Wet Air Oxidation

The catalytic batch wet air oxidation of maleic acid over 5wt.% Ru/CeO2 catalysts was investigated at temperatures of 160 and 200°C and an oxygen partial pressure of 20bar. Additional uncatalysed experiments were carried out and the results are compared to those of the catalytic runs. The concentration profiles of liquid phase reaction intermediates were followed by means of high-performance liquid

Phenol laden waste streams are not considered suitable for conventional biological treatment if phenol is present in amounts higher than 70 mg/L. Catalytic wet air oxidation (CWAO) is one of the potential methods for treating waste streams containing such compounds. The present work investigated the degradation of synthetic wastewater contaminated with phenol (1 g/L) using a CWAO process in the presence of homogenous (CuSO4) and heterogeneous (LaCoO3, CuX and CuO-ZnO/CeO2) catalysts. The reaction was conducted at low temperature (90°C) and atmospheric pressure, and at moderate operating conditions (T ⪈ 160°C and total pressure ⪇0.8 MPa). Among all the tested catalysts, CuSO4, LaCoO3 and CuX were used for WAO reactions performed at atmospheric pressure conditions, whereas the performance of CuO-ZnO/CeO2 catalyst was tested in above atmospheric pressure studies. At atmospheric conditions, homogeneous CuSO4 was found to be the best showing ca. 90% phenol degradation and ca. 83% chemical oxygen demand (COD) reduction after a 24 h reaction period. The above atmospheric studies with catalyst showed ca. 82% phenol reduction and 54% COD removal within 3 h reaction time. The effect of pH, catalyst concentration, pressure and temperature are also reported. Finally, the major results are summarized and the recommendations for future work are given.

Shake flask experiments were conducted to determine the biodegradability of aqueous linear alkylbenzene sulfonate (LAS) and LAS (1600 mg dm−3) subjected to wet air oxidation (WAO), to assess the suitability of WAO as a pre-treatment for biological degradation. The effects of WAO temperature (180–240 °C) and the concentration of the orthophosphoric acid catalyst (0–1.0 mol dm−3) were investigated. Results showed that a higher WAO temperature increased the biodegradability of the WAO effluent. This was due to a greater removal of both recalcitrant sulfonated organics and organic concentration (TOC and COD). Conversely, greater orthophosphoric acid concentrations decreased the biodegradability of the WAO effluents. This was because the higher acid concentration increased the ionic strength and changed the WAO intermediate and product distribution, inhibiting microbial action. Nevertheless, the effluents from both variations of WAO were still more biodegradable than LAS at equivalent concentrations. However, since higher WAO temperatures can substantially increase capital costs, future work should focus on developing a WAO catalyst that both desulfonates and mildly oxidises LAS at moderate temperatures (200 °C).© 2002 Society of Chemical Industry

Shake flask experiments were conducted to determine the biodegradability of aqueous linear alkylbenzene sulfonate (LAS) and LAS (1600 mg dm−3) subjected to wet air oxidation (WAO), to assess the suitability of WAO as a pre-treatment for biological degradation. The effects of WAO temperature (180–240 °C) and the concentration of the orthophosphoric acid catalyst (0–1.0 mol dm−3) were investigated. Results showed that a higher WAO temperature increased the biodegradability of the WAO effluent. This was due to a greater removal of both recalcitrant sulfonated organics and organic concentration (TOC and COD). Conversely, greater orthophosphoric acid concentrations decreased the biodegradability of the WAO effluents. This was because the higher acid concentration increased the ionic strength and changed the WAO intermediate and product distribution, inhibiting microbial action. Nevertheless, the effluents from both variations of WAO were still more biodegradable than LAS at equivalent concentrations. However, since higher WAO temperatures can substantially increase capital costs, future work should focus on developing a WAO catalyst that both desulfonates and mildly oxidises LAS at moderate temperatures (200 °C).© 2002 Society of Chemical Industry

The influence of temperature (140 and 160 °C) and oxygen partial pressure (2 and 9 bar) on the continuous catalytic wet air oxidation (CWAO) over activated carbon (AC) has been investigated for the removal of phenol, o-cresol and 2-chlorophenol in aqueous solutions. Seventy-two-hour tests were performed in a fixed bed reactor in trickle flow regime. A commercial AC was employed as catalyst. The pollutant elimination effectiveness, the distribution of major reaction products and the AC behaviour were studied for each compound. Also, respirometric screening tests were completed to the effluents before and after each CWAO experiment to assess the biodegradability enhancement. The results show that pollutant disappearance, chemical oxygen demand (COD) removal, total organic carbon (TOC) abatement and biodegradability enhancement (fraction of COD readily biodegradable, %CODRB) were very sensitive to temperature but rather independent of the oxygen partial pressure (PO2PO2). On the contrary, AC catalyst preservation was strongly influenced by both temperature and pressure. For instance, for phenol CWAO at 2 bar of PO2PO2, as temperature increased from 140 to 160 °C, phenol conversion increased from 45% to 78%, COD removal from 33% to 65%, TOC abatement from 21% to 62% and %CODRB from 4% to 36%. Similar behaviour was found for the other model compounds tested, although the level of refractoriness was rather different.

ABSTRACT The wet air oxidation of aqueous solutions of aniline over a Ru/CeO2 catalyst was investigated. Batch oxidation experiments were performed at temperatures between 160 and 230̊C 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.

Continuous catalytic wet air oxidation (CWAO) was investigated as a suitable precursor for the biological treatment of industrial wastewater that contained phenols (phenol, o-cresol, 2-chlorophenol and p-nitrophenol), aniline, sulfolane, nitrobenzene or sodium dodecylbenzene sulfonate (DBS). Seventy-two-hour tests were carried out in a fixed bed reactor in trickle flow regime, using a commercial activated carbon (AC) as catalyst. The temperature and total pressure were 140 °C and 13.1 bar, respectively. The influence of hydroxyl-, methyl-, chloride-, nitro-, sulfo- and sulfonic-substituents on the oxidation mechanism of aromatic compounds, the occurrence of oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate elimination) were established. The results show that the AC without any supported active metal behaves bifunctionally as adsorbent and catalyst, and is active enough to oxidate phenol, o-cresol, 2-chlorophenol and DBS, giving conversions between 30 and 55% at the conditions tested. The selectivity to the production of carbon dioxide was considerable with total organic carbon (TOC) abatement between 15 and 50%. The chemical oxygen demand (COD) reduction was between 12 and 45%. In turn, aniline, sulfolane, p-nitrophenol and nitrobenzene conversions were below 5% and there was almost no TOC abatement or COD reduction, which shows the refractory nature of these compounds.