System and method for reducing ethylene loss of light cracking raw material
By combining multi-stage cooling separation and expansion compressor units, the problem of high ethylene loss rate of light feedstock was solved, and an ethylene cracking process with high ethylene yield and low energy consumption was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-30
- Publication Date
- 2026-04-07
AI Technical Summary
During ethylene cracking, the reduction of light feedstocks leads to high ethylene loss rates, affecting the economic efficiency of the plant and potentially causing coking in dryers and reactors. Existing methods increase equipment investment and energy consumption.
A multi-stage cooling and separation unit and an expansion compressor unit are used to separate a liquid phase rich in methane and a gas phase rich in hydrogen through staged cooling and separation. The cold energy is recovered by expansion refrigeration to reduce ethylene loss, and the separation efficiency of the demethanizer is improved by circulating and enriching methane.
It effectively reduces the ethylene loss rate from 0.4% to 0.1%, thereby reducing equipment investment and energy consumption, and improving ethylene yield and the economic benefits of the plant.
Smart Images
Figure CN121801599A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of petrochemical technology, and in particular to a system and method for reducing ethylene loss in light cracking feedstocks. Background Technology
[0002] In the existing ethylene cracking industry, the reduction of feedstock weight is an important development direction. Lighter feedstock can increase ethylene yield, reduce energy consumption, decrease carbon emissions, and improve the economic efficiency of the plant. However, with the reduction of feedstock weight, the hydrogen content in the cracking products increases. Under the same operating process, this leads to a higher ethylene loss rate at the top of the demethanizer. A high ethylene loss rate reduces the economic efficiency of the plant, and the lost ethylene mixed with fuel gas (methane hydrogen) can cause coking and blockage at the cracking furnace burners. As methane hydrogen is used as regeneration gas for dryers or reactors in ethylene plants, the increased ethylene content can cause coking in the dryer and reactor beds during the regeneration heating process, affecting the lifespan of the dryer and catalyst. Therefore, controlling ethylene loss is crucial for the long-term operation of ethylene plants.
[0003] Traditional approaches to this process include increasing the outlet pressure of the cracked gas compressor to maintain the condensation temperature at the top of the demethanizer and thus increase its operating pressure. However, this increases the wall thickness of valves and equipment along the pipeline from the compressor outlet to the demethanizer, particularly raising the pressure rating of pipeline valves from 300 psi to 600 psi, leading to increased investment and energy consumption. Another approach is to lower the demethanizer condensation temperature. The classic process at the top of the demethanizer uses -100°C ethylene refrigerant as the condensation energy source. Since the lowest temperature ethylene refrigerant can reach is -100°C, further lowering the condensation temperature requires adding a methane refrigeration compressor to provide refrigerant below -100°C, increasing equipment investment and energy consumption. Therefore, reducing ethylene loss in the methane hydrogen from the cracking of light feedstocks is a pressing technical problem that needs to be solved. Summary of the Invention
[0004] This application provides a system and method for reducing ethylene loss in light pyrolysis feedstocks to solve the following technical problem: how to reduce ethylene loss in methane hydrogen during light feedstock pyrolysis.
[0005] In a first aspect, embodiments of this application provide a system for reducing ethylene loss in light pyrolysis feedstocks, the system comprising: A compression unit, wherein the inlet of the compression unit is fed with pyrolysis gas to pressurize the pyrolysis gas; A multi-stage cooling and separation unit includes at least three stages of coolers and corresponding separation tanks. The inlet of the multi-stage cooling and separation unit is connected to the outlet of the compression unit, and is used to cool the cracked gas stage by stage and separate the liquid phase stage by stage. The last stage separates a liquid phase rich in methane and a gas phase rich in hydrogen. The demethanizing tower has its inlet connected to the liquid phase outlet of each stage in the multi-stage cooling and separation unit, and is used to demethanize the liquid phase to obtain methane-rich gas. The first expansion compressor unit is connected to the gas phase outlet of the last stage of the multi-stage cooling and separation unit and the top gas outlet of the demethanizing tower. It is used to perform two-stage expansion refrigeration and compression treatment on the hydrogen-rich gas phase and the first part of the methane-rich gas to obtain methane hydrogen product. The second expansion compressor unit has its inlet connected to the top gas outlet of the demethanizer tower and its outlet connected to the inlet of the compression unit. It is used to pressurize the second portion of methane-rich gas and return it to the middle of the compression unit, where it is mixed with cracked gas.
[0006] Optionally, the multi-stage cooling separation unit includes: The first demethanizer feed cooler has its inlet connected to the outlet of the compression section, and is used to cool the cracked gas to -40°C to -32°C using propylene refrigerant. The first demethanizer feed separator has an inlet connected to the gas phase outlet of the first demethanizer feed cooler, and a liquid phase outlet connected to the bottom feed inlet of the demethanizer. The second demethanizer feed cooler has its inlet connected to the gas phase outlet of the first demethanizer feed separator, and is used to cool the gas phase to -52°C to -44°C. The inlet of the second demethanizer feed separator is connected to the outlet of the second demethanizer feed cooler, and the liquid phase outlet of the second demethanizer feed separator is connected to the lower feed inlet of the demethanizer. The third demethanizer feed cooler, the inlet of which is connected to the gas phase outlet of the second demethanizer feed separator, is used to cool the gas phase to -75°C to -70°C using ethylene refrigerant. The inlet of the third demethanizer feed separator is connected to the outlet of the third demethanizer feed cooler, and the liquid phase outlet of the third demethanizer feed separator is connected to the middle layer feed inlet of the demethanizer. The fourth demethanizer feed cooler is connected to the gas phase outlet of the third demethanizer feed separator, and is used to cool the gas phase to -102°C to -94°C using ethylene refrigerant. The fourth demethanizer feed separator has an inlet connected to the outlet of the fourth demethanizer feed cooler, and a liquid phase outlet connected to the upper feed inlet of the demethanizer. The low-temperature heat exchange unit has its first inlet connected to the gas phase outlet of the feed separator of the fourth demethanizer, in order to cryogenically cool the gas phase to ≤-130℃. The fifth demethanizer feed separator has its inlet connected to the first outlet of the cryogenic heat exchange unit, and its liquid phase outlet connected to the second inlet of the cryogenic heat exchange unit for reheating the liquid phase to -103°C to -95°C. The second outlet of the cryogenic heat exchange unit is connected to the uppermost feed inlet of the demethanizer, and the gas phase outlet of the fifth demethanizer feed separator is connected to the inlet of the first expansion compressor unit.
[0007] Optionally, the first expander compressor unit includes: The first expander has a first inlet connected to the gas phase outlet of the feed separator of the fifth demethanizer, a second inlet connected to the second gas phase outlet at the top of the demethanizer, and an outlet connected to the third inlet of the low-temperature heat exchange unit to provide cooling capacity. The first compressor has its inlet connected to the third outlet of the low-temperature heat exchange unit, and its outlet is used to output methane hydrogen products. The first expander is coaxially connected to the first compressor.
[0008] Optionally, the inlet pressure of the first expander is 2.0 MPaG to 3.8 MPaG, the outlet pressure of the first expander is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the first compressor is 0.1 MPaG to 0.3 MPaG, and the outlet pressure of the first compressor is 0.3 MPaG to 0.5 MPaG.
[0009] Optionally, the second expander compressor unit includes: The second expander has its inlet connected to the first gas phase outlet at the top of the demethanizer, and its outlet connected to the fourth inlet of the low-temperature heat exchange unit to provide cooling capacity. The second compressor has its inlet connected to the fourth outlet of the low-temperature heat exchange unit, and its outlet connected to the middle inlet of the compression unit. The second expander is coaxially connected to the second compressor.
[0010] Optionally, the inlet pressure of the second expander is 2.8 MPaG to 3.1 MPaG, the outlet pressure of the second expander is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the second compressor is 0.3 MPaG to 0.5 MPaG, and the outlet pressure of the second compressor is 0.6 MPaG to 1.0 MPaG.
[0011] Optionally, the operating pressure of the demethanizer is 2.8 MPaG to 3.1 MPaG, and the top condensation temperature of the demethanizer is -90℃ to -99℃.
[0012] Secondly, embodiments of this application provide a method for reducing ethylene loss in light pyrolysis feedstocks, the method being adaptable to the system described in any embodiment of the first aspect, the method comprising: Pressurize the pyrolysis gas; The pyrolysis gas is cooled in stages and separated into liquid phases in stages, with the last stage separating a liquid phase rich in methane and a gas phase rich in hydrogen. The liquid phase is subjected to demethanization separation to obtain methane-rich gas; The hydrogen-rich gas phase and the first part of the methane-rich gas are subjected to two-stage expansion refrigeration and compression treatment to obtain the methane hydrogen product. The second portion of methane-rich gas is pressurized and returned to the middle of the compression unit, where it is mixed with cracked gas.
[0013] Optionally, the flow rate of the first portion of methane-rich gas is 75% to 85% of the total flow rate of the methane-rich gas; The flow rate of the second part of the methane-rich gas is 15% to 25% of the total flow rate of the methane-rich gas.
[0014] Optionally, the volume fraction of hydrogen in the hydrogen-rich gas phase is ≥90%; The volume fraction of hydrogen in the methane-rich liquid phase is ≤2%; The volume fraction of ethylene in the methane hydrogen product is ≤0.2%.
[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a system for reducing ethylene loss in light pyrolysis feedstock. The system includes: a compression unit, the inlet of which is fed with pyrolysis gas to pressurize it; a multi-stage cooling and separation unit, including at least three coolers and corresponding separation tanks, the inlet of which is connected to the outlet of the compression unit, for progressively cooling the pyrolysis gas and progressively separating the liquid phase, wherein the last stage separates a methane-rich liquid phase and a hydrogen-rich gas phase; and a demethanizer, the inlet of which is connected to the liquid phase outlet of each stage in the multi-stage cooling and separation unit. The liquid phase is used for demethanization separation to obtain methane-rich gas; a first expansion compressor unit, which is connected to the gas phase outlet of the last stage of the multi-stage cooling and separation unit and the top gas outlet of the demethanization tower, is used to perform two-stage expansion refrigeration and compression treatment on the hydrogen-rich gas phase and the first part of the methane-rich gas to obtain methane hydrogen product; a second expansion compressor unit, whose inlet is connected to the top gas outlet of the demethanization tower and whose outlet is connected to the inlet of the compression unit, is used to pressurize the second part of the methane-rich gas and return it to the middle of the compression unit and mix it with cracked gas.
[0016] The multi-stage cooling and separation unit uses a combination of at least three coolers and separation tanks to progressively cool the pressurized cracked gas. The first-stage cooling and separation stage can gradually separate the liquid phase rich in C2 and above components such as ethylene and ethane, reducing the amount of ethylene entering the subsequent cryogenic system; the final stage separation uses deep cooling to precisely separate the liquid phase rich in methane (where ethylene is mainly retained) and the gas phase rich in hydrogen (reducing the entrainment of ethylene in hydrogen), thereby reducing the possibility of ethylene entering the methane hydrogen product with hydrogen from the source; Meanwhile, the liquid phase from each separation tank enters the demethanizer in stages, and the deep separation of methane and ethylene is achieved through distillation. The bottom of the tower produces the target component rich in ethylene, while the top of the tower only produces methane-rich gas (containing a small amount of ethylene), thus avoiding excessive enrichment of ethylene in the methane hydrogen product. In addition, the first expansion compressor unit processes the hydrogen-rich gas phase and the first part of the methane-rich gas separated in the last stage. While recovering the cold energy through expansion refrigeration, it compresses the gas into methane hydrogen product, reducing the volatilization loss of ethylene under low pressure. The second expansion compressor unit pressurizes the second part of the methane-rich gas and returns it to the middle of the compression unit, forming a methane circulation enrichment effect, increasing the methane concentration in the feed of the demethanizer, enhancing the separation efficiency of ethylene and methane in the tower, and indirectly reducing the amount of ethylene entrained in the methane-rich gas at the top of the tower.
[0017] Therefore, the system provided in this application reduces the loss of ethylene with hydrogen from both separation accuracy and circulation optimization, ultimately reducing the loss of ethylene in methane hydrogen products. Attached Figure Description
[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.
[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0020] Figure 1 This is a schematic diagram of a system for reducing ethylene loss in light pyrolysis feedstock, provided in Embodiment 1 of this application. Figure 2 A schematic flowchart illustrating a method for reducing ethylene loss in light pyrolysis feedstock, provided as an embodiment of this application; Figure 3 This is a schematic diagram of a system for reducing ethylene loss in light pyrolysis feedstock, provided in Embodiment 2 of this application. Figure 4 This is a schematic diagram of a system for reducing ethylene loss in light pyrolysis feedstock, provided in Embodiment 3 of this application. Figure label: 1-Compression unit, 2-Multi-stage cooling and separation unit, 201-First demethanizer feed cooler, 202-First demethanizer feed separator, 203-Second demethanizer feed cooler, 204-Second demethanizer feed separator, 205-Third demethanizer feed cooler, 206-Third demethanizer feed separator, 207-Fourth demethanizer feed cooler, 208-Fourth demethanizer feed separator, 209-Cryogenic heat exchange unit, 210-Fifth demethanizer feed separator, 3-Demethanizer, 4-First expansion compressor unit, 41-First expander, 42-First compressor, 5-Second expansion compressor unit, 51-Second expander, 52-Second compressor, 6-Deethanerizer, 7-Cracked gas compressor, 8-Deethanerizer reflux tank. Detailed Implementation
[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0022] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.
[0023] Figure 1 This is a schematic diagram of a system for reducing ethylene loss from light pyrolysis feedstock, provided in an embodiment of this application.
[0024] like Figure 1 As shown in the figure, this application provides a system for reducing ethylene loss in light pyrolysis feedstock, the system comprising: Compression unit 1, the inlet of which is fed with pyrolysis gas to pressurize the pyrolysis gas; The multi-stage cooling and separation unit 2 includes at least three stages of coolers and corresponding separation tanks. The inlet of the multi-stage cooling and separation unit 2 is connected to the outlet of the compression unit 1. It is used to cool the cracked gas stage by stage and separate the liquid phase stage by stage. The last stage separates the liquid phase rich in methane and the gas phase rich in hydrogen. The demethanizer 3 has its inlet connected to the liquid phase outlet of each stage in the multi-stage cooling and separation unit 2, and is used to demethanize the liquid phase to obtain methane-rich gas. The first expansion compressor unit 4 is connected to the gas phase outlet of the last stage of the multi-stage cooling and separation unit 2 and the top gas outlet of the demethanizer tower 3. It is used to perform two-stage expansion refrigeration and compression treatment on the hydrogen-rich gas phase and the first part of the methane-rich gas to obtain methane hydrogen product. The second expansion compressor unit 5 has its inlet connected to the top gas outlet of the demethanizer tower 3 and its outlet connected to the inlet of the compression unit 1. It is used to pressurize the second part of the methane-rich gas and return it to the middle of the compression unit 1 and mix it with the cracked gas.
[0025] It should be noted that compression unit 1 receives cracked gas with a high hydrogen content, and pressurizes it through a compressor and auxiliary separation equipment to provide a sufficient pressure basis for subsequent cryogenic cooling separation. After increasing the pressure of the cracked gas, compression unit 1 can reduce the temperature requirement for subsequent cooling separation and reduce cooling energy consumption; at the same time, it separates some heavy components, so that the outlet gas is mainly composed of light components of C2 and above, simplifying the subsequent separation process.
[0026] The feed coolers for the first to fourth demethanizers progressively lower the temperature using propylene refrigerant (-40℃ to -32℃) and ethylene refrigerant (-52℃ to -44℃, -75℃ to -70℃, -102℃ to -94℃), utilizing the boiling point differences of different components (e.g., ethylene boiling point -103.7℃, ethane -88.6℃) to achieve initial separation. The first to fourth stage separators separate the cooled gas-liquid mixture. The liquid phase (rich in ethylene, ethane, and other C2 and above components) enters the demethanizer 3 (from the bottom to the top) in stages, while the gas phase (rich in hydrogen and methane) enters the next stage for deep cooling, preventing heavy components from interfering with subsequent low-temperature separation. The low-temperature heat exchange unit 209 deeply cools the gas phase in the fourth separator to ≤-130℃, far exceeding the -100℃ limit of the ethylene refrigerant, causing a large amount of methane (boiling point -161.5℃) to condense, while hydrogen (boiling point -252.8℃) remains in the gas phase. The fifth separator separates a hydrogen-rich gas phase (hydrogen ≥ 90%) and a methane-rich liquid phase (hydrogen ≤ 2%). The former enters the first expander group 41, while the latter is heated back and then added to the demethanizer 3, which greatly reduces the interference of hydrogen on the separation of the demethanizer 3.
[0027] Demethanizer 3 performs distillation separation on the liquid phase of each stage of the separator. The top of the tower produces methane-rich gas (containing a small amount of ethylene), and the bottom of the tower produces C2 and above components (target products such as ethylene and ethane).
[0028] The first expansion compressor unit 4 consists of a first expander 41 and a first compressor 42. The first expander 41 receives the hydrogen-rich gas phase from the fifth separator and the first portion of the methane-rich gas from the top of the demethanizer 3. Through expansion and depressurization (inlet 2.0–3.8 MPaG → outlet 0.1–0.8 MPaG), it generates low-temperature cooling capacity to supply cooling for the low-temperature heat exchange unit 209, thus achieving cold energy recovery. The first compressor 42 is coaxially connected to the expander. It uses the energy output from the expander to compress the expanded low-pressure gas (outlet 0.3–0.5 MPaG) and collect it as methane hydrogen product, avoiding the emission of ethylene with the low-pressure waste gas, thereby preventing the loss of ethylene products.
[0029] The second expander compressor unit 5 consists of a second expander 51 and a second compressor 52. The second expander 51 receives the second portion of methane-rich gas (accounting for 75% to 85% of the total) from the top of the demethanizer 3, expands and cools it to supply cooling to the low-temperature heat exchange unit 209, and further recovers the cold energy. The second compressor 52 compresses the expanded gas (outlet 0.6 to 1.0 MPaG) and returns it to the middle of the compression unit 1, increasing the methane concentration in the feed of the demethanizer 3, improving the separation efficiency of methane and ethylene in the tower, and forming a cyclic enrichment effect.
[0030] In some embodiments, the multi-stage cooling separation unit 2 includes: First demethanizer feed cooler 201, the inlet of which is connected to the outlet of the compression section, is used to cool the cracked gas to -40°C to -32°C using propylene refrigerant. The first demethanizer feed separator 202 has its inlet connected to the gas phase outlet of the first demethanizer feed cooler 201, and its liquid phase outlet connected to the bottom feed inlet of the demethanizer 3. The second demethanizer feed cooler 203 has its inlet connected to the gas phase outlet of the first demethanizer feed separator 202, and is used to cool the gas phase to -52°C to -44°C. The inlet of the second demethanizer feed separator 204 is connected to the outlet of the second demethanizer feed cooler 203, and the liquid phase outlet of the second demethanizer feed separator 204 is connected to the lower feed inlet of the demethanizer 3. The third demethanizer feed cooler 205 is connected to the gas phase outlet of the second demethanizer feed separator 204, and is used to cool the gas phase to -75°C to -70°C using ethylene refrigerant. The third demethanizer feed separator 206 has its inlet connected to the outlet of the third demethanizer feed cooler 205, and its liquid phase outlet connected to the middle layer feed inlet of the demethanizer 3. The fourth demethanizer feed cooler 207 is connected to the gas phase outlet of the third demethanizer feed separator 206 at its inlet, and is used to cool the gas phase to -102°C to -94°C using ethylene refrigerant. The fourth demethanizer feed separator 208 has its inlet connected to the outlet of the fourth demethanizer feed cooler 207, and its liquid phase outlet connected to the upper feed inlet of the demethanizer 3. The low-temperature heat exchange unit 209 has its first inlet connected to the gas phase outlet of the fourth demethanizer feed separator 208, in order to cryogenically cool the gas phase to ≤-130℃. The fifth demethanizer feed separator 210 has its inlet connected to the first outlet of the low-temperature heat exchange unit 209, and its liquid phase outlet connected to the second inlet of the low-temperature heat exchange unit 209, for warming the liquid phase to -103°C to -95°C. The second outlet of the low-temperature heat exchange unit 209 is connected to the uppermost feed inlet of the demethanizer 3, and the gas phase outlet of the fifth demethanizer feed separator 210 is connected to the inlet of the first expansion compressor unit 4.
[0031] In this embodiment, the liquid phases from the first four stages of cooling and separation enter different stages of the demethanizer 3 according to their boiling points from high to low (ethane → ethylene), and come into countercurrent contact with the rising gas phase inside the tower, thereby improving the ethylene recovery rate. The methane-rich liquid phase from the fifth separator is replenished from the top of the tower after being warmed up, enhancing the condensation effect at the top of the tower and reducing ethylene entrainment. The cooling energy generated by the expander from the hydrogen-rich gas phase in the fifth separator and the methane-rich gas at the top of the tower is entirely used in the low-temperature heat exchange unit 209, eliminating the need for an additional methane refrigeration compressor, achieving "cooling self-sufficiency," and reducing energy consumption.
[0032] The first to fourth coolers cool the gas phase to -40℃ to -32℃, -52℃ to -44℃, -75℃ to -70℃, and -102℃ to -94℃ respectively, gradually reducing the temperature and separating liquid phase components with different boiling points to prepare for subsequent deep cooling. The low-temperature heat exchange unit 209 deeply cools the gas phase to ≤-130℃, achieving full condensation of methane and enrichment of hydrogen, significantly reducing the hydrogen content in the liquid phase and reducing the risk of ethylene loss with hydrogen.
[0033] In some embodiments, the operating temperature of the fifth demethanizer feed separator 210 is ≤-130°C.
[0034] The operating temperature of the feed separator 210 of the fifth demethanizer is limited to ≤-130℃, which can ensure the effective separation of the methane-rich liquid phase and the hydrogen-rich gas phase. The methane-rich liquid phase is warmed to -103℃ to -95℃ before entering the demethanizer 3, which avoids the impact of low temperature on the tower body and can also replenish the condensate at the top of the tower.
[0035] In some embodiments, the first expander compressor unit 4 includes: The first expander 41 has a first inlet connected to the gas phase outlet of the feed separator 210 of the fifth demethanizer, a second inlet connected to the second gas phase outlet at the top of the demethanizer 3, and an outlet connected to the third inlet of the low-temperature heat exchange unit 209, for providing cooling capacity. The first compressor 42 has its inlet connected to the third outlet of the low-temperature heat exchange unit 209, and its outlet is used to output methane hydrogen products. The first expander 41 is coaxially connected to the first compressor 42.
[0036] In some embodiments, the inlet pressure of the first expander 41 is 2.0 MPaG to 3.8 MPaG, the outlet pressure of the first expander 41 is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the first compressor 42 is 0.1 MPaG to 0.3 MPaG, and the outlet pressure of the first compressor 42 is 0.3 MPaG to 0.5 MPaG.
[0037] The first expander 41 has an inlet pressure of 2.0–3.8 MPaG and an outlet pressure of 0.1–0.8 MPaG, achieving expansion and refrigeration through a large pressure difference to provide cooling capacity for the low-temperature heat exchange unit 209. The first compressor 42 has an inlet pressure of 0.1–0.3 MPaG and an outlet pressure of 0.3–0.5 MPaG, compressing the expanded low-pressure gas to a suitable pressure for output as a methane hydrogen product. For example, the inlet pressure of the first expander 41 can be 2.0 MPaG, 2.4 MPaG, 2.8 MPaG, 3.0 MPaG, 3.3 MPaG, 3.5 MPaG, 3.7 MPaG, 3.8 MPaG, etc. The outlet pressure of the first expander 41 can be 0.1 MPaG, 0.2 MPaG, 0.3 MPaG, 0.4 MPaG, 0.5 MPaG, 0.6 MPaG, 0.7 MPaG, 0.8 MPaG, etc. The inlet pressure of the first compressor 42 can be 0.1 MPaG, 0.15 MPaG, 0.2 MPaG, 0.22 MPaG, 0.25 MPaG, 0.27 MPaG, 0.29 MPaG, 0.3 MPaG, etc. The outlet pressure of the first compressor 42 can be 0.3 MPaG, 0.35 MPaG, 0.4 MPaG, 0.43 MPaG, 0.45 MPaG, 0.47 MPaG, 0.49 MPaG, 0.5 MPaG, etc.
[0038] In some embodiments, the second expander compressor unit 5 includes: The second expander 51 has its inlet connected to the first gas phase outlet at the top of the demethanizer 3, and its outlet connected to the fourth inlet of the low-temperature heat exchange unit 209, for providing cooling capacity. The second compressor 52 has its inlet connected to the fourth outlet of the low-temperature heat exchange unit 209, and its outlet connected to the middle inlet of the compression unit 1. The second expander 51 is coaxially connected to the second compressor 52.
[0039] In some embodiments, the inlet pressure of the second expander 51 is 2.8 MPaG to 3.1 MPaG, the outlet pressure of the second expander 51 is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the second compressor 52 is 0.3 MPaG to 0.5 MPaG, and the outlet pressure of the second compressor 52 is 0.6 MPaG to 1.0 MPaG.
[0040] The inlet pressure of the second expander 51 is limited to 2.8-3.1 MPaG (matching the top pressure of the demethanizer 3) and the outlet pressure is 0.1-0.8 MPaG. It also generates cooling capacity through expansion. The inlet pressure of the second compressor 52 is 0.3-0.5 MPaG and the outlet pressure is 0.6-1.0 MPaG. It compresses the gas to match the pressure in the middle of the compression unit 1, so as to facilitate circulation back to the system.
[0041] For example, the inlet pressure of the second expander 51 can be 2.8 MPaG, 2.85 MPaG, 2.9 MPaG, 2.95 MPaG, 3.0 MPaG, 3.03 MPaG, 3.07 MPaG, 3.1 MPaG, etc. The outlet pressure of the second expander 51 can be 0.1 MPaG, 0.25 MPaG, 0.4 MPaG, 0.5 MPaG, 0.6 MPaG, 0.7 MPaG, 0.75 MPaG, 0.8 MPaG, etc. The inlet pressure of the second compressor 52 can be 0.3 MPaG, 0.35 MPaG, 0.4 MPaG, 0.42 MPaG, 0.45 MPaG, 0.48 MPaG, 0.49 MPaG, 0.5 MPaG, etc. The outlet pressure of the second compressor 52 can be 0.6MPaG, 0.65MPaG, 0.7MPaG, 0.75MPaG, 0.8MPaG, 0.85MPaG, 0.9MPaG, 1.0MPaG, etc.
[0042] In this embodiment, the expander and compressor are coaxially connected. The mechanical energy generated during the expansion process directly drives the compressor, reducing external energy consumption (such as motor drive) and saving more than 30% energy compared to the traditional process. The second expander group 51 circulates the methane-rich gas back to the compression unit 1, increasing the methane concentration in the feed of the demethanizer tower 3 (forming a "methane enrichment cycle"), which increases the methane partial pressure in the tower, suppresses ethylene volatilization, and forms a "double insurance" with low-temperature separation, reducing the ethylene loss rate from 0.4% to 0.1%.
[0043] In some embodiments, the operating pressure of the demethanizer 3 is 2.8 MPaG to 3.1 MPaG, and the top condensation temperature of the demethanizer 3 is -90℃ to -99℃.
[0044] The operating pressure of the demethanizer tower 3 is limited to 2.8–3.1 MPaG. This pressure range ensures efficient separation of methane and ethylene while avoiding a surge in equipment investment due to excessive pressure. The tower top condensation temperature is limited to -90℃ to -99℃, matching the pressure to ensure effective condensation and separation of methane-rich gas and reduce ethylene entrainment.
[0045] For example, the operating pressure of the demethanizer 3 can be 2.8 MPaG, 2.88 MPaG, 2.92 MPaG, 2.95 MPaG, 3.0 MPaG, 3.05 MPaG, 3.08 MPaG, 3.1 MPaG, etc. The top condensing temperature of the demethanizer 3 can be -90℃, -92℃, -94℃, -95℃, -96℃, -97℃, -98℃, -99℃, etc.
[0046] In this embodiment, the compression unit 1 pressurizes, the demethanizer 3 operates at medium pressure (2.8-3.1 MPaG), and the cryogenic heat exchange unit 209 operates at ultra-low temperature (≤-130℃). This not only avoids increasing the pressure of all pipelines and valves by 300 to 600 pounds, but also eliminates the need for an additional methane refrigeration system, achieving efficient separation at low cost.
[0047] Figure 2 This is a schematic flowchart of a method for reducing ethylene loss in light pyrolysis feedstock, provided in an embodiment of this application.
[0048] like Figure 2 As shown, this application provides a method for reducing ethylene loss in light pyrolysis feedstocks. The method is adaptable to the system described in any of the above embodiments, and includes: Pressurize the pyrolysis gas; The pyrolysis gas is cooled in stages and separated into liquid phases in stages, with the last stage separating a liquid phase rich in methane and a gas phase rich in hydrogen. The liquid phase is subjected to demethanization separation to obtain methane-rich gas; The hydrogen-rich gas phase and the first part of the methane-rich gas are subjected to two-stage expansion refrigeration and compression treatment to obtain the methane hydrogen product. The second portion of methane-rich gas is pressurized and returned to the middle of the compression unit, where it is mixed with cracked gas.
[0049] In some embodiments, the flow rate of the first portion of the methane-rich gas is 75% to 85% of the total flow rate of the methane-rich gas; The flow rate of the second part of the methane-rich gas is 15% to 25% of the total flow rate of the methane-rich gas.
[0050] The first part of the methane-rich gas is processed as a product, while the second part is recycled back into the system. This ensures product output while also increasing the methane concentration in the demethanizer feed through recycling, thus optimizing separation efficiency.
[0051] For example, the first part of the methane-rich gas flow rate can be 75%, 77%, 79%, 80%, 82%, 83%, 84%, 85%, etc. The second part of the methane-rich gas flow rate can be 15%, 17%, 19%, 20%, 22%, 23%, 24%, 25%, etc.
[0052] In some embodiments, the volume fraction of hydrogen in the hydrogen-rich gas phase is ≥90%; The volume fraction of hydrogen in the methane-rich liquid phase is ≤2%; The volume fraction of ethylene in the methane hydrogen product is ≤0.2%.
[0053] A hydrogen fraction of ≥90% in the hydrogen-rich gas phase indicates good hydrogen separation, reducing interference from components such as methane in subsequent processing and minimizing ethylene entrainment in the hydrogen. A hydrogen fraction of ≤2% in the methane-rich liquid phase ensures a high methane concentration in the liquid phase entering the demethanizer, reducing separation difficulty and improving ethylene recovery. An ethylene volume fraction of ≤0.2% in the methane-hydrogen product directly reflects the system's effectiveness in reducing ethylene loss, significantly lower than the 0.4% of traditional methods.
[0054] This application addresses the issue of ethylene loss due to high hydrogen content in the cracking of light feedstocks, which leads to hydrogen loss from methane. It reduces ethylene loss through three core aspects: improved separation precision, efficient utilization of cold energy, and material recycling and enrichment. The specific mechanisms are as follows: (1) Ultra-low temperature deep separation, precise ethylene interception, and reduced hydrogen entrainment: Multi-stage cooling pre-separation and step-by-step methane concentration. The first four cooling separation units use propylene refrigerant (-40℃ to -32℃), ethylene refrigerant (-52℃ to -44℃, -75℃ to -70℃, -102℃ to -94℃) to gradually lower the temperature. Utilizing the boiling point differences between ethane (boiling point -88.6℃), ethylene (boiling point -103.7℃), methane (boiling point -161.5℃), and hydrogen (boiling point -252.8℃), the liquid phase rich in C2 and above components is gradually separated, reducing the amount of ethylene entering the subsequent cryogenic system. The liquid phase enters the demethanizer in stages, contacting the rising gas phase in the countercurrent direction, improving the ethylene recovery rate at the bottom of the tower. Simultaneously, ultra-low temperature cryogenic enrichment separates hydrogen-rich and methane-rich gases. The low-temperature heat exchange unit cryogenically cools the gas phase after the fourth-stage separation to ≤-130℃, far exceeding the limit temperature of ethylene refrigerant, causing a large amount of methane to condense into a liquid phase, while the hydrogen remains in the gas phase, achieving the desired effect through the fifth separation tank. (2) Expander unit works together to recover cold energy and reduce energy consumption and secondary losses: The first expander compressor unit processes the hydrogen-rich gas phase to reduce ethylene emissions; at the same time, the hydrogen-rich gas phase from the fifth separator and the first part of the methane-rich gas at the top of the demethanizer (accounting for 15% to 25% of the total) enter the first expander, and generate low-temperature cold energy through expansion and pressure reduction (inlet 2.0 to 3.8 MPaG → outlet 0.1 to 0.8 MPaG) to supply cooling for the low-temperature heat exchange unit and realize cold energy recovery; in addition, the expanded gas is compressed into methane hydrogen product (ethylene ≤ 0.1%) by the coaxial compressor to avoid ethylene being emitted with the waste gas under low pressure. The second expansion compressor unit circulates methane-rich gas to improve separation efficiency. The second portion of methane-rich gas (accounting for 75% to 85% of the total) at the top of the demethanizer enters the second expander. After expansion and cooling, it supplies cooling to the low-temperature heat exchange unit. Then, it is compressed by the compressor and returned to the middle of the compression unit, forming a "methane enrichment cycle". The circulating gas increases the methane concentration in the feed to the demethanizer, increases the methane partial pressure in the tower, inhibits ethylene volatilization (according to Henry's Law, an increase in methane concentration can reduce the solubility of ethylene in the gas phase), and reduces ethylene entrainment at the top of the tower.
[0055] (3) Optimization of the demethanizer operation, balancing pressure and temperature, and enhancing separation effect: The demethanizer is designed with medium-pressure operation (2.8~3.1MPaG) and top condensation temperature (-90℃~-99℃). The medium-pressure environment increases the relative volatility of methane and ethylene, reducing the loss of ethylene with the top gas. The top temperature is matched with the methane-rich liquid phase (-103℃ to -95℃) after the low-temperature heat exchange unit is warmed up, enhancing the condensation effect and further retaining ethylene. C2 and above components are produced at the bottom of the tower, avoiding secondary losses caused by ethylene retention in the tower.
[0056] (4) From “passive emission reduction” to “active recovery”, through the above design, the ethylene content in the methane hydrogen product is reduced from 0.4% in the traditional process to ≤0.1%, which is manifested in the following ways: reducing direct losses, the ultra-low temperature separation reduces the amount of ethylene entrained in the hydrogen-rich gas phase, and the first expander unit directly outputs methane hydrogen products with low ethylene content after processing; reducing indirect losses, the methane recycling enrichment improves the separation efficiency of the demethanizer tower, and the ethylene recovery rate at the bottom of the tower is improved; low cost is achieved, without the need to add a methane refrigeration compressor or upgrade the equipment pressure level, and through cold energy recovery and circulation design, high-efficiency separation is achieved while reducing energy consumption.
[0057] In summary, this application addresses the problem of ethylene loss in high-hydrogen environments by employing a synergistic process of "staged pre-separation → ultra-low temperature cryogenics → cyclic enrichment → cold energy recovery," making it particularly suitable for the retrofitting of ethylene plants with lighter feedstocks.
[0058] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.
[0059] In this embodiment, the outlet components of compression unit 1 include hydrogen, methane, ethylene, ethane, C3, etc., as shown in Table 1: Table 1 Composition at the outlet of the compression unit
[0060] The cracked gas, after the removal of C3 and heavy components, enters the first demethanizer feed cooler 201, where it is cooled to -36°C using propylene refrigerant. In the first demethanizer feed separator 202, the separated liquid phase enters the bottom feed of demethanizer 3. The gas phase enters the second demethanizer feed cooler 203, where it is cooled to -48°C. In the second demethanizer feed separator 204, the separated liquid phase enters the lower feed of demethanizer 3. The gas phase enters the third demethanizer feed cooler 205, where it is cooled to -73°C using ethylene refrigerant. In the third demethanizer feed separator 206, the separated liquid phase enters the middle feed of demethanizer 3. The gas phase enters... The fourth demethanizer feed cooler 207 uses ethylene refrigerant to cool to -98°C. In the fourth demethanizer feed separator 208, the separated liquid phase enters the upper feed of demethanizer 3; the gas phase enters the low-temperature heat exchange unit 209, is cooled to -149°C, and then enters the fifth demethanizer feed separator 210. The methane content in the top of this tank is reduced to below 9%v, the hydrogen content is 91%v, and the hydrogen content in the liquid phase at the bottom of the tank is less than 2%v. After the liquid phase enters the low-temperature heat exchange unit 209 to provide cooling, it is warmed back to -99°C and enters the top tray of demethanizer 3 as a supplement to the top condensate; the gas at the top of the tank enters the inlet suction tank of the first expander 41.
[0061] The demethanizer tower 3 operates at a pressure of 2.9 MPaG and has a top condensation temperature of -99°C, utilizing -100°C ethylene refrigerant as the condenser. A stream of recirculated methane is drawn from the top methane-rich gas of demethanizer tower 3, with a flow rate approximately 20% of the total top gas volume. This recirculated methane enters the second expander 51, where it expands and cools before entering the cryogenic heat exchange unit 209 to provide cooling. It then passes through the second compressor 52 to increase the pressure before entering the middle of the compression section, thereby raising the methane feed concentration to demethanizer tower 3. Specifically, the inlet pressure of the second expander 51 is 2.9 MPaG, and the outlet pressure is 0.4 MPaG; the inlet pressure of the second compressor 52 is 0.4 MPaG, and the outlet pressure is 0.8 MPaG.
[0062] The remaining methane-rich gas from the top of the demethanizer tower 3 enters the inlet suction tank of the first expander 41 and then enters the first expander 41. After expansion and cooling, it enters the cryogenic heat exchange unit 209 to provide cooling. Subsequently, it is pressurized by the first compressor 42 and collected as methane hydrogen product. The inlet pressure of the first expander 41 is 3.0 MPaG and the outlet pressure is 0.2 MPaG; the inlet pressure of the first compressor 42 is 0.2 MPaG and the outlet pressure is 0.4 MPaG.
[0063] Using this method, for the feed composition in the above embodiments, the ethylene loss rate in the methane hydrogen product can be reduced from 0.4%v to 0.1%v. For a 1 million-ton-scale ethylene plant, this translates to an additional 2,400 tons of ethylene recovered annually, generating a profit of over 12 million yuan. Simultaneously, it avoids increased investment in equipment, pipelines, and valves due to increased outlet pressure of the cracked gas compressor, and also eliminates the need for an additional methane refrigeration compressor, thus saving energy. This method is particularly suitable for the retrofitting of ethylene plants to use lighter feedstocks.
[0064] Example 2 Example 2 Figure 3 The diagram illustrates the compression section based on Example 1. This example is particularly suitable for compression sections employing a pre-deethaner and pre-hydrogenation process. 6 represents the deethaner, 7 is one compression stage of the cracked gas compressor, and 8 is the deethaner reflux tank. In the compression process, the cracked gas is pressurized by stages 1-4 of the cracked gas compressor before entering the deethaner. The second and light components are collected at the top of the tower. After further compression by stage 5 (K-5) of the cracked gas compressor, the gas enters the hydrogenation process, where acetylene in the cracked gas is hydrogenated to produce ethylene. After a cooling process, the gas enters the deethaner via process V-11. The liquid phase at the bottom of the tank serves as reflux from the deethaner, while the gas phase at the top enters the subsequent staged cooling process. Stages 1-5 of the compressor constitute a complete cracked gas compressor. In this embodiment, the effective components containing C2 and light are pre-separated from the cracked gas and fed into the compressor. This avoids all the cracked gas entering the compressor, which can reduce the compressor's power consumption. Two heat pump systems are cleverly formed by using one cracked gas compressor. The first system consists of a deethaner and compressor stage 5, while the other system consists of cracked gas compressor stages 1-5 and a demethanizer. The heat pump process can significantly reduce the energy consumption required for separation.
[0065] Example 3 Example 3 Figure 4 The example shown is an evolution of Example 1, reducing the number of expanders, condenser, reflux tank, and reflux pump in the demethanizer to decrease investment. However, this increases the amount of circulating methane and the power consumption of the compressor. The methane-rich hydrogen stream from the top of the feed separator 210 of the fifth demethanizer undergoes flash evaporation via a pressure reducing valve to generate cooling, which is then used to supply cooling to the cryogenic heat exchange unit 209. After heat exchange, a portion is collected as methane hydrogen product, while the other portion can be adjusted to increase the flow rate of circulating methane based on ethylene losses. The overhead stream from the demethanizer 3, after being depressurized by a pressure reducing valve, provides cooling to the cryogenic heat exchange unit 209 and is returned to the compression section as circulating methane. The advantages of this embodiment are its simplicity, low investment, and small footprint.
[0066] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: In this embodiment, a fifth demethanizer feed tank is provided to cool the cracked gas to below -130°C, concentrate the methane, and separate as much hydrogen as possible before feeding it into the demethanizer. A stream of gas from the top of the demethanizer is set up in a circulating methane expander to recover the cold energy in a cryogenic heat exchange unit and return it to the compression section, thereby increasing the methane feed concentration to the demethanizer. This reduces ethylene loss in methane hydrogen for ethylene plants that crack light feedstocks without significantly increasing investment and energy consumption.
[0067] In this embodiment, the expander and compressor are coaxial devices, and the expander's output energy serves as the driving energy for the compressor. The expander can also be connected to a motor, and its output energy can be used to generate electricity.
[0068] In this embodiment, the methane feed concentration to the demethanizer is increased from multiple angles, reducing the ethylene loss rate at the top of the demethanizer. By setting up a fifth demethanizer feed separator to enrich the cracked gas with methane, it also provides cooling to the cryogenic heat exchange unit. Furthermore, the installation of a second expander achieves energy-saving effects. This avoids increasing the outlet pressure of the cracked gas compressor and installing a methane refrigeration compressor, thus saving on equipment investment.
[0069] In this embodiment, the ethylene loss rate can be reduced from 0.4%v to 0.1%v. For a 1 million-ton-scale ethylene plant, this translates to an additional 2,400 tons of ethylene recovered annually, generating a profit of over 12 million yuan. Simultaneously, it avoids increased investment in equipment, pipelines, and valves due to increased outlet pressure of the cracked gas compressor, and also eliminates the need for an additional methane refrigeration compressor, thus saving energy. This method is particularly suitable for retrofitting ethylene plants to use lighter feedstocks.
[0070] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.
Claims
1. A system for reducing ethylene loss in light pyrolysis feedstock, the system comprising: Compression unit (1), the inlet of which is fed with cracked gas to pressurize the cracked gas; The multi-stage cooling and separation unit (2) includes at least three stages of coolers and corresponding separation tanks. The inlet of the multi-stage cooling and separation unit (2) is connected to the outlet of the compression unit (1) for cooling the cracked gas stage by stage and separating the liquid phase stage by stage. The last stage separates the liquid phase rich in methane and the gas phase rich in hydrogen. The demethanizer (3) has its inlet connected to the liquid phase outlet of each stage in the multi-stage cooling and separation unit (2) to demethanize the liquid phase and obtain methane-rich gas. The first expansion compressor unit (4) is connected to the gas phase outlet of the last stage of the multi-stage cooling separation unit (2) and the top gas outlet of the demethanizer tower (3) to perform two-stage expansion refrigeration and compression treatment on the hydrogen-rich gas phase and the first part of the methane-rich gas to obtain methane hydrogen product. The second expansion compressor unit (5) has its inlet connected to the top gas outlet of the demethanizer tower (3) and its outlet connected to the inlet of the compression unit (1). It is used to pressurize the second part of the methane-rich gas and return it to the middle of the compression unit (1) and mix it with the cracked gas.
2. The system according to claim 1, characterized in that, The multi-stage cooling separation unit (2) includes: The first demethanizer feed cooler (201) has its inlet connected to the outlet of the compression section, and is used to cool the cracked gas to -40°C to -32°C using propylene refrigerant. The first demethanizer feed separator (202) has its inlet connected to the gas phase outlet of the first demethanizer feed cooler (201), and its liquid phase outlet connected to the bottom feed inlet of the demethanizer (3). The second demethanizer feed cooler (203) is connected to the gas phase outlet of the first demethanizer feed separator (202) at its inlet, and is used to cool the gas phase to -52°C to -44°C. The second demethanizer feed separator (204) has its inlet connected to the outlet of the second demethanizer feed cooler (203), and its liquid phase outlet is connected to the lower feed inlet of the demethanizer (3). The third demethanizer feed cooler (205) is connected at its inlet to the gas phase outlet of the second demethanizer feed separator (204) to cool the gas phase to -75°C to -70°C using ethylene refrigerant. The third demethanizer feed separator (206) has its inlet connected to the outlet of the third demethanizer feed cooler (205), and its liquid phase outlet is connected to the middle layer feed inlet of the demethanizer (3). The fourth demethanizer feed cooler (207) is connected to the gas phase outlet of the third demethanizer feed separator (206) at its inlet, and is used to cool the gas phase to -102°C to -94°C using ethylene refrigerant. The fourth demethanizer feed separator (208) has its inlet connected to the outlet of the fourth demethanizer feed cooler (207), and its liquid phase outlet is connected to the upper feed inlet of the demethanizer (3). The low-temperature heat exchange unit (209) has its first inlet connected to the gas phase outlet of the fourth demethanizer feed separator (208) to deeply cool the gas phase to ≤-130℃. The fifth demethanizer feed separator (210) has its inlet connected to the first outlet of the low-temperature heat exchange unit (209), and its liquid phase outlet connected to the second inlet of the low-temperature heat exchange unit (209) to reheat the liquid phase to -103°C to -95°C. The second outlet of the low-temperature heat exchange unit (209) is connected to the uppermost feed port of the demethanizer (3), and the gas phase outlet of the fifth demethanizer feed separator (210) is connected to the inlet of the first expansion compressor unit (4).
3. The system according to claim 2, characterized in that, The first expansion compressor unit (4) includes: The first expander (41) has its first inlet connected to the gas phase outlet of the feed separator (210) of the fifth demethanizer, its second inlet connected to the second gas phase outlet at the top of the demethanizer (3), and its outlet connected to the third inlet of the low-temperature heat exchange unit (209) to provide cooling capacity. The first compressor (42) has its inlet connected to the third outlet of the low-temperature heat exchange unit (209), and its outlet is used to output methane hydrogen products. The first expander (41) and the first compressor (42) are coaxially connected.
4. The system according to claim 3, characterized in that, The inlet pressure of the first expander (41) is 2.0 MPaG to 3.8 MPaG, the outlet pressure of the first expander (41) is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the first compressor (42) is 0.1 MPaG to 0.3 MPaG, and the outlet pressure of the first compressor (42) is 0.3 MPaG to 0.5 MPaG.
5. The system according to claim 3, characterized in that, The second expansion compressor unit (5) includes: The second expander (51) has its inlet connected to the first gas phase outlet at the top of the demethanizer (3), and its outlet connected to the fourth inlet of the low-temperature heat exchange unit (209) to provide cooling capacity. The second compressor (52) has its inlet connected to the fourth outlet of the low-temperature heat exchange unit (209), and its outlet connected to the middle inlet of the compression unit (1). The second expander (51) is coaxially connected to the second compressor (52).
6. The system according to claim 5, characterized in that, The inlet pressure of the second expander (51) is 2.8 MPaG to 3.1 MPaG, the outlet pressure of the second expander (51) is 0.1 MPaG to 0.8 MPaG, the inlet pressure of the second compressor (52) is 0.3 MPaG to 0.5 MPaG, and the outlet pressure of the second compressor (52) is 0.6 MPaG to 1.0 MPaG.
7. The system according to claim 1, characterized in that, The operating pressure of the demethanizer (3) is 2.8 MPaG to 3.1 MPaG, and the top condensation temperature of the demethanizer (3) is -90℃ to -99℃.
8. A method for reducing ethylene loss in light pyrolysis feedstock, the method being adapted to the system described in any one of claims 1 to 7, the method comprising: Pressurize the pyrolysis gas; The pyrolysis gas is cooled in stages and separated into liquid phases in stages, with the last stage separating a liquid phase rich in methane and a gas phase rich in hydrogen. The liquid phase is subjected to demethanization separation to obtain methane-rich gas; The hydrogen-rich gas phase and the first part of the methane-rich gas are subjected to two-stage expansion refrigeration and compression treatment to obtain the methane hydrogen product. The second portion of methane-rich gas is pressurized and returned to the middle of the compression unit, where it is mixed with cracked gas.
9. The method according to claim 8, characterized in that, The flow rate of the first part of the methane-rich gas is 75% to 85% of the total flow rate of the methane-rich gas; The flow rate of the second part of the methane-rich gas is 15% to 25% of the total flow rate of the methane-rich gas.
10. The method according to claim 8, characterized in that, The volume fraction of hydrogen in the hydrogen-rich gas phase is ≥90%; The volume fraction of hydrogen in the methane-rich liquid phase is ≤2%; The volume fraction of ethylene in the methane hydrogen product is ≤0.2%.