Collision Efficiency Research Articles

Effects of soluble organics on the settling rate of modified clay and development of improved clay formulations for harmful algal bloom control

For many years, the dispersal of modified clay (MC) has been used to control harmful algal blooms (HABs) in coastal waters of China. MC flocculation efficiency can be influenced by many factors in variable and complex natural environments, including high concentrations of dissolved organic matter (DOM) in the water to be treated. Since many HABs occur in nearshore waters where DOM concentrations are high, this is a significant problem that requires urgent attention. This study involved the use of humic acid as a representative form of DOM to study the influence of organic matter on the MC flocculation process. At high concentrations, humic acid was adsorbed onto MC particles, resulting in a decrease in surface potential and an increase in electrostatic repulsion between the clay particles; this decreased the MC settling rate and increased the water clarification time. Flocs were characterized by their relatively small particle size, high particle concentration, and low collision efficiency, which together resulted in slow clarification of the water after MC spraying. Based on the mechanism of the DOM-MC interaction and combined with the Derjaguin-Landau-Verwey-Overbeek theory and theoretical considerations of clay surface modification, the “ionic atmosphere compression” method was used to improve MC flocculation efficiency in high-organic water. This method increased the ionic strength of the clay stock solution by adding salt, thereby compressing the ionic atmosphere of MC particles and lowering the potential barrier, allowing the MC particles in the treated water to flocculate rapidly and form large flocs, followed by further floc growth and rapid settling via differential sedimentation. The settling rate of MCs improved by a factor of two and the removal efficiency of the HAB cells increased by 7–28%. This study provides important baseline information that will extend the application of MC to HAB control in water bodies with high organic loadings.

The significance of positive and negative inertial forces in Particle-Bubble interaction and their role in the general flotation kinetics model

In this study, a theoretical evaluation of the effect of inertial forces in particle-bubble interactions during the flotation process is presented and supported by the experimental data. The effects of positive and negative inertial forces were analyzed by comparing the differences between the models, which either consider or neglect the inertial forces. The Sutherland collision model and the Nguyen attachment model that completely ignore the effect of particle’s inertial forces (inertialess models) were implemented into the general flotation kinetic model. The modified model was then compared with one of the most accurate inertial models, which considers the Generalized Sutherland Equation (GSE) for collision efficiency along with the Dobby-Finch model for attachment efficiency. The flotation kinetics of chalcopyrite and galena particles were estimated using the general flotation kinetic model in order to demonstrate the effect of particle density on the model, which emphasizes the effect of inertial forces. The influence of positive and negative inertial forces on flotation kinetics was evaluated for various explicit parameters such as particle density, turbulence (energy dissipation), and bubble size and velocity. Obtained theoretical results clearly showed the potential of the particle density to counterbalance the negative effects of the inertial forces. The capability of the positive inertial forces for galena particles (high density) to overcome its negative effect was shown when the general flotation kinetic model was used. Theoretical calculations were further confirmed by experimental bubble loading measurements. It was shown that the inertial forces should not be omitted in any flotation model amidst concerns over the complexity.

A Hybrid Device for Enhancing Flotation of Fine Particles by Combining Micro-Bubbles with Conventional Bubbles

Flotation in the mining industry is a very significant separation technique. It is known that fine and ultra-fine particles are difficult to float, leading to losses of valuable minerals, mainly due to their low collision efficiency with bubbles. Flotation of fine particles can be enhanced either by increasing the apparent particle size or by decreasing the bubble size. Literature review reveals that electroflotation resulted in higher recoveries of ultrafine particles as compared with dispersed-air flotation, because electrolytic bubbles are smaller in size. To this end, the best practical approach is to combine conventional air bubbles and micro-bubbles from water electrolysis. Therefore, the design, fabrication, and operation of a bench-scale micro-bubble generator through water electrolysis is proposed. Moreover, this electrolysis unit is adapted in a mechanical Denver-type flotation cell. The resulting hybrid flotation device is capable of producing bubbles within a wide range of diameters. The significance of this process is that micro-bubbles, attached tothe surface of fine particles, facilitate the attachment of conventional-sized bubbles and subsequently increase the flotation recovery of particles. Experimental flotation results so far on the hybrid device indicate the enhancement of fine particle recovery by approximately 10% with the addition of micro-bubbles.

Formation of Oil-Particle-Aggregates: Numerical Model Formulation and Calibration

When oil spills occur in turbid waters, the oil droplets and mineral grains can combine to form oil-particle aggregates (OPAs). The formation of OPAs impacts the vertical transport of both the oil and the mineral grains; especially increasing deposition of oil to the seabed. Though the coastal oceans can be very turbid, to date, few numerical ocean models have accounted for aggregation processes that form OPAs. However, interactions between oil and mineral aggregates may be represented using techniques developed to account for sediment aggregation. As part of Consortium for Simulation of Oil Microbial Interactions in the Ocean (CSOMIO), we modified an existing, population dynamics-based sediment flocculation model to develop OPAMOD, a module that accounts for the formation of OPAs. A zero-dimensional model using OPAMOD is shown to be capable of reproducing the size distribution of aggregates from existing laboratory experimental results. Also using the zero-dimensional model, sensitivity tests were performed on two model parameters, the fractal dimension and collision efficiency. Results showed that fractal dimension played a role in the OPA size distribution by influencing the effective particle density, which modified the number concentration of flocs for a given mass concentration. However, the modeled particle characteristics and oil sequestration were relatively insensitive to collision efficiency. To explore OPA formation for an outer continental shelf site, two simulations were conducted using a one-dimensional (vertical) implementation of the model. One scenario had high sediment concentration near the seabed to mimic storm-induced resuspension. The other scenario represented river plume sediment delivery by having high sediment concentration in surface waters. Results showed that OPA formation was sensitive to the vertical distribution of suspended sediment, with the river plume scenario creating more OPA, and sequestering more oil within OPA than the storm resuspension scenario. OPAMOD was developed within the Coupled Ocean-Atmosphere-Wave-and-Sediment Transport (COAWST) modeling system, therefore the methods and parameterizations from this study are transferrable to a three-dimensional coupled oil-sediment-microbial model developed by CSOMIO within the COAWST framework.

Crystal Growth, Dissolution, and Agglomeration Kinetics of Sodium Chlorate

The analysis, development, and implementation of novel complex industrial crystallization processes requires kinetic knowledge of not only crystal growth and nucleation but also breakage, dissolution, and agglomeration processes. In this work, the crystal growth, dissolution, and agglomeration kinetics of sodium chlorate (NaClO3) in aqueous solutions are estimated via seeded batch experiments with in-line particle size distribution measurements. Contrary to previous works, the growth/dissolution kinetics are expressed in terms of the fundamental driving force of crystallization calculated from the activity of supersaturated solutions. The activity-based driving force is roughly 2-fold higher than the commonly used representation, which assumes ideal solutions. By fitting experimental desupersaturation data to mechanistic and empirical growth models, we show that the growth of sodium chlorate is surface integration controlled and is best described by a two-dimensional birth and spread surface nucleation mechanism. The dissolution of sodium chlorate crystals is diffusion controlled and is ∼4 times faster than the growth at an equal initial driving force. Particle agglomeration is substantial in the early stages of crystallization experiments, likely due to a strong increase in the particle number due to initial breeding secondary nucleation upon seeding. The agglomeration rate constant increases with supersaturation and decreases at higher energy dissipation rate (by increased agitation) due to a strong decrease in the efficiency of interparticle collisions. Seeding with material of different sizes does not influence the agglomeration rate constant, although substantial amount of small particles was present in all seeding materials.

Experimental and modeling investigations of temperature effect on chemical inhibitors of asphaltene aggregation

In this work, both experimental and theoretical procedures were performed to investigate the effect of temperature, ranging from 25 to 65 °C, on the performances of dodecyl benzene sulfonic acid (DBSA), benzoic acid (BA) and salicylic acid (SA) as chemical inhibitors of asphaltene aggregation in an undiluted dead crude oil. DBSA was found the most efficient inhibitor in size reduction of aggregates at all applied temperatures due to its acid-base interactions with asphaltenes. The size reduction by DBSA was 19% compared to free-additive oil at 65 °C. But SA increased asphaltene aggregates size by 2% at 65 °C. The temperature rise significantly accelerates the rate of asphaltene aggregation and so the effectiveness of inhibitors was reduced at high temperatures. The population balance modeling was employed in which a novel size-dependent relation was considered for collision efficiency. Fractal dimension, as an input of the model, was measured from experimental data at different temperatures and time intervals. Due to minor changes in fractal dimensions, an average value of 2.5 was considered for fractal dimension. The modeling results revealed that the collision efficiency is decreased by both efficient inhibitors and decrement of temperature. Considering a size-dependent collision efficiency halved the error of the model on average in comparison to a size-independent collision efficiency.

Understanding the coagulation mechanism and floc properties induced by Fe(VI) and FeCl3: population balance modeling.

Coagulation kinetics and floc properties are of great fundamental and practical importance in the field of water treatment. To investigate the performance of Fe(VI) and Fe(III) salt on particle coagulation, Malvern Mastersizer 2000 was employed to continuously and simultaneously monitor the kaolin floc size and structure change, and population balance modeling was used to investigate the coagulation mechanism. The results show dosage increase had positive effect on collision efficiency and floc strength and negative effect on restructure rate. Low shear rate resulted in higher collision efficiency and stronger floc. Low water temperature had a pronounced detrimental effect on coagulation kinetics. Temperature increase showed the most significant positive effect on collision efficiency, floc strength and restructure rate. The optimum pH zone for the coagulation was found to be between 6 and 8. Further pH increase lowered the collision efficiency and floc strength and increased the restructure rate. FeCl3 resulted in a larger ratio of the mass to volume of kaolin flocs (compactness) than those induced by ferrate.

Effect of hydrothermal pretreatment on surface physicochemical properties of lignite and its flotation response

Flotation is the common method for fine low rank coal upgrading while it is always inefficient due to the rough surface morphology and rich oxygen-containing functional groups. Hydrothermal pretreatment is widely used in dewatering for low rank coal. The physicochemical properties of coal surface considerably vary following the hydrothermal pretreatment. In this study, hydrothermal pretreatments at different temperatures for different times were applied to enhance lignite flotation. The hydrothermal pretreatment at 200 °C can significantly improve the flotation efficiency, while the temperature raised to 300 °C, the flotation recovery did not increase or even decrease. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Zeta potential, Brunauer–Emmett–Teller, and particle size distribution analysis were conducted to explain the underlying mechanism. The contents of pore and crack on coal surface considerably decreased and more fresh and smooth surfaces exposed following hydrothermal pretreatment, according to SEM analysis. The FTIR spectroscopy and XPS showed that the contents of oxygen-containing functional groups were decreased, while the hydrophobicity of lignite was increased following pretreatment. The decrease of Zeta potential also indicated the decrease of oxygen-containing functional groups and increase of hydrophobicity on coal surface. However, excessive temperature of hydrothermal pretreatment could increase the pore diameter and fine particle fraction, leading to a higher collector consumption and lower bubble–particle collision efficiency. Consequently, the flotation recovery was not improved or even decreased at the hydrothermal pretreatment temperature of 300 °C.

Turbulent effects of vortex generators on the separation of fine particles

The separation of fine particles has always been challenging in flotation. Strong turbulence is considered to be beneficial. In our previous study, vortex generator (VG), commonly used to improve heat transfer by enhancing turbulence, was proved to be effective for fine particle flotation. However, the turbulent effect of VG on different sized fine particles is still poorly understood, which hinders the industrial application. Flotation tests and flow field simulations were conducted on a pipe flow unit with VGs and without VGs under different Reynolds numbers (Re). Results showed that, VGs improved the flotation of particles with a diameter less than 37 μm and especially for particles less than 15 μm. Nevertheless, VGs were detrimental to the flotation of particles in the size range of 37–74 μm when Re exceeds 10,000. These results are attributed to the high turbulent energy dissipation (ε) caused by counter-rotation vortex pairs induced by VGs. The increased ε enhances the collision efficiency while reduces the stability efficiency for increased particle size, which indicates there is a correlation between particle size and turbulence at which the optimal recovery can be achieved. In the experimental range of this study, the optimal flotation rate was observed when the ratio of turbulent Kolmogorov eddy scale and particle diameter is 0.3.

Hybrid path planning using positioning risk and artificial potential fields

Most conventional path generation algorithms search for an optimal path that avoids collisions with obstacles under the constraint of platforms' kino-dynamics. These conventional algorithms usually assume that a user position and obstacle locations are accurately known at any point in navigation environments. However, in a positioning network, the accuracy of a position estimate varies depending on, e.g., the ranging accuracy, network geometry, multipath error, and signal blockages which may lead to unexpected situations, including collision and low efficiency path planning. Therefore, positioning accuracy must be considered in path generation to ensure a reliable navigation capability and collision avoidance. To consider positioning accuracy in path planning, the proposed method in this paper uses a mixture of potential and positioning risk fields that generates a hybrid directional flow to guide an unmanned vehicle (UV) in a safe and efficient path. The results of simulations showed that the proposed method generated successful paths for around 90% percent of the tested routes, while using only the potential field method failed for around 50%. To demonstrate the effectiveness of the proposed local hybrid path planning method, we perform an experiment using a small-size quadcopter, and the results are analyzed and discussed.

Effective picture of bubble expansion

Recently the thermal friction on an expanding bubble from the cosmic first-order phase transition has been calculated to all orders of the interactions between the bubble wall and thermal plasma, leading to a γ2-scaling instead of the previously estimated γ1-scaling for the thermal friction exerted on a fast-moving bubble wall with a Lorentz factor γ. We propose for the first time the effective equation of motion (EOM) for an expanding bubble wall in the presence of an arbitrary γ-scaling friction to compute the efficiency factor from bubble collisions, which, in the case of γ2-scaling friction, is found to be larger than the recently updated estimation when the bubble walls collide after starting to approach a constant terminal velocity, leading to a slightly larger signal of the gravitational waves background from bubble collisions due to its quadratic dependence on the bubble collision efficiency factor, although the γ2-scaling friction itself has already suppressed the contribution from bubble collisions compared to that with γ1-scaling friction. We also suggest a phenomenological parameterization for the out-of-equilibrium term in the Boltzmann equation that could reproduce the recently found (γ2-1)-scaling of the friction term in the effective EOM of an expanding bubble wall, which merits further study in future numerical simulations of bubble expansion and collisions.

Hydrothermal Synthesis of TiO2 Aggregates and Their Application as Negative Electrodes for Lithium-Ion Batteries: The Conflicting Effects of Specific Surface and Pore Size.

TiO2 aggregates of controlled size have been successfully prepared by hydrothermal synthesis using TiO2 nanoparticles of different sizes as a building unit. In this work, different techniques were used to characterize the as-prepared TiO2 aggregates, e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer, Emmett and Teller technique (BET), field emission gun scanning electron microscopy (FEGSEM), electrochemical measurements etc. The size of prepared TiO2 aggregates varied from 10–100 nm, and their pore size from around 5–12 nm; this size has been shown to depend on synthesis temperature. The mechanism of the aggregate formations was discussed in terms of efficiency of collision and coalescence processes. These newly synthetized TiO2 aggregates have been investigated as potential negative insertion electrode materials for lithium-ion batteries. The influence of specific surface areas and pore sizes on the improved capacity was discussed—and conflicting effects pointed out.

Modelling the Effect of Electrification on Volcanic Ash Aggregation

The fine ash released into the atmosphere (particles <63 μm) during explosive volcanic eruptions represents a significant threat for both the ecosystem and many sectors of society. In order to mitigate the associated impact, ash dispersal models need to accurately estimate ash concentration through time and space. Since most fine ash sediments in the form of aggregates, ash dispersal models require a quantitative description of ash aggregation. The physical and chemical processes involved in the collision and sticking of volcanic ash have been extensively studied in the last few decades. Among the different factors affecting volcanic particle aggregation (e.g., turbulence, particle-particle adhesion, presence of liquid and solid water), the charge carried by volcanic particles has been found to play a crucial role. However, Coulomb interactions are not yet taken into account in existing models. In order to fill this gap, we propose a strategy to take charge into account. In particular, we introduce a quantitative model for aggregation of oppositely charged micron—to millimetre-sized objects settling in still air. Our results show that the presence of charge considerably enhances the collision efficiency when one of the colliding objects is very small (<20 µm), and that the sticking efficiency is not affected by particle charge if colliding objects are either small enough (<20 µm) or large enough (>200 µm). Besides providing a theoretical framework to quantify the effect of charge, our findings demonstrate that aggregation models that do not account for electrification significantly underestimate the amount of fine ash that sediments in the form of aggregates, leading to an overestimation of the residence time of fine ash in the atmosphere after explosive volcanic eruptions.

Importance of salinity-induced stratification on flocculation in tidal estuaries

Flocculation of suspended particles in tidal estuaries exhibits large spatiotemporal variability due to an interplay of various physical and biogeochemical drivers. Salinity (S) is known to promote flocculation of fine-grained suspended particulate matter (SPM). However, the influence of salinity and salinity-induced stratification on flocculation has not been sufficiently investigated yet. This study aims to understand how these two factors, interactively with turbulent shear (G) and SPM concentration (C), control the vertical variation of floc size and flocculation process in different depth layers in a typical tide-dominated estuarine environment. Analysis of field observation data shows that flocculi (diameter < 20 μm) are mainly affected by C and originate primarily from local resuspension. Macroflocs (> 200 μm) are mainly controlled by stratification that greatly improves aggregate collision efficiency; Microflocs (20–200 μm), as a transition group between flocculi and macroflocs, are affected by dynamics of both sides. They are influenced jointly by C, G and stratification. Besides, the fresh water-dominated surface layer is dominated by small particles (flocculi and microflocs), confined in a relatively narrow particle size range between Ο (100) and Ο (101) as a result of the low level of both C (13–20 mg/L) and S (< 2 practical salinity units). Below the surface layer, floc size increases drastically along with an increased salinity-induced density gradient and achieves maximum particle size (Ο (102)) within the stratified layer. Because of its high efficiency in promoting flocculation and formation of macroflocs, the stratified layer around the halocline can be regarded as an optimal flocculation zone. The benthic layer is characterized by high C (> 30 mg/L), gentle G (~5/s), and periodic stratification, which result in a wide size range between Ο (101) and Ο (102) with microflocs as the dominant group. Finally, we found that the accuracy of flocculation modeling can be significantly improved by integrating a simple relationship between particle collision and stratification.

Modelling of particle capture by expanding droplets

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Collision rate of bidisperse, hydrodynamically interacting spheres settling in a turbulent flow

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Feasibility Evaluation on Single-Collector Collision Model to Separate Microplastics in Micro Bubble Flotation Process

Objectives:The single collector collision (SCC) model is a model that predicts the separation efficiency of particles based on the collision efficiency between microbubbles and particles in the particle separation process. In order to remove micro-plastics (MPs) particles, which have recently been known to be harmful, we tried to analyze the impact and separation efficiency of MPs based on the SCC model and evaluate its application feasibility.Methods:Based on the SCC model, the collision efficiency prediction for MPs particle removal was simulated and the separation efficiency was evaluated. In addition, a series of flotation experiments were conducted using a flotation device that injects microbubbles to remove MPs particles suspended in water, and the experimental results and predicted values were compared.Results and Discussion:Using the SCC model, the collision efficiency according to the size distribution of MPs particles was not significantly different from that of typical particles (clay, kaolin) in water. Based on this collision efficiency, the maximum removal efficiency of MPs particles separated from the water body is predicted to reach about 90%. On the other hand, the initial collision-adhesion coefficient () of the MPs particles based on the SCC model was 0.03 to 0.1, showing a slight difference from the clay particles having the range of 0.3 to 0.4. The flotation separation efficiency of the MPs particles evaluated and predicted by applying this initial collision-adhesion coefficient showed a range similar to the measured value. In addition, the particle separation efficiency according to the increase of the measured bubble volume concentration to improve the particle separation efficiency also showed a pattern similar to the predicted value.Conclusions:In the flotation process, the initial collision-adhesion efficiency of the MPs particles predicted by the SCC model was lower than that of the kaolin particles. Based on the simple and simple model SCC theory, it was confirmed that it can be applied to the flotation separation analysis of MPs particles.

Particle interactions with permeable drops in shear flow

Motivated by particle removal from suspension by agglomeration with drops, the behavior of a spherical solid particle as it approaches a spherical drop covered with a rigid, permeable film is examined numerically. The particle and drop are in an external shear flow where viscous forces dominate. First, we revisit the classical problem of a particle interacting with a non-permeable drop, which includes closed orbits and open trajectories, but no collisional trajectories (without attractive forces present). Next, permeability is introduced, which modifies the two-sphere mobility functions for the system. For permeable drops, four types of trajectories are found, including infinity to drop (collision), infinity to infinity (miss), finite drop-to-drop, and closed-orbit trajectories. The collision efficiency as a function of the particle/drop size ratio and film permeability is then calculated. The efficiency dramatically increases with permeability, and it is maximum when the particle and drop are of nearly equal size.

Numerical investigations into the effect of turbulence on collision efficiency in flotation

At present, the theoretical understanding of the collisional interaction between particles and a single bubble under well-defined laminar flow conditions is relatively sound. However, there is a lack of experimental data and predictions on details of the interaction processes in stirred turbulent conditions, as exists in mineral flotation cells. This paper investigates the effect of turbulence on the efficiency of collision between particles and bubbles from a fundamental point of view. A computational fluid dynamics method is used to establish a bubble–particle collision efficiency model system for turbulent flow. The 3D model is used to systematically study the effect of fluid turbulence on bubble–particle collision efficiency. In order to isolate the phenomena caused by turbulent flow physics, the study is conducted in two parts: large-scale turbulence and small-scale turbulence, which are respectively larger and smaller than the bubble size. Large-scale turbulence is assumed to increase the speed of bubbles relative to the slurry, and in most cases, increase in the speed increases collision efficiency, though in some cases the gravitational effect can cause decrease in overall efficiency. Small-scale turbulence is treated through the effect of fluctuations on particle trajectories. Again, such turbulence increases collision efficiency when the turbulent particle Stokes number is less than or about unity, but has little effect for Stokes number much greater than unity. An equation for the increase in efficiency in the limit of small Stokes number is obtained from the simulations.

Effects of coarse chalcopyrite on flotation behavior of fine chalcopyrite

The flotation of ultrafine particles is marked with low recovery due to low collision efficiency between particles and bubbles. Carrier-flotation is a method to enhance the recovery by the attachment of fine particles onto larger carrier particles. In this study, the effects of coarse chalcopyrite particles as a carrier on flotation behavior of fine chalcopyrite was investigated by the flotation experiments using 20 g fine chalcopyrite (particle size D50 = 2.3 µm) with a varied amount of carrier particles of different sizes (–75 + 38 µm and –106 + 75 µm) using a 400 mL-mechanically agitated flotation cell. The addition of carrier particles improved the recovery of ultrafine particles into the froth from around 25% (without carrier) to around 80% (with 20 g of carrier, –75 + 38 µm size). Recovery of fine particles was higher with smaller carrier size (–75 + 38 µm) compared with larger carrier size (–106 + 75 µm), though the difference was not significant. Theoretical calculations showed that hydrophobic interaction played a more dominant role compared with van der Waals force in the attachment of fine chalcopyrite particles on the carrier (larger chalcopyrite particles).