Mirdrikvand, Mojtaba
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Item-typ:Veröffentlichung, Distribution of water in ceramic green bodies during drying(Elsevier, 2019-04-30); ; ; ; In order to investigate drying mechanisms at different stages, the distribution of water within the ceramic green bodies at different scales has been examined. The experimental measurements, using a simple weighing technique and Magnetic Resonance Imaging (MRI), show that during the first stage of drying involving shrinkage the material is constituted of uniquely solid and water with no gradient in water content within the sample. Then, during the second stage of drying, significant differences of water content as a function of position appear. As a complement, at the grain scale, observations using environmental scanning electron microscopy were made giving useful information on the solid–liquid–gas interfaces in the near surface part of the green body. Finally, the gradients in the water distribution were exploited to make a simple estimate of the diffusion coefficient of water with its dependence on the moisture content.Wissenschaftlicher ArtikelBand:39Heft:10156 102 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Diffusion weighted magnetic resonance imaging for temperature measurements in catalyst supports with an axial gas flow(Royal Society of Chemistry, 2019-07-24); ; ; 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.Wissenschaftlicher ArtikelBand:4109 137 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, NMR methods for the characterization of mass transport and reaction processes in porous materials(2020-06-17); ; ; NMR imaging (MRI) and localized NMR spectroscopy are powerful techniques for non-invasively characterizing fluids in opaque porous materials, particularly those used for gas phase catalytic reactions. Transport processes of gases such as diffusion, dispersion, and flow can be investigated by fast NMR methods despite short effective transverse relaxation times in porous media. This project aims at developing novel and optimizing existing NMR measurement methods for the non-invasive characterization of liquids and gases in porous materials. The project comprises the development and application of MRI, in particular diffusiometry and velocimetry techniques that allow investigations required for optimizing heterogeneous catalytic gas phase reactions. Thus, optimized NMR methods were applied to determine the local velocity fields, molecular diffusion, dispersion, and temperature. Two major components of mass transport, diffusion and flow of gases, were investigated as preparatory studies for the analysis of gas phase reactions. To perform these investigations, measurement techniques were developed and optimized based on the specific demands in catalytic gas phase reactions taking place in monolithic structures. The measurements enabled the spatially resolved characterization of mass transport in such opaque systems by determining vital engineering parameters including temperature, composition of substances, velocity fields as well as molecular diffusion and dispersion of gas in catalyst supports. A 7-Tesla NMR imaging system (Bruker Biospec 70/20 USR) was used to develop methods and to perform measurements. A spatially resolved NMR method for measuring the probability function of molecular displacement was developed to characterize diffusion and dispersion of thermally polarized gases in open-cell foams with different pore densities. The apparent diffusion coefficients and dispersion coefficients of thermally polarized methane were measured under off-flow and flow conditions, respectively. Additionally, the influence of mechanical and diffusional dispersion at various flow rates (0.1-2.25 L·min-1, sample diameter: 25 mm) was investigated. The 3D MR velocimetry (MRV) measurements of gas flow in regular and irregular monolithic catalyst supports were conducted using an optimized spin-echo based phase-contrast MRV sequence. The obtained MRV data of thermally polarized methane gas were compared to numerical simulations performed for the identical samples. Finally, an optimized diffusion-weighted (DW) MRI technique was used for the in situ analysis of temperature in the catalyst supports. Using 3D DW-MRI to measure the temperature dependent diffusion coefficients of ethylene glycol, glycerol, and the temperature stable ionic liquid Pyr13 [TFSI] allowed to use capillaries filled with these liquids as NMR thermometers for a broad temperature range. Measurements were performed in a temperature range from 20 to 160 °C and allowed to obtain the axial temperature profiles in catalyst supports.Dissertation335 247 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Spatially Resolved Characterization of the Gas Propagator in Monolithic Structured Catalysts Using NMR DiffusiometryGas diffusivity measurements in opaque porous media were performed using nuclear magnetic resonance. An optimized pulsed-field gradient stimulated echo method with free volume selection was used to investigate the propagator of thermally polarized methane gas within commercial monolithic catalyst supports. Since signal losses due to T2 relaxation were minimized by using a short echo time, diffusion processes could be characterized by the measured propagator functions and effective diffusion coefficients were determined for a broad range of observation times and in different spatial directions. The study of this noninvasive characterization of gas diffusion found a clear effect of the monolith type and its pore size and coating on the effective gas diffusion coefficient and the apparent tortuosity for a given observation time.Wissenschaftlicher ArtikelBand:41Heft:9705 157 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Pore-scale analysis of axial and radial dispersion coefficients of gas flow in macroporous foam monoliths using NMR-based displacement measurements(Elsevier, 2020-02-06); ; ; ; 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.Wissenschaftlicher ArtikelBand:388132 98