Apparatus and membrane processes for separating gas components from gas streams having different compositions or flow rates

Abstract

An apparatus for separating gas streams includes a compressor and three membrane separation units connected in series to deliver retentate streams from each of the first two units to a subsequent membrane separation unit. The apparatus also includes conduits for recirculating permeate streams from the second and third membrane separation units upstream of the compressor, and control means for controlling the fraction of the second permeate stream being recirculated. Adjusting the fraction of the second permeate stream to be recirculated can be used to maintain a target composition of the retentate obtained in the third membrane separation unit when the flow rate or composition of the gas stream changes.

Description

Technical Field

[0001] The present invention relates to separating gas streams having different compositions or flow rates, and containing a first gas component and a second gas component that permeates more slowly, by membrane methods, to provide a product gas stream rich in the second gas component with a substantially constant composition. Background Technology

[0002] Membrane methods for gas separation are widely used because they require fewer process chemicals, mobile machinery, and energy compared to gas separation methods that use gas liquefaction, absorption onto solids, or absorption into liquids. Existing membrane methods are highly efficient for separating gas streams with essentially constant composition and flow rates, but they have certain drawbacks for separating gas streams with varying compositions or flow rates. When membrane methods are used to provide product gases rich in slower-permeating gas components (which have lower levels of faster-permeating gas components than specified limits), such as in the production of biomethane (also known as renewable natural gas) from biogas or pipeline-grade natural gas from crude natural gas, the membrane separation unit must be designed to provide the required specifications at the expected maximum gas flow rate and maximum faster-permeating gas component content. Otherwise, operating such units at increased gas flow rates or with gas streams containing higher levels of faster-permeating gas components will cause the concentration of faster-permeating gases in the product gas to increase above specified limits. However, designing membrane separation units to accommodate the maximum possible gas flow rate, which only occurs occasionally, not only leads to high membrane investment costs but also has the following drawback: when the unit operates at gas flow rates below the maximum, the recovery rate of slower-permeating gas components will decrease. This is the case for most of the operating time for biogas from anaerobic digesters or landfills, where gas production rates vary throughout the day and up to a season.

[0003] WO 2014 / 075850 discloses a two-stage, two-step membrane separation device comprising a feed stream separation unit, a retentate separation unit, and a permeate separation unit. When the composition or flow rate of the gas feed changes, it maintains a constant product stream composition by controlling the permeate pressure in the retentate separation unit and the retentate pressure in the permeate separation unit. However, this concept cannot be applied to single-stage membrane separation devices.

[0004] WO 2014 / 183977 discloses a single-stage two-step membrane separation apparatus that uses control of permeate pressure in a first separation step based on the flow rate or composition of the feed gas stream or the composition of the product gas stream.

[0005] Adjusting permeate pressure to respond to changes in gas flow rate or composition requires operating the device at a pressure higher than necessary for most of the time in order to be effective against increases in gas flow rate or the content of faster-permeating gas components.

[0006] Four Korean patents propose alternative concepts for using diversion in multi-step, multi-stage methods to compensate for changes in feed gas flow rate or biogas composition.

[0007] KR 1840337 B discloses a three-step, two-stage method that uses a controller to guide different proportions of permeate obtained in the first stage to the second membrane stage based on the methane content in the first-stage permeate. The retentate from the second stage of the first step is combined with the retentate from the first stage of the first step to provide feed for the first stage of the second step.

[0008] KR 1840340 B discloses a three-step, two-stage method using a first controller and a second controller. The first controller guides different proportions of the retentate obtained in the first-stage second step to the first-stage third step based on the methane content in the feed stream. The second controller guides different proportions of the retentate from the second-stage first step to the second-stage second step based on the carbon dioxide content in the retentate. The second-stage first step receives the permeate from the first-stage second step as feed.

[0009] KR 1840343 B discloses a three-step, two-stage method of KR 1840340 B that does not have a first controller and does not divert the first-stage, second-step intercepted logistics.

[0010] KR 1863058 B discloses a three-step, two-level method of KR1840340 B that does not have a second controller and does not divert the intercepted material in the first step of the second level.

[0011] All four methods described above require a device with at least five membrane units in two stages.

[0012] US 6,197,090 discloses a two-step single-stage membrane separation method in which a portion of the first-step permeate is recycled to the feed stream, the proportion of which is controlled based on the concentration of the faster-permeating gas component in the first-step permeate, or controlled to maintain a constant pressure in the feed stream. Summary of the Invention

[0013] The inventors of this invention have now discovered that variations in the composition or velocity of the gas flow can be compensated for to provide a product gas rich in the slower-permeating gas component, which has a substantially constant purity relative to the faster-permeating gas component, without altering the pressure in the method by using a single-stage three-step arrangement of the membrane unit with the third-step permeate recirculated to the feed stream and by changing the fraction of the second-step permeate recirculated to the feed stream. Adjusting the fraction of the second-step permeate recirculated to the feed stream to maintain the target composition of the third-step retentate can further compensate for membrane efficiency variations, for example, caused by membrane fouling (which may occur when liquid enters the membrane module or gas components condense on the membrane), or due to fiber blockage in the hollow fiber membrane module. Attached Figure Description

[0014] Figure 1 An embodiment of the apparatus and method of the present invention is shown, wherein a gas analyzer (22) connected to a control device (16) is used to control the fraction of the second permeate stream being recirculated in order to maintain the gaseous properties of the third retentate stream.

[0015] Figure 2 An embodiment of the apparatus and method of the present invention is shown, wherein an additional vacuum pump (19, 20) is used to provide negative pressure on the permeate side of the second membrane separation unit (6) and the third membrane separation unit (9).

[0016] Figure 3 An embodiment of the apparatus and method of the present invention is shown, wherein the third membrane separation unit (9) includes two membrane modules (9a, 9b) and a shut-off valve for cutting off the flow of the membrane module (9b) when the flow rate of the gas stream (1) to be separated is reduced to below a threshold. Detailed Implementation

[0017] The apparatus of the present invention is designed to separate gas streams (1) comprising a first gas component and a second gas component and having different compositions or flow rates. The apparatus of the present invention includes three membrane separation units and a gas compressor, as well as conduits connecting them and a control device for controlling the diversion of specific process streams.

[0018] The apparatus of the present invention includes a first membrane separation unit (2) that receives a gas stream (1) via a feed conduit (3) and includes a gas separation membrane having a higher permeability to a first gas component than to a second gas component. The first membrane separation unit (2) provides a first permeate stream and a first retained stream. A first permeate conduit (4) is connected to the first membrane separation unit (2) to receive the first permeate stream, and a first retained stream conduit (5) is connected to the first membrane separation unit (2) to receive the first retained stream.

[0019] The term permeate here refers to a gas mixture containing gas components of the gas mixture fed into the membrane separation unit, which have passed through the gas separation membrane due to the partial pressure difference across the membrane. The term retentate refers to the gas mixture remaining after the gas components that form the permeate have passed through the gas separation membrane. Because the gas separation membrane has a higher permeability to the first gas component than to the second gas component, the permeate will be richer in the first gas component than the gas mixture fed into the first membrane separation unit (2), while the retentate will deplete the first gas component.

[0020] Permeability is defined as the gas flow rate per unit time, area, and pressure difference through a membrane, and is typically based on volumetric flow rate in gas permeation units (GPU, 10). -6 cm 3 cm -2 s -1 cm(Hg) -1 The permeability P of a specific membrane and gas component in the GPU was determined by a pure gas permeation experiment and was P = 10. 6 *Q / (RT*Δp), where Q is the velocity (in cm) passing through the membrane under standard conditions. 3 The standardized gas flow rate is expressed in units of / s, R is the gas constant, T is the temperature, and Δp is the transmembrane pressure difference in cm (Hg).

[0021] The pure gas selectivity S of the membrane for the first gas component relative to the second gas component is defined as S = P1 / P2, where P1 is the permeability of the first gas component and P2 is the permeability of the second gas component.

[0022] The separation capacity of a membrane separation unit is defined as the product of the total membrane area of ​​the membrane separation unit and the permeability of the membrane used in the membrane separation unit.

[0023] Suitable gas separation membranes are known in the prior art. Gas separation membranes containing a separation layer of a glassy polymer (i.e., a polymer having a glass transition point at a temperature above the operating temperature of the membrane separation unit) are preferred because they generally provide higher selectivity than membranes with separation layers of different polymer types. This glassy polymer can be polyetherimide, polycarbonate, polyamide, polybenzoxazole, polybenzimidazole, polysulfone, or polyimide, and the gas separation membrane preferably contains at least 80% by weight of polyimide or a mixture of polyimides.

[0024] In a preferred embodiment, the gas separation membrane of the first membrane separation unit comprises at least 50% by weight of a polyimide prepared by reacting a dianhydride with a diisocyanate. The dianhydride is selected from 3,4,3',4'-benzophenone tetracarboxylic dianhydride, 1,2,4,5-benzenetetracarboxylic dianhydride, 3,4,3',4'-biphenyltetracarboxylic dianhydride, oxophthalic dianhydride, sulfonylphthalic dianhydride, 1,1,1,3,3,3-hexafluoro-2,2-propylene phthalic dianhydride, and mixtures thereof. The diisocyanate is selected from 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-methylene diphenyl diisocyanate, 2,4,6-trimethyl-1,3-phenylene diisocyanate, 2,3,5,6-tetramethyl-1,4-phenylene diisocyanate, and mixtures thereof. The dianhydride is preferably 3,4,3',4'-benzophenone tetracarboxylic dianhydride or a mixture of 3,4,3',4'-benzophenone tetracarboxylic dianhydride and 1,2,4,5-benzenetetracarboxylic dianhydride. The diisocyanate is preferably a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, or a mixture of 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and 4,4'-methylene diphenyl diisocyanate. Such suitable polyimides are available from Evonik Fibres GmbH under the trade name... Type 70, commercially available, has CAS number 9046-51-9 and is a polyimide made from a mixture of 3,4,3',4'-benzophenone tetracarboxylic dianhydride, 64 mol% 2,4-toluene diisocyanate, 16 mol% 2,6-toluene diisocyanate, and 20 mol% 4,4'-methylene diphenyl diisocyanate, and is marketed under the trade name... The polyimide, commercially available from HT and bearing CAS number 134119-41-8, is prepared from a mixture of 60 mol% 3,4,3',4'-benzophenone tetracarboxylic dianhydride and 40 mol% 1,2,4,5-benzenetetracarboxylic dianhydride with 80 mol% 2,4-toluene diisocyanate and 20 mol% 2,6-toluene diisocyanate. The gas separation membranes of this embodiment are preferably as described in WO 2014 / 202324 A1 and have been heat-treated in an inert atmosphere to improve their long-term stability in the methods of the present invention.

[0025] The gas separation membrane can be a flat sheet membrane or a hollow fiber membrane, and is preferably an asymmetric hollow fiber membrane comprising a dense polyimide layer on a porous support. The term "dense layer" here refers to a layer that substantially does not contain macropores extending through it, and the term "porous support" here refers to a support material having macropores extending through it. The asymmetric hollow fiber membrane can be prepared by coating porous hollow fibers with polyimide to form a dense polyimide layer on the support. In a preferred embodiment, the asymmetric hollow fiber membrane is prepared by a reverse process of spinning with an annular bicomponent spinneret, passing a solution of polyimide through the annular opening, and passing a non-solvent liquid containing polyimide through a central opening. This process provides an asymmetric hollow fiber membrane with a dense layer on a porous support, both composed of the same polyimide.

[0026] The gas separation membrane preferably comprises a dense separation layer of a glassy polymer coated with an additional dense layer of a rubbery polymer, which has higher gas permeability than the glassy polymer. A preferred gas separation membrane comprising a polyimide separation layer is preferably coated with a polydimethylsiloxane elastomer.

[0027] The apparatus of the present invention further includes a second membrane separation unit (6) connected to a first retentate conduit (5) to receive a first retentate stream as feed. The second membrane separation unit (6) includes a gas separation membrane having a higher permeability to the first gas component than to the second gas component, and provides a second retentate stream and a second permeate stream. A second retentate conduit (7) is connected to the second membrane separation unit (6) to receive the second retentate stream, and a second permeate conduit (8) is connected to the second membrane separation unit (6) to receive the second permeate stream.

[0028] The apparatus of the present invention further includes a third membrane separation unit (9) connected to a second retentate conduit (7) to receive a second retentate stream as feed. The third membrane separation unit (9) includes a gas separation membrane having a higher permeability to the first gas component than to the second gas component, and provides a third retentate stream and a third permeate stream. A product conduit (10) is connected to the third membrane separation unit (9) to receive the third retentate stream.

[0029] Each of the three membrane separation units may include multiple membrane modules arranged in parallel, and may also include several membrane modules arranged in series. When the gas separation membrane is a flat sheet membrane, the membrane separation unit preferably includes one or more helically wound membrane modules containing flat sheet membranes. When the gas separation membrane is a hollow fiber membrane, the membrane separation unit preferably includes one or more membrane modules, each containing a bundle of hollow fiber membranes. When the membrane separation unit includes membrane modules arranged in series, the retentate provided by the membrane modules is fed as feed to the membrane modules following the series of membrane modules, the last membrane module in the series providing the retentate of the membrane separation unit, and the permeate from all membrane modules in the series is combined to provide the permeate of the membrane separation unit. When the membrane separation unit includes several hollow fiber membrane modules arranged in series, the membrane modules are preferably removable membrane cartridges arranged in series as a cartridge chain in a common pressure vessel and connected to each other by a central permeate collection tube, as detailed in WO 2016 / 198450 A1.

[0030] The three membrane separation units may include the same type of gas separation membrane. In this case, the second and third membrane separation units include the same type of gas separation membrane as further described above with respect to the first membrane separation unit.

[0031] In a preferred embodiment, the first membrane separation unit (2) includes a gas separation membrane that has a higher pure gas selectivity for the first gas component relative to the second gas component at 20°C compared to the gas separation membrane included in the second membrane separation unit (6), and has a lower permeability for the first gas component at 20°C than the gas separation membrane included in the second membrane separation unit (6). The third membrane separation unit (9) may then include the same gas separation membrane as the second membrane separation unit (6) or a different gas separation membrane. Preferably, the gas separation membrane of the first membrane separation unit (2) has a pure gas selectivity for carbon dioxide relative to methane at 20°C that is 1.05 to 2 times that of the pure gas selectivity of the gas separation membrane included in the second membrane separation unit (6) measured at the same temperature and for the same gas component. When the first membrane separation unit (2) comprises a preferred polyimide gas separation membrane prepared from dianhydride and diisocyanate as further described above, the second membrane separation unit (6) preferably comprises a gas separation membrane containing at least 50% by weight of block-copolyimide, as described in WO2015 / 091122, page 6, line 20 to page 16, line 4. The block-copolyimide preferably comprises at least 90% by weight of polyimide blocks having a block length of 5 to 1000, preferably 5 to 200.

[0032] In another preferred embodiment that can be combined with the foregoing embodiments, the second membrane separation unit (6) includes a gas separation membrane that has a higher pure gas selectivity for the first gas component relative to the second gas component at 20°C compared to the gas separation membrane included by the third membrane separation unit (9), and has a lower permeability for the first gas component at 20°C than the gas separation membrane included by the third membrane separation unit (9). Preferably, the gas separation membrane of the second membrane separation unit (6) has a pure gas selectivity for carbon dioxide relative to methane at 20°C that is 1.05 to 3 times that of the pure gas selectivity of the gas separation membrane included by the third membrane separation unit (9) measured at the same temperature and for the same gas component. The third membrane separation unit preferably also includes a gas separation membrane containing at least 50% by weight of block copolymer polyimide, as described in WO 2015 / 091122, page 6, line 20 to page 16, line 4. Higher permeability compared to the membrane used in the second membrane separation unit can be provided by selecting different polymer blocks or using different block lengths.

[0033] The apparatus of the present invention further includes a first recirculation conduit (11) connected to a third membrane separation unit (9) to receive a third permeate stream, and connected to a first recirculation feed point (12) on a feed conduit (3) for recirculating the third permeate stream.

[0034] The apparatus of the present invention further includes a gas compressor (13) disposed in a feed conduit (3) between the first recirculation feed point (12) and the first membrane separation unit (2), or disposed in the first recirculation conduit (11). When the gas stream (1) to be separated is received at or slightly above ambient pressure and needs to be compressed to a pressure sufficient to operate the first membrane separation unit (2), the gas compressor (13) is placed between the first recirculation feed point (12) and the first membrane separation unit (2). When the gas stream (1) to be separated is received at a pressure sufficient to operate the first membrane separation unit (2), only the gas to be recirculated needs to be compressed, and the gas compressor (13) may be placed in the first recirculation conduit (11). Any gas compressor compatible with the composition of the gas stream (1) can be used, such as a turbo compressor, piston compressor, or preferably a screw compressor. The screw compressor may be an oil-free compressor or a fluid-cooled compressor cooled by water or oil. When using an oil-cooled compressor, the device preferably also includes a droplet separator between the gas compressor (13) and the first membrane separation unit (2) to prevent oil droplets from entering the first membrane separation unit (2). Preferably, a cooler is placed in the feed duct (3) between the gas compressor (13) and the first membrane separation unit (2) to cool the compressed gas before it enters the first membrane separation unit (2). The cooler may also include a condenser for condensing moisture or other condensable components, and an additional heater may be placed between the condenser and the first membrane separation unit (2) to prevent condensable gas components from condensing on the gas separation membrane of the membrane separation unit.

[0035] The apparatus of the present invention further includes a second recirculation conduit (14) connected to the second permeate conduit (8) to receive all or part of the second permeate stream, and connected to a second recirculation feed point (15) on the feed conduit (3) or the first recirculation conduit (11) for recirculating a portion of the second permeate stream it receives. The second recirculation feed point (15) is located upstream of the gas compressor (13), allowing all or part of the second permeate stream to be recirculated without additional equipment. A control device (16) is configured to control how much of the second permeate stream is delivered to the second recirculation conduit (14). The remaining second permeate stream is delivered to the discharge conduit (17). The control device can implement the diversion of the second permeate stream into a portion fed to the second recirculation conduit (14) and a remainder fed to the discharge conduit (17) by operating a three-way diverter valve or by operating two separate valves, namely a first valve in the second recirculation conduit (14) and a second valve in the discharge conduit (17). For this purpose, the control valve is preferred over the on / off valve to prevent fluctuations in pressure and flow rate during operation of the apparatus.

[0036] The control device (16) may be configured to provide feed-forward control of a fraction of the second permeate stream recirculated through the second recirculation conduit (14). This can be achieved by providing a flow meter and / or gas analyzer on the feed conduit (3) upstream of the first recirculation feed point (12) to determine the velocity and / or composition of the gas stream (1) to be separated, connecting the control device (16) to the flow meter and / or gas analyzer, and configuring the control device (16) to set a control valve for diverting the second permeate stream into a fraction fed into the second recirculation conduit (14) and a remainder fed into the discharge conduit (17). In this embodiment, additional flow meters may be used in the second permeate conduit (8), the second recirculation conduit (14), the discharge conduit (17), or any combination thereof, which are connected to the control device (16) to set the desired diversion rate for the second permeate stream based on the velocity and / or composition of the gas stream (1) to be separated.

[0037] In an alternative and preferred embodiment, the control device (16) is configured to provide feedback control over a portion of the second permeate stream recirculated through the second recirculation conduit (14). For this purpose, a gas analyzer (22) is connected to the product conduit (10) to determine the gas properties of the third entrapment stream, the gas analyzer (22) is connected to the control device (16) to transmit information about the gas properties, and the control device (16) is configured to maintain the gas properties at target values ​​by controlling the fraction of the second permeate stream transmitted to the second recirculation conduit (14), preferably via a control valve. The gas properties measured by the gas analyzer (22) are preferably the content of a first gas component, the content of a second gas component, or the calorific value.

[0038] In a preferred embodiment, the first permeate conduit (4) and the discharge conduit (17) are connected to a combined discharge conduit (18), which then receives a portion of the first permeate stream and a portion of the second permeate stream that has not been recirculated. This is advantageous when the permeate stream leaving the device contains gaseous components that should be monitored or require further treatment, such as the removal of organic components by a thermal oxidizer. When there is a connection between the first permeate conduit (4) and the discharge conduit (17) via the combined discharge conduit (18), it is preferable to arrange a check valve (19) in the product discharge conduit (17) to prevent any transfer of the first permeate stream from the first permeate conduit (4) to the second recirculation conduit (14).

[0039] In another preferred embodiment, the apparatus of the present invention further includes a first vacuum pump (20) disposed in the first recirculation conduit (11), which provides negative pressure on the permeate side of the third membrane separation unit (9). This allows the third membrane separation unit (9) to operate at a higher pressure differential across the gas separation membrane, which reduces the membrane area required in the third membrane separation unit (9). The first vacuum pump (20) may be a variable displacement vacuum pump or a blower. The additional first vacuum pump (20) is preferably positioned upstream of the second recirculation feed point (15) to ensure that the pressure in the discharge conduit (17) does not drop below ambient pressure.

[0040] In another preferred embodiment that can be combined with the foregoing embodiments, the apparatus of the present invention further includes a second vacuum pump (21) disposed in a second permeate conduit (8) upstream of the discharge conduit (17), which provides negative pressure on the permeate side of the second membrane separation unit (6). This allows the second membrane separation unit (6) to operate at a higher pressure differential across the gas separation membrane, which reduces the membrane area required in the second membrane separation unit (6).

[0041] In another preferred embodiment, the third membrane separation unit (9) includes at least two membrane modules (9a, 9b) arranged in parallel, and at least one shut-off valve (23) for shutting off the flow from one or more membrane modules (9b). In this embodiment, the membrane module (9b) to be shut off preferably includes a shut-off valve (23) connected to the membrane module (9b) in the feed conduit. A check valve may be placed in the conduit that receives retentate from the membrane module (9b) to prevent backflow from the product conduit (10) through the gas separation membrane of the membrane module (9b) to the first recirculation conduit (11). In a preferred alternative, the same result is achieved by using a second shut-off valve in the conduit that receives permeate from the membrane module (9b) to be shut off. When the apparatus of the present invention includes a membrane module (9b) to be cut off, the apparatus preferably includes a flow meter on a feed duct (3) upstream of a first recirculation feed point (12) for measuring the flow rate of the gas flow (1) to be separated, and an additional control device configured to open or close a shut-off valve (23) according to the flow rate of the gas flow (1) to be separated, thereby connecting or cutting off the flow of the membrane module (9b).

[0042] The separation capacity of the membrane separation unit of the apparatus of the present invention is preferably selected such that, when the apparatus is operated with 80% to 100% of the second permeate stream recycled to the second recirculation feed point (15), it provides a specified target composition of the third retentate stream and a specified recovery rate of the second gas component recovered from the third retentate stream under the conventional load of the apparatus, which is the flow rate (also known as the nameplate capacity) of the gas flow (1) designed for the apparatus. Preferably, the separation capacity of the membrane separation unit is selected such that, when the apparatus is operated without any recirculation of the second permeate stream, it provides a specific maximum load higher than the conventional load, thereby providing substantially the same target composition as the third retentate stream, although at a lower recovery rate. A suitable separation capacity can be determined using process simulation software, thereby simulating membrane separation based on experimentally determined membrane permeability and selectivity values.

[0043] The method of the present invention for separating a gas stream (1) comprising a first gas component and a second gas component is carried out in the apparatus of the present invention. The method of the present invention includes feeding the gas stream (1) into a feed line (3) of the apparatus of the present invention, preferably upstream of a gas compressor (13), removing a third retentate stream from a product conduit (10) as a product gas stream rich in the second gas component, and removing a waste gas stream rich in the first gas component. The waste gas stream is removed by removing the first permeate stream from a first permeate conduit (4), or, if a combined discharge conduit (18) is present, by removing the stream derived from the stream that combines the first permeate stream with the remaining second permeate stream conveyed to the discharge conduit (17) from the combined discharge conduit (18).

[0044] The method of the present invention is preferably carried out using a gas stream (1) comprising carbon dioxide as a first gas component and methane as a second gas component. The gas stream (1) may then be natural gas or biogas, preferably having a combined content of more than 90% by volume of methane and carbon dioxide, i.e., containing less than 10% by volume of components other than methane and carbon dioxide. The gas stream (1) is preferably biogas from a landfill, wastewater treatment plant, or anaerobic digester.

[0045] In the method of the present invention, when the flow rate of the gas flow (1) changes, or when the composition of the gas flow (1) changes, or both change, the fraction of the second permeate stream delivered to the second recirculation conduit (14) is preferably adjusted. Preferably, the fraction of the second permeate stream delivered to the second recirculation conduit (14) is increased when the flow rate of the gas flow (1) decreases, and decreased when the flow rate of the gas flow (1) increases. Alternatively or in combination, the fraction of the second permeate stream delivered to the second recirculation conduit (14) is increased when the fraction of the first gas component in the gas flow (1) decreases, and decreased when the fraction of the first gas component in the gas flow (1) increases. The fraction of the second permeate stream delivered to the second recirculation conduit (14) can vary from a fraction of 0 (meaning all the second permeate stream is delivered to the discharge conduit (17)) up to a fraction of 1 (meaning all the second permeate stream is delivered to the second recirculation conduit (14)), and any value between 0 and 1 is possible.

[0046] When applying the convention, the minimum number of membrane units through which the permeate product of the membrane separation process must pass is counted as the membrane stage number, and the minimum number of membrane units through which the retentate product of the membrane separation process passes to provide permeates with different compositions is counted as the membrane step number. The method of the present invention, with a recirculation fraction of the second permeate stream between 0 and 1, is a single-stage three-step membrane separation method. However, for a recirculation fraction of 0, the method effectively becomes a single-stage two-step membrane separation method with a large membrane area in the first step because the permeate from the first and second membrane separation units is combined, while for a recirculation fraction of 1, the method effectively becomes a single-stage two-step membrane separation method with a large membrane area in the second step because the permeate from the second and third membrane separation units is combined.

[0047] In a preferred embodiment of the method of the present invention, the gas properties of the third retentate stream are monitored by an analyzer (22), and the fraction of the second permeate stream delivered to the second recirculation conduit (14) is controlled to maintain substantially constant gas properties. The gas properties monitored by the analyzer (22) are preferably the content of the first gas component in the third retentate stream, the content of the second gas component in the third retentate stream, or the calorific value of the third retentate stream. The content of the first or second gas component is preferably kept constant with a deviation from the target value of less than 0.5% by volume, and the calorific value is preferably kept constant with a deviation from the target value of less than 2%.

[0048] In another preferred embodiment of the method of the invention, which can be combined with the foregoing preferred embodiments, the apparatus of the invention is used, wherein the third membrane separation unit (9) comprises a plurality of membrane modules arranged in parallel that can be individually cut off from the flow stream, and the membrane modules of the third membrane separation unit (9) are cut off from the flow stream when the flow rate of the gas flow (1) decreases. Preferably, when the method is operated at the maximum flow rate of the gas flow (1), all membrane modules are connected to the flow stream. The membrane modules can be cut off from the flow stream based on a measurement of the actual flow rate of the gas flow (1). Alternatively, the membrane modules can be cut off from the flow stream when the concentration of carbon dioxide in the third retentate flow stream drops below a preset first threshold, and can be returned to the flow stream when the concentration of carbon dioxide in the third retentate flow stream increases to a value above a preset second threshold.

[0049] Example

[0050] Example 1

[0051] The separation of biogas from a landfill was calculated using process simulation software for a membrane separation module with a pure gas selectivity of approximately 55% for methane relative to carbon dioxide. For example... Figure 1 The three-stage membrane separation in the device shown calculates to 1260 Nm. 3 The device separates biogas containing 58.7 vol% methane, 40.0 vol% carbon dioxide, 1.0 vol% nitrogen and 0.3 vol% oxygen per hour, and has 21 membrane modules in the first membrane separation unit (2), 22 membrane modules in the second membrane separation unit (6) and 95 membrane modules in the third membrane separation unit (9).

[0052] The feed to the first membrane separation unit (2) is compressed to 13.1 bar, and all three membrane separation units operate at a pressure of 1.03 bar on the permeate side. 84% of the second permeate stream is recycled to the second recycle feed point (15), and the remaining 16% is combined with the first permeate stream, providing 742 Nm³. 3 The third retentate stream contains 97.0% by volume methane, 1.1% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.3% by volume oxygen. The third retentate stream contains 97.3% methane from the biogas fed into the unit, and the process requires compression of 1712 Nm³. 3 / h of gas (36% dual compression).

[0053] For up to 1400 Nm 3 With a higher biogas flow rate of / h, and by recirculating less of the second permeate stream, the methane content of the third retained stream can be maintained at the same value. 1400 Nm³ can be separated without recirculating any of the second permeate stream. 3 The same biogas provides 804 Nm³ / h.3 The third retentate stream contains 97.0% by volume methane, 1.2% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.2% by volume oxygen. This third retentate stream then contains 95.0% of the methane from the biogas fed into the unit, a process requiring compression of 1636 Nm³. 3 / h of gas (17% double compression).

[0054] Separation 1000Nm 3 / h reduced biogas gas flow rate, and recycles all second permeate streams to the second recycle feed point (15) to provide 582 Nm 3 The third retentate stream contains 97.7% by volume methane, 0.4% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.2% by volume oxygen. The third retentate stream contains 97.0% methane from the biogas fed into the unit, and the process requires compression of 1359 Nm³. 3 / h of gas (36% dual compression).

[0055] Comparative Example 1

[0056] When membrane separation unit 2a comprises the same number and type of membrane modules (i.e., 43 membrane modules) as the first and second membrane units combined with the apparatus of the present invention, and membrane separation unit 2b comprises the same number and type of membrane modules (i.e., 95 membrane modules) as the third membrane unit of the apparatus of the present invention, operation of US 6,197,090 is possible without recirculation from membrane separation unit 2a. Figure 3 The apparatus shown provides the present invention for operation without recirculating any second permeate stream. Figure 1 The device achieves the same separation effect. Therefore, at 1400 Nm 3 Operating at a maximum flow rate of / h and without recirculated material G7, US 6,197,090 is equivalent to the device of Example 1. Figure 3 The apparatus shown provides the same product stream G7 as the third entrapment stream obtained using the apparatus of the present invention.

[0057] At 1260Nm 3 At a nameplate capacity flow rate of / h, to simulate the same biogas separation in the same unit, the recirculation rate of recirculated stream G7 needs to be adjusted to 28% to provide the same methane content of 97.0 vol% methane in product stream G6. Then, at 735 Nm... 3Product stream G6 is obtained at a flow rate of / h, which has a composition of 97.0 vol% methane, 1.1 vol% carbon dioxide, 1.6 vol% nitrogen, and 0.3 vol% oxygen, and contains 96.4% methane from the biogas fed into the unit. The process then requires compression to 1718 Nm³. 3 / h of gas (36% dual compression).

[0058] Separate 1000 Nm using the same apparatus 3 The reduced biogas gas flow rate of / h provides the same 97.0% methane content with a recirculation rate of 64% for the G7 recirculation stream. Then, at 595 Nm³ / h... 3 Product stream G6 is obtained at a flow rate of / h, which has a composition of 97.0 vol% methane, 1.0 vol% carbon dioxide, 1.6 vol% nitrogen, and 0.3 vol% oxygen, and contains 98.3% methane from the biogas fed into the unit. The process then requires compression to 2032 Nm³. 3 / h of gas (103% double compression).

[0059] Example 1 and Comparative Example 1 demonstrate that the method of the present invention provides a better methane yield than prior art methods at the nameplate capacity, and requires much less compression energy when the biogas flow rate drops below the nameplate capacity, with only a slight loss in methane yield.

[0060] Example 2

[0061] The separation of biogas from landfills was calculated using process simulation software, such as... Figure 2 The separation is performed using a device that lacks a second vacuum pump (21). Three different membrane types used in the three membrane separation units were calculated: a type A membrane with a pure gas selectivity of 56 for carbon dioxide relative to methane used in the first membrane separation unit (2); a type B membrane with a pure gas selectivity of 50 for carbon dioxide relative to methane and a permeability to carbon dioxide twice that of the type A membrane used in the second membrane separation unit (6); and a type C membrane with a pure gas selectivity of 25 for carbon dioxide relative to methane and a permeability to carbon dioxide four times that of the type A membrane used in the third membrane separation unit (9). The first membrane separation unit (2), the second membrane separation unit (6), and the third membrane separation unit (9) have a total membrane area ratio of 2:1:1. The feed to the first membrane separation unit (2) is compressed to 13.5 bar. The first and second membrane separation units operate at a pressure of 1.0 bar on the permeate side, and the third membrane separation unit operates at a pressure of 0.6 bar on the permeate side generated by the first vacuum pump (20).

[0062] Separating 1000 Nm of material while completely recycling the second permeate stream to the second recycle feed point (15).3 The biogas produced per hour (nameplate capacity) was the same as in Example 1, providing 591 Nm³. 3 The third retentate stream contains 97.1% by volume methane, 0.9% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.3% by volume oxygen. The third retentate stream contains 97.7% methane from the biogas fed into the unit, and the process requires compression of 1455 Nm³. 3 / h of gas (46% dual compression).

[0063] Without recycling the second permeate stream to the second recycling feed point (15), at 1145 Nm 3 When operating at its maximum biogas feed capacity of / h, the same unit provides 641 Nm³. 3 The third retentate stream contains 97.2 vol% methane, 1.0 vol% carbon dioxide, 1.6 vol% nitrogen, and 0.2 vol% oxygen. This third retentate stream then contains 92.8% methane from the biogas fed into the unit, a process requiring compression of 1370 Nm³. 3 / h of gas (20% double compression). For any biogas flow rate between the nameplate capacity and the maximum capacity, the methane content of the third retentate stream can be kept constant by adjusting the fraction of the second permeate stream recirculated to the second recirculation feed point (15).

[0064] Example 3

[0065] The calculations of Example 2 were repeated, except that a C-type membrane was used in the second membrane separation unit (6), and the ratio of the total membrane area of ​​the first membrane separation unit (2), the second membrane separation unit (6), and the third membrane separation unit (9) was 2:0.6:1.

[0066] Separating 1000 Nm of material while completely recycling the second permeate stream to the second recycle feed point (15). 3 / h (nameplate capacity), then provides 589Nm 3 The third retentate stream contains 97.3% by volume methane, 0.8% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.3% by volume oxygen. The third retentate stream contains 97.7% methane from the biogas fed into the unit, and the process requires compression of 1518 Nm³. 3 / h of gas (52% double compression).

[0067] Without recycling the second permeate stream to the second recycling feed point (15), at 1200 Nm 3 The maximum capacity operating unit of / h provides 626Nm 3The third retentate stream contains 97.3 vol% methane, 0.7 vol% carbon dioxide, 1.7 vol% nitrogen, and 0.2 vol% oxygen. This third retentate stream then contains 86.6% methane from the biogas fed into the unit, a process requiring compression of 1411 Nm³. 3 / h of gas (18% dual compression).

[0068] Example 4

[0069] The calculations of Example 2 are repeated, except that a type A membrane is used in the second membrane separation unit (6), and the total membrane area of ​​the first membrane separation unit (2), the second membrane separation unit (6), and the third membrane separation unit (9) has a ratio of 2:2:1.

[0070] Separating 1000 Nm of material while completely recycling the second permeate stream to the second recycle feed point (15). 3 / h (nameplate capacity), then provides 591Nm 3 The third retentate stream contains 97.2% by volume methane, 0.9% by volume carbon dioxide, 1.6% by volume nitrogen, and 0.3% by volume oxygen. The third retentate stream contains 97.7% methane from the biogas fed into the unit, and the process requires compression of 1454 Nm³. 3 / h of gas (45% double compression).

[0071] Without recycling the second permeate stream to the second recycling feed point (15), at 1145 Nm 3 The maximum capacity operating unit of / h provides 644Nm 3 The third retentate stream contains 97.2 vol% methane, 1.0 vol% carbon dioxide, 1.6 vol% nitrogen, and 0.2 vol% oxygen. This third retentate stream then contains 93.2% methane from the biogas fed into the unit, a process requiring compression of 1369 Nm³. 3 / h of gas (20% double compression).

[0072] A comparison of Example 4 with Examples 1 and 3 demonstrates that when a membrane with lower carbon dioxide selectivity but higher permeability is used in the third membrane separation unit (9) compared to the membrane used in the second membrane separation unit (6), better methane recovery is provided at flow rates above the nameplate capacity.

[0073] Comparing Examples 2 and 3 with Example 4 demonstrates that using a membrane with lower carbon dioxide selectivity but higher permeability in the second membrane separation unit (6) compared to the membrane used in the first membrane separation unit (2) allows the device to be operated with a lower total membrane area while achieving the same product purity and methane recovery rate at the nameplate capacity.

[0074] Example 5

[0075] The calculations of Example 2 are repeated, except that the third membrane separation unit (9) of this device has the following characteristics: Figure 3 The additional membrane module (9b) shown can be used to cut off the flow, which provides an additional 50% membrane area in the third membrane separation unit (9).

[0076] The same separation results as in Example 2 were provided by operating the device at nameplate capacity with the second permeate stream fully recycled to the second recycle feed point (15) and the additional membrane module (9b) cut off the stream.

[0077] Without recycling the second permeate stream to the second recirculation feed point (15) and the additional membrane module (9b) connecting the streams, at 1400 Nm 3 The maximum capacity operating unit provides 797 Nm / h 3 The third retentate stream contains 97.3 vol% methane, 0.9 vol% carbon dioxide, 1.6 vol% nitrogen, and 0.2 vol% oxygen. This third retentate stream then contains 94.3% methane from the biogas fed into the unit, a process requiring compression of 1772 Nm³. 3 / h of gas (27% dual compression).

[0078] A comparison of Example 5 with Example 2 demonstrates that the apparatus having a third membrane separation unit comprising a parallel membrane module (part of which can be connected or disconnected depending on the flow rate of the gas to be separated) can provide the required product purity at a higher maximum capacity and provide better methane recovery at flow rates above the nameplate capacity.

[0079] Reference tag list:

[0080] 1. Gas streams to be separated

[0081] 2 First Membrane Separation Unit

[0082] 3 Feed pipe

[0083] 4 First permeate conduit

[0084] 5 First retentate catheter

[0085] 6 Second Membrane Separation Unit

[0086] 7 Second effluent catheter

[0087] 8 Second permeate conduit

[0088] 9 Third Membrane Separation Unit

[0089] 9a, 9b Membrane modules of the third membrane separation unit (9)

[0090] 10 Product delivery tube

[0091] 11 First recirculation catheter

[0092] 12 First Recycle Feed Point

[0093] 13 Gas compressor

[0094] 14 Second recirculation catheter

[0095] 15 Second Recycle Feed Point

[0096] 16. Control device

[0097] 17. Discharge duct

[0098] 18 Combined emission duct

[0099] 19 Check valve

[0100] 20 First Vacuum Pump

[0101] 21 Second Vacuum Pump

[0102] 22 Gas Analyzer

[0103] 23. Stop valve

Claims

1. An apparatus for separating a gas stream (1) comprising a first gas component and a second gas component, comprising: (a) A first membrane separation unit (2) receives the gas flow through a feed conduit (3), the first membrane separation unit comprising a gas separation membrane having a higher permeability to the first gas component than to the second gas component, thereby providing a first permeate stream rich in the first gas component and a first entrapment stream; (b) A first permeate conduit (4) is connected to the first membrane separation unit (2) to receive the first permeate stream; (c) A first retentate conduit (5) is connected to the first membrane separation unit (2) to receive the first retentate stream; (d) A second membrane separation unit (6) is connected to the first retentate conduit (5) to receive the first retentate stream as feed, the second membrane separation unit (6) including a gas separation membrane that has a higher permeability to the first gas component than to the second gas component, thereby providing a second retentate stream and a second permeate stream; (e) A second retentate conduit (7) is connected to the second membrane separation unit (6) to receive the second retentate stream; (f) A second permeate conduit (8) is connected to the second membrane separation unit (6) to receive the second permeate stream; (g) A third membrane separation unit (9) is connected to the second retentate conduit (7) to receive the second retentate stream as feed, the third membrane separation unit (9) including a gas separation membrane that has a higher permeability to the first gas component than to the second gas component, thereby providing a third retentate stream and a third permeate stream; (h) Connected to the third membrane separation unit (9) to receive the product conduit (10) of the third retentate stream; (i) A first recirculation conduit (11) is connected to the third membrane separation unit (9) to receive the third permeate stream and is connected to a first recirculation feed point (12) on the feed conduit (3); (j) A gas compressor (13) is arranged in the feed duct (3) between the first recirculation feed point (12) and the first membrane separation unit (2), or in the first recirculation duct (11); (k) A second recirculation conduit (14) which is connected to the feed conduit (3) or a second recirculation feed point (15) on the first recirculation conduit (11); as well as (l) A control device (16) that controls the fraction of the second permeate flow transmitted to the second recirculation conduit (14) and transmits the remaining second permeate flow to the discharge conduit (17). Furthermore, its characteristic is The second recirculation feed point (15) is located upstream of the gas compressor (13), and the second recirculation conduit (14) is connected to the second permeate conduit (8) to receive all or part of the second permeate stream; The first membrane separation unit (2) includes a gas separation membrane that has a higher pure gas selectivity for the first gas component relative to the second gas component at 20°C compared to the gas separation membrane included in the second membrane separation unit (6), and has a lower permeability for the first gas component at 20°C. The second membrane separation unit (6) includes a gas separation membrane that has a higher pure gas selectivity for the first gas component relative to the second gas component at 20°C compared to the gas separation membrane included by the third membrane separation unit (9), and has a lower permeability for the first gas component at 20°C.

2. The apparatus according to claim 1, wherein the first permeate conduit (4) and the discharge conduit (17) are connected to a combined discharge conduit (18); and a check valve (19) is disposed in the discharge conduit (17) to prevent the first permeate stream from being transferred from the first permeate conduit (4) to the second recirculation conduit (14).

3. The apparatus according to claim 1 or 2, further comprising a first vacuum pump (20) disposed in the first recirculation conduit (11) to provide negative pressure on the permeate side of the third membrane separation unit (9).

4. The apparatus according to claim 3, wherein the first vacuum pump (20) is arranged upstream of the second recirculation feed point (15).

5. The apparatus according to claim 1 or 2, further comprising a second vacuum pump (21) disposed in the second permeate conduit (8) upstream of the discharge conduit (17) to provide negative pressure on the permeate side of the second membrane separation unit (6).

6. The apparatus according to claim 1 or 2, further comprising a gas analyzer (22) connected to the product conduit (10) for determining the gas properties of the third entrapment stream, the gas properties being selected from the content of the first gas component, the content of the second gas component, and the calorific value, the gas analyzer (22) being connected to the control device (16) to transmit information about the gas properties, and the control device (16) being configured to maintain the gas properties at a target value by controlling the fraction of the second permeate stream transmitted to the second recirculation conduit (14).

7. The apparatus according to claim 1 or 2, wherein the third membrane separation unit (9) comprises at least two membrane modules (9a, 9b) arranged in parallel, and at least one shut-off valve (23) for shutting off one of the at least two membrane modules (9a, 9b) arranged in parallel.

8. A method for separating a gas stream (1) comprising a first gas component and a second gas component, characterized in that... The method includes: The gas stream (1) is fed into the feed conduit (3) of the device according to any one of claims 1 to 6, thereby feeding the gas stream (1) into the feed conduit (3) upstream of the gas compressor (13); A third retentate stream is extracted from the product conduit (10) as a product gas stream rich in the second gas component; and The first permeate stream is taken out from the first permeate conduit (4) as a waste gas stream rich in the first gas component.

9. The method according to claim 8, wherein the gas stream (1) is fed into the feed conduit (3) in which the gas compressor (13) is arranged between the first recirculation feed point (12) and the first membrane separation unit (2).

10. The method according to claim 8, wherein the stream derived from combining the first permeate stream with the remaining second permeate stream delivered to the discharge duct (17) is removed from the combined discharge duct (18) as a waste gas stream rich in the first gas component.

11. The method according to claim 8 or 9, wherein the first gaseous component is carbon dioxide and the second gaseous component is methane.

12. The method according to claim 8 or 9, wherein the fraction of the second permeate stream delivered to the second recirculation conduit (14) increases when the flow rate of the gas stream (1) decreases and decreases when the flow rate of the gas stream (1) increases.

13. The method according to claim 8 or 9, wherein the fraction of the second permeate stream delivered to the second recirculation conduit (14) increases when the fraction of the first gas component in the gas stream (1) decreases, and decreases when the fraction of the first gas component in the gas stream (1) increases.

14. The method according to claim 8 or 9, wherein the gas properties of the third entrapped stream are monitored by an analyzer (22), the gas properties being selected from the content of the first gas component, the content of the second gas component, and the calorific value, and the fraction of the second permeate stream delivered to the second recirculation conduit (14) is controlled to keep the gas properties substantially constant.

15. The method according to claim 8 or 9, wherein the third membrane separation unit (9) comprises a plurality of membrane modules arranged in parallel and individually capable of cutting off the flow stream, wherein the membrane modules of the third membrane separation unit (9) are cut off from the flow stream when the flow rate of the gas stream (1) decreases.

16. The method according to claim 8 or 9, wherein the gas stream is biogas from a landfill, wastewater treatment plant, or anaerobic digester.

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