EP1973842A2 - Chlorine dioxide generator - Google Patents

Abstract

A chlorine dioxide gas generator includes a chlorine dioxide gas source and a cooling system for cooling a chlorine dioxide gas obtained from the chlorine dioxide gas source. An electrochemical chlorine dioxide generator includes an anolyte loop and a catholyte loop that is fluidly connected to a neutralization system. The neutralization system can neutralize waste products from the anolyte loop or the catholyte loop. A chlorine dioxide solution generator includes a chlorine dioxide gas source capable of producing a chlorine dioxide gas. The generator further includes an eductor system for effecting the dissolution of the chlorine dioxide gas into an aqueous liquid stream. The eductor system is fluidly connected to the chlorine dioxide gas source. The generator further includes an absorption system fluidly connected to the eductor system capable of effecting additional dissolution of the chlorine dioxide gas into the aqueous liquid.

Description

CHLORINE DIOXIDE GENERATOR

Cross-Reference to Related Application(s)

[0001] This application claims priority to the following pending patent applications: (a) U.S. Patent Application Serial No. 11/289,813 filed on November 30, 2005, entitled "High-Capacity Chlorine Dioxide Generator"; (b) U.S. Patent Application Serial No. 11/418,741 filed on May 4, 2006, entitled "Neutralization System for Electrochemical Chlorine Dioxide Generators"; and (c) U.S. Patent Application Serial No. 11/548,611 filed on October 11, 2006, entitled "Chlorine Dioxide Solution Generator". The '813, 741 and '611 applications are each hereby incorporated by reference herein in their entirety.

Field of the Invention

[0002] The present invention relates generally to chlorine dioxide generators. More particularly, the present invention relates to chlorine dioxide generators capable of operating at high currents and also capable of enhanced operational reliability and waste product handling.

Background of the Invention

[0003] Chlorine dioxide (ClO₂) has many industrial and municipal uses. When produced and handled properly, ClO₂ is an effective and powerful biocide, disinfectant and oxidizer. [0004] ClO₂ is used extensively in the pulp and paper industry as a bleaching agent, but is gaining further support in such areas as disinfection in municipal water treatment. Other end-uses include disinfection in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfection of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry and uses in the oil and gas industry.

[0005] In water treatment applications, ClO₂ is primarily used as a disinfectant for surface waters with odor and taste problems. It is an effective biocide at low concentrations and over a wide pH range. ClO₂ is desirable because when it reacts with an organism in water, chlorite results, which studies to date have shown does not pose a significant adverse risk to human health at a concentration of less than 0.8 parts per million (ppm) of chlorite. The use of chlorine, on the other hand, can result in the creation of chlorinated organic compounds when treating water. Such chlorinated organic compounds are suspected to increase cancer risk.

[0006] Producing ClO₂ gas for use in a chlorine dioxide water treatment process is desirable because there is greater assurance of ClO₂ purity when in the gas phase. ClO₂ is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl₂), oxygen gas (O₂) and heat. The high reactivity OfClO₂ generally requires that it be produced and used at the same location. ClO₂ is, however, soluble and stable in an aqueous solution.

[0007] ClO₂ can be prepared a number of ways, generally via a reaction involving either chlorite (ClO₂ ^") or chlorate (Clθ₃ ^~ ) solutions. The ClO₂ created through such a reaction is often refined to generate ClO₂ gas for use in the water treatment process. The ClO₂ gas is then educed into the water selected for treatment. Eduction occurs where the ClO₂ gas, in combination with air, is mixed with the water selected for treatment.

[0008] The production of ClO₂ can be accomplished both by electrochemical and reactor-based chemical methods. Electrochemical methods have an advantage of relatively safer operation compared to reactor-based chemical methods. In this regard, electrochemical methods employ only one precursor, such as a chlorite solution, unlike the multiple precursors that are employed in reactor-based chemical methods. Moreover, in reactor-based chemical methods, the use of concentrated acids and chlorine gas poses a safety concern.

[0009] Electrochemical cells are capable of carrying out selective oxidation reaction of chlorite to ClO_2. The selective oxidation reaction product is an solution containing dissolved ClO₂ and residual reactants. To further purify the ClO₂, the ClO₂ gas is separated from the solution using a stripper column. The solution is sprayed at the top of the stripper column while air flows in a counter current direction. The ClO₂ that is in solution exchanges from solution to air at a solution-air interface.

[0010] Suction of ClO₂ gas and air from the stripper column can be accomplished using an eductor or a vacuum gas transfer pump. However, the use of a traditional eductor system to deliver a ClO₂ solution directly to a pressurized water system can raise reliability concerns as described in PCT International Publication Number WO 2006/015071 published on February 9, 2006 entitled "Chlorine Dioxide Solution Generator." A vacuum gas transfer pump can alternatively be employed. Electrolytic cells can, however, have increased maintenance issues for vacuum gas transfer pumps as the ClO₂ gas generation rate increases. For instance, as the current increases in an electrochemical ClO₂ generator, more vacuum gas transfer pumps between the stripper column and the absorption loop may be needed as ClO₂ gas production increases.

[0011] Unlike vacuum gas transfer pumps, which have moving parts, eductors operate on a Venturi principle where a liquid is forced through a nozzle at a high velocity to create a pressure drop without moving parts. Eductors consist of two basic components: a motive nozzle for converting pressure energy to kinetic (velocity) energy, and a suction chamber where entrainment and mixing may occur. Thus, the use of an eductor in a ClO₂ gas generation system typically increases system reliability over the use of a vacuum gas transfer pump. However, the gas suction rate of an eductor depends on the differential pressure between the inlet and outlet water pressure. Depending on the end-use application for a ClO₂ solution, discharge pressures varying from 0 psig to 200 psig (101 kPa to 1,480 kPa) can be encountered. As the discharge pressure varies for the end-use application, the differential pressure in the eductor will also vary and cause changes in the air suction rate. Changes in the air suction rate lead to varying concentrations of ClO₂ in air instead of a desired ratio of ClO₂ to air that is relatively constant. Thus, the use of an eductor system to directly feed a ClO₂ solution to a pressurized water system can lead to decreased generator reliability.

[0012] Electrochemical ClO₂ generators can be utilized to obtain higher yields of ClO₂ gas or ClO₂ solution by applying more current to the electrochemical cell. Applying a greater current to the cell increases the rate of the selective oxidation reaction of chlorite to ClO₂, which results in a higher yield of ClO₂ gas. A higher yield of ClO₂ gas ultimately results in a higher yield of ClO₂. However, when more current is applied to the electrochemical ClO₂ generator cell to increase the production of ClO₂ gas, more heat is generated in the electrolytic cell anolyte loop. It is known that ClO₂ is unstable and capable of decomposing, in an exothermic reaction, to chlorine and oxygen. Due to this instability, an operating temperature greater than about 163⁰F (73⁰C) can result in potentially hazardous and less efficient operation of the ClO₂ generator. [0013] While electrochemical generators are suited for generating chlorine dioxide, such generators produce undesirable waste products. These waste products are produced both in the anolyte and catholyte loops of the electrochemical generator. Anolyte waste products typically result from residual reactants, side reaction products, and chlorine dioxide that remains unstripped from the stripper column. Other anolyte impurities can include sodium chlorite, sodium chlorate, sodium chloride, sodium sulfate, hypochlorous acid and chlorine dioxide. The operation of the catholyte loop in an electrochemical chlorine dioxide generator also produces caustic soda (that is, sodium hydroxide). Although sodium hydroxide can be used in other reactions, it can be more economical to dispose of it when catholyte production is low.

[0014] It would be desirable to provide a ClO₂ generator capable of operating at varying gas generation rates including at high currents for an electrochemical ClO₂ generator. It would further be desirable to provide a ClO₂ generator than can reliably deliver a ClO₂ solution into an end-use system that is pressurized. Moreover, it would be desirable to provide effective neutralization of waste products from the anolyte and catholyte loops of an electrochemical ClO₂ generator. Suinmarv of the Invention

[0015] The present chlorine dioxide generator includes an anolyte loop for generating chlorine dioxide gas and a cooling system connected to the anolyte loop.

[0016] In one embodiment, the cooling system is made up of an inner tube through which reactant feedstock or chlorine dioxide solution is directed, an outer jacket surrounding the inner tube and a coolant material within the outer jacket. This cooling system can be interposed between a reactant feedstock stream and an electrochemical cell that is fluidly connected to that reactant feedstock stream such that the reactant feedstock is directed through the cooling system. Alternatively this cooling system could also be interposed between an electrochemical cell having a positive end and a negative end and a stripper column that is fluidly connected to the electrochemical cell such that a chlorine dioxide solution directed from the positive end of the electrochemical cell is directed through the stripper column. Alternatively this cooling system could be interposed between the negative end of an electrochemical cell and a byproduct tank.

[0017] In another embodiment, the cooling system is made up of a coiled tube placed within the chlorine dioxide gas generator and a coolant material within the coiled tube. This cooling system could be located in the interior space of a stripper column.

[0018] Another embodiment has a cooling system made up of a chamber in proximity with a surface of an electrochemical cell so as to effectuate temperature conditioning and a coolant material within the chamber.

[0019] Yet another embodiment has a cooling system having a fluid circulation apparatus directing fluid flow onto a surface of an electrochemical cell. This apparatus can be enhanced with a plurality of fins protruding from the surface of the electrochemical cell.

[0020] In one embodiment, the chlorine dioxide solution generator has an electrochemical cell operating at a current greater than 120 A.

[0021] In another embodiment, the chlorine dioxide solution generator cooling system maintains a chlorine dioxide gas temperature of less than 130⁰F (54.4°C).

[0022] In another aspect, the present chlorine dioxide solution generator includes a chlorine dioxide gas source, an absorption loop fluidly connected to the chlorine dioxide gas source for effecting the dissolution of chlorine dioxide into a liquid stream, and a cooling system that functions in the chlorine dioxide gas source or the absorption loop.

[0023] In one embodiment, the chlorine dioxide solution generator has an electrochemical cell operating at a current greater than 120 A. [0024] In another embodiment, the chlorine dioxide solution generator produces a chlorine dioxide gas with a temperature of less than 130⁰F (55°C).

[0025] Other embodiments have a cooling system functioning within the absorption loop that is made up of at least one water flush injector fluidly connected before or after a gas transfer pump to allow for intermittent water injection. The water flush injector can include at least one solenoid valve. The water flush injector can be controlled by a program logic controller (PLC) or a standalone timer.

[0026] A method of generating chlorine dioxide solution includes: providing a chlorine dioxide gas source, dissolving chlorine dioxide into a liquid stream by employing an absorption loop fluidly connected to the chlorine dioxide gas source and cooling occurring within the chlorine dioxide gas source or the absorption loop.

[0027] An electrochemical chlorine dioxide generator comprises an anolyte loop fluidly connected to a neutralization system. A catholyte loop is fluidly connected the neutralization system and the neutralization system treats (that is, neutralizes) waste products from the anolyte loop and/or the catholyte loop. In a preferred embodiment, an absorption loop is fluidly connected to the anolyte loop.

[0028] In an embodiment, the electrochemical chlorine dioxide generator has a stripper column within the anolyte loop that is fluidly connected to the neutralization system. In a further embodiment, a probe monitors the pH of the waste products in the neutralization system. In another embodiment, at least one of the electrochemical chlorine dioxide generator and the neutralization system is controlled by a PLC.

[0029] In a preferred embodiment, the neutralization system comprises a pH treatment tank for receiving waste products that is fluidly connected to at least one of the anolyte loop and the catholyte loop. The neutralization system can further comprise a neutralization solution tank fluidly connected to the pH treatment tank. In a further embodiment, a recirculation pump is fluidly connected to the pH treatment tank, and the recirculation pump mixes the waste products in the pH treatment tank.

[0030] In a preferred embodiment, the electrochemical chlorine dioxide generator further comprises an effluent holding tank fluidly connected to the pH treatment tank. The effluent holding tank receives neutralized waste products. The electrochemical chlorine dioxide generator can further comprise a transfer pump for transferring the neutralized waste products to the effluent holding tank. A diverting valve controls the fluid connection between the pH treatment tank and the effluent holding tank.

[0031] In a preferred embodiment, an acidic solution is contained in the neutralization solution tank and is used to neutralize the waste products in the pH treatment tank. The acidic solution can comprise hydrochloric acid. The acidic solution can further comprise a chlorite neutralizing compound. In a further embodiment, the chlorite neutralizing compound is selected from the group of ferrous chloride tetra hydrate, sodium sulfite, sodium metabisulfite, and sodium thiosulfate pentahydrate.

[0032] A method for neutralizing waste products from an electrochemical chlorine dioxide generator comprises collecting waste products from the electrochemical generator in a pH treatment tank. A neutralizing solution is added to the waste product in the pH treatment tank. The waste products are recirculated to achieve at least one of the removal of substantially all chlorite and a pH between 4 and 10. The waste products are transferred from the pH treatment tank for storage and/or disposal. A preferred embodiment further comprises controlling the neutralizing of waste products with a PLC.

[0033] The present chlorine dioxide solution generator includes a chlorine dioxide gas source and an eductor system for at least partially effecting the dissolution of chlorine dioxide gas into an aqueous liquid stream. The eductor system is fluidly connected to the chlorine dioxide gas source. An absorption system is fluidly connected to the eductor system that is capable of effecting additional dissolution of the chlorine dioxide gas into the aqueous liquid. In a preferred embodiment, the aqueous liquid stream for the eductor system is at least one of a chlorine dioxide solution and dilution water recirculated between the eductor system and the absorption system.

[0034] In other embodiments, the chlorine dioxide gas source further includes a cooling system. The absorption system of the chlorine dioxide solution generator can also be fluidly connected to a dosing pump capable of delivering a chlorine dioxide solution from the absorption system into a pressurized water system that operates at pressures up to approximately 200 psig (1,480 kPA). The chlorine dioxide solution generator can be operated using a single precursor chemical. Furthermore, the absorption system can be fluidly connected to the chlorine dioxide gas source to recirculate residual chlorine dioxide gas into the chlorine dioxide gas source. In another embodiment, the dilution water can be fluidly connected with the absorption system.

[0035] In further preferred embodiments, the chlorine dioxide solution generator includes an anolyte loop and a catholyte loop, where the catholyte loop is fluidly connected to the anolyte loop via a common electrochemical component. The anolyte loop can further include a precursor chemical feedstock stream with at least one electrochemical cell fluidly connected to the feedstock stream. The electrochemical cell system has a positive end and a negative end where the precursor chemical feedstock stream can be directed through the electrochemical cell to produce a chlorine dioxide solution. The chlorine dioxide solution generator can further include a stripper column, where the chlorine dioxide solution is directed from the positive end of the electrochemical cell into the stripper column, and the stripper column produces a chlorine dioxide gas stream. The chlorine dioxide gas stream can exit the stripper column directed toward the eductor system. In a further embodiment, the chlorine dioxide gas stream can be a mixture of less than 10 percent chlorine dioxide gas in air.

[0036] In other embodiments, a cooling system for the chlorine dioxide solution generator maintains a chlorine dioxide gas temperature of less than 130⁰F (54.4°C). The electrochemical component of the chlorine dioxide solution generator can also operate at currents of from 120 amperes to 300 amperes and greater.

[0037] A method of generating chlorine dioxide solution that includes providing a chlorine dioxide gas source and effecting at least partial dissolution of chlorine dioxide gas into an aqueous liquid stream by employing an eductor system fluidly connected to the chlorine dioxide gas source. Additional dissolution of the chlorine dioxide gas can be effected into the aqueous liquid employing an absorption system fluidly connected to the eductor system.

[0038] In another embodiment, the method can include cooling the chlorine dioxide gas within the chlorine dioxide gas source. In further embodiments, the cooling can result in a chlorine dioxide gas temperature of less than 130⁰F (54.4°C). The chlorine dioxide gas source can further preferably operate at currents of 300 amperes or greater.

[0039] In a further embodiment, the method can include recirculating an aqueous liquid stream between the eductor system and the absorption system wherein the aqueous liquid stream for the eductor system is at least one of a chlorine dioxide solution and a dilution water. In other embodiments, the chlorine dioxide gas source can further produce a mixture of less than 10 percent chlorine dioxide gas in air.

Brief Description of the Drawings

[0040] FIG. 1 is a process flow diagram of an embodiment of a chlorine dioxide generator.

[0041] FIG. 2 is a process flow diagram of an anolyte loop of a chlorine dioxide generator.

[0042] FIG. 3. is a process flow diagram of a catholyte loop of a chlorine dioxide generator.

[0043] FIG. 4 is a process flow diagram of an absorption loop of a chlorine dioxide generator.

[0044] FIG. 5 is a graph showing a relationship between current applied and pounds of chlorine dioxide generated for a typical 10 lb/day cell. [0045] FIG. 6 is a side view and flow diagram of a cooling system embodiment for piping within a chlorine dioxide gas source.

[0046] FIG. 7 is a side view and flow diagram of a cooling system embodiment for operation within an interior of a stripper column of a chlorine dioxide gas source.

[0047] FIG. 8 is a side view and flow diagram of a cooling system embodiment for operation in proximity to a surface of an electrochemical cell of a chlorine dioxide gas source.

[0048] FIG. 9 is a frontal view of a cooling system embodiment using a plurality of fins on an electrochemical cell.

[0049] FIG. 10 is a side view of a cooling system embodiment using a plurality of fins on an electrochemical cell.

[0050] FIG. 11 is a process flow diagram of a cooling system embodiment operating within an absorption loop using intermittent water injection.

[0051] FIG. 12 is a process flow diagram of an electrochemical chlorine dioxide generator with a neutralization system for waste products.

[0052] FIG. 13 is a process flow diagram of a neutralization system for waste products from an electrochemical chlorine dioxide generator. [0053] FIG. 14 is a process flow diagram of a chlorine dioxide generator with an eduction and absorption system.

Detailed Description of Preferred Embodiment s)

[0054] The embodiments disclosed herein are intended to be illustrative and should not be read as limitations of the current disclosure.

[0055] FIG. 1 is a process flow diagram of an embodiment of the present ClO₂ generator 100. The process flow of FIG.1 consists of three sub-processes: an anolyte loop 102, a catholyte loop 104 and an absorption loop 106. The purpose of the anolyte loop 102 is to produce a chlorine dioxide (ClO₂) gas by oxidation of chlorite, and the process can be referred to as a ClO₂ gas generator loop. The ClO₂ gas generator loop can be described as a ClO₂ gas source. The catholyte loop 104 of the ClO₂ gas generator loop produces sodium hydroxide and hydrogen gas by reduction of water. The anolyte loop and catholyte loop together can also be referred to as a ClO₂ gas source. Once the ClO₂ gas is produced in the ClO₂ gas generator loop, the ClO₂ gas can be transferred to the absorption loop 106 where the gas is dissolved or infused into a liquid. Here, ClO₂ gas, which is produced in the ClO₂ gas generator loop, can be dissolved or infused into an aqueous liquid stream directed through absorption loop 106. [0056] The anolyte loop 102 may include a reactant feedstock 108 fluidly connected to an electrochemical cell 116. The reactant feedstock is delivered to the positive end of the electrochemical cell 110 and is oxidized to form ClO₂ gas, which is dissolved in an electrolyte solution along with other side products to form a chlorine dioxide solution. The chlorine dioxide solution is directed to a stripper column 112 where the pure chlorine dioxide gas is stripped off from other side products. The pure chlorine dioxide gas can then be directed to a gas transfer pump 118 using a vacuum or other similar means.

[0057] The catholyte loop 104 handles byproducts produced from the electrochemical reaction of the reactant feedstock 108 solution in the anolyte loop 102. These byproducts react at the negative end of the electrochemical cell 114 and then proceed to the byproduct tank 122 that is fluidly connected to the electrochemical cell. For example, where a sodium chlorite (NaClO₂) solution is used as the reactant feedstock 108, water in the catholyte loop 104 is reduced to produce hydroxide and hydrogen gas. The reaction of the anolyte loop 102 and catholyte loop 104 where sodium chlorite is used as the reactant feedstock 108 is represented by the following net chemical equation:

2NaClO_2(aq) + 2H₂O -» 2C10_2(gas) + 2NaOH_(aq) + H_2(gas)

[0058] The absorption loop 106 dissolves the chlorine dioxide gas from the anolyte loop or the ClO₂ gas source into an aqueous chlorine dioxide solution. The chlorine dioxide gas is directed from the stripper column 112 using a gas transfer pump 118. The gas transfer pump 118 can be a part of the anolyte loop 102 or the absorption loop 106. After passing through the gas transfer pump the chlorine dioxide gas can be directed to an absorber tank 120. Before ClO₂ gas is directed to the absorber tank 120, the tank 120 can be filled with water to approximately 0.5 inch (13 mm) below a main level control. The flow switch controls the amount of liquid delivered to the absorber tank 120. A process delivery pump feeds the ClO₂ solution from the absorption tank 120 to the end process without including air or other gases. The process delivery pump is sized to deliver a desired amount of water per minute. The amount of ClO₂ gas delivered to the absorber tank 120 is set by the vacuum and delivery rate set by the gas transfer pump 118.

[0059] FIG. 2 is a process flow diagram of an anolyte loop 102 in an embodiment of a chlorine dioxide generator 100. The contribution of anolyte loop 102 to the ClO₂ solution generator is to produce a ClO₂ gas that is directed to absorption loop 106 for further processing. The anolyte loop embodiment of FIG. 2 is for a ClO₂ gas produced using a reactant feedstock 202. In a preferred embodiment, a 25 percent by weight sodium chlorite (NaClO₂) solution can be used as reactant feedstock 202. However, feedstock concentrations ranging from 0 percent to a maximum solubility (40 percent at 17°C in the embodiment involving NaClO₂), or other suitable method of injecting suitable electrolytes, can be employed.

[0060] The reactant feedstock 202 can be connected to a chemical metering pump 204, which delivers the reactant feedstock 202 to a recirculating connection 206 in the anolyte loop. Recirculating connection 206 in anolyte loop connects a stripper column 208 to an electrochemical cell 210. The delivery of the reactant feedstock 202 can be controlled using PLC system 108. PLC system 108 can be used to activate chemical metering pump 204 according to signals received from a pH sensor 212. pH sensor 212 is generally located along recirculating connection 206. A pH set point can be established in PLC system 108, and once the set point is reached, the delivery of reactant feedstock 202 can either start or stop.

[0061] Reactant feedstock 202 can be delivered to a positive end 214 of electrochemical cell 210 where the reactant feedstock is oxidized to form a ClO₂ gas, which is then dissolved in an electrolyte solution along with other side products. The ClO₂ solution with the side products is directed away from electrochemical cell 210 to the top of stripper column 208 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 is then removed from stripper column 208 under a vacuum induced by gas transfer pump 216, or analogous gas or fluid transfer device (such as, for example, a vacuum-based device), where it is delivered to an adsorption loop. The remaining solution is collected at the base of stripper column 208 and recirculated back across the pH sensor 212 where additional reactant feedstock 202 can be added. The process with the reactant feedstock and/or recirculation solution being delivered into positive end 214 of electrochemical cell 210 is then repeated.

[0062] 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. In a further example, ClO₂ can be generated by purely chemical generators and transferred to an absorption loop for further processing.

[0063] FIG. 3 is a process flow diagram of a catholyte loop 104 in an embodiment of a chlorine dioxide generator 100. Catholyte loop 104 contributes to the chlorine dioxide generator 100 by handling byproducts produced from the electrochemical reaction of reactant feedstock 202 solution in anolyte loop 102. As an example, where a sodium chlorite (NaClO₂) solution is used as reactant feedstock 202, sodium ions from the anolyte loop 102 migrate to catholyte loop through a cationic membrane 302, in electrochemical cell 210, to maintain charge neutrality. Water in the catholyte is reduced to produce hydroxide and hydrogen (H₂) gas. The resulting byproducts in catholyte loop, in the example of a NaClO₂ reactant feedstock, are sodium hydroxide (NaOH) and hydrogen gas. The byproducts are directed to a byproduct tank 304.

[0064] In an embodiment of catholyte loop, in the example of a NaClO₂ reactant feedstock, a soft (that is, demineralized) water source 306 can be used to dilute the byproduct NaOH using a solenoid valve 308 connected between soft water source 306 and the byproduct tank 304. Solenoid valve 308 can be controlled with PLC system 108. In a preferred embodiment, PLC system 108 can use a timing routine that maintains the NaOH concentration in a range of 5 percent to 20 percent. When byproduct tank 304 reaches a predetermined level above the base of byproduct tank 304, the diluted NaOH byproduct above that level is removed from catholyte loop.

[0065] 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 306. The H₂ gas rises up in byproduct tank 304 where there is a hydrogen disengager 310. The H₂ gas can be diluted with air in hydrogen disengager 310 to a concentration of less than 0.5 percent. The diluted H₂ gas can be discharged from catholyte loop 104 and chlorine dioxide solution generator 100 using a blower 312.

[0066] 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.

[0067] FIG. 4 is a process flow diagram of an absorption loop 106 of an embodiment of a chlorine dioxide generator 100. The absorption loop processes the ClO₂ gas from anolyte loop 102 into a ClO₂ solution that is ready to be directed to the water selected for treatment.

[0068] ClO₂ gas is removed from stripper column 208 of anolyte loop 102 using gas transfer pump 216. In a preferred embodiment, a gas transfer pump 216 can be used that is "V" rated at 75 Torr (10 kPa) with a discharge rate of 34 liters per minute. The vacuum and delivery rate of gas transfer pump 216 can vary depending upon the free space in stripper column 208 and desired delivery rate OfClO₂ solution.

[0069] The ClO₂ gas removed from stripper column 208 using gas transfer pump 216 is directed to an absorber tank 402 of the absorption loop. In a preferred embodiment, discharge side 404 of gas transfer pump 216 delivers ClO₂ gas into a 0.5 -inch (13-mm) poly(vinyl chloride) (PVC) injection line 406 external to absorber tank 402. Injection line 406 is an external bypass for fluid between the lower to the upper portions of the absorber tank 402. A gas injection line can be connected to injection line 406 using a T-connection 408. Before ClO₂ gas is directed to absorber tank 402, the tank 402 is filled with water to approximately 0.5 inch (13 mm) below a main level control 410. Main level control 410 can be located below where injection line 406 connects to the upper portion of absorber tank 402. Introducing CIO₂ gas into injection line 406 can cause a liquid lift that pushes newly absorbed ClO₂ solution up past a forward-only flow switch 412 and into absorber tank 402. Flow switch 412 controls the amount of liquid delivered to absorber tank 402. Absorber tank 402 has a main control level 410 to maintain a proper tank level. In addition to main control level 410, safety control levels can be employed to maintain a high level 414 and low level 416 of liquid where main control level 410 fails. A process delivery pump 418 feeds ClO₂ solution from absorber tank 402 to the end process without including air or other gases. Process delivery pump 418 is sized to deliver a desired amount of water per minute. The amount OfClO₂ gas delivered to absorber tank 402 is set by the vacuum and delivery rate set by gas transfer pump 216.

[0070] PLC system 108 can provide a visual interface for the operation of a chlorine dioxide generator 100. PLC system 108 can automatically control the continuous operation and safety of the production of a ClO₂ solution. PLC system 108 can set flow rates for anolyte loop 102 and catholyte loop 104. The safety levels of absorber tank 402 can also be enforced by PLC system 108. PLC system 108 can also control the power for achieving a desired current in an embodiment using an electrochemical cell 210. In a preferred embodiment, the current can range from 0 to 100 amperes, although currents higher than this average are possible. The amount of current determines the amount OfClO₂ gas that is produced in anolyte loop 102. The current of the power supply can be determined by the amount OfClO₂ that is to be produced. PLC system 108 can also be used to monitor the voltage of electrochemical cell 210. In a preferred embodiment, electrochemical cell 210 can be shut down when the voltage exceeds a safe voltage level. In another preferred embodiment, 5 volts can be considered a safe voltage level.

[0071] Another operation that can be monitored with PLC system 108 is the temperature of electrochemical cell 210. If overheating occurs, PLC system 108 shuts down electrochemical cell 210. PLC system 108 can also monitor the pH of the anolyte using a pH sensor 212 (shown in FIG. 2). During operation of electrochemical cell 210, the pH of the solution circulating in anolyte loop decreases as hydrogen ions are generated. In the exemplary embodiment of the NaClO₂ reactant feedstock, when the pH goes below 5, additional reactant feedstock is added using PLC system 108. Control of pH can also be handled by adding a reactant that decreases the pH when the pH is too high. [0072] In another embodiment, the transfer line from gas transfer pump 216 can be connected to absorber tank 402 directly without injection line 406, and can allow for increasing the pump transfer rate. Other embodiments can include a different method of monitoring the liquid level in absorber tank 402. For example, an oxidation and reduction potential (ORP) can be dipped in absorber tank 402. ORP can be used to monitor the concentration OfClO₂ in the solution in absorber tank 402. PLC system 108 can be used to set a concentration level for the ClO₂ as monitored by ORP, which provides an equivalent method of controlling the liquid level in absorber tank 402. Optical techniques such as photometers can also be used to control the liquid level in absorber tank 402. Absorption loop 106 can be a part of the chlorine dioxide solution generator or it can be installed as a separate unit outside of the chlorine dioxide solution generator. In another embodiment, process water can be fed directly in absorber tank 402 and treated water can be removed from the absorber tank 402. The process water can include a demineralized, or soft, water source 420 and the process water feed can be controlled using a solenoid valve 422.

[0073] The chlorine dioxide generator 100 can be utilized to obtain a higher yield Of ClO₂ gas, or a ClO₂ solution, by applying a higher current to the electrochemical cell than those previously applied. FIG. 5 illustrates a relationship between current and pounds Of ClO₂ generated for a typical 10 lb/day cell. As the current applied to the cell is increased the pounds of ClO₂ that can be generated increases. The high-capacity current can be greater than 50 A, but a desirable embodiment contemplates cooling for a system that operates on the order of greater than 120 A. Applying a higher current to the cell increases the rate of the selective oxidation reaction of, for example, chlorite to ClO₂, which can result in a higher yield of ClO₂ gas. A higher yield of ClO₂ gas can result in a higher yield OfClO₂ solution.

[0074] FIG. 6 is a side view and flow diagram of a cooling system 600 embodiment for use, for example, with piping that may be used within a chlorine dioxide gas source or anolyte loop 102. This cooling system 600 can be interposed between a reactant feedstock 108 stream and an electrochemical cell 116 that is fluidly connected to the reactant feedstock stream such that reactant feedstock 108 is directed through the cooling system 600 before entering the electrochemical cell 116. The cooling system 600 can also be interposed between the positive end of an electrochemical cell 110 and a stripper column 112 that is fluidly connected to the electrochemical cell 116 such that a chlorine dioxide solution directed from the positive end of the electrochemical cell 110 is directed through the cooling system 600. The cooling system 600 can also be interposed between the negative end of an electrochemical cell 114 and a byproduct tank 122 that is fluidly connected to the electrochemical cell 116 such that a byproduct stream directed from the negative end of the electrochemical cell 114 is directed through the cooling system 600.

[0075] The cooling system 600 can have an inner tube 602. The reactant feedstock 108 or chlorine dioxide solution can enter the inner tube 602 through an inlet 604, pass through the inner tube 602 and exit through an outlet 606. The inner tube 602 can be made out of material that is inert to chlorine dioxide. Metals such as titanium and tantalum can be used or inert plating materials may also be used.

[0076] The inner tube 602 is surrounded by an outer jacket 608. Coolant enters the outer jacket through a coolant inlet 610 and exits through a coolant outlet 612. The outer jacket 608 can be made of an insulating material such as poly(vinyl chloride) (PVC), chlorinated poly(vinyl chloride) (CPVC) or poly(tetrafluoroethylene) (trade name Teflon^®). A coolant material, such as water or silicon oil, can be cooled with Freon^® or equivalent materials and then pumped through the outer jacket 608. The coolant material then cools the reactant feedstock 108 or chlorine dioxide solution inside the inner tube 602. It is desirable that the coolant temperature is such that it cools the reactant feedstock 108 or chlorine dioxide solution to a temperature of less than 130⁰F (54.4°C) and allows downstream ClO₂ gas to also be below 130⁰F (54.4°C). However, it is desirable that the coolant material does not freeze the reactant feedstock 108 or chlorine dioxide solution. It is further desirable that the Freon^® or equivalent material does not lower the temperature of the coolant material to a point where the coolant material cannot be pumped through the outer jacket 608.

[0077] FIG. 7 illustrates another embodiment of the present cooling system 700 for operating within the interior of a stripper column of the chlorine dioxide gas source 102. The cooling system is made up of a coiled tube 703 placed within a chlorine dioxide gas source or anolyte loop 102. The coiled tube 703 can comprise a material that is inert to chlorine dioxide such as titanium or tantalum. A coolant material is directed through the coiled tube 703. The coolant enters through a coolant inlet 704 is directed through the coiled tube 703 and exits from a coolant outlet 706. Possible coolant materials can include water and silicon oil. The coolant material can be cooled by Freon^® or equivalent materials and pumped through the coiled tube 703.

[0078] The cooling system 700 can be located in the interior space of a stripper column 112. The coolant material flows through the coiled tube 703 and cools the chlorine dioxide solution in the stripper column 112. It is desirable that the coolant temperature is such that it cools the chlorine dioxide solution to a temperature of less than 130⁰F (54.4⁰C) and allows downstream ClO₂ gas to also be below 130⁰F (54.4°C). However, the coolant material temperature should also be such that it does not cause the chlorine dioxide solution to freeze. [0079] FIG. 8 is a side view and flow diagram of a cooling system 800 operating in proximity with the surface of an electrochemical cell 116. The cooling system comprises a chamber 802 in proximity with the positive end of the electrochemical cell 110 and/or the negative end of the electrochemical cell 114. It is preferred that the proximity of the chamber 802 to the electrochemical cell is such that the chamber 802 effects cooling of the electrochemical cell. The chamber 802 may be in direct contact or adjacent to the electrochemical cell. Coolant material enters the chamber 802 through a coolant inlet 808, is directed through the chamber 802 and exits from a coolant outlet 810. The coolant material can be a non-conducting material such as pure water or silicon oil. The coolant material can be cooled using Freon^® or equivalent materials and then pumped through the chamber 802.

[0080] The precursor chemical or reactant feedstock 108 enters the positive end of the electrochemical cell 110 through an anolyte inlet 812, where the reactant feedstock 108 can be 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 can be directed out of the electrochemical cell at the anolyte outlet 814. The ClO₂ solution can then be cooled by the coolant material in chamber 802. It is desirable for the coolant material temperature to be such that the coolant material cools the downstream ClO₂ gas and the chlorine dioxide solution or the catholyte solution to a temperature of less than 130⁰F (54.4°C). However, it is further desirable that the coolant temperature does not freeze the chlorine dioxide solution or catholyte solution.

[0081] Another embodiment of the present cooling system is where a fluid circulation apparatus is located so as to direct fluid flow onto the surface of the electrochemical cell 900, such as shown by the example in FIG. 9. A fluid can be a liquid or gas tending to flow or conform to the outline of its container. Examples of fluids include water, air, oil and an inert gas. One embodiment blows air onto the surface of an electrochemical cell 900. This cools the chlorine dioxide solution as it passes through the electrochemical cell 900.

[0082] In order to increase the effectiveness of cooling by the fluid circulation apparatus a plurality of fins 902 can be added to the surface of the electrochemical cell 900. FIG. 9 illustrates a frontal view of the surface of an electrochemical cell 900 having a plurality of fins 902 in accordance with an embodiment of the present disclosure. The plurality of fins 902 can be made of a metal such as stainless steel or copper or other such material that may be used to build the structure of the electrochemical cell. The plurality of metal fins 902 increases the total cooling surface area of the electrochemical cell, resulting in more effective cooling.

[0083] FIG. 10 illustrates a side view of an electrochemical cell having a plurality of fins 902 in accordance with an embodiment of the present disclosure.

[0084] As an example of the ClO₂ cooling using coolant material, when a current applied to an electrochemical cell in a ClO₂ gas generator increases, the temperature of the ClO₂ solution increases. However, by using the cooling coil or jacket as outlined in this disclosure a temperature of 65°F -85⁰F (18.3⁰C - 29.4°C) can be maintained.

[0085] FIG. 11 is a process flow diagram of a cooling system 1100 operating, for example, within the absorption loop 106 using intermittent water injection. Chorine dioxide gas exits the stripper column 112 and is directed through the a gas transfer pump 1102. In this embodiment, the chlorine dioxide gas entering the gas transfer pump 1102 is cooled by intermittently injecting water into the ClO₂ gas flow. At least one water flush injector 1104 is fluidly connected to the gas transfer pump 1102. The water flush injector 1104 could be a solenoid valve or other method of controlling water flow. The water flush injector 1104 can be placed either before the gas transfer pump 1102 in the piping system 1106 as shown in FIG. 11 or it can be placed in the piping system 1106 after (not shown) the gas transfer pump 1102.

[0086] It is desirable to add water to adequately cool the ClO₂ gas to less than 130⁰F (54.4°C). However, it is not necessarily preferred to add water constantly because this may diminish the vacuum in the gas transfer pump 1102. Therefore, it is preferred to add water at intermittent intervals. In this context, the term intermittent contemplates that the time between water injections and the length of each injection can, but need not, be constant. In one embodiment, at least 30 seconds is allowed between each water injection. Each water injection can last for approximately 1 to 30 seconds.

[0087] The water flow injectors 1104 can be operated through a program logic control (PLC) system that can include displays. Alternatively, the water flow injectors 1104 can be controlled by a standalone timer.

[0088] The water flush injectors 1104 can also be used to extend the life of pump heads. Chlorine dioxide gas carries unreacted salts, which build deposits on the pump heads over time. Intermittently injecting water helps keep the pump heads clean.

Table 1

Effect of Cell Current and Water Injection on Cooling ClO₂ Gas Discharged from Pump for a 10 lb/day Unit

[0089] Table 1 shows an example Of ClO₂ cooling using water injection. Increasing the current that is applied to an electrochemical cell in a chlorine dioxide gas generator from 100 A to 200 A raises the temperature of the ClO₂ gas coming out of the gas transfer pump 118 from 12O⁰F to 15O⁰F (48.9°C to 65.6°C). The water flush injection technique outlined above can be used to lower the temperature of the ClO₂ gas coming out of the gas transfer pump. By injecting water for 2 seconds every 300 seconds the temperature of the ClO₂ gas coming out of the gas transfer pump at 200 A is lowered from 15O⁰F (65.6⁰C) to 128°F (53.3⁰C).

Table 2 Cooling Effect of Coolants on a 10 lb/day Unit

[0090] Table 2 shows another example of ClO₂ cooling using coolant material. As a the current applied to an electrochemical cell in a chlorine dioxide gas generator increases, the temperature of the ClO₂ solution increases. However, by using the cooling coil or jacket as outlined above in this disclosure a temperature of 65⁰F -85⁰F (18.3°C - 29.4⁰C) can be maintained.

[0091] FIG. 12 is a process flow diagram of an electrochemical chlorine dioxide generator 1200 with a neutralization system for waste products. The process flow of FIG. 12, similar to the process flow described for FIG. 1, consists of three sub-processes including an anolyte loop 102, a catholyte loop 104 and an absorption loop 106. Once a ClO₂ gas is produced in the anolyte loop 102, the ClO₂ gas can be transferred to an absorption loop 106 where the gas can be further conditioned for water treatment end-uses. The anolyte loop 102 and catholyte loop 104 can be fluidly connected to a neutralization system 1300 that can process wastes produced by the anolyte loop 102 and catholyte loop 104. The process can be operated at least partially through a PLC-based system that can include visual and/or audible displays.

[0092] In this application, the term "absorb" 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, is "absorbed" (that is, dissolved or infused) into an aqueous liquid stream directed through absorption loop 106. The neutralization system 1300 can process wastes for electrochemical ClO₂ generators that produce ClO₂ gas (for example, without an absorption loop) or ClO₂ solution (for example, with an adsorption loop).

[0093] FIG. 13 is a process flow diagram of an embodiment of a neutralization system 1300 for waste products from ClO₂ generator 100. As illustrated in FIG. 13, wastes from the anolyte loop 102 can be fluidly connected to the neutralization system 1300 by way of a connection from the stripper column. Another embodiment illustrates the wastes from the catholyte loop 104 fluidly connected to the neutralization system 1300 by way of a connection from a byproduct tank. Hydrogen byproduct wastes from the catholyte loop typically exit the byproduct tank by way of the blower 312 as shown in FIG. 3, where the hydrogen can be diluted with air and discharged into the atmosphere at a preferred concentration of less than 0.1 percent hydrogen. Caustic soda or sodium hydroxide from the catholyte loop is also typically collected in the byproduct tank 304, such as, for example, where a chlorite reactant feedstock is used in the anolyte loop 102.

[0094] FIG. 13 illustrates anolyte waste and catholyte waste directed into a pH treatment tank 1310. In a preferred embodiment, both waste streams are combined into the same tank. The pH treatment tank 1310 is fluidly connected to a number of pumps including a recirculation and transfer pump 1320 and a neutralization feed pump 1330. A single pump can perform one or more functions. For example, the recirculation and transfer pump 1320 can perform both recirculation and transfer operations or two separate pumps can be used to individually perform each separate operation. The neutralization feed pump 1330 can allow uninterrupted flow to the pH treatment tank 1310 using a pH control system 1340 that can continuously modulate the neutralization feed pump 1330. The recirculation and transfer pump 1320 can serve a dual function of treating (that is, mixing or neutralizing) waste products in the pH treatment tank 1310 and transferring or discharging the treated waste products to an effluent holding tank 1350, which can have a drain 1355 to remove waste products. The effluent holding tank can also accept dilution water.

[0095] A waste product that can result in the anolyte loop 102 process is unseparated CIO₂ from the stripper column operation. In one embodiment, a typical concentration range of a chlorine dioxide waste product from the anolyte loop is 700 ppm to 1500 ppm. For a ClO₂ concentration at this level, it is difficult to tolerate the odor and the overall effects can be toxic, thus making neutralization of the toxic effects of the chlorine dioxide waste a desired outcome. Since the waste from the catholyte loop of an electrochemical chlorine dioxide generator includes sodium hydroxide, the catholyte waste can neutralize the chlorine dioxide waste produced from the anolyte loop, as represented by the following chemical equation:

2ClO₂ + 2NaOH -» NaClO₂ + NaClO₃ + H₂O

[0096] The quantity of chlorine dioxide waste that is produced in the electrochemical reaction is typically much less that the quantity of sodium hydroxide. As the anolyte and catholyte waste streams enter the pH treatment tank 1310 and combine, the overall mixture is typically alkaline in nature where the pH can range from approximately 12 to 13. This high pH range is due to the larger quantity of catholyte waste compared to the anolyte waste.

[0097] In consideration of various disposal options, the waste products can be neutralized with acid prior to disposal. In a preferred embodiment, hydrochloric acid or muriatic acid, such as that which is commercially available for use in swimming pools, can be used as a neutralizing agent. By way of the neutralization solution tank 1320, hydrochloric acid can be added to the pH treatment tank 1310. The addition of the hydrochloric acid to the combined anolyte and catholyte wastes in the pH treatment tank is preferably done with sufficient mixing to limit the formation of chlorine dioxide, which can occur when the pH in the treatment tank 1310 decreases to less than approximately 4. In a preferred embodiment, the neutralization system 1300 illustrated in FIG. 13 can have the pH control system 1340 monitor the pH so that the addition of acid is stopped when the pH of the treated waste stream drops below 9. In another embodiment, the pH level of the treated waste stream is maintained above 4.

[0098] In addition to the alkalinity of the wastes from an electrochemical chlorine dioxide generator, the levels of chlorite from the anolyte loop wastes can also preferably be converted or neutralized. This can be done by converting the chlorite into chloride by reacting the sodium chlorite waste product with any of a number of compounds. For example, sodium chlorite can be reacted with ferrous chloride as demonstrated by the following chemical equation: NaClO₂ + 4FeCl₂ + 2H₂O -> 4 Fe(OH)₂ + NaCl

In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 795.24 grams of ferrous chloride tetra hydrate.

[0099] As another example, sodium chlorite can be reacted with sodium sulfite as demonstrated by the following equation:

NaClO₂ + 2Na₂SO₃ → 2Na₂SO₄ + NaCl

In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 252.08 grams of sodium sulfite.

[00100] In another embodiment, sodium chlorite can be reacted with sodium metabisulfite as demonstrated by the following equation:

NaClO₂ + Na₂S₂O₅ + H₂O → 2Na₂SO₄+ NaHSO₄ + HCl

In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 190 grams of sodium metabisulfite.

[00101] Sodium chlorite can also be reacted with sodium thiosulfite as demonstrated by the following equation:

NaClO₂ + 4HCl + 4Na₂S₂O₃ -» 2Na₂S₄O₆ + 2H₂O + 5NaCl

In a preferred embodiment, 67.45 grams of chlorite will react in the above equation with 992.68 grams of sodium thiosulfate pentahydrate. [00102] Table 3 summarizes preferred chlorite neutralizing compounds and the concentration of neutralizer compound that will neutralize 1 ppm of chlorite into chloride.

Table 3

Chemical concentration that will neutralize 1 ppm of chlorite

[00103] In a preferred embodiment, the chlorite neutralizing compound, such as those described in Table 3, can be combined with a dilute acid (for example, a solution of 5 percent acid) for subsequent neutralizing of the alkaline anolyte and catholyte waste mixture held in the pH treatment tank 1310. The chlorite neutralizing compound and dilute acid can be combined in the neutralization solution tank 1360. The amount of neutralizer compound can be calculated based on the amount of chlorite present in the waste stream. A typical concentration range of chlorite is 0 to 0.3 moles/liter (that is, molarity (M)) in an electrochemical chlorine dioxide generator.

[00104] FIG. 13 illustrates the chlorine dioxide generator 100 fluidly connected to the pH treatment tank 1310 by way of connection to the anolyte and catholyte waste discharges from the chlorine dioxide generator 100. Following the mixing of the catholyte and anolyte waste streams in the pH treatment tank 1310, the pH of the combined wastes typically ranges from around 12 to 14. A recirculation or mixing process can be performed with the pH treatment tank 1310 using the recirculation and transfer pump 1320. Recirculation is typically performed when a diverting valve 1370 between the recirculation and transfer pump 1320 and the effluent holding tank 1350 is closed. A recirculation process is preferred to help keep the concentration of the waste products uniform and diluted so that the wastes in the pH treatment tank 1310 do not become acidic and cause ClO₂ to form.

[00105] In a preferred embodiment, residual chlorine dioxide from the anolyte waste stream reacts with sodium hydroxide from the catholyte waste stream to form chlorite and chlorate when the pH in the pH treatment tank 1310 is maintained between approximately 12 and 14. Following the reaction of the chlorine dioxide waste, the waste product in the pH treatment tank 1310 can be adjusted to have a pH of less than 10 using a dilute hydrochloric acid (HCl concentration less than 30 percent). Adjusting and maintaining a pH of less than 10 can be accomplished by monitoring the pH of the waste product in the pH treatment tank 1310 using a probe 1345 connected to the pH control system 1340 and automatically adding the neutralizing hydrochloric acid with the neutralizing feed pump 1330 which is also connected to the pH control system 1340. Although the probe can be placed anywhere in the neutralization system 1300 where it can monitor the pH of the product in pH treatment tank 1310, in a preferred embodiment the pH probe 1345 is placed in the recirculation loop. The recirculation loop includes the recirculation and transfer pump 1320 and the pH treatment tank 1310. The waste products in the pH treatment tank 1310 are recirculated in the recirculation loop using the pump 1320 to mix the waste products.

[00106] In a further embodiment, chlorite neutralizing chemicals such as those identified in Table 3 can be mixed with the dilute hydrochloric acid in the neutralization solution tank 1360. Preferably, the chlorite neutralizing chemicals are in excess of the concentrations identified in Table 3. As the neutralization feed pump 1330 adds neutralization solution to the pH treatment tank 1310, the remaining chlorite in the wastes in the pH treatment tank 1310 will be converted into chloride. In a preferred embodiment, the neutralization solution tank 1360 can have double wall jacketing to provide leak containment.

[00107] In a preferred embodiment, level sensors are used to control the level of the waste products in the pH treatment tank 1310. A level sensor arrangement can include a level sensor high (LSH) 1380 and a level sensor low (LSL) 1385. LSH 1380 can open diverting valve 1370 when the level in the pH treatment tank 1310 is high and needs to be lowered by diverting some of the waste product to the effluent holding tank 1350. When the level in the pH treatment tank decreases to a level that triggers LSL 1385, the diverting valve 1350 will close. Any additional diversion of waste product to the effluent holding tank 1350 occurs by opening the diverting valve 1370. The diverting valve can also be controlled by a pH sensor 1347 that limits waste products of high or low pH from being discharged into the effluent holding tank 1350.

[00108] In a preferred embodiment, the neutralization system 1300 and/or electrochemical chlorine dioxide generator 100 can be controlled with a PLC 1390. For example, for the neutralization system 1300, the PLC 1390 can be used to control the neutralization feed pump 1330, the recirculation and transfer pump 1320, the level sensors 1380, 1385, the diverting valve 1370, the pH probe 1345, pH sensor 1347 and/or the pH control system 1340. The PLC 1390 can be further used to control the chlorine dioxide generator 100.

[00109] FIG. 14 is a process flow diagram of an embodiment of the present chlorine dioxide (ClO₂) generator 100. The process flow of FIG.14 consists of a ClO₂ gas source 1410, an eductor system 1450 and an absorption system 1460. In a preferred embodiment, chlorine dioxide gas source 1410 can provide an up to 10 percent pure CIO₂ gas in air. In a another preferred embodiment, the concentration of ClO₂ gas in air is 5 percent or lower. ClO₂ gas source 1410 can be fluidly connected to an eductor system 1450 in which the ClO₂ gas/air mixture can be introduced into an aqueous liquid stream. The eductor system can be fluidly connected to an absorption system 1460 in which the ClO₂ gas/air mixture can be further absorbed, that is dissolved or infused, into the aqueous liquid being circulated through eductor system 1450. In a preferred embodiment, the pressure in absorption system 1460 can be maintained at a relatively constant pressure of approximately 0 to 2 psig (101 to 115 kPa). The resulting ClO₂ solution can then be pumped from absorption system 1460 using a dosing pump 1470 and discharged at a pressure suitable for the end-use system.

[00110] In a preferred embodiment, the ClO₂ gas can be produced electrochemically through the oxidation of chlorite obtained from a precursor chemical feedstock 1412, such as sodium chlorite. An electrochemical chlorine dioxide gas generator can include an anolyte loop and a catholyte loop both of which are connected to an electrochemical cell 1414 or a series of such cells.

[00111] The anolyte loop can include precursor chemical feedstock 1412 fluidly connected to electrochemical cell 1414. In a preferred embodiment, the fluid connection between precursor chemical feedstock 1412 and electrochemical cell 1414 is through a stripper column 1416 that is fluidly connected to an anolyte recirculation pump 1418 that is further fluidly connected to electrochemical cell 1414. Precursor chemical feedstock 1412 can be fluidly connected to stripper column 1416 with a precursor feed pump 1420. Precursor chemical feedstock 1412 is delivered to the positive end of electrochemical cell 1414 and is oxidized to form ClO₂ gas, which is dissolved in an electrolyte solution along with other side products. The electrolyte solution can be directed to a stripper column 1416 where the ClO₂ gas is stripped from the electrolyte solution to form a substantially pure ClO₂ gas in air. The substantially pure ClO₂ gas in air can then be directed to an absorption system 1460 using an eductor system 1450. The remaining electrolyte solution in stripper column 1416 can be mixed with additional precursor chemicals from precursor chemical feedstock 1412 and can then be directed back into electrochemical cell 1414 using anolyte recirculation pump 1418. The stripping of the ClO₂ gas in stripper column 1416 can be effected using a heat exchanger 1422 at the base of stripper column 1416. In further preferred embodiments, a dilution water source 1440 and an electrolyte solution overflow 1424 can be fluidly connected to stripper column 1416.

[00112] The catholyte loop can handle byproducts produced from the electrochemical reaction of precursor chemical feedstock 1412 in the anolyte loop. These byproducts react at the negative end of electrochemical cell 1414 and from there can be directed to a catholyte tank 1422 fluidly connected to electrochemical cell 1414. For example, where a sodium chlorite (NaClO₂) solution is used as precursor chemical feedstock 1412, water in the catholyte loop is reduced in the reaction in electrochemical cell 1414 to hydroxide and hydrogen gas. The hydroxide can remain in catholyte tank 1422 while the hydrogen gas can be removed using a hydrogen gas dilution blower 1428.

[00113] The reaction of the anolyte loop and catholyte loop of preferred electrochemical chlorine dioxide gas generator where sodium chlorite is used as precursor feedstock 1412 is represented by the following net chemical equation:

2NaC10_2(aq) + 2H₂O -> 2C10_2(gas) + 2NaOH_(aq) + H_2(gas)

[00114] Eductor system 1450 can include a gas eductor 1452. Gas eductor 1452 works on a Venturi principle in which an aqueous liquid is forced through an orifice at high velocity using a recirculation pump 1454 or similar means. Gas eductor 1452 has at least two inlets 1456, 1457 and one outlet 1458. One inlet 1456 can be fluidly connected to stripper column 1416 from ClO₂ gas source 1410 and other inlet 1457 can receive an aqueous liquid pumped from fluidly connected recirculation pump 1454. The pumping of the aqueous liquid through the orifice causes a pressure drop in gas eductor 1452. The pressure drop creates a vacuum in the fluid connection between ClO₂ gas source 1410 and gas eductor 1452, which causes the ClO₂ gas from stripper column 1416 to be drawn into gas eductor 1452. The ClO₂ gas and aqueous liquid are combined in eductor 1452 and ejected from outlet 1458 where the combined gas and liquid are discharged into absorption system 1460. The ClO₂ gas can be at least partially dissolved into the aqueous liquid in eductor system 1450. In a preferred embodiment, the combined ClO₂ gas and aqueous liquid are discharged from gas eductor 1452 into absorption system 1460 at or near atmospheric pressure.

[00115] In further embodiments, absorption system 1460 can be fluidly connected to eductor system 1450 and to the ClO₂ gas source 1410. For example, a fluid connection can be made between an absorber tank 1462, recirculation pump 1454 and gas eductor 1452. The aqueous liquid from gas eductor 1452 is discharged into absorber tank 1462 and can be at least partially pumped back into eductor system 1450 through recirculation pump 1454. In absorber tank 1462, the ClO₂ gas from the combined ClO₂ gas and aqueous liquid discharged from gas eductor 1452 can be further absorbed into the aqueous liquid to a desired concentration of ClO₂ solution.

[00116] The absorption system 1460 can also be fluidly connected to stripper column 1416 of chlorine dioxide gas source 1410 to allow unabsorbed ClO₂ gas to return for reintroduction into eductor system 1450. Absorption system 1460 can also include a vent 1464 to relieve pressure. In further preferred embodiments, dilution water 1440 can be fluidly connected to absorber tank 1462 to allow an initial quantity of aqueous liquid into absorption system 1460 for initial aqueous liquid circulation and subsequent recirculation through eductor system 1450 and to allow for dilution of the ClO₂ solution being stored in absorption tank 1462. In a preferred embodiment, absorber tank 1462 can be sized at 30 gallons (113.6 liters) and include level switches operating when the tank drops to 15 gallons (56.8 liters) or approaches 30 gallons (113.6 liters).

[00117] The absorption system 1460 can be fluidly connected with a dosing pump 1470 or similar delivery device sized for feeding the ClO₂ solution from absorption system 1460 to an end process. Dosing pump 1470 is sized to deliver a desired liquid flow rate. As ClO₂ solution is removed from absorption system 1460, the same amount of dilution water can be introduced into absorber tank 1462 to maintain the liquid level in absorber tank 1462. In a preferred embodiment, dosing pump 1470 is capable of delivering the ClO₂ solution to a pressurized water system at a delivery pressure up to approximately 200 psig (1,480 kPa). In a further preferred embodiment, a diaphragm pump can be used to minimize corrosive effects to dosing pump 1470 in the ClO₂ environment.

[00118] An eductor is chosen based on the suction air flow rate that is desired for the gas exiting stripper column 1416. In a preferred embodiment, the desired flow rate between stripper column 1416 and gas eductor 1452 is such that the concentration Of ClO₂ in air is 10 percent or less. In another preferred embodiment, the ClO₂ gas concentration in air between stripper column 1416 and gas eductor 1452 is 5 percent or less. Safe and stable operation can generally be achieved with a ClO₂ concentration of 5 percent or less. In further embodiments, absorption tank 1462 can be fluidly connected to stripper column 1416 to facilitate the introduction of air to meet the desired ClO₂ gas concentration.

[00119] The desired air flow rate that can result in a 5 percent concentration Of ClO₂ in air is balanced with the ClO₂ gas production rate. In a preferred embodiment, air flow rates with corresponding ClO₂ production rates that result in a 5 percent concentration of ClO₂ in air are listed below in Table 4.

Table 4

Air Flow and ClO₂ Production Rates for Maintaining a Five Percent ClO₂ Concentration in Air

[00120] In a preferred embodiment, a Fox liquid eductor available from Fox Valve Development Corp. of Dover, New Jersey, USA can be used in eductor system 1450. The operation of the Fox Model 129 eductor with 1.5 -inch (3.81 -cm) suction and discharge connections yielded the water pressure, water flow rate and air flow rate data listed below in Table 5. The operation of the Fox Model 129 eductor with 1-inch (2.54-cm) suction and discharge connections yielded the water pressure, water flow rate and air flow rate data listed below in Table 6. The water pressure and water flow were monitored at the inlet 1458 of gas eductor 1452. The air flow rate was monitored at inlet 1456.

Table 5 Flow Rates Using 1.5-inch Fox Model 129 Eductor

Table 6 Flow Rates Using 1-inch Fox Model 129 Eductor

[00121] In another preferred embodiment, the Model 485 eductor from Schutte & Koerting of Trevose, Pennsylvania, USA with 0.75-inch (1.91 -cm) suction and discharge connections can be used in eductor system 1450. Water pressure, water flow rate and air flow rate data for the Schutte & Koerting eductor are listed below in Table 7. The water pressure and water flow were monitored at inlet 1457 of gas eductor 1452. The air flow rate was monitored at inlet 1456. Table 7

Flow Rates Using 0.75-in. Schutte & Koerting Model 485 Eductor

[00122] In a preferred embodiment, ClO₂ generator 100 can be utilized to obtain higher yields Of ClO₂ gas, and therefore ClO₂ solution, by applying a higher current to electrochemical cell 1414 than those previously applied. As the current applied to the cell is increased the quantity Of ClO₂ that can be generated increases. Applying a higher current to the cell increases the rate of the selective oxidation reaction of, for example, chlorite to ClO₂, which can result in a higher yield of ClO₂ gas. A higher yield of ClO₂ gas can result in a higher yield Of ClO₂ solution. The high- capacity current can be greater than 50 amperes, but a preferred embodiment contemplates cooling for a generator system that operates at a current greater than 120 amperes. In another preferred embodiment, the electrochemical cell can operate at a current of 300 amperes or greater.

[00123] ClO₂ gas can be made using many different processes and the present chlorine dioxide generator can be used with a variety of such processes. Such processes include, but are not limited to, using electrochemical cells and a sodium chlorite solution, acidification of chlorite, reduction of chlorates by acidification, and reduction of chlorates by sulfur dioxide.

[00124] While particular elements, embodiments and applications of the present disclosure have been shown and described, it will be understood, of course, that the disclosure 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.

Claims

What is claimed is:

1. A chlorine dioxide gas generator comprising:

(a) a chlorine dioxide gas source; and

(b) a cooling system for cooling a chlorine dioxide gas obtained from said chlorine dioxide gas source.

2. The chlorine dioxide gas generator of claim 1 wherein said cooling system is interposed between

(a) a reactant feedstock for an anolyte loop in said chlorine dioxide gas source; and

(b) an electrochemical cell that is fluidly connected to said reactant feedstock stream wherein said electrochemical cell is located at least partially within said anolyte loop and a reactant feedstock stream is directed from said reactant feedstock through said cooling system.

3. The chlorine dioxide gas generator of claim 2 wherein said interposed cooling system comprises:

(a) an inner tube through which said reactant feedstock stream is directed;

(b) an outer jacket surrounding said inner tube; and

(c) a coolant material within said outer jacket.

4. The chlorine dioxide gas generator of claim 1 wherein said cooling system is interposed between

(a) an electrochemical cell operating within said chlorine dioxide gas source; and (b) a stripper column that is fhiidly connected to a positive end of said electrochemical cell, wherein said electrochemical cell and said stripper column are located at least partially within an anolyte loop of said chlorine dioxide gas source and wherein a chlorine dioxide solution is directed from said positive end of said electrochemical cell through said cooling system to said stripper column.

5. The chlorine dioxide gas generator of claim 4 wherein said interposed cooling system comprises:

(a) an inner tube through which chlorine dioxide solution is directed;

(b) an outer jacket surrounding said inner tube; and

(c) a coolant material within said outer jacket.

6. The chlorine dioxide gas generator of claim 1 wherein said cooling system is interposed between

(a) an electrochemical cell operating within said chlorine dioxide gas source; and

(b) a byproduct tank that is fluidly connected to a negative end of said electrochemical cell, wherein said electrochemical cell and said byproduct tank are located at least partially within a catholyte loop and wherein a byproduct solution is directed from said negative end of said electrochemical cell through said cooling system to said byproduct tank.

7. The chlorine dioxide gas generator of claim 6 wherein said interposed cooling system comprises:

(a) an inner tube through which said byproduct solution is directed;

(b) an outer jacket surrounding said inner tube; and

(c) a coolant material within said outer jacket.

8. The chlorine dioxide gas generator of claim 1 wherein said cooling system comprises:

(a) a coiled tube placed within said chlorine dioxide gas source; and

(b) a coolant material within said coiled tube.

9. The chlorine dioxide gas generator of claim 1 wherein said cooling system is located in the interior space of a stripper column, said stripper column located within said chlorine dioxide gas source.

10. The chlorine dioxide gas generator of claim 1 wherein said cooling system comprises:

(a) a cooling chamber in proximity with a surface of an electrochemical cell operating within said chlorine dioxide gas source; and

(b) a coolant material within said cooling chamber.

11. The chlorine dioxide gas generator of claim 1 wherein said cooling system comprises a fluid circulation apparatus directing fluid flow onto at least one surface of an electrochemical cell operating within said chlorine dioxide gas source.

12. The chlorine dioxide gas generator of claim 11 wherein a plurality of fins protrude from said surface of said electrochemical cell.

13. The chlorine dioxide gas generator of claim 1 wherein said cooling system maintains a chlorine dioxide gas temperature ofless than l30°F (54.4°C).

14. The chlorine dioxide gas generator of claim 1 wherein an electrochemical cell operates within said chlorine dioxide gas source.

15. The chlorine dioxide gas generator of claim 14 wherein said electrochemical cell operates at a current greater than 120 A.

16. The chlorine dioxide gas generator of claim 1 further comprising an absorption loop for effecting the dissolution of said chlorine dioxide gas, wherein said absorption loop is fluidly connected to said chlorine dioxide gas source and wherein said cooling system operates within at least one of said chlorine dioxide gas source and said absorption loop.

17. The chlorine dioxide gas generator of claim 16 wherein said cooling system operating within said absorption loop comprises at least one water flush injector fluidly connected before a gas transfer pump, said gas transfer pump located within said absorption loop and after said chlorine dioxide gas source.

18. The chlorine dioxide gas generator of claim 16 wherein said cooling system operating within said absorption loop comprises at least one water flush injector fluidly connected after a gas transfer pump, said gas transfer pump located within said absorption loop and after said chlorine dioxide gas source.

19. The apparatus of claim 18 wherein said water flush injector periodically introduces water into a chlorine dioxide gas stream from said chlorine dioxide gas source.

20. An electrochemical chlorine dioxide generator comprising: >

(a) an anolyte loop fluidly connected to a neutralization system; and

(b) a catholyte loop fluidly connected said neutralization system; wherein said neutralization system neutralizes waste products from at least one of said anolyte loop and said catholyte loop.

21. The electrochemical chlorine dioxide generator of claim 20, wherein an absorption loop is fluidly connected to said anolyte loop.

22. The electrochemical chlorine dioxide generator of claim 20, wherein a stripper column within said anolyte loop is fluidly connected to said neutralization system.

23. The electrochemical chlorine dioxide generator of claim 20, further comprising a pH control system to monitor the pH of said waste products in said neutralization system.

24. The electrochemical chlorine dioxide generator of claim 20, wherein at least one of said electrochemical chlorine dioxide generator and neutralization system is controlled with a programmable logic controller.

25. The electrochemical chlorine dioxide generator of claim 20, wherein said neutralization system comprises a pH treatment tank for receiving waste products, wherein said pH treatment tank is fluidly connected to at least one of said anolyte loop and said catholyte loop.

26. The electrochemical chlorine dioxide generator of claim 25, wherein said neutralization system further comprises a neutralization solution tank fluidly connected to said pH treatment tank.

27. The electrochemical chlorine dioxide generator of claim 25, further comprising a recirculation pump fluidly connected to said pH treatment tank, wherein said recirculation pump mixes said waste products in said pH treatment tank.

28. The electrochemical chlorine dioxide generator of claim 25, further comprising an effluent holding tank fluidly connected to said pH treatment tank, wherein said effluent holding tank receives neutralized waste products.

29. The electrochemical chlorine dioxide generator of claim 28, further comprising a transfer pump for transferring said neutralized waste products to said effluent holding tank, wherein a diverting valve controls said fluid connection between said pH treatment tank and said effluent holding tank.

30. The electrochemical chlorine dioxide generator of claim 26, wherein an acidic solution contained in said neutralization solution tank is used to neutralize said waste products in said pH treatment tank.

31. The electrochemical chlorine dioxide solution generator of claim 30, wherein said acidic solution comprises hydrochloric acid.

32. The electrochemical chlorine dioxide generator of claim 30, wherein said acidic solution further comprises a chlorite neutralizing compound.

33. The electrochemical chlorine dioxide generator of claim 32, wherein said chlorite neutralizing compound is selected from the group of ferrous chloride tetra hydrate, sodium sulfite, sodium metabisulfite, and sodium thiosulfate pentahydrate.

34. A chlorine dioxide solution generator comprising:

(a) a chlorine dioxide gas source capable of producing a chlorine dioxide gas;

(b) an eductor system for at least partially effecting the dissolution of said chlorine dioxide gas into an aqueous liquid stream wherein said eductor system is fluidly connected to said chlorine dioxide gas source; and

(c) an absorption system fluidly connected to said eductor system that is capable of effecting additional dissolution of said chlorine dioxide gas into said aqueous liquid.

35. The chlorine dioxide solution generator of claim 34, wherein said aqueous liquid stream for said eductor system is at least one of a chlorine dioxide solution and a dilution water recirculated between said eductor system and said absorption system.

36. The chlorine dioxide solution generator of claim 34, wherein said chlorine dioxide gas source further comprises a cooling system.

37. The chlorine dioxide solution generator of claim 34, wherein said absorption system is fluidly connected to a dosing pump capable of delivering a chlorine dioxide solution from said absorption system into a pressurized water system that operates at pressures up to approximately 200 psig (1,480 kPA).

38. ^■ The chlorine dioxide solution generator of claim 34, wherein said chlorine dioxide gas source operates using a single precursor chemical.

39. The chlorine dioxide solution generator of claim 34, wherein said absorption system is fluidly connected to said chlorine dioxide gas source to recirculate residual chlorine dioxide gas into said chlorine dioxide gas source.

40. The chlorine dioxide solution generator of claim 34, wherein said chlorine dioxide gas source comprises an anolyte loop and a catholyte loop, said catholyte loop fluidly connected to said anolyte loop via a common electrochemical component.

41. The chlorine dioxide solution generator of claim 40, wherein said anolyte loop comprises: (a) a precursor chemical feedstock stream;

(b) at least one electrochemical cell fluidly connected to said feedstock stream, said electrochemical cell system having a positive end and a negative end, said precursor chemical feedstock stream directed through said at least one electrochemical cell to produce a chlorine dioxide solution; and

(c) a stripper column, said chlorine dioxide solution directed from said positive end of said at least one electrochemical cell into said stripper column, said stripper column producing a chlorine dioxide gas stream, said chlorine dioxide gas stream exiting said stripper column directed to said eductor system.

42. The chlorine dioxide solution generator of claim 41 , wherein said chlorine dioxide gas stream is a mixture of less than 10 percent chlorine dioxide gas in air.

43. The chlorine dioxide solution generator of claim 36, wherein said cooling system maintains a chlorine dioxide gas temperature of less than 130° F (54.4°C).

44. The chlorine dioxide solution generator of claim 36, wherein said chlorine dioxide gas source comprises an electrochemical component operating at a current between 120 amperes and 300 amperes.

45. The chlorine dioxide solution generator of claim 36, wherein said chlorine dioxide gas source comprises an electrochemical component operating at a current greater than 300 amperes.