Solids Settling in High Concentration Slurries under Static and Sheared Conditions
Creators: name: Lotfiman, Samaneh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400403
subject: Chemical engineering design
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)
classificationCode: 400406
subject: Powder and particle technology

description: Electrical Resistance Tomography (ERT) has been used previously in multiphase systems to detect different phases based on differences in their conductivities. ERT measures particles' position in a settling vessel using solid-liquid mixture conductivities, which determine solids concentration at different heights. Developing mathematical models for estimating solids settling velocity in settling vessels relies on detecting the motion of particles rather than tracking the solid-liquid interface. For the first time, ERT is used in this work to study solids settling velocity in slurries under static and sheared conditions.In this work, a custom-designed ERT cup, equipped with three sensor planes, was used in solids settling experiments. Glass particles and glycerol solution were used as the solid and liquid phases in settling experiments. Settling experiments were carried out under static conditions using solids concentrations ranging from 5 to 30 vol% in glycerol-water solutions with glycerol concentrations ranging from 80 to 90 vol%. Experimental results showed that the solids settling velocity decreases with increasing solids concentration and liquid viscosity. A semi-empirical model was developed using experimental results and the well-known Richardson and Zaki's model to incorporate the effects of hindered settling and slurry viscosity. The predictions from the model agreed closely with the experimental results. In addition to the solids settling rate, the buildup of the settled bed at the bottom is also an important parameter to be determined in settling tanks and slurry transportation in pipelines. In this work, the temporal variations in solids concentration obtained from ERT measurements were used to determine the volume and height of the settled bed at the vessel bottom using solids mass balance. The settled bed buildup velocity estimated from the above measurement was found to increase with increasing solids concentration in suspension and decrease with an increase in liquid viscosity. An empirical model was developed for settled-bed buildup velocity, and its predictions agreeed closely with the experimental results. A comparison of settling and settled bed buildup velocities under static conditions showed that, with increasing solids concentration, solids settling velocity decreases, whereas the settled bed buildup velocity increases. Additionally, with an increase in liquid phase viscosity, both settling and settled bed buildup velocities decrease. The above findings indicate that, as a direct and non-invasive method, ERT could simultaneously measure the solids settling and settled bed buildup velocities in slurries.Study of solids settling velocity under sheared conditions is important for designing and operating many slurry handling equipment, including pipelines for slurry transportation. In this work, ERT was used for the first time to determine solids settling velocity under sheared conditions using an HR3 rheometer. The ERT cup was used as the rheometer cup in a cup-and-bob arrangement. The solids settling velocity of glass particles descending in 90 vol% glycerol solution located in the annular space between the rheometer cup and bob was evaluated by tracking particles' downward movement using the changes in slurry conductivity. The shear rate applied by the rheometer was varied from 5 to 125 s-1 by changing the bob rotational speed. Solids concentration was varied from 5 to 25 vol%. Experimental results showed that solids settling velocity decreases with increasing solids concentration, slurry viscosity and shear rate. An empirical settling velocity model was developed using a modified Richardson-Zaki model incorporating the effects of solids concentration, slurry viscosity and shear rate. The prediction from the model agreed well with the experimental values. The empirical correlation developed for solids settling velocity under sheared conditions was used to develop a model to predict the settling velocity of solids in slurry transported in a pipeline under laminar flow conditions. The predictions from the proposed model were verified using solids settling velocity data available from the literature. The model was used  to develop a follow-up model to predict the critical pipe length beyond which accumulated solids settling could lead to eventual blockage in laminar flow conditions. The model’s predictions showed that the critical pipe length increases with increasing solids concentration in the slurry, pipe diameter and slurry viscosity.