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Keywords:

  • spout-fluidized beds;
  • granulation;
  • elevated spout;
  • spout (nozzle);
  • height;
  • particle image velocimetry;
  • discrete particle model

Abstract

The effect of elevating the spout on the dynamics of a spout-fluidized bed, both numerically and experimentally is studied. The experiments were conducted in a pseudo-two-dimensional (2-D) and a cylindrical three dimensional (3-D) spout-fluidized bed, where positron emission particle tracking (PEPT) and particle image velocimetry (PIV) were applied to the pseudo-2-D bed, and PEPT and electrical capacitance tomography (ECT) to the cylindrical 3-D bed. A discrete particle model (DPM) was used to perform full 3-D simulations of the bed dynamics. Several cases were studied, that is, beds with spout heights of 0, 2, and 4 cm. In the pseudo-2-D bed, the spout-fluidization and jet-in-fluidized-bed regime, were considered first, and it was shown that in the spout–fluidization regime, the expected dead zones appear in the annulus near the bottom of the bed as the spout is elevated. However, in the jet-in-fluidized-bed regime, the circulation pattern of the particles is affected, without the development of stagnant zones. The jet-in-fluidized-bed regime was further investigated, and additionally the experimental results obtained with PIV and PEPT were compared with the DPM simulation results. The experimental results obtained with PIV and PEPT agreed mutually very well, and in addition agreed well wtih the DPM results, although the velocities in the annulus region were slightly over predicted. The latter is probably due to the particle-wall effects that are more dominant in pseudo-2-D systems compared with 3-D systems. In the jet-in-fluidized-bed regime, the background gas velocity is relatively high, producing bubbles in the annulus that interact with the spout channel. In the case of a non elevated spout, this interaction occurs near the bottom of the bed. As the spout is elevated, this interaction is shifted upwards in the bed, which allows the bubbles to remain undisturbed providing the motion of the particles in the annulus near the bottom of the bed. As a result, no dead zones are created and additionally, circulation patterns are vertically stretched. These findings were also obtained for the cylindrical 3-D bed; although, the effects were less pronounced. In the cylindrical 3-D bed the PEPT results show that the effect on the bed dynamics starts at hspout =1 4 cm, which is confirmed by the ECT results. Additionally, ECT measurements were conducted for hspout =1 6 cm to verify if indeed the effect happens at larger spout heights. The root mean square of the particle volume fraction slightly increased at hspout =1 2 cm, whereas a larger increase is found at hspout = 4 and 6 cm, showing that indeed more bubbles are formed. The presented results have not been reported so far and form valuable input information for improving industrial granulators. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2524–2535, 2012