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Στην εργασία αυτή μελετήθηκε η ρόφηση και η εκρόφηση του παρασιτοκτόνου ατραζίνη (atrazine) σε πρότυπες εδαφικές ουσίες καθώς και σε ένα τυπικό δείγμα εδάφους, τόσο σε αντιδραστήρες διαλείποντος έργου (batch) όσο και σε πληρωμένες κλίνες (bed). Ως πρότυπες ουσίες επιλέχθηκαν η πυριτία (silica-gel, SiO2), η αλούμινα (γ-alumina, Al2O3), το ανθρακικό ασβέστιο (calcite, CaCO3) και το χουμικό οξύ (humic acid). Η ατραζίνη ροφείται σημαντικά στο χουμικό οξύ στην πυριτία και στο έδαφος, όμως βρέθηκε ότι έχει πολύ μεγαλύτερη συγγένεια για το χουμικό οξύ σε σύγκριση με την πυριτία. Η ατραζίνη δεν έδειξε να ροφείται στη γ-αλούμινα και το CaCO3, ανεξαρτήτως από τις πειραματικές συνθήκες που χρησιμοποιήθηκαν. Σε όλες τις περιπτώσεις, η κινητική μελέτη έδειξε δύο διακριτά στάδια: ένα πρώτο γρήγορο στάδιο ρόφησης της ατραζίνης, διαδεχόμενο από ένα δεύτερο πιο αργό στάδιο. Η κινητική της ρόφησης υπακούει ικανοποιητικά στο μοντέλο Elovich. Στα χρονικά πλαίσια που μελετήθηκε η ρόφηση και η εκρόφηση, η διεργασία είναι αντιστρεπτή και η ατραζίνη εκροφείται ποσοτικά. Οι ισόθερμοι ρόφησης της ατραζίνης λήφθηκαν σε διαφορετικές τιμές ιοντικής ισχύος, pH και θερμοκρασίας και υπακούουν στο μοντέλο Freundlich. Σε όλες τις περιπτώσεις που μελετήθηκαν, η αύξηση της ιοντικής ισχύος του διαλύματος προκάλεσε αύξηση της ροφημένης ποσότητας της ατραζίνης. Η ρόφηση της ατραζίνης μειώνεται καθώς το pH του διαλύματος αυξάνεται. Όσον αφορά στην πυριτία, η ρόφηση της ατραζίνης φαίνεται ότι γίνεται κυρίως μέσω δεσμών υδρογόνου με τις υδροξυλομάδες της επιφάνειας. Όσον αφορά στο χουμικό οξύ, το σημαντικότερο ρόλο στη ρόφηση παίζει η διάχυση της ατραζίνης προς το εσωτερικό του στερεού όπου η ρόφηση επιτυγχάνεται κυρίως μέσω υδρόφοβων αλληλεπιδράσεων. Η προσρόφηση της ατραζίνης στην πυριτία αυξάνεται σημαντικά καθώς αυξάνεται η θερμοκρασία του διαλύματος, μια τάση που δεν παρατηρείται κατά τη ρόφηση της ατραζίνης στο χουμικό οξύ. Η θερμοδυναμική ανάλυση έδειξε ότι η ρόφηση στα μοντέλα εδάφους είναι φυσική αφού παρατηρήθηκαν τιμές της ενθαλπίας ρόφησης στην περιοχή των 10 kJ mol-1. Οι λαμβανόμενες ισόθερμοι ρόφησης από τα πειράματα σε πληρωμένες κλίνες ήταν σε καλή συμφωνία με αυτές που λήφθηκαν από τα πειράματα στους αντιδραστήρες διαλείποντος έργου. Η παρουσία χουμικού οξέος στις κλίνες προκαλεί με το χρόνο δραστική μείωση της διαπερατότητας. / Adsorption and desorption of the herbicide atrazine was investigated on the principal inorganic constituents of soil, as well as on a typical Greek soil sample. The studies were conducted both in batch, stirred reactors and in packed beds. Silica-gel (SiO2), γ-alumina (Al2O3) and calcite (CaCO3), were selected as model inorganic substances. Humic acid was selected as a model substance representative of the organic part of soil. Significant adsorption of atrazine was measured on the humic acid, silica and on the soil suspensions in electrolyte solutions. Atrazine exhibited higher affinity for humic acid rather than for silica. Atrazine did not adsorb on γ-alumina and on CaCO3 regardless the experimental conditions (pH range or total solid available for adsorption). In all cases, kinetic results have shown two distinct features: a first, fast sorption step, followed by a second, slow step. The kinetics data of atrazine uptake on both substrates yielded satisfactory fit to the Elovich model. Atrazine was found to be completely desorbed from both the humic acid and the silica substrates. Adsorption isotherms for atrazine were obtained at different values of ionic strength, pH and temperature. The adsorption data gave the best fit to the Freundlich model. In all cases investigated, the amount of adsorbed atrazine increased upon increasing the ionic strength of the solution. The adsorption of atrazine decreased with increasing solution pH. The adsorption of atrazine on silica was primarily dominated by the formation of hydrogen bonds with the surface hydroxyl groups. In the case of humic acid, the diffusion of atrazine to the interior of the solid seemed to play the most significant role. Inside the organic substance particles, sorption took place mainly through hydrophobic interactions. The sorption of atrazine on silica surface increased considerably with increasing temperature, a trend not found in the case of humic acid. The thermodynamic analysis yielded adsorption energy values of the order of 10 kJ mol-1 suggesting physical adsorption. The isotherms obtained from the packed bed experiments were in a good agreement with those obtained from batch experiments. Finally, humic acid grains, mixed with silica in packed beds, were found to change morphology upon hydration which resulted to swelling. The humic substances clogged a large portion of the pores of the packed beds, decreasing drastically their permeability.

The thesis, through experimental and numerical investigations reports on the work related to packed bed reactors in co-current configuration for biomass gasification. This study has extensively focused on the gasification operating regimes and addressing the issues of presence of tar, an undesirable component for engine application. Systematically, the influence of fuel properties on the gasification process has been studied using single particle analysis and also in packed bed reactors. Studies related to the effect of fuel properties - size, surface area volume ratio and density on the reactor performance are addressed. The influence of these parameters on the propagation rate which indirectly influences the residence time, tar generation, gas compositions is explicitly elucidated. Most of the reported work in literature primarily focuses on counter-current configurations and analysis on propagation flame front/ignition mass flux and temperature profiles mostly under the combustion regime. In this work, flame propagation front movement, bed movement and effective movement for a co-current packed bed reactor of different reactor capacities and a generalized approach towards establishing ‘effective propagation rate’ has been proposed. The work also reports on the importance of particle size and sharing of air from the top and through nozzles on tar generation in the open top down draft reactor configuration. Firstly, pyrolysis, an important component of the thermochemical conversion process has been studied using the flaming time for different biomass samples having varying size, shape and density. The elaborate experiments on the single particle study provides an insight into the reasons for high tar generation for wood flakes/coconut shells and also identifies the importance of the fuel particle geometry related to surface area and volume ratio. Effect of density by comparing the flaming rate of wood flakes and coconut shells with the wood sphere for an equivalent diameter is highlighted. It is observed that the tar level in the raw gas is about 80% higher in the case of wood flakes and similar values for coconut shells compared with wood pieces. The analysis suggests that the time for pyrolysis is lower with a higher surface area particle and is subjected to nearly fast pyrolysis process resulting in higher tar fraction with low char yield. Similarly, time for pyrolysis increases with density as observed from the experimental measurements by using coconut shells and wood flakes and concludes the influence on the performance of packed bed reactors. Studies on co-current reactor under various operating conditions from closed top reactor to open top reburn configuration suggests improved residence time reduces tar generation. This study establishes, increased residence time with staged air flow has a better control on residence time and yields lower tar in the raw gas. Studies on the influence of air mass flux on the propagation rate, peak temperature, and gas quality, establishes the need to consider bed movement in the case of co-current packed bed reactor. It is also observed that flame front propagation rate initially increases as the air mass flux is increased, reaches a peak and subsequently decreases. With increase in air mass flux, fuel consumption increases and thereby the bed movement. The importance of bed movement and its effect on the propagation front movement has been established. To account for variation in the fuel density, normalized propagation rate or the ignition mass flux is a better way to present the result. The peak flame front propagation rates are 0.089 mm/s for 10 % moist wood at an air mas flux of 0.130 kg/m2-s and while 0.095 mm/s for bone-dry wood at an air mass flux of 0.134 kg/m2-s. These peak propagation rates occur with the air mass flux in the range of 0.130 to 0.134 kg/m2-s. The present results compare well with those available in the literature on the effective propagation rate with the variation of air mass flux, and deviations are linked to fuel properties. The propagation rate correlates with mass flux as ̇ . during the increasing regime of the front movement. The extinction of flame propagation or the front receding has been established both experimentally supported from the model analysis and is found to be at an air mass flux of 0.235 kg/m2-s. The volume fraction of various gaseous species at the reactor exits obtained from the experiment is 14.89±0.28 % CO2, 15.75±0.43 % CO and 11.09±1.99 % H2 respectively with the balance being CH4 and N2. The model analysis using an in-house program developed for packed bed reactor provide a comprehensive understanding with respect to the performance of packed bed reactor under gasification conditions. The model addresses the dependence on air mass flux on gas composition and propagation rate and is used to validate the experimental results. Based on the energy balance in the reaction front, the analysis clearly identifies the reasons for stable propagation front and receding front in a co-current reactor. From the experiments and modelling studies, it is evident that turn-down ratio of a downdraft gasification system is scientifically established. Both the experimental and the numerical studies presented in the current work establishes that the physical properties of the fuel have an impact on the performance of the co-current reactor and for the first time, the importance of bed movement on the propagation rate is identified.

<div>Adsorbent technology is widely used in many industrial applications including waste heat recovery, water purification, and atmospheric revitalization in confined habitations. Astronauts depend on adsorbent-based systems to remove metabolic carbon dioxide (CO₂) from the cabin atmosphere; as NASA prepares for the journey to Mars, engineers are redesigning the adsorbent-based system for reduced weight and optimal efficiency. These efforts hinge upon the development of accurate, predictive models, as simulations are increasingly relied upon to save cost and time over the traditional design-build-test approach. Engineers rely on simplified models to reduce computational cost and enable parametric optimizations. Amongst these simplified models is the axially dispersed plug-flow model for predicting the adsorbate concentration during flow through an adsorbent bed. This model is ubiquitously used in designing fixed-bed adsorption systems. The current work aims to improve the accuracy of the axially dispersed plug-flow model because of its wide-spread use. This dissertation identifies the critical model inputs that drive the overall uncertainty in important output quantities then systematically improves the measurement and prediction of these input parameters. Limitations of the axially dispersed plug-flow model are also discussed, and recommendations made for identifying failure of the plug-flow assumption.</div><div><br></div><div>An uncertainty and sensitivity analysis of an axially disperse plug-flow model is first presented. Upper and lower uncertainty bounds for each of the model inputs are found by comparing empirical correlations against experimental data from the literature. Model uncertainty is then investigated by independently varying each model input between its individual upper and lower uncertainty bounds then observing the relative change in predicted effluent concentration and temperature (<i>e.g.</i>, breakthrough time, bed capacity, and effluent temperature). This analysis showed that the LDF mass transfer coefficient is the largest source of uncertainty. Furthermore, the uncertainty analysis reveals that ignoring the effect of wall-channeling on apparent axial dispersion can cause significant error in the predicted breakthrough times of small-diameter beds.</div><div><br></div><div>In addition to LDF mass transfer coefficient and axial-dispersion, equilibrium isotherms are known to be strong lever arms and a potentially dominant source of model error. As such, detailed analysis of the equilibrium adsorption isotherms for zeolite 13X was conducted to improve the fidelity of CO₂ and H₂O on equilibrium isotherms compared to extant data. These two adsorbent/adsorbate pairs are of great interest as NASA plans to use zeolite 13X in the next generation atmospheric revitalization system. Equilibrium isotherms describe a sorbent’s maximum capacity at a given temperature and adsorbate (<i>e.g.</i>, CO₂ or H₂O) partial pressure. New isotherm data from NASA Ames Research Center and NASA Marshall Space Flight Center for CO₂ and H₂O adsorption on zeolite 13X are presented. These measurements were carefully collected to eliminate sources of bias in previous data from the literature, where incomplete activation resulted in a reduced capacity. Several models are fit to the new equilibrium isotherm data and recommendations of the best model fit are made. The best-fit isotherm models from this analysis are used in all subsequent modeling efforts discussed in this dissertation.</div><div><br></div><div>The last two chapters examine the limitations of the axially disperse plug-flow model for predicting breakthrough in confined geometries. When a bed of pellets is confined in a rigid container, packing heterogeneities near the wall lead to faster flow around the periphery of the bed (<i>i.e.</i>, wall channeling). Wall-channeling effects have long been considered negligible for beds which hold more than 20 pellets across; however, the present work shows that neglecting wall-channeling effects on dispersion can yield significant errors in model predictions. There is a fundamental gap in understanding the mechanisms which control wall-channeling driven dispersion. Furthermore, there is currently no way to predict wall channeling effects a priori or even to identify what systems will be impacted by it. This dissertation aims to fill this gap using both experimental measurements and simulations to identify mechanisms which cause the plug-flow assumption to fail.</div><div><br></div><div>First, experimental evidence of wall-channeling in beds, even at large bed-to-pellet diameter ratios (<i>d</i>_bed/<i>d</i>_p=48) is presented. These experiments are then used to validate a method for accurately extracting mass transfer coefficients from data affected by significant wall channeling. The relative magnitudes of wall-channeling effects are shown to be a function of the adsorption/adsorbate pair and geometric confinement (<i>i.e.</i>, bed size). Ultimately, the axially disperse plug-flow model fails to capture the physics of breakthrough when nonplug-flow conditions prevail in the bed.</div><div><br></div><div>The final chapter of this dissertation develops a two-dimensional (2-D) adsorption model to examine the interplay of wall-channeling and adsorption kinetics and the adsorbent equilibrium capacity on breakthrough in confined geometries. The 2-D model incorporates the effect of radial variations in porosity on the velocity profile and is shown to accurately capture the effect of wall-channeling on adsorption behavior. The 2-D model is validated against experimental data, and then used to investigate whether capacity or adsorption kinetics cause certain adsorbates to exhibit more significant radial variations in concentration compared than others. This work explains channeling effects can vary for different adsorbate and/or adsorbent pairs—even under otherwise identical conditions—and highlights the importance of considering adsorption kinetics in addition to the traditional <i>d</i>_bed/<i>d</i>_p criteria.</div><div><br></div><div>This dissertation investigates key gaps in our understanding of fixed-bed adsorption. It will deliver insight into how these missing pieces impact the accuracy of predictive models and provide a means for reconciling these errors. The culmination of this work will be an accurate, predictive model that assists in the simulation-based design of the next-generation atmospheric revitalization system for humans’ journey to Mars.</div>