Critical impeller speed for solid suspension in mechanically agitated contactors

Abstract

Critical impeller speed for the suspension of solid particles, N,,, has been measured in 0.3, 0.4,0.57, 1 .O, and 1.5 m ID mechanically agitated contactors. Tap water and quartz particles (100, 340, 700, 850, 2,000 pm) were used as liquid and solid phase, respectively. The impeller speed was varied from 3.5 to 13.3 r / s and solid loading from 0 to 50 wt. YO.Three types of impellers were employed: disk turbine, pitched turbine downflow, and pitched turbine upflow. The impeller diameter to vessel diameter ratio was varied in the range 0.175 to 0.58 and the impeller blade width to impeller diameter ratio was varied in the range 0.25 to 0.4. The impeller clearance from the tank bottom was varied from 0.5 to 0.167 of tank diameter. The effect of impeller blade thickness was also studied.The pitched-blade impellers were found to be more efficient than a conventional disk turbine, and the pitched turbine downflow type was found to be more efficient than a pitched turbine upflow impeller. An attempt has been made to explain the mechanism of suspension on a rational basis and a correlation has been proposed for the estimation of Ncs that is expected to be useful in design.

The mean flow field generated by a pitched blade turbine: Changes in the circulation pattern due to impeller geometry

“…Jaworski and co-authors ( 199 1 ) may have been the first to present precise flow patterns in a stirred tank equipped with a PTD, which results replaced the relatively rough sketches used in early papers (Raghav Rao and Joshi, 1988;Rewatkar et al, 1990). Jaworski et a].…”

“…Therefore, compared with the disc turbine, information on the PTD flow field is less detailed. Early researchers (Joshi et al, 1982;Raghav Rao and Joshi, 1988) considered the PTD as a type of axial flow impeller and they thought that the primary circulation loop existed throughout the entire tank. So, they conducted a series of studies on modelling these estimated flow patterns, but the deviation of the prediction and experiment was somewhat regular and large.…”

“…The characteristics of the flow strongly depend on the types and geometries of the impellers. Recently, pitched blade downflow type turbines (PTD) have been found to be more efficient with respect to liquid phase mixing and solid suspension than other impellers (Pandit and Joshi, 1983;Raghav Rao et al, 1988). The aim of this work is to study the three dimensional, circulating turbulent flow field formed by an impeller in order to be able to predict the flow field formed by a given impeller in the new geometric configuration, and to be able to characterize the important features of the flow field in all parts of the tank.…”

Suspension of ultrahigh concentration solids in an agitated vessel

“…The extra energy required at higher solid loadings is to compensate for the energy dissipation due to increased solid–liquid friction, particle–particle collisions, and particle‐equipment collisions 3. Raghava Rao et al4 and Bubbico et al3 agreed that the energy loss due to this mechanism is negligible at low‐solids concentration (< 0.04 v/v), but becomes appreciable at high‐solids loading. To achieve complete off‐bottom suspension at higher solids loading and minimize the energy dissipation at the same time, significant modifications in vessel/impeller geometry or vessel operating conditions are required.…”

An Eulerian/Lagrangian study of solid suspension in stirred tanks

“…These include the diameter and height of the tank, the type, diameter, height, width, pitch, and number of the blades. There have been a number of mostly experimental studies of suspension and empirical criteria have been evolved over the past several decades for the design and for determining the critical impeller speed of impellers 1–6. A large amount of literature dealing with solid suspension using different types of impellers is available.…”