(261b) Theoretical and Experimental Studies of the Dynamics of Bubbling and Slugging Fluidised Beds

Fluidized bed processing is one of the most important techniques used in many chemical and process industries. Fluidized bed drying, catalityc reactors and combustion systems are of significant economic and strategic importance. Particularly, circulating fluidized beds (CFB) are widely used in the chemical and petroleum industries. Their future applications in the next generation of power plants have been extensively studied.

The efficiency of heat and mass transfer processes within a fluidized bed is a function of operational parameters, such as gas flow rates, particle size and density and the settled bed height. Also it strongly depends on the gas-solid flow pattern. It is well known that the dynamics of the fluidized bed can be characterized by two time-scales. Large-scale fluctuations occur owing to formation of bubbles during fluidization and arising from the circulating motion of the particles. Small fluctuations are induced by the inter- phase interaction between individual particles or collections of particles. Particularly the flow pattern just above the air distributor at the base of a fluidized bed is determined by particle-gas interaction, particle-particle interaction and bubble coalescence. These interactions, in a time domain, can therefore be characterized by both large and small fluctuations. The rapid fluctuations (over milliseconds intervals or less) of voidage in the bed normally negate the possibility of using radiation-based tomographic sensors but such phenomena can be sensed using electrical capacitance tomography methods.

A qualitative and quantitative description of the gas-solids hydrodynamics just above the air distributor is fundamental for understanding the mechanisms responsible for transition between bubbling and slugging flow regimes. This is also critical to allow the optimum design of the fluidized bed system which should be characterized by a high efficiency of heat and mass transfer processes between gas and solids phases.

The aim of this paper is to investigate the hydrodynamics of a fluidized bed using both numerical and experimental approaches. The numerical approach is based on the Euler-Euler model which is currently considered to be the highest possible level of continuum modelling, as it allows for dynamic interactions between the two phases by writing a separate set of mass, momentum and energy equations for each phase.

Electrical capacitance tomography (ECT) is useful for examining processes and structures where density or temperature gradients exist. In gas-solid system density gradients are caused by a non-uniform distribution of solids in a pipe cross-section as, for example, in a dense pneumatic conveying or a bubbling fluidization. Bubble diameters and rise velocities within a bubbling fluidized bed have been measured by Halow et al, Wang et al., Makkawi and Wright, White and McKeen and Pugsley. Knowledge of these two parameters is critical for a better modelling of gas-solid interactions as discussed by McKeen and Pugsley . They applied their results to modify the gas-solids drag term used in a numerical two-fluid model. Liu et al compared the data of solids concentration in a fluidized bed measured by ECT with the data measured by a pressure difference method, showing a good agreement under high concentration conditions. White and Zakhari also investigated the internal structures in fluidized beds of various diameters and demonstrated the capability of ECT for beds with a diameter up to 0.6 m. The experimental results obtained in the vicinity of the air distributor will be validated against the data obtained from a two-fluid model. The paper will discuss the transition between the bubbling and slugging flow modes.