Low-temperature membrane method for biogas purification

Abstract

A method for separating an unprocessed feed gas stream utilizes a compressor, a deep freezer, a membrane drying stage, and a plurality of membrane separation stages. The unprocessed feed gas stream may include biogas. In one example, the stream of unprocessed feed gas is supplied to the deep freezer, in which the stream of unprocessed feed gas is cooled to a target operating temperature to separate condensed water. The gas stream is then supplied to a membrane drying stage to separate water vapor. The off-gas from one of the membrane module stages may be recycled as a low pressure purge gas at the low pressure side of the membrane drying stage to increase the driving force for water penetration within the membrane drying stage. The gas stream is then supplied to the plurality of membrane separation stages in which the gas stream is purified to a high purity methane stream.

Description

Technical Field

[0001] The present application relates to a multi-stage membrane process and system for recovering methane from biogas. Background Art

[0002] Multi-stage membrane systems are well-known for purifying raw biogas streams into high-purity methane streams. Current pre-purification systems produce pressurized feed gas streams by supplying gas streams to compressors. The compressed gas stream is fed to subsequent membrane separation stages, in which the compressed gas stream is separated into a non-permeate stream rich in slow gas and a permeate stream rich in fast gas, to recycle a final product stream with a high concentration of slow non-permeate gas such as methane. Membrane systems are typically operated at ambient temperature or warmer temperatures, and ambient temperature or higher temperatures are selected to provide a certain degree of superheat to prevent condensation on the membrane material. Such multi-stage membrane systems can achieve high methane recovery and purity, but generally require relatively high capital expenditures (CapEx) associated with high membrane area (or number), and high operating expenses (OpEx) associated with high compressor power costs. In addition, since high recirculation flow rates require larger and more expensive compressors, the operating expenses for such current purification systems may be relatively high.

[0003] Therefore, there is a need for an efficient, reliable and cost-effective multi-stage membrane process and system that can achieve the desired methane product recovery and purity while reducing overall capital expenditures and operating expenses. Summary of the invention

[0004] This summary is provided to introduce in a simplified form a selection of concepts that are further described in the detailed description below. 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. Several aspects of the system and method are summarized as follows.

[0005] The disclosed embodiments meet the needs in the art by providing a multi-stage membrane method and system for recovering methane from biogas, which reduces the total cost of ownership and operating costs. It has been found that a system of polymer membranes with high carbon dioxide ("CO2") permeability, low CO2 activation energy, and high methane ("CH4") activation energy can be operated at low temperatures to utilize higher CO2 / CH4 selectivity while maintaining high CO2 permeability, which can replace some high selectivity membranes that typically have low CO2 permeability. In this way, the recycle flow is reduced (which reduces the required compressor power) and the number of membranes is reduced. In addition, in order to protect the separation membrane from liquid condensation, a cryogenic refrigerator and a membrane drying stage are provided upstream of the separation membrane. The membrane drying stage can be purged with a portion of the recycle gas from one of the separation membranes. This design flexibility is not available in the current separation membrane systems with the lowest temperature sensitivity.

[0006] Several aspects of the systems and methods are summarized below.

[0007] Aspect 1: A method comprising:

[0008] (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;

[0009] (b) cooling the pressurized feed gas stream to a target operating temperature in a cryocooler to form a condensed water stream and a saturated gas stream;

[0010] (c) feeding the saturated gas stream to at least one membrane drying stage, each of the at least one membrane drying 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 purge port and a permeate port;

[0011] (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 2,000 ppm;

[0012] (e) purging and discharging a water vapor permeate stream from the low pressure side using a purge gas stream fed in a flow direction opposite to the flow direction of the saturated gas stream fed to the at least one membrane drying stage in step (c);

[0013] (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 greater percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module;

[0014] (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 greater percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module;

[0015] (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 greater percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module;

[0016] (i) combining the third non-permeate stream with the water vapor permeate stream and optionally a first portion of the purge gas stream to form a recycle stream; and

[0017] (j) combining the recycle stream with the feed gas stream prior to carrying out step (a);

[0018] The target operating temperature is between 0°C and 20°C.

[0019] Aspect 2: The method according to aspect 1, wherein the target operating temperature is between 3°C and 7°C.

[0020] Aspect 3: The method according to aspect 1, wherein the target operating temperature is about 5°C.

[0021] Aspect 4: The method according to aspect 1, wherein the feed gas stream contains at least 40% CH4.

[0022] Aspect 5: A method according to Aspect 1, wherein the first-stage CO2 / CH4 selectivity of each first-stage membrane component in the at least one first-stage membrane component is greater than the second-stage CO2 / CH4 selectivity of each second-stage membrane component in the at least one second-stage membrane component and the third-stage CO2 / CH4 selectivity of each third-stage membrane component in the at least one third-stage membrane component.

[0023] Aspect 6: The method according to aspect 1, wherein the first-stage CO2 / CH4 selectivity of each of the at least one first-stage membrane module is greater than 30 at the target operating temperature.

[0024] Aspect 7: The method according to Aspect 1, wherein the CO2 / CH4 selectivity of each of the at least one second-stage membrane module is greater than 20 at the target operating temperature.

[0025] Aspect 8: A method according to Aspect 1, wherein each of the at least one first-stage membrane components comprises a polymer membrane having a first capacity, and wherein each of the at least one second-stage membrane components and each of the at least one third-stage membrane components comprises a polymer membrane having a capacity greater than the first capacity.

[0026] Aspect 9: A method according to Aspect 1, wherein each of the at least one first-stage membrane components comprises a polymer membrane having a first selectivity, and wherein each of the at least one second-stage membrane components and each of the at least one third-stage membrane components comprises a polymer membrane having a selectivity greater than the first selectivity.

[0027] Aspect 10: The method according to 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.

[0028] Aspect 11: The method according to aspect 1, further comprising:

[0029] (k) controlling the split between the first portion of the purge gas flow and the purge gas flow.

[0030] Aspect 12: The method according to aspect 1, wherein the second non-permeate stream contains CO2 in a range between about 0.5% and about 5%.

[0031] Aspect 13: A system comprising:

[0032] (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;

[0033] (b) a cryocooler for cooling the pressurized feed gas stream to a target operating temperature between 0° C. and 20° C. to form a condensed water stream and a saturated gas stream;

[0034] (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 the feed port to the non-permeate port, the low pressure side being in fluid flow communication with the purge port and the 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 purge and discharge the water vapor stream from the low pressure side using the purge gas stream, the water content of the dry gas stream being less than 2000 ppm;

[0035] (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 greater percentage of methane than the first permeate stream, the first separation stage comprising at least one first stage membrane module;

[0036] (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 greater percentage of methane than the sweep gas stream, the second separation stage comprising at least one second stage membrane module;

[0037] (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 greater percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; and

[0038] (g) a first mixing junction for combining the third non-permeate stream with the water vapor stream to form a recycle stream to be combined with the feed gas stream.

[0039] Aspect 14: The system according to aspect 13, further comprising a second mixing joint for optionally combining the first portion of the purge gas stream with the third non-permeate stream.

[0040] Aspect 15: The system according to aspect 13, wherein the target operating temperature is between 3°C and 7°C.

[0041] Aspect 16: The system of aspect 13, wherein the target operating temperature is approximately 5°C.

[0042] Aspect 17: The system according to Aspect 13, wherein the feed gas stream comprises at least 40% CH4.

[0043] Aspect 18: The system of aspect 13, wherein the second non-permeate stream comprises CO2 in a range between about 0.5% and about 5%.

[0044] Aspect 19: The system according to aspect 13, wherein the purge gas stream is fed in a flow direction opposite to the flow direction of the saturated gas stream fed into the at least one membrane dryer. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention will hereinafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements.

[0046] Figure 1 is a process flow diagram of an exemplary three-stage biogas separation system;

[0047] Figure 2 is a cross-sectional view illustrating the internal structure of an exemplary membrane drying stage;

[0048] Figure 3 It is an explanation Figure 1 and a graph showing performance characteristics of an exemplary three-stage biogas separation system of FIG. 1 and comparing it to two biogas separation systems of standard design; and

[0049] Figure 4 Compared with two biogas separation systems of standard design, Figure 1 Graphs of relative membrane costs, relative system capital expenses, and relative estimated 10-year cost of ownership for an exemplary three-stage biogas separation system. DETAILED DESCRIPTION

[0050] The detailed description that follows only provides preferred exemplary embodiments and is not intended to limit the scope, applicability or configuration of the present invention. On the contrary, the detailed description that follows of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the present invention. It should be understood that various changes may be made to the functions and arrangements of the elements without departing from the spirit and scope of the present invention.

[0051] To help describe the invention, directional terms may be used in the specification and claims to describe parts of the invention (e.g., top, bottom, left, right, etc.). These directional terms are intended only to help describe and claim the invention and are not intended to limit the invention in any way. In addition, reference numerals associated with figures of the accompanying drawings introduced in the specification may be repeated in one or more subsequent figures without additional description in the specification to provide context for other features.

[0052] In the claims, letters are used to identify claimed steps (e.g., (a), (b), and (c). These letters are used to aid in referencing method steps and are not intended to indicate the order in which the claimed steps may be performed, unless and only to the extent such an order is specifically recited in the claims.

[0053] Unless otherwise indicated, the articles "a / an" as used herein mean one or more when applied to any feature of the embodiments of the present invention described in the specification and claims. The use of "a / an" does not limit the meaning to a single feature unless such limitation is specifically stated. The article "the / said" preceding a singular or plural noun or noun phrase means one or more specific specified features and can have a singular or plural meaning depending on the context in which it is used.

[0054] Unless otherwise indicated herein, any and all percentages identified in the specification, drawings, and claims should be understood to be on a mole percentage basis. Unless otherwise indicated herein, any and all pressures identified in the specification, drawings, and claims should be understood to mean gauge pressures.

[0055] The term "biogas" as used in the specification and claims refers to a renewable fuel produced by the decomposition of organic matter, including but not limited to food waste and animal manure.

[0056] The term "membrane module" used in the specification and claims refers to a device for selectively separating gases by allowing the feed gas to flow through one or more conduits (also referred to as the high pressure side) contained in the 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 inside the shell and outside each membrane, which is maintained at a relatively low pressure. The shell side is connected to the permeate port fluid flow, and the gas that permeates the membrane leaves the shell through the permeate port. Optionally, a purge port can also be provided, which supplies a purge gas to the shell space and helps the permeate gas flow through the permeate port. The membrane material is selected so that one or more gases in the feed stream can pass through the membrane material at a higher rate than other gases in the feed stream. The membrane module can be a hole-side feed design, wherein the membrane module is pressurized by introducing the feed gas flow into its hole side, or it can be a shell-side feed design, wherein the membrane module is pressurized by introducing the feed gas flow into its shell side.

[0057] The term "sweep gas" as used in the specification and claims means a gas stream that is supplied to the low pressure side of the membrane and dilutes the concentration of fast permeating gas in the permeate stream, lowering its partial pressure and increasing the driving force for separation.

[0058] The term "waste gas" as used in the specification and claims means evolved gases, especially gases emitted as a by-product of a chemical process.

[0059] Figure 1 An exemplary embodiment of a three-stage membrane separation system 100 is shown, which includes a feed compressor 103, a cryogenic refrigerator 105, a membrane drying stage 107 and three membrane separation stages 110, 113 and 120. The raw feed gas stream 101 generally contains carbon dioxide (CO2) and methane (CH4) as main components, and generally has other minor components, such as oxygen (O2), nitrogen (N2) and water (H2O). The membrane used in the system 100 is generally selective for CO2 to CH4, which means that CO2 is considered to be a fast permeating gas that preferentially passes through the membrane at a relatively high rate, while CH4 is a slow permeating gas that passes through the membrane at a relatively low rate. The gas permeability through the membrane depends on the solution-diffusion migration mechanism, in which the permeation rate is a function of the molecular size (diffusivity) and the molecular solubility in the polymer, and is proportional to the driving force. The driving force for gas separation is the partial pressure difference between the high-pressure side and the low-pressure side of the permeating substance. The gas permeability of each gas component of a gas mixture will generally be different, and the partial pressure of each gas component will be proportional to its relative concentration in the gas mixture and the total pressure of the gas mixture.

[0060] In the system 100, the recycle stream 125 is mixed into the raw feed gas stream 101 at the mixing joint 122 to form a combined feed stream 102. The raw feed gas stream can be or contain a biogas stream and can be obtained from a variety of sources, such as a raw biogas stream produced by a fermentation process. The raw feed gas stream can also be obtained from a landfill. A typical biogas raw feed gas stream will contain about 40% to 70% CH4, about 30% to 60% CO2, will be almost or completely saturated with water vapor, and will contain low concentrations of hydrogen sulfide (0 ppm to 5000 ppm), nitrogen (0% to 20%) and oxygen (0% to 5%). The combined feed stream 102 is compressed to a pressure in the range of about 7 bar to 35 bar in the feed compressor 103 to produce a pressurized, combined feed stream 104. For example, the combined feed stream 102 can be compressed to a pressure of about 14 bar.

[0061] The pressurized, combined feed stream 104 is fed into a cryogenic refrigerator 105, which reduces the temperature of the pressurized, combined feed stream 104 to a target operating temperature. The target operating temperature may be between about 0°C and 20°C. Alternatively, the target operating temperature may be between about 3°C ​​and 7°C or about 5°C. The cryogenic refrigerator 105 may include its own refrigeration unit that cools the circulating cryogenic refrigerator 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, thereby producing a cryogenically cooled saturated gas stream 106. Reducing the feed temperature of the pressurized, combined feed stream 104 also enables the membrane stages 110, 113, and 120 to operate with more favorable selectivity. The condensed water stream 127 may optionally be collected and discharged to a condensed water discharge pipe (not shown). The cryogenically cooled saturated gas stream 106 is then supplied to a membrane drying stage 107 which removes water vapor from the cryogenically cooled saturated gas stream 106 by selective permeation of water molecules through a polymer membrane located within the membrane drying stage 107. A suitable membrane dryer may be a membrane dryer such as that of Air Products Membrane Solutions. GreenDry Flex, which eliminates the need to heat the stream above its dew point.

[0062] Reference now Figure 1 and Figure 2 , some structural details of an exemplary membrane drying stage 107 are shown. The membrane drying 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 drying stage 107 also includes a permeate port 138 and a purge port 142 located along the sidewall of the shell 126. Figure 2 , the purge 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 is not intended to be construed as a Figure 2 The location of the purge port 142 shown should not be limiting. The purge port 142 can be positioned at other locations on the shell 126. For example, the purge port 142 can be positioned on the side of the shell 126 opposite the permeate port 138, for example, at Figure 2 to the right of the mark 126 in order to improve the purge efficiency. Figure 2 The length of the shell 126 shown is exemplary only and should not be limiting. The shell can be provided with a length less than the membrane stages 110, 113 and 120. The membrane drying stage 107 includes a selectively permeable membrane fiber 144 contained within the shell 126, which acts as a molecular filter, wherein water vapor in the cryogenically cooled saturated gas stream 106 passes through the membrane faster than other gas molecules and exits the membrane drying stage through the permeate port 138 as a water vapor stream 109. The drying stage 107 must have a higher selectivity for water than other gas components in the cryogenically cooled saturated gas stream 106. As shown in FIG. Figure 2 As best shown, the membrane fibers 144 may consist of bundles of hollow fibers whose ends are potted in epoxy or other resins. The membrane fiber bundles 144 are enclosed within a housing 126 that protects the membranes and properly routes the gases.

[0063] As the cryogenically cooled saturated gas stream 106 flows into the feed port 130 and flows along the high pressure side (or "feed" side) of the membrane drying 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 drying stage 107 through the permeate port 138 as a water vapor stream 109. As the cryogenically cooled saturated gas stream 106 flows along the length of the membrane drying stage 107 from the feed port 130 to the non-permeate port 134, the water vapor content of the cryogenically cooled saturated gas stream 106 gradually decreases. A low pressure purge gas stream 119 (discussed below) may be injected into the purge port 142 to purge the permeate side of the membrane drying stage 107 to increase the driving force for water permeation. At the opposite end, the dry gas stream 108 exits the non-permeate port 134 of the membrane drying stage 107. The water content of the drying gas stream 108 may preferably be in the range of about 200 ppm to 500 ppm, but the water content may also be about 1000 ppm, or even about 2000 ppm. The membrane drying stage 107 has minimal effect on the composition of the drying gas stream 108, for example, a change of less than 0.5 mol% of CO2 is expected.

[0064] The dry gas stream 108 is supplied to the first membrane stage 110. The stream containing mainly fast permeating gas permeates and leaves the first membrane stage 110 as the first stage permeate stream 112. The stream containing mainly slow permeating gas is rejected and taken out as the first stage non-permeate stream 111. In the case of biogas, CO2 will permeate much faster than CH4. The first membrane stage 110 may include a membrane that exhibits high selectivity but low capacity (wherein capacity is defined as the product of membrane permeability and active membrane area). Studies have determined that there is a behavioral trade-off between permeability and selectivity (LM Robeson, "Journal of Membrane Science (J. Membr. Sci.)" 320,390-400,2008), which has been quantified by studying many polymer membranes. It is generally known in the art that these two properties cannot be high at the same time in polymer membranes. For example, the first membrane stage may include a membrane (membrane type "A") using a polymer with a CO2 permeability between 0.5 and 3 bar and a CO2 / CH4 selectivity in the range of 30 to 60 at the target operating temperature. The first membrane stage 110 may include one or more membrane modules in which a plurality of membranes are arranged in series or in parallel.

[0065] The first stage non-permeate stream 111 is supplied to the second membrane stage 113. The second membrane stage 113 may include a membrane that exhibits high capacity but low selectivity when used at a standard operating temperature (such as 30° C.). For example, the second membrane stage may include a gas separation membrane (membrane type "B") using a polymer having a CO2 permeability between 5 bar and 50 bar and a CO2 / CH4 selectivity in the range of 20 to 30 under standard warmer operating conditions. A stream containing mainly slow permeating gases may be rejected and taken out as a second stage non-permeate stream 114. The second stage non-permeate stream 114 may be taken out of the system 100 as a final product stream using a pressure control valve 115. A stream containing mainly fast permeating gases permeates and leaves 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 drying stage 107 and used as a low pressure purge gas stream 119. A control valve 117 may be provided to control the flow of the low pressure purge gas stream 119 into the purge port 142. The remainder of the second stage permeate stream 118 may be returned to a point upstream of the feed compressor 103 for recirculation through the system. The second membrane stage 113 may include one or more membrane modules in which multiple membranes are arranged in series or in parallel.

[0066] The first stage permeate stream 112 can be supplied to the third membrane stage 120. The third membrane stage 120 is used as the last step for collecting methane, so it is important to include a highly selective membrane in this stage. In a conventionally designed system, such a highly selective membrane also generally has a low capacity, so a large number of membranes (the number of membranes) is required in the third membrane stage 120. However, in an exemplary embodiment of the present invention, due to a lower operating temperature, a membrane type that exhibits high capacity, generally low selectivity, and low CO2 activation energy can replace a membrane that exhibits high selectivity but low capacity. For example, the third membrane stage 120 can include a gas separation membrane of membrane type "B", whose CO2 activation energy is in the range of about 1000 cal / mol to 3000 cal / mol, and CH4 activation energy is about 5000 cal / mol. This membrane type responds well to lower operating temperatures to achieve higher selectivity while substantially maintaining capacity, as if 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. Therefore, the number and cost of the membranes used in the third membrane stage 120 can be greatly reduced. In addition, the recycle / feed ratio can be significantly reduced, thereby reducing the flow rate of the recycle stream 125 and improving the overall process efficiency. A stream containing primarily fast permeating gases permeates the membrane and leaves the third membrane stage 120 as a third stage permeate stream 121, which can be a discharge stream or a waste stream. A stream containing primarily slow permeating gases can be withdrawn as a third stage non-permeate stream 129. The third stage non-permeate stream 129 leaves the third membrane stage 120 via a pressure control valve 123 and can be combined with the remainder of the second stage permeate stream 118 and the water vapor stream 109 into a single recycle stream 125 and returned to a point upstream of the feed compressor 103 for recycle through the system 100. The CO2 concentration of the third stage non-permeate stream 129 is higher than that of CH4 because the third membrane stage 120 is fed with a first stage permeate stream 112 containing 90 mol% to 95 mol% CO2. The third membrane stage 120 can contain one or more membranes, wherein multiple membranes are arranged in series or in parallel.

[0067] Reference now Figure 3 and Table 1, Figure 3Graphs showing modeled performance curves for two "standard design" (identified as "standard design A" and "standard design B") biogas separation systems (i.e., without membrane drying stages) operating at 20°C and a biogas separation system 100 of the present invention ("new design") including a membrane drying stage 107 and operating at 5°C. Standard design A is a conventionally designed system including a first membrane stage including membranes of type "A" (high selectivity / low capacity), a second membrane stage including membranes of type "B" (low selectivity / high capacity), and a third membrane stage having membranes of type "A" (high selectivity / low capacity). Standard design B is also a conventional design in which all three membrane stages include membranes of type "A" (high selectivity / low capacity). All simulations were performed using the following operating parameters: a feed gas composition of 60% CH4 and 40% CO2 on a dry basis, which is saturated with water vapor, the feed gas stream is compressed to a pressure of 14 bar by the feed compressor 103, and the feed gas stream is supplied at a flow rate of 1000 standard cubic meters per hour (ncmh). The target CH4 recovery for the system is 99.5%, wherein the CH4 recovery is defined as the amount of methane in the product stream (in the system 100, the second stage non-permeate stream 114) divided by the amount of methane in the feed gas stream (in the system 100, the unprocessed feed gas stream 101). Each simulation system in the simulation system includes two curves on the graph. The first curve for each simulation system shows the membrane cost coefficient. The membrane cost coefficient for standard design A has been normalized to a value of 1.00 so that it can be compared with the membrane cost coefficient for standard design B and the membrane cost coefficient for the biogas separation system 100 of the present invention. Figure 3 It is shown that the new design achieves a target product recovery of 99.5% CH4 while reducing the relative membrane cost for most CO2 percentages in the product gas stream compared to the standard design A, and reducing the relative membrane cost for all CO2 percentages in the product gas stream compared to the standard configuration B. The membrane costs are also listed in Table 1 below.

[0068] The second curve for each simulated system is the applicable recycle / feed ratio (ratio of recycle stream 125 to feed gas stream 101) as a function of product gas purity (e.g., % CO2 in second stage non-permeate stream 114). Figure 3 As shown in Table 1 below, by lowering the target operating temperature, the new design achieves a 99.5% target product recovery of CH4 while reducing the recycle flow (which reduces the required compressor power) 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 the increased use of high-capacity polymer membranes with low CO2 activation energy in the membrane separation stage. When operating at lower temperatures, the high-capacity membranes gain selectivity and unexpectedly lose very little capacity. The recycle / feed ratio is also listed in Table 1.

[0069] Reference now Figure 4 and Table 1, provide membrane costs, system capital costs, and estimated 10-year cost of ownership 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). For Standard Design A, these cost and expense values ​​have been normalized to a value of 1.00 to facilitate comparison with the performance of Standard Design B and the biogas separation system 100. Normalization is achieved by dividing each value for the system 100 and for Standard Design B by the corresponding value for Standard Design A as a reference, so that the values ​​for the new design and Standard Design B are expressed as coefficients of the corresponding values ​​for Standard Design A. Figure 4 Table 1 shows that by reducing the target operating temperature, the new design achieves a 99.5% target product recovery of CH4 while reducing the membrane cost for most CO2 percentages in the product gas stream. In addition, by reducing the target operating temperature, the new design system achieves a 99.5% target product recovery of CH4 while achieving lower capital costs (including capital costs of all equipment) compared to either standard design system. This is due to the reduction in the recycle / feed ratio without increasing the membrane cost. Figure 4 Also shown are the modeled 10-year cost of ownership for an exemplary embodiment of the new design system relative to two standard design systems for different CO2 percentages in the product gas stream (second stage non-permeate stream 114). Figure 4 As shown, the new design system provides a significantly lower expected cost of ownership over the expected life of the system of ten years than either standard design system. As energy costs continue to rise, it is expected that the biogas separation system of the present invention will further reduce relative operating expenses.

[0070] Table 1

[0071]

[0072] The present invention has been disclosed in terms of preferred embodiments and alternative embodiments thereof. Of course, various changes, modifications and variations to the teachings of the present invention may be envisioned by those skilled in the art without departing from the spirit and scope of the present invention. It is intended that the present invention be limited only by the terms of the appended claims.

Claims

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 respect to water; (b) cooling the pressurized feed gas stream to a target operating temperature in a cryogenic refrigerator to form a condensed water stream and a saturated gas stream; (c) feeding the saturated gas stream to at least one membrane drying stage, each of the at least one membrane drying 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 purge 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) purging and exhausting the water vapor permeate stream from the low pressure side using a purge gas stream fed in a flow direction opposite to the flow direction of the saturated gas stream fed 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 greater 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 the sweep gas stream in a second separation stage, the second non-permeate stream having a greater 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 greater 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 with the water vapor permeate stream and optionally a first portion of the sweep gas stream to form a recycle stream; as well as (j) combining the recycle stream with the feed gas stream prior to carrying out step (a); Wherein, the target operating temperature is between 0°C and 20°C.

2. The method according to claim 1, wherein: The target operating temperature is between 3°C and 7°C.

3. The method according to claim 1, wherein: The target operating temperature was about 5°C.

4. The method according to claim 1, wherein: The feed gas stream comprises at least 40% CH4.

5. The method according to claim 1, wherein: Each of the at least one first-stage membrane assembly has a first-stage CO2 / CH4 selectivity greater than the second-stage CO2 / CH4 selectivity of each of the at least one second-stage membrane assembly and the third-stage CO2 / CH4 selectivity of each of the at least one third-stage membrane assembly.

6. The method according to claim 1, wherein: Each of the at least one first stage membrane module has a first stage CO 2 / CH 4 selectivity greater than 30 at the target operating temperature.

7. The method according to claim 1, wherein: Each of the at least one second stage membrane module has a CO 2 / CH 4 selectivity greater than 20 at the target operating temperature.

8. The method according to claim 1, wherein: Each of the at least one first-stage membrane assembly comprises a polymer membrane having a first capacity, and wherein each of the at least one second-stage membrane assembly and each of the at least one third-stage membrane assembly comprise a polymer membrane having a capacity greater than the first capacity.

9. The method according to claim 1, wherein: Each of the at least one first-stage membrane components includes a polymer membrane having a first selectivity, and wherein each of the at least one second-stage membrane component and each of the at least one third-stage membrane component include a polymer membrane having a selectivity smaller than the first selectivity.

10. The method according to claim 1, wherein: Step (j) includes 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. The method according to claim 1, further comprising: (k) controlling a split of the first portion of the purge gas flow from between the purge gas flows.

12. The method according to claim 1, wherein: The second non-permeate stream comprises CO2 in a range between about 0.5% and about 5%.

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 respect to water; (b) a cryocooler for cooling the pressurized feed gas stream to a target operating temperature between 0° C. and 20° C. to form a condensed water 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 purge 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 purge and discharge the water vapor stream from the low pressure side using a purge 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 greater 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 greater 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 greater percentage of methane than the third permeate stream, the third separation stage comprising at least one third stage membrane module; as well as (g) a first mixing joint for combining the third non-permeate stream with the water vapor stream to form a recycle stream to be combined with the feed gas stream.

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

15. The system of claim 13, wherein: The target operating temperature is between 3°C and 7°C.

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

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

18. The system of claim 13, wherein: The second non-permeate stream comprises CO2 in a range between about 0.5% and about 5%.

19. The system of claim 13, wherein: The sweep gas stream is fed in a flow direction opposite to the flow direction of the saturated gas stream fed into the at least one membrane dryer.

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