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Title: Fluidization of cohesive powders
Author: Wong, A. C. Y.
Awarding Body: University of Bradford
Current Institution: University of Bradford
Date of Award: 1983
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The fundamental differences in fluidization characteristics between free-flowing Group A and cohesive Group C powders have been studied. Results from various experiments revealed that the differences are basically due to the difference in magnitude of their interparticle forces. The relatively weak interparticle forces in free-flowing Group A powders are responsible for the typical Group A characteristics such as homogeneous bed expansion and constant deaeration rate of the dense phase. The strong interparticle forces present in cohesive Group e powders are responsible for the poorer fluidization characteristics in the form of severe channelling and/ or agg~meration. What proved to be the best parameter (i.e. Urnb/Urnf) for differentiating Group A from Group B powders has been found to be inappropriate in the demarcation of the A/e boundary. The significance of Urnf and Urnb vanishes with increasing cohesiveness. The difference in the shape of bed expansion curve between Group A and e powders has further supported the belief that fluidization mechanisms in the two groups of powders are not the same. The difference between the two groups in the shapes of curves from experiments of entrainment and bed collapse may, again, be taken as an indication of whether a powder is cohesive. The experiments on solids discharge through an orifice from a fluidized bed supported the speculation that the non-Bernoulli type discharge flow of cohesive powders at low bed height is caused by agglomeration. Observations made from the behaviour of a powder in the rotating fluidized bed enabled us to define some qualitative criteria for differentiating' Group A from C powders. The effect of the relative humidity of the fluidizing gas on the fluidization characteristics of a powder has been extensively studied; non-porous powders are much more sensitive to increasing relative humidity than porous powders. Gas viscosity has also been found to be an important parameter especially for powders near the A/C boundary. Correla~~ons for the prediction of collapse curves of cohesive powders and bed expansion have been developed based on the Hausner ratio (i.e. the ratio of tapped bulk density to aerated bulk density) of the powder. This ratio has also been used to distinguish quantitatively between Group A and Group C powders. A transition or intermediate group (Group AC) has also been identified. Finally, the original entirely empirical demarcation of the A/c boundary has been modified by taking cohesive forces into consideration.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical engineering Chemical engineering