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Gas absorption in cocurrent turbulent bubble flow

University of British Columbia

Mass transfer rates have been measured for streams of CO₂ bubbles of controlled frequency being absorbed into water in cocurrent pipeline flow. Superficial liquid Reynolds number varied from 1810 to 24000. Mass transfer coefficients based on equivalent spherical areas were between 0.6 and 4.5 cm/min. For 5/16- and 5/8 inch I.D. tubes oriented both horizontally and vertically, the mass transfer coefficients were proportional to (Reynolds number)⁰•⁵² and (tube diameter) ⁻⁰•⁸⁵ at high Reynolds number. Bubble velocities were measured for all test sections and flow conditions. Photographs of bubbles in turbulent flow were obtained by a high speed flash technique. The mass transfer results support a postulated mechanism of surface renewal by turbulent eddies which result from the mean flow of liquid through the tube. Two theoretical approaches have been described in an attempt to relate the surface renewal rate to the pipe flow turbulence. A model based on mixing length theory gives good agreement with the experimental results. In this model the larger scales of motion dominate. A second model was based on the assumption that the very small scales dominate. The flow and convective diffusion equations were solved for idealized viscous eddy cells which represent the small motions. The size, velocity and mass transfer rate of these cells were linked to the turbulent energy spectrum for both solid/liquid and gas/liquid interfaces. The predicted dependence of mass transfer coefficient on Schmidt number and energy dissipation is identical with experimental results for solid surfaces. However, the Reynolds number dependence (Re•⁶⁹) is higher than for the present experiments. Nevertheless, the eddy cell model maybe valid for bubbles and solids in sufficiently highly developed turbulence.

Text

2011-08-24

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http://hdl.handle.net/2429/36860

Chemical and Biological Engineering

University of British Columbia

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