Effect of Pressure and Temperature on the Hydrodynamics of a Polyethylene Fluidized Bed.

摘要

Gas-Solid fluidized beds are among the most important reactor systems in the chemical industry both because of excellent mixing ability and high heat and mass transfer rates, in situations of relatively low pressure drops. The presence of bubbles in the reactor produces a vigorous mixing, that enhances the gas-particle contact, improving the heat and mass transfer rates. Transport mechanisms in a gas-solid fluidized bed are dominated by bubble behavior. The purpose of this study was to experimentally investigate the effect of high pressures and temperatures on fluidization. Experiments were carried out in a bubbling bed over the range of operating pressures of about 100-2760 kPag and in two different temperatures, 50°C and 80°C. The bed material selected was Polyethylene, and it was fluidized by Nitrogen in an aluminum column (30 cm in diameter and 280 cm in height). Non-intrusive techniques such as pressure fluctuation were used to analyze the results. Superficial gas velocity up to 6 Umf was used in order to avoid significant entrainment problems. Operating temperature was selected less than 85°C to prevent agglomeration.;Detailed statistical methods such as Standard Deviations, Time Series, Periodograms and Power Spectral Density Functions (PSD) were applied to analyze the pressure fluctuations. The results confirm Multiple Bubble Regime in different pressures and temperatures for this system, meaning that bubbles are from different sizes and velocities. Moreover, minimum fluidization velocity (U mf) decreases with both increasing pressure and temperature for Polyethylene particles under experimental conditions. At constant P and T, bubble sizes increase with bed height and excess gas velocities due to coalescence and more gas flowing upward the system. Bubble diameters decrease with increasing pressure because of more breakage and splitting and less coalescence and gas flow. At the same operating conditions, bubble sizes increase with temperature due to viscous forces and more coalescence.