Hydrodynamics of a pressurized fluidized bed with horizontal tubes: Influence of pressure, fluidization velocity and tube-bank geometry
Journal article, 1995
Measurements of the bubble hydrodynamics were carried out in a cold pressurized bed with horizontal tubes. The mean bubble rise velocity, the bubble frequency, the mean pierced length, the bubble volume fraction, and the visible bubble flow rate were measured using capacitance probes. The absolute gas velocity through the bubbles was measured using Pitot-static pressure probes. The bed expansion ratio was determined by measuring the pressure difference between the freeboard and the bed at different heights and extrapolating the pressure difference down to zero. The fluctuations in the pressure drop over the entire bed height were also measured, and the power spectral density distribution of these fluctuations was calculated. The influence of pressure, fluidization velocity, and tube-bank geometry on the bubble behaviour and gas-flow distribution were studied. The bed has a cross-section of 0.2 m x 0.3 m. It was operated at pressures between 0.1 and 1.6 MPa, at excess gas velocities of 0.2 and 0.6 m/s. Three different tube-bank geometries were used, one with a fairly dense pitch and two with more sparse configurations, and comparisons are also made with previous results obtained without tubes in the bed. The bed material was silica sand with a mean particle diameter of 0.7 mm. For the three tube banks investigated here, all the measured parameters except the mean pierced length consistently increased with increasing excess gas velocity. The mean pierced length increased with increasing excess gas velocity at low pressures, while the velocity effect at high pressures was less obvious. When the pressure was increased, the mean pierced length first increased to a maximum value, at p = 0.4 MPa for the low excess gas velocity and at p = 0.2 MPa for the high excess gas velocity, then decreased again as the pressure was increased further. The bubble frequency and the bed expansion increased as the pressure was increased. The absolute gas velocity through the bubbles as well as the gas velocity relative to the bubbles decrease as the pressure was increased. For the sparse tube banks at the lower excess gas velocity, the mean bubble rise velocity, the bubble volume fraction and the visible bubble flow rate increased as the pressure was increased. For the dense tube bank, however, these parameters showed a maximum at p = 0.4 MPa. At the higher excess gas velocity, these parameters showed a maximum at about p = 0.2-0.4 MPa for all three tube banks. It appears that, at this velocity, the presence of tubes prevents a further increase in these parameters. This behaviour differs significantly from the behaviour without tubes. From the power spectral density distributions of the pressure fluctuations over the entire bed height and from visual observation, it appears that the bed is slugging, or close to slugging, at atmospheric pressure for all the tube configurations. When the pressure is increased, the power spectral density distribution becomes wider as the large bubbles/slugs break down and the bed moves toward a more dispersed bubbling behaviour.