a b s t r a c tThe motion of a large object in a bubbling fluidized bed was experimentally studied using digital image analysis (DIA). The experiments were performed in a 2 D bubbling fluidized bed with glass spheres as bed material. The object motion was measured using non intrusive tracking techniques, while independent measurements of the dense phase velocity (using Particle Image Velocimetry (PIV)) and bubble velocity (using DIA) were carried out. The effect of the dimensionless gas velocity on the object motion was also analyzed.This work characterizes the circulation patterns of an object with a density similar to that of the bed, but much larger in size. Object size and density remained constant throughout the experiments. A comparison between the motion of sinking objects and the motion of the dense phase provided evidence of the feeble effect of buoyant forces on the motion of sinking objects. In contrast, the motion of rising objects is linked to the motion of bubbles. It was found that objects may be raised to the surface of the bed either by the action of a single bubble (one jump) or by several passing bubbles (multiple jumps). Based on these results, the circulation time of objects throughout the bed is a function of two parameters: the maximum depth attained by an object and the number of jumps during its rising path. This relationship is presented along and the multiple jumps phenomenon is studied in detail. Finally, an estimate of the circulation time of an object based on semi empirical expressions is presented for different dimensionless gas velocities. The probability density function of the circulation time shows two different modes as the object was less prone to be raised at moderate depths. The estimate of the circulation time was found to be in good agreement with our experimental data.

a b s t r a c tThis work compares simulation and experimental results of the hydrodynamics of a two dimensional, bubbling air fluidized bed. The simulation in this study has been conducted using an Eulerian Eulerian two fluid approach based on two different and well known closure models for the gas particle interaction: the drag models due to Gidaspow and Syamlal & O'Brien. The experimental results have been obtained by means of Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV) techniques applied on a real bubbling fluidized bed of 0.005 m thickness to ensure its two dimensional behaviour. Several results have been obtained in this work from both simulation and experiments and mutually compared. Previous studies in literature devoted to the comparison between two fluid models and experiments are usually focused on bubble behaviour (i.e. bubble velocity and diameter) and dense phase distribution. However, the present work examines and compares not only the bubble hydrodynamics and dense phase probability within the bed, but also the time averaged vertical and horizontal component of the dense phase velocity, the air throughflow and the instantaneous interaction between bubbles and dense phase. Besides, quantitative comparison of the time averaged dense phase probability as well as the velocity profiles at various distances from the distributor has been undertaken in this study by means of the definition of a discrepancy factor, which accounts for the quadratic difference between simulation and experiments The resulting comparison shows and acceptable resemblance between simulation and experiments for dense phase probability, and good agreement for bubble diameter and velocity in two dimensional beds, which is in harmony with other previous studies. However, regarding the time averaged velocity of the dense phase, the present study clearly reveals that simulation and experiments only agree qualitatively in the two dimensional bed tested, the vertical component of the simulated dense phase velocity being nearly an order of magnitude larger than the one obtained from the PIV experiments. This discrepancy increases with the height above the distributor of the two dimensional bed, and it is even larger for the horizontal component of the time averaged dense phase velocity. In other words, the results presented in this work indicate that the fine agreement commonly encountered between simulated and real beds on bubble hydrodynamics is not a sufficient condition to ensure that the dense phase velocity obtained with two fluid models is similar to that from experimental measurements on two dimensional beds.

Abstract:The circulation time is defined as the time required for a group of particles to reach the freeboard from the bottom of a fluidized bed and return to their original height. This work presents an estimation and validation of the circulation time in a 2D gas solid bubbling fluidized bed under different operating conditions. The circulation time is based on the concept of the turnover time, which was previously defined by Geldart [1] as the time required to turn the bed over once. The equation t c,est = 2 Ah′/Q b is used to calculate the circulation time, where A is the cross section of the fluidized bed, h′ is the effective fluidized bed height and Q b is the visible bubble flow. The estimation of the circulation time is based on the operating parameters and the bub ble phase properties, including the bubble diameter, bubble velocity and bed expansion. The experiments for the validation were carried out in a 2D bubbling fluidized bed. The dense phase velocity was measured with a high speed camera and non intrusive techniques such as particle image velocimetry (PIV) and digital image analysis (DIA), and the experimental circulation time was calculated for all cases. The agreement between the theoretical and experimental circulation times was satisfactory, and hence, the proposed estimation can be used to reliably predict the circulation time.

a b s t r a c tThis work presents an investigation of the perturbations induced by the bubbles in a 2 D fluidized bed. A combination of Digital Image Analysis (DIA) and Particle Image Velocimetry (PIV) techniques was developed to characterize the dense and bubble phases. The analysis of the mean and the instantaneous fluctuations of the velocity of the dense phase, together with the solid movement around bubbles, allowed for the measurement of the influence region, distinguishing an upward moving dense phase in the nose and the wake of the bubble (drift) and a downward moving dense phase in the sides of the bubble. For an isolated bubble, we measured the drift area within the total influence region and related the size of these regions to the equivalent diameter of the bubble. This work also presents results on the volumetric dissipation of kinetic energy, where we concluded that the energy dissipation in the dense phase is proportional to the square of the bubble velocity.

a b s t r a c tIn the present study, a new correlation for the determination of the minimum fluidization velocity in 2D fluidized beds was developed. The proposed correlation was based on the experimental results obtained in 2D fluidized beds with different particle sizes, bed thicknesses and bed heights. Thus, the proposed correlation depends only on the nondimensional variable t/d p , where t is the bed thickness and d p is the particle size. The proposed correlation was compared with other experimental results that can be found in the literature, and two different trends were observed. Namely, one set of experimental results was in accordance with the proposed correlation, while the other set deviated from the theoretical results. In particular, the minimum fluidization velocities of the experimental results were greater than the predicted values of the proposed correlation. In view of the differences in the experimental conditions, the observed discrepancies may be attributed to the effects of electrostatic charge and particle shape. In addition, the experimental fluidization defluidization curves were compared to the theoretical results of Jackson's model, and the parameters were fitted to the experimental data. However, Jackson's model is based on a 1D bed; thus, general parameters could not be obtained for a bed with a fixed particle size and thickness due to the two dimensional voidage distribution in the bed and bed cohesion effects, which are a result of electrostatic forces and are not considered in Jackson's model.

This work studies the defluidization time and the agglomerates generation in a Bubbling Fluidized Bed (BFB) reactor during Cynara Cardunculus L. gasification using, separately, two different bed materials, silica sand and sepiolite (Mg 8 Si 12 O 30 (OH) 4 (OH 2 ) 4 8H 2 ). The high adsorption capacity and the elemental composition of the sepiolite make it suitable as an alternative bed material in order to reduce agglomeration. Experiments were performed on a stainless steel lab-scale BFB reactor operating with air as gasifying agent at different air excess ratios (u/u mf ). A quartz reactor was alternatively used for the visualization of bed material and biomass during gasification, allowing to observe the agglomerate formation process. Pressure signals were analyzed both in time and frequency domain to determine the defluidization time.Furthermore, the shape and size of the bed material after the experiments were evaluated.Higher defluidization times in the case of sepiolite were measured. Particle sizes were affected by the type of bed material and the air excess and agglomerates of different shapes were formed for sepiolite and silica sand.

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Gasification of Cynara cardunculus L. was performed in a bubbling fluidized bed (BFB) using air as the gasifying agent and magnesite and olivine as different bed materials. Temperature was varied during the experiments (700-800 ºC) with fixed biomass feeding and air flow rates.The effect of using the magnesite and olivine on the gas and tar composition, carbon and biomass conversion, and cold gas efficiency was investigated. The product gas showed high hydrogen content (13-16 %v/v) for both magnesite and olivine in the temperature range studied.Higher heating value and gas yield were improved with increasing the temperature from 700 to 800 ºC. Biomass and carbon conversion were greater than 75%, giving values higher than 90 % for both 700 and 800 ºC in magnesite and for 800 ºC in olivine. Indane and cresols were the main tar compounds at low temperature while naphthalene was the dominant tar species at the high temperatures. Gasification performance was better with magnesite at 700 ºC while olivine showed better properties at 800 ºC.