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Multi-scale modeling of mass and heat transfer in dense gas-solid flows

Final Report Summary - MULTI-SCALE FLOWS (Multi-scale modeling of mass and heat transfer in dense gas-solid flows)

Multi-scale modeling of mass and heat transfer in dense gas-solid flows

Dense gas-solid flows have been the subject of intense research over the past decades, owing to its wealth of scientifically interesting phenomena. Many processes in chemical industry and biotechnology make use of so-called fluidized bed reactors. Here small particles in a large column are ‘fluidized’ by a gas stream entering from below and reactions occur inside the particles. This means that chemical species are transported to and from these particles and heat is produced or consumed. The ability to model these types of transport phenomena is important to optimally design new processes needed in, e.g. the biobased economy.

Dense gas solid flows are notoriously complex and its phenomena difficult to predict. This finds its origin in the large range of relevant scales: particle-particle and particle-gas interactions at the microscale (< 1 mm) dictate the phenomena that occur at the macroscale (> 1 meter), the fundamental understanding of which poses a huge challenge for both the scientific and technological community.

The aim of the ERC project led by Prof. Hans Kuipers at Eindhoven University of Technology (The Netherlands) was to provide a comprehensive understanding of large-scale dense gas-solid flow based on first principles. In the past multi-scale modeling of gas-solids interactions was mainly focused on the flows of the solid particles and the gas. In this project, the team of prof. Kuipers extended this modeling to include heat and mass transfer. This is important because gas-solid flows are applied in many chemical reactor designs, where the heat generated by (or needed for) reactions must be removed or distributed quickly.

Transport phenomena are determined by the exchange of mass, momentum and heat at the surface of the individual solid particles. The phenomena below the millimeter scale influence dynamics on the reactor scale. In order to model the full range of relevant scales Prof. Kuipers’ team employed a multi-scale approach, where the gas-solid flow was described by three different models: Direct Numerical Simulation (DNS; the most detailed with fully resolved gas-solid coupling), Computational Fluid Dynamics - Discrete Element Models (CFD-DEM; where correlations are used for the coupling between gas and solid phase), and Two-Fluid Models (TFM; where both phases are treated on a continuum level). These models were validated by one-to-one experiments.

At the most detailed level of modeling (DNS) a so-called Immersed Boundary Method was used. Simulations were compared with detailed experiments of a small fluidized bed, and measured heat transfer in small arrays of particles. The validated code was used to propose new improved correlations for the drag and heat-transfer between particles and gas. At the intermediate modeling level (CFD-DEM) heat and mass transfer correlation were implemented and validated. Novel experimental techniques were used to measure heat-transfer by combining infrared thermography with visual techniques. Also individual particles were tracked in reactors that are visually inaccessible. Lastly, a new efficient and accurate TFM simulation code was developed for the simulation of industrial scale reactors. The results have been validated by ultra-fast X-ray tomography in collaboration with scientists at HZDR in Dresden, Germany.