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Foamed electrolyte in flow-by fixed-bed electrodes : study of fluid dynamics & transport processes Mohandes, Abdolhosein


Experiments were carried out in bench scale electrochemical packed-bed cells to study the “fluid dynamics and transport processes” with upward flow-by foamed electrolyte.In this context, the pressure gradient, liquid holdup, axial liquid dispersion coef- ficient, flowing foam electric conductivity, and overall mass transfer capacity were measured in packed rectangular cells. All the runs were conducted at 22 degree Celsius inlet foam temperature. The cells were packed with graphite fiber felt (comprised of 20 micron diameter fibers) and the liquid phase of the flowing foam was composed of water (for dispersion experiments) and 1 N NaOH solution (for all other experiments) with 0.1 % v/v surfactant, Tergitol. The gas phase of the foam was oxygen for mass transfer experiments and nitrogen for all other experiments. The studies were performed with respect to the three independent variables of bed porosity (at three levels, except for the mass transfer measurements which were done at two levels), liquid load and gas load (at numerous levels). Empirical second order correlations including interaction terms were developed for each parameter, in terms of the three independent variables. The overall results are summarized as follows: I - Absolute value of the pressure gradient ranged from 20 to 80 bar.m⁻¹ , and is a factor of 50-100 higher than in nonfoaming systems with similar fluid loads. II - Liquid holdup ranged from 7 to 20 %, which is lower than the minimum of about 40 % observed in nonfoaming systems at similar fluid loads. III - Axial dispersion coefficient ranged from 0.01 to 0.46 cm².s⁻¹, which is below prev- iously reported values of packed bed processors with nonfoaming liquids. IV - The flowing foam effective electric conductivity ranged from 6.8 to 11.1 mho.m⁻¹, which is equivalent to the effective electrolyte conductivity in nonfoaming gas/liquid flow at 3 to 5 times the liquid holdup, as estimated by the Neale and Nader correlation. V - Overall mass transfer capacity ranged from 4.7 to 7.6 s⁻¹ which is a factor of 1.8 times the values in similar nonfoaming systems, assuming the total bed is electrochemi- cally active. Apart from the high pressure gradient, the graphite fiber packed-bed cell with foamed electrolyte shows promising performance for electrochemical processing involving gaseous reactants.

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