Effect of particle size distribution on mixing and segregation in a gas-solid fluidized bed with binary system
Abstract
The mixing and segregation characteristics were investigated in gas-solid fluidized beds with binary solid mixture. Bed materials were constituted with binary solids, having different size and density. To investigate the effect of particle size distribution on the mixing characteristics, two other binary solid mixtures were used, which have similar mean particle size and particle density, but their particle size distribution was different to each other. Bed column has ID=0.14 m and H=2.14m. Bed aspect ratio was 3.0. Bed materials were two sets: one of bed materials was the mixture of ilmenite (dp=153 m, ρs=3,860 kg/m3 ) and coke (dp =582 m, ρs =1,762 kg/m3 ), which has wide size distribution. The other bed materials was the mixture of ceramic beads (dp =122 m, ρs =3,800 kg/m3 ) and plastic beads (dp =813 m, ρs =1,500 kg/m3 ), which has narrow size distribution. Bed composition of ilmenite-coke mixture was determined to 0.7:0.3 by mass ratio. And, bed composition of ceramic beads-plastic beads was 0.75:0.25 by mass ratio. Axial bed pressure drop was measured according to gas velocity. Bed composition was measured according to axial bed height by sampling. Bed pressure drop of ilmenite-coke mixture was maximized above Uo=0.15 m/s, and fully fluidization was occurred. However, criterion of mixing to segregation was not found in the axial bed composition according to gas velocity. In the case of ceramicbeads-plastic beads mixture, bed pressure drop was maximized at Uo=0.05 m/s, and the criterion of mixing to segregation was found at the same gas velocity.
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Introduction
The contents of each section may be provided to understand easily about the paper. Particle mixing and segregation characteristics are important in industrial fluidized beds, where particles of wide size distribution or particles of different density are usually handled. Therefore, to analyze gas-solid fluidized beds with binary solids, the degree of these properties should be evaluated. Past studies on particles in a gas fluidized bed have concentrated primarily on the mixing aspect of the phenomenon, notably those by Rowe and Nienow [1] using two separate layers of flotsam and jetsam as a starting mixture. The flotsam is the lighter or smaller particles; which tend to float at the top of the bed, while the jetsam is those heavier or larger particles, which tend to settle to the bottom part of the fluidized bed. These words were coined originally by Rowe et al. [2] and now have become widely accepted terminology. There are two primary objectives for investigating the particle segregation characteristic in gas fluidized beds. In one respect, the fluidized beds are studied to determine the operating conditions required to promote bed mixing and minimize particle segregation. The other objective is to study the optimum conditions under which clean separation can be accomplished between different materials in the bed [3].Takeover velocity is the superficial gas velocity which the gas-solid fluidized bed with binary solids is transformed from segregated to a solid mixing region. Therefore, mixing or segregation in binary solid beds is classified if takeover velocity could be estimated. However, in case of binary solids mixture dealing with wide size distribution has different range that certain or every particle can be fluidized. The mixing phenomena are beneficial for the process which perfect mixing has to be required. In contrast, it could be harsh to separate each particle for separation process.
Conclusion
In the ilmenite-coke binary solid mixture, takeover velocity was not found significantly, because of their wide particle size distribution. However, in the ceramic beads-plastic beads mixture which has narrow size distribution, takeover velocity was found unlike to ilmenite-coke mixture. From these results, we figured out that particle size distribution of binary mixture strongly affect to mixing characteristics of beds.