Τμήμα Χημικών Μηχανικών (ΔΔ)

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    Treatment of sanitary landfill leachates
    (2022-11-29) Γενεθλίου, Χριστιάνα; Genethliou, Christiana
    Sanitary landfill leachate (SLL) is produced extensively in sanitary landfills and is loaded with hazardous substances, causing serious risks to both living organisms and the environment. In recent years, numerous processes, either single or combined, have been implemented to treat this hazardous wastewater. However, the treatment of raw leachate is very challenging due to the complexity of its composition. In almost all processes applied to treat raw landfill leachate, the quality of the final effluent did not fully comply with the discharge limits imposed by environmental regulations for release into a water body, while the combined systems developed consisted of mostly expensive processes. The objective of the current PhD thesis is to treat real sanitary landfill leachates using single and combined processes, which are environmentally friendly, easy to operate, cost-effective and highly effective in removing pollutants from raw landfill leachate. Initially, a naturally occurring zeolite was examined for the simultaneous removal of ammonium nitrogen (NH4+-N), dissolved chemical oxygen demand (d-COD) and color from raw SLL in batch adsorption (AD) experiments, where several operational parameters were applied. Saturation, desorption and regeneration tests of zeolite were performed. Mechanistic information for NH4+-N was also obtained by fitting adsorption/desorption data to kinetic and isotherm models using both linear and non-linear methods. Additional information regarding the assessment of the pathway of pollutants uptake was obtained by zeolite particles after adsorption, saturation, regeneration and desorption processes using scanning electron microscopy (SEM), powder X- ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). In a second stage, the efficiency of a naturally occurring palygorskite as an adsorbent for the simultaneous removal of color, d‐COD and NH4+-N from raw SLL was investigated. Operational parameters for adsorption process were also examined in batch adsorption experiments. Saturation and desorption studies of palygorskite were also performed for all three pollutants simultaneously. Natural palygorskite was then combined with natural zeolite in both single and sequential adsorption arrangements to evaluate possible enhancement of the treatment efficiency in terms of the simultaneous removal of color, d-COD and ΝΗ4+-Ν. After adsorption, saturation and desorption, the palygorskite solids were characterized using XRD, Fourier-transform infrared spectroscopy (FTIR), XPS, SEM, energy dispersive spectroscopy (EDS), measurements of BET specific surface area (SSA), and the zeta potential of the suspended particles. Next, a parametric evaluation of the electrocoagulation (EC) process on raw SLL was performed to optimize the process. Then, hybrid systems consisting of EC and AD (with natural zeolite and/or natural palygorskite) were developed using the optimal conditions of both processes to effectively treat raw SLL. The efficiency of the systems was examined in terms of simultaneous removal of color, d- COD, nitrate nitrogen (NO3--N) and NH4+-N. In particular, the optimal arrangement of the combined EC and AD system using zeolite (ADzeo) was assessed by changing the sequence of the two processes. Then, AD process using palygorskite (ADpal) was integrated into the first or middle stage of the optimum hybrid EC and ADzeo system determined, thus resulting in the implementation of two more hybrid systems: ADpal-ADzeo-EC and ADzeo-ADpal-EC. Lastly, the effectiveness of a three-stage pilot approach using adsorption (AD), electrocoagulation (EC) and biological (BIO) processes for the treatment of raw SLL was investigated. Initially, column adsorption experiments with natural zeolite were performed to examine the removal of toxic NH4+-N using different initial NH4+-N concentrations and recirculation flow rates. Two sequential treatment scenarios were then examined, i.e., AD-EC-BIO and AD-BIO-EC, to determine which achieved the highest removal of pollutants and leachate toxic potential based on Thamnocephalus platyurus bioassay.
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    Φυσικοχημική επεξεργασία αγροτοβιομηχανικών αποβλήτων
    (2022-11-28) Ζαμπέτα, Χαρίκλεια; Zampeta, Charikleia
    Ο υπερπληθυσμός παράλληλα με τον έντονα καταναλωτικό τρόπο ζωής, έχει οδηγήσει σε δραματική αύξηση των εκλυόμενων υγρών αποβλήτων, τα οποία είναι πιθανό να καταλήξουν στο περιβάλλον χωρίς επεξεργασία. Τα μη επεξεργασμένα υγρά απόβλητα όταν διατεθούν στους υδάτινους αποδέκτες είναι πιθανόν να προκαλέσουν ένα άσχημο αισθητικό αποτέλεσμα. Επιπλέον, είναι δυνατόν να προκαλέσουν κίνδυνο για την ανθρώπινη υγεία και να οδηγήσουν στο φαινόμενο του ευτροφισμού. Στην παρούσα Διατριβή, επεξεργάσθηκαν τρία πραγματικά βιομηχανικά υγρά απόβλητα (απόβλητο μελανιού χαρτονιοβιομηχανίας, απόβλητο επιτραπέζιας ελιάς και απόβλητο τυρογάλακτος), τα οποία προμηθεύτηκαν από Ελληνικές βιομηχανίες. Η μελέτη εστιάσθηκε κυρίως στην επεξεργασία του αποβλήτου μελανιού διότι το συγκεκριμένο απόβλητο παρουσιάζει μεγάλο βιομηχανικό ενδιαφέρον, ενώ παράλληλα δεν έχει μελετηθεί σε βάθος από άλλες ερευνητικές ομάδες. Οι κλασσικές βιολογικές μέθοδοι επεξεργασίας αν και είναι οικονομικές και οικολογικές δεν θα αποτελέσουν κύριο αντικείμενο μελέτης της παρούσας εργασίας, λόγω των φυσικοχημικών χαρακτηριστικών των υπό μελέτη αποβλήτων. Θα μελετηθούν τρείς φυσικοχημικές μέθοδοι: η ηλεκτροκροκίδωση, η κροκίδωση/συσσωμάτωση, η υδροδυναμική σπηλαίωση και ο συνδυασμός τους, για την επεξεργασία των υγρών αποβλήτων. Τα αρχικά πειράματα αφορούσαν τον αποχρωματισμό βιομηχανικών μελανιών και του αραιωμένου αποβλήτου μελανιού με τη χρήση της υδροδυναμικής σπηλαίωσης, με σκοπό την εύρεση των βέλτιστων συνθηκών λειτουργίας του αντιδραστήρα. Ωστόσο, παρατηρήθηκε ότι έπρεπε να εφαρμοστεί ο συνδυασμός με μία μέθοδο μετεπεξεργασίας για τη μεγιστοποίηση των απομακρύνσεων των ρύπων. Ως εκ τούτου, εφαρμόστηκε το σύστημα υδροδυναμικής σπηλαίωσης και καθίζησης εκμεταλλεύοντας τις συνθήκες τις οποίες επικρατούν στον αντιδραστήρα υδροδυναμικής σπηλαίωσης και τη φύση των μελανιών ως κολλοειδή αιωρήματα. Είναι γεγονός πως το μη αραιωμένο απόβλητο μελανιού, είναι δύσκολο στην επεξεργασία λόγω των υψηλών συγκεντρώσεων σε ρύπους. Στην παρούσα Διατριβή, αρχικά το μη αραιωμένο απόβλητο μελανιού, υποβλήθηκε σε επεξεργασία με χρήση αντιδραστήρα ηλεκτροκροκίδωσης συνεχούς λειτουργίας χρησιμοποιώντας ηλεκτρόδια αλουμινίου. Παρατηρήθηκε αύξηση της αποτελεσματικότητας της διεργασίας τόσο με τη μείωση της παροχής του αποβλήτου όσο και με την αύξηση της εξεταζόμενης πυκνότητας ρεύματος. Η επόμενη μέθοδος επεξεργασίας του αποβλήτου μελανιού αφορούσε την επεξεργασία του, με το συνδυασμό υδροδυναμικής σπηλαίωσης και ρύθμισης pH. Παρατηρήθηκαν υψηλές απομακρύνσεις ρύπων και μείωση της ποσότητας του οξέος με τη χρήση του αντιδραστήρα υδροδυναμικής σπηλαίωσης σαν προεπεξεργασία. Επιπλέον, το συγκεκριμένο απόβλητο, υποβλήθηκε σε επεξεργασία με τη μέθοδο της υδροδυναμικής σπηλαίωσης και κροκίδωσης/συσσωμάτωσης. Η προσθήκη κροκιδωτικού και ανιονικού πολυακριλαμιδίου ως μέσο συσσωμάτωσης οδήγησε σε υψηλές απομακρύνσεις ρύπων, ενώ η προσθήκη της υδροδυναμικής σπηλαίωσης σαν προεπεξεργασία, οδήγησε σε μείωση του απαιτούμενου κροκιδωτικού. Τέλος, οι χρησιμοποιούμενες μέθοδοι συγκρίθηκαν ως προς την αποτελεσματικότητα τους στην απομάκρυνση των ρύπων, ως προς το κόστος, την τοξικότητα και την παραγόμενη λάσπη. Η μέθοδος της συνδυαστικής χρήσης της υδροδυναμικής σπηλαίωση και ρύθμισης pH, θα μπορούσε να αποτελέσει μία εξαίρετη επιλογή για την επεξεργασία έντονα χρωματισμένων αποβλήτων με υψηλή συγκέντρωση κολλοειδών αιωρημάτων. Επίσης, με τις ανωτέρω μεθόδους εξετάσθηκαν και δύο απόβλητα αγροτικής προέλευσης. Παρατηρήθηκε πως η ηλεκτροκροκίδωση πιλοτικής κλίμακας μπορεί να αποτελέσει μία εναλλακτική, μη κοστοβόρα μέθοδο για την επεξεργασία αποβλήτου επιτραπέζιας ελιάς το οποίο είχε ήδη προεπεξεργαστεί με βιολογική διεργασία. Παράλληλα, για την επεξεργασία του αποβλήτου τυρογάλακτος αποδείχθηκε ότι ο συνδυασμός υδροδυναμικής σπηλαίωσης και κροκίδωσης/συσσωμάτωσης σαν πρώτο βήμα επεξεργασίας και η χρήση φιλτρόπρεσας σαν επόμενο στάδιο επεξεργασίας, οδήγησαν σε ένα άχρωμο διαυγές υγρό με μειωμένες συγκεντρώσεις ρύπων. Στόχος της παρούσας εργασίας μέσα από τη μελέτη διαφόρων μεθόδων, είναι η πρόταση μίας μεθόδου επεξεργασίας του εκάστοτε πραγματικού αποβλήτου, η οποία να συνδυάζει αποτελεσματικότητα ως προς τα ποσοστά απομάκρυνσης ρύπων, ευκολία εφαρμογής, οικονομική βιωσιμότητα και μείωση των περιβαλλοντικών επιπτώσεων του επεξεργασμένου υγρού. Προκειμένου, να είναι δυνατή η εφαρμογή της εκάστοτε μεθόδου σε πραγματικές συνθήκες στην βιομηχανία.
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    Modelling mixed matrix membranes and ceramic foams with application to separation processes
    Στιάπης, Χρήστος; Stiapis, Christos
    In several critical engineering applications, such as gas particulate filtration, coal bio-conversion process, CO2 geological storage, and biomedical processes, heterogeneous porous materials are ubiquitous. These applications rely on specific physical mechanisms, such as transport (e.g., mass, momentum, or energy) and chemical reactions for their functioning. The physical mechanisms in the interior of the porous media are strongly dependent upon the porous medium structure and morphology. Therefore, detailed elucidation of the internal structure of such porous media is needed to understand and improve the physical processes in which they are used. A detailed microstructure description can be used to find the physical properties and estimate and enhance their performance. This thesis uses statistical properties extracted from scanning electron microscopy (SEM) images to explore and digitally re-create porous media structures. First, a stochastic approach for reconstructing porous media is chosen because it captures their stochastic nature in reasonable cost and time ranges. Then, a digital reconstruction model and a computer program are created to recreate porous media structures with various statistical properties. The main objective of this thesis is the digital characterization of PES/ PVP membranes used in hemodialysis processes and the development of a model that can predict their blood purification performance. Those membranes comprise multiple layers, and usually, a foam-like structure is formed in the center path of those membranes affecting the separation performance and providing superior mechanical properties to the whole membrane structure. In this framework, a novel method for digitally reconstructing foams was developed utilizing the Laguerre Tessellation approach, which shows a remarkable ability to describe foamy materials comprised of macrovoids with polyhedral shapes [1]. Furthermore, to describe foamy materials with macrovoids of spherical shapes and increased connectivity, an alternative method was developed based on the generation of packings of hollow spheres [2]. Finally, a model to describe the removal of protein-bound toxins during the hemodialysis process using mixed matrix membranes was developed and validated against experimental data [3]. This model was further extended and utilized to predict the performance of a multilayer mixed matrix membrane during creatinine removal [4]. In addition, the incorporation of the reconstruction processes developed into the model further reduced the required experimental data. The utilization of those reconstruction methods into the newly developed model provided insight into the membrane characteristics and process conditions. Thus, it could assist in optimizing the mass transfer through the hemodialysis membrane. Furthermore, this procedure can be extended into different separation technologies paving the way towards designing tailored porous media structures with desired transport properties avoiding exhaustive physical experimentation.
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    Production & characterization of graphene-based polymer nanolaminates
    Παύλου, Χρήστος; Pavlou, Christos
    Graphene, with its superior mechanical, electrical and thermal properties, is the perfect candidate as reinforcement in lightweight, high strength composite materials with interesting multi-functionalities. Since now, graphene has been adopted mainly in the form of separate flakes (e.g. GNPs) for the production of large-scale composites. Nonetheless, the overall physical performance (e.g. mechanical, electrical and thermal properties) of graphene-flake reinforced composites may be far below the expectations and this has been attributed to the small lateral size of the particles that leads to inefficient stress transfer with the polymer matrix. An alternative way to overcome this issue is represented by the incorporation of large size CVD graphene sheets in polymer laminates. In this PhD thesis, it is proposed a novel bottom-up approach for the production of macro-scale CVD graphene/polymer nanolaminates based on the combination of ultra-thin casting, wet transfer and floating deposition. Actually, by casting ultra-thin polymer films, it was possible to produce macroscale nanolaminates in a wide range of graphene volume fractions with the potential to outperform the current state-of-the-art graphene-based composite materials in both mechanical properties, electrical conductivity and other multi-functionalities (e.g. thermal conductivity, EMI shielding, Joule heating). Freestanding graphene/poly (methyl methacrylate) nanolaminates (Gr/PMMA) have been produced with layer numbers ranging from 10 to 100 and volume fractions of 0.044 to 1%. Uniaxial tensile tests have highlighted the effective reinforcement provided by graphene in the nanolaminate configuration, with effective contribution in both the Young’s modulus (with increase up to 250%) and the ultimate strength (up to 100%) of the produced Gr/PMMA nanolaminates. Mechanical behaviour have been probed also by using in-situ Raman spectroscopy combined with uniaxial tensile tests, that shed light on some important differences of CVD graphene reinforcement compared to perfect exfoliated monolayer graphene, in terms of strain sensitivities and Grüneisen parameter. Also, mechanical properties of the nanolaminates at zero strain have been estimated through Brillouin Light Scattering, which is a non-invasive technique, thus eliminating the possibility of strain hardening upon loading. A systematic characterization of physical properties of the produced Gr/PMMA nanolaminates has been carried out and has highlighted excellent behavior in both electrical and thermal conduction. In fact, the in-plane electrical conductivity has been found to increases with graphene content up to 25000 S/m for 1% vol in graphene, and the in-plane thermal conductivity can achieve values of 15 W/mK for the same graphene content. Furthermore, other impressive multifunctionalities have been observed for the produced systems, such as a record EMI shielding behavior in the THz range and Joule heating phenomena.
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    Integration of process design & control based on economics
    Πατήλας, Χρήστος; Patilas, Christos
    In the preliminary design stage of an arbitrary process, the designer must be able to express the process characteristics through mathematical formulations, and study the dynamic behavior of such systems to fully comprehend the over-all operation under any conditions. Simultaneously, they must be able to operate the process in optimal or near-optimal conditions based on an appropriate objective function. Following this trend, researchers over the years followed different approaches and techniques in order to address the design and control problem and produce plants that operate at optimal conditions in different time scales. These formulations may utilize different methodologies based on optimization techniques or heuristics but overall they can be divided in two main categories based on the addressed problem: (a) Sequential Design and Control and (b) Integrated Design and Control. This Thesis contributes to both of these categories with a variety of formulations that are based on the concept of back-off. For almost three decades, the back-off methodology has been extensively developed and refined for addressing the control structure selection problem. Previous work has been based on a linear and a non-linear formulation. The linear formulation ensures quick determination of the optimal solution but with a potentially significant loss of accuracy in complex non-linear processes. The non-linear formulation offers higher accuracy and the opportunity of simultaneous consideration of design and control. This improved accuracy usually comes with an increase in computational cost and complexity. A new methodology based on linear models and a quadratic approximation of economics is proposed in this work which demonstrates improved accuracy when compared with the linear approximation and reasonable increase in computational effort compared with non-linear counterpart. The importance of the simultaneous consideration of process design and control at the early design stages has been identified in the past and a lot of effort has been expended towards the development of systematic and practically applicable methodologies to address it. The complex nature of chemical processes combined with the stochastic nature of uncertainty are the main reason for the slow development in this field. This Thesis introduces a novel methodology for the simultaneous solution of the design and control problems. The back-off methodology is employed in order to avoid solving complex dynamic optimization problems. This method is based on the solution of a static optimization problem where the information related to process dynamics is captured in the necessary back-off from the constraints to ensure feasibility. A novel algorithmic approach is also presented in this Thesis which offers improvements and overall advantages compared to previous efforts of addressing the integrated design and control problem. This new approach utilizes the back-off concept and methodology to address dynamic optimization issues. Furthermore, this new procedure can be used in large-scale or even plant-wide applications. The practical usefulness and effectiveness of both methodologies are exhibited in several examined applications.