Low carbon dioxide emission ethane decomposition device

Abstract

A treatment method and system with low carbon dioxide emissions for steam cracking of hydrocarbons for producing products such as ethylene will be described. This treatment method and system include decomposing a raw material in a furnace configured to burn a high-hydrogen fuel with less carbon dioxide emissions than methane, which is usually used as fuel for such a furnace. The high-hydrogen stream is separated from the waste gas of the decomposition device and can be reused.

Description

Detailed Description of the Invention

[0001] [Cross - Reference to Related Applications] This application claims priority based on U.S. Non - Provisional Patent Application 17 / 880,973, filed on August 4, 2022, the content of which is incorporated herein by reference.

[0002] [Field of the Invention] This application relates to the steam cracking of hydrocarbon feeds, particularly ethane. More specifically, this application relates to a steam cracking system with low carbon dioxide emissions.

[0003] Ethylene is an important petrochemical intermediate used to manufacture a variety of products. Ethylene can be produced from high - ethane hydrocarbon feeds by a process known as steam cracking. During steam cracking, the hydrocarbon feed is mixed with steam (known as dilution steam) and heated to the cracking temperature, where the hydrocarbons in the feed are cracked into smaller molecules, producing ethylene and by - products.

[0004] In an ethane steam cracking plant, the ethane and dilution steam used in this cracking reaction are heated using multiple furnaces. A portion of the waste heat from the furnaces is used to supply heat for the production of dilution steam and for the production of steam used to supply heat to other components of the process, such as a cooling turbine, for example. When the furnaces provide sufficient heat for all of the steam required by the system, the system is said to be in a "steam balance" state.

[0005] When a high-ethane hydrocarbon raw material is decomposed, a decomposition stream is produced that contains ethylene along with other products such as methane (CH4), hydrogen (H2), and carbon dioxide (CO2). The decomposition stream can be separated into an ethylene product stream and a waste gas stream containing methane and other components. A portion of the waste gas stream can be used as fuel for the furnace burner. In other words, the decomposition reaction can provide some or all of the fuel required for the burner. When sufficient fuel (in the form of waste gas) is produced by the decomposition reaction to maintain the operation of the burner, the system is said to be in a "fuel balance" state.

[0006] In a typical ethane decomposition apparatus, the waste gas may contain about 80 to 85 mol% H2, with the remainder being mostly CH4. When the waste gas is burned in the furnace, a significant amount of CO2 is produced due to the CH4 present in the fuel / waste gas. A reduction in the amount of CO2 produced during ethane decomposition is required.

[0007] [Overview] This specification discloses a processing method for steam cracking a raw material containing a hydrocarbon raw material and steam. The processing method includes heating the raw material in a furnace to generate a cracked gas stream containing methane (CH4), ethylene, and hydrogen (H2), separating the cracked gas stream into a product stream rich in ethylene and a waste gas stream rich in CH4 and H2, separating the waste gas stream into a high-CH4 stream and a high-H2 stream rich in H2, recycling at least a portion of the high-H2 stream as a fuel stream for a plurality of furnace burners in the furnace, mixing the fuel stream with combustion air to generate a combustion mixture, and burning the combustion mixture with a plurality of furnace burners. According to some embodiments, the hydrocarbon raw material contains ethane. According to some embodiments, recycling at least a portion of the high-H2 stream in the furnace includes expanding the high-H2 stream in an expander to generate an expanded high-H2 stream and recycling the expanded high-H2 stream as a fuel stream in the furnace. According to some embodiments, the pressure of the high-H2 stream is greater than 20 barg and the pressure of the expanded high-H2 stream is less than 10 barg. According to some embodiments, expanding the high-H2 stream includes reducing the temperature of the high-H2 stream such that the expanded high-H2 stream is cooler than the high-H2 stream. According to some embodiments, separating the waste gas stream into a high-hydrocarbon stream and a high-H2 stream includes cooling the waste gas stream using heat exchange with the expanded high-H2 stream. According to some embodiments, the high-hydrocarbon stream is not used at all as fuel for the furnace burners. According to some embodiments, the fuel stream contains more than 90 mol% H2. According to some embodiments, the furnace includes a radiant section and a convection section. According to some embodiments, the processing method further includes preheating the combustion air using heat from the combustion exhaust gas in the convection section before the combustion air is mixed with the fuel stream. According to some embodiments, before the combustion air is mixed with the fuel stream, the combustion air is preheated to at least 350 degrees.According to some embodiments, the processing method further includes heating the raw material by heat exchange with the cracked gas stream. According to some embodiments, the processing method further includes heating the raw material in a raw material preheater by utilizing heat from the combustion exhaust gas in the convection section. According to some embodiments, the processing method further includes heating the raw material to at least 350 degrees by heat exchange with the cracked gas stream, and then heating the raw material to at least 650 degrees in the raw material preheater by utilizing the combustion exhaust gas heat in the convection section. According to some embodiments, the fuel requirement of the furnace is satisfied by using only the high-H2 stream as the fuel stream for the furnace burner. According to some embodiments, the processing method further includes superheating steam in one or more steam superheaters by utilizing heat from the combustion exhaust gas in the convection section.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0009] [Detailed Description] Aspects of the present disclosure relate to reducing CO2 emissions from furnace burners used in hydrocarbon steam cracking processes. The present disclosure is particularly relevant to ethylene production processes, in which high ethane streams are steam cracked to produce ethylene. FIG. 1 is a high-level schematic of a representative steam cracking process 100. As described above, the feed gas is mixed with steam and cracked in furnace 102. The heat of the furnace is also used to produce steam. The steam produced is dropped through a plurality of turbines and used as diluent steam for the cracking reaction. The steam is also utilized as process steam for other aspects of the system, such as in a refrigeration compressor. The off-gas from the cracking apparatus (i.e., the cracked gas) obtained by the cracking reaction is provided to a recovery section 104. The recovery section is configured to separate the ethylene product from other components in the off-gas of the cracking apparatus. In other words, the recovery section produces a high ethylene stream and a waste gas stream depleted in ethylene. The waste gas stream contains CH4, H2, and small amounts of other components. As described above, some or all of the waste gas stream can be recycled in the furnace and used as fuel for the furnace burner.

[0010] FIG. 2 is a more detailed schematic view of an embodiment of a portion of the recovery section 104 in a representative ethane cracking apparatus. The cracked gas (line 202) containing CH4, H2, and ethylene is gradually cooled, and a series of knockout drums 204 (i.e., a plurality of drums 204A - 204D) are used to remove ethylene from the exhaust gas stream. The ethylene (and a small amount of methane) removed by each drum 204A - 204D is mixed into a mixed high - ethylene stream 206. In the illustrated embodiment, the high - ethylene stream 206 is sent through a cooling box 208 (e.g., a plate - fin heat exchanger), compressed by a turbo - expander 210 / turbo - compressor 212, and provided from the recovery section as an ethylene recovery stream 214. The ethylene recovery stream 214 may be reused in a process for recovery. The top stream of each knockout drum is rich in CH4 and H2 and depleted in ethylene. CH4 and H2 are discharged from the system as a single, low - pressure waste gas stream 214. Typically, the waste gas contains from about 80 to about 85 mol% of H2, with the remainder being mostly CH4.

[0011] As described above, the waste gas may be reused in a furnace as fuel for a burner. Since the waste gas contains a significant amount of CH4, a significant amount of CO2 is generated by the combustion of the waste gas in the burner. The inventors have discovered that the amount of CO2 generated by the burner can be reduced by separating a stream rich in H2 (and depleted in CH4) from the waste gas and using the H2 - rich waste gas as fuel for the burner. Specifically, embodiments of the present disclosure provide a process for generating a low - pressure high - H2 waste gas stream as fuel for a burner in an ethane steam cracking furnace.

[0012] Figure 3 shows an embodiment of an improved recovery section process 300 in the ethane cracking process. The cracked process gas (line 202) from the steam cracker contains H2, CH4, and ethylene. The cracked gas enters the illustrated process at a temperature of typically about -73 degrees. The cracked gas process stream 202 is cooled in a cooling box 308. It should be noted here that the temperatures and pressures in the description of this process 300 are only illustrative of a particular embodiment of the process. Depending on design constraints and considerations, the temperature / pressure may vary as would be understood by one of ordinary skill in the art. It should also be noted that the cooling box 308 may include a plurality of other hot and cold lines not shown here for clarity.

[0013] Process gas stream 202 is gradually cooled and ethylene is removed from the process gas using knockout drums 302 and 304. The cooling temperature can be configured to manage the approach temperature within the cooling box. According to some embodiments, the temperature of the first knockout drum 302 may be about -115 degrees ± 10 degrees, and the temperature of the second knockout drum may be from about -130 degrees to about -145 degrees. The bottom streams of knockout drums 302 and 304 are rich in ethylene and can be mixed into high ethylene stream 306. High ethylene stream 306 can be recompressed using turboexpander / turbocompressor 310 to provide ethylene rich ethylene recovery stream 312. The top streams of knockout drums 302 and 304 are rich in CH4 and H2 (i.e., waste gas), are further cooled in the cooling box, and can be provided to a third knockout drum 314. According to some embodiments, the temperature of the third knockout drum 314 may be about -163 degrees ± 10 degrees. The top stream 316 from the third knockout drum 314 is rich in H2. The bottom stream 318 from the third knockout drum 314 is rich in CH4. The amount of CH4 removed is determined by the temperature of the third knockout drum 314. That is, the purity of the H2 stream is determined by the temperature of the third knockout drum 314. The bottom stream 318 rich in CH4 is reheated in the cooling box and discharged from the system as a high CH4 stream. The top stream 316 rich in H2 is reheated in the cooling box to provide reheated high H2 stream 322. According to some embodiments, the temperature of the reheated high H2 stream 322 can be -140 degrees and its pressure can be from about 20 to about 35 barg. Stream 322 is expanded using turboexpander / turbocompressor 310 to produce expanded high H2 stream 324. This causes the temperature and pressure of the expanded high H2 stream 324 to decrease.For example, according to some embodiments, the temperature of stream 324 may be about -177 degrees, the pressure may be less than about 10 barg, for example about 6 barg. The expanded high-H2 stream 324 is reintroduced into the cooling box 308, thereby providing a cooling stream within the cooling box that can provide an appropriate temperature approach to cause separation of CH4 and H2 in the knockout drum 314. The expanded high-H2 stream 324 is ultimately discharged from the cooling box as the high-H2 fuel stream 326, and the high-H2 fuel stream 326 can be sent back to the furnace to provide high-H2 fuel for the burner. According to some embodiments, the high-H2 fuel stream 326 may contain more than 90 mol% H2, or more than 95 mol% H2, and most of the remainder may contain CH4. According to some embodiments, the recovery section process 300 may recover more than 90%, or more than 95%, of the H2 recoverable in the cracked gas process stream.

[0014] The representative recovery section process 104 (Figure 2) generates a single waste gas stream 214 containing both H2 and CH4, while the improved recovery section process 300 (Figure 3) separates the waste gas into a high hydrocarbon stream (e.g., high CH4 stream) 320 and a high H2 stream 326. This allows the high H2 stream to be preferentially used as fuel for the furnace burner, thus reducing the CO2 emissions from the furnace. It should be noted that the prior art describes processes for separating H2 from the decomposition process gas. These processes described in the prior art are typically used to separate H2 as a gas for sale and not for use as fuel for a burner. The processes described in the prior art typically sacrifice a portion of the H2 by reintroducing it into the high CH4 stream to lower the liquid stream evaporation temperature. As a result, the prior art processes can typically recover only about 80 to 85% of the H2 recoverable from the waste gas. In a typical process, there is no stream in the cooling box having a temperature low enough to provide the thermal driving force to effectively separate CH4 and H2, thus necessitating the sacrificial use of a portion of the H2. The inventors have discovered that an appropriate temperature approach can be achieved by expanding the reheated high H2 stream 322 from a high pressure of about 20 barg or more to a low pressure of about 10 barg or less and returning the expanded (and thus cooled) H2 stream to the cooling box. It should also be noted that in applications where the purpose of separating H2 is to obtain H2 as a product stream, the expansion / cooling process used in process 300 is likely not appropriate. In such H2 recovery processes, typically, the high H2 stream would be provided to a recovery operation such as pressure swing adsorption (PSA) treatment. Such a treatment requires a high pressure high H2 stream as input. Thus, a pressure drop of the stream as performed in process 300 would not be appropriate for H2 recovery. However, since the high H2 fuel stream 326 in process 300 is to be burned in the furnace burner, it is desirable for the pressure to be lower.

[0015] Table 1 shows a comparison of simulations performed using a recovery section process similar to that of step 104 and step 300. In the simulation, step 104 produces a single waste gas stream containing 84.6 mol% H2 (14,899 kilograms per hour) and 14.86 mol% CH4 (20,816 kilograms per hour). By burning the waste gas from step 104 in a furnace burner, 689.6 gigacalories of heat work is provided per hour, and 88.6 kilograms of CO2 is produced per gigacalorie generated. Step 300 produces a high-H2 stream containing 14,857 kilograms of H2 per hour and 6,287 kilograms of CH4 per hour. By burning the high-H2 stream produced in step 300 in a furnace burner, 501.1 gigacalories of heat work is provided per hour, and 34.7 kilograms of CO2 is produced per gigacalorie generated.

[0016] [Table 1]

[0017] As shown in Table 1, in the simulation, by separating a high-H2 fuel stream from the waste gas and using the separated stream as fuel for the furnace burner, more than 50% of the CO2 generated by the burner was reduced. However, it should be noted that when only the high-H2 stream is used as fuel, the available heat work is also reduced. Therefore, some embodiments of the steam reforming process of the present disclosure may include optimizing the system to compensate for the loss of heat work.

[0018] FIG. 4 shows an embodiment of a furnace system 400 configured to burn high-H2 fuel as described above. Temperatures are shown in degrees Celsius (°C) at various locations within the system. The temperatures shown are merely examples, and it should be understood that other temperatures may be used in other embodiments.

[0019] The furnace system 400 includes a radiant section (known as the firebox) 402 where fuel is burned by a burner (not shown), and a convection section 404 where heat from the hot combustion exhaust gas can be recovered for various heating processes as described below. Fuel for the burner is supplied via line 406. As described above, the fuel may include a high-H2 stream separated from the recovery section process 300 (Figure 3). Thus, as is known in the art, the burner should be configured to burn H2.

[0020] Combustion air is supplied to the furnace via line 416. One way to optimize the system to compensate for the reduced available firing when using only high-H2 fuel is to preheat the combustion air. In the illustrated furnace system, the combustion air is supplied from blower 418 at 21 degrees. According to some embodiments, the air is preheated to at least 350 degrees. The air is preheated in an air preheater 420 (to 425 degrees in the figure) using heat from the combustion exhaust gas within the convection section. By heating the combustion air, some of the heat available for other heating requirements, such as heating process feedstocks or generating steam, which would otherwise have been provided by the firing of the furnace burner, is reduced.

[0021] A feedstock containing steam and high ethane hydrocarbons enters the furnace system 400 via feedstock line 408. In the illustrated embodiment, the feedstock is first preheated using a feedstock / exhaust gas exchanger 410 and further heated using a feedstock preheater 412 within the convection section 404. According to some embodiments, the feedstock preheater may heat the feedstock to at least 650 degrees. In the illustrated embodiment, the feedstock is heated to 710 degrees. Next, the preheated feedstock passes through the radiant section of the furnace via tubes 414A and 414B where a cracking reaction occurs to produce products. Although two rows of radiant heat are depicted in the figure, it should be understood that more or fewer rows of radiant heat may be used.

[0022] If the feedstock passes through the radiant section of the furnace, the product stream is cooled in primary quench exchangers 422A and 422B. The product stream is combined in a combined stream 424 that is used to preheat the feedstock stream 408 entering the feedstock / gas exhaust exchanger 410. The product stream is discharged from the system as product output stream 426.

[0023] In the illustrated embodiment, combustion exhaust gas is used to heat the combustion air in the air preheater 420 and the feedstock in the feedstock preheater 412. The combustion exhaust gas is also used to heat the steam in the steam superheater 428 to produce a superheated stream (SHS). In the illustrated embodiment, a saturated stream is supplied to the steam superheater 428 by heating boiler feedwater (BFW) from the steam drum 430. The quench exchangers 422A and 422B can be used to heat the BFW and provide steam, which is then superheated to provide the SHS. In the illustrated embodiment, the BFW is provided to the quench exchanger 422B via line 431a, and the partially evaporated BFW / steam returns to the steam drum 430 via line 431b. Similar piping going to and returning from the quench exchanger 422A is omitted for clarity. As described above, since the furnace burner burns only the high-H2 fuel stream instead of the entire flue gas stream, the firing of the furnace is reduced. This results in less combustion exhaust gas heat being provided in the convection section 404 in performing the required heating operation.

[0024] Another way the system 400 can be optimized to adapt to reducing firing operations is by balancing the raw material preheating operations between the raw material / gas exchanger 410 and the raw material preheater 412. In conventional systems that do not perform combustion air preheating with more firing operations, the primary quench exchangers 422A and 422B may be used to extract a significant amount of heat from the reaction raw materials to generate steam. The secondary quench exchanger may also be used to extract additional heat from the raw materials to increase steam production. However, in the system 400, the quench exchangers may be configured to extract less heat, and thus, the combined stream 424 will have more heat available to heat the raw material stream 408 in the raw material / gas exchanger 410. According to some embodiments, the raw material / gas exchanger 410 may be configured to heat the raw material stream to at least 350 degrees. In the illustrated embodiment, the raw material stream 408 is heated from 137 degrees to 488 degrees. As a result of this preheating using the raw material / gas exchanger, the load on some of the heating requirements in the raw material preheater 412 is reduced, and instead, more heat remains in the convection section for superheating process steam in the superheater 428 and for combustion air preheating 420.

[0025] The inventors have discovered that by utilizing aggressive combustion air preheating and raw material heating in the raw material / gas exchanger, the heating requirements of the modeled furnace can be reduced within the range of available firing operations provided by the high H2 fuel stream generated using process 300 (Table 1). That is, the firing operation requirements of the furnace can be met using only the high H2 fuel separated from the waste gas. This enables the system to operate with emissions of 30% or less of the CO2 emissions of the basic (conventional) system.

[0026] As described above, according to some embodiments, the steam generated from the furnace is significantly reduced. As an option to compensate for this and maintain the steam balance, changing one or more of the main compressors from steam turbine drive to electric motor drive can be mentioned. According to some embodiments, ideally, in order to maximize the overall impact of CO2 emissions, the power may be supplied from renewable resources.

[0027] Although specific embodiments of the present invention have been shown and described, it should be understood that the foregoing discussion is not intended to limit the present invention to these embodiments. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention is intended to embrace alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.

Claims

1. A processing method for steam reforming a raw material containing a hydrocarbon raw material and steam, comprising: heating the raw material in a furnace to generate a cracked gas stream containing methane (CH4), ethylene, and hydrogen (H2); separating the cracked gas stream into a product stream rich in ethylene and a waste gas stream rich in CH4 and H2; separating the waste gas stream into a high-CH4 stream and a high-H2 stream rich in H2; reusing at least a part of the high-H2 stream as a fuel stream for a plurality of furnace burners in the furnace; mixing the fuel stream with combustion air to generate a combustion mixture; burning the combustion mixture in the plurality of furnace burners. A processing method comprising the above steps.

2. The processing method according to Claim 1, wherein the hydrocarbon raw material contains ethane.

3. The processing method according to Claim 1, wherein reusing at least a part of the high-H2 stream in the furnace includes expanding the high-H2 stream in an expander to generate an expanded high-H2 stream, and reusing the expanded high-H2 stream as the fuel stream in the furnace.

4. The processing method according to Claim 3, wherein the pressure of the high-H2 stream is greater than 20 barg, and the pressure of the expanded high-H2 stream is less than 10 barg.

5. The processing method according to Claim 3, wherein expanding the high-H2 stream includes reducing the temperature of the high-H2 stream such that the expanded high-H2 stream is cooler than the high-H2 stream.

6. The processing method according to Claim 5, wherein separating the waste gas stream into the high-hydrocarbon stream and the high-H2 stream includes cooling the waste gas stream by utilizing heat exchange with the expanded high-H2 stream.

7. The processing method according to Claim 1, wherein the high-hydrocarbon stream is not used at all as fuel for the plurality of furnace burners.

8. The processing method according to Claim 1, wherein the fuel stream contains more than 90 mol% H2.

9. The processing method according to Claim 1, wherein the furnace comprises a radiant section and a convection section.

10. The processing method according to Claim 9, further comprising preheating the combustion air before the combustion air is mixed with the fuel stream by utilizing heat from the combustion exhaust gas in the convection section.

11. The combustion air is preheated to at least 350 degrees before being mixed with the fuel stream, and the processing method according to claim 10.

12. The processing method according to claim 9, further comprising heating the raw material by heat exchange with the decomposition gas stream.

13. The processing method according to claim 12, further comprising heating the raw material in the raw material preheater by utilizing the heat from the combustion exhaust gas in the convection section.

14. The processing method according to claim 9, further comprising heating the raw material to at least 350 degrees by heat exchange with the decomposition gas stream, and then heating the raw material to at least 650 degrees in the raw material preheater by utilizing the combustion exhaust gas heat in the convection section.

15. The fuel requirement of the furnace is satisfied by using only the high-H2 stream as the fuel stream for the plurality of furnace burners, and the processing method according to claim 1.

16. The processing method according to claim 9, further comprising superheating steam in one or more steam superheaters by utilizing the heat from the combustion exhaust gas in the convection section.

Images