Onset of cohesive behaviour in gas fluidized beds: a numerical study using DEM simulation

Process modeling in the pharmaceutical industry using the discrete element method

The discrete element method (DEM) is widely used to model a range of processes across many industries. This paper reviews current DEM models for several common pharmaceutical processes including material transport and storage, blending, granulation, milling, compression, and film coating. The studies described in this review yielded interesting results that provided insight into the effects of various material properties and operating conditions on pharmaceutical processes. Additionally, some basic elements common to most DEM models are overviewed. A discussion of some common model extensions such as nonspherical particle shapes, noncontact forces, and interstitial fluids is also presented. While these more complex systems have been the focus of many recent studies, considerable work must still be completed to gain a better understanding of how they can affect the processing behavior of bulk solids.

Additive decomposition of shear strength in cohesive granular media from grain-scale interactions

We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and tensile strength) between grains in the framework of the contact dynamics method. We find that, for a wide range of bond parameters, the macroscopic angle of friction at the peak state can be split into three distinct terms of collisional, frictional and dilational origins. Remarkably, the macroscopic tensile strength depends only on the bond tensile strength, and the friction angle at the peak state is proportional to the dilatancy angle which varies linearly with sliding friction.

Shear strength properties of wet granular materials

We investigate shear strength properties of wet granular materials in the pendular state (i.e., the state where the liquid phase is discontinuous) as a function of water content. Sand and glass beads were wetted and tested in a direct shear cell and under various confining pressures. In parallel, we carried out three-dimensional molecular dynamics simulations by using an explicit equation expressing capillary force as a function of interparticle distance, water bridge volume, and surface tension. We show that, due to the peculiar features of capillary interactions, the major influence of water content over the shear strength stems from the distribution of liquid bonds. This property results in shear strength saturation as a function of water content. We arrive at the same conclusion by a microscopic analysis of the shear strength. We propose a model that accounts for the capillary force, the granular texture, and particle size polydispersity. We find fairly good agreement of the theoretical estimate of the shear strength with both experimental data and simulations. From numerical data, we analyze the connectivity and anisotropy of different classes of liquid bonds according to the sign and level of the normal force as well as the bond direction. We find that weak compressive bonds are almost isotropically distributed whereas strong compressive and tensile bonds have a pronounced anisotropy. The probability distribution function of normal forces is exponentially decreasing for strong compressive bonds, a decreasing power-law function over nearly one decade for weak compressive bonds, and an increasing linear function in the range of tensile bonds. These features suggest that different bond classes do not play the same role with respect to the shear strength.

Phase diagrams for cohesive particle mixing and segregation

By taking a discrete view of cohesion, we develop a particle-level model which can accurately predict the extent of particle mixing and segregation in cohesive (wet) granular systems. Our model is based on a discrete characterization tool and is used to generate phase diagrams of the predicted particle behavior. These phase diagrams exhibit both mixed and segregated phases where the boundary is determined by the mechanical and surface properties of the particles, such that manipulation of surface properties and/or size/density ratios provides a method to control cohesive particle mixing and segregation. A detailed description of the phase diagram development process as well as quantitative validation of the theoretical results are reported here.

Effect of particle size and interparticle force on the fluidization behavior of gas-fluidized beds

Gas-fluidized powders of fine particles display a fluidlike regime in which the bed does not have a yield strength, it expands uniformly as the gas velocity is increased and macroscopic bubbles are absent. In this paper we test the extension of this fluidlike regime as a function of particle size and interparticle attractive force. Our results show that for sufficiently large particles, bubbling initiates just after the solidlike fluidized regime as it is obtained experimentally by other workers. A scaling behavior of the solid-phase pressure in the fluidlike regime and a predictive criterion for the onset of macroscopic bubbling are analyzed in the light of these results.