Circulation in gas-slurry column reactors

Turbulent bubbly air/water two-phase up and down flows in bubble column were investigated. Important flow quantities such as local void fraction, liquid velocity and the turbulence intensity were measured using both LDV and PIV. the presence of the bubbles increased the level of turbulence in the flow. Measurements of liquid velocities in bubble columns, of different superficial gas velocities, and of different height, the flow structures show tours vortices and axisymetric flow. Key words: turbulent flow; gas-liquid flow; laser velocitymenter

Experimental determination of the drag coefficient in a swarm of bubbles

Measurement of bubble size, velocity and concentration in flashing flow behind a sudden constriction

A triple electroresistivity probe was newly developed to detect the bubbles whose centers moved upwards through it and to measure their rise velocities and sizes (central chord lengths). The probe was designed based on the directional coincidence of the probe axis with the bubble motion, as required to determine the bubble velocity, as well as on the positional determination of the bubble chord detected, as required to obtain the chord length. The conventional double probe has a difficulty in sampling its signals for the bubbles moving vertically and in determining which chord of the individual bubble is detected. The measurements of the behaviors of bubbles with the triple probe were made for two different sizes of normal bubble columns and an external loop airlift bubble column with the liquid and solid phases performed batchwise under a wide range of operating conditions. An analysis of the signal sequences obtained by the probe was proposed which considered the effects exerted by the complicated behaviors of both bubbles and particles. The method gave the results on the local bubble frequency and gas holdup, as well as on distributions of the bubble central chord length and bubble rise velocity at a given radial position of the column. These results suggested an enhanced interaction between bubbles, such as their coalescence and breakup, due to the presence of solid particles. The radial profiles of the bubble frequency, the gas holdup and the mean bubble size and rise velocity, were found to depend on the bubble column type, operating conditions and physical properties of liquid and solid particles.

Influence of gas distribution and regime transitions on liquid velocity and turbulence in a 3-D bubble column

Influence of coalescence behaviour of the liquid and of gas sparging on hydrodynamics and bubble characteristics in a bubble column

Bubble columns are used in various industrial plants: e.g. chemical plants, bioreactors, wastewater treatments and so on. Bubbly flows in the bubble columns fluctuate in time and space. The flow structures in the bubbly flows should be considered in order to achieve the safety operation and high mass transfer rate of the bubble columns. Few researchers, however, clarified the flow structures and the mass transfer mechanisms in the flows. One of the major problems is that researchers currently possess no instrument to simultaneously measure the bubble characteristics and local solution concentration. In the present study, we overcame this problem by developing a new optical fiber probe (POFP) and will clarify the relationship between the flow structures and mass transfer mechanism. The time-series void fractions, local CO_2 concentration and liquid-phase velocities were simultaneously measured by using the POFP and Laser Doppler Velocimeter (LDV). We calculated the spatial scale of the bubble swarms and liquid-phase motion based on the thinking of the integral length scale. As a result, the spatial scale of the bubble swarms and liquid-phase motion have changed by height. Moreover, the liquid-phase flows were not isotropic around the center of the column. The liquid-phase flows appeared to be isotropic in a region near the wall.

Bubble properties measurement in bubble columns: From homogeneous to heterogeneous regime

Experiments were performed to measure the velocities of both phases in a monodispersed bubbly flow in a vertical column. Using water and water-glycerin mixtures, measurements were obtained for a range of Reynolds numbers from 10 to 500. For all cases, the Weber number was below 2. To generate a uniform stream of bubbles, an array of identical capillaries was used. To avoid coalescence, a small amount of salt was added to the interstitial fluid, which did not affect the fluid properties significantly. Measurements of the bubble phase velocity were obtained using a dual impedance probe and through high-speed digital video processing. A measurement of the vertical component of the fluctuating liquid velocity was obtained using a flying hotwire technique, which resolved the deficiency of this technique for flows with zero-mean velocity. To the best of our knowledge, this technique has not been used to study bubbly liquids in the past. It was found that for all cases, the bubble velocity decreases as the mean gas volume fraction increases. For vanishing gas volume fraction, the mean bubble velocity was found to be smaller than that calculated for an isolated bubble. The flow agitation, characterized by the velocity variance both in the liquid and the bubble phases, increases with bubble concentration. The bubble velocity variance was found to rapidly increase with gas volume fraction but to reach a nearly constant value for concentrations higher than 1.5% for all cases. The liquid velocity variance was found to increase linearly with gas concentration for large Re; for low Re the increase was at a smaller rate. The variance, normalized with the mean bubble velocity squared, increased as the Reynolds number decreased. Direct comparisons are performed with recent theoretical and computational studies for the same range of conditions.

An integral model for vertical bubbly jets with nearly monodisperse bubble sizes is presented. The model is based on the Gaussian type self-similarity of mean liquid velocity, bubble velocity and void fraction, as well as on functional relationships for initial liquid jet velocity and radius, bubble diameter and relative velocity. Adjusting the model to experimental data available in the literature for a wide range of densimetric Froude numbers provide constant values for the entrainment coefficient, momentum amplification factor, and spreading ratio of the bubble core for different flow conditions. Consistency and sensitivity of key model parameters are also verified. Overall, the deviations between model predictions and axial/radial profiles of mean liquid velocity, bubble velocity and void fraction are lower than about 20%, which suggests that bubbly jets tend to behave as self-preserving shear flows, similarly to single-phase jets and plumes. Furthermore, model simulations indicate a behavior similar to those of single-phase buoyant jets and slurry jets, but some differences with respect to confined bubbly jets are highlighted. This article provides not only a contribution to the problem of self-similarity in two-phase jets, but also a comprehensive model that can be used for analysis of artificial aeration/mixing systems involving bubbly jets.

Fischer-Tropsch synthesis (FTS), an exothermic reaction where hydrogen and carbon monoxide synthesis gas are converted to hydrocarbon products, has been under development since the 1930's. The interest in FTS depends on current and perceived future prices of crude oil but is increasingly viewed as an option for exploiting stranded natural gas. Other advantages of FTS hydrocarbons include the absence of sulphur, nitrogen, heavy metal contaminants, low aromatic content and the ability to produce high value middle distillates/fuels. Current interest is directed towards slurry bubble processes – comprising gas, liquid, and solid phases. Industrial slurry phase FTS reactors may range in size from 6 – 10 m in diameter and upwards of 30 m in height and include multiple internal heat transfer tubes. Such systems offer numerous advantages including high heat transfer rates, good mixing, and ease of online catalyst addition and withdrawal. However, one disadvantage is the complex hydrodynamics associated with slurry bubble columns, which make scale-up difficult. A literature review on heat transfer studies and correlations has been completed focusing on previous experimental setups, the synthesis of the key findings/parameters, and the identification of the necessary criteria required for reactor design and scale-up.The parameters having the most pronounced impact on heat transfer in slurry bubble columns and three-phase fluidized beds are the superficial gas velocity and liquid properties such as viscosity and surface tension, which significantly alter the bubble properties and the column hydrodynamics. The effect of particles is poorly understood and is a complex function of particle diameter and concentration. The experimental results and correlations reported here from the majority of studies are dependent upon the equipment and properties of the three phases studied – resulting in very limited applicability to other systems or for scale-up. Other concerns include the use of relatively low gas velocities, ambient temperature and pressure, relatively large particles, and relatively small columns employed in most studies, which are not relevant to industrial operating conditions. Furthermore, studies involving multiple internals were relatively few. Most columns were only equipped with a single tube or small heat flux probe thereby measuring only the local heat transfer and not taking into account the effect on column hydrodynamics of multiple internals. Of these studies only a few tubes were equipped with heaters (that did not run the entire tube length) and heat flux probes while the remaining probes were inactive.

The current paper presents steady and unsteady flow data of a transonic test turbine stage operating under flow conditions similar to modern highly loaded gas turbines. Measurements were performed between stator and rotor as well as downstream of the rotor in planes perpendicular to the rotor axis. Time-resolved axial and tangential velocities were measured by a two-component laser doppler velocimeter (LDV) to investigate unsteady phenomena, while time-averaged flow properties were measured by means of a pneumatic seven-hole probe for all three spatial directions. The time-resolved investigation done by LDV allows to present velocity fields, flow angles and turbulence data at different stator-rotor positions during one blade passing period. Averaging these results enabled comparison with the pneumatic multihole probe measurement. LDV data and stage geometry can be obtained per email request and used for computational fluid dynamics (CFD) code verification.

In the second year of the project, the Eulerian-Lagrangian formulation for analyzing three-phase slurry flows in a bubble column is further developed. The approach uses an Eulerian analysis of liquid flows in the bubble column, and makes use of the Lagrangian trajectory analysis for the bubbles and particle motions. An experimental set for studying a two-dimensional bubble column is also developed. The operation of the bubble column is being tested and diagnostic methodology for quantitative measurements is being developed. An Eulerian computational model for the flow condition in the two-dimensional bubble column is also being developed. The liquid and bubble motions are being analyzed and the results are being compared with the experimental setup. Solid-fluid mixture flows in ducts and passages at different angle of orientations were analyzed. The model predictions were compared with the experimental data and good agreement was found. Gravity chute flows of solid-liquid mixtures is also being studied. Further progress was also made in developing a thermodynamically consistent model for multiphase slurry flows with and without chemical reaction in a state of turbulent motion. The balance laws are obtained and the constitutive laws are being developed. Progress was also made in measuring concentration and velocity of particles of different sizes near a wall in a duct flow. The technique of Phase-Doppler anemometry was used in these studies. The general objective of this project is to provide the needed fundamental understanding of three-phase slurry reactors in Fischer-Tropsch (F-T) liquid fuel synthesis. The other main goal is to develop a computational capability for predicting the transport and processing of three-phase coal slurries. The specific objectives are: (1) To develop a thermodynamically consistent rate-dependent anisotropic model for multiphase slurry flows with and without chemical reaction for application to coal liquefaction. Also establish the material parameters of the model. (2) To provide experimental data for phasic fluctuation and mean velocities, as well as the solid volume fraction in the shear flow devices. (3) To develop an accurate computational capability incorporating the new rate-dependent and anisotropic model for analyzing reacting and nonreacting slurry flows, and to solve a number of technologically important problems related to Fischer-Tropsch (F-T) liquid fuel production processes. (4) To verify the validity of the developed model by comparing the predicted results with the performed and the available experimental data under idealized conditions.

Experiments using nickel magnetostrictive oscillators as transducers that generate acoustic standing waves in a bubble column were conducted in this study. The acoustic frequencies employed are 16 and 20 kHz with power up to 400 W. The column is operated at a constant room temperature, with the heat generated from acoustic excitation removed by the use of a water jacket. The study examines the fundamental behavior of initial bubble size, bubble rise velocity and movement of a swarm of mesobubbles (2−8 mm in diameter), gas−liquid mass transfer, and turbulence under the influence of the Bjerknes force in the acoustic field. The results indicate a smaller initial bubble size, a slower bubble rise velocity, and more concentrated bubble aggregates in the presence, rather than in the absence, of the acoustic field. Furthermore, the gas−liquid mass transfer rate and the liquid-phase turbulence are found to be significantly enhanced when an acoustic field is present.

Instrumentation development in two-phase flow