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

Non-invasive and non-intrusive diagnostic techniques for gas-solid fluidized beds – A review

Gas-solid fluidized-bed systems offer great advantages in terms of chemical reaction efficiency and temperature control where other chemical reactor designs fall short. For this reason, they have been widely employed in a range of industrial application where these properties are essential. Nonetheless, the knowledge of such systems and the corresponding design choices, in most cases, rely on a heuristic expertise gained over the years rather than on a deep physical understanding of the phenomena taking place in fluidized beds. This is a huge limiting factor when it comes to the design, the scale-up and the optimization of such complex units. Fortunately, a wide array of diagnostic techniques has enabled researchers to strive in this direction, and, among these, non-invasive and non-intrusive diagnostic techniques stand out thanks to their innate feature of not affecting the flow field, while also avoiding direct contact with the medium under study. This work offers an overview of the non-invasive and non-intrusive diagnostic techniques most commonly applied to fluidized-bed systems, highlighting their capabilities in terms of the quantities they can measure, as well as advantages and limitations of each of them. The latest developments and the likely future trends are also presented. Neither of these methodologies represents a best option on all fronts. The goal of this work is rather to highlight what each technique has to offer and what application are they better suited for

Multiphase flow in spout fluidized bed granulators

Spout fluidized beds are frequently used for the production of granules or particles through granulation, which are widely applied, for example, in the production of detergents, pharmaceuticals, food and fertilizers (M¨orl et al. 2007). Spout fluidized beds have a number of advantageous properties, such as high mobility of the particles preventing undesired agglomeration and enabling excellent heat transfer control. Additionally, liquid can easily be sprayed into the bed through the spout, making spout fluidized beds very suitable for coating and layer wise growth of particles. During the granulation process, particles contain different loadings of melt which results in altered collision properties in time and space across the bed. This change in collision properties influences the bed dynamics, and consequently the granule quality. To improve the performance of the spout fluidized bed granulator, it is very important to understand the interplay of collision properties and bed dynamics, and is therefore studied in this work. The particle-particle interactions were first studied in a 3D system, using the Discrete Element Model (DEM). Several test cases were defined, where the particles possessed a different restitution coefficient for each case, and the examined flow regimes comprised the intermediate / spout-fluidization regime (B1), spouting-with-aeration regime (B2) and the jet-in-fluidized-bed regime (B3). The pressure drop and the vertical particle velocity were compared to experimental data obtained by Link et al. (2007). The computed results with en = 0.97 resembled the experimental results very well. It was shown that a decreasing restitution coefficient produces more vigorous bubbling and more pronounced heterogeneity (instability). The particle velocity and RMS (root mean square) profiles confirm the effect on the stability of the bed and reveal that the spout channel for cases B1 and B3 becomes unstable when the restitution coefficient decreases. For case B2, a transition occurred from the spouting-with-aeration to the intermediate/spout-fluidization regime at low restitution coefficient. These findings demonstrate the profound influence of the restitution coefficient on the dynamics of the bed. During the granulation process, when the particles contain different moisture contents, regions in the bed exist that contain particles with different restitution coefficients. These regions thus experience different dynamics, resulting in complex overall dynamic behaviour of the spout fluidized bed granulator. To verify if the same features are observed in experiments, different particle systems with a.o. different restitution coefficients were investigated in a pseudo-2D spout fluidized bed. This was done for different flow regimes: the spoutfluidization regime (case B1), the spouting-with-aeration regime (case B2) and the jet-in-fluidized-bed regime (case B3). The considered particle systems comprise glass beads, -alumina oxide and zeolite 4A particles, which are all classified as Geldart D particles. A non-intrusive measurement technique was used, viz. particle image velocimetry (PIV) to obtain the particle flow field in a pseudo twodimensional (2D) spout fluidized bed. Additionally, digital images were analyzed using a newly developed digital image analysis (DIA) algorithm to evaluate the particle volume fraction. It is demonstrated that the new proposed DIA algorithm provides reliable information on the particle volume fraction distribution, showing that it is a powerful tool when combined with PIV. The added value of DIA is confirmed by comparing the particle velocity fields and volumetric particle fluxes. The particle flux obtained with the combined PIV/DIA technique was used to validate DEM simulation results of the jet-in-fluidized-bed regime (case B3) for all three particle systems. It was found that the vertical particle fluxes obtained from the simulations were slightly overpredicted higher up in the bed and in the annulus region, which most likely is due to the more pronounced wall effect in pseudo-2D beds. Simulations with a larger friction coefficient for particle-wall interactions with glass beads showed (for this examined system) a better resemblance to the computed downward flux in the annulus compared to the experimental results. The effect of the collision properties for glass beads, -alumina oxide and zeolite 4A particles has been studied in the three flow regimes and for each flow regime, the particle volume fraction profiles show small differences among the different particle systems. For the -alumina oxide and zeolite 4A particles, the spout channel is less stable for the cases B1 and B2. The particle fluxes also display small differences between the particle systems for each flow regime. The simulated cases mimicked different stages of wetting during granulation processes, and they revealed that the bed dynamics is highly affected by differences in the restitution coefficient. During granulation processes, however, regions of wet particles and dry particles prevail at different locations inside the bed and at different time-scales. Therefore, a variable restitution coefficient was considered, to study the effect of the inter-particle interaction on the bed dynamics. The restitution coefficient is varied in time and space depending on the moisture content due to the particle-droplet interaction and evaporation. For this study, the DEM was extended by incorporating the moisture content into the (effective) restitution coefficient where both droplets and particles were considered as discrete elements. The same flow regimes were examined and for all flow regimes, the averaged bed height increased with decreasing restitution coefficient. Moreover, the averaged bed height for a variable restitution coefficient was larger for all flow regimes compared to a case with a constant restitution coefficient, indicating that the spatial distribution of the restitution coefficient influences the bed dynamics. The effect of evaporation on the distribution of the restitution coefficient was only observed for the jet-in-fluidized-bed regime (B3), where the background velocity is relatively high leading to enhanced evaporation from the particles in the annulus region. This is reflected in the averaged bed height for the evaporation test case, which is larger compared to a test case without evaporation. A larger bed height for cases with variable restitution coefficient is due to the pressure build up in the spout region caused by the longer closing period of the spout channel. This is confirmed by the recorded pressure fluctuation signal and its root mean square which are larger for the cases with the variable restitution coefficient. To the author’s knowledge, most of the research on spout fluidized beds done so far had been focussed on single-spout fluidization. However, multiple spouts are present in industrial granulators, and little was known about the effect of multiple spouts on the bed dynamics. Therefore, the objective of this work was to study the effect of two and three spouts on the bed dynamics of a pseudo-2D spout fluidized bed, by employing the DEM and applying Particle Image Velocimetry (PIV) and Positron Emission Particle Tracking (PEPT) techniques on a pseudo-2D spout fluidized bed. A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agreed very well for all the three cases, showing the good performance of these techniques. The DEM simulation results slightly deviated from the experiments which was attributed to particle-wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds. A second geometric feature in industrial spout fluidized bed granulators is that the spouts are slightly elevated from the bottom plate to facilitate efficient the injection of the liquid. The influence on the bed dynamics was investigated as well. The experiments were conducted in a pseudo-2D and a cylindrical 3D spout fluidized bed, where Positron Emission Particle Tracking (PEPT) and Particle Image Velocimetry (PIV) were applied to the pseudo-2D bed, and PEPT and Electrical Capacitance Tomography (ECT) to the cylindrical 3D bed. A discrete element model (DEM) was used to perform full 3D simulations of the bed dynamics. Several cases were studied, i.e. beds with spout heights of 0, 2 and 4 cm. In the pseudo-2D bed the spout-fluidization and jet-in-fluidized-bed regime were considered first, and it was shown that in the spout-fluidization regime the expected dead zones appeared in the annulus near the bottom of the bed in case the spout is elevated. However, in the jet-in-fluidized-bed regime the circulation pattern of the particles is affected, without the development of stagnant zones. The jet-in-fluidized-bed regime was further investigated, and additionally the experimental results obtained with PIV and PEPT were compared with the DEM simulation results. The experimental results obtained with PIV and PEPT agreed mutually very well, and in addition agreed well wtih the DEM results, although the velocities in the annulus region were slightly overpredicted. The latter is probably due to the particle-wall effects that are more dominant in pseudo-2D systems compared to 3D systems. In the jet-in-fluidized-bed regime the background gas velocity is relatively high, producing bubbles in the annulus that interact with the spout channel. In case of a non-elevated spout, this interaction occurs near the bottom of the bed. As the spout is elevated, this interaction is shifted upwards in the bed, which allows the bubbles to remain undisturbed providing the motion of the particles in the annulus near the bottom of the bed. As a result, no dead zones are created and additionally, circulation patterns are vertically stretched. These findings were also obtained for the cylindrical 3D bed, though, the effects were less pronounced. In the cylindrical 3D bed the PEPT results show that the effect on the bed dynamics starts at hspout = 4 cm, which is confirmed by the ECT results. Additionally, ECT measurements were conducted for hspout = 6 cm to verify if indeed the effect prevails at larger spout heights. The root mean square of the particle volume fraction slightly increased at hspout = 2 cm, while a larger increase is found at hspout = 4 and 6 cm, showing that indeed more bubbles are formed. The presented results have not been reported so far and form valuable input information for improving industrial granulators

A Review of X-Ray Flow Visualization With Applications to Multiphase Flows

Flow visualization and characterization of multiphase flows have been the quest of many fluid mechanicians. The process is fairly straight forward only when there is good optical access (i.e., the vessel is not opaque or there are appropriate viewing ports) and the flow is transparent, implying a very low volume fraction of the dispersed phase; however, when optical access is not good or the fluid is opaque, alternative methods must be developed. Several different noninvasive visualization tools have been developed to provide high-quality qualitative and quantitative data of various multiphase flow characteristics, and overviews of these methods have appeared in the literature. X-ray imaging is one family of noninvasive measurement techniques used extensively for product testing and evaluation of static objects with complex structures. X-rays can also be used to visualize and characterize multiphase flows. This paper provides a review of the current status of X-ray flow visualization and details various X-ray flow visualization methods that can provide qualitative and quantitative information about the characteristics of complex multiphase flows

Estimation and experimental validation of the circulation time in a 2D gas-solid fluidized beds

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 tc,est =2Ah′/Qb 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 Qb 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.Publicad

Use of a-shapes for the measurement of 3D bubbles in fluidized beds from two-fluid model simulations

A geometrical technique based on shape construction was employed to reconstruct the simulated domain of 3D bubbles in a gas-solid fluidized bed, from two-fluid model (TFM) simulations. The Delaunay triangulation of the cloud of points that represent volume fraction iso-surfaces in transient TFM simulations was filtered by means of the so-called a-shapes, allowing a topologically accurate description of 3D bubbles within a fluidized bed. Consequently, individual 3D bubble properties such as size and velocity were measured. Simulated bubble characteristics were further compared to those measured on pseudo-2D bed facilities by image techniques in order to illustrate the effect of the bed geometry on the bubbling behavior under mimicked operational conditions

Hydrodynamic Characterization of 3D Fluidized Beds Using Noninvasive Techniques

Fluidized beds are useful processing systems that are employed by many industries for their relatively unique operating properties. Low pressure drops, uniform temperature distributions, and high heat/mass transfer rates occur through the action of vertical gas injection into a column of solid particles. Although these properties give fluidized beds great advantages over other processing systems, the hydrodynamic characterization of fluidized beds is important for the efficient processing of many consumer products. However, fluidized bed hydrodynamics are difficult to visualize and quantify because most fluidized beds are opaque. Traditionally, the monitoring of local fluidized bed hydrodynamics has been done with intrusive probes that disturb local structure and the collection of data over large areas is time consuming. X-ray computed tomography (CT), as a noninvasive technique, can quantify local time-average phase fractions in highly dynamic multiphase systems without disturbing local structure. Using X-ray visualization techniques, methods have been developed in this study to: 1) test the repeatability of calculating local time-average gas holdup values using X-ray CTs; 2) find the fluidization uniformity of a non-reactive cold-flow fluidized bed; 3) compare local time-average gas holdup values in various bed materials, diameters, and operating conditions; and 4) compare annular hydrodynamic structures within the beds. Tests for the first two objectives were completed using a 15.2 cm ID reactor, while varying between two bed materials (crushed walnut shell and glass beads) of the same size and two gas flow rates. The third objective used a 10.2 cm and 15.2 cm ID reactor, varied between three bed materials (ground corncob, crushed walnut shell, and glass beads) of the same size, and over four and five relative superficial gas velocities and side-air injection gas flow rates respectively. The fourth objectives mirrored the third, however, did not use side-air injection. Observations show that local time-average gas holdup values can be calculated through the use of multiple X-ray CTs. The method of calculation is shown to be highly repeatable over the various flow rates, bed materials used, and ambient environmental conditions. Axisymmetric fluidization uniformity of the bed is also confirmed using the same method, while some differences are observed with varying materials and flow rates. Uniformity is observed to increase with bed height and increased gas flow rates, due to increased dispersion of gas into the bed and mixing rates respectively. Local time-average gas holdup is observed to differ somewhat between reactors. However, the overall results show that the hydrodynamic structures, i.e. aeration jets, bubble coalescence zones, bubble rise zones, particle shearing zones, and the side-air injection plume, within the fluidized beds for each reactor are very similar. These structures coupled with axisymmetric fluidization uniformity indicate that gas flow and material circulation tend to be annular in shape. Moreover, changes in the shape, size, number, and location occur with changes in superficial gas velocity, bed diameter, and bed material density. It is also suspected that the aeration scheme of the bed and the bed material properties i.e. shape factor, coefficients of restitution, and porosity play a role in the development of these structures. The aeration jets are similar in length in all beds regardless of material density or bed diameter. They also tend to decrease in height and become increasingly wall leaning as superficial gas velocity increases. The coalescence of bubbles tends to occur in regular locations near the reactor wall just above the aeration jets within all beds regardless of material density, bed diameter, and gas flow rates. The rise paths of bubbles through all beds emanate from the coalescence zones with relatively small widths and increasing in width as bed height increases. Particle shear zones occur in differing size, shape, number outside of all other hydrodynamic structures while migrating around the bed with changing material density, bed diameter, and superficial gas velocity. The diffusion of gas into the fluidized bed from the side-air injection plume in each bed is similar, due to advection dominance within the plume. Gas dispersion does not seem to occur by similar means between materials though, because crushed corncob and ground walnut shell are natural systems and have a higher porosity and lower density than glass beads. The natural materials also have non-uniform shape factors causing behavior differences with the fluidization gas. The time-average bed height between bed diameters is different for each material density and gas flow rate, where the height in the 10.2 cm diameter reactor is observed to be greater on average in all tests than in the 15.2 cm reactor, due to wall effects. Lastly, the techniques used for analysis in this study are valuable to computational fluid dynamicists for direct comparison to simulation and models of fluidized beds

Dynamic viscoplastic granular flows: A persistent challenge in gas-solid fluidization

Fluidization is a prime example of complex granular flows driven by fluid-solid interactions. The interplay of gravity, particle-particle and fluid-particle forces leads to a rich spectrum of hydrodynamic behavior. A number of complex mathematical formulations exist to describe granular flows. At a macroscopic scale, Eulerian models based on the Kinetic Theory of Granular Flow (KTGF) have been successfully employed to simulate dilute and moderately dense systems, such as circulating fluidized bed reactors. However, their applications to dense flows are challenging, because sustained particle contacts are important. As solid fraction rises, the behavior of granular media responds dramatically to particle properties and changes in concentration. Lacking a coherent transition between formulations of dilute, dense and quasi-static flow behavior, kinetic models are incapable of describing how microstructure emerges and affects the rheology. The behavior of transitional granular flows, such as pulsed fluidized beds, for which the particulate phase transitions between the viscous and plastic regimes, are good reminders of this limitation. In recent years, tremendous effort has been devoted to finding new ways to describe the effects of sustained solids friction and dense flow rheology. This article provides a perspective on this matter from the viewpoint of gas-solid fluidization and discusses advances in describing the dilute-to-dense transition in a continuum framework. Four innovative approaches prevail to extend or supersede the existing kinetic theory: (i) including effective restitution coefficients, (ii) coupling local granular rheological correlations, (iii) introducing rotational granular energy, and (iv) combining non-local laws. While their reliability is still far from that of a Eulerian-Lagrangian approach, they lay a promising foundation for developing a rigorous description of granular media that merges the classical frameworks of continuous fluid and soil mechanics. The progress of continuum formulations does not compete with multi-scale modeling platforms with an applied focus. Ultimately, combining both is a prerequisite to developing new solid stress models that will improve not only the performance of macroscopic models, but also our understanding of granular physics

Investigation of Particle Motion in a Swirling Fluidized Bed using Particle Imaging Velocimetry

Fluidized bed is an advanced technology which possesses a number of characteristics ideal for a wide variety of industrial applications due to its advantages over many existing technologies in industry. However, conventional fluidized bed used in most of the industry today has certain drawbacks which affect the efficiency of the bed. Swirling Fluidized Bed (SFB) is one of the evolutions of fluidized bed, which has the potential for solving many drawbacks of conventional fluidized bed. Nonetheless, limited research has been done on this bed as compared to other versions of fluidized bed, thus a lot of problems occurred when come to scaling up to commercial size. This was mainly due to the lack of understanding of the particle dynamic characteristics of the bed. Most of the research studied the overall bed characteristics especially the pressure drop. There is limited study on the velocity and particle motion. Furthermore, available literature concentrates only analytical model and simulation results. No published experimental information is available for the analysis of the particle velocity. Thus, the objectives of the present study are to investigate particle motion in a swirling fluidized bed and to study the effect of air flow rate, bed weight, blade angle, particle size and particle shape on the fluidized particle velocity. The particle motion of the SFB is studied by using Particle Imaging Velocimetry (PIV) in an experimental model of SFB. The experiments were carried out with bed weight varied from 500 g to 1500 g with only stable swirling regime was studied and the velocity of the top layer particles was evaluated. From the study, it is found that the particle velocity increases with air flow rate at shallow bed and as bed weight is increased, particle velocity decreases by higher occurrence of vigorous bubbles. It is also observed that particle velocity decreases less than 18% with 3o increment in the blade angle. Small particle yields lower minimum swirling air superficial velocity which means preferable for saving energy, but with constraint of shallow bed. Particle with elongated shape possesses short range of stable swirling due to easier occurrence of bubbling. The results of this project give a better understanding of the particle velocity and motion which can provide a great contribution towards designing the fluidized bed especially for catalyst activity, coating and drying