Hydrodynamic and flow regime characteristics have been determined in a circulating fluidized bed ($0.1m ID \times 5.3 m high$) of FCC particles ($d_p = 65 ㎛, \rho_s = 1720 kg/㎥$) and silica sand particles ($d_p = 125 \mum, \rho_s = 3055 kg/m^3$).
The hydrodynamics in the bottom and upper regions of the riser have been determined. The transport velocity to the fast fluidized bed has been determined by the emptying time method. Solid holdup in the upper region of the riser exhibits their maximum values with variation of gas velocities at a constant solid circulation rate. When the solid holdup exhibits a maximum value in the upper dilute region, a dense region forms in the bottom region of the riser. The gas velocity at which solid holdup in the upper dilute region exhibits a maximum value is defined as the fast transition velocity ($U_{FT}$).
The fast fluidization regime can be divided into the two regions depending on the operating conditions based on $U_{FT}$. One is the existence of S-shaped solid holdup profile or fully developed fast fluidization region, which indicates dense region forms in the bottom of the riser. The other is the fast transition region defined in this study. Although the axial solid holdup distribution exists at this condition, the dense region in the bottom is not formed.
The fast transition velocity and the transition velocity ($U_{FD}$) to pneumatic transport in terms of Reynolds numbers have been correlated as a function of Archimedes number and a dimensionless group (Gs/$ρ_gU_t$) as:
$Re_{FT}=0.395Ar^{0.572}\left(\frac{G_s}{\rho_gU_t}\right)^{0.345}$
\\
$Re_{FD}=0.440Ar^{0.563}\left(\frac{G_s}{\rho_gU_t}\right)^{0.359}$
Consequently, the fast fluidization regions can be defined by the choking correlation, $U_{FD}$ and $U_{FT}$.
The effects of operating variables (gas velocity and solid circulation rate) and secondary air injection on radial gas dispersion coefficient have been determined. The radial gas dispersion coefficient ($D_r$) increases with increasing solid circulation rate, while $D_r$ decreases with increasing gas velocity at a given solid circulation rate. The radial gas dispersion coefficient increases with increasing column diameter.
Effect of secondary air injection (secondary air injection ratio, secondary air injection type) on $D_r$ have been determined. The $D_r$ increases with increasing secondary air injection ratio. The effect of secondary air injection on $D_r$ is greatly pronounced in case of tangential air injection than that of radial air injection.
The Peclet number based on particle diameter has been correlated using the isotropic turbulence theory as:
- Bubbling and turbulent fluidized bed
$Pe_{r,dp}=1.704\times10^3\left(\frac{d_p}{D_t} \right)^{1.735}\left(\frac{U_g}{U_g-U_{mf}}\right)^{2.345}$
- Circulating fluidized bed
$Pe_{r,dp}=1.075\times10^3\left(\frac{d_p}{D_t} \right)^{1.148}\left(\frac{U_g}{U-g-U_p}\right)^{36.047)$
The gas backmixing characteristics have been determined in a circulating fluidized bed. The gas backmixing coefficient ($D_b$) decreases with increasing gas velocity at a given solid circulation rate, while $D_b$ increases with increasing solid circulation rate at a constant gas velocity.
In the dilute region, when the tracer gas was injected near the wall, a very sharp radial concentration profile is observed, which may indicate a considerable gas backmixing occurs near the wall due to the downflow of solid. However, when the tracer gas was injected at the center region, the amount of backmixed tracer gas is very low because of high upward gas-solid flow in the core region of the bed. It may reflect the existence of core-annulus flow structure in the dilute region of a CFB. The gas mixing behaviour in the annulus region is quite different from that of the core region. Therefore, the core-annulus model in the dilute region of a CFB is proposed to calculate $D_b$ and mass transfer coefficient (k) between the core and annulus region. The equation of core-annulus model is as:
core region : $\epsilon_cU_c\frac{dC_c}{dx}+\frac{2k}{r_c}(C_c-C_a)=0$
annulus region : $-D_b\epsilon_a\frac{d^2C_a}{dx^2}+\frac{2r_c}{R^2-r^2_c}k(C_a-C-c)=0$
The $D_b$ and k increase with increasing the particle to gas $[U_p(=G_s/ρ_s)/U_g]$ velocities.
The effect of adsorbed $CO_2$ tracer on the gas backmixing coefficient has been determined. The gas backmixing coefficients determined by adsorbed $CO_2$ tracer in the transition and dense regions exhibit much higher values compared to the $D_b$ values obtained by He tracer gas. However, the difference of $D_b$ determined by using the $CO_2$ and He tracer gases in the dilute region is comparatively small due to the very low solid holdup.
