COAL CONVERSION WASTEWATER TREATMENT BY CATALYTIC OXIDATION IN SUPERCRITICAL WATER

Abstract

Wastewaters from coal-conversion processes contain phenolic compounds in appreciable concentrations. These compounds need to be removed so that the water can be discharged or re-used. Catalytic oxidation in supercritical water is one potential means of treating coal-conversion wastewaters, and this project examined the reactions of phenol over different heterogeneous oxidation catalysts in supercritical water. More specifically, we examined the oxidation of phenol over a commercial catalyst and over bulk MnO{sub 2}, bulk TiO{sub 2}, and CuO supported on Al{sub 2} O{sub 3}. We used phenol as the model pollutant because it is ubiquitous in coal-conversion wastewaters and there is a large database for non-catalytic supercritical water oxidation (SCWO) with which we can contrast results from catalytic SCWO. The overall objective of this research project is to obtain the reaction engineering information required to evaluate the utility of catalytic supercritical water oxidation for treating wastes arising from coal conversion processes. All four materials were active for catalytic supercritical water oxidation. Indeed, all four materials produced phenol conversions and CO{sub 2} yields in excess of those obtained from purely homogeneous, uncatalyzed oxidation reactions. The commercial catalyst was so active that we could not reliably measure reaction rates that were not limited by pore diffusion. Therefore, we performed experiments with bulk transition metal oxides. The bulk MnO{sub 2} and TiO{sub 2} catalysts enhance both the phenol disappearance and CO{sub 2} formation rates during SCWO. MnO{sub 2} does not affect the selectivity to CO{sub 2}, or to the phenol dimers at a given phenol conversion. However, the selectivities to CO{sub 2} are increased and the selectivities to phenol dimers are decreased in the presence of TiO{sub 2} , which are desirable trends for a catalytic SCWO process. The role of the catalyst appears to be accelerating the rate of formation of phenoxy radicals, which then react in the fluid phase by the same mechanism operative for non-catalytic SCWO of phenol. The rates of phenol disappearance and CO{sub 2} formation are sensitive to the phenol and O{sub 2} concentrations, but independent of the water density. Power-law rate expressions were developed to correlate the catalytic kinetics. The catalytic kinetics were also consistent with a Langmuir-Hinshelwood rate law derived from a dual-site mechanism comprising the following steps: reversible adsorption of phenol on one type of catalytic site, reversible dissociative adsorption of oxygen on a different type of site, and irreversible, rate-determining surface reaction between adsorbed phenol and adsorbed oxygen.