Solids recycle and heat transfer characteristics have been determined in atmospheric and pressurized solids recycle systems in a circulating fluidized bed of silica sand particles ($d_p$ = 240μm, $\rho_s$ = 2582 kg/㎥).
Solids recycle characteristics through conventional loop-seal (0.10 m i.d.) systems are determined in a circulating fluidized bed of silica sand particles. Solids mass fluxes increase linearly with increasing aeration rate but it reaches a maximum value. At a constant solid inventory and gas velocity in riser, pressure drops across the riser and the downcomer increase with increasing solid circulation rate to maintain pressure balance.
Loop-seal : $\Delta P_{Is}=\Delta P_w + \Delta_{va}$
\\
Weir section : $\big(\frac{\Delta P}{L}\big)_w=
\rho_s(1-\epsilon_{mf})g$
\\
Vertical aeration section: \\
$\big(\frac{\Delta P}{L}\big)_{va}=1.8\times10^{-5}
\big[\frac{(G_s/\rho_s)^2}{d_pg}\big]^{0.40}
\big[\frac{G_sd_p}{\mu}\big]^(-0.36}
\big[\frac{\rho_s}{\rho_g}\big]^{2.88}$
The effects of particle size and density on solids recycle characteristics of loop-seal (0.08 m-i.d.) have been determined in a circulating fluidized bed (CFB; 0.1m-i.d. 5.3 m-high). Five different particles (silica sands: 78, 101, 157, 239μm and FCC: 65μm) were used to determine the effects of particle properties on solids recycle characteristics. As the particle size is increased, aeration requirement in the loop-seal increases to attain the same mass flux of silica sands with different size. The effects of aeration rate, particle size and density on pressure balance around the CFB have been determined. As particle size is increased, solid holdup in the riser decreases at a given mass flux and gas velocity. Pressure drops in the weir section of loop-seal increases with larger and denser particles. In the vertical aeration section, pressure drop increases with decreasing particle size and increasing particle density. Split of vertical aeration to the downward direction increases with increasing mass flux. The pressure drops across vertical aeration section in the loop-seal have been correlated with Froude, Reynolds numbers and the density ratio of gas and solid phases. The pressure drops across the loop-seal can be obtained by proposed equations.
The effects of system pressure (P$_{sys}) on solids flow characteristics of loop-seal (0.10 m-I.D.) have been determined for the application in a pressurized CFB (PCFB). Experiments were carried out in a solids recycle system incorporating with two hoppers and a lift line. The solids recycle system is composed of a downcomer (0.10 m i.d. 2.25 m high) and a loop-seal (0.10 m i.d.). Silica sand ($d_p$ = 240μm, $\rho_s$ = 2582 kg/㎥) particles were transported at room temperature and P$_{sys} up to 0.71 MPa using air. Solids mass flux (G$_s$) increases with increasing system pressure at constant aeration rate. Pressure gradient, solids velocity and actual gas velocity increase with increasing Psys at constant aeration rate. The Pressure drop number (φ) on pressure gradient in downcomer can be obtained by following equation with Transportation number (Tr).
$\Phi = 0.91 Tr^{0.32}$
Pressure drop across the loop-seal increases with increasing of G$_s$ irrespective of variation of P$_{sys}. The obtained G$_s$ and Transportation number (Tr) are correlated with the experimental variables as:
$G_s=1.67\big(\frac{U_A}{U_{mf}}\big)^{1.62}Ar^{0.49}$
\\
$Tr=1.33\times10^{-4}\big(\frac{U_A}{U_{mf}}\big)^{0.53}Ar^{0.72}$
A pressure balance model has been developed for describing the dynamics around a circulating fluidized bed (CFB) loop. The developed model describes the exit effect on axial pressure profile in riser with abrupt exit. In the model, the decay constant and reflux constant have been correlated with pertinent dimensionless numbers and geometry for describing exit effect in riser of CFB as:
-decay constant:\\
$a_eD_r=1.27\big[\frac{(U_g-U_t)^2}{gD_r}\big]^{1/2}
\big[\frac{G_s}{\rho_s(U_g-U_t)}\big^{-1/2}
\big[\frac{D_e}{D_r}\big]^{-1/2}
\big[\frac{\rho_s-\rho_g}{\rho_g}\big]^{-1}$
\\
-reflux constant:
\\
$C_e=0.046\big[\frac{(U_g-U_t)^2}{gD_r}\big]^{1/2}
\big[\frac{G_s}{\rho_s(U_g-U_t)}\big]^{-1/3}
\big[\frac{H_e}{d_p}\big]^{-1/3}
\big[\frac{D_e}{D_r}\big]^{-3/4}$
Practical operating condition in solid recycle system with loop-seal was considered for CFB combustor application. The predicted results from the proposed model show a good agreement with the observed data.
The effect of gas velocity on the average and local heat transfer coefficients between a submerged horizontal tube (25.4 mm-O.D.) and a fluidized bed has been determined in a fluidized-bed-heat-exchanger (FBHE: 0.34 X 0.50 ㎡; 0.6 m-high) of silica sand particles. The heat transfer coefficients and the properties of bubble and emulsion phases were simultaneously measured at the same location around the tube circumference by thermocouples and an optical probe. The average heat transfer coefficient ($h_avg$) exhibits a maximum value with variation of gas velocity. The local heat transfer coefficient ($h_i$) exhibits maximum values at the side of the tube ($0^o$). As the gas velocity is increased, the bubble frequency ($f_b$) increases and the emulsion contacting time ($t_e$) decreases. The $h_i$ increases with increasing fb and decreasing $t_e$. The fb exhibits higher values and $t_e$ is shorter at the bottom (under each side) than at the top section of the tube. The $t_e$ and bubble fraction ($δ_b$) have been correlated with the Froude number.
The predicted $H_avg$ values based on the packet renewal model and the emulsion contacting characteristics around the tube well accord to the experimental data.
The loop-seal with horizontal tube bundle (25.4 mm i.d.) and downward tangential aeration has been developed. The maximum Gs of silica sand (dp = 240μm, $ρ_s$ = 2582 kg/㎥) was enhanced by 1.3 times by using the developed loop-seal compared to conventional loop-seal. More split of aeration to the downward direction in flow control section of the loop-seal is obtained with downward tangential aeration at a given aeration rate compared to vertical aeration. Maximum heat transfer coefficient of 400 W/㎡K is obtained with gas velocity in heat exchanger of the loop-seal in atmospheric condition. The heat transfer coefficient is higher by 3.5 times than in moving bed flow. The Gs increases with increasing Psys (0.10 - 0.74 MPa) at constant aeration rate. Pressure gradient, solids velocity and actual gas velocity increase with increasing Psys at constant aeration rate. The Pressure drop number (φ) on pressure gradient in downcomer can be obtained by following equation with Transportation number (Tr).
Pressure drop across the developed loop-seal increases with increasing of Gs irrespective of variation of Psys. The obtained Gs and Transportation number (Tr) are correlated with the experimental variables.
The average and local heat transfer coefficient increases with increase of Psys at gas velocity of constant fluidizing number. With Psys, local heat transfer coefficient exhibits higher values at bottom of the tube. The heat transfer coefficient shows a constant value in given range of solids mass flux (0 - 140 kg/㎡s). The obtained maximum Nusselt numbers have been correlated with Archimedes and Reynolds numbers..
