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Item-typ:Veröffentlichung, Colloid deposition in monolithic porous media – Experimental investigations using X-ray computed microtomography and magnetic resonance velocimetry(Elsevier, 2017-11-14); ; ; ; For experimental investigations of colloid retention in porous media, also denoted as deep bed filtration, X-ray computed microtomography (µCT) has become a basic tool within the last decade. On the one hand, µCT can spatially resolve particle deposition at discrete points of filtration time. On the other hand, the topological information of the porous media including the porosity and the pore size distribution can be obtained. Aside from structural parameters, the velocity field of the fluid within the pores, which cannot be measured by means of µCT, plays an important role in the underlying mechanisms of particle transport and immobilization. In a given structure, a high flow rate will result in increased velocity gradients as well as increased shear forces compared to a lower flow rate. High shear forces are in turn unfavorable for particle deposition. Another imaging modality, magnetic resonance velocimetry (MRV), is capable of quantifying the desired velocity maps. We demonstrate an experimental approach that combines both, MRV and µCT. In contrast to the majority of other investigations about colloid retention, the porous media investigated in this work are monolithic foam-like structures. The evaluation of colloid deposition in those monolithic filters is based on analyzing individual pores. Particle deposition in a pore is expressed by the volumetric fraction of particles while the pore flow is described by the Reynolds number. Results indicate that pores with high Reynolds numbers are not among the pores with the highest or lowest volume fraction of particles for a given time. The particle volume fraction in pores with low Reynolds numbers is mainly a function of the axial position of the pore.Wissenschaftlicher ArtikelBand:175116 125 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Microparticle trajectories in a high-throughput channel for contact-free fractionation by dielectrophoresisContinuous, 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 Veff 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.Wissenschaftlicher ArtikelBand:153147 108 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Delayed binary and multicomponent gas diffusion in conical tubesCatalyst 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.Wissenschaftlicher ArtikelBand:148112 126 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Predicting optimal temperature profiles in single-stage fixed-bed reactors for CO2-methanationThe 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.Wissenschaftlicher ArtikelBand:13294 203