MX2007009830A

MX2007009830A - Plant and method for liquefying natural gas.

Info

Publication number: MX2007009830A
Application number: MX2007009830A
Authority: MX
Inventors: Jolinde Machteld Van De Graaf
Original assignee: Shell Int Research
Priority date: 2005-02-17
Filing date: 2006-02-15
Publication date: 2007-09-04
Also published as: NO20074699L; AU2006215629B2; AU2006215629A1; AU2006215629C1; KR20070114751A; BRPI0608158A2; EP1848945A2; KR20070111531A; WO2006087330A3; WO2006087331A1; MY141434A; JP2008530505A; CN101120220A; MX2007009824A; CN101120219A; RU2395764C2; US20080156037A1; AU2006215630B2; AU2006215630A1; US20080156036A1

Abstract

The present invention relates to a plant (10) for liquefying natural gas (90) , the plant (10) at least comprising: - a pre-cooling heat exchanger train (1) comprising a final heat exchanger (2a) for cooling the natural gas stream (90); - a distributor (4) located upstream of the final heat exchanger for splitting the natural gas stream (90) into at least first and second natural gas substreams; - at least first and second main cryogenic systems (200,200') , each system (200,200') comprising an outlet for liquefied natural gas (95,95').

Description

PLANT AND METHOD FOR LIQUEURING NATURAL GAS FIELD OF THE INVENTION The present invention relates to a plant and a method for liquefying natural gas. BACKGROUND OF THE INVENTION US Patent 6,389,844 describes a plant and method. This comprises a single common pre-cooling cycle followed by two main liquefaction cycles arranged in parallel that operate simultaneously, where the natural gas flowing through the plant is liquefied and subcooled. A problem with the plant and previous method is the possibility of a maldistribution occurring, as the natural gas is partially condensed in the pre-cooling cycle. It is complex to realize an equal distribution of the partially condensed current in the main liquefaction cycles arranged in parallel and additional equipment and controls are required, resulting in an increase in pressure drop in the system and with it a reduction in the efficiency of liquefaction. SUMMARY OF THE INVENTION It is an object of the present invention to minimize the above problem. Additionally, it is an object of the present invention to provide a plant and a less complex method for liquefying gas.

Ref.: 184231 It is a still further object of the present invention to provide an alternative plant and method for liquefying gas for the purpose of satisfying different specifications of liquefied natural gas, in particular with respect to calorific value, in different markets. One or more of the above or other different objects are achieved in accordance with the present invention by providing a plant for liquefying natural gas, the plant at least comprising: a train of pre-cooling heat exchangers comprising a final heat exchanger optionally preceded by one or more heat exchangers, the final heat exchanger is conditioned with a precooling refrigerant circuit to extract heat from the natural gas stream; a distributor for dividing the natural gas stream into at least a first and a second natural gas substream; at least one first and second major cryogenic systems, each system comprises a main heat exchanger having a first hot side having an inlet arranged to respectively receive the first and second substreams of natural gas and an outlet for liquefied natural gas , and each system comprises a main refrigerant circuit to extract heat of the natural gas flowing through the first hot side of the corresponding main heat exchanger; where the distributor is located upstream of the final heat exchanger. It has been surprisingly discovered that placing the distributor to divide the natural gas stream into first and second subcurrents upstream of the pre-cooling heat exchanger reduces the probability of maldistribution as the natural gas stream can be divided into a point at which it is substantially in a single phase. One aspect of the present invention is that the probability of maldistribution is reduced in a surprisingly simple manner. A further advantage of the present invention is that - in that the division of the natural gas stream takes place at a point that occurs substantially in a single phase the division will not cause significant pressure drops in the natural gas circuit, as As a result, the efficiency of the liquefaction plant is increased. The above advantages can also be achieved in a plant and method for liquefying natural gas in accordance with what is currently claimed but which also comprises less two units of extraction of natural gas liquids downstream of the distributor but upstream of the main cryogenic systems. The natural gas liquid extraction units can be located upstream or downstream of the final heat exchanger of the pre-cooling heat exchanger train. Where the train of pre-cooling heat exchangers comprises two or more heat exchangers arranged in series, or the pre-cooling is performed in two or more serial stages, the distributor can be located between two heat exchangers in the train to divide the natural gas stream between the consecutive pre-cooling stages. In the mode where the natural gas liquid extraction units are present, the pre-cooling refrigerant circuit assists two main refrigerant circuits, but each main refrigerant circuit is assisted by its own natural gas liquid extraction unit . In this way the liquefaction capacity is not limited by the capacity to extract liquids from natural gas. Another advantage of this mode is that the extraction units of natural gas have not had to be increased in size to receive the highest flow rate. It has been discovered that in high productions of natural gas, it is reaches the feasibility limit of construction and transport of high pressure separation columns. This problem is treated with the supply of two smaller columns arranged in parallel and operated simultaneously. This can even mean that the additional capital cost resulting from the supply of two or more relatively small natural gas liquid extraction units in parallel is less than that of a large natural gas liquids extraction unit equipped to handle the full flow of natural gas. The invention includes not only a first group of embodiments wherein each of the main heat exchangers receives the vaporized upper light fraction exclusively from one of the natural gas liquid extraction units, but also a second group of embodiments wherein each one of the main heat exchangers receives parts of the vaporized upper light fraction from two or more natural gas liquid extraction units. An advantage of the first group of modalities is that the configuration of the equipment is relatively simple; An advantage of the second group of modalities is that a possible maldistribution is eliminated in the form of small variations in for example the composition or temperature of the respective vaporized upper light fractions.

In a further aspect, the present invention provides a method for liquefying a natural gas stream, the method at least comprising: (a) precooling the natural gas stream in one or more stages including a final stage in a train of exchangers of heat against a pre-cooling refrigerant which is circulated in a pre-cooling refrigerant circuit; (b) dividing the natural gas stream into at least one first and second natural gas substreams; (c) additionally cooling the first and second natural gas substreams obtained in step (b) in a complete condensation against a main refrigerant in at least two main cryogenic systems, wherein in each main cryogenic system the main refrigerant is circulated in a main refrigerant circuit; and (d) extracting a liquefied natural gas stream from the main cryogenic systems; wherein the division of the natural gas stream into a first and a second natural gas substream is carried out upstream of the final pre-cooling step. BRIEF DESCRIPTION OF THE FIGURES The invention will now be described by way of example in greater detail with reference to the accompanying non-limiting Figures, wherein: Figure shows a general schematic flow diagram of a first group of embodiments of the invention; Figure lb shows a general schematic flow diagram of a second group of embodiments of the invention; Figure lc shows a general schematic flow diagram of a third group of embodiments of the invention; Figure 2 schematically shows the plant and liquefaction process according to the present invention; Figure 3 schematically shows a more specific embodiment of the plant and process according to the present invention; and Figure 4 schematically shows a final expansion unit to be used in combination with the modalities. DETAILED DESCRIPTION OF THE INVENTION For the purposes of this description, an individual reference number will be assigned to a line as well as to a current carried on that line. The same reference numbers will refer to similar components. The reference is made with respect to Figures la-lc. The plant 10 for liquefying natural gas according to the present invention comprises a train of natural gas pre-cooling heat exchangers 1, a distributor 4, two main cryogenic systems 200 and 200 J and optionally two gas liquid extraction units natural 100 and 100 X The pre-cooling heat exchanger train 1 has an inlet line 90 for natural gas and outlet lines 27, 27 'for pre-cooled natural gas. In the embodiment shown in Figures la-lc the pre-cooling heat exchanger train 1 comprises two heat exchangers 2a, 2b, wherein the heat exchanger 2a is the final heat exchanger. Those skilled in the art will readily understand that train 1 may comprise more than two heat exchangers. If desired (and as is preferably the case) the heat exchangers of the train 1 may be part of the same refrigerant circuit. The distributor 4 is located upstream of the final heat exchanger 2a. If the train 1 comprises more than two heat exchangers 2a, 2b the distributor can also be located upstream. Preferably, the distributor 4 is located between two heat exchangers that are part of the precooling heat exchanger train 1. The final heat exchanger 2a can be a single heat exchanger (see Figures la and lb) but can also be a set of two or more parallel heat exchangers (2al and 2a2 in Figure 1c). It is understood that in case - in which the distributor 4 is also placed upstream of the heat exchanger 2b - the heat exchanger 2b it can also comprise two or more parallel heat exchangers. In the embodiment of the Figures la-lc the distributor 4 has at least two outputs 22, 23 and output lines 19, 19 X According to that shown in Figures la-lc, the currents in the output lines 19, 19 'are both further cooled in the final heat exchanger 2a. Alternatively, the output lines 19, 19 'may be connected to separate, the final parallel heat exchangers (2al and 2a2, according to that shown in Figure 1c). In the embodiments according to Figure la and lb, each of the natural gas liquid extraction units 100, 100 'is connected to a line 27 or 27 J and has a discharge line 108, 108' to discharge a heavy fraction, a discharge line 127, 127 'to discharge a higher light fraction. The heavy fraction comprises a natural gas liquid that is enriched with heavier components such as C3 + components, the upper light fraction comprises a poorer unenriched mixture of these heavier components, and must be liquefied. Each main cryogenic system 200, 200 'is associated with a discharge line 95, 95' for discharging liquefied natural gas. In Figure la, a generic modality is shown where each of the main cryogenic systems 200, 200 'is exclusively associated with one of the natural gas liquid extraction units 100, 100'. In Figure lb, a generic embodiment is shown in which the product streams from the natural gas liquid extraction units 100 and 100 'in the respective lines 127 and 127' are gathered together and redistributed in a second distributor 44. In this mode, each main cryogenic system 200 and 200 'thereby receives portions of the vaporized upper light fraction from both natural gas liquid extraction units 100 and 100'. In Figure 1c, the natural gas liquid extraction units 100 and 100 'are placed upstream of the final heat exchangers 2al and 2a2 of the pre-cooling heat exchanger train 1. Now with reference to a more detailed embodiment of according to that shown in Figure 2, the natural gas precooling heat exchanger train 1 may comprise a precooling heat exchanger 2a, but conveniently comprises a set of two or more heat exchangers arranged in series and / or parallel , wherein the precooling refrigerant is allowed to evaporate at one or more pressure levels. For the purpose of simplification, hereinafter the pre-cooling heat exchanger train 1 will be illustrated with the use of the precooling heat exchanger 2a; he The above heat exchanger 2b has only been indicated schematically in the figures. The natural gas precooling heat exchanger 2a has a hot side shown schematically in the form of tubes 12, 12 'having inlets 13, 13' for natural gas and outlets 14, 14 'for precooled natural gas. The tubes 12, 12 'are arranged on the cold side 15, which may be a cover side 15, of the natural gas precooling heat exchanger 2a. The plant 10 according to the invention typically also comprises a pre-cooling refrigerant circuit 3. It is understood that the same applies for other pre-cooling heat exchangers present. The precooling refrigerant circuit 3 comprises a precooling refrigerant compressor 31 having an inlet 33 and an outlet 34. The outlet 34 is connected via the conduit 35 to a cooler 36, which may be an air cooler or an air cooler. water cooler. The conduit 35 is extended by an expansion device, provided here in the form of a regulating valve 38, to the inlet 39 of the cold side 15 of the natural gas precooling heat exchanger 2. The outlet 40 of the cold side 15 is connected by means of the return duct 41 to the inlet 33 of the precooling refrigerant compressor 31.

Conveniently, the precooling refrigerant circuit 3 comprises four pressure levels for precooling the natural gas stream in two or three or four stages. The pre-cooling refrigerant configuration can be provided in accordance with US Patent 6,637,238, which is incorporated herein by reference. The distributor 4 has an inlet line 18 for receiving precooled natural gas in the above heat exchanger 2b, and two outlets 22 and 23. The two outlets 22 and 23 of the distributor 4 are connected to the inlets of two hot parallel sides in the final precooling stage 2a whereby the streams flowing through these parallel hot sides can exchange heat against the precooling refrigerant in the precooling refrigerant circuit 3. Each main cryogenic system 200, 200 'contains a main heat exchanger 5, 5 ', and a main refrigerant circuit 9, 9'. Each main heat exchanger 5, 5 'comprises a first hot side 25, 25' having an inlet 26, 26 '. The inlet 26 of the first hot side 25 is connected to the outlet 14 of the final heat exchanger 2a by the natural gas liquid extraction unit 100 by means of the conduits 27 and 127, and the inlet 26 'of the first hot side 25 'is connected to the outlet 14 'by means of the natural gas liquid extraction unit 100' by means of the conduits 27 'and 127 J Each first hot side 25, 25' has an outlet 28, 28 'in the upper part of the main heat exchanger 5 , 5 'for liquefied natural gas. The first hot side 25, 25 'is located on the cold side 29, 29' of the main heat exchanger 5, 5 ', whose cold side 29, 29' has an outlet 30, 30 '. The main heat exchangers 5 and 5 'are each associated with a liquefaction refrigerant circuit 9 respectively 9'. Each liquefaction refrigerant circuit 9, 9 'comprises a liquefying refrigerant compressor 50, 50' having an inlet 51, 51 'and an outlet 52, 52'. The inlet 51, 51 'is connected by means of a return duct 53, 53' to the outlet 30, 30 'of the cold side 29, 29' of the main heat exchanger 5, 5 '. The outlet 52, 52 'is connected by means of a conduit 54, 54' to a cooler 56, 56 ', which can be an air cooler or a water cooler, and the hot side 57, 57' of an exchanger of refrigerant heat 58, 58 'to a separator 60, 60'. Each separator 60 has an outlet 61, 61 'for liquid at its lower end and an outlet 62, 62' for gas at its upper end. Each refrigerant heat exchanger 58, 58 'includes a cold side 85, 85' having an inlet 139, 139 'and a outlet 140, 140 'to allow the entry of an auxiliary refrigerant and the discharge of spent auxiliary refrigerant. The cold side 85 is included in an auxiliary refrigerant cycle for which many options are feasible, among which are the following: One option is that the auxiliary refrigerant cycle be incorporated as a parallel cycle according to that described in US Patent 6,389,844, incorporated herein by reference, which utilizes the precooling refrigerant compressor 31 and a cooler 36, wherein the inlet 139, 139 'is connected to the line 37 by an expansion device such as a regulating valve, and the outlet 140, 140 'is connected to line 41. In another option, a separate auxiliary refrigerant circuit is provided such as that described in U.S. Patent Application Publication 2005/0005635, incorporated herein by reference, which uses either a auxiliary refrigerant compressor to feed to each of the refrigerant heat exchangers 58, 58 'in parallel or using a comp auxiliary refrigerant resistor adapted for each refrigerant heat exchanger 58, 58 '. In yet another option, for which reference is made to Figures 2 and 3 of US Patent 6,389,844 already incorporated in the present specification, the natural gas precooling heat exchanger 2a and the heat exchangers refrigerants 58 and 58 'shown in Figure 2 are combined in an integrated heat exchanger, whereby the hot sides 57 and 57' are incorporated in the form of additional hot tube packages in one or more of the heat exchangers of precooling 2a, 2b of the precooling heat exchanger train 1. Instead of a stage, the integrated precooling heat exchanger train 1 may comprise two or three or more stages in series, as described with specific reference to Figure 3 in US Patent 6,389,844 already incorporated by reference. Each liquefying refrigerant circuit 9, 9 'further includes a first conduit 65, 65' extending from the outlet 61, 61 'to the inlet of a second hot side 67, 67' extending to a midpoint of the exchanger of main heat 5, 5 ', a conduit 69, 69', an expansion device 70, 70 'and an injection nozzle 73, 73'. Each liquefying refrigerant circuit 9, 9 'further includes a second conduit 75, 75' extending from the outlet 62, 62 'to the inlet of a third hot side 77, 77' extending to the top of the exchanger of main heat 5, 5 ', a conduit 79, 79', an expansion device 80, 80 'and an injection nozzle 83, 83'.

The two natural gas liquid extraction units 100 and 100 'each comprise a distillation column 105 respectively 105'. The distillation column 105, 105 'is conditioned with a distillation column inlet 107, 107', which in the present embodiment is at the same time the extraction unit inlet which is connected to the pre-cooling heat exchanger train 1. Specifically, the distillation column input 107 is connected to the outlet 14 of the final heat exchanger 2a of the train 1 via the conduit 27, and the distillation column inlet 107 'is connected to the outlet 14' via the conduit 27 ' . The extraction unit outputs are supplied in the form of lines 127 and 127 'respectively. The distillation column 105, 105 'further has a heavy fraction outlet 109, 109' to discharge a liquid separated from the natural gas stream pre-cooled in the corresponding line 27, 27 ', and an upper outlet of light fraction 111, 111 'to discharge a vapor separated from the pre-cooled natural gas stream in the corresponding line 27, 27'. A fractionation unit (not shown), whether operating in the parallel heavy fractions or in the combined heavy fractions, can be connected to the heavy fraction output 109, 109 '.

The distillation column 105, 105 'is in accordance with that shown in Figure 2, it is conditioned only with a rectification section. Although not required by this invention, the distillation column can also be conditioned with a rectification section and an etching section, by adding an evaporator to raise the temperature at the bottom of the column. Also, an absorption section can be conditioned in the distillation column if necessary. The distillation column can be a forced separation column. The natural gas liquid extraction unit 100, 100 'further comprises an upper heat exchanger unit 113, 113', an upper separator 117, 117 'in the shape of a reflux drum, and a reflux pump 119, 119 ' The reflux drum 117, 117 'comprises a liquid reflux outlet 121, 121', and a vapor outlet 123, 123 '. The upper output of light fraction 111, 111 'is connected to a hot side 116, 116' of the upper heat exchanger unit 113, 113 ', from which the cold side 112, 112' can be exposed to a cold stream 115, 115 '. The hot side outlet of the upper heat exchanger 113, 113 'is connected to the reflow drum 117, 117'. The liquid reflux outlet 121, 121 'is connected to a suction side of the reflux pump 119, 119' of which a pressure side is connected to a reflux inlet 125, 125 'conditioned in the corresponding distillation column 105, 105 J The steam outlet 123, 123' is connected to the line 127, 127 '. Conveniently the main refrigerant circuits 9 and 9 'are identical to one another and thus are the main heat exchangers 5 and 5' and the liquid extraction units of the natural gas 100 and 100 '.

During normal operation, the natural gas 90 is supplied to the precooling heat exchanger train 1, is pre-cooled in stages in the heat exchanger 2b, is divided in the distributor 4 in the at least one first and one second sub-circulating natural gas precooled, and supplied as parallel streams 19, 19 'via the inlets 13, 13' to the natural gas precooling heat exchanger 2a. Normally, depending on the composition of the natural gas, the natural gas is partially condensed in the precooling heat exchanger train 1. The precooling refrigerant is withdrawn from outlet 40 on the cold side 15 of the natural gas precooling heat exchanger 2a, compressed in the precooling refrigerant compressor 31 to a high pressure, condensed in the condenser 36 and allowed to expand in the expansion device 38 to a low pressure. On the cold side 15 the expanded precooling refrigerant is lets evaporate at low pressure and in this way the heat is extracted from natural gas. The precooled natural gas withdrawn from the outlet 14 of the heat exchanger 2a is conducted through the conduits 27, 27 '. The amounts of natural gas that pass through conduits 27 and 27 'are conveniently equal with one another. Through the conduits 27 and 27 'the respective first and second pre-cooled natural gas streams are supplied to the inlets 107 and 107' of the natural gas liquid extraction units 100 and 100 '. Here, each of the first and second precooled natural gas substreams are fed into their respective distillation columns 105 and 105 'where they are simultaneously separated, typically by distillation or forced separation, into a heavy fraction comprising the condensed part of the corresponding undercurrent, and a vaporized upper light fraction. Depending on the temperature in the distillation column, the vaporized upper light fraction is de-enriched for C3 + components that include propane and contains predominantly methane, and frequently also C2 components including ethane, and nitrogen. The vaporized light overhead stream leaves the distillation column 105, 105 'through the upper outlet of light fraction 111, 111' after which it is fed into the the hot side 116, 116 'of the upper heat exchanger 113, 113' where it is partially condensed in a partially condensed upper stream comprising a mixture of light condensate and light steam. The partially condensed upper stream is fed to the reflow drum 117, 117 'where the light condensate is separated from the light steam. The light condensate is removed from the reflux drum 117, 117 'by the liquid reflux outlet 121, 121', and fed to a reflux of cold liquid into the distillation column 105, 105 '. The light steam is extracted from the steam outlet 123, 123 'and fed to the inlets 26 and 26' of the first hot sides 25 and 25 'of the main heat exchangers 5 and 5'. On the first hot side 25, 25 'the vaporized light fraction of the natural gas is liquefied and subcooled. Subcooled natural gas is withdrawn through conduits 95 and 95 '. Subcooled natural gas is conducted to a unit for further treatment, of which some options will be discussed later in this specification, and to tanks for storing liquefied natural gas (not shown). The main refrigerant is withdrawn from the outlet 30, 30 'of the cold side 29, 29' of the main heat exchanger 5, 5 ', compressed to a high pressure in the Liquefaction refrigerant compressor 50, 50 '. The heat from the compression is removed in the cooler 56, 56 'and additional heat is removed from the main coolant in the coolant heat exchanger 58, 58' to obtain partially condensed coolant. The partially condensed main refrigerant is then separated in the separator 60, 60 'in a liquid, heavy fraction, and a light, gaseous fraction, whose fractions are further cooled in the second and third hot sides 67, 67' and 77, 77 ' respectively to obtain liquefied and subcooled fractions at elevated pressure. The subcooled refrigerants are then allowed to expand in the expansion devices 70, 70 'and 80, 80' to a lower pressure. In this pressure the refrigerant is allowed to evaporate on the cold side 29, 29 'of the main heat exchanger 5, 5' to extract heat from the natural gas passing through the first cold side 25, 25 '. The cold stream 115, 115 ', or upper stream of refrigerant 115, 115', required to condense the reflux of liquid out of the vaporized upper light fraction may come from any suitable source. For example, it can be fed with a current derived from cycle 3, or it can be integrated as a pressure level in cycle 3. Alternatively, the upper refrigerant stream 115, 115 'can be fed with a current derived from the main coolant, for example from line 65, 65 '. This can be achieved with an arrangement where the cold side 115, 115 'of the upper heat exchanger is in fluid communication with at least one of at least two main refrigerant circuits 9, 91. An advantage of indirect heat exchanging the upper light fraction vaporized with the main refrigerant in at least one of at least two main refrigerant circuits 9, 9 'is that the temperature of the pre-cooled natural gas stream is at a low point as possible which helps to reach a deeper extraction of C3 + in the separation of natural gas liquids. In addition, the temperature of the liquid reflux stream leaving the outlet 121, 121 'may be lower to increase the recovery of C3 +. Other options are formed by any combination of two or more of the described options for cooling the vaporized upper light fraction, in particular a combination involving an integration of the hot side 116, 116 'in another heat exchanger followed by a separate upper unit of heat exchanger 113, 113 'arranged downstream of the integrated. It has been found that the pre-cooled natural gas temperature is around -25 ° C when the compressor drive power for each of the circuits of main coolant 9, 9 'and the drive power of the compressor for the pre-cooling refrigerant circuit 3 are equal and the plant is operated at a total capacity. The pressure of the pre-cooled natural gas is typically between 4 Pa and 6 MPa (40 and 60 bar). Preferably the temperature of the liquid reflux stream is between -25 ° C and -65 ° C, whereby upon lowering the temperature most of the C3 + components are separated out of the pre-cooled natural gas. More preferably, the temperature of the reflux stream of liquid is lower than -31 ° C. A percentage of 40 to 45% propane recovery is feasible with a cold reflux temperature of approximately -45 ° C, using a main refrigerant for the upper cooling in the upper heat exchanger 113, 113 J This depends on the pressure and composition of the gas. Reference is now made to Figure 3 which shows a modality involving a specific example of the use of main refrigerant from one of the main refrigerant circuits 9, 9 'to cool the vaporized upper light fraction extracted from the upper separator 117, 117 '. The hot side 116, 116 'is integrated into the main heat exchanger. Figure 3 largely corresponds to Figure 2 but in this the natural gas liquid extraction units 100, 100 'have been replaced by an alternative embodiment of natural gas liquid extraction unit 110, 110 J Since Figure 3 corresponds to Figure 2 it will not be described again, but the general reference is made instead to the corresponding parts of Figure 2. The main cryogenic heat exchangers 5, 5 'have been replaced by a modified version 55, 55', where the hot face 25, 25 'is divided into an upstream part 24, 24' and a part downstream 24a, 24a '. In the alternative mode, the upper output of light fraction 111, 111 'is connected to the inlet 26, 26' of the corresponding upstream part 24, 24 'via the conduit 126, 126'. The outlet of the upstream part 24, 24 'is connected to the reflux drum 117, 117' and the steam outlet 123, 123 'of the reflux drum 117, 117' is connected to the corresponding inlet of the downstream part 24a, 24a 'on the hot side 25, 25' through the duct 127, 127 '. As with the main heat exchanger 5, 5 ', the downstream part 24a, 24a' has an outlet 28, 28 'in the upper part of the main heat exchanger 55, 55' for liquefied natural gas. During the normal operation of the alternative mode, the cold required to condense the reflux of liquid out of the vaporized upper light fraction is provided by the main refrigerant. In another embodiment (not shown) the natural gas liquid extraction unit 100, 100 'and / or 110, 110' and the separation of the partially condensed natural gas substreams into a heavy fraction comprising the condensed part of the corresponding undercurrent, and a vaporized upper light fraction, takes a form in accordance with the embodiments thereof as described in Publication WO 2004/069384, incorporated herein by reference. In particular, the reflux of cold liquid in such embodiments is divided into first and second reflux streams of which the first is introduced into the upper part of the forced separation column and the second at a mid-point. In the embodiments described above, the precooling refrigerant is conveniently a single-component refrigerant, such as propane, or a mixture of hydrocarbon components or other convenient refrigerant used in a compression-cooling cycle or in an absorption-cooling cycle. . The main refrigerant is conveniently a multi-component refrigerant comprising nitrogen, methane, ethane, propane and butane. Conveniently, the refrigerant heat exchangers 58 and 58 'comprise a set of two or more heat exchangers arranged in series, where the pre-cooling refrigerant is allowed to evaporate at one or more pressure levels. The main heat exchangers 5 and 5 'and 55 and 55' may be of any suitable design, such as a rotary heat exchanger or a plate-fin heat exchanger. In the embodiments according to that described with reference to Figures 2 and 3, the main heat exchangers 5, 5 ', 55, 55' have a second and a third hot side, 67, 67 'and 77, 77', respectively. In an alternative embodiment, the main heat exchanger has only one hot side in which the second and third hot sides are combined. In this case the partially condensed main coolant is supplied directly to the third hot side 77, 77 ', without separating it into a liquid, heavy fraction, and a light, gaseous fraction. The compressors 31, 50 and 50 'can be multi-stage compressors with an intercooling, or a combination of compressors in series with an intercooling between two compressors, and / or a combination of compressors in parallel. The compressors 31, 50 and 50 'in the pre-cooling refrigerant circuit 3 and the two main refrigerant circuits 9 and 9' can be driven by turbine or driven by electric motor, or a combined turbine / electric motor drive. Conveniently the turbine (not shown) in the precooling refrigerant circuit is a steam turbine. In this case conveniently, the steam required to drive the steam turbine is generated with the heat released from the cooling of the gas turbine extractors (not shown) of the main refrigerant circuits. The present invention provides a plant for liquefying natural gas that can be expanded, wherein in a first stage a single train is constituted with a 100% liquefaction capacity, and wherein in a second stage the second main heat exchanger and the second liquefaction refrigerant circuit of the same size equal to the first can be added to expand the liquefaction capacity to between approximately 140 and approximately 160%, while the C3 + components of natural gas can be controlled. An advantage of the present invention is that the pre-cooling and liquefaction conditions, for example the refrigerant compositions, can be easily adapted so that an efficient operation is achieved. Furthermore, in the event that one of the liquefaction circuits has to be left out of operation, the conditions may adapt to work efficiently with a single liquefaction train. The calculations have further shown that the liquefaction efficiency (amount of liquefied gas produced per unit of work performed by the compressors) is not adversely affected by the use of a pre-cooling refrigerant circuit that assists two main refrigerant circuits. The liquefaction capacity can be further extended by the supply of at least one final expansion unit, connected to the outlet conduits 95, 95 'for liquefied natural gas. Figure 4 shows an embodiment of such a final expansion unit that can be added to any of the plants described above. Each conduit 95, 95 'is connected to a final expansion expander 97, 97' and a regulating valve 99, 99 '. The low pressure ends discharge into conduits 101, 101 'which both connect to a final expansion gas separator 103. Alternatively, the junction where the liquefied natural gas in conduits 95 and 95' is combined upstream of a single final expansion expander (not shown). The final expansion gas separator is conditioned with a final expansion gas outlet 133 and a liquefied natural gas outlet 135. The final expansion separator can also be a distillation column or a forced separation column or any convenient alternative to achieve an optimum separation efficiency between expanded gas and liquefied natural gas. An optional pump 137 may be conditioned to carry the liquefied natural gas to any desired pressure before discharging it into line 138 for transport or storage. Expanded gas outlet 133 is connected to a compressor 139. The high pressure outlet of compressor 139 is connected to a cooler 141, which may be an environmental cooler. A heat exchanger 143 is supplied upstream of the compressor 139 to have the ability to maintain the cold created in the final expansion gas. During normal operation, the pressure in the liquefied natural gas is lowered in the final expansion expander 97, 97 'and the regulating valve 99, 99', preferably at atmospheric or near atmospheric conditions. This expansion lowers the temperature of the liquefied natural gas, and final expansion gas is also formed in the process. Typically, the temperature is lowered by approximately 10 ° C when the expanded pressure drops from 5 MPa (50 bar) to atmospheric pressure. Due to an additional drop in temperature, more liquefied natural gas can be produced with a certain cooling energy in the pre-cooling train 1 and the main cryogenic systems 200, 200 J The final expansion gas is separated from the liquefied natural gas in the final expansion gas separator 103. The final expansion gas leaving the final expansion gas separator 103 it is compressed to a pressure whereby it can be discharged through line 145 for further use, for example as fuel gas. The cold present in the final expansion gas can be maintained by the heat exchanger 143, for example to precool the main coolant. In that case, the heat exchanger 143 could be included in the main refrigerant circuits 9, 9 '. For the purpose of further increasing the capacity of the plant, an optional final expansion gas feedback loop can be supplied through which a portion of the final expansion gas in line 145 is at least partially condensed and reinjected inside. of the liquefied natural gas stream upstream of the final expansion separator 103. For this purpose, the optional feedback circuit may comprise an additional compressor 147, of which the low pressure terminal is connected to line 145. The high-end terminal pressure of the additional compressor 147 is connected to a line upstream of the final expansion gas separator, consecutively by means of an optional additional cooler 149, a heat exchanger 143 and an expansion device such as a regulating valve 151. With optional reinjection, the additional compressors 139 and 147 provide extra points where the cooling service can be introduced. in the process and the cooling temperature in the main refrigerant circuits can be increased. Due to the extra cooling service added in this way a higher quantity of liquid natural gas can be produced. Calculations have shown that 4 to 5% additional liquefaction capacity can be achieved with the final expansion system that includes optional recycling. Other final expansion or expanded systems may be used instead of those described here. The final expansion system according to what is described in the US Patent 5,893,274 is included as a reference. Those skilled in the art will readily understand that the present invention can be modified in many different ways without departing from the scope of the appended claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims

CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A plant for liquefying natural gas, characterized in that it at least comprises: - a train of pre-cooling heat exchangers comprising a heat exchanger At the end, preceded by one or more heat exchangers, the final heat exchanger is conditioned with a precooling refrigerant circuit to extract heat from the natural gas stream; a distributor for dividing the natural gas stream into at least a first and a second natural gas substream; - at least the first and second main cryogenic systems, each system comprises a main heat exchanger having a first hot side having an input arranged to respectively receive the first and second substreams of natural gas and an outlet for liquefied natural gas , and each system comprises a main refrigerant circuit for extracting heat from the natural gas flowing through the first hot side of the corresponding main heat exchanger; where the distributor is located upstream of the final heat exchanger.
2. The plant according to claim 1, characterized in that it also comprises: - at least two extraction units of natural gas liquids each conditioned with an extraction unit inlet arranged to receive one of the natural gas sub-currents, and each comprises a heavy fraction outlet, and an upper outlet of light fraction; the upper outputs of the light fraction are connected to the inlets of the main cryogenic systems. The plant according to claim 2, characterized in that each of the natural gas liquid extraction units is conditioned with a reflux inlet arranged to receive a reflux of liquid from a liquid reflux outlet of a top separator , the upper separator is conditioned with an inlet in fluid communication with the upper outlet of light fraction and a vapor outlet in fluid communication with the corresponding main cryogenic heat exchanger. The plant according to claim 3, characterized in that an upper heat exchanger is supplied upstream of the upper separator to extract heat from the upper light fraction, from the upper heat exchanger the cold side is located in the upper heat exchanger. fluid communication with at least one of the at least two main refrigerant circuits. 5. A plant according to claim 1, characterized in that the final heat exchanger comprises two parallel heat exchangers, the plant further comprising: - at least two extraction units of natural gas liquids that are present upstream of The final heat exchangers are parallel. A plant according to any of the preceding claims, characterized in that it also comprises at least one final expansion unit, connected to the outlets for liquefied natural gas of the at least two heat exchangers and comprising at least one output for final expansion gas and an outlet for liquefied natural gas. 7. A plant for liquefying natural gas according to any of the preceding claims, characterized in that the distributor has at least two outputs. 8. A method for liquefying a stream of natural gas, characterized in that it at least comprises: (a) precooling the stream of natural gas in one or more stages including a final stage in a heat exchanger train against a precooling refrigerant which is circulated in a circuit pre-cooling refrigerant, where the heat exchanger train comprises a final heat exchanger preceded by one or more heat exchangers; (b) dividing the natural gas stream into at least one first and second natural gas substreams; (c) additionally cooling the first and second natural gas substreams obtained in step (b) in a complete condensation against a main refrigerant in at least two main cryogesystems, wherein in each main cryogesystem the main refrigerant is circulated in a main refrigerant circuit; and (d) extracting a liquefied natural gas stream from the main cryogesystems; wherein the division of the natural gas stream into a first and a second natural gas substream is carried out upstream of the final pre-cooling step. The method according to claim 8, characterized in that the first and second subcurrents of natural gas obtained in step (b) are simultaneously separated into a liquid heavy fraction and a vaporized upper light fraction, before further cooling of the fraction light vaporized top in step (c) in a complete condensation. The method according to claim 9, characterized in that the additional cooling in step (c) it comprises partially condensing each of the vaporized upper light fractions to form a light condensate and a light vapor, separating the light condensate from the light steam, feeding the light condensate as a cold reflux into the step of simultaneously separating each of the first and second partially condensed natural gas substreams and additionally cooling the light steam in a complete condensation. The method according to claim 10, characterized in that the partial condensate of each of the vaporized upper light fractions comprises an indirect heat exchange with the main refrigerant in at least one of the at least two refrigerant circuits main. The method according to any of the preceding claims 8-11, characterized in that the stream of liquefied natural gas obtained in step (d) is subsequently expanded thereby obtaining a mixture comprising a liquefied natural gas even further cooled and a expanded steam, wherein the expanded vapor is separated from the liquefied natural gas even further cooled, compressed, at least partially condensed and reinjected into the liquefied natural gas stream upstream of the expanded vapor separation.