CN111032831B - Cracking furnace system and process for cracking hydrocarbon feedstock therein - Google Patents

Abstract

A cracking furnace system for converting a hydrocarbon feedstock into a cracked gas, the cracking furnace system comprising a convection section, a radiant section, and a cooling section, wherein the convection section comprises a plurality of convection tube bundles configured to receive and preheat the hydrocarbon feedstock, wherein the radiant section comprises a combustion chamber comprising at least one radiant coil configured to heat the feedstock to a temperature that allows for pyrolysis reactions, wherein the cooling section comprises at least one transfer line heat exchanger.

Description

Cracking furnace system and process for cracking hydrocarbon feedstock therein

The present invention relates to a cracking furnace system.

Conventional cracking furnace systems, such as disclosed in document US 4479869, typically include a convection section wherein a hydrocarbon feedstock is preheated and/or partially vaporized and mixed with dilution steam to provide a feedstock-dilution steam mixture. The system also includes a radiant section comprising at least one radiant coil in the combustion chamber, wherein the feedstock-dilution steam mixture from the convection section is converted to product and byproduct components by pyrolysis at elevated temperatures. The system also includes a cooling section that includes at least one quench heat exchanger, such as a transfer line heat exchanger, configured to rapidly quench the product or cracked gas exiting the radiant section to stop the pyrolysis side reactions and maintain a reaction equilibrium favorable to the product. Heat from the transfer line heat exchanger can be recovered as high pressure steam.

A disadvantage of the known system is that a lot of fuel needs to be supplied for the pyrolysis reaction. To reduce this fuel consumption, the combustor efficiency, the percentage of heat released in the combustor that is absorbed by the radiant coils, can be significantly increased. However, heat recovery schemes in the convection section of conventional cracker furnace systems with increased furnace efficiency have only a limited ability to heat the hydrocarbon feedstock to reach an optimum temperature for entry into the radiant section. Therefore, it is almost impossible to reduce fuel consumption in conventional cracking furnace systems, thereby reducing CO2 emissions.

It is an object of the present invention to solve or alleviate the above problems. In particular, it is an object of the invention to provide a more efficient system, reducing the need for energy supply and thus reducing CO2 emissions.

To this end, according to a first aspect of the present invention, a cracking furnace system is provided, characterized by the features of claim 1. In particular, a cracking furnace system for converting a hydrocarbon feedstock into a cracked gas includes a convection section, a radiant section, and a cooling section. The convection section includes a plurality of conventional rows configured to receive and preheat the hydrocarbon feedstock. The radiant section includes a combustion chamber including at least one radiant coil configured to heat the feedstock to a temperature that allows for pyrolysis reactions. The cooling section comprises at least one transfer line heat exchanger as a heat exchanger. In an inventive manner, the system is configured such that the feedstock is preheated by the transfer line exchanger prior to entering the radiant section.

The transfer line heat exchanger is a heat exchanger arranged to cool or quench cracked gas. This quenched recovered heat or waste heat can then be recovered and used in a cracking furnace system, such as steam generation as is generally known in the art. According to the present invention, using the waste heat of the cracked gas in the transfer line heat exchanger instead of heating the feedstock in the convection section, as is done in prior art systems, heating the feedstock in the cooling section, can allow for a significant increase in combustor efficiency, resulting in a reduction in fuel gas by up to, or even more than, about 20%. The combustor efficiency is the ratio between the amount of heat absorbed by at least one radiant coil by pyrolysis, which is an endothermic reaction, converting the hydrocarbon feedstock into cracked gas and the amount of heat released by the combustion process in the combustion zone, which is based on the lower heating value of 25 ℃. This definition conforms to fuel efficiency equation 3.25 as defined in API standard 560 (fired heater in general refineries). The higher the efficiency, the lower the fuel consumption, but the lower the amount of heat available in the convection section for preheating the feedstock. Preheating of the feedstock in the cooling section may overcome this obstacle. Thus, in the cracking furnace system according to the present invention, there is a first feedstock preheating step and a second feedstock preheating step. The first feedstock preheating step includes preheating the hydrocarbon feedstock by hot flue gas of the cracking furnace system, for example, in one of a plurality of convection banks in a convection section. Preheating also includes partial vaporization in the case of liquid feedstocks and superheating in the case of gaseous feedstocks. The second feedstock preheating step comprises further preheating the feedstock by waste heat of cracked gases of the cracking furnace system before the feedstock enters the radiant section of the cracking furnace system. The second feed preheating step is performed using a transfer line heat exchanger in the cooling section. As known to those skilled in the art, the optimum inlet temperature of the feedstock into the radiant section is determined by the thermal stability of the feedstock. Ideally, the feedstock enters the radiant section at a temperature just below the start of the pyrolysis reaction. If the feedstock inlet temperature is too low, additional heat is required to heat the feedstock in the radiant section, increasing the heat supplied in the radiant section and the corresponding fuel consumption. If the feedstock inlet temperature is too high, pyrolysis may already begin in the convection section, which is undesirable because the reactions are associated with the formation of coke on the tube inner surfaces, which cannot be easily removed from the convection section during decoking. Another advantage of the cracker system of the present invention is that in the transfer line heat exchanger according to the present invention, condensation of the heavy (asphaltene) tails to form scale is almost impossible. In the case of gas to boiling steam heat transfer, for example when the transfer line heat exchanger is configured to produce saturated steam as in prior art systems, boiling water has a higher heat transfer coefficient than that of the gas. This results in a wall temperature very close to that of boiling water. The temperature of the boiler water in the cracking furnace is typically about 320 ℃, for most cold ends of the heat exchangers the wall temperature at the cold side of the exchanger is only slightly above this temperature, while for most liquid feeds the dew point of the cracked gas is above 350 ℃, resulting in condensation of heavy tail components on the tube surfaces and fouling of the equipment. For this reason, the heat exchanger needs to be cleaned regularly. This is partly achieved during decoking of the radiant coils, but at regular time intervals the furnace must be taken out for operation of mechanical cleaning of the transfer line heat exchanger. This may take several days as it not only involves hydraulic injection of the heat exchanger but also controls slow cooling and heating of the furnace to avoid damage. In the case of gas-to-gas heat transfer, as in the present system of the invention, the two heat transfer coefficients are of equal magnitude, and the wall temperature of the transfer line heat exchanger is higher than in the case of gas-to-boiling water heat exchange, the wall temperature being approximately the average of the two media on each side of the wall. In the system according to the invention, the expected wall temperature is about 450 ℃ on the coldest part and increases rapidly to about 700 ℃ in the hotter part. This means that the temperature in the entire heat exchanger always exceeds the hydrocarbon dew point, so that no condensation can occur.

In a preferred embodiment, the convection section may include a boiler coil configured to produce saturated steam. The boiler coil may generate steam so that all of the waste heat of the flue gas that is not used to preheat the feedstock may be recovered by generating steam. This increases the overall efficiency of the furnace. In fact, the system according to this preferred embodiment may allow for variations in the heat recovery of the system by partially transferring the heat in the effluent for preheating of the feedstock in order to reach the optimum temperature of the feedstock before entering the radiant section, while transferring the heat in the flue gas to produce high pressure steam. More heat may be transferred for heating of the feedstock than is transferred to produce saturated high pressure steam, which may reduce high pressure steam production, thereby facilitating increased feedstock heating. The boiler coil may advantageously be located at the bottom of the convection section. The temperature in the bottom region of the convection section is higher than the temperature in the top region of the convection section, which may provide a relatively high efficiency in the heating of boiler water. At the same time, the boiler coil can protect the high pressure steam superheater in the convection section from overheating.

The convection section may also preferably be configured for mixing the hydrocarbon feedstock with a diluent to provide a feedstock-diluent mixture, wherein the transfer line heat exchanger is configured to preheat the feedstock-diluent mixture prior to entering the radiant section. The diluent may preferably be steam. Alternatively, methane may be used as a diluent instead of steam. The mixture may also be superheated in the convection section. This is to ensure that the feed mixture no longer contains any droplets. The amount of superheating must be sufficient to ensure that the dew point is exceeded with sufficient margin to prevent condensation of the diluent or hydrocarbons. At the same time, the higher risk of feedstock decomposition and coke formation in the convection section and coke formation due to high temperatures in the transfer line exchanger can be prevented. Furthermore, since the specific heats of both the feed-diluent mixture and the cracked gas are very similar, the resulting heat flows on both sides of the wall of the heat exchanger (i.e., the transfer line heat exchanger) are also similar to the specific heats of the feed-diluent mixture and the cracked gas. This means that the heat exchanger can be operated at almost the same temperature difference from the cold side to the hot side throughout the heat exchanger. This is advantageous both from a process point of view and from a mechanical point of view.

The system may also include a secondary transfer line heat exchanger, wherein the secondary transfer line heat exchanger is configured to produce saturated high pressure steam. Depending on the combustor efficiency and thus on the available heat in the cooling section, a secondary transfer line heat exchanger may be placed in series after the primary transfer line heat exchanger to further cool the cracked gas from the radiant section. While the primary transfer line heat exchanger is configured to heat the feedstock prior to entering the radiant section, the secondary transfer line heat exchanger may be configured to partially evaporate the boiler water. The system may include one or more secondary heat exchangers, but the primary heat exchanger is always configured to preheat the feedstock, rather than to produce high pressure saturated steam. The system may also include a steam drum connected to the boiler coil and/or the secondary transfer line heat exchanger. For example, boiler water may flow from a steam drum of the cracking furnace system to the secondary transfer line heat exchanger and/or boiler coil. Where the system includes a secondary transfer line heat exchanger and a boiler coil, they may simultaneously produce saturated high pressure steam. After the steam and water mixture is partially vaporized within one of the secondary transfer line heat exchanger and the boiler coil, the steam and water mixture may be redirected to a steam drum where the steam may be separated from the remaining liquid water. Thus, compared to prior art systems, an additional parallel circuit is created so that boiler water can be conveyed from the steam drum of the cracking furnace system to the boiler coils in the convection section of the cracking furnace system, wherein the boiler water is partially evaporated by the hot flue gas. The mixture of water and steam may then be returned to the steam drum.

The combustion chamber may preferably be configured such that the combustion chamber efficiency is higher than 40%, preferably higher than 45%, more preferably higher than 48%. As already explained above, the combustor efficiency is the ratio between the amount of heat absorbed by the at least one radiant coil by pyrolysis to convert the hydrocarbon feedstock into cracked gas and the amount of heat released during combustion. The normal combustion chamber efficiency of prior art cracking furnaces is about 40%. If we are above this efficiency, the feedstock cannot be reheated to the optimum temperature because the heat in the flue gas is insufficient: increasing the combustor efficiency from about 40% to about 48% will decrease the proportion of heat available in the convection section from about 50-55% to about 42-47%. The system according to the invention can cope with this reduction of the available heat in the convection section compared to prior art systems. By increasing the combustor efficiency from about 40% by about 20% to about 48%, about 20% of the fuel can be saved. The combustor efficiency may be increased in different ways, for example by increasing the adiabatic flame temperature in the combustor and/or by increasing the heat transfer coefficient of at least one radiant coil. Increasing the efficiency of the combustion chamber without increasing the adiabatic flame temperature has the following advantages: NOx emissions are not significantly increased, which may be the case with oxy-fuel combustion or preheated air combustion, among other ways to increase combustor efficiency, as will be discussed further below. For example, the combustion chamber may be configured such that ignition is limited to the hot side of the combustion chamber, i.e., the region near the bottom of the tank in the case of a bottom burner, or near the region near the top in the case of a top burner. The furnace preferably has a sufficient heat transfer area, more specifically, the heat transfer surface area of the at least one radiant coil is high enough to transfer the heat required to convert the feedstock inside the at least one radiant coil to the desired conversion level, while cooling the flue gas to a temperature at the furnace outlet, or convection section inlet, which is low enough to obtain a furnace efficiency of greater than 40%, preferably greater than 45%, more preferably greater than 48%. The at least one radiant coil of the combustion chamber preferably comprises highly efficient radiant tubes, for example vortex flow tubes as disclosed in EP1611386, EP2004320 or EP2328851 or winding loop radiant tubes as described in UK 1611573.5. More preferably, the at least one radiant coil has an improved radiant coil layout, for example a three-channel layout as disclosed in US 2008142411.

The convection section may advantageously include an economizer configured to preheat boiler feedwater, preferably prior to the feedwater entering the steam drum of the system, for the production of saturated steam. This can increase the overall efficiency of the system, which is the ratio of the heat absorbed by the at least one radiant coil to convert the hydrocarbon feedstock to cracked gas by pyrolysis and the heat absorbed by the plurality of convection tube bundles (excluding any oxidant preheater and/or fuel preheater) in the convection section, to the heat released by the combustion process in the combustion zone, based on the lower heating value of 25 ℃.

In another embodiment of the invention, the convection section may comprise an oxidant preheater, preferably located downstream of the convection section, i.e. the flue gas is coldest, configured to preheat the oxidant, e.g. combustion air and/or oxygen, prior to introducing the oxidant into the combustion chamber. In this case, the heat for the pyrolysis reaction in the combustion chamber may be provided by combustion of the fuel gas and preheated air in a burner, e.g. a combustion chamber. Preheating of the oxidant may increase the adiabatic flame temperature and may make the combustion chamber more efficient.

The system may further be configured for introducing oxygen into the radiant section. Preferably, for example, a limited amount of oxygen may be introduced directly into the radiant section burner, particularly with the combustion air, to increase the adiabatic flame temperature in the radiant section, which may increase the combustor efficiency. Thus, in the absence of a flue gas recirculation loop, as will be discussed later with the convention of complete oxy-fuel combustion, may be considered a separate invention. By way of example, the flue gas may typically be cooled from an adiabatic flame temperature of about 1900 ℃ to a reference temperature of about 25 ℃. At an adiabatic flame temperature, 100% of the heat will be available in the flue gas, whereas at a reference temperature no heat is left in the flue gas. To simplify the example, it is necessary to cool the inside of the combustion chamber from 1900 ℃ to 1150 ℃ to achieve 40% efficiency, assuming a constant specific heat over the entire temperature range. To achieve 50% efficiency while keeping the flue gas temperature away from the combustion chamber at 1150 ℃, we needed to raise the adiabatic flame temperature from 1900 ℃ to 2275 ℃, which is an increase of 375 ℃. This can be done by injecting pure oxygen in the burner together with the combustion air. Injection of oxygen at a weight ratio of oxygen to combustion air of about 7% is sufficient to increase the combustor efficiency by 25%. This can be achieved by supplying oxygen at each individual burner, preferably away from the fuel tip to minimize NOx formation, or directly in the combustion zone, for example through the walls of the combustion chamber. The main advantage is the significantly increased combustion chamber efficiency, which leads to a reduced fuel gas consumption and also to an equally reduced emission of the greenhouse gas CO2 to the atmosphere. Another advantage is that, as discussed later, the pure oxygen required is limited compared to oxy-fuel combustion, where oxygen is combusted as an oxidant rather than combustion air. Injecting 7wt% oxygen in the combustion air can increase the oxygen content from 20.7vol% to 25.2vol% and can decrease the nitrogen content from 77vol% to 72.6vol%. Higher adiabatic flame temperatures may result in higher NOx production. NOx abatement measures may need to be taken, for example by installing selective catalytic NOx reduction beds in the convection section or in the stack.

In preferred embodiments, the system may additionally include an external flue gas recirculation loop configured to recover at least a portion of the flue gas and recirculate the flue gas to the radiant section to control flame temperature. This allows the oxygen injection in the oxidant to be increased and thus the nitrogen concentration in the oxidant to be reduced for a given adiabatic flame temperature. The higher the oxygen concentration in the oxidant, the higher the flue gas recirculation required to maintain the same adiabatic flame temperature. In the extreme case, the oxidant is pure oxygen, effectively depleting the nitrogen. This is known as oxy-fuel combustion. In the absence of nitrogen, no NOx can be formed. When pure oxygen combustion raises the adiabatic flame temperature above the optimum, it may be preferable to add sufficient external flue gas recirculation to quench the flame and maintain it at the desired temperature level. The flue gas is preferably recirculated from downstream of the convection section of the system. In this way, the adiabatic flame temperature in the radiant section can be reduced. As described above, external flue gas recirculation is introduced to mitigate adiabatic flame temperature increases caused by increased oxygen content in the oxidant. The higher the flue gas recirculation rate, the lower the recirculated flue gas temperature, the cooler the flame and the less NOx formation.

The external flue gas recirculation loop may advantageously comprise a first flue gas injector configured to introduce oxygen into the recirculated flue gas prior to entering the combustion chamber. In this case, the heat for the highly endothermic pyrolysis reaction in the combustion chamber comes from the combustion of fuel gas and oxygen, preferably highly nitrogen-depleted oxygen, or a combination of fuel gas and oxygen and combustion air, in the presence of recirculated flue gas. The injector may be placed upstream of the combustor burner such that the recirculated flue gas and oxygen are supplied to the combustor in a common line. Advantageously, the ejector may generate lower pressure in the outer flue gas recirculation duct and may reduce the power requirements of a recirculation device (e.g., an induced draft fan), which may be located downstream of the convection section of the cracking furnace system.

An advantageous embodiment of the system may further comprise a heat pump circuit comprising an evaporator coil located in the convection section and the 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 the boiler feed water. Such a heat pump loop can reduce stack temperatures by about 40-50 c depending on the specific furnace feed composition and operating conditions. Reducing the stack temperature may then result in an increase in the overall efficiency of the system. It is known to preheat boiler feed water by recovering heat from the flue gas to improve the overall efficiency of the system. However, especially in the case of oxy-fuel combustion in the furnace combustion chamber, the waste heat of the flue gases may not be sufficient to directly preheat the boiler feedwater, as the temperature of the flue gases may be lower than the temperature of the boiler feedwater. Boiler feed water is typically supplied directly from the deaerator at a temperature of about 120-130 ℃, while the flue gas leaving the feed preheat train is typically below this temperature, so that direct preheating of the feed water is not possible. The heat pump loop can provide a solution for indirect heat exchange, so that the temperature of the chimney can be further reduced, and the overall efficiency of the system can be further improved.

The heat pump loop (which may itself be considered the present invention) used to preheat the boiler feed water of the cracking furnace system can do this preheating indirectly and does not require an economizer in the convection section, thereby increasing the overall efficiency of the system. The organic fluid circulated in the loop may comprise, for example, one of butane, pentane or hexane or any other suitable organic fluid. Furthermore, as a further advantage, the heat pump circuit may be embodied as an additional module, such that an existing cracker furnace system may be equipped with such a heat pump circuit after installation, without requiring major modifications of the existing system. Furthermore, the heat pump may be configured such that it can serve multiple cracker systems, thereby reducing the required equipment items and reducing the associated costs.

According to one aspect of the present invention, a process for cracking a hydrocarbon feedstock in a cracking furnace system is provided that provides one or more of the above-described advantages.

The invention will be further elucidated with reference to the drawings of an exemplary embodiment. Wherein the content of the first and second substances,

FIG. 1 shows a schematic diagram of a first preferred embodiment of a cracker system according to the present invention;

FIG. 2 shows a schematic view of a second embodiment of a cracking furnace system according to the invention;

FIG. 3 shows a schematic view of a third embodiment of a cracker furnace system according to the present invention;

FIG. 4 shows a schematic view of a fourth embodiment of a cracker furnace system according to the present invention;

FIG. 5 shows a schematic view of a fifth embodiment of a cracking furnace system according to the invention

FIG. 6 shows a schematic view of a sixth embodiment of a cracker furnace system according to the present invention;

FIG. 7 shows a schematic view of a seventh embodiment of a cracker furnace system according to the present invention;

fig. 8 shows a graph of relative oxygen flow rate versus relative air flow rate.

It is noted that the figures are given by way of schematic representations of embodiments of the invention. Corresponding elements have been designated with corresponding reference numerals.

Fig. 1 shows a schematic diagram of a cracker furnace system 40 according to a preferred embodiment of the invention. The cracking furnace system 40 includes a convection section that includes a plurality of convection banks 21. The hydrocarbon feedstock 1 can enter a feed preheater 22, and the feed preheater 22 can be one of a plurality of convection tube bundles 21 in the convection section 20 of the cracking furnace system 40. The hydrocarbon feedstock 1 may be any kind of hydrocarbon, preferably paraffinic or naphthenic, but small amounts of aromatics and olefins may also be present. Examples of such starting materials are: ethane, propane, butane, natural gasoline, naphtha, kerosene, natural condensates, gas oils, vacuum gas oils, hydrotreated or desulfurized or hydrodesulfurized (vacuum) gas oils, or combinations thereof. Depending on the state of the feedstock, the feed is preheated and/or partially or completely vaporized in a preheater before being mixed with a diluent (e.g., dilution steam 2). The dilution steam 2 may be directly injected or, alternatively, as in the preferred embodiment, the dilution steam 2 may first be superheated in the dilution steam superheater 24 prior to mixing with the feedstock 1. There may be a single steam injection point or multiple steam injection points, for example for heavier feedstocks. The mixed feed/dilution steam mixture can be further heated in high temperature coil 23 and, in accordance with the present invention, heated in primary transfer line heat exchanger 35 to reach the optimum temperature for introduction into radiant coil 11. The radiant coil may for example be of the eddy current type, as disclosed in EP1611386, EP2004320 or EP2328851, or of the three-channel radiant coil design (as disclosed in US2008 142411), or of the winding loop type (UK 1611573.5) or of any other type maintaining a reasonable running length, as known to the person skilled in the art. In the radiant coil 11, the hydrocarbon feedstock is rapidly heated to the point where the pyrolysis reaction begins, such that the hydrocarbon feedstock is converted into products and byproducts. These products include hydrogen, ethylene, propylene, butadiene, benzene, toluene, styrene and/or xylene. Byproducts include methane and fuel oil. The resulting mixture of diluents (e.g., dilution steam, unconverted feedstock and converted feedstock, which is the reactor effluent referred to as "cracked gas") is rapidly cooled in transfer line heat exchanger 35 to freeze in favor of the reaction equilibrium in the product. In the manner of the present invention, waste heat in the cracked gas 8 is first recovered in the transfer line heat exchanger 35 by heating the feedstock or feedstock-diluent mixture prior to passing it to the radiant coils 11. According to the present invention, high pressure steam may be generated in the convection section, such as by boiler coils 26 configured to at least partially evaporate boiler water from the steam drum 33 to produce saturated high pressure steam. The boiler coil 26 may be located in the bottom of the convection section and connected with the steam drum 33 such that boiler water 9a may flow from the steam drum 33 to the boiler coil 26 and such that partially evaporated boiler water 9b may flow from the boiler coil 26 back to the steam drum 33 through natural circulation. The boiler feed water 3 may be delivered directly to the steam drum 33. In the steam drum 33, the boiler feed water 3 is mixed with boiler water already present in the steam drum. In the steam drum 33, the produced saturated steam is separated from the boiler water and may be sent to the convection section 20 to be superheated, which may be accomplished by at least one high pressure steam superheater 25 (e.g., by first and second superheaters 25 in the convection section 20). The boiler coil 26 at the bottom of the convection section can recover excess heat from the flue gas and can protect downstream convection section rows, particularly the at least one high pressure steam superheater row 25, from overheating. The at least one superheater 25 may preferably be located upstream of the dilution steam superheater 24, and preferably downstream of the boiler coil 26. To control the high pressure steam temperature, additional boiler feed water 3 may be injected into the super superheater 34 located between the first and second superheaters 25.

As known to those skilled in the art, the heat of reaction for the highly endothermic pyrolysis reaction can be provided in many different ways by the combustion of fuel (gas) 5 in radiant section 10, also referred to as a furnace combustion chamber. The combustion air 6 may, for example, be introduced directly into the burner 12 of the furnace combustion chamber, wherein the burner 12 fuel gas 5 and combustion air 6 are ignited to provide heat for the pyrolysis reaction. In the combustion zone 14 in the furnace combustion chamber, the fuel 5 and the combustion air 6 are converted into combustion products, such as water and CO2, so-called flue gas. Various types of convection bank 21 are used to recover waste heat from the flue gas 7 in the convection section 20. A portion of the heat is used on the process side, i.e., preheating and/or vaporization and/or superheating of the hydrocarbon feed and/or feed-diluent mixture, and the remainder is used on the non-process side, e.g., high pressure steam generation and superheating as described above.

In an embodiment, such as the schematic of the second embodiment of the cracking furnace system shown in fig. 2, any excess heat in the cracked gas may be recovered, such as in at least an additional transfer line heat exchanger, a secondary transfer line heat exchanger 36, configured to produce saturated high pressure steam. The steam is generated from boiler water 9a from the steam drum 33, which is partially vaporized by the secondary transfer line heat exchanger 36. The partially evaporated boiler water 9b passes through a natural circulation flow to the steam drum 33. In this way, additional loops from and to the steam drum 33 are provided to increase high pressure steam generation and improve overall furnace efficiency. The boiler feedwater 3 may be delivered directly to the steam drum 33, as in FIG. 1, or may be first preheated, such as by excess heat available in the convection section 20 that is not required by the boiler coils 26. Additionally, additional convection bank 21 (e.g., economizer 28) may be added to the furnace convection section 20. The convection bank 28 may be configured to preheat the boiler feed water 3 prior to entering the steam drum 33 in order to improve overall furnace efficiency and provide a more cost-effective convection section. The embodiment in FIG. 2 further shows an induced draft fan 30 (also referred to as a flue gas fan) and a stack 31 located at the downstream end of the convection section to discharge flue gas from the convection section 20.

With the new inventive arrangement, as shown in fig. 1 and 2, the amount of non-process load, i.e., the load recovered in the cracked gas and convection section for high pressure steam generation, can be reduced independently of the amount of process load required to preheat the diluted steam-hydrocarbon mixture to an optimal temperature for entry into the radiant coils. This means that for the new solution as shown in fig. 1 and 2, the combustor efficiency can be increased from 40% of the conventional solution up to 48%, thereby reducing fuel consumption by about 17%. The reduced fuel consumption also reduces the flue gas flow rate and associated convection section load by about 17%. The new scheme allows preferential use of this heat for process purposes at the expense of non-process use, resulting in optimized process inlet temperature of the radiant coil, but with lower high pressure steam production. Maintaining an optimized radiant coil inlet temperature is important because a lower inlet temperature of the feedstock will increase the radiant load and reduce the combustor efficiency and increase fuel consumption, while a higher inlet temperature can result in conversion of the feedstock inside the convection section and associated deposition of coke on the inside surface of the convection section tubes. In radiant coils, this coke deposit cannot be removed during a conventional decoking cycle because the tube temperature is too low to burn the coke in the convection section, eventually requiring long furnace shut downs and being expensive to cut the affected tubes in the convection section and mechanically remove the coke.

The combustion in the furnace combustion chamber 10 may be accomplished by a bottom burner 12 and/or a side wall burner and/or by a top burner and/or a side wall burner in a top fired furnace. In the exemplary embodiment of the furnace 10 shown in fig. 2, ignition is limited to the lower portion of the combustion chamber by using only the bottom burner 12. This may improve combustor efficiency and may substantially reduce fuel gas consumption by up to about 20% compared to conventional approaches. For example, high combustor efficiency may be achieved using only bottom burners (as shown) or rows of sidewall burners placed near the bottom in the case of bottom combustion, or by using only top burners or rows of sidewall burners placed near the top in the case of top combustion. Making the combustion chamber taller or placing the more efficient radiation coils are other examples of achieving this. Since the heat distribution in this case is more concentrated on the portion of the radiation coil, the local heat flux increases, thereby reducing the run length. To counteract this effect, applications of radiant coils with enhanced heat transfer, such as vortex tube types or winding loop radiant tube types, may be required in radiant coils in order to maintain reasonable run lengths. Other means for achieving better performance (e.g., a three-way coil design) may also be used alone or in conjunction with other means to increase run length. Advantageously, this embodiment has substantially no problems with NOx emissions compared to conventional furnaces, since the adiabatic flame temperature is not increased by oxy-fuel combustion or air preheating.

FIG. 3 shows a schematic of a third embodiment of a cracker furnace system. In this embodiment, the heat of the pyrolysis reaction in the furnace combustion chamber 10 is provided by the fuel gas 5 combusted in the burner 12 and the preheated combustion air 50. The combustion air 6 may be introduced via a forced draft fan 37 and may then be heated in the convection section 20, for example by a convection bank implemented as an air preheater 27 located downstream of the convection section 20, preferably downstream of all other rows of convection sections in the convection section. Preheating of the combustion air can increase the adiabatic flame temperature and make the combustion chamber more efficient than the system presented in fig. 2. A reduction of more than 25% of fuel gas is feasible compared to conventional solutions. However, depending on the degree of combustion air preheating, higher adiabatic flame temperatures may also increase NOx emissions. Depending on the environmental regulations permitting maximum 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. Since the combustor efficiency may be higher than the system shown in fig. 2, the convection section load is lower and the excess heat in the convection section for preheating boiler feed water may no longer be available as the combustor efficiency increases. Eventually, the economizer may become redundant, and boiler feed water may be sent to the steam drum without preheating in the economizer, as shown in FIG. 3.

FIG. 4 shows a schematic of a fourth embodiment of a cracker furnace system. In this embodiment, the heat of the pyrolysis reaction in the furnace combustion chamber 10 is provided by the fuel gas 5, combustion air 6 and high nitrogen depleted combustion oxygen 51 combusted in the burner 12. The introduction of oxygen in the combustion zone 14 may also raise the adiabatic flame temperature as an alternative to the scheme presented in fig. 3. Also with this solution, a reduction of fuel gas of more than 25% is feasible compared to the conventional solution. However, depending on the degree of oxygen injection, higher adiabatic flame temperatures may also increase NOx emissions. Depending on the environmental regulations permitting maximum NOx emissions, this may require NOx abatement measures to be taken, for example by installing selective catalytic NOx reduction beds in the convection section 20.

FIG. 5 shows a schematic of a fifth embodiment of a cracker furnace system. In this embodiment, the heat of the pyrolysis reaction in the furnace combustion chamber 10 is provided by fuel (gas) 5, combustion air 6 and high nitrogen-depleted combustion oxygen 51 combusted in the burner 12 in the presence of externally recirculated flue gas 52. The combustion oxygen 51 may be mixed with the recirculated flue gas upstream of the combustor 12 in a common line to the combustor 12 using an injector 55. To obtain recirculated flue gas 52, the flue gas exiting convection section 20 may be divided into produced flue gas 7 and flue gas 52 for external recirculation by, for example, a flue gas splitter 54. The produced flue gas 7 may be discharged through a stack 31 using an induced draft fan 30. The same fan 30 may be configured to recirculate flue gas from the outside to the combustor 12. Alternatively, the fan 30 may be implemented as two or more fans depending on parameters such as the pressure drop difference of the downstream system (e.g., the stack 31 or the flue gas recirculation loop 52).

Fig. 6 shows a schematic view of a sixth embodiment of a cracking furnace system. In this embodiment, the heat of the pyrolysis reaction in the furnace combustion chamber 10 is provided by the fuel (gas) 5 and the high nitrogen depleted combustion oxygen 51 combusted in the burner 12 in the presence of the externally recirculated flue gas 52. This solution is practically identical to the one given in fig. 5, except that all the combustion air 6 is replaced by combustion oxygen 51. This is the highest consumption scheme for combustion oxygen 51, but the lowest amount of flue gas leaves the stack. The flue gas is very rich in CO2, making it ideal for carbon capture, and minimizes NOx emissions due to the absence of nitrogen, except for nitrogen along with air leaking into the convection section. This solution is most environmentally friendly.

The relationship between fig. 4, 5 and 6 may be further explained with reference to fig. 8, which shows the relative air flow rate (on the horizontal axis) as a function of the relative oxygen flow rate (on the vertical axis). The relative oxygen flow rate is the flow rate relative to the oxygen demand at 100% oxy-fuel combustion (i.e., in the absence of any combustion air). FIG. 4 is a schematic diagram of a cracking furnace system for partial oxy-fuel combustion and without external flue gas recirculation, while FIG. 6 is a schematic diagram of a cracking furnace system for oxy-fuel combustion with external flue gas recirculation to moderate adiabatic flame temperatures. FIG. 5 is a schematic of a cracking furnace system for an intermediate case. For the scheme shown in fig. 4, the oxygen demand for oxy-fuel combustion is 25% as shown in fig. 6, one extreme shown in fig. 6, represented by "y" in the figure, and 100% for the scheme of fig. 6, which is represented as "x" in the graph of fig. 8. The scheme of fig. 5 is between these two extremes. The fig. 6 scheme produces the lowest NOx in the three schemes, lower than the NOx emission levels of the current prior art scheme, while the fig. 4 scheme has significantly higher NOx emission levels than the other two schemes. The scheme of fig. 5 is between these two extremes. If there is no requirement for carbon capture, and only for better fuel efficiency, the fig. 4 scheme may be the most economical of the three schemes. As previously mentioned, the fig. 6 scheme may be most environmentally friendly and suitable for carbon capture. The introduction of combustion air may provide a significant reduction in the oxygen demand, which may be reduced from 100% to about 25% in terms of relative air flow. For the fig. 6 version, the relative oxygen flow rate is 100%, and for the fig. 4 version, the relative oxygen flow rate is about 25%. The scheme of fig. 5 is between these two extremes. The relative air flow rate is the flow rate required relative to the combustion air at partial oxy-fuel combustion according to the scheme of fig. 4 at about 7wt% oxygen injection to increase the adiabatic flame temperature and no external flue gas recirculation. In the fig. 6 version, the relative combustion air requirement is 0%. The scheme of fig. 5 is between these two extremes.

FIG. 7 shows a schematic of a seventh embodiment of a cracker furnace system. This embodiment of the cracker furnace system is based on the embodiment of fig. 6 and therefore includes a flue gas recirculation loop with oxygen introduction and no combustion air is introduced. To further improve 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, thereby increasing the production of high pressure steam. The heat source of the heat pump circuit 70 includes an evaporator coil 77 located in the convection section 20 of the cracking furnace 40. The evaporator coil 77 is connected to a vapor-liquid separation device 76, such as a vapor-liquid separation tank, via downcomers and risers. The organic fluid 60, such as butane, pentane or hexane, flows under natural circulation through the downcomer to the evaporator coil 77 where it is partially evaporated by the heat recovered from the flue gas. The organic liquid/vapor mixture 61 is refluxed to the vapor-liquid separation device via a riser. In the vapor-liquid separation device, vapor 62 is separated from liquid/vapor mixture 61. The vapor 62 separated from the mixture 61 is then superheated in the feed effluent exchanger 74 to increase the loop efficiency. The superheated steam 63 is sent to a compressor 71. The compressor 71 is configured to raise the pressure of the superheated steam 63 to a level at which the condensing temperature at the outlet of the compressor 71 exceeds the temperature level at which the boiler feed water 3 needs to be preheated with sufficient margin. This requires a proper choice of compressor efficiency. The compressed high pressure vapor 64 from the compressor 71 is completely condensed in the condenser 72. The heat of condensation is used to preheat boiler feed water 3. Condensed organic liquid 65 accumulates in condensate container 73. From the condensate vessel 73, the saturated liquid 66 is sent to the feed effluent exchanger 74 to be subcooled. Subcooled liquid 67 is flashed to a lower pressure in pressure reducing valve 75. The more cold the liquid is in the feed effluent exchanger 74, the higher the proportion of liquid at the outlet of this valve 75 and the lower the required circulation rate of the organic heat pumping fluid. The low pressure liquid vapor mixture 68 is sent to a vapor-liquid separation device 76 where the liquid and vapor are separated from each other, completing the circuit, in vapor-liquid separation device 76.

In the case where the evaporator coil 77 is the heat source of the circuit, the condenser 72 can be considered as a radiator of the circuit. The load that needs to be condensed in the condenser 72 is the heat recovered by the flue gas in the evaporator and the heat supplied by the driver of the compressor 71. This means that the power supplied by the drive is also used for generating high-pressure steam. This heat increases the circuit efficiency because there is no heat loss in driving the compressor. However, it is still beneficial to select an efficient compressor and apply the feed effluent exchanger 74 to keep the flow rates of all items in the circuit and the corresponding equipment size as small as possible. In the case of a cracking furnace train, the compressor 71, condensate vessel 73 and feed effluent exchanger 74 can be configured to service the cracking furnace train.

Achievement the project of the present application was funded by the european union horizon H2020 project (H2020-SPIRE-2016) under the scientific funding agreement n ° 723706.

For clarity and conciseness of description, technical features are described herein as being part of a single embodiment or separate embodiments, however, it should be understood that the scope of the invention may include embodiments having combinations of all or some of the described features. It is to be understood that the illustrated embodiments have identical or similar components, except for the differences that they have been described.

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 "only" should not be construed as limited to "only one," but rather 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 advantage. Many variations will be apparent to those skilled in the art. All such variations are understood to be included within the scope of the invention as defined in the following claims.

Reference numerals

1. Hydrocarbon feedstock

2. Dilution steam

3. Boiler feed water

4. High pressure steam

5. Fuel gas

6. Combustion air

7. Flue gas

8. Cracking gas

9a boiler water

9b boiler water partially evaporated

10. Radiant section/furnace combustion chamber

11. Radiation coil pipe

12. Bottom burner

14. Combustion zone

20. Convection section

21. Convection bank

22. Feed preheater

23. High-temperature coil pipe

24. Dilution steam superheater

25. High-pressure steam superheater

26. Boiler coil pipe

27. Air preheater

28. Energy-saving device

30. Draught fan

31. Chimney (chimney)

33. Steam drum

34. Super heater

35. Heat exchanger for main conveying pipeline

36. Secondary transfer line heat exchanger

37. Forced fan

40. Cracking furnace system

50. Preheated combustion air

51. Oxygen gas

52. Externally recirculating flue gas

54. Flue gas flow divider

55. Flue gas injector

60. Organic liquid

61. Organic liquid-vapor mixture

62. Steam generation

63. Superheated steam

64. High pressure steam

65. Condensing organic liquids

66. Saturated liquid

67. Supercooled liquid

68. Low pressure liquid-vapor mixture

70. Heat pump circuit

71. Compressor with a compressor housing having a plurality of compressor blades

72. Condenser

73. Condensation container

74. Feed effluent exchanger

75. Pressure reducing valve

76. Vapor-liquid separation device

77. Evaporator coil

Claims (34)

1. A cracking furnace system for converting a hydrocarbon feedstock into a cracked gas, said cracking furnace system comprising a convection section, a radiant section, and a cooling section,

wherein the convection section comprises a plurality of convection tube banks configured to receive and preheat a hydrocarbon feedstock,

wherein the radiant section comprises a combustion chamber comprising at least one radiant coil configured to heat the feedstock to a temperature that allows for pyrolysis reactions,

wherein the cooling section comprises at least one transfer line heat exchanger,

wherein the system is configured such that the transfer line heat exchanger preheats the feedstock by gas-to-gas heat transfer from a cooling or quenching cracked gas to the waste heat of the hydrocarbon feedstock prior to the feedstock entering the radiant section.

2. The cracker furnace system of claim 1, wherein the convection section comprises a boiler coil configured to produce saturated steam, wherein the boiler coil is located at a bottom of the convection section.

3. The cracker furnace system of claim 1, wherein the convection section is further configured for mixing the hydrocarbon feedstock with a diluent to provide a feedstock-diluent mixture, wherein the transfer line heat exchanger is configured to preheat the feedstock-diluent mixture prior to entering the radiant section.

4. The cracking furnace system of claim 2, further comprising a secondary transfer line heat exchanger, wherein the secondary transfer line heat exchanger is configured to produce saturated high pressure steam.

5. The cracking furnace system of claim 4, further comprising a steam drum connected to the boiler coil and/or the secondary transfer line heat exchanger.

6. The cracker furnace system of claim 1, wherein the furnace is configured such that the furnace efficiency is greater than 40%.

7. The cracker furnace system of claim 1, wherein the convection section comprises an economizer configured to preheat boiler feed water to produce saturated steam.

8. The cracker furnace system of claim 1, wherein the convection section comprises an oxidant preheater located downstream of the convection section, the oxidant preheater configured to preheat an oxidant prior to the introduction of combustion air into the combustion chamber.

9. The cracker furnace system of claim 8, wherein said oxidant is combustion air and/or oxygen.

10. The cracking furnace system of claim 1, wherein the system is configured to introduce oxygen into the radiant section in the absence of external flue gas recirculation.

11. The cracking furnace system of claim 1, further comprising an external flue gas recirculation loop configured to recover at least a portion of the flue gas and recirculate the flue gas to the radiant section to control flame temperature.

12. The cracking furnace system of claim 11, wherein the external flue gas recirculation loop comprises a flue gas injector configured to introduce oxygen into the recirculated flue gas prior to entering the combustion chamber.

13. The cracking furnace system of any one of claims 1-12 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 the heat to boiler feed water.

14. A heat pump circuit for preheating boiler feed water of a cracking furnace system according to any one of claims 1-12, the heat pump circuit comprising an evaporator coil arranged to recover heat from flue gas in a convection section of the cracking furnace system, and a condenser configured to transfer the heat to boiler feed water.

15. The heat pump circuit of claim 14, further comprising a vapor-liquid separation device connected to the evaporator coil, the vapor-liquid separation device being arranged to separate vapor from a liquid-vapor mixture from the evaporator coil.

16. The heat pump circuit of any preceding claim 14 to 15, further comprising a feed effluent exchanger arranged to superheat steam produced in a heat source and subcool liquid produced in a radiator of the heat pump circuit.

17. A heat pump circuit according to any one of the preceding claims 14-15, further comprising a compressor arranged to increase the steam pressure such that the condensation temperature of the steam exceeds the desired temperature to be delivered to the boiler feed water.

18. Process for cracking a hydrocarbon feedstock in a cracking furnace system, in a cracking furnace system according to any of claims 1-13, said process comprising a first feedstock preheating step and a second feedstock preheating step,

wherein the first feedstock preheating step comprises preheating a hydrocarbon feedstock by hot flue gases of a cracking furnace system,

wherein said second feedstock preheating step comprises further preheating said feedstock by gas-to-gas heat transfer from cooled or quenched cracked gases to waste heat of the hydrocarbon feedstock prior to entry of said feedstock into the radiant section of said cracking furnace system.

19. The method of claim 18, wherein the second feedstock preheating step is performed using a transfer line heat exchanger.

20. The method of any of the preceding claims 18-19, wherein boiler feed water is conveyed from a steam drum of the cracking furnace system to a boiler coil in a convection section of the cracking furnace system, wherein the boiler feed water is heated by hot flue gas, and wherein a mixture of water and steam is returned to the steam drum.

21. The process according to any of the preceding claims 18-19, wherein the hydrocarbon feedstock is mixed with a diluent to provide a feedstock-diluent mixture prior to the second feedstock preheating step.

22. The method of any of the preceding claims 18-19, wherein high pressure steam is generated from waste heat of cracked gas of the cracking furnace system using a secondary transfer line heat exchanger located downstream of the transfer line heat exchanger.

23. The method of any of the preceding claims 18-19, wherein boiler feed water is preheated by hot flue gas prior to entering the steam drum of the cracking furnace system.

24. The method of any of the preceding claims 18-19, wherein the adiabatic flame temperature in the radiant section of the cracking furnace system is increased by introducing an oxidant directly into the radiant section.

25. The method of any of the preceding claims 18-19, wherein the adiabatic flame temperature in the radiant section is increased by introducing combustion air as a primary oxidant and oxygen as a secondary oxidant directly into the radiant section of the cracking furnace system in the absence of a flue gas recirculation loop.

26. The method of claim 25, wherein highly nitrogen-depleted oxygen is used as the secondary oxidant.

27. The method of claim 25, wherein the primary oxidant and the secondary oxidant are preheated prior to introduction into the radiant section.

28. The method of claim 27 wherein the primary oxidant and the secondary oxidant are preheated by flue gas of the cracking furnace system.

29. A method according to any one of the preceding claims 18-19, wherein the adiabatic flame temperature in the radiant section of the cracking furnace system is controlled by recycling at least part of the flue gas.

30. The method of claim 29, wherein oxygen is mixed with the recirculated flue gas prior to entering a furnace combustion chamber.

31. The process of claim 18, wherein boiler feed water is preheated by a heat pump circuit prior to entering the steam drum of the cracking furnace.

32. The method of claim 31, wherein organic liquid is heated by hot flue gas from the cracker furnace system and returned to a vapor-liquid separation device of the heat pump loop.

33. A method according to any of claims 31-32, wherein heat from the high pressure steam is transferred to the boiler feed water by a condenser of the heat pump circuit.

34. The method according to any of the preceding claims 31-32, wherein heat from condensed liquid produced in the heat sink of the heat pump circuit is transferred to saturated steam produced in the heat source of the heat pump circuit by a feed effluent exchanger.

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