DEM simulation of polydisperse systems of particles in a fluidized bed

Numerical simulations based on three-dimensional discrete element model （DEM） are conducted for mono-disperse, binary and ternary systems of particles in a fluidized bed. Fluid drag force acting on each particle depending on its size and relative velocity is assigned. The drag coefficient corresponding to Ergun＇s correlation is applied to the system of fluidized bed with particle size ratios of 1：1 for the mono-disperse system, 1：1.2, 1：1.4 and 1：2 for the binary system and 1：1.33：2 for the ternary system by keeping total volume and surface area of the particles constant. Results indicated that a reasonable estimation of drag force based on individual particle diameters as compared to that of the mean diameter of the particles is achieved in the fluid cells. The total translational kinetic energy of the particles is found to increase as the particle size ratio increases, suggesting an enhanced momentum transfer in polydisperse particle systems. Systems with wide particle size distribution exhibited higher particle velocities around bubbles, resulting in faster bubble growth and its subsequent rise through the fluidized bed.

Fluidization science,its development and future

By revisiting the three stage theory for the progress of science proposed by Taketani in 1942, the footmarks of fluidization research are examined. The bubbling and fast fluidization issues were emphasized so that the future offluidization research can be discussed among scientists and engineers in a wider perspective. The first cycle of fluidization research was started in the early 1940s by an initial stage of phenomenology. The second stage of structural studies was kicked off in the early 1950s with the introduction of the two phase theory. The third stage of essential studies occurred in the early 1960s in the form of bubble hydrodynamics. The second cycle, which confirmed the aforementioned three stages closed at the turn of the century, established a general understanding of suspension structures including agglomerating fluidization, bubbling, turbulent and fast fluidizations and pneumatic transport; also established powerful measurement and numerical simulation tools.After a general remark on science, technology and society issues the interactions between fluidization technology and science are revisited. Our future directions are discussed including the tasks in the third cycle, particularly in its phenomenology stage where strong motivation and intention are always necessary, in relation also to the green reforming of the present technology. A generalized definition of ＇fluidization＇ is proposed to extend fluidization principle into much wider scientific fields, which would be effective also for wider collaborations.

COLLISION PROCESS DETAILS CONTROL THE HYDRODYNAMICS AND HEAT TRANSFER IN FLUIDIZED BED REACTORS

In the '90s DEM simulation research was successful in exploring its potential in the simulation of fluidization phenomena and in its application to the design of fluidized-bed processes. Nevertheless, not much progress has been made regarding the realistic treatment of collision processes that are critical in determining macroscopic mode of fluidization. All through the second half of the '90s, the author investigated/demonstrated the issue by introducing different surface interactions caused by formation of liquid and/or solid bridges, van der Waals force and the existence of surface roughness. In the first part of the presentation these are to be summarized and the tasks remaining are discussed. In the second part, are presented the results from a newly developed force-deformation meter to demonstrate the significance of the surface roughness and its elasto-plastic characteristics.