A model for Kolbe electrolysis in a parallel plate reactor

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A parallel-plate reactor model is developed for the Kolbe electrolysis of acetate to ethane and carbon dioxide with hydrogen evolution as the counterelectrode reaction. The parallel-plate reactor is considered to consist of three zones: a turbulent bulk region in which streamwise convection is the dominant mass-transport mechanism (plug-flow model) and a thin diffusion layer at each electrode where diffusion and migration mass transport are dominant (Nernst diffusion-layer model). The acetic acid solution is supported with sodium hydroxide, and the reactor is under steady cell-potential control. Gaseous products are tracked by a hypothetical gas layer which increases in thickness in the streamwise direction. The gas phase is assumed to be an ideal, three-component mixture of hydrogen, carbon dioxide and ethane; the liquid phase consists of acetate, proton, acetic acid, and sodium and hydroxyl ions. The model predicts streamwise profiles of concentration, current density, gas-void fraction, and gas and liquid velocities in addition to reactant conversion, and cell-polarization characteristics. The average current density exhibits a maximum at a base-to-acid ratio of 0.96 due to the weak-acid/strong-base chemistry and a broad maximum at an interelectrode spacing of 0.37 cm resulting from minimized ohmic losses.

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