ring.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. Several methods of defluidization behavior have been presented. The simplest is an empirical approach which relates the excess gas fluidizing velocity needed to maintain fluidization to the difference between the bed temperature, T, and the sintering temperature, Ts. The sintering temperature is the observed temperature at which defluidization occurs at the minimum fluidization velocity.In the case of single-phase materials, sintering can potentially occur by a number of mechanisms: flow of the material, surface diffusion, volume diffusion, and vaporization/condensation. In fluidized beds, the first of these mechanisms is most important because it is the most rapid. Sintering by diffusion (surface and volume) requires the movement of atoms of the material and is relatively slow. Because of the flow of gas, vaporization/condensation of the material is probably the least significant mechanism. Glassy materials flow by viscous of plastic deformation. In the case of crystalline materials, such as metals, visco-plastic flow occurs by slip at dislocations.The sintering of two particles has been modeled by representing the particles as smooth spheres. For two such particles in contact, the rate of neck growt...
