Membrane process and system for high recovery rate of non-permeable gas using sweep gas

Abstract

To provide a membrane process and system for high recovery of a nonpermeating gas utilizing a sweep gas.SOLUTION: A method for separating a raw feed gas stream using a plurality of membrane module stages. The raw feed gas stream may be from a biogas process. Off-gas from another unit process in the system, such as a temperature swing adsorption unit or liquefaction unit, may be used as a low-pressure sweep gas on the low-pressure side of at least one of the membrane module stages. In one example, the sweep gas is used in a first membrane module stage. In another example, a stripping membrane module stage is provided and the sweep gas is used in the stripping membrane module stage. Optionally, portions of the off-gas can be directed to other streams in the system for the purpose of balancing compressor power requirements.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] This application relates to a multi-stage membrane process and system for methane recovery from biogas.

Background Art

[0002] Multi-stage membrane systems are known methods for upgrading a raw biogas stream to a high-purity methane stream. Such multi-stage membrane systems can achieve high methane recovery rates and purities, but often require a relatively high membrane area (or count) to do so.

[0003] In some membrane-based biogas separation processes, there is also a low-pressure off-gas with a low carbon dioxide (CO2) concentration generated from other unit operations, such as tail gas from a temperature swing adsorption (TSA) system or a liquefaction unit. It may be desirable to insert the low-pressure off-gas into the separation process. For example, it may be desirable to recover methane (CH4) or CO2 from the off-gas stream. However, inserting the off-gas stream into the separation process can increase the membrane area required to achieve the desired product purity.

[0004] Accordingly, there is a need for an effective, reliable, and cost-efficient multi-stage membrane method and system that can achieve the desired methane product recovery rate and purity while reducing the required membrane size and capital cost. There is also a need to process external gas streams generated from other unit operations using a multi-stage membrane process system that does not increase the membrane area and associated capital costs.

Summary of the Invention

[0005] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0006] The disclosed embodiments meet the needs in the art by providing a multi-stage membrane method and system for methane recovery from biogas that utilizes a gas to sweep a membrane responsible for producing the final biomethane product. The sweep gas is applied along the low-pressure (or permeate) side of the membrane, dilutes the partial pressure of the species permeating on the low-pressure side, serves to increase the driving force, improves the performance of the membrane process, and reduces the capital cost of the membrane.

[0007] Some aspects of the system and method are outlined below.

[0008] Aspect 1: A method comprising: (a) compressing a feed gas stream to form a pressurized feed gas stream; and (b) supplying the pressurized feed gas stream to at least one first-stage membrane module, each of the at least one first-stage membrane module having a first high-pressure side and a first low-pressure side, the first high-pressure side extending from a first supply port to a first retentate port, and the first low-pressure side being in fluid flow communication with a first sweep port and a first permeate port; and (c) separating the pressurized feed gas stream in each of the at least one first-stage membrane module into a first retentate stream and a first permeate stream; and (d) using a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the pressurized feed gas stream is supplied to the at least one first-stage membrane module in step (a) to sweep the first permeate stream and discharge it from the first permeate port; and (e) discharging the first retentate stream from each of the at least one first-stage membrane module through the first retentate port; and (f) compressing the first permeate stream in a first compressor to form a compressed first permeate stream; and (g) Supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid flow communication with a second sweep port and a second permeate port; (h) Separating the compressed first permeate stream within the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (i) Discharging the second non-permeate stream from each of the at least one second-stage membrane module through a second non-permeate port; (j) Discharging the second permeate stream from each of the at least one second-stage membrane module through a second permeate port; (k) Mixing the second non-permeate stream with a compressed hybrid feed stream to form a pressurized feed gas stream; (l) Supplying the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third supply port to a third non-permeate port, and the third low-pressure side being in fluid flow communication with a third sweep port and a third permeate port; (m) Separating the second permeate stream within the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; (n) Discharging the third non-permeate stream from each of the at least one third-stage membrane module through a third non-permeate port; (n) Discharging the third permeate stream from each of the at least one third-stage membrane module through a third permeate port; (o) Mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; (p) Compressing the mixed feed stream in a second compressor to form a compressed mixed feed stream, the method comprising.

[0009] Aspect 2: The method according to aspect 1, wherein step (d) further comprises introducing the sweep gas at a pressure of less than 1.00 barg.

[0010] Aspect 3: The method according to aspect 1 or 2, wherein the sweep gas contains less than 15% carbon dioxide.

[0011] Aspect 4: The method according to any one of aspects 1 to 3, wherein the sweep gas contains at least 85% methane.

[0012] Aspect 5: The method according to any one of aspects 1 to 4, wherein the sweep gas provides nitrogen at a first molar flow rate that is 5% or less of a second molar flow rate of the pressurized feed gas stream.

[0013] Aspect 6: The method according to any one of aspects 1 to 5, wherein the sweep gas contains off-gas.

[0014] Aspect 7: The method according to any one of aspects 1 to 6, wherein the off-gas contains off-gas from a liquefaction process or a temperature swing adsorption process.

[0015] Aspect 8: The method according to any one of aspects 1 to 7, wherein the sweep gas contains tail gas from a thermal swing adsorption (TSA) process.

[0016] Aspect 9: The method according to any one of aspects 1 to 8, further comprising directing at least a portion of the off-gas to one or more selected from the group consisting of (1) the feed gas stream and (2) the first permeate stream.

[0017] Aspect 10: The method according to any one of aspects 1 to 9, wherein at least one first-stage membrane module comprises a plurality of membrane modules arranged in series and / or in parallel.

[0018] Aspect 11: The method according to any one of aspects 1 to 10, wherein at least one second-stage membrane module comprises a plurality of membrane modules arranged in series and / or in parallel.

[0019] Aspect 12: The method according to any one of Aspects 1 to 11, wherein at least one third-stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel.

[0020] Aspect 13: The method according to any one of Aspects 1 to 12, wherein the raw feed gas stream is a product stream from an anaerobic digester.

[0021] Aspect 14: The method according to any one of Aspects 1 to 13, wherein the raw feed gas stream contains at least 40% methane.

[0022] Aspect 15: The method according to any one of Aspects 1 to 14, wherein step (f) further includes compressing the first permeate stream to a pressure of at least 10 barg in a first compressor to form a compressed first permeate stream.

[0023] Aspect 16: The method according to any one of Aspects 1 to 15, wherein the first non-permeate stream contains at least 70% methane.

[0024] Aspect 17: The method according to any one of Aspects 1 to 16, wherein the third permeate stream contains less than 1% methane.

[0025] Aspect 18: A method comprising: (a) compressing a feed gas stream to form a pressurized feed gas stream; (b) supplying the pressurized feed gas stream to at least one first-stage membrane module, each of the at least one first-stage membrane module having a first high-pressure side and a first low-pressure side, the first high-pressure side extending from a first supply port to a first non-permeate port, and the first low-pressure side being in fluid communication with a first sweep port and a first permeate port; (c) separating the pressurized feed gas stream in each of the at least one first-stage membrane module into a first non-permeate stream and a first permeate stream; (d) discharging the first non-permeate stream from each of the at least one first-stage membrane module through a first non-permeate port; (e) Supplying the first non-permeate stream to at least one stripping membrane module, each of the at least one stripping membrane module having a fourth high-pressure side and a fourth low-pressure side, the fourth high-pressure side extending from a fourth supply port to a fourth non-permeate port, and the fourth low-pressure side being in fluid flow communication with a fourth permeate port; (f) Separating the first non-permeate stream in each of the at least one stripping membrane module into a fourth non-permeate stream and a fourth permeate stream; (g) Discharging the fourth permeate stream from the fourth permeate port; (h) Discharging the fourth non-permeate stream from each of the at least one stripping membrane module through the fourth non-permeate port; (i) Sweeping the fourth permeate stream with a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the first non-permeate stream is supplied to the at least one stripping membrane module in step (e), and discharging the fourth permeate stream from the fourth permeate port; (j) Compressing the first permeate stream in a first compressor to form a compressed first permeate stream; (k) Supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid flow communication with a second sweep port and a second permeate port; (l) Separating the compressed first permeate stream in the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (m) Discharging the second non-permeate stream from each of the at least one second-stage membrane module through the second non-permeate port; (n) Discharging the second permeate stream from each of the at least one second-stage membrane module through the second permeate port; (o) Mixing the second non-permeate stream with the compressed hybrid feed stream to form a pressurized feed gas stream; (p) Supplying the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third supply port to a third non-permeate port, and the third low-pressure side being in fluid flow communication with a third sweep port and a third permeate port; (q) Separating the second permeate stream in the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; (r) Discharging the third non-permeate stream from each of the at least one third-stage membrane module through a third non-permeate port; (s) Discharging the third permeate stream from each of the at least one third-stage membrane module through a third permeate port; (t) Mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; (u) Compressing the mixed feed stream in a second compressor to form a compressed mixed feed stream, the method comprising.

[0026] Aspect 19: The method according to aspect 18, wherein step (i) further comprises introducing the sweep gas at a pressure of less than 1.00 barg.

[0027] Aspect 20: The method according to aspect 18 or 19, wherein the sweep gas comprises off-gas from a liquefaction process or a temperature swing adsorption process.

Brief Description of the Drawings

[0028]

Figure 1

[0029]

Figure 2

[0030]

Figure 3

[0031]

Figure 4

[0032]

Figure 5

[0033]

Figure 6

Mode for Carrying Out the Invention

[0034] The following detailed description provides only exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the present invention. Rather, the following detailed description of the exemplary embodiments provides an effective explanation for those skilled in the art to implement the exemplary embodiments of the present invention. Various changes can be made to the functions and arrangements of the elements without departing from the spirit and scope of the present invention.

[0035] To assist in explaining the present invention, terms indicating directions may be used in this specification and the claims to describe parts of the present invention (e.g., above, below, left, right, etc.). These terms indicating directions are merely intended to assist in explaining and claiming the present invention and are not intended to limit the present invention in any way. In addition, the reference numbers introduced in this specification in relation to the drawings may be repeated in one or more subsequent drawings without additional explanation in the specification to provide the context of other features.

[0036] In the claims, letters are used to identify the claimed steps (e.g., (a), (b), and (c)). These letters are used to assist in referring to method steps and are not intended to indicate the order in which the claimed steps are to be performed, except where such order is specifically recited in the claims.

[0037] Unless otherwise indicated, as used herein, the articles "a" and "an" mean one or more when applied to any feature in embodiments of the invention described herein and in the claims. The use of "a" and "an" does not limit the meaning to a single feature unless such a limitation is specifically recited. The article "the" preceding a singular or plural noun or noun phrase refers to a particular recited feature or particular recited features and may have a singular or plural connotation depending on the context in which it is used.

[0038] As used herein and in the claims, the term "biogas" means a renewable fuel produced by the decomposition of organic matter, such as food waste or animal manure.

[0039] As used herein and in the claims, the term "sweep gas" means a gas stream that is fed to the low-pressure side of a membrane, dilutes the permeate gas, reduces its partial pressure, and further aids in the removal of the permeate gas from the membrane.

[0040] As used herein and in the claims, the term "membrane" means an interface between two adjacent phases that acts as a selective barrier and regulates the transport of gas between gas mixtures.

[0041] As used herein and in the claims, the term "offgas" means a gas that is produced or emitted, particularly as a byproduct of a chemical process.

[0042] As used in this specification and the claims, the term "biomethane product" means purified biogas concentrated to at least 95% methane.

[0043] As used in this specification and the claims, the term "thermal swing adsorption" means a separation process that utilizes the thermodynamic properties of an adsorbent.

[0044] As used in this specification and the claims, the term "membrane module" means a device used to selectively separate a gas by flowing a feed gas through one or more conduits (also referred to as the high-pressure side) contained within a shell at a relatively high pressure. The conduits are at least partially defined by a membrane material that provides a barrier between each conduit and the shell space (also referred to as the low-pressure side). The shell space is the internal volume within the shell that is maintained at a relatively low pressure and outside of each of the membranes. The shell side is in fluid flow communication with a permeate port through which the gas that permeates the membrane exits the shell. Optionally, a sweep port may also be provided to feed a sweep gas into the shell space to assist the flow of the permeate gas through the permeate port. The membrane material is selected such that one or more gases in the feed stream (referred to as the permeate gas) pass through the membrane material at a higher rate than other gases in the feed stream (referred to as the non-permeate or product gas).

[0045] FIG. 1 is an exemplary three-stage membrane biogas separation system 10 that includes two compressors 12 and 14. Biogas typically contains carbon dioxide (CO2) and methane (CH4) as main components, and often includes other minor components such as oxygen (O2) and nitrogen (N2). The membranes used in system 10 are typically more selective for CO2 than for CH4, which means that CO2 is considered the fast permeating gas that preferentially passes through the membrane at a relatively high rate, while CH4 is the slow permeating gas that crosses the membrane at a relatively low rate. Hereinafter, the fast permeating gas and the slow permeating gas may be referred to as the fast gas and the slow gas, respectively. The gas permeability through the membrane is controlled by the solution-diffusion transport mechanism, and the permeation rate is a function of the molecular size (diffusion rate) and molecular solubility in the polymer and is proportional to the driving force. The driving force for gas separation is the partial pressure difference of the permeating species between the high-pressure side and the low-pressure side of the membrane. The gas permeability of each gas component in the gas mixture typically varies, and the partial pressure of each gas component is proportional to its relative concentration in the gas mixture and the total pressure of the gas mixture.

[0046] In system 10, a third non-permeate stream 48 (discussed below) is mixed with the raw feed gas stream 30 to form a mixed feed stream 32. The raw feed gas stream can be obtained from several sources, and one such source is biogas resulting from the biological fermentation of organic solids during anaerobic digestion. A typical composition of the biogas raw feed gas stream is 40 - 70% CH4 saturated with water vapor, 30 - 60% CO2, and low concentrations of hydrogen sulfide (0 - 5 ppm), nitrogen (0 - 5%), and oxygen (0 - 5%). The mixed feed stream 32 is compressed to a higher pressure in the feed compressor 12 to yield a compressed mixed feed stream 34. A second recycle stream 44 (also referred to as the second non-permeate stream 44, as discussed below) is mixed with the compressed mixed feed stream 34 to form a pressurized feed gas stream 36.

[0047] The pressurized feed gas 36 containing a mixture of low-speed gas and high-speed gas is fed to the first-stage membrane 20. A gas mainly containing high-speed gas and additionally a trace amount of low-speed gas permeates through the first-stage membrane 20 and exits as the first permeate stream 40, while a gas that cannot permeate through the membrane, mainly containing low-speed gas and additionally a trace amount of high-speed gas, is rejected and taken out as the first non-permeate stream 38. In the case of biogas, CO2 permeates much faster than CH4. The first non-permeate stream 38 can be taken out as the final product stream of low-speed gas having a high concentration of low-speed gas and a very low concentration of high-speed gas. The first permeate stream 40 is recompressed in the compressor 14 to produce the compressed first-stage permeate stream 42, which is fed to the second-stage membrane 22, where additional low-speed gas is rejected and taken out as the second-stage non-permeate stream 44, and the high-speed gas permeates through the membrane to produce the second-stage permeate stream 46. The second-stage non-permeate stream 44 is mixed with the compressed hybrid feed stream 34 at a downstream point of the supply compressor 12. The second-stage permeate stream 46 is fed to the third-stage membrane 24, where the low-speed gas is rejected and taken out as the third-stage non-permeate stream 48, and the high-speed gas permeates through the membrane 24 to produce the waste stream 50. The third-stage non-permeate stream 48 is returned to an upstream point of the supply compressor 12, where it is mixed with the original feed gas stream 30 and recycled through the system 10 as described above. Each of the membranes 20, 22, 24 may include one or more membranes. When multiple membranes are used at each stage, they can be arranged in series, in parallel, or both.

[0048] It is not uncommon for a biogas generation system to have a low-pressure off-gas having a low CO2 concentration generated from other unit operations such as a thermal swing adsorption (TSA) system or tail gas from a liquefaction unit. In the system 10, the off-gas stream 18 is added to the hybrid feed stream 32 upstream of the supply compressor 12. In many applications, adding the off-gas stream 18 to the hybrid feed stream 32 increases the membrane area required to achieve the desired product purity. Additionally, the low-pressure off-gas stream 18 often has a higher purity than the original feed gas stream 30. Therefore, mixing the two streams is thermodynamically unfavorable.

[0049] Figure 2 shows an exemplary embodiment of a three-stage membrane biogas separation system 100. System 100 is substantially similar to system 10 of FIG. 1, except that the offgas stream (shown schematically as coming from offgas source 156) is introduced as a low-pressure sweep gas stream 118 for the first-stage membrane 120 instead of being added to the mixed feed stream 132.

[0050] Similar to system 10, the membranes 120, 122, 124 used in this system 100 are also more selective for CO2 than for CH4. In system 100 of FIG. 2, a raw feed gas stream 130 (e.g., biogas containing CO2 and CH4) containing a mixture of a high-speed gas and a low-speed gas is supplied to system 100. For example, similar to system 10, the raw feed gas can be obtained from the biological fermentation of organic solids during anaerobic digestion. A first recycle stream 148 (also referred to as the third non-permeate stream 148) is mixed with the raw feed gas stream 130 to form a mixed feed stream 132. The mixed feed stream 132 is compressed to a higher pressure in the feed compressor 112 to yield a compressed mixed feed stream 134. A second recycle stream 144 (also referred to as the second non-permeate stream 144) is mixed with the compressed mixed feed stream 134 to form a pressurized feed gas stream 136 containing a mixture of a low-speed permeate gas and a high-speed permeate gas, and is fed to a first-stage membrane 120 that is more selective for the high-speed gas than for the low-speed gas. In the case of this biogas, CO2 permeates much more rapidly than CH4. The first-stage membrane 120 can include one or more membranes.

[0051] The composition of the low-pressure sweep gas stream 118 is preferably 0-5% CO2 and less than 1% undesired low-speed gas (such as nitrogen). The sweep gas stream 118 is supplied to the first-stage membrane 120 to sweep the shell side of the first-stage membrane 120. A gas containing mainly the high-speed gas and, in addition, a trace amount of the low-speed gas permeates through the first-stage membrane 120 and exits as a first permeate stream 140, while a gas containing mainly the low-speed gas and, in addition, a trace amount of the high-speed gas that cannot permeate through the first-stage membrane 120 is rejected and taken out as a first non-permeate stream 138. A control valve 116 can be used to regulate the pressure when the first non-permeate stream 138 exits the system 100.

[0052] The first non-permeate stream 138 can be taken out as the final product stream of the low-speed gas, which has a high-concentration low-speed gas and a very low-concentration high-speed gas. The first permeate stream 140 is recompressed in the first-stage permeate compressor 114 to produce a compressed first permeate stream 142, that is, a pressurized mixture of a high-speed permeate gas and a certain amount of low-speed permeate gas, which is supplied to the second-stage membrane module 122 that may include one or more membranes. A gas containing mainly a high-speed gas and, in addition, a trace amount of low-speed gas permeates through the second-stage membrane module 122 and exits as the second permeate stream 146, while a low-speed gas and a gas containing a low-speed gas that cannot permeate through the membrane are rejected and taken out as the second non-permeate stream 144. The second non-permeate stream 144 is recycled and returned to the compressed mixed feed stream 134 downstream of the supply compressor 112.

[0053] The second permeate stream 146 is supplied to the third-stage membrane 124. In the third-stage membrane 124, a gas containing mainly a high-speed gas and, in addition, a trace amount of low-speed gas permeates through the third-stage membrane 124 and exits as the third permeate stream 150, while the low-speed gas is rejected and taken out as the third non-permeate stream 148. The third non-permeate stream 148 is recycled and returned to the raw feed gas stream 130 upstream of the supply compressor 112. A control valve 113 is used to maintain an appropriate pressure across the third-stage membrane 124 and can be used to affect the pressure on the shell side of the second-stage membrane module 122. The third permeate stream 150 (also referred to as the exhaust stream 150) is a high-speed gas-rich stream, which is mainly CO2 in the case of biogas and can be further exhausted or processed according to the application. The third-stage membrane 124 may include one or more membranes. Each membrane stage 120, 122, and 124 may include one or more membranes, and multiple membranes are arranged in series or in parallel.

[0054] The system 100 can be used to separate any one of several high-speed gas / low-speed gas pairs in the feed stream 130, including but not limited to: CO2 / CH4, H2 / CO, H2 / CO2, CO2 / N2, O2 / N2, He / CH4, H2 / CH4, and H2 / N2.

[0055]

[0056]

[0057] In the stripping membrane 252, a gas containing a mixture of a high-permeation gas and a low-permeation gas permeates through the stripping membrane 252 and exits as a fourth permeate stream 258, while the low-speed gas is rejected and taken out as a stripped non-permeate stream 260. A control valve 216 can be used to adjust the pressure when the stripped non-permeate stream 260 exits the system 200. The stripped permeate stream 254, which mainly contains a mixture of a high-permeation gas and a low-permeation gas, exits through the stripping membrane 252. All or part of the stripped permeate stream 254 can be directed to one or more flows within the system 200. For example, part 254A can be recycled into the compressed feed stream 234 upstream of the supply compressor 212, part 254B can be mixed with the sweep gas 218 and then flowed to the shell side of the first-stage membrane 220, and / or part 254C can be supplied to the inlet of the first-stage permeate compressor 214.

[0058] In most applications, the sweep gas is fed only to the low-pressure side of the membrane module stage providing the final product gas stream. Generally, the product gas stream is the stream having the highest concentration of the product gas (CH4 in the examples provided herein) of any stream within the system. In system 100, the first non-permeate stream 138 is the final product stream (also referred to as the biomethane product). Thus, in system 100, the sweep gas stream 118 is fed to the low-pressure side of the first-stage membrane module 120. In system 200, the stripped non-permeate stream 260 is the product gas stream. Thus, the sweep gas 218C is fed to the low-pressure side of the stripping membrane module 252.

[0059] Referring now to FIG. 3, the structural details of an exemplary first stage membrane 120 are shown. A similar structure can be used for any of the membranes 120, 122, 124 used in the system 100. The first stage membrane 120 includes a core having a bundle of open hollow fibers 160a - 160h extending along the length of the shell 162. The shell 162 includes a supply port 164 to which the pressurized feed gas stream 136 is supplied, a non-permeate port 166 through which the first non-permeate stream 138 exits the first stage membrane 120, and a permeate port 168 through which the first permeate stream 140 exits the first stage membrane 120.

[0060] The bundle of hollow fibers 160a - 160h is held together at both ends by tube sheets 170, 172 made of a thermoplastic or thermosetting material. Examples of tube sheet materials include cured epoxy or polyurethane-based formulations. One of the tube sheets 170 is located at the feed end 174 of the first stage membrane 120 (adjacent to the supply port 164), and the other tube sheet 172 is located at the non-permeate end 176 of the first stage membrane 120 (adjacent to the non-permeate port 166).

[0061] To provide countercurrent flow between the pressurized feed gas stream 136 and the first permeate stream 140, the permeate port 168 is located inside the feed end tube sheet 170, typically at a distance of not more than one-third of the distance from the inner edge of the feed end tube sheet 170 to the non-permeate end tube sheet 172. Similarly, the sweep port 178 is located inside the non-permeate end tube sheet 172, typically at a distance of not more than one-third of the distance from the inner edge of the non-permeate end tube sheet 172. The sweep port 178 can be in fluid flow communication with a feed rate of sweep gas 118.

[0062] The fibers 160a - 160h have a semi - permeable wall that is intended to be permeable to high - speed gas and less permeable to low - speed gas. Thus, when the pressurized feed gas stream 136, which is a mixture of low - speed gas and high - speed gas, flows through the fibers 160a - 160h, the high - speed gas passes through the walls of the fibers 160a - 160h and flows through the permeate port 168, while the low - speed gas remains within the fibers 160a - 160h and flows to the non - permeate port 166.

[0063] The lumen side of the first - stage membrane 120 is defined herein as the path that the fluid follows when it is introduced through the supply port 164, passes through the lumens or lumen sides of the fibers 160a - 160h (indicated by arrows in FIG. 3), and exits through the non - permeate port 166. The shell side of the first - stage membrane 120 is defined herein as the internal volume within the shell 162, between the tube sheets 170 and 172, and outside the fibers 160a - 160h. In the system 100, the pressurized feed gas stream 136 flows into the lumen side of the first - stage membrane 120 and exits the lumen side as the first non - permeate stream 138. After passing through the walls of the fibers 160a - 160h, the high - speed gas portion of the pressurized feed gas stream 136 enters the shell side, where it is optionally mixed with the sweep gas 118 and is swept from the shell side through the permeate port 168 as the first permeate stream 140.

[0064] The sweep gas stream 118 can be introduced at a low pressure of less than 1.00 barg, for example, 0.89 barg. The sweep gas stream 118 can be provided with a CO2 concentration of less than 15%, or less than 10%, or less than 5%. The sweep gas stream 118 can be provided with a CH4 concentration higher than 85%, or higher than 90%, or higher than 95%. The sweep gas stream 118 can be provided with a nitrogen content in a molar flow rate that is 5% or less of the total molar flow rate of the raw feed gas in the stream 130. The sweep gas stream 118 can be provided with an oxygen content in a molar flow rate that is 5% or less of the total molar flow rate of the raw feed gas in the stream 130.

[0065] In system 100, each membrane stage 120, 122, and 124 may include one or more membranes, and the plurality of membranes are arranged in series and / or in parallel. Each membrane may be in the form of a flat sheet or a hollow fiber, and the membrane module may be either a spiral-wound flat sheet or a bundle of hollow fibers. In each membrane stage 120, 122, and 124, it is not necessary to use the same number and / or type of membranes. For example, in some embodiments, all three stages may use membranes of the same permeability and selectivity. In other embodiments, the membrane permeability and selectivity of each stage may be different from those of the other stages. In still other embodiments, two stages may use membranes of the same permeability and selectivity, and the remaining stage may use membranes of different permeability and selectivity. Each membrane may be made of a single polymer selected from a number of polymers known in the art or that are determined in the future to be suitable for the desired separation, or each membrane may be a composite membrane made of a plurality of polymers.

[0066] It should be understood that other types of membrane modules can be used. Examples of membrane configurations include membrane bundles, flat sheets, or hollow fibers packaged as helical wraps, as well as plate and frame configurations. Membranes are generally formed from polymers. Examples of polymers used to make membranes include, but are not limited to, polystyrene, polysulfone, polyethersulfone, polyvinyl fluoride, polyvinylidene fluoride, polyetheretherketone, polycarbonate, polyphenylene oxide, polyethylene, polypropylene, cellulose acetate, polyimide (such as Matrimid 5218 or P-84), polyamide, polyvinyl alcohol, polyvinyl acetate, polyethylene oxide, polydimethylsiloxane, copolymers, block copolymers, or polymer blends. The hollow fiber membrane may be non-porous and asymmetric, or may include a porous support with a non-porous coating. The coating can be applied to the inner or outer surface of the hollow fiber. In a membrane (such as membrane 120 in Figure 3) where a bundle of hollow fibers is held together at one or both ends by a tube sheet, the tube sheet can be made of a thermoplastic or thermosetting material. Examples of tube sheet materials include cured epoxy or polyurethane-based formulations. The container can be constructed of plastic, metal, or other suitable materials.

[0067] In embodiments where a bundle of hollow fibers is provided (such as membrane 120 in Figure 3), the feed gas can be supplied through the lumen side of the fibers (as shown in Figure 3). Alternatively, the feed gas can be supplied through the shell space. In this case, the shell space becomes the high-pressure side of the membrane, the lumen space becomes the low-pressure side of the membrane, the sweep port 178 and the permeate port 168 become the feed port and the product port, respectively, and the feed port 164 and the non-permeate port 166 become the sweep port and the permeate port.

[0068] The tables of FIGS. 4 and 5 compare the performance characteristics of a three-stage biogas separation system 10 (FIG. 1) that adds an off-gas stream to the mixed feed stream 32 with the performance characteristics of a three-stage biogas separation system 100 (FIG. 2) that uses the off-gas stream as the sweep gas stream 118 for the first-stage membrane 120. Simulations were performed using the conditions shown in Table 1 below. [Table 1]

[0069] As shown in FIGS. 4 and 5 and summarized in Table 2 below, both system 10 and system 100 produce the target product composition and target product recovery rate. However, by utilizing the sweep gas stream 118, system 100 achieves the same result using 75% of the membrane area required under system 10 for the above conditions. [Table 2]

[0070] As shown in FIG. 4, a process simulation was performed using the configuration of system 10 having three membrane stages 20, 22, and 24, a feed compressor 12, and a first stage permeate compressor 14. In this process simulation, low pressure off-gas is fed to an upstream point of the compressor, unlike in the sweep gas system 10. A raw biogas stream 30 containing 60% CH4 and 40% CO2 at a feed rate of 500 NMH is provided to the system. The feed stream 30 is mixed with a third non-permeate stream 48 having a flow rate of 20.2 NMH and containing 56.12% CH4 and 43.88% CO2 and a low pressure off-gas having a flow rate of 200 NMH and containing 95.00% CH4 and 5.00% CO2 to form a mixed feed stream 32, which is compressed by compressor 12. The resulting compressed mixed feed stream 34 is mixed with a second non-permeate stream 44 at 250.8 NMH and containing 62.17% CH4 and 37.83% CO2 to produce a pressurized feed gas stream 36 at about 13.00 barg and 40° C. and containing 67.69% CH4 and 32.31% CO2 at 971.1 NMH. The compressed mixed feed stream 36 is fed to the first stage membrane 20 to produce a first non-permeate stream 38 and a first permeate stream 40. The first non-permeate stream 38 has a molar flow rate of 495.4 NMH, contains 98.5% CH4 and 1.5% CO2, and is taken out as a product gas.

[0071] The first permeate stream 40 has a molar flow rate of 475.7 NMH and enters the first stage permeate compressor 14. The compressed feed gas stream 42 is fed to the second stage membrane 22 to produce a second non-permeate stream 44 and a second permeate stream 46. The second permeate stream 46 is fed to the third stage membrane, and a third stage permeate stream 50 (or waste stream) is taken out at 204.6 NMH and contains exactly 1.00 mole % CH4 and 99.0% CO2.

[0072] For comparison, FIG. 5 shows a process simulation performed using the configuration of system 100 that utilizes low-pressure off-gas as sweep gas stream 118. System 100 includes three stages of membranes 120, 122, and 124, a feed compressor 112, and a first-stage permeate compressor 114. A raw biogas stream 130 containing 60% CH4 and 40% CO2 at a feed rate of 500 NMH is provided to system 100. Stream 130 is mixed with a third non-permeate stream 148 having a flow rate of 28.8 NMH and containing 46.85% CH4 and 53.15% CO2 to form a mixed feed stream 112, which is compressed by compressor 1112. The resulting compressed mixed feed stream 134 is mixed with a second non-permeate stream 144 at 463.4 NMH and containing 59.57% CH4 and 40.43% CO2 to produce a pressurized feed gas stream 136 at about 13.00 barg and 40 °C and containing 59.42% CH4 and 40.58% CO2 at 992.2 NMH. The pressurized feed gas stream 136 is supplied to the first-stage membrane 120 to produce a first non-permeate stream 138 and a first permeate stream 140.

[0073] Simultaneously, a sweep gas stream 118 containing 95.00% CH4 and 5.00% CO2 is supplied to the first-stage membrane module 120. The introduction of sweep gas stream 118 enables the target product composition and target product recovery rate to be achieved at only 75% of the area required for system 10. The first non-permeate stream 138 contains 98.5% CH4 and 1.5% CO2 and is taken out as the product gas.

[0074] The first permeate stream 140 has a molar flow rate of 696.8 NMH and enters the first-stage permeate compressor 114. The compressed first permeate stream 142 is supplied to the second-stage membrane module 122 to produce a second non-permeate stream 144 and a second permeate stream 146. The second permeate stream 146 is supplied to the third-stage membrane module 124, and a third-stage permeate stream 150 (or waste stream) is taken out at 204.6 NMH and contains exactly 1.00 mol% CH4 and 99.0% CO2.

[0075] The present invention is not limited by the specific aspects or embodiments disclosed in the examples intended as illustrations of some aspects of the present invention, and any embodiments that are functionally equivalent are within the scope of the present invention. In addition to those shown and described herein, various modifications of the present invention will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims. Examples of embodiments of the present invention are listed in the following items [Aspect 1] to [Aspect 20]. [Aspect 1] A method comprising: (a) Compressing a supply gas stream to form a pressurized supply gas stream; (b) Supplying the pressurized supply gas stream to at least one first-stage membrane module, each of the at least one first-stage membrane module having a first high-pressure side and a first low-pressure side, the first high-pressure side extending from a first supply port to a first non-permeate port, and the first low-pressure side being in fluid flow communication with a first sweep port and a first permeate port; (c) Separating the pressurized supply gas stream in each of the at least one first-stage membrane module into a first non-permeate stream and a first permeate stream; (d) Using a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the pressurized supply gas stream is supplied to the at least one first-stage membrane module in step (a), to sweep the first permeate stream and discharge it from the first permeate port; (e) Discharging the first non-permeate stream from each of the at least one first-stage membrane module through the first non-permeate port; (f) Compressing the first permeate stream in a first compressor to form a compressed first permeate stream; (g) Supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid flow communication with a second sweep port and a second permeate port; (h) Separating the compressed first permeate stream in the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (i) Discharging the second non-permeate stream from each of the at least one second-stage membrane module through the second non-permeate port; (j) Discharging the second permeate stream from each of the at least one second-stage membrane module through the second permeate port. (k) Mixing the second non-permeate stream with the compressed mixed feed stream to form the pressurized feed gas stream; (l) Supplying the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third supply port to a third non-permeate port, and the third low-pressure side being in fluid flow communication with a third sweep port and a third permeate port; (m) Separating the second permeate stream within the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; (n) Discharging the third non-permeate stream from each of the at least one third-stage membrane module through the third non-permeate port; (n) Discharging the third permeate stream from each of the at least one third-stage membrane module through the third permeate port; (o) Mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; (p) Compressing the mixed feed stream in a second compressor to form a compressed mixed feed stream, the method comprising. [Aspect 2] The method according to aspect 1, wherein step (d) further comprises introducing the sweep gas at a pressure of less than 1.00 barg. [Aspect 3] The method according to aspect 1, wherein the sweep gas contains less than 15% carbon dioxide. [Aspect 4] The method according to aspect 1, wherein the sweep gas contains at least 85% methane. [Aspect 5] The method according to aspect 1, wherein the sweep gas provides nitrogen at a first molar flow rate that is 5% or less of a second molar flow rate of the pressurized feed gas stream. [Aspect 6] The method according to aspect 1, wherein the sweep gas contains off-gas. [Aspect 7] The method according to aspect 6, wherein the off-gas contains off-gas from a liquefaction process or a temperature swing adsorption process. [Aspect 8] The method according to aspect 1, wherein the sweep gas contains tail gas from a thermal swing adsorption (TSA) process. [Aspect 9] The method according to aspect 7, further comprising directing at least a portion of the off-gas to one or more selected from the group consisting of (1) the feed gas stream and (2) the first permeate stream. [Aspect 10] The method according to aspect 1, wherein the at least one first-stage membrane module comprises a plurality of membrane modules arranged in series and / or in parallel. [Aspect 11] The method according to aspect 1, wherein the at least one second stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel. [Aspect 12] The method according to aspect 1, wherein the at least one third stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel. [Aspect 13] The method according to aspect 1, wherein the raw feed gas stream is a product stream from an anaerobic digester. [Aspect 14] The method according to aspect 1, wherein the raw feed gas stream contains at least 40% methane. [Aspect 15] The method according to aspect 1, wherein step (f) further includes compressing the first permeate stream to a pressure of at least 10 barg in the first compressor to form a compressed first permeate stream. [Aspect 16] The method according to aspect 1, wherein the first non-permeate stream contains at least 70% methane. [Aspect 17] The method according to aspect 1, wherein the third permeate stream contains less than 1% methane. [Aspect 18] A method comprising: (a) compressing a feed gas stream to form a pressurized feed gas stream; (b) supplying the pressurized feed gas stream to at least one first stage membrane module, each of the at least one first stage membrane module having a first high pressure side and a first low pressure side, the first high pressure side extending from a first supply port to a first non-permeate port, and the first low pressure side being in fluid flow communication with a first sweep port and a first permeate port; (c) separating the pressurized feed gas stream in each of the at least one first stage membrane module into a first non-permeate stream and a first permeate stream; (d) discharging the first non-permeate stream from each of the at least one first stage membrane module through the first non-permeate port; (e) supplying the first non-permeate stream to at least one stripping membrane module, each of the at least one stripping membrane module having a fourth high pressure side and a fourth low pressure side, the fourth high pressure side extending from a fourth supply port to a fourth non-permeate port, and the fourth low pressure side being in fluid flow communication with a fourth permeate port; (f) separating the first non-permeate stream in each of the at least one stripping membrane module into a fourth non-permeate stream and a fourth permeate stream. (g) discharging the fourth permeate stream from the fourth permeate port; (h) discharging the fourth non-permeate stream from each of the at least one stripping membrane module through the fourth non-permeate port; (i) using a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the first non-permeate stream is supplied to the at least one stripping membrane module in step (e), to sweep the fourth permeate stream and discharge it from the fourth permeate port; (j) compressing the first permeate stream in a first compressor to form a compressed first permeate stream; (k) supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid communication with a second sweep port and a second permeate port; (l) separating the compressed first permeate stream in the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (m) discharging the second non-permeate stream from each of the at least one second-stage membrane module through the second non-permeate port; (n) discharging the second permeate stream from each of the at least one second-stage membrane module through the second permeate port; (o) mixing the second non-permeate stream with a compressed mixed feed stream to form the pressurized feed gas stream; (p) supplying the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third supply port to a third non-permeate port, and the third low-pressure side being in fluid communication with a third sweep port and a third permeate port; (q) separating the second permeate stream in the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; discharging the third non-permeate stream from each of the at least one third-stage membrane module through the third non-permeate port; discharging the third permeate stream from each of the at least one third-stage membrane module through the third permeate port; mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; compressing the mixed feed stream in a second compressor to form a compressed mixed feed stream, the method comprising. [Aspect 19] The method according to aspect 18, wherein step (i) further comprises introducing the sweep gas at a pressure of less than 1.00 barg. [Aspect 20] The method according to aspect 18, wherein the sweep gas comprises offgas from a liquefaction process or a temperature swing adsorption process.

Claims

【Claim 1】 A method comprising: (a) compressing a feed gas stream in a second compressor to form a pressurized feed gas stream; (b) supplying the pressurized feed gas stream to at least one first-stage membrane module, each of the at least one first-stage membrane module having a first high-pressure side and a first low-pressure side, the first high-pressure side extending from a first supply port to a first non-permeate port, and the first low-pressure side being in fluid flow communication with a first sweep port and a first permeate port; (c) separating the pressurized feed gas stream in each of the at least one first-stage membrane module into a first non-permeate stream and a first permeate stream; (d) using a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the pressurized feed gas stream is supplied to the at least one first-stage membrane module in step (a) to sweep the first permeate stream and discharge it from the first permeate port; (e) discharging the first non-permeate stream from each of the at least one first-stage membrane module through the first non-permeate port; (f) compressing the first permeate stream in a first compressor to form a compressed first permeate stream; (g) supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid flow communication with a second sweep port and a second permeate port; (h) separating the compressed first permeate stream in the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (i) discharging the second non-permeate stream from each of the at least one second-stage membrane module through the second non-permeate port; (j) discharging the second permeate stream from each of the at least one second-stage membrane module through the second permeate port; (k) mixing the second non-permeate stream with the compressed hybrid feed stream to form the pressurized feed gas stream; (l) feeding the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third feed port to a third non-permeate port, and the third low-pressure side being in fluid flow communication with a third sweep port and a third permeate port; (m) separating the second permeate stream within the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; (n) discharging the third non-permeate stream from each of the at least one third-stage membrane module through the third non-permeate port; (n) discharging the third permeate stream from each of the at least one third-stage membrane module through the third permeate port; (o) mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; (p) compressing the mixed feed stream in the second compressor to form a compressed mixed feed stream, comprising: The method wherein the sweep gas comprises offgas from a liquefaction process or a temperature swing adsorption process. **Claim 2** The method according to claim 1, wherein step (d) further comprises introducing the sweep gas at a pressure of less than 1.00 barg. **Claim 3** The method according to claim 1, wherein the sweep gas comprises less than 15% carbon dioxide. **Claim 4** The method according to claim 1, wherein the sweep gas comprises at least 85% methane. **Claim 5** The method according to claim 1, wherein the sweep gas has nitrogen at a first molar flow rate that is 5% or less of the total molar flow rate of the pressurized feed gas stream. **Claim 6** The method according to claim 1, wherein the sweep gas includes tail gas from a temperature swing adsorption (TSA) process. **Claim 7** The method according to claim 1, further comprising directing at least a portion of the off-gas to one or more selected from the group consisting of (1) the feed gas stream and (2) the first permeate stream. **Claim 8** The method according to claim 1, wherein the at least one first stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel. **Claim 9** The method according to claim 1, wherein the at least one second stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel. **Claim 10** The method according to claim 1, wherein the at least one third stage membrane module includes a plurality of membrane modules arranged in series and / or in parallel. **Claim 11** The method according to claim 1, wherein the raw feed gas stream is a product stream from an anaerobic digester. **Claim 12** The method according to claim 1, wherein the raw feed gas stream includes at least 40% methane. **Claim 13** The method according to claim 1, wherein step (f) further comprises compressing the first permeate stream to a pressure of at least 10 barg in the first compressor to form a compressed first permeate stream. **Claim 14** The method according to claim 1, wherein the first non-permeate stream includes at least 70% methane. **Claim 15** The method according to claim 1, wherein the third permeate stream includes 1% or less methane. **Claim 16** A method, (a) Compressing a supply gas stream in a second compressor to form a pressurized supply gas stream; (b) Supplying the pressurized supply gas stream to at least one first-stage membrane module, each of the at least one first-stage membrane module having a first high-pressure side and a first low-pressure side, the first high-pressure side extending from a first supply port to a first non-permeate port, and the first low-pressure side being in fluid communication with a first sweep port and a first permeate port; (c) Separating the pressurized supply gas stream in each of the at least one first-stage membrane module into a first non-permeate stream and a first permeate stream; (d) Discharging the first non-permeate stream from each of the at least one first-stage membrane module through the first non-permeate port; (e) Supplying the first non-permeate stream to at least one stripping membrane module, each of the at least one stripping membrane module having a fourth high-pressure side and a fourth low-pressure side, the fourth high-pressure side extending from a fourth supply port to a fourth non-permeate port, and the fourth low-pressure side being in fluid communication with a fourth permeate port; (f) Separating the first non-permeate stream in each of the at least one stripping membrane module into a fourth non-permeate stream and a fourth permeate stream; (g) Discharging the fourth permeate stream from the fourth permeate port; (h) Discharging the fourth non-permeate stream from each of the at least one stripping membrane module through the fourth non-permeate port; (i) Sweeping the fourth permeate stream using a sweep gas supplied in a first flow direction that is countercurrent to a second flow direction in which the first non-permeate stream is supplied to the at least one stripping membrane module in step (e) and discharging the fourth permeate stream from the fourth permeate port; (j) Compressing the first permeate stream in a first compressor to form a compressed first permeate stream; (k) Supplying the compressed first permeate stream to at least one second-stage membrane module, each of the at least one second-stage membrane module having a second high-pressure side and a second low-pressure side, the second high-pressure side extending from a second supply port to a second non-permeate port, and the second low-pressure side being in fluid communication with a second sweep port and a second permeate port; (l) Separating the compressed first permeate stream in the at least one second-stage membrane module into a second non-permeate stream and a second permeate stream; (m) Discharging the second non-permeate stream from each of the at least one second-stage membrane module through the second non-permeate port; (n) Discharging the second permeate stream from each of the at least one second-stage membrane module through the second permeate port; (o) Mixing the second non-permeate stream with a compressed mixed feed stream to form the pressurized feed gas stream; (p) Supplying the second permeate stream to at least one third-stage membrane module, each of the at least one third-stage membrane module having a third high-pressure side and a third low-pressure side, the third high-pressure side extending from a third supply port to a third non-permeate port, and the third low-pressure side being in fluid communication with a third sweep port and a third permeate port; (q) Separating the second permeate stream in the at least one third-stage membrane module into a third non-permeate stream and a third permeate stream; (r) Discharging the third non-permeate stream from each of the at least one third-stage membrane module through the third non-permeate port; (s) Discharging the third permeate stream from each of the at least one third-stage membrane module through the third permeate port; (t) mixing the third non-permeate stream with the raw feed gas stream to form a mixed feed stream; (u) compressing the mixed feed stream in the second compressor to form a compressed mixed feed stream, The sweep gas includes off-gas from a liquefaction process or a temperature swing adsorption process, the method. **Claim 17** The method according to claim 16, wherein step (i) further includes introducing the sweep gas at a pressure of less than 1.00 barg.

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