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 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 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, wherein the system is configured such that the transfer line exchanger preheats the feedstock before entry into the radiant section using waste heat from cooling down or quenching the cracked gas be a gas-to-gas heat transfer from the cracked gas to the feedstock, and wherein the transfer line exchanger raises feedstock temperature over a majority of a remaining deviation from the temperature allowing the pyrolysis reaction.
2. Cracking furnace system according to claim 1, wherein the convection section comprises a boiler coil configured to generate saturated steam.
3. Cracking furnace system according to claim 1, wherein the convection section is configured for mixing said hydrocarbon feedstock with a diluent, providing a feedstock-diluent mixture, wherein the transfer line exchanger is configured to preheat the feedstock-diluent mixture before entry into the radiant section.
4. Cracking furnace system according to claim 1, further comprising a secondary transfer line exchanger, wherein the secondary transfer line exchanger is configured to generate saturated high pressure steam.
5. Cracking furnace system according to claim 2, further comprising a steam drum which is connected to the boiler coil.
6. Cracking furnace system according to claim 1, wherein the firebox is configured such that a firebox efficiency is higher than at least one of 40%, 45%, or 48%.
7. Cracking furnace system according to claim 1, wherein the convection section comprises an economizer configured to preheat boiler feed water for the generation of saturated steam.
8. Cracking furnace system according to claim 1, wherein the convection section comprises an oxidant preheater, configured to preheat oxidant before introduction of said combustion air into the firebox.
9. Cracking furnace system according to claim 1, wherein the system is configured for oxygen introduction into the radiant section.
10. Cracking furnace system according to claims 1, further comprising an external flue gas recirculation circuit configured to recover at least part of the flue gas and to recirculate said flue gas to the radiant section to control flame temperature.
11. Cracking furnace system according to claim 10, wherein the external flue gas recirculation circuit comprises a flue gas ejector configured to introduce oxygen into the recirculated flue gas prior to entry into the firebox.
12. Cracking furnace system according to claim 1, further comprising a heat pump circuit including an evaporator coil located in the convection section and a condenser, wherein the heat pump circuit is configured such that the evaporator coil recovers heat from the convection section and the condenser transfers said heat to boiler feed water.
13. Cracking furnace system according to claim 2, wherein said boiler coil is located in a bottom part of the convection section.
14. Cracking furnace system according to claim 8, wherein the oxidant preheater is located downstream in the convection section.
15. Cracking furnace system according to claim 9, wherein the system is configured for oxygen introduction into the radiant section in the absence of external flue gas recirculation.
16. Cracking furnace system according to claim 2, further comprising a steam drum which is connected the secondary transfer line exchanger or connected the secondary transfer line exchanger and the boiler coil.
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) FIG. 5 shows a schematic representation of a fifth embodiment of a cracking furnace system according to the invention
(7) FIG. 6 shows a schematic representation of a sixth embodiment of a cracking furnace system according to the invention;
(8) FIG. 7 shows a schematic representation of a seventh embodiment of a cracking furnace system according to the invention;
(9) FIG. 8 shows a graph representing relative oxygen flow rate versus relative air flow rate.
(10) 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.
(11) 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 can be further heated in a high temperature coil 23 and, according to the invention, in the primary transfer line exchanger 35 to reach an optimum temperature for introduction into the radiant coil 11. The radiant coil can for example be of the swirl flow type, as disclosed in EP1611386, EP2004320 or EP2328851, or a three lane radiant coil design (as disclosed in US 2008 142411), or a winding annulus tube type (UK 1611573.5) 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 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 favour of the products. In an inventive way, 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 before it is sent to the radiant coil 11. According to the present invention, high pressure steam can be generated in the convection section, for example by a boiler coil 26 configured to at least partly evaporate boiler water from the steam drum 33 to generate saturated high pressure steam. The boiler coil 26 can be located in a bottom part of the convection section and is connected with the steam drum 33, such that boiler water 9a can flow from the steam drum 33 to the boiler coil 26 and such that partly vaporized boiler water 9b can flow back from the boiler coil 26 to the steam drum 33 by natural circulation. Boiler feed water 3 can be delivered directly to the steam drum 33. In the steam drum 33, boiler feed water 3 is mixed with boiler water already present in the steam drum. In the steam drum 33 the generated saturated steam is separated from boiler water and can be sent to the convection section 20 to be superheated, which can be done by at least one high pressure steam super heater 25, for example by a first and a second super heater 25 in the convection section 20. Said boiler coil 26 located in a bottom part of the convection section can recover excess heat from the flue gas and can protect the downstream convection section banks, especially the at least one high pressure steam super heater bank 25, from overheating. Said at least one super heater 25 can preferably be located upstream of the dilution steam super heater 24, and preferably downstream of the boiler 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.
(12) 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. In the combustion zones 14 in the furnace firebox, fuel 5 and combustion air 6 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.
(13) In one embodiment, such as illustrated in FIG. 2 showing a schematic representation of a second embodiment of a cracking furnace system, any excess heat in the cracked gas can for example be recovered in at least an additional transfer line exchanger, the secondary transfer line exchanger 36, which is configured to generate saturated high pressure steam. This steam is generated from boiler water 9a coming from the steam drum 33, which boiler water is partly vaporized by the secondary transfer line exchanger 36. This partly vaporized boiler water 9b is flowing to the steam drum 33 by natural circulation. In this way, an additional loop from and to the steam drum 33 is provided to increase high pressure steam generation and improve the overall furnace efficiency. Boiler feed water 3 can be delivered directly to the steam drum 33, as in FIG. 1, or can first be preheated, for example by excess heat available in the convection section 20 not required by the boiler coil 26. Thereto, a further convection bank 21, for example an economizer 28, can be added to the furnace convection section 20. This convection bank 28 can be configured to preheat the boiler feed water 3 before entering the steam drum 33, with the purpose to raise overall furnace efficiency and provide a more cost-effective convection section. The embodiment in FIG. 2 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.
(14) With the new inventive arrangement, as shown in FIGS. 1 and 2, the amount of non-process duty, i.e. the duty recovered in the cracked gas and the convection section for the high pressure steam generation, can be reduced independently of the amount of process duty required to preheat the dilution steam hydrocarbon mixture to the optimum temperature to enter the radiant coil. This means that the firebox efficiency can be increased from 40% for a conventional scheme to as high as 48% for the new scheme as is shown in FIGS. 1 and 2, reducing the fuel consumption by approximately 17%. The reduced fuel consumption also reduces the flue gas flow rate and the associated convection section duty with roughly 17%. The new scheme allows this heat to be prioritized for the process usage at the cost of the non-process usage, resulting in an optimized process inlet temperature for the radiant coil, but with a lower high pressure steam production. 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. This coke deposition cannot be removed during the regular decoking cycle for the removal of cokes in the radiant coil as the tube temperature is too low for combustion of the cokes in the convection section, ultimately requiring a prolonged and costly furnace shut-down for cutting the affected tubes in the convection section and the mechanical removal of the cokes.
(15) 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. 2, 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. Advantageously, this embodiment does not substantially have issues with NOx emissions, compared with a conventional furnace as the adiabatic flame temperature is not increased due to oxy-fuel combustion or air preheat.
(16) FIG. 3 shows a schematic representation of a third embodiment of a cracking furnace system. In this embodiment, heat for the pyrolysis reaction in the furnace firebox 10 is provided by fuel gas 5 and preheated combustion air 50 fired in the burners 12. Combustion air 6 can be introduced via a forced draft fan 37, and can then be heated up 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. Preheating of the combustion air can raise the adiabatic flame temperature and make the firebox even more efficient than the system presented in FIG. 2. Fuel gas reduction in excess of 25% as compared with conventional schemes is feasible. However, the higher adiabatic flame temperature may also raise the NOx emission, depending on the extent of the combustion air preheat. Depending on the environmental regulations on maximum allowable NOx emissions, this may require NOx abatement measures to be taken, for example by installing a selective catalytic NOx reduction bed in the convection section 20. As the firebox efficiency can be higher than in the system shown in FIG. 2, the convection section duty is lower and excess heat in the convection section for preheating boiler feed water might no longer be available as the firebox efficiency is increased. Eventually the economizer can become redundant and the boiler feed water can be sent to the steam drum without being preheated in an economizer, as is shown in FIG. 3.
(17) FIG. 4 shows a schematic representation of a fourth embodiment of a cracking furnace system. 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. 3. Also with this scheme, fuel gas reduction in excess of 25% as compared with conventional schemes is feasible. However, the higher adiabatic flame temperature may also raise the NOx emission, depending on the extent of the oxygen injection. Depending on the environmental regulations on maximum allowable NOx emissions, this may require NOx abatement measures to be taken, for example by installing a selective catalytic NOx reduction bed in the convection section 20.
(18) FIG. 5 shows a schematic representation of a fifth embodiment of a cracking furnace system. 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.
(19) FIG. 6 shows a schematic representation of a sixth embodiment of a cracking furnace system. 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. 5, 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.
(20) The relation between FIGS. 4, 5 and 6 can be further explained with reference to FIG. 8, the graph showing the relative oxygen flow rate (on the vertical axis) as a function of relative air flow rate (on the horizontal axis). The relative oxygen flow rate is the flow rate relative to the oxygen requirement at 100% oxy-fuel combustion, i.e. in the absence of any combustion air. FIG. 4 is a schematic representation of a cracking furnace system for partial oxy-fuel combustion without any need for external flue gas recirculation, while FIG. 6 is a schematic representation of a cracking furnace system for full oxy-fuel combustion with external flue gas recirculation to temper the adiabatic flame temperature. FIG. 5 is a schematic representation of a cracking furnace system for an intermediate situation. The oxygen requirement relative to full oxy-fuel combustion as shown in FIG. 6 is 25% for the scheme as shown in FIG. 4 as one extreme, indicated by “y” in the graph, and 100% for the FIG. 6 scheme, which is indicated as “x” in the graph of FIG. 8. The FIG. 5 scheme is in between these two extremes. The FIG. 6 scheme produces the lowest NOx of the three schemes, lower than that of current state-of-the-art schemes, while the FIG. 4 scheme has a substantially higher NOx emission level than the other two schemes. The FIG. 5 scheme is in between these two extremes. The FIG. 4 scheme may be the most economical of the three schemes if there is no requirement for carbon capturing, but only for better fuel efficiency. As mentioned before, the FIG. 6 scheme may be the most environmentally friendly and suitable for carbon capturing. The introduction of combustion air can provide a significant reduction of the need for oxygen, the oxygen requirement reducing from 100% to approximately 25% as a function of the relative air flow. For the FIG. 6 scheme the relative oxygen flow rate is 100%, and for the FIG. 4 scheme this is approximately 25%. The FIG. 5 scheme is in between these two extremes. The relative air flow rate is the flow rate relative to the combustion air requirement at partial oxy-fuel combustion as per FIG. 4 scheme, at approximately 7 wt % oxygen injection to raise the adiabatic flame temperature and no external flue gas recirculation. In the FIG. 6 scheme the relative combustion air requirement is 0%. The FIG. 5 scheme is in between these two extremes.
(21) FIG. 7 shows a schematic representation of a seventh embodiment of a cracking furnace system. This embodiment of the cracking furnace system is based on the embodiment of FIG. 6, thus including a flue gas recirculation circuit with oxygen introduction, and without introduction of combustion air. In order to further increase the furnace efficiency, a heat pump circuit 70 is added to the system 40. The heat pump circuit 70 is configured to recover heat from the flue gas and use it to preheat boiler feed water thus increasing the production of high pressure steam. The heat source of the heat pump circuit 70 comprises an evaporator coil 77 located in the convection section 20 of the cracking furnace 40. This evaporator coil 77 is connected to a vapour-liquid separating device 76, such as for example a knock-out drum, via down comers and risers. Organic fluid 60, such as for example butane, pentane or hexane, is flowing under natural circulation via the down comers to the evaporator coil 77 where it is partially evaporated by the heat recovered from the flue gas. The organic liquid/vapour mixture 61 is flowing back to the vapour-liquid separating device via the risers. In the vapour-liquid separating device the vapour 62 is separated from the liquid/vapour mixture 61. The vapour 62 separated from the mixture 61 is then superheated in a feed effluent exchanger 74 in order to increase loop efficiency. The superheated vapour 63 is sent to a compressor 71. This compressor 71 is configured to raise the pressure of the superheated vapour 63 to such a level that the condensing temperature at the outlet of the compressor 71 exceeds with sufficient margin the temperature level to which the boiler feed water 3 needs to be preheated. This requires a proper selection of the compressor efficiency. The compressed high pressure vapour 64 from the compressor 71 is fully condensed in the condenser 72. The condensation heat is used to preheat boiler feed water 3. The condensed organic liquid 65 is accumulated in the condensate vessel 73. From the condensate vessel 73 the saturated liquid 66 is sent to the feed effluent exchanger 74 to be subcooled. The subcooled liquid 67 is flashed to a lower pressure in a pressure reduction valve 75. The more the liquid is subcooled in the feed effluent exchanger 74, the higher the liquid fraction at the outlet of this valve 75 and the lower the required circulation rate of the organic heat pumped fluid. The low pressure liquid vapour mixture 68 is sent to the vapour-liquid separating device 76, where the liquid and vapour are separated from each other, completing the circuit.
(22) Where the evaporator coil 77 is the heat source of the circuit, the condenser 72 can be considered as the heat sink of the circuit. The duty that needs to be condensed in the condenser 72 is that of the heat recovered from the flue gas in the evaporator and the heat supplied by a driver of the compressor 71. This means that the power supplied by the driver is also used to generate high pressure steam. This heat improves loop efficiency as no heat is lost in driving the compressor. Yet, it is still beneficial to select a high efficiency compressor and to apply a feed effluent exchanger 74 to keep the flow rate and corresponding equipment size of all items in the circuit as small as possible. In case of a train of cracking furnaces, the compressor 71, the condensate vessel 73 and the feed effluent exchanger 74 can be configured to serve said train of cracking furnaces.
(23) The project leading to this application has received funding from the European Union Horizon H2020 Programme (H2020-SPIRE-2016) under grant agreement n°723706.
(24) 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.
(25) 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.
(26) 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
(27) 1. Hydrocarbon feedstock
(28) 2. Dilution steam
(29) 3. Boiler feed water
(30) 4. High pressure steam
(31) 5. Fuel gas
(32) 6. Combustion air
(33) 7. Flue gas
(34) 8. Cracked gas
(35) 9a. Boiler water
(36) 9b. Partly vapourized boiler water
(37) 10. Radiant section/furnace firebox
(38) 11. Radiant coil
(39) 12. Bottom burner
(40) 14. Combustion zone
(41) 20. Convection section
(42) 21. Convection bank
(43) 22. Feed preheater
(44) 23. High temperature coil
(45) 24. Dilution steam super heater
(46) 25. High pressure steam super heater
(47) 26. Boiler coil
(48) 27. Air preheater
(49) 28. Economizer
(50) 30. Induced draft fan
(51) 31. Stack
(52) 33. Steam drum
(53) 34. De-super heater
(54) 35. Primary transfer line exchanger
(55) 36. Secondary transfer line exchanger
(56) 37. Forced draft fan
(57) 40. Cracking furnace system
(58) 50. Preheated combustion air
(59) 51. Oxygen
(60) 52. Externally recycled flue gas
(61) 54. Flue gas splitter
(62) 55. Flue gas ejector
(63) 60. Organic liquid
(64) 61. Organic liquid-vapour mixture
(65) 62. Vapour
(66) 63. Super heated vapour
(67) 64. High pressure vapour
(68) 65. Condensed organic liquid
(69) 66. Saturated liquid
(70) 67. Subcooled liquid
(71) 68. Low pressure liquid-vapour mixture
(72) 70. Heat pump circuit
(73) 71. Compressor
(74) 72. Condenser
(75) 73. Condensate vessel
(76) 74. Feed effluent exchanger
(77) 75. Pressure reduction valve
(78) 76. Vapour-liquid separating device
(79) 77. Evaporator coil
