Catalyst activity, stability, and transformations during oxidation in supercritical water (original) (raw)
Related papers
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.
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.
Supercritical water oxidation of high concentrations of phenol
Journal of Hazardous Materials, 2000
Decomposition of phenol at a concentration as high as 2 wt.% was effected by supercritical water oxidation at 25 MPa. Reaction temperatures ranged from 623-723 K and residence times were varied from 6.5 to 26 s. Oxygen was added in an equivalent amount to investigate reaction intermediates. The degree of phenol decomposition and reaction product were measured. Although tarry material production was observed, phenol decomposition conversion was predicted well by the reaction rate equations developed by previous researchers who conducted experiments at lower concentrations. Difference from low concentration phenol oxidation was found in the reaction product distribution and tarry material production. One possible explanation for this result is that the initiation of phenol decomposition is the same regardless of phenol concentration but that the succeeding radical reactions are different. The additive reaction between aromatic compounds was enhanced by high phenol concentration. q
Environmental Science and Pollution Research
During the past few decades, the treatment of hazardous waste and toxic phenolic compounds has become a major issue in the pharmaceutical, gas/oil, dying, and chemical industries. Considering polymerization and oxidation of phenolic compounds, supercritical water oxidation (SCWO) has gained special attention. The present study objective was to synthesize a novel in situ Fe2O3nano-catalyst in a counter-current mixing reactor by supercritical water oxidation (SCWO) method to evaluate the phenol oxidation and COD reduction at different operation conditions like oxidant ratios and concentrations. Synthesized nano-catalyst was characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM). TEM results revealed the maximum average particle size of 26.18 and 16.20 nm for preheated and non-preheated oxidant configuration, respectively. XRD showed the clear peaks of hematite at a 2θ value of 24, 33, 35.5, 49.5, 54, 62, and 64 for both catalysts treated preheated a...
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.
Oxidation catalysis in a supercritical fluid medium
Industrial & Engineering Chemistry Research, 1987
The supercritical fluid (SCF) extraction technique was extended by its application to SCF-solidcatalyzed reactions that otherwise take place in a two-fluid-phase reactor. By use of COz as a solvent, toluene was contacted with air in the presence of redox or acid catalysts and underwent partial oxidation t o primarily benzaldehyde. A screening study of several oxide and mixed-metal oxide catalysts revealed that supported COO, partly oxidized to Co(III), was the most active (10-5/s turnover number) and selective (no multiring condensation products) catalyst. The relative activity of the supported COO catalyst was not unexpected, because the Co2+/Co3+ redox couple is the most effective catalyst for this reaction in solution. The oxide was capable of duplicating the liquid-phase behavior, although at lower activity levels than promoted (for example, with Br-ions) homogeneous cobalt catalysts.
Physicochemical Treatment of Hazardous Wastes, 2003
Supercritical water oxidation (SCWO) reaction is effective in destruction of polychlorinated biphenyls (PCBs) to mineral products of CO 2 , HCl, and H 2 O such that 99.95% conversion of Aroclor 1248 is possible in 54.5 seconds residence time at 823 K and 25.3 MPa. Yet to design and operate SCWO reactors to avoid harmful products formation such as polychlorinated dibenzofurans and dioxins, it is necessary to understand the PCB reaction pathway leading to final products. This study shows the dechlorination reaction pathway of 3344-Tetrach lorobiphenyl (a surro gate for Aroclo r 1248) in methanol solutions. It is suggested that hydrogen species from methanol act as reaction rate enhancers and lead to a series reaction pathway of dechlorination to biphenyl followed by ring-opening oxidation to mineral products. Also, reaction kinetics for this system are presented. These results are of value for reactor design to ensure oxidation of these recalcitrant pollutants in an environmentally safe manner.
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...
Symmetry
This work reports supercritical water oxidation (SCWO) of organic pollutants in industrial wastewater in the absence and presence of catalysts. To increase the efficiency of the oxidation process, the SCWO of organic compounds in industrial wastewater was performed in the presence of various iron- and manganese-containing heterogeneous catalysts (Fe-Ac, Fe-OH, and Mn-Al). The catalytic and non-catalytic SCWO of organic compounds in wastewater from PJSC “Nizhnekamskneftekhim”, generated from the epoxidation of propylene with ethylbenzene hydroperoxide in the process of producing propylene oxide and styrene (PO/SM), was performed. The effect of operational parameters (temperature, pressure, residence time, type of catalysts, oxygen excess ratio, etc.) on the efficiency of the process of oxidation of organic compounds in the wastewater was studied. SCWO was studied in a flow reactor with induction heating under different temperatures (between 673.15 and 873.15 K) and at a pressure of 2...