Computer Automated Radioactive Particle Tracking Research Articles

Application of wavelet filtering to the radioactive particle tracking technique

The Computer Automated Radioactive Particle Tracking (CARPT) technique has been used for the investigation of fluid dynamics, material dispersion and mixing in various multiphase rectors. The accuracy of the CARPT measurement depends on the properties of the tracer particle, the algorithm and the technique for calibration and signal processing. In this paper, a filtering technique based on the wavelet theory for the removal of white noise from the data is presented. It is shown, experimentally, that the adopted wavelet-based filtering algorithm reduces the level of noise in the data by 80–90%. The suitability and necessity of wavelet filtering are further illustrated by performing a group of typical CARPT experiments in an air-water bubble column operated at different superficial gas velocities. It is demonstrated that the use of the unfiltered data results in significant over-estimation of turbulent parameters while the influences on the mean velocities are minor.

Optimal design of radioactive particle tracking experiments for flow mapping in opaque multiphase reactors

In the past decade, radioactive particle tracking techniques have emerged in the field of chemical engineering and have become increasingly popular for non-invasive flow mapping of the hydrodynamics in multiphase reactors. Based on γ-ray sensitization of an array of scintillation detectors, the Computer Automated Radioactive Particle Tracking (CARPT) technique measures flow fields by monitoring the actual motion path of a single discrete radioactive flow follower which has the physical properties of the phase whose motion is being followed. A limitation to the accuracy of CARPT lies in the error associated with the reconstruction of the tracer particle position which affects the space-resolution capability of the technique. It is of interest, therefore, to minimize this error by choosing wisely the best hardware and an optimal configuration of CARPT detectors’ array. Such choices are currently based on experience, without firm scientific basis. In this paper, through theoretical modeling and simulation, we describe how the accuracy of a radioactive particle tracking setup may be assessed a priori. Through an example of a proposed implementation of CARPT on a gas–solids riser, we demonstrate how this knowledge can be used for choosing the hardware required for the experiment. Finally, we show how the optimal arrangement of detectors can be effected for maximum accuracy for a given amount of monetary investment for the experiment.

Experimental validation of computational fluid dynamic codes (CFD) for liquid-solid risers in clean alkylation processes

This manuscript, based on the presentation given by one of the authors (M.P. Dudukovic) at the Technological and Engineering Forum in Pancevo, May 21 2002, summarizes the use of the computer automated radioactive particle tracking (CARPT) and gamma computed tomography (CT) in obtaining the data needed to validate the Euler-Euler based CFD simulations for solids distribution, flow pattern and mixing in a liquid-solid riser. The riser is one of the reactors considered for acid solid catalyst promoted alkylation. It is shown that CFD calculations, validated by CARPT-CT data, show promise for scale-up and design of this novel reactor type.

Characterization of Single Phase Flows in Stirred Tanks via Computer Automated Radioactive Particle Tracking (CARPT)

Single phase flows in stirred tanks have been extensively characterized using different experimental techniques like Hot Wire Anemometry1, Laser Doppler Anemometry2, and Digital Particle Imaging Velocimetry3. None of these techniques however, show much promise for the interrogation of opaque multiphase flows. Hence, little or no information of the local fluid dynamics of multiphase flows in stirred tanks is available. Non-optical techniques like Computer Automated Radioactive Particle Tracking (CARPT) and Computed Tomography (CT) have been successfully applied to probe a variety of multiphase reactors4–5 such as bubble columns6–7, risers8 etc., over a range of dispersed phase holdups. CARPT provides the local fluid dynamic information such as velocities and the turbulence parameters throughout the system that is investigated. CT provides time averaged local dispersed phase holdup profiles in various planes of the entire reactor. In this study, it is proposed to extend these techniques to characterize gas-liquid flows in stirred tank reactors. As a first step, CARPT is implemented in characterization of single phase flows in stirred tanks. CARPT experiments have been performed with water at 150 rpm in a 0.20m cylindrical tank quipped with a six bladed Rushton turbine (0.067m dia) conforming to the standard Holland and Chapman9 configuration. The CARPT technique is shown to capture some of the important flow phenomena observed in such flows, like the two recirculating loops above and below the impeller and the dead zones at the bottom of the tank. Radial pumping numbers determined by CARPT (0.67 near the impeller tip) compare reasonably well with data reported in the literature. Comparison of the complete three dimensional mean velocity profiles from CARPT with similar PIV, LDA and other data reported in the literature reveals that CARPT captures the right order of magnitude of the radial and the tangential velocities. Comparisons of the fluctuating velocity components, like the root mean squared (rms) velocity and the turbulent kinetic energy, suggest that the CARPT experiments were limited by large tracer particle size (dp = 2.3 mm) from sampling the high frequency fluctuations of the fluid. In addition, the three dimensional profiles of the components of the Reynolds stress tensor are measured. The detailed comparisons, even with the large tracer particle, indicate that CARPT measurements capture all the important qualitative features of the flow and quantitatively capture the right order of magnitude of the mean flow parameters. The quantitative comparisons suggest that the current size of the tracer particle restricts it from responding completely to the fluid phase fluctuations. Some Lagrangian measures of the fluid dynamics like the ‘Sojourn’ time distributions (STDs) in different zones of the reactor, Circulation Time Distributions (CTDs), Particle Return Maps to specific planes, Poincarre sections and Hurst exponents are evaluated from the collected CARPT data.

Experimental study of gas‐induced liquid‐flow structures in bubble columns

AbstractThe computer‐automated radioactive‐particle tracking (CARPT) technique is used to investigate liquid recirculation and turbulence in bubble columns. Experiments are conducted in air–water bubble columns of different sizes, with various gas distributors, and operated over a range of superficial gas velocities. This study covers gas–liquid flows from the bubbly regime to the churn turbulent regime. The time‐averaged spatial flow structure, axial liquid velocity profiles, Reynolds shear stress, and turbulence intensities profiles are analyzed for different columns and flow conditions. Supplementary experiments, using the hot‐probe anemometry technique, are performed at selected flow conditions, and the results are compared with the corresponding ones from CARPT measurements. The Reynolds stress determined by CARPT was compared with that obtained by hot‐film anemometry in an independent investigation by Menzel et al. (1990).

Particle motion in packed/ebullated beds by CT and CARPT

AbstractComputed tomography (CT) and computer‐automated radioactive particle tracking (CARPT) were used to study the gas distribution and incipient particle motion in a packed/ebullated bed in which gas and liquid are in cocurrent upflow. CT scans were performed to evaluate the gas–liquid distributor and gas sparger for the experiments. Using a perforated plate gas–liquid distributor (with 96 1‐mm holes, open area of 0.1%) and a gas sparger (cross‐shaped, with 16 3‐mm holes), the cross‐sectional gas holdup distribution in the packed/ebullated bed was relatively uniform, with gas holdup of about 0.11 in the center and 0.09 near the wall of the bed at superficial gas and liquid velocities of 2 and 0.3 cm/s, respectively. The cross‐sectional averaged gas holdup was 0.095. CARPT experiments were utilized in an air–water–ethanol– (10 wt. %) solids system to identify the operating conditions at which solid particles first start to move in the bed. At a superficial gas velocity of 1.7 cm/s and superficial liquid velocity of 0.3 cm/s, solids particles in the bed started to move and travel long distances in the axial direction. CARPT experiments in the same system at superficial gas velocity of 2.0 cm/s and superficial liquid velocity of 0.3 cm/s showed that the solid particles in the bed were moving significantly throughout the column, generating a recirculation pattern with upflow in the center and downflow near the wall of the column. The time‐averaged maximum upward and downward velocity of the tracer particle was about 0.47 and 0.57 mm/s, respectively.

A Lagrangian description of flows in stirred tanks via computer-automated radioactive particle tracking (CARPT)

In this study, computer-automated radioactive particle tracking (CARPT) is implemented for the first time in the characterization of flows in stirred tanks. Both the experimental technique and the experimental set-up are discussed. The CARPT technique is seen to capture qualitatively most of the important flow phenomena observed in stirred tank flows, like the two recirculating loops above and below the impeller and the dead zones at the bottom of the tank. The CARPT data is also used to extract “Sojourn” time distributions in different zones of the reactor. These distributions are used to partially quantify the observed dead and active zones in the tank.

Numerical simulation of gas–liquid dynamics in cylindrical bubble column reactors

In this paper, we have attempted to validate a transient, two-dimensional axisymmetric simulation of a laboratory-scale cylindrical bubble column, run under bubbly flow and churn turbulent conditions. The experimental data was obtained via gamma-radiation based non-invasive flow monitoring methods, viz., computer automated radioactive particle tracking (CARPT) provided the data on liquid velocity and turbulence, and computed tomography (CT) determined the gas holdup profiles. The numerical simulation was done using the FLUENT software and compares the results from the algebraic slip mixture model, and the two-fluid Euler–Euler model. Reasonably, good quantitative agreement was obtained between the experimental data and simulations for the time-averaged gas holdup and axial liquid velocity profiles, as well as for the kinetic energy profiles. The favorable results suggest that the simple two-dimensional axisymmetric simulation can be used for reasonable engineering calculations of the overall flow pattern and gas holdup distributions.

Comparative hydrodynamics study in a bubble column using computer-automated radioactive particle tracking (CARPT)/computed tomography (CT) and particle image velocimetry (PIV)

The hydrodynamics of a 10-cm-diameter cylindrical bubble column at the superficial gas velocity of 2, 4, and 8 cm/s are investigated by computer-automated radioactive particle tracking (CARPT), particle image velocimetry (PIV), and computed tomography (CT). These experimental techniques are capable of providing the knowledge of velocity and holdup fields in a bubble column system, which are essential as the experimental benchmark for modeling of such systems. The flow field of liquid phase, as well as the Reynolds stresses, obtained by CARPT and PIV in an air–water system are compared in detail. The results indicate that CARPT and PIV complement each other well. Further, the profile of gas holdup obtained by gamma ray based on CT compares favorably to the independently determined holdup.

Gas holdup distributions in large-diameter bubble columns measured by computed tomography

Using the computed tomography (CT) and computer automated radioactive particle tracking (CARPT) facilities at the Chemical Reaction Engineering Laboratory (CREL), time-averaged gas holdup distributions and liquid recirculation velocities were measured in a 44 cm diameter bubble column for air–water and air–drakeoil systems at 2, 5, and 10 cm/s superficial gas velocities, which cover bubbly, transition and churn-turbulent flow regimes, respectively. Gas holdup was found to increase only slightly with the increase in axial distance from the distributor, but increased significantly with the increase in superficial gas velocity, as expected. A lower gas holdup was observed in the air–drakeoil system than in the air–water system. This could be predominantly attributed to the formation of large bubbles in the former case due to the higher viscosity of drakeoil (approximately 0.03 Pas (=30 cp)). At high superficial gas velocities, the time-averaged cross-sectional gas holdup distributions were almost symmetric for both air–water and air–drakeoil systems. However, at 2 cm/s superficial gas velocity, an asymmetry in the holdup distribution was observed, which manifested itself in an asymmetric liquid recirculation pattern. At all gas velocities, the radial gas holdup distribution for the air–water system was steeper than that for the air–drakeoil system, yielding steeper radial liquid velocity profiles. Comparison of the gas holdup obtained in the 44 cm diameter column and that obtained in a 10 cm diameter column is discussed.

Tomographic and Particle Tracking Studies in a Liquid−Solid Riser

A liquid−solid circulating fluidized bed is a potential reactor of interest in a variety of industrial processes, such as petroleum refining, and in the synthesis of fine chemicals, petrochemicals, and foodstuffs. Rapid deactivation of the solid catalyst in these processes necessitates regeneration and recirculation of the solids into the riser section in which the principal reaction is accomplished. In this study we show that computer-automated radioactive particle tracking (CARPT) can be used to obtain solids velocity patterns in the riser and that backflow of solids exists at the tested liquid velocities. γ-ray computed tomography (CT) reveals slightly higher solids concentrations in the center of the column. This is in contrast to gas−solid riser reactors in which the concentration of solids is higher at the walls.

Liquid mixing based on convection and turbulent dispersion in bubble columns

A phenomenological model for liquid mixing, that captures the essence of the observed physical phenomena in bubble columns, is presented. In this model, mixing of the liquid phase is considered to be primarily due to convective recirculation and turbulence caused by the rising gas bubbles. The mixing phenomena is thus linked with the hydrodynamics of the flow. The input parameters of the model are obtained using noninvasive experimental techniques, such as Computer Automated Radioactive Particle Tracking (CARPT) and Computer assisted Tomography (CT) to measure the local liquid velocities and void fraction distribution in the system. In addition, it is shown that model parameters can be related to the turbulent eddy diffusivity measurements obtained by CARPT. The model is shown to compare well with tracer data and to reflect a variety of experimental observations. A brief comparison with the axial dispersion model (ADM) and recycle with cross-flow (RCF) model is presented.

Effect of scale on liquid recirculation in bubble columns

Successful scale-up of liquid hydrodynamics in bubble columns using a one dimensional model requires an adequate closure scheme for the Reynolds shear stress. The existing correlations for the prescription of the eddy viscosity or the mixing length scale are demonstrated to be applicable only for a limited range of conditions and consequently cannot be used for scale-up predictions. A method for estimating the mixing length scale has been explored and an attempt at unifying a wide range of data available in the literature within the purview of the method has been made. The futility of such an attempt is attributed to the nonreproducibility of the flow in different laboratories and the consequent lack of data obtained under identical conditions. It is demonstrated, however, that scale-up based on the mixing length distribution is possible when it is obtained from a consistent set of data for liquid velocity and gas void fraction profiles. Such a data set was obtained in the author's laboratory using CARPT (Computer Automated Radioactive Particle Tracking) and CT (Computed Tomography). Using the present method for prescribing the mixing length scale, model predictions for scale-up compare satisfactorily for the data that were obtained in the author's laboratory.

Liquid backmixing in bubble columns via computer-automated radioactive particle tracking (CARPT)

A single, fully wettable, neutrally buoyant, small radioactive particle and 16 scintillation detectors are utilized to monitor particle motion in gas-liquid bubble columns of different heights and diameters and at different gas superficial velocities. Instantaneous velocities, time-averaged velocities, Reynolds stresses and eddy diffusivities are computed from the particle position versus time data. Anisotropy, indicated by the data, was confirmed by Hurst's rescaled R/S analysis of the fluctuating velocities. For the radial direction, Hurst's exponent is about 0.5 in agreement with a random walk (diffusion)-type mechanism, while in the axial direction, Hurst's exponent of about 0.7 indicates the persistence of long-range effects.

Radioactive particle tracking technique for investigation of phase recirculation and turbulence in multiphase systems

The development of a noninvasive PC-based computer automated radioactive particle tracking (CARPT) facility for investigation of phase recirculation and turbulence in multiphase systems such as fluidized beds and bubble columns is presented. In this facility, the motion of a single radioactive particle, which is dynamically similar to the recirculating phase, is monitored by an array of scintillation detectors which surround the test section. An on-line computer is used to map the flow field of the recirculating phase. The data acquisition is achieved by commercially available nuclear instrumentation via a modular, high-speed GPIB-CAMAC system through assembly language software. Using CARPT, solids’ motion in gas-fluidized beds and liquid motion in bubble columns have been investigated. The capabilities and versatility of the CARPT facility is described by illustrating some typical results for mean recirculation in gas-fluidized beds with and without internals and in a gas-liquid bubble column. The results include the mean circulation profiles and turbulence parameters such as the Reynolds normal and shear stresses and the turbulent eddy dispersion coefficients. Potential applications of CARPT technique to other recirculating systems are also discussed.

Liquid backmixing in bubble columns

A noninvasive Computer Automated Radioactive Particle Tracking (CARPT) technique is used to quantify liquid backmixing in bubble columns of three different diameters over the range of gas velocities that span the bubble, transitional and churn turbulent flow regimes. The obtained axial dispersion coefficients (eddy diffusivities) are much larger than radial values and exhibit a stronger dependence on gas velocity. Column averaged axial diffusivities differ from the lumped effective axial dispersion coefficients, calculated from correlations, especially in smaller columns. Hurst's rescaled range (R/S) statistical analysis of the fluctuating velocities indicates different mixing mechanisms in the radial and axial direction. For the radial direction Hurst's exponent is about 0.5 indicating a diffusion type mixing mechanism. For the axial direction, Hurst's exponent is larger than 0.5 indicating the persistence of long range effects due to random turbulent eddies.

Flow mapping in bubble columns using CARPT

A noninvasive Computer Automated Radioactive Particle Tracking (CARPT) Facility is used for the investigation of liquid recirculation and turbulence in a bubble column. The motion of a single neutrally buoyant radioactive particle is monitored by an array of scintillation detectors and analyzed by an on-line computer to map the flow field. Results for the mean flow patterns and turbulence in a 12″ diameter column are reported for the air-water system. This communication marks the first application of CARPT for tracing the liquid flow in bubble columns.