CFD study of the minimum bubbling velocity of Geldart A particles in gas-fluidized beds

Flow Behavior of Nanoparticle Agglomerates in a Fluidized Bed Simulated with Porous-Structure-Based Drag Laws

Fluidization bed reactor is an attractive method to synthesize and process quantities of functional nanoparticles, due to the large gas-solid contact area and its potential scalability. Nanoparticles fluidize not individually but as a form of porous agglomerates with a typical porosity above 90%. The porous structure has a significant effect on the hydrodynamic behavior of a single nanoparticle agglomerate, but its influence on the flow behavior of nanoparticle agglomerates in a fluidized bed is currently unclear. In the present study, a drag model was developed to consider the porous structure effects of nanoparticle agglomerates by incorporating porous-structure-based drag laws in the Eulerian-Eulerian two-fluid model. Numerical simulations were performed from particulate to bubbling fluidization state to evaluate the applicability of porous-structure-based drag laws. Results obtained for the minimum fluidization and bubbling velocities, bed expansion ratio, and agglomerate dispersion coefficient show that, compared with the drag law of solid sphere, the porous-structure-based drag laws, especially the drag law of fractal porous spheres, provide a closer fit to the experimental data. This indicates that the pore structures have a great impact on gas-solid flow behavior of nanoparticle agglomerates, and the porous-structure-based drag laws are more suitable for describing flows in nanoparticle agglomerate fluidized beds.

Reinterpretation of the Geldart A powder classification based on Eulerian–Eulerian CFD simulation

Geldart classified powders into four categories and assigned each category its own unique characteristic. Geldart A particles, being easily aeratable, show a unique feature of ‘Homogenous expansion’ before bubbling. In this work, an additional feature for the Geldart chart is proposed which adds significant utility for the processing of Geldart A particles. CFD was used to characterize the entire Geldart A region of the Geldart chart based on detailed fluidization behavior. For this, Eulerian–Eulerian Two-fluid model (TFM) simulations were conducted for 25 particle systems across the entire span of the Geldart A region. The simulations (Solid volume fraction (SVF) contours) of bed evolution, taken before the appearance of multiple bubbles, were analyzed in detail. The particle systems were then sub-categorized into Red (5% average bed expansion), Orange (12.5% average bed expansion), and Green (30% average bed expansion) sub-types. The sub-types were plotted on Geldart chart, and for the first time a continuum heat map was generated, from which the ‘level of fluidizability’ of all Geldart A powders can be conveniently gaged. The map can be used for a more informed choice of powder for various industrial applications. Also, the A/B boundary proposed by Verloop was found to be a better fit for our proposed continuum when compared to the original Geldart A/B boundary. The 2D Simulation results performed in this work, found adequate validation against experimental findings in literature. Further, fine mesh 2D simulation results compared well with 3D simulations for dense bed, and were thereby deemed adequate for revealing dense bed behavior before onset of multiple bubbles.

Effect of Fines Content on Fluidity of FCC Catalysts for Stable Operation of Fluid Catalytic Cracking Unit

Effect of fines content (weight % of particles with diameter less than 45 μm) on bed fluidity was determined to get a base for good fluidization quality in the fluid catalytic cracking (FCC) unit. The fines content in equilibrium FCC catalysts (Ecat) from commercial units were controlled by adding or removing the fines to simulate commercial situation. To get the fluidity values (Umb/Umf) of seven different FCC catalysts (2 Ecats and 5 fresh catalysts) and their mixture, minimum fluidization velocity (Umf) and minimum bubbling velocity (Umb) were measured in a fluidized bed reactor (0.05 m ID). The fluidity decreased with loss of fines content and increased with increments of makeup of fresh catalysts or additive with the controlled fines content. The fluidities of catalysts increase with increases of normalized particle diameter variation by the fines addition. The obtained fluidities have been correlated with the fines contents and the catalyst and gas properties. The proposed correlation could guide to keep good catalyst fluidity in the FCC unit.

Pyrolysis of textile dyeing sludge in fluidized bed and microwave-assisted auger reactor: Comparison and characterization of pyrolysis products

This paper investigated fluidized bed pyrolysis (FBP) and microwave-assisted auger pyrolysis (MWAP) for treatment and disposal of textile dyeing sludge (DS), and the products were analyzed and compared. MWAP achieved higher yields of char and condensate, and lower non-condensable gas yields compared to FBP. The yields of CO₂ from FBP were much higher than those from MWAP at 450-850 °C. Whereas the yields of H₂, CO and CH₄ from MWAP were greater than those from FBP at higher temperature (e.g. 850 °C). The maximum condensate yields of FBP and MWAP were observed at 650 °C. Pyrolysis oil of MWAP contained less of macromolecules compared to FBP. Pyridine, phenol, aniline and their derivatives were major components in MWAP oil. Pyridine was the dominant oil component at 850 °C for FBP. Most of nitrogen-, sulfur- and chlorine-containing compounds were retained in FC (FBP char) and MC (MWAP char), and higher relative proportion (RP) of nitrogen, sulfur and chlorine were observed in the condensate and non-condensable gas from MWAP in comparison with FBP. FBP and MWAP both decreased risk degrees of heavy metals compared to raw DS, and heavy metals in FC and MC posed slight risk to the environment based on national standards in China.

Coupling CFD-DEM and microkinetic modeling of surface chemistry for the simulation of catalytic fluidized systems

In this work, we propose numerical methodologies to combine detailed microkinetic modeling and Eulerian-Lagrangian methods for the multiscale simulation of fluidized bed reactors. In particular, we couple the hydrodynamics description by computational fluid dynamics and the discrete element method (CFD-DEM) with the detailed surface chemistry by means of microkinetic modeling. The governing equations for the gas phase are solved through a segregated approach. The mass and energy balances for each catalytic particle, instead, are integrated adopting both the coupled and the operator-splitting approaches. To reduce the computational burden associated with the microkinetic description of the surface chemistry, in situ adaptive tabulation (ISAT) is employed together with operator-splitting. The catalytic partial oxidation of methane and steam reforming on Rh are presented as a showcase to assess the capability of the methods. An accurate description of the gas and site species is achieved along with up to 4 times speed-up of the simulation, thanks to the combined effect of operator-splitting and ISAT. The proposed approach represents an important step for the first-principles based multiscale analysis of fluidized reactive systems.