Showing papers on "Wet oxidation published in 2003"

Wet Air Oxidation of nitrogen-containing organic compounds and ammonia in aqueous media

Abstract: Treatment of toxic nitrogen-containing compounds is one of the major applications of the Wet Air Oxidation (WAO) processes. The aim of this paper is to review the literature dealing with the Catalytic Wet Air Oxidation (CWAO) of these nitrogenous compounds, mainly produced in chemical and pharmaceutical industries. Many studies deal with oxidation of aniline, often chosen as a model molecule of pollutant of dye industries. First, the results obtained with CWAO are compared with those obtained with other oxidation processes. Particular attention is paid to the selectivity towards organic by-products (specially, azo, nitroso and nitro compounds, phenolic compounds and carboxylic acids) as well as towards several inorganic forms of nitrogen (NH 4 + , N 2 , NO 2 − , NO 3 − ). In a second part, the review focuses on the mechanism of chemical reactions that can explain the formation of the observed products. Usually, similar catalysts can be used for CWAO of oxygen-containing (phenol, carboxylic acids) and nitrogen-containing organic compounds. Ammonia is one of the most refractory by-product formed during catalytic WAO of the nitrogen-containing organic pollutants and is itself a pollutant. For this reason, recent reports about its oxidation by the CWAO process are finally reviewed. Very high selectivities to dinitrogen can be obtained on certain noble metal catalysts. As a rule, catalysts active and selective for ammonia oxidation are different (nature of active phase, support, etc.) from the solids proven to be the best catalysts for CWAO of organic compounds. Multifunctional catalysts are, thus, required for the treatment of nitrogenous organic compounds.

Catalytic Wet Oxidation of Phenol by Hydrogen Peroxide over Pillared Clay Catalyst

Abstract: Extrudates of Al−Fe pillared clay catalyst suitable for packed-bed operations are evaluated for wastewater treatment via a wet oxidation process employing hydrogen peroxide as the oxidant. The reaction was carried out in a semibatch basket reactor under rather mild conditions. Operational parameters were studied under the following conditions: temperature from 25 to 90 °C, atmospheric pressure, initial phenol concentration from 100 to 2000 ppm of the liquid phase, catalyst loading from 0 to 10 g/L, and input H2O2 concentration from 0.15 to 0.6 mol/L. Under these conditions, the Al−Fe pillared clay catalyst achieves a total elimination of phenol and significant total organic carbon (TOC) removal. This catalyst can be used several times without any change in its catalytic properties, and hence, it would be a promising catalyst for industrial wastewater treatment. The reaction takes place to a significant extent both in the liquid phase and on the catalyst surface. Hence, apparent kinetic models were develo...

State of Ru on CeO2 and its catalytic activity in the wet oxidation of acetic acid

Abstract: The relationship between the state of Ru on CeO 2 and catalytic activity in the wet oxidation of acetic acid was investigated for Ru/CeO 2 catalysts prepared by different methods. The temperature programmed reduction (TPR) experiments of Ru/CeO 2 showed that the oxygen species of RuO 2 was reduced at different temperatures depending upon the methods of preparation. Ru species reduced at low temperatures could not be observed by TEM and XRD. It was concluded that RuOCe bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the wet oxidation.

Activity and resistance to leaching of Cu-based catalysts in the wet oxidation of phenol

Abstract: The activity and resistance to leaching of a co-precipitated CuCeOx (Cuat:Ceat=1:9; TC, 500 and 800 °C) catalyst in the wet oxidation (CWO) of phenol (TR, 150 °C; PR, 12 atm), with reference to a CuZnAlOx commercial system, has been investigated using a semi-batch “slurry” reactor with continuous feeding of oxygen. The role of homogeneous catalysis by leached Cu2+ ions on the activity pattern of solid catalysts is addressed. Thermal and chemical (“acid washing”) treatments of the CuCeOx catalyst induce marked modifications on the structure and redox properties also altering its CWO activity-selectivity pattern. The whole set of experimental findings leads to infer that the CWO of phenol on Cu-based catalysts proceeds via a homogeneous–heterogeneous path promoted by the leaching of Cu2+ ions.

Catalytic wet air oxidation of aqueous ammonia with activated carbon

Abstract: The oxidation of aqueous ammonia to nitrogen by the catalytic wet air oxidation (CWAO) process with activated carbon as catalyst was studied. A commercial activated carbon CUDU1000 chemically modified by oxidation with HNO3 and/or reduction with H2 was used. The characterization of the activated carbons was carried out by N2 adsorption, thermal programmed decomposition (TPD), Fourier transformed infrared (FTIR) spectroscopy, Boehm titration and zero point charge (ZPC) techniques. Studies of aqueous ammonia adsorption on the activated carbons under atmospheric conditions showed that carboxylic, lactonic and anhydride surface groups increase both the rate and capacity of adsorption. The oxidized carbons had lower activity towards selective aqueous ammonia oxidation in CWAO process because of a strong ammonia adsorption. However, hydrogenated activated carbons had higher activity for selective aqueous ammonia oxidation. It is establish that a strong ammonia adsorption takes place onto carboxylic, lactonic and/or anhydride surface groups while the quinonic surface groups are responsible of the catalytic activity shown by these carbons.

Role of inherent water in low-temperature oxidation of coal

Abstract: The role of water content in coal oxidation was studied using an isothermal flow reactor at atmospheric pressure and temperatures below 100°C. Transient rates of consumption of oxygen and production of CO 2 and CO were measured during oxidation experiments, by means of an online dual-column micro gas chromatograph and an oxygen analyzer. Experiments were carried out with a bituminous coal at three levels of initial water content, i.e., 0.8, 2.0, and 3.0%. Comparisons of the rates of production of carbon oxides during the oxidation experiments indicated that inherent water plays a roleinchemicalreactions occurring during coal oxidation. It was also found that the rateof oxygen consumption decreases with increasing water content of a sample. The current observations suggest that inherent water present in coal pores may react with carbonyl species to form carboxyl species during the oxidation process.

Catalytic wet oxidation of H2S to sulfur on Fe/MgO catalyst

Abstract: The room temperature wet catalytic oxidation was conducted in a batch reactor with Fe/MgO catalyst. Fe/MgO catalyst was prepared by the dissolution–precipitation method. XRD and temperature-programmed reductions (TPR) indicate that Fe oxide in the Fe/MgO is finely dispersed in the MgO support. The high H 2 S removal capacities of Fe/MgO can be explained by the finely dispersed iron oxide MgO. The H 2 S removal capacities of Fe/MgO are dependent on oxygen partial pressure (1.0 g H 2 S/g cat in air and 2.6 g H 2 S/g cat in oxygen). The valence state analysis of Fe/MgO catalyst suggests that the H 2 S oxidation on Fe/MgO can occur by a redox couple reaction, reducing Fe 3+ into Fe 2+ by H 2 S and oxidizing Fe 2+ to Fe 3+ by O 2 .

Kinetics of Wet Air Oxidation of Phenol over a Novel Catalyst

Abstract: The wet air oxidation kinetics of an aqueous solution of phenol was studied over extrudates of an Al−Fe pillared clay catalyst in the temperature range of 90−150 °C and air pressure range of 0.8−2.5 MPa. The variables studied included reaction temperature, air pressure, solution pH, initial phenol concentration, and catalyst loading. The obtained findings are compared with those obtained using hydrogen peroxide oxidation. A group of kinetic models considering both the power-law and Langmuir−Hinshelwood approaches was evaluated to describe the catalytic kinetics of the phenol disappearance. The nonlinear dependence of the phenol conversion rate on the catalyst concentration was taken into account via the empirical power-law function of this variable. Finally, a kinetic model was discriminated on the basis of equilibrium adsorption of phenol and dissociated oxygen on two distinct types of active sites. The rate-controlling step was assumed to be the surface reaction between adsorbed reactant species. This m...

Platinum catalysed wet oxidation of phenol in a stirred slurry reactor: A practical operation window

Abstract: The catalytic performance of graphite supported platinum (5 wt.%) catalyst in liquid phase oxidation has been studied using a continuous flow stirred tank slurry reactor (CSTR) in order to determine the proper operation window. The study was carried out in a temperature range of 120–180 °C and in a total pressure range of 1.5–2.0 MPa. Other operational variables employed were oxygen partial pressure (0.01–0.8 MPa), initial phenol feed concentration (0.005–0.07 M), and catalyst concentration from 1 to 10 kg m−3. It was found that the extent of oxygen coverage on the platinum surface determines the reaction pathway and selectivity to CO2 and H2O. Complete oxidation of phenol to CO2 and H2O could be achieved at 150 °C when the reaction proceeds within the range of weight specific oxygen loads of 0.15–0.35 mol s−1 kgPt−1 and at stoichiometric oxygen excess in the range of 0–80%. The activity of the platinum catalyst remained high when the residual partial pressure of oxygen in the reactor was kept below 150 kPa. Higher residual oxygen partial pressure resulted into deactivation of the platinum catalyst (over-oxidation), which was temporary and could be reversed at reducing conditions. The formation of p-benzoquinone, followed by the formation of polymeric products was also favoured at higher oxygen load, which resulted into permanent deactivation of the platinum catalyst (poisoning). While the platinum surface was vulnerable to poisoning by carbonaceous compounds when insufficient oxygen was used, a fully reduced platinum surface favoured the formation of acetic and succinic acids which are difficult to oxidize. Higher temperatures can enhance the activity of the platinum catalyst, while at lower temperatures catalyst deactivation occurs with increased formation of polymeric products and lower selectivity to CO2 and H2O. In order to maintain the catalyst within the proper operation window, a CSTR is the preferred reactor.

Thiocyanate Wet Oxidation

Abstract: Aqueous solutions of thiocyanate were oxidized in a semi-batch reactor at temperatures between 170 and 210 °C and system total pressures ranging from 70 to 100 atm. The initial pH of the solution was set at 12, while initial concentrations have ranged between 20 and 1000 ppm according to the typical concentrations of thiocyanate-containing wastewater. The kinetic data discussion was based on the establishment of a kinetic regime control that guaranteed the validity of the experimental data and that oxygen excess was assured for all the runs. An attempt to establish a thiocyanate wet oxidation pathway has been made on the basis of the reaction intermediates and final oxidation products identified during the experimental work. SO42- and CO3- have been pointed out as the main final reaction products whereas (SCN)2 and SCNO- are suggested as possible short-life oxidation intermediates. The oxidation reaction results were fitted to a first-order kinetic equation with respect to thiocyanate and to 0.81 with res...

Catalytic and Noncatalytic Wet Oxidation of Formaldehyde. A Novel Kinetic Model

Abstract: Formaldehyde is a carcinogen compound and one of the most important pollutants contained in wastewaters. Three different treatments were evaluated in a high-pressure batch reactor for the total organic carbon (TOC) degradation of formaldehyde solutions: thermolysis, noncatalytic wet oxidation, and catalytic wet oxidation over a CuO−ZnO/Al2O3 catalyst. The absence of the catalyst leads to a predominant induction period (30 min) without changes in TOC concentrations, while the catalytic treatment leads to significant enhancement in TOC reduction. In the catalytic experiments, an asymptotic behavior was observed with a final TOC reduction of approximately 80%, with the remaining nonoxidizable TOC being due to methanol, a refractory compound contained in the formaldehyde solution that is resistant to oxidation even with increasing temperature and pressure. Formic acid was identified as an intermediary compound, and a new kinetic model was developed, designed as the modified generalized kinetic model, to acco...

Method and apparatus for generating H20 to be used in a wet oxidation process to form SiO2 on a silicon surface

Abstract: Chemical generator and method for generating a chemical species at a point of use such as the chamber of a reactor in which a workpiece such as a semiconductor wafer is to be processed. The species is generated by creating free radicals, and combining the free radicals to form the chemical species at the point of use.

Experimental and predictive approach for determining wet air oxidation reaction pathways in synthetic wastewaters

Abstract: The wet air oxidation of aqueous solutions of aniline over a Ru/CeO 2 catalyst was investigated. Batch oxidation experiments were performed at temperatures between 160 and 230C and an oxygen partial pressure of 2 MPa. Liquid phase reaction intermediates were identified and their concentration-time profiles were followed by means of high-performance liquid chromatography. Gas chromatography was also used to follow the concentration of carbon dioxide formed; therefore the extent of total oxidation that had occurred was measured. Based on the experimentally determined reaction intermediates a reaction mechanism for the oxidation of aniline was proposed. In addition a computational approach based upon thermodynamics was used to determine possible reaction pathways. It appears that the theoretical and experimental approaches are in good agreement and provide complementary information that can be used for refining reaction pathways.

Oxidation Behavior of High-speed Steels in Dry and Wet Atmospheres

Abstract: The oxidation behavior of high-speed steel (HSS), which is used as the work rolls in hot strip mills, was examined under both wet and dry oxidation conditions. In a dry atmosphere, carbides as well as the martensite matrix were oxidized, while only the matrix was oxidized in the wet atmosphere. After dry oxidation, the M 2 C-, M 6 C- and M 7 C 3 -type carbides maintained their original shapes, while the MC-type carbides were oxidized into parallelepiped (orthorhombic) crystals. The parallelepiped oxides were easily removed from the sample surface due to their low adhesion strength. Double-layered oxides were formed after oxidizing the matrix, in dry as well as wet atmospheres. The outer layer showed a dense structure after dry oxidation, while a columnar and porous layer was formed in the wet atmosphere. In the early stages of oxidation, the high-speed steels oxidized following the parabolic rate law in both the dry and wet atmospheres. The parabolic weight change transformed to a linear one, when the oxide thickness was >1.3 μm by dry oxidation. The transition to a linear weight change was not observed in the wet oxidation. It is believed that, in a dry atmosphere, cracks occurred due to stress accumulation in the oxide layer, while the porous oxide layer hindered crack formation during wet oxidation.