Τμήμα Χημικών Μηχανικών (ΔΔ)

Permanent URI for this collection

https://hdl.handle.net/10889/31

Browse

Now showing 1 - 20 of 337

Open Access
Air quality forecasting in an urban area at high spatial resolution
(2023-06-02) Σιούτη, Ευαγγελία; Siouti, Evangelia
Τμήμα Χημικών Μηχανικών (ΔΔ)
Air quality forecasting systems can contribute to the eventual improvement of atmospheric quality on regional or urban scales and to reduce the risk of people being exposed to high air pollutant levels. PM2.5 is one of the most important air pollutants that can penetrate deep into our lungs causing premature deaths, cardiovascular and respiratory problems, but also can damage ecosystems. Gas-phase pollutants such as O3 and NOx have also a negative effect on human health causing lung diseases. Air pollution forecasting systems are becoming an increasingly useful tool, as they can predict the future status of the atmosphere and potentially prevent human health risks. The air quality forecasting system that we have developed called SmartAQ (Smart Air Quality), combines state-of-the-art meteorological and chemical transport models to produce high-resolution predictions of 1x1 km2 for an urban area for the next 3-day period. The system simulates the air pollution in Europe using 36x36 km2 spatial resolution and through three nested grids with increasing resolution it focuses on the urban area of interest with 1x1 km2 resolution. The WRF (Weather Research and Forecasting) model prepares high-resolution predictions of meteorology, the MEGAN (Model of Emissions of Gases and Aerosols from Nature) model provides aerosol and gas-phase emissions and the chemical transport model PMCAMx (Particulate Matter Comprehensive Air quality Model with extensions) is used to simulate air quality. The PSAT (Particulate matter Source Apportionment Technology) algorithm is used together with PMCAMx to determine the source contributions for each pollutant for the urban area. SmartAQ operates automatically, in real-time and predicts the pollutant concentration of tens of gases (NOx, SO2, CO, O3, volatile organic compounds, etc.), the complete aerosol size and chemical composition together with PM1, PM2.5 and PM10 levels and the pollutant sources. The urban area of Patras in Greece was used for the first application of our system due to high-dense network of low-cost sensors that measure PM2.5 concentrations in the city. Field measurements have indicated that cooking and residential biomass burning sources are of primary importance in urban areas, but the corresponding emissions are often neglected or underestimated by emission inventories and chemical transport models leading to uncertain air quality predictions. In Patras, wood burning in fireplaces is the dominant source of pollution during wintertime, while cooking is the most important local source of pollution during the summer and especially during the nighttime based on field measurements. For these reasons, bottom-up inventories for cooking and residential biomass burning were developed at 1x1 km2 resolution for the city of Patras. The spatial distribution of cooking emissions was based on the exact location of restaurants in the city and the temporal distribution on measured cooking OA diurnal profile. The spatial distribution of residential biomass burning emissions was based on the density of houses in the urban area and the temporal patterns on field measurements. Biomass burning organic aerosols were considered volatile and chemically reactive and their emissions were temperature dependent. PMCAMx reproduced well the PM2.5 concentrations for the area of high cooking OA emissions, while it tended to overestimate the PM2.5 concentrations for the outskirts of the city. During the wintertime, the model reproduced the measured PM2.5 profile in the high-biomass-burning-emission area, but it tended to overpredict PM2.5 in the suburbs. Driving variables for the discrepancies between model and measurements were uncertainties in cooking and biomass burning OA emissions, errors in meteorology and uncertainties in the low-cost sensor measurements used for the evaluation. The overprediction at the outskirts was probably due to overestimation of regional pollution. SmartAQ predicted for PM2.5 that cooking was the dominant local source of pollution during summer and biomass burning for residential heating was the dominant one during winter, which are consistent with measurements. The developed air quality forecasting system can be applied to any European urban area to produce high resolution atmospheric and weather predictions.
Open Access
Apatite based materials for solid oxide fuel cell (SOFC) and catalytic applications
(2012-10-01) Gasparyan, Hripsime; Μπεμπέλης, Συμεών; Μπεμπέλης, Συμεών; Μπογοσιάν, Σογομών; Νικολόπουλος, Παναγιώτης; Νεοφυτίδης, Στυλιανός; Κουτσούκος, Πέτρος; Κονταρίδης, Δημήτριος; Κατσαούνης, Αλέξανδρος
Τμήμα Χημικών Μηχανικών (ΔΔ)
Low cost silicates with apatite-structure (general formula of apatite A10-xM6O26±δ, where A = rare earth or alkaline earth and M= Si, Ge, P, V..) have been proposed recently as promising solid electrolyte materials (oxygen ion conductors) for use at intermediate temperature solid oxide fuel cells (SOFCs). These materials exhibit sufficiently high ionic conductivity (e.g. ~ 0.01 S cm-1 at 700 oC), which is dominated by the interstitial site mechanism and can exceed that of yttria-stabilized-zirconia (YSZ), the solid electrolyte used in state-of-the-art SOFCs. The apatite structure is tolerant to extensive aliovalent doping, which has been applied for improving ionic conductivity. In this work are presented results concerning synthesis, conductivity and catalytic characterization of Fe- and/or Al-doped apatite type lanthanum silicates (ATLS) of the general formula La10-zSi6-x-yAlxFeyO26±δ as well as electrochemical characterization of interfaces of ATLS pellets with perovskite and Ni-based electrodes. The aim was to investigate the properties of these ATLS material, in particular as it concerns their potential use as SOFC components or as catalysts in oxidation reactions. The conductivity of pellets prepared from ATLS powders synthesized via four different methods and having different grain size was measured under air and at different temperatures in the range 600 -850 oC, aiming to identification of the effect of composition (doping), method of synthesis, grain size and pellet sintering conditions. For electrolytes of the same composition, those prepared via mechanochemical activation exhibited the highest conductivity, which was improved with increasing Al- and decreasing Fe-content. In state-of-the-art SOFCs perovskite electrodes are used as cathodes and Ni-based electrodes as anodes, thus electrochemical characterization of perovskite and Ni-based/ATLS interfaces was carried out. As it concerns perovskite/ATLS interfaces, the characterization focused on the study of the open circuit AC impedance characteristics of a La0.8Sr0.2Ni0.4Fe0.6O3-δ/La9.83Si5Al0.75Fe0.25O26±δ interface, at temperatures 600 to 800 oC and oxygen partial pressures ranging from 0.1 to 20 kPa. Under the aforementioned conditions, it was observed that the impedance characteristics of the interface were determined by at least two different processes, corresponding to two partially overlapping depressed arcs in the Nyquist plots. The polarization conductance of the interface was found to increase with increasing temperature as well as with increasing oxygen partial pressure, following a power law dependence. The electrochemical characterization of Ni-based electrodes/ATLS interfaces involved study of the electrochemical characteristics of NiO-apatite cermet electrodes as well as a Ni sputtered electrode interfaced with Al- or Fe-doped apatite electrolytes, under hydrogen atmospheres. The impedance characteristics of these electrodes were found to be determined by up to three different processes, their relative contribution depending on the electrode microstructure, Ni content (as it concerns the cermet electrodes), temperature, hydrogen partial pressure and applied overpotential. Aiming to investigation of potential catalytic properties of ATLS materials the catalytic activity for CO combustion of a series of ATLS powders was studied. For this purpose, two series of apatite-type lanthanum silicates La10-xSi6-y-zAlyFezO27-3x/2-(y+z)/2 (ATLS), undoped or doped with Al and/or Fe, were synthesized via sol-gel and modified dry sol-gel methods and tested as catalysts for CO combustion. The experiments revealed that the ATLS powders were catalytically active for CO combustion above approximately 300 oC, with light-off temperatures T50 (50% conversion of CO) ranging from 505 to 629 oC. The study focused on the effect on catalytic activity of the synthesis method and doping with Al and/or Fe. Non-doped ATLS with stoichiometric structure, namely La10Si6O27 prepared via the sol-gel method, exhibited the highest catalytic activity for CO oxidation among all tested compositions, the comparison being based on the measured catalytic rate (expressed per surface area of the catalyst) under practically differential conditions. Compared to La-Sr-Mn-O and La-Sr-Co-Fe-O perovskite powders, the tested ATLS powders exhibited lower catalytic activity for CO oxidation.
Open Access
Atmospheric acidity and aerosol nitrate formation
Ζακούρα, Μαρία; Πανδής, Σπυρίδων; Τσαμόπουλος, Ιωάννης; Παρασκευά, Χριστάκης; Νένες, Αθανάσιος; Κορνάρος, Μιχάλης; Βαγενάς, Δημήτρης; Μαντζαβίνος, Διονύσης; Zakoura, Maria
Τμήμα Χημικών Μηχανικών (ΔΔ)
Atmospheric particles, also known as atmospheric aerosols, are suspended particles (liquid or solid) with diameters ranging from a few nanometers to 100 μm. Atmospheric aerosols affect the Earth’s radiant budget and hence the global climate through their direct and indirect radioactive effects, and also have a negative impact on human health. They can be classified as primary (emitted directly in the particulate phase) or secondary (formed in the atmosphere via chemical reactions involving gas-phase precursors). Atmospheric particles consist of a mixture of inorganic and organic chemical compounds, including nitrate, sulfate, ammonium, organic compounds, elemental carbon, sea salt, soil and water dust, with nitrates being one of the most important inorganic compounds of particles in polluted areas. Acidity is an important atmospheric aerosol property that drives a series of processes related to gas-particle partitioning and heterogeneous chemistry. pH affects the nitrogen cycle through the HNO3/NO3- and NH3/NH4+ gas-particle partitioning. Adverse health outcomes have been linked to strong aerosol acidity by some studies, like respiratory diseases and lung and laryngeal cancers in humans. Chemical transport models are tools well suited for the simulation and detailed study of atmospheric processes. Historically, chemical transport models have had major problems in reproducing the observed aerosol nitrate concentrations in both the US and Europe. Also, it is important that even though aerosol pH affects many processes, the size-dependence of the aerosol pH is not simulated in detail by chemical transport models. This thesis uses the 3-D chemical transport model PMCAMx over US with high grid resolution and in combination with a Plume-in-Grid sub-model to improve the aerosol nitrate predictions. Also, size-resolved aerosol pH predictions over Europe during May 2008 were made for the first time and their variation with time, height, presence of dust is studied, along with their impact on inorganic nitrate.
Open Access
Atmospheric acidity and secondary inorganic aerosol formation
(2022-07-22) Κακαβάς, Στυλιανός; Kakavas, Stylianos
Τμήμα Χημικών Μηχανικών (ΔΔ)
Secondary inorganic aerosol components (sulfate, nitrate, and ammonium) constitute a significant part of atmospheric PM mass and impact aerosol acidity. Aerosol nitrate and ammonium are mainly formed through gas-to-particle conversion processes of nitric acid (HNO3) and ammonia (NH3), while for sulfate the multiphase oxidation of sulfur dioxide (SO2) is mainly responsible. Chemical transport models (CTMs) have the tendency to overpredict fine nitrate aerosol levels in both U.S and Europe. This in turn can also affect fine ammonium aerosol predictions since in fine PM fraction nitrate partitions to the aerosol phase together with ammonium. Responsible for these errors in CTMs predictions may be the effects of non-volatile cations (NVCs) on fine PM levels and composition, which usually are not quantified correctly due to urban dust levels underestimation, but also the errors in the prediction of aerosol acidity. In addition, several Earth System Models (ESMs) usually neglect inorganic aerosol thermodynamics due to the additional computational burden. In this work, we use the PMCAMx CTM to quantify the effects of NVCs on fine PM levels and composition and to gain a better understanding of aerosol acidity and its various dependences. Also, ISORROPIA-lite, a simplified and lean version of the widely used ISORROPIA-II inorganic aerosol thermodynamics model is presented. Compared to its parent model, ISORROPIA-lite can simulate the effects of secondary organic aerosol water on aerosol thermodynamics. These effects are also examined. The first part of this thesis tests the hypothesis that errors in PM predictions by CTMs, such as PMCAMx, occur at least partially due to the urban dust emissions underestimation. The simulations suggest that the corresponding emissions are underestimated in the official pan-European reported emissions by a factor of ten. This hypothesis leads to improved PM10 predictions in all sites in Europe and especially in urban areas reducing the PM10 bias by 23% and the error by 13%. Simulations with the improved urban dust emissions indicate that PM1 nitrate, sulfate and ammonium levels can decrease on average within 20% over the modeling domain, while at the same time coarse levels can increase on average within 15% due to the higher levels of urban dust. In the second part of this thesis, aerosol acidity was simulated depending on particle size, location and altitude over Europe during summer using the hybrid version of PMCAMx for the simulation of inorganic aerosol formation. Simulations indicate that pH changes more with particle size in northern and southern Europe with differences up to 1−4 pH units between sub- and super-micron particles, while the average pH of PM1-2.5 can be as much as 1 unit higher than that of PM1. PM1 has the most water over the continental region of Europe, while coarse particles have the most water content in the marine and coastal areas due to the relatively higher levels of sea salt. Particles acidity increases with altitude (0.5-2.5 units pH decrease over 2.5 km) due to the decrease in aerosol liquid water content. Aerosol pH affects inorganic nitrate with the highest average nitrate levels predicted for the PM1-5 range and over locations where the pH exceeds 3. Dust increases aerosol pH for all particle sizes and nitrate concentrations for supermicron range particles. This effect of dust depends on calcium content. Τhe hybrid version of aerosol dynamics in PMCAMx is also used in the third part of this thesis to quantify aerosol acidity over the U.S during a wintertime and a summertime period as a function of particle size and altitude. Average PM1 pH can be higher up to 2 units during winter than summer due to the higher aerosol water levels in the cold periods. For the supermicron range, pH values are predicted to be higher during summer due to the higher concentrations of alkaline dust. Sub-micron aerosol is more acidic than supermicron for both seasons with pH differences of up to 1−4 units. Acidity is predicted to increase with altitude by up to 1−1.5 units for PM1, and 2−2.5 units for PM1−10 in the first two kilometers due to the decrease of liquid water content with height. In the fourth part, ISORROPIA-lite, an accelerated and simplified version of ISORROPIA-II aerosol thermodynamics model is presented and evaluated. ISORROPIA-lite assumes that the aerosol exists in liquid form even at low relative humidities (metastable state) and treats the aerosol thermodynamics using binary activity coefficients from precalculated look-up tables. These assumptions speed up the thermodynamic calculations by 35%. Application of ISORROPIA-lite in the PMCAMx CTM accelerates the simulations by about 10% with changes in the concentrations of the major aerosol components of less than 10% over Europe. Compared to ISORROPIA-II, ISORROPIA-lite also simulates the effects of organic water on aerosol thermodynamics. Simulation of these effects indicates an increase of fine nitrate and ammonium concentrations within 1 μg m−3 in places where the organic aerosol and RH levels are high. In the fifth part, the effects of secondary organic aerosol water (SOAW) on inorganic aerosol thermodynamics are studied using ISORROPIA-lite in PMCAMx for a full year over United States. SOAW can increase annual average fine aerosol water levels up to a factor of two when secondary organic aerosol (SOA) is a major PM1 component. Total dry PM1 can increase up to 2 μg m−3 due to increased partitioning of nitrate and ammonium (nitrate levels increase up to 200%) because of the additional SOAW mass when RH levels and PM1 components concentrations are high.
Open Access
Atmospheric organic aerosol - water interactions
(2014-08-26) Ψυχουδάκη, Μαγδαλινή; Πανδής, Σπύρος; Κουτσούκος, Πέτρος; Λυμπεράτος, Γεράσιμος; Νένες, Αθανάσιος; Μιχαλόπουλος, Νικόλαος; Παπαευθυμίου, Ελένη; Παρασκευά, Χρηστάκης; Psichoudaki, Magdalini
Τμήμα Χημικών Μηχανικών (ΔΔ)
Atmospheric aerosols are responsible for adverse health effects and uncertain climate forcing. Depending on their composition, they can directly affect climate by scattering or absorbing solar radiation and they can also indirectly affect by serving as cloud condensation nuclei (CCN). While the chemistry and physical properties of the inorganic components of the aerosols are more or less known, the same does not stand for the organic components. Hygroscopic water soluble organic material can enhance the water absorption of the particles, affecting their climate forcing. This dissertation explores the hygroscopic properties of atmospheric organic aerosol, the first part of the thesis is dedicated to the development and analysis of methods for the measurement of water soluble organic aerosol, while the second part investigates the hygroscopic properties and CCN activity of organic particulate matter emitted by different sources or produced in the atmosphere through oxidation of volatile organic compounds.
Open Access
Atomistic molecular dynamics simulations of polymer melt viscoelasticity
(2009-12-19T19:54:56Z) Χαρμανδάρης, Ευάγγελος; Θεοδώρου, Δ.; Θεοδώρου, Δ.; Κυπαρισσίδης, Κ.; Παπαθεοδώρου, Γ.; Τσαμόπουλος, Ι.; Φωτεινός, Δ.; Βλασσόπουλος, Δ.; Τοπρακτσιόγλου, Χ.; Harmandaris, Evangelos
Τμήμα Χημικών Μηχανικών (ΔΔ)
-
Open Access
Atomistic simulation of self-organization in semiconducting polymers and polypeptides with molecular dynamics and Monte Carlo methodologies
Τσούρτου, Φλώρα; Μαυραντζάς, Βλάσιος; Μαυραντζάς, Βλάσιος; Τσιτσιλιάνης, Κωνσταντίνος; Αμανατίδης, Ελευθέριος; Θεοδώρου, Δώρος; Χαρμανδάρης, Ευάγγελος; Περιστεράς, Λουκάς; Κούρνια, Ζωή; Tsourtou, Flora
Τμήμα Χημικών Μηχανικών (ΔΔ)
In soft nanostructured materials based on chain molecules (typical examples include organic semiconducting polymers and polypeptides), chain self-organization at the nanoscale and mesoscale completely controls their macroscopic behavior and functionality. Although the equilibrium structure of many of these systems is rather well-known today thanks to advanced experimental techniques, their molecular modelling remains challenging, currently preventing the use of computer simulations as a tool for the rational design of new nanostructured materials with tailored or modulated properties. The objective of the current dissertation is to investigate self-organization in soft nanostructured materials by means of atomistic simulations based on Molecular Dynamics (MD) and Monte Carlo (MC) methods. The latter involves the design and implementation of new stochastic (i.e., non-dynamic) algorithms capable of overcoming the problem of long relaxation times governing chain dynamics and plaguing MD methods. Through the utilization of some very powerful ‘unhysical’ moves, MC can increase dramatically the rate of sampling new microstructures. State-of-the-art MC moves, originally proposed for simpler polymer structures, were thus redesigned for two important classes of soft nanostructured materials: (a) thiophene-based semiconducting oligomers and polymers and (b) polypeptides, which exhibit spectacular structures at the nanoscale and have tremendous technological applications. First, a powerful MC algorithm was developed for the simulation of bulk models of α-unsubstituted oligo- and poly-thiophenes using moves that account for the rigid ring structure of the thiophene moiety. We also introduced two new united-atom models for the simulation of these materials: a rigid model for MC simulations and a more flexible model for MD simulations. We were able to predict the high temperature phase behavior of an important α-unsubstituted oligo-thiophene (α-nΤ with n denoting the number of rings), α-sexithiophene (α-6T), in good agreement with available literature data. Furthermore, the MD simulations were extended to other members of the family of α-nTs (n = 5, 7 and 8) to gain an insight into the dependence of their phase behavior on chain length. Upon cooling from the isotropic phase, spontaneous successive phase transitions were observed giving rise to liquid crystalline phases; an odd-even structural phenomenon was also observed. For the design of a MC algorithm in the future for the simulation of alkyl-substituted poly-thiophenes such as regioregular poly(3-hexylthiophene) (or RR-P3HT), the availability of a promising force field (FF) will also be of importance. We thus carried out a systematic evaluation of available all-atom FFs that can reliably describe the physical properties of RR-P3HT oligomers and polymers in their amorphous phase. By selecting the most accurate FF, large scale MD simulations of RR-P3HT with quite long chains were conducted in order to shed light into their structural behavior in the amorphous phase. Our results indicated that relatively short RR-P3HT chains are semiflexible but adopt random coil conformations at higher temperatures and for higher molecular weights. As far as the polypeptides are concerned, a new MC algorithm was designed and implemented using an all-atom approximation for homo-polypeptides based on the L-alanine amino acid residue. The new methodology was capable of predicting the secondary structure of homo-polypeptides consisting of a few decades of residues, characterized by the formation of a significant population of α-helix secondary structure elements under vacuum, starting from a random configuration as an initial condition. These first simulation results are very promising rendering possible the extension of the proposed methodology to melts and solutions of poly-L-alanine peptides.
Open Access
Atomistic simulation of weak polyelectrolytes in aqueous solutions
Μιντής, Δημήτρης; Mintis, Dimitris
Τμήμα Χημικών Μηχανικών (ΔΔ)
The present Ph.D. Thesis has received funding from the European Union’s Horizon 2020 Program (Marie Skłodowska-Curie project titled: “Training in Bio-Inspired Design of Smart Adhesive Materials, BioSmartTrainee”, Grant agreement No. 642861) for carrying out computational work that can aid the rational design and development of new, bio-inspired materials capable of bonding and de-bonding according to the micro-environmental conditions on wet, rough, and fouled surfaces. This project falls under the BioSmartTrainee ETN program (for more information please refer to the following link: http://biosmarttrainee.eu/) and established a network of 10 full partners from academia and industry (BASF, AkzoNobel, and URGO) for developing new links between polymer science, adhesion technology, and biomechanics. In the framework of the BioSmartTrainee ETN program, different material strategies have been suggested for the design of switchable adhesives capable of bonding and de-bonding on demand under wet conditions, including among others: a) polymer brushes, b) micro-patterned surfaces, and c) polyelectrolyte gels (complex coacervates). The objective of the present Ph.D. Thesis was to develop and implement computational algorithms capable of providing quantitative predictions of the microstructure, state of hydration, and dynamics (segmental and terminal) of bulk aqueous solutions of weak polyelectrolytes, and investigate their response to relevant physicochemical parameters (such as pH, total polymer concentration, and chain length) starting from the molecular level. Additional work was carried out for determining the phase boundary of aqueous solutions containing symmetrical (in terms of charge density and molecular length), oppositely charged polyelectrolytes undergoing complex coacervation (that leads to liquid-liquid phase association) by computing the salt-polymer binodal phase diagram. Suitable modelling methodologies at different levels, such as Quantum Mechanics (QM) and Molecular Dynamics (MD), were employed to gain improved insight into the connection between detailed molecular structure and macroscopic properties of aqueous solutions of the following weak polyelectrolytes: • poly(acrylic acid) (PAA) • poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), and • poly(ethylene imine) (PEI) The global and local conformation as well as the dynamics of PAA chains in infinitely dilute solutions were first examined as a function of pH (equivalent to acid, neutral, and basic pH conditions) and chain degree of polymerization N (= 20, 23, 46, 70 and 110). Our predictions were compared to previous experiments and already established scaling theories for flexible polyelectrolytes. The effect of the total polymer concentration on the state of hydration, structure, and dynamics of PDMAEMA aqueous solutions was investigated next. In collaboration with one of our BIOSMART partners (Wageningen University), we also carried out rheological measurements of PDMAEMA solutions as a function of concentration. Aqueous solutions containing PEI of either a linear or a short chain branched architecture were also considered in the present Ph.D. Thesis in order to study the effect of pH and molecular weight on local rigidity, global conformation, and diffusive behaviour of PEI. These systems were simulated in infinitely dilute solution at ambient conditions. A detailed comparison to established theories and previous studies was reported. In the final stages of the Thesis, we computed (using fully atomistic models and free energy calculations) the phase boundary of an aqueous solution of two oppositely and fully charged weak polyelectrolytes, poly(acrylic acid) (PAA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). Such a system, under certain conditions (temperature, salt concentration, and molecular length) undergoes a particular type of liquid-liquid phase separation (LLPS) that leads to the formation of two distinct phases co-existing in thermodynamic equilibrium: a dense polymer phase (the coacervate) showing up in the form of liquid droplets and a dilute or polymer-deficient phase (the supernatant). The salt-partitioning in the two phases was also studied, followed by a detailed analysis of the specific interactions between certain pairs of atoms or groups of atoms in the two polyelectrolytes driving complexation and eventually phase separation.
Open Access
Bioprocess development and design for the production of succinic acid via continuous fermentation of spent sulphite liquor
Λαδάκης, Δημήτριος; Ladakis, Dimitrios
Τμήμα Χημικών Μηχανικών (ΔΔ)
The scope of this thesis is the evaluation of spent sulphite liquor (SSL) as the main fermentation carbon source for succinic acid (SA) production in continuous cultures. Subsequently, a techno-economic analysis of a process that produces SA from SSL under continuous and fed-batch fermentations strategies is presented. The purpose of the techno-economic analysis is to investigate the economic feasibility of the succinic acid that derived from the utilization of SSL and factors in which the succinic acid production cost has high dependence. SSL is the liquid by-product stream derived from the acidic sulphite process of mainly hardwoods. This waste stream contains solubilised lignin in the form of lignosulphonates and sugars which arise after hemicelluloses degradation. The type of the using wood and the processing conditions employed in the plant are the two main elements that define the sugars contained in SSL. In this study the waste stream that was used resulting from the pulp and paper industry of the Eucalyptus globulus wood. This work was initiated with the evaluation of SSL as substrate in continuous cultures using the wild-type rumen bacterial strains Actinobacillus succinogenes and Basfia succiniciproducens. These strains are considered as two of the most promising strains for industrial implementation. The cultures were initially carried out at constant dilution rate (0.04 h-1) and varying initial commercial xylose concentrations (23-55 g/L) or constant xylose concentration (40 g/L) and varying dilution rates (0.02-0.25 h-1) showing that dilution rates of 0.02-0.15 h-1 led to satisfactory succinic acid production by both microbial strains. In continuous cultures using nanofiltrated SSL, maximum yields of SA were achieved at dilution rate of 0.02 h-1 (0.48 g/g for A. succinogenes and 0.55 g/g for B. succiniciproducens) while maximum values of productivity were obtained at dilution rate of 0.04 h-1 in the case of A. succinogenes (0.67 g/(L⸱h)) and 0.1 h-1 (1.6 g/(L⸱h)) in the case of B. succiniciproducens. During the fermentation of both strains observed biofilm creation at the solid parts of the reactor. Due to this the biomass concentration was impossible to measure, so the biomass concentrations at different dilution rates for both strains was estimated using metabolic flux analysis. In the techno-economic analysis of SA, the design of the entire SA production process was based on data obtained from experimental results and the material and energy balances were carried out using spreadsheets and validated through UniSim software. Bacterial fermentations were conducted with Basfia succiniciproducens as the SA producer. The down stream design based on direct crystallization process using ion-exchange resins for acidification step, with final purified SA crystals equal to 99% and with a recovery yield of 97% which achieved by recirculation of the side sterams in down stream process. Process simulation was performed for a range of plant capacities, 5, 30, 100 kt of SA per year. The study indicated the feasibility of fermentative SA production from SSL and the minimum selling price (MSP) for 10% annual return on investment was calculated at 2.70 $/kg for continuous and 3.06 $/kg for fed-batch strategy when commercial nitrogen sources were applied. The MSP was reduced to 2.16 $/kg and 2.76 $/kg for continuous and fed-batch respectively assuming that commercial nitrogen sources are substituted with hydrolysates of corn steep liquor (CSL). The fermentation and evaporation units were identified as significant capital and operating cost contributors and therefore the optimization of bioreactor geometrical characteristic and alternative more cost-effective evaporation technics should be considered in future research.
Open Access
Calculation of multi-time correlation functions in dissipative quantum φ^4 theory
(2023-05-04) Αλατάς, Παναγιώτης; Alatas, Panagiotis
Τμήμα Χημικών Μηχανικών (ΔΔ)
In quantum field theories, Feynman diagrams depict visually the interaction of elementary particles and involve closed loops, corresponding to integration over all possible combinations of energy and momentum of the virtual particles traveling in the space-time continuum, leading often to infinities. To treat these infinities, renormalization of the parameters of the theory is required. Renormalization specifies relationships between parameters in the theory whose values at large length scales are different from those at short length scales. Behind renormalization lies the problem of the proper elimination of degrees of freedom in order to bridge the gap between different length and time scales. Nonequilibrium thermodynamics offers an alternative approach to the problem of the time evolution of an open quantum system through a time-evolution master equation for the density matrix of the system, which includes reversible and irreversible contributions. In particular, irreversibility introduces dissipative smoothing mechanisms for the terms responsible for the infinities, providing the proper regularization of the theory. The objective of the present doctoral dissertation is the implementation of the dissipative quantum field theory (DQFT) developed by Prof. Dr. ‪Hans Christian Öttinger to calculate the perturbative expansions up to third order in the interaction parameter of the two- (propagator) and four-point (effective interaction vertex) correlation functions of the scalar theory with quartic interactions (φ^4 theory) in the limit of zero temperature, as well as to extract the respective Feynman diagrams where (in the context of DQFT) collision events have a well-defined arrow of time. To achieve this, we developed a powerful symbolic code which allowed us to organize and simplify the outcome of the commands performing the calculation of terms contained in each function within a given order so that to lead to a compact final expression, thus rendering the identification of the relevant Feynman diagrams more transparent. The use of symbolic computation was motivated by the fact that the perturbative DQFT in d dimensions (d denotes the number of space dimensions) is considerably more complicated than the usual calculations in D = d + 1 dimensions (D the number of space + time dimensions) of the Lagrangian quantum field theory, generating a huge number of terms with increasing order of perturbation. As expected, the resulting expressions for the propagator and the effective interactive vertex contain terms corresponding to zeroth-, first-, second- and third-order contributions. The terms up to second order have already been reported by Prof. Dr. Hans Christian Öttinger. In this Thesis, we computed the third-order contributions to both of them in terms of connected and unconnected Feynman diagrams and studied their topology. In the limit of vanishing dissipation (zero friction coefficient), the final expressions for the two correlation functions analyzed in this Thesis coincide with those obtained from the Lagrangian quantum field theory. This is achieved by integrating out the time or energy component in the D-dimensional integrals of the Lagrangian approach to obtain the d-dimensional integrals of the dissipative approach. The final expression for the propagator up to third order is regularized with two different schemes, one of them being the so-called “on shell” scheme, thus obtaining fully convergent results in d = 3 dimensions. Even more important, by combining our results for the second- and third-order contributions at the long-time or low-energy limit, we confirm that the value of the critical coupling constant λ* as a function of space dimensionality d is correctly reproduced, which demonstrates the power of the new, dissipative approach to quantum field theory and the correct implementation (in its entirety) of the powerful symbolic code developed in this Thesis. The corresponding dissipative result for the effective interaction vertex is expressed in terms of a parameter describing the level of energy conservation violation. A compact and rather friendly expression is obtained by assuming that the external legs are amputated and the mass is equal to zero, demonstrating the finiteness of the terms appearing in the vertex, in the limit of vanishing friction, which is a great outcome of the dissipative approach followed. Moreover, many striking similarities are pointed out and discussed with the corresponding Lagrangian result at the same order of perturbation.
Open Access
Characterization and sources of atmospheric particles in different population density environments
(2013-12-06) Πικριδάς, Μιχαήλ; Πανδής, Σπυρίδων; Πανδής, Σπυρίδων; Κουτσούκος, Πέτρος; Μιχαλόπουλος, Νικόλαος; Κορνάρος, Μιχαήλ; Παπαευθυμίου, Ελένη; Γιαννόπουλος, Παναγιώτης; Παρασκευά, Χρηστάκη; Pikridas, Michael
Τμήμα Χημικών Μηχανικών (ΔΔ)
In order to reduce uncertainty of atmospheric particle emissions and to examine the mechanism of new particle formation from precursor gases, measurements were conducted in a megacity (Paris, France), an urban area (Patras, Greece) and a remote location (Finokalia, Greece). At Finokalia, the composition of particles with diameter smaller than 1 μm (PM1) depended on air mass origin. The highest concentrations, and most frequent, were observed when air masses were coming from Europe. Organic aerosol was found to be 80% water soluble and the increased organic to elemental carbon ratio correlated with ozone concentration. These findings indicate that particulate matter (PM) at Finokalia was not emitted near the site but was transported from source regions hunderd of kilometers away and thus the area can be considered as a background of Europe. At Finokalia, atmospheric nucleation was observed more frequently during winter when sunlight intensity was below average and favored by air masses that crossed land before reaching the site. This behavior was explained by ammonia involvement in the nucleation process. PM1 was mainly acidic during summer and consumed all available ammonia, contrary to winter when, due to the lower sunlight intensity, particles were neutral and ammonia was available. During both seasons nucleation would only occur if particles were neutral which resulted in higher frequency of events during winter. Air masses that crossed land before reaching the site were enriched with ammonia, thus it was more likely for nucleation to occur. Number size distributions were monitored in Paris, France at fixed and mobile ground stations along with airborne measurements. The Paris plume was identified at a distance of at least 200 km from the city center and the number concentration was found to increase even by a 3-fold when air masses crossed Paris. During summer nucleation was observed approximately half of the campaign days; when the condensational sink was lower than average contrary to winter when no event was identified due to higher sink. Increased number concentration was observed at an altitude outside of the Paris plume simultaneously with new particle formation observed on the ground and was attributed to that phenomenon. At Patras, the legislated by E.U. daily PM10 standards were found to be violated. Exceedances were more frequent (58 of a total of 75) during the colder months (October to March) of the year. The warmer months (April to September) 80% of the PM2.5 was transported from other areas. Contrary during the colder months the contribution of transported PM reduced to 70% during autumn and 50% during winter, when the highest concentrations were observed on average. Local traffic contributed approximately 15% during winter and the remaining 35% was primarily due to domestic heating. PM2.5 and PM1 concentrations were found to exceed 100 μg m-3 on several occasions during nighttime due to domestic heating, either diesel or biomass combustion. Potassium, a tracer of biomass combustion, correlated well (R2=0.79) with PM2.5 during winter indicating a biomass source. Potassium concentrations were higher within the urban premises than a rural area located 36 km away from the city, indicating that at least a portion of the biomass combustion related PM2.5 were emitted locally.
Open Access
Chemical characterization of fine PM and source apportionment of organic aerosol from ambient and laboratory measurements
Φλώρου, Καλλιόπη; Πανδής, Σπυρίδων; Πανδής, Σπυριδών; Παρασκευά, Χριστάκης; Μιχαλόπουλος, Νικόλαος; Βαγενάς, Δημήτριος; Κορνάρος, Μιχαήλ; Κουζούδης, Δημήτριος; Νένες, Αθανάσιος; Florou, Kalliopi
Τμήμα Χημικών Μηχανικών (ΔΔ)
Atmospheric aerosols, also known as atmospheric particles, are suspended particles (solid or liquid) in the air with diameters ranging from 1 nm to about 100 μm. Atmospheric aerosols affect the Earth's radiant budget and hence the global climate through its so-called direct and indirect radioactive effects, and also have a negative impact on human health. They can be classified as primary (emitted directly into the particle phase) or secondary (formed in the atmosphere through a series of chemical reactions). Typically, atmospheric particles consist of a mixture of inorganic and organic chemicals, including nitrates, sulfates, ammonia, organic compounds, elemental carbon, sea salt, crystalline compounds and water. The organic aerosol represents a significant fraction of the mass of atmospheric particles, but its sources and chemical composition have not yet been elucidated. Real-time high resolution aerosol mass spectroscopy was the central measurement technique used in this work. The Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) can continuously measure the chemical composition and size distribution of non-refractory submicron aerosol (NR-PM1). The high-resolution mass spectra provided by the instrument every few minutes contain information about both the organic aerosol sources and processes. This thesis presents the first HR-ToF-AMS measurements in two major Greek cities (Athens and Patras) and a remote site (Finokalia, Crete) and quantifies the contributions of the various sources to the corresponding organic aerosol levels. In addition, the formation of secondary organic aerosol during the photo-oxidation of m- and p-xylene, two important atmospheric aromatic hydrocarbons, is investigated in the laboratory using an atmospheric simulation chamber.
Open Access
Composites of carbon nanotubes and two-dimensional nanomaterials : development and technology applications
(2023-02) Κωσταράς, Χρήστος; Kostaras, Christos
Τμήμα Χημικών Μηχανικών (ΔΔ)
The fabrication of carbon nanotubes (CNT) from Iijima in 1991 constitutes a landmark for the development of nanocomposites and paved the path for employing carbon-based nanomaterials as the reinforcement phase. The next decade was marked by the isolation of monolayer graphene by means of micromechanical cleavage (2004). Henceforth, composite materials that employ CNTs and graphene as reinforcement are the epicenter of intense research interest for improvement of the mechanical properties of continuous matter, mainly of polymeric nature. At the same time, carbon-based nanomaterials display a very low percolation threshold for imparting a polymer matrix with electrical properties, thus drawing comparable amounts of interest for their use in lightning protection, pressure and mechanical stress sensing and electromagnetic shielding. A particular use of graphene/CNT composites is resistive (Joule) heating – i.e. the tendency of materials to heat up under electrical current load. This property is particularly prominent in de-icing applications in the aeronautical and energy sector such as airplane wings and wind turbine blades, but also in de-fogging and personalized heating applications. The electrical and thermal properties of graphene and its allotropes render them attractive materials for use in Joule heating-related applications. Composites that employ only small amounts of nanomaterials exhibit limited flexibility, small electrical and thermal conductivities – in most cases of orders of magnitude smaller than the filler material itself – and slow heating/cooling rates resulting from the small thermal conductivity of polymer materials combined with interface resistance effects. Increasing filler content to achieve bigger weight ratios (<10 - 20 % wt. in filler content) is burdened by poor dispersion in the matrix which, in turn, downgrades the composite’s mechanical performance. Various processing methods, which aim at the enhancement of dispersion nanofiller in polymer matrices involve calendaring, shear-mixing or ball-milling. However, such methods tend to reduce nanofiller size, which also reduces property transfer. During the 90s Richard Smalley demonstrated that filtered CNT suspensions form free-standing structures consisting of the nanomaterial forming an intertwined microstructure resembling cellulose in paper. The popularity of the then-newly discovered Buckminster fullerene (C-60), commonly termed “buckyball”, inherited the name of “buckypaper” to the films. Later on, it was shown that these paper-like films can be infiltrated by a polymeric matrix allowing the formation of composites with high filler content and without the issues of poor dispersion. Since then, filtration of suspensions of well dispersed nanomaterials have also been used in hybrid structures of CNTs and graphene with great success. Graphene, well exfoliated flakes of which are hard to create stable suspensions alone, can be supported within the CNT network. A sibling material however, graphene oxide (GO), has been shown to exfoliate readily to form good aqueous suspensions. Its stacked flakes can also form paper-like materials with excellent mechanical properties that are only diminished by flake quality (degree of functionalization, flake size and defects) and the presence of voids within the lamellar structure. While the primer 4 parameter is determined and can be controlled through thorough characterization of the nanomaterial, inter-lamellar void filling can be achieved with the inclusion of polymeric material forming a biomimetic microstructure known as brick-and-mortar. This structure that is found in nacre (mother of pearl) is a composite of calcite platelets (brick) bound by a biopolymer (mortar). Thus, with a relatively smaller polymer weight ratio (10 – 20 % wt.) a strengthening of the paper-like structure is achieved, while maintaining flexibility and only slightly affecting electrical conductivity. Moreover, the polymer is part of the initial dispersion, thus the need for insertion of nanomaterial in the matrix and vice versa is circumvented. The present thesis reports the development and mechanical and resistive heating characterisation of 10 different types of hybrid paper-like films synthesized from multi-walled CNTs (MWCNT) and different types of graphenes at variable concentrations. Mechanical properties of the papers were determined by tensile testing. Small amounts of polymeric materials (10 wt. %) were added to the papers to investigate mechanical strengthening of brick-and-mortar structures in films consisting of MWCNTs with increasing graphene weight ratio. Three different types of graphene were used, as well as GO and epoxidized reduced graphene oxide erGO in order to investigate the effect of lateral size, number of layers, dispersibility in solvents and interaction between nanomaterials on the papers’ mechanical properties. Dispersion of nanomaterials in solvents was assisted by two methods, either chemical modification or a common non-ionic surfactant (Triton X-100). Furthermore, reduction processes to remove the functional groups from the materials are described along with their effects in papers that consist of the corresponding materials. Finally, the resistive heating of the produced materials was tested. Performance was gauged as the temperature achieved from the samples when working as heating elements as a function of the power drawn when under direct current load. As the materials tested are tens of microns thick and of small mass, large heating rates were expected while under load, and thus tested. Moreover, since additional mass of a material that is not a good electrical or heat conductor is included, heating performance as well as heating rates are expected to change.
Open Access
Crystallization and dissolution of electrolyte salts
(2010-04-12T06:25:58Z) Βαβουράκη, Αικατερίνη; Κουτσούκος, Πέτρος; Παρασκευά, Χριστάκης; Κλεπετσάνης, Παύλος; Κέννου, Στυλιανή; Κοντογιάννης, Χρίστος; Ντάλας, Ευάγγελος; Γκοντελίτσας, Αθανάσιος; Vavouraki, Aikaterini
Τμήμα Χημικών Μηχανικών (ΔΔ)
Η κρυστάλλωση και η διάλυση αλάτων αποτελούν σημαντικές διεργασίες οι οποίες συνεισφέρουν στην φθορά των μνημείων της πολιτιστικής μας κληρονομιάς, τα οποία έχουν κατά κύριο λόγο κατασκευασθεί από δομικά υλικά όπως ο ασβεστόλιθoς και το μάρμαρο. Η κρυστάλλωση ευδιάλυτων αλάτων (π.χ. θειϊκό νάτριο, θειϊκό μαγνήσιο, χλωριούχο νάτριο) σε πορώδη υλικά έχει καταστροφικές επιπτώσεις τόσο στις ιστορικές όσο και στις σύγχρονες κατασκευές από σκυρόδεμα. Το πιο κατεστρεπτικό άλας για την ακεραιότητα των κατασκευών έχει αποδειχθεί, ότι είναι το θειίκό νάτριο. Η κατανόηση του μηχανισμού κρυστάλλωσης του άλατος αυτού είναι απαραίτητη προϋπόθεση για τον περιορισμό ή τον έλεγχο του σχηματισμού του σε ατμοσφαιρικές συνθήκες. Για τον σκοπό αυτό, έγινε συστηματική μελέτη της κρυσταλλικής ανάπτυξης του δεκαένυδρου θειϊκού νατρίου (Μιραμπιλίτης) σε υπέρκορα διαλύματά του. Αναπτύχθηκε μεθοδολογία βασισμένη στον εξώθερμο χαρακτήρα της κρυσταλλικής ανάπτυξης του μιραμπιλίτη. Η πειραματική μελέτη περιορίσθηκε στην ετερογενή κρυσταλλική ανάπτυξη τόσο σε φύτρα Μιραμπιλίτη, όσο και σε ξένα υποστρώματα. Τα υποστρώματα τα οποία μελετήθηκαν περιλάμβαναν ασβεστόλιθο από την Γρανάδα (ασβεστιτικός κυρίως) καθώς και ψαμμόλιθο (Πράγα, Τσεχίας) πυριτικής κατά κύριο λόγο σύστασης. Η μελέτη της κινητικής της κρυσταλλικής ανάπτυξης του Μιραμπιλίτη, έδειξε ότι το καθορίζον την ταχύτητα στάδιο είναι η διάχυση των δομικών μονάδων στην επιφάνεια των κρυσταλλικών φύτρων του Μιραμπιλίτη. Το συμπέρασμα αυτό οδήγησε στην δοκιμή οργανοφωσφορικών ενώσεων, ως προς την επίδρασή τους στην κινητική της κρυσταλλικής ανάπτυξης του μιραμπιλίτη. Οι ενώσεις αυτές ιονίζονται και αλληλεπιδρούν αποτελεσματικά με την κρυσταλλική επιφάνεια δηλητηριάζοντας τα ενεργά κέντρα κρυστάλλωσης. Ο βαθμός ιονισμού βρέθηκε ότι είναι καθοριστικός για την ανασταλτική τους δράση. Πειράματα ταχείας καταβύθισης ευδιάλυτων αλάτων ηλεκτρολυτών σε ασβεστολιθικά και ψαμμιτικά δοκίμια τα οποία έγιναν τόσο με εμβάπτιση, όσο και με έκθεση σε θάλαμο αλατονέφωσης επιβεβαίωσαν τα αποτελέσματα της μελέτης της κινητικής της κρυστάλλωσης του Μιραμπιλίτη και της σχετικής αποτελεσματικότητος των αναστολέων που χρησιμοποιήθηκαν. Πλην της κρυσταλλικής ανάπτυξης ευδιάλυτων αλάτων, σημαντική συνεισφορά στην αποδόμηση των δομικών υλικών παίζει και η διάλυση του ανθρακικού ασβεστίου, το οποίο και αποτελεί τη βασική συστατική τους ένωση. Για τη μελέτη της διεργασίας της κρυσταλλικής ανάπτυξης και διάλυσης του ανθρακικού ασβεστίου τόσο απουσία όσο και παρουσία ανιόντων όπως τα θειϊκά και τα ανιόντα φθορίου χρησιμοποιήθηκε η μικροσκοπία ατομικής δύναμης,η οποία έδωσε την δυνατότητα in situ μέτρησης του ρυθμού κρυσταλλικής ανάπτυξης και διάλυσης σε συνθήκες σταθερού κορεσμού. Η παρουσία θειϊκών ανιόντων έδειξε ότι η κρυσταλλική ανάπτυξη του ανθρακικού ασβεστίου αναστέλλεται ενώ η παρουσία φθορίου επιταχύνει τη διάλυση. Τα αποτελέσματα των μετρήσεων της κινητικής των διεργασιών έδειξαν ότι οι επιμολύνσεις στα υπέρκορα διαλύματα δρούν στην κινητική λόγω προσρόφησης και δέσμευσης των ενεργών κέντρων κρυσταλλικής ανάπτυξης διάλυσης.
Open Access
Development of carbon nanotube membranes for wastewater treatment and surface enhanced Raman scattering study of the membrane efficiency and eventual contamination caused
Αναστασόπουλος, Ιωάννης; Βογιατζής, Γεώργιος; Μαυραντζάς, Βλάσης; Μαυραντζάς, Βλάσης; Βογιατζής, Γεώργιος; Παρασκευά, Χριστάκης; Γιαννόπουλος, Σπυρίδων; Μπόκιας, Γεώργιος; Αμανατίδης, Ελευθέριος; Κουρούκλης, Γεράσιμος; Anastasopoulos, Ioannis
Τμήμα Χημικών Μηχανικών (ΔΔ)
Membrane Bioreactors (MBRs) are well established preferably in industrial wastewater treatment and were introduced aiming at the coupling of membrane separation properties simultaneously with a biochemical reaction. The solid-liquid separation that is conventionally carried out in gravity-based clarifier is replaced by membrane filtration in a MBR system thus combining the strength of biological treatment processes and efficiency of membrane filtration. MBRs have been implemented across a number of industrial sectors such as the food and beverage sector, chemical, pharmaceutical and cosmetics, textile industry as well as in laundries, and have seen extensive take-up of this technology. However, the commonly employed MBRs combined to nanofiltration membrane systems have a high operating efficiency with respect to cost and quality for treatment of wastewater containing high biodegradable organic compounds. The successful fusion of nanotechnology and membrane technology has been stated to lead to efficient next generation separation systems. A novel technology with regards to MBR and membrane systems for efficient wastewater treatment is proposed for the development of a new class of functional low fouling membranes showing enhanced properties such as high water flux and high rejection of organic matter with low molecular weight, by the subsequent inclusion of carbon nanotubes (CNTs) into porous polymeric membranes. The hollow CNT structure provides frictionless transport of water molecules, a feature that makes them suitable for the development of high flux separation systems. The type and quality of CNTs, the filling/host/substrate materials, the processing, and the fabrication methods used for the synthesis of CNT-membranes are the main factors influencing their performances. Different approaches concerning the fabrication of CNT-membranes were studied in the context of the present thesis. The study of the experimental parameters influencing the efficient incorporation of CNTs in the thin selective layer of the ultra-filtration membrane with pore diameters of ~40 nm in order to transform it to a nano-filtration one with pores to be defined exclusively by the hollow CNT-internal diameters, aiming at the rejection of a variety of organic pollutants of industrial wastewaters was the main target of the thesis. Additionally, the immersion precipitation phase separation method was studied and employed for the preparation of porous membranes of tailored morphological features. Mixed matrix membranes prepared by the subsequent mixing of CNTs during the preparation processes were investigated as well, while, the incorporation of vertically aligned CNTs, grown on silicon substrates, to polymer matrix was also examined for the preparation of a CNT-membrane. A basic principle of the CNT-membranes is the efficient binding of CNTs in the membranes to eliminate probable health risk associated with chances of product water getting contaminated with CNTs. Provided that health issues are important concerns to be addressed, the potent release of CNTs into water was investigated by the use of Surface Enhanced Raman Scattering (SERS) technique on the detection and quantification of multi-walled CNTs functionalized with pyridine moieties. In addition, given that extremely small amounts of substances can be detected and further quantified via SERS, the method applied on the investigation of the dye molecules Methylene blue and Remazol Brilliant Blue R, potent wastewater effluents.
Open Access
Development of catalysis and processes for electrochemical energy technologies
Shroti, Nivedita; Shroti, Nivedita
Τμήμα Χημικών Μηχανικών (ΔΔ)
The objective of this study is to study new electrocatalyst for high temperature water electrolysis and well as high temperature proton exchange membrane fuel cell. More specifically, to understand and improve the electrochemical interface of anodic electrode in HT electrolysis, as well as optimize the catalyst layer structure for operation under high temperature electrolysis conditions. For that reason the effect of catalyst layer, effect of the catalyst’s substrate and alternative membrane material were investigated on performance of water electrolysis for high temperature application. Initially IrO2 and RuO2 and there different compositions investigated for anodic electrode. Stability of electrocatalyst material were evaluated as anodic material for acid doped TPS® membrane provided by Advent Technologies for high temperature water electrolysis. It was observed that IrxRu1-xO2 gives better performance compare to pure IrO2 but is not stable for high temperature water electrolysis condition. More specifically, under electrolysis conditions in presence of acid, oxidative environment IrOx and RuOx undergoes changes in oxidation state and new formed species that are not stable under electrolysis condition. Pin hole formation is observed for different MEA’s. This can be attributed to catalyst and membrane interaction in presence of acid at high temperature. RuO2 is converting to RuO4, newly formed species may be reacting with pyridine present in membrane making unstable interface. A new concept double layer electrolyte introduce where two membranes, acid doped and solid acid based works as electrolyte for water electrolysis system. By introducing double layer of membranes extra resistance added to system, which doesn’t contribute towards better performance for water electrolysis. For fuel cell Pt based catalyst till now gives better performance. In order to reduce cost of catalyst and to enhance catalytic activity for fuel cell system Pt alloyed catalyst synthesized and tested for high temperature fuel cell. Alloyed catalyst attributed to structure (change in Pt-Pt bond distance) as well as changing Pt d-electron valance. In order to increase the performance and increase the three phase boundary, a newly synthesized electrocatalyst was evaluated, and compared to the commercial 30wt%Pt/C. The new catalyst is based on Pt alloy with Cobalt (Co) on oxidized carbon nanotubes, ox.MWCNT and pyridine functionalized carbon nanotubes (ox.MWCNT)-Py more specifically Pt3Co/f-MWCNT. The aim of studied catalyst is to achieve fine dispersion, quantitative deposition and alloy formation on functional carbon nanotubes. CL employing the new catalyst were formulated and tested at the cathode. Initially different reaction conditions were studied for deposition of Pt and Co on ox.MWCNT as well as for (ox.MWCNT)-Py. It was found that the better Pt deposition and dispersion found on both substrate in acidic pH, while Co deposition takes place in basic pH. To deposits Pt-Co as alloy different parameters varied during reaction like pH, temperature etc. It was found that basic pH conditions favours Pt-Co alloy formation but have negative influence on dispersion. By varying reaction time at basic pH favours alloy formation as well as good dispersion. Prepared catalyst tested in-situ for fuel cell performance in comparison with commercial Pt/C, also optimization of the in-situ ECSA evaluation procedure at the using CO as a probe molecule, without damaging the catalyst distribution was studied. Effect of H3PO4, temperature and different CO stripping methods were studied for ECSA measurements. Low PA amounts in the catalyst layer (<2gPA/gPt) corresponds to low ESCA, while (>2 gPA/gPt) have poisoning effect on catalyst layer which also effect ECSA measurement. ECSA measurements were carried out for Pt3CO/functionalize MWCNT in comparison with commercial Pt/C catalyst. It was found that Pt3CO alloyed catalyst have similar performance compare to Pt/C and Pt/functionalize MWCNT in terms of I-V performance but shows less ECSA values at all studied conditions that may attributed to presence of Co on surface.
Open Access
Development of electrocatalysts/electrodes for their application in H2O (or H2O + CO2) co-electrolysis processes in high temperature solid oxide electrolysis cells
Ιωαννίδου, Ευαγγελία; Ioannidou, Evangelia
Τμήμα Χημικών Μηχανικών (ΔΔ)
In the present thesis targeted modifications with Au, MoOx and FexOy were applied on commercially available NiO/GDC (65 wt.% NiO – 35 wt.% Ce0.9Gd0.1O2-x) cermet, by means of deposition precipitation and deposition co-precipitation. Extensive physicochemical characterization was performed for the prepared materials, in the form of powders and as electrode films, with various surface and bulk techniques in order to extract information about their surface and bulk structure. The oxidation properties of the powders were examined in the presence of H2O and CO2, in the TGA, at 650−800 oC. Furthermore, there are reports on comparative electrocatalytic measurements of the developed materials as fuel electrodes under high temperature (800−900 oC) Η2Ο electrolysis. The single SOECs comprised a circular shaped, planar − electrolyte (8YSZ) with GDC10 (Gd0.10Ce0.90O2-x) │ LSCoF (La0.6Sr0.4Co0.8Fe0.2O3-δ) as oxygen electrode. In regards to the H2O/CO2 co-electrolysis process, it is widely accepted that the fuel solid oxide electrodes meet a complex environment, where catalytic reactions, such as the Reverse Water Gas Shift (RWGS) reaction, are coupled with electrochemical processes, such as Η2Ο and CO2 electrolysis. The extent of each reaction determines the composition of the products, i.e. H2/CO ratio. Up today, there are various H2O/CO2 co-electrolysis scenarios about the extent of CO production, resulting from the RWGS reaction or/and the CO2 electrochemical reduction. This research topic was another key part of the presented thesis. In this respect, the examined modified 3 wt.% Au-Ni/GDC, 3 wt.% Mo-Ni/GDC, 3 wt.% Au – 3 wt.% Mo-Ni/GDC and 2 wt.% Fe-Ni/GDC electrocatalysts were also investigated, in the form of half-electrolyte supported cells, for their performance in the RWGS reaction through catalytic-kinetic measurements at 800−900 oC. The samples were tested at open circuit potential conditions (OCP), in order to elucidate their catalytic activity towards the production rate of CO (rco), which is one of the products of the H2O/CO2 co-electrolysis reaction. Through the latter approach a reference profile for the catalytic performance of the candidate electrodes was created, by applying co-electrolysis feed conditions. In continuation to the catalytic investigation, this research focused on further elucidating the extent of the occurring electro-catalytic processes during H2O/CO2 co-electrolysis. For this reason, the electrolyte supported Ni-Ce0.9Gd0.1O2-x||ZrO2(8 mol% Y2O3)||Gd0.10Ce0.90O2-x|La0.6Sr0.4Co0.8Fe0.2O3-δ SOC was examined in Η2Ο/CO2 co-electrolysis mode at 800−900 oC, by applying various pΗ2Ο/pCO2 feed ratios, in the range of 0 ≤ pΗ2Ο/pCO2 ≤ 1 and two pΗ2 values (2 and 21 kPa). The main objective was to discriminate the occurrence of individual CO2 electrolysis during polarization and the contribution of the RWGS reaction on the production rate of CO (rCO).
Open Access
Development of scale-bridging methodologies and algorithms founded on the outcome of detailed atomistic simulations for the reliable prediction of the viscoelastic properties of polymer melts
(2011-08-11T07:22:38Z) Στεφάνου, Παύλος; Μαυραντζάς, Βλάσιος; Μπερής, Αντώνης; Edwards, Brian; Τσαμόπουλος, Ιωάννης; Τσιτσιλιάνης, Κωνσταντίνος; Θεοδώρου, Θεόδωρος; Βλασσόπουλος, Δημήτριος; Stefanou, Pavlos
Τμήμα Χημικών Μηχανικών (ΔΔ)
In this thesis we design and develop algorithms for predicting the rheological behavior of polymer melts based on the results of detailed atomistic simulations and guided by theories of the Dynamics of Polymers and fundamental Principles of Science of the Non-Equilibrium Thermodynamics. More specifically: 1) We propose a new rheological constitutive model for the time evolution of the tensor conformation tensor C of chains in a polymer melt (and hence the stress tensor τ) using the generalized bracket formalism of Beris and Edwards. The new constitutive model includes terms that describe a whole range of phenomena and are successfully used to describe the rheological properties of commercial polyethylene resins. 2) We developed a new methodology that allows direct connection of the results of atomistic simulations with molecular reptation theory for entangled polymers. The final result of the methodology is the calculation of the function ψ(s,t) which expresses the probability that the segment s along the contour of the primitive path remain in the original tube after time t. 3) We extended the Rouse theory for systems without polymer chain ends, as the polymer rings. While there have been previous theoretical work, a comprehensive analysis of the Rouse model of cyclic polymers was still lacking; here we develop the theory in its entirety.
Open Access
Development, characterization, and performance assessment of modified Ni/GDC electrocatalysts for the reversible operation of solid oxide cells (rSOCs)
(2023-09-21) Ζαραβέλης, Φώτιος; Zaravelis, Fotios
Τμήμα Χημικών Μηχανικών (ΔΔ)
The operation of solid oxide cells (SOC) at high temperatures offers several advantages, which are reflected in better performance and reduced losses due to favorable thermodynamics and kinetics of the reactions. These cells provide flexibility to switch between electrolysis and fuel oxidation processes, making them ideal for both energy production (X-to-Power) and chemical production and storage (Power-to-X).In this dissertation, cermet electrocatalysts were developed based on commercially available NiO/GDC powder (65 wt.% NiO - 35 wt.% Ce0.9Gd0.1O2-x) modified through chemical methods with transition metals such as Au, Fe, and Mo, aiming for their operation under reverse solid oxide fuel cell (rSOC) conditions. The initial efforts focused on optimizing the already studied electrode 0.4 wt.% Mo - 3 wt.% Au - Ni/GDC, with the primary goal of reducing the Au content and then finding the optimal ratio of Au to Mo. Subsequently, compositions of new electrocatalysts were created by depositing Au and Fe metals with different weight percentages (0.5 wt.% and 3 wt.% Au) to investigate the effect of metal concentration on the electrocatalyst's performance. Suitable pastes were prepared from the modified NiO/GDC powders and deposited as fuel electrodes on ceramic electrolyte 8YSZ (ZrO2 stabilized with 8 mol.% Y2O3) using screen printing. An LSCoF (La0.6Sr0.4Co0.8Fe0.2O3-δ) commercial perovskite was used as an oxygen electrode.To determine the impact of each modification on the physical-chemical and electrochemical characteristics of Ni/GDC, a series of methods were applied under different experimental conditions. Extensive physicochemical analysis of the electrodes, both in powder and half-cell forms, was conducted using techniques such as BET, SEM, XRD, H2-TPR, H2O-TPO, and in-situ H2O XPS. Electrochemical characterization was carried out over a range of temperatures from 900 to 800 °C, with the introduction of different gas mixtures (H2O/H2), switching between solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOE) operation. Data of voltage-current density (V-i) were recorded under these conditions with simultaneous analysis through Electrochemical Impedance Spectroscopy (EIS). The modified Fe-Au-Ni/GDC electrode with the best electrochemical performance was studied in a comparative stability experiment with Ni/GDC under reverse solid oxide fuel cell (rSOC) conditions. Finally, a detailed electrokinetic study of these two electrodes was performed, both in fuel cell and electrolysis cell operation, using comprehensive EIS spectra under different experimental conditions. The goal of these experiments was to calculate key kinetic parameters, such as the apparent order of the studied reaction, and to determine the effect of modifications with Fe and Au.
Open Access
Dynamic analysis of elastic instabilities in flows of complex fluids
Βαρχάνης, Στυλιανός; Varchanis, Stylianos
Τμήμα Χημικών Μηχανικών (ΔΔ)
Hydrodynamic instabilities are encountered during the motion of non-Newtonian fluids at low flow rates and in the absence of inertia, buoyancy, and surface tension. These unexpected flow configurations, called elastic instabilities, do not arise in the corresponding flows of Newtonian fluids at the same flow rates, and stem from the interaction of the macroscopic flow with the internal microstructure of the complex fluid. Given the plethora of materials that can be classified as complex fluids (polymer solutions and melts, crude oil, blood, foams, emulsions, lava, soft media, etc.), one can envision that such elastic instabilities play a crucial role in the evolution of a wide range of physical, biological and industrial processes. Considering the fact that such elastic instabilities arise at high values of the Weissenberg number (Wi quantifies the level of elasticity in complex fluids), and that current finite element methods cannot reach such values of Wi, because of a notoriously famous numerical instability referred to as the “High Weissenberg Number Problem” (HWNP); we developed a novel finite element formulation that circumvents the HWNP and at the same time yields an extreme reduction in the cost of transient simulations in 2 and 3 dimensions. Using this numerical formalism, we simulated flows of viscoelastic solutions, elasto-visco-plastic materials and entangled polymer melts under conditions that trigger elastic instabilities, which have been observed experimentally but have never been captured theoretically. By means of parametric analysis, we investigated in detail the impact of the rheological properties on the onset criteria of such elastic instabilities. In some cases, we accessed regions of the parameter space where inertial and capillary effects become comparable to elastic effects and studied their interplay on the flow configuration. More specifically, such techniques were employed to: 1) Correlate the presence of certain proteins in human blood plasma with in vitro observed elastic instabilities during its flow, 2) Study the effect of the rheological properties of polymer solutions on the preferential asymmetric passage of the fluid in totally symmetric geometries, 3) Derive experimental protocols for the characterization of the stress-induced transition from solid to liquid state of gels and emulsions under pure extensional deformations, and 4) Investigate the role of the rheological properties of pressure sensitive adhesives on their adhesion energy. Through our analysis, we provided a deeper understanding of the underlying physical mechanisms that cause these elastic instabilities, and aimed at the development of improved, built-to-order materials for various applications.