Search

The generation of high-quality volume meshes out of µCT data can be a challenging task that is mandatory to perform CFD simulations of fluid flows within or around scanned objects. Despite the growing relevance of CT-based CFD simulations, there is still a lack of a standardized, free-to-use workflow. In this work, an open-source based workflow is presented, which covers all steps from the CT image stack to a volume mesh in the end. The software packages ImageJ, ParaView, and Blender are used for surface reconstruction and processing of µCT data; resulting meshes are evaluated regarding geometric parameters, surface fidelity, and used memory. For volume meshing, the mesh generator snappyHexMesh implemented in OpenFOAM is used. A detailed description of this workflow is provided in the Supplementary Materials. We demonstrate how potential pitfalls and shortcomings can be avoided and how surface smoothing is especially important to preserve the surface area of scanned objects. We use CT data of a 10 ppi open cell foam to illustrate that data decimation can reduce the required time for the volume meshing by up to 45% and RAM up to 75%. The resulting meshes are used to simulate flow fields and heat transport with a surface heat source. The resulting temperature fields show that differences in the surface area and recesses in an unsmoothed mesh can affect the outcome of heat transport simulations, highly relevant for reactive CFD simulations. The results highlight the quality of our workflow’s outcome, which can help engineers that rely on CT reconstruction processes, also for other applications.

Recovery of noble materials from waste is essential for industries around the globe. Dielectrophoretic (DEP) filtration, an electrically switchable particle separation technique, can be applied to tackle this challenge. It is highly selective regarding particle size, material, or shape. Expanding the scope of DEP towards high throughput and improving the trapping efficiency are vital to make DEP a viable robust alternative to conventional separation methods. DEP filtration works by selective immobilisation of particles in a porous medium by the action of an inhomogeneous electric field. The field inhomogeneity comes from scattering an electric field at the phase boundary between the particle suspension and the filer surface. In this article, we show how the filter structure affects the DEP separation. We study fixed bed filters of three different grain types and find that the morphology of the grains highly influences the DEP filter efficiency. Specifically, grains with irregular surface structure and high perceived angularity show high separation efficiency. We believe insights into the design of DEP filtration will pave the way towards its application in, e.g., the recovery of valuable materials from electronic waste dust.

Electrochemical NAD+ reduction is a promising method to regenerate NADH for enzymatic reactions. Many different electrocatalysts have been tested in the search for high yields of the 1,4-isomer of NADH, the active NADH, but aside from electrode material, other system parameters such as pH, electrode potential and educt concentration also play a role in NADH regeneration. The effect of these last three parameters and the mechanisms behind their influence on NADH regeneration was systematically studied and presented in this paper. With percentages of active NADH ranging from 10 to 70% and faradaic efficiencies between 1 and 30%, it is clear that all three system parameters drastically affect the reaction outcome. As a proof of principle, the NAD+ reduction in the presence of pyruvate and lactate dehydrogenase was performed. It could be shown that the electrochemical NADH regeneration can also be done successfully in parallel to enzymatically usage of the regenerated cofactor.

The separation of particles with respect to their intrinsic properties is an ongoing task in various fields such as biotechnology and recycling of electronic waste. Especially for small particles in the lower micrometer or nanometer range, separation techniques are a field of current research since many existing approaches lack either throughput or selectivity. Dielectrophoresis (DEP) is a technique that can address multiple particle properties, making it a potential candidate to solve challenging separation tasks. Currently, DEP is mostly used in microfluidic separators and thus limited in throughput. Additionally, DEP setups often require expensive components, such as electrode arrays fabricated in the clean room. Here, we present and characterize a separator based on two inexpensive custom-designed printed circuit boards (80 × 120 mm board size). The boards consist of interdigitated electrode arrays with

Electrochemical NAD+ reduction is a promising method to regenerate NADH for enzymatic reactions. Many different electrocatalysts have been tested in the search for high yields of the 1,4-isomer of NADH, the active NADH, but aside from electrode material, other system parameters such as pH, electrode potential and educt concentration also play a role in NADH regeneration. The effect of these last three parameters and the mechanisms behind their influence on NADH regeneration was systematically studied and presented in this paper. With percentages of active NADH ranging from 10% - to 70% and faradaic efficiencies between 1% and 30%, it is clear that all three system parameters drastically affect the reaction outcome. The optimised reaction conditions were applied to test the consumption of the regenerated NADH by an enzymatic reaction. Electrochemical NAD + reduction in the presence of pyruvate and lactate dehydrogenase revealed that the 1,4-isomer of NADH, the physiological substrate of oxidoreductases, accounted for around 45% of the detectable electrochemically regenerated NADH.

Electrochemical NAD

The present study introduces a methodology to model electricity based hydrogen supply systems as a Mixed Integer Linear Programming (MILP) problem. The novelty of the presented approach lies especially in the linear formulations of the models for electrolysis and salt cavern storage. The proposed linear electrolysis model allows for an accurate consideration of operating limits and operating point-specific efficiencies, while the two-dimensional cavern model treats the cavern volume as a decision variable. The developed formulations are implemented in the open energy modeling framework (oemof) and applied to representative case studies with 2020 marginal conditions. Thereby, it has been confirmed that the individual consideration of power supply and hydrogen demand is crucial for optimal system design and operation. If electricity is drawn exclusively from the German grid, hydrogen costs of 2.67 € kg H 2 − 1 $€{\text{kg}}_{{\text{H}}_{2}}^{-1}$ are identified along with an increased CO2 footprint compared to natural gas based hydrogen. By contrast, a significantly reduced CO2 footprint results from autarkic wind power supply at costs of at least 4.28 € kg H 2 − 1 $€{\text{kg}}_{{\text{H}}_{2}}^{-1}$ . Further findings on autarkic operation include optimal ratios of electrolyzer and wind farm nominal power, as well as power curtailment strategies. Evidence is provided that salt cavern interim storage is beneficial. With grid connection, it serves to exploit electricity price fluctuations, while with renewable autarkic operation, it is essential to compensate for seasonal fluctuations in generation.

Open-cell foams are promising catalyst supports as they provide a low pressure drop, radial mixing, and exceptional heat transport properties. Even though their large potential for the design of small-scale, dynamically operated reactors with strongly exothermic reactions is known, their application is not yet common. To design efficient and safe structured reactors in the future, the understanding of structure-heat transport relations is key. Fully resolved CFD simulations of non-isothermal structured reactors including chemical surface reactions require a high modeling effort and are computationally expensive. In a previous study we therefore implemented volumetrically distributed heat sources in the solid to mimic the heat production during an exothermal reaction, and evaluated the resulting heat flows and temperature distributions via CFD. The previous analysis, however, was limited to one specific open-cell foam geometry. In this study, we extend the conjugate heat transfer problem including heat production in the solid to five periodic open-cell foams (Kelvin cell-lattices) with defined but different structural parameters to establish structure-heat transport relations. We confirmed conduction being the dominant heat removal mechanism and found the strut diameter and the solid thermal conductivity being the key parameters to improve heat transport and reduce hot spots.

Membranes showing monomodal pore size distributions with mean pore diameters of 23, 33, and 60 nm are chemically functionalized using silanes with varying chain length and functional groups like amino, alkyl, phenyl, sulfonate, and succinic anhydrides. Their influence on the morphology, pore structure, and gas flow is investigated. For this, single-gas permeation measurements at pressures around 0.1 MPa are performed at temperatures ranging from 273 to 353 K using He, Ne, Ar, N2, CO, CO2, CH4, C2H4, C2H6, and C3H8. Results show pore size and pore volume linearly depending on the length of functional molecules, as expected for monolayer deposition. However, the gas flow through functionalized membranes is disproportionally decreased up to a factor of around 10. Hence, the decreased pore size and pore volume cannot explain the large decrease in flow. Furthermore, there is no specific dependency between the decrease in flow and temperature or gas type other than the relation proposed by Knudsen (√RTM)-1. Considering the large variety of functional molecules used, it is very surprising that no correlations between the type of functional group and the flow have been found. The decrease in flow, however, is strongly dependent on the chain length of the silanes (factor of 10 at ∼2 nm length). This leads to the conclusion that the observed effect is not caused by sorption driven processes. It is proposed that steric interactions between functional groups and gas molecules lead to increased residence times on the surface and longer molecular trajectories, which, in turn, lead to a decrease in flow. In membrane design, any surface modification should, therefore, make use of functionalizing agents with chain length as short as possible.

Martensitic stainless steels are widely used materials. Their mechanical and corrosion properties are strongly influenced by their microstructure and thereby can be affected by heat treatment. In the present study, the effect of different austenitizing temperatures on the passive film growth kinetics of martensitic stainless steel is studied by electrochemical impedance spectroscopy. The data was further fitted by the point defect model to determine kinetic parameters. We show that an increasing austenitizing temperature leads to a more protective passive film and slows down passive film dissolution in sulfuric acid.

This work investigates the change of the flow topology of Taylor flow and qualitatively relates it to the excess velocity. Ensemble-averaged 3D2C-$$\upmu$$ μ PIV measurements simultaneously resolve the flow field inside and outside the droplets of a liquid–liquid Taylor flow that moves through a rectangular horizontal microchannel. While maintaining a constant Capillary number Ca = 0.005, the Reynolds number ($$0.52 \le {\text{Re}} \le 2.14$$ 0.52 ≤ Re ≤ 2.14 ), the viscosity ratio ($$0.24 \le \lambda \le 2.67$$ 0.24 ≤ λ ≤ 2.67 ) and surfactant concentrations of sodium dodecyl sulfate (0–3 CMC) are varied. We experimentally identified the product of the Reynolds number Re and the viscosity ratio $$\lambda$$ λ to indicate the momentum transport from the continuous phase (slugs) into the droplets (plugs). The position and size of the droplet’s main vortex core as well as the flow topology in the cross section of this vortex core changed with increased momentum transfer. Further, we found that the relative velocity of the Taylor droplet correlates negatively with the evoked topology change. A correlation is proposed to describe the effect quantitatively. Graphical abstract

In numerous of studies, dielectrophoresis (DEP) has proven that its high selectivity and versatility make it a promising separation technique in a variety of fields. So far, however, only a few processes in the bioanalytics have made it to commercial use. One of the main challenges is to achieve a technically relevant throughput while maintaining a high selectivity. We present a novel approach of a mesh-based DEP filter with ordered field disturbing structures that has a high potential for upscaling because low-cost and commercially available materials are used. In this filter, we firstly trap a mixture of particles and then selectively remobilize them via a frequency shift, which allows for multidimensional separation. Shape-selective separation is demonstrated using ellipsoidal and spherical polystyrene particles, first in established microchannels and subsequently in the mesh-based filter. Hence, particles were trapped at flow rates up to 120 mLh-1 and then selectively remobilized according to their shape. These results pave the way for high-throughput multitarget separations in a single and scalable device.

This study outlines the design, construction and characterization of a tailored low-cost linear birdcage (BC) resonator for magnetic resonance imaging at 7T. Typically, different BC designs found in literature are well described in theory but lack crucial information for practical realization. This is challenging, as theoretical and practical aspects often differ greatly from each other, especially in the field of high frequency technology. We propose a simple and open-source 3D printable design which results in a working BC if the instructions in this publication are followed. The aim is to open up the possibility of building a functioning BC with simple means and a budget below 750 €, even for users without a great deal of expertise in MRI coil building. We demonstrate that the BC can achieve a good

Recently, in situ studies using nuclear magnetic resonance (NMR) have shown the possibility to monitor local transport phenomena of gas-phase reactions inside opaque structures. Their application to heterogeneously catalyzed reactions remains challenging due to inherent temperature and pressure constraints. In this work, an NMR-compatible reactor was designed, manufactured, and tested, which can endure high temperatures and increased pressure. In temperature and pressure tests, the reactor withstood pressures up to 28 bars at room temperature and temperatures over 400 °C and exhibited only little magnetic shielding. Its applicability was demonstrated by performing the CO2 methanation reaction, which was measured operando for the first time by using a 3D magnetic resonance spectroscopic imaging sequence. The reactor design is described in detail, allowing its easy adaptation for different chemical reactions and other NMR measurements under challenging conditions.

COx hydrogenation reactions for hydrocarbon synthesis, such as methane, are becoming more and more important in terms of the energy transition. The formation of the byproduct water leads to a hydrothermal environment, which necessitates stable catalyst materials under harsh reaction conditions. Therefore, novel nanostructured core-shell catalysts are part of scientific discussion, since these materials offer an exceptional resistance against thermal sintering. Here we report on a core-shell catalyst - Co@mSiO2 - for the hydrogenation of CO/CO2 mixtures towards methane. CO methanation experiments reveal a rapid temperature-depended deactivation for temperatures above 350 °C caused by coking and possible blocking of the pores. In comparison to a Co/mSiO2 reference catalyst with the same Co particle size a significantly higher methane selectivity was found for CO2 hydrogenation, which we attribute to the confinement effect of the core-shell structure and therefore a higher probability of CO readsorption. Finally, the simultaneous CO/CO2 co-methanation experiments show a high flexibility of the catalyst materials on different gas feed compositions.

In situ thermometry of catalytic gas phase reactions allows the determination of temperature profiles in catalyst beds. In NMR imaging systems used for measuring the chemical composition of species in model reactors, temperature measurements by NMR spectroscopy are technically challenging and confined to a rather low temperature range. In this study, an optimized NMR in situ technique is proposed, which will allow the determination of the temperature distribution in highly exothermic reactions on structured catalysts. Diffusion weighted magnetic resonance imaging (DW-MRI) was successfully applied as an alternative method for temperature measurements commonly performed by chemical shift measurements using ethylene glycol. DW-MRI applied with different diffusion sensitizing gradients allows high-resolution imaging of the temperature dependent diffusion coefficient, without the need for high spatial homogeneity of the magnetic field. Using 3D DW-MRI on ethylene glycol, glycerol, and the temperature stable ionic liquid Pyr13 [TFSI] (decomposition temperature of 400 °C) as NMR thermometers, measurements were performed in a temperature range from 20 to 160 °C. The proposed method can be used in reaction engineering approaches performed in NMR systems.

Due to the finite nature and scarcity of crude oil-based fuels, increasing attention is being directed towards various forms of renewable energy. To obtain a fully functional renewable energy system, some compensation for the spatiotemporal fluctuations in renewable energy sources is necessary. In this context, the so-called liquid organic hydrogen carrier (LOHC) systems are promising from a technological perspective, although their impact on the environment and on animal/human health has not been considered in great detail. Hence, we present a proactive, comparative environmental hazard assessment of LOHC systems based on three alkylcarbazoles and quinaldine, including H2-rich, H2-lean and intermediate (i.e., partially hydrogenated) forms as well as on benzene and toluene, also including H2-rich forms (cyclohexane and methylcyclohexane, respectively). Specifically, the present study reports on the enzyme inhibitory (acetylcholinesterase), mutagenic (Ames test) and cytotoxic (IPC-81 cell line) activities of the above-mentioned compounds. Furthermore, the aquatic toxicity of the test compounds to marine bacteria (Vibrio fischeri), green algae (Raphidocelis subcapitata), aquatic plants (Lemna minor) and water fleas (Daphnia magna) was assessed in addition to their biodegradability, using inocula from a wastewater treatment plant. To complete the picture, we compared the effects of the LOHCs investigated herein with those of diesel oil. In this context, our results suggest that the quinaldine-based LOHC system is comparable with diesel oil in terms of ecotoxicity but is less biodegradable. The alkylcarbazoles appear to be more toxic and poorly biodegradable. Lastly, the benzene and toluene-based LOHC systems give serious reasons for concern in terms of human and aquatic toxicity. Nevertheless, due to their less complex composition, the assessment of the LOHC systems carries much lower levels of uncertainty regarding adverse effects. Additionally, because of more favourable physicochemical properties (e.g., higher boiling points), some LOHCs are safer for handling and transportation. Finally, the possibility of storing and using renewable energy offers a significant environmental and economic benefit.

Separation of (biological) particles (<< 10 µm) according to size or other properties is an ongoing challenge in a variety of technical relevant fields. Dielectrophoresis is one method to separate particles according to a diversity of properties, and within the last decades a pool of dielectrophoretic separation techniques has been developed. However, many of them either suffer selectivity or throughput. We use simulation and experiments to investigate retention mechanisms in a novel DEP scheme, namely, frequency-modulated DEP. Results from experiments and simulation show a good agreement for the separation of binary PS particles mixtures with respect to size and more importantly, for the challenging task of separating equally sized microparticles according to surface functionalization alone. The separation with respect to size was performed using 2 µm and 3 µm sized particles, whereas separation with respect to surface functionalization was performed with 2 µm particles. The results from this study can be used to solve challenging separation tasks, for example to separate particles with distributed properties.

The tailorable and often unique properties of ionic liquids (ILs) drive their implementation into a broad variety of seminal technologies. The modular design of ILs allows in this context a proactive selection of structures that favor environmental sustainability─ideally without compromising their technological performance. To achieve this objective, the whole life cycle must be taken into account and various aspects considered simultaneously. In this review, we discuss how the structural design of ILs affects their environmental impacts throughout all stages of their life cycles and scrutinize the available data in order to point out knowledge gaps that need further research activities. The design of more sustainable ILs starts with the selection of the most beneficial precursors and synthesis routes, takes their technical properties and application specific performance into due account, and considers its environmental fate particularly in terms of their (eco)toxicity, biotic and abiotic degradability, mobility, and bioaccumulation potential. Special emphasis is placed on reported structure-activity relationships and suggested mechanisms on a molecular level that might rationalize the empirically found design criteria.

In today's industrial processes reactions in dispersed gas–liquid systems are of major importance. Many products originate from gas–liquid reactions inside the bubble wake, acting as a mixing zone. High reaction yields are mainly influenced by the hydrodynamics within these zones. However, undisturbed hydrodynamic measurements of low viscous systems inside the bubble wakes are lacking. In this work we report on non-invasive MRI of gas–liquid Taylor flows. A detailed explanation of the developed MRI setup and sequence is given.

In the last decade, in-situ studies using nuclear magnetic resonance (NMR) showed the possibility to monitor local transport phenomena of gas-phase reactions inside opaque structures. While spatial species concentration is known to be accessible operando during heterogeneously catalyzed gas-phase reactions, the spatial acquisition of gas temperatures has proven to be challenging. So far, temperature information is gathered by auxiliary liquid-filled capillaries with temperature-dependent NMR-properties. Here, we present a method for spatially resolved temperature and density measurements of gases using the temperature dependence of the signal amplitude and the longitudinal relaxation time (T1). In order to support the proposed method, temperature measurements of methane-filled tubes are carried out and compared to state-of-the-art temperature measurements using glycerol-filled capillaries. To demonstrate the applicability of the method during heterogeneously catalyzed gas-phase reactions, we measured the temperature operando inside a catalytically active honeycomb structure at standard pressure. The results show that direct temperature measurements of gases are possible, even though the low pressure inside the methane-filled tubes led to a low signal-to-noise ratio which compromised the accuracy of the data yielding relative errors of up to 7 %. This study focusses on temperature measurements of pure methane only. When applied to mixtures of gases, however, the proposed method has the potential to measure gas temperature and concentration at the same time.

Abstract Pattern of fluid flow through open-cell foams is important because of its influence on the performance of processes such as filtration, adsorption and heterogeneous catalysis that make use of such foams. So far, however, the experimental verification of velocity profiles obtained by computational fluid dynamics (CFD) simulation was insufficient. Here, the effect of morphology of ceramic foams on local gas flow patterns is observed via the noninvasive magnetic resonance velocimetry (MRV) technique. In order to cross-validate the simulations with the experimental flow mapping results, micro-computed tomography (µCT) data of the entire foams were used for generating the computational network required for 3D CFD simulations of velocity fields within the pores. The results of CFD simulations and MRV measurements of gas flow showed a remarkable agreement with deviations mainly below 10 percent if the whole foam structure was utilized in CFD simulations. The qualitative and quantitative agreement between CFD and MRV results underlines the reliability of CFD simulations that are based on µCT data and underpins the capability of NMR-based measurements for in situ velocity measurements. Graphic abstract

The knowledge of the influence of swarm effects on gas-liquid mass transfer is still limited. In this article, an optical measurement method for the identification of local mixing properties, mass flow rates, and mass transfer coefficients in the wake of rising bubbles with and without swarm turbulence is presented. Two-dimensional concentration data is temporally rearranged to reconstruct a quasi tri-dimensional concentration wake. By this means, the path of the bubble-wake-complex can now be visualized. The analysis shows that mixing time decreases strongly with superimposed swarm turbulence and becomes independent of bubble size. This is expected to have a major influence on chemical reactions. Especially slow parallel and consecutive reaction steps are promoted by poor mixing. Additionally based on locally measured data, mass transfer coefficients of bubbles rising in counter-current flow and with superimposed swarm turbulence are calculated. With the presented method, concentration distributions are now accessible in bubble flows with high spatiotemporal resolution. This paves the way for validating numerical simulations and developing physically determined models.

The local mass transfer of a bubble under swarm turbulence influence is the key to the performance of gas-liquid contactors. For measurements, optical accessibility is reduced due to an increased void fraction. The performance of a flexible grid of rigid particles to mimic the hydrodynamic conditions of bubble-induced swarm turbulence is assessed using spectral energy density distribution and an approach to connect the turbulent intensity to the according length scale of the flow. Despite the significant physical property differences to real bubbly flow the flexible grid is found to realistically mimic the swarm turbulence of a co-current bubble column. Hence, a single bubble approaching the proposed flexible grid experiences hydrodynamic conditions comparable to real bubble induced turbulence.

Insulator-based dielectrophoresis (iDEP) is a powerful particle analysis technique based on electric field scattering at material boundaries which can be used, for example, for particle filtration or to achieve chromatographic separation. Typical devices consist of microchannels containing an array of posts but large scale application was also successfully tested. Distribution and magnitude of the generated field gradients and thus the possibility to trap particles depends apart from the applied field strength on the material combination between post and surrounding medium and on the boundary shape. In this study we simulate trajectories of singe particles under the influence of positive DEP that are flowing past one single post due to an external fluid flow. We analyze the influence of key parameters (excitatory field strength, fluid flow velocity, particle size, distance from the post, post size, and cross-sectional geometry) on two benchmark criteria, i.e., a critical initial distance from the post so that trapping still occurs (at fixed particle size) and a critical minimum particle size necessary for trapping (at fixed initial distance). Our approach is fundamental and not based on finding an optimal geometry of insulating structures but rather aims to understand the underlying phenomena of particle trapping. A sensitivity analysis reveals that electric field strength and particle size have the same impact, as have fluid flow velocity and post dimension. Compared to these parameters the geometry of the post's cross-section (i.e. rhomboidal or elliptical with varying width-to-height or aspect ratio) has a rather small influence but can be used to optimize the trapping efficiency at a specific distance. We hence found an ideal aspect ratio for trapping for each base geometry and initial distance to the tip which is independent of the other parameters. As a result we present design criteria which we believe to be a valuable addition to the existing literature.

The use of silver nanoparticles (AgNPs) as an antimicrobial agent has increased significantly over the past decade which potentiates their release to the environment. The antimicrobial effect of AgNPs is generally considered to be due to the release of silver ions (Ag+). Here we describe their bioavailability under environmental conditions and demonstrate its influence on (eco-)toxicity for the AgNP NM-300K and a wide variety of aquatic organisms (green algae, plants and crustaceans), terrestrial organisms (soil bacteria) and mammalian cells. Since the bioavailability of AgNPs is largely determined by Ag speciation, this paper focuses on the Ag speciation in the test media for these organisms. We predicted the Ag speciation in aqueous test media by equilibrium speciation calculation and validated the results by comparison with experimental speciation using ultracentrifugation and membrane filtration. Silver amounts were quantified using GF-AAS, ICP-OES/MS and UV-vis. The dissolved Ag concentrations were controlled by the fast initial release of a limited amount of Ag+. After this initial release, the media components, chloride and proteins, controlled the available dissolved Ag by precipitation and complexation. Further release of Ag+ due to oxidation was not observed in the time frame of our experiments, except for media with very high chloride content. Apparently, the stabilisers of these AgNPs prevented any further release accounting for an enhanced redox stability. These findings facilitated the prediction of the bioavailability of Ag in the test media and, based on literature toxicity data, also of its toxic effects (EC50) on the respective organisms. The toxic effects of the AgNP NM-300K depended solely on the amount of Ag+ that was already present in the stock dispersion and not from further release due to later oxidation processes.

Mesoporous inorganic structures with mean pore diameters of 26 nm are prepared by extrusion based on a yttria stabilized zirconia nanopowder. The sintered capillary membranes serve as model structures to investigate the influence of an alkyl-chain (C 16 ) surface functionalization on the gas diffusion kinetics of argon (Ar), nitrogen (N 2 ) and carbon dioxide (CO 2 ) in mesopores. The density of the C 16 alkyl-chains immobilized on the membrane surface has an effect on both, gas flow as well as gas selectivity. For low functional group densities ( −2 ), the gas flow is reduced without having an effect on the selectivity. In contrast, for high alkyl-chain densities (>4 groups nm −2 ) the mean distance between the C 16 -chains is reduced to the order of magnitude of the gas molecules leading to a reduction in gas flow and a significant change of the gas selectivity. The selectivity is found to be influenced depending on the molecular diameter of the gas species, being more evident for CO 2 compared to Ar and N 2 , suggesting a separation mechanism more comparable to molecular sieving than to surface diffusion.

The separation of microparticles with respect to different properties such as size and material is a research field of great interest. Dielectrophoresis, a phenomenon which is capable of addressing multiple particle properties at once, can be used to perform a chromatographic separation. However, the selectivity of current dielectrophoretic particle chromatography (DPC) techniques is limited. Here we show a new approach for DPC based on differences in the dielectrophoretic mobilities and the crossover frequencies of polystyrene particles. Both differences are addressed by modulating the frequency of the electric field to generate positive and negative dielectrophoretic movement to achieve multiple trap and release cycles of the particles. A chromatographic separation of different particle sizes revealed a voltage dependency of this method. Additionally, we showed the frequency bandwidth influence on separation using one example. The DPC method developed was tested with model particles but offers possibilities to separate a broad range of plastic and metal microparticles or cells and to overcome currently existing limitations in selectivity.

Droplets of microscopic Taylor flows can act as stirred batch microreactors, while the continuous phase compartments supply good mixing and mass transfer to the channel walls. The design of reliable reactors, however, requires a priori information about droplet formation frequency and droplet length, which is difficult to obtain. Here, we report about a new method to reliably describe the droplet deformation, which could act as a flow indicator. The flow-related interface shape deformation of liquid–liquid Taylor flows in square micro channels for $$Re < 5$$ and $$Ca < 0.02$$ is quantified by experimental studies and ensemble image averaging. We deduce an easy-to-access droplet cap deformation model, which allows reducing the complex interface curvature information to an ellipse and provide correlation functions to link this deformation ratio to the flow conditions quantitatively. In addition, we introduce an experimental approach using a ternary liquid–liquid material system that allows adjusting $$Re$$ and $$Ca$$ numbers independently without surfactant addition by changing the mass fraction of two liquids forming the continuous phase. In combination, we suggest an image processing method to overcome the optical issues arising from the poor image contrast of Taylor droplets with low differences of refractive index to the bulk material. The presented approach opens the possibility to benchmark numerical studies of the interface deformation and paves the way for improved flow modeling, e.g., pressure drop or relative velocity of Taylor droplets, since the flow-induced curvature could serve as an indicator of the forces acting on the droplet front and rear.

Continuous, contact-free fractionation of sensitive microparticles at high throughput is a challenge. For this purpose, we developed a sheath flow assisted dielectrophoretic (DEP) field-flow separator with a tailored arrangement of cylindrical interdigitated electrodes (cIDE) and observed size-dependent trajectories of dispersed particles. Using a voltage input of 200 V eff at a frequency of 200 kHz, polystyrene particles (45, 25, and 11 µm in diameter) levitated to different heights along the channel length due to a negative DEP force. Experimental observations agree well with simulated particle trajectories that were obtained by a modified Lagrangian particle tracking model in combination with Laplace's and Navier–Stokes equations. By exploiting the size-dependent levitation height difference the desired particle size fraction can be collected at a specific channel length. The required channel length of the proposed cIDE separator increases with decreasing particle size to be separated. The quality of theoretical fractionation, which we quantify by resolution, improves strongly with reduced collector width, reduced volume flow rate and increased voltage input. The sensitivity of these dependencies increases with decreasing particle size. We calculated a theoretical throughput of up to 47 mL min −1 when trading-off design and operation parameters, allowing for contact-free fractionation of sensitive microparticles with negligible shear stress.

Catalyst pores are typically non-uniform along their longitudinal axis, and the transport of gaseous reactants and products takes place in a somehow tapered confinement. In a previous study we observed a diffusion delay in single tapered pores by means of a transient two-bulb-diffusion-cell (Veltzke et al., 2015). Processes in heterogeneous catalysis, however, are typically operated under steady state conditions. Hence also the diffusion processes are non-transient and reactant species are permanently consumed while product species steadily emerge. To mimic steady-state multicomponent diffusion in a cone, we developed a novel two-pipe-diffusion-cell and described the mass transport by an analytical model. Here we can show that the delay effect, which is caused by volumetric changes in longitudinal direction, also exists for steady-state binary and multicomponent diffusion. It is experimentally confirmed that the diffusion hindrance increases with conicity of the test tube. Also the results are transferable to those of the transient two-bulb-diffusion-cell. The measurement of steady-state experiments, however, is much faster.

Dielectrophoresis (DEP), a measure to manipulate motion trajectories of suspended particles, has a high potential for solving difficult particle–liquid separation problems. Applications of DEP so far have been limited to micro-channels and lab-on-a-chip devices. However, for designing DEP separators with sufficiently high throughput to reach preparative scale, an understanding of the interplay of channel geometry and electrode concept with respect to induced particle velocity is required. The objective of tailored design is a control of particle motion trajectories predominantly by DEP while avoiding electrothermal interference. It is Joule heating, which gives rise to temperature gradients in the liquid phase and, thus, induces thermal convection. In this work we demonstrate that a solution of this Joule heating problem in large scale DEP systems is a tailored ratio of electrode diameter, electrode distance, and channel height. Based on model calculations we predicted the influence of both DEP force and drag force through thermal convection on particle trajectories for a case study, a channel with rectangular cross section and an array of cylindrical interdigitated electrodes (IDE) at the bottom. These theoretical results were verified experimentally by measured velocities of polyelectrolytic resin microparticles located at the subsurface of demineralized water. This allowed for a qualitative sensitivity analysis of the impact of voltage input, particle size and medium properties on the critical design parameter. From this, design criteria were deduced for the IDE–DEP system that allow for minimizing the influence of Joule heating. The findings demonstrate that, even if high voltages are applied, Joule heating problems can be effectively suppressed in DEP system scale-up.

A micro-scale analysis of mass transport in ceramic foams that are used as catalyst supports in gas phase reactions is of high interest. Although the effects of flow rate and foam parameters on the radial and axial dispersion are known for liquid flows, no pore-scale experimental analysis has been yet reported to correlate the mechanical and diffusional dispersion of gas flows to the geometry of open-cell foams. Here, a spatially resolved Pulsed Field Gradient NMR method is applied to determine dispersion coefficients of thermally polarized gas along axial and transversal directions of open-cell foams. The comparative study of three commercial foam samples with different morphologies shows the effect of open porosity, window size, and flow rate on gas dispersion. Additionally, the influence of mechanical and diffusional dispersion at each flow rate is investigated for individual samples. By observing the transition from diffusional dispersion to mechanically driven dispersion of gas, it is found that diffusional dispersion plays an important role, even at higher flow rates after a transition from Darcy to Darcy-Forchheimer regime occurs. The measured values for dispersion coefficients of methane can be directly used in pseudo-heterogeneous models for the methanation reaction.

This study aims to investigate the effect of dielectrophoretic (DEP) force on microorganisms' viability in a membrane bioreactor (MBR). The impact of different electrical potentials (5-150 V) and different exposure durations (20-120 min) on the viability of microorganisms were studied. An interdigitated cylindrical electrode (IDE) configuration was used in the membrane module. Each electrical potential application was operated intermittently with 10 s of the electric field on and 15 s of the electric field off. It was found that the bacteria were able to withstand voltage up to 50 V, and their activity even increased with time with the application of 5 V. At high electrical potentials (100 and 150 V), the microbial activity decreased as a result of the increased current flow and temperature build up due to the Joule heating effect. The decrease of the microbial activity caused the increase of the total organic carbon (TOC) and ammonium (NH4+) concentrations in the bulk solution. A comparison between the continuous and intermittent voltage supply for the 50 V further proved the Joule heating impact on the bacteria viability. This research is made possible by NPRP award (NPRP7-089-2-044) from Qatar National Research Fund (QNRF). The statements made herein are solely the responsibility of the authors. The support from Mr. Naveen Francis and Mr. Steve Green from Doha South Waste Water Treatment Plant is also gratefully acknowledged. Scopus

Dielectrophoresis (DEP) is a versatile technique for the solution of difficult (bio-)particle separation tasks based on size and material. Particle motion by DEP requires a highly inhomogeneous electric field. Thus, the throughput of classical DEP devices is limited by restrictions on the channel size to achieve large enough gradients. Here, we investigate dielectrophoretic filtration, in which channel size and separation performance are decoupled because particles are trapped at induced field maxima in a porous separation matrix. By simulating microfluidic model porous media, we derive design rules for DEP filters and verify them using model particles (polystyrene) and biological cells (S. cerevisiae, yeast). Further, we bridge the throughput gap by separating yeast in an alumina sponge and show that the design rules are equally applicable in real porous media at high throughput. While maintaining almost 100% efficiency, we process up to 9 mL min-1, several orders of magnitude more than most state-of-the-art DEP applications. Our microfluidic approach provides new insight into trapping dynamics in porous media, which even can be applied in real sponges. These results pave the way toward high-throughput retention, which is capable of solving existing problems such as cell separation in liquid biopsy or precious metal recovery.

Mesoporous ceramic capillary membranes with mean pore sizes of about 20 nm are prepared as model structures to investigate the influence of an altered surface chemistry on the flow behavior of gases. To modify the membrane surface, a wet chemical silanization process with hexadecyltrimethoxysilane (HDTMS) is used to gain an alkyl-functionalized surface. Structural and surface characterizations show that the surface chemistry is altered without affecting the mean pore diameter. For the non-functionalized membrane, single gas permeation measurements at 20 °C reveal ideal permselectivities which are in good agreement with the Knudsen theory. In contrast, the HDTMS-functionalized membrane shows permselectivities regarding carbon dioxide (CO 2 ) which deviate about 20% from Knudsen theory. The gas permeation measurements further indicate a relative flow enhancement for CO 2 in comparison to nitrogen (N 2 ), argon (Ar) and methane (CH 4 ). Adsorption and desorption isotherms of CO 2 and N 2 at 20 °C show a decreased specific adsorption capacity for both gases, while the adsorption selectivity for CO 2 /N 2 is increased. This indicates a weaker interaction of gas molecules and membrane surface due to HDTMS functionalization. This weaker gas–solid interaction along with the increased adsorption selectivity is proposed as reason for the experimentally observed deviation of the permselectivities from Knudsen theory.

Pressure driven flows of rarefied single gases through long tapered rectangular channels are studied computationally. A part of the results is compared to the available experimental data. The flow is modeled by the linearized Bhatnagar-Gross-Krook kinetic equation. The mass flow rate and the distributions of the pressure and the rarefaction parameter are deduced. Calculations are performed in the whole range of the gaseous rarefaction. The diodicity has a maximum at intermediate values of the Knudsen number and increases with large to small cross section ratio. The diodic effect is explained phenomenologically. The pressure profile depends on the flow direction and varies strongly near the small height. The mass flow rates of CO 2 , N 2 and Ar gases in a mini/microchannel at moderate rarefactions are compared to the numerical results. An end-correction to the numerical calculation is applied. A relatively good agreement is found between the computational and experimental flow rates. The agreement is improved if the end-correction is used. The experimental diodicity is reproduced well with the numerical simulation. The results presented in the paper might be useful for the development and optimal design of pumping systems based on the diode effect.

In situ analysis of heterogeneously catalyzed gas phase reaction systems is becoming a valuable aid to their modeling and optimization. The commonly applied methods are either invasive, do not provide spatial information or are not applicable for optically inaccessible systems. This work investigates the possibility to use NMR imaging to study gas phase reaction processes in situ, spatially resolved and non-invasively. A multislice NMR spectroscopic imaging pulse sequence, which was optimized to realize ultrashort echo time TE, was employed to study the ethylene hydrogenation reaction in an NMR-compatible packed bed flow reactor. The catalyst bed, containing inactive γ -Al 2 O 3 pellets and Pt-Al 2 O 3 pellets, was subdivided into several sections in order to identify reaction zones that depend on initial conditions. Spatial mapping of the chemical composition was demonstrated on the basis of two experiments with varying initial volume flow and ethylene conversion. The inlet and outlet temperature of the catalyst bed was simultaneously detected by analyzing the spectra of inserted glycol capsules. The resulting spatial shift of the reactive zones in both experiments could be proven by the spatially resolved concentration measurements and the temperature measurements. The locations of single active catalyst pellets were also detectable by the same measure. The quantitative results of product gas composition of both experiments were in good agreement with accompanying mass spectrometric measurements. The results demonstrate the applicability of NMR imaging methods to investigate gas phase reaction processes and can help to establish these methods as a standard tool to map chemical transformations in gas flow reactors.

The catalytic conversion of carbon dioxide into methane, known as Sabatier process, is a promising option for chemical storage of excess renewable energy and greenhouse gas emission control. Typically externally cooled fixed-bed reactors (FBR) using supported nickel or ruthenium catalyst are applied. The Sabatier process, however, is strongly exothermic and leads to substantial hot spots within the reactor at stoichiometric feed ratios. Although high temperatures increase the reaction rate in general, they thermodynamically limit the achievable methane-yield in the Sabatier process. Here, we present an easy-to-use method based on a Semenov number optimization (SNO) to compute optimal axial temperature profiles in single-stage fixed-bed reactors that account for kinetic and thermodynamic limitations simultaneously, and thus result in maximized yield for a fixed reactor length. In a case study on CO2-methanation, these temperature profiles result in a twofold improvement of the methane-yield compared to isothermal and adiabatic operation, and thus demonstrate the high potential of thermal optimization that lies in the Sabatier process. The SNO-method provides a valuable tool to compute optimal temperature profiles, and allows intuitive insight into the key parameters for thermal process intensification. Further, it can readily be transferred to other processes that suffer from the dilemma between kinetic and thermodynamic limitations. Our findings illustrate the attractiveness of the SNO-method to compute optimal temperature profiles in fixed-bed reactors, and the need for catalyst supports with enhanced and tailorable heat transport properties.

Shallow vertical ground-source heat pump systems (GSHPSs) have become a popular alternative to conventional heating systems. Typically more than one vertical ground heat exchanger (GHE) is required along with an increasing heat demand. The higher the number of GHEs, the more a system may benefit from optimal design and operation strategies that focus on costs and efficiencies. However, an optimisation of the heat and fluid flows in these systems, based on discretised models, can be computationally time-consuming and sometimes infeasible. To meet this challenge, one might apply simplified models and identify suitable constraints. In this work an analytical finite line source (FLS) model is compared by RockFlow, a finite element approach. The average absolute difference for a long-term investigation between these approaches is obtained at only approx. 0.2 °C, which was evaluated as sufficient. Subsequently, the FLS model is successfully applied to demonstrate the existence of borehole-specific heat flux distributions. For all case studies optimal solutions were found. These results confirmed the useful application of novel optimisation methods. The impact of the GHE specific heat flux distributions on the time-dependent and spatial temperature course in the vicinity of the GHE is impressively shown. The investigation of the soil and heat pump cycle revealed the system efficiency potential and costs. The efficiency improvement potential, caused by different optimal heat flux distributions, was approx. 2%. The best energy extraction improvement was nearly 20%, which equated to a monetary saving of 12%.