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Bed Materials (bed + material)
Selected AbstractsIn Situ CO2 Capture in a Dual Fluidized Bed Biomass Steam Gasifier , Bed Material and Fuel VariationCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2009G. Soukup Abstract The AER (absorption enhanced reforming) process, based on the dual fluidized bed steam gasification process, uses a bed material which, apart from its function as heat carrier, acts as a CO2 transporting agent and selectively transfers CO2 from the gasification reactor to the combustion reactor. Thus, a hydrogen-rich gas is produced (H2 55...70 vol.-%db, CH4 8...13 vol.-%db, CO 5...11 vol.-%db as well as CO2 7...20 vol.-%db). Seven different CaO containing bed materials were tested for their principal applicability to the AER process with special focus on their influence on tar production. The amount of tars was generally found to be five times lower than with conventional gasification (at 850,°C) despite the low gasification temperatures (600,700,°C). Due to these low gasification temperatures, biomass with high alkali contents (e.g., straw), indicating a low ash melting point (< 800,°C), can be used under AER conditions. In summary, the AER process is very effective owing to the integration of the heat of the chemical reaction of CO2 with CaO and the water-gas shift reaction, which are both exothermic, into the gasification and the internal reforming of primary and secondary tars, which cuts off the formation of higher tars. [source] Structural patterns in coarse gravelriver beds: typology, survey and assessment of the roles of grain size and river regimeGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2002Lea Wittenberg The concept of river-bed stability as indexed by the occurrence of stable bed forms was examined in humid-temperate perennial streams and in Mediterranean ephemeral streams. The study examined the structural patterns of bed forms and their spatial distribution between temperate-humid and Mediterranean streams. Study sites in Northumberland, UK, and Mt. Carmel, Israel, were selected for their morphometric similarity, despite the contrast in climate, vegetation and hydrological regime. Fieldwork was based on a large number of Wolman grain size distributions and structure measurements along cross-sections at seven sites; Differences in mean grain size of bed structures were estimated using the general linear model (GLM) procedure and Duncan's multiple range test. Based on field evidence, river-bed configurations were divided into structural categories, according to the depositional setting of each measured particle on the river bed. Statistical analysis confirmed former qualitative descriptions of small-scale bed forms. The study identified spatial segregation in bed form distribution. In general, 30,40%of the bed material in the surveyed perennial streams was clustered, in contrast to approximately 10%in the ephemeral counterparts. The sorting index revealed higher values for the perennial streams, namely 2.39,3.59 compared with 1.73,2.07 for the ephemeral counterparts. It is suggested that the degree of sediment sorting and the proportion of clusters are strongly related. Sediment sorting, sediment supply and the hydrological regime explain the mechanism of cluster formation. It is assumed that climate shifts or human interference within river basins might affect the regional characteristic flood hydrograph, and consequently alter the sedimentary character of the river bed. In the case where river bed stability is reduced owing to changes in cluster bed form distribution, rivers that normally do not yield a significant amount of sediment might be subject to notable sedimentation problems. [source] The formation of dunes, antidunes, and rapidly damping waves in alluvial channelsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 7 2001L.-H. Huang Abstract Under the effect of a constant current for a long time, a water channel of infinitely long and constant depth interacting with a uniform sandbed of infinite thickness is used to simulate the formation of dunes, antidunes and rapidly damping waves in alluvial channels. The theory of potential flow is applied to the channel flow, while Biot's theory of poroelasticity is adopted to deal with erodible bed material. The governing equations, together with free surface, bed surface, and far field boundary conditions, form a complete boundary-value problem without applying empirical sediment discharge formulas as in conventional researches. The comparison of the present result with Kennedy's (Journal of Fluid Mechanics, 1963; 16: 521,544) instability analysis not only indicates the appropriateness of the present work, but also reveals the advantage of the present study due to its ability to find all kinds of bed forms (including the rapidly damping waves that Kennedy could not find) and of solving for the unclear lagged distance , introduced in Kennedy's work. Copyright © 2001 John Wiley & Sons, Ltd. [source] Downstream Fining and Sorting of Gravel Clasts in the Braided Rivers of mid-Canterbury, New ZealandNEW ZEALAND GEOGRAPHER, Issue 2 2004Greg Browne ABSTRACT Gravel clast size dimensions have been determined in the Rakaia, Ashburton, and Rangitata rivers by measuring 100 clasts at representative sample locations along each river. In all rivers, gravel size decreases and sorting improves downstream for mean, D50, and D90 fractions of the bed material. Clast size entering the sea is similar in all rivers (30,40 mm b-axis dimension), despite large variations in transport distance, input size of clasts at their gorges, and discharge. The greatest size reduction occurs in the Rangitata River which has the shortest transport distance and steepest gradient. Rates of downstream clast size reduction are greater than would be assumed from Sternberg's Law, suggesting that additional factors, other than physical abrasion, such as sorting and selective entrainment operate. [source] In Situ CO2 Capture in a Dual Fluidized Bed Biomass Steam Gasifier , Bed Material and Fuel VariationCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2009G. Soukup Abstract The AER (absorption enhanced reforming) process, based on the dual fluidized bed steam gasification process, uses a bed material which, apart from its function as heat carrier, acts as a CO2 transporting agent and selectively transfers CO2 from the gasification reactor to the combustion reactor. Thus, a hydrogen-rich gas is produced (H2 55...70 vol.-%db, CH4 8...13 vol.-%db, CO 5...11 vol.-%db as well as CO2 7...20 vol.-%db). Seven different CaO containing bed materials were tested for their principal applicability to the AER process with special focus on their influence on tar production. The amount of tars was generally found to be five times lower than with conventional gasification (at 850,°C) despite the low gasification temperatures (600,700,°C). Due to these low gasification temperatures, biomass with high alkali contents (e.g., straw), indicating a low ash melting point (< 800,°C), can be used under AER conditions. In summary, the AER process is very effective owing to the integration of the heat of the chemical reaction of CO2 with CaO and the water-gas shift reaction, which are both exothermic, into the gasification and the internal reforming of primary and secondary tars, which cuts off the formation of higher tars. [source] Size distributions of suspended particles in open channel flow over bed materialsENVIRONMETRICS, Issue 2 2005B. S. Mazumder Controlled experiments have shown that the grain-size distribution of suspended sediments is related to bed material, flow velocity and height of suspension above the sand bed in an open channel flow. A theoretical model has been developed for computation of suspended grain-size distribution on the basis of continuity equations of sediment and water, using the computed bed-layer concentration as a reference. The proposed model includes the effect of suspension concentration into the mean velocity, turbulent and viscous shear stresses owing to the dynamic coupling between the flow and sediments in suspension. The effect of hindered settling due to the increased concentration in suspension is also taken into account. The model is considered to be a more general one than the existing models, and the results of the present model compare well with the experimental data. Copyright © 2004 John Wiley & Sons, Ltd. [source] Synthesis of multiwalled carbon nanotubes on Al2O3 supported Ni catalysts in a fluidized-bedAICHE JOURNAL, Issue 1 2010Jun Liu Abstract Multiwalled carbon nanotubes (MWNTs) were synthesized on Al2O3 supported Ni catalysts from C2H2 and C2H4 feedstocks in a fluidized bed. The influence of the ratio of superficial gas velocity to the minimum fluidization velocity (U/Umf), feedstock type, the ratio of carbon in the total quantity of gas fed to the reactor, reaction temperature, the ratio of hydrogen to carbon in the feed gas, and nickel loading were all investigated. Significantly, the pressure drop across the fluidized-bed increased as the reaction time increased for all experiments, due to the deposition of MWNTs on the catalyst particles. This resulted in substantial changes to the depth and structure of the fluidized bed as the reaction proceeded, significantly altering the bed hydrodynamics. TEM images of the bed materials showed that MWNTs, metal catalysts, and alumina supports were predominant in the product mixture, with some coiled carbon nanotubes as a by-product. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Experimental and computational study of the bed dynamics of semi-cylindrical gas,solid fluidized bedTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009A. Sahoo Abstract With computational fluid dynamics (CFD) it is possible to get a detailed view of the flow behaviour of the fluidized beds. A profound and fundamental understanding of bed dynamics such as bed pressure drop, bed expansion ratio, bed fluctuation ratio, and minimum fluidization velocity of homogeneous binary mixtures has been made in a semi-cylindrical fluidized column for gas,solid systems, resulting in a predictive model for fluidized beds. In the present work attempt has been made to study the effect of different system parameters (viz., size and density of the bed materials and initial static bed height) on the bed dynamics. The correlations for the bed expansion and bed fluctuations have been developed on the basis of dimensional analysis using these system parameters. Computational study has also been carried out using a commercial CFD package Fluent (Fluent, Inc.). A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas,solid flow. CFD simulated bed pressure drop has been compared with the experimental bed pressure drops under different conditions for which the results show good agreements. La simulation par ordinateur de la dynamique des fluides (CFD) permet de décrire le comportement des écoulements dans les lits fluidisés. Une étude fondamentale et approfondie de la dynamique de lit, tels la perte de charge de lit, le taux d'expansion de lit, le taux de fluctuation de lit et la vitesse de fluidisation minimale de mélanges binaires homogènes, a été réalisée dans une colonne fluidisée semi-cylindrique pour des systèmes solides de gaz, permettant d'obtenir un modèle prédictif pour les lits fluidisés. Dans le présent travail, on a tenté d'étudier l'effet de différents paramètres de système (à savoir, la taille et la masse volumique des matériaux de lit et la hauteur statique initiale de lit) sur la dynamique de lit. Des corrélations ont été établies pour l'expansion de lit et les fluctuations de lit en s'appuyant sur l'analyse dimensionnelle de ces paramètres de système. Une étude par ordinateur a également été menée à l'aide du logiciel commercial de CFD Fluent (Fluent, Inc.). Un modèle eulérien multifluide faisant appel à la théorie cinétique pour les particules solides a été utilisé afin de simuler l'écoulement gaz-solides. La perte de charge de lit simulée par la CFD a été comparée à la perte de charge de lit expérimentale dans différentes conditions et les résultats montrent un bon accord. [source] Solids Circulation Flux and Gas Bypassing in a Pressurized Spout-fluid Bed with a Draft TubeTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2002Rui Xiao Abstract An experimental study on solids circulation flux and gas bypassing of a spout-fluid bed with a draft tube at elevated pressures up to 600 kPa was performed in a 200 mm diameter cylindrical steel column with a 608 conical distributor. Glass beads with mean diameter 2.067 mm were used as bed materials to investigate the effect of operating conditions and geometric configuration on the solids circulation flux and the gas distribution between the annulus and the draft tube. A novel technique has been developed to measure the solids fluxes under pressure, and gas (CO2) traces have been employed to investigate gas bypassing characteristics. The solids circulation flux is greatly enhanced when operating pressure and auxiliary gas flowrate are increased, and it is also strongly influenced by geometric configuration. Two experimental relations are proposed for predicting solids circulation flux enhancement factors. Une étude expérimentale sur le flux de circulation des solides et la dérivation du gaz d'un lit jaillissant muni d'un tube d'aspiration à des pressions élevées jusqu'à 600 kPa, a été réalisée dans une colonne d'acier cylindrique de 200 mm de diamètre avec un distributeur conique à 608. Des billes de verre d'un diamètre moyen de 2,067 mm ont été utilisées comme matériaux de lit pour étudier l'effet des conditions opératoires et de la configuration géométrique sur le flux de circulation des solides et la distribution du gaz entre l'espace annulaire et le tube d'aspiration. Une nouvelle technique a été élaborée pour mesurer les flux de solides sous pression et les caractéristiques de la dérivation du gaz ont été étudiées à l'aide de gaz traceurs (CO2). Le flux de circulation des solides est grandement amélioré lorsque la pression de fonctionnement et le débit de gaz auxiliaire sont augmentés, et il est également fortement influencé par la configuration géométrique. Deux relations expérimentales sont proposées pour la prédiction des facteurs d'amélioration du flux de circulation des solides. [source] Mathematical model for mixing index in gas,solid fluidized bed: an analysisASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010Abanti Sahoo Abstract An available theoretical model for mixing index in gas-solid fluidized bed has been analyzed for further modification and improvement. Assumption of 50% bed material moving in the upward direction and rest 50% in the downward direction considered for the above model (the counter-flow circulation model) has been analyzed for optimum result. A computer program was run for different bed conditions by varying operating parameters, namely, the mixture composition, fractions of bed materials, minimum fluidization velocity of the jetsam particles and that of the bed materials (i.e. mixture of the jetsam and the flotsam particles). The developed model was solved by finite difference (central differencing). Fraction of the bed material moving in the upward direction was varied from 0 to 100% and the minimum fluidization velocity was varied within the Umf of the flotsam to that of the jetsam particles (i.e. 0.465,1.0335 m/s). Optimum fraction of bed materials with respect to its distribution (as per the earlier proposed model) in the upward and downward streams during the fluidization process was found to be 20 and 80%, respectively. The modified model was validated by conducting experiments on fluidization and studying the mixing characteristics of regular homogeneous materials (sago) in a 15 × 100 cm cylindrical column. Fairly good agreement was observed between the values of mixing index obtained from the proposed modified model and the experimental observations. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] | |||||||||||||