Many industries produce, or encounter during processing, dry powders having mean sizes in the range 20-150pm; others, notably the petroleum and petrochemical industries, deliberately choose such powders as catalysts for use in fluided bed reactors. These Group A powders, as they are known, are, on the whole, aeratable; that is, they retain gas in the interparticle voids, and this property gives them to a greater or lesser extent good flowability. However, flowability and other related properties are influenced by interparticle forces (IPFs) and hydrodynamic forces (HDFs) which in turn are affected by the physical and physico-chemical properties of the gas and particles. It is believed that it is the balance of these which determines the behaviour of fine powders in a fluidized bed and in powder flow and handling operations.
The overall objectives of this research programme were to understand better (a) the nature of these forces and their relative importance and, in particular (b) the influence which temperature, addition of fine particles, and the gas itself have on them.
Experiments were carried out in 152 mm diameter columns with cracking catalyst (a spherical alumino-silicate) to which much finer catalyst particles were added. These were fluidized at temperatures up to SOOC with air, argon, neon, carbon dioxide, and freon-12. Measurements were made of bubble sizes , bed expansion, and collapse times using specially developed purged pressure probes.
The results show that the behaviour of Group A powders is caused by a combination of IPFs and HDFs, and that their relative magnitudes change with mean particle size. For cracking catalyst HDFs dominate above about 70 urn and IPFs below about 60 l.t m. Strongly adsorbing gases such as CO2 can increase the IPFs at temperatures below about 1OOC so that even relatively coarse powders may exhibit cohesive behaviour, Dimensional analysis shows that the fluidization behaviour can be characterized by a Cohesion number, and the Galileo and Density numbers. As yet it is not possible to make predictions of the Cohesion number a priori because it depends on the Van der Waals forces which in turn depend on the size of the asperities, and on the Hamaker constant, i.e.on the nature of the material from which the particle is formed. For FCC-FCC contact, particles can be treated as smooth when the asperities are smaller than 0.01 l.trn; however, the interactions between the asperities begin to dominate the IPFs rather than the parent particles when the asperities are larger than 0.1 l,trn. This critical size could be as large as 10 pm for FCC-polymer contacts because the polymer particles are soft and deform easily. For FCC-FCC the theoretical predictions agree well with our experimental results and those of other researchers. Although the Cohesion number cannot readily be predicted for most powders in Group A, other parameters which are relatively easy to measure can be used to characterize their fluidized behaviour, notably the ratio of minimum bubbling to minimum fluidization velocity, and the standardized collapse time. The influence of temperature on these parameters has been measured and has been incorporated into new and existing correlations. Measurements of bubble size confirm that there is an equilibrium size which is sensitive to particle size, but virtually independent of bed level and gas velocity. It appears to change relatively little with increasing temperature.
The presence of small amounts of fine particles influences strongly the behaviour of aerated and fluidized Group A powders, and the percentage present in powders used in any given industrial process may increase due to attrition of coarser components, or may reduce as elutriation occurs. In either case the performance of the process may be affected adversely.