| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | August 3, 1999 |
| PATENT TITLE |
Chlorine dioxide generation for water treatment |
| PATENT ABSTRACT | Chlorine dioxide is generated by electrochemical oxidation of sodium chlorite in an anode compartment of a cation-exchange membrane-divided cell and is recovered in a suitable recipient medium by passing the chlorine dioxide through a hydrophobic microporous membrane. Water balance in a continuous operation is maintained by removing water from the anolyte by transporting the same partly across the hydrophobic microporous membrane in vapor form and partly across the cation-exchange membrane |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | March 13, 1996 |
| PATENT CT FILE DATE | May 12, 1995 |
| PATENT CT NUMBER | This data is not available for free |
| PATENT CT PUB NUMBER | WO94/26670 |
| PATENT CT PUB DATE | November 24, 1994 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What we claim is: 1. A method of removing at least one dissolved gas and water from an aqueous solution of the at least one gas, which comprises: contacting said aqueous solution with one face of hydrophobic microporous membrane, and providing a differential of partial pressure of both said at least one gas and water vapor between said aqueous solution and a recipient medium in contact with the opposite face of said hydrophobic microporous membrane, whereby both the at least one gas in gaseous form and water vapor pass through the membrane. 2. The method of claim 1 wherein said at least one dissolved gas is chlorine dioxide. 3. The method of claim 2 wherein said chlorine dioxide is formed by electrochemical generation from an aqueous solution of an alkali metal or alkaline earth metal chlorite. 4. The method of claim 3 wherein said alkali metal chlorite is sodium chlorite. 5. The method of claim 4 wherein said sodium chlorite solution is substantially free from impurities. 6. The method of claim 4 wherein said electrochemical generation is effected by feeding said aqueous solution of sodium chlorite to an anodic compartment of an electrochemical cell divided by at least one ion-permeable separator from a cathode compartment to which an aqueous electrolyte is fed. 7. The method of claim 6 wherein said aqueous solution of sodium chlorite present in said anodic compartment has substantially neutral pH. 8. The method of claim 6 wherein said electrochemical cell is a two-compartment cell and said ion-permeable separator is a cation-exchange membrane. 9. The method of claim 6 wherein said electrochemical cell is a three-compartment cell and said at least one ion-permeable separator comprises two cation-exchange membranes. 10. The method of claim 6 wherein said electrochemical cell is a three-compartment cell and said at least one ion-permeable separator comprises an anion-exchange membrane separating the anode compartment and a central compartment and a cation-exchange membrane separating the central compartment and the cathode compartment and said aqueous sodium chlorite solution is fed to said central compartment. 11. The method of claim 6 wherein backmigration of hydroxyl ions from said cathode compartment to said anode compartment occurs through said cation-exchange membrane and hydrogen ions are generated in said anode compartment to effect at least partial neutralization of said backmigrating hydroxyl ions as well as any hydroxyl ions introduced into said anolyte compartment with said sodium chlorite feed. 12. The method of claim 11 wherein said hydrogen ions are formed by electrochemical decomposition of water at the anode in accordance with the equation: 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e and the co-produced oxygen is vented from the anode compartment. 13. The method of claim 6 wherein a buffer is present in said anode compartment to effect neutralization of said backmigrating hydroxyl ions as well as any hydroxyl ions introduced into said anolyte compartment with said sodium chlorite feed and hydrogen ions produced in said anode compartment. 14. The method of claim 6 wherein sodium ions are transferred through the cation-exchange membrane into the cathode compartment and form sodium hydroxide therein, which is recovered from the cathode compartment. 15. The method of claim 6 which is carried out continuously by effecting said removal of chlorine dioxide from electrolyzed aqueous sodium chlorite solution, recycling electrolyzed sodium chlorite solution following said chlorine dioxide removal to said anodic compartment and feeding make-up sodium chlorite to said recycled sodium chlorite solution, and wherein water is removed from said electrolyzed aqueous sodium chlorite solution to maintain a substantially uniform concentration of aqueous sodium chlorite solution entering said anode compartment. 16. The method of claim 15 wherein said electrolyzed aqueous sodium chlorite having chlorine dioxide dissolved therein is passed from said anode compartment to a gaseous transfer zone comprising said hydrophobic microporous membrane dividing the zone into a first chamber through which said electrolyte aqueous sodium chlorite solution is passed, and a second chamber through which said chlorine dioxide recipient medium is passed, and said chlorine dioxide and water vapor are passed through said hydrophobic microporous membrane to said recipient medium. 17. The method of claim 16 wherein said electrolyzed aqueous sodium chlorite solution is hotter by from about 0.1.degree. to about 100.degree. C. than said recipient medium to facilitate said water vapor passage, to provide said partial pressure differential with respect to water vapor. 18. The method of claim 17 wherein said electrolyzed aqueous sodium chlorite solution has a temperature of about 5 to about 100.degree. C. and said recipient medium has a temperature of about 1.degree. to about 80.degree. C. 19. The method of claim 16 wherein the recipient medium is water requiring treatment by the passed chlorine dioxide. 20. The method of claim 16 wherein said hydrophobic microporous membrane is constructed of expanded polytetrafluoroethylene. 21. The method of claim 6 wherein sodium hydroxide produced in the cathode compartment is added to said recipient medium before or after receipt of chlorine dioxide therein. 22. The method of claim 6 wherein said aqueous electrolyte fed to the cathode compartment contains bicarbonate or carbonate ions or a mixture thereof and the product of cathodic reaction is enriched in carbonate ions . -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF INVENTION The present invention relates to an enviromentally-friendly, continuous process for the production of chlorine dioxide for water treatment applications in a very pure form, substantially free from contaminants, such as chlorine, chlorite, chlorate and chloride. BACKGROUND TO THE INVENTION In U.S. Pat. No. 4,683,039, assigned to the applicant and the disclosure of which is incorporated herein by reference, there is described the production of chlorine dioxide and its separation using gas membranes, i.e. microporous hydrophobic membranes which permit gas or vapor of chlorine dioxide to pass therethrough but resist the passage of liquid and ions therethrough. U.S. Pat. No. 2,163,793 describes an electrochemical chlorine dioxide generating process in which a mixture of alkali metal chlorite and alkali metal chloride is electrolyzed in an electrolytic cell equipped with a porous diaphragm separating the anode and the cathode compartments. British Patent No. 714,828 describes a process for the production of chlorine dioxide by electrolysing an aqueous solution containing chlorite and a water soluble salt of an inorganic oxy-acid other than sulfuric acid while U.S. Pat. No. 2,717,237 discloses a method for producing chlorine dioxide by electrolysis of chlorite in the presence of sulfate ions. Japanese Patent Publication No. 81-158883, published Dec. 7, 1981, describes an electrolytic process for producing chlorine dioxide by electrolysis of chlorite in which the electrolysed solution, at a pH of 2 or less, is fed to a stripping tank where air is introduced to recover the chlorine dioxide. U.S. Pat. No. 4,542,008 describes an electrolytic process for chlorine dioxide production in which the sodium chlorite concentration in the anolyte is monitored and controlled by means of a photometric cell. Published International patent application WO 91/09158 discloses a method of producing chlorine dioxide from chlorite in an ion exchange compartment of a multi-compartment cell in which hydrogen ions generated in the anode compartment enter the ion exchange compartment through a cation exchange membrane, causing chlorite ions decomposition to form chlorine dioxide and other by-products. Published International patent application WO 91/09990 teaches an electrochemical process for producing chlorine dioxide from dilute alkali metal chlorite solution in a single pass mode using a porous flow-through anode in which the unconverted chlorite together with other components of the anolyte constitute an effluent. A disadvantage of all of the above described electrolytic processes for the production of chlorine dioxide is that they are not suitable for a highly efficient, continuous, effluent-free operation in which all the components of the chlorite feed are safely and very efficiently removed with formation of an essentially pure chlorine dioxide, whereby no undesired or harmful by-products or contaminants are generated or accumulated. SUMMARY OF INVENTION In the present invention, there is employed a combination of highly efficient electrochemical oxidation of sodium chlorite to chlorine dioxide and membrane separation of the chlorine dioxide so produced in continuous production of chlorine dioxide. Accordingly, in one embodiment of the present invention, there is provided a method for the production of chlorine dioxide, which comprises electrochemically generating chlorine dioxide from an aqueous solution of an alkali metal chlorite, particularly sodium chlorite, and recovering the chlorine dioxide so produced by passing the chlorine dioxide through a hydrophobic, microporous membrane to a recipient medium. However, a broad aspect of the present invention provides a method of removing at least one dissolved gas and water from an aqueous solution of the at least one gas, which comprises contacting the aqueous solution with one face of hydrophobic microporous membrane; and providing a differential of partial pressure of both the at least one gas and water vapor between the aqueous solution and a recipient medium in contact with the opposite face of the hydrophobic microporous membrane, whereby both the at least one gas in gaseous form and water vapor pass through the membrane. BRIEF DESCRIPTION OF DRAWING FIG. 1 is a schematic flow sheet of a chlorine dioxide generator and utilization process provided in accordance with one embodiment of the invention. GENERAL DESCRIPTION OF INVENTION Electrochemical production of chlorine dioxide is effected in an electrochemical cell, divided by an ion-permeable separator, generally a cation-exchange membrane, into an anode compartment and a cathode compartment. The electrochemical reaction to form chlorine dioxide is effected in the anode compartment of the cell from a feed thereto of an aqueous solution of alkali metal chlorite, usually sodium chlorite. A suitable aqueous electrolyte is provided to the cathode compartment. Chlorine dioxide and water vapor are transferred to the recipient medium through a hydrophobic microporous membrane. The production of chlorine dioxide from sodium chlorite by electrochemical oxidation in the anode compartment in accordance with the equation: NaClO.sub.2 .fwdarw.ClO.sub.2 +e+Na.sup.+ (1) enables highly efficient production of chlorine dioxide to be achieved, essentially uncontaminated with chlorine. The use of a gas membrane process (or pervaporation as it is termed in the above-noted U.S. Patent) enables highly pure chlorine dioxide to be recovered from the cell liquor and transferred to a recipient medium, as desired. An efficient chlorite oxidation process preferably is carried out at approximately neutral pH. Since the typical technical product which can be used as a feed to the chlorine dioxide generation system, for example, about 37 wt % sodium chlorite solution, usually contains some sodium hydroxide and/or carbonate/bicarbonate as a stabilizer, it is beneficial to adjust the pH of the sodium chlorite feed accordingly. Another source of hydroxide ions that can be present in the anolyte is the so-called "backmigration" of hydroxyl ions originating in the cathode compartment of the electrochemical cell employed to effect reaction (1) and entering the anolyte through the cation exchange membrane or another ion-permeable separator. While a cation exchange membrane is, in principle, permeable only to cations, its anion rejection characteristics is usually not perfect, thus allowing a limited quantities of anions, such as hydroxyl ions to penetrate to the anode compartment. Some cation exchange materials are known to have a better anion rejection characteristics than others but in general a certain degree of hydroxyl ions backmigration can be anticipated. It is known, for example, that cation exchange membranes based on the composite of perfluorinated carboxylic and sulfonic cation exchange copolymers have a better selectively than the polymers equipped with sulfonic groups only. The former membranes, however, are much more sensitive to the presence of impurities, such as hardness forming ions, than the latter membranes. The anion rejection characteristics of the cation exchange material is important, not only from the viewpoint of the minimization of the hydroxyl ion backmigration to the anolyte but also with regard to potential contamination of the catholyte by the anionic species present in the anolyte, such as chlorite ions. The backmigration of hydroxyl ions to the anolyte may also be influenced by the concentration of such ions in the catholyte. The presence of excess alkali in the anolyte may lead to a well-known chlorine dioxide disproportionation reaction and should be avoided. A pH adjustment of the anolyte may be made by any convenient method, including an electrochemical method. Such electrochemical method is based on the occurrence of the oxygen evolution reaction at the anode: 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e (2) in addition to the primary electrode reaction depicted by equation (1). Since reaction (1) is thermodynamically more favourable than reaction (2), one possible method of enabling the occurrence of reaction (2) is to impose an anodic current density which cannot be fully sustained by reaction (1), i.e. by operating the process at a current density exceeding that corresponding to a mass transport limitation of reaction (1). The mass transport limitation of electrode reaction (1) is believed to be a function of such factors as chlorite ion concentration, anolyte composition, flow rate (velocity), presence of turbulence, temperature and anolyte viscosity. Relative contributions of reactions (1) and (2) to the overall current can also be modified, for example, by the pH of the anolyte and by the selection of the anode material. It is known that certain oxygen evolving anodes, such as DSA-O.sub.2 .RTM. anodes (trademark of ELTECH CORP.), are characterized by a lower oxygen evolution overpotential than other anode materials, such as, for example, graphite, thus facilitating the occurrence of reaction (2). The oxygen evolution characteristic of the anode is one of many factors to be considered for proper selection of the anode material. Other factors are, for example, the anode effects on the decomposition of chlorite ions and chlorine dioxide. Anode material facilitating such decomposition or oxidation of chlorine dioxide to chlorate ions should be avoided. When using a plurality of electrochemical cells it is possible to combine cells equipped with anodes made from different electrode materials. For example, some cells may be equipped with anodes promoting reaction (2), such as DSA-O.sub.2 .RTM. anodes, while other with anodes enabling a highly efficient (i.e. with no undesired by-product formation) reaction (1), such as graphite, which, on the other hand, may not be sufficiently stable under conditions of oxygen evolution reaction (2). The current density imposed may differ for different anode materials serving different objectives in the multi-cell assembly. For example, if oxygen evolution is to be avoided on graphite anodes, the current density on such anodes should not exceed that governed by mass transport limitations. The costs of the anode material should also be considered during optimization. There are numerous anode materials in various shapes and forms which may be considered for the proper balancing of reactions (1) and (2). Typical anode materials include not only above mentioned graphite (or any other carbon material, such as glassy carbon) and DSA-O.sub.2 .RTM. but also lead dioxide, platinum or other noble metal (both taken alone as well as connected to a suitable substrate), ruthenium dioxide on titanium (known e.g. as DSA-Cl.sub.2 .RTM.--Trademark of ELTECH CORP.), platinum/iridium on titanium, for example. The effective surface of the anodes may be enhanced, if required, by utilizing their three-dimensional structures, the latter being particularly useful for generation of chlorine dioxide from dilute anolyte streams. A method of preparing such structures has recently been described in U.S. Pat. No. 5,298,280. The extend to which reaction (2) is permitted to occur may be calculated based on the overall input of hydroxyl ions to the anolyte. Thus, for example, a more pronounced hydroxyl ions backmigration from the catholyte (resulting, for example, from a higher concentration of hydroxyl ions in the catholyte) would usually require, but not always, a higher anodic current density in order to increase the contribution of reaction (2). It is possible to employ a pH measurement in the anolyte as a feedback for the adjustment of current density. At a given anolyte composition (in terms of chlorite concentration) and for a given flow conditions, an upward trend of pH would trigger, in such case, an increased current density by either increasing the overall current at a given overall surface area of the anodes or, if an approximately constant production rate of chlorine dioxide is desired, the current may be maintained at a constant level while the overall surface area of the anodes is decreased (by, for example, switching off anolyte flow to the part of electrochemical cells comprising a multi-cell assembly). Alternatively, a feedback originating from the pH measurement of the anolyte may also lead to an automatic or manual adjustment of the anolyte composition (chlorite concentration), flow conditions, temperature etc. Similarly, a downward trend in pH may automatically trigger the required adjustments in current density, chlorite concentration, flow conditions, temperature etc. It is believed that lasting excursions of the anolyte pH to the acidic range may result in a decomposition of chlorite ions leading to the formation, in addition to chlorine dioxide, of at least one of the following impurities, namely chloride ions, chlorate ions and chlorine. Chlorine may originate, for example, from a secondary reaction involving electrooxidation of chloride ions at the anode. Such reaction may not be entirely undesired since chlorine is known to react rapidly with chlorite ions to form chlorine dioxide and chloride ions. Chlorine evolution characteristics of the anode is yet another factor to be considered for the proper anode selection. Such excursions, therefore, should generally be avoided. It is further believed that small, temporary excursions from the optimum pH of about 7 of the anolyte to either alkaline or acidic range are generally acceptable. The impact of such excursions is a function of such factors as chlorite concentration, chlorine dioxide concentration, temperature, time, etc. For example, the higher the chlorine dioxide concentration, the more pronounced would be its decomposition during pH excursions to the alkaline pH range. The excursions to the slightly acidic pH range are believed to be generally more acceptable than those to the alkaline pH range. It has been reported in the literature that the occurrence of the undesired electrooxidation reaction of chlorine dioxide to chlorate can be minimized by maintaining the pH of the anolyte in the slightly acidic range. Since hydroxyl ions originating from the catholyte and entering the anolyte through the cation exchange membrane as a result of the backmigration may create a strongly alkaline boundary layer on the surface of the membrane facing the anode and since the compensating (neutralization) reaction producing hydrogen ions (reaction 2) takes place at the anode, which may create a strongly acidic boundary layer in the proximity of the anode which is located at a certain, finite distance from the alkaline boundary layer, it may be beneficial to create conditions facilitating the neutralization reaction between the hydroxyl and hydrogen ions in order to minimize the occurrence of possible undesired reactions involving, for example, chlorite ions in a localized acidic medium or chlorine dioxide in a localized alkaline medium. It is possible, for example, to improve the mixing within the anode compartment by improving the flow characteristics and employing turbulence promoters. Any suitable pH buffer, such as phosphate buffer, phthallate buffer, citrate buffer or a combination thereof, may be employed in order to moderate the pH changes within the anodic compartment. Such buffer should be characterized by a sufficient buffering capacity in terms of its ability to "absorb" hydroxyl ions and hydrogen ions without introducing any kinetic limitations on the neutralization process. More concentrated buffers are generally known to have higher buffering capacity. The effect of an alkaline boundary layer in the proximity of the surface of the cation exchange membrane facing the anode can also be minimized by employing a slightly acidic anolyte, if desired. Accordingly, in a further aspect of the invention, there is provided an electrochemical process in an anode compartment of an electrochemical cell divided by at least one ion-permeable separator from a cathode compartment, which comprises feeding a first electrolyte to the anode compartment and effecting electrolysis of the electrolyte to form hydrogen ions; feeding a second electrolyte to the cathode compartment and effecting electrolysis of the electrolyte to form hydroxyl ions, at least some of the hydroxyl ions backmigrating across the at least one ion-permeable separator into the anode compartment; and providing a buffer in the anode compartment to neutralize both the electrochemically-produced hydrogen ions and the back-migrating hydroxyl ions as well as the hydroxyl ions which may be introduced to the anode compartment with the first electrolyte feed. While reaction (2) is a preferred reaction generating the necessary hydrogen ions required for the neutralization of hydroxyl ions entering the anolyte loop, some other electrochemical reactions may also be employed. An example of an electrochemical reaction resulting in the generation of hydrogen ions without a formation of any persistent byproducts is the electrooxidation of hydrogen peroxide. H.sub.2 O.sub.2 .fwdarw.2H.sup.+ +2e+O.sub.2 (3) Alternatively, metal peroxides or superoxides, preferably sodium peroxide, may be employed to substitute hydrogen peroxide in reaction (3). There are many other reagents, in particular organic compounds, which may generate hydrogen ions during oxidation without co-production of any persistent by-products. Preferably, simple organic compounds such as methanol or other simple alcohols, aldehydes, ketones, acids or their combination can be employed whereby the electrooxidation reaction results in the formation of hydrogen ions and carbon dioxide. An example of such reaction is depicted by equation (4): organic compound+water.fwdarw.pCO.sub.2 +nH.sup.+ +me.sup.-(4) where the coefficients p, n and m are based on the stoichiometry of reaction (4). Carbon dioxide generated in reaction (4), if not stripped from the reaction medium, may form bicarbonate or carbonate ions which could comprise a persistent impurity. Nitrogen based reagents such as ammonia, urea, hydrazine and hydroxylamine may also be employed, whereby the electrooxidation reaction preferably produces nitrogen gas and hydrogen ions. While it is preferred to operate the process of the invention continuously without generating any undesired effluent or by-products and thus it is beneficial to employ a hydroxide neutralization method which fulfills such requirements, it is still possible to utilize a conventional acid or acid anhydride (such as sulfuric or phosphoric acid or carbon dioxide, the latter being equivalent to carbonic acid) addition, especially if the build-up of persistent contaminants, such as sulfate, phosphate, bicarbonate or carbonate (or other ions, if a different acid or acid anhydride is employed), is counteracted by periodic removal of impurities by any convenient method, such as bleed-out, precipitation, chemical decomposition and membrane separation. Such approach may be particularly acceptable when a non-continuous, batch-wise operation is employed. Any of the above mentioned methods also can be employed to counteract the build-up of impurities in a continuous operation involving a preferred, electrochemical method of hydroxyl ions neutralization. Even during such operation some of the undesired contaminants, such as chlorate ions, may be formed or may be added with the chlorite feed and hence a suitable method for the anolyte purification may be incorporated into the process, if necessary. Since the typical, commercially available sodium chlorite usually may contain, in addition to the previously mentioned hydroxide, small quantities of various contaminates, such as carbonate, chlorate, sulfate and chloride, which may accumulate to unacceptable levels during a prolong, continuous operation, it is beneficial to modify the sodium chlorite manufacturing process in order to produce a sufficiently pure substrate intended for use as a feed to the chlorine dioxide generation process. Alternatively, the commercially available sodium chlorite may be subjected to a suitable purification prior to being use as a feed. While various above-described methods for pH adjustment, specifically the electrochemical method depicted by reactions (2) to (4), have been disclosed in the context of the electrooxidation of chlorite ions to chlorine dioxide, their applicability is not limited to this process only and can be extended to any electrochemical process in which the proper balancing of pH is required. Accordingly, in another aspect of the invention, there is provided an electrochemical process in an anode compartment of an electrochemical cell divided by at least one ion-permeable separator from a cathode compartment to which an aqueous electrolyte is fed, which comprises effecting electrochemical oxidation of an aqueous salt solution in the anode compartment while transferring the cation species of the salt to the cathode compartment; and simultaneously effecting electrochemical acidification of the aqueous salt solution to effect pH constancy of the aqueous salt solution. An analogous approach can also be applied to electroreduction reactions when the primary electrochemical reaction occurs at the cathode and when the pH balancing can be achieved by, for example, electrogeneration of hydroxyl ions at the cathode to any desired degree. Again, no persistent impurities are accumulated during such operation which is of great importance for continuous environmentally-friendly processes. A major problem associated with operating the chlorine dioxide generation process according to the invention and depicted by equation (1) on a continuous basis is maintaining a proper water balance. For example, using a concentrated (37 wt %) sodium chlorite solution as a feed introduces water to the electrochemical cell at a mole ratio of NaClO.sub.2 :H.sub.2 O of about 1:9. Under conditions of a 100% efficient sodium chlorite electrooxidation process effected in a two-compartment compartment cell equipped with a cation-exchange membrane to produce chlorine dioxide, approximately 2 to 5 moles of water is transported through the membrane to the cathode compartment with each mole of Na.sup.+ ions, which corresponds to 2 to 5 moles of water removed from the anolyte for each mole of ClO.sub.2 produced. However, to maintain the proper water balance required for a continuous operation of the chlorine dioxide generation process, an additional 4 to 7 moles of water per mole of chlorine dioxide produced must be removed from the anolyte. The amount of water to be removed from the anolyte may be higher if a less concentrated feed of sodium chlorite is used. The amount of water also may be lower, for example, if crystalline sodium chlorite is employed and the amount of water fed to the system is smaller. In accordance with the present invention, the further volume of water required to be removed is removed by a gas membrane process, similar to that used for separation of chlorine dioxide from cell liquor. While the hydrophobic microporous membrane used in this process does not allow liquid water transport, it has been found that, it is possible to transfer water through this membrane in a vapor form. Accordingly, the chlorine dioxide-containing anolyte from the electrochemical oxidation of sodium chlorite is maintained at a positive water vapor pressure gradient with respect to the chlorine dioxide receiving solution (i.e. the water treated by the chlorine dioxide transferred from the anolyte) by employing, for example, a higher temperature of the chlorine dioxide donating solution, in order to effect transfer of water vapor through the gas membrane along with the chlorine dioxide. The temperature differential between the donor and recipient medium to facilitate the water vapor passage may vary from about 0.1.degree. to about 100.degree. C., preferably about 1.degree. to about 50.degree. C., to facilitate the transfer of water vapor. Accordingly, the donor medium may have a temperature from about 5 to about 100.degree. C., preferably about 15 to about 80.degree. C. while the recipient medium may have a temperature of about 1 to about 80.degree. C., preferably about 5 to about 40.degree. C. A pressure differential may be used alternatively to or in conjunction with the temperature differential to provide the vapor transfer driving force. The composition of the chlorine dioxide donating solution may affect the water vapor gradient thus influencing the effectiveness and the rate of water transfer. Other important factors to consider in this respect is the flow characteristics of both the donating and receiving solutions and their temperatures. The hardware design of the gas transfer module equipped with the gas membrane plays a very important role in ensuring a proper flow characteristics by allowing a desired flow rate (velocity) and turbulence. The removal of water from the chlorine dioxide generator, therefore, is effected by a combination of two membranes, one a cation-exchange membrane in the electrolysis step and the other a gas membrane, to maintain continuous chlorine dioxide production for water treatment. While it is usually beneficial to employ a common gas membrane for both chlorine dioxide and water vapor transfer, it is also possible to utilize separate gas transfer modules for each of the processes, whereby membrane characteristics may be optimized according to the requirements of each process. Alternative water removal procedures may be adopted, if desired, such as membrane distillation, reverse osmosis, and evaporation. A combination of various methods may also be employed. It is further believed that analogously to the case of water vapor transfer where the water vapor pressure gradient is considered to be the main driving force for such process, the transfer of chlorine dioxide between the donating and receiving solutions is also governed by a similar driving force, i.e. the chlorine dioxide vapor pressure differential. Similarly, as in the case of water vapor transfer, several factors should be considered during the optimization of the chlorine dioxide transfer. The chlorine dioxide vapor pressure and gradient (and resulting flux and transfer rate) can be manipulated by adjusting chlorine dioxide concentration, chlorite concentration and flow characteristics (flow rate, velocity and turbulence) of both donating and receiving solutions as well as the temperatures of both the solutions. Addition of any suitable salt, such as sodium chloride or sulfate as well as pH buffers may also affect the vapor pressure of both chlorine dioxide and water. The gas membrane material used in the gaseous transfer steps may affect the effectiveness and the rate of the gas transfer. The important features to be considered in this respect are the hydrophobicity of the microporous material, pore size, thickness, chemical stability towards the attack of chlorine dioxide, chlorine, chlorite, chlorate, chloride, acid and base, a so-called "bubble point" (related to a maximum pressure differential between the donating and receiving solutions that can be attained before the contact between both solutions is effected, leading to highly undesired contamination of the receiving solution by the ions present in the donating solution). Examples of such materials and their characteristics have been described in the aforementioned U.S. Pat. No. 4,683,039. One material specifically recommended in the above mentioned U.S. Patent is expanded polytetrafluoroethylene, which is commercially-available under the trademark "GORE-TEX". Another material which exhibits a satisfactory performance is known as polyvinylidenefluoride (PVDF). Other materials may also be employed. However, at this time, the polytetrafluoroethylene material appears to have a superior performance, especially in terms of its chemical resistance towards the attack of strong oxidizers, such as chlorine dioxide, chlorine and chlorite ions. The hydrophobic microporous materials can be manufactured in various forms, such as sheets, hollow fibers, tubes and spirals and sealed in the appropriate modules. It is possible to design a module in which an anodic compartment in an electrochemical cell is adjacent to the gas membrane so that the donor compartment of the gas membrane unit and the anode compartment of the electrochemical cell are combined into one, common chamber. If desired, the content of the cathodic compartment of the electrolytic chlorine dioxide production, comprising mainly aqueous alkali, may be added to the recipient medium, before or after chlorine dioxide treatment. The relatively small volume of catholyte effluent compared to volume of water treated should result in an insignificant change in pH in the treated water. Since the produced hydroxide has many applications in the water treatment facilities, the production of hydroxyl ions by an electroreduction of water is a preferred cathodic reaction. However, any other suitable cathodic reaction may be employed, for example, the electroreduction of oxygen, resulting in a mixture of perhydroxyl and hydroxyl ions and at the same time allowing a lower cell voltage. Perhydroxyl produced in this reaction may be used as a disinfecting agent in water treatment (separately or together with chlorine dioxide) or other suitable application. The catholyte may comprise also an acidic electrolyte, such as sulfuric acid, sulfurous acid, phosphoric acid, carbonic acid, hydrochloric acid or any other acid, the corresponding salts or their mixtures with acids and/or acid anhydrides. The selection of the cathode material depends on the nature of the catholyte. In general, the cathode materials suitable for alkaline medium, such as, mild steel or nickel, may not be sufficiently resistant against the corrosive attack of acidic electrolytes. In the latter case, the cathode materials such as graphite, lead, lead dioxide, Ebonex.RTM., titanium, tantalum, zirconium, various metalloids, such as metal carbides or nitrides, as well as various noble metals or noble metals coated electrodes, may be employed. Various pH buffers, such as phosphate, citrate, borate, phthallate, carbonate, acetate, ammonium or other buffers, may also be employed as catholyte solutions. Maintaining the pH of the catholyte below the strongly alkaline range corresponding to the presence of free hydroxide may be beneficial having regard to the minimization of the previously mentioned backmigration of the hydroxyl ions to the anolyte. Since in most cases the primary reaction occurring at the cathode generates hydroxyl ions via decomposition of water or corresponds to the cathodic discharge of hydrogen ions present in the acidic catholyte, the pH of the catholyte may increase in the course of electrolysis. In such case, it may be beneficial to maintain the pH of the catholyte approximately constant by employing any suitable method, such as acid or acid anhydride (e.g. carbon dioxide) addition, by dilution, by chemical decomposition or by precipitation. The content of the catholyte loop may be continuously or periodically discharged in any suitable manner, either to the sewage or to the treated water. For example, by employing a catholyte containing bicarbonate, carbonate or their mixtures, it is possible to co-produce a very useful by-product, such as soda ash, which is readily applicable in the water treatment facilities, while, at the same time, maintaining the catholyte pH below the strongly alkaline range, so that the concentration of free hydroxyl ions and hence their back-migration to the anolyte is minimized. The rejection of both bicarbonate and carbonate ions by the cation exchange membrane is usually much more efficient than that of hydroxyl ions. The pH of the bicarbonate/carbonate mixture can easily be adjusted by either continuous or periodic addition of carbon dioxide or other acid/acid anhydride. It is also possible to employ a continuous, single-pass (or with recirculation) cathodic process in which bicarbonate or bicarbonate/carbonate mixture is fed to the cathode compartment and the product enriched in carbonate is continuously withdrawn from the compartment. While using an acidic catholyte, it is possible to utilize the effect of hydrogen ions "leak" to the anolyte through the cation exchange membrane. The extent of the hydrogen ion "leak" may be adjusted to match the quantity of hydroxyl ions entering the anolyte loop with the sodium chlorite feed. The cathodic reaction may result in the co-production of other useful by-products. For example, a catholyte containing bisulfite or sulfite ions or their mixtures with sulfur dioxide may be utilized for the co-production of sodium dithionite, which is a known bleaching agent. It is also possible to utilize a special cell design in which the distance between the cathode and the cation exchange membrane is minimized (a so-called "zero gap" approach), so that a very dilute electrolyte, even water containing very small quantities of ions, resulting for example, from the dissociation of the carbonic acid being in equilibrium with the naturally present, dissolved carbon dioxide, may be employed without requiring an unacceptably high cell voltage. Small additions of any additive which produces ions in water, i.e. acids, bases, salts, acid anhydrides, oxides etc., may be employed, if desired, in order to improve the conductivity of the catholyte. A so-called "single pass" continuous operation may be employed, whereby the catholyte is not recycled, thus allowing for simplification of the equipment required. It is possible, for example, that the treated water be subjected to cathodic reaction in a once-through system without being recycled. Similarly to the "zero gap" approach, a so-called solid polymer electrolyte (SPE) configuration also may be employed, whereby the cathode and/or the anode or both remains in an intimate contact with the cation exchange membrane. Alternatively to an electrolytic cell equipped cation exchange membrane, a yet another approach can be taken in which the catholyte and the anolyte compartments are separated by means of a conducting ceramic material which preferably allows a specific ionic transfer of sodium ions. Numerous examples of such materials are described in the prior art, e.g. beta-alumina or various ceramic materials described in U.S. Pat. No. 5,290,405. Selectivity of the ceramic material towards the sodium ion transfer is required when sodium chlorite is employed as a source of chlorite ions in the anolyte. However, the utilization of other chlorites, preferably alkali metal or alkali earth metal chlorites, is also possible and, in such a case, the selection of an appropriate ceramic separator allowing an ionic transfer of a desired metal ion may be required. While a two-compartment electrolytic cell equipped with one separator between the anode and the cathode compartment is preferred for effecting the electrooxidation of chlorite ions to chlorine dioxide, a multicompartment cell containing at least one additional compartment between the cathode and anode compartments also may be employed. A centre compartment so-provided is preferably separated from the adjacent cathode compartment by means of a cation exchange membrane. The separation of the centre compartment from an adjacent anode compartment may preferably be effected by means of either a cation-exchange membrane or an anion-exchange membrane. In the latter case, the sodium chlorite feed should be directed to the centre compartment from which the chlorite ions and sodium ions resulting from the dissociation of the sodium chlorite are transferred through the anion exchange membrane to the anode compartment and through the cation exchange membrane to the cathode compartment, respectively. The anion-exchange membrane selected for such three-compartment cell should preferably be resistant to the attack of strong oxidizers, such as chlorine dioxide, chlorine and chlorite. When the three-compartment cell is equipped with two cation exchange membranes the sodium chlorite feed should be directed to the anode compartment, while the centre compartment can be utilized as a buffering compartment preventing penetration of the anode compartment by hydroxyl ions generated in the cathode compartment. Elimination of the hydroxyl ion back-migration to the anode compartment by employing a multi-compartment cell allows to increase the contribution of reaction (1) to the overall current, but at the same time, it is associated with an increased cell voltage and higher cost of the equipment required. The optimization of the overall process comprising two basic operations, i.e. chlorine dioxide electrogeneration and chlorine dioxide and water vapor transfer through the gas membrane requires careful balancing of process parameters which may differently affect each of above mentioned operations. For example, the steady state concentration of chlorine dioxide in the anolyte/donating solution, while having positive effect on the rate of gas transfer through the gas membrane, may accelerate chlorine dioxide decomposition reactions to undesired by-products. Chlorine dioxide concentrations may typically be varied between 0.01 gram per liter and 20 grams per liter, preferably 1 to 10 grams per liter. The chlorine dioxide concentration in the receiving solution will typically be lower than in the donating solution. Chlorite ion concentration in the anolyte/donating solution may affect both the electrochemical cell performance and the gas membrane module operation in several ways. In general, higher chlorite ion concentration results in an increased partial pressure of chlorine dioxide while decreasing the partial pressure of water vapor, thus enhances the transfer rate of chlorine dioxide while decreasing the transfer rate of water vapor. Increased chlorite ion concentration also improves the conductivity of the anolyte thus resulting in a reduction of the cell voltage. This latter effect, i.e. an increase of conductivity may also be achieved by an addition of any suitable electrolyte to the anode compartment. Such electrolyte may be either electrochemically inert, such as, sodium sulfate, nitrate, carbonate, phosphate, perchlorate, etc, or electroactive, such as sodium chloride. Since the mass transport limited current corresponding to reaction (1) is proportional to chlorite ion concentration, higher current densities are generally easier to attain for more concentrated anolytes. On the other hand, however, the decomposition rate of chlorite ions is generally enhanced by increasing chlorite ion concentration. It is believed that the decomposition of chlorite ion is usually preceded by its protonation in which an unstable chlorous acid intermediate is formed. It is further believed that the source of chlorous acid may be the well-known hydrolysis reaction of chlorite shown in the following equation : ClO.sub.2.sup.- +H.sub.2 O.revreaction.HClO.sub.2 +OH.sup.-(5) The pH of resulting solution can be approximated by the following equation: pH=7+1/2 pKa+1/2 log C (6) where Ka denotes the dissociation constant of chlorous acid (pKa approx. 2) and C denotes the molar concentration of chlorite ions. Using formula (6), it can be shown that, at even moderately concentrated (>1 molar) chlorite ion solution, the "natural" pH is higher than 8. An adjustment of pH to a lower value, such as pH=7, would trigger an approximately ten fold increase in the equilibrium concentration of chlorous acid, which, in turn, may result in an increase of the undesired decomposition of chlorite in which at least one of chlorate ions and chloride ions are formed along with some chlorine dioxide and chlorine. The higher the chlorite ion concentration the more decomposition can be expected upon the adjustment of pH to approximately neutral. The concentration of chlorite ions may also affect the extent of the formation of a complex compound between chlorite ion and chlorine dioxide. The "natural" pH value for solutions containing a lower chlorite ion concentration is closer to 7 and hence less decomposition can be anticipated under such conditions. In general, the steady state concentration of chlorite ions should be optimized based on the proper balance of all the above described effects while maintaining the rate of the chlorine dioxide formation corresponding the chlorite oxidation current density at a commercially-acceptable level. The current density standardized on the membrane area is usually in the range between about 0.5 and about 10.0 kA/m.sup.2, preferably between about 1 and about 4 kA/m.sup.2. Lower concentrations of chlorite ions do not necessarily translate into lower current density attainable, since there are other variables available to manipulate its value. For example, high surface area anodes may be employed for processing of dilute chlorite ion solutions, whereby the conductivity of the anolyte may optionally be enhanced by an addition of any suitable electrolyte. A mass transport limited current may also be varied by manipulating the flow characteristics of the anolyte. When employing a dilute chlorite ion solution as an anolyte, it may be economical to operate the process in a "single pass" mode, whereby the anolyte is not recycled or, alternatively, is recycled only as long as the level of impurities accumulated during the operation is acceptable. Another parameter that requires optimization is temperature. While higher temperatures generally enhance the gas transfer rates as well as reduce the cell voltage, they may also facilitate the rate of undesired decomposition reactions involving either chlorine dioxide or chlorite ions or a combination thereof. Higher temperatures generally also increase the conductivity, thus lowering the cell voltage. Yet another variable to be considered during the process optimization is the flow characteristics of the anolyte/donating solution as compared to the flow characteristics of the receiving solution. While it may generally be beneficial to increase the velocity of the anolyte/donating solution from the viewpoint of the mass transport limited current density achievable as well as an enhancement of the gas transfer rate, there may be some limitations associated with gas membrane ability to sustain higher pressures resulting from higher velocities ("bubble point"). DESCRIPTION OF PREFERRED EMBODIMENT Referring to the drawing, concentrated sodium chlorite solution is fed by line 10 to a feed tank 12 wherein the sodium chlorite is mixed with recirculated cell liquor to form a sodium chlorite feed stream which is fed by line 14 to an anode compartment 16 of an electrolytic cell 18. The cell 18 comprises a cathode compartment 20 separated from the anode compartment 16 by a cation-exchange membrane 22. In the anode compartment 16, the sodium chlorite undergoes electrolytic oxidation to form chlorine dioxide, while sodium ions are transferred, along with some water, from the anode compartment 16 to the cathode compartment 20 through the cation-exchange membrane 22. An aqueous catholyte is fed to the cathode compartment 20 by line 24, resulting in the formation of an aqueous sodium hydroxide effluent stream in line 26. Alternatively, the sodium hydroxide stream may be recirculated through the cathodic compartment. During the recirculation, any desired concentration of sodium hydroxide may be achieved and maintained, for example, by adding water, as required. The aqueous solution of chlorine dioxide in the anolyte effluent is removed from the anode compartment 16 and forwarded by line 28 to a gas membrane unit 30. The gas membrane unit 30 contains a microporous, hydrophobic gas membrane 32 dividing the interior of the unit into two compartments 34 and 36 and which permits gaseous vapors to transfer therethrough under suitable driving force. The chlorine dioxide-containing solution in line 28 is received in compartment 34 of the gas membrane unit 30 while water to be treated is fed by line 38 to the other compartment 36. In view of the difference in the partial pressure of chlorine dioxide on the two sides of the gas membrane, chlorine dioxide is transferred from the chlorine dioxide-containing solution in compartment 34 to the water in compartment 36 by gaseous phase transfer through the membrane 32 so as to treat the water, removed by line 40. While the invention is particularly useful for the treatment by chlorine dioxide of water for consumption, the invention includes the use of chlorine dioxide for the treatment of any aqueous medium, including sewage and other waste water treatments. In addition, the chlorine dioxide may be used in the treatment of non-aqueous medium, for example, in tallow bleaching. Generally, the present invention may be employed in any disinfection, bleaching, sterilization or oxidation application involving chlorine dioxide. The aqueous phase in compartment 34 preferably is maintained at a higher temperature or pressure than the water in compartment 36 to provide a vapor pressure driving force, as a result of which water vapor is transferred along with the chlorine dioxide through the gas membrane 32 from compartment 34 to compartment 36 (gas phase ClO.sub.2, vapor phase delivery). The residual aqueous phase in compartment 34 containing unreacted chlorite and residual chlorine dioxide is recycled by line 42 to the feed tank 12 for mixing with concentrated sodium chlorite feed in line 10 and the process is repeated. The aqueous alkaline solution in line 26 may be added to the treated water in line 40. The drawing illustrates a continuous process of effecting water treatment using pure chlorine dioxide, in which steps are taken to effect water extraction from the chlorine dioxide generating process. As mentioned above, using a concentrated (37 wt %) solution feed of sodium chlorite, about 9 moles of water need to be removed for each mole of chlorine dioxide produced. This water removal is effected by transfer through the cation-exchange membrane 22, generally in an amount of 2 to 5 moles, and by transfer through the gas membrane 32, in an amount of about 4 to 7 moles, or more or less, as required. Alternatively, vacuum or partial vacuum can be applied to the receiving side in order to facilitate the transfer of chlorine dioxide gas and water vapour. Such a system also may be employed to deliver gaseous chlorine dioxide, optionally in mixtures with water vapour and other diluting gases, such as nitrogen-oxygen or air, if required. A delivery system producing gaseous chlorine dioxide may have application not only to water treatment but also in other areas, such as in the bleaching of pulp, where gaseous chlorine dioxide can be delivered directly to the treated pulp without being previously dissolved in water. While the two-step removal of water employed in the present invention has been described with respect to the generation of chlorine dioxide electrolytically from sodium chlorite, the principles hereof are applicable to other chlorine dioxide generating processes, for examples, those based on chlorate ion reduction, wherein the chlorate ions originate from chlorate salts, chloric acid and mixtures thereof. Further, while the present invention has been described with respect to the simultaneous transfer of chlorine dioxide and water vapor to a recipient medium, the principles hereof are applicable to the transfer of other gases, such as chlorine monoxide, hypochlorous acid or chlorine, along with water vapor to a recipient medium. Gases, such as carbon dioxide, oxygen or hydrogen, which may be formed during the operation of the process of the invention can also be removed from the donor medium to the recipient medium, if desired. |
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