Churn-turbulent Flow Regime Research Articles

Implementation of an improved bubble breakup model for TFM-PBM simulations of gas–liquid flows in bubble columns

An improved bubble coalescence and breakup model was implemented in the inhomogeneous TFM-PBM model for buoyancy-driven gas–liquid bubbly flows. The bubble coalescence was modeled by considering bubble collisions induced by turbulent fluctuations, buoyancy driven, wake entrainment, and viscous shear, and the liquid film drainage model was used for the description of the coalescence efficiency of collisions. The bubble breakup was analyzed in terms of bubble interactions with turbulent eddies which coupled the restriction of surface energy with the capillary pressure. A generic TFM-PBM model was developed and applied for the simulations of bubble columns operated in different flow regimes. The evolutions of the bubble size distributions were simulated and validated for literature data of a D=0.14m cylindrical bubble column at gas superficial velocities 0.03 and 0.45m/s, respectively. Furthermore, a well documented D=0.44m bubble column was simulated, and the predicted distributions of the gas holdup and liquid velocity were compared with the experimental measurements of Chen et al. (1998). The model showed the capacity of describing the gas–liquid fluid dynamics of bubble columns operated in both bubbly and churn-turbulent flow regimes.

Impacts of dense heat exchanging internals on gas holdup cross-sectional distributions and profiles of bubble column using gamma ray Computed Tomography (CT) for FT synthesis

The effect of heat exchanging internals that represent Fischer–Tropsch (FT) synthesis in bubble column has been investigated for the first time using gamma ray Computed Tomography (CT) in a 5.5″ (0.14m) inner diameter Plexiglas bubble column operated at atmospheric condition with air water system. Thirty vertical Plexiglas rods of 0.5″ outer diameter were used which covered ∼25% of the total cross-sectional area and were arranged in a triangular pitch of 0.84″ (2.14cm). The superficial gas velocities applied were based on both total cross-sectional area as well as free cross-sectional area available for the flow and were in the range of 5–45cm/s covering the bubbly through churn-turbulent flow regimes.New knowledge and findings have been obtained which have not been reported in previous studies. In churn turbulent flow regime, the overall gas holdup and the profiles of gas holdup obtained in bubble columns without internals can be extrapolated to those with internals in the central region of the column if the superficial gas velocity is based on the free cross-sectional area (CSA) available for the flow of the phases provide that symmetric time averaged gas holdup cross-sectional distributions are achieved. The results show that a significant increment in overall and local gas holdup are obtained upon inserted of the internals when the velocity measured based on the total cross sectional area of the column. The gas holdup distribution over the cross-sectional area (CSA) of the column in the presence of internals has a symmetrical shape at low gas velocities and asymmetry at higher ones for the honeycomb configuration and its installation used in this study. At a high superficial gas velocity that is based on free CSA for the flow, the influence of dense internal becomes insignificant at the central region of the column. However, the profiles of the gas holdup in the column with internals become less steeper compared to those that are like parabolic shape at high gas velocity in the column without internals due to higher gas holdup was obtained in the region near the wall with internals.

Experimental studies of the transition velocity in a slurry bubble column at high gas temperature of a helium–water–alumina system

In this paper, the transition velocity is investigated experimentally for a helium gas at 90°C injected through a slurry of water at 22°C and alumina solid particles in a slurry bubble column reactor. This paper examines the effects of superficial gas velocity, static liquid height and solid particles concentration, on the transition velocity of the SBCR. From the experimental work, it is found that the transition velocity between homogeneous and churn turbulent flow regimes, decreases by increasing the static liquid height and/or the solid concentration. It is also found that there is no slug flow regime in the industrial slurry bubble column reactors.

CFD Simulation of Hydrodynamic of a Bubble Column Reactor Operating in Churn-Turbulent Regime and Effect of Gas Inlet Distribution on System Characteristics

Abstract CFD simulation of cylindrical bubble column including air as dispersed phase and water as continuous phase operating in churn-turbulent flow regime with diameter of 0.49 m, height of 3.6 m and gas superficial velocity of 0.14 m/s have been conducted. All simulations have been carried out in a 2D axisymmetric, unsteady and Euler/Euler framework with the aid of commercial software FLUENT v. 14.5. Simulations were validated by our experimental results through residence time distribution (RTD) data. Effect of bubble size distribution at inlet on column hydrodynamic was investigated and results clearly showed that equilibrium bubble size distribution in most parts of column is independent of bubble size distribution at inlet. In addition, liquid axial velocity and gas hold-up profiles results in column center were approximately the same for cases with different inlet distribution and confirmed similarity of their hydrodynamic.

New methods for flow regime identification in bubble columns and fluidized beds

New methods for flow regime identification were developed and applied to photon count time series measured in a bubble column (0.162m in ID) and fluidized bed (0.438m in ID). The signals in the bubble column (operated with an air-therminol system) were measured by means of Computed Tomography (CT), whereas the data in the fluidized bed (operated with an air-polyethylene system) were recorded by means of Nuclear Gauge Densitometry (NGD). The hidden information in the time series was extracted by means of two new parameters: entropy (bit/s) and information entropy (bit). Both of them were calculated on the basis of multiple reconstructions of the time series. In the case of the bubble column, the well-pronounced local minima were used for identification of three transition velocities (0.04, 0.08 and 0.13m/s). They distinguished the boundaries of the bubbly flow, transition and churn-turbulent flow regimes. In the case of the fluidized bed, the minimum fluidization velocity (0.086m/s) and minimum bubbling velocity (0.12m/s) were also identified on the basis of the well-pronounced local minima in the profiles of the new parameters. They distinguished the boundaries of both the transition and bubbling fluidization regimes.

Characteristics of Gas-liquid Mass Transfer and Interfacial Area in a Bubble Column

Characteristics of gas-liquid mass transfer and interfacial area were investigated in a bubble column of diameter and height of 0.102 m and 2.5 m, respectively. Effects of gas and liquid velocities on the volumetric gas-liquid mass transfer coefficient (<TEX>$k_La$</TEX>), interfacial area (a) and liquid side true mass transfer coefficient (<TEX>$k_L$</TEX>) were examined. The interfacial area and volumetric gas-liquid mass transfer coefficient were determined directly by adopting the simultaneous physical desorption of <TEX>$O_2$</TEX> and chemical absorption of <TEX>$CO_2$</TEX> in the column. The values of <TEX>$k_La$</TEX> and a increased with increasing gas velocity but decreased with increasing liquid velocity in the bubble column which was operated in the churn turbulent flow regime. The value of <TEX>$k_L$</TEX> increased with increasing gas velocity but did not change considerably with increasing liquid velocity. The liquid side mass transfer was found to be related closely to the liquid circulation as well as the effective contacting frequency between the bubbles and liquid phases.

Bubble Coalescence Modeling in the Population Balance Framework

In the churn-turbulent bubbly flow regime with highly nonuniform bubble size distributions, bubble breakage and coalescence are important processes because they govern the bubble size distribution and consequently directly affect the interfacial mass, momentum, and heat transfer fluxes through the renewal bubble surfaces. At present, accurate prediction of bubble size distributions of dispersed gas–liquid flows by use of the population balance (PB) equation is a difficult task. The modeling of bubble breakup and coalescence rates is very complex and is based on the knowledge of collision and breakup frequencies, breakage daughter size distributions, and probability of coalescence. In this work, we focus on the coalescence phenomenon. The coalescence models are still on an empirical level and the mechanisms are not fully understood. This motivates the analysis of the suitability of the coalescence closures for the prediction of experimental data obtained from coalescence dominated gas–liquid flows. For this task, a cross-sectional averaged combined multifluid-PB model is adopted. Based on different theories for the coalescence efficiency, the simulation results show a similar trend in the prediction of the experimental data. Good prediction of the Sauter mean diameter is achieved although the shape of the bubble size distribution is not completely reproduced. The second aim of this work is to review the PB framework. Here, focus is placed on the coalescence term and the combined multifluid-PB model based on kinetic theory approach.

Hydrodynamics of Pilot-Scale Bubble Columns: Effect of Internals

The gas holdup and bubble characteristics are key parameters that affect the performance of bubble columns. An experimental study of the effect of heat exchanging internals on the gas holdup, bubble dynamics, and their radial profiles is performed. An air–water system is used in a Plexiglas column 45 cm (18 in.) in diameter. Internals configurations mimicking those typical for the liquid-phase methanol synthesis and Fischer–Tropsch process and covering 5% and 25%, respectively, of the total cross-sectional area of the column, are investigated. High superficial gas velocities that fall within the churn turbulent flow regime, which are of interest for most industrial applications, are applied. An increase in the gas holdup is observed for the dense internals structure. A decrease in the bubble chord length and a vigorous recirculation behavior are also obtained as a result of the insertion of the internals.

0206 水平管群を横切る気液二相流の管周りボイド率分布計測(OS2-2 混相流の多次元可視化計測,OS2 混相流の多次元可視化計測)

Measurements of local void-fraction around tube in liquid-gas two-phase flow were carried out using a void probe technique. Horizontal tube bundle was set in a vertical rectangular channel with 90 X 90 mm^2, and the tube layout was in-line. Outer diameter of the tube was 15 mm, and the pitch to the diameter ratio was 1.5. Void fraction distributions were compared between 1^<st>, 4^<th> and 7^<th> rows at the center column. Furthermore, the differences of the void fractions were compared around between the center and wall side. Void fraction distributions at inlet of the tube bundle strongly affected on the void fraction distributions in the bundle. Void fraction deviation increased with gas-flow rate. The flow regime changed from bubbly to churn-turbulent flow with increasing of the gas-flow rate, and it increased bubbles' horizontal movement in the bundle. Therefore, the void fraction at 0° and 180° increased with gas-flow rate. As a result, the void fraction distributions around a tube became flat in churn-turbulent flow regime.

Interfacial area concentration in gas–liquid bubbly to churn-turbulent flow regime

There are very few established correlations to predict the interfacial area concentration beyond the bubbly flow regime in cap-slug and churn-turbulent flow regimes. Present study shows a systematic approach to estimate the interfacial area concentration in bubbly, cap-slug and churn-turbulent flow regimes. Ishii and Mishima’s (1980) formulation and the two group approach for categorizing bubbles (Group-1: spherical or distorted bubble, Group-2: cap bubble) are used to estimate the interfacial area concentration. The key parameters in this framework are the estimation of Group-1 bubble size and the amount of void in the liquid slug, which is a function of Group-1 void fraction. Hibiki and Ishii’s (2002) correlation is utilized to predict the size of the Group-1 bubbles. A correlation is developed to estimate the Group-1 void fraction. The developed model for the estimation of interfacial area concentration is compared with the three existing datasets. These are data for air–water flow taken in annular geometry and round tube and also for air–NaOH solution taken in round tube. The estimation accuracies for these data sets are ±36.4%, ±26.5% and ±37.4%, respectively. These datasets cover a wide range of flow regimes and different physical properties.

Impact of Internals on the Heat-Transfer Coefficient in a Bubble Column

The effect of heat exchanging internals on the heat-transfer coefficient has been investigated in a 0.19 m diameter bubble column. An air–water system was used with superficial gas velocities varying from 3 to 20 cm/s, which cover both the bubbly and churn-turbulent flow regimes. Internals occluding 0% (empty column), 5% (simulating methanol synthesis), and 22% (simulating Fischer–Tropsch synthesis) of the column cross-sectional area were implemented. A fast response heat-transfer probe was utilized to measure the radial profile of the heat-transfer coefficient in the fully developed flow region. The results indicate that the presence of a high percentage of internals causes an increase in the heat-transfer coefficient at the same actual gas velocity on the basis of the open cross-sectional area. Also, the heat-transfer coefficient increases with an increase in the gas velocity. The heat-transfer coefficients in the column center for all of the studied conditions are larger than those near the wall of the column.

Development of a computational multiphase flow model for Fischer Tropsch synthesis in a slurry bubble column reactor

The Hybrid Energy Systems Testing (HYTEST) Laboratory at the Idaho National Laboratory was established to develop and test hybrid energy systems with the principal objective of reducing dependence on imported fossil fuels. A central component of the HYTEST is the slurry bubble column reactor (SBCR) in which the gas-to-liquid reactions are performed to synthesize transportation fuels using the Fischer Tropsch (FT) process. These SBCRs operate in the churn-turbulent flow regime, which is characterized by complex hydrodynamics, coupled with reacting flow chemistry and heat transfer. Results, our team is developing a research tool to aid in understanding the physicochemical processes occurring in the SBCR. A robust methodology to couple reaction kinetics and mass transfer into a four-field model (consisting of the bulk liquid, small bubbles, large bubbles and solid catalyst particles) consisting of thirteen species, which are CO reactant, H 2 reactant, hydrocarbon product, and H 2O product in small bubbles, large bubbles, and the bulk fluid plus catalyst is outlined. Mechanistic submodels for interfacial momentum transfer in the churn-turbulent flow regime are incorporated, along with bubble breakup/coalescence and two-phase turbulence submodels. The absorption and kinetic models, specifically changes in species concentrations, have been incorporated into the mass continuity equation. The reaction rate is based on the macrokinetic model for a cobalt catalyst developed by Yates and Satterfield. The model includes heat generation produced by the exothermic chemical reaction, as well as heat removal from a constant temperature heat exchanger. A property method approach is employed to incorporate vapor–liquid equilibrium (VLE) in a robust manner. Physical and thermodynamic properties as functions of changes in both pressure and temperature are obtained from VLE calculations performed external to the computational multiphase fluid dynamics (CMFD) solver. The novelty of this approach is in its simplicity, as well as its accuracy over a specified temperature and pressure range.

Local gas holdup in a draft tube airlift bioreactor

Airlift column bioreactors are gas–liquid contact devices characterized by a rising channel and a down flow channel due to gas holdup differences in these two channels. Local gas holdup distribution strongly affects the overall gas–liquid flow dynamics in airlift columns. In this work, local gas holdup distributions in a draft tube airlift column covering both bubbly flow and churn–turbulent flow regimes have been studied using computed tomography (CT) technique as well as conventional techniques. The radial and axial evolutions of the gas holdup distribution will be discussed, together with the effects of superficial gas velocity and geometry parameters. The obtained gas holdup results will also be used to verify various empirical and semi-empirical correlations in the literature. Moreover, the obtained gas holdup information, combined with liquid flow dynamic information reported in Luo and Al-Dahhan (2008a, b) , forms a benchmark database for the design and scale-up of airlift column bioreactors and for computational fluid dynamic (CFD) modeling validations.

Influence of Operating Pressure on Gas Holdup and Flow Regime Transition in a Bubble Column

The effect of elevated pressures on the gas holdup and flow regime transition in a gas–liquid bubble column is examined. Experiments are carried out in a bubble column of 0.3 m diameter at system pressures between 0.1 and 0.6 MPa, using different liquids sparged with nitrogen. The following results are obtained. (1) The superficial gas velocity at the flow regime transition point (Utrans) increases with the system pressure. (2) In the bubbly flow regime, the gas holdup shows no notable change when the superficial gas velocity is maintained at a low value. However, in the churn-turbulent flow regime, the gas holdup increases with the system pressure. These phenomena are in agreement with those predicted on the basis of the equations for Utrans and gas holdup, proposed previously.

Mathematical Modeling of Fischer-Tropsch Synthesis in an Industrial Slurry Bubble Column

The increase in society's need for fuels and decrease in crude oil resources are important reasons to make more interest for both academic and industry in converting gas to liquids. Fischer-Tropsch synthesis is one of the most attractive methods of Gas-to-Liquids (GTL) processes and the reactor in which, this reaction occurs, is the heart of this process. This work deals with modeling of a commercial size slurry bubble column reactor by two different models, i.e. single bubble class model (SBCM) and double bubble class model (DBCM). The reactor is assumed to work in a churn-turbulent flow regime and the reaction kinetic is a Langmuir-Hinshelwood type. Cobalt-based catalyst is used for this study as it plays an important role in preparing heavy cuts and the higher yield of the liquid products. Parameter sensitivity analysis was carried out for different conditions such as catalyst concentration, superficial gas velocity, H2 over CO ratio, and column diameter. The results of the SBCM and DBCM revealed that there is no significant difference between single and double bubble class models in terms of temperature, concentration and conversion profiles in the reactor, so the simpler SBCM with less number of model parameters can be a good and reliable model of choice for analyzing the slurry bubble column reactors.

Interfacial Area Transport Equation and Implementation Into Two-Fluid Model

A dynamic treatment of interfacial area concentration has been studied over the last decade by employing the interfacial area transport equation. When coupled with the two-fluid model, the interfacial area transport equation replaces the flow regime dependent correlations for interfacial area concentration and eliminates potential artificial bifurcation or numerical oscillations stemming from these static correlations. An extensive database has been established to evaluate the model under various two-phase flow conditions. These include adiabatic and heated conditions, vertical and horizontal flow orientations, round, rectangular, annulus, and 8×8 rod-bundle channel geometries, and normal-gravity and reduced-gravity conditions. Currently, a two-group interfacial area transport equation is available and applicable to comprehensive two-phase flow conditions spanning from bubbly to churn-turbulent flow regimes. A framework to couple the two-group interfacial area transport equation with the modified two-fluid model is established in view of multiphase computational fluid dynamics code applications as well as reactor system analysis code applications. The present study reviews the current state-of-the-art in the development of the interfacial area transport equation, available experimental databases, and the analytical methods to incorporate the interfacial area transport equation into the two-fluid model.

Computed Tomographic Investigation of the Influence of Gas Sparger Design on Gas Holdup Distribution in a Bubble Column

The effect of gas sparger design on the gas holdup radial profile in a bubble column (with a diameter of 0.162 m) has been studied using γ-ray computed tomography (CT). Six different configurations of gas spargers were examined, using an air−water system for selected superficial gas velocities from 2 cm/s to 30 cm/s, covering the homogeneous and heterogeneous (churn-turbulent) flow regimes. Two operating pressures were used: 1 and 4 atm. Differences were found between the gas holdup distributions produced by different spargers at dimensionless radii of r/R < 0.8 in the central region of the column. The cross and single nozzle spargers produced closely similar gas holdup distributions, while the perforated plate sparger produced a higher gas holdup when compared to other spargers with the same percentage open area (POA). At 4 atm, the sparger design did not have a significant effect on the gas holdup profiles, compared to atmospheric pressure, except for the case of the single-hole sparger, which was found to yield a higher gas holdup.

Multicompartment hydrodynamic model for slurry bubble columns

A core-annulus multicompartment two-dimensional two-bubble class model accounting for slurry recirculation and coupled with catalyst transport was developed as a part and parcel of the analysis of the behavior of slurry bubble column reactors at high gas throughputs corresponding to the churn turbulent flow regime. The model analyzed the contributions of bubble-induced turbulence closures, bubble coalescence and breakup phenomena, and catalyst axial distribution as the resultant of sedimentation, advection via liquid–solid slip, per-compartment axial dispersion and core-annulus lateral exchange of catalyst by bubble-induced turbulence. The model was also used to analyze the effects of catalyst loading, gas density and superficial velocity, and column diameter and vessel aspect ratio on the hydrodynamics of slurry bubble column reactors, namely, the per-compartment phase holdups and interstitial velocities, pressure gradient, bubble coalescence and break-up rates, and loci of velocity inversion for the gas and slurry profiles.

Prediction of flow regimes transitions in bubble columns using passive acoustic measurements

Passive acoustic sound, measured by a hydrophone in an air–water bubble column, is used to study the hydrodynamics of the unit. The recorded measurements taken at different superficial gas velocities are processed using both spectral and chaos-based techniques in order to characterize the column flow regimes and to predict the transitions points. These processing tools were supported by digital video imaging of bubbles motion inside the column. The results of data analysis indicate the applicability of passive sound measurements to identify flow regimes in the bubble column. In this regard the analysis of sound spectra gives a useful qualitative comparison of flow regimes. Chaos-based techniques, on the other hand, are more successful in predicting the transition points between the homogenous and the churn-turbulent flow regimes in the column. The critical gas velocities of the transition are associated with a marked change in some of the calculated chaotic invariants of sound pressures. The calculated superficial gas velocities of the critical points are also found to be consistent with experimental observations. Moreover, a useful visualization of the dynamics induced in the column by the alternation of small and large bubbles is possible through the inspection of phase-space trajectories reconstructed from time-series measurements. The shape and size of the trajectories are closely linked to the size distribution of bubbles, and they change as the flow moves from one regime to an other.

An algorithm for predicting the hydrodynamic and mass transfer parameters in bubble column and slurry bubble column reactors

A large number of experimental data points obtained in our laboratory as well as from the literature, covering wide ranges of reactor geometry (column diameter, gas distributor type/open area), physicochemical properties (liquid and gas densities and molecular weights, liquid viscosity and surface tension, gas diffusivity, solid particles size/density), and operating variables (superficial gas velocity, temperature and pressure, solid loading, impurities concentration, mixtures) were used to develop empirical as well as Back-Propagation Neural Network (BPNN) correlations in order to predict the hydrodynamic and mass transfer parameters in bubble column reactors (BCRs) and slurry bubble column reactors (SBCRs). The empirical and BPNN correlations developed were incorporated in an algorithm for predicting gas holdups (εG, εG-Small, εG-Large); volumetric liquid-side mass transfer coefficients (kLa, kLa-Small, kLa-Large); Sauter mean bubble diameters (dS, dS-Small, dS-Large); gas–liquid interfacial areas (a, aSmall, aLarge); and liquid-side mass transfer coefficients (kL, kL-Large, kL-Small) for total, small and large gas bubbles in BCRs and SBCRs.The developed algorithm was used to predict the effects of reactor diameter and solid (alumina) loading on the hydrodynamic and mass transfer parameters in the Fisher–Tropsch (F–T) synthesis for the hydrogenation of carbon monoxide in a SBCR, and to predict the effects of presence of organic impurities (which decrease the liquid surface tension) and air superficial mass velocity in the Loprox process for the wet air oxidation of organic pollutants in a BCR. In the F–T process, the predictions showed that increasing the reactor diameter from 0.1 to 7.0 m and/or increasing the alumina loading from 25 to 50 wt.% significantly decreased εG, kLaH2 and kLaCO and increased dS. The decrease of the total gas holdup was found to be controlled by the holdup of small gas bubbles. The increase of the Sauter mean bubble diameter increased both kLH2 and kLCO, however, the decrease of the total gas holdup coupled with the increase of dS resulted in a dramatic decrease of the gas–liquid interfacial area, a, and subsequently kLaH2 and kLaCO. Thus, in the churn-turbulent flow regime, the hydrodynamic and mass transfer behaviors of the F–T SBCR were controlled by the holdup and the gas–liquid interfacial area of small bubbles. In the Loprox process, the predictions showed that increasing the liquid surface tension (removal of organic impurities from water) significantly increased dS and decreased both εG and kLaO2. The decrease of the total gas holdup with increasing liquid-phase surface tension was due mainly to the decrease of the liquid-phase foamability which led to the decrease of the holdup of small gas bubbles. The increase of the Sauter mean bubble diameter and the decrease of the total gas holdup resulted in a strong decrease of the gas–liquid interfacial area, and subsequently kLaO2. Increasing the air superficial mass velocity increased εG, dS, a, kL-O2 and kLaO2. Within the conditions used in the Loprox BCR, the hydrodynamics and mass transfer parameter behaviors of the process appeared also to be controlled by the gas holdup of small gas bubbles; and the gas–liquid interfacial area.