Study On Gas-Liquid Dispersion And Fluidynamics In Aerated Stirred Tanks

Though mechanically agitated gas-liquid tanks/ reactors are widely used in chemical and processing industries, yet it shows a lack of thorough researches on it. There are only some researches at macro scale. Now the study should be at a transition from the empirical stage to the semi-empirical stage, to meet the purpose for better development and design. A deeper understanding of the fluidynamics is necessary for this aim. This study is a preliminary work along this line.A microcomputerized automatic measurement system with dual electric conductivity probe was developed first in China in this study for determining the bubble size distribution. The photographic method for measuring bubble size was used to calibrate the dual electric conductivity probe. The measurement system has the advantages of fast response, high sensitivity and convenience in use, especially of real-time data acquisition and processing.A hot-film anemometer was used in the measurement of local gas-holdup, liquid mean velocity and turbulence parameters in gas-liquid stirred tanks. An improved method for measuring local gas holdup was developed according to the characteristics of cyclindrical hot-film probe and fluid flow in aerated stirred tanks. Based on the method, the liquid mean velocity and root-mean-square(r.m.s) value of the fluctuating velocity can be measured . The influences of yaw angles of hot—film probe and fluid temperature on the measuring accuracy of hot-film anemometer were studied, and the effect of fluid temperature was corrected.All experiments were carried out in geometrically similar vessels with diameters of 0.287m, 0.495m and 1.1m respectively. Bubble size distributions were measured by the dual electric conductivity probe in upper, lower circulation regions and impeller region in the tanks. Local gas holdups were measured by the hot-film anemometer in the same regions as mentioned above. The overall gas holdup was measured by the spillover method. The effects of operating conditions on the bubble size and gas holdup and the bubble size distribution and the local gas holdup have been studied, and the correlation equations for bubble size and gas holdup are propored.Liquid mean velocity, root-mean-square(r.m.s) value of the fluctuating velocity, turbulence intensity were measured by the hot-film anemometer in the upper, lower circulation regions and the impeller region in the tank too. Turbulence energy spectra were measured in the upper and lower circulation regions. Also the effects of operation conditions on liquid mean velocity, root—mean—square(r.m.s) value of the fluctuating velocity, turbulence intensity have been studied, and the empirical correlation equations for these parameters have been obtained.