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Abstract
In a Confined Plunging Liquid Jet Contactor (CPLJC) a jet of liquid is introduced into an enclosed cylindrical column (downcomer) that generates fine gas bubbles that are contacted with the bulk liquid flow. The region where the liquid jet impinges the receiving liquid and expands to the wall of the downcomer is called the Mixing Zone (MZ). In the MZ, the energy of the liquid jet is dissipated by the breakup of the entrained gas into fine bubbles, and the intense recirculation of the two-phase mixture. The study presented here was undertaken to quantify the ozone-water mass transfer performance of the MZ through the determination of the volumetric mass transfer coefficient, kLa (s−1), and to produce a model for predicting kLa based on the specific energy dissipation rate. It was found experimentally that kLa in the MZ increased with increasing superficial gas velocity. A maximum experimental kLa value of 0.84 s−1 was achieved which compares well to other contactors used in water treatment. Such a large kLa value combined with the small volume of the reactor, favorable energy requirements and safety features of the system, suggests that the CPLJC provides an attractive alternative to conventional ozone contactors. The relatively large mass transfer rates were found to be a function of the high gas holdup and fine bubble size generated in the MZ, which results in an almost froth-like consistency. A model based on the specific energy dissipation rate of the water jet, E (kg · m−1· s−3), and MZ bubble size was used to predict kLa in the MZ. Using E, the number average bubble size was predicted which was then used to calculate the liquid phase mass transfer coefficient kL. The bubble size was also used with the predicted mixing zone gas holdup to calculate the specific interfacial area, a (m−1), which was then combined with kL to determine a predicted value of kLa. The average deviation between experimental and predicted kLa was 6.2%.