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[0001] The invention is in the field of chemical and physical processes for the production and use of stabilized calcium oxide aerogel sorbents for CO2 capture at high temperature.


[0002] Alternative methods have been described for the preparation and use of various metal oxide aerogels, such as silica, alumina (AI2O3) aerogels, magnesia and calcium oxide nanoparticles (Teichner et al., Inorganic oxide aerogels, Advances in Colloid and Interface Science (1976), 5(3), 245-73; US4550093; US4717708). These materials generally have very high surface areas and are often excellent sorbents for a range of substances. In general, metal oxide aerogels and nanoparticles show much more reactivity than the corresponding metal oxides. These materials may be prepared as a single metal oxide, or as composites (see US3963646, US4469816, and US6770584).

[0003] Increasing amounts of CO2 in the environment have given rise to the need to find easy and effective ways to capture this gas without producing toxic by products. CO2 has extremely high thermal stability, and can be converted to value-added products by known processes or stored in underground fields. For example, the capture capacity of calcium oxide has been utilized for CO2 removal at high temperatures through carbonate formation. However, the weak structure of calcium oxide is prone to sintering, thereby significantly decreasing capacity during extended cyclic performance. Many approaches have been proposed to maintain this capacity for long periods of time, but problems remain.

[0004] While capturing techniques appear promising and have favorable thermodynamics, the cost of these techniques has been considerable, owing in part to the fact that, to be effective, the capture reagents must be very finely divided for maximum surface area. Moreover, these reactions are non-catalytic and depend entirely on molecular reaction at the surface of the reagents. There is accordingly a real and unsatisfied need for reagents with enhanced capture efficiencies.

[0005] US2010/0139486A disclosed the design and development of novel calcium-oxide-based refractory sorbents synthesized by flame spray pyrolysis (FSP) for CO2 capture. FSP, as used herein, refers to a technique for converting precursor droplets into solid nanoparticles in flames. FSP allows for the controlled synthesis of nanoparticles with high specific surface areas. Ca-naphthenate precursor and the calculated refractory dopant precursor are dissolved in xylene and fed by a syringe pump through the spray nozzle. The most stable sorbent (40 wt.% ZrO2-60 wt.% CaO) gives a CO2 capacity of 10.76 mole/kg in an extended cyclic operation of 50 without any activity loss. Carbonation was conducted at 700 °C in 30% C02 for 30 min.

[0006] US2010/0311577 describes a method for making inert nanoparticle-doped porous CaO sorbent by physically dry mixing and decomposing calcium acetate (Ca(CFl3COO)2) or calcium oxalate (CaC204) with inert nanoparticles, to form a nitrate-free mixture, and calcining this mixture to form a stable porous microstructure with CO2 sorbent properties. The C02-capture performance of MgO-doped CaO (42 wt. % MgO-58 wt. % CaO) sorbents has been described as having a capacity of 8.5 mole/kg in a multicyclic operation of 100 without any activity loss. Carbonation and decarbonation (calcination) were performed at 758° C in 100% CO2 for 30 min and at 758° C in 100% Fie for 30 min respectively. US6087294 describes the preparation of calcium oxide nanoparticles by the sol-gel method followed by supercritical extraction of solvents from the gel. These nanoparticles were then coated with reactive elements. US6740141 disclosed that iron-oxide-coated calcium oxide nanoparticles (Fe203.Ca0) were excellent adsorbents for a variety of target substances, such as CO2 and chlorinated hydrocarbons from gas streams at low temperatures.


[0007] Highly reactive nanosized calcium oxide particle compositions are provided, including compositions comprising materials that are oxides or hydroxides of the elements of groups IIA or the transition metals. The materials may be intimately mixed on a nanosized scale. Select compositions showed very small average particle sizes and consistently large surface areas. Methods for the synthesis and fabrication of these compositions are provided, along with methods for the use of these compositions as sorbents. In select embodiments, nanosized zirconia-stabilized calcium oxide particle sorbents with a BET surface area on the order of 150 m2/g are disclosed. Methods for preparing calcium hydroxide and oxide compositions are described herein and in CA3044223 (claiming priority to U.S. provisional Application No. 62/675,505, filed May 23, 2018). Calcium oxide nanoparticles are synthesized in a high surface area alumina aerogel (2000 m2/g), as follows:


Calcium methoxide + Methanol + co-solvent > Calcium hydroxide Alcogel

Supercritical Vacuum Dehydration

Drying (450°C)

=> Calcium hydroxide Nanoparticles => Dehydrated Calcium

[0008] In select embodiments, solid compositions may be prepared by mixing solid oxides and/or hydroxides with large surface areas. These compositions exhibited excellent performance in the high-temperature carbon capture process. These compositions may for example include materials selected from the oxides and/or hydroxides of elements of Groups IIA, IMA, and the transition metals. The compositions used as base sorbents were synthesized by preparing calcium alkoxide slurries, which were then hydrolyzed to obtain an alcogel. The alcogel was first supercritically dried and then calcined at high temperature to yield calcium oxide. The exemplified calcium oxide nanoparticles comprise a uniform dispersion of nanosized particles of calcium oxide, characterized as substantially fluffy clusters of particles, having a BET surface area in the range of 80 to 150 m2/g, a pore volume in the range of 0.5 to 2.5 cm3/g, and a bulk density in the range of 0.01 to 0.05 g/cm3

[0009] Calcined calcium oxide was impregnated and/or core-shelled with an inorganic and organic metal precursor to exemplify alternative solid compositions. These compositions were used as sorbents for carbon dioxide capture from flue gases at high temperature.

[0010] The prepared sorbent of calcium oxide nanoparticles stabilized with zirconia exhibited an almost 100% C02-capture efficiency. The loss of CO2 capacity of the zirconia-stabilized sorbents was shown to be near 10-20% during multi-cycle operations.


[0011] Figure 1 is a graph showing the particle size distribution of Sample NC1 (average size, 27.5 nm).

[0012] Figure 2 is a graph showing the particle size distribution of NC1/10% Zr02 (average size, 348.5 nm).

[0013] Figure 3 is a graph showing thermogravimetric analysis (TGA) performance of NC1 sample at 675 °C (CO2 capture).

[0014] Figure 4 is a graph showing thermogravimetric analysis (TGA) performance of NC1/10% Zr02 sample at 675 °C (CO2 capture).

[0015] Figure 5 is a graph showing thermogravimetric analysis (TGA) performance of NC1/10% Zr02 -P123 sample at 675 °C (CO2 capture).


[0016] Methods are provided for preparing a series of calcium compounds, including nanoscale oxide and hydroxide particulates with very high surface areas. In an initial step, calcium alkoxide solutions are prepared in a suitable solvent. Calcium alkoxide may for example have the formula (RO)3Ca, where each R is a C1 -C2 straight chain alkyl group. Exemplary alkoxides comprise methyl and ethyl groups. The calcium alkoxide solution is then hydrolyzed to yield a calcium hydroxide alcogel. Thereafter, the alcogel is dried under supercritical conditions, at a temperature over the supercritical point of the solvent, to yield a calcium hydroxide nanoparticle. Supercritical drying may for example be carried out for a period of from 1 .5-3.5 hours.

[0017] The calcium hydroxide nanoparticles may in turn be subject to thermal dehydration, to provide a dehydrated calcium oxide nanoparticle comprising calcium hydroxide. The thermal dehydration may for example be carried out at a temperature of 300-500° C, for example for a period of 1-3 hours under vacuum. The nanosized calcium hydroxide prepared in this way has a high BET surface area, in some embodiments of at least 140 m2/g.

[0018] The dried or dehydrated nanoparticle compositions may be used as sorbents, for example for chemical adsorption of gases. The dehydrated nanoparticles may be calcined to provide particulate calcium oxide compositions. The calcination may for example be performed at a temperature of 750-850° C for a period of 2-5 hours. The nanosized calcium oxide prepared in this way has a higher BET surface area, in some embodiments of at least 20 m2/g compared with a commercial calcium oxide with BET surface area less than 1 m2/g.

[0019] Compositions provided by the foregoing methods may be used as solid sorbents, for example for the removal of target materials through physisorption or chemisorption. Exemplified processes of this kind involve contacting the selected compositions with target materials such as CO2 and SO2 (exemplary of flue gases containing CO2). The following examples also illustrate that nanoparticles-derived compositions with high surface areas calcined at high temperatures have sufficient surface and stability to provide solid sorbents as part of the calcium-looping process.

[0020] The following examples describe select compositions and methods, illustrating only select aspects of the present innovation. Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Numeric ranges are inclusive of the numbers defining the range. The word "comprising" is used herein as an open-ended term, substantially equivalent to the phrase "including, but not limited to", and the word "comprises" has a corresponding meaning. As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a thing" includes more than one such thing. Citation of references herein is not an admission that such references are prior art to the present invention. Any priority document(s) and all publications, including but not limited to patents and patent applications, cited in this specification, and all documents cited in such documents and publications, are hereby incorporated herein by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein and as though fully set forth herein. The invention includes all embodiments and variations substantially as herein before described and with reference to the examples and drawings.

EXAMPLE 1 : Preparation and Characterization of Calcium Oxide Nanoparticle Compositions

[0021] In this example, calcium oxide nanosized powder without/with zirconia stabilizer was synthesized and stored. The synthesis consisted of four main steps, and was followed by characterization of the materials:

Synthesis of calcium oxide alcogel without/with zirconia stabilizer

[0022] This step comprises the hydrolysis of a calcium alkoxide solution (Ca(RO)3 + alcohol + co-solvent). The chemicals used in the synthesis were directly obtained from a commercial source without further purification. Calcium methoxide (Aldrich) was added to a 500 ml beaker to prepare 10% wt. calcium methoxide slurry in methanol (Aldrich). The Ca(CH30)2 slurry was dispersed in a solution of toluene co solvent (Aldrich) (Vol. of tolueneA/ol. of methanol of 5-8) to form a grey slurry. A specific amount of Dl water (1.5-3 moles of H2O per mole of Ca(CH30)2) was then added drop-wise to the slurry to form the calcium hydroxide alcogel. The reaction mixture was then stirred at room temperature for 1 -3 days for ageing. During this time, the mixture remained an opaque gel, but was dilute enough to maintain a semi-liquid state. The step of admixing zirconia stabilizer comprises of adding the specific amounts of zirconium precursors (depending on zirconia percentages in the final sorbents) such as ethanolic zirconium tetra-butoxide solution to calcium alkoxide solution prior to hydrolysis step.

Supercritical drying of the alcogel

[0023] The hydroxide alcogel was transferred to a 100 ml glass liner of a Parr high-pressure batch reactor. The reactor was first flushed with nitrogen and then pressurized to 200-250 Psi with nitrogen. The reactor was slowly heated without stirring from room temperature to 250-270°C for a period of 1-3 hours. As the reactor was heating, the pressure was increased from 200-250 Psi to 600-800 Psi. After the reactor reached the target temperature, it was kept at that temperature for a while and then flashed to the atmosphere quickly to remove the solvent vapors. Afterward, the heating jacket was removed, and the reactor was flushed with nitrogen for 5 minutes to remove the remaining solvent vapors. The reactor was then allowed to cool down to the room temperature.

Thermal dehydration of calcium hydroxide

[0024] Data from thermogravimetric analysis (TGA) confirmed that calcium hydroxide lost the highest weight at a temperature of 400-450°C to convert to dehydrated calcium oxide. The fluffy white calcium hydroxide powder was placed into a BET tube connected to a degassing vacuum line of the BET instrument. The tube was evacuated at room temperature for a while to 10 pHg vacuum. Afterward, the tube was slowly heated from room temperature to 400-450°C at a ramp of 10°C/min under dynamic vacuum. After the heat treatment was complete, the degassing line was turned off and the tube cooled down to room temperature under dynamic vacuum. After this step, the dehydrated calcium oxide had a light white color.


[0025] The fresh and heat-treated samples were calcined to the temperature up to 850 °C to obtain nanoparticle-derived calcium oxide, which exhibited a higher surface area than that of the commercial calcium oxide. Specifically, calcium oxide nanoparticles were calcined in a muffle furnace without gas flow and at high temperatures. In an exemplary procedure, first, sample NC1 was subjected to a dynamic vacuum using the BET instrument degassing port at 450°C. The BET surface area without vacuum dehydration measured 117 m2/g as shown in Table 1 . Generally, at temperature dehydration of 450°C, the surface area was highest (140 m2/g). The significant decrease in surface areas at temperatures above 500°C can be explained by sintering. Calcination of NC1 at 850°C, after vacuum dehydration reduced its surface area to 28 m2/g. The additional five samples vacuum dehydrated at 450°C showed different surface areas. The highest surface area belonged to sample NC1/10% Zr02, as shown in Table 1. Two samples of NC2 and NC3 calcined at the higher temperature of 850°C showed low surface areas of less than 20 m2/g Characterization

[0026] The Brunauer-Emmett-Teller (BET) surface areas and pore size distributions were measured using nitrogen adsorption and desorption isotherms at -196°C on a Micromeritics 2020 volumetric adsorption analyzer, using pressure values ranging from 1 to 760 mmHg. The samples were degassed at 150-450°C for at least 2-5 hours. The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) pore size and volume analysis method.

[0027] Thermogravimetric Analysis (TGA) was used to determine the cyclic performance of Ca(OH)2and the Zr02-stabilized Ca(OH)2 during CO2 capture. These studies were conducted under nitrogen and CO2 flows. To measure weight loss and gain, the samples were placed in a crucible and heated at a rate of 40 min from room temperature to 850°C. The instrument used was a thermogravimetric analyzer, the TGA-STA-6000 from the PerkinElmer Company.

[0028] The Malvern zetasizer (nano-series, Nano-ZS) dynamic light scattering (DLS) instrument was used to measure the size of alumina aerogel particles. This instrument uses a 633 nm wavelength laser through which the sample particles scattered light in all directions, including towards a detector. The change in the movement of the particle and a correlation function were used in the software (version 7.12) to draw size distribution graphs.

[0029] In accordance with the foregoing methods, several embodiments were prepared using calcium methoxide as a starting material, methane solvent and toluene co-solvent, and by varying the concentration of the solutions, time of ageing and duration of supercritical drying. All these parameters influence the surface area and particle sizes of the resulting samples. Results are shown in Table 1.

Table 1. Result of BET surface areas of the prepared samples

[0030] Samples NC1 and NC1/10% ZrC , in which calcium tri-methoxide and zirconium tetra-butoxide were used as the starting material, exhibited the highest surface areas. In order to obtain samples with the highest surface areas, sufficient co solvent/solvent ratios are required to be provided for the solutions. The ratios of alcoholic solvent (e.g. methanol) to aromatic co-solvent (e.g. toluene) were varied and found to have an effect on the surface area. This ratio was over 5 for samples samples NC1 and NC1/10% Zr02, and below 3 for samples NC2 and NC2/10% Zr02 which resulted in lower surface areas. It was found that decreasing the ratio resulted in a significant decrease in the surface area, changing from 200 to 80 m2/g as for sample NC3. This sample had a methanol/toluene ratio of less than 2. The amount of calcium alkoxide used ranged from 0.5 to 1.3 g and solution mixture concentration varied

between 10 and 26 g alkoxide/L, corresponding to 0.3-0.7g CaO/L. The lowest concentrations resulted in the highest surface areas as for all samples in Table 1 .

[0031] Solution concentration plays a significant role in increasing surface area. The slurry of calcium methoxide in the mixture solution was much clearer than in the individual alcohol solution. Using different alcohols resulted in the same surface areas. The main difference among the samples prepared by the same alcohol was the ageing time and the toluene/alcohol ratio. It was found that changing the solvent resulted in reducing the surface area by half. The other important factor that affected the performance of the samples was storage time. A long storage time resulted in the degradation of the fresh sample due to the very reactive surface triggering the particle growth and strong adsorption of gases. The surface area of sample NC1 decreased from 116 to 75 m2/g after one week of storage. Therefore, to avoid the effects of this instability, a fresh sample obtained immediately after the supercritical drying process may beneficially be thermally converted to more stable phases. Particle sizes of three samples NC1 , NC2 and NC1/10% ZrC were measured by the zetasizer particle analyzer in a range of 20400 nm (Figures 1 ,2 and 3). The particles of the calcined CaO nanoparticles grew to larger values (range of 500-800 nm) due to sintering.

EXAMPLE 2: High temperature carbon capture

[0032] In this example, the prepared calcium oxide aerogel was used for high-temperature CO2 capture from flue gases, and embodiments were exemplified that used zirconia (Zr02) stabilized calcium oxide CaO for high-temperature CO2 capture. This sorbent adsorbs CO2 at 650-700 °C by the following reaction to form CaC03: CaO + CO2 <® CaC03 (1 ).

[0033] Regeneration of CaC03 occurs at 850-900°C with the release of CO2. Based on reaction (1 ), the theoretical amount of CO2 adsorbed per kilogram of calcium oxide is calculated at 17.857 mole or 785.7 gram. Pure CaO is a weak substance that must be modified to achieve high C02-capture efficiency for long cyclic operations. CaO stabilized by zirconia ceramic was found to be an efficient sorbent due to the zirconia’s high thermal barrier coating, high physical strength and significant resistance to sintering. The C02-capture efficiency of 10%-20% CaO-stabilized zirconia was illustrated using various preparation methods such as: (1 ) mixing stabilizer precursor during alcogel preparation, (2) incipient wetness impregnation (IWI, also called capillary impregnation or dry impregnation) of nanoparticles with stabilizer precursor solution, comprises of zirconium oxynitrate dissolved in sufficient amount of water which only covers the particles pores and surfaces. Next, the partially wet particles are dried and (3) shelling the nanoparticles or calcined nanoparticles surface by the core-shell method using two types of surfactants, P123 and TMA, comprises of an ethanolic zirconium precursor solution containing zirconium tetra-butoxide, acetylacetone, surfactant and water is used for shelling. Nanoparticles are dipped in this solution several times, between each dip, nanoparticles are dried.

[0034] All sorbents were tested at a carbonation temperature of 675°C and decarbonation temperature of 850°C. The exemplary CaO sorbent showed high reactivity compared to other calcium-based sorbents tested at the higher calcination temperature of 850°C. The sorbent fabricated by mixing zirconia precursor (zirconium n-butoxide) with calcium hydroxide alcogel solution with a surface area of 190 m2/g showed the highest C02-capture efficiency and stability compared with the other fabricated sorbents by different preparation methods. The pure CaO sorbent prepared by the same sol-gel method with a surface area of 140 m2/g showed the lowest CO2-capture stability due to having a high sintering characteristic. Higher sintering of the sorbent resulted in converting the particulate-active CaO precursor to an agglomerated inactive precursor of CaC03 that cannot be completely regenerated at the selected calcination conditions.

[0035] The zirconia-stabilized calcium oxide sorbent (sample NC1 ) provided higher surface area to disperse the active component (zirconium) on the surface more effectively. These results explain that the highest C02-capture efficiency and stability (less activity loss) was displayed by the 10% Zr02 stabilized CaO. Results are shown in Table 2. The pelleting of samples at a size of 1 mil using the Parr pellet press (up to 1000kg. total force can be exerted on the punch) decreases the activity to 10-15% due to the gas diffusion barrier. Figures 4,5 and 6 show the cyclic operations of three sorbents NC1 , NC1/10% Zr02 and NC1/10% Zr02 (P123). As can be seen in Figure 5, the stabilized sorbent remained stable after 20 carbonation/calcination cycles.

Table 2. Result of CO2 capture performance of the prepared sorbents