(664e) Modelling of a Continuous Solar Tubular Aerogel Reactor for the Thermochemical Reduction of CeO2: The Effect of Residence Time, Particle Diameter and Particle Loading

A simulation is developed and implemented to describe a continuously operated and indirectly irradiated solar reactor for the thermochemical reduction of CeO₂ to Ce₂O₃. An aerogel, consisting of N₂ and CeO₂ (particle diameter between 1-20 microns), enters the reactor at 900 K. The aerogel flows upward through a 30 cm long by 2.54 cm diameter tube, made of Tungsten coated with Silicon Carbide at 2500 K. The model aims at calculating the concentration and temperature profiles within the reaction tube at steady state, the conversion of CeO₂ particles and the Ce₂O₃ particles mass flow exiting the reactor.

The radiative transfer model within the reactor is implemented by considering a gray, non-isothermal, absorbing, emitting and scattering media. Scattering is considered to be anisotropic, as described by the Henyey-Greenstein phase function. Optical parameters where calculated from Mie Scattering theory, weighted according the blackbody spectra at 2500 K. The thermochemical reduction of CeO₂ is modeled by considering an endothermic, first order, source term. Temperature dependence follows the Arrhenius equation. Convective heat and mass transport mechanisms disregard any thermo-hydrodynamic effects, and assume instead plug flow. Finally, gas conduction is accounted and CeO₂ particle diffusivity is estimated from the Einstein-Smoluchowsky equation for brownian motion of spherical particles.

Simulations consider axisymmetric fields, i.e. dependent on the axial and radial position within the tube. First, the radiation heat transfer mechanisms where implemented using an implicit hybrid Monte Carlo-Finite Volume scheme, linearizing the fourth order emission term using a Taylor series expansion. On the other hand, the heat and mass convection, diffusion and reaction where implemented using an explicit iterative scheme. The steady state distributions of all combined physical phenomena are obtained by using the false transient-state approximation, while considering an iterative-splitting scheme.

The parameters considered within this study where the average flow velocity, the CeO₂ particle loading and the particle diameter. Simulations are carried out considering particle loadings of 10 and 100 mg/l, particle diameter of 1,5,10 and 20 microns and average flow velocities of 1,5 and 10 mm/s. Obtained results suggest that high particle loadings tend to favour temperature gradients, which hinder the overall reactor performance. Higher gas velocity lowers the conversion, but slightly increases the mass Ce₂O₃ flow rate. Finally, it is interesting to point out, that higher particle diameter, presents low specific extinction coefficients, which allow relatively high radiation heat transfer rates throughout all the reactor, minimizing temperature gradients and yielding overall higher CeO₂ conversion and mass Ce₂O₃ flow rate.