Powder Flow Pattern Research Articles

An experimental study on flow behaviour of pharmaceutical powders during suction filling

Die filling is a crucial step in the pharmaceutical tablet manufacturing process. For industrial-scale production using rotary presses, suction filling is typically employed due to its significant efficiency advantages over gravity filling. Despite its widespread use, our understanding of the suction filling process remains limited. Specifically, there is insufficient comprehension of how filling performance is influenced by factors such as suction velocity, filling velocity, and the properties of the powder materials. Building on our previous research, this study aims to further investigate the effects of powder properties and process parameters (e.g., filling velocity, suction velocity, fill depth) on suction filling behaviour. A systematic experimental investigation was conducted using a model suction filling system, considering both cohesive and free-flowing pharmaceutical powders. The effect of fill depth on suction filling of these powders was examined at different filling and suction velocities. The results demonstrate that two distinctive flow regimes for suction filling can be identified: slow filling and fast filling. These regimes are delineated by a critical filling-to-suction velocity ratio. In the slow filling regime, the filling-to-suction velocity ratio is lower than the critical ratio, meaning that the filling phase is slower than the suction phase. Conversely, the fast filling regime occurs when the filling-to-suction velocity ratio exceeds the critical ratio, implying that the filling phase is faster than the suction phase. This study reveals, for the first time, that when the powder flow pattern during suction filling is dominated by plug flow, full die fill (i.e., the fill ratio equals unity) is achieved in the slow filling regime. However, in the fast filling regime, incomplete die fill is obtained. It is also found that when plug flow prevails during fast filling, the fill ratio has an inverse correlation with the filling-to-suction velocity ratio. This study further reveals that when the plug flow assumption is valid, the filling ratio at various fill-to-suction velocity ratios can be well predicted mathematically. Furthermore, it is also found that once the powder flow pattern differs from the ideal plug flow, which could be induced by the filling conditions and powder cohesion, the fill ratio can be overpredicted.

Effect of carrier gas type on feeding characteristics of powder supply system

The new powder engine is an efficient power system with powder fuel as propellant. Its excellent performance is inseparable from stable and efficient powder feeding assisted by carrier gas. In this paper, we investigate the effect of carrier gas type (air, N2, CO2 and He, gas viscosity: CO2<N2<air<He) on the powder feeding characteristics of the powder supply system. The experimental results show that the change in carrier gas types has an insignificant effect on the powder flow pattern, but greatly affects the powder mass flow rate, pressure distribution and gas velocity in the silo, which is owing to the difference in the gas viscosity. Moreover, the analysis of powder column stress state reveals that the powder axial stress is related to the gas phase pressure gradient in the powder column. Specifically, under the same differential pressure, the smaller the gas viscosity, the higher the permeability, the greater the gas phase pressure gradient, the greater the powder axial stress pressure transferred to the outlet, thus obtaining a higher mass flow rate. However, energy analysis indicates that the smaller the gas viscosity, the higher the energy consumption required to transport the same mass powder. We hope our findings can provide ideas and references for efficient powder feeding.

Influence of weak electrostatic charges and secondary flows on pneumatic powder transport

AbstractThe dynamics of the transported powder determines the functionality and safety of pneumatic conveying systems. The relation between the carrier gas flow, induced by the flown‐through geometry, and the powder flow pattern is not clear yet for electrostatically charged particles. This paper highlights the influence of relatively minor cross‐sectional secondary flows and electrostatic forces on the concentration and dynamics of the particles. To this end, direct numerical simulations (DNS) capture the interaction of the continuous and dispersed phases using a four‐way coupled Eulerian–Lagrangian strategy. The transport of weakly charged particles in channel flows, where turbopheresis defines the particle concentration, is compared to duct flows, where additional cross‐sectional vortices form. For both geometries, the Stokes number () and the electrical Stokes number () are varied, and the turbulent carrier flow was fixed to . The presented simulations demonstrate that secondary flows, for the same , , and , dampen the effect of particle charge. In a duct flow, vortical secondary flows enhance the cross‐sectional particle mobility against the direction of electrostatic forces. Compared to a duct flow, in a channel, the wall‐normal aerodynamic forces are weaker. Thus, electrical forces dominate their transport; the local particle concentration at the walls increases. Further, electrostatic charges cause a stronger correlation between the gas and particle velocities. In conclusion, despite being weak compared to the primary flow forces, secondary flow and electrostatic forces drive particle dynamics during pneumatic transport.

Elucidation of the powder flow pattern in a twin-screw LIW-feeder for various refill regimes

The importance of residence time distribution modeling is acknowledged as a tool for enabling material tracking and control within a continuous manufacturing line in order to safeguard both product quality and production efficiency. One of the first unit-operations into a continuous direct compression line (i.e. CDC-line) worthwhile doing extensive RTD-analysis upon are the LIW-feeders since they dose the ingredients in a controlled way following the label claim and hence can directly influence critical quality attributes like content uniformity. An NIR measurement method was developed determining the RTD of selected powders at specific feeder settings. Step-change experiments using sodium saccharin as a tracer were conducted. In order to gain and in depth understanding of the material flow, spatial samples throughout the hopper were taken at predefined timepoints during the step change experiments. This revealed the presence of a bypass trajectory along the edges of the agitator, while in the center of the agitator an inner mixing volume in which the tracer concentration lags behind seemed to be present. Finally, a model based on a plug flow and continuous stirred tank reactor was evaluated. The fitted model was not able to capture this complex flow behavior and shows the need for an extended compartmental model distinguishing between a bypass trajectory formed by the agitator and an inner mixing volume.

Dispersion of Irregular Particles during Free Fall

This paper describes an experimental set up to examine the relationship between powder properties on powder flow pattern during filling operation. For this purpose, the free fall velocities of four types of agricultural dusts (i.e. castor sugar, oat, semolina and tea powder) in a rectangular enclosure were captured using a Particle Image Velocimetry (PIV). The result reveals that oat powder portrayed greater cloud dispersion due to its flaky shape and a lower bulk density. The characteristics of oat powder allowed more air to be dragged into the particle stream and forced the particle to be dispersed and suspended in air due to drag force and terminal velocity. The result presented here may facilitate improvements in dust control and measures in powder industries.

Investigations of powder flow patterns in hopper model through discrete element method

In this paper, a comparative study was conducted to observe the flow patterns of bulk powder materials through a conical shaped hopper. Hoppers were designed based on Jenike charts and further analyzed using discrete element method. The paper focuses on the flow of micro-sized powder materials and how their area flow pattern changes along with velocity distribution variation with a change in hopper angle. Discrete element method was used to predict the flow patterns for different hopper angles and to study the transition of flow from funnel flow to mass flow. A comparison was made among these hoppers based on their average velocity and average mass flow rate to have an optimum design for mass flow. It was observed that a decrease in hopper angle resulted in an increase in both the mass flow rate and average velocity.

Optical diagnostics and optimization of the gas-powder flow in the nozzles for laser cladding

The results of investigation of gas and gas-powder flows in various cladding nozzles are presented and discussed. Schlieren-imaging, high-speed imaging and laser Doppler anemometry were used for optical diagnostics of the flows. It is demonstrated that diameter of the outlet in removable cyclone cap has significant impact on the powder flow pattern in an off-axis nozzle. It provides a convenient way to optimize focusing of the powder flow. It is found that stable vortices that appear near the surface of the substrate significantly reduce efficiency of powder material use when off-axis nozzle is applied. It is demonstrated that excessive shielding gas flow rate in four-stream and coaxial nozzles may lead to defocusing of gas-powder flows. A possibility to increase the efficiency of powder use in various laser cladding nozzles through modification of their design as well as shielding and carrier gases flow rates is discussed.

Numerical simulation of powder flow in a pharmaceutical tablet press lab-scale gravity feeder

The die filling process within a lab-scale rotary tablet press is simulated using the Discrete Element Method (DEM). The particle motion is captured as particles travel from the gravity feeder consisting of a chute connected to the shoe represented by a rectangular box to the dies/cavities of the turret. The particles fill the dies mostly due to gravity. The process conditions such as die table speed, die diameter as well as the material characteristics e.g. cohesion, coefficient of friction, coefficient of restitution, etc. influence the die filling process. The process conditions and material characteristics are systematically investigated by varying one of those parameters at a time while keeping other conditions constant. Eventually process conditions as well as material properties were changed simultaneously to analyze interaction relationships. The results give both qualitative (powder flow pattern from the feeder to the dies) and quantitative (tablet mass variation, mass flow rate etc.) insights into the die filling process. The numerical results indicate that faster die table speeds and smaller die diameters give rise to lower tablet mass with a significantly increased mass variation, which would eventually increase the challenges in meeting the specifications. On the other hand, cohesion between the particles significantly reduces the tablet mass and increases the mass variation. The influence of particle-wall friction coefficient on the particle rearrangement is analyzed both qualitatively and quantitatively, which shows that increasing the particle-wall friction reduces the dead zones within the shoe. The coefficient of restitution does not exhibit any influence on the die filling which may be attributed to the very confined space of the feeding system. Eventually this study emphasizes the importance of tablet blend characterization prior to tableting and provides operators with optimal process parameters and material attributes to ensure a high tableting process performance.

Scale-up effects on flow patterns in the high shear mixing of cohesive powders

Processing of granular material often requires mixing steps in order to blend cohesive powders, distribute viscous liquids into powder beds or create agglomerates from a wet powder mass. For this reason, using bladed, high-speed mixers is frequently considered a good solution by many types of industry. However, despite the importance of such mixers in powder processing, the granular flow behavior inside the mixer bowl is generally not totally understood. In this work extensive experimentation was performed comparing the behavior of a lab-scale mixer (1.9l vessel volume) to that of a pilot-scale mixer (65l vessel volume) with a mixture of some pharmaceutical excipients (e.g. lactose, cellulose). The aim was to propose a new and more detailed method for describing the complex powder rheology inside an high shear mixer using impeller torque, current consumption and particle image velocimetry (PIV) analysis. Particularly, a new dimensionless torque number is proposed for the torque profile analysis in order to isolate the contributions of mass fill and blade clearance at the vessel base. Impeller torque and motor current consumption were integrated with PIV to obtain more detailed information about the surface velocity and flow pattern changes in the pilot-scale mixer. Mass fill resulted to be one of the most critical variables, as predicted by the torque model, strongly affecting the powder flow patterns. An additional mixing regimes was furthermore defined according to the observation of the surface velocity of the powder bed.

Study of powder flow patterns in a Couette cell with axial flow using tracers and solid fraction measurements

We present different aspects of dense granular flows in a Couette geometry using a variety of particulate materials with shape and size distributions. Tracer studies point to an apparent coupling of particle size with flow and stress field gradients. While there is a clear industrial motivation to use “real” materials as a means to expand basic physical and engineering research in granular dynamics, the current study suggests additional academic motivations. Indeed, particles with distributed characteristics uncover rich interactions between flow and stress fields that might otherwise go un-noticed with model materials such as spherical glass beads. Distribution of size and shape play a strong role in how stress is transmitted in granular media (Kheiripour Langroudi et al. in Powder Technol 203:23–32, 2010) and how particle pattern arrangements evolve. Direct solid fraction measurements, using a capacitance probe, show that dense particle flows exhibit significant variations in solid fraction in both sheared and stagnant layers. Furthermore, these measurements also show different dependence of the solid fraction on shearing rate: solid fraction decreases in sheared layers and increases in stagnant layers as the shear rate is increased. From these results the thickness of the shear band could be estimated and was found to vary as a function of particle shape and the roughness of the container walls. The main result is that shear stress (or torque) (see also Kheiripour Langroudi et al. in Powder Technol 197:91–101, 2010) and solid fraction profiles depend on particle shape and whether or not an extra degree of freedom in their movement is provided so that the system can dilate under various shear states in the Couette cell. This extra degree of freedom is assured in the present experimental work by allowing a slight axial outflow from the Couette device while the driven shear fields are in the radial and tangential directions.

The effects of air and particle density difference on segregation of powder mixtures during die filling

Segregation of mono-disperse binary mixtures with different particle densities during die filling in the presence of air was numerically analysed using a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach. Die filling with powders of different particle density ratios (i.e. the ratio of the heavy particles to the light particles) at various shoe speeds was simulated, in order to explore the effects of air and particle density difference on segregation. For die filling from a stationary shoe, the air can induce significant segregation by hindering the deposition of light particles (i.e., air-sensitive particles). As the particle density ratio increases, the light particles are deposited into the die at even lower speeds compared with the heavy ones due to the effect of air drag, resulting in an increase in the degree of segregation. For die filling with a moving shoe, segregation occurs due to different post-collisional velocities resulting from different particle inertia; and the degree of segregation increases as the particle density ratio increases due to the increasing difference in particle inertia. It is found that, as the shoe velocity increases, the powder flow pattern changes from nose flow dominated to bulk flow dominated and the degree of segregation generally decreases. The effect of air is limited for nose flow dominated die filling because the air can easily evacuate through the gap between the die walls and flowing powder stream. When bulk flow dominates in die filling, the air can be entrapped in the die, which has a significant impact on the powder flow and segregation behaviours. Finally, the effect of interparticle friction on segregation was investigated.

Foam granulation: Binder dispersion and nucleation in mixer-granulators

Traditional wet granulation method involves spraying of liquid binder onto a moving powder bed to granulate the powder particles in the granulator. A new alternative method of wet granulation has been developed where foam delivery of binder is used to granulate the powder particles. This study investigated binder distribution in wet granulation and focused on the nucleation stage, where nuclei are formed during the initial binder distribution. Nucleation experiments were used to study the formation of nuclei by the foam and spray delivery methods in a high shear mixer-granulator. The distribution of foams on a dynamic powder bed were also investigated by filming small portion of foams as they penetrated into moving powder beds under different powder flow conditions in a high shear mixer-granulator. Nucleation experiments in this study show that foam delivery tends to create a narrower nuclei size distribution during the early stage of wet granulation process compared to spray delivery at the same processing conditions, demonstrating the potential of foam granulation in achieving improved binder distribution. For foam delivery, the nuclei formation is influenced by the foam properties and powder flow conditions in the granulator. The experiments show that the narrowest nuclei size distribution is obtained by granulating with high-quality foam and intensive powder mixing conditions. Coarser nuclei are formed when low-quality foam is dispersed in a less intensively agitated powder. The interactions of foam quality and the powder flow pattern are discussed and two distinct wetting and nucleation mechanisms are proposed: (1) under bumping flow, a low-quality foam tends to induce localised wetting and nucleation. The wetting and nucleation is “foam drainage” controlled. (2) Under roping flow, foam will be dispersed by the motion of the agitated powder. The wetting and nucleation is “mechanical dispersion” controlled.

Numerical analysis of density-induced segregation during die filling

In this study, segregation behaviour of binary granular mixtures with the same particle size but different densities during die filling in the presence of air was investigated using a combined discrete element method (DEM) and computational fluid dynamics (CFD) approach, in which the kinematics of particles was modelled using DEM, the motion of air was analysed using CFD and a two-way coupling of the particles and the air was incorporated. The depositions of powder from stationary and moving shoes into the die cavities of different geometries were simulated and the corresponding segregation behaviours were analysed. It has been found that, for die filling from a stationary shoe, the concentration distributions of the heavy and light particles along the die width mainly depend on the initial spatial distribution of the granular mixture in the shoe. For die filling from a moving shoe, a low concentration of light particles on the leading side of the die (referring to the direction of the shoe motion) is observed for die filling with a square die, in which the process is dominated by nose flow. The density difference can cause segregation along the die depth with a low concentration of light particles at the bottom. The presence of air enhances this segregation tendency by resisting the flow of light particles into the bottom of the die and causes a higher concentration of the light particles at the top. Finally, the segregation index, defined as the volume weighted root-mean-square deviation in the content of light particles, was introduced to quantify the degree of segregation in the horizontal and vertical directions. It has been found that the degree of segregation is determined by the presence of air and also the powder flow pattern.

The effect of lubrication on density distributions of roller compacted ribbons

Roller compaction is a continuous dry granulation process for producing free flowing granules in order to increase the bulk density and uniformity of pharmaceutical formulations. It is a complicated process due to the diversity of powder blends and processing parameters involved. The properties of the produced ribbon are dominated by a number of factors, such as the powder properties, friction, roll speed, roll gap, feeding mechanisms and feeding speed, which consequently determine the properties of the granules (size distribution, density and flow behaviour). It is hence important to understand the influence of these factors on the ribbon properties. In this study, an instrumented roller press developed at the University of Birmingham is used to investigate the effect of lubrication on the density distribution of the ribbons. Three different cases are considered: (1) no lubrication, (2) lubricated press, in which the side cheek plates of the roller press are lubricated, and (3) lubricated powder, for which a lubricant is mixed into the powder. In addition, how the powders are fed into the entry region of the roller press and its influence on ribbon properties are also investigated. It is found that the method of feeding the powder into the roller press plays a crucial role in determining the homogeneity of the ribbon density. For the roller press used in this study, a drag angle (i.e., the angle formed when the powder is dragged into the roller press) is introduced to characterise the powder flow pattern in the feeding hopper. It is shown that a sharper drag angle results in a more heterogeneous ribbon. In addition, the average ribbon density depends upon the peak pressure and nip angle. The higher the peak pressure and nip angle are, the higher the average ribbon density is. Furthermore, the densification behaviour of the powder during roller compaction is compared to that during die compaction. It has been shown that the densification behaviour during these two processes is similar if the ribbons and the tablets have the same thickness.

Granular Flow in a Planetary Mixer

Wet massing granulation is widely used within the particle-forming industries. However, the process is difficult to monitor and control and the structure of the granular flow is poorly understood. Positron emission particle tracking was used to study powder flow patterns in a planetary mixer. Prior to studying granulating systems, a dry system of a model pharmaceutical formulation was studied to develop understanding of the interaction between mixer parameters such as the speed, level of fill, and the structure of the flow. Two quite different regimes of tracer motion exist. Within the central core of the vessel the powder behaviourwas determined by the rapid movement of the agitator. In contrast, in the regions close to the wall, the material was found to exhibit ascending steps dictated by the planetary rotation. These two forms of behaviour were observed at all levels of fill and agitator speeds. The angular velocity and total speed scaled with the agitator speed. Further analysis showed that the radial and axial velocities of the tracer were up to an order of magnitude lower than the tangential component.

Scale-up of mixer granulators for effective liquid distribution

There is considerable anecdotal evidence from industry that poor wetting and liquid distribution can lead to broad granule size distributions in mixer granulators. Current scale-up scenarios lead to poor liquid distribution and a wider product size distribution. There are two issues to consider when scaling up: the size and nature of the spray zone and the powder flow patterns as a function of granulator scale. Short, nucleation-only experiments in a 25L PMA Fielder mixer using lactose powder with water and HPC solutions demonstrated the existence of different nucleation regimes depending on the spray flux Ψ a—from drop-controlled nucleation to caking. In the drop-controlled regime at low Ψ a values, each drop forms a single nucleus and the nuclei distribution is controlled by the spray droplet size distribution. As Ψ a increases, the distribution broadens rapidly as the droplets overlap and coalesce in the spray zone. The results are in excellent agreement with previous experiments and confirm that for drop-controlled nucleation, Ψ a should be less than 0.1. Granulator flow studies showed that there are two powder flow regimes—bumping and roping. The powder flow goes through a transition from bumping to roping as impeller speed is increased. The roping regime gives good bed turn over and stable flow patterns. This regime is recommended for good liquid distribution and nucleation. Powder surface velocities as a function of impeller speed were measured using high-speed video equipment and MetaMorph image analysis software. Powder surface velocities were 0.2 to 1 ms −1—an order of magnitude lower than the impeller tip speed. Assuming geometrically similar granulators, impeller speed should be set to maintain constant Froude number during scale-up rather than constant tip speed to ensure operation in the roping regime.

Motion in a particle bed agitated by a single blade

AbstractStudies on the agitation of granular materials were conducted in a horizontal cylindrical shell stirred by a single long flat blade located on radial arms fixed to a rotor shaft. Positron emission particle tracking, a noninvasive method of investigating opaque systems, permitted the motion of a single particle to be followed. Axial flow patterns showed two loops of circulation inside each compartment defined by the radial arms of the rotating blades. Results revealed the presence at 20% of fill of a circulation zone in the cross‐sectional plane directly beneath the rotating shaft where little agitation occurred. This zone decreased in size and moved toward the agitator shaft as fill increased from 20% to 60%. Velocity profiles and Fourier analysis of particle displacement showed that particle movement was controlled by the number of blade passes. Torque measurements on the agitator shaft were correlated with distribution of material in the cross section of the mixer and distribution of tangential velocity. This opens the prospect of relating torque to powder flow patterns. The information can now be incorporated into models for heat transfer, chemical reaction, or agglomeration.

Continuous Mixing of Solids

Powder mixing is common in many industrial processes, yet traditional methods of investigation provide only very limited information on the influence of agitator geometry and important operational parameters on flow patterns. In this work, a particular technique called Positron Emission Particle Tracking (PEPT) is used to investigate powder flow patterns within a continuous mixer. The motion of a single positron-emitting particle is followed and the technique yields spatial coordinates of this tracer as a function of time.

Some aspects of flow in granular materials in hoppers

The main part of the paper is devoted to the description of the localized deformation zones in plane (slow) flow of granular materials from hoppers. Specifically, two types of zones are discussed i...

Static electrification phenomena in pneumatic transportation of coal

For the safe handling of powders in an industrial scale, special attention needs to be paid for electrostatic charge generation. Electrostatic charge is generated by contact, friction, separation, etc. between particles, and between particles and pipe walls. The powder flow pattern in a pipe greatly influences the charge generation. The flow pattern in small piping systems is considered to be different from the one in practical pneumatic systems where 50 ≈ 300 mmφ transport pipes are usually used. Bearing this in mind, the authors used relatively large pipes in order to study electrostatic phenomena in industries. An attempt was made to investigate the relationship between charge generation characteristics and transport conditions using a large straight pipe line system (53 mmφ) as a fundamental study. It was found that static electrification phenomena are affected by the powder concentration and the transporting velocity and that the electrostatic charge increases, peaks and decreases with increasing powder concentrations for transporting velocities.