(8f) The Effect of Impeller off-Wall Clearance On the Performance of Pulp Suspension Mixing in a Side-Entering Cylindrical Stock Chest

Impeller clearance from the vessel wall, characterized by E/D, affects the mixing quality attained in a side-entering stock chest. In the agitation of pulp suspensions, the mixing can be characterized 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 Cm = 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 by dynamic mixing tests in continuous operation, with cavern size increasing with increasing E/D. At E/D = 0.14, throttling of the impeller suction occurs which reduced cavern size. CFD simulations for steady state operation under-predicted the cavern size, but correctly captured the trend in 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.