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Various drag models were implemented in a superquadric CFD-DEM code and validated for the simulation of spherical and ellipsoidal particles in spouted beds. The most suitable drag models were identified by comparing the predicted local particle velocity with those measured by particle tracking velocimetry (PTV). The model by Beetstra et al. is the one that best reproduces the experimental results for the spouting of spherical particles, whereas the one by Sanjeevi et al. is the most suitable for ellipsoidal irregular ones. Their capability for the prediction of key operating parameters was demonstrated in both conical and prismatic spouted beds, as they correctly predict the minimum spouting velocity (ums) and fountain height in different configurations (without draft tube and different draft tubes) for particles of different size and shape. The CFD-DEM model predicts the preferential orientation of ellipsoidal particles at each location in the bed and the influence of internal devices on this parameter, which is of the utmost importance in the design of reliable coating apparatuses in the pharmaceutical industry.

Fluid–solid systems play a major role in a wide variety of industries, from pharmaceutical and consumer goods to chemical plants and energy generation. Along with this variety of fields comes a diversity in apparatuses and applications, most prominently fluidized and spouted beds, granulators and mixers, pneumatic conveying, drying, agglomeration, coating, and combustion. The most promising approach for modeling the flow in these systems is the CFD-DEM method, coupling computational fluid dynamics (CFD) for the fluid phase and the discrete element method (DEM) for the particles. This article reviews the progress in modeling particle–fluid flows with the CFD-DEM method. A brief overview of the basic method as well as methodical extensions of it are given. Recent applications of this simulation approach to separation and classification units, fluidized beds for both particle formation and energy conversion, comminution units, filtration, and bioreactors are reviewed. Future trends are identified and discussed regarding their viability.

In this work, a method to predict the surface structures of particles produced by fluidized bed spray layering granulation using the CFD-DEM method was developed. A simple state-variable/event tracking approach was implemented to capture indirect quantifiers of the progression of structure-forming microprocesses. The state of the droplet at the time of impact on the particle surface, as well as the time required for drying, is correlated to product properties that quantify surface structure morphology such as roughness. A workflow for scale-up of fluidized bed granulation guided by product-property predictors is presented. The approach was tested on a demonstration case from the literature, where a particle core is coated with sodium benzoate solution. The experiment was scaled-up by a factor of eight to pilot-scale using the developed method. Varying the number of nozzles in use in the pilot-scale granulation affected the particle surface roughness due to the differing drying conditions encountered. On this basis, the ability of the tracked-quantity approach to capture the relationship between product properties and geometric feature or process conditions is demonstrated. In this work, a method to predict the surface structures of particles produced by fluidized bed spray layering granulation using the CFD-DEM method was developed. A simple state-variable/event tracking approach was implemented to capture indirect quantifiers of the progression of structure-forming microprocesses. The state of the droplet at the time of impact on the particle surface, as well as the time required for drying, is correlated to product properties that quantify surface structure morphology such as roughness. A workflow for scale-up of fluidized bed granulation guided by product-property predictors is presented. The approach was tested on a demonstration case from the literature, where a particle core is coated with sodium benzoate solution. The experiment was scaled-up by a factor of eight to pilot-scale using the developed method. Varying the number of nozzles in use in the pilot-scale granulation affected the particle surface roughness due to the differing drying conditions encountered. On this basis, the ability of the tracked-quantity approach to capture the relationship between product properties and geometric feature or process conditions is demonstrated.

A workflow for developing a multidimensional, linear correlation between the process conditions during fluidized bed spray granulation and the surface morphology of the resulting granules is presented. Spray coating experiments with Cellets®500 particles and sodium benzoate solution were performed in a lab-scale fluidized bed varying liquid spray rate, fluidization air flow rate, fluidization air temperature, spray air temperature and spray atomization pressure. To characterize the surface structure, the surface roughness of the coated particles was quantified using confocal laser-scanning microscopy. The roughness was correlated to the process conditions, and the resulting correlation was rigorously analyzed for the importance and co-linearities of the individual process parameters using a principal component analysis. The surface roughness is strongly dependent on the spray rate of the coating solution, the fluidization air temperature and the atomization pressure at the nozzle. In general, wet process conditions and large droplets with a low initial velocity favor the formation of particles with a rough surface structure, while dry conditions and fine droplets with a high velocity result in granules with a smooth and compact coating layer.

In this contribution, a three-dimensional prismatic spouted bed for coating and granulation purposes is investigated. To determine the influence of a particle size distribution on the spouting behavior, binary and ternary mixtures of different particle sizes (γ-Al2O3 particles) are used. The stability is quantified using the Fourier transforms of the pressure drop signals. The stability range of binary and ternary systems differs from the monodisperse systems. Both the lower and the upper velocity limits of stable spouting can be described by a power function dependent on the Reynolds number and the Archimedes number. A larger Archimedes number results in an increased Reynolds interval size of stable spouting, which refers to a larger size of the velocity interval. This correlation was found to be not valid for the mass equivalent mixture of three particle fractions where no stable spouting state was obtained. In addition, clustering and segregation effects were observed: Coarse particles accumulate in the spout and fine particles in the annulus region of the regarded prismatic spouted bed in case of the instable spouting regime. This is problematic for granulation and coating processes as a homogeneous droplet distribution and a proper mixing between spraying and drying zones is disabled if some particle fractions are inhibited to pass the spray zone.

This work studies the particle exchange rates in horizontal fluidized beds equipped with different weir designs between compartments. These particle exchange rates provide information on the axial dispersion of the solid material within the process. For this purpose discrete particle modelling (DPM) was used to determine the particle exchange on microscopic level. This method uses a coupled CFD-DEM approach to observe particle dynamics in a fluid field. The model was validated against exchange rates in a lab-scale setup as determined by Particle Tracking Velocimetry (PTV) with very good quantitative agreement, showing the suitability of the method for the evaluation of weir designs. Simulations were performed for different weir designs and under variation of the hold-up mass, the feed rate and gas velocity to predict their transport behavior in a pilot-scale 3D horizontal fluidized bed. The results indicate that the solids transport behavior is strongly dependent on the used weir design and the main driving force for the particle transport that can be influenced by the process conditions. The installation of weirs between two compartments induces a transport resistance, while the base type without the installation of a weir between the two chambers represents the fastest possibility for mixing the particles of a two-compartment system. It has been observed that the general trend shows higher particle recirculation rates for the overflow weir and base configuration (no weir), whereas the underflow and sideflow weir applications improve the solids transport through the horizontal fluidized bed.

Gas–solid fluidized beds exhibit characteristic, reappearing patterns. External parameter variations or coupling between internal processes superpose a transient behavior on top of these recurrences. We demonstrate how recurrence CFD (rCFD) can cope with such scenarios by interpolating between multiple, pre-generated databases corresponding to short (2.5 s) time series in different flow regimes. The methodology is applied to a fluidized bed of approximately 80 000 solid particles subject to internal heat production and fluidization with cool gas for a duration of 800 s. The fluid’s thermal expansion due to heat transfer ([290 K; 430 K]) and the varying inlet velocities ([0.4 m/s; 0.9 m/s]) cause the desired transiency. We find excellent agreement for particle mean temperatures over time and qualitative similarity for their standard deviation between detailed, established simulation techniques and rCFD with speed-ups of more than two orders of magnitude. Furthermore, we present implementation advancements concerning parallel data exchange and compare all-to-all with one-to-one communication schemes. Analysis of the code’s performance points the way for future optimization measures.

In order to measure the residence time distribution (RTD) in continuous gas-solid processes tracer particles are used. In this contribution, a novel production concept for magnetizable particles has been developed and optimized. The process includes coating of a fraction of the investigated bed material with a magnetizable paint in a laboratory spouted bed. The coated particles fulfill the requirements of tracer material, are resistant, harmless to health and reusable. After passing a continuously operated process, the tracer material can be separated by a magnetic rod from the sample, which allows the fast and easy calculation of the exit age distribution as demonstrated in this contribution with a horizontal fluidized bed.

In this contribution, coupled CFD-DEM simulations of a three-dimensional prismatic spouted bed with two tangential, parallel gas inlets are presented. Contact model parameters were calibrated. Simulations with different inlet conditions were performed, whereby an additional degree of freedom by merging the two inlets was found to match the experimentally observed bed behavior. Additionally, it was shown that the inlet slit pressure drop has an important influence on spouting stability. By adjusting the inlet conditions, calibrating the particle interaction model parameters and applying fluid-coarsening correction of the Beetstra drag model, an experimentally validated model was obtained.

Coupled CFD-DEM simulations of a three-dimensional prismatic spouted bed with liquid injection were performed. Spouting stability is investigated by means of pressure drop fluctuation analysis and high speed camera recording. To determine the influence of flow stability on coating quality, droplets are injected in a post-processing step and deposited on colliding solid parcels. In order to estimate liquid holdup in the bed, heat transfer and subsequent evaporation of the surface liquid film were modelled. The coating quality of different process conditions is compared for a simulation time of 10 s and found to be broader distributed in the dense stable spouting operating regime. The analysis of particle circulation frequencies shows slower circulation under stable spouting conditions, resulting in a slower coating progress. This explains the inhomogeneous coating quality in the first 10 s for these cases. Nevertheless, the frequencies are less broadly distributed compared to the instable regime, which is advantageous for obtaining a homogeneous layer in real-time coating processes.

Although numerical models such as the computational fluid dynamics–discrete element method (CFD–DEM) have enabled the accurate simulation of laboratory-scale apparatuses, the application of these methods to large-scale apparatuses with many particles and time scales ranging from minutes to hours remains a challenge. The recently developed recurrence CFD (rCFD) method seeks to overcome these issues in pseudo-periodic processes by extrapolating globally recurring patterns in a physically meaningful way and describing the transport and interaction of passive scalars using Lagrangian tracers. Spouted beds represent an interesting target because of the associated variety of flow regimes. They can be effectively described by CFD–DEM on the time scale of tens of seconds, whereas industrially relevant processes typically take hours. In this contribution, we established the validity of applying the Lagrangian rCFD method to spouted beds by demonstrating the accurate reproduction of the particle residence time distribution in a fictitious spray zone. The deposition of spray droplets onto tracer particles was simulated for 1 h, and the particle surface coverage distribution was estimated using a statistical approach for both an unstabilized prismatic spouted bed and one stabilized by draft plates.