Production of biomethane using a high recovery module
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 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.
1. A method of producing biomethane, comprising:
separating a feed stream containing methane and impurities at a first purification stage comprising at least one membrane selective for carbon dioxide over methane into a first retentate stream and a first permeate stream;
separating the first retentate stream at a second purification stage comprising at least one membrane selective for carbon dioxide over methane into a second retentate stream and a second permeate stream, the second retentate stream being a product stream comprising biomethane;
feeding the first permeate stream to a high recovery module comprising a second compressor and a third purification stage comprising one or more membranes in parallel or in series;
compressing the first permeate stream at the secondary compressor; and
separating the compressed first permeate stream at the third purification stage into a third retentate stream and a third permeate stream.
2. The method of claim 1 , further comprising a step of feeding the second permeate stream and a raw biogas stream to a main compressor and compressing them so as to form a combined stream, the raw biogas stream comprising 40-75% methane and impurities of: greater than 0% and up to 10% nitrogen, greater than 0% and up to 3% oxygen, 20-55% carbon dioxide, greater than 0 ppm and up to 5,000 ppm hydrogen sulfide, 0 to 100 ppm siloxanes, greater than 0 ppm and up to 1,000-2,000 ppm of C6+ hydrocarbons and VOCs, and greater than 0% water.
3. The method of claim 2 , wherein the first retentate stream comprises at least 60% methane, the first permeate stream comprises less than 20% methane, the second retentate stream comprises at least 94% methane, the second permeate stream comprises less than 80% methane, the third retentate stream comprises at least 40% methane.
4. The method of claim 3 , wherein an amount of water is removed from the combined stream at a dehydration unit or through cooling after compression prior to combination with the third retentate stream.
5. The method of claim 2 , further comprising the steps of:
feeding the second permeate stream and a raw biogas stream to a main compressor and compressing them so as to form a combined stream; and
forming the feed stream is formed by combining the third retentate stream with the combined stream, wherein the second permeate stream comprises at least 40% methane, at least 20% carbon dioxide, less than 1,000 ppm hydrogen sulfide, less than 1000 ppm of C 6+ hydrocarbons, VOCs and siloxane, and less than 0.05% water and the first permeate stream comprises 5-15% methane, greater than 0 ppm but up to 10,000 ppm of H 2 S, greater than 0% but up to 3% water, and up to 95% CO 2 .
6. The method of claim 4 , wherein, after water removal at the dehydration unit and prior to combination with the third retentate stream, H 2 S levels in the combined stream are reduced to less than 4 ppm using H 2 S scavenger media.
7. The method of claim 5 , further comprising the steps of:
feeding the combined stream to an adsorption-based purification unit where amounts of C 6+ hydrocarbons present in the combined stream are removed by the adsorption-based purification unit, the adsorption-based purification unit comprising an adsorbent bed, the adsorption-based purification unit being selected from the group consisting of a non-regenerative media unit, a pressure swing adsorption unit, a temperature swing adsorption unit, and a pressure-temperature swing adsorption unit; and
optionally, when the adsorption-based purification unit is a pressure swing adsorption unit, a temperature swing adsorption unit or a pressure-temperature swing adsorption unit, regenerating the adsorption bed with the third permeate stream.
8. The method of claim 1 , wherein:
the third purification stage comprises a first group of one or more membranes that is in series with a second group of one or more membranes;
the first permeate stream is separated by the first group into an intermediate retentate stream and an intermediate permeate stream;
the intermediate retentate stream is fed to the second group where it is separated into the third permeate stream and the third permeate stream.
9. The method of claim 1 , wherein each of said at least one membrane of said first purification stage comprises a porous polymeric substrate having at least one separation layer, the substrate being selected from the group consisting of polyimides, poly sulfones, polyether ether ketones, and mixtures thereof.
10. The method of claim 9 , wherein the substrate is a polyether ether ketone.
11. The method of claim 9 , wherein the separation layer is made of a copolymer or block polymer of the formula:
where PA is an aliphatic polyamide having 6 or 12 carbon atoms and PE is either poly(ethylene oxide) or poly(tetramethylene oxide).
12. The method of claim 9 , wherein the separation layer is made of repeating units of the following monomers:
13. The method of claim 9 , wherein the separation layer is made of a copolymer or block polymer of tetramethylene oxide, propylene oxide, and/or ethylene oxide.
14. The method of claim 1 , wherein the at least one membrane of the first purification stage is also selective for H 2 S, C 6+ hydrocarbons, siloxane and water over methane.
15. The method of claim 1 , wherein the at least one membrane of the second purification stage is comprised of a material selected from the group consisting of cellulose acetate, a polysulfone, a polyimide, and mixtures thereof.
16. The method of claim 1 , wherein a pressure differential over the membrane between the first permeate stream and the feed stream is less than 50 psig.
17. The method of claim 1 , wherein the second permeate stream comprises at least 40% methane, at least 20% carbon dioxide, less than 1,000 ppm hydrogen sulfide, less than 1000 ppm of C 6+ hydrocarbons and siloxane, and less than 0.05% water.
18. The method of claim 1 , wherein the first permeate stream comprises 5-15% methane, greater than 0 ppm but up to 10,000 ppm of H 2 S, greater than 0% but up to 3% water, and up to 95% CO 2 .
19. The method of claim 1 , further comprising the steps of compressing the second permeate stream and a raw biogas stream at a main compressor so as to form a combined stream and forming the feed stream by combining the third retentate stream with the combined stream, wherein the raw biogas stream has a methane concentration of 40-75% and at least 98% of methane in the raw biogas stream is recovered in the second retentate stream.
20. The method of claim 1 , wherein the at least one membrane of the third stage is the same as the at least one membrane of the first stage.
21. The method of claim 1 , wherein the at least one membrane of the third stage is the same as the at least one membrane of the second stage.
22. The method of claim 1 , wherein said product stream is comprised of at least 94% methane, below 3% of CO 2 , below 150 ppm of H 2 S, below 100 ppm of VOC's, and below 0.01% of H 2 O.
23. The method of claim 1 , further comprising the step of removing amounts of H 2 S from the second retentate stream with a H 2 S scavenger media so as to achieve a H 2 S concentration in the second retentate stream of less than 4 ppm of H 2 S.