Hydrodynamics Of Bubble Columns Research Articles

Axial and Radial Investigation of Hydrodynamics in a Bubble Column; Influence of Fluids Flow Rates and Sparger Type

A detailed investigation of local hydrodynamics in a pilot plant bubble column has been performed using various techniques, exploring both axial and radial variations of the gas hold-up, bubble average diameter and frequency, surface area. A wide range of operating conditions has been explored up to large gas and liquid flow rates, with two sparger types. Two main complementary techniques were used: a quasi local measurement of gas hold-up via series of differential pressure sensors to get the axial variation and a double optic probe giving radial variations of gad hold-up, bubble average size and frequency and surface area.According to axial evolutions, three zones, where radial evolutions have been detailed, have been separated: at the bottom the gas injection zone, the large central region or column bulk and the disengagement zone at the column top. It was found that significant axial and radial variations of the two phase flow characteristics do exist even in the so called homogeneous regime. The normalized profiles of bubble frequency appear sparger and gas velocity independent contrary to bubble diameter, gas hold-up and interfacial area normalized profiles. In any case bubbles are larger in the sparger zone than elsewhere.The main result of this work is the very strong effect of liquid flow on bubble column hydrodynamics at low gas flow rate. First the flow regime map observed in batch mode is dramatically modified with a drastic reduction of the homogeneous regime region, up to a complete heterogeneous regime in the working conditions (uG> 0.02 m/s). On the contrary, liquid flow has limited effects at very high gas flow rates.A large data bank is provided to be used for example in detailed comparison with CFD calculations.

Identification of hydrodynamics characteristics in bubble columns through analysis of acoustic sound measurements—Influence of the liquid phase properties

The effects of liquid properties on the hydrodynamics of bubble columns were investigated experimentally through analysis of acoustic sound measurements, using coalescence and non-coalescence mediums. The hydrodynamics parameters such as gas holdup, average bubble radius, gas bubbling rate, root mean square of the sound pressure and damping ratio of the bubble pulsation were investigated at the sparger and bulk regions. The acoustic study revealed that the addition of carboxymethyl cellulose (CMC) and xanthan gum (XG) in small percentage increased the overall gas holdup and reduced the average bubble radius. Moreover, the bubbling rate for these solutions is lower than that for water at low superficial gas velocities. These observations were more apparent in the CMC case. The injection of KCl and silicon polymer substances however, resulted in reduction of the gas holdup and enlargement of the bubble size. In addition, the bubbling rate for the KCl and silicon polymer solutions is found to be superior to that for water. In addition, it is found that acoustic measurements can be used to detect sparger activity for both air–water and non-Newtonian solutions systems. At low superficial gas velocity, the sparger acts as a nozzle, hence heterogeneous bubble size distribution was observed and detected. At high superficial gas velocity, the sparger becomes fully activated and consequently homogeneous size distribution was detected.

Hydrodynamics of slurry bubble column during dimethyl ether (DME) synthesis: Gas–liquid recirculation model and radioactive tracer studies

Radioactive tracer measurements, using impulse injections of Ar 41, powdered oxide of Mn 56 and real catalyst particles doped with an oxide of Mn 56, conducted at the Advance Fuels Development Unit (AFDU) slurry bubble column (BC) reactor during dimethyl ether (DME) synthesis (reactor pressure of 5.27 MPa, reactor temperature of T = 250 ∘ C , inlet superficial gas velocity of 17.1 cm/s, and a catalyst loading of 36 wt%) at LaPorte, Texas, are interpreted. The differences in the responses obtained by the catalyst and fine powdered Mn 2O 3 tracer injections are minimal indicating the validity of the pseudo-homogeneous assumption for the liquid plus solid (catalyst) phase mixtures. The gas–liquid recirculation model [Gupta et al., 2001a. Comparison of single- and two-bubble class gas–liquid recirculation models—application to pilot-plant radioactive tracer studies during methanol synthesis. Chemical Engineering Science 56(3), 1117–1125. 2001b. Hydrodynamics of churn turbulent bubble columns: gas–liquid recirculation and mechanistic modeling. Catalysis Today 64(3–4), 253–269], based on a constant bubble size, describing gas–liquid mass transfer superimposed on turbulent mixing of the gas and liquid phases, is used to simulate the gas, liquid and catalyst tracer responses acquired at the AFDU. The model is able to predict the characteristic features of the experimental responses observed for gas, slurry powder and catalyst tracers at different reactor elevations. The fact, that the same model was previously shown capable of predicting both gas and liquid radioactive tracer responses during methanol and Fischer–Tropsch (FT) synthesis, indicates that this model offers a relatively simple tool for assessing mixing and transport in bubble (BCs) for a variety of gas conversion processes and provides a phenomenologically based framework for BC reactor modeling.

Stability analysis of bubble columns: Predictions for regime transition

A criterion for the transition from the homogeneous to the heterogeneous regime in a bubble column is developed based on the theory of linear stability. Hydrodynamics of bubble column is described by two-fluid model incorporating the interphase forces like drag force and added mass force. Added mass force affects the hydrodynamics of gas–liquid flows significantly and is formulated by taking into account the bubble deformation. A proper understanding of the nature of gas–liquid interface (clean or contaminated) is desired for the reliable predictions of the added mass coefficient. Data from the literature on the transition in bubble columns is critically analyzed. A good agreement has been obtained between the experimental transition gas hold-up and the predictions of the same obtained by the theory developed in this work.

CFD Modeling of Bubble Column Reactor Including the Influence of Gas Contraction

AbstractIn this paper a CFD model for a bubble column reactor undergoing a first order reaction A → B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter DT = 1 m and height HT = 5 m. The incoming gas stream contains inerts, varying in proportion from 10 % to 90 %. Three‐dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity UG = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts. The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.

Some Aspects of the Hydrodynamics of Bubble Columns

Bubble columns have been the object of much attention these last 20 years; it has become a kind of benchmarking reactor for both advanced measuring techniques and for CFD. However, the classical approaches are still of interest, and improving these reactors is still a field of research. In the present article, first some reminders on classical approaches of bubble columns and airlift reactors are presented with recent applications, then the different aspects of CFD as applied to bubble columns are presented, then highlights on some features specific to non-Newtonian fluids are given, finally some attempts to improve bubble columns are presented.

Nonlinear Bubbling Hydrodynamics in a Gas-Liquid Bubble Column with a Single Nozzle

In this work, the chaotic bubbling mechanism in a gas-liquid bubble column with a single nozzle was investigated. The signal for the analysis was the time series of pressure fluctuations measured from a pressure transducer probe placed in the bubble column close to the nozzle. In order to study the bubbling process, statistical analysis, qualitative and quantitative non-linear analyses were carried out for the pressure fluctuations. Power spectra used as standard statistical measures provided preliminary evidence that bubbling in the middle values of gas flow rates may be chaotic in nature. Phase plots provided a qualitative means of analyzing the fine geometry structure of the attractor reconstructed from the bubbling time signal. Positive finite estimates of the Kolmogorov entropy provided a quantitative evidence of behavior consistent with chaos. Besides previous diagnostic tools, the local non-linear short-term prediction was also used as a supplement method. It was found that the bubbling process exhibits a deterministic chaotic behavior in a certain range of the gas flow rate. When increasing the gas flow rate, the sequence of periodic bubbling, primary and advanced chaotic bubbling, and jetting or random bubbling were successively observed. However, no clear period doubling sequence leading to chaotic behavior was observed. The sharp loss of the ability to predict the pressure signal successfully with the non-linear prediction method provides a strongest evidence of the presence of the chaotic bubbling. The variations of the non-linear invariants, such as the Kolmogorov entropy and the correlation dimension together with the plot of the correlation integral with the operation conditions, might be developed as potential and effective quantitative tools for flow regime identification of the bubbling process.

Effect of Gas Density on the Hydrodynamics of Bubble Columns and Three‐Phase Fluidized Beds

AbstractExperiments were performed at ambient temperature and pressure in a 127 mm inner diameter column with a 55% wt. aqueous glycerol solution, 6‐mm spherical borosilicate beads and four gases — helium, air, carbon dioxide and sulphur hexafluoride — giving a 35‐fold gas density range. The dispersed bubble flow regime was sustained to higher gas velocities and gas holdups for denser gases. This finding appears to be due to the reduction of the maximum stable bubble size (i.e. enhanced bubble break‐up), rather than to formation of smaller bubbles at the distributor with increasing gas density. The effect of gas density was significant both with and without the particles present, with gas holdup increasing, bed voidage increasing and liquid holdup decreasing with increasing gas density. The holdup correlations of Han et al. (1990) have been modified to incorporate the effect of gas density.

Comparison, Combination and Validation of Measuring Techniques for Local Flow and Turbulence Analysis in Bubble Columns and Airlift Reactors

AbstractThe applicability of velocimetry techniques based on the Doppler effect — such as laser and ultrasound Doppler velocimetry — for investigating local hydrodynamics in bubble columns and airlift reactors have been extended to non‐coalescing media. Their limitations are highlighted, especially as a function of gas sparger and reactor type. The ultrasound technique was shown to be able to measure either bubble or liquid velocity. Differences in local hydrodynamics due to coalescence behaviour were used to support the analysis. Data validation was carried out both by mass balance and by comparison with other techniques, such as electrochemical probes, Pavlov tubes and optical probes.

Long-term prediction of nonlinear hydrodynamics in bubble columns by using artificial neural networks

An artificial neural network (ANN) model was proposed for the long-term prediction of nonlinear dynamics underlying holdup fluctuations in bubble columns with three different diameters of 200, 400 and 800 mm. Local holdup fluctuations were measured with an optical probe in the bubble columns. The superficial gas velocity was varied in the range of 33–90 mm/s. The time intervals between successive bubbles were extracted from the time series of holdup fluctuations to represent hydrodynamic behaviors in the system and used as training and validation data sets. The effect of data preprocessing as well as the numbers of nodes in input and hidden layers on the ANN training behavior was systematically investigated. The prediction capability of the ANN was evaluated in terms of time-averaged characteristics, power spectra and Lyapunov exponents. It was observed that: the ANN model, which was trained with experimental time series and gas velocity, can be used for the long-term prediction of dynamic characteristics in bubble columns by using random data as the initial input. The results indicate that the trained ANN models have the potential of modeling nonlinear hydrodynamic behaviors in bubble columns.

Influence of operating pressure on the gas hold-up in bubble columns for high viscous media

In industrial practice bubble column reactors are often operated at elevated pressures. Despite numerous studies on hydrodynamics in bubble columns, the number of experimental studies performed under high pressure is limited. This study reports the combined effect of high liquid viscosity, column diameter and operating pressure on the total gas hold-up, which has not been published previously in the open literature. The experiments presented in this study are performed under pressure (0.1 to 1 MPa ) with Newtonian viscous liquid media up to η L =0.55 Pa . It was already shown in literature that increasing the viscosity of the liquid results in a pronounced decrease in the total gas hold-up (Urseanu, Scaling up bubble column reactors, Ph.D. Dissertation in Chemical Engineering, University of Amsterdam, Amsterdam, 2000; Krishna et al., A scale up strategy for bubble column slurry reactors, Catalysis Today 66 (2001a) 199). The same authors found that total gas hold-up reduces with increasing column size, both for low- and high-viscosity liquids. Increasing the operation pressure leads to a considerable increase in the total gas hold-up. In this study it was found that the effect of the pressure gradually disappears as the liquid viscosity increases.

An advanced LIF-PLV system for analysing the hydrodynamics in a laboratory bubble column at higher void fractions

Bubble columns are widely used in the chemical industry and biotechnology. Flow and turbulence in such an apparatus are induced by the bubble rise, and the bubble behaviour is strongly affected by swarm effects (i.e. the interaction between bubbles). For analysing the bubble swarm behaviour and simultaneously evaluating the flow structure and bubble-induced turbulence, a bubble column of 140 mm diameter and a height of 650 mm or 1,400 mm (initial water level) were considered. The bubble column was aerated with relatively fine bubbles having a mean size between about 0.5 and 4.0 mm. The gas hold-up was varied in the range between 0.5 and 19%. A two-phase pulsed-light velocimetry (PLV) system was developed to evaluate instantaneous flow fields of both rising bubbles and the continuous phase. The measurement of the liquid velocities in the bubble swarm was achieved by adding fluorescing seed particles. Images of bubbles and fluorescing tracer particles were acquired by two CCD cameras. Hence, the images from tracers and bubbles were easily separated by optical interference filters with a bandwidth corresponding to the emitting wavelength of the fluorescing tracer particles and the wavelength of the applied Nd-YAG pulsed laser, respectively. To improve the phase separation of the system, the CCD cameras were additionally placed in a non-perpendicular arrangement with respect to the light sheet. The acquired images were evaluated with the minimum-quadratic-difference algorithm. The potential of this technique for the analysis of bubbly flows with higher void fraction was explored. In order to obtain averaged velocity maps of bubble and fluid within the entire column, about 1,000 image pairs were recorded and evaluated for each phase. In addition, turbulence intensities of the fluid were deduced from the measurements. The turbulence properties were used to characterise bubble-induced turbulence for various bubble mean diameters and gas hold-ups. Moreover, the determination of the average bubble slip velocity within the bubble swarm was possible.

CFD Simulations of a Bubble Column Operating in the Homogeneous and Heterogeneous Flow Regimes

Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: and bubbles. For the air-water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag-, coefficients, which differ for the and bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k-∈ model. For a 0.1 m diameter column operating with the air-water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006-0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.

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.

Effect of a new high porosity packing on hydrodynamics of bubble columns

The purpose of this work was to study the effect of a new high porosity packing (solid fraction corresponds to 0.5%) on the hydrodynamics of two bubble columns of different diameters (15 and 20 cm) operating with co-current up-flow of gas and liquid. This specific packing is a stainless steel welded grid with a mesh size of 12.5 mm. Two types of gas sparger were used. Gas hold up, gas hold up profiles bubble size, slip velocity and liquid axial dispersion coefficients were determined with and without packing. The results obtained show that this type of packing has a considerable effect: it delays regime transition and can maintain a homogeneous regime over a large range of superficial gas velocities. Liquid axial mixing was also greatly affected by the presence of this packing, tending toward plug flow.

Hydrodynamics and ozone mass transfer in a tall bubble column

In the paper, major hydrodynamic parameters such as gas hold-up, phase velocities and axial dispersion as well as the ozone mass transfer coefficients in the liquid phase have been investigated in a tall bubble column for co-current, counter-current and semi-batch modes of operation. The major emphasis has been placed on evaluation of the dynamic characteristics of the combined system of experimental column and measuring sensors, which was applied in the subsequent determination of the axial dispersion and ozone mass transfer coefficients in the liquid phase. The ozone mass transfer coefficients have been estimated using two treatment methods of the recorded changes of ozone concentration in the liquid and gas phases with time during ozone absorption or stripping to an inert gas.

Application of multiresolution analysis for simultaneous measurement of gas and liquid velocities and fractional gas hold-up in bubble column using LDA

A new strategy using wavelet transforms for multiresolution analysis of bubble column hydrodynamics from laser Doppler anemometer measurements of the axial velocity has been formulated. The advantage of the methodology is that we may infer intrinsic features of the bubble column hydrodynamics by accurately detecting the passage of bubbles at local positions in the column. The detection of bubbles was possible by denoising local time series data and analyzing for its intermittent behavior. The methodology allows for calculating the changes in the local average liquid velocities and local gas hold-up and was related to the intermittent nature of turbulence in the system. The variation in these properties and flow behavior as a function of radial distance have been studied.

Experimental study of bubble column hydrodynamics

This paper is devoted to the experimental study of bubble column hydrodynamics. A bubble column flow was performed in a vertical rectangular channel, at liquid velocities of approximately 0.10 m/ s and high void fractions. First, void fraction, bubble size and bubble velocity were measured with a dual optical probe. Liquid velocities were obtained using hot film velocimetry. Some characteristics of homogeneous bubbly flow were clearly highlighted. Secondly, mean and fluctuating wall shear stress measurements were made using the electrochemical method. The presence of bubbles is known to increase wall-friction. This increase was found to be much more important than that at the higher values of liquid velocity usually investigated in fluid mechanics. Space–time analysis was used to calculate integral length scale, coherence time and convection velocity. With these techniques, wall-turbulence features were investigated, namely the non-validity of the logarithmic law, very high turbulent kinetic energy and very intense turbulent dissipation.

Hydrodynamics in Bubble Columns Staged with Structured Fibrous Catalytic Layers

Hydrodynamics in Bubble Columns Staged with Structured Fibrous Catalytic Layers

Scaling up Bubble Column Reactors with the Aid of CFD

Bubble column reactors, used widely in industry, often have large column diameters (up to 6 m) and are operated at high superficial gas velocities (in the range of 0.1 to 0.4 ms −1 ) in the churn-turbulent flow regime. Experimental work on bubble column hydrodynamics is usually carried out on a scale smaller than 0.3 m, at superficial gas velocities lower than 0.25 ms −1 . The extrapolation of data obtained in such laboratory scale units to the commercial scale reactors requires a systematic approach based on the understanding of the scaling principles of bubble dynamics and of the behaviour of two-phase dispersions in large scale columns. We discuss a multi-tiered approach to bubble column reactor scale up, relying on a combination of experiments, backed by Computational Fluid Dynamics (CFD) simulations for physical understanding. This approach consists of the following steps: (a) description of single bubble morphology and rise dynamics (in this case both experiments and Volume-of-Fluid (VOF) simulations are used); (b) modelling of bubble-bubble interactions, with experiments and VOF simulations as aids; (c) description of behaviour of bubble swarms and the development of the proper interfacial momentum exchange relations between the bubbles and the liquid; and (d) CFD simulations in the Eulerian framework for extrapolation of laboratory scale information to large-scale commercial reactors.