Gas mixing in conical spouted beds

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2020-12-28
Doğan, Neslin
Conical spouted beds are relatively new generation gas-solid flow reactors used in various applications such as drying, catalytic polymerization, treatment of wastes, particle coating and chemical vapor deposition due to their advantages over cylindrical spouted beds. For complete characterization of conical spouted beds, determination of the local flow structure based on gas velocity and gas mixing is of paramount importance. There are limited studies on local gas behavior in conical spouted beds, especially operating with high-density particles. Therefore, the objective of this thesis study was to investigate the effect of particle density, bed size and conical angle on the local gas behavior and the axial gas mixing behavior in conical spouted beds. Local gas velocity measurements were conducted utilizing a Pitot tube in two small scale (Dc = 150 mm, γ = 30º, 60º) and one large scale (Dc = 250 mm, γ = 66º) conical spouted beds. Three different types of particles; zirconia (ρp = 6050 kg/m3), zirconia toughened alumina (ρp = 3700 kg/m3) and glass beads (ρp = 2460 kg/m3), were used in the experiments. Careful calibration of the Pitot tube for determination of gas velocities in the annulus section was carried out in a separate loosely packed bed set-up, which represents the flow conditions in the annulus section of the spouted bed. Effects of axial position, conical angle, bed size and particle density on gas velocity profiles were investigated. It is shown that the amount of air flows through the annulus increases as the axial distance and particle density increases; however, it decreases with increasing cone angle. Dispersion of the gas into the annulus is found to be significantly higher in the large-scale unit than the small-scale unit. Local gas velocity measurements conducted in small scale conical spouted beds were used in a 1-D convection-diffusion gas mixing model to quantify the axial gas mixing characteristics of conical spouted beds. . The developed gas mixing model based on San Jose et al (1995) was implemented in MATLAB and dispersion coefficients for all operating conditions were determined. The results show that the dispersion coefficients obtained in this study have an order of 〖10〗^(-2) m2/s and are higher than the corresponding dispersion coefficients of fixed beds, lower than the corresponding dispersion coefficients of fluidized beds and similar with the cylindrical spouted beds reported in the literature. In addition, Peclet numbers found in the range of 0.56-7.84 revealed that the convective transport was more dominant than axial dispersion for light particles compared to heavy particles.

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Citation Formats
N. Doğan, “Gas mixing in conical spouted beds,” M.S. - Master of Science, Middle East Technical University, 2020.