Hydrodynamics, erosion and heat transfer in a pressurized fluidized bed: influence of pressure, fluidization velocity, particle size and tube bank geometry

Measurements of hydrodynamics, local tube erosion and local instantaneous bed-to-tube heat transfer were carried out in a cold pressurized fluidized bed, with two horizontal tube banks having different tube packings. 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 d(p) = 0.7 mm and one with d(p) = 0.45 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 results show that, if plotted vs the excess gas velocity, the hydrodynamic behaviour is similar for the two different particle sizes. However, the smaller particles generally give rise to less erosion than the larger particles, as an effect of their momentum being lower at a given particle velocity. The small particles also give a higher heat transfer than the large particles, as a result of a higher particle convection. The hydrodynamic behaviour, erosion levels and local heat transfer differ significantly between the two tube banks. The denser tube bank causes an earlier transition to a turbulent bed behaviour with increasing pressure or fluidization velocity. The dense tube bank gives rise to considerably less erosion but also gives a somewhat lower heat transfer than the more sparse tube bank, at corresponding operating conditions. 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 dense tube bank 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