ETHYLENE PLANT WITH GENERATION OF ELECTRIC POWER WITH RENEWABLE SOURCE

Abstract

 Ethylene plant comprising :
- A cracking furnace for converting a hydrocarbon feedstock into a cracked gas stream;
- A separation section configured to provide at least an ethylene-enriched product stream, and a methane-enriched fuel stream from the cracked gas stream;
- Means for producing liquefied methane from the methane enriched fuel stream;
- Means for storing the liquefied methane;
- Evaporator for gasifying liquefied methane;
- A first passageway for feeding evaporator with liquefied methane provided by means for storing;
- A second passageway for feeding the gas turbine with methane provided by the evaporator
- A gas turbine configured to be fed by the methane provided by the evaporator and to generate intermittently electric power used in the ethylene plant;
- A renewable source configured to generate intermittently electric power used in the ethylene plant;
- Means for closing the first and/or the second passageways when the renewable source generates electric power greater than a threshold value;
- Means for opening the first and/or the second passageways when the renewable source does not generate electric power or generates electric power less than a threshold value;
With means for producing liquefied methane comprising heat exchangers organized in cascade cycles.

Description

[0001] The present invention relates to an ethylene plant and to a process for producing ethylene in such a plant. The ethylene plant comprises in particular means for the intermittent storage of a methane enriched fuel stream when renewable electric power is available.

[0002] Olefins (ethylene, propylene and butenes) production is a very energy-intensive process. Current steam cracking technology involves a process furnace to provide energy to crack hydrocarbon feedstock to produce olefinic products, heat recovery from the products, a large compressor to pressurize the product stream to a relatively high pressures (20- 35 barg), and progressive 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 in the cracking reactions. The remainder of the furnace heat is recovered in the furnace convective section and integrated with the process gas heat recovery systems to provide high pressure steam to drive the reactor effluent and refrigeration compressors. Any additional energy (in the form of high pressure steam) is typically provided by auxiliary boilers. Steam cracking suffers from the disadvantage that providing compressor energy through such a steam cycle is thermodynamically inefficient, converting only about 25% of the thermal energy of the fuel into useful shaftwork. This, combined with the low efficiency of the process furnace, makes the production of olefins very fuel-intensive.

[0003] One way of improving the efficiency of such processes is to provide the heat for cracking by cogeneration (using both gas and steam turbines to provide energy), which is up to 55% efficient in converting fuel thermal energy into usable shaftwork. An example of this is disclosed in WO 01/04236, in which the steam cracking process is characterised in that the energy source to heat the hydrocarbon mixture is provided by a cogeneration unit. The cogeneration unit simultaneously produces thermal energy and mechanical energy by combustion of fuel provided from the cracked hydrocarbons; the mixture of hydrocarbons and steam are subjected to preheating by the thermal energy, whilst the mechanical energy is converted to electricity by an alternator or energy generator, which is then the used to heat the hydrocarbon mixture to the required cracking temperature.

[0004] However, as renewable power sources become more and more important, ways to buffer energy are as well.

[0005] So, the present invention aims to provide an ethylene plant with a new way to use renewable power.

[0006] A solution of the present invention is an ethylene plant comprising :

• A cracking furnace for converting a hydrocarbon feedstock into a cracked gas stream;
• A separation section configured to provide at least an ethylene-enriched product stream, and a methane-enriched fuel stream from the cracked gas stream;
• Means for producing liquefied methane from the methane enriched fuel stream;
• Means for storing the liquefied methane;
• Evaporator for gasifying liquefied methane;
• A first passageway for feeding evaporator with liquefied methane provided by means for storing;
• A second passageway for feeding the gas turbine with methane provided by the evaporator
• A gas turbine configured to be fed by the methane provided by the evaporator and to generate intermittently electric power used in the ethylene plant;
• A renewable source configured to generate intermittently electric power used in the ethylene plant;
• Means for closing the first and/or the second passageways when the renewable source generates electric power greater than a threshold value;
• Means for opening the first and/or the second passageways when the renewable source does not generate electric power or generates electric power less than a threshold value;

With means for producing liquefied methane comprising heat exchangers organized in cascade cycles.

[0007] In this plant, the power generation is continuously even if the production of the electric power coming from the renewable source is intermittently.

[0008] By "cascade cycles" we mean a cascade refrigeration cycles comprising at least two refrigeration circuits thermally connected by a cascade condenser, which is the condenser of the low-temperature circuit and the evaporator of the high-temperature circuit. A cascade system utilizes one refrigerant to condense the other primary refrigerant, which is operating at the desired evaporator temperature.

[0009] The time during which the renewable source generates electric power can be regular or irregular and the time during which the renewable source does not generate electric power can be regular or irregular.

[0010] The threshold value depends on the size of the ethylene plant, typically the threshold value is comprised between 5% and 20%, more preferably between 5 and 10%.
In the solution of the present invention, the ethylene plant is modified to use the electric power generated by the renewable source and to store the methane-enriched fuel stream when the renewable source generates little or no electric power. For this purpose, the methane needs to be liquified, such that it can be stored. It is this liquefaction process that is of interest here. Specifically, the intermittent nature of this particular application. The methane needs to be stored during the time that the renewable power is available and has to be used when the renewables are reducing or even absent to meet the required power demand. All of this without upsetting the ethylene plant operation. This, to make optimum advantage of the stored energy in the form of stored excess fuel gas, basically methane-rich gas. Practically all this stored fuel gas can be used to keep the ethylene plant running and maximize the power export when little to none of the energy is used to keep the liquefaction plant running.

[0011] This liquefaction in cascade cycles facilitate the temporary storage of excess fuel gas during times of excess renewable power. One can imagine that when operating a solar power plant, the availability of this power depends on the emerging climatic condition and the length of the day time and the night time during the various seasons. Similar concerns are there for wind power, where the length of the day time and night time might not be relevant, but more the emerging strength of the wind. Given that these energy sources are fluctuating in nature, it is important to be able to store energy for times when these resources are not available. In the case of ethylene plants, an excess of fuel gas can be generated when the pyrolysis reactors are improved in efficiency, either by reducing the fuel requirement or by fully electrifying them. In this case, the production of fuel gas from the plant is no longer fully utilized by the pyrolysis reactors, the so-called cracking furnaces, but needs to be used elsewhere. One of the options is to use this excess fuel gas for generating power. For this reason excess fuel gas needs to be stored during excess renewable power availability, without upsetting the ethylene plant operation.

[0012] Advantageously, the means for producing liquefied methane from the methane enriched fuel stream, as above described, are secondary means and the ethylene plant according to the present invention comprises primary means for producing liquefied methane from the methane enriched fuel stream. This primary means operates continuously whereas the secondary means operates intermittently when the renewable source generates electric power greater than a threshold value. In this case, the ethylene plant according to the present invention comprises means for closing and opening the passageway feeding the secondary means of liquefaction. In other words, the present invention offers a decoupled solution, meaning that the liquefaction train is independent from the ethylene plants own continuously operating liquefaction train. The secondary means has a very high turn-down ratio, such that renewable power loads can be handled very smoothly and power requirements at full turn-down are very low. This, to make optimum advantage of the stored energy in the form of excess fuel gas, basically methane-rich gas. Practically all this stored fuel gas can be used to provide power for the ethylene plant itself and maximize the power export. Little to none of the energy is used to keep the liquefaction plant running in the absence of renewable energy.

[0013] Advantageously, means for closing the first and/or the second passageways and means opening the first and the second passageways are control valves.

[0014] The separation section for separating cracked gas in different fractions are generally known in the art. E.g. one can use conventional distillation, such as cryogenic distillation, to obtain an ethylene-enriched product stream, a hydrogen enriched fuel stream and a methane-enriched fuel stream.

[0015] When desired, the separation section can comprise one or more units to further treatment, e.g. to increase the concentration/purity of a certain component or to remove undesired components before further use. If hydrogen or methane is to be used for a different purpose than for energy production in the plant, this hydrogen or methane can be obtained from a hydrogen-enriched stream or a methane-enriched stream after further separation/purification. Thus, e.g. hydrogen can be obtained for use in a hydrogenation process.

[0016] Note that a part of the methane-enriched fuel obtained from the cracked gas may be returned to a burner of the cracking furnace.

[0017] The gas turbine generally operates as follows: the fuel gas (methane-enriched fuel) is combusted in the combustion chamber of the gas turbine by combustion air supplied to the combustion chamber by a combustion air compressor. The produced flue gas is typically let down over a gas turbine to generate electric power via a generator. Further heat recovery of the hot flue gas exiting the turbine is generally performed in a downstream waste heat boiler. The recovered heat is used to generate steam for additional power generation, thus increasing the heat-to-power efficiency to more than 50%, so called co-generation.

[0018] The electrical power system providing electricity from a renewable source comprises usually one or more power systems selected from the group 5 consisting of wind power systems, solar energy systems, hydropower systems, geothermal energy systems and osmotic power systems (blue energy).

[0019] Depending on the embodiment, the ethylene plant according to the present invention can comprise one or more of the following features:

• the means for producing liquefied methane comprises a methane cycle, an ethylene or ethane cycle and a propylene or propane cycle.
• 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 the ambient temperature.
• the means for producing liquefied methane comprises, between the ethylene or ethane cycle and the methane cycle, means for the separation of a liquid fraction and a gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -35°C and -102°C. And preferably, the means for producing liquefied methane comprises means for injection of said gaseous fraction in the fuel gas network of ethylene plant after recovering of the cold of said gaseous fraction in the ethylene or ethane cycle and in the propylene or propane cycle.
• the means for producing liquefied methane comprises, after the methane cycle, means for the separation of a liquid fraction and of a gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between - 160°C and -100°C, and means for the mixing of said gaseous fraction with the methane-enriched fuel stream from hydrocarbon feedstock or of the ethylene plant.
• the separation section is configured to provide two methane enriched fuel streams at two different pressures and means for producing liquefied methane is configured to receive both methane enriched fuel streams.
• The means for producing liquefied methane comprises between 3 and 11 heat exchangers, preferably between 5 and 9 heat exchangers. The exchangers are special type of exchangers, namely plate fin exchangers. These exchangers allow the heat transfer between multiple hot and cold streams at very small temperature approaches for maximum recovery of heat/cold and not restricted to only two streams as are conventional exchangers.
• The means for producing liquefied methane each cycle comprises compressors, expansion valves, and coolers. The compressors are screw compressors. This type of device offers a much higher turn-down than the centrifugal compressor and ensures that at all the time the various refrigeration cycles remain coupled. The latter is essential. If the heat can't be conveyed from one cycle to the next, refrigeration is not possible.
• means for producing liquefied methane comprises a separator vessel to separate the liquid fraction from the vapor fraction.
• The ethylene plant comprises a processor able to create a first signal when the renewable source generates electric power greater than a threshold value and a second signal when the renewable source does not generate electric power or generates electric power less than a threshold value, and to send these signals to means for closing and opening the first and/or the second passageways respectively. Preferably said processor can compare the electric power generated by the renewable source with the threshold value.

[0020] The methane cycle preferably has 1 - 4 levels, preferably 3, each level fitted with an expansion valve and a downstream accumulation vessel for separating the flashed liquid into a liquid and a gaseous phase. Each of the levels has a compressor for the gaseous phase. The compressor discharge coming from the highest level is fitted with at least a heat exchanger for the condensation of the refrigerant, in this case methane, and preferably at least one exchanger for desuperheating the gaseous compressor discharge. Optionally a heat exchanger for subcooling the liquified refrigerant can be provided.

[0021] The ethane or ethylene cycle preferably has 1 - 4 levels, preferably 3, each level fitted with an expansion valve and a downstream accumulation vessel for separating the flashed liquid into a liquid and a gaseous phase. Each of the levels has a compressor for the gaseous phase. The compressor discharge coming from the highest level is fitted with at least a heat exchanger for the condensation of the refrigerant, in this case ethane or ethylene, and preferably at least one exchanger for desuperheating the gaseous compressor discharge. Optionally a heat exchanger for subcooling the liquified refrigerant can be provided.

[0022] The propane or propylene cycle preferably has 2 - 5 levels, preferably 4, each level fitted with an expansion valve and a downstream accumulation vessel for separating the flashed liquid into a liquid and a gaseous phase. Each of the levels has a compressor for the gaseous phase. The compressor discharge coming from the highest level is fitted with at least a heat exchanger for the condensation of the refrigerant, in this case propane or propylene, and preferably at least one exchanger for desuperheating the gaseous compressor discharge. Optionally a heat exchanger for subcooling the liquified refrigerant can be provided.

[0023] Another object of the present invention is a process for producing ethylene from a hydrocarbon feedstock implementing an ethylene plant as defined in the present invention, and comprising:

a) Cracking step of the hydrocarbon feedstock in the cracking furnace to produce a cracked gas stream;

b) Separating step of the cracked gas stream to provide at least an ethylene-enriched product stream, and a methane-enriched fuel stream,

c) Liquefaction step of the methane-enriched fuel stream to produce liquefied methane,

d) Storing step of the methane liquefied,

e) Evaporation step of the methane liquefied coming from the storing step in the evaporator,

f) Production step of electric power intermittently in a gas turbine with the evaporated methane coming from the evaporator,

g) Production step of electric power intermittently by a renewable source,

h) Stop step of the evaporation of the liquified methane and of production of electric power by the gas turbine when the renewable source stops producing electric power greater than a threshold value, and

i) Resumption step of the evaporation of the methane liquefied and of production of electric power by the gas turbine when the renewable source does not generate electric power or generates electric power less than a threshold value,

With the liquefaction step realized by exchange with the refrigerants of a cascade cycle.

[0024] Depending on the embodiment, the process for producing ethylene from a hydrocarbon feedstock according to the present invention can comprise one or more of the following features:

• the separating step provide two methane enriched fuel streams at two different pressures.
• the liquefaction step comprises the following sub-steps:
  i) the methane enriched fuel streams is pre-cooled at a temperature between -40°C and the ambient temperature in a propylene or propane cycle,

  ii) the pre-cooled methane enriched fuel stream is cooled at a temperature between -35°C and -102°C in a ethylene or ethane cycle, and

  iii) the cooled methane enriched fuel stream is liquefied at a temperature between -160°C and -100°C in a methane cycle.
• the liquefaction step comprises between the sub-steps ii) and iii) a separation sub-step of liquid fraction and of the gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -35°C and - 102°C, and an injection sub-step of gaseous fraction in the fuel gas network of the ethylene plant after recovering of the cold of said gaseous fraction in the ethylene or ethane cycle and in the propylene or propane cycle.
• the liquefaction step comprises after the sub-step iiii) a separation sub-step of liquid fraction and of the gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -160°C and -100°C, and a mixing sub-step of said gaseous fraction with the methane-enriched fuel stream from hydrocarbon feedstock.
• in the propylene or propane cycle, the propylene or propane is compressed at a pressure comprised between 15 and18 bar; in the ethylene or ethane cycle, the ethylene or ethane is compressed at a pressure comprised between 25 and 28 bar; in the methane cycle, the methane is compressed at a pressure comprised between 33 and 36 bar.
• The process for producing ethylene comprises a comparison step of the electric power generated by the renewable source with the threshold value; a production step of a first signal when the renewable source generates electric power greater than a threshold value and a second signal when the renewable source does not generate electric power or generates electric power less than a threshold value; and a sending step of these signals to means for closing and opening the first and the second passageways respectively. The intermittent generation of power results in excess fuel gas production. The pressure in the fuel gas system will fluctuate as a result of that and by using control signals as pressure and/or level controllers this can be used to send fuel gas to storage or retrieve fuel gas from storage. Two systems can be used HP and MP fuel gas to send the fuel gas to storage. For retrieval of the stored methane, the existing cold box in the separation section of the ethylene plant can be used as evaporator via the route that is generating the MP methane fuel as this is suitable for flashed liquid.

[0025] With the three cycles in cascade, the excess fuel gas from the ethylene plant can progressively be refrigerated to the boiling point of the excess fuel required for the storage of the excess fuel gas at atmospheric condition, approximately -160°C. Atmospheric storage is preferred, but storage at higher pressure is also possible with this scheme, in which case a higher methane compressor suction pressure can be used. To minimize exergy losses, the various refrigeration cycles have multiple levels. This minimizes the power requirements. Using screw compressors with slide valve capacity control, turn-down can be as high as 100% to 10%. In addition, VSD (VSD= variable speed drive) can further reduce the power requirement during minimum load handling, when renewable power is absent.
As the pressure ratio for screw compressors is limited, each refrigeration level is fitted with its own compressor.
The number of refrigeration levels per loop can be optimized. The fact that multiple compressors need to be used due to this limitation, makes the use of mixed refrigerants less attractive.

[0026] The figure 1 describes an example of the cascade cycles used in the ethylene plant according to the present invention.
In the ethylene plant, usually more than one methane-rich gas source is available for liquefaction. In this scheme there are two sources, high and medium pressure (HP and MP), both at roughly ambient temperature but at different pressures. Both are progressively refrigerated and fed at individual compressors depending on the available pressure. The MP at the suction of M-2 and the HP at the suction of M-3. These streams are successively compressed in M-2 and M-3 to a pressure high enough to be able to use the next refrigeration loop as heat sink. In the case of this example, to roughly 34 bar such that when the methane-rich gas is progressively desuperheated in the air cooler, the CW cooling water cooler and the various propylene and ethylene refrigeration exchangers, the major part of the methane-rich gas can be condensed at a temperature of approximately -99 to -100°C at the outlet of the cold box, just above the boiling point of ethylene of -102°C at the suction pressure of the ethylene compressor E-1. At the outlet of the cold box the partly condensed methane-rich gas is separated in a vessel (not shown for simplicity, basically each branch in the flow scheme is a separation vessel). The light ends are returned to the fuel gas network after progressively recovering the cold in the various cold box exchangers of the ethylene and propylene loop. The liquid fraction is depressurized in stages to reduce the power requirement of the methane compressors. At each stage the flashed liquid is separated in a flash vessel and the vapor fraction is sent to the connected methane compressor. The last stage at -160°C yields the liquefied methane at practically atmospheric pressure. This is sent to storage. The remaining vapor is sent to compressor M-1 to be compressed successively in M-1, M-2 and M-3 combined with the MP and HP methane rich gas from the fuel gas network and the flashed vapours from the staged decompression train.

[0027] The ethylene loop is the heat sink of the methane loop. Condensed ethylene is fully evaporated at the suction pressure of the ethylene compressor E-1 to be compressed in compressors E-1, E-2 and E-3 successively to a pressure high enough to be able to use the next refrigeration loop as heat sink. In the case of this example, to roughly 26 bar such that when the ethylene is progressively desuperheated in the air cooler, the CW cooling water cooler and the various propylene refrigeration exchangers, all the ethylene can be condensed at a temperature of approximately -21 to -24°C in the cold box and receive a bit of subcooling down to -35°C by the time it leaves the cold box, high enough above the boiling point of propylene of -40°C at the suction pressure of the propylene
compressor P-1. The subcooled liquid is depressurized in stages to reduce the power requirement of the ethylene compressors. At each stage the flashed liquid is separated in a flash vessel and the vapor fraction is sent to the connected ethylene compressor. In addition, each refrigeration level is connected to the cold box to act as a heat sink, by evaporating liquid ethylene at each level. Any remaining liquid is evaporated in the last stage at -102°C. This vapor is sent to compressor E-1 to be compressed successively in E-1, E-2 and E-3 combined with the flashed vapours from the staged decompression train.
The propylene is also a closed loop system just like the ethylene loop and operates very much in the same way. The propylene loop is the heat sink of the ethylene loop. Condensed propylene is fully evaporated at the suction pressure of the propylene compressor P-1 to be compressed in compressors P-1, P-2, P-3 and P-4 successively to a pressure high enough to be able to use a heat sink operating slightly above ambient temperature. In the case of this example, to roughly 16 bar such that when the propylene is progressively desuperheated in the air cooler, all the propylene can be condensed in the CW cooling water cooler. The condensed liquid is depressurized in stages to reduce the power requirement 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. In addition, each refrigeration level is connected to the cold box to act as a heat sink, by evaporating liquid propylene at each level. Any remaining liquid is evaporated in the last stage at -40°C. This vapor is sent to compressor P-1 to be compressed successively in P-1, P-2, P-3 and P-4 combined with the flashed vapours from the staged decompression train.

[0028] As indicated above, this solution of the present invention is meant to store the excess fuel gas from ethylene plants during the time that renewable power is available. This requires a high turn-down not available in conventional schemes. In addition, the flow scheme is particularly set up to handle methane-rich gas. This gas contains lighter fractions that prevent full condensation of the feed gas in the open methane loop. With the plant of the invention, these lighter fractions can be removed efficiently. Another advantage of means for producing liquefied methane as defined in the present invention is the reduced energy consumption for the liquefaction of methane.

Claims

1. Ethylene plant comprising :

- a cracking furnace for converting a hydrocarbon feedstock into a cracked gas stream;

- a separation section configured to provide at least an ethylene-enriched product stream, and a methane-enriched fuel stream from the cracked gas stream;

- means for producing liquefied methane from the methane enriched fuel stream;

- means for storing the liquefied methane;

- evaporator for gasifying liquefied methane;

- a first passageway for feeding evaporator with liquefied methane provided by means for storing;

- a second passageway for feeding the gas turbine with methane provided by the evaporator

- a gas turbine configured to be fed by the methane provided by the evaporator and to generate intermittently electric power used in the ethylene plant;

- a renewable source configured to generate intermittently electric power used in the ethylene plant;

- means for closing the first and/or the second passageways when the renewable source generates electric power greater than a threshold value;

- means for opening the first and/or the second passageways when the renewable source does not generate electric power or generates electric power less than a threshold value;

with means for producing liquefied methane comprising heat exchangers organized in cascade cycles.

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

3. 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 the ambient temperature.

4. Ethylene plant according to claim 3, wherein the means for producing liquefied methane comprises, between the ethylene or ethane cycle and the methane cycle, means for the separation of a liquid fraction and a gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -35°C and -102°C.

5. Ethylene plant according to any one of claims 1 to 4, wherein the separation section is configured to provide two methane enriched fuel streams at two different pressures and means for producing liquefied methane is configured to receive both methane enriched fuel streams.

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

7. Ethylene plant according to any one of claims 1 to 6, wherein the means for producing liquefied methane comprises a separator vessel to separate the liquid fraction from the vapor fraction.

8. Ethylene plant according to any one of claims 1 to 7, comprising a processor able to create a first signal when the renewable source generates electric power greater than a threshold value and a second signal when the renewable source does not generate electric power or generates electric power less than a threshold value, and to send these signals to means for closing and opening the first and/or the second passageways respectively.

9. Process for producing ethylene from a hydrocarbon feedstock implementing ethylene plant as defined in any one of claims 1 to 8, and comprising:

a) a cracking step of the hydrocarbon feedstock in the cracking furnace to produce a cracked gas stream;

b) a separating step of the cracked gas stream to provide at least an ethylene-enriched product stream, and a methane-enriched fuel stream,

c) a liquefaction step of the methane-enriched fuel stream to produce liquefied methane,

d) a storing step of the methane liquefied,

e) an evaporation step of the methane liquefied coming from the storing step in the evaporator,

f) a production step of electric power intermittently in a gas turbine with the evaporated methane coming from the evaporator,

g) a production step of electric power intermittently by a renewable source,

h) a stop step of the evaporation of the liquified methane and of production of electric power by the gas turbine when the renewable source stops producing electric power greater than a threshold value, and

i) a resumption step of the evaporation of the methane liquefied and of production of electric power by the gas turbine when the renewable source does not generate electric power or generates electric power less than a threshold value, with the liquefaction step realized by exchange with the refrigerants of a cascade cycle.

10. Process for producing ethylene according to claim 9, wherein the separating step provides two methane enriched fuel streams at two different pressures.

11. Process for producing ethylene according to claim 9 or claim 10, wherein the liquefaction step comprises the following sub-steps:

i) the methane enriched fuel streams is pre-cooled at a temperature between -40°C and the ambient temperature in a propylene or propane cycle,

ii) the pre-cooled methane enriched fuel stream is cooled at a temperature between -35°C and -102°C in a ethylene or ethane cycle, and

iii) the cooled methane enriched fuel stream is liquefied at a temperature between -160°C and -100°C in a methane cycle.

12. Process for producing ethylene according to claim 11, wherein the liquefaction step comprises between the sub-steps ii) and iii) a separation sub-step of liquid fraction and of the gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -35°C and -102°C, and an injection sub-step of gaseous fraction in the fuel gas network of the ethylene plant after recovering of the cold of said gaseous fraction in the ethylene or ethane cycle and in the propylene or propane cycle.

13. Process for producing ethylene according to claim 11 or claim 12, wherein the liquefaction step comprises after the sub-step iiii) a separation sub-step of liquid fraction and of the gaseous fraction from the pre-cooled methane enriched fuel stream cooled at a temperature between -160°C and -100°C, and a mixing sub-step of said gaseous fraction with the methane-enriched fuel stream from hydrocarbon feedstock.

14. Process 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 comprised between 15 and 18 bar;

- in the ethylene or ethane cycle, the ethylene or ethane is compressed at a pressure comprised between 25 and 28 bar;

- in the methane cycle, the methane is compressed at a pressure comprised between 33 and 36 bar.

15. Process for producing ethylene according to any one of claims 11 to 14, wherein it comprises a comparison step of the electric power generated by the renewable source with the threshold value; a production step of a first signal when the renewable source generates electric power greater than a threshold value and a second signal when the renewable source does not generate electric power or generates electric power less than a threshold value; and a sending step of these signals to means for closing and opening the first and the second passageways respectively.

Drawing