Method for obtaining a helium-rich fraction

Abstract

A process for obtaining a helium-rich fraction is described, in which a hydrocarbon-rich feed fraction (1) containing nitrogen and helium is taken and separated into a methane-rich liquid fraction (4) and a nitrogen-rich gas fraction (3) by rectification in a double-column process consisting of a high-pressure column (T1) and a low-pressure column (T2) that are thermally coupled (E2). According to the invention, the helium-enriched, gaseous overhead fraction (5) of the high-pressure column (T1) is partially condensed (E3) and rectified (T3) into a helium-rich gas fraction (6) and a nitrogen-rich liquid fraction (7), wherein the nitrogen-rich liquid fraction (7) is at least partially fed back to the low-pressure column (T2) as reflux (9, 10).

Description

[0001] The invention relates to a process for obtaining a helium-rich fraction having a helium content of at least 60 mol% from a hydrocarbon-rich feed fraction containing nitrogen and helium, wherein the feed fraction is rectified into a methane-rich liquid fraction and a nitrogen-rich gas fraction by means of a double column process consisting of a high-pressure column and a low-pressure column which are thermally coupled.

[0002] Helium is commercially produced from a mixture of volatile components of natural gas, which, in addition to helium, typically contains methane, nitrogen, and traces of hydrogen, argon, and other noble gases. To prevent unwanted freezing during a helium liquefaction process, the concentration of impurities in the helium stream to be liquefied must not exceed 100 ppm, preferably 10 ppm, due to the extremely low temperatures required for helium liquefaction. For this reason, the helium liquefaction process is preceded by a helium purification process. This usually consists of a combination of cryogenic processes and / or membrane units, which are generally based on partial condensation and adsorption processes, the latter of which can be regenerated by pressure and / or temperature changes.

[0003] In a typical process, unmodified for helium recovery, components with a lower condensation temperature compared to methane, including helium, are separated from the natural gas in a nitrogen separation process, also known as a Nitrogen Rejection Unit (NRU). Various NRU process configurations exist.

[0004] In NRU processes, helium enters the nitrogen product stream, either at approximately atmospheric pressure in the case of a dual-column NRU or at pressures between 10 and 40 bara in the case of a single-column NRU. The helium can be recovered from this helium-enriched nitrogen stream in a separate helium separation process downstream of the NRU. This helium separation process typically consists of a combination of cryogenic processes based on partial condensation and / or membrane units and adsorptive processes, the latter of which can be regenerated by pressure and / or temperature changes.

[0005] US patent 20120090355 discloses a dual-column NRU process in which the overhead stream of the high-pressure (HP) column is completely condensed against evaporating methane from the bottom of the low-pressure (LP) column. The resulting liquid phase is split into two streams. One portion is returned to the HP column, while the remaining liquid is returned to the LP column after being supercooled in a separate heat exchanger against the product streams of the LP column.

[0006] If helium is present in the feed gas to the NRU, the topstream of the HP column is enriched with helium. If a significant amount of helium is present in the feed gas to the NRU, the topstream of the HP column can only be partially condensed at typical temperatures. The resulting liquid phase continues to be fed to the HP or LP column, while the helium-rich gas phase is directed to the LP column, and all the helium is ultimately discharged via the low-pressure nitrogen product, enabling high helium recovery rates (> 99%). However, the helium concentration in the feed gas to the helium recovery unit is very low—typically well below 10 mol%—leading to a comparatively high compression power requirement and higher investment costs for the downstream equipment of the helium recovery unit due to the higher flow rates, as the nitrogen concentration remains high, usually > 90 mol%.

[0007] Alternatively, the helium-rich gas can be processed directly in the downstream helium recovery unit after heating in the nitrogen recovery unit (NRU), instead of in the low-pressure (LP) column. This allows for a reduction in investment costs in the helium recovery unit by processing lower volumetric flow rates, as a large portion of the nitrogen is removed in the NRU. Furthermore, the compression step can be eliminated, or the compression power can be significantly reduced due to the higher pressure of the helium-rich gas. The disadvantage of this alternative process is the reduction of the helium recovery rate to < 90% for the same NRU efficiency. Recovery can be increased to 95% by adjusting the process parameters in the NRU, but this leads to increased power consumption by the product gas or methane product compressor.

[0008] In a single-column NRU process, the head gas is also enriched with helium under high pressure. This stream is typically heated in the NRU heat exchangers and treated in a separate helium recovery process.

[0009] The object of the present invention is to provide a generic process for obtaining a helium-rich fraction that makes it possible to generate a helium-rich stream with high purity, preferably > 60 mol%, under high pressure, preferably > 20 bar. Furthermore, the process should have a comparatively low power requirement with reduced plant costs and enable a high helium recovery rate, preferably > 99%.

[0010] To solve this problem, a process for obtaining a helium-rich fraction is proposed, characterized in that the helium-enriched, gaseous overhead fraction of the high-pressure column is partially condensed and rectified to separate it into a helium-rich gas fraction and a nitrogen-rich liquid fraction, wherein the nitrogen-rich liquid fraction is at least partially fed back to the low-pressure column.

[0011] Further advantageous embodiments of the inventive method for obtaining a helium-rich fraction are characterized in that The helium-enriched, gaseous overhead fraction is partially condensed against the nitrogen-rich liquid fraction, preferably to a temperature of less than -170 °C; at least a partial stream of the liquid fraction obtained in the head region of the high-pressure column is also fed to the rectification; a partial stream of the nitrogen-rich liquid fraction obtained by rectification is evaporated and fed to the rectification as stripping vapor; a partial stream of the nitrogen-rich liquid fraction obtained by rectification is subcooled and fed to the low-pressure column, preferably in its head region, as reflux; a partial stream of the nitrogen-rich liquid fraction obtained by rectification is subcooled and fed to the high-pressure column, preferably in its head region, as reflux; and the rectification takes place at a pressure that is less than 5 bar, preferably less than 2 bar, below the operating pressure of the high-pressure column.

[0012] The inventive method for obtaining a helium-rich fraction is described below with reference to the one described in the Figure The illustrated embodiment is explained in more detail.

[0013] A hydrocarbon-rich feed fraction 1 containing nitrogen and helium, preferably natural gas, is cooled in heat exchanger E1 against process streams to be heated, which will be discussed in more detail below, and fed via valve V1 to the lower section of high-pressure column T1. High-pressure column T1 and the low-pressure column T2 located above it, which are thermally coupled via heat exchanger E2, form a so-called double-column process. While high-pressure column T1 is preferably operated at a pressure between 13 and 33 bara, the pressure in low-pressure column T2 is preferably between 1 and 3 bara.

[0014] A methane-rich liquid fraction 2 is drawn off from the bottom of high-pressure column T1, cooled in heat exchanger E4 against process streams to be heated (which will be discussed in more detail below), and fed via valve V2 to the middle section of low-pressure column T2. ​​A nitrogen-rich gas fraction 3 is drawn off at the top of low-pressure column T2, heated in heat exchangers E4 and E1, and fed to its further use. A methane-rich (product) liquid fraction 4 is drawn off from the bottom of low-pressure column T2, pumped P, and heated in heat exchangers E4 and E1 against process streams to be cooled, where it is then evaporated. This fraction is subsequently compressed to the desired discharge pressure by compressor C; the aftercooler E5 serves to dissipate the heat of compression generated during this process.

[0015] In heat exchanger E2, located in the bottom of low-pressure column T2, the helium-enriched, methane- and nitrogen-containing overhead fraction a of high-pressure column T1 is partially condensed against the evaporating, methane-rich bottom fraction of low-pressure column T2. ​​The resulting helium-enriched, gaseous overhead fraction 5 of high-pressure column T1 is partially condensed in heat exchanger E3, preferably to a temperature of less than -170 °C, and fed via valve V3 to the overhead section of stripping column T3. There, it is rectified into a helium-rich gas fraction 6 and a nitrogen-rich liquid fraction 7. Stripping column T3 is preferably operated at a pressure less than 5 bar, and preferably less than 2 bar, below the operating pressure of high-pressure column T1.

[0016] The helium-rich gas fraction 6 obtained at the head of the stripping column T3 is preferably heated in the heat exchangers E4 and E1 against process streams to be cooled and then fed to its further use or processing.

[0017] A partial stream of the liquid phase 12, which accumulates in the top section of the high-pressure column T1, is fed back to the high-pressure column T1 via valve V5. Another partial stream 11 of the liquid phase 12 is fed to the stripping column T3 in the middle section via valve V4.

[0018] To improve the rectification separation in the stripping column T3, a partial stream 8 of the nitrogen-rich liquid fraction 7 is evaporated in heat exchanger E4 and fed to the stripping column T3 as stripping vapor. Another partial stream 10 of the nitrogen-rich liquid fraction 7 is subcooled in heat exchanger E4 and fed to the low-pressure column T2 as reflux via valve V8 in its head section. Yet another partial stream 9 of the nitrogen-rich liquid fraction 7 is sufficiently expanded in valve V6 to provide the cooling capacity required in heat exchanger E3. This partial stream is then also fed to the low-pressure column T2 via valve V7 below the feed point of the reflux stream 10.

[0019] The high-pressure column T1 can have a heating system (not shown in the figure) to reduce helium losses via the sump of the high-pressure column T1 and thus improve helium recovery. A separate heat exchanger can be provided for this heating system, or it can be integrated, for example, into the heat exchanger E1.

[0020] The helium-rich gas fraction 6 obtained at the head of the stripping column T3, which has a helium content of at least 60 mol%, preferably at least 70 mol%, requires post-treatment before it can be liquefied. Thanks to the inventive process for obtaining a helium-rich fraction, however, this post-treatment requires less process engineering effort.

[0021] A comparatively high helium recovery rate is achieved by separating helium from the helium-enriched, gaseous overhead fraction 5 of the high-pressure column T1 before this fraction is fed to the low-pressure column T2. ​​The inventive process for obtaining a helium-rich fraction thus enables helium recovery rates of over 99%.

[0022] A further advantage is that the helium-rich gas fraction 6 withdrawn from stripping column T3 has a comparatively high pressure, since the helium-enriched feed fraction 5 of stripping column T3 is taken from high-pressure column T1 and stripping column T3 is operated at approximately the same pressure as high-pressure column T1. This eliminates the need for further compression of the helium-rich gas (product) fraction 6 in any subsequent helium treatment and liquefaction that might otherwise be required.

[0023] Integrating cryogenic helium recovery into the known double-column process results in no or only a slight increase in power consumption compared to a double-column process without helium recovery. The inventive process for obtaining a helium-rich fraction can therefore be retrofitted to existing double-column processes relatively easily.

Claims

1. Process for obtaining a helium-rich fraction (6) having a helium content of at least 60 mol% from a hydrocarbon-rich feed fraction (1) containing nitrogen and helium, wherein the feed fraction is rectified into a methane-rich liquid fraction (4) and a nitrogen-rich gas fraction (3) by means of a double column process consisting of a high-pressure column (T1) and a low-pressure column (T2) which are thermally coupled (E2), characterized by the fact that The helium-enriched, gaseous overhead fraction (5) of the high-pressure column (T1) is partially condensed (E3) and rectified (T3) into a helium-rich gas fraction (6) and a nitrogen-rich liquid fraction (7), with the nitrogen-rich liquid fraction (7) being fed at least partially to the low-pressure column (T2) as reflux (9, 10).

2. Method according to claim 1, characterized by the fact thatThe high-pressure column (T1) is operated at a pressure between 13 and 33 bara and the low-pressure column (T2) at a pressure between 1 and 3 bara.

3. Method according to claim 1 or 2, characterized by the fact that the helium-enriched gaseous head fraction (5) is partially condensed against a partial stream (9) of the nitrogen-rich liquid fraction (7) (E3), preferably to a temperature of less than -170 °C.

4. Method according to any one of the preceding claims 1 to 3, characterized by the fact that at least a partial stream (11) of the liquid fraction generated in the head region of the high-pressure column (T1) is also fed to the rectification (T3).

5. Method according to any one of the preceding claims 1 to 4, characterized by the fact that a partial stream (8) of the nitrogen-rich liquid fraction obtained by rectification (T3) is evaporated (E4) and fed to the rectification (T3) as stripping steam.

6. Method according to any one of the preceding claims 1 to 5, characterized by the fact that a partial stream (10) of the nitrogen-rich liquid fraction obtained by rectification (T3) is subcooled (E4) and fed to the low-pressure column (T2), preferably in its head region, as reflux.

7. Method according to any one of the preceding claims 1 to 6, characterized by the fact that A partial stream of the nitrogen-rich liquid fraction (7) obtained by rectification (T3) is subcooled (E4) and fed back to the high-pressure column (T1), preferably in its head region.

8. Method according to any one of the preceding claims 1 to 7, characterized by the fact that The rectification (T3) takes place at a pressure that is less than 5 bar, preferably less than 2 bar, below the operating pressure of the high-pressure column (T1).

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