Abstract
Swelling is an important process in many natural materials and industrial products, such as swelling clays, paper, and Super Absorbent Polymer (SAP) particles in hygienic products. SAP particles are capable to absorb large amounts of fluid. Each grain of SAP can absorb water 30 to 1000 times its initial mass, depending on the water composition.To gain insight in the swelling behaviour of a bed of SAP particles, we have developed a grain-scale model and have tested it by comparing it to experiments. The grain-scale model is based on a combination of the Discrete Element Method (DEM) and the Pore Finite Volume (PFV) method, which we have extended to account for the swelling of individual SAP particles. Using this model, we can simulate the behaviour of individual particles inside a water-saturated bed of swelling SAP particles while taking into account the hydro-mechanical effect arising from the presence of pore water. The model input includes physical parameters such as particle stiffness and friction angle, which were found in the literature, as well as particle size distribution and diffusion coefficients, which were measured experimentally. A swelling rate equation was developed to simulate the swelling of individual particles based on water diffusion into a spherical particle. We performed experiments to measure the rise of the surface of a bed of initially dry SAP particles, which were put inside a glass beaker that contained sufficient amount of water for the SAP particles to swell and to remain saturated at all times. We used our model to simulate the swelling of that SAP particle bed as a function of time. Simulations show that the numerical model is in accordance with the experimental data. We have also verified the model with Terzaghi's analytical solution for a small swelling event. Finally, a sensitivity analysis was performed to study the effects of main grain-scale parameters on the larger-scale behaviour of a bed of particles.
Highlights
Swelling of porous media occurs in many natural and industrial materials e.g. clays, foods, biological tissues, papers, and absorbent polymers in hygienic products [1,2,3,4,5]
The decrease of the swelling rate in experiments can be related to i) limited water availability inside the Super Absorbent Polymers (SAP) particle bed due to clogging inside the bed, which is not present in simulations, ii) the SAP particles swell heterogeneously during swelling whereas we assume particles to remain spherical, or iii) the local confining pressure acting on the SAP particles reduces the swelling rate
A particle model has been developed to simulate the swelling of granular materials
Summary
Swelling of porous media occurs in many natural and industrial materials e.g. clays, foods, biological tissues, papers, and absorbent polymers in hygienic products [1,2,3,4,5]. Tong et al [35] found that the permeability values, which were predicted by the DEM-PFV model for packings of spheres, were in good agreement with experiments In those studies, absorption of water and swelling of particles were not included. The aim is twofold i) to investigate whether DEM-PFV is capable to reproduce experiments of a bed of swelling particles, and ii) to study the effect of mechanical parameters on the swelling behaviour of a bed of SAP particles. We first describe the Discrete Element Method (DEM) and the Pore Finite Volume (PFV) model and how we included swelling of individual particles. The model is used for performing sensitivity analysis on how changes in various particlescale properties would affect the macro-scale behaviour of a bed of particles
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