Hygroscopicity issues in powder and grain technology

The effect of hygroscopicity on flowability of powders and bulk solids with applications in the packaging industry is experimentally and numerically investigated. Firstly, four granular materials are tested at different water contents to study the impact of relative humidity on some hydro-mechanical properties, namely the hydraulic diffusivity on wetting, as well as the shear strength and compressibility properties of the materials. Next, a capillary model covering a wider water content range –compared to the previous tests– is applied to discrete element simulations of a granular column collapse set-up. These simulations give further insight into important aspects of grain hygroscopicity in packaging and other industrial applications (such as the kinematics of flow and flowability issues), which are outside the scope of conventional experimental testing.Postprint (published version

Anàlisi estructural d’elements de formigó reciclat

[ANGLÈS] Experiences with recycled aggregates, recycled aggregate concrete and recycled reinforced concrete regarded in the literature, show that sufficient strength is achievable in these materials to consider its use for structural reasons taking into good account the variations observed with respect to conventional concrete. It is announced a calculation method which, by means of the determination of coefficients for several comparative mechanical properties observed in recycled and conventional concretes, establishes modifications to be applied to the formulations used in conventional concrete analysis, for different values of the replacement of natural aggregates by recycled aggregates.[CATALÀ] Les experiències amb els àrids reciclats, el formigó reciclat en massa i armat que recull la bibliografia mostren que són assolibles en aquests materials propietats resistents suficients per plantejar-ne l’ús en estructures bo i tenint el compte les variacions que s’observen respecte del formigó convencional. Es proposa una metodologia de càlcul que per, mitjà de la determinació d’uns coeficients comparatius de diverses propietats mecàniques observades en formigons reciclats i convencionals, estableix modificacions en les formulacions que s’empren en el dimensionament del formigó produït només amb àrids naturals, per a diferents valors del percentatge dels àrids naturals per àrids reciclats

A new column collapse apparatus for the characterisation of the flowability of granular materials

The packaging industry is lacking a standard methodology to characterise the granular flow of a wide range of powders and grains in actual handling conditions. We present a new fully-instrumented granular column collapse apparatus for the experimental investigation of granular flow phenomena, by a quasi-two-dimensional set-up with novel features including: a lifting gate activated by a parallelogram mechanism for material release; a reversible pneumatic circuit to impose fluidised and vacuum conditions to the initial granular column; a set of load cells to monitor the basal load distribution during flow propagation; a 3D laser line profile sensor to scan the free surface morphology of the samples at rest; and a high-speed video recording set to capture near-wall flow visualisations and relevant kinematic measures by particle image velocimetry. The selected results on dry flows of oat flakes, copper sulphate fertiliser, and talc powder samples show their distinctive flow dynamics, indicating the good flowability of fertiliser compared to the poor flowability of talc. This research has implications for the selection and design of bulk solids handling equipment, and the calibration and validation of mechanical and numerical models.Peer ReviewedPostprint (author's final draft

A new granular column collapse device to characterise flowability of bulk materials

A wide range of bulk materials with different physical properties are nowadays handled in the packaging industry using different material conveying techniques. Nevertheless, experimental methodologies to characterise flowability of granular materials in actual handling conditions are still under development. This paper presents a new fully instrumented device for flowability assessment by granular column collapse of bulk materials. The generated granular flow is monitored by load cells that register the flow heights and by a high-speed video camera that captures the bulk flow kinematics through particle image velocimetry analysis. The 3D surface morphology of the final condition is determined with a 2D laser profile scanner. Results show the effect of varying the initial column aspect ratio on flow response.Peer ReviewedPostprint (published version

Modeling the homogenization of a heterogeneous granular bentonite mixture

A hydro-mechanical model of a heterogeneous granular bentonite mixture is presented based on discretization into a finite number of “bentonite units”, systems in which “megapores” are considered between bentonite grains, which, in turn, contain micro- and macropores. A macroscopic approach is used, where pore levels are associated with superposed homogeneous continua. Mixture heterogeneity is incorporated into the model considering the potentially very different percentage of megapores and grains in adjacent bentonite unit pairs. Mechanical contact is therefore affected, so a strategy is proposed based on porosity values, controlling deformability by the most deformable of the two units in contact. The constitutive formulation was implemented on a numerical solver to simulate a confined hydration test on a granular bentonite mixture with a notable initial local heterogeneity. The reproduced experimental evolution of the swelling pressure, relative humidity distribution, and water inflow was satisfactory. Porosity variation data were synthesized through histograms, describing not only the variation of the maximum and minimum porosity values but also the evolution of the porosity distribution in the mixture. This provides a quantitative description of the homogenization process, allowing an assessment of its scope and technological interest.This work is part of the project PID2020-118291RB-I00 funded by MCIN/AEI/ 10.13039/501100011033. This work was also supported by a ‘Margarita Salas’ grant from Ministerio de Universidades – Gobierno de España, awarded to Joel Torres-Serra.Peer ReviewedPostprint (published version

Discrete element modelling of granular column collapse tests with industrial applications

Describing the behaviour of granular materials is a challenging issue for the industry. Our work concerns packaging industries where packing equipment is designed to handle a wide range of powders and bulk solids with varying physical and mechanical properties. While packaging, a variety of material conveying techniques are used ranging from air fluidisation to discharge of material through a hopper. Thereby even a small improvement in their efficiency can lead to significant benefits, both financial and environmental. Flowability of powders and bulk solids is often experimentally investigated using granular column collapse, as this test provides deep insights into the kinematics of granular flow both at particle and bulk levels [1]. Here, we consider a quasi-two-dimensional set-up with a reservoir containing the granular pile which is instantaneously released onto a channel where run-out takes place. Instead of experiments, we use discrete particle simulations allowing us to quantitatively link bulk-level observations to particle-level properties of the materials, besides enabling inverse analysis leading to indirect measures of micro-scale parameters. We present a simulation strategy aimed at controlling several particle parameters influencing the run-out: - Polydispersity in size, using different particle size distributions; and also in shape, comparing the use of spherical and non-spherical particles, namely cylinders and ellipsoids. - Mechanical properties of the contacts, comprising normal stiffness and dissipation, as well as sliding, rolling and torsion coefficients. Specifically, hygroscopic behaviour of bulk materials is inspected modifying the contact law parameters. Additionally at the bulk level, air fluidisation of the columns before release is studied through the initial packing state by changing the volume fraction of the piles. Numerical simulations are implemented with the open-source code MercuryDPMPostprint (published version

Analysis of the mobilization of an unsaturated infinite natural slope due to strength reduction

This work analyses the mobilization of infinite natural slopes that are initially partially saturated. Starting from dry conditions (the water table is assumed to be deep, far from the slope surface), the soil shear strength will decrease when the wetting front advances from the surface. First, the geometry of the failure mechanism that can develop because of such strength reduction is determined. Second, after the mobilization causes the progressive reduction in the soil strength, the slide speed reached the minimum strength is determined, obtaining an indicator of the risk derived from failure.Peer ReviewedPostprint (published version

Density of water adsorbed on bentonites: determination and effect on microstructural void ratio modelling

The ability of helium gas displacement pycnometry to characterise the reduction of adsorbed water density with increasing bentonite water content was tested. For this purpose, after obtaining the grain density (which was assumed to be constant), an intensive campaign of 45 experiments was carried out with two bentonites for a total of 90 determinations, obtaining very consistent water density results. Furthermore, the water retention curve of one of the two bentonites was fully characterised by 95 tests with a chilled-mirror dew-point psychrometer. The water content results were used to determine the microstructural void ratio, both assuming a constant water density and considering the variation of the adsorbed water density obtained with the helium pycnometer. It was verified that with the constant value, by assuming a lower water density (1 g/cm3) than the experimental (values of 1.20 g/cm3 were obtained under hygroscopic conditions), higher values of microstructural void ratios are predicted, up to 15% greater for a relative humidity of 70% (0.46 instead of 0.40). For higher relative humidities the comparison loses its consistency, since the presence of capillary water cannot be disregarded, and the calculation of the microstructural void ratio is based on the assumption that the water present in bentonite is mainly in the form of adsorbed water. However, this does not compromise the analysis performed, as the results obtained for lower relative humidity values provide enough information to demonstrate the relevance of the errors that can occur when deriving models of the microstructural void ratio without taking into account the variation of the density of adsorbed water.This study was funded by the Junta de Comunidades de Castilla-La Mancha and the European Regional Development Fund (European Union) through project SBPLY/19/180501/000222.Peer ReviewedPostprint (published version

Compacted bentonite hydro-mechanical modelling when interaggregate porosity tends to zero

A formulation is proposed for reproducing the hydro-mechanical behaviour of compacted bentonites as interaggregate porosity becomes negligible. In that case, the model assumes the bentonite deformability to be controlled only by the microstructural modelling level in a double porosity approach. Hence, the bulk stiffness of the system is defined by the relationship between the microstructural void ratio and the thermodynamic swelling pressure, requiring no additional parameters to simulate the overall behaviour. An experimental study has been conducted to check the conceptual consistency of the formulation, with favourable results. The quantitative potential of the model has also been satisfactorily verified by numerical simulation of the tests carried out in this study. A displacement-based finite-element model developed for double porosity systems has been modified to run the simulations. The proposed formulation has been implemented using a simple strategy that can be easily replicated for similar numerical models. Both the simulation capabilities and the computational cost efficiency of the proposed formulation confirm its practical interest.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This publication is part of Grant PID2020-118291RB-I00 funded by MCIN/AEI/https://doi.org/10.13039/501100011033.Peer ReviewedPostprint (published version