BUBBLE-SIZE DISTRIBUTIONS IN A MECHANICALLY AGITATED GAS-LIQUID CONTACTOR

Detailed studies of bubble-size distributions in mechanically agitated contactors are non-existent and there is a need for a better description of the internal dispersion structure of such systems. The gas-liquid dispersion in a 1.0 m diameter stirred vessel has been explored on a quasi-point grid using an automated capillary suction probe which was designed for bubble-size measurement in pilot-scale contactors. Local bubble-size distributions have been measured at a total of 50 different positions in the vertical mid-plane between two adjacent baffles, the vertical baffle plane and the vicinity of a baffle. Measurements were obtained mainly for an air-water system under different agitation and gassed conditions. The effect of elctrolytes was also investigated using a 0.15 M NaCl solution. The measurement technique achieved very good reproducibility and fast data processing. Results are presented in the form of spatial distribution maps of Sauter mean bubble diameters and histograms of spherical bubble diameters. The bubble-size distributions vary widely within the contactor, deviating considerably from a standard normal distribution. The effects of varying the impeller speed and the gas flow rate are analysed in each of the different regions of the vessel. The salt solution produced considerable bubble coalescence retardation which was reflected by a substantial reduction in the bubble size and more uniform dispersions. Simulated overall bubble-size distributions revealed that the coalescing and "non-coalescing" systems are well represented by a Weibull and an exponential-type distribution, respectively.