Mixing And Segregation Characteristics Of Binary Mixture System In A Circulating Fluidized Bed

Circulating fluidized bed is widely used as a kind of advanced multiphase flow reactor, in which particles flow and mixing/segregation characteristics play a significant influence for different physical properties particles on the heat, mass transfer and chemical reaction. Particles mixing and segregation characteristics have studied by lots of domestic and foreign scholars in a fluidized bed of binary mixture system, but a few studies in the circulating fluidized bed with high gas velocity and high circulation rate. This thesis presents a detailed experiment on the flow characteristics of binary mixture about Geldart B-B (silica sand-resin) and Geldart B-D (silica sand-millet) in the cylindrical fluidized bed of167mm inner diameter and2250mm high and the circulating fluidized bed of80mm inner diameter and5500mm high.The experiments on the basic fluidization characteristics of mono-particle and binary systems show that the effect of initial static bed height on the minimum fluidization velocity of mono-particle systems can be ignored in the range of operational conditions in this work. With increasing the small particle (silica sand) mass fraction, the minimum fluidization velocity of binary mixture decreases, and the changes in scope of partial fluidization state also decreases. The minimum fluidization velocity of Geldart B-B (silica sand-resin) system shows a decreasing trend to more and more slower, and the minimum fluidization velocity of Geldart B-D (silica sand-millet) system shows a decreasing trend to faster and faster. In addition, silica sand-millet (B-D) binary mixture appears secondary fluidized phenomena.The experiments on the flow characteristics of binary mixture system show that the mixing characteristics is the main flow characteristics in the riser of the circulating fluidized bed, but show segregation characteristics in the regions of the bottom feeding and the top outlet. Compared to the B-B (silica sand-resin) system, the height of segregation characteristics becomes larger in the top and bottom of riser and the full development height in the middle of rise becomes smaller for the B-D (silica sand-millet) system. Decreasing the gas velocity (above10Umf) and increasing particles circulating rate is beneficial for the mixing quality in axial direction. Moreover, when the mass fraction of small particles is25%in the feed have a better mixing effect. Because of strong restriction of export structure in the riser, the bed density along the axial appears a C-type distribution which low in the middle and high at the ends. The particle concentration decreases along the axial direction of the overall riser with increasing superficial gas velocity, decreasing the particle circulation rate, and decreasing the mass fraction of small particles in the feed of binary mixture system.