Low-temperature membrane process for biogas upgrading

Abstract

To provide a low-temperature membrane process for biogas upgrading.SOLUTION: A method for separating a raw feed gas stream utilizes a compressor, a chiller, a membrane drying stage, and a plurality of membrane separation stages. The raw feed gas stream may comprise biogas. In one example, the raw feed gas stream is supplied to the chiller, where it is cooled to a target operating temperature to separate out condensed water. The gas stream is then supplied to a membrane drying stage to separate out water vapor. An off-gas from one of the membrane module stages may be recycled as a low-pressure sweep gas on the low-pressure side of the membrane drying stage to increase the driving force for water permeation within the membrane drying stage. The gas stream is then supplied to the plurality of membrane separation stages, where it is upgraded into a high-purity methane stream.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] This application relates to a multi-stage membrane process and system for methane recovery from biogas. Multi-stage membrane systems are known for upgrading a raw biogas stream into a high purity methane stream. Current upgrading systems generate a pressurized feed gas stream by feeding the gas stream to a compressor. The compressed gas stream is fed to a subsequent membrane separation stage where it is separated into a non-permeate stream that is rich in slow gases, and a permeate stream that is rich in fast gases to recover an end product stream having a high concentration of slow non-permeate gases such as methane. Membrane systems often operate at ambient or warmer temperatures selected to provide a few degrees of superheat to prevent condensation on the membrane material. While such multi-stage membrane systems may achieve high methane recovery and purity, they often require a relatively high capital expenditure (CapEx) associated with a high membrane area (or number of membranes) and a high operating expenditure (OpEx) associated with high compressor power costs. Also, the operating expenditure of such current upgrading systems can be relatively high due to high recirculation flow rates that require larger and more expensive compressors.

[0002] Thus, a need exists for effective, reliable, and cost-effective multi-stage membrane processes and systems that can achieve desired methane product recovery and purity while reducing overall capital and operating expenditures. Summary of the Invention

[0003] 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. Certain aspects of the systems and methods are summarized below.

[0004] The disclosed embodiments fulfill the need in the art by providing a multi-stage membrane method and system for methane recovery from biogas that reduces the overall cost of ownership and operation. It has been found that a system with a polymeric membrane with high carbon dioxide ("CO2") permeability, low CO2 activation energy, and high methane ("CH4") activation energy can operate at low temperatures to take advantage of higher CO2 / CH4 selectivity while maintaining high CO2 permeability that can replace some highly selective membranes that typically have low CO2 permeability. In this way, the recycle flow is reduced (which reduces the compressor power required) and the number of membranes is reduced. Also, a chiller and a membrane dryer stage are provided upstream of the separation membrane to protect the separation membrane from liquid condensation. The membrane dryer stage can be swept with a portion of the recirculated gas from one of the separation membranes. This design flexibility is not available in current separation membrane systems that have minimal temperature sensitivity.

[0005] Several aspects of the systems and methods are outlined below.

[0006] Aspect 1: A method comprising: (a) compressing a feed gas stream to form a pressurized feed gas stream, the pressurized feed gas stream comprising CH4 and being fully saturated with water; (b) cooling the pressurized supply gas stream in a cooling device to a target operating temperature to form a condensate stream and a saturated gas stream; (c) supplying the saturated gas stream to at least one membrane dryer stage, each of the at least one membrane dryer stage having a high pressure side and a low pressure side, the high pressure side extending from a feed port to a non-permeate port, and the low pressure side in fluid flow communication with a sweep port and a permeate port; (d) separating the saturated gas stream in at least one membrane drying stage to form a water vapor permeate stream and a dry gas stream, the dry gas stream having a water content of less than 2000 ppm; (e) sweeping and exhausting the water vapor permeate stream from the low pressure side using a sweep gas stream supplied in a flow direction countercurrent to a flow direction of the saturated gas stream supplied to the at least one membrane drying stage in step (c); (f) separating the dry gas stream into a first non-permeate stream and a first permeate stream in a first separation stage, the first non-permeate stream having a higher percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module; (g) separating the first non-permeate stream into a second non-permeate stream and a sweep gas stream in a second separation stage, the second non-permeate stream having a higher percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module; (h) separating the first permeate stream into a third non-permeate stream and a third permeate stream in a third separation stage, the third non-permeate stream having a higher percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; (i) combining the third non-permeate stream, the water vapor permeate stream, and optionally a first portion of the sweep gas stream to form a recycle stream; (j) combining the recycle stream with the feed gas stream prior to carrying out step (a); The method wherein the target operating temperature is 0 to 20°C.

[0007] Aspect 2: The method of aspect 1, wherein the target operating temperature is 3 to 7°C.

[0008] Aspect 3: The method of aspect 1, wherein the target operating temperature is about 5°C.

[0009] Aspect 4: The method of aspect 1, wherein the feed gas stream comprises at least 40% CH4.

[0010] Aspect 5: The method of aspect 1, wherein each of the at least one first stage membrane modules has a first stage CO2 / CH4 selectivity that is greater than the second stage CO2 / CH4 selectivity of each of the at least one second stage membrane module and the third stage CO2 / CH4 selectivity of each of the at least one third stage membrane module.

[0011] Aspect 6: The method of aspect 1, wherein each of the at least one first stage membrane modules has a first stage CO2 / CH4 selectivity of greater than 30 at the target operating temperature.

[0012] Aspect 7: The method of aspect 1, wherein each of the at least one second stage membrane modules has a CO2 / CH4 selectivity greater than 20 at the target operating temperature.

[0013] Aspect 8: The method of aspect 1, wherein each of the at least one first stage membrane module comprises a polymeric membrane having a first capacity, and each of the at least one second stage membrane module and each of the at least one third stage membrane module comprise a polymeric membrane having a capacity greater than the first capacity.

[0014] Aspect 9: The method of aspect 1, wherein each of the at least one first stage membrane modules comprises a polymeric membrane having a first selectivity, and each of the at least one second stage membrane module and each of the at least one third stage membrane module comprise a polymeric membrane having a selectivity less than the first selectivity.

[0015] Aspect 10: The method of aspect 1, wherein step (j) comprises combining the third non-permeate stream, the water vapor permeate stream, and the first portion of the sweep gas stream to form a recycle stream.

[0016] Aspect 11: 2. The method of embodiment 1, further comprising: (k) controlling a flow split between the first portion of the sweep gas stream and the sweep gas stream.

[0017] Embodiment 12: The method of embodiment 1, wherein the second non-permeate stream comprises CO2 in the range of about 0.5% to about 5%.

[0018] Aspect 13: A system comprising: (a) a compressor for compressing a feed gas stream to form a pressurized feed gas stream, the pressurized feed gas stream comprising CH4 and being fully saturated with water; (b) a cooling device for cooling the pressurized feed gas stream to a target operating temperature of 0-20° C. to form a condensate stream and a saturated gas stream; (c) at least one membrane dryer, each of the at least one membrane dryer having a high pressure side and a low pressure side, the high pressure side extending from a feed port to a non-permeate port, and the low pressure side in fluid flow communication with a sweep port and a permeate port, the at least one membrane dryer being provided for separating the saturated gas stream to form a dry gas stream and a water vapor stream, and for sweeping and discharging the water vapor stream from the low pressure side using the sweep gas stream, the dry gas stream having a water content of less than 2000 ppm; (d) a first separation stage for separating the dry gas stream into a first non-permeate stream and a first permeate stream, the first non-permeate stream having a higher percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module; (e) a second separation stage for separating the first non-permeate stream into a second non-permeate stream and a sweep gas stream, the second non-permeate stream having a higher percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module; (f) a third separation stage for separating the first permeate stream into a third non-permeate stream and a third permeate stream, the third non-permeate stream having a higher percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; (g) a first mixing junction for combining the third non-permeate stream with the water vapor stream to form a recycle stream that is combined with the feed gas stream.

[0019] Example 14: The system of Example 13, additionally comprising a second mixing junction for optionally combining the first portion of the sweep gas stream with a third non-permeate stream.

[0020] Aspect 15: A system as described in aspect 13, wherein the target operating temperature is 3 to 7°C.

[0021] Aspect 16: The system of aspect 13, wherein the target operating temperature is approximately 5°C.

[0022]

[0041] Aspect 17: The system of aspect 13, wherein the feed gas stream comprises at least 40% CH4.

[0023] Embodiment 18: The system of embodiment 13, wherein the second non-permeate stream comprises CO2 in the range of about 0.5% to about 5%.

[0024] Aspect 19: The system of aspect 13, wherein the sweep gas stream is supplied in a flow direction countercurrent to a flow direction in which the saturated gas stream is supplied to the at least one membrane dryer. [Brief description of the drawings]

[0025] The present invention is hereinafter described in conjunction with the accompanying drawings, in which like numerals refer to like elements and in which:

[0026] [Figure 1]1 is a process flow diagram of an exemplary three-stage biogas separation system.

[0027] [Diagram 2] FIG. 2 is a cross-sectional view illustrating the internal structure of an exemplary membrane drying stage.

[0028] [Diagram 3] 2 is a graph illustrating the performance characteristics of the exemplary three-stage biogas separation system of FIG. 1 and comparing it with two biogas separation systems of standard design.

[0029] [Figure 4] 2 is a graph showing the relative membrane costs, relative system capital costs, and relative estimated 10-year ownership costs of the exemplary three-stage biogas separation system of FIG. 1 compared to two biogas separation systems of standard design. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The following detailed description provides only preferred exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the present invention. Rather, the following detailed description of preferred exemplary embodiments provides those skilled in the art with an enabling description for practicing preferred exemplary embodiments of the present invention. It will be understood that various changes can be made in the function and arrangement of elements without departing from the spirit and scope of the present invention.

[0031] To aid in the description of the invention, directional terms may be used in the specification and claims to describe portions of the invention (e.g., top, bottom, left, right, etc.). These directional terms are intended merely to aid in the description and claims of the invention and are not intended to limit the invention in any manner. In addition, reference numbers introduced herein in connection with a drawing may be repeated in one or more subsequent drawings without further description herein to provide context for other features.

[0032] 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 the method steps and are not intended to dictate the order in which the claimed steps are performed, unless and only if such order is specifically recited in the claims.

[0033] Unless otherwise indicated, as used herein, the articles "a" and "an" mean one or more when applied to any feature in the embodiments of the invention described herein and claimed. The use of "a" and "an" does not limit the meaning to a single feature unless such a limitation is specifically stated. The article "the" preceding a singular or plural noun or noun phrase indicates the particular named feature or features and may have the connotation of singular or plural depending on the context in which it is used.

[0034] Unless otherwise stated herein, any and all percentages specified in the specification, drawings, and claims should be understood to be on a mole percentage basis. Unless otherwise stated herein, any and all pressures specified in the specification, drawings, and claims should be understood to mean gauge pressure.

[0035] As used herein and in the claims, the term "biogas" means a renewable fuel produced by the decomposition of organic material, including but not limited to food scraps and animal waste.

[0036] As used herein and in the claims, the term "membrane module" refers to a device used to selectively separate gases by flowing a feed gas at a relatively high pressure through one or more conduits contained within a shell (also referred to as the high pressure side). The conduits are defined at least in part 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 an internal volume within the shell and outside each of the membranes that is maintained at a relatively low pressure. The shell side is in fluid flow communication with a permeate port through which gases that permeate the membrane exit the shell. Optionally, a sweep port may also be provided that supplies a sweep gas to the shell space and aids in the flow of the permeate gas through the permeate port. The membrane material is selected to allow one or more gases in the feed stream to pass through the membrane material at a higher rate than other gases in the feed stream. The membrane module may be of a bore-side feed design, in which the membrane module is pressurized by the introduction of a feed gas stream into its bore side, or may be of a shell-side feed design, in which the membrane module is pressurized by the introduction of a feed gas stream into the shell side.

[0037] As used herein and in the claims, the term "sweep gas" means a stream of gas supplied to the low pressure side of a membrane to dilute the concentration of the fast permeate gas in the permeate stream, reducing its partial pressure and increasing the driving force for separation.

[0038] As used herein and in the claims, the term "off-gas" means gases that are released, especially gases that are released as a by-product of a chemical process.

[0039] FIG. 1 shows an exemplary embodiment of a three-stage membrane separation system 100, including a feed compressor 103, a chiller 105, a membrane drying stage 107, and three membrane separation stages 110, 113, and 120. The feed gas stream 101 typically contains carbon dioxide (CO) and methane (CH) as major components, often with other minor components such as oxygen (O), nitrogen (N), and water (H). The membranes used in the system 100 are typically selective for CO over CH, meaning that CO is considered a fast permeable gas that preferentially crosses the membrane at a relatively high rate, while CH is a slow permeable gas that crosses the membrane at a relatively low rate. Gas permeability through the membrane is governed by a solution diffusion transport mechanism, with the permeation rate being a function of the molecular size (diffusivity) and the molecular solubility in the polymer, and proportional to the driving force. The driving force for gas separation is the partial pressure difference of the permeant species between the high pressure and low pressure sides of the membrane. The gas permeability of each gaseous component of a gas mixture is typically different, and the partial pressure of each gaseous component is proportional to its relative concentration in the gas mixture, as well as the total pressure of the gas mixture.

[0040] In the system 100, the recycle stream 125 is mixed into the feed gas stream 101 at the mixing junction 122 to form the combined feed stream 102. The feed gas stream may be or may include a biogas stream and may be obtained from a number of sources, such as a raw biogas stream obtained from a fermentation process. The feed gas stream may also be obtained from a landfill. A typical biogas feed gas stream contains about 40-70% CH4, about 30-60% CO2, is nearly or completely saturated with water vapor, and contains low concentrations of hydrogen sulfide (0-5000 ppm), nitrogen (0-20%), and oxygen (0-5%). The combined feed stream 102 is compressed in the feed compressor 103 to a pressure in the range of about 7-35 Barg, resulting in the pressurized combined feed stream 104. For example, the combined feed stream 102 may be compressed to a pressure of about 14 Barg.

[0041] The pressurized combined feed stream 104 is fed to a chiller 105 which reduces the temperature of the pressurized combined feed stream 104 to a target operating temperature. The target operating temperature may be about 0-20° C. Alternatively, the target operating temperature may be about 3-7° C. or about 5° C. The chiller 105 may include its own cooling unit that cools a circulating chiller fluid or may be cooled by indirect heat exchange provided by a refrigerant. Reducing the temperature of the combined feed stream reduces the dew point at the same pressure by forming a condensed water stream 127, resulting in a cooled saturated gas stream 106. Reducing the feed temperature of the pressurized combined feed stream 104 also allows the membrane stages 110, 113, and 120 to operate at a more favorable selectivity. The condensed water stream 127 may be optionally collected and discharged to a condensate drain (not shown). The cooled saturated gas stream 106 is then fed to a membrane dryer stage 107 to remove water vapor from the cooled saturated gas stream 106 by selectively permeating water molecules through a polymer membrane located within the membrane dryer stage 107. A suitable membrane dryer can be one such as the Air Products Membrane Solutions PRISM® GreenDry Flex, which eliminates the need to heat the stream above its dew point.

[0042] 1 and 2, some of the structural details of an exemplary membrane dryer stage 107 are shown. The membrane dryer stage 107 includes a generally cylindrical shell or housing 126, a feed port 130 located at a first end of the shell 126, and a non-permeate port 134 located at a second end of the shell 126 opposite the first end. The membrane dryer stage 107 also includes a permeate port 138 and a sweep port 142 located along a sidewall of the shell 126. In FIG. 2, the sweep port 142 is shown as being located on the same side of the shell 126 as the permeate port 138. This location is merely exemplary, and the location of the sweep port 142 as shown in FIG. 2 should not be limiting. The sweep port 142 may be located at other locations on the shell 126. For example, the sweep port 142 may be located on the side of the shell 126 opposite the permeate port 138, for example to the right of the number 126 in FIG. 2, to improve sweep efficiency. Also, the length of the shell 126 as shown in FIG. 2 is merely exemplary and should not be limiting. The shell may be provided in a length shorter than the length of the membrane stages 110, 113, and 120. The membrane dryer stage 107 includes selectively permeable membrane fibers 144 housed within the shell 126 that act as a molecular filter such that water vapor in the cooled saturated gas stream 106 passes through the membrane faster than other gas molecules and exits the membrane dryer stage through permeate port 138 as water vapor stream 109. The dryer stage 107 must be highly selective for water in the cooled saturated gas stream 106 over other gas components. As best shown in FIG. 2, the membrane fibers 144 may consist of a bundle of hollow fibers with the ends encapsulated in epoxy or other resin. The bundle of membrane fibers 144 is encapsulated within the shell 126, which protects the membrane and properly directs the gas.

[0043] As the cooled saturated gas stream 106 enters the feed port 130 and flows along the high pressure side (or "feed" side) of the membrane dryer stage 107, water vapor permeates through the walls of the membrane fibers 144 to the low pressure side (or "permeate" side) and exits the membrane dryer stage 107 through the permeate port 138 as water vapor stream 109. The water vapor content of the cooled saturated gas stream 106 becomes progressively lower as the cooled saturated gas stream 106 flows along the length of the membrane dryer stage 107 from the feed port 130 to the non-permeate port 134. A low pressure sweep gas stream 119 (discussed below) may be injected into the sweep port 142 to sweep the permeate side of the membrane dryer stage 107 to increase the driving force for water permeation. At the opposite end, the dry gas stream 108 exits the membrane dryer stage 107 through the non-permeate port 134. The drying gas stream 108 may preferably have a water content in the range of about 200-500 ppm, but may also have a water content of approximately 1000 ppm, or even approximately 2000 ppm. The effect of the membrane dryer stage 107 on the composition of the drying gas stream 108 is minimal, e.g., a change of less than 0.5 mole % of CO2 is expected.

[0044] The dry gas stream 108 is fed to the first membrane stage 110. The stream containing mainly fast permeate gases permeates and exits the first membrane stage 110 as the first stage permeate stream 112. The stream containing mainly slow permeate gases is rejected and removed as the first stage non-permeate stream 111. In the case of biogas, CO2 permeates much faster than CH4. The first membrane stage 110 may contain membranes that exhibit high selectivity but low capacity (capacity is defined as the product of membrane permeance and active membrane area). Studies have confirmed that there is a trade-off behavior between permeability and selectivity (LM Robeson, J. Membr. Sci., 320, 390-400, 2008), which has been quantified by studying a large number of polymeric membranes. It is generally known in the art that both properties cannot be high simultaneously in a polymeric membrane. For example, the first membrane stage may include a membrane using a polymer with a CO2 permeability of 0.5-3 Barrels at the target operating temperature and a CO2 / CH4 selectivity in the range of 30-60 (membrane type "A"). The first membrane stage 110 may include one or more membrane modules in which multiple membranes are arranged in series or parallel.

[0045] The first stage non-permeate stream 111 is fed to a second membrane stage 113. The second membrane stage 113 may include a membrane that exhibits high capacity but low selectivity when used at standard operating temperatures, such as 30° C. For example, the second membrane stage may include a gas separation membrane using a polymer with a CO2 permeability of 5-50 barr and a CO2 / CH4 selectivity in the range of 20-30 at standard warmer operating conditions (membrane type "B"). A stream containing primarily slow permeate gases may be rejected and removed as a second stage non-permeate stream 114. The second stage non-permeate stream 114 may be removed from the system 100 using a pressure control valve 115 as a final product stream. A stream containing primarily fast permeate gases permeates through and exits the second membrane stage 113 as a second stage permeate stream 116. A portion of the second stage permeate stream 116 may be returned to the membrane dryer stage 107 and utilized as a low pressure sweep gas stream 119. A control valve 117 may be provided to control the flow of the low pressure sweep gas stream 119 to the sweep port 142. The remaining portion of the second stage permeate stream 118 may be returned to a point upstream of the feed compressor 103 and recirculated through the system. The second membrane stage 113 may include one or more membrane modules in which multiple membranes are arranged in series or parallel.

[0046] The first stage permeate stream 112 may be fed to a third membrane stage 120. Since the third membrane stage 120 serves as the final step for collecting methane, it is important to include a membrane that is highly selective in this stage. In systems of conventional design, such highly selective membranes also typically have low capacity, thus requiring a large membrane count in the third membrane stage 120. However, in an exemplary embodiment of the present invention, a membrane type that exhibits high capacity, typically low selectivity, and low CO2 activation energy due to the lower operating temperature may be replaced with a membrane that exhibits high selectivity but low capacity. For example, the third membrane stage 120 may include a gas separation membrane of membrane type "B" having a CO2 activation energy in the range of about 1000-3000 cal / mol and a CH4 activation energy of about 5000 cal / mol. This membrane type responds well to lower operating temperatures to achieve higher selectivity while substantially maintaining capacity as if it were operating at a higher temperature. This favorable response to low temperatures can only occur when the CO2 activation energy is low and the CH4 activation energy is high. Thus, the number and cost of membranes utilized in the third membrane stage 120 can be substantially reduced. Furthermore, the recycle / feed ratio can be substantially reduced, thus reducing the flow rate of the recycle stream 125 and increasing the overall process efficiency. A stream containing primarily fast permeate gases permeates the membrane and exits the third membrane stage 120 as a third stage permeate stream 121, which can be a vented stream or a waste stream. A stream containing primarily slow permeate gases can be removed as a third stage non-permeate stream 129. The third stage non-permeate stream 129 exits the third membrane stage 120 via a pressure control valve 123 and is combined with the remaining portion of the second stage permeate stream 118 and the water vapor stream 109 into a single recycle stream 125 that can be returned to a point upstream of the feed compressor 103 and recirculated through the system 100.Because the third membrane stage 120 is fed with the first stage permeate stream 112 containing 90-95 mol % CO2, the third stage non-permeate stream 129 has a higher concentration of CO2 than CH4. The third membrane stage 120 may contain one or more membranes, with multiple membranes arranged in series or parallel.

[0047] Referring now to FIG. 3 and Table 1, FIG. 3 is a graph showing modeled performance curves for two "standard designs" (identified as "Standard Design A" and "Standard Design B") biogas separation systems (i.e., no membrane dryer stage) operating at 20° C., and a biogas separation system 100 of the present invention ("New Design") including a membrane dryer stage 107 and operating at 5° C. Standard Design A is a conventionally designed system including a first membrane stage with a type "A" (high selectivity / low capacity) membrane, a second membrane stage with a type "B" (low selectivity / high capacity) membrane, and a third membrane stage having a type "A" (high selectivity / low capacity) membrane. Standard Design B is also of conventional design with all three membrane stages including type "A" (high selectivity / low capacity) membranes. All of the simulations were performed using the following operating parameters: feed gas composition of 60% CH4 and 40% CO2 on a dry basis saturated with water vapor, the feed gas stream is compressed to a pressure of 14 Barg by the feed compressor 103, and the feed gas stream is fed at a flow rate of 1000 normal cubic meters per hour (ncmh). The target CH4 recovery for the system is 99.5%, where CH4 recovery is defined as the amount of methane in the product stream (in system 100, the second stage non-permeate stream 114) divided by the amount of methane in the feed gas stream (in system 100, the raw feed gas stream 101). Each of the simulated systems includes two curves on the graph. The first curve for each simulated system shows the membrane cost factor. The membrane cost factor of standard design A has been normalized to a value of 1.00 to allow comparison with the membrane cost factor of standard design B and the membrane cost factor of the biogas separation system 100 of the present invention. Figure 3 demonstrates that the new design achieves a CH4 target product recovery of 99.5% while lowering the relative membrane cost for most of the CO2 percentage in the product gas stream compared to standard design A and for all of the CO2 percentage in the product gas stream compared to standard design B. The membrane costs are also listed in Table 1 below.

[0048] The second curve for each simulated system is the applicable recycle / feed ratio (recycle stream 125 to feed gas stream 101) as a function of product gas purity (e.g., CO2% in the non-permeate stream 114 of the second stage). As shown in FIG. 3 and Table 1 below, by lowering the target operating temperature, the new design achieves a CH4 target product recovery of 99.5% while reducing the recycle flow (reducing the compressor power required) when compared to the standard design systems, i.e., Standard Design A and Standard Design B. In addition, the lower operating temperature of the new design allows for increased use of high-capacity polymeric membranes with low CO2 activation energy in the membrane separation stage. When operated at lower temperatures, the high-capacity membranes gained selectivity and unexpectedly sacrificed little capacity. The recycle / feed ratio is also listed in Table 1.

[0049] Referring now to FIG. 4 and Table 1, membrane costs, system capital costs, and estimated 10-year cost of ownership are provided for Standard Design A, Standard Design B, and the new design for different CO2 percentages in the product gas stream (second stage non-permeate stream 114). These cost and expense values ​​have been normalized to a value of 1.00 for Standard Design A to allow comparison with Standard Design B and the performance of the biogas separation system 100. Normalization is achieved by dividing each value of the system 100 and Standard Design B by the corresponding value from the benchmark Standard Design A, and the values ​​of the new design and Standard Design B are expressed as a factor of the corresponding value of Standard Design A. FIG. 4 and Table 1 demonstrate that by lowering the target operating temperature, the new design achieves a CH4 target product recovery of 99.5% while lowering the membrane cost for most CO2 percentages in the product gas stream. Also, by lowering the target operating temperature, the new design system achieves a CH4 target product recovery of 99.5% while achieving a lower capital cost (including all equipment capital costs) when compared to either standard design system. This is due to the reduced recycle / feed ratio without increasing membrane costs. FIG. 4 also shows the modeled 10-year ownership costs of an exemplary run of the new design system compared to two standard design systems with different CO2 percentages in the product gas stream (second stage non-permeate stream 114). As demonstrated in FIG. 4, the new design system offers a substantially lower expected cost of ownership than either of the standard design systems over the system's 10-year expected life. As energy costs continue to rise, the biogas separation system of the present invention is expected to further reduce relative operating costs.

[0050] [Table 1]

[0051] The present invention is disclosed herein with respect to preferred and alternative embodiments thereof. Naturally, various changes, modifications, and variations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. The present invention is intended to be limited only by the terms of the appended claims.

Claims

1. 1. A method comprising: (a) compressing a feed gas stream to form a pressurized feed gas stream, the pressurized feed gas stream comprising CH4 and fully saturated with water; (b) cooling the pressurized feed gas stream in a cooling device to a target operating temperature to form a condensate stream and a saturated gas stream; (c) feeding the saturated gas stream to at least one membrane dryer stage, each of the at least one membrane dryer stage having a high pressure side and a low pressure side, the high pressure side extending from a feed port to a non-permeate port, the low pressure side being in fluid flow communication with a sweep port and a permeate port; (d) separating the saturated gas stream in the at least one membrane drying stage to form a water vapor permeate stream and a dry gas stream, the dry gas stream having a water content of less than 2000 ppm; (e) sweeping and exhausting the water vapor permeate stream from the low pressure side using a sweep gas stream supplied in a countercurrent flow direction relative to the flow direction of the saturated gas stream supplied to the at least one membrane drying stage in step (c); (f) separating the dry gas stream into a first non-permeate stream and a first permeate stream in a first separation stage, the first non-permeate stream having a higher percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module; (g) separating the first non-permeate stream into a second non-permeate stream and a sweep gas stream in a second separation stage, the second non-permeate stream having a higher percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module; (h) separating the first permeate stream into a third non-permeate stream and a third permeate stream in a third separation stage, the third non-permeate stream having a higher percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; (i) combining the third non-permeate stream, the water vapor permeate stream, and optionally a first portion of the sweep gas stream to form a recycle stream; (j) combining the recycle stream with the feed gas stream prior to carrying out step (a); The method, wherein the target operating temperature is between 0 and 20°C.

2. The method of claim 1 , wherein the target operating temperature is between 3 and 7° C.

3. The method of claim 1 , wherein the target operating temperature is approximately 5° C.

4. The method of claim 1 , wherein the feed gas stream comprises at least 40% CH4.

5. 2. The method of claim 1, wherein each of the at least one first stage membrane module has a first stage CO2 / CH4 selectivity that is greater than a second stage CO2 / CH4 selectivity of each of the at least one second stage membrane module and a third stage CO2 / CH4 selectivity of each of the at least one third stage membrane module.

6. 2. The method of claim 1, wherein each of the at least one first stage membrane module has a first stage CO2 / CH4 selectivity of greater than 30 at the target operating temperature.

7. 2. The method of claim 1, wherein each of the at least one second stage membrane module has a CO2 / CH4 selectivity greater than 20 at the target operating temperature.

8. 2. The method of claim 1, wherein each of the at least one first stage membrane module comprises a polymeric membrane having a first capacity, and each of the at least one second stage membrane module and each of the at least one third stage membrane module comprise a polymeric membrane having a capacity greater than the first capacity.

9. 2. The method of claim 1, wherein each of the at least one first stage membrane module comprises a polymeric membrane having a first selectivity, and each of the at least one second stage membrane module and each of the at least one third stage membrane module comprise a polymeric membrane having a selectivity less than the first selectivity.

10. 2. The method of claim 1, wherein step (j) comprises combining the third non-permeate stream, the water vapor permeate stream, and the first portion of the sweep gas stream to form a recycle stream.

11. 10. The method of claim 1, further comprising: (k) controlling a flow split between the first portion of the sweep gas stream and the sweep gas stream.

12. The method of claim 1 , wherein the second non-permeate stream comprises in the range of about 0.5% to about 5% CO2.

13. 1. A system comprising: (a) a compressor for compressing a feed gas stream to form a pressurized feed gas stream, the pressurized feed gas stream comprising CH4 and fully saturated with water; (b) a cooling device for cooling the pressurized feed gas stream to a target operating temperature of 0-20° C. to form a condensate stream and a saturated gas stream; (c) at least one membrane dryer, each of the at least one membrane dryer having a high pressure side and a low pressure side, the high pressure side extending from a feed port to a non-permeate port, the low pressure side being in fluid flow communication with a sweep port and a permeate port, the at least one membrane dryer being configured to separate the saturated gas stream to form a dry gas stream and a water vapor stream, and to sweep and discharge the water vapor stream from the low pressure side using a sweep gas stream, the dry gas stream having a water content of less than 2000 ppm; (d) a first separation stage for separating the dry gas stream into a first non-permeate stream and a first permeate stream, the first non-permeate stream having a higher percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module; (e) a second separation stage for separating the first non-permeate stream into a second non-permeate stream and the sweep gas stream, the second non-permeate stream having a higher percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module; (f) a third separation stage for separating the first permeate stream into a third non-permeate stream and a third permeate stream, the third non-permeate stream having a higher percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; (g) a first mixing junction for combining the third non-permeate stream with the water vapor stream to form a recycle stream that is combined with the feed gas stream.

14. 14. The system of claim 13, further comprising a second mixing junction for optionally combining a first portion of the sweep gas stream with the third non-permeate stream.

15. The system of claim 13, wherein the target operating temperature is between 3 and 7 degrees Celsius.

16. The system of claim 13 , wherein the target operating temperature is approximately 5° C.

17. 14. The system of claim 13, wherein the feed gas stream comprises at least 40% CH4.

18. 14. The system of claim 13, wherein the second non-permeate stream comprises in the range of about 0.5% to about 5% CO2.

19. 14. The system of claim 13, wherein the sweep gas stream is supplied in a countercurrent flow direction relative to a flow direction in which the saturated gas stream is supplied to the at least one membrane dryer.

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