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Understanding of particle flow behaviour as a function of strain rate is of great interest in many items of equipment of industrial processes, such as screw conveyors, impeller mixers, and feeders. The traditional commercial instruments for bulk powder flow characterisation, such as shear cells, operate at low shear strain rates, and are not representative of unit operations under dynamic conditions. In recent years, the FT4 powder rheometer of Freeman Technology has emerged as a widely used technique for characterising particle flow under dynamic conditions of shear strain rate; yet little is known about its underlying powder mechanics. We analyse the effect of gas flow on the flow behaviour of cohesionless particles in FT4 both experimentally and by numerical simulations using the combined discrete element method (DEM) and computational fluid dynamics (CFD). The results show that the effect of gas flow on the flow energy could be described by the resultant fluid-induced drag on the particles above the blade position as the impeller penetrates the bed. The strain rate in front of the blade is mainly determined by the impeller tip speed, and is not sensitive to the gas flow and particle size. The flow energy correlates well with the shear stress in front of the blade. They both increase with the strain rate and are significantly reduced by the upward gas flow. [ABSTRACT FROM AUTHOR]

A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements. [ABSTRACT FROM PUBLISHER]

In this study, a steady state, three-dimensional, multiphase CFD modeling of a pilot-plant counter-current spray drying tower is carried out to study the drying of detergent slurry and to predict spray-dried detergent powder characteristics. The coupling between the two phases is achieved using the Eulerian-Lagrangian approach. The continuous phase turbulence is modeled using the Reynolds stress transport model. The droplet drying kinetics is studied using a semi-empirical droplet/particle drying model. Emphasis is given on the modeling of particle-wall interaction by considering only the rebound effect and specifying the coefficient of restitution as a function of impact angle with wall surface roughness taken into account using a stochastic approach, as well as a function of moisture content. This influences the post-wall collision trajectories of particles, residence time distribution and the overall exchange of heat and mass transfer. The model predictions agree well with the measured outlet values of powder average temperature, moisture content and exhaust air temperature considering the complexity of the process and the measurements accuracy. [ABSTRACT FROM AUTHOR]

Spiral jet mills are ubiquitous in the pharmaceutical industry. Breakage and classification in spiral jet mills occur due to complex interactions between the fluid and the solid phases. The study of these interactions requires the use of computational fluid dynamics (CFD) for the fluid phase coupled with discrete element models (DEM) for the particle phase. In this study, we investigate particle dynamics in a 50-mm spiral jet mill through coupled CFD-DEM simulations. The simulations showed that the fluid was significantly decelerated by the presence of the particles in the milling chamber. Furthermore, we study the particle dynamics and collision statistics at two different operating conditions and three different particle loadings. As expected, the particle velocity was affected by both the particle loading and operating pressure. The particles moved slower at low pressures and high loadings. We also found that particle–particle collisions outnumbered particle–wall collisions. [ABSTRACT FROM AUTHOR]

Understanding of the rheological behaviour of fully three-dimensional and non-uniform particle flow is of great interest. We analyse the resistance exhibited by a granular bed as a rotating impeller is penetrated into it, from which the rheological characteristics of the bed are deduced. For this end, the transient rheological response of both spherical and rodlike particles is simulated by Discrete Element Method. Transition from quasi-static to intermediate flow regime of rodlike particles is found to occur at a much larger shear strain rate than that of spherical particles. The relationship between the bulk friction coefficient and the inertial number is not monotonic. The viscosity of particle flow is inferred from the blade torque and is related to the inertial number and granular temperature, for which a power law is obtained covering both quasi-static and intermediate flow regimes. It can be used for obtaining flow field in complex geometry and dynamics using continuum mechanics by Computational Fluid Dynamics. [Display omitted] • Rheological characteristics of a dense granular system is investigated. • An impeller with twisted blades is penetrated into a particle bed. • Different flow regimes may prevail during the penetration. • Viscosity of granular flow is related to inertial number and granular temperature. • A power law model of viscosity is proposed that is obtainable from impeller torque. [ABSTRACT FROM AUTHOR]

[Display omitted] Milling is widely used in various industries to tailor the particle size distribution for desired attributes. The ability to predict milling behaviour by testing the breakability of a small quantity of material is of great interest. In this paper, a widely available aerodynamic dispersion method, i.e. the Aero S disperser of Malvern Mastersizer 3000 has been evaluated for this purpose. This device is commonly used for dispersion of fine and cohesive powders, as the particles are accelerated and impacted at a bend, but here its use for assessing particle breakability is explored. The fluid flow field is modelled using one-way coupled Computational Fluid Dynamics approach, as the particle concentration is low, following which the particle impact velocity is calculated by Lagrangian tracking and used in the analysis of particle breakage. Experimental work on the breakability is carried out using aspirin, paracetamol, sucrose and α–lactose monohydrate particles. The relative shift in the specific surface area is determined and together with the calculated particle impact velocity and physical properties, they are used to calculate the breakability index. A good agreement is obtained with the single particle impact testing and aerodynamic dispersion by Scirocco disperser, indicating the breakability could also be inferred from this method. [ABSTRACT FROM AUTHOR]

Triboelectric charging can strongly influence bulk powder flow behaviour, and hence its characterization is of great interest for safe manufacturing operations. In a recent development, the use of an aerodynamic disperser, employing a pressure pulse to disperse a small powder quantity, shows a great potential for inducing triboelectric charge transfer. We analyse this process by coupled Discrete Element Method and Computational Fluid Dynamics (DEM-CFD) simulations, incorporating triboelectric charge transfer. The simulations are based on property data of glass ballotini as model particles, together with those of α-lactose monohydrate (α-LM) and aspirin, as powders of practical interest. The characteristics of particle-particle and particle-wall collisions are analysed in detail. The analysis shows that pharmaceutical particles charge significantly more than glass ballotini. The charge-to-surface area ratio is remarkably constant and close to its equilibrium value for each test material. Overall, the analysis provides a great insight on the triboelectric charging by aerodynamic dispersion. Unlabelled Image • Triboelectric charging of small samples can be characterized with aerodispersion. • Discrete element simulations can be a valuable tool to study triboelectric charging. • During aerodynamical dispersion, the number of particle-wall impacts is significant. • High specific charge levels for fine particles are observed. • Pharmaceutical powders exhibit strong charge accumulation. [ABSTRACT FROM AUTHOR]

• Generic multi-zonal modelling methodology is proposed for spray dryers. • Multi-zonal modelling methodology is applied to a counter-current dryer. • CFD results are used for the basis of different zones in the dryer. • Multi-zonal modelling results are compared with CFD and plug-flow models. A generic multi-zonal modelling methodology for spray drying towers is proposed to provide relatively quick (seconds) simulations on desktop computers for decision making on changing process parameters for automatic process control. A multi-zonal model comprises combinations of all or some of six differing zones: plug-flow, semi-plug-flow and well-mixed zones in either co- or counter-current flow. This approach is demonstrated by predicting the dried powder characteristics of a detergent powder from a pilot-scale counter-current spray drying tower. The types, sizes and locations of the different zones are obtained by a detailed analysis of predictions from a previous 3-dimensional Computational Fluid Dynamics (CFD) simulation of gas and particle flow dynamics and drying kinetics within the tower. The multi-zonal model consists of seven zones – five in counter-current and two in co-current flow. The trends of the predicted gas temperature profiles are close to those from the CFD results. The particle exit temperature, moisture content and residence times over the full particle size distribution (PSD) range are very close to the CFD values unlike the previous simulated results from our plug-flow model. The outcome clearly demonstrates that the prerequisite to have a sound conceptual model of spray drying towers, such as the multi-zonal model developed here, is a detailed knowledge of the gas and particle flow fields within the tower. [ABSTRACT FROM AUTHOR]

Acicular crystals are frequently encountered in pharmaceutical and fine chemical industries. They are prone to breakage, as a result of which their bulk behaviour changes drastically. It is therefore of great interest to be able to assess their breakage propensity quickly and preferably using a small quantity. An integrated experimental and modelling method is proposed to quantify the breakability of acicular crystals by aerodynamic dispersion using the disperser of Morphologi G3, in which a pulse of compressed gas induces particle impact. Needle-shaped crystals of β l -glutamic acid, benzothiazin and potassium phosphate are used as the model test materials. The extent of breakage as a function of dispersion pressure is obtained by quantifying the shift in particle size distribution provided by Morphologi G3. It increases as the dispersion pressure is increased for β l -glutamic acid and benzothiazin particles, while not noticeably for potassium phosphate crystals. The impact velocity of the particles at different pressures is estimated by computational fluid dynamics (CFD) calculations. Its effect on the extent of breakage is used to infer the ease with which the crystals break, expressed by a lumped parameter as the ratio of hardness over square of toughness. Benzothiazin breaks by fracture along both crystal length and width due to presence of cleavage planes, whilst β l -glutamic acid breaks only perpendicular to the length. Potassium phosphate undergoes minor chipping from the edges and corners. Impact breakage of acicular crystals due to aerodynamic dispersion by a compressed air pressure pulse Unlabelled Image • Impact breakage of acicular crystals by aerodynamic dispersion is analysed. • Dispersion is achieved by an air pressure pulse using Morphologi G3. • Impact velocity against walls is related to pulse pressure by CFD. • Shift in particle size distribution is used to estimate breakage extent. • Dependence of breakage extent on velocity is used to infer breakability. [ABSTRACT FROM AUTHOR]

The performance of two new Vertical Roller Mills (VRM) has been analysed using Computational Fluid Dynamics (CFD). The results show notable air ingress from the bottom of the mill, due to moving components in the mill. This is detrimental to mill performance as the milled powder gets recirculated back into the milling region and reduces the overall milling efficiency. Optimisation of the mill design has been carried out based on CFD modelling results. Proposed reconfigurations produce a positive pressure upstream, suppressing air ingress. Design modifications were implemented in a pilot-scale mill, resolving the air ingress problem. Additionally, erosion patterns were computed for the mill geometry and qualitatively confirmed by visual inspection of the mill. [Display omitted] • The performance of a vertical roller mill is analysed with CFD. • Adverse air circulation is detected through CFD simulations. • Design modifications were evaluated and implemented. • The detected issues were resolved, improving the performance. • The erosion pattern inside the mill was calculated with CFD. [ABSTRACT FROM AUTHOR]

The particle residence time in counter-current spray drying towers has a significant influence on the moisture content of the powder exiting the tower. Therefore, the reliability of predictions of residence time by numerical methods is highly desirable. A combined experimental and computational fluid dynamics investigation is reported for the prediction of the residence time distributions of glass beads with a narrow size range of 300–425 μm in a counter-current tower with isothermal swirling flows of air. The particle-wall collision is taken into account using a rough-wall collision model. Overall, a reasonably good agreement is obtained between the measurements and predictions. Consideration of wall roughness results in greater axial dispersion of particles in the tower compared to a smooth wall assumption. The rough particle-wall collision is important for a reliable prediction of residence time distributions. In addition, analysis of the results infers that the clustering effect of particles on drag and particle-particle interactions is important and should be investigated in a future study. [ABSTRACT FROM AUTHOR]

Spiral jet milling is commonly used for size reduction of high value particulate solids, such as pharmaceutical ingredients, for which low contamination is critical. The mill utilises high-pressure gas nozzles to form an internal vortex and induce particle breakage through inter-particle collisions. Due to the centrifugal flow field, the particles can move radially inwards towards the mill centre and escape the mill only when their size is sufficiently reduced for the fluid drag to exceed the centrifugal force. Large particles move radially towards the outer wall and form a dense particle bed. The bed itself circulates in the milling chamber due to the induced effect of the inclined gas jets. In this study, we analyse the implementation of a coarse-graining (CG) approach on a coupled Computational Fluid Dynamic-Discrete Element Method simulation. Along with the actual particle size, four CG scale cases are compared (CG-2, CG-4, CG-8 and CG-10). To analyse the success of the approach in predicting the dynamics of fluid and particle motion, the characteristic features of the particle bed and fluid field, i.e. the fluid and particle velocity distributions and dissipated energy through particle collisions are analysed. There is good agreement between the original particle size and the two smallest scaled cases (CG-2, CG-4) for the above characteristics. However, modelling the lean phase is less successful, as there are fewer particles that reside there at any given time. There is also good agreement between these three cases in terms of dissipated energy through particle collisions. An average value for dissipated energy of 0.6 mJ is recorded for each of the three lowest cases. This proves beneficial for simulation time required, as the particle number is reduced 2-fold and 4-fold, respectively, and interparticle collision rate is reduced 60% each time. Application of the Coarse Grain method in a spiral jet mill using coupled Computational Fluid Dynamics and Discrete Element Method Modelling. Top: Original particle size system and four scaled systems. Bottom: Resulting temporal average dissipated energy through interparticle collisions and particle wall collisions. [Display omitted] • Coarse graining method is applied in the spiral jet mill using a CFD-DEM framework. • Particle and fluid motion is similar in the bed at smaller scales. • Insufficient particles were present in the lean phase to model the original system. • Temporal dissipated energy remained constant for low coarse grain numbers. [ABSTRACT FROM AUTHOR]