Dr Roberto Moreno-Atanasio

This work presents a computational study based on Discrete Element Method (DEM) to investigate the capture of particles by bubbles in the presence of electrical double layer repulsion. In the DEM model a fully mobile boundary condition was assumed for the gas-liquid (bubble) interface. The forces acting on the particle were gravitational, buoyancy, hydrodynamic, as well as elastic and damping forces if/when the particle penetrated inside the bubble. Surface forces were considered and were evaluated through the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. A preliminary theoretical analysis of the surface forces was carried out in order to determine the possibility of particle capture. This analysis included the determination of the interaction potential energy. Five different types of interaction energy curves were found. They were characterised by (1) a monotonic increase with distance; (2) the presence of a primary minimum; (3) the presence of an energy barrier and a primary minimum, (4) the presence of an energy barrier and a primary and a secondary minimum, and (5) a monotonic decrease with distance. DEM modelling was conducted to investigate the induction time for a single particle-bubble system. It was found that the induction time decreased with increasing contact angle and decreasing height of the energy barrier. In contrast, the primary minimum had very limited impact on induction time. Additionally, modelling of a more complex system consisting of a single bubble and multiple particles was also carried out. The multiple particle-single bubble simulation results showed that a decrease in induction time considerably enhanced collection efficiency. Finally, the concept of ¿contactless¿ flotation, which occurs in the case in which only a primary minimum exists, was also demonstrated through the use of DEM modelling.

Colloidal silica is used in many applications including catalysis, pharmaceuticals, and coatings. Although naturally formed silica materials are widely available, they are often in forms that are difficult to process or are even harmful to health. Therefore, uniform colloidal silicas are generally manufactured using synthetic chemical processes. While established high temperature gaseous synthesis methods fall out of favor in our energy conscious society, liquid synthesis methods are current industrial leaders. The precipitated silica method provides the majority share of commercially produced specialty silicas with its economic advantages predicted to continue to grow in the future. The biomimetic method and microemulsion methods of synthesis provide a superior level of surface chemistry and morphological control than current industrial processes and are the major focus of current silica synthesis research. Movement toward more tailor-made products and ecologically friendly production methods will likely provide incentive for biomimetic methods, in particular, to take more of a market share. However, the lack of procedures to viably scale up the biomimetic and microemulsion methods still forms significant gaps in the literature. In this review, the current methods of colloidal silica synthesis are discussed alongside significant models and mechanisms of silica formation.

This paper presents a computer simulation analysis of the selective capture of binary particle mixtures by a central bubble, as influenced by the relative strength of the hydrophobic interaction assigned to each type of particle. The analysis was carried out for a quiescent fluid using two different configurations of initial particle positions, namely: spherical (particles released from within a spherical shell surrounding the bubble) and top (particles released from a horizontal plane located above the bubble) distributions. The top distribution was also used to study the effect of fluid velocity (< 0.05 m/s). The results show that in the case of a quiescent fluid the collection efficiency was greater for the top distribution than for the spherical one. In addition, when the strength of the hydrophobic force was less than the net particle weight, particles easily detached from the bubble surface. In the presence of fluid flow the collection efficiency followed an exponential decay with the fluid velocity and a quadratic relationship with an effective cross-section for the particle-bubble collision. The latter closely follows the collision models in the literature. Importantly, we have shown that selective capture only occurs when one type of particle possesses a hydrophobic force magnitude close to or less than the net particle weight, while the hydrophobic force for the second type needs to be much larger than the net weight of the particle. Therefore, we have concluded that selectivity does not depend solely on the hydrophobicity differences, but also requires that one type of particle has to be weakly interacting with the bubble.

Rapid advancement in computer technology makes it possible to perform simulations of large particulate systems using Discrete Element Method (DEM). However, it is still a challenge to adjust the DEM simulation parameters, especially the interparticle forces, in order to obtain rational results. Many of these forces diverge as the distance between particle surfaces approaches zero. Therefore, a cut-off distance needs to be considered in order to avoid such a problem. This work presents the results of the computational analysis of the influence of the cut-off distance, which is required to avoid the divergence of the van der Waals force, on the collision of a single particle against a flat surface. Hence, the effect of the cut-off distance on the coefficient of restitution, collision duration and maximum overlap has been studied. In addition, we have theoretically derived an expression for the minimum velocity under which the particle remains adhered to the surface (critical velocity) as a function of the cut-off distance. The simulation predictions of the critical velocity are within the range of experimental data published in the literature. We demonstrate that the cut-off distance has a profound influence on particle rebound and therefore, a careful selection of this parameter should take place when simulating bulk particle behaviour. Given that the hydrophobic force is usually simulated using the same expression as the van der Waals force, the results presented here could also be considered in the context of the simulation of the hydrophobic force.

Polyamine-derived silica particles are proposed to grow due to primary particle (<25 nm) aggregation. On the basis of material balance, we propose an aggregation model to predict the kinetics of polyamine-derived silica formation. The model is based on a rate equation consisting of two terms, representing both the production and aggregation of primary particles. The modeled rate constants were found to be a function of the concentration of silica precursor and biomimetic catalyst. Our experimental results agree with our model and suggest that the cube root of the silica precursor concentration is linearly proportional to particle diameter, as well as that below a critical concentration (~25 mM) no particles will be formed. If the concentration of reagents was high enough to produce particles with diameters greater than ~350 nm, other populations of particles, each of them growing at different rates, were necessary to describe the overall particle diameter observed, which was modeled as the average diameter of these different populations.

A high internal phase (HIP) water-in-oil emulsion was used as the binder in the selective agglomeration of fine coal from an aqueous suspension of coal and mineral particles. Traditionally, this agglomeration is achieved by a pure oil, hydrophobic, binder. However, the high cost associated with using pure oil makes the process economically unfeasible. Therefore, the emulsion binder introduced in this work was motivated by the economic need to reduce the amount of organic liquid required in the process. The effect of the agitation time during the agglomeration process and the composition of the emulsion on its performance as a binder were investigated. The best result obtained was for a HIP emulsion made from 3 wt % aqueous NaCl and diesel oil with sorbitan monooleate as the emulsifier. This emulsion had a dispersed phase volume fraction of 0.94 and achieved a 7.5-fold reduction in the amount of organic liquid required to achieve agglomeration.

Numerical simulations have been carried out to investigate the formation and motion of single bubble in liquids using volume-of-fluid (VOF) method using the software platform of FLUENT 6.3. Transient conservation mass and momentum equations with considering the effects of surface tension and gravitational force were solved by the pressure implicit splitting operator (PISO) algorithm to simulate the behavior of gas-liquid interface movements in the VOF method. The simulation results of bubble formation and characteristics were in reasonable agreement with experimental observations and available literature results. Effects of fluid physical properties, operation conditions such as orifice diameter on bubble behavior, detachment time, bubble formation frequency and bubble diameter were numerically studied. The simulations showed that bubble size and bubble detachment times are linear functions of surface tension and decrease exponentially with the increase in liquid density. In contrast, only a small influence of the fluid viscosity on bubble size and detachment time was observed. Bubble collapse at a free surface simulation with VOF method was also investigated. © 2014 Korean Institute of Chemical Engineers, Seoul, Korea.

High-resolution dynamic light scattering (DLS), scanning electron microscopy (SEM), time-lapse photography, and attenuated total reflectance Fourier transform infrared spectroscopy were used to analyze the growth kinetics of polyethyleneimine (PEI)-silica particles fabricated from the condensation of hydrolyzed trimethoxymethylsilane (TMOMS) and PEI/phosphate buffer (PEI/PB). Depending on the concentration of hydrolyzed TMOMS and PEI/PB, three stages were identified. We observed the existence of a nucleation time that has never been reported in the literature when TMOMS has been used. During this nucleation time, particles of less than 25 nm were detected using in situ DLS measurements taken every 15 s (high resolution), a DLS time-scale resolution not previously reported. In addition, the length of the nucleation time depended mainly on the PEI/PB concentration, but also TMOMS concentration. The growth stage was evident from the rapid increase of particle size with time. Due to the high resolution of the DLS measurements, a peak could be observed in the particle diameter during particle growth, which corresponds to a secondary population of particles required for the larger particles to further increase in size. Finally, during the equilibrium region, particles reached their maximum diameter that was independent of the concentration of PEI/PB and only changed with concentration of hydrolyzed TMOMS. © 2014 Springer-Verlag Berlin Heidelberg.

This study was concerned with the interaction between a gaseous dispersion of fine particles travelling in the horizontal direction and discrete drops of water falling vertically through the dispersion. A simple analytical model of the particle-drop collision was developed to describe the particle recovery by the drops as a function of the water flux, covering two extremes of relative velocity between the particles and drops. The Discrete Element Method was used to validate the analytical model. Further validation of the model and insights were obtained through experimental studies. The physical process of wetting was observed to be important in influencing the tendency of particles to become engulfed by the drops of water, or to either adhere to the drops or by-pass the drops altogether. Hydrophilic particles were readily engulfed while hydrophobic particles, at best, adhered to the surface of the drop, or failed to attach. Moreover, the recovery of the hydrophilic silica particles was significantly higher than the recovery of hydrophobic coal particles, with the selectivity ratio approximately 1.5. Spherical ballotini particles were the most sensitive, with a notable increase in recovery when cleaned, and evidence of increased recovery with increasing particle size. The recovery of irregular shaped silica flour particles, however, was largely independent of the particle size. A similar result was observed for irregular coal particles, though the recoveries were all lower than relatively more hydrophilic ballotini or silica flour. Crown Copyright © 2014.

The objective of this paper is to present a co-precipitation technique with calcium carbonate for removal of heavy metals ions from oil field brine. Metals such as Mercury, Copper and Lead can be removed by the co-precipitation technique. In this work, parameters such as pH, injection time and concentration of reactants were optimized in other to obtain maximum removal efficiency. The results showed that pH has a significant effect while the injection time and reactant concentrations were less effective to remove the metals. Based on the SEM, ICP and hydrometer analysis, the optimum experimental conditions were the pH of 10, the injection time of 20 min and the reactant concentration of 0.5 mol percent. Approximately, more than 90 percent of mercury, lead and copper was removed employing co-precipitation techniques with calcium carbonate. Scanning Electron Microscopy analysis indicates that 200 rpm is the optimum rotational speed which leads to the formation of small particles. Finally, this study showed especially that the co-precipitation technique is a good alternative to remove the heavy metals especially mercury from the contaminated oil field brine.

Copyright 2015, Society of Petroleum Engineers. The objective of this paper is to present a co-precipitation technique with calcium carbonate for removal of heavy metals ions from oil field brine. Metals such as Mercury, Copper and Lead can be removed by the co-precipitation technique. In this work, parameters such as pH, injection time and concentration of reactants were optimized in other to obtain maximum removal efficiency. The results showed that pH has a significant effect while the injection time and reactant concentrations were less effective to remove the metals. Based on the SEM, ICP and hydrometer analysis, the optimum experimental conditions were the pH of 10, the injection time of 20 min and the reactant concentration of 0.5 mol percent. Approximately, more than 90 percent of mercury, lead and copper was removed employing co-precipitation techniques with calcium carbonate. Scanning Electron Microscopy analysis indicates that 200 rpm is the optimum rotational speed which leads to the formation of small particles. Finally, this study showed especially that the co-precipitation technique is a good alternative to remove the heavy metals especially mercury from the contaminated oil field brine.

Recent findings on the influence of single-particle properties on the bulk behaviour of dense particulate assemblies are analysed by using computer simulations based on 3-D Distinct Element Method (DEM). The micromechanical behaviour of dense systems depends on the ability of the particles to deform along the normal and tangential directions on the contact plane, i.e., the values of normal and tangential particle contact stiffnesses. In the present study, we investigate the effect of the stiffness ratio, at the macroscopic and microscopic scales, on the behaviour of three systems subjected to quasi-static shearing. These three systems are made of sphere, oblate and prolate particles, respectively. We show that, the variation in the contact stiffness ratio affects the micromechanical characteristics of non-sphere particulate systems more dominantly than the sphere particulate systems. Hence, attention must be paid to measure both the normal and tangential contact stiffnesses when characterising non-sphere fine particulates to estimate their assembly strength characteristics during shearing. © 2006 Civil-Comp Press.

In this work the effect of contact stiffness and surface energy on fluidization behavior of cohesive powders is analysed using a combined continuum and discrete model (CCDM). Four batches of particles with different combinations of contact stiffness (50 N/m and 50000 N/m) and surface energy (0.37 mJ/m 2 and 3.7 mJ/m 2) were studied. The analysis has been carried out in terms of the number of interparticle contacts, average contact forces and deformations. The simulation results indicate that for low values of surface energy (0.37 mJ/m 2) the contact stiffness does not influence appreciably the fluidization behavior. In contrast, for large values of surface energy (3.7 mJ/m 2) the contact stiffness influences the fluidization behavior significantly where poor fluidization occurs for systems with small contact stiffness whilst good fluidization is observed for the system with larger contact stiffness. © 2005 Taylor & Francis Group.

The current study adapts a novel approach to directly measure the effect of particle adhesion on the bulk powder flow characteristics. This is achieved by modifying the adhesion of spherical glass particles (mean size: 38 µm) through a protocol that involves the deposition of surface silane monolayers on individual glass particles. The increase in particle surface energy has been characterized by particle-particle pull off force measurements using Atomic Force Microscopy. An annular shear cell has been used to measure flow properties and to quantify the effect of surface energy of the silanized glass particles on their bulk cohesion. Computer simulations using the Distinct Element Method (DEM) have also been carried out. The DEM simulations use JKR adhesion model to incorporate the surface energy values of glass beads corresponding to the AFM pull-off force measurements. The Unconfined Yield Stress (UYS) and Flow Factor (ff) obtained by the simulations have been compared with the experiments. © 2005 Taylor & Francis Group.