Interparticle Forces in Fine Particle Fluidization

Publication Reference: 
15-06
Author Last Name: 
Geldart
Authors: 
D Geldart H Y Xie
Report Type: 
ARR
Research Area: 
Particle Formation
Publication Year: 
1993
Publication Month: 
01
Country: 
United Kingdom

Executive Summary

- Previous annual reports concentrated on the presentation of experimental data relating to the behaviour of fluidized beds of catalyst (Group A) powders. These related to the influence of mean particle size, addition of fines, type of gas, and temperature, on bubbling, bed expansion/density, minimum fluidization and mimimum bubbling velocities/voidages, and bed collapse characteristics.

- In this report we focus attention on the theoretical background of non- bubbling fluidized/aerated beds of powder in order (a) to provide a sound foundation for the interpretation of our experimental data and (b) to allow generalisations to be made for other powders and operational conditions

- Both hydrodynamic and interparticle forces play a role in the behaviour of fine particle fluidization, with the latter assuming increasing importance as the mean particle size of the catalyst powder is reduced below about 70um. The van der Waals forces are evaluated with respect to particle size, particle roughness, and,in particular, gas adsorption. It is shown, theoretically and experimentally, that the use of CO2 and other strongly adsorbing gases at room temperature causes a considerable increase in the interparticle forces, to the extent that the catalyst can not be fluidized. As expected, the effect of adsorption disappeared at temperatures above about lOOoC, and this finding has consequences for experimental work in cold models, especially at high pressures.

- The trends shown by the experimental data are, on the whole, in accord with the theoretical predictions; however, because of the lack of fundamental data on, for example, the size of asperities on the particles, the Hamaker constant for FCC, (and the change in the valuejf any, with temperature), it is not possible to make accurate quantitative predictions.

- By using experimental data, theoretical equations, and dimensional analysis, semi-empirical correlations which include powder cohesion have been developed. Work to improve these is continuing but much remains to be done.