Abstract
Abstract The research project MODEM, Next-generation multiscale MOdelling of Dense EMulsions for enhanced multiphase flow processes, aims to advance the fundamental understanding and predictive modeling of dense liquid-liquid emulsions for application to a wide range of industrial multiphase processes. The project, supported by the European Union through the NextGenerationEU programme, involved the Universities of Udine, Bologna, and Naples, as well as the Polytechnic University of Turin. Dense emulsions, involving two immiscible liquid phases dispersed at high volume fractions, are encountered in critical operations such as petroleum extraction and transport, chemical production, food and pharmaceutical manufacturing, and liquid-liquid separations. Despite their ubiquity and importance, their behavior remains difficult to predict because of the complex, multiscale mechanisms at play, including turbulence, droplet deformation and interactions, interfacial phenomena influenced by surfactants, and the emergence of non-Newtonian rheology at higher concentrations. MODEM addresses these challenges through an integrated multiscale strategy that combines advanced experiments, high-fidelity simulations at different scales, and robust modeling approaches that bridge the gap between microscale droplet physics and macroscale process conditions. Achieved Results Within the MODEM project, the experimental activity carried out at UNIBO aimed at providing high-quality drop size distribution (DSD) and liquid hold-up data for liquid-liquid dispersions in mechanically agitated systems under fully turbulent conditions. Accurate DSD characterization is essential since interfacial area and process performance depend on the full spectral distribution rather than on averaged metrics. Liquid hold-up measurements allow identification of the transition between different dispersion regimes. To address these objectives, a comprehensive experimental campaign was completed, investigating the combined effects of impeller speed, dispersed-phase viscosity, and volume fraction on the steady-state DSD. The study spans dilute conditions, where breakup mechanisms can be examined in isolation, up to denser regimes in which collective effects progressively alter the distribution morphology. The adopted approach enables consistent comparison across operating parameters and provides a reference dataset for the multi-scale modeling activities developed within the project. The analysis of the liquid hold-up distribution was carried out in denser regimes, changing the working conditions to move from stratified to completely dispersed regimes. Both DSD and hold-up experiments were carried out in a cylindrical, flat-bottomed, fully baffled stirred tank of diameter and height T = H = 232 mm, operated with a single Rushton turbine. In the DSD experiments, the tank operated in the fully turbulent regime, Re = 5 x 104 to 7 x 104, with the selected speed range preventing phase segregation and air entrainment from the free surface. In the liquid hold-up experiments, the Reynolds number was varied in the range Re = 4 x 103 to 4 x 104. The continuous phase was demineralized water, ρC = 1000 kg m-3, µC = 1.0 x 10-3 Pa s, refractive index n = 1.33, while the dispersed phase consisted of three Newtonian polydimethylsiloxane (PDMS) silicone oils with kinematic viscosities of 2 cSt, 10 cSt, and 100 cSt, corresponding to densities of 867, 941, and 974 kg m-3, respectively, refractive index n = 1.43, and interfacial tension sigma = 0.021 N m-1. The selected system provides a controlled variation of dispersed-phase viscosity over two orders of magnitude while maintaining comparable density ratios and interfacial properties. DSD experiments The experimental campaign was structured to systematically explore the combined influence of impeller speed, N = 500, 600, 700 rpm, dispersed-phase viscosity, and volume fraction α. Volume fractions ranged from dilute conditions, α= 0.1%, to progressively denser regimes, α= 10%, in which collective effects become increasingly relevant. A total of 27 independent operating conditions were characterized in the campaign. The experimental procedure was developed building on previous research on similar systems [1]. Drop size distributions were measured using a Spraytec laser-diffraction system (Malvern Panalytical) equipped with a wet dispersion unit. Samples were diluted to α= 0.1% prior to measurement to maintain obscuration below recommended limits [2]. Concerning the effect of dispersed-phase viscosity, at lower viscosities the experimental results show that the distributions remain unimodal across the investigated range and exhibit a systematic shift toward smaller diameters with increasing rotational speed. At higher viscosities, multiple peaks appear, suggesting a more complex breakup pathway involving the formation of satellite droplets. With respect to the effect of dispersed-phase volume fraction, the distributions at lower volume fractions are very similar, suggesting that, within this range, droplet-droplet interactions do not substantially alter the spectral morphology and that the hydrodynamic breakup mechanism remains dominant. A more pronounced shift toward larger diameters is observed at α= 10%. The change in the distribution position and spread highlights the sensitivity of the steady-state DSD to dispersed-phase concentration under otherwise identical hydrodynamic conditions. The limited variation between 0.1% and 1%, contrasted with the more substantial shift at 10%, indicates an increasing influence of droplet-droplet interactions at higher phase loading. The analysis also explored distributions normalized by the Hinze diameter obtained under the different operating conditions. For the lowest viscosity, 2 cSt, the distribution is unimodal and approximately bell-shaped, with a well-defined peak and limited dispersion. Increasing the viscosity to 10 cSt, a secondary peak emerges at smaller normalized diameters, indicating the formation of additional populations of fine droplets. At 100 cSt, the distribution becomes broader and distinctly multimodal, with multiple peaks partially overlapping and an extended left-hand tail. The normalization with respect to the Hinze diameter collapses the upper bound of the distributions to the same order of magnitude across all viscosities, confirming that the largest stable drop sizes remain governed by the turbulent stress-interfacial tension balance. However, the marked evolution of the distribution shape demonstrates that viscosity primarily alters the internal structure of the DSD rather than its maximum extent. While classical correlations may adequately predict a single characteristic diameter, the present results highlight the added value of full spectral measurements. The detailed morphology of the DSD, including multimodality and satellite-rich tails, provides a substantially stronger validation benchmark for numerical simulations and reduced-order modeling frameworks. Liquid hold-up and mixing time experiments The experimental campaign was structured to systematically explore the combined influence of impeller speed, N = 50 to 500 rpm, dispersed-phase viscosity, and volume fraction α. The volume fraction was analysed at 10% for the three different oils, while 20% was analysed only for the 10 cSt oil. The technique used for the oil hold-up and mixing time analysis was Electrical Resistance Tomography (ERT) [3]. This technique is based on the injection of current between two electrodes and the measurement of potential differences between the remaining electrodes in the measuring plane. The experimental data allow quantification of the axial and radial distribution of the dispersed phase, oil in the present experiment. The measurements of liquid hold-up at different axial measuring planes allow identification of regime transitions, as pointed out by Maluta et al. (2020) [3]. Finally, measurements of the time required to achieve homogenization of a tracer added at the gas-liquid interface were performed. In detail, 5 mL of salted water were added and the time trace was analysed to identify the time required for homogenization, t95. The experiments allow quantification of the effect of the presence of the oil phase in the tank when the viscosity of the dispersed phase is varied. The experimental data show that when the oil, or part of it, is stratified at the top of the tank, a significant increase in the time required to achieve homogenization is observed. The increase, with respect to the pure water case, depends on the viscosity of the oil phase to be dispersed. Overall, the ERT measurements complement the DSD analysis by identifying the spatial distribution of the dispersed phase, the transition from stratified to fully dispersed regimes, and the corresponding impact on mixing performance. References [1] Maluta, F., Buffo, A., Marchisio, D., Montante, G., Paglianti, A., & Vanni, M. (2021). Effect of turbulent kinetic energy dissipation rate on the prediction of droplet size distribution in stirred tanks. International Journal of Multiphase Flow, 136, 103547. [2] Triballier, K., Dumouchel, C., & Cousin, J. (2003). A technical study on the Spraytec performances: influence of multiple light scattering and multi-modal drop-size distribution measurements. Experiments in Fluids, 35(4), 347-356. [3] Maluta, F., Montante, G., & Paglianti, A. (2020). Analysis of immiscible liquid-liquid mixing in stirred tanks by Electrical Resistance Tomography. Chemical Engineering Science, 227, 115898.
Dettagli del progetto
Responsabile scientifico: Alessandro Paglianti
Strutture Unibo coinvolte:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinatore:
Università degli Studi di NAPOLI Federico II(Italy)
Contributo totale Unibo: Euro (EUR) 50.631,00
Durata del progetto in mesi: 24
Data di inizio
28/09/2023
Data di fine:
28/02/2026
