Experimental observations on the different mechanisms for solid ejection in gas-fluidized beds

Almendros-Ibáñez, J.A.; Sánchez-Delgado, S.; Sobrino, C.; Santana, D.

doi:10.1016/j.cep.2008.09.008

Cited by 20 publications

(15 citation statements)

References 32 publications

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“…3 shows that at the nose of the bubble typical instabilities appear and the particles rain down into the bubble in the form of "stalactites". These elongated group of particles can reach the bottom of the bubble and split the bubble in two [25,26]. Yet, the effect may be more common in 2D beds than in 3D beds because of wall effects.…”

Section: Resultsmentioning

confidence: 99%

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Voidage distribution around bubbles in a fluidized bed: Influence on throughflow

et al. 2010

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a b s t r a c tIn this work, a new method for measuring void fraction distribution around endogenous bubbles in a 2D fluidized bed is presented. The technique is based on illuminating a transparent wall 2 dimensional bed with diffuse light from the rear and recording the distribution of light that penetrates the bed. The recording is made with a high speed video camera, which gives frames with grey level corresponding to the light penetration and from which the voidage distribution around the bubbles can be determined. In this way, voidage distribution in the region very close to the bubble contour (r/R b ≲ 1.2) is obtained, which was not possible in previous studies due to limitations in spatial resolution. A correlation is proposed for the voidage at the contour of the bubble, with the voidage depending on the radial position and the polar angle ε(r, θ). In addition, the effect of the voidage distribution on the throughflow crossing the bubbles was studied and an increase of 20% was determined for the average bubble geometry of the more than 100 bubbles analysed.

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Section: Resultsmentioning

confidence: 99%

“…(1) and (2) respectively. This fact is the result of the instabilities appearing at the nose of the bubbles, which gives the rain of particles in the form of the typical "stalactites" [26]. The value of k 1 (θ) in Eq.…”

Section: Discussionmentioning

confidence: 99%

Voidage distribution around bubbles in a fluidized bed: Influence on throughflow

et al. 2010

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“…When a pair of bubbles is rising vertically, the trailing bubble is accelerated under the influence of the leading bubble; eventually, the dome of the trailing bubble enters into the wake of the leading bubble and they become a single bubble [7,20]. During coalescence, it is difficult to establish the influence region of each bubble.…”

Section: Resultsmentioning

confidence: 99%

“…Interrogation windows of 16 × 16 pixels with a 50% overlap were typically used in the PIV analyses performed to compute the velocity fields of the dense phase. Recently, several authors have used this application on 2 D fluidized beds to calculate particle velocities [7,17]. For these experiments, the fluidized bed column was filled with a total weight of 1600 g, 1568 g of white particles and 32 g of black ones to improve the measuring technique [17].…”

Section: Methodsmentioning

confidence: 99%

“…Recently, Sanchez Delgado et al [6] studied the time averaged bubble concentration and time averaged particle velocity in a 2 D fluidized bed to characterize the behavior of fluidized beds with different bed aspect ratios and superficial gas velocities. These techniques were applied by Almendros Ibáñez et al [7] to study the different mechanisms for solid ejection in gas fluidized beds. Busciglio et al [8] presented a digital image analysis technique developed to study the fluidization dynamics of a lab scale bubbling bed.…”

Section: Introductionmentioning

confidence: 99%

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Dense-phase velocity fluctuation in a 2-D fluidized bed

Sánchez-Delgado¹,

Marugán-Cruz²,

Acosta-Iborra³

et al. 2010

Powder Technology

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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.

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Mixing dynamics in bubbling fluidized beds

2017

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Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017

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Experimental observations on the different mechanisms for solid ejection in gas-fluidized beds

Cited by 20 publications

References 32 publications

Voidage distribution around bubbles in a fluidized bed: Influence on throughflow

Voidage distribution around bubbles in a fluidized bed: Influence on throughflow

Dense-phase velocity fluctuation in a 2-D fluidized bed

Mixing dynamics in bubbling fluidized beds

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