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Mixing dynamics of pulp suspensions in cylindrical vessels Hui, Kwok Wai Leo

Abstract

Cylindrical agitated chests are frequently used to facilitate manufacturing processes in pulp and paper industry and one of their main functions is to attenuate any process disturbances. However, owing to the inherited non-Newtonian nature of pulp suspensions, it is not easy to achieve complete mixing and with the improper chest design, these agitated chests do not always perform ideally or satisfactorily. The cavern formation in incomplete mixing may induce bypassing and dead zones, which significantly affect the chest performance. A study of cavern formation in a cylindrical agitated chest was thus carried out. Also, a dynamic model developed by Soltanzadeh et al. (2009) was used to quantify the mixing dynamics of the cylindrical chest. In addition, using computational fluid dynamics (CFD), the simulated results of the flow in the chest were compared with the experimental results to verify the applicability of the CFD model on the study of pulp suspension agitation. To investigate the cavern formation in a lab-scale cylindrical chest, electrical resistance tomography (ERT) and ultrasonic Doppler velocimetry (UDV) were applied to estimate the cavern shape and size. Both methods gave satisfactory results and as expected, the cavern size was found to increase with impeller speeds. The cavern shape was best described as a truncated right-circular cylinder. Based on this observation, a model considering the interaction between the cavern and chest walls was developed to calculate the cavern volume. With the dynamic model, a series of dynamic tests were carried out to characterize the mixing behavior of the lab-scale cylindrical chest. It was found that the proposed flow configuration with the outlet close to the cavern could minimize the bypassing which affects mixing quality. Also, ERT verified the presence of cavern and dead zone when the chest was not completely agitated in continuous-flow operation. Numerical simulations using CFD were compared with the experimental results under different operating conditions. Pulp suspensions are a mixture of water and wood fibres that can entangle each other to form flocs affecting the mixing flow. Owing to this complex rheology, it is not easy to model the agitation precisely in CFD using a homogeneous fluid model. The floc formation and air entrapment observed in experiments were difficult to be numerically taken into account in the simulations. Although the CFD model could not exactly predict the mixing situation of pulp suspensions, it still can be used to estimate the mixing flow patterns, e.g., flow directions, in the proposed chest designs.

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