Space Velocity (space + velocity)

Distribution by Scientific Domains


Selected Abstracts


A C1 microkinetic model for methane conversion to syngas on Rh/Al2O3

AICHE JOURNAL, Issue 4 2009
Matteo Maestri
Abstract A microkinetic model capable of describing multiple processes related to the conversion of natural gas to syngas and hydrogen on Rh is derived. The parameters of microkinetic models are subject to (intrinsic) uncertainty arising from estimation. It is shown that intrinsic uncertainty could markedly affect even qualitative model predictions (e.g., the rate-determining step). In order to render kinetic models predictive, we propose a hierarchical, data-driven methodology, where microkinetic model analysis is combined with a comprehensive, kinetically relevant set of nearly isothermal experimental data. The new, thermodynamically consistent model is capable of predicting several processes, including methane steam and dry reforming, catalytic partial oxidation, H2 and CO rich combustion, water-gas shift and its reverse at different temperatures, space velocities, compositions and reactant dilutions, using the measured Rh dispersion as an input. Comparison with other microkinetic models is undertaken. Finally, an uncertainty analysis assesses the effect of intrinsic uncertainty and catalyst heterogeneity on the overall model predictions. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]


Kinematics and metallicity analysis for nearby F, G and K stars

ASTRONOMISCHE NACHRICHTEN, Issue 1 2009
S. Vidojevi
Abstract A sample containing 1 026 stars of spectral types F, G, and K, mainly dwarfs, from the solar neighbourhood with available space velocities and metallicities is treated. The treatment comprises a statistical analysis of the metallicity and velocity data and calculation of galactocentric orbits. Sample stars identified as members of the galactic halo are detached from the rest of the sample based on the values of their metallicities, velocity components and galactocentric orbits. In identifying halo stars a new, kinematical, criterion is proposed. Except one, these halo stars are the metal-poorest ones in the sample. Besides, they have very high velocities with respect to LSR. On the other hand, the separation between the thin disc and thick one is done statistically based on LSR space velocities, membership probability (Schwarzschild distribution with assumed parameters) and galactocentric orbits. In the metallicity these two groups are not much different. For each of the three subsamples the mean motion and velocity ellipsoid are calculated. The elements of the velocity ellipsoids agree well with the values found in the literature, especially for the thin disc. The fractions of the subsystems found for the present sample are: thin disc 93%, thick disc 6%, halo 1%. The sample stars established to be members of the thin disc are examined for existence of star streams. Traces of both, known and unknown, star streams are not found (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Kinetic Reaction Models for the Selective Reduction of NO by Methane over Multifunctional Zeolite-based Redox Catalysts

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 12 2004
T. Sowade
Abstract Kinetic measurements of the selective catalytic reduction (SCR) of NO by methane were performed over CeO2/H-ZSM-5, In-ZSM-5, and CeO2/In-ZSM-5 catalysts. The parameter space covered NO, CH4, and O2 concentrations varying from 250 to 1000 ppm, from 500 to 2000 ppm, and from 0.5 to 10,vol.-%, respectively, space velocities between 5000 and 90000 h,1 and temperatures between 573 and 873 K depending on the catalyst activities. With CeO2/In-ZSM-5 an additional series of measurements was performed with moistened feed gas (0.5,10,vol.-% H2O). On the basis of a pseudo-homogeneous, one-dimensional fixed-bed reactor model, the data were fitted to a kinetic model that includes power rate laws for the reduction of NO and for the unselective total oxidation of methane. From analyses of isothermal data sets, almost all reaction orders were found to vary significantly with changing temperature, which indicates that the simple kinetic model cannot reflect the complex reaction mechanism correctly. Nevertheless, the data measured with In-ZSM-5 could be modeled with good accuracy over a wide range of reaction temperatures (150 K) while the accuracy was less satisfactory with the remaining data sets, in particular for data with the moist feed over CeO2/In-ZSM-5. With the latter catalyst it was not possible to represent the data measured in dry and in moist feed in a single model even upon confinement to fixed reaction temperatures. A comparison of the separate models established showed strong changes in the reaction orders in the presence of water, which occur apparently already at a very low water content (,,0.5,vol.-%). The kinetic parameters found are in agreement with earlier conclusions about the reaction mechanisms. With In-ZSM-5, both reaction orders and the activation energy show a rate-limiting influence of NO oxidation on the NO reduction path which is removed by the presence of the CeO2 promoter. A difference in the reaction mechanism over CeO2/In-ZSM-5 and CeO2/H-ZSM-5 is reflected in different kinetic parameters. The differences of the kinetic parameters between dry-feed and moist-feed models for CeO2/In-ZSM-5 reflect adsorption competition between the reactants and water. [source]


Effluent treatment using a bipolar electrochemical reactor with rotating cylinder electrodes of woven wire meshes

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2009
Javier M Grau
Abstract BACKGROUND: The behaviour of a bipolar electrochemical reactor consisting of one or more rotating cylinder electrodes of woven wire meshes is reported using copper and cadmium deposition from dilute solutions as test reactions. RESULTS: The best performance related to electrode number was determined for copper deposition and was achieved by an arrangement with two bipolar electrodes, for which the conversion in a single pass was approximately 47%. The specific energy consumption was 3.27 kWh kg,1 with a normalised space velocity of 23.05 h,1. The copper powder obtained showed a nodular and dendritic surface morphology. This reactor configuration was also analysed for cadmium deposition, in which hydrogen evolution takes place simultaneously as a side cathodic reaction, considering the effect of flow rate and total current. The maximum conversion per pass for cadmium removal was 38.91%. In this case the reactor with two bipolar electrodes showed a performance similar to that of a monopolar reactor operated at a rotation speed three times higher. CONCLUSION: A continuous electrochemical reactor with two rotating bipolar electrodes of woven wire meshes presents a good performance for copper or cadmium removal from dilute solutions. Copyright © 2009 Society of Chemical Industry [source]


Comparison of the surface and catalytic properties of rare earth-promoted CaO catalysts in the oxidative coupling of methane

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2006
Vilas H Rane
Abstract Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650,800 °C, CH4/O2 ratios, 2.0,8.0 and space velocity, 51 360 cm3 g,1 h,1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm-promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO2 chemisorbed at 50° and 500 °C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below ,700 °C, whereas at higher temperature no further effect is seen. The La2O3CaO, Nd2O3CaO and Yb2O3CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd-promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH4 conversion with 70.8% C2+ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO2 chemisorbed at 50° and 500 °C respectively) and the C2+ selectivity and/or C2+ yield has been observed. Copyright © 2005 Society of Chemical Industry [source]


Performance study of modified ZSM-5 as support for bimetallic chromium,copper catalysts for VOC combustion

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2004
Ahmad Zuhairi Abdullah
Abstract The catalytic performance of bimetallic chromium,copper supported over untreated ZSM-5 (CrCu/Z), ZSM-5 treated with silicon tetrachloride (Cr,Cu/SiCl4 -Z) and ZSM-5 treated with steam (Cr,Cu/H2O-Z) is reported. The activity is based on the combustion of ethyl ethanoate and benzene at a feed concentration of 2000 ppm and a gas hourly space velocity (GHSV) of 32 000 h,1. Due to higher reactivity and larger molecular size compared with that of water molecules, SiCl4 reacted at the external surface of ZSM-5 crystals. Cr,Cu/SiCl4 -Z and Cr,Cu/H2O-Z both gave slightly lower conversion and carbon dioxide yield compared with Cr,Cu/Z. This was attributed to larger active metal crystallites formed in the mesopores and narrowing pore mouth and pore intersection by extraframework species. Cr,Cu/SiCl4 -Z and Cr,Cu/H2O-Z both had reduced concentration and strength of acid sites, thus making them less susceptible to deactivation by coking. The coke accumulated by these two catalysts was relatively softer and more easily decomposed in oxygen during catalyst regeneration. Copyright © 2004 Society of Chemical Industry [source]


Light FCC gasoline olefin oligomerization over a magnetic NiSo4/,-Al2o3 catalyst in a magnetically stabilized bed

AICHE JOURNAL, Issue 3 2009
Ying Peng
Abstract Magnetic NiSO4/,-Al2O3 catalysts were prepared by impregnating NiSO4 solutions onto the ,-Al2O3 support containing a magnetic material of Fe3O4. Characterization by XRD, NH3 -TPD, and thermal analysis showed that the magnetic NiSO4/,-Al2O3 catalyst with a nickel content of 7.0% by weight had a monolayer dispersion of NiSO4 and the largest number of moderate strength acid sites, and a high specific saturation magnetization. The magnetic catalyst was evaluated for light FCC gasoline olefin oligomerization in both fixed-bed and magnetically stabilized bed (MSB) reactors. Comparing with that in the fixed-bed reactor, the optimal reaction temperature in the MSB lowered to 443 K, and its space velocity ranged broadly from 2.0 to 6.0 h,1. The sulfur-free diesel distillate produced by operation of the MSB for 100 h had higher cetane number and good low-temperature flow property, which illuminates a promising application of the MSB to manufacture clean diesel fuels with high productivity and flexibility. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]


Effects of Transition-Metal Substitution on the Catalytic Properties of Barium Hexaaluminogallate

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002
Toshiaki Yamaguchi
The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaaluminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(Al,Ga)11O19, M = transition metal, Al/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100°C. The direct NOx reduction was observed over BaM(Al,Ga)11O19 to be around 10%. The NOx removal activity of BaM(Al,Ga)11O19 powders was improved by addition of C3H6 as a reductant gas. Co-, Ni- and Cu-substituted BaM(Al,Ga)11O19 catalysts exhibited about 40% NOx reduction with C3H6 in excess oxygen at a high space velocity of 10 000 h,1. The NOx reduction on Mn- and Fe-substituted BaM(Al,Ga)11O19 catalysts was less than 10% even in the presence of C3H6. The temperature of the effective NOx reduction on BaM(Al,Ga)11O19 catalysts could be adjusted from 350° to 500°C by the selection of the transition-metal substitution in the catalysts. The catalysts hold high activities for NOx reduction even at 500°C in water vapor produced in the combustion system of reductant gases. [source]


Is the sky falling?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
RAVE surveys, Searching for stellar streams in the local Milky Way disc in the CORAVEL
ABSTRACT We have searched for in-falling stellar streams on to the local Milky Way disc in the CORrelation RAdial VELocities (CORAVEL) and RAdial Velocity Experiment (RAVE) surveys. The CORAVEL survey consists of local dwarf stars (Nördstrom et al. Geneva,Copenhagen survey) and local Famaey et al. giant stars. We select RAVE stars with radial velocities that are sensitive to the Galactic vertical space velocity (Galactic latitude b < ,45°). Kuiper statistics have been employed to test the symmetry of the Galactic vertical velocity distribution functions in these samples for evidence of a net vertical flow that could be associated with a (tidal?) stream of stars with vertically coherent kinematics. In contrast to the ,Field of Streams' found in the outer halo, we find that the local volumes of the solar neighbourhood sampled by the CORAVEL dwarfs (complete within ,3 × 10,4 kpc3), CORAVEL giants (complete within ,5 × 10,2 kpc3) and RAVE (5,15 per cent complete within ,8 kpc3) are devoid of any vertically coherent streams containing hundreds of stars. This is sufficiently sensitive to allow our RAVE sample to rule out the passing of the tidal stream of the disrupting Sagittarius (Sgr) dwarf galaxy through the solar neighbourhood. This agrees with the most-recent determinations of its orbit and dissociates it from the Helmi et al. halo stream. Our constraints on the absence of the Sgr stream near the Sun could prove a useful tool for discriminating between Galactic potential models. The lack of a net vertical flow through the solar neighbourhood in the CORAVEL giants and RAVE samples argues against the Virgo overdensity crossing the disc near the Sun. There are no vertical streams in the CORAVEL giants and RAVE samples with stellar densities ,1.6 × 104 and 1.5 × 103 stars kpc,3, respectively, and therefore no evidence for locally enhanced dark matter. [source]


Rate equations for the fischer-tropsch reaction on a promoted iron catalyst

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2001
Montazer Rahmati
Abstract Intrinsic rates for the Fischer-Tropsch synthesis reaction over a promoted iron catalyst fabricated at the Research Institute of the Petroleum Industry (RIPI) have been obtained in the temperature range of 290°C to 310°C, pressure range of 1500 to 2300 kPa, molar hydrogen to carbon monoxide ratio of 0.76 to 1.82, and a space velocity of 3300 h,1 under conditions of constant catalyst activity. To this end, the initial reaction rates have been measured at constant temperature (±1°C) in the absence of diffusion limitations, and power-law equations have been fitted in terms of the hydrogen and carbon monoxide partial pressures for the reaction rates. Des vitesses intrinsèques pour la réaction de synthèse de Fischer-Tropsch sur un catalyseur de fer activé fabriqué à l'Institut de recherche de l'industrie du pétrole (RIPI) ont été obtenues pour des températures entre 290°C et 310°C, des pressions entre 1500 et 2300 kPa, un rapport hydrogène molaire/oxyde de carbone entre 0.76 et 1.82 et une vitesse spatiale de 3300 h,1 avec une activité constante du catalyseur. À cette fin, les vitesses de réaction initiales ont été mesurées à température constante (±1°C) en l'absence de limitations par la diffusion, et des équations de loi de puissance ont été calées par rapport aux pressions partieiles de l'hydrogène et de l'oxyde de carbone pour les vitesses de réaction. [source]


Production of hydrogen via glycerol steam reforming in a Pd-Ag membrane reactor over Co-Al2O3 catalyst

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2010
A. Iulianelli
Abstract Generally, biodiesel fuel, when converted from vegetables oils, produces around 10 wt% of glycerol as a byproduct, which could be used for producing hydrogen by a steam-reforming reaction. Different scientific works have been realized in conventional reactors on the steam reforming of glycerol (GSR) in the aqueous or the gas phase. High reaction pressure and a relatively small catalyst deactivation are noticed when GSR is carried out in an aqueous phase, whereas the catalyst deactivation is the main disadvantage in the gas phase. In this work, GSR reaction was performed in a perm-selective Pd-Ag membrane reactor (MR) packed with a Co-Al2O3 commercial catalyst in order to extract a CO-free hydrogen stream and also enhance the performances in terms of glycerol conversion and hydrogen yield with respect to a traditional reactor (TR), both working at weight hourly space velocity (WHSV) = 1.01 h,1, 400 °C and H2O/C3H8O3 = 6/1. In MR, a maximum glycerol conversion of around 45.0% was achieved at 1.0 bar as reaction pressure, whereas it was around 94% at 4.0 bar. Moreover, as best value, more than 60.0% of CO-free hydrogen recovery was achieved in the MR at 4.0 bar and 22.8 of sweep factor (sweep gas to glycerol ratio). Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source]


Study on surface morphology and selectivity of precipitated iron catalysts of FTS

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009
Wang Xingjun
Abstract The precipitated iron catalyst was prepared by co-precipitation. The surface morphology of the catalyst was investigated under different reduction conditions by SEM (S-250, USA). Under H2 -reduction, the surface morphology of the catalyst had the obvious changes, which the diameter reduced, adhered together, came into being wads considered as a group. But the surface morphology of the catalyst had almost no change under CO reduction. The crystal structure of the catalyst was studied under different reduction conditions by X-ray diffraction (XRD) (Rigaku D/max, Japanese). It was found that the catalyst was reduced completely with H2, but it was reduced partly with CO. The crystal structure of the catalyst converted into the metallic phase with H2 reduction. However, most of the iron converted into iron oxide (Fe3O4) with CO reduction. And the predominant phase in a sample of a mature catalyst is ,-Fe5C2, which is the active phase in the Fischer-Tropsch synthesis (FTS). The experimental results showed that CO conversion and H2 conversion increase with the change of reaction temperature from 260 to 300 °C, under the conditions of pressure P = 2.6 MPa, space velocity = 0.86 Nl h,1 g-Fe,1, n(H2)/n(CO) = 2/3, and most of the hydrocarbon products are C5,11 which hold half of the hydrocarbon products. The next content is C2,4 which holds the quarter of hydrocarbon products. Then it is C12+, which is equal to 18%. And the last is C1, which is equal to 7%. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source]


Nearby stars of the Galactic disk and halo.

ASTRONOMISCHE NACHRICHTEN, Issue 1 2004

Abstract High-resolution spectroscopic observations of about 150 nearby stars or star systems are presented and discussed. The study of these and another 100 objects of the previous papers of this series implies that the Galaxy became reality 13 or 14 Gyr ago with the implementation of a massive, rotationally-supported population of thick-disk stars. The very high star formation rate in that phase gave rise to a rapid metal enrichment and an expulsion of gas in supernovae-driven Galactic winds, but was followed by a star formation gap for no less than three billion years at the Sun's galactocentric distance. In a second phase, then, the thin disk , our "familiar Milky Way" , came on stage. Nowadays it traces the bright side of the Galaxy, but it is also embedded in a huge coffin of dead thick-disk stars that account for a large amount of baryonic dark matter. As opposed to this, cold-dark-matter-dominated cosmologies that suggest a more gradual hierarchical buildup through mergers of minor structures, though popular, are a poor description for the Milky Way Galaxy , and by inference many other spirals as well , if, as the sample implies, the fossil records of its long-lived stars do not stick to this paradigm. Apart from this general picture that emerges with reference to the entire sample stars, a good deal of the present work is however also concerned with detailed discussions of many individual objects. Among the most interesting we mention the blue straggler or merger candidates HD 165401 and HD 137763/HD 137778, the likely accretion of a giant planet or brown dwarf on 59 Vir in its recent history, and HD 63433 that proves to be a young solar analog at , , 200 Myr. Likewise, the secondary to HR 4867, formerly suspected non-single from the Hipparcos astrometry, is directly detectable in the highresolution spectroscopic tracings, whereas the visual binary , Cet is instead at least triple, and presumably even quadruple. With respect to the nearby young stars a complete account of the UrsaMajor Association is presented, and we provide as well plain evidence for another, the "Hercules-Lyra Association", the likely existence of which was only realized in recent years. On account of its rotation, chemistry, and age we do confirm that the Sun is very typical among its G-type neighbors; as to its kinematics, it appears however not unlikely that the Sun's known low peculiar space velocity could indeed be the cause for the weak paleontological record of mass extinctions and major impact events on our parent planet during the most recent Galactic plane passage of the solar system. Although the significance of this correlation certainly remains a matter of debate for years to come, we point in this context to the principal importance of the thick disk for a complete census with respect to the local surface and volume densities. Other important effects that can be ascribed to this dark stellar population comprise (i) the observed plateau in the shape of the luminosity function of the local FGK stars, (ii) a small though systematic effect on the basic solar motion, (iii) a reassessment of the term "asymmetrical drift velocity" for the remainder (i.e. the thin disk) of the stellar objects, (iv) its ability to account for the bulk of the recently discovered high-velocity blue white dwarfs, (v) its major contribution to the Sun's ,220 km s,1 rotational velocity around the Galactic center, and (vi) the significant flattening that it imposes on the Milky Way's rotation curve. Finally we note a high multiplicity fraction in the small but volume-complete local sample of stars of this ancient population. This in turn is highly suggestive for a star formation scenario wherein the few existing single stellar objects might only arise from either late mergers or the dynamical ejection of former triple or higher level star systems. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]