Modeling of Non-isothermal Reacting Flow in Fluidized Bed Reactors

Abstract: We investigate a prototype concept of a back-up electricity device where we use liquid formic acid (FA) to produce a mixture of carbon dioxide (CO2) and hydrogen (H2) which is used in a PEM fuel cell, Fig. 1. In the fluidized bed reactor the liquid FA is decomposed to a gaseous mixture of CO2 and H2 in the presence of microscopic floating solid catalytic particles. We describe the system, contained in a fixed control volume, as a mixture composed of five constituents liquid FA, gaseous FA (FAg), catalyst micro-particles (Cat), CO2 and H2. For the individual mixture components, we distinguish partial densities and momenta, while we only consider one common temperature field for the mixture as a whole. We reduce the fiveconstituents model to a binary mixture model of liquid phase (Cat + FA) and gaseous phase (CO2 + H2 +FAg) which forms bubbles. The liquid phase is considered as a compressible viscous fluid with temperature-dependent density and viscosity depending on both the temperature (Arrhenius model) and the volume fraction of the catalyst particles. The gaseous phase is considered as an ideal gas mixture where the molar-ratio of H2 and CO2 is 1:1. Since we assume local gas/liquid equilibrium where the liquid is already saturated by all three gaseous constituents, the amount of FAg in the bubbles depends on the saturation pressure (i.e. temperature and ambient pressure) and any interfacial mass flow is neglected. Chemical rates satisfy massaction law and follow the Arrhenius kinetics. The quasi-steady model was implemented numerically in COMSOL Multiphysics and we present several simulations addressing primarily the role of saturation pressure and another important dependencies. We investigate a prototype concept of a back-up electricity device where we use liquid formic acid (FA) to produce a mixture of carbon dioxide (CO2) and hydrogen (H2) which is used in a PEM fuel cell, Fig. 1. In the fluidized bed reactor the liquid FA is decomposed to a gaseous mixture of CO2 and H2 in the presence of microscopic floating solid catalytic particles. We describe the system, contained in a fixed control volume, as a mixture composed of five constituents liquid FA, gaseous FA (FAg), catalyst micro-particles (Cat), CO2 and H2. For the individual mixture components, we distinguish partial densities and momenta, while we only consider one common temperature field for the mixture as a whole. We reduce the fiveconstituents model to a binary mixture model of liquid phase (Cat + FA) and gaseous phase (CO2 + H2 +FAg) which forms bubbles. The liquid phase is considered as a compressible viscous fluid with temperature-dependent density and viscosity depending on both the temperature (Arrhenius model) and the volume fraction of the catalyst particles. The gaseous phase is considered as an ideal gas mixture where the molar-ratio of H2 and CO2 is 1:1. Since we assume local gas/liquid equilibrium where the liquid is already saturated by all three gaseous constituents, the amount of FAg in the bubbles depends on the saturation pressure (i.e. temperature and ambient pressure) and any interfacial mass flow is neglected. Chemical rates satisfy massaction law and follow the Arrhenius kinetics. The quasi-steady model was implemented numerically in COMSOL Multiphysics and we present several simulations addressing primarily the role of saturation pressure and another important dependencies.