Oscillatory Baffled Column Research Articles

Effect of hydrodynamics on mass transfer in a gas–liquid oscillatory baffled column

In this paper, we present an experimental study on mass transfer of oxygen into water in an oscillatory baffled column (OBC). The objective of this work is to establish the contribution of individual parameters, such as the size of bubbles and the gas holdup, to the overall volumetric mass transfer coefficient ( k L a). Our results show that the gas holdup is the most important factor. The liquid-side mass transfer coefficient ( k L) is calculated directly from the experimental measurements of gas holdup, Sauter mean diameter and k L a, and an increase of k L with D 32 is observed. The results also show that, above a critical level of fluid oscillation, the mass transfer coefficients are mainly governed by the oscillatory operating conditions, while independent of the type of gas sparger.

Evaluation of Turbulent Integral Length Scale in an Oscillatory Baffled Column Using Large Eddy Simulation and Digital Particle Image Velocimetry

In phase-separated synthesis, two fundamental factors control the formation of droplets in a reactor system: the turbulent integral length scale and the rate of droplet breakage and coalescence. In this paper, we report the evaluation and validation of the former in an oscillatory baffled column (OBC) using large eddy simulation (LES) and digital particle image velocimetry (DPIV), respectively. In LES the Navier–Stokes equations are spatially filtered, which eliminates eddies whose scales are smaller than a filter width. Large eddies are directly resolved, while small eddies are modelled using sub-grid scale models that describe the interactions between the large eddies and the unresolved smaller scales. Eddies are the essential ingredient for mixing in OBC and the methodology of the LES is particularly suited for flows in OBC. In parallel to the LES work, a high-resolution DPIV is also employed. Since it works as a low-pass filtering system, which is analogous to the spacious filtering in LES, it is therefore possible to validate the numerical simulation using the DPIV technique. The results show that the turbulent integral length scale in the OBC is of an order around about 10 –3 m, and the Kolmogorov's time scale of a range from 0.2 to 24 ms depending on the oscillatory Reynolds numbers. Such information is vitally important in controlling chemical processes whose reaction time has an equal or similar value as the time scales shown here. We believe that the drop formation and subsequently size distribution in the OBC are also affected by these scales.

On modelling turbulent flow in an oscillatory baffled column – RANS model or large‐eddy simulation?

AbstractOscillatory flow in a baffled column can achieve intensive and uniform mixing with lower shear rate in comparison to the traditional stirred tank reactors (STR). The flow in an oscillatory baffled column (OBC) is unsteady with periodicity in both time and space. Mixing is achieved by the formation of eddies. It is therefore very important to understand the turbulence in an OBC as it determines the degree of segregation, breakage of droplets and bubbles, and the efficiency of chemical/biochemical reactions. Modelling of turbulence is usually associated with the traditional Reynolds Averaged Navier–Stokes (RANS) model. Our results show that the RANS turbulence models, which are sufficiently good for simulating flows in STR, are very poor in predicting periodic flows in an OBC as the methodology of averaging in time in RANS has effectively removed the turbulence. In OBC, eddies of various sizes are the main ingredient for mixing, the large‐eddy simulation (LES) is particularly suitable for such type of flows. In this paper, we describe our understanding on the way in which the LES works.© 2003 Society of Chemical Industry

Using non‐intrusive laser‐induced fluorescence in the characterisation of mixing in an oscillatory baffled column

AbstractTraditionally the characterisation of mixing in a system is achieved by injecting a tracer solution and monitoring the changes of tracer concentration with time using conductivity probes. In this paper we demonstrate that a non‐intrusive laser‐induced fluorescence (LIF) technique can be used to quantify the behaviour of viscous fluids in a model oscillatory baffled column (OBC). The OBC is an alternative‐mixing device in which longitudinal periodic fluid oscillations are transposed onto a column with evenly spaced ‘doughnut’ shaped baffles. This combination of baffles and oscillation generates an enhanced yet uniform mixing system. In LIF, a dye fluoresces when it is induced by a laser. The intensity of the light emitted can be converted into the concentration of the dye. The timed profile of average concentration (within a baffled cell) leads to the determination of the mixing time and axial dispersion coefficient. This paper attempts to show that LIF is a powerful tool in studying mixing.© 2003 Society of Chemical Industry

A study of velocity and residence time of single bubbles in a gassed oscillatory baffled column: effect of oscillation amplitude

AbstractIn this paper, the experimental evaluation of velocity and residence time of single bubbles of different sizes in a gassed oscillatory baffled column (OBC) using a high‐speed imaging technique is reported. This work is particularly concentrated on the effect of oscillatory amplitude. The results show that the instantaneous axial bubble velocities vary up to four times with respect to the mean value at an oscillatory amplitude greater than 2 mm. The ratio of the radial to axial bubble velocity increases beyond unity for the highest oscillation amplitude investigated. The direct measurement of residence time of the bubbles within a baffled cell is also reported.© 2003 Society of Chemical Industry

A Study of Bubble Velocity and Bubble Residence Time in a Gassed Oscillatory Baffled Column: Effect of Oscillation Frequency

In this paper, we report an experimental evaluation of velocity and residence time of single bubbles of different sizes in a gassed oscillatory baffled column (OBC) using a high-speed imaging technique. This work is particularly concentrated on the effect of oscillation frequency. The results show that the axial bubble velocities span several orders of magnitude with respect to the mean at an oscillatory frequency greater than 2 Hz. The ratio of the axial to radial bubble velocity components approaches unity at elevated oscillation frequencies. Combining the high-speed imaging with the high-resolution digital particle image velocimetry, we are able to examine the influence of the fluid flow on the bubble motion within the OBC.We also report the direct measurement of residence time of the bubbles within a baffled cell.

On the determination of axial dispersion coefficient in a batch oscillatory baffled column using laser induced fluorescence

We report a novel method of obtaining tracer concentration profiles and determining the axial dispersion coefficient in a batch oscillatory baffled column (OBC) utilising the laser induced fluorescence (LIF) technique. In LIF, a dye fluoresces when it is induced by a laser. By receiving the wavelength of the light emitted by the dye, the intensity of the dye can be converted into the concentration. This technique is non-intrusive and has a number of unique advantages in comparison with the traditional tracer method. One of such advantages is that it can be used to investigate the concentration profiles in different areas of the cell axially and radially, which has led to, for the first time, the quantitative determination of the radial dispersion in the OBC. In short, LIF is a viable and powerful tool in quantifying mixing and dispersion in both the OBC and other reactor systems.

Computational fluid dynamic modelling of flow patterns in an oscillatory baffled column

Numerical simulations to date within the context of oscillatory flow in a baffled column have been limited to flows in a symmetrical regime, i.e. eddies are generated symmetrical to the central line of the column where the oscillatory Reynolds numbers are below 400. In this paper, 3-D computational fluid dynamic (CFD) simulation of flow patterns of oscillatory flow in a baffled column has, for the first time, been carried out and the results extended to all regimes of oscillatory Reynolds numbers covering from symmetric to asymmetric flows. The flow patterns simulated have also been validated by both direct flow visualisation and by digital particle image velocimetry measurements. The success of such CFD simulations opens doors for many potential studies, from optimisation of geometry for plug flow to suspension of particles, and from droplet breakage and coalescence to mass/heat transfer of particles.

On the effect of tracer density on axial dispersion in a batch oscillatory baffled column

In this paper, we report our modelling evaluation on the effect of tracer density on axial dispersion in a batch oscillatory baffled column (OBC). Tracer solution of potassium nitrite, its specific density ranged from 1.0 to 1.5, was used in the study, and was injected to the vertical column from either the top or bottom. Local concentration profiles are measured using conductivity probes at two locations along the height of the column. Using the experimental measured concentration profiles together with both ‘Tank-in-Series’ and ‘Plug Flow with Axial Dispersion’ models, axial dispersion coefficients were determined and used to describe the effect of specific tracer density on mixing in the OBC. The results showed that the axial dispersion coefficients evaluated by the two models are very similar in both magnitudes and trends, and the range of variations in such coefficients is generally larger for the bottom injection than for the top one. Empirical correlations linking the mechanical energy for mixing, the specific density of tracer and axial dispersion coefficient were established. Using these correlations, we identified the enhancements of up to 269% on axial dispersion for various specific tracer densities.

Gas hold-up and bubble diameters in a gassed oscillatory baffled column

In this paper attempts are made to address how bubble behaviour in a batch oscillatory baffled column (OBC) contributes to the overall measured enhancement in mass transfer. A CCD camera is used to measure the bubble size distribution and the gas hold-up in the OBC. The experimental results of Sauter mean diameter and gas hold-up are correlated as a function of the power dissipation and superficial gas velocity, in order to allow for comparisons with published correlations for the mass transfer coefficient. In general, an increase in the oscillatory velocity causes an increase in the hold-up and a decrease in the Sauter mean diameter. Furthermore, we are able to show that the changes in the gas hold-up contribute more than the mean bubble size to the control of the mass transfer coefficient.

Characterisation of flexible baffles in an oscillatory baffled column

AbstractIn our previous studies the volume‐averaged strain rates, of 2–22 s−1, were obtained in an oscillatory baffled column (OBC) based on velocity measurements over a half baffled cell for oscillatory Reynolds numbers of 1000–4030. These values are very low compared with those in a traditional stirred tank vessel (at least >100 s−1) for similar operational conditions and the same power consumption. It was also observed that the volume‐averaged strain rates in the OBC fluctuated with the phase of oscillation over any cycle, with high values coinciding with eddy generation, and low values with eddy cessation. The objective of this study is to show that such fluctuations can be attenuated by employing innovative flexible baffles, whose inside edges move with the fluid oscillation. In this paper experimental measurements of velocity vector maps and strain rate distributions using Digital Particle Image Velocimetry (DPIV) are presented for both conventional and flexible baffles in an OBC. Mixing characterisation, in terms of axial dispersion coefficients, are compared for both baffle designs. The results show that the flexible baffles can reduce the fluctuations and magnitudes of the volume‐averaged strain rates in the OBC without compromising the mixing performance. Low and uniform strain rate distributions in time and space are essential biochemical, biomedical and pharmaceutical applications where shear sensitive cultures are involved.© 2001 Society of Chemical Industry

On the measurement of strain rate in an oscillatory baffled column using particle image velocimetry

We report, for the first time, our direct experimental measurement of velocity vectors and strain-rate distributions in an oscillatory baffled column (OBC) using time-resolved particle image velocimetry (PIV). The technique allowed a time series of spatial velocity maps to be obtained for several phases per oscillation cycle and the results, comprising several thousand such maps, illustrate in detail the variations of velocity and strain rate in a baffled region over a range of oscillation amplitudes and oscillation frequencies and provide a deep insight into the mixing, vortex convection and transport mechanism in such a device. The results show that an OBC not only provides enhanced mixing, but also offers low strain rates, which are lower than those in stirred tank vessels. We have also reported for the first time the quantitative correlation between the strain rate and the power dissipation in an OBC. Using the PIV technique we are able to quantify the strain rates experienced in an OBC.

Flow patterns and oil-water dispersion in a 0.38 m diameter oscillatory baffled column

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Experimental Investigation of Bentonite Flocculation in a Batch Oscillatory Baffled Column

ABSTRACT We report, for the first time, the preliminary results of flocculation of bentonite particles in a batch oscillatory baffled column. The effects of floe settling time, mixing intensity, polymer dose, and polymer injecting mode on percentage of flocculation were investigated. The optical density of bentonite suspension prior to and after polymer injection was measured as the means to quantify the percentage of flocculation. The results show clearly that the intensity of oscillation is an important factor to the flocculation rate, and the two-stage mixing mode produced a higher percentage of flocculation than the single-stage mode for low polymer dosages up to 10 ppm. The advantages of the incremental polymer injection method over the slug method are also discussed.