CFD simulation for reduction of pyrolusite in fluidized beds

In this study, a CFD model coupled with heterogeneous flow structure, mass transfer equations, and chemical reaction kinetics is established to forecast the pyrolusite reduction reaction behavior. Compared with the previous studies which ignore the volume change of solids phase, the influence of volume shrinkage on reaction and flow behavior is explored in this research. Volume shrinkage of pyrolusite is proved to be non-negligible in predicting the conversion rate. The negligence of volume shrinkage leads to the overestimation of conversion rate for its inaccurate estimation of surface area for reaction. Besides, the influence of volume shrinkage on the reaction is found smaller in the scaled-up reactor.     

In-situ experiment tests and particulate simulations on powder paving process of additive manufacturing

In this work, in-situ experimental tests are first performed to investigate the powder spreading process of additive manufacturing, where different kinds of scrapers and spreading speeds are employed. Detailed kinetic behaviours of individual powder particles are discussed by discrete element method simulations. It is found that the decrease of inclination angle of the scraper improves the powder pressure and compaction in the spreading process, leading to a denser powder flow and thus a denser powder bed. The increase of spreading speed also improves the powder pressure and compaction in the spreading process. However, the powder flow becomes looser due to the volume dilation, and thus the quality of the paved powder bed decreases. In industrial applications, if the higher powder spreading speed is employed to improve the processing efficiency, the scraper with a smaller inclination angle can be used to ensure the powder bed quality.     

Carbon dioxide desorption behavior of potassium carbonate supported on gamma-alumina solid sorbent in wet fluidized bed under steam atmosphere

Since the carbon dioxide (CO₂) capture using solid sorbent is a reversible reaction, the solid sorbent can be regenerated by the desorption process. Therefore, the desorption process is one of the key important processes for the CO₂ capture system. Traditionally, most of the literature studies focus on the desorption of solid sorbent under an N₂ atmosphere. However, the desorption process of the solid sorbent is inappropriate in the real system because the system will need another process to separate CO₂ and nitrogen (N₂) after the desorption process. This study focused on the CO₂ desorption of potassium carbonate supported on gamma-alumina (K₂CO₃/γ-Al₂O₃) in a wet fluidized bed under a steam atmosphere by using the multiphase computational fluid dynamics (CFD) simulation. The effects of water thickness and dry restitution coefficient on CO₂ desorption rate were investigated to provide a realistic particle collision behavior and to explore their effects on CO₂ desorption phenomena. Moreover, the effect of steam velocity on the hydrodynamic behaviors of fluidization which on CO₂ desorption rate was studied. The simulated results demonstrated that all the parameters, water thickness, dry restitution coefficient, and steam velocity had significantly affected system hydrodynamics and CO₂ desorption rate in the wet fluidization desorption process. Furthermore, the effect of desorption temperature on CO₂ desorption rate was evaluated for finding the appropriate temperature for CO₂ desorption process of K₂CO₃/γ-Al₂O₃. The results showed that the appropriate desorption temperature for CO₂ desorption under steam atmosphere was the temperature over 150 °C.     

Predictions of the P1 approximation for radiative heat transfer in heterogeneous granular media

The P1 approximation is a computationally efficient model for thermal radiation. Here, we present a P1 formulation in the context of the combined computational fluid dynamics and discrete element method (CFD-DEM), including closures for dependent scattering and coarse-graining. Using available analytical and semi-analytical solutions, we find agreement for steady-state and transient quantities in size-disperse systems. Heat flux is identified as the most sensitive quantity to predict, displaying unphysical spatial oscillations. These oscillations are due to a temperature slip at the locations of abrupt change in solid fraction. We propose two techniques that mitigate this effect: smoothing of the radiative properties, and pseudo-scattering. Furthermore, using up to a million times enlarged particles, we demonstrate practically limitless compatibility with coarse-graining. Finally, we compare predictions made with our code to experimental data for a pebble bed under vacuum conditions, and in presence of nitrogen. We find that a carefully calibrated simulation can replicate trends observed in experiments, with relative temperature error of less than 10%.