화학공학소재연구정보센터
Industrial & Engineering Chemistry Research, Vol.47, No.7, 2420-2427, 2008
Solid suspension in a boiling stirred tank with radial flow turbines
This paper considers conditions in reactors in which solids must be maintained in suspension in a boiling liquid. The situation is analogous to, but differs from, the more commonly studied conditions when the liquid is well-below its boiling point in a gas-liquid-solid reactor. Impeller performance in both cases is usually controlled by the development of gas or vapor cavities behind the impeller blades. Power consumption, critical off-bottom suspension speed, and vapor holdup measurements are reported for impellers working in a threephase model system of 0.1 mm glass beads at concentrations of 3, 6, 9, 15, and 21 vol %, suspended in deionized boiling water. The vessel was electrically heated, dished based, and of 0.48 in diameter. It was agitated by single 0.24 in diameter impellers. The three radial-flow disc turbines studied were a flat-blade Rushton turbine, RT-6, a hollow half-pipe blade turbine, Chemineer CD-6, and a hollow semi-elliptical blade turbine, HEDT. The modem CD-6 and HEDT impellers are highly efficient in handling high gas-phase loading and in maintaining high levels of power input throughout a range of impeller tip speeds. For all three impellers, the relative power demand (RPD, i.e., the ratio between the gassed and ungassed power at a given impeller speed) was only slightly affected by solids, even up to 21 vol %. In contrast to the effect of varying the gas sparge rate in a cold aerated system, the boil-off rate had only a limited effect on the RPD, the minimum impeller speed needed to maintain suspension, or the retained void fraction. When boiling, the void fraction was, as might have been expected, much lower than in cold systems at similar power input and off-gas rates. Correlations of vapor holdup and RPD for the three radial flow turbines are suggested as a reference for future industrial applications in which knowledge of both the power requirements and the actual liquid fraction retained in the reactor may be of commercial importance. The HEDT impeller is recommended for both vapor dispersion and solid suspension in ungassed boiling three-phase systems.