Gas flow in fluidized beds of large particles : experiments and theory Public Deposited


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  • Gas mixing is studied in fluidized beds of large particles. The bed is 0. 483 m by 0. 127 m (19 in. by 5 in. ) in cross section and has transparent plexiglass panels on the front and back. A tube matrix made of twenty-seven 50.8 mm (2 in.) diameter plexiglass cylinders fixed in an equitriangular pitch (with the pitch to diameter ratio equal to 2) is used to study the effects of tubes. Experiments are done in a bed with and without tube array. Sand and dolomite particles with mean particle diameter of 1.3 mm (0. 051 in. ) to 4.0 mm (0. 157 in.) are used. Gas velocities are varied from minimum fluidization velocity to an excess velocity of 2.5 m/s. Two large particle fluidization regimes are described. The experimental evidence is presented in support of the existence of slow bubble and exploding bubble regimes. A semi-theoretical equation establishing the boundary between these two modes of fluidization is derived. A slow bubble regime is encountered in the bed in which the interstitital velocity of the gas exceeds the rising velocity of bubbles. Here the gas uses the bubble as a convenient shortcut on its way through the bed. The exploding bubble regime is reached at higher superficial gas velocities when the bubble growth rate is of the same magnitude as the bubble rise velocity. Large pressure drop oscillations, gross gas bypassing, defluidization of some of the particles and rapid bubble growth are characteristics of the exploding bubble regime. A new criterion is suggested for distinguishing between the fast and the slow bubble regimes. The criterion is derived as a relationship between two non-dimensional groups. The expansion of the bed of large particles with and without tube array is also studied. Theoretical equations are proposed for correlating relative bed voidage versus relative excess gas velocity. They are based on the two-phase theory and all are developed as a special case of one general equation. The equation derived for the slow bubble regime fits the experimental data of this study better than other existing correlations. The equation developed for the fast bubble regime compares favorably with literature data for fine particles. A special case of the general equation can be developed for stationary bubbles and for the slugging regime. It is found that there is little difference in expansion of beds with and without a tube array at low excess gas velocities. However, for higher excess gas velocities expansion is considerably greater for beds with a tube array. Exploding bubbles in a bed without tubes are responsible for this difference. The dispersion of tracer gas injected continuously through a line source above the distributor plate is determined for time-averaged concentration measurements. The tracer concentration at points within the bed is successfully predicted using a single-phase model with interstitial gas velocity (based on average bed voidage) as a characteristic fluidization velocity. The model is insensitive to axial dispersion and depends only on radial dispersion coefficients. The radial dispersion coefficient does not depend on either horizontal or vertical position in the bed and is a strong function of excess gas velocity. Considerable difference is found in tracer dispersion in beds with and without tube array. A new model, called the meandering flow model, is proposed for gas flow through fluidized beds of large particles. The concept of meandering flow is developed on the basis of actual physical movement of gas. The series of peaks observed in tracer concentration data records are explained by a bulk lateral movement of gas induced by large bubbles. A simple mathematical technique is suggested for the analysis of tracer data. As a result of the application of the meandering flow model the turbulent and meandering dispersion coefficients are defined. Meandering of fluid through the bed does not contribute to gas mixing, and consequently the meandering dispersion coefficient has to be subtracted from the overall radial dispersion coefficient. Only the turbulent dispersion should be used in the evaluation of the extent of gas mixing in fluidized beds of large particles, which contributes to gas phase combustion.
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