Method for liquefying nitrogen-containing natural gas (variants)

Abstract

The invention relates to a method for liquefying nitrogen-containing natural gas. The invention can be used for removing nitrogen from natural gas during liquefaction. The proposed method for liquefying nitrogen-containing natural gas includes the following steps: introducing a feed stream of natural gas into a mass transfer column in which the gas feed stream is compressed and separated into nitrogen-rich gas, which is withdrawn from the top of the column, and liquefied natural gas, which is withdrawn from the bottom of the column; heating the nitrogen-rich gas withdrawn from the top of the column in at least one heat exchanger and subsequently compressing the gas in a compressor, then cooling the gas in a cooling apparatus and the heat exchanger, expanding the gas and introducing the resulting two-phase flow into the top part of the mass transfer column; wherein the flow rate of the compressor is selected so that the flow rate of the gas therethrough is at least 2.5 times greater than the flow rate of the natural gas feed stream.

Description

[0001] METHOD FOR LIQUEFYING NATURAL GAS CONTAINING NITROGEN (OPTIONS)

[0002] The present invention relates to a method for liquefying natural gas containing nitrogen. The invention can be used to remove nitrogen from liquefied natural gas.

[0003] In natural gas liquefaction processes, it is often desirable or even necessary, for example due to requirements for the resulting liquefied natural gas (LNG), to remove nitrogen from the natural gas. Typical LNG specifications often include a nitrogen content of less than 1% for Class A LNG and less than 5% for Class B LNG. Nitrogen concentration requirements in LNG stem from the risk of stratification and spontaneous mixing of LNG in LNG storage tanks with high nitrogen content. This can lead to an uncontrolled increase in tank pressure and even damage to the tank's internal capacity.

[0004] In cases of very high nitrogen concentrations (typically greater than 10 mol%, in some cases up to 20 mol% or even higher) in the natural gas feed, the use of a dedicated nitrogen recovery unit (NRU) is a reliable method for efficiently removing nitrogen and producing a pure (>99 mol%) nitrogen product. However, in most cases, natural gas contains between 1 and 10 mol% nitrogen. If the nitrogen concentration in the feedstock is in this range, the use of NRUs is difficult due to high capital costs.

[0005] U.S. Patent No. 9,945,604 discloses a simple and efficient method for removing nitrogen even from natural gas feeds with relatively low nitrogen concentrations. In this document's method, the natural gas feed stream is cooled and liquefied in a primary heat exchanger through heat exchange with a evaporating mixed refrigerant. The resulting LNG stream exits the primary heat exchanger at a temperature of approximately -150°C. The LNG stream is then further cooled in a reboiler heat exchanger, which provides heat for boiling in a mass transfer column. The LNG stream is introduced into the column and separated into a nitrogen-rich overhead gas and a nitrogen-depleted bottoms liquid. The bottoms liquid stream is withdrawn as nitrogen-depleted finished LNG.The overhead vapor stream is heated to near ambient temperature in the overhead heat exchanger and then separated into two portions, namely a nitrogen bleed stream which is vented to the atmosphere and a recycle stream which is compressed to high pressure and then cooled and condensed in the overhead heat exchanger to provide reflux to the distillation column.

[0006] The disadvantage of the proposed method is the need to use a large amount of complex, expensive cryogenic heat exchange equipment to obtain C1P.

[0007] In U.S. Patent 9,945,604, nitrogen from the column overhead is recycled back to the column to provide reflux to the top of the column, with additional reflux cooling in the overhead heat exchanger provided by a portion of the mixed refrigerant used in the main heat exchanger. The primary difference between U.S. Patent 9,816,754 and U.S. Patent 9,945,604 is that in U.S. Patent 9,945,604, the feed stream to the column is provided by a stream of boil-off gas from an LNG storage tank, which is first compressed and recycled through the main heat exchanger, where it is completely condensed before being fed to the column.

[0008] A disadvantage of the presented invention is the need to use a complex multi-flow main heat exchanger to obtain LNG fed to the column for processing.

[0009] The closest to the claimed invention is the method for liquefying natural gas containing nitrogen, known from US application No. 2023076428, which includes the following stages: feeding a stream of inlet natural gas into a distillation column, in which the inlet gas stream is liquefied and separated into a nitrogen-enriched gas taken from the top of the column and liquefied natural gas taken from the bottom of the column; heating the nitrogen-enriched gas taken from the top of the column in a heat exchanger, followed by its compression, sequential cooling in an air cooling apparatus and a heat exchanger, expansion and feeding the resulting two-phase stream to the top of the column.

[0010] The disadvantage of the presented method is that cold is used to liquefy natural gas, which must be produced using special refrigeration cycles. This greatly complicates the entire gas liquefaction plant and leads to high capital and operating costs.

[0011] The objective of the invention is to increase the efficiency of nitrogen removal from the natural gas feed stream while simultaneously reducing capital expenditures on a plant for producing nitrogen-depleted finished gas. The technical result achieved thereby consists of simplifying the design of the gas liquefaction plant.

[0012] BRIEF DESCRIPTION OF THE INVENTION

[0013] The proposed method for liquefying natural gas containing nitrogen can be implemented in several ways.

[0014] In a first embodiment, the proposed method for liquefying natural gas containing nitrogen includes the following stages: feeding an input natural gas stream into a mass-exchange column, in which the input gas stream is liquefied and separated into a nitrogen-enriched gas taken from the top of the column and liquefied natural gas taken from the bottom of the mass-exchange column; heating the nitrogen-enriched gas taken from the top of the column in at least one heat exchanger, followed by compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expansion and feeding the resulting two-phase stream into the top of the mass-exchange column; wherein the capacity of the compressor is selected such that the gas flow through it is at least 2.5 times greater than the flow rate of the input natural gas stream.

[0015] In a second embodiment, the proposed method for liquefying natural gas containing nitrogen includes the following steps: feeding an input natural gas stream into a separator as a first input stream, collecting nitrogen-enriched gas from the top of the separator and liquefied natural gas from the bottom of the separator; heating the nitrogen-enriched gas collected from the top of the separator in at least one heat exchanger, followed by compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expansion and feeding the resulting two-phase stream into the separator as a second input stream or feeding the resulting two-phase stream for mixing with the input natural gas stream before feeding it into the separator; wherein the capacity of the compressor is selected such that the gas flow rate through it is at least 2.5 times greater than the flow rate of the input natural gas stream.

[0016] In a third embodiment, the proposed method for liquefying natural gas containing nitrogen includes the following stages: feeding an input natural gas stream into a separator or mass transfer column, collecting the gas phase from the top of the separator or mass transfer column and the liquid and / or crystalline phase from the bottom of the separator or mass transfer column;feeding the gas phase from the top of the separator or mass-exchange column into an additional separator or an additional mass-exchange column, collecting liquefied natural gas from the bottom of the additional separator or an additional mass-exchange column, heating the nitrogen-enriched gas collected from the top of the additional separator or an additional mass-exchange column in at least one heat exchanger, followed by its compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expanding and separating the resulting two-phase flow into first and second parts, feeding the first part of the resulting two-phase flow for mixing with the flow of incoming natural gas, feeding the second part of the resulting two-phase flow for mixing with the gas phase from the top of the separator or mass-exchange column, wherein the capacity of the compressor is selected such that the gas flow through it is at least 2.5 times greater than the flow rate of the incoming natural gas flow.

[0017] In all embodiments of the proposed methods, it is possible to select a portion of the nitrogen-enriched gas at any stage of processing the nitrogen-enriched gas.

[0018] In this case, the selected portion of the nitrogen-enriched gas can be subjected to additional processing to isolate pure nitrogen.

[0019] The inlet natural gas stream is processed to remove moisture, mercury, acidic components, and heavy hydrocarbons from the natural gas before being fed to the mass transfer column or separator.

[0020] In the first and second variants of the proposed method, the inlet natural gas stream can be cooled before being fed into the mass transfer column or separator by mixing with liquefied natural gas, and the resulting condensate and / or solid products formed during the cooling process can be separated.

[0021] In all three variants, expansion of the nitrogen-enriched gas flow can be carried out in a throttle or turbine.

[0022] Liquefied natural gas taken from the bottom of the mass exchange column (options 1 and 3) or separator (options 2 and 3) can be cooled in an additional heat exchanger.

[0023] Liquefied natural gas taken from the bottom of the mass exchange column (option 1) or from the bottom of the separator (option 2) may contain solidified fractions of natural gas.

[0024] In the third variant of the proposed gas liquefaction method, mixing the first portion of the resulting two-phase stream with the incoming natural gas flow can be accomplished within a separator or mass-transfer column. Similarly, mixing the second portion of the resulting two-phase stream with the gas phase from the overhead of the separator or mass-transfer column can sometimes be accomplished within an additional separator or mass-transfer column.

[0025] BRIEF DESCRIPTION OF THE FIGURES

[0026] Fig. 1 is a schematic diagram illustrating a method and system for liquefying and removing nitrogen from a natural gas stream in accordance with embodiment 1 of the present invention.

[0027] Fig. 2 is a schematic diagram illustrating a method and system for liquefying and removing nitrogen from a natural gas stream in accordance with embodiment 2 of the present invention.

[0028] Fig. 3 is a schematic diagram illustrating a method and system for liquefying and removing nitrogen from a natural gas stream in accordance with Embodiment 3 of the present invention.

[0029] Fig. 4 is an enlarged schematic diagram illustrating a method and system for liquefying and removing nitrogen from a natural gas stream in accordance with the present invention.

[0030] List of designations:

[0031] 1 - input natural gas,

[0032] 2-nitrogen-enriched gas taken from the top of the column or separator,

[0033] 3 - gas flow entering the compressor inlet,

[0034] 4- mass transfer column or separator,

[0035] 5 - the flow that has been cooled in the cooling apparatus,

[0036] 6-stream entering the input of the expansion device,

[0037] 7- two-phase flow,

[0038] 8 - liquefied natural gas,

[0039] 9 - mixing device,

[0040] 10 - the selected portion of nitrogen-enriched gas,

[0041] 11 - liquid and / or crystalline phase collected from the bottom of the separator or mass transfer column,

[0042] 12 - gas expansion device,

[0043] 13 - compressor,

[0044] 14 - cooling apparatus,

[0045] 15 - heat exchanger, 16 - additional mass exchange column or separator,

[0046] 17 - gas phase from the top of the separator or mass transfer column,

[0047] 18 - the first part of the two-phase flow,

[0048] 19 - the second part of the two-phase flow,

[0049] 20 - mixing and separation unit.

[0050] DETAILED DESCRIPTION OF THE INVENTION

[0051] Unless otherwise expressly stated herein, the singular form of a noun means one or more with respect to any feature in the embodiments of the present invention described in the specification and claims. Use of the singular form does not limit the meaning to a single feature unless such a limitation is expressly stated in the text.

[0052] As used herein, the term "natural gas feed stream" further includes gases and streams containing synthetic and / or natural gas substitutes, as well as natural gas recycle streams, such as, for example, a stream containing or consisting of boil-off gas from an LPG storage tank. The primary component of natural gas is methane, and the natural gas feed stream typically contains at least 50% methane (by mole). Other typical components of the feed or raw natural gas, which may be present in the feed stream in smaller quantities, include other heavier hydrocarbons (such as ethane, propane, butanes, pentanes, etc.), helium, hydrogen, carbon dioxide, hydrogen sulfide and / or other gases, and mercury.However, the natural gas input stream may be pre-treated as necessary to reduce the (relative) levels of any high freeze point components, such as moisture, acid gases, mercury, and / or heavier hydrocarbons, to levels necessary to prevent freezing or other operational problems in the liquefaction plant before being fed to the patented gas processing circuit.

[0053] A stream or gas is "nitrogen-rich" if the nitrogen concentration in the stream or gas is higher than the nitrogen concentration in the nitrogen-containing natural gas inlet stream.

[0054] As used in this document, the term "heat exchanger" refers to any device or system that exchanges heat between two or more streams. Unless otherwise stated, a heat exchanger may consist of one or more heat exchanger sections arranged in series and / or parallel, where a "heat exchanger section" is the portion of the heat exchanger in which heat is exchanged between two or more streams. Each such section may constitute a separate module with its own housing, but the sections may equally well be combined into a single heat exchanger module with a common housing. Unless otherwise stated, a heat exchanger may be of any suitable type, such as, but not limited to, a shell-and-tube, wound-wound, or plate-and-fin heat exchanger.

[0055] The term "liquefaction" as used in this document refers to the transformation (usually by cooling) of a fluid or fluid stream from the gas phase to the liquid phase.

[0056] The term "mass transfer column" refers to an apparatus (or group of apparatuses) containing one or more mass transfer sections, where each mass transfer section consists of one or more mass transfer stages (which may, but do not necessarily, contain inserts such as packing, trays, or mesh) that increase the contact area and thus improve mass transfer between the ascending vapor and the descending liquid flowing through the section within the apparatus. Thus, the concentration of lighter components (such as nitrogen) increases toward the top of the column, while the concentration of heavier components (such as methane and other hydrocarbons) increases toward the bottom of the column.

[0057] The term "expand" implies that the gas is expanded (the gas pressure is reduced). Expansion can occur either in a conventional throttle without performing any work or in a turbine, extracting mechanical energy from the turbine. The turbine can be either a turbogenerator, where the turbine shaft is connected to an electric generator, or a turboexpander unit, which contains a turbine and a compressor section. The compressor section can be included in the proposed gas processing scheme at any point, for example, before or after the compressor. In cases of low inlet gas flow rates, a piston expander can also be used for gas expansion.

[0058] Various other types of devices that reduce gas pressure can also be used to expand the gas, such as an ejector, a vortex tube, a supersonic separator, and other similar devices. The term "cooling unit" refers to any unit that cools the flow. This unit is often an air or water cooler, meaning that the gas is cooled by heating the surrounding air or pumped water. A special heat exchanger can also be used as a cooling unit, using any flow according to the proposed gas liquefaction scheme, or a special coolant (freon or other coolants).

[0059] The term "separator" refers to any unit that separates the gas phase from the liquid phase (or solid phase). The liquid phase may contain a solid phase, such as solidified components of natural gas such as heavy hydrocarbons, CO2, H2S, etc. Gas-liquid separators used in gas processing are divided according to their operating principle into the following main types: gravity, inertial, louver, centrifugal, mesh filter-separators, or combined separators. In-line separators, as well as separators with simultaneous flow expansion, such as supersonic separators, can also be used as separators. Separation of the liquid phase (or solid phase) can be carried out simultaneously with the recovery of a portion of the gas, in the form of a gas-liquid flow. Liquefied natural gas storage tanks can also be used as separators. A detailed description of the various types of separators is given in the book by Berlin M.A., Gorechenkov V.G., Volkov N.P. “Processing of oil and natural gases”, M., Chemistry, 1981. - 472 p.

[0060] The term "compressor" refers to any unit that increases gas pressure. The proposed methods for liquefying natural gas may use centrifugal, piston, or screw compressors, or a combination of these. A detailed description of various compressor types is provided in the book "Processing of Oil and Natural Gases" by M.A. Berlin, V.G. Gorechenkov, and N.P. Volkov, Moscow: Khimiya, 1981, 472 pages.

[0061] The term "mixing" refers to the mixing of streams. Mixing can occur simply by using a tee, for example, into which two streams are fed, and at the outlet of the tee, the combined streams are withdrawn into a single pipeline. Mixing can also be accomplished using mixing devices, such as various types of mixers, such as ejectors, jet pumps, etc. Mixing can also be accomplished by feeding the streams to be mixed into a unit or group of units in which the mixing process is combined with other processes, such as separation, fractionation, cooling or heating, expansion, compression, etc. In these cases, within the context of the proposed liquefaction methods, the unit or group of units is considered to perform the mixing process and is referred to as a "mixing device."

[0062] The primary difference between the proposed invention and its prototype and similar products is that the flow rate of nitrogen-enriched gas withdrawn from the top of the mass-transfer column (or separator) is greater than the flow rate of the inlet gas. This difference is precisely why, in known gas liquefaction methods, already liquefied natural gas is fed to the mass-transfer column (or separator), and complex heat exchange equipment is used for its liquefaction. In the proposed method, gas can be fed to the column (or separator) in the gas phase, and liquefaction of the natural gas occurs within the column (or separator) by evaporating the liquid phase from the two-phase flow fed to the mass-transfer column (or separator). The two-phase flow is formed by heating the nitrogen-enriched gas withdrawn from the top of the mass-transfer column (or separator) in a heat exchanger, followed by compression in a compressor, cooling in a cooling unit and heat exchanger, and subsequent expansion.Inside the mass transfer column (or separator), natural gas, which consists primarily of methane, is condensed by evaporating liquid nitrogen fed into the mass transfer column (or separator) as part of a two-phase stream fed into the top of the mass transfer column (or separator).

[0063] The key to successfully implementing the proposed process is feeding a two-phase stream containing liquefied nitrogen to the mass-transfer column (or separator). In the proposed invention, all or part of the nitrogen-enriched gas withdrawn from the mass-transfer column (or separator) is used as the two-phase stream fed to the mass-transfer column (or separator). To condense the nitrogen-enriched gas withdrawn from the mass-transfer column (or separator), the following sequence of operations is used: heating the nitrogen-enriched gas withdrawn from the mass-transfer column (or separator) in a heat exchanger, followed by compression in a compressor, cooling in a refrigeration unit and heat exchanger, and expansion.

[0064] The necessary conditions for the formation of a two-phase flow are ensured by selecting the gas pressure after compression in the compressor. The required gas pressure after compression depends on the gas pressure in the mass-transfer column (or separator) and can be calculated using existing software packages for calculating natural gas processing processes (e.g., AEROSYM). For example, with a gas pressure in the column of 5 atm, the gas pressure after compression should be close to 60 atm.

[0065] Because nitrogen circulates at the top of the column and the nitrogen-containing inlet stream is supplied continuously, upon unit startup, nitrogen will accumulate in the nitrogen-enriched gas withdrawn from the mass-transfer column (or separator) to the required design nitrogen concentration, after which the nitrogen concentration stabilizes at the design level. Because nitrogen circulates from the mass-transfer column (or separator) back to the mass-transfer column (or separator), its flow rate may be higher than the inlet gas flow rate and is limited only by the capacity of the compressor compressing the gas.

[0066] Inside the mass transfer column (or separator), natural gas, which consists primarily of methane, is condensed by evaporating liquid nitrogen, which is fed to the mass transfer column (or separator) as part of the two-phase stream fed to the mass transfer column (or separator). Since the heat of vaporization of nitrogen (198 kJ / kg) is less than the heat of condensation of methane (510 kJ / kg), for an ideal process, the compressor capacity should be such that the gas (nitrogen) flow rate through it is at least 2.5 times greater than the flow rate of the inlet natural gas stream. This amount of nitrogen would be required for an ideal process in which the inlet natural gas stream has already been cooled to a temperature close to the methane condensation temperature, and the two-phase stream fed to the top of the column consists entirely of the liquid phase. In reality, the flow rate of nitrogen-enriched gas taken from the top of the column typically far exceeds this threshold (2.5 times) than the flow rate of the inlet gas, due to the fact that the temperature of the inlet natural gas is far from the condensation temperature of methane, and the two-phase flow fed to the top of the column includes some nitrogen in a gaseous state.

[0067] For the presented implementation options of the natural gas liquefaction method, taking into account the actual efficiency of the compressors and reasonable temperature undercoupling in the heat exchanger, the flow rate of nitrogen-enriched gas taken from the top of the column is approximately 15 times greater than the flow rate of the input natural gas.

[0068] In the proposed application, a necessary and mandatory condition for implementing the described natural gas liquefaction process is that the compressor capacity be selected such that the gas flow rate through it is at least 2.5 times greater than the inlet natural gas flow rate. No process is known to maintain such a flow ratio. Fulfilling this condition allows the liquefaction process to be carried out without additional refrigeration cycles using mixed refrigerants or pure nitrogen, significantly reducing the capital costs of implementing the liquefaction process.

[0069] Fig. 1 shows the process according to variant 1, in which the nitrogen content in the liquefied gas is lower than in the input gas due to the fact that part of the nitrogen (stream 10) is taken from the installation.

[0070] The method according to the first embodiment of the invention is carried out as follows.

[0071] The feed natural gas stream 1 is fed to the bottom of the mass-exchange column 4, in which the feed gas stream is liquefied and separated into nitrogen-enriched gas 2, taken from the top of the column, and liquefied natural gas 8, taken from the bottom of the mass-exchange column. The nitrogen-enriched gas 2 is heated in the heat exchanger 15, compressed in the compressor 13, cooled sequentially in the cooling apparatus 14 (for example, in the air cooling apparatus) and the heat exchanger 15, expanded in the gas expansion device 12 (for example, in the turbine) and the resulting two-phase stream 7 is fed to the top of the mass-exchange column 4; wherein the compressor capacity is selected such that the gas flow rate through it is at least 2.5 times greater than the flow rate of the feed natural gas stream 1.

[0072] Table 1 shows the flow data for the proposed installation according to embodiment 1 of the invention, shown in Fig. 1. The flow data contains detailed information on flow rates, pressures, temperatures, and flow compositions. Based on the flow data presented, it is easy to understand how the installation operates and what equipment is required to implement the proposed method.

[0073] Fig. 2 shows the process according to option 2.

[0074] The method according to the second embodiment of the invention is carried out as follows.

[0075] A stream of input natural gas 1 is fed directly into a separator 4 as a first input stream (or fed into a mixing device 9 installed in front of the separator 4), nitrogen-enriched gas 2 is withdrawn from the top of the separator and liquefied natural gas 8 is withdrawn from the bottom of the separator; the nitrogen-enriched gas 2, taken from the top of the separator 4, is heated in at least one heat exchanger 15, followed by its compression in a compressor 13, sequential cooling in a cooling apparatus 14 and a heat exchanger 15, expansion in an expansion device 12 (for example, in a turbine) and feeding the resulting two-phase stream 7 into the separator 4 as a second input stream or feeding the resulting two-phase stream 7 for mixing with the stream of input natural gas 1 into the mixing device 9 before feeding it into the separator 4; wherein the capacity of compressor 13 is selected so that the gas flow through it is at least 2.5 times greater than the flow rate of the input natural gas 1.

[0076] Table 2 shows the flow data for the proposed installation, according to embodiment 2 of the invention, shown in Figure 2. The flow data contains detailed information on flow rates, pressures, temperatures, and flow compositions. Based on the flow data presented, it is easy to understand how the installation operates and what equipment is required to implement the proposed method.

[0077] Fig. 3 shows the process according to option 3.

[0078] The method according to the third embodiment of the invention is carried out as follows.

[0079] A flow of inlet natural gas 1 is fed into a separator or mass-exchange column 4, a gas phase 17 is collected from the top of the separator or mass-exchange column 4 and a liquid and / or crystalline phase 11 from the bottom of the separator or mass-exchange column 4; feeding gas phase 2 from the top of separator or mass-exchange column 4 into additional separator or additional mass-exchange column 16, collecting liquefied natural gas 8 from the bottom of additional separator or mass-exchange column 16, heating nitrogen-enriched gas collected from the top of additional separator or additional mass-exchange column 16 in at least one heat exchanger 15 with subsequent compression in compressor 13, sequential cooling in cooling apparatus 14 and heat exchanger 15, expansion in expansion device 12 (for example, in a throttle) and dividing the resulting two-phase flow 7 into first 18 and second 19 parts,the first portion 18 of the resulting two-phase flow is supplied for mixing with the flow of inlet natural gas 1, the second portion 19 of the resulting two-phase flow is supplied for mixing with the gas phase 17 from the top of the separator or mass-exchange column, wherein the capacity of the compressor 13 is selected such that the gas flow rate through it is at least 2.5 times greater than the flow rate of the inlet natural gas flow 1. Table 3 shows the flow data for the proposed installation according to embodiment 3 of the invention shown in Fig. 3. The flow data contain detailed information on flow rates, pressures, temperatures and flow compositions. Based on the presented flow data, it is easy to understand how the installation operates and what equipment is needed to implement the proposed method.

[0080] The calculation models for the installations were developed using the AEROSYM software package, designed for modeling gas processing plants and liquefied natural gas facilities. This software package is included in the Russian Ministry of Communications's register of Russian software.

[0081] Based on the data provided in Tables 1, 2, 3, any qualified specialist will be able to fully understand how to implement the proposed options for the method of liquefying natural gas.

[0082] Fig. 4 is an enlarged schematic diagram illustrating a method and system for liquefying and removing nitrogen from a natural gas stream in accordance with the present invention.

[0083] In this diagram, the mixing and separation unit is designated by the number 20. In the first embodiment of the proposed method for liquefying natural gas, this mixing and separation unit is a mass-exchange column within which the two-phase flow 7 and the inlet natural gas flow are mixed, and the liquefied natural gas 8 is separated. In the second embodiment of the proposed method for liquefying natural gas, this mixing and separation unit 20 is a separator 4 or a combination of the mixing unit 9 and the separator 4. In the third embodiment of the proposed method for liquefying natural gas, this mixing and separation unit 20 is a combination of the mass-exchange column (separator) 4 with an additional mass-exchange column (additional separator) 16 and mixing units into which the two-phase flow is fed. In all three proposed methods, the two-phase flow 7 and the inlet natural gas 1 are mixed, and the liquefied natural gas 8 is separated.Thus, in accordance with the proposed methods, the mixing and separation unit may comprise a single unit or a group of units in which the process of mixing the input natural gas and the two-phase flow and the process of separating the liquefied natural gas is combined with other processes, such as fractionation, cooling or heating, expansion, compression, etc. In these cases, within the framework of the proposed liquefaction methods, the unit or group of units is considered to perform the process of mixing the input natural gas and the two-phase flow and separating the liquefied natural gas. In all three embodiments of the proposed method for liquefying natural gas, to ensure a reduction in the nitrogen concentration in the liquefied gas, it is advisable to extract a portion of the nitrogen-enriched gas at any stage of processing the nitrogen-enriched gas. In Figs. 1 and 2, the extracted portion of the nitrogen-enriched gas is shown by the number 10.Tables 1 and 2 respectively indicate the volumes of the sampled portion of nitrogen-enriched gas.

[0084] In all three variants of the presented natural gas liquefaction method, the recovered portion of nitrogen-enriched gas can be subjected to additional processing to isolate pure nitrogen. As shown in Tables 1 and 2, the molar concentration of nitrogen in the recovered portion of nitrogen-enriched gas exceeds 93%. Meanwhile, the molar concentration of methane in the recovered portion of nitrogen-enriched gas exceeds 6%. Therefore, in cases where the volume of recovered nitrogen-enriched gas is large, as well as in cases where the gas contains helium or other light components, it is advisable to condense the hydrocarbon fractions and separate the nitrogen from the helium using additional cooling. When cooling the recovered portion of nitrogen-enriched gas, hydrocarbon gases (methane, etc.) will condense first, followed by nitrogen. Thus, by cooling the recovered portion of nitrogen-enriched gas, pure nitrogen and helium can be obtained.To ensure high frequency of the specified gases, fractionation units can be additionally used.

[0085] In all proposed natural gas liquefaction options, it is advisable to treat the inlet natural gas stream before feeding it to the mass-exchange column or separator to remove moisture, mercury, acidic components, and heavy hydrocarbons. Adsorption units, such as those based on zeolites, can be used for this treatment. An absorption unit, such as one based on amines, can be used to remove acidic components, hydrogen sulfide, and carbon dioxide.

[0086] In the first and second embodiments of the proposed method, the inlet natural gas stream can be cooled before entering the mass-transfer column or separator, mixing it with liquefied natural gas, and the resulting condensate and / or solids formed during the cooling process can be separated. The condensate and / or solids will contain heavy hydrocarbons and acidic components.

[0087] In all three gas liquefaction options, expansion of the nitrogen-enriched gas flow can be accomplished in a throttle or turbine. When using a turbine, the temperature of the resulting two-phase flow is lower than when using a throttle, so a larger amount of input natural gas can be liquefied using the same compressor.

[0088] In all natural gas liquefaction variants, the liquefied natural gas withdrawn from the bottom of the separator or mass-exchange column can be cooled in an additional heat exchanger. It is advisable to perform this cooling in an additional heat exchanger in which the cooling medium is a two-phase flow, such as that withdrawn after the expansion unit. Additional cooling of the liquefied gas is beneficial because it allows for longer-distance transportation of the additionally cooled liquefied gas.

[0089] In the first and second embodiments of the proposed natural gas liquefaction method, the liquefied natural gas withdrawn from the bottom of the separator may contain solidified fractions of the natural gas. These solidified fractions can be removed from the liquefied natural gas by settling in liquefied natural gas storage facilities. The storage facilities can be periodically cleaned of the settled solidified fractions. Solidified fractions can also be removed from the liquefied natural gas using hydrocyclone units, in which the separation of the solidified fractions occurs in a centrifugal force field.

[0090] In the third variant of the proposed natural gas liquefaction method, mixing the first portion of the resulting two-phase stream with the inlet natural gas stream can be accomplished within a separator or mass-transfer column. Although the inlet natural gas stream and the first portion of the resulting two-phase stream can be fed to the separator or mass-transfer column as separate streams, mixing will occur automatically within the separator or mass-transfer column. Similar mixing of the second portion of the resulting two-phase stream with the gas phase from the overhead of the separator or mass-transfer column can also be accomplished within an additional separator or mass-transfer column.

[0091] In cases where natural gas contains low levels of nitrogen, the proposed liquefaction method can be implemented by injecting pure nitrogen into any stream of the proposed unit. This injection of pure nitrogen can occur either continuously or intermittently.

[0092] In all variants of the proposed method, the input natural gas can be pre-cooled and subjected to additional processing.

[0093] It should be appreciated that the invention is not limited to the details described above with reference to the preferred embodiments; numerous modifications and variations can be made without departing from the spirit or scope of the invention as defined by the appended claims. In particular, any stream in the proposed method can be subjected to further processing in one or more units. The units may include cooling / heating units, fractionators, separators, compression and expansion devices, mixing units, etc.

[0094] Table 1

[0095] Table 2

[0096] Table 3

Claims

CLAUSES OF THE INVENTION 1. A method for liquefying natural gas containing nitrogen, comprising the following steps: feeding an input natural gas stream into a mass-exchange column, in which the input gas stream is liquefied and separated into a nitrogen-enriched gas taken from the top of the column and liquefied natural gas taken from the bottom of the column; heating the nitrogen-enriched gas taken from the top of the column in at least one heat exchanger, followed by compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expansion and feeding the resulting two-phase stream to the top of the mass-exchange column; wherein the capacity of the compressor is selected such that the gas flow rate through it is at least 2.5 times greater than the flow rate of the input natural gas stream.

2. The method according to paragraph 1, characterized in that a portion of the nitrogen-enriched gas is collected at any stage of processing the nitrogen-enriched gas.

3. The method according to paragraph 2, characterized in that the selected portion of the nitrogen-enriched gas is subjected to additional processing to isolate pure nitrogen.

4. The method according to claim 1, characterized in that the input natural gas stream is processed before being fed into the mass exchange column to remove moisture, mercury, acidic components and heavy hydrocarbons from the natural gas.

5. The method according to paragraph 1 or 4, characterized in that the flow of input natural gas is cooled before being fed into the mass transfer column by mixing with liquefied natural gas, and the resulting condensate and / or solid products formed during the cooling process are separated.

6. The method according to claim 1, characterized in that the expansion of the nitrogen-enriched flow is carried out in a throttle or turbine.

7. The method according to paragraph 1, characterized in that the liquefied natural gas taken from the bottom of the mass exchange column is cooled in an additional heat exchanger.

8. The method according to claim 1, characterized in that the liquefied natural gas taken from the bottom of the mass exchange column contains solidified fractions of natural gas.

9. A method for liquefying natural gas containing nitrogen, comprising the following stages: feeding a stream of inlet natural gas into a separator as a first input stream, collecting nitrogen-enriched gas from the top of the separator and liquefied natural gas from the bottom of the separator; heating the nitrogen-enriched gas collected from the top of the separator in at least one heat exchanger, followed by compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expansion and feeding the resulting two-phase stream into the separator as a second input stream, or feeding the resulting two-phase stream for mixing with the stream of inlet natural gas before feeding it into the separator; wherein the capacity of the compressor is selected such that the gas flow rate through it is at least 2.5 times greater than the flow rate of the inlet natural gas stream.

10. The method according to paragraph 9, characterized in that a portion of the nitrogen-enriched gas is collected at any stage of processing the nitrogen-enriched gas.

11. The method according to claim 10, characterized in that the selected portion of the nitrogen-enriched gas is subjected to additional processing to isolate pure nitrogen.

12. The method according to claim 9, characterized in that the flow of input natural gas is processed before being fed to the separator to remove moisture, mercury, acidic components and heavy hydrocarbons from the natural gas.

13. The method according to claim 9, characterized in that the flow of input natural gas is cooled before being fed to the separator, mixing it with liquefied natural gas, and the resulting condensate and / or solid products formed during the cooling process are separated.

14. The method according to paragraph 9, characterized in that the expansion of the flow of nitrogen-enriched gas is carried out in a throttle or turbine.

15. The method according to paragraph 9, characterized in that the liquefied natural gas taken from the bottom of the separator is cooled in an additional heat exchanger.

16. The method according to claim 9, characterized in that the liquefied natural gas taken from the bottom of the separator contains solidified fractions of natural gas.

17. A method for liquefying natural gas containing nitrogen, comprising the following steps: feeding an input stream of natural gas into a separator or mass-exchange column, collecting the gas phase from the top of the separator or mass-exchange column and the liquid and / or crystalline phase from the bottom of the separator or mass-exchange column; feeding the gas phase from the top of the separator or mass-exchange column into an additional a separator or an additional mass-exchange column, the extraction of liquefied natural gas from the bottom of the additional separator or the additional mass-exchange column, the heating of nitrogen-enriched gas, extracted from the top of the additional separator or the additional mass-exchange column, in at least one heat exchanger, followed by its compression in a compressor, sequential cooling in a cooling apparatus and a heat exchanger, expansion and separation of the resulting two-phase flow into first and second parts, the supply of the first part of the resulting two-phase flow for mixing with the flow of inlet natural gas, the supply of the second part of the resulting two-phase flow for mixing with the gas phase from the top of the separator or the mass-exchange column, wherein the capacity of the compressor is selected such that the gas flow through it is at least 2.5 times greater than the flow rate of the inlet natural gas flow.

18. The method according to paragraph 17, characterized in that a portion of the nitrogen-enriched gas is collected at any stage of processing the nitrogen-enriched gas.

19. The method according to claim 18, characterized in that the selected portion of the nitrogen-enriched gas is subjected to additional processing to isolate pure nitrogen.

20. The method according to claim 17, characterized in that the flow of input natural gas is processed before being fed into an additional separator or an additional mass transfer column to remove moisture, mercury, acidic components and heavy hydrocarbons from the natural gas.

21. The method according to paragraph 17, characterized in that the expansion of the flow of nitrogen-enriched gas is carried out in a throttle or turbine.

22. The method according to paragraph 17, characterized in that the liquefied natural gas taken from the bottom of the separator or mass exchange column is additionally cooled in an additional heat exchanger.

23. The method according to claim 17, characterized in that the mixing of the first part of the resulting two-phase flow with the flow of input natural gas is carried out inside a separator or mass-exchange column.

24. The method according to paragraph 17, characterized in that the mixing of the second part of the resulting two-phase flow with the gas phase from the top of the separator or mass-exchange column is carried out inside an additional separator or an additional mass-exchange column.

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