Longitudinal Dispersion of the Dispersed Phase in Spray Extraction Columns

Abstract

The longitudinal dispersion of the dispersed phase was studied in spray columns with mechanical dispersion of a heavy phase (water) in kerosene or a light phase (kerosene) in water. The columns were operated in the loose, close, and stirred close drop packing modes [1, 2]. The specific total liquid flow rate was varied in the range of Q = 15‐40 m 3 /(m 2 h); the peripheral velocity of the stirrers, in the range of u st = 0.690‐1.265 m/s. In the close drop packing mode, the peripheral velocity of the stirrers located in the close drop packing layer was varied from 0.6 to 1.0 m/s. In the Q , dispersed-phase holdup Φ , and mean drop diameter d 32 ranges examined, Pe disp = 4‐6 and, correspondingly, E disp = (100 - 150) 〈 10 —4 m 2 /s (see figure). The relatively low values of e disp and the high values of E disp indicate an intensive longitudinal dispersion of the dispersed phase in this drop packing mode. In conventional spray columns (with nozzle spraying) the Peclet number e disp is much higher [3], that is, there is only a slight longitudinal dispersion of the dispersed phase in the loose drop packing mode. The high intensity of longitudinal dispersion in columns with mechanical dispersion is apparently due to the formation of a more polydisperse emulsion, from which a large amount of small drops is carried away by the continuous phase. In the close drop packing mode, the entrainment of small drops is reduced owing to some enlargement of the drops and some narrowing of the drop-size range; the close-packing layer itself can prevent the small drops from being carried away by the continuous phase. In this case, we observe (figure) lower e disp values of 2.5‐4.0 ( E disp = (50—75) 〈 10 —4 m 2 /s), which indicate that the longitudinal dispersion intensity is lower in the close drop packing mode than in the loose one. Comparing these e disp and E disp values with the values of e ae ∪ 18 and E cont ∪ 6 〈 10 —4 m 2 /s [3] for the close drop packing demonstrates that the intensity of the longitudinal dispersion of the continuous phase is higher than that of the dispersed phase. Comparing the observed e disp values for the close and stirred close drop packings (figure) shows that