AU2010331784A1

AU2010331784A1 - Method for the Production of Liquefied Natural Gas Having an Adjusted Gross Calorific Value

Info

Publication number: AU2010331784A1
Application number: AU2010331784A
Authority: AU
Inventors: Jean-Claude Garcel; Christophe Thomas
Original assignee: Total SE
Current assignee: TotalEnergies SE
Priority date: 2009-12-18
Filing date: 2010-12-16
Publication date: 2012-06-28
Anticipated expiration: 2030-12-16
Also published as: FR2954345B1; CN102762700B; CN102762700A; BR112012014829A2; WO2011073931A1; AU2010331784B2; FR2954345A1; EP2513267A1

Abstract

The invention relates to a method for treating natural gas containing propane and/or butane, including the following steps: extracting at least one portion of the propane and/or butane from the natural gas to provide a light natural gas; cooling and liquefying the light natural gas to provide a light liquefied natural gas; mixing a portion of the light liquefied natural gas with the extracted propane and/or butane to provide a heavy liquefied natural gas; the mixing step being performed in a liquefied natural gas production facility. The invention also relates to a liquefied natural gas production facility suitable for implementing said method.

Description

IA METHOD FOR THE PRODUCTION OF LIQUEFIED NATURAL GAS HAVING AN ADJUSTED GROSS CALORIFIC VALUE 5 FiELD OF THE INVENTION The present invention relates to a method for the production of liquefied natural gas in which the gross calorific value can be adjusted 10 according to the application. The invention also relates to an installation suitable for the implementation of this method, as well as a liquefied natural gas composition enriched in C 3

-C

4 hydrocarbons. TECHNOLOGICAL BACKGROUND 15 Natural gas is essentially used as a fuel in boiler burners, gas turbines for the production of electricity, or simply in domestic cookers. All these items of equipment must be able to burn the gas safely and reliably. It is therefore essential for the combustion characteristics of the gas to be constant, in order to be compatible with users' equipment over the long term. 20 One of the sources of gas supply in consumer countries is liquefied natural gas (LNG) produced in liquefaction plants from natural gas originating from deposits. Among the properties of the gas on the market, the gross calorific value (GCV) parameter is particularly important for defining the burners- This 25 is the thermal energy given off during combustion- It is measured in kcal/Nm 3 or, in imperial units, in BTU/scf. Natural gas contains various hydrocarbons, firstly methane, but also ethane, propane, butane and traces of heavier hydrocarbons. Not all these constituents have the same GCV and so the GCV of the gas depends on its 30 composition. The GCV of a hydrocarbon is a function of the length of its carbon chain; the longer the chain, the higher the GCV. Conversely, a non fuel gas (nitrogen, etc.) has a zero GCV. A natural gas consequently has a higher GCV, the higher its heavy hydrocarbons content. As a general rule, the GCV of the extracted natural gas exceeds the specifications stipulated in 35 the various consumer countries. Moreover, the specifications of the gas in terms of GCV differ significantly from one consumer country to another, and even from one distribution network to another within the same country. For example, the 2 GCV must be relatively high in Korea, Japan or Taiwan (of the order of 1100 to 1200 btu/scf), and relatively low in the United States or the United Kingdom (of the order of 1075 btu/sof), with continental Europe specifying intermediate values. 5 There are two main means of reducing the GCV value of a gas: (1) the injection of a ballast gas, the calorific contribution of which is zero, such as nitrogen or even air in some cases, and (2) the extraction of the heavier hydrocarbons which have the greater calorific contribution. The injection of nitrogen is frequently used in the LNG reception 10 terminals but the maximum nitrogen content is limited in the distribution networks. As a result, for certain gases, injection of nitrogen is insufficient, as the maximum acceptable nitrogen content is reached before the GCV of the gas is sufficiently reduced. The injection of nitrogen is therefore not a promising route. 15 On the other hand, the extraction of liquefied petroleum gas or LPG (constituted essentially of propane and butane), at the liquefaction stage, as co-product of the LNG, allows a separate upgrading of this co-product while reducing the GCV of the LNG. However, the extraction of LPG can lead to a LNG having a GCV value that is too low for some consumer countries (Asian 20 in particular); in this case, LPG must be re-injected into the gas, typically at the reception terminals. Moreover, if this method is used, the storage and transport of the LPG require special equipment, different from those dedicated to the LNG. This is particularly disadvantageous in the context of offshore exploitation, since the 25 dimension of the equipment is a critical factor in such a context; moreover, the presence of a large stock of LPG on a maritime installation can raise safety issues, in particular due to the risks of explosion. Thus a real need exists to develop an improved process for the production of LNG, in which the storage, handling and transport of the 30 products are simplified, in particular in an offshore context; and in which it is possible to supply both markets where the GCV specification is high, and markets where the GCV specification is low. SUMMARY OF THE INVENTION 35 Firstly, the invention relates to a method for processing a natural gas containing propane and/or butane, comprising the following steps: - extracting at least a portion of the propane and/or the butane from the natural gas in order to provide a light natural gas; 3 - cooling and liquefaction of the light natural gas in order to provide a light liquefied natural gas; - mixing a portion of the light liquefied natural gas with the extracted propane and/or butane in order to provide a heavy liquefied 5 natural gas; the mixing step being carried out in an installation for the production of liquefied natural gas. According to an embodiment, the method comprises a step of deacidification, dehydration and optionally demercurization of the natural gas 10 before the step of extraction of the propane and/or the butane; and the step of extraction of the propane and/or the butane is accompanied by a step of extraction of the natural gas condensates. According to an embodiment, the method comprises a step of cooling of the extracted propane and/or butane before it is mixed with a portion of the 15 light liquefied natural gas, preferably to a temperature comprised between 100*C and -160*C, and, more particularly preferred, to a temperature comprised between -135 0 C and -145*C, said cooling step being optionally preceded by a step of addition to the extracted propane and/or butane of natural gas or of light natural gas or of methane and/or of ethane. 20 According to an embodiment, the method comprises: - storing the light liquefied natural gas; and - storing the heavy liquefied natural gas or storing the propane and/or butane extracted before the mixing step. According to an embodiment, the method comprises: 25 - optionally, a flash expansion of the light liquefied natural gas and the recovery of a first flash gas originating from this flash expansion; - optionally, a flash expansion of the heavy liquefied natural gas and the recovery of a second flash gas originating from this flash 30 expansion; - recovering a first evaporation gas originating from the storage of the light liquefied natural gas; - recovering a second evaporation gas originating from the storage of the heavy liquefied natural gas; 35 - supplying a fuel gas obtained from the first evaporation gas, the second evaporation gas, optionally from the first flash gas and optionally from the second flash gas. According to an embodiment: 4 - the first evaporation gas and the second evaporation gas are compressed by same compression means, separately or in a mixture and/or the first flash gas and the second flash gas are compressed by same compression means, separately or in a 5 mixture; or - the first evaporation gas, the second evaporation gas, the first flash gas and the second flash gas are compressed by same compression means, separately or in a mixture. According to an embodiment, the portion of the light liquefied natural 10 gas which is mixed with the extracted propane and/or butane is sampled before the flash expansion of the light liquefied natural gas or is sampled after the flash expansion of the light liquefied natural gas. According to an embodiment, the method comprises: - transferring the light liquefied natural gas and/or the heavy 15 liquefied natural gas to at least one carrier; and - optionally transporting the light liquefied natural gas and/or the heavy liquefied natural gas in the carrier; and the installation for the production of liquefied natural gas is preferably offshore. 20 According to an embodiment, the method comprises a step of mixing light liquefied natural gas and heavy liquefied natural gas in order to provide intermediate liquefied natural gas, said step: - being carried out in a carrier reception terminal; or - being carried out in the installation for the production of liquefied 25 natural gas and being optionally followed by the transfer of the intermediate liquefied natural gas to at least one carrier. According to an embodiment, during the step of mixing a portion of the light liquefied natural gas with the extracted butane and/or propane, the proportion of extracted butane and/or propane is comprised between 4 and 30 50%, preferably between 5 and 25%, more particularly preferably between 6 and 12%, with respect to the heavy liquefied natural gas obtained. A subject of the invention is also an installation for the production of liquefied natural gas comprising: - a unit for the extraction of at least a portion of the propane and/or 35 the butane from the natural gas; - a supply line for light natural gas and a supply line for propane and/or butane originating from the extraction unit; 5 - a unit for the cooling and liquefaction of the light natural gas fed by the supply line for light natural gas; - a light liquefied natural gas supply line originating from the cooling and liquefaction unit; 5 - a light liquefied natural gas branch line originating from the light liquefied natural gas supply line; and - a supply line for heavy liquefied natural gas, fed on the one hand by the light liquefied natural gas branch line and on the other hand by the supply line for propane and/or butane. 10 According to an embodiment, the installation comprises a unit for deacidification, dehydration and optionally demercurization of the natural gas, as well as means for the extraction of condensates of the natural gas. According to an embodiment, the installation comprises means of cooling propane and/or butane on the supply line for propane and/or butane. 15 According to an embodiment, the installation comprises: - a storage tank of light liquefied natural gas fed by the light liquefied natural gas supply line; and - a storage tank of heavy liquefied natural gas fed by the supply line for heavy liquefied natural gas or a storage tank of propane and/or 20 butane on the supply line for propane and/or butane. According to an embodiment, the installation comprises: - optionally a flash drum for light liquefied natural gas on the light liquefied natural gas supply line; - optionally a first flash gas collection line, originating from the flash 25 drum for light liquefied natural gas; - optionally a flash drum for heavy liquefied natural gas on the supply line for heavy liquefied natural gas; - optionally a second flash gas collection line, originating from the flash drum for heavy liquefied natural gas; 30 - a first line for the collection of evaporation gas, originating from the storage tank of light liquefied natural gas; - a second line for the collection of evaporation gas originating from the storage tank of heavy liquefied natural gas; - means for the production of energy, fed by the first line for the 35 collection of evaporation gas, the second line for the collection of evaporation gas, optionally the first flash gas collection line and optionally the second flash gas collection line. According to an embodiment, the installation comprises: 6 - means for compressing fuel gas upstream of the means for the production of energy, fed by the first line for the collection of evaporation gas and by the second line for the collection of evaporation gas; and/or 5 - means for compressing fuel gas upstream of the means for the production of energy, fed by the first flash gas collection line and by the second flash gas collection line; or - means for compressing fuel gas upstream of the means for the production of energy, fed by the first line for the collection of 10 evaporation gas, by the second line for the collection of evaporation gas, by the first flash gas collection line and by the second flash gas collection line. According to an embodiment, the light liquefied natural gas branch line is branched upstream or downstream of the flash drum for light liquefied 15 natural gas. According to an embodiment, the installation is preferably an offshore installation, and the installation comprises a system for the transfer of liquefied natural gas to a carrier, said system for the transfer of liquefied natural gas being capable of being fed: 20 - by the storage tank of light liquefied natural gas and by the supply line for heavy liquefied natural gas; or - by the storage tank of light liquefied natural gas and by the storage tank of heavy liquefied natural gas. A subject of the invention is also a liquefied natural gas composition 25 comprising from 5 to 90% of propane and butane, preferably from 5 to 60% of propane and butane, more particularly preferably from 10 to 30% of propane and butane and ideally from 10 to 15% of propane and butane. A subject of the invention is also a process for the production of liquefied natural gas comprising mixing a composition of light liquefied natural 30 gas with a composition of heavy liquefied natural gas, the total propane and butane content in the heavy liquefied natural gas being greater than the total butane and propane content in the light liquefied natural gas. According to an embodiment: - the light liquefied natural gas contains propane and butane at a 35 total content less than or equal to 2%, preferably less than or equal to 1%, ideally less than or equal to 0.5%; and - the heavy liquefied natural gas contains propane and butane at a total content comprised between 5 and 90%, preferably between 5 7 and 60%, more particularly preferably between 10 and 30% and ideally between 10 and 15%. The present invention allows the drawbacks of the state of the art to be overcome. It provides more particularly an improved process for the 5 production of LNG, in which the storage, handling and transport of the products are simplified, in particular in an offshore context; and in which it is possible to supply both markets where the GCV specification is high, and markets where the GCV specification is low. This is accomplished by producing two separate streams of LNG from 10 a single cooling and liquefaction process, namely a stream called "light" iLe. relatively low in LPG (propane and butane) and a stream called "heavy" i.e. relatively rich in LPG (propane and butane). The two streams can be processed in a similar manner. Moreover, it is possible to produce LNG having an intermediate LPG concentration (i.e. having a GCV adjusted 15 according to commercial requirements), by mixing the two above-mentioned streams, in order to be able to satisfy all markets. According to certain particular embodiments, the invention also has one or preferably several of the advantageous features detailed below. - The storage, handling and transport of the light LNG and the 20 heavy LNG (and optionally the intermediate LNG) can be carried out by using a single type of equipment. Indeed, the properties of the light LNG and the heavy LNG (temperature, evaporation, etc.) are similar. - The same loading line can be used for transferring the heavy LNG 25 and the light LNG in the carriers. This simplifies the construction and operation of the installation, in particular within the context of offshore production. No dedicated line for loading the LPG is required. - Similarly, processing the pressure-reduction and evaporation 30 gases originating from the storage of the heavy LNG and the light LNG can be carried out with common equipment. - The invention optionally makes it possible to dispense with all storage of LPG, significantly reducing the safety constraints, and allows a reduction in investment and operating costs, in particular 35 in the offshore context.

8 BRIEF DESCRIPTION OF THE FIGURES Figure 1 diagrammatically represents an embodiment of the installation according to the invention; 5 DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention is now described in greater detail and non-limitatively in the following description. All the stated concentrations are expressed, unless otherwise mentioned, in molar percentages. With reference to Figure 1, the natural gas originating from the 10 exploitation site is provided by a supply line for natural gas 1. The natural gas undergoes a step of deacidification, dehydration (or drying) and optionally demercurization in a deacidification, dehydration and optionally demercurization unit 2. Thus, the natural gas is purified of the major part of the acid gasses (in particular CO 2 and H 2 8), water and mercury that it 15 contains. The deacidification can be based on the use of aqueous solutions of amines. The dehydration can be based on the use of solutions of glycol compounds and/or molecular sieves. The drying of the gas is intended in particular to avoid the crystallization of water in the cryogenic equipment. The deacidified and dried natural gas (optionally freed from mercury 20 contained therein) then undergoes an extraction of the heavy hydrocarbons, and more particularly an extraction of the hydrocarbons of type C3+, within an extraction unit 3, in order to provide a "light natural gas". In general, this extraction step is carried out by fractionation of the natural gas. In this case, the extraction unit 3 is a fractionation unit. The 25 fractionation units are constituted by a set of distillation columns allowing the different cuts of the natural gas to be isolated The fractionation usually comprises a separation of the natural gas into a C1 cut and a G2+ cut, then the C2+ cut undergoes several successive separations in order to provide a C2 cut, a C3 cut, a C4 cut and a C5+ cut. If 30 necessary the C3 and/or C4 cuts are processed in order to remove the acids and sulphur-containing impurities (mainly COS and mercaptans). The light natural gas is essentially obtained from the C1 cut and optionally from all or part of the C2 cut (ethane). Optionally, all or part of the ethane originating from the fractionation can also be used as a fuel gas or for 35 petrochemical applications. The C5+ cut (condensates) is recovered in a condensates collection line 4 (which feeds means for storing the condensates, not represented)- The extraction of the condensates such as cyclohexane or benzene (if they are 9 present in significant quantity in the original natural gas) is important in order to avoid any crystallization in the cryogenic equipment. Moreover, the C3 and C4 cuts constitute the LPG, i.e. a stream essentially comprising butane and/or propane, which is recovered in a 5 propane and/or butane supply line 5. The C3-C4 hydrocarbons (propane and butane) can be extracted essentially in their entirety from the starting natural gas, by fractionation. The LPG streams can contain a proportion of ethane less than or equal to 1% and a proportion of C5+ less than or equal to 5%, Preferably, the 10 LPG stream contains at least 92% propane and butane, or at least 95% propane and butane, or at least 97% propane and butane or at least 98% propane and butane. But it is also possible to retain, in the light natural gas, a quantity of propane and/or butane less than or equal to 2%, preferably less than or 15 equal to 1% or even less than or equal to 0.5%. The quantity of LPG optionally left in the light natural gas is adjusted as a function of the desired GCV for the light LNG which is produced. After the extraction of the condensates and the LPG (and optionally the ethane), the light natural gas is transported by a supply line for light 20 natural gas 25 to a cooling and liquefaction unit 6. At this stage, the light natural gas (optionally already pre-cooled typically to between -30 and -70*C in the extraction unit 3) is cooled and liquefied, typically to a temperature of approximately -150*C, according to means known in the field. Thus, a LNG called "light ", is obtained, transported in a supply line for light LNG 7. 25 The light LNG generally undergoes a flash expansion (or final flash) in a flash drum for light LNG 8 placed on the supply line for light LNG 7. The flash gases are recovered in a first flash gas collection line 9. This expansion makes it possible to release the nitrogen contained in the light LNG, the latter tending to be concentrated in the flash gasses. In order to satisfy the nitrogen 30 specification of the light LNG (typically 1 % molar) the flash drum for LNG can be replaced by a column called a denitrogenation column. However, if the nitrogen content allows, it is possible that there is no final flash. In this case the light LNG is further sub-cooled (to a temperature of approximately -160*C for example) in the cooling and liquefUcliurn unit 6. 35 After the flash expansion, the light LNG is typically at a temperature of approximately -160*C, at a pressure slightly higher than atmospheric pressure (for example at pressure of 1.26 bar absolute) and it preferably 10 ulaiiis less than 1% nitrogen. It is then sent to a light LNC storage tonk 10 (it is understood that several storage tanks can be provided if necessary). A portion of the light LNG is sampled before its storage, in a light LNG branch line 11, 11a. This light LNG branch line can be connected before the 5 flash drum for light LNG 8 (i.e. upstream of the latter, line marked 11 on the diagram), or after the flash drum for light LNG 8 (i.e. downstream of the latter, line marked 11 a on the diagram). The light LNG thus sampled is mixed with the stream of the extracted propane and/or butane, in order to provide a so-called "heavy" LNG, 10 transported in the heavy LNG supply line 13. It is understood that the stream of extracted propane and/or butane, which is mixed with the light LNG sampled, can contain compounds different from propane and butane, generally in a minor quantity. For example, it can contain C5+ hydrocarbons, ethane and methane. The flow of light LNG sampled and redirected is 15 adjusted as a function of the composition desired for the heavy LNG, and as a function of the flow of extracted propane and/or butane. Generally, it is desired to avoid storing or processing the LPG, and therefore it is generally desired to use all the LPG extracted in order to produce the heavy LNG. The proportion of extracted LPG which is added to the light LNG can 20 for example be comprised between 4 and 50%, preferably between 5 and 25%, more particularly preferably between 6 and 12%, with respect to the heavy liquefied natural gas obtained. Before the mixing, it is generally desirable to provide a step of cooling the stream of propane and/or butane, by providing cooling means 12 on the 25 propane and/or butane supply line 5. For example, the LPG originating from the extraction step typically has a temperature of 30 to 40'C, and it can be cooled to a temperature comprised between -100"C and -150"C, for example to a temperature of approximately -140"C. Cooling the LPG is preferably incorporated with the liquefaction unit 6. It can also be carried out in a 30 dedicated separate exchanger, fed by a portion of the coolant or one of the coolants used for the liquefaction, or even with a coolant independent from the main liquefaction. This cooling of the LPG makes it possible to ensure that the heavy LNG is at a sufficiently low temperature and does not release too much gas during its subsequent flash expansion. 35 When the stream of LPG is cooled in the cooling means 12, it may be necessary to adjust the chemical composition of the latter in order to avoid the risks of crystallization or icing of the heavy products (propane, butane and minor quantity of C5+). To this end, the LPG can be diluted before 11 cooling by an addition of natural gas (before fractionation) and/or by an addition of the C1 cut originating from the fractionation and/or by an addition of the C2 cut originating from the fractionation and/or by an addition of light natural gas. 5 Once formed, the heavy LNG generally undergoes a step of flash expansion in a flash drum for the heavy LNG 14. Flash gases are recovered in a second line for the collection of the flash gas 15. The nitrogen contained in the heavy LNG tends to concentrate in these flash gases. After the flash expansion, the heavy LNG is typically at a temperature 10 comprised between -150*C and -160*C, at a pressure slightly greater than atmospheric pressure (for example at a pressure of 1.26 bar absolute) and it preferably contains less than 1% nitrogen. It is then sent to a heavy LNG storage tank 16 (it is understood that several storage tanks can be provided if necessary). 15 The light LNG contained in the light LNG storage tank 10 typically comprises: - at least 88% of methane, preferably at least 90% of methane, or even at least 92% of methane; - a quantity of propane and butane less than or equal to 2%, 20 preferably less than or equal to 1%, or even less than or equal to 0.5%; - a quantity of nitrogen less than or equal to 1%. - a quantity of ethane less than or equal to 10%, preferably less than or equal to 8%; 25 - a quantity of C5+ hydrocarbons less than 0.1%. The heavy LNG contained in the heavy LNG storage tank 16 typically comprises: - from 10 to 90% of methane, preferably from 30 to 88% of methane, and more particularly preferably from 80 to 85% of 30 methane; - from 5 to 90% of propane and butane, preferably from 5 to 60% of propane and butane, and more particularly preferably from 10 to 30% of propane and butane and ideally from 10 to 15% of propane and butane; 35 - a quantity of nitrogen generally less than or equal to 1%; - a quantity of ethane less than or equal to 10%, preferably less than or equal to 8%; - a quantity of C5+ hydrocarbons less than 0.1%.

12 Generally, it is advantageous for the concentration of LPG in the heavy LNG to be as high as possible, in order to minimize the quantity of heavy LNG produced. However, on the other hand, it can be desirable for the heavy LNG to have a density less than or equal to 500 kg/m, so that it can 5 be transported without difficulty by a standard methane carrier. On average, this corresponds to a butane and propane concentration less than or equal to approximately 12% (see the example below; the exact threshold however depends on the propane/butane ratio and the other compounds present, and in particular on the proportion of ethane). 10 It is nevertheless possible, if the density of the heavy LNG is greater than that accepted by standard vessels (approximately 500 kg/m 3 maximum) to design specific vessels for transporting products that have a density greater than the standard generally accepted by the marine transport companies. 15 At the light LNG storage tank 10, recovery of the evaporation gas is provided in a first evaporation gas collection line 17. Similarly, at the heavy LNG storage tank 16, recovery of the evaporation gas is provided in a second evaporation gas collection line 18. The chemical composition of the respective evaporation gases is 20 sufficiently close, and therefore their mean molecular weight is sufficiently similar, to allow these evaporation gasses (generally at a pressure of the order of 1.06 bar absolute) to be compressed by common gas compression means 19 (the pressure in the consumer devices being of the order of 4 to 50 bar or even more as a function of the envisaged energy system: steam 25 boilers, gas-; or even gas-export turbines/engines). The evaporation gasses can preferably be mixed before their compression but can also optionally be processed separately (for example in parallel) by the gas compression means 19. After compression, these gasses supply a portion of the fuel gas which is consumed in order to provide energy 30 to the entire installation. Similarly, the chemical composition of the respective flash gases is sufficiently close, and therefore their mean molecular weight is sufficiently similar, to allow these flash gases to be compressed by common gas compression means 26 (the pressure in the consumer devices being of the 35 order of 4 to 50 bar or even more as a function of the envisaged energy system: steam boilers, gas-, or even gas-export turbines/engines). The flash gases can preferably be mixed before their compression but can also optionally be processed separately (for example in parallel) by the gas 13 compression means 26. After compression, these gases also supply a portion of the fuel gas which is consumed in order to provide energy to the entire installation. It can also be envisaged to have a single compression instead of the 5 two compressions 19 and 26. However, taking account of the distance between the storage tanks area and the liquefaction train, it can be economic to boost the gases originating from the LNG storage tanks by compressors of the blower type. Moreover, it is possible to store the heavy LNG in a sub-cooled liquid 10 state, i.e. not causing any emission of flash vapour under normal operation. This makes it possible to use a storage tank of a smaller size. The method according to the invention can be implemented, and the installation according to the invention can be positioned, on shore or offshore, preferably offshore, for example on a floating support anchored to 15 the sea bed or on a fixed platform. It is also possible to provide that some (upstream) operations are carried out on shore and other (downstream) operations offshore. Advantageously, after storage the heavy LNG and the light LNG are transferred to a carrier 23 for their marine transport to a reception terminal 20 (not shown). The transfer to the carrier 23 is carried out via a LNG transfer system 22, preferably a single system, capable of being fed by a light LNG supply line 20 originating from the light LNG storage tank 10 and by a withdrawal line 21 for heavy LNG originating from the heavy LNG storage tank 16. 25 According to an alternative embodiment, the LNG transfer system 22 is fed directly by the heavy LNG supply line 13: in this case, any heavy LNG storage tank 16 is dispensed with. According to this embodiment (not shown), the extracted LPG is stored in a storage tank, and mixed with the light LNG in order to produce the heavy LNG directly when loading the 30 carrier. In this embodiment, the storage of the heavy LNG is replaced by storage of LPG; however the LPG remains in the installation and is not transferred to carriers. Still according to this embodiment, it can be advantageous to sub-cool the extracted LPG before its storage in order to avoid the emission of evaporation gas and in order to reduce the size of the 35 storage tank. In the two cases in question, the heavy LNG and the light LNG are advantageously conveyed by the same transfer system 22 to the carrier 23 (optionally providing a purge phase between the transfer of one type of LNG 14 uid it tuiinri uF 1t ul.liti ype of LNG), which constitutes a considerable simplification in the context of an offshore installation. Still within the context of an offshore installation, the transfer system 22 typically comprises loading arms or cryogenic joints/cargo hoses capable of withstanding the very low 5 temperatures of the LNG. During loading of the carrier 23, a gas evaporation return line 24 is provided. The evaporation gas collected during loading is used in the installation as fuel gas. It can for example be mixed with the evaporation gases originating from the storage tank of the light LNG 10 and/or the 10 storage tank of the heavy LNG 16. It is understood that each carrier 23 can be loaded either with light LNG only, or with heavy LNG only, or successively with light LNG and heavy LNG, each type of LNG then being stored in different tankers of the carrier. In order to provide LNG having an intermediate GCV value between 15 the GCV value of the light LNG and that of the heavy LNG produced according to the invention, it is sufficient to mix the light LNG and the heavy LNG in the adequate proportions in order to obtain the desired GCV. This mixing can be carried out: - in a receiving terminal for methane carriers; or 20 - directly in the installation according to the invention, in which case the LNG originating from the mixture can be loaded directly into the carriers, still using the transfer system 22. EXAMPLE 25 The following example illustrates the invention without limiting it. The method of the invention is applied to a natural gas having the following composition (after deacidification, demercurization and dehydration): - nitrogen 1%; 30 - C1: 87.6%; - C2: 5.93%; - C3: 1.80%; - C4 (isobutane and n-butane): 0.87%; - C5: 0.47%; 35 - C6+ 2.33%. The condensates (C5+ molecules) are extracted, as well as a portion of the LPG. In this way a light natural gas is obtained. This light natural gas is liquefied and stored in the form of light LNG. A portion of the light LNG is 15 redirected before the flash expansion (line marked 11 on Figure 1) and mixed with the stream of propane and butane extracted from the natural gas in order to constitute the heavy LNG. Three possible cases can be envisaged: 5 - heavy LNG formed from approximately 90% light LNG and 10% LPG (hereafter "10% heavy LNG"); - heavy LNG formed from approximately 75% light LNG and 25% LPG (hereafter "25% heavy LNG"); and - heavy LNG formed from approximately 50% light LNG and 50% 10 LPG (hereafter "50% heavy LNG"). Also considered by way of reference is the LNG obtained from the same natural gas after separation of the condensates but without extraction of the LPG ("LNG with LPG"). The compositions and properties of the different types of LNG compositions envisaged (streams of LNG ready at the 15 stage of transfer to a carrier) are summarized in Table 1 below (the difference between the proportion of LPG used for production of the heavy LNG and the proportion of final LPG present in the heavy LNG being explained by the loss of methane and nitrogen during the final flash): LNG with LPG 10% heavy 25% heavy 50% heavy (reference) LNG LNG LNG

N

2 0.19% 0.24% 0.14% 0.06% 0.02% C1 90.44% 92.38% 82.01% 66.09% 40.25% C2 6.73% 6.9% 6.33% 5.46% 3,84% C3 2.05% 0.48% 7.41% 18% 35.3% C4 0.98% 0% 4.08% 10.31% 20.49% C5+ 0% 0% 0.04% 0,1% 0.1% GCV 1115 btu/scfd 1070 1266 1567 2060 btu/scfd btu/scfd btu/scfd btu/scfd Density 456 kg/m 3 442 kg/m 3 500 kg/m 3 564 kg/m 3 640 kg/m3 T1 -158.1*C -158.7*C -157.6*C -155.9"C -151-0C T2 -160.15"C -160.5*C -159.4*C -157,5 0 C -152.6"C 20 Table 1: composition and properties of the LNG streams produced In the above table, T, represents the temperature of the LNG in the final flash drum and T 2 represents the temperature of the LNG at the storage 16 level. The temperatures obtained for the heavy LNGs are compatible with the existing equipment and procedures for the production and transport of LNG. By way of reference, the density of the pure LPG under the storage conditions (approximately 1080 mbar and temperature of -42 0 C for the 5 propane and -- 5C for the butane) is approximately 580 kg/m 3 . Moreover, the characteristics of the different gases recovered in order to provide fuel gas are summarized in the following Table 2:

N

2 C1 C2+ Flow content content content (kmolf (%) (%) (%) h) Fuel gases originating from Flash (F) 11.05 88.94 0.01 2053 the production of LNG Evaporation 5.75 94.24 0.01 1570 without extraction of LPG (E) (reference) Total (F)+(E) 8.75 91.24 _0.01 3623 Fuel gases originating from Flash (F) 8,22 91.76 0.02 3099 the production of LNG with Evaporation 6.36 93.63 0.01 682 extraction of LPG (E) (reference) Total (F)+(E) 7.8_9 92.1 _ 0.01 3781 Flash (F) 8-63 91.46 0.01 2303 Fuel gases originating from Evaporation 6.61 93.38 0.01 568 light LNG (if production of (E) 10% heavy LNG) Total (F)+(E) 8.15 91.84 0.01 2871 Flash (F) 7.91 92.08 0.01 761 Fuel gases originating from Evaporation 5.37 94.62 0.01 149 10% heavy LNG (E) ____ Total (F)+(E) 7.5 92.49 0.01 910 Flash (F) 8.38 91.61 0.01 3064 originating from light LNG Evaporation 6.35 93.64 0.01 717 and 10% heavy LNG. (E) Total (F)+(E) 7.99 92 0.01 3781 Flash (F) 8.53 91.46 0.01 2723 Fuel gases originating from Evaporation 6.61 93.38 0.01 644 light LNG (if production of (E) 25 eavy LNG) () ____ Total (F)+(E) 8.16 91.83 0.01 3367 10 17 Flash (F) 6.6 93.39 0.01 337 Fuel gases originating from Evaporation 3.53 94.46 0.01 72 25% heavy LNG (E) Total (F)+(E) 6.05 93.94 __ 0.01 410 Flash (F) 8.32 91.67 0.01 3060 Coiinti frmlghe LEvaporation 6.35 93.69 0.01 716 originating from light LNG (E) and 25% heavy LNG.(E Total (F)+(E) 7.93 92.06 0.01 3776 Flash (F) 8.44 91.54 0.02 2906 FuelEvaporation 6.54 93.45 0.01 669 light LNG (if production of (E) 50% heavy LNG) Total (F)+(E) 8.09 91.9 0.01 3575 Flash (F) 5.07 94.91 0.02 155 Fuel gases originating from Evaporation 1.78 98.21 0.01 45 50% heavy LNG (E) ____ I Total (F)+(E) 4.33 95.65 0.02 200 Flash (F) 8.27 91.71 0.02 3061 Combined fuel gases Evaporation 624 93.75 0.01 715 originating from light LNG (p) 62 937 00_1 Total (F)+(E) 7.89 92.1 0.01 3775 Table 2: characteristics of the different sources of fuel gas produced during the production of the LNG In each case, the total production of LNG (including LPG) is of the 5 order of 4.93 million tonnes annually. The operational parameters are adjusted so that the total quantity of fuel gas is similar in each case. It should be noted that, except in the case where there is no extraction of LPG, the compositions of evaporation gas and flash gas are close for all the LNGs produced. As a result, the equipment with respect to the flash gas, 10 the evaporation gas and the loading and transfer systems can be shared for the light LNG and the heavy LNG.

Claims

1. Installation for the production of liquefied natural gas 5 comprising: - a unit for the extraction of at least a portion of the propane and/or the butane (3) from the natural gas; - a light natural gas supply line (25) and a propane or butane supply line (5) originating from the extraction unit (3); 10 - a unit for the cooling and liquefaction of the light natural gas (6) fed by the light natural gas supply line (25); - a light liquefied natural gas supply line (7) originating from the cooling and liquefaction unit (6); - a light liquefied natural gas branch line (11, 11a) originating 15 from the light liquefied natural gas supply line (7); and - a heavy liquefied natural gas supply line (13), fed on the one hand by the light liquefied natural gas branch line (11, 11a) and on the other hand by the propane or butane supply line (5). 20

2. Installation according to claim 1, comprising a unit for the deacidification, dehydration and optionally demercurization of the natural gas (2), as well as means of extraction of condensates from the natural gas (3). 25

3. Installation according to claim 1 or 2, comprising means of cooling propane and/or butane (12) on the propane or butane supply line (5). 30

4. Installation according to one of claims 1 to 3, comprising: - a storage tank for light liquefied natural gas (10) fed by the light liquefied natural gas supply line (7); and - a storage tank for heavy liquefied natural gas (16) fed by the heavy liquefied natural gas supply line (13) or a storage 35 tank of propane and/or butane on the propane or butane supply line (5).

5. Installation according to one of claims I to 4, comprising: 19 - optionally a light liquefied natural gas flash drum (8) on the light liquefied natural gas supply line; - optionally a first flash gas collection line (9), originating from the light liquefied natural gas flash drum (8); 5 - optionally a heavy liquefied natural gas flash drum (14) on the heavy liquefied natural gas supply line (13); - optionally a second flash gas collection line (15), originating from the heavy liquefied natural gas flash drum (14); - a first evaporation gas collection line (17), originating from 10 the light liquefied natural gas storage tank (10); - a second evaporation gas collection line (18), originating from the heavy liquefied natural gas storage tank (16); - means for the production of energy, fed by the first evaporation gas collection line (17), the second evaporation 15 gas collection line (18), optionally the first flash gas collection line (9) and optionally the second flash gas collection line (15).

6. Installation according to claim 5, comprising: 20 - fuel gas compression means (19) upstream of the means for the production of energy, fed by the first evaporation gas collection line (17) and by the second evaporation gas collection line (18); and/or - fuel gas compression means (26) upstream of the means 25 for the production of energy, fed by the first flash gas collection line (9) and by the second flash gas collection line (15); or - means for compressing fuel gas upstream of the means for the production of energy, fed by the first evaporation gas 30 collection line (17), by the second evaporation gas collection line (18), by the first flash gas collection line (9) and by the second flash gas collection line (15).

7. Installation according to claim 5 or 6, wherein the light liquefied 35 natural gas branch line (11, 11a) is branched upstream (11) or downstream (11a) of the light liquefied natural gas flash drum (8). 2Q

8. Installation according to one of claims 4 to 7, which is preferably an offshore installation, said installation comprising a system for the transfer of liquefied natural gas (22) to a carrier (23), said system for the transfer of liquefied natural gas being capable of 5 being fed: - by the light liquefied natural gas storage tank (10) and by the heavy liquefied natural gas supply line (13); or - by the light liquefied natural gas storage tank (10) and by the heavy liquefied natural gas storage tank (16). 10

9. Method for processing a natural gas containing propane and/or butane, comprising the following steps: - extracting at least a portion of the propane and/or the butane from the natural gas in order to provide a light 15 natural gas; - cooling and liquefaction of the light natural gas in order to provide a light liquefied natural gas; - mixing a portion of the light liquefied natural gas with the extracted propane and/or butane in order to provide a 20 heavy liquefied natural gas; the mixing step being carried out in an installation for the production of liquefied natural gas.

10. Method according to claim 9, comprising a step of 25 deacidification, dehydration and optionally demercurization of the natural gas before the step of extraction of the propane and/or the butane; and wherein the step of extraction of the propane and/or the butane is accompanied by a step of extraction of the natural gas condensates. 30

11. Method according to claim 9 or 10, comprising a step of cooling of the extracted propane and/or butane before it is mixed with a portion of the light liquefied natural gas, preferably to a temperature comprised between -100"C and -160'C, and, more 35 particularly preferably, to a temperature comprised between 1354C and -145*C, said cooling step being optionally preceded by a step of addition to the extracted propane and/or butane of 21 natural gas or of light natural gas or of methane and/or of ethane.

12. Method according to one of claims 9 to 11, comprising: 5 - storing the light liquefied natural gas; and - storing the heavy liquefied natural gas or storing the propane and/or butane extracted before the mixing step.

13. Method according to one of claims 9 to 12, comprising: 10 - optionally, a flash expansion of the light liquefied natural gas and the recovery of a first flash gas originating from this flash expansion; - optionally, a flash expansion of the heavy liquefied natural gas and the recovery of a second flash gas originating from 15 this flash expansion; - recovering a first evaporation gas originating from the storage of the light liquefied natural gas; recovering a second evaporation gas originAtino from the storage of the heavy liquefied natural gas; 20 - supplying a fuel gas obtained from the first evaporation gas, the second evaporation gas, optionally from the first flash gas and optionally from the second flash gas.

14. Method according to claim 13, wherein: 25 - the first evaporation gas and the second evaporation gas are compressed by the same compression means, separately or in a mixture and/or the first flash gas and the second flash gas are compressed by the same compression means, separately or in a mixture; or 30 - the first evaporation gas, the second evaporation gas, the first flash gas and the second flash gas are compressed by the same compression means, separately or in a mixture.

15. Method according to claim 13 or 14, wherein the portion of the 35 light liquefied natural gas which is mixed with the extracted propane and/or butane is sampled before the flash expansion of the light liquefied natural gas or is sampled after the flash expansion of the light liquefied natural gas. 22

16. Method according to one of claims 9 to 15, comprising: - transferring the light liquefied natural gas and/or the heavy liquefied natural gas to at least one carrier; and 5 - optionally transporting the light liquefied natural gas and/or the heavy liquefied natural gas in the carrier; and wherein the installation for the production of liquefied natural gas is preferably offshore. 10

17. Method according to one of claims 9 to 16, comprising a step of mixing light liquefied natural gas and heavy liquefied natural gas in order to provide intermediate liquefied natural gas, said step: - being carried out at a reception terminal for carriers; or - being carried out in the installation for the production of 15 liquefied natural gas and being optionally followed by the transfer of the intermediate liquefied natural gas to at least one carrier.

18. Method according to one of claims 9 to 17, wherein, during the 20 step of mixing a portion of the light liquefied natural gas with the extracted butane and/or propane, the proportion of extracted butane and/or propane is comprised between 4 and 50%, preferably between 5 and 25%, more particularly preferably between 6 and 12%, with respect to the heavy liquefied natural 25 gas obtained.