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

Gas fluidization of nanoparticles

The primary objective of this study is to perform a systematic investigation on the gas fluidization of various nanoparticle agglomerates. Firstly, the gas fluidization characteristics and regime classifications without any additional external force fields are identified using both experimental measurements and modeling. Secondly, the effect of introducing certain external force fields on nanoparticle fluidization is experimentally investigated. Two external force fields were applied: sound waves from a loud speaker (acoustic assistance) and in-bed magnets that were excited by an external oscillating magnetic field (magnetic assistance). Thirdly, exploratory experimental research on the use of nanoparticle agglomerates as a granular filtration media for airborne fine particles is conducted. The last part of this dissertation is an exploratory modeling study to interpret the newly-found core-annulus-wall flow structure in gas fluidization. The experimental study on the gas fluidization of nanoparticles shows that most nanoparticles can be fluidized in the form of nanoparticle agglomerates. For those agglomerates (fluffy carbon black and very large agglomerates) that are difficult to fluidize, channeling always occurs. For those nanoparticle agglomerates that can be fluidized, the fluidization behaviors can be classified into two general categories, namely, agglomerate particulate fluidization (APF) and agglomerate bubbling fluidization (ABF). The classification appears to be depend mainly on the primary nanoparticle size and the bulk density. Nanoparticle agglomerates have a special structure with extremely high porosity. In this study, an analytical model is developed to calculate the flow partition through and around the porous agglomerates, as well as the drag force on an agglomerate of nanoparticles in a swarm of other similar agglomerates. Also, an analytical model based on the Richardson-Zaki equation has been developed to predict the fluidizing agglomerate size, the voidage around the agglomerates, and the minimum fluidization velocities of APF nanoparticles. The introduction of an external field such as sound excitation and magnetic excitation with in-bed magnets can significantly change the fluidization characteristics of nanoagglomerates, including a significant reduction in the minimum fluidization velocity and agglomerate size. The intensity and frequency of the external sound and magnetic fields will influence the fluidization quality of the nanoparticles. In this study, a series of exploratory experiments have been conducted to remove sub-micron particles (including solid particles and liquid droplets) generated by burning incense. The results show that nanoparticle agglomerates in a packed bed can be used successfully as a filter media for airborne submicron particulates. In addition, this study interprets the formation mechanism of the recently discovered core-annulus-wall structure in a circulating fluidized bed, which originates from the wall region mixing of a down flow of solids from the top section of a riser and the upward solids flow near the bottom of the riser, and the strong solid particle collisions in the dense phase suspension. A mathematic model of this phenomenon has been successfully developed and solved numerically

Phase Distribution in an Upflow Monolith Reactor using Computed Tomography

Computed Tomography (CT) is Known to Be a Viable Technique for Determining Flow Maldistribution in Two-Phase Flow through Packed Beds. in This Study, Gamma Ray Computed Tomography Has Been Used to Quantify the Flow Distribution in a Monolith Catalytic Bed, with Water as the Liquid Phase and Air as the Gas Phase, Flowing Co-Currently Upward. the Flow Conditions Were Selected to Bracket Some Commercially Viable Operating Conditions for Such Reactors. in the Monolith Core Region, Fairly Uniform Flow Distribution Has Been Obtained for All the Flow Conditions Used. This Distribution is Quantified using the Standard Deviation of the Holdup Distribution. However, Maldistribution of Air and Water in the Monolith Bed Wall Region Due to Wall Effects at the Monolith Entrance Has Been Observed and Quantified by CT. the Obtained Results Confirm that the Entrance and Exit Regions of the Monolith Bed Need to Be Carefully Designed and to Be Free of Obstacles and Vortex Creating Devices. © 2005 American Institute of Chemical Engineers

Investigation of local velocities and phase holdups, and flow regimes and maldistribution identification in a trickle bed reactor

Trickle bed reactors are packed beds of catalyst on which gas and liquid reactants flow concurrently downward. In this work, the experimental work was carried out in 0.14 m diameter Plexiglas column using air-water system flowing over a packed bed of 3 mm glass bead particles. The local liquid and gas velocities, phase saturation and their time series have been investigated for the first time by developing, validating, and implementing a new two-tip optical fiber probe technique. It was found the radially and axially the liquid and gas velocities and their saturation vary and they also vary with times. In various locations, due to non-uniform distribution of the flowing phases, there are windows of time where the gas phase does not pass through that location where the optical fiber probe was put at. The non-invasive gamma-ray densitometry (GRD) technique has been implemented for the first time in trickle bed reactor as in online monitoring technique to identify flow regime, gross maldistribution and liquid distribution. The GRD technique was able to identify trickle and pulse flow regime and their transition. The findings have been consistent with what have been reported in the literature. The measurement of these techniques was conducted at various axial and radial positions with the superficial liquid velocity varies in the range 0.004 - 0.016 m/s and the superficial gas velocity varies in the range of 0.03-0.27 m/s covering trickling and pulsing flow regime. The results obtained confirm that these techniques can be used with fidelity for measurements and the studies mentioned above and can be employed in various sizes of reactor operated at industrial conditions including harsh conditions of corrosion materials, high pressure, and high temperatures --Abstract, page iv

Local Bubble Hydrodynamics in Multiphase Stirred Tanks

This research serves to understand local bubble hydrodynamics of multiphase stirred tanks using optical probe measurements. The potential of probe to measure gas phase and liquid/solid phase was demonstrated using high speed imaging and PIV techniques. Detailed measurements were then carried out in stirred tanks to understand the impact of operating conditions, baffles, solid loading and impeller geometry on gas phase hydrodynamics. Reported data would be invaluable for CFD model validation, design and scale up

Effects of Internals Configurations on Heat Transfer and Hydrodynamics in Bubble Columns - With and Without Solid Particles

Internals of different types are required in a number of industrial applications of bubble columns to achieve the desired mixing or to remove the heat of reaction to maintain desired temperature and isothermal conditions of operation. Some of these applications include Fischer-Tropsch synthesis, methanol synthesis, and production of dimethyl ether (DME). The presence of internals however can alter the column hydrodynamics and mixing patterns which could influence reactor performance. A fast response probe capable of capturing bubble dynamics, as well as detecting flow direction is used to study the effect of internals on local heat transfer and column hydrodynamics in a bubble column with and without solid particles. It captured the temporal variations in heat transfer coefficients due to changes in local hydrodynamic conditions. Measurements obtained in presence of different configurations and combinations of internals are compared with those without internals to elucidate the effects of internals design and configurations. Comparisons are based on average values and fluctuating component of local temporal variations of the heat transfer coefficient obtained with the fast response probe. The average gas holdup, center line liquid, and bubble rise velocities obtained with and without internals are also compared. The observed differences are discussed based on the insights provided by these comparisons. The heat transfer coefficient and gas holdup increases in presence of internals. Relationships between local heat transfer measurements and hydrodynamic conditions with internals are shown and discussed. The observed increase in heat transfer coefficients with scale can be related to increase in liquid circulation velocity with column diameter, which in turn is related to an increase in large bubbles rise velocity

Optical Measurements in Gas-Liquid Stirred Tanks

This dissertation outlines the development of novel, in-situ and relatively inexpensive optical measurement techniques for use in opaque multiphase reactors at elevated temperature: 350 ┬░C) and pressure: 180 bar) environments where conventional measurement techniques either cannot be used or are difficult or expensive to implement. Important parameters: such as gas holdup, specific interfacial area, bubble velocity, bubble chord lengths, liquid level, and phase transition) in opaque, multiphase reactors at industrially relevant conditions are lacking in the literature. A miniaturized 4-point probe is developed and methodology outlined that can simultaneously capture local gas holdup, interfacial area, size, and velocities of bubbles in a multiphase stirred tank reactor where small bubble sizes can be expected, especially at elevated pressures and/or high agitation rates. The miniaturized 4-point probe accurately captures bubble dynamics of bubbles as small as 850 microns at elevated temperature and pressure. Single-point probes are also developed that are moveable under high pressure that can measure liquid level in a reactor as well as the volumetric expansion of carbon dioxide expanded liquids: CXLs are an emerging green technology). A reflectance-based probe: a 7-fiber, hexagonally packed bundle) that detects critical opalescence and thus the phase transition of complex, multicomponent systems from the subcritical to the supercritical state is also developed for the investigation of CXLs. Most importantly, detailed instructions for construction of all of the above optical probe technologies are provided in a step-by-step manner

Investigations in Hydrodynamics and Mixing Pattern in the Bubble Column Equipped with Internals

Bubble column reactors, with or without solid particles, have a number of applications in the chemical, petrochemical, biochemical and environmental industries. A number of these industrial applications require internals such as baffles, heat transfer surfaces and special distributors to meet demands. Proper selection and design of these internals can lead to the improved performance and efficiency of a bubble column reactor. Several experiments are carried out in a bubble column equipped with a concentric tube bundle (CT) and an internal combination consisting of a concentric tube bundle and concentric baffle (or static mixer) (CTB) respectively. Neutrally buoyant particles are used to determine the effect of the CT and CTB internals on the local flow structures in the equipped column respectively. More upward, near-linear particle movements are observed with the CTB internal over the CT internal. Several non-linear particle movements are also observed. Overall bulk liquid circulation flow patterns are proposed for the intermediate to high gas velocity range based on the observed local flow structures for both internals. Comparisons are made between the gas holdups obtained during internal equipment and that of a comparable hollow bubble column from the literature. Both internals increase the gas holdup of a hollow bubble column. However, the increases with the CT internal are higher by more than 25% of that obtained with the CTB internal on average. The effect of the internals on average bubble size is investigated for the small bubble class. Smaller average diameters are obtained when the CT internal is used. The interfacial area in the presence of the two internals is determined respectively. Higher interfacial areas are obtained with the CT internal. The average difference in interfacial area is 54.0 m2/m3. The effect of the internals on mixing time is determined through dye and aqueous salt tracer studies. In both instances, higher mixing times are obtained with the CTB internal. Liquid backmixing is quantified through the axial dispersion coefficients obtained from the salt tracer studies. The axial dispersion coefficients obtained with the CT internal are higher than that of the CTB internal by about 15% on average

Annular gap bubble column: Experimental investigation and computational fluid dynamics modeling

This paper investigates the countercurrent gas-liquid flow in an annular gap bubble column with a 0.24 m inner diameter by using experimental and numerical investigations. The two-phase flow is studied experimentally using flow visualizations, gas holdup measurements, and double fiber optical probes in the following range of operating conditions: superficial air velocities up to 0.23 m/s and superficial water velocities up to -0.11 m/s, corresponding to gas holdups up to 29%. The flow visualizations were used to observe the flow patterns and to obtain the bubble size distribution (BSD). The gas holdup measurements were used for investigating the flow regime transitions, and the double fiber optical probes were used to study the local flow phenomena. A computational fluid dynamics (CFD) Eulerian two-fluid modeling of the column operating in the bubbly flow regime is proposed using the commercial software ansys fluent. The three-dimensional (3D) transient simulations have been performed considering a set of nondrag forces and polydispersity. It is shown that the errors in the global holdup and in the local properties are below 7% and 16%, respectively, in the range considered. Copyright © 2016 by ASME