Ethane cracker and start-up abatement method

Abstract

The present application relates to chemical technology field, especially a kind of ethane cracking device and start-up emission reduction method, including cracking section, compression section, separation section, separation section includes deethanizer, cold area, hot area, with deethanizer as limit, deethanizer top connects cold area, bottom connects hot area, cold area includes carbon two hydrogenator, demethanizer, ethylene column, cold box, refrigeration system, deethanizer top pipeline is connected with carbon two hydrogenator, hydrogenated material is cooled in cold box, one way output hydrogen, another way is connected with demethanizer, demethanizer top outputs methane, bottom is connected with ethylene column, ethylene column top exports ethylene, bottom exports ethane, refrigeration system is cooled to carbon two hydrogenator export material, demethanizer top, bottom material, ethylene column top, bottom material of ethylene column, the present application has the following beneficial effects: not only can shorten the product qualified time of each system after cracking furnace feed, reduce start-up emission, but also can improve the safety of operation.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to an ethane cracking unit and a method for reducing emissions during startup. Background Technology

[0002] The world's mainstream ethylene production processes use naphtha and coal as feedstocks. Since the US shale gas revolution, ethane cracking to ethylene has become a rising star in ethylene manufacturing. Compared to naphtha cracking and coal-to-olefins, ethane cracking has advantages such as lower energy consumption, higher yield, and shorter process flow. Most of these plants are located in the US and Europe. In China, ethane cracking plants are relatively few. By 2025, China's ethylene production capacity will exceed 50 million tons, accounting for 22% of global ethylene production capacity. However, currently, China's ethane cracking to ethylene production capacity is less than 5 million tons. Exploring a method for reducing emissions during ethane cracking to ethylene production would be a significant contribution to the ethylene industry.

[0003] The most common start-up procedure for an ethylene plant using naphtha as feedstock is as follows: First, start the ethylene and propylene refrigeration compressors to provide sufficient preparation for the plant's operation. Next, feed is introduced into the cracking furnace to ensure the material required for the operation of the cracked gas compressor, simultaneously generating high-pressure steam to drive the cracked gas compressor turbine. Then, the cracked gas compressor is started to provide power to the separation section. Subsequently, the cold box and demethanizing feed equipment are pre-cooled to meet standard requirements. Finally, the ethylene plant is started up. During the start-up process, there are several flare discharge points for non-conforming products. There are three main discharge points: First, after approximately 2 hours of operation in the cracking furnace, all cracked gas from the cracked gas compressor must be discharged into the flare. Second, when the cold box operates at -165℃, pre-cooling the cold box requires 12 hours, during which a large amount of non-conforming products are generated and must be discharged into the flare. Third, each distillation column in the separation unit should only be fed after the column operation is stable and the column output is qualified. During this period, a large amount of non-conforming products are generated and are forced to be discharged into the flare.

[0004] Similarly, in ethane cracking to ethylene plants, during startup, materials from each system before reaching product quality are directly discharged into the flare system, burned, and released into the atmosphere. This is especially true during the period from when the cracking furnace feeds in until the cracking gas compressor starts, before the C2 hydrogenation is completed, and during the cooling process after the cracking gas enters the cold box; all of these periods result in significant amounts of cracking gas being emitted into the flare. Therefore, reducing flare emissions during ethylene plant startup has significant economic and environmental benefits. Summary of the Invention

[0005] The purpose of this invention is to provide an ethane cracking unit to replace the traditional ethylene unit that uses naphtha as feedstock, which has the advantages of low energy consumption, high yield, and short process.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] An ethane cracking unit includes a cracking section, a compression section, and a separation section connected in sequence. The cracking section includes a cracking furnace and a quench tower. The compression section includes a cracked gas compressor, an alkaline scrubbing tower, and a dryer. The separation section includes an ethane stripper, a cold zone, and a hot zone. The ethane stripper serves as the boundary, with the top of the ethane stripper connected to the cold zone and the bottom connected to the hot zone. The cold zone includes a C2 hydrogenator, a demethanizer, an ethylene tower, a cold box, and a refrigeration system. The top pipeline of the demethanizer is connected to the C2 hydrogenator. The outlet pipeline of the C2 hydrogenator is cooled by the cold box and then split into two paths: one outputs hydrogen, and the other connects to the demethanizer. The top of the demethanizer outputs methane, and the bottom of the demethanizer is connected to the ethylene tower. The top of the ethylene tower outputs ethylene, and the bottom outputs ethane. The refrigeration system cools the feed to the demethanizer, the outlet of the C2 hydrogenator, the top and bottom materials of the demethanizer, and the top and bottom materials of the ethylene tower.

[0008] Preferably, the refrigeration system includes an ethylene refrigeration system and a propylene refrigeration system. The ethylene refrigeration system provides cooling capacity to the ethylene tower, the demethanizer tower, and the cold box, while the propylene refrigeration system provides cooling capacity to the deethanizer tower and the cold box.

[0009] Preferably, the system also includes a flare gas system, a flare gas recovery system, and a fuel gas system. The flare gas recovery system has a flare recovery compressor. A pipeline for external hydrogen delivery at the cold box outlet is connected to the flare gas system. The top pipeline of the demethanizer is also connected to the flare gas system. The flare gas system is connected in sequence to the flare gas recovery system and the fuel gas system. The fuel gas system is also connected to the discharge pipeline of the quench tower in the cracking section.

[0010] Preferably, the system includes a start-up unit and an operating unit, both of which include a cracking section, a compression section, and a separation section. The start-up unit is connected to a flare gas system, a flare gas recovery system, and a fuel gas system. The operating unit is connected to the start-up unit and is used to provide hydrogen, methane, and ethylene during the start-up phase of the start-up unit.

[0011] Preferably, an ethylene supply line is led from the top pipeline of the ethylene tower in the operating unit to the start-up unit and connected to the top pipeline of the ethylene tower in the start-up unit.

[0012] Preferably, the top pipeline of the demethanizer of the operating unit is connected to the methane supply line to the quench tower of the start-up unit, and the ethylene supply line is connected to the quench tower of the start-up unit.

[0013] As a preferred option, each of the hydrogen and methane product lines of the start-up unit is equipped with a circulation line to the quench tower of the start-up unit, and a hydrogen mutual supply line is also provided from the operating unit to the start-up unit.

[0014] As a preferred option, the system also includes a spherical tank for ethylene defective products. The bottom pipeline of the demethylation tower in the start-up unit is connected to the spherical tank for ethylene defective products, and the spherical tank for ethylene defective products is connected to the feed end of the deethanerization tower in the start-up unit via an ethylene transfer pump.

[0015] A method for emission reduction during startup of an ethane cracking unit, characterized by performing the following steps on the startup unit:

[0016] Step 1: The flare system and fuel gas system are put into operation, while the flare recovery compressor is in standby mode.

[0017] Step two: Nitrogen and natural gas purging are performed on all systems except the pyrolysis section;

[0018] Step 3: After the propylene refrigeration system is pressurized with nitrogen, the propylene phase is pressurized and replaced for later use.

[0019] Step 4: After nitrogen is introduced into the ethylene refrigeration system and ethylene tower for pressurization, ethylene gas is introduced from the ethylene product delivery line of the operating unit for pressurization and replacement, and then kept on standby.

[0020] Step 5: The propylene refrigeration system is started to pre-cool the cold box and replenish the cooling capacity;

[0021] Step 6: Start the ethylene refrigeration system, adjust it to a stable state, fill it with liquid ethylene, and establish reflux in the ethylene tower;

[0022] Step 7: Start the pyrolysis gas compressor;

[0023] Step 8, Pre-cooling the cold box:

[0024] Hydrogen is continuously supplied from the operating unit to the start-up unit, so that the hydrogen circulation volume reaches 50-60 tons / hour. At the same time, the precooling rate of the cold box is ensured to be no more than 25℃ / h, and the temperature difference between the cold medium and the hot medium in the cold box is limited to <25℃. The purity of hydrogen reaches more than 85%.

[0025] Under the premise of controlling the cooling rate, hydrogen is continuously introduced from the operating unit to the quench tower, thereby continuously pressurizing the process system. The pressure of the deethanizer is controlled at 1.4 MPa, the demethanizer at 3.0 MPa, and the quench tower at 90 kPa by adjusting the valves in the system. After the pressure of each tower stabilizes, as the temperature of each tower and reflux tank continues to decrease, methane and ethylene are sequentially added to the inlet of the cracked gas compressor, so that the demethanizer reflux tank, the demethanizer bottom, and the deethanizer reflux tank can accumulate sufficient liquid phase. At the same time, liquid phase mixed C3 is injected into the deethanizer, realizing the total reflux of the demethanizer and deethanizer.

[0026] Step 9, Feeding the pyrolysis furnace:

[0027] After the cracking furnace is fed, the entire cracking process is opened up. After the furnace is fed, the reflux from the deethaner is first adjusted to stabilize the reflux of the deethaner.

[0028] Step 10, subsequent system adjustments:

[0029] The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy fraction content at the top of the deethanizer is below 0.1%. Once the content is within acceptable limits, the C2 hydrogenator is started. The C2 hydrogenator is heated for 2 hours. Before the acetylene content in the demethanizer feed drops to the lower limit of 500 ppm, the bottom of the demethanizer can still normally collect C2 components and send them to the ethylene non-conforming product spherical tank. The ethylene non-conforming product spherical tank has a capacity of 6-10 hours of C2 production. Once the acetylene content in the bottom of the demethanizer drops below 500 ppm, the ethylene tower is fed, and the ethylene product is sent out of the boundary. The entire unit is then declared to have started up successfully.

[0030] As a preferred option, the specific steps for step two are as follows:

[0031] Close all valves supplying external products. Open and connect all valves in the entire process system, including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, methanization, and ethylene distillation, and connect them in series. Close all valves discharging flare gas to form a closed system. Introduce nitrogen into the closed system to pressurize it to 0.7 MPa, then open the flare release valve to depressurize it to 0 MPa. Repeat this process several times to ensure the oxygen content is <0.2%. Then begin natural gas replacement, again by introducing gas to 0.7 MPa and depressurizing it to 0 MPa. Repeat this pressure replacement process several times. The natural gas pressure replacement is controlled and orderly discharged into the flare gas system. At the same time, activate the flare recovery compressor to recover the gas and supply it to the fuel gas system for fuel gas users.

[0032] As a preferred option, step six involves introducing ethylene as a refrigerant through an ethylene interconnection line to start the ethylene refrigeration system.

[0033] As a preferred option, step seven is as follows: using a hydrogen supply line, hydrogen is introduced into the quench tower, and the entire process system, including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, and methanization, is pressurized to 0.2 MPa. The cracked gas compressor is started, and the compressor inlet pressure is controlled at 90 kPa. At this time, the entire device forms a large hydrogen circulation.

[0034] As a preferred option, after the unit is successfully started up in step ten, the substandard ethylene with high acetylene content in the bottom of the demethanizer is returned from the substandard ethylene product tank to the deethanizer to reduce the unit's start-up losses.

[0035] In summary, the present invention has the following beneficial effects:

[0036] 1. Using ethane as the cracking feedstock to replace the traditional ethylene plant using naphtha as feedstock has advantages such as low energy consumption, high yield, and short process.

[0037] 2. It can not only shorten the product qualification time of each system after feeding into the cracking furnace and reduce start-up emissions, but also improve operational safety;

[0038] 3. The system included in this invention also includes a flare gas system, a flare gas recovery system, and a fuel gas system. The flare gas recovery system can recover the flare gas emitted in an organized manner within the device and reuse it in the fuel gas system, which greatly saves start-up costs.

[0039] 4. The spherical tank for non-conforming ethylene products in this invention is used to receive non-conforming ethylene with high acetylene content in the bottom of the demethanizer during the start-up phase, and it is returned to the deethanizer after the unit is running normally, thereby reducing start-up losses of the unit;

[0040] 5. This invention replaces the entire process system with hydrogen before starting work and achieves orderly discharge, recovering the hydrogen to the flare gas system, thus saving a large amount of materials;

[0041] 6. The present invention uses two sets of ethane cracking units to cooperate in the emission reduction. The two units are connected by a mutual supply line and can introduce one or more materials as needed. In particular, hydrogen, methane, ethylene and C3 are introduced in sequence according to the pre-cooling stage, which can realize the full reflux of the tower in a timely manner.

[0042] 7. This invention replaces the entire process system with hydrogen before starting work and achieves orderly discharge, and promptly recovers the hydrogen to the flare gas recovery system, saving a lot of materials;

[0043] 8. This invention utilizes hydrogen circulation precooling from adjacent devices, which, compared to traditional nitrogen precooling, reduces the replacement time after startup by 10 hours.

[0044] 9. The method described in this invention only involves flare discharge in three places: after the cracking furnace is fed into the cracking gas compressor system, the C3 content at the top of the deethaner tower is adjusted, and the cold box outlet pressure is adjusted. The total duration is less than 6 hours, which is far less than the tens of hours of flare discharge in traditional processes and traditional start-up methods. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the structure of Example 1;

[0046] Figure 2 This is a structural schematic diagram of Example 2;

[0047] Figure 3 This is a schematic diagram of the structure of Example 3;

[0048] Figure 4This is a structural schematic diagram of Example 4. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to the accompanying drawings.

[0050] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

[0051] Example 1:

[0052] like Figure 1 As shown, it includes a pyrolysis section, a compression section, and a separation section.

[0053] The pyrolysis section includes a pyrolysis furnace and a quench tower.

[0054] The compression section includes a pyrolysis gas compressor, an alkaline scrubbing tower, and a dryer.

[0055] The separation section includes an ethane stripper, a cold zone, and a hot zone, with the ethane stripper serving as the boundary. The top of the ethane stripper connects to the cold zone, and the bottom connects to the hot zone. The cold zone includes a cold zone compressor, a C2 hydrogenator, a demethanizer, a methane generator, an ethylene tower, a cold box, and a refrigeration system. The cold box is connected to the outlet pipeline of the C2 hydrogenator and the top pipeline of the demethanizer. The refrigeration system includes an ethylene refrigeration system and a propylene refrigeration system. Both the ethylene and propylene refrigeration systems are connected to the cold box, providing different temperature levels. The ethylene refrigeration system also provides cooling for the demethanizer, and the propylene refrigeration system also provides cooling for the ethane stripper.

[0056] Workflow:

[0057] The gaseous ethane feedstock enters the cracking furnace and undergoes a thermal cracking reaction at 800°C. The resulting cracked gas contains: hydrogen, 1-carbon methane, carbon monoxide, and carbon dioxide, 2-carbon acetylene, ethylene, and ethane, 3-carbon propyne, propylene, propane, and propadiene, a complex 4-carbon substance named mixed C4, a complex 5-carbon mixture named mixed C5, and C6 / C7 / C8 / C9 / C10 and above components, of which the target product ethylene accounts for 80% by mass.

[0058] The pyrolysis gas enters the quench tower from the pyrolysis furnace, where it is rapidly cooled by water spraying. This quickly terminates the reaction and reduces side reactions. Simultaneously, C9 and higher C9 components are precipitated through water spraying and discharged from the quench tower. Pyrolysis gas with lower C9 content enters the pyrolysis gas compressor for pressurization. The purpose of pressurization is to raise the boiling point of the lighter components in the pyrolysis gas, thus allowing the refrigeration system to liquefy the pyrolysis gas without requiring a low temperature (like atmospheric pressure boiling point), simplifying the fractionation process. The pressurized pyrolysis gas then enters the alkaline scrubbing tower to remove acidic gases such as carbon dioxide, preventing corrosion of downstream pipelines and the formation of dry ice that could cause blockages in the low-temperature cold box. Finally, it enters the dryer to remove moisture, also preventing moisture from entering the refrigeration system and forming ice. The dried pyrolysis gas enters the deethanizer. Based on the different boiling points of the various substances in the pyrolysis gas, the light components (C2 and below) are separated from the top of the deethanizer, while the heavy components (C3 and above) are separated from the bottom. (The pyrolysis gas is divided at the deethanizer; the light components at the top enter the cold zone for cryogenic liquefaction and distillation separation, while the heavy components at the bottom enter the hot zone for conventional steam reboiling and water condensation fractionation). The pyrolysis gas separated from the top of the deethanizer is compressed again by the compressor in the cold zone and then enters the C2 hydrogenator. Here, acetylene in the C2 gas reacts with a catalyst to produce ethylene, removing the acetylene byproduct and increasing the ethylene product. The gas then enters the cold box from the C2 hydrogenator. Hydrogen, with its lowest boiling point, does not liquefy in the cold box and is delivered as a gaseous product. The C1 and C2 mixture is liquefied in the cold box and enters the demethanizer. In the demethanizer, C1 and C2 are separated. C1, i.e., methane, is sent out as a gaseous phase from the top of the tower via the methane generator. C2 (ethylene and ethane) liquid phase goes to the ethylene tower from the bottom of the tower. In the ethylene tower, ethylene and ethane are separated. The gaseous ethylene flows out from the top of the tower, and after being condensed and liquefied, the liquid phase is sent out of the boundary area. The ethane at the bottom of the ethylene tower is returned to the cracking furnace for re-cracking.

[0059] The aforementioned cold zone includes an ethylene refrigeration system and a propylene refrigeration system. The ethylene refrigeration system uses ethylene as a refrigerant and has an ethylene refrigeration compressor. The compressor compresses the ethylene refrigerant, which then undergoes condensation, expansion, and evaporation to provide three temperature levels: -100℃, -80℃, and -60℃. The propylene refrigeration system uses propylene as a refrigerant and has a propylene refrigeration compressor. This compressor compresses the refrigerant, which then undergoes condensation, expansion, and evaporation to provide three temperature levels: -37℃, -18℃, and 12℃. The propylene refrigeration system and the binary refrigeration system together form a cascade refrigeration system, meaning the propylene refrigeration system also needs to provide cooling capacity to the ethylene refrigeration system. Since cascade refrigeration is existing technology, it will not be elaborated upon here.

[0060] In summary, the process is as follows: cracking, quenching, compression, alkaline washing, drying, initial separation in the deethanizer, compression, and the C2 and lower light components from the top of the deethanizer enter the C2 hydrogenator, cold box, demethanizer, and ethyleneizer, where hydrogen (H2), methane (C1), ethylene (C2), and ethane product (C2) are separated. The C3 and higher heavy components from the bottom of the deethanizer then enter the depropaneizer and debutaneizer in sequence to separate mixed C3, mixed C4, and cracked gasoline C5+ product.

[0061] After ethane completes its reaction in the cracking section, the cracked gas is separated in the separation section according to the different boiling points of each substance. The separation of heavy components with C3 and above is a hot zone separation, which is not related to the refrigeration system. The most conventional start-up method, namely feeding, starting reflux and reboiling, is a routine operation, so it will not be described in detail.

[0062] Example 2:

[0063] This embodiment includes all the technical features of Embodiment 1, except that it also includes a flare gas system, a flare gas recovery system, and a fuel gas system, wherein the flare gas recovery system has a flare recovery compressor.

[0064] like Figure 2 As shown, the pipeline for venting hydrogen after exiting the C2 hydrogenator and passing through the cold box is connected to the flare gas system. Simultaneously, the top pipeline of the demethanizer is also connected to the flare gas system. The flare gas system is connected to the flare gas recovery system, which in turn is connected to the fuel gas system.

[0065] Workflow:

[0066] The flare gas system and fuel gas system are in operation, while the flare recovery compressor is in standby mode.

[0067] Nitrogen and natural gas purging were performed on all systems except the pyrolysis section. Specifically, the pyrolysis gas main valve (valve 7) was closed, and all valves for external product delivery were closed. Figure 2 Valves 1, 2, 13, and 14, as well as pipeline valves in the cold zone boundary area, should be connected in series. All valves (not shown in the diagram) within the entire process system, including the quench tower, cracked gas compressor, alkali scrubber, dryer, ethane stripper, C2 hydrogenator, methanogen stripper, and ethylene tower, should be opened and connected. All valves discharging flare gas should be closed. Figure 2The system is closed by valves 3 and 4. After opening nitrogen valve 11 to pressurize nitrogen to 0.7 MPa, the flare release valves (valve 3 and 4) are opened to release pressure to 0 MPa. This process is repeated twice to ensure the oxygen content is <0.2%. Then, valve 8 is opened to begin natural gas replacement. Again, the system is pressurized to 0.7 MPa and then released to 0 MPa. This pressure replacement process is repeated three times. The natural gas pressure replacement is controlled and orderly discharged into the flare gas system. Simultaneously, the flare recovery compressor is activated to recover the natural gas into the fuel gas system for use by fuel gas users, significantly reducing start-up costs.

[0068] Example 3:

[0069] like Figure 3 As shown, this embodiment includes an operating device and a start-up device, wherein the operating device is the device that is currently running, and the start-up device is the device that is ready to start operation. Both the operating device and the start-up device include all the technical features of Embodiment 1, and the start-up device is connected to the flare gas system, flare gas recovery system, and fuel gas system described in Embodiment 2.

[0070] An ethylene supply line is drawn from the top pipeline of the ethylene tower in the operating unit to the start-up unit and connected to the top pipeline of the ethylene tower in the start-up unit.

[0071] Workflow:

[0072] During the start-up phase of the refrigeration system, after the nitrogen inlet area of ​​the propylene refrigeration system is pressurized three times, the gaseous phase of propylene is pressurized and purged three times from the inlet area before being put into standby mode. After the nitrogen inlet area of ​​the ethylene refrigeration system is pressurized three times, the gaseous phase of ethylene is pressurized and purged three times from the ethylene product delivery line of the operating unit using the ethylene interconnection line before being put into standby mode.

[0073] After the ethylene tower is isolated separately (valve 13, valve 14, and valve 12 are closed), the ethylene gas phase is pressurized and replaced three times from the ethylene product delivery line of the operating unit using the ethylene interconnection line, and then put into standby mode.

[0074] The propylene refrigeration system is started to pre-cool the cold box and replenish its cooling capacity.

[0075] Ethylene is supplied from the operating unit's ethylene product delivery line to the ethylene tower and ethylene refrigeration system of the start-up unit. The ethylene refrigeration system is started up using ethylene refrigerant. The load is gradually adjusted until it stabilizes.

[0076] Example 4:

[0077] like Figure 4 As shown, in this embodiment, based on embodiment 3, the methane supply line and the ethylene supply line are connected to the quench tower of the start-up unit.

[0078] The hydrogen and methane product lines of this start-up unit each have a circulation line leading to the upstream quench tower, referred to as the main circulation line. Additionally, a hydrogen interconnection line is provided between the operating unit and the start-up unit.

[0079] This embodiment also includes a spherical tank for non-conforming ethylene products, with the bottom pipeline of the deacetylator in the start-up unit connected to the spherical tank. The spherical tank for non-conforming ethylene products is connected to the feed end of the deethaner in the start-up unit via an ethylene transfer pump.

[0080] Workflow:

[0081] Cracking gas compressor startup: Using the hydrogen interconnection line, hydrogen is introduced into the quench tower to pressurize the entire process system, including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, and demethanization, to 0.2 MPa (excluding the ethylene tower, which has been fully refluxed and disconnected from the demethanization tower valves). The cracking gas compressor is started, and the inlet pressure of the cracking gas compressor is controlled at 90 kPa. At this point, the entire unit forms a large hydrogen circulation loop.

[0082] Cold Box Pre-cooling: Continuously replenish hydrogen from the operating unit into the main circulation system, ensuring a hydrogen circulation rate of 50-60 tons / hour. Simultaneously, ensure the cold box pre-cooling rate does not exceed 25℃ / h, and that the temperature difference between the cold and hot media in the cold box is limited to <25℃. When initially introducing hydrogen, ensure its purity is above 85%. This is to prevent impure hydrogen, such as that containing C1 components, from liquefying when the cold box cools to -100℃, thus clogging the cold box's fins. It also prevents a decrease in cold box outlet pressure due to liquefaction of heavy components in the hydrogen, which could affect the insufficient inlet pressure of the cracked gas compressor.

[0083] Under controlled cooling rate, hydrogen is continuously introduced from the operating unit into the quench tower, thereby continuously pressurizing the process system. Valve adjustments within the system maintain the deethanizer pressure at 1.5 MPa, the demethanizer pressure at 3.0 MPa, and the quench tower pressure at 90 kPa. Once the pressures in each tower stabilize, as the temperatures of each tower and reflux tank continue to decrease, methane and ethylene are sequentially added to the inlet of the cracked gas compressor. This ensures sufficient liquid phase accumulates in the demethanizer reflux tank, the demethanizer bottom, and the deethanizer reflux tank. Simultaneously, liquid-phase mixed C3 is injected into the deethanizer, achieving total reflux in both the demethanizer and deethanizer towers.

[0084] Cracking furnace feeding: After feeding into the cracking furnace, open valve 7 to start the entire cracking process. After feeding into the furnace, first adjust the reflux from the deethaner to stabilize the deethaner reflux.

[0085] Post-system adjustments: The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy fraction (C3) content at the top of the deethanizer is below 0.1%. Once the content is within acceptable limits, the C2 hydrogenator is started. The C2 hydrogenator is heated for 2 hours. Before the acetylene in the demethanizer feed drops to the lower limit of 500 ppm, the demethanizer bottom can still normally collect C2 components and send them to the ethylene non-conforming product spherical tank. The ethylene non-conforming product spherical tank has a capacity of 6-10 hours of C2 production. Once the acetylene content in the demethanizer bottom drops below 500 ppm, valve 12 is opened to feed into the ethylene tower, and the ethylene product is simultaneously sent out of the boundary area. The entire unit is then declared to have successfully started up.

[0086] Previously, substandard ethylene with high acetylene content in the bottom of the demethanizer was returned from the substandard ethylene product tank to the deethanizer, reducing the plant's operating losses.

[0087] Example 5: A method for emission reduction during start-up of an ethane cracking unit, comprising the following steps:

[0088] Step 1: The flare system and fuel gas system are put into operation, while the flare recovery compressor is in standby mode.

[0089] Step two involves nitrogen and natural gas purging of all systems except the pyrolysis section. The specific procedures are as follows:

[0090] Close the cracked gas main valve 7, and close all valves for external product delivery. Figure 2 Valves 1, 2, 13, and 14 in the diagram, as well as the valves in the cold zone boundary area, are connected in series. All valves (not shown in the diagram) in the entire process system, including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, methanization removal, and ethylene distillation, are opened and connected in series. All valves for flare gas exhaust are closed. Figure 2 The system is closed by valves 3 and 4. After opening nitrogen valve 11 to pressurize nitrogen to 0.7 MPa, the flare release valves (valve 3 and 4) are opened to release pressure to 0 MPa. This process is repeated twice to ensure the oxygen content is <0.2%. Then, natural gas replacement is started, again by pressing to 0.7 MPa and releasing pressure to 0 MPa. This pressure replacement is repeated three times. The natural gas pressure replacement is controlled and orderly discharged into the flare gas system. At the same time, the flare recovery compressor is activated to recover the gas into the fuel gas system for use by fuel gas users, which greatly saves start-up costs.

[0091] Step 3: After the nitrogen inlet of the propylene refrigeration system is pressurized three times, the gas phase of propylene is pressurized and replaced three times before it is ready for use.

[0092] Step 4: After the ethylene tower is isolated separately (valve 13, valve 14, and valve 12 are closed), the nitrogen inlet zone of the ethylene refrigeration system is pressurized three times. Then, using the ethylene interconnection line, the ethylene gas phase is pressurized and replaced three times from the ethylene product delivery line of the operating unit and put into standby mode.

[0093] Step 5: Start the propylene refrigeration system:

[0094] Start the propylene refrigeration system to pre-cool and replenish the cold box.

[0095] Step 6: Using the ethylene interconnection line, ethylene is drawn from the ethylene product delivery line of the operating unit to replace the ethylene refrigeration system and ethylene tower (heat pump system). The ethylene refrigeration system is started with ethylene refrigerant and slowly filled with liquid after it is running stably. The ethylene tower is established with reflux.

[0096] Step 7: Start the pyrolysis gas compressor:

[0097] Using a hydrogen supply line, hydrogen is introduced into the quench tower, pressurizing the entire process system—including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, and demethanization—to 0.2 MPa (excluding the ethylene tower, which has been fully refluxed and disconnected from the demethanization tower valve). The cracked gas compressor is started, and the compressor inlet pressure is controlled at 90 kPa, at which point the entire unit forms a large hydrogen circulation loop.

[0098] Step 8, Pre-cooling the cold box:

[0099] Hydrogen is continuously replenished from the operating unit into the main circulation system, maintaining a circulation rate of 50-60 tons / hour. Simultaneously, the pre-cooling rate of the cold box is ensured to be no greater than 25°C / h, and the temperature difference between the cold and hot media in the cold box is limited to <25°C. When initially introducing hydrogen, its purity is ensured to be above 85%. This is to prevent impurities such as C1 components in the hydrogen from liquefying at -100°C and clogging the cold box's fins. It also prevents a decrease in cold box outlet pressure due to the liquefaction of heavy components in the hydrogen, which could affect the insufficient inlet pressure of the cracked gas compressor.

[0100] Under controlled cooling rate, hydrogen is continuously introduced from the operating unit into the quench tower, thereby continuously pressurizing the process system. Valve adjustments within the system maintain the deethanizer pressure at 1.5 MPa, the demethanizer pressure at 0.65 MPa, and the quench tower pressure at 90 kPa. Once the pressures in each tower stabilize, as the temperatures of each tower and reflux tank continue to decrease, methane and ethylene are sequentially added to the inlet of the cracked gas compressor. This ensures sufficient liquid phase accumulates in the demethanizer reflux tank, the demethanizer bottom, and the deethanizer reflux tank. Simultaneously, liquid-phase mixed C3 is injected into the deethanizer, achieving total reflux in both the demethanizer and deethanizer towers.

[0101] Step 9, Feeding the pyrolysis furnace:

[0102] After the cracking furnace is fed, valve 7 is opened, thus starting the entire cracking process. After the furnace is started, the reflux from the deethaner is first adjusted to stabilize the reflux in the deethaner.

[0103] Step 10, subsequent system adjustments:

[0104] The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy component (C3) content at the top of the deethanizer is below 0.1%. Once the content is within acceptable limits, the C2 hydrogenator is started. The C2 hydrogenator is heated for 2 hours. Before the acetylene content in the demethanizer feed drops to the lower limit of 500 ppm, the bottom of the demethanizer can still normally collect C2 components and send them to the ethylene non-conforming product spherical tank. The ethylene non-conforming product spherical tank has a capacity of 6-10 hours of C2 production. Once the acetylene content in the bottom of the demethanizer drops below 500 ppm, valve 12 is opened to feed into the ethylene tower, and the ethylene product is simultaneously sent out of the boundary area. The entire unit is then declared to have successfully started up.

[0105] Previously, substandard ethylene with high acetylene content in the bottom of the demethanizer was returned from the substandard ethylene product tank to the deethanizer, reducing the plant's operating losses.

Claims

1. An ethane cracking apparatus, characterized in that, The system includes start-up and operation units. Both units consist of a cracking section, a compression section, and a separation section connected in sequence. The cracking section includes a cracking furnace and a quench tower. The compression section includes a cracked gas compressor, an alkaline scrubbing tower, and a dryer. The separation section includes an ethane stripper, a cold zone, and a hot zone. The ethane stripper serves as the boundary, with the top of the stripper connected to the cold zone and the bottom connected to the hot zone. The cold zone includes a C2 hydrogenator, a demethanizer, an ethylene tower, a cold box, and a refrigeration system. The top pipeline of the demethanizer is connected to the C2 hydrogenator. The outlet pipeline of the C2 hydrogenator is cooled by the cold box and then split into two paths: one outputs hydrogen, and the other connects to the demethanizer. The top of the demethanizer outputs methane, and the bottom connects to the ethylene tower. The top of the ethylene tower outputs ethylene, and the bottom outputs ethane. The refrigeration system cools the feed to the demethanizer, the outlet of the C2 hydrogenator, the top and bottom materials of the demethanizer, and the top and bottom materials of the ethylene tower. It also includes a flare gas system, a flare gas recovery system, and a fuel gas system. The flare gas recovery system has a flare recovery compressor. The pipeline for sending hydrogen to the cold box outlet is connected to the flare gas system. The top pipeline of the demethanizer is also connected to the flare gas system. The flare gas system is connected to the flare gas recovery system and the fuel gas system in sequence. The fuel gas system is also connected to the discharge pipeline of the quench tower in the cracking section. The start-up unit is connected to the flare gas system, flare gas recovery system, and fuel gas system. The operating unit is connected to the start-up unit and is used to provide hydrogen, methane, and ethylene during the start-up phase of the start-up unit. Each of the hydrogen and methane product lines of the start-up unit is equipped with a circulation line to the quench tower of the start-up unit. At the same time, a hydrogen mutual supply line is also set up from the operating unit to the start-up unit. It also includes spherical tanks for non-conforming ethylene products. The bottom pipeline of the demethanizer in the start-up unit is connected to the spherical tanks for non-conforming ethylene products. The spherical tanks for non-conforming ethylene products are connected to the feed end of the deethaner in the start-up unit through an ethylene transfer pump.

2. An ethane cracking apparatus according to claim 1, characterized in that, The refrigeration system includes an ethylene refrigeration system and a propylene refrigeration system. The ethylene refrigeration system provides cooling capacity to the ethylene tower, the demethanizer tower, and the cold box, while the propylene refrigeration system provides cooling capacity to the deethanizer tower and the cold box.

3. An ethane cracking apparatus according to claim 2, characterized in that, An ethylene supply line is drawn from the top pipeline of the ethylene tower in the operating unit to the start-up unit and connected to the top pipeline of the ethylene tower in the start-up unit.

4. An ethane cracking apparatus according to claim 3, characterized in that, The top pipeline of the demethanizer of the operating unit leads to the methane supply line to the quench tower of the start-up unit, and the ethylene supply line is connected to the quench tower of the start-up unit.

5. The method for reducing emissions during the start-up of an ethane cracking unit according to claim 4, characterized in that, Perform the following steps on the starting device: Step 1: The flare system and fuel gas system are put into operation, while the flare recovery compressor is in standby mode. Step two: Nitrogen and natural gas purging are performed on all systems except the pyrolysis section; Step 3: After the propylene refrigeration system is pressurized with nitrogen, the propylene phase is pressurized and replaced for later use. Step 4: After nitrogen is introduced into the ethylene refrigeration system and ethylene tower for pressurization, ethylene gas is introduced from the ethylene product delivery line of the operating unit for pressurization and replacement, and then kept on standby. Step 5: The propylene refrigeration system is started to pre-cool the cold box and replenish the cooling capacity; Step 6: Start the ethylene refrigeration system, adjust it to a stable state, fill it with liquid ethylene, and establish reflux in the ethylene tower; Step 7: Start the pyrolysis gas compressor; Step 8, Pre-cooling the cold box: Hydrogen is continuously supplied from the operating unit to the start-up unit, so that the hydrogen circulation volume reaches 50-60 tons / hour. At the same time, the precooling rate of the cold box is ensured to be no more than 25℃ / h, and the temperature difference between the cold medium and the hot medium in the cold box is limited to <25℃. The purity of hydrogen reaches more than 85%. Under the premise of controlling the cooling rate, hydrogen is continuously introduced from the operating unit to the quench tower, thereby continuously pressurizing the process system. The pressure of the deethanizer is controlled at 1.4 MPa, the demethanizer at 3.0 MPa, and the quench tower at 90 kPa by adjusting the valves in the system. After the pressure of each tower stabilizes, as the temperature of each tower and reflux tank continues to decrease, methane and ethylene are sequentially added to the inlet of the cracked gas compressor, so that the demethanizer reflux tank, the demethanizer bottom, and the deethanizer reflux tank can accumulate sufficient liquid phase. At the same time, liquid phase mixed C3 is injected into the deethanizer, realizing the total reflux of the demethanizer and deethanizer. Step 9, Feeding the pyrolysis furnace: After the cracking furnace is fed, the entire cracking process is opened up. After the furnace is fed, the reflux from the deethaner is first adjusted to stabilize the reflux of the deethaner. Step 10, subsequent system adjustments: The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy fraction content at the top of the deethanizer is below 0.1%. Once the content is within acceptable limits, the C2 hydrogenator is started. The C2 hydrogenator is heated for 2 hours. Before the acetylene content in the demethanizer feed drops to the lower limit of 500 ppm, the bottom of the demethanizer can still collect C2 components normally and send them to the ethylene non-conforming product spherical tank. The ethylene non-conforming product spherical tank has a capacity of 6-10 hours of C2 production. Once the acetylene content in the bottom of the demethanizer drops below 500 ppm, the ethylene tower is fed, and the ethylene product is sent out of the boundary. The entire unit is then declared to have started up successfully.

6. The emission reduction method for starting operations according to claim 5, characterized in that, The specific steps for step two are as follows: Close all valves supplying external products. Open and connect all valves in the entire process system, including quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, methanization, and ethylene distillation, and connect them in series. Close all valves discharging flare gas to form a closed system. Pressurize the closed system with nitrogen to 0.7 MPa, then open the flare release valve to depressurize to 0 MPa. Repeat this process several times to ensure the oxygen content is <0.2%. Then begin natural gas replacement, similarly by pressing to 0.7 MPa and depressurizing to 0 MPa. Repeat this pressure replacement process several times. The natural gas pressure replacement is controlled and orderly discharged into the flare gas system. At the same time, activate the flare recovery compressor to recover the gas and supply it to the fuel gas system for fuel gas users.

7. The emission reduction method for starting operations according to claim 5, characterized in that, The specific procedure for step six is ​​as follows: introduce ethylene as a refrigerant using the ethylene interconnection line to start the ethylene refrigeration system.

8. The emission reduction method for starting operations according to claim 5, characterized in that, Step seven is as follows: Using the hydrogen supply line, hydrogen is introduced into the quench tower, and the entire process system of quenching, compression, alkaline washing, drying, ethane removal, C2 hydrogenation, and methanization is pressurized to 0.2 MPa. The cracked gas compressor is started, and the compressor inlet pressure is controlled at 90 kPa. At this time, the entire unit forms a large hydrogen circulation.

9. The emission reduction method for starting operations according to claim 5, characterized in that, After the unit is successfully started up in step ten, the substandard ethylene with high acetylene content in the bottom of the demethanizer is returned from the substandard ethylene product tank to the deethanizer to reduce the unit's start-up losses.

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