Mixing In Bubble Column Research Articles

CFD investigation of the gas dispersion and liquid mixing in bubble columns with dense vertical internals

Bubble columns are widely equipped with vertical internals for heat removal in slurry/bubble column reactors or providing the light source in photobioreactors. Understanding the effect of internals on the hydrodynamics of a bubble column reactor is crucial for the reactor design and scale-up. The gas dispersion and liquid mixing are two important parameters in design of bubble columns. In this study, the Eulerian-Eulerian simulation coupled with the population balance model was developed for the bubble column with dense vertical internals. The impulse tracer injection was applied to study the gas dispersion. The effect of the gas flow fluctuations, the time span of tracer injection, and the presence of internals was assessed at different superficial gas velocities. The results showed that the presence of internals has a notable effect on gas behavior. A stronger gas velocity gradient with less turbulence and dispersion were observed in the presence of internals. To study the liquid mixing, the tracer technique was applied, and the sensitivity of results to the tracer injection and detector points have been discussed. The presence of internals reduced the fluctuating liquid velocities, which led to a lower mixing performance in the bubble column.

CFD-based scale-up of hydrodynamics and mixing in bubble columns

Unsteady and three-dimensional Eulerian–Eulerian CFD simulations of bubble column reactors under operating conditions of industrial interest are discussed in this work. The flow pattern in this equipment depends strongly on the interactions between the gas and liquid phases, mainly via the drag force. In this work, a correlation for the drag force coefficient is tested and improved to consider the so-called swarm effect that modifies the drag force at high gas volume fractions. The improved swarm factor proposed in this work is the adjustment of the swarm factor proposed by Simonnet et al. (2008). This new swarm factor is suitable for very high gas volume fractions without generating stability problems, which were encountered with the original formulation. It delivers an accurate prediction of gas volume fraction and liquid velocity in a wide range of tested operating conditions. Results are validated by comparison with experimental data on bubble column reactors at different scales and for several operating conditions. Hydrodynamics is well predicted for every operating condition at different scales. Several turbulence models are tested. Finally, the contribution of Bubble Induced Turbulence (BIT), as proposed by Alméras et al. (2015), on mixing is evaluated via an analysis of the mixing time.

Investigations of flow structure and liquid mixing in bubble column equipped with selected internals

Flow structures and liquid mixing effects are examined in a gas sparged column equipped with internals selected to achieve desired effects. The internals used in this study include a concentric tube bundle, a baffle designed to alter flow of bubbles across column cross-section and a multi-arm gas distributor with downward facing holes. The selection represents consideration of heat removal system, minimal obstruction to flow and desirable mixing effects. Measurements have been made in a 0.15m ID column in two-phase air-water and three phase air-water-glass beads systems. The air flow rate is varied from 0.03 to 0.3m/s to cover a wide range of velocities and the particles are 40µm glass beads. Liquid phase mixing effects are examined with tracer injections from which mixing time and axial dispersion coefficients are obtained. For more detailed survey of flow structure neutrally buoyant particles are employed. Measurements with a fast response heat flux probe in two and three phase systems are compared and analyzed to demonstrate its potential use for screening of internals. Averaged flow structure in the presence of the internals is proposed from observations with different techniques.

Mixing in bubble column reactors: Experimental study and CFD modeling

We have examined the effect of superficial velocity, tracer injection location and measurement location on the mixing time in a bubble column. It was found that for the range of superficial velocities examined (0.07–0.29m/s) there was little change in mixing time with superficial velocity. In contrast, it was found that the tracer injection point and measurement location had a large impact on the measured mixing time, with measured values ranging between 3s and 25s. Additionally, it was found that it was possible to introduce the tracer in such a way as to produce zones of poor mixing, leading to a high local tracer concentration. Such a finding is important in the design of bubble columns used in the bio-processing industry as these conditions can lead to a reduction in the yield of industrial bioprocesses. The second half of this work examines the use of Computational Fluid Dynamics (CFD) as a tool to model mixing in bubble columns. It was found that a computationally efficient CFD model correctly predicted the trends in mixing time for all superficial velocities and tracer addition locations examined, with the model being able to correctly predict the mixing time within the margin of variation.

Liquid-Phase Back mixing in Bubble Columns

Liquid-phase axial dispersion coefficients have been measured for air-water system in bubble columns of 10, 15 and 30 cm diameter. The experiments are carried out using a transient method (the tracer response method). Dispersion coefficient is obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The experimental results show that one-dimensional axial dispersion coefficient, Dax,L, reveal strong scale dependence. Backmixing of liquid phase increases with the increase of reactor diameter and superficial gas velocity. Axial dispersion coefficient for large column reactors can be easily predicted from the developed relation . Comparison of calculated with the experimental data and with the published data of other authors shows good agreement which ensure the reliability and confusability of the adopted correlations to be used in further design and scale-up purposes.

INVESTIGATION MEASUREMENT OF DISPERSION COEFFICIENT AND MIXING TIMES IN BUBBLE COLUMN

A complete model of liquid-phase dispersion was used to simultaneously characterize axial and radial mixing in bubble column of 0.15 m inside diameter and plastic plate distributor with holes of 2 mm diameter. Axial and radial dispersion coefficients and mixing times were determined in tap water for superficial gas velocities in the range 0.6-5.36 cm/s. The experiments were carried out using a transient method (the tracer response method). The dispersion coefficient was obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The measured axial dispersion coefficients (Dax,L) were generally consistent with the predictions of the well established correlations, thus validating the complete dispersion model used in the analysis. The Dax,L values ranged from 110 to 200 cm2/s. There was evidence that the existing literature data on Dax,L in bubble columns are slightly underestimated, as consistent underestimation was found to be a characteristic of the widely used dispersion model that disregards radial dispersion. The value of the radial dispersion coefficient was typically about 1 to 2% of the Dax,L value under any given condition. The mixing time data were generally consistent with the existing literature.
 The results of this study are compared with experimental result and upper bound theory and have shown a good agreement with a minimum discrepancy

THE DEVELOPMENT OF TECHNICAL MEANS TO UPDATA, RESTORE AND TO RESERVE THE MAPS. AS APPLICATION: THE BASE MAP OF ENGINEERING COLLEGE IN DIYALA UNIVERSITY

A complete model of liquid-phase dispersion was used to simultaneously characterize axial and radial mixing in bubble column of 0.15 m inside diameter and plastic plate distributor with holes of 2 mm diameter. Axial and radial dispersion coefficients and mixing times were determined in tap water for superficial gas velocities in the range 0.6-5.36 cm/s. The experiments were carried out using a transient method (the tracer response method). The dispersion coefficient was obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The measured axial dispersion coefficients (Dax,L) were generally consistent with the predictions of the well established correlations, thus validating the complete dispersion model used in the analysis. The Dax,L values ranged from 110 to 200 cm2/s. There was evidence that the existing literature data on Dax,L in bubble columns are slightly underestimated, as consistent underestimation was found to be a characteristic of the widely used dispersion model that disregards radial dispersion. The value of the radial dispersion coefficient was typically about 1 to 2% of the Dax,L value under any given condition. The mixing time data were generally consistent with the existing literature.
 The results of this study are compared with experimental result and upper bound theory and have shown a good agreement with a minimum discrepancy

Liquid-Phase Back mixing in Bubble Columns

Liquid-phase axial dispersion coefficients have been measured for air-water system in bubble columns of 10, 15 and 30 cm diameter. The experiments are carried out using a transient method (the tracer response method). Dispersion coefficient is obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The experimental results show that one-dimensional axial dispersion coefficient, D ax,L, reveal strong scale dependence. Backmixing of liquid phase increases with the increase of reactor diameter and superficial gas velocity. Axial dispersion coefficient for large column reactors can be easily predicted from the developed relation . Comparison of calculated with the experimental data and with the published data of other authors shows good agreement which ensure the reliability and confusability of the adopted correlations to be used in further design and scale-up purposes.

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1–0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298–323 K and 0.01–0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient ( D L ). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas–liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data.

Effect of bubble deformation on stability and mixing in bubble columns

The paper deals with hydrodynamics in bubble columns. The objective of the paper is to study stability and mixing in a bubble column. The modeling of parameters such as stationary drag and added mass is addressed. In addition, the effect of bubble deformation in terms of eccentricity is highlighted. In a previous paper, the transition between homogeneous and heterogeneous regimes in bubble column without liquid flow has been shown to be driven by the deformation of the bubbles associated to drag and added mass. In the present paper, this work is generalized to bubble column with liquid flow and to the transition from bubble flow to slug flow in a vertical pipe. Numerical simulations of gas–liquid reactors are presented. The numerical simulations are validated in the case of gas plume after the Becker et al. data (Becker, S., Sokolichin, A., & Eigenberg, G. (1994) Gas–liquid flow in bubble columns and loop reactors: Part II. Comparison of detailed experiments and flow simulations. Chemical Engineering Science, 49 (24B), 5747–5762. The numerical simulations are finally applied to a bubble column. The simulations of residence time distribution coupled to transient hydrodynamics are shown to be very sensitive to the modeling of interfacial transfer of momentum from the bubbles to the liquid in terms of drag and added mass, including the effect of bubble deformation.

Modelling of fluid dynamics and mixing in shallow bubble column reactors: influence of sparger design

Fluid dynamics and mixing in shallow bubble column reactors is controlled by sparger configuration. In this paper, we evaluate the applicability of using computational fluid dynamics (CFD)-based models to simulate transient fluid dynamics and mixing in shallow bubble columns. A two-fluid model along with the standard k– ε turbulence model was used to simulate flow generated by gas sparged through two different spargers in a bubble column with height to diameter ratio of 2. Different mixing time definitions were evaluated using the CFD-based simulations of mixing of passive tracer. The role of unsteady fluid dynamics and implications of possible simplifications in the geometry of bubble columns for simulating mixing in bubble columns were studied. The simulated results were analysed with reference to the experimental data of Haque et.al. (1986. Chemical Engineering Journal, 33, 63–69). The presented results and analysis will be useful to extend the application of CFD models for screening alternative sparger designs.

A networks-of-zones analysis of mixing and mass transfer in three industrial bioreactors

The original version of the networks-of-zones (N-o-Z) model developed for the description of gas–liquid flow in stirred-vessel reactors (R. Mann, Gas–liquid stirred vessel mixers: towards a unified theory based on network of zones, Transactions of the Institution of Chemical Engineers 64 (1986) 23–34) has been extended and enhanced to cover distributed bubble sizes, gas–liquid mass transfer, bioreaction kinetics and multiple-impeller operation. In addition, a modified version of the N-o-Z model for bubble columns has been simply derived from the impeller version, assuming the existence of a two-loop axisymmetrical circulation pattern induced by the non-uniform distribution of gas holdup in bubble columns. The liquid circulation velocity has been expressed as a function of gas flow rate and the density difference between the gas and liquid phases, based on Zehner's circulation model (P. Zehner, G. Schuh, A concept for the description of gas phase mixing in bubble columns, German Chemical Engineering 5 (1985) 282–289). These two variants of the N-o-Z model have been used for modelling three different industrial fermenters: 3 and 31 m 3 triple-impeller stirred reactors, and a 236 m 3 bubble column reactor. The performance of these three reactors, typical of the fine chemicals, bioprocessing and pharmaceutical process industries was evaluated and compared in terms of geometry/size, gas flows, power inputs, pressure, liquid mixing, oxygen mass transfer, reaction speed and spatial variability of behaviour. This provides potentially valuable insights into the relative factors influencing the selection of an appropriate reactor type.

Back-mixing of the continuous phase in bubble columns

Most authors publishing numerical calculations for the flow field claim that scientific progress on liquid mixing in bubble columns must be based on the fluid dynamics. In a previous paper (Millies, M. and Mewes, D., 1995, Calculation of circulating flows in bubble columns. Chem. Engng Sci. 50, 2093–2106) a calculation method was presented, which allows the prediction of the flow field including several consecutive circulation cells. The conclusions for liquid mixing are drawn in the present paper. The main transport resistance of mixing of liquid phase is located between the circulation cells, a region which was completely neglected up to now. A structured model and even the one-dimensional dispersion model may be obtained from simplifying the transport equation of tracer under certain assumptions. Thus, the coefficients of the structured model and the one-dimensional dispersion model are obtained as results of the numerical calculation. Even though the stirred tank model appears convincing on first view, it is shown to fail as a link between the fluid dynamics and the liquid mixing.

Prediction of fluid dynamics and liquid mixing in bubble columns

A simple model for flow behavior and liquid mixing in bubble columns containing Newtonian (water and salt solutions) and non-Newtonian (CMC at different concentrations) liquids was developed. Based on an energy balance which takes into account the energy losses at the gas—liquid interface, and on the Baird and Rice relation for axial dispersion, the model predicts experimental data of gas holdup, liquid velocities profiles and liquid axial dispersion coefficient with satisfactory accuracy in a wide range of column sizes, fluid systems and experimental condition. Model predictions were compared with recommended correlations for different types of fluids and a good agreement was found. Therefore, the present model may be a more general tool for fluid dynamic and mixing description of bubble columns.

Multiphase Reactors: Models and Experimental Verification

Identification and quantification of flow regimes, phase holdup distributions, flow patterns and backmixing is essential for proper design and scale-up of multiphase reactors. Existing models often suffer from inadequate experimental confirmation. Here, we describe recent progress made in our laboratory in characterizing liquid circulation and backmixing in bubble columns and in quantifying, via a phenomenological model, the behavior of trickle-beds in the low interaction regime. The need for imaging techniques such as fan-beam tomography and PET is illustrated. Liquid mixing in bubble columns caused by global liquid convection and by turbulent eddies is usually described by the axial dispersion model (ADM) which, at least in the churn turbulent regime, lacks a theoretical basis. In the ADM the two mixing mechanisms are lumped into a single effective dispersion coefficient. An alternative backmixing description assumes multiple liquid circulation cells, with height equal to column diameter, to occur in the column. We have successfully demonstrated that instantaneous and time averaged liquid velocity profiles in the column can be determined by monitoring the motion of a neutrally bouyant tracer particle via a Computer Aided Radioactive Particle Tracking (CARPT) technique. It can now be shown that a single strong liquid circulation cell exists in columns of a variety of diameters and at different operating conditions but that smaller circulation cells can form in the distributor region. Model predicted one dimensional axial time averaged liquid velocity profile agrees well with the data and applies to the middle section of the column. The CARPT technique allows the computation of Lagrangian autocorrelation coefficients, the rms fluctuating velocities, Lagrangian integral time scales and the turbulent dispersion coefficients. Thus CARPT provides the necessary information i. e. velocity profiles and turbulent dispersion coefficients, for implementation of a convection dispersion model. The planned implementation of fan-beam tomography would quantify the two-dimensional holdup distribution and together with CARPT provide all the data necessary for verification of fundamental two phase flow models. Pressure drop, holdup, flow regime transition and phase distribution in trickle-beds has received considerable attention but no generally accepted theory for prediction of these quantities exists. Here we describe a phenomenological model for the low interaction regime which views the bed as an array of slits with liquid film flow. Universal velocity profile is used to describe the flow in both liquid films and the gas core and a large data bank for pressure drop and holdup was used to confirm that in the low interaction regime each of the two phases ignores the presence of the other one. The final model contains only two parameters, which are determined from single phase flow experiments, and contains no constants fitted to two phase flow data. The model predicts pressure drop and holdup in the uniform low interaction regime better than any of the existing models. Furthermore, introducing the Kapitza's criterion for laminar liquid film instability into the model the flow regime transition to pulsing is predicted well for all the data that satisfy the conditions for the theory to apply. Other pore level transition mechanisms are suspected for other data. The above phenomenological model is also used as a basis for predicting liquid distribution in a cell model of a trickle-bed. Experimental confirmation awaits the application of PET technology.

Gas-phase mixing in bubble columns.

Residence time distributions of gas phase in bubble columns were obtained by excluding end effects from observed responses to pulse inputs of tracer, using Fourier transforms. Based on the behavior of the residence time distributions, a gas-phase mixing model was proposed in which bubble swarm was assumed to be composed of two bubble groups, one in the core (or central) region and the other in the annular (or peripheral) region of the column. As a result, it was found that about 80% of all the bubbles are rising in the core region and that gas-phase mixing is more intensive in the core region than in the annular region. Axial dispersion coefficients in each bubble group were correlated empirically.

Liquid phase mixing in bubble columns with Newtonian and non-Newtonian fluids

A hydrodynamic model for the liquid phase in bubble columns is developed. The proposed model for fully developed turbulent Newtonian and non-Newtonian fluids is based on an energy balance and the mixing length theory. The predictions of the model are in reasonably good agreement with data on liquid velocity at the column axis and the axial dispersion coefficient. The liquid velocity data in an inverted conical bottom gas—liquid column contactor have also been measured. They are correlated by the proposed model reasonably well.

Axial mixing in bubble column with suspended solid particles.

Etude experimentale de l'influence du distributeur de gaz sur les coefficients de dispersion des phases solide et liquide, dans une colonne a bulle de diametre relativement faible

Diffusion model of gas and liquid mixing in bubble columns

Diffusion model of gas and liquid mixing in bubble columns

Holdup and Axial Mixing in Bubble Columns Containing Screen Cylinders

Holdup and Axial Mixing in Bubble Columns Containing Screen Cylinders