Heterogeneous catalyzed macromolecular hydrogenations in oscillating systems

An examination of novel oscillatory (alternating gas and liquid phase) reactors and heterogeneous catalysts for multi-phase macromolecular reactions was carried out. A monolith-containing square die with Pd/Al2O3 catalysts was used to successfully hydrogenate poly(styrene) (PS). The inherent pulse behavior of the extruder was found to be sufficient to approach intrinsic kinetics at low polymer concentrations. At higher (10 wt% PS) concentrations, forced pulsing was shown to have a greater impact on observed reaction rates. Selectivity with this extruder-fed reactor was better than with a stirred tank in all cases, due to a more plug-flow-like residence time distribution. While accurate control over the exit distribution of forced pulses was difficult due to gas back mixing, an optimal frequency of forced pulsation was observed for the 10 wt% PS system. Mesoporous catalysts for macromolecular hydrogenations were synthesized and tested for PS hydrogenation. They were shown to be more active than microporous catalysts. The type of support was shown not to have a large influence on activity, but high dispersion of the active metal was critical. The addition of a second inactive metal did improve hydrogenation selectivity, but it was observed that having a chloride-free support is even more important in achieving high activity. For the hydrogenation of a low molecular weight species (α-methylstyrene) (AMS) in a piston oscillating monolith reactor (POMR), oscillations gave improvements in reaction rate of up to 84%. With no oscillations, the activity was still higher than in a stirred tank operated at an equivalent power per unit volume. Selectivity in the POMR was as good as or better than in a stirred tank. It was also found that the Pd crystallite size had a large influence on activity. For the hydrogenation of soybean oil, the POMR gave a higher activity than a stirred tank at identical conditions. The hydrogenation rate increased by as much as 112% with oscillations. It was shown that this improvement was unrelated to external mass transfer; rather it arised from improved intraparticle mass transfer limitations or surface wetting. Sereo-selectivity was largely unaffected by the reactor system but was instead dependent on intraparticle diffusion lengths

Hydrodynamics study of the bubble columns with intense vertical heat-exchanging tubes using gamma-ray computed tomography and radioactive particle tracking techniques

Understanding the hydrodynamics of bubble columns with and without vertical heat-exchanging tubes is a necessity for the proper design, scale-up, and operation of these reactors. To achieve this goal, systematic experiments were performed to visualize and quantify the influence of the presence of vertical internal tubes on the gas holdup distributions and their profiles, axial liquid velocity, and turbulent parameters (i.e., normal and shear stresses; turbulent kinetic energy) by using advanced gamma-ray computed tomography (CT) and radioactive particle tracking (RPT). In this study, the experiments were conducted in 6- and 18-inch bubble columns with an air-water system as the working fluid, under a wide range of superficial gas velocities (5-45 cm/s). Three configurations of vertical internals (i.e., hexagonal, circular without a central tube, and circular with a central tube plus vertical internals), as well as the vertical internals sizes, were examined in this study. These three configurations were designed to cover 25% of the column\u27s cross-sectional area (CSA) to represent the percentage of the covered area utilized in the Fischer-Tropsch process. Reconstructed CT images reveal that the configurations of the vertical internal tubes significantly impacted the gas holdup distribution over the CSA of the column. Additionally, the bubble column equipped with 1-inch vertical internals exhibited a more uniform gas holdup distribution than the column with 0.5-inch internals. Moreover, a remarkable increase in the gas holdup values at the wall region was achieved in the churn turbulent flow regime due to the insertion of vertical internals inside the column. Furthermore, pronounced peaks of the gas holdup and axial liquid velocity were observed in the inner gaps between the vertical internals --Abstract, page iv

Temperature control in a multi-tubular fixed bed Fischer-Tropsch reactor using encapsulated phase change materials

The Fischer-Tropsch synthesis is a highly exothermic, indirect, catalytic, gas (syngas) liquefaction chemical process. Temperature control is particularly critical to the process in order to ensure longevity of the catalyst, optimise the product distribution, and to ensure thermo-mechanical reliability of the entire process. This thesis proposes and models the use of encapsulated, phase change material, in conjunction with a supervisory temperature control mechanism, as diluents for the catalytic, multi-tubular fixed bed reactor in order to help mitigate the heat rejection challenges experienced in the process. The modelling was done using the Finite Element Analysis (FEA) software, COMSOL Multiphysics. In the main, three studies were considered in this thesis. In the first study, a two dimensional quasi-homogeneous, reactor model, without and with the dissipation of the enthalpy of reaction into a near isothermal phase change material (silica encapsulated tin metal) heat sink, in a wall-cooled, single-tube fixed bed reactor was implemented and the results were presented. The encapsulated phase change material was homogeneously mixed with the active catalyst pellets. The thermal buffering provided by the phase change material were found to induce up to 7% increase in selectivity towards the C5+ and a 2.5% reduction in selectivity towards CH4. Although there was a reduction in the conversion per pass of the limiting reactant and hydrocarbon productivity due to a reduction in reactor temperature, it was observed that for a unit molar reduction in the productivity of C5+, there was a corresponding 1.5 moles reduction in methane production. In the second study, a modified, one dimensional, α-model was derived which accounted for the heat sink effect of the phase change material diluent. The resulting, less computationally cumbersome, yet sufficiently accurate model was benchmarked against the more rigorous two-dimensional quasi-homogeneous model in order to check its fidelity in predicting the reactor performance. As in the first case study, a homogeneous distribution of the phase change material and active catalyst pellets was assumed. The α-model was able to approximate the reactor temperature profile of the 2D-quasi-homogeneous reactor model to within 4% error, and consistently, slightly over-predicted the limiting reactant conversion by about 3%. Based on these comparisons, the α-model was deemed sufficiently accurate to predict the reactor performance in place of the 2D model for the optimisation simulation in the third study. The third case study entailed simultaneously maximising the production of long chain hydrocarbon molecules and ensuring proper heat rejection from the reacting system, two desirable yet often conflicting operational requirements. The homogeneous distribution of the active catalyst pellets and the phase change material diluents was abandoned for a multi-zonal axial distribution in which, individual zones of the catalyst bed were diluted to varying extents. The best dilution and distribution “recipe” was determined using optimisation techniques and the previously derived modified α-model. The multi-zonal axial dilution of the catalyst bed brought about a marked increase (up to 19%) in the productivity of the long chain hydrocarbons, while ensuring a more judicious use of the catalyst bed in contrast to the homogeneous catalyst/phase change material arrangement in the previous two studies. The latent enthalpy of the metallic phase change material combined with its good thermal conductivity helped push the limits of the catalyst bed by increasing the conversion per pass beyond the typical 20-30% reported in literature, with less likelihood of either early catalyst deactivation or thermal unreliability of the reacting system. In the main, it was observed that the overall productivity of the desired C5+ could be enhanced by reducing the quantity of the catalyst pellets by a pre-defined reactor volume. In addition, the reactor productivity benefits from a highly active zone situated at the reactor entrance, immediately followed by a less reactive zone. This arrangement has the effect of ramping the reaction rate (and in effect the reactor temperature) early on, and this is kept in check by the less reactive zone immediately adjacent to the reactive one at the reactor entrance

Heterogeneously catalysed aerobic oxidation of alcohols in microstructured reactors

The goal of this thesis research was to develop microfluidic platforms for the study of gas-solid and gas-liquid-solid alcohol oxidation reactions. The desired products of these reactions are of great importance industrially due to their value as intermediates in industries such as the fine chemical and pharmaceutical sectors. The application of microreaction technology to these reactions is proving to be beneficial due to their high surface area-to-volume ratio, resulting in fast heat and mass transfer and an ability to circumvent problems such as high exothermicity, mass transfer limitations, and poor control of reaction conditions. Two types of reaction systems were developed to facilitate this research; a three-phase micro-packed bed reactor for the study of benzyl alcohol oxidation on supported gold-palladium catalyst and a wall-coated microreactor for the study of methanol oxidation to formaldehyde on silver catalyst. Reaction and deactivation flow studies were first conducted in continuous flow microfluidic setups to understand catalyst activation and deactivation behaviour, culminating in the selection of the most stable catalyst formulation. These reaction studies were followed by a series of hydrodynamic and mass transfer investigations, where differences in hydrodynamics to conventional macroscale systems were identified, and a classification of flow regimes applicable to micro-packed bed reactors presented. An understanding of the influence of hydrodynamics on mass transfer, catalyst deactivation and reaction performance has been developed for benzyl alcohol oxidation, resulting in enhancement in flow reactor performance in comparison to batch. Exploration of different microreactor designs, to cope with challenging process conditions, as well as the application of novel methods for reactor characterization (such as Raman spectroscopy) are also presented

Fluid Dynamics And Scale-Up Of Bubble Columns With Internals

Bubble columns and slurry bubble columns, as multiphase reactors, are favored for a wide range of applications in the chemical, biochemical, petrochemical and metallurgical industries. They are considered the reactor of choice for the Fischer Tropsch synthesis, among other applications, offering an alternative energy source and providing clean liquid fuels as compared to other reactors. Most of the industrial applications of bubble column reactors require the utilization of heat exchanging tubes, the effect of which on the reactor\u27s performance is not fully understood. This study proposes detailed investigations of selected local hydrodynamics in bubble columns with and without internal heat exchanging tubes. The main focus of this dissertation is to enhance the understanding of the phenomena associated with the local gas holdup and the bubble dynamics: specific interfacial area, frequency, velocity, and chord length) and their radial profiles via detailed experimentations by means of the four-point optical fiber probe as a measuring technique. In addition, the liquid phase mixing is investigated. The effects of the presence of cooling tubes, which are commonly used in industrial applications of bubble columns, are thoroughly investigated in columns of different diameters to assess the effect of scale. Based on the insights gained from the above, one of the main limitations in bubble columns, scale up, is to be tackled in this study. A new approach, yet simple, for designing the reactor in order to reduce the scale-up risk is developed making use of the necessary heat exchanging vertical internals in controlling the effect of scale through reactor compartmentalization leading to an optimized, yet efficient, design of large scale bubble columns. The main findings of this work can be summarized as follows: The impact of vertical internals on bubble dynamics and liquid phase mixing is assessed: Increase in gas holdup, interfacial area. Decrease in bubble size due to higher break-up rates. Enhancement in the large scale recirculation cells. Increase in the liquid phase mixing. The new scaling methodology was proposed and proven viable

Survey of gas-liquid mass transfer in bioreactors

Bioreactors are becoming more important in the production of biobased products such as proteins, medicines, and renewable fuels. The economic viability of these processes is dependent on the bioreactor\u27s ability to aid the microorganism and provide a friendly environment. One of the important microorganism requirements is proper gas concentrations so that the microorganism has the necessary inputs for proper metabolism. These gas concentrations are obtained and maintained through optimized gas-liquid mass transfer and mixing, also known as hydrodynamics. Other bioreactor responsibilities include damage mitigation and bioreactor volume utilization. A proper bioreactor design should also maximize profitability through ease of use, maintenance, and construction. This thesis work provides a survey of gas-liquid mass transfer theories, applications, and dependencies in major bioreactor and several novel designs. The major reactor designs include the stirred tank bioreactor, bubble column, airlift bioreactor, and fixed bed bioreactor. Variations of these major designs are also considered such as the slurry bubble column, internal- and external-loop airlift, draught-tube bioreactor, and trickle, packed, and flooded bed bioreactor. Since the microorganisms used in biological processes are diverse, a best or preferred bioreactor design is not feasible. Rather, bioreactor options can be presented based on the microorganism properties and production scale. Stirred tank bioreactors generally produce the largest gas-liquid mass transfer rates, but they also tend to cause high shear rates and variations, which can be very harmful to microorganisms. The impeller often limits the operating range, scale, process time, especially with non-Newtonian liquids. The bubble column and internal-loop airlift bioreactor have similar gas-liquid mass transfer rates; however, the bubble column has significant backmixing while the airlift bioreactor has lower bioreactor volume utilization. The external-loop airlift bioreactor provides more process and mixing control and generally has lower shear rates, but the attainable gas-liquid mass transfer rate and volume utilization tend to be lower. The fixed bed bioreactors protect and support microorganisms very well. On the other hand, the phase flow rates are much lower than in the other bioreactor designs. In other words, each bioreactor design has important advantages and disadvantages, and the microorganism may very well determine the optimal design. The bioreactor designs may be described as complementary rather than competitive. Each design and design variation has been implemented to fill a void caused by the original form. This design mentality has led to highly complex bioreactor relationships and the inability to identify the single best bioreactor because that was not the intent. Future research and development can be taken into two different directions. First, a design variation could be approached with the clear intent of superiority for biological processes. Such a device could possible use a mixture of airlift and stirred tank bioreactor attributes. Second, research could be oriented towards the continued niche creation. Each design improvement would be implemented with the intent of improving a certain bioreactor attribute or application with a specific type of microorganism. For example, the fixed bed bioreactor research could investigate new packing that would provide better support and shear protection for very sensitive microorganisms such mammalian cells

Novel reactors for multiphase processes

Process intensification tools, such as the capillary reactor, offer several benefits to the chemical process industries due to the well-defined high specific interfacial area available for heat and mass transfer, which increases the transfer rates, and due to low inventories, they also enhance the safety of the process. This has provided motivation to investigate three such tools, namely the capillary microreactor, spinning disc and rotating tube reactors, in this study.The gas-liquid slug flow capillary microreactor intensifies reactor performance through internal circulation caused by the shear between the continuous phase/wall surface and the slug axis, which enhances the diffusivity and consequently increases the reaction rates. However, integrating the complex hydrodynamics of this reactor with its chemical kinetics is a mathematically challenging task. Therefore, in this study, a simple-to-complex approach, using a set of state-of-the-art computational fluid dynamic tools, has been used. Firstly, simulations were performed without any chemical reaction to ascertain the extent of slug flow regime. The model also clearly captured the slug flow generation mechanism which can be used to structurally optimize the angle of entry in these reactors. Finally, the hydrodynamic model was also capable of estimating the pressure drop and slug lengths. After successfully simulating the hydrodynamics of the system, a reaction model was incorporated to study the chemical reaction kinetics. The results were compared with the published experimental work and were found to be in good agreement.The spinning disc reactor utilizes the centrifugal and shear forces to generate thin liquid films characterized with intense interfering waves. This enables a very high heat transfer coefficients to be realized between the disc and liquid, as well as very high mass transfer between the liquid and the bulk gas phase. The waves formed also produce an intense local mixing with very little back mixing. This makes a spinning disc reactor an ideal contactor for multiphase processes. The focus of this study has been to elucidate the hydrodynamic behaviour of the liquid film flow over the horizontal spinning disc. Investigations were also performed to elaborate the local and overall hydrodynamic characteristics of a fully developed spinning disc reactor. Simulation results showed a continuous linear liquid film on the horizontal spinning disc and intense mixing performance in the annulus of the reactor around the disc surface. Finally, the film thickness data from the simulations were compared with the limited amount of data available for this novel process.Rotating tube reactor also uses centrifugal forces to generate the liquid film and a high degree of mixing along with an improved control over the reactant retention times. In this work we have conducted a CFD analysis to understand the hydrodynamics of this new technology for future developments

Ultra-fast Imaging of Two-Phase Flow in Structured Monolith Reactors; Techniques and Data Analysis

This thesis will address the use of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques to probe the “monolith reactor”, which consists of a structured catalyst over which reactions may occur. This reactor has emerged as a potential alternative to more traditional chemical engineering systems such as trickle bed and slurry reactors. However, being a relatively new design, its associated flow phenomena and design procedures are not rigorously understood, which is retarding its acceptance in industry. Traditional observations are unable to provide the necessary information for design since the systems are opaque and dynamic. Therefore, NMR is proposed as an ideal tool to probe these systems in detail. The theory of NMR is summarised and the development of novel NMR techniques is presented. Novel techniques are validated in simple systems, and tested in more complex systems to ascertain their quantitative nature, and to find their limitations. These techniques are improvements over existing techniques in that they either decrease the acquisition time (allowing the observation of dynamically-changing systems) or allow us to probe systems in different ways to extract useful information. The goal of this research is to better understand the flow phenomena present in such systems, and to use this information to design better, more efficient, more controllable industrial reactors. The analysis of the NMR data acquired is discussed in detail, and several novel image-processing techniques have been developed to aid in the quantification of features within the images, and also to measure quantities such as holdup and velocity. These novel techniques are validated, and then applied to the systems of interest. Various configurations of monolith reactor, ranging from low flow rate systems to more challenging (and more industrially relevant) turbulent systems, are probed using these methods, and the contrasting flow phenomena and performance of these systems are discussed, with a view to optimisation of the choice of design parameters