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

Abstract

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.

Computational simulation of mixing flow of shear thinning non-Newtonian fluids with various impellers in a stirred tank

“…1). The E/D ratio (clearance from rear-wall to impeller diameter ratio) was set to 0.56 based on the results presented by [9,33,44].…”

“…Due to the wide variety of mixing requirements, many variations of the constituent parts of the stirred tank have arisen. It has been shown that operating conditions and geometrical properties of the impeller and vessel define the fluid dynamic inside the tank [9,25,34,42]. For instance, flow patterns of axial and hydrofoil impellers are characterized by one main circulation loop, and high velocities near the blade tip.…”

“…To date, most of the attention has been caught by stirred tanks equipped with top entry impellers, and little has been done to elucidate the hydrodynamics and the mixing mechanisms in tanks with side entry impellers. This configuration is suitable for large storage vessels commonly used in the pulp and paper industry as well as blending and sludge control tanks in the oil industry, among others [9,19,21].…”

Effect of mixer geometry and operating conditions on flow mixing of shear thinning fluids with yield stress

“…Another important parameter, affecting power consumption and flow pattern is the impeller location. There are some guidelines in literature for impeller clearance (distance between the impeller and vessel rear wall) based on empirical work . However, their applicability might be limited to specific conditions, since the optimum dimensionless clearance E/D (where E is the clearance from rear‐wall, and D is the impeller diameter) is a function of operating conditions, rheology, and mechanical constraints.…”

“…However, their applicability might be limited to specific conditions, since the optimum dimensionless clearance E/D (where E is the clearance from rear‐wall, and D is the impeller diameter) is a function of operating conditions, rheology, and mechanical constraints. Bhole et al used electrical resistance tomography to identify dead zones inside a stock chest and showed that for a 3% hardwood pulp suspension the optimal E/D ratio is close to 0.5. Moreover, they found a substantial restriction of the flow to the impeller suction at E/D = 0.14, which leads to an increase of the power requirements.…”