Method for separating carbon dioxide from natural gas

Abstract

A process is described for separating a hydrocarbon-rich feed fraction (1), preferably natural gas, containing nitrogen and carbon dioxide, in which a) the feed fraction (1) is permeatively (M) separated into a nitrogen-depleted and carbon dioxide-enriched fraction (2) and a nitrogen-enriched and carbon dioxide-depleted fraction (3), b) the nitrogen-enriched and carbon dioxide-depleted fraction (3, 3') is separated in a cryogenic separation process (NRU) into a nitrogen-rich fraction (4) and a methane-rich product fraction (5), and c) the nitrogen-depleted and carbon dioxide-enriched fraction (2) is fed to the product fraction (5).

Description

[0001] The invention relates to a process for separating a hydrocarbon-rich feed fraction containing nitrogen and carbon dioxide, preferably natural gas.

[0002] If natural gas is piped through a natural gas network, the maximum nitrogen content may be between 2.0 and 7.0 mol%. If the natural gas is liquefied, this limit is 1 mol% nitrogen in the liquid natural gas product. If the nitrogen content exceeds the permitted limit, the nitrogen must be separated before further use, e.g. in the natural gas network, or before further processing or liquefaction of the natural gas. This nitrogen separation is usually carried out in a so-called nitrogen rejection unit (NRU). The hydrocarbon-rich feed gas fed to an NRU typically contains, in addition to the main components methane and nitrogen, other hydrocarbons such as ethane, propane, butane and higher hydrocarbons. In addition, it often contains a non-negligible amount of carbon dioxide. Traces of other components such as oxygen, water and sulfur may also be present.In an NRU, separation takes place using a cryogenic process, typically at temperatures below -150 °C. To avoid blockages in the NRU with components that freeze at these temperatures, these components, including carbon dioxide, must be removed to an acceptable level before the feed gas is fed into the NRU. An NRU or the cryogenic process implemented therein is described, for example, in EP 0095739 as a double-column process or in US 5257505 as a single-column process. Nitrogen removal, carbon dioxide removal, and NGL recovery can also be carried out in an integrated process with multiple process units and process streams, as described, for example, in GB 2500830 or WO 2012 / 177405.

[0003] To remove carbon dioxide, an amine scrubber is typically used. However, this requires considerable process engineering and design effort, and also results in the feed gas extracted from the amine scrubber being water-saturated. For this reason, a drying unit is usually installed upstream of the NRU, in which the water content of the feed gas extracted from the NRU is reduced to a level acceptable for NRU operation. This drying is typically performed adsorptively using a so-called TSA (temperature swing adsorption) process, as described, for example, in FR 3066258.

[0004] The object of the present invention is to provide a generic process for separating a hydrocarbon-rich feed fraction containing nitrogen and carbon dioxide, preferably natural gas, in which the above-described amine scrubbing and, in many cases, drying by means of a TSA process can be dispensed with.

[0005] To solve this problem, a generic process for separating a hydrocarbon-rich feed fraction containing nitrogen and carbon dioxide is proposed, in which a) the feed fraction is permeatively separated into a nitrogen-depleted and carbon dioxide-enriched fraction and a nitrogen-enriched and carbon dioxide-depleted fraction, b) the nitrogen-enriched and carbon dioxide-depleted fraction, hereinafter referred to as retentate, is separated in a cryogenic separation process into a nitrogen-rich fraction and a methane-rich product fraction, and c) the nitrogen-depleted and carbon dioxide-enriched fraction, hereinafter referred to as permeate, is fed to the product fraction.

[0006] Further advantageous embodiments of the process according to the invention for separating a hydrocarbon-rich feed fraction containing nitrogen and carbon dioxide are characterized in that the permeative separation takes place by means of one or more membrane stages connected in series, rubber-like and / or glass-like membranes are used in the membrane stage(s), the nitrogen-enriched and carbon dioxide-depleted fraction obtained in the permeative separation is subjected to an adsorption process to separate carbon dioxide, water and / or C 2+ hydrocarbons before being fed to the cryogenic separation process, the adsorption process is designed as a PSA, TSA or iTSA process, provided that the adsorption process comprises a rinsing step, the rinsing gas obtained in the rinsing step is fed to the nitrogen-depleted and carbon dioxide-enriched fraction and the methane-rich product fraction and / or the nitrogen-depleted and carbon dioxide-enriched fraction are compressed.

[0007] According to the invention, the previously required amine scrubbing is now replaced by at least one membrane unit or stage in which a permeative separation of the feed fraction into a nitrogen-depleted and carbon dioxide-enriched fraction and a nitrogen-enriched and carbon dioxide-depleted fraction takes place. A so-called rubber-like membrane is preferably used for this purpose.

[0008] In general, a membrane separation step or process uses a membrane that is semi-permeable or selectively permeable to certain components of a gas mixture to be separated. The membrane can be made of ceramic materials or one or more polymers, particularly glassy or rubbery polymers. See, for example, H. Strathmann, "Membrane Separation Processes, 1. Principles," Ullmann's Encyclopedia of Industrial Chemistry, online publication October 15, 2011, Wiley VCH, https: / / doi.org / 10.1002 / 14356007.a16_187.pub3.

[0009] Rubber-like membranes are characterized by the fact that their permeability increases with increasing permeation size and the material is selective for large molecules rather than small molecules. This means that rubber-like materials favor the passage of methane over nitrogen. In addition, the passage of carbon dioxide is favored over nitrogen in rubber-like membranes, which is why such a membrane is preferably used in the process according to the invention. The rubber-like membrane supplies a retentate stream in which the carbon dioxide content is significantly reduced compared to the carbon dioxide content of the feed fraction, while the nitrogen content is significantly increased compared to the nitrogen content of the feed fraction. The feed fraction typically contains between 3 and 20 mol% nitrogen and the retentate between 4 and 70 mol%, ideally more than 30 mol%.The permeate ideally has a nitrogen content of less than 7 mol%, preferably less than 3 mol%. The feed fraction contains, for example, approximately 3 mol% carbon dioxide, with the retentate having a carbon dioxide content of less than 500 ppmv, in particular less than 300 ppmv. According to the invention, the retentate stream is fed to the NRU and separated therein into a nitrogen-rich fraction and a methane-rich product fraction. While the nitrogen-rich fraction is usually released to the atmosphere, the methane-rich product fraction is fed for further use. However, the process according to the invention not only makes it possible to dispense with amine scrubbing but, in many cases, also makes it possible to dispense with a nitrogen enrichment column provided in the NRU, provided the nitrogen content in the retentate is sufficiently high.Furthermore, in many cases, a separation step for the critical higher hydrocarbons can be omitted in the NRU if these can be reduced below the level critical for the NRU by permeative separation in the retentate.

[0010] The permeate stream withdrawn from the permeative separation, depleted of nitrogen and enriched with carbon dioxide, ideally already meets the desired product specifications. In this case, it can be directly added to or mixed with the product fraction withdrawn from the NRU. Due to the selected membrane, the permeate stream also contains other components critical to the NRU, such as water and C2+ hydrocarbons, which no longer burden the NRU.

[0011] If the retentate stream has a carbon dioxide, water and / or C2+ hydrocarbon content that does not allow direct feeding of this stream to an NRU, one embodiment of the invention provides for drying of the retentate stream before it is fed to the NRU. In this case, the drying is preferably designed as a TSA process. Alternatively, carbon dioxide, water and / or higher hydrocarbons can be removed by a PSA or an iTSA process. An iTSA process is understood to be a TSA process in which cooling takes place indirectly by means of a temperature transfer fluid. Since the retentate stream has a lower mass flow than the feed stream and, in addition, the contents of the aforementioned components are already significantly reduced, any drying or removal of carbon dioxide and / or higher hydrocarbons that may be required can be correspondingly smaller orcan be carried out in a less complex process.

[0012] The process according to the invention for separating a hydrocarbon-rich feed fraction containing nitrogen and carbon dioxide as well as further advantageous embodiments thereof are described below with reference to the Figures 1 and 2 illustrated embodiments are explained in more detail.

[0013] In the Figure 1 According to the process shown, the hydrocarbon-rich feed fraction 1 containing nitrogen and carbon dioxide is fed, according to the invention, instead of an amine wash, to a membrane separation unit M having a rubber-like membrane. In the retentate stream 3 withdrawn from the membrane separation unit M, the carbon dioxide content is significantly reduced compared to the carbon dioxide content of the feed fraction 1, while the nitrogen content is significantly increased compared to the nitrogen content of the feed fraction 1.

[0014] According to the invention, the retentate stream 3 can thus be fed directly to the cryogenic separation process NRU. In this process, the retentate stream 3 is separated into a nitrogen-rich fraction 4 and a methane-rich product fraction 5. The nitrogen-rich fraction 4 is typically released to the atmosphere, while the methane-rich product fraction 5 is fed for further use, such as feeding into the natural gas grid. If necessary, the product fraction 5 must be compressed to the desired discharge pressure V2.

[0015] The nitrogen-depleted and carbon dioxide-enriched permeate stream 2 withdrawn from the membrane separation unit M can, provided it meets the desired product specifications, be admixed with the product fraction 5 withdrawn from the NRU. This may require compression V1 of the permeate stream 2. Since permeate stream 2 also contains other components critical to the NRU, such as water and C 2+ hydrocarbons, it contributes to reducing the load on the NRU.

[0016] If the retentate stream 3 is not yet suitable for direct feeding into the NRU due to its content of carbon dioxide, water and / or higher hydrocarbons, the Figure 2The process is shown in Figure 1. In this process, the retentate stream 3 is preferably freed of carbon dioxide, water, and / or higher hydrocarbons by means of a temperature swing adsorption process T, which are discharged via line 6. Depending on the design of the TSA process, the stream leaving the TSA must not exceed a carbon dioxide content of up to 500 ppmv, ideally not exceeding 300 ppmv. As an alternative to this TSA process, a suitable iTSA or pressure swing adsorption (PSA) process can be used to separate the aforementioned undesirable components. Typically, the adsorption process T comprises a purge step, with the purge gas produced in this purge step preferably being fed to the nitrogen-depleted and carbon dioxide-enriched fraction 2 prior to its compression V1. The purified retentate stream 3' withdrawn from the TSA process T can now be fed to the cryogenic separation process NRU.The further procedure is analogous to that in the . Figure 1 Described.

[0017] The Figures 1 and 2 The membrane separation unit M shown can have one or more membrane stages. If the required nitrogen and / or carbon dioxide concentrations cannot be achieved with just one membrane stage, two or more membrane stages connected in series can be provided, whereby an intermediate compression of the process stream (permeate) fed to the next membrane stage usually takes place. Each individual membrane stage can have different membranes or membrane types, e.g., a combination of rubber-like and glass-like membranes. Which of the aforementioned variants is implemented and which membrane type(s) are used depends on the desired separation task of the membrane separation unit M.

Claims

1. A process for separating a hydrocarbon-rich feed fraction (1), preferably natural gas, containing nitrogen and carbon dioxide, wherein a) the feed fraction (1) is permeatively (M) separated into a nitrogen-depleted and carbon dioxide-enriched fraction (2) and a nitrogen-enriched and carbon dioxide-depleted fraction (3), b) the nitrogen-enriched and carbon dioxide-depleted fraction (3, 3') is separated in a cryogenic separation process (NRU) into a nitrogen-rich fraction (4) and a methane-rich product fraction (5), and c) the nitrogen-depleted and carbon dioxide-enriched fraction (2) is fed to the product fraction (5).

2. Method according to claim 1, characterized in that the permeative separation (M) is carried out by means of one or more membrane stages connected in series.

3. Method according to claim 2, characterized in thatrubber-like and / or glass-like membranes are used in the membrane stage(s) (M).

4. Method according to claim 1 to 3, characterized in that the nitrogen-enriched and carbon dioxide-depleted fraction (3, 3') obtained in the permeative separation (M) is subjected to an adsorption process (T) before being fed into the cryogenic separation process (NRU), which adsorption process separates carbon dioxide, water and / or C 2+ -hydrocarbons.

5. Method according to claim 4, characterized in that the adsorption process (T) is designed as a PSA, TSA or iTSA process.

6. The method according to claim 4 or 5, wherein the adsorption process (T) comprises a rinsing step, characterized in that the purge gas produced in the purge step is fed to the nitrogen-depleted and carbon dioxide-enriched fraction (2).

7. Method according to one of claims 1 to 6, characterized in thatthe methane-rich product fraction (5) and / or the nitrogen-depleted and carbon dioxide-enriched fraction (2) are compressed (V1, V2).

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