(585a) Multiscale Characterization and CFD Simulation of W/O Emulsions

Juan Pablo Gallo-Molina, Nicolás Ratkovich, Óscar Álvarez.

Process and Product Design Group (GDPP).

Department of Chemical Engineering, Universidad de los Andes, Bogotá, Colombia. 

Emulsions are a type of metastable colloids composed by two or more immiscible liquids. Both oil-in-water (O/W) and water-in-oil (W/O) emulsions are widely used in a variety of applications, such as cosmetics, drug delivery, food, etc. Although there exist theoretical foundations which offer insights into these systems, industry practices often focus on formulation and process; commonly using empirical methods. In this work, a multiscale approximation was used for the study of W/O emulsions. This approach allows for the analysis of interrelationships among macroscopic, microscopic, process and formulation variables. Additionally, the emulsions were modelled with Computational Fluid Dynamics (CFD), which permitted a better understanding of the role process variables plays. Using the incorporated energy during the emulsification process as a transversal factor, it was possible to establish relationships between rheological measurements (i.e. the macroscopic response) and droplet size distribution observations (i.e. the microscopic variable). Additionally, stability measurements were made and differences among impeller types used during preparation were established.

In accordance to previous literature (Pradilla et al. 2015), average droplet size was found to be inversely proportional to the dispersed phase concentration, while the opposite behavior was observed in incorporated energy. This was explained by the increments in the elasticity of the interphase associated with increases in the dispersed phase concentration (Langevin, 2000). Results also showed that an increase in the incorporated energy leads to larger values of the elastic modulus in the linear viscoelastic region. That is: more elasticity at the interphase and more solid-like behavior in the product. Furthermore, larger amounts of incorporated energy tended to generate more stable emulsions with lower average droplet diameters. This was attributed to the lower sensitivities to gravitational effects and the reduction in Ostwald ripening associated with smaller droplets in the system (Leal-Calderon et al., 2007).

Emulsions were prepared using four impeller types: propeller, straight paddles turbine, 45º pitched blade turbine and Rushton turbine. Sizable differences in the response variables were observed among the mentioned types. For instance, significantly different average mean droplet diameters were observed in function of the impeller type, which led to a tendency towards different degrees of stability. This phenomenon occurs because different impeller geometries add different amounts of energy and make different quantities of shear available for the emulsification process (Torres & Zamora, 2002). In turn, the divergences in the incorporated energies generated generally slight differences in the values of the elastic modulus.

CFD simulations were developed using commercial software STAR-CCM+ v11.04 (Siemens). The modelling was limited to emulsions prepared with the propeller type impeller and only the homogenization stage (steady state) was considered. An Eulerian approach was preferred and no turbulence model was implemented, for the reason that flow regimes were laminar under all conditions. The Morris and Boulay model was used for describing the rheological behavior of the systems, while the S-gamma approximation was chosen for describing the drop size distribution as well as breakage and coalescence. There was good agreement between experimental data and CFD results for relative viscosity and incorporated energy and it was possible to observe three dimensional profiles of relevant variables. Gradients in droplet diameter, relative viscosity and dispersed phase volume fraction were found to exist in the mixing tank, which indicated that the chosen impeller and tip velocity don’t provide optimal mixing, while generating effects in the final macroscopic and microscopic properties of the product.

REFERENCES

Langevin, D. (2000). Influence of interfacial rheology on foam and emulsion properties. Advances in Colloid and Interface Science, 88(1-2), 209–222. https://doi.org/10.1016/S0001-8686(00)00045-2-

Leal-Calderon, F., Schmitt, V., & Bibette, J. (2007). Emulsion science, (Second Edi). New York: Springer. https://doi.org/10.1007/978-0-387-39683-5

Pradilla, D., Vargas, W., & Alvarez, O. (2015). The application of a multi-scale approach to the manufacture of concentrated and highly concentrated emulsions. Chemical Engineering Research and Design, 95(February), 162–172. https://doi.org/10.1016/j.cherd.2014.10.016

Torres, L. G., & Zamora, E. R. (2002). Preparation and power consumption of surfactant-fuel oil-water emulsions using axial, radial, and mixed flow impellers. Fuel, 81(17), 2289–2302. https://doi.org/10.1016/S0016-2361(02)00137-0