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A critical comparison between local heat and mass transfer coefficients of horizontal cylinders immersed in bubbling fluidised beds
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EROSION RATE OF A CYLINDER IN A BUBBLINGFLUIDISED BED
No full textThis paper reports experiments on the erosion of a horizontal cylinder immersed in a bubbling fluidised bed at ambient pressure and temperature. Experiments
were carried out using a commercial resin for the sampling eroded surface. This
material couples the advantages of a relatively high erosion rate with a rigid structure. Tests were carried out at different gas velocities and include both the evaluation of the total mass eroded and of the erosion rate as a function of angular position on the cylindrical surface
Recovery of tungstate from aqueous solutions by ion exchange
No full textA commercial anionic resin has been tested to evaluate its suitability for tungsten recovery from aqueous solutions. Equilibrium runs at fixed temperature have been carried out at different pH levels, showing a higher exchange capacity at acid pH. Kinetics studies consist of the analysis of the breakthrough curves at different tungsten concentrations, flow rates, pHs, and bed depths. At basic pH, the breakpoint and the saturation times of the system are almost constant with pH whatever the flow rates and the tungsten concentrations are. Moreover, the length of the mass transfer zone remains almost unchanged with bed depth, feed flow rate and concentration. At acid pH a critical working condition appears, as the breakpoint time progressively decreases and the saturation time strongly increases. The different behaviors of the exchange resin at acid and basic pHs can be connected to the different forms of tungsten present in solution
HEAT, MASS AND EROSION RATES BETWEEN BUBBLING FLUIDIZED BEDS AND IMMERSED SURFACES
No full textSeveral industrial applications of fluidization technology consider the presence of surfaces immersed in bubbling fluidized beds. For example, tube banks in bed boilers, tubular membranes to enhance specific chemical reactions and food pieces to be freezed or dehydrated. Moreover, surface treatments, as hardening, coating or chemical vapor depositions, can be reliably carried out in fluidized beds. The surface-to-bed heat, mass and erosion rates control the actual efficiency as well as the velocity of the aforementioned industrial processes, and this explains the paramount importance of experimental and modeling studies to support a reliable design and operation of industrial processes.
The fluid dynamic field around the immersed surface presents different peculiarities respect to the average characteristics of the fluidized bed. These peculiarities are responsible for the actual values of the mass and heat transfer between the surfaces and the fluidized beds. Similarly, they governed the momentum transfer, whose main aspect is the surface erosion caused by the impacts with the fluidized particles.
This chapter aims to analyze the transport phenomena among surfaces immersed in fluidized beds in light of the most recent experimental and modeling results reported in literature. The main goal of this contribution is to shed light on the characteristics of each transport phenomena and to point out the real analogies among them
HEAT AND MASS TRANSFER CORRELATIONS FOR SURFACES IN FLUIDISED BEDS
No full textThis paper presents a comparison of heat and mass transfer coefficient around a horizontal cylinder immersed in a 500 μm glass beads bubbling fluidised bed.
In particular, experimental results on the local heat transfer coefficient, h, derived from Di Natale et al., (2008) and local mass transfer coefficient, k, given by Garim et. al., (1999) are used as the starting point for this analysis.
The heat transfer coefficient measures pointed out the occurrence of different heat transfer zones which directly reflect the existence of different hydrodynamic conditions around the cylindrical surface (e.g. Buyevich et al. 1986; Rong et al., 1999) studies. The highest value of the heat transfer coefficient is observed at the boundary of the lateral mixing and the upper sliding zones, as the result of a counterbalance between particle surface renewal frequency and particle concentration.
On the contrary, the mass transfer coefficient is the highest in the lower and lateral zones of the cylinder, where high mixing occurs.
Indeed, mass and heat transfer phenomena in fluidised beds are substantially different but they are both linked to the void fraction profile near the surface, εw: mass transfer increases with εw while the heat transfer coefficient show the opposite behaviour.
Surface void fraction profile can be obtained from the heat transfer coefficient measures by applying the Di Natale et al. (2008). The model results consistent with former investigations and the void fraction profile and provide for a good description of mass transfer coefficient profile near the surfac
A single particle model for surface-to-bed heat transfer in fluidized beds
No full textThis paper presents a semi empirical single particle model for the description of heat transfer coefficient between a submerged surface and a fluidized bed. The model is applied to several experimental data and gives a satisfactory description of the effect of pressure, temperature and bed material properties on the heat transfer coefficient either in bubbling or slugging fluidized beds. The model considers the averaged surface void fraction as the only regression parameter for the description of experimental data. Surface void fraction results to be a function of Archimedes number and minimum fluidization bed voidage and its value is consistent with the numerical and experimental data reported in literature
RIMOZIONE DI PCB DA OLI MINERALI
No full textScopo di questa nota è fornire una sintetica descrizione delle tecnologie dedicate alla bonifica degli oli minerali contaminati da PCB. Le soluzioni tecniche sono confrontate in base alle indicazioni della letteratura tecnico-scientifica
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