Influence of Bubble-Bubble Interactions on the Macroscale Circulation Patterns in a Bubbling Gas-Solid Fluidized Bed

The macro-scale circulation patterns in the emulsion phase of a gas-solid fluidized bed in the bubbling regime have been studied with a 3D Discrete Bubble Model. It has been shown that bubble-bubble interactions strongly influence the extent of the solids circulation and the bubble size distribution

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

Modelling of large-scale dense gas–solid bubbling fluidised beds using a novel discrete bubble model

In order to model the complex hydrodynamic phenomena prevailing in industrial scale gas–solid bubbling fluidised bed reactors and especially the macro-scale emulsion phase circulation patterns induced by bubble–bubble interactions and bubble coalescence, a discrete bubble model (DBM) has been developed. In the DBM, the (larger) bubbles are modelled as discrete elements and are tracked individually during their rise through the emulsion phase, which is considered as a continuum. The DBM, originally developed for the description of gas–liquid flows, has been adapted to cope with bubbles with a diameter larger than the size of an Eulerian cell, which is required in view of the large bubble size distribution at higher gas flow rates. Moreover, a new drag model for a single bubble rising in a fluidised bed derived from empirical correlations has been implemented, as well as a simple model to account for bubble coalescence and break-up. The strong advantage of the DBM compared to other models previously reported in the literature for the description of large-scale fluidised beds is that it fully accounts for the two-way coupling between the bubbles and the emulsion phase, which enables direct computation of the emulsion phase velocity profiles. Comparison of the results of simulations ignoring bubble coalescence and simulations taking bubble coalescence properly into account demonstrated the significant effect of bubble coalescence on the large-scale circulation patterns prevailing in bubbling fluidised beds. The simulation results for the lateral profiles of the visible bubble flow rate have been compared qualitatively with experimental results reported by Werther [1974. Influence of the bed diameter on the hydrodynamics of gas fluidized beds. A.I.Ch.E. Symposium Series 70(141), 53–62]. The effect of the superficial gas velocity on the velocity and porosity profiles has been studied. In general, it can be concluded that the DBM is able to capture the salient features of the hydrodynamics of bubbling fluidised beds. However, further research is required to improve the closure equations for the bubble behaviour, bubble–bubble interactions and bubble coalescence and break-up to enable a complete quantitative description

On the hydrodynamics in gas phase polymerization reactors

Polyolefins are polymers produced from olefins such as ethylene and/or propylene. Although polyolefins can be produced via different production methods, the gas-phase polymerization process based on fluidized bed reactor technology is the most important method for the production of polyethylene since the 1980’s and also polypropylene is increasingly produced via the gas-phase polymerization process. Although fluidized bed reactors have been employed for several decades in the chemical industry, quantitative information on solids motion and macroscopic circulation patterns is still incomplete. To investigate the macroscopic circulation patterns in a freely bubbling, gas-solid fluidized bed, first the hydrodynamics in two pseudo-2D columns of different width filled with glass beads and Linear Low Density Polyethylene (LLDPE) particles have been investigated (both exhibiting Geldart B type behavior) experimentally with two optical non-invasive measuring techniques. Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA) has been developed to determine simultaneously the emulsion phase circulation patterns, bubble hold-up, bubble size and velocity distributions and visual bubble flow rate profiles. 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 number-averaged emulsion phase circulation patterns have been measured as a function of fluidization velocity, bed aspect ratio, bed width and bed material. Moreover, with DIA the average bubble diameter and averaged bubble velocity as a function of height and fluidization velocity have been determined and found to correspond reasonably well with literature correlations. However, the difference in averaged bubble diameter as a function of the height in the fluidized bed for the two different particle types could not be explained by the currently available correlations for the bubble diameter. The difference in observed bubble properties is attributed to differences in the particle collisional properties (coefficients of restitution and the particle friction coefficient). To verify this hypothesis, the influence of microscopic particle properties on the hydrodynamics in a bubbling fluidized bed have be investigated in detail using the Discrete Particle Model (DPM) and the Two-Fluid Model (TFM). It was concluded that, for the conditions investigated, indeed bubbles are formed due to collisional dissipation. Furthermore, the nature (i.e. due to restitution or friction) of the energy dissipation is important for the shape of the bubbles. In addition it is shown that in a bubbling fluidized bed, the energy is mainly dissipated by friction between particles and particles and the wall. The influence of the normal restitution coefficient on the macroscopic circulation pattern was investigated with the Two-Fluid Model. The observed influence of the coefficient of restitution in the normal direction agreed with the influence of the coefficient of restitution in the normal direction in the DPM. Also the experimental results obtained with the PIV combined with DIA measurements for the solids phase and DIA measurements for the bubble behavior were compared with simulations performed with the DPM and the TFM. It was shown that the trends for the emulsion phase and the bubble phase can be predicted with the DPM. The solids and bubble behavior in a freely bubbling, three dimensional, gas-solid fluidized bed has been experimentally investigated using different bed materials, different bed aspect ratios at different superficial gas velocities by performing Positron Emission Particle Tracking (PEPT) experiments. The fluidized bed was filled with either glass beads or with linear low density polyethylene (LLDPE). At lower superficial gas velocities two distinct vortices appear above each other for both types of bed material; when the superficial gas velocity is increased, the lower vortex disappears and the top vortex spans the entire length of the bed. Although qualitatively the same phenomena were observed, the timeaveraged solids phase circulation rate in the fluidized bed filled with LLDPE particles was higher than the time-averaged solids phase velocity in the fluidized bed filled with glass beads. When the bed aspect ratio is increased from 1 to 1.5, the vortices become elongated without altering the solids circulation rate. Differences in the particle-particle collisional properties (coefficients of restitution and friction particle coefficients) are believed to be the cause of the observed quantitative differences in the bed hydrodynamics via their influence on the bubble properties. Finally, the hydrodynamic behavior of industrial scale bubbling fluidized bed reactors, a 3D Discrete Bubble Model (DBM) has been used. In the DBM, an Euler-Lagrange model, the bubbles are treated as discrete elements and the bubble trajectories are tracked individually, while the emulsion phase is considered as a continuum and is described with the continuity and Navier-Stokes equations. The main advantage of the DBM is that it fully accounts for the two-way coupling, allowing computation of the prevailing macroscopic circulation patterns in large scale gas-fluidized beds. We have examined the effects of bubble-bubble interactions on the macro-scale velocity profiles using the DBM. It has been found that the extent of the macroscopic circulation is significantly increased by the bubble-bubble interaction forces

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 and computational investigation on the macroscopic circulation patterns in a bubbling gas-solid fluidized bed

The hydrodynamics of a freely bubbling, pseudo 2-D fluidized bed has been investigated experimentally for different bed aspect ratios at different superficial gas velocities by using Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA). Coupling of both non-invasive measuring techniques allows us to obtain information on both the bubble behavior and emulsion phase circulation patterns simultaneously. In particular, the combination of DIA with PIV allows to correct for the influence of particle raining through the roof of the bubbles on the time-averaged emulsion phase velocity profiles

Improvement of Sodium Status to Optimize the Efficacy of Renin-Angiotensin System Blockade

Blockade of the renin-angiotensin-aldosterone system (RAAS) offers superior renoprotection in the treatment of patients with hypertension, but the efficacy of RAAS inhibition strongly depends on sodium status, presumably in relation to extracellular volume status. Because assessing volume status by physical examination is challenging, 24-hour urine collection and NT-proBNP levels are useful tools for guiding volume management and achieving sodium status targets