A predictive hydrodynamic model for circulating fluidized bed risers

ANNUAL LITERATURE SURVEY 1996 Summary paper on fluidization and transport phenomena Fan L.-S., Powder Technology, 1996, 88/3 (245-253). In English. The papers presented in the sessions on fluidization and transport phenomena cover a wide range of subjects pertaining to fluid-particle systems. Examples of these subjects include bubble coalescence mechanisms, entrainment, pressure effects and chaos in bubbling and turbulent fluidized beds, wave phenomena and cluster structure in circulating fluidized beds, and spectral and fractal characteristics in gas-liquid-solid fluidizcd beds. In this summary paper, an overview based on these papers is given and salient results are highlighted. A predictive hydrodynamic model for circulating fluidized bed risers Pugstey T.S. & Berruti F., Powder Technology, 1996, 89/1 (57-69). In English. A predictive mathematical model, able to characterize and quantify all facets of the time-averaged gas and solids flow structure and Properties within circulating fluidized bed (CFB) risers, is proposed. The model can be used as a tool to assess a-priori the operation of a riser and can be easily coupled to kinetic models for process simulation. The model postulates the existence of a core-annulus type of flow structure and is based on both fundamental principles and empirical relationships. The model is successfully vertified against experimental data from CFB units of various sizes and operating under different regimes of fluidization. The model outputs, consisting of axial pressure drop profiles, axial and radial voidage profiles, radial solids velocity and mass flux profiles, average gas velocity and core radius, are compared to existing data and are assessed critically. Characterization of gas fluidization regimes using pressure fluctuations Bai D., Shibuya E., Nakagawa N. & Kato K., Powder Technology, 1996, 87/2 (105-111). In English. Flow regimes varying from bubbling fluidization to pneumatic transport were characterized by analyzing the differential pressure fluctuations. Two kinds of particles, typifying those from group A and group B, were employed in the experiments. It is found, under all the operating conditions and at various axial positions, the standard deviation of the differential pressure fluctuations almost coincides, if the solids holdup is the same. For the two particles employed, the dimensionless standard deviations normalized by the maximum standard deviation at Uc showed a unique relationship with the solids holdup, giving a favorable characteristics that allows the determination of the boundaries between the flow regimes of gas-solids fluidization. Based on the experimental results, we find approximate values of: ~sb > cs > 0.35, for bubbling fluidization; 0.15 < Cs< 0.35, for turbulent fluidization; 0.05 < E, < 0.15 for fast fluidization; and % < 0.05 for pneumatic transport. Bed expansion and average bubble rise velocity in a gas-solid fluidized bed A1-Zahrani A.A. & Daous M.A., Powder Technology, 1996, 87/3 (255-257). In English. A model for bed expansion of a gas-solid fluidized bed is proposed. Bed expansion is found to depend on excess fluidization velocity, and the geometry and physical properties of the bed and the particles. An expression for predicting the average velocity of an isolated bubble in a gas-solid fluidized bed, which is based on the bed expansion model, is also presented. The predictions of the proposed models agree well with experimental measurement. Novel gas fluidized bed stirred media mill: design and performance of a prototype Sadler L.Y., Odom J.M. & Hood E., Powder Technology, 1996, 89/1 (37-43). In English. The performance of a prototype novel gas fluidized bed stirred media mill was compared with that of conventional stirred media mills. The gas fluidized bed stirred media mill concept is thought to offer the advantages of reduced viscous dissipation of energy into the suspending fluid, continuous removal of product fines, and comminution in a dry environment. The test results confirm reduced power consumption relative to the liquid suspension stirred media mill. The energy efficiency of the prototype was found to be less than that of the liquid suspension mill, but it was greater than when the mill was operated without liquid suspension or air fluidization. Resources did not permit optimization of the mechanical design or operating conditions of the mill. Particle granular temperature in gas fluidized beds Cody G.D., Goldfarb D.J., Storch G.V. Jr & Norris A.N., Powder Technology, 1996, 87/3 (211-232). In English. This paper introduces and validates a novel non-intrusive probe of the average kinetic energy, or granular temperature, of the particles at the wall of a gas fluidized bed. Data is presented on the granular temperature of monodispersed glass spheres which span region B, and extend into region A, of the Geldart powder classification. The underlying physics of the measurement is the acoustic shot noise excitation of the surface of the fluid bed vessel by random particle impact. The concept and calibration of this acoustic shot noise probe is validated in the frequency range 10-20 kHz, through a comprehensive series of laboratory measurements with gasses and cylinders of significantly different acoustic properties. Data is presented on the dependence of the granular temperature on gas flow and particle diameter and on the change in the character of the fluidization transition from first order (hysteretic and discontinuous) to second order (reversible and continuous) for Geldart B glass spheres as the A/B boundary is approached. A striking difference in the dependence of the granular temperature on gas flow between Geldart B and A glass spheres is noted. The vibrational probe was used to study the time dependence of the granular temperature under bed collapse conditions when fluidizing gas is withdrawn rapidly from the system. Pipeline friction losses of coarse sand slurries. Comparison with a design model Sundqvist A., Sellgren A. & Addie G., Powder Technology, 1996, 89/I (9-18). In English. Friction losses in 0.2-0.3 m i.d. pipelines were in vestigated for three coarse sands with mass median particle sizes of 0.6-0.7 mm and size distributions of 1.4, 5.4 and 27.3, respectively, when expressed in terms of the ratio of particle diameters 85 to 15'/o by mass finer. The partially-stratified friction loss model proposed by K.C.

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