In the shear deformation of powder beds beyond the quasi-static regime the shear stress is dependent on the strain rate. Extensive work has been reported on the rapid chute flow of large granules but the intermediate regime has not been widely addressed particularly in the case of cohesive powders. However in industrial powder processes the powder flow is often in the intermediate regime. In the present work an attempt is made to investigate the sensitivity of the stresses in an assembly of cohesive spherical particles to the strain rate in ball indentation using the Distinct Element Method. This technique has recently been proposed as a quick and easy way to assess the flowability of cohesive powders. It is shown that the hardness, deviatoric and hydrostatic stresses within a bed, subjected to ball indentation on its free surface, are dependent on the indentation strain rate. These stresses are almost constant up to a dimensionless strain rate of unity, consistent with trends from traditional methods of shear cell testing, though fluctuations begin to increase from a dimensionless strain rate of 0.5. For dimensionless strain rates greater than unity, these stresses increase, with the increase in hardness being the most substantial. These trends correlate well with those established in the literature for the Couette device. However quantitative value of the strain rate boundaries of the regimes differ, due to differences in the geometry of shear deformation band. Nevertheless, this shows the capability of the indentation technique in capturing the dynamics of cohesive powder flow.
As the size of individual particles is reduced below several microns, the interparticle cohesive forces begin to play a major role in the bulk powder behavior. Fine powders generally exhibit poor flowability as well as an affinity to agglomerate and form clusters due to this cohesion. This clustering behavior of dry, binderless particles is known as auto-granulation and can often cause difficulties in processing and handling of powders. In this study, a titania powder is vibrated under controlled conditions to induce clustering and promote agglomerate growth. The amplitude and frequency of the mechanical vibration is varied to view the effect of the input energy on the equilibrium agglomerate size. Furthermore, the densities of the formed agglomerates are measured to investigate the role of consolidation as a mechanism of auto-granulation. Given that the size of the agglomerates formed by this auto-granulation process is affected by the balance between the cohesive energy of the particles and the disruptive energy of vibration, this work provides insight into the mechanism controlling the growth of these agglomerates to an equilibrium size.
Zafar U, Hare C, Calvert G, Ghadiri M, Girimonte R, Formisani B, Quintanilla MAS, Valverde JM (2015) Comparison of cohesive powder flowability measured by Schulze Shear Cell, Raining Bed Method, Sevilla Powder Tester and new Ball Indentation Method, POWDER TECHNOLOGY 286 pp. 807-816
ELSEVIER SCIENCE BV
The Freeman FT4 Powder Rheometer has been reported to describe well the powder
flow behaviour in instances where other techniques fail. We use DEM to simulate the FT4
operation for slightly cohesive large glass beads at a range of strain rates. The curved impeller is
shown to be beneficial in comparison to a flat blade as the variation of shear stress across the blade
is reduced. The shear stress in front of the blade correlates well with flow energy (which the device
measures) for a range of tip speeds and is shown to increase approximately linearly with tip speed
when operating beyond the quasi-static regime.
The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels. For filling and dosing of small quantities of powders in capsules and for dispersion in dry powder inhalers, the interest is on the behaviour of loosely-compacted powders in small quantities and under very low applied loads. Furthermore at the early stages of drug development, the quantity of the powder available is often very small and the traditional bulk testing methods are neither possible nor applicable. In this work we evaluate a method to infer powder flowability by ball indentation. This technique provides a measure of flow resistance which can be related to the unconfined yield stress. It can be applied at very low loads and requires only a small sample quantity, typically a few mm3 . The operational window in the ball indentation method in terms of minimum sample size, penetration depth and indenter properties (such as size, shape, friction and Young?s modulus) has been analysed and reported here.
Fine cohesive powders are often dry granulated to improve their flowability. Roller compaction is commonly used to produce dense ribbons which are then milled. The material properties of the powder and the conditions in the roller compactor affect the strength of the ribbons, however there is no method in the literature to predict the size distribution of the product of ribbon milling. Here we introduce a method, by using the Distinct Element Method (DEM) to determine the prevailing impact velocities and stresses in the mill, with bonded spheres representing the ribbons. The bond strength is calibrated by matching experimental results of three point bend measurements and predictions from numerical simulations. The ribbons are then exposed to the dynamic conditions predicted by the DEM, by dropping them from a controlled height to cause fragmentation, and subsequently stressing them in a shear cell under the conditions again predicted by the DEM. The fragments are sheared under these conditions to represent repeated passage of bars over the fragments at the mill base. Sieve analysis is used here to determine the particle size distribution under given mill conditions. The predicted size distribution of the mill product compares well with the plant data. It is found that the mill speed and length of ribbons fed to the mill have no significant influence on the product size distribution for the range tested.
Auto-granulation is the growth of particle clusters within a dry, fine powder bed due to the bulk powder cohesion. This clustering occurs without the addition of any binder to the system due to simple agitation of a powder, such as during storage or handling. For this reason, it is important in powder processing to be able to characterize this behavior. In this study, a sub-micron titania powder is mechanically vibrated under controlled conditions to induce clustering and promote auto-granulation. The amplitude and frequency of the vibration is varied to view the effect on the equilibrium granule size. A statistical model of the effect is also developed to determine that the granule size increases linearly with vibrational energy. Furthermore, imaging of cross-sections of the granules is conducted to provide insight into to the internal microstructure and measure the packing fraction of the constituent particles. It is found that under all vibrational conditions investigated the particles exhibit a core-rim microstructure.
Particles are frequently exposed to shear stresses during manufacturing, which leads to
breakage. This is particularly relevant to weak active pharmaceutical ingredients and is
prevalent in pharmaceutical and food industries. The attrition of Paracetamol and Aspirin
caused by shear deformation at very low stresses is investigated here. The extent of breakage
of these particles is related to the prevailing shear stresses and strains. In contrast to the
expected trend, smaller particles exhibited increased breakage rates. At the onset of shearing
at low stresses Aspirin particles experienced slightly more breakage than the Paracetamol,
however prolonged shearing resulted in greater breakage of Paracetamol. Breakage occurred
initially through chipping with some fragmentation, particularly more noticeable for Aspirin,
with an increase in abrasion after extensive shear strain for Paracetamol. Empirical breakage
relationships are proposed and when combined with process stresses and strain analyses the
extent of breakage occurring in process equipment can be estimated.
In the seed processing industry, rotary batch seed coaters are widely used for providing a protective coating layer (consisting of various ingredients including fertilisers and crop protection chemicals) on the seeds. Seed motion and mixing are important in ensuring uniform coating. In the batch seed coater, the base of a cylindrical vessel rotates, whilst the cylindrical wall is stationary and two baffles turn the bed over for mixing. In the present study, the Discrete Element Method (DEM) is used to simulate the effect of particle shape on motion and mixing in this device. Corn seed is used as a model material and the effect of its shape on motion is analysed by considering two approaches: (1) manipulation of rolling friction to account for shape as it is commonly used in the field; (2) approximation of the actual shape by a number of overlapping spheres of various sizes. The geometry of corn seeds is captured using X-Ray micro tomography and then the ASG2013 software (Cogency, South Africa) is used to generate and optimise the arrangement of the overlapping spheres. A comparison is made of the predicted tangential and radial velocity distributions of the particles from DEM and those measured experimentally. It is concluded that for rapid shearing systems with short collisional contacts a small number of clumped spheres suffices to provide a reasonable agreement with experimental results. Equally well, manipulating the rolling friction coefficient can provide results that match experiments but its most suitable value is unknown a priori, hence the approach is empirical rather than predictive.
During agitated drying and mixing processes, particle beds are exposed to shear deformation. This leads to particle attrition, the extent of which is dependent on the prevailing stresses and strains in the bed. The distributions of shear stresses and strain rates within the bed are highly non-uniform, requiring attention to localised conditions. Therefore a narrow angular sector of the bed is divided radially and vertically into a number of measurement cells, within which the stresses and strain rates are calculated throughout one rotation by the Distinct Element Method. These are then used in an empirical relationship of material breakage to predict the extent of attrition due to agitation. Here we investigate the influence of the measurement cell size on the estimated stresses and strain rates, and the subsequent effect on the predicted attrition. The measurement cell size is altered by varying the measurement sector size and the number of radial and vertical divisions within it. The median particle size is also varied to establish its influence on the predicted attrition. An increase in the average number of particles in a given cell, by varying the particle size or measurement cell dimensions, leads to a reduction in the estimated stresses and strain rates, and therefore a reduction in the predicted attrition. Comparison of the predicted attrition with the experimental breakage in the agitated vessel shows that the prediction method is accurate when the cell dimensions are comparable to the width of a naturally occurring shear band.
The paper entitled ?Analysis of the Dynamics of the FT4 Powder Rheometer? contains calculation errors that lead to an underestimate of deviatoric stresses by a factor of ten and average compressive stresses by a factor of approximately three. The trends remain unchanged from those shown in the paper, with compressive stress and deviatoric stress both increasing approximately linearly with blade penetration depth. Here, the corrected values of compressive and deviatoric stress are given.
Measurement of the adhesive force is of great interest in a large number of applications, such as powder coating and processing of cohesive powders. Established measurement methods such as Atomic Force Microscopy (AFM) and the centrifugal method are costly and time consuming. For engineering applications there is a need to develop a quick test method. The drop test method has been designed and developed for this purpose. In this test method particles that are adhered to a substrate are mounted on and are subjected to a tensile force by impacting the stub against a stopper ring by dropping it from a set height. From the balance of the detachment force and adhesive force for a critical particles size, above which particles are detached and below which they remain on the substrate, the interfacial specific energy is calculated. A model of adhesion is required to estimate the adhesive force between the particles and the surface, and in this work we use the JKR theory. The detachment force is estimated by Newton?s second law of motion, using an estimated particle mass, based on its size and density and calculated particle acceleration. A number of materials such as silanised glass beads, Avicel, ±-lactose monohydrate and starch have been tested and the adhesive force and energy between the particle and the substrate surface have been quantified. Consistent values of the interface energy with a narrow error band are obtained, independent of the impact velocity. As the latter is varied, different particle sizes detach; nevertheless similar values of the interface energy are obtained, an indication that the technique is robust, as it is in fact based on microscopic observations of many particles. The trends of the results obtained with the drop test method are similar to those shown in studies by other researchers using established methods like the AFM and the centrifuge method.
Coating of particulate solids by a thin film layer is of interest in many industrial applications such as seed and tablet coating. In seed processing, seeds are commonly coated with a protective coating layer consisting of fertilisers and disease control agents, such as pesticides and fungicides. Batch coaters are commonly used for this purpose. A typical coater consists of a vertical axis cylindrical vessel with a rotating base and a spray disc in the centre, onto which the coating liquid is fed to atomise and spray-coat the seeds. The seeds are driven around the vessel by its rotating base, and are mixed by two baffles; one on either side of the vessel. In the present study, Distinct Element Method (DEM) simulations are used to model the seed coating process. Corn seed are used as a model material and their shape is captured using X-Ray Tomography (XRT), which is approximated in the DEM by clumped spheres. The coating uniformity of the seeds is predicted by implementing a coating model in the DEM, whereby the coating droplets are simulated as very fine spheres projecting tangentially from a ring at the edge of the spinning disk. The size and velocity of droplets leaving the spray disk are measured using high speed video imaging and implemented into DEM simulations. The coating mechanism is represented in the DEM by considering that once a droplet contacts a corn seed, it is removed from the simulation and its mass is attributed to the coating of the corn seed. The distribution of mass of sprayed spheres on the corn seeds and their coefficient of variation are evaluated for a range of process conditions, such as the base rotational speed, atomiser disc position relative to the base and baffle arrangement and designs. It is found that the atomiser disc vertical position, baffle angle and clearance to the wall are most influential, whilst the base rotational speed and baffle width and curvature have only minimal effect.
Radjai F, Dogbe S, Ghadiri M, Hassanpour A, Hare C, Wilson D, Storey R, Crosley I, Nezamabadi S, Luding S, Delenne J (2017) Fluid-particle energy transfer in spiral jet milling, EPJ Web of Conferences 140 pp. 09040-1
EDP Sciences: EPJ Open Access
Spiral jet milling is a size reduction process driven by the fluid energy of high velocity gas jets. Inter-particle and particle-wall interactions are responsible for size reduction. The process is energy intensive, but inefficient. The underlying mechanisms for size reduction in the mill are also not very well understood. The optimum grinding conditions are still currently found by trial and error experimentation.
In this work, the Discrete Element Method coupled with Computational Fluid Dynamics is used to investigate the effects of different parameters on the particle collisional behaviour in a spiral jet mill. These include the particle concentration in the grinding chamber, the particle size, and the fluid power input. We report on our work analysing the efficiency of energy transfer and how it can be improved by changing the milling conditions and particle properties.
The inability of cohesive powders to flow consistently and reliably is a major cause of process downtime and reduced efficiency across a wide range of powder processing industries. Most methods to assess powder flowability fail at low consolidation pressures (
Poor and inconsistent flow of cohesive powders is a major issue in powder processing. A common solution is to coat the surfaces of the cohesive particles with finer particles, referred to as flow-aids. Such particles adhere to sticky surfaces and act as spacers preventing them from contacting each other and thus reducing the inter-particle forces and bulk powder cohesion. A question which naturally arises is how much flow-aid is needed to enhance the flowability to an optimum level. This work aims to establish a relationship between the degree of Surface Area Coverage (SAC) of flow-aids and the flowability, the latter as determined by a quasi-static shear cell method, as well as the angle of repose test and the FT4 powder rheometer. Glass beads of 90-150 ¼m sieve cut are made cohesive by silanising their surfaces with a commercial chemical reagent, Sigmacote® and are used as host particles. Two types of zeolite particles are used as flow aids. The mass fraction of the flow aids required to achieve a theoretical SAC of 1, 5, 10, 20, 50 and 100% is first estimated and then the host particles are coated in a pan mixer. The SAC is measured by Scanning Electron Microscopy, coupled with image analysis, and found to correlate well with the estimated value. The optimum surface coverage is found to be when SAC is 10-20%, as this provides the greatest flowability. An increase in SAC beyond this range leads to a gradual reduction in flowability.
Tablets are the most common solid dosage form of pharmaceutical active ingredients due to their ease of use. Their dissolution behaviour depends on the particle size distribution and physicochemical properties of the formulation, and the compression process, which need to be optimised for producing consistently robust tablets, as weaker tablets are often prone to breakage during production, transport and end use. Tablet strength is typically determined by diametric compression and friability tests. The former gives rise to propagation of a crack on a plane along the compression axis, whilst the latter, carried out in a rotating drum, incurs surface damage and produces chips and debris. These tests produce different measures of strength, neither of which have been correlated with mechanical properties that are accountable for breakage, i.e. hardness, elastic modulus and fracture toughness. We propose a new method based on single tablet impact testing, following the work of Ghadiri and Zhang (2002), who analysed particle damage by propagation of sub-surface lateral cracks and identified the fundamental form accountable for impact surface damage to be a lumped parameter related to hardness and fracture toughness. Microindentation, carried out separately, to determine fracture toughness led to complete failure of the tablets, hence an unreliable measurement of fracture toughness and no correlation with the experimental trend. In addition, by assuming the fracture toughness to be proportional to the square root of Young?s modulus, the indentation measurements do not correlate well with the impact breakage. The discrepancy between the impact and indentation methods is expected to be due to mechanical property variation across the tablet surface, and with strain rate. The impact method is a more suitable test to describe tablet propensity for attrition as it directly represents the failure mode tablets may experience during processing under well-defined conditions. In contrast, the friability test subjects tablets to a similar breakage mechanism but under less well-defined conditions, whilst the compression test represents a different failure mode that is not representative of stresses incurred during processing.
Die filling is a critical process step in pharmaceutical tablet manufacturing. Mass and content uniformity of the tablets as well as production throughput depend upon the die filling performance of the formulations. The efficiency of the die filling process is influenced by powder properties, such as flowability, cohesion, particle size and morphology, as well as the process conditions. It is hence important to understand the influence of powder properties on the die filling performance. The purpose of the present study is to identify the critical material attributes that determine the efficiency of die filling. For this purpose, a model rotary die filling system was developed to mimic the die filling process in a typical rotary tablet press. The system consists of a round die table of 500/mm diameter, equipped with a rectangular die. The die table can rotate at an equivalent translational velocity of up to 1.5/m/s. The filling occurs when the die passes through a stationary shoe positioned above the die table. Using this system, die filling behaviours of 7 commonly used pharmaceutical excipients with various material characteristics (e.g. particle size distribution, sphericity and morphology) and flow properties were examined. The efficiency of die filling is evaluated using the concept of critical filling velocity. It was found that the critical filling velocity is strongly dependent on such properties as cohesion, flowability, average particle size and air sensitivity index. In particular, the critical filling velocity increases proportionally as the mean particle size, flow function, air permeability and air sensitivity index increase, while it decreases with the increase of specific energy and cohesion.
Die filling is an important process step in manufacturing of
tablets. An important mechanism in the die filling process is suction
that is developed with the downward motion of the bottom punch once the
die is covered by powder. However, the contribution of suction to the die
filling performance is still poorly understood. Hence, the present study
aimed to experimentally investigate the flow behaviour of powders during
suction filling. Four different types of pharmaceutical powders were used
and a model suction filling system was developed. Effects of filling and
suction velocities, as well as powder properties, on the efficiency of
die filling were systematically investigated. Cohesive and free-flowing
powders behaved differently at various filling-to-suction velocity
ratios. The filling behaviour of cohesive powders was improved at high
filling-to-suction ratios due to acceleration-induced densification.
Free-flowing powders performed better at low filling-to suction ratios.
The present study investigated particle size-induced segregation during die filling of binary pharmaceutical blends, consisting of fine and coarse particles in various fractions. Coarse fraction was made of milled and sieved acetylsalicylic acid, whereas the fine fraction was mannitol. The die filling process was carried out in gravity filling and suction filling. The segregation was assessed through determination of the coarse component concentration using UV?Visible spectrophotometry. The obtained values of concentration, determined for ten units
of identical volume inside the die, were used to calculate the segregation index (SI), which was an indicator of uniformity of the powder blend deposited into the die. It was found that high segregation tendency was generally observed during gravity filling at a low velocity, due to the effect of air drag, and during gravity filling at a high velocity, as it was carried out through three consecutive filling steps. The lowest segregation tendency was generally observed during suction filling at a low velocity. The horizontal segregation was mostly observed in
the top layers of the die, due to mainly two mechanisms: coarse particles cascading down the heap formed by the powder in the final steps of die filling, which produces higher coarse concentration at the near side of the die, observed at low coarse concentration; or coarse particle cascading down the top surface of the flowing powder stream into the die, which increases the coarse concentration at the far end of the die.
The inability of cohesive powders to flow consistently and reliably is a major cause of process downtime and product wastage across a wide range of powder processing industries. Extensive work has been carried out characterising powder flowability using a wide array of techniques, with the most established method of powder flow measurement at moderate to high stresses being shear cells, with theories developed for silo and hopper design. Many processes of great industrial interest though, such as filling and dosing of powders in capsules and dispersion in dry powder inhalers (DPI), expose particles to very low consolidation stresses (d 1 kPa), at which the determination of unconfined yield strength by shear testing is often marred with inconsistencies in the measurement, or in comparison to the observed behaviour. Therefore, there is a need for an alternative approach in order to develop an understanding of powder flow for weakly consolidated powders, such as ball indentation, which is a penetration technique capable of assessing powder flowability down to very low stresses, whilst requiring only a very small amount of material. In this technique, a powder bed is consolidated to any desired stress that provides a reasonably flat surface, and then penetrated by a spherical indenter. The flowability is determined by calculating the hardness of the powder bed, from the force-displacement response of the bed. Hardness can be linked to the unconfined yield strength, commonly derived by shear testers, via the constraint factor, which is dependent on particle properties, although cannot yet be determined a priori.
In this work, the constraint factor of a broad class of powders was quantified from indentation and shear cell experiments at moderate to high stresses and was found to be generally independent of the applied stress, while for four out of the twenty-five materials it exhibited fluctuations. In order to infer the unconfined yield strength from hardness measurements at low stresses, it was assumed that the constraint factor remains constant at lower stresses. Distinct Element Method (DEM) modelling was also utilised to simulate the ball indentation system, allowing the powder bed internal failure stresses to be realised in order to elucidate the behaviour of the constraint factor at low stresses. The simulations validated the assumption that the constraint factor remains constant throughout the applied stress range.
Furthermore, the applicability and reliability of both ball indentation and the FT4 shear cell were assessed in a wide range of both low and moderate to high stresses. Ball indentation gave very repeatable results throughout the whole range of stresses applied, whilst the FT4 shear cell was deemed unreliable for most materials at pre-shear normal stresses of 1 kPa and below. For all materials except the three powders that remain very cohesive throughout the stress range tested, the increase of hardness (and therefore also the unconfined yield strength inferred from ball indentation) with stress was observed to be much steeper at low stresses, as compared to higher stresses, due to a more rapid increase in packing fraction. For all model glass beads tested, except for the 0 - 20 ¼m samples, hardness was found to be independent of penetration depth in a certain depth range. In contrast, for most ?real? materials, plus the aforementioned very fine model glass beads, hardness was found to continually increase with depth, with a gradient that is independent of the applied stress and similar for all materials tested. The powders that are prone to stick-slip deviated from the above behaviour and exhibited a fluctuating force response.
The influence of a variety of particle properties on the constraint factor, and subsequently powder flowability, was also determined both experimentally and computationally. The effects of particle size, size distribution, and single particle and agglomerate shape were investigated exper
Ball indentation is a technique capable of assessing powder flowability down to very low consolidation stresses (d1 kPa). With this method, powder flowability is determined by measuring the hardness of a powder bed, which allows the unconfined yield strength to be inferred via the constraint factor. The latter is well established for continuum materials, whereas for particulate systems its dependency on stress level and powder properties is not well defined. This work investigates these factors by simulating the ball indentation method using DEM. The constraint factor is shown to be independent of pre-consolidation stress. Constraint factor generally increases with interface energy for relatively cohesion-less powders, though not for cohesive powders. An increase in plastic yield stress leads to a decrease in the constraint factor. Increasing the coefficient of interparticle static friction reduces the constraint factor, while increasing the coefficient of inter-particle rolling friction significantly increases the constraint factor.
In industrial processing and manufacturing, characterizing the flowability of particulate solids is of particular importance both for reliable powder flow and for a consistent production rate. Shear testing is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions. However, this method is not suitable for measuring the powder flow properties occurring in dynamic systems, such as powder mixers and screw conveyors. In this study, the rheological behaviour of powders at high shear rates has been evaluated by the ball indentation method. The technique, which simply consists of dropping a ball onto a cylindrical bed of previously consolidated powder, directly measures the material hardness, which is related to the unconfined yield stress by the constraint factor. The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The hardness against the strain rate is considered for four different materials. Because of their difference in particle size, and by using a range of drop heights and a range of indenter densities, the intermediate regime of flow has been fully analyzed. Although hardness is constant in the quasi-static condition, it results to be strain rate dependent in the intermediate regime of flow. Finally, a predictive correlation that allows the operator to choose the best operating conditions for achieving the desired flow regime is proposed, and the unconfined yield strength of the materials is inferred.