Patent Application
20080003507
Once the ClO2 gas is produced in the ClO2 gas generator loop, the ClO2 gas can be transferred to, for example, an absorption loop 206 where the gas can be further conditioned for water treatment end-uses. In this application, the term “absorption” refers to the process of dissolving or infusing a gaseous constituent into a liquid, optionally using pressure to effect the dissolution or infusion. Here, ClO2 gas, which is produced in the ClO2 gas generator loop, can be “absorbed” (that is, dissolved or infused) into an aqueous liquid stream directed through absorption loop 206.
The ClO2 gas can also be transferred to other water treatment end-uses without additional processing in an absorption loop. For example,
The contribution of anolyte loop 202, 302 to the ClO2 solution generator is to produce a ClO2 gas that can be directed to absorption loop 206 for further processing or to other water treatment end uses, as illustrated, for example, in
The reactant feedstock 210 can be connected to a chemical metering pump 214, which can deliver the reactant feedstock 210 to a recirculating connection 216 in the anolyte loop. Recirculating connection 216 in anolyte loop connects a stripper column 218 to an electrochemical cell 212. The delivery of the reactant feedstock 212 can be controlled using PLC system 208. PLC system 208 can be used to activate chemical metering pump 214 according to signals received from a pH sensor. The pH sensor is generally located along recirculating connection 216. A pH set point can be established in PLC system 208, and once the set point is reached, the delivery of reactant feedstock 210 can either start or stop.
Reactant feedstock 210 can be delivered to a positive end 220 (that is, the anode) of electrochemical cell 212 where the reactant feedstock is oxidized to form a ClO2 gas, which is dissolved in an electrolyte solution along with other side products. The ClO2 solution with the side products is directed away from electrochemical cell 212 to the top of stripper column 218 where a pure ClO2 is stripped off in a gaseous form from the other side products. Side products or byproducts can include chlorine, chlorates, chlorites and/or oxygen. The pure ClO2 gas can then be removed from stripper column 218 under a vacuum induced by gas transfer pump, or analogous gas or fluid transfer device (such as, for example, a vacuum-based device), where it is delivered to an absorption loop 206, as illustrated in
The pure ClO2 gas can also be removed from stripper column 218 where it is directed to an eductor 306 that combines the stripped ClO2 gas with a process water desired for ClO2 treatment, as illustrated in
As described in the '681 application, modifications to the anolyte loop process can be made that achieve similar results. As an example, an anolyte hold tank can be used in place of a stripper column. In such a case, an inert gas or air can be blown over the surface or through the solution to separate the ClO2 gas from the anolyte. As another example, chlorate can be reduced to produce ClO2 in a cathode loop instead of chlorite. The ClO2 gas would then similarly be transferred to the absorption loop or eductor. In a further example, ClO2 can be generated by chemical generators and transferred to an absorption loop for further processing or to an eductor.
Catholyte loop 204 contributes to the chlorine dioxide generator 200 by handling byproducts produced from the electrochemical reaction of reactant feedstock 210 solution in anolyte loop 202, 302. As an example, where a NaClO2 solution is used as reactant feedstock 210, sodium ions from the anolyte loop 202, 302 migrate to catholyte loop 204, 304 through a cationic membrane 222 in electrochemical cell 212 to maintain charge neutrality. Water in the catholyte is reduced to produce hydroxide (OH−) and hydrogen (H2) gas. The resulting byproducts in catholyte loop, in the example of a NaClO2 reactant feedstock, are sodium hydroxide (NaOH) and H2 gas. The byproducts can be directed to a byproduct tank 224.
In an embodiment of catholyte loop in the example of a NaClO2 reactant feedstock, a soft (that is, demineralized) water source can be used to dilute the byproduct NaOH using a solenoid valve connected between soft water source and the byproduct tank 224. The solenoid valve can be controlled with PLC system 208. In a preferred embodiment, PLC system 208 can use a timing routine that maintains the NaOH concentration in a range of 5 percent to 20 percent. When byproduct tank 224 reaches a predetermined level above the base of byproduct tank 224, the diluted NaOH byproduct above that level is removed from catholyte loop.
In the example of a NaClO2 reactant feedstock, the catholyte loop self-circulates using the lifting properties of the H2 byproduct gas formed during the electrochemical process and forced water feed from soft water source. The H2 gas rises up in byproduct tank 224 where there is a hydrogen disengager. The H2 gas can be diluted with air in hydrogen disengager to a concentration of less than 0.5 percent. The diluted H2 gas can be discharged from catholyte loop 204, 304 and chlorine dioxide generator 200 using a blower.
As described in the '681 application, in another embodiment, dilute sodium hydroxide can be fed instead of water to produce concentrated sodium hydroxide. Oxygen or air can also be used as a reductant instead of water to reduce overall operation voltage since oxygen reduces at lower voltage than water.
The reaction of anolyte loop 202, 302 and catholyte loop 204, 304 in the embodiments illustrated in
NaClO2(aq)+H2O→ClO2(gas)+NaOH(aq)+½H2(gas)
The NaClO2 is provided by reactant feedstock 202 of anolyte loop 202, 302. The NaOH and H2 gas are byproducts of the reaction in catholyte loop 204, 304. The ClO2 solution along with the starting unreacted NaClO2 and other side products are directed to the stripper column for separating into ClO2 gas as part of anolyte loop 202, 302 process. Chlorite salts other than NaClO2 can be used in anolyte loop 202, 302.
Hardness control of the reactant feedstock 210 (that is, the anolyte) prior to entering the anode compartment is desirable to extend the operating life of the membrane 222 of the electrochemical cell 212. In an embodiment for a NaClO2-anolyte based electrochemical chlorine dioxide generator, accelerated tests can be made to determine an acceptable concentration of Ca2+ for the operation of the generator. An acceptable concentration can be one in which the electrochemical generator functions for a one to three year operating time, that is, one to three years of cumulative actual operation of the electrochemical cell, without fouling the membrane. Therefore, if the electrochemical cell were to operate half of the time over a period of two to six years without fouling the membrane, that could qualify as an acceptable concentration of Ca2+. Another manner of determining an acceptable concentration uses the cell voltage; fewer impurities create a lower cell voltage. Theoretically, the Ca2+ concentration in the feed electrolyte is such that the ionic product of Ca2+ and OH− is less than the solubility product constant at the operating temperature. At this theoretical Ca2+ concentration, calcium ions can generally pass through the membrane 222 without precipitating.
A method for assessing an acceptable concentration of Ca2+ in the actual operation of a chlorine dioxide generator is described.
Chlorine dioxide electrochemical cells have their lowest cell voltage (as shown by the minimum voltage on an electrolysis time vs. voltage graph) for a given current density if there are no impurities in the electrolyte solution. As the impurity level increases, the cell voltage can correspondingly increase though the current density remains generally the same. The increasing impurities can cause the resistance along the membrane 222 to increase, which is the cause of the increased cell voltage. In a preferred embodiment, a membrane made of the Nafion® product is used.
In the example of a 25 percent sodium chlorite solution, if the calcium ion concentration is in the range of 1,000 ppm, cell voltage increases rapidly after only a few hours of cell operation. An anolyte with a known concentration of the target impurity, for example Ca2+, can be fed into the anode compartment 226 of an electrochemical cell 212. A TTD can be determined for a given Ca2+ concentration by plotting cell voltage vs. electrochemical cell operating time and determining the time necessary to reach 5 V. The TTD determination can be repeated at varying known concentrations of Ca2+, for example, from 1,000 ppm to 10 ppm. A plot of TTD vs. Ca2+ concentration can then be made and used to estimate the acceptable levels of Ca2+. In the TTD example, the Ca2+ concentration value can be extrapolated using the desired operating time for replacement of the membrane as the TTD value, for example, one to three years. These methods for assessing the acceptable concentration of Ca2+ can be used for other ions that are anticipated to precipitate in the membrane 222, for example, Mg2+, Fe2+, Ni2+ and Al3+.
An electrolyte feed formulation can then be made after the levels of acceptable hardness-causing impurities is assessed. In the example of Ca2+ concentration in a 25 percent sodium chlorite solution, a concentration of Ca2+ of less than 1 part per million will allow an acceptable operating life for the membrane of the electrochemical cell. In a preferred embodiment, the Ca2+ concentration in the electrolyte solution is less than 50 ppb. In another preferred embodiment, the Ca2+ concentration in the electrolyte solution is less than 20 ppb.
A method of reducing the levels of hardness-causing impurities from a sodium chlorite electrolyte solution is described. The method is described for removal of Ca2+ impurities, but it would be understood that the described method can be used to remove other multivalent ion impurities. A preferred method is the ion exchange resins method. Sodium chlorite salt is typically supplied as 80 percent solid with the remaining 20 percent of the salt comprising primarily water and secondarily other impurities such as hardness-causing impurities. The sodium chlorite salt is first dissolved in water. The majority of the divalent and trivalent ions can be removed through one or more passes of the solution through an ion exchange column. Examples of commercially available ion exchange resins that may be used include Amberlite™ IRC747, Lewatit TP207, and Duolite™ C467 resins. Ion exchange membranes can also be used. The amount of resin and the flow rate that reduces the level of impurity to an acceptable level, as determined by the methods described previously, can then be utilized.
The impurities in the electrolyte solution can also be lowered by passing the electrolyte solution, that is, the reactant feedstock 210, through the ion exchange column 228. In a further embodiment, one or more ion exchange columns 228 can be installed in line with the feed before entering the electrochemical cell 212.
Table 2 illustrates the reduction of calcium ion concentrations by treating a reactant feedstock with Amberlite™ IRC 747 ion exchange.
The example of Table 2 illustrates that the amount ion exchange resin and duration of treatment of the NaClO2 solution can affect the calcium ion concentration. Varying the amount of ion-exchange resin and treatment duration can reduce the level of calcium ion to a desired level. It would be understood that other hardness-causing ions can be removed by similar methods. It would be further understood that different commercially available ion-exchange resins can be used.
Another preferred embodiment for removing multivalent impurities from an electrolyte solution, such as the reactant feedstock 210, is the precipitation method. The precipitation method is described for removal of Ca2+ impurities for a solution containing calcium ions, but it would be understood that the described method can be used to remove other multivalent ion impurities, such as Mg2+, Fe2+, Ni2+ and Al3+. In the example of calcium ion impurities, calcium phosphate can be precipitated by adding a common ion such as phosphate to the electrolyte solution with Ca2+ impurities. The addition of the phosphate will lead to the precipitation of the calcium phosphate, which results in the removal of hardness-causing calcium ions from the reactant feedstock. In this example, an alkaline phosphate, such as sodium phosphate, sodium mono hydrogen phosphate, sodium dihydrogen phosphate or other soluble phosphates, can be used to effect the precipitation. Sodium can, for instance, be substituted by potassium or ammonium. The method can also be effective for other anions that have low solubility product constants with the impurity cations whose concentrations are desired to be reduced. The resulting precipitate from the precipitation method can be separated from the electrolyte solution using filtration. A centrifuge can also be used to separate the precipitate.
Table 3 illustrates decreasing calcium ion concentrations for various additives using the precipitation method.
It will be understood that various soluble salts can be used to effect the precipitation of a target impurity. The desired salt will be generally soluble in water, but will react and be effectively insoluble when the salt is combined with the target impurity in the feed electrolyte (that is, the reactant feedstock 210).
The above treatments produce electrolytes that increase electrochemical cell efficiency because they are lower in hardness ions. Efficiency of the electrochemical cell can be further increased by maintaining the pH of the electrolyte less than about 10. The pH of an electrolyte has an effect on the efficiency of chlorine dioxide production as shown by
2NaOH+2ClO2→NaClO2+NaClO3
Because of the effect of pH on efficiency, it is advantageous to use electrolytes with a pH of less than about 10. Since they electrolytes produced by ion-exchange resins typically have pH levels greater than about 10, an additional treatment of the electrolyte can be added. After removing hardness ions using the ion exchange resin, dilute acid, such as sulfuric acid, can be added to lower pH to less than about 10. A gaseous carbon dioxide can also be bubbled through the electrolyte to reduce pH. Another method of reducing pH of the electrolyte would be to pass the hardness removed electrolyte through an acid ion exchange column. Yet another method is to remove hardness by adding sodium hydrogen phosphate. While phosphate removes hardness ions, the protons in the sodium hydrogen phosphate partially neutralize the hydroxide present in commercial electrolytes. This lowers the pH of the electrolyte to the desired level.
As mentioned above, migration of hydroxide into the anolyte compartment, also known as caustic migration, lowers chlorine dioxide yield because of the byproduct reaction between chlorine dioxide and sodium hydroxide discussed above. In order to increase efficiency of the electrochemical cell, it is desirable to decrease migration of hydroxide into the anolyte compartment. Another method of decreasing migration of hydroxide across the membrane is to attach a polymeric membrane with carboxylic groups to the cathode side of the membrane. Commercially available examples of such composite membranes include Dupont's N-series membranes such as N961, N962 and N966. A coated cathode side of the membrane can be used in conjunction with the electrolytes discussed above to further increase efficiency.
Efficiency of chlorine dioxide production can further be increased by variances in the anode material. Platinum anodes offer increased efficiency when used with electrolyte that is treated according to the methods above. Another possible electrode could be made of noble metal oxides, such as iridium, ruthenium oxide, and palladium oxide. Other mixed oxide metal oxides like iridium and tantalum oxide can also be used. The effect of using a platinum electrode is shown in the example below.
In one example, a 28 inch ion exchange column is prepared by loosely loading about 108 grams of Lewatit TP-207. The electrolyte is passed through the column at a flow rate of about 40 bed volumes per hour. The resulting electrolyte has less than 20 ppb calcium ions and less than 20 ppb magnesium ions.
In another example, about 166 grams of disodium phosphate is added to 5 kilograms of electrolyte and stirred. The solution is then decanted before using in a cell. The calcium level was about 50 ppb and the magnesium level was less than about 20 ppb.
3 to 4 kilograms of electrolyte prepared according to either of the methods described above is then loaded into the reservoir of a generator similar to that shown in
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.
This application relates to and claims priority benefits from U.S. Provisional Patent Application Ser. No. 60/806,380, filed Jun. 30, 2006, entitled “Formulation of Electrolyte Solutions for Electrochemical Chlorine Dioxide Generators.” The '380 provisional application is hereby incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60806380 | Jun 2006 | US |
