H.T. Bi

A state-of-the-art review of gas–solid turbulent fluidization

Abstract 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.

Flow regime diagrams for gas-solid fluidization and upward transport

Abstract Flow regime maps are presented for gas-solids fluidized beds and gas-solids upward transport lines. For conventional gas solids fluidization, the flow regimes include the fixed bed, bubbling fluidization, slugging fluidization and turbulent fluidization. For gas solids vertical transport operation, solids flux must be incorporated in the flow regime diagrams. The flow regimes then include dilute-phase transport, fast fluidization or turbulent flow, slug/bubbly flow, bubble-free dense-phase flow and packed bed flow. In practical circulating fluidized beds and transport risers, operation below the fast fluidization regime is commonly impossible due to equipment limitations. Practical flow regime maps are proposed with the flow regimes, including homogeneous dilute-phase flow, core-annular dilute-phase flow (where there are appreciable lateral gradients but small axial gradients) and fast fluidization (where there are both lateral and axial gradients). The boundary between fast fluidization and dilute-phase pneumatic transport is set by the type A choking velocity, at which the uniform suspension collapses and particles start to accumulate in the bottom region of the transport line, while the mechanism of transition from fast fluidization to dense-phase flow depends on the column and particle diameters.

Suspension densities in a high-density circulating fluidized bed riser

Flow behaviour of fluid catalytic cracking particles were investigated in a riser with volumetric solids concentrations of 20% and more. The air velocity and solids circulation flux had only a small influence on the solids hold-up once high-density conditions were attained. At these high-density conditions, refluxing of solids near the riser wall, commonly observed in low-density CFB risers, disappeared and was replaced by a more homogeneous flow structure. The slip velocity increased with solids hold-up and, for constant superficial gas velocity, there appears to be a unique relationship between the two. Slip factors as high as 10 were obtained in the developed region of the riser, compared to values of about 2 to 5 reported in the literature for more dilute fully developed flows in the fast fluidization flow regime. Differential pressure fluctuations increased with increasing suspension density and were not significantly affected by superficial gas velocity and height. A transition point is defined to distinguish dilute from high density conditions in the CFB riser.

Types of choking in vertical pneumatic systems

Abstract Choking is examined in terms of its definitions. Three choking initiation mechanisms are identified: type A (accumulative) choking occurs when solids start to accumulate at the bottom of the conveyor as the saturation gas carrying capacity is reached; type B (blower-/standpipe-induced) choking results from instabilities due to gas blower-conveyor or solids feeder-conveyor interactions where there is insufficient pressure or too limited solids feed capacity to provide the needed solids flow; and type C (classical) choking corresponds to a transition to severe slugging. Approaches for predicting the onset of each of these type of choking are recommended. Implications for regime transitions in fast fluidization are also identified.

Propagation of pressure waves and forced oscillations in gas-solid fluidized beds and their influence on diagnostics of local hydrodynamics

Abstract Experiments were conducted in a 50-mm diameter gas-fluidized bed to investigate the origin and propagation behaviour of pressure waves. The attenuation and amplification of pressure waves during propagation away from their sources are explained by the interaction between particles and the fact that forced oscillations of fluidized beds are coupled with propagating pressure waves. Both the pseudo-homogeneous compressible wave theory and the separated flow compressible wave theory are shown to give good predictions of the propagation velocity of pressure waves in gas-fluidized beds. The dramatic increase in wave velocity when the superficial gas velocity is decreased below the minimum fluidization velocity is attributed to a change in the forms of waves. Due to the existence of pressure waves from many locations in gas-fluidized beds, absolute pressure probes are not suitable for determining local bubble behaviour. Closely-spaced differential pressure probes filter out most pressure waves so that their signals mainly reflect local void and particle behaviour. However, a differential pressure probe does not provide the same local measurement as an optical fibre probe due to its large measurement volume and distortion from nearby bubbles.

Effect of measurement method on the velocities used to demarcate the onset of turbulent fluidization

The transition velocity Uc corresponding to the maximum rms amplitude of pressure fluctuations in a 102 mm diameter fluidized bed was found to be a strong function of measurement method, while the other commonly used transition velocity Uk depends on the solids return system and is not a well defined parameter. Different results and trends are obtained for Uc depending on whether one uses absolute or differential pressure fluctuations and whether or not one normalizes by the time-mean local or differential pressure. For differential pressure fluctuation measurements, a higher Uc was obtained from the dimensionless standard deviation normalized by the local pressure drop, and the value decreased with height. For local voidage fluctuation measurements using an optical fibre probe, a maximum point also appeared in the standard deviation curve, although the curve was much flatter than corresponding pressure fluctuation curves. Uc corresponding to zero skewness of voidage fluctuations on the axis of the column gave a value close to Uc from differential pressure fluctuation measurements at the same level.

Variable-gas-density fluidized bed reactor model for catalytic processes

Abstract A generalized fluidized bed reactor model which covers the three fluidization flow regimes most commonly encountered in industry (bubbling, turbulent and fast fluidization) is proposed. The model is based on probabilistic averaging shown previously (Thompson, Bi, & Grace, Chem. Eng. Sci. 54 (1999) 2175; Grace, Abba, Bi, & Thompson, Can. J. Chem. Eng. 77 (1999) 305) to be applicable over a range of superficial gas velocities. In this paper, we extend the model to cases where the volumetric gas flow changes appreciably due to variations in molar flow, pressure and temperature. For the air-based oxy-chlorination process as a case study, it is shown that the volume change affects both the hydrodynamics and the reactor performance. Because the reactions are rapid, almost complete conversion of ethylene is attained immediately above the distributor resulting in an ∼25% reduction in volumetric flowrate. Using the probabilistic averaging technique, the model tracks the probability of being in the bubbling, turbulent and fast fluidization regimes along the reactor height. The impacts of temperature and pressure variations are also examined. The variable density model gives predictions which compare well with commercial data; ignoring density variations leads to significant underprediction.

NUMERICAL SIMULATIONS OF HYDRODYNAMIC BEHAVIORS IN CONICAL SPOUTED BEDS

The axial and radial distributions of static pressures and vertical particle velocities of conical spouted beds have been simulated and compared with experimental data. Simulation results show that, among all factors investigated, the Actual Pressure Gradient (the APG term) in conical spouted beds, introduced as the default gravity term plus an empirical axial solid phase source term, has the most significant influence on static pressure profiles, followed by the restitution coefficient and frictional viscosity, while other factors almost have no effect. Apart from the solid bulk viscosity, almost all other factors affect the radial distribution of the axial particle velocity, although the influence of the APG term is less significant. For complex systems such as conical spouted beds where a fluidized spout region and a defluidized annulus region co-exist, the new term introduced in this work can improve the CFD simulation. Furthermore, for other systems with the Actual Pressure Gradient different from either fluidized beds or packed beds, the new approach can also be applied.

Regime transitions: Analogy between gas-liquid co-current upward flow and gas-solids upward transport

Abstract Flow regime transitions in gas-liquid co-current upward transport are analyzed by analogy to gas-solids upward transport. The transition from bubbly flow to slug flow of gas-liquid transport is well predicted by extending an analysis used for gas-solid systems. The transition from slug to churn flow in gas-liquid systems is analogous to the transition from slugging to turbulent fluidization. The transition velocity in gas-liquid systems based on visual observation is found to be close to the point where the maximum of the standard deviation of absolute pressure fluctuations reaches a maximum, the criterion commonly employed to define the onset of turbulent fluidization. The transition from churn to annular flow is also analogous to the transition from fast fluidization to dilute pneumatic transport in gas-solids systems. The data on transition from churn to annular flow is analyzed systematically and compared with available correlations.

Radial pressure differences and their fluctuations in dense fluidized beds

The local instantaneous pressures in both the wall and the core regions have been measured using pressure transducers in a fluidized bed operated in the bubbling and turbulent fluidization regimes. The local time-mean pressures and the standard deviations of pressure fluctuations were then calculated and used to evaluate the radial variation of local pressures