Patent Publication Number: US-6712951-B2

Title: Integrated ozone generator process

Description:
This is a continuation of U.S. Application Ser. No. 09/568,680, filed May 11, 2000, now U.S. Pat. No. 6,309,521, which is a continuation and claimed the benefit of U.S. Application Ser. No. 08/821,419, filed Mar. 21, 1997, now U.S. Pat. No. 6,287,431. 
    
    
     This invention was made with government support under contract F41624-96-C-2001 awarded by the Air Force. The government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the production of ozone for use in a variety of processes such as decontamination of water. More specifically, the invention relates to an electrochemical cell and a reprocess for generating ozone in the electrochemical cell. 
     2. Background of the Related Art 
     Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. Ozone has also been shown to be useful in both gas and aqueous phase oxidation reactions which may be carried out by advanced oxidation processes (AOPs) in which the formation of OH. radicals is enhanced by exposure to ultraviolet light. Certain AOPs may even involve a catalyst surface, such as a porous titanium dioxide photocatalyst, that further enhances the oxidation reaction. There is even evidence that ozone will destroy viruses. Consequently, it is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. Ozone may also be employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid. 
     Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. In addition, since ozone is explosive when concentrated as either a gas or liquid, or when dissolved into solvents or absorbed into cells, its transportation is potentially hazardous. Therefore, its is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations. 
     On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone through the following reaction: 
     
       
         3/2O 2 ═O 3 ; ΔH° 298 =34.1 kcal 
       
     
     Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O 3  by weight. Such O 3  concentrations, while quite poor, in an absolute sense, are still sufficiently high to furnish usable quantities of O 3  for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful NO x  otherwise known as nitrogen oxides. Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O 3 . 
     However O 3  may also be produced by the electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte, i.e., electrically conducting, fluid. The electrolyte includes water, which, in the process dissociates into its respective elemental species, O 2  and H 2 . Under the proper conditions, the oxygen is also evolved as the O 3  species. The evolution of O 3  may be represented as: 
     
       
         3H 2 O═O 3 +3H 2 ; ΔH° 298 =207.5 kcal 
       
     
     It will be noted that the ΔH° in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage. 
     More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high yields have not been realized in any forseeably practical electrolytic system. 
     The evolution of O 3  by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O 2 /O 3  gases evolved at the anode are comprised of 35% O 3  by volume. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about −30° C. to about −65°. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation. 
     Ozone, O 3 , is present in large quantities in the upper atmosphere in the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes, is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known. 
     Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste water may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl (OH.) and peroxyl (HO 2 .) radicals. These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O 3 . Most of the OH. radicals are produced in chain reactions where OH. itself or HO 2 . act as initiators. 
     Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide. 
     Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell. 
     The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode. 
     Recent ozone research has been focused primarily on methods of using ozone, as discussed above, or methods of increasing the efficiency of ozone generation. For example, research in the electrochemical production of ozone has resulted in improved catalysts, membrane and electrode assemblies, flowfields and bipolar plates and the like. These efforts have been instrumental in making the electrochemical production of ozone a reliable and economical technology that is ready to be taken out of the laboratory and placed into commercial applications. 
     However, because ozone gas has a very short life, it is preferably generated in close proximity to where the ozone will be consumed and at a rate substantially equal to the rate of consumption. Because so many of the present applications for ozone deal with the oxidation of contaminants in water streams, air streams and soil, it is typically impractical to bring the contaminant to a centralized ozone processing plant. Rather, it is imperative that the ozone be generated at the site of the contamination. This may be an active or abandoned industrial site or a remote location where little or no utilities are available. Furthermore, the rate of ozone consumption will vary according to the type of decontamination process and the nature of the site itself. 
     Unfortunately, there has been very little attention given to the development of self-contained and self-controlled support systems and utilities for ozone producing electrochemical cells. In order for these systems to be commercially successful, the systems must be reliable, require low maintenance, operate efficiently and be able to operate on standard utilities, such as 110 V, 60 Hz AC electricity provided by a standard gasoline powered generator. Furthermore, these objectives must be met while providing a simple system that can be used to decontaminate a site in a cost-effective manner. 
     Therefore, there is a need for an ozone generator system that operates efficiently on standard AC electricity and water to deliver a steady and reliable stream of ozone gas. It would be desirable if the system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system could provide a continuous supply of ozone at a rate dependent upon demand. 
     SUMMARY OF THE INVENTION 
     The present invention provides an ozone generating system that includes one or more electrolytic cells comprising an anode flow field and a cathode flow field. The system also includes an anode reservoir in fluid communication with the anode flow field, the anode reservoir comprising a gas discharge valve; and a cathode reservoir in fluid communication with the cathode flow field, the cathode reservoir comprising a gas discharge valve. The anode and cathode reservoirs may comprise a water inlet port. The anode reservoir preferably comprises a water cooling member in thermal communication with the anode reservoir and a water recirculating member. The anode reservoir may comprise a stand pipe having a small hole for equalizing water levels in the stand pipe and the anode reservoir. The anode reservoir may be in fluid communication through a control valve to the cathode reservoir. The system may further comprise a pump having an inlet in fluid communication with the anode reservoir and an outlet in fluid communication with the anode. The anode reservoir is preferably elevated above the anode flowfield and the anode reservoir inlet preferably communicates with the top of the anode flowfield. A system controller may be included in the system and be programmed to operate the anode reservoir gas discharge valve based on the water level in the anode reservoir. The system controller may also be programmed to operate a cathode reservoir gas discharge valve based on the water level in the cathode reservoir. 
     In another aspect of the invention, a process for generating ozone is provided comprising the steps of: electrolyzing water in one or more electrolytic cells comprising an anode flowfield and a cathode flowfield which separate ozone and oxygen from hydrogen; recirculating water between the anode flowfield and an anode reservoir; separating ozone and oxygen from water in the anode reservoir; discharging oxygen and ozone from the anode reservoir; receiving water from the cathode flowfield in a cathode reservoir; separating hydrogen from water in the cathode reservoir; discharging hydrogen from the cathode reservoir; and adding water to each reservoir as needed to maintain continuous production of ozone. The process may also include cooling water in the anode reservoir. It is preferred that water from the anode flowfield be recirculated to the anode reservoir through a stand pipe in the anode reservoir. A preferred stand pipe has a small hole at its base for equalizing water levels. Water can be added to the anode reservoir from the cathode reservoir. The anode reservoir and cathode reservoir may be operated at the same or different pressures and be maintained at separate setpoint pressures and a substantially constant water level. Most preferably, the anode reservoir operates at lower pressure than the cathode reservoir, such as about 30 psig and about 40 psig, respectively. A gas stream comprising between about 10% and about 18% by weight of ozone is discharged from the anode reservoir. 
     The ozone generator may comprise: one or more electrolytic cells comprising an anode and cathode; a power supply electronically coupled to the electrolytic cells; an anode reservoir in fluid communication with the anode, the anode reservoir comprising a gas releasing member; a recirculating member in fluid communication between the anode reservoir and the anode; a cathode reservoir in fluid communication with the cathode; a system controller in electronic communication with the power supply, the recirculating member, and the anode gas releasing member; and a memory device coupled to the system controller, the memory device comprising a readable program code for selecting a process comprising the steps of electrolyzing water in the electrolytic cells, recirculating water between the anode cell and the anode reservoir, separating ozone and oxygen from water in the anode reservoir, discharging oxygen and ozone from the anode reservoir, receiving water from the cathode cell in the cathode reservoir, and adding water from the cathode reservoir to the anode reservoir as needed to maintain continuous production of ozone. The ozone generator may further comprise a cooling member disposed in thermal communication with the water in the anode reservoir and/or a battery backup in electronic communication with the electrolytic cells. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the above recited features and advantages of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 is a schematic diagram of a self-controlled ozone generator which operates solely on electricity and distilled water. 
     FIG. 2 is a schematic diagram of an alternate ozone generator which operates without a controller, valves or level sensors. 
     FIG. 3 is an exploded perspective view of an electrolytic cell for the production of ozone. 
     FIG. 4 is a front view of a cell frame suitable for use in the electrolytic cell of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an ozone generator useful for supplying ozone to many industrial processes such as the photocatalytic oxidation of organic compounds in a non-organic solvent such as groundwater. The ozone generator includes one or more electrolytic cells comprising an anode, a cathode and a proton exchange membrane (PEM) disposed between the anode and cathode. The PEM is not only proton conducting, but also electronically insulating and gas impermeable to maintain separation of ozone and oxygen gases generated at the anode from hydrogen or other gases generated at the cathode. The ozone generator also comprises an anode reservoir having a gas discharge valve for venting oxygen and ozone, a recirculating member for recirculating water between the anode reservoir and the anode flowfield, and, optionally, a cooling member for cooling water in the anode reservoir. The ozone generator further comprises a cathode reservoir having a gas discharge valve for venting gases produced at the cathode. While both reservoirs may have a separate water filling port, it is preferred that the reservoirs communicate through an isolation valve so that the anode reservoir can be filled from the cathode reservoir while continuing to produce ozone. The ozone generator is readily configured for self-control using a system controller programmed to generate ozone while operating the anode reservoir and the cathode reservoir at constant pressures. 
     The ozone source preferably generates a gas stream comprising from about 10% to about 18% by weight of ozone in oxygen. Such electrolytic cells, including depolarizing electrolytic cells, are described in U.S. Pat. No. 5,460,705 which description is incorporated by reference herein. A fully self-controlled electrolytic cell for producing ozone is most preferred for use at remote locations such as a groundwater treatment facility. 
     In one aspect of the present invention, the anode reservoir comprises a cooling member which cools the water in the reservoir. Since the cooled water is recirculated to the anode compartment, the electrolytic cell is maintained at a temperature below about 30° C., where the cell operates most efficiently. Without the cooling member, the electrical resistances in the electrolytic cell generate heat that increases the temperature of the cell and effects the cell operation. 
     In another aspect of the present invention, a battery backup system is provided to maintain a potential across the cells during periods of power loss or idle operation. A preferred battery backup system includes a battery connected to the electrolyzer in parallel with the main power supply through a forward biased diode to provide backup power without interruption of signal processing. Maintaining a potential across an electrolytic cell has been found to increase the life of the lead dioxide electrocatalyst, which experiences an irreversible decrease in ozone production capacity following a complete loss of electrical potential. 
     In yet another aspect of the present invention, the ozone generator is provided with an ozone destruction unit or “ozone destruct”. The ozone destruct is disposed in communication with the ozone discharge of the anode reservoir. The amount of ozone that is produced and separated, but not used by some ozone consuming process, is catalytically destroyed on contact. The ozone destruct comprises a catalyst, such as Fe 2 O 3 , MnO 2  or a noble metal (e.g., platinum and palladium), most preferably MnO 2  or platinum. 
     Another aspect of the invention provides a simplified anode reservoir in which the ozone control valve and level sensor are eliminated. The simplified anode reservoir comprises a membrane that selectively allows the passage of ozone and oxygen gas while retaining water. The membrane is preferably a porous polytetratluoroethylene (PTFE) membrane, available from W. L. Gore &amp; Associates, Inc., Elkton, Md. under the trade name GORETEX®. The simplified anode reservoir also allows eliminates the need for a shut off valve in the tubing that connects the anode and cathode reservoirs. Without the shut off valve, water from the cathode reservoir flows freely to the anode reservoir to keep the anode reservoir full of water. 
     Yet another aspect of the present invention provides an electrolytic cell that efficiently produces ozone. The electrolytic cell uses a proton exchange membrane (PEM), such as a perfluorinated sulfonic acid polymer sheet, in intimate contact between the anode and cathode catalysts. The anode and cathode catalysts are also in intimate contact with an anode flowfield and a cathode flowfield, respectively. The flowfields make electrical contact with either a bipolar plate disposed between each cell or a current collector plate at the two ends of the cell stack. The anode flowfield is preferably made from a valve metal such as titanium. However, because the valve metals become embrittled from exposure to hydrogen, the cathode flowfield is preferably made from a metal other than the valve metals, such as stainless steel, nickel, copper or combinations thereof. 
     Another aspect of the invention provides anode and cathode flowfields each comprising a first region adjacent the PEM that is flat, smooth and porous and a second region that is more open and provides a low-resistance flow path therethrough. The first region provides substantially continuous and even support of the membrane and electrocatalysts so that the membrane and electrocatalysts are not damaged when the cell stack is compressed. The preferred anode flowfield has a first region made of porous, sintered titanium and a second region made of rolled, expanded titanium with each sheet rotated 90 degrees from the next sheet. The anode catalyst, such as lead dioxide (PbO 2 ), may be deposited either on the porous, sintered titanium surface of the anode flowfield or the surface of the PEM. The preferred cathode flowfield has a first region made of stainless steel felt or wool and porous stainless steel and a second region made of rolled, expanded stainless steel. Where the second regions are made of expanded metal, it is preferred that at least two sheets of the expanded metal be used and that each of the sheets be turned relative to the previous sheet, most preferably at about 90 degrees. The use of multiple expanded metal sheets substantially eliminates blockages to fluid flow that can occur with a single expanded metal sheet. 
     Because stainless steel felt can be so easily compressed, a most preferred cathode flowfield comprises stainless steel felt, at least two sheets of rolled expanded stainless steel, and a rigid perforated stainless steel sheet disposed between the felt and the expanded stainless steel. The preferred rigid perforated stainless steel sheet has holes therethrough which are larger than the passages in the felt and smaller than the openings in the expanded stainless steel. The rigid perforated stainless steel sheet provides support for the stainless steel felt and prevents the rolled expanded stainless steel sheet from damaging the stainless steel felt. 
     FIG. 1 is a schematic diagram of a self-controlled ozone generator  10  which operates solely on electricity and distilled water. The heart of the ozone generator  10  is a stack  12  of electrolytic cells (two shown)  14  separated by bipolar plates (one shown)  16  and sandwiched between a positive end plate  18  and a negative end plate  20 . Each of the two cells have an anode compartment  22  and a cathode compartment  24  separated by a proton exchange membrane  26 . The cells are constructed in a similar manner as those cells described in U.S. Pat. No. 5,460,705, which description is incorporated by reference herein, with the primary difference being that the preferred cathode and anode compartments of the present invention include rolled, expanded metal flowfields and the cathode compartment is filled with water instead of gas. The flow of fluid in and out of the anode and cathode compartments is schematically shown in FIG. 1 as passing through framing members  28  for purposes of simplicity. However, it should be recognized that the fluids actually pass through manifolds formed by adjacent framing members  28 , bipolar plates  16 , proton exchange membranes  26  and the like which communicate the fluid to openings in the end plates  18 ,  20 . 
     The anode compartment  22  is provided with water from an anode reservoir  30 . The anode reservoir  30  also serves as a liquid/gas separator wherein oxygen and ozone generated in the anode compartment  22  diffuses from the deionized water and collects at the top of the reservoir  30 . The reservoir  30  preferably includes a stand pipe  32  which enhances the liquid/gas separation. A preferred stand pipe  32  includes a small hole  34  in its sidewall below the water line, most preferably near the bottom of the anode reservoir  30 , which allows the water level in the stand pipe  32  to drop when the ozone generator  10  is in a low flow idle mode, such as when the recirculation pump  36  is turned off, so that gases will continue to rise from the anode compartment  22  through natural means. The small hole  34  does not interfere with flow of the gases up the stand pipe  32  during normal operation. 
     The water in the anode reservoir  30  is recirculated by a pump  36  back to the anode compartments  22 . As water is consumed by the electrochemical reaction which produces oxygen and ozone, water may be added to the anode reservoir from a deionized water source  38  or from the cathode reservoir  40 , as will be described in greater detail below. The gases accumulating in the upper portion of the anode reservoir  30 , comprising essentially oxygen and ozone, are released through an ozone control valve  42 . The ozone control valve  42  controls the flow of gases from the reservoir  30  either mechanically or in accordance with instructions from the system controller  44  which may be programmed in various manners. However, the control valve  42  is preferably opened to maintain a water level above the level sensor  46 . 
     The cathode reservoir  40  holds deionized water which rises from the cathode compartments  24 . The cathode reservoir  40  also serves as a liquid/gas separator wherein hydrogen generated in the cathode compartments  24  diffuses from the deionized water and collects at the top of the reservoir  40 . A hydrogen control valve  48  controls the flow of gases from the top of the reservoir  40  in co-operation with various system sensors, preferably the high/low liquid level indicators  50 ,  52 . 
     The anode reservoir  30  and the cathode reservoir  40  are preferably in communication with each other and a source of deionized (DI) water  38 . While these components may be communicated in a variety of ways, it is preferred that the system remain simple and include a minimal number of valves and couplings. One preferred configuration is shown in FIG. 1 having tubing that includes a first shut-off valve  54  between the reservoirs  30 ,  40  and a second shut-off valve  56  between the DI water source  38  and the cathode reservoir  40 . It is also preferred to have tubing that provides a drain loop having a third shut-off valve  58  between the anode reservoir  30  and the drain  62  and a fourth shut-off valve  60  between the DI source  38  and the drain  62  for bypassing the first and second shut-off valves for flushing or draining the system. 
     A cooling member  64  is disposed in a thermal relationship with the water in the anode reservoir  30 . Preferably, the cooling member  64  is a cooling coil disposed within the anode reservoir  30  that circulates a cooling fluid through a cooling cycle that includes a condenser  66  and a compressor  68 . While only about three windings of the cooling coil  64  are shown, any number of windings may be used. 
     The ozone generator  10  also includes a main power supply unit  70  and a power converter  72  for converting AC current to DC current for operation of the array of electrolytic cells  12 . The main power supply unit  70  preferably provides electrical power to all electrically powered devices in the generator  10  through appropriate electrically conducting wires. The generator  10  preferably includes a battery  74  which is used to backup the main power supply unit  70  during electrical interruptions and to provide smooth DC power to the system controller  44 . The battery  74  is preferably continuously charged by a battery charger  76  in order to maintain the battery  74  at a full charge. The main power supply unit  70  and the battery charger  76  are directly connected through electrical line  78  to some external source of AC electrical power, such as a standard household electrical line or a gasoline powered generator for remote use. 
     The ozone generator  10  is preferably self-controlled by a system controller  44  which receives various signals from sensors and switches and sends control signals to valves, pumps, switches and other devices shown in FIG.  1 . The system controller  44  executes system control software stored in a memory. The software is programmed to monitor the various signals indicating the operating conditions of the system and to control various devices in accordance with those conditions. It should be recognized that the programming of the system controller may take on any of a great number of schemes within the scope of the present invention and include additional, non-essential programming, such as system diagnostics, communications, data storage and the like. Further, the system may include additional devices and monitors not shown or described herein, such as an on/off switch. 
     In operation of the ozone generator  10 , the DI water source  38  preferably provides water at a pressure higher than the normal operating range of the anode reservoir  30  and the cathode reservoir  40  so that deionized water can be added to the system during normal operation. Reservoirs  30 ,  40  are preferably designed to operate in a range between about 0 and about 30 psig, and deionized water is conveniently provided to the system at about 50 psig. During initial start-up of the generator, the valves  54 ,  56  connecting the reservoirs  30 ,  40  and the DI water source  38  are open, but the valves  58 ,  60  leading to a system drain  62  remain closed. Deionized water fills the anode compartments  22  and the cathode compartments  24 . Prior to filling the anode reservoir  30  and the cathode reservoir  40 , the gas valves  42 ,  48  are closed to allow the pressure in the system to rise up to about 30 psig. Providing additional DI water into the system raises the level of water in either reservoir  30 ,  40  to the high level sensors  46 ,  50  by letting trapped air escape through the gas valves  42 ,  48  on the reservoirs  30 ,  40 , respectively, to maintain system pressure below about 30 psig. When the reservoirs are filled, the flow of deionized water is stopped by the second shut-off valve  56 . Recirculation of water in the anode reservoir  30  by the pump  36  and cooling of the water within the reservoir  30  by the cooling member  64  commences when electric current is applied to the array of electrolytic cells  12 . The first shut-off valve  54  will typically remain open so that water carried through the proton exchange membranes  26  from the anode compartments  22  to the cathode compartments  24  can rise into the cathode reservoir  40  and eventually return to the anode reservoir  30 . 
     Initial operation of the ozone generator with the gas valves closed causes oxygen, ozone, and water vapor to accumulate in the anode reservoir  30  and hydrogen and water vapor to accumulate in the cathode reservoir  40  until the system pressure reaches a desired level of about 30 psig. The gas valves  42 ,  48  are operated by the system controller  44  to maintain the desired system pressure while sending the wet oxygen/ozone gas through line  80  to some ozone consuming process, such as an advanced oxidation process. The wet hydrogen can be collected and used or flared. The ozone generator can continue operation while deionized water is added to the system by temporarily increasing the flow of gases through the gas valves  42 ,  48  to compensate for added water without raising the system pressure. Alternatively, the system pressure can be reduced prior to adding the deionized water. 
     FIG. 2 is a schematic diagram of an alternate ozone generator  140  which operates without a controller, valves or level sensors. The ozone generator  140  operates in a similar manner to the ozone generator  10  of FIG. 1, but has been modified to operate in a completely passive manner without the requirement of a control system, valves, or level sensors. The generator  140  eliminates all solenoid valves  54 ,  56 ,  58 ,  60 ,  42 ,  48 , all level sensors  46 ,  50 ,  52  and the control system  44  that are part of generator  10  of FIG.  1 . 
     The passive generator system  140  provides all water handling requirements and maintains a full water level in the anode reservoir  142  and cathode reservoir  144 . These reservoirs  142 ,  144  are both placed in direct communication with the deionized water source  146 . The fluid line  148  between the cathode reservoir  144  and the anode reservoir  142  is small in diameter to provide a sufficiently rapid fluid flow from the cathode reservoir  144  to the anode reservoir  142  so that ozone dissolved in the anode water is not allowed to diffuse into the cathode reservoir  144 . A back flow prevention device  150  prevents water or gas flow from the anode reservoir  142  to the cathode reservoir  144 . 
     The gas vent control valves  42  and  48  in FIG. 1 are replaced with hydrophobic membranes or phase separators  152 ,  154  that prevent the liquid water from escaping out of the tops of the reservoirs  142 ,  144 . These hydrophobic phase separators  152 ,  154  provide a barrier to water in its liquid state, but allows the free transmission of gases such as water vapor, hydrogen gas, oxygen gas, and ozone gas. The separators  152 ,  154  allow water from the deionized water source  146  to displace any gases in the reservoirs  142 ,  144  during initial filling. After all the gases are eliminated from the reservoirs  142 ,  144  and the water is in direct contact with the hydrophobic membranes  152 ,  154 , then the transfer of water ceases as the pressures in the reservoirs  142 ,  144  equalize with that of the water source  146 . The water in each reservoir  142 ,  144  continuously remains at this level during all phases of operation. 
     During normal operation of the ozone generation system  140 , gas bubbles are generated in the electrolyzer  156  and then transfer to the water reservoirs. Oxygen and ozone gas bubbles generated in the anode compartments  158  of the electrolyzer  156  are transferred to the anode reservoir  142  and hydrogen gas bubbles generated in the cathode compartments  160  are transferred to the cathode reservoir  144  where the gas bubbles rise to the top surface of their respective reservoirs into contact with the hydrophobic membranes  152 ,  154 . The hydrophobic membranes provide little or no restriction to the transmission of gas and water vapor from inside the reservoirs, at elevated pressure, to the vent lines  162 ,  164 . The separators  152 ,  154  are suitably supported by support structures  166 ,  168  which provide free flow of gas and any condensed liquid, but provide sufficient support of the membranes so that pressure differentials between the water in the reservoirs and the gas in the vent lines may possibly exceed about 100 psi. The membrane  152  and the support  166  are in turn provided with mechanical support and liquid and gas sealing by the vessel top  170 . The ozone/oxygen vent  162  is in direct communication with the dry side of the membrane  152  allowing the gas previously contained in the bubbles to leave the anode reservoir. Likewise, the hydrogen vent  164  is in direct communication with the dry side of its membrane  154  allowing the hydrogen gas previously entrained in bubbles to leave the cathode reservoir  144 . 
     A pressure regulator  172  may be added to allow the pressure of the oxygen and ozone gas on the dry side of the membrane  152  to reach any value up to the pressure of the liquid within the vessel  142 . In a similar manner, a pressure regulator  174  may be added to the hydrogen vent  164  to control the hydrogen delivery pressure. The pressure regulators  172 ,  174  may be operated independently of each other allowing the gases from the anode reservoir  142  and the cathode reservoir  144  to be regulated individually at gas pressures from sub-ambient up to the pressure of the water which is common to both the anode reservoir  142  and the cathode reservoir  144 . Overpressure regulators  176 ,  178  may be added to prevent overpressurizing the system in the event that the main discharge vents  162 ,  164  become blocked or surplus gas is produced. Ozone exiting the pressure release valve  176  may be destroyed using a catalytic destruct unit  178  before the gas is released through vent  180  to the atmosphere. Surplus hydrogen, or that resulting from overpressure gas, may be destroyed in a catalytic destruct unit  182  that reacts the hydrogen gas with oxygen from the air provided by an air pump  184 . The resulting water vapor and surplus air is released through a vent  186  to the atmosphere. The two destruct units  178 ,  182  may be placed in thermal communication with each other so the waste heat from the hydrogen/oxygen combination reaction will assist in the destruction of the ozone gas. 
     An optional boost pump  188  may be added between the deionized water source  146  and the water reservoirs  142 ,  144 . To further condition the water, a resin bed  190  may be added to the water source line. It is preferred to further include a return loop containing flow rate adjusting means  192  in order to continuously polish the incoming water. A back flow prevention device  194  is useful to prevent water from returning to the source  146 . 
     An auxiliary vent system in the cathode reservoir  144  prevents the transfer of hydrogen gas from the cathode reservoir  144  to the anode reservoir  142  in the event of an interruption of the water supply. This is accomplished using a dip tube  196  that extends downward in the cathode reservoir  144  to a point  198  above the bottom of the reservoir which defines the minimum acceptable water level. The dip tube  196  extends upward out of the reservoir and communicates with a hydrophobic membrane  200  with suitable support and housing  202 . When the water level is above point  198  and the reservoir is under pressure, water forces any gas in the tube  196  through the hydrophobic membrane  200  and out the vent  203  which is at atmospheric pressure or below. Should the water in the cathode reservoir drop below point  198 , the water presently in the tube will drain back out of the tube  196  into the reservoir  144  allowing the gas within the cathode reservoir  144  to escape up the dip tube and out the vent  203 . In this manner, the pressure in the reservoir  144  is reduced down to ambient pressure to prevent any further transfer of liquid from the cathode reservoir to the anode reservoir. 
     The anode reservoir  142  preferably includes a similar auxiliary vent system having a dip tube  204 , hydrophobic phase separator  206 , housing  208 , and vent  210 . Through some event, such as pressure fluctuations in the incoming water, if the pressure in the anode reservoir  142  is higher than the pressure in the cathode reservoir  144 , then the pressure driven transfer of water from the cathode reservoir to the anode reservoir will stop. When the water level in the anode reservoir falls below the lower opening  212  of the dip tube  204 , the pressure within the anode system is reduced and the pressure driven water transfer from the cathode reservoir to the anode reservoir is reestablished. 
     The rest of the system  140  may remain unchanged from that of generator  10  of FIG.  1 . Therefore, system  140  may include an anode recirculation pump  36 , a power supply  70 , a cooling system  66 , and a standpipe  32  with a level equaling hole  34  for natural circulation of the water during periods when the anode pump  36  is off and the water level falls below the top of the standpipe  32 . The cooling system  64 ,  66 ,  68  may be operated by an electrical or mechanical temperature controller  106  and a temperature sensor  35  in direct communication with the condenser system and the body to be temperature regulated, shown as the anode reservoir water in FIGS. 1 and 2 but which body may be the anode end plate or any other location representative of the electrolyzer temperature. The power supply unit  70  may also operate in an autonomous mode with self-control of the power output to match ozone demand. 
     Electrolytic Cells 
     Ozone gas is preferably generated by an electrolytic method which offers both process and cost benefits. In the electrolytic method, ozone is generated by the electrolysis of water using a special electrolytic cell. Sources of electrical power and water are the only requirements for producing O 3  electrochemically. Unlike the ozone gas produced by the corona process, electrolytically generated ozone does not contain toxic by-products. The electrolytic reactions occur by applying DC power between the anode and cathode which are placed on either side of a proton-exchange membrane (PEM), preferably a perfluorinated sulfonic acid polymer membrane (such as NAFION 117 available from DuPont de Nemours, Wilmington, Del.). Water is fed to the anode catalyst where water oxidation takes place resulting in both the thermodynamically favored O 2  evolution reaction and the O 3  formation reaction. 
     Utilization of high overpotentials and certain electrode materials selectively enhance O 3  formation at the expense of O 2  evolution. The water oxidation reactions yield protons and electrons which are recombined at the cathode. Electrons are conducted to the cathode via the external circuit. The protons and electrons are recombined at the cathode in the presence of water to form hydrogen gas. 
     The use of a PEM instead of a liquid electrolyte offers several advantages. First, fluid management is simplified and the potential for leakage of corrosive liquids is eliminated. Second, the PEM/anode interface provides a chemical environment which is well-suited to the electrochemical O 3  reaction. A PEM based on a fluoropolymer, such as a perfluorinated sulfonic acid polymer, displays very high resistance to chemical attack. 
     FIG. 3 is an exploded perspective view of the electrolytic cell stack  12  for the production of ozone. The cell stack  12  may include any number of individual cells, but is shown here with two cells  90  which are similar in construction and operation. Each cell  90  comprises an expanded titanium flowfield  107 , a porous titanium member  108  having a lead dioxide catalyst deposited on its surface facing the PEM  110 , and a cell frame  109  disposed around the flowfield  107  and member  108 . The PEM  110  may be either coated with a cathodic catalyst, such as platinum, facing the porous stainless steel sheet  111  or be placed in contact with a carbon fiber paper (not shown) that has the cathodic catalyst formed thereon. A porous stainless steel sheet  111  is placed against the cathodic catalyst surface, followed by a rolled, expanded stainless steel flowfield  112  which may include a plurality of sheets. Another cell frame  109  is disposed around the sheet  111  and flowfield  112 . A bipolar plate  113  is disposed between the two cells  90  to allow electronic conduction between the adjacent stainless steel flowfield  112  and the adjacent titanium flowfield  107 . 
     The positive terminal of the cell stack  12  (shown at the top of FIG. 3) includes a current collector face plate  106  and a current collector  105  which is coupled to a cable  92  attached to the positive terminal of the power converter  72  (shown in FIG.  1 ). An insulator plate  102  is disposed against the current collector  105  to isolate the end plate  101 , the water recycle bushing  104 , which delivers water from the anode reservoir  30  through the tubing  94  to the anode compartment, and the hydrogen/water bushing  103 , which communicates water and hydrogen from the cathode through tubing  96  to the cathode reservoir  40 . 
     The negative terminal of the cell stack  12  (shown at the bottom of FIG. 3) includes a current collector face plate  114  and a current collector  115  which is coupled to a cable  98  attached to the negative terminal of the power converter  72  (shown in FIG.  1 ). An insulator plate  102  is disposed against the current collector  115  to isolate the end plate  116  and the water/oxygen/ozone bushing  99 , which delivers water, oxygen and ozone from the anode compartment through the tubing  100  to the anode reservoir  30 . The current collector  115  is therefore cooled by the anode water passing through the cell stack  12 . The primary heat dissipating components of the power supply are preferably in thermal contact with the cooled current collector  115 . 
     The two endplates  101 ,  116  are drawn together to compress all the components of the electrolytic cell stack  12  into a filter press type arrangement in which adjacent components are in intimate contact. The cell frames  109 , membranes  110 , bipolar plate  113 , the current collector face plate  106 , and the like are sufficiently compressed to provide a sealing engagement and collectively form manifolds for the delivery and withdrawal of fluids in the cell stack  12 . 
     FIG. 4 is a front view of the cell frame  109  suitable for use in the electrolytic cell of FIG.  3 . The cell frame  109  has a plurality of bolt holes  120  around its perimeter edge for aligning and securing the cell frame in place with adjacent membranes  110 , bipolar plates  113  or current collector face plates  106 . The cell flame  109  has a center region  122  that is open to receive a flowfield and electrode, such as the expanded titanium flowfield  107 , the porous titanium sheet  108  and the electrocatalyst formed on the sheet  108 . A first manifold is provided by the row of holes  124  which may, for example, supply water to the center region  122  through the slots  126 . The water flowing through the center region  122  is then preferably collected in the opposing manifold, which is comprised of the holes  128  and slots  130 , and withdrawn from the cell stack. It should be recognized that the holes  124 ,  128  in both manifolds are lined up with and communicate with similar holes through adjacent components of the cell stack  12  (See FIG.  3 ). In the example just given, the water is delivered through holes  124  and slots  126  and passed through the titanium flowfield  107  and the porous titanium sheet  108  to the electrocatalyst where oxygen and ozone are produced. The ozone containing water is withdrawn through the slots  130  and holes  128  out of the cell stack to the anode reservoir. Conversely, the manifold formed by holes  132  and the manifold formed by holes  134  allow passage of fluids therethrough to another cell fame (not shown), such as a cell frame around a stainless steel flowfield  112  and a porous stainless steel sheet  111 . 
     EXAMPLE 
     An ozone generator was designed in accordance with FIGS. 1,  3  and  4  to produce about 5 pounds per day of ozone from about 5 gallons per day of deionized water. A stack of 10 electrolytic cells were used to generate a continuous output of about 7 liters/minute of a wet oxygen stream having about 15 wt % of ozone. Each cell had an active area of about 100 square centimeters. The anode flowfield was provided by three rolled, expanded sheets of titanium and a layer of sintered titanium in electrical contact with the expanded titanium. The sintered titanium layer had a thin layer of a lead dioxide catalyst deposited onto its surface and the lead dioxide was placed in face-to-face contact with a proton exchange membrane (PEM). The PEM was a sheet of perfluorinated sulfonic acid polymer, NAFION 117. The cathodic electrocatalyst was provided by a carbon fiber paper impregnated with a platinum catalyst. The fiber paper was placed against the second side of the PEM. The cathode flowfield was then assembled adjacent the carbon fiber paper and included a sheet of compressible, stainless steel felt, a perforated stainless steel sheet, and three sheets of rolled, expanded stainless steel where the diamond shaped openings of the three sheets were oriented 90 degrees from each adjacent sheet. 
     The anode reservoir and cathode reservoir were made from cylinders of borosilicate glass (PYREX® glass available from Corning Glass Works, Corning, N.Y.) bolted between two stainless steel endplates that were machined to receive the cylinders and communicate with various tubes and devices. The clear cylinder allowed visual inspection of the liquid/gas separation processes carried out therein. Each reservoir was oriented vertically and had a volume of about 2 gallons. The anode water was cooled with about 70 feet of ½ inch diameter tubing disposed in the anode reservoir. The tubing was coupled to a condenser unit rated at 26,900 BTU at 100° F. ambient and 35° F. suction temperature (such as model F 3AD-A325, available from Copeland of Sidney, Ohio). The power source received up to 70 amps of 208 volt three-phase current to power the various components. The power converter was a six pulse, midpoint converter consisting of six thyristors (model 110RK180, available from International Rectifier of El Segundo, Calif.). The system controller provided the thyristors with a phase angle which allowed for an increase or decrease of the power output. 
     While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims which follow.