Turbulent Fluidization Regime Research Articles

Hydrodynamic behavior of a binary solids fluidized bed.

This paper presents a study on the hydrodynamic behavior of a fluidized bed containing mixtures of solids of different sizes and densities. The transition velocity from bubbling to turbulent fluidization and solids holdup of the dense bed in different fluidization regimes were experimentally determined. The onset velocity to turbulent fluidization was found to increase with increasing fraction of coarse solids. Solids holdups changed differently with gas velocity and fraction of coarse particles in bubbling and turbulent fluidization regimes. As a quantitative measure for describing the complexity or the self-affine property of the time series signals, a fractal analysis was introduced and related to the hydrodynamics of the fluidized beds. A correlation to predict the transition velocity of a binary solids fluidized bed was also developed in this paper.

Quantitative estimation of bubble size in PFBC

This paper presents a study on the quantitative estimation of bubble size in pressurized fluidized bed combustors (PFBC). A generalized correlation of bubble size which fits the experimental data quite well has been developed for PFBC on the basis of a comprehensive analysis of previous work. The correlation takes into account the different flow regimes at different pressures and gas velocities, as well as the special variation of bubble size within the lower pressure range of the bubbling regime. Using this correlation, an ideal operating area of the PFBC can also be defined from the viewpoint of hydrodynamics and mass transfer. In addition, the correlation can be applied to atmospheric operations to overcome problems encountered by the previous correlations under high gas velocities (e.g. turbulent fluidization regime).

On the nature of turbulent and fast fluidized beds

Fluidization characteristics have been investigated over the range from bubbling to fast fluidization by using a circulating fluidized bed cold model (riser diameter: 50 mm i.d., riser height: 2 390 mm, powder: fluidized cracking catalyst (FCC) and silica sand). Special focus was placed on the concepts of turbulent and fast fluidization regimes. Based on careful measurements of flow structure and cluster behavior, it is demonstrated that the turbulent fluidization regime is a transition regime between bubbling and fast fluidization. Experimental evidence supports the postulate that in fast fluidization the clusters adjust their size so that the gas drag force acting on them compensates with their gravity.

Regime classification of macroscopic gas—solid flow in a circulating fluidized bed riser

A conceptual flow regime diagram for a circulating fluidized bed riser is proposed, combining existing investigations with experimental data obtained under idealized conditions in which a fully independent control of gas velocity and solid circulation rate was conducted by use of a screw feeder for solid feed into the riser. The diagram classifies the flow state into five regimes by qualitative transition lines which describe the relationship between gas velocity and solid circulation rate. These regimes are particulate fluidization, bubbling fluidization, turbulent fluidization, dense-phase transport and dilute-phase transport. The diagram suggests that S-shaped bed-density distribution or dense/dilute region interface appears only at limited conditions in the bubbling and turbulent fluidization regimes. These experimental findings were generalized by further experiments in a conventional circulation system with a ball valve between the riser and the downcomer which permits changes in the solid circulation rate and the bed height in the downcomer. The experimental results showed that the bed height in the downcomer has no particular effect on the bed density distribution or the height of the dense/dilute region interface, but an appreciable effect on the lowest gas velocity to maintain steady solid circulation at a given rate. These results are consistent with the above diagram.

Effect of particle size distribution in different fluidization regimes

AbstractThe effect of particle size distribution (PSD) on the performance of a fluidized bed reactor was investigated in different hydrodynamic regimes: bubbling, slugging, turbulent and fast fluidization, with three particle size distributions, all with the same mean diameter and nearly the same particle density and BET surface area. Regime transitions were examined by measuring pressure fluctuations. Void sizes tended to be smaller and the transition from bubbling or slugging to turbulent fluidization was achieved earlier for the wide distribution powder than for the narrow PSD. At gas velocities ≤ 0.2 m/s, the conversion and reactor efficiency were not affected greatly by the PSD. However, at higher gas velocities, PSD played a significant role. For particles of wide size distribution, the conversion in the turbulent and fast fluidization regimes was usually higher than in the bubbling fluidization regime at the same dimensionless rate constant, kf′. On the other hand, for particles of narrow size distribution, the dependence of conversion on the regime is small, except for the fast fluidization regime.

High-velocity fluidized bed reactors

There is increasing interest in high-velocity fluidized bed reactors operated in the turbulent and fast fluidization regimes. Understanding of the hydrodynamics of these fluidization regimes has improved greatly in recent years, and there are prospects for applications beyond those practiced in industry at this time. Reactors operated in these regimes offer some unique features for gas-solid contacting. However, considerable work is required to achieve a better understanding of these high-velocity systems and to allow them to be optimized with respect to reactor geometry and operating conditions.

Predicting fluid‐bed reactor efficiency using adsorbing gas tracers

AbstractGas tracers have been used frequently to predict conversion in chemical reactors. But in heterogeneous systems such as fluid‐bed reactors, tracer residence time data usually overpredict actual contact time and conversion. In this work, the general two‐phase countercurrent flow model is extended to include tracer adsorption, which is important in predicting contact time. The tracer used was SF6, whose adsorption on the solids could be varied over a wide range. The apparent reactor efficiency, calculated directly from tracer response, is shown to approach that estimated from the two‐phase model when the fluid bed approached homogeneity. This is the case for many large‐scale reactors that are operated in the turbulent fluidization regime. The use of horizontal baffles in methanol‐to‐gasoline cold flow models is shown to improve bed efficiency by staging the bed.

Solids mixing in an expanded top fluid bed

AbstractThis study investigated solids mixing of a group A fine powder in a 0.15 m ID expanded top fluid bed with a ferromagnetic tracer. The superficial gas velocity was raised from 0.075 to 1.1 m/s, causing the bed to go through bubbling, slugging, and turbulent fluidization regimes. A countercurrent flow model described the data well at low gas velocities. The bed assumed a more homogeneous appearance at higher gas velocities; a one‐dimensional axial dispersion model was used to correlate the data. Axial dispersion coefficients increased with gas velocity. The data agree well with literature data for low gas velocities.