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.