1. Catalytic hydro(deoxy)genation of furfural and modelling of its reaction kineticsRok Šivec, 2024 Opis: In recent decades, there has been a growing interest in producing biofuels and biochemicals from renewable sources. Furfural stands as one of the ligno(hemi)cellulosic biomass derived platform chemical, which can be transformed into numerous value-added products.
The goal of this PhD was to systematically study hydrotreatment reactions of furfural under varying operating conditions and to gain insights into the reaction mechanism and kinetics. An extensive experimental and computational study of hydrogenation, hydrodeoxygenation, oligomerisation and etherification of furfural in a three-phase batch reactor was performed. The goals were divided into three consecutive objectives.
In the first part, hydrotreatment of furfural over Pd/C catalyst under various reaction conditions, including the solvent selection (solventless conditions, tetrahydrofuran, isopropanol), atmosphere (nitrogen, hydrogen), temperature (100–200 °C), pressure (25–75 bar) and stirring speed, was studied. A reaction pathway network and a micro-kinetic model were developed, incorporating thermodynamics (hydrogen solubility), mass transfer, adsorption, desorption, and surface reactions. These phenomena and their contribution to the surface coverages, TOF’s and global reaction rates were studied. The hydrogen presence on the catalyst surface was found to influence the main reaction pathway, leading to ring, aldehyde group or full hydrogenation.
In the second part, various monometallic catalysts (Pd/C, Pt/C, Re/C, Ru/C, Rh/C, Ni/C, Cu/C) were tested at 100 -200 °C with 60 bar of hydrogen and tetrahydrofuran as solvent. A generalized reaction pathway network was developed. H2 temperature-programmed reduction (H2-TPR) and CO temperature-programmed desorption (CO-TPD) were conducted, and a regression analysis of the results was subsequently performed by numerical modelling and optimisation. The obtained adsorption and desorption kinetic parameters for active metallic sites were further used in a generalized micro-kinetic model, applicable to all tested catalysts. Pd/C exhibited high activity and non-selective hydrogenation of furfural, while other catalysts showed selective aldehyde group hydrogenation followed by deoxygenation, consistent with density functional theory (DFT) calculations. Ru/C uniquely produced 2 methyltetrahydrofuran and ring-opening products at 200 °C. In silico optimization of reaction conditions for promising catalysts ((Pd/C, Pt/C, Re/C, Ni/C) aimed to maximize the yield of the target product.
In the third part, the influence of support on catalytic activity was studied. Hydrotreatment of furfural over Pd/Al2O3, Pd/SiO2, Ru/Al2O3, Ru/SiO2, Ni/Al2O3, and Ni/SiO2 was performed between 150 - 200 °C, using 60 bar of hydrogen and tetrahydrofuran as solvent. The strength and rate of adsorption and desorption to/from acidic, metallic and interface site structures were determined, using H2-TPR, CO-TPD and NH3-TPD and subsequent regression analysis of the results by numerical modelling and optimisation. The resulting parameters were sequentially used in the generalized micro-kinetic model to quantify the contribution of the active metal (Ni, Pd, or Ru), support (Al2O3 or SiO2), interphase sites and their relationship on catalyst activity and selectivity. Evaluation of morphological and structural characteristics, adsorption/desorption and intrinsic reaction kinetics has indicated that the coverage of acidic sites (on alumina or silica) facilitated yielding ring hydrogenation and inhibited deoxygenation, decarbonylation and cyclic compound opening. The rates for aromatics or aldehyde functional groups were, nonetheless, affected in a different order.
The used and developed methods and findings of this PhD offer useful guidelines for transforming furfural into high-value chemicals through catalytic hydrotreatment, with significant implications for future research and industrial applications. Ključne besede: Lignocellulosic biomass, furfural, catalytic hydrogenation, micro-kinetic mass transfer model, reaction kinetics, first-principle methods, furfuryl alcohol, tetrahydrofurfuryl alcohol. Objavljeno v RUNG: 08.11.2024; Ogledov: 131; Prenosov: 1 Celotno besedilo (9,23 MB) |
2. Lens parameters for Gaia18cbf – a long gravitational microlensing event in the Galactic planeK. Kruszyńska, Ł. Wyrzykowski, K. A. Rybicki, M. Maskoliūnas, E. Bachelet, N. Rattenbury, P. Mróz, P. Zieliński, K. Howil, Z. Kaczmarek, S. T. Hodgkin, N. Ihanec, I. Gezer, M. Gromadzki, P. Mikołajczyk, A. Stankevičiūtė, V. Čepas, E. Pakštienė, K. Šiškauskaitė, J. Zdanavičius, V. Bozza, M. Dominik, R. Figuera Jaimes, A. Fukui, M. Hundertmark, N. Narita, R. Street, Y. Tsapras, Mateusz Bronikowski, M. Jabłońska, A. Jabłonowska, O. Ziółkowska, 2022, izvirni znanstveni članek Opis: Context. The timescale of a microlensing event scales as a square root of a lens mass. Therefore, long-lasting events are important candidates for massive lenses, including black holes.
Aims. Here, we present the analysis of the Gaia18cbf microlensing event reported by the Gaia Science Alerts system. It exhibited a long timescale and features that are common for the annual microlensing parallax effect. We deduce the parameters of the lens based on the derived best fitting model.
Methods. We used photometric data collected by the Gaia satellite as well as the follow-up data gathered by the ground-based observatories. We investigated the range of microlensing models and used them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using a known mass-brightness relation, we determined how likely it is that the lens is a main-sequence (MS) star.
Results. This event is one of the longest ever detected, with the Einstein timescale of tE = 491.41−84.94+128.31 days for the best solution and tE = 453.74−105.74+178.69 days for the second best. Assuming Galaxy priors, this translates to the most probable lens masses of ML = 2.65−1.48+5.09 M⊙ and ML = 1.71−1.06+3.78 M⊙, respectively. The limits on the blended light suggest that this event was most likely not caused by a MS star, but rather by a dark remnant of stellar evolution. Ključne besede: gravitational lensing: micro, Galaxy: stellar content, stars: black holes, stars: neutron, Astrophysics - Solar and Stellar Astrophysics Objavljeno v RUNG: 13.11.2023; Ogledov: 1527; Prenosov: 5 Celotno besedilo (9,87 MB) Gradivo ima več datotek! Več... |
3. Numerical simulations of nozzles with gas and liquid focusing for production of micro-jets : dissertationGrega Belšak, 2022, doktorska disertacija Ključne besede: numerical simulations, OpenFOAM, liquid sheets, micro-jets, multiphase flow, converging nozzles, double flow focusing nozzle, gas compressibility, vacuum conditions, atmospheric conditions, operational parameters, liquid properties, dissertations Objavljeno v RUNG: 07.12.2022; Ogledov: 2373; Prenosov: 46 Celotno besedilo (21,26 MB) |
4. X-ray absorption spectroscopy set-up for unstable gases: A study of 5p HydridesRobert Hauko, Jana Padežnik Gomilšek, Alojz Kodre, Iztok Arčon, 2020, izvirni znanstveni članek Opis: An absorption cell is constructed for x-ray absorption spectroscopy of reactive, unstable or hazardous gases at
room temperature. In conjunction with in-situ micro-synthesis technique relying on handling the gas in syringes
it enabled a first measurement of x-ray absorption spectra in the region of K and L edges for the series of hydrides
of 5p elements (SnH4, SbH3, TeH2, HI). The signal-to-noise ratio above 103 was achieved, whereby fine detail is
discerned in the spectra, in particular the small sharp features above each absorption edge, testifying of coexcitations
of outer electrons in the core photoeffect. Ključne besede: X-ray absorption spectroscopy
Micro-synthesis absorption cell
Gaseous hydrides
Multielectron photoexcitations Objavljeno v RUNG: 10.02.2020; Ogledov: 4354; Prenosov: 0 Gradivo ima več datotek! Več... |
5. Simulation Of Gas Focused Liquid JetsRizwan Zahoor, 2018, doktorska disertacija Opis: The main aim of dissertation is to develop an experimentally verified computational fluid dynamic (CFD) model of micron-sized liquid jet, produced by an injection molded Gas Dynamic Virtual Nozzle (GDVN). In these nozzles, liquid jets are efficiently orientedly transporting mass and momentum. They are produced by intelligently projecting hydrodynamic focusing effect from a high-speed stream of a co-flowing lower density and lower viscosity gas on a stream of liquid from a feeding capillary. Liquid micro-jets are used for delivery of protein crystal samples in a hard X-ray beam in serial femtosecond crystallography experiments. The diffraction patterns of crystals are collected just before their destruction. The samples are hard to crystallize and very precious, so a thorough knowledge of the jet used in delivering them is required. The jet characteristics are analyzed as a function of operating parameters, geometry and material properties.
The physical model is described by mixture formulation and Navier-Stokes equations for transient, Newtonian, two-phase, compressible flow. Multiphase flow problem is solved with finite volume method (FVM), where fluid-fluid interface tracking is obtained with volume of fluid (VOF). The implementation of FVM-VOF CFD model is available in open source codes OpenFOAM and Gerris. They are validated by performing a series of standard interface advection and multiphase flow test cases. Both open source codes are compared for their abilities in solving GDVN flow problem. Due to the compressible nature of the focusing gas flow, OpenFOAM was chosen for GDVN simulations, since Gerris has no compressible flow option.
Constant effective material properties are used in the phases together with ideal gas density constitutive relation. A mixture model of the two-phase system is solved in axisymmetry. The discretization of the nozzle and chamber system uses approximately 300 000 finite volumes. Mesh independent results are obtained with the finite volumes of the size 0.25 µm in the vicinity of the jet and drops. The simulations are compared with experimental results according to the jet thickness and length for distilled water jet and helium focusing gas, discharging into low-pressure environment of 150 Pa. Reynolds numbers of the liquid and gas are in the range 413-3828 and 17-1222, respectively and Weber number in the range 3-353. A reasonably good agreement with experimental and scaling results is found for the range of nozzle operating parameters never tackled before.
Subsequently, a numerical study of effects of nozzle geometry on stability, shape and flow characteristics of micron-sized liquid jets is performed. The jet characteristics are described as a function of (i) capillary-to-orifice distance, (ii) nozzle outlet orifice diameter and (iii) liquid feeding capillary angle. The study is performed for two sets of liquid flow rates while keeping the gas flow rate unchanged. It is observed that for each value of capillary-to-orifice distance and nozzle outlet diameter, there exists a minimum liquid flow rate below which the jet stability cannot be achieved. It is found that the changes in the nozzle outlet diameter have the biggest influence on the jet diameter, length and velocity, while the liquid capillary angle has no observable effect on the jet characteristic. Change in capillary-to-orifice distance does not affect the flow field around micro jet, so the jet stability and shape is found to be affected by the way liquid-gas interacts near meniscus.
The same numerical model is used to additionally analyze the jet performance under the influence of Argon, Carbon dioxide and Nitrogen focusing gases. The study shows that the helium gas at the same mass flow rate provides twice the length of the jet compared to other gases. The jet focused with helium is also much thinner, faster and interestingly shows no considerable temperature drop at the nozzle outlet.
This work for the first time discuss the computational model of an injection molded micron-sized nozzle and produces valuable information for their design. Ključne besede: Microfluidics, gas dynamic virtual nozzle, flow focusing, micro-jet, convective instability, absolute instability, compressible multiphase flows, dripping, spurting, jetting, jet thickness, jet length, computational fluid dynamics, finite volume method, volume of fluid method Objavljeno v RUNG: 27.03.2018; Ogledov: 7608; Prenosov: 180 Celotno besedilo (11,47 MB) |
6. Influence of Gas Dynamic Virtual Nozzle Geometry on Micro-Jet CharacteristicsRizwan Zahoor, Saša Bajt, Božidar Šarler, 2018, izvirni znanstveni članek Opis: In this paper we present a numerical study investigating the effects of nozzle geometry on stability, shape and flow characteristics of micron-sized liquid jets, produced by injection molded gas dynamic virtual nozzles (GDVNs) operating in vacuum. The jet characteristics are described as a function of (i) capillary-to-orifice distance, (ii) nozzle outlet orifice diameter, and (iii) liquid feeding capillary angle. An experimentally verified numerical model of GDVN with laminar two-phase Newtonian compressible flow, based on finite volume method and volume of fluid interface tracking, is used to assess the changes. The study is performed for two sets of liquid flow rates while keeping the gas flow rate constant. It is observed that for each value of capillary-to-orifice distance and nozzle outlet diameter there is a minimum liquid flow rate below which the jet is unstable. We find that the nozzle outlet diameter has the biggest influence on the jet diameter, length and velocity, while liquid capillary angle has no observable effect on jet characteristic. Varying capillary-to-orifice distance does not affect the flow field around micro-jet. It is found that the liquid and the gas interaction near the meniscus primarily affect the jet stability and shape Ključne besede: Gas dynamic virtual nozzle, Micro-jet, Compressible multiphase flow, Finite volume method, Volume of fluid, Jetting, Dripping Objavljeno v RUNG: 09.03.2018; Ogledov: 5876; Prenosov: 0 Gradivo ima več datotek! Več... |
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