CFD SIMULATION OF GAS-SOLID FLUIDIZED BED

Abstract

Fluidized beds are widely used in chemical and physical processes in large-scale operations like manufacturing of polyethylene and polypropylene, synthesis of fuels and chemicals, and in coating, drying, roasting, and for heat exchange. The widespread applications of fluidized beds are due to their effective mixing characteristics and high contact surface areas between the phases. The fundamental problem in modeling of these reactors is due to the complex motions of two phases where the interface is unknown and transient and to the interaction between the phases which is understood only for a limited range of conditions. Also, a large number of independent variables such as particle size, shape and density, as well as gas velocity and turbulence intensity, significantly affect the hydrodynamics behavior of fluidized bed reactors. • For example, for particles with smaller diameters, the cohesive forces become more important and possibly dominant; and when the gas velocity increases, the shape of bubbles, as well as the flow regime, will change. Computational fluid dynamics (CFD) has emerged as an effective tool for investigation of gas—solid fluidized bed hydrodynamics. In the literature, CFD has been used for modeling of multiphase flows to reduce the design time and cost. The Eulerian—Eulerian multiphase model treats both phases including the particulates as interpenetrating continua. To describe the particulate phase stress in the Eulerian—Eulerian approach, the kinetic theory of granular flow (KTGF) is a best method to deal with these kinds of problems. In this dissertation, to investigate hydrodynamics and heat transfer phenomenon in Gas-Solid Fluidized bed a Gas- Solid Fluidized bed system having a diameter of 0.285 m and height of 1 m containing glass beads of average diameter equal to 500 um has been simulated using commercial computational fluid dynamics (CFD) code Fluent. For simulation different drag models are used and results are compared. It is found that for the present application Syamlal-Obrien drag model provided best result. To study heat transfer coefficient (HTC) from wall to bed in the state of fluidization a multi-fluid Eulerian model incorporating the kinetic theory for solid particles has been used. Obtained results are compared with standard penetration theory as well as simulation results due to [Kuiper's et al. (1992)] for wall to bed HTC. The results showed that CFD simulation offers better agreement with results of theoretical investigations within maximum deviation of 5% and 10% with penetration theory and results obtained by Kuiper's et iii al. (1992) respectively. Further, effect of parameters like superficial velocity of gas (SVG), height of bed, diameter of Particle, and solid phase volume fraction on wall to bed HTC has been studied. In addition to it, effects of SVG on pressure drop, expansion of bed, and hydrodynamics of gas bubble are also discussed.