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Scale-up of the perforated bipole trickle bed electrochemical reactor for the generation of alkaline peroxide Gupta, Neeraj

Abstract

Conventional electrochemical reactors generating alkaline hydrogen peroxide by electro-reduction of oxygen use three-dimensional electrodes in mono-polar cell stacks that operate near atmospheric pressure. The available commercial electrochemical process (eg. the Dow-Huron trickle-bed cathode) is limited to a current density of about 1 kA m-², while other systems under development (eg. the Kvaerner-Chemetics gas diffusion cathode) run at current density up to about 2 kA m-². This relatively low current density results in a high capital cost that limits the use of the electrochemical process as an alternative to the commercial thermochemical process that obtains hydrogen peroxide by the auto-oxidation of anthraquinols. The limitations to the current density in the electrochemical processes operating near atmospheric pressure are largely due to oxygen mass transfer constraints. To increase the oxygen mass transfer rate work has been done at UBC with a bipolar electrochemical reactor that runs at 800-1200 kPa. As opposed to other systems the UBC process uses a relatively simple cell configuration in which a single electrolyte flows with oxygen gas in a graphite felt cathode, sandwiched between a microporous diaphragm and a bipolar electrode plate. To compete with the commercial thermochemical process such an electrochemical reactor should operate with good current efficiency and low voltage (e.g. > 80 %, < 3 Volt) at current densities above 3 kA m-². The anodic generation of oxygen in the UBC system at current density above ca. 2 kA m-² is a problem as it inhibits the passage of current and compromises the performance of the reactor. To circumvent this problem of anode resistance experimental work was done on a perforated bipole electrochemical reactor that allows oxygen disengagement on the anodes through the perforations into the adjacent cathode bed. These perforations also allow current by-pass that translates in to a loss in current efficiency. As a guide to the development and scale-up of this system a two-cell bipolar electrochemical reactor was modelled with trickle-bed cathodes and the current by-pass through the perforated bipole accounted for. The predictions of this model were compared to the performance of a bench scale reactor operating at current density up to 5 kA m-² and used to optimize the bipole configuration. The reactor was eventually scaled-up from small scale (120 mm length by 25mm width and superficial electrode area 30e-4 m² ) to medium scale (630 mm length by 40 mm width and superficial electrode area 200e-4 m2) for two cells. The current efficiency for peroxide generation on the two-cell medium scale reactor was very encouraging (~ 80% at 5 kA m-²) and the voltages obtained were also in the desired range (~ 3.2 V per cell at 5 kA m-²).

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