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.     

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.     

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 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 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 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.     

A model for two-phase turbulent mixing in a jet bubble column

Mixing behavior of the two phase air-water turbulent flow in a jet bubble column is examined. The time evolution of the mixing behavior of a liquid tracer in a turbulent air-water flow within a jet bubble column is predicted using a model based on the fundamental governing equations of fluid motion. The predictions of the model are compared with experimental measurements. Measured residence time distributions (RTD) of the liquid tracer within the cone agree well with the predicted values given by the model. For the range of parameters considered in the study, lack of radial mixing and large axial mixing are evident within the cone of the jet bubble column. Use of fundamental mathematical models for the study of hydrodynamics in a two-phase conventional bubble column has been reported earlier (Torvik, 1990; Jakobsen et al., 1993). The present paper extends the use of such models to predict the mixing characteristics in a jet bubble column.     

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.     

Bubble size distribution in the sparger region of bubble columns

It is well known that the gas distributor can play an important role on the evolution of the bubble size distribution (BSD) in gas-liquid reactors, strippers and absorbers. Therefore, the main subject of the present work was to study the influence of sparger design and process parameters on the BSD in the sparger region of the considered apparatus. For this purpose, both detailed measurements and prediction of the size of bubbles produced at the sparger were carried out in three different experimental apparatuses.The unique set of BSD curves were obtained by analyzing a large amount of bubbles with a measurement based on image analysis technique.Additionally, Colella's model of BSD evolution in bubble columns was further developed by implementing a detailed physical model for predicting the initial BSD at the sparger where the model input is only based on design/process parameters. A validation of the model was carried out using data from two different columns. The comparison between calculated and experimental BSD shows good agreement.     

Single bubble formation in high pressure liquid—solid suspensions

Bubble formation from a single nozzle is investigated analytically and experimentally in nonaqueous liquid and liquid—solid suspensions at pressures up to 17.3 MPa. A mechanistic model is proposed to predict the initial bubble size in liquid—solid suspensions, by taking into account the various forces affecting the bubble growth including those induced by the presence of the particles, such as the suspension inertial force and the particle-bubble collision force. It is found that the initial bubble size in the suspensions is generally larger than that in the liquid mainly due to the inertia effect of the suspension. The initial bubble size increases with the solids holdup. The pressure has an insignificant effect on the initial bubble size in both the liquid and liquid—solid suspensions under the conditions of this study. The model can reasonably predict the initial bubble sizes obtained in this study and those reported in the literature.     

A correlation of two-phase pressure drops in screen-plate bubble column

The pressure drop, ΔP_TP, was measured for two-phase flow through screen-plate bubble columns. Based on a statistical analysis, an empirical correlation of the present data was obtained which indicates that ΔP_TP is dependent only on the liquid kinematic viscosity and screen hydraulic radius over the ranges of variables studied.     

Numerical simulation of bubble and particles motions in a bubbling fluidized bed using direct simulation Monte-Carlo method

Flow behavior of bubbles and particles in a bubbling fluidized bed were numerically computed using Euler-Lagrange approach. Particle collision was simulated by means of the direct simulation Monte-Carlo (DSMC) method and hard-sphere model. The computed velocities and fluctuations of particles were in agreement with experimental data of Yuu et al. [S. Yuu, H. Nishikawa, T. Umekage, Numerical simulation of air and particle motions in group-B particle turbulent fluidized bed, Powder Technol. 118 (2001) 32-44]. The distributions of velocity, concentration, granular temperature and collision frequency of particles in a bubbling fluidized bed were analyzed. The wavelet multi-resolution analysis was used to investigate flow behavior of bubbles and particles. The bubble frequency of random-like bubble fluctuation was determined from the wavelet multi-resolution analysis over a time-frequency plane.