Experimental study on solids circulation patterns and bubble behavior using particle imagevelocimetry combined with digital image analysis

The hydrodynamics, viz. the solids circulation patterns and\ud bubble behavior, of a freely bubbling gas-solid fluidized bed\ud has been investigated experimentally using Particle Image\ud Velocimetry (PIV) combined with Digital Image Analysis\ud (DIA). Coupling of these non-invasive measuring techniques\ud allows us to obtain information on both the bubble behavior\ud and emulsion phase circulation patterns simultaneously, in\ud order to study in detail their intricate interaction. In\ud particular, the combination of DIA with PIV allows correcting\ud for the influence of particle raining through the roof of the\ud bubbles on the time-averaged emulsion phase velocity\ud profiles. Because of the required visual access, this technique\ud can only be applied for pseudo-2D fluidized beds.\ud The bubble rise velocity as a function of the equivalent\ud bubble diameter and the average bubble diameter as a\ud function of the position above the distributor were\ud determined with DIA and compared with literature\ud correlations. Subsequently, the importance was demonstrated\ud of filtering the instantaneous emulsion phase velocity profiles\ud obtained with PIV for particle raining, using DIA, to obtain\ud the time-averaged emulsion phase velocity profiles. The timeaveraged\ud solids circulation patterns have been studied as a\ud function of the superficial gas velocity and bed aspect rati

Experimental study on solids mixing and bubble behavior in a pseudo-2D, freely bubbling, gas-solid fluidized bed using PIV and DIA

The hydrodynamics of a freely bubbling, gas-solid fluidized bed has been investigated experimentally with non-invasive measuring techniques in a pseudo-2D column filled with glass beads of 400-600 μm fluidized with air. Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA) has been used to determine simultaneously the emulsion phase circulation patterns, bubble hold-up and bubble size and velocity distributions. The combination of DIA with PIV allows correcting for the influence of particle raining through the roof of the bubbles on the time-averaged emulsion phase velocity profiles. The time-averaged emulsion phase circulation patterns have been measured as a function of fluidization velocity. Moreover, with DIA the average bubble diameter and bubble velocity as a function of height and fluidization velocity have been determined and found to correspond reasonably well with literature correlations. The experimental data provides a basis for development and validation of CFD models to describe the solids-mixing in gas-solid fluidized beds

Investigating the performance of different fluidized bed membrane reactor geometries

The hydrodynamics and mass transfer phenomena occurring in different fluidized bed membrane reactors have been studied with a Two Fluid Model (TFM). The present work focuses on the in-situ selective extraction of hydrogen from a fluidized bed membrane reactor, aiming to study and quantify the membrane performance, including possible concentration polarization. Using a hydrogen-nitrogen gas mixture as fluidizing gas, various fluidized bed geometries containing vertically or horizontally immersed membranes were simulated. The hydrodynamics and mass transfer phenomena of a fluidized bed can be strongly affected by the membrane configuration. Previous work by the group of Van Sint Annaland (1) showed the appearance of densified particle zones near the membranes, which could affect their performance. Furthermore, so called gas pockets (solids free non-rising bubbles, attached to the membrane) are formed underneath horizontal membrane tubes, see Medrano et al. (2). These phenomena are identified, their adverse effect on the membrane flux is quantified and possible remedies are discussed. Hydrogen fluxes of a membrane placed vertically in a fluidized bed were obtained from the TFM, experiments and a 1D model that does not take concentration polarization into account. These fluxes are compared in Figure 1. Concentration polarization is clearly very important in fluidized bed reactors with state-of-the-art high-flux membranes. The TFM predicts the fluxes quite accurately, whereas the model that does not account for concentration polarization severely overpredicts them. Please click Additional Files below to see the full abstract

The clustering morphology of freely rising deformable bubbles

We investigate the clustering morphology of a swarm of freely rising deformable bubbles. A three-dimensional Vorono\"i analysis enables us to quantitatively distinguish between two typical clustering configurations: preferential clustering and a grid-like structure. The bubble data is obtained from direct numerical simulations (DNS) using the front-tracking method. It is found that the bubble deformation, represented by the aspect ratio \chi, plays a significant role in determining which type of clustering is realized: Nearly spherical bubbles with \chi <~ 1.015 form a grid-like structure, while more deformed bubbles show preferential clustering. Remarkably, this criteria for the clustering morphology holds for different diameters of the bubbles, surface tension, and viscosity of the liquid in the studied parameter regime. The mechanism of this clustering behavior is connected to the amount of vorticity generated at the bubble surfaces.Comment: 10 pages, 5 figure