Catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors

Abstract

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 isothermal fixed-bed reactors at T = 298 K and atmospheric pressure. Experiments carried out in a bubble-column fixed-bed reactor in the presence of distilled water as a reaction medium, demonstrate that nitrates can be efficiently removed from the liquid-phase, and that the maximum contaminant level for ammonium ions in drinking water is not exceeded. The measured nitrate conversions are considerably influenced by the variation of volumetric flow rate of either the gas- or liquid-phase. The order of magnitude analysis of apparent rate constant and mass transfer coefficients confirms that the observed reaction rate is governed by the mass transfer of hydrogen from the gas- into the bulk liquid-phase. Due to shorter mean residence times, lower nitrate conversions are measured in a trickle-bed reactor. At the given reaction 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, the nitrate disappearance rate as well as reaction selectivity decrease appreciably, which is attributed to the presence of dissolved ionic species. Additionally, traces of nitrites were detected in the reactor effluent.