Effects of Height to Diameter Ratio and Aeration rate on Liquid Mixing and Hydrodynamic Properties in a Bubble Column

Experimental measurements of overall gas holdup (εg), residence time distribution (RTD) and liquid mixing time (tm) have been carried out in air-water system in a 17 lit bubble column with 11 cm diameter, over a wide range of superficial gas velocity (0.14-1.46 ms-1) and height to diameter ratio (1.36-8.84). The bed expansion method was used to obtain holdup values. The change of slope in εg versus superficial gas velocity (Usg) indicates the transition point from homogeneous to heterogeneous regime at Usg= 0.7-0.9 ms-1. Because the bubble generated had a similar size to the equilibrium bubble size, the H/D ratio had no effect on holdup values. The axial dispersion model was used with semi-closed boundary conditions for prediction of RTD and hence the axial dispersion coefficients (Dax). Fitting results of the model and experimental RTD curves achieved from tracer injection method attained the model parameter, Dax in three H/D ratio of 4.73, 6.36 and 8.84. Results show that an elevation in H/D ratio causes an increase in Dax. The mixing time data decrease with increasing Usg, but enhancement of H/D ratio cause an increase in tm.

کلیدواژه ها:

bubble column ، overall gas holdup ، residence time distribution ، axial dispersion coefficient ، mixing time