For decades, CFD (computational fluid dynamics) has been applied for numerical calculations of the rather complex flow in bubble columns using the two-fluid approach (Euler/Euler method) as well as the Euler/Lagrange approach, both based on the point-mass approximation. Numerous different models and closures have been proposed and used for describing the fluid dynamic forces acting on bubbles, modelling bubble induced turbulence (BIT) and transport of bubbles by turbulence structures (Sommerfeld, 2004). However, the dynamics of bubbles, i.e., oscillations and tumbling motion, is mostly neglected in such calculations since a physically based model in the frame of a point-particle approximation is not readily available. Such a model was developed, implemented in the frame of LES-Euler/Lagrange calculations (LES: Large Eddy Simulations) and validated based on detailed experiments (Sommerfeld and Bröder, 2009). The temporal evolution of bubble eccentricity was randomly generated based on measured correlations for the mean and rms (root mean square) values of eccentricity in connection with a theoretical oscillation time scale. Naturally, the state-of-the-art fluid forces acting on the bubbles were considered and the transport of bubbles by sub-grid-scale-turbulence (SGS) was modelled by a Langevin approach, as well as turbulence modification by the bubbles being considered in the frame of the LES context. Similar to experimental observations, such a thorough model yielded a bubble tumbling motion even in the point-particle approximation. The correct anisotropic bubble fluctuating velocities were thereby reproduced and good agreement with experiments was obtained for all other velocities. Note that bubble velocity fluctuations were so far never considered for the validation of calculations, but they are of immense importance when also considering mass transfer. Hence, the bubbly dynamics model provided the correct bubble residence time and, as a result, a gas hold-up identical to measured values.
With the recent developments in microfluidic instruments and devices, various types of micrometer-sized materials have been produced by employing multiphase flow patterns formed in the microchannel. In particular, microparticles and microfibers, which are compatible with biomolecule incorporation or living cell encapsulations, have been gaining significant attention as new tools for biochemical analysis, cellular physiological studies, tissue engineering, cell transplantation, and controlled drug delivery. Herein, we introduce recent developments in microfluidic systems to produce alginate-based hydrogel microparticles and microfibers. By utilizing droplet dispersions either in equilibrium or non-equilibrium states, or by employing parallel laminar flows, microengineered functional materials that are difficult to generate using conventional devices and operations can be obtained. New and interesting multiphase phenomena are reviewed, together with the pros and cons of these systems and their applications. Furthermore, the fundamentals of multiphase microfluidics and the materials used to prepare particles and fibers are briefly introduced.
Optical fiber probing (OFP) is a very useful and practical technique for investigating and monitoring multiphase systems, and it is particularly suitable for simultaneously measuring a bubble’s/droplet’s chord length, velocity and number density in an industrial-scale apparatus, as well as a laboratory-scale setup. Here, we outline the principles of OFP and propose several types of optical fiber probes that meet the requirements for particular purposes of the multiphase systems in chemical engineering processes. We describe measurement methods that use an optical fiber probe suitably tuned for liquid film and foam, as well as for bubble measurement. The basic measurement principle of OFP is very simple: the probe tip’s detection of changes in the refraction indices from a gas phase to a liquid phase or vice versa. For example, to precisely measure a bubble’s properties such as the chord length and velocity, it is necessary to determine the precise relationship between the process of the optical fiber probe’s penetration into the bubble and the optical signal. Since the probing signal provides a variety of information due to the complicated interaction of the laser beams, optical fiber and gas–liquid interfaces, it is necessary to use both experimental and computational approaches in order to extract the physical meanings of the probe’s signals. Using our own fully-3D ray-tracing simulator and well-arranged high-speed visualization setups, we discuss how to improve the measurement accuracy of OFP. We first computationally analyze the OFP signals under several penetration conditions, and we explain our recommendations regarding how to improve the accuracy of a single-tip optical fiber probe used for measurement in multiphase systems. On the basis of our present findings and the improvement in measurement accuracy, we then propose several applications of OFP to chemical engineering processes.
The Integrated coal Gasification Combined Cycle (IGCC) is one successful technology to replace conventional coal-fired power plants. In order to achieve better performance in the IGCC, the knowledge and behavior of volatile products are necessary. In the present study, experimental analysis of coal pyrolysis in a novel downer reactor was conducted. Dried Loy Yang coal particles (particle size Dp=0.250–0.300 mm) were pyrolyzed with a novel quartz glass downer reactor (1.0 m in length and 20 mm in inner diameter). The effect of activated carbon (AC) on coal pyrolysis was investigated. The main importance of this study is to understand the effects of pure carbon itself in a pyrolysis process focusing on tar capturing and reforming. Moreover, the study focuses on understanding how the pores of AC (i.e., micro and mesopores) interact with tar. During the experiment, AC was preheated at 1173 K in a fluidized bed heater before feeding to the downer reactor, and then the pyrolysis reaction and its product yields were observed. The results from the study indicate that AC can eliminate the light tar and significantly reduce the heavy tar. This can be attributed to the decomposition of aromatic hydrocarbons into coke.
Solid–liquid micro-fluidized beds (FBs), i.e., fluidization of micro-particles in sub-centimetre beds, hold promise in applications in the microfluidics and micro-process technology context. This is mainly due to fluidized particles providing enhancement of mixing, mass and heat transfer under the low Reynolds number flows that dominate in micro-devices. Although there are quite few studies of solid–liquid micro-fluidized beds, we are presenting the first study of a micro-circulating fluidized bed. The present experimental research was performed in a micro-circulating fluidized bed which was made by micro-machining channels of 1 mm2 cross section in Perspex. PMMA and soda lime glass micro-particles were used as the fluidized particles and tap water as the fluidizing liquid to study flow regime transition for this micro-circulating fluidized bed. The results are in line with the macroscopic observation that the critical transition velocity from fluidization to circulating regime is very dependent on solid inventory, but once the inventory is high enough it is approximately equal to the particle terminal velocity. However, the transitional velocity is weakly dependent on wall effect and surface forces confirming the importance of these two properties in a micro-fluidized bed system. Similarly, the transitional velocity to transporting regime is a strong function of surface forces. Finally, combining these results with our previous result on conventional fluidization the map of solid–liquid fluidization in a micro-circulating fluidized bed system is constructed showing conventional fluidization, circulating fluidization and a transport regime.
The investigation of transient reactive bubbly flow simulation data offers a variety of data mining challenges due to the interaction of mass transfer, hydrodynamic and local concentration. Modern visualization techniques for Euler–Euler based multiphase simulations are used to study the hydrodynamic influence on the reactive mass transfer in a cylindrical bubble column at transient conditions, as it is given during start up of the column. We apply a recently new bubble sampling approach to study the bubble dynamics. The approach is able to conserve the number of bubbles, volume of bubbles and shape of bubbles, while an overlapping of bubbles is avoided. The performed streamline investigations of the continuous phase allow for an investigation of the transient movement of the continuous phase at column start up condition. Especially at different volumetric flow rates, the lambda2 criterion is an appropriate tool to identify mixing effects. With the use of ensemble visualization, different transient cases can be compared. For the investigated bubble sizes, it could be shown that the main difference in contact of bubbles with liquid elements is close to the inlet.
The present study investigates the effect of structured packing on the global and local phase distributions in a technical scale bubble column operated in a semi-batch mode. MellapakPlus N252Y manufactured by Sulzer ChemTech GmbH (Switzerland) was used as a structured packing. Stationary integral, cross-sectional and radial gas holdups were determined by means of camera and wire-mesh sensor. Integral and cross-sectional gas holdups were comparable for the empty and packed bubble column. On a local scale, the installation of packing led to an increase in gas holdup in the column center compared to the empty bubble column. Additionally, the gas phase centroid was calculated from wire-mesh data for both setups. The position of the gas phase centroid was closer to the column axis for the empty bubble column, indicating a more uniform gas phase distribution. Liquid phase distribution was analyzed via tracer studies. Time-dependent cross-sectional and radial tracer responses were analyzed via camera and wire-mesh sensor for the bubble column with and without packing. In comparison, the number and intensity of tracer peaks were reduced in the presence of packing. It was found that, in the case of a packed bubble column, the time until complete tracer distribution in the liquid phase decreased.
The present study investigates the main flow regime boundaries in a bubble column with internals based on a novel statistical-chaotic method. The latter was applied to gas holdup fluctuations recorded by means of a wire-mesh sensor (8×8 wires). The bubble column (0.1 m in I.D.) was operated with an air-deionized water system at ambient conditions. Thirty-seven vertical tubes (arranged in a square pitch with a diameter of 8×10−3 m) were installed as internals.
Based on an original combination of statistical and chaotic parameters, it was found that in the core of the bubble column with internals, the first transition velocity Utrans-1 (end of homogeneous regime) occurred at a superficial gas velocity UG of 0.06 m/s, the second transition velocity Utrans-2 (end of heterogeneous regime) appeared at UG=0.13 m/s. At these critical velocities, the new parameters exhibited well pronounced minima. In the column’s core, the existence of a transition flow regime was not identified. In the annulus of the bubble column with internals, three transition velocities (at UG=0.03, 0.06 and 0.10 m/s) were identified. The first transition velocity identified the end of the gas maldistribution regime. The second and third critical velocities distinguished the ends of the homogeneous and heterogeneous regimes, respectively.
The processing of the gas holdup data in the entire cross-section of the column revealed that the Utrans-1 and Utrans-2 values occurred at somewhat lower UG values (0.05 and 0.08 m/s). These critical gas velocities identified both the lower and upper boundaries of the transition regime.
Microemulsion systems consisting of water, oil and non-ionic surfactants can develop up to three liquid phases as a function of composition and temperature. Under three phase conditions, Population Balance Equations (PBEs) derived from two phase systems need to be extended to account for the influence of the third phase on the droplet size distributions. PBE fitting parameters can not be simply transferred from two phase to three phase conditions, which is attributed to the changing composition of the liquid phases induced by solubility variation. A method to derive PBE fitting parameters for the three phase conditions and describe the droplet sizes in these systems is presented herein.