UBC Theses and Dissertations
Manufacture of hydroxylamine by reduction of nitric oxide in trickle-bed electrochemical reactors Bathia, Mahendra Liladhar
The manufacture of hydroxylamine by electrolytic reduction of nitric oxide was experimentally investigated. The experiments were carried out in a two-compartment packed bed electrochemical reactor consisting of a cathodic bed of tungsten carbide and a lead plate anode. The compartments were separated by an anion exchange membrane which was supported by a bed of glass beads on the anode side. The reduction occurred as the nitric oxide gas and sulphuric acid electrolyte trickled down cocurrently over the tungsten carbide bed. Sulphuric acid was also circulated separately through the anode chamber. The efficiency of tungsten carbide particles for nitric oxide reduction was observed to drop between successive runs. When in operation the activity remained fairly stable for a period of twelve hours. The drop in activity between the runs appeared to be the result of surface oxidation of the particles by dissolved or free oxygen. Tungsten carbide nevertheless gave better performance than a graphite bed electrode. The operating parameters studied over the stabilized portion of the activity of fresh tungsten carbide included catholyte flowrate and composition, catholyte recycle, gas flowrate and composition, particle size, bed dimensions and reactor pressure and temperature. In typical operation at atmospheric pressure, a current efficiency of 62% and a hydroxylamine concentration of 0.03 M were obtained under a current density of 213 A m⁻² . Doubling the pressure approximately doubled the current density under which similar current efficiencies were observed. The hy-droxylamine concentration decreased sharply with increase in catholyte flowrate. Increases in gas flow-rate produced only a moderate increase in current efficiency above a certain current density. In recycling catholyte, the hydroxylamine concentration was built up almost linearly with number of passes of catholyte through the reactor without appreciable drop of current efficiency. The rate of reduction of nitric oxide to hydroxylamine appeared to be controlled by mass transfer. Appropriate mass transfer correlations for trickle-bed reactors were suitably corrected for the present system and applied to predict the theoretical limiting current densities.
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