The axial pressure profiles and the radial gas mixing characteristics have been determined in a downer reactor (0.1 m-I.D. × 3.6 m-high) of a circulating fluidized bed loop (0.1 m-I.D. × 7.6 m-high). The solid particles used in this study was silica sand particles of four different sizes ($d_p$ = 84, 103, 163, 236 ㎛) with a density of 3120 kg/㎥.
The effects of gas velocity, solid circulation rate, and particle size on the axial pressure profiles and pressure gradients in the downer reactor have been determined. The axial pressure gradients and pressure variation increase with increasing solid circulation rate but they decrease with increasing gas velocity. However, the effect of particle size on the pressure gradients and variation is insignificant at the given gas velocities and solid circulation rates.
In the fully developed flow region (H ＞ 2.0 m), pressure gradients estimated from a single particle motion are higher than that of the experimental value at lower gas velocities (＜ 3.0 m/s) whereas, they are similar to the experimental value at higher gas velocities (＞ 3.0 m/s). These behavior are caused by particle agglomeration or cluster formation that has been confirmed by the comparison of slip and terminal velocities of a single and cluster particles. The degree of cluster formation is defined as the cluster ratio (α) as:
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where α increases with increasing solid circulation rate, while α decreases with increasing gas velocity and particle size.
The cluster ratio (α) has been correlated as a function of Reynolds number, Froud number and the ratios of solid to gas velocities and particle size to column size as: ◁수식 삽입▷(원문을 참조하세요)
The effect of gas velocity, solid circulation rate and particle size on the radial gas dispersion coefficient ($D_r$) in the downer reactor have been determined. In the reactor with gas flow alone, $D_r$ increases with increasing gas velocity whereas, $D_r$ decreases with increasing gas velocity at a given solid circulation rate. From these data, $D_r$ is higher than that of gas flow alone at lower gas velocities (＜ 3.0 m/s) but a reverse trend was observed at higher gas velocities (＞ 3.0 m/s). With increasing gas velocity, variation of $D_r$ is more pronounced with smaller particle than that of larger ones. At lower gas velocities (＜ 3.0 m/s), $D_r$ increase with increasing solid circulation rate but it decreases with solid circulation rate at higher gas velocities (＞ 3.0 m/s) due to the particle agglomeration or cluster formation that enhances turbulence intensity in the reactor.
The radial dispersion coefficients in terms of Peclet number has been correlated with the dimensionless numbers with the data of the present and previous studies as:
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Based on the Dr values in the downer and riser reactors, it can be claimed that gas-solid contacting in the downer is superior to that in the riser reactor.