Chemical and Biomolecular Engineering - Theses
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ItemDisintegration of powder agglomerates in a flash furnace shaft( 2002-12)The disintegration of agglomerates of solid charge used in a nickel flash furnace has been investigated. The in-flight agglomerate size of solid charge with different characteristics has been measured using turbulent conditions similar to a full-scale flash furnace. Plant observations indicated that under certain conditions solid charge was able to pass through the furnace shaft unreacted. The motivation for this work was to improve the understanding and the modelling of a flash furnace. A review of the literature revealed that the disintegration of agglomerates of solid charge is very important to the performance of a flash furnace, however, there has been no detailed study of the factors governing the disintegration of in-flight agglomerates of solid charge. A laboratory-scale experimental rig was constructed to non-intrusively measure the in-flight agglomerate size distribution of solid charge and to visualise the powder injection process, while using turbulent conditions similar to a flash furnace. A high-speed video technique and a laser diffraction technique were used to measure the wide range of in-flight agglomerate sizes present during powder injection. A range of variables including the particle size, turbulence level, packing density, particle shape, solid concentration, and moisture content were investigated. Experimental results indicated that agglomerate sizes from 200 to 7000 μm of solid charge were present under turbulent conditions similar to a flash furnace. Particle size and turbulence level both significantly affected the disintegration of in-flight agglomerates, while packing density and particle shape had a smaller effect. Sample moisture and solids concentration also significantly affected the agglomeration of solid charge, but the investigation of these variables was left as future work. Visualisations of the powder injection process showed that agglomerates undergo break-up by erosion and fragmentation mechanisms. A mathematical model, calculating the agglomerate strength and hydrodynamic stress, was formulated. The model calculated the range of agglomerate strengths for the powders considered in this work to be from 0.01 to 38.7 Pa, which was comparable with measurements from previous work. The model also calculated the range of inter-particle forces to be from 2.2×10-12 to 1.5×10-10 N, which were used to evaluate the types of force that may be holding agglomerates together. It was found that both Van der Waals and electrostatic forces are likely to be contributing to the agglomeration of concentrate, while magnetic forces may also play a part in the agglomeration of dust. Capillary forces do not contribute to the agglomeration. The contribution of each force depends on the material properties, which include the Hamaker constant, the contact potential and the magnetic susceptibility, as well as the separation distance between particles. The model offers an explanation for the prevailing disintegration mechanisms. Furthermore, changes to the agglomerate size distribution measured for different levels of turbulence were explained using turbulent eddy theory. Recommendations for improving the break-up of agglomerates of solid charge in a flash furnace include: removing some of the fine material; increasing the turbulence level of the gas at the entry region of a flash furnace by improving the burner design; decreasing the packing density by changing the solid delivery system or the method of solid charge storage; and smoothing the feed rate of the solid charge to the furnace. It is also recommended that the modelling approach developed in this thesis be incorporated into existing computational fluid dynamics models of concentrate combustion. This will allow a more accurate determination of agglomerate size in the reaction shaft, leading to improved understanding of reaction shaft behaviour.