Supercritical water oxidation of high concentrations of phenol (original) (raw)
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Kinetic comparison between subcritical and supercritical water oxidation of phenol
Chemical Engineering Journal, 2001
Wet air oxidation (WAO) and supercritical water oxidation (SCWO) processes have been studied by numerous researchers, proving their effectiveness to treat a wide variety of wastes and presenting the kinetics involved in each case. As a result, a substantial amount of kinetic information describing organic reactions in those environments has been accumulated. In most cases, predictions from kinetics models obtained below and above the critical point of water are completely different. Furthermore, predictions from kinetic expressions obtained in the same range of operating conditions vary considerably.
Supercritical water oxidation of phenol with air. Experimental results and modelling
Chemical Engineering Journal, 2009
Hydrothermal oxidation is an efficient and clean way for the treatment of wastewater containing organic matter. Because of its specific properties, supercritical water ensures high conversion of a wide range of organic load in the presence of an oxidant. The purpose of this work is to develop a mathematical model for a continuous flow tubular reactor devoted to hydrothermal oxidation. This reactor has a low ratio diameter length with one air injection. The mathematical model is based on plug flow assumption. The governing equations are: momentum, mass, species and energy balances. According to this model, the profiles of temperature and concentration of chemical species are computed along the reactor. The numerical predictions of the model are compared to experimental profiles obtained in the case of supercritical oxidation of phenol. These comparisons show very good agreement.
Reaction Engineering Model for Supercritical Water Oxidation of Phenol Catalyzed by Activated Carbon
Industrial & Engineering Chemistry Research, 2003
Supercritical water oxidation is an efficient technology for the ultimate destruction of organic waste materials. We previously reported that the addition of activated carbon catalyst promoted the oxidation of phenol in supercritical water and that yield of tarry materials was remarkably suppressed at 400°C and 25 MPa. In this study, reaction kinetics of the carbon-catalyzed phenol oxidation in supercritical water was studied, and especially, the influence of mass-transfer limitation inside and outside of the catalyst particles was investigated. Experimental results indicated that mass-transfer limitation between bulk fluid and the catalyst surface was negligible whereas mass transfer within the pores of the activated carbon catalyst limited the overall reaction rate. This was in agreement with the result of the calculation of Mears' and Weisz-Prater's criteria. We then developed model equations considering the influence of mass transfer to investigate the intrinsic reaction rate and to describe the temporal change of reaction kinetics. In the model, three reactions were taken into account: homogeneous phenol oxidation, heterogeneous phenol oxidation on the catalyst surface, and combustion of carbon catalyst. The parameter values were determined by curve fitting with the experimental data. By this model, temporal changes of the mass-transfer effect and the reaction rate profile in the packed bed were determined.
Phenol oxidation over CuO/Al2O3 in supercritical water
Applied Catalysis B: Environmental, 2000
Phenol was oxidized in supercritical water at 380-450 • C and 219-300 atm, using CuO/Al 2 O 3 as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO 2 formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO 2 formation are sensitive to the phenol and O 2 concentrations, but insensitive to the water density. A dual-site Langmuir-Hinshelwood-Hougen-Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars-van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO 2 formation than did bulk MnO 2 or bulk TiO 2. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.
Influencing Parameters on Supercritical Water Reactor Design for Phenol Oxidation
2021
For accurate and reliable process design for phenol oxidation in a plug flow reactor with supercritical water, modeling can be very insightful. Here, the velocity and density distribution along the reactor have been predicted by a numerical model and variations of temperature and phenol mass fraction are calculated under various flow conditions. The numerical model shows that as we proceed along the length of the reactor the temperature falls from above 430℃ to approximately 380℃. This is because the generated heat from the exothermic reaction is less that the amount lost through the walls of the reactor. Also, along the length, the linear velocity falls to less than one-third of the initial value while the density more than doubles. This is due to the fall in temperature which results in higher density which in turn demands a lower velocity to satisfy the continuity equation. Having a higher oxygen concentration at the reactor inlet leads to much faster phenol destruction; this lea...
Kinetics of wet air oxidation of phenol
Chemical Engineering Journal, 1997
Aqueous solutions of phenol were oxidized in a batch reactor at temperatures between 150 and 300 "C and pressures from 100 to 200 bar. The initial phenol concentrations were between 460 and 1650 ppm and the initial oxygen concentration was always above 800% excess. The oxidation experiments covered essentially the entire range of phenol conversions and included all temperature ranges studied by previous workers. The reduction of COD during oxidation was also measured. Furthermore, pyrolysis experiments were carried out to verify that phenol is not degraded by this mechanism in the conditions studied. Due to disagreement in the previous published data about the activation energy, an effort has been made to obtain reliable kinetic data, assuring great oxygen excess and minimum disturbance in the sampling procedure. The oxidation reaction was found to be pseudo-first order with respect to phenol, with an activation energy of 34.4 kJ mol-'. The influence of pressure and temperature on the induction time was also studied.
The process of deep phenol oxidation in electrochemically activated medium has been studied in an effort to develop the process for neutralizing the wastes of highly toxic matter. The known scheme of phenol destruction to carbon dioxide and water through the stage of p-benzoquinone and carboxylic acid formation has been confirmed. The comparative an alysis of specific reaction rates of phenol oxidation has been conducted using the various oxidizers including oxygen, hydrogen peroxide in the presence of Fe 2+ , and ozone and also by means of electrochemical oxidation.
Catalytic Supercritical Water Oxidation: Phenol Conversion and Product Selectivity
Environmental Science & Technology, 1995
This work investigated a novel nonpetroleum-based catalytic process of methanol to phenol. The idea was to convert methanol to produce a main product stream having a molar ratio of propylene to benzene/toluene of unity along with relatively higher-value products including para-xylene and alkenes. Such a product mix would be ideal for the manufacturing of phenol. This was achieved using a catalyst of 1.5 wt% zinc impregnated on a silica-deposited HZSM-5 zeolite at 0.1 MPa, 430 C and 1.2 h À1 weight hourly space velocity. HZSM-5, with its acidic sites predominately being Brønsted acid, produced mainly alkanes and aromatics, of which a good fraction was undesirable nine-or more-nine-carbon higher aromatics. Silica deposition on HZSM-5 passivated the catalytic activity outside the HZSM-5 pores, resulting in an increase of alkenes selectivity, a sharp decrease of nine-or more-nine-carbon higher aromatics selectivity, and a shift of the xylene product from an equilibrium mixture of meta-xylene, para-xylene, and ortho-xylene to mostly para-xylene. Impregnation of 1.5 wt% zinc on silica-deposited HZSM-5 generated more Lewis acid sites and further increased alkene selectivity, which, with the proper selection of process conditions, led to the production of the target stream. A detailed analysis of the effects of silica deposition, zinc impregnation, acidic sites, and process conditions on the catalyst performance was presented.