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Measurements of flow pattern in a 150 mm i.d. bubble column were carried out using a laser doppler anemometer (LDA) in a forward scatter mode. A superficial gas velocity of 20 mm/s was used in all the experiments. Two different spargers (perforated plate and porous plate) were employed. The liquid flow starts developing from the sparger where non-uniformities in the gas sparging exist. From z/D ≥ 2 (z = axial location; D = column diameter), the fully developed axial liquid velocity profiles are seen. The radial variation of mean axial velocity shows gross liquid circulation in the column. Circulation velocities are smaller for the bubble column with porous plate sparger mainly due to the fact that the mean bubble size is small and the gas distribution is relatively uniform over the column cross-section. The difference in the hydrodynamic behavior with two different spargers is also apparent from the profiles of the turbulent kinetic energy and Reynolds stress measured from the velocity−time series.

Axial flow impellers are commonly used for pulp suspension agitation. Pulp fiber suspensions are non-Newtonian and exhibit a yield stress. In mixing operations, a 'cavern' (region of active motion) is created around the impeller, with the size of the cavern affecting the quality of mixing attained. In this work, the cavern size produced by four different axial flow impellers in a C m ) 3% (mass concentration) hardwood pulp suspension was measured using electrical resistance tomography (ERT) and by analysis of dynamic mixing tests. Cavern size is shown to depend on impeller performance as characterized by power number, N P , and axial force number, N f . At an equal power consumption of 0.53 kW/m 3 the largest cavern was produced by the impeller having the largest values of N P and N f . The measured cavern volumes compared well with predictions of the axial force model developed by Hui et al. [Hui, L. K.; Bennington, C. P. J.; Dumont, G. A. Cavern formation in pulp suspensions using side-entering axial-flow impellers. Chem. Eng. Sci. 2009, 64, 509], which accounted for interaction between the cavern and the vessel walls. When the cavern just filled the vessel volume, the time constants determined using the dynamic mixing test data reached 90% of their theoretical values (with the estimated standard deviation of (10%), indicating that the chest approached an ideal dynamic response (complete mixing) with the onset of complete motion in the chest.

For a side-entering impeller of diameter D, the clearance from the off-wall (E) is characterised by E/D and it affects the mixing quality attained in a pulp stock chest. For agitation of pulp suspensions, mixing can be characterised by the size of cavern produced by the impeller. In this work, we have investigated cavern size as a function of the impeller off-wall clearance in a laboratory-scale cylindrical stock chest. Hardwood pulp suspensions of C m = 2%, 3%, and 4% (fibre mass concentration) were agitated using an axial flow impeller with E/D varied from 0.14 to 0.68. Cavern size was measured using electrical resistance tomography (ERT) in batch operation and dynamic mixing tests in continuous operation, with cavern size increasing with increasing E/D. At E/D = 0.14, throttling of the impeller suction occurred which reduced cavern size. Computational fluid dynamic (CFD) simulations for steady state operation under-predicted the cavern size, but correctly captured the trend in cavern size variation with E/D. The measured cavern volumes compared well with predictions of an axial force model that accounted for interaction between the cavern and the vessel walls only when impeller throttling was absent.

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A population balance model is developed for the prediction of the average gas holdup and axial gas holdup profile in the bubble columns operating in the homogeneous regime. The model takes into account the effect of bubble expansion along the column height and thus it is applicable to tall columns such as flotation columns in the mineral processing industries. The model captures the effect of superficial gas and liquid velocities, bubble rise velocity, operating pressure, and column height on the gas holdup. In the case of flotation columns, because of the presence of surfactants in the liquid phase, froth is observed at the top of the column. Typically, froth shows the axial profile of gas holdup and the mean bubble size, indicating that the coalescence is an important phenomenon to describe the dynamics of the froth. A probabilistic perspective is developed to address the bubble coalescence process occurring in the froth and the bubble transition probability for coalescence is obtained from experimental data in the literature.