Composition and process for manufacture of a high temperature carbon-dioxide separation membrane

A membrane composition and process for its formation are disclosed from the removal of carbon dioxide (CO2) from mixed gases, such as flue gases of energy production facilities. The membrane includes a substrate layer comprising inorganic oxides, a barrier layer of in-situ formed Li2ZrO3, a Li2ZrO3 sorbent layer and an inorganic oxide cap layer. The membrane has a feed side for introduction of mixed gases containing nitrogen (N2) and a sweep side for recovery of CO2 wherein the membrane has a relatively high selectivity for CO2 transport at temperatures in the range of 400° to 700° C.

FIELD OF THE INVENTION

The present invention relates to a membrane composition and process for its formation which may then be employed for removal of carbon-dioxide (CO2) such as the removal of CO2from flue gases of energy production facilities. The removal and capture of CO2may be accomplished at relatively high temperature and without the need for cooling flue gas thereby increasing the efficiency of carbon capture and sequestration.

BACKGROUND

Reduction in carbon dioxide (CO2) emission from the coal-fired power plants has become a focal point of international efforts for climate control in recent years. Carbon dioxide emission from coal-fired power plants constitutes a relatively large portion (˜40%) of total CO2emissions. Developing a means to control coal-derived CO2emission is imperative in this effort; however, the cost for such carbon capture and sequestration (CCS) can be up to 50% of the energy production cost of a coal-fired power plant. A major portion of the penalty for CCS is the energy loss in cooling the flue gas (typically up to 750° C. exiting from the burner) to a lower temperature (100-300° C.), which the CO2separation system can tolerate. One reported method of reducing the energy penalty for CCS is to separate CO2at the existing flue gas temperature.

Relatively early studies of CO2capture with membranes indicated that the cost was 30% higher than the cost of the traditional amine chemical absorption process. The limitations of the studied membrane processes was identified as coming from the high cost of compressing low pressure flue gas and the low purity of the permeate, which resulted in the need for multistage processing to achieve the most economic arrangement for CCS in such systems. Recent analyses show that considerable CO2removal rates can be achieved with gas membrane separation systems, and the economic competitiveness of such systems in comparison with amine-based systems depends on the characteristics of both of membrane and feed gas. When membrane selectivity and permeability are improved, the CO2capture and total CCS costs may be reduced by up to 15% compared to the amine process.

CO2separation using membranes is a topic of great commercial interest with most of the reported membranes operating at relatively low temperatures. From the viewpoints of mechanism of separation and material stability, a growing need exists for membrane materials that are useful at temperatures of 400° C. or above.

SUMMARY

The present disclosure relate to a method of forming a membrane for separation of carbon dioxide (CO2) from a mixture of gases comprising supplying a substrate layer of inorganic oxides with average pore sizes of 3.0 μm to 10.0 μm which is 20-80% porous with a thickness of 6.0 mm to 15 mm. One may then deposit on the substrate layer precursors for the formation of Li2ZrO3and react the precursors and form a Li2ZrO3barrier layer wherein the barrier layer is formed at a thickness of 10 μm to 100 μm with a porosity of 0% to 30%. One may then deposit on the barrier layer a sorbent layer comprising Li2ZrO3at a thickness of 100 μm to 500 μm followed by deposition of a cap layer on the sorbent layer comprising inorganic oxides wherein the cap layer has a thickness of 50 μm to 250 μm. One may then expose such membrane to carbon dioxide at elevated temperatures (400° C. to 700° C.) and separate carbon dioxide from the gas mixture without the need for gas mixture cooling.

The present disclosure also relates to a membrane for separation of carbon dioxide (CO2) from a mixture of gases comprising a substrate layer comprising inorganic oxides with average pore sizes of 3.0 μm to 10.0 μm which is 20-80% porous with a thickness of 6.0 mm to 15 mm; a barrier layer of in-situ Li2ZrO3at a thickness of 10 μm to 100 μm with a porosity of 0% to 30%; a Li2ZrO3sorbent layer at a thickness of 100 μm to 500 μm; and an inorganic oxide cap layer at a thickness of 50 μm to 250 μm. The membrane is configures such that it defines a feed side for introduction of mixed gases containing nitrogen (N2) and a sweep side for recovery of CO2and the membrane has a selectivity a according to the following:
α=(YA/YB)/((PhXA−P1YA)/(PhXB−P1YB))
where YA=mole % CO2in the sweep side, XA=mole % CO2in the feed side, Ph=backpressure in the feed side, XB=mole % N2in the feed side; YB=mole % N2in the sweep side, and P1=backpres sure in the sweep side; and wherein the value of α at 500° C. is in the range of 2.0 to 20.

DETAILED DESCRIPTION

The present invention provides a membrane to separate CO2from, e.g., the flue gas of coal-fired power plants and similar hot sources, such as hydrogen production plants using a hydrocarbon steam reforming process, or any industrial processes that produce relatively large quantities of CO2at relatively high temperatures. The membranes herein are therefore configured to separate CO2from a given environment at temperatures at or above 400° C., and in the preferred range of 400° C. to 700° C.

Attention is directed toFIG. 1which illustrates a preferred configuration for the CO2separation membrane. As can be seen, the membrane10may preferably be in tubular form12and may include an internal sweep within the tube to assist in CO2transport. The membrane is preferably sourced from inorganic oxide base substrate materials, such as ZrO2, Al2O3, and lithium zirconate (Li2ZrO3) may then be applied as a solid absorbent to the substrate layer, as discussed further herein. Accordingly, the membrane preferably includes a substrate layer14of inorganic oxides which preferably has pores (openings) at an average pore size of 3.0 μm to 10.0 μm (linear dimension of the openings) and the substrate layer itself is preferably 20-80% porous. The substrate layer14may preferably have a thickness of 6.0 mm to 15 mm, and all values therein, in 0.1 mm increments. A more preferably thickness is in the range of 6.0 mm to 8.0 mm. This therefore provides what may be understood as a porous support layer containing lithium zirconate sorbent material.

A relatively thin and dense Li2ZrO3barrier layer16is provided herein by in-situ procedures and presents a restriction to prevent any other gases except CO2, if entered into a dual-phase sorbent layer18(described below) from transporting through the substrate by its relatively lower porosity than that of the dual phase sorbent layer as well as the optional use of selective eutectic carbonates layers on the Li2ZrO3. Barrier Layer16may therefore preferably have a thickness of 10 μm to 100 μm and a porosity form 0 to 30%

At relatively high temperature in the CO2environment, e.g. at temperature in the range of 400° C.-700° C. porous Li2ZrO3sorbent may form a dual-phase layer18comprising a solid ZrO2framework and molten lithium carbonate (Li2CO3). Such dual-phase system may then only allow CO2to diffuse through and thus be transported while leaving the majority of any other gases behind. That is, 90% or more of gases other than CO2will not transport through the membrane10, more preferably 95% or more, and in a most preferred configuration, 99% or more of the gases other than CO2will not transport through the membrane.

A porous ceramic ZrO2or Al2O3cap layer20may be deposited on the dual phase sorbent layer18and used to improve the mechanical strength of the entire membrane system as well as serving as a protective coating to prevent other flue gas contaminants from entering into the membrane. Cap layer20may preferably have a thickness of 50 μm to 250 μm.

FIG. 2provides a composite microscopic view of the membrane10herein. Layer14comprises the porous ZrO2and at22can be seen the region now infiltrated with liquid Li2CO3. The relatively dense Li2ZrO2layer formed in-situ is again shown at16and the dual-phase sorbent layer is again identified at18along with porous cap layer20.

It may now be noted that the CO2absorption and transport through the membrane is based in part on the reversible reaction of Li2ZrO3and CO2as shown in the following:
Li2ZrO3(s)+CO2(g)Li2CO3(I)+ZrO2(s)[Eq. 1]
Furthermore, at high temperatures, such as temperatures in the range of 400-700° C., and under a certain CO2concentration in the flue gas, a steady-state reaction will be established and the CO2will be transported through the membrane containing Li2ZrO3, while other gases, such as N2, being rejected.

In-Situ Formation of Li2ZrO3Barrier Layer

As noted above, the substrate layer14may comprise an inorganic oxide such as porous zirconia oxide or porous alumina oxide. The preferred embodiment herein utilizes an in-situ formed Li2ZrO3based barrier layer16. The role of this layer is to serve as a final barrier against the penetration of gases other than CO2through the membrane. Layer16may control the rate of CO2transport as well as the efficiency of the CO2selectivity of the assembled membrane. It may preferably be formed within the porous ceramic substrate layer14, with or without pore size reduction, via in-situ procedures, using chemical vapor deposition (CVD), physical vapor deposition (PVD), pressurized chemical infiltration, and similar processes.

CVD processes utilize appropriate precursor compounds for deposition of Li2ZrO3coating via decomposition and activation of the precursor in an oxygen containing gaseous atmosphere. Reference to precursor compounds for Li2ZrO3herein may be understood as one or more compounds which may react when deposited within the substrate14and form Li2ZrO3. In the CVD process the substrate temperature is established in the range of thermodynamically stable formation of Li2ZrO3via a heterogeneous reaction between the reactive gas atmosphere and the substrate to be coated. Plasma assisted CVD processes using inductively coupled thermal plasma (ICTP) or electron cyclotron resonance (ECR) plasma heating can be utilized for further activation of the reactive vapor plasma environment, which allows reducing the deposition temperature and improving the coating density and morphology by reduction of the coating grains and producing ultra-fine coating morphology using deposition under relatively intense ion bombardment. In addition, plasma activation of the CVD process via ionization and excitation of atoms, molecules and radicals by collisions with excited particles and electrons allows depositing near stoichiometric coatings on substrates having temperatures below the temperature determined by thermodynamically equilibrated conditions of the coating material. This results in substantial reduction of coating stress due to thermal expansion mismatch between coating and substrate materials.

Turning to the pressed infiltration method, also referred to as press wet deposition, the precursors of Li2ZrO3, lithium nitrate (LiNO3) and zirconia nitrate (ZrO(NO3)2), in stoichiometric ratios, are mixed in a solvent. A porous soft medium is saturated with the mixture and is placed on a substrate. The Li2ZrO3precursors are then forced to infiltrate into the substrate by a rubber roller following by calcination at high temperatures, such as temperatures of 700° C. to 900° C., to again form in-situ Li2ZrO3in the substrate.

Preferably, in order to obtain a relatively higher CO2absorption efficiency, Li2ZrO3herein may also be modified to form a potassium doped sorbent in the formula of KxLi2-xZrO3, with an optimal crystalline structure. Reference to an optimum crystal structure may be best defined in terms of CO2absorption efficiency, and therefore, the crystal structure is configured such that the absorption by weight of the KxLi2-xZrO3is such that is absorbs at or above 25% by weight CO2, up to about theoretical absorption of 27.8% by weight. It should also be noted that the value of “x” in the indicated formula is in the range of 0.2-1.0.

The K-doped Li2ZrO3yields eutectic carbonate mixtures of Li2CO3and K2CO3which will be in a molten stage at high temperature (to facilitate the diffusion of CO2for faster rate of CO2transport reaction). The optimal CO2absorption of the tetragonal crystalline phase is achieved by a calcination process.

The in-situ chemical synthesis of Li2ZrO3according to the pressed infiltration method can be shown in the equation below. It should be noted that in the equation below, the addition of KNO3will readily provide the potassium doped sorbent noted herein.
2LiNO3+ZrO(NO3)2→Li2ZrO3+2N2O5[Eq. 2]

The Li2ZrO3formed by the press infiltration method, according to the in-situ approach becomes physically attached to the inner pores of the substrates to provide for the boost in CO2separation disclosed herein.FIG. 3shows the SEM of the uncoated substrate.FIG. 4shows the SEM of the Li2ZrO3crystals deposited on the surface of the pores in the substrate14by the press infiltration method. The kinetics of CO2absorption of the resulting Li2ZrO3is comparable to Li2ZrO3prepared by traditional solid state methods. SeeFIG. 5. In order for the membrane to possess the property of relatively high selectivity of CO2a relatively close encounter of CO2and Li2ZrO3is preferably obtained for the chemical reaction to take place as shown above in Equation 1. The selectivity characteristics are discussed further below.

That is, preferably, to minimize the transport of gases other than CO2, channels, open pores or voids in dense barrier layer16(seeFIGS. 1 and 2) may be minimized to restrict the non-reactive flow of flue gas, which may be understood as a physical flux. Therefore, prior to the process of dense layer deposition, pore size reduction of the upper section of the substrate14may preferably be performed to assist in the maximum pore filling of Li2ZrO3precursors to assist in the achievement of relatively low physical flux. Reference to upper section of the substrate is reference to the feature that for a given thickness of the substrate14, the top ⅓ of such thickness including the surface layer may be understood as the upper section. Commercially available nanometer particle suspension solutions, such as ZrO2sol and organosilicate sols may therefore be applied to the substrate, by repeating dipping, drying, and sintering process. Reference to organosilicate may be understood as silica wherein a portion of the Si—O bonds have been replaced by organic groups.FIG. 6presents a SEM image depicting a completed dense layer with no visible channels on the same substrate as shown inFIG. 3. That is, the upper section is such that it predominantly (50% or more) of the pores that may be present are less than or equal to 5.0 μm, more preferably, less than or equal to 1.0 μm. This process may therefore increase CO2contact with the Li2ZrO3according to Equation 1 above.

Furthermore, a layer of eutectic mixture of Li2CO3:K2CO3(80:20 mole ratio) may be preferably applied on the surface of the substrate14before application of layer18(see below). This layer may have a thickness of less than or equal to 10.0 μm, or in the range of 0.1-10.0 μm. It is also worth noting that such carbonates may preferably be prepared in an organic solvent as a suspension and applied on the substrate by repeating dipping sintering at the melting point to seal all the minor cracks and pinholes that might be formed during the process of providing dense barrier layer16. Gas tightness may be evaluated before the presence of layer18. A substrate layer14so treated with the referenced eutectic mixture may preferably have a gas flow through rate of close to 0%, +/−1.0% of that in the feed, and within the range of measurement sensitivity.

Layer18generally serves as the initial layer to retain CO2from the flue gas and minimize the entry of other gases. Layer18may preferably have a thickness of 100 μm to 500 μm. The CO2absorbed in layer18may build a high partial pressure as the driving force for CO2to travel through the membrane. Two deposition methods for producing layer18on barrier layer16may be employed, include dip coating and chemical spray deposition, as more fully described below. This also may provide for control of the particle sizes and thickness of the deposited Li2ZrO3.

Dip Coating

Synthesis of Li2ZrO3sorbent with controlled particle size is preferred for layer18within the high temperature CO2separation membrane herein. A solid state method of preparing K-doped Li2ZrO3with the optimal CO2absorption kinetics is preferably employed. It may be a mixture of reagent grade K2CO3, Li2CO3and ZrO2solids of small particle sizes, in proper molar ratios, or a mixture of water solutions of K2NO3, LiNO3and zirconyl nitrate [ZrO(NO3)2.xH2O], in proper molar ratios, and calcined at 700-800° C. to yield Li2ZrO3sorbent in the favorable tetragonal phase. The particle size of the resulting solid sorbent may be further reduced to <1.0 μm by wet milling. Organic binder, such as polyvinyl alcohol (PVA) may be used to mix with the solid sorbent in water as a dip coating mixture. Dense layer coated substrate is manually dipped into the mixture for a specified time, typically 1 second, and dried at 80° C. The dipping and drying process is repeated 5-7 times before the product is then sintered. The particle size of Li2ZrO3will be controlled by the particle size of starting ZrO2, since during the calcination only ZrO2particles remain in a solid state due to its high melting point (2700° C.), while Li2CO3forms a eutectic molten carbonate. The low calcination temperature applied, 700-800° C., favors the formation of the tetragonal phase for the optimal CO2absorption kinetics.

Chemical Spray Deposition

The precursors of Li2ZrO3, LiNO3, KNO3, and ZrO(NO3)2, in proper molar ratio, are dissolved in organic solvent and sprayed in fine droplets through an atomizing nozzle, with a stream of warm air, to deposit on the surface of the dense layer16deposited substrate at the boiling point of the solvents, approximately 60° C. After repeated spraying to reach the desired thickness, the spray-deposited ceramic substrates are then calcined in air. The calcination condition is the same as that for the dip coating process. The Li2ZrO3sorbents formed in the spray coating has CO2absorption kinetics that are again comparable to that by solid state as shown inFIG. 5. The chemical spray deposition technique combines the formation and deposition of Li2ZrO3in one step instead of two as in the dip coating method.

Outer Layer Deposition

The porous cap outer layer coating20can be optionally deposited on top of Li2ZrO3sorbent layer18to further improve the structural integrity of the membrane at high temperature reactive interaction with CO2. This coating having porosity ranging from 10 to 90% will consist of nonreactive ceramics such as alumina (Al2O3) or zirconia-based materials ZrO2or yttria-stabilized zirconia (YSZ). This layer preferably has a thickness ranging from 50 μm to 250 μm and may be deposited by high temperature high velocity powder plasma spray, HVOF, thermal spray, detonation (D-gun), CVD, or alternatively, by sol-gel, wet spray and other suitable powder application techniques followed by sintering in a suitable gas atmosphere at appropriate temperature and pressure. The SEM images of the cross section of a typical completely assembled membrane is shown inFIGS. 7 and 8for dip coating and chemical spray deposition, respectively.

Membrane Performance

The performance of the membrane as a high temperature CO2separation membrane may be evaluated in a test system as shown inFIG. 9. Membranes may be evaluated at temperature of 400-630° C. under the conditions of anticipated flue gas in a coal-fired power plant; CO2of 10-20%, and differential pressure 0-5 psig. Selectivity of CO2/N2and CO2permeability are the two parameters for measuring the effectiveness of a membrane as a CO2separator.

For the evaluation of the membranes, two parameters were measured: the CO2gas permeability and CO2/N2selectivity. The quantity of CO2permeated through the membrane via various mechanisms is defined as the permeability or permeation:
Permeability (permeation)=(Mp×F1×(P/(R×T))/(A×P1×60)   [Eq. 3]

in the units of mol−2s−1mPa, or
Permeability (GPU)=(((Mp×F1×(P/(R×T))*V)/(A×P1×60))/(1 E−6)   [Eq. 4]

in the units of cm3(STP)cm−2s−1cmHg−1as conventionally referred to as gas permeation unit (GPU)

The ability of a membrane to separate gases A and B is defined as Selectivity:
Selectivity(α)=(YA/YB)/((PhXA−P1YA)/(PhXB−P1YB))   [Eq. 5]

In the literature, a more conventional expression of the separation of two gases through the membrane is called the membrane separation factor (SAB), which is a measure of the quality of separation that the membrane provides:
SAB=(YA/YB)/(XA/XB)   [Eq. 6]

When the sweep side pressure approaches to zero, i.e, P1→0, the selectivity of the membrane for gases A and B is equal to the separation factor. In this disclosure, SABis used as the selectivity and the GC responses (the peak areas) of CO2and N2in the gas streams for the computation of permeability.

Membrane Testing Results

Tables 1 to 4 presents typical selectivity and permeability data for the assembled membranes herein illustrated generally inFIGS. 1 and 2and evaluated under different conditions expected in a coal-fired power plant: CO2at 10-20% by volume, and the differential pressure between the feed and sweep (permeate) sides in 0-15 psig. Comparisons of the data are depicted inFIGS. 10 to 13. The selectivity values of CO2/N2obtained are temperature and differential pressure dependent. A relatively high selectivity, 2 to 20 at 500° C. is observed.

TABLE 1Selectivity and Permeability of Membranes - Performance under VariousDifferential Pressure between the Feed and Permeate Sides(Test Condition: CO2at 10.9% in N2, 500° C.)Differential Pressure (psig)Membrane051015CX2Selectivity6.895.003.162.51Permeability (GPU)25.737.753.365.6CX3Selectivity20.012.310.18.81Permeability (GPU)24.838.946.456.9Membranes:CX2: 50% porosity substrate with dip coating for layer 18 (FIG. 1).CX3: 50% porosity substrate with chemical spray coating for layer 18 (FIG. 1).

TABLE 2Selectivity and Permeability of Membranes - Performance under VariousDifferential Pressure between the Feed and Permeate Sides(Test Condition: CO2at 18.8% in N2, 500° C.)Differential Pressure (psig)Membrane051015CX2Selectivity2.081.751.591.71Permeability (GPU)29.838.969.983.9CX3Selectivity5.533.932.192.63Permeability (GPU)21.026.930.228.4MembranesCX2: 50% porosity substrate with dip coating for layer 18 (FIG. 1)..CX3: 50% porosity substrate with chemical spray coating for layer 18 (FIG. 1).

The foregoing description of several methods and embodiments has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the claims to the precise steps and/or forms disclosed, and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.