2008, entitled “Three Compartment Electrochemical Process for Production of Sodium Hypochlorite,” the entire disclosures of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61091627 | Aug 2008 | US | |
| 61120737 | Dec 2008 | US |
