Soot aerosols are known to be an important atmospheric constituent. The physical and chemical properties of soot allows it to act as a precursor of gas-surface heterogeneous reactions, providing active sites for the reduction and oxidation of trace species in the atmosphere, potentially affecting atmospheric composition. In this work the heterogeneous reaction of NO2 on soot leading to nitrous acid (HONO) formation was studied through a series of kinetic uptake experiments and HONO yield measurements. The soot was collected from a diffusion flame using propane and kerosene fuels using two different methods. A low pressure fast-flow reactor coupled to a Chemical Ionization Mass Spectrometer (CIMS) was used to monitor NO2 and HONO signals evolution using atmospheric-level NO2 concentration. HONO yields up to 100 percent were measured and NO2 uptake coefficients varying from 5.6x10-6 to 1.6x10-4 were obtained. Heating soot samples before exposure to NO2 increased HONO yield and the NO2 uptake coefficient on soot due to the removal of the organic fraction from the soot backbone unblocking active sites, which become accessible for the heterogeneous reaction. From the kinetic uptake curves and the effect observed in the HONO yield and NO2 uptake coefficient measurements by heating the soot samples, our results support a complex oxidation-reduction mechanism of reaction. This heterogeneous reaction mechanism involves a combination of competitive adsorptive and reductive centers on soot surface where NO2 is converted into HONO, and the presence of processes on soot where HONO can be decomposed producing other products. Atmospheric soot "aging" effect on the reactivity of soot toward NO2 and HONO yield was studied by coating the soot surface with glutaric acid, succinic acid, sulfuric acid, and pyrene. Glutaric and succinic acid increased both HONO yield and the NO2 uptake coefficients, while sulfuric acid decreased both. However, pyrene did not show any particular trend.

This study was to investigate the effect on conversion, deactivation of long-term, selectivity of product, and kinetics in deep oxidation of toluene over copper catalyst. The copper catalyst is supported on honeycomb of ceramic monolith (400 cell/inch2). The operation parameters in heterogeneous reactor were performed as follows: 1000 ppm initial concentration of toluene, temperature of reaction in ranging from 200 ¢J to 400 ¢J, 15 % of oxygen concentration, and 4000 hr-1 of space velocity. In the selection of catalyst, we decided to use 20% Cu catalyst for its high conversion, high selectivity and low cost in oxidation of toluene. The conversion of toluene in catalytic reaction was increased with the increasing both of reaction temperature and influent concentration of oxygen, and decreased with the going up of initial concentration of toluene and of space velocity. In the catalyst stability of long-term test, Cu catalysts had a good stability after 7 days reaction in heterogeneous reactor. The tests such as BET, SEM and EA were also determined to verify the stability from surface of catalyst. The kinetics of heterogeneous reactor over Cu catalyst supported on ceramic honeycomb in oxidation of toluene was found that a pseudo-first order could be described by both Power-rate law and Mars-Van Krevelen model. The apparent reaction order and activated energy were obtained in this work.

This study was to investigate the effect on the conversion, long-term test, product selectivity and kinetics in oxidation of ammonia over copper catalyst supported on slag powder. The operation parameters in heterogeneous reactor were performed as follows: 1000 ppm initial concentration of ammonia, temperature of reaction in ranging from 100 ¢J to 500 ¢J, 10 % of oxygen concentration, and 80000hr-1 of space velocity. In the experiments of catalyst selection, we decided to use 15% Cu catalyst for its high conversion and selectivity in oxidation of ammonia. The conversion of ammonia in catalytic reaction increased with the increasing both of reaction temperature and influent concentration of oxygen but decreased with the going up of initial concentration of ammonia and of space velocity. In the long-term test of catalyst stability, Cu catalyst had a good stability after 48 hours reaction in heterogeneous reactor. The tests such as XRD, SEM and EA were also determined. The kinetics of heterogeneous reactor over Cu catalyst supported on slag powder in oxidation of ammonia was found that a pseudo-first order could be described by Mars-Van Krevelend model. The apparent reaction order and activated energy were obtained.

At least three poorly soluble compounds can be formed in the CuSO4 – K4P2O7 – H2O system: a single salt Cu2P2O7.5H2O and two dimorph’s both having chemical formula Cu3K2(P2O7)2.3H2O. A previously known form of Cu3K2(P2O7)2.3H2O (we named it Dimorph A) transforms into a novel Dimorph B, which has a different structure. The similarities between the XRD patterns and vibrations spectra of copper–ammonium and copper–potassium Dimorph’s B imply that they are isostructural. The values of the angle for copper–potassium and copper–ammonium salts and Cu2P2O7.5H2O are relatively low and vary within a narrow range (123.1 – 127.1°). The formation of the compounds significantly depends on the conditions of the experiment. A double salt in the CuSO4 – K4P2O7 – H2O system may be formed after some time in the reaction mixture as a result of the following heterogeneous reaction: 3Cu2P2O7∙5H2O↓ + H2P2O72– + 4K+ → 2Cu3K2(P2O7)2∙3H2O↓ + 2H+ + 9H2O This reaction has not been previously described for any polyphosphate system. Therefore, novel results of scientific investigation are presented. The rate, duration and yield of heterogeneous reaction substantially depend on pH of the solution, [Cu2+ + P2O74–] and n ([P2O74–] / [Cu2+]). At a relatively low pH the induction time is high, the reaction rate is low and duration is long. The maximal yields close to 100% can be achieved when [Cu2+ + P2O74–] = 0.1 M. At increased pH values, the induction time, the yield and duration of the reaction diminish... [to full text] / Naudojant IR-spektrometrijos, rentgenofazinės analizės, atominės absorbcinės spektroskopijos, kolorimetrijos analizės metodus buvo ištirta sistemose CuSO4 – K4P2O7 – H2O ir CuSO4 – (NH4)4P2O7 – H2O susidarančių mažai tirpių junginių sudėtis, jų susidarymo dėsningumai ir vykstančios reakcijos, įvairių veiksnių įtaką nuosėdų sudėčiai ir struktūrai. Nustatyta, kad sistemoje CuSO4 – K4P2O7 – H2O, be jau žinomų mažai tirpių junginių (paprastosios druskos Cu2P2O7 ∙ 5H2O ir dvigubosios druskos Cu3K2(P2O7)2 ∙ 3H2O), susidaro ir naujas mokslinėje literatūroje neaprašytas junginys (Dimorfas B), dvigubajai druskai patiriant polimorfinį perėjimą. Polimorfinio perėjimo trukmė labai priklauso nuo pH ir reaguojančių jonų koncentracijų. FTIR duomenų analizė parodė, kad difosfato jonų energetinė būsena Cu – K ir Cu – NH4 Dimorfuose B panaši. Jų rentgenogramos yra panašios, tikėtina, kad šie junginiai – izostruktūriniai, jų kristalinės gardelės tipas – monoklininis. Naudojant Lazarevo lygtį, bei Rulmonto priklausomybę buvo apskaičiuoti Cu – K ir Cu – NH4 Dimorfų A ir B bei paprastosios druskos P – O – P kampai, gauta, kad jie mažai keičiasi siaurame 123,1 – 127,1 º intervale. Nustatyta, kad dviguboji druska susidaro iš paprastosios, vykstant heterogeninei reakcijai: 3Cu2P2O7 ∙ 5H2O↓ + H2P2O72– + 4K+ → 2Cu3K2(P2O7)2∙ 3H2O↓ + 2H++ 9H2O. Mokslinėje literatūroje tokia reakcija neaprašyta jokiai difosfatų sistemai, jos tyrimo duomenys yra nauji. Taip pat nustatyta, kad heterogeninės reakcijos... [toliau žr. visą tekstą]

Disertaciniame darbe naudojant IR-spektrometrijos, rentgenofazinės analizės, atominės absorbcinės spektroskopijos, kolorimetrijos analizės metodus buvo ištirta sistemose CuSO4 – K4P2O7 – H2O ir CuSO4 – (NH4)4P2O7 – H2O susidarančių mažai tirpių junginių sudėtis, jų susidarymo dėsningumai ir vykstančios reakcijos, įvairių veiksnių įtaka nuosėdų sudėčiai ir struktūrai. Nustatyta, kad sistemoje CuSO4 – K4P2O7 – H2O, be jau žinomų mažai tirpių junginių (paprastosios druskos Cu2P2O7 5H2O ir dvigubosios druskos Cu3K2(P2O7)2 H2O, susidaro ir naujas mokslinėje literatūroje neaprašytas junginys (Dimorfas B), dvigubajai druskai patiriant polimorfinį perėjimą. Polimorfinio perėjimo trukmė labai priklauso nuo pH ir reaguojančių jonų koncentracijų. FTIR duomenų analizė parodė, kad difosfato jonų energetinė būsena Cu – K ir Cu – NH4 Dimorfuose B panaši. Jų rentgenogramos yra panašios, tikėtina, kad šie junginiai – izostruktūriniai, jų kristalinės gardelės tipas – monoklininis. Naudojant Lazarevo lygtį bei Rulmonto priklausomybę buvo apskaičiuoti Cu – K ir Cu – NH4 Dimorfų A ir B bei paprastosios druskos P – O – P kampai, gauta, kad jie mažai keičiasi siaurame 123,1 – 127,1 º intervale. Nustatyta, kad dviguboji druska susidaro iš paprastosios, vykstant heterogeninei reakcijai: 3Cu2P2O7 5H2O↓ + H2P2O72– + 4K+ → 2Cu3K2(P2O7)2∙3H2O↓ + 2H++ 9H2O. Mokslinėje literatūroje tokia reakcija neaprašyta jokiai polifosfatų sistemai, jos tyrimo duomenys yra nauji. Taip pat nustatyta, kad heterogeninės... [toliau žr. visą tekstą] / At least three poorly soluble compounds can be formed in the CuSO4 – K4P2O7 – H2O system: a single salt Cu2P2O7.5H2O and two dimorph’s both having chemical formula Cu3K2(P2O7)2.3H2O. A previously known form of Cu3K2(P2O7)2.3H2O (we named it Dimorph A) transforms into a novel Dimorph B, which has a different structure. The similarities between the XRD patterns and vibrations spectra of copper–ammonium and copper–potassium Dimorph’s B imply that they are isostructural. The values of the angle for copper–potassium and copper–ammonium salts and Cu2P2O7.5H2O are relatively low and vary within a narrow range (123.1 – 127.1°). The formation of the compounds significantly depends on the conditions of the experiment. A double salt in the CuSO4 – K4P2O7 – H2O system may be formed after some time in the reaction mixture as a result of the following heterogeneous reaction: 3Cu2P2O7∙5H2O↓ + H2P2O72– + 4K+ → 2Cu3K2(P2O7)2∙3H2O↓ + 2H+ + 9H2O This reaction has not been previously described for any polyphosphate system. Therefore, novel results of scientific investigation are presented. The rate, duration and yield of heterogeneous reaction substantially depend on pH of the solution, [Cu2+ + P2O74–] and n ([P2O74–] / [Cu2+]). At a relatively low pH the induction time is high, the reaction rate is low and duration is long. The maximal yields close to 100% can be achieved when [Cu2+ + P2O74–] = 0.1 M. At increased pH values, the induction time, the yield and duration of the reaction diminish... [to full text]

Oxidative coupling of methane (OCM) represents a potentially viable method to convert methane directly into more desirable products such as ethane, and ethylene. In this dissertation, a comprehensive kinetic study of oxidative coupling of methane was performed over La2O3-based catalysts. An accurate and reliable gas-phase model is critical for the entire mechanism. The gas-phase kinetics was first studied using a jet-stirred reactor without catalyst. Both experiments and simulations were conducted under various operating conditions using different gas-phase models. Quantities of interest and rate of production analyses on hydrocarbon products were also performed to evaluate the models. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics and used for the next study. Next, microkinetic analysis on La2O3-based catalysts with different dopants was performed. The Ce addition has the greatest boost over the performance. The kinetics at low conversion regimes were analyzed and correlated to the catalysts’ properties. The activation energy for methane hydrogen abstraction was estimated, with the formation rate of primary products, which suggested that the initiation reaction steps were similar for La2O3-based catalyst. A homogeneous-heterogeneous kinetic model for La2O3/CeO2 catalyst was then constructed. By applying in situ XRD, the doping of CeO2 not only enhanced catalytic performance but also improved catalyst stability from CO2 and H2O. A wide range of operating conditions was investigated experimentally and numerically, where a packed bed reactor model was constructed based on the dimensions of experimental setup and catalyst characterization. The rate of production (ROP) was also performed to identify the important reactions and prove the necessity of surface reactions for the OCM process. Laser-induced fluorescence was implemented to directly observe the presence of formaldehyde. The last section includes the implementation of in situ laser diagnosis techniques at the near-surface region to solve the existing challenges. Raman scattering was implemented to quantitate the concentration profiles of major stable species near the surface and measure the in situ local temperatures at different heights above the catalyst surface, to study the kinetics transiting from the surface edge to the near-surface gas phase and provide a new perspective in OCM kinetic studies.

This thesis addresses three relevant macroeconomic issues: (i) why Central Banks behave so cautiously compared to optimal theoretical benchmarks, (ii) do monetary variables add information about future Euro Area inflation to a large amount of non monetary variables and (iii) why national saving and investment are so correlated in OECD countries in spite of the high degree of integration of international financial markets. The process of innovation in the elaboration of economic theory and statistical analysis of the data witnessed in the last thirty years has greatly enriched the toolbox available to macroeconomists. Two aspects of such a process are particularly noteworthy for addressing the issues in this thesis: the development of macroeconomic dynamic stochastic general equilibrium models (see Woodford, 1999b for an historical perspective) and of techniques that enable to handle large data sets in a parsimonious and flexible manner (see Reichlin, 2002 for an historical perspective). Dynamic stochastic general equilibrium models (DSGE) provide the appropriate tools to evaluate the macroeconomic consequences of policy changes. These models, by exploiting modern intertemporal general equilibrium theory, aggregate the optimal responses of individual as consumers and firms in order to identify the aggregate shocks and their propagation mechanisms by the restrictions imposed by optimizing individual behavior. Such a modelling strategy, uncovering economic relationships invariant to a change in policy regimes, provides a framework to analyze the effects of economic policy that is robust to the Lucas'critique (see Lucas, 1976). The early attempts of explaining business cycles by starting from microeconomic behavior suggested that economic policy should play no role since business cycles reflected the efficient response of economic agents to exogenous sources of fluctuations (see the seminal paper by Kydland and Prescott, 1982} and, more recently, King and Rebelo, 1999). This view was challenged by several empirical studies showing that the adjustment mechanisms of variables at the heart of macroeconomic propagation mechanisms like prices and wages are not well represented by efficient responses of individual agents in frictionless economies (see, for example, Kashyap, 1999; Cecchetti, 1986; Bils and Klenow, 2004 and Dhyne et al., 2004). Hence, macroeconomic models currently incorporate some sources of nominal and real rigidities in the DSGE framework and allow the study of the optimal policy reactions to inefficient fluctuations stemming from frictions in macroeconomic propagation mechanisms. Against this background, the first chapter of this thesis sets up a DSGE model in order to analyze optimal monetary policy in an economy with sectorial heterogeneity in the frequency of price adjustments. Price setters are divided in two groups: those subject to Calvo type nominal rigidities and those able to change their prices at each period. Sectorial heterogeneity in price setting behavior is a relevant feature in real economies (see, for example, Bils and Klenow, 2004 for the US and Dhyne, 2004 for the Euro Area). Hence, neglecting it would lead to an understatement of the heterogeneity in the transmission mechanisms of economy wide shocks. In this framework, Aoki (2001) shows that a Central Bank maximizing social welfare should stabilize only inflation in the sector where prices are sticky (hereafter, core inflation). Since complete stabilization is the only true objective of the policymaker in Aoki (2001) and, hence, is not only desirable but also implementable, the equilibrium real interest rate in the economy is equal to the natural interest rate irrespective of the degree of heterogeneity that is assumed. This would lead to conclude that stabilizing core inflation rather than overall inflation does not imply any observable difference in the aggressiveness of the policy behavior. While maintaining the assumption of sectorial heterogeneity in the frequency of price adjustments, this chapter adds non negligible transaction frictions to the model economy in Aoki (2001). As a consequence, the social welfare maximizing monetary policymaker faces a trade-off among the stabilization of core inflation, economy wide output gap and the nominal interest rate. This feature reflects the trade-offs between conflicting objectives faced by actual policymakers. The chapter shows that the existence of this trade-off makes the aggressiveness of the monetary policy reaction dependent on the degree of sectorial heterogeneity in the economy. In particular, in presence of sectorial heterogeneity in price adjustments, Central Banks are much more likely to behave less aggressively than in an economy where all firms face nominal rigidities. Hence, the chapter concludes that the excessive caution in the conduct of monetary policy shown by actual Central Banks (see, for example, Rudebusch and Svennsson, 1999 and Sack, 2000) might not represent a sub-optimal behavior but, on the contrary, might be the optimal monetary policy response in presence of a relevant sectorial dispersion in the frequency of price adjustments. DSGE models are proving useful also in empirical applications and recently efforts have been made to incorporate large amounts of information in their framework (see Boivin and Giannoni, 2006). However, the typical DSGE model still relies on a handful of variables. Partly, this reflects the fact that, increasing the number of variables, the specification of a plausible set of theoretical restrictions identifying aggregate shocks and their propagation mechanisms becomes cumbersome. On the other hand, several questions in macroeconomics require the study of a large amount of variables. Among others, two examples related to the second and third chapter of this thesis can help to understand why. First, policymakers analyze a large quantity of information to assess the current and future stance of their economies and, because of model uncertainty, do not rely on a single modelling framework. Consequently, macroeconomic policy can be better understood if the econometrician relies on large set of variables without imposing too much a priori structure on the relationships governing their evolution (see, for example, Giannone et al., 2004 and Bernanke et al., 2005). Moreover, the process of integration of good and financial markets implies that the source of aggregate shocks is increasingly global requiring, in turn, the study of their propagation through cross country links (see, among others, Forni and Reichlin, 2001 and Kose et al., 2003). A priori, country specific behavior cannot be ruled out and many of the homogeneity assumptions that are typically embodied in open macroeconomic models for keeping them tractable are rejected by the data. Summing up, in order to deal with such issues, we need modelling frameworks able to treat a large amount of variables in a flexible manner, i.e. without pre-committing on too many a-priori restrictions more likely to be rejected by the data. The large extent of comovement among wide cross sections of economic variables suggests the existence of few common sources of fluctuations (Forni et al., 2000 and Stock and Watson, 2002) around which individual variables may display specific features: a shock to the world price of oil, for example, hits oil exporters and importers with different sign and intensity or global technological advances can affect some countries before others (Giannone and Reichlin, 2004). Factor models mainly rely on the identification assumption that the dynamics of each variable can be decomposed into two orthogonal components - common and idiosyncratic - and provide a parsimonious tool allowing the analysis of the aggregate shocks and their propagation mechanisms in a large cross section of variables. In fact, while the idiosyncratic components are poorly cross-sectionally correlated, driven by shocks specific of a variable or a group of variables or measurement error, the common components capture the bulk of cross-sectional correlation, and are driven by few shocks that affect, through variable specific factor loadings, all items in a panel of economic time series. Focusing on the latter components allows useful insights on the identity and propagation mechanisms of aggregate shocks underlying a large amount of variables. The second and third chapter of this thesis exploit this idea. The second chapter deals with the issue whether monetary variables help to forecast inflation in the Euro Area harmonized index of consumer prices (HICP). Policymakers form their views on the economic outlook by drawing on large amounts of potentially relevant information. Indeed, the monetary policy strategy of the European Central Bank acknowledges that many variables and models can be informative about future Euro Area inflation. A peculiarity of such strategy is that it assigns to monetary information the role of providing insights for the medium - long term evolution of prices while a wide range of alternative non monetary variables and models are employed in order to form a view on the short term and to cross-check the inference based on monetary information. However, both the academic literature and the practice of the leading Central Banks other than the ECB do not assign such a special role to monetary variables (see Gali et al., 2004 and references therein). Hence, the debate whether money really provides relevant information for the inflation outlook in the Euro Area is still open. Specifically, this chapter addresses the issue whether money provides useful information about future inflation beyond what contained in a large amount of non monetary variables. It shows that a few aggregates of the data explain a large amount of the fluctuations in a large cross section of Euro Area variables. This allows to postulate a factor structure for the large panel of variables at hand and to aggregate it in few synthetic indexes that still retain the salient features of the large cross section. The database is split in two big blocks of variables: non monetary (baseline) and monetary variables. Results show that baseline variables provide a satisfactory predictive performance improving on the best univariate benchmarks in the period 1997 - 2005 at all horizons between 6 and 36 months. Remarkably, monetary variables provide a sensible improvement on the performance of baseline variables at horizons above two years. However, the analysis of the evolution of the forecast errors reveals that most of the gains obtained relative to univariate benchmarks of non forecastability with baseline and monetary variables are realized in the first part of the prediction sample up to the end of 2002, which casts doubts on the current forecastability of inflation in the Euro Area. The third chapter is based on a joint work with Domenico Giannone and gives empirical foundation to the general equilibrium explanation of the Feldstein - Horioka puzzle. Feldstein and Horioka (1980) found that domestic saving and investment in OECD countries strongly comove, contrary to the idea that high capital mobility should allow countries to seek the highest returns in global financial markets and, hence, imply a correlation among national saving and investment closer to zero than one. Moreover, capital mobility has strongly increased since the publication of Feldstein - Horioka's seminal paper while the association between saving and investment does not seem to comparably decrease. Through general equilibrium mechanisms, the presence of global shocks might rationalize the correlation between saving and investment. In fact, global shocks, affecting all countries, tend to create imbalance on global capital markets causing offsetting movements in the global interest rate and can generate the observed correlation across national saving and investment rates. However, previous empirical studies (see Ventura, 2003) that have controlled for the effects of global shocks in the context of saving-investment regressions failed to give empirical foundation to this explanation. We show that previous studies have neglected the fact that global shocks may propagate heterogeneously across countries, failing to properly isolate components of saving and investment that are affected by non pervasive shocks. We propose a novel factor augmented panel regression methodology that allows to isolate idiosyncratic sources of fluctuations under the assumption of heterogenous transmission mechanisms of global shocks. Remarkably, by applying our methodology, the association between domestic saving and investment decreases considerably over time, consistently with the observed increase in international capital mobility. In particular, in the last 25 years the correlation between saving and investment disappears.

El entendimiento de los fenómenos de transferencia de calor y de masa en medios porosos implica el estudio de modelos de transporte de fluidos en la fracción vacía del medio; este hecho es de fundamental importancia en muchos sistemas de Ingeniería Química, tal como en procesos de extracción o en reactores catalíticos. Los estudios de flujo realizados hasta ahora (teóricos y experimentales) usualmente tratan al medio poroso como un medio efectivo y homogéneo, y toman como válidas las propiedades medias del fluido. Este tipo de aproximación no tiene en cuenta la complejidad del flujo a través del espacio vacío del medio poroso, reduciendo la descripción del problema a promedios macroscópicos y propiedades efectivas. Sin embargo, estos detalles de los procesos locales de flujo pueden llegar a ser factores importantes que influencien el comportamiento de un proceso físico determinado que ocurre dentro del sistema, y son cruciales para entender el mecanismo detallado de, por ejemplo, fenómenos como la dispersión de calor, la dispersión de masa o el transporte entre interfaces.La Dinámica de Fluidos Computacional (CFD) como herramienta de modelado numérico permite obtener una visión mas aproximada y realista de los fenómenos de flujo de fluidos y los mecanismos de transferencia de calor y masa en lechos empacados, a través de la resolución de las ecuaciones de Navier - Stokes acopladas con los balances de materia y energía y con un modelo de turbulencia si es necesario. De esta forma, esta herramienta permite obtener los valores medios y/o fluctuantes de variables como la velocidad del fluido, la temperatura o la concentración de una especie en cualquier punto de la geometría del lecho empacado.El objetivo de este proyecto es el de utilizar programas comerciales de simulación CFD para resolver el flujo de fluidos y la transferencia de calor y de masa en modelos bi/tri dimensionales de lechos empacados, desarrollando una estrategia de modelado aplicable al diseño de equipos para procesos de extracción o de reacción catalítica. Como referencia se tomaran procesos de tecnología supercrítica debido a la complejidad de los fenómenos de transporte involucrados en estas condiciones, así como a la disponibilidad de datos experimentales obtenidos previamente en nuestro grupo de investigación. Estos datos experimentales se utilizan como herramienta de validación de los modelos numéricos generados, y de las estrategias de simulación adoptadas y realizadas durante el desarrollo de este proyecto. / An understanding of the heat and mass transfer phenomena in a porous media implies the study of the fluid transport model within the void space; this fact is of fundamental importance to many chemical engineering systems such as packed bed extraction or catalytic reaction equipment. Experimental and theoretical studies of flow through such systems often treat the porous medium as an effectively homogeneous system and concentrate on the bulk properties of the flow. Such an approach neglects completely the complexities of the flow within the void space of the porous medium, reducing the description of the problem to macroscopic average or effective quantities. The details of this local flow process may, however, be the most important factor influencing the behavior of a given physical process occurring within the system, and are crucial to understanding the detailed mechanisms of, for example, heat and mass dispersion and interface transport.Computational Fluid Dynamics as a simulation tool allows obtaining a more approached view of the fluid flow and heat and mass transfer mechanisms in fixed bed equipment, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. In this way, this tool permit to obtain mean and fluctuating flow and temperature values in any point of the bed. The goal of this project is to use commercial available CFD codes for solving fluid flow and heat and mass transfer phenomena in two and three dimensional models of packed beds, developing a modeling strategy applicable to the design of packed bed chemical reaction and extraction equipment. Supercritical extraction and supercritical catalytic reaction processes will be taken as reference processes due to the complexity of the transport phenomena involved within this processes, and to the availability of experimental data in this field, obtained in the supercritical fluids research group of this university. The experimental data priory obtained by our research group will be used as validation data for the numerical models and strategies dopted and followed during the developing of the project.

L’étude multi-échelle d’un lit fluidisé gaz-particule est réalisée afin de comprendre les raisons de la mauvaise prédiction de la combustion dans ce type de réacteur. Dans un premier temps, des simulations numériques directes à l’échelle de quelques particules montrent le couplage entre la fraction volumique de particules et la fraction massique des espèces dans l’écoulement. Ensuite, une analyse des équations flitrées en LES montre qu’un terme de corrélation fluide-particule apparaît lorsque l’on explicite le taux de réaction gaz-particules. On comprend alors que si ce couplage n’est pas pris en compte correctement dans les simulations aux grandes échelles, le résultat ne peut pas être bon. Des simulations numériques à l’échelle du réacteur sont alors réalisées avec différents maillages pour tenter de mettre en évidence les effets de sous-maille liés à ce couplage à l’échelle des petites structures solides dans l’écoulement. / The multiscale study of a reactive gas-solid fluidized bed is performed to understand the reason of the bad prediction of the combustion in such a reactor. First, Direct Numerical Simulations at the particle array lenght scale show the dependency of the species mass fraction released in the gas phase on the solid volume fraction. Then, the analysis of the filtered continuity equations for the eulerian granular model highlights that a fluid-particle coupling term appears when expliciting the heterogeneous reaction rate. Therefore, we understand the need to take this coupling into account in large scales simulations to obtain good results. Computations at the lab-scale reactor are eventually performed using different grid refinements in order to try to highlight the subgrid terms due to this coupling at the small solid structure scale.

In this thesis, a new aerosol module is developed for the STEM model (the Sulfur Transport and dEposition Model) to better understand the chemical aging of dust during long range transport and assess the impact of heterogeneous reactions on tropospheric chemistry. The new aerosol module is verified and first applied in a box model, and then coupled into the 3-Dimentional STEM model. In the new aerosol model, a non-equilibrium (dynamic or kinetic) approach to treat gas-to-particular conversion is employed to replace the equilibrium method in STEM model. Meanwhile, a new numerical method solving the aerosol dynamics equation is introduced into the dynamic aerosol model for its improved computational efficiency and high accuracy. Compared with the equilibrium method, the new dynamic approach is found to provide better results on predicating the different hygroscopicity and chemical aging patterns as a function of size. The current modeling study also takes advantage of new findings from laboratory experiments about heterogeneous reactions on mineral oxides and dust particles, in order to consider the complexity of surface chemistry (such as surface saturation, coating and relative humidity). Modeling results show that the impacts of mineralogy and relative humidity on heterogeneous reactions are significant and should be considered in atmospheric chemistry modeling with first priority. Finally, the upgraded 3-D STEM model is utilized to explore the observations from the Intercontinental Chemical Transport Experiment - Phase B (INTEX-B). The new dynamic approach for gas-to-particular conversion and RH-dependent heterogeneous uptake of HNO3 improve the model performance in term of aerosol predictions under different conditions. It is shown that these improvements change the modeled nitrate and sulfate concentrations, but also modify their size distributions significantly.