Cracking furnace system and method for cracking hydrocarbon feedstock therein

Abstract

Cracking furnace system for converting a hydrocarbon feedstock into cracked gas comprising a convection section, a radiant section and a cooling section, wherein the convection section includes a plurality of convection banks, including a first high temperature coil, configured to receive and preheat hydrocarbon feedstock, wherein the radiant section includes a firebox comprising at least one radiant coil configured to heat up the feedstock to a temperature allowing a pyrolysis reaction, wherein the cooling section includes at least one transfer line exchanger.

Claims

1. Cracking furnace system for converting a hydrocarbon feedstock into cracked gas comprising a convection section, a radiant section and a cooling section, wherein the convection section includes a plurality of convection banks, including a first heated coil, operable in use to receive and preheat a hydrocarbon feedstock-diluent mixture, wherein the radiant section includes a firebox comprising at least one radiant coil configured to heat up the feedstock to a temperature allowing a pyrolysis reaction, wherein the cooling section includes at least one transfer line exchanger, characterised in that the system is operable in use to mix a hydrocarbon feedstock with a diluent in the form of a superheated steam to provide said hydrocarbon feedstock-diluent mixture in the convection section, upstream of the first heated coil, wherein the system is operable in use to further preheat the feedstock-diluent mixture after exit from the first heated coil by waste heat of cracked gas of the cracking furnace system in the transfer line exchanger in the transfer line exchanger before entry into the radiant section, wherein the convection section includes a second heated coil operable in use to further preheat the feedstock-diluent mixture after exit of the feedstock from the transfer line exchanger and before entry into the radiant section.

2. Cracking furnace system according to claim 1, wherein the second heated coil is located in a bottom part of the convection section.

3. Cracking furnace system according to claim 1, wherein the cracking furnace system comprises a provision, configured to mix the feedstock with diluent steam, upstream of said first heated coil.

4. Cracking furnace system according to claim 3 wherein the convection section includes at least one dilution steam super heater configured to superheat dilution steam to add to the feedstock or the feedstock-diluent mixture.

5. Cracking furnace system according to claim 3, wherein the cracking furnace system comprises a further provision configured to add additional diluent steam to the hydrocarbon feedstock-diluent steam mixture, which provision is configured to introduce the additional diluent steam into the hydrocarbon feedstock-diluent steam mixture between the outlet for hydrocarbon feed-diluent steam mixture from said first heated coil and the inlet for hydrocarbon feed-diluent steam mixture into said transfer line exchanger.

6. Cracking furnace system according to claim 1, further comprising a steam drum configured to generate saturated heated pressurized steam.

7. Cracking furnace system according to claim 6, further comprising a secondary transfer line exchanger which is located downstream from the at least one transfer line exchanger and which is connected to the steam drum, and which is configured to at least partly vaporize boiler water coming from the steam drum.

8. Cracking furnace system according to claim 6 wherein the convection section includes at least one heated pressure steam superheater configured to superheat heated pressurized steam coming from the steam drum.

9. Cracking furnace system according to claim 1, wherein the plurality of convection banks further includes a feed preheater configured to preheat the hydrocarbon feedstock prior to a provision, configured to mix the preheated feedstock with part or all of the diluent, which provision is situated between said feed preheater and said first heated coil.

10. Cracking furnace system according to claim 1, wherein the plurality of convection banks includes a further provision configured to mix further diluent into the feedstock-diluent mixture which further provision is located downstream of the first heated coil and upstream of the transfer line exchanger.

11. Method for cracking hydrocarbon feedstock in a cracking furnace system, the method comprising mixing a hydrocarbon feedstock with a diluent in the form of a superheated steam to provide a hydrocarbon feedstock-diluent mixture in a convection section of the cracking furnace system upstream of a first heated coil, and subjecting the hydrocarbon feedstock-diluent mixture to a first feedstock preheating step, a second feedstock preheating step, and a third preheating step before entry of the hydrocarbon feedstock-diluent mixture into a radiant section of the cracking furnace system, in which radiant section the hydrocarbon feedstock is cracked, wherein the first feedstock preheating step includes preheating the hydrocarbon feedstock-diluent mixture by hot flue gasses of a cracking furnace system using the first heated coil, wherein the second feedstock-diluent mixture preheating step includes further preheating of the feedstock-diluent mixture by waste heat of cracked gas of the cracking furnace system using a transfer line exchanger after exit from the first heated coil before entry into the radiant section, wherein the third feedstock-diluent mixture preheating step includes further preheating of the feedstock by flue gasses of the cracking furnace system using a second heated temperature coil after exit from the transfer line exchanger and before entry into the radiant section.

12. Method according to claim 11, wherein after said first preheating step, further dilution steam is added to the feedstock-diluent mixture, before subjecting the feedstock-diluent mixture to said further preheating of the feedstock-diluent mixture by waste heat of cracked gas of the cracking furnace system using the transfer line exchanger.

13. Method according to claim 11, wherein heated pressurized steam is generated by waste heat of cracked gas of the cracking furnace system, using a secondary transfer line exchanger located downstream of the transfer line exchanger.

14. Method according to claim 11, wherein the hydrocarbon feedstock-diluent mixture is superheated in a convection section.

15. Method according to claim 11, wherein the feedstock is subjected to preheating prior to mixing the feedstock with diluent.

16. Method according to claim 15, wherein the feedstock is preheated prior to the mixing with the diluent to temperature whereby upon mixing with diluent a feedstock-diluent mixture, to be fed into the first heated coil, is obtained having a temperature exceeding the water dew point.

17. Method according to claim 11, wherein the feedstock-diluent mixture enters the first heated coil at a temperature above the dew point of water.

18. Method according to claim 17, wherein the feedstock-diluent mixture enters the first heated coil at a temperature of 30-70? C. above the dew point of water.

19. Method according to claim 11, wherein the feedstock-diluent mixture is preheated in the first heated coil and the feedstock-diluent mixture already has a temperature exceeding the feedstock's hydrocarbon dew point at the start of the second feedstock-diluent preheating step.

20. Method according to claim 11, wherein the method is carried out in the cracking furnace system, further comprising a steam drum configured to generate saturated heated steam.

21. Method according to claim 20, wherein the cracking furnace system further comprises a secondary transfer line exchanger which is located downstream from the transfer line exchanger and which is connected to the steam drum, and which is configured to at least partly vaporize boiler water coming from the steam drum.

22. Method according to claim 20, wherein the hydrocarbon feedstock-diluent mixture is superheated in a convection section that includes at least one heated pressurized steam superheater configured to superheat heated pressurized steam coming from the steam drum.

23. Method according to claim 11, wherein the hydrocarbon feedstock-diluent mixture is superheated in a convection section that includes at least one dilution steam super heater configured to superheat dilution steam to add to the feedstock or the feedstock-diluent mixture.

Description

(1) The present invention will be further elucidated with reference to figures of exemplary embodiments. Therein,

(2) FIG. 1 shows a schematic representation of a first preferred embodiment of a cracking furnace system according to the invention;

(3) FIG. 2 shows a schematic representation of a second embodiment of a cracking furnace system according to the invention;

(4) FIG. 3 shows a schematic representation of a third embodiment of a cracking furnace system according to the invention;

(5) FIG. 4 shows a schematic representation of a fourth embodiment of a cracking furnace system according to the invention.

(6) It is noted that the figures are given by way of schematic representation of embodiments of the invention. Corresponding elements are designated with corresponding reference signs.

(7) FIG. 1 shows a schematic representation of a cracking furnace system 40 according to a preferred embodiment of the invention. The cracking furnace system 40 comprises a convection section including a plurality of convection banks 21. Hydrocarbon feedstock 1 can enter a feed preheater 22, which can be one of the plurality of convection banks 21 in the convection section 20 of the cracking furnace system 40. This hydrocarbon feedstock 1 can be any kind of hydrocarbon, preferably paraffinic or naphthenic in nature, but small quantities of aromatics and olefins can also be present. Examples of such feedstock are: ethane, propane, butane, natural gasoline, naphtha, kerosene, natural condensate, gas oil, vacuum gas oil, hydro-treated or desulphurized or hydro-desulphurized (vacuum) gas oils or combinations thereof. Depending on the state of the feedstock the feed is preheated and/or partly or fully evaporated in the preheater before being mixed with a diluent, such as dilution steam 2. Dilution steam 2 can be injected directly or, alternatively, as in this preferred embodiment, dilution steam 2 can first be superheated in a dilution steam super heater 24 before being mixed with the feedstock 1. There can be a single steam injection point or multiple steam injection points, for example for heavier feedstock. The mixed feedstock/dilution steam mixture 13 can be further heated in a first high temperature coil 23, and then in the primary transfer line exchanger 35. After exit of the mixed feedstock/dilution steam mixture 13 from the transfer line exchanger 35 and before entry into the radiant section 10, the feedstock or the mixture, is further preheated, according to the invention, by a second high temperature coil 26 in the convection section 20 to reach an optimum temperature for introduction into the radiant coil 11. The radiant coil can for example be of one of the types mentioned before or of any other type maintaining a reasonable run length, as known to the person skilled in the art. In the radiant coil 11 the hydrocarbon feedstock is quickly heated up to the point where the pyrolysis reaction starts so that the hydrocarbon feedstock is converted into products and by-products. Such products are amongst others hydrogen, ethylene, propylene, butadiene, benzene, toluene, styrene and/or xylenes. By-products are amongst others methane, aromatics and fuel oil. The resulting mixture of a diluent such as dilution steam, unconverted feedstock and converted feedstock, which is the reactor effluent called cracked gas, is cooled quickly in the transfer line exchanger 35, to freeze the equilibrium of the reactions in favor of the products. The waste heat in the cracked gas 8 is first recovered in the transfer line exchanger 35 by heating up the feedstock or feedstock-diluent mixture 13 before it is sent back to the convection section for further preheating in the second high temperature coil 26 before entry into the radiant section 10. Any further excess waste heat in the cracked gas 8 may then further be recovered in at least an additional transfer line exchanger, the secondary transfer line exchanger 36, which is located downstream from the primary transfer line exchanger 35, and which is configured to generate saturated high pressure steam from boiler water 9a by at least partly vaporizing boiler water 9a. The system can comprise a steam drum 33 configured to generate saturated high pressure steam 4. Boiler feed water 3 may be fed to the steam drum 33. Boiler water 9a may then be fed to the secondary transfer line exchanger 36, where it is partly vaporized. The at least partly vaporized boiler water 9b may then flow back to the steam drum by natural circulation. In the steam drum 33, the generated saturated steam can then be separated from the boiler water and sent to the convection section 20 to be superheated by at least one high pressure steam superheater 25, for example by a first and a second super heater 25 in the convection section 20. Said at least one super heater 25 can preferably be located upstream of a dilution steam super heater 24, and preferably downstream of the second high temperature coil 26. To control the high pressure steam temperature, additional boiler feed water 3 can be injected into a de-super heater 34 located between a first and a second super heater 25.

(8) The heat of reaction for the highly endothermic pyrolysis reaction can be supplied by the combustion of fuel (gas) 5 in the radiant section 10, also called the furnace firebox, in many different ways, as is known to the person skilled in the art. Combustion air 6 can for example be introduced directly into burners 12 of the furnace firebox, in which burners 12 fuel gas 5 and combustion air 6 is fired to provide heat for the pyrolysis reaction. Alternatively, combustion air 6 may first be preheated in the convection section 20, for example by a convection bank embodied as an air preheater 27 located to a downstream side of the convection section 20, preferably downstream all the other convection section banks in the convection section, as shown. The combustion air 6 may be introduced into the air preheater 27 by for example a forced draft fan 37. Preheating of the combustion air can raise the adiabatic flame temperature and make the firebox more efficient. In the combustion zones 14 in the furnace firebox, fuel 5 and (preheated) combustion air are converted to combustion products such as water and CO2, the so-called flue gas. The waste heat from the flue gas 7 is recovered in the convection section 20 using various types of convection banks 21. Part of the heat is used for the process side, i.e. the preheating and/or evaporation and/or superheating of hydrocarbon feed and/or the feedstock-diluent mixture, and the rest of the heat is used for the non-process side, such as the generation and superheating of high pressure steam, as described above. The combustion in the furnace firebox 10 can be done by means of bottom burners 12 and/or sidewall burners and/or by means of roof burners and/or sidewall burners in a top fired furnace. In the exemplary embodiment of the furnace 10 as shown in FIG. 1, firing is restricted to the lower part of the firebox by using bottom burners 12 only. This can raise firebox efficiency and can drastically reduce fuel gas consumption by up to approximately 20% compared with a conventional scheme. A high firebox efficiency can be achieved among others using for instance only bottom burners (as shown) or a number of rows of side wall burners placed close to the bottom in case of bottom firing, or by using only roof burners or a number of rows of side wall burners placed very close to the roof in case of top firing. Making the firebox taller or placing more efficient radiant coils are other examples to reach this objective. As the heat distribution in this case is rather focused on part of the radiant coil, the local heat flux is increased, reducing run length. To counteract this effect, the application of heat transfer enhancing radiant coil tubes, such as for example swirl flow tube types or winding annulus radiant tube types may be required in the radiant coil in order to maintain a reasonable run length. Other means to gain better performance, such as a three lane coil design, can also be used to increase run length, either separately or in combination with other means. The embodiment in FIG. 1 further shows an induced draft fan 30, also called a flue gas fan, and a stack 31 located at a downstream end of the convection section to evacuate the flue gas from the convection section 20.

(9) With the new inventive arrangement, an optimized radiant coil inlet temperature can be maintained while the logarithmic mean temperature difference in the primary transfer line exchanger can be enlarged, which can accelerate the freezing of the reaction equilibrium and limit the conversion of products to by-products, leading to an improvement of the yield of the system. As an example, the feedstock may enter the transfer line exchanger 35 at a cold side inlet temperature of around 350? C. and be preheated to a cold side outlet temperature of around 555? C. instead of approximately 610? C. previously, whereas at the same time, the effluent may enter the transfer line exchanger 35 with a hot side inlet temperature of approximately 810? C. and be cooled to a hot side outlet temperature of around 630? C. instead of approximately 575? C. in a prior art design. This results in an increase of the logarithmic mean temperature difference from 213? C. to 267? C., which corresponds to an increase of 25% in the logarithmic mean temperature difference in the primary transfer line exchanger, improving the yield of the system with a factor of approximately 0.1% to more or less 2.0%, which may be significant for large production capacities of products such as ethylene, propylene, or butadiene. As mentioned before, maintaining an optimized radiant coil inlet temperature is important as a lower inlet temperature of the feedstock would raise the radiant duty and lower the firebox efficiency and raise the fuel consumption, while a higher inlet temperature could result in conversion of feedstock inside the convection section and associated deposition of cokes on the internal surface convection section tubes.

(10) The invention of a three-step preheating of hydrocarbon feedstock by a first high temperature coil in the convection section, a transfer line exchanger in the cooling section and by a second high temperature coil in the convection section can also be advantageously applied to alternative cracking furnace systems and methods for cracking hydrocarbon feedstock therein. FIG. 2 shows a schematic representation of a second embodiment of a cracking furnace system according to the invention. In this embodiment, heat for the pyrolysis reaction in the furnace firebox 10 is provided by fuel gas 5, combustion air 6 and highly nitrogen depleted combustion oxygen 51 fired in the burners 12. Introduction of oxygen in the combustion zone 14 can also raise the adiabatic flame temperature as an alternative method to the scheme presented in FIG. 1.

(11) FIG. 3 shows a schematic representation of a third embodiment of a cracking furnace system according to the invention. In this embodiment, heat for the pyrolysis reaction in the furnace firebox 10 is provided by fuel (gas) 5, combustion air 6 and highly nitrogen depleted combustion oxygen 51 fired in the burners 12 in the presence of externally recirculating flue gas 52. The combustion oxygen 51 can be mixed with recirculated flue gas 52 upstream of the burners 12 in a common line to the burners 12 using an ejector 55. To obtain the recirculated flue gas 52, the flue gas exiting the convection section 20 can be split by for example a flue gas splitter 54 into produced flue gas 7 and flue gas 52 for external recirculation. The produced flue gas 7 can be evacuated through a stack 31 using an induced draft fan 30. The same fan 30 can be configured to recirculate the flue gas externally to the burners 12. Alternatively, the fan 30 may be embodied as two or more fans, depending on parameters such as pressure drop difference of a downstream system, e.g. stack 31 or flue gas recirculation circuit 52.

(12) FIG. 4 shows a schematic representation of a fourth embodiment of a cracking furnace system according to the invention. In this embodiment, heat for the pyrolysis reaction in the furnace firebox 10 is provided by fuel (gas) 5 and highly nitrogen depleted combustion oxygen 51 fired in the burners 12 in the presence of externally recirculating flue gas 52. This scheme is practically the same as the one presented in FIG. 3, except that all the combustion air 6 is replaced by combustion oxygen 51. This is the scheme with the highest consumption of combustion oxygen 51, but the lowest quantity of flue gas leaving the stack. This flue gas is very rich in CO2 making it ideal for carbon capturing, and the NOx emission is the lowest due to the absence of nitrogen, except for the nitrogen associated with air leakage into the convection section. This scheme is the most environmentally friendly.

(13) The work leading to this invention has received funding from the European Union Horizon H2020 Programme (H2020-SPIRE-04-2016) under grant agreement no. 723706.

(14) For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. It may be understood that the embodiments shown have the same or similar components, apart from where they are described as being different.

(15) In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word comprising does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words a and an shall not be construed as limited to only one, but instead are used to mean at least one, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage. Many variants will be apparent to the person skilled in the art. All variants are understood to be comprised within the scope of the invention defined in the following claims.

REFERENCES

(16) 1. Hydrocarbon feedstock 2. Dilution steam 3. Boiler feed water 4. High pressure steam 5. Fuel gas 6. Combustion air 7. Flue gas 8. Cracked gas 9a. Boiler water 9b. Partly vaporized boiler water 10. Radiant section/furnace firebox 11. Radiant coil 12. Bottom burner 13. Feedstock/dilution steam mixture 14. Combustion zone 20. Convection section 21. Convection bank 22. Feed preheater 23. First high temperature coil 24. Dilution steam super heater 25. High pressure steam super heater 26. Second high temperature coil 27. Air preheater 30. Induced draft fan 31. Stack 33. Steam drum 34. De-super heater 35. Primary transfer line exchanger 36. Secondary transfer line exchanger 37. Forced draft fan 40. Cracking furnace system 50. Preheated combustion air 51. Oxygen 52. Externally recycled flue gas 54. Flue gas splitter 55. Flue gas ejector