The effect of off‐wall clearance of a side‐entering impeller on the mixing of pulp suspensions in a cylindrical stock chest

Abstract

AbstractFor 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 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 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.