From the powder mixing of pharmaceutical ingredients, to fuel particles inside a fluidized bed nuclear reactor; from the mixing of yeast into dough in a bakery operation, to gravel inside an asphalt plant; the search for increasing homogeneity of a system is a common need in many industries.
Mixing is a unit operation that involves the manipulation of a heterogeneous system in order to reduce its non-uniformity or gradients in composition, properties, or temperature. It can also be applied to enhance heat and mass transfer rates, and control reactions and structural changes.
What if mixing fails?
Malfunctioning or poorly designed mixing equipment can significantly raise production costs by increasing the time and/or energy spent on achieving the desired homogeneity. In the event mixing failure, the whole batch of product may be discarded and a production plant stopped for some time.
Despite the large number of equipment options available in the market, efficient mixing can be difficult on an industrial scale. In cases with very different sizes or densities, additional complexity appears since gravitational forces tend to segregate particles. Dealing with organic materials also adds some difficulties regarding material degradation and sanitary issues.
Why Rocky DEM?
Rocky DEM can be used to evaluate the behavior of particles inside mixing devices, helping engineers to design or optimize equipment. Computational simulations can reduce costs significantly by studying several configurations before actually testing it at lab and then scaling up from lab to real-scale models.
Rocky simulations are also very useful in assessing the equipment efficiency, especially in cases of changing operational conditions, increasing production or dealing with unanticipated material properties.
High numbers of particles
Previously, one drawback of using DEM to evaluate mixing was unfeasible simulation run times due to the number of particles. However, with advancements in computational power and parallelization of the codes, it is not a limitation anymore for most cases.
The simulation below shows a very quick and efficient mixing of the product in a plow mixer at 30 RPM and 1.2 million particles. Details of the mixing near the paddles as well as high mixing and dead zones within the mixer can be visualized, which can aid in optimizing mixer performance and understanding process efficiency. (Learn more about the comprehensive post-processing capabilities included in Rocky DEM by reviewing this webinar.)
In some cases, shape matters and the approach of using spherical particles to mimic real shape behavior is not enough. In this way, the Rocky DEM ability to run non-spherical particles can help a lot.
The following demo presents the mixing of pecans with peanuts using a kitchen hand mixer. It shows that the maximum mixing efficiency is reached before 7s. Some segregation can be observed near the bottom which suggests some redesign to improve the device.
Particles can be cohesive-forming agglomerates and can also stick to the walls. In these cases, the Rocky DEM adhesion model can be used to model these phenomena.
In the example below, the rotating drum section of an asphalt plant equipment was evaluated, where binding material and fine particles are mixed with gravel particles.
Simulations were able to reproduce the general behavior of wet particles inside the equipment, emulating the formation of particle clusters, as well as the adhesion of particles to the walls, assessing the impact of new scraping fins on the residence time and mass retention in this region.
(More information about this particular rotating mixer can be found here.)
Heat transfer modeling
Very often, heat transfer accompanies the mixing operation. In these cases, the mixing is critical to ensuring adequate heat and/or mass transfer between particles.
In the following example, Rocky DEM was used to model the heat transfer in a rotary calciner, a very common mixing device in the metallurgical and catalyst industries. The curved wall is quickly heated up to 1298K, and the evolution of the particles’ temperature is monitored over time, assessing the impact upon different speed drums and the inclusion of lifters. You can find more details about this particular simulation in this post.
Rocky-Fluent coupling capabilities enable users to account for the mixing within fluid-solid cases, enlarging the range of processes that can be modeled using DEM.
The video below shows a two-way coupling simulation of a fluidized bed with increasing gas flow rate. The intimate mixing behavior shown is one of the main advantages of using fluidized beds in industrial processes.
Lucilla holds a BE (Chemical) undergraduate degree, an M.Sc. in Chemical Engineering and a Ph.D. in Nuclear engineering from the Federal University of Rio de Janeiro. She joined ESSS in 2008 and has spent 5 years focused on applying CFD to solve common engineering problems in the Oil and Gas industry, dealing with turbulent and multiphase flow simulations. Since 2013, she is an Application Engineer for Rocky DEM Business Unit, supporting users, working on consultancy projects and validating models implemented for the CFD-DEM coupling.