Please read the abstract in the dissertation / Dissertation (MEng)--University of Pretoria, 2012. / gm2014 / Chemical Engineering / unrestricted

Trickle bed reactors (TBRs) exhibit complex hydrodynamics and this study is aimed at giving insight into whether liquid-solid mass transfer and wetting are influenced by bed depth in a 5 m trickling column using 4 mm glass spheres as random packing. Measurements were made using the novel electrochemical technique developed by Joubert and Nicol (2013). Using this technique the wetting and mass transfer could be measured simultaneously. The study proves that the liquid-solid mass transfer and wetting efficiency do not stabilise at a minimum bed depth. The parameters were found to continue decreasing until the bottom of the bed. For the upper branch of the hydrodynamic envelope, the rate of decrease for the wetting efficiency was slow at the top of the bed and decreased rapidly closer to the bottom. However, only the wetting efficiency decreased significantly as a function of bed length; the liquid-solid mass transfer exhibited only a slight decrease of 14%. This compared well with the results of Du Toit et al. (2014), who found an 11% decrease in the liquid-solid mass transfer in a column with an x/D value of 29,4. The lower branch of the hydrodynamic envelope showed a linear decrease with respect to bed length for both wetting and mass transfer. The liquid-solid mass transfer decreased by 50% from the top of the bed to the bottom. These results are also in agreement with those of Du Toit et al. (2014)1 who found a decrease of 30% for a 1,6 m column. The wetting efficiency for the Levec mode decreased by 52%, whereas Du Toit et al. (2014)2 found a decrease of 20%. / Dissertation (MEng)--University of Pretoria, 2014. / lk2014 / Chemical Engineering / MEng / Unrestricted

The limiting current technique has widely been used to study liquid-solid mass transfer in various reactor configurations. In the present contribution several underlying physical aspects have been investigated in order to improve the design of mass transfer experiments. Experimentally, the significant influence of electrolyte composition and hydrodynamic conditions have been studied and quantified to ensure conditions of high reproducibility. In the course of single phase COMSOL simulations, different electrode configurations have been examined with emphasis on concentration fields and electric current distribution showing a large sensitivity of the experimental configuration on the absolute current values.

The limiting current technique has widely been used to study liquid-solid mass transfer in various reactor configurations. In the present contribution several underlying physical aspects have been investigated in order to improve the design of mass transfer experiments. Experimentally, the significant influence of electrolyte composition and hydrodynamic conditions have been studied and quantified to ensure conditions of high reproducibility. In the course of single phase COMSOL simulations, different electrode configurations have been examined with emphasis on concentration fields and electric current distribution showing a large sensitivity of the experimental configuration on the absolute current values.

Several types of reactors are encountered in industry where reagents in a gas and a liquid phase need to be catalysed by a solid catalyst. Common reactors that are used to this end, are trickle-bed reactors, where gas and liquid flow cocurrently down a packed bed of catalyst. Apart from the catalytic process itself, several mass transfer steps can influence the rate and/or selectivity of a solid catalysed gas-liquid reaction. In trickle-bed reactors, flow morphology can have a major effect on these mass transfer steps. This study investigates the interaction between liquid flow morphology and mass transfer in trickle-bed reactors from three different angles. The primary focus is on liquid-solid mass transfer and internal diffusion as affected by the contacting between the liquid and the catalyst. First, the contacting between the liquid and the solid in trickleflow, or wetting efficiency, is characterised using colorimetry. Though this investigation is limited to the flow of nitrogen and water over a packed bed at ambient conditions, it provides useful information regarding liquid flow multiplicity behaviour and its influence on the distribution of fractional wetting on a particle scale. The colorimetric study also provides descriptions of the geometry of the liquid-solid contacting on partially wetted particles. These are used in a second investigation, for the numerical simulation of reaction and diffusion in partially wetted catalysts. This second investigation uses numerical simulations to evaluate and develop simple theoretical descriptions of liquid-solid contacting effects on catalyst particle efficiency. Special attention is given to the case where external and intraparticle mass transfer rates of both a volatile and non-volatile reagent affect the overall rate of reaction. Also, since these are not often considered in theoretical studies, some suggestions are made for the evaluation of the particle efficiency of eggshell catalyst. Finally, liquid-solid contacting is investigated in a high-pressure pilot reactor. Wetting efficiency is measured with a useful technique that does not rely on descriptions of particle kinetics or liquid-solid mass transfer rates. Liquid-solid mass transfer coefficients are also measured and results agree well with the colorimetric investigation, suggesting the existence of different types of flow within in the hydrodynamic multiplicity envelope of trickle-flow. Since it consists of different investigations of liquid-solid contacting from different angles, the study highlights several aspects of liquid-solid contacting and how it can be expected to influence trickle-bed reactor performance. / Thesis (PhD)--University of Pretoria, 2009. / Chemical Engineering / unrestricted

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