RU2743086C1 - Extraction of helium from natural gas - Google Patents

Abstract

FIELD: gas industry.

SUBSTANCE: present invention relates to a method of extracting helium from raw natural gas containing methane, nitrogen and helium. Method includes cooling natural gas to obtain partially condensed cooled natural gas, separation of cooled natural gas in the system of the distillation column to obtain helium-enriched steam removed from the top of the column, and helium-depleted still liquor, cooling of the helium-rich vapor, by indirect heat exchange to obtain partially condensed head stream, Separation of partially condensed overhead stream in separator of overhead for production of crude helium vapor and recycled liquid and expanding the portion of the helium-depleted still residue to obtain a first fraction of the helium-depleted still residue. At that, cooling load for helium-enriched steam is partially provided due to indirect heat exchange with first fraction of helium depleted still residue.

EFFECT: invention provides maximum total output of helium using an energy-saving method.

13 cl, 2 dwg, 2 tbl, 2 ex

Description

LEVEL OF TECHNOLOGY

[0001] The present invention relates to methods and apparatus for recovering helium. In particular, the invention relates to the separation of helium from a natural gas stream containing methane, nitrogen and helium using cryogenic distillation.

[0002] Helium is present in many natural gas fields around the world, but the growing interest in the efficient extraction of helium from natural gas fields with low concentrations of helium, for example, less than about 2000 ppm ^1. Recovering helium from natural gas at such low levels has long been considered uneconomical. Typically, helium is extracted from natural gas as a by-product of liquefied natural gas (LNG) production or nitrogen emissions. In both cases, the methane condenses and the lighter helium is easily released as a gas. The present invention relates to the case where the natural gas stream does not require liquefaction or nitrogen removal. In this case, the gas may still contain a significant amount of nitrogen, but this is not enough to prevent the use of natural gas in a pipeline or gas turbine.

[0003] It is known to extract helium from natural gas. Gottier (US5011521) describes the recovery of helium using a stripper to enrich the concentration of helium relative to the composition of the feed gas. In the example shown, the helium enrichment is limited by the stripping column by about one order of magnitude, from 0.44% to 5.16% helium. The goal of overhead helium enrichment is to reduce the flow to the helium purifier by increasing the helium mole fraction. No additional means for enriching the helium in the stream leaving the top of the stripper before entering the purifier is not disclosed.

[0004] Additionally, Gottier discloses the use of a dense fluid expander (DFE) to extract energy from the expansion of a higher pressure stream to a lower pressure for feeding to a distillation column. Operating the distillation column at a higher pressure is associated with higher capital costs due to the difficulty of performing separation at high pressure and the difficulty of feeding the reboiler load to the distillation column. Complex separation results in higher reboiler loading for a given helium output, resulting in higher steam consumption. Higher steam flow rates combined with unfavorable surface tension and vapor-to-liquid density ratios result in increased column diameters. To avoid these disadvantages, the pressure of the feed is reduced before entering the distillation column.

[0005] Oeflke (US2014 / 0137599) describes additional separation to further enrich the helium content of the overhead stream from the stripper. The overhead stream is cooled and the pressure reduced to provide a helium-rich vapor stream and a helium-depleted liquid stream. A helium-depleted liquid stream, which still contains some helium, is pumped and combined with helium-depleted natural gas from the bottom of the stripper. Helium not recovered from the helium-depleted liquid stream reduces the overall yield by 0.4% in accordance with the above example. In addition, in this example, the pressure of the helium-rich steam stream decreases from 550 psi. d / sq. inch to 100 lb. d / sq. inch, which may require re-compression to enter the downstream helium purification stage.

[0006] Mitchell et al. (US4758258) describe multistage separation for recovering helium from natural gas while separating ethane, propane and heavier hydrocarbons from the base material, methane. There are two similarities to Oeflke. First, cooling for the final separation of helium and nitrogen from methane is achieved by lowering the feed pressure to the separator to form a crude helium stream. Second, the helium contained in the liquid stream from the separator is not recovered, which reduces the overall helium yield.

[0007] Agrawal (US5167125) describes a process in which light gases such as helium are removed by partial condensation of overhead vapor from a distillation column. The resulting liquid stream provides reflux in the distillation column and the helium-rich vapor stream can be further purified.

[0008] To minimize the power required for the helium recovery methods described in the prior art, intermediate streams that contain small but significant amounts of helium are diverted into helium-depleted finished natural gas, reducing the overall helium yield. There is a need to achieve the highest possible overall helium yield by recovering helium from intermediate streams in an energy efficient manner.

SUMMARY OF THE INVENTION

[0009] The present invention relates to a multi-stage process for recovering helium from a natural gas stream optimized for high helium yield and low energy consumption. First, contaminants are removed as needed, for example CO ₂ by amine absorption, water and heavy hydrocarbons by adsorption at variable temperature, and / or mercury by adsorption on activated carbon. The helium is then recovered using a cryogenic distillation column system. The column overhead helium is increased with a condenser to recover nitrogen and methane, which increases the methane yield and reduces the flow rate for downstream helium purification. The raw helium stream undergoes a cryogenic partial condensation process to further increase the concentration of helium before removing hydrogen by catalytic combustion. Final purification is carried out by pressure swing adsorption (PSA), after which the tail gas is re-compressed, dried and recirculated. The pure PSA helium product can then be liquefied for shipping and sale.

[0010] The helium-depleted liquid from the bottom of the distillation column system is used to provide cooling to the process. Several pressures have been selected for the refrigerant to optimize the cooling curves and therefore the heat transfer efficiency. A portion of the helium-depleted liquid is pumped to minimize the overall recompression power. All returning natural gas streams are recompressed to match the feed pressure when returned to the pipeline, or compressed to the pressure required to recover natural gas, for example by combustion in a gas turbine.

[0011] The pressure in the distillation column system is selected to reduce the risk of poor separation as a result of operating at too high a pressure. To reduce the increased demand for power, a dense fluid expander (DFE) can be used to generate power that can be used in the process by expanding the feed stream to column pressure. Isentropic expansion of the fluid through the DFE also results in a lower temperature in the outlet stream than would be the case with isenthalpic expansion through the valve. The use of DFE saves power to increase capital costs and should be optimized accordingly. In addition, the process may use an expander in one or more return streams to reduce overall power consumption and provide cooling to the process.

BRIEF DESCRIPTION OF THE GRAPHIC MATERIALS

[0012] The present invention will now be described with reference to the accompanying figures, in which like reference numerals designate like elements:

[0013] FIG. 1 is a process flow diagram illustrating a method for pretreating, recovering, purifying and liquefying helium from a natural gas stream.

[0014] FIG. 2 is a flow diagram illustrating a method for recovering helium in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The following detailed description illustrates only preferred exemplary embodiments of the invention and is not intended to limit the scope, scope, or configuration of the invention. Rather, the following detailed description of the preferred exemplary embodiments of the invention will provide those skilled in the art with a description to enable the preferred exemplary embodiments of the invention to be practiced. Various modifications may be made to the function and arrangement of elements without departing from the spirit and scope of the invention, which are defined by the appended claims.

[0016] As used herein, the singular form of the noun means one or more in relation to any feature in the embodiments of the present invention described in the description and the claims. The use of the singular forms does not limit the meaning to one feature, unless such limitation is expressly indicated in the text. The definite article preceding singular or plural nouns or expressions denotes a specific specified feature or specific specified features and may denote singular or plural depending on the context in which it is used.

[0017] As used herein, the expression "and / or" placed between the first object and the second object includes any of the values of (1) only the first object, (2) only the second object, and (3) the first object, and second object. The term "and / or", placed between the last two items of a list of 3 or more items, denotes at least one of the items in the specified list, including any particular combination of items in this list. For example, “A, B and / or C” has the same meaning as “A and / or B and / or C” and includes the following combinations of A, B and C: (1) only A, (2 ) only B, (3) only C, (4) A and B but not C, (5) A and C but not B, (6) B and C but not A and (7) A and B and C.

[0018] The term "multiple (plural)" means "two or more of two".

[0019] The adjective "any" means one, several or all, regardless of the number.

[0020] The phrase "at least part" means "part or all". "At least part of the stream" has the same composition, with the same concentration of each of the types of molecules, as the stream from which it is obtained

[0021] As used herein, "first", "second", "third", etc. are used to distinguish from a variety of stages and / or features and are not indicative of a total number or relative position in time and / or space, unless it is expressly indicated in the text as such.

[0022] All composition values will be indicated in mole percent.

[0023] The terms "lean" or "lean" mean that the concentration of the specified component in molar percentage is less than in the original stream from which it was obtained. "Depleted" and "poor" does not mean that the specified component is completely absent in the stream.

[0024] The terms "enriched" or "rich" mean that the concentration of the specified component in mole percent is higher than in the original stream from which it was obtained.

[0025] The terms "downstream" and "upstream" refer to the intended direction of flow of the transferred process fluid. If the direction of flow of the process fluid from the first device to the second device is assumed, then the second device is located downstream of the first device. In the case of recycle flow, downstream and upstream refer to the first process fluid pass.

[0026] The term "dense fluid expander," abbreviated DFE, also known as a liquid expander, refers to equipment that derives mechanical work from the depressurization of a dense fluid, such as a liquid or supercritical fluid, similar in function to a gas expander. This expansion is best approximated as an isentropic process, as opposed to a valve, which is best approximated as an isenthalpic process.

[0027] The term "indirect heat transfer" refers to the process of transferring physical heat and / or potential heat between two or more fluids without physical contact between said fluids. Heat can be transferred through the wall of the heat exchanger or using an intermediate heat exchange fluid. The term "hot stream" refers to any stream that exits a heat exchanger at a temperature lower than entering the heat exchanger. Conversely, the term "cold stream" is a stream that leaves the heat exchanger at a temperature higher than when entering the heat exchanger.

[0028] The term "distillation column" includes fractionation columns, fractionation columns, and stripping columns. A distillation column may refer to a single column or a plurality of columns, connected in series or in parallel, where the plurality of columns can be any combination of the above. Each column may contain one or more trays and / or stacks.

[0029] The term "reboil" refers to the partial evaporation of liquid present in a distillation column by indirect heat exchange with a hotter process stream. This produces steam which aids in mass transfer in the distillation column. The liquid can come from the bottoms liquid or from an intermediate stage in the column. The heat load for reboiling can be directed to the distillation column through the use of an in situ reboiler or an external heat exchanger designed for this purpose, or part of a larger heat exchange system. Vapor-liquid separation can also take place in a distillation column or in an external flash tank.

[0030] The present apparatus and method are described with reference to the drawings. As used herein, a single reference numeral may be used to identify a process gas stream and a process gas transfer line that carries a specified process gas stream. Which attribute the reference position refers to will be clear from the context.

[0031] For the sake of simplicity and clarity, detailed descriptions of prior art devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

[0032] The natural gas supply described in the present invention refers to a gas containing hydrocarbons typically originating from underground in a geological formation. Natural gas is typically produced at pressures ranging from about 1 to about 200 bar. All pressures indicated are absolute, not gauge pressure. The natural gas pressure is preferably from about 10 to about 100 bar.

[0033] The methane content of natural gas is typically from about 50% to about 99%. All composition percentages indicated are on a volume or molar basis and not on a weight basis.

[0034] The nitrogen content of natural gas is typically from about 1% to about 50%, or from about 10% to about 35%.

[0035] The helium content of natural gas is typically from about 0.01 to about 10%. Some embodiments of the present invention are directed to recovering helium from natural gas containing from about 0.05% to about 1.0%, or from about 0.05% to about 0.2% helium.

[0036] Figure 1 illustrates a general method for producing helium from a natural gas source. Raw natural gas 1 enters an acid gas removal unit A to remove gases such as CO ₂ , H ₂ S and COS that would freeze in downstream cryogenic plants. Acid gases in stream 2 can be vented to atmosphere or, if necessary, directed to sulfur removal. There are several options for removing acid gases, including pressure swing adsorption, pressure swing adsorption, or methanol absorption, which in the following examples presented herein is assumed to be a steam regenerated amine scavenger.

[0037] The pollutant-depleted natural gas that exits stream A in stream 3 now contains acceptably low levels of acid gases, typically less than about 100 ^ppmv . If an amine scavenger is used, stream 3 will be saturated with water vapor, which would solidify in the downstream cryogenic process. Stream 3 will therefore be fed to a feed dehydration module D , which preferably contains a variable temperature absorber (TSA) and an anti-mercury layer containing activated carbon, both well known in the art for removing water and mercury, respectively. TSA removes water, CO ₂ and aromatics such as benzene, toluene and xylene (collectively known as BTX). Specifications are established to prevent the formation of a solid phase in the cryogenic process; for example, the specification for water is often around 1 ^ppmv .

[0038] Impurities are adsorbed and then removed during TSA regeneration. Regeneration requires both heat, which can be supplied by electric heaters or process steam, and a process stream to transport impurities from the TSA, such as nitrogen or a portion of the helium-depleted stream, from the Helium Recovery Module X. Depending on the pressure required to regenerate the TSA, this stream may be at least part of the low pressure return stream 27 or the helium-depleted natural gas stream 29 . Figure 1 illustrates a case where a low pressure stream can be used, in which case stream 27 'is used as regeneration gas. The contaminated natural gas stream 27 ″ can then be re-combined with the remainder of stream 27 prior to recompression. If nitrogen is used to regenerate TSA, then the contaminated nitrogen gas can be vented to the atmosphere or sent to further processing to remove hydrocarbons in accordance with air pollution regulations. If vapor phase mercury is present in the feed stream, the TSA requires a protective layer against mercury to prevent vapor phase mercury from attacking aluminum in downstream heat exchangers.

[0039] The natural gas feed stream 5 enters the helium recovery module X , which is the subject of the present invention. A helium recovery unit is a cryogenic process that separates at least 99% helium, or at least 99.5% helium, from a natural gas feed stream, which may contain from 0.05% to 1.0% helium by volume or from 0.05%. up to 0.2% helium by volume, with obtaining a stream of crude helium, which can contain from 4 to 20% helium by volume. The cryogenic process is designed for maximum efficiency with high helium yields, which requires careful heat integration to reduce the overall power consumption of the process.

[0040] The low pressure return stream 27 is recompressed in compressor R if it is to be returned to the pipeline, burned in a gas turbine, or otherwise disposed of as helium-depleted natural gas 29 . The low pressure return stream can exit the recovery module as one or more streams at different pressures. For example, see streams 27 and 28 in Figure 2. The higher pressure streams can enter a later stage of compression, or bypass compression entirely and combine with helium-depleted natural gas.

[0041] A crude helium stream 16 is directed to a helium P purification module to obtain a pure helium stream 30 with a typical specification of 99.99% or 99.999%. Inside the helium purification module (and therefore not shown in Figure 1), stream 16 is cooled to cryogenic temperatures, partially condensing the feed stream, such that most of the nitrogen and substantially all of the methane condenses, leaving a vapor stream containing about 50 to 90 % helium, or about 70 to 85% helium, and usually about 80% helium. This steam stream is heated and then the hydrogen is removed in a catalytic combustion process before it enters the helium PSA. The pressure of the liquid stream leaving the partial condensation process is reduced to recover the helium by purging and removing the vapor stream. The steam stream is reheated, re-compressed and combined with the crude helium stream entering the purifier. By decreasing the pressure of the liquid stream, the resulting lower temperature liquid stream provides cooling for the partial condensation process. This small stream 31 exits the helium purification process and can be re-compressed into a commercial gas stream as stream 31 ' or vented to the atmosphere as stream 31 after being catalytically combusted to remove methane, if desired. The tail gas from PSA helium is dried with TSA to remove water generated by catalytic combustion of hydrogen with air or oxygen-enriched stream, then re-compressed and mixed with stream 16 as it enters the purifier. In addition, the helium collected in the "gas bag" is reused for recirculation to the purifier to minimize the overall helium loss in the process.

[0042] Alternative embodiments of the helium purification module are well known in the art. Blackwell and Kalman (US 3599438) describe the purification of helium in more detail, including the steps of removing hydrogen by catalytic oxidation, dehydration by adsorption and enrichment of helium by partial condensation. In addition, Blackwell and Kalman demonstrate the recirculation of an intermediate pressure helium flow (16). Additionally, Kirk-Othmer in the Encyclopedia of Chemical Technology, "Cryogenic technology" (2012), describes alternative process flowsheets for helium purification. For example, Figure 13 in this chapter illustrates a single pressure burst at the cold end of helium that causes a higher loss of helium due to the dissolution of helium in the fluid stream leaving the system. Figure 14 in the same chapter illustrates a different order of operations: first partial condensation followed by catalytic oxidation and final purification with PSA, the PSA residual gas being recompressed, dehydrated and recycled to the partial condensation stage. Gottier and Herron (US 5017204) describe a helium purification cycle using a reflux condenser that combines heat transfer and mass transfer steps in a single heat exchanger. Any of these methods, or similar purification methods, can be used to obtain a pure helium product from a crude helium stream.

[0043] The pure helium stream 30 can be sold as a gaseous product, but more often it is liquefied in a helium liquefier L to produce a liquid helium stream 32 that can be transported more efficiently over long distances. Additionally, the liquefier removes traces of neon if present. The liquefier can use liquid nitrogen for cooling at the warm end of the process, supplied by a small nitrogen generator or imported by truck in liquid form, or it can use any other refrigeration option known in the art. The cold end of the process typically uses recirculated helium in a heat pump process for cooling.

[0044] If desired, at least a portion of the acid gas stream 2 ' and / or tail gas 31' may be mixed with the helium-depleted natural gas 29 prior to recompression or an intermediate recompression step. This may be advantageous if the helium-depleted natural gas has been designed for a given mass flow rate, for example for a gas turbine. If a small amount of H ₂ S is present in the acid gas stream, re-compression and dilution will avoid the complications associated with venting the H ₂ S-containing CO ₂ , or the cost of H ₂ S oxidation or the cost of a tall vent pipe. Likewise, recompression of stream 31 ' helps to avoid the additional cost of oxidizing the remaining methane in the tail gas stream, if necessary, prior to discharge.

[0045] Figure 2 illustrates in detail the helium recovery module X. The natural gas feed stream 5 is supplied to heat exchanger 101 after exiting pretreatment modules A and D in Figure 1. The heat exchanger is typically a brazed aluminum finned plate heat exchanger common in the cryogenic industry and may be in the form of one or more heat exchangers, serial or parallel. The stream is cooled in a heat exchanger by indirect heat exchange with streams returning from the cryogenic distillation section, at least partially condensed and exits the heat exchanger in the form of cooled natural gas feed 6 . If necessary, the pressure of stream 6 can be reduced to ensure efficient separation in the distillation column. The parameter for providing effective separation can be the ratio of the density of the liquid phase to the density of the vapor phase, the desired ratio being greater than 4 or greater than 6 or greater than 8. In addition, the parameter may be the surface tension of the liquid phase, the desired value being greater than 0.5 dyne / cm or more than 1 dyne / cm or more than 2 dyne / cm. If a pressure reduction is required, it is shown in FIG. 2 as occurring in valve 102 , but can also be achieved with a dense fluid expander. Then the feed stream fed to the column 7 enters the distillation column 103 , preferably in the upper stage.

[0046] A distillation column 103 separates helium from a column feed stream 7 , which leaves the top of the column as helium-rich vapor 8 discharged from the top of the column. The top of the distillation column requires a reboiler, which is shown in Figure 2 as an external reboiler. In this configuration, the liquid stream 9 leaves the bottom of the column and is then heated by indirect heat exchange with the natural gas feed 5 to heat exchanger 101 . The partially vaporized stream 10 is then separated in a reboiler separator 104 . The distillation column 103 , the reboiler separator 104, and the portion of the heat exchanger 101 used to transfer heat to stream 9 constitute the distillation column system. The vapor stream 11 is returned to the distillation column 103 and the helium-depleted bottoms liquid exits the distillation column system as stream 12 .

[0047] The distillation column system is illustrated in Figure 2 as a flow diagram with an external reboiler, where 104 is a reboiler separator. In addition, the reboiler may be internal to the column, or the external reboiler may be a separate heat exchanger rather than integrated into a multi-flow heat exchanger with other hot and cold streams, as shown as 101 in Figure 2. The reboiler provides steam to the bottom the column by boiling part of the liquid leaving the bottom of the column in the form of stream 9 . As is known in the art, this can be done in several ways. A reboiler, such as a thermosyphon reboiler, may be located in a liquid sump to boil liquid inside the sump. In this case, a stream with a temperature intermediate between streams 5 and 6 will be fed to the reboiler to provide the necessary heating, and the parameters of the liquid stream leaving the settling tank of the column will be the same as for stream 12 in Figure 2. In the distillation system columns can use one of the reboiler configurations described above, or any other known reboiler.

[0048] The helium-rich vapor 8 discharged from the top of the column is then partially condensed in the heat exchanger 105 . The partially condensed overhead strand 13 enters the top separator 106 . The specified overhead separator 106 can be a simple flash tank or a distillation column with several stages. The overhead stream of 106 is a crude helium vapor stream 14 . The crude helium vapor stream 14 provides cooling by passing through both heat exchangers 105 and 101 before exiting the helium recovery module as stream 16 . Said crude helium vapor 14 is now at a sufficiently high concentration level, typically 4% to 20% by volume, for injection into a helium purifier, represented as module P in Figure 1.

[0049] The recycle liquid stream 17 exits the bottom of the overhead separator 106 and returns to the distillation column 103 . Although stream 17 appears at the same location in the process flow as a reflux stream would appear in a conventional distillation process, the recycle liquid in the present invention is not suitable for backflow for two reasons. Firstly, the flow rate of stream 17 is small compared to the feed stream 7 into the column, in contrast to the reflux stream, which must have a sufficiently high flow rate to flush out the steam flowing up the column. Since the recycle liquid stream 17 has a relatively low flow rate, it does not affect the separation and is returned to the distillation column only to recover the helium contained in stream 17 . Second, the distillation column 103 operates as a stripping column with feed to the column at the upper stage. The recycle liquid 17 can enter the upper stage or any of the lower stages, so it cannot be used to flush the vapor exiting the upper stage.

[0050] The pressure in the upper separator 106 should be kept as close as possible to the pressure of the distillation column system so that the pressure at the top of the liquid in stream 17 is sufficient to overcome the pressure drop and flow into the distillation column 103 . This reduces the overall power consumption of the process, since the crude helium vapor stream from the overhead separator 106 thus does not need to be recompressed. It should be noted that the higher pressure in the overhead separator results in more helium being captured in stream 17 , but that liquid phase helium is recovered by recirculating stream 17 back to distillation column 103 .

[0051] The helium-depleted bottoms liquid 12 can be split into at least two streams, each of which provides cooling at a different pressure and thus a different temperature. Stream 12 can be divided into as many streams as there are one more than the number of compression stages available in the recompressor R. This is because each compression stage can receive one flow at its own suction pressure, and one additional flow can bypass R if it is at the same pressure as the outlet R. In the embodiment of the invention illustrated in Figure 2, stream 12 is divided into three streams: 18 , 21, and 23 . The use of product streams to cool the process is called auto-cooling. and improves efficiency compared to external cooling. The use of multiple pressure levels of the return process streams minimizes temperature differences throughout the heat exchanger system, increasing efficiency and resulting in a lower overall recompression power requirement. The pressure of the first helium-depleted bottoms fraction 18 is reduced to give stream 19 . The pressure reduction is represented as valve 105 , but can also be achieved using the DFE. Stream 19 provides cooling to partially condense stream 8 in heat exchanger 105 , after which stream 20 provides additional cooling for heat exchanger 101 . The pressure of the second fraction 21 of the helium-depleted bottoms may be reduced to form stream 22 if needed for additional cooling. If required, the pressure reduction can be carried out in valve 107 or using the DFE. Stream 22 provides cooling to heat exchanger 101 . Since its pressure is reduced to an intermediate pressure that is higher than that of stream 19 , the temperature of stream 22 is not as low as that of stream 19 . The pressure of the third fraction 23 of the helium-depleted bottoms may be increased in pump 108 to produce stream 24 . Stream 24 then provides evaporative cooling in heat exchanger 101 . Pumping the liquid stream before vaporizing it, as shown here, is more efficient than vaporizing the liquid stream and then compressing the vapor, since the liquids cannot be effectively compressed.

[0052] After stream 22 is heated in heat exchanger 101 , the resulting preheated second helium-depleted bottoms fraction 25 may optionally be expanded in expander 109 , which simultaneously cools the stream and generates power. The resulting expanded second helium-depleted bottoms fraction 26 can be returned to heat exchanger 101 to provide additional cooling, then combined with stream 20 and finally discharged from the heat exchanger as low pressure return stream 27 . Stream 27 is then re-compressed in a return compressor R. Stream 24 exits heat exchanger 101 as a medium pressure return stream 28 which can be re-compressed by being fed to an interstage return compressor R. Depending on the pressure required for the final helium-depleted finished natural gas, pump 108 could pressurize stream 23 to a high enough pressure that no further compression is required.

[0053] Heat exchangers 101 and 105 are a heat exchanger system that in various embodiments of the invention may be represented by one or more heat exchangers, or may be divided into two or more heat exchangers arranged in series or in parallel. For example, heat exchanger 101 may be split into two separate heat exchangers at the point where the expanded second fraction 26 of the helium-depleted bottoms stream returns to the exchanger and mixes with stream 20 as it returns from 105 . In addition, the load required for the reboiler can be provided by a separate heat exchanger, either in parallel with 101 or at the cold end 101 , exchanging heat exclusively between stream 6 and stream 9 to simplify operation of the distillation column system. In general, the more integrated the heat exchanger system, the more efficient the heat exchange between all desired streams. However, the heat exchanger is often split, which reduces efficiency, since a small increase in total energy consumption provides advantages such as simplified operation, smaller heat exchanger system, simpler heat exchanger system design, or reduced risk to the process.

[0054] The return compressor R may be a single compressor with one or more stages, with or without intercoolers between stages, or a plurality of compressors connected in series or in parallel. In a series arrangement, stream 27 could enter the first of the compressors and stream 28 could enter a compressor further down the row. In parallel arrangement, the individual compressors could compress streams 27 and 28 to the desired final discharge pressure. The re-compressed gas exits the helium recovery module as a helium-depleted natural gas stream 29 , which can then be piped, incinerated or otherwise disposed of. If the waste streams 2 ' and / or 31' are to be recompressed and combined with the helium-depleted natural gas stream, they also enter R.

[0055] There are situations in which recompression of the medium pressure return flow may not be required. All pressures of the return streams 20 , 22, and 24 must be lower than the pressure of the feed stream 5 because the return streams must boil at a lower pressure than the pressure at which the feed stream condenses to ensure efficient operation of the heat exchanger 101. If the required pressure of stream 29 is less than the pressure of stream 5 , then stream 24 can be pumped to a pressure equal to the pressure of stream 29 and does not need additional compression. In this case, the medium pressure return stream 28 ' may instead bypass the return compressor and mix directly with stream 29 .

[0056] Several embodiments and features of the invention have been described using a set of numerical upper limits and a set of numerical lower limits. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, it should be understood that ranges from any lower limit to any upper limit are contemplated, unless the text clearly indicates otherwise. Likewise, ranges from any lower limit can be combined with any other lower limit to indicate a range not explicitly stated in the text, and ranges from any upper limit can be combined with any other upper limit to indicate a range not specified in the text. text explicitly. In addition, the range includes every point or individual value between its endpoints, even if not explicitly stated in the text. Thus, each point or individual value can serve as its own lower or upper limit in combination with any other point or individual value or any other lower or upper limit to denote a range not explicitly stated in the text. All numerical values are values that indicate "approximately" or "about" the specified value and take into account the experimental error and variance that might be expected by one of ordinary skill in the art.

[0057] Aspects of the present invention include:

# 1: Method for recovering helium from raw natural gas containing methane, nitrogen and helium, said method comprising:

cooling said natural gas feed to obtain a cooled natural gas feed that is at least partially condensed;

separating the cooled natural gas feed in a distillation column system to obtain a helium-rich vapor discharged from the top of the column and a helium-depleted bottom liquid;

cooling said helium-rich vapor discharged from the top of the column to obtain a partially condensed overhead stream;

separating said partially condensed overhead stream in an overhead separator to obtain crude helium vapor and recycle liquid;

expanding at least a portion of the helium-depleted bottoms liquid to provide a first fraction of the helium-depleted bottoms;

wherein the cooling load for said helium-enriched steam discharged from the top of the column is at least partially provided by indirect heat exchange with said first fraction of helium-depleted bottoms.

No. 2. The method according to No. 1, characterized in that the pressure of said cooled natural gas feed is lowered to achieve a liquid to vapor density ratio in the distillation column above 4.

No. 3. A method according to any one of Nos. 1-2, characterized in that the pressure of said cooled natural gas feed is lowered to achieve a surface tension of the liquid phase in the distillation column above 0.5 dynes / cm.

No. 4. A method according to any one of Nos. 1-3, characterized in that the difference between the pressure at the top of the distillation column system and the pressure in the overhead separator is not more than 1 bar.

No. 5. A method according to any one of Nos. 1-4, characterized in that the reboil load for said distillation column system is at least partially provided by indirect heat exchange with natural gas feed.

No. 6. A method according to any one of Nos. 1-5, characterized in that said recycle liquid is introduced into a distillation column.

No. 7. The method according to No. 6, characterized in that the recycle liquid is introduced into the distillation column at the same or lower stage as the place where the cooled natural gas is fed into the distillation column.

No. 8. A method according to any of Nos. 1-7, further comprising the step of expanding at least a portion of the helium-depleted bottoms residue to form a second helium-depleted fraction.

No. 9. The method according to No. 8, characterized in that the pressure of said second fraction of helium-depleted bottoms is higher than the pressure of said first fraction of helium-depleted bottoms.

No. 10. The method according to any of Nos. 8-9, further comprising the steps of heating said second fraction of helium-depleted bottoms to provide at least a portion of the cooling load for cooling and condensing said natural gas feed and obtaining a heated second fraction of helium-depleted VAT residue;

and expanding said preheated second fraction of the helium-depleted bottoms to provide power and obtain an expanded second fraction of the helium-depleted bottoms.

No. 11. A method according to any of Nos. 8-10, further comprising combining and compressing said first and second helium-depleted bottoms fractions or streams obtained therefrom to obtain a helium-depleted natural gas stream.

No. 12. The method according to any of Nos. 8-11, further comprising the steps of increasing the pressure of at least a portion of the specified helium-depleted bottoms liquid to obtain a third fraction of helium-depleted bottoms;

and heating said third helium-depleted bottoms fraction to provide at least a portion of the cooling load for the cooling and condensation processes of said natural gas feed.

No. 13. The method of No. 12, further comprising combining and compressing said first, second, and third fractions of helium-depleted bottoms or streams obtained therefrom to obtain a helium-depleted natural gas stream.

No. 14. Installation for processing natural gas for the extraction of helium from raw natural gas containing methane, nitrogen and helium, and the specified installation contains:

heat exchanger system;

a distillation column system having a vapor outlet and a liquid outlet;

a first conduit for transferring cooled feed gas from said heat exchanger system to said distillation column;

a second conduit for transferring helium-enriched steam discharged from the top of the column from said vapor outlet of said distillation column to said heat exchanger system;

an overhead separator containing a vapor outlet and a liquid outlet;

a third conduit for transferring the partially condensed overhead from said heat exchanger system to said overhead separator;

a fourth conduit for transferring a first fraction of helium-depleted bottoms from said liquid outlet of said distillation system to said heat exchanger system;

and the fourth conduit includes a pressure reducing device.

No. 15. Installation for processing natural gas according to No. 14, characterized in that said first pipeline contains a device for reducing the pressure.

# 16. A natural gas processing plant according to any one of # 14-15, further comprising a fifth conduit for transferring recirculating liquid from said liquid outlet of said overhead separator to said distillation column.

No. 17. Plant for processing natural gas according to No. 16, characterized in that said fifth line is connected to said distillation column at the same stage or at a lower stage than the place where said first line is connected to said distillation column.

No. 18. Installation for processing natural gas according to any one of No. 14-17, further comprising a sixth pipeline for transferring the second fraction of helium-depleted bottoms from said liquid outlet of said distillation system to said heat exchanger system, wherein said sixth pipeline further comprises a reducing pressure device.

No. 19. Installation for processing natural gas according to No. 18, additionally containing:

expander;

a seventh pipeline for transferring the heated second fraction of the helium-depleted bottoms from said heat exchanger to said expander;

and an eighth conduit for transferring the expanded second helium-depleted bottoms fraction from said expander to said heat exchanger system.

No. 20. Installation for processing natural gas No. 18-19, additionally containing:

pump;

a ninth conduit for transferring a third fraction of helium-depleted bottoms from said liquid outlet of said distillation system to said pump;

and a tenth conduit for transferring the pressurized third fraction of the helium-depleted bottoms from said pump to said heat exchanger system.

No. 21. Installation for processing natural gas according to any of No. 18-20, additionally containing:

return compressor;

and an eleventh conduit for transferring a low pressure return stream from said heat exchanger system to said return compressor.

No. 22. Installation for processing natural gas according to No. 21, additionally containing a twelfth pipeline for moving the return flow of medium pressure from the specified heat exchanger system to the specified return compressor.

No. 23. Installation for processing natural gas according to No. 21, additionally containing:

mixing device;

a thirteenth pipeline for moving a stream of compressed, helium-depleted natural gas from said return compressor to said mixing device;

and a fourteenth conduit for transferring a medium pressure return stream from said heat exchanger system to said mixing device.

EXAMPLE 1

[0058] Computer simulations of the method illustrated in Figures 1 and 2 were performed in Aspen Plus, a commercially available method simulation software package. The natural gas feed stream contains 35% nitrogen and 0.14% helium. The main flow parameters such as composition, pressure, temperature and flow rate are shown in Table. 1, together with the total power consumption.

[0059] For the purposes of Example 1, two changes were made to the method shown in Figures 1 and 2. In this example, it is assumed that the steam 31 ' in Figure 1 is recompressed with a helium-depleted natural gas stream, but stream 2 is vented to atmosphere. Additionally, this example assumes that stream 5 in Figure 2 is cooled, condensed, and expanded through the DFE instead of valve 102 .

[0060] As shown in Table. 1, the helium recovery model X produces a crude helium stream 16 , containing more than 12% helium, at a concentration high enough to feed a helium P purifier while maintaining a 99.9% yield in the helium recovery module. The output in module X is defined as the helium content of stream 16 leaving the module divided by the helium content of stream 5 entering the module. This high yield is possible because the recycle liquid stream 17 , which contains 6.8% of the helium contained in the condensed helium-rich overhead stream 12 , is returned to the distillation column. In prior art processes in which the distillation column overhead is further concentrated, this liquid phase helium would not be lost since the equivalent of stream 17 would be directed to the equivalent of helium-depleted natural gas stream 29 . A 99.9% helium yield in the X helium recovery module provides an overall helium yield of 99.6% due to the small loss of helium in stream 31 ' , where the total helium yield is defined as the helium content of the pure helium stream 30 divided by the helium content of the crude natural stream gas 1 .

[0061] This method provides flexibility over the composition of the raw helium stream 16 . The helium mole fraction in stream 16 can be increased either by increasing the flow rate or by lowering the pressure of the low pressure return stream 19 . Either option results in a higher concentration of helium in stream 16 due to an increase in power consumption during compression of stream 27 .

[0062] If the waste stream from the helium purification process is to be discarded as stream 31 , then an optimization that minimizes power would increase the methane flow rate in stream 16 to avoid recompression in compressor R. The optimization would have to include the methane value at vent 31 to balance the increase in flow 16 .

Table 1

Flow one 2 five 8 12 14 17 19 Component Composition He mol% 0.14 0.00 0.15 2.81 0.00 12.80 0.24 0.00 N ₂ mol% 35.09 0.00 36.74 75.26 36.22 81.45 73.67 36.22 CO ₂ mol% 4.01 88.79 0.00 0.00 0.00 0.00 0.00 0.00 CH ₄ mol% 60.16 0.00 62.99 21.78 63.65 5.17 26.05 63.65 C ₂ H ₆ mol% 0.10 0.00 0.10 0.00 0.11 0.00 0.00 0.11 C ₃ H ₈ mol% 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.01 H ₂ O mol% 0.47 11.14 0.00 0.00 0.00 0.00 0.00 0.00 H ₂ mol% 0.01 0.00 0.01 0.15 0.00 0.58 0.04 0.00 Temperature ° C 67.2 58.9 26.7 -140.6 -139.3 -155.9 -155.9 -160.1 Pressure bar (a.d.) 39.3 1.7 37.6 19.7 19.8 19.7 19.7 5.5 Flow rate (total) kmol / hour 22679.6 1024.4 21662.9 1212.6 21415.0 247.8 964.7 1054.3 Flow 22 24 25 26 31 ' 29 16 thirty Component Composition He mol% 0.00 0.00 0.00 0.00 0.01 0.00 12.80 100,00 N ₂ mol% 36.22 36.22 36.22 36.22 94.06 36.81 81.45 0.00 CO ₂ mol% 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CH ₄ mol% 63.65 63.65 63.65 63.65 5.90 63.07 5.17 0.00 C ₂ H ₆ mol% 0.11 0.11 0.11 0.11 0.00 0.11 0.00 0.00 C ₃ H ₈ mol% 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 H ₂ O mol% 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 H ₂ mol% 0.00 0.00 0.00 0.00 0.02 0.00 0.58 0.00 Temperature ° C -139.3 -137.3 -36.1 -92.3 26.4 67.2 25.0 51.1 Pressure bar (a.d.) 19.8 34.9 19.3 5.0 1.8 38.3 19.2 17.4 Flow rate (total) kmol / hour 1216.5 19144.3 1216.5 1216.5 217.4 21632.4 247.8 31.7 Re-compression power 8.95 MW Pump power 0.71 MW Expander power -0.47 MW DFE power -0.20 MW Total net power 9.01 MW

EXAMPLE 2

[0063] Computer simulations of the method illustrated in Figures 1 and 2 were performed in Aspen Plus, a commercially available method simulation software package. The natural gas feed stream contains 10% nitrogen and 0.065% helium. Table. 2 shows the conditions for the flows in Figures 1 and 2, as well as the total power consumption.

[0064] For the purposes of Example 2, two changes were made to the method shown in Figures 1 and 2. In this example, it is assumed that the steam 31 'of Figure 1 is recompressed to return natural gas, but stream 2 is vented to the atmosphere. In addition, these examples assume that stream 5 in Figure 2 is cooled, condensed and expanded using a DFE instead of valve 102 .

[0065] Example 2 has many of the same characteristics as Example 1, the same high overall helium yield, but differs in the nitrogen content of the feed. The lower nitrogen content in Example 2 results in higher temperatures in distillation column 103 , as represented by stream 8 , which is 20 ° C warmer than its counterpart in Example 1. Since the distillation column does not require such a low temperature, there is no need to depressurize flow 19 to such a low pressure: 7.7 bar, as opposed to 5.5 bar. Stream 19 , operating at a higher pressure, reduces the recompression load, which results in a decrease in net power to 7.75 MW compared to 9.01 MW in Example 1.

table 2

Flow one 2 five 8 12 14 17 19 Component Composition He mol% 0.065 0.000 0.068 1.404 0.000 4.721 0.055 0.000 N ₂ mol% 10.03 0.00 10.51 36.45 9.72 64.04 25.24 9.72 CO ₂ mol% 4.01 88.79 0.00 0.00 0.00 0.00 0.00 0.00 CH ₄ mol% 85.29 0.00 89.30 61.97 90.16 30.71 74.67 90.16 C ₂ H ₆ mol% 0.10 0.00 0.11 0.00 0.11 0.00 0.00 0.11 C ₃ H ₈ mol% 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.01 H ₂ O mol% 0.47 11.14 0.00 0.00 0.00 0.00 0.00 0.00 H ₂ mol% 0.01 0.00 0.01 0.17 0.00 0.53 0.02 0.00 Temperature ° C 67.2 58.9 26.7 -120.7 -119.2 -135.2 -135.2 -136.9 Pressure bar (a.d.) 39.3 1.7 37.6 19.7 19.8 19.7 19.7 7,7 Flow rate (total) kmol / hour 22679.6 1025.2 21662.1 1079.5 21350.2 311.9 767.6 741.4 Flow 22 24 25 26 31 ' 29 16 thirty Component Composition He mol% 0.000 0.000 0.000 0.000 0.002 0.000 4.721 99.995 N ₂ mol% 9.72 9.72 9.72 9.72 67.89 10.53 64.04 0.00 CO ₂ mol% 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CH ₄ mol% 90.16 90.16 90.16 90.16 32.07 89.35 30.71 0.00 C ₂ H ₆ mol% 0.11 0.11 0.11 0.11 0.00 0.11 0.00 0.00 C ₃ H ₈ mol% 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 H ₂ O mol% 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 H ₂ mol% 0.00 0.00 0.00 0.00 0.02 0.00 0.53 0.00 Temperature ° C -119.2 -117.1 -32.0 -78.2 23.3 67.2 25.0 51.1 Pressure bar (a.d.) 19.8 35.0 19.3 7.2 1.8 38.3 19.2 17.4 Flow rate (total) kmol /
hour 1319.6 19289.1 1319.6 1319.6 298.6 21648.8 311.9 14.7 Re-compression power 7.65 MW Pump power 0.77 MW Expander power -0.43 MW DFE power -0.24 MW Total net power 7.75 MW

[0066] Although the principles of the invention have been described above in connection with the preferred embodiments of the invention, it should be clearly understood that such description is made by way of example only and is not intended to limit the scope of the invention.

Claims (21)

1. A method for extracting helium from a natural gas feed containing methane, nitrogen and helium, the method comprising: cooling the natural gas feed to obtain a cooled natural gas feed that is at least partially condensed; separating the cooled natural gas feed in a distillation column system to obtain a helium-rich vapor discharged from the top of the column and a helium-depleted bottom liquid; cooling the helium-rich steam discharged from the top of the column by indirect heat exchange to obtain a partially condensed overhead stream; separating the partially condensed overhead stream in an overhead separator to obtain crude helium vapor and recycle liquid; expanding at least a portion of the helium-depleted bottoms residue to provide a first helium-depleted bottoms fraction; wherein the cooling load for the helium-rich steam discharged from the top of the column is at least partially provided by indirect heat exchange with the first fraction of the helium-depleted bottoms. 2. The method according to claim 1, characterized in that the pressure of the cooled natural gas feed is lowered to provide a liquid to vapor density ratio in the distillation column above 4. 3. The method according to claim 1, characterized in that the pressure of the cooled natural gas feed is lowered to ensure the surface tension of the liquid phase in the distillation column is higher than 0.5 dynes / cm. 4. A process according to claim 1, wherein the difference between the pressure at the top of the distillation column system and the pressure in the overhead separator is not more than 1 bar. 5. A method according to claim 1, wherein the reboil load for the distillation column system is at least partially provided by indirect heat exchange with the natural gas feed. 6. A method according to claim 1, wherein the recycle liquid is introduced into the distillation column. 7. A method according to claim 6, characterized in that the recycle liquid is introduced into the distillation column at the same or lower stage as the location where the cooled natural gas is fed into the distillation column. 8. The method of claim 1, further comprising the step of expanding at least a portion of the helium-depleted bottoms residue to form a second helium-depleted bottoms fraction. 9. The method according to claim 8, characterized in that the pressure of the second fraction of the helium-depleted bottoms is higher than the pressure of the first fraction of the helium-depleted bottoms. 10. The method according to claim 8, further comprising the steps of heating a second fraction of helium-depleted bottoms to provide at least a portion of the cooling load for cooling and condensation of the raw natural gas and obtaining a heated second fraction of helium-depleted bottoms; and expanding the heated second fraction of the helium-depleted bottoms residue to provide power and obtaining an expanded second fraction of the helium-depleted bottoms. 11. The method of claim 8, further comprising combining and compressing the first and second helium-depleted bottoms fractions or streams derived therefrom to form a helium-depleted natural gas stream. 12. The method according to claim 8, further comprising the steps of increasing the pressure of at least a portion of the helium-depleted bottoms liquid to obtain a third fraction of the helium-depleted bottoms; and heating the third fraction of the helium-depleted bottoms to provide at least a portion of the cooling load for the natural gas feed cooling and condensation processes. 13. The method of claim 12, further comprising combining and compressing the first, second, and third helium-depleted bottoms or streams derived therefrom to produce a helium-depleted natural gas stream.

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