Alumina Pellets Research Articles

Study of Stark broadening of Li i 460 and 497 nm spectral lines with independent plasma diagnostics by Thomson scattering

We present results of experimental and theoretical studies of the Stark broadening of the Li i 460 nm spectral line with forbidden components and of the isolated 497 nm line. Plasma was induced by Nd:YAG laser radiation at 1064 nm with pulse duration ∼4.5 ns. Laser-induced plasma was generated in front of the alumina pellet, with some content of Li2CO3, placed in a vacuum chamber filled with argon under reduced pressure. Plasma diagnostics was performed using the laser Thomson scattering technique, free from assumptions about the plasma equilibrium state and its composition and so independently of plasma emission spectra. Spatially resolved spectra with Li lines were obtained from the measured, laterally integrated ones applying the inverse Abel transform. The Stark profiles were calculated by computer simulation method assuming a plasma in the local thermodynamic equilibrium. Calculations were performed for experimentally-inferred electron densities and temperatures, from 1.422 × 1023 to 3.55 × 1022 m−3 and from 1.96 eV to 1.04 eV, respectively. Our studies show very good agreement between experimental Stark profiles and those computer simulated.

Ni or Ru supported on MgO/γ-Al2O3 pellets for the catalytic conversion of ethanol into butanol.

Ni- and Ru-based catalytic pellets supported on commercial γ-Al2O3 pellets modified with magnesium oxide have been prepared for application in the catalytic conversion of ethanol into butanol. MgO/γ-Al2O3 pellets with or without added metals have been characterized by XRD, SEM/EDX, TGA, N2 physisorption, H2 TPR, and CO2 TPD in order to investigate the effect of MgO coverage and metal distribution on the surface and red-ox properties of the materials and, in turn, their effects on the catalytic performance. The conversion of ethanol into butanol has been investigated in a continuous flow reactor at 350–400 °C under diluted conditions (3% ethanol) in order to rank the different catalytic pellets and identify the best formulations and preparation procedures via a comparison with powder catalysts previously proposed in the literature with similar compositions. Results show enhanced catalytic performance for MgO-covered alumina pellets with respect to a pure MgO powder catalyst in spite of the lower MgO load. A significant further positive effect is found when Ni or Ru enters a solid solution with MgO.

Determination of Pore Network Accessibility in Hierarchical Porous Solids

This paper validates the hypothesis that the supposedly nonspecific adsorbates nitrogen and argon wet heavy metals differently and shows how this unexpected effect can be actively utilized to deliver information on pore interconnectivity. To explore surface chemistry influences on differential adsorbate wetting, new findings for a mixed silica–alumina material were compared with data for pure silica and alumina materials. The new structural characterization described can determine the distribution of the particular subset of meso- and micropores that connect directly to macropores that entrap mercury following porosimetry, as mapped by computerized X-ray tomography. Hence, it elucidates the spatial organization of the network and measures the improved accessibility to smaller pores provided by larger pores. It was shown that the silica–alumina pellets have a hierarchical pore-size arrangement, similar to the optimal blood vessel network architecture in animals. The network architecture derived from the ne...

Demonstration of Ethanol-Blended Hydrogen Peroxide Gas Generator for Ramjet Combustor Flow Simulation

A gas generator using the catalytic decomposition of ethanol-blended hydrogen peroxide was investigated for application in experimental research on high-speed airbreathing engines. The gas generator should supply product gas with an oxygen concentration of 21 mol % at a given pressure of 3–12 bar and a temperature near 500°C, which are the flow conditions in a ramjet combustor flying at Mach 3. To avoid detonation of the liquid monopropellant, an appropriate mixture ratio of hydrogen peroxide, water, and ethanol was selected. In the batch reactor test, the tested catalysts were manganese oxide and platinum with gamma-alumina pellets. The temperature measurements indicated that the platinum with alumina pellets was more appropriate for decomposing propellant than manganese oxide with alumina pellets. In addition, product gas species with an oxygen concentration of 21 mol % were confirmed through the test. In the demonstration test, the system was operated with a propellant mass flow rate near 100 g/s. A catalytic reaction was sustained for the whole operation time with an adiabatic decomposition temperature efficiency of over 0.98. The measured flow temperature was near 500°C and the chamber pressure was near 5.5 bar, which satisfied the flow conditions in a ramjet combustor flying at Mach 3.

Flame front inclination instability in the porous media combustion with inhomogeneous preheating temperature distribution

The present work examines effects of the initial inhomogeneous preheating temperature distribution on the flame front inclination instability in porous media combustion experimentally and numerically. The inclination evolution is observed in a vertical burner with a packed bed of alumina pellets experimentally. Correspondingly, a 2D two-temperature mathematical model is developed in simulation. To understand the inclination mechanism, the whole process of the inclination evolution is divided into three stages according to the inclination angle growth rate. Different performances of the inclination are obtained and discussed for different factors, such as the initial preheating temperature perturbation, the gas inlet velocity and the equivalence ratio. Conclusively, a larger initial preheating temperature perturbation, a greater gas inlet velocity and a smaller equivalence ratio can lead to a more serious inclination. The results are helpful to guide the design and operation of the porous media burner.

Additive Manufacturing of Catalyst Substrates for Steam\u2013Methane Reforming

Steam–methane reforming is a highly endothermic reaction, which is carried out at temperatures up to 1100 °C and pressures up to 3000 kPa, typically with a Ni-based catalyst distributed over a substrate of discrete alumina pellets or beads. Standard pellet geometries (spheres, hollow cylinders) limit the degree of mass transfer between gaseous reactants and catalyst. Further, heat is supplied to the exterior of the reactor wall, and heat transfer is limited due to the nature of point contacts between the reactor wall and the substrate pellets. This limits the degree to which the process can be intensified, as well as limiting the diameter of the reactor wall. Additive manufacturing now gives us the capability to design structures with tailored heat and mass transfer properties, not only within the packed bed of the reactor, but also at the interface between the reactor wall and the packed bed. In this work, the use of additive manufacturing to produce monolithic-structured catalyst substrate models, made from acrylonitrile–butadiene–styrene, with enhanced conductive heat transfer is described. By integrating the reactor wall into the catalyst substrate structure, the effective thermal conductivity increased by 34% from 0.122 to 0.164 W/(m K).

Numerical modeling of spark plasma sintering—Discussion on densification mechanism identification and generated porosity gradients

A multi-physical numerical model of the SPS process has been developed in order to evaluate electrical, thermal and mechanical fields undergone by the powder during sintering. The reliability of this numerical model has been validated by comparison with experimental data carried out on a submicrometric alpha alumina powder. This numerical approach is applied to achieve a critical analysis of the main methodologies used in the literature for the identification of the densification parameters, and the possible misinterpretations of involved deformation mechanisms. Moreover, as shown during sintering of alumina pellets of larger diameter (50mm in diameter), the induced porosity gradient seems to be mainly correlated to the existence of thermal gradient within the powder, and in a lesser level to stress gradient. Consequently, the temperature gradient seems to be a crucial point to be controlled during SPS process of large size samples in order to obtain fine and homogeneous microstructure.

Combustion in a Hybrid Porous Burner Packed with Alumina Pellets and Silicon Carbide Foams with a Gap

AbstractCombustion of premixed methane/air in a hybrid porous burner is experimentally studied. The burner is filled with alumina pellets in the preheating zone and silicon carbide foams with a gap...

Preparation of egg-shell-type Ni/Ru bimetal alumina pellet catalysts: Steam methane reforming for hydrogen production

Egg-shell-type pellet catalysts were prepared by selectively placing nickel and/or ruthenium on the outer region (shell) of alumina pellets so that the active components can be utilized effectively in steam methane reforming (SMR) reaction with high gas-hourly space velocity conditions. To ensure the reproducibility of catalyst preparation, we evaluated two types of commercial alumina pellets. And, we finally selected one commercial alumina pellet, which had uniform pores distribution. The thickness of the ruthenium-shell can be controlled by optimizing the evaporation temperature and rotating speed while preparing an ‘egg-shell-type’ 1 wt. % Ru/alumina catalyst. When applied to a SMR reaction, as the space velocity of the reactant increased in SMR reaction, ‘egg-shell-type’ 1 wt. % Ru/alumina catalyst showed a higher methane conversion than a ‘homo-type’ 1 wt. % Ru/alumina pellet catalyst, in which the active metal was uniformly dispersed in the whole region of pellet. Since ruthenium is a costly noble metal, we prepared a Ni/Ru bimetal catalyst [‘egg-shell-type’ 5 wt. % Ni/0.7 wt. % Ru bimetal catalyst] substituting nickel for some portion of the ruthenium in order to increase the economic feasibility. The bimetal Ni/Ru catalyst showed even better CH4 conversion than the egg-shell-type catalyst containing ruthenium only. We also confirmed that the egg-shell-type catalyst effectively utilized its active components in the SMR reaction.

Facile preparation of egg-shell-type pellet catalysts using immiscibility between hydrophobic solvent and hydrophilic solution: Enhancement of catalytic activity due to position control of metallic nickel inside alumina pellet

Egg-shell-type catalysts, in which active components were selectively located outer region of pellet, have been prepared to effectively utilize catalytically active components in the reactions with high gas-hourly space velocity conditions. We developed a new and facile preparation method for egg-shell catalysts using immiscibility between hydrophobic solvent preoccupied inside pellet and aqueous hydrophilic precursor solution. A series of alcohols, propanol, butanol, pentanol and so on, were used as hydrophobic solvents to retard the penetration of aqueous metal precursor solutions. After drying solvents and proper calcination and reduction, catalytically active components, metallic nickel particles, were selectively located in the outer region of pellet. Owing to the selective deposition of metallic particles, the egg-shell catalysts showed higher conversion of methane in steam methane reforming than homo-type catalysts, which has homogeneous distribution of metal particles on the entire range of pellet, as the space velocity increased from 6000 to 30,000h−1.

Elementary Reaction Modeling and Experimental Characterization on Methane Partial Oxidation within a Catalyst-Enhanced Porous Media Combustor

In this study, the fuel-rich combustion of methane in a two-layer porous media burner consisting of dense alumina pellets of different diameters was investigated experimentally and numerically. For a fixed inlet gas velocity of 0.15 m/s, methane-rich flames were stabilized near the interface of two layers for equivalence ratios from 1.4 to 1.6. It was found that 40% of the methane was converted to syngas at the equivalence ratio of 1.6 using a reforming efficiency based on low heating values. To further increase the hydrogen yield and make the burner more suitable for applications in fuel cells, a portion of the downstream layer was coated with 0.08 wt % Ni catalyst. The reforming efficiency of methane to hydrogen increased from 18.2% to 23.9% after the catalytic enhancement. A combined homogeneous and heterogeneous elementary reaction mechanism was developed for methane partial oxidation in the porous media burner with catalytic enhancement. A one-dimensional model was explored by coupling the combined m...

Bending testing and characterization of surrogate nuclear fuel rods made of Zircaloy-4 cladding and aluminum oxide pellets

Behavior of surrogate nuclear fuel rods made of Zircaloy-4 (Zry-4) cladding with alumina pellets under reversed cyclic bending was studied. Tests were performed under load or moment control at 5 Hz. The surrogate rods fractured under moment amplitudes greater than 10.16 Nm with fatigue lives between 2.4 × 103 and 2.2 × 106 cycles. Fatigue response of Zry-4 cladding was characterized by using flexural rigidity. Degradation of flexural rigidity was shown to depend on the moment and the prefatigue condition of specimens. Pellet-to-pellet interface (PPI), pellet-to-cladding interface (PCI), and pellet condition affect surrogate rod failure. Both debonding of PPI/PCI and pellet fracturing contribute to surrogate rod bending fatigue. The effect of sensor spacing on curvature measurement using three-point deflections was studied; the method based on effective gauge length is effective in sensor spacing correction. The database developed and the understanding gained in this study can serve as input to analysis of SNF (spent nuclear fuel) vibration integrity.

Entrainment characteristics of fine particles in fluidized bed under preheating conditions

The entrainment characteristics of fine silicon particles from solid mixtures (Geldart groups C and B) are investigated in a cylindrical fluidized bed with an inner diameter of 30mm and a height of 450mm and that is equipped with an agitator and an electromagnetic vibration table. Silicon particles (mean size of 2.7μm) are used as entrained materials (group C), hollow alumina pellets (mean size of 1200μm) are used as coarse particles (group B), and nitrogen gas is applied as carried gas. The effects of the superficial velocity of nitrogen gas at room temperature (U, 0.39m/s to 0.98m/s), the initial loading quantity of fine particles (M, 5g to 20g), the vibration intensity (Γ, 1.3 to 4.83), the stirring speed of agitator (V, 75rpm to 195rpm), the mass ratio of coarse to fine particles (N, 0 to 1.5), and the preheating temperature of nitrogen gas (T, 20°C to 170°C) on the entrainment characteristics (entrainment rate Wi and entrained powder–gas ratio R) are experimentally studied under atmospheric pressure. Significance analyses of U, M, Γ, and V are performed via the analysis of variance. M and U both significantly affect Wi, but only M significantly affects R. The experimental results show that an increase in U, M, N, and T constantly improves the entrainment characteristics, an increase in Γ deteriorates such characteristics, whereas there exists an optimal value for V to obtain the optimum entrainment characteristics. This study also determines that an optimal operating condition can result in optimal entrainment characteristics (Wi, 9.27g/min and R, 0.19g/g), which can be achieved with a U of 0.98m/s, M of 20g, Γ of 1.3, V of 155rpm, N of 1.0, and T of 170°C.

Crack formation in cladding under LOCA quench conditions

Loss-of-Coolant-Accident (LOCA) is a design basis accident that is considered in safety analyses for LWR. This paper discusses crack formation in one-side oxidized Zircaloy-4 cladding with LOCA oxidation quench experimental data. Two types of test pins were used in these experiments. One consisted of a Zircaloy-4 cladding tube and alumina pellets. Another comprised a Zircaloy-4 cladding tube alone. The experimental results showed that the fracture load in the latter cases was larger by about 1000N, and cracks in the oxide layer were clearly observed in the former cases but not in the latter cases. This trend suggested that the order of cracks formed in cladding during LOCA quench conditions should be, first in the α-Zr(O) layer, and then in the oxide, finally in the prior-β layer when the fracture of cladding occurs. The formation of cracks in the oxide was considered to be related to the maximum thermal stress generated in the cladding during quench. RANNS computation suggested that the maximum radial temperature difference in the former case tended to be larger than that in the latter case, and the radial temperature difference increases considerably with off-center pellets, which might also affect crack formation.

Study of a thermoelectric generator based on a catalytic premixed meso-scale combustor

The recent advances in miniaturized mechanical devices open exciting new opportunities for combustion, especially in the field of micro power generation, allowing the development of power-supply devices with high specific energy. The development of a device based on a catalytic combustor coupled with thermoelectric modules is particularly appealing for combustion stability and safety. Furthermore, when implemented in micro/meso scale devices, catalytic combustion allows full utilization of hydrocarbon fuel’s high energy densities, but at notably lower operating temperatures than those typical of traditional combustion. These conditions are more suitable for coupling with conventional thermoelectric modules since they prevent the modules’ degradation.In this work a novel catalytic meso-scale combustor fueled with propane/air mixture has been coupled with two conventional thermoelectric modules. The wafer-like combustor is filled with commercially available catalytic pellets of alumina with Platinum (1% weight). Temperature measurements, in terms of point values and 2D distribution across the combustor surfaces, were carried out in different operating conditions. Exhaust gases concentration measurements and pellet aging investigation were performed to determine the combustor efficiency and stability. Starting from these results the combustor has been coupled to bismuth telluride-based thermoelectric modules using a water cooled heat exchanger at the cold side. The system obtained produces 9.86W of electrical power reaching an overall efficiency up to 2.85%: these results represent an improvement in portable-scale electrical power production from hydrocarbon fuels state-of-art. Moreover, the voltage and current characteristics allow using such generator for small portable devices power supplying.

Study of the performance of a catalytic premixed meso-scale burner

Portable electric devices have been object of interest of the scientific community for their application in different areas. The development of catalytic combustors coupled with thermo-electric devices is particularly attracting for combustion stability and safety. In this work a novel premixed catalytic meso-scale combustor fueled with propane/air mixture has been designed and tested. The wafer-like combustor is filled up with commercially-available catalytic pellets of alumina covered with platinum (1% weight). As a preliminary investigation the stability limits have been investigated. The analysis of the temperature values and distribution across the combustor surfaces have been carried out with thermocouples, for local measurements, as well as with an IR camera, for 2D measurements. Moreover, concerning combustion processes, the chemical composition of the exhaust gases have been derived by performing FT-IR measurements. This quantitative analysis allows obtaining the overall chemical efficiency of the catalytic combustor. As a further analysis, the effect of aging of catalytic coating on the overall combustor performances has been investigated. The configuration under study has been proved to be suitable for the coupling with thermo-electric devices.

Combustion characteristics of low-concentration coal mine methane in ceramic foam burner with embedded alumina pellets

Ceramic foam as a rigid matrix is difficult to be embedded with various shapes of heat exchange tubes. In this paper, a ceramic foam burner with embedded alumina pellets was designed, which can set different shapes of tubes by taking advantage of the discrete pellets. An experimental system was built to study the effects of the pellet diameter and pellet location on the combustion of low-concentration coal mine methane (LCM). Results indicate that the heat transfer features of 13-mm pellets are more similar to those of 10-PPI ceramic foam compared with 6-mm pellets and 9-mm pellets. When arranged in upper section of the burner, the 13-mm pellets preserve the combustion heat, which helps to move the flame stabilization position to the upstream of the burner. In addition, the start-up time in the burner with the upper 13-mm pellets is longer than that with the intermediate 13-mm pellets. The total NOx emissions of the studied burners are improved with the increasing inlet velocity, while the CO emissions first decrease and then increase. The HC emissions of the ceramic foam burner with embedded pellets are between those of the pellet burner and the ceramic foam burner.

In situ magnetic resonance imaging study of the impregnation of γ-alumina pellets

In petroleum refining, γ-alumina is used as a solid support of molybdenum nickel/cobalt promoted catalysts for the hydrotreatment processes, such as hydrodesulfurization. The ever decreasing norms for the sulfur content in gasoline require better and more efficient catalysis, prompting extensive research on the materials and the factors influencing the activity, selectivity and stability of the catalytic processes. In the present study, Magnetic Resonance Imaging Single Point Imaging is tested as a tool to monitor the impregnation of γ-alumina pellets with Ni(NO3)2 aqueous solutions at varying Ni2+ concentration. The method enables a study in the presence of paramagnetic elements in the alumina support, in the conditions of very fast T1 and T2. It is shown that at higher concentrations of metal ions a homogenous distribution in the support is attained faster. A non-linear dependence is observed and a minimum ion concentration of 0.2 M is necessary for achieving short impregnation times of alumina pellets of millimeter size scale.

Covalently Immobilized Laccase for Decolourization of Glucose-Glycine Maillard Products as Colourant of Distillery Wastewater.

Maillard reaction products like melanoidins are recalcitrant, high-molecular-weight compounds responsible for colour in sugarcane molasses distillery wastewater. Conventional biological treatment is unable to break down melanoidins, but extracellular laccase and manganese peroxidase of microbial origin can degrade these complex molecules. In this work, laccase was covalently immobilized on alumina pellets activated with aminopropyltriethoxysilane (APTES). The immobilization yield was 50-60 %, and the enzyme activity (886 U/L) was 5-fold higher compared to the soluble enzyme (176 U/L). The immobilized enzyme also showed higher tolerance to pH (4-6) and temperature (35-60 °C), as well as improved storage stability (49 days) and operational stability (10 cycles). Degradation of glucose-glycine Maillard products using immobilized laccase led to 47 % decolourization in 6 h at pH 4.5 and 28 °C. A comprehensive treatment scheme integrating enzymatic, microbial and membrane filtration steps resulted in 90 % decolourization.

Experimental and Simulation Investigations of DBD Plasma Reactor at Normal Environmental Conditions

In this paper, a cylindrical dielectric barrier discharge (DBD) plasma reactor is analyzed and investigated at normal environmental conditions. The stressed voltage is varied up to 40 kV peak-to-peak with a variable frequency of up to 4 kHz. In order to compare the experimental findings with the simulation results, an equivalent electrical circuit model is used. To simulate the dynamic microdischarge inside the plasma reactor, a voltage-controlled current source is implemented. The reactor is filled with aluminum oxide to enhance its operation. An equivalent configuration is proposed to model the region of alumina pellets inside the reactor and to calculate the corresponding equivalent capacitance. It is found that the measured voltage, discharge current, and voltage-charge waveforms are consistent with the simulated results. The quantitative comparison of the experimental and simulated data confirms the validity of the developed electrical circuit model of the plasma reactor.