Τμήμα Χημικών Μηχανικών (ΔΔ)

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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)
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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
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
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
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
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.
Open Access
Electrocatalytic investigation of high temperature PEM fuel cells
Ορφανίδη, Αλίν; Μπεμπέλης, Συμεών; Νεοφυτίδης, Στυλιανός; Βαγενάς, Κωνσταντίνος; Βερύκιος, Ξενοφώντας; Κατσαούνης, Αλέξανδρος; Καλλίτσης, Ιωάννης; Κονταρίδης, Δημήτριος; Orfanidi, Alin
Τμήμα Χημικών Μηχανικών (ΔΔ)
The objective of this study is to shed more light on the electrochemical interface of HTPEM fuel cell. More specifically, to understand and improve the electrochemical interface of both the anodic and cathodic electrode in HTPEM fuel cells, as well as optimize the catalyst layer structure for operation under various challenging conditions. For that reason the effect of the PA amount in the catalyst layer and the effect of the catalyst’s substrate on the fuel cell’s performance were investigated. Initially the poisoning effect of PA on the anodic electrode was investigated. The PA amount was altered in the anodic catalyst layer and its effect on the ECSA and the anode’s performance were evaluated. It was observed that the reversible performance loss of the anodic electrode was a function of the PA amount in the catalyst layer. More specifically, under low PA loading (<3 gPA/gPt) on the anodic electrode, < 10% of the Pt active surface is electrochemically active under fuel cell operating conditions. This was attributed to the blockage of the Pt surface by pyrophosphoric acid or poly-phosphates, H2 reduced polyphosphoric acid species and the shrinkage of the interface due to the displacement of the H3PO4 by the adsorbed H2 species. High PA loadings reduced the poisoning effect of these reduced PA species ( >3 gPA/gPt). It was found that the controlled and increased PA content within the catalytic layer can result even up to the tenfold decrease in the Pt loading when the anode operates under H2 rich conditions. In order to increase the fuel cell 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 pyridine functionalized carbon nanotubes ,30wt%Pt/oxMWCNT-Py. Pyridine groups are known to interact with PA and thus it is expected to increase the TPB and lower the Pt loading. CL employing the new catalyst were formulated and tested at the anodes. It was found that the presence of pyridine groups homogeneously distributed PA in the catalyst layer, resulting in high ECSA values, 40m2/gPt. As a result the MEA employing 30wt% Pt oxMWCNT-Py showed the same performance as the 30%Pt/C (having 1.3mgPt/cm2), for Pt loading loadings as low as 0.2mgPt/cm2. The performance of the anodic electrode was also found to be largely depended on the PA amount imbedded in the CL, when low Pt loading were used. The latter was an effect of the shrinkage of the ECSA as a result of the formation of PA poisoning species, as also mentioned in the previous paragraph. Since 30wt% Pt/oxMWCNT-Py exhibited very promising results and high ECSA values, its performance under harsh synthetic reformate gas was also evaluated. The synthetic reformate gas that was used comprised of 50.7kPa of H2, 2 kPa of CO and 33.5kPa of H2O balanced with Ar. It was found that the 30wt%Pt/oxMWCNT-Py electrocatalyst are ideal candidates for operation under those harsh reformates conditions, as they exhibited smaller voltage losses and higher stability under these conditions. The interaction of pyridine groups with phosphoric acid not only promotes its uniform distribution on the CL but also stabilizes the EI under high partial pressure of water. Additionally, the use of pyridine functionalized MWCNT based electrocatalyst gives the opportunity of lowering the Pt loading in the electrodes without sacrificing the overall cell’s performance under reformate conditions. The observed voltage loss under synthetic reformate gas rich in CO and steam was found to be a multi-step process and a function of the hydrophobicity of catalyst substrate, the PA loading in the CL as well as the water and CO molar fraction in the reformate gas. In order to optimize the cathodic catalyst layer, CL were formulated using the newly synthesized electrocatalyst (30wt%Pt/oxMWCNT-Py) and compared to the commercial 30wt%Pt/C. A full parametric analysis with respect to catalyst type, PA loading and Pt loading was conducted. It was found that the presence of pyridine groups homogeneously distributes PA in the catalyst layer minimizing the blockage of the pores of the catalyst layer and increase the three phase boundary. As a result the MEA employing 30wt% Pt oxMWCNT-Py showed the same performance as the 30%Pt/C for half the Pt loading. Despite the hard operating conditions the Pt particles attached to the ox.MWCNT-Py substrate exhibit the same stability as the commercial catalyst and the pyridine groups were found to be stable, at least for short term operation at the cathodic and anodic electrode. Also optimization of the ECSA evaluation procedure at the cathodic electrode, using CO as a probe molecule, without damaging the Pt distribution was found. It is clear that the use of this newly synthesized electrocatalyts 30wt%Pt/oxMWCNT-Py , at both electrodes, has major advantages as it increase the catalyst utilization and there is no need to use a polymer-binder inside the catalytic layer. Thus avoiding problems of inhomogeneous binder distribution and/or electronic insulation of catalyst nanoparticles. Using 30wt%Pt/oxMWCNT-Py electrocatalyst opens the possibility of significant reduction of the amount of Pt on both electrodes, under various operation conditions, without sacrificing the performance and stability of the fuel cell.
Open Access
Emissions and atmospheric reactions of gas and particulate pollutants
Καλτσονούδης, Χρήστος; Πανδής, Σπύρος; Πανδής, Σπύρος; Κουτσούκος, Πέτρος; Μιχαλόπουλος, Νικόλαος; Βαγενάς, Δημήτριος; Μαντζαβίνος, Διονύσιος; Παρασκευά, Χριστάκης; Κονταρίδης, Δημήτριος; Kaltsonoudis, Christos
Τμήμα Χημικών Μηχανικών (ΔΔ)
Every day vast quantities of volatile organic compounds (VOCs) are emitted in the atmosphere from anthropogenic and natural sources. These compounds are the ‘fuel’ that promotes atmospheric photochemistry influencing the quality of ambient air locally, regionally and globally. Some of these compounds are considered dangerous for human health while others affect the climate due to their ability to form particles when they are oxidized. In this study the organic emissions (gas and particulate) of several major sources of air pollution in Greece were characterized through laboratory experiments and field measurements. The emissions quantified included those from the burning of olive tree branches, from traffic, from biomass burning for residential heating, and from meat cooking. Our analysis indicates that the contribution of ‘traditional’ pollution sources such as transportation has decreased during the last two decades due to the control of their emissions and now other sources such as biomass burning or cooking are at least as important for organic pollutants in Greece. The burning of olive tree branches is a common agricultural waste management technique after the pruning of olive trees during the months November to February. It is estimated that there are 125 million olive trees in Greece and that 6500 tones of fine particles are released in the atmosphere during these months. This number is a lot higher than the annual emissions from all the passenger cars in Greece (150 tones). In this study, the aerosol composition and the emission factors for particulate matter (PM) and VOCs were measured. These emission factors should be included in the relevant data bases concerning atmospheric pollution in Europe and they should be used in future atmospheric simulations. The chemical composition of the particles emitted during our experiments was similar to that determined by the analysis of the aerosol mass spectrometer (AMS) measurements confirming that the burning of olive tree branches is an important source during the winter in Greece. The recent economic crisis in Greece had as a use of wood as fuel for residential heating. Measurements conducted in Athens during the winter of 2013 characterized the particulate and gas phase emissions from this practice. Aromatic VOCs such as benzene, toluene and the xylenes, were emitted from biomass burning in amounts similar to that from traffic. Moreover, several compounds that are associated mainly with biogenic sources, and are known to produce organic aerosol when oxidized, were also emitted. During summer, besides the emissions due to transportation in these urban environments, a large fraction of the VOCs was related to biogenic sources. Emissions for cooking can contribute up to 30% of the organic aerosol in urban environments. In this study the fraction of the organic aerosol and organic vapors due to the charbroiling of meat in major Greek cities was estimated. Smog chamber experiments were used to characterize the composition of the aerosol and the VOCs that are emitted. Additionally the atmospheric chemical evolution of these emissions was examined by exposing them to sunlight and the typical oxidants of the atmosphere. Ambient measurements conducted in Athens and Patras during summer and winter as well as in Patras during Fat Thursday showed that the AMS spectra are significantly similar to those of the smog chamber experiments. The freshly emitted particles correspond more to the ambient winter measurements while the aged particles to the ambient summer measurements. The emissions from meat cooking are a significant source of organic aerosol in Greek cities and their chemical evolution needs to be accounted during source apportionment. In the last phase of this work a new mobile dual smog chamber system was developed and tested. Our objective is to facilitate future source characterization and chemical aging studies moving the laboratory close to various pollution sources and certain environments that have been so far out of reach. The system was evaluated and was deployed in the field in order to assess its capabilities and limitations.
Open Access
Energy valorization of agro-industrial wastes and sweet sorghum for the production of gaseous biofuels through anaerobic digestion
Δαρειώτη, Μαργαρίτα; Κορνάρος, Μιχαήλ; Κορνάρος, Μιχαήλ; Λυμπεράτος, Γεράσιμος; Παύλου, Σταύρος; Μαντζαβίνος, Διονύσιος; Κούκος, Ιωάννης; Παρασκευά, Χρηστάκης; Σταματελάτου, Αικατερίνη; Dareioti, Margarita
Τμήμα Χημικών Μηχανικών (ΔΔ)
It is clear that renewable resources have received great interest from the international community during the last decades and play a crucial role in the current CO2-mitigation policy. In this regard, energy from biomass and waste is seen as one of the most dominant future renewable energy sources. Thus, organic waste i.e. animal wastes, wastewaters, energy crops, agricultural and agro-industrial residues are of specific importance since these sources do not compete with food crops in agricultural land usage. The various technologies that are available for power generation from biomass and waste can be subdivided into thermochemical, biochemical and physicochemical conversion processes. Anaerobic digestion (AD), classified within the biochemical conversion processes, is a robust process and is widely applied. Various types of biomass and waste, can be anaerobically co-digested to generate a homogeneous mixture increasing both process and equipment performance. This study focused on the valorization of agro-industrial wastes (such as olive mill wastewater (OMW), cheese whey (CW) and liquid cow manure (LCM)) and sweet sorghum stalks. Olive mills, cheese factories and cow farms are agro-industries that represent a considerable share of the worldwide economy with particular interest focused in the Mediterranean region. These industries generate millions of tons of wastewaters and large amounts of by-products, which are in many cases totally unexploited and thus dangerous for the environment. On the other hand, sweet sorghum as a lignocellulosic material represents an interesting substrate for biofuels production due to its structure and composition. Anaerobic co-digestion experiments using different mixtures of agro-industrial wastes were performed in a two-stage system consisting of two continuously stirred tank reactors (CSTRs) under mesophilic conditions (37°C). Subsequently, more mixtures were studied, where sweet sorghum was added, in order to simulate the operation of a centralized AD plant fed with regional agro-wastes which lacks OMW or/and CW due to seasonal unavailability. Two operational parameters were examined in a two-stage system, including pH and HRT. Batch experiments were performed in order to investigate the impact of controlled pH on the production of bio-hydrogen and volatile fatty acids, whereas continuous experiments (CSTRs) were conducted for the evaluation of HRT effect on hydrogen and methane production. Moreover, further exploitation of digestate from an anaerobic methanogenic reactor was studied using a combined ultrafiltration/nanofiltration system and further COD reduction was obtained. On the other hand, vermicomposting was conducted in order to evaluate the sludge transformation to compost and as a result, good results in terms of increased N-P-K concentration values were obtained. Furthermore, simulation of mesophilic anaerobic (co)-digestion of different substrates was applied, using the ADM1 modified model, where the results indicated that the modified ADM1 was able to predict reasonably well the steady-state experimental data.
Open Access
Film flow over solid substrates : the effect of fluid rheology and substrate geometry and the prediction of formation of gas inclusions
(2010-04-12T06:26:38Z) Παυλίδης, Μιχαήλ; Τσαμόπουλος, Ιωάννης; Αικατερινάρης, Ιωάννης; Γογγολίδης, Ευάγγελος; Μαυραντζάς, Βλάσσιος; Μιτσούλης, Ευάγγελος; Μπουργανός, Βασίλειος; Πελεκάσης, Νικόλαος; Τσαμόπουλος, Ιωάννης; Pavlidis, Michael
Τμήμα Χημικών Μηχανικών (ΔΔ)
Surface coating is widely used in microelectronic industry to produce thin films over surfaces with uneven topography. Such processes are used in fabricating integrated circuits, storage devices, such as magnetic disks, memory devices and optical disks as well as for manufacturing adhesives, magnetic tapes, magazines which can produce thicker films over patterns of similar depth and width at higher speeds. Other applications of film flow over uneven surfaces come from specific designs of surfaces of heat-exchangers and the surfaces of various structured packings used to improve heat and mass transfer operations. The one-dimensional, gravity-driven film-flow of a linear or exponential PTT liquid, flowing either on the outer or on the inner surface of a vertical cylinder or over a planar wall is analyzed. Numerical solution of the governing equations is generally possible. Analytical solutions are derived only for: (i) linear PTT model in cylindrical and planar geometries in the absence of solvent and the affinity parameter set at zero; (ii) linear or exponential PTT model in a planar geometry in the absence of solvent and the affinity parameter the affinity parameter obtains nonzero values; (iii) exponential PTT model in planar geometry in the absence of solvent and the affinity parameter set at zero. Then, the two-dimensional, steady flow of a viscoelastic film over a periodic topography under the action of a body force is studied. It is examined the interplay of elastic, viscous, inertia and capillary forces on the film thickness and planarization efficiency over steep topographical changes of the substrate. The code is validated by verifying that in isolated topographies the periodicity conditions result in fully developed viscoelastic film flow at the inflow/outflow boundaries and that its predictions for Newtonian fluids over 2D topography under creeping flow conditions coincide with those of previous works. Finally, the steady film-flow of a Newtonian fluid has been studied over a trench examining the various types of inclusions that can be formed. It can be distinguished three possible flow configurations when (a) the triple contact points are ‘pinned’ at the lips of the cavity, (b) the triple contact points are at the left side and the bottom of the cavity so that the cavity is not filled with liquid only around its left concave corner and (c) the two triple contact points are at the two sides of the cavity so that its bottom remains empty.
Open Access
Generalized integral transforms related to the theory of potential and stokes flow
(2011-07-29T05:54:57Z) Δόσχορης, Μιχαήλ; Δάσιος, Γεώργιος; Παπαθεοδώρου, Θεόδωρος; Κοτσιώλης, Αθανάσιος; Αθανασιάδης, Χριστόδουλος; Στρατής, Ιωάννης; Χαραλαμπόπουλος, Αντώνιος; Χατζηνικολάου, Μαρία; Δάσιος, Γεώργιος; Doschoris, Michael
Τμήμα Χημικών Μηχανικών (ΔΔ)
The main concern of this Dissertation is focused on the derivation of novel integral formulation for simple problems. This alternative integral representations display a rapid decay as the complex parameter involved tends to infinity and are therefore suitable for numerical computations and for the study of the asymptotic properties of those solutions. There is also another important advantage attached to the novel formulae presented. These integral representations are useful for solving changing-type boundary value problems (such as Dirichlet data on part of the boundary and Neumann data on the complementary of the boundary). The following problems are analyzed: (a) The Laplacian operator in the interior of a Square, (b) the Laplacian operator in the interior and exterior of a Sphere and, (c) the Stokes' operator concerning the irrotational flow of an incompressible, viscous fluid. Moreover, the behaviour of the Gegenbauer functions of the first and second kind of general complex degree and order on the cut (-1, +1) are examined.