PRODUCTION OF BIOMETHANE USING MULTIPLE TYPES OF MEMBRANE

Abstract

The invention relates to a process for recovering methane from digester biogas or landfill gas. More specifically, the invention pertains to a method for producing biomethane that removes impurities from a compressed digester biogas with staged membrane modules of at least two different types, to produce a biomethane having at least 94% CH.sub.4, below 3% of CO.sub.2, and below 4 ppm of H.sub.2S.

Claims

1. A system for producing biomethane, comprising: a source of a biogas feed comprising 40-75% of methane, 20-55% of carbon dioxide, up to 5,000 ppm of hydrogen sulfide (“H.sub.2S”), an amount of water, and up to 2,000 ppm of volatile organic compounds (“VOC's”); a compressor fluidly communicating with said source and being adapted and configured to receive and compress said biogas feed; a first separation stage in fluid communication with said compressed feed, comprising one or more gas separation membranes for processing said compressed feed being selective for VOCs over CH.sub.4 and having a selectivity of at least 10 for H.sub.2S over CH.sub.4; a second separation stage in fluid communication with said first separation stage, comprising one or more polymeric gas separation membranes for processing a retentate from said first separation stage of membrane to produce biomethane, wherein said one or more polymeric gas separation membranes of said second separation stage has a selectivity of at least 20 for CO.sub.2 over CH.sub.4, and an inlet for recycling a permeate from said second separation stage back to said compressor inlet, thereby commingling said biogas feed with said second stage permeate stream as a mixed gas feed to said compressor so that said compressor compresses and discharges said compressed feed to said first separation stage of membrane, wherein no regenerable adsorbent beds are in fluid communication between said compressor and said second membrane stage.

2. The system of claim 1, further comprising a water removal apparatus adapted and configured to remove water from said compressed biogas feed prior to said feeding said feed into said first separation stage of membrane.

3. The system of claim 1, wherein each of said at least one membrane of said first separation stage exhibits a pressure drop between a pressure of the feed gas and a pressure of the retentate gas is less than 50 psi (3.45 bar).

4. The system of claim 1, wherein said at least one membrane of said first separation stage has a separation layer made of a copolymer or block polymer of the formula: ##STR00015## where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) poly(tetramethylene oxide).

5. The system of claim 1, wherein said at least one membrane of said first membrane stage has a separation layer made of repeating units of the following monomers: ##STR00016##

6. The system of claim 1, wherein each of said at least one membrane of said first separation stage has a separation layer that is supported by a support layer.

7. The system of claim 6, wherein each of said support layers is made of a polyimide, polysulfone, polyether ether ketone, or mixtures thereof.

8. The system of claim 7, wherein each of said support layers is porous and is made of polyether ether ketone.

9. The system of claim 1, wherein each of said membranes of said second separation stage is made of cellulose acetate, a polysulfone, a polyimide, or mixtures thereof.

10. The system of claim 1, further comprising a H.sub.2S scavenger media in fluid communication between said first and second separation stages that is adapted and configured to remove at least some H.sub.2S from a retentate from said first separation stage before said retentate is sent to said second separation stage.

11. The system of claim 1, further comprising a H.sub.2S scavenger media located on a high-pressure retentate side of said second separation stage, said media being in fluid communication with said second separation stage and being adapted and configured to remove at least some H.sub.2S from a retentate from said second separation stage to produce biomethane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 illustrates a biomethane production by a two-stage membrane system.

[0063] FIG. 2 illustrates a three-stage membrane system for biomethane production, and a H.sub.2S scavenger media.

[0064] FIG. 3 illustrates relative permeation rates for a rubbery first separation stage membrane.

[0065] FIG. 4 illustrates relative permeation rates for a glassy second separation stage membrane.

DETAILED DESCRIPTION OF THE INVENTION

[0066] There is a disclosed method for producing biomethane from biogas, comprising: compressing a stream of the biogas in a compressor; removing water from the compressed feed by cooling; and sending the dehydrated and compressed feed into a first separation stage containing at least one polymeric gas separation membrane having a selectivity of at least 10, preferably at least 30, for H.sub.2S over CH.sub.4 to permeate a first low quality gas mixture having less than 20% methane and impurities such as H.sub.2S, water, siloxane, CO.sub.2, and VOC's, then passing the retentate or first gas mixture, having at least 60% methane from the first separation stage, to a second separation stage containing at least one polymeric gas separation membrane having a selectivity of at least 20, preferably at least 35, for CO.sub.2 over CH.sub.4 to produce a biomethane having at least 94% methane, below 3% of CO.sub.2, below 100 ppm of H.sub.2S, below 100 ppm of VOC's, below 100 ppm of siloxanes, and below 0.01 wt. % of water.

[0067] The membranes of the first separation stage are substantially different from the membranes in the second separation stage. More particularly, the polymeric membranes of the first separation stage may have a selectivity for H.sub.2S over CH.sub.4 that is higher than the polymeric membranes of the second separation stage. More particularly, the polymeric membranes of the first separation stage may have a selectivity for CO.sub.2 over CH.sub.4 that is lower than the polymeric membranes of the second separation stage. More particularly, the polymeric membranes of the first separation stage may be rubbery membranes while the polymeric membranes of the second separation stage may be glassy membranes.

[0068] The permeate from the first separation stage is comprised of about 15% of methane, up to 10,000 ppm of H.sub.2S, up to 3 wt. % of water and up to 85% of CO.sub.2. The first permeate from the first separation stage is optionally fed into a third separation stage containing at least one polymeric gas separation membrane having a selectivity of at least 4, preferably at least 6, for H.sub.2S and CO.sub.2 over CH.sub.4, to concentrate the impurities within permeate to be vented or sent to an oxidizer for further treatment. The permeate from the second separation stage is comprised of about 50% to 80% methane, at least 20% CO.sub.2, below 1,000 ppm of H.sub.2S, below 1,000 ppm of VOC's and below 0.05 wt. % of H.sub.2O. The permeate from the second separation stage and the retentate of the third separation stage may be recycled back to the main compressor, to be compressed and sent into the first separation stage to produce biomethane. Specifically, the second permeate gas mixture and the retentate from the third separation stage may be fed to the suction inlet of the main compressor, or combined with the biogas feed upstream of the suction inlet so that the combined stream is compressed along with the biogas feed.

[0069] The present invention excludes the use of regenerable adsorption systems, such as PSA, TSA, and VSA. Specifically, no regenerable adsorbent beds are used from an initial step of obtaining the biogas from the landfill (or digester) to a final step of obtaining the product biomethane gas. The exclusion of regenerable adsorbent beds reduces the cost, as well as eliminates the need for clean, low pressure gas and pre-conditioning facilities.

[0070] As used herein, the term “biogas” typically refers to a mixture of different gases produced from the breakdown of organic matter in the absence of oxygen in an anaerobic digestion process. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste.

[0071] Biogas typically comprises as the main components 50-70% of methane (CH.sub.4) and 20 to 50% carbon dioxide (CO.sub.2) with lower levels of other components such as N.sub.2 and O.sub.2, from up to 5,000 ppm or more of hydrogen sulfide (H.sub.2), up to 100 ppm of siloxanes, up to 1,000-2,000 ppm of volatile organic compounds (VOC's), and is saturated with water. Biogas also includes landfill gas (LFG), which is derived from solid waste landfills that decompose to organic waste with time, and microbe digestion of a variety of the organic waste to produce a methane and CO.sub.2 with the wide variety of decomposition products above. In either case biogas includes high concentrations of methane and carbon dioxide, water vapor, and lesser concentrations of VOC's and other contaminants

[0072] The composition of digester biogas (digester gas) or landfill gas varies depending upon the substrate composition, as well as the conditions within the anaerobic reactor (temperature, pH, and substrate concentration). The biogas or landfill gas of the present invention is entirely distinct from natural gas extracted from a subterranean or subsea geological formation, or that of a producing well. Specifically, the digester biogas or landfill gas of the present invention is essentially free (contains <100 ppm) of butane, ethane and propane,

[0073] As used herein, the term “biomethane” refers to renewable natural gas (RNG) which is a pipeline-quality gas that is fully interchangeable with conventional natural gas and can be used in natural gas vehicles. Biomethane is essentially biogas (the gaseous product of the decomposition of organic matter) that has been processed to purity standards. Like conventional natural gas, biomethane can be used as a transportation fuel in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). Biomethane qualifies as an advanced biofuel under the Renewable Fuel Standards. Typically, the biomethane produced according to the disclosed method and system meets the requirements of SoCal Gas® Rule 30 or PG&E Rule 21, predetermined requirements of the delivery pipeline or the predetermined requirements of the CNG station requirements and has at least 94%, preferably at least 97% , of methane, less than 3% CO.sub.2, and less than 100 ppm H.sub.2S and VOC's.

[0074] Each component in a landfill gas or digester biogas stream, once contacted with polymeric membranes, has an intrinsic solubility in the polymers. Once dissolved in the polymeric matrix of the membranes, the components diffuse across the polymers from the high pressure side to the low pressure side at different rates. The permeability for a given gas component is thus a combination of solubility and diffusivity in a given polymer.

[0075] A given membrane may have selectivity for (i.e., is more permeable to), one gas over another gas. As used herein, the term “selectivity” refers to the ratio of two gas permeabilities in permeance, and the measure of the ability of a membrane to separate two gases. The selectivity (α), of CO.sub.2 over CH.sub.4 is calculated according to the below formula:

[00001]${\mathrm{\alpha CO}}_2/{\mathrm{CH}}_4=\frac{P^{*}{\mathrm{CO}}_2}{P^{*}{\mathrm{CH}}_4}$

wherein P is the permeance or the flow flux of the given gas component through membranes and is expressed as 1 gas permeation unit (gpu)=10.sup.−6cm.sup.3(S.T.P)/(s.cm.sup.2.cm Hg). It is derived from the following equation:

[00002]$J=\frac{P^{*}}{\delta }\left(x{P}_f-y{P}_p\right)=\stackrel{.Math.}{P^{*}}\left(x{P}_f-y{P}_p\right)$ [0076] Where: [0077] J=the volume flux of a component (cm.sup.3(S.T.P)/cm.sup.2.s); [0078] P*=membrane permeability that measures the ability of the membrane to permeate gas (cm.sup.3(S.T.P).cm/(s.cm.sup.2.cm Hg)); [0079] =membrane permeance (cm.sup.3(S.T.P.)/(s.cm.sup.2.cm Hg))*; [0080] δ=the membrane thickness (cm); [0081] χ=the mole fraction of the gas in the feed stream; [0082] y=the mole fraction of the gas in the permeate stream; [0083] P.sub.f=the feed-side pressure (cm Hg); [0084] P.sub.p=the permeate-side pressure (cm Hg).

[0085] More details of the calculation of permeance can be found in “Technical and Economic Assessment of Membrane-based Systems for Capturing CO.sub.2 from Coal-fired Power Plants” by Zhai, et al. in Presentation to the 2011 AlChE Spring Meeting, Chicago, IL, which is incorporated by reference in its entirety.

Membranes of the First Separation Stage

[0086] The membranes of the first separation stage are selective for H.sub.2S over CH.sub.4 and also for CO.sub.2 over CH.sub.4. Specifically, the membranes of the first separation stage have a selectivity of at least 10, preferably at least 30, for H.sub.2S over CH.sub.4. These membranes also have a selectivity of at least 4, preferably at least 6, for CO.sub.2 over CH.sub.4.

[0087] While these membranes may be asymmetric membranes and comprised of a single polymeric material or polymeric blend, typically the membranes are comprised of a porous polymeric substrate having an additional separation layer or coating.

[0088] While the polymeric material making up the substrate is not limited, it is typically selected from the group consisting of polyimides, polysulfones, and polyether ether ketones. More typically, it is made of polyether ether ketones (PEEK). The separation layer is supported by the substrate, which provides mechanical strength and may also separate gases. On the other hand, the separation layer is either wholly or primarily responsible for performing the desired separation. The membranes of the first separation stage typically known as “rubbery” membranes. The membranes of the first separation stage typically have a specific surface area above 20 m.sup.2/g, preferably above 100 m.sup.2/g, and a pore size of below 1 micrometer, preferably below 0.25 micrometer, and more preferably below 0.015 micrometer. The membranes of the first separation stage are in the form of a flat film, or as a plurality of hollow fibers.

[0089] In the context of composite hollow fibers, the separation layer may be configured as a sheath surrounding a core made of the support layer. In the case of hollow fibers, the fiber preferably possesses an outside diameter from about 50 to about 50,000 micrometers, more preferably from about 80 to about 1,000 micrometers, with a wall thickness from about 10 to about 1,000 micrometers, preferably from 20 to 500 micrometers. In the case of film, the film preferably possesses a thickness of from about 10 to about 1,000 micrometers, most preferably from about 25 to abut 500 micrometers. The film may be optionally supported by a permeable cloth or a screen.

[0090] Alternatively, the membrane is in the form of spirally round sheets.

[0091] The separation layer for the first separation stage membrane is optionally made of a copolymer or block polymer of the formula:

##STR00005## [0092] where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) poly(tetramethylene oxide). These copolymers are commercially available as poly(ether-b-amide) multiblock copolymers from Arkema under the trade name of PEBAX®, and poly(butylene terephthalate) ethylene oxide copolymer available under the trade name of Polyactive®. Typically, the PEBAX polymers from Arkema include PEBAX 7233, PEBAX 7033, PEBAX 6333, PEBAX 2533, PEBAX 3533, PEBAX 1205, PEBAX 3000, PEBAX 1657, or PEBAX 1074. PEBAX 1657 exhibits a methane permeability of 5.12; see Barrer. H. Rabiee, et al., J. Membrane Sci. vol. 476, pp. 286-302 (2015).

[0093] Alternatively, the separation layer is made of repeating units of the following monomers, also known as Polyactive® multiblock copolymers:

##STR00006##

[0094] Alternatively, the separation layer of the first membrane stage is made of a copolymer or block polymer of tetramethylene oxide, and/or propylene oxide, or ethylene oxide. These copolymers or block polymers of tetramethylene oxide, and/or propylene oxide, or ethylene oxide may be conveniently synthesized, such as the polyester ether disclosed in U.S. Pat. No. 6,860,920, the polyester ethers of which are incorporated by reference;

##STR00007##

wherein PE may be one or more of the following structures:

##STR00008##

[0095] Other copolymers or block polymers of tetramethylene oxide, and/or propylene oxide, or ethylene oxide may be conveniently synthesized, such as polyimide ether disclosed in U.S. Pat. No. 5,776,990, the polyimide ethers of which are incorporated by reference.

[0096] The copolymers can be further obtained by copolymerization of acrylated monomers containing oligomeric propylene oxide, ethylene oxide, or tetramethyelene oxide.

[0097] Without being bound by any particularly theory, we believe that the rubbery membrane operates as follows: the product methane primarily remains on the retentate, high pressure side as a slow gas while water, H.sub.2S, CO.sub.2 and/or heavy hydrocarbons or VOC's are fast permeating gases that are permeated and removed at the low-pressure permeate side. The permeation of the impurities is due to their higher solubility in the polymeric separation layer, while CH.sub.4 permeates at a slower speed than the impurities. Overall H.sub.2S, CO.sub.2, VOC's, siloxanes and water are “fast” gases while methane is a “slow” gas. Therefore, the rubbery membrane preferentially permeates water, H.sub.2S, CO.sub.2 and/or heavy hydrocarbons and VOC's from high pressure to low pressure, leaving behind at high pressure a lean product stream, enriched in methane, with less than about 0.1 wt. % of water. As a result, there is no need for recompression of the first retentate before it is fed to the second separation stage. Typically, the pressure drop between the feed gas and the retentate gas is less than 50 psi (3.45 bar), preferably less than 30 psi (2.07 bar), or more preferably less than 20 psi (1.38 bar).

[0098] The membrane is robust and is operable with coalescing filters in condensing environments. The rubbery membrane fiber withstands exposure to VOC's such as benzene, toluene, and xylene (BTX), water, mercaptans or acid gases. An example of relative gas permeabilities for such a rubbery membrane is shown in FIG. 3.

Membranes of the Second Separation Stage

[0099] These membranes are also known as “glassy” membranes. For the common glassy polymers, methane (CH.sub.4) is a fairly slow gas, meaning it is not very permeable and substantially remains on the high pressure side (retentate) of the membrane, while CO.sub.2 is a faster gas and, thus, more freely permeates from the high pressure to the low pressure side. Glassy membranes take advantage of the faster rate of CO.sub.2 permeance to remove CO.sub.2 from residual product gas. These membranes have a selectivity of at least 15, preferably at least 30, for H.sub.2S over CH.sub.4. These membranes also have a selectivity of at least 20, preferably at least 35, for CO.sub.2 over CH.sub.4. A representation of the relative rates of permeation for glassy polymers is shown in FIG. 4.

[0100] It is noted that for the second stage glassy membrane, VOC's and siloxanes are slow gases and, thus, permeating slower than methane, such that they remain with the methane and only minimally permeate through the second stage, which is opposite of the first stage rubbery membrane where VOC's and siloxanes will permeate through the membrane.

[0101] Typically, the membrane of the second separation stage is made of cellulose acetate, a polysulfone, or a polyimide. The polyimide essentially consists of repeating units of dianhydride-derived units of formula (I) and diamine-derived units

##STR00009##

[0102] Each R is a molecular segment independently selected from the group consisting of formula (1), formula (2), formula (3), and formula (4):

##STR00010##

Each Z is a molecular segment independently selected from the group consisting of formula (5), formula (6), formula (7), formula (8), and formula (9).

##STR00011##

Each diamine-derived unit is a diamine-derived moiety independently selected from the group consisting of formula (A), formula (B), formula (C), formula (D), formula (E), formula (F), formula (G), and formula (H):

##STR00012##

Each X, X.sub.1, X.sub.2, X.sub.3, X.sub.4, X.sub.5, X.sub.6, X.sub.7, and X.sub.8 is independently selected from the group consisting of hydrogen, an aromatic group, and a straight or branched C.sub.1 to C.sub.6 alkyl group. Each R.sub.a is a straight or branched C.sub.1 to C.sub.6 alkyl group having either a terminal hydroxyl group, a terminal carboxylic acid group, or a terminal carbon to carbon double bond. Each Z′ is a molecular segment selected from the group consisting of formula (a), formula (b), formula (c), and formula (d):

##STR00013##

Each Z″ is a moiety selected from the group consisting of formula (U) and formula (V):

##STR00014##

Each X.sub.9 is selected from the group consisting of hydrogen, a straight or branched alkyl group having 1 to 6 carbon atoms, and a straight or branched pefluoroalkyl group having 1 to 6 carbon atoms.

[0103] Suitable membranes for the second gas separation membrane stage are commercially available from Medal®, a unit of Air Liquide Advanced Technologies, US.

Biomethane Production

[0104] As illustrated in FIG. 1, a biogas or landfill gas feed 60 having about 5,000 ppm H.sub.2S at 100° F. is compressed in a main compressor 1 to a compressed feed 75, and a water removal apparatus 5 cools compressed feed 75 and separates water from the cooled feed 75 to produce a vapor phase stream 3. Stream 3 has a pressure of at least 100 psig, and is sent to a first separation stage of rubbery membranes 10, which permeates water and impurities such as H.sub.2S and CO.sub.2 and VOCs to an output stream 20, having a low pressure of 2 psig, to be optionally rejected for flaring, or a controlled burning in a flaring system consists of a flare stack and pipes that feed the rejected gas to the stack.

[0105] The retentate from rubbery membrane 10 is sent as an output stream 15 to a second separation stage of glassy membranes 30, wherein a retentate output stream is extracted as biomethane (product gas) having at least 94% methane, and containing 20 ppm or less of H.sub.2S at 190 psig. A permeate containing CO.sub.2 and at least 40% CH.sub.4 from the second separation stage of membranes 30 is withdrawn as output stream 25, which is and recycled back to compressor 1. Alternatively, the permeate 85 from the second stage is compressed in a second compressor 90, and the compressed permeate 95 is fed to the first separation stage.

[0106] Alternatively, as shown in FIG. 2, a H.sub.2S scavenger media 40 is used to process the retentate output stream 35 that may contain up to 100 ppm H.sub.2S, and produce a product gas stream 80 (biomethane) from the scavenger media 40 having at least 94% methane, 4 ppm or less of H.sub.2S, and less than or about 0.05 wt. % of water.

[0107] Alternatively, the H.sub.2S scavenger media 40 is located between first separation stage 10 and second separation stage 30 (not shown), to remove H.sub.2S prior to the retentate output stream 15 entering second separation stage of glassy membrane 30. Alternatively, a third separation stage containing rubbery or glassy membranes 45 (such as used in stage 1 or stage 2) is incorporated to process the permeate output stream 20, wherein a retentate output stream 50 is produced from the third stage 45. Output stream 50 is combined with the output stream 25 to form a recycle stream 55, which is then combined with feed 70 to be sent to compressor 1. The permeate from the third stage 45 is rejected as an output stream 65 at 2 psig to be flared.

PROPHETIC EXAMPLE

[0108] A raw biogas stream 60 has the following gas composition: [0109] 6.68% by volume water [0110] 1,000 ppm by volume H.sub.2S [0111] 33.22% by volume CO.sub.2 [0112] 60% by volume CH.sub.4

[0113] As shown in the process scheme of FIG. 1, the gas is compressed to 232 psia through a main compressor 1 and then fed into stage I, or first membrane stage 10, which contains a first separation stage of rubbery membranes exhibiting a selectivity for H.sub.2S over CH.sub.4 of 31, and a selectivity for CO.sub.2 over CH.sub.4 of 4.6. The retentate output stream 15 from 10 is fed into a second membrane stage 30 containing glassy membrane exhibiting a selectivity for H.sub.2S over CH.sub.4 of 35.5, and selectivity for CO.sub.2 over CH.sub.4 of 46.9. The permeate pressures for first membrane stage 10 is 41 psia, and for membrane stages 10 and 45, the permeate pressures are 15 psia. The total gas flow for the biomethane 35 or 80 is 0.54 MMSCFD (CO.sub.2 concentration is below 2% by volume and H.sub.2S concentration is below 4 ppm). The material balance is shown in Table 1.

TABLE-US-00001 TABLE 1 60 70 3 20 15 35 or 80 25 Vapour Fraction 1.0000 1.00 1.0000 1.0000 1.0000 1.0000 1.0000 Temperature (F.) 100 100 100.0 100.0 91.7 81 91.7 Pressure (psia) 15 15 215 18 195 190 15 Molar Flow 1.00 1.905 184.7 0.4015 1.446 0.54 0.9052 (MMSCFD) CO.sub.2 0.3322 0.4135 0.4264 0.8271 0.6846 <0.0200 0.5034 CH.sub.4 0.6000 0.5507 0.5679 0.1478 0.3152 0.98+ 0.4962 H.sub.2S 0.0010 0.0007 0.0007 0.0025 0.0002 0.000004 0.0003 H.sub.2O 0.0668 0.0351 0.0049 0.0226 0.0001 0.000150 0.0001