Hydrodynamics of beds of small particles in the voids of coarse particles

Uniformly sized particulate solids often fluidize poorly with bubbling and slugging. Slugging is a phenomenon associated with the geometry of the vessel, gas flow rate and properties of gas solid particles. It is severe with Geldart Group B particles especially in bed with high aspect ratio and beyond the minimum bubbling velocity. Geldart B particles fluidize well, but bubbles appear as soon as the operating gas velocity exceeds the minimum fluidization velocity. Fluidization of small Geldart B particles in the interstitial void space of large and stationary particles, called packing, overcomes these drawbacks and changes the hydrodynamic characteristics of the bed. Experiments were carried out to investigate these characteristics as a function of the size of the small particles, fraction of voids of packing filled by the small particles and bed wall temperature. Small particles in the voids of packing start to fluidize at a lower superficial velocity (flow-rate divided by empty bed cross-sectional area) than small particles in unary beds. This velocity decreases with increase in wall temperature. A correlation is proposed to estimate the minimum fluidization velocity of the small particles in the voids as a function of measurable variables.

The minimum fluidization velocity of small particles in packed fluidized bed decreases by almost 50% and it decreases with increase in bed wall temperature, as shown in the figure. It was found that in packed fluidized bed minimum fluidization velocity depends on some dimensionless parameters. A correlation is proposed to estimate the minimum fluidization velocity in packed fluidized bed.Figure optionsDownload as PowerPoint slideHighlights
► Effect of various parameters on the minimum fluidization velocity was investigated.
► In packed-fluidized bed, minimum fluidization velocity of particles is about 50% low.
► In packed fluidized bed minimum fluidization velocity decreases with temperature.
► Dimensionless parameters were identified to find the minimum fluidization velocity.
► A correlation is proposed to find min. fluidization velocity in packed fluidized bed.