Solid Suspension in a Boiling Stirred Tank with Radial Flow Turbines

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• 作者：Yuyun Bao ; Zhengming Gao ; Zhipeng Li ; Ding Bai ; John M. Smith ; Rex B. Thorpe
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2008
• 出版时间：April 2, 2008
• 年：2008
• 卷：47
• 期：7
• 页码：2420 - 2427
• 全文大小：257K
• 年卷期：v.47,no.7(April 2, 2008)
• ISSN：1520-5045

文摘

This paper considers conditions in reactors in which solids must be maintained in suspension in a boilingliquid. The situation is analogous to, but differs from, the more commonly studied conditions when the liquidis well-below its boiling point in a gas-liquid-solid reactor. Impeller performance in both cases is usuallycontrolled by the development of gas or vapor cavities behind the impeller blades. Power consumption, criticaloff-bottom suspension speed, and vapor holdup measurements are reported for impellers working in a three-phase model system of 0.1 mm glass beads at concentrations of 3, 6, 9, 15, and 21 vol %, suspended indeionized boiling water. The vessel was electrically heated, dished based, and of 0.48 m diameter. It wasagitated by single 0.24 m diameter impellers. The three radial-flow disc turbines studied were a flat-bladeRushton turbine, RT-6, a hollow half-pipe blade turbine, Chemineer CD-6, and a hollow semi-elliptical bladeturbine, HEDT. The modern CD-6 and HEDT impellers are highly efficient in handling high gas-phase loadingand 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 impellerspeed) was only slightly affected by solids, even up to 21 vol %. In contrast to the effect of varying the gassparge rate in a cold aerated system, the boil-off rate had only a limited effect on the RPD, the minimumimpeller speed needed to maintain suspension, or the retained void fraction. When boiling, the void fractionwas, 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 forfuture industrial applications in which knowledge of both the power requirements and the actual liquid fractionretained in the reactor may be of commercial importance. The HEDT impeller is recommended for bothvapor dispersion and solid suspension in ungassed boiling three-phase systems.