High-Temperature Sorption Process for Air Separation and Oxygen

Ind. Eng. Chem. Res. 2002, 41, 2775-2784

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High-Temperature Sorption Process for Air Separation and Oxygen Removal Zhaohui Yang,† Y. S. Lin,*,† and Y. Zeng‡ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171, and Group Technical Center, The BOC Group, 100 Mountain Avenue, Murray Hill, New Jersey 07974

This paper describes the concept of a new, efficient high-temperature oxygen sorption process based on a perovskite-type ceramic sorbent for oxygen removal and air separation. The new sorption process takes advantage of the unique properties of certain perovskite-type ceramics that can adsorb a large quantity of oxygen, but not other gases, at high temperatures (300-800 °C). The essential principle of this new sorption process is based on the changing of oxygen nonstoichiometry of the perovskite-type ceramics with temperature and oxygen partial pressure. Two highly oxygen-deficient perovskite oxides, La0.1Sr0.9Co0.5Fe0.5O3-δ, and La0.1Sr0.9Co0.9Fe0.1O3-δ, were examined as candidate materials for the oxygen sorption process. Oxygen sorption equilibrium properties were studied by thermogravimetric analysis (TGA) at 500 and 600 °C and oxygen pressures ranging from 1.3 × 10-4 to 1 atm. The oxygen removal performance at 500 and 600 °C was also investigated in a fixed-bed adsorption column. An infinitely large selectivity, a relatively high oxygen sorption capacity, and fast sorption kinetics are the main characteristics of this new type of sorbent. The process can be used to remove trace oxygen from other gases or to produce high-purity nitrogen and ultrapure oxygen from air. Introduction Air separation is mainly carried out by three types of unit operations: (1) cryogenic distillation, by exploiting the difference of relative volatilities of oxygen and nitrogen; (2) membrane separation, by taking advantage of the difference in solubility and diffusivity of oxygen and nitrogen in membranes; and (3) gas adsorption, by utilizing the preferential adsorption of one component over the other on solid sorbents, either by equilibrium or by kinetics. Cryogenic distillation is the most developed process, particularly useful for air separation with high-purity products on a large production scale. Membrane and adsorption processes are suitable for on-site production, which is common for small and medium throughputs. For nitrogen production, membrane process offers the best choice at small scale, whereas adsorption processes are preferred at relatively large scale. For oxygen production, membrane separation is limited to a single-stage process for economic considerations; therefore, the purity of oxygen is limited to 50 mol %. At larger throughputs and higher purities, the economic advantage shifts to adsorption processes.1 With the introduction of pressure swing adsorption (PSA), adsorption processes have gained wide acceptance in industry for gas separation. Among the diversity of the adsorbents and process designs for air separation, two basic types of separations exist: equilibrium-controlled and kinetic-controlled separations. Equilibrium-controlled separation is achieved by exploiting the difference in the adsorption isotherms or the adsorption capacities of the two major components, N2 and O2. In this case, both oxygen and nitrogen have rapid diffusion rates in the adsorbents, and separation * Corresponding author. Tel.: +1 (513) 556-2761. Fax: +1 (513) 556-3473. Email: [email protected]. † University of Cincinnati. ‡ The BOC Group.

depends on the preferential adsorption of nitrogen. For kinetic-controlled processes, separation is accomplished by taking advantage of the faster adsorption rate for oxygen than nitrogen because of the difference in molecular size. In kinetic-controlled processes, the choice of cycle time is critical for the performance of the adsorption process. In air separation, equilibriumcontrolled processes are used mainly for the production of oxygen, whereas the production of nitrogen is very often a kinetic-controlled process. The performance of adsorption system depends highly on the properties of adsorbents. Generally, good adsorbents require: (1) high selectivities, (2) large adsorption capacities, (3) fast sorption and desorption kinetics, and (4) ease of regeneration. For air separation, much progress has been made in developing adsorbents with better sorption properties. In the production of oxygen, zeolite 5A and X, especially the latter, have been extensively used. It was found that zeolites with low silicon/aluminum ratios are more selective for the adsorption of nitrogen.2 Nitrogen has a greater quadrupole than oxygen. Thus, the interaction between nitrogen and the cations in the zeolite is much stronger than that between oxygen and the cations. Moreover, the adsorption capacity of nitrogen is much more sensitive to the choice of cation present in the zeolite. With these considerations, extensive studies have been reported on the synthesis of ion-exchanged zeolites with different cations and improved properties.3,4 Chao et al.5 and Baksh et al.6 reported Li-X zeolites with excellent N2 adsorption capacities and selectivities. Ca- and/or Sr-exchanged lithium-X zeolites exhibit good sorption properties for nitrogen.7 Binary Li- and Ag-exchanged zeolite X sorbents showing good nitrogen to oxygen selectivities (∼10) and large working capacities were also recently reported by Yang and co-workers.8,9 Carbon molecular sieve (CMS) is another type of adsorbent commonly used in air separation, particularly

10.1021/ie010736k CCC: $22.00 © 2002 American Chemical Society Published on Web 04/20/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002

in the production of nitrogen. Typically, a carbon molecular sieve has an average pore diameter of 0.4 nm and a high surface area up to 1000 m2/g. One characteristic of such materials is their narrow pore size distribution, which is important in separating gas or liquid molecules with very similar molecular sizes. In the mid-1970s, researchers from different groups independently reported the preparation of CMSs and their application in separating oxygen from nitrogen.10-12 Further improvements on synthesis of CMS sorbents and design of CMS-based adsorption separation processes were reported by researchers at Calgone and Air Products.13-16 Ruthven and co-workers investigated adsorption on CMSs for PSA processes and analyzed the diffusion of nitrogen and oxygen in CMSs.17,18 In the preparation of the adsorbent, Braymer et al. reported a granulation process and the adsorption properties of the prepared granules.19 Despite the progress made on both process design and adsorbent development, current industrial adsorption processes for air separation still suffer from the following drawbacks: (1) The selectivity of nitrogen to oxygen on zeolite sorbents is still not high enough for efficient separation. At present, the highest thermodynamic selectivity achieved is around 10. (2) Adsorption properties often deteriorate dramatically in the presence of other components, especially water and carbon dioxide. Pretreatment is definitely required to maintain the performance of the major adsorbents in the system, which makes process design more complicated. (3) For most zeolite sorbents, there is no thermodynamic selectivity of adsorption between oxygen and argon under normal conditions because of their weak or nonpolar nature and comparable polarizabilities.17 By using equilibrium-controlled adsorption processes not in combination with other processes, the highest purity of oxygen achieved is about 95-96%, which is not suitable for many applications. Furthermore, it would be highly desirable to develop an adsorption process in which the adsorbed component is oxygen with very high purity. New and more efficient sorption process can be developed only when new sorbents are discovered. Recently, Lin et al. disclosed in a U.S. patent a new group of sorbents for air separation and oxygen removal for which all of the above drawbacks had possibly been eliminated.20 These sorbents are oxygen-deficient perovskite-type ceramics that can selectively adsorb a considerable amount of oxygen at high temperatures (>300 °C). This group of the sorbents theoretically has an infinitely high selectivity for oxygen over nitrogen or other non-oxygen species. The effects of the presence of other gases on the separation properties of these new sorbents are expected to be minimum. Unlike all previously reviewed sorbents, these perovskite-type ceramic sorbents should be operated at high temperatures because of the inherent thermodynamics and kinetics involved in the oxygen sorption process. It should be noted that for some applications, it is desirable to separate oxygen from oxygen-containing gas streams by adsorption at temperatures above 300 °C. For example, many industrial processes (such as in the glass and steel industries) require oxygen-enriched air at high temperatures.21 High-temperature separation of oxygen has recently received increasing interest from industrial gas companies.21-23 The objective of the present paper is to report experimental data supporting the concept of the

high-temperature sorption process for oxygen removal and air separation. Concept of High-Temperature Sorption Process Perovskite-type ceramics are a group of metal oxides having the general formula ABO3. They have a simple cubic lattice with a B cation in the center, eight A cations on the corners, and six oxygen anions in the face centers. Oxygen nonstoichiometry occurs in some perovskite-type ceramics with B-site cations of variable oxidation states and A-site cations partially substituted by another cation with a lower oxidation state. The occurrence of oxygen nonstoichiometry or oxygen vacancies leads to a modification of the properties, especially the conductivity. Doped perovskite-type ceramics, socalled “mixed conductors”, exhibit high ionic and electronic conductivities. The oxygen equilibrium and kinetic properties of perovskite-type ceramics have been studied primarily for applications as fuel cell electrodes and oxygenpermeable membranes. Mizusaki and co-workers studied the oxygen nonstoichiometry, diffusion, and electrical properties of several perovskite-type ceramics with the general formula of LaxSr1-xBO3-δ (B ) Al, Zr, Bi, Cr, Mn, Fe, Co) by thermogravimetric method and standard four-probe method.24-29 Carter et al. investigated the oxygen transport in perovskite-type ceramics and the A- and B-site doping effects on conductivity and other properties.30 Perovskite-type ceramics, especially the lanthanum cobaltite series, have received increasing attention as materials for membrane separation and membrane reactor applications. Several research groups, including our laboratory, have systematically studied the oxygen permeation properties of these ceramic membranes.31-36 Perovskite-type mixed conducting ceramic membranes offer very high oxygen permeance with infinitely large permselectivities for oxygen. Because of this unique property, significant efforts have been made toward the research and development of these dense ceramic membranes for high-temperature air separation and partial oxidation of methane.21,37 Despite many inherent advantages associated with these membrane processes for air separation, perovskite-type ceramic membranes have not yet been successfully applied in the area because of their chemical/mechanical stability problems.21,37 Furthermore, these high-temperature dense ceramic membrane processes still face many technical challenges,21 including (1) difficulties in the fabrication of the membranes with the desired structure at a large scale; (2) uncertainties in the long-term chemical and structural stability of the membranes; (3) unavailability of reliable high-temperature sealing materials and techniques; and (4) complexities in the design and fabrication of large-scale dense membrane modules and processes that can ensure safe operation. Nevertheless, it is the recent advances in perovskitetype ceramic membranes, especially those achieved in our laboratory, that have promoted a new concept of high-temperature sorption processes for oxygen removal and air separation, which will be described next. The oxygen nonstoichiometry δ of perovskite-type ceramics A1-xA′xB1-yB′yO3-δ, where A′ and B′ are dopants on the A and B sites, respectively, is a function of temperature (T) and oxygen partial pressure (PO2). Therefore, by changing T or PO2, the value of δ, or the degree of oxygen vacancy in the material, can be made

Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2777

Figure 1. Schematic description of TSA process on perovskitetype oxide LSCF for oxygen sorption: (A) LSCF-1, PO2 ) 1 atm; (B) LSCF-2, PO2 ) 0.01 atm.

to change. Within certain ranges of T and PO2, the change in oxygen nonstoichiometry does not affect the perovskite structure of the material, and the change in the oxygen content in the material is reversible.38 The oxygen nonstoichiometry of perovskite-type ceramics is determined by the thermodynamic equilibrium of defect reaction between gas-phase oxygen and solid oxide. Whether a metal oxide can maintain its perovskite structure in a specific range of temperature and PO2 is dictated by the basic properties of the metal ions, such as size and chemical activity.38 Figure 1 shows the change in oxygen nonstoichiometry with temperature at PO2 ) 1 atm for La0.1Sr0.9Co0.5Fe0.5O3-δ (LSCF-1, Figure 1A) and at PO2 ) 0.01 atm for La0.1Sr0.9Co0.9Fe0.1O3-δ (LSCF-2, Figure 1B). The basis for selecting these two perovskite-type ceramics and their synthesis will be described in the Experimental Studies section. As seen from Figure 1B, initially, sample LSCF-2 achieved equilibrium at 600 °C with a δ value of 0.440, and then, with a decrease in temperature to 500 and 400 °C, the oxygen nonstoichiometry (value of δ) changed to 0.384 and 0.324, respectively. The decrease of the oxygen nonstoichiometry corresponds to the increase of the oxygen content in the perovskite solid. Therefore, this can be considered as the sorption part of the TSA process. In the desorption part, with an increase in temperature from 400 to 500 to 600 °C, δ returns to the original values. As shown in the figure, the sorption and desorption processes are fully reversible. A similar temperature dependency of δ is also observed for the LSCF-1 sample, as shown in Figure 1A. Similarly, Figure 2 shows the change in oxygen nonstoichiometry of the LSCF samples as the flowing gas was switched between air (or oxygen) and helium. The temperature was kept at 600 °C, and the total pressure was held at 1 atm. Desorption occurred when

Figure 2. Schematic description of VSA process on perovskitetype oxide LSCF for oxygen sorption at 600 °C: (A) LSCF-1, (B) LSCF-2.

the gas flow was switched from pure oxygen (PO2 ) 1 atm) to He (PO2 ) 1.3 × 10-4 atm). For sample LSCF-1 (Figure 2A), the δ value increased from 0.297 to 0.488 during this period. Sorption started when the gas flow was switched from He to air, and the δ value subsequently returned to 0.297. This completed one whole cycle of desorption and sorption processes. Similar results on LSCF-2 are shown in Figure 2B, in which gas flow was switched between helium and air. As a conclusion from the results shown in Figures 1 and 2, a high-temperature oxygen sorption process could be established by either TSA or PSA. The obvious advantage of this new process over conventional oxygen adsorption processes lies in the infinitely large selectivity for oxygen. This high-temperature oxygen sorption process might allow for the production of both oxygen and nitrogen with higher purity than can be obtained with established sorption processes. However, this new sorption process is only suitable for high-temperature separation. In comparison with perovskite-type ceramic membrane separation, this high-temperature sorption process would have fewer challenges, such as the stability problem and the requirement for a hightemperature sealing technique. Furthermore, sorption technology, including sorbent and adsorber fabrication and process design and operation, is well-established compared to membrane technology.37 These factors will make such high-temperature sorption processes more attractive from an industrial point of view. Experimental Studies Selection of Sorbent Materials. Among the different perovskite oxides studied, the La1-xSrxCo1-yFeyO3-δ

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Table 1. Dependence of δ on Temperature and PO2 for Some LSCF Materials material a

LaxSr1-xCo0.2Fe0.8O3-δ LaxSr1-xCoO3-δb LaxSr1-xCoO3-δb LaxSr1-xFeO3-δc

a

X

experimental conditions

∆δ

0.6 0.4 0.2 0.8 0.6 0.3 0.8 0.6 0.3 0.9 0.75 0.5

PO2 ) 0.21 atm, T increases from 300 to 1200 °C

0.14 0.25 0.32 0.032 0.075 0.083 0.045 0.092 0.092 0.039 0.092 0.131

T ) 800 °C, PO2 changes from 0.2 to 0.001 atm PO2 ) 0.01 atm, T increases from 725 to 950 °C T ) 1000 °C, PO2 changes from 1 to 10-5 atm

Reference 46. b Reference 47. c Reference 25.

(LSCF) series has attracted increasing attention.24-26,39-46 For the LSCF series, it is known that A-site substitution of La by Sr enhances the amount of oxygen adsorbed, whereas B-site substitution of Co by Fe changes the properties of the adsorbed oxygen or, more strictly, the bonding strength of the adsorbed oxygen. Also, the presence of Fe helps to preserve the single-phase perovskite structure, especially at high Sr content. Recent work by Stevenson et al. has further showed that LSCF materials have large oxygen deficiencies while still maintaining the perovskite structure.46 Furthermore, with an increase in Sr and Co content, especially the former, the oxygen nonstoichiometry becomes more sensitive to the temperature and/or to the oxygen partial pressure. Without a detailed knowledge of the kinetic properties, a high sensitivity of the nonstoichiometry to the operating conditions and a good stability of the material structure are two major criteria in selecting the composition of the sorbent material. A high sensitivity of the nonstoichiometry means a large variation in oxygen nonstoichiometry (and oxygen content) with respect to a given change in the operating conditions, i.e., temperature or oxygen partial pressure. A material with good structural stability can maintain the same crystal structure over a large range of operating conditions, ensuring the reversible operation of sorption and desorption processes. Table 1 summarizes the results reported in the literature on the effects of Sr and Co contents on the oxygen nonstoichiometry in LSCF materials for certain ranges of temperature and oxygen partial pressure. As shown, with an increase in Sr content at the A site, the change in oxygen nonstoichiometry to a given change in temperature or oxygen partial pressure increases. On the other hand, as mentioned above, the presence of Fe in the materials enhances their structural stability. On the basis of these considerations, two materials with high Sr contents at the A site, La0.1Sr0.9Co0.5Fe0.5O3-δ (LSCF-1) and La0.1Sr0.9Co0.9Fe0.1O3-δ (LSCF-2), were selected as candidate materials. Different Fe contents at the B sites of these two materials can be used to investigate the effect of Fe on the oxygen sorption properties. Synthesis and Characterization of Sorbent Materials. The liquid citrate method followed by sintering at elevated temperature was used to prepare these perovskite-type ceramics. This method has advantages in preparing samples with high surface areas and precise stoichiometries. In the synthesis, metal nitrate precursors were dissolved in deionized water according to the stoichiometry. The amount of citric acid added

into the above solution was a 50% excess of that required for the reactions. The system was under heating and stirring during the polymerization and condensation reactions, which were carried out at 100105 and 105-110 °C, respectively. Water was gradually evaporated during the condensation process to facilitate gelation. At the end of the condensation, viscous gellike products were obtained. Self-ignition was performed at 400 °C to burn the organics from the products after they had been dried at 110 °C for 24 h. Finally, as-prepared powders were sintered at 1250 °C for 25 h with a ramping rate of 60 °C/h. As-prepared samples were ground into fine powders for characterization and TGA measurements. XRD analysis (Siemens D-50, Cu KR1 radiation) was performed to examine the crystalline structure of the as-prepared perovskite-type ceramics. The shape and particle size of the samples were observed by SEM (Hitachi S-4000). Measurement of Oxygen Sorption Isotherms. The oxygen nonstoichiometry δ of the perovskite samples was measured gravimetrically at different temperatures and oxygen partial pressures. As discussed later, the oxygen nonstoichiometry can be directly used to calculate the oxygen sorption capacity once the initial state (zero sorption capacity) of the material is defined. The experiments were performed on a Cahn electronic microbalance (Cahn-1000).36 In an experiment, a small amount of LSCF powder (100-200 mg) was placed in the sample pan of the microbalance. The sample weight at room temperature in air (PO2 ) 0.21 atm) was first measured. This weight was then used as a reference in calculating the weight change under various experimental conditions. The sample was then heated to 500 or 600 °C under a flow of air/He mixture at a fixed PO2, controlled by mass flow controllers. The steady-state weight of the sample at a given T and PO2 was recorded. All experiments were conducted at 500 and 600 °C with PO2 ranging from 1.3 × 10-4 to 1 atm. The value of δ0, the oxygen nonstoichiometry of a fresh sample at room temperature in air, was obtained experimentally by the following method. The perovskitetype ceramic sample was reduced under 5% hydrogen flow at 850 °C. The weight change was recorded by microbalance. For both samples, equilibrium weights under such reducing condition were achieved within about 100 min. Perovskite-type ceramics La1-xSrxCo1-yFeyO3-δ reduce to metals Fe and Co and metal oxides La2O3 and SrO under the above-mentioned reducing conditions.34 This behavior was also confirmed here, as shown in the XRD patterns of Figure 5.


Figure 3. Schematic diagram of fixed-bed system for studying air separation on perovskite-type sorbent.

The oxygen nonstoichiometry δ0 at the reference conditions (room temperature and in air) for the perovskite oxide was calculated from the weight change between the reducing and reference conditions according to the equation

Figure 4. SEM photograph of LSCF-2 powder.

δ0 ) 1.95MO - 3xMO - x(0.1MLa + 0.9MSr + 0.9MCo + 0.1MFe) (1 - x)MO

(1) where x ) (Wf - Wi)/Wi, with Wf being the sample weight after reduction and Wi being the sample weight under the reference condition (room temperature and air), and Mi is the atomic weight of element i. The oxygen nonstoichiometry at any given T and PO2 was calculated from the sample weight under these conditions, Wx, and that under reference condition Wi, by using the equation

δ ) δ0 +

(Wi - Wx)/MO Wi/Mw,i

(2)

where Mw,i is the molecular weight of sorbent under the initial conditions. Fixed-Bed Experiments for Oxygen Removal from Air. The sorption breakthrough curves of the fixed-bed packed LSCF sorbents were measured to study the feasibility of this new sorption process for oxygen removal and air separation. The experimental setup is illustrated in Figure 3. It includes a gas delivery system, an adsorber column, an oxygen analyzer (Ceramatec, model 1100), and a computer data acquisition system. The column is made from a dense alumina tube of 6-mm i.d. and 9-mm o.d. packed with 3.66 g of the perovskite LSCF powder. Atmospheric air and helium were used, respectively, as the feed gases for sorption and regeneration. The flow rate of the feed gas (air or He) was maintained at 4.88 mL/min. The oxygen concentration in the effluent of the column was continuously monitored by an oxygen analyzer. The temperatures were kept the same during the adsorption and desorption processes and limited to 500 and 600 °C. Results and Discussion Sorbent Materials Characterization. As-prepared LSCF powder is in black color with an irregular shape and nearly uniform size. The average size of the aggregates was estimated to be 180 µm from SEM photography, as shown in Figure 4. Figure 5A is the XRD pattern of the fresh LSCF-2 powder (after sintering in air). It shows a perfect perovskite phase structure

Figure 5. XRD patterns of LSCF powder for (A) fresh sample, (B) sample quenched in He, and (C) sample quenched after reduction in H2: 1, SrO; 2, La2O3; 3, Co; 4, Fe.

under the above conditions. To examine its phase stability under medium to low PO2, sample LSCF-2 was exposed to helium at 600 °C for over 4 h and then quickly quenched to room temperature in helium. The quenched sample was then exposed to air for XRD analysis. The XRD pattern of this sample (Figure 5B) reveals that it still maintains the major characteristic peaks of the perovskite structure. However, these peaks, with slightly decreased 2θ values, are split and become broader and less intense than those of the fresh sample. The smaller 2θ values indicate that the quenched sample has a larger lattice parameter. As will be shown in the TGA data, the quenched sample contains a considerable number of oxygen vacancies. The XRD results suggest that the presence of oxygen vacancies might slightly distort the lattice structure of the perovskite, and increase the lattice size. When this sample was re-exposed to air at high temperature, it returned to the same perovskite structure as shown in Figure 5A. To study the phase structure of the sample after reduction in hydrogen, the sample was initially reduced in 5% H2 at 850 °C until equilibrium was achieved and then quickly quenched to room temperature in a H2 environment. XRD analysis was then conducted in air, and the results are shown in Figure 5C. Apparently, the sample after reduction no longer has the perovskite phase structure, and the major peaks of phases of metals Co and Fe and metal oxides La2O3 and SrO are observed in the XRD pattern. The perovskite phase

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Figure 7. Changes in oxygen nonstoichiometry δ with PO2 at 600 °C for LSCF-1 and LSCF-2.

Figure 6. Weight loss of LSCF-2 in (A) H2 and (B) He at 850 °C.

structure could not be restored by exposing the sample to air at high temperature, which suggests that the reduction is irreversible. To determine the oxygen nonstoichiometry δ0 under reference condition (room temperature, air), a sample was reduced in 5% hydrogen at 850 °C. Under such reducing conditions, LSCF-2 was reduced to La2O3, SrO, Co, and Fe, as confirmed by the XRD analysis discussed previously. Compared to the reference weight, the total weight loss after reduction was 14.42%, as shown in Figure 6A. By using eq 1, the initial oxygen nonstoichiometry was calculated to be 0.178. Similarly, the initial oxygen nonstoichiometry for a fresh sample of LSCF-1 was found to be 0.159. For the reason of comparison, the weight change between reference conditions (room temperature and air) and 850 °C, the helium atmosphere was also measured. In Figure 6B, the sample was first heated to 850 °C in air until equilibrium was achieved. The corresponding weight loss was 1.22%. Further weight loss was observed when the gas feed was switched from air to He. The total weight loss was less than 2.5% for both samples when equilibrated in He. Oxygen Nonstoichiometry and Sorption Processes. The oxygen nonstoichiometry δ of the LSCF samples at 600 °C was measured by TGA in PO2 range from 1.3 × 10-4 (He) to 1 atm. Figure 7 shows that δ decreases with increasing oxygen partial pressure in the gas phase. The equilibrium oxygen content of the sample, 3-δ, is determined by the oxygen partial pressure PO2. Generally, the higher the PO2 in the gas phase, the larger the oxygen content in the material or, in other words, the smaller the value of δ. At 600 °C, LSCF-1 exhibits an almost linear dependence of δ on log PO2 in the pressure range of the

investigations, as shown in Figure 7A. However, sample LSCF-2 shows three distinct regions in the δ vs log PO2 curve, as will be discussed next. This difference might stem from the different Fe doping amounts in the two samples, which results in different structural stabilities. Three distinct regions can be observed for LSCF-2 in Figure 7B, in terms of the sensitivity of δ with respect to PO2. Region 1 is a plateau, in which no perceivable change of δ is present in the pressure range from 1.3 × 10-4 to 0.002 atm. This is the so-called “electronic stoichiometry” region,25 in which the average valance of B-site ions is close to the stoichiometric valance (3+), although three ions, B2+, B3+, and B4+, are present. This region, in practice, is not useful for VSA processes as no change in oxygen stoichiometry was observed. Region 3, the low vacuum zone (PO2 > 0.1 atm), shows a small degree of change in oxygen content with PO2. It should be noted that for the selected material LSCF-2, even at room temperature and in air, the level of oxygen vacancy is high. This suggests that oxygen stoichiometric condition cannot be achieved in the temperature range of our investigation at subatmospheric pressure. Thus, in Figure 7B, a plateau for an “oxygen stoichiometry” zone cannot be observed. However, with increasing oxygen partial pressure, the oxygen content in the material will approach the oxygen stoichiometry zone, as seen region 3 in the figure. A small change of oxygen nonstoichiometry δ will not provide enough working sorption capacity for practical applications; therefore, region 3 is also not very helpful for VSA processes. In region 2 of Figure 7B, a straight line with a steep slope is observed in the half-logarithmic coordinate. We call this region the “logarithmic zone” because the change in oxygen nonstoichiometry is proportional to the change in the logarithm of PO2. Region 2 is in the medium range of PO2 investigated in this study, from 0.002 to 0.1 atm. The change in oxygen nonstoichiometry in this region is due to the mixed valance of the


Figure 8. Change in oxygen nonstoichiometry δ with temperature in air.

B-site ions. Compared to region 1, the average valance of B-site ions decreases ( 0.02 atm), a small increase in sorption capacity is shown. This presents the three regions of Figure 7B from a different aspect. Figure 10A compares the oxygen and nitrogen sorption equilibrium properties of the perovskite-type ceramic sorbent with those of a representative conventional adsorbent, zeolite Li-X. Zeolite Li-X shows preferable sorption of nitrogen and gives an equilibrium selectivity (RN2/O2) of around 10 in the pressure range of investigation.8 LSCF-2 sorbent gives a relatively large sorption capacity for oxygen but zero sorption for nitrogen. This mono-sorption property gives an infinitely large selectivity for oxygen over nitrogen, argon, or any other gas in air. Of course, it should be noted that the operating conditions are different, zeolite Li-X is at room temperature, whereas LSCF-2 is at high temperature. To further evaluate the excellence of sorbents for oxygen sorption, a term qs/qg, the ratio of the oxygen concentration in the solid phase to that in the gas phase, was adopted. In the calculation of qs and qg (mol/m3) for both sorbents, the density of zeolite was assumed as 2.0 g/cm3. For LSCF-2, considering the small amount of La and Fe in the composition, the theoretical density of SrCoO3, 5.605 g/cm3, was used. Figure 10B gives a comparison of qs/qg for these two sorbents. Throughout the pressure range, LSCF has a much higher (1 order of magnitude or higher) oxygen partition amount in the solid phase than zeolite Li-X,

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Figure 10. Comparison of sorption capacities of two types of adsorbents in air separation.

which suggests that this new type of sorbent offers better sorption equilibrium properties. Removal of Oxygen from Air in a Fixed Bed. Oxygen removal by the LSCF samples was conducted at 500 and 600 °C in a fixed-bed adsorption column under the conditions described in Experimental Studies section. Figure 11 gives the breakthrough curves of the sorption process for LSCF-1 and LSCF-2. For LSCF-1, at 600 °C, no outgoing oxygen was detected by the oxygen sensor before 1570 s, and therefore, high-purity nitrogen (>98%) can easily be collected as a product. Sharp breakthrough curve indicates fast sorption process. After 1730 s, the sorbent was saturated with oxygen, and the outlet oxygen concentration equaled the concentration of the feed stream, 20.9%. In our study, inert gas, helium or nitrogen, was used for desorption. For industrial applications, evacuation can be employed to collect ultra-high-purity oxygen as the main product. Sorption capacity can also be calculated from the breakthrough curve with the following mass balance equation considering the pressure drop and change in flow rate of the effluent due to the change in composition

Q)

1 RTms

∫0t (FiXiPi - Fo,tXo,tPo) dt f

(4)

with

Fo,t )

Fi(1 - Xi) 1 - Xo,t

(5)

where ms is the amount of sorbent packed; F, X, and P are the flow rate, oxygen molar percentage, and total pressure, respectively; and the subscripts i and o stand for the inlet and outlet, respectively. The sorption capacities calculated from the breakthrough curves agree well with those derived from equilibrium measurements by TGA. For LSCF-1, at 500 and 600 °C, the

Figure 11. Breakthrough curves for air separation on LSCF at 500 and 600 °C: (A) LSCF-1, (B) LSCF-2.

sorption capacities are, respectively, 0.345 and 0.316 mmol/g from the breakthrough curve and 0.358 and 0.319 mmol/g from TGA. Good agreement was also found for sample LSCF-2. Desorption was conducted in the fixed bed by passing an inert gas, He in this study, at the same temperature as the sorption process. It was found that the sorbent could be fully regenerated by helium. More than 10 cycles of sorption and desorption were conducted at each temperature, and no noticeable difference in sorption capacity was observed. This indicates the good structural stability of the sorbent. As seen from the breakthrough curves shown in Figure 11, this new type of sorbent is extremely efficient in removing oxygen from gas mixtures. For air separation, high-purity nitrogen product can be obtained by collecting the outlet stream before the breakthrough time, and the adsorbed oxygen on the sorbent can be collected under vacuum as the second product. Another example of a potential application of this high-temperature oxygen sorption process is in coal gasification for power generation. Steam can be used as the sweeping gas to regenerate the adsorption bed. A carefully controlled mixture of oxygen and steam reacts with coal at high temperature and high pressure to form hydrogen and carbon monoxide. Conclusions In this study, by comparing the change in oxygen content of the material for the sorption or desorption process, a concept of new oxygen sorption process was described. This work is among a few efforts aimed at developing a novel high-temperature oxygen sorption


process for oxygen removal and air separation. The process concepts, sorbent selection, and applications for VSA and TSA processes were discussed. Two perovskitetype ceramics, La0.1Sr0.9Co0.5Fe0.5O3-δ and La0.1Sr0.9Co0.9Fe0.1O3-δ, were selected for the study. Their highly oxygen-deficient structures caused by the large substition of La with Sr at the A site is responsible for their good oxygen sorption properties. The oxygen nonstoichiometry δ and oxygen sorption isotherms of these two materials were calculated from the weight change measured by TGA. The adsorption and desorption behaviors of the new sorbents were also investigated in a fixed-bed adsorption column. Very sharp oxygen breakthrough curves were observed. The sorption capacity calculated from the breakthrough curves matched very well with that from TGA measurements. Compared to conventional sorbents for air separation, this new type of sorbent exhibits an infinitely large selectivity, relatively high sorption capacity for oxygen, and fast sorption kinetics. As a conclusion, this new type of sorbent has great potential for being used in many industrial applications, for example, air separation, trace oxygen removal, coal gasification, and oxygen storage and transport. Further research is required to better understand the sorption properties of this new type of sorbent, including equilibrium, kinetics, and heat of sorption. Acknowledgment The authors acknowledge support from The BOC Group and the NSF (CTS-9502434, CTS-0132694) on this project. Notation F ) gas flow rate, mL/min M ) molecular weight ms ) sorbent amount packed in the fixed bed, g M h w ) average molecular weight of adsorbent during sorption process, g/mol PO2 ) oxygen partial pressure in the gas phase, atm q ) oxygen sorption capacity, mol/g of adsorbent qg ) oxygen concentration in the gas phase, mol/m3 qs ) oxygen concentration in the solid phase, mol/m3 T ) temperature, °C W ) weight, mg X ) oxygen molar percentage, % Greek Letters R ) separation factor δ ) oxygen nonstoichiometry of perovskite-type oxide δ0 ) oxygen nonstoichiometry of fresh adsorbent at room temperature and 1 atm air

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Received for review September 4, 2001 Accepted March 13, 2002 IE010736K

High-Temperature Sorption Process for Air Separation and Oxygen

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