STUDIES ON MULTIPHASE AGITATION

Abstract

The present study deals with the measurement of critical impeller, speed for suspension of solid particles (NCR) in mechanically agitated two-phase (solid-liquid) contactor, as well as critical impeller speed for suspension of solid particles (NCG) in mechanically agitated three-phase (solid-liquid-gas) contactor of 0.2032 m i.d. Tap Water and glass beads of average particle size 1 mm and 3mm were used as the liquid and solid phases, respectively. Compressed air was used as the gas phase passed through a ring sparger. The impeller speed was varied. The superficial gas velocity and solid loading were varied in the ranges 0-0.0343 m/s and 1-10% w/w, respectively. Three types of impellers, namely, flat blade turbine (FBT), vaned disc (VD), and pitched turbine downflow (PTD), were employed. Readings were taken at three clearances. The experiments were carried in two tank bottom environment, namely-flat and hybrid. Power measurements were also carried out. The critical impeller speed for suspension of solid particles was found to be always more for the gas-liquid-solid system as compared to the solid-liquid system. The difference in the impeller speeds (LNCR=NCG-NCR) was found to be dependent on the superficial gas velocity, solid loading particle settling velocity, and the impeller diameter. Empirical correlation has been presented for this increase in critical impeller speed for solid suspension in three phase systems in case of hybrid bottomed tank using PTD impeller: oNCR = 110.3X0.088 0.875 -0.628 Vset1.234 As a final conclusion, compared with the standard flat bottomed mixing tank, hybrid bottomed tank was much more efficient in suspending the solids and pitched turbine downflow impeller at a clearance of 6th the tank diameter was the best out of the three agitators studied.