Bar Of Oxygen Partial Pressure Research Articles

Wet air oxidation (WAO) as a precursor to biological treatment of substituted phenols: Refractory nature of the WAO intermediates

Abstract Wet air oxidation (WAO) was investigated as a suitable precursor for the biological treatment of industrial wastewaters that contained high concentrations of phenol, o-cresol or 2-chlorophenol. Two hours WAO semi-batch experiments were conducted at 2 and 9 bar of oxygen partial pressure ( P O 2 ) and at 215, 240 and 265 °C with total organic carbon (TOC) abatement up to 86%. The influence of hydroxyl-, methyl- and chloride ring substitution on the oxidation mechanism of aromatic compounds was established. Also, the readily biodegradable COD fraction (%CODRB) of the WAO effluents was found by respirometric techniques. The maximum %CODRB was 24% for phenol WAO at 265 °C and 9 bar of P O 2 , 10% for o-cresol WAO at 265 °C and 9 bar of P O 2 and 19% for 2-chlorophenol WAO at 215 °C and 2 bar of P O 2 . These results allow comparison of whether or not the WAO effluents were suitable for a conventional activated sludge plant with non-acclimated biomass. Also, the results were compared to those obtained from a previous study for the same model compounds but using catalytic WAO (CWAO) with activated carbon (AC) as a catalyst [1]. This comparison allows us to establish whether or not the WAO effluents were less biodegradable or had more refractory intermediates than CWAO effluents.

MnCu Oxides for Wet Oxidation of Orange II

The catalytic wet oxidation of an azo dye was investigated at 180°C and 15 bar of oxygen partial pressure. The effects of different catalysts were examined under operating conditions selected in pre-runs of non-catalytic oxidation and thermolysis (absence of oxygen). Manganese - copper catalysts with different molar ratios were tested and MnCu5 (50% Mn and 50% Cu) was found to be the most effective, giving almost complete dye degradation and 56.8% TOC removal after 60 minutes of reaction. Moreover, this catalyst revealed to be stable regarding metal leaching and catalyst poisoning due to sulphur or carbon depositions.

Effect of gas feed flow and gas composition modulation on activated carbon performance in phenol wet air oxidation

Modulation of gas feed composition (air/N 2 cycling) and gas feed flow (on–off air cycling) was investigated in the catalytic wet air oxidation of phenol over activated carbon (AC). Fifty hours lasting experiments were conducted in a laboratory trickle bed reactor at 140–160 °C, 2 bar of oxygen partial pressure and different splits and periods to determine the set of cycling parameters that optimise the periodic reactor operation. To follow the dynamic behaviour of the phenol oxidation, temperature and conversion were continuously monitored by means of computerised data acquisition and automatic liquid sampling. Several long term tests over 144 h were also run using both periodic operating strategies to compare the activity and stability of AC with those obtained in a steady state operation at otherwise same conditions. The results show that, depending on the selection of split and period, modulation of the gas phase significantly improves the stability of AC compared to steady state operation, thereby performing a superior long term phenol conversion.

Biodegradability enhancement of phenolic compounds by Hydrogen Peroxide Promoted Catalytic Wet Air Oxidation

The aim of this work is to study the viability of the H 2O 2 Promoted Catalytic Wet Air Oxidation (PP-CWAO) process, using activated carbon (AC) as catalyst, to increase the biodegradability of phenolic aqueous solutions. Seventy-two hours experiments were performed in a trickle bed reactor at 140 °C and 2 bar of oxygen partial pressure. Feed concentrations, in terms of theoretical chemical oxygen demand (ThCOD), were 11.8 g COD l −1 for phenol, 12.6 g COD l −1 for o-cresol and 8.0 g COD l −1 for p-nitrophenol. Air was used as main oxidant and 20% of the stoichiometric amount of H 2O 2 needed for pollutant complete mineralisation was added as oxidation promoter. Adding H 2O 2 to the CWAO process not only increases pollutant removal but also leads to higher mineralisation of the remaining oxidation products. For instance, removal of phenol, o-cresol and p-nitrophenol increase from 45, 33 and 15% in the CWAO process to 64, 64 and 49% in the PP-CWAO process. In addition, the PP-CWAO process leads to better biodegradability enhancements, when compared to CWAO, as demonstrated by the respirometric tests. However, it is still necessary to improve the oxidation step in order to assure more biodegradable effluents that could be combined with a subsequent biological wastewater plant.

Long-term stability of an Au/Al 2O 3 catalyst prepared by incipient wetness in continuous-flow glucose oxidation

A 0.3% Au/Al 2O 3 catalyst prepared by the incipient wetness (IW) method was investigated in the continuous-flow liquid-phase glucose oxidation. Therefore, a continuous stirred tank reactor (CSTR) system equipped with an ultrasonic separator was used. The continuous-flow glucose oxidation was carried out at 40 °C, pH 9 and 1 bar oxygen partial pressure. Residence time and glucose concentration were varied. The IW gold catalyst showed very high activity and selectivity to gluconic acid within its 110 days of operation and, thus, an excellent long-term stability. Even after severe microbial contaminations of the catalyst, its activity could be completely restored by in situ regeneration with 2-propanol.

Experimental evaluation and continuous catalytic process for fine aldehyde production from lignin

The catalytic process of aldehyde production from alkaline lignin extracted from sugarcane bagasse was evaluated in a batch slurry reactor and operated in a continuous three-phase fluidized-bed reactor with a palladium catalyst, at the operating conditions 373–413 K under 2–10 bar oxygen partial pressure. The scale-up of the fine aldehyde production process from batch to continuous operation presented a yield increase of approximately 15 times higher in 2 h of operation.

Catalytic and non-catalytic wet air oxidation of sodium dodecylbenzene sulfonate: Kinetics and biodegradability enhancement

Wet air oxidation (WAO) and catalytic wet air oxidation (CWAO) were investigated as suitable precursors for the biological treatment of industrial wastewater containing sodium dodecylbenzene sulfonate (DBS). Two hours WAO semi-batch experiments were conducted at 15 bar of oxygen partial pressure ( P O 2 ) and at 180, 200 and 220 °C. It was found that the highest temperature provides appreciable total organic carbon (TOC) and chemical oxygen demand (COD) abatement of about 42 and 47%, correspondingly. Based on the main identified intermediates (acetic acid and sulfobenzoic acid) a reaction pathway for DBS and a kinetic model in WAO were proposed. In the case of CWAO experiments, seventy-two hours tests were done in a fixed bed reactor in continuous trickle flow regime, using a commercial activated carbon (AC) as catalyst. The temperature and P O 2 were 140–160 °C and 2–9 bar, respectively. The influence of the operating conditions on the DBS oxidation, the occurrence of oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate removal) were established. The results show that the AC without any supported active metal behaves bi-functional as adsorbent and catalyst, giving TOC conversions up to 52% at 160 °C and 2 bar of P O 2 , which were comparable to those obtained in WAO experiments. Respirometric tests were completed before and after CWAO and to the main intermediates identified through the WAO and CWAO oxidation route. Then, the readily biodegradable COD (COD RB) of the CWAO and WAO effluents were found. Taking into account these results it was possible to compare whether or not the CWAO or WAO effluents were suitable for a conventional activated sludge plant inoculated with non adapted culture.

Selective oxidation of carbohydrates with gold catalysts: Continuous-flow reactor system for glucose oxidation

For the first time, a continuous-flow liquid-phase oxidation using a gold catalyst is described. A 0.25% Au/Al 2O 3 catalyst was investigated in the continuous-flow carbohydrate oxidation using glucose as the model compound. To achieve this, a continuous stirred tank reactor (CSTR) system was developed in which the powdered catalyst was separated by a combination of an ultrasonic separator and a separation vessel. The continuous-flow glucose oxidation was carried out at 40 °C, pH 9 and 1 bar oxygen partial pressure and maintained for 70 days. During this run-time the residence time and glucose concentration were varied. The alumina-supported gold catalyst showed very high activity and selectivity to gluconic acid. The long-term stability of the gold catalyst was excellent as no loss in activity or selectivity occurred during 70 days of continuous operation.

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO 3, (NH 4) 2S 2O 8, or H 2O 2 and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH 4) 2S 2O 8 treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.

Carbon supported platinum catalysts for catalytic wet air oxidation of refractory carboxylic acids

Carbon supported platinum (1% wt) catalysts were prepared by the incipient wetness impregnation method and by organometallic chemical vapor deposition. Catalyst characterization was carried out by means of adsorption and thermogravimetric techniques, and by electron microscopy. The catalyst with higher metal dispersion was produced by incipient wetness impregnation. The catalysts were tested in the catalytic wet air oxidation (200°C and 6.9 bar of oxygen partial pressure) of aqueous solutions containing low molecular weight (C2 to C4) carboxylic acids. Significant conversions (greater than 60% over 2 h) and 100% selectivity towards water and non-carboxylic acid products were observed for both systems. The initial reaction rate was used to compare the performance of the two catalytic materials and direct correspondence to the metal dispersion was found. No metal leaching was observed during reaction and no significant deactivation occurred in three successive catalytic oxidation runs. A kinetic model based on the Langmuir–Hinshelwood formulation was applied and the results were analyzed in terms of a heterogeneously catalyzed free radical mechanism.

Platinum catalysts supported on MWNT for catalytic wet air oxidation of nitrogen containing compounds

Multi-walled carbon nanotubes (MWNT) activated by nitric acid oxidation were used to prepare supported platinum (1% Pt) catalysts by the excess solution impregnation method. Three different platinum precursors were used to prepare the impregnation solutions, namely diiodo dicarbonyl platinum(II) [PtI 2(CO) 2], dimethyl (1,5-cyclooctadiene) platinum(II) [Pt(CH 3) 2(C 8H 12)] and hydrogen hexachloroplatinate(IV) hexahydrate H 2PtCl 6·6H 2O. The catalysts were characterized by transmission electron microscopy (TEM) analysis and H 2 chemisorption. They were tested in the catalytic wet air oxidation (CWAO) of aniline aqueous solutions (200 °C and 6.9 bar of oxygen partial pressure). The Pt/MWNT catalyst prepared from H 2PtCl 6·6H 2O showed the best performance with respect to combined activity and selectivity for aniline conversion to non-organic compounds. This catalyst was used in CWAO (150 °C and 6.9 bar of oxygen partial pressure) of aqueous solutions of various textile dyes such as the monoazo dye chromotrop 2R (C2R), the diazo dye erionyl red B (ERB) and the triazo dye solophenyl green BLE (SG) 155%. It was found that the selectivity for dye removal decreased in the order diazo > monoazo > triazo. The CWAO of a textile wastewater was also performed using the same catalyst and identical reaction conditions. Complete color removal and 51.2% TOC abatement was obtained under these conditions.

Catalytic wet oxidation of ethylene glycol: kinetics of reaction on a Mn–Ce–O catalyst

Wet oxidation (WO) is an ambitious key technology looking for the treatment of different industrial effluents, safeguarding environmental problems. Ethylene glycol (EG) has been found to be a refractory compound. Different heterogeneous catalysts in slurry conditions using a high-pressure batch reactor at 160–220 °C and 15–25 bar of oxygen partial pressure were studied for enhancing WO efficiency. The activity of commercial catalysts (Pt/Al 2O 3, CuO–ZnO/Al 2O 3, CuO– MnO x / Al 2 O 3 , Fe 2 O 3 – MnO x and CuO– MnO x ) was compared with the one related to various catalysts prepared in our laboratory (Ag–Ce–O, Mn–Ce–O and Ce–O). The Mn–Ce–O catalyst showed very high activity in terms of total oxidation of EG as well as total organic carbon reductions (up to 99.3%). Formic acid was identified as the only intermediate compound and the pH evolution was related with its formation/oxidation. Kinetic studies revealed activation energies of 31.8 and 45.0 kJ/mol concerning the two step mechanism of EG degradation by means of Mn–Ce–O. The catalyst stability was observed in terms of metal leaching and carbon adsorption, and its texture and morphology were analyzed by SEM, TEM, EDXS and SAED. Whiskers of MnOOH and/or β - MnO 2 were detected.

CWAO of Butyric Acid Solutions: Catalyst Deactivation Analysis

Carbon-supported platinum and iridium catalysts were prepared for catalytic wet air oxidation (CWAO) applications. These catalysts were tested in the CWAO of butyric acid aqueous solutions at 200 °C and 6.9 bar of oxygen partial pressure, and significant conversions above 70% were obtained depending on the catalyst used. The long-term deactivation resistance of the Pt/C catalyst is higher than that of the Ir/C catalyst. The deactivation observed with the Ir/C catalyst was attributed to the overoxidation of the surface of iridium by molecular oxygen. A deactivation study of the catalysts was conducted using N2 adsorption, thermogravimetric analysis, temperature-programmed reduction, and X-ray photoelectron spectroscopy. A kinetic model describing the observed deactivation phenomena in the Ir/C catalyst is proposed and tested using the experimental data obtained in the CWAO of the systems studied.

Catalysts based in cerium oxide for wet oxidation of acrylic acid in the prevention of environmental risks

Acrylic acid is a refractory compound for the non-catalytic wet oxidation (WO) process and can seriously damage the environment when released in industrial effluents. Oxidation of acrylic acid by catalytic wet oxidation (CWO) was studied in slurry conditions in a high-pressure batch reactor at 200°C and 15bar of oxygen partial pressure. Several solid cerium-based catalysts prepared in our laboratory were used (Ag/Ce, Co/Ce, Mn/Ce, CeO, MnO) and evaluated in terms of activity, selectivity and stability. Mn/Ce shows the higher activity in 2h with 97.7% reduction of total organic carbon (TOC) followed by: MnO(95.5%)>Ag/Ce(85.0%)>Co/Ce(65.1%)>CeO(61.2%). Attempts were also carried out to analyze the influence of different Mn/Ce molar ratios. High percentages of Mn lead to practically total organic carbon concentration (TOC) abatements while low ratios lead to the formation of non-oxidizable compounds. Acrylic acid was readily degraded by all the catalysts pointing out the high importance of using a catalytic process. pH was an indicator of the reaction pathway and acetic acid was found as the major reaction intermediate compound; however it is completely oxidized after 2h with exception for Co/Ce, CeO and MnO. Carbon adsorption and leaching of metals were poorly found for Mn/Ce indicating high stability. The catalyst microstructure after the reaction was analyzed and formation of whiskers of β-MnO2 (or less probably MnOOH) were observed at the catalyst surface. Therefore, Mn/Ce revealed to be a promising catalyst for the treatment of effluents containing acrylic acid; nevertheless, its commercialization depends on further research.

Catalytic Wet Oxidation of Acrylic Acid: Studies with Manganese-based Oxides

Development of active, stable and economical catalysts for oxidation of acrylic acid contained in industrial effluents is nowadays of great importance. Several manganese-based solids catalysts supported in Ce, Zr, Ti and Al oxides were prepared in our laboratory by incipient wetness impregnation for oxidation of acrylic acid in a batch high-pressure reactor at slurry conditions under 200°C and 15 bar of oxygen partial pressure. The Mn-Ce-O catalyst was the most active leading to high conversions (92.3%) in terms of total organic carbon (TOC) and total degradation of acrylic acid in the first 30 min of reaction. The TOC reductions were lower with Mn-Zr-O (45.6%), Mn-Ti-O (43.6%) and Mn-Al-O (28.2%) as well as acrylic acid was not completely degraded even after 180 min of reaction. The impregnation method used for catalyst preparation was compared with the co-precipitation procedure, being the last one less effective for TOC reduction (87.0%) although the same activity to oxidation of acrylic acid was observed. Acetic and formic acids were found as reaction intermediates being refractory compounds for the catalysts supported in Zr, Ti and Al. Catalyst stability was also evaluated and Mn-Ce-O prepared by impregnation showed low leaching of Mn to the liquid phase as well as carbon adsorption was not found strengthening the catalyst potentialities in the treatment of wastewaters containing acrylic acid. Since under non-catalytic conditions acrylic acid is refractory up to high temperatures (260°C), this work offers a step forward in the design of an economical catalyst able to promote acrylic acid oxidation in the liquid-phase at lower operating conditions than those used in non-catalytic systems.

Catalytic studies in wet oxidation of effluents from formaldehyde industry

Development and design of active catalysts for the oxidation of formaldehyde present in wastewaters is of great importance. In this context, catalytic performance studies for oxidation of high formaldehyde containing solutions (1500 ppm) were carried out in a semibatch high-pressure reactor at 190–220°C and 15–35 bar of oxygen partial pressure. The removal efficiency of total organic carbon (TOC) was evaluated experimentally under different solid catalysts, using several heterogeneous composite oxides prepared in our laboratory (Mn/Ce, Co/Ce and Ag/Ce), as well as a commercial catalyst (CuO–ZnO/Al 2O 3). The Mn/Ce catalyst was the more active leading to high TOC conversions (99.4%). The TOC reductions were lower using Co/Ce, Ag/Ce and CuO–ZnO/Al 2O 3, respectively, 71.3%, 54.2% and 78.7%. Attempts were made to identify the influence of different Mn/Ce ratios. A high molar fraction of Mn leads to high TOC abatements. During the preparation procedure of catalysts the drying temperature does not affect significantly the catalytic area while increasing calcination temperature leads to lower surface areas. Carbon capacity adsorption was not observed for the Mn/Ce and CuO–ZnO/Al 2O 3 catalyst and leaching of the correspondent metals, Mn, Cu and Zn, was not significant. The catalytic wet oxidation (CWO) process was studied with an industrial high formaldehyde concentration effluent (800 ppm) . Using an Mn/Ce catalyst the formaldehyde concentration decreased from 800 to 0.1 ppm and ammoniacal nitrogen from 420 to 155 ppm while 91.7% in TOC reduction was observed. These conversions were higher than those obtained with the commercial CuO–ZnO/Al 2O 3 catalyst. Therefore, Mn/Ce catalysts seem to be interesting options for the treatment of effluents from formaldehyde industry by CWO process.

Catalytic wet air oxidation of butyric acid solutions using carbon-supported iridium catalysts

Aqueous solutions of butyric acid were treated by catalytic wet air oxidation using carbon-supported iridium catalysts in a stirred reactor. Under the operating conditions of 6.9bar of oxygen partial pressure and 200°C of temperature, conversions up to 52.9% after 2h were obtained depending on the type of catalyst used. The effects of butyric acid initial concentration, loading of catalyst, oxygen partial pressure and temperature were investigated and the empirical rate law for acid conversion is presented. Oxidation intermediates such as propionic and acetic acid were identified. The heterogeneous catalyzed free-radical oxidation of butyric acid is discussed.

CWO of phenol on two differently prepared CuO–CeO2 catalysts

The activity and selectivity of the Ce1−xCuxO2−δ catalysts prepared by co-precipitation and by sol–gel methods were studied in the catalytic wet oxidation (CWO) of phenol at 432K and at 7.3bar oxygen partial pressure in a semibatch CST reactor. Both types of catalysts are very stable under hydrothermal reaction conditions at low pH values. It has been found that activity and selectivity increase with the dispersion of copper oxide phase on the cerium oxide. While the Cu content was equal in both types of catalysts, the samples prepared by sol–gel technique are up to four times more active and about 25% more selective toward CO2 formation than the samples prepared by co-precipitation at given reaction conditions. The heterogeneously initiated radical mechanism results in autocatalytic type kinetics. The activity and selectivity depend on the initial concentration of heterogeneously catalytically formed radicals in each subsequent oxidation step. Between these steps the radicals react further on in liquid phase.

Catalytic wet air oxidation of low molecular weight carboxylic acids using a carbon supported platinum catalyst

Aqueous solutions of low molecular weight carboxylic acids, such as acetic, propionic and butyric acids, were treated by catalytic wet air oxidation (CWAO) using a carbon supported platinum catalyst. Oxidation in the presence of the catalyst, in a stirred reactor, was carried out at 200°C and 6.9bar of oxygen partial pressure, with conversions (after 2h) ranging from 59.4 to 75%, and selectivities to gaseous products of up to 100%. Initial rates for conversion varied from 184 (butyric acid) to 260mmolh−1gPt−1 (propionic acid). The activation energy for butyric acid conversion was found to be 56.7kJmol−1.

Effect of oxygen partial pressure on the lower solubility limit of Nd 1+ xBa 2− xCu 3O 7

The lower solubility limits of Nd 1+ x Ba 2− x Cu 3O 7 (Nd123ss) in 1 bar, 0.2 bar, 0.01 bar, and 0.001 bar oxygen partial pressure ( P O2) were studed by differential thermal analysis (DTA) and X-ray diffraction (XRD). Large relative changes in stability with decreasing P O2 explain why sharp superconducting transitions can be achieved in P O2 ≤ 0.01 bar while processing in 0.2 to 1 bar P O2 tends to produce broad transitions. While the lower solubility limits in 0.01 bar and 0.001 bar P O2 remain at x = 0.00, the solubility limits in 0.2 bar and 1 bar P O2 show a narrowing with decreasing temperature. Because of the narrowing of the solubility range in 1 bar P O2, oxygen annealing of Nd123 initially processed in low P O2 will result in precipitation of second phases which act as flux pinning centers.