Coalescence Process Of Droplets Research Articles

Unstable Coalescence Mechanism and Influencing Factors of Heterogeneous Oil Droplets.

The use of a surfactant solution during oil and gas field development might improve the recovery rate of oil reservoirs. However, the serious emulsification of the produced liquid will bring challenges to the subsequent treatment process and storage and transportation. It is urgent to understand the coalescence mechanism of crude oil under the action of surfactant solution. This research investigates the coalescence mechanism of numerous oil droplets under liquid flow perturbation. The model was established to study the coalescence process of multiple oil droplets. The effects of the number of oil droplets under homogeneous conditions, the size of oil droplets, and the distance between oil droplets under non-homogeneous conditions on the coalescence process were analyzed. Meanwhile, the change rules of the completion time of oil droplet coalescence were drawn. The results show that the smaller the size of individual oil droplets under non-homogeneous conditions, the longer the coalescence completion time is, and when the size of individual oil droplets reaches the nanometer scale, the time for coalescence of oil droplets is dramatically prolonged. Compared to static circumstances, the time it takes for oil droplets to coalesce is somewhat shorter under gravity. In the fluid flow process, in the laminar flow zone, the coalescence time of oil droplets decreases with the increase of the liquid flow rate. However, in the turbulent flow zone, the coalescence time of oil droplets increases with the increase in the liquid flow rate. The coalescence time is in the range of 600~1000 ms in the flow rate of 0.05~0.2 m/s. In the presence of surfactants, the oil content in the emulsion system increases under the influence of pumping flow. The change in oil content rate with various surfactants is less impacted by flow rate, owing to the stable emulsion structure created by the extracted fluid within the reservoir. The study findings presented in this research provide technical assistance for effective crude oil storage and transportation.

Molecular Dynamics Study on the Effect of Polyacrylamide on Electric Field Demulsification of Oil-in-Water Emulsion.

The effect of the water-soluble polymer (partially hydrolyzed polyacrylamide, HPAM) in produced water on the demulsification process of the electric field was studied by molecular dynamics simulations. By comparing the coalescence process of oil droplets in the electric field environment with or without HPAM, we find that HPAM in the water phase can promote the coalescence of nearly oil droplets but hinder the deformation and migration of oil droplets. By analyzing the radial distribution function and interaction energy between molecules, we conclude that the existence of HPAM molecules can reduce the hydrophilicity of other molecules through their strong interaction with water, and sodium ions (Na+) have strong interaction with bound water in the process of breaking away from HPAM, thus leading the movement of water molecules. At the same time, the influence of HPAM molecules located between the two oil droplets on the demulsification process was also studied. The HPAM molecules and sodium ions located between the two oil droplets also affected the coalescence process of oil droplets under an electric field by interacting with water.

Demulsification law of polyether demulsifier for W/O crude oil emulsion containing hydrophobically modified polyacrylamide in water

Hydrophobically modified polyacrylamide (HMPAM) is one of the commonly used polymers for polymer flooding. Because it is water soluble, there are many reports on the impact of HMPAM on the stability and treatment of produced water (it is O/W emulsion), but few reports on its influence on the demulsification of W/O crude oil emulsion. In this paper, firstly, three types of HMPAM containing cationic, anionic, and nonionic hydrophobic monomers (abbreviated as P(AM-AANa-N)Cn, P(AM-AANA-S)Cn, and P(AM-AANA-O)Cn, respectively) were synthesized; Then, demulsification experiments and interface behavior experiments (including dynamic interfacial tension, dilatational rheology and dynamic coalescence time measurement) were conducted to investigate the influence of HMPAM on the demulsification performance of a W/O heavy oil emulsion demulsifier D1. The demulsification results showed that HMPAM reduced the dehydration speed of D1, the type of hydrophobic monomer and the length of hydrophobic chain in HMPAM can affect the final water separated of D1. P(AM-AANa-S)Cn, P(AM-AANa-N)Cn with long hydrophobic chain, and P(AM-AANa-O)Cn with short hydrophobic chains had the negative effect on performance of D1. The results of dynamic interfacial tension experiments indirectly showed that HMPAM had the influence on the D1′s conformation and speed of rearrangement at the W/O interface during the water droplet coalescence process, thereby affecting the reduction of elastic modulus (G′) of W/O interface and the coalescence rate of water droplets. When there was HMPAM in water, the coalescence time of water droplets increased, resulting in the decrease of dehydration rate. At the meantime, HMPAM had an impact on the lg(G1′/Gdemulsifier′)/lgG1′, thereby affecting the final water separated of D1.

Study on the mechanism of surfactant droplet wetting and coagulation of respiratory dust: The case of AEO-9

In industrial production, spraying is the most widely used dust control method. However, the traditional pure water spray has poor dust removal effect, and the principles of droplet wetting and dust encapsulation are unclear, resulting in poor dust removal effect. In this study, based on the CLSVOF method, we investigated the dynamic wetting and coalescence process of surfactant droplets and dust particles from a microscopic perspective. By changing the size ratio between the droplets and dust particles, as well as the initial velocity of the droplets, we simulated the spreading and wetting of surfactant droplets on the dust surface and found the optimal parameter range for encapsulating respirable dust. Through the experimental determination of spray atomization characteristics, it was found that the spray pressure of 6 MPa and the use of a 1.2 mm fine atomization nozzle produced a spray field with droplet sizes ranging from 14 to 21 μm, accounting for approximately 62.4 % of the distribution. Around 60 % of the droplets had velocities between 20 and 30 m/s. Dust settling efficiency experiments were conducted under these optimal spray parameters, and the settling efficiency of respirable dust reached 84.33 % within the favorable parameter range. Numerical simulations of the dynamic wetting process of dust-fog coupling at the microscopic level, as well as macroscopic spray experiments, were carried out to validate the reliable results. This method has significant implications for the control of respirable dust.

Dynamics of two unequal micron-sized viscous emulsion drops flowing in a two-dimensional constricted microcapillary channel: A numerical study

Emulsions containing unequal-sized droplets are ubiquitous. Compared to the equal-sized emulsion droplet coalescence process in porous media, unequal-sized emulsion droplets show distinctly different dynamics and features when flowing through the capillary channel. In this paper, we numerically studied the flow of two unequal small micron-size emulsion drops flowing in a two-dimensional (2D) half-sine pulse shape constricted tube. The finite element method solves the momentum, continuity, and level set equations. The oil–water interface is tracked using the level set method. Effects of drop size, capillary number (interfacial tension), viscosity ratio, and geometrical shape constriction effects on the droplets' flow behavior across the throat, its flow velocities, and average viscous pressure across the tube were investigated. It is found when there is a significant difference in diameters of the two un-equalized droplets, they will flow separately through the throat, whereas when the two un-equalized droplets’ size is close to each other, they coalescence with each other after passing through the constriction. Higher interfacial tension leads unequal-sized droplets to coalescence more readily while droplets pass without coalescence at low interfacial tension cases. The highest pressure drop is recorded at the critical point where the two unequal-sized droplets begin to merge in the constriction throat. Increasing droplet viscosity reduces the chances of coalescence and increases the average viscous pressure drop hence leading to mobility reduction. In the case of geometrical constrictions, circular-shaped contracted tubes offer the least resistance to fluid flow and are more appropriate for enhanced oil recovery processes. We hope our numerical findings will provide more information to facilitate the enhanced oil recovery process.

Droplet size distribution in annular flow affected by the breakup and coalescence processes

The present work studies the influence of droplet breakup and coalescence processes on droplet size distribution in annular flow. The influence of the breakup and coalescence processes on droplet number is reflected in four factors, i.e., breakup-induced droplet birth, breakup-induced droplet death, coalescence-induced droplet birth and coalescence-induced droplet death. The four effects jointly shape the probability density distribution of the droplet size after reaching a dynamic equilibrium. A numerical procedure is compiled using the microscopic mechanisms of the droplet breakup and coalescence process to calculate the droplet size distribution and characteristic droplet sizes. The numerical predictions are fully validated by comparison with the experimental data available in the literature. In addition, the birth and death of droplets in the breakup and coalescence processes are discussed in detail.

Lattice Boltzmann simulation of droplet coalescence in fuel cell flow channel with mechanical vibration

Fuel cells are the promising power source for future green vehicles and water management is an important issue for proton exchange membrane (PEM) fuel cells, which affects the lifetime and durability of fuel cells critically. In the available studies, the key parameters such as channel shape, airflow rate and material wettability have been extensively studied for water management. However, so far there is no open research on the effect of mechanical vibration on water management although most fuel cells work under vibration conditions. To bridge the knowledge gap, based on the Shan-Chen lattice Boltzmann model, this paper aims to reveal the water droplet coalescence process in an airflow channel under vibration conditions and several interesting observations are obtained for the first time. A large vibration amplitude or a small vibration frequency will not only hinder the movement of droplets, but also accelerate the coalescence of droplets. Also by comparing the simulation results it can be found that the droplet movement velocity decreases significantly after coalescence. Therefore, it is important to avoid large amplitude and small frequency for the operation of PEM fuel cells.

Mechanisms of microstructural deformation governing Vickers hardness in phase‐separated calcium aluminosilicate glasses

AbstractThe impact of microstructure on hardness in phase‐separated calcium aluminosilicate glasses is investigated. Changes in hardness are governed by microstructure deformations that occur during indentation. Phase separation leads to decreased hardness due to the incongruent yielding of the droplet and matrix phases. Moreover, the deformation of microstructures possessing dilute, spherical droplets did not have a significant impact on hardness. Microstructures characterized by concentrated, acicular droplets were found to deform through a process of droplet coalescence. This process absorbs additional energy during yielding and results in glasses that deform through droplet coalescence possessing improved hardness.

Investigation of droplet pairs’ movement under the electric field generated by matrix electrodes

Electric dehydration technology has gained lots of attention as an oil-water separation technology that has only been around for a short time. Many research scholars have studied the laws of droplet motion in electric fields. However, previous research on the droplet motion law in a non-uniform electric field has only focused on the single droplet, ignoring the dipole-dipole interaction between droplets, which plays an important role in the droplet coalescence process. In this paper, the motion of droplet pairs in a non-uniform matrix electric field is studied by means of theoretical analysis and numerical simulation. A mathematical model of the migration velocity and coalescence time of droplet pairs was established. The results show that the migration velocity of droplet pairs under dielectrophoresis and dipole-dipole interactions is significantly greater than that of single droplets and that the droplet velocity is related to the electric field strength, continuous phase viscosity, droplet diameter, and droplet pair center distance. This study increases our understanding of droplet electrocoalescence kinetics, enriches the electrocoalescence mechanism of droplets under non-uniform electric fields, and provides a basis for the design of subsequent electrodehydration equipment.

Effect of salt addition on the coalescence process of droplets on fiber

Fiber coalescence is an economical and effective technology for the treatment of oily wastewater. In this paper, the microscopic coalescence process of salt droplets on the fiber is studied using a high-speed camera and PIV, and the effect of salt on hydrodynamics and morphological evolution is discussed. The results show that the increase of salt concentration leads to a decrease of double layer force, a larger liquid bridge expansion, and a higher capillary wave propagation velocity. When the salt concentration is too high, the capillary wave propagation velocity decreases. A catchment region with a high velocity is formed near the traveling region of the capillary wave, and the velocity can reach 0.03 m/s. The attenuation coefficient of the oscillation process increases with the increase of salt content. By combining fiber and salt, the rate of droplet coalescence can be increased, and the production of secondary droplets can be reduced.

Inner circulation flow characteristics of coalescence droplets in microchannel

Controllable coalescence of multiple droplets is an important part of droplet manipulation, which is used to add samples to existing droplets, complete multi-step biological and chemical reactions inside droplet. It is of great importance to study the internal circulation characteristics for enhancing heat and mass transfer inside droplets. In this paper, microchannel experiments and volume of fluid method are conducted, and the evolution process of droplet coalescence is analyzed. The velocity distribution inside droplet is used as the main index to study the internal circulation characteristics under different surface velocities conditions. A characterization method of internal circulation strength based on central radial velocity and axial velocity inside the droplet is proposed, which can characterize the internal circulation region inside droplet caused by shearing action of microchannel wall of continuous phase. Results show that with the decrease of droplet length, the velocity distribution inside the droplet changes from being dominated by shearing action of microchannel wall to shearing action of continuous phase. In the process of droplet coalescence, there is a convective trend in the contact area between two droplets, and the maximum radial and axial velocities reach 3.5 times and 7.5 times of the droplet surface velocity respectively. This work can provide a reference for improving droplet control accurancy and mixing efficiency.

Increased aerosols can reverse Twomey effect in water clouds through radiative pathway

Aerosols play important roles in modulations of cloud properties and hydrological cycle by decreasing the size of cloud droplets with the increase of aerosols under the condition of fixed liquid water path, which is known as the first aerosol indirect effect or Twomey-effect or microphysical effect. Using high-quality aerosol data from surface observations and statistically decoupling the influence of meteorological factors, we show that highly loaded aerosols can counter this microphysical effect through the radiative effect to result both the decrease and increase of cloud droplet size depending on liquid water path in water clouds. The radiative effect due to increased aerosols reduces the moisture content, but increases the atmospheric stability at higher altitudes, generating conditions favorable for cloud top entrainment and cloud droplet coalescence. Such radiatively driven cloud droplet coalescence process is relatively stronger in thicker clouds to counter relatively weaker microphysical effect, resulting the increase of cloud droplet size with the increase of aerosol loading; and vice-versa in thinner clouds. Overall, the study suggests the prevalence of both negative and positive relationships between cloud droplet size and aerosol loading in highly polluted regions.

Coalescence law of microdroplet swarms in microchannels

Droplet microfluidic technology refers to the technique of preparing and controlling microscale monodisperse droplets by shear force and interfacial tension of two immiscible phases accurately, which has a wide range of applications in biomedicine, chemical reaction enhancement, nanoparticle preparation, and other fields due to its efficient mass and heat transfer, easy manipulation, rapid response, and high throughput. In the application of droplet microfluidic technology, coalescence often occurs due to the structural complexity of microchannels, which limits the practical application performance. The mechanism of droplet coalescence has been widely studied in conventional reactors, among which the liquid film drainage theory is the most widely accepted. To achieve accurate control of the droplet coalescence process, there have been many studies on droplet coalescence in microchannels. A variety of microchannel structures have been proposed and the mechanism and influencing factors of double droplets coalescence have been explored. However, most of the existing studies on coalescence are based on cross structures, mainly studying the coalescence of double droplets in confined systems. There is still a lack of research on the coalescence process of droplet swarms, which is more widespread in practical applications. Therefore, this work used the rectangular expansion structure to realize the droplet coalescence and explored the effects of two-phase flow rate, continuous phase viscosity, surfactant concentration, and the size of expanding channel on the droplet coalescence behavior. The coalescence types, probabilities, and size distributions of microdroplet swarms under different factors were summarized. The time distribution of liquid film drainage in the droplet coalescence process and the influence of various factors on it were further analyzed based on the liquid film drainage theory.

Lattice Boltzmann simulation of binary three-dimensional droplet coalescence in a confined shear flow

Small-scale microscopic phenomena determine the behavior of large-scale droplets, which brings great challenges to accurately simulate the droplet coalescence process. In this paper, the mesoscopic lattice Boltzmann method based on the phase field theory is used to simulate the collision and coalescence of binary three-dimensional droplets in a confined shear flow. The numerical prediction of droplet coalescence behavior was first compared with the experimental result, and good agreement was reported. Then, we investigated the influences of a comprehensive range of capillary numbers (0.01≤Ca≤0.5) and Reynolds numbers (0.01≤Re≤10) on the shearing dynamics of binary droplets and also provided a quantitative description of droplet behavior in terms of the droplet deformation parameter and relative trajectory. A shearing regime diagram is further constructed based on the coupling effect of Ca and Re, which reveals three distinct types of droplet behaviors, including coalescence, breakup after the coalescence, and non-coalescence. Concretely, three different patterns of droplets can be completely captured with the variation of Ca at low Re; only two types of coalescence and non-coalescence can be observed for a medium Re, and two droplets just slide over each other without the occurrence of the coalescence when Re is sufficiently large. Also, we identified two critical capillary numbers in the lower Re region and one critical capillary number in the middle Re region, respectively, characterizing flow type transitions from the coalescence to breakup, from the breakup to the non-coalescence, and from the coalescence to the non-coalescence. It is found that all the capillary numbers decrease with Re.

Molecular dynamics study of droplet electrocoalescence in the oil phase and the gas phase

• Dispersive attraction and steric effect of oil molecules affect droplet coalescence. • Solvent accessible surface area could determine when droplets contact and coalesce. • The migration of molecules causes uneven distribution of charges in water droplets. • The electro-coalescence behavior of droplets in oil and gas are highly similar. Under a uniform electric field, water droplets in a water-in-oil (W/O) emulsion approach and coalesce by dipole–dipole forces, which fundamentally affect the efficiency of electric dehydration. In order to unravel the molecular mechanism of droplet electrocoalescence, we explored the coalescence behavior of droplets in the oil phase and the simplified oil phase (nitrogen) under an external electric field using molecular dynamics (MD) simulations. The results indicated that the coalescence process of droplets can be divided into three stages based on the solvent accessible surface area (SASA), before coalescence, coalescing and after coalescence. The two droplets completely coalesced when the center-of-mass distance between droplets was 2.73 nm. Weak interaction and interaction energy analysis revealed that the electrostatic attraction between water molecules dominated the coalescence process of water droplets due to the significant characteristics of electrostatic potential (ESP). The dispersion attraction of C 6 H 14 H 2 O and C 6 H 14 C 6 H 14 was very pronounced while the interactions related to nitrogen molecules were negligible, which was the underlying reason for the differences between droplet coalescence behaviors in the two media. Nevertheless, the coalescence behaviors of a droplet pair in oil and gas had a high level of similarity, which implied that the MD simulation results of the electric coalescence based on nitrogen is acceptable to some extent. The field-induced hydrogen and oxygen atoms migrated toward both ends of the droplet, causing the redistribution of charges and changing the overall dipole moment of the droplets. This study made it possible to use oil-phase molecules for MD simulation study on the electrostatic coalescence behavior of droplets in the W/O emulsion, and confirmed the feasibility of nitrogen replacing with the oil phase.

Study on demulsifier crude oil interactions at oil-water interface for crude oil dehydration

Stabilization mechanisms of water-in-oil emulsions have been well studied. The process of dehydration with demulsifiers in these previous works have focused primarily on macro scale bottle tests or on microscale investigations of asphaltene film. A connection between the two scales, however, is necessary to better understand the mechanisms of dehydration using demulsifiers. In our present work, results from bottle tests of undiluted crude oil were compared with the coalescence process of water droplets in solvent diluted crude oil and with visualization experiments of the interfacial film rigidity through observing the crumpling of water droplets in diluted crude oil. To gain further insight into the molecular interactions between the native surface active materials and three different polymeric demulsifiers at the solvent-water interface, Langmuir interfacial isotherms were measured using diluted crude oil-water systems with and without the addition of demulsifiers. The highest water removal percentage was obtained though the addition of a random biopolymer demulsifier, butyl acrylate/2-(dimethylamino) ethyl acrylate (P1), that also rendered the shortest coalescence time of the three polymers tested, indicating the importance of coalescence behavior in dehydration. In addition, the smallest crumpling ratio which corresponds to lowest rigidity was detected for the P1 treated interface. For the first time, we observed that the diluted crude oil-water interfacial film collapsed easily upon compression with the administration of the very efficient demulsifier P1. It was found that the collapse pressure can be used as a measure of the rigidity of very soft interfacial films that may not be measurable directly by compression energy. Based on the close relation between the dehydration process and the interaction between demulsifiers and natural active surfactants, it can be deduced that, efficient demulsifiers essentially disrupt the continuity of the asphaltenic interfacial films and reduce the strength of anchoring of these natural surfactants at the interface, allowing for the collapse of segments and resulting in efficient dehydration.

Study of Stratus-Lowering Marine-Fog Events Observed During C-FOG

Two stratus-lowering marine fog events observed on 28 September and 4 October 2018 during the Coastal Fog (C-Fog) field campaign that took place offshore of eastern Canada from 1 September to 6 October 2018 are described. In situ, profiling, and remote sensing observations were made at selected land sites in eastern Newfoundland, Nova Scotia, and aboard the research vessel Hugh R. Sharp cruising in adjoining coastal waters. Synoptic-scale analysis shows that both fog episodes result from the interaction between synoptic-scale surface-level low-pressure systems and a contiguous high-pressure system. At the same time, back trajectories reveal that the bulk of the fog layer is formed due to differential advection. The diameter of the fog droplets at the surface gradually decreases from the centre of the fog layer to its leading/trailing edges. The bimodal fog-droplet diameter distribution with peaks at 5–10 µm and 20–25 µm provide clues on droplet collision and coalescence processes. The observed difference between microphysical variables and droplet distribution between the two fog events and within the same fog layer might be governed by the atmospheric-boundary-layer (e.g., humidity conditions and turbulence) that prevailed in the fog layer. Overall, it is concluded that the life cycle of observed stratus-lowering coastal-fog episodes depends on synoptic conditions and atmospheric-boundary-layer characteristics such as stability, cloud-top cooling, and entrainment.

Enhancing oil-in-water emulsion separation performance of polyvinyl alcohol hydrogel nanofibrous membrane by squeezing coalescence demulsification

The membrane fouling is the obstruction that limits the efficiency and durability of the nanofibrous membrane in oil-water emulsion separation. Herein, a polyvinyl alcohol (PVA) hydrogel nanofibrous membrane could continuously separate emulsion by using squeezing coalescence demulsification (SCD). During this SCD process, the small oil droplets can coalesce into bigger ones under the mandatory compression of the pore wall. And then the coalesced oil droplets will pass through the nanofibrous membrane and form an oil layer, and the oil layer can be easily removed. Furthermore, a two-phase numerical model is proposed to demonstrate this coalescence process of oil droplets in the pore. The numerical results also indicate that the pore size, wettability of membrane, and the interface tension coefficient between oil and water can control the efficiency of SCD. This SCD method endows PVA hydrogel nanofibrous membrane with a sustained separation capability, which implys that this method has great potential in practical application.

Coating flows down a vertical fibre: towards the full Navier–Stokes problem

Abstract

Modeling of carbon dioxide dissolution in an injection well for geologic sequestration in aquifers

Carbon dioxide (CO2) sequestration is considered to be one of the most effective technologies of mitigating greenhouse gas emissions. In this technology, single phase supercritical CO2 is injected into an underground geological formation such as a deep saline aquifer. Existing sequestration projects demonstrate that successful implementations are possible; however, significant uncertainties associated with the risks of leakage remain an obstacle for broader use of this technology. The security of underground disposal could be considerably increased by dissolving the CO2 in a brine produced from the aquifer, then re-injecting the mixture underground. The dissolution process occurs before the mixture reaches the aquifer; this significantly reduces or completely eliminates the risks of CO2 leakage. This technique can drastically extend the amount of worldwide aquifers available for carbon sequestration. As was previously shown, complete dissolution could be achieved in a surface pipeline operating under the pressure of a target aquifer, where CO2 is injected. In this paper, a comprehensive model of CO2 droplet dissolution in a vertical injection well is presented. The model accounts for droplet breakup, coalescence, and dissolution processes as well as temperature and pressure variations over well depth. Feasibility and results are discussed and compared with surface dissolution options.