Methods for Producing Sodium Hypochlorite with a Three-Compartment Apparatus Containing a Basic Anolyte

Abstract

An electrochemical method for the production of a chlorine-based oxidant product, such as sodium hypochlorite, is disclosed. The method may potentially be used to produce sodium hypochlorite from sea water or low purity un-softened or NaCl-based salt solutions. The method utilizes alkali cation-conductive ceramic membranes, such as membranes based on NaSICON-type materials, and organic polymer membranes in electrochemical cells to produce sodium hypochlorite. Generally, the electrochemical cell includes three compartments and the first compartment contains an anolyte having a basic pH.

1 . An electrolytic cell, comprising:
 an anolyte compartment holding an anolyte, the anolyte compartment comprising an anode in contact with the anolyte;   a catholyte compartment containing a catholyte, the catholyte compartment comprising a cathode in contact with the catholyte;   a middle compartment in operative communication with the anolyte compartment and the catholyte compartment, the middle compartment further comprising an electrolyte;   a polymeric anion-conducting membrane positioned between the anolyte compartment and the middle compartment; and   an alkali cation-conductive ceramic membrane selective to one type of cation, the cation-conductive membrane positioned between the middle compartment and the catholyte compartment, and   wherein the anolyte comprises a pH greater than about 6, such that the electrolytic cell produces a halogen-based oxidant product.   
   
   
       2 . The electrolytic cell of  claim 1 , wherein the alkali cation-conductive ceramic membrane comprises a MSICON membrane selective to M +  ions, wherein M comprises one or more of lithium, sodium, and potassium. 
   
   
       3 . The electrolytic cell of  claim 1 , wherein the alkali cation-conductive ceramic membrane comprises NaSICON. 
   
   
       4 . The electrolytic cell of  claim 1 , wherein the electrolyte comprises a pH greater than about 5.5. 
   
   
       5 . The electrolytic cell of  claim 1 , wherein the anolyte comprises alkali-chloride salt solution. 
   
   
       6 . The electrolytic cell of  claim 1 , wherein the anolyte comprises a pH between about 6 and about 12, the electrolyte comprises a pH between about 6 and about 13, and the catholyte comprises a pH between about 5.5 and about 14. 
   
   
       7 . The electrolytic cell of  claim 1 , wherein the halogen-based oxidant product comprises alkali hypohalite. 
   
   
       8 . The electrolytic cell of  claim 7 , wherein the catholyte comprise alkali hydroxide and/or alkali halide. 
   
   
       9 . An electrolytic cell for producing sodium hypochlorite, comprising:
 an anolyte compartment holding an anolyte, the anolyte compartment comprising an anode in contact with the anolyte;   a catholyte compartment containing a catholyte, the catholyte compartment comprising a cathode in contact with the catholyte;   a middle compartment in operative communication with the anolyte compartment and the catholyte compartment, the middle compartment comprising an electrolyte;   a polymeric anion-conducting membrane positioned between the anolyte compartment and the middle compartment; and   a NaSICON alkali cation-conductive ceramic membrane selective to sodium ions, the NaSICON membrane positioned between the middle compartment and the catholyte compartment,   wherein the anolyte comprises aqueous sodium chloride solution having a concentration of between about 1 wt % and about 25 wt % sodium chloride, and has a pH greater than about 7, and   wherein the catholyte comprises an aqueous sodium chloride solution having a concentration of between about 1 wt % and about 25 wt % sodium chloride, and   wherein the catholyte further comprises sodium hydroxide at a concentration of between about 1 wt % and about 26 wt %.   
   
   
       10 . The electrolytic cell of  claim 9 , wherein the anolyte comprises a pH between about 7 and about 12, the electrolyte comprises a pH between about 5.5 and about 13, and the catholyte comprises a pH between about 5.5 and about 14. 
   
   
       11 . A method for creating sodium hypochlorite, the method comprising:
 providing an electrolytic cell having:
 an anolyte compartment for holding an anolyte, the anolyte compartment comprising an anode positioned to contact the anolyte; 
 a catholyte compartment for containing a catholyte, the catholyte compartment comprising a cathode positioned to contact the catholyte; 
 a middle compartment for holding an electrolyte, the middle compartment being in operative communication with the anolyte compartment and the catholyte compartment; 
 a polymeric anion-conducting membrane positioned between the anolyte compartment and the middle compartment; and 
 an alkali cation-conductive ceramic membrane selective to one type of material, the cation-conductive membrane positioned between the middle compartment and the catholyte compartment, 
   introducing an aqueous, sodium chloride solution into the electrolytic cell;   applying a current between the anode and the cathode;   maintaining a pH of the first aqueous fluid greater than about 7; and   producing a product comprising sodium hypochlorite in the anolyte.   
   
   
       12 . The method of  claim 11 , wherein the aqueous sodium chloride solution is selected from brine, sea water, another solution comprising water and sodium chloride, and mixtures thereof. 
   
   
       13 . The method of  claim 11 , further comprising operating the electrochemical cell at a temperature between about 5° C. and about 30° C. 
   
   
       14 . The method of  claim 11 , further comprising producing the solution comprising sodium hypochlorite, wherein the solution comprises sodium hypochlorite at a concentration between about 0.1 and about 30 wt %. 
   
   
       15 . The method of  claim 11 , wherein the product comprises sodium hypochlorite present at a concentration between about 0.1 wt. % and about 15 wt. %. 
   
   
       16 . The method of  claim 10 , wherein the product comprises sodium hypochlorite present at a concentration between about 0.1 wt. % and about 8 wt. %. 
   
   
       17 . The method of  claim 11 , wherein the pH of the anolyte is maintained above 7 by addition of sodium hydroxide from either catholyte or external supply 
   
   
       18 . The method of  claim 11 , wherein the introducing the aqueous sodium chloride solution comprises introducing the sodium chloride solution into the anolyte compartment and the catholyte compartment, and wherein the method further comprises feeding effluent from the anolyte in the anolyte compartment and effluent from the catholyte in the catholyte compartment into the middle compartment. 
   
   
       19 . The method of  claim 11 , wherein the introducing the aqueous sodium chloride solution comprises introducing the sodium chloride solution into the catholyte compartment, and wherein the method further comprises:
 feeding effluent from the catholyte in the catholyte compartment into the middle compartment; and   feeding effluent from the electrolyte in the middle compartment into the anolyte compartment.   
   
   
       20 . The method of  claim 11 , wherein the introducing the aqueous sodium chloride solution comprises introducing the sodium chloride solution into the middle compartment, and wherein the method further comprises:
 feeding an effluent from the electrolyte in the middle compartment into the anolyte compartment and the catholyte compartment; and   mixing an effluent from the anolyte compartment and an effluent from the catholyte compartment to form the solution comprising sodium hypochlorite.   
   
   
       21 . The method of  claim 11 , further comprising:
 maintaining a pH of the anolyte between about 7 and about 12;   maintaining a pH of the electrolyte between about 5.5 and about 13; and   maintaining a pH of the catholyte between about 5.5 and about 14.   
   
   
       22 . The method of  claim 21 , wherein the usage of anolyte of pH above about 5.5 will prevent precipitation of scale forming salts present in the electrolyte onto the cationic membrane. 
   
   
       23 . The method of  claim 11 , wherein the aqueous sodium chloride solution comprises sodium chloride at a concentration between about 1% and about 25%.