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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.

Liquid phase mixing time (Ornix) was measured in mechanically agitated contactors of internal diameter 0.57 m, 1.0 m and 1.5 m. Tap water was used as the liquid phase. The impeller speed was varied in the range of 0.4-9.011s. Three types of impellers, namely disc turbine (DT), pitched blade downflow turbine (PTD) and pitched-blade upflow turbine (PTU) were employed. The ratio of impeller diameter to vessel diameter (DIT) and the ratio of impeller blade width to impeller diameter (WID) were varied over a wide range. The effects of impeller clearance from the tank bottom (C), the blade angle ($), the number of blades (n,), the blade thickness ( k ) and the total liquid height (HIT) were studied in detail. Mixing time was measured using the conductivity method.Mixing time was found to have a strong dependance on the flow pattern generated by the impeller. Mixing time was found to decrease by decreasing the impeller clearance in the case of DT and PTU. However in the case of PTD it increases with a decrease in the impeller clearance. Similar trend of the effect of impeller clearance on Omi, was observed for all the other PTD impellers with different diameter, number of blades and blade angle (except 60' and 90"). All the impeller designs were compared on the basis of power consumption and on this basis optimum design recommendations have been made. For PTD impellers, a correlation has been developed for the dimensionless mixing time. KEYWORDS Mixing time Row pattern and pumping capacity Pitched bladed turbine Impeller geometry Optimum impeller design.

Fractional gas holdup was measured in 0.57, 1.0, and 1.5 m i.d. vessels. Pitched blade downflow turbines (PTD) were used as the impeller. Design details of the impeller, such as the impeller diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.4 D), were studied. The effect of sparger type, geometry and size on fractional gas hold‐up has been investigated in detail. Four different types of spargers (pipe, conical, ring and concentric ring spargers) were used. Sparger location was varied for all the types studied. Further, design details of the ring sparger, which gave the highest hold‐up were then studied in detail. These included ring diameter, number of holes and hole size. All the reported correlations for fractional gas hold‐up in mechanically agitated gas‐liquid reactors were tested and compared. A better correlation has been developed for pitched blade turbines.

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