Evaluation of the hydrodynamics of opaque multi-phase flows remains a challenging task, with implications for various industrial processes such as chemical processing, pharmaceutical, and mineral processing. Understanding how design and operational variables affect the complex behavior of multi-phase flow systems is essential for optimizing processing conditions and improving efficiency. Radioactive particle tracking (RPT) has been a proven measurement technique to evaluate hydrodynamics in multi-phase flow systems. However, a limitation of the classical RPT technique exists in the assumptions made in the simulation of the count rate received by the detectors in correcting for varying flow-induced fluctuations in the volume fraction of the dispersed phase, often encountered in industrial multi-phase flow systems. In this paper, we introduce a fundamentally novel experimental RPT method that directly uses detected incident photon hit locations for the reconstruction of the three-dimensional radioactive tracer particle position. We argue that this approach is inherently more robust as varying attenuation does not affect the reconstruction. The RPT setup consists of three identical γ-radiation slit collimator detectors that are placed equidistantly at 120° intervals. A subsequent calibration-experimentation procedure is established that allows reconstruction of the tracer particle position with spatial accuracy and precision in the order of 1 mm. We demonstrate the applications of this technique in evaluating hydrodynamics in multi-phase systems by characterizing the flow field of industrial-grade polypropylene reactor powder in a laboratory-scale horizontal stirred bed reactor.

The fractal structure of aggregates consisting of primary nanoparticles naturally arises during their synthesis. While typically considered to be a fully stochastic process, we suspect long-range interactions, in particular van der Waals forces, to induce an active pull on particles, altering the clustering process. Using an off-grid 3D model, we show that an active pull decreases the density and fractal dimension of formed clusters. These findings could not be reproduced by 2D models, which underestimate screening effects. Additionally, we determined the range within which van der Waals forces dominate the aggregation process.

Noble metal nanoparticles (NPs), particularly platinum (Pt), are widely used in heterogeneous catalysis due to their exceptional activity. However, controlling their size and preventing sintering during synthesis remains a major challenge, especially when aiming for high dispersion and stability on supports such as graphene. Atomic layer deposition (ALD) has emerged as a promising method to address these issues, yet conventional processes often lead to broad particle size distributions (PSDs). This work introduces a new approach for the deposition of size-controlled and sintering-resistant Pt NPs on graphene by atmospheric-pressure ALD using MeCpPtMe₃and O₂. In this approach, the deposition temperature varies in a cyclic manner in accordance with the Pt precursor and the O₂exposure steps. In every ALD cycle, the MeCpPtMe₃exposure is carried out at either 150 or 200 °C, and the O₂exposure is at room temperature. The room-temperature step hinders the diffusion and coalescence of Pt NPs, resulting in significantly narrower PSDs compared to those achieved by the conventional ALD processes at 150 and 200 °C. Importantly, Pt NPs with narrower PSDs exhibit higher catalytic activity and improved stability, which are demonstrated for the propene oxidation reaction, despite having a significantly lower Pt loading. Our approach may open a new avenue toward the size-selection synthesis of noble metal NPs for catalytic applications.

Chelator-impregnated resins have been studied earlier for the chemical separation of elements in aqueous solutions, but issues with their chemical stability have limited their use in the separation of (medical) radionuclides from their respective irradiated targets. We developed a polydimethylsiloxane (PDMS)-based chelator-impregnated resin that showed a high chemical stability against leaching. Several different chelators were tested in this study. After impregnation of the PDMS beads with the di-2-ethylhexylphosphoric acid (D2EHPA) chelator, an in-flow separation study with various radionuclides (Y-90, La-140, and Ac-225) was conducted. These three radionuclides have potential use in nuclear medicine and a production route through irradiation of Sr-, Ba-, and Ra-targets respectively, necessitating their chemical separation. The D2EHPA-impregnated beads achieved high adsorption efficiencies of 99.89% ± 0.14%, 99.50% ± 0.10%, and 98.51% ± 0.25%, for Y-90, La-140, and Ac-225, respectively, while co-adsorption of minor amounts (< 3%) of the targets were reported. These results, together with the high chemical stability of the PDMS-based resin, highlight the potential of chelator-impregnated resins in the rapidly growing field of (medical) radionuclide production.

Conventional fluidization of cohesive powders is challenging due to their strong interparticle forces, often requiring assistance methods. In this study, the hydrodynamics of pulsed and vibrated beds of cohesive Geldart C silica powder (Sauter mean diameter d₃₂=7.9μm) in a 19.2cm diameter column were investigated using X-ray imaging. The results show that low-frequency, moderate-amplitude gas pulsation improves fluidization by disrupting long, persistent gas channels. Higher-frequency pulsation is dampened throughout the bed, resulting in negligible improvement over unassisted systems. When coupled with mechanical vibration, gas pulsation slightly mitigates solid compaction at the bottom section, but the overall flow pattern remains largely unchanged compared to vibration alone. The findings highlight the potential of integrating gas pulsation with other assistance methods to enhance fluidization in practical applications.

An integrated battery-electrolyzer stores renewable electricity as a battery and produces hydrogen when overcharged. This dual application requires electrode concepts that ideally enhance both battery and electrolysis operation without compromising either. One such concept is 3D structured electrodes including channels that improve ionic conductivity and material utilization as well as facilitating bubble removal during electrolysis. In this work, we first develop a 1D model of a porous sintered nickel electrode that takes the void fraction of the 3D geometry into account and allows for the determination of the current and potential distribution for both battery charging and oxygen evolution. An optimized void fraction that maximizes the reactive surface area for oxygen evolution is determined, and we discuss under what circumstances a 3D geometry is beneficial. Finally, we show how the improved ionic conductivity of 3D electrodes also results in more homogeneous battery charging, increasing charging efficiency in nickel electrodes.

Electrochemical CO2 reduction presents an opportunity to transform waste flue gas with water and renewable electricity into chemicals or fuels. However, the energy-intensive nature of purification of flue gas underscores the appeal of directly utilizing the flue gas streams containing impurities. In this study, we investigate the impact of SO2 impurities on CO2 electroreduction in two electrochemical cell geometries: an H-cell and a membrane electrode assembly (MEA) cell. We observe distinctly different behavior of the Ag on carbon black (Ag/CB) catalyst under SO2 impurities in the H-cell compared to the MEA cell, where SO2 impurities exhibit a more pronounced effect on Ag/CB catalysts in the H-cell than in the MEA cell. This difference is attributed to the higher solubility of SO2 in the electrolyte compared to CO2, resulting in an accumulation effect and causing differences in the SO2 concentration near the electrode between the H-cell and the MEA system. By depositing a very thin SiO2 coating on the outermost surface of the Ag/CB catalyst using atomic layer deposition (ALD), the impact of SO2 on the catalyst's selectivity is diminished. This is attributed to the permeability difference between CO2 and SO2 through the SiO2 coatings and results in a local SO2 concentration difference between samples with and without SiO2 coatings.

Atomic layer deposition (ALD) of platinum (Pt) has gained significant interest in the recent years due to its capability of depositing various Pt nanostructures for applications in different fields, such as Pt nanoparticles (NPs) for catalytic reactions and energy devices and Pt thin films for microelectronic technology. Among various developed processes, Pt ALD using MeCpPtMe₃as the precursor has been most popularly employed owing to the high reactivity, volatility, and thermal stability of the precursor, which enable controlled deposition of Pt nanostructures in a broad range of temperatures. Typical MeCpPtMe₃-based Pt ALD processes use O₂and H₂as the coreactants. In this study, we explore atomic hydrogen as an alternative and reveal its exceptional reactivity that outperforms H₂and O₂. Specifically, atomic hydrogen enables the deposition of highly dispersed Pt NPs with narrow particle size distributions (i.e., standard deviation <0.3 nm) on various oxide surfaces, including TiO₂, SiO₂, CeO₂and V₂O₅, which is unattainable with H₂under identical experimental conditions. In addition, it facilitates the deposition of Pt NPs with improved size uniformity and accelerates the closure of Pt films compared to ALD processes using O₂as the coreactant. The results demonstrate a significant potential of atomic hydrogen as a highly effective coreactant for ALD of Pt NPs and thin films.

Stirring has been recognized in the literature as a promising technique for facilitating fluidization of cohesive powders, via inputting additional energy to counteract interparticle forces. However, the influence of operating conditions and stirrer configurations on flow behavior remains largely unknown, which impedes the practical implementation of stirred fluidization. Utilizing X-ray imaging, this research demonstrates that stirring enhances fluidization in cohesive micron-silica powder (Sauter mean diameter [Formula presented]) by collapsing the powder packing structure, and transitioning channeling beds into bubbling states. Comb-like configurations featuring fewer stirrers and blades, placed in the bottom region, have shown to be highly effective. Excessive stirring can lead to air pockets and a compacted phase of particles on the column walls, undermining the interaction between particles and stirrers. Additionally, the experiments show that maximizing the sweeping coverage, employing complex asymmetrical configurations, and avoiding tortuous gas pathways are preferable.

The most common way to protect metallic structures from corrosion is through the use of passive and active corrosion protection with coatings containing dispersed corrosion inhibitor particles. Current approaches use inorganic microparticles containing mostly toxic and/or critical elements (e.g. CrVI, Li-salts). Organic inhibitors have been identified as a potential replacement technology due to their high inhibiting efficiency in solution, high versatility and lower toxicity. Nevertheless, when brought into organic coatings these inhibitors lose their efficiency due to unwanted side reactions with the surrounding organic matrix (coating). In this work we propose a novel strategy to isolate the organic corrosion inhibitor microparticles from the surrounding matrix. The new approach is based on the gas-deposition of an oxide nanolayer on the microparticles using gas deposition in a fluidized bed reactor. As a result, the organic particles are better dispersed in the coating and do not react with the surrounding matrix. Upon coating damage the particles are exposed to water and release sufficiently high amounts of the organic corrosion inhibitor at the damaged location. The work introduces a technique that can be used in other applications with similar challenges and a new technology that enables for the first time to store large amounts of active organic corrosion inhibitors in reactive organic coatings for efficient protection of metallic infrastructure. This opens the path to the practical use of highly efficient organic inhibitors in coatings for corrosion protection.

In this study, we investigate the correlation between fluidization behavior and flow properties of 10 commercially available cohesive powders through fluidization and rotating drum experiments. The rotating drum was operated at high speeds to aerate the powder, creating flow fields and stress conditions comparable to those in gas–solid fluidized beds. We introduce a fluidization quality index (FQI) computed from X-ray imaging, which addresses the limitations of conventional analyses for assessing fluidization quality of cohesive powders. By analyzing flow patterns, the FQI integrates gas holdup and its temporal variation, providing a reliable measure of fluidization quality. The findings establish a positive correlation between flowability and fluidization quality, demonstrating how flowability measurements can predict the effectiveness of mechanical vibration in enhancing fluidization characteristics. These results suggest that fast, user-friendly flowability assessments in a rotating drum can effectively predict fluidization potential, contributing to process optimization and advancing fluidization studies for cohesive powders.

In this study, we investigated the wettability and agglomeration characteristics of polymer microspheres coated with low-temperature deposited SiO₂ in a fluidized bed atomic layer deposition (ALD) setup. Surface characterization revealed the presence of a significant amount of deposited Si-OH groups within the first cycles. A drastic decrease in agglomerate size, water contact angle (WCA), and droplet absorption time of the powder was observed when coating was applied. Furthermore, we observed an increase in the amount of Si-OH present on the particle surface with increasing coating cycles, while no significant improvement in water affinity was found after the first coating cycles. Our findings suggest that surface coverage is the primary factor in improving the colloid stability of particles, coated at low temperatures. The low temperature operation of our system introduced a chemical vapor deposition (CVD) component to our coating process, which allowed full surface coverage to be achieved within the first two coating cycles.

Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

Horizontal stirred bed reactors (HSBRs) are widely used in the commercial production of polypropylene (PP). Despite their commercial significance, a comprehensive understanding of the flow behavior in HSBRs remains elusive, primarily due to the lack of detailed experimental data. This study investigates the influence of operating parameters on the particle flow behavior of two types of PP reactor powder in a laboratory-scale HSBR using X-ray imaging. Our results indicate that the overall flow behavior and phase holdup in the HSBR are dominated by agitation. Moreover, gas injection through the inlet points at the bottom of the HSBR results in spouting behavior, which can lead to reduced gas–solid contacting and, in extreme cases, complete bypass. Finally, the presence of liquid (in this study, isopropyl alcohol) adversely affects the flow behavior of the PP reactor powder due to liquid bridging at the contact points of particles. Powders that comprise particles with relatively small sizes and dense surface morphology are particularly prone to reduced flow behavior when exposed to liquid.

Stirred gas–solid fluidized bed reactors are commercially employed in polyolefin manufacturing, but the complex gas–solid contacting dynamics pose challenges in design, scale-up, and operation. In this study, the influence of agitation on the fluidization performance of Geldart B particles was investigated experimentally by X-ray imaging and pressure drop measurements and numerically by Computational Fluid Dynamics (CFD) - Discrete Element Method (DEM) - Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop show good qualitative agreement with the simulation results. Visual observations underscore that an increase in the angular velocity of the agitator results in reduced bubble size and improved bed homogeneity, as further evidenced by reduced pressure fluctuations. Furthermore, the simulations reveal that while the impeller enhances solids agitation, a proper design study is imperative, considering that static immersed bodies, such as the stirrer shaft, can adversely impact solids motion.

To make green hydrogen more economically attractive, the energy losses in alkaline electrolysis need to be minimized while operating at high current densities (1 A cm⁻²). At these current densities the ohmic resistance and gas bubbles effects contribute largely to the energy losses. To mitigate the gas bubbles losses, we demonstrate, for the first time, a pressure swing to remove gas bubbles in a zero-gap alkaline water electrolyzer. The pressure swing leverages the ideal gas law to increase the volume of gas in the system periodically, for a short duration (<2 s). This temporal volume increase effectively removes bubbles from the electrolyzer. We show that pressure swing can be used to measure the effect of bubbles on the ohmic resistance (R_Bubbles). Our results reveal that foam electrodes have a significantly larger R_Bubbles than perforated plate electrodes (1.8 Ω cm² vs 0.3 Ω cm²). The time-averaged cell voltage reduces by 170 mV when applying pressure swings to an electrolyzer operating at 200 mA cm⁻² in 1 M KOH with foam electrodes. The bubble resistance further depends on the electrolyte conductivity (inversely proportional) and is only moderately affected by operating pressure (25 % lower when increasing pressure amplitude from 1–2 to 1–5 bar). By implementing these findings in a model, we estimate that the pressure swing could reduce the cell voltage by ∼0.1 V for an electrolyzer operating at industrial conditions (6 M KOH, 80 °C, 1 A cm⁻²) for foam electrodes. For perforated plate electrodes, however, the reduced cell voltage is lower and does not outweigh the additional compression energy.

Nanoparticles are usually fluidized as agglomerates, which are in dynamic states of agglomeration and fragmentation. It is critical to consider the size distribution of agglomerates in modeling of the fluidization of nanoparticle agglomerates. In this article, the fluidization behavior of nanoparticle agglomerates is investigated using a two-fluid model─population balance model. The model includes the agglomeration and breakage kernel functions based on the continuum theory of cohesive particles developed by Kellogg et al. (J. Fluid Mech. 2017;832:345-382). The ratio of the critical breakage velocity to the critical agglomeration velocity is defined to represent the cohesion of nanoparticles. The predictions of bed pressure drop, bed expansion ratio, and bed collapse curves agree well with those of experiments. By changing the critical agglomeration velocity and the ratio between the critical velocities, the transition from almost defluidization to uniform fluidization is predicted. Finally, the model’s ability to simulate the fluidization of fine particles with a few micrometers is also shown. This study provides a practical tool for simulating the fluidization of nanoparticle agglomerates.

In this study, the impact of different vibrational modes on the fluidization characteristics of cohesive micro- and nano-silica powder was examined. Fractional pressure drop, bed expansion measurements, and X-ray imaging were utilized to characterize the fluidization quality. The densities of the emulsion phase at the top and bottom of the column were quantified and compared, providing insights into the solid distribution within the fluidized bed. In the absence of vibration, neither powder could be fluidized within the considered range of superficial gas velocities. Vertical vibration was found to initiate fluidization for both powders. In contrast, elliptical vibration failed to overcome the channelling behavior when fluidizing the micro-powder. For nano-powder, combined channelling and powder compaction occurred when the bed was subjected to elliptical vibration. For the micro-powder, it was observed that bed homogeneity was independent of vertical vibration intensity but improved with increasing superficial gas velocity. For nano-powder, intensifying vertical vibration led to segregation, likely due to agglomerate densification. Furthermore, fractional pressure drop measurements proved to be a strong tool in assessing fluidization quality, providing insights that could not be attained by conventional indicators.

Horizontal stirred bed reactors are widely used in the commercial manufacturing of polypropylene. However, a comprehensive understanding of the particle dynamics in horizontal stirred bed reactors remains elusive, primarily due to the lack of detailed experimental data. In this work, we studied the influence of operating parameters on the particle flow dynamics in a laboratory-scale horizontal stirred bed reactor using single-photon emission radioactive particle tracking. The results show that the general solids flow behavior is strongly affected by both the agitator rotation speed and reactor fill level. Operation at low rotation speed and low fill level results in solids flow with poor radial and circumferential distribution due to internal bed circulation. On the contrary, at increased rotation speeds and fill levels, solids motion throughout the bed is continuous resulting in excellent solids distribution. The solids circulation was found to increase for both an increase in rotation speed and reactor fill level. The axial dispersion coefficient, on the other hand, shows a linear relation with the rotation speed, but no conclusive relation between the axial dispersion coefficient and the reactor fill level was found.

We explore the low-temperature limit of atomic layer deposition of Pt using MeCpPtMe3 and O2. We reveal that by supplying a sufficiently high O2 exposure, highly dispersed and thermally stable Pt sub-nanometer clusters can be deposited onto the surface of P25 TiO2 nanoparticles even at room temperature by atmospheric-pressure ALD.