Rozprawy doktorskie na temat „Liquid holdups”

Agitated dispersions of one liquid in another immiscible liquid are widely used in chemical industry in operations such as liquid-liquid extraction, suspension polymerisation, and blending of polymers. When holdup of the dispersed phase is increased, in an effort to increase the productivity, at a critical holdup, the dispersed phase catastrophically becomes the continuous phase and vice versa. This phenomenon is known as phase inversion. Although the inversion phenomenon has been studied off and on over the past few decades, the mechanism of phase inversion (PI) has yet not become clear. These studies have however brought out many interesting aspects of PI, besides unravelling the effect of physical and operational variables on PL Experiments show that oil-in-water (o/w) and water-in-oil (w/o) dispersions behave very differently, e.g water drops in w/o dispersions contain oil droplets in them, but oil drops in o/w dispersions contain none, dispersed phase hold up at which inversion occurs increases with agitation speed for w/o dispersions but decreases for o/w dispersions. A common feature of both types of dispersions however is that as agitation speed is increased to high values, inversion holdups reach a constant value. A further increase in agitation speed does not change inversion hold up. Although this finding was first reported a long time ago, the implications it may have not received any attentions. In fact, the work reported in the literature since then does not even mention it. The present work shows that this finding has profound implications. Starting with the finding that at high agitation speed inversion hold up does not change with agitation speed, the present work shows that inversion hold up also does not change with agitator diameter, type of agitator and vessel diameter. In these experiments, carried out in agitated vessel, energy was introduced as a point source. The experiments carried out with turbulent flow in annular region of two coaxial cylinders, inner one rotating, in which energy is introduced nearly uniformly throughout the system, show that the inversion holdup remains unchanged. These results indicate that constant values of inversion holdups for a given liquid-liquid systems (o/w and w/o) are properties of the liquid-liquid systems alone, independent of geometrical and operational parameters. A new hypothesis is proposed to explain the new findings. Phase inversion is considered to occur as a result of imbalance between breakup and coalescence of drops. Electrolytes, which affect only coalescence of drops, were therefore added to the system to investigate the effect of altering coalescence of drops on phase inversion. The experiments performed in the presence of electrolyte KI at various concentrations indicate that addition of electrolyte increases the inversion holdup for both o/w and w/o dispersions for three types of systems: non polar-water, polar-water and immiscible organic-organic. Higher the concentration of electrolyte used, higher was the holdup required for phase inversion. These findings indicate that while the addition of electrolyte increases coalescence of drops in lean dispersions, it has exactly opposite effect on imbalance of breakage and coalescence of drops at high holdups near phase inversion point. The opposite effect of electrolytes in lean and concentrated dispersions could be explained qualitatively, but only in part in the light of a new theory, involving multi-particle interactions. The phase inversion phenomenon is quantified in a simple manner by testing the breakage and coalescence rate expressions available in literature. It has been found that, equilibrium drop size (where breakage and coalescence events are in dynamic equilibrium) approaches infinity near phase inversion holdup which is not an ex perimentally observed fact. To capture the catastrophic nature of phase inversion, two steady state approach is proposed. The two steady states namely the stable steady state and unstable steady state, are achieved by modifying the expression for coalescence frequency on the basis of (i) shear coalescence mechanism and, (ii) recognising the fact that at high dispersed phase holdup the droplets are already in contact with each other at all times and hence rendering the second order coales cence process to a first order one. Using two steady states approach, catastrophic phase inversion is shown to occur at finite drop size.

Agitated dispersions of one liquid in another immiscible liquid are widely used in chemical industry in operations such as liquid-liquid extraction, suspension polymerisation, and blending of polymers. When holdup of the dispersed phase is increased, in an effort to increase the productivity, at a critical holdup, the dispersed phase catastrophically becomes the continuous phase and vice versa. This phenomenon is known as phase inversion. Although the inversion phenomenon has been studied off and on over the past few decades, the mechanism of phase inversion (PI) has yet not become clear. These studies have however brought out many interesting aspects of PI, besides unravelling the effect of physical and operational variables on PL Experiments show that oil-in-water (o/w) and water-in-oil (w/o) dispersions behave very differently, e.g water drops in w/o dispersions contain oil droplets in them, but oil drops in o/w dispersions contain none, dispersed phase hold up at which inversion occurs increases with agitation speed for w/o dispersions but decreases for o/w dispersions. A common feature of both types of dispersions however is that as agitation speed is increased to high values, inversion holdups reach a constant value. A further increase in agitation speed does not change inversion hold up. Although this finding was first reported a long time ago, the implications it may have not received any attentions. In fact, the work reported in the literature since then does not even mention it. The present work shows that this finding has profound implications. Starting with the finding that at high agitation speed inversion hold up does not change with agitation speed, the present work shows that inversion hold up also does not change with agitator diameter, type of agitator and vessel diameter. In these experiments, carried out in agitated vessel, energy was introduced as a point source. The experiments carried out with turbulent flow in annular region of two coaxial cylinders, inner one rotating, in which energy is introduced nearly uniformly throughout the system, show that the inversion holdup remains unchanged. These results indicate that constant values of inversion holdups for a given liquid-liquid systems (o/w and w/o) are properties of the liquid-liquid systems alone, independent of geometrical and operational parameters. A new hypothesis is proposed to explain the new findings. Phase inversion is considered to occur as a result of imbalance between breakup and coalescence of drops. Electrolytes, which affect only coalescence of drops, were therefore added to the system to investigate the effect of altering coalescence of drops on phase inversion. The experiments performed in the presence of electrolyte KI at various concentrations indicate that addition of electrolyte increases the inversion holdup for both o/w and w/o dispersions for three types of systems: non polar-water, polar-water and immiscible organic-organic. Higher the concentration of electrolyte used, higher was the holdup required for phase inversion. These findings indicate that while the addition of electrolyte increases coalescence of drops in lean dispersions, it has exactly opposite effect on imbalance of breakage and coalescence of drops at high holdups near phase inversion point. The opposite effect of electrolytes in lean and concentrated dispersions could be explained qualitatively, but only in part in the light of a new theory, involving multi-particle interactions. The phase inversion phenomenon is quantified in a simple manner by testing the breakage and coalescence rate expressions available in literature. It has been found that, equilibrium drop size (where breakage and coalescence events are in dynamic equilibrium) approaches infinity near phase inversion holdup which is not an ex perimentally observed fact. To capture the catastrophic nature of phase inversion, two steady state approach is proposed. The two steady states namely the stable steady state and unstable steady state, are achieved by modifying the expression for coalescence frequency on the basis of (i) shear coalescence mechanism and, (ii) recognising the fact that at high dispersed phase holdup the droplets are already in contact with each other at all times and hence rendering the second order coales cence process to a first order one. Using two steady states approach, catastrophic phase inversion is shown to occur at finite drop size.

Gravity driven trickle flow of a liquid over a fixed bed in the presence of a gaseous phase is widely encountered throughout the process industry. It is one of the most common ways of contacting multi-phase fluids for reaction or mass transfer purposes. The presence of three phases greatly complicates the mathematical modelling of trickle-bed reactors and makes a description from first principles difficult. Trickle flow performance is usually characterized in terms of hydrodynamic parameters. One such parameter is the liquid holdup. The value and morphology (shape or texture) of the holdup influences the catalyst contacting, wetting, mass transfer characteristics and ultimately the performance of the trickle flow unit. This study is limited to the air-water-glass spheres system with no gas flow. It is partitioned into three sections. An investigation into the nature of the residual liquid holdup in beds of spherical particles revealed that the general assumption that all residual liquid is held in the form of pendular rings at particle contact points proves to be untrue. Instead, indication is that 48 % of the residual holdup is present in the form of agglomerated liquid globules in interstices of low local porosity. Theoretical residual liquid holdup models and residual liquid holdup-based mass transfer models should include this phenomenon. In a subsequent section, the influence of the prewetting procedure on the operating holdup is investigated. Three distinct limiting cases are identified: Kan-wetted, Levec-wetted and non-wetted. A volumetric utilization coefficient that describes the extent to which the bed is irrigated is developed. It indicates that large fractions of the bed remain non-irrigated in the Levec- and non-wetted modes. A momentum balance-based model is adopted to predict the Kan-wetted mode holdup. This model was successfully extended to predicting the holdup in the Levec- and non-wetted modes by simple incorporation of the volumetric utilization coefficient. The predictive capability of this model is highly satisfactory, especially in light of it using only the classical Ergun constants and no fitted parameters (AARE = 9.6 %). The differences in the hysteresis behaviour of holdup and pressure drop in the different modes are attributed to differences in the morphology of the operating holdup. The existence of the three limiting prewetted modes is confirmed by residence time distribution (RTD) analysis of the stimulus-response behaviour of the system. This behaviour was quantified using a NaCl tracer and conductivity measurements at both the inlet and outlet of a bench scale bed. The analyses show that: · There are large fractions of the holdup that is inaccessible to the tracer in the Levec-wetted and non-wetted modes. · The mixedness in the three prewetted modes differ appreciably, with the Kan-wetted mode clearly less mixed than the Levec-wetted mode. The RTD analyses also confirm the existence of the three prewetting modes in a porous system (spherical a-alumina), with a large fraction of the holdup being inaccessible to the tracer in the Levec-wetted mode. This study emphasizes the role of the morphology of the various types of liquid holdup on the hydrodynamic performance of a trickle flow unit. It is apparent that aspects of the morphology depend strongly on phenomena like globule formation, hysteresis and flow and prewetting history that have not been adequately recognized to date. The visualization of the various modes of trickle flow is an intellectual platform from which future studies may be directed.
Dissertation (MEng)--University of Pretoria, 2004.
Chemical Engineering
Unrestricted

L'etude experimentale porte sur plusieurs systemes : 4 types de solides (3 supports de catalyseur et des billes de verres, 2 liquides (eau ou cyclohexane) et 2 gaz (air ou azote) ainsi que 2 tailles de colonne (5 ou 15 cm). On suit l'influence de ces parametres sur le regime d'ecoulement, la retenue liquide et gazeuse et sur les pertes de charges

Le fonctionnement de contacteurs gaz-liquide alimentés en gaz par différentes turbines autoaspirantes associées à un arbre creux a été étudié. Une étude comparative a été menée sur l'hydrodynamique globale et locale du réacteur de 0,6 m de diamètre équipé des turbines suivantes: (1) turbine carénée radiale de D/T = 1/3 ; (2) turbine Rushton modifiée D/T = 0,25 ; D/T = 1/3 ; D/T = 0,4 ; (3) turbine à pales inclinées modifiée D/T = 0,35. Plusieurs techniques de mesure de l'hydrodynamique locale ont été testées dans l'optique d'une modélisation du réacteur en terme de modèle de zones interconnectées. L’étude de la turbine carénée a montré que le réacteur ne peut pas être décrit par un modèle de réacteur parfaitement agité du fait de la mauvaise circulation du liquide liée à la géométrie de la turbine. Ainsi, les caractéristiques du champ de vitesses des deux phases ont été déterminées et ont permis de délimiter les différentes zones du réacteur. L'étude concernant la turbine Rushton autoaspirante a permis de mettre en évidence les différents régimes de fonctionnement liés aux phénomènes de formation de différents types et structures de cavités derrière les pales du mobile ; les transitions de régime étant alors identifiées grâce au nombre de Froude modifié. En ce qui concerne l'hydrodynamique locale de ce mobile, la cartographie des vitesses des deux phases a montré que le caractère parfaitement agité correspondait à un volume du réacteur nettement plus important que dans le cas du mobile précédent. La géométrie de la turbine à pales inclinées défavorise l'entrainement du gaz par autoaspiration mais offre en revanche une bonne circulation du liquide à puissance consommée modérée. Ce mobile peut être de ce fait aussi attractif que les deux turbines précédentes

On étudie la faisabilité du procédé de biolixiviation de minerais sulfurés métalliques dans une colonne à bulles à solide suspendu, à priori moins énergivore que le réacteur mécaniquement agité utilisé habituellement dans l'industrie minéralurgique. Afin de donner les bases du dimensionnement du réacteur, des études hydrodynamique, de transfert de matière gaz-liquide et de transfert de chaleur à un tube de transfert ont été réalisées sur une colonne pour deux types de particules (billes de verre et particules de pyrite). La rétention de gaz est plus petite en milieu triphasique qu'en système biphasique et elle n'est pas affectée par la vitesse du liquide ni par la concentration en solide dans la colonne. Même aux plus grandes vitesses utilisées, la concentration en billes de verre n'est pas homogène dans le réacteur. Pour la pyrite, aux vitesses de gaz élevées, le profil de concentration en solide est moins marqué, alors qu'aux faibles vitesses, les particules se concentrent en bas de colonne. L’écoulement de la phase liquide peut être modélisé par deux réacteurs agités en série. Pour le transfert de chaleur sonde/lit, on constate une différence de comportement entre les deux types de particules. Dans le cas des billes de verre, les valeurs de h sont plus faibles qu'en colonne à bulles ; dans le cas de la pyrite, le coefficient de transfert de chaleur est plus important qu'en système eau-air. L’étude du transfert de matière gaz-liquide a montré que les valeurs du coefficient volumique de transfert de matière gaz-liquide en milieu triphasique sont plus faibles qu'en gaz-liquide. Enfin, quelques essais en maquette chaude ont montré que la réaction de biolixiviation avait bien lieu dans la colonne à bulles à solide suspendu, même à des concentrations en solide assez élevées. Au maximum de la croissance des micro-organismes, la réaction de biolixiviation est limitée par le transfert de matière gaz-liquide

U okviru doktorske disertacije izvedena su eksperimentalna istraživanja osnovnih hidrodinamičkih i maseno-prenosnih karakteristika airlift reaktora sa spoljnom recirkulacijom sa ugrađenom višekanalnom cevnom membranom u silaznu cev (ALSRM). ALSRM je radio na dva načina rada: bez mehurova u silaznoj cevi (način rada A) i sa mehurovima u silaznoj cevi (način rada B) u zavisnosti od nivoa tečnosti u gasnom separatoru. Ispitivani su uticaji prividne brzine gasa, površinskih osobina tečne faze, tipa distributora gasa i prisustva mehurova gasa u silaznoj cevi na sadržaj gasa, brzinu tečnosti u silaznoj cevi i zapreminski koeficijent prenosa mase u tečnoj fazi u ALSRM. Rezultati su poređeni sa vrednostima dobijenim u istom reaktoru ali bez membrane (ALSR). Sadržaj gasa u uzlaznoj i silaznoj cevi određivan je pomoću piezometarskih cevi merenjem hidrostatičkog pritiska na dnu i vrhu uzlazne i silazne cevi. Brzina tečnosti merena je pomoću konduktometrijskih elektroda dok je zapreminski koeficijent prenosa mase dobijen primenom dinamičke metode merenjem promene koncentracije kiseonika u vremenu optičkom elektrodom. Eksperimentalni rezultati pokazuju da sadržaj gasa, brzina tečnosti i zapreminski koeficijent prenosa mase zavise od prividne brzine gasa, vrste alkohola i tipa distributora gasa kod oba reaktora. Višekanalna cevna membrana u silaznoj cevi uzrokovala je povećanje ukupnog koeficijenta trenja za 90% i time dovela do smanjenja brzine tečnosti u silaznoj cevi do 50%. Smanjena brzina tečnosti u silaznoj cevi povećala je sadržaj gasa do 16%. Predložene neuronske mreže i empirijske korelacije odlično predviđaju vrednosti za sadržaj gasa, brzinu tečnosti i zapreminski koeficijent prenosa mase.
An objective of this study was to investigate the hydrodynamics and the gas-liquid mass transfer coefficient of an external-loop airlift membrane reactor (ELAMR). The ELAMR was operated in two modes: without (mode A), and with bubbles in the downcomer (mode B), depending on the liquid level in the gas separator. The influence of superficial gas velocity, gas distributor’s geometry and various diluted alcohol solutions on hydrodynamics and gas-liquid mass transfer coefficient of the ELAMR was studied. Results are commented with respect to the external loop airlift reactor of the same geometry but without membrane in the downcomer (ELAR). The gas holdup values in the riser and the downcomer were obtained by measuring the pressures at the bottom and the top of the riser and downcomer using piezometric tubes. The liquid velocity in the downcomer was determined by the tracer response method by two conductivity probes in the downcomer. The volumetric mass transfer coefficient was obtained by using the dynamic oxygenation method by dissolved oxygen probe. According to experimental results gas holdup, liquid velocity and gas-liquid mass transfer coefficient depend on superficial gas velocity, type of alcohol solution and gas distributor for both reactors. Due to the presence of the multichannel membrane in the downcomer, the overall hydrodynamic resistance increased up to 90%, the liquid velocity in the downcomer decreased up to 50%, while the gas holdup in the riser of the ELAMR increased maximally by 16%. The values of the gas holdup, the liquid velocity and the gas-liquid mass transfer coefficient predicted by the application of empirical power law correlations and feed forward back propagation neural network (ANN) are in very good agreement with experimental values.

L’objectif de notre recherche est d'une part, l'approfondissement des connaissances hydrodynamiques et de transfert gaz-liquide d'un réacteur triphasé mécaniquement agité et d'autre part, la recherche des conditions optimales des réactions d'hydrogénolyse catalytique du glucose et de l'amidon. Les caractéristiques hydrodynamiques et du transfert gaz-liquide sont obtenues à l'aide de méthodes physiques en régime dynamique. Les paramètres étudiés sont: le débit gazeux, la vitesse d'agitation et la pression totale. Les grandeurs déduites sont: l'état du macro-mélange, le taux de rétention de la phase gazeuse et la conductance globale du transfert gaz-liquide. L’hydrogénolyse directe d'une suspension aqueuse d'amidon avec un catalyseur à base ruthénium imprégné sur charbon actif peut être effectuée en présence d'acide acétique. La sélectivité de la transformation en glycérol et diols dépend du cation associe à l'ion acétate. Les meilleurs résultats sont obtenus pour les cations calcium et sodium associé à l'ion acétate. Une addition supplémentaire d'hydroxyde de calcium est également favorable. Toutefois, dans les conditions opératoires étudiées, le catalyseur au ruthénium reste peu sélectif et il conduit en parallèle à la formation d'acides organiques. Enfin, l'hydrogénolyse directe de l'amidon, en présence d'acide acétique, peut être modélisée convenablement à l'aide d'un modèle cinétique phénoménologique ou les quatre étapes réactionnelles sont du 1er ordre

The lower part of the iron-making blast furnace is very important for operational stability and productivity of it. It represents a multiphase flow, including a cohesive zone. In order to reduce the coke consumption and hence reduce the environmental problems, pulverized coal is injected laterally through the tuyeres. This multiphase (gas-powder-liquid) flow system has been studied at room temperature in the presence of a raceway using new methods to estimate the static and dynamic powder holdups. Reproducible results have been obtained, which were lacking in the previous studies carried out by other researchers. The effect of powder size, gas flow rate, packing size, and liquid flow on fluid flow behaviour and powder holdup has been studied in detail. The pressure drop profiles were compared and explained for both the rectangular and cylindrical beds. Correlations for the static and dynamic powder holdups have been proposed for the various conditions. One of the aims was to study the combination of cross and counter-current flow in a multiphase system and quantify the liquid and powder holdups in presence of raceway. The study revealed localized flooding in the packed bed, which increased with the gas and liquid flow rates. The quantification of holdups revealed higher liquid accumulation and low powder accumulation with increase in the gas velocities.

碩士
國立臺灣大學
化學工程學研究所
90
This experiment was focused on local gas holdup distribution in three-phase fluidized beds. The fluidized bed is consisted of Plexiglas cylindrical column of 50 cm high and 8 cm i.d.. The particles used are glass bead with average diameter of 1.7 mm. Water and CMC (carboxymethyl cellulose) solution (0.3wt%, 0.5wt% and 1.0wt%) are used as liquid phase. Air is used as gas phase. Optical fiber method and valve technique are used to detect radial gas holdup and overall gas holdup individually. The effect of different superficial gas velocities, superficial liquid velocities and concentration of CMC solutions on local gas holdup distribution was studied. The result shows that local gas holdup in gas-liquid-solid three-phase fluidized beds was increased with increasing superficial gas velocity and decreasing superficial liquid velocity and viscosity of liquid phase. Furthermore, shape and position of sparger in the column affected gas holdup distribution. Two higher gas holdup values near the wall of column detected by optical fiber probe in 0.5wt% CMC and 1.0 wt% CMC system were affected by the sparger. The higher viscosity liquid phase system was influenced by the sparger obviously.

The prewetting of a trickle-bed reactor has important implications in the design and operation of these reactors. This is because the prewetting changes the flow morphology (shape and texture) of the liquid flowing through the bed and leads to the existence of multiple hydrodynamic states. The extent of this change in flow morphology can be seen in the effect the prewetting of the reactor has on the pressure drop, liquid holdup and gas-liquid mass transfer. The following prewetting procedures were used: -- Levec-wetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flow is reintroduced -- Kan-wetted: the bed is operated in the pulse flow regime and liquid and gas flow rates are reduced to the desired set point -- Super-wetted: the bed is flooded and gas and liquid flow are introduced once draining commences For the pressure drop: -- The different prewetting procedures resulted in two distinct regions (Upper region Kan and Super-wetted, Lower region Dry and Levec-wetted) -- There was no significant difference between the Dry and Levec-wetted beds -- The pressure drop in the Kan and Super-wetted beds can be as much as seven times greater than the pressure drop in the Dry and Levec-wetted beds For the liquid holdup: -- The different prewetting procedures resulted in four distinct regions (Kan-wetted, Super-wetted, Levec-wetted, Dry bed) -- The liquid holdup in the Kan-wetted bed can be as much as four times greater than the liquid holdup in the Dry bed -- The liquid holdup in the Levec-wetted can be as much as thirty percent lower than the liquid holdup in the Kan-wetted bed For the gas-liquid mass transfer: -- The different prewetting procedures resulted in three distinct regions (Kan and Super-wetted, Levec-wetted, Dry bed) -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as six times greater than the mass transfer coefficient in the Dry bed -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as two and a half times greater than the mass transfer coefficient in the Levec-wetted bed While an increase in the liquid flow rate results in an increase in the pressure drop, liquid holdup and gas-liquid mass transfer for all of the experiments, the effect of increasing gas flow on the measured variables were more pronounced for the prewetted beds. In a prewetted bed (Kan, Super and Levec-wetted) an increase in the gas flow rate causes an increase in the volumetric gas-liquid mass transfer coefficient and a decrease in the liquid holdup. The decrease in the liquid holdup is due to the fact that the increased gas flow rate causes the films around the particles to thin and spread out. In the dry bed the flow is predominantly in the form of rivulets and the increase in gas flow rate does not affect the liquid holdup. In the case of the volumetric gas-liquid mass transfer coefficient the increased gas flow rate causes an increase in the mass transfer coefficient regardless of the prewetting procedure. This increase is due to the effect that the gas flow rate has on the liquid holdup as well as the increase in the gas-liquid interfacial area due to the increased gas-liquid interaction. If the pulsing in the Kan-wetted bed is induced by increasing the gas flow rate and keeping the liquid flow rate constant the results are significantly different. The pressure drop in the gas-pulsing experiments was lower than the pressure drop in the recorded in the Kan and Super-wetted beds, but higher than the pressure drop in the dry and Levec-wetted beds. However, the liquid holdup in the gas-pulsing experiments was higher than the liquid holdup in any of the other beds. The volumetric gas-liquid mass transfer coefficient in the gas-pulsing experiments was lower than the mass transfer coefficients of the Kan and Super-wetted beds, but higher than the mass transfer coefficients in the dry and Levec-wetted beds. The multiple operating points obtained from the different prewetting procedures are by no means the only possible operating points. By simply decreasing the draining time in the Levec-wetted bed steady state operating points can be found between those of the Super and Levec-wetted beds. This alludes to the fact that the operating conditions determined from the different prewetting modes are only boundaries and that the actual operating point can lie anywhere between these boundaries. The existence of these multiple hydrodynamic states complicates things further when a correlation is developed to determine the pressure drop, liquid holdup or the volumetric gas-liquid mass transfer coefficient. No correlation tested was able to accurately predict the pressure drop, liquid holdup or volumetric gas-liquid mass transfer coefficient in the dry or prewetted beds.
Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2007.
Chemical Engineering
unrestricted

Trickle Bed Reactors have etched a ubiquitous presence in chemical processing sector. From petroleum and petrochemical products, fine chemicals to biochemical, wastewater treatment, they are almost everywhere. Products worth of 300 billion US $ are processed by these reactors on an annual average. A complete understanding of hydrodynamics, fluid phase mixing, interphase and interparticle heat and mass transfer and reaction kinetics of TBR can help us to extract the full potential of TBR. Studying the variation of pressure drop and liquid holdup is crucial for evaluation of performance of trickle bed reactors and can help in further optimizing their performance. This project focuses on the effect of gas and liquid velocities on the pressure drop and liquid holdup in a trickle-Bed reactor operating at ambient temperature and atmospheric pressure. Pressure drop and liquid holdup are two critical hydrodynamics parameters that influence other parameters directly and indirectly and hence, these two parameters are preferred for hydrodynamic study of TBR. Their variation along longitudinal and transverse direction is the focus of this project. A comparison of results from different simulation scenarios (using different pressure values as patching values) made in this project helps in understanding how different initial guess can affect the final solution in simulating real-life TBR operation. It is found that pressure ranging up to 10000 Pa as patching pressure value can lead to a converging solution. Afterwards, solution instability creeps in leading to impractically higher values of pressure and liquid holdup and sometimes ending up with divergence. Even the effect of gas and liquid velocity is studied on the two parameters. The variation of the two hydrodynamic parameters with changing liquid velocities and gas velocities are also studied.

The pressure drop, liquid holdup and overall mass transfer capacity have been studied in a graphite fibre electrode of porosity 0.90and fibre diameter 22 micrometer with cocurrent upward gas-liquid flow. A U-tube mercury manometer was used to measure the pressure drop in a graphite fibre bed 356 mm long by 38 mm wide by 3 mm thick. The gas and liquid were oxygen and water. For gas load 0 5 G 5 0.43kg/m2s and liquid load 1.46 5 L 5 7.30 kg/m2s, the pressure gradient ranged from 0.24 and 2.09 bar/m. The correlation for the pressure gradient is AP = L [ 0.36 + 1.182 (G/L ) 0.618 ]2 where AP = pressure gradient^bar/m L = liquid load^kg/m2s G = gas load^kg/m2s The quick closing valve method was used to measure the total liquid holdup in a graphite fibre bed 356 mm long by 38 mm wide by 3mm thick. Oxygen and 1M aqueous sodium hydroxide were the fluids used for the total liquid holdup measurements. For gas load 0 5 G 5 0.35 kg/m2s and liquid load 1.53 5 L 5 7.62 kg/m2s the liquid holdup ranged from 0.44 to 1.0. The correlation for the total liquid holdup is hL = 1 - 0.907 L 0.362 G 0.301 where hL = liquid holdup. The overall mass transfer capacity was determined by the electrochemical reaction method with the electro-reduction of oxygen to peroxide. The electrochemical reactor used consisted of graphite fibre cathode bed of dimension 89 mm long by 38 mm wide by3 mm thick. The cathode was separated from the anode by a cation membrane. Oxygen gas and 1M aqueous sodium hydroxide were the fluids used. For gas load 0.04 5 G 5 0.36 and liquid load 3.05 5L 5 7.62 the overall mass transfer capacity ranged from 3.4 to 9.0 -1 s . The correlation for the overall mass transfer capacity is Ka = 5 . 9 L0.371 G0.233 where K = overall mass transfer coefficient^m/s a = effective interfacial area for gas to solid^m-1 L = superficial liquid load^kg/m2s G = superficial gas load^kg/m2s

Residence time distributions have become an important analytical tool in the analysis of many types of flow systems. Residence time distributions have proven to be effective for analysing trickle bed reactors, as it allows determination of parameters under operating conditions allowing no interference of these conditions. By studying the residence time distribution a great amount of information can be obtained and therefore used to determine a number of hydrodynamic parameters. Due to recent findings that prewetting has a tremendous effect on a number of hydrodynamic parameters such as holdup, wetting efficiency and pressure drop, it is therefore the aim of this study to investigate the effect of trickle flow morphology or prewetting on a trickle bed reactor. The residence time distribution is obtained whereby hydrodynamic parameters are determined and therefore the effect the flow morphology has on various hydrodynamic parameters is highlighted. A number of methods were used to determine these parameters, namely that of the best-fit method, whereby the PDE model was used, and the method of moments. Operating conditions included varying gas and liquid flow rates for porous and non-porous catalyst particles at atmospheric pressure. The different prewetting procedures used during this work included the following:

• Non-wetted
• Levec-wetted
• Super-wetted

From this investigation the following conclusions were made:

Prewetting has a great effect on the hydrodynamic parameters of trickle bed reactors

The differences in prewetting can be attributed to differing flow morphologies for the different prewetted beds i.e. the dominant flow morphology for a non-wetted bed is that of rivulets and for prewetted beds that of film flow

It was also found that at low liquid flow rates the flow morphology in prewetted beds changes from film flow to a combination of rivulet and film flow

The different flow morphologies for prewetted and non prewetted beds was confirmed by the residence time distributions and various parameters obtained there from

At low liquid flow rates the flow morphology becomes a more predominant factor in creating the tailing effect present in residence time distribution for prewetted beds

The tailing effect in residence time distributions is a result of both internal diffusion and liquid flow morphology, where the liquid flow morphology is the more dominant factor

The use of residence time distributions to determine a number of hydrodynamic parameters proved to be very useful and accurate by means of different methods, i.e. method of moments and best-fit method

Differences in the liquid holdup determined from the method of moments and the weighing method confirmed that different flow morphologies exist for different prewetted beds

An increase in the dispersion coefficient with prewetting was observed indicating that the amount of micro mixing is different for the different prewetted beds

Differences in residence times and high values for the dynamic holdup, for the porous packing, confirmed that the PDE model does not model well the porous packing response curves due to the lack of internal diffusion and internal holdup in this model

The dynamic-static mass transfer showed that film flow, as in prewetted beds, results in slower mass transfer as opposed to rivulet flow and therefore it is concluded that prewetting results in different flow morphologies.

Following this study it is recommended that a residence time distribution model be used or developed that incorporates the effects of internal diffusion and internal holdup as present in porous catalyst particles. In addition, it was found that very few correlations could accurately predict hydrodynamic parameters due to the absence of the effect of prewetting and therefore it is recommended that correlations be developed that incorporate the effect of prewetting.
Dissertation (MEng)--University of Pretoria, 2008.
Chemical Engineering
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