Solid-liquid mixing in mechanically agitated vessels
Experimental data are reported for solids suspension and distribution in four geometrically similar vessels with diameters equal to 0.31, 0.61, 1.83 and 2.67 m. Agitation was provided by a series of pitched blade turbines with impeller to vessel diameter ratios from 0.3 to 0.6 and pitched angles between 30° and 90°. The effect of impeller clearance on solids suspension was examined for a clearance range of T/4 to T/8. Dual impeller systems were also studied, covering two combinations (dual pitched and flat/pitched) and impeller spacing of half to two diameters apart. The majority of the experiments were carried out with 150-210 μm round-grained sand (density: 2630 kg m-3 and settling velocity: 0.015 m s-1) and tap water. Solids concentration was varied between 0.1 to 40% by weight. Four parameters were measured; impeller speed, using an optical tachometer, power input, calculated from the shaft torque given by strain gauges, just suspension speed, ascertained both visually and by use of an ultrasonic Doppler flowmetering (UDF) technique and the local solids concentration, measured by a in-house solids concentration probe. In addition extensive flow visualisations were made with the 0.61 m vessel in order to establish both liquid and particles flow patterns during the experiments. Results from this study were compared with previous publications in order to examine the effects of some of the important geometrical variables on solids suspension and distribution. This work revealed that for the range of parameters covered, the smallest (D/T=0.3) and the largest (D/T=0.6) impellers are the most and least efficient ones for solids suspension. Distribution tests with the three geometrically similar impellers show that the results are neither correlated in terms of tip speed nor power input but are best described by the thrust force generated by the impellers. In general, dual impeller systems improve solids distribution but require more power to just suspend solids compared with a single impeller. The scaling effect proposed by Zwietering (1958) for solids suspension has been confirmed by this study for vessel up to 2.67 m in diameter. The constant tip speed rule for solids distribution, which is based on one-dimensional dispersion models was found to underestimate the power requirement in large scale applications. This study indicates that equal power per unit volume is required to achieve the same degree of homogeneity.