Aqueous-biphasic organometallic catalysis is, as illustrated by the industrial hydroformylation of propene and butene, one of the most promising ways to overcome the intrinsic problem of catalyst separation in organometallic catalysis. However, for poorly water-soluble substrates, mass transfer limitations bring the reaction rate below any that could be economically viable, greatly limiting the scope of this elegant technology. We have studied three different strategies to overcome this limitation. We developed additives that speed up the reaction whilst retaining fast phase separation and good metal retention. Evidence suggests that those additives affect the reaction by forming emulsions with poor stability under the reaction conditions These emulsions increase the interfacial surface area but break after settling for a short time. We also developed ligands that allow the catalyst to be reversibly transported between an aqueous and an organic phase upon addition and removal of carbon dioxide. This allows the reaction to be carried out under homogeneous conditions, only limited by intrinsic kinetics, and the catalyst to be separated by aqueous extraction triggered by carbon dioxide. The catalyst can be returned to a fresh organic phase by flushing out the carbon dioxide. By applying this methodology for the hydroformylation of medium chain length alkenes, very high reaction rates were obtained and the catalyst could be recycle three times with excellent retention of activity and low metal leaching. This methodology could also be reversed with the reaction being carried out in an aqueous phase in the presence of carbon dioxide and extracting the catalyst into an organic solvent using nitrogen flushing. Finally, we briefly investigated the use of an oscillatory baffled reactor as a mean for mass transfer improvement for aqueous-biphasic hydroformylation. This new type reactor did not improve the performance of the system under the investigated conditions, but may require less energy input for equivalent agitation and mixing.

Due to the ever increasing concerns about pollutants and contaminants found in water, new treatment technologies have been developed. Ozonation is one of such technologies. It has been widely applied in the treatment of pollutants in water and wastewater treatment processes. Ozone has many applications such as oxidation of organic components, mineral matter, inactivation of viruses, cysts, bacteria, removal of trace pollutants like pesticides and solvents, and removal of tastes and odours. Ozone is the strongest conventional oxidant that can result in complete mineralisation of the organic pollutants to carbon dioxide and water. Because ozone is unstable, it is generally produced onsite in gas mixtures and is immediately introduced to water using gas/liquid type reactors (e.g. bubble columns). The ozone reactions are hence of the type gas liquid reactions, which are complex to model since they involve both chemical reactions, which occur in the liquid phase, and mass transfer from the gas to the liquid phase. This study focuses on two aspects: mass transfer and chemical reactions in multicomponent systems. The mass transfer parameters were determined by experiments under different conditions and the chemical reactions were studied using single component and multicomponent systems. Two models obtained from the literature were adapted to the systems used in this study. Mass transfer parameters in the semi-batch reactor were determined using oxygen and ozone at different flow rates in the presence and absence of t-butanol. t-Butanol is used as a radical scavenger in ozonation studies and it has been found to affect the gas-liquid mass transfer rates. An experimental study was carried out to investigate the effects of t-butanol concentrations on the physical properties of aqueous solutions, including surface tension and viscosity. It was found that t-butanol reduced both properties by 4% for surface tension and by a surprising 30% for viscosity. These reductions in the solution physical properties were correlated to enhancement in the mass transfer coefficient, kL. The mass transfer coefficient increased by about 60% for oxygen and by almost 50% for ozone. The hydrodynamic behaviour of the system used in this work was characterised by a homogeneous bubbling regime. It was also found that the gas holdup was significantly enhanced by the addition of t-butanol. Moreover, the addition of t-butanol was found to significantly reduce the size of gas bubbles, leading to enhancement in the volumetric mass transfer coefficient, kLa. The multicomponent ozonation was studied with two systems, slow reactions when alcohols were used and fast reactions when endocrine disrupting compounds were used. ii These experiments were simulated by mathematical models. The alcohols were selected depending on their volatilization at different initial concentrations and different gas flow rates. The degradation of n-propanol as a single compound was studied at the lowest flow rate of 200 mL/min. It was found that the degradation of n-propanol reached almost 60% within 4 hours. The degradation of the mixture was enhanced with an increase in the number of components in the mixture. It was found that the degradation of the mixture as three compounds reached almost 80% within four hours while the mixture as two compounds reached almost 70%. The effect of pH was studied and it was found that an increase in pH showed slight increase in the reaction. Fast reactions were also investigated by reacting endocrine disrupting chemicals with ozone. The ozone reactions with the endocrine disrupters were studied at different gas flow rates, initial concentrations, ozone concentrations and pH. The degradation of 17β-estradiol (E2) as a single compound was the fastest, reaching about 90% removal in almost 5 minutes. However estrone (E1) degradation was the lowest reaching about 70% removal at the same time. The degradation of mixtures of the endocrine disruptors was found to proceed to lower percentages than individual components under the same conditions. During the multicomponent ozonation of the endocrine disruptors, it was found that 17β-estradiol (E2) converted to estrone (E1) at the beginning of the reaction. A MATLAB code was developed to predict the ozone water reactions for single component and multicomponent systems. Two models were used to simulate the experimental results for single component and multicomponent systems. In the case of single component system, good simulation of both reactions (slow and fast) by model 1 was obtained. However, model 2 gave good agreement with experimental results only in the case of fast reactions. In addition, model 1 was applied for multicomponent reactions (both cases of slow and fast reaction). In the multicomponent reactions by model 1, good agreement with the experimental results was also obtained for both cases of slow and fast reactions.

Cette étude porte sur la modélisation dynamique d'un pilote d'extraction liquide-liquide avec des contacteurs membranaires à fibres creuses. L'objectif de ce travail est donc le développement d'un outil de modélisation afin d'optimiser des contacteurs membranaires à fibres creuses pour l'extraction liquide-liquide dans deux applications différentes : l'extraction d'arômes et de cuivre des solutions aqueuses. Un modèle axial-radial et à fibre unique basé sur une analyse des résistances en série ont été décrits. Il a aussi été développé un modèle en régime transitoire capable de prédire la concentration en soluté dans les réservoirs d'alimentation en fonction du temps. Ces deux modèles ont été couplés afin de former un modèle dynamique intégré pour l'extraction liquide-liquide avec une unité membranaire. Une étude expérimentale a permis de déterminer le mécanisme d'extraction avec plusieurs systèmes et solvants et ainsi valider le modèle dynamique intégré. Le modèle a permis ensuite la détermination de l'influence de divers paramètres comme la configuration, la structure des membranes, l'hydrodynamique et les conditions de transport ou opératoires sur l'efficacité de la vitesse d'extraction. / The current study deals with the dynamic modelling of hollow fiber membrane contactor extraction plant. The objective of the study is to develop a simulation tool in order to optimize the membrane contactors for liquid-liquid extraction in two different applications: aroma and copper extraction from aqueous solutions. Axial-radial stage and single-fiber model of hollow fiber membrane contactor have been proposed based on resistance-in-series model. A separate dynamic model across reservoir has been developed based on macroscopic unsteady state mass transfer balance. Both models are, then, coupled to consolidate into an integrated dynamic model of the membrane-based solvent extraction plant. Experiments have been carried out to verify the reaction mechanism of various solvent extraction systems and to validate the integrated dynamic model. Model has been used to determine the influence of different module configurations, membrane structural para meters, hydrodynamic, transport and operating conditions upon the extraction efficiency and speed of extraction.

Malgré l'ampleur que prend la technologie sous atmosphère modifiée pour le conditionnement des fruits et légumes frais, l'adéquation des matériaux d'emballage avec les besoins des produits est insuffisante. D'une part parce que la conception de ces emballages repose encore sur des approches empiriques de type essai-erreur, et d'autre part parce que la plupart des matériaux disponibles sur le marché présentent des propriétés de transfert aux gaz trop restreintes compte tenu de la gamme de propriétés nécessaire pour couvrir les besoins de ces produits. En vue d'apporter des solutions à ces deux verrous, les travaux de thèse ont porté sur :-la mise en place et la validation d'une approche basée sur l'ingénierie reverse visant à identifier les propriétés de transfert requises ou cibles pour un végétal donné ;-l'étude des procédés de structuration à différentes échelles pour moduler les propriétés de transferts de papiers enduits de protéines de blé, issus de ressources renouvelables.Une démarche d'ingénierie reverse organisée en 5 étapes dont l'identification des besoins des produits et la prédiction des propriétés de transferts requises a été proposée et validée à travers la conception d'un papier enduit actif pour l'emballage sous atmosphère modifiée adapté à la conservation de fraises. Les différents procédés de structuration étudiés ont permis de produire des matériaux couvrant des gammes de permsélectivité allant de 5 à 18 et de perméance à l'oxygène allant de 0,02x10-10 à 2x10-10 mol.Pa-1.m-2.s-2, ce qui répond parfaitement aux besoins d'une sélection représentative de produits frais respirant. Que ce soit des papiers enduits par des procédés conventionnels (couche épaisse) ou des procédés innovants (nano-structuration couche par couche en présence de feuillets d'argile, montmorillonites), il est possible de moduler les propriétés de transferts aux gaz des papiers enduits soit en jouant sur le raffinage des papiers supports, le nombre de paires de couche (gluten/montmorillonite), ou encore en soumettant les matériaux ainsi obtenus à un balayage gazeux de CO2. Cette nouvelle approche et l'efficacité des matériaux protéiques à couvrir les besoins des fruits et légumes ouvrent la voie à la conception de nouveaux emballages mieux adaptés aux produits respirant. / Despite the growing importance of the modified atmosphere packaging (MAP) technology for fresh fruits and vegetable preservation, the adequacy of the packaging materials with the produce needs remains a problem. On one hand because the packaging development approaches are still based on empirical trial and error methods, and on the other hand, because of the non-adequate gas transfer properties (too restricted to cover the large game of gas transfer properties required) of the vast majority of conventional synthetic plastics currently used for fresh food packaging. In order to overcome these hindrances, the thesis work was split on two parts:-the establishment and the validation of a new approach based on reverse engineering aiming to identify the optimal (targeted) properties for optimal preservation of a selected produce;-the study of the various structuration processes at different scales to modulated the gas transfer properties of bio-sourced wheat gluten coated papers.The new reverse engineering approach for MAP conception consisted in 5 steps including the definition of the produce needs and the prediction of the optimal gas transfer properties of the packaging, and was validated through the conception of an active optimal packaging for strawberry preservation. The different structuration processes studied allowed production of a gluten based materials able to cover ranges of permselectivity and oxygen permeations going from 5 to 18 and 0.02x10-10 à 2x10-10 mol.Pa-1.m-2.s-2, which perfectly matched the needs of a representative selection of fresh produce. Be it for conventional coating techniques (thick layer) or innovative processes (layer-by-layer nano-structuring of gluten/montmorillionites layers), it was possible to modulate the transfer properties of coated papers by changing the refining degree of the support paper, the number of layer pairs (gluten/montmorillionites) deposited, or even by submitting such materials to a CO2 treatment.This new approach and the efficiency of the protein-based materials for covering the fresh fruits and vegetable needs open the way for conception of new optimal packaging for respiring produces.

Les matériaux hygroscopiques, et tout particulièrement le bois et ses dérivés possèdent des propriétés complexes rendant difficile la modélisation des transferts couplés de chaleur et de masse dans les parois incluant ces matériaux. De ce fait, très peu d'outils numériques sont aujourd'hui capables de prédire correctement la performance hygrothermique de l'habitat bois. L'objectif de ce travail est de caractériser expérimentalement les transferts chaleur-masse dans les parois des constructions bois afin de valider un outil numérique destiné à simuler le comportement hygrothermique des parois comportant des matériaux hygroscopiques. Dans un premier temps, les notions théoriques et les études antérieures sur les transferts couplés chaleur - masse sont présentés. Ensuite, nous donnons un descriptif détaillé du dispositif expérimental conçu pour caractériser les transferts couplés chaleur-masse dans les parois. Les expériences de caractérisation des performances hygrothermiques des parois fournies par les industriels partenaires du projet TRANSBATIBOIS dans lequel s'inscrit cette thèse sont également abordées. Nous détaillons par la suite les expériences réalisées ainsi que la phase de confrontation des résultats expérimentaux avec les résultats prédits par le code numérique BuildingPore et l'outil commercial WUFI. La troisième partie de ce travail est consacrée aux expérimentations à l'échelle de l'enveloppe. Nous y présentons une analyse de la performance hygrothermique et des consommations énergétiques des constructions bois à travers le suivi de modules-test exposés au climat extérieur. La dernière partie du travail est consacrée aux dispositifs de suivi de bâtiments. / Coupled heat and moisture transfer through hygroscopic materials, particularly wood and wood-based products are difficult to model. This is partly due to some specific and complex properties of these materials that are often not included in numerical models. Currently, only a few numerical models are able to predict accurately the hygrothermal performance of wooden building envelope. The aim of this work is to assess the heat and moisture transfer in wooden building envelope through experiments and validate the prediction capacity of a numerical model developed to simulate hygrothermal behavior of envelope including wooden materials. After giving a theoretical reminder of the coupled heat and moisture transfer through building envelope and reporting the results of previous studies in this field, we will give details of the experimental investigation on heat and moisture transfer through timber walls. Firstly, the experimental apparatus used for the wall tests is presented. Then, we will analysis the hygrothermal performance of wooden walls provided by the partners of the TRANSBATIBOIS project in which this work was achieved. Experimental works achieved for Buildingpore model validation and results of the comparisons between experimental assessment and numerical predictions with Buildingpore and WUFI are also reported. The third part of this study deals with the experimental assessment of wooden building envelopes exposed to climatic conditions. An analysis of the hygrothermal performance and the energy consumption of wooden test-cells is performed and reported in this part. The latest part concerns experimental works on buildings.

Le transport par voie maritime s’est développé très rapidement durant la dernière décennie. Il constitue la solution idéale pour déplacer à faibles coûts d’importantes quantités de marchandises sur de longues distances. L’accroissement de ces échanges entraine inévitablement une augmentation de la fréquentation des routes maritimes, de la densité du trafic et donc du risque d’accident et de naufrage.Les quantités impliquées lors d’un accident étant importantes, un rejet sous-marin de produit chimique issu d’une épave peut provoquer de graves conséquences environnementales et humaines. Or, les mécanismes physico-chimiques apparaissant lors du transfert du produit à la surface sont encore mal compris. Ces mécanismes dépendent de nombreux paramètres dont les propriétés physico-chimiques intrinsèques au produit et celles propres à l’environnement de l’accident.Ce travail de recherche, réalisé en collaboration entre l’Ecole des Mines d’Alès et le Cedre, s’inscrit dans l’amélioration de la compréhension du comportement d’un rejet de produit chimique liquide, flottant et soluble issu d’une épave. Cette étude expérimentale a été réalisée selon 3 points :- La phénoménologie du rejet à la brèche dont l’objectif est de quantifier le débit et les modes de rejet,- Le comportement hydrodynamique du produit libéré et son évolution lors de la remontée dans la colonne d’eau,- Le transfert de matière du produit lors de sa solubilisation dans la colonne d’eau.Pour chaque point, les résultats obtenus sont comparés à ceux prédits par les corrélations disponibles dans la littérature afin de proposer une modélisation globale des conséquences adaptée à la gestion des pollutions marines. / Maritime transport quickly increases during the last decade. It is the cheapest way to transport large quantities of goods over long distances. The growth of this trade necessarily leads to an increase in the traffic density and therefore the risk of accident and sinking increases as well.The large amount of chemicals involved in an accident may threaten the environment and human health in case of shipwreck with underwater chemical release. Unfortunately the physicochemical mechanisms occurring during the transfer of a chemical to the sea surface are still poorly understood. Those mechanisms depend on various parameters such as the physicochemical properties of the chemicals and the specific properties of the accident surroundings. This research was conducted in collaboration with the Ecole des Mines d'Alès and Cedre in a global approach to understand the behaviour of liquid, floating and soluble chemicals released after a shipwreck. This experimental study has been carried out according to 3 main points:- The phenomenology of a release at the breach. The objectives are to quantify the flow rate and the release pattern,- The hydrodynamic behaviour of the released chemicals and their fate in the water column,- The mass transfer of the chemical during its solubilization in the water column.For each point, the results are compared to those predicted by correlations in the literature in order to propose a global model of consequences that is adapted to the marine pollution management.

La digestion anaérobie est un processus de transformation de la matière organique en biogaz riche en méthane. Les déchets solides sont le plus souvent traités par une technologie par voie sèche qui est caractérisée par des teneurs en matières sèches supérieures à 20%. Du fait de la faible teneur en eau, la digestion sèche présente de nombreuses inconnus scientifiques, tant au niveau microbiologique et physico-chimique. Le but de cette thèse est de comprendre l'influence des parametres opératoires spécifiques à la digestion anaérobie par voie sèche sur les performances et le comportement des communautés microbiennes de ce processus. Les résultats obtenus montrent que l'augmentation de la teneur en matières sèches au-delà de 30% et la diminution de la surface spécifique d'échange ont un effet négatif sur le processus de digestion anaérobie. En effet lorsque la teneur en matière sèche est supérieure à 30%, le transfert des gaz dissous du milieu réactionnel vers le ciel gazeux peut limiter la méthanisation. L'analyse des populations microbiennes montre la présence de bio-indicateurs bactériens liés au non fonctionnement de la digestion anaérobie. Ces bio-indicateurs sont des clostridium et sont présents quel que soit l'inoculum utilisé. L'étude des populations méthanogènes montre une sensibilité de ces microorganismes à leur environnement : des shifts de populations méthanogènes ont été observés en fonction de la concentration en matières sèches. Ce travail a permis d'accroître les connaissances dans le domaine de la digestion anaérobie sèche, ces connaissances pouvant contribuer à améliorer les performances des procédés. / Anaerobic digestion is the transformation of organic matter into biogas rich in methane. Solid waste is treated by dry anaerobic digestion process with a total solid content higher than 20%. The high solids content make the process difficult to control. The aim of this work is to study the influence of operating parameters specific to dry anaerobic digestion on performances and behavior of microbial communities. The results show that increasing the solids content higher than 30% and decreasing the specific surface area of change have a negative effect on the anaerobic digestion process. Indeed, when the solids content exceeds 30%, the transfer of dissolved gas from the reaction mixture to the headspace can limit anaerobic digestion. The analysis of microbial populations demonstrates the presence of bacterial bio-indicators related to no-functioning anaerobic digestion. These bio-indicators correspond to the clostridium genus and are present regardless of the inoculum source. The study of methanogenic Archaea shows a sensitivity of these microorganisms. Shifts in the methanogenic populations related to the concentration of total solids were observed. This work has improved the fundamental knowledge of dry anaerobic digestion which can help improve the performance of this process.

Master of Science / Department of Mechanical and Nuclear Engineering / Steven J. Eckels / Membrane dehumidification and evaporative cooling applications have the potential to significantly improve the energy efficiency of air conditioning equipment. The use of wire mesh media in such membrane applications is feasible but has not been studied extensively. Therefore, the aim of this work is to investigate the heat and mass transfer performance of several different wire mesh media in membrane based dehumidification and evaporative cooling. There were six wire mesh membranes tested in an experimental facility. The wire mesh membranes vary with respect to percent open area, wire diameter, pore size and material. Two non-permeable, solid membranes were also tested in the facility and compared with the wire mesh membranes. The test section of the experimental facility consists of a narrow air duct and a plate apparatus. The membrane samples were fashioned into rectangular plates and installed into the test section. The plate membranes separate liquid water and air flow streams. The inlet air temperature and humidity are altered to produce condensation or evaporation at the membrane surface. The average convective heat and mass transfer coefficient of the air boundary layer is measured for each of the experimental plates. Membrane based dehumidification and evaporative cooling were accomplished using the wire mesh media. However, the wire mesh membranes did not exhibit any significant differences in their performance. The mesh plates were compared with the solid plate membranes and it was discovered that the solid plates exhibited significantly higher heat transfer coefficients during condensation conditions. This result most likely is due to the formation of large water droplets on the solid plates during condensation. The experimental data is then compared to analytical predictions of the heat and mass transfer coefficients developed from several heat transfer correlations and by invoking the heat and mass transfer analogy. The experimental data is also compared directly with the heat and mass transfer analogy. It was found that the data did not compare well with the heat and mass transfer analogy. This result is attributed to the fact that the membrane surface limits the amount of direct exposure to the gas-liquid interface.

Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p><p> </p>

<p>Cilj doktorske disertacije je pobolj&scaron;anje hidrodinamičkih i<br />masenoprenosnih karakteristika airlift reaktora sa<br />spolja&scaron;njom recirkulacijom umetanjem samo-rotirajućih<br />impelera u uzlaznu cev. Uticaj ugradnje samo-rotirajućih<br />impelera na osnovne hidrodinamičke i masenoprenosne<br />osobine ispitivan je pri radu sa različitim tečnostima i<br />distributorima gasa. Rezultati su tumačeni poređenjem<br />vrednosti za sadržaj gasa u uzlaznoj cevi, za brzinu<br />tečnosti u silaznoj cevi, i za zapreminski koeficijent<br />prenosa mase, dobijenih za dve konfiguracije reaktora (sa<br />i bez impelera).<br />Rezultati disertacije ukazuju da ugradnja samo-rotirajućih<br />impelera dovodi do znatnog razbijanja mehurova i<br />smanjenja srednjeg prečnika mehurova gasa, naročito u<br />viskoznim rastvorima karboksimetilceluloze. Prividna<br />brzina gasa, vrsta tečne faze i tip distributora gasa u<br />velikoj meri utiču na efikasnost samo-rotirajućih impelera.<br />Dobijene vrednosti sadržaja gasa u uzlaznoj cevi su do<br />47% veće u konfiguraciji sa impelerima u odnosu na<br />konfiguraciju bez impelera. Iako samo-rotirajući impeleri<br />predstavljaju dodatni otpor proticanju tečnosti, njihovom<br />ugradnjom je u svim ispitivanim sistemima postignuto<br />relativno malo smanjenje brzine tečnosti (oko 10%).<br />Zahvaljujući impelerima, zapreminski koeficijent prenosa<br />mase uvećan je do 82% pri manjim protocima gasa. Pri<br />većim protocima, koji odgovaraju uslovima rada koji se<br />sreću u većini fermentacionih procesa, postignute su oko<br />20-30% veće vrednosti zapreminskog koeficijenta prenosa<br />mase.<br />Pored empirijskih modela, u ovom radu uspe&scaron;no su<br />razvijeni i modeli ve&scaron;tačkih neuronskih mreža kojim se<br />predviđaju sadržaj gasa u uzlaznoj cevi, brzina tečnosti i<br />zapreminski koeficijent prenosa mase za obe konfiguracije<br />airlift reaktora sa spolja&scaron;njom recirkulacijom.</p> / <p>The aim of this study was intensification of<br />hydrodynamic and mass transfer properties of<br />external-loop airlift reactor by means of self-agitated<br />impellers mounted in the riser section. The influence<br />of impellers&rsquo; insertion on the main hydrodynamic<br />and mass transfer characteristics was investigated for<br />various liquid phases and sparger types. The values<br />of riser gas holdup, downcomer liquid velocity and<br />volumetric mass transfer coefficient obtained in<br />configurations with and without impellers were<br />compared.<br />Results showed that the installment of self-agitated<br />impellers led to a significant bubble breakage and<br />decrease of bubble size, especially in viscous<br />carboxymethylcellulose solutions. The efficiency of<br />self-agitated impellers was immensely influenced by<br />superficial gas velocity, liquid phase, and distributor<br />type. Obtained riser gas holdup values were up to<br />47% higher in the configuration with impellers, in<br />comparison to the configuration wthout impellers.<br />Despite the fact that self-agitated impellers<br />represented an obstacle to liquid flow, relatively low<br />reduction of downcomer liquid velocity was attained<br />(about 10%). Furthermore, the insertion of impellers<br />induced up to 82% higher values of volumetric mass<br />transfer coefficient at lower superficial gas velocities.<br />At higher superficial gas velocities, i.e. conditions<br />mainly operated in various fermentation processes,<br />about 20-30% higher values of volumetric mass<br />transfer coefficient were achieved with impellers.<br />In addition to empirical correlations, artificial neural<br />network models were sucessfully developed to<br />predict riser gas holdup, downcomer liquid velocity<br />and volumetric mass transfer coefficient in both<br />external-loop airlift configurations.</p>