Removal of nitrogenous compounds by catalytic wet air oxidation. Kinetic study.

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200-325 degrees C) and pressure (up to 150 bar). However, organic nitrogen can be relatively resistant to oxidation and can be harmful to the environment. In the course of treatment, organic nitrogen (N-Org) is converted into ammonia (NH(3)), while organic carbon (C-Org) is converted mainly into carbon dioxide (CO(2)). This can be done without catalysts. In the presence of Mn/Ce composite oxides, it is possible to transform ammonia into molecular nitrogen at a temperature close to 260 degrees C. The direct conversion of organic nitrogen into molecular nitrogen also can be achieved using the same catalyst. This paper discusses the results obtained during the treatment of nitrogenous compounds like aniline, nitrophenol, beta-alanine and ammonia. Laboratory investigations were conducted in a stirred batch reactor with Mn/Ce composite oxides as catalysts. Very limited amounts of nitrites and nitrates were observed with amines, but more significant quantities were found with nitro-compounds. The kinetics of oxidation of ammonia, organic compounds, and more particularly aniline, were investigated. The treatment of a real waste (process wastewater) was also investigated. The dependence of the transformation rate on various parameters (amount of catalyst, temperature, etc.) was established. The rates of oxidation are described by first-order kinetic laws with respect to the various nitrogen species (aniline, NH(3)). Several parallel pathways are considered for the transformation of organic nitrogen, amongst which is an interaction with the catalyst surface. The orders with respect to oxygen and catalyst are established.

Treatment of organic aqueous wastes: Wet air oxidation and Wet Peroxide Oxidation.

There is growing concern about the problems of waste elimination. We should consider our environment as being borrowed from future generations and refrain from leaving a legacy of problems we are not able to solve. Various oxidation techniques are suited for the elimination of organic aqueous wastes, but because of the environmental drawbacks of incineration, enclosed processes, like liquid phase oxidation should be preferred. Wet air oxidation (WAO) under high temperature (200-325 degrees C) and pressure (50-150 bar) is suited to such liquid wastes and various catalysts, including hydrogen peroxide, can be used in order to increase the efficiency without increasing temperature and pressure. Wet Peroxide Oxidation (WPO) is a similar process. A comparable oxidation efficiency is obtained when using hydrogen peroxide as the oxidising agent instead of oxygen, at a temperature of only 100-120 degrees C. As opposed to WAO, which is capital intensive, WPO needs limited capital but generates higher running costs. The paper reviews the major results obtained for both processes and assesses the field of possible application of each of them. TOC removal efficiencies typically obtained range from 65 to 90% or more for most of the pollutants.

Subcritical wet oxidation of municipal sewage sludge: comparison of batch and continuous experiments.

Wet oxidation in subcritical conditions is a new alternative to usual routes for sewage sludge treatment and it complies with environmental standards. The paper presents tests carried out on a batch reactor and on a continuous pilot unit, treating municipal sewage sludge. A method is proposed that shows that the oxidation efficiency in a continuous reactor can only be easily predicted from the residence time distribution and batch tests results. Nevertheless, a partial settling of the solid residue in the continuous bubble column reactor is evident, and it increases the solid residence time and then decreases its organic content with respect to a similar batch test. In addition, these results highlight the considerable influence of temperature in the oxidation reactor and of the type of sewage sludge which is treated. At temperatures around 240 degrees C, foaming can seriously impair the operation of the continuous reactor, because of the presence of non-degraded fatty compounds and surfactants. Moreover, the COD reduction is limited to 70%. On the contrary, at 300 degrees C, COD removal efficiency greater than 80% is achieved without any catalyst additive and, in addition, only highly biodegradable compounds remain in the oxidised liquor.

Mass transfer in VOC adsorption on zeolite: experimental and theoretical breakthrough curves.

From experimental results of adsorption of volatile organic compounds (VOCs) on zeolite, we propose simulations of the breakthrough curves based on the Linear Driving Force model. Experiments were run on fixed beds of hydrophobic commercial zeolites. Pollutants chosen are from several chemical classes with different polarities. A good agreement between experimental and numerical results is found when an adjustable value of the internal mass-transfer coefficient is used. A constant value of effective diffusivity is found independent of the nature and the amount of VOCs adsorbed. A relation linking intrapellet mass-transfer coefficient and equilibrium constant is proposed, including the average effective diffusivity, to make predictions of breakthrough curves for any kind of volatile organic pollutant in gaseous effluents.