Improved two-phase membrane for separating carbon from a carbon-containing feed gas and separation method using the membrane

Abstract

The two-phase membrane comprises a porous support having a solid phase with a matrix of interconnected pores and a liquefiable ion transport phase within the pores of the porous support. The ion transport phase is formed from at least one alkali metal hydroxide and at least one oxide ion transport agent providing an ion source selected from the group consisting of borate, nitrate, phosphate, vanadate, niobate, or sulfate. The at least one alkali metal hydroxide may be selected from the group consisting of NaOH, KOH, LiOH, RbOH, CsOH, and mixtures thereof. The oxide ion transport agent is preferably present in the ion transport phase in an amount of about 1 to about 30 mole percent. When the membrane is used to separate CO2 from a feed gas, substantially lower operating temperatures can be achieved.

Description

Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 068,081 (Attorney Docket No. 4271-0575), filed August 20, 2020, the entire contents of which are expressly incorporated herein by reference. Government Rights

[0002] This invention was made with government support under the following government contract: Department of Energy Contract No. DE-SC0017124. The government has certain rights in this invention.

[0002]

[0003] FIELD OF THE INVENTION Embodiments disclosed herein generally relate to the use of two-phase separation membranes for high-performance carbon capture and separation from feed gases (e.g., flue gases from industrial facilities). Certain embodiments described herein relate to incorporating oxide ion transport agents (e.g., borates, nitrates, phosphates, vanadates, niobates, and / or sulfates) into the composition of the ionic liquid (e.g., molten salt) separation phase of the two-phase membrane to enable higher performance and / or extend the temperature range over which the gas separation membrane can be operated. BACKGROUND

[0003]

[0004] Greenhouse gas reduction through carbon capture and separation remains a key challenge for the chemical, petroleum, and power generation industries, as well as for a low-carbon global economy. The cost-effective capture and separation of carbon dioxide (CO2) from any gas mixture is expected to enable the benefits of reducing greenhouse gas emissions through the use or sequestration of the captured carbon dioxide.

[0004]

[0005] The economic and energy costs of conventional carbon capture and separation techniques, such as those using polymer membranes or solvent-based solutions, are often too high to allow widespread application of cost-effective CO2 capture. New technologies are needed to separate CO2 from gas mixtures that may be generated as part of various industrial processes, including chemical, petroleum, and power plants. For example, traditional polymer membrane-based technologies are sometimes applied to capture carbon from the flue gas of fossil-fuel power plants. Dual-phase membranes are an alternative technology that can capture CO2 through a molten carbonate phase, typically at temperatures above 550°C (Rui, Z. et al., "Ionic conducting ceramic and carbonate dual phase membranes for carbon dioxide separation," J. Memb. Sci. 417-418, 174-182 (2012), the entire contents of which are expressly incorporated herein by reference).

[0005]

[0006] Recently, molten hydroxides have been introduced into the ion separation phase to increase CO2 permeation rates and reduce operating temperatures to 400°C or higher (see Ceron, MR et al., "Surpassing the Conventional Limitations of CO2 Separation Membranes with Hydroxide / Ceramic Dual-Phase Membranes," J. Memb. Sci. 567, 191-198 (2018), the entire contents of which are expressly incorporated herein by reference). For example, U.S. Pat. No. 10,464,015 (expressly incorporated herein by reference in its entirety) proposes providing a separation membrane that, in use, comprises at least one molten alkali metal hydroxide disposed within the pores of a solid porous support structure. Thus, the molten separation phase may comprise appropriate concentrations of NaOH, KOH, LiOH, RbOH, CsOH, and mixtures thereof. When used as a ternary mixture for separating CO from flue gas, such a ternary mixture of molten alkali metal hydroxides will thereby provide a molten separate phase within the membrane having an average concentration of KOH, NaOH, LiOH, KCO, NaCO, and / or LiCO, depending on the particular hydroxide species contained in the mixture.

[0006]

[0007] However, the overall process efficiency using two-phase membranes in the area of ​​post-combustion carbon capture and flue gas applications would benefit greatly from the ability to operate even at temperatures as low as 125-300° C. Accordingly, embodiments disclosed herein are directed to providing such improvements, e.g., lower operating temperatures, in two-phase membrane separation of CO from flue gas.

[0007]

[0008] In general, embodiments of the invention disclosed herein are directed to an improved two-phase membrane for separating CO2 from a feed gas, which membrane necessarily comprises a solid porous support phase having a porous solid matrix of interconnected open pores and a molten salt phase contained within the pores of the solid porous support phase, wherein the liquid phase, in use, comprises at least one molten alkali metal hydroxide and one or more oxide transport agents dispersed throughout the alkali metal hydroxide. In this regard, these oxide ion transport agents can function in the liquid phase as oxide ion transport catalysts to affect gas adsorption reactions or to control the acid / base equilibrium of oxide ions. Furthermore, these oxide ion transport agents can also chemically couple the permeation of carbon dioxide from the feed gas with water vapor in the vapor sweep, enabling an active transport membrane mechanism. This active transport mechanism therefore allows for the use of a large concentration gradient (partial pressure) of water vapor to facilitate carbon dioxide separation, rather than relying solely on the concentration gradient of carbon dioxide to facilitate gas separation through the membrane. In this manner, therefore, the two-phase active membrane embodiments described herein enable high performance separation of CO2 from feed gas over a relatively wide range of relevant application temperatures and other operating conditions.

[0008]

[0009] According to certain embodiments, a two-phase membrane includes a porous support having a solid phase with a matrix of interconnected pores and a liquefiable ion transport phase within the pores of the porous support, the ion transport phase being formed from at least one alkali metal hydroxide and at least one oxide ion transport agent providing an ion source selected from the group consisting of borate, phosphate, vanadate, and niobate. The at least one alkali metal hydroxide may be selected from the group consisting of NaOH, KOH, LiOH, RbOH, CsOH, and mixtures thereof. The oxide ion transport agent is preferably present in the ion transport phase in an amount of about 1 to about 30 molar percent (molar %).

[0009]

[0010] In some embodiments, the ion transport phase may comprise a molar amount of a melting temperature lowering component sufficient to lower the operating temperature of the membrane to about 100° C. or higher, e.g., from about 100° C. to about 500° C., e.g., from about 125° C. to about 300° C. The melting temperature lowering component may be comprised of at least one compound selected from the group consisting of at least one alkaline earth metal hydroxide, at least one alkali metal nitrate, and / or at least one alkaline earth metal nitrate, e.g., Be(OH)2, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, LiNO3, NaNO3, KNO3, RbNO3, CsNO3, Be(NO3)2, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2, and mixtures thereof.

[0010]

[0011] The porous support may be composed of a metal or ceramic material, such as a nickel-chromium alloy, stainless steel, zirconium oxide, cerium oxide, magnesium oxide, aluminum oxide, lanthanum oxide, samarium oxide, gadolinium oxide, iron oxide, calcium carbonate, silicon oxide, and silicon carbide. The pores of the porous support may have an average pore size of about 10 nm to about 1 mm.

[0011]

[0012] When used to separate gaseous species (e.g., CO) from a feed gas, the two-phase membrane surface may be contacted with a feed gas containing the gaseous species to be separated at an operating temperature sufficient to melt the ion transport phase, and the gaseous species may be transported through the melted ion transport phase to the opposite surface of the membrane. A sweep gas, e.g., steam, may be applied to the opposite surface of the membrane. In certain embodiments, the concentration of CO in the steam sweep gas may be greater than the concentration of CO in the feed gas.

[0012]

[0013] These and other aspects of the present invention will become more apparent after careful consideration of the following detailed description of the presently preferred exemplary embodiments.

[0013]

[0014] Reference is made to the accompanying drawings in which: [Brief explanation of the drawings]

[0014] [Figure 1]

[0015] FIG. 1 is a highly schematic cross-sectional view of a two-phase active separation membrane according to an embodiment of the invention described herein. [Figure 2]

[0016] FIG. 2 is a graph plotting CO2 permeability (mol / m s Pa) versus temperature (°C) through a two-phase membrane using various liquid phases according to the prior art (comparative example) and according to specific specific embodiments of the present invention as described in the Examples below. Detailed Description

[0015]

[0017] As used herein and in the appended claims, the term "two-phase membrane" refers to a gas separation membrane that necessarily contains a solid phase and a liquid phase at the membrane's operating temperature. As shown in FIG. 1, a two-phase membrane 10 comprises a solid phase 12 and a liquid phase 14 at the membrane's operating temperature. Thus, the solid phase 12 provides the membrane with a structural form factor, provides mechanical support, and provides a continuous pore structure that retains the liquid phase by capillary action. The liquid phase 14 is a non-volatile liquid at the membrane's operating temperature that selectively transports gas from one side 10a of the membrane (the feed gas or retentate side) to the other side 10b of the membrane (the sweep gas or permeate side) through absorption, diffusion / conduction, and desorption processes. This allows the feed gas (indicated by arrow A) flowing in contact with the feed gas side 10a of the membrane 12. F ) carbon dioxide (CO2) in carbonate (CO3 2- ), bicarbonate (HCO3 - ), or associated ionic species, permeate the liquid phase 14 of the membrane 12 and flow in contact with the sweep gas side 10b of the membrane 12 (arrow A S ), which is usually recovered as steam.

[0016]

[0018] The specific contents of the solid and liquid phases are described in more detail below. A. Porous support / solid phase

[0019] The porous support or solid phase 12 of the two-phase membranes according to embodiments disclosed herein can be virtually any solid porous structure that can be utilized in a solid / liquid membrane at the operating temperature and pressure conditions. Thus, for example, the porous support or solid phase 12 can be formed of a suitable metal or ceramic material that can withstand the operating temperatures and pressure gradients (e.g., temperatures up to about 700°C) in the system in which the two-phase membrane will be used. For example, the porous support or solid phase 12 can include a porous metal or metal aerogel formed from or including Inconel® nickel-chromium based superalloy, stainless steel (e.g., Grade 316SS), etc., or a porous ceramic or ceramic aerogel formed from or including zirconium oxide, cerium oxide, magnesium oxide, aluminum oxide, lanthanum oxide, samarium oxide, gadolinium oxide, iron oxide, calcium carbonate, and silicon oxide or silicon carbide.

[0017]

[0020] Regardless of the material forming the solid phase 12, the solid phase 12 defines a large number of interconnected open-cell pores, thereby providing a continuous, tortuous path from one side or face 10a of the solid phase 12 to the other opposite side or face 10b. The pores may have substantially the same average diameter throughout the thickness of the solid phase. Alternatively, the pores may have different pore sizes throughout the cross-sectional thickness of the solid phase 12. For example, for embodiments utilizing a pore size gradient, pores having relatively larger pore sizes may be located in a region of the solid phase 12 adjacent one of the two outer faces 10a, 10b, while pores having relatively smaller pore sizes may be located on the opposite face.

[0018]

[0021] The particular pore size utilized is not critical, and the selection of any particular pore size will depend on many operational factors well known to those skilled in the art, including the operating temperature conditions and pressure gradients, if any, utilized during use, which will impart stresses to the solid phase. Thus, the solid phase may advantageously comprise pore sizes ranging from about 1 nm to about 100 μm, e.g., from about 3 nm to about 10 μm, or from about 10 nm to about 1 μm, or from about 30 nm to about 300 nm.

[0019]

[0022] The thickness of the solid phase 12, and thus the membrane, can likewise be selected to withstand the temperature and pressure conditions of the membrane during use and to facilitate mass transport through the membrane. Thus, the solid phase (and thus the membrane itself) can have a thickness of about 30 μm to about 1 cm, advantageously about 100 μm to about 3 mm, for example, about 300 μm to about 1 mm. B. Ion transport (liquefiable) phase

[0023] As previously mentioned, ion transport phase 14 is contained within the pores of solid porous support phase 12. According to embodiments disclosed herein, ion transport phase 14 is necessarily molten (liquid) at the operating temperature conditions of membrane 10, e.g., temperatures above 100°C, typically above 125°C. Thus, in use, ion transport phase 14 comprises at least one molten alkali metal hydroxide and one or more oxide transport agents dispersed throughout the alkali metal hydroxide.

[0020]

[0024] The molten alkali metal hydroxide may actually be one or more molten alkali metal hydroxides selected from the group consisting of NaOH, KOH, LiOH, RbOH, CsOH, and mixtures thereof. Preferably, the mixture of alkali metal hydroxides is eutectic so that the entire mixture melts as a whole, as opposed to the individual components melting individually at the lowest possible melting temperature under different conditions, e.g., temperature and pressure, across all possible mixing ratios for the species involved.

[0021]

[0025] The operating temperature of membrane 10 may be further reduced, for example, to about 125° C. or even to about 100° C., by adding a melting temperature lowering component to ion transport phase 14 that lowers the melting temperature through a solvation or dilution effect. Such a preferred melting temperature lowering component may be, for example, an alkaline earth metal hydroxide, such as Be(OH), Mg(OH), Ca(OH), Sr(OH), Ba(OH), and mixtures thereof. Additionally, alkali and alkaline earth metal nitrates, such as LiNO, NaNO, KNO, RbNO, CsNO, Be(NO), Mg(NO), Ca(NO), Sr(NO), Ba(NO), and mixtures thereof, may also be added to the molten alkali metal hydroxide of ion transport phase 14 to provide a low melting temperature molten liquid for CO.

[0022]

[0026] One technical challenge to achieving CO separation at low temperatures, between about 125°C and about 300°C, relates to CO absorption by molten hydroxides in the ion transport phase 14. However, the conversion of alkali and alkaline earth metal hydroxides to alkali and alkaline earth metal carbonates upon CO absorption is thermodynamically highly favorable. Therefore, a distinctive feature of the presently disclosed embodiments is the inclusion of an oxide ion transport agent that catalyzes or buffers the oxide ion transport reactions associated with the absorption, diffusion, and desorption of CO for transport through the ion transport phase 14 of membrane 10 as carbonate-based species.

[0023]

[0027] Accordingly, embodiments disclosed herein necessarily include such oxide ion transport agents in addition to a liquid phase based on molten alkali metal hydroxide and / or molten alkali metal hydroxide-nitrate. Accordingly, the oxide ion transport agent may be provided by virtually any compound that, in use, provides a source of anions selected from the group consisting of borate, nitrate, phosphate, vanadate, niobate, and / or sulfate. Accordingly, preferred oxide ion transport agents may be alkali metal borates, nitrates, phosphates, vanadates, niobates, and / or sulfates. Accordingly, oxide ion transport agents utilized herein have favorable solubility at reasonably low temperatures, O 2- They have the appropriate lux-flood acid / base chemistry for ions, catalytic effects for CO2 absorption and desorption, and other effects that enhance CO2 permeation rates. Generally, these oxide ion transport agents are dispersed throughout the liquid phase.

[0024]

[0028] The borate, nitrate, phosphate, vanadate, niobate, and / or sulfate oxide ion transport agents are present in the ion transport (liquefiable) phase in a molar amount typically of about 1 to about 30 mol %, typically about 4 to about 20 mol %, and most preferably about 8 to about 16 mol %. C. Separation method

[0029] As briefly described above, the liquid phase molten salt selectively sorbs and transports the target gas across the membrane 10. For example, when separating carbon dioxide from a gas mixture, CO2 is generally separated as CO2 + O 2- →CO3 2- through CO3 2- It is absorbed as an ion, but polycarbonate, bicarbonate, and other species related to the carbonate state may also be present under various conditions. CO3 2- As CO is transported from one side 10a to the other side 10b of the membrane 10 and desorbed as CO, some mechanism must be used to maintain mass and charge balance across the membrane 10. 2- Ions must be transported in the opposite direction.

[0025]

[0030] Because molten salts, primarily hydroxides and carbonates, are among the fastest conductors of ions through the condensed (solid or liquid) phase at a given temperature, two-phase membranes can achieve significantly improved separation properties in terms of permeability and selectivity compared to all other currently known membrane proposals in the art. Two-phase membranes properly designed, prepared, and operated according to the presently disclosed embodiments can achieve selectivities of over 1,000 for CO relative to relatively inert gases, such as nitrogen (N), oxygen (O), and argon (Ar).

[0026]

[0031] Operation of the two-phase membrane 10 according to the presently disclosed embodiments may be advantageously combined with a steam sweep. Steam is used to sweep CO2 from the permeate side 10b of the membrane 10, lowering the CO2 concentration in the permeate and thereby promoting further CO2 permeation. In some cases, the presence of H2O can enhance CO2 permeation through chemical effects. Two-phase membranes can also increase CO2 permeation through the equilibrium reaction K2CO3 + H2O ⇔ 2KOH + CO2. 2- and OH - When relying on countercurrent transport of CO3 in the liquid phase, the vapor 2- or OH - It may be useful to balance the carbonate to hydroxide ratio so that neither concentration is too low and significantly limits the separation rate. In some cases, CO2 and HO gas absorption and CO3 2- and OH - The combination of transport may enable advancement towards a newly disclosed active transport membrane mechanism, in which the coupled transport of a highly concentrated species (vaporous HO) is used to thermodynamically force or accelerate membrane separation of a less concentrated species (exhaust gas CO).

[0027]

[0032] Thermodynamics theoretically dictates that CO permeation can be driven until the ratio of CO to HO in the steam sweep matches the ratio of CO to HO in the feed gas, although this ratio may actually be lower. For example, the exhaust of nearly complete natural gas combustion has a ratio of approximately 1 CO per 2 HO, so the composition of the steam sweep may approach a ratio of 1 CO per 2 HO. At a minimum, any CO concentration in the steam sweep higher than that in the feed gas is sufficient to indicate the presence of an active transport mechanism. The temperature of the CO-rich steam sweep can be lowered to induce subsequent separation of CO and HO by condensing the steam to water. Depending on how the two-phase membrane and steam sweep as disclosed herein are integrated into a power plant or other CO source, carbon can be captured at significantly lower capital and operating (e.g., energy) costs than state-of-the-art carbon capture technologies.

[0028]

[0033] As briefly mentioned above, the operating temperature of conventional biphasic membranes for carbon capture was initially limited to temperatures above 550 °C due to the thermal requirements of solid oxide ion conduction. The incorporation of alkali metal hydroxides and vapor sweeps reduces the operating temperature to above 400 °C. This generally occurs when the electrolyte hydroxide is completely converted to carbonate (Li 43.5 Na 31.5 K 25 This corresponds to the temperature at which the ternary eutectic of 2CO3 solidifies. Two-phase membranes function reasonably well when the liquid phase is partially or completely solidified and mass transport through it is inhibited. The acid / base equilibrium of the Lux-Flood must be buffered or otherwise controlled at low temperatures to maintain the liquidus temperature below the operating temperature by limiting the amount of hydroxide converted to carbonate.

[0029]

[0034] The targeted high permeability and low operating temperature are achieved by using alkali metal (Li + , Na + , K. + , Rb + , Cs +As mentioned above, in some cases, the alkali earth metal (Be) hydroxides were used in combination with other hydroxides. 2+ , Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) hydroxides can be incorporated and mixed with alkali metal hydroxides and alkali metal hydroxide-nitrate mixtures to lower the operating temperature. For example, a liquid phase based on a mixture of barium, sodium, and potassium may allow membrane operating temperatures above 300°C. In such cases, O 2- Ionic catalysts can improve gas separation rates by avoiding the kinetic barrier to CO2 absorption at low operating temperatures. The Lux-Flood acid / base equilibrium controls the degree of hydroxide to carbonate conversion so that the carbonate concentration is large enough to support a fast CO2 permeation rate but not so large that the liquid phase temperature rises above the operating temperature. 2- Ionic buffers are required. 2- Ionic buffers are suitable for catalytic oxidation as long as they dissolve in molten hydroxide and hydroxide-nitrate systems at concentrations of several mole percent or more and do not significantly increase the liquidus temperature. 2- It may also have an ion transport effect.

[0030]

[0035] O 2- A greater complement of liquid phase components and additives is required to catalyze transport and adapt the Lux-Flood acid / base equilibrium at a given operating temperature. Accordingly, the presently disclosed embodiments encompass a relevant range of electrolyte compositions suitable for various carbon capture applications. As previously discussed, compounds providing ion sources selected from the group consisting of borates, nitrates, phosphates, vanadates, niobates, and sulfates may be added to the ion transport (liquid) phase of the two-phase membrane as oxide ion transport agents to improve performance or function. Thus, the liquid phase is primarily composed of molten hydroxide and / or nitrate solvents with a concentration of carbonate during operation or use.

[0031]

[0036] The liquid phase may also contain significant concentrations of nitrates or sulfates, either through preparation or through use, when exposed to feed gases that also contain nitrogen oxide or sulfur oxide gases. Apart from the use of alkali metal and alkaline earth metal cations to manage the liquid phase melting temperature and acid / base effects, CO2 + O 2- ⇔CO3 2- O for optimal CO2 sorption via 2- To control catalytic activity and the Lux-Flood acid / base equilibrium, borate, nitrate, phosphate, vanadate, niobate, and / or sulfate anions have been added. These components dissolve well in molten hydroxide and nitrate solvents and affect the transport and equilibrium of oxide ions through the meta-, ortho-, and pyro- (or related poly) states. For example, metaphosphate (PO3 - ) accepts oxide ions and converts them into PO3 - +O 2- ⇔PO4 3- and 2PO4 3- ⇔P2O7 4- +O 2- By this, orthophosphate (PO4 3- ) or pyrophosphate (P2O7 4- ) respectively. In vanadates and niobates, the same reaction as in phosphates occurs under slightly more acidic conditions. In boron, metaborate (BO2 - ) accepts oxide ions and converts them into BO2 - +O 2- ⇔BO3 3- and 2BO3 3- ⇔B2O5 4- +O 2- By this, orthoborate (BO3 3- ) or pyroborate (B2O5 4- ), which occurs in a similar manner. Membrane performance is usually improved when two catalysts are paired at similar concentrations. This result may indicate that one catalyst promotes CO2 adsorption and the other promotes CO2 desorption. Alternatively, the improvement may be due to the interaction of the two catalysts, e.g., PO4 3- +VO4 3- ⇔PVO7 4-+O 2- These reactions should be considered representative and may be more complicated by the actual presence of CO2 and H2O in the melt phase.

[0032]

[0037] Oxide ion transport agents can be incorporated into liquid-phase formulations through various combinations of components to achieve meta-, ortho-, and pyro- (or related poly-) states in the final formulation. The choice of component may depend on commercial availability, cost, cation pairing, or other factors. For example, sodium borate is commercially available as sodium metaborate, NaBO2, and borax, Na2B4O7·10H2O. These borates can react with hydroxides in the formulation to form sodium orthoborate, Na3BO3, via the representative reactions NaBO2 + 2NaOH ⇔ Na3BO3 + H2O or Na2B4O7·10H2O + 10NaOH ⇔ 4Na3BO3 + 15H2O. Similarly, trisodium orthophosphate, Na3PO4, may be an initial component or may be formed by reacting monosodium orthophosphate, NaH2PO4, or disodium orthophosphate, Na2HPO4, with excess hydroxide in the formulation, via the representative reactions NaH2PO4 + 2NaOH ⇔ Na3PO4 + 2H2O or Na2HPO4 + NaOH ⇔ Na3PO4 + H2O. Alternatively, sodium metaphosphate, NaPO3, may be formed by dehydration of monosodium orthophosphate with NaH2PO4 ⇔ NaPO3 + H2O. Reactions similar to those described for phosphates are expected to occur with vanadates, niobates, and nitrates. These reactions will depend on the specific composition and concentration of the formulation, the temperature at which the liquid phase is processed, and other factors.

[0033]

[0038] Further advantages and aspects of embodiments of the present invention will become more apparent after consideration of the following non-limiting examples.

[0034] [example]

[0039] Several liquefiable ion transport phase compositions, identified below in Table 1, were formulated and incorporated into a porous solid phase consisting of a nanoporous zirconia ceramic tube having a wall thickness of approximately 1 mm, an inner diameter of approximately 5 mm, a porosity of approximately 25-35%, and a particle size distribution near 100 nm, and were obtained from Media and Process Technology Inc., Pittsburgh, PA.

[0035] [Table 1]

[0036]

[0040] Membranes with the CE2 and E1-E6 liquid phase compositions identified in Table 1 above were exposed to a feed gas consisting of 5 vol% CO2 and 95 vol% N2 at a flow rate of 200 sccm, and a sweep gas consisting of 50 vol% HO and 50 vol% Ar at a flow rate of 200 sccm. Both the feed and sweep gases were used at pressures near ambient pressure, ranging from 0 to 3 PSIG, and temperatures ranging from 200 to 500°C. The conditions for the CE1 membrane tested by Rui, Z. et al. in the above-cited publication (J. Memb. Sci. 417-418, 174-182 (2012)) are described in detail therein. CO2 permeability (mol / m s Pa) was determined under steady-state conditions by measuring the CO2 concentration in the sweep gas using a non-dispersive infrared (NDIR) sensor from CO2meter.com, taking into account the membrane surface area, membrane thickness, and the temperature (15-17 °C) and pressure (0.3 PSIG) of the sweep gas fed to the CO2 sensor after removing water from the sweep gas by condensation. The CO2 permeability results are shown in Figure 2 as a function of various membrane operating temperatures.

[0037]

[0041] As can be seen in Figure 2, the presence of oxide ion transport agents, such as nitrates, phosphates, vanadates, borates, and niobates, in combination with one or more alkali metal hydroxides significantly improves the CO2 permeation rate of the membrane while also significantly lowering the operating temperature range.

[0038]

[0042] Those skilled in the art will understand that certain liquid phase components are degenerate, in that different combinations of components may result in the same final molar composition of the liquid phase. For example, homogeneously mixing one mole of sodium hydroxide (NaOH) with one mole of potassium nitrate (KNO) will result in the exact same liquid phase composition as mixing one mole of potassium hydroxide (KOH) with one mole of sodium nitrate (NaNO). While Table 1 lists representative components used to prepare the liquefiable phase formulations of the examples provided herein, such components are intended to be illustrative only and are therefore not limited to the embodiments disclosed herein. Therefore, it will be understood that the embodiments disclosed herein relate to blended formulations, and are not limited solely to the representative list of components identified in Table 1, from which pure formulations can be obtained.

[0039]

[0043] While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention.

Claims

1. A two-phase separation membrane having an operating temperature in the range of 100°C to 500°C, wherein the two-phase separation membrane comprises: a porous support comprising a solid phase having a matrix of interconnected pores; a liquefiable ion transport phase within the pores of the porous support that is molten at the operating temperature, the ion transport phase comprising: (i) at least one alkali metal hydroxide; (ii) at least one oxide ion transport agent providing an ion source selected from the group consisting of alkali and alkaline earth metal salts that are vanadates, present in the ion transport phase in an amount of 1 to 30 mole percent.

2. 10. The two-phase membrane of claim 1, wherein the at least one alkali metal hydroxide is selected from the group consisting of NaOH, KOH, LiOH, RbOH, CsOH, and mixtures thereof.

3. 10. The two-phase membrane of claim 1, wherein the ion transport phase further comprises a molar amount of a melting temperature lowering component sufficient to lower the operating temperature of the two-phase separation membrane by 100°C or more.

4. 4. The two-phase membrane of claim 3, wherein the melting temperature lowering component comprises at least one alkaline earth metal hydroxide, at least one alkali metal nitrate, and / or at least one alkaline earth metal nitrate.

5. The melting temperature lowering component is Be(OH) ２ , Mg(OH) ２ , Ca(OH) ２ , Sr(OH) ２ , Ba(OH) ２ , LiNO ３ , NaNO ３ , KNO ３ , RbNO ３ , CsNO ３ , Be(NO ３ ) ２ , Mg(NO ３ ) ２ , Ca(NO ３ ) ２ , Sr(NO ３ ) ２ , Ba(NO ３ ) ２ 5. The two-phase membrane of claim 4, comprising at least one compound selected from the group consisting of: and mixtures thereof.

6. The two-phase membrane according to claim 1, wherein the operating temperature of the two-phase separation membrane is in the range of 125°C to 300°C.

7. The two-phase membrane of claim 1 , wherein the porous support is composed of a metal or ceramic material.

8. 8. The two-phase membrane of claim 7, wherein the porous support is made of at least one material selected from the group consisting of nickel-chromium based alloys, stainless steel, zirconium oxide, cerium oxide, magnesium oxide, aluminum oxide, lanthanum oxide, samarium oxide, gadolinium oxide, iron oxide, calcium carbonate, silicon oxide, and silicon carbide.

9. 2. The two-phase membrane of claim 1, wherein the pores have an average pore size of 10 nm to 1 mm.

10. A method for separating carbon from a feed gas at an operating temperature in the range of 100°C to 500°C, comprising: (a) contacting a surface of the two-phase membrane of claim 1 with a feed gas containing the gas species to be separated at an operating temperature sufficient to melt the ion transport phase; and (b) transporting said gas species through said molten ion transport phase to an opposite surface of said two-phase membrane.

11. The feed gas is CO ２ 11. The method of claim 10, comprising a concentration of

12. The method of claim 10 further comprising contacting the opposing surface of the two-phase membrane with a sweep gas.

13. The method of claim 12 wherein the sweep gas is steam.

14. The method of claim 13, wherein a vapor sweep gas is used to remove CO in the vapor sweep gas exiting the two-phase membrane. ２ The concentration of CO in the feed gas ２ The method of claim 13, wherein the concentration is greater than 1000 ppm.

15. 11. The method of claim 10, wherein steps (a) and (b) are carried out at an operating temperature in the range of 125°C to 300°C.

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