Ethylene plant for generating electricity using renewable sources

Abstract

An ethylene plant comprising:-a cracking furnace for converting a hydrocarbon feedstock into a cracking gas stream; -a separation section configured to provide at least an ethylene-rich product stream and a methane-rich fuel stream from the cracked gas stream; means for producing liquefied methane from the methane-rich fuel stream; means for storing liquefied methane; -an evaporator for gasifying liquefied methane; -a first passage for feeding the evaporator with liquefied methane provided by the means for storage; -a second passage for feeding the gas turbine with methane provided by the evaporator; -a gas turbine configured to be fed with methane provided by the evaporator and to intermittently produce electric power for use in the ethylene plant; -a renewable source configured to intermittently generate electrical power for use in an ethylene plant; -means for closing the first path and / or the second path when the renewable source generates power greater than a threshold; -means for opening the first path and / or the second path when the renewable source does not produce power or produces power less than a threshold value; wherein the device for producing liquefied methane comprises heat exchangers organized in a cascade cycle.

Description

[0001] This invention relates to an ethylene plant, and more specifically to a method for producing ethylene in such a plant. The ethylene plant specifically includes apparatus for intermittently storing a methane-rich fuel stream when renewable electricity is available.

[0002] Olefin (ethylene, propylene, and butene) production is a highly energy-intensive process. Current steam cracking technology involves a process furnace that provides energy to crack hydrocarbon feedstocks to produce olefin products, heat recovery from the products, large compressors that pressurize the product stream to relatively high pressures (20-35 bar), and incremental distillation to separate and purify the products. The process furnace is a relatively inefficient way to provide the heat of cracking: only about 40% to 50% of the heat released in the process furnace is used for the cracking reaction. The remaining furnace heat is recovered in the furnace convection section and integrated with a process gas heat recovery system to provide high-pressure steam to drive the reactor effluent and refrigeration compressor. Any additional energy (in the form of high-pressure steam) is typically provided by an auxiliary boiler. A disadvantage of steam cracking is that providing compressor energy through this steam cycle is thermodynamically inefficient, converting only about 25% of the fuel heat energy into useful shaft work. This, combined with the inefficiency of the process furnace, makes olefin production highly fuel-intensive.

[0003] One way to improve the efficiency of such processes is to provide heat for cracking through combined heat and power (CHP), which uses both gas turbines and steam turbines to provide energy. This achieves efficiencies of up to 55% in converting fuel thermal energy into usable shaft work. An example of this approach is disclosed in WO 01 / 04236, in which the steam cracking process is characterized by the energy for heating the hydrocarbon mixture being provided by a CHP unit. The CHP unit simultaneously generates thermal and mechanical energy by burning fuel supplied by cracked hydrocarbons; the mixture of hydrocarbons and steam is preheated by thermal energy, while the mechanical energy is converted into electricity by an AC generator or energy generator, which is then used to heat the hydrocarbon mixture to the desired cracking temperature.

[0004] However, as renewable energy sources become increasingly important, so do ways of buffering energy.

[0005] Therefore, the present invention aims to provide a new way for ethylene plants to use renewable electricity.

[0006] The solution of the present invention is an ethylene plant, which includes:

[0007] - A cracking furnace used to convert hydrocarbon feedstocks into cracked gas streams;

[0008] - A separation section configured to provide at least an ethylene-rich product stream and a methane-rich fuel stream from a cracked gas stream;

[0009] - A device for producing liquefied methane from a methane-rich fuel stream;

[0010] - A device for storing liquefied methane;

[0011] - An evaporator used for vaporizing liquefied methane;

[0012] - A first passage for feeding liquefied methane supplied by the storage device into the evaporator;

[0013] -A second passage for feeding methane supplied by the evaporator into the gas turbine;

[0014] - A gas turbine configured to be fed with methane supplied by an evaporator and intermittently generate electricity used in an ethylene plant;

[0015] -Configured to intermittently generate a renewable source of electricity used in ethylene plants;

[0016] -A means for shutting off a first and / or second pathway when a renewable source generates more electricity than a threshold;

[0017] - Used to activate the first power source when the renewable energy source is not generating electricity or is generating less than a threshold amount of electricity.

[0018] Devices for a primary and / or secondary pathway;

[0019] The apparatus used to produce liquefied methane includes heat exchangers organized in a cascaded cycle.

[0020] In this factory, even though the generation of electricity from renewable sources is intermittent, power generation is continuous.

[0021] A "cascaded cycle" refers to a cascaded refrigeration cycle that includes at least two refrigeration loops thermally connected by a cascaded condenser, which serves as the condenser for the low-temperature loop and the evaporator for the high-temperature loop. The cascaded system utilizes one refrigerant to condense another primary refrigerant, and the system operates at a desired evaporator temperature.

[0022] The time when a renewable source generates electricity can be regular or irregular, and the time when a renewable source does not generate electricity can also be regular or irregular.

[0023] The threshold depends on the size of the ethylene plant, and is typically between 5% and 20%, more preferably between 5% and 10%.

[0024] In the solution of this invention, the ethylene plant is modified to use electricity generated by a renewable source and to store a methane-rich fuel stream when the renewable source generates little or no electricity. For this purpose, the methane needs to be liquefied so that it can be stored. This liquefaction process is of interest here. Specifically, the intermittent nature of this particular application. The methane needs to be stored while renewable electricity is available and must be used to meet the required electricity demand when renewable energy is reduced or even absent. All of this should not disrupt the operation of the ethylene plant. This is to optimally utilize the stored energy in the form of excess fuel gas (essentially methane-rich gas). In fact, when little or no energy is available to keep the liquefaction plant running, all of this stored fuel gas can be used to keep the ethylene plant running and maximize power output.

[0025] This liquefaction in a cascaded cycle helps to temporarily store excess fuel gas during periods of renewable electricity surplus. Imagine the availability of this electricity when operating a solar power plant, dependent on emerging weather conditions and the length of day and night during each season. Similar concerns exist with wind power, where the length of day and night may be less important than the intensity of wind. Given the inherently fluctuating nature of these energy sources, it is crucial to be able to store energy when these resources are unavailable. In the case of an ethylene plant, excess fuel gas can be generated when the pyrolysis reactors are made more efficient by reducing fuel demand or by energizing them fully. In this situation, the production of fuel gas from the plant is no longer fully utilized by the pyrolysis reactors (so-called degraded furnaces) and needs to be used elsewhere. One option is to use this excess fuel gas to generate electricity. For this reason, excess fuel gas needs to be stored during periods of abundant renewable electricity availability without disrupting the operation of the ethylene plant.

[0026] Advantageously, the apparatus described above for producing liquefied methane from a methane-rich fuel stream is an auxiliary unit, and the ethylene plant according to the invention includes a main unit for producing liquefied methane from a methane-rich fuel stream. This main unit operates continuously when the renewable source generates electricity exceeding a threshold, while the auxiliary unit operates intermittently. In this case, the ethylene plant according to the invention includes means for shutting down and opening the feed passage to the auxiliary liquefaction unit. In other words, the invention provides a separate solution, meaning the liquefaction line operates independently of the continuously operating liquefaction line of the ethylene plant itself. The auxiliary unit has a very high regulation ratio, allowing the renewable power load to be handled very smoothly, and the power demand at full regulation is very low. This is to optimally utilize the stored energy in the form of excess fuel gas (essentially methane-rich gas). In fact, all of this stored fuel gas can be used to power the ethylene plant itself and maximize power output. In the absence of renewable energy, little or no energy is used to keep the liquefaction plant running.

[0027] Advantageously, the means for closing the first passage and / or the second passage and the means for opening the first passage and the second passage are control valves.

[0028] Separation sections for separating cracked gases into different fractions are generally known in the art. For example, conventional distillation, such as cryogenic distillation, can be used to obtain an ethylene-rich product stream, a hydrogen-rich fuel stream, and a methane-rich fuel stream.

[0029] When needed, the separation section may include one or more units for further processing, such as increasing the concentration / purity of a component or removing unwanted components before further use. If the hydrogen or methane is used for purposes different from energy production in the plant, it may be obtained from a hydrogen-rich stream or a methane-rich stream after further separation / purification. Thus, hydrogen may be obtained, for example, for use in a hydrogenation process.

[0030] It should be noted that a portion of the methane-rich fuel obtained from the cracked gas can be returned to the cracker burner.

[0031] Gas turbines typically operate as follows: fuel gas (methane-rich fuel) is burned in the combustion chamber of the gas turbine by combustion air supplied to the chamber by a combustion air compressor. The resulting flue gas is typically discharged through the gas turbine to generate electricity via a generator. Further heat recovery of the hot flue gas leaving the turbine usually takes place in a downstream waste heat boiler. The recovered heat is used to generate steam for additional power generation, thereby increasing the thermal-electric efficiency to over 50%, a process known as combined heat and power (CHP).

[0032] Electricity systems that provide electricity from renewable sources typically include one or more of the following groups of 5: wind power systems, solar power systems, hydropower systems, geothermal power systems, and infiltration power systems (blue energy).

[0033] According to an embodiment, the ethylene plant according to the present invention may include one or more of the following features:

[0034] - Devices for producing liquefied methane include methane cycles, ethylene or ethane cycles, and propylene or propane cycles.

[0035] The methane cycle is configured to operate between -160°C and -100°C, the ethylene or ethane cycle is configured to operate between -35°C and -102°C, and the propylene or propane cycle is configured to operate between -40°C and ambient temperature.

[0036] - The apparatus for producing liquefied methane includes, between the ethylene or ethane cycle and the methane cycle, means for separating liquid and gaseous fractions from a pre-cooled methane-rich fuel stream cooled at a temperature between -35°C and -102°C. Preferably, the apparatus for producing liquefied methane includes means for injecting the gaseous fraction into the fuel gas network of the ethylene plant after recovering the coldness of the gaseous fraction in the ethylene or ethane cycle and the propylene or propane cycle.

[0037] - The apparatus for producing liquefied methane includes, after the methane cycle, a process for refining the methane from a source at -160°C and...

[0038] Apparatus for separating liquid and gaseous fractions from a pre-cooled methane-rich fuel stream cooled at a temperature between -100°C, and apparatus for mixing the gaseous fraction with a methane-rich fuel stream from a hydrocarbon feedstock or ethylene plant.

[0039] - The separation section is configured to provide two methane-rich fuel streams at two different pressures, and the device for generating liquefied methane is configured to receive the two methane-rich fuel streams.

[0040] - The apparatus for producing liquefied methane includes between 3 and 11 heat exchangers, preferably between 5 and 9. These exchangers are a special type, namely plate-fin exchangers. These exchangers allow heat transfer between multiple hot and cold streams at very small temperature differences to achieve maximum heat / cold recovery, and are not limited to two streams as in conventional exchangers.

[0041] Each cycle of liquefied methane production consists of a compressor, an expansion valve, and a cooler. The compressor is a screw compressor. This type of equipment offers much higher regulation than a centrifugal compressor and ensures that the various refrigeration cycles remain connected at all times. The latter is necessary. If heat cannot be transferred from one cycle to the next,

[0042] Therefore, refrigeration is impossible.

[0043] - Apparatus for producing liquefied methane includes a separator vessel to separate the liquid fraction from the vapor fraction.

[0044] The ethylene plant includes a processor capable of generating a first signal when the renewable source produces more electricity than a threshold, and a second signal when the renewable source does not produce electricity or produces less than the threshold, and sending these signals respectively to means for closing and / or opening the first and / or second pathways. Preferably, the processor can compare the electricity generated by the renewable source with the threshold.

[0045] The methane cycle preferably has 1-4 stages, preferably 3 stages, each stage equipped with an expansion valve and a downstream accumulation vessel for separating the flash liquid into liquid and gas phases. Each stage has a compressor for the gas phase. The discharge section from the highest-level compressor is equipped with at least one heat exchanger for condensing the refrigerant (in this case, methane), and preferably at least one exchanger for cooling the gas compressor discharge section. Optionally, a heat exchanger for subcooling the liquefied refrigerant may be provided.

[0046] The ethane or ethylene cycle preferably has 1-4 stages, preferably 3 stages, each stage equipped with an expansion valve and a downstream accumulation vessel for separating the flash liquid into liquid and gas phases. Each stage has a compressor for the gas phase. The discharge section from the highest-level compressor is equipped with at least one heat exchanger for condensing the refrigerant (in this case, ethane or ethylene), and preferably at least one exchanger for cooling the gas compressor discharge section. Optionally, a heat exchanger for subcooling the liquefied refrigerant may be provided.

[0047] The propane or propylene cycle preferably has 2-5 stages, preferably 4 stages, each stage equipped with an expansion valve and a downstream accumulation vessel for separating the flash liquid into liquid and gas phases. Each stage has a compressor for the gas phase. The discharge section from the highest-level compressor is equipped with at least one heat exchanger for condensing the refrigerant (in this case, propane or propylene), and preferably at least one exchanger for cooling the gas compressor discharge section. Optionally, a heat exchanger for subcooling the liquefied refrigerant may be provided.

[0048] Another object of the present invention is a method for producing ethylene from hydrocarbon feedstock, the method realizing an ethylene plant as defined in the present invention, and comprising:

[0049] a) The step of cracking hydrocarbon feedstock in a cracker to generate a cracked gas stream;

[0050] b) The step of separating the cracked gas stream to provide at least an ethylene-rich product stream and a methane-rich fuel stream.

[0051] c) The step of liquefying a methane-rich fuel stream to produce liquefied methane.

[0052] d) Steps for storing liquefied methane,

[0053] e) The step of evaporating the liquefied methane from the storage step in an evaporator.

[0054] f) The step of intermittently generating electricity in a gas turbine using evaporated methane from an evaporator.

[0055] g) The step of intermittently generating electricity from a renewable source.

[0056] h) The steps to stop the evaporation of liquefied methane and the power generation of the gas turbine when the renewable source stops producing more electricity than a threshold, and

[0057] i) When the renewable source does not generate electricity or generates less than a threshold amount of electricity, restore liquefied petroleum gas (LPG).

[0058] The steps of alkyl evaporation and gas turbine electricity generation,

[0059] The liquefaction step is achieved by exchanging refrigerant with that of the cascaded cycle.

[0060] According to an embodiment, the method for producing ethylene from a hydrocarbon feedstock according to the present invention may include one or more of the following features:

[0061] - The separation step provides two methane-rich fuel streams at two different pressures.

[0062] - The liquefaction step includes the following sub-steps:

[0063] i) Methane-rich fuel streams are pre-cooled in the propylene or propane cycle at a temperature between -40°C and ambient temperature.

[0064] ii) The pre-cooled methane-rich fuel stream is cooled in an ethylene or ethane cycle at temperatures between -35°C and -102°C, and

[0065] iii) The cooled, methane-rich fuel stream is liquefied in the methane cycle at temperatures between -160°C and -100°C.

[0066] The liquefaction step, between sub-steps ii) and iii), includes a sub-step of separating liquid and gaseous fractions from a pre-cooled methane-rich fuel stream cooled at a temperature between -35°C and -102°C, and a sub-step of injecting the gaseous fractions into the fuel gas network of an ethylene plant after recovering the coldness of the gaseous fractions in an ethylene or ethane cycle and a propylene or propane cycle.

[0067] - The liquefaction step, following sub-step (ii) includes a sub-step of separating liquid and gas fractions from a pre-cooled methane-rich fuel stream cooled at a temperature between -160°C and -100°C, and a step of mixing the gas fraction with a methane-rich fuel stream from a hydrocarbon feedstock.

[0068] - In the propylene or propane cycle, propylene or propane is compressed at a pressure between 15 bar and 18 bar; in the ethylene or ethane cycle, ethylene or ethane is compressed at a pressure between 25 bar and 28 bar; in the methane cycle, methane is compressed at a pressure between 33 bar and 36 bar.

[0069] - A method for producing ethylene includes the steps of comparing electricity generated by a renewable source with a threshold; generating a first signal when the renewable source generates electricity greater than the threshold and a second signal when the renewable source does not generate electricity or generates electricity less than the threshold; and sending these signals to devices for respectively closing and opening the first and second pathways. The intermittent generation of electricity results in the generation of excess fuel gas. The pressure in the fuel gas system will fluctuate as a result, and this can be achieved by using control signals as pressure and / or level controllers, which can be used to send fuel gas to or retrieve fuel gas from the storage unit. Both systems can use HP and MP fuel gases to send fuel gas to the storage unit. To retrieve stored methane, an existing cold box in the separation section of the ethylene plant can be used as an evaporator via the route that generates MP methane fuel, as this is suitable for flashing liquids.

[0070] Through three cascaded cycles, excess fuel gas from an ethylene plant can be gradually cooled to the boiling point of the excess fuel required for storage under atmospheric conditions, approximately -160°C. Atmospheric storage is preferred, but storage at higher pressures is also possible using this method, in which case higher methane compressor suction pressures can be used. To minimize exergy losses, the various refrigeration cycles have multiple stages. This minimizes power demand. Using screw compressors with slide valve capacity control, regulation can be as high as 100% to 10%. Furthermore, VSD (Variable Speed ​​Drive) further reduces power demand during minimum load processing when renewable electricity is unavailable.

[0071] Because the pressure ratio of a screw compressor is limited, each refrigeration stage is equipped with its own compressor.

[0072] The number of refrigeration stages in each loop can be optimized. The fact that this limitation necessitates the use of multiple compressors makes the use of mixed refrigerants less attractive.

[0073] Figure 1 An example of a cascaded cycle used in an ethylene plant according to the present invention is described.

[0074] In ethylene plants, there is typically more than one source of methane-rich gas available for liquefaction. In this scheme, there are two sources, high pressure and medium pressure (HP and MP), both at approximately ambient temperature but at different pressures. Both are gradually cooled and fed at separate compressors based on available pressure. MP is at the suction port of M-2, and HP is at the suction port of M-3. These streams are continuously compressed in M-2 and M-3 to a sufficiently high pressure to allow the next refrigeration loop to be used as a radiator. In this example case, approximately 34 bar is reached, such that as the methane-rich gas is gradually cooled in the air cooler, CW cooling water cooler, and various propylene and ethylene refrigeration exchangers, the majority of the methane-rich gas can condense at the cold box outlet at a temperature of approximately -99°C to -100°C, just above the boiling point of ethylene at the suction pressure of the ethylene compressor E-1 (-102°C). At the cold box outlet, the partially condensed methane-rich gas is separated in containers (not shown for simplicity; essentially, each branch in the process scheme is a separation container). After the refrigeration is gradually recovered in various cold box exchangers in the ethylene and propylene loops, the light fraction is returned to the fuel gas network. The liquid fraction is depressurized in stages to reduce the power demand of the methane compressor. At each stage, the flashed liquid is separated in a flash vessel, and the vapor fraction is sent to the connected methane compressor. The final stage at -160°C produces liquefied methane at near atmospheric pressure. This is then sent to a storage unit. The remaining vapor is sent to compressor M-1, where it is successively compressed in M-1, M-2, and M-3 in combination with MP and HP-rich methane gas from the fuel gas network and flash vapor from the staged depressurization line.

[0075] The ethylene loop acts as a radiator for the methane loop. The condensed ethylene is completely evaporated at the suction pressure of the ethylene compressor E-1 to be successively compressed in compressors E-1, E-2, and E-3 to a pressure high enough to allow the next refrigeration loop to function as a radiator. In this example, this is approximately 26 bar, such that all the ethylene can condense in the cold box at a temperature of approximately -21°C to -24°C as it gradually cools in the air cooler, CW cooling water cooler, and various propylene refrigeration exchangers, and receives a small amount of subcooling down to -35°C upon exiting the cold box, which is sufficiently higher than the -40°C boiling point of propylene at the suction pressure of the propylene compressor P-1. The subcooled liquid is depressurized in stages to reduce the power demand of the ethylene compressor. At each stage, the flashed liquid is separated in a flash vessel, and the vapor fraction is sent to the connected ethylene compressor. Furthermore, each refrigeration stage is connected to the cold box to act as a radiator by evaporating liquid ethylene at each stage. Any remaining liquid is evaporated at -102°C in the final stage. The vapor is sent to compressor E-1, where it is successively compressed in E-1, E-2 and E-3 in combination with flash vapor from the staged vacuum production line.

[0076] Like the ethylene loop, propylene is a closed-loop system and operates in a very similar manner. The propylene loop acts as a radiator for the ethylene loop. The condensed propylene is completely evaporated at the suction pressure of the propylene compressor P-1 to be successively compressed in compressors P-1, P-2, P-3, and P-4 to a pressure high enough to allow operation of the radiator at slightly above ambient temperature. In this example, this is approximately 16 bar, such that all the propylene can be condensed in the CW cooling water cooler as it gradually cools in the air cooler. The condensate is depressurized in stages to reduce the power demand of the propylene compressors. At each stage, the flashed liquid is separated in a flash vessel, and the vapor fraction is sent to the connected propylene compressor. Additionally, each refrigeration stage is connected to a cold box to act as a radiator by evaporating liquid propylene at each stage. Any remaining liquid is evaporated at -40°C in the final stage. This vapor is sent to compressor P-1 to be successively compressed in P-1, P-2, P-3, and P-4 in combination with the flash vapor from the staged depressurization line.

[0077] As indicated above, this solution of the present invention aims to store excess fuel gas from an ethylene plant during the period when renewable electricity is available. This requires high regulation, which is not available in conventional schemes. Furthermore, the process scheme is specifically configured to handle methane-rich gas. This gas contains lighter fractions, which prevent the feed gas from completely condensing in the open methane loop. Using the plant of the present invention, these lighter fractions can be effectively removed. Another advantage of the apparatus for producing liquefied methane as defined in the present invention is the reduced energy consumption for methane liquefaction.

Claims

1. An ethylene plant, including: - A cracking furnace used to convert hydrocarbon feedstocks into cracked gas streams; - A separation section configured to provide at least an ethylene-rich product stream and a methane-rich fuel stream from the cracked gas stream; - An apparatus for producing liquefied methane from the methane-rich fuel stream; - A device for storing the liquefied methane; - An evaporator used for vaporizing liquefied methane; - A first passage for feeding liquefied methane supplied by the storage device into the evaporator; - A second passage for feeding methane supplied by the evaporator into the gas turbine; - A gas turbine configured to be fed with methane provided by the evaporator and intermittently generate electricity for use in the ethylene plant; - A renewable source configured to intermittently generate electricity used in the ethylene plant; -A means for shutting down the first and / or the second pathway when the renewable source generates power greater than a threshold; -A means for opening the first and / or the second pathway when the renewable source does not generate electricity or generates less than a threshold amount of electricity; The apparatus used to produce liquefied methane includes heat exchangers organized in a cascaded cycle.

2. The ethylene plant according to claim 1, wherein the apparatus for producing liquefied methane comprises a methane cycle, an ethylene or ethane cycle, and a propylene or propane cycle.

3. The ethylene plant according to claim 2, wherein: The methane cycle is configured to operate between -160°C and -100°C. The ethylene or ethane cycle is configured to operate between -35°C and -102°C, and The propylene or propane cycle is configured to operate between -40°C and ambient temperature.

4. The ethylene plant of claim 3, wherein the apparatus for producing liquefied methane includes, between the ethylene or ethane cycle and the methane cycle, a means for separating liquid and gaseous fractions from a pre-cooled, methane-rich fuel stream cooled at a temperature between -35°C and -102°C.

5. The ethylene plant according to any one of claims 1 to 4, wherein the separation section is configured to provide two methane-rich fuel streams at two different pressures, and the apparatus for generating liquefied methane is configured to receive the two methane-rich fuel streams.

6. The ethylene plant according to any one of claims 1 to 5, wherein the apparatus for producing liquefied methane comprises between 3 and 11 heat exchangers, preferably between 5 and 9 heat exchangers.

7. The ethylene plant according to any one of claims 1 to 6, wherein the apparatus for producing liquefied methane includes a separator vessel to separate the liquid fraction from the vapor fraction.

8. The ethylene plant according to any one of claims 1 to 7, comprising a processor capable of generating a first signal when the renewable source generates power greater than a threshold, and generating a second signal when the renewable source does not generate power or generates power less than a threshold, and sending these signals respectively to means for shutting down and / or opening the first and / or the second pathway.

9. A method for producing ethylene from hydrocarbon feedstock, said method implementing an ethylene plant according to any one of claims 1 to 8, and comprising: a) The step of cracking the hydrocarbon feedstock in the cracking furnace to generate a cracked gas stream; b) The step of separating the cracked gas stream to provide at least an ethylene-rich product stream and a methane-rich fuel stream. c) The step of liquefying the methane-rich fuel stream to produce liquefied methane. d) The step of storing the liquefied methane, e) The step of evaporating the liquefied methane from the storage step in the evaporator. f) The step of intermittently generating electricity in a gas turbine using evaporated methane from the evaporator. g) The step of intermittently generating electricity from a renewable source. h) stopping the evaporation of liquefied methane and the power generation of the gas turbine when the renewable source stops generating more electricity than a threshold, and i) resuming the evaporation of liquefied methane and the power generation of the gas turbine when the renewable source does not generate electricity or generates less than a threshold. The liquefaction step is achieved by exchanging refrigerant with that of the cascaded cycle.

10. The method for producing ethylene according to claim 9, wherein the separation step provides two methane-rich fuel streams at two different pressures.

11. The method for producing ethylene according to claim 9 or claim 10, wherein the liquefaction step comprises the following sub-steps: i) The methane-rich fuel stream is pre-cooled in a propylene or propane cycle at a temperature between -40°C and ambient temperature. ii) The pre-cooled methane-rich fuel stream is cooled in an ethylene or ethane cycle at temperatures between -35°C and -102°C, and iii) The cooled, methane-rich fuel stream is liquefied in the methane cycle at temperatures between -160°C and -100°C.

12. The method for producing ethylene according to claim 11, wherein the liquefaction step comprises, between sub-steps ii) and iii), a sub-step of separating liquid and gaseous fractions from the pre-cooled methane-rich fuel stream cooled at a temperature between -35°C and -102°C, and a sub-step of injecting the gaseous fractions into the fuel gas network of the ethylene plant after recovering the coldness of the gaseous fractions in the ethylene or ethane cycle and the propylene or propane cycle.

13. The method for producing ethylene according to claim 11 or claim 12, wherein the liquefaction step, after said sub-step (iii), includes a sub-step of separating a liquid fraction and a gaseous fraction from said pre-cooled methane-rich fuel stream cooled at a temperature between -160°C and -100°C, and a step of mixing said gaseous fraction with said methane-rich fuel stream from hydrocarbon feedstock.

14. The method for producing ethylene according to any one of claims 11 to 13, wherein: - In the propylene or propane cycle, the propylene or propane is compressed at a pressure between 15 bar and 18 bar; - In the ethylene or ethane cycle, the ethylene or ethane is compressed at a pressure between 25 bar and 28 bar; - In the methane cycle, the methane is compressed at a pressure between 33 bar and 36 bar.

15. The method for producing ethylene according to any one of claims 11 to 14, wherein the method comprises the steps of comparing the electricity generated by the renewable source with the threshold; generating a first signal when the renewable source generates electricity greater than the threshold and generating a second signal when the renewable source does not generate electricity or generates electricity less than the threshold; and sending these signals to means for respectively closing and opening the first and second pathways.

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