Modeling of the performance of BSCF capillary membranes in four-end and three-end integration mode
Introduction
Mixed ionic–electronic conducting (MIEC) perovskite membranes have been attracting significant research interest in recent years due to their potential application in oxy-fuel and pre-combustion CO2 capture routes in fossil fuel power plants. Their capability to separate oxygen from air within high temperature combustion processes, has the potential to minimize the plant's efficiency losses to around 2–8% points [1], [2], [3], [4], [5], [6]. If they are gas-tight, such membranes show 100% oxygen selectivity in the presence of an oxygen partial pressure gradient and at high temperatures (>700 °C) since only oxygen ions can travel through the oxygen vacancies of the crystal lattice from the high pO2 side of the membrane to the low. At the same time, electrons travel in the opposite direction through the membrane to maintain charge neutrality, eliminating the need for electrodes and external electrical circuits [7], [8], [9], [10], [11]. A standard fossil fuel power plant with oxy-fuel or pre-combustion capture requires an oxygen flow rate of ∼8000 t per day [1], [5]. Therefore, in order for the MIEC technology to be economically viable, both high oxygen fluxes and high surface-to-volume ratios need to be obtained. For power plant applications, an oxygen flux of 10 N ml cm−2 min−1 is postulated, limiting the total required membrane area to ∼38850 m² [12], [13]. In theory, the capillary and hollow fiber geometry provide the largest membrane area in relation to the volume of the membrane module, potentially minimizing the required number of membranes in a module. For large-scale gas separation applications, capillaries and hollow fibers fulfill this requirement more adequately than flat sheet and tubular membranes. However, most modeling and design studies are performed with the latter type of geometries [14], [15], [16], [17], [18], [19], [20], [21]. There is little data in the literature on the performance of MIEC capillaries and hollow fibers under industrial working conditions. From a technological point of view, MIEC membranes can be integrated with fossil fuel power plants using a four-end or three-end integration mode [22], [23]. In the four-end integration mode, CO2 and H2O rich flue gasses are typically used to sweep away the permeated oxygen. In the three-end mode, the permeated pure oxygen is drawn out by a vacuum pump, avoiding contact with the flue gasses [1], [5], [6], [22]. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), which has the potential to achieve the commercially interesting oxygen flux of 10 N ml cm−1 min−1, is nowadays considered a first generation material and potential candidate for operation in a three-end design in oxy-fuel or precombustion processes, because BSCF membranes can only operate in the presence of an oxygen chemical potential gradient and at elevated temperatures [1], [5], [18], [22], [24]. Its instability in high CO2 concentrations excludes the use of BSCF in the four-end integration mode. The gradual transformation from the cubic structure to the hexagonal, limits its use to temperatures above 850 °C [23], [25], [26], [27], [28].
The work herein reports the results of modeling studies to scope the impact of the geometry of MIEC capillaries (outer and inner diameter, membrane length) on their performance in both three-end and four-end modes with a view to their integration in fossil power plant processes. The model's parameters were derived from lab-scale experiments using gas-tight, macrovoid-free and sulfur-free BSCF capillary membranes with an outer diameter of ∼3.6 mm and a wall thickness of 0.4 mm prepared by a phase-inversion spinning technique [29], [30].
Section snippets
Oxygen flux measurements through BSCF capillaries under four-end lab-scale conditions
Oxygen permeation fluxes through gas-tight, macrovoid-free and sulfur-free BSCF capillaries with a ca. 3.6 mm outer diameter and 0.4 mm wall thickness were studied as a function of temperature and argon (Ar) sweep gas flow rate at 750–950 °C using the experimental setup described in [29], [31], [32]. BSCF capillaries of about 3 cm in length were hermetically sealed between two gas-tight YSZ tubes in the hot zone of the furnace using high temperature glass so as to ensure isothermal conditions. The
Modeling of the oxygen flux through sulfur-free BSCF capillaries measured under lab-scale conditions
Fig. 2 plots both the measured and modeled oxygen flux through a sulfur-free BSCF capillary against the argon sweep gas flow rate for temperatures between 750 and 950 °C. The model, which is based on bulk-diffusion limitation of the oxygen flux, shows generally good agreement with the experimental values. The corresponding values are listed in Table 1. However, at temperatures <850 °C, the measured oxygen fluxes are found to be nearly independent of the sweep gas flow rate (>70 Nml min−1),
Conclusions
In the work described herein, the performance of BSCF capillaries with a wall thickness of 0.4 mm was modeled under power plant conditions for both a three-end and four-end integration mode assuming bulk diffusion limitation of the oxygen flux and using parameters of sulfur-free BSCF capillaries derived from lab-scale experimental values. The modeling revealed that, in the four-end mode, higher average oxygen fluxes and smaller total membrane areas are obtained compared to the three-end mode.
Acknowledgments
The authors wish to thank all the VITO staff involved in the project for their continued support, and in particular B. Molenberghs, W. Doyen, H. Beckers and S. Mullens. C. Buysse would like to acknowledge funding from VITO and the University of Antwerp for a Ph.D. studentship. This work has been performed in the framework of the German Helmholtz Alliance Project “MEM-BRAIN”, aiming at the development of gas separation membranes for zero-emission fossil fuel power plants.
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