Membrane development for separation of CO2 - Η2 mixtures

Development of inorganic membranes for gas separation has attracted increased attention recently, as it can lead to more efficient separation processes in industrial, energy and environmental applications such as: CO2 capture from thermal power plant emissions, H2 separation from the product gas of fuel reforming and removal of CO2 from natural gas and biogas. Depending on the nature and chemical composition of the membrane, several methods have been developed for the synthesis of inorganic membranes such as chemical vapor deposition (CVD) and pyrolysis. In the present thesis, the separation performance of three types of inorganic membranes in separation of mixtures containing H2, CO2, CH4 and N2 was studied: (a) carbon membranes, (b) MoO3 membranes, (c) DD3R membranes. The membrane performance was evaluated by measuring the permeance of N2, H2, CO2, CH4 and He as a function of temperature. Carbon membranes were synthesized from a furfuryl alcohol-furfural precursor solution. The aim was to examine the effect of the type of catalyst (p-toluenesulfonic acid or iron chloride, FeCl3) to the polymerization of precursors, and as a consequence to the synthesis of the carbon membrane. In particular, FeCl3 is reduced to metallic iron during pyrolysis, which acts as a catalyst to the graphitization of the carbon membrane, modifying its structure in the proximity of iron particles. The obtained results showed that the presence of iron has no beneficial effect on the membrane separation properties for the tested gases. MoO3 membranes were developed on porous Vycor tubular supports using three synthesis procedures: (a) dip-coating in a MoCl5 solution, (b) MoO3 sublimation, and (c) Chemical Vapor Deposition (CVD) with reactants: MoO2Cl2 or MoCl5 and water vapor. The synthesized MoO3 membranes were not stable. Nevertheless, the obtained permeation results showed the following gas permeation order: He> H2> CO2> CH4 ≈ N2. Finally, the performance of the tectosilicate material of Deca-Dodecasil-3Rombohedral (DD3R) was studied in the separation of CO2 from gas mixtures containing He, H2, N2, CH4 and CO. Permeation measurements of H2, CO2, N2, CH4 and CO were performed as a function of temperature, composition and pressure difference. The effect of humidity and the presence or absence of carrier gas at the permeation side to the permeance of the gas components were examined. In all cases the membrane was selective to CO2 permeation and the gas permeation order was: CO2> H2> He> CO ≥ N2> CH4. The best performance of the membrane was found at low temperatures (273 K), while the presence of humidity did not significantly affect its performance. The selectivity of the membrane is attributed to differences in the adsorption and diffusion coefficients of the tested gases.Τα τελευταία χρόνια η ανάπτυξη ανόργανων μεμβρανών για διαχωρισμό αερίων μιγμάτων έχει αποκτήσει ιδιαίτερο ερευνητικό και τεχνολογικό ενδιαφέρον, καθώς μπορεί να οδηγήσει σε πιο αποδοτικές διεργασίες διαχωρισμού σε βιομηχανικές, ενεργειακές και περιβαλλοντικές εφαρμογές, όπως η δέσμευση του CO2 προερχόμενου από θερμικές μονάδες, ο διαχωρισμός υδρογόνου από το αέριο προϊόν της αναμόρφωσης καυσίμων και η απομάκρυνση του CO2 από το φυσικό αέριο και το βιοαέριο. Η σύνθεση ανόργανων μεμβρανών βασίζεται σε διάφορες μεθόδους, όπως η χημική εναπόθεση από ατμό (CVD) και η πυρόλυση, ανάλογα με το είδος και τη χημική σύσταση της μεμβράνης. Στην παρούσα εργασία μελετήθηκαν τρεις τύποι ανόργανων μεμβρανών ως προς τη συμπεριφορά τους στο διαχωρισμό αερίων μιγμάτων H2, CO2, CH4, και N2: (α) μεμβράνες άνθρακα, (β) μεμβράνες MoO3, (γ) μεμβράνες Deca-dodecasil 3 Rombohedral (DD3R). Η απόδοση των μεμβρανών αξιολογήθηκε με μετρήσεις διαπέρασης Ν2, Η2, CO2, CH4 και He συναρτήσει της θερμοκρασίας διαπέρασης. Στην περίπτωση των μεμβρανών άνθρακα, χρησιμοποιήθηκαν διαλύματα φουρφουρυλικής αλκοόλης-φουρφουράλης ως πρόδρομες ενώσεις και μελετήθηκε η επίδραση του είδους του καταλύτη, π-τολουόλο σουλφονικό οξύ (p-toluenesulfonic acid, PTSΑ) ή τριχλωριούχος σίδηρος (FeCl3), στον πολυμερισμό των πρόδρομων ενώσεων, και κατ’ επέκταση στη σύνθεση των μεμβρανών άνθρακα. Ειδικότερα, κατά τη διαδικασία της πυρόλυσης ο FeCl3 ανάγεται σε μεταλλικό σίδηρο, ο οποίος λειτουργεί ως καταλύτης γραφιτοποίησης κατά το σχηματισμό της μεμβράνης άνθρακα, τροποποιώντας τη δομή της στην περιοχή γειτνίασης με τα σωματίδια σιδήρου. Βρέθηκε ότι η παρουσία του σιδήρου δεν έχει ευεργετική επίδραση στις ιδιότητες της μεμβράνης ως προς το διαχωρισμό των αερίων που μελετήθηκαν. Οι μεμβράνες ΜοΟ3 παρασκευάσθηκαν σε πορώδεις σωλήνες Vycor. Εξετάσθηκαν τρεις μέθοδοι παρασκευής: (α) εμβάπτιση των σωλήνων σε διάλυμα που περιέχει MoCl5, (β) εξάχνωση του ΜοΟ3 και (γ) χημική εναπόθεση με ατμό (CVD) μέσω της αντίδρασης MoO2Cl2 ή MoCl5 και υδρατμού. Οι συντεθείσες μεμβράνες ΜοΟ3 δεν είναι σταθερές. Παρόλα αυτά, οι ληφθείσες μετρήσεις έδειξαν την ακόλουθη σειρά διαπέρασης αερίων: He > H2 >CO2> CH4 ≈ N2. Τέλος, εξετάσθηκε η απόδοση του τεκτοπυριτικού υλικού Deca- dodecasil-3Rombohedral (DD3R) στο διαχωρισμό CO2 από μίγματα που περιέχουν He, H2, N2, CH4 και CO. Πραγματοποιήθηκαν μετρήσεις διαπέρασης των H2, CO2, N2, CH4 και CO ως συνάρτηση της θερμοκρασίας, της σύστασης και της πίεσης του ρεύματος εισόδου. Επίσης εξετάσθηκε η επίδραση της παρουσίας υγρασίας καθώς και της χρήσης ή μη φέροντος αερίου στην πλευρά του διηθήματος στην ειδική ροή μάζας των συστατικών. Σε όλες τις περιπτώσεις η μεμβράνη επέτρεψε την εκλεκτική μεταφορά του CO2 με σειρά διαπέρασης των αερίων: CO2>H2>Ηe>CO≥N2>CH4. Η βέλτιστη απόδοση της μεμβράνης εμφανίζεται σε χαμηλές θερμοκρασίες (273 Κ), ενώ η παρουσία υγρασίας στην τροφοδοσία δεν επηρέασε σημαντικά την απόδοσή της. Η εκλεκτικότητα της μεμβράνης μπορεί να αποδοθεί σε διαφορές στην προσρόφηση και διάχυση των αερίων που εξετάσθηκαν

Carbon Membranes Prepared from Poly (Furfuryl Alcohol–Furfural) Precursors: Effect of FeCl3 Additive

Thermosetting resins, such as poly (furfuryl alcohol), are efficient precursors for preparation of carbon membranes with molecular sieving properties. Polymerization of furfuryl alcohol is catalyzed by Bronsted or Lewis acids. FeCl3, showing Lewis-acid behavior, is an interesting polymerization catalyst, because it gets reduced into metallic iron during pyrolysis of the resin, promoting transformation of amorphous carbon into graphitic domains. The goal of the present work was to examine whether use of FeCl3 as a polymerization catalyst of furfuryl alcohol–furfural mixtures could lead to preparation of carbon membranes with improved gas separation performance compared to those prepared with use of p-toluenesulfonic acid. The resins were deposited onto tubular porous ceramic supports and pyrolyzed at temperatures in the range of 500–1000 °C. Material characterization was carried out by X-Ray Diffraction, N2 physisorption, Raman spectroscopy and Scanning Electron Microscopy. The membrane performance was examined using H2, CO2 and CH4 as probe molecules. It was found that the membranes operate mainly via the molecular sieving mechanism and the use of FeCl3 instead of p-toluenesulfonic acid does not lead to an improvement in the permeation characteristics of the respective membranes

Estimating the Statistical Significance of Cross–Correlations between Hydroclimatic Processes in the Presence of Long–Range Dependence

Hydroclimatic processes such as precipitation, temperature, wind speed and dew point are usually considered to be independent of each other. In this study, the cross–correlations between key hydrological-cycle processes are examined, initially by conducting statistical tests, then adding the impact of long-range dependence, which is shown to govern all these processes. Subsequently, an innovative stochastic test that can validate the significance of the cross–correlation among these processes is introduced based on Monte-Carlo simulations. The test works as follows: observations obtained from numerous global-scale timeseries were used for application to, and a comparison of, the traditional methods of validation of statistical significance, such as the t-test, after filtering the data based on length and quality, and then by estimating the cross–correlations on an annual-scale. The proposed method has two main benefits: it negates the need of the pre-whitening data series which could disrupt the stochastic properties of hydroclimatic processes, and indicates tighter limits for upper and lower boundaries of statistical significance when analyzing cross–correlations of processes that exhibit long-range dependence, compared to classical statistical tests. The results of this analysis highlight the need to acquire cross–correlations between processes, which may be significant in the case of long-range dependence behavior

Estimating the Statistical Significance of Cross–Correlations between Hydroclimatic Processes in the Presence of Long–Range Dependence

Hydroclimatic processes such as precipitation, temperature, wind speed and dew point are usually considered to be independent of each other. In this study, the cross–correlations between key hydrological-cycle processes are examined, initially by conducting statistical tests, then adding the impact of long-range dependence, which is shown to govern all these processes. Subsequently, an innovative stochastic test that can validate the significance of the cross–correlation among these processes is introduced based on Monte-Carlo simulations. The test works as follows: observations obtained from numerous global-scale timeseries were used for application to, and a comparison of, the traditional methods of validation of statistical significance, such as the t-test, after filtering the data based on length and quality, and then by estimating the cross–correlations on an annual-scale. The proposed method has two main benefits: it negates the need of the pre-whitening data series which could disrupt the stochastic properties of hydroclimatic processes, and indicates tighter limits for upper and lower boundaries of statistical significance when analyzing cross–correlations of processes that exhibit long-range dependence, compared to classical statistical tests. The results of this analysis highlight the need to acquire cross–correlations between processes, which may be significant in the case of long-range dependence behavior