Liquid phase axial disporsion characteristics in three-phase fluidized beds

Abstract

The axial dispersion characteristics in two (gas-liquid, liquid-solid) and three (gas-liquid-solid) phase fluidized beds have been studied in a 14.5cm I.D. column. A wide variety of liquids and three different solid particles (1.7, 3.0 and 6.0 mm glass beads) were employed. Air was used as the gas phase throughout the study. The effects of liquid velocity (2-13 cm/sec), gas velocity (0-12 cm/sec), liquid viscosity (1-27 cp), surface tension (38.5-76 dyne/cm) and particle size on the axial mixing of liquid phase have been examined. Liquid phase axial dispersion coefficients were measured using a pulse tracer technique and individual phase holdups were measured using a pressure-profile technique. In gas-liquid system, liquid phase axial dispersion coefficient increased with increasing gas and liquid velocities and gas phase holdup increased with increasing gas flow rate. However, gas holdup was nearly independent from liquid flow rate. In liquid-solid systems, the liquid mixing intensity increased with liquid flow rate in the region of smaller liquid holdup, however, above the liquid holdup of 0.65, somewhat decreasing trend of the mixing intensity was observed. In three phase systems, the axial dispersion coefficient increased with increasing gas flow rate and decreasing trend was observed with particle size. The effect of liquid velocity on the dispersion coefficient depended on the gas flow rate and it was more pronounced with increase of gas flow rate. Thus, the mixing intensity increased with the ratio of gas to liquid flow rate. Liquid viscosity generally reduced the dispersion intensity at high gas flow rates, however at small gas flow rates, the reverse trend was observed. Considerable bubble disintegrating phenomenon was observed when the liquid surface tension decreased and the dispersion magnitude increased with increasing surface tension. Liquid phase axial dispersion in terms of Peclet number has been correlated with Froude numbers of liquid and gas ...