Cette thèse se focalise sur les écoulements gaz-liquide dans un milieu poreux, problématique rencontrée dans des domaines variés allant de la physique fondamentale à la chimie appliquée. Nous avons caractérisé expérimentalement les régimes hydrodynamiques dans deux géométries différentes : un canal millifluidique (écoulement quasi-1D) et une cellule de Hele-Shaw (écoulement quasi-2D). L’originalité de ce travail est d’analyser l’effet du milieu poreux (lits de billes polydisperses ou mousses solides à cellules ouvertes), du confinement (1D/2D) et de la gravité en couplant des approches locales et globales développées dans les communautés de physique expérimentale et de génie chimique. D’une part, une analyse globale a permis de quantifier les pertes de charge [1] et, basée sur le transport d’un traceur fluorescent, les distributions de temps de séjour [2] et le transfert gaz-liquide dans l’expérience 1D ; d’autre part, une analyse locale de la fraction liquide et l’évolution spatio-temporelle de son contenu fréquentiel ont permis de mettre en évidence deux régimes hydrodynamiques dans le canal millifluidique [3-5] : un régime pseudo-Taylor, où les caractéristiques de l’écoulement périodique amont sont conservées, et un régime modulé, pour lequel l’écoulement se désorganise à l’entrée du milieu poreux. Un modèle phénoménologique basé sur la propagation des bulles dans le milieu est proposé, et rend compte de l’existence de ces deux régimes [4,5]. Enfin, ces deux analyses sont couplées pour étudier les écoulements dans la cellule de Hele-Shaw, et une analyse dimensionnelle de l’effet du confinement et de la gravité sur les écoulements gaz-liquide dans un milieu poreux est proposée.Références –[1] M. Serres, R. Philippe & V. Vidal, to be submitted to Geophys. Res. Lett. (2017). [2] M. Serres, D. Schweich, R. Philippe & V. Vidal, to be submitted to Chem. Eng. Sci. (2017).[3] M. Serres, R. Philippe & V. Vidal, Compte-rendus de la 19e Rencontre du Non-Linéaire, Eds. E. Falcon, M. Lefranc, F. Pétrélis & C.-T. Pham, Non-Linéaire Publications, 109-114 (2016).[4] M. Serres, M.-L. Zanota, R. Philippe & V. Vidal, Int. J. Multiphase Flow 85, 157-163 (2016).[5] M. Serres, T. Maison, R. Philippe & V. Vidal, to be submitted to Int. J. Multiphase Flow (2017). / This thesis focuses on gas-liquid flow in porous media, a common problem encountered in various domains from fundamental physics to applied chemical engineering. We have characterized the hydrodynamic regimes based on two different experimental devices geometry: a millichannel (1D flow) and a Hele-Shaw cell (2D flow). The originality of this work is to analyze the influence of the porous medium (monodisperse micro-packed beds or open cell solid foams), confinement (1D/2D) and gravity by coupling global and local analysis from either chemical engineering or fundamental physics community. On the one hand, a global analysis made it possible to quantify pressure drops, residence time distributions (RTD) based on fluorescent dye transport and gas-liquid mass transfer on the 1D device. On the other hand, a local analysis of the liquid fraction and the spatio-temporal evolution of its frequency pointed out the existence of two hydrodynamic regimes: a Taylor-like regime in which the characteristics of the periodic flow upstream are conserved in the porous medium and a modulated regime characterized by the flow disorganization at the porous medium entrance. A phenomenological model is developed based on bubbles propagation inside the medium and reproduces well both regimes. These two analyses are finally coupled to study multiphase flows inside the Hele-Shaw cell. The effects of gravity and confinement are discussed.

Gas-liquid two-phase flows occur in many industrial applications and apparatuses. The design and optimization of such apparatuses and processes requires the numerical simulation of two-phase flows. However, two-phase flow simulations are still a challenging task, especially for industrial scales. Here, the simulation of large flow domains and high Reynolds number flows require a reduction of the resolved time-scales and length-scales by a high level of modeling to decrease the computational effort. Therefore, physics-based models are needed to depict the complex transport processes between the phases. Thus, two-phase flows are the object of ongoing research. Up to now, the majority of closure models for turbulence, interfacial forces or bubble breakup and coalescence were validated against experimental data derived from experiments in simple flow channel geometries like straight pipes. Their application for the simulation of two-phase flows with three-dimensional flow structures like e.g. recirculating areas, flow separation or strong velocity gradients requires constant experimental validation and further development. Hence, improved experimental methods are required for investigations of gas-liquid two-phase flows to provide reliable data for further development and validation of numerical flow simulation models. Therefore, experiments were performed in a constricted pipe under bubbly two-phase flow conditions. Three-dimensional flow structures were created by two types of flow constrictions for a variety of gas and liquid superficial velocities up to jg = 0.1400 m⋅s-1 and jl = 1.6110 m⋅s-1. The flow fields upstream and downstream of the flow constrictions were studied using ultrafast X-ray tomography and hot-film anemometry to obtain cross-sectional phase distribution, bubble characteristics and liquid velocity. The analysis of the ultrafast X-ray tomography image data was significantly improved by development of a histogram-based gas holdup calculation. Furthermore, the spatial dependence of the axial image plane distance was studied to improve the determination of axial bubble velocities and, thus, bubble sizes. The experimental method was advanced by simultaneous application of ultrafast X-ray tomography and hot-film anemometry. Eventually, the experimental data was compared to state-of-the-art Euler/Euler two-fluid simulations. The simulations were performed in the framework of a parallel doctoral thesis in the Experimental Thermal Fluid Dynamics department at the Helmholtz-Zentrum Dresden – Rossendorf by Ms. Sibel Tas-Koehler following the baseline approach. The results were compared in terms of the phase distribution, bubble sizes and gas velocity for two operating conditions using the homogeneous multiple size group model. / Zweiphasenströmungen aus Gasen und Flüssigkeiten treten in vielen industriellen Anwendungen und Apparaten auf. Um einen sicheren, zuverlässigen und optimalen Betrieb einzelner Komponenten und gesamter Anlagen zu gewährleisten, sind die Strömungen Gegenstand zahlreicher Untersuchungen. Numerische Simulationen sind ein unverzichtbares Instrument, um Prozesse unter diesen Aspekten zu bewerten. Die Simulation von Zweiphasenströmungen, insbesondere im industriellen Maßstab, ist jedoch nach wie vor eine anspruchsvolle Aufgabe. Um den Rechenaufwand zu verringern und die Simulation von großen Strömungsgebieten und Strömungen mit hohen Reynoldszahlen zu ermöglichen, ist ein hohes Maß an Modellierung notwendig. Gleichzeitig wurden die meisten Schließungsmodelle zur Beschreibung von Turbulenz, Grenzflächenkräften oder Blasenzerfall und -koaleszenz für einfache Geometrien wie beispielsweise gerade Rohre entwickelt. Die Anwendung dieser Modelle für die Simulation von Zweiphasenströmungen mit dreidimensionalen Strömungsstrukturen, wie z.B. Rezirkulationsgebieten, Strömungsablösungen oder starken Geschwindigkeitsgradienten, erfordert eine ständige experimentelle Validierung und Weiterentwicklung. Dies wiederum erfordert eine immer höhere Auflösung der eingesetzten Messsysteme und steigende Qualität der experimentellen Daten. Um verlässliche Daten für die Weiterentwicklung und Validierung von Modellen für die numerische Strömungssimulation zu erhalten sind daher verbesserte experimentelle Methoden zur Untersuchung von Gas-Flüssig-Strömungen erforderlich. Aus diesem Grund wurden Experimente an einer Blasenströmung in einem Rohr mit einer Strömungsverengung durchgeführt. Zwei Arten von Verengungen wurden genutzt, um dreidimensionale Strömungsstrukturen für eine Vielzahl von Betriebsbedingungen zu erzeugen. Diese sind durch Gas- und Flüssigkeitsleerrohrgeschwindigkeiten bis zu jg = 0.1400 m⋅s-1 und jl = 1.6110 m⋅s-1 definiert. Um die Phasenverteilung im Querschnitt der Strömung, Blaseneigenschaften und die Flüssigphasengeschwindigkeit stromauf- und -abwärts der Verengung zu ermittelt, wurde die Strömung mit Hilfe der ultraschnellen Röntgentomographie und Heißfilm-Anemometrie untersucht. Die Datenanalyse für die Bilddaten der ultraschnellen Röntgentomographie wurde durch die Entwicklung einer Histogramm-basierten Gasgehaltsberechnung erheblich verbessert. Um die Bestimmung der axialen Blasengeschwindigkeiten und damit der Blasengrößen zu verbessern, wurde außerdem die räumliche Abhängigkeit des axialen Bildebenenabstands untersucht. Die experimentellen Methoden wurden durch die gleichzeitige Anwendung von ultraschneller Röntgentomographie und Heißfilm-Anemometrie weiterentwickelt. Die experimentellen Daten wurden mit dem Stand der Technik von Euler/Euler-Zweiphasen-Simulationen verglichen. Die Simulationen wurden im Rahmen eines parallelen Promotionsvorhabens in der Abteilung Experimentelle Thermofluiddynamik am Helmholtz-Zentrum Dresden – Rossendorf von Frau Sibel Tas-Köhler durchgeführt und folgten der Baseline-Modell Strategie. Die Ergebnisse wurden unter Verwendung des homogenen Modells mehrerer Größenklassen bezüglich der Phasenverteilung, der Blasengrößen und der Gasgeschwindigkeit für zwei Betriebsbedingungen verglichen.

[pt] O estudo do escoamento intermitente de gás e líquido é de enorme relevância. Devido a sua vasta ocorrência em diversos problemas industriais, como na indústria do petróleo, em escoamentos de água e vapor em trocadores de calor e em sistemas de refrigeração de usinas nucleares, diversos estudos experimentais e numéricos buscam obter um conhecimento mais profundo deste complexo fenômeno. Devido a esta complexidade, se fazem necessários experimentos detalhados de maneira a dar suporte aos modelos matemáticos desenvolvidos. O presente trabalho descreve um estudo experimental de um escoamento turbulento gás-líquido no regime intermitente para tubo horizontal. As técnicas de velocimetria por imagem de partícula estereoscópica de alta frequência (SPIV ) e fluorescência induzida por laser (LIF) foram usadas para medir todos os três componentes do vetor de velocidade em diferentes seções do tubo, referenciadas pelo nariz da bolha de gás. A seção de testes consistiu de uma tubulação de acrílico com diâmetro interno de 40mm e 17,7m de comprimento. Os fluidos de trabalho utilizados foram água e ar, com velocidades superficiais de jL igual 0,3 0,4 e 0,5m/s e jG igual 0,5m/s que formaram o padrão de escoamento intermitente. Um sistema de sensores fotossensíveis foi utilizado para medir a velocidade de translação da bolha de gás, além de acionar o sistema SPIV. Desta forma, foi possível determinar os campos médios das três componentes de velocidade da fase líquida do escoamento turbulento gás-líquido em regiões de interesse na vizinhança da bolha alongada. Os dados obtidos revelaram a influência das bolhas de gás de maior velocidade, na dinâmica do campo de velocidade do líquido. Tais dados contém informações valiosas que contribuem não apenas para uma melhor compreensão da física que rege o escoamento, mas também como forma de validação e aprimoramento de modelos numéricos. / [en] The analysis of gas-liquid intermittent flow in horizontal pipes is of great relevance importance due its applications in many industrial problems, such as in the petroleum industry, boiler and heat exchanger tubes and cooling systems of nuclear power plants. A considerable number of experimental and analytical studies have been carried out on the pursuit of a deeper knowledge of this complex phenomenon. The present work describes an experimental study of a horizontal, gas-liquid pipe flow in the intermittent regime. Experimental techniques such as high frequency stereoscopic particle image velocimetry (SPIV ) and laser induced fluorescence (LIF), were applied in order to obtain all three components of the velocity vector at different pipe sections, referred to the gas bubble nose tip. A 40mm inner diameter, 17.7m long acrylic pipe was used as test section (L/D approximately 450). The working fluids, water and air formed the intermittent flow pattern, with superficial velocities of jL equal 0.3, 0.4 and 0.5 m/s and jG equal 0.5 m/s. A set of three photogate sensors, equally-spaced along the pipe, were used to measure the bubble translational velocities, and to trigger the SPIV system, allowing for the determination of ensemble-averaged, threecomponent velocity fields of the turbulent liquid flow in cross-stream planes around the gas bubble. The original data obtained revealed the influence of the faster-moving gas bubbles on the dynamics of the liquid velocity field, providing valuable information that contribute to a better understanding of the physics governing the flow, also serving for the validation of numerical simulations.

O presente trabalho apresenta um estudo associado à separação de gás para escoamento gás-líquido em um duto anular inclinado. Esse tipo de escoamento e geometria são encontrados em separadores de gás do tipo shroud invertido na indústria de petróleo quando utilizadas técnicas de bombeamento para a exploração de poços. A presença de gás livre a montante da bomba é uma das maiores limitações dos sistemas de bombeamento, por acarretar cavitação e falhas dinâmicas nos equipamentos. O presente estudo tem por objetivo garantir a separação total de gás livre a montante da bomba através da proposição de um separador do tipo shroud invertido em tubulação inclinada para aplicação na exploração de petróleo em poços direcionais. Um estudo experimental com ar e água como fluidos de trabalho a pressão quase atmosférica foi desenvolvido visando a compreensão da fenomenologia do separador shroud invertido. Foram observados escoamentos em duto anular do tipo: bifásico gás-líquido ascendente, monofásico em canal livre e bifásico gás-líquido descendente devido ao fenômeno de aeração; foram coletados também dados de eficiência de separação em função do ângulo de inclinação, vazão do líquido e queda de pressão entre o shroud e a saída do tubo de produção. Encontrou-se uma variável de extrema importância no fenômeno de separação até agora não reportada na literatura: o nível do anular interno do shroud (NAI). Um modelo fenomenológico que prevê a separação total do gás foi desenvolvido a partir da interpretação dos fenômenos físicos observados experimentalmente. Uma correlação inédita para a modelagem do fenômeno de dissipação de energia cinética turbulenta vinculado à separação do gás é proposta. O modelo foi validado qualitativamente com dados da literatura e ajustado com os dados coletados neste trabalho, mostrando boa concordância. / This paper presents a study associated with gas separation in an inclined gas-liquid annular-duct flow. This type of flow and geometry are found in shroud-inverted gas separators applied to petroleum industries when using pumping technique for oil production. High void fraction at the pump suction of is one of the most important limitations of the SCP technique, causing cavitation and dynamics failures in the equipment. The present study aims to provide a solution for the total gas separation through the use of an innovative inclined inverted-shroud separator for directional wells. An experimental study, where air and water at near atmospheric pressure constituted the working fluids, was carried out to understand the phenomenology of the inclined inverted-shroud separator. Different annular-duct flows were observed: upward gas-liquid flow, single-phase open channel flow, downward gas-liquid flow due to the phenomenon of aeration; also new data of separation efficiency were collected as a function of inclination angle, liquid flow rate and pressure drop between the shroud and production pipe outlet. One of the most significant findings is that the liquid level of the inner annular channel (NAI) of the shroud is a very important variable in the phenomenon of separation; so far this was not reported in the literature. Based on the observations, a phenomenological model that predicts total gas separation is proposed. A new correlation for the modeling of the dissipation of turbulent kinetic energy associated with the gas separation is presented. The model was qualitatively compared with available data from the literature and quantitatively adjusted against the new experimental data obtained in this work, and the agreement was quite good.

Gas, liquid and powder flow in the lower part of a blast furnace is complex phenomena. In order to understand the aerodynamics of the blast furnace properly, these phenomena must be included in their advanced form. Previous studies have shown that the conditions of blast furnace resemble the cold model experiments which have been done in decreasing gas velocities. Also, the recent studies have shown that liquid flow in a blast furnace can be represented more realistically considering it discrete in nature. In the current study, both the phenomena have been considered along with the injection of powder through a nozzle while studying the fluid flow behaviour in a packed bed. The situation resembles the lower part of an ironmaking blast furnace. In this study, gas flow has been modelled using k-ε turbulent model and has been coupled with previously developed stress model to calculate the raceway size. Coal powder is treated as continuum and has been modelled in the similar way as gas flow. After this gas and powder flow model were coupled with previously developed discrete liquid flow model. Liquid flow model has been considered for structured bed only. The governing equations for gas phase were discretized. Finite Volume method was used for the solution. Co-located grid is used for the simulation. Blending of upwind difference scheme and central difference scheme (deferred correction approach) is used to achieve the stability of upwind scheme and accuracy of central difference scheme. Similar treatment was employed for powder phase. For the solution of volume fraction of powder, powder phase continuity equation was used along with pseudo time step scheme. Results obtained from gas and powder models have been validated against published experimental data. Similarly, gas-liquid flow results have been validated against published experimental data on gas-powder flow. Results obtained by gas-powder-liquid model could not be validated against any experimental or theoretical data as they are not available in the literature. The effect of various parameters on the fluid flow (gas/liquid/powder) behaviour have been studied like the effect of increasing and decreasing gas velocities, flow rates of liquid, gas and powder, size of powder and packing etc. It is found that the above mentioned phenomena have significant effect on the fluid flow behaviour in a packed bed.

O presente trabalho apresenta um estudo associado à separação de gás para escoamento gás-líquido em um duto anular inclinado. Esse tipo de escoamento e geometria são encontrados em separadores de gás do tipo shroud invertido na indústria de petróleo quando utilizadas técnicas de bombeamento para a exploração de poços. A presença de gás livre a montante da bomba é uma das maiores limitações dos sistemas de bombeamento, por acarretar cavitação e falhas dinâmicas nos equipamentos. O presente estudo tem por objetivo garantir a separação total de gás livre a montante da bomba através da proposição de um separador do tipo shroud invertido em tubulação inclinada para aplicação na exploração de petróleo em poços direcionais. Um estudo experimental com ar e água como fluidos de trabalho a pressão quase atmosférica foi desenvolvido visando a compreensão da fenomenologia do separador shroud invertido. Foram observados escoamentos em duto anular do tipo: bifásico gás-líquido ascendente, monofásico em canal livre e bifásico gás-líquido descendente devido ao fenômeno de aeração; foram coletados também dados de eficiência de separação em função do ângulo de inclinação, vazão do líquido e queda de pressão entre o shroud e a saída do tubo de produção. Encontrou-se uma variável de extrema importância no fenômeno de separação até agora não reportada na literatura: o nível do anular interno do shroud (NAI). Um modelo fenomenológico que prevê a separação total do gás foi desenvolvido a partir da interpretação dos fenômenos físicos observados experimentalmente. Uma correlação inédita para a modelagem do fenômeno de dissipação de energia cinética turbulenta vinculado à separação do gás é proposta. O modelo foi validado qualitativamente com dados da literatura e ajustado com os dados coletados neste trabalho, mostrando boa concordância. / This paper presents a study associated with gas separation in an inclined gas-liquid annular-duct flow. This type of flow and geometry are found in shroud-inverted gas separators applied to petroleum industries when using pumping technique for oil production. High void fraction at the pump suction of is one of the most important limitations of the SCP technique, causing cavitation and dynamics failures in the equipment. The present study aims to provide a solution for the total gas separation through the use of an innovative inclined inverted-shroud separator for directional wells. An experimental study, where air and water at near atmospheric pressure constituted the working fluids, was carried out to understand the phenomenology of the inclined inverted-shroud separator. Different annular-duct flows were observed: upward gas-liquid flow, single-phase open channel flow, downward gas-liquid flow due to the phenomenon of aeration; also new data of separation efficiency were collected as a function of inclination angle, liquid flow rate and pressure drop between the shroud and production pipe outlet. One of the most significant findings is that the liquid level of the inner annular channel (NAI) of the shroud is a very important variable in the phenomenon of separation; so far this was not reported in the literature. Based on the observations, a phenomenological model that predicts total gas separation is proposed. A new correlation for the modeling of the dissipation of turbulent kinetic energy associated with the gas separation is presented. The model was qualitatively compared with available data from the literature and quantitatively adjusted against the new experimental data obtained in this work, and the agreement was quite good.

Parmi les différents outils de laboratoire, les réacteurs agités triphasiques à panier catalytique sont souvent utilisés pour l'acquisition de données cinétiques avec des catalyseurs mis en forme. Malgré sa large utilisation, très peu d'auteurs se sont intéressés à la caractérisation de l'hydrodynamique et des transferts de matière de ces outils. Or, dans les cas de réactions rapides, des limitations hydrodynamiques et/ou au transfert peuvent conduire à des difficultés à discriminer les catalyseurs ou à obtenir des paramètres cinétiques. L'objectif de cette étude était de connaître le domaine d'applicabilité de ces outils et présenter des pistes d'optimisation. Une méthodologie de caractérisation qui couple une étude hydrodynamique et une étude de transfert de matière a été mise en place. L'étude hydrodynamique a permis d'établir une cartographie des régimes d'écoulement gaz/liquide selon les conditions opératoires et les configurations géométriques. Cette étude a permis d'expliquer les résultats obtenus au niveau du transfert de matière, notamment, l'influence de la présence du panier et des bulles de gaz. Dans la configuration actuelle, cet outil semble être limité par le transfert liquide/solide. Ainsi, avec ce système, des études cinétiques pour des réactions avec une constante cinétique cin k pouvant aller jusqu'à 0,02 s-1 pourront être réalisées. Au-delà, l'outil sera inadapté. Pour améliorer cet outil de test, il faut optimiser le réacteur en modifiant le design du panier et de la turbine, et le diamètre de la cuve de manière à maximiser la vitesse radiale à travers du milieu poreux. Il faut aussi éviter la présence d'un régime de contournement du panier par le liquide / Stationary catalytic basket stirred reactors are often used among the various three-phase laboratory reactors for primary screening of commercial shaped catalysts. Until today, hydrodynamics and mass transfer studies concerning the impact of the presence of the basket in the flow are scarce which can be an obstacle to catalyst screening mainly in the case of fast reactions. The aim of this study is to know the range of applicability of these devices and optimize them if necessary. A characterization methodology that couples hydrodynamics and mass transfer was developed. The hydrodynamic studies allowed establishing a flow regime map of the gas/liquid flow for different reactor designs and operational conditions. This study has allowed as well understanding the influence of the basket and gas bubbles on gas/liquid and solid/liquid mass transfer. For the studied reactor, the liquid/solid mass transfer is the limiting phenomena. This system can however be used for catalyst screening for reaction rate constants smaller than 0.02 s-1. For faster reactions, these devices must be improved by changing the design of basket and impeller and the tank diameter. The optimized configuration should improve de radial flow through the porous medium and avoid the flow bypassing around the basket

L’extrapolation des réacteurs catalytiques nécessite l’acquisition des données cinétiques sur des réacteurs à petite échelle dans les conditions opératoires industrielles. Le critère de dimensionnement utilisé lors de la réduction d’échelle est la conservation de la vitesse volumique horaire, ce qui conduit à des vitesses de circulation très faibles dans les réacteurs pilotes à lit-fixe. A ces vitesses, les flux de transfert de matière externes peuvent devenir limitant par rapport au flux de réaction. Dans ce contexte, une nouvelle géométrie de réacteur a été imaginée pour intensifier les transferts de matière et chaleur et pour augmenter les vitesses de circulation des fluides : le réacteur "filaire". Il s’agit d’un réacteur dont le diamètre est égal ou proche de celui des grains de catalyseur et avec un ratio longueur sur diamètre très élevé. Le principal objectif de cette thèse est de caractériser ce réacteur en termes d’hydrodynamique et de transferts de matière externes pour définir ses limites d’utilisation. En écoulement diphasique, ce réacteur est relativement piston et la rétention liquide est élevée, ce qui permet d'assurer un mouillage total du catalyseur. En ce qui concerne les vitesses des transferts de matière externe, celles-ci sont proches de celles d'un réacteur agité avec panier et sont supérieures à celles caractéristiques d'un réacteur pilote à lit-fixe conventionnel. Cette observation est liée à l’augmentation des vitesses locales du liquide et à la présence d'un écoulement du type Taylor modifié. En conclusion, le réacteur "filaire" constitue une alternative efficace aux réacteurs pilotes à lit-fixe pour l’étude de catalyseurs mis en forme. / Small size fixed-bed reactors are a common choice for testing industrial supported catalyst under industrial operating conditions. The most common criterion for reactor’s scale-down is based on the conservation of the liquid hourly space velocity which leads to a very low fluid flow velocity at the laboratory scale. Under these conditions, the external mass transfer flux can become the limiting step of the process. In this context, a new reactor geometry was proposed to intensify mass and heat transfers and to increase fluid flow velocities: the single pellet string reactor. This reactor is composed of a tube with an internal diameter close to that of the catalyst particles and with a high length over diameter ratio. The main goal of this thesis is to characterise the hydrodynamic and external mass transfer performances of this new reactor in order to define its application domain. In two-phase gas-liquid flow, the reactor flow is plug flow and the liquid hold-up values are high, which insures a complete wetting of the catalyst particles. The mass transfer coefficients were quantified and the measured rates are much higher than those observed in conventional pilot fixed-bed reactors, which can be explained by the increased local liquid velocities and by the modified Taylor flow regime. Catalytic tests with a very fast model reaction revealed that the external mass transfer performances of the single pellet string reactor are close to those measured in a stirred tank reactor equipped with a catalytic basket. In conclusion, the single pellet string reactor represents a new and efficient alternative to fixed-bed pilot reactors to study shaped catalysts.