Ethane cracking device and start-up emission reduction method

Abstract

The invention relates to the technical field of chemical engineering, in particular to an ethane cracking device and a start-up emission reduction method.The ethane cracking device comprises a cracking workshop section, a compression workshop section and a separation workshop section, the separation workshop section comprises a dethanizing column, a cold area and a hot area, the dethanizing column serves as a boundary, the top of the dethanizing column is connected with the cold area, and the bottom of the dethanizing column is connected with the hot area; the cold area comprises a C2 hydrogenator, a demethanizer, an ethylene tower, a cold box and a refrigerating system, a tower top pipeline of the deethanizer is connected with the C2 hydrogenator, hydrogenated materials enter the cold box to be cooled, one path outputs hydrogen, the other path is connected with the demethanizer, methane is output from the tower top of the demethanizer, the tower bottom is connected with the ethylene tower, ethylene is output from the tower top of the ethylene tower, and the refrigerating system is connected with the refrigerating system. And the refrigerating system is used for refrigerating the material at the outlet of the C2 hydrogenator, the material at the top and the bottom of the demethanizer and the material at the top and the bottom of the ethylene tower, and the method has the following beneficial effects: not only can the product qualification time of each system after feeding of the cracking furnace be shortened, the start-up emission be reduced, but also the operation safety can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of chemical industry, and in particular to an ethane cracking device and a start-up emission reduction method. Background Art

[0002] The mainstream ethylene process in the world uses naphtha and coal as raw materials. Since the shale gas revolution in the United States, ethane cracking to produce ethylene has become a rising star in the ethylene manufacturing process. Compared with naphtha cracking and coal-to-olefins, ethane cracking has the advantages of low energy consumption, high yield, and short process. Most of the devices are distributed in the United States and Europe. In China, there are relatively few ethane cracking devices. By 2025, China's ethylene production capacity will exceed 50 million tons, accounting for 22% of the global ethylene production capacity. However, the current domestic ethane cracking production capacity of ethylene is less than 5 million tons. Exploring a method for starting ethane cracking to produce ethylene to reduce emissions is a huge contribution to the ethylene industry.

[0003] The most common start-up process of an ethylene unit using naphtha as raw material is: first, start the ethylene and propylene refrigeration compressors to provide sufficient preparation for the operation of the unit, then feed the cracking furnace to ensure the materials required for the operation of the cracking gas compressor, and at the same time generate high-pressure steam to drive the cracking gas compressor turbine, then start the cracking gas compressor to provide power for the separation part, then pre-cool the cold box and demethanization feed equipment to make their temperature meet the standard requirements, and finally start the ethylene unit. During the start-up process of the ethylene unit, there will be some flare emission points for unqualified products. There are three main emission points, namely: First, when the cracking furnace is fed and operated for about 2 hours, the cracking gas in the cracking gas compressor must be discharged into the flare. Second, when the operating temperature of the cold box equipment is -165℃, it takes 12 hours to pre-cool the cold box. During this period, a large number of unqualified products will be produced, all of which must be discharged into the flare. Third, each distillation tower of the separation equipment should be fed only after the tower operation is stable and the tower discharge is qualified. During this period, a large number of unqualified products will be produced and forced to be discharged into the flare.

[0004] Similarly, in the ethane cracking device to produce ethylene, during the start-up process, the materials before the products of each system are qualified are directly discharged into the flare system and discharged into the atmosphere after combustion, especially during the period from the time when the cracking furnace is fed to the time when the cracking gas compressor is started, before the carbon dioxide hydrogenation is qualified, and during the process of cooling the cracking gas after entering the cold box, a large amount of cracking gas is discharged into the flare. Therefore, reducing the flare emissions during the start-up of the ethylene device has significant economic and environmental benefits. Summary of the invention

[0005] The purpose of the present invention is to provide an ethane cracking device to replace the traditional ethylene device using naphtha as raw material, which has the advantages of low energy consumption, high yield, short process and the like.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] An ethane cracking device comprises a cracking section, a compression section and a separation section which are connected in sequence. The cracking section comprises a cracking furnace and a quenching tower. The compression section comprises a cracking gas compressor, an alkali washing tower and a dryer. The separation section comprises a deethanizer, a cold zone and a hot zone. The deethanizer is used as a boundary. The top of the deethanizer is connected to the cold zone, and the bottom of the deethanizer is connected to the hot zone. The cold zone comprises a C2 hydrogenator, a demethanizer, an ethylene tower, a cold box and a refrigeration system. The top pipeline of the deethanizer is connected to the C2 hydrogenator. The discharge pipeline of the C2 hydrogenator is divided into two paths after being cooled in the cold box, one path outputs hydrogen, and the other path is connected to the demethanizer. The top of the demethanizer outputs methane, and the bottom of the tower is connected to the ethylene tower. The top of the ethylene tower outputs ethylene, and the bottom of the tower outputs ethane. The refrigeration system refrigerates the feed of the deethanizer, the discharge of the C2 hydrogenator, the materials at the top and bottom of the demethanizer, and the materials at the top and bottom of the ethylene tower.

[0008] Preferably, the refrigeration system comprises an ethylene refrigeration system and a propylene refrigeration system, wherein the ethylene refrigeration system provides cooling for the ethylene tower, the demethanizer and the cold box, and the propylene refrigeration system provides cooling for the deethanizer and the cold box.

[0009] Preferably, 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 out at the outlet of the cold box is connected to the flare gas system, and 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.

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

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

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

[0013] Preferably, the hydrogen product line and methane product line of the start-up device are each provided with a circulation line to the quenching tower of the start-up device, and a hydrogen mutual supply line is also provided from the operating device to the start-up device.

[0014] Preferably, it also includes a spherical tank for unqualified ethylene products, the bottom pipeline of the demethanization tower in the start-up device is connected to the spherical tank for unqualified ethylene products, and the spherical tank for unqualified ethylene products is connected to the feed end of the deethanizer tower of the start-up device through an ethylene delivery pump.

[0015] A method for reducing emissions from the start-up of an ethane cracking device, characterized in that the following steps are performed on the start-up device:

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

[0017] Step 2: nitrogen replacement and natural gas replacement are performed on all systems except the cracking section;

[0018] Step 3, after the propylene refrigeration system is pressurized by introducing nitrogen, the gas phase propylene is pressurized and replaced for standby use;

[0019] Step 4: After the ethylene refrigeration system and the ethylene tower are pressurized by introducing nitrogen, the ethylene product delivery line of the operating device is used to introduce gas-phase ethylene for pressure expansion and replacement for standby use;

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

[0021] Step 6: The ethylene refrigeration system is started, adjusted to a stable state, filled with liquid phase ethylene, and the ethylene tower is refluxed;

[0022] Step 7, starting the cracking gas compressor;

[0023] Step 8: Precooling of cold box:

[0024] Continuously replenish hydrogen from the operating device to the operating device to make the circulation volume of hydrogen reach 50-60 tons / hour, and ensure that the precooling rate of the cold box is no more than 25℃ / h, and ensure that the temperature difference between the cold medium and the hot medium in the cold box is limited to <25℃, and the purity of hydrogen reaches more than 85%;

[0025] Under the premise of controlling the cooling rate, hydrogen is continuously introduced from the operating device to the quench tower, thereby continuously pressurizing the process system. The pressure of the deethanizer is controlled at 1.4Mpa, the pressure of the demethanizer is controlled at 3.0Mpa, and the pressure of the quench tower is controlled at 90Kpa through valve adjustment in the system. When the pressure of each tower is stable, along with the continuous decrease in the temperature of each tower and the reflux tank, methane and ethylene are added to the inlet of the cracking gas compressor in turn, so that the demethanizer reflux tank, the demethanizer kettle, and the deethanizer reflux tank can accumulate enough liquid phase, and at the same time, the liquid phase mixed C3 is injected into the deethanizer to achieve full reflux of the demethanizer and the deethanizer;

[0026] Step nine, feeding the cracking furnace:

[0027] After the cracking furnace is charged, the entire cracking process is opened. After the furnace is charged, the reflux tank of the deethanizer is first used to adjust the reflux stability of the deethanizer;

[0028] Step 10, post-system adjustment:

[0029] The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy content at the top of the deethanizer is below 0.1%. When the content is qualified, the C2 hydrogenator is turned on and the temperature is raised for 2 hours for C2 hydrogenation. Before the acetylene in the feed of the demethanizer drops to the lower limit of 500ppm, the bottom of the demethanizer can still collect C2 components normally and enter the ethylene unqualified product spherical tank. The capacity of the ethylene unqualified product spherical tank is 6-10 hours of C2 production. Once the acetylene content in the bottom of the demethanizer drops below 500ppm, the ethylene tower is fed and the ethylene product is sent out of the boundary area. The entire device is declared successfully started.

[0030] As a preferred embodiment, the specific method of step 2 is:

[0031] Close all valves for product delivery, open all valves in the entire process system, including quenching, compression, alkali washing, drying, deethaneization, carbon dihydrogenation, demethanization, and ethylene distillation, and connect them in series. Close all valves for discharging flare gas to form a closed system. Fill the closed system with nitrogen to increase the pressure to 0.7Mpa, then open the flare valve to release the pressure to 0Mpa. Repeat several times to ensure that the oxygen content is less than 0.2%, and then start natural gas replacement. Similarly, fill it to 0.7Mpa, then release the pressure to the gauge pressure of 0Mpa, and repeat the pressure increase and replacement several times. The pressure increase and replacement of natural gas is controllable and orderly discharged to the flare gas system. At the same time, activate the flare recovery compressor and recover it to the fuel gas system for use by fuel gas users.

[0032] Preferably, the specific method of step six is: using the ethylene mutual supply line to introduce ethylene as the refrigerant to start the ethylene refrigeration system.

[0033] Preferably, the specific method of step seven is: using the hydrogen mutual supply line to introduce hydrogen into the quenching tower, pressurizing the entire process system including quenching, compression, alkali washing, drying, deethaneization, carbon two hydrogenation, demethanization to 0.2Mpa, starting the cracking gas compressor, and controlling the compressor inlet pressure at 90Kpa. At this time, the entire device forms a large cycle of hydrogen.

[0034] Preferably, after the successful start-up of the device in step 10, unqualified ethylene with high acetylene content in the demethanizer kettle is returned from the unqualified ethylene product spherical tank to the deethanizer to reduce the start-up loss of the device.

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

[0036] 1. Using ethane as cracking raw material to replace the traditional ethylene unit using naphtha as raw material has the advantages of low energy consumption, high yield and short process;

[0037] 2. It can not only shorten the qualified time of each system after the cracking furnace is fed, reduce the start-up emissions, but also improve the safety of operation;

[0038] 3. The system included in the present 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 discharged in an organized manner in the device and enter the fuel gas system for reuse, thereby greatly saving the start-up cost;

[0039] 4. The unqualified ethylene product spherical tank in the present invention is used to receive unqualified ethylene with high acetylene content in the demethanizer kettle during the start-up phase, and refluxes to the deethanizer after the device is operating normally, thereby reducing the start-up loss of the device;

[0040] 5. The present invention replaces the entire process system with hydrogen before starting the operation, and realizes orderly discharge, and recovers the hydrogen to the flare gas system, saving a lot of materials;

[0041] 6. The present invention uses two sets of ethane cracking devices to cooperate with each other to achieve emission reduction. The two devices are connected by a mutual supply line, and one or more materials can be introduced at any time according to needs, especially hydrogen, methane, ethylene, and C3 can be introduced in sequence according to the precooling stage, and the full reflux of the tower can be achieved in time;

[0042] 7. The present invention replaces the entire process system with hydrogen before starting, and realizes orderly discharge, and recovers the hydrogen to the flare gas recovery system in time, saving a lot of materials;

[0043] 8. The present invention utilizes hydrogen circulation precooling of adjacent devices, which can reduce the replacement time after start-up by 10 hours compared with traditional nitrogen precooling;

[0044] 9. The method described in the present invention has flare emissions only in three places: the pyrolysis gas compressor system is incorporated after the cracking furnace is charged, the C3 content at the top of the deethanizer is adjusted, and the cold box outlet pressure is adjusted. The total duration is less than 6 hours, which is much shorter than the flare emissions of dozens of hours in traditional processes and traditional start-up methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a schematic structural diagram of Example 1;

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

[0047] Figure 3 is a schematic structural diagram of Example 3;

[0048] Figure 4It is a structural schematic diagram of Example 4. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0050] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

[0051] Embodiment 1:

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

[0053] The cracking section includes a cracking furnace and a quenching tower.

[0054] The compression section includes cracking gas compressor, alkali washing tower and dryer.

[0055] The separation section includes a deethanizer, a cold zone and a hot zone. The deethanizer is used as the boundary. The top of the deethanizer is connected to the cold zone, and the bottom of the deethanizer is connected to the hot zone. The cold zone includes a cold zone compressor, a C2 hydrogenator, a demethanizer, a methane machine, 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 refrigeration system and the propylene refrigeration system are connected to the cold box to provide low temperatures of different temperature levels. The ethylene refrigeration system also provides cooling for the demethanizer, and the propylene refrigeration system also provides cooling for the deethanizer.

[0056] Workflow:

[0057] The gaseous ethane raw material enters the cracking furnace and undergoes thermal cracking reaction at a temperature of 800°C. The generated cracking gas contains: hydrogen, methane, carbon monoxide, and carbon dioxide with 1 carbon, acetylene, ethylene, and ethane with 2 carbons, propyne, propylene, propane, and propadiene with 3 carbons, a complex substance with 4 carbons is named mixed carbon four, a complex mixture with 5 carbons is named mixed carbon 5, as well as C6 / C7 / C8 / C9 / C10 and above components, among which the mass percentage of the target product ethylene is 80%.

[0058] The cracked gas enters the quench tower from the cracking furnace, and the temperature of the cracked gas is quickly reduced by water spraying, so that the reaction is quickly terminated and side reactions are reduced. At the same time, the components C9 and above are precipitated by water spraying and discharged from the quench tower. The cracked gas below C9 enters the cracked gas compressor for pressurization. The purpose is to increase the boiling point of the light components in the cracked gas by pressurizing the cracked gas, so that the refrigeration system does not need to provide a temperature as low as the normal pressure boiling point, and the cracked gas can be liquefied, making the cracked gas fractionation simpler. The pressurized cracked gas enters the alkali washing tower to remove acidic gases such as carbon dioxide to avoid corrosion of the downstream pipeline and the formation of dry ice in the low-temperature cold box to cause blockage, and then enters the dryer to remove moisture, which also prevents moisture from entering the cold system and freezing. The dried cracked gas enters the deethanizer. According to the different boiling points of various substances in the cracked gas, light components C2 and below are separated from the top of the deethanizer, and heavy components C3 and above are separated from the bottom of the deethanizer (the cracked gas is bounded by the deethanizer. The light components at the top of the tower enter the cold zone and are liquefied at low temperature and separated by distillation. The heavy components at the bottom of the tower enter the hot zone and are fractionated by conventional steam reboiling and water condensation). The cracked gas separated from the top of the deethanizer is compressed again by the cold zone compressor and enters the C2 hydrogenator. The acetylene in C2 is reacted with a catalyst to generate ethylene, removing the by-product acetylene and increasing the ethylene product. It enters the cold box from the C2 hydrogenator. Hydrogen has the lowest boiling point and is not liquefied in the cold box. It is sent out in the form of gas as a hydrogen product. The C1 and C2 mixture is liquefied in the cold box and enters the demethanizer. C1 and C2 are separated in the demethanizer. C1, i.e. methane, is sent out from the top of the tower through a methane generator in the gas phase. C2 (ethylene and ethane) liquid phase goes to the ethylene tower from the bottom of the tower. Ethylene and ethane are separated in the ethylene tower. The gaseous ethylene flows out from the top of the tower, is condensed and liquefied, and is sent out of the boundary area in the liquid phase. The ethane at the bottom of the ethylene tower returns to the cracking furnace and is cracked again.

[0059] The cold zone described above 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 ethylene refrigeration compressor compresses the ethylene refrigerant and provides three temperature levels of -100°C, -80°C, and -60°C through condensation, expansion, and evaporation. The propylene refrigeration system uses propylene as a refrigerant and has a propylene refrigeration compressor. The propylene refrigeration compressor compresses the refrigerant and provides three temperature levels of -37°C, -18°C, and 12°C through condensation, expansion, and evaporation. The propylene refrigeration system and the binary refrigeration system form a cascade refrigeration system, that is, the propylene refrigeration system also needs to provide cooling capacity for the ethylene refrigeration system. Since cascade refrigeration belongs to the prior art, it will not be described here.

[0060] In summary, the process is: cracking, rapid cooling, compression, alkaline washing, drying, initial separation in the deethanizer, compression, C2 and below light components at the top of the deethanizer enter the C2 hydrogenator, cold box, demethanizer, and ethylene tower to separate hydrogen (H2), methane (C1), ethylene (C2), and ethane products (C2), and C3 and above heavy components at the bottom of the deethanizer enter the depropanizer and debutanizer in turn to separate mixed C3, mixed C4 and pyrolysis gasoline C5+ products.

[0061] After ethane completes the reaction in the cracking section, the cracking gas is separated in the separation section according to the different boiling points of each substance. The separation of heavy components C3 and above belongs to hot zone separation and is not related to the refrigeration system. It follows the most conventional start-up method, i.e. feeding, starting reflux and reboiling, which is a routine operation and will not be elaborated.

[0062] Embodiment 2:

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

[0064] like Figure 2 As shown, the pipeline for discharging hydrogen after exiting the carbon dihydrogenator and passing through the cold box is connected to the flare gas system, and at the same time, the top pipeline of the demethanizer is connected to the flare gas system. The flare gas system is connected to the flare gas recovery system, and the flare gas recovery system is connected to the fuel gas system.

[0065] Workflow:

[0066] The flare gas system and fuel gas system are put into use, and the flare recovery compressor is in standby status.

[0067] All systems except the cracking section are replaced with nitrogen and natural gas. The specific method is: close the cracking gas valve 7, and all valves for product delivery are closed ( Figure 2 All valves in the entire process system (not shown in the figure) including valves 1, 2, 13, 14 in the cold zone and the pipeline valves in the cold zone outbound zone) are opened and connected in series, including the quench tower, cracking gas compressor, alkali washing tower, dryer, deethanizer, carbon two hydrogenator, demethanizer, ethylene tower, etc. All valves (not shown in the figure) in the entire process system are opened and connected in series, and all valves for exhausting flare gas are closed ( Figure 2Valve 3, valve 4) in the middle to form a closed system. After opening nitrogen valve 11 to charge nitrogen and increase the pressure to 0.7Mpa, open the flare valve (valve 3, valve 4) to release the pressure to a gauge pressure of 0Mpa, repeat 2 times to ensure that the oxygen content is less than 0.2%, then open valve 8 to start natural gas replacement, also charge to 0.7Mpa, and then release the pressure to a gauge pressure of 0Mpa, and replace 3 times with pressure increase. The pressure increase replacement of natural gas is controllable and orderly discharged to the flare gas system. At the same time, the flare recovery compressor is activated to recover the natural gas to the fuel gas system for use by fuel gas users, which greatly saves the start-up cost.

[0068] Embodiment 3:

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

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

[0071] Workflow:

[0072] During the start-up phase of the refrigeration system, the propylene refrigeration system draws nitrogen from the boundary area to increase the pressure three times, and then draws gas-phase propylene from the boundary area to increase the pressure and replace it three times before it is ready for use. The ethylene refrigeration system draws nitrogen from the boundary area to increase the pressure three times, and then uses the ethylene mutual supply line to draw gas-phase ethylene from the ethylene product delivery line of the operating device to increase the pressure and replace it three times before it is ready for use.

[0073] After the ethylene tower is isolated (valve 13, valve 14, valve 12 are closed), the ethylene mutual supply line is used to draw gas phase ethylene from the ethylene product delivery line of the operating unit, and the ethylene is pressurized and replaced three times for standby use.

[0074] The propylene refrigeration system starts to pre-cool the cold box and add cold capacity.

[0075] Using the ethylene mutual supply line, ethylene is introduced from the ethylene product delivery line of the operating unit to the ethylene tower and ethylene refrigeration system of the operating unit. The ethylene refrigeration system is started using ethylene refrigerant. The load is gradually adjusted to a stable state.

[0076] Embodiment 4:

[0077] like Figure 4 As shown, based on Example 3, this example further connects the methane mutual supply line and the ethylene mutual supply line to the quench tower of the start-up device.

[0078] The hydrogen product line and methane product line of this start-up device are each equipped with a circulation line to the upstream quench tower, which is called the large circulation line. At the same time, a hydrogen mutual supply line is also set from the operating device to the start-up device.

[0079] This embodiment also includes an ethylene unqualified product spherical tank, and the bottom pipeline of the demethylation tower in the start-up device is connected to the ethylene unqualified product spherical tank. The ethylene unqualified product spherical tank is connected to the feed end of the deethanizer of the start-up device through an ethylene delivery pump.

[0080] Workflow:

[0081] Start the cracking gas compressor: Use the hydrogen mutual supply line to introduce hydrogen into the quenching tower, and pressurize the entire process system including quenching, compression, alkali washing, drying, deethaneization, carbon two hydrogenation, demethanization to 0.2Mpa (excluding the ethylene tower, which has been fully refluxed and disconnected from the demethanization tower valve). Start the cracking gas compressor and control the inlet pressure of the cracking gas compressor to 90Kpa. At this time, the entire device forms a large cycle of hydrogen.

[0082] Cold box precooling: continuously add hydrogen from the operating device to the large cycle, so that the circulation volume of hydrogen reaches 50-60 tons / hour. At the same time, ensure that the cold box precooling rate is no more than 25℃ / h, and ensure that the temperature difference between the cold medium and the hot medium of the cold box is limited to <25℃. When hydrogen is introduced, ensure that the purity of hydrogen reaches more than 85%, in order to avoid impure hydrogen such as C1 components from liquefying when the cold box is cooled to -100℃, blocking the plate fins of the cold box, and also to avoid the cold box outlet pressure reduction caused by the liquefaction of heavy components in hydrogen, which affects the insufficient inlet pressure of the cracking gas compressor.

[0083] Under the premise of controlling the cooling rate, the process system is continuously pressurized by continuously introducing hydrogen from the operating device to the quench tower, and the pressure of the deethanizer is controlled at 1.5Mpa, the pressure of the demethanizer is controlled at 3.0Mpa, and the pressure of the quench tower is controlled at 90Kpa through the valve adjustment in the system. When the pressure of each tower is stable, along with the continuous decrease in the temperature of each tower and the reflux tank, methane and ethylene are added to the inlet of the cracking gas compressor in turn, so that the demethanizer reflux tank, the demethanizer kettle, and the deethanizer reflux tank can accumulate enough liquid phase, and at the same time, the liquid phase mixed C3 is injected into the deethanizer to achieve full reflux of the demethanizer and the deethanizer.

[0084] Feeding the cracking furnace: After feeding the cracking furnace, open valve 7 to open the entire cracking process. After feeding the furnace, first adjust the reflux stability of the deethanizer from the deethanizer reflux tank.

[0085] After system adjustment: the deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy content (C3) at the top of the deethanizer is below 0.1%. When the content is qualified, the carbon two hydrogenator is turned on, and the carbon two hydrogenation is heated for 2 hours. Before the acetylene in the feed of the demethanizer drops to the lower limit of 500ppm, the tower kettle of the demethanizer can still normally collect the C2 component and enter the ethylene unqualified product spherical tank. The tank capacity of the ethylene unqualified product spherical tank is 6-10 hours of C2 production. Once the acetylene content in the demethanizer kettle drops below 500ppm, the valve 12 is opened to feed the ethylene tower, and the ethylene product is sent out of the boundary area, and the whole device is declared to be started successfully.

[0086] Previously, the unqualified ethylene with high acetylene content in the demethanizer kettle was returned to the deethanizer from the unqualified ethylene product spherical tank, reducing the start-up losses of the unit.

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

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

[0089] Step 2: Perform nitrogen replacement and natural gas replacement on all systems except the cracking section. The specific steps are as follows:

[0090] Close the cracking gas valve 7, and all valves for product delivery are closed ( Figure 2 All valves (not shown) in the entire process system, including valves 1, 2, 13, 14 in the cold zone, and valves in the cold zone outbound zone, are opened and connected in series. All valves for exhausting flare gas are closed ( Figure 2 Valve 3, valve 4) in the middle to form a closed system. After opening the nitrogen valve 11 and filling in nitrogen to increase the pressure to 0.7Mpa, open the flare valve (valve 3, valve 4) to release the pressure to a gauge pressure of 0Mpa, and repeat 2 more times to ensure that the oxygen content is less than 0.2%. Then start the natural gas replacement, also filling to 0.7Mpa, and then releasing the pressure to a gauge pressure of 0Mpa. The pressure increase replacement is repeated 3 times. The pressure increase replacement of natural gas is controllable and orderly discharged to the flare gas system. At the same time, the flare recovery compressor is activated and recovered to the fuel gas system for use by fuel gas users, which greatly saves the start-up cost.

[0091] Step 3: After the propylene refrigeration system introduces nitrogen into the boundary area to increase the pressure three times, it introduces gas-phase propylene from the boundary area to increase the pressure and replace it three times before use.

[0092] Step 4: After the ethylene tower is isolated from the process (close valves 13, 14 and 12), the ethylene refrigeration system is connected to the boundary area to increase the nitrogen pressure three times, and then the ethylene mutual supply line is used to draw gas phase ethylene from the ethylene product delivery line of the operating device to increase the pressure and replace it three times for standby use.

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

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

[0095] Step 6: Using the ethylene mutual supply line, ethylene is introduced from the ethylene product delivery line of the operating device to replace the ethylene refrigeration system and the ethylene tower (heat pump system). The ethylene refrigeration system is started with ethylene refrigerant, and after stable operation, the liquid is slowly filled, and the ethylene tower is established to reflux;

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

[0097] Using the hydrogen mutual supply line, hydrogen is introduced into the quenching tower, and the entire process system including quenching, compression, alkali washing, drying, deethaneization, carbon two hydrogenation, and demethanization is pressurized to 0.2Mpa (excluding the ethylene tower, which has been fully refluxed and disconnected from the demethanization tower valve). Start the cracking gas compressor and control the compressor inlet pressure at 90Kpa. At this time, the entire device forms a large cycle of hydrogen.

[0098] Step 8: Precooling of cold box:

[0099] Continuously add hydrogen from the operating device to the large cycle to make the circulation volume of hydrogen reach 50-60 tons / hour. At the same time, ensure that the precooling rate of the cold box is no more than 25℃ / h, and ensure that the temperature difference between the cold medium and the hot medium of the cold box is limited to <25℃. When starting to introduce hydrogen, ensure that the purity of hydrogen reaches more than 85%, in order to avoid impure hydrogen containing impurities such as C1 components, which will liquefy when cooled to -100℃ and block the plate fins of the cold box. At the same time, it also avoids the reduction of the outlet pressure of the cold box due to the liquefaction of the heavy components in the hydrogen, which affects the insufficient inlet pressure of the cracking gas compressor.

[0100] Under the premise of controlling the cooling rate, the process system is continuously pressurized by continuously introducing hydrogen from the operating device to the quench tower. The pressure of the deethanizer is controlled at 1.5Mpa, the pressure of the demethanizer is controlled at 0.65Mpa, and the pressure of the quench tower is controlled at 90Kpa through the valve adjustment in the system. When the pressure of each tower is stable, along with the continuous decrease in the temperature of each tower and the reflux tank, methane and ethylene are added to the inlet of the cracking gas compressor in turn, so that the demethanizer reflux tank, the demethanizer kettle, and the deethanizer reflux tank can accumulate enough liquid phase, and at the same time, the liquid phase mixed C3 is injected into the deethanizer to achieve full reflux of the demethanizer and the deethanizer.

[0101] Step nine, feeding the cracking furnace:

[0102] After the cracking furnace is charged, valve 7 is opened to open the entire cracking process. After charging, the deethanizer reflux tank is first used to adjust the deethanizer reflux stability.

[0103] Step 10, post-system adjustment:

[0104] The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy content (C3) of the deethanizer top is below 0.1%. When the content is qualified, the carbon two hydrogenator is opened, and the carbon two hydrogenation is heated for 2 hours. Before the acetylene in the feed of the demethanizer drops to the lower limit of 500ppm, the tower kettle of the demethanizer can still normally collect the C2 component and enter the ethylene unqualified product spherical tank. The tank capacity of the ethylene unqualified product spherical tank is 6-10 hours of C2 production. Once the acetylene content of the demethanizer kettle drops below 500ppm, the valve 12 is opened to feed the ethylene tower, and the ethylene product is sent out of the boundary area at the same time, and the whole device is declared to be started successfully.

[0105] Previously, the unqualified ethylene with high acetylene content in the demethanizer kettle was returned to the deethanizer from the unqualified ethylene product spherical tank, reducing the start-up losses of the unit.

Claims

1. An ethane cracking device, characterized in that: The invention comprises a cracking section, a compression section and a separation section which are connected in sequence. The cracking section comprises a cracking furnace and a quenching tower. The compression section comprises a cracking gas compressor, an alkali washing tower and a dryer. The separation section comprises a deethanizer, a cold zone and a hot zone. The deethanizer is used as a boundary. The top of the deethanizer is connected to the cold zone, and the bottom of the deethanizer is connected to the hot zone. The cold zone comprises a C2 hydrogenator, a demethanizer, an ethylene tower, a cold box and a refrigeration system. The top pipeline of the deethanizer is connected to the C2 hydrogenator. The discharge pipeline of the C2 hydrogenator is divided into two paths after being cooled by the cold box, one path outputs hydrogen, and the other path is connected to the demethanizer. The top of the demethanizer outputs methane, and the bottom of the tower is connected to the ethylene tower. The top of the ethylene tower outputs ethylene, and the bottom of the tower outputs ethane. The refrigeration system refrigerates the feed of the deethanizer, the discharge of the C2 hydrogenator, the materials at the top and bottom of the demethanizer, and the materials at the top and bottom of the ethylene tower.

2. An ethane cracking device 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 for the ethylene tower, the demethanizer, and the cold box, while the propylene refrigeration system provides cooling capacity for the deethanizer and the cold box.

3. An ethane cracking device according to claim 2, characterized in that: 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 out at the outlet of the cold box is connected to the flare gas system, and 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 turn. The fuel gas system is also connected to the discharge pipeline of the quench tower in the cracking section.

4. An ethane cracking device according to claim 3, characterized in that: It includes a start-up device and an operating device. Both the start-up device and the operating device include a cracking section, a compression section, and a separation section. The start-up device is connected to a flare gas system, a flare gas recovery system, and a fuel gas system. The operating device is connected to the start-up device to provide hydrogen, methane, and ethylene during the start-up phase of the start-up device.

5. An ethane cracking device according to claim 4, characterized in that: An ethylene mutual supply line is led from the tower top pipeline of the ethylene tower of the operating device to the start-up device and connected to the tower top pipeline of the ethylene tower in the start-up device.

6. An ethane cracking device according to claim 5, characterized in that: The top pipeline of the demethanizer of the operating unit leads the methane mutual supply line to the quenching tower of the started unit, and the ethylene mutual supply line is connected to the quenching tower of the started unit.

7. An ethane cracking device according to claim 6, characterized in that: The hydrogen product line and methane product line of the started unit are each equipped with a circulation line to the quenching tower of the started unit. At the same time, a hydrogen mutual supply line is also set from the operating unit to the started unit.

8. An ethane cracking device according to claim 7, characterized in that: It also includes a spherical tank for unqualified ethylene products. The bottom pipeline of the demethanization tower in the start-up device is connected to the spherical tank for unqualified ethylene products. The spherical tank for unqualified ethylene products is connected to the feed end of the deethanizer tower of the start-up device through an ethylene transfer pump.

9. The method for reducing emissions during the start-up of an ethane cracking device according to claim 8, characterized in that: Perform the following steps for the startup device: Step 1: The flare system and fuel gas system are put into use, and the flare recovery compressor is in standby mode; Step 2: nitrogen replacement and natural gas replacement are performed on all systems except the cracking section; Step 3, after the propylene refrigeration system is pressurized by introducing nitrogen, the gas phase propylene is pressurized and replaced for standby use; Step 4: After the ethylene refrigeration system and the ethylene tower are pressurized by introducing nitrogen, the ethylene product delivery line of the operating device is used to introduce gas-phase ethylene for pressure expansion and replacement for standby use; Step 5: The propylene refrigeration system is started to pre-cool the cold box and provide additional cold capacity; Step 6: The ethylene refrigeration system is started, adjusted to a stable state, filled with liquid phase ethylene, and the ethylene tower is refluxed; Step 7, starting the cracking gas compressor; Step 8: Precooling of cold box: Continuously replenish hydrogen from the operating device to the operating device to make the circulation volume of hydrogen reach 50-60 tons / hour, and ensure that the precooling rate of the cold box is no more than 25℃ / h, and ensure that the temperature difference between the cold medium and the hot medium in the cold box is limited to <25℃, and the purity of hydrogen reaches more than 85%; Under the premise of controlling the cooling rate, hydrogen is continuously introduced from the operating device to the quench tower, thereby continuously pressurizing the process system. The pressure of the deethanizer is controlled at 1.4Mpa, the pressure of the demethanizer is controlled at 3.0Mpa, and the pressure of the quench tower is controlled at 90Kpa through valve adjustment in the system. When the pressure of each tower is stable, along with the continuous decrease in the temperature of each tower and the reflux tank, methane and ethylene are added to the inlet of the cracking gas compressor in turn, so that the demethanizer reflux tank, the demethanizer kettle, and the deethanizer reflux tank can accumulate enough liquid phase, and at the same time, the liquid phase mixed C3 is injected into the deethanizer to achieve full reflux of the demethanizer and the deethanizer; Step nine, feeding the cracking furnace: After the cracking furnace is charged, the entire cracking process is opened. After the furnace is charged, the reflux tank of the deethanizer is first used to adjust the reflux stability of the deethanizer; Step 10, post-system adjustment: The deethanizer and demethanizer gradually establish stable reflux. During this process, the heavy content at the top of the deethanizer is below 0.1%. When the content is qualified, the C2 hydrogenator is turned on and the temperature is raised for 2 hours for C2 hydrogenation. Before the acetylene in the feed of the demethanizer drops to the lower limit of 500ppm, the bottom of the demethanizer can still collect C2 components normally and enter the ethylene unqualified product spherical tank. The capacity of the ethylene unqualified product spherical tank is 6-10 hours of C2 production. Once the acetylene content in the bottom of the demethanizer drops below 500ppm, the ethylene tower is fed and the ethylene product is sent out of the boundary area. The entire device is declared successfully started.

10. The method for reducing emissions during the start-up of an ethane cracking device according to claim 9, characterized in that: The specific steps of step 2 are: Close all valves for product delivery, open all valves in the entire process system, including quenching, compression, alkali washing, drying, deethaneization, carbon dihydrogenation, demethanization, and ethylene distillation, and connect them in series. Close all valves for discharging flare gas to form a closed system. Fill the closed system with nitrogen to increase the pressure to 0.7Mpa, then open the flare valve to release the pressure to 0Mpa. Repeat several times to ensure that the oxygen content is less than 0.2%, and then start natural gas replacement. Similarly, fill it to 0.7Mpa, then release the pressure to the gauge pressure of 0Mpa, and repeat the pressure increase and replacement several times. The pressure increase and replacement of natural gas is controllable and orderly discharged to the flare gas system. At the same time, activate the flare recovery compressor and recover it to the fuel gas system for use by fuel gas users.

11. The method for reducing emissions during the start-up of an ethane cracking device according to claim 9, characterized in that: The specific method of step six is: use the ethylene mutual supply line to introduce ethylene as the refrigerant to start the ethylene refrigeration system.

12. The method for reducing emissions during the start-up of an ethane cracking device according to claim 9, characterized in that: The specific method of step seven is: use the hydrogen mutual supply line to introduce hydrogen into the quenching tower, pressurize the entire process system including quenching, compression, alkali washing, drying, deethaneization, carbon two hydrogenation, demethanization to 0.2Mpa, start the cracking gas compressor, and control the compressor inlet pressure at 90Kpa. At this time, the whole device forms a large cycle of hydrogen.

13. The method for reducing emissions during the start-up of an ethane cracking device according to claim 9, characterized in that: Step 10: After the device is successfully started up, the unqualified ethylene with high acetylene content in the demethanizer kettle is returned from the unqualified ethylene product spherical tank to the deethanizer to reduce the start-up loss of the device.

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