PROCESS AND APPARATUS FOR TREATING SPENT CAUSTIC SOLUTION

Abstract

A process for treating a spent caustic solution and an apparatus for treating a spent caustic solution. The treatment provides an environmentally acceptable solution for discharge into a conventional wastewater treatment plant or for further processing. A spent caustic solution enters a reactor and is cycled through the reactor and into a process liquid line where it is contacted with ozone and with carbon dioxide to form a treated solution of pH 7.0 to 11.0.

Claims

1. A process for treating spent caustic solution, the process comprising: contacting a spent caustic solution with (i) a gas comprising ozone; and (ii) carbon dioxide; to form a treated solution having a pH of 7.0 to 11.0; and discharging at least a portion of the treated solution.

2. The process of claim 1, wherein the spent caustic solution comprises sodium hydroxide.

3. The process of claim 1, wherein the spent caustic solution is contacted with a gas comprising ozone in a first reactor to form a partially treated solution and the partially treated solution is contacted with carbon dioxide in a second reactor to form the treated solution.

4. The process of claim 3, wherein at least a portion of the treated solution is recycled from the second reactor to the first reactor.

5. The process of claim 1, wherein the spent caustic solution is contacted with a gas comprising ozone and carbon dioxide in a first reactor to form the treated solution.

6. The process of claim 5, wherein at least a portion of the spent caustic solution is contacted with the gas comprising ozone, carbon dioxide, or a mixture of ozone and carbon dioxide, prior to introducing the portion of the spent caustic solution and the gas into the first reactor.

7. The process of claim 1, wherein the gas comprising ozone consists of oxygen and ozone.

8. The process of claim 1, wherein the gas comprising ozone comprises a percentage volume ratio of ozone to oxygen of 10% to 15%.

9. The process of claim 1, wherein contacting the spent caustic solution with (i) a gas comprising ozone or (ii) carbon dioxide is carried out at a temperature of 20 C. to 140 C.

10. The process of claim 1, wherein the spent caustic solution is heated to a temperature of 20 C. to 140 C. before being contacted with (i) a gas comprising ozone and (ii) carbon dioxide.

11. The process of claim 1, wherein the process is carried out at a pressure of 1.0 barg to 2.5 barg.

12. The process of claim 1, wherein the process is carried out in a continuous process.

13. The process of claim 1, wherein the process is carried out in a batch process.

14. The process of claim 1, wherein the treated solution has a pH of 7.0 to 9.0.

15. The process of claim 1, wherein the spent caustic solution comprises heavy metals.

16. The process of claim 15, wherein the heavy metals comprise cobalt, molybdenum, or a combination of cobalt and molybdenum.

17. The process of claim 15, further comprising removing heavy metal carbonates and heavy metal oxides as precipitate from the treated solution prior to discharging at least a portion of said treated solution; wherein the treated solution has a pH of 10.0 to 11.0 prior to removing the heavy metal carbonates and heavy metal oxides as precipitate.

18. The process of claim 17, wherein the treated solution has a pH of 10.25 to 10.75 prior to removing the heavy metal carbonates and heavy metal oxides as precipitate.

19. The process of claim 17, wherein heavy metal carbonates and heavy metal oxides are removed via coagulation, filtration, gravity settlement, one or more hydrocyclones, centrifugation or evaporation.

20. The process of claim 19, wherein heavy metal carbonates and heavy metal oxides are removed via nano-filtration or ultra-filtration.

21. The process of claim 17, further comprising heating the removed heavy metal carbonates to a temperature of 400 C. to 800 C. to recover heavy metals and heavy metal oxides.

22. The process of claim 21, wherein the removed heavy metal carbonates are heated to a temperature of 500 C. to 700 C.

23. The process of claim 17, further comprising reducing the pH of the treated solution after removing heavy metal carbonates and heavy metal oxides.

24. The process of claim 1, wherein at least a portion of the treated solution is discharged as a discharge solution having a pH of 7.0 to 9.0.

25. The process of claim 24, wherein the discharge solution has a pH of 7.0 to 8.5.

26. The process of claim 25, wherein the discharge solution is discharged to a wastewater plant.

27. The process of claim 1, wherein (i) the spent caustic solution comprises volatile organic components and the process produces an off-gas comprising at least a portion of said volatile organic components; (ii) at least a portion of the off-gas is combusted in the presence of hydrogen to produce a recycle stream comprising carbon dioxide; and (iii) the spent caustic solution is contacted with at least a portion of the recycle stream.

28. The process of claim 27, further comprising mixing at least a portion of the recycle stream with the gas comprising ozone, carbon dioxide, or a mixture of ozone and carbon dioxide before contacting the spent caustic solution.

29. The process of claim 27, wherein the recycle stream further comprises one or more of H.sub.2SO.sub.4, H.sub.2SO.sub.3, S.sub.2O.sub.3, SO.sub.3, N.sub.2O.sub.5, NH.sub.4NO.sub.3 and NO.sub.2.

30. The process of claim 1, wherein at least a portion of (i) the gas comprising ozone and (ii) carbon dioxide are mixed to provide a gas mixture comprising ozone and carbon dioxide before contacting the spent caustic solution and the partially treated solution.

31. The process of claim 1, wherein the gas comprising ozone comprises 5% to 20% by volume of ozone based on the total volume of the gas.

32. An apparatus for treating a spent caustic solution, the apparatus comprising: a first reactor; wherein the first reactor has an inlet for introducing a solution and an outlet for removing solution; and the first reactor comprises a means for introducing a gas comprising ozone into the reactor and a means for introducing carbon dioxide into the reactor.

33. An apparatus for treating a spent caustic solution, the apparatus comprising: a first reactor, and a second reactor, wherein each of the first reactor and the second reactor has an inlet for introducing solution and an outlet for removing solution; and wherein the first reactor and the second reactor are in fluid communication with one another, such that, in use, solution can be transferred from the first reactor to the second reactor; the first reactor comprising a means for introducing a gas comprising ozone into the reactor, and the second reactor comprising a means for introducing carbon dioxide into the reactor.

34. The apparatus of claim 33, further comprising a means for recycling solution from the second reactor to the first reactor.

35. The apparatus of claim 32, further comprising an ozone source or an ozone generator in fluid communication with the first reactor.

36. The apparatus of claim 32, further comprising a means to monitor the pH of solution present in the first reactor.

37. The apparatus of claim 32, further comprising a pressure monitor in the first reactor.

38. The apparatus of claim 32, further comprising a pressure valve to control the pressure in the first reactor.

39. The apparatus of claim 32, further comprising a combustion vessel in fluid communication with the first reactor, for combusting off-gas produced in the first reactor with hydrogen.

40. The apparatus of claim 39, further comprising a means to recycle gas produced in the combustion vessel to the first reactor.

41. The apparatus of claim 39, further comprising a means to monitor the heat generated in the combustion vessel or a means to monitor combustion products.

42. The apparatus claim 32, further comprising a means to monitor the total organic carbon (TOC) of the solution present in the first reactor.

43. The apparatus claim 32, further comprising a means to monitor the chemical oxygen demand (COD) of the solution present in the first reactor.

44. The apparatus of claim 32, further comprising a programme logic controller (PLC) configured to control the amount or the make-up of the gas comprising ozone, carbon dioxide, or a mixture of ozone and carbon dioxide, provided to the first reactor.

45. The apparatus of claim 44, wherein the apparatus comprises one or more components selected from: (i) a means to monitor the pH of solution present in the first reactor; or (ii) a means to monitor the heat generated in the combustion vessel; or (iii) a means to monitor combustion products; or (iv) a means to monitor the total organic carbon of the solution present in the first reactor; or (v) a means to monitor the chemical oxygen demand of the solution present in the first reactor; wherein the programme logic controller is in communication with each component.

46. (canceled)

47. (canceled)

48. The apparatus of claim 33, further comprising an ozone source or an ozone generator in fluid communication with the first reactor.

49. The apparatus of claim 33, further comprising a means to monitor the pH of solution present in the first reactor, the second reactor or both the first reactor and the second reactor.

50. The apparatus of claim 33, further comprising a pressure monitor in the first reactor, the second reactor or both the first rector and the second reactor.

51. The apparatus of claim 33, further comprising a pressure valve to control the pressure in the first reactor, the second reactor or both the first reactor and the second reactor.

52. The apparatus of claim 33, further comprising a combustion vessel in fluid communication with the first reactor, the second reactor or both the first reactor and the second reactor, for combusting off-gas produced in the first reactor, the second reactor or both the first reactor and the second reactor with hydrogen.

53. The apparatus of claim 52, further comprising a means to recycle gas produced in the combustion vessel to the first reactor, the second reactor or both the first reactor and the second reactor.

54. The apparatus of claim 52, further comprising a means to monitor the heat generated in the combustion vessel or a means to monitor combustion products.

55. The apparatus claim 33, further comprising a means to monitor the total organic carbon (TOC) of the solution present in the first reactor, the second reactor or both the first reactor and the second reactor.

56. The apparatus claim 33, further comprising a means to monitor the chemical oxygen demand (COD) of the solution present in the first reactor, the second reactor or both the first reactor and the second reactor.

57. The apparatus of claim 33, further comprising a programme logic controller (PLC) configured to control the amount or the make-up of the gas comprising ozone, carbon dioxide, or a mixture of ozone and carbon dioxide provided to the first reactor, the second reactor or both the first reactor and the second reactor.

58. The apparatus of claim 57, wherein the apparatus comprises one or more components selected from: (i) a means to monitor the pH of solution present in the first reactor, the second reactor or both the first reactor and the second reactor; or (ii) a means to monitor the heat generated in the combustion vessel; or (iii) a means to monitor combustion products; or (iv) a means to monitor the total organic carbon of the solution present in the first reactor, the second reactor or both the first reactor and the second reactor; or (v) a means to monitor the chemical oxygen demand of the solution present in the first reactor, the second reactor or both the first reactor and the second reactor; wherein the programme logic controller is in communication with each component.

Description

[0153] These and other aspects of the invention will now be described with reference to the accompanying Figures, in which:

[0154] FIG. 1 is a schematic diagram of a multi-reactor apparatus and process according to the present invention.

KEY TO FIG. 1

[0155] 1=ozone generator [0156] 2=gas pressure control valve [0157] 3=pressure indicator/transmitter [0158] 4=pH control meter [0159] 5=treated solution outlet [0160] 6=CO.sub.2 dosing panel [0161] 7=recycle pump [0162] 8=primary ozone gas/liquid contactor [0163] 9=secondary ozone gas liquid off-gas contactor [0164] 10=CO.sub.2 fine bubble diffusers [0165] 11=first reactor [0166] 12=second reactor [0167] 13=inlet feed line [0168] 14=pH probe [0169] 15=gas outlet [0170] 16=gas space [0171] 17=spent caustic solution for treatment [0172] 18=feed pump [0173] 19=off-gas recycle [0174] 20=O.sub.3/O.sub.2 [0175] 21=CO.sub.2 gas [0176] 22=interconnecting pipe

[0177] FIG. 1 depicts an apparatus comprising a first reactor 11, and a second reactor 12. Inlet feed line 13 allows spent caustic solution 17 to be fed to the first reactor 11 and subsequently the second reactor 12, via interconnecting pipe 22. The first reactor 11 has a means for introducing a gas comprising ozone into the reactor via a gas liquid contacting device, such as a venturi/in-line static mixer or similar device 8. Off-gas from reactor 11 may be recycled back to reactor 11 via a secondary gas/liquid contacting device 9 connected in line with recycle pump 7. Contents from the second reactor 12 can be recycled back to first reactor 11 and the contents of first reactor 11 are connected by a gravity fed interconnecting pipe 22 to the second reactor 12. The second reactor 12 comprises a means 10 for introducing carbon dioxide. The second reactor 12 comprises a treated solution outlet 5. The second reactor 12 may also comprise a pH probe 14 optionally connected to a pH meter 4. In this embodiment the first reactor 11 comprises a means and a pump 7 to recycle solution from the second reactor 12 to first reactor 11. The first reactor 11 is in fluid communication with second reactor 12. In this schematic the first reactor 11 and the second reactor 12 respectively have a gas outlet 15 to allow excess gas to be removed.

[0178] FIG. 2 is a schematic diagram of an apparatus and process for single-reactor batch processing according to the present invention.

KEY TO FIG. 2

[0179] 1=spent caustic delivery pump [0180] 2=vessel gas-liquid contacting packing material [0181] 3=reaction vessel [0182] 4=treated caustic solution transfer pump [0183] 5=venturi gas/liquid contactor [0184] 6=process recycle pump [0185] 7=heat exchanger [0186] 8=process mixer and distributor [0187] 9=gaseous effluent outlet [0188] 10=process liquid recycle line [0189] 11=gaseous recycle line [0190] 12=mixing ejector [0191] 13=to wastewater plant [0192] 14=CO.sub.2 [0193] 15=O.sub.3/O.sub.2 [0194] 16=Heat [0195] 17=Temperature 20 to 140 C.

[0196] In one embodiment, the present invention may be operated as a batch process, for example using the apparatus/process shown in FIG. 2. The spent caustic solution enters the first reactor 3 by transfer pump 1 and is controlled by PLC/SCADA (not shown). The spent caustic solution to be treated is then cycled through the first reactor 3 and into the process fluid line 10, where it is optionally heated by a heat exchanger 7, for example a steam device or other heating medium. The spent caustic solution is pumped by pump 6 and enters the venturi gas/liquid contactor 5, where it is contacted with a gas comprising ozone and oxygen 15 and carbon dioxide 14. Recycle gas may also contact the spent caustic solution in the venturi gas/liquid contactor 5 via gaseous recycle line 11 or via an alternative mixing device. The liquid/gas mixture, i.e. the spent caustic solution and the gas comprising ozone, oxygen and carbon dioxide, is then injected into the first reactor 3 by means of a distributor or nozzle array 8. A portion of the first reactor 3 may be filled to a defined height (from 25% to 100%) with gas-liquid contacting packing material 2 (random or structured) to improve the contact between the gas phase and the liquid phase. To control and maintain the pressure in the first reactor 3, the exhaust gaseous are removed from the first reactor 3 and recycled via recycle line 11 and/or emitted via gaseous effluent outlet 9 to a thermal destruct unit (not shown) to breakdown any remaining ozone to oxygen.

[0197] When introducing elements of the present disclosure or the preferred embodiments(s) thereof, the articles a, an, the and said are intended to mean that there are one or more of the elements. The terms comprising, including and having are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0198] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.