1,721,084 research outputs found
Market potential and productivity: Evidence from Finnish regions
No full textNew economic geography (NEG) stresses the role of various types of linkages as agglomeration forces. We show that a typical NEG model can be used to design an empirical methodology to assess whether linkages are relevant at all and, if so, whether they are more important for firms or workers in terms of productivity or amenity respectively. Applying the proposed methodology to Finnish NUTS 4 regions from 1977 to 2002, we find that linkages are indeed relevant and that firm-related demand and cost linkages are more important than worker-related cost-of-living linkages. © 2006 Elsevier B.V. All rights reserved
Biodegradation of PAHs in aggregates of a low permeability soil
No full textIn the bioremediation of low-permeability soils, pollutant and, especially, the oxygen bioavailability are often the rate limiting steps. In cases when a biopile treatment can represent an applicable technique, pretreatment of the excavated soil is often necessary to attain an adequate air-filled porosity in the soil and to avoid the presence of large soil aggregates. The present work was performed to evaluate the influence of soil aggregate size in the bioremediation of a silt-clay type soil contaminated by PAHs. Microcosms were arranged with spherical soil aggregates of different diameter in near water-saturation conditions. Concentration of two and three aromatic ring PAHs, total biomass, and respiration rates were monitored. PAH concentration profiles inside the particles were also obtained. A simple and quick way to estimate the critical dimension of the soil aggregates was developed based on the evaluation of an oxygen penetration depth, that is, the distance from the external surface to the aggregate core beyond which oxygen concentration is practically zero. A very different time course of PAHs consumption was found in the external layer and the inner core of the aggregates as well as in aggregates of different dimensions. The results suggest that only the 3 mm external layer of the sphere is not limited by oxygen diffusion
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
Numerical evaluation of airfoil friction drag
No full textThe capabilities of a Navier-Stokes solver to predict the friction drag over an airfoil were studied. In particular, the sensitivity to the grids resolution and to different Reynolds-averaged Navier-Stokes equations (RANS) closure models was investigated for three different Reynolds: the standard κ-ε, Renormalization Group (RNG) κ-ε, and Reynolds stress model. The results indicate that in general, the RANS calculations seem to require computational resources, both memory and computing time
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
In situ bioremediation of a soil contaminated by mineral oil: A case study
No full textLaboratory tests were conducted to study the feasibility of a bioremediation treatment. The first phase of the work was exploratory and comprised slurry condition microcosm tests. The second phase consisted of solid-phase microcosm tests to evaluate the effectiveness of the treatment and to estimate the time needed to reach the target level of 100 mg.kg fixed by the local environmental agency. The third phase consisted of helping the remediation company to select the appropriate technology, of choosing the strategy for its application in the field and of monitoring the site during treatment. From the microcosm tests data, the depletion rate became slower as a residual concentration of pollutants of ~ 500 mg/kg was reached. This residual contamination part was hardly bioavailable to the microorganisms that proved to be very active in using hydrocarbons as a carbon source. The results of the tests showed very low contaminant depletion rates. All the attempts to increase the bioremediation rate failed
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
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