PRODUCTION OF BIOMETHANE USING A HIGH RECOVERY MODULE

Abstract

The invention relates to a process for recovering methane from digester biogas or landfill gas. More specifically, the invention pertains to biomethane production that substantially removes carbon dioxide from a digester biogas or landfill gas using first, second, and third purification stages each comprising one or more membranes selective for carbon dioxide over methane. A retentate from the first stage is separated by the one more membranes of the second stage into a second state retentate, forming a biomethane product gas. A permeate from the first stage is separated by the one or more membranes of the third stage into a third stage retentate and a third stage permeate. Recovery of methane from the the biogas is boosted by feeding the third stage retentate to the first purification stage. The recovery may be optionally further boosted by compressing the second stage permeate with the biogas at a main compressor.

Claims

1. A installation for producing biomethane containing at least 94% methane, comprising: a source of a biogas; a main compressor, an inlet of which is in fluid communicating with the source; a first purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the first purification stage being in fluid communication with an outlet of the main compressor; a second purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the second purification stage being in fluid communication with a retentate outlet of the first purification stage; and a high recovery module comprising a third purification stage comprising one or more membranes in parallel or in series that are selective for carbon dioxide over methane and a secondary compressor in fluid communication between a permeate outlet of the first purification stage and an inlet of the third purification stage, wherein a retentate outlet of the third purification stage is in fluid communication with the first purification stage inlet so as to allow a retentate stream from the third purification stage to be recycled to the first purification stage.

2. The installation of claim 1, wherein: a permeate outlet of the second purification stage is in fluid communication with the main compressor inlet so as to allow a permeate stream from the second purification stage to be recycled to the main compressor.

3. The installation of claim 1, further comprising a dehydration unit in fluid communication between the main compressor outlet and the first purification stage inlet that is adapted and configured to remove amounts of water from a combination stream received from the compressor outlet prior to the combination stream being fed to the first purification stage inlet.

4. The installation of claim 3, wherein said dehydration unit is a condenser or dehydration media.

5. The installation of claim 1, wherein the at least one membrane of the first purification stage comprises a porous polymeric substrate and at least one separation layer, the polymeric substrate being selected from the group consisting of polyimides, polysulfones, polyether ether ketones, and mixtures thereof.

6. The installation of claim 5, wherein said polymeric substrate is a polyether ether ketone.

7. The installation of claim 5, wherein the separation layer is made of a copolymer or block polymer of the formula: ##STR00013## where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) or poly(tetramethylene oxide).

8. The installation of claim 5, wherein the separation layer is made of repeating units of the following monomers: ##STR00014##

9. The installation of claim 5, wherein the separation layer is made of a copolymer or block polymer of tetramethylene oxide, propylene oxide, and/or ethylene oxide.

10. The installation of claim 1, wherein the at least one membrane of the first purification stage is in the form of flat films or a plurality of hollow fibers.

11. The installation of claim 1, wherein the at least one membrane of the first purification stage is selectivity for H.sub.2S, CO.sub.6+ hydrocarbons, carbon dioxide, siloxane and water over methane.

12. The installation of claim 1, wherein said stream of said first gas mixture has a pressure drop of less than 50 psi from said feed gas.

13. The installation of claim 1, wherein each of said membranes of said second stage is made of cellulose acetate, a polysulfone, a polyimide, polyamide, or mixtures thereof.

14. The installation of claim 1, further comprising a H.sub.2S removal unit comprising a H.sub.2S scavenger media in fluid communication between the main compressor and the first purification stage, the H.sub.2S removal unit being adapted and configured to remove amount of H.sub.2S from a combination stream received from the compressor.

15. The installation of claim 1, further comprising a H.sub.2S removal unit comprising a H.sub.2S scavenger media in fluid communication downstream of a retentate outlet of the second stage purification unit, the H.sub.2S unit being adapted and configured to remove amounts of H.sub.2S from a second stage retentate stream produced by the second purification stage.

16. The installation of claim 1, wherein said at least one membrane of said third purification stage is the same as the membrane of the first purification stage.

17. The installation of claim 1, wherein said at least one membrane of the third purification stage is the same as the membrane of the second purification stage.

18. The installation of claim 1, wherein: the high recovery module further comprises a fourth purification stage comprising at least one membrane selective for carbon dioxide over methane; an inlet of the fourth purification being in downstream flow communication with a retentate outlet of the third purification stage; a permeate outlet of the second purification stage is in fluid communication with the main compressor inlet so as to allow a permeate stream from the second purification stage to be recycled to the main compressor; and a retentate outlet of the fourth purification stage is in fluid communication with the first purification stage inlet so as to allow a retentate stream from the fourth purification stage to be recycled to the first purification stage.

19. A method for increasing a recovery of methane in biomethane product gas produced by an existing installation for producing biomethane comprising a source of a biogas, a main compressor, a first purification stage comprising at least one membrane selective for carbon dioxide over methane, a second purification stage comprising at least one membrane selective for carbon dioxide over methane, an inlet of the main compressor being in fluid communication downstream of the source, an inlet of the first purification stage being in fluid communication downstream of an outlet of the main compressor, an inlet of the second purification stage being in fluid communication downstream of a retentate outlet of the first purification stage, a permeate outlet of the second purification stage being in fluid communication with the inlet of the main compressor, said method comprising the steps of: providing a high recovery module comprising a secondary compressor and a third purification stage comprising one or more membranes in series or in parallel that are selective for carbon dioxide over methane; placing an inlet of the secondary compressor in downstream flow communication with a permeate outlet of the first purification stage; placing an inlet of the third purification stage in downstream flow communication with an outlet of the secondary compressor; and placing a retentate outlet of the third purification stage in fluid communication with the inlet of the first purification stage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 illustrates a first embodiment of biomethane production containing a high recovery module and a H.sub.2S removal media.

[0063] FIG. 2 illustrates a second embodiment of biomethane production containing a high recovery module and a H.sub.2S removal media.

[0064] FIG. 3 illustrates relative permeation rates for rubbery membranes.

[0065] FIG. 4 illustrates relative permeation rates for glassy membranes.

DETAILED DESCRIPTION OF THE INVENTION

[0066] The present invention is directed towards a novel process of producing biomethane from biogas with improved recoveries of biogas methane in the biomethane product gas. A raw biogas stream may optionally be dehydrated, pretreated to remove impurities, and compressed.

[0067] Whether combined with one or more gases, the raw biogas stream is ultimately fed to a first purification stage that includes one more gas separation membranes (typically polymeric membranes) each of which is selective for carbon dioxide over methane. At the first purification stage, the feed stream (ordinarily at a pressure of about 150-200 psig) is separated into a first permeate stream (ordinarily at a pressure of about 1-15 psig, more typically about 1-7 psig) and a first retentate stream. The first retentate stream (which is at a pressure often around 10 psig lower than that of the feed stream) is fed to a second purification stage including one or more gas separation membranes (again typically polymeric membranes) each of which is also selective for carbon dioxide over methane. At the second purification stage, the first retentate stream is separated into a second permeate stream (ordinarily at a pressure of around 0-7 psig) and a second retentate stream (ordinarily at a pressure about 10 psig lower than that of the first retentate stream). Whether the second retentate stream is optionally treated for further removal of impurities, the second retentate stream constitutes the biomethane product.

[0068] The first permeate stream typically comprises at least 5% (often 5-15%) methane, up to 10,000 ppm of H.sub.2S, up to 3% water, and up to 95% CO.sub.2. In order to recover amounts of methane present in the first permeate stream that might otherwise not be recovered, it is fed to a high recovery module that includes a (secondary) compressor and a third purification stage that includes one or more gas separation membranes (typically polymeric membranes) in parallel or in series, each of which is selective for carbon dioxide over methane. At the third purification stage, the first permeate stream is compressed by the secondary compressor (ordinarily to a pressure about 10 psig higher than that of the feed gas fed to the first purification stage) is separated into a third permeate stream and third retentate stream (which is at a same pressure as that of the feed gas fed to the first purification stage). While the third permeate stream is ordinarily at a pressure as low as 0-1 psig in the case of rejection by flaring or thermal oxidation, it is typically at a pressure of about 3-6 psig when it is used to regenerate an adsorbent bed (as describe below). Because valuable amounts of methane (typically 50-75%) are present in the third retentate stream, it is fed back to the first purification stage. Because the third retentate stream is already at a relatively high pressure (by virtue of the first permeate stream being compressed in the high recovery module), it need not be recompressed.

[0069] In one embodiment, the third purification stage may include a first group of one or more membranes which separate the first permeate stream into an intermediate retentate stream and an intermediate permeate stream. The third purification stage also includes a second group of one or more membranes in series that receive the intermediate retentate from the first group and separate it into the third permeate stream and the third retentate stream. This particular embodiment is advantageous because the intermediate permeate stream contains very high amounts of CO.sub.2 and may be recovered as a useful product gas containing as much as 96% CO.sub.2. The third permeate stream, which is quite low in methane, may be vented or flared.

[0070] The second permeate stream typically comprises at least 40% 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% of H.sub.2O. Optionally, in order to recover additional amounts of methane present in the second permeate stream, the second permeate stream may be fed to the compressor upstream of the first purification where the second permeate stream and the raw biogas stream are compressed.

[0071] Raw biogas from a landfill or digester is ordinarily pressurized a little above atmospheric pressure. Assuming that it is not otherwise provided at relatively higher pressures, it may optionally be compressed by a main compressor, typically to a pressure of about 150-200 psig. The compressed raw biogas is then cooled and amounts of condensed water are removed via phase separation and/or with a dehydration media.

[0072] Before it is combined with the third retentate stream (and optionally after it has been combined with the second permeate stream) and fed to the first purification stage, the raw biogas may be optionally processed to remove impurities. For example, the H.sub.2S levels of the raw biogas (or combined raw biogas and third stage retentate) may be lowered by passing the biogas through a H.sub.2S removal unit that includes H.sub.2S scavenger media. In addition to, or as an alternative to such purification of the raw biogas (or combined raw biogas and third stage retentate), H.sub.2S may be removed from the second retentate stream by an H.sub.2S removal unit.

[0073] Also, before the raw biogas is combined with the third retentate stream (and optionally after it has been combined with the second permeate stream) and fed to the first purification stage, VOCs such as C.sub.6+ hydrocarbons may be removed from the raw biogas (or the combined raw biogas and third stage retentate) with an adsorption-based purification unit (containing an adsorbent bed) such as a one having a non-regenerative media bed, a pressure swing adsorption unit, a temperature swing adsorption unit, or a pressure-temperature swing adsorption unit.

[0074] In any of these cases, the adsorbent bed may be regenerated with a regeneration gas such as air or optionally the third stage permeate stream may be used as a regeneration gas.

[0075] In addition to, or instead of, use of the third stage permeate stream as a regeneration gas, the third stage permeate stream may be vented, flared, thermally oxidized at a thermal oxidation unit, or recovered as a useful CO.sub.2 product gas.

[0076] Each of the first, second, and third purification stages is selective for carbon dioxide over methane. This means that a stream of gas fed to such a purification stage will produce a permeate stream (from the associated one or more gas separation membranes of that purification stage) and a retentate stream (from the associated one or more gas separation membranes of that purification stage) where the permeate is enriched in carbon dioxide and deficient in methane compared to the associated retentate stream. This leaves open the possibility to include one or more gas separation membranes in the first stage, in the second stage, and/or in the third stage that is selective for methane over carbon dioxide. While this is not typically preferred, there may be an advantage to include one or more membranes that are highly selective for one or more impurities despite the fact that they may otherwise permeate more methane than desired. Optionally, the membranes may also be selective for H.sub.2S over methane, for C.sub.6+ hydrocarbons over methane, and/or for water over methane.

[0077] 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, such as in an aerobic digester, from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. As the main components, biogas typically comprises 50-70% methane and 20 to 50% carbon dioxide. Biogas includes lower levels of other components such as N.sub.2 and O.sub.2 (typically from air infiltration), up to 5,000 ppm or more of hydrogen sulfide (H.sub.2S), may contain measurable amounts of siloxanes, up to 1,000-2,000 ppm of volatile organic compounds (VOC's). It is also typically 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 methane and CO.sub.2 with the wide variety of other components described above. In either case, biogas includes high concentrations of methane and carbon dioxide, water vapor, and lesser concentrations of VOC's and other contaminants

[0078] The composition of the biogas sourced from a 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 from butane, ethane and propane.

[0079] As used herein, the term biomethane refers to renewable natural gas (RNG) or biomethane. It is a pipeline-quality gas that is interchangeable with conventional natural gas and, thus, can be used in natural gas vehicles. Biomethane is essentially biogas 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. For the present invention, biomethane is the product gas that fits the requirements of SoCal Gas Rule 30 or the requirements of the delivery pipeline (when it is injected into a natural gas pipeline) or the requirements of the CNG station (when it is provided to a CNG station). It has at least 94%, preferably at least 98% methane, less than 3% carbon dioxide, and less than 100 ppm H.sub.2S, water, and VOC's (such as C.sub.6+ hydrocarbons).

Membranes of the First Stage

[0080] The membranes of the first stage are selective for carbon dioxide over methane. They are typically also relatively highly selective for H.sub.2S over methane. The membrane may be asymmetric or composite, in which case one or more separation layers is supported by a porous support. In the case of composite membranes, the substrate is often a polyimide, polysulfone, and polyether ether ketone, and typically a polyether ether ketone. The separation layer performs the desired separation while the substrate provides mechanical strength. The membranes of the first 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 membrane is in the form of a flat film, or as a plurality of hollow fibers. The first purification stage typically has a CO.sub.2 to methane selectivity of 5-40.

[0081] In the context of hollow fibers, the separation layer is 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 about 500 micrometers. The film may be optionally supported by a permeable cloth or a screen.

[0082] Skilled artisans will recognize that the dense outer layer of an asymmetric membrane or the separation layer supported by a support layer (hereinafter separation layer) may be made of a glassy or rubbery polymer. The relative rates of permeation for common gases through rubber membranes and glassy membranes are graphically illustrated in FIG. 3 (rubbery membranes) and FIG. 4 (glassy membranes). Optionally, the membranes of the first stage are made of rubbery membranes as rubbery membranes exhibit satisfactory resistance to degradation upon exposure to benzene, toluene, and xylene (BTX), water, mercaptans, or acid gases. Rubbery membranes preferentially permeate water, H.sub.2S, CO.sub.2 and/or heavy hydrocarbons and VOCs from high pressure to low pressure, leaving behind, at relatively high pressure, a water, H.sub.2S, carbon dioxide, and VOC-lean retentate that is enriched in methane, typically with less than about 0.1% of water. As a result, there is no need for recompression of the first retentate stream before it is fed to the second stage. Typically, the pressure drop between the feed gas (in the case of the first stage) 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).

[0083] Particular types of rubbery polymers for separation layer include the copolymer or block polymer of the formula:

##STR00003##

[0084] 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).

[0085] Another type of rubbery polymer for the separation layer include those made of repealing units of the following monomers, also known as Polyactive multiblock copolymers:

##STR00004##

[0086] Other suitable polymers for separation layer include those 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;

##STR00005##

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

##STR00006##

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

[0087] The membrane is robust and is operable with coalescing filters in the condensing environments.

[0088] Alternatively, the separation layer may be made of a glassy polymer. Common glassy polymers for the membrane separation layer include cellulose acetate, polysulfones, and polyimides. Polyimides essentially consists of repeating units of dianhydride-derived units of formula (I) and diamine-derived units

##STR00007##

[0089] Each R is a molecular segment independently selected from the group consisting of formula (1), formula (2), formula (3), and formula (4):

##STR00008##

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

##STR00009##

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):

##STR00010##

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):

##STR00011##

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

##STR00012##

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.

[0090] Alternatively, the separation layer in the first stage, the membranes are of a mixture of rubbery and glassy membrane.

[0091] Suitable hollow fiber membranes for the first stage are commercially available from Air Liquide Advanced Technologies, US.

Membranes of the Second Stage

[0092] While the membranes of the second stage may have a separation layer made of a glassy or rubbery polymer, typically the separation layer is made of a glassy polymer, non-limiting members of which cellulose acetate, polysulfones, polyamides, and polyimides as described above. Glassy membranes often exhibit superior CO.sub.2 rates.

[0093] In one particular embodiment, rubbery membranes available from Air Liquide Advanced Technologies, US are used for the first stage while glassy membranes available from Air Liquide Advanced Technologies, US are used for the second stage. Those skilled in the art will recognize that rubbery membranes refers to membranes having a rubbery separation layer while glassy membranes refers to membranes having a glassy separation layer. This particular combination of membranes for the first and second stages exhibits several advantages. While the carbon dioxide/methane selectivity of the first stage may not be as high as that of the second stage, the relatively high flux of carbon dioxide, H.sub.2S, C.sub.6+ hydrocarbons, and water allows the production of a more purified first retentate stream, even if it comes at the cost of losing some of the methane (in comparison to a glassy polymer) to the first permeate stream. On the other hand, since the H.sub.2S, C.sub.6+ hydrocarbons, and water are largely removed by the first stage, the superior carbon dioxide/methane selectivity of the second stage allows the production of a second stage permeate stream (i.e., the biomethane product gas) having a relatively high methane purity.

High Recovery Module (HRM)

[0094] The HRM includes a (secondary) compressor and a third purification stage comprising one or more membranes in parallel or in series.

[0095] Because the HRM compressor receives far less gas than any main compressor that is used to compress the raw biogas, and can have a higher suction pressure, a much smaller compressor for the HRM may be used. As a result, the capital expense (i.e., the sales price of the compressor) and the operating expense (i.e., the electrical energy consumed by the compressor) are far lower than those of any main compressor. Because the first permeate stream is compressed by the secondary compressor before being separated by the membranes of the third stage, the third stage retentate stream has a relatively high pressure, thereby allowing it to be fed to the first stage without first being compressed by the main compressor. As a result, the capital expense and size of the main compressor and the operating expense and energy consumed by the main compressor may be correspondingly decreased.

[0096] Similar to those of the first and second stages, the membranes of the third purification stage may include a separation layer made of either rubbery or glassy polymers, including those described above. Because the first permeate stream (fed to the HRM) has a much smaller mass flow rate than those of the feed gas stream (fed to the first stage) and the first retentate stream, skilled artisans will recognize that the total surface area of (i.e., the number of) membranes used in the third stage is significantly lower than those in each of the first and second stages.

[0097] As the name suggests, the HRM helps boost the recovery of methane (from the raw biogas) realized in the biomethane product gas to 98% or higher. It also can produce a third permeate gas with a relatively high carbon dioxide concentration of 96% or more.

Retrofitting a Two-Stage Membrane System with a HRM

[0098] Instead of a full replacement of an existing two-stage membrane purification system for producing biomethane from biogas, a HRM can be added onto an existing two-stage gas separation membrane system so as to boost recovery of methane from the raw biogas while producing a biomethane product gas containing at least 94% methane. Specifically, an existing two-stage membrane purification system includes a first purification stage (that includes one or more membranes selective for carbon dioxide over methane) which separates a feed stream of raw biogas stream (or optionally also including a permeate stream from a downstream purification stage) into a first permeate stream and a first retentate stream. The existing system also includes a second purification stage (that includes one or more membranes selective for carbon dioxide over methane) that separates the first retentate stream into a second permeate stream and a second retentate stream that ordinarily constitutes biomethane product gas. The materials that the membranes are comprised of may be those of the first and second stages as described above. An HRM (comprising a secondary compressor and a third purification stage comprising one or more membranes in parallel or in series) is integrated with the existing system by fluidly connecting a permeate outlet of the first stage with an inlet of the third stage. A retentate outlet of the third stage is fluidly connected to an inlet of the first stage. In operation, the first permeate stream is compressed by the secondary compressor of the HRM and fed to the one or more membranes of the third stage where it is separated into a third permeate stream and third retentate stream. The methane recovery of the existing system is boosted by the fact that the methane otherwise lost in the first permeate stream is recovered by the HRM and recycled to the first stage where much of the recovered methane is retained as a first stage retentate stream.

Biomethane Production

[0099] I will now proceed to describe particular embodiments.

[0100] As best illustrated in FIG. 1, a raw biogas feed 1 of primarily methane and carbon dioxide and including up to about 5,000 ppm H.sub.2S at 100 F. is compressed in a main compressor 10 along with a second stage permeate stream 60 so as to provide a compressed combined feed 12. If desired, the water level of the combined feed 12 is optionally reduced by a condenser 15 which cools the combined feed 12. The resultant condensed water is removed by phase separation, thereby producing a dried, combined feed 20, typically at a pressure of 150-200 psig. Alternatively, the water level may be reduced with a dehydration media. Also if desired, H.sub.2S levels in the dried, combined feed 20 may be subsequently lowered using an H.sub.2S removal media 25. After this optional H.sub.2S treatment, stream 20 is combined with a retentate stream 75 from the HRM 85 to form a feed stream 30.

[0101] The feed stream 30 is fed to a first purification stage 35 which includes one or more gas separation membranes (glassy or rubbery) that are selective for water, H.sub.2S, and carbon dioxide over methane (and in the case of rubbery membranes, selective for C.sub.6+ hydrocarbons over methane). The feed stream 30 is separated by the first purification stage 35 into a first stage retentate stream 40 and a first stage permeate stream 45 that is deficient in methane but enriched in carbon dioxide, water and other impurities such as H.sub.2S in comparison to the first stage retentate stream.

[0102] The first stage retentate stream 40 is subsequently fed to a second purification stage 50 which includes one or more glassy gas separation membranes that are selective for water, H.sub.2S, and carbon dioxide over methane and for methane over C.sub.6+ hydrocarbons. The first stage retentate stream 40 is separated by the second purification stage 50 into a second stage retentate stream 55 and a second stage permeate stream 60 that is deficient in methane but enriched in carbon dioxide and water in comparison to the second stage retentate stream 55. The second stage retentate stream 55 constitutes the biomethane product gas. Typically at about 150 psig, the second stage retentate stream 55 has at least 94% methane and contains 150 ppm or less of H.sub.2S. While the second stage permeate stream 60 contains less than 80% methane and also contains significant amounts of carbon dioxide, it still contains a valuable amount of methane and is preferably recycled back to an inlet 5 of the main compressor 10 for compression with the raw biogas stream 1.

[0103] The first permeate stream 45, typically at a relatively low pressure such as about 2 psig, is fed to the HRM 85 where it is compressed by a secondary compressor 70 to produce a compressed first permeate stream 72. The compressed first permeate stream 72 is then fed to a third purification stage 65 where it is separated into a third retentate stream 75 and a third permeate stream 80. The third retentate stream 75 is combined with the feed stream 30 and the combined stream is then fed to the first purification stage 35. The third permeate stream 80 may be flared, vented, thermally oxidized, or recovered as a valuable CO.sub.2 product gas.

[0104] Alternatively, as shown in FIG. 2, the H.sub.2S scavenger media 25 is placed downstream of the second purification stage 50 so as to remove remaining undesirable amounts of H.sub.2S from the second retentate stream 55. After treatment by the the H.sub.2S scavenger media 25, a stream of biomethane product gas 62 is yielded that contains 4 ppm or less of H.sub.2S and 150 ppm or less of water.

[0105] While not illustrated in FIGS. 1-2, one of ordinary skill in the art can easily envisage an HRM that contains a first group of one or more membranes in series with a second group of one or more membranes where the first permeate stream 45 is separated by the first group into an intermediate permeate stream and an intermediate retentate stream. The intermediate retentate stream is separated by the second group into the third permeate stream and the third retentate stream. In this scenario, the intermediate permeate stream may be recovered as a useful CO2 product gas and the third permeate stream is vented, flared or thermally oxidized at a thermal oxidation unit.

Prophetic Example

[0106] With reference to the process conditions shown in Table I, raw biogas stream (A) has the following gas composition: 6.69% by volume water, 100 ppm by volume H.sub.2S, 32.08% by volume CO.sub.2, and 61.23% by volume CH.sub.4. After being combined with the second permeate stream, the combined stream (B) is compressed to 200 psig with a main compressor and subsequently dehydrated and also treated by H.sub.2S scavenger media to reduce the H.sub.2S down to trace levels. The reduced-H.sub.2S stream is combined with the third retentate stream (C) to yield a feed stream (D). This feed stream (D) is fed to the first purification stage where it is separated into a first permeate stream (E) and a first retetate stream. The first retentate stream is fed to the second purification stage where it is separated into a second permeate stream (which is combined with the raw biogas stream (A) upstream of the main compressor) and a stream of product biomethane (F). The first permeate stream (E) is fed to the HRM where it is first compressed and then separated by the third purification stage into a third permeate stream (G) and a third retentate stream (C) (which is combined with the reduced-H.sub.2S stream). pressures for first membrane stage 10 is 2 psig, and the first membrane permeate is pressurized to 210 psig then enters a third membrane stage of rubbery or glassy membranes. For membrane stages 65 and 50, the permeate pressures are 2 psig. The total gas flow for the biomethane 55 is 432 SCFM (CO.sub.2 concentration is 2% by volume and H.sub.2S concentration is below 4 ppm). The material balance according to FIG. 1 is shown in Table 1.

TABLE-US-00001 TABLE I Process Conditions for Prophetic Example Diagram Label A B C D E F G Flow, SCFM 694 949 42 948 261 432 217 Pressure, psig 0 0 200 200 2 150 2 Temperature ( F.) 100 100 100 100 100 100 100 CH.sub.4 (mol %) 61.23 58.50 51.69 60.84 8.96 98.00 0.83 CO.sub.2 (mol %) 32.08 36.51 48.27 38.67 89.63 2.00 98.57 H.sub.2S (mol %) 100 ppm ~70 ppm Trace Trace Trace Trace Trace H.sub.2O (mol %) 6.69 4.99 0.05 0.48 1.42 Dry 0.60

[0107] The invention provides several advantages.

[0108] A relatively high methane recovery (in the second retentate stream from the raw biogas stream) may be achieved at a capital expense that is lower than many conventional schemes such as the one disclosed by U.S. Pat. No. 8,999,038. In U.S. Pat. No. 8,999,038, biogas is purified in a cascaded manner in which the retentate from the feed stage is fed to the retentate stage, which is in turn yields the product biomethane. Because the low pressure permeate from the feed stage is fed to the permeate stage without first being compressed, the pressure difference across the membrane from the feed side to the permeate side (i.e., the driving force) is quite low. Since the driving force is quite low, a significantly greater number of membranes must be used in the permeate stage of U.S. Pat. No. 8,999,038.

[0109] In contrast, the fairly high mixed gas carbon dioxide/methane selectivities required by U.S. Pat. No. 8,999,038 (such as the selectivity of 45 described in the examples of U.S. Pat. No. 8,999,038) are not necessary in the instant invention. Because the first permeate stream is compressed (according to the invention) before it is fed to the third purification stage, there is a relatively high driving force across the membrane(s) of the third purification stage. Since the driving force is relatively high, a more moderate mixed gas carbon dioxide/methane selectivity is more than satisfactory for practicing the invention.

[0110] Also, although the invention requires the use of a compressor in the HRM, the cost of this secondary compressor is more than offset by two factors: 1) the reduced number of membranes required by the scheme of the invention, 2) the reduced size of the secondary compressor that need only have the capacity to compress a fairly low flow of gas from the first purification stage, and 3) the first stage permeate pressure can be higher than the raw gas pressure leading to a higher suction pressure and smaller compressor due to the resultant reduced compression pressure ratio

[0111] In order to better illustrate the above point regarding the number of membranes required, computer simulations were performed comparing the three-stage scheme of U.S. Pat. No. 8,999,038 (which does not compress the feed stage permeate before being fed to the permeate stage) to the system according to the invention. The following assumptions were made about the U.S. Pat. No. 8,999,038 scheme: the carbon dioxide content of the raw biogas treated was 40%, the selectivity of the membranes was equal to 40, the 98.5% of the methane in the raw biogas was recovered in the biomethane product gas, and the carbon dioxide content of the biomethane product gas was only 0.38%. Based upon these assumptions, 36 membrane modules per 1 MM SCFD of raw biogas treated were required by the U.S. Pat. No. 8,999,038 scheme. Using the same assumptions (except for compression of the first permeate stream), only 16 modules per 1 MM SCFD of raw feed were required by the invention. Thus, the number of membrane modules required dropped by 56%. Using the same assumptions (except for a more moderate selectivity of 16 and compression of the first permeate stream), only 11 modules per 1 MM SCFD of raw feed were required by the invention. Thus, the number of membrane modules required dropped by 69%.

[0112] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0113] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0114] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0115] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0116] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0117] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0118] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.