Faı̈çal Larachi

Ce 3d XPS study of composite CexMn1 xO2 y wet oxidation catalysts

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state of cerium in unsupported composite Ce x Mn 1− x O 2− y catalysts. Manganese–cerium composite oxides are being widely used in sub- and supercritical catalytic wet oxidation for the treatment of wastewater containing toxic organic pollutants. The performance of such catalysts depends, among others, on the redox reactions involving CeO 2− x suboxides and manganese oxides. In order to investigate the surface chemistry of such environmental catalysts, 10 samples with Mn/Ce ratios ranging from 0/10 to 9/1—molar basis—were synthesized by co-precipitation and characterized as-prepared and after gold coating. Principal component analysis and curve fitting of Ce 3d and O 1s core-level spectra were used to determine the types of valence states and their proportions on the catalysts’ surface. The study revealed valence state modifications in cerium and changes in the amount of lattice oxygen depending on the Mn/Ce ratio and on whether or not the as-prepared catalysts were subjected to gold deposition. The proportion of surface Ce 3+ species in the non-stoichiometric CeO 2− x oxides was successfully quantified through linear correlation between v 2 ′ peak area and integral of the Ce 4+ 3d spectrum.

Composition–activity effects of Mn–Ce–O composites on phenol catalytic wet oxidation

Abstract Mn–Ce–O composite catalysts have been widely used in sub- and supercritical catalytic wet oxidation of toxic organics contained in aqueous streams. In order to investigate their composition–activity relationship, 11 samples with Ce/(Mn+Ce) atomic bulk ratios ranging from 0 to 100% were prepared by co-precipitation. Phenol was selected as a model pollutant and the catalytic oxidation was carried out in a batch slurry reactor using oxygen as the oxidizing agent under mild reaction conditions. The results showed that the catalytic activity was greatly influenced by the catalyst composition. The catalyst with Mn/Ce ratio=6/4 was found to be the most active in reducing both phenol concentration and total organic carbon (TOC). All catalysts were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and nitrogen adsorption techniques. Systematic shifts in binding energy, diffraction angle, and reduction temperature were observed in the XPS, XRD and TPR spectra, respectively. XPS and XRD data revealed the occurrence of significant interactions between Mn and Ce oxides, resulting in the evolution of textural, structural and oxidation state with composition. TPR analysis showed that the interaction between Mn and Ce greatly improved the oxygen storage capacity of manganese and cerium oxides as well as oxygen mobility on the surface of catalyst. Catalytic active sites have been ascribed to manganese oxide species exhibiting higher oxidation state. Furthermore, XPS revealed that the most active catalyst, i.e. Mn/Ce 6/4, exhibits an electron-rich surface which may be very important in the activation of adsorbed oxygen.

Experimental study of a trickle-bed reactor operating at high pressure: two-phase pressure drop and liquid saturation

Abstract The effect of pressure on the hydrodynamics of trickle-bed reactors is investigated. The two-phase pressure drop and the liquid hold-up (liquid RTD determination) were measured for pressures up to 8.1 MPa. The influence of pressure, gas and liquid flow rates, viscosity, the coalescence behaviour of the liquid, and the particle size was examined. The experimental results were compared to correlations from the literature and two new correlations for the pressure drop and the liquid hold-up for non-foaming liquids are proposed; they are based on 1500 experimental results. Consideration of systems exhibiting non-foaming behaviour shows that the two-phase pressure drop is correctly described by the introduction of the modified Lockhart and Martinelli parameter. The liquid saturation data analysis shows that this hydrodynamic parameter is pressure-independent for very low gas superficial velocities allowing for an acceptable estimation at atmospheric pressure.

Pressure drop through structured packings: Breakdown into the contributing mechanisms by CFD modeling

Determination of dry pressure drops is often the preliminary diagnostic tool for characterizing structured packing-containing columns. One conventional approach that ushered in this area evolves around the use of Ergun expressions along with mandatory experimental pressure drops for the fitting of some empirical constants characterizing a given packing. This method is strictly representational, and incapable of predicting the impact on bed pressure drop of changes in packing geometry, e.g., corrugation angle, channel size, or packing topography. In this work, a combined mesoscale—microscale predictive approach was developed to apprehend the aerodynamic macroscale phenomena in structured packings. The proposed method consists in identifying recurrent mesoscale patterns (the representative elementary units, REU) wherein the constitutive microscale dissipation mechanisms occur. The dissipative phenomena that were identified to be important are: the elbow loss and jet splitting at the packed bed entrance, the elbow loss at the column wall, the elbow loss at the jump from one layer to another, and the collisional losses at the criss-crossing junctions. Each mechanism was simulated over a wide Reynoids range spanning the pure creeping flow to the fully developed turbulent flow using three-dimensional computational fluid dynamics (CFD). Postulating additiveness of dissipation, the overall pressure drop was reconstructed. The approach was validated using experimental dry pressure drop data for five packing types (Flexipac, Gempak, Mellapak, Sulzer BX and Montz-Pak) having different channel sizes, corrugation angles, and surface topography. Our goal was to advocate CFD as a quicker and cheaper means for design and optimization, in terms of energy dissipation, of new structured packing shapes.

Catalytic oxidation of aqueous phenolic solutions catalyst deactivation and kinetics

Abstract The catalytic oxidation of aqueous phenol over MnO 2 /CeO 2 mixed oxide catalyst was shown to be accompanied with undesirable formation of heavy polymers on the catalyst surface leading thereby to its deactivation. This deactivation was proved experimentally through comprehensive quantitative and qualitative experimental measurements such as elemental analysis of deactivated cata-lyst, X-ray photoelectron spectroscopy, and temperature-programmed oxidation coupled with mass spectrometry. Using the Langmuir–Hinshelwood–Hougen–Watson approach, a deactivation-reaction network kinetic model was developed to predict the fate of the various carbon lumps involved in phenol wet oxidation. It was postulated that deactivation was the resultant of blockage of active sites by fouling species issued from polymerization of parent reactant (phenol) and its daughter intermediate dissolved partially oxidized products. The kinetic model was verified by comparing the experimental results with those foreseen in the simulation for different experimental conditions.

Gas}liquid interfacial mass transfer in trickle-bed reactors: state-of-the-art correlations

The state-of-the-art of the gas-liquid mass transfer characteristics in trickle-bed reactors was summarized and its quantification methods were reevaluated based on a wide-ranging data base of some 3200 measurements. A set of three unified whole-flow-regime dimensionless correlations for volumetric liquid- and gas-side mass transfer coefficients, and gas–liquid interfacial area, each of which spanned four-order-of-magnitude intervals, were derived. The correlations involved combination of artificial neural networks and dimensional analysis. The dimensionless interfacial area, ShL and ShG were expressed as a function of the most pertinent dimensionless groups: ReL, ReG, WeL, WeG, ScL, ScG, StL, XG, MoL, FrL, Eom, Sb.

Hydrodynamics and mass transfer in trickle-bed reactors: an overview

Abstract The fluid dynamic and the gas–liquid mass transfer characteristics of trickle-bed reactors were revisited and their quantification methods reevaluated based on extensive experimental historic flow databases (22,000 experiments) set up from the open literature published over the last 50 years. The state-of-the-art of trickle-bed fluid dynamics was summarized and a set of unified and updated estimation methods relying on neural network, dimensional analysis and phenomenological hybrid approaches were discussed.

Tailoring the pressure drop of structured packings through CFD simulations

A computational fluid dynamic methodology is proposed to breakdown into elementary dissipation mechanisms the overall single-phase gas flow bed pressure drop in towers containing corrugated sheet structured packings. The goal behind was to allow piecewise geometry optimization of such packings in terms of capacity enlargement and efficiency enhancement. The dissipations sorted in order of decreasing importance were the collision losses by jet streams at criss-crossing junctions within corrugated channels, elbow loss by form drag at interlayer transition, elbow loss by jets striking wall and subsequent flow redirection to upper channels, and elbow loss in bed entrance. Replacement of sharp bends at the interlayer junctions by progressive direction change was beneficial for the reduction of the dissipations at the wall and the interlayer junction thus stretching capacity of the structured packing. However, this improvement was not spectacular because the most energy-intensive component (criss-crossing) remained unaffected by such modifications. Computational fluid dynamics is foreseen to be a successful, rapid and economic tool to theoretically explore new geometries coping with this limitation.

Residence time, mass transfer and back-mixing of the liquid in trickle flow reactors containing porous particles

Abstract A residence time distribution (RTD) model to describe the liquid trickle flow in a trickle-bed reactor packed with porous particles and operated both under partially and fully wetted conditions was proposed based on a simple representation of the liquid flow structure. The model views the external liquid stream as divided into a dynamic zone where an axially dispersed plug flow pattern prevails, and an external stagnant (or static) zone contiguous to both the dynamic zone and the partially wetted porous particles. The model incorporates mass transfer between (i) external dynamic and stagnant zones, (ii) dynamic zone and nearby partially wetted porous particles, (iii) stagnant zone and adjacent partially wetted particles, and (iv) finally intraparticle diffusion. The model parameters were derived from liquid residence time distribution tests with various air/Newtonian and air/non-Newtonian systems. Analysis of the dynamic tracer impulse–response data of the liquid revealed the significance of the mass transfer resistance between static liquid and adjacent wetted particles, intraparticle diffusion resistance, as well as partial wetting. By properly accounting for intraparticle diffusion, peculiarly high liquid axial dispersion coefficients were obtained for low liquid velocities and high carboxymethyl-cellulose (CMC) concentrations. Finally, the deficiency of lumping static liquid–solid mass transfer, internal diffusion, and partial wetting in the Peclet number and the number of transfer units was discussed.

Wet oxidation of acetic acid by H2O2 catalyzed by transition metal-exchanged NaY zeolites†

During the wet oxidation of contaminated wastewaters, the destruction of low molecular weight carboxylic acid intermediates such as acetic, glyoxalic, and oxalic acids is often the rate-controlling step. Oxidation of acetic acid, a very recalcitrant intermediate, requires compelling treatment severity. Heterogeneous catalytic wet oxidation of model acetic acid aqueous solutions was conducted under mild conditions (below the normal boiling point of water) using hydrogen peroxide over various transition metal-exchanged NaY zeolites. Treatment of Cu 2+ -NaY with oxalic acid [OA] led to a catalyst, Cu 2+ -NaY [OA], with significantly improved properties in terms of total organic carbon (TOC) removal efficiency and catalyst stability against leaching. This catalyst outperformed homogeneous Cu 2+ by a factor of 2-2.5 times. Continuous feeding of H 2 O 2 reduced its undesirable decomposition. Improvement of the TOC-degradation performance by Cu 2+ -NaY [OA] was tentatively attributed to the removal of sodium and possibly aluminium in the zeolite.