Effects of Equipment and Process Variables on the Suspension of Buoyant Particles in Gas-Sparged Vessels

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• 作者：Yuyun Bao ; Zhengming Gao ; Zhigang Hao ; Jiangang Long ; Litian Shi ; John M. Smith ; and Norman F. Kirkby
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2005
• 出版时间：September 28, 2005
• 年：2005
• 卷：44
• 期：20
• 页码：7899 - 7906
• 全文大小：321K
• 年卷期：v.44,no.20(September 28, 2005)
• ISSN：1520-5045

文摘

Previous work (Bao et al., 2005) reported the influence of solid concentration and agitatorselection on the just-drawdown agitation speed and gas holdup in three-phase reactors containingnonwetting buoyant solids. This paper reports experimental results for the unaerated and aeratedaqueous suspension of buoyant (polypropylene and polyethylene) particles in a tall vessel of0.476 m diameter. The composite agitators used a concave blade dispersing impeller surmountedby one or two up-pumping wide-blade hydrofoils. To find the influence of the density and size ofparticles on the drawdown of particles, all solids were cleaned up to four times with pure alcoholuntil they were hydrophilic. The influence on suspension and gas retention of solid density (900-955 kg·m^-3) and size (0.5-4 mm), baffle width, and agitator centering have been studied. Thereis little difference in the gas retention behavior in the presence of similarly sized differentparticles. Gas holdup is relatively unaffected by particle size, though particles <1 mm tend toreduce gas retention. Baffle width has significant effects: drawdown is easier with either narrow-or full-width baffles than with midwidth baffles, which need higher agitator speeds. In a fullybaffled three-phase system, the drawdown of buoyant particles results from the bulk velocityand turbulence intensity, while with narrow baffles, conditions appear to be controlled by thedevelopment of a central vortex. However, gas dispersion in such a partially baffled system isnot as effective as when fully baffled, so the fully baffled conditions are recommended for thesparged drawdown of buoyant particles. Despite the possible mechanical drawbacks, off-centeredagitators achieve effective drawdown of buoyant particles in both sparged and unaerated systems.Off-centering the agitator does not affect the retained void fraction except at low gassing ratesand specific power input.