Hydrodynamic characteristics such as the transition velocity to the turbulent flow regime, the rise velocity of voids and the bed expansion in turbulent fluidized beds of glass beads ($d_p$ = 0.362 mm) and coal particles ($d_p$ = 0.507, 0.987, 1.147 mm) have been studied from the pressure fluctuations in a 0.1 m -ID × 3.0 m high Plexiglas column. In addition, the mixing characteristics of gas and solids phases in the slugging and turbulent flow regimes have been determined.
The transition velocities from the slugging to turbulent flow regimes nave been determined from the statistical properties of pressure fluctuations in the bed such as mean amplitude, fluctuation interval, standard deviation, skewness and flatness.
Variation of the velocity ratio between the transition velocity to the turbulent flow regime and the terminal velocity of an individual particle can be quantitatively demarcated by the criterion between group A and B of Geldart's classification. The column size has no appreciable influence on the transition velocity to the turbulent flow regime. A correlation of the transition velocity to the turbulent flow regime has been derived based on the data of the present and previous studies in terms of the Reynolds and Archimedes numbers as:
$Re_c=0.700Ar^{0.485}$
An attempt has been made to develop a model of slug breakdown in the turbulent flow regime based on the bubble stability in terms of the Froude number criterion in which the slug breakdown in the turbulent flow regime is mainly caused by the instability of a maximum stable slug which comes from the inertial force exceeding the gravitational force of solids refluxing in the bed. This Froude number criterion provides the transition velocity to the turbulent flow regime as:
$Re_c=[(33.7)^2+0.0408Ar]^{1/2}+0.598Ar^{1/2}-33.7$
A flow regime map has been proposed to demarcate flow regimes quantitatively from the bubbling to dilute phase flows in fluidized beds in terms of the Reynolds number and the Archimedes number.
The rise velocities of slugs and voids in the slugging and turbulent flow regimes have been determined from the cross-correlation function between the two pressure fluctuation signals. The slug rise velocity in the slugging flow regime increases with an increase in gas velocity, but the void rise velocity in the turbulent flow regime remains almost constant with the variation of gas velocity. The rise velocities of slugs and voids have been correlated with the relevant dimensionless parameters and operating variables as:
$U_{s}=85.7(\frac{d_p}{D_t})^{-0.093}(\frac{\rho_s}{\rho_\textrm{g}})^{-0.616}(U_\textrm{g}-U_\textrm{mf})+0.35(gD_t)^{1/2}$
$V_t=40.66~Ar^{-0.280}[0.35(gD_t)^{1/2}]-U_{mf}$
The rate of bed expansion with gas velocity is more pronounced in the turbulent flow regime than in the slugging flow regime. The bed expansion of coarse particles cannot be predicted by the two-phase theory.
The gas phase mixing characteristics of the radial dispersion and backmixing in the slugging and turbulent flow regimes have been determined by the $CO_2$ steady-state tracer injection technique. The radial dispersion coefficient is nearly constant with the variation of gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime. The extent of gas backmixing in the turbulent flow regime is lower than that in the slugging flow regime. The radial and backmixing coefficients of gas phase in terms of the pellet number have been correlated with the relevant dimensionless parameters as:
$Pe_r=1.472\times10^3(\frac{U_{g}}{U_{mf}})^{-1.012}(\frac{\rho_{g}}{\rho_{s}})^{-0.446}(\frac{d_{p}}{D_{t}})^{0.614}$
$Pe_b=2.566\times10^-2(\frac{U_{g}}{U_{mf}})^{-0.546}(\frac{\rho_{g}}{\rho_{s}})^{0.067}(\frac{d_{p}}{D_{t}})^{-0.588}$
The gas flow pattern in the slugging and turbulent flow regimes of coarse particles has been predicted by a simplified model based on the gas phases in the dilute and interstitial phases in the bed as plug flows. In the slugging and turbulent flow regimes, the overall gas exchange coefficient, the interstitial gas flow rate, the fraction of dilute phase, and the fraction of interstitial gas in the dense phase can be predicted from the developed model.
Axial mixing characteristics of coarse particles in the slugging and turbulent flow regimes have been determined by means of the axial transport of heat at steady state. The effective solids axial dispersion coefficient remains almost constant with an increase in gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime. The effective solids axial dispersion coefficient in the turbulent flow regime of coarse particles is lower than that of fine particles.
The perfectly mixed volume fraction in the dense phase has been predicted by solids mixing in the slugging and turbulent flow regimes which is assumed to be the mixing tanks in series coupled with perfectly mixed and plug flows. The perfectly mixed volume fraction remains almost constant in the slugging flow regime, and it increases with n increase in gas velocity in the turbulent flow regime.
