Sintering in Fluidized Bed :: essays papers

Sintering in Fluidized Bed Processes for cleaner and more efficient energy generation from feed stocks such as coals, lignites, peats, and waste liquors use fluidized beds that are operated at high temperatures and pressures. These processes involve systems that are multi-phase and have complex chemical reactions. Research work has tackled a number of aspects, including mechanical engineering aspects of the reactors, reaction chemistry and products, characterization and physical properties of the ash, fouling by ash deposits and the phenomenon of defluidization by agglomeration or sintering of the ash particles. It is with this latter aspect, the phenomenon of defluidization, that this contribution is concerned. Defluidization is also a problem in a number of other elevated temperature fluidized bed production processes, including size enlargement by agglomeration, fluidized bed processes for poly-olefin production and metallurgical processes. Defluidization occurs when the particles in the bed adhere. When two particles touch, material at the point of contact migrates forming a neck that is strong enough to withstand the disruptive forces in the fluidized bed. Two categories of adhesion can be discerned. The first type is visco-plastic sintering and it occurs with glassy materials. With these materials, migration is limited by the ability of the material to flow. With increasing temperature the viscosity of the material is reduced and hence the material flows and the size of the neck is increased. At some point, the necks are sufficiently large and strong enough to cause defluidization. The second type occurs when a large quantity of liquid is formed by melting or by chemical reaction. With an increase in temperature, a phase boundary may be crossed bring about rapid defluidization. The liquid formed has a relatively low viscosity and defluidization is caused by the amount of liquid present. This section will primarily f ocus on visco-plastic sintering. Model systems, in which chemical reactions do not occur, have been used to investigate the relationship between process conditions and the tendency to defluidize due to visco-plastic sintering. Materials used have included low-density ploy-ethylene and poly-propylene, soda glasses, metals, and inorganic crystalline salts. Under sintering conditions, the fluidizing velocity has to be increased above the minimum fluidizing velocity, Umf, to a higher velocity, U; at which pronounced bubbling occurs. A fluidizing velocity, Umfs, may be defined as a velocity just sufficient to prevent sintering. This characterizes a different fluidization state from Umf, since the in the latter the bed is in an expanded but not bubbling state.