Session 6 - Theme: Suspensions, slurries, emulsions (2)

Solids suspensions must be processed using mixing tanks in many industrial operations and some knowledge of the just-suspended speed (Njs) is required for tank design and operation. In this study, experiments were conducted using three geometrically similar mixing tanks (T = 0.4 m, 1 m, 2.5 m) to measure Njs for concentrated, coarse particle suspensions in steel mixing tanks. A range of sand particle diameters, mixture concentrations and suspending fluid properties was tested. Measurements of Njs measurements were obtained using a number of methods, i.e. (i) each of these tanks was custom-built to include a clear viewing window that extended from the bottom of the tank almost to the top so visual indication of the presence of a settled bed could be obtained; (ii) using a traversing gamma ray densitometer positioned just above the tank bottom; side-wall pressure taps (located at 0.2T, 0.4T, 0.6T, 0.8T and T off the tank bottom) so that the pressure gauge measurement (PGM) for Njs could be tested; and (iii) a novel “bed detector” placed on the external (outer) side of the tank bottom. This device, which was developed at SRC, is sensitive to the reduction in the rate heat transfer that would occur once a stationary bed of solids formed. We will compare the different Njs measurement methods tested during this study, and evaluate their performance over a wide range of test conditions. We also will compare the Njs results with predictions obtained using existing Njs correlations.

Understanding solid suspension mechanisms in oscillatory baffled reactors (COBRs) has not been extensively explored in the literature; however, it is crucial for transitioning from empirical operation to predictive design. This study addresses the mechanisms of solid suspensions in a NiTech® glass COBR using phase-resolved Particle Image Velocimetry. Experiments used glass beads (dp= 150 and 500 µm) at concentrations of 0.2 % and 1.0 % v/v suspended in calcium chloride solution. The flow conditions were investigated over net Reynolds numbers of 0-20 and oscillatory Reynolds number of 415-1583, achieving different solid-liquid flow regimes (settled bed, moving bed and full suspension). Results show the dispersed phase predominantly attenuates velocity fluctuations relative to single-phase flow. Under identical oscillatory conditions resulting in a settled bed, increased solids concentration enhances turbulence kinetic energy (TKE) via boundary roughening and particle collisions. However, in the fully suspended regime, inertial damping dominates with increased solids concentration, reducing TKE. Larger particle diameters amplify turbulence in settled beds but decrease TKE when fully suspended. To better understand the mechanism governing particle suspension in the COBR, a modified Strouhal number, which compares local and convective acceleration, is proposed to assess the role of local acceleration on solid suspension. The analysis reveals transitions at a critical St of 830–1200 for the settled to moving bed and 2247–4015 for full suspension. A dimensionless parameter (Shields criterion) relating the instantaneous axial and radial velocities to the particle terminal velocity is employed to further elucidate the mechanisms governing solid suspension.

Even though mixing of poly-disperse liquid-solid systems is occasionally studied in literature both numerically and experimentally, comprehensive understanding of this issue is still lacking. In presented work, systematic experiments were conducted on bi-disperse mixed batches using the new Selective Resistance Tomography (SRT) method (an extension of the conventional ERT) to measure volume fraction compositions over two horizontal planes in a model mixing vessel. The vessel was cylindrical (D = 0.3 m), with a flat-bottom and four evenly spaced baffles. It was filled by water and model solid particles (≥10% total mean volume fraction) and mixed by a PBT impeller (d = 0.1 m) at higher than critical impeller speed. The new SRT method enables distinction between two model particle species through successive measurements tailored to the species' differing electrical properties. This way, volume fraction distributions of individual particle species and corresponding individual homogeneity indices were found for different combinations of the species’ sizes, densities and loads. The results provide new insights into the general behaviour of poly-disperse mixed batches in engineering praxis and support development of hypotheses regarding how impeller power input is partitioned among particle species with differing physical properties.

Double emulsions (DEs) consist of a simple emulsion dispersed within a continuous phase, forming a two-compartment structure. They are widely used for encapsulation and controlled-release applications, such as in the pharmaceutical industry. Their preparation typically involves two steps, where the second step is mainly performed in batch devices (stirred tanks or rotor-stator, for instance). However, these systems exhibit highly non-uniform energy dissipation, often leading to broad droplet size distribution (DSD). In this study, a continuous alternative is proposed where the second step of preparation is carried out in open-cell solid foams (OCSF) used as static mixers. Thanks to their random structure and high porosity, these materials are cheap and result in reduced pressure drop and energy consumption, while promoting local drop deformation. Single-phase Computational Fluid Dynamics (CFD) simulations are carried out to compare their performance with that of SMX+ (a structured commercial static mixer). The simulations show that OCSFs generate high local shear and elongational rates, whereas the SMX+ mixer produces stronger overall energy dissipation. To connect local flow information with droplet-scale behavior, a population balance model accounting for the effects of both turbulence and elongation-induced breakage is employed to predict the DSD of these DEs. The influence of the operating conditions (flow rate of the continuous phase, mixer type) and the formulation parameters (oil viscosity, water fraction, etc.) on the DSD of water-in-oil-in-water DEs and their encapsulation efficiency is investigated. The results demonstrate that OCSF can achieve finer emulsions at lower energy cost.

Key targets of United Nations Sustainable Development Goal 2, namely the increase of agricultural productivity whilst ensuring sustainable food production to end hunger and malnutrition, aim to address the growing gap between food demand and supply. Plant based biopesticides are therefore receiving increasing attention to minimise crop losses whilst observing sustainable agricultural practices. Neem oil, obtained from the seeds of the neem tree, is an attractive candidate as it contains azadirachtin- a bioactive which is effective on hundreds of species of insects, and is also biodegradable with very low toxicity to mammals. This study has been performed taking the process intensification route to obtain fine emulsions of neem oil with the objective of determining breakup kinetics relevant to process design and scale-up. Neem oil (10%w:w)-in-deionised water emulsions in the presence of surfactants have been prepared using VMI Turbotest® Rayneri batch rotor–stator equipped with slotted and toothed mixer head. Covering a power input range of around 19-52 W, the process was monitored through drop size measurements. Results were compared to those with the Hielscher UP200S ultrasonic processor. Comparable Drops Size Distributions (DSD) from different parts of the emulsion demonstrated that the VMI alone can ensure spatial homogeneity; with the ultrasonic processor an impeller was also needed. The evolution of DSDs were similar with the kinetics being significantly slower when using the VMI. Equilibrium d32 values below 200 nm could be obtained. Detailed comparisons, also including equipment combinations/configurations will be elaborated in relation to process design and scale-up.

Gas scrubbing plays a critical role in ensuring clean industrial emissions and meeting increasingly stringent environmental regulations. Maximizing its performance depends strongly on the efficiency of the underlying mass transfer processes. The utilization of static mixers is known to significantly enhance efficiency of such processes. Sulzer has long-standing expertise in designing high‑performance scrubbing systems for removing species such as H₂S, SO₂, and acids from gas streams. Sulzer's SMVTM static mixer technology offers particularly attractive characteristics for this application, providing high interfacial contact area and efficient mass transfer in a compact configuration. The combination of high F-factor capability and short installation length enables dramatic footprint reduction compared to packed columns, while the technology exhibits superior tolerance to ash and particulate matter, eliminating the need for upstream wash sections. The vertical in-line configuration with co-current downward flow and spray nozzle liquid introduction further simplifies system design. Recently, Sulzer’s R&D performed extensive CO₂ absorption tests using NaOHaq, covering a wide range of gas and liquid loads, droplet size measurements, concentration analyses, and pressure‑drop characterization that demonstrate the efficiency of the Sulzer mixers for gas scrubbing applications. Furthermore, to complement the experimental work, computational fluid dynamics (CFD) simulations provided further insight into the main driving forces of the flow and identified the fundamental mechanisms responsible for mass transfer. This presentation examines Sulzer's systematic approach to advancing static mixer scrubbing performance through integrated experimental, computational, and analytical methods. The experimental program encompasses a purpose-built pilot facility designed to characterize mass transfer performance across industrially relevant operating conditions. Key experimental aspects include advanced droplet size characterization, real-time concentration measurement techniques, pressure drop quantification, and the operational challenges encountered when working with reactive liquid-gas systems. Complementing the physical testing, computational fluid dynamics simulations provide fundamental insight into the flow physics governing mass transfer, including interfacial area generation mechanisms, local concentration gradients, and droplet-gas interaction dynamics. A critical enabler of this development effort is the integration of structured data management practices that allow systematic correlation of experimental observations with CFD predictions, accelerating the identification of performance-limiting factors and optimization pathways. This multi-disciplinary approach demonstrates how combining physical experimentation, numerical simulation, and data-driven analysis accelerates technology development cycles while building fundamental process understanding that extends beyond individual test conditions to inform broader design principles for static mixer scrubbing systems.