Electrochemical methods for production of alkali metal hydroxides without the co-production of chlorine

• US 5,246,551 A
• Filed: 02/11/1992
• Issued: 09/21/1993
• Est. Priority Date: 02/11/1992
• Status: Expired due to Fees

• Current Assignee: Electrosynthesis Co., Inc. (RWE AG)
• Original Assignee: Chemetics International Co. Ltd. (Nouryon BV)
• Inventors: Genders, J. David, Spiegel, Ella F., Weinberg, Norman L., Pletcher, Derek
• Primary Examiner(s): Niebling, John
• Assistant Examiner(s): IGOE, PATRICK
• Application Number:
  US07/835,147
• Time in Patent Office: 588 Days
• Field of Search: 204/96, 204/98
• US Class Current: 205/510
• CPC Class Codes: C25B 1/16 Hydroxides by simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine C25B1/34

Abstract

Methods for the electrolysis of aqueous solutions of alkali metal carbonates and bicarbonates for the production of alkali metal hydroxides at current efficiencies of >85 percent without the simultaneous co-production of halogen or acid can be performed at very low, commercially attractive cell voltages and at high current densities in single or two solution compartment cells with carbon dioxide as the only substantive co-product by maintaining cell pH at >7. The methods are also especially suitable for retrofitting existing chlor-alkali facilities for shifting the balance of production in favor of caustic soda at peak demands. The methods may also be performed with fuel cell configurations for even more attractive operating economics.

Claims

1. A method for producing an alkali metal hydroxide without the simultaneous production of chlorine, which comprises the steps of:

a) providing an electrochemical cell comprising a hydrogen consuming anode and an alkali metal hydroxide producing cathode;

b) introducing an electrolyte solution into said electrochemical cell, said solution comprising a salt selected from the group consisting of an alkali metal carbonate, alkali metal bicarbonate, and mixtures thereof;

c) impressing a voltage across said anode and cathode to produce alkali metal hydroxide and hydrogen at the cathode;

d) feeding a source of hydrogen to said hydrogen consuming anode while maintaining said electrolyte solution in said electro-chemical cell at a pH>

7 to produce carbon dioxide and water, and

e) facilitating the discharge of carbon dioxide at said anode at a sufficient rate to maintain cell voltages at <

2.6 V and at a current density of at least 100 mA/cm².

2. The method of claim 1 including the step of feeding hydrogen to said hydrogen consuming anode from a source other then said electrochemical cell.

3. The method of claim 1 including the step of feeding at least a portion of the hydrogen from said cathode to said hydrogen consuming anode.

4. The method of claim 1 including the step of feeding at least a portion of the hydrogen to said hydrogen consuming anode from another electrochemical cell.

5. The method of claim 1 including the step of providing an electrochemical cell comprising a cell divider positioned between said anode and cathode to form an anolyte compartment and a catholyte compartment, said divider being selected from the group consisting of a porous diaphragm and a cation exchange perm-selective membrane.

6. The method of claim 5 wherein said electrochemical cell is a two solution compartment type cell.

7. The method of claim 5 wherein the cell divider is a fluorinated cation exchange permselective membrane comprising chemical functionality selected from the group consisting of carboxylic acid groups, sulfonic acid groups and both carboxylic and sulfonic acid groups.

8. The method of claim 5 wherein the cell divider is a cation exchange permselective membrane comprising a material selected from the group consisting of perfluorosulfonic acid and perfluoro-carboxylic acid types.

9. The method of claim 5 wherein the cell divider is a non-fluorinated cation exchange membrane comprising a material having chemical functionality selected from the group consisting of carboxylic acid groups and sulfonic acid groups.

10. The method of claim 5 wherein the cell divider is a porous diaphragm comprising a material selected from the group consisting of PTFE, polypropylene and asbestos.

11. The method of claim 5 wherein the membranes are bi-layered.

12. The method of claim 11 wherein the membranes are bi-layered and comprise a perfluorocarboxylic acid layer adjacent to the anolyte compartment and a perfluorosulfonic acid layer adjacent to the catholyte compartment.

13. The method of claim 5 including the step of maintaining a sufficient concentration of alkali metal salt in said electrolyte solution in the anolyte compartment to provide a alkali metal hydroxide current efficiency of at least 85 percent.

14. The method of claim 5 including the step of spacially separating the anode and cell divider sufficiently to facilitate the discharge of carbon dioxide at the anode and provide a cell voltage of <

2.6 V at a current density of at least 100 mA/cm².

15. The method of claim 1 including the step of discharging carbon dioxide at the anode at a sufficient rate to maintain the cell voltage at <

1.8 V and at a current density of at least 200 mA/cm².

16. The method of claim 14 including the step of providing a hydrogen consuming anode comprising a dry side and a wet anolyte side wherein a substantial portion of the carbon dioxide generated at the anode is discharged on said dry side.

17. The method of claim 1 wherein the electrochemical cell is undivided and the hydrogen consuming anode comprises a dry side and a wet anolyte side wherein a substantial portion of the carbon dioxide generated at the anode is discharged on said dry side.

18. The method of claim 16 including the step of feeding a source of hydrogen to the hydrogen consuming anode under sufficient pressure and flow rate to enable discharge of a substantial portion of the carbon dioxide on the dry side of said anode, but below the gas breakthrough point of said anode so as to minimize discharge of carbon dioxide on the wet side of said anode.

19. The method of claim 17 including the step of feeding a source of hydrogen to the hydrogen consuming anode under sufficient pressure and flow rate to enable discharge of a substantial portion of the carbon dioxide on the dry side of said anode, but below the gas breakthrough point of said anode so as to minimize discharge of carbon dioxide on the wet side of said anode.

20. The method of claim 13 including the step of providing an electrochemical cell in which at least one of the electrodes comprises a substoichiometric titanium oxide.

21. A method for producing an alkali metal hydroxide without the simultaneous production of chlorine, which comprises the steps of:

a) providing an electrochemical cell comprising a hydrogen consuming anode in an anolyte compartment, a high performance cathode in a catholyte compartment and a cell divider positioned therebetween;

b) introducing an electrolyte solution into said anolyte compartment, said solution comprising a salt selected from the group consisting of an alkali metal carbonate, alkali metal bicarbonate, and mixtures thereof;

c) introducing an aqueous solution into said catholyte compartment, said aqueous solution comprising alkali metal cartions from said anolyte compartment;

d) impressing a voltage across said anode and cathode to produce alkali metal hydroxide and hydrogen at the cathode;

e) feeding a source of hydrogen to said hydrogen consuming anode while maintaining the electrolyte solution in said anolyte compartment at a pH >

7 to produce carbon dioxide and water, and

f) maintaining a sufficient concentration of said alkali metal salt in solution in said anolyte compartment and at a sufficiently high temperature while facilitating the discharge of carbon dioxide at said anode to provide a cell voltage of <

2.0 V at a current density of at least 100 mA/cm² and a alkali metal hydroxide current efficiency in the range of about 90 to about 95 percent.

22. The method of claim 21 wherein said electrochemical cell is a two solution compartment type cell.

23. The method of claim 21 including the step of providing an electrochemical cell with a hydrogen consuming anode which is carbon based and comprises a catalyst.

24. The method of claim 23 wherein the catalyst comprises a platinum group metal.

25. The method of claim 21 wherein the cell divider is a member selected from the group consisting of a porous diaphragm and a cation exchange permselective membrane.

26. The method of claim 21 wherein the high performance cathode is an electrode capable of lowering cell voltages by at least 100 mV below that of a conventional steel cathode.

27. A method for producing an alkali metal hydroxide without the simultaneous production of chlorine, which comprises the steps of:

a) providing an electrochemical cell comprising a hydrogen consuming anode and a gas consuming cathode;

b) introducing an electrolyte solution into said electro-chemical cell, said solution comprising a salt selected from the group consisting of an alkali metal carbonate, alkali metal bicarbonate, and mixtures thereof;

c) feeding a source of gas to said gas consuming cathode selected from the group consisting of air, oxygen and mixtures thereof;

d) impressing a voltage across said anode and cathode to produce alkali metal hydroxide at the cathode, and

e) feeding a source of hydrogen to said hydrogen consuming anode while maintaining said electrolyte solution in said electro-chemical cell at a pH >

7 to produce carbon dioxide and water at said anode.

28. The method of claim 27 including the step of providing an electrochemical cell comprising a hydrogen consuming anode, a gas consuming cathode, and a cell divider positioned therebetween.

29. The method of claim 28 wherein the cell divider is a member selected from the group consisting of a porous diaphragm and a cation exchange permselective membrane.

30. The method of claim 27 including the step of feeding a source of hydrogen to said hydrogen consuming anode from another electro-chemical cell.

31. The method of claim 30 wherein said source of hydrogen is from a chlor-alkali cell.

32. The method of claim 27 including the step of providing an electrochemical cell having a hydrogen consuming anode comprising a dry side and a wet side wherein at least a substantial portion of the carbon dioxide generated at the anode is discharged on said dry side.

33. The method of claim 27 wherein the electrochemical cell produces at least a portion of the power required for alkali metal hydroxide production.

34. The method of claim 27 wherein the electrochemical cell is a fuel cell.

35. In a method of producing alkali metal hydroxide without the simultaneous production of chlorine in an electrochemical cell comprising a hydrogen consuming anode and an alkali metal hydroxide producing cathode in which an electrolyte solution is introduced into said electrochemical cell comprising a salt selected from the group consisting of alkali metal carbonate, alkali metal bicarbonate and mixtures thereof;

impressing a voltage across said anode and cathode to produce alkali metal hydroxide and hydrogen at said cathode, and feeding the hydrogen to said hydrogen consuming anode to produce carbon dioxide and water while maintaining said electrolyte solution at a pH >

7,

the improvement comprising operating said electrochemical cell with a hydrogen consuming anode comprising a polymer selected from the group consisting of non-ionic and ionic charged types to provide a cell voltage of <

2.6 V and at a current density of at least 100 mA/cm².

36. The method of claim 35 wherein said hydrogen consuming anode is a composite type electrode having a coating of said ionic charged type polymer.

37. The method of claim 36 wherein said hydrogen consuming composite type electrode comprises a coating of said ionic charged type polymer adjacent to said electrolyte solution.

38. The method of claim 35 wherein said ionic-type polymer containing hydrogen consuming anode includes an electrocatalyst for electrochemical dissociation of hydrogen.

39. The method of claim 35 wherein said electrochemical cell is operated with a hydrogen consuming anode comprising the polymer so as to provide a cell voltage of <

1.8 V and at a current density of at least 100 mA/cm².