Patent Application
20230110470
This invention relates to systems and methods for separation of oxygen from a gas mixture.
Production of renewable energy can address increasing energy demand and global climate concerns. One suitable technology is the concentration and storage of solar power (CSP) for generating electricity. Thermal energy from the sun can be stored as chemical energy in a process called solar thermochemical energy storage (TCES). TCES can be used to store energy while the system is capturing solar radiation, and release energy at other times for continuous power generation. TCES uses reversible chemical reactions for these store-and-release energy cycles. Reversible redox reactions involving oxygen gas constitute one class of these TCES reactions.
This disclosure describes systems and methods for a temperature swing adsorption (TSA) process in which oxygen is captured from a gas mixture to produce substantially inert gas streams with low pO2 during the sorption step of the process. The sorbents can then be regenerated at a higher temperature. The TSA process includes adsorption, heating, counter-current desorption, and cooling. The oxygen separation is achieved using an oxygen selective sorbent, such as YBaCo4O7+δ (YBC114, where oxygen non-stoichiometry (δ) represents the amount of oxygen adsorbed).
YBC114 is capable of capturing O2 within a narrow temperature range from 275° C. to 325° C. and desorbing most of O2 at a temperature higher than 400° C. These sorption and regeneration temperatures are both lower than other TCES materials such as perovskites or redox metal oxides. YBC114 also shows consistent sorption and desorption performance during cycles and is thus suitable for temperature swing adsorption TCES processes.
Oxygen can be removed from the TCES reactor as it is released, thereby decreasing the oxygen partial pressure (pO2) and increasing the reduction capacity of the redox material to a sufficient extent at an acceptable temperature. Oxygen removal is achieved by purging the reactor with an inert sweep gas (ISG). Oxygen separation is characterized by relatively low cost, flexible operation, and efficient regeneration. Oxygen can then be separated from the sweep gas (typically a mixture containing N2 and O2) with an oxygen-selective sorbent. The sorption and desorption (regeneration) can be operated in a cyclic manner with a multiple bed configuration, so that the desired oxygen-depleted product is substantially continuous.
In a first general aspect, an oxygen separation method includes a) contacting a sorbent with a first gas stream and b) adsorbing oxygen in the first gas stream with the sorbent. The sorbent is selective for oxygen and the adsorbing occurs at an adsorbing temperature between 275° C. and 325° C. The first general aspect further includes c) heating the sorbent to a desorbing temperature greater than 400° C., and d) desorbing a majority of the oxygen to yield a second gas stream. A difference between the desorbing temperature and the adsorbing temperature is less than 400° C.
Implementation of the first general aspect can include one or more of the following features.
Some implementations further include cooling the sorbent to a temperature between 275° C. and 325° C. after desorbing the majority of the oxygen. Some cases further include repeating a) through d) after cooling the sorbent. In some cases, contacting the sorbent with the first gas stream includes flowing the first gas stream in a first direction through the sorbent. In some implementations, desorbing the majority of oxygen includes subjecting the sorbent to a vacuum. In some implementations, desorbing the majority of oxygen includes contacting the sorbent with a purge gas. The purge gas can include carbon dioxide, steam, or flue gas. In some cases, the flue gas is recycled flue gas. In some implementations, desorbing the majority of the oxygen includes flowing the purge gas in a second direction through the sorbent. In some cases, the second direction is opposite the first direction. In some implementations, desorbing the majority of the oxygen includes regenerating the sorbent. Certain implementations further include providing the second gas stream to an energy storage reactor. In some cases, adsorbing oxygen in the first gas stream further includes increasing or decreasing a pressure of the first gas stream. Some implementations further include contacting an additional sorbent with an additional gas stream during b), c), or d).
In a second general aspect, an oxygen separation system includes a sorption bed including a sorbent. The sorbent is oxygen-selective. The second general aspect further includes a heater configured to heat the sorption bed, an oxygen analyzer, a first conduit configured provide an input gas to a sorption bed, a second conduit configured to provide processed input gas from the sorption bed to the oxygen analyzer, a third conduit configured to provide a purge gas to the sorption bed, and a fourth conduit configured to provide processed purge gas to the oxygen analyzer. The first conduit and the third conduit are configured to flow the input gas and the purge gas flow in opposite directions through the sorption bed.
Implementations of the second general aspect can include one or more of the following features.
In some cases, the sorption bed further includes an inert packing material. In certain implementations, a particle size of the inert packing material exceeds a particle size of the sorbent. In some implementations, the sorbent includes YBaCo4O7+δ. In certain cases, the sorption bed is a fixed bed. Some implementations further include an additional sorption bed configured to operate in parallel to the sorption bed. In some cases, an energy storage reactor can include the second general aspect.
The disclosed methods and systems provide multiple advantages related to energy and cost, performance, operation, versatility, and safety. In particular, the disclosed methods and systems provide an economical and efficient option to create and maintain a low pO2 during the thermal reduction step and reduce the overall energy cost for energy storage. Other advantages include the cost effectiveness of the fixed bed system and materials of resources for regeneration of the sorbent, and relatively low sorption temperature. In addition, oxygen sorption selectivity exceeds that of over other gas components, owing at least in part to the oxygen storage properties of the sorbent material. Oxygen can be separated effectively even with an incoming mixture with low oxygen concentration. The desired low pO2 can be reached quickly, and the production of inert sweep gas can be continuous with a multiple bed configuration. Oxygen separation performance is consistent during TSA cycles. The TSA process can be automatically operated with electrical or pneumatic valves, and the temperature and gas flow rate can be remotely controlled with a commercially available platform. The separation unit can be used with various sorbents or for other fixed bed sorption processes. The process has a comparatively low operating temperature for a TSA process, the gases are at near ambient pressure, and the overall risk of the oxygen separation system is low.
This disclosure describes systems and methods for a temperature swing adsorption (TSA) process in which oxygen is captured from a gas mixture to produce substantially inert gas streams with low pO2 during the sorption step of the process. The sorbents can then be regenerated at a higher temperature. The TSA process includes adsorption, heating, counter-current desorption, and cooling. The process can be carried out in a sorption system with one or more sorption units. For a system containing a single sorption unit, the sorption step can be intermittently or periodically stopped to permit regeneration of the sorbent bed. When multiple sorption units are employed in parallel, one unit can be in sorption operation while other units are undergoing heating, desorption, or cooling. In some embodiments, adsorption cycles are repeatedly carried out in a manner such that production of the desired oxygen-depleted product is substantially continuous.
An example of a suitable sorbent is YBaCo4O7+δ (YBC 114, where oxygen non-stoichiometry (δ) represents the amount of oxygen adsorbed), an oxygen ion conductor. Due at least in part to its crystal structure and the changeable valence of its Co ions, the material is capable of capturing oxygen within a narrow temperature range from 275° C. to 325° C. and desorbing most of the oxygen at a temperature higher than 400° C. For a TSA operation between 275° C. and 400° C., at a pO2 of 21 kPa, the working capacity of YBC114 is 0.45 mol O2/mol YBC114. The sorbent material also shows consistent sorption and desorption performance during cycles, thus advantageous for the TSA process.
The oxygen separation system includes one or multiple sorption units depending on the design, one or multiple heaters for controlling the temperature of the sorption unit(s), measurement devices for monitoring or controlling the pressure, temperature, and gas flow rates at the inlets or outlets of the sorption unit(s), and pneumatic valves for automatic operation of the process. The input of the sorption unit(s) connects to the reduction sweep gas coming from the energy storage reactor. During the sorption step, the input gas mixture flows through the sorption bed for oxygen sorption. The resulting processed input gas exits the sorption bed as an inert gas stream. For desorption, the sorption bed can be regenerated at a higher temperature with a counter-current purge flow. The oxygen concentration of the product inert gas can be monitored by an oxygen analyzer.
A schematic of an oxygen separation system 100 is shown in
The equation below shows the reaction for sorption and desorption of O2 on YBC114:
The four-step temperature swing absorption (TSA) process includes adsorption, heating, counter-current desorption, and cooling. The TSA process can be carried out in a sorption system containing multiple sorption units in parallel, where one unit can be in sorption operation while other units are undergoing heating, desorption, or cooling. The adsorption cycles are repeatedly carried out in a manner such that the production of the desired oxygen-depleted product is substantially continuous.
A process flow diagram (PFD) of the oxygen separation design is shown in
The properties used for the adsorber design calculation include sorbent densities, void fractions, sorption isotherms, kinetics, and fixed bed dynamics. Since the system will be operated in TSA mode, heat transfer modeling was used to optimize the column dimensions to minimize the thermal resistance from the heating source to the sorbent material. A 1-D transient heat transfer calculation can be performed on the radial direction of the sorption bed. The modeling equation is shown as follows:
Where T is temperature, t is time, k is thermal conductivity, c is heat capacity, p is density, and x is the radial distance from the center of the pipe.
In one example, a TSA process was conducted in a single-bed lab oxygen separation system with the following parameters. The sorption column had an internal diameter of 0.85 inches and a length of 24 inches. The column was packed with 40 g of YBC114 and 160 g of inert packing material (silica beads) to increase the average particle size and reduce the column pressure drop. The sorption step was carried out at a temperature of 300° C., and the regeneration was carried out at a temperature of 500° C. The input gas was a gas mixture with 10% oxygen and 90% nitrogen to mimic the reduction sweep gas. The flow rate of the gas mixture into the system was 200 ml/min (STP). The TSA cycle consisted of 20 min sorption, 20 min heating, 40 min desorption, and 40 min cooling. The oxygen concentration of the processed input gas was lower than 100 ppmv (equal to a pO2 of 10 Pa if the total pressure is 1 bar) without an obvious increase during the whole sorption step, and the performance was consistent during cycles.
Implementations of the oxygen separation system include providing an inert sweep gas with low pO2 for thermal reduction in TCES reactors, and production of oxygen-free inert gas (product of sorption) or oxygen-concentrated gas (product of desorption) for use in industrial or agricultural applications. Since the process is operated at relatively high temperatures, the oxygen-concentrated gas can be produced with a sweep gas like CO2 and/or steam for gasification systems or recycled flue gas for oxy-combustion systems. When the oxygen sorption capacity of the sorption material varies with pO2, a pressure swing adsorption (PSA) or a vacuum swing adsorption (VSA) can also be applied to the separation unit to increase or decrease the pressure of the input gas, respectively. Pure oxygen can be produced using a vacuum instead of a sweep gas during the desorption step of the process.
The oxygen separation system is illustrated in detail by the following examples. Unless otherwise indicated, parts and percentages are on a volume basis.
The YBC114 sample was synthesized with an EDTAsol-gel method. Stoichiometric amounts of Y2O3 (Alfa Aesar, 99.995%), Ba(NO3)2 (Acros Organics, 99+%), Co(NO3)2 6H2O (Sigma-Aldrich, 99+%) were mixed and dissolved in excess HNO3 solution (Acros Organics, 68-70%), followed by the addition of EDTA (Honeywell, 99+%). The molar ratio of EDTA and all the metal ions (M+) is 1.5:1. The solution was then neutralized with NH3H2O (Thomas Scientific, 28% in water) until the pH of the solution was about 8. The solution was dried on a heating plate with stirring until it self-burned to fluffy ashes. The ashes were transferred to a muffle furnace and calcined at 1000° C. in air for 10 hours. After sintering, the resulting samples were cooled back to room temperature rapidly and milled. A thermogravimetric analyzer (TGA, Netzsch TG 209 F1 Libra) was used to investigate the oxygen uptake and release characteristics. The mass of the sample for each measurement was 20-50 mg. All the samples were fully reduced at 400° C. in 20 ml/min nitrogen flow for 2 hours before the measurements. The measurements were performed in a mixed gas flow of oxygen (Matheson, UHP) and nitrogen (Matheson, UHP). The pO2 was controlled by adjusting the ratio of oxygen and nitrogen in the mixed gas flow.
A thermogravimetric analyzer (TGA, Netzsch TG 209 F1 Libra) was used to investigate the oxygen uptake and release characteristics. The mass of the sample for each measurement was 20-50 mg. All the samples were fully reduced at 400° C. in 20 ml/min nitrogen flow for 2 hours before the measurements. The measurements were performed in a mixed gas flow of oxygen (Matheson, UHP) and nitrogen (Matheson, UHP). The pO2 was controlled by adjusting the ratio of oxygen and nitrogen in the mixed gas flow.
The oxygen separation system shown in
The sorption bed was first regenerated with N2 for 24 hours to remove the initial adsorbed O2 content in the particles. The TSA cycles, including sorption, heating, desorption, and cooling, were initiated. For the sorption step, V1 and V2 were opened, and a mixture of N2 and O2 was fed to the sorption bed. For the heating step, V2 was opened, and the sorption bed was heated to the desired desorption temperature For the desorption step, V3 and V4 were opened and sorbent was regenerated by purging in the counter-current direction. For the cooling step, V4 was opened and the sorbent bed was cooled to the desired sorption temperature.
The baseline sorbent material used in the oxygen separation process was YBC114. To assess the oxygen sorption isotherms of YBC114 at different temperatures, the isotherm measurements were conducted at various pO2s with TGA at 275° C., 300° C., and 325° C. A Langmuir-like isotherm model was developed (Sips model) to fit the experimental isotherm data for future process simulation. The equation of the isotherm model is shown as follows:
where q is the uptake amount, qsat is the saturation uptake amount, and b and n are isotherm constants. Values of qsat, b, and n are determined with a non-linear regression model based on the experimental isotherm data at different temperatures and summarized in Table 1. The experimental and simulated isotherm curves are compared in
To determine the working capacities in the TSA process, the isotherm measurement was repeated at 400° C. (potential desorption temperature) and compared with the isotherm profile at 275° C., as shown in
Use of small particles in a packed column can cause a large column pressure drop that may reduce process performance. To mitigate the potential risk of a high pressure drop, the sorbent material was mixed with inert packing material with a larger particle size. The particle mixture was then loaded into the sorption column.
Oxygen sorption breakthrough experiments were performed with YBC114 to determine the processed input gas oxygen concentration profile of sorption and desorption. A breakthrough occurs when the processed input gas oxygen concentration has a significant increase during the sorption step. By definition a breakthrough occurred when the outlet oxygen concentration reached 100 ppmv. The shape of the breakthrough curve contains information about the mass transfer properties of the sorption system. The working capacity of the sorbent material for a dynamic sorption process was also determined by integrating the breakthrough data.
Three consecutive sorption-desorption cycles were performed with identical sorption conditions but different desorption temperatures from 400° C. to 500° C. As shown in
To fully assess the effect of other operating variables except for temperatures (such as pO2 and flow rate) on process performance, a series of breakthrough experiments were performed with varying oxygen concentrations or flow rates. The sorption and desorption temperatures are fixed at 300° C. and 500° C., respectively. Table 2 summarizes oxygen sorption performance at different pO2s or flow rates. The static capacity was measured in previous TGA characterizations, and the dynamic capacity was obtained from the oxygen sorption breakthrough experiments. The mass transfer coefficients were estimated for different pO2s with a linear driving force (LDF) model that typically has a good kinetics prediction of cyclic sorption systems. The equation of the LDF model is shown as follows:
where k is a lumped mass transfer coefficient that includes all the mass transfer mechanisms during fixed-bed sorption, and qsat is the saturation uptake amount that is accessible from the isotherm model. As evident in the table, both capacities and mass transfer coefficients increase with pO2, while the breakthrough time decreases with pO2. As for the effect of flow rate, the sorption capacity inferred from breakthrough data at different flow rates are close, and all of them are less than the static oxygen capacity, which should be expected due to the existence of a mass transfer zone in the fixed bed. The breakthrough time and pressure drop also show reasonable correlations with the flow rates.
To analyze the performance of the oxygen separation unit with YBC114, a TSA operation was performed with the same setup used in the breakthrough experiments. In this process, the sorption bed was first regenerated with N2 at 500° C. for 24 hours, to remove the initial adsorbed oxygen content in the particles. The temperature was then cooled to 275° C. for sorption, and the flow switched to a mixture of N2 and O2 with a pO2 of 10 kPa (i.e., 10% O2). For the next cycle, the TSA was repeated between 300° C. and 500° C. for sorption and desorption.
Process simulations were performed to simulate the TSA operation between 300° C. and 500° C. with an input gas mixture containing 1% oxygen. The simulation model was based on the isotherm data obtained from TGA characterizations and mass transfer coefficients inferred from sorption breakthrough measurements. The TSA process was simulated for 20 cycles to verify if it reached a cyclic steady state (CSS). As seen from F IG. 11, the oxygen concentration profile was consistent from cycle to cycle. The peak value of each cyclic oxygen concentration curve decreases from cycle to cycle, and the rate of decrease gradually levels off. For example, the decreasing rate during the first 5 cycles is in a range of 1 -2%. For the last five cycles (i.e., cycles 16-20), the rate of decrease is reduced to less than 0.2%, where the TSA was considered to have reached CSS.
Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.
Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.
Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.
This application claims the benefit of U.S. Pat. Application 63/240,685 filed on Sep. 3, 2021, which is incorporated herein by reference in its entirety.
This invention was made with government support under DE-EE0008991 awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63240685 | Sep 2021 | US |
