Process for the separation of a gas from a gas mixture

Abstract

A process for the separation of CO2 from a gas mixture comprising the same using at least one membrane with a feed side and a permeate side, said process comprising: - contacting said gas mixture with the feed side of the membrane; and - contacting a sweep gas with the permeate side of the membrane, wherein the sweep gas comprises water vapour; wherein the pressure of the sweep gas is 0.95 bar or less.

Description

[0001] 174471 / 01

[0002] Process for the separation of a gas from a gas mixture

[0003] Field of the Invention

[0004] This invention relates to a process for the separation of carbon dioxide from gas mixtures containing the same; and to an apparatus for the same. In particular, the present invention provides a process for the separation of carbon dioxide from a mixture containing the same using a sweep gas at a pressure of 0.95 bar or less on the permeate side of a membrane.

[0005] Background

[0006] The global climate crisis post industrialisation is primarily due to post industrialisation and anthropogenic factors. Of all the causes, the tremendous increase in greenhouse gas emissions, most importantly, the emission of CO2, over the past few decades, could be directly coupled to the current global warming scenario. Implementation of Carbon Capture, Utilization and Seguestration (CCUS) represents the most effective solution for the coming decade to undergo the transition to a more sustainable energy system. However, limitations relating to the cost and technical viability in the CO2 capture technologies (i.e., absorption, membrane separation and adsorption) still pose as the main challenges to be addressed in the implementation of CCUS.

[0007] Polymeric gas separation membranes have been explored widely for CO2 separation applications due to their low-cost, high modularity and easy scalability. Membrane materials with preeminent permeation properties (permeability and selectivity), as well as good chemical and mechanical properties, increase efficiency of separation processes significantly. WO2021 / 219887 describes composite membranes with a selective layer comprising: a polymeric matrix comprising an amine polymer; graphene oxide nanofiller; and a mobile carrier selected from an ionic liguid or an amino acid salt.

[0008] With regards to the process for separating a gas from a gas mixture, a key aspect is the continuous removal of the permeate (e.g. the gas to be removed). A sweeping flow of a gas / liquid is known to be used to boost the driving force. Alternatively, suction from a vacuum source is known to drive the removal of the permeate, since a difference in pressures across the membrane is established. However, systems which operate under vacuum typically have an irregular or unpredictable flow pattern on the permeate side which can reduce the efficiency of the gas separation process.

[0009] The present inventors have surprisingly found that the provision of a sweep gas under reduced pressure (i.e. the combination of a vacuum with a sweep gas) improves the performance of gas separation processes since it can improve continuous removal of the permeates, drive the separation of a gas from a mixture via a transmembrane pressure differential to help permeation, and / or provide a more efficient flow pattern and better performance for the separation of the gas compared to the irregular crossflow current when only vacuum is used. The combination of a vacuum with a sweep gas (e.g. a steam sweep) may additionally allow for lower operational temperatures to be used, thus saving energy and reducing the carbon footprint, whilst also minimising degradation of the gas separation membrane.

[0010] When a sweep gas comprising water vapour (e.g. a steam sweep) is used, one or more of the following advantages may be obtained: the driving force for CO2 permeation is additionally increased and / or the driving force for water vapour permeation is reduced. As a result, the efficiency of the process may be improved. Water permeation retardation can be useful for facilitated transport membranes to prevent drying of the membranes.

[0011] Summary of Invention

[0012] In a first aspect, a process is provided for the separation of CO2 from a gas mixture comprising the same, using at least one membrane with a feed side and a permeate side, said process comprising:

[0013] - contacting said gas mixture with the feed side of the membrane; and

[0014] - contacting a sweep gas with the permeate side of the membrane, wherein the sweep gas comprises water vapour; wherein the pressure of the sweep gas is 0.95 bar or less.

[0015] In a further aspect, there is provided a gas separation apparatus for the separation of CO2 from a gas mixture, comprising:

[0016] - at least one membrane with a feed side and a permeate side; and - a reduced pressure-generating apparatus, configured to generate a reduced pressure on the permeate side of the membrane

[0017] - a sweep gas supply apparatus, configured to supply sweep gas to the permeate side of the membrane.

[0018] In a further aspect, there is provided the use of a sweep gas (e.g. steam sweep) under reduced pressure conditions in a facilitated transport membrane for the separation of CO2 from a gas mixture comprising the same.

[0019] The features of the aspects and / or embodiments indicated herein are usable individually and in combination in all aspects and embodiments of the invention where technically viable, unless otherwise indicated.

[0020] Description of Figures

[0021] Figure 1 compares the flow pattern for the gas separation process using sweep gas (e.g. a steam sweep) in combination with a vacuum in a counter-current process (right hand figure) with a process where a vacuum is used alone (left hand figure). When a vacuum is used alone, a mixed / crossflow system is established which does not allow for an efficient recovery of the gas from the gas mixture. However, when a sweep gas is used in combination with a vacuum, a counterflow current is established which is a regular flow pattern allowing greater separation efficiency which enables higher gas (e.g. CO2) flux and purity to be achieved. When a sweep gas comprising water vapour (e.g. a steam sweep) is used, the driving force for gas (e.g. CO2) permeation is additionally increased, and the driving force for water vapour permeation is reduced, thus further improving the efficiency of the process. Water permeation retardation can be useful for facilitated transport membranes to prevent drying of the membranes.

[0022] Figure 2 shows a mixed gas permeation rig suitable for use in the present process. MFC = mass flow controller, VP = vacuum pump, TT = temperature sensor, PT = pressure sensor, HT = humidity sensor. The setup is contained within an oven to allow for temperature control, e.g. between 20°C - 75°C. Humidification tanks are installed in line with the feed and sweep streams, with an additional heating jacket equipped on the sweep side to maximize water vapor content. A vacuum pump downstream of the permeate provides the vacuum suction for the system. Boiling is done externally in a vacuum rated pressure vessel with a heating coil. Vacuum is generated by the same pump downstream of the permeate side of the module. Under vacuum steam sweep operation, a condenser and knock out drum is used to fully condense and remove the outgoing steam.

[0023] Figure 3 shows the CO2 flux across various pressure ratios in a pre-pilot model for the separation of CO2 for a 10 vol% CO2 / 90 vol% N2 under various sweep conditions, namely: a sweep gas of humidified helium (95% relative humidity) at flow rate of 90 and 180 mL / min; a steam sweep under vacuum; and vacuum in the absence of a sweep gas. For Figure 3 and subsequent figures which refer to ‘bore side pressure’, bore side pressure is permeate side pressure (the membrane is in the form of a hollow fiber, with the bore side being the permeate side and the shell side being the feed side - see Figure 9). The data is shown in Table 1. The membrane used is a facilitated transport membrane in the form of a module comprising 85 hollow fibers and an area of 242.32 cm2. The selective layer of the membrane comprises a sterically hindered poly(allylamine) / polyvinyl alcohol polymer, porous graphene oxide nanofiller (0.2 wt% based on the weight of the selective layer), and potassium prolinate mobile carrier (20 wt% based on the weight of the selective layer). The support material is a poly(p-phenylene) oxide. For the humidified He sweep gas at a flow rate of 90 and 180 mL / min, a feed gas flow rate of 440 mL / min is used at a temperature of 35°C and a pressure of 1.2 bar. For the vacuum only, the feed gas flow rate is also at 440 mL / min and a temperature of 35°C and a pressure of 1.2 bar. For the vacuum in combination with a steam sweep, the feed gas has a flow rate of 306 mL / min, a temperature of 65°C and a pressure of 1.2 bar. The flow rate for the steam sweep is in the range of 4,000 to 10,000 mL / min. The temperature of the sweep gas is the same as that of the feed gas. From Figure 3, it is evident that under the same conditions, the addition of a humidified He sweep gas shows improved CO2 flux, especially at higher flow rates compared to vacuum only. Moreover, if we compare the vacuum only sweep to the vacuum and steam sweep, the CChflux for the combination of vacuum and steam sweep is much improved. It is also apparent for the humidified He gas sweeps that the best CO2 flux is observed at a pressure ratio of around 3:1.

[0024] Figure 4 shows the CO2 / N2 recovery across various pressure ratios in a pre-pilot model for the separation of CChfrom a 10 vol% CO2 / 90 vol% N2 mixture under various sweep conditions. The sweep conditions, membrane, feed gas flow rate, temperature, and pressures are in line with those in Figure 3 (see Table 1). From Figure 4, it is evident that under the same conditions, humidified He sweep gas (95% RH) shows improved CO2 recovery compared to vacuum only, especially at higher flow rates and a pressure ratio of around 3: 1. Moreover, if we compare the vacuum only sweep and the vacuum and steam sweep, the CO2 recovery for the combination of vacuum and steam sweep is improved. Recovery herein refers to CO2 capture rate, which is the amount of CO2 in permeate / amount of CO2 in feed.

[0025] Figure 5 shows the CO2 and N2 flux across various pressure ratios in a prepilot model using a steam sweep (see conditions from Figure 3 discussion above). Here, the feed gas is 10 vol% CO2 / 90 vol% N2 mixture. The data is shown in Table 1 The membrane used is a hollow fiber membrane in line with that used in Figure 3. The feed gas has a flow rate of 306 mL / min and is at a temperature of 65°C and a pressure of 1.2 bar. The sweep gas is a steam sweep gas set in the range of 4,000 to 10,000 mL / min. Figure 5 shows that the CO2 flux is optimum at pressure ratio of 4.4:1 feed gas / sweep gas.

[0026] Figure 6 shows the CO2 recovery and purity across various pressure ratios in a pre-pilot model. Here, the feed gas is 10% CO2 in N2. The membrane used is a hollow fiber membrane in line with that used in Figure 3. The feed gas has a flow rate, temperature and pressure in line with that described for Figure 5. The sweep gas flow rate is also in line with that described for Figure 5. Figure 6 shows that the CO2 recovery is optimum at pressure ratio of 4.4:1 feed gas / sweep gas.

[0027] Figures 7a-b compare the CO2 flux, recovery and purity for various sweep conditions. For these results, a feed gas of 20% CO2 in N2 is used at a pressure of 1.2 bar. The vacuum used for the sweep is set at a pressure of 0.4 bar in Figure 7a. The pressure of the sweep gas is varying in Figure 7b; for data at 70°C, a pressure of 0.27 bar is used, and for data at 80°C a pressure in the range of 0.27 to 0.41 bar is used. The membrane used is 2 modules in parallel with 2 m2of membrane area each. Each 2 m2module is made of 2000 fibers. The selective layer of the membrane comprises a sterically hindered poly(allylamine) / polyvinyl alcohol polymer, porous graphene oxide nanofiller (0.2 wt% based on the weight of the selective layer), and potassium prolinate mobile carrier (20 wt% based on the weight of the selective layer). The support material is a poly(p-phenylene) oxide. The humidified system (HMS) refers to the use of a steam sweep gas. The feed gas is applied at a flow rate of 42.8 L / min for 70°C tests and 85.5 L / min for 80°C test , and the sweep gas (where used) at a flow rate of 4.6 to 64.3 L / min. For the case where no steam sweep gas is used (i.e. Figure 7a), the feed flow rate was varied from 16.9 to 67.5 L / min, and the CO2 flux ranges from 5 NL / m2 / h to a maximum of 30.5 NL / m2 / h with purity ranging from 52-58%. However, when the steam sweep gas is used, a stark increase in CO2 and recovery is observed, and purities of over 65% are achieved. Additionally, when a steam sweep gas is used, the operation range of the membranes could be increased in terms of temperature, since the drying possibility (i.e. drying of the membrane) is significantly reduced.

[0028] Figure 8 further compares CO2 flux, recovery and purity for humidified systems. The data in Figure 8 reflects the CO2 separation performance when a steam sweep gas (flow rate of 4.6 to 64.3 L / min) is used at 70 and 80°C compared to when only a vacuum is used at 60°C. Here the membrane is the same as that used for Figure 7a-b. The feed gas used here is 20% CO2 in N2 at 1.2 bar pressure. The vacuum used is set at 0.27 bar pressure. From the data for without the HMS at 60°C there is no major increase in CO2 flux, recovery or purity compared to the data in Figure 7a. It is evident that managing the water content is important to maximise the temperature effect to allow the facilitated transport mechanism and improve performance.

[0029] Figure 9 shows a hollow fiber membrane module for use in the present process. The feed gas / retentate are on the shell side and the sweep gas / permeate are on the bore side. The module is in a counter-current configuration.

[0030] Figure 10 shows an upscaled module with 2000 fibers, as used in the rig for obtaining the data of Figures 7 and 8.

[0031] Figure 11 shows a fixed carrier facilitated transport mechanism for CO2 separation using an amine polymer in the selective layer a facilitated transport membrane.

[0032] Figure 12 is a graph illustrating the empirical model used for applying a pressure drop correction factor on the permeate side of the membrane for the hollow fibre membranes used with the steam sweep systems of Figures 3-6 herein, wherein the “pressure set point” is the pressure measured using a pressure gauge, and the “actual pressure” is the pressure after the correction factor has been applied. Detailed Description of the Invention

[0033] Process

[0034] Viewed from one aspect, the present invention refers to a process for the separation of CO2 from a gas mixture comprising the same using at least one membrane with a feed side and a permeate side, said process comprising: contacting said gas mixture with the feed side of the membrane; and contacting a sweep gas with the permeate side of the membrane; wherein the sweep gas comprises water vapour, wherein the pressure of the sweep gas is 0.95 bar or less.

[0035] Typically, the process of the present invention further comprises transmembrane permeation of the gas i.e. from the feed side of the membrane to the permeate side of the membrane. A combined permeate stream is generated downstream from the permeate side of the membrane, and may be seen as the combination of the permeate stream from the membrane and the sweep gas stream (see Figure 1).

[0036] Gas Mixture

[0037] The gas mixture as described herein may also be referred to as the feed gas. The feed gas thus refers to the gas mixture which is contacted with the feed side of the membrane in any aspect of the present invention.

[0038] The terms “feed gas”, “feed” and “feed stream” may be used interchangeably. The term ‘feed module’ refers to a module configured to supply a feed gas to the feed side of the membrane.

[0039] The gas mixture (e.g. feed gas) of the present invention comprises at least two different gases. The gas mixture (e.g. feed gas) is not particularly limited but contains CO2.

[0040] Generally the amount of CO2 that is present in the gas mixture may be in the range of 0.04 to 90 vol% such as 0.04 to 80 vol%. Generally, the preferred amount of CO2 present in the gas mixture is in the range of 5 to 80 vol%, such as 10 to 75 vol%. Typically, the amount of CO2 present in the gas mixture will be in the range of 5 to 25 vol% or 15 vol% or less. The amount of CO2 present in the gas mixture may there be 0.04 vol% or more, such as 0.5 vol% or more, 1 vol% or more, 5 vol% or more or 10 vol% or more. The amount of CO2 present in the gas mixture may be 90 vol% or less, 80 vol% or less, 75 vol% or less, 50 vol% or less or 25 vol% or less. Unless otherwise indicated, all % values described above, below or in the claims refer to vol % when referring to mixtures of gases.

[0041] The gas mixture of the present invention may comprise other gases in addition to carbon dioxide, such as oxygen, nitrogen, hydrogen, methane, carbon monoxide, nitrogen / nitrous oxides, sulfur oxides (e.g. sulfur dioxide), ammonia, hydrogen sulphide, steam and / or inert gases (e.g. helium, argon, neon, krypton, xenon etc.). The gas mixture of the present invention may additionally comprise particulate matter such as dust. However, it is generally preferred that any solid particulate matter (where present) is removed prior to contacting the gas mixture on the feed side of the membrane. Particulate matter may be removed using a filter, e.g. with pores of 2 pm diameter or less, prior to contacting the gas mixture on the feed side of the membrane. Typically, the gas mixture comprises carbon dioxide, oxygen, nitrogen, methane and / or hydrogen, preferably at least carbon dioxide, oxygen and nitrogen.

[0042] Oxygen will typically be present in an amount of 0 to 50 vol% of the gas mixture, such as 5 to 30 vol%, 10 to 25 vol% or 8.5 to 10 vol%. The amount of oxygen may be 5 vol% or less, such as 0 to 1 vol%. Alternatively, the amount of oxygen in the gas mixture may be 5 vol% or more, such as 10 vol% or more, 20 vol% or more, or 30 vol% or more.

[0043] Nitrogen will typically be present in an amount of 0 to 95 vol% of the gas mixture, such as 50 vol% or more, 70 vol% or more, e.g. in the range of 70 to 85 vol%. The amount of nitrogen in the gas mixture may alternatively be in the range of 25 to 75 vol%, such as 30 to 50 vol%. The amount of nitrogen in the gas mixture may alternatively be in the range of 0 to 25 vol% such as 10 to 20 vol%.

[0044] Methane will typically be present in an amount of 0 to 95 vol% of the gas mixture, such as 30 to 90 vol% or 45 to 75 vol%. The amount of methane be 5 vol% or less of the total gas mixture, such as 1 vol% or less or 0 to 5 vol% (e.g. 0 to 1 vol%).

[0045] Hydrogen may typically be present in an amount of 0 to 95 vol% of the gas mixture, such as 10 to 50 vol% or 25 to 30 vol%. The amount of hydrogen present in the gas mixture may be 5 vol% or less, such as 1 vol% or less or 0 to 1 vol%. When referring to the composition of the gas mixture, the total amount of all gaseous components will not exceed 100 vol%.

[0046] The gas mixture may be selected from a flue gas, air, biogas, fuel cell exhausts or syngas. In certain cases, the gas mixture may be from the exhaust of a fossil fuel combustion engine, e.g. from the engine of a sea-faring vessel.

[0047] It is preferred if the feed gas is at a pressure of 1 bar or more, such as 1.05 bar or more, typically 1.2 bar or more. The pressure of the feed gas is typically measured using a WIKA IUT-10 pressure gauge (WIKA Alexander Wiegand GmbH & Co. KG). It is typically measured inline directly after the membrane on the retentate side of the module. The pressure of the feed gas may be in the range of 1 bar to 40 bar, such as 1-20 bar or 1-5 bar. Preferably the pressure of the feed gas is in the range of 1.0 to 1.3 bar, preferably 1.0 to 1.2 bar. The pressure of the feed gas should be greater than the pressure of the sweep gas, to allow a pressure differential to be established across the membrane, thus providing a driving force for the separation of the gas from the gas mixture. The pressure of the feed gas is therefore typically greater than the pressure of the sweep gas.

[0048] Alternatively put, the pressure on the feed side of the membrane is preferably in the above ranges, e.g. in the range of 1.0 to 1.3 bar, preferably 1.0 to 1.2 bar.

[0049] If the feed gas pressure is higher than 1.3 bar, there may be disadvantages from an economical and environmental perspective, as it will typically require a compressor or blower which consumes a high amount of energy and adds complexity to the process.

[0050] The ratio between the pressure of the feed gas and the pressure of the sweep gas may be of 6:1 or less, such as 5:1 or less.

[0051] However, the ratio between the pressure of the feed gas and the pressure of the sweep gas is preferably in the range of 4.5:1 or less, preferably 4.4:1 or less, such as 4.0:1 or less or 3:1 or less. Ideally, the ratio between the pressure of the feed gas and the pressure of the sweep gas is in the range of 1.1 :1 to 4.5:1 , preferably 1.1 :1 to 4.4:1 , such as 1.5:1 to 3:1. If the pressure ratio between the feed side and the permeate side is high (e.g. higher than 4.5:1), the process may require a compressor on the feed side and / or or a larger degree of vacuum on the permeate side, both of which are undesirable on an industrial level, and can add complexity and cost to the process. High pressure differentials across the membrane also require highly mechanically-robust membranes, whereas low pressure ratios are suitable for a broader range of membranes. On the other hand, if the pressure ratio between the feed side and the permeate side is too low, such as less than 1.1:1, then the driving force for separation may be too weak.

[0052] The above pressure ratios can also be seen as the ratio between the pressure on the feed side of the membrane and the pressure on the permeate side of the membrane. Therefore, the ratio between the pressure on the feed side of the membrane and the pressure on the permeate side of the membrane may be of 6:1 or less, such as 5:1 or less. Preferably, the ratio is in the range of 4.5:1 or less, preferably 4.4:1 or less, such as 4.0:1 or less or 3:1 or less. Ideally, the ratio is in the range of 1.1 : 1 to 4.5: 1 , such as 1.5: 1 to 3: 1.

[0053] The above pressure ratios may alternatively be used as the ratios between the pressure of the feed gas and the pressure of the combined permeate stream.

[0054] The flow rate of the feed gas typically depends on the scale of the process. However, the flow rate of the feed gas is preferably the same or higher, preferably higher, than the flow rate of the sweep gas. The ratio of the flow rates of the feed gas to the sweep gas may be in the range of 20:1 to 1:5, such as 20:1 to 1.5:1 and is preferably in the range of 10:1 to 1:1, preferably 10:1 to 1.5:1.

[0055] Sweep Gas

[0056] The sweep gas of the present invention is a gas which is contacted with the permeate side of the membrane. The terms “sweep gas”, “sweep” and “sweep stream” may be used interchangeably. The term “sweep module” may be used to refer to a module configured to supply a sweep gas to the sweep side of the module. The sweep gas has a pressure below atmospheric pressure. The pressure of the sweep gas used herein is 0.95 bar or less.

[0057] The low pressure of the sweep gas is typically obtained through vacuum suction on the permeate side of the membrane. Vacuum, low pressure, or reduced pressure may be used interchangeably herein. The use of vacuum suction on the permeate side of the membrane is known in the art, and it is typically used to create a pressure differential between the permeate side and the feed side of the membrane. For example, the reduced pressure may be provided by a vacuum pump, from engine suction or from ejectors downstream of the permeate side of the membrane. Generally speaking, the reduced pressure is generated downstream of the permeate side of the membrane (e.g. via a reduced pressure generating apparatus such as a vacuum pump, from engine suction or from ejectors). The pressure differential can act as a driving force for the separation of the gas from the gas mixture. By combining the use of vacuum suction with the use of a sweep gas on the permeate side of the membrane, an efficient flow current (e.g. counter flow current) can surprisingly be established which aids the separation of the gas from the gas mixture. Vacuum suction may be applied on the permeate side of the membrane so that the sweep gas has a pressure of 0.95 bar or less, such as in the range of 0.1 to 0.95 bar, preferably 0.2 to 0.8 bar, preferably 0.25-0.8 bar such as 0.3 to 0.6 bar. The pressure of the sweep gas is typically measured using WIKA IUT-10 pressure gauge (WIKA Alexander Wiegand GmbH & Co. KG). It is typically measured inline directly after the membrane before the vacuum pump on the permeate side of the module.

[0058] The application of vacuum may be used on the permeate side so that the pressure of the sweep gas is in the range of 0.4 to 0.95 bar. Low pressures on the permeate side of the membrane are less preferable on an industrial scale, from a cost and environmental perspective due to technical constraints with creating vacuum on a large rig. However, pressures above 0.95 bar are also insufficient to establish a differential of pressures, and the driving force for separation is thus too weak. The lower the pressure of the sweep gas, the greater the pressure differential across the membrane and hence the better the separation.

[0059] The sweep gas pressures described herein may alternatively, and preferably, be seen as the pressure on the permeate side of the membrane. The pressure on the permeate side of the membrane may be of 0.95 bar or less, such as in the range of 0.1 to 0.95 bar, preferably 0.2 to 0.8 bar, preferably 0.25-0.8 bar such as 0.3 to 0.6 bar. Another suitable range is 0.4-0.95 bar. Alternatively put, the sweep gas pressures described herein may be seen as the pressures in the permeate side within the membrane module.

[0060] The pressure of the sweep gas is typically measured using WIKA IUT-10 pressure gauge (WIKA Alexander Wiegand GmbH & Co. KG). It is typically measured inline directly after the membrane before the vacuum pump on the permeate side of the module.

[0061] For hollow fiber modules, since the bore volume of the fibers is fixed, a pressure drop can be expected on the permeate side of the membrane, which is a function of total flow rate including the amount of sweep gas. The pressure drop may be accounted for by applying a correction factor to the measured pressure, said correction factor being based on an empirical model (see Figure 12, for example), where cross flow was sent in to the hollow fibres and a response of pressure was obtained, after which a model fit was performed. Through applying the empirical model to the measured pressure, a more accurate prediction of the equilibrium pressure on the permeate side of the membrane can be determined.

[0062] Accounting for pressure drop in hollow fibers is standard practice in the art for hollow fiber technology. In addition to the empirical model referred to herein (e.g. Fig 12), a correction factor can also be calculated through Bernoulli’s principles (see in this regard ‘Transport Processes and Unit Operations’, Geankopolis C.J., Third edition, Prentice-Hall International, Inc., ISBN 0-13-045253- X, pg 101-102 in particular) to establish an adjusted pressure on the permeate side of the membrane.

[0063] For example, a correction factor which is dependent on total mass flux (G) and inner diameter of the fiber, may be determined following the below equation, in particular for hollow fiber membranes:

[0064] Wherein f is the friction factor; G is the total mass flux (flow of permeate and sweep); vi is specific volume of the gas mixture; Pi and P2 are the pressures before and after the flow boundary (i.e. module); D is inner diameter of the fiber; and Leis active length of the fiber in the module.

[0065] For the humidified Helium sweep systems used in the present examples (e.g. as in Figures 3 and 4), the sweep flow is low (and therefore the total mass flux relative to the membrane area is also low), and so the actual values of pressure on the permeate side of the membrane are essentially the same as the pressure set points. The systems of Figures 7 and 8 do not require correction factors, because of the low total mass flux relative to membrane area.

[0066] Example sweep gases which may be used include, for example, steam, helium, argon, air, nitrogen, and combinations thereof. The sweep gas according to the invention preferably comprises or consists of water vapour. The presence of water vapour in facilitated transport membranes has shown to increase the flux and purity of the gas to be separated (e.g. CO2). Without being bound by theory, when a sweep gas which comprises or consists of water vapour is used, the permeation of water from the feed side of the membrane to the permeate side of the membrane is reduced. The vapour pressure of the water vapour on the permeate side of the membrane is effectively manipulated so as to reduce the driving force for water in the retentate side of the membrane. Consequently, water permeation is severely hindered which thus conserves water vapour in the membrane which is important for CO2 transport in particular. Moreover, the use of the sweep gas at a low pressure (i.e. 0.95 bar or less) can also achieve a fully developed counterflow of the permeate which aids the recovery of gas (e.g.CCh) along the length of the module. The term “membrane module” as used herein typically refers to a gas separation module comprising the membrane . The apparatus preferably further comprises a sweep module and a feed module. The feed module may be connected to the feed side of the membrane / membrane module via a conduit. The sweep module may be connected to the permeate side of the membrane / membrane module via a conduit. The general effect of a sweep gas comprising water vapour (e.g. a steam sweep) at a pressure of 0.95 or less is illustrated in Figure 1.

[0067] The sweep gas is typically separate to the feed gas and the retentate. The sweep gas is typically a gas which is different, and / or does not originate, from the retentate. It is particularly preferable that the retentate is not recycled for use as a sweep gas. The process typically comprises venting of the retentate (i.e. removal of the retentate after contact of the feed gas with the membrane). The process typically comprises a step of using a feed gas outlet, downstream from the feed gas inlet, to provide removal of retentate gas after contact with the feed side of the membrane.

[0068] Additionally, in the case of membranes comprising a polyamine selective layer (e.g. also containing amino acid mobile carriers), the presence of water in the polymer matrix of a membrane may also aid the reversible reaction of CO2 with the amino functional groups, where a carbonate / bicarbonate may form following a zwitterionic mechanism.

[0069] In certain cases, the sweep gas consists of steam, i.e. is a steam sweep. The terms “steam” and “water vapour” may be used interchangeably.

[0070] In certain cases, the sweep gas is a moisture-containing sweep gas. The terms water vapour-containing sweep gas, moisture-containing sweep gas, humid sweep gas, humidified sweep gas, all refer to a mixture of water vapour and another gas, which may be referred to herein as a sweep carrier gas. A water vapour-containing sweep gas as referred to herein typically has a water content of 5 to 50 vol%, such as 10 to 30 vol%. Generally, the water content of the moisturecontaining sweep gas is 0.5 vol% or more, such as 1 vol% or more or 5 vol% or more. Generally, the water content of the moisture-containing sweep gas is 50 vol% or less, such as 30 vol% or less or 15 vol% or less. In some cases, the water content of the moisture-containing sweep gas may be 50 vol% or more, such as 50 to 99 vol% or 60 to 80 vol%.

[0071] Alternatively, the water vapour-containing sweep gas may have a relative humidity (RH) of 20-99 %. Preferably, the water vapour-containing sweep gas has a relative humidity (RH) of 50 % or more, preferably 50-99 %, more preferably 50- 98% RH.

[0072] A moisture-containing sweep gas will typically comprise 50 to 95 vol% or 50 to 90 vol% of at least one gas which is not water vapour / steam, which may be referred to herein as a sweep carrier gas. Generally, the content at least one gas which is not water vapour / steam in the moisture-containing sweep gas is 50 vol% or more, such as 75 vol% or more or 90 vol% or more. Generally, the content at least one gas which is not water vapour / steam in the moisture-containing sweep gas is 95 vol% or less, such as 80 vol% or less or 75 vol% or less. In some cases, the content at least one gas which is not water vapour / steam in the moisture-containing sweep gas may be 50 vol% or less, such as 5 to 50 vol% or 20 to 40 vol%. The at least one gas which is not water vapour is not limited but may include helium, argon, nitrogen, oxygen, ammonia, methane or a combination thereof.

[0073] Typically, when the sweep gas is a moisture-containing sweep gas or when the sweep gas consists of steam (i.e. is a steam sweep) the recovery of the gas to be separated is improved. Compared to when the sweep gas is a dry sweep gas (e.g. is a pure inert gas), when a moisture-containing sweep gas or a steam sweep is used, lower pressures can typically be tolerated without dehydrating the membrane (e.g. the selective layer). Without being bound by theory, in cases where no sweep gas or a dry sweep gas is used, the relative humidity of the feed compared to the retentate has been shown to rapidly decline, suggesting drying of the membrane along the module length. In cases where no sweep gas or a dry sweep gas is used, higher temperatures are considered to be less tolerable, as this may cause cracks in the crystalline selective layer due to the crystalline nature of the polymer under dry conditions. Moisture-containing membranes are particular useful in facilitated transport membranes, e.g. hybrid facilitated transport membranes, used in CO2 separation. See Figure 11. If the membrane (e.g. the selective layer) becomes dehydrated, its ability to separate gases is worsened, resulting in a lower flux and permeance of CO2. The use of a sweep gas comprising water vapour, in particular when the sweep gas is steam, reduces water permeation from the feed to the permeate side by lowering the driving force for water. Water, being a smaller molecule, diffuses faster than CO2, especially at higher temperatures. Drying of the membrane on the retentate side decreases the permeance of the selective layer, reducing flux. The system adjusts water vapor pressure on the permeate side, limiting water permeation and conserving humidity for better CO2 transport. A counterflow of permeate also aids in CO2 recovery, improving flux and purity through optimized flow and water management across the membrane.

[0074] In view of the above, it is preferred that the water content of the sweep gas is greater than the water content of the feed gas. When the water content of the sweep gas is higher than that of the feed gas (i.e. the gas mixture) the separation of CO2 from the gas mixture is improved and dehydration of the membrane is avoided. Generally, when the water content of the sweep gas is greater than the water content of the feed gas, the permeation of water vapour (from the feed side to the permeate side) is avoided. The avoidance of dehydration or dehumidification of the gas mixture is especially preferred.

[0075] Since the permeation of water vapour across the membrane is preferably avoided, the water content of the feed gas and the retentate is preferably consistent or essentially consistent during the process. The variability of the water content in the feed gas and the retentate during the process is preferably 25 vol% or less, such as 10 vol% of less, especially 5 vol% or less or 1 vol% or less. Essentially, during the process, the water content of the feed gas and retentate typically changes by 25 vol% or less, such as 10 vol% of less, especially 5 vol% or less or 1 vol% or less.

[0076] In effect a reduced pressure sweep gas comprising water vapour has multiple benefits. It can: provide continuous removal of the permeates, drive the separation of a gas from a mixture via a transmembrane pressure differential to help permeation, provide a more efficient flow pattern and better performance for the separation of the gas allow for lower operational temperatures to be used (thus saving energy and reducing the carbon footprint, whilst also minimising degradation of the gas separation membrane); prevent dehydration of gas separation membranes (i.e. moisture-containing membranes such as facilitated transport membranes for CO2 separation) thus leading to improved performance.

[0077] The use of a steam sweep is particularly advantageous when used at a pressure of 0.95 or lower. Since the steam sweep is saturated with water vapour, concerns with dehydrating the selective layer can be further eliminated which allows even lower pressures to be used in comparison to a sweep gas containing lower levels of water vapour. As the transmembrane pressure difference correlates with the driving force for gas separation, decreasing the pressure of the sweep gas (i.e. increasing the vacuum suction) leads to an increase in CO2 flux and purity. Without being bound by theory, for membranes which function with a hydrated polymeric selective layer, when a steam sweep is used it is believed that the polymer matrix is sufficiently hydrated and relaxed and the permeability of the gas across the membrane is improved.

[0078] Typically, when the sweep gas consists of steam, the pressure of the sweep gas must be low enough at a particular operating temperature to prevent partial condensation across the membrane, as the presence of liquid water may negatively impact the performance of the membrane. It is desired that the sweep gas provides continuous removal of permeates from the membrane, and thus the sweep gas should remain in the gaseous phase.

[0079] The temperature of the sweep gas may be in the range of 20°C to 200°C, such as 25-150°C, 30-100°C or 60-80°C. The temperature of the sweep gas may be the same as the temperature of the feed gas. Alternatively, the sweep gas may be at a temperature which is different to the temperature of the feed gas. In certain cases, a higher sweep gas temperature may be used which could enhance the stripping of CO2. In certain cases, the sweep gas consists of steam and has a temperature in the range of 100°C or less, preferably 95°C, such as 50-90°C or 60- 80°C. Alternatively, the sweep gas may be a moisture-containing sweep gas and has a temperature of 200°C or less, such as 150°C, preferably 50-150°C or 75- 125°C. The temperature of the sweep gas refers to the temperature of the sweep gas at the given pressure (e.g. of 0.95 or less). Generally, temperatures greater than 20°C are preferred for the sweep gas, particularly for membranes where the selective layer comprises a mobile carrier.

[0080] In certain cases, the sweep gas is heated to achieve the desired temperature. Typically, the formation and / or heating of the sweep gas is conducted in a separate vessel to the membrane module, and the sweep gas at the desired temperature is then contacted with the permeate side of the membrane. The process may therefore comprise a step of generating water vapour to be supplied to, or as, the sweep gas. The sweep gas (e.g. steam) may be generated in a vacuum rated pressure vessel with a heating coil, such as a vacuum-steam kettle.

[0081] Alternatively, the sweep gas (e.g. steam) may be derived from industrial sources, such as from recycling the by-product of an industrial process, optionally using pressure reduction equipment. For example, the sweep gas may be derived from a combustion process, e.g. the by-product from a combustion process. The sweep gas may be process steam, e.g. coming from an upstream process.

[0082] In certain cases, the heat energy used to generate the sweep gas (e.g. to generate steam) may be in the form of excess heat energy from a separate process (e.g. a separate industrial process). For example, in the case of the application of this process in a sea-faring vessel, excess heat generated from a process on the sea-faring vessel (e.g. from the combustion engine) may be used to heat the sweep gas to the desired temperature. The heat source therefore may be excess heat from a combustion engine. Such a process has the benefits of reducing, or even eliminating energy (e.g. electrical energy) consumption for the heating of the sweep gas.

[0083] It is particularly preferred if the sweep gas (e.g. steam sweep) can be prepared by heating under reduced pressure. Typically, the sweep gas (e.g. steam sweep) is generated at a pressure of 0.1 to 0.95 bar, such as 0.2 to 0.8 bar or 0.3 to 0.6 bar. The reduced pressure used to generate the sweep gas may be provided by a vacuum pump, engine suction or ejectors for example. The source of the vacuum may be the same as the vacuum which is applied downstream of the permeate side of the membrane. Since the sweep gas (e.g. steam sweep) may be generated at reduced pressures, lower heating temperatures may be employed. Using lower temperatures for the heating of the sweep gas brings the additional advantages of improving energy efficiency and thus reducing environmental impact and saving on costs. The process for the separation of a gas from a gas mixture as defined in any aspect herein may further include the step of separating a permeate comprising the gas (e.g. CO2) from the sweep gas. The permeate, as referred to herein, refers to the gas / gases which have permeated from the feed side to the permeate side of the membrane. The method of separating a permeate from the sweep gas may include any of the following: condensation of sweep gas; cryogenic or fractional distillation; adsorption; absorption; or diffusion. The method of separating the permeate from the sweep gas typically depends on the physical and chemical properties of the gases, the desired purity of the separated gases and other factors such as costs and efficiency. In certain cases where the sweep gas consists of steam, e.g. is a steam sweep, condensation may be used to separate the permeate from the sweep gas. The separation of the permeate from the sweep gas (e.g. through condensation) is typically conducted in the absence of the membrane. For example, separation of the permeate from the sweep gas typically takes place downstream of the permeate.

[0084] Membrane

[0085] The membrane as referred to across any aspect of the present invention is not particularly limited, provided it has a feed side and a permeate side. The terms “membrane” and “membrane module” refer to the membrane as described herein which partitions a feed side from a permeate side. The membrane is a gas separation membrane, preferably a CO2 separation membrane. Typically, the membrane is a composite membrane which comprises a selective layer coated on a support layer. The selective layer itself may also be referred to as the membrane or the membrane may be an unsupported membrane.

[0086] The membrane is typically a moisture-containing membrane. During the process, it is typically beneficial for the moisture to remain in the membrane, i.e. as opposed to a membrane acting as a water separation membrane (with a membrane which may thus be moist, but with a flux of water across the membrane).

[0087] Typically, the selective layer or membrane comprises a polymer / polymeric matrix. The membrane may be selected from a solution-diffusion membrane or a facilitated transport membrane (FTM). Preferably, the membrane is a facilitated transport membrane, such as a hybrid facilitated transport membrane (HFTM). Facilitated transport membranes differ from solution-diffusion membranes, as facilitated transport membranes transfer the gas through an additional reactive pathway than convention membranes which follow a solution-diffusion mechanism.

[0088] The term “hybrid” when referring to transport membranes (e.g. facilitated transport membranes) means that the membrane comprises a polymeric selective layer with a compound dispersed therein. The compound is typically a nanofiller, such as graphene oxide.

[0089] The membrane of the present invention has a feed side and a permeate side. The feed side and permeate side are on opposite sides of the membrane. The feed side of the membrane is the side of the membrane which is contacted with the gas mixture to be separated. The permeate side of the membrane is the side of the membrane which is contacted with the sweep gas. The permeate side of the membrane may alternatively be referred to as the sweep side of the membrane. The “permeate” as referred to herein refers to the portion of the gas mixture which is withdrawn on the permeate side of the membrane, exclusive of other fluids, such as the sweep gas, which may be present on the permeate side of the membrane. The permeate typically comprises CO2.

[0090] The membrane as referred to herein may be a composite membrane suitable for separating a gas from a gas mixture as described herein, said composite membrane comprising a selective layer coated on a support, wherein said selective layer comprises: a) a polymeric matrix comprising an amine polymer; b) a graphene oxide nanofiller; c) a mobile carrier selected from an ionic liquid or an amino acid salt.

[0091] Alternatively, the membrane as referred to herein may be a composite membrane suitable for separating a gas from a gas mixture as described herein, said composite membrane comprising a selective layer coated on a support, wherein said selective layer comprises: a) a polymeric matrix comprising an amine polymer; b) a porous graphene oxide or PEG-modified graphene oxide nanofiller, and optionally c) a mobile carrier selected from an ionic liquid or an amino acid salt.

[0092] Alternatively, the membrane as referred to herein may be a composite membrane suitable for separating a gas from a gas mixture comprising a selective layer coated on a support, e.g. a hollow fiber or flat sheet support, wherein said selective layer comprises: a) a polymeric matrix comprising an amine polymer; b) a porous graphene oxide nanofiller or chemically-modified graphene oxide nanofiller, optionally wherein the chemically-modified graphene oxide nanofiller is graphene oxide with an organic unit grafted thereon, preferably wherein said organic unit is selected from a nitrogen and / or oxygen-containing organic unit, a polymer, or a nitrogen- and / or oxygen-containing polymer, preferably said chemically-modified graphene oxide nanofiller is PEG-modified graphene oxide nanofiller, and optionally c) a mobile carrier selected from an ionic liquid or an amino acid salt.

[0093] Support

[0094] Gas separating membranes can typically take two forms, supported or unsupported. Preferably the membranes referred to herein are carried on a support. As noted below, the support can be in the form of a flat sheet, tubular, capillary or a hollow fiber support.

[0095] Suitable supports are known in the art and most are ones which are porous to the gas being transported. Typically, the support is porous, therefore. Suitable supports include polyethersulfone (PES), polytetrafluoroethylene (PTFE), polypropylene, sulphonated polysulfone, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN) and related block copolymers, cellulosics such as cellulose acetate (CA), polyimide, polyether imide (PEI), aliphatic polyamides, polyetheretherketone (PEEK), polyphenylene oxide (PPG) and polysulfone (PSf). Such supports are available commercially from suppliers such as Osmonics. In a preferred embodiment the support is PVDF, especially where the support is in the form of a flat sheet. Where the support is a hollow fiber both PSf and especially PPO are preferred, in particular PPO.

[0096] Most of these supports are typically ultrafiltration supports where the size of the pores in the support is of the order of 20 to 1000 Angstroms although it is more common to express pore sizes in terms of molecular weight cut off values.

[0097] In some embodiments of the invention, it is also within the scope of the invention to employ microporous support structures. Such supports have much bigger pores sizes, e.g. 0.10 to 10 pm making gas transport there through very rapid. It is not normal to express pore sizes of these supports in MWCO terms but in this invention, microporous supports are considered to have MWCO values of greater than 100,000.

[0098] Microporous supports can be formed from any suitable material including those mentioned above in connection with ultrafiltration supports and inorganic materials such as ceramics (alumina, zirconium oxide), glass membranes such as silica and so on. These can be prepared by sintering, sol gel or leaching techniques known in the art.

[0099] Conventionally, it has been assumed that the use of these microporous supports in gas separation membranes using selective layer polymers was not possible as the pores of the support are so large that the polymer would simply collapse into the pores. This can be overcome by utilising high molecular weight polymers in the selective layer, which have been found not only to possess excellent permeance and selectivity but also excellent mechanical strength. Alternatively, a pore filler such as 3M™ Fluorinert™ Electronic Liquid FC-72 (perfluorohexane) can be used to fill the pores to avoid pore penetration of the casting solution. This is a low viscosity, low VOC fluorinated compound. The use of a fluorinated hydrocarbon is preferred as a pore filler. The mechanical strength of high Mw polymers allows them to be used without the problem of filling in even when the pores in the support material are micron sized.

[0100] The molecular weight cut off (MWCO) of the support is preferably kept as high as possible. MWCO is essentially a measure of the pore size in a support with larger MWCO values representing higher pore sizes. The MWCO in this invention is preferably more than 20,000, e.g. at least 35,000, preferably more than 50,000, more preferably at least 60,000, especially at least 75,000. In a highly preferred embodiment the MWCO is at least 100,000. In fact, the invention enables the use of supports having MWCO of up to 300,000, e.g. 30,000 to 300,000. In one embodiment, the MWCO may be less than the molecular weight Mw of the polymer from the selective layer cast on top.

[0101] It has been found that when a composite membrane of the invention was prepared using a selective layer of high molecular weight, the problem of "filling in" is minimised even if using a high molecular weight support. This then allows the use of a high MWCO support and can therefore lead to an improvement in permeance and selectivity.

[0102] Without wishing to be limited by theory, when using porous supports with larger pores, whether ultrafiltration or microfiltration supports, the increased pore size not only decreases the mass transfer resistance towards a gas to be separated but also changes the support separation mechanism itself. An ultrafiltration support with low pore size (low MWCO) may present selectivity towards nitrogen, for example, via Knudsen diffusion and not towards carbon dioxide.

[0103] As noted in more detail below, using a high Mw selective layer polymer allows the use of porous support with larger pores and consequently low mass transfer resistance towards the gas molecules separated by the selective layer without affecting mechanical stability.

[0104] In a preferred embodiment the support used for any aspect defined herein can have a porous lower layer with a thin dense top layer. By dense is meant that there are no pores in the dense top layer.

[0105] The dense top layer is preferably no more than 60 nm in thickness, e.g. around 40 nm or less in thickness. It is however within the scope of the invention for the dense layer to have a greater thickness e.g. 100 to 1000 nm, such as 200 to 700 nm, e.g. 600 nm.

[0106] Supports with dense top layers are preferably hollow fiber supports and ideally can be formed from PPO. The dense top layer is formed during the spinning process. In this case, a pore filler is not required.

[0107] Polymeric matrix

[0108] Preferably, the membranes described herein comprise a selective layer which comprises a hydrophilic polymer. In some cases, the selective layer may comprise at least one of an amine polymer, a polyether block amide resin, a polyamide resin, a polyimide resin, a cellulose acetate resin, a silicone resin, a fluorine resin and / or a polyimidazole-derived polymer or imidazole-derived polymer.

[0109] Preferably, the membranes described herein comprise a selective layer which comprises an amine polymer. The polymeric matrix can comprise one or more polymers. By ‘polymeric matrix’ is meant the polymer component a) in the selective layer of the composite membrane as referred to herein. The polymeric matrix comprises an amine-based polymer, e.g. a polymer with a hydrocarbon backbone with pendant amine groups or a polyamine (i.e. with amine groups in the backbone). Preferably, the polymeric matrix comprises a polymer with a hydrocarbon backbone with pendant amine groups. Without being bound by theory, polymeric matrices based on amine-based polymers are considered to be better than imidazole-based polymers, since the reactivity with CO2 is better as cyclic n-containing groups are generally less reactive.

[0110] More preferably, this is a polyallylamine-based or polyvinylamine-based polymer. Polyallylamine-based or polyvinylamine-based polymers cover polyallyamines (‘PAA’s) or polyvinylamines (‘PVAm’s) which have been modified, e.g. structurally modified at the amino group. Polyallylamine-based polymers are particular preferred.

[0111] In a particular embodiment, the polymeric matrix comprises a sterically hindered polymer of the following formula (I): wherein R1 and R2 are independently selected from hydrogen or a C1-C10 hydrocabyl group, preferably Ci-Ce hydrocarbyl group, preferably Ci-Ce alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl, more preferably isopropyl or sec-butyl. Preferably one of R1 and R2 is hydrogen and the other is a C1-C10 hydrocabyl group, preferably Ci-Ce hydrocarbyl group, preferably Ci-Ce alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl, more preferably isopropyl or sec-butyl.

[0112] The integer m is typically 0-6, preferably 0-2, more preferably 0-1 , most preferably 1.

[0113] The integer n is used to denote the polymeric nature of the structure, and the value of n is typically such that the polymeric matrix has a Mw as defined below or in the claims. This definition of n is valid for the definitions of the structures below.

[0114] Steric hindrance of amine-based polymers in solid phase has been demonstrated to increase gas permeation performance of facilitated transport membranes.

[0115] In a particular embodiment, the polymeric matrix comprises a sterically hindered polyallylamine (‘SHPAA’) of formula (II): wherein R is a C1-C10 hydrocabyl group, preferably Ci-Ce hydrocarbyl group, preferably Ci-Ce alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl, more preferably R is isopropyl or sec-butyl. In a particular embodiment, therefore, SHPAA is selected from:

[0116] SHPAA(A) SHPAA(B)

[0117] Typically, SHPAA(A) is used with a flat sheet support and SHPAA(B) is used with a hollow fiber support. In a particular embodiment, SHPAA(B) is preferred.

[0118] Similarly, the selective layer polymer can be a sterically hindered polyvinylamine polymer (SHPVAm) of formula (III):

[0119] SHPVAm (H wherein R is defined as above for sterically hindered polyallylamine (SHPAA).

[0120] The polymeric matrix can comprise a single polymer or a combination of two or more polymers. In a particular embodiment, the polymeric matrix consists of at least one polymer. Typically, the polymer matrix comprises at least 60 wt%, preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 85 wt% of the amine based-polymer as defined herein or in the claims. Typically, the polymer matrix comprises 60-99 wt%, preferably 70-98 wt%, 80-95 wt% of the amine-based polymer. In some embodiments, the amine polymer is the only polymer in the polymer matrix.

[0121] Typically, the polymeric matrix makes up at least 50 wt% of the selective layer, e.g. at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, or at least 95 wt% of the selective layer. Typically, the polymeric matrix makes up 50-99 wt% of the selective layer, preferably 60-95 wt%, more preferably 70-95 wt% of the selective layer.

[0122] In a particular embodiment, the amine-based polymer (e.g. SHPAA) can be combined with another polymer, in a particular an oxygen-containing polymer, e.g. polyvinylalcohol (PVA). The oxygen containing polymer can comprise oxygen based functional groups in the backbone of the polymer or as pendant functional groups. The oxygen group is preferably hydroxyl. The oxygen group is preferably pendant. The use of PVA is preferred.

[0123] The combination of the amine-based polymer with a second polymer, in particular PVA, results in a reduction of the brittleness of the selective layer, and superior transport properties due to the water-swelling nature of the PVA.

[0124] Viewed from another aspect the invention provides a composite membrane suitable for separating a gas from a gas mixture comprising a selective layer coated on a support, wherein said selective layer comprises: a) a polymeric matrix comprising an amine polymer and PVA; b) a graphene oxide nanofiller; and c) a mobile carrier selected from an ionic liquid or an amino acid salt.

[0125] This advantage is most prominent when the second polymer, e.g. PVA, is present in an amount of 2-20 wt%, preferably 5-15 wt% based on the combined weights of the amine-based polymer and second polymer. This ensures low brittleness without compromising the separation properties of the selective layer.

[0126] The weight average molecular weight (Mw) of the polymeric matrix used in this invention can range from 10,000 to 3,000,000, preferably 20,000 to 750,000 more preferably 30,000 to 500,000, more preferably 80,000 to 300,000 (molecular weights are herein given in g / mol unless indicated otherwise).

[0127] The weight average molecular weight (Mw) of the selective layer polymer is typically at least 50,000. Preferably, the Mw of the selective layer polymer is at least 100,000. The weight average molecular weight (Mw) of the amine polymer is typically at least 50,000. Preferably, the Mw of the selective layer polymer is at least 100,000, such as 80,000 to 300,000.

[0128] The weight average molecular weight (Mw) of the oxygen polymer such as PVA is typically at least 50,000. Preferably, the Mw of the selective layer polymer is at least 100,000, such as 80,000 to 300,000.

[0129] It has been found that using higher Mw gives the selective layer strength. This allows the use of a support with a much higher MWCO. In a particular embodiment, the invention therefore provides a selective layer polymer having a Mw of at least 100,000 with a support having a MWCO of at least 60,000.

[0130] It has also been observed that even when using a higher molecular weight polymer, this does not result in a reduction in permeance or selectivity when used for any process for the separation of CO2 from a gas mixture as described herein. The use of a higher Mw polymer means that the actual selective layer used will tend to be denser than selective layers formed from lower Mw polymers. Surprisingly, the inventors have found that even at higher densities the permeance values of the composite membranes remain very high and the gas selectivity is good.

[0131] A further benefit of the use of a higher Mw polymer matrix concerns water uptake. The higher Mw polymer has more densely packed molecular chains meaning more densely packed amino groups. This leads to greater water uptake in comparison to lower Mw polymers which promotes reactivity of the amino groups to, inter alia, carbon dioxide.

[0132] The skilled person might also expect that increased water uptake would lead to selective layer swelling and hence lower permeance values as thicker selective layers are obviously harder for gases to cross. However, any swelling which does occur is limited and counter balanced by the increase in carbon dioxide transfer which the higher uptake of water facilitates. A high Mw polymer matrix therefore has good compatibility with a sweep gas comprising water vapour, such as steam.

[0133] The combination therefore of a higher molecular weight polymeric matrix with high MWCO supports provides composite membranes with excellent properties.

[0134] A further benefit of the use of higher Mw polymer matrix is their ability to withstand greater pressures. The membranes of the prior art are conventionally used at low gas pressures. Flue gases from industrial plants can however be at relatively high pressures, e.g. up to 15 bars and ideally any composite membrane would be able to carry out gas separation on such higher pressure gases. In particular, it is preferred that the permeance and selectivities obtained at higher gas pressures are not reduced (or not significantly reduced) relative to operation at lower pressures. It is a further feature of this invention that the composite membranes claimed are able to handle gases which are under pressure, e.g. at a pressure of up to 40 bars, e.g. up to 15 bars, such as 2 to 15 bars or 2 to 10 bars.

[0135] Also, despite the use of a higher Mw polymeric matrix causing an overall densification of selective layer relative to a lower polymeric matrix, no reduction of permeance or selectivity caused by the use of a higher Mw polymeric matrix is observed.

[0136] It is generally preferred if the selective layer is cross-linked. Therefore, the selective layer may comprise a cross-linked polymer matrix. Crosslinking the selective layer results in a more robust membrane which is highly suitable for industrial gas separation conditions. Cross-linking may be achieved through any known means in the art, such as thermal cross-linking (or thermal treatment) and / or chemical cross-linking. Cross-linking may occur in the presence or absence of a cross-linking agent. Thermal crosslinking may be preferred due to simplicity.

[0137] Thermal treatment of the composite membrane may provide advantageous properties, especially where the composite membrane will operate at elevated gas pressures, e.g. above 10 bars.

[0138] By thermal treatment (e.g. thermal cross-linking) is meant exposing the composite membrane (i.e. selective layer on the support) to heat to induce strength therein. Suitable thermal treatment (e.g. thermal cross-linking) conditions encompass heating to 50 to 150°C, e.g. 80 to 120°C, especially 90 to 110°C. This thermal cross-linking imparts additional strength to the composite membrane, perhaps by encouraging intermolecular interaction between polymer chains, e.g. by hydrogen bonding, and between the polymeric matrix and the porous support.

[0139] Chemical cross-linking of the composite membrane may provide advantageous properties, especially where the composite membrane will operate at elevated gas pressures, e.g. above 10 bars.

[0140] Example cross-linkers may be polar small molecules such as low molecular weight polymers such as PEI, PEG, glutaraldehyde, other small molecules that contain more than one amine or N-containing groups with ability to form hydrogen bonding. The effect of chemical cross-linking is considered to strengthen the composite membrane. It will be clear that thermal treatment of the selective layer takes place when this is supported. Without wishing to be limited by theory, it is believed that the thermal treatment step also modifies the support thus allowing improved permeance values. It may be that the interaction between the support and the dense layer of amine polymer is improved.

[0141] Graphene oxide nanofiller

[0142] Preferably, the membranes described herein comprise a selective layer which comprises a graphene oxide nanofiller, preferably a 2D graphene oxide filler. The term 2D implies that one of the dimensions of the filler is very small, e.g. 10 nm or less. The graphene oxide fillers are therefore in the form of flakes or can be considered planar with width and height but very low thickness. Graphene oxide (GO) is therefore a two-dimensional material and thus the terms nanofiller, nanoplatelets, or nanosheets, can be used interchangeably herein. The term graphene oxide also covers graphene oxide that has been modified, either physically or chemically.

[0143] The membrane of the invention is typically a ‘hybrid’ membrane. The graphene oxide nanofiller is therefore typically dispersed within the polymeric matrix comprising an amine polymer.

[0144] The terms ‘nanofiller’, ‘nanoplatelets’, or ‘nanosheets’ indicate that the average dimension of the graphene oxide is of 1000 nm or less, e.g. is in the range 10- 1000nm, preferably 100-1000 nm, more preferably 300-900 nm, more preferably 400-800 nm. These dimensions refer to the average lateral dimensions, i.e. in the 2D plane of the nanosheets. These dimensions can be measured using Atomic Force microscopy (AFM). For the AFM analysis, ultra dilute solutions (cone 0.0005- 0.001 wt%) of graphene oxide in water is prepared and a drop is added to a freshly cleaved mica sheet and left for the water to evaporate. Since the mica sheets are atomically flat, the dimensions of the 2d material are observed through varying thickness in the graphene oxide (the method automatically records the change in surface height from the boundary of a graphene oxide flake due to its thickness on the atomically flat surface).

[0145] Several images of the graphene oxide sheets are taken and image analysis technique (e.g. using imaged software) is used to identify the average lateral dimension. It has surprisingly been found that the size of the GO flakes influences the gas permeation performance when used in a process for any aspect described herein, and the best results have been observed for average dimensions in the range 400-800 nm. Average particle sizes of more than 1000 nm (i.e. ‘micro’ fillers) lead to a decrease in performance. The average thickness of the graphene oxide nanosheets is typically 10 nm or less, such as 2.0 nm or less, especially 1.0 nm or less. In a particular embodiment, the nanosheets are at least one layer of graphene oxide thick, e.g. at least two layers thick. In a particular embodiment, the graphene oxide nanofiller is in the form of graphene oxide monolayers.

[0146] In a particular embodiment, the nanofiller is size-controlled, i.e. the particles are uniform or substantially uniform in their size distribution. The size distribution profile is typically unimodal. Typically, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles are within ±50%, preferably within ±25%, more preferably within ±10% of the average dimension. Alternatively, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles can have (average) lateral dimensions of less than 1000 nm. Alternatively, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles can have (average) lateral dimensions in the range 10- 1000 nm. Alternatively, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles can have (average) lateral dimensions in the range 100-1000 nm. Alternatively, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles can have (average) lateral dimensions in the range 300- 900 nm. Alternatively, at least 75%, preferably at least 90%, more preferably at least 95%, more preferably at least 99% of the nanofiller particles can have (average) lateral dimensions in the range 400-800 nm.

[0147] Different suppliers of GO provide dispersions of different flake size. The size of the flakes is typically optimized to get the best performance. The present inventors used GO from commercial supplier Graphene-XT for certain hollow fiber composite membranes and prepared their own nanofillers for flat sheet composite membranes. The nanofiller from either source can be exfoliated by means of sonication to obtain monolayers of GO. Different sonication times can be used to control the flake size and effect the gas permeation performance. The use of sonication to control flake size is potentially valuable. Longer sonication leads to smaller flakes. Sonication for 4 to 8 hrs appears to lead to ideal GO flake sizes.

[0148] Graphene oxide can furthermore be engineered to improve the gas permeation in a composite membrane. Both physically and chemically modified GO nanoplatelets with an array of properties can be synthesized and successfully dispersed in polymer matrices. These physically and chemically modified GO nanoplatelets can also be subject to sonication to control flake size a priori.

[0149] In a particular embodiment, the graphene oxide nanofiller has been physically modified, e.g. using hydrogen peroxide, to render it porous. Such modification may occur at high temperature, e.g. 100 to 250°C. Thus, in a particular embodiment, the graphene oxide is porous. Typically, the average size of the pores is 1-200 nm, such as 2 to 20 nm.

[0150] The physical modification to render the GO porous is expected to create defects in the plane perpendicular to the direction of gas transport while preserving the 2D morphology of the original platelet. More generally, increased pore content and defects across the thickness generated during the treatment process trigger nanoscopic changes in polymer packing resulting in enhanced permeation.

[0151] GO nanofillers of the invention can be subject to both sonication to control particle size and physical modification to impart pores in the nanofiller.

[0152] In another embodiment, the graphene oxide is chemically modified such that it has an organic unit grafted thereon (preferably an oxygen- and / or nitrogencontaining organic unit), preferably a polymer, preferably a nitrogen- and / or oxygencontaining organic polymer, more preferably an oxygen-containing organic polymer, e.g. a polyether, e.g. a polyalkylene polyol such as polyethylene glycol (PEG) grafted thereon. In a particular embodiment, therefore, the graphene oxide is modified with polyalkylene glycol groups such as PPG (polypropylene glycol) and / or PEG (polyethylene glycol) groups. The nitrogen-containing polymer could be, for example, a polyamine polymer, e.g. polyethylenimine (PEI). The polyether could comprise, for example, a polyether backbone (e.g. based on a polyglycerol such as hexaglycerol), with pendant PEG groups. Where the term polyalkylene polyol is used, a mixture of polyalkylene polyols (e.g. PPG and PEG) could be used. An example of an oxygen- and nitrogen-containing polymer is a PEG / PEI hybrid.

[0153] The oxygen and / or nitrogen introduce polarity to the grafted polymer which can be beneficial for membrane performance. In a particular embodiment, the oxygen is in the repeating unit (e.g. ether) and / or terminal (e.g. -OH), typically in the repeating unit. In a particular embodiment, the nitrogen is in the repeating unit (e.g. amine) and / or terminal (e.g. terminal amine groups such as -NH2), typically terminal.

[0154] In a particular embodiment, the graphene oxide nanofiller is a polymer- modified graphene oxide nanofiller. Preferably, the polymer that is grafted onto the graphene oxide nanofiller is a polymer selected from or comprising a polyamine and / or a polyether, preferably a polymer selected from or comprising a polyethylenimine and / or a polyalkylene glycol, preferably a polymer selected from or comprising PEI, PPG and / or PEG, preferably a polymer selected from or comprising PPG and / or PEG, preferably a polymer which comprises PEG.

[0155] The organic units or polymers that are grafted on the graphene oxide nanofiller can be branched or linear. In the case of branched nitrogen-containing polymers, e.g. branched polyethylenimine, the polymer can have primary, secondary or tertiary amine groups, as is well known in the art.

[0156] Other groups (e.g. terminal groups such as terminal -NH2 groups) may be present in the polymer unit grafted to the graphene oxide nanofiller. For example, if a PEG-containing polymer is grafted to the graphene oxide, it is within the scope of the invention for this PEG unit to contain other groups, e.g. terminal -NH2 groups, or a hexaglycerol core, as in the following 8-arm PEG:

[0157] (n is such that the Mn is as defined in the ranges below).

[0158] The organic groups (e.g. PEG) can be grafted onto the graphene oxide via any typical coupling reaction, e.g. EDC coupling reaction. There can therefore be linker groups between the graphene oxide and the organic moiety. Typically, the molecular weight of the grafted groups (e.g. the nitrogen and / or oxygen- containing polymer) is in the range 1,000-500,000, preferably 1,000-100,000, preferably 2,000 to 50,000, preferably 5,000 to 20,000 g / mol. The molecular weight for the grafted group is typically given as the number average molecular weight, Mn. A suitable example is commercially-available 8-arm PEG, with an Mnof 10,000.

[0159] GO nanofillers of the invention can be subject to both sonication to control particle size and chemical modification. In a particular embodiment, the graphene oxide nanofiller is either porous (i.e. physically modified) or has PEG groups grafted thereon (i.e. chemically modified).

[0160] The amount of graphene oxide nanofiller in the selective layer is typically 5 wt% or less, preferably 1 wt% or less, preferably 0.5 wt% or less. Other suitable ranges include less than 5 wt%, preferably less than 1 wt%, more preferably less than 0.5 wt%. Suitable ranges include 0.05 wt% to 5.0 wt%, preferably 0.1 to 1.0 wt%, preferably 0.1 to 0.5 wt% or preferably 0.1 to 0.3 wt%. These wt% are determined on a dry weight basis.

[0161] The inventors surprisingly found that at very low loadings (e.g. down to 0.2 wt% nanofiller), graphene oxide could efficiently increase the CO2 permeance by about 200% compared to that of the non-modified selective layer with no significant changes in selectivity, when used in a process for the separation of CO2 from a gas mixture as described for any aspect herein. At low loadings, graphene oxide (whether graphene oxide, porous graphene oxide or chemically-modified graphene oxide) effectively disrupts polymer chain packing while simultaneously increasing CO2 sorption and reorienting water distribution in the matrix.

[0162] In a particular embodiment, the nanofillers are aligned or substantially aligned in the selective layer, i.e. the planes formed by the 2D shape of the nanofiller particles are parallel or substantially parallel, and preferably parallel or substantially parallel to the plane formed by the selective layer. In other words, the nanofiller particles are typically aligned or parallel along their larger two dimensions. Alternatively put, the nanofillers can be coplanar or substantially coplanar, e.g. coplanar or substantially coplanar with the plane formed by the selective layer. ‘Substantially’ aligned, parallel or coplanar herein typically means that 75% or more, preferably 90% or more, e.g. 95% or more of the nanofiller particles deviate from the plane formed by the selective layer by ±45° or less, preferably ±25° or less, more preferably ±10° or less. In-plane alignment of GO is attributed to the lowering of surface free energies of GO-based fillers, and leads to enhanced gas permeation. Moreover, it can often be a necessity to have the nanofillers in alignment for selective layers that are in the range of a few hundred nanometers, since the lateral dimensions can exceed the thickness of the selective layer otherwise.

[0163] In the case of porous graphene oxide, the nanofiller particles typically have evenly distributed pores. Typically, the pores in each nanofiller particle are distributed evenly throughout each layer of graphene oxide. In a particular embodiment, therefore, each layer of graphene oxide in the nanofiller particles is porous, preferably each layer of graphene oxide has a similar number of pores (e.g. within ± 50%, e.g within ± 25% or within ± 10% of the average number of pores per layer of the particle). In a particular embodiment, the number of pores per 100 nm2deviates by at most ± 50%, e.g at most ± 25%, e.g. at most ± 10%. The even distribution of pores can be achieved by generating the porous nanoparticles in a dispersion state (e.g. hydroxide treatment on already cleaved monolayers in GO dispersion). The creation of pores in the dispersion state ensures that pores are created in all monolayers of graphene oxide nanofiller. If the porous graphene oxide is generated in the solid state (e.g. thermal annealing of aggregated GO powder), then the porosity is not typically uniform. Typically, therefore, the porosity in the graphene oxide is generated in a dispersion. In a particular embodiment, the pores in the graphene oxide are not prepared by solid-state treatment of the graphene oxide (e.g. thermal annealing of solid GO).

[0164] Mobile carriers

[0165] For membranes with a selective layer, in order to increase the number of reactive sites for the CO2 to interact in the selective layer, low molecular weight CO2-philic components can be added. These are referred to as mobile carriers herein as they diffuse through the polymer matrix and enhance permeation. The use of mobile carriers in the selective layer increases the performance and helps especially with CO2 / CH4 separations to resist the carrier saturation phenomena. The mobile carriers are typically dispersed in the polymeric matrix comprising an amine polymer, when present.

[0166] A surprising aspect of the present invention is that the composite membranes comprising the mobile carriers have increased CO2 permeance and increased CO2 flux compared to the same modules without the mobile carriers, at industrially relevant conditions, when used in a process for the separation of gas from a gas mixture as described herein.

[0167] In use, the amine groups of the polymeric matrix reversibly react with CO2 in the presence of water to transport CO2 across the composite membrane. Although polymers like polyvinylamine and polyallylamine contain high density of amine groups relative to the hydrocarbon content in the repeating unit of the polymer, their effect to contribute to increased CO2 transport relies on the access to the amine groups for CO2 and the proximity to form continuous channels for CO2 reaction and transfer. The amine groups in the polymer matrix are however locked in the polymer, restricting their mobility.

[0168] To increase diffusivity, mobile carriers which are also CC>2-philic are added in the matrix. The addition of these mobile carriers not only enhances the density of CC>2-philic moieties in the polymer matrix but also increases the mobility of CO2- reacted species, thus increasing CO2 diffusivity in the host matrix. Important characteristics of such mobile carriers include (1) low molecular weight (higher mobility), (2) high CO2 uptake capacity, and (3) capability of formation of weak bond with CO2 that enhances the transport of CO2 through water-swollen composite membrane matrices (reversible CO2 association / dissociation) and facilitate its release at the permeate side.

[0169] The mobile carrier is an ionic liquid or an amino acid salt.

[0170] An ionic liquid is salt which is a liquid at 25 °C and atmospheric pressure. Room-temperature ionic liquids comprise bulky and asymmetric organic cations often based on heterocycles such as 1-alkyl-3-methylimidazolium, 1- alkylpyridinium, fluorosulfonyl-trifluoromethanesulfonylimide (FTFSI) N-methyl-N- alkylpyrrolidinium and ammonium ions. Phosphonium cations are also possible. A wider range of anions are employed, ranging from simple halides to inorganic anions such as tetrafluoroborate and hexafluorophosphate, and to small or large organic anions like bistriflimide, acetate, cyanamide, triflate or tosylate.

[0171] Suitable ionic liquids include [Emin][OAc], [Emim][CI], [Emim][dicyanamide], and 1-butyl-3,5-dimethylpyridinium bromide. In a particular embodiment, the ionic liquid comprises the cation 1-Ethyl-3-methylimidazolium ([Emim]) or 1 -butyl-3- methylimidazolium. 1-Ethyl-3-methylimidazolium acetate ([Emim][0Ac]) is particularly preferred. Typically, the ionic liquid has a melting point in the range 25 to 100 °C.

[0172] Amino acid salts are salts of compounds comprising a COOH group and a amino group which may be primary, secondary or tertiary. The salt preferably forms with the acid part, i.e. the salt is a cation and the amino acid forms the anion.

[0173] Suitable amino acid salts are salts of any naturally occurring amino acid, e.g. any essential amino acid, preferably Gly, Arg, Cys or Pro. Preferred amino acid salts are salts of proline (e.g. i-proline), such as a potassium salt of proline (i.e. ‘ProK’). The cation in the salt is ideally an alkali metal. [Emim][OAc], a room-temperature ionic liquid, reacts with CO2 via carbene route and forms a carbene-CCh adduct. One of the main advantages of using [Emim][OAc] as mobile carrier is that its carbene-routed interaction with CO2 does not influence the viscosity of the solution, which may be beneficial in reducing the mass transfer resistance in the selective layer upon the sorption of CO2.

[0174] The carbene-CO2 adduct (as shown in Scheme A below) has also been reported to have faster diffusion. ProK (potassium L-prolinate), a secondary amino acid, reacts with CO2 to form carbamate and bicarbonate / carbonate species represented in Scheme B above).

[0175] PZEA-Sarc is an amino acid salt containing one primary amine, two secondary amines (one from sarcosine) and one tertiary amine. CO2 interacts with this mobile carrier to form primary and secondary mono-carbamates (Scheme C above).

[0176] The mobile carrier is typically present in the selective layer in an amount of 1.0-40 wt%, preferably 2.0-30 wt%, more preferably 5.0-25 wt% (dry weight). Particularly suitable ranges for ionic liquids include 2.0-40 wt%, preferably 5.0-15 wt%, and particularly suitable ranges for amino acid salts include 5.0-40 wt%, preferably 15-25 wt%.

[0177] Viewed from another aspect the invention provides a composite membrane suitable for separating a gas from a gas mixture comprising a selective layer coated on a support, wherein said selective layer comprises: a) at least 50 wt% of a polymeric matrix comprising an amine polymer; b) 0.05 to 5.0 wt% of a graphene oxide nanofiller; and c) 1.0 to 40 wt% of a mobile carrier selected from an ionic liquid or an amino acid salt.

[0178] In one embodiment, the invention relates to a composite membrane suitable for separating a gas from a gas mixture comprising a selective layer coated on a hollow fiber or flat sheet support, wherein said selective layer comprises: a) a polymeric matrix comprising an amine polymer and preferably PVA; b) a porous graphene oxide or PEG-modified nanofiller, and optionally c) a mobile carrier selected from an ionic liquid or an amino acid salt.

[0179] All preferred embodiments discussed above also apply to this embodiment.

[0180] Other selective layer components

[0181] For membranes which comprise a selective layer, it is preferred if the selective layer consists essentially of the polymeric matrix, the graphene oxide nanofiller, and the mobile carrier. Typically, therefore, these materials are the only materials used in the selective layer other than minor levels of any necessary additives such as stabilisers, anti-oxidants or residuals solvents etc. The combination of the polymeric matrix, the graphene oxide nanofiller, and the mobile carrier preferably form at least 95 wt% of the selective layer, such as at least 98 wt%, especially at least 99 wt% of the selective layer. It is preferred if the selective layers of the invention consist of the polymeric matrix, the graphene oxide nanofiller, and the mobile carrier.

[0182] The membrane components, such as the polymeric matrix, the graphene oxide nanofiller, the mobile carriers and other selective layer components as described above apply to all aspects of the disclosed invention.

[0183] Composite membrane The thickness of the membrane or selective layer will vary depending on the concentration of the solute in the casting solution with higher concentration solutions giving thicker membranes. Thickness can also be adjusted however using a casting knife or is reduced using the bar coating method. In a typical embodiment, therefore, the membrane is prepared by casting. When a polymeric matrix is present, the polymeric matrix component is typically dissolved in the casting solution prior to evaporation. The polymeric matrix comprising an amine polymer is typically not formed by interfacial polymerisation, for example.

[0184] It will be evident that the polymeric matrix comprising an amine polymer, the graphene oxide nanofiller, and the mobile carrier all typically form a single layer.

[0185] The thickness of the membrane or selective layer of the invention may be of less than 100 pm, preferably less than 10 pm, more preferably less than 1 pm, more preferably less than 500 nm. Typically, the thickness of the membrane or selective layer is in the range 20 nm to 100 pm, preferably 50 nm to 10 pm, preferably 100 nm to 5 pm, more preferably 100 nm to 1 pm, more preferably 100 nm to 500 nm. Thin membranes or selective layers tend to have higher permeance values but are also less strong. Membranes or selective layers that are less than 200 nm in thickness are especially preferred.

[0186] It will also be appreciated that any membrane or selective layer is ideally defect free.

[0187] The thickness of the support on which the selective layer can be carried can vary although this may be of the order of 50 to 500 pm, e.g. around 100 pm. It will be appreciated, however, that this invention covers the use of a flat support, a hollow fiber support, a tubular support or a capillary support. When the support is a hollow fiber support the thickness of the support is regarded as the wall thickness of hollow fiber. The support should be porous.

[0188] It is generally preferred if the membrane of the present invention is a hollow fiber membrane. The hollow fiber membrane preferably comprises a selective layer as defined herein. In some embodiments, the membrane is a hollow fiber membrane and the feed side of the membrane is the shell side of the hollow fiber membrane and the permeate side of the membrane is the bore side of the hollow fiber. Alternatively, for a membrane which is a hollow fiber membrane, the feed side of the membrane may be the bore side of the membrane and the permeate side of the membrane is may be the shell side of the membrane. A configuration is shown in Figures 9 and 10. A plurality of hollow fiber membranes are typically assembled in a module.

[0189] After formation of the selective layer on the support, the solvent is removed, e.g. by evaporation. This can be achieved using gentle heat if necessary, e.g. about 60°C.

[0190] To avoid any possible loss of selective layer forming material into the support, it is normal if there is a reasonable difference between the average molecular weight of the selective layer polymer(s) and the molecular weight cut-off of the support structure. Such a difference may be larger than about 10,000, such as larger than about 15,000, for example larger than about 20,000, especially more than 50,000. Alternatively, a pore filling material can be used prior to casting with the casting solution containing the selective layer components.

[0191] The formed selective layer can then be cross-linked if desired. Cross-linking could be effected chemically using cross-linking agents such as glutaraldehyde or ammonium fluoride.

[0192] It is also at this stage of the manufacturing process that thermal treatment of the composite membrane can be effected. The selective layer can be crosslinked thermally.

[0193] The resulting membrane acts as a fixed site carrier (FSC) for gas, e.g. carbon dioxide, transport due to the high concentration of amino groups.

[0194] The composite membranes of the invention can be fashioned into modules for use in gas separation systems.

[0195] Apparatus

[0196] Viewed from an alternative aspect, the present invention refers to a gas separation apparatus for the separation of a gas from a gas mixture, comprising:

[0197] - at least one membrane with a feed side and a permeate side; and

[0198] - a reduced pressure-generating apparatus, configured to generate a reduced pressure on the permeate side of the membrane

[0199] - a sweep gas supply apparatus, configured to supply sweep gas to the permeate side of the membrane.

[0200] The sweep gas supply apparatus may be seen as the sweep gas module. The terms “gas’, “gas mixture”, “membrane”, “sweep gas”, “feed side” and “permeate side” used in relation to the gas separation apparatus are the same as described for any aspect of the present invention.

[0201] Typically, a reduced pressure generating apparatus (e.g. a vacuum pump) is applied downstream of the permeate side of the membrane. The reduced pressure generating apparatus may be a vacuum-steam kettle which comprises a heating coil. A vacuum-steam kettle is configured to generate steam through boiling water, but the temperature required for boiling is lower when conducted under reduced pressure, thus saving energy. The apparatus will typically comprise: a boiler configured to supply water vapour as, or as part of, the sweep gas to be supplied to the permeate side of the membrane, typically wherein the boiler is upstream from the membrane on the permeate side, and / or a source of humid gas or steam (e.g. humidified inert gas or process steam) to be supplied as the sweep gas to the permeate side of the membrane, typically wherein the source of humid gas or steam is upstream from the membrane on the permeate side.

[0202] The boiler and source of humid gas or steam may be part of the sweep gas module. The vacuum-steam kettle may be integrated as part of the sweep module. In the case of the vacuum-steam kettle, the vacuum source may come from downstream or upstream thereof.

[0203] Alternatively, a heat exchange unit may be used in combination with a reduced pressure generating source. The heat exchange unit may therefore be integrated as part of the sweep module. The heat exchange unit is configured to boil water to generate a water vapour-comprising sweep gas. The feed gas on the bore side can therefore be used as a source of heat to boil the water in the sweep module and generate a water vapour-containing sweep gas.

[0204] In a typical embodiment, a reduced pressure generating apparatus is used downstream and provides a reduced pressure for the whole of the permeate side of the membrane.

[0205] The gas separation apparatus of the present invention typically will have a feed gas inlet for the gas mixture (e.g. feed gas) on the feed side of the membrane. The gas separation apparatus will further typically comprise a sweep gas inlet to provide the sweep gas on the permeate side of the membrane. Both the feed gas and the sweep gas inlet may be configured to allow for the required flow rate for each of the gas streams. ‘Upstream’ and ‘downstream’ are herein used to define relative positions, and reflect the flows of the gases when in use, or refer to the position of items when referring to the apparatus per se.

[0206] The gas separation apparatus of the present invention typically will also comprise a feed gas outlet, downstream from the feed gas inlet. The feed gas outlet will allow the removal of the retentate gas after contact with the feed side of the membrane. The gas separation apparatus of the present invention will typically also comprise a sweep gas outlet, which allows the removal of the permeate.

[0207] The gas separation apparatus for the separation of a gas from a gas mixture as referred to herein may comprise:

[0208] - at least one membrane with a feed side and a permeate side;

[0209] - a feed gas inlet for providing the gas mixture to the feed side of the membrane;

[0210] - a feed gas outlet, downstream from the feed gas inlet, for providing removal of retentate gas after contact with the feed side of the membrane;

[0211] - a sweep gas inlet for providing a sweep gas to the permeate side of the membrane;

[0212] - a sweep gas outlet for removing the permeate gas; and

[0213] - a reduced pressure generating source, such as a pump, on the permeate side of the membrane, e.g. downstream from the sweep gas outlet.

[0214] The gas separation apparatus of the present invention may further comprise a condenser. The condenser is typically downstream from the reduced pressure generating source. The use of the condenser allows the separation of the gas / permeate from the sweep gas. When a steam sweep gas is used, the addition of the condenser is highly beneficial for the recovery of the gas.

[0215] The gas separation apparatus of the present invention may further comprise at least one storage unit. The at least one storage unit may be used to store the gas which has been separated from the gas mixture, for example to store CO2.

[0216] It is further preferred if the gas separation apparatus is provided with an insulating cover to maintain the required temperature. The gas separation apparatus may further be integrated with at least one heat recovery vessel, wherein the heat recovery vessel can store energy from a separate process which may be used to heat the sweep gas. The gas separation apparatus may additionally be provided with a calibrated gas chromatograph (e.g. 490 Micro GC, Agilient) which may be used to analyse the gas composition of the permeate and the separation performance of the membrane.

[0217] Since the avoidance of dehydration of the gas mixture is preferred, the gas separation apparatus is generally not suitable for the separation of water / water vapour from the gas mixture. Therefore, the gas which is separated from a gas mixture using a gas separation apparatus as defined herein does not include water vapour.

[0218] It is preferable that the gas separation apparatus described herein does not comprise a plurality of (i.e. two or more) membranes connected in series. The apparatus may comprise, however, a plurality of hollow fibers arranged in parallel, as shown in Figure 8 for example.

[0219] Application

[0220] The process comprises separating, i.e. removing, CO2 from the gas mixture comprising CO2.

[0221] Viewed from a further aspect, the present invention refers to the use of a sweep gas (e.g. steam sweep) under reduced pressure conditions in a facilitated transport membrane for the separation of CO2 from a gas mixture comprising the same.

[0222] The process and gas separation apparatus as described for any aspect herein is particularly beneficial for sea-faring vessels. For example, the process and gas separation apparatus as described herein may be used for onboard carbon capture and storage (OCCS). In certain embodiments, the process and gas separation apparatus as described herein may be used to capture CO2 from the exhaust of a fuel combustion engine onboard a sea-faring vessel. The CO2 may be stored on the vessel or pressurised / liquified.

[0223] The process and gas separation apparatus therefore may provide an efficient way to reduce carbon emissions in the shipping industry.

[0224] Examples

[0225] Materials

[0226] The composite membrane chosen for this investigation consists of a dense polymeric layer containing mobile and fixed carrier groups deposited on a porous support material. Surface modifiers were also introduced to improve dense layer polymer adhesion and water distribution. Poly(allylamine hydrochloride) (MW = 120,000 - 200,000) acquired from Thermo Fischer Scientific, Sweden and 2- bromobutane (>= 98%, MW = 137.02) from Sigma-Aldrich, Norway were utilized in the synthesis of the dense layer polymer; sterically hindered poly(allylamine) (SHPAA). Aqueous solutions of SHPAA (5 wt%) were pre-synthesized by AquaLung Carbon Capture AS, Norway and were used as is in membrane preparation. The SHPAA was the following:

[0227] L-Proline Reagentplus® and Potassium Hydroxide (KOH) (Pellets, 99.9%) were purchased from Sigma-Aldrich, Norway to prepare the mobile carrier. Polyvinyl Alcohol (PVA) (MW = 89,000 - 98,000, 89% hydrolyzed) was also obtained from Sigma-Aldrich and an aqueous solution of porous Graphene Oxide (pGO) (1 mg / ml) was premade and provided by AquaLung Carbon Capture AS, Norway. The porous support utilized in the study were poly(p-phenylene oxide) (PPO) hollow fibers (HF) from Parker AS, Norway. Premixed bottles of CO2 / N2 (10% CO2 in N2) and He (99.996%) supplied by AGA, Norway were used as the performance testing gas and sweep gas respectively.

[0228] Casting Solution Preparation

[0229] Preparation of the mobile carrier, Potassium Prolinate (ProK) is done by mixing equimolar amounts of L-proline Reagentplus® with KOH in De-lonized (DI) water to form a solution of 10wt% solids. Mixing is conducted at 600 rpm over a 12 hour period at room temperature. Prior to use, pGO is transferred to a glass container and submerged in an ultrasonic bath for a minimum of 1 hour. The casting solution can then be made by first measuring the required amount of DI water for the desired final volume, followed by the introduction of respective amounts of dilute PVA, pGO, and mobile carrier. PVA is added at a 9:1 SHAPAA to PVA ratio, while mobile carrier and pGO concentration is reported relative to the total weight of the organic phase, as expressed in Equation 1 and 2 below. This mixture is stirred at 600 rpm for a minimum of 1 hour before a drop wise addition of the the SHPAA, maintaining an overall solids content (polymer + mobile carrier) at 0.15 wt%. This solution is then stirred overnight at 600 rpm at room temperature. where cmcand cnf is the concentration of the mobile carriers and nanofillers to the total organic phase (wt%) respectively, wmcis the weight of the mobile carrier (g), Wnf is the weight of the nanofiller (g), and wpoiis the total weight of the dry polymer (g). The resulting casting solution is stored at 5°C and is left to equalize to room temperature before use.

[0230] Coating of Hollow Fiber Membranes and Module Preparation

[0231] Poly (p-phenylene oxide) (PPO) hollow fibers used for hollow fiber supports with inside diameter of 350 pm and outside diameter of 540 pm were obtained from Parker A / S Norway.

[0232] Two coating methods were used in the coating of the HF membranes; dip coating and scoop coating. Both methods begin with cutting pre-washed and dried PPO HF membranes into equal lengths. In dip coating, the segments are hung vertically with a paperclip on the free hanging end to seal the fiber and maintain tension throughout the coating process. Each fiber is then submerged into the casting solution bath by hand at a constant speed and is withdrawn at a similar rate. The fibers are air dried for 1 hour before each one is flipped and dipped once more.

[0233] After a final air drying step, the fibers are cured in an oven at 60°C for a minimum of 6 hours. For scoop coating, the casting solution is transferred to a horizontal container with a wide opening. The fibers are laid across the container, submerging 3 / 4 of its length in the casting solution. Each fiber is then pulled at a constant speed by hand from the exposed end. Similarly, the fibers are hung vertically and air dried for 1 hour before the step is repeated from the opposite end. Finally the fibers are cured in an oven at following the same conditions and length of time as dip coating. 85 - 100 hollow fibers are inserted into a stainless steel assembly consisting of a 15 - 25 cm, 1 / 2- inch diameter pipe and 1 / 2-inch to 3 / 4-inch SwagelokTM fittings. The open ends of each fiber is first individually sealed with a quick setting epoxy adhesive (Loctite® 3090) to prevent ingress of adhesive in the following step. A segment of silicone tubing is then attached to one end of the module and filled with a low viscosity, long setting adhesive (Loctite® 9483) that separates the bore and shell side. The adhesive is left to cure overnight, after-which the silicone tubing is cut off the protruding segment of cured adhesive. This segment is carefully broken off with the aid of a bench-vice to re-open the tube side of the module. This sealing process is then repeated on the opposite end of the module. A schematic of the completed module can be seen in Figure 9.

[0234] Experimental The conditions used for the data in Figures 3-6 are shown in Table 1 below.

[0235]

[0236] Table 1

Claims

Claims1. A process for the separation of CO2 from a gas mixture comprising the same using at least one membrane with a feed side and a permeate side, said process comprising:- contacting said gas mixture with the feed side of the membrane; and- contacting a sweep gas with the permeate side of the membrane, wherein the sweep gas comprises water vapour; wherein the pressure of the sweep gas is 0.95 bar or less.

2. The process of any preceding claim, wherein the pressure ratio between the feed gas and the sweep gas is in the range of 4.5:1 or less, preferably 4.4:1 or less, preferably 4.0:1 or less.

3. The process of any preceding claim, wherein the pressure of the feed gas is in the range of 1.0 to 1.2 bar, preferably 1.1 to 1.2 bar.

4. The process of any preceding claim, wherein the membrane comprises a selective layer, wherein said selective layer is cross-linked.

5. The process of any preceding claim, wherein the membrane comprises a selective layer on a support, said selective layer comprising: a) a polymeric matrix comprising an amine polymer; b) graphene oxide nanofiller; and c) a mobile carrier selected from an ionic liquid or an amino acid salt.

6. The process of claim 5, wherein the polymeric matrix comprises a polyallylamine-based polymer or a polyvinylamine-based polymer, preferably polyallylamine-based polymer.

7. The process of any preceding claim, wherein the sweep gas consists of water vapour and is a steam sweep gas, or wherein the sweep gas comprises water vapour and is a water vapour-containing sweep gas.

8. The process of any preceding claim, wherein the gas mixture is air, flue gas, biogas, natural gas, fuel cell exhaust or syngas.

9. The process of any preceding claim, wherein the gas mixture comprises 0.4 to 90 vol% CO2, 0 to 50 vol% of O2, and 0 to 95 vol% of N2, CH4 and / or H2, wherein the total percentage does not exceed 100 vol%.

10. The process of any preceding claim, wherein the sweep gas is at a pressure of 0.1 to 0.95 bar, preferably 0.2 to 0.8 bar, preferably 0.25 to 0.8 bar, preferably 0.4-0.8 bar, such as 0.3 to 0.6 bar.

11. The process of any preceding claim, wherein the sweep gas is a steam sweep gas at a temperature of 100°C or less, preferably 95°C or less, such as 60- 80°C; or wherein the sweep gas is a water vapour-containing sweep gas at a temperature of 150°C or less, such as 75-125°C.

12. The process of any preceding claim, wherein the sweep gas has a water content of 0.5 vol% or more, such as 1 vol% or more or 5 vol% or more.

13. The process of any preceding claim, wherein the sweep gas has a relative humidity (RH) of 20-99%, such as 50-98%.

14. The process of any preceding claim, wherein the membrane is a solutiondiffusion membrane or a facilitated transport membrane, preferably a facilitated transport membrane such as a hybrid facilitated transport membrane.

15. The process of any preceding claim, wherein the membrane is selected from a hollow fiber membrane, a flat sheet membrane, a tubular membrane, or a capillary membrane, preferably a hollow fiber membrane.

16. The process of any preceding claim, wherein the flow rate ratio between the feed gas and the sweep gas is in the range of 1 :5 to 20:1, preferably 1:1 to 10:1.

17. The process of any preceding claim, wherein the sweep gas is generated in a vacuum rated pressure vessel with a heating coil.

18. The process of any preceding claim, wherein the sweep gas is a by-product from an industrial process (e.g. process steam) or wherein the sweep gas is generated from excess heat energy from a separate process (e.g. an industrial process such as from a combustion process).

19. The process of any preceding claim, further comprising separating a permeate comprising the gas from the sweep gas e.g. through condensation of the sweep gas.

20. Process as claimed in any preceding claim, wherein the process comprises transmembrane permeation of CO2, i.e. from the feed side to the permeate side.

21. The process of any preceding claim, wherein the gas mixture and sweep gas flow in a counterflow current pattern.

22. A gas separation apparatus for the separation of CO2 from a gas mixture, comprising:- at least one membrane with a feed side and a permeate side; and- a reduced pressure-generating apparatus, configured to generate a reduced pressure on the permeate side of the membrane- a sweep gas supply apparatus, configured to supply sweep gas to the permeate side of the membrane.

23. A gas separation apparatus as claimed in claim 22, wherein the feed side of the membrane is configured to receive the gas mixture, e.g. wherein the gas mixture is a flue gas, air, biogas, natural gas, fuel cell exhausts or syngas.

24. A gas separation apparatus as claimed in any of claims 22-23, wherein the reduced pressure generating apparatus is downstream of the permeate.

25. A gas separation apparatus as claimed in any of claims 22-24, wherein on the feed side of the membrane there is a feed flow inlet and on the permeate side of the membrane there is a sweep gas inlet.

26. A gas separation apparatus as claimed in any of claims 22-25, comprising:- at least one membrane with a feed side and a permeate side;- a feed gas inlet for providing the gas mixture to the feed side of the membrane- a feed gas outlet, downstream from the feed gas inlet, for providing removal of retentate gas after contact with the feed side of the membrane;- a sweep gas inlet for providing a sweep gas (e.g. a steam sweep) to the permeate side of the membrane- a sweep gas outlet for removing the permeate gas; and- a reduced pressure generating source, such as a pump, on the permeate side of the membrane, e.g. downstream from the sweep gas outlet.

27. A gas separation apparatus as claimed in any of claims 22-27 wherein the membrane comprises a selective layer on a support, said selective layer comprising: a) a polymeric matrix comprising an amine polymer; b) graphene oxide nanofiller; and c) a mobile carrier selected from an ionic liquid or an amino acid salt.

28. The use of a sweep gas (e.g. steam sweep) under reduced pressure conditions in a facilitated transport membrane for the separation of CO2 from a gas mixture comprising the same.