Solid sponges as support for heterogeneous catalysts in gas-phase reactions

Solid sponges combine large specific surface areas and low pressure losses with excellent heat transport properties. Thus, they are promising catalyst supports for endo- and exothermic processes. Nevertheless, design tradeoffs regarding the porosity and window diameter of solid sponges with respect to high catalyst densities, low pressure losses, and high effective thermal conductivities remain unsolved. Therefore, a 2-d pseudo-homogeneous multi-scale reactor model for solid sponges is developed in this work. The model is validated against polytropic lab-scale experiments for the methanation of carbon dioxide in a fixed-bed reactor. In order to quantify and analyze the design tradeoffs, the model is used to solve the outlined multi-objective optimization problem. Moreover, tailored graded solid sponges with an optimal porosity distribution in the radial direction are introduced to successfully resolve the existing design tradeoffs

Feste Schwämme als Träger für heterogene Katalysatoren in Gasphasenreaktionen

Solid sponges combine large specific surface areas and low pressure losses with excellent heat transport properties. Thus, they are promising catalyst supports for endo- and exothermic processes. Nevertheless, design tradeoffs regarding the porosity and window diameter of solid sponges with respect to high catalyst densities, low pressure losses, and high effective thermal conductivities remain unsolved. Therefore, a 2-d pseudo-homogeneous multi-scale reactor model for solid sponges is developed in this work. The model is validated against polytropic lab-scale experiments for the methanation of carbon dioxide in a fixed-bed reactor. In order to quantify and analyze the design tradeoffs, the model is used to solve the outlined multi-objective optimization problem. Moreover, tailored graded solid sponges with an optimal porosity distribution in the radial direction are introduced to successfully resolve the existing design tradeoffs

Solid Sponges as Monolithic Catalyst Supports for CO₂Methanation – Experimental Realization and Structure Optimization

Solid sponges are applied as catalyst supports in a bench-scale fixed-bed reactor (25 x 100 mm) for CO2 methanation to demonstrate their potential for process intensification. The structure e.g. porosity pore size spatial distribution of the porosity and pore size is optimized to tune the heat transport properties locally and to determine the optimized temperature profiles for process intensification.</p

Predicting optimal temperature profiles in single-stage fixed-bed reactors for CO₂-methanation

The catalytic conversion of carbon dioxide into methane, known as Sabatier process, is a promising option for chemical storage of excess renewable energy and greenhouse gas emission control. Typically externally cooled fixed-bed reactors (FBR) using supported nickel or ruthenium catalyst are applied. The Sabatier process, however, is strongly exothermic and leads to substantial hot spots within the reactor at stoichiometric feed ratios. Although high temperatures increase the reaction rate in general, they thermodynamically limit the achievable methane-yield in the Sabatier process. Here, we present an easy-to-use method based on a Semenov number optimization (SNO) to compute optimal axial temperature profiles in single-stage fixed-bed reactors that account for kinetic and thermodynamic limitations simultaneously, and thus result in maximized yield for a fixed reactor length. In a case study on CO2-methanation, these temperature profiles result in a twofold improvement of the methane-yield compared to isothermal and adiabatic operation, and thus demonstrate the high potential of thermal optimization that lies in the Sabatier process. The SNO-method provides a valuable tool to compute optimal temperature profiles, and allows intuitive insight into the key parameters for thermal process intensification. Further, it can readily be transferred to other processes that suffer from the dilemma between kinetic and thermodynamic limitations. Our findings illustrate the attractiveness of the SNO-method to compute optimal temperature profiles in fixed-bed reactors, and the need for catalyst supports with enhanced and tailorable heat transport properties.</p

Multiscale modeling of monolithic sponges as catalyst carrier for the methanation of carbon dioxide

Solid sponges provide large surface areas, low pressure drops, and excellent heat transport properties and are thus promising monolithic catalyst carriers. Their potential compared to randomly packed beds at industrial scales, however, is largely unknown. To facilitate scale-up and future simulation studies, we present a 2-d mulstiscale reactor model for catalytic sponges. Therefore, we couple a 2-d pseudo-homogeneous reactor model with a 1-d reaction–diffusion model to explicitly consider heat and mass transfer and diffusional limitations at the catalyst scale. A comparison of simulated temperature profiles with experimental ones during CO2 methanation at the lab scale demonstrates the validity of the developed model. Further, the results show that it is necessary to include heat and mass transport at the catalyst scale to adequately simulate concentration and temperature distributions in solid sponges although the applied catalyst layers are typically thinner than 100 μm. The presented model thus allows to obtain insights into the interplay between heat and mass transport at both, the reactor and the catalyst scale, and provides a solid foundation for scale-up and techno-economic studies to assess the performance of solid sponges as catalyst carrier at industrial scales.</p

Pareto-optimal design and assessment of monolithic sponges as catalyst carriers for exothermic reactions

Monolithic sponges combine low pressure losses and excellent heat transport properties and are consequently considered as promising catalyst carriers for fixed-bed reactors. Insights on how to design porosity and window size of monolithic sponges to resolve conflicting relations between low pressure losses, high thermal conductivities, and high space-time-yields (STY), i.e., a high catalyst inventory, are still unknown, especially at pilot or production scales. This study quantifies the outlined tradeoffs and assesses the potential of monolithic sponges as catalyst carriers compared to conventional packed beds of pellets. A state-of-the-art heterogeneous reactor model was applied in combination with a genetic multi-objective optimization algorithm to predict Pareto-optimal sets of sponge designs (max. STY, min. Δp,ΔTmax⩽ΔTtol). As example, the methanation of CO2 was chosen. The Pareto-optimal set of sponge designs shows that small windows are necessary to obtain high space-time-yields comparable to the ones of conventional packed beds. As a consequence, the expected low pressure loss cannot be achieved. Because of excellent heat transport properties, which are weakly dependent on the throughput, monolithic sponges however allow stable operation under varying gas loads. The results demonstrate that monolithic sponges will probably not replace packed pellet beds of pellets for the steady-state production of chemicals. Instead, they provide a competitive option for small-scale, decentralized production for example within chemical energy storage and CO2 utilization.</p