ARR - Annual Report

During the first term of the project, our investigation on screw feeder performance had three components. The first was to formulate two mechanics-based models for the kinematics and mechanics of non-cohesive powders: a simple model that relies on several simplifying approximations, and a more detailed continuum model that predicts the variations of velocity, packing fraction and stress within the feeder. Both models make the interesting prediction that the feed rate is maximum for a specific value of p/(2R!), the ratio of the screw pitch to barrel diameter of the feeder. The second component of our work was to conduct DEM simulations to validate the models and guide experimental efforts. The third component was to conduct experiments on a custom-built feeder assembly to test the model predictions and to extend/refine the model. Overall, we find good agreement between the experimental data, DEM simulations, and model predictions for non-cohesive powders (such as glass beads). In particular, the simulations and experiments verify the model predictions of the feed rate being maximum at a certain ratio of pitch to diameter of the screw. The experiments also identify the free surface at the feeder exit as being responsible for feed rate fluctuations for dry powders, and show that the fluctuations may be mitigated by appropriate end-cap design. The combination of theoretical analysis, DEM simulations, and experiments yielded substantial insight.

In the current (renewed) term of the project, we have focused on understanding the flow of cohesive powders in feeders. To quantify the effect of cohesion, we first created powders of controlled cohesion by combining a small quantity of glycerol to dry glass beads. Interestingly, the experiments show that the feed rate as a function of p/(2R!) shows the same qualitative trend as for dry powders. However, the feed rate fluctuations for cohesive powders are quite different from those seen in dry powders. A lacuna in the current understanding of cohesive powders is that there is no reliable constitutive model. To address this, we have initiated a study to determine the flow rule, a relationship between the strain rate and stress, for cohesive powders. Our initial studies on horizontal rotating drums point to the formation of clusters or agglomerates that crucially affect the flow of cohesive powders. On the modelling front, we have shown that the non-local model correctly predicts a complex dilatancy-driven secondary flow seen in experiments and DEM simulations, thereby increasing confidence on the model.

In ongoing work, we are conducting DEM simulations and experiments to quantify the formation and persistence of clusters in cohesive powders and determine how they influence their flowability.

We present the results of an experimental and theoretical study on the atomization process of high viscosity and polymeric fluids. The current year’s study was on the development of a model for the atomization process in swirl and fan nozzles. The primary atomization in such nozzles results in the formation of filaments and long ligaments, which breakup into droplets. Therefore, the first part of the current study was to determine the size distribution of the droplets that form by the breakup of such filaments. The second part of the current study was to develop a model to predict the droplet sizes using knowledge developed based on our experiments.

Atomization Model for Swirl and Fan Nozzles:

We propose a primary atomization model based on the interaction of two breakup mechanisms: the growth of surface wave and the formation of perforations, as shown by figure 1. At the nozzle orifice (zone 1), small scale surface waves are formed due to a high relative velocity between the liquid sheet and ambient gas. As the surface waves grow (zone 2), the liquid sheet forms alternating thick and thin regions due to the nonlinearity of the surface wave. The thin region becomes thinner as the sheet expands and perforations appear. As these perforations expand (zone 3), streamwise filaments form as the boundaries of these perforations approach each other. Close to the breakup position (zone 4), the growth of these perforations will eventually be stopped by the thick regions on the liquid sheet. As these streamwise filaments detach from the liquid sheet, the liquid sheet breaks up and forms filaments. As a result, in zone 5, there are two types of filaments formed: the thin streamwise filaments formed from the breakup of thin regions due to perforations, and the thick spanwise filaments formed from the thick regions due to the surface wave. These filaments become thinner due to the lateral velocity and eventually in zone 6, these two types of filaments break up into droplets due to surface waves and form the spray with a wide range of droplet sizes. The breakup of the liquid sheet is accompanied by the growth in wavelength of the surface waves: the distances between the thick regions at zone 4 are much larger than those in zone 1.

The production of fine powders and particles, such as milk powder, food additives like vitamins, pharmaceutical ingredients, and industrial ceramics, heavily relies on spray drying technologies. In standard spray drying, a liquid or slurry feed is atomized into fine droplets using high-pressure nozzles within a chamber filled with hot gas. This process achieves rapid drying by subjecting droplets to intense energy transfer, facilitating liquid evaporation in a short time. However, modern industrial requirements demand advancements in energy efficiency as well as product quality, necessitating improvements in spray drying technology. Energy efficiency is a crucial factor in spray drying, as conventional systems operate with high inlet gas temperatures between, consuming significant energy for gas heating. Spray drying systems can optimize energy utilization by reducing drying time without compromising product quality. Alongside energy efficiency, product quality in spray drying is paramount. Specific attributes, such as particle porosity, size, and density distribution, must be controlled to meet industry standards. Achieving precise product characteristics necessitates a detailed understanding of the drying process and its parameters, requiring advanced control strategies to meet strict quality specifications.

One promising technique to improve spray drying efficiency is foam spray drying, which involves injecting inert gas (e.g., nitrogen) into the feedstock at high pressures (typically above 50 bar). This pressurized feed is then atomized, releasing pressure at the nozzle outlet and allowing the feed to expand [1-2]. The rapid pressure depletion causes gas-saturated droplets to form bubbles, which alter the drying dynamics and the characteristics of the final product. Compared to standard spray drying, foam spray drying increases dryer throughput, reduces residence time, and modifies product properties. Although foam spray drying has been empirically explored, existing studies often focus on specific products without providing a reliable physics-based model to generalize findings across different parameters and scales.

Foam spray drying involves complex heat, mass, and momentum transfers between three phases (liquid, gas, and solid) within each droplet/particle, as well as interactions with the surrounding hot gas environment [3]. Compared to conventional spray drying, foam spray drying introduces additional complexities due to the presence of gas bubbles within the liquid matrix. These bubbles affect heat transfer by creating localized regions of lower thermal conductivity, alter mass transfer by influencing the diffusion of water vapor and dissolved gases, and impact momentum transfer by disrupting the liquid flow. Furthermore, bubble dynamics including nucleation, growth, and collapse can significantly modify the drying process by redistributing liquid and altering the capillary flow. These unique factors make foam spray drying more challenging to fully understand compared to non-foamed spray drying [4]. Therefore, focusing on a single slurry droplet, typically ranging from 20 to 180 μm in diameter, allows to study physical effects behind drying dynamics, which can be scaled up to the entire dryer using computational models.

In foam spray drying, individual solution or slurry (the focus of present study) droplets, containing liquid saturated with gas and dispersed solid particles, undergo rapid phase changes as they encounter atmospheric pressure within the drying chamber. The sudden pressure depletion initiates bubble nucleation within the droplets. At the same time, the gas solubility in the liquid phase decreases sharply, prompting the release of dissolved gas into those nucleated bubbles. This process varies based on nozzle type, which controls the pressure release rate, the magnitude of the pressure depletion, and droplet size, influencing both bubble nucleation and growth mechanisms [1].

The drying process of foam spray drying is influenced by the unique presence of bubbles within the slurry droplets, raising critical questions about their role in flow dynamics, drying kinetics and the eventual formation of solid structures and porosity. This study investigates these questions by exploring the coupled effects of heat, mass, and momentum transfer during foam spray drying. Through a dynamic pore network model, we examine the interactions between bubbles and liquid flow, highlighting their impact on drying kinetics and bubble dynamics. The findings provide new insights into the complex physics of foam spray drying and establish a foundation for optimizing this process for industrial applications.

Over the past decades our understanding of the wet granulation process and our ability to predict it computationally have made significant strides. However, despite these advancements we have yet to fully leverage models for granulation and other particulate processes to optimize the predictive design of granular product performance.

The goal of this research is to bridge this gap by linking process models with product performance models for wet granulation. To this end, a novel granule performance model has previously been developed within this project. This multi-scale model, which simulates swelling-driven granule disintegration and dispersion, has been specifically designed to integrate with existing wet granulation process models.

To validate these models, novel experiments were conducted in collaboration with the University of Strathclyde. The experimental results provided essential data for parameterizing the models, and have also offered deeper insights into the rate processes governing granule disintegration and dispersion.

Recent work has focussed on the recruitment of a new researcher who will support the project, Amir Arjmandi-Tash. The development of a surrogate model using Gaussian Process regression has commenced, to enable the eventual solution of the inverse problem.

Overall the work in our IFPRI project has focused on how, moving away from model systems containing spherical colloids with near hard interactions, we can widen the range of rheological responses by changing the properties of the building blocks of the suspensions, so that even in simple formulations a wide range of behaviors can be ”built in”, i.e. obtaining formulation guidelines to do “more with less” or simplifying formulations from within. At the same time we use novel experimental methods. We also developed a constitutive model for simplified industrial suspensions, based on insights from the advanced rheological methods (stress de-convolution) and using models taking into account plastic flow behaviour using an Eyring like approach and a viscoelastic upper convected Maxwell model.

For the this second IFPRI period we now focus on :

1. Particle roughness has been identified to generate surprising effects in colloidal gels and could both be tool to engineer materials from within for rheology or gravitational stability. The effect of roughness will now be explored more systematically. For gravitational stability, we will also study combined effects of roughness and shape, and investigate synergistic effects.
2. A full study of the structural evolution of systems in (1) will be investigated by 4D confocal rheology, with an emphasis on understanding the yielding transition.
3. To understand the role of non-central forces we would intensify the measurements of local scale mechanics using AFM (to characterize static friction) and then go to the micro mechanics of model aggregates (using optical tweezers).
4. we propose to also study these systems in 2D as this also has application to engineer strong interfaces or understand what happens in protein solutions, as a model system for lock and key interactions. The 2D nature of these systems makes them also a stepping stone for doing the micromechanics not immediately on the 3D systems.
5. we propose to continue the investigation of simplified industrial dispersion by industrial partners especially in light of systems with roughness and shape variations.

Project Phases

In the first phase of this project (2019 – 2023), three types of grinding aid additives in their pure form (n-Heptanoic acid, Diethylene glycol, and 1-Hexanol), which promote material bulk properties to various levels, were selected. The research aimed to assess the influence of these grinding aids by diverse dosages on both, grinding in ball mills and size classification in dynamic air classifiers. The focus was placed on determining how the grinding aid additives influence the grinding and classification process environment and on modifying selected models from literature for both, ball mills and deflector wheel classifiers. The models should account for the presence of grinding aids during simulating a dry-operated grinding plant in a flowsheet simulation.

Second Phase

During the second phase of the project (2023 – 2026), the research is focusing on identifying appropriate powder macroscopic bulk properties that accurately reflect both, the characteristics of particles and the presence of a particular type and dosage of grinding aid additives, as illustrated in Figure 1. In addition, the study aims to establish correlations between important microscopic particles properties, such as particle size, specific surface area, and particles specific surface energy, and bulk behavior of the powder. These correlations can be used to account for the impact of grinding aids within modeling grinding processes. Moreover, the bulk properties of the powder influence specific process characteristics, such as the discharge rate that can be achieved, the quantity of material hold-up in the grinding drum, and the residence time within the mill. Further, the knowledge about the material hold-up and residence time is required for modeling continuous grinding processes. These factors subsequently affect the quality of the final product (see Figure 1).

Experimental Approach

In order to understand the relationship between the powder’s bulk properties, the respond of the process in terms of resulting process characteristics, and the outcome of the grinding process, dry grinding experiments in an open-circuit continuous tumbling ball mill were performed. The trials were conducted until a stable operational state was reached. The outcomes of this series of trials provided data on the hold-up mass, discharge rate, and samples from product as well as hold-up material in the grinding drum. The gained samples underwent measurement to evaluate the characteristics of the particles and the powder and to correlate it to the system response. Based on this series of tests, couple of tests were selected for measuring residence time distribution. The results demonstrated that Diethylene glycol (DEG) led to a higher material hold-up compared to the product that did not contain grinding aid by the same feed rate. However, DEG resulted in a shorter median residence time with the highest maximum residence time and much finer product than that without grinding aid. On the other hand, Hexanol produced a product size and powder-to-void-ratio of almost 1 similar to that of the product without grinding aids, but this was achieved at a higher feed rate than that of the product without grinding aids. The comparison of the experimental data regarding residence time measurements with the results obtained from the one-dimensional axial dispersion model provided a reliable approximation, especially up to 80 % of the cumulative residence time distribution, showing slight deviations for all product formulations. The influence of grinding aids on the macroscopic properties of powder, as well as their incorporation into flowsheet modeling for a grinding plant, has revealed a dependable correlation between the powder flowability index (ffc) and the specific surface area. This correlation, which illustrate the effects of various grinding aids, can be represented by a power function. This function can be used in the ball mill model to predict dynamically changes in powder flowability as the particle size evolves during the grinding process.

The objective of this research project is to assess and enhance the ability of a recently-advanced high-fidelity modeling framework for spray formation to model complex liquid break-up and predict drop size distributions in high viscosity and non-Newtonian liquid atomization systems, such as found in spray drying applications. This framework, dubbed enhanced Volume of Fluid (eVOF), was developed by the PI’s research group and hinges on two key components:

1. a fully conservative Eulerian interface capturing technique with the ability to capture sub-grid scale liquid features such as thin films and thin ligaments, known to be of critical importance in the break-up of viscous and non-Newtonian fluids, and
2. simple physics-based break-up models to convert these thin liquid features into spray droplets that can be tracked in a Lagrangian fashion.

In the first year of the project, non-Newtonian constitutive models were implemented and tested in the PI’s research flow solver, and shown to influence spray formation in simple flow configurations. Moreover, a simple pressure-swirl configuration was explored to demonstrate the ability of eVOF to preserve thin liquid films and virtually eliminate numerical break-up. In year 2, the non-Newtonian constitutive models were further refined and validated against bubble rise experiments. A realistic pressure-swirl nozzle was selected for study based on availability of nozzle geometry and drop size measurements. Despite the complexity of the resulting flow, eVOF was again shown to preserve sub-grid scale liquid films successfully. Advances were made on sub-grid scale modeling of film and ligament break-up, allowing preliminary comparisons of drop sizes against experiments to be shown.

Going forward, the sub-grid scale modeling of film and ligament break-up will be further improved, in particular in the limit of high-viscosity liquids, and a more comprehensive comparison of drop sizes will be made against experiments.

The main objective of this project is to apply a pneumatic nozzle design, the Air-Core-Liquid-Ring (ALCR)-nozzle, for spray-drying of highly viscous liquids and pastes. The project is divided into three main working packages (WP). WP 1 aims to validate the ACLR atomizer technology to enable spraying of highly viscous liquids, using both experimental measurements and CFD simulations. WP 2 aims to evaluate the impact of the composition and morphology of the atomized droplets on the drying kinetics, for highly concentrated feeds. WP 3 aims to join the results of both packages to investigate the applicability of the ACLR nozzle for spray-drying of highly viscous liquids. The followings findings were achieved in the present funding period.

WP 1: Atomization with the ACLR nozzle

• The ACLR can achieve stable atomization with feed viscosities as high as 3 Pa·s, at relatively low pressures (7 bar) and low air-to-liquid ratios (0.8).
• The internal flow and the external spray instabilities can be directly correlated with each other.
• A CFD model was successfully adapted in STAR-CCM+ v.2206 to predict the internal flow of non-Newtonian maltodextrin solutions being atomized with an ACLR nozzle. In general, the predicted ALRs from the simulations agree with what was observed in experiments. Additionally, the liquid lamella thickness inside the nozzle follows the same trend in the simulations as what is observed in experiment: A smaller internal lamella variation is observed as the ALR increases.
• The possibility of using simulations to evaluate operating conditions outside of experimental capabilities was evidenced. The lamella variation can be severely reduced by increasing the operating pressure to 15 bar, which is still far below the 50-250 bar that is common in pressure swirl nozzles.

WP 2: Evaluation of the impact of the composition and morphology on the drying kinetics and model development by single droplet drying

• A method for the analysis of the mass data and calculation of the drying kinetics was developed.
• Experiments were conducted to evaluate the impact of initial solids concentrations of up to 45 wt%. The results for the particle size, mass and drying kinetics showed good agreement with theoretical considerations.
• The impact of the drying temperature was evaluated. While the impact of the drying temperature on the drying time agrees well with expectation, its influence on the drying kinetics showed no apparent trend.

WP 3: Proof-of-concept of industrial applicability of the ACLR nozzle for spray-drying of highly viscous liquids

The simulation plan to optimize the geometrical design of the nozzle has been formulated, and it is currently being carried out.

This study assesses the use of acoustic waves as a non-destructive accelerated aging method to be tested on multiple formulations including particle dispersions and emulsions. Two Ph.D. students in the PI's group, who are partially supported by this IFPRI grant, are working on the project. The method serves as an alternative to conventional techniques such as centrifugation and thermal aging, which either fail to capture essential destabilization mechanisms or may harm sensitive components. The project was initiated based on feedback from our industry liaison panel, which expressed interest in developing methods to expedite particle aggregation and subsequent sedimentation in colloidal dispersions. Selected samples from IFPRI members were tested as part of the project.

To address specific industry needs on stability testing, we examined model poly(methyl methacrylate) colloidal gels and commercially available agricultural dispersions provided by the panel, in addition to a model oil-in-water nanoemulsion system used in our group. We initially compared samples that were naturally aged, heat aged, and exposed to high and low powered ultrasound waves. Heating induced chemical degradation, solute particle melting, and solvent evaporation—each of which deviated mechanistically from the original natural aging pathway. High powered ultrasound waves caused irreversible yielding of the samples. Based on these initial results, a decision was made to focus solely on comparing natural and low-power acoustic aging methods.

A major technical goal in Y1 is the design of acoustic transducers compatible with confocal microscopy and rheometry, enabling detailed imaging and precise measurement of particle dynamics during acoustic treatment. We have achieved this goal for ex situ testing of small volume samples (<20 mL). Temperature measurements showed that the low-power acoustic transducers generated little dissipative heat and that the samples could be kept at ±3°C of room temperature without a need for cooling devices.

This custom experimental setup produced finely tuned acoustic waves at much lower power (<2 W) and pressures (<200 kPa) and allowed for controlled, efficient aging acceleration without compromising system integrity. While initial results on the IFPRI agricultural dispersion were mixed, accelerated phase separation was induced reproducibly for PMMA colloidal gels. For nanoemulsion samples, dynamic light scattering measurements suggest that a moderate acoustic pressure hastened both Ostwald ripening and coalescence processes. Additional tests are underway to determine the reproducibility and viability of these results through a more careful analysis of the intensity and number-weighted particle size distributions. Another set of IFPRI samples that are analogous to these emulsion systems have been obtained and will be tested.

Efforts in Y1 were primarily targeted at establishing method viability. In Y2, we plan to systematically check the macroscopic and microscopic effects of acoustic aging on both classes of materials, and to begin identifying the destabilization mechanisms that acoustic aging targets. We plan to continue providing regular updates to our IFPRI industry partner as well as to the IFPRI liaison meetings scheduled every 2 months.

The state of the art of capillary suspensions was described in a review paper published in Current Opinion in Colloid & Interface Science [1]. Our model system for capillary suspensions consists of fairly monodisperse, spherical, silica particles fluorescently labeled with rhodamine B isothiocyanate in a mixture of 1,2-cyclohexane dicarboxylic acid diisononyl ester (Hexamoll DINCH) and n-dodecane, with added aqueous glycerol. Capillary suspensions are prepared using a two-step ultrasonication: an emulsification of both liquids after which the particles are added and sonicated again. The three components are all index matched and the silica contact angle can be modified [2]. The attractive interaction strength can be modified by tuning the contact angle, and fraction of secondary liquid, and, by changing the particle roughness. This lets us access both granular-like systems with weak interactions and strong attractive gels using the same model system. During the project, we switched from using porous silica particles to nonporous particles as there were problems noted with adsorption of the secondary liquid into the pores. This means the particles are detected on the images as red rings rather than filled circles. To detect the particles, a self-coded particle detection algorithm based on edge detection and Hough transform was designed. The algorithm includes a simple graphical user interface for both local detection and manual addition or removal of missing or misdetected particles to improve the final detection efficiency. Using consecutive images on the rheoconfocal, the displacement of the particles can be detected as a function of the applied shear. AI tools have also been used to detect the particles.

Initial Goal

Our initial goal of the project was to track microstructural changes in the network in response to external shear applied via a linear shear cell. These structural changes were correlated with the rheological response of the material. Application of external shear via the linear shear cell, however, was unsuitable. Due to the very low yield strain in capillary suspensions, the applied shear was often above the flow point and specific changes during yielding could not be adequately captured. Furthermore, the prior setup only allowed for the deformation profile to be captured in one shear plane. While this has provided valuable information, proving that capillary suspensions tend to undergo solid-body movement, where the rotation of particles around their respective bridges is resisted through both the structure of the network and the extra torque provided by the contact angle pinning and/or the contact angle hysteresis, full 3D tracking is necessary. Therefore, a rheometer was mounted onto a highspeed confocal microscope in 2022. The improved setup has allowed us to directly compare bulk, rheological changes with local, microscopic changes to the clusters and network, as discussed further in Section 1.6.