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Acid mine drainage is wastewater from a mine having a low pH and an elevated level of dissolved heavy metals. These metals are harmful to aquatic, animal and human life. This paper looks at the removal of copper from acid mine drainage using ion exchange to less than 1 mg/l. A weak acidic cation resin was used. Spectrophotometric determination of copper with sodium diethyldithiocarbamate was used to determine the copper concentrations in the treated water. Using regression analysis, the experimental results gave a correlation coefficient of 0.977 and a coefficient of determination of 99.5%. Results indicated that the higher the flows rate the shorter the period after which the copper concentration in the treated water reaches 1 mg/l. At pH 3.85 and 5.09, the resin performed better and at pH above 6.62 and between pH 3.0 and below the resin’s does not perform well. The higher the resin height the greater is the resin exchange capacity and the longer it takes for the copper concentration to reach 1 mg/l in the treated water. The higher the wastewater copper concentration the shorter the time it takes the resin to reach 1 mg/l. The results for this experiment indicated that acid mine drainage can be treated well by ion exchange resins, but it is also very important to establish suitable operating conditions.

The open source code MFIX is used to perform CFD-DEM and TFM simulations based on the small scale challenge problem 1 fluidized bed set up. The basic flow features such as the core-annulus structure and slugging dynamics are well predicted by both modeling approaches. Similarly the first four statistical moments of the particle velocity distributions from HsPIV measurements are equally reproduced qualitatively by the models. The predicted mean pressure drop is independent of the drag correlation used in both modeling frameworks but the CFD-DEM model gives predictions closer to the experimental values. Predicted fluctuating quantities such as rms and granular temperature show sensitivity to the drag model used. The TFM demonstrates greater accuracy over the CFD-DEM in prediction of the particle granular temperature while the opposite is true for the bubble granular temperature. CFD-DEM simulations on average take twice the time required to perform a TFM simulation.

In this work, experimental data from the novel high speed particle image velocimetry (HSPIV) measurements and pressure fluctuations complemented by numerical predictions from TFM simulations are processed using advanced signal processing protocols to identify and characterize coherent structures in a gas–solid fluidized bed with Geldart D particles. The time-frequency properties of the bed dynamics were studied using the wavelet coherent analysis (WCA) and the Hilbert Huang transform. The WCA of numerical pressure drop signals at different measurement positions showed the existence of spatial-temporal coherent structures. At close measurement positions phase locked coherent phenomenon due to bubble generation dominate most of the frequency and time scales due the proximity to the distributor. At a wider measurement spacing the fast traveling waves are attenuated due to gas bubble/void coalescence and acceleration and only pockets of highly coherent oscillations are visible mostly in the frequency range of 0.5–3 Hz at certain times. Multi-resolution of the particle fluctuations realized with the Hilbert–Huang transform. The structures were resolved into the micro, meso and macro scales of fluidization based on the energy and frequency distributions. The meso scale structures form the main contribution to the normal axial Reynolds stresses and bubble granular temperature and ultimately the mixing.

It is common to characterize fluidized bed dynamics using spectral analysis and nonlinear methods such as correlation entropy (Kolmogorov entropy). In this paper, an alternative method, the Hilbert...

Accurate prediction of the onset of turbulent fluidization still remains elusive owing to the dependence of the transition velocity on several factors including measurement methods and interpretation of results. In this work, numerical simulations using the two fluid model (TFM) are performed in an attempt to predict the regime change reported by Gopalan et al. (2016) in a small scale pseudo-2D gas–solid fluidized bed containing Geldart D particles. Various time and frequency domain analyses were applied on predicted absolute and differential pressure time series data to reveal the bed dynamics. Numerical predictions of the transition velocity, Uc are in reasonably good agreement with experimental results from the small scale challenge problem. The literature correlations completely fail to predict the transition velocity for the system considered in this work. This work thus provides a different approach for validating the CFD model against experimental measurements.

In many industrial processes, electrostatic charges are inevitable and affect the hydrodynamic behavior and heat transfer ability of chemical equipment. A comprehensive study of the electrostatic effect on bubble behavior, particle fluctuation velocity and heat transfer coefficient in the fluidized bed with a central jet has been evaluated in this paper by Eulerian-Eulerian two-fluid model coupled with electrostatic model and energy model. The simulated voidage profiles at different positions, bubble detachment time and initial bubble diameter are compared with experimental results from the literature without charge. The bubble behaviors including bubble frequency and bubble numbers, combined with particle fluctuation parameters are analyzed in both charged and uncharged system. The electrostatic effect on two kinds of heat transfer coefficients is quantitatively compared, namely bubble to emulsion phase heat transfers based on the gas throughflow velocity and gas-solid local heat transfer coefficient. Simulation results show that electrostatic charges decrease bubble numbers and granular temperature, whereas the averaged heat transfer coefficients are enhanced. Overall, the electrostatic effect on the hydrodynamic and heat transfer characteristics can be revealed. This work was supported by the Project of National Natural Science Foundation of China (No. 91434205), the National Science Fund for Distinguished Young Scholar (No. 21525627), the Science Fund for Creative Research Groups of National Nature Science Foundation of China (No. 61621002).

This work explores the ability of the two-fluid-model (TFM) to model the dynamical and turbulent features of a pseudo 2D gas-solid fluidized bed operated under slugging conditions. 2D and 3D numerical simulations are performed to investigate the effect of the bed thickness on predicted quantities. Hi fidelity raw pressure drop and particle velocity data from the NETL small scale challenge problem is processed and used to validate the CFD model. Our work shows that the differences between 2D and 3D simulations in predicting the fluidized bed dynamics using pressure fluctuation data is minimal. However the effect of the bed thickness on turbulent properties namely the normal Reynolds stresses, turbulent kinetic energy, granular temperatures is significant. Taking into account the bed thickness does not necessarily improve the model predictions of all the dynamic and turbulent features. Furthermore mean profiles alone are not sufficient to validate TFM models as is quite common in the open literature. Mixing in the slugging bed is predominantly due to coherent meso-scale structures (voids and slugs) rather than individual particles as revealed from computed granular temperatures.

Vortex and electrostatic charges in the gas-solid fluidized bed have a significant influence on its transport abilities and hydrodynamics. In this work, the electrostatic model coupled with energy model has been applied to reveal the electrostatic effect on hydrodynamics, vorticity and local heat transfer coefficients based on the kinetic theory of granular flow. The results indicate that particle vortices change the gas and solid phase interaction around the bubble and enhance the local heat transfer coefficients. Gas interchange decreases by 6.5% compared to Davidson model at the jet velocity of 10 m·s−1 and 13% of 5 m·s−1. After adding electrostatic charges, bubble diameter decreases with the increasing specific charges. Furthermore, vorticity at the initial stage of bubble formation is larger and the particle vortex diffuses to a large extent. The simulation results can be applied to modify and estimate the overall heat transfer coefficient of the fluidized bed reactor and provide the basis for studying the effect of electrostatic effect on heat transfer.

Electric fields have great potential to control meso-scale structures, typically bubbles and agglomerates in gas-solid fluidized beds. However, previous research only considered the effects of polarization forces and ignored the electrostatic effects. This work, by means of cold model experiments, for the first time investigates the effects of DC electric fields on meso-scale structures in a 3-D fluidized bed with electrostatic effects. Results show that particles will also experience the Coulomb force under the effects of electrostatics besides of polarization forces, and the effects of these two electric field forces are different. The Coulomb forces make particles migrate at lower field strengths, then increases the bubble size and breaks up agglomerates. While at higher field strengths, the polarization forces result in formation of agglomerates and reduction of bubble size. Therefore, the effects mechanism of electric fields is the competition effects between the Coulomb force and the polarization forces. The critical transition field strength, at which the predominant position of Coulomb and polarization forces on bubbles and agglomerates changes, is significant in the control of meso-scale structures by DC electric fields.

Bubble and particle motion behaviors are investigated experimentally in a gas solid fluidized bed with liquid spray on the side wall. The particles used in the experiment are classified as Geldart B particles. The results reveal that when the fluid drag force is less than the liquid bridge force between particles, liquid distribute all over the bed. Bubble size increases as the increase of inter-particle force, then decreases owing to the increase of particle weight with increasing liquid flow rate. When the fluid drag force is greater than the liquid bridge force, liquid mainly distribute in the upper part of the bed. And it is difficult for the wet particles to form agglomerates. Bubble size decreases with increasing liquid flow rate due to the increasing of minimum fluidization velocity. Besides, the acoustic emission (AE) measurements illustrate that the liquid adhesion and evaporation on particles could enhance the particles motion intensity. Consequently, the bubble and particle behaviors change due to the variation in fluidized gas velocity and liquid flow rate should be seriously considered when attempting to successfully design and operate the side wall liquid spray gas solid fluidized bed.

Unsuccessful prediction of particle elutriation in fluidized beds is in part due to the negligence of electrostatic effects. In addition, lack of properly designed experiments makes it difficult to quantify the effects of electrostatics on particle elutriation. In this work, the CFD-DEM modeling approach has been used for the first time to investigate particle elutriation in a 2D fluidized bed taking into account electrostatic effects. Model predictions show that electrostatic charges on particles suppress particle elutriation and even eliminate elutriation completely at relatively higher charge levels. The study further shows that electrostatic effects result in the increase of the fluidized bed height and axial velocity of small particles in the free board which ultimately promotes particle elutriation. However, the presence of agglomerates caused by electrostatic attractions is dominant, thus the decrease in the concentration of small particles in the free board suppresses particle elutriation.

Particles and bubbles are important discrete elements in gas–solid fluidized beds. The interactions between them affect the evolution of agglomerates. Based on the analyses of microscopic physical processes, mathematical models of particle coalescence and agglomerate breakage caused by bubble expansion were developed. Coupling with the constant-number Monte Carlo method, the evolution of particle size distribution was simulated. Modeling results matched well with cold-mode experiment results, which demonstrated that the approach is a promising way to predict agglomeration behavior in the fluidized beds. Also, the coalescence efficiency β0 and breakage efficiency βb extracted from Monte Carlo results can provide rate constants for the kernels in population balance modeling. Further investigations of the effects of process parameters show that decreasing bed temperature or increasing superficial gas velocity reduces the coalescence efficiency and increases the breakage efficiency, which are beneficial to pr...

Solids circulation establishment in a cold model and industrial scale reactor has been investigated via visual observation, bed density, pressure drop, particle loss and acoustic emission (AE) techniques. The gas-solid flow pattern in the cold model and industrial unit share similar features based on the visual observation and bed density detection respectively. The circulating fluidized bed (CFB) with sweeping bend return initially exhibits stable fluidization in the riser and downer. As time elapses, the resistance fluctuation in the riser and downer is induced by the imbalance between the inlet and the outlet solids flow in the downer, leading to the unstable gas-solids flow pattern in riser and downer. The alternation between fluidized bed and moving bed is observed in the downer during the unstable stage, and further induces high-pressure drop fluctuations, heavy particle loss. Besides, the detection of pressure drop, particle loss rate and corresponding AE energy could be used to determine the time range of unstable stage.

The unstable solids circulation behavior in a circulating fluidized bed (CFB) with sweeping bend return is investigated by monitoring pressure fluctuations in a bench-scale model. The pressure fluctuations at the top and bottom of the riser and downer are processed using the Fast Fourier Transform (FFT) technique to extract the dominant frequency and its amplitude in order to quantify the effects of riser inlet gas velocity, valve opening and bed inventory on the operation of the unit. The differential pressure drop fluctuations between riser and downer are processed in a similar manner. The study reveals that the unstable solids circulation behavior is cyclical vis-a-vis gas–solids flow pattern and solids exchange between riser and downer, named as solids oscillation circulation behavior. A dominant frequency of 0.25 Hz is observed for all pressure fluctuation signals corresponding to the cyclical change of bed inventory and gas flow rate in riser and downer induced by the oscillation circulation behavior. An additional dominant frequency for the pressure fluctuation signals at the riser and downer bottom is also observed related to the gas–solid flow pattern. The solids exchange frequency is strongly related to the dominant frequency of the differential pressure drop. The work also demonstrates that the oscillation behavior is activated by a critical inlet gas velocity or valve opening and that the oscillation frequency increases with the decrease of inlet gas velocity, while it does not change with the valve opening. The solids exchange amount decreases with the decrease of gas flow and small percentage valve opening. The oscillation behavior changes from sine model to rectangular with the increase of bed inventory and thus slugging solids circulation is established.

Numerical simulations using the Gidaspow and Syamlal-O’Brien drag models have been performed based on the National Energy Technology Laboratory small scale challenge problem for three test cases. Validation tools including statistical moments, spectral analysis, and the autocorrelation function are used to analyze the pressure drop fluctuations. The analyses using these tools reveal a regime change from bubbling/slugging to turbulent flow with increasing superficial gas velocity. 1D discrete wavelet analysis is utilized in the study of the distribution of energy between the macro-, meso-, and microscales and also in the characterization of turbulence quantified in terms of the flatness factor. 1D fast Fourier transform energy spectra are constructed from the predicted and measured particle velocity data and compared. Predicted and measured granular temperatures are found to be in reasonable agreement with literature values.

This work for the first time shows that both falling polyethylene sheets and small agglomerates significantly affect the electrostatic behaviors in a fluidized bed. By cold model experiments, this work found that V-shaped fluctuations of induced electrostatic potentials were observed as a sheet fell to a certain position, and polarity reversals of induced electrostatic potentials were discovered as some small agglomerates were added and fluidized in the lower part of the bed. Further analysis found that the falling sheet could affect the particle concentration distribution in the bed as well as the surface charges of particles, and these two factors always had opposing effect on the induced electrostatic potential and thus caused V-shaped fluctuations to appear. The reason for the reversal of polarity as small agglomerates were added was the appearance of the positively charged agglomerates in the measuring sensitivity zone. This work opens up new possibilities for agglomerates detection.

For the first time the bubble-based EMMS drag model developed for a mono-disperse system has been extended to simulate a bi-disperse fluidized bed in conjunction with the multi-fluid model. Simulations with the EMMS drag model reveal that the system attains a fluidized state earlier. Furthermore the model predicts bed height and voidage in better agreement with data from the literature. Inclusion of the sub-grid drag correction is found to have a significant effect on the predicted axial pressure drop profile, mixing and segregation behavior, granular temperatures and turbulent kinetic energy. The effect of the specularity coefficient on the mixing index is also investigated.

An attempt has been made to model the flow structure and to predict heat transfer coefficients in a gas–solid bubbling fluidized bed operated with a central jet using a two fluid model with closures from the kinetic theory of granular flow. Quantities such as the fluid-to-particle heat transfer coefficient are not easily obtained practically from experiments with a high degree of accuracy thereby making computational methods attractive. The CFD model has been verified using experimental bubble properties obtained from the literature. Axial and lateral normal Reynolds stresses, energy spectra and granular temperatures have been computed. The simulations show that the maximum local instantaneous fluid-to-particle heat transfer coefficient occurs in the wake of the bubble. Heat transfer coefficients at the center of the bed exhibit high oscillations compared to the near wall region. However time averaged values in the near wall region are larger compared to the bed center. The average heat transfer coefficient exhibits a maximum value with the variation of the jet velocity.

Numerical simulations based on the Eulerian-Eulerian approach have been performed in the study of interphase heat transfer in a gas solid fluidized bed. The kinetic theory of granular flow (KTGF) has been used to describe the solid phase rheology. An assessment of drag models in the prediction of heat transfer coefficients shows that no major difference is observed in the choice of the drag model used. Fluctuations of the interphase heat transfer coefficient have been found to be closely related to the bubble motion in the bed. Effects of the wall boundary condition, inlet gas velocity, initial bed height and particle size on the predicted heat transfer coefficient have also been investigated. Typical temperature profiles in the bed show that thermal saturation is attained instantaneously close to the gas distributor. Simulated results of the coefficients are in fair agreement with those reported in literature.

A novel approach based on the passive acoustic emission (AE) monitoring technique has been established for analyzing particle–wall collision and friction separately in the present work. Using power spectrum density analysis, the main frequency of AE signal caused from particle–wall collision is found to be higher than that generated by particle–wall friction. Besides, a method for quantitatively extracting the information on particle–wall collision and friction has been set up by wavelet transform analysis. On the basis of these analyses, a theoretical approach has been established for relating the AE signals and solids loading ratio in a vertical pneumatic conveying pipe. The model predictions are verified using experimental data and are in good agreement. Particle mass flow rates obtained using this model give errors less than 6.62%. Conclusions can be drawn that the AE technique has great potential in the measurement of hydrodynamics in pneumatic conveying as well as similar particulate processes.