Investigation axial mixing coefficients

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Finally, a proper investigation about the effect, on axial mixing, pressure drop and heat transfer coefficient, of the end pulsing valve is missing. 

Solids mixing in beds with an assembly of vertical tubes has not been widely investigated. Ramamoorthy and Subramanian (1981), applying a one-dimensional diffusion model, determined an empirical correlation to predict the axial diffusion coefficient at given fluidizing velocity and packing density of vertical rods. The diffusivity is substantially decreased in the presence of internals and further decreased with a reduction in the spacing of internals. 

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