Water desalination process and apparatus

• US 20060060532A1
• Filed: 09/13/2005
• Published: 03/23/2006
• Est. Priority Date: 09/13/2004
• Status: Active Grant

• Current Assignee: South Carolina University
• Original Assignee: University of South Carolina
• Inventors: Davis, Thomas A.
• Granted Patent: US 7,459,088 B2
• Time in Patent Office: Days
• US Class Current: 210/652
• CPC Class Codes: B01D 61/025 Reverse osmosis; Hyperfiltration
• CPC Class Codes: B01D 61/026 comprising multiple reverse osmosis steps
• CPC Class Codes: B01D 61/027 Nanofiltration
• CPC Class Codes: B01D 61/029 Multistep processes comprising different kinds of membrane processes selected from reverse osmosis, hyperfiltration or nanofiltration
• CPC Class Codes: B01D 61/422 Electrodialysis
• CPC Class Codes: B01D 61/423 comprising multiple electrodialysis steps
• CPC Class Codes: B01D 61/44 Ion-selective electrodialysis
• CPC Class Codes: B01D 61/445 with bipolar membranes; Water splitting
• CPC Class Codes: B01D 61/48 having one or more compartments filled with ion-exchange material , e.g. electrodeionisation
• CPC Class Codes: B01D 61/52 Accessories; Auxiliary operation
• CPC Class Codes: B01D 61/58 Multistep processes
• CPC Class Codes: C01D 3/06 Preparation by working up brines; seawater or spent lyes
• CPC Class Codes: C01F 11/46 Sulfates dehydration of gypsum for the production of calcium sulfate cements C04B11/02
• CPC Class Codes: C02F 1/441 by reverse osmosis
• CPC Class Codes: C02F 1/469 by electrochemical separation, e.g. by electro-osmosis, electrodialysis, electrophoresis
• CPC Class Codes: C02F 1/4693 electrodialysis
• CPC Class Codes: C02F 1/5236 using inorganic agents
• CPC Class Codes: C02F 2101/103 Arsenic compounds
• CPC Class Codes: C02F 2103/08 Seawater, e.g. for desalination
• CPC Class Codes: C02F 5/00 Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents softening using ion-exchange C02F1/42
• CPC Class Codes: Y02A 20/124 : Water desalination
• CPC Class Codes: Y02A 20/131 : Reverse-osmosis

Abstract

A process and system for purifying water is disclosed. For example, in one embodiment, the process may be used to remove a divalent salt, such as calcium sulfate, from a water source in order to prevent the divalent salt from precipitating during the process. The water source, for instance, may be fed to an ion separating device, such as an electrodialysis device. In the electrodialysis device, an ion exchange takes place between the divalent salt and another salt, such as a monovalent salt to produce two concentrated salt streams that contain salts having greater solubility in water than the divalent salt. In one embodiment, the two salt streams that are produced may then be combined to precipitate the divalent salt in a controlled manner. During the process, various other components contained within the water feed stream may also be removed from the stream and converted into useful products. In one particular embodiment, the process is configured to receive a byproduct stream from a reverse osmosis process.

Claims

1. A process for treating a water stream comprising:

feeding a water stream through a desalination process to produce a byproduct stream, the byproduct stream containing dissolved calcium ions and sulfate ions;

passing the byproduct stream in between a cation-removal device and an anion-removal device to remove a substantial portion of the calcium ions and the sulfate ions, the calcium ions passing into a first salt stream, the sulfate ions passing into a second salt stream, the first salt stream containing chlorine ions, the second salt stream containing sodium ions;

separately feeding the first salt stream and the second salt stream to a calcium sulfate precipitation chamber for precipitating calcium sulfate; and

collecting the precipitated calcium sulfate.

2. A process as defined in claim 1 wherein the cation-removal device and the anion-removal device are contained in an electrodialysis stack, the cation-removal device and the anion-removal device comprising membranes.

3. A process as defined in claim 1 wherein the first salt stream contains the calcium ions at a concentration that is at least about twice the concentration of calcium ions contained in the byproduct stream and the second salt stream contains sulfate ions at a concentration that is at least about twice the concentration of sulfate ions in the byproduct stream.

4. A process as defined in claim 2 wherein the electrodialysis stack further contains a cation-permeable membrane and an anion-permeable membrane, and wherein the process further comprises the step of feeding an aqueous solution comprising sodium chloride in between the cation-permeable membrane and the anion-permeable membrane for providing chloride ions to the first salt stream and sodium ions to the second salt stream respectively.

5. A process as defined in claim 4 wherein at least a portion of the sodium chloride in the aqueous solution is derived from the original water stream.

6. A process as defined in claim 4 wherein the aqueous solution comprising sodium chloride is completely derived from the original water stream.

7. A process as defined in claim 5 wherein the sodium chloride is derived from the water stream prior to formation of the byproduct stream.

8. A process as defined in claim 5 wherein the sodium chloride derived from the water stream is obtained by passing the water stream or the byproduct stream through a desalination device.

9. A process as defined in claim 4 wherein at least a portion of the sodium chloride contained in the aqueous solution is obtained from a supernatant stream exiting the calcium sulfate precipitation chamber.

10. A process as defined in claim 4 wherein the cation-permeable membrane and the anion-permeable membrane are both monovalent selective.

11. A process as defined in claim 4 wherein the electrodialysis stack comprises repeating cells containing the cation removal device, the anion-removal device, the cation-permeable membrane, and the anion-permeable membrane.

12. A process as defined in claim 11 wherein the byproduct stream is continuously recirculated through the electrodialysis stack while being combined with new amounts of the byproduct stream in order to maintain the supply of calcium ions to the first salt stream and sulfate ions to the second salt stream, and wherein portions of the first salt stream and portions of the second salt stream are diverted to the calcium sulfate precipitation chamber for precipitating the calcium sulfate.

13. A process as defined in claim 1 wherein at least portions of the second salt stream are fed to a sodium sulfate crystallization chamber upstream from the calcium sulfate precipitation chamber, and wherein sodium sulfate is crystallized and collected in the sodium sulfate crystallization chamber thereby producing a supernatant that is recombined with the second salt stream.

14. A process as defined in claim 1 wherein at least one of the sulfate ion concentration or the calcium ion concentration is monitored in the calcium sulfate precipitation chamber and wherein, based upon the monitored concentration, the relative amounts of the first salt stream and the second salt stream being fed to the calcium sulfate precipitation chamber are controlled to maintain excess calcium ions in the chamber.

15. A process as defined in claim 1 wherein the calcium sulfate precipitation chamber produces a supernatant containing sodium ions and chlorine ions and wherein the supernatant from the calcium sulfate precipitation chamber is used to provide chlorine ions and sodium ions for the first salt stream and the second salt stream respectively.

16. A process as defined in claim 1 wherein the calcium sulfate precipitation chamber produces a supernatant containing calcium ions that were not precipitated within the chamber, and wherein the process further comprises the step of combining the supernatant with a carbonate salt to produce a calcium carbonate precipitate.

17. A process as defined in claim 16 wherein the carbonate salt comprises sodium carbonate that, after being combined with the supernatant, is neutralized with an acid after removal of the calcium carbonate precipitate.

18. A process as defined in claim 1 wherein the flow rate of the byproduct stream through the process is at least 50,000 gallons per day.

19. A process as defined in claim 4 wherein the sodium chloride solution contains sodium chloride at a concentration of from about 0.01 eq/l to about 5.5 eq/l.

20. A process as defined in claim 4 wherein the sodium chloride solution contains sodium chloride at a concentration that is not substantially less than a total concentration of ions contained in the byproduct stream.

21. A process as defined in claim 1 wherein the calcium sulfate precipitation chamber produces a supernatant and wherein the supernatant is fed in between a monovalent-cation-permeable membrane and a monovalent-anion-permeable membrane to remove sodium monovalent ions, the remaining supernatant stream containing at least one multivalent metal ion, the supernatant stream being fed to a metal crystallization chamber for crystallizing the metal as a salt.

22. A process as defined in claim 21 wherein the metal comprises magnesium and the magnesium is crystallized either as magnesium sulfate or magnesium chloride.

23. A process as defined in claim 1 wherein the calcium sulfate precipitation chamber produces a supernatant, the supernatant containing ions comprising selenium, the supernatant stream being combined with a salt causing a selenium salt to precipitate.

24. A process as defined in claim 23 wherein the salt combined with the supernatant comprises a barium salt causing barium selenate to precipitate.

25. A process as defined in claim 2 wherein the desalination process to produce the byproduct stream comprises a reverse osmosis device that is maintained at a pressure and wherein the electrodialysis stack is operated at substantially the same pressure.

26. A process as defined in claim 1 wherein at least portions of the second salt stream are fed to a bipolar electrodialysis device for removing sulfate ions from the second salt stream prior to the calcium sulfate precipitation chamber.

27. A process as defined in claim 1 wherein the desalination process comprises a nanofiltration process.

28. A process for purifying water by removing a low soluble salt, MY, from a water stream to prevent the low soluble salt from precipitating within the water stream, the process comprising:

feeding a salt solution containing a dissolved salt, NX, to an ion-separating device for forming a first salt stream containing an anion, X, and a second salt stream containing a cation, N;

feeding the water stream containing the low soluble salt, MY, to an ion-separating device for adding the cation, M, to the first salt stream and for adding the anion, Y, to the second salt stream; and

wherein N comprises Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺, NH₄⁺ or an amine, and wherein X comprises Cl⁻

, Br⁻

, I⁻

, NO₃⁻

or an organic ion, and wherein both salts NY and MX are more soluble in water than the salts MY.

29. A process as defined in claim 28 wherein the salt solution containing the dissolved salt, NX, is derived from the water stream containing the low soluble salt, MY.

30. A process as defined in claim 29 wherein the concentration of NX in the water stream is less than the concentration of the low soluble salt, MY, in the water stream.

31. A process as defined in claim 30 wherein the concentration of NX in the water stream is at least less than one-half the concentration of MY in the water stream.

32. A process as defined in claim 28 wherein M comprises a barium ion or a strontium ion.

33. A process as defined in claim 28 wherein the low soluble salt, MY, comprises calcium sulfate.

34. A process as defined in claim 28 wherein the salt, NX, comprises sodium chloride.

35. A process as defined in claim 28 wherein both the salt solution and the water stream are fed to an electrodialysis stack for separating the anions from the cations.

36. A process as defined in claim 35 wherein the electrodialysis stack produces a product stream containing residual amounts of MY, the product stream being fed to a desalination device for producing a purified water stream and a salt stream, the salt stream being recycled back to the electrodialysis stack and being combined with the water stream that is being fed to the electrodialysis stack.

37. A process as defined in claim 35 wherein the electrodialysis stack comprises a repeating cell including alternately arranged anion permeable membranes and cation permeable membranes.

38. A process as defined in claim 28 wherein the water stream comprises a byproduct stream obtained from a desalination process.

39. A process as defined in claim 38 wherein the desalination process comprises reverse osmosis, nanofiltration, electrodialysis, ion exchange, evaporation, or combinations thereof.

40. A process as defined in claim 38 wherein, after the water stream containing the low soluble salt, MY, passes through the ion separating device, at least a portion of the water stream is returned to the desalination process.

41. A process as defined in claim 28 further comprising the step of feeding the first salt stream and the second salt stream to a precipitation chamber for precipitating and collecting the low soluble salt, MY.

42. A process as defined in claim 41 wherein the precipitation chamber produces a supernatant and wherein the process further comprises the step of feeding the supernatant through an electrodialysis stack for forming a more concentrated salt solution containing the dissolved salt, NX.

43. A process as defined in claim 42 wherein the more concentrated salt solution is used as the salt solution fed to the ion-separating device for forming the first salt stream and the second salt stream.

44. A process as defined in claim 41 wherein the precipitation chamber produces a supernatant that contains unprecipitated cations, M, and wherein the process further includes the step of combining the supernatant with a salt that causes the cation, M, to form a salt and precipitate.

45. A process as defined in claim 44, wherein the salt that causes the cation, M, to precipitate is a soluble carbonate.

46. A process as defined in claim 35, wherein the salt solution containing the dissolved salt, NX, is fed in between a cation-permeable membrane and an anion-permeable membrane contained within the electrodialysis stack wherein one or both of the membranes are monovalent selective.

47. A process as defined in claim 41, wherein the precipitation chamber produces a supernatant, the supernatant containing at least one cation selected from the group including magnesium, copper, and cobalt, and wherein the process further comprises the step of feeding the supernatant to a crystallization chamber for crystallizing the cation as a salt.

48. A process as defined in claim 47, wherein the cation comprises magnesium and magnesium is crystallized as magnesium sulfate or as magnesium chloride.

49. A process as defined in claim 41, wherein the precipitation chamber produces a supernatant, the supernatant containing at least one anion selected from the group consisting of selenate and arsenate and wherein the supernatant is combined with a salt that causes the anion to precipitate as a salt.

50. A process as defined in claim 49, wherein the anion is selenate and the salt that is combined with the supernatant comprises a barium salt and wherein selenium is precipitated as a barium selenate.

51. A process as defined in claim 49, wherein the anion is arsenate and the salt that is combined with the supernatant comprises a zinc salt and wherein arsenate is precipitated as a zinc arsenate.

52. A process as defined in claim 46 wherein the salt solution exits the electrodialysis stack containing residual amounts of the dissolved salt, NX, the salt solution exiting the electrodialysis stack being fed to a desalination device for producing a purified water stream and a salt stream containing the dissolved salt, NX, the salt stream being combined with the salt solution containing the dissolved salt, NX, as it is being fed to the ion-separating device.

53. A process for purifying water by removing a low soluble salt, MY, from a water stream to prevent the low soluble salt from precipitating within the water stream, the process comprising;

feeding the water stream containing the low soluble salt, MY, to a ion-separating device for adding the cation, M, to a first salt stream and for adding the anion, Y, to a second salt stream;

feeding a salt solution containing a dissolved salt, NX, to the ion-separating device for adding the anion, X, to the first salt stream and for adding the cation, N, to the second salt stream, the salt solution being derived from the water stream containing the low soluble salt; and

wherein N comprises Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺, NH₄⁺ or an amine, and wherein X comprises Cl⁻

, Br⁻

, I⁻

, NO₃⁻

or an organic ion, and wherein both salts NY and MX are more soluble in water than the salts MY.

54. A process as defined in claim 53 wherein the water stream contains the low soluble salt, MY, at a concentration that is higher than the concentration at which the water stream contains the salt, NX.

55. A process as defined in claim 54 wherein the concentration of MY in the water stream is at least about twice the concentration of NX in the water stream.

56. A process as defined in claim 53 wherein the salt solution containing NX is derived from the water stream by feeding the water stream through a desalination device in order to remove the salt, NX.

57. A process as defined in claim 56 wherein the desalination device comprises a nanofiltration device.

58. A process as defined in claim 53 further comprising the step of feeding the water stream exiting the ion-separating device to a desalination device for producing a purified water stream and a salt stream containing MY, the salt stream containing MY being recycled back to the water stream.

59. A process as defined in claim 53 further comprising the step of feeding the salt solution exiting the ion-separating device to a desalination device for producing a purified water stream and a salt stream containing NX, the salt stream containing NX being recycled back to the salt solution.

60. A process as defined in claim 53 wherein MY comprises calcium sulfate and NX comprises sodium chloride.

61. A process as defined in claim 53 wherein the ion-separating device comprises an electrodialysis stack.

62. A process as defined in claim 61 wherein the electrodialysis stack comprises a repeating cell including alternately arranged anion permeable membranes and cation permeable membranes.

63. A process as defined in claim 53 further comprising the step of feeding the first salt stream and the second salt stream to a precipitation chamber for precipitating and collecting the low soluble salt, MY.

64. A process as defined in claim 63 wherein the precipitation chamber produces a supernatant and wherein the process further comprises the step of feeding the supernatant through an electrodialysis stack for forming a more concentrated salt solution containing the dissolved salt, NX.

65. A process as defined in claim 64 wherein the more concentrated salt solution is used as the salt solution fed to the ion-separating device.

66. A process as defined in claim 63 wherein the precipitation chamber produces a supernatant that contains unprecipitated cations, M, and wherein the process further includes the step of combining the supernatant with a salt that causes the cation, M, to form a salt and precipitate.

67. A process as defined in claim 61, wherein the salt solution containing the dissolved salt, NX, is fed in between a cation-permeable membrane and an anion-permeable membrane contained within the electrodialysis stack wherein one or both of the membranes are monovalent selective.

68. A process as defined in claim 63, wherein the precipitation chamber produces a supernatant, the supernatant containing at least one cation selected from the group including magnesium, copper, and cobalt, and wherein the process further comprises the step of feeding the supernatant to a crystallization chamber for crystallizing the cation as a salt.

69. A process as defined in claim 63, wherein the precipitation chamber produces a supernatant, the supernatant containing at least one anion selected from the group consisting of selenate and arsenate and wherein the supernatant is combined with a salt that causes the anion to precipitate as a salt.