Theoretical, numerical and experimental model of a microreactor prototype for CO2 methanation as an alternative to Power-to-Gas

Abstract

In this thesis work, the advantages of microreactors for the production of methane from the hydrogenation of CO2 were demonstrated, models derived from the resolution of transport phenomena for future optimizations were presented, and a kinetic study applicable to Power-to-Gas (PtG) technology. The kinetics were experimentally modeled in a catalytic microreactor using a 12%Ni/Al2O3 catalyst, considering the momentum balance and three-dimensional species continuity. The microreactor consisted of 80 microchannels with dimensions 0.45x0.15x50 (mm3), and a catalyst thickness of 0.04 (mm). 60 experimental tests were carried out in triplicate under various conditions of volumetric flow (110, 130 and 140 mL/min), temperature (275, 300, 325 and 325°C) and partial pressure of reagents. The kinetic parameters were fitted numerically using COMSOL Multiphysics 5.6 and Matlab 2021. It was found that a power kinetic model with a low kinetic exponent (0.1-0.2) is adequate to represent a wide range of CO2 conversion experimental (up to 86%) and CH4 yield (up to 86%) data, and that it could be useful in PtG applications. An activation energy in the range of 79-97 (kJ/mol) was calculated. In addition, a rigorous analytical model of velocity was proposed based on the application of the finite Fourier transform method to the multidimensional Navier-Stokes equation, which not only allows accurate velocity profiles to be obtained both in the catalytic and hollow zones within channels of a microreactor used for CO2 methanation, but also constitutes a robust tool for future scaling and optimization of the catalytic efficiency of this process. Finally, the numerical results showed a higher conversion of CO2 (and H2) to CH4 under conditions where diffusion forces prevailed, while an increase in flow rate decreased catalytic activity when only advection forces were considered. In a laminar regime, the diffusive forces in a microreactor are higher than those of a conventional reactor due to its configuration (high surface/volume ratio and number of microchannels), which represents a comparative advantage in catalytic activity. Thus, this research determined that the use of microreactors is a promising alternative to increase the conversion of H2 into CH4, and thus facilitate the storage of renewable energy and green hydrogen in natural gas infrastructure.