Browsing by Author "Nieken, Ulrich"

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Open Access
Abluftoxidation in Monolith-Katalysatoren mit periodischem Wechsel der Strömungsrichtung
(1988) Eigenberger, Gerhart; Nieken, Ulrich
Mit steigenden gesetzlichen Anforderungen zur Reduzierung des Schadstoffgehalts gewinnen Verfahren zur oxidativen Umwandlung von Schadstoffen in Abluft zunehmend an Bedeutung. Typische Aufgaben der Abluftreinigung sind gekennzeichnet durch hohe Luftdurchsätze und niedrige Schadstoffkonzentrationen; die adiabate Temperaturerhöhung der Totaloxidation beträgt häufig weniger als 100°C. Das bedeutet, daß der Druckverlust in der Reinigungsanlage niedrig und der Warmerücktausch zwischen Zu- und Ablauf hoch sein muß, um die Betriebskosten niedrig zu halten. Im folgenden wird eine neuartige Betriebsweise der katalytischen Oxidation untersucht, die sich durch effektiven Warmerücktausch, niedrigen Druckverlust und geringes Bauvolumen auszeichnet. Sie beruht auf der Benutzung von Monolith-Katalysatoren in Verbindung mit der von Macros und Mitarb. entwickelten Prozeßführung mit periodischem Wechsel der Strömungsrichtung. Dabei wirkt jeweils der vordere und der hintere Teil des Katalysatorbettes als Regenerator-Wärmeaustauscher.
Open Access
Acid-base flow battery, based on reverse electrodialysis with bi-polar membranes : stack experiments
(2020) Xia, Jiabing; Eigenberger, Gerhart; Strathmann, Heinrich; Nieken, Ulrich
Neutralization of acid and base to produce electricity in the process of reverse electrodialysis with bipolar membranes (REDBP) presents an interesting but until now fairly overlooked flow battery concept. Previously, we presented single-cell experiments, which explain the principle and discuss the potential of this process. In this contribution, we discuss experiments with REDBP stacks at lab scale, consisting of 5 to 20 repeating cell units. They demonstrate that the single-cell results can be extrapolated to respective stacks, although additional losses have to be considered. As in other flow battery stacks, losses by shunt currents through the parallel electrolyte feed/exit lines increases with the number of connected cell units, whereas the relative importance of electrode losses decreases with increasing cell number. Experimental results are presented with 1 mole L^-1 acid (HCl) and base (NaOH) for open circuit as well as for charge and discharge with up to 18 mA/cm2 current density. Measures to further increase the effciency of this novel flow battery concept are discussed.
Open Access
An application of the Cahn-Hilliard approach to smoothed particle hydrodynamics
(2014) Hirschler, Manuel; Huber, Manuel; Säckel, Winfried; Kunz, Philip; Nieken, Ulrich
The development of a methodology for the simulation of structure forming processes is highly desirable. The smoothed particle hydrodynamics (SPH) approach provides a respective framework for modeling the self-structuring of complex geometries. In this paper, we describe a diffusion-controlled phase separation process based on the Cahn-Hilliard approach using the SPH method. As a novelty for SPH method, we derive an approximation for a fourth-order derivative and validate it. Since boundary conditions strongly affect the solution of the phase separation model, we apply boundary conditions at free surfaces and solid walls. The results are in good agreement with the universal power law of coarsening and physical theory. It is possible to combine the presented model with existing SPH models.
Open Access
Carbon‐methanol based adsorption heat pumps : identifying accessible parameter space with carbide‐derived carbon model materials
(2020) Träger, Lisa; Gläsel, Jan; Scherle, Marc; Hartmann, Julian; Nieken, Ulrich; Etzold, Bastian J. M.
In adsorption heat pumps, the properties of the porous adsorbent and the refrigerant determine the performance. Major parameters for this working pair are the total uptake of the adsorptive, its kinetics, and the heat transfer characteristics. In the technical application despite powdered adsorbents, thin consolidated layers of the adsorbent can be attractive and obtained by a binder‐based approach but likely result in competing material properties. Thus, for a process optimization, the accessible parameter space and interdependencies have to be known and were deduced in this work for model porous carbons (carbide‐derived carbons derived from TiC and ZrC) and methanol as well as the addition of different amounts of boron nitride, silver, and graphite as heat‐conductive agents and the use of two binders.
Open Access
Catalytic cleaning of polluted air : reaction engineering problems and new solutions
(1994) Eigenberger, Gerhart; Nieken, Ulrich
Integration of regenerative heat exchange into a catalyst bed enables auto-thermal operation of catalytic purification of polluted air with a low content of combustible pollutants. Concentrations corresponding to an adiabatic temperature rise of less than 20°C can be handled without introduction of additional fuel. For higher concentrations, a technique involving side-stream withdrawal allows utilization of the total heat of combustion at the highest reactor temperature. Heat recovery by integrated heat exchange gives rise to an unusual behavior of the reactor. Based upon an analogy to fixed-bed reactors with integrated countercurrent heat exchange, simple equations are derived for reactor design and operation. Substantial reduction in pressure loss and in the volume of the packed bed can be obtained by replacing conventional catalyst packings by monolithic catalysts. The corresponding relationships are briefly discussed.
Open Access
Catalytic combustion with periodic flow reversal
(1988) Eigenberger, Gerhart; Nieken, Ulrich
Operation of fixed-bed reactors with periodic flow reversal as proposed and demonstrated by Matros and co-workers is a novel mode of operation for weakly exothermic or equilibrium limited reactions. In the present paper catalytic combustion in monolith type catalysts is considered. The influence of the operating parameters upon the shape of the temperature profiles in the stationary state als well as the stability of the ignited state are studied by model calculations. The advantages of inert front- and end sections and of hot gas withdrawal from the centre of the reactor are discussed. Experimental results for the catalytic combustion of traces of methane and propene in air show general agreement with the simulations.
Open Access
Constitutive correlations for mass transport in fibrous media based on asymptotic homogenization
(2023) Maier, Lukas; Kufferath-Sieberin, Lars; Pauly, Leon; Hopp-Hirschler, Manuel; Gresser, Götz T.; Nieken, Ulrich
Mass transport in textiles is crucial. Knowledge of effective mass transport properties of textiles can be used to improve processes and applications where textiles are used. Mass transfer in knitted and woven fabrics strongly depends on the yarn used. In particular, the permeability and effective diffusion coefficient of yarns are of interest. Correlations are often used to estimate the mass transfer properties of yarns. These correlations commonly assume an ordered distribution, but here we demonstrate that an ordered distribution leads to an overestimation of mass transfer properties. We therefore address the impact of random ordering on the effective diffusivity and permeability of yarns and show that it is important to account for the random arrangement of fibers in order to predict mass transfer. To do this, Representative Volume Elements are randomly generated to represent the structure of yarns made from continuous filaments of synthetic materials. Furthermore, parallel, randomly arranged fibers with a circular cross-section are assumed. By solving the so-called cell problems on the Representative Volume Elements, transport coefficients can be calculated for given porosities. These transport coefficients, which are based on a digital reconstruction of the yarn and asymptotic homogenization, are then used to derive an improved correlation for the effective diffusivity and permeability as a function of porosity and fiber diameter. At porosities below 0.7, the predicted transport is significantly lower under the assumption of random ordering. The approach is not limited to circular fibers and may be extended to arbitrary fiber geometries.
Open Access
Convective drying of porous media : comparison of phase-field simulations with microfluidic experiments
(2024) Maier, Lukas; Brosch, Sebastian; Gaehr, Magnus; Linkhorst, John; Wessling, Matthias; Nieken, Ulrich
Convective drying of porous media is central to many engineering applications, ranging from spray drying over water management in fuel cells to food drying. To improve these processes, a deep understanding of drying phenomena in porous media is crucial. Therefore, detailed simulation of multiphase flows with phase change is of great importance to investigate the complex processes involved in drying porous media. While many studies aim to access the phenomena solely by simulations, here we succeed to compare comprehensively simulations with an experimental methodology based on microfluidic multiphase flow studies in engineered porous media. In this contribution, we propose a Navier-Stokes Cahn-Hilliard model coupled with balance equations for heat and moisture to simulate the two-phase flow with phase change. The phase distribution of the two fluids air and water is modeled by the Phase-Field equation. Comparisons with experiments are rare in the literature and usually involve very simple cases. We compare our simulation with convective drying experiments of porous media. Experimentally, the interface propagation of the water-air interface was visualized in detail during drying in a structured microfluidic cell made from PDMS. The drying pattern and the drying time in the experiment are very well reproduced by our simulation. This validation will enable the application for the presented Navier-Stokes Cahn-Hilliard model in more complex cases focused more on applications, e.g., in the field of fibrous materials.
Open Access
Coupled electrohydrodynamic and thermocapillary instability of multi-phase flows using an incompressible smoothed particle hydrodynamics method
(2022) Almasi, Fatemeh; Hopp-Hirschler, Manuel; Hadjadj, Abdellah; Nieken, Ulrich; Safdari Shadloo, Mostafa
This paper concerns the study of coupled effects of electrohydrodynamic (EHD) and thermocapillary (TC) on the dynamic behaviour of a single liquid droplet. An incompressible Smoothed Particle Hydrodynamic (ISPH) multiphase model is used to simulate EHD-TC driven flows. The complex hydrodynamic interactions are modeled using the continuum surface force (CSF) method, in which the gradient of the interfacial tension and the Marangoni forces are calculated with an approximated error or 0.014% in the calculation of Marangoni force compared to the analytical solutions which is a significant improvement in comparison with previous SPH simulation studies, under the assumption that the thermocapillarity generates sufficiently large stress to allow droplet migration, while the electrohydrodynamic phenomena influences the droplet morphology depending on the electrical and thermal ratios of the droplet and the ambient fluid. This study shows that, when applying a vertical electric field and thermal gradient, the droplet starts to stretch horizontally towards a break-up condition at a high rate of electrical permitivity. The combined effect of thermal gradient and electric field tends to push further the droplet towards the break-up regime. When the thermal gradient and the electric field vector are orthogonal, results show that the droplet deformation would take place more slowly and the Marangoni forces cause the droplet to migrate, while the stretching in the direction of the electric field is not seen to be as strong as in the first case.
Open Access
Electrically heated oxide ceramic tubes for high temperature reactions
(2023) Matthies, Jörn; Schall, Thomas; Pritzkow, Walter; Tuttlies, Ute; Nieken, Ulrich
Endothermic high temperature reactions are usually carried out in metal tubes heated by gas burners. Electrical heating allows substantial reduction of CO2 emissions. We propose the usage of a composite tube, where a thin metallic layer is embedded between an inner and outer ceramic layer. While monolithic ceramics suffer from brittleness and low tolerance to thermal stress, only the inner layer is made from monolithic ceramics, while the outer layer is made of fiber reinforced oxide ceramics. In first tests the hybrid ceramic tube was electrically heated to 1250 °C with a maximum heat release of 85 kW m-2.
Open Access
Experimental analysis of inductive heating of catalytic beds
(2024) Kleschew, Daniel; Tuttlies, Ute; Nieken, Ulrich
To reduce CO2 emissions in the chemical industry, already established endothermic reactors should be heated with electrical energy. One promising possibility is inductive heating, in which electrical energy can be transferred to the susceptor (material to be heated). The uniform heating of the susceptor material is necessary for stable operation in chemical reactors and is therefore investigated in this work. For this purpose, a non‐reactive test setup was built that enables the investigation of inductive heating of different susceptor materials and measurement of the temperature distribution under defined non‐reactive conditions. In the first tests, temperatures of up to 800 °C were reached and an uneven heat input in a bed of stainless‐steel rings was observed.
Open Access
Fixed bed reactors with periodic flow reversal : experimental results for catalytic combustion
(1994) Nieken, Ulrich; Kolios, Grigorios; Eigenberger, Gerhart
The influence of design and operating parameters on the behavior of a fixed-bed reactor with periodic flow reversal has been studied in a laboratory set-up for the case of catalytic total oxidation. The results are in accordance with detailed model simulations published elsewhere. They show that the periodic operation is completely dominated by the regenerative heat exchange and that steady-state kinetics can be used. Like any other autothermal reactor, a fixed-bed reactor with periodic flow reversal has to be operated in the ignited steady state. It was shown that totally and partially ignited steady states may exist under the same operating conditions if several combustible components with different ignition temperature are present in the feed. Hot gas withdrawal from the middle of the packed bed proved to be a suitable method to utilize almost all of the heat of reaction at the highest temperature in the reactor and to prevent high temperature peaks at the respective exit valves. Together with an appropriate design of the fixed bed, composed of inert front and end sections with low effective axial conductivity and an active portion with large axial conductivity, hot gas withdrawal allows for an efficient control of the reactor under the conditions of both high and low feed concentrations.
Open Access
Katalytische Abluftreinigung : verfahrenstechnische Aufgaben und neue Lösungen
(1991) Eigenberger, Gerhart; Nieken, Ulrich
Neue Verfahren der katalytischen Abluftreinigung mit im Reaktor integriertem regenerativem Wärmeaustausch erlauben die autotherme Abluftreinigung bis zu Schadstoffkonzentrationen entsprechend einer adiabaten Temperaturerhöhung von weniger als 20°C. Bei höheren Konzentrationen erlaubt ein Mittenabzug die nahezu vollständige Auskopplung der Verbrennungswärme auf dem höchsten im Reaktor vorliegenden Temperaturniveau. Infolge der Rückkopplung durch den integrierten Wärmeaustausch besitzt der Abluftreaktor ein ungewöhnliches Betriebsverhalten. Wie gezeigt wird, läßt es sich aber über eine Analogie zur Reaktionsführung mit integriertem Gegenstromwärmeaustausch mit einfachen Gleichungen abschätzen. Werden statt herkömmlicher Katalysatorschüttungen Monolithkatalysatoren eingesetzt, so lassen sich Stromungsdruckverluste und/oder Bauvolumen drastisch reduzieren. Auf die dafür maßgebenden Zusammenhänge wird eingegangen.
Open Access
Katalytischer Brenner für ein wärmeintegriertes Motorabgasreinigungssystem
(2008) Nieken, Ulrich; Eigenberger, Gerhart; Tuttlies, Ute; Bernnat, Jens
Inhalt dieses Projektberichts sind Arbeiten zur Entwicklung und Erprobung von Dieselbrennersystemen, die als aktives Regelelement in wärmeintegrierten, multifunktionalen Dieselabgasreinigungssystemen eingesetzt werden sollen.
Open Access
Modeling of diffusive transport of polymers moments using limiting cases of the Maxwell-Stefan model
(2022) Welzel, Stefan; Säckel, Winfried; Nieken, Ulrich
A polymer distribution is usually represented by its moments. Thus, to calculate transport in a polymer system, a formulation for the transport of moments of the polymer is needed. This is only possible if the moments close or if there is a suitable closing condition. To archive this, two simplifications of the Stefan-Maxwell diffusion are derived, which convert the transport equation of polymeric species to a closed set of transport equations for the polymer moments. The first approach corresponds to an infinitely diluted polymer system, whereas the second one describes a highly concentrated polymer system. Both formulations are compared with the full Stefan‐Maxwell model of a ternary mixture of a solvent and two polymer species of different chain length.
Open Access
Modeling of the branching point distribution during the polymerization of N‐vinylpyrrolidone
(2022) Welzel, Stefan; Zander, Christian; Hungenberg, Klaus‐Dieter; Nieken, Ulrich
To gain insights into the microstructure of polyvinylpyrrolidone (PVP), a detailed reaction mechanism is developed, which characterizes the polymer along the property coordinate chain length, terminal double bonds (TDB), and branching points. For practical purposes, calculations with three property coordinates are unfeasible, and model reduction is needed. Here, a reduced model with only one single property coordinate without significant loss of accuracy is derived. In the first step, the coordinate TDBs are reduced by a linear relationship between TDBs and chain length. As the parameters of this relation are state dependent, they are dynamically adjusted from a parallel calculated 0D model. In a second step, the pseudodistribution approach is used to reduce the 2D distribution to chain length as the only property coordinate and calculate moments of branching points as a function of chain length. A 2D class model is set up for validation. To demonstrate the benefits of the model, the chain length distribution and moments of branching points are calculated for different average residence times and monomer concentrations in a stirred tank reactor. In a future publication, the model will be validated by experimental data in terms of chain length distribution and branching points.
Open Access
Modeling strategies for the propagation of terminal double bonds during the polymerization of N‐vinylpyrrolidone and experimental validation
(2020) Zander, Christian; Hungenberg, Klaus‐Dieter; Schall, Thomas; Schwede, Christian; Nieken, Ulrich
Based on a recently suggested reaction mechanism, which involves the production and propagation of terminal double bonds (TDBs), kinetic models for the polymerization of N‐vinylpyrrolidone in aqueous solution are developed. Two modeling strategies, the classes and the pseudodistribution approach, are applied to handle the multidimensional property distributions that result from this reaction mechanism and to get detailed structural property information, e.g., on the chain length distribution and the distribution of TDBs. The structural property information is then used to develop reduced models with significantly lower computational effort, which can be used for process design, on‐line applications or coupled to computational fluid dynamic simulations. To validate the derivations, the models are first compared against each other and finally to experimental results from a continuous stirred tank reactor. The evolution of monomer conversion and molecular weight average data as well as molecular weight distributions can be represented very well by the models that are derived in this article. These results support the correctness of the reaction mechanism predicted by quantum mechanical simulations.
Open Access
Regelung eines instationär betriebenen Festbettreaktors mit Fuzzy-Kontrollregeln
(1993) Nieken, Ulrich; Kolios, Gregorios; Eigenberger, Gerhart
Katalytiscbe Abluftreinigung in Reaktoren mit periodischer Strömungsumkehr erlaubt einen autothermen Betrieb bis zu niedrigen Schadstoffkonzentrationen. Bei höheren Schadstoffkonzentrationen ermöglicht ein Mittenabzug die vollständige Auskopplung der Verbrennungswärme. Aufgabe der Regelung ist es, den gezündeten Zustand bei geringen Rohgaskonzentrationen durch Zugabe von Stützgas aufrechtzuerhalten und bei höheren Konzentrationen Reaktionswärme in Form eines heißen Teilgassstromes zurückzugewinnen. Ein konventioneller Reglerentwurf gestaltet sich in diesem Fall als schwierig, da der Reaktor keinen definierten Arbeitspunkt besitzt, sondern nur im gezündeten Zustand gehalten werden muß, wobei er sich aufgrund der direkten Kopplung von chemischer Reaktion und Wärmetausch stark nichtlinear verhält. Aus diesem Grund werden die Methoden der Fuzzy-Regelung ausgewählt und an einem Reaktor im Labormaßstab erprobt. Im folgenden wird zunächst kurz auf das Prinzip der katalytischen Oxidation mit periodisch wechselnder Strömungsrichtung eingegangen, um die Wahl der Reglerein- und Ausgangsgrößen zu begründen. Anschließend werden der Aufbau des Fuzzy-Reglers skizziert und die erzielten Versuchsergebnisse diskutiert.
Open Access
Simultaneous optimization of process operational and material parameters for a 2-bed adsorption refrigeration process
(2020) Scherle, Marc; Nieken, Ulrich
In process engineering, optimization is usually carried out without the simultaneous consideration of material and process. This issue is addressed in the following contribution. A model-based optimization is presented to improve the performance of adsorption heat pumps. Optimization is carried out in two steps. First, we optimize the operational parameters, the cycle time, and the thickness of the adsorbent for a given adsorption material. In a second step we use a material model to predict heat and mass transfer and adsorption capacity from structural material parameters. This allows us to vary the structural material parameters and calculate the optimal operational parameters for each adsorbent. The two-step optimization thus identifies optimal material properties together with corresponding optimal operational parameters. As constraints, a minimum specific cooling power (SCP) and the passive mass of heat transfer pipes are used. The coefficient of performance (COP) is taken as the objective function. We exemplarily demonstrate the approach for a two-bed adsorption chiller, carbide-derived carbon as the adsorbent, methanol as the sorptive and boron-nitrate as additive to improve heat conductivity. The approach can be easily extended to multi-bed installations and more sophisticated material models.
Open Access
Thin organic‐inorganic anti‐fouling hybrid‐films for microreactor components
(2022) Neßlinger, Vanessa; Welzel, Stefan; Rieker, Florian; Meinderink, Dennis; Nieken, Ulrich; Grundmeier, Guido
Deposit formation and fouling in reactors for polymer production and processing especially in microreactors is a well‐known phenomenon. Despite the flow and pressure loss optimized static mixers, fouling occurs on the surfaces of the mixer elements. To improve the performance of such parts even further, stainless steel substrates are coated with ultra‐thin films which have low surface energy, good adhesion, and high durability. Perfluorinated organosilane (FOTS) films deposited via chemical vapor deposition (CVD) are compared with FOTS containing zirconium oxide sol‐gel films regarding the prevention of deposit formation and fouling during polymerization processes in microreactors. Both film structures led to anti‐adhesive properties of microreactor component surfaces during aqueous poly(vinylpyrrolidone) (PVP) synthesis. To determine the morphology and surface chemistry of the coatings, different characterization methods such as X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy as well as microscopic methods such as field‐emission scanning electron microscopy (FE‐SEM) and atomic force microscopy (AFM) are applied. The surface free energy and wetting properties are analyzed by means of contact angle measurements. The application of thin film‐coated mixing elements in a microreactor demonstrates a significant lowering in pressure increase caused by a reduced deposit formation.