Method and system for separating carbon dioxide from a flue gas stream

Abstract

The present application provides an integrated method for the separation and recovery of CO2 from a flue gas stream, particularly from a gas stream containing less than 35 vol% CO2. The method particularly comprises the steps of pressure swing adsorption-desorption, membrane separation, liquefaction and low temperature distillation / flashing and, optionally, stripping. The present application also relates to a system for the separation of CO2 contained in a flue gas stream, configured and adapted for the implementation of the method according to the present application. The present method and system combine high CO2 recovery and purity and significant reduction in CAPEX and energy consumption.

Description

[0001] METHOD AND SYSTEM FOR SEPARATING CARBON DIOXIDE FROM A GAS STREAM

[0002] Technical field

[0003] The present invention generally relates to method and system for separating carbon dioxide from a gas stream, particularly from a flue gas stream from combustion sources.

[0004] Background

[0005] A wide range of industrial activities are emitting significant amounts of CO2 emissions into the atmosphere. In particular, combustion processes yield a flue gas, typically comprising on dry gas basis, 1-30 vol% CO2, 70-99 vol% nitrogen, 0-5 vol% oxygen and traces of impurities, such as SO2 or NOX. In this context, several technologies have been developed to capture CO2 from flue gas. Examples include adsorption-desorption processes, absorptionregeneration processes - such as amine-based absorption-regeneration process (MDEA, MEA etc.), membrane separation, low temperature distillation systems and calcium looping. All these separation methods suffer from high operational costs and capital costs when they are used independently, especially when the CO2 concentration in the exhaust gas is below 15vol%, and when very high CO2 recovery and purity is desired.

[0006] The amine-based absorption-regeneration process requires a significant amount of steam, which is typically produced using fossil energy. It also involves the release of chemicals, corrosion, and can be unstable due to amine degradation. Additionally, it is not well-suited for frequent start-and-stop sequences. Vacuum Swing Adsorption Desorption (VPSA) requires no large amounts of vapor and can be fully electrified. However, its drawbacks include the substantial unit sizes and complex processes required for achieving 99.9% CO2 concentrations, which are the required specifications for further transport and storage. Although membrane technologies require much less energy to concentrate carbon dioxide, a large membrane surface and several subsequent membranes materials with specific permeance and selectivity properties are required to achieve both high CO2 recovery and CO2 purity. In addition, only a few membranes are industrially available, and they can be easily degraded by impurities in the gas stream. Low temperature distillation is used to treat gases with concentrations exceeding 70% and generally above 90%. The CO2 recovery efficiency decreases significantly when the concentration of gas other than CO2 increases. It is linked to the physical features of the mixture of CO2 with other gases (N2, CO, H2, O2, CH4) and the triple point of CO2 which does not allow temperatures below -50°C. Calcium Looping (CaL) technology typically operates at 850-900°C for the calcination and carbonation reactions. Fossil fuel is used to reach these high temperatures which reduces the CO2 recovery rate. In addition, this process requires high-purity oxygen for sorbent regeneration under oxy-fuel conditions. This dependency on oxygen is a limitation. Last but not least, calcium containing sorbents show a significant activity decrease over time.

[0007] The combination of adsorption-desorption and low temperature distillation (EP1869385) and the combination of membrane and low temperature distillation (US8734569) technologies to separate CO2 from flue gas has been reported. However, when multiple methods are used, integration of the two methods to obtain more efficient energy and CAPEX savings have been overlooked. WO2011097162 discloses the combination of three technologies for biomethane production and purification. In this document, the gas flow to be treated is composed of, in average, on dry gas basis, 30 to 60 vol% of methane, 15 to 50 vol% CO2, 0 to 30 vol% N2, 0 to 6 vol% O2, 0 to 1 vol% H2S, and the goal is to recover a methane- rich flow. However, the method and system of WO2011097162 will not be efficient at all to separate CO2 from a flue gas with composition as mentioned above, or from any gas stream with similar composition to a flue gas. In US11 ,344,842 B2, a gas stream containing carbon dioxide is cooled down and partially condensed to produce a liquid stream and gaseous stream. The gaseous stream is further purified on a membrane. A part of the stream can be further purified on adsorption material to trap the remaining impurities. However, this process requires a significant amount of energy as the gas stream has to be cooled down to negative temperatures prior purification.

[0008] There is thus a need in the art for improved methods and systems for separating and recovering carbon dioxide from a gas stream, particularly from a flue gas stream, with improved energy efficiency and CAPEX savings.

[0009] Summary

[0010] It is a particular object of the present invention to overcome one or more of the drawbacks of the prior art and to propose solutions to combine high carbon dioxide recovery and purity and significant reduction in CAPEX and energy consumption. In particular, it is an object of the present invention to obtain more than 90% recovery and more than 98% purity from a CO2 stream containing less than 20 vol% CO2.

[0011] The present invention generally relates to a method for the separation and recovery of CO2 from a gas stream, particularly a flue gas stream, particularly comprising the steps of adsorption-desorption, membrane separation, liquefaction and low temperature distillation / liquid-vapor separation, and, optionally, stripping. The present invention also relates to a system for the separation of CO2 contained in a gas stream, particularly a flue gas stream, configured and adapted for the implementation of a method according to the present application. More in particular, in the present invention, carbon dioxide is separated and recovered from a gas stream, particularly from a gas stream having a low CO2 content, in particular a flue gas stream, by subjecting the carbon containing gas stream to an adsorption-desorption process in an adsorption-desorption unit, particularly in a pressure swing adsorptiondesorption unit, to membrane separation in a membrane unit, and to low temperature liquefaction in a distillation unit, and, optionally to a stripping process in a stripping unit, wherein carbon dioxide depleted gas streams, particularly generated in the adsorptiondesorption unit, in the membrane unit, and / or in the distillation unit, are recycled to earlier steps in the process. Advantageously, by combining different separation steps, including recycling of carbon dioxide depleted gas streams, the present invention allows to efficiently separate carbon dioxide from a gas stream containing less than 20 vol% CO2, such as a flue gas stream. Additionally, by ensuring that the energy recovery between different gas streams is maximized, the present invention allows a significant reduction of carbon dioxide capture cost while obtaining a CO2 recovery exceeding 90% and a purity higher than 98%, surprisingly, even when starting from a carbon dioxide containing gas stream having less than 20 vol% carbon dioxide. Advantageously, the recovered CO2 stream is meeting the specifications for geological disposal (aquifers, coal seams, ...). Furthermore, the innovative process tailors each technology to operate under its most suitable conditions, such as CO2 concentrations, pressure, temperature, flow, recycling ratio, energy recovery, etc., allowing drastic CAPEX and OPEX cuts.

[0012] A first aspect of the present invention provides a method for the separation of carbon dioxide from a gas stream, comprising the steps of:

[0013] (A) providing a carbon dioxide containing gas stream, particularly comprising less than 35 vol% of carbon dioxide, relative to the total volume of the gas stream, in particular a flue gas stream;

[0014] (B) separating a feed gas stream comprising the gas stream of step (A) in an adsorptiondesorption unit, particularly in a pressure swing adsorption-desorption unit, into at least two gas streams, wherein the at least two gas streams comprise:

[0015] (i) a first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and

[0016] (ii) a first carbon dioxide enriched gas stream, particularly comprising at least 15 vol% carbon dioxide, such as between 15 and 60 vol% carbon dioxide, relative to the total volume of the gas stream;

[0017] (C) separating at least part of the first carbon dioxide enriched gas stream in a membrane unit into a second carbon dioxide depleted retentate gas stream, particularly comprising less than 20 vol% carbon dioxide, relative to the total volume of the gas stream, and into a second carbon dioxide enriched gas permeate stream, particularly comprising at least 60 vol% or at least 80 vol% carbon dioxide, relative to the total volume of the gas stream;

[0018] (D) liquefying the second carbon dioxide enriched gas stream, particularly by compressing and cooling the second carbon dioxide enriched gas stream, thereby obtaining a liquid carbon dioxide containing stream;

[0019] (E) separating the carbon dioxide containing liquid stream from step (D) into a third carbon dioxide depleted gas stream, particularly comprising less than 50 vol% carbon dioxide, relative to the total volume of the gas stream, and into a liquid carbon dioxide stream, particularly comprising at least 95 vol% carbon dioxide, relative to the total volume of the liquid stream;

[0020] (F) combining the second carbon dioxide depleted gas stream of step (C) with the gas stream of step (A), particularly flue gas stream of step (A), thereby obtaining the feed gas stream of step (B); and combining the third carbon dioxide depleted gas stream of step (E) with the first carbon dioxide enriched gas stream of step (B) and providing the combined gas stream to the membrane unit; and, optionally,

[0021] (G) subjecting the liquid carbon dioxide stream of step (E) to a gas stripping process in a stripping unit, thereby obtaining a liquid carbon dioxide stream comprising at least 98 vol% carbon dioxide and a loaded stripping gas; and

[0022] (H) combining the loaded stripping gas of step (G) with the second carbon dioxide enriched gas stream of step (C).

[0023] Optionally, step (A) further comprises subjecting the carbon dioxide containing gas stream to a pretreatment cleaning step, particularly in a pretreatment dryer, for removing water and other oxygenated compounds from the carbon dioxide containing gas stream.

[0024] In particular embodiments, the adsorption-desorption unit of step (B) is a vacuum pressure swing adsorption-desorption unit.

[0025] In particular embodiments, the membrane unit comprises a membrane having a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70.

[0026] In particular embodiments, in step (B), the feed gas stream is separated into two gas streams, wherein the two gas streams are:

[0027] (i) the first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and

[0028] (ii) the first carbon dioxide enriched gas stream, particularly wherein step (B) comprises measuring the carbon dioxide concentration of the first carbon dioxide enriched gas stream, wherein

[0029] (a) when the carbon dioxide concentration of the first carbon dioxide enriched gas stream is below 40 vol% or below 50 vol% carbon dioxide, particularly between 15 vol% and 40 vol% carbon dioxide or between 15 vol% and 50 vol% carbon dioxide, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is provided to and separated in the membrane unit of step (C), and

[0030] (b) when the carbon dioxide concentration of the first carbon dioxide enriched gas stream is above 40 vol% or above 50 vol%, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is combined with the second carbon dioxide enriched gas stream, thereby obtaining a combined carbon dioxide enriched gas stream, and step (D) comprises liquefying the combined carbon dioxide enriched gas stream; and, optionally, wherein step (H) comprises combining the loaded stripping gas of step (G) with the combined carbon dioxide enriched gas stream.

[0031] In certain embodiments, in step (B), the feed gas stream is separated into two gas streams, wherein the two gas streams are

[0032] (i) the first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and

[0033] (ii) the first carbon dioxide enriched gas stream, particularly following homogenization of the first carbon dioxide enriched gas stream, comprising between 30 and 60 vol% carbon dioxide, relative to the total volume of the gas stream.

[0034] In certain embodiments, the carbon dioxide containing gas stream of step (A), particularly flue gas stream, comprises between 1 and 30 vol% carbon dioxide, particularly between 3 and 25 vol%, more particularly between 3 and 20 vol% or between 3 and 15 vol% carbon dioxide, even more particularly between 4 and 10 vol% or between 5 and 10 vol% carbon dioxide and yet even more particularly between 7 and 9 vol% carbon dioxide, relative to the total volume of the gas stream.

[0035] In particular embodiments, step (B) comprises the step of cooling the feed gas to a temperature below 30 °C, preferably below 10 °C, more preferably below 3°C, prior to separating the feed gas in the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit; and / or wherein step (C) comprises the step of cooling the first carbon dioxide enriched stream to a temperature below 30 °C, preferably below 10 °C, more preferably below 3°C, prior to separating the first carbon dioxide enriched stream in the membrane unit.

[0036] In particular embodiments, the ratio between the pressure of the feed gas at an inlet of the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, and the pressure of the first carbon dioxide enriched stream at the outlet of the adsorptiondesorption unit, particularly the pressure swing adsorption-desorption unit, is lower than 40, preferably lower than 20 and more preferably lower than 10; and / or wherein the ratio between the pressure of the first carbon dioxide enriched gas stream at an inlet of the membrane unit and the pressure of the second carbon dioxide enriched permeate gas stream at an outlet of the membrane unit is lower than 15, preferably lower than 10, and more preferably lower than 5.

[0037] In particular embodiments, step (D) comprises compressing at a pressure between 15 and 30 bara, particularly between 15 and 25 bara, and cooling the second carbon dioxide enriched gas stream or the combined carbon dioxide enriched gas stream, thereby obtaining a carbon dioxide containing liquid stream.

[0038] In particular embodiments, the first, second and / or third carbon dioxide depleted gas streams are expanded in an expander or turbine to recover energy. In certain embodiments, at least part of the energy needed for cooling and / or compression of a gas stream is provided by expansion of the first, second and / or third carbon dioxide depleted gas streams and / or by expansion of the gasified liquid carbon dioxide stream of step G. In particular, the first, second and / or third carbon dioxide depleted gas stream, optionally after expansion thereof, are used as a cooling medium in a heat exchanger for cooling a gas stream in any one of steps (A) to

[0039] (H), in particular for cooling the first and / or second carbon dioxide enriched gas stream and / or to regenerate a pretreatment dryer for removing water and other oxygenated compounds from the carbon dioxide containing gas stream.

[0040] A second aspect of the present application provides a system for the separation of carbon dioxide from a gas stream, particularly a flue gas stream, comprising:

[0041] (I) an adsorption-desorption unit, particularly a pressure swing adsorption-desorption unit, configured for separating a feed gas stream comprising carbon dioxide into at least two gas streams, wherein the at least two gas streams comprise (i) a first carbon dioxide depleted gas stream, and (ii) a first carbon dioxide enriched gas stream;

[0042] (II) a membrane unit, downstream of the adsorption-desorption unit, comprising one or more membranes, separating a retentate side and a permeate side, configured for separating the first carbon dioxide enriched gas stream into a second carbon dioxide depleted retentate gas stream, and into a second carbon dioxide enriched permeate gas stream;

[0043] (III) a liquefaction unit or liquefaction means, particularly comprising a compressor and cooling means, downstream of the membrane unit, configured for converting the second carbon dioxide enriched permeate gas stream into a carbon dioxide containing liquid vapor mixture; and

[0044] (IV) a distillation column or liquid vapor separation unit, such as a flash vessel, downstream of the liquefaction unit, configured for separating the carbon dioxide containing liquid vapor mixture into a gas stream comprising non-condensable gases and a liquid carbon dioxide stream, wherein the distillation column or liquid vapor separation unit comprises an outlet for the gas stream comprising non-condensable gases, and an outlet for the liquid carbon dioxide stream; wherein the retentate side of the membrane unit is in fluid communication with the adsorptiondesorption unit, particularly the pressure swing adsorption-desorption unit, for providing the retentate gas stream to the adsorption-desorption unit; and wherein the outlet for the gas stream comprising non-condensable gases is in fluid communication with the membrane unit, for providing the gas stream comprising non-condensable gases to the membrane unit.

[0045] In particular embodiments, the system according to the present application further comprises one or more units selected from

[0046] (V) a combustion source of a flue gas in fluid communication with the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit;

[0047] (VI) a cleaning unit, downstream of the combustion source of a flue gas and upstream of the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, configured for removing water and other oxygenated compounds from the flue gas,

[0048] (VII) a stripping unit, downstream of the distillation column or liquid vapor separation unit, configured for stripping the liquid carbon dioxide stream with a stripping gas, the stripping unit comprising an inlet for the liquid carbon dioxide stream, an outlet for the purified liquid carbon dioxide stream, an inlet for the stripping gas, particularly carbon dioxide, and an outlet for the loaded stripping gas, wherein the outlet of the loaded stripping gas is in fluid communication with the liquefaction unit;

[0049] (VIII) one or more compressors configured for compressing one or more gas streams between two consecutive units;

[0050] (IX) one or more expanders or turbines, configured for recovering energy from a gas stream by expansion thereof, particularly from the first carbon dioxide depleted gas stream, the second carbon dioxide depleted retentate gas stream, the gas stream comprising noncondensable gases; and / or a gasified liquid carbon dioxide stream; and / or

[0051] (X) one or more heat exchangers, particularly wherein the first carbon dioxide depleted gas stream, the second carbon dioxide depleted retentate gas stream and / or the gas stream comprising non-condensable gases are used as the cooling medium.

[0052] In particular embodiments, the membrane unit comprises one or more polymeric membranes, mixed matrix membranes, carbon membranes, ceramic membranes, or a combination thereof; and / or the membrane unit comprises a membrane having a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70.

[0053] In particular embodiments, the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, comprises a vessel containing solid sorbent material, wherein the solid sorbent material comprises metal-organic frameworks (MOFs), zeolites, activated carbon, amino-acid based adsorbents, or a combination thereof, and wherein the sorbent material has an adsorption capacity higher than 1 .5 mmol / g (determined at room temperature), more preferably an adsorption capacity higher than 2.5 mmol / g (determined at room temperature).

[0054] Description of the Figures

[0055] Figure 1 illustrates an embodiment of the present invention depicting the various gas flows and energy recovery steps, the main purifications units as well as the main expanders or turbines, pumps and compressors.

[0056] Figure 2 illustrates another embodiment of the present invention depicting the various gas flows and energy recovery steps, the main purifications units as well as the main expanders or turbines, pumps and compressors.

[0057] In the figures, features A, F, H and K depict a compressor; features C, M and P depict an expander with turbine or a Joule-Thomson expansion valve; features E and I depict a vacuum pump; feature Q depicts a pump; feature B is a dryer; feature D is a pressure swing adsorption-desorption unit, comprising one or more adsorption-desorption beds; feature G is a membrane unit; feature L is a distillation column; feature O represents a heat exchanger or a plurality of heat exchangers; feature J is a buffer tank; and feature N is a stripping unit.

[0058] Detailed description of invention

[0059] Before the present system and method of the invention are described, it is noted that this invention is not limited to particular systems and methods or combinations described, since such systems and methods and combinations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0060] As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise.

[0061] The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open- ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of', "consists" and "consists of".

[0062] The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

[0063] The term "about" or “approximately” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / -10% or less, preferably + / -5% or less, more preferably + / -1% or less, and still more preferably + / -0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" or “approximately” refers is itself also specifically, and preferably, disclosed.

[0064] Whereas the terms “one or more” or “at least one”, such as one or more or at least one member(s) of a group of members, is clear per se, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any >3, >4, >5, >6 or >7 etc. of said members, and up to all said members.

[0065] All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

[0066] In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0067] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

[0068] It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

[0069] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0070] Unless explicitly stated otherwise, all the gas compositions mentioned in the present application are expressed as volume percentage, i.e. relative to the total volume of the respective gas stream.

[0071] As used herein, the recovery efficiency of a compound in a separation unit is the ratio between the quantity of that compound entering the unit and the quantity of that compound exiting the unit in the desired produced form.

[0072] As used herein, the term “carbon dioxide depleted gas stream” refers to a gas stream obtained after a purification step, which has a lower carbon dioxide content / concentration than the carbon dioxide containing stream prior to said purification step. Similarly, as used herein, the term “carbon dioxide enriched gas stream” refers to a gas stream obtained after a purification step, which has a higher carbon dioxide content / concentration than the carbon dioxide containing stream prior to said purification step.

[0073] In general, the inventors have developed an integrated method and system for the efficient separation and recovery of carbon dioxide from a gas stream, by the subsequent separation of a carbon dioxide containing gas stream, particularly a flue gas stream, using different separation technologies in different separation units, in particular CO2 adsorptiondesorption in an adsorption-desorption unit, particularly a pressure swing adsorptiondesorption unit, followed by membrane separation in a membrane unit, CO2 liquefaction and liquid vapor separation in a (low temperature) distillation unit or flash vessel and the like, and optionally gas stripping in a stripping unit. More in particular, the integrated method and system further comprises recycling of carbon dioxide depleted streams generated by a purification unit to an earlier purification unit in the process, which allows to increase the recovery efficiency, and is adapted to maximize the energy recovery between different gas streams in the process, such as by using carbon dioxide depleted gas streams to cool down a carbon dioxide containing stream prior to the latter being introduced in a separation unit or by expanding a pressurized gas stream, such as in a turbine. This allows to obtain carbon dioxide from a gas stream with a low carbon dioxide concentration, such as a flue gas stream, with a recovery exceeding 90% and a purity higher than 98%, with a significant reduction in carbon dioxide capture costs. In particular, the present invention typically also comprises gas compression, gas expansion and vacuum creation. In a gas compression step, the increase in gas pressure is accompanied by an increase in gas temperature. Hence, the compressed gas may be cooled via one or more heat exchangers. More in particular, a gas stream leaving the adsorption-desorption unit, the membrane unit, and / or the distillation unit, particularly following expansion of said gas stream, may be used to cool the compressed gas streams entering these units. Advantageously, this allows to minimize the high operating cost of the liquefaction step. Also, as further discussed below, the method as envisaged herein is operating at a rather low pressure, thereby further contributing to a reduced energy consumption.

[0074] Furthermore, each separation technology is particularly operated under its most suitable conditions (CO2 concentrations, pressure, temperature, flow, recycling ratio, energy recovery, etc.) allowing to drastically cut CAPEX and OPEX, particularly compared to carbon dioxide recovery processes and methods based on isolated adsorption-desorption technology, isolated membrane separation technology or isolated liquefaction technology, as well as compared to coupled adsorption-desorption / membrane, membrane / liquefaction and adsorption-desorption / liquefaction technologies. For instance, as no heat is used in the methods according to the present invention, energy consumption and CAPEX are further lowered and minimized.

[0075] A first aspect of the present invention provides a method for the separation of carbon dioxide from a gas stream. The method comprises the steps of

[0076] (A) providing a gas stream, such as a flue gas stream, said gas stream also referred to herein as the carbon dioxide containing gas stream to be treated;

[0077] (B) separating a feed gas stream comprising the gas stream of step (A) in an adsorptiondesorption unit into at least two gas streams, wherein the at least two gas streams comprise:

[0078] (i) a first carbon dioxide depleted gas stream; and

[0079] (ii) a first carbon dioxide enriched gas stream; (C) separating the first carbon dioxide enriched gas stream in a membrane unit into a second carbon dioxide depleted retentate gas stream, and into a second carbon dioxide enriched permeate gas stream;

[0080] (D) liquefying the second carbon dioxide enriched gas stream, particularly by compressing and cooling the second carbon dioxide enriched gas stream, thereby obtaining a carbon dioxide containing liquid-vapor mixture;

[0081] (E) separating the carbon dioxide containing liquid-vapor mixture from step (D) into a third carbon dioxide depleted gas stream, also referred to herein as the non-condensable gas stream, and into a liquid carbon dioxide stream;

[0082] (F) combining the second carbon dioxide depleted gas stream of step (C) with the gas stream of step (A), thereby obtaining the feed gas stream of step (B); and combining the third carbon dioxide depleted gas stream of step (E) with the first carbon dioxide enriched gas stream of step (B) and providing the combined gas stream to the membrane unit.

[0083] Advantageously, the combination of different separation technologies and the recycling of carbon dioxide depleted streams into the feed gas stream allows to obtain carbon dioxide from a gas stream with a low carbon dioxide concentration, such as a flue gas stream, with a recovery exceeding 90% and a purity higher than 98%.

[0084] Additionally, the method may be adapted to maximize the energy recovery between different gas streams in the process, such as by using carbon dioxide depleted gas streams to cool down a carbon dioxide containing gas before the latter is subjected to a purification step. Accordingly, in particular embodiments, the method further comprises:

[0085] Optionally, expanding the first, second, and / or third carbon dioxide depleted gas streams, particularly in a turbine or a Joule-Thomson valve, thereby recovering energy;

[0086] Optionally, compressing the feed gas stream, and / or the first and / or second carbon dioxide enriched gas stream; and

[0087] Cooling the feed gas stream, and / or the first and / or second carbon dioxide enriched gas stream, particularly cooling the compressed feed gas stream and / or the compressed first and / or second carbon dioxide enriched gas stream with the first, second, and / or third carbon dioxide depleted gas streams as cooling medium, particularly with the expanded first, second, and / or third carbon dioxide depleted gas streams as cooling medium.

[0088] The carbon dioxide liquid stream produced in step (E) may optionally be expanded to cool a heat exchanger, such as via a Joule-Thomson valve. This expanded CO2 can be further compressed to 111 bara for pipe transportation. The exchanger may also optionally be cooled via an external cooling system, such as by using a refrigerant such as ammonia.

[0089] In certain embodiments, the gas stream or flue gas stream of step (A), i.e. the CO2 containing gas stream to be treated, comprises less than 35 vol% or less than 30 vol% carbon dioxide, particularly less than 25 vol%, or less than 20%, or even less than 15 vol% or less than 10 vol% of carbon dioxide. In certain embodiments, the CO2 containing gas stream to be treated comprises between 1 and 30 vol% carbon dioxide, particularly between 3 and 25 vol%, more particularly between 3 and 20 vol% or between 3 and 15 vol% carbon dioxide, even more particularly between 4 and 10 vol% or between 5 and 10 vol% carbon dioxide and yet even more particularly between 7 and 9 vol% carbon dioxide.

[0090] In particular embodiments, the CO2 containing gas stream to be treated is a flue gas stream. In addition to carbon dioxide, a flue gas stream contains other combustion by-products depending on the fuel used (biogas, natural gas, coal, ...) such as oxygenated compounds, including but not limited to water, carbon monoxide, sulfur oxides or SOx, and nitrogen oxides or NOx. A flue gas stream typically comprises other gases as well, such as nitrogen, oxygen and argon and contains at most traces of hydrogen or methane. In certain embodiments, a flue gas stream as considered herein comprises, on a dry gas basis, between 1.0 and 35.0 vol% carbon dioxide, between 65 and 99 vol% of nitrogen and between 0 and 10 vol% oxygen. An example of a flue gas composition from a power plant comprises 4-5 vol% carbon dioxide, 10-15 vol% water, 5-10 vol% oxygen, 70-75 vol% nitrogen, 2-5 ppmv SOx and 15-30 ppmv NOx. Depending on the concentration of these by-products and other components, a pretreatment in a purification or cleaning unit may be implemented to pre-clean the gas mixture. Suitable treatment methods are disclosed in W02009010690, W02009095581 , US2009013717, US2009013868, or US2009013871 , which are incorporated herein by reference.

[0091] Accordingly, in particular embodiments, the method, particularly step (A) of the method, further comprises the step of subjecting the CO2 containing gas stream to be treated to a pretreatment cleaning step, particularly in a cleaning unit. Advantageously, this allows to reduce moisture and other oxygenated compounds to a low level, thereby promoting an efficient CO2 separation in the next steps, in particular in the adsorption-desorption unit, and / or the membrane unit, and avoiding certain potential problems in the next steps, such as sorbent poisoning, freezing in the low temperature distillation unit, condensation in a heat exchanger, corrosion and the like. Stated differently, in certain embodiments, step (A) comprises removing water and other oxygenated compounds from the CO2 containing gas stream or flue gas stream to be treated in a cleaning or purification unit. Suitable cleaning units include dryers or units based on an adsorbent-based process, wherein water and oxygenated compounds are removed by adsorption on solid materials. In certain embodiments, such cleaning units comprise alumina, silica or molecular sieves. Additionally or alternatively, the cleaning unit may comprise a condenser, wherein water in the CO2 containing gas stream to be treated is lowered by condensation. Prior to the pretreatment cleaning step, the pressure of the carbon dioxide containing gas may be adjusted by a compression unit to a pressure between 3 and 6 bara.

[0092] In step (B), the feed gas is treated in an adsorption-desorption unit, particularly in a pressure swing adsorption-desorption unit, and is separated in at least two streams, i.e. (i) a first carbon dioxide depleted gas stream and (ii) a first carbon dioxide enriched gas stream. In particular, the first carbon dioxide depleted gas stream comprises less than 5.0 vol% CO2. Preferably, the first carbon dioxide depleted gas stream comprises less than 4.0 vol% CO2, more preferably less than 3.0 vol% or less than 2.0 vol% CO2, and even more preferably less than 1.0 vol% CO2. In particular, the first carbon dioxide enriched gas stream comprises at least 15 vol% carbon dioxide, such as between 15 and 60 vol% carbon dioxide, or between 20 and 60 vol% carbon dioxide, particularly following homogenization of the first carbon dioxide enriched gas stream.

[0093] In particular embodiments, the feed gas is separated in the adsorption-desorption unit, particularly in a pressure swing adsorption-desorption unit into two streams, i.e. the first carbon dioxide depleted gas stream and the first carbon dioxide enriched gas stream, wherein, depending on the carbon dioxide concentration of the first carbon dioxide enriched gas stream, this stream is either treated in the membrane unit or is subject to a liquefaction step, such as, by way of example, illustrated in exemplary embodiment 1 and in Figure 1 . The carbon dioxide concentration of the first carbon dioxide enriched gas stream may vary over time during the desorption step of the adsorption-desorption cycle, which can be monitored and measured. In particular, as discussed further below, when the carbon dioxide concentration in the first carbon dioxide enriched gas stream is below a particular concentration, such as below 40 vol%, below 50 vol%, below 60 vol%, below 70 vol%, or below 80 vol% carbon dioxide, the first carbon dioxide enriched gas stream is treated in the membrane unit, and when the carbon dioxide concentration in the first carbon dioxide enriched gas stream is above said particular concentration, such as above 40 vol%, above 50 vol%, above 60 vol%, above 70 vol%, or above 80 vol% carbon dioxide, the first carbon dioxide enriched gas stream is subject to the liquefaction step.

[0094] Alternatively, the feed gas is separated in the adsorption-desorption unit, particularly in a pressure swing adsorption-desorption unit into two streams, i.e. the first carbon dioxide depleted gas stream and the first carbon dioxide enriched gas stream, wherein the first carbon dioxide enriched gas stream, particularly following homogenization such as in a buffer tank, is treated in the membrane unit, such as, by way of example, illustrated in exemplary embodiment 2 and in Figure 2.

[0095] In particular embodiments, the method thus comprises the steps of

[0096] (A) providing a carbon dioxide containing flue gas stream, particularly comprising less than 30 vol%, such as between 3 and 25 vol%, of carbon dioxide;

[0097] (B) separating a feed gas stream comprising the gas stream of step (A) in pressure swing adsorption-desorption unit into two gas streams, wherein the two gas streams are (i) the first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream, and (ii) the first carbon dioxide enriched gas stream, and, particularly, measuring the carbon dioxide concentration in the first carbon dioxide enriched gas stream,

[0098] (C) when or as long as the carbon dioxide concentration of the first carbon dioxide enriched gas stream is below 40 vol%, below 50 vol%, below 60 vol%, below 70 vol%, or below 80 vol% carbon dioxide, particularly between 15 vol% and 40 vol% carbon dioxide, between 15 vol% and 50 vol% carbon dioxide or between 15 vol% and 60 vol% carbon dioxide, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is provided to and separated in a membrane unit into a second carbon dioxide depleted retentate gas stream, particularly comprising less than 20 vol% carbon dioxide, and into a second carbon dioxide enriched permeate gas stream, particularly comprising at least 60 vol% or at least 80 vol% carbon dioxide;

[0099] (D) when or as long as the carbon dioxide concentration of the first carbon dioxide enriched gas stream is above 40 vol% or above 50 vol% carbon dioxide, particularly above 60 vol%, above 70 vol% or above 80 vol% carbon dioxide, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is combined with the second carbon dioxide enriched gas stream, thereby obtaining a combined carbon dioxide enriched gas stream and liquefying the combined carbon dioxide enriched gas stream, particularly by compressing and cooling the combined CO2 enriched gas stream, thereby obtaining a carbon dioxide containing liquid-vapor mixture;

[0100] (E) separating the carbon dioxide containing liquid-vapor mixture from step (D) into a third carbon dioxide depleted gas stream, particularly comprising less than 50 vol% carbon dioxide and into a carbon dioxide liquid stream, particularly comprising at least 95 vol% carbon dioxide; (F) combining the second carbon dioxide depleted gas stream of step (C) with the gas stream of step (A), thereby obtaining the feed gas stream of step (B).

[0101] Stated differently, step (B) may be considered as the separation of the feed gas in the adsorption-desorption unit into three gas streams, i.e. one carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide; and two carbon dioxide enriched gas streams, including a moderately CCh-enriched gas stream, particularly comprising below 40 vol%, below 50 vol%, or below 60 vol% carbon dioxide, particularly between 15 and 40 vol%, between 15 and 50 vol% or between 15 and 60 vol% carbon dioxide, and a highly CCh-enriched gas stream, particularly comprising at least 50 vol%, at least 60 vol% or at least 80 vol% carbon dioxide.

[0102] In other particular embodiments, the method comprises the steps of

[0103] (A) providing a carbon dioxide containing flue gas stream, particularly comprising less than 30 vol%, such as between 3 and 25 vol%, of carbon dioxide;

[0104] (B) separating a feed gas stream comprising the gas stream of step (A) in a pressure swing adsorption-desorption unit into two gas streams, wherein the two gas streams are (i) the first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide; and (ii) the first carbon dioxide enriched gas stream, particularly comprising between 30 and 60 vol% carbon dioxide, such as following homogenization of the first carbon dioxide enriched gas stream particularly in a buffer tank;

[0105] (C) separating the first carbon dioxide enriched gas stream in a membrane unit into a second carbon dioxide depleted gas stream, particularly comprising less than 20 vol% carbon dioxide, and into a second carbon dioxide enriched gas stream, particularly comprising at least 60 vol% or at least 80 vol% carbon dioxide;

[0106] (D) liquefying the third carbon dioxide enriched gas stream, particularly by compressing and cooling the second carbon dioxide enriched gas stream, thereby obtaining a carbon dioxide containing liquid vapor mixture;

[0107] (E) separating the carbon dioxide containing liquid stream from step (D) into a third carbon dioxide depleted gas stream, particularly comprising less than 50 vol% carbon dioxide and into a carbon dioxide liquid stream, particularly comprising at least 95 vol% carbon dioxide;

[0108] (F) combining the second carbon dioxide depleted gas stream of step (C) with the gas stream of step (A), thereby obtaining the feed gas stream of step (B).

[0109] The feed gas is typically a mixture of a carbon dioxide gas stream or flue gas to be treated with recycled CO2 containing gas streams from the method as envisaged, in particular a mixture with the second carbon-dioxide depleted gas stream. The CO2 concentration in the feed gas stream conveyed to the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, is particularly lower than 35 vol%.

[0110] In particular embodiments, the pressure of the feed gas stream is adjusted to a pressure between 1 and 6 bara, particularly to a pressure between 1 and 4 bara or between 1 and 3 bara, to perform the separation step in the pressure swing adsorption-desorption unit at a suitable or optimal pressure. This may be done by compression of the feed gas stream in a compressor, thereby obtaining a pressurized or compressed feed gas stream. Alternatively, particularly when the carbon dioxide containing gas to be treated was subject to a pretreatment cleaning step, the feed gas stream may be expanded in a suitable expander or turbine.

[0111] In particular embodiments, the separation step in the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, is performed at a temperature below 80 °C, or below 70°C, below 60°C, below 50°C, or below 45°C, such as at a temperature between -30 °C and 80°C, between -30 °C and 70°C, between -30 °C and 60°C, between -30 °C and 50°C, or between -30°C and 45°C, preferably at a temperature below 30 °C, such as at a temperature between -30 °C and 30 °C, more preferably at a temperature below 10 °C, below 5 °C or below 3 °C, such as at a temperature between -20 °C and 5 °C. Accordingly, the method as envisaged herein may further comprise the step of cooling the feed gas stream or the compressed feed gas stream, prior to separating the feed gas stream in the adsorptiondesorption unit, to a temperature between -30 °C and 45°C, preferably to a temperature below 30 °C, such as to a temperature between -30 °C and 30 °C, more preferably to a temperature below 10 °C, below 5 °C or below 3 °C, such as to a temperature between -20 °C and 5 °C, to provide the feed gas at a suitable or optimal temperature to the pressure swing adsorptiondesorption unit.

[0112] The cooling step is typically performed in a heat exchanger, preferably using the first, the second and / or the third carbon dioxide depleted stream, particularly following expansion thereof, as cooling medium.

[0113] In particular embodiments, the adsorption-desorption unit as disclosed herein is a pressure swing adsorption unit, more particularly is a vacuum pressure swing adsorptiondesorption unit. More in particular, in a pressure swing adsorption unit used in the present invention, gas molecules, in particular carbon dioxide molecules, are adsorbed on solid surfaces at elevated pressures. By dropping the pressure, in particular by employing a vacuum, the gas molecules, in particular carbon dioxide molecules are desorbed from the solid surfaces. This process allows selective separation of different gases based on their affinity for the adsorbent material. It is understood that the concentration of the carbon dioxide in the desorbed gas may change during the desorption step. The carbon dioxide concentration can easily be monitored and measured as known to the skilled person. In certain embodiments, a desorbed gas with a homogeneous carbon dioxide concentration may be obtained by providing the desorbed gas to a buffer tank prior to the further treatment of the desorbed gas, in particular the first carbon dioxide enriched gas stream as described elsewhere herein.

[0114] In particular, the pressure at the outlet of the pressure swing adsorption-desorption unit, in particular the pressure of the first and second carbon dioxide enriched gas stream, is lower than 0.4 bara absolute pressure, preferably lower than 0.2 bara, and more preferably lower than 0.1 bara. In certain embodiments, the ratio between the pressure of the feed gas at an inlet of the pressure swing adsorption-desorption unit and the pressure of first carbon dioxide enriched stream at the outlet of the pressure swing adsorption-desorption unit is lower than 40, preferably lower than 20 and more preferably lower than 10.

[0115] In certain embodiments, the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit, comprises a vessel containing a sorbent material, such as in the form of adsorption-desorption beds, particularly wherein the sorbent material comprises metal-organic frameworks (MOFs), zeolites, activated and porous carbon materials, amino-acid based adsorbents, or a combination thereof. The sorbent material preferably has an adsorption capacity higher than 1.0 mmol / g (determined at 25 °C and 100 kPa), more preferably with an adsorption capacity higher than 2.0 mmol / g or 2.5 mmol / g, such as higher than 3.0 mmol / g (determined at 25 °C and 100 kPa).

[0116] The determination and measurement of the adsorption capacity of a material is known to the skilled person, and includes the determination of an adsorption isotherm. Briefly, the porous material is typically first activated by heating to remove any adsorbed moisture or gases. This is done under vacuum or in an inert gas atmosphere at a specified temperature. The adsorption isotherm can be either obtained via gravimetric method or a volumetric method. In the gravimetric method, the sample is placed in a balance within a controlled environment chamber. The weight change is monitored as CO2is introduced at a constant pressure and temperature. The increase in weight corresponds to the amount of CO2adsorbed. In the volumetric method, the sample is placed in a sealed chamber, and the volume of CO2gas adsorbed is measured by monitoring the pressure change in the chamber. For both methods, the adsorption capacity is measured at different pressures to generate an adsorption isotherm. This plot shows the amount of CO2adsorbed as a function of pressure at a constant temperature.

[0117] In certain embodiments, the first carbon dioxide depleted gas stream obtained after the separation step in the adsorption-desorption unit is released to the atmosphere. Prior to releasing this gas mixture to the atmosphere, the first carbon dioxide depleted gas stream may be expanded in an expander or turbine to recover energy and / or may be used as a cooling medium for another gas stream in the process, such as to cool the first and / or second carbon dioxide enriched gas stream.

[0118] Step (C) of the method as envisaged herein comprises a membrane separation of a carbon dioxide containing membrane feed stream into a retentate and a permeate. The retentate refers to the part of the membrane feed stream retained by the membrane during the membrane separation process, whereas the permeate is the product stream that passes through the membrane.

[0119] More in particular, in step (C) of the method as envisaged herein, the first carbon dioxide enriched gas stream is separated in a membrane unit into a carbon dioxide depleted retentate gas stream and a carbon dioxide enriched permeate gas stream. The retentate stream, also referred to herein as the second carbon dioxide depleted gas stream or the second carbon dioxide depleted retentate gas stream particularly comprising less than 20 vol% carbon dioxide, is particularly recycled, by mixing it with the carbon dioxide gas to be treated, such as flue gas, to prepare the feed gas stream of the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit. The permeate gas stream, also referred to herein as the second carbon dioxide enriched gas stream, particularly comprises at least 80 vol% carbon dioxide.

[0120] The CO2 concentration in the membrane feed gas stream, which is mainly the first carbon dioxide enriched gas stream coming from the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, is generally higher than 15 vol%, such as higher than 20 vol%, and up to 60vol% or up to 80 vol%. In certain embodiments, the CO2 concentration in the membrane feed gas stream is higher than 30 vol%, such as higher than 40 vol%. In certain embodiments, the first carbon dioxide enriched gas stream is first homogenized prior to providing it to the membrane unit, thereby obtaining a carbon dioxide concentration of between 20 and 60 vol%, between 25 and 60 vol%, or between 30 and 60 vol% carbon dioxide, relative to the total volume of the gas stream.

[0121] In particular embodiments, the pressure of the first carbon dioxide enriched gas stream is adjusted to a pressure between 1 and 3 bara, particularly by compression of said gas stream in a compressor, thereby obtaining a pressurized or compressed first carbon dioxide enriched as stream. In certain embodiments, the ratio between the pressure of the first carbon dioxide enriched gas stream at an inlet of the membrane unit (Pf) and the pressure of the permeate, or the second carbon dioxide enriched gas stream at an outlet of the membrane unit, is lower than 15, preferably lower than 10, and more preferably lower than 5.

[0122] In particular embodiments, the separation step in the membrane unit is performed at a temperature below 30 °C, such as at a temperature between -30 °C and 30 °C, preferably at a temperature below 20 °C, below 10 °C, below 5 °C or below 3 °C, such as at a temperature between -20 °C and 5 °C. Accordingly, the method as envisaged herein may further comprise the step of cooling the first carbon dioxide enriched gas stream or the compressed first carbon dioxide enriched gas stream, prior to separating the feed gas stream in the membrane to a temperature below 30 °C, such as to a temperature between -30 °C and 30 °C, preferably to a temperature below 20 °C, below 10 °C, below 5 °C or below 3 °C, such as at a temperature between -20 °C and 5 °C. The cooling step is typically performed in a heat exchanger, preferably using the first, the second and / or the third carbon dioxide depleted stream, particularly following expansion thereof, as cooling medium.

[0123] A sweep gas may be used at the permeate side, such as a carbon dioxide depleted gas stream, to lower the CO2 permeate partial pressure and to increase the membrane separation driving force, thus increasing the efficiency of the membrane separation step.

[0124] In particular embodiments, the membrane unit as disclosed herein comprises one or more membranes, made up of a material that selectively allows CO2 to pass through, generating a carbon dioxide enriched permeate, while retaining other molecules, generating a carbon dioxide depleted retentate. The membrane typically has a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70. The membrane has CO2 permeance expressed in GPU (1 GPU = 10'6Ncm3.cnr2.s'1.cmHg'1) higher than 800, preferably higher than 1500 and more preferably higher than 2000. The CO2 / N2 selectivity may be evaluated as described in Huang et al. 2014, Journal of Membrane science, 463, 33-40.

[0125] The membrane may be a membrane made up of a polymeric material, a carbon membrane, a mixed matrix membrane (MMM), a ceramic membrane or a combination thereof. Examples include polymeric membranes made up of polyimide or fluoropolysulfones, poly(phenylene oxides), poly(fluorocarbonates), or polymers of fluoroacetophenone; carbon molecular sieves membranes, or mixed matrix membranes as described in “Tan et al., Science 378, 1189-1194 (2022)”.

[0126] The membrane unit may comprise flat sheet membranes, tubular tubes membranes, hollow fiber membranes and / or other membranes commonly used in industry or later developed. Depending upon a variety of factors, including the concentration of CO2, the membrane unit may utilize one or more gas separation modules or membranes. If more than one gas separation module or membrane is utilized, they may be arranged in series, parallel, cascade or in recycle formation.

[0127] After the permeation through the membrane, the permeate gas stream is further purified by the compression and liquefaction, which is supplemented with one or more distillations or liquid vapor separation steps, such as flashing, to increase the CO2 purity of the end product. In particular, the carbon dioxide enriched gas stream from the membrane unit, i.e. the second carbon dioxide enriched gas stream, or from the membrane unit and the adsorption-desorption unit is liquefied, by compressing and cooling the CO2 enriched gas stream(s), thereby obtaining a carbon dioxide containing liquid-vapor mixture, more in particular to a pressure between 15 and 35 bara and to a temperature between - 50°C and - 20 °C;

[0128] In this partial condensation step, the carbon dioxide rich stream is particularly compressed by a compressor to a pressure ranging from approximately 15 bara to approximately 35 bara, such as between 15 and 30 bara, particularly between 15 and 25 bara. The compression may be performed by one or more compressors or by a multistage compressor. The compressed carbon dioxide rich stream is then cooled to a temperature to get a liquid vapor mixture. It is well known that the temperature to reach liquid CO2 is depending on the concentration of carbon dioxide in the stream.

[0129] The energy needed for the compression and cooling of the carbon dioxide enriched gas stream may at least partly be provided by energy released and recovered by expansion or depressurization of various gas streams in the process, such as in a turbine. For instance, the energy may be provided by expansion of the purified carbon dioxide end product stream of the method according to the present application, in particular a gasified purified carbon dioxide liquid stream. This improves the process efficiency and decreases the overall energy consumption of the process.

[0130] The carbon dioxide containing liquid vapor mixture of step (D) is further treated in a distillation column or liquid vapor separation unit, such as a flash vessel, into a third carbon dioxide depleted gas stream, essentially containing the non-condensable gases, and into a liquid carbon dioxide stream. The third carbon dioxide depleted gas stream particularly comprises less than 50 vol% CO2. The liquid carbon dioxide stream particularly comprises at least 95 vol% CO2.

[0131] In particular embodiments, the third carbon dioxide depleted gas stream obtained after the low temperature distillation step or flashing step in a flash vessel or after another liquid vapor separation step, comprises the non-condensable gases, and typically has a CO2 concentration of less than 50 vol%. Preferably, this gas stream is recycled to earlier separation steps. In particular, it is mixed with the first carbon dioxide enriched gas stream upstream of the membrane unit. Prior to recycling this gas mixture, the third carbon dioxide depleted gas stream may be expanded in a turbine to recover energy and / or may be used as a cooling medium for another gas stream in the process, such as to cool the first and / or second carbon dioxide enriched gas stream. This improves the process efficiency and decreases the overall energy consumption of the process.

[0132] The present invention is particularly defined by integrating the different separation steps, not only with respect to the carbon dioxide containing gas streams, but also with respect to the energy balance. This improves the process efficiency and decreases the overall energy consumption of the process.

[0133] In particular, the method as envisaged herein comprises recycling one or more carbon dioxide depleted gas streams generated by a particular separation unit to an upstream separation unit.

[0134] In particular, the second carbon dioxide depleted gas stream, i.e. the membrane retentate stream is mixed with the carbon dioxide containing gas stream or flue gas to be treated, thereby obtaining the feed gas stream of the adsorption-desorption unit.

[0135] Additionally or alternatively, the third carbon dioxide depleted gas, i.e. the noncondensable gas stream, particularly with a CO2 concentration of less than 50 vol%, is mixed with the first carbon dioxide enriched gas stream, particularly having a CO2 concentration of between 15 and 60 vol%, such as between 30 and 60 vol%, and the combined mixture is subsequently treated in the membrane separation unit.

[0136] The overall energy consumption may be decreased by expanding a pressurized gas stream, particularly a carbon dioxide depleted gas stream, such as in a turbine or a Joule- Thomson valve, and / or by exchanging heat between warm gas streams, such as warm, pressurized carbon dioxide enriched streams, and cold streams, such as cold, expanded carbon dioxide depleted gas streams.

[0137] In certain embodiments, the first carbon dioxide depleted gas stream exiting the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, may be expanded in an expander or a turbine to recover energy. The first carbon dioxide depleted gas stream, particularly following expansion in a turbine and prior to release to the atmosphere, may further be used as a cooling medium in a heat exchanger to cool another gas stream in the process, such as to cool the first and / or second carbon dioxide enriched gas stream.

[0138] In certain embodiments, the second carbon dioxide depleted gas stream exiting the membrane unit, prior to mixing with the carbon dioxide containing gas stream to be treated, may be expanded in an expander or turbine to recover energy. The second carbon dioxide depleted gas stream, particularly following expansion in a turbine, may further be used as a cooling medium in a heat exchanger to cool another gas stream in the process, such as to cool the first and / or second carbon dioxide enriched gas stream. In certain embodiments, the third carbon dioxide depleted gas stream of step (E) exiting the membrane unit, prior to mixing with the first carbon dioxide enriched gas stream, may be expanded in a turbine to recover energy. The third carbon dioxide depleted gas stream, particularly following expansion in a turbine, may further be used as a cooling medium in a heat exchanger to cool another gas stream in the process, such as to cool the first and / or second carbon dioxide enriched gas stream.

[0139] In certain embodiments, the purified liquid carbon dioxide stream obtained at the end of the method as envisaged herein, said purified liquid carbon dioxide stream particularly comprising at least 95 vol% or at least 98 vol% carbon dioxide, may be expanded in a turbine or in a Joule-Thomson valve in order to cool and / or compress other gas streams, such as the first and / or second carbon dioxide enriched gas stream.

[0140] In order to further increase the purity of the liquid carbon dioxide stream of step (E), a gas stripping purification step may be performed.

[0141] In a particularly preferred embodiment, the method as envisaged herein further comprises the steps of

[0142] (G) subjecting the liquid stream of step (E), particularly comprising at least 95 vol% carbon dioxide, to a gas stripping process in a stripping unit, thereby obtaining a liquid carbon dioxide stream comprising at least 98 vol% carbon dioxide and a loaded stripping gas; and

[0143] (H) combining the loaded stripping gas of step (G) with the second carbon dioxide enriched gas stream of step (C).

[0144] In particular, in the stripping unit, the remaining impurities in the liquid carbon dioxide stream comprising at least 95 vol% carbon dioxide are stripped using carbon dioxide vapor as stripping gas. Advantageously, the stripping step of step G allows to achieve a carbon dioxide purity higher than 98 vol%. In particular, the loaded stripping gas, comprising carbon dioxide and the impurities that have been removed from the liquid carbon dioxide stream comprising at least 95 vol% carbon dioxide, is mixed with the second carbon dioxide enriched stream and is further purified by compression and liquefaction in steps (D) and (E).

[0145] In certain embodiments, the method as envisaged herein may comprise one or more homogenization or buffering steps, wherein cyclic variations of a stream issuing from the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, and / or from the membrane unit are attenuated by providing the respective gas stream to a buffer unit. In particular, a buffer unit or tank may be a vessel or reservoir having a variable volume at substantially constant pressure, or it may be a vessel or reservoir having a fixed volume and equipped with a pressure control system for maintaining a substantially constant pressure. Advantageously, in case of interruption of the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit, the membrane unit, or the distillation unit, the buffer unit allows to decouple the particular unit involved.

[0146] The successive gas treatments typically require compression and / or expansion of a gas stream and / or a vacuum creation step. The compressor for compression of a gas stream is not particularly limited. The compressor may be a centrifugal compressor, a screw compressor, a reciprocating compressor, an axial compressor, and the like, or a combination thereof.

[0147] As indicated above, the successive gas treatments typically require cooling of one or more gas streams, particularly a compressed gas stream. Such cooling can be performed by one or more heat exchangers, such as a plate fin heat exchanger, a shell-in-tube heat exchanger, a spiral wound heat exchanger, a brazed aluminum plate heat exchanger, or a combination thereof. In such heat exchanger, a compressed gas stream, prior to its introduction in the adsorption-desorption unit, in the membrane unit or in the low temperature distillation unit, is cooled by a gas stream leaving such unit.

[0148] When vacuum creation needs to be performed, the means for creating the vacuum is not particularly limited. Examples include rotary van pumps, diaphragm pumps, turbomolecular pumps, roots blowers, etc.

[0149] A second aspect of the present invention provides a system for the separation of carbon dioxide, particularly configured for performing an embodiment of the method according to the present invention.

[0150] The system for the separation of carbon dioxide from a gas stream according to the present invention comprises multiple separation units. In particular, the system for the separation of carbon dioxide at least comprises (I) an adsorption-desorption unit, particularly a pressure swing adsorption-desorption unit, (II) a membrane unit, (III) a liquefaction unit or a liquefaction means and (IV) a distillation column or liquid vapor separation means, as separation units.

[0151] (I) The adsorption-desorption unit, particularly the pressure swing adsorptiondesorption unit, is configured for separating a feed gas stream comprising carbon dioxide, particularly comprising less than 30 vol% carbon dioxide, into at least two gas streams, wherein the at least two gas streams comprise (i) a first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide; and (ii) a first carbon dioxide enriched gas stream, particularly comprising at least 15 vol% carbon dioxide. The adsorption- desorption unit, particularly the pressure swing adsorption-desorption unit, comprises an inlet for the feed gas and at least one outlet for the separated gas streams. In particular embodiments, the adsorption-desorption unit is a pressure swing adsorption unit, more particularly a vacuum pressure swing adsorption-desorption unit. In certain embodiments, the adsorption-desorption unit comprises a vessel containing a sorbent material, particularly wherein the sorbent material comprises metal-organic frameworks (MOFs), zeolites, activated porous carbon material, amino-acid based adsorbents, or a combination thereof. The sorbent material preferably has an adsorption capacity higher than 1.0 mmol / g (determined at 25 °C and 100 kPa), more preferably with an adsorption capacity higher than 2.0 mmol / g or 2.5 mmol / g, such as higher than 3.0 mmol / g (determined at 25 °C and 100 kPa).

[0152] (II) The membrane unit is located downstream of the adsorption-desorption unit, particularly the pressure swing adsorption-desorption unit. In particular, an inlet of the membrane unit is in fluid communication or connected to an outlet of the adsorption-desorption unit. The membrane unit comprises a retentate side with an outlet for the retentate gas stream and a permeate side with an outlet for the permeate gas stream, separated by a membrane. The membrane unit is configured for separating the first carbon dioxide enriched gas stream into a retentate gas stream, particularly comprising less than 20 vol% carbon dioxide, i.e. the second carbon dioxide depleted gas stream, and into a permeate gas stream, particularly comprising at least 60 vol% or at least 80 vol% carbon dioxide, i.e. the second carbon dioxide enriched gas stream. The retentate side of the membrane unit, particularly the outlet thereof, is in fluid communication with or fluidly connected to the adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit, particularly the feed gas inlet thereof, for providing the retentate gas stream to the adsorption-desorption unit. This promotes the separation efficiency and carbon dioxide recovery of the system.

[0153] As indicated above, the membrane is made up of a material that selectively allows CO2 to pass through. The membrane typically has a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70. The membrane has CO2 permeance expressed in GPU (1 GPU = 10'6Ncm3.cm'2.s'1.cmHg'1) higher than 800, preferably higher than 1500 and more preferably higher than 2000. The membrane may be a membrane made up of a polymeric material, a carbon membrane, a mixed matrix membrane (MMM), a ceramic membrane, or a combination thereof. Examples include polymeric membranes made up of polyimide or fluoropolysulfones, poly(phenylene oxides), poly(fluorocarbonates), or polymers of fluoroacetophenone; carbon molecular sieves membranes, or mixed matrix membranes as described in “Tan et al., Science 378, 1189-1194 (2022)”. The membrane unit may comprise flat sheet membranes, tubular tubes membranes, hollow fiber membranes and / or other membranes commonly used in industry or later developed. (III) The liquefaction unit or liquefaction means is located downstream of the membrane unit, particularly downstream of the permeate side of the membrane unit. The liquefaction unit / means particularly comprises a compressor and cooling means. This unit is configured for converting the permeate gas stream into a carbon dioxide containing liquid vapor mixture.

[0154] (IV) The distillation column or liquid vapor separation unit, such as a flash vessel, is located downstream of the liquefaction unit and is configured for separating the carbon dioxide containing liquid vapor mixture into a gas stream comprising non-condensable gases and a liquid carbon dioxide stream. The distillation column or liquid vapor separation unit, such as a flash vessel, typically comprises an inlet for the carbon dioxide containing liquid vapor- mixture, an outlet for the gas stream comprising non-condensable gases, and an outlet for the purified liquid carbon dioxide stream. The outlet for the gas stream comprising non-condensable gases is in fluid communication with or fluidly connected to the membrane unit, particularly to the inlet thereof, for providing the gas stream comprising non-condensable gases to the membrane unit. This promotes the separation efficiency and carbon dioxide recovery of the system.

[0155] In particular embodiments, the system further comprises one or more further units:

[0156] (V) a combustion source of a flue gas. This unit provides the carbon dioxide containing gas to be treated to the system as envisaged herein. Accordingly, the combustion source is in fluid communication with the adsorption-desorption unit, particularly with the inlet thereof;

[0157] (VI) a cleaning unit for removing water and other oxygenated compounds from the flue gas. The cleaning unit is located downstream of the combustion source and upstream of the adsorption-desorption unit, with an inlet of the cleaning unit in fluid communication with or connected to the combustion source of a flue gas, and an outlet of the cleaning in fluid communication with or connected to the inlet of the adsorption-desorption unit. Suitable cleaning units include dryers or units based on an adsorbent-based process, wherein water and oxygenated compounds are removed by adsorption on solid materials. In certain embodiments, such cleaning units comprise alumina, silica or molecular sieves. Additionally or alternatively, the cleaning unit may comprise a condenser, wherein water in the CO2 containing gas stream to be treated is lowered by condensation.

[0158] (VII) a stripping unit. The stripping unit is located downstream of the distillation column and is configured for stripping the liquid carbon dioxide stream with a stripping gas. The stripping unit typically comprises an inlet for the liquid carbon dioxide stream, an outlet for the purified liquid carbon dioxide stream, an inlet for the stripping gas, particularly carbon dioxide, and an outlet for the loaded stripping gas, i.e. the stripping gas comprising the gaseous impurities removed from the carbon dioxide liquid stream. The outlet of the loaded stripping gas is in fluid communication with or connected to the liquefaction unit.

[0159] (VIII) one or more compressors configured for compressing one or more gas streams between two consecutive separation units;

[0160] (IX) one or more means for expanding a gas stream, such as a turbine or a Joule Thomson valve, configured for recovering energy from a gas stream by expansion thereof, particularly from the first carbon dioxide depleted gas stream, the retentate gas stream and / or the gas stream comprising non-condensable gases; and / or

[0161] (X) one or more heat exchangers, particularly wherein the first carbon dioxide depleted gas stream, the retentate gas stream and / or the gas stream comprising non-condensable gases are used as the cooling medium.

[0162] In certain embodiments, the system further comprises one or more buffer units, typically downstream of the adsorption-desorption unit and / or the membrane unit, for attenuating cyclic variations of a stream issuing from the adsorption-desorption unit and / or from the membrane unit are attenuated. In particular, a buffer unit or tank may be a vessel or reservoir having a variable volume at substantially constant pressure, or it may be a vessel or reservoir having a fixed volume and equipped with a pressure control system for maintaining a substantially constant pressure. Advantageously, in case of interruption of the adsorptiondesorption unit, the membrane unit, or the distillation unit, the buffer unit allows to decouple the particular unit involved.

[0163] The successive gas treatments in the different separation units typically require compression and / or expansion of a gas stream, cooling or heating of a gas stream, and / or a vacuum creation step. The compressor, turbine, heat exchanger and / or vacuum pump necessary for the implementation thereof are thus located between two successive purification units for adapting the conditions of a carbon dioxide containing gas to suitable or optimal conditions for carbon dioxide separation and recovery in the respective purification unit.

[0164] The compressor for compression of a gas stream is not particularly limited. The compressor may be a centrifugal compressor, a screw compressor, a reciprocating compressor, an axial compressor, and the like, or a combination thereof.

[0165] Cooling of a gas streams, particularly a compressed gas stream, can be performed by one or more heat exchangers, such as a plate fin heat exchanger, a shell-in-tube heat exchanger, a spiral wound heat exchanger, a brazed aluminum plate heat exchanger, or a combination thereof. In such heat exchanger, a compressed gas stream, prior to its introduction in the adsorption-desorption unit, in the membrane unit or in the low temperature distillation unit, is cooled by a gas stream leaving such unit. When vacuum creation needs to be performed, the technology is not particularly limited. Examples include rotary van pumps, diaphragm pumps, turbomolecular pumps, roots blowers, etc.

[0166] The following exemplary embodiments further illustrate the present invention

[0167] Exemplary embodiment 1

[0168] Figure 1 represents a flow diagram of a first exemplary embodiment of the present invention. Although this embodiment depicts only one heat exchanger (O), one of ordinary skill in the art will recognize that multiple heat exchangers may replace the exchanger drawn in Figure 1.

[0169] A gas stream containing CO2 (1), in particular a flue gas, is compressed by a compressor (A) to produce a compressed gas stream containing CO2 (2). Suitable compressing ranges are 3 to 6 bara. The compressed gas stream (2), particularly compressed flue gas, is optionally conveyed through a dryer (B) to trap and remove undesired particles and molecules such as oxygenated compounds. This promotes an efficient CO2 separation in the next steps and potential problems are avoided, such as sorbent poisoning, water freezing, condensation in heat exchanger, corrosion, and the like. Impurities can be removed from the dryer to improve dryer efficiency via impurity stream (18).

[0170] The dried and purified compressed gas stream can be combined with gas stream (11), which is the retentate of the membrane unit (G), and, optionally, with gas stream (20) which corresponds to the uncondensed gas from the distillation unit (L) (not shown), to obtain feed gas stream (3). The gas stream (20) contains less than 50 vol% carbon dioxide. The gas stream (11) typically contains less than 20 vol% carbon dioxide. The dried compressed feed gas stream (3) is then cooled down in exchanger O and may also be expanded (not shown), to obtain a cooled, dried gas stream (19). For instance, the dried, compressed gas stream (3) is optionally expanded to 3 bara and is cooled down to 3 °C.

[0171] The cooled, dried stream (19) is conveyed to the vacuum pressure swing adsorptiondesorption unit (D), comprising one or more adsorption-desorption beds to produce a CO2 depleted stream (4) containing less than 5 vol% carbon dioxide and a CO2 enriched stream (6) via vacuum pump E. Initially, vacuum pump E generates a highly CO2 enriched stream (8) containing more than 60 vol% or, preferably, more than 80 vol% carbon dioxide; next, a moderately CCh-enriched stream (7) containing less than 60 vol% carbon dioxide, particularly between 15 and 40 vol% carbon dioxide, is obtained as well. The CO2 depleted stream (4) is expanded and, subsequently, used as a cooling medium in the heat exchanger (O). Subsequently, the first CO2 depleted stream is released to the atmosphere as a clean flue gas (5).

[0172] The highly CCh-enriched stream (8) is pumped to the buffer tank (J).

[0173] The moderately CCh-enriched stream (7) is compressed in compressor (F) to 2.5 bara and cooled to 3°C, generating a cooled, compressed moderately CCh-enriched stream (9) and conveyed to the membrane unit (G). The cooled, compressed moderately CCh-enriched stream (9) is then further separated in the membrane unit (G) into a CO2 depleted retentate stream (10), containing less than 20 vol% CO2, which is the retentate of the membrane, and a highly CO2 enriched permeate stream (12) containing more than 80 vol% CO2, which is the permeate stream of the membrane. The retentate stream (10) can optionally be compressed to 3 to 6 bara using compressor (H), resulting in compressed retentate stream (11) and subsequently mixed with the dried compressed gas stream (3).

[0174] The permeate stream (12) containing more than 80 vol% carbon dioxide, is pumped to the buffer tank (J) and combined with the highly CO2-enriched stream (8). The mixture (13) of the permeate stream (12) and the highly CO2-enriched stream (8) is pumped via pump (I), compressed to 25 bara via compressor K and subsequently cooled in the heat exchanger (O) to -50°C, resulting in cooled, CO2 rich stream (14), which is conveyed to the distillation column (L) (CO2 vapor / liquid separation). A liquid stream containing more than 95 vol% CO2 (17) is conveyed from the distillation column (L) to the CO2 stripping unit (N) where remaining impurities are removed by stripping with carbon dioxide. The CO2 gas stream (21) used for the stripping may be obtained from the pure carbon dioxide downstream of the stripping unit

[0175] (N), and can optionally pass through the heat exchanger (O). A purified liquid CO2 stream containing more than 98 vol% carbon dioxide (22) is pumped out of the stripping unit N and is either now ready for transport & storage (27) or is expanded (24) via expander or turbine (P) to cool the exchanger. The expanded CO2 (25) can be further compressed up to 111 bara for transportation via pipelines. The gas stream containing non-condensable gases (15), containing less than 50 vol% CO2, obtained from the distillation column (L), is expanded to 3 to 6 bara in expander or turbine (M), and used as a cooling medium in the heat exchanger

[0176] (O), thereby generating stream (20). The CO2 containing gas stream (20) is combined with the gas stream 9 entering in the membrane unit. The gas stream (23) exiting from the top of the of the stripping unit N is first subject to a heat exchange step in heat exchanger (O) for cooling down the heat exchanger, and following compressing in compressor K is recycled to the distillation column L for further purification. Exchanger (O) may also, optionally, be cooled down via an external cooling system (26). Exemplary embodiment 2.

[0177] Figure 2 represents a flow diagram of a second exemplary embodiment of the present invention. Similar as for exemplary embodiment 1 , although this embodiment depicts only one heat exchanger (O), one of ordinary skill in the art will recognize that multiple heat exchangers may replace the exchanger drawn in Figure 2.

[0178] In general, exemplary embodiment 2 does not differ from exemplary embodiment 1 , until the gas stream 19 is conveyed to the vacuum pressure swing adsorption-desorption unit (D). Briefly, a gas stream containing CO2 (1), in particular a flue gas, may be subsequently subject to a compression by a compressor (A), thereby obtaining a compressed gas (2); a drying / purification step in a dryer (B) to obtain a dried gas stream (3), and a cooling step in heat exchanger (O) to obtain a cooled, dried gas stream (19), which is further treated in the adsorption-desorption unit (D). In the present exemplary embodiment, the following gas streams are recovered from the adsorption-desorption unit (D):

[0179] • a CO2 depleted stream (4) containing less than 5 vol% carbon dioxide; and

[0180] • a CC>2-enriched stream (6*), via pump (E).

[0181] The CO2 depleted stream (4) is expanded, used as cooling medium in the exchanger (O) and released to the atmosphere, as a clean flue gas (5).

[0182] The moderately CCh-enriched stream (6*) is pumped to the buffer tank (J), such as for homogenization purposes. The moderately CCh-enriched stream (7*) containing between 30 and 60 vol% exiting the buffer tank (J) is compressed to 2.5 bara by compressor (F), thereby obtaining a compressed CO2 enriched stream (8*) and cooled to 3°C in heat exchanger (O), thereby generating cooled, compressed moderately CCh-enriched stream (9*), which is conveyed to the membrane unit (G). The gas stream (9*) is then further separated in the membrane unit G into a CO2 depleted retentate stream (10) containing less than 20 vol% CO2, which is the retentate of the membrane, and a CO2 rich permeate stream (12) containing more than 60 vol%, or preferably more than 80 vol% CO2, which is the permeate of the membrane. The retentate stream can be optionally compressed to 3 to 6 bara in compressor H, to obtain compressed CO2 depleted retentate stream (11), which is mixed with the dried compressed gas stream to obtain feed gas stream (3).

[0183] The CO2 rich permeate stream (12) containing more than 60 vol% or more than 80 vol% carbon dioxide, is pumped via pump (I), compressed to 25 bara in compressor (K) and cooled in heat exchanger (O) to -50°C and the resulting cooled, compressed CO2 rich stream 14 is then conveyed to the low temperature distillation column (CO2 vapor / liquid separation) (L). A liquid stream containing more than 95 vol% CO2 (17) is conveyed out of the distillation column (L) to the CO2 stripping unit (N) where remaining impurities are removed. The CO2 gas stream (21) used for the stripping may be obtained from the pure carbon dioxide downstream of the stripping unit N, and can optionally pass through the heat exchanger (O). A pure liquid CO2 stream containing more than 98 vol% carbon dioxide (22) is pumped out of the stripping unit N and is either ready for transport & storage (27) or is expanded (24) to cool the exchanger (O). The expanded CO2 (25) can be further compressed up to 111 bara for transportation via pipelines. The gas stream containing non-condensable gases (15), containing less than 50 vol% CO2, obtained from the distillation column (L), is expanded to 3 to 6 bara in expander M, and used as a cooling medium in the heat exchanger O, thereby generating stream (20). The CO2 containing gas stream (20) is combined with the gas stream 9* entering in the membrane unit. The gas stream (23) exiting from the top of the of the stripping unit N is first subject to a heat exchange step in heat exchanger O, and following compressing in compressor K is recycled to the distillation column L for further purification. Exchanger O is optionally cooled down via an external cooling system (26).

Claims

Claims1. A method for the separation of carbon dioxide from a flue gas stream, comprising the steps of:(A) providing a carbon dioxide containing flue gas stream, particularly comprising less than 35 vol% of carbon dioxide, relative to the total volume of the gas stream;(B) separating a feed gas stream comprising the flue gas stream of step (A) in a pressure swing adsorption-desorption unit, particularly in a vacuum pressure swing adsorptiondesorption unit, into at least two gas streams, wherein the at least two gas streams comprise:(i) a first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and(ii) a first carbon dioxide enriched gas stream, particularly comprising at least 15 vol% carbon dioxide, relative to the total volume of the gas stream;(C) separating at least part of the first carbon dioxide enriched gas stream in a membrane unit into a second carbon dioxide depleted retentate gas stream, particularly comprising less than 20 vol% carbon dioxide, relative to the total volume of the gas stream, and into a second carbon dioxide enriched permeate gas stream, particularly comprising at least 60 vol% or at least 80 vol% carbon dioxide, relative to the total volume of the gas stream;(D) liquefying the second carbon dioxide enriched gas stream, particularly by compressing and cooling the second carbon dioxide enriched gas stream, thereby obtaining a carbon dioxide containing liquid stream;(E) separating the carbon dioxide containing liquid stream from step (D) into a third carbon dioxide depleted gas stream, particularly comprising less than 50 vol% carbon dioxide, relative to the total volume of the gas stream, and into a liquid carbon dioxide stream, particularly comprising at least 95 vol% carbon dioxide, relative to the total volume of the liquid stream;(F) combining the second carbon dioxide depleted gas stream of step (C) with the flue gas stream of step (A), thereby obtaining the feed gas stream of step (B); and combining the third carbon dioxide depleted gas stream of step (E) with the first carbon dioxide enriched gas stream of step (B) and providing the combined gas stream to the membrane unit; and, optionally,(G) subjecting the liquid carbon dioxide stream of step (E) to a gas stripping process in a stripping unit, thereby obtaining a liquid carbon dioxide stream comprising at least 98 vol% carbon dioxide and a loaded stripping gas; and(H) combining the loaded stripping gas of step (G) with the second carbon dioxide enriched gas stream of step (C).

2. The method according to claim 1 , wherein, in step (B), the feed gas stream is separated into two gas streams, wherein the two gas streams are:(i) the first carbon dioxide depleted gas stream, particularly comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and(ii) the first carbon dioxide enriched gas stream, wherein step (B) comprises measuring the carbon dioxide concentration of the first carbon dioxide enriched gas stream, wherein(a) when the carbon dioxide concentration of the first carbon dioxide enriched gas stream is below 40 vol% or below 50 vol% carbon dioxide, particularly between 15 vol% and 40 vol% carbon dioxide or between 15 vol% and 50 vol% carbon dioxide, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is provided to and separated in the membrane unit of step (C), and(b) when the carbon dioxide concentration of the first carbon dioxide enriched gas stream is above 40 vol% or above 50 vol% carbon dioxide, relative to the total volume of the gas stream, the first carbon dioxide enriched gas stream is combined with the second carbon dioxide enriched gas stream, thereby obtaining a combined carbon dioxide enriched gas stream, and step (D) comprises liquefying the combined carbon dioxide enriched gas stream; and, optionally, wherein step (H) comprises combining the loaded stripping gas of step (G) with the combined carbon dioxide enriched gas stream.

3. The method according to claim 1 , wherein, in step (B), the feed gas stream is separated into two gas streams, wherein the two gas streams are:(i) the first carbon dioxide depleted gas stream, comprising less than 5 vol% carbon dioxide, relative to the total volume of the gas stream; and(ii) the first carbon dioxide enriched gas stream, wherein the first carbon dioxide enriched gas stream, particularly following homogenization of the first carbon dioxide enriched gas stream, comprises between 30 and 60 vol% carbon dioxide, relative to the total volume of the gas stream.

4. The method according to any one of claims 1 to 3, wherein the flue gas stream of step (A) comprises between 1 and 30 vol% carbon dioxide, particularly between 3 and 25 vol%, more particularly between 3 and 20 vol% or between 3 and 15 vol% carbon dioxide, relative to the total volume of the gas stream.

5. The method according to claim 4, wherein the flue gas stream of step (A) comprises between 4 and 10 vol% carbon dioxide or between 5 and 10 vol%, more particularly between 7 and 9 vol% carbon dioxide, relative to the total volume of the gas stream.

6. The method according to any one of claims 1 to 5, wherein step (B) comprises the step of cooling the feed gas to a temperature below 30 °C, preferably below 10 °C, more preferably below 3°C, prior to separating the feed gas in the pressure swing adsorption-desorption unit, particularly in a vacuum pressure swing adsorption-desorption unit; and / or wherein step (C) comprises the step of cooling the first carbon dioxide enriched stream to a temperature below 30 °C, preferably below 10 °C, more preferably below 3°C, prior to separating the first carbon dioxide enriched stream in the membrane unit.

7. The method according to any one of claims 1 to 6, wherein the ratio between the pressure of the feed gas at an inlet of the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit, and the pressure of the first carbon dioxide enriched stream at the outlet of the pressure swing adsorption-desorption unit is lower than 40, preferably lower than 20 and more preferably lower than 10; and / or wherein the ratio between the pressure of the first carbon dioxide enriched gas stream at an inlet of the membrane unit and the pressure of the second carbon dioxide enriched gas stream at an outlet of the membrane unit is lower than 15, preferably lower than 10, and more preferably lower than 5.

8. The method according to any one of claims 1 to 7, wherein step (D) comprises compressing at a pressure between 15 and 30 bara, particularly between 15 and 25 bara, and cooling the second carbon dioxide enriched gas stream or the combined carbon dioxide enriched gas stream, thereby obtaining a carbon dioxide containing liquid stream.

9. The method according to any one of claims 1 to 8, wherein the first, second and / or third carbon dioxide depleted gas streams are expanded in an expander or turbine to recover energy.

10. The method according to any one of claims 1 to 9, wherein at least part of the energy needed for cooling and / or compression of a gas stream is provided by expansion of the first,second and / or third carbon dioxide depleted gas streams and / or by expansion of the gasified liquid carbon dioxide stream of step G.11 . The method according to any one of claims 1 to 10, wherein the first, second and / or third carbon dioxide depleted gas stream, optionally after expansion thereof, are used as a cooling medium in a heat exchanger for cooling a gas stream in any one of steps (A) to (H), in particular for cooling the first and / or second carbon dioxide enriched gas stream.

12. The method according to any one of claims 1 to 11 , wherein step (A) further comprises subjecting the carbon dioxide containing flue gas stream to a pretreatment cleaning step, particularly in a pretreatment dryer, for removing water and other oxygenated compounds from the carbon dioxide containing gas stream.

13. The method according to claim 12, wherein at least part of the energy to regenerate the pretreatment dryer is provided by expansion of the first, second and / or third carbon dioxide depleted gas streams and / or by expansion of the gasified liquid carbon dioxide stream of step G.

14. The method according to any one of claims 1 to 13, wherein the membrane unit comprises a membrane having a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70.

15. A system for the separation of carbon dioxide from a flue gas stream comprising:(I) a pressure swing adsorption-desorption unit, particularly a vacuum pressure swing adsorption-desorption unit, configured for separating a feed gas stream comprising carbon dioxide into at least two gas streams, wherein the at least two gas streams comprise (i) a first carbon dioxide depleted gas stream, and (ii) a first carbon dioxide enriched gas stream;(II) a membrane unit, downstream of the adsorption-desorption unit, comprising one or more membranes, separating a retentate side and a permeate side, configured for separating the first carbon dioxide enriched gas stream into a second carbon dioxide depleted retentate gas stream, and into a second carbon dioxide enriched permeate gas stream;(III) a liquefaction unit or liquefaction means, particularly comprising a compressor and cooling means, downstream of the membrane unit, configured for converting the second carbon dioxide enriched permeate gas stream into a carbon dioxide containing liquid vapor mixture; and(IV) a distillation column or liquid vapor separation unit, such as a flash vessel, downstream of the liquefaction unit, configured for separating the carbon dioxide containing liquid vapor mixture into a gas stream comprising non-condensable gases and a liquid carbon dioxide stream, wherein the distillation column or liquid vapor separation unit comprises an outlet for the gas stream comprising non-condensable gases, and an outlet for the liquid carbon dioxide stream; wherein the retentate side of the membrane unit is in fluid communication with the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorptiondesorption unit, for providing the retentate gas stream to the pressure swing adsorptiondesorption unit; and wherein the outlet for the gas stream comprising non-condensable gases is in fluid communication with the membrane unit, for providing the gas stream comprising non-condensable gases to the membrane unit.

16. The system according to claim 15, further comprising one or more of:(V) a combustion source of a flue gas in fluid communication with the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit;(VI) a cleaning unit, downstream of the combustion source of a flue gas and upstream of the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit, configured for removing water and other oxygenated compounds from the flue gas,(VII) a stripping unit, downstream of the distillation column or liquid vapor separation unit, configured for stripping the liquid carbon dioxide stream with a stripping gas, the stripping unit comprising an inlet for the liquid carbon dioxide stream, an outlet for the purified liquid carbon dioxide stream, an inlet for the stripping gas, particularly carbon dioxide, and an outlet for the loaded stripping gas, wherein the outlet of the loaded stripping gas is in fluid communication with the liquefaction unit;(VIII) one or more compressors configured for compressing one or more gas streams between two consecutive units;(IX) one or more expanders or turbines, configured for recovering energy from a gas stream by expansion thereof, particularly from the first carbon dioxide depleted gas stream, the second carbon dioxide depleted retentate gas stream, the gas stream comprising non- condensable gases; and / or a gasified liquid carbon dioxide stream; and / or(X) one or more heat exchangers, particularly wherein the first carbon dioxide depleted gas stream, the second carbon dioxide depleted retentate gas stream and / or the gas stream comprising non-condensable gases are used as the cooling medium.

17. The system according to claim 15 or 16, wherein the membrane unit comprises one or more polymeric membranes, mixed matrix membranes, carbon membranes, ceramic membranes, or a combination thereof, and / or wherein the membrane unit comprises a membrane having a CO2 / N2 selectivity higher than 25, preferably higher than 45 and more preferably higher than 70.

18. The method according to any one of claims 1 to 14, or system according to any one of claims 15 to 17, wherein the pressure swing adsorption-desorption unit, particularly the vacuum pressure swing adsorption-desorption unit, comprises a vessel containing solid sorbent material, wherein the solid sorbent material comprises metal-organic frameworks (MOFs), zeolites, activated carbon, amino-acid based adsorbents, or a combination thereof, and wherein the sorbent material has an adsorption capacity higher than 1.5 mmol / g (determined at room temperature), more preferably an adsorption capacity higher than 2.5 mmol / g (determined at room temperature).

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