Abstract
In the last decade, a few of the early attempts to bring CFD-DEM of fluidized beds beyond the limits of small, lab-scale units to larger scale systems have become popular. The simulation capabilities of the Discrete Element Method in multiphase flow and fluidized beds have largely benefitted by the improvements offered by coarse graining approaches. In fact, the number of real particles that can be simulated increases to the point that pilot-scale and some industrially relevant systems become approachable. Methodologically, coarse graining procedures have been introduced by various groups, resting on different physical backgrounds. The present review collects the most relevant contributions, critically proposing them within a unique, consistent framework for the derivations and nomenclature. Scaling for the contact forces, with the linear and Hertz-based approaches, for the hydrodynamic and cohesive forces is illustrated and discussed. The orders of magnitude computational savings are quantified as a function of the coarse graining degree. An overview of the recent applications in bubbling, spouted beds and circulating fluidized bed reactors is presented. Finally, new scaling, recent extensions and promising future directions are discussed in perspective. In addition to providing a compact compendium of the essential aspects, the review aims at stimulating further efforts in this promising field.
Highlights
The fluidized bed technology is at the very heart of a very broad number of industrial processes, ranging from chemical transformations in reactors (energy conversion and storage,oil refining, chemical synthesis, polymerization) to physical operations [1]
I.e., the small-scale interaction between the fluid and individual particles, discrete-continuum simulations based on the discrete element method appeared, thanks to the progress in the computational power, giving rise to the CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) approach [6]
The results indicate that: the particle minimum distance and gas layer length must be kept constant in dimensionless terms, i.e., they should scale with the coarse graining degree f ; the dimensionless particle-wall distance scales with f −1; so, overall, the particle-fluid-wall transfer mechanism calculated using the grain properties must be scaled by
Summary
The fluidized bed technology is at the very heart of a very broad number of industrial processes, ranging from chemical transformations in reactors (energy conversion and storage, (bio-)oil refining, chemical synthesis, polymerization) to physical operations (solids mixing or separation, drying, coating, agglomeration) [1]. Even from a lexical point of view, the terms “fluidization”, “bubbling”, “emulsion phase”, “floating” and “sinking” of objects in the suspended particle bed recall the behavior of liquids Based on this idea, the Two-Fluid Model (TFM) concept [4] was introduced, complemented by the kinetic theory of granular flow (KTGF), giving rise to groundbreaking success in the simulation of large-scale systems, as summarized in the book by Gidaspow [5]. From the physical point of view, Lu et al [16] noted that in most fluidized bed applications, only the collective behavior of the particles is of primary interest, rather than their individual trajectories This forms the basis of practical demand for the application of coarse-graining to DEM. The summation is on all torque contributions generated by non-collinear collisions, Tc,ij, and the corresponding rolling friction torque, Tr,ij, and the fluid-particle torque, T f p,i
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