319 research outputs found

    Analysis of the fluidization behaviour and application of a novel spouted bed\ud apparatus for spray granulation and coating

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    Spouted beds are well known for their good mixing of the solid phase and for their intensive heat\ud and mass transfers between the fluid phase and the solid phase. Nearly isothermal conditions are\ud enabled which is of advantage for the treatment of granular solid materials in granulation,\ud agglomeration or coating processes. In this work the hydrodynamic behaviour of a novel spouted\ud bed apparatus with two horizontal and slit-shaped gas inlets is investigated by high-frequency\ud recordings of the gas phase pressure fluctuations over the entire bed. The hydrodynamic stable\ud operation domain, which is of importance for operating the apparatus, will be identified and\ud depicted in the Re-G-Ar-diagram by Mitev [1]. Another focus of this work is the simulation of the\ud spouting process by application of a continuum approach in FLUENT 6.2. The effect of the\ud frictional stresses on the hydrodynamic behaviour is examined by performing simulations with and\ud without consideration of friction. The angle of internal friction fi in Schaeffer`s [10] model will be\ud varied and the simulation results will be compared with experiments. It was found that the influence\ud of friction is not very big by application of the quite simple and empirical frictional viscosity model\ud by Schaeffer [10] basing on soil mechanical principles. Also the simulation results under negligence\ud of friction were similar to those under consideration of friction. Another part of this work is the\ud industrial application of the novel spouted bed in granulation and coating processes. Compared to\ud classical fluidized beds, a much narrower particle size distribution, a higher yield and a higher\ud product quality was obtained in the novel spouted be

    Characterization of the pneumatic behavior of a novel spouted bed apparatus

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    Recently the importance of spouted bed technology has significantly increased in the context of drying processes as well as granulation, agglomeration or coating processes. Particulate systems concerning very fine or non spherical particles that are difficult to fluidize, often cannot be treated in conventional fluidized beds. In contrast to those fluidized beds, the spouted bed technology with its specific flow structure offers the opportunity of stable fluidization under controlled conditions. Within this work the fluid dynamics of a novel spouted bed with two adjustable gas inlets is investigated. By analysis of gas fluctuation spectra by means of a fast Fourier transformation algorithm, different operation regimes are identified and depicted graphically. Furthermore, continuum CFD-modeling of the granular solid phase motion by means of an Euler/Euler approach and comparisons with experimental obtained velocity vector fields by means of particle image velocimetry (PIV) measurements will be presented in this work

    Characterizing and Modeling the Hydrodynamics of Shallow Spouted Beds

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    The hydrodynamics of shallow, conical spouted beds of heavy particles were experimentally studied to determine how they differ from previous spouted bed reports in the literature. Key experimental measurements included minimum spouting velocity, time-average and time-varying (dynamic) pressure drop, time-average fountain height and time-average gas velocity profile in the bed. New correlations were developed for minimum spouting velocity, time-average pressure drop and fountain height based on the experimental data. The time-average gas velocity profile measurements confirmed that the beds in the present study exhibited gas flow features that were at least qualitatively similar to those previously reported for other experimental conical spouted beds and predicted by detailed computational fluid dynamics models. At least some of the major features of the observed spouted bed pulsation behavior appear to be captured by a simple zone-based model of ordinary differential equations. The equations are derived from time-differential mass and momentum balances over 4 spatial zones: entrainment, spout, fountain, and annulus. The dynamic behavior of the model is dominated by the entrainment zone, which includes the effects of 3 key processes: 1) Granular particle flow from the annulus into the area immediately above the gas inlet; 2) Radial leakage of gas outward from the inlet zone in response to the inward flowing particles and; 3) Upward flow of the main part of the inlet gas and subsequent particle entrainment in response to the gas-particle drag. Recommendations are made for further improvements to the model

    Solids and gas hydrodynamic characteristics in square-based spouted beds for thermal applications

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    Gasification is considered a favourable method to convert a biomass or other organic materials to a multipurpose gas; the syngas produced can be addresses to the chemical industry as a synthesis reactant or considered a mere fuel. New concepts based on the valorisation of non-conventional resources have been developed in the recent years to save fossil fuels and reduce the generation of civil and industrial waste. According to these ideas, refuses can be transformed into a valuable energy source instead of being disposed. In conformity with these perspectives, updated technologies should be investigated, both to optimize gasification processes and obtain a medium heating value gas. The quality of syngas depends on the properties of the raw material used, the operative conditions and the design of the gasifier. In case the fuel used is a by-product of a certain industrial operation, the chemical composition cannot kept constant, and only an average value can be estimated to predict the composition of the syngas generated by the thermal reaction. According to these considerations, the gasification reaction should be run in a unit adaptable to a variable source of materials. The present work aims at improving the understanding of spouted beds, which display advantages if compared to more common fluidization techniques; a spouted bed can be operated with a better hydrodynamic control, thanks to lower pressure drop and gas flow rate, slightly exceeding the minimum spouting condition, rather than a much higher flow rate required by bubbling or turbulent fluidization. Additionally, the hydrodynamic features of a spouted bed offer optimal mixing of solid phases characterized by very different density or shape; specifically, a light particulate reacting waste, continuously added to a stationary buffer inert, is not affected by segregation. Spouted beds appear to go through a revival, proved by a very recent and comprehensive book on the topic (Epstein and Grace, 2011). This renewed interest arises by implementing new concepts in scaling-up spouting contactors and devising potential applications to high temperature processes, noticeable examples being given by pyrolysis and gasification of biomass, kinetically controlled drying of moist seeds to guarantee the requested qualities and polymer upgrading processes. Moving to our area of interest, a textile district produces waste fibres and fibre fragments at a noticeable mass rate. As a case study, a survey within the Biella district has revealed a daily generation close to 5000 kg. This by-product represent a cost for the sector because they have to be compacted, stored and disposed, while a small-scale thermal process can be a prompted reasonable solution of valorisation. A square-based spouted bed pilot unit was designed and constructed to investigate the gasification of various types of textile waste, fed to the reactor upon pelletization. To guarantee a reducing or low oxidizing atmosphere in the reaction zone at gas flow rate proper for spouting, steam was in-line generated by gas combustion to close the mass and enthalpy balance

    Scaleup and hydrodynamics study of gas-solid spouted beds

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    A thorough understanding of the complex flow structure of gas-solid spouted bed is crucial for design, scale-up and performance. Advanced gas-solid optical probes were developed and used to evaluate different hydrodynamic parameters of spouted beds. These optical probes measure solids concentration, velocity and their time series fluctuations. Since solids concentration needs to be converted to solids holdup through calibration, for meaningful interpretation of results, a novel calibration method was proposed (which is inexpensive and reliable compared to the current reported methods) and validated in the present study. The reported dimensionless groups approach of spouted bed scale-up was assessed and was found that the two different spouted beds were not hydrodynamically similar. Hence, a new scale-up methodology based on maintaining similar or close radial profiles of gas holdup was proposed, assessed and validated. CFD was used after it was validated as an enabling tool to facilitate the implementation of the newly developed scale-up methodology by identifying the new conditions for maintaining radial profiles of gas holdup while scaling up. It can also be implemented to quantify the effect of various variables on their hydrodynamic parameters. Gamma Ray Densitometry (GRD), a non-invasive radioisotope based technique, was developed and demonstrated to montior [sic]on-line the conditions for the scale-up, flow regime and spouted beds operation. The solids holdup in spout region increases with axial height due to movement of solids from the annulus region. However, solids velocity in the spout region decreases with axial height. In the annulus region the solids move downward as a loose packed bed and the solids velocity and holdup do not change with axial height. Using factorial design of experiments it was found that solids density, static bed height, particle diameter, superficial gas velocity and gas inlet diameter had significant effect on the identification of spout diameter. Flow regimes in spouted bed were determined with the help of optical probes, pressure transducers and GRD. It was found that the range of stable spouting regime is higher in 0.152 m beds and the range of stable spouting decreases in the 0.076 m beds. The newly developed non-invasive radioisotope technique (GRD) was able to successfully identify different flow regimes and their transition velocities besides scale-up conditions and operation --Abstract, page iii

    Investigation of the hydrodynamics and scale-up of advanced TRISO nuclear fuel manufacturing using sophisticated measurement techniques

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    The successful performance and safety of the Very-High-Temperature Nuclear Reactors (VHTR) extremely depend on the quality of the TRISO nuclear fuel coated particles. However, the coating of the TRISO particles is delicate process and impacted by the hydrodynamics of spouted beds. Therefore, in this work, we applied advanced non-invasive measurement techniques which are gamma-ray computed tomography (CT) and radioactive particle tracking (RPT), to investigate foremost the local parameters to advance the fundamental understanding of the hydrodynamics and scale-up of gas-solid spouted beds. The CT technique has been applied to study the effects of particle density, particle size, bed size, and superficial gas velocity on the gas-solid cross-sectional distributions of spouted beds. The CT results demonstrated that the summation that operating spouted beds at stable spouting state would lead to achieving proper coating layers of the particles in the TRISO fuel coating process is not adequate. On the other hand, the RPT technique has been applied to evaluate the hydrodynamics, and mixing and segregation behavior of binary solids mixture spouted beds with particles of same size but different densities encountered in the TRISO nuclear fuel particles manufacturing process. The RPT results demonstrated that for the hydrodynamics of binary solids mixture spouted beds having particles of similar size but different densities, particle collisions by the particle-particle interaction plays an important role. At last, we evaluated the new mechanistic scale-up methodology that has been developed in our laboratory based on matching the radial profile of gas holdup at the region of the developed flow since the gas dynamics dictate the hydrodynamics of the gas-solid spouted beds. The measured local parameters obtained in this part confirm the validation of our new methodology of scale-up of gas-solid spouted beds --Abstract, page iii

    Hydrodynamic study of fine metallic powders in an original spouted bed contactor in view of chemical vapor deposition treatments

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    An original gas–solid contactor was developed so as to treat by chemical vapor deposition, fine (mean diameter 23 μm) and dense (bulk density 7700 kg/m3) NiCoCrAlYTa powders with large size distribution. In order to avoid the presence of a distributor in the reactive zone, a spouted bed configuration was selected, consisting in a glass cylindrical column associated through a 60° cone to an inlet tube, connected at its bottom to a grid so as to support the powders at rest. A hydrodynamic study was conducted at ambient temperature and pressure, combining pressure drop measurements and visual observations as a function of gas velocity and of the ratio H/D of the height of the bed at rest over the bed diameter. Using conventional alumina particles belonging to Geldart's group B, it was shown that this equipment is able to ensure conventional spouted bed behavior, especially for H/D ratio equal to 1. From numerous experiments conducted with the fine metallic powders of interest, it was shown that (i) conventional pressure drop curves for spouted beds are obtained for H/D ratios between 1 and 1.8, (ii) due to the large grain size distribution of particles, minimum spouted bed velocities occur in a range rather than at precise values. Visual observations reveal the presence of the spout and fountain at the minimum spouted bed velocity and for H/D equal to 1

    Investigation of hydrodynamic scaling relationships in shallow spouted beds

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    Important global hydrodynamic relationships for shallow spouted beds of high-density particles were characterized in terms of three features: minimum spouting velocity, overall bed pressure drop at minimum spouting velocity; and fountain height. Spouted bed literature is sparse for shallow beds (static particle depth to bed diameter ≤ 1) and beds with heavy particles (density \u3e 3000 kg/m3). Correlations for such beds were developed here by varying column diameter, static bed height, particle diameter, particle density, gas density and gas flow in an ambient temperature and pressure bed. The degree of correlation between each of the observed hydrodynamic features and a set of selected dimensionless groups from the literature was evaluated with principal components analysis. The minimum spouting velocity correlated strongly with the ratios of particle to bed diameter, of particle to gas density, and of static bed height to particle diameter, and weakly with Archimedes number. Overall bed pressure drop at minimum spouting correlated strongly with Archimedes number, the ratio of static bed height to particle diameter and Froude number. Fountain height correlated strongly with the ratios of the superficial gas velocity to minimum spouting velocity, of static bed height to particle diameter and of the particle to the bed diameter. Principal component regression models were developed for minimum spouting velocity, bed pressure drop, and fountain height with respect to a selected set of dimensionless parameters. All models have regression coefficient values exceeding 85%. Predictions using models developed in this study were compared with correlations in the literature and found to give better results for the experimental conditions studied. Most likely the literature models were less accurate because they were extrapolated. Distinct bed pressure drop relationships with gas flow were observed for certain ranges of particle diameter and static bed height. In addition three dynamical spouting modes were observed, and named as regular, erratic and bimodal. A spouting regime map is proposed based on the spouting regimes defined in this investigation. The correspondence between bed pressure drop relationships and spouting regimes is still unclear
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