USING THE DYNAMIC GAS DISENGAGEMENT TECHNIQUE TO CHARACTERIZE GAS/LIQUID CONTACTING IN MICROBUBBLE-AERATED COLUMNS
Gas-liquid contacting plays a significant role in the chemical, petrochemical, mineral processing, and biochemical industries, where it is encountered in a wide range of operations. Generally, high volumetric mass transfer coefficients (kLa) are required to achieve better performance in such operations. Unfortunately, the contaminants present in virtually all natural and industrial streams result in lowering kL values that are only a fraction of their original value in clean waters. This situation can be partially compensated for by using fine bubbles that generate large specific interfacial contact areas of contact between the phases. This approach is widely used to intensify multiphase operations. Regrettably, the techniques usually used to measure bubble size distributions, BSD, and the associated specific interfacial contact areas do not perform well for microbubble systems with large gas holdups and interfacial areas. An advanced version of the dynamic gas disengagement technique (often used to obtain a crude two-class approximation of the BSD) was therefore developed. It is capable of generating relatively reasonable and reproducible estimates of the fine BSDs and the ensuing large interfacial contact area encountered in microbubble-aerated columns. This investigation was conducted using a pilot-scale bubble column, and contaminated systems exhibiting a wide range of coalescence-retarding characteristics were used to ensure the relevance of the findings to industrial practice. Microbubbles were generated using an innovative adjustable dual-phase venturi (ADPV) sparger operating over a broad range of conditions. It can also uninterruptedly alter the quality of the dispersion, thereby meeting the process control needs. The newly developed advanced DGD data analysis approach was applied to determine the BSDs prevailing in the two hydrodynamic regions observed in microbubble-aerated columns (the sparger region and that across the column). It is based on the use of several models describing the drag forces acting on a single bubble, combined with those accounting for the interaction between adjacent bubbles, to obtain highly-reproducible estimates of several multiphase characteristics. This technique was used to successfully analyze 313 experimental runs with relatively high reproducibility. These experiments covered gas holdups varying between 0.2 - 35 % and estimated Sauter mean bubble diameter varying between 140 and 2,400 microns. Although this approach is somewhat limited in providing accurate values for the gas/liquid dispersion characteristics, most of these uncertainties disappear when addressing the relative changes achieved by varying operating and design parameters. As reported by many previous investigators, the estimated BSDs encountered in the microbubble-aerated column closely fit the Log-Normal, with the mean and variance of the distribution being affected by the system's physical properties and operating conditions of the gas/liquid contactor. Finally, the proposed approach for intensifying gas/liquid contacting was found to be capable of generating interfacial areas as high as 5,470 m2/m3 and bubble population densities as high as 7.4*107 m-3 in the region close to the sparger. Somewhat smaller values were observed in the whole column.