Catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors

Liquid-phase hydrogenation using a solid Pd-Cu bimetallic catalyst offers a promising technique for the removal of nitrates from contaminated drinking water, In this study, catalytic nitrate reduction was investigated in isot hermal fixed-bed reactors at T = 298 K and atmospheric pressure. Experiment s carried out in a bubble-column fixed-bed reactor in the presence of disti lled water as a reaction medium, demonstrate that nitrates can be efficient ly removed from the Liquid-phase, and that the maximum contaminant level fo r ammonium ions in drinking water is not exceeded. The measured nitrate con versions are considerably influenced by the variation of volumetric flow ra te of either the gas- or liquid-phase. The order of magnitude analysis of a pparent rate constant and mass transfer coefficients confirms that the obse rved reaction rate is governed by the mass transfer of hydrogen from the ga s- into the bulk liquid-phase. Due to shorter mean residence times, lower n itrate conversions are measured in a trickle-bed reactor. At the given reac tion conditions, catalyst particles were directly exposed to the gas-phase in this reactor system, which drastically enhanced ammonia production. When drinking water is used as a reaction medium instead of distilled water, th e nitrate disappearance rate as well as reaction selectivity decrease appre ciably, which is attributed to the presence of dissolved ionic species. Add itionally, traces of nitrites were detected in the reactor effluent. (C) 19 99 Elsevier Science B.V. All rights reserved.