Apparatus and method for automatically achieving and maintaining congruent control in an industrial boiler

• US 5,696,696 A
• Filed: 10/11/1994
• Issued: 12/09/1997
• Est. Priority Date: 10/11/1994
• Status: Expired due to Term

• Current Assignee: BetzDearborn Inc. (Veolia Environnement SA)
• Original Assignee: BetzDearborn Inc. (Veolia Environnement SA)
• Inventors: Worrell, Norman B., Boyette, Scott M., Burgmayer, Paul R., Gunther, John C., Thungstrom, Eric A.
• Primary Examiner(s): Voeltz, Emanuel T.
• Assistant Examiner(s): Wachsman, Hal D.
• Application Number:
  US08/321,338
• Time in Patent Office: 1,155 Days
• Field of Search: 364/496, 364/497, 364/499, 364/500, 364/502, 364/149, 364/150, 364/153, 364/164, 364/165, 364/148, 364/166, 364/509, 364/510, 364/578, 364/492, 364/468.1, 364/157 , 364/132, 364/159, 364/478.08, 364/152, 364/151, 210/743, 210/742, 210/739, 210/96.1, 210/103, 210/69.6, 210/311-313.85, 422/62, 422/12, 422/14, 422/16, 422/110, 422/111, 422/81, 122/45 R, 122/451, 122/451.1, 122/2, 122/401, 395/906, 395/907, 395/914, 395/915, 436/55, 436/56, 436/50, 376/216, 376/217, 237/8 A
• US Class Current: 700/266
• CPC Class Codes: C02F 5/025 Hot-water softening devices
• CPC Class Codes: G01N 33/18 Water
• CPC Class Codes: G05D 21/02 characterised by the use of electric means

Abstract

A control system for automatically achieving and maintaining a desired sodium/phosphate ratio and phosphate concentration of the boiler water in an industrial boiler for minimizing corrosion. The system uses an adaptive controller that models the boiler which enables the system to predict boiler pH and phosphate concentrations at any future time given the feed rates, feed concentrations of high and low sodium/phosphate stocks, blowdown rate, mass of the boiler water, initial boiler phosphate concentration and initial pH. Once these future concentrations are determined, the controller then determines which feed rates will return the current boiler water state to, and then maintain the boiler water at, the desired sodium/phosphate congruency ratio in the least amount of time. Where the controller determines that it is not possible to move the current boiler water state to the desired congruency setpoint, the controller determines a feed rate program that will move the current boiler water state to the attainable steady state point closest to the desired setpoint. An alternative embodiment fixes the phosphate concentration by ratioing the phosphate feed with the blowdown rate while continuously monitoring blowdown pH. This arrangement allows for the control of sodium by switching between the high and low ratio sodium/phosphate stocks based on the measured pH with respect to the pH setpoint.

Claims

1. An automatic control system for controlling at least two interdependent chemicals in the fluid of a continuously stirred tank reactor system having an effluent flow, said control system comprising input means for receipt of fluid parameters and control means responsive to said input means, said control means using non-proportional control for automatically minimizing the time that said at least two interdependent chemicals in the fluid spend outside of a target region, and away from a setpoint, of said at least two interdependent chemicals in the fluid without controlling said effluent flow.

2. The control system of claim 1 wherein said continuously stirred tank reactor system is an industrial boiler having a boiler fluid and said effluent flow is a blowdown flow.

3. The control system of claim 2 wherein one of said fluid parameters comprises the pH of the boiler fluid and wherein said input means input means comprises means for determining the pH value of the fluid.

4. The control system of claim 3 wherein said means for determining the pH value of the fluid provides an on-line measurement of the pH value of the fluid.

5. The control system of claim 3 wherein said control means comprises a first feedstream and a second feedstream for feeding first and second fluid treatment materials, respectively, to the boiler fluid at respectively determined feed rates for achieving and maintaining said target region, said first material comprising a mixture of sodium and phosphate having a first predetermined sodium-to-phosphate ratio and said second material comprising a mixture of sodium and phosphate having a second predetermined sodium-to-phosphate ratio.

6. The control system of claim 5 wherein a second of said fluid parameters comprises the phosphate concentration of the boiler fluid and wherein said input means further comprises means for determining the phosphate concentration of the boiler fluid.

7. The control system of claim 6 wherein said means for determining the phosphate concentration of the boiler fluid provides an on-line measurement of the phosphate concentration of the boiler fluid.

8. The control system of claim 6 wherein said target region comprises a desired predetermined ratio of the sodium concentration in the boiler fluid to the phosphate concentration in the boiler fluid.

9. The control system of claim 8 wherein said target region has an upper ratio control limit, a lower ratio control limit, an upper phosphate control limit and a lower phosphate control limit, said limits forming a closed region, said target region having target region vertices.

10. The control system of claim 9 wherein said control means further comprises an adaptive controller, said adaptive controller comprising a model of the industrial boiler for determining said respectively determined feed rates of said first and second fluid treatment materials.

11. The control system of claim 10 wherein said adaptive controller comprises means for accounting for dead time of the boiler during boiler operation.

12. The control system of claim 10 wherein said adaptive controller further comprises means for updating said model responsive to said input means.

13. The control system of claim 12 wherein said input means comprises a blowdown flowmeter for providing an on-line measurement of the blowdown flow.

14. The control system of claim 12 wherein said adaptive controller further comprises means for calculating the blowdown flow.

15. The control system of claim 14 wherein said adaptive controller further comprises means for calculating the sodium concentration in the boiler fluid.

16. The control system of claim 15 wherein said input means further comprises means for measuring ammonia in the boiler fluid and wherein said means for calculating the sodium concentration in the boiler fluid accounts for the ammonia in the boiler fluid.

17. The control system of claim 16 wherein said adaptive controller further comprises means for determining a feedwater contaminant ingress.

18. The control system of claim 15 wherein said adaptive controller further comprises means for determining a feedwater contaminant ingress.

19. The control system of claim 18 wherein said means for calculating the blowdown flow and said means for calculating a feedwater contaminant ingress are based on a series of phosphate and pH measurements in an industrial boiler system using small sample intervals.

20. The control system of claim 18 wherein said adaptive controller further comprises means for developing a pumpable region that defines a first range of steady state boiler fluid sodium and phosphate concentrations that are attainable from the current boiler fluid sodium and phosphate concentrations using a second range of first feedstream feed rates and second feedstream feed rates, said pumpable region having pumpable region vertices and a pumpable region perimeter.

21. The control system of claim 20 wherein said adaptive controller further comprises means for analyzing said target region, said pumpable region, and said current boiler fluid sodium and phosphate concentrations in a boiler state space.

22. The control system of claim 21 wherein said adaptive controller comprises means for determining that said target region cannot be reached from said boiler state space by any feed program.

23. The control system of claim 22 wherein said means for determining that said target region cannot be reached warns the operator and implements a feed program that will drive said current boiler fluid concentrations towards an area of said pumpable region that is closest to said target region.

24. The control system of claim 21 wherein said means for analyzing determines an optimum feed program, said optimum feed program controlling said respectively determined feed rates of said first and second feedstreams in order to achieve said target region within the boiler fluid in the least amount of time.

25. The control system of claim 24 wherein said optimum feed program comprises first means for bringing said current boiler fluid concentrations within said target region in the least amount of time thereby establishing new boiler fluid concentrations and second means for bringing said new boiler fluid concentrations to a predetermined setpoint within said target region in the least amount of time.

26. The control system of claim 25 wherein said optimum feed program further comprises means for maintaining said respectively determined feed rates corresponding to said setpoint once said setpoint has been achieved.

27. The control system of claim 25 wherein said first means for bringing said current boiler fluid concentrations within said target region in the least amount of time comprises means for analyzing a first set of feed rate trajectories that form lines between the current boiler fluid concentrations and said pumpable region vertices, and a second set of feed rate trajectories that form lines between the current boiler fluid concentrations and said target region vertices, said second set of feed rate trajectories being projected until they intersect said pumpable region perimeter if at all, thereby defining a third set of feed rate trajectories.

28. The control system of claim 27 wherein said means for analyzing selects one feed rate trajectory from said first and said third set of feed rate trajectories that reaches said target region in the least amount of time.

29. The control system of claim 25 wherein said second means for bringing said new boiler fluid concentrations to said setpoint in the least amount of time comprises means for determining the feed rate trajectory that coincides with a line formed between said new boiler fluid concentrations and said setpoint, said feed rate trajectory being projected until it intersects said pumpable region perimeter, if at all.

30. The control system of claim 25 wherein said second means for bringing said new current boiler fluid concentrations to said setpoint in the least amount of tile comprises means for establishing a new target region around said setpoint and having new target region vertices.

31. The control system of claim 30 wherein said second means for bringing said new boiler fluid concentrations to said setpoint in the least amount of time further comprises means for analyzing a fourth set of feed rate trajectories that form lines between the new boiler fluid concentrations and said pumpable region vertices, and a fifth set of feed rate trajectories that form lines between the new boiler fluid concentrations and said new target region vertices, said fifth set of feed rate trajectories being projected until they intersect said pumpable region perimeter if at all, thereby defining an sixth set of feed rate trajectories.

32. The control system of claim 31 wherein means for analyzing selects one feed rate trajectory from said fourth and said sixth set of feed rate trajectories that reaches said new target region in the least amount of time, thereby establishing newer boiler fluid concentrations.

33. The control system of claim 32 wherein said second means further comprises means for repeating the establishment of additional target regions and corresponding feed rate trajectories based on said newer boiler fluid concentrations, said means for repeating operating until said setpoint is achieved.

34. A method for controlling at least two interdependent chemicals in the fluid of a continuously stirred tank reactor system having an effluent flow, said method comprising the steps of:

(a) establishing a mathematical model of said continuously stirred tank reactor system;

(b) monitoring the concentration of one of said at least two interdependent chemicals in the fluid, the pH of the fluid and the temperature at which the pH is measured;

(c) updating the model based on the concentration of said one of said at least two interdependent chemicals in the fluid, the pH of said fluid and the temperature at which the pH is measured;

(d) providing a feedstream of a high-pH fluid treatment material comprising a mixture of said at least two interdependent chemicals and a feedstream of a low-pH fluid treatment material comprising a mixture of said at least two interdependent chemicals for feeding to the fluid at respective feed rates; and

(e) developing an optimum feed rate program for controlling said feedstreams to automatically achieve and maintain a setpoint of said at least two interdependent chemicals in the fluid of the continuously stirred tank reactor system in the least amount of time from the current concentrations of said chemicals in the fluid.

35. The method of claim 34 wherein said step of developing an optimum feed rate program includes generating a pumpable region in a system state space that defines a first range of all steady state fluid concentrations of said at least two interdependent chemicals in the fluid that are attainable from the current concentrations of said at least two interdependent chemicals in the fluid depending on a second range of first feedstream feed rates and second feedstream feed rates, said pumpable region having pumpable region vertices and a pumpable region perimeter that corresponds to said second range of first feedstream feed rates and second feedstream feed rates.

36. The method of claim 35 wherein said step of developing an optimum feed rate program further includes:

(a) defining a new target region about said setpoint and having new target region vertices;

(b) determining the time required to drive the new concentrations of said at least two interdependent chemicals in the fluid along a fourth set of feed rate trajectories formed between the new concentrations of said at least two interdependent chemicals and said pumpable region vertices;

(c) determining the time required to drive the new concentrations of said at least two interdependent chemicals in the fluid along a fifth set of feed rate trajectories formed between the new concentrations of said at least two interdependent chemicals and said new target region vertices, said fifth set of feed rate trajectories being projected until they intersect said pumpable region perimeter if at all, thereby defining a sixth set of feed rate trajectories;

(d) selecting one feed rate trajectory from said fourth and sixth set of feed rate trajectories that requires the least amount of time to reach said new target region and then pumping said first and second feedstreams along said selected feed rate trajectory, thereby establishing newer concentrations of said at least two interdependent chemicals in the fluid;

(e) repeating steps a-d until said setpoint is achieved; and

(f) maintaining the feed rates of said first and said second feedstreams that correspond to said setpoint when said setpoint is achieved.

37. The method of claim 35 wherein said step of developing an optimum feed rate program further includes defining a target region about said setpoint of said at least two interdependent chemicals, said target region having target region vertices.

38. The method of claim 37 wherein said step of developing an optimum feed rate program further includes determining the time required to drive the current concentrations of said at least two interdependent chemicals in the fluid along a first set of feed rate trajectories formed between the current concentrations of said at least two interdependent chemicals and said pumpable region vertices.

39. The method of claim 38 wherein said step of developing an optimum feed rate program further includes determining the time required to drive the current concentrations of said at least two interdependent chemicals in the fluid along a second set of feed rate trajectories formed between the current concentrations of said at least two interdependent chemicals and said target region vertices, said second set of feed rate trajectories being projected until they intersect said pumpable region perimeter if at all, thereby defining a third set of feed rate trajectories.

40. The method of claim 39 wherein said step of developing an optimum feed rate program further includes selecting one feed rate trajectory from said first and third set of feed rate trajectories that requires the least amount of time to reach said target region and then pumping said first and second feedstreams along said selected feed rate trajectory, thereby establishing new concentrations of said at least two interdependent chemicals in the fluid.

41. The method of claim 40 wherein said step of developing an optimum feed rate program further includes determining a second feed rate trajectory that coincides with a line formed between the new concentrations of said at least two interdependent chemicals in the fluid and said setpoint, said second feed rate trajectory being projected until it intersects said pumpable region perimeter, if at all.

42. The method of claim 41 wherein said step of developing an optimum feed rate program further includes maintaining said respective feed rates of said first and second feedstreams that correspond to said setpoint when said setpoint has been achieved.

43. The method of claim 34 wherein said continuously stirred tank reactor system is an industrial boiler having a boiler fluid and said effluent flow is a blowdown flow.

44. The method of claim 43 wherein said one of said at least two interdependent chemicals is phosphate.

45. The method of claim 44 wherein said one of said at least two interdependent chemicals is sodium.

46. The method of claim 45 wherein said method further includes the step of estimating the blowdown flow.

47. The method of claim 46 wherein said method further includes the steps of calculating the phosphate concentration and the sodium concentration in the boiler fluid.

48. The method of claim 47 wherein said method further includes the step of estimating a feedwater contaminant ingress.

49. The method of claim 48 wherein said steps of estimating a blowdown flow and a feedwater contaminant ingress are based on a series of phosphate and pH measurements of the boiler fluid wherein the system uses small sample intervals.

50. The method of claim 47 wherein said step of calculating the sodium concentration includes a step for measuring ammonia in the boiler fluid and accounting for the ammonia in determining the sodium concentration.

51. The method of claim 50 wherein said method further includes the step of estimating a feedwater contaminant ingress.

52. The method of claim 43 wherein said method further includes measuring the blowdown flow to obtain blowdown flow values.

53. The method of claim 52 wherein said one of said at least two interdependent chemicals is phosphate.

54. The method of claim 53 wherein said one of said at least two interdependent chemicals is sodium.

55. The method of claim 54 wherein said method further includes the steps of calculating the phosphate concentration and the sodium concentration in the boiler fluid.

56. The method of claim 55 wherein said method further includes the step of estimating a feedwater contaminant ingress.

57. The method of claim 56 wherein said method further includes calculating a boiler phosphate mass imbalance.

58. The method of claim 54 wherein said method further includes providing on-line measurement of the pH of the boiler fluid and on-line measurement of the phosphate concentration in the boiler fluid to provide pH values:

and phosphate concentration values.

59. The method of claim 58 wherein said method further includes calculating a feedwater contaminant ingress.

60. The method of claim 34 wherein said method further includes a step that accounts for dead time in the continuously stirred tank reactor system.

61. A system for controlling the sodium-to-phosphate ratio and the phosphate concentration of a boiler fluid in an industrial boiler having a feedwater flow and a blowdown flow, said system comprising input means for receipt of a boiler fluid parameter and a parameter indicative of said phosphate concentration and control means responsive to said input means for automatically achieving and maintaining a predetermined fixed phosphate concentration of the boiler fluid and for automatically achieving and maintaining a predetermined desired sodium-to-phosphate ratio of the boiler fluid without controlling said blowdown flow and independent of the feedwater flow rate without the need to measure any reactant concentration in the feedwater flow.

62. The system of claim 61 wherein said boiler fluid parameter comprises the pH of the boiler fluid, the pH of the boiler fluid being defined by the sodium-to-phosphate ratio and the phosphate concentration and wherein said input means comprises a pH meter for determining the pH value of the boiler fluid and providing the pH value to said control means.

63. The system of claim 62 wherein said input means further comprises a blowdown flowmeter for monitoring the blowdown flow and providing a signal indicative of said blowdown flow, said parameter indicative of said phosphate concentration being calculated from said signal indicative of said blowdown flow.

64. The system of claim 63 wherein said control means comprises a first feedstream and a second feedstream for feeding first and second fluid treatment materials, respectively, to the boiler fluid, said first material comprising a mixture of sodium and phosphate having a first predetermined sodium-to-phosphate ratio and a first predetermined concentration of phosphate, said second material comprising a mixture of sodium and phosphate having a second predetermined sodium-to-phosphate ratio and a second predetermined concentration of phosphate.

65. The system of claim 64 wherein said first predetermined concentration of phosphate and said second predetermined concentration of phosphate are identical.

66. The system of claim 65 wherein said control means includes maintenance means for maintaining said predetermined fixed phosphate concentration in the boiler fluid, said maintenance means being arranged to deliver said first and second fluid treatment materials to the boiler fluid in proportion to the blowdown flow.

67. The system of claim 66 wherein said control means operates upon a pH setpoint, said pH setpoint being defined by the predetermined desired sodium-to-phosphate ratio and the predetermined fixed phosphate concentration, and wherein said control means further comprises comparator means for comparing the pH value of the boiler fluid with the pH setpoint.

68. The system of claim 67 wherein said maintenance means feeds a selected one but not the other of said first and second fluid treatment materials from its associated feedstream into the boiler fluid at one time, said selected one being selected depending upon whether the pH value is greater than or less than the pH setpoint.

69. The system of claim 68 wherein said input means further comprises a phosphate analyzer for determining a boiler phosphate mass imbalance.

70. A system for maintaining a fixed concentration of a fluid treatment material in the boiler water of an industrial boiler having a blowdown flow and a feedwater flow, said system comprising at least one feedstream for delivering said fluid treatment material to the boiler fluid, said at least one feedstream delivering said fluid treatment material in proportion to said blowdown flow but without controlling the blowdown flow, said system operating independent of the feedwater flow rate without the need to measure any reactant concentration in the feedwater flow.

71. A method for controlling the sodium-to-phosphate ratio and the phosphate concentration of a boiler fluid in an industrial boiler having a blowdown flow and a feedwater flow, said method comprising the steps of:

(a) selecting a predetermined fixed phosphate concentration and a predetermined desired sodium-to-phosphate ratio;

(b) monitoring a boiler fluid parameter;

(c) comparing said boiler fluid parameter with a preestablished setpoint;

(d) monitoring a parameter indicative of said phosphate concentration;

(e) providing a supply of a first sodium phosphate fluid treatment material, said first sodium phosphate fluid treatment material having a first predetermined sodium-to-phosphate ratio and a first known phosphate concentration;

(f) providing a supply of a second sodium phosphate fluid treatment material, said second sodium phosphate fluid treatment material having a second predetermined sodium-to-phosphate ratio and a second known phosphate concentration;

(g) automatically feeding said first sodium phosphate fluid treatment material to the boiler fluid when said monitored parameter is less than or equal to said pre-established setpoint and automatically feeding said second sodium phosphate fluid treatment material to the boiler fluid when said monitored parameter is greater than said pre-established setpoint, said first sodium phosphate fluid treatment material and said second sodium phosphate fluid treatment material defining a selected sodium phosphate fluid treatment material whenever one of these is being automatically fed; and

(h) said selected fluid treatment material being fed, in proportion to the blowdown flow without controlling the blowdown flow and independent of the feedwater flow rate without the need to measure any reactant concentration in the feedwater flow, to the boiler fluid based on said parameter indicative of said phosphate concentration to automatically achieve and maintain said predetermined fixed phosphate concentration.

72. The method of claim 71 wherein said monitored parameter is the pH of the boiler fluid.

73. The method of claim 72 wherein said first known phosphate concentration is identical to said second known phosphate concentration.

74. A method for maintaining a fixed concentration of a fluid treatment material in the boiler fluid of an industrial boiler having a blowdown flow and a feedwater flow, said method comprising the steps of:

(a) selecting a predetermined fluid treatment material concentration for the boiler fluid;

(b) providing a supply of the predetermined fluid treatment material having a known concentration;

(c) monitoring the blowdown flow;

(d) feeding the fluid treatment material to the boiler fluid in proportion to the blowdown flow to automatically achieve and maintain the predetermined fluid treatment material concentration in the boiler fluid but without controlling the blowdown flow independent of the feedwater flow rate and without the need to measure any reactant concentration in the feedwater flow.