Chemical Engineering Science, Vol.55, No.21, 4789-4825, 2000
A state-of-the-art review of gas-solid turbulent fluidization
Turbulent fluidization has only been widely recognized as a distinct flow regime for the past two decades, even though it is commonly utilized in industrial fluidized-bed reactors due to vigorous gas-solids contacting, favourable bed-to-surface heat transfer, high solids hold-ups (typically 25-35% by volume), and limited axial mixing of gas. Despite its practical importance, turbulent fluidization has received much less attention than the adjacent flow regimes of bubbling, slugging and fast fluidization, due to the challenges of experimental and theoretical work related to this flow regime. However, recent years have seen an upsurge in interest in turbulent fluidization. Various methods - pressure fluctuations, visual observations, capacitance signals, optical fibre probes and bed expansion - have been used to determine the transition velocity, usually denoted U-c, at which turbulent fluidization begins. Different methods tend to give different results. There appear to be as many as three different types of turbulent fluidization, depending on such factors as mean particle size, particle size distribution, column diameter and internal baffles, if any. When turbulent fluidization is preceded by bubbling, U-c denotes a change from closed laminar bubble wakes to open turbulent wakes. The upper boundary of turbulent fluidization occurs when a distinct upper bed surface disappears due to substantial entrainment. Much of the literature regarding the turbulent fluidization flow regime adopts the terminology of the bubbling regime, ascribing such properties as bubble diameter and bubble rising velocity, despite the transitory and distorted nature of the voids. Turbulent beds exhibit non-uniform radial voidage distributions, with lower time-mean voidages near the wall than in the interior of the column. Axial mixing of both gas and solids is usually characterized by axial dispersion coefficients and Peclet numbers which depend on the column dimensions, as well as the gas and particle properties. Empirical equations are presented for prediction of these quantities for both gas and solids. Surface-to-bed convective heat transfer coefficients tend to reach a maximum in the turbulent fluidization regime. When turbulent beds are represented by two-phase models, interphase mass exchange is rapid. Reactor models vary widely, some treating the turbulent bed as a single phase homogeneous suspension subject to axial dispersion, while others assume two-phase behaviour. A probabilistic approach that merges these approaches as the gas velocity increases shows promise. While considerable progress has been made, substantial challenges remain in understanding and characterizing the turbulent fluidization flow regime.