Process and apparatus for recovering and liquefying carbon dioxide

Abstract

Liquid carbon dioxide is produced from a gaseous mixture comprising carbon dioxide and methane by separating the gaseous mixture into a carbon dioxide-enriched gas and a methane-enriched gas, compressing the carbon dioxide-enriched gas, cooling and partially condensing the compressed gas, phase separating the partially condensed fluid, reducing the pressure of the crude carbon dioxide liquid and distilling the reduced pressure liquid to produce liquid carbon dioxide product. Costs are reduced and efficiency improved by using the vaporous products from the phase separation and distillation to help cool and partially condense the compressed gas.

Description

TITLE OF THE INVENTIONProcess and Apparatus for Recovering and Liquefying Carbon DioxideTECHNICAL FIELD OF THE INVENTION

[0001] The present invention is in the field of carbon dioxide recovery and liquefaction, specifically from gaseous mixtures comprising carbon dioxide and methane. The invention has particular application in recovering and liquefying carbon dioxide from such gaseous mixtures that are generated by fermentation and / or from biogas.BACKGROUND OF THE INVENTION

[0002] Carbon dioxide (CO2) generated in biological processes, or "biogenic" carbon dioxide, has only recently been considered as a potential source for high quality, food grade liquid carbon dioxide. Biogenic carbon dioxide can also be combined with renewable hydrogen to make eFuels such as eMethanol or Synthetic Natural Gas (SNG).

[0003] One of the main challenges to generation 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 carbon dioxide concentrations above 95 mol. %. Such feeds are generally compressed to pressures in a range from about 22 bar to about 24 bar and then liquefied with external refrigerant, typically liquid ammonia.

[0004] Such standard processes are not applied to mixtures of carbon dioxide and methane (CH4) with concentrations of carbon dioxide below 90 mol. % because higher compression power would be required (as the compressor must now also compress methane and other impurities, e.g., nitrogen (N2) and oxygen (O2), instead of only carbon dioxide with low levels of impurities) and a significant vent stream would be created from the cryogenic separation of carbon dioxide and methane. In this regard, the vent stream would not only contain most of the methane introduced into the liquefaction unit, but also significant amounts of carbon dioxide. The vent stream would have to be vented to the atmosphere as it could not be recycled to the feed of liquefaction unit.

[0005] Previous attempts at recovering carbon dioxide generated in biological processes include CN111256431 A (published 2020). In this process, biogas is pre-treated by desulfurization, drying and filtration. The pre-treated biogas is then compressed and fed to a membrane separation system to produce a permeate gas enriched in carbon dioxide and a retentate gas enriched in methane. The permeate gas is compressed, cooled and then fed to a rectification column to produce liquid carbon dioxide and impurity-enriched overhead vapour. Refrigeration duty is provided by an external mixed refrigerant circuit.

[0006] There is a need for an improved process and apparatus for generating liquid carbon dioxide from biogas and similar gaseous mixtures.BRIEF SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, there is provided a process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said process comprising separating the gaseous mixture in a membrane separation system to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; compressing the permeate gas, or a combined gas mixture comprising the permeate gas, in a compression system to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide which is recycled to the compression system; and, after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour, wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.

[0008] While part of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, may be provided by heat exchange with either the tail gas or the methane-enriched overhead vapour, this part of the refrigeration duty is typically provided by heat exchange with the tail gas, optionally in combination with the methane-enriched overhead vapour.

[0009] Conventional liquid carbon dioxide plants produce liquid carbon dioxide at a rate in a range from about 100 to about 400 tonnes / day. The invention may be applied within plants producing liquid carbon dioxide at or above conventional rates although the invention has particular application in plants producing liquid carbon dioxide at a rate in a range from about 50 to about 100 tonnes / day.

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

[0011] In addition, the use of a phase separator at feed pressure removes the bulk of the impurities to allow them to be recycled to the membrane separator without further compression.

[0012] Further, while the distillation column may be conventional, the overhead can be recycled back to the compression system as impurities are being recovered in the phase separation.

[0013] Moreover, the recovery of carbon dioxide and methane is improved compared with equivalent conventional processes.

[0014] According to a second aspect of the present invention, there is provided apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said apparatus comprising a membrane separation system comprising an inlet for the 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 in fluid flow communication with the first outlet of the membrane separation system and an outlet for compressed gas comprising carbon dioxide; a phase separator comprising an inlet in fluid flow communication with the outlet of the compression system, a first outlet for crude carbon dioxide liquid and a second outlet for tail gas; a first pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the phase separator and an outlet for crude carbon dioxide liquid at reduced pressure; and a distillation system comprising an inlet in fluid flow communication with the outlet of the first pressure reduction unit, a first outlet for liquid carbon dioxide product, a second outlet for methane-enriched overhead vapour and a reboiler, wherein the apparatus comprises a heat exchange system arranged and located for cooling and partially condensing compressed gas comprising carbon dioxide by heat exchange with tail gas from the phase separator and / or methane-enriched overhead vapour from the distillation system, and at least one other refrigerant.

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

[0016] Preferred embodiments of the present invention are distinguished over the art by:• Recovery of heat from the heated tail gas to the feed to the DeOXO unit reduces the additional duty required from a heater to heat the DeOXO feed to the temperature required for the DeOXO reaction;• Low dew point recycle from the cold box may be mixed with feed to membrane units to increase the dew point margin;• Lower carbon dioxide content in the compressed permeate gas;• Return of tail gas from flash separator to membrane separator unit to improve recovery of carbon dioxide and methane;• Pressure to which the permeate gas is compressed is higher than the operating pressure of the distillation column;• A liquid-gas separation is performed at high pressure after compression and cooling but before distillation;• Use of external refrigerant for condensation of feed stream; and• Use of multiple pass brazed aluminum heat exchanger.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a simplified flowsheet depicting a first embodiment of an integrated process according to the present invention.

[0018] FIG. 2 is a simplified flowsheet depicting a second embodiment of an integrated process according to the present invention.

[0019] FIG. 3 is a simplified flowsheet depicting a third embodiment of an integrated process according to the present invention.

[0020] FIG. 4 is a simplified flowsheet depicting a fourth embodiment of an integrated process according to the present invention.

[0021] FIG. 5 is a simplified flowsheet depicting a fifth embodiment of an integrated process according to the present invention.

[0022] FIG. 6 is a simplified flowsheet depicting a sixth embodiment of an integrated process according to the present invention.

[0023] FIG. 7 is a simplified flowsheet of a reference process in which carbon dioxide liquefier is not integrated with the carbon dioxide separation system.

[0024] FIG. 8 is a bar chart depicting the estimated annualized cost (MM$) of the reference process of FIG. 7 ("Bolt-on CO2 liquefier" with low or medium N2 content in feed) compared with the integrated process of FIG. 3 ("Integrated" CO2 liquefier with low or medium N2 content in feed).

[0025] FIG. 9 is a simplified flowsheet depicting a seventh embodiment of an integrated process according to the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0026] Throughout the specification, any references to pressure are references to absolute pressure unless otherwise stated. In addition, all percentages are calculated on the basis of molarity, i.e. mol. %, unless otherwise stated.

[0027] The expression "super-atmospheric pressure" is intended to refer to pressures that are significantly greater than atmospheric pressure, e.g., at least 10 bar and preferably at least 20 bar. Operation at super-atmospheric pressures is limited only by the apparatus. Typically, a super-atmospheric pressure could be up to 70 bar, particularly in cases where the carbon dioxide content in the feed to the membrane separation systems is low, e.g., about 20 mol. %, or perhaps up to 40 bar in other embodiments.

[0028] The expression "methane-enriched" in the context of a methane-enriched gas (or other fluid) is intended to describe the composition of a product fluid in which the proportion of methane is greater than in the feed from which the product fluid is generated. Corresponding expressions involving different gases, e.g., the expression "carbon dioxide-enriched", are to be interpreted accordingly.

[0029] The expression "oxygen-depleted" in the context of an oxygen-depleted gas (or other fluid) is intended to describe the composition of a product fluid in which the proportion of oxygen is less than in feed from which the product fluid is generated. The expression "oxygenlean" in the context of an oxygen-lean gas (or other fluid) is intended to describe the composition of a product fluid in which there is no oxygen, or substantially (or essentially) no oxygen. Corresponding expressions involving different gases are to be interpreted accordingly.

[0030] The expression "partially condensed" in the context of a partially condensed fluid is intended to refer to the fluid having both a vapor phase and a liquid phase.

[0031] The expression "refrigeration duty" is intended to refer to the cooling duty by transfer of latent heat and / or sensible heat by heat exchange between fluids at different temperatures.

[0032] The expression "in fluid flow communication" will be understood to mean that piping or other suitable conduits will be used to convey fluid from one specified location to another. During passage between the two locations, the fluid may flow through one or more other units which may be designed and / or arranged to alter the physical condition, e.g., temperature (e.g., a heat exchanger) and / or pressure (e.g., a compressor, a pump, a pressure reduction valve or an expander) of the fluid, or the composition of the fluid through reaction of components within the fluid (e.g., a catalytic reactor). The expression "in direct fluid flow communication" will be understood to mean that the fluid flows directly from the one location to the other, i.e., does not flow through another such unit during its passage and hence there is at least essentially no change to the composition or physical condition of the fluid.

[0033] The term "downstream" will be understood to mean in the direction of flow of a fluid under normal operation. The term "upstream" is to be interpreted accordingly.

[0034] The expression "located between" in the context of a heat exchanger being located between two other apparatus units in fluid flow communication with each other will be understood to mean that the heat exchanger is provided at a position intermediate to the positions of the other apparatus units where it is able to alter the temperature of a process fluid flowing from one apparatus unit to the other by heat exchange with another fluid. For the avoidance of doubt, the fluids within the heat exchanger are not mixed and remain independent of each other.

[0035] The "gaseous mixture" is the feed to the membrane separation system and, in some embodiments, will therefore be the tail gas from the phase separation.

[0036] The gaseous mixture may be a binary mixture of carbon dioxide and methane. However, other components may be present in the gaseous mixture as impurities depending on the source of the gaseous mixture. In this regard, the gaseous mixture is typically derived from biogas. Biogas is produced from raw organic materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, waste-water and food waste, typically by anaerobic digestion with anaerobic organisms and / or methanogens. Biogas is also produced by fermentation from a landfill site.

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

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

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

[0040] Gaseous mixtures that are particularly suitable for use with the present invention are derived from biogas generated by anaerobic digestion as these mixtures typically have a "low" nitrogen content and hence processing units to remove the nitrogen, e.g., NRUs, are not typically required.

[0041] The present invention is intended primarily to recover and liquefy carbon dioxide from gaseous mixtures comprising no more than about 60 mol. % carbon dioxide, e.g., from about 30 mol. % to about 50 mol. % carbon dioxide or from about 20 mol. % to about 40 mol. % carbon dioxide. The gas mixtures typically comprise from about 30 mol. % to about 80 mol.% methane, e.g., from about 30 mol. % to about 70 mol. % methane or from about 60 mol. % to about 80 mol. % methane.

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

[0043] The gaseous mixture being fed to the membrane separation system is typically at a super-atmospheric pressure suitable to drive the separation across the membrane, e.g., at least 10 bar or at least 15 bar. The pressure of the gaseous mixture is typically in a range from about 10 bar to about 70 bar, e.g., from about 15 bar to about 20 bar, or about 18 bar, when membranes operating at lower pressures are used; or from 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 comprises from about 50 mol. % to about 90 mol. %, e.g., from about 70 mol. % to about 85 mol. %, or about 80 mol. %, carbon dioxide.

[0045] The methane-enriched retentate gas typically comprise from about 60 mol. % to about 100 mol. %, e.g., from about 85 mol. % to about 100 mol. % or from about 90 mol. % to about 99 mol. %, or about 96 mol. %, methane.

[0046] The carbon dioxide-enriched gas is typically at a pressure in a range from about 0.1 bar gauge ("barg") to about 15 barg, e.g., from about 1 barg to about 10 barg, or from about 2 barg to about 5 barg, or about 3 barg to about 4 barg.

[0047] The carbon dioxide-enriched permeate gas (or a combined gas mixture comprising the carbon dioxide-enriched permeate gas) is then compressed in a compression system, e.g., a liquefier feed compressor, to produce a compressed gas comprising carbon dioxide which is typically at a pressure in a range from about 20 bar to about 40 bar, e.g., from about 25 bar to about 35 bar, or from about 30 bar to about 32 bar.

[0048] Compression of the gas may require more than one compression stage. In conventional arrangements, the gas being compressed is cooled between the stages of compression, resulting 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, thereby reducing the water content in the gas.

[0049] Compressed gas leaving the compression system is typically cooled before being further processed. Cooling can be performed in a heat exchanger that is a part of the compression system or in a heat exchanger that is separate from but in direct fluid flow communication with the compression system. Such a heat exchanger is usually referred to as an aftercooler. Cooling the compressed gas often results in the formation of a condensatewhich again is mostly water but may contain some dissolved components such as carbon dioxide. The condensate is typically separated and removed from the compressed gas, thereby reducing the water content in the gas further.

[0050] If sufficiently dry already, e.g., the water concentration in the compressed gas is no more than 100 ppm, the compressed gas (or a compressed gas derived therefrom comprising carbon dioxide) may be fed directly to a heat exchanger where it is cooled and partially condensed by heat exchange to produce a partially condensed fluid comprising carbon dioxide.

[0051] In cases where there is a higher concentration of water in the gas at this stage, 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 water is removed from the gas by adsorption using one or more suitable adsorbents. Use of a TSA unit removes water from the compressed gas to a level below 100 ppm, e.g., below 50 ppm and preferably below 5 ppm or even more preferably below 1 ppm, to prevent blockages in the heat exchanger and other problems resulting from water freezing out of the gas when cooled.

[0052] The water removed by the TSA unit is left behind on the adsorbent(s). The TSA operates with multiple beds - one or more online whilst other(s) are being regenerated. Regeneration is the process of removing water from the adsorbent(s) so that the bed can be brought back into service to remove water from the compressed gas. Many options are available for the gas used to regenerate the adsorbent beds in the TSA, and some of these options are also well known in the art. For example, a separate stream of nitrogen can be provided and, after heating, used to remove water. Part of the dried stream from the TSA unit itself can be heated and used to drive water off the adsorbent(s). This water would then be removed by condensation. The wet feed can also be used, after being suitably heated. Other process streams (either full or part) can also be used, such as the stream being recycled back to the membrane process, or the column overhead stream and / or the boiloff from the storage tanks. In the case of using the column overhead or boiloff, the stream would pick up water from the beds in the TSA and then be fed to the compression system, e.g., the liquefier feed compressor - the water is then at least partially removed in the intercooler(s) and / or aftercooler of the compression system. It would also be possible to vaporize part of the crude carbon dioxide liquid and use this gas to regenerate the beds. This stream will go to the compression system after use as the regeneration gas.

[0053] While TSA may be preferred for use with the present invention, other drying processes capable of lowering the water content to the required levels may be used, such as a membrane dryer.

[0054] Other impurities may be removed from the compressed gas at this stage, if required.

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

[0056] The partially condensed fluid is phase separated, e.g., the liquid and vapour phases are separated in a vessel, to produce a crude carbon dioxide liquid, and a tail gas comprising methane and residual carbon dioxide.

[0057] The crude carbon dioxide liquid typically comprises from about 80 mol. % to about 98 mol. %, e.g., about 93 mol. %, carbon dioxide.

[0058] The tail gas typically comprises from about 20 mol % to about 60 mol. %, e.g., about 55 mol. %, carbon dioxide, and from about 40 mol % to about 60 mol. %, e.g., about 45 mol. %, methane.

[0059] Residual carbon dioxide is recovered from at least part of the tail gas to produce recovered carbon dioxide which is recycled, either directly or indirectly, to the compression system, e.g., the liquefier feed compressor.

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

[0061] In other embodiments, the initial (or bulk) separation of the carbon dioxide and methane takes place in a phase separator (see FIG. 6). These embodiments may be referred to as the "phase separator-first" embodiments. In these embodiments, all (or part) of the tail gas forms the whole feed to the membrane separation system producing the carbon dioxideenriched permeate gas and methane-enriched retentate gas. Since it forms the feed to the membrane separation unit, the tail gas in these embodiments is the "gaseous mixture" defined in the claims.

[0062] In all embodiments, the tail gas is typically at or above the pressure of the gas being fed to the membrane separation system, e.g., at a pressure up to about 40 bar. In embodiments where the membrane separation system is operating at a higher pressure, e.g., in a range from about 25 bar to about 35 bar, (re)compression of the tail gas prior to feeding to the membrane separation system is not typically required. However, in embodiments where the membrane separation system is operating at a lower pressure, e.g., in a range from about 15 bar to about 20 bar, it is typically necessary to reduce the pressure of the tail gas, e.g., across a pressure reduction valve, before the tail gas is fed to the membrane separation system.

[0063] In some "membrane-first" embodiments, the tail gas may be split with a first part being fed into the feed to a DeOXO unit (to increase the temperature of the feed to that unit) and with the second part being fed directly to feed to the membrane separation system (to increase the dew point of the feed to that system).

[0064] The pressure of the crude carbon dioxide liquid is reduced to a pressure typically in a range from about 15 bar to about 20 bar, e.g., about 19 bar. The crude carbon dioxide liquid is then fed at the reduced pressure to a distillation (or stripping) column system comprising at least one column, where it is purified to form liquid carbon dioxide and methane-enriched overhead vapour. The skilled person would appreciate that the feed to the column will usually be at the saturation temperature for the operating pressure of the column.

[0065] The liquid carbon dioxide typically comprises from about 98 mol. % to 100 mol. %, e.g., about 99.999 mol. %, carbon dioxide.

[0066] The overhead vapour typically comprises from about 60 mol. % to about 70 mol %, e.g., about 65 mol. %, carbon dioxide, and from 30 mol % to about 50 mol. %, e.g., about 40 mol. %, methane. Any light gases, e.g., nitrogen and / or oxygen, that are present in the biogas feed in the "phase separator-first" or have permeated through the membrane in the “membrane-first” case, will also be present in this stream. Thus, the overhead vapour may further comprise up to about 1 mol. % nitrogen and / or up to about 1 mol. % oxygen.

[0067] The present invention is characterized by the thermal integration of the carbon dioxide liquefier. In this regard, the inventors realized that several process fluids generated in the cold box of the liquefier are in an appropriate physical condition such that they would be suitable 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 recycling, the tail gas is typically used to provide a first part, e.g., from about 5% to about 15%, or from about 10% to about 12%, of the refrigeration duty required to cool and partially condense the compressed gas.

[0069] Another such fluid is the overhead vapour generated in the distillation (or stripping) step. Thus, the overhead vapour may be used to provide a part of the refrigeration duty required to cool and partially condense the compressed gas, typically (but not necessarily) together with a first part provided by tail gas. The part of the refrigeration duty provided by the overhead vapour is typically from about 1% to about 3% of the refrigeration duty in question, which is significantly less than the part of the refrigeration duty provided by the tail gas.

[0070] The resultant warmed overhead gas may be recycled within the process to recover methane and residual carbon dioxide. In this regard, the warmed overhead gas is typically at the operating pressure of the distillation column system, e.g., at a pressure in the range from about 15 bar to about 20 bar. In embodiments in which the compression system is a singlestage compressor, the warmed overhead gas may be let down in pressure and fed to the feed to the compression system. In embodiments in which the compression system is a multistage compressor, the warmed overhead gas may be fed to an appropriate interstage of the compressor, after pressure let down as appropriate, where it would be recompressed to the feed pressure to the main heat exchanger of the liquefier.

[0071] Additional process fluids that may be used to provide refrigeration duty to cool and partially condense the compressed gas include the crude carbon dioxide liquid generated in the phase separation step and the liquid carbon dioxide generated in the distillation (or stripping) step.

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

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

[0074] Boil-off gas and / or displacement gas from the storage vessel may be sufficiently cold to provide refrigeration duty to the compressed gas. Thus, if there is a sufficient amount, this gas may be heated by heat exchange with compressed gas and recycled to the carbon dioxide-enriched gas at an intermediate stage of the liquefier feed compressor.

[0075] The balance of the refrigeration duty is typically provided by heat exchange with a refrigerant in an external refrigerant circuit. The refrigerant may be a mixed refrigerant or 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] Regarding the apparatus, the membrane separation system comprises an inlet for the gaseous mixture, a first outlet for carbon dioxide-enriched gas and a second outlet for methane-enriched fluid. The membrane separation system may include one or more stages with each comprising one or more membrane separation units. Conventional membrane separation units may be used involving any suitable carbon dioxide-selective polymeric membrane in any suitable configuration, e.g., hollow fiber, spiral wound fiber, etc.

[0077] The compression system comprises an inlet in fluid flow communication with the first outlet of the membrane separation system and an outlet for compressed gas comprising carbon dioxide. Positive displacement, e.g. reciprocating, compressors or centrifugal compressors may be used. However, use of an oil-flooded screw compressor is typically preferred. The compression system may be single stage compressor, or a multistage compressor comprising an inlet to an intermediate stage of the compression system in fluid flow communication with the second outlet of the distillation (or stripping) column and / or an inlet to an intermediate stage of the compression system in fluid flow communication with the second outlet of the storage vessel. The compression system typically further comprises an aftercooler to cool the compressed gas.

[0078] The apparatus may further comprise a gas-liquid separator to remove water (or aqueous) condensate from cooled compressed gas. The separator comprises an inlet in fluid flow communication with the outlet of the aftercooler of the compression system, a first outlet in fluid flow communication with the inlet of a downstream unit, e.g., a TSA unit for drying the compressed gas, and a second outlet for water condensate. The separator may be equipped with a demister unit which eliminates water droplets from the cooled compressed gas. In case an oil-flooded screw compressor is used, the gas-liquid separator may also comprise an oil removal system to catch droplets of oil originating from compressor but entrained within the gas.

[0079] The apparatus may comprise a drier, e.g., a TSA unit or a membrane dryer, for drying cooled compressed gas. Such a drier unit comprises an inlet in fluid flow communication with the outlet of an upstream unit, e.g., the gas-liquid separator, and an outlet in fluid flow communication with the inlet of a downstream unit, e.g., the heat exchanger.

[0080] The phase separator, e.g., a separator vessel, comprises an inlet in fluid flow communication with the outlet of the compression system, a first outlet for crude carbon dioxide liquid and a second outlet for tail gas. The second outlet is typically in fluid flow communication with the inlet of the membrane separation system.

[0081] The pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the phase separator and an outlet for crude carbon dioxide liquid at reduced pressure. The unit is typically a pressure reduction valve.

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

[0083] The storage vessel comprises an inlet in fluid flow communication with the first outlet of the distillation (or stripping) column, a first outlet for liquid carbon dioxide and a second outlet for boil off gas and / or displacement gas.

[0084] The apparatus comprises a heat exchange system located and arranged for cooling and partially condensing compressed gas by heat exchange against either tail gas from the phase separator or overhead vapor from the distillation (or stripping) column or both, together with at least one other refrigerant.

[0085] The heat exchange system typically comprises at least one heat exchanger located between the second outlet of the phase separator and the compression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with tail gas. In some embodiments, there are two such heat exchangers arranged in series, e.g., a first heat exchanger comprising demister to remove oil entrailed in the compressed gas from the compression system and a second heat exchanger downstream (with respect to the flow of compressed gas) of the first heat exchanger.

[0086] Alternatively (or, more usually, additionally), the heat exchange system may comprise a heat exchanger located between the second outlet of the distillation (or stripping) column and the compression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with methane-enriched overhead vapor.

[0087] The reboiler of the distillation column may be part of the heat exchange system. In these embodiments, the reboiler is arranged for reboiling liquid carbon dioxide by heat exchange with compressed gas comprising carbon dioxide.

[0088] The heat exchange system may comprise a heat exchanger located between the first outlet of the distillation column and the inlet of the storage vessel and arranged for subcooling liquid carbon dioxide produce by heat exchange against at least one refrigerant, e.g., an external refrigerant.

[0089] The apparatus typically comprises an external refrigerant circuit to provide at least the balance of the refrigeration duty. The external refrigerant typically provides the majority, e.g., over 50%, preferably over 75%, and more preferably over 80%, of the refrigeration duty for cooling and partially condensing the compressed gas. Any suitable refrigerant may be used but liquid ammonia is preferred.

[0090] The apparatus may further comprise a second pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the distillation column and an outlet in fluid flow communication with the inlet of the storage vessel. In these embodiments, the heat exchange system typically comprises a heat exchanger for subcooling 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 in which the compression system is a multistage compressor, the compressor may comprise a second inlet to an intermediate stage of the compressor in fluid flow communication with the second outlet of the storage vessel.

[0092] The heat exchange system may further comprise a heat exchanger located between the outlet of the compression system and the second outlet of the storage vessel and arranged for cooling compressed gas by heat exchange with boil off gas and / or displaced gas.

[0093] The heat exchange system may comprise a plurality of individual heat exchangers or alternatively may be a single heat exchanger having multiple passages for the various process streams to be cooled by heat exchange with one or more of the (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 invention include:#1. A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said process comprising: separating the gaseous mixture in a membrane separation system to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; compressing the permeate gas, or a combined gas mixture comprising the permeate gas, in a compression system produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide which is recycled to the compression system; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour; wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.#2. A process according to #1 , wherein the partially condensed fluid is at a pressure in a range from about 20 bar to about 40 bar, or from about 25 bar to about 33 bar, or from about 28 bar to about 31 bar.#3. A process according to #1 or #2, wherein the crude carbon dioxide liquid after pressure reduction is at a pressure in a range from about 12 to about 20 bar.#4. A process according to any of #1 to #3, wherein a first portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with the tail gas.#5. A process according to #4, wherein the first portion is from about 5% to about 15%.#6. A process according to any of #1 to #5, wherein a second portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with the methane-enriched overhead vapour.#7. A process according to #6, wherein the second portion is from about 1% to about 3%.#8. A process according to #6 or #7, wherein the compression system is a multistage compressor and the methane-enriched overhead vapour is recycled after the heat exchange to the permeate gas at an intermediate stage of the multistage compressor.#9. A process according to any of #1 to #8, wherein a third portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with crude carbon dioxide liquid and / or liquid carbon dioxide.#10. A process according to any of #1 to #9, wherein the balance, e.g., over 50% or over 75% or over 80%, of the refrigerant duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with an external refrigerant.#11. A process according to #10, wherein the external refrigerant is liquid ammonia.#12. A process according to #11 , wherein the liquid ammonia is evaporated in an external refrigerant circuit at about 1 bar, 5 bar and 10 bar.#13. A process according to any of #1 to #12, wherein the liquid carbon dioxide is fed to storage.#14. A process according to #13, wherein the liquid carbon dioxide is subcooled by heat exchange against, e.g., external refrigerant and / or crude carbon dioxide fluid, before being fed to storage.#15. A process according to #13 or #14, wherein the liquid carbon dioxide is stored at a pressure lower than the pressure at which the crude carbon dioxide liquid is purified by distillation.#16. A process according to #15, wherein the liquid carbon dioxide is stored at a pressure in a range from about 11 bar to about 15 bar.#17. A process according to any of #1 to #16, wherein the compression system is a multistage compressor and boil-off and / or displacement gas from storage is heated by heat exchange with the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, and recycled to the permeate gas at an intermediate stage of the multistage compressor.#18. A process according to any of #1 to #17, wherein the tail gas is recycled to the membrane separation system to recover the residual carbon dioxide.#19. A process according to any of #1 to #18, wherein liquid carbon dioxide is reboiled by heat exchange against the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide.#20. A process according to any of #1 to #19, wherein the gaseous mixture is derived from biogas, preferably generated by anaerobic digestion.#21. A process according to any of #1 to #20, wherein the gaseous mixture comprises no more than about 60 mol. % carbon dioxide.#22. A process according to any of #1 to #21 , wherein the gaseous mixture comprises from about 30 mol. % to about 80 mol. % methane.#23. A process according to any of #1 to #22, wherein the gas being fed to the compression system comprises no more than about 90 mol. % carbon dioxide.#24. A process according to any of #1 to #23, wherein the gas being fed to the compression system comprises from about 5 mol. % to about 50 mol. % methane.#25. A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said process comprising: separating the gaseous mixture in a membrane separation system to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas;compressing the permeate gas, or a combined gas mixture comprising the permeate gas, in a liquefier feed compressor to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide which is recycled to the liquefier feed compressor; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour, wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and the methane-enriched overhead vapour.#26. A process according to #25, wherein the gaseous mixture comprises from about 30 mol. % to about 50 mol. % carbon dioxide.#27. A process according to #25 or #26, wherein the gaseous mixture comprises from about 30 mol. % to about 70 mol. % methane.#28. A process according to any of #25 to #27, wherein the gas being fed to the liquefier feed compressor comprises from about 70 mol. % to about 90 mol. % carbon dioxide.#29. A process according to any of #25 to #28, wherein the gas being fed to the liquefier feed compressor comprises from about 5 mol. % to about 30 mol. % methane.#30. A process according to any of #25 to #29, wherein the residual carbon dioxide is recovered from the tail gas by recycling the tail gas to the membrane separation system.#31 . A process for producing liquid carbon dioxide from biogas comprising carbon dioxide and methane, said process comprising: combining the biogas with a carbon dioxide-enriched permeate gas to form a combined gas mixture comprising the permeate gas;compressing the combined gas mixture, or a gas comprising the combined gas mixture, in a compression system to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; separating the tail gas in a membrane separation system to produce the carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour; wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.#32. A process according to #31 , wherein the tail gas comprises from about 20 mol. % to about 40 mol. % carbon dioxide.#33. A process according to #31 or #32, wherein the tail gas comprises from about 60 mol. % to about 80 mol. % methane.#34. A process according to any of #31 to #33, wherein the gas being fed to the compression system comprises from about 50 mol. % to about 70 mol. % carbon dioxide.#35. A process according to any of #31 to #34, wherein the gas being fed to the compression system comprises from about 30 mol. % to about 50 mol. % methane.#36. Apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said apparatus comprising: a membrane separation system comprising an inlet for the 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 in fluid flow communication with the first outlet of the membrane separation system and an outlet for compressed gas comprising carbon dioxide;a phase separator comprising an inlet in fluid flow communication with the outlet of the compression system, a first outlet for crude carbon dioxide liquid and a second outlet for tail gas; a first pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the phase separator and an outlet for crude carbon dioxide liquid at reduced pressure; and a distillation column comprising an inlet in fluid flow communication with the outlet of the first pressure reduction unit, a first outlet for liquid carbon dioxide product, a second outlet for methane-enriched overhead vapour and a reboiler, wherein the apparatus comprises a heat exchange system arranged and located for cooling and partially condensing compressed gas comprising carbon dioxide by heat exchange with tail gas from the phase separator and / or methane-enriched overhead vapour from the distillation column, and at least one other refrigerant.#37. An apparatus according to #36, wherein the heat exchange system comprises at least one heat exchanger located between the second outlet of the phase separator and the compression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with tail gas.#38. An apparatus according to #36 or #37, wherein the second outlet of the phase separator is in fluid flow communication with the inlet of the membrane separation system.#39. An apparatus according to any of #36 to #38, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage in fluid flow communication with the second outlet of the distillation column.#40. An apparatus according to #39, wherein the heat exchange system comprises a heat exchanger located between located between the second outlet of the distillation column and the inlet to the intermediate stage of the compression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with second gas- enriched overhead vapour.#41. An apparatus according to any of #36 to #40, wherein the reboiler of the distillation column is part of the heat exchange system and is arranged for reboiling liquid carbon dioxide by heat exchange with compressed gas comprising carbon dioxide.#42. An apparatus according to any or #36 to #41 comprising a storage vessel having an inlet in fluid flow communication with the first outlet of the distillation column, afirst outlet for liquid carbon dioxide product and a second outlet for boil off and / or displacement gas.#43. An apparatus according to #42, wherein the heat exchange system comprise a heat exchanger located between the first outlet of the distillation column and the inlet of the storage vessel and arranged for subcooling liquid carbon dioxide produce by heat exchange against at least one refrigerant, e.g., an external refrigerant.#44. An apparatus according to any of #36 to #43, wherein the heat exchange system comprises an external refrigerant circuit.#45. An apparatus according to #44, wherein the external refrigerant is liquid ammonia.#46. An apparatus according to any of #36 to #45, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage of the compressor in fluid flow communication with the second outlet of the storage vessel.#47. An apparatus according to any of #36 to #46 comprising a second pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the distillation column and an outlet in fluid flow communication with the inlet of the storage vessel.#48. An apparatus according to any of #36 to #47 wherein the heat exchange system comprises a multiple pass brazed aluminium heat exchanger.

[0095] The invention will now be described by way of example only with reference to the figures.

[0096] According to FIG. 1 , stream 10 of biogas comprising about 37 mol. % carbon dioxide and about 55 mol. % methane at atmospheric pressure is fed to a block 12 where it is upgraded to produce a stream 14 of crude carbon dioxide gas comprising about 81% carbon dioxide, and stream 16 of renewable natural gas (RNG), or "biomethane", comprising about 96% methane.

[0097] Block 12 typically contains a compression unit (not shown) for compressing biogas, together with one or more units (not shown) for pre-treating the compressed biogas such as an adsorption unit for drying the 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 the biogas.

[0098] The RNG stream 16 is fed to a natural gas pipeline (not shown) whereas the stream 14 of crude carbon dioxide gas, which is typically a permeate gas generated in the membrane unit(s) in block 12, is processed to produce liquid carbon dioxide.

[0099] Stream 14 of crude carbon dioxide gas at about 4 bar is fed to a multistage compressor K101 where it is compressed to produce a stream 18 of compressed gas at about 30 bar. Water is condensed out of the compressed gas in one or more intercoolers (not shown) and the condensate removed (shown by dashed line).

[0100] Stream 18 of compressed gas is cooled in an aftercooler (not shown) and condensate formed during cooling is removed in knock-out drum S101 to produce a stream 20 of condensate and a stream 22 of compressed gas. Stream 22 is fed to a temperature swing adsorption (TSA) unit D101 which dries the gas to form a stream 24 of dried, compressed gas. Water is removed from the TSA unit D101 (shown by a dashed line) during regeneration of the beds using a purge gas.

[0101] The dried, compressed gas is fed 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 fed to phase separator S102 where it is separated into a stream 28 of carbon dioxide-enriched liquid (comprising about 94 mol. % carbon dioxide) and a stream 30 of tail gas (comprising about 41 mol. % methane and about 57 mol. % residual carbon dioxide).

[0102] The pressure of the carbon dioxide-enriched liquid in stream 28 is reduced across valve V101 to produce stream 32 of carbon dioxide-enriched liquid at a pressure of about 19 bar which is fed to a stripping column C101 where it is separated into liquid carbon dioxide (having a purity of at least about 99.9 mol. %) and methane-enriched overhead vapour (comprising about 34 mol. % methane and about 65 mol. % carbon dioxide).

[0103] Stream 34 of liquid carbon dioxide is removed from the stripping column C101 , reboiled by heat exchange against the compressed gas in the main heat exchanger E101 and returned as stream 36 to the stripping column C101 to provide the vapour for the separation.

[0104] Stream 38 of liquid carbon dioxide is removed from the stripping column C101 , subcooled in the main heat exchanger E101 by heat exchange against liquid ammonia refrigerant in an external refrigeration circuit 40 and then fed as stream 42 to a storage vessel T101 where it is stored at a pressure lower than the operating pressure of the stripping column C101 , e.g., about 14 bar. Liquid carbon dioxide is removed from the storage vessel T101 in stream 44.

[0105] Stream 30 of tail gas is removed from the phase separator S102 and fed to the main heat exchanger E101 to help cool the compressed gas by heat exchange. The warmed tail gas is recycled as stream 46 to the block 12 for recovery of residual carbon dioxide and methane.

[0106] Stream 48 of methane-enriched overhead vapor is removed from the stripping column C101 and fed to the main heat exchanger to help cool and partially condense the compressed gas by heat exchange. The warmed overhead gas at a pressure of about 18 bar is recycled as stream 50 to an intermediate stage of the multistage compressor K101.

[0107] Boil off gas / displacement gas is removed from the storage vessel T101 and recycled as stream 52 to an intermediate stage of the multistage compressor K101.

[0108] The main heat exchanger E101 as depicted is a multi-pass brazed aluminum heat exchanger. However, one or more individual heat exchangers could be used to provide the necessary heat exchange between the compressed gas and one or more of the cold streams depicted, i.e., stream 30 of tail gas, stream 48 of overhead vapor, stream 34 of liquid carbon dioxide, and ammonia refrigerant from the external refrigeration circuit 40.

[0109] FIG. 2 is a simplified flowsheet depicting a second embodiment of the present invention. The flowsheet is similar to that depicted in FIG. 1 and features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0110] An additional heat exchanger E102 has been added to provide further cooling to stream 18 of compressed gas to produce a stream 19 of further cooled compressed gas. The cooling duty in heat exchanger E102 is provided by stream 46 of tail gas produced in the phase separator S102 after passage through the main heat exchanger E101. Additional cooling of the compressed gas may be provided using external refrigeration 40.

[0111] The use of the additional heat exchanger E102 to cool stream 18, e.g., to below 65°C, prior to feeding the stream to the main heat exchanger E101 simplifies the design and construction of the main heat exchanger E101. In addition, the heat exchanger E102 may be integrated with an oil and condensate removal system, which protects the main heat exchanger E101 against oil transfer from the multistage compressor K101 , should the compressor be of an oil-flooded type.

[0112] FIG. 3 is a simplified flowsheet depicting a possible arrangement of processing units within block 12 of the flow sheet depicted in FIG. 2. The flowsheet is similar to that depicted in FIG. 2 and the features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0113] Stream 10 of "low nitrogen" biogas, e.g., biogas generated 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 cooling duty, stream 54 of warmed tailed gas is combined with stream 56 to form a combined stream 58 which is fed to a DeOXO unit I1101 .

[0114] Biogas usually contains a small amount, e.g., up to about 2.5 mol. %, oxygen. However, such amounts of oxygen exceed the maximum amount allowed in natural gas forindustrial or domestic use. Therefore, the oxygen cannot be allowed to follow the methane into the RNG. In FIG. 3, oxygen is removed from the compressed biogas by catalytic combustion in DeOXO unit U101 to produce a stream 60 of oxygen-depleted biogas comprising less than 0.2 mol. % oxygen.

[0115] The oxygen-depleted biogas in stream 60 is fed to a membrane separation system M101 where it is separated into the carbon dioxide-enriched permeate gas (stream 14) and a first retentate gas. Stream 14 is fed to the multistage compression unit K101 and processed accordingly to FIG. 2. The retentate gas is fed to a second membrane separation system M102 where it is separated into a second permeate gas and the methane-enriched retentate gas. The second permeate gas is recycled in stream 62 to the biogas feed 10 to recover residual carbon dioxide and methane. The methane-enriched retentate gas is compressed in a compressor K103 to form the stream 16 of methane-enriched retentate gas as a pressure of about 41 bar.

[0116] The skilled person would appreciate that the location and / or arrangement of the DeOXO unit 11101 in FIG. 3 is just one possible location / arrangement for this unit. In the model presented here, the DeOXO unit 11101 removes just enough oxygen to meet the oxygen specification in the RNG product (stream 16). In reality, however, the DeOXO unit 11101 may be designed to remove all, or at least essentially all, of the oxygen in the feed to the unit U 101 , partly due to practical considerations over the control of partially removing the oxygen. In another arrangement, however, part of stream 58 could bypass the DeOXO unit 11101 so that just enough oxygen is removed to meet specification in the RNG.

[0117] Alternatively, the DeOXO unit could be placed further upstream in the process, such as before the point at which the recycle (stream 62) from the second membrane separation system M102 is combined with the compressed biogas feed (stream 56) or downstream of a "blower" unit (that is used as part of the compression system K102) to pressurize the feed biogas to an intermediate pressure to allow pre-treatment (e.g., removal of hydrogen sulfide, VOCs and / or siloxanes) of the biogas or, where the compression system K101 is a multistage compression system, the DeOXO unit may be located at an intermediate stage of the compression system, if that is desirable and feasible. The DeOXO unit may even be placed on the product RNG, although a dryer such as a TSA unit or a membrane dryer would also be required to remove the resultant water from the oxygen-depleted gas.

[0118] The DeOXO unit will typically include heating of the feed to the unit to the operating temperature of the DeOXO catalyst, cooling of the resultant oxygen-depleted gas, and condensation and separation of the condensed water (which is not shown in FIG. 3). The membrane separation system M101 will also typically include heating in order to ensure that that no liquid phase forms in the membrane unit(s). Typically, the feed to the membrane separation system is heated so that it is about 10°C above its dew point. Since the feed leavesthe separator of the DeOXO unit at its dew point, this would simply mean increasing the temperature by about 10°C. However, if some of warmed tail gas (stream 46) - which is dry - is mixed downstream of the DeOXO unit, the dew point of the feed to the membrane separation system M101 will drop, as the recycled stream contains no water or heavy components (already dew-pointed down to about -28°C in separator S102). This arrangement is not shown in the figures but does represent a potential integration, particularly if the DeOXO unit removes substantially all of the oxygen in the feed and so stream 54 will be oxygen-lean and so can be recycled to downstream of 11101 .

[0119] The membrane separation systems M101 & M 102 in this embodiment operate at "low" pressure, e.g., have a feed pressure in a range from about 15 bar to about 20 bar. Thus, the pressure of the tail gas (stream 29) from the flash drum S102 is reduced across a valve V102 prior to being sent to the main heat exchanger E101 to provide cooling duty.

[0120] FIG. 4 is a simplified flowsheet depicting a fourth embodiment of the present invention. The flowsheet is similar to that depicted in FIG. 3 and the features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0121] A third membrane separation system M 103 is used to recover further carbon dioxide from the methane retentate produced in the second membrane separation system M102. The carbon dioxide permeate from the third membrane separation system M 103 is recycled to the biogas feed 10. The retentate that is further enriched in methane is them compressed in the RNG compressor K103 to produce stream 16 of compressed RNG.

[0122] The membrane modules in this embodiment operate at a higher pressure than the membrane modules in FIG. 3, e.g., having a feed pressure in a range from about 25 bar to about 30 bar. Thus, the pressure of the tail gas (stream 30) from the flash drum S102 does not need to be reduced prior to being sent to the main heat exchanger E101 to provide cooling duty.

[0123] FIG. 5 is a simplified flowsheet depicting a fifth embodiment of the present invention. The flowsheet is similar to that depicted in FIG. 4 and the features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0124] This flowsheet is proposed for processing biogas having a higher concentration of nitrogen, e.g., from about 5 mol. % to about 30 mol. % nitrogen, e.g., biogas from landfill. Nitrogen typically comes out of membrane separation systems with the methane in the retentate gas. The nitrogen limit in natural gas is typically less than 3 mol. %. Accordingly, it is important that the nitrogen is removed before the methane is added to a natural gas pipeline. Typically, a nitrogen rejection unit (NRU) is used in this context.

[0125] The retentate gas from the third membrane separation unit M103 is fed to a carbon dioxide TSA unit D102 to remove residual carbon dioxide to produce a carbon dioxidedepleted methane gas which is then fed to an NRU in which nitrogen is rejected to produce nitrogen-depleted methane gas which is then compressed in the RNG compressor K103 before being fed in stream 16 to a natural gas pipeline (not shown).

[0126] The exact configuration of the NRU is not critical for the present invention. A cryogenic NRU could have one or more columns, and bottoms liquid methane is typically evaporated (after pumping or pressure reduction) at one or more pressures to form the NRG. In FIG. 5, the rejected nitrogen is simply vented ( / .e., not used) and part of the RNG product is used for regeneration, so the carbon dioxide ends up in the product (the membrane separation system has already removed it to the specification of the RNG and the TSA unit is just to remove it for cryogenic processing).

[0127] It would also be possible to use an adsorption-based NRU, for example a PSA.

[0128] FIG. 6 is a simplified flowsheet depicting a different configuration of a process according to the present invention. The flowsheet is similar to that depicted in FIG. 4 and the features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0129] A stream 10 of "low nitrogen" biogas, e.g., biogas generated by anaerobic digestion having less than 1 mol. % nitrogen, is compressed in compressor K102 and fed to DeOXO unit U101 to reduce the oxygen content in the gas so that the RNG stream 16 is within specification for oxygen. Oxygen-depleted gas from the DeOXO unit U101 is fed as stream 18 to heat exchanger E102 where it is cooled by heat exchange against ammonia in the external refrigerant circuit 40.

[0130] Water from the biogas feed and generated in the DeOXO unit U101 is condensed when the oxygen-depleted gas is cooled in heat exchanger E102. Therefore, the cooled gas is fed as stream 19 to phase separator S101 where the condensate is separated and removed as stream 20. The cooled gas is then fed as stream 22 to the TSA unit D101 where it is dried.

[0131] Stream 24 of dried gas from TSA unit D101 is fed to the main heat exchanger E101 where it is cooled and partially condensed and then fed (as stream 26) into flash drum S102 to produce impure carbon dioxide liquid, together with the tail gas comprising methane and residual carbon dioxide.

[0132] The impure carbon dioxide liquid is split into two portions, a first portion and a second portion.

[0133] The first portion (stream 28) is typically the minor portion, e.g., from about 30% to about 45% of the total mass flow of the impure carbon dioxide liquid, and is reduced in pressureacross valve V101 before being fed as stream 32 to the stripper column C101 where the carbon dioxide is purified.

[0134] The second portion, typically the major portion, e.g., from about 55% to 70% of the total mass flow of the impure carbon dioxide liquid, is fed as stream 64 to the main heat exchanger E101 where it is warmed by heat exchange with the compressed gas in stream 24 to produce stream 66 of impure carbon dioxide fluid which is then let down in pressure across valve V103 to reduce the temperature of the fluid. The degree by which stream 64 is heated before the stream is let down in pressure across V103 would be determined during the process optimization, and it may be that the optimal amount of heating is minor or zero and it would be better to feed stream 64 to V103 without preheating. Preheating of stream 64 is particularly necessary if stream 68 would otherwise be below the freezing point of carbon dioxide. Stream 68 of impure carbon dioxide fluid at the lower temperature is fed 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, stream 70 of impure carbon dioxide gas is recycled back to the biogas compressor K102.

[0135] Stream 30 of tail gas from phase separator S102, which contains carbon dioxide and methane, is passed through the main heat exchanger E101 to provide cooling duty and the resultant warmed stream 51 is fed into the membrane separation systems M101 , M102. Carbon dioxide-enriched permeate gases in streams 72 and 74 from the membrane separation systems, M101 , M102 are recycled back to the biogas compressor K102 in combined stream 62.

[0136] Overhead from the distillation column system C101 is recycled as streams 48 and 54 back to the biogas compressor K102 via the main heat exchanger E101 where it provides cooling duty. Stream 52, containing the boiloff gas from the carbon dioxide storage vessel T101 , is also recycled back to the biogas compressor K102.

[0137] RNG compressor K103 is optional depending on the RNG export pressure requirement.

[0138] The feeds to compressor K102 are indicated in Table 4 below as being at different pressures. As would be readily appreciated by the skilled person, these pressures would need to be optimized to match the compression system chosen for the required compression duty. For example, the compression system may have two compressors (or compression stages) in series and so there would be a first feed pressure to the first compressor (or stage) and a second pressure to the second compressor (or stage). Streams that were too low in pressure to be fed to the interstage, i.e., the feed pressure to the second stage, would be reduced in pressure, normally by a valve, and fed to the first stage. Streams with a pressure higher than the interstage pressure would similarly be dropped in pressure and fed to the interstage. There may be further stages, either due to more compressors in series, or a compressor mayhave an interstage feed. The pressures of streams 52, 54, 70, 72 and 74 would be optimized to reduce the power and / or capital cost of the process where this was preferable to just dropping the pressure of the stream at the feed to a lower pressure stage of the compressor.

[0139] One advantage of this configuration compared with the other configurations depicted herein is that only the biogas feed compressor K102 is required compared with the two compressors K101 and K102 in the other configurations. RNG compressor K103 may also be eliminated if biogas compressor K102 discharge pressure can be increased to RNG export pressure, if membrane systems M101 and M102 can operate at this higher pressure. The configuration in Figure 6 is a particular advantage at smaller flowrates as compressors at low flowrates can be more expensive or difficult to obtain. In this alternative configuration, the biogas compressor K102 will be larger than an equivalent configuration based on the other figures, but there is no requirement for a separate liquefier feed compressor K101.

[0140] However, the DeOXO unit 11101 in this case will need to remove oxygen down to a lower level, and the TSA unit D101 will be larger since it is processing all of the feed flowrate, plus the recycle. The power and number of membrane modules will be about the same - the major advantage being in reducing the number of compressors and enlarging the remaining compressor, which is an advantage in liquefied carbon dioxide plants producing less than 100 metric tons per day.

[0141] As mentioned above, some of the impure carbon dioxide liquid from the phase separator S102 in the liquefied carbon dioxide plant is used to provide extra refrigeration in the main heat exchanger E101 , colder than that provided by unit 40. It should be noted that this is also an option in the other configurations.

[0142] FIG. 7 is a simplified flowsheet depicting a reference process in which a carbon dioxide liquefier (block 100) is not integrated with the carbon dioxide separation system (block 12). The features common to this flowsheet and other flowsheets depicted herein have been given the same reference numerals. The following is a discussion of the different features.

[0143] Oxygen-depleted, "low" nitrogen biogas is fed as stream 60 to the first membrane separation system M101 where it is separated into a carbon dioxide-enriched permeate gas (stream 14) and a methane-enriched retentate gas. The retentate gas is fed to the second membrane separation system M102 where it is separated into a second carbon dioxide- enriched permeate gas 62 which is recycled to the feed to the biogas compressor K102, and a second retentate gas that is further enriched in methane. The second retentate gas is fed to the RNG compressor K103 where it is compressed to natural gas pipeline pressure.

[0144] The permeate gas (stream 14) from the first membrane separation system M101 is fed to a further membrane separation system M104 where it is separated into a further carbon dioxide-enriched permeate gas and a further methane-enriched retentate gas. The further retentate gas is recycled as stream 76 to the biogas feed compressor K102 but the furtherpermeate gas is fed as stream 78 to the compression system K101 to produce the stream 22 of compressed permeate gas at a pressure in a range from about 15 bar to about 25 bar.

[0145] The compressed permeate gas (stream 22) is dried in TSA unit D101 and then fed to a carbon dioxide liquefier 100 to produce liquefied carbon dioxide (stream 44) and methane- enriched overhead vapour (stream 50).

[0146] FIG. 8 is a bar chart depicting the estimated annualized costs in millions of US dollars (MM$) of producing 50 tonnes / day of liquid carbon dioxide using the Reference Process of FIG. 7 ("Bolt-on CO2 liquefier") compared with the integrated process of FIG. 3 ("Integrated" CO2 liquefier). Estimates are provided for biogas feed having either "low", specifically 1 mol. %, or "medium", specifically 10 mol. %, nitrogen content.

[0147] The elements contributing to the estimated cost include the capital expense (“capex”) of the compression system, together with the costs of the power and operating the membrane separation systems. Where the biogas has "medium" nitrogen content, the estimated cost additionally includes the costs of the NRU and TSA where appropriate.

[0148] According to FIG. 8, the estimated cost of the process according to the present invention is significantly less than that of the Reference Process, irrespective of whether biogas has a "low" or "medium" nitrogen content.

[0149] Specifically, for biogas having a "low” nitrogen content, the estimated cost of the identified elements for the Reference Process is about 1.36 MM$ whereas the equivalent figure for the process according to the invention is about 1.08 MM$. In other words, total estimated cost of the identified elements for the integrated process of FIG. 3 is less than 80% of the total estimated cost for the equivalent elements of the Reference Process. In addition, each of the individual elements contributing to the total estimated cost of the process according to the invention is less than the equivalent element for the Reference Process.

[0150] In addition, for biogas having a "medium" nitrogen content, the total estimated cost of the identified elements of the Reference Process is about 2.24 MM$ whereas the equivalent figure for the process according to the present invention is about 1.64 MM$. In other words, total estimated cost of the identified elements for the integrated process of FIG. 3 is less than 74% of the total estimated cost of the equivalent elements of the Reference Process, despite the need for a TSA unit. In addition, each of the individual elements contributing to the total estimated cost of the process according to the present invention are again less than the equivalent elements that are common for the Reference Process.

[0151] It should be noted that these costs do not represent the total costs of the processes. In contrast, the costs are the differentiating costs only. Both processes have other costs but these other costs do not differentiate the two processes in a meaningful way and hence, for simplicity, the other costs have not been presented.

[0152] FIG. 9 is a simplified flowsheet depicting a different configuration of a process according to the present invention. The flowsheet is similar to that depicted in FIG. 1 and the features common to both flowsheets have been given the same reference numerals. The following is a discussion of the different features.

[0153] In FIG.1 , stream 46 of warmed tail gas comprising carbon dioxide and methane is recycled to the carbon dioxide separation system in block 12. In contrast, in FIG. 9, this stream is fed to a further membrane separation system M 105 to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas.

[0154] The permeate gas is recycled (as stream 80) to an intermediate stage of the liquefier feed compressor K101 or (as stream 82) to the feed to the liquefier feed compressor, depending on the pressure of the permeate gas.

[0155] Depending on its composition, the retentate gas may be recycled (as stream 84) to an appropriate point within the carbon dioxide separation system in block 12 such as the feed to the DeOXO unit (not shown), the feed to the membrane stages (not shown) or to a suitable point between membrane stages. Alternatively, the retentate gas may be recycled (as stream 86) to the feed 10 to block 12 or, if the gas is methane having a sufficiently level of purity, the retentate gas may be fed (as stream 88) directly to the RNG stream 16.

[0156] In alternative arrangements of the flowsheet of FIG. 9, only a portion of warmed tail gas is sent to the further membrane separation system M 105 as stream 46, with the remaining portion being fed to the feed to the DeOXO unit (not shown) or to the feed to the primary membrane separation system (not shown) in block 12.

[0157] The invention will now be described with reference to the following Reference Example and non-limiting examples of the invention.REFERENCE EXAMPLE

[0158] The process depicted in FIG. 7 has been simulated by computer (Aspen Plus, ver. 12.1 , Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream 44) from a biogas feed stream containing 50 tonnes / day of CO2 using low pressure membrane separation units.

[0159] The results are depicted in Table 1.TABLE 1

[0160] Thus, the process takes about 3411 kg / h of "low" nitrogen biogas comprising about 37 mol. % carbon dioxide and about 54 mol. % methane and produces about 1878 kg / h of liquefied carbon dioxide (99.99 mol. % purity) with about 1201 kg / h of methane (about 96 mol. % purity).EXAMPLE 1

[0161] The process depicted in FIG. 3 has been simulated by computer (Aspen Plus, ver. 12.1 , Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream 44) from a biogas feed stream containing 50 tonnes / day of CO2 using low pressure membrane separation units.

[0162] The results are depicted in Table 2.TABLE 2

[0163] Thus, the process takes about 3411 kg / h of "low" nitrogen biogas comprising about 37 mol. % carbon dioxide and about 54 mol. % methane and produces about 2027 kg / h of liquefied carbon dioxide (99.99 mol. % purity) with about 1211 kg / h of methane (about 96 mol. % purity), which is an increase in production of about 8% and 1% respectively.EXAMPLE 2

[0164] The process depicted in FIG. 4 has been simulated by computer (Aspen Plus, ver. 12.1 , Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream 44) from a biogas feed stream containing 50 tonnes / day of CO2 using higher pressure membrane separation units.

[0165] The results are depicted in Table 3.TABLES

[0166] Thus, the process takes about 3411 kg / h of "low" nitrogen biogas comprising about 37 mol. % carbon dioxide and about 54 mol. % methane and produces about 2027 kg / h of liquefied carbon dioxide (99.99 mol. % purity) with about 1211 kg / h of methane (about 96 mol. % purity).EXAMPLE 3

[0167] The process depicted in FIG. 6 has been simulated by computer (Aspen Plus, ver. 12.1 , Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce liquid carbon dioxide (stream 44) from a biogas feed stream containing 50 tonnes / day of CO2 using higher pressure membrane separation units.

[0168] The results are depicted in Table 4.TABLE 4

[0169] Thus, the process takes about 3411 kg / h of "low" nitrogen biogas comprising about 37 mol. % carbon dioxide and about 54 mol. % methane and produces about 2027 kg / h of liquefied carbon dioxide (99.99 mol. % purity) with about 1211 kg / h of methane (about 96 mol. % purity).

[0170] The feeds to compressor K102 are at different pressures in Table 4. As would be readily appreciated by the skilled person, these pressures would need to be optimized as described above to match the compression system chosen for the required compression duty.

[0171] While the invention has been described with reference to the preferred embodiments depicted in the figure, it will be appreciated that various modifications are possible within the spirit or scope of the invention as defined in the following claims.

[0172] In this specification, unless expressly otherwise indicated, the word "or" is used in the sense of an operator that returns a true value when either or both of the stated conditions are met, as opposed to the operator "exclusive or" which requires only that one of the conditions is met. The word "comprising" is used in the sense of "including" and incorporates "consisting of" rather than meaning "consisting of' exclusively.

[0173] All prior teachings above are hereby incorporated herein by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Australia or elsewhere at the date thereof.

Claims

CLAIMS1. A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said process comprising: separating the gaseous mixture in a membrane separation system to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; compressing the permeate gas, or a combined gas mixture comprising the permeate gas, in a compression system to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide which is recycled to the compression system; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour; wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.

2. The process of Claim 1 , wherein the partially condensed fluid is at a pressure in a range from about 20 bar to about 40 bar, or from about 25 bar to about 33 bar, or from about 28 bar to about 31 bar.

3. The process of Claim 1 , wherein the crude carbon dioxide liquid after pressure reduction is at a pressure in a range from about 12 to about 20 bar.

4. The process of Claim 1 , wherein a first portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with the tail gas.

5. The process of Claim 4, wherein the first portion is from about 5% to about 15%.

6. The process of Claim 1 , wherein a second portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefromcomprising carbon dioxide, is provided by heat exchange with the methane-enriched overhead vapour.

7. The process of Claim 6, wherein the second portion is from about 1% to about 3%.

8. The process of Claim 6, wherein the compression system is a multistage compressor and the methane-enriched overhead vapour is recycled after the heat exchange to the permeate gas at an intermediate stage of the multistage compressor.

9. The process of Claim 1 , wherein a third portion of the refrigeration duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with crude carbon dioxide liquid and / or liquid carbon dioxide.

10. The process of Claim 1 , wherein the balance, e.g., over 50% or over 75% or over 80%, of the refrigerant duty required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, is provided by heat exchange with an external refrigerant.

11. The process of Claim 10, wherein the external refrigerant is liquid ammonia.

12. The process of Claim 11 , wherein the liquid ammonia is evaporated in an external refrigerant circuit at about 1 bar, 5 bar and 10 bar.

13. The process of Claim 1 , wherein the liquid carbon dioxide is fed to storage.

14. The process of Claim 13, wherein the liquid carbon dioxide is subcooled by heat exchange, e.g., against external refrigerant and / or crude carbon dioxide fluid, before being fed to storage.

15. The process of Claim 13, wherein the liquid carbon dioxide is stored at a pressure lower than the pressure at which the crude carbon dioxide liquid is purified by distillation.

16. The process of Claim 15, wherein the liquid carbon dioxide is stored at a pressure in a range from about 11 bar to about 15 bar.

17. The process of Claim 1 , wherein the compression system is a multistage compressor and boil-off and / or displacement gas from storage is heated by heat exchange with the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, and recycled to the permeate gas at an intermediate stage of the multistage compressor.

18. The process of Claim 1 , wherein the tail gas is recycled to the membrane separation system to recover the residual carbon dioxide.

19. The process of Claim 1 , wherein liquid carbon dioxide is reboiled by heat exchange against the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide.

20. The process of Claim 1 , wherein the gaseous mixture is derived from biogas, preferably generated by anaerobic digestion.

21. The process of Claim 1 , wherein the gaseous mixture comprises no more than about 60 mol. % carbon dioxide.

22. The process of Claim 1 , wherein the gaseous mixture comprises from about 30 mol. % to about 80 mol. % methane.

23. The process of Claim 1 , wherein the gas being fed to the compression system comprises no more than about 90 mol. % carbon dioxide.

24. The process of Claim 1 , wherein the gas being fed to the compression system comprises from about 1 mol. % to about 50 mol. % methane.

25. A process for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said process comprising: separating the gaseous mixture in a membrane separation system to produce a carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; compressing the permeate gas, or a combined gas mixture comprising the permeate gas, in a liquefier feed compressor to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; recovering residual carbon dioxide from the tail gas to produce recovered carbon dioxide which is recycled to the liquefier feed compressor; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour, wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partiallycondensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.

26. The process of Claim 25, wherein the gaseous mixture comprises from about 30 mol. % to about 50 mol. % carbon dioxide.

27. The process of Claim 25, wherein the gaseous mixture comprises from about 30 mol. % to about 70 mol. % methane.

28. The process of Claim 25, wherein the gas being fed to the liquefier feed compressor comprises from about 70 mol. % to about 90 mol. % carbon dioxide.

29. The process of Claim 25, wherein the gas being fed to the liquefier feed compressor comprises from about 5 mol. % to about 30 mol. % methane.

30. The process of Claim 25, wherein the residual carbon dioxide is recovered from the tail gas by recycling the tail gas to the membrane separation system.31 . A process for producing liquid carbon dioxide from biogas comprising carbon dioxide and methane, said process comprising: combining the biogas with a carbon dioxide-enriched permeate gas to form a combined gas mixture comprising the permeate gas; compressing the combined gas mixture, or a gas comprising the combined gas mixture, in a compression system to produce a compressed gas comprising carbon dioxide; cooling and partially condensing the compressed gas, or a compressed gas derived therefrom comprising carbon dioxide, by heat exchange to produce a partially condensed fluid comprising carbon dioxide; phase separating the partially condensed fluid to produce a crude carbon dioxide liquid comprising methane, and a tail gas comprising methane and residual carbon dioxide; separating the tail gas in a membrane separation system to produce the carbon dioxide-enriched permeate gas and a methane-enriched retentate gas; and after pressure reduction, purifying the crude carbon dioxide liquid by distillation to produce liquid carbon dioxide and a methane-enriched overhead vapour; wherein refrigeration duty is required to cool and partially condense the compressed gas, or the compressed gas derived therefrom comprising carbon dioxide, to produce the partially condensed fluid, and wherein part of the refrigeration duty is provided by heat exchange with the tail gas and / or the methane-enriched overhead vapour.

32. The process of Claim 31 , wherein the tail gas comprises from about 20 mol. % to about40 mol. % carbon dioxide.

33. The process of Claim 31 , wherein the tail gas comprises from about 60 mol. % to about 80 mol. % methane.

34. The process of Claim 31 , wherein the gas being fed to the compression system comprises from about 50 mol. % to about 70 mol. % carbon dioxide.

35. The process of Claim 31 , wherein the gas being fed to the compression system comprises from about 30 mol. % to about 50 mol. % methane.

36. Apparatus for producing liquid carbon dioxide from a gaseous mixture comprising carbon dioxide and methane, said apparatus comprising: a membrane separation system comprising an inlet for the 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 in fluid flow communication with the first outlet of the membrane separation system and an outlet for compressed gas comprising carbon dioxide; a phase separator comprising an inlet in fluid flow communication with the outlet of the compression system, a first outlet for crude carbon dioxide liquid and a second outlet for tail gas; a first pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the phase separator and an outlet for crude carbon dioxide liquid at reduced pressure; and a distillation column comprising an inlet in fluid flow communication with the outlet of the first pressure reduction unit, a first outlet for liquid carbon dioxide product, a second outlet for methane-enriched overhead vapour and a reboiler, wherein the apparatus comprises a heat exchange system located and arranged for cooling and partially condensing compressed gas comprising carbon dioxide by heat exchange with tail gas from the phase separator and / or methane-enriched overhead vapour from the distillation column, and at least one other refrigerant.

37. The apparatus of Claim 36, wherein the heat exchange system comprises at least one heat exchanger located between the second outlet of the phase separator and thecompression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with tail gas.

38. The apparatus of Claim 36, wherein the second outlet of the phase separator is in fluid flow communication with the inlet of the membrane separation system.

39. The apparatus of Claim 36, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage in fluid flow communication with the second outlet of the distillation column.

40. The apparatus of Claim 39, wherein the heat exchange system comprises a heat exchanger located between located between the second outlet of the distillation column and the inlet to the intermediate stage of the compression system and arranged for cooling compressed gas comprising carbon dioxide by heat exchange with second gas-enriched overhead vapour.41 . The apparatus of Claim 36, wherein the reboiler of the distillation column is part of the heat exchange system and is arranged for reboiling liquid carbon dioxide by heat exchange with compressed gas comprising carbon dioxide.

42. The apparatus of Claim 36 comprising a storage vessel having an inlet in fluid flow communication with the first outlet of the distillation column, a first outlet for liquid carbon dioxide product and a second outlet for boil off and / or displacement gas.

43. The apparatus of Claim 42, wherein the heat exchange system comprise a heat exchanger located between the first outlet of the distillation column and the inlet of the storage vessel and arranged for subcooling liquid carbon dioxide produce by heat exchange against at least one refrigerant, e.g., an external refrigerant.

44. The apparatus of Claim 36, wherein the heat exchange system comprises an external refrigerant circuit.

45. The apparatus of Claim 44, wherein the external refrigerant is liquid ammonia.

46. The apparatus of Claim 36, wherein the compression system is a multistage compressor comprising an inlet to an intermediate stage of the compressor in fluid flow communication with the second outlet of the storage vessel.

47. The apparatus of Claim 36 comprising a second pressure reduction unit comprising an inlet in fluid flow communication with the first outlet of the distillation column and an outlet in fluid flow communication with the inlet of the storage vessel.

48. The apparatus of Claim 36 wherein the heat exchange system comprises a multiple pass brazed aluminum heat exchanger.

Images