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 (eg), residence time distribution (RTD) and liquid mixing time (tm) have been carried out in an air-water system in a 17 lit bubble column with an 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 and the change of slope related to eg versus superficial gas velocity (Usg) indicated the transition point from homogeneous to heterogeneous regime at Usg= 0.7-0.9 ms -1 . Besides, the experiments illustrated that 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). Moreover, fitting results of the model and experimental RTD curves achieved from tracer injection method attained the model parameter, Dax , in three H/D ratios of 4.73, 6.36 and 8.84. Results showed that an elevation in H/D ratio, caused a rise in Dax. On the other hand, the mixing time data declined with an increase in Usg; however enhancement of H/D ratio caused an increase in tm.
Effect of internals and sparger design on mixing behavior in sectionalized bubble column
Abstract An experimental study has been carried out to investigate the effect of internals and sparger design on mixing time ( θ mix ) and fractional gas hold-up ( ɛ G ) in a batch mode sectionalized bubble column. Air and water were used as the gas–liquid phases, respectively. In the present work, sparger with the percent free area (% FA) of 0.136 and 0.6% has been used and the superficial gas velocity ( V G ), liquid height to column diameter ( H c / D ), percent free area of the sectionalizing plate was varied from 0.06 to 0.295 m s −1 , 3 to 4 and 4 to 23%, respectively. It was found that there is no significant effect of the sparger design on the mixing time but it does strongly depend on V G , H c / D and % FA of the sectionalizing plate. The one-dimensional dispersion model successfully predicts the tracer concentration profile and the longitudinal dispersion coefficient. Also, the effect of the presence of the electrolyte too has been studied by adding a known volume of the tracer solution (NaCl). Correlations have been developed for the estimation of the fractional gas hold-up, mixing time, longitudinal dispersion coefficient ( D L ) and the intercell exchange velocity ( u B ).
Experimental studies on kLa in a mechanically agitated contactor from transient electrical conductivity response data
A new dynamic response method for the measurement of volumetric mass transfer coefficient, k L a, in a mechanically agitated contactor is proposed. This involves bubbling pure CO 2 continuously into a batch stirred aqueous liquid phase and measuring continuously, as a function of time, the change in the electrical conductivity of the aqueous phase caused by the transformation of carbonic acid. A mathematical model is proposed with the help of which the value of k L a can be determined by using a nonlinear least squares method. The results obtained by using the present method do not depend on the RTD of the gas phase, as pure gas is employed. Experiments were conducted in 0.164, 0.295, 0.41 m diameter standard tanks agitated by six flat blade disk mounted turbines. The following correlation was obtained:k L a = 4.193X10 -2 (P g /V) 0.495 V S 0.75 where P g /V is power consumption in watts/m 3 and V S is superficial gas velocity in m/s