Time-Series Analysis of Optical Measurements in Gas-Liquid Stirred Tanks

This work reports findings on both the time-series analysis and the frequency domain analysis of optical probe measurements in a lab scale gas-liquid stirred tank reactor. Optical signals are collected from the tapered (conical) end optical fibers at five radial positions (r=0.4R, 0.5R, 0.6R, 0.7R, 0.8R) on the impeller discharge plane at all practical operating conditions achievable in our Chemical Reaction Engineering Laboratory (CREL). The data are then processed and analyzed in both the time and the frequency domains via MATLAB algorithms developed in house. The time-domain analysis provided useful information such as local gas phase holdup, and the frequency domain analysis revealed invaluable information regarding the chaotic nature of bubble occupancy as a function of position and operating conditions. It is found that beyond certain distance from the impeller, the distinct pattern of bubble generation by the blades becomes chaotic, i.e. the bubbles are detected at random frequencies. This characteristic distance may be applied as a criterion for the impeller design. These results prove optical probe technique’s ability to serve as a potent tool for investigation/verification of bubbles' chaotic behavior in complex gas-liquid multiphase systems.