Experimental measurement of bubble breakup in a jet bubbling reactor

The bubble breakups in a jet bubbling reactor are captured using a high-speed camera and the velocity field is measured by particle image velocimetry. Two typical breakup patterns, jet breakup and jet-vortex breakup are observed. The breakup time interval of the jet-vortex breakup is two orders of magnitude higher than the jet breakup. The probability of the jet-vortex breakup and the jet breakup accounting in the total breakup events increases and decreases with the jet velocity and the mother bubble size, respectively. The bubble breakup region increases with the jet velocity. The bubble breakup frequency increases with the turbulent dissipation rate and the mother bubble size. The average number of daughter bubbles increases with the Weber number. An L-shaped daughter bubble size distribution is observed. Empirical correlations are established for the bubble breakup frequency, the average number of daughter bubbles and daughter bubble size distribution, and fitted well with the experimental results.     

Bubble behavior in corrugated‐wall bubbling fluidized beds—Experiments and CFD simulations

A new concept to harness bubble dynamics in bubbling fluidization of Geldart D particles was proposed. Various geometrical declinations of a cold‐prototype corrugated‐wall bubbling fluidized bed were compared at different flow rates (U_g) to conventional flat‐wall fluidized bed using high‐speed digital image analysis. Hydrodynamic studies were carried out to appraise the effect of triangular‐shaped wall corrugation on incipient fluidization, bubble coalescence (size and frequency), bubble rise velocity, and pressure drop. Bubble size and rise velocity in corrugated‐wall beds were appreciably lower, at given U_g/U_mb, than in flat‐wall beds with equal flow cross‐sectional areas and initial bed heights. The decrease (increase) in size (frequency) of bubbles during their rise was sustained by their periodic breakups while protruding through the necks between corrugated plates. Euler‐Euler transient full three‐dimensional computational fluid dynamic simulations helped shape an understanding of the impact of corrugation geometry on lowering the minimum bubbling fluidization and improving gas distribution. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Bubble properties and pressure fluctuations in bubble columns

The effects of gas (0.02-0.1 m/s) and liquid velocities (0.0-0.10 m/s) on the bubble properties and pressure fluctuations have been determined in a 0.376 m-IDx 2.1 m-high bubble column. The pressure fluctuations have been analyzed by resorting to the Fractal analysis; the time series of pressure fluctuation signals have been analyzed by means of the Rescaled range analysis and the Hurst exponent has been obtained. The bubble chord length and its rising velocity increase but the Hurst exponent decreases with increasing gas velocity. Whereas, the bubble chord length decreases, but the Hurst exponent increases with an increase in liquid velocity in the continuous bubble column (U_L>0.02 m/s). The Hurst exponent has been found to have a definite relationship with the bubble chord length and its standard deviation.     

Novel phenomenological discrete bubble model of freely bubbling dense gas–solid fluidized beds: Application to two‐dimensional beds

A phenomenological discrete bubble model has been developed for freely bubbling dense gas–solid fluidized beds and validated for a pseudo‐two‐dimensional fluidized bed. In this model, bubbles are treated as distinct elements and their trajectories are tracked by integrating Newton's equation of motion. The effect of bubble–bubble interactions was taken into account via a modification of the bubble velocity. The emulsion phase velocity was obtained as a superposition of the motion induced by individual bubbles, taking into account bubble–bubble interaction. This novel model predicts the bubble size evolution and the pattern of emulsion phase circulation satisfactorily. Moreover, the effects of the superficial gas velocity, bubble–bubble interactions, initial bubble diameter, and the bed aspect ratio have been carefully investigated. The simulation results indicate that bubble–bubble interactions have profound influence on both the bubble and emulsion phase characteristics. Furthermore, this novel model may become a valuable tool in the design and optimization of fluidized‐bed reactors. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

The measurement of bubble parameters in two-phase dispersions—II: The structure of sieve tray froths

An improved method of measuring bubble properties in swarms has been applied to an air-water froth on a sieve-tray. The distributions of bubble size and velocity have been measured and mean froth properties such as gas flow, gas-liquid interfacial area and local liquid content have been derived therefrom. The gas- and liquid-phase mass transfer efficiencies have been calculated directly from analytical forms of the distributions of size and velocity and well known theories of mass transfer. The derived values of these froth parameters have been found to be in excellent agreement with the present independently observed and previously published data.     

The measurement of bubble parameters in two-phase dispersions—I: The development of an improved probe technique

An improved probe technique for the measurement of bubble size and velocity in turbulent bubble dispersion has been developed. The technique makes use of fast real-time data processing methods to identify the bubble orientation during measurement and to accept readily interpretable data for only those bubbles whose central axes are coincident with the probe axis. The method is evaluated experimentally using well defined single bubbles and shown to be accurate as well as rapid.     

Characterization of the bubble swarm trajectory in a jet bubbling reactor

The bubble swarm trajectory in the jet bubbling reactor is measured through the bubble image velocimetry (BIV) technique. The result shows that the bubble swarm rises straightly when the jet Reynolds number is lower than 7,000. However, when the jet Reynolds number exceeds 14,000, the bubble swarm exhibits vortex-like motion, and the bubble vortices oscillate periodically. The oscillating frequency of bubble vortices under the gas bubbling condition is lower than the flapping frequency of pure liquid jet. Moreover, the moving region and oscillating frequency of bubble vortices increase with the jet Reynolds number. The superficial gas velocity has little effect on the bubble swarm trajectory and the oscillating frequency. An empirical correlation between the oscillating frequency of bubble vortices and the jet Reynolds number is built based on the simple harmonic vibration theory.     

Transient multi gas—solid reactions in a bubbling fluidized bed

A “multimodel” for gas‐solid reactions in a reacting particle has been applied to a bubbling fluidized bed reactor. The particle is tracked and bed and particle variables are determined continuously. The conservation equations of mass and heat with auxiliary relations are solved in an accelerating particle, which may rise or fall. The effects of bulk pressure, velocity and temperature, and particle diameter are studied. Heat and mass transfer coefficients may fluctuate up to 75% and 148% respectively. Doubling the pressure changes h_c by 75% and k_c by ?45%. Increase in pellet diameter reduces both h_c and k_c.     

An experimental study of solids mixing in a freely bubbling two-dimensional fluidized bed

An experimental study of solids mixing in a freely bubbling two-dimensional bed of 600 to 850 μm glass particles was performed. Vertical and horizontal particle mixing were studied using heated particles as tracers. The steady-state temperature patterns around a heated wire and the transient response to an injected pulse of heated particles were measured. The bubbling behavior of the bed was recorded with a high-speed video camera and an optical bubble probe.Particle motion was found to be closely related to the random bubble motion in the bed. Mixing experiments must, therefore, be repeated numerious times to achieve meaningful results.Vertical particle transport is asymmetrical. Upward displacement is characterized by a mixing length of the order of the bubble diameter, whereas downward displacement is more uniform, and at a much lower velocity level. Horizontal solids mixing is partially due to mixing in the bubble wakes. In a freely bubbling bed, horizontal mixing is considerably augmented by the lateral motion of bubbles.     

Bubble properties and mixing characteristics in an airlift bubble column with redistributor

The effects of gas velocity liquid velocity redistributor and their free surface area on bubble properties (size rising velocity size distribution) and mixing characteristics (mixing time circulation time) were studied in an internal airlift bubble column Bubble properties were measured by resistivity probe method, and mixing characteristics were obtained by salt-base impulse method in air-water system. The empirical correlations between bubble properties, mixing characteristics and various operating parameters were proposed The disk type redistnbutor was more effective to redisperse coalesced bubbles than the perforated plate type Especially, the disk type with 19.5% free surface area had given the best result.     

Hydrodynamics of bubble columns in the uniform bubbling regime

A relation which defines the drag of spherical particles in more or less concentrated suspensions was employed to describe bubble column behaviour in the uniform bubbling regime. General validity of the relation under low density ratios of dispersed to continuous phase was shown by evaluation of fluidization experiments with spherical foamed polystyrene particles. Experiments performed by the authors as well as evaluation of measurements found in literature also confirmed the suitability of the relation in question for bubble columns. In particular, mean bubble sizes can be predicted from simultaneous measurements of the superficial velocities of gas and liquid and of the gas holdup, respectively. Furthermore, limiting conditions for uniform bubbling can be read off a state diagram of the uniform bubbling regime.     

Gamma ray tomography design for the measurement of hold-up profiles in two-phase bubble columns

A gamma ray tomography system was designed, developed and employed for the measurement of radial hold-up profiles in bubble columns. Measurements were of gamma ray attenuation. For the given source strength the collimator designs for both source and detectors were optimized to get the maximum number of counts with the least scattering effects. For the given dwell and scanning times, excellent reproducibility was obtained. Both fan beam and pencil beam scanning were employed to minimize the error due to scattering of the photons near to the wall regions. Experiments were performed in a 0.385 m internal diameter bubble column. Chordal hold-up profiles were used to find the radial hold-up profile using Abel’s inversion method.     

Bubble sizes produced by shear and turbulence in a bubble column

Measurements of bubble sizes for co-current upward air-water flow in a recirculating loop bubble column are described. The bubble sizes were determined with a two electrode conductivity detector. The effectiveness of two different designs of baffle in reducing the bubble size is assessed. The first type of baffle was a perforated plate placed across the air-water flow, which broke the bubbles by the shear flow and turbulence it caused. The measurements with this type of baffle are consistent with a recently developed theory of bubble break-up in shear flow. The second type of baffle consisted of a series of wire arrays placed across the flow. These broke the bubbles by a cutting action. The rate of bubble coalescence downstream of this baffle was determined, over a range of gas hold-up.     

Bubble size and velocity measurement in gas—liquid systems: Application of fiber optic technique to pilot plant scale

A new fiber optic probe was developed for the detection and measurement of bubble sizes and velocities in a gas-liquid column. The probe is made of a single 1 mm plastic fiber with a sharp wedge tip. The calibration was carried out on a laboratory scale unit using a cine-photographic technique to verify the measurements. Bubble velocities were calculated using two identical parallel fibers and the cross-correlation technique. With the velocities and the passage time determined, it was then possible to determine the bubble diameter. Bubble diameters were within 10% of the measurements obtained with the cine-photographic technique. After calibration, measurements were carried out on a pilot scale unit.     

Bubble and gas holdup characteristics in a bubble column of CMC solution

The local bubble behavior such as holdup, bubble frequency, bubble size and rising velocity in a bubble column of CMC solution was measured using the eleclroresislivity probe technique, and the effects of gas velocity and CMC concentration on the behavior were investigated. Also, the total gas holdup was measured from the liquid level in the column, and its relation with gas velocity and CMC concentration was studied. Two correlations of mean bubble size and total gas holdup with dimensionless groups, composed of gas velocity and physical properties of gas and liquid, were obtained from the experimental results.     

The oxidative coupling of methane in a fluidised-bed reactor

A small fluidised-bed reactor has been used by the CSIRO Division of Coal Technology to study the oxidative coupling of methane to higher hydrocarbons. Methane conversions of 9.6 to 13.5% were obtained in preliminary experiments using a lithium-promoted magnesium oxide catalyst at 850°C and with feed gases containing 5.6 to 10.7% v/v oxygen. Total hydrocarbon selectivity declined from 82 to 72% with increasing methane conversion. When operating with ethane in the feed at concentrations found in natural ethylene, the fluidised-bed reactor converted the ethane with good selectivity to ethylene, a key result in the context of using oxidative coupling for natural gas conversion. In view of these promising results, current work is directed towards increasing methane conversion and hydrocarbon selectivity in fluidised-bed reactors by development of more active and selective catalysts.     

The investigation of gas holdup distribution in a two-phase bubble column using ultrasonic computed tomography

In this study, time-averaged gas holdup distributions were investigated in a 16 cm diameter bubble column for two-phase dispersed system of air–water and air–glycerol solution of 10 wt% by using ultrasonic computed tomography (UCT). A quantitative result of UCT – as a coupling of the ultrasonic transmission method and the iterative filtered backprojection (IFBP) image reconstruction technique – is presented. The UCT results are in a good agreement with those by the bed expansion method. A higher gas holdup in the air–glycerol 10 wt% solution than in the air–water system was observed. The distribution of gas holdup in the column with an attached baffle is also investigated by UCT.