Process and apparatus for recovering and liquefying carbon dioxide

Abstract

Liquid carbon dioxide is produced from a gaseous mixture containing carbon dioxide and methane by the steps of separating the gaseous mixture into carbon dioxide-enriched gas and methane-enriched gas, compressing the carbon dioxide-enriched gas, cooling and partially condensing the compressed gas, phase separation of the partially condensed fluid, reducing the pressure of the raw liquid carbon dioxide, and distilling the reduced-pressure liquid to produce the liquid carbon dioxide product. Costs are reduced and efficiency is improved by using various products from phase separation and distillation to aid in cooling and partially condensing the compressed gas.

Description

Technology Field

[0001] The present invention relates to the field of carbon dioxide recovery and liquefaction, particularly the recovery and liquefaction of carbon dioxide from gaseous mixtures containing carbon dioxide and methane. The present invention is particularly applicable to the recovery and liquefaction of carbon dioxide from such gaseous mixtures produced by fermentation and / or from biogas. Background Technology

[0002] Carbon dioxide (CO2) produced by biological processes, or "biogenic" carbon dioxide, has only recently begun to be considered as a potential source of high-quality food-grade liquid carbon dioxide. Biogenic carbon dioxide can be combined with renewable hydrogen to produce efuels such as eMethanol or Synthetic Natural Gas (SNG).

[0003] One of the major challenges in the production of liquid carbon dioxide from these sources is the low carbon dioxide content, typically less than 90 mol%, in the feed to the liquefier. Standard carbon dioxide liquefaction processes use carbon dioxide feeds with a carbon dioxide concentration of 95 mol% or more. These feeds are typically compressed to a pressure in the range of about 22 bar to about 24 bar and then liquefied with an external refrigerant, typically liquid ammonia.

[0004] These standard processes are not applicable to mixtures of carbon dioxide and methane (CH4) with a carbon dioxide concentration of less than 90 mol%, because higher compression power would be required (since the compressor would now have to compress methane and other impurities, such as nitrogen (N2) and oxygen (O2), instead of just carbon dioxide with low impurity levels), and a significant emission stream could be generated from the cryogenic separation of carbon dioxide and methane. In this regard, the emission stream would contain a significant amount of carbon dioxide as well as most of the methane introduced into the liquefaction unit. Since the emission stream cannot be recirculated as a feed for the liquefaction unit, it would have to be released into the atmosphere.

[0005] Previous attempts to recover carbon dioxide generated from biological processes include CN111256431A (published in 2020). In this process, biogas is pre-treated by desulfurization, drying, and filtration. The pre-treated biogas is then compressed and fed into a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residue gas. The permeate gas is compressed and cooled and then fed into a rectification column to produce liquid carbon dioxide and impurity-rich top vapor. Refrigeration duty is provided by an external mixed refrigerant circuit.

[0006] There is a need for improved processes and devices to produce liquid carbon dioxide from biogas and similar gaseous mixtures.

[0007] According to a first aspect of the present invention, a process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane is provided, the process comprising: separating the gaseous mixture in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residual gas; compressing the permeate gas or the combined gas mixture comprising the permeate gas in a compressor system to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas derived from the compressed gas or the compressed gas comprising carbon dioxide by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a raw carbon dioxide liquid comprising methane and a tail gas comprising methane and residual carbon dioxide; and recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide that is recirculated to a compressor system. and after pressure reduction, the method includes a step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched top vapor, and a refrigeration duty is required to cool and partially condense the compressed gas derived from the compressed gas containing carbon dioxide to produce a partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or methane-enriched top vapor.

[0008] A portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived therefrom containing carbon dioxide may be provided by heat exchange with the tail gas or methane-enriched top steam, but this portion of the refrigeration duty is typically provided by heat exchange with the tail gas, optionally with the methane-enriched top steam.

[0009] Conventional liquid carbon dioxide plants produce liquid carbon dioxide at a rate ranging from about 100 tons to about 400 tons per day. Although the present invention may be applied in plants that produce liquid carbon dioxide at a rate higher than the conventional rate, the present invention has applications particularly in plants that produce liquid carbon dioxide at a rate ranging from about 50 tons to 100 tons per day.

[0010] The inventors determined that the use of a high-pressure feed stream enables the condensation of carbon dioxide from feed streams having lower carbon dioxide concentrations, thereby enabling the commercial production of liquid carbon dioxide from biogas.

[0011] Additionally, the use of a phase separator at supply pressure removes large amounts of impurities, making it possible to recirculate them to a membrane separator without additional compression.

[0012] Additionally, while the distillation column may be conventional, the overhead material can be recirculated back into the compression system as impurities are recovered from phase separation.

[0013] In addition, the recovery of carbon dioxide and methane is improved compared to equivalent conventional processes.

[0014] According to a second aspect of the present invention, an apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane is provided, the apparatus comprising: a membrane separation system comprising an inlet for the gaseous mixture, a first outlet for a carbon dioxide-enriched permeate gas, and a second outlet for a methane-enriched gas; a compression system comprising an inlet communicating with the first outlet of the membrane separation system via fluid flow and an outlet for a compressed gas comprising carbon dioxide; a phase separator comprising an inlet communicating with the outlet of the compression system via fluid flow, a first outlet for a raw carbon dioxide liquid, and a second outlet for a tail gas; and a first pressure reduction unit comprising an inlet communicating with the first outlet of the phase separator via fluid flow and an outlet for a raw carbon dioxide liquid at reduced pressure. The apparatus comprises a distillation column including an inlet communicating fluid flow with the outlet of a first pressure reduction unit, a first outlet for a liquid carbon dioxide product, a second outlet for a methane-enriched top vapor, and a reboiler, and the apparatus comprises a heat exchange system positioned and arranged to cool and partially condense a compressed gas containing carbon dioxide by heat exchange with the tail gas from a phase separator and / or the methane-enriched top vapor from the distillation column and at least one refrigerant.

[0015] The apparatus of the second embodiment of the present invention is particularly suitable for carrying out the process of the first embodiment.

[0016] Preferred embodiments of the present invention are distinguished from the prior art by the following:

[0017] ● The recovery of heat from the heated tail gas to the feed for the DeOXO unit reduces the additional duty required from the heater to heat the DeOXO feed to the temperature required for the DeOXO reaction;

[0018] ● Low dew point recirculation from the cold box can be mixed with the feed to the membrane units to increase the dew point margin;

[0019] ● Lower carbon dioxide content in the compressed permeate gas;

[0020] ● Return of tail gas from a flash separator to a membrane separator to improve the recovery of carbon dioxide and methane;

[0021] ● The pressure at which the permeate gas is compressed is higher than the operating pressure of the distillation column;

[0022] ● Liquid-gas separation is performed at high pressure after compression and cooling, but before distillation;

[0023] ● Use of external refrigerant for condensation of the supply stream; and

[0024] ● Use of multi-pass brazed aluminum heat exchangers. Brief explanation of the drawing

[0025] FIG. 1 is a simplified flowchart illustrating a first embodiment of an integrated process according to the present invention. FIG. 2 is a simplified flowchart illustrating a second embodiment of an integrated process according to the present invention. FIG. 3 is a simplified flowchart illustrating a third embodiment of an integrated process according to the present invention. FIG. 4 is a simplified flowchart illustrating a fourth embodiment of an integrated process according to the present invention. FIG. 5 is a simplified flowchart illustrating a fifth embodiment of an integrated process according to the present invention. FIG. 6 is a simplified flowchart illustrating a sixth embodiment of an integrated process according to the present invention. Figure 7 is a simplified flowchart of a reference process in which the carbon dioxide liquefier is not integrated with the carbon dioxide separation system. Figure 8 is a bar chart comparing the estimated annual cost (MM$) of the integrated process of Figure 3 (an "integrated" CO2 liquefier with low or medium N2 content in the feed) and the reference process of Figure 7 (a "bolt-on" CO2 liquefier with low or medium N2 content in the feed). FIG. 9 is a simplified flowchart illustrating a seventh embodiment of an integrated process according to the present invention. Specific details for implementing the invention

[0026] Throughout this specification, any reference to pressure refers to absolute pressure unless otherwise specified. Also, all percentages are calculated based on molar concentration, i.e., mol%, unless otherwise specified.

[0027] The expression “super-atmospheric pressure” is intended to indicate a pressure significantly greater than atmospheric pressure, e.g., at least 10 bar, preferably at least 20 bar. Operation at super-atmospheric pressure is limited only by the device. Typically, super-atmospheric pressure may be up to 70 bar, particularly when the carbon dioxide content in the feed for membrane separation systems is low, e.g., 20 mol%, or up to 40 bar in other embodiments.

[0028] In the context of methane-enriched gases (or other fluids), the expression "methane-enriched" is intended to describe a composition of a product fluid in which the proportion of methane is greater than that of the feed from which the product fluid is produced. Corresponding expressions involving different gases, for example, the expression "carbon dioxide-enriched," should be interpreted accordingly.

[0029] In the context of oxygen-depleted gases (or other fluids), the expression "oxygen-depleted" is intended to describe a composition of the product fluid in which the proportion of oxygen is lower than that of the feed from which the product gas is generated. In the context of oxygen-lean gases (or other fluids), the expression "oxygen-lean" is intended to describe a composition of the product fluid in which there is no oxygen at all or is substantially (or essentially) oxygen-lean. Corresponding expressions involving different gases should be interpreted accordingly.

[0030] In the context of partially condensed fluids, the expression "partially condensed" is intended to refer to a fluid that has both a gaseous and a liquid phase.

[0031] The expression "refrigeration duty" is intended to indicate the cooling duty resulting from the transfer of latent heat and / or sensible heat due to heat exchange between fluids of different temperatures.

[0032] The expression "in communication with fluid flow" will be understood to mean that piping or other suitable conduits will be used to transfer a fluid from one designated location to another. During the transfer between the two locations, the fluid may flow through one or more other units that may be designed and / or arranged to change the physical state of the fluid, e.g., temperature (e.g., heat exchanger) and / or pressure (e.g., compressor, pump, pressure reducing valve, or expander), or the composition of the fluid through the reaction of phases within the fluid (e.g., catalytic reactor). The expression "directly in communication with fluid flow" will be understood to mean that the fluid flows directly from one location to another, that is, it does not flow through other such units during the passage, and consequently, there is at least essentially no change in the composition or physical state of the fluid.

[0033] It will be understood that the term "downstream" refers to the direction of fluid flow under normal operation. The term "upstream" should be interpreted accordingly.

[0034] In the context of a heat exchanger located between two different devices that communicate fluid flow with each other, it will be understood that the term "located between" means that a heat exchanger capable of changing the temperature of a process fluid flowing from one device unit to another device unit by heat exchange with another fluid is provided at an intermediate location relative to the locations of the other devices. To avoid misunderstanding, the fluids within the heat exchanger are kept independent and not mixed with each other.

[0035] "Gaseous mixture" is a feed for the membrane separation system, and therefore, in some embodiments, it will be a tail gas from the phase separation.

[0036] Gaseous mixtures can be binary mixtures of carbon dioxide and methane. However, depending on the source of the gaseous mixture, one or more other components may be present in the gaseous mixture as impurities. In this regard, gaseous mixtures are typically derived from biogas. Biogas is produced from organic feedstocks such as agricultural waste, livestock manure, municipal solid waste, plant matter, sewage, green waste, wastewater, and food waste by anaerobic digestion using anaerobic microorganisms and / or methane-producing bacteria. Biogases are also produced by fermentation from landfills.

[0037] The main components of the biogas are carbon dioxide (e.g., about 30 mol% to about 50 mol%) and methane (e.g., about 40 mol% to about 70 mol%), but the biogas is typically saturated with water. As a result of its biological origin, the biogas further comprises one or more impurities selected from typically non-condensable gases, depending on the material from which the biogas is derived and the method by which the biogas is produced, such as nitrogen (e.g., 0 mol% to about 30 mol%) and oxygen (e.g., 0 mol% to about 10 mol%); hydrogen sulfide (e.g., 0 mol% to about 0.5 mol%); carbon monoxide (e.g., 0 mol% to about 0.2 mol%); non-methane organic compounds (e.g., 0 mol% to about 0.6 mol%); thiols; ammonia; and siloxanes.

[0038] Biogas is typically pre-treated to remove at least most impurities. For example, the gas may be dried, desulfurized, and decarbonized at appropriate points in the overall process, as is known in the prior art (though not necessarily in this order). However, it is important that water is removed before the gaseous mixture is cooled and partially condensed.

[0039] Biogas typically has a "low" nitrogen content, for example, about 0.1 mol% to 2 mol%, where the biogas is produced by anaerobic digestion. However, due to the presence of air, the nitrogen content is much higher in biogas produced at landfills, for example, up to 30 mol%, for example, about 5 mol% to about 30 mol%. Because nitrogen tends to pass through selective membrane units along with methane, a nitrogen rejection unit (NRU) is typically required to separate nitrogen from methane recovered from biogas with a "high" methane content before it is added to the natural gas pipeline, in order to ensure that the methane meets nitrogen specifications.

[0040] Gaseous mixtures particularly suitable for use with the present invention are derived from biogas produced by anaerobic digestion, because these mixtures typically have a "low" nitrogen content and, accordingly, typically do not require processing units, e.g., NRUs, to remove nitrogen.

[0041] The present invention is primarily intended to recover and liquefy carbon dioxide from a gaseous mixture comprising about 60 mol% or less of carbon dioxide, for example, about 30 mol% to about 50 mol% of carbon dioxide or about 20 mol% to about 40 mol% of carbon dioxide. The gaseous mixture typically comprises about 30 mol% to about 80 mol% of methane, for example, about 30 mol% to about 70 mol% of methane or about 60 mol% to about 80 mol% of methane.

[0042] The gaseous mixture is separated in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residue gas. Such a membrane separation system may include one or more stages, each stage comprising one or more membrane separation units. Types of membranes suitable for use in membrane separation systems are known in the art and include polymer membranes.

[0043] The gaseous mixture supplied to the separation system is typically at an ultra-atmosphere pressure suitable for driving separation across the membrane, e.g., at least 10 bar or at least 15 bar. The pressure of the gaseous mixture is typically in the range of about 10 bar to about 70 bar, e.g., about 15 bar to about 20 bar, or about 18 bar when membranes operating at lower pressure are used; or in the range of about 30 bar to about 40 bar, or about 31 bar when membranes operating at higher pressure are used.

[0044] The carbon dioxide-enriched permeate gas typically contains about 50 mol% to about 90 mol% carbon dioxide, for example, about 70 mol% to about 85 mol% carbon dioxide or about 80 mol% carbon dioxide.

[0045] The methane-enriched residual gas typically contains about 60 mol% to about 100 mol% of methane, for example, about 85 mol% to about 100 mol% of methane or about 90 mol% to about 99 mol% of methane, or about 96 mol% of methane.

[0046] The carbon dioxide-enriched gas is typically at a pressure within the range of about 0.1 barg ("bar gauge") to about 15 barg, for example, about 1 barg to 10 barg, or about 2 barg to about 5 barg, or about 3 barg to about 4 barg.

[0047] Then, the carbon dioxide-enriched permeate gas (or a combined gas mixture containing carbon dioxide-enriched permeate gas) is compressed by a compression system, for example, a liquefier feed compressor, to produce a compressed gas containing carbon dioxide at a pressure typically in the range of about 20 bar to about 40 bar, for example, about 25 bar to about 35 bar, or about 30 bar to about 32 bar.

[0048] Compression of the gas may require two or more compression stages. In typical arrangements, the gas being compressed is cooled between the compression stages, which leads to the formation of condensate, which is mostly water but may contain some dissolved components such as carbon dioxide. The condensate is typically separated and removed from the compressed gas to reduce the moisture content in the gas.

[0049] Compressed gas leaving the compression system is typically cooled before further processing. Cooling can be performed in a heat exchanger within a part of the compression system or in a heat exchanger that is separated from the compression system but in direct fluid flow communication with it. Such heat exchangers are generally referred to as aftercoolers. Cooling the compressed gas often results in the formation of condensate, which is mostly water but may contain some dissolved components such as carbon dioxide. The condensate is typically separated and removed from the compressed gas to further reduce the moisture content in the gas.

[0050] When it is already sufficiently dry, for example, when the moisture concentration in the compressed gas is 100 ppm or less, the compressed gas (or the compressed gas containing carbon dioxide and derived therefrom) can be supplied directly to a heat exchanger, where the compressed gas is cooled by heat exchange and partially condensed to produce a partially condensed fluid containing carbon dioxide.

[0051] In such stages, if the concentration of moisture in the gas is high, a drying process is typically included to dry the compressed gas. An example of a suitable drying process is thermal swing adsorption (TSA), in which moisture is removed from the gas by adsorption using one or more suitable adsorbents. The use of a TSA unit removes moisture from the compressed gas to a level of less than 100 ppm, e.g., less than 50 ppm, preferably less than 5 ppm, and more preferably less than 1 ppm, in order to prevent blockage of the heat exchanger and other problems caused by freezing of moisture in the gas when cooled.

[0052] Moisture removed by the TSA is left on the adsorbent(s). The TSA operates with multiple beds, where one or more are online while the others are regenerated. Regeneration is the process of removing moisture from the adsorbent(s) so that the bed can once again perform the function of removing moisture from compressed gas. Several options are available for the gas used to regenerate the adsorbent beds in the TSA, some of which are also well known in the art. For example, a separate stream of nitrogen may be provided and, after heating, used to remove moisture. A portion of the dry stream from the TSA unit itself may be heated and used to remove moisture from the adsorbent(s). This moisture can then be removed by condensation. A moist feed may also be used after being appropriately heated. Other process streams (in whole or in part), such as streams recirculated to the membrane process, or column overhead streams and / or boiloff from storage tanks, may also be used. In the case of using the column top or evaporator, the stream will pick up moisture from the beds within the TSA and then be fed into a compression system, e.g., a liquefied feed compressor—the moisture is then removed at least partially in the intercooler(s) and / or aftercooler of the compression system. It will also be possible to vaporize a portion of the raw carbon dioxide and use this gas to regenerate the beds. This stream will then be transferred to the compression system after use as a regeneration gas.

[0053] While TSA may be preferable to use with the present invention, other drying processes capable of lowering moisture content to the required level, such as membrane dryers, may be used.

[0054] Other impurities can be removed from the compressed gas in this stage if necessary.

[0055] A compressed gas (or a compressed gas derived therefrom) is cooled and partially condensed by heat exchange to produce a partially condensed fluid containing carbon dioxide. Two or more refrigerants are used to achieve the cooling and partial condensation of the compressed gas, as further discussed below.

[0056] The partially condensed fluid is phase-separated to produce a tail gas containing methane and residual carbon dioxide and a raw carbon dioxide liquid, for example, the liquid phase and the gas phase are separated within the container.

[0057] Raw liquid carbon dioxide typically contains about 80 mol% to about 98 mol%, for example, about 93 mol% of carbon dioxide.

[0058] The tail gas typically contains about 20 mol% to about 60 mol%, for example, about 55 mol% of carbon dioxide, and about 40 mol% to about 60 mol%, for example, about 45 mol% of methane.

[0059] Residual carbon dioxide is recovered from at least a portion of the tail gas to produce recovered carbon dioxide that is recirculated directly or indirectly to a compression system, for example, a liquefier supply compressor.

[0060] In some embodiments, the initial (or "bulk") separation of carbon dioxide and methane occurs in a membrane separation system (see FIGS. 1 through 5 and FIG. 9). These embodiments may be referred to as "membrane-first" embodiments. In these embodiments, all (or part) of the tail gas may be combined with a fresh feed to the membrane separation system, and thus residual carbon dioxide is recovered within the initial separation of carbon dioxide and methane in the separation system.

[0061] In other embodiments, the initial (or bulk) separation of carbon dioxide and methane occurs in a phase separator (see FIG. 6). These embodiments may be referred to as "phase separator-first" embodiments. In these embodiments, all (or part) of the tail gas forms the entire feed for a membrane separation system that produces carbon dioxide-enriched permeate gas and methane-enriched gas. Because the tail gas forms the feed for the membrane separation unit, in these embodiments the tail gas is the "gaseous mixture" defined in the claims.

[0062] In all embodiments, the tail gas is typically at a pressure of gas supplied to the membrane separation system, e.g., up to about 40 bar. In embodiments where the membrane separation system operates at a higher pressure, e.g., in the range of about 25 bar to about 35 bar, (re)compression of the tail gas is typically not required before supplying it to the membrane separation system. However, in embodiments where the membrane separation system operates at a lower pressure, e.g., in the range of about 15 bar to about 20 bar, it is typically necessary to reduce the pressure of the tail gas, e.g., through a pressure reducing valve, before the tail gas is supplied to the membrane separation system.

[0063] In some "membrane-first" embodiments, the tail gas is divided into a first portion supplied into the feed to the DeOXO unit (to increase the temperature of the feed to the unit) and a second portion supplied directly into the feed to the membrane separation system (to increase the dew point of the feed to the system).

[0064] The pressure of the raw carbon dioxide liquid is typically reduced to a pressure within the range of about 15 bar to about 20 bar, for example, about 19 bar. Then, the raw carbon dioxide liquid is fed at the reduced pressure into a distillation (or stripping) column system comprising at least one column, where the raw carbon dioxide liquid is purified to form liquid carbon dioxide and methane-enriched top vapor. Those skilled in the art will understand that the feed to the column will generally be at the saturation temperature for the operating pressure of the column.

[0065] Liquid carbon dioxide typically contains about 98 mol% to about 100 mol%, for example, about 99.999 mol% of carbon dioxide.

[0066] The top steam typically contains about 60 mol% to about 70 mol%, e.g., about 65 mol% of carbon dioxide, and about 30 mol% to about 50 mol%, e.g., about 40 mol% of methane. Any light gases present in the biogas in the "phase separator-first" case or permeated through the membrane in the "membrane-first" case, e.g., nitrogen and / or oxygen, will also be present in this stream. Thus, the top steam may further contain up to about 1 mol% of nitrogen and / or up to about 1 mol% of oxygen.

[0067] The present invention features the thermal integration of a carbon dioxide liquefier. In this regard, the inventors recognized that some process fluids generated in the cold box of the liquefier are in a suitable physical state to contribute to the refrigeration (or "cooling") duty required to cool and partially condense the compressed gas.

[0068] One such process fluid is the tail gas generated in the phase separation step. Thus, prior to recirculation, the tail gas can typically be used to provide a first portion of the refrigeration duty required to cool and partially condense the compressed gas, for example, about 5% to about 15%, or about 10% to about 12%.

[0069] Another such fluid is the top vapor generated during the distillation (or stripping) step. Thus, the top vapor can be used to provide a portion of the refrigeration duty required to cool and partially condense the compressed gas, together with a first portion typically (but not necessarily) provided by the tail gas. The portion of the refrigeration duty provided by the top vapor is typically about 1% to about 3% of the refrigeration duty under discussion, which is significantly less than the portion of the refrigeration duty provided by the tail gas.

[0070] The resulting warmed top gas can be recirculated within the process to recover methane and residual carbon dioxide. In this regard, the warmed top gas is typically at an operating pressure of the distillation column system, for example, within the range of about 15 bar to about 20 bar. In embodiments where the compression system is a single-stage compressor, the warmed top gas can be pressure-reduced and fed to the compression system. In embodiments where the compression system is a multi-stage compressor, the warmed top gas can be fed to a suitable interstage of the compressor after the pressure has been appropriately reduced, where it will be recompressed to a feed pressure to the main heat exchanger of the liquefier.

[0071] Additional process gases that can be used to provide a refrigeration duty to cool and partially condense the compressed gas include raw carbon dioxide liquid generated in the phase separation stage and liquid carbon dioxide generated in the distillation (or stripping) stage.

[0072] The vapor for the distillation (or stripping) column is typically provided by reboiling liquid carbon dioxide. In this regard, the liquid carbon dioxide is removed from the distillation (or stripping) system, reboiled by heat exchange with the compressed gas, and then can be fed back to the distillation (or stripping) column. In alternative embodiments, the reboiler may be located at the bottom of the distillation (or stripping) column, and the compressed gas may be fed to the reboiler to provide heat for reboiling as is known in the art.

[0073] A portion of the liquid carbon dioxide is also removed as a product from the distillation (or stripping) system, supercooled by heat exchange, and transferred to a storage vessel. The liquid may be supercooled prior to storage to minimize the amount of instantaneous vapor when the liquid pressure is reduced to match the storage vessel. In this regard, the liquid is typically stored at a pressure lower than that of the distillation (or stripping) column, for example, a pressure in the range of about 11 bar to about 15 bar, or a pressure of about 14 bar.

[0074] The evaporated gas and / or displacement gas from the storage container may be cold enough to provide a refrigeration duty to the compressed gas. Therefore, if a significant amount is present, this gas can be heated by heat exchange with the compressed gas and recirculated as carbon dioxide-enriched gas at the intermediate stage of the liquefier supply compressor.

[0075] The remainder of the refrigeration duty is typically provided by heat exchange with the refrigerant in the external refrigerant circuit. The refrigerant may be a mixed refrigerant, propane, or ammonia, with ammonia being preferred. In some embodiments, the external refrigerant is provided by an external liquid ammonia refrigeration circuit. In these embodiments, the liquid ammonia in the circuit may evaporate at about 1 bar, about 5 bar, and about 10 bar.

[0076] In relation to the device, the membrane separation system comprises an inlet for a gaseous mixture, a first outlet for a carbon dioxide-enriched gas, and a second outlet for a methane-enriched fluid. The membrane separation system may comprise one or more stages, wherein each stage comprises one or more membrane separation units. A conventional membrane separation unit may be used with any suitable configuration, e.g., hollow fibers, spirally wound fibers, etc., comprising any suitable carbon dioxide-selective polymer membrane.

[0077] The compression system includes an inlet communicating with the fluid flow of a first outlet of a membrane separation system and an outlet for the compressed gas, including carbon dioxide. Positive displacement compressors, for example, reciprocating compressors or centrifugal compressors may be used. However, the use of oil-flooded screw compressors is typically preferred. The compression system may be a single-stage compressor or a multi-stage compressor including an inlet to an intermediate stage of the compression system communicating with the fluid flow of a second outlet of a distillation (or stripping) column or an inlet to an intermediate stage of the compression system communicating with the fluid flow of a second outlet of a storage vessel. The compression system typically further includes an aftercooler for cooling the compressed gas.

[0078] The device may further include a gas-liquid separator for removing moisture (or aqueous) condensate from the cooled compressed gas. The separator includes an inlet communicating fluid flow with the outlet of the aftercooler of the compression system, a first outlet communicating fluid flow with the inlet of a downstream unit for drying the compressed gas, e.g., a TSA, and a second outlet for moisture condensate. The separator may be equipped with a demister unit for removing droplets from the cooled compressed gas. Where an oil-lubricated screw compressor is used, the gas-liquid separator may also include an oil removal system for catching oil droplets generated from the compressor and mixed into the gas.

[0079] The device may include a dryer for drying cooled compressed gas, for example, a TSA unit or a membrane dryer. Such a dryer unit includes an inlet communicating fluid flow with an upstream unit, for example, an outlet of a gas-liquid separator, and an outlet communicating fluid flow with an downstream unit, for example, an inlet of a heat exchanger.

[0080] A phase separator, for example, a separator vessel includes an inlet communicating fluid flow with the outlet of a compression system, a first outlet for raw carbon dioxide liquid, and a second outlet for tail gas. The second outlet typically communicates fluid flow with the inlet of a membrane separation system.

[0081] The pressure reduction unit includes an inlet communicating with the first outlet of the phase separator and fluid flow, and an outlet for the raw carbon dioxide liquid at reduced pressure. This unit is typically a pressure reduction valve.

[0082] The distillation (or stripping) column (or column system) comprises an outlet of a pressure reduction unit and an inlet communicating with the fluid flow, a first outlet for liquid carbon dioxide, a second outlet for methane-enriched upper steam, and a reboiler. As mentioned above, the reboiler may be located within the lower section of the column. Alternatively, the reboiler may be integrated with a main heat exchanger or a separate heat exchanger outside the column and may be heated by at least a portion of the compressed gas.

[0083] The storage vessel includes an inlet communicating fluid flow with a first outlet of the distillation (or stripping) column, a first outlet for liquid carbon dioxide, and a second outlet for evaporated gas and / or replacement gas.

[0084] The device includes a heat exchange system positioned and arranged to cool and partially condense the compressed gas by heat exchange with one or both of the tail gas from the phase separator and / or the upper vapor from the distillation (or stripping) column, together with at least one other refrigerant.

[0085] The heat exchange system typically includes at least one heat exchanger positioned between the second outlet of the phase separator and the compression system and arranged to cool the compressed gas containing carbon dioxide by heat exchange with the tail gas. In some embodiments, there are two such heat exchangers arranged in series, namely, a first heat exchanger including a demister for removing oil mixed into the compressed gas from the compression system and a second heat exchanger downstream of the first heat exchanger (downstream with respect to the flow of the compressed gas).

[0086] Alternatively (or more generally, additionally), the heat exchange system may include a heat exchanger positioned between the second outlet of the distillation (or stripping) column and the compression system and arranged to cool the compressed gas containing carbon dioxide by heat exchange with the methane-enriched upper steam.

[0087] The reboiling unit of the distillation column may be part of a heat exchange system. In these embodiments, the reboiling unit is arranged to reboil liquid carbon dioxide by heat exchange with a compressed gas containing carbon dioxide.

[0088] The heat exchange system may include a heat exchanger positioned between the first outlet of the distillation column and the inlet of the storage vessel, arranged to supercool the liquid carbon dioxide product by heat exchange with at least one refrigerant, for example, an external refrigerant.

[0089] The device typically includes an external refrigerant circuit to provide at least the remainder of the cooling duty. The external refrigerant typically provides most of the refrigeration duty to cool and partially condense the compressed gas, for example, more than 50%, preferably more than 75%, more preferably more than 80%. Any suitable refrigerant may be used, but liquid ammonia is preferred.

[0090] The device may further include a second pressure reduction unit comprising an inlet communicating with the first outlet of the distillation column via fluid flow and an outlet communicating with the inlet of the storage vessel via fluid flow. In these embodiments, the heat exchange system typically includes a heat exchanger for supercooling liquid carbon dioxide by heat exchange with an external refrigerant located between the first outlet of the distillation (or stripping) column and the inlet of the storage vessel.

[0091] In embodiments where the compression system is a multi-stage compressor, the compressor may include a second inlet to an intermediate stage of the compressor that communicates fluid flow with a second outlet of the storage vessel.

[0092] The heat exchange system may further include a heat exchanger positioned between the outlet of the compression system and the second outlet of the storage vessel and arranged to cool the compressed gas by heat exchange with the evaporated gas and / or replacement gas.

[0093] The heat exchange system may comprise a plurality of individual heat exchangers, or alternatively, may be a single heat exchanger having a plurality of passages for various process streams to be cooled by heat exchange with one or more of (internal and / or external) refrigerant streams. In preferred embodiments, the heat exchange system is a multiple-pass brazed aluminum heat exchanger.

[0094] Aspects of the present invention include the following:

[0095] #1. As a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane,

[0096] A step of separating the gaseous mixture in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residual gas;

[0097] A step of compressing the permeate gas, or a combined gas mixture including the permeate gas, in a compressor system to produce a compressed gas containing carbon dioxide;

[0098] A step of cooling and partially condensing the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide;

[0099] A step of phase separation of the partially condensed fluid to produce a raw carbon dioxide liquid containing methane and a tail gas containing methane and residual carbon dioxide;

[0100] A step of recovering residual carbon dioxide from the tail gas to generate recovered carbon dioxide that is recirculated to the compressor system; and

[0101] After pressure reduction, the method includes the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched upper vapor, and

[0102] A process in which a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched top steam.

[0103] #2. In #1, the partially condensed fluid is in a process at a pressure within the range of about 20 bar to about 40 bar, or about 25 bar to about 33 bar, or about 28 bar to about 31 bar.

[0104] #3. In #1 or #2, the process in which the raw carbon dioxide liquid is at a pressure within the range of about 12 to about 20 bar after pressure reduction.

[0105] #4. A process in any one of #1 to #3, wherein the first portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide is provided by heat exchange with the tail gas.

[0106] #5. In #4, the above first portion is about 5% to about 15%, a process.

[0107] #6. A process in any one of #1 to #5, wherein the second portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide is provided by heat exchange with the methane-enriched upper steam.

[0108] #7. In #6, the process, wherein the second portion is about 1% to about 3%.

[0109] #8. In #6 or #7, the compression system is a multistage compressor, and the methane-enriched upper steam is recirculated to the permeate gas in the intermediate stage of the multistage compressor after the heat exchange, in a process.

[0110] #9. A process in any one of #1 to #8, wherein the third portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide is provided by heat exchange with raw carbon dioxide liquid and / or liquid carbon dioxide.

[0111] #10. A process in any one of #1 to #9, wherein the remainder of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas including carbon dioxide, e.g., 50% or more, 75% or more, or 80% or more, is provided by heat exchange with an external refrigerant.

[0112] #11. In #10, the external refrigerant is liquid ammonia, in the process.

[0113] #12. In #11, the liquid ammonia is evaporated in an external refrigerant circuit at about 1 bar, 5 bar, and 10 bar, in a process.

[0114] #13. A process in which, in any one of #1 to #12, liquid carbon dioxide is supplied to a storage unit.

[0115] #14. In #13, the liquid carbon dioxide is supercooled, for example, by heat exchange with an external refrigerant and / or raw carbon dioxide fluid, before being supplied to a storage unit.

[0116] #15. In either #13 or #14, the liquid carbon dioxide is stored at a pressure lower than the pressure at which the raw carbon dioxide liquid is purified by distillation, in a process.

[0117] #16. In #15, the liquid carbon dioxide is stored at a pressure within the range of about 11 bar to about 15 bar, in a process.

[0118] #17. A process in any one of #1 to #16, wherein the compression system is a multi-stage compressor, and the evaporated gas and / or displacement gas from the storage unit is heated by heat exchange with the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide, and is recirculated to the permeate gas in an intermediate stage of the multi-stage compressor.

[0119] #18. A process in any one of #1 to #17, wherein the tail gas is recirculated to the membrane separation system to recover the residual carbon dioxide.

[0120] #19. A process in any one of #1 to #18 in which liquid carbon dioxide is reboiled by heat exchange with the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide.

[0121] #20. A process in any one of #1 to #19, wherein the gaseous mixture is biogas, preferably biogas produced by anaerobic digestion.

[0122] #21. A process in any one of #1 to #20, wherein the gaseous mixture comprises about 60 mol% or less of carbon dioxide.

[0123] #22. A process in any one of #1 to #21, wherein the gaseous mixture comprises about 30 mol% to about 80 mol% of methane.

[0124] #23. A process in any one of #1 to #22, wherein the gas supplied to the compression system comprises about 90 mol% or less of carbon dioxide.

[0125] #24. A process in any one of #1 to #23, wherein the gas supplied to the compression system comprises about 5 mol% to about 50 mol% of methane.

[0126] #25. A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane,

[0127] A step of separating the gaseous mixture in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residual gas;

[0128] A step of compressing the permeate gas, or a combined gas mixture containing the permeate gas, in a liquefier supply compressor to produce a compressed gas containing carbon dioxide;

[0129] A step of cooling and partially condensing the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide;

[0130] A step of phase separation of the partially condensed fluid to produce a raw carbon dioxide liquid containing methane and a tail gas containing methane and residual carbon dioxide;

[0131] A step of recovering residual carbon dioxide from the tail gas to generate recovered carbon dioxide that is recirculated to the liquefier supply compressor; and

[0132] After pressure reduction, the method includes the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched upper vapor, and

[0133] A process in which a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and the methane-enriched top steam.

[0134] #26. In #25, the gaseous mixture comprises about 30 mol% to about 50 mol% of carbon dioxide, in the process.

[0135] #27. In #25 or #26, the gaseous mixture comprises about 30 mol% to about 70 mol% of methane, in the process.

[0136] #28. A process in which, in any one of #25 to #27, the gas supplied to the liquefier supply compressor comprises about 70 mol% to about 90 mol% carbon dioxide.

[0137] #29. A process in which, in any one of #25 to #28, the gas supplied to the liquefier supply compressor comprises about 5 mol% to about 30 mol% of methane.

[0138] #30. A process in any one of #25 to #29, wherein the residual carbon dioxide is recovered from the tail gas by recirculating the tail gas to the membrane separation system.

[0139] #31. As a process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane,

[0140] A step of combining the biogas with carbon dioxide-enriched permeate gas to produce a combined gas mixture containing permeate gas;

[0141] A step of compressing the combined gas mixture or the gas containing the combined gas mixture in a compression system to produce a compressed gas containing carbon dioxide;

[0142] A step of cooling and partially condensing the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide;

[0143] A step of phase separation of the partially condensed fluid to produce a raw carbon dioxide liquid containing methane and a tail gas containing methane and residual carbon dioxide;

[0144] A step of separating the tail gas in a membrane separation system to generate the carbon dioxide-enriched permeate gas and methane-enriched residual gas; and

[0145] After pressure reduction, the method includes the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched upper vapor, and

[0146] A process in which a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched top steam.

[0147] #32. In #31, the tail gas comprises about 20 mol% to about 40 mol% of carbon dioxide.

[0148] #33. A process in which, in #31 or #32, the tail gas comprises about 60 mol% to about 80 mol% methane.

[0149] #34. A process in which, in any one of #31 to #33, the gas supplied to the compression system comprises about 50 mol% to about 70 mol% carbon dioxide.

[0150] #35. A process in any one of #31 to #34, wherein the gas supplied to the compression system comprises about 30 mol% to about 50 mol% of methane.

[0151] #36. As an apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane,

[0152] A membrane separation system comprising an inlet for the above-mentioned gaseous mixture, a first outlet for carbon dioxide-enriched permeate gas, and a second outlet for methane-enriched gas;

[0153] A compression system comprising an inlet communicating with the first outlet of the membrane separation system and fluid flow, and an outlet for a compressed gas including carbon dioxide;

[0154] A phase separator comprising an inlet communicating with the outlet of the above-mentioned compression system for fluid flow, a first outlet for raw carbon dioxide liquid, and a second outlet for tail gas;

[0155] A first pressure reduction unit comprising an inlet communicating with the first outlet of the phase separator and a fluid flow outlet for raw carbon dioxide liquid at reduced pressure; and

[0156] It comprises a distillation column including an inlet communicating with the outlet of the first pressure reduction unit and fluid flow, a first outlet for liquid carbon dioxide product, a second outlet for methane-enriched upper steam, and a reboiling unit,

[0157] The above device comprises a heat exchange system positioned and arranged to cool and partially condense a compressed gas containing carbon dioxide by heat exchange with the tail gas from the phase separator and / or the methane-enriched top vapor from the distillation column and at least one refrigerant.

[0158] #37. In #36, the heat exchange system comprises at least one heat exchanger positioned between the second outlet of the phase separator and the compression system and arranged to cool the compressed gas containing carbon dioxide by heat exchange with the tail gas.

[0159] #38. A device in which, in #36 or #37, the second outlet of the phase separator is in fluid flow communication with the inlet of the membrane separation system.

[0160] #39. An apparatus in any one of #36 to #38, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage that communicates fluid flow with the second outlet of the distillation column.

[0161] #40. In #39, the heat exchange system comprises a heat exchanger positioned between the second outlet of the distillation column and the inlet of the intermediate stage of the compression system, arranged to cool the compressed gas containing carbon dioxide by heat exchange with the second gas-enriched upper steam.

[0162] #41. In any one of #36 to #40, the reboiling device of the distillation column is part of a heat exchange system and is arranged to reboil liquid carbon dioxide by heat exchange with a compressed gas containing carbon dioxide.

[0163] #42. An apparatus comprising, in any one of #36 to #41, an inlet communicating fluid flow with the first outlet of the distillation column, a first outlet for a liquid carbon dioxide product, and a second outlet for an evaporated gas and / or replacement gas, the apparatus comprising a storage vessel.

[0164] #43. In #42, the heat exchange system comprises a heat exchanger positioned between the first outlet of the distillation column and the inlet of the storage vessel, arranged to supercool the liquid carbon dioxide product by heat exchange with at least one refrigerant, e.g., an external refrigerant.

[0165] #44. A device comprising, in any one of #36 to #43, the heat exchange system including an external refrigerant circuit.

[0166] #45. In #44, the device, wherein the external refrigerant is liquid ammonia.

[0167] #46. A device in any one of #36 to #45, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage that communicates fluid flow with the second outlet of the storage container.

[0168] #47. An apparatus comprising, in any one of #36 to #46, a second pressure reduction unit comprising an inlet communicating fluid flow with the first outlet of the distillation column and an outlet communicating fluid flow with the inlet of the storage vessel.

[0169] #48. In any one of #36 to #47, the heat exchange system comprises a multiple pass brazed aluminum heat exchanger.

[0170] The present invention will now be described merely as an example with reference to the drawings.

[0171] According to FIG. 1, a stream of biogas (10) containing about 37 mol% carbon dioxide and about 55 mol% methane at atmospheric pressure is supplied to block (12), where this is upgraded to produce a stream of raw carbon dioxide gas (14) containing about 81% carbon dioxide and a stream (16) of renewable natural gas (RNG) or "biomethane" containing about 96% methane.

[0172] Block (12) typically includes one or more units (not shown) for pre-treating compressed biogas, such as a compression unit (not shown) for compressing biogas, an adsorption unit for drying gas, a filtration unit for removing particulates, a catalytic deoxygenation (or "DeOXO") unit for removing oxygen, a desulfurization unit for removing hydrogen sulfide, a unit for removing volatile organic compounds (VOCs), and / or a unit for removing siloxanes, followed by one or more membrane units (not shown) for upgrading biogas.

[0173] The RNG stream (16) is supplied to a natural gas pipeline (not shown), while the stream (14) of raw carbon dioxide gas, which is permeated gas typically produced in membrane unit(s) within the block (12), is processed to produce liquid carbon dioxide.

[0174] A stream (14) of raw carbon dioxide gas at about 4 bar is supplied to a multistage compressor (K101), where this stream is compressed to produce a stream (18) of compressed gas at about 30 bar. Moisture in the compressed gas is condensed in one or more intercoolers (not shown), and the condensate is removed (indicated by a dashed line).

[0175] A stream of compressed gas (18) is cooled in an aftercooler (not shown), and the condensate formed during cooling is removed in a knock-out drum (S101) to produce a stream of condensate (20) and a stream of compressed gas (22). The stream (22) is fed to a temperature swing adsorption (TSA) unit (D101) that dries the gas to form a stream of dried compressed gas (24). Moisture is removed from the TSA unit (D101) during the regeneration of beds using the purge gas (indicated by a dashed line).

[0176] The dried compressed gas is supplied to a main heat exchanger (E101), where the gas is cooled and partially condensed by heat exchange to produce a stream (26) of cooled and partially condensed gas. The partially condensed gas is supplied to a phase separator (S102), where the gas is separated into a stream (28) of carbon dioxide-enriched liquid (containing about 94 mol% carbon dioxide) and a stream (30) of tail gas (containing about 41 mol% methane and about 57 mol% residual carbon dioxide).

[0177] The pressure of the carbon dioxide-enriched liquid in the stream (28) is reduced as it passes through the valve (V101) to produce a stream (32) of carbon dioxide-enriched liquid at a pressure of about 19 bar, which is fed to a stripping column (C101), where the stream is separated into liquid carbon dioxide (having a purity of at least about 99.9 mol%) and methane-enriched top steam (containing about 34 mol% methane and about 65 mol% carbon dioxide).

[0178] The stream (34) of liquid carbon dioxide is removed from the stripping column (C101), reboiled by heat exchange with the compressed gas in the main heat exchanger (E101), and returned to the stripping column (C101) as a stream (36) to provide steam for separation.

[0179] A stream (38) of liquid carbon dioxide is removed from the stripping column (C101) and supercooled in the main heat exchanger (E101) by heat exchange with the liquid ammonia refrigerant in the external refrigerant circuit (40), and then supplied to a storage vessel (T101) as a stream (42), where this stream is stored at a pressure lower than the operating pressure of the stripping column (C101), for example, about 14 bar. Liquid carbon dioxide is removed from the storage vessel (T101) as a stream (44).

[0180] The tail gas stream (30) is removed from the phase separator (S102) and supplied to the main heat exchanger (E101) to help cool the compressed gas by heat exchange. The heated tail gas is recirculated to the block (12) as a stream (46) for the recovery of residual carbon dioxide and methane.

[0181] The stream (48) of methane-enriched upper steam is removed from the stripping column (C101) and supplied to the main heat exchanger to help cool and partially condense the compressed gas by heat exchange. The heated upper gas at a pressure of about 18 bar is recirculated as a stream (50) to the intermediate stage of the multistage compressor (K101).

[0182] The evaporated gas / replacement gas is removed from the storage container (T101) and recirculated as a stream (52) to the intermediate stage of the multistage compressor (K101).

[0183] The main heat exchanger (E101) as described is a multi-pass brazed aluminum heat exchanger. However, one or more individual heat exchangers may be used to provide the necessary heat exchange between the compressed gas and one or more of the described cold streams, namely the tail gas stream (30), the upper steam stream (48), the liquid carbon dioxide stream (34), and the ammonia refrigerant of the external refrigeration circuit (40).

[0184] FIG. 2 is a simplified flowchart illustrating a second embodiment according to the present invention. The flowchart is similar to the flowchart illustrated in FIG. 1, and features common to both flowcharts are given the same reference numerals. The following is a discussion of the different features.

[0185] An additional heat exchanger (E102) is added to provide additional cooling to the stream (18) of compressed gas to produce an additionally cooled stream (19) of compressed gas. The cooling duty in the heat exchanger (E102) is provided by the stream (46) of tail gas produced in the phase separator (S102) after passing through the main heat exchanger (E101). Additional cooling of the compressed gas can be provided using external refrigeration (40).

[0186] The use of an additional heat exchanger (E102) to cool the stream (18) to, for example, below 65°C before supplying the stream to the main heat exchanger (E101) simplifies the design and configuration of the main heat exchanger (E101). Additionally, the heat exchanger (E102) can be integrated with an oil and condensate removal system that protects the main heat exchanger (E101) from oil inflow from a multi-stage compressor (K101) if the compressor is of the oil-lubricated type.

[0187] FIG. 3 is a simplified flowchart illustrating a possible arrangement of processing units within a block (12) of the flowchart illustrated in FIG. 2. The flowchart is similar to the flowchart illustrated in FIG. 2, and features common to both flowcharts are given the same reference numbers. The following is a discussion of the different features.

[0188] A stream (10) of “low nitrogen” biogas, for example, biogas produced by anaerobic digestion having less than 1 mol% nitrogen, is compressed in a compressor (K102) to form a stream (56) of compressed biogas at a pressure of about 17 bar. After providing a cooling duty, a stream (54) of heated tailing gas is combined with stream (56) to form a combined stream (58) supplied to a DeOXO unit (U101).

[0189] Biogas generally contains a small amount of oxygen, for example, up to about 2.5 mol%. However, this amount of oxygen exceeds the maximum amount allowed in natural gas for industrial or domestic use. Therefore, oxygen cannot be allowed to follow methane into RNG. In FIG. 3, oxygen is removed from the compressed biogas by catalytic combustion in a DeOXO unit (U101) to produce a stream (60) of oxygen-depleted biogas containing less than 0.2 mol% oxygen.

[0190] Oxygen-depleted biogas in stream (60) is fed to a membrane separation system (M101), where it is separated into carbon dioxide-enriched permeate gas (stream (14)) and a first residual gas. Stream (14) is fed to a multi-stage compression unit (K101) and processed according to FIG. 2. The residual gas is fed to a second membrane separation system (M102), where it is separated into a second permeate gas and a methane-enriched residual gas. The second permeate gas is recirculated to the biogas feed (10) as stream (62) to recover residual carbon dioxide and methane. The methane-enriched residual gas is compressed in a compressor (K103) to form a stream (16) of methane-enriched residual gas at a pressure of about 41 bar.

[0191] Those skilled in the art will understand that the location and / or arrangement of the DeOXO unit (U101) in FIG. 3 is only one possible location / arrangement for such a unit. In the model presented herein, the DeOXO unit (U101) removes only enough oxygen from the RNG product (stream (16)) to meet coral specifications. However, in practice, the DeOXO unit (U101) may be designed to remove all or at least essentially all oxygen from the feed to the unit (U101), partly due to practical considerations regarding the control of partial oxygen removal. However, in other arrangements, a portion of the stream (58) may bypass the DeOXO unit (U101) so that only enough oxygen is removed to meet RNG specifications.

[0192] Alternatively, the DeOXO unit may be located further upstream in the process, for example, before the point where the recirculated material (stream (62)) from the second membrane separation system (M102) is combined with the compressed biogas feed (stream (56)), or downstream of a "blower" unit (used as part of the compression system (K102)) for pressurizing the feed biogas to an intermediate pressure to enable pre-treatment of the biogas (e.g., removal of hydrogen sulfide, VOCs and / or siloxanes), or, if the compression system (K101) is a multi-stage compression system, the DeOXO unit may be located at an intermediate stage of the compression system, if desirable and feasible. The DeOXO unit may also be located on the product RNG, but a dryer, such as a TSA unit or a membrane dryer, may also be required to remove the resulting moisture from the oxygen-depleted gas.

[0193] The DeOXO unit will typically involve heating the feed to the unit to the operating temperature of the DeOXO catalyst, cooling the resulting oxygen-depleted gas, and condensing and separating the condensed moisture (not shown in FIG. 3). The membrane separation system (M101) will also typically involve heating to ensure that no liquid phase is formed on the membrane unit(s). Typically, the feed to the membrane separation system is heated to a temperature about 10°C higher than its dew point. Since the feed leaves the separator of the DeOXO unit at its dew point, this simply means increasing the temperature by about 10°C. However, if some of the heated tail gas (stream (46)) (dry) is mixed downstream of the DeOXO unit, the dew point of the feed to the membrane separation system (M101) will drop as the recirculated stream does not contain moisture or heavy components (the dew point in the separator (S102) is already lowered to about -28°C). This arrangement is not illustrated in the drawings but represents a potential integration, particularly in the case where the DeOXO unit can remove substantially all oxygen from the feed and thus the stream (54) becomes oxygen-lean and can be recirculated downstream of U101.

[0194] The membrane separation systems (M101 and M102) of these embodiments operate at a "low" pressure and have a supply pressure in the range of, for example, about 15 bar to about 20 bar. Accordingly, the pressure of the tail gas (stream (29)) from the flash drum (S102) is reduced as it passes through the valve (V102) before being delivered to the main heat exchanger (E101) to provide a cooling duty.

[0195] FIG. 4 is a simplified flowchart illustrating a fourth embodiment according to the present invention. The flowchart is similar to the flowchart illustrated in FIG. 3, and features common to both flowcharts are given the same reference numerals. The following is a discussion of the different features.

[0196] A third membrane separation system (M103) is used to further recover carbon dioxide from the methane produced in the second membrane separation system (M102). The carbon dioxide permeated from the third membrane separation system (M103) is recirculated to the biogas feed (10). The residue, further enriched with methane, is then compressed in an RNG compressor (K103) to produce a stream (16) of compressed RNG.

[0197] The membrane module of this embodiment operates at a higher pressure than the membrane module of FIG. 3, and has a supply pressure in the range of, for example, about 25 bar to about 30 bar. Accordingly, the pressure of the tail gas (stream (30)) from the flash drum (S102) does not need to be reduced before being transferred to the main heat exchanger (E101) to provide a cooling duty.

[0198] FIG. 5 is a simplified flowchart illustrating a fifth embodiment according to the present invention. The flowchart is similar to the flowchart illustrated in FIG. 4, and features common to both flowcharts are given the same reference numerals. The following is a discussion of the different features.

[0199] This flowchart is proposed for processing biogas with higher concentrations of nitrogen, for example, biogas from landfills, with nitrogen content ranging from about 5 mol% to 30 mol%. Nitrogen is typically discharged as residual gas along with methane from a membrane separation system. The nitrogen limit in natural gas is typically less than 3 mol%. Therefore, it is important that nitrogen is removed before methane is added to the natural gas pipeline. Typically, a nitrogen rejection unit (NRU) is used in this context.

[0200] The residual gas from the third membrane separation unit (M103) is supplied to the carbon dioxide TSA unit (D102) to remove residual carbon dioxide and produce carbon dioxide-depleted methane gas, which is then supplied to the NRU, where nitrogen is removed to produce nitrogen-depleted methane gas, which is then compressed in the RNG compressor (K103) before being supplied to the natural gas pipeline (not shown) as a stream (16).

[0201] The exact configuration of the NRU is not critical to the present invention. The cryogenic NRU may have one or more columns, and the liquid methane at the bottom typically evaporates at one or more pressures (after pumping or pressure reduction) to form NRG. In FIG. 5, the removed nitrogen is simply discharged (i.e., not used), and a portion of the RNG product is used for regeneration, so the carbon dioxide eventually remains in the product (the membrane separation system has already removed carbon dioxide to meet the specifications of the RNG, and the TSA unit serves only to remove carbon dioxide for cryogenic processing).

[0202] It would also be possible to use adsorption-based NRUs, for example, PSA.

[0203] FIG. 6 is a simplified flowchart illustrating a different configuration of a process according to the present invention. The flowchart is similar to the flowchart illustrated in FIG. 4, and features common to both flowcharts are given the same reference numerals. The following is a discussion of the different features.

[0204] A stream (10) of “low nitrogen” biogas, for example, biogas produced by anaerobic digestion having less than 1 mol% nitrogen, is compressed in a compressor (K102) and supplied to a DeOXO unit (U101) that reduces the oxygen content so that the RNG stream (16) is within the specifications for oxygen. The oxygen-depleted gas from the DeOXO unit (U101) is supplied as a stream (18) to a heat exchanger (E102), where the oxygen-depleted gas is cooled by heat exchange with ammonia in an external refrigerant circuit (40).

[0205] Moisture from the biogas feed and generated in the DeOXO unit (U101) condenses when the oxygen-depleted gas is cooled in the heat exchanger (E102). Thus, the cooled gas is supplied to the phase separator (S101) as a stream (19), where the condensate is separated and removed as a stream (20). Then, the cooled gas is supplied to the TSA unit (D101) as a stream (22) to be dried.

[0206] A stream (24) of dried gas from the TSA unit (D101) is supplied to a main heat exchanger (E101), where the stream is cooled and partially condensed and then supplied to a flash drum (S102) (as stream (26)) to produce liquid carbon dioxide mixed with impurities, along with a tail gas containing methane and residual carbon dioxide.

[0207] Liquid carbon dioxide mixed with impurities is divided into two parts, namely the first part and the second part.

[0208] The first portion (stream (28)) is typically a small portion, for example, about 30% to about 45% of the total mass flow of liquid carbon dioxide mixed with impurities, and the pressure is reduced as it passes through valve (V101) before being supplied as a stream (32) to a stripper column (C101) where the carbon dioxide is purified.

[0209] Typically, a second portion, which is the main portion, for example, about 55% to 70% of the total mass flow of the liquid carbon dioxide mixed with impurities, is supplied to the main heat exchanger (E101) as a stream (64), where this stream is heated by heat exchange with the compressed gas in the stream (24) to produce a stream (66) of the carbon dioxide fluid mixed with impurities, and this stream is then lowered in pressure as it passes through valve (V103) to reduce the temperature of the fluid. The degree to which the stream (64) is heated before the stream is lowered in pressure as it passes through V103 will be determined during process optimization, which may be that the optimal amount of heating is negligible or zero, in which case it would be better to supply the stream (64) to V103 without preheating. Preheating of the stream (64) is particularly necessary if the stream (68) is lower than any point of carbon dioxide that would otherwise be so. A stream (68) of carbon dioxide fluid mixed with impurities at a lower temperature is supplied back to the main heat exchanger (E101) to provide colder refrigeration to the heat exchanger (E101) than is available from the ammonia in the external refrigerant circuit (40). After the heat exchange, a stream (70) of carbon dioxide gas mixed with impurities is recirculated back to the biogas compressor (K102).

[0210] A stream (30) of tail gas from a phase separator (S102) containing carbon dioxide and methane passes through a main heat exchanger (E101) to provide a cooling duty, and the resulting heated stream (51) is supplied to membrane separation systems (M101, M102). The carbon dioxide-enriched permeate gases in the streams (72 and 74) from the membrane separation systems (M101, M102) are recirculated back to the biogas compressor (K102) as a combined stream (62).

[0211] The upper portion from the distillation column system (C101) is streams (48 and 54), which are recirculated back to the biogas compressor (K102) through the main heat exchanger (E101) that provides the cooling duty. The stream (52) containing the evaporated gas from the carbon dioxide storage vessel (T101) is also recirculated back to the biogas compressor (K102).

[0212] The RNG compressor (K103) is optional depending on the RNG export pressure requirements.

[0213] The feed to the compressor (K102) is indicated in Table 4 below as being at different pressures. As will be readily understood by those skilled in the art, these pressures should be optimized to match the selected compression system for the required compression duty. For example, the compression system may have two compressors (or compression stages) in series, and thus there will be a first supply pressure for the first compressor (or stage) and a second pressure for the second compressor (or stage). Streams that are too low a pressure to be supplied to the interstage, that is, streams that do not meet the supply pressure for the second stage, will generally have their pressure reduced by a valve and be supplied to the first stage. Streams that have a pressure higher than the interstage pressure will similarly have their pressure reduced and be supplied to the interstage. There may be additional stages due to more compressors connected in series or because the compressor may have an interstage supply. The pressures of the streams (52, 54, 70, 72 and 74) will be optimized to reduce the power and / or capital costs of the process, provided that it is desirable to lower the pressure of the stream in the feed to a lower pressure stage of the compressor.

[0214] One advantage of this configuration compared to other configurations illustrated herein is that only a biogas supply compressor (K102) is required, compared to the two compressors (K101 and K102) in other configurations. If the discharge pressure of the biogas compressor (K102) can be increased to the RNG export pressure, the RNG compressor (K103) can also be eliminated if the membrane systems (M101 and M102) can operate at this higher pressure. The configuration of FIG. 6 is particularly advantageous at smaller flow rates, as compressors for lower flow rates may be more expensive or difficult to obtain. In this alternative configuration, the biogas compressor (K102) will be larger than in the equivalent configuration based on other drawings, but there is no requirement for a separate liquefier supply compressor (K101).

[0215] However, in this case, the DeOXO unit (U101) will need to remove oxygen to a lower level, and the TSA unit (D101) will be larger because it processes the entire feed flow rate with added recirculation. The power and number of membrane modules will be approximately the same, and the main advantage lies in reducing the number of compressors and expanding the remaining compressors, which is advantageous for plants producing less than 100 tons of liquefied carbon dioxide per day.

[0216] As mentioned above, the impurity-mixed carbon dioxide liquid from the phase separator (S102) in the liquefied carbon dioxide plant is used to provide additional refrigeration that is colder than that provided by the unit (40) in the main heat exchanger (E101). It should be noted that this is optional in other configurations as well.

[0217] FIG. 7 is a simplified flowchart illustrating a reference process in which a carbon dioxide liquefier (block (100)) is not integrated with a carbon dioxide separation system (block (12)). Features common to this flowchart and other flows illustrated herein are given the same reference numerals. The following is a discussion of the different features.

[0218] Oxygen-depleted "low" nitrogen biogas is fed as a stream (60) to a first membrane separation system (M101), where it is separated into carbon dioxide-enriched permeate gas (stream (14)) and methane-enriched residual gas. The residual gas is fed to a second membrane separation system (M102), where it is separated into a second carbon dioxide-enriched permeate gas (62) and a second residual gas further enriched with methane, which are recirculated as feed to a biogas compressor (K102). The second residual gas is fed to an RNG compressor (K103), where the second residual gas is compressed to natural gas pipeline pressure.

[0219] The permeate gas (stream (14)) from the first membrane separation system (M101) is supplied to an additional membrane separation system (M104), where the permeate gas is separated into additional carbon dioxide-enriched permeate gas and additional methane-enriched residual gas. The additional residual gas is recirculated to the biogas supply compressor (K102) as stream (76), but the additional permeate gas is supplied to the compression system (K101) as stream (78) to produce a stream (22) of compressed permeate gas at a pressure within the range of about 15 bar to about 25 bar.

[0220] The compressed permeated gas (stream (22)) is dried in the TSA unit (D101) and then supplied to the carbon dioxide liquefier (100) to produce liquefied carbon dioxide (stream (44)) and methane-enriched upper steam (stream (50)).

[0221] FIG. 8 is a bar chart comparing the estimated annual cost (million US dollars, MM$) of producing 50 tons of liquid carbon dioxide per day using the integrated process of FIG. 3 ("integrated" CO2 liquefier) ​​and the reference process of FIG. 7 ("bolt-on CO2 liquefier"). The estimates are provided for biogas feedstocks with a nitrogen content of "low," specifically 1 mol%, or "medium," specifically 10 mol%.

[0222] Factors contributing to the estimated cost include the capital cost (“capex”) of the compression system, along with electricity costs and the operating costs of the membrane separation systems. If the biogas has an “intermediate” nitrogen content, the estimated cost additionally includes the costs of NRU and TSA, where appropriate.

[0223] According to FIG. 8, the estimated cost of the process according to the present invention is significantly less than that of the reference process, regardless of whether the biogas has a "low" or "medium" nitrogen content.

[0224] Specifically, for biogas with a "low" nitrogen content, the estimated cost of the elements identified for the reference process is approximately 1.36 MM$, whereas the corresponding cost for the process according to the present invention is approximately 1.08 MM$. In other words, the total estimated cost for the elements identified for the integrated process of FIG. 3 is less than 80% of the total estimated cost for the equivalent elements of the reference process. Additionally, each individual element contributing to the total estimated cost of the processing according to the present invention is less than the equivalent elements for the reference process.

[0225] Furthermore, for biogas with "medium" nitrogen content, the estimated cost of the identified elements of the reference process is approximately 2.24 MM$, whereas the corresponding cost for the process according to the present invention is approximately 1.64 MM$. In other words, the total estimated cost for the identified elements for the integrated process of Fig. 3 is less than 74% of the total estimated cost for the equivalent elements of the reference process, despite the need for TSA units. Additionally, each individual element contributing to the total estimated cost of the processing according to the present invention is once again less than the equivalent elements for the reference process.

[0226] It should be noted that these costs do not represent the total cost of the process. In contrast, these costs include only distinct costs. Both processes include other costs, but these other costs do not distinguish the two processes in a meaningful way, and therefore, for the sake of simplicity, the other costs are not presented.

[0227] FIG. 9 is a simplified flowchart illustrating a different configuration of a process according to the present invention. The flowchart is similar to the flowchart illustrated in FIG. 1, and features common to both flowcharts are given the same reference numerals. The following is a discussion of the different features.

[0228] In FIG. 1, a stream (46) of heated tail gas containing carbon dioxide and methane is recirculated to the carbon dioxide separation system of block (12). In contrast, in FIG. 9, this stream is fed to an additional membrane separation system (M105) to produce carbon dioxide-enriched permeate gas and methane-enriched residue gas.

[0229] The permeate gas is recirculated to the intermediate stage of the liquefier supply compressor (K101) (as stream (80)) or to the feed to the liquefier supply compressor (as stream (82)), depending on the pressure of the permeate gas.

[0230] Depending on its composition, the residual gas may be recirculated (as stream (84)) to a feed for a DeOXO unit (not shown), to a feed for membrane stages (not shown), or to a suitable point within the carbon dioxide separation system of block (12), such as a suitable point between membrane stages. Alternatively, the residual gas may be recirculated (as stream (86)) to a feed (10) for block (12), or, if the gas is methane with a sufficient level of purity, the residual gas may be fed directly to the RNG stream (16) (as stream (88)).

[0231] In alternative arrangements of the flowchart of FIG. 9, only a portion of the heated tail gas is delivered as a stream (46) to an additional membrane separation system (M105), where the remainder is fed to a feed for a DeOXO unit (not shown) or to a feed for a primary membrane separation system (not shown) of block (12).

[0232] The present invention will now be described with reference to the following reference examples and non-limiting examples of the invention.

[0233] See example

[0234] The process illustrated in FIG. 7 was simulated by a computer (Aspen Plus, ver. 12.1, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream (44)) containing 50 tons of CO2 per day from a biogas supply stream using low-pressure membrane separation units.

[0235] The results are shown in Table 1.

[0236] Table 1

[0237]

[0238] Therefore, the process takes about 3411 kg / h of “low” nitrogen biogas containing about 37 mol% carbon dioxide and about 54 mol% methane to produce about 1878 kg / h of liquefied carbon dioxide (99.99 mol% purity) and about 1201 kg / h of methane (about 96 mol% purity).

[0239] Example 1

[0240] The process illustrated in FIG. 3 was simulated by a computer (Aspen Plus, ver. 12.1, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream (44)) containing 50 tons of CO2 per day from a biogas supply stream using low-pressure membrane separation units.

[0241] The results are shown in Table 2.

[0242] Table 2

[0243]

[0244] Therefore, the process takes about 3,411 kg / h of “low” nitrogen biogas containing about 37 mol% carbon dioxide and about 54 mol% methane to produce about 2,027 kg / h of liquefied carbon dioxide (99.99 mol% purity) and about 1,211 kg / h of methane (about 96 mol% purity), which is an increase of about 8% and 1% of production, respectively.

[0245] Example 2

[0246] The process illustrated in FIG. 4 was simulated by a computer (Aspen Plus, ver. 12.1, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream (44)) containing 50 tons of CO2 per day from a biogas supply stream using higher pressure membrane separation units.

[0247] The results are shown in Table 3.

[0248] Table 3

[0249]

[0250] Therefore, the process takes about 3411 kg / h of “low” nitrogen biogas containing about 37 mol% carbon dioxide and about 54 mol% methane to produce about 2027 kg / h of liquefied carbon dioxide (99.99 mol% purity) and about 1211 kg / h of methane (about 96 mol% purity).

[0251] Example 3

[0252] The process illustrated in FIG. 6 was simulated by a computer (Aspen Plus, ver. 12.1, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream (44)) containing 50 tons of CO2 per day from a biogas supply stream using higher pressure membrane separation units.

[0253] The results are shown in Table 4.

[0254] Table 4

[0255]

[0256] Therefore, the process takes about 3411 kg / h of “low” nitrogen biogas containing about 37 mol% carbon dioxide and about 54 mol% methane to produce about 2027 kg / h of liquefied carbon dioxide (99.99 mol% purity) and about 1211 kg / h of methane (about 96 mol% purity).

[0257] The feed to the compressor (K102) is at different pressures in Table 4. As will be readily understood by those skilled in the art, these pressures should be optimized as described above to match the selected compression system for the required compression duty.

[0258] Although the present invention has been described with reference to preferred embodiments illustrated in the drawings, it will be understood that various modifications are possible within the spirit or scope of the invention as defined in the following claims.

[0259] In this specification, unless otherwise explicitly indicated, the word "or" is used to mean an operator that returns a true value when one or both of the specified conditions are met, in contrast to the operator "exclusive or," which requires that only one of the conditions be satisfied. The word "comprising" is used to mean "including" and does not exclusively mean "comprising" but includes "comprising".

[0260] All prior teachings for the foregoing are incorporated herein by reference. Any acknowledgment in this specification of any previously published document shall not be deemed an acknowledgment or representation that the teachings of such document were generally known knowledge in Australia or elsewhere on that date.

Claims

Claim 1 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, comprising: separating said gaseous mixture in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residual gas; compressing said permeate gas, or a combined gas mixture containing said permeate gas, in a compressor system to produce a compressed gas containing carbon dioxide; cooling and partially condensing said compressed gas or a compressed gas derived from said compressed gas containing carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide; phase separating said partially condensed fluid to produce raw carbon dioxide liquid containing methane, and a tail gas containing methane and residual carbon dioxide; and recovering the residual carbon dioxide from said tail gas to produce recovered carbon dioxide recirculated to said compressor system. A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, comprising the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched top vapor after pressure reduction, wherein a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched top vapor. Claim 2 A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the partially condensed fluid is at a pressure within the range of about 20 bar to about 40 bar, or about 25 bar to about 33 bar, or about 28 bar to about 31 bar. Claim 3 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein, after pressure reduction, the raw carbon dioxide liquid is at a pressure within the range of about 12 to about 20 bar. Claim 4 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein, in claim 1, the first portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide is provided by heat exchange with the tail gas. Claim 5 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the first portion is about 5% to about 15% in the first portion. Claim 6 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein, in claim 1, the second portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide is provided by heat exchange with the methane-enriched upper steam. Claim 7 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the second portion is about 1% to about 3%. Claim 8 In paragraph 6, the compression system is a multi-stage compressor, and the methane-enriched upper steam is recirculated to the permeate gas in the intermediate stage of the multi-stage compressor after the heat exchange, a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane. Claim 9 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein, in claim 1, a third portion of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide is provided by heat exchange with raw carbon dioxide liquid and / or liquid carbon dioxide. Claim 10 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the remainder of the refrigeration duty required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide, for example, 50% or more, 75% or more, or 80% or more, is provided by heat exchange with an external refrigerant. Claim 11 In paragraph 10, the above external refrigerant is a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, the external refrigerant being liquid ammonia. Claim 12 In claim 11, the process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the liquid ammonia is evaporated in an external refrigerant circuit at about 1 bar, 5 bar, and 10 bar. Claim 13 In claim 1, the process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, supplied to a storage unit. Claim 14 In paragraph 13, a process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the liquid carbon dioxide is supercooled by, for example, heat exchange with an external refrigerant and / or raw carbon dioxide fluid before being supplied to a storage unit. Claim 15 In paragraph 13, the liquid carbon dioxide is a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the liquid carbon dioxide is stored at a pressure lower than the pressure at which the raw liquid carbon dioxide is purified by distillation. Claim 16 In claim 15, a process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the liquid carbon dioxide is stored at a pressure within the range of about 11 bar to about 15 bar. Claim 17 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein, in claim 1, the compression system is a multi-stage compressor, and the boil-off gas and / or displacement gas from the storage unit is heated by heat exchange with the compressed gas or the compressed gas derived from the compressed gas containing carbon dioxide, and is recirculated to the permeate gas in an intermediate stage of the multi-stage compressor. Claim 18 A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the tail gas is recirculated to the membrane separation system to recover the residual carbon dioxide. Claim 19 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the liquid carbon dioxide is reboiled by heat exchange with the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide. Claim 20 In claim 1, the process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the gaseous mixture is derived from biogas, preferably biogas produced by anaerobic digestion. Claim 21 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gaseous mixture contains about 60 mol% or less of carbon dioxide. Claim 22 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gaseous mixture comprises about 30 mol% to about 80 mol% methane. Claim 23 A process for producing liquid carbon dioxide from a gaseous mixture of carbon dioxide and methane, wherein the gas supplied to the compression system comprises about 90 mol% or less of carbon dioxide. Claim 24 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gas supplied to the compression system comprises about 1 mol% to about 50 mol% methane. Claim 25 A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, comprising: separating said gaseous mixture in a membrane separation system to produce carbon dioxide-enriched permeate gas and methane-enriched residual gas; compressing said permeate gas, or a combined gas mixture containing said permeate gas, in a liquefier supply compressor to produce a compressed gas containing carbon dioxide; cooling and partially condensing said compressed gas or a compressed gas derived from said compressed gas containing said carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide; phase separating said partially condensed fluid to produce raw carbon dioxide liquid containing methane, and a tail gas containing methane and residual carbon dioxide; and recovering the residual carbon dioxide from said tail gas to produce recovered carbon dioxide recirculated to said liquefier supply compressor. A process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, comprising the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched top vapor after pressure reduction, wherein a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched top vapor. Claim 26 In paragraph 25, a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gaseous mixture contains about 30 mol% to about 50 mol% of carbon dioxide. Claim 27 In paragraph 25, a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gaseous mixture contains about 30 mol% to about 70 mol% methane. Claim 28 In paragraph 25, a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gas supplied to the liquefier supply compressor contains about 70 mol% to about 90 mol% carbon dioxide. Claim 29 In paragraph 25, a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the gas supplied to the liquefier supply compressor contains about 5 mol% to about 30 mol% methane. Claim 30 In paragraph 25, the residual carbon dioxide is a process for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, which is recovered from the tail gas by recirculating the tail gas to the membrane separation system. Claim 31 A process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, comprising: combining the biogas with carbon dioxide-enriched permeate gas to produce a combined gas mixture containing permeate gas; compressing the combined gas mixture or the gas containing the combined gas mixture in a compression system to produce a compressed gas containing carbon dioxide; cooling and partially condensing the compressed gas or the compressed gas containing carbon dioxide by heat exchange to produce a partially condensed fluid containing carbon dioxide; phase separating the partially condensed fluid to produce raw liquid carbon dioxide containing methane, and a tail gas containing methane and residual carbon dioxide; and separating the tail gas in a membrane separation system to produce the carbon dioxide-enriched permeate gas and the methane-enriched residual gas. A process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, comprising the step of purifying the raw carbon dioxide liquid by distillation to produce liquid carbon dioxide and methane-enriched top steam after pressure reduction, wherein a refrigeration duty is required to cool and partially condense the compressed gas or the compressed gas derived from the compressed gas containing the carbon dioxide to produce the partially condensed fluid, and a portion of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched top steam. Claim 32 In claim 31, a process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, wherein the tail gas contains about 20 mol% to about 40 mol% carbon dioxide. Claim 33 In claim 31, a process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, wherein the tail gas contains about 60 mol% to about 80 mol% methane. Claim 34 In claim 31, a process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, wherein the gas supplied to the compression system contains about 50 mol% to about 70 mol% carbon dioxide. Claim 35 In claim 31, a process for producing liquid carbon dioxide from biogas containing carbon dioxide and methane, wherein the gas supplied to the compression system contains about 30 mol% to about 50 mol% methane. Claim 36 An apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, comprising: a membrane separation system comprising an inlet for said gaseous mixture, a first outlet for carbon dioxide-enriched permeate gas, and a second outlet for methane-enriched gas; a compression system comprising an inlet communicating fluid flow with said first outlet of the membrane separation system and an outlet for a compressed gas comprising carbon dioxide; a phase separator comprising an inlet communicating fluid flow with said outlet of the compression system, a first outlet for raw carbon dioxide liquid, and a second outlet for a tail gas; and a first pressure reduction unit comprising an inlet communicating fluid flow with said first outlet of the phase separator and an outlet for raw carbon dioxide liquid at reduced pressure. An apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, comprising a distillation column including an inlet communicating fluid flow with the outlet of the first pressure reduction unit, a first outlet for a liquid carbon dioxide product, a second outlet for a methane-enriched top vapor, and a reboiling unit, wherein the apparatus comprises a heat exchange system positioned and arranged to cool and partially condense a compressed gas containing carbon dioxide by heat exchange with the tail gas from the phase separator and / or the methane-enriched top vapor from the distillation column and at least one refrigerant. Claim 37 In paragraph 36, the apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane comprises, wherein the heat exchange system comprises at least one heat exchanger positioned between the second outlet of the phase separator and the compression system and arranged to cool the compressed gas containing carbon dioxide by heat exchange with the tail gas. Claim 38 In paragraph 36, the second outlet of the phase separator is in fluid flow communication with the inlet of the membrane separation system, an apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane. Claim 39 In paragraph 36, the apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage that communicates fluid flow with the second outlet of the distillation column. Claim 40 In paragraph 39, the heat exchange system comprises a heat exchanger positioned between the second outlet of the distillation column and the inlet of the intermediate stage of the compression system, arranged to cool the compressed gas containing carbon dioxide by heat exchange with the second gas-enriched upper steam, for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane. Claim 41 In paragraph 36, the reboiling device of the distillation column is part of a heat exchange system and is arranged to reboil liquid carbon dioxide by heat exchange with a compressed gas containing carbon dioxide, an apparatus for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane. Claim 42 In paragraph 36, the apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane comprises a storage vessel including an inlet communicating fluid flow with the first outlet of the distillation column, a first outlet for the liquid carbon dioxide product, and a second outlet for the evaporated gas and / or replacement gas. Claim 43 An apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the heat exchange system comprises a heat exchanger positioned between the first outlet of the distillation column and the inlet of the storage vessel and arranged to supercool the liquid carbon dioxide product by heat exchange with at least one refrigerant, e.g., an external refrigerant. Claim 44 In paragraph 36, the above heat exchange system comprises an external refrigerant circuit, a device for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane. Claim 45 In paragraph 44, the device for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein the external refrigerant is liquid ammonia. Claim 46 In paragraph 36, the device for producing liquid carbon dioxide from a gaseous mixture containing carbon dioxide and methane, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage of the compressor that communicates fluid flow with the second outlet of the storage vessel. Claim 47 An apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, wherein, in paragraph 36, a second pressure reduction unit comprising an inlet communicating with the first outlet of the distillation column via fluid flow and an outlet communicating with the inlet of the storage vessel via fluid flow. Claim 48 In paragraph 36, the heat exchange system comprises a multiple pass brazed aluminum heat exchanger, an apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane.

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