Electrochemical System and Method for On-Site Generation of Oxidants at High Current Density

Abstract

An electrochemical system and method are disclosed for On Site Generation (OSG) of oxidants, such as free available chlorine, mixed oxidants and persulfate. Operation at high current density, using at least a diamond anode, provides for higher current efficiency, extended lifetime operation, and improved cost efficiency. High current density operation, in either a single pass or recycle mode, provides for rapid generation of oxidants, with high current efficiency, which potentially allows for more compact systems. Beneficially, operation in reverse polarity for a short cleaning cycle manages scaling, provides for improved efficiency and electrode lifetime and allows for use of impure feedstocks without requiring water softeners. Systems have application for generation of chlorine or other oxidants, including mixed oxidants providing high disinfection rate per unit of oxidant, e.g. for water treatment to remove microorganisms or for degradation of organics in industrial waste water.

1 . A system for On-Site Generation (OSG) of an oxidant solution for chemical water treatment and disinfection comprising:
 an electrochemical cell comprising an anode and a cathode defining an active area of the cell, at least the anode being a conductive diamond anode;   an input port and flow control means for injecting an aqueous electrolyte feedstock between the electrodes, said electrolyte feedstock consisting essentially of a salt solution having a concentration in the range greater than 0.5 Molar to 3 Molar, said salt being selected for electrochemical generation of an oxidant comprising one or more of hypochlorite, other free and available chlorine (FAC), hydrogen peroxide, ozone, mixed oxidant and peroxydisulfate;   current means for supplying current for electrolyzing the aqueous feedstock electrolyte at a current density in the range from 150 mA/cm 2  to 1000 mA/cm 2  to produce an oxidant solution containing said oxidant; and   an output port for supplying the oxidant solution from the electrochemical cell.   
     
     
         2 . A system according to  claim 1  operable for electrolyzing the aqueous feedstock electrolyte at a current density≧200 mA/cm 2 . 
     
     
         3 . A system according to  claim 1  operable for electrolyzing the aqueous feedstock electrolyte at a current density in the range from 300 to 600 mA/cm 2 . 
     
     
         4 . A system according to  claim 3  further comprising control means for controlling the electrolyte temperature and flow rate and wherein, dependent on electrolyte feedstock concentration, during an operational cycle the current density is selected to provide a current efficiency at least 20% greater than the current efficiency at 100 mA/cm 2  at the selected temperature and flow rate. 
     
     
         5 . A system according to  claim 1  wherein the conductive diamond anode comprises an Ultra Nano Crystalline Diamond (UNCD) electrode. 
     
     
         6 . A system according to  claim 1  wherein the cathode comprises one of conductive diamond, tungsten, graphite, stainless steel, zirconium or titanium. 
     
     
         7 . A system according to  claim 1  wherein the anode and cathode are a matched pair of conductive diamond electrodes comprising UNCD on a metal substrate. 
     
     
         8 . A system according to  claim 1  wherein at least the diamond anode is rated for an operational lifetime at a current density of >300 mA/cm 2  of greater than 1 year, preferably greater than 2 years, and more preferably greater than 5 years. 
     
     
         9 . A system according to  claim 1  wherein operation at >300 mA/cm 2  provides a current efficiency that is at least 10% higher than the current efficiency for operation at 150 mA/cm 2  with other substantially similar operating parameters. 
     
     
         10 . A system according to  claim 1  wherein operation at >300 mA/cm 2  provides a current efficiency greater than 50%, with a salt conversion efficiency of greater than 10%. 
     
     
         11 . A system according to  claim 1  having an output capacity of more than 1 pound (0.45 kg) per day of free and available chlorine for each 100 cm 2  of active area. 
     
     
         12 . A system according to  claim 1  which is further operable in reverse polarity for a cleaning cycle. 
     
     
         13 . A system according to  claim 1  further comprising temperature control means for maintaining the electrolyte at a temperature in a selected range. 
     
     
         14 . A system according to  claim 1  further comprising control means for selecting operating parameters comprising a current density in the range from 300 to 600 mA/cm 2 , a flow rate in a range from 0.2 to 10 L/min per 200 cm 2  of active area, and an operating temperature below 30° C. 
     
     
         15 . A system according to  claim 1  configured for operation in a recycle mode or a batch mode, wherein the system further comprises a feedstock electrolyte reservoir coupled between the output port and the input port for recirculation or recycling of a volume/batch of the electrolyte during electrolysis to increase the oxidant concentration and/or percentage conversion of the salt. 
     
     
         16 . A system according to  claim 1  configured for operation in a single pass mode, such that the pumping means injects aqueous electrolyte from the input port, at a selected/desired flow rate between the electrodes, to the output port of the cell. 
     
     
         17 . A system according to  claim 1  having an output capacity of more than 1 pound per day of free and available chlorine for each 100 cm 2  of active area. 
     
     
         18 . A system according to  claim 1  further comprising means for applying a high current density reverse polarity cleaning cycle for 15 seconds to several minutes after operation at forward polarity for an extended operational period. 
     
     
         19 . A system according to  claim 1  wherein the controller provides for periodic reverse polarity cleaning cycle of less than 5 minutes per 24 hour operational period, and more preferably for approximately 1 to 3 minutes or less during a 24 hour operational period. 
     
     
         20 . A system according to  claim 1  wherein the pump means provides for injection of feedstock electrolyte at a rate from 0.2 L to 10 L per minute for each 100 cm 2  of active area. 
     
     
         21 . A system according to  claim 1  wherein the pump means provides for injection of feedstock electrolyte at a plurality of different rates for controlling the rate of reaction or production of oxidant or other reactive species in the cell. 
     
     
         22 . A system according to  claim 1  for generation of an oxidant comprising hypochlorite or other free available chlorine (FAC) from an electrolyte feedstock comprising one of sodium chloride (salt), solar salt, or other salt comprising more than 0.1% by weight of impurities. 
     
     
         23 . A system according to  claim 1  for generation of an oxidant comprising peroxydisulfate (peroxodisulfate or persulfate) from a sulfate of sodium or potassium, other alkali metal sulfate, or other alkaline earth metal sulfate or sulfuric acid. 
     
     
         24 . A system according to  claim 1  wherein the electrodes are spaced by an inter electrode separation in the range from 3 mm to 15 mm and preferably in the range from 4 mm to 8 mm. 
     
     
         25 . A system according to  claim 1  wherein one or both electrodes comprise a plurality of holes or perforations to enable flow through operation 
     
     
         26 . A method for On-Site Generation (OSG) of an oxidant solution for chemical water treatment and disinfection comprising in an electrochemical cell comprising a conductive diamond anode and a cathode, wherein the anode and cathode define an active area of the cell, performing the steps of:
 supplying to the electrochemical cell an aqueous feedstock electrolyte, consisting essentially of a salt solution having a concentration in the range greater than 0.5 Molar to 3 Molar, said salt being selected for electrochemical generation of an oxidant comprising one or more of hypochlorite, other free and available chlorine (FAC), hydrogen peroxide, ozone, mixed oxidant and peroxydisulfate;   in an operational cycle, electrolyzing the aqueous feedstock electrolyte at a current density in the range from 150 mA/cm 2  to 1000 mA/cm 2  to produce an oxidant solution comprising said oxidant; and   supplying said oxidant solution from the electrochemical cell.   
     
     
         27 . A method according to  claim 26  comprising electrolyzing the aqueous feedstock electrolyte at a current density above 200 mA/cm 2 . 
     
     
         28 . A method according to  claim 26  comprising electrolyzing the aqueous feedstock electrolyte at a current density in the range from 300 mA/cm 2  to 600 mA/cm 2 . 
     
     
         29 . A method according to  claim 26  comprising electrolyzing the aqueous feedstock electrolyte at a current density that provides a current efficiency at least 10% greater, and
 preferably at least 20% greater than the current efficiency achievable below 150 mA/cm 2 , with other substantially similar operating parameters. 
 
     
     
         30 . A method according to  claim 26  comprising electrolyzing the aqueous feedstock electrolyte at a current density that provides a current efficiency at least 10% greater than the current efficiency at 200 mA/cm 2 . 
     
     
         31 . A method according to  claim 26  comprising electrolyzing the aqueous feedstock electrolyte using conductive diamond electrodes having an operational lifetime of greater than 1 year, and preferably greater than 2 years and more preferably greater than 5 years. 
     
     
         32 . A method according to  claim 26  wherein the operational cycle comprises recycling and electrolyzing a batch volume of electrolyte to increase the concentration of oxidant (percentage conversion of the feedstock salt into oxidant). 
     
     
         33 . A method according to  claim 26  wherein the operational cycle comprises electrolyzing the electrolyte in a single pass or continuous flow system. 
     
     
         34 . A method according to  claim 26  further comprising: periodically operating the cell in a reverse polarity cleaning cycle. 
     
     
         35 . A method according to  claim 26  further comprising: after an operational cycle at a selected current density, operating the cell in a reverse polarity cleaning cycle for a period of 15 seconds to several minutes for operational cycle. 
     
     
         36 . A method according to  claim 26  comprising electrolyzing the feedstock wherein the diamond anode comprises ultrananocrystalline diamond (UNCD) on a metal substrate comprising niobium or tantalum. 
     
     
         37 . A method according to  claim 26  comprising: selecting the flow rate to control the rate of production of the oxidant. 
     
     
         38 . A method according to  claim 26  wherein the electrode separation (gap) is between 3 mm and 15 mm, more preferably between 4 mm and 8 mm 
     
     
         39 . A method according to  claim 26  wherein the aqueous feedstock electrolyte is injected into the cell at a flow rate of between 0.2 L per minute and 10 L per minute per 100 cm 2  of active area, and preferably between 1 and 5 L per minute per 200 cm 2  of active area. 
     
     
         40 . A method according to  claim 26  wherein the temperature of the electrolyte is maintained below 30° C. (86 F), and preferably about 20° C. (68 F). 
     
     
         41 . A method according to  claim 26  wherein the salt comprises a chloride for producing an oxidant comprising one of hypochlorite, ozone, hydrogen peroxide, and mixed oxidant. 
     
     
         42 . A method according to  claim 26  comprising converting at least 25% of the feedstock electrolyte salt into the oxidant. 
     
     
         43 . A method according to  claim 26  comprising converting at least 10% of the electrolyte feedstock salt into the oxidant at a current efficiency at least 10% greater than the current efficiency at a current density of less than 200 mA/cm 2 , with other substantially similar operating parameters. 
     
     
         44 . A method according to  claim 26  for generation of an oxidant comprising chlorine or other form of free available chlorine (FAC), wherein the feedstock comprises one of sodium chloride (salt), solar salt, and other salt comprising greater than 0.1% by weight of impurities, or the aqueous feedstock electrolyte comprises seawater. 
     
     
         45 . A method according to  claim 26  wherein the feedstock electrolyte solution comprises an aqueous solution having a salt concentration in the range from 0.6 M to 2 M. 
     
     
         46 . A method according to  claim 26  comprising producing more than 1 pound (0.45 kg) per day of free available chlorine per 100 cm 2  active area. 
     
     
         47 . A method according to  claim 26  wherein a solution of the resultant oxidant provides a disinfection rate at least 30% faster than that of a solution of sodium hypochlorite of the same FAC concentration. 
     
     
         48 . A method according to  claim 26  for water treatment wherein a solution of the resultant oxidant provides a disinfection rate at least 30% faster than that of hypochlorite or chlorine bleach solution of the same FAC concentration. 
     
     
         49 . A method according to  claim 26  for generation of an oxidant comprising peroxydisulfate (persulfate) wherein the salt comprises a sulfate of sodium, a sulfate of potassium, other alkali metal sulfate or other alkaline earth metal sulfate. 
     
     
         50 . A method according to  claim 26  comprising controlling the temperature and flow rate of the electrolyte, and selecting the current density for electrolyzing the feedstock to provide a current efficiency at the selected current density at least 20% greater than the current efficiency for operation below 150 mA/cm2, with other substantially similar operating parameters. 
     
     
         51 . An electrochemical On-Site Generation (OSG) system comprising a diamond anode for generating an oxidant by the method step of  claim 26 . 
     
     
         52 . An oxidant solution produced by electrolysing an aqueous feedstock electrolyte solution by the method step of  claim 26  comprising a solution of a mixed oxidant that provides a rate of disinfection at least 30% faster than that of hypochlorite solution of the same FAC concentration.