Medium pressure steam intervention in an olefin cracking furnace decoke procedure

Abstract

Described herein is a method for removing coke deposits in radiant tubes of an olefin cracking furnace and removing accumulated spalled coke from one or more outlet elbows of the olefin cracking furnace without performing a cold shutdown of the furnace.

Claims

1. A method for removing coke deposits in radiant tubes of an olefin cracking furnace and removing accumulated spalled coke from one or more outlet elbows of the olefin cracking furnace without performing a cold shutdown of the furnace, the method comprising the steps of: (a) introducing decoking air and low pressure steam into a plurality of radiant tubes of the olefin cracking furnace, and maintaining flow through the plurality of radiant tubes for a first period of time; (b) identifying a first outlet elbow with restricted flow therethrough due to accumulated spalled coke resulting from step (a); and (c) lining-up a supply of medium pressure steam by opening a block valve then opening a control valve, and introducing the medium pressure steam through a first subset of the plurality of radiant tubes, the first subset of tubes comprising (i) a plurality of tubes upstream of one or more outlet elbows through which flow is not restricted and (ii) a second subset of tubes upstream of the first outlet elbow identified in step (b) through which flow is restricted, and maintaining flow therethrough for a second period of time, thereby removing accumulated spalled coke from the first outlet elbow.

2. The method of claim 1, wherein the medium pressure steam is from about 300 psig to about 600 psig.

3. The method of claim 2, wherein the medium pressure steam is about 400 psig.

4. The method of claim 1, wherein the low pressure steam is from about 100 psig to about 200 psig.

5. The method of claim 1, wherein the first outlet elbow is located at a primary transfer line exchanger (PTLE) or Transfer Line Exchanger (TLE) outlet.

6. The method of claim 1, wherein the second subset of tubes upstream of the first outlet elbow comprises at least two radiant tubes.

7. The method of claim 1, wherein the second subset of tubes upstream of the first outlet elbow consists of two or four radiant tubes.

8. The method of claim 1, wherein the plurality of radiant tubes comprises a plurality of octants, wherein the first subset of the plurality of radiant tubes is a first octant of tubes comprising the second subset of tubes and a plurality of other radiant tubes upstream of one or more outlet elbows through which flow is not restricted, wherein the octants other than the first octant do not comprise tubes upstream of an outlet elbow with restricted flow therethrough.

9. The method of claim 8, wherein the plurality of radiant tubes consists of at least eight octants.

10. The method of claim 8, wherein each octant consists of from 20 tubes to 50 tubes.

11. The method of claim 10, wherein the first octant of tubes consists of 36 tubes including four tubes upstream of the first outlet elbow with restricted flow therethrough and 32 tubes upstream of 8 outlet elbows which do not have restricted flow therethrough.

12. The method of claim 8, wherein the decoking air and the low pressure steam continue to flow through octants other than the first octant while medium pressure steam is introduced through the first octant during step (c).

13. The method of claim 1, wherein the plurality of radiant tubes have internal fins.

14. The method of claim 1, wherein the plurality of radiant tubes have weld-bead type static mixing elements.

15. The method of claim 1, wherein the plurality of radiant tubes have a round equivalent internal diameter from about 1 inch to about 2 inches.

16. The method of claim 15, wherein the plurality of radiant tubes have a round equivalent internal diameter of about 1.55 inches.

17. The method of claim 1, wherein the first outlet elbow has an entrance internal diameter and/or exit internal diameter of less than about 3 inches.

18. The method of claim 1, wherein step (a) comprises introducing decoking air at a rate from about 1000 lb/hr to about 5000 lb/hr and low pressure steam at a rate from about 7000 lb/hr to about 10,000 lb/hr into the plurality of radiant tubes.

19. The method of claim 1, wherein step (c) comprises introducing medium pressure steam through the first subset of radiant tubes at a rate from about 5000 lb/hr to about 40,000 lb/hr.

20. The method of claim 1, wherein at least one of the low pressure steam, the decoking air, and the medium pressure steam is at a temperature from about 400 F. to about 800 F.

21. The method of claim 1, wherein step (a) comprises maintaining the flow of decoking air and low pressure steam through the plurality of radiant tubes for a period of time from about 16 hours to about 24 hours.

22. The method of claim 1, wherein step (c) comprises maintaining the flow of medium pressure steam through the first subset of tubes until the accumulated spalled coke is dislodged and removed.

23. The method of claim 1, wherein step (b) comprises identifying the first outlet elbow with restricted flow therethrough by visual inspection of the radiant tubes upstream of the first outlet elbow.

24. The method of claim 1, wherein step (b) comprises identifying the first outlet elbow with restricted flow therethrough by monitoring a temperature of one or more tubes upstream of the first outlet elbow.

25. A method for removing coke deposits in radiant tubes of an olefin cracking furnace and removing accumulated spalled coke from one or more outlet elbows of the olefin cracking furnace without performing a cold shutdown of the furnace, the method comprising the steps of: (a) introducing decoking air and low pressure steam into a plurality of radiant tubes of the olefin cracking furnace, and maintaining flow through the plurality of radiant tubes for a first period of time; (b) identifying a first outlet elbow with restricted flow therethrough due to accumulated spalled coke resulting from step (a); and (c) introducing medium pressure steam through a first subset of the plurality of radiant tubes, the first subset of tubes comprising (i) a plurality of tubes upstream of one or more outlet elbows through which flow is not restricted and (ii) a second subset of tubes upstream of the first outlet elbow identified in step (b) through which flow is restricted, and maintaining flow therethrough for a second period of time, thereby removing accumulated spalled coke from the first outlet elbow, wherein step (c) comprises opening a medium pressure steam control valve while simultaneously blocking-in the low pressure steam and air supply to the first subset of tubes, thereby avoiding furnace trip due to low flow to an affected plurality of radiant tubes.

26. The method of claim 25, comprising the step of closing the medium pressure steam control valve while simultaneously opening a low pressure steam valve to permit flow of low pressure steam to the first subset of tubes after it has been determined the accumulated spalled coke has been removed from the first outlet elbow.

27. The method of claim 25, wherein the plurality of radiant tubes comprises a plurality of octants, wherein the first subset of the plurality of radiant tubes is a first octant of tubes comprising the second subset of tubes and a plurality of other radiant tubes upstream of one or more outlet elbows through which flow is not restricted, wherein the octants other than the first octant do not comprise tubes upstream of an outlet elbow with restricted flow therethrough and wherein step (c) comprises providing a check valve downstream of the medium pressure steam control valve to limit the impact of the medium pressure steam on the octants other than the first octant.

28. The method of claim 1, comprising blocking-in and depressurizing the medium pressure steam system following step (c).

29. The method of claim 1, further comprising resetting fuel gas valves for resumption of olefin cracking operation of the furnace.

30. The method of claim 1, wherein the olefin cracking furnace has olefin production capacity of at least about 100,000 metric tons per annum (at least about 100 kTA).

31. The method of claim 30, wherein the olefin cracking furnace has olefin production capacity from about 100 kTA to about 300 kTA.

32. The method of claim 30, wherein the olefin cracking furnace has olefin production capacity of at least about 220,000 metric tons per annum (at least about 220kTA).

33. The method of claim 1, wherein the coke deposits result from cracking a feedstock used in the production of olefins via pyrolysis.

34. The method of claim 25, wherein the medium pressure steam is from about 300 psig to about 600 psig.

35. The method of claim 34, wherein the medium pressure steam is about 400 psig.

36. The method of claim 25, wherein the low pressure steam is from about 100 psig to about 200 psig.

37. The method of claim 25, wherein the first outlet elbow is located at a primary transfer line exchanger (PTLE) or Transfer Line Exchanger (TLE) outlet.

38. The method of claim 25, wherein the second subset of tubes upstream of the first outlet elbow comprises at least two radiant tubes.

39. The method of claim 25, wherein the second subset of tubes upstream of the first outlet elbow consists of two or four radiant tubes.

40. The method of claim 25, wherein the plurality of radiant tubes comprises a plurality of octants, wherein the first subset of the plurality of radiant tubes is a first octant of tubes comprising the second subset of tubes and a plurality of other radiant tubes upstream of one or more outlet elbows through which flow is not restricted, wherein the octants other than the first octant do not comprise tubes upstream of an outlet elbow with restricted flow therethrough.

41. The method of claim 40, wherein the plurality of radiant tubes consists of at least eight octants.

42. The method of claim 40, wherein each octant consists of from 20 tubes to 50 tubes.

43. The method of claim 42, wherein the first octant of tubes consists of 36 tubes including four tubes upstream of the first outlet elbow with restricted flow therethrough and 32 tubes upstream of 8 outlet elbows which do not have restricted flow therethrough.

44. The method of claim 40, wherein the decoking air and the low pressure steam continue to flow through octants other than the first octant while medium pressure steam is introduced through the first octant during step (c).

45. The method of claim 25, wherein the plurality of radiant tubes have internal fins.

46. The method of claim 25, wherein the plurality of radiant tubes have weld-bead type static mixing elements.

47. The method of claim 25, wherein the plurality of radiant tubes have a round equivalent internal diameter from about 1 inch to about 2 inches.

48. The method of claim 47, wherein the plurality of radiant tubes have a round equivalent internal diameter of about 1.55 inches.

49. The method of claim 25, wherein the first outlet elbow has an entrance internal diameter and/or exit internal diameter of less than about 3 inches.

50. The method of claim 25, wherein step (a) comprises introducing decoking air at a rate from about 1000 lb/hr to about 5000 lb/hr and low pressure steam at a rate from about 7000 lb/hr to about 10,000 lb/hr into the plurality of radiant tubes.

51. The method of claim 25, wherein step (c) comprises introducing medium pressure steam through the first subset of radiant tubes at a rate from about 5000 lb/hr to about 40,000 lb/hr.

52. The method of claim 25, wherein at least one of the low pressure steam, the decoking air, and the medium pressure steam is at a temperature from about 400 F. to about 800 F.

53. The method of claim 25, wherein step (a) comprises maintaining the flow of decoking air and low pressure steam through the plurality of radiant tubes for a period of time from about 16 hours to about 24 hours.

54. The method of claim 25, wherein step (c) comprises maintaining the flow of medium pressure steam through the first subset of tubes until the accumulated spalled coke is dislodged and removed.

55. The method of claim 25, wherein step (b) comprises identifying the first outlet elbow with restricted flow therethrough by visual inspection of the radiant tubes upstream of the first outlet elbow.

56. The method of claim 25, wherein step (b) comprises identifying the first outlet elbow with restricted flow therethrough by monitoring a temperature of one or more tubes upstream of the first outlet elbow.

57. The method of claim 25, comprising blocking-in and depressurizing the medium pressure steam system following step (c).

58. The method of claim 25, further comprising resetting fuel gas valves for resumption of olefin cracking operation of the furnace.

59. The method of claim 25, wherein the olefin cracking furnace has olefin production capacity of at least about 100,000 metric tons per annum (at least about 100 kTA).

60. The method of claim 59, wherein the olefin cracking furnace has olefin production capacity from about 100 kTA to about 300 kTA.

61. The method of claim 59, wherein the olefin cracking furnace has olefin production capacity of at least about 220,000 metric tons per annum (at least about 220kTA).

62. The method of claim 25, wherein the coke deposits result from cracking a feedstock used in the production of olefins via pyrolysis.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates a typical olefin cracking furnace having a primary transfer line exchanger (PTLE) outlet.

(2) FIG. 2 is a schematic view of a tight 90 radius elbow of a PTLE outlet.

(3) FIG. 3 shows a typical cross-section of an internally finned radiant tube where valley coke forms during normal operation of a cracking furnace.

(4) FIG. 4 shows four passes of a radiant tube assembly for an olefin cracking furnace that has a total of eight passes (octants), each pass having 36 tubes that end in nine harps, each harp having 4 tubes and a tetra fitting for single conduit to a primary transfer line exchanger.

DETAILED DESCRIPTION

(5) It should be understood that the order of steps or order for performing certain actions is immaterial so long as the invention remains operable. Moreover, two or more steps or actions may be conducted simultaneously.

(6) The mention herein of any publication or technique, for example, in the Background section, is not an admission that the publication or technique serves as prior art with respect to any of the claims presented herein. The Background section is presented for purposes of clarity and is not meant as a description of prior art with respect to any claim.

(7) Coke typically contains about 80% to 95% carbon by weight with the balance comprising sulfur, nitrogen inorganic materials, ash, and small amounts of oxygen. Coke deposits which form in radiant tubes, heat exchangers, reactors, etc. degrade heat transfer and fuel flow characteristics and, if left unchecked, can lead to system failure.

(8) It is found that introducing medium pressure (MP) steam through a pass of radiant tubes affected by an obstructed outlet (e.g., a PTLE outlet) is particularly effective as an adjunct, or backup, step in a steam-air decoking procedure conducted on an olefin cracking furnace. The unique arrangement of radiant tubes and outlets in an olefin cracking furnace allows for safe flow of the medium pressure steam through an affected pass while maintaining sufficient hydraulic surge to effectively dislodge the accumulated coke without requiring a cold shutdown of the furnace and without disrupting the ongoing decoking of other passes of radiant tubes of the furnace. Methods and systems described herein are helpful to overcome excessive down time caused by an obstructed PTLE outlet elbow.

(9) Without wishing to be bound by any theory, it appears that the need for MP steam intervention during steam-air decoke (SAD) to dislodge accumulated coke from PTLE outlets is exacerbated by: a) an under-sized PTLE internal diameter; b) a PTLE outlet with short turn 90 radius; c) the use of 41 tetra fittings to one PTLE rather than 21 wyes to one PTLE; d) the use of ethane or ethane-rich cracking process feed streams, which are found to produce hard coke; and/or e) the use of radiant tubes that have internal fins or weld-bead type static mixing elements, such that long shards of coke form in the valleys or crevices of such fins or elements and break off during decoking, accumulating at a PTLE outlet.

(10) Referring to FIG. 1, a typical olefin cracking furnace has a primary quench exchanger (PTLE) outlet connected to four radiant tubes 107. The PTLE 105 has a Boiler Feed Water (BFW) inlet 106 and coil supports 103. The circled area is the PTLE outlet including a steam outlet 104, an erosion resistant elbow 102 and process outlet manifold 101.

(11) FIG. 2 depicts an exemplary PTLE outlet elbow 201 from FIG. 1 with a short turn tight radius of 90. It is believed this structure may contribute to the coke accumulation problem that the MP steam intervention procedure addresses.

(12) Now turning to FIG. 3, a cross-section of a radiant tube is shown. For example, a round equivalent internal diameter of a radiant tube can be from about 1 to about 2 inches, e.g., 1.55 inches. As illustrated, there are 10 internal fins 301 which are provided to increase heat transfer area inside the tube. Long shards of coke form in the valleys of such fins during olefin cracking and break off during decoking, accumulating at a PTLE outlet elbow 102.

(13) Hard catalytic coke builds in the valleys of internally finned tubes in cracking furnaces that employ ethane or ethane-rich feedstocks, forming long shards, e.g., 8-12 inches in length. Plugging can occur whenever the size or amount of coke that is spalled or otherwise released from the tube wall is too great to pass through the PTLE outlet elbow 102. It is not possible to combust the coke in this area of piping via Steam Air Decoke (SAD) or Steam Water Decoking (SWD) because the temperature is too low. Normal conditions of decoking steam (7-10 klb/hr, 105-175 psig) and air (1-5 klb/hr, 110-120 psig) lack sufficient pressure and/or flow to decoke valley coke from Primary Transfer Line Exchange (PTLE) outlet elbows. By comparison, it is found that the use of Medium Pressure (MP) steam during SAD supplies sufficient pressure and flow to crush, break, or otherwise physically move or dislodge valley coke from PTLE outlet elbows.

(14) FIG. 4 shows four passes 401 of a radiant tube assembly for an olefin cracking furnace that has a total of eight passes (octants), each pass having 24 radiant tubes that end in six harps 402, each harp having 4 tubes and a tetra fitting for single conduit to a primary transfer line exchanger (PTLE) 105. Only one PTLE is shown in FIG. 4, but it is understood that each 4-tube harp has an associated PTLE. An octant, as the term is used herein, is one of a plurality of (e.g., eight, but not necessarily eight) radiant tube banks making up a flow pass. The octant includes sets of tubes whose outlets are connected, e.g., via tetra fittings, for entry into primary transfer line exchangers (PTLE) or transfer line exchangers (TLE). The unique arrangement of radiant tubes and outlets in an olefin cracking furnace allows for the safe flow of the MP steam through an affected pass while maintaining sufficient hydraulic surge to effectively dislodge the accumulated coke without requiring a cold shutdown of the furnace and without disrupting the ongoing decoking of other passes of radiant tubes of the furnace. Because MP steam is transmitted through radiant tubes whose PTLE outlets are not obstructed as well as tubes whose PTLE outlet is obstructed, the danger posed by pressure build up or backflow of MP steam is minimized. Furthermore, it is found that the MP steam pressure is great enough such that the hydraulic surge is sufficient to remove the accumulated coke from the PTLE outlet in relatively short order (e.g., from 10 to 20 minutes).

(15) The present disclosure provides a method for removing coke deposits in radiant tubes of an olefin cracking furnace and removing accumulated spalled coke from one or more outlet elbows of the furnace without performing a cold shutdown of the furnace, the method comprising the steps of: (a) introducing decking air and low pressure steam into a plurality of radiant tubes of the olefin cracking furnace, and maintaining flow through the plurality of radiant tubes for a first period of time; (b) identifying a first outlet elbow with restricted flow therethrough due to accumulated spalled coke resulting from step (a); and (c) introducing medium pressure steam through a first subset of the plurality of radiant tubes, the first subset of tubes comprising (i) a plurality of tubes upstream of one or more outlet elbows through which flow is not restricted and (ii) a second subset of tubes upstream of the first outlet elbow identified in step (b) through which flow is restricted, and maintaining flow therethrough for a second period of time, thereby removing accumulated spalled coke from the first outlet elbow.

(16) In accordance with the present disclosure, MP steam piping and controls are provided. In some embodiments, relevant sections of feed, dilution steam, and decoking air piping are designed and rated for MP steam temperature, pressure and flow. Additionally, a check valve to automatically protect feed and decoke air piping can be used to eliminate the requirement of manually blocking the common air source and individual feed pass controllers to the affected octant. In some embodiments, Safety Instrumented System (SIS) for the furnace interlock system may be modified such that the furnace will not trip on low-flow with the use of MP steam intervention during steam-air decoke (SAD). Alternatively or additionally, manually blocking decoke air and/or normal decoking steam flow to the affected octant can be used with the methods and systems described herein. Some embodiments include resetting low fire trip and/or Fuel Gas (FG) valves.

(17) In some embodiments, use of MP steam during SAD is helpful for quickly addressing aggressive exotherms of COTs above 1700 F. and/or cause/promote mechanical spalling before air addition to expedite decoking A person of ordinary skill in the art would be able to optimize controls and procedures accordingly. Large bore PTLEs may be used as well.

Exemplifications

(18) In this example, while in SAD mode, when four consecutive radiant tubes associated with a single PLTE appear plugged, the following procedures are used. First, a MP steam system is warmed up and a MP steam is lined up to control value on affected octant. MP steam control valve is then opened slowly while simultaneously a normal decoke steam and air supply to the affected octant is blocked-in. This should be done if there is a danger the furnace may trip on low-flow to the affected octant, due to configuration of the Safety Interlock System (SIS).

(19) During a 10-20 minute period, the MP steam is increased to a rate of up to about 40klb/hr, as needed. Plugged radiant tubes are monitored for visual signs of clearing. In some embodiments, the MP steam flow is adjusted from 5 to 40 k in steps to aid in hydraulically dislodging the bridged coke.

(20) As the tubes clear, the MP steam control valve is closed and simultaneously the normal decoke steam and air flow is restarted only in the affected octant for 20-30 minutes to ensure the tubes are fully decoked. The MP steam system is then blocked-in and depressured.

(21) A flow fire trip condition is cleared and, if necessary, FG valves are reset. The cleared tubes are monitored for signs of active burning and proceed accordingly. After the tubes are fully decoked and uniform in color to the surrounding tubes, normal SAD procedures resume for completion of SAD, and the furnace may be returned to normal cracking operations.

Other Embodiments and Equivalents

(22) While the present disclosures have been described in conjunction with various embodiments and examples, it is not intended that they be limited to such embodiments or examples. On the contrary, the disclosures encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art. Accordingly, the descriptions, methods and diagrams of should not be read as limited to the described order of elements unless stated to that effect.

(23) Although this disclosure has described and illustrated certain embodiments, it is to be understood that the disclosure is not restricted to those particular embodiments. Rather, the disclosure includes all embodiments that are functional and/or equivalents of the specific embodiments and features that have been described and illustrated.