Background: Electrohydrodynamic (EHD) drying relies on the generation of a corona wind that is created with a high electric potential between an emitter and a ground electrode. The impinging corona wind enhances convective heat and mass transport close to the sample, which is positioned on the ground electrode. Although EHD drying is reported to have large potential for milder and energy efficient drying, most studies have only evaluated its potential at the lab scale and for specific applications. Scope and approach: This review focusses on discussing the underlying physical phenomena that may be expected to influence the performance of EHD drying processes. Specifically, we discuss how corona wind changes due to discharge behaviour as it is influenced by moisture release during drying, how the microstructural properties of the product could influence electromigration in the drying material and how EHD drying could be efficiently and safely scaled up for biobased & food products. We conclude with an outlook to the research that still is required to concretize the potential of EHD drying in practice. Key findings and conclusions: In EHD drying, electrical discharge induces airflow which enhances external convective drying. Studies coupling physical phenomena are still lacking, although some possible couplings are mentioned in chapter 2. Internal mass transfer is enhanced due to presence of an electrical potential over the product, which can cause several electrically-driven transport phenomena as discussed in chapter 3. Several drying setups have been explored in industry, of which the characteristics are summarized in chapter 4.

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CO2 conversion is an important part of the transition towards clean fuels and chemicals. However, low solubility of CO2 in water and its slow diffusion cause mass transfer limitations in aqueous electrochemical CO2 reduction. This significantly limits the partial current densities towards any desired CO2-reduction product. We propose using flowable suspension electrodes to spread the current over a larger volume and alleviate mass transfer limitations, which could allow high partial current densities for CO2 conversion even in aqueous environments. To identify the requirements for a well-performing suspension electrode, we use a transmission line model to simulate the local electric and ionic current distributions throughout a channel and show that the electrocatalysis is best distributed over the catholyte volume when the electric, ionic and charge transfer resistances are balanced. In addition, we used electrochemical impedance spectroscopy to measure the different resistance contributions and correlated the results with rheology measurements to show that particle size and shape impact the ever-present trade-off between conductivity and flowability. We combine the modelling and experimental results to evaluate which carbon type is most suitable for use in a suspension electrode for CO2 reduction, and predict a good reaction distribution throughout activated carbon and carbon black suspensions. Finally, we tested several suspension electrodes in a CO2 electrolyzer. Even though mass transport limitations should be reduced, the CO partial current densities are capped at 2.8 mA cm−2, which may be due to engineering limitations. We conclude that using suspension electrodes is challenging for sensitive reactions like CO2 reduction, and may be more suitable for use in other electrochemical conversion reactions suffering from mass transfer limitations that are less affected by competing reactions and contaminations.@en

Hydrogen carriers are attractive alternative fuels for the shipping sectors. They are zero-emission, have high energy densities, and are safe, available, and easy to handle. Sodium borohydride, potassium borohydride, dibenzyltoluene, n-ethylcarbazole, and ammoniaborane are interesting hydrogen carriers, with high theoretical energy densities. The exact energy density of these hydrogen carriers depends on the integration of heat and mass with the energy converters. This combination defines the energy efficiency and, thus, the energy density of the system. Using a 0D model, we combined the five carriers with two types of fuel cells (PEM and SOFC), an internal combustion engine and a gas turbine. This resulted in 20 combinations. Despite the limitations of the 0D model and the occasional difficulty of validating input values, this model still produces exciting findings, which are valuable for further research. For the dehydrogenation of both dibenzyltoluene and n-ethylcarbazole, an external hydrogen burner is required if no waste heat resources from the integrated system are available. For the borohydrides, on the other hand, energy integration is essential for reducing cooling power. Dehydrogenation produces substantial energy, but only a fraction of this energy can be used for internal preheating. Dehydrogenation of ammoniaborane produces less energy. Among all hydrogen carriers, both ammoniaborane and sodium borohydride provide energy densities comparable to that of marine diesel oil. In particular, ammoniaborane possesses a remarkably high energy density. Thus, we conclude, that hydrogen carriers are attractive alternative fuels that deserve more attention, including their potential performance for hydrogen imports. @en

Sodium borohydride (NaBH4) is considered as an alternative fuel for the maritime industry [1]. In contrast to conventional fuels, NaBH4 is a granular material. To use a simulation-supported design for assessing the feasibility of equipment designs for storing and handling this material, its mechanical characteristics are required. These are then used to calibrate and verify simulations using the Discrete Element Method (DEM). However, as this is a novel application for this material, virtually no bulk characteristics are known yet. Therefore extensive testing has been done to extract required mechanical characteristics, such as cohesion, adhesion, internal friction, wall friction, and the Angle of Repose (AoR). These experiments showed that NaBH4 is initially free-flowing, but an increase in the moisture content because of an increase in relative humidity leads to an increase in cohesion, effectively reducing the flowability of the bulk material. Furthermore, our experimental results showed plastic deformation of individual NaBH4 particles.
This work focuses on capturing both the free-flowing and cohesive behaviour of NaBH4 in DEM. To this end, the two-step calibration approach introduced by Grima [2] is adopted and adjusted. First, the free-flowing behaviour is calibrated using a non-cohesive contact model, Hertz-Mindlin (HM). Second, the cohesive material is calibrated using the appropriate cohesive parameters of the Edinburgh Elasto-Plastic Adhesion contact model (EEPA), while the (calibrated) non-cohesive parameters are kept constant. The novelty of this work is the use of EEPA for the second calibration step, which allows the modelling of both the cohesive behaviour and the plastic deformation of the individual particles in the bulk material.


[1] M.C. van Benten, J.T. Padding, and D.L. Schott. “Towards Hydrogen-Fuelled Marine Vessels using Solid Hydrogen Carriers”. In: The 14th International Conference on Bulk Materials Storage, Handling and Transportation. Wollongong, Australia, July 2023.
[2] A. Grima. “Quantifying and modelling mechanisms of flow in cohesionless and cohesive granular materials”. In: University of Wollongong Thesis Collection 1954- 2016 (Jan. 2011).@en

Reducing the use of fossil fuels in shipping requires new, alternative maritime fuels. Hydrogen carriers offer a safe and energy-dense solution for storing hydrogen, a zero-emission alternative fuel. This research focuses on ammonia borane, NaBH4, n-ethylcarbazole and dibenzyltoluene. Applying hydrogen carriers influences ship design significantly, as they require additional specialised equipment to remove hydrogen from the hydrogen carrier. This research estimates the size of the equipment. As this equipment will need to be stored and maintained on the ship, the exact sizing and sequence of the additional equipment will likely influence ship design. Results show that the reactor size is significant for all hydrogen carriers. The mixing tank is considerably sized for NaBH4 and ammonia borane, while the heat exchangers are large for dibenzyltoluene and n-ethylcarbazole.@en

The maritime sector accounts for approximately 3% of the global emissions, and to limit these emissions, a transition towards alternative fuels is ongoing. In this work, a novel circular bunkering process for marine vessels is considered, such that a renewable fuel economy for the maritime industry can be realised. The proposed bunkering process is shown in Figure 1, and uses sodium borohydride (NaBH4) as the fuel, due to its favourable gravimetric and volumetric energy density compared to other alternatives [1]. NaBH4 is fed into a reactor during vessel operation, where it reacts with water to form hydrogen and sodium metaborate (NaBO2). While the hydrogen can be used in e.g. fuel cells to power the ship, the NaBO2, also referred to as spent fuel, has to be stored for the remainder of the voyage. Both NaBH4 and NaBO2 are bulk solids with a particle size distribution ranging from a hundred micrometres to several millimetres.
To this moment, NaBH4 has predominantly been used in the chemical industry as a reducing agent [2], and consequently, the mechanical characteristics and the effects of operational conditions such as humidity, temperature, and stress on the behaviour of the material are virtually unknown. However, this knowledge is essential to be able to design the required storage and handling equipment to realise the aforementioned circular bunkering process. Therefore, this work focuses on acquiring the relevant data using experiments. While most experiments show that NaBH4 is initially free-flowing, particular combinations of operational conditions show a significant change in the materials flow characteristics, some results showing very cohesive material or even non-flowing characteristics. Using the acquired experimental data, the Discrete Element Method (DEM) will be used to calibrate, verify, and validate material models, such that the flow characteristics of this novel fuel can be captured numerically. These models can then be used to conceptualise the bunkering process in a virtual environment. Finally, an outlook on how to use the gained insights to develop and design the storage and handling equipment for the proposed circular bunkering process is presented.

References
[1]: M.C. van Benten, J.T. Padding, and D.L. Schott. “Towards Hydrogen-Fuelled Marine Vessels using Solid Hydrogen Carriers”. In: The 14th International Conference on Bulk Materials Storage, Handling and Transportation. Wollongong, Australia, July 2023
[2]: Kaiqiang Zhang et al. “Recent Advances in the Nanocatalyst-Assisted NaBH 4 Reduction of Nitroaromatics in Water”. DOI: 10.1021/acsomega.8b03051
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Correction to: Scientific Reportshttps://doi.org/10.1038/s41598-024-68971-x, published online 13 August 2024 In the original version of this Article a previous rendition of Figure 2B, Figure 4 and Figure 5D was published. The original Figure 2, 4 and 5 and accompanying legends appear below. (Figure presented.) (Figure presented.) (Figure presented.) (A) Representative bubble dynamics for different channel geometries. (B) Universal motion of bubbles within channels with different size, shape and length. The dashed line represents the developed theory, Eq. (2). The marker colors represent the hydraulic diameters (d_h), the shapes represent the cross-section and the facecolor represent the lengths (L). The graphical marker symbols and colors established here are followed throughout this article. The black arrow represents the region of deviation(s) from the expected dynamics. The threshold laser energy absorbed for bubble formation estimated from experiments (E_th,exp) against theory (E_th,theory) presented in Eq. (5). (A,B) Representative dynamic bubble size curves illustrating the emergence of instabilities. The zones of the instabilities are highlighted using a shaded rectangular area. The arrows represent if the instabilities occur before or after X_max. (A) Illustrates the experimental data for different d_h with similar oscillation time. The instabilities emerge with increasing d_h. (B) Illustrates the data for d_h = 200 µm with increasing laser energies. The instabilities disappear with increasing E_abs. (C) Flow stability diagram with the transition line at W_o = 734. The markers represent the experiments and the lines represent the analytical estimate. The numbers correspond to the channel hydraulic diameters (in µm) with the dashed and solid lines representing the channel lengths L = 25 and 50 mm, respectively. (D) The dimensionless convective timescale against the L/d_h aspect ratio. The partition line is a linear relation between the x and y axes with 45 × 10⁻⁶ as the slope and the origin as the intercept. The original Article has been corrected.

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Oscillatory flow in confined spaces is central to understanding physiological flows and rational design of synthetic periodic-actuation based micromachines. Using theory and experiments on oscillating flows generated through a laser-induced cavitation bubble, we associate the dynamic bubble size (fluid velocity) and bubble lifetime to the laser energy supplied—a control parameter in experiments. Employing different channel cross-section shapes, sizes and lengths, we demonstrate the characteristic scales for velocity, time and energy to depend solely on the channel geometry. Contrary to the generally assumed absence of instability in low Reynolds number flows (<1000), we report a momentary flow distortion that originates due to the boundary layer separation near channel walls during flow deceleration. The emergence of distorted laminar states is characterized using two stages. First the conditions for the onset of instabilities is analyzed using the Reynolds number and Womersley number for oscillating flows. Second the growth and the ability of an instability to prevail is analyzed using the convective time scale of the flow. Our findings inform rational design of microsystems leveraging pulsatile flows via cavitation-powered microactuation.

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The local conditions inside a gas diffusion electrode (GDE) pore, especially in the electrical double layer (EDL) region, influence the charge transfer reactions and the selectivity of desired CO₂ER products. Most GDE computational models ignore the EDL or are limited in their applicability at high potentials. In this work, we present a continuum model to describe the local environment inside a catalytic pore at varying potentials, electrolyte concentrations and pore diameters. The systems studied in this work are based on an Ag catalyst in contact with KHCO₃ solution. Our study shows that steric effects dominate the local environment at high cathodic potentials (≪−25 mV vs pzc at the OHP), leading to a radial drop of CO₂ concentration. We also observe a drop in pH value within 1 nm of the reaction plane due to electrostatic repulsion and attraction of OH⁻ and H⁺ ions, respectively. We studied the influence of pore radii (1-10 nm) on electric field and concentrations. Pores with a radius smaller than 5 nm show a higher mean potential, which lowers the mean CO₂ concentration. Pores with a favourable local environment can be designed by regulating the ratio between the pore radius and Debye length.

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In flow-by capacitive deionization (CDI) brackish water flows between two electrodes that capacitively remove salt. We assume low inlet concentrations so “salt shocks” appear in the electrodes and the process becomes diffusion-limited. For unit charge efficiency, a simplified model is derived consisting of two coupled partial differential equations. We obtain approximations, and exact solutions in terms of the Lambert W function, for the salt concentration as a function of time and space and for the equilibrium charge-voltage relation. These surprisingly simple solutions compare well with the results from comprehensive two-dimensional simulations. Useful analytical expressions are obtained for optimal geometrical and operational parameters that maximize the productivity and minimize the specific energy losses. By making cells much thinner the productivity can be increased an order of magnitude compared to typical values in the literature. The optimal electrode is found to be roughly six times thinner than the spacer. The associated pressure drop is around 0.4 bar per 1 mM of inlet salt concentration, making our recommendations practically feasible only for relatively low concentrations. The obtained model and analytical expressions provide useful guidance to strongly improve the design process.

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Electrochemical oxygen reduction is a promising and sustainable alternative to the current industrial production method for hydrogen peroxide (H₂O₂), which is a green oxidant in many (emerging) applications in the chemical industry, water treatment, and fuel cells. Low solubility of O₂ in water causes severe mass transfer limitations and loss of H₂O₂ selectivity at industrially relevant current densities, complicating the development of practical-scale electrochemical H₂O₂ synthesis systems. We tested a flow-by and flow-through configuration and suspension electrodes in an electrochemical flow cell to investigate the influence of electrode configuration and flow conditions on mass transfer and H₂O₂ production. We monitored the H₂O₂ production using Cu-tmpa (tmpa = tris(2-pyridylmethyl)amine) as a homogeneous copper-based catalyst in a pH-neutral phosphate buffer during 1 h of catalysis and estimated the limiting current density from CV scans. We achieve the highest H₂O₂ production and a 15-20 times higher geometrical limiting current density in the flow-through configuration compared to the flow-by configuration due to the increased surface area and foam structure that improved mass transfer. The activated carbon (AC) material in suspension electrodes, which have an even larger surface area, decomposes all produced H₂O₂ and proves unsuitable for H₂O₂ synthesis. Although the mass transfer limitations seem to be alleviated on the microscale in the flow-through system, the high O₂ consumption and H₂O₂ production cause challenges in maintaining the initially reached current density and Faradaic efficiency (FE). The decreasing ratio between the concentrations of the O₂ and H₂O₂ in the bulk electrolyte will likely pose a challenge when proceeding to larger systems with longer electrodes. Tuning the reactor design and operating conditions will be essential in maximizing the FE and current density.

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Greenhouse gas emissions drive global warming, posing significant risks to ecosystems and human society. The maritime sector contributes approximately 3% of global emissions, leading the International Maritime Organization (IMO) to target a 40% reduction in emissions by 2030 and 70% by 2050, compared to 2008 levels. This reduction can be achieved by improving vessel efficiency or adopting alternative fuels such as hydrogen. Traditionally, hydrogen is stored as a gas, liquid, or cryocompressed, but these methods have drawbacks, including high pressure and energy demands. A promising alternative is sodium borohydride (NaBH4), a solid hydrogen carrier that offers high volumetric energy density and safe storage at ambient conditions. When used in maritime vessels, NaBH4 reacts with water to produce hydrogen and a byproduct, which is stored and later regenerated onshore. Both NaBH4 and its spent fuel are granular materials, requiring specific handling equipment. This paper aims to identify the mechanical characteristics of NaBH4 and its spent fuel to design appropriate storage and handling systems using the Discrete Element Method (DEM).@en

Electrochemical reduction of CO2 heavily depends on the reaction conditions found near the electrode surface. These local conditions are affected by phenomena such as electric double layer formation and steric effects of the solution species, which in turn impact the passage of CO2 molecules to the catalytic surface. Most models for CO2 reduction ignore these effects, leading to an incomplete understanding of the local electrode environment. In this work, we present a modeling approach consisting of a set of size-modified Poisson–Nernst–Planck equations and the Frumkin interpretation of Tafel kinetics. We introduce a modification to the steric effects inside the transport equations which results in more realistic concentration profiles. We also show how the modification lends the model numerical stability without adopting any separate stabilization technique. The model can replicate experimental current densities and faradaic efficiencies till −1.5 vs. SHE/V of applied electrode potential. We also show the utility of this approach for systems operating at elevated CO2 pressures. Using Frumkin-corrected kinetics gels well with the theoretical understanding of the double layer. Hence, this work provides a sound mechanistic understanding of the CO2 reduction process, from which new insights on key performance controlling parameters can be obtained.@en

Green hydrogen combined with PEM fuel cell systems is a viable option to meet the demand for alternative maritime fuels. However, hydrogen storage faces challenges, including low volumetric density, fire and explosion risks and transport challenges. We assessed over fifteen hydrogen carriers based on their maritime performance characteristics to determine their suitability for shipboard use. Evaluation criteria included energy density, locally zero-emission, circularity of process, safety, dehydrogenation process, logistic availability and handling. Thus, excluding ammonia and methanol because of these constraints, we found that borohydrides, liquid organic hydrogen carriers and ammoniaborane are the most promising hydrogen carriers to use on ships with PEM fuel cells. Borohydrides, specifically sodium borohydride, have high energy densities but face regeneration issues. The liquid organic hydrogen carrier dibenzyltoluene has a lower energy density but exhibits easy hydrogenation and good handling. Given varying operational demands, we developed a framework to assess the suitability of hydrogen carriers for use in different ship categories. Evaluating the three types of hydrogen carriers, using our framework and considering current practices, shows that these are viable options for almost all ship types. Thus, we have identified three types of hydrogen carriers, which should be the focus of future research.

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The use of gas diffusion electrodes that supply gaseous CO₂ directly to the catalyst layer has greatly improved the performance of electrochemical CO₂ conversion. However, reports of high current densities and Faradaic efficiencies primarily come from small lab scale electrolysers. Such electrolysers typically have a geometric area of 5 cm², while an industrial electrolyser would require an area closer to 1 m². The difference in scales means that many limitations that manifest only for larger electrolysers are not captured in lab scale setups. We develop a 2D computational model of both a lab scale and upscaled CO₂ electrolyser to determine performance limitations at larger scales and how they compare to the performance limitations observed at the lab scale. We find that for the same current density larger electrolysers exhibit much greater reaction and local environment inhomogeneity. Increasing catalyst layer pH and widening concentration boundary layers of the KHCO₃ buffer in the electrolyte channel lead to higher activation overpotential and increased parasitic loss of reactant CO₂ to the electrolyte solution. We show that a variable catalyst loading along the direction of the flow channel may improve the economics of a large scale CO₂ electrolyser.

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Crystallization abounds in nature and industrial practice. A plethora of indispensable products ranging from agrochemicals and pharmaceuticals to battery materials are produced in crystalline form in industrial practice. Yet, our control over the crystallization process across scales, from molecular to macroscopic, is far from complete. This bottleneck not only hinders our ability to engineer the properties of crystalline products essential for maintaining our quality of life but also hampers progress toward a sustainable circular economy in resource recovery. In recent years, approaches leveraging light fields have emerged as promising alternatives to manipulate crystallization. In this review article, we classify laser-induced crystallization approaches where light-material interactions are utilized to influence crystallization phenomena according to proposed underlying mechanisms and experimental setups. We discuss nonphotochemical laser-induced nucleation, high-intensity laser-induced nucleation, laser trapping-induced crystallization, and indirect methods in detail. Throughout the review, we highlight connections among these separately evolving subfields to encourage the interdisciplinary exchange of ideas.

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Solid hydrogen carriers, such as sodium borohydride or potassium borohydride, are considered promising options to enable the use of hydrogen as a fuel for marine vessels, because of their favourable gravimetric and volumetric energy density compared to compressed or liquefied hydrogen. When using solid hydrogen carriers, in the form of granules or powder, as fuel for marine vessels, a ’spent fuel’ forms which has to be stored on the vessel for the remainder of the voyage. The spent fuel has to be regenerated upon arrival at the destination port to achieve circularity. From an operational perspective, both the fuel and the spent fuel have to be stored for at least the duration of one vessel trip. To design the required storage and handling equipment to realize a circular bunkering process, the mechanical characteristics of both the fuel and the spent fuel e.g. particle size distribution, internal friction, cohesion, wall-friction, and flowability are required. However, little is known about these mechanical characteristics. Consequently, this paper aims to identify the relevant mechanical characteristics of solid hydrogen carriers in the context of bunkering marine vessels. Therefore, an extensive experimental plan using, amongst others, a ring shear tester and a ledge test is presented together with preliminary results of mechanical characteristics including time consolidation effects. The paper concludes with an outlook on the use of the results in DEM-supported design for storage and handling equipment, both onboard the vessel and in the port.@en

Lubrication forces play a major role in the behaviour of fluid–solid systems, where they affect the collisions between particles. Current implementations of lubrication forces in unresolved simulations often suffer from shortcomings, such as neglecting parts of the physics or relying on arbitrarily defined parameters. In this short communication, we propose a novel implementation, rigorously defined based on physical and numerical factors. Both particle roughness and deformation are considered, and the model accuracy is demonstrated through comparison with experimental results.

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Sodium borohydride (NaBH₄) has a high hydrogen (H₂ ) gravimetric capacity of 10.7 wt %. NaBH₄ releases H₂ through a hydrolysis reaction in which aqueous NaB(OH)₄ is formed as a byproduct. NaB(OH)₄ strongly influences the thermophysical properties of aqueous solutions (i.e., densities, viscosities, and electrical conductivities) and the hydrolysis reaction kinetics and conversion of NaBH₄. Here, molecular dynamics (MD) simulations are performed to compute viscosities, electrical conductivities, and self-diffusivities of H₂ , Na⁺, and B(OH)₄^- for a temperature and concentration range of 298-353 K and 0-5 mol NaB(OH)₄/kg water, respectively. Continuous fractional component Monte Carlo (CFCMC) simulations are used to compute the solubilities of H₂ and activities of water in aqueous NaB(OH)₄ solutions for the same temperature and concentration range. A new force field is developed (Delft force field of B(OH)₄^-: DFF/B(OH)₄^-) in which B(OH)₄^- is modeled as a tetrahedral structure with a scaled charge of −0.85. The OH group in B(OH)₄^- is modeled as a single interaction site. This force field is based on TIP4P/2005 water and the Madrid-2019 Na⁺ force field. The MD simulations can accurately capture the densities and viscosities within 2.5% deviation from available experimental data at 298 K up to a concentration of 5 mol NaB(OH)₄/kg water. The computed electrical conductivities deviate by ca. 10% from experimental data at 298 K for the same concentration range. Based on the molecular simulations results, engineering equations are developed for shear viscosities, self-diffusivities of H₂, Na⁺, and B(OH)₄^-, and solubilities of H₂, which can be used to design and model NaBH₄ hydrolysis reactors.

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