Dr. Andrea Santomaso is active in research at the University of Padova where he directs the APTLab (Advanced Particle Technology Laboratory). His research activities cover particulate and multiphase systems and are focused on: - powder mixing/de-mixing problems - wet agglomeration processes - powder mechanics (dense granular flows) - flowability and wettability characterization - on-line characterization. He is member of the EFCE Working Party on the Mechanics of Particulate Solids since 2012.
Research Items (54)
- Sep 2018
Mixing grains with different properties is a remarkably challenging process, relevant to many industrial applications. Rotating drums have been used extensively as model systems to study granular media flow and mixing and segregation. Numerous studies considered the mixing of grains with different sizes, but only few studies considered shape and elongation, which have been already identified as important characteristics affecting packing and inducing segregation. In this contribution, the mixing of binary mixtures of grains having the same volume, but different elongations was studied experimentally. The mixing dynamics of a layered granular medium was characterized in a rotating drum, highlighting the impact of the grain shape, of the drum angular speed and of the drum filling ratio. A mixed or segregated state is reached very rapidly, but experiments are continued to verify that the state reached is a steady state. The experiments demonstrated that below a critical difference of elongation, the grains can be mixed effectively. Conversely, when grain elongations are very different, a central core is formed, rich in the more elongated grains. In this case only limited mixing can be achieved. These results can guide the formulation of mixtures of grains with different elongations.
- Jul 2018
Dry mixing is a common operation in many industrial processes. The unloading of the mixer is an essential aspect of the process which actively contributes to the final quality of the mixture. Its effect however is often underestimate or even ignored. In the present work the consequences of the unloading process on mixture quality were investigated in three different types of tumbling mixers: a symmetric double cone, an asymmetric double cone and a conic mixer were considered. The symmetric double cone mixer is a standard geometry in the pharmaceutical industry, while the other two geometries are unconventional. The effects of mixer geometry on the mixture quality were studied and related to differences in the unloading flow patterns. A segregating polydispersed mixture was used during the experiments in order to emphasize the different performances of the three tumblers. Additional quasi-2D experiments and FEM simulation of the unloading process clearly showed the ability of the unconventional geometries to mitigate the effect of segregation improving the mixture quality evaluated at the outlet of the mixer.
- Jun 2018
It is well known that even small amounts of liquid can strongly modify the mechanical behavior of granular packings in static and dynamic conditions. This experimental work, therefore, focuses on the collapse of columns of wet granular materials in the pendular wetting regime. Different from previous studies, where idealized spherical materials (glass beads) are typically used, here experiments on irregular wet calcium carbonate particles (coarse sand) were carried out and compared with glass sphere results. Particles of different sizes (in the range 0.8-5 mm) were mixed with water from 0% to 4% w/w and poured in a rectangular box. Flow was then triggered by removing a lateral wall of the box. The measured runout distances showed marked differences between the two types of materials which could not be explained only in terms of particle shape or capillary forces. Ring shear tests and 3D tomographic reconstructions of the liquid distribution in the materials highlighted the role of additional mechanisms related to liquid spreading at the particle surface, inter-particle friction, and contact lubrication.
- Jan 2018
- 28th European Symposium on Computer Aided Process Engineering
In this work, a methodology is presented to develop an artificial vision system to characterize the particle size distribution of granular products in motion on a conveyor belt. This methodology exploits the wealth of information stored in videos of the bulk material and, in particular, the information on the particle size distribution is extracted by multivariate and multiresolution image texture analysis. The method is applied to the case of a granulated microcrystalline cellulose directly discharged from a hopper on a conveyor belt. The effectiveness of the presented methodology to estimate the particle size distributions is demonstrated in terms of estimation accuracy, velocity, reliability, and non-invasiveness of the system.
- May 2017
Sugars and sweeteners are common ingredients in foods and also in pharmaceutical industries. They are soluble and sticky ingredients and their processing in granulators may be difficult since they can easily adhere to mixer walls or lead to uncontrolled granule growth. The purpose of this research was to evaluate the feasibility of the high shear wet granulation process and to find the optimal amount of binder by studying the granulation performances of four sugars: mannitol, sorbitol, xylitol and sucrose in different amounts. Sugar interactions with the liquid binder and solid carrier were carefully characterized. The increased viscosity of the liquid bridges, due to the partial solubilization of the sugars, was found to play a major role in the agglomeration process since it controls the liquid mobility and its distribution. Experiments demonstrate the feasibility of the process and the ability of mixer torque rheometer to predict the optimal binder amount.
- Feb 2017
This study shows how to develop a fast, reliable, and non-invasive artificial vision system to quantitatively estimate the particle size distribution of granular products. The system, based on multivariate and multiresolution texture analysis, uses digital images of the bulk material to extract quantitative information on the particle size ranges appearing in each image and on their weight proportion independently of the shape of the particle distribution (mono- or multi-modal). The method is applied to a wet-granulated product (namely, microcrystalline cellulose), and it is shown that the size distributions can be estimated accurately. The system performance is discussed in the light of an application in the automated monitoring of particle size distribution in industrial processes.
- Jan 2017
A simulation study was carried out on the free surface granular flow occurring in partially filled tumbler mixers. Three different 2D geometries were considered: a drum, a double cone mixer and an asymmetric double cone mixer. The solid-air two phases flow was simulated using an Eulerian continuum approach, including a suitable rheology for the granular flow. We focused on the kinematics of the active layer where mixing occurs. The modification of the surface active layer, as a consequence of mixer geometry and rotation, was studied. The analysis of (a) the active layer depth and area, (b) the velocity field and (c) the level of granular activation (given by granular temperature) suggested that the less conventional geometry, i.e. the asymmetric double cone mixer, has the potential to give better mixing performances than the traditional tumblers.
- Nov 2016
Near infrared spectroscopy is proposed as a method for continuous characterization of ground oilseed concentration in diluted solid liquid dispersions. The effect of operational parameters, such as flow rate and temperature on the stability and reliability of the in-line measurement was investigated. Moreover, in order to widen the applicability of the approach to different materials, the variety of oilseed, the particle size distribution, and the chemistry of the suspending liquid carrier were all changed, and their influence assessed through principal component analysis. Simple multivariate techniques were efficiently used to regress spectral features against known values of dispersed solids concentration. The calibration, obtained after internal cross validation, proved to give values of coefficient of determination always higher than 0.95, ratio of performance to deviation higher than 4, and comparably reduced root mean squared error of cross validation. Instantaneous concentration prediction were characterized by oscillations much dependent on the dispersed solids particle size, being their amplitude lower in the case of finer particles. Those oscillations were characterized through the coefficient of variation that, at steady state, gave values small enough to constitute an effective parameter to tackle mixing end-point, hence constituting a novel way to assess homogeneity during powder incorporation in liquids.
- Apr 2016
In this paper we present a model to predict the onset and evolution of segregation during the discharge of binary mixtures of granular materials. The model accounts for the multi-phase and multi-component nature of the granular mixtures, to simulate the main flow regimes occurring in the discharge of silos (funnel and mass flow) and how they affect segregation. The new comprehensive model for segregation follows a continuum Eulerian approach and results from the coupling between an ad-hoc rheology for granular flow and a percolation model for multi-component mixtures. Predictions are compared with independent literature experimental data, for short and tall silos and prove to be quite accurate, after a tuning of the percolation flux sub-model. The larger segregation in short flow paths with smaller amount of fines reported by the experiments is quantitatively predicted. The model also predicts the three phases observed in experiments during the discharge of tall silos.
- Aug 2015
We quantitatively studied the advantages of improving the compaction of a powder waste by several techniques, including its pelletization. The goal is increasing the mass storage capacity in a given storage volume, and reducing the permeability of air and moisture, that may trigger exothermic spontaneous reactions in organic waste, particularly as powders. The study is based on dried sludges from a wastewater treatment, mainly from tanneries, but the indications are valid and useful for any waste in the form of powder, suitable to pelletization. Measurements of bulk density have been carried out at the industrial and laboratory scale, using different packing procedures, amenable to industrial processes. Waste as powder, pellets and their mixtures have been considered. The bulk density of waste as powder increases from 0.64t/m(3) (simply poured) to 0.74t/m(3) (tapped) and finally to 0.82t/m(3) by a suitable, yet simple, packing procedure that we called dispersion filling, with a net gain of 28% in the compaction by simply modifying the collection procedure. Pelletization increases compaction by definition, but the packing of pellets is relatively coarse. Some increase in bulk density of pellets can be achieved by tapping; vibration and dispersion filling are not efficient with pellets. Mixtures of powder and pellets is the optimal packing policy. The best compaction result was achieved by controlled vibration of a 30/70wt% mixture of powders and pellets, leading to a final bulk density of 1t/m(3), i.e. an improvement of compaction by more than 54% with respect to simply poured powders, but also larger than 35% compared to just pellets. That means increasing the mass storage capacity by a factor of 1.56. Interestingly, vibration can be the most or the least effective procedure to improve compaction of mixtures, depending on characteristics of vibration. The optimal packing (30/70wt% powders/pellets) proved to effectively mitigate the onset of smouldering, leading to self-heating, according to standard tests, whereas the pure pelletization totally removes the self-heating hazard. Copyright © 2015 Elsevier Ltd. All rights reserved.
- Apr 2015
Improvements of the oral bioavailability of lipophilic drugs can be obtained using lipidic formulations such as the self-emulsifying drug delivery systems. The high shear wet granulation (HSWG), using microemulsions as binder, is a viable process to produce self-emulsifying granules. However only few information are present in the literature on the effect of process variables on the properties of the granules obtained with these binders. Consequently, this article compares the effects of some relevant experimental variables (impeller speed and massing time) on the final technological and pharmaceutical properties of the granules produced using simple water, or alternatively, a microemulsion as binder and containing simvastatin (SV) as model drug. The effects of the variables were determined by evaluating the granule median diameter, their particle size distribution, roundness, disintegration time and dissolution rate of SV. Results clearly demonstrated that the microemulsion-based process was less sensitive to operating conditions than the water-based process. With microemulsion the nucleation process and growth regimes were more difficult to control, resulting in products with broader PSDs. At the same operating conditions microemulsion-based granules were more brittle but rounder and showed smaller median diameter compared to water-based granules. The dissolution rate of simvastatin was not significantly affected by the operating conditions.
- Mar 2015
In this work wet granulation experiments were carried out in a planetary mixer with the aim to develop a novel analytical tool based on surface texture analysis. The evolution of a simple formulation (300g of microcrystalline cellulose with a solid binders pre-dispersed in water) was monitored from the very beginning up to the end point and information on the kinetics of granulation as well as on the effect of liquid binder amount were collected. Agreement between texture analysis and granules particle size distribution obtained by sieving analysis was always found. The method proved to be robust enough to easily monitor the process and its use for more refined analyses on the different rate processes occurring during granulation is also suggested. Copyright © 2015. Published by Elsevier B.V.
- Oct 2014
Mixer agglomeration, in particular high shear wet granulation, is a unit operation typically used in the pharmaceutical industry to improve the flowability, the compressibility, the dosing accuracy during tableting or the content uniformity of a blend. Thanks to its advantages (production of spherical and dense granules, reduction of production time), this technique can be potentially successful also in the food industry as for example in the production of dietary supplements. In this work four thickening agents (povidone, maltodextrin, k-carrageenan and xanthan gum) have been tested to study their effects on the granule growth behavior and on some technologically relevant granule properties (size, shape, strength and flowability). Experiments highlighted the full feasibility of the process and the possibility of using these agents to get products with satisfactory technological properties. The dependence of product properties on the formulation variables (water and binder amount) has been analyzed according to a multivariate approach and a robust predictive tool for the granule size has been developed. Furthermore it was observed that a reduced amount of binding liquid (water) can be used in the presence of strongly thickening binders with a reduction up to 25%. This would decrease drying time and energy requirement and be beneficial especially in the food and food supply industry where products have generally lower added value than in the pharmaceutical one and reducing production costs is critical.
Small quantities of liquid in a granular material control the flow dynamics as well as the triggering and jamming phases. In order to study this problem, some experimental collapse tests conducted in a rectangular box were reproduced with a 1:1 scale numerical model using the Discrete Element Method. In simulations the effect of the capillary bridges has been investigated implementing a mid-range attractive force between particles based on the minimum energy approach. Also a bonding-debonding mechanism was incorporated in the algorithm and the volume of each sessile drop on the particle surface was considered during its motion. The influence of some variables was investigated with respect to the final slope profiles and the runout lengths: the initial liquid content, the particle size, the solid density, the liquid surface tension, and the liquid-solid contact angle. Also the crucial effect of the confinement walls on the collapse phenomenon was investigated: wet particles adhere to the lateral walls providing a higher flow resistance in comparison to the same material in dry conditions. It was observed that particles with largest path-lengths are localized near the movable wall at a middle-height of the initial column sample. Other particles at the surface moves in a rigid way especially if they were wet and with a low solid density. The “fidelity” of each particle with respect to the nearest neighbours was evaluated allowing to recognize the emergence of clusters of particles and rigid parts, to extract the failure surface and to localize where debonding mechanisms concentrate in the wet case.
- Sep 2013
Wettability is an important property involved in the industrial use of granular solids and powders. It is commonly described with the contact angle and an experimental method for its determination in dynamic conditions is proposed in this work. The method is based on the capillary rise of the wetting liquid into a packed bed of the material under analysis. Differently from the classical Washburn method, the packed bed is closed to the atmosphere and the air pressure increase is measured allowing to evaluate the powder contact angle through a dynamic balance of the pressure forces. In the expression of such forces a new equivalent capillary radius for the powder bed is used based on an alternative definition of the particle equivalent diameter. This diameter is closely related to the length of the three phase line which divide the wet portion of the bed from the dry one and mirrors the physics of wetting process better than the classical Sauter diameter. A way to determine it with optical microscopy is given. Also the measure of the packed bed porosity (entering in the equivalent capillary radius definition) has been improved by using the effective porosity concept [Hapgood et al., J. Coil. Interface Sci. 253(2002)353-366] and by modifying the way of estimating it. The proposed experimental technique, coupled to the theoretical model for the packed bed, can describe accurately the packed bed geometry and the wetting dynamics by following the changes of the contact angle from its initial maximum value up to the final equilibrium one.
In this paper the results from simulations performed using a hydrodynamic model proposed by Artoni et al. [Chem. Eng. Sci. 64 (2009a), 4040-4050] have been compared with published data of an extensive experimental investigation carried out at the Tel-Tek Research Institute in Porsgrunn, Norway. The experiments collected several data and observations on the wall stresses and the flow patterns observed during discharge of a full-scale funnel flow silo with and without inserts. The comparison between simulation and experiments showed the ability of the model to capture quantitatively the main features of both the flow and of the wall stress profiles when flow corrective inserts are put in the hopper of the silo in order to convert the discharge regime to a mass flow regime. Moreover information such as the stresses on the internals, which are difficult or impossible to get experimentally, have been collected from the simulations and discussed
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 (1.9 l vessel volume) to that of a pilot-scale mixer (65 l 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.
- Apr 2013
In this paper, the process of seeded granulation in a high shear mixer is simulated by the discrete element method (DEM). A 5 litre Cyclomix granulator manufactured by Hosokawa Micron B.V. was simulated at different impeller rotational speeds. It has been observed that the seeded granules form by a continuous growth and reduction in size during the granulation process. Quantitative analysis shows that in general a higher number of seeded granules form at lower impeller rotational speeds; however it is found that for all the seeded granules the seed surface coverage by fines is from 5% to 60%. Further analyses revealed that seeded granules with the seed surface coverage higher than 50% are more frequently formed at high impeller rotational speeds. The work demonstrates the capability of DEM for modelling granulation processes, as a tool to explore the underlying mechanisms of granulation in general and seeded granulation in particular.
- Mar 2013
Axial segregation in horizontal rotating drums is studied focusing on the effect of wall roughness and geometry. The problem is tackled both numerically and experimentally. Simulations are based on a cellular automata (CA) approach using rules derived from experiments and the experimental validation of the simulated phenomenology is given for noteworthy cases. Notwithstanding the simple model used to implement CA simulations, good qualitative agreement with experiments is observed and many features of the segregation process are captured. These include the major role of fluctuations of concentration on the onset of axial segregation, the overall exponential decrease of the number of segregated bands due to their progressive merging and the long lasting character of the segregation dynamics. Simulations and experiments also suggest a simple but effective strategy to reduce axial segregation which can be potentially used to contrast segregation in industrial mixing of free-flowing powders.
This paper deals with the experimental characterization of the collapse of wet granular columns in the pendular state, with the purpose of collecting data on triggering and jamming phenomena in wet granular media. The final deposit shape and the runout dynamics were studied for samples of glass beads, varying particle diameter, liquid surface tension, and liquid amount. We show how the runout distance decreases with increasing water amount (reaching a plateau for w>1%) and increases with increasing Bond number, while the top and toe angles and the final deposit height increase with increasing water amount and decrease with decreasing Bond number. Dimensional analysis allowed to discuss possible scalings for the runout length and the top and toe angles: a satisfying scaling was found, based on the combination of Bond number and liquid amount.
- Aug 2012
Characterization of powder wettability is a prerequisite to the understanding of many processes of industrial relevance such as agglomeration which spans from pharmaceutical and food applications to metallurgical ones. The choice of the wetting fluid is crucial: liquid must wet the powder in order for agglomeration to be successful. Different methods for wettability assessment of powders were reported in the literature, however the sessile drop method and capillary rise test remain among the most widely employed because they are easy to perform and inexpensive. In this paper, the application and limitations of sessile drop method and capillary rise test on mineral and metallic surface were discussed. This work provides a collection of wettability measurements using several powders and binders which are involved in the manufacturing process of welding wires. Moreover a new reference liquid for the calibration of capillary rise method was proposed.
In this Letter, the two-dimensional dense flow of polygonal particles on an incline with a flat frictional inferior boundary is analyzed by means of contact dynamics discrete element simulations, in order to develop boundary conditions for continuum models of dense granular flows. We show the evidence that the global slip phenomenon deviates significantly from simple sliding: a finite slip velocity is generally found for shear forces lower than the sliding threshold for particle-wall contacts. We determined simple scaling laws for the dependence of the slip velocity on shear rate, normal and shear stresses, and material parameters. The importance of a correct determination of the slip at the base of the incline, which is crucial for the calculation of flow rates, is discussed in relation to natural flows.
- Nov 2011
We extended the standard approach to countercurrent gas–solid flow in vertical vessels by explicitly coupling the gas flow and the rheology of the moving bed of granular solids, modelled as a continuum, pseudo-fluid. The method aims at quantitatively accounting for the presence of shear in the granular material that induces changes in local porosity, affecting the gas flow pattern through the solids. Results are presented for the vertical channel configuration, discussing the gas maldistribution both through global and specific indexes, highlighting the effect of the relevant parameters such as solids and gas flowrate, channel width, and wall friction. Non-uniform gas flow distribution resulting from uneven bed porosity is also discussed in terms of gas residence time distribution (RTD). The theoretical RTD in a vessel of constant porosity and Literature data obtained in actual moving beds are qualitatively compared to our results, supporting the relevance under given circumstances of the coupling between gas and solids flow.
The purpose of this research was to determine the effects of some important drug properties (such as particle size distribution, hygroscopicity and solubility) and process variables on the granule growth behaviour and final drug distribution in high shear wet granulation. Results have been analyzed in the light of widely accepted theories and some recently developed approaches. A mixture composed of drug, some excipients and a dry binder was processed using a lab-scale high-shear mixer. Three common active pharmaceutical ingredients (paracetamol, caffeine and acetylsalicylic acid) were used within the initial formulation. Drug load was 50% (on weight basis). Influences of drug particle properties (e.g. particle size and shape, hygroscopicity) on the granule growth behaviour were evaluated. Particle size distribution (PSD) and granule morphology were monitored during the entire process through sieve analysis and scanning electron microscope (SEM) image analysis. Resistance of the wet mass to mixing was furthermore measured using the impeller torque monitoring technique. The observed differences in the granule growth behaviour as well as the discrepancies between the actual and the ideal drug content in the final granules have been interpreted in terms of dimensionless quantity (spray flux number, bed penetration time) and related to torque measurements. Analysis highlighted the role of liquid distribution on the process. It was demonstrated that where the liquid penetration time was higher (e.g. paracetamol-based formulations), the liquid distribution was poorer leading to retarded granule growth and selective agglomeration. On the other hand where penetration time was lower (e.g. acetylsalicylic acid-based formulations), the growth was much faster but uniformity content problem arose because of the onset of crushing and layering phenomena.
New sampling probes and methods for investigating cohesive powders are conceived, designed and characterized. Probes are made of two metallic shells (a slide and a cover) which need to be inserted sequentially into the bed of powder in order to extract representative samples. The thin profile of the shells, combined with a particular insertion procedure, is intended to minimize stresses on the powder; thereby reducing both the invasiveness and the dragging of material through the bed. Probes of similar design with different shape and size have been tested on stratified beds of cohesive powders of different colors. Sampling performances are quantitatively compared among different probes (for size and shape) and also with literature data. The comparison has indicated that the new sampling devices effectively improved sampling efficiency, reliability and possibilities. The simple construction and use suggest they can be viable and effective alternatives to traditional probes for cohesive mixtures.
- Jun 2011
- May 2011
A hydrodynamic model for dense granular flows, previously developed for confined flows, has been extended to address free surface flow down an inclined chute. Results show that the model can predict the existence of two critical inclination angles, namely, the avalanche starting angle θ(start) above which the granular bed begins flowing from an initially jammed configuration, and an avalanche stopping angle θ(stop), which is the minimum to maintain flowing conditions, in agreement with experiments and numerical simulations available from the literature. The dependence of these critical angles on the bed depth is also analytically formulated, reflecting the expected qualitative behavior. Such a hysteretic behavior is specific of granular flow and its prediction provides indications of consistence of the modeling approach. The improved model also captures the scaling of the velocity profiles down the bed depth.
- Feb 2011
A comparison of the predictions of a rheological model that we recently developed with experimental results of stress and flow profiles in a pilot scale silo is presented in this work. Experiments were performed to collect information on the flow field by means of a tracer method and on wall normal stresses at several different positions along the vessel. The silo (2.5 m high, 0.5 m wide) had the possibility of inserting internal devices; the model was first validated on data without internals and then used to predict the profiles for the case with them. Both stress and flow profiles with and without internals agree with the experimental results within the experimental error that locally could be rather significant due to the difficulty of large scale experiments with granular materials.
High-shear wet granulation is commonly used in many industries such as in the pharmaceutical industry to convert fine cohesive powders into dense and round granules. The purpose of this work was to determine the effects of some important powder properties (crystalline or amorphous nature, hygroscopicity, solubility and particle size) and process variables (liquid addition rate, impeller speed) on the early stages of the granulation process and on drug distribution in granules obtained by high-shear wet granulation. The glass transition concept coupled with on-line impeller torque monitoring and measurements of the time evolution of the particle size distribution were used to study mixtures of pharmaceutical excipients and some common active ingredients. In particular a formulation map for estimating the minimum amount of liquid binder required to induce appreciable granule growth is presented, thus outlining a new method to considerably increase the predictability of the behaviour of different formulations on the basis of the physical properties of each single component. The description of the effects of the wetting condition on drug uniformity content in some formulations with hydrophobic active ingredients is given as well.
- Jan 2011
This work aims at investigating the effect of triggering and jamming due to the addition of a small quantity of fluid to the material. Collapse of dry and wet granular columns is studied both from the experimental and the numerical point of view. Wet samples of glass beads of different grain-sizes in the pendular state were packed in a rectangular box and then allowed to flow by removing a lateral wall. The dependence of the kinematics and the final state of the system on grain size and water content was particularly investigated. DEM numerical simulations were carried out in a 1:1 scale. A good qualitative agreement between experiments and DEM simulations was found with respect to the kinematic and the final slope profile. In particular, both the techniques highlight the strong effect of the liquid which decreases the run-out distance and time even for small liquid contents. This work demonstrates the suitability of the DEM approach also for the study of wet granular materials in static as well as in dynamic conditions, however it highlights that the water redistribution model is critical for the model outcome.
- Nov 2010
- 2010 AIChE Annual Meeting
Scale-up of high-shear wet granulators is currently difficult to perform and there are no universally applicable scale-up rules. It is not completely clear how changes in granulator geometry and processing conditions can affect the granule growth . An investigation on the scale-up effects on the growth behaviour in high-shear wet granulation is here described. Firstly, a mixture of some common-used pharmaceutical powders was processed using a top-driven, small scale granulator (2 l vessel volume). The processing conditions were systematically changed in order to arrange reproducible liquid dispersion conditions and nucleation regimes . Secondly, the same powder mixture was processed using a pilot plant scale, bottom-driven granulator (65 l vessel volume). The dimensionless spray flux concept was used in order to keep the same liquid dispersion conditions . The Focused Beam Reflectance Measurement Probe (FBRM) was used in combination with other particle size distribution measurement techniques (e.g. sieve analysis, laser diffraction methods) in order to carry out a detailed analysis of the growth behaviour. As already demonstrated by Cavinato et al. , this technique presents a good degree of accuracy and reproducibility. Results clearly showed the effect of the change in the granulator geometry and scale on the liquid distribution and, thus, the growth kinetics. A modification of the dimensionless spray flux number was furthermore proposed in order to best describe the phenomena and improve the predictability of the method.  P. R. Mort, Scale-up of binder agglomeration processes, Powder Technology 150 (2005) 86-103.  J.D. Litster, K.P. Hapgood, J.N. Michaels, A. Sims, M. Roberts, S.K. Kameneni, T. Hsu, Liquid distribution in wet granulation: dimensionless spray flux, Powder Technology 114 (2001) 3239.  M. Cavinato, D. Kayrak-Talay, J.D. Litster, Predicting the growth kinetics based on the formulation properties in high shear wet granulation, World Congress on Particle Technology, Nuremberg 25-29 April 2010.
The effects on granule shape of binders of different viscosities have been compared in the high shear wet granulation process. Water and different emulsions were used as liquid binders. The observed differences in shape have been explained in terms of the granule growth regime map and show that it is easier to control the shape of granules obtained using emulsions as binder. Moreover, evidences have been collected showing that high shear wet granulation is a viable solution for solid self-emulsifying drug delivery systems.
With a view to describing the powder agglomeration process, particles have often been considered as inert material and the solid–liquid interactions have rarely been contemplated.The present research aims to fill the gap in understanding how the nucleation and the early stage of the granule growth depend on some important formulation properties.The glass transition concept coupled with on-line impeller torque monitoring and measurements of the time evolution of the particle size distribution was used to give a description of the early stage of the agglomeration process in high shear wet granulation. A mixture of commonly-used pharmaceutical powders, which are amorphous and crystalline in nature, was processed.Accordingly, a new formulation map is presented which describes the onset of significant granule growth as a function of the key formulation components (i.e. diluent, dry and liquid binder).From this map, the minimum amount of liquid binder required to induce appreciable granule growth is determined as a function of the type, quantity, hygroscopicity and particle size distribution of the diluent and the solid binder. This map can be obtained from a priori glass transition measurement using a static humidity conditioning system and by fitting the experimentally obtained data using a modified Gordon–Taylor equation.
- Dec 2009
An alternative procedure for achieving formulation design in a high-shear wet granulation process has been developed. Particularly, a new formulation map has been proposed which describes the onset of a significant granule growth as a function of the formulation variables (diluent, dry and liquid binder). Granule growth has been monitored using on-line impeller torque and evaluated as changes in granule particle size distribution with respect to the dry formulation. It is shown how the onset of granule growth is denoted by an abrupt increase in the torque value requires the amount of binder liquid added to be greater than a certain threshold that is identified here as 'minimum liquid volume'. This minimum liquid volume is determined as a function of dry binder type, amount, hygroscopicity and particle size distribution of diluent. It is also demonstrated how this formulation map can be constructed from independent measurements of binder glass transition temperatures using a static humidity conditioning system.
- Nov 2009
- 2009 AIChE Annual Meeting
Controlling granule shape may be desirable for many reasons; among these are for example the flow properties. A spherical shape possesses a minimum surface area to volume ratio resulting in reduced cohesive forces and mechanical interlocking thereby resulting in an improved flowability of the bulk powder. Obtaining spheronisated shape is a desired prerequisite also when a subsequent coating or drug layering of the granules is necessary. The advantage of wet granulation process in high shear mixer is that mixing, massing and granulation are performed in few minutes in the same equipment. However the high shear wet granulation (HSWG) process does not always warrant spheronisated granules. The process needs to be controlled with care as the granulation progresses so rapidly that a usable granule can be transformed very quickly into an unusable system. The focus of this paper is therefore on the influence of process parameters on granule shape. Two different systems have been considered and results compared. Both systems include a fixed mixture of Microcrystalline Cellulose and Monohydrate Lactose as powder mass but two different binder were considered. In the first case water was considered whereas in the second case an oil-in-water emulsion was used as liquid binder in order to produce a solid self-emulsifying systems. All experiments were performed in the same vertical, bottom driven high shear granulator. The results obtained through DOE plus data statistical analysis showed that some of the process variables studied (impeller speed, granulation time, amount of initial powder and of binder) were able to induce different granule shapes. In particular a clear dependence on the binder quantity and on its nature has been observed and two main behaviours were individuated. Final structure of granules depended on structural changes which occurred during the granulation process and these were determined by the rheology of the wet bulk. Granules must be strong enough to resist the forces in the granulator, and deformable enough to generate spherical granules. Using an excess of liquid both binders produced a loss of roundness. Different surface characteristics were also observed. In particular using water, pellets resulted smaller, more rough ad with irregular shape. Composition of the second system containing an oil-in-water emulsion produced longer, more flat and smooth aggregates. These differences have been quantified using various shape factors and explained in terms of MCC granulation mechanisms where phenomena such as powder wettability, binder adsorption/desorption, intra- and inter-particle lubrication play a major role. Measures of granules strength were used in addition to corroborate our conclusions.
- Sep 2009
A model to simulate the dense flow of granular materials is presented. It is based on continuum, pseudo-fluid approximation. Balance equations and constitutive relations account for fluctuations in the velocity field, through the ‘granular temperature’ concept. Partial wall slip is also allowed by means of a slip-length approach. The model is applied to an industrial silo geometry, though not limited in its formulation to any geometry or flow configuration. It predicts realistic flow patterns, requiring quantitative validation with detailed measurements. This work focuses on the prediction of the normal stress at the wall during discharge. Profiles closely match available correlations by Jannsen and Walker, including prediction of peak pressure where section changes. Connections with literature correlations together with a sensitivity analysis provide clues to link model parameters to intrinsic material properties.
We derive an effective boundary condition for dense granular flow taking into account the effect of the heterogeneity of the force network on sliding friction dynamics. This yields an intermediate boundary condition which lies in the limit between no slip and Coulomb friction; two simple functions relating wall stress, velocity, and velocity variance are found from numerical simulations. Moreover, we show that this effective boundary condition corresponds to Navier slip condition when the model of G. D. R. Midi [Eur. Phys. J. E 14, 341 (2004)] is assumed to be valid, and that the slip length depends on the length scale that characterizes the system, viz. the particle diameter.
- Jan 2009
- Traffic and Granular flow 07
We address the slow, dense flow of granular materials as a continuum with the incompressible Navier-Stokes equations plus the fluctuating energy balance for granular temperature. The pseudo-fluid is given an apparent viscosity, for which we choose an Arrhenius-like dependence on granular temperature; the fluctuating energy balance includes a 'mobility enhancing' term due to shear stress and a jamming, dissipative term which we assume to depend on the isotropic part of the stress tensor and on shear rate. After having proposed a 'chemical' interpretation of the phenomenology described by the model in terms of reaction rates, we report results for some 2-D standard geometries of How, which agree semi-quantitatively with experimental and DENT observations. In particular, our model well reproduces the formation of stagnant. zones of a characteristic shape (e.g. wedge-shaped static zones in a silo with flat bottom) without prescribing them a-priori with erosion techniques.
- Nov 2008
Composition quantification in granular mixtures through colorimetric imaging is addressed. Digital images of binary mixtures have been analysed with three different colour spaces: gray scale, L⁎a⁎b⁎ and HSV. Experiments have been carried out on a small scale drum mixer. After blending, the mixtures have been impregnated with a binder, solidified and sliced. The colorimetric analysis has been carried out on the interior of the granular bed. Results obtained using the three colour-spaces have been performed and compared. The HSV colour space yields better results in terms of accuracy and computational effort. Effectiveness of HSV relies on its independence from shadows that are a distinctive feature of pictures of the three-dimensional granular surface. Very important, HSV does not require a calibration curve to convert colour information into composition, differently from the other colour spaces.
- Jul 2008
- THE XV INTERNATIONAL CONGRESS ON RHEOLOGY: The Society of Rheology 80th Annual Meeting
We address the slow, dense flow of granular materials as a continuum with the incompressible Navier-Stokes equations plus the fluctuating energy balance including granular temperature. The pseudo-fluid is given an apparent viscosity which depends on an order parameter which we choose to be granular temperature. We derive the fluctuating energy balance following Babic ; this balance includes a 'mobility enhancing' term due to 'viscous heating' effects and a dissipative term which we assume, as a constitutive hypothesis, dependent on the isotropic part of the stress tensor and on shear rate, based on local friction considerations. For its particular structure the model is called Dissipative Coulomb Model. Solving the equation system we get an appreciable agreement with experiments in complex geometries (silos, hoppers). The model predicts also the typical behaviour of stresses in these configurations, thus predicting phenomena like the hourglass effect.
We discuss the advantages and results of using a mixing-length, compressible model to account for the shear banding behaviour in granular flow. We formulate a general approach based on two functions of the solid fraction to be determined. Studying the vertical chute flow, we show that the shear band thickness is always independent of flow rate in the quasistatic limit, for Coulomb wall boundary conditions. The effect of bin width is addressed using the functions developed by Pouliquen and coworkers, predicting a linear dependence of shear band thickness on channel width, while the literature reports contrasting data. We also discuss the influence of wall roughness on shear bands. Through a Coulomb wall friction criterion we show that our model correctly predicts the effect of increasing wall roughness on the thickness of shear bands. Then a simple mixing-length approach to steady granular flows can be useful and representative of a number of original features of granular flow.
- Feb 2007
Fluid-dynamic conditions that are compatible with tensile stress on the bonds between platelet glycoprotein Ibalpha and immobilized Von Willebrand factor A1 domain (VWF-A1) led to Ca++ release from intracellular stores (type alpha/beta peaks), which preceded stationary platelet adhesion. Raised levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate inhibited these [Ca++]i oscillations and prevented stable adhesion. Once adhesion was established through the integrin alphaIIbbeta3, new [Ca++]i oscillations (type gamma) of greater amplitude and duration, and involving a transmembrane ion flux, developed in association with the recruitment of additional platelets into aggregates. We have defined the distinct roles that the two ADP receptors, P2Y1 and P2Y12, play in the early events that follow the initial platelet interaction with immobilized VWF-A1 under high flow conditions. We have examined the consequences of specific pharmacologic inhibition of P2 receptors and our findings demonstrate a differential role of P2Y1 and P2Y12, respectively, in platelet adhesion and aggregation onto immobilized VWF under elevated shear stress, and highlight the distinct contribution of signaling pathways dependent on Src family kinases, PLC, and phosphoinositide 3-kinase (PI 3-K) to these processes. Results have been achieved through original experiments under flow, thoroughly characterized by ad hoc image analysis techniques and quantitative kinetic analysis.
We experimentally study the mixing of binary granular systems in a horizontal rotating cylinder. When materials have the same size and differ by dynamic angle of repose only, we observe an axial transport of matter that generates transient radial segregation. The system then evolves towards homogeneity. If materials differ by density also radial segregation becomes steady. A mechanism is suggested where radial segregation is promoted by axial differences of dynamic angle of repose. This differs from the free surface segregation suggested so far to explain radial segregation.
- Jun 2006
A versatile and automated image processing technique and data extraction procedure from videomicroscopic data is presented. The motivation is a detailed quantification of blood platelet adhesion from laminar flow onto a surface. The characteristics of the system under observation (type of cells, their speed of movement, and the quality of the optical image to analyze) provided the criteria for developing a new procedure enabling tracking for long image sequences. Specific features of the novel method include: automatic segmentation methodology which removes operator bias; platelet recognition across the series of images based on a probability density function (two-dimensional, Gaussian-like) tailored to the physics of platelet motion on the surface; options to automatically tune the procedure parameters to explore different applications; integrated analysis of the results (platelet trajectories) to obtain relevant information, such as deposition and removal rates, displacement distributions, pause times and rolling velocities. Synthetic images, providing known reference conditions, are used to test the method. The algorithm operation is illustrated by application to images obtained by fluorescence microscopy of the interaction between platelets and von Willebrand factor-coated surfaces in parallel-plate flow chambers. Potentials and limits are discussed, together with evaluation of errors resulting from an inaccurate tracking.
- Aug 2005
- BIOMEDICINE 2005
Platelet tethering to injured vascular surfaces exposed to rapidly flowing blood relies on the interaction with the adhesive protein von Willebrand factor (VWF). By perfusing blood through a flow chamber and analyzing surface events using a tailored/specific image processing and particle tracking algorithms, we derived non-conventional information, such as deposition and removal rates, supporting kinetics and thermodynamics speculations. Statistical reasoning allowed the formulation of single-event observations into mean-field behaviour, suitable for a macroscopic, clinical scale application. The arrest and release frequencies were quantified through association and dissociation rate constants, kon and koff. Variables influencing the arrival/removal rate were identified and their dependence on different experimental factors is illustrated in detail. It is shown how variation of hydrodynamic forces expressed as wall shear rate can be used to sample adhesion rate and bond resistance, ultimately resulting in different kon/off values and equilibrium adhesion constant. The issue of establishing whether multiple bonds are synergistic in opposing tensile stress is addressed. Potentials, applicability and limitations of the proposed characterization of platelet adhesion to VWF-coated surfaces are discussed. Developments in this area of research can considerably help to elucidate important aspects of normal hemostasis and pathological arterial thrombosis. Keywords: platelets, kinetics, association and dissociation rate constant, residence time, thermodynamics, adhesion equilibrium constant, image analysis.
A method for quantitative analysis of platelet deposition under flow is discussed here. The model system is based upon perfusion of blood platelets over an adhesive substrate immobilized on a glass coverslip acting as the lower surface of a rectangular flow chamber. The perfusion apparatus is mounted onto an inverted microscope equipped with epifluorescent illumination and intensified CCD video camera. Characterization is based on information obtained from a specific image analysis method applied to continuous sequences of microscopical images. Platelet recognition across the sequence of images is based on a time-dependent, bidimensional, gaussian-like pdf. Once a platelet is located,the variation of its position and shape as a function of time (i.e., the platelet history) can be determined. Analyzing the history we can establish if the platelet is moving on the surface, the frequency of this movement and the distance traveled before its resumes the velocity of a non-interacting cell. Therefore, we can determine how long the adhesion would last which is correlated to the resistance of the platelet-substrate bond. This algorithm enables the dynamic quantification of trajectories, as well as residence times, arrest and release frequencies for a high numbers of platelets at the same time. Statistically significant conclusions on platelet-surface interactions can then be obtained. An image analysis tool of this kind can dramatically help the investigation and characterization of the thrombogenic properties of artificial surfaces such as those used in artificial organs and biomedical devices.
- Apr 2005
Most of the earlier studies on mixing have been focused on drums operated in or close to rolling regime, considered the most convenient for metallurgical, cement and mining applications in rotary kilns. However, many other industrial applications deal with powder mixing in rotating drums or other tumblers such as in pharmaceutical, detergent or food industry. In these cases, effectiveness of mixing may be given by other regimes. Here we compare mixing efficiency and kinetics of two different regimes, i.e. rolling and cataracting. The attention has been specifically focused on both the internal composition patterns and the mixing kinetics, aiming at optimising the operation time and the final homogeneity of the mixtures. The internal structure of the bed, after mixing, has been investigated through a solidification technique. Images of the transverse plane of the mixture at the ends of the drum provide information on the mixture composition there, during mixing. Both information from the interior and the ends have been used to point out differences in the mixing patterns and kinetics of the two regimes considered. It is known since a long time in the industry that the evolution of the mixing process strongly depends on the history whom the bulk has been subjected to before mixing begins or during the early stages of the process. This work aims at providing some mechanistic and quantitative explanation of this knowledge and shows that not a single regime, but a proper combination of the two regimes allows to achieve a better mixing quality more rapidly.
- Aug 2004
Experimental investigations on mixing of non-ideal powders (granular tetraacetylendiamine (TAED)) are described. The evolution of mixing in rotating batch cylinders, in rolling regime has been addressed. Characterization and quantification of the local mixture composition have been obtained through an efficient solidification technique, coupled with computerized image analysis.Starting from a completely segregated configuration, the formation of a temporary, poorly mixed core at low rotation speed has been observed. Investigation of intermediate configurations during the mixing process allows to identify some unexpected granular mixing mechanism. The observed core has been explained in terms of transient axial convective fluxes superimposed on diffusive motion. Small differences of dynamic angle of repose between the two granular materials have been suggested to drive the axial convection, similarly to the mechanism reported in the literature to explain axial segregation phenomena. The differences in repose angle result from surface and shape irregularities typical of actual (i.e. non-ideal) granules.Convective fluxes due to the friction of powder with the end plates are also identified at the extremities of the mixer. Short-circuiting zones are created that hinder both axial diffusion and convection from the center of the vessel. Eventually, we suggest a mixing mechanism of non-ideal granular material where convection plays a major role.
- Sep 2003
Rotating drums are extensively used in the chemical and process industries as mixers, dryers, granulators and reactors for processing granular materials. As a result, granular behaviour in rotating drums has attracted numerous research efforts from both engineering and physics communities over the past few decades. Most of these studies have been focused on drums operated in or close to the rolling mode. However, there are many industrial cases where drums are operated in other modes, e.g. the cascading and cataracting modes, which forms the main motivation for this work. Comprehensive experiments have been carried out to investigate granular behaviour in a drum operated over a wide range of rotational speed with solids motion across the rolling, cascading and cataracting modes. A digital recording device was used to capture images of the transverse plane of the material bed. Analyses of the images were carried out to extract the bed behaviour as a function of rotational speed, drum fill level and particle size. This has led to three relationships between the surface shape expressed in terms of three characteristic lengths, operating conditions, as well as the friction properties of both particles and drum wall. These relations are found to apply approximately to the whole range of rotational speed used in this work. The generality of these relationships and possible application of them for drum scaling are discussed.
- Jul 2003
The propensity of powders to flow under given circumstances (flowability) affects a large number of industrial applications. A single, reliable and widely applicable flowability test does not exist, because of the variety of both granular materials and influence of handling on the measurements results. Here we critically examined the results provided by Hausner's method, based on apparent densities ratio, with several granular materials. Major limitations appeared to be the achievement and measurement of a dense packing condition, provided by the tapped density in the Hausner's ratio. After a detailed discussion of standard and modified techniques to measure bulk density, we eventually suggest a new flowability criterion based on a novel technique to determine a high packing density. The proposed criterion is more sensitive to differences in flowability, as quantified by the repose angle. In order to investigate also the domain of cohesive powders, we developed a novel procedure to measure the repose angle of such powders. Eventually, the new criterion was able to account consistently for free-flowing and cohesive powders. It also stimulates the discussion on subtle issues involved in the determination and use of elementary powder's properties.
- Jun 2001
This experimental investigation deals with the observation of the behaviour that dense granular materials present when they flow in steady regime on a rough chute, focusing the attention on the transition to movement of the bed and on quantities involved as the internal friction angle. An important aspect of the study is the identification of parameters that distinguish granular from fluid flows, aiming to verify the possibility to describe a granular bed as it was a pseudo-fluid having a particular rheological behaviour. In the experiments we have not used idealised particles (spheres, rods or disks) but sieved powders of ethylenediaminetetraacetic acid (EDTA), constituted of non-spherical particles with polydisperse size distribution and surface roughness. A static and a flowing (dynamic) layer are clearly identified. The thickness of the observed layers (static and dynamic) along the chute has been measured for different inclination, finding out that they collapse into a single curve when considered in non-dimensional scale. On the ground of the experimental data we propose a direct way of measuring the dynamic friction angle from chute observations and a simple constitutive law for granular materials in the frictional regime of motion. The law has been tested using velocity profiles obtained by filming the flowing granular bed.
The simulation of the behaviour of a bulk solid continuously flowing through a silo with internal flow feeders has been performed by means of a dissipative hydrodynamic model which has been already tested on simple geometries (flat bottomed silo, silo with converging hopper) showing good results . The model will be now tested on more complex geometries, like silos with internal inserts useful to modify the flow profiles in some particular industrial applications with a continuous material flow through the silo. This test will be carried out with a comparison between simulation and experimental results coming from an experimental campaign developed at Centro Ricerche Danieli. The results obtained by these calculations and those found experimentally agree, not only with regard to the velocity profiles, but also relatively to the pressure profiles on silo's walls.
In previous works [1, 2], a dissipative hydrodynamic model was used to simulate the behaviour of a dense granular solid flowing through silos with simple geometries or with internal devices, showing good agreement with experimental results. That model has been upgraded taking into account the interaction between the solid itself and a nonreactive gaseous stream flowing countercurrent through it. This has been made by solving for the gas phase the Brinkman equation where the porosity and the permeability of the porous media are derived from the flow field of bulk solid, and for the solid phase solving the same equations of [1,2] in which a volume force raising from the gas pressure gradient is computed. This upgraded model has been compared with the results coming from an experimental campaign developed at Danieli R&D, and it will be shown that it still gives good agreement with the experimental results obtained.