Hydrodynamics, Erosion and Heat Transfer in Pressurized Fluidized Beds

Abstract: Measurements of hydrodynamics, local tube erosion and local instantaneous bed-to-tube heat transfer were carried out in a cold, pressurized, fluidized bed with different horizontal tube banks. The influence of pressure, fluidization velocity, particle size and tube-bank geometry was studied. Two size distributions of silica sand were used, one with a mean particle diameter of dp = 0.45 mm and one with dp = 0.70 mm. The hydrodynamics were also studied without tubes in the bed for the silica sand with dp = 0.45 mm, and comparisons were made with previous results obtained with particles of dp = 0.70 mm. The bed has a cross-section of 0.2 m x 0.3 m and was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities of 0.2 and 0.6 m/s.

The hydrodynamics was measured using capacitance probes and Pitot-static pressure probes. The bed expansion ratio was determined by pressure drop measurements. The fluctuations in the pressure drop over the entire bed height were also measured, and the power spectral density distributions of these fluctuations were calculated. The erosion tests were carried out at different locations within the tube banks using target tubes coated with a thin layer of stearin. The bed-to-tube heat transfer was measured at a position in the tube bank near the centre line of the bed using a heat-flux sensor-equipped tube.

At corresponding operating conditions, the tube bank with the most dense horizontal pitch gives rise to considerably less erosion, but also somewhat lower heat transfer, than the tube banks with a more sparse horizontal pitch. The tube erosion is strongly related to the bubble rise velocity. The heat transfer coefficient is generally coupled to the bubble frequency, except for the high excess gas velocity with the most dense horizontal pitch where, at high pressures, the bed assumes a strongly turbulent behaviour and no distinct bubble pattern exists. The results indicate that the most severe erosion will occur in sparsely packed parts of a tube bank. For the sparse tube bank investigated, at high pressures, the erosion decreases with increasing pressure. The bed-to-tube heat transfer coefficient generally increases with increasing pressure. Thus, it should be favourable to operate a bed at high pressure levels.

Results obtained without tubes in the bed show that the bed expansion, bubble rise velocity, bubble volume fraction and visible bubble flow rate fall on single curves, if plotted vs a dimensionless available drag force. This drag force is a suitable scaling parameter as long as the particles do not respond to the gas phase velocity fluctuations and as long as the dense phase does not expand.

An increased gas-particle interaction at high pressures, in combination with turbulent fluctuations in the gas phase, might explain the increased instability of bubbles, with a corresponding increased bubble splitting and dense phase expansion at high pressures.

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