Radial and axial density fluctuations in a high-velocity fluidized bed.

摘要

The use of gas-solid high velocity fluidized beds as catalytic chemical reactors in industrial practice is well known. A typical fast fluidized bed is operated with two distinct flow regions, a dense phase in the bottom and a dilute phase on top giving an s-shaped axial profile of the solid fraction. Recent publications show also the existence of a radial solid distribution profile with a dense wall region and a dilute core in both of the regions. Also, the occurrence of pressure waves in the lower dense region have been reported. These pressure waves, with frequencies of the order of one Hz, were interpreted as resulting from density waves. At this point in time only the first steps in describing the flow pattern for a fluidized bed operating in the high velocity fluidization regimes have been discussed.;In this work a technique is developed to directly measure instantaneous solid fractions and its radial distribution in a riser flow of air and fluid cracking catalyst. The method is based on an x-ray chordal absorptometry technique using phototransistor sensors hardwired to a computerized data acquisition system. Complementary to the x-ray measurements density fluctuations were studied using instantaneous pressure signals. Measurements were made in a high velocity fluidization unit with an inner diameter of 0.152 m and 8.4 m high. The solid used in this investigation was a Zeolite FCC catalyst, HFZ-33 (dp = 59 ;The data confirmed that the pressure readings are dependent on the spacing of the pressure probes. Decreasing the spacing of the probes leads to an increase in fluctuation amplitude and mean time average of the pressure readings, while the power spectrum shows no dependence on the spacing. To obtain reproducible data, a time average of three minutes for the pressure readings and one minute for the x-ray measurements are necessary.;Radial density profiles detected with an x-ray system in the dilute and dense phase show the existence of a core-annulus flow with a dilute core surrounded by a denser wall region. The density profiles indicate clearly that a fully developed flow pattern exists only in the dense phase.;Pressure fluctuation readings converted directly to solid fraction are larger in amplitude than the solid fraction fluctuation detected with the x-ray system. Pressure fluctuation readings are a sum of the pressure drop caused by the solid fraction in a height segment of the riser and acceleration and deceleration of the solid. The power spectra for both the pressure readings and x-ray measurements show a superimposed low frequency.