Towards Tailored Monolithic Sponges for Highly Exothermic Catalytic Processes in Chemical Energy Storage
Lars Kiewidt (firstname.lastname@example.org) and Jorg Thming (email@example.com)
Center for Environmental Research and Sustainable Technology (UFT), University of Bremen, Leobener Straµe, 28359 Bremen, Germany
Monolithic ceramic and metallic sponges, also known as open-cell foams, have been generating substantial interest as catalyst support for fixed-bed reactors in current and recent years. In contrast to conventional packed beds composed of pellets or extrudates, they combine low pressure drops and large volumetric surface areas with superior heat and mass transport properties, and are thus well-suited as catalyst support for highly endo- or exothermic processes. Three prominent examples of exothermic processes, which are currently discussed as promising options for effective storage of excess renewable energy, are the Sabatier process (Power to Gas, PtG), the Fischer-Tropsch process (Power to Liquid, PtL), and the synthesis of methanol (PtL) from green syngas. The performance of these processes, however, is vitally dependent on the effective removal of heat from the reaction zone to avoid hot spots that induce thermodynamic limitations on conversion and yield, promote undesirable side reactions, and decrease catalyst lifetime by thermal sintering. In addition, thermal runaway has to be prevented for safe operation. Consequently, catalyst supports with enhanced and adjustable heat transport properties are required to enable efficient small-scale units for decentralized Chemical Energy Storage as proposed in our former study .
Figure 1: Sensitivity of the volumetric surface area, the effective thermal conductivity, and the pressure drop of monolithic sponges on variations of porosity and pore count.
In this study we demonstrate how the structure of monolithic sponges, represented by their pore count and open porosity, and further their spatial distribution can be utilized to tune the heat transport properties locally, and thus intensify heat transport where necessary to avoid hot spots. In order to show the potential of tuning the volumetric surface area, the effective thermal conductivity, and the pressure drop by adjusting the sponge structure, we conducted a sensitivity analysis at typical reaction conditions (300 °C, 10 bar, 4:1 H2/CO2-mixture) using recently published correlations [2–4]. Figure 1 shows that the volumetric surface area and the pressure drop of monolithic sponges can effectively be tuned by adjusting the pore size, whereas the effective thermal conductivity can be altered by changing the porosity. Consequently, the heat transport properties of monolithic sponges and the release of heat can be tuned locally by acutely adjusting the structure of the sponge.
Figure 2: Qualitative temperature profiles for graded (A), uniform (B), and tailored (C) monolithic sponge catalyst supports in the yield-temperature plane.
Therefore, we propose tailored catalyst supports based on monolithic sponges with a graded structure to balance high heat transport, low pressure drops, and high space-time yields (see Fig. 2). As a first proof of principle we utilized a 2-d fixed-bed reactor model to simulate the methanation of CO2 over a 20 wt.-% Ni/Al2O3 catalyst. Figure 3 shows the temperature distributions in a 2.5 m long and 25 mm in diameter single-tube reactor with external cooling and a 4:1 H2/CO2-mixture at 325 °C and 5 bar(a) as feed, for a uniform sponge (Fig. 1a; 40 ppi, 80 % porosity), and a graded sponge with linear radial porosity profile (Fig. 1b; 40 ppi, center: 75 % porosity; wall: 90 % porosity). The axial temperature profiles are qualitatively depicted in Fig. 2 (profiles A and B). The hot spot close to the inlet can effectively be reduced from 625 °C to 360 °C using the graded sponge. Because of the lower temperature, the CH4-yield drops simultaneously from 80 % to 30 % for the same reactor length. Therefore, we use the developed model to find the optimal porosity profile that leads to the optimal temperature profile (Fig. 2, profile C)  and provides high yields, tolerable hot spots, and low pressure drops in order to intensify the catalytic process. Further, we give an outlook on the experimental realization of graded sponges and their application in CO2-methanation.
Figure 3: Temperature distribution in a sinlge-tube methanation reactor for (a) a uniform sponge (80 % porosity, 40 ppi), and (b) a graded sponge (40 ppi) with linear porosity profile (axis: 75 % porosity; wall 90 % porosity) at 325 °C inlet temperature and 5 bar(a) (isobaric). The tube is 25 mm in diameter and 2.5 m long.
 L. Kiewidt, J. Thming, Predicting optimal temperature profiles in single-stage fixed-bed reactors for CO2-methanation, Chem. Eng. Sci. (2015). doi:10.1016/j.ces.2015.03.068.
 A. Inayat, H. Freund, T. Zeiser, W. Schwieger, Determining the specific surface area of ceramic foams: The tetrakaidecahedra model revisited, Chem. Eng. Sci. 66 (2011) 1179–1188. doi:10.1016/j.ces.2010.12.031.
 E. Bianchi, T. Heidig, C. Visconti, G. Groppi, H. Freund, E. Tronconi, An appraisal of the heat transfer properties of metallic open-cell foams for strongly exo-/endo-thermic catalytic processes in tubular reactors, Chem. Eng. J. 198-199 (2012) 512–528. doi:10.1016/j.cej.2012.05.045.
 E. Bianchi, T. Heidig, C.G. Visconti, G. Groppi, H. Freund, E. Tronconi, Heat transfer properties of metal foam supports for structured catalysts: Wall heat transfer coefficient, Catal. Today. 216 (2013) 121–134. doi:10.1016/j.cattod.2013.06.019.