Particle Settling Through Emulsions: System Stability and Prediction of Settling Velocity

  • Author / Creator
    Fozooni Kangarshahi, Aref
  • Particle settling in emulsions is encountered in a number of industrial processes. For instance, multiphase separators are used to separate oil from mixtures that also contain water, gas, and particles. In such separators, an “emulsion layer” forms through which the solid particles must settle. To design these separators, the prediction of the particle settling rate is required. If the particle settling velocity prediction through any medium (including the emulsion layer) is incorrect, the settler may not operate effectively.

    Past experimental works on particle settling in emulsions have shown that these systems can become unstable when the droplet and particle sizes are similar, which leads to the formation of finger-shaped streams. These “fingering streams” include both downward-flowing liquid streams transporting solids and upward-flowing streams transporting emulsion droplets. When the fingering phenomenon occurs, the actual particle settling velocity is significantly higher than predicted values from correlations appropriate for batch sedimentation.
    In other settling systems comprised of suspensions of light and heavy particles, a similar phenomenon takes place. Studies on these heavy-light particle systems show that the size, concentration, and density of both heavy and light particles determine the system stability. According to the similarities between the two settling systems, the same properties of the settling particles and emulsion droplets have been considered as controlled variables for studying emulsion-particle settling systems stability in the current project. In other words, the emulsion droplets and solid particles have been considered as light and heavy particles, respectively.
    Two main objectives of this project are: 1- To develop a tool for defining the stability status of emulsion-particle settling systems under a variety of conditions; 2- To study the velocity enhancements caused by the fingering phenomenon and improve the understanding of unstable emulsion-particle settling systems.
    A series of experimental studies have been conducted in this project. The settling of glass beads with different sizes in oil-in-water emulsions was observed through the transparent wall of a Plexiglas settler. The results include a set of ‘stability maps,’ which show the stability status of the emulsion-particle settling systems under different conditions, including oil droplet density, size and concentration, and particle size and concentration. Since standard settling velocity correlations cannot be used for unstable settling, the stability maps prepared in this project could be used to predict the stability status of the emulsion-particle settling systems.
    In addition to the stability observations, particle settling velocities were measured for each settling test. Settling velocities predicted using the Richardson-Zaki equation for both stable and unstable tests were compared to the measured values. In all of the velocity predictions, the emulsion is considered to be a “continuous phase.” The analysis of the results shows that the Richardson-Zaki equation can only predict the settling velocities of the stable systems. In other words, the emulsion cannot be considered as a continuous phase when a system is unstable.
    The final task in this project was developing a model for fingering conditions by considering the emulsion as a non-continuous phase and assuming that fluid movements inside the fingers carry particles and emulsion droplets. The model predicts the behavior of the system with high accuracy. One key conclusion from this study is that emulsion droplets act like rising particles moving through water during the fingering phenomenon. Another significant conclusion is that fluid movement inside the fingers is the main reason for settling velocity enhancements. This achievement expands the understanding of unstable emulsion-particle settling systems and broadens the knowledge in this area of research.
    At the end, two recommendations are provided for the future work in this area: 1- Conducting emulsion-particle settling experiments using advanced methods like particle image velocimetry (PIV) to more accurately measure phase velocities during unstable settling; and 2- Conducting stability tests for particle settling through water-in-oil emulsions (since oil-in-water emulsions were tested here).

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.