Toward a reliable model for industrial gas-fluidized bed reactors with polydisperse particles

Summary: Dispersed particulate flows are frequently encountered in a variety of large scale manufacturing processes such as the production of fuels, base chemicals, polymers, fertilizers, detergents, and many other products. These flows are unfortunately very poorly understood, which for a large extent can be attributed to the complex flow structure prevailing at the macroscopic scale, i.e. the scale of the process equipment. As a direct consequence thereof the design and scale-up of these processes is extremely cumbersome in practice (requiring extensive and time-consuming experimentation). In addition, the process operation is often far from optimal in the sense of energy and feedstock utilization. Therefore significant improvements are required to boost the sustainability of these processes, which demands for a better understanding of the fundamentals of these flows. For this reason, numerical models of gas-fluidized beds have become an indispensable tool in the design and scale up gas-solid chemical reactors. The main problem in this is the wide range of scales: catalyst particles can have the size of 50 micrometer or less,whereas an industrial size fluidized bed can be as large as 10 meters. Therefore, we have adopted a multi-level modeling strategy, where we consider dense gas-solid flows at three distinctive levels of modeling, namely the lattice Boltzmann model, the discrete particle model and the continuum model based on the kinetic theory of granular flow. The idea is essentially that fundamental models, taking into account the relevant details of fluid-particle and particle-particle interactions, are used to develop closure laws to feed continuum models which can be used to compute the flow structures on a much larger (industrial) scale. For monodisperse systems, this approach has proven to be very successful. In the proposed research we intend to focus on an extremely important aspect from the application point of view, namely the treatment of polydispersity, which has so far not been tackled in the modeling of gas- fluidized beds. Polydisperse systems are encountered in practically all large scale processes involving gas-fluidized beds, yet predictive models are not available. For the development of a reliable continuum model for polydisperse dense particulate flows it is of crucial importance to have accurate representations (closures) for both fluid-particle and particle-particle (including particle-wall) interaction. This will be the main focus of our research. The representation of the effective fluid-particle interaction (drag closures) will be obtained from detailed simulations using a lattice Boltzmann model (LBM). Particle-particle interactions are incorporated at the continuum level via representations for the effective solids pressure and viscosity, obtained from both the kinetic theory for polydisperse granular flow, and direct measurement in the discrete particle simulations. At each distinct level of modeling we intend to experimentally validate our computational results using state-of-the-art non-invasive optical measuring techniques. Our work will generate results (f.i. fluid-particle drag closures) which are also relevant for many other fields in science and engineering. In the past five years, our group has become one of the leading research groups on numerical modeling of multiphase flows. In particular, the integrated approach with three levels of modeling is unique in the world. This grant would allow us to further strengthen this successful and internationally recognized line of research.