Janez Levec

Catalytic oxidation of organics in aqueous solutions: I. Kinetics of phenol oxidation

Abstract Liquid-phase oxidation using a solid catalyst provides a potential method for removal of dissolved toxic organic pollutants from waste waters. Catalytic oxidation of phenol in an aqueous solution was studied in a semibatch slurry reactor. A catalyst comprising ZnO, CuO, and A1203 was found to be effective for converting phenol to nontoxic compounds via different intermediate products at pressures slightly above atmospheric and temperatures below 130°C. Rate measurements showed that the reaction progressed autocatalytically and involved a heterogeneous-homogeneous free radical mechanism and stepwise addition polymerization. The proposed rate equation for the phenol disappearance, which exhibits linear behavior with respect to the phenol concentration and one-fourth power of the oxygen partial pressure, is expressed in terms of the initial phenol concentration as well as the catalyst concentration. Apparent activation energy for thecatalytic oxidation of phenol was found to be 84 kJ/mol in a temperature range of 105 to 130°C.

Kinetics of the Catalytic Liquid-Phase Hydrogenation of Aqueous Nitrate Solutions

Abstract Liquid-phase reduction using a solid Pd/Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. Kinetic measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi-batch slurry reactor operating at atmospheric pressure. The effects of catalyst loading and initial nitrate concentration on the reaction rate were also investigated. The proposed intrinsic rate expression for nitrate disappearance is based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. The apparent activation energy for catalytic liquid-phase nitrate reduction and the heat of nitrate adsorption, in the temperature range 280.5–293 K, were found to be 47 and 22 kJ/mol, respectively. It is confirmed that the process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions undergoes a redox mechanism.

Catalytic oxidation of aqueous solutions of organics. An effective method for removal of toxic pollutants from waste waters

Abstract Catalytic liquid-phase oxidation of aqueous solutions of organics is presented as a potential, advanced waste water treatment technology. Catalysts are briefly reviewed first, followed by mechanistic speculations and kinetics that have been proposed for liquid-phase oxidation of some model pollutants. Subsequently, oxidation reactors and potential process schemes are discussed.

Catalytic liquid-phase oxidation of refractory organics in waste water

Abstract Liquid-phase oxidation of phenol, p - Chlorophenol, and p- nitrophenol by oxygen to carbon dioxide over a catalyst comprising CuO,ZnO, and λ-alumina in a slurry system undergoes heterogeneous - homogeneous free radical mechanism and behaves as an autocatalytic reaction system. The rate of pollutant disappearance exhibits a linear behavior with respect to pollutant concentration, but oxygen dependence differs for each pollutant

Catalytic denitrification : direct and indirect removal of nitrates from potable water

Abstract A nitrate removal process that drastically reduces salt consumption and waste discharge has been developed on a bench scale. Nitrate is removed by chloride ion exchange, and the strong-base anion resin is completely regenerated at mild reaction conditions (i.e., ambient temperature, atmospheric pressure) in a closed circuit containing a single-flow fixed-bed reactor packed with a Pd–Cu/γ-Al 2 O 3 catalyst. The combined treatment system avoids direct contact between the denitrification reactor and the water to be treated, and minimizes operational problems associated with each separate technique. No dissolution of Pd and Cu metallic-phases was observed at the given operating conditions.

Integrated ion exchange/catalytic process for efficient removal of nitrates from drinking water

A nitrate removal process that drastically reduces salt consumption and waste discharge has been developed on a bench scale. Nitrate is removed by chloride ion-exchange, and the strong-base anion resin is completely regenerated at mild reaction conditions (i.e., ambient temperature, atmospheric pressure) in a closed circuit containing a single-flow fixed-bed reactor packed with a Pd-Cu/γ-Al 2 O 3 catalyst. The combined treatment system avoids direct contact between the denitrification reactor and the water to be treated, and minimizes operational problems associated with each separate technique. No dissolution of Pd and Cu metallic phases was observed at the given operating conditions.

Catalytic liquid-phase oxidation of aqueous phenol solutions in a trickle-bed reactor

Aqueous-phase deep oxidation at comparatively low temperatures and pressures made possible by the use of heterogeneous catalysts is a promising technique for the destruction of organic water pollutants. Catalytic liquid-phase oxidation of aqueous phenol solutions was investigated in an isothermal trickle-bed reactor at T=403-423 K and oxygen partial pressure of 7 bar. The results obtained in the presence of a catalyst composed of supported copper, zinc, and cobalt oxides show that during the reaction course only small amounts of aromatic and aliphatic hydrocarbons are accumulated in the liquid phase, thus resulting to a constant pH value of the aqueous solution along the axial reactor coordinate. In the off-gas, no carbon monoxide was detected at any operating conditions. Process simulation using one-dimensional axial dispersion and plug-flow models demonstrates that efficiency of the catalyst bed for phenol removal is influenced by the mass-transfer rate of oxygen from the gas phase to the bulk liquid phase, and by resistance due to a surface reaction step. It is believed that partial wetting of catalyst particles in a trickle-bed reactor increases phenol conversion to intermediates and CO 2 as the main reaction product, through the formation of a larger number of active sites on the catalyst surface. Finally, it has been observed that at the given operating conditions metal oxide phases are leached into the aqueous solution.

Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions

Abstract Catalytic liquid-phase oxidation of aqueous solutions of substituted phenols ( p -chlorophenol and p -nitrophenol) was studied in a differential, liquid-full operated fixed bed reactor. Experiments were performed in a wide concentration range of both reactants, i.e. dissolved model pollutant and oxygen, at reaction temperatures between 150 and 190°C and total pressure of 30 bar. A proprietary catalyst containing supported copper, zinc and cobalt oxides was found to be effective for converting phenols via intermediates such as benzenedioles, quinones, and the C-4 compounds to carbon dioxide. A proposed intrinsic rate expression for disappearance of phenols is based on the Langmuir-Hinshelwood kinetic formulation, accounting for equilibrium model pollutant as well as dissociative oxygen adsorption processes on different types of active sites, and an initial hydroxyl hydrogen abstraction reaction to be the rate controlling step. The oxidation rates of phenols decrease as the Hammett constants of the substituents become more positive, reflecting trends in the basicity and nucleophilicity of substituted phenols. It is believed that heterogeneously catalysed aqueous-phase oxidation of phenols undergoes a combined redox and nonbranched-chain free-radical mechanism which involves initiation on the catalyst surface and, due to a high solid to liquid ratio in a fixed bed reactor, predominant heterogeneous propagation and termination. The involvement of a free-radical mechanism is indicated by the intermediates formed, and free-radical initiator and inhibitor effects on observed disappearance rates of phenols.

Trickle-Bed Oxidation Reactors

Abstract For reactions necesitating a solid catalyst and which invoive both reisuvely volatile and nonvclarile reactants, three-phase reactirs are required. Equipment used to achieve intimate contacting of the three phases has been procominantly in the form of slurry reactors, analogous to the stirred-tank homogenous system, or fixed-bed reactors in which the two fluid phases flow through a stationary bed of catalyst particles. Trickle-bed reactors are a type ofthe second classification in which both gas and liquid flow downward through the catalyst bed. Such systems avoid the disadvantage of separating small catalyst particles from the fluid product streams associated with slurry reactors, and also avoid the limitation of flow rates uncountered with upflow or countercurrent flow through fixed beds.

The role of catalyst in supercritical water oxidation of acetic acid

Abstract The oxidation kinetics of acetic acid in supercritical water (1.02≤ T r ≤1.15 and 1.04≤ P r ≤1.13) was examined in the homogeneous phase as well as in the presence of a solid catalyst consisting of supported copper, zinc, and cobalt oxides. For the conditions studied, the uncatalyzed oxidation reaction was found to be first order in acetic acid and 0.3 order in oxygen, with an activation energy of 182 kJ mol −1 . The rate of catalyzed oxidation was found to be well described by means of the power-law kinetic formulation based on non-uniform surfaces. It is postulated that oxygen is adsorbed on active sites and that a reaction between adsorbed species and organic molecules from the van der Waals sublayer forms a carbonate complex which further decomposes to carbon dioxide and water. The apparent activation energy of catalyzed oxidation is 110 kJ mol −1 . The observed products in uncatalyzed supercritical water oxidation were carbon monoxide, carbon dioxide, and water. The oxidation of acetic acid over transition metal oxides favors the production of carbon dioxide over carbon monoxide.