Quantitative Measurements of the Critical Impeller Speed for Solid‐Liquid Suspensions

Abstract

A quantitative methodology for particle suspension assessment is presented. A new parameter, f mov/tot , the ratio of the mean number of moving particles to the total number of particles, is introduced to evaluate the minimum speed required to just suspend solids. This approach is tested to investigate the impact of impeller clearance on the minimum impeller speed, N js , in a vessel when using a radial flow Rushton turbine. Flow patterns and power numbers obtained experimentally and computationally support the suspension findings. Image analysis is an appropriate method for determining N js . Lowering the impeller clearance reduces the speed required for particle suspension with a change of flow pattern from a radial discharge with two loops to a single loop scouring the vessel base. The power number also falls markedly at the two-to-one loop transition as does the strain rate near the base.

Determination of the just‐suspended speed, Njs, in stirred tanks using electrical resistance tomography (ERT)

“…In flat‐bottom vessels, particles tend to be suspended primarily from the periphery of vessels in the presence of an axial impeller, which is to be expected 67 . However, Figure 10 also shows that the N js values for the PBT vary more significantly with C / T than for the other impellers, which is consistent with the fact that the PBT is more a “mixed‐flow” impeller rather than a true axial flow type 6,29 . Therefore, the flow it generates is oriented downward but at an angle, and the location where solid suspension off the vessel bottom takes place moves from the periphery of the vessel toward the center as the impeller clearance is increased, thus requiring higher N js values as well as higher power dissipation 68,69 .…”

“…Therefore, the flow it generates is oriented downward but at an angle, and the location where solid suspension off the vessel bottom takes place moves from the periphery of the vessel toward the center as the impeller clearance is increased, thus requiring higher N js values as well as higher power dissipation 68,69 . Although these observations have been made by previous researchers and are not new, 6,29,68,69 they are relevant here because they show that the ERT method can be used irrespectively of the location where the suspension originates at the vessel bottom, even when the same impeller type (PBT) is used while C / T is varied.…”

“…Below this off‐bottom particle suspension state, the solid–liquid interfacial surface area is not completely or efficiently utilized. Therefore, it is important to determine the impeller agitation speed, N js , at which the just‐suspended state is achieved by the solids, 5 and a significant amount of work has been reported already on the experimental determination of N js and the development of equations correlating N js to relevant geometric and operational variables 5–29 …”

“…Several experimental methods have been reported in the literature to measure N js and they have been summarized elsewhere 29–31 . The most common techniques include solids sampling, 17,20,32 visual observation, 5,31 pressure gauge technique, 33–39 and steady cone radius measurements, 30,40,41 positron emission particle tracking, 28 each one with different advantages and disadvantages 29,31,42 .…”

“…Several experimental methods have been reported in the literature to measure N js and they have been summarized elsewhere 29–31 . The most common techniques include solids sampling, 17,20,32 visual observation, 5,31 pressure gauge technique, 33–39 and steady cone radius measurements, 30,40,41 positron emission particle tracking, 28 each one with different advantages and disadvantages 29,31,42 . A review of the literature shows that the most common method to experimentally measure N js is the visual method originally developed by Zwietering, 5 consisting in inspecting the tank bottom and visually determining the impeller agitation speed at which the solids are observed to rest on the tank bottom for no more than 1–2 s before being swept away.…”