1,720,990 research outputs found
Analysis of the fluid dynamic behavior of the liquid and gas phases in reactors stirred with multiple hydrofoil impellers
No full textLiquid- and gas-phase macromixing behavior was studied in gas - liquid high-aspect-ratio reactors stirred with multiple hydrofoil impellers pumping downward. Water, a sodium sulfate solution, and poly(vinylpyrrolidone) solutions of viscosity up to 110 mPa·s were used as the liquid. For characterizing the liquid phase, mixing time experiments were conducted at various operating conditions, while detecting the response curves at several positions inside the tank. Comparison of the experimental curves with the theoretical ones provided by simple fluid dynamic models showed that the axial dispersion model is quite acceptable. The influence of impeller speed, gas flow rate, and viscosity on the model parameter was studied, and dimensionless relationships are given. The gas behavior was studied by means of the RTD and modeled with the axial dispersion model, which proved good for water and acceptable with coalescence-inhibiting electrolyte solutions. The model parameter dependence on the operating conditions was studied. Comparison between hydrofoil impellers and radial Rushton turbines is also attempted
Solids distribution in stirred slurry reactors: Influence of some mixer configurations and limits to the applicability of a simple model for predictions
No full textThe features of solids concentration distribution were investigated in baffled and unbaffled tanks of high aspect ratio, stirred with multiple radial, mixed-flow or axial impellers. In the baffled tanks the average profiles can be predicted with the sedimentation-dispersion model regardless of impeller type - in spite of slight systematic departures from the average trend at the impeller plane and/or midway between the impellers. This is hardly possible for the unbaffled tanks due to lack of physical foundation of this simple model. When the unbaffled tank is stirred with hydrofoil impellers, an inverted profile is even obtained. The mentioned departures and this last anomalous behaviour are qualitatively discussed with reference to particle-turbulence interaction
Scale-up criteria for the solids distribution in slurry reactors stirred with multiple impellers
No full textScale-up criteria for obtaining the same vertical concentration profiles in agitated suspensions are discussed. The experiments were carried out in reactors of two scales (V=39.6 and 261 l) characterised by a high aspect ratio and stirred with multiple, evenly spaced impellers of two different types. The profiles were determined under different conditions: at constant tip speed, at constant specific power consumption, and at an intermediate condition (i.e., N∝D-0.93). The experimental profiles were compared with different approaches, namely on a qualitative basis, in terms of standard deviation and the parameter of the axial dispersion model with sedimentation. In all cases, the same criterion based on the aforementioned intermediate condition (closer to constant tip speed) was confirmed. The experimental data were also examined in terms of effective particle settling velocity, which is a basic parameter for modelling, and fair agreement of the data obtained at the two scales with the different impellers was obtained. © 2003 Elsevier Ltd. All rights reserved
Diagnosis of solid distribution in vessels stirred with multiple PBTs and comparison of two modelling approaches
No full textThe features of solids concentration distribution were investigated in two baffled vessels of different scale. The vessels were of high aspect ratio and were stirred with multiple PBTs. Both steady- and unsteady-state experiments were performed. The experimental data were compared with the previsions of the one-dimensional sedimentation-dispersion model and of CFD tools. The former approach provides good estimates only of the average, steady-state vertical profiles, while the latter describes the local variations much more accurately. Both approaches give fairly good estimates of the transient solids concentration distribution. The dynamic CFD simulations allowed us also to tune the value of the turbulent Schmidt number as a relevant parameter. Finally, both simulation approaches have confirmed that the particle settling velocity in a stirred liquid is a correct parameter to be used instead of the terminal settling velocity
Experiments and CFD predictions of solid particle distribution in a vessel agitated with four pitched blade turbines
No full textThe distribution of solid particles in a high aspect-ratio baffled tank agitated with four 45° pitched blade turbines (PBT) was investigated using both experimental measurements and CFD simulations. Dilute suspensions of glass beads in water and moderately viscous liquids were considered. The measurement of axial particle concentration profiles was conducted by means of a light attenuation technique. Fully predictive simulations of solid-liquid suspensions were performed using a Sliding-Grid approach coupled with the Eulerian-Eulerian Two Fluid Model and the 'homogeneous' two-phase k-ε turbulence model. The simulated particle axial concentration profiles were compared with the experimental data and good agreement was found
An environmental impact assessment of exported wood pellets from Canada to Europe
No full textThere have been increased interests on exporting wood pellets from Canada to Europe to meet the increased demand on biofuels in European countries. The wood pellet industry in Canada, especially in the west coastal region, has grown at an annual rate of more than 20% averaged over last 5 years due to the steady supply of wood residues. This paper attempted to analyze the fuel consumption and air emissions associated with the wood pellet production in British Columbia and export to Sweden based on a streamlined life cycle analysis, starting from tree harvesting for wood residue production to the shipping of wood pellets from Vancouver to Stockholm in Sweden. The results showed that about 7.2 GJ of energy is consumed for each tonne of wood pellets produced and shipped to Europe, representing about 39% of the total energy content of the wood pellets. Among those energies consumed over the life cycle, about 2.6 GJ is associated with long-distance ocean transportation. The ocean transportation is also the major contributor to environmental and health impacts, followed by the pellet production processes. The fossil fuel content, which quantifies the amount of fossil fuel consumed over the life cycle, for exported wood pellets ranged from 19% to 35%, depending on whether natural gas or wood residue is used in the drying operation during the wood pellet production stage. To reduce the fossil fuel content and the environmental impacts, wood residues should be used in the drying operation and, if possible, local market should be explored to reduce the energy consumption associated with wood pellet transportation over long distances
Comparison of experimental techniques for the measurement of mixing time in gas-liquid systems
No full textMeasurements of the homogenisation characteristics during the agitation of a liquid and the mixing time by simple in situ conductivity probes are very well established. However, unless special precautions are taken, in the presence of the second phase such as gas, the conductivity trace becomes distorted to a greater or lesser extent, so that it is not possible to follow the transient change of concentration in the liquid phase or estimate the mixing time. In this paper it is confirmed that, without special precautions, simple in situ probes are unsatisfactory. However, by shielding the probe with a "cage", the ingress of bubbles into the probe region is essentially prevented and satisfactory results can be obtained in situ with responses having as little noise as in the case without gas. A second technique involves elimination of the gas from a small sample stream and measurement of the stream's conductivity transient. By suitable and rather simple treatment of the response, results equivalent to that from the in situ shielded probes can be obtained. The latter technique is especially useful where the placement of in situ probes is difficult. It is also suggested that recent results, which disagree with much of the literature on liquid phase mixing times in gassed systems, arose due to the use of in situ unshielded conductivity probes
On the Simulation of Solid Particle Distribution in Multiple Impeller Agitated Tanks via Computational Fluid Dynamics
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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