Axial dispersion characteristics in two-(gas-liquid, liquid-solid) and three-(gas-liquid-solid) phase fluidized beds have been studied in 0.254 and 0.376 m I.D. Plexiglas columns. The axial dispersion coefficients have been measured using a pulse response technique and analyzed by means of the axially-dispersed plug flow model.
Tap water, Carboxyl methyl cellulose (CMC), TritonX-100 and ethanol aqueous solutions were used as the liquid phase; air as the gas phase and glass beads of 1.0, 2.3, 3.0, and 6.0 mm in diameter as the solid phase.
The effects of liquid velocity (0.02~0.2 m/s), gas velocity (0.0~0.12 m/s), liquid viscosity (1~72.5 $mPa\cdot s^n$), surface tension (30~72.8 mN/m), particle size (1~6 mm) and column diameter (0.254, 0.376 m) on the axial dispersion coefficient of liquid phase have been determined. The liquid axial dispersion coefficient ($D_z$) increases with increasing gas and liquid velocities and liquid phase surface tension in two (gas-liquid) and three phase fluidized beds. Whereas, the effect of liquid viscosity on $D_z$ is found to be small. The axial dispersion coefficient shows a maximum value with an increase in particle size and it sharply increases with increasing column size in three phase fluidized beds.
The axial dispersion coefficients in terms of the Peclet number have been well represented by a correlation equation based on the concept of isotopic turbulence theory involving the ratio of fluid velocities and the ratio of particle to column diameter.
$Pe,_z = \frac{d_pU_1}{D_z} = 1.82 \Bigg( \frac{d_p}{D_c} \Bigg)^{1.15}\Bigg( \frac{U_1}{U_1+U_g} \Bigg)^{0.935}$
