METHOD FOR PRODUCING A PURIFIED, METHANE-CONTAINING GAS STREAM

Description

[0001] The field of the present invention is that of processes for producing a purified, methane-containing gas stream from a raw gas stream containing methane and contaminated, among other things, with carbon dioxide.

[0002] Such a process is known from DE 20 2019 001 415 U1. The device disclosed therein, which comprises two membrane separation stages and a hydrogen sulfide adsorber with an activated carbon bed, can be used to separate methane from a gas mixture containing methane, carbon dioxide, and hydrogen sulfide. The gas mixture is fed into the feed line of the device, and retentate is drawn off from the second membrane separation stage as a product gas enriched with methane. The gas mixture is, for example, natural gas, landfill gas, or biogas from an anaerobic digester.

[0003] WO 2014 / 043730 A2 describes a device and a method for separating a gas mixture into product gas and off-gas by means of gas permeation.

[0004] US patent 2022 / 0203294 Al describes a process for gas separation with at least three membrane stages.

[0005] US patent 2020 / 0308494 Al discloses a method for separating hydrocarbons using membranes.

[0006] Biogas is a combustible gas that can be produced by the fermentation of biomass of any kind. Suitable feedstocks for fermentation are primarily biogenic materials, such as fermentable, biomass-containing residues like sewage sludge, organic waste or food scraps, manure (liquid manure, solid manure), previously unused plants and plant parts (e.g., cover crops, plant residues, and the like), as well as specifically cultivated energy crops as renewable resources. Biogas is produced through the microbial decomposition of organic matter, mostly under anoxic conditions, whereby microorganisms convert carbohydrates, proteins, and fats into methane and carbon dioxide.

[0007] Depending on the substrates used for fermentation, biogas can contain not only short-chain hydrocarbons but also higher hydrocarbons (especially from C6 upwards) as impurities, which can clog a gas permeation membrane. These impurities are partly fermentation products and partly originating directly from the substrate. Currently, these impurities are primarily removed by filters upstream of the membrane separation device, particularly activated carbon filters. Other methane-containing gases, such as natural gas, landfill gas, or associated petroleum gas, can also contain such impurities.

[0008] Despite the use of a pre-filter to remove these impurities, higher hydrocarbons also enter the membrane separation device over time. WO 2015 / 071257 A1 states that the most common impairment of a gas separation membrane's performance in practice is blockage or clogging by long-chain liquid, solid, or gaseous molecules, mostly hydrocarbons. These are introduced into the membrane by the gas stream being separated. This blockage, caused by the deposition of long-chain hydrocarbons, can begin as early as the porous support layer, but the most significant negative impact occurs when these long-chain molecules block the selective separation layer.

[0009] To solve this problem, WO 2015 / 071257 A1 proposes a method for cleaning a gas permeation membrane, wherein the removal of impurities takes place against the operating direction of the gas permeation membrane, characterized in that a suitable gas or gas mixture is introduced on the permeate side of a gas permeation membrane unit to be cleaned, wherein the gas or gas mixture loaded with impurities after passing through the gas permeation membrane is discharged via the feed gas inlet and / or retentate outlet of the gas permeation membrane unit.

[0010] However, this regeneration process inherently causes undesirable idle times. The object of the present invention is therefore to improve the process for producing a purified, methane-containing gas stream of the type mentioned above in such a way that these idle times for membrane regeneration are reduced. Therefore, the present invention provides a process for producing a purified, in particular with regard to carbon dioxide content, methane-containing gas stream from a raw gas stream containing methane and contaminated with higher hydrocarbons, carbon dioxide, and optionally other impurities (such as particulate impurities). The raw gas stream may additionally contain other gases, in particular fuel gases such as ethane and / or inert gases such as nitrogen. The process comprises the following steps: Separation of particulate impurities (e.g., impurities that would clog the membrane of the membrane separation device and could not be removed by any regeneration of the membrane) from the raw gas stream (in particular by filtration, for example with a filter with a pore size of 0.1–0.005 µm, preferably 0.03–0.007 µm, particularly 0.01 µm) and preferably separation of a portion of the higher hydrocarbons from the raw gas stream (optionally also by the same filtration) to obtain a pretreated, in particular pre-filtered, gas stream; conditioning, namely heating, to a temperature T₂ (at which the clogging of the membrane of the membrane separation device by higher hydrocarbons still present in the gas stream is reduced, or at which the condensation of higher hydrocarbons still present in the gas stream on the surface of this membrane is reduced, e.g.compared to ambient temperature) to obtain a conditioned gas stream, wherein the temperature T2 is adjusted such that the permeance of the membrane separator with respect to carbon dioxide remains constant on average over a time interval t, and introducing the conditioned gas stream into a membrane separator (which has a membrane with a higher permeability for carbon dioxide than for methane) to separate some of the carbon dioxide contained therein as permeate, thereby obtaining the purified, methane-containing gas stream as retentate.

[0011] Typically, separation (at least partially) is carried out at temperature T1, with temperature T2 being higher than temperature T1. However, it is also possible, for example, to perform the conditioning step before separation and to carry out both separation and introduction at temperature T2.

[0012] The raw gas stream preferably contains (or consists of) biogas, landfill gas, process gas, natural gas, or associated petroleum gas. The methane content in the raw gas stream is typically at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, and particularly at least 90%. The content of higher hydrocarbons (especially C6 and above) in the raw gas stream is preferably 0.1% to 10%, more preferably 0.5% to 5%. The carbon dioxide content in the raw gas stream is preferably 1% to 20%, more preferably 2% to 10%.

[0013] In the course of the present invention, it has been shown that conditioning the gas stream to a certain temperature (usually at least 30°C, preferably at least 40°C, more preferably at least 50°C, even more preferably at least 60°C, and in particular at least 70°C or even at least 80°C) reduces membrane fouling. Surprisingly, continuous operation at higher temperatures does not lead to an excessive impairment of the membrane's stability or to an excessive deterioration of the separation performance. The inventive process allows the proportion of carbon dioxide in the purified, methane-containing gas stream to be reduced to below 10%, preferably below 5%, and even more preferably below 2.5%.

[0014] Various processes can be used for the optional removal of some of the higher hydrocarbons from the raw gas stream. Known in the art are, for example, cryogenic separation, adsorptive separation (using adsorbents such as zeolites or activated carbon that bind higher hydrocarbons), absorptive separation (using a solvent that absorbs higher hydrocarbons while methane remains in the gas stream), or the combination of filtration and condensation. The process according to the invention is particularly well suited for adsorptive separation (e.g., via an activated carbon filter), which, although it leaves a higher proportion of impurities in the pretreated gas stream, is significantly more cost-effective.

[0015] Advantageously, the temperature T1 therefore corresponds to the ambient temperature (in particular, the outside air temperature). In other words, the raw gas stream for the separation of some of the higher hydrocarbons is preferably not subjected to cooling (for condensation). Consequently, the temperature T2 is preferably above the ambient temperature.

[0016] Conventional processes from the prior art can be used to condition, in particular to heat, the gas stream to temperature T2. For example, the pretreated gas stream can be heated in a flow heater. The use of a heat exchanger is also possible.

[0017] A larger temperature difference between T1 and T2 has proven advantageous in preventing the membrane from becoming blocked. Thus, the temperature T2 is preferably at least 10°C, more preferably at least 20°C, more preferably at least 30°C, even more preferably at least 40°C, and in particular at least 50°C higher than the temperature T1. It is also preferred that the temperature T2 be at least 30°C, more preferably at least 40°C, more preferably at least 50°C, even more preferably at least 60°C, and in particular at least 70°C or even at least 80°C.

[0018] To increase energy efficiency, it is therefore particularly preferable if the membrane separation device has thermal insulation (for example, insulation with polyurethane foam, mineral wool and / or reflective films) from the environment.

[0019] According to the invention, the temperature T₂ is set such that the permeance of the membrane separation device with respect to carbon dioxide remains constant on average over a time interval t, preferably wherein the time interval t is at least 12 h, more preferably at least 24 h, more preferably at least 36 h, even more preferably at least 48 h, and particularly at least 72 h. In other words, the temperature T₂ is set such that, during operation, an equilibrium is maintained between higher hydrocarbons adhering to the membrane and higher hydrocarbons evaporating from the membrane.

[0020] In a preferred embodiment, the membrane separator can be regenerated at a predetermined time (for example, at regular intervals or when it is found that the permeance of the membrane separator has decreased by a certain value on average over the time interval t) by heating the membrane separator to a temperature T3, wherein the temperature T3 is higher than the temperature T2. Advantageously, the temperature T3 is at least 10°C, preferably at least 20°C, more preferably at least 30°C, even more preferably at least 40°C, and in particular at least 50°C higher than the temperature T2. It is also preferred that the temperature T3 is at least 50°C, preferably at least 70°C, more preferably at least 90°C, even more preferably at least 110°C, in particular at least 130°C, or even at least 150°C. The method disclosed in WO 2015 / 071257 A1 is preferably used for regeneration.

[0021] Preferably, regeneration takes place against the (ordinary) operating direction of the membrane separation device, preferably wherein a gas or gas mixture, for example nitrogen or air, is introduced on the permeate side, in particular wherein the gas or gas mixture loaded with impurities after passing through the membrane of the membrane separation device is discharged via a feed gas inlet and / or a retentate outlet of the membrane separation device.

[0022] The membrane of the membrane separation device can be, for example, a flat membrane or a hollow fiber membrane. Membranes suitable for the membrane separation device with a higher permeability for carbon dioxide than for methane are known from the prior art. These are preferably certain polymer membranes. In general, membranes with a separation layer made of a glassy polymer, i.e., a polymer with a glass transition temperature above the operating temperature of the membrane, will provide a higher permeability for carbon dioxide than for methane. The polymer can be a polyetherimide, a polycarbonate, a polyamide, a polybenzoxazole, a polybenzimidazole, a polysulfone, or a polyimide. The membrane of the membrane separation device preferably comprises at least 80% by weight of a polyimide or a mixture of polyimides.

[0023] According to a further preferred embodiment, the proportion of methane in the purified, methane-containing gas stream is at least 60%, preferably at least 70%, more preferably at least 80%, even more preferably at least 90%, and in particular at least 95%. It is understood that this proportion is higher than the original methane content in the raw gas stream.

[0024] According to a preferred definition, "higher hydrocarbons" are C6 hydrocarbons and above, meaning that these compounds contain at least six carbon atoms in their molecular structure. This group includes alkanes, alkenes, and alkynes with a chain length of C6 or more. Examples of such higher hydrocarbons are hexane, heptane, and octane, as well as their unsaturated analogs such as hexene and hexyne. Aliphatic alcohols and ketones with a corresponding chain length also fall into this group.

[0025] According to a preferred definition, "particulate impurities" are impurities consisting of solid or liquid particles, such as dust or aerosols. Preferably, these are solid particles. The solid or liquid particles have, for example, an average particle diameter of 100 µm to 1 µm.

[0026] All percentage figures relating to the proportions of gases in gas mixtures are given in vol%.

[0027] In this context, all pressure specifications given in "bar" refer to absolute pressure, i.e., "bar absolute".

[0028] The present invention will now be explained in more detail in the following example, to which, however, it is not limited.

[0029] The raw gas stream is sourced from a biogas plant. The composition of the biogas raw gas stream varies depending on the operating conditions, but on average it is: 60% methane, 30% carbon dioxide, 9% nitrogen, traces of hydrogen and about 1% higher hydrocarbons.

[0030] In the first step of the process, the raw gas stream is passed through an activated carbon filter at a pressure of approximately 9 bar to remove coarse impurities (e.g., dust) that would clog the membrane and cannot be removed by regeneration, as well as some of the higher hydrocarbons. This pretreatment reduces the proportion of these hydrocarbons to around 0.25%. The separation takes place at an ambient temperature of 15°C, which corresponds to temperature T1.

[0031] The pre-treated gas stream is then heated to a temperature of 70°C (T 2 ) in a flow heater.

[0032] The heated, pretreated gas stream is then introduced into the membrane separation device, which has a hollow fiber membrane made of polyimide. During the separation process, which takes place at a pressure of approximately 10 bar, a large portion of the carbon dioxide is separated as permeate, and the retentate, i.e., the purified, methane-containing gas stream, contains less than 5% carbon dioxide, depending on the residence time of the gas stream in the membrane and the approximately constant permeance for carbon dioxide.

[0033] The membrane's permeance to carbon dioxide is continuously monitored. As soon as a drop in permeance (average over 1 hour) below a threshold is detected, the temperature T2 is gradually increased to maintain a constant permeance.

[0034] If the permeance continues to decrease despite repeated increases in temperature T2, a regeneration cycle is performed in which the membrane is regenerated with hot air at a temperature of 120°C or higher. Details of the regeneration procedure are described in WO 2015 / 071257 A1. Regular regeneration helps to maintain the membrane's performance over longer periods, thereby reducing costs and effort, as well as increasing the plant's operational reliability.

[0035] After the regeneration cycle, the purification process continues from the raw gas stream.

Claims

1. Method for producing a purified methane-containing gas stream, particularly reduced in terms of carbon dioxide content, from a raw gas stream, wherein the raw gas stream contains methane and is contaminated with higher hydrocarbons, carbon dioxide and possibly further impurities, wherein the method comprises at least the following steps: - Separating particulate impurities and preferentially a portion of the higher hydrocarbons from the raw gas stream to obtain a pre-treated, particularly pre-filtered, gas stream, - Conditioning, namely heating, to a temperature T2, to obtain a conditioned gas stream, wherein the temperature T2 is adjusted such that the permeance of the membrane separation device with respect to carbon dioxide remains constant on average over a time period t, and - Introducing the conditioned gas stream into a membrane separation device to separate a portion of the carbon dioxide contained therein as permeate, whereby the purified, methane-containing gas stream is obtained as retentate.

2. Method according to claim 1, wherein the separation is carried out at least partially at a temperature T1, wherein the temperature T2 is higher than the temperature T1; preferentially wherein the temperature T2 is at least 10 °C, preferably at least 20 °C, more preferably at least 30 °C, even more preferably at least 40 °C, particularly at least 50 °C higher than the temperature T1 and / or preferentially wherein the temperature T1 corresponds to the ambient temperature.

3. Method according to claim 1 or 2, wherein the temperature T2 is above the ambient temperature.

4. Method according to any one of claims 1 to 3, wherein the temperature T2 is at least 30 °C, preferably at least 40 °C, more preferably at least 50 °C, even more preferably at least 60 °C, particularly at least 70 °C or even at least 80 °C.

5. Method according to any one of claims 1 to 4, wherein the time period t is at least 12 h, preferably at least 24 h, more preferably at least 36 h, even more preferably at least 48 h, particularly at least 72 h.

6. Method according to claim 5, wherein the temperature T3 is at least 10 °C, preferably at least 20 °C, more preferably at least 30 °C, even more preferably at least 40 °C, particularly at least 50 °C higher than the temperature T2.

7. Method according to claim 5 or 6, wherein the temperature T3 is at least 50 °C, preferably at least 70 °C, more preferably at least 90 °C, even more preferably at least 110 °C, particularly at least 130 °C or even at least 150 °C.

8. Method according to any one of claims 1 to 7, wherein the membrane separation device has a thermal insulation against the environment.

9. Method according to any one of claims 1 to 8, wherein the raw gas stream contains biogas, landfill gas, process gas, natural gas or associated petroleum gas.

10. Method according to any one of claims 1 to 9, wherein the proportion of methane in the raw gas stream is at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, particularly at least 90%.

11. Method according to any one of claims 1 to 10, wherein the proportion of higher hydrocarbons in the raw gas stream is 0.1% to 10%, preferably 0.5% to 5%.

12. Method according to any one of claims 1 to 11, wherein the proportion of carbon dioxide in the raw gas stream is 1% to 30%, preferably 2% to 20%, particularly 3% to 15%.

13. Method according to any one of claims 1 to 12, wherein the proportion of carbon dioxide in the purified, methane-containing gas stream is below 10%, preferably below 5%, even more preferably below 2.5%.

14. Method according to any one of claims 1 to 13, wherein the membrane separation device has a polymer membrane, preferably a polyimide membrane.