Molecular Dynamics Simulation for the Demulsification of O/W Emulsion under Pulsed Electric Field

A bidirectional pulsed electric field (BPEF) method is considered a simple and novel technique to demulsify O/W emulsions. In this paper, molecular dynamics simulation was used to investigate the transformation and aggregation behavior of oil droplets in O/W emulsion under BPEF. Then, the effect of surfactant (sodium dodecyl sulfate, SDS) on the demulsification of O/W emulsion was investigated. The simulation results showed that the oil droplets transformed and moved along the direction of the electric field. SDS molecules can shorten the aggregation time of oil droplets in O/W emulsion. The electrostatic potential distribution on the surface of the oil droplet, the elongation length of the oil droplets, and the mean square displacement (MSD) of SDS and asphaltene molecules under an electric field were calculated to explain the aggregation of oil droplets under the simulated pulsed electric field. The simulation also showed that the two oil droplets with opposite charges have no obvious effect on the aggregation of the oil droplets. However, van der Waals interactions between oil droplets was the main factor in the aggregation.     

Stability of Water/Crude Oil Systems Correlated to the Physicochemical Properties of the Oil Phase

A characterization of 30 crude oils has been performed to determine the relative level of influence that individual parameters have over the overall stability of w/o emulsions. The crude oils have been analyzed with respect to bulk and interfacial properties and the characteristics of their w/o emulsions. The parameters include compositional properties, acidity, spectroscopic signatures in the infrared and near‐infrared region, density, viscosity, molecular weight, interfacial tension, dilational relaxation, droplet size distribution, and stability to gravitationally and electrically induced separation. As expected, a strong covariance between several physicochemical properties was found. Near‐infrared spectroscopy proved to be an effective tool for crude oil analysis. In particular, we have showed the importance of the hydrodynamic resistance to electrically‐induced separation (static) in heavy crude oil‐water emulsions. A rough estimate of the drag forces and dielectrophoretic forces seemed to capture the difference between the 30 crude oils. Given enough time, water‐in‐heavy oil emulsions could be destabilized even at very low electric field magnitude (d.c.). When droplets approach each other in an inhomogeneous electric field, strong dielectrophoretic forces disintegrate the films and result in coalescence. The relative contribution from film stability to the overall emulsion stability may therefore be very different in a gravitational field compared to that in an electrical field.     

Emulsions of Heavy Crude Oils. II. Viscous Responses and Their Influence on Emulsion Stability Measurements

The stability of 30 heavy crude oil emulsions was studied in a parallel-plate laboratory coalescer (DC field). Particularly, viscous responses and their influence on the emulsion stability measurements were investigated. In addition to highlighting previous results from the same experimental setup and discussing these based on recent experience, new results at different temperatures and volume fractions of water were presented. A new semi-empirical model for the characteristic time of the destabilization process was presented. The electrical forces were modelled with a point-dipole approximation and the hydrodynamic resistance to droplet transport was modelled with an empirical term including the logarithmic viscosity of the oil phase. The new model clearly performed much better than the previous model, particularly for very viscous crude oils. Studies of the performance of industrial electrocoalescers have showed that simple electrostatic theory can potentially explain complex separation phenomena when the resistance to the coalescence step is reduced by an efficient demulsifier. The ultimate goal is to build a model for both the laboratory setup and the industrial coalescer so that laboratory experiments can be used to predict the behavior of the industrial process.     

Investigations into the electrical and coalescence behaviour of water-in-crude oil emulsions in high voltage gradients

The stability of water-in-crude oil emulsions when subjected to high voltage electric fields depends on the nature of the crude oil and the presence of chemical additives. Optical microscopy, conductivity and coalescence measurements have revealed two distinct types of behaviour, designated type I and type II. These are shown to be related to the crude oil/water interfacial rheological properties. For incompressible crude oil/water films, droplet—droplet coalescence is hindered and chains of water droplets are established. These increase the electrical conductivity of the emulsion (type I behaviour). On the other hand, efficient droplet—droplet coalescence accompanied by minimal conduction occurs in electric fields if the interfacial film is compressible (type II).     

An Electrorheological Study on the Behavior of Water‐in‐Crude Oil Emulsions Under Influence of a DC Electric Field and Different Flow Conditions

The influence of an applied DC electric field on viscosity and droplet size distribution of different water‐in‐crude oil emulsions was monitored in order to investigate the induction of coalescence of the water droplets. The effects caused by the voltage imposition were studied by rheological analysis and the validity of the obtained results was discussed, comparing with the features of real electrocoalcscer systems. A low field NMR technique (CPMG NMR) and digital video microscopy (DVM) were used to elucidate the behavior of the emulsions. Experiments performed at low shear rate with increasing electric field magnitude showed an increase in viscosity until a critical value. E_CRIT was reached. Thereafter coalescence occurred and viscosity decreased irreversibly below its initial value. The electrorheological behavior of the emulsions can be attributed to the organization (flocculation) of water droplets induced by the electric field, accompanied by an increase in viscosity. The structure breaks down as the shear rate is increased, leading to a decrease in viscosity. Experiments performed at high shear showed only a small decline in the viscosity. Although it was evident that coalescence took place, it did not involve the whole sample, because the electrodes were uncoated. As a direct consequence, the mean value of the droplet size within the emulsion did not change noticeably. Nonetheless this mean value was less recurrent and the formation of droplets of very large diameter occurred.     

利用酯化淀粉和油酸甲酯制备高效氯氟氰菊酯水乳剂

以辛烯基琥珀酸淀粉钠和油酸甲酯分别为替代乳化剂和溶剂,采用浓缩乳化法制备了高度稳定的2.5％高效氯氟氰菊酯水乳剂,通过测定乳液油滴粒径分布,结合乳液外观研究了乳化方法、预处理液中辛烯基琥珀酸淀粉钠质量分数、转速和剪切时间等工艺条件对乳液稳定性的影响.研究结果表明,辛烯基琥珀酸淀粉钠对油酸甲酯具有较好乳化效果,以其为乳化剂可制备高度稳定的2.5％高效氯氟氰菊酯水乳剂,油滴平均粒径在1.2 μm左右,且加速试验[即(54±2)℃密封14 d]和常温储存6个月后平均粒径仅增长了0.1～0.3μm,外观无变化;采用浓缩乳化法且预处理液中辛烯基琥珀酸淀粉钠质量分数在15％～25％时乳液稳定性较好,提高转速可降低油滴平均粒径,但对乳液均一性无显著影响,延长剪切时间对油滴平均粒径影响不大,但有利于提高乳液均一性;辛烯基琥珀酸淀粉钠为乳化剂制备的高效氯氟氰菊酯水乳剂稳定性优于常规水乳剂.     

Molecular dynamics simulation on influence of temperature effect on electro-coalescence behavior of nano-droplets

In electric dehydration of crude oil, the dewatering efficiency can be improved by raising emulsion temperature properly which reduces the viscosity of crude oil. However, it should be noticed that the emulsion temperature does not only affect the emulsion viscosity but also nano-droplets dynamics behavior which impacts the coalescence efficiency either. Therefore the influence of temperature effect on the electro-coalescence of nano-droplets is studied by a molecular dynamics method. The results show that the temperature presents an active or negative effect, depending on the competitive relation between electrostatic interaction and thermal motion. Two stages are distinguished according to the dominant mechanism. During stage I, governed by the electrostatic interaction, lower temperature promotes the polarization and leads to an acceleration of the droplets coalescence, but higher temperature restrains the coalescence process due to molecules thermal motion breaking the polarization process. During stage II, governed by the thermal motion, lower temperature improves the coalescence because of a diffusion effect, but higher temperature deteriorates electro-coalescence because of a violent molecular thermal motion. Additionally, hydrogen bond and radial distribution functions are obtained by statistics to describe droplets micromorphology, which explains the reason why the droplet forms longer chain structure at the critical electric field.     

低场核磁共振法对油基钻井液乳状液乳滴稳定性的研究

采用低场核磁共振技术,针对油基钻井液油包水型乳状液乳滴的稳定性进行研究。引入弛豫试剂Mn Cl2·4H2O对W/Q型乳状液的T2分布曲线进行定性分析,位于10~1 000 ms之间的弛豫峰对应于中度可自由移动水和白油弛豫峰的叠合峰,定义为乳状液弛豫峰;1 000~10 000 ms之间的峰为高度可自由移动水的弛豫峰。基于此,以弛豫峰峰形为定性指标,弛豫峰面积比率和弛豫峰间距为定量指标,针对弛豫试剂、油水比和老化温度等因素对乳状液横向弛豫时间T2分布曲线的影响进行了分析,进而深入研究了其对油基钻井液乳状液乳滴稳定性的影响。还将低场核磁共振分析技术运用于油基钻井液乳状液体系相对含油率的测量。结果表明,低场核磁共振是一种高效、快捷、准确反映油基钻井液乳状液稳定性的分析测试技术,同时,还可用于油基钻井液乳状液或原油相对含油率的测量。     

FLOW PROPERTIES OF WEATHERED CRUDE OILS AND THEIR EMULSIONS

The present paper proposes the emulsification of weathered crude oils in water as a competitive and cost effective method for reducing their viscosities. Weathered crude oil samples were collected from major Kuwaiti oil lakes. Emulsion preparation involved using, either a nonionic surfactant or alkali, as well as both alkali and fatty acid. The obtained emulsions were characterized by measuring the droplet size distribution of the dispersed phase using optical microscopy. Emulsion stability was also examined in terms of the system breakdown. The rheological properties were measured using a concentric cylinder rotary rheometer. The emulsion rheological behavior has been studied as a function of composition, temperature, and shear rate. A constitutive model was developed to characterize the pseudoplastic behavior of the crude oil and the emulsion systems. The model fitted well the experimental results with a correlation coefficient higher than 95%. Associated with the pseudoplastic behavior, viscoelastic behavior has been observed with emulsions and some oils at high shear rates.

The results of this investigation indicated that the examined weathered crude oils can be transported through pipelines as emulsions of up to 80 vol.% oil concentrations. The proposed method of treatment with NaOH and oleic acid offers several advantages over the surfactant treatment. It exhibited comparable rheological behavior at lower cost and less mixing energy. It also provided higher emulsion stability, which favors oil transportation for longer distances.     

三元复合驱碱/表面活性剂/聚合物模拟原油乳状液稳定动力学特性

基于两相分离的乳状液稳定模型,研究了三元复合驱模拟原油乳状液稳定动力学特性;通过液膜强度和油水界面张力探讨了碱/表面活性剂/聚合物对模拟原油乳状液稳定动力学特性的影响机理。 结果表明,乳状液稳定模型可以很好的评价乳状液的稳定性,并得到乳状液的稳定动力学特性;碱浓度小于900 mg/L有利于乳状液的稳定,碱浓度大于900 mg/L不利于乳状液的稳定;表面活性剂和聚合物浓度的增加使得形成的模拟原油乳状液更加稳定;模拟原油乳状液的稳定作用主要是通过碱、表面活性剂降低油水界面张力并增加油水界面膜强度,聚合物通过提高界面膜强度实现的,三者存在协同效应。     

Selective membrane test for crude oil based model emulsions by use of video-enhanced microscopy

 A method for testing water/oil emulsion droplet membranes selectively has been demonstrated. The method uses electric fields to induce attraction, membrane thinning and coalescence between aqueous droplets deposited in an oil continuum. The coalescence process is monitored visually by the use of videomicroscopy. A set of model oils containing indigenous surfactants (asphaltenes) from a crude oil has been studied, and the effects of asphaltene concentration, oil phase aromaticity, aging of oils and interfacial exposure time have been investigated. The strength of the field at the point of coalescence is defined as the critical parameter describing membrane strength. In the current experiments a.c. fields were used and droplet sizes were of the order of 500–600 μm. Received: 8 October 1998 Accepted in revised form: 11 January 1999     

Effect of Emulsifier Type on the Characterization of O/W Emulsions Using a Spectroscopy Technique

The droplet size distribution (DSD) of emulsions is the result of two competitive effects that take place during emulsification process, i.e., drop breakup and drop coalescence, and it is influenced by the formulation and composition variables, i.e., nature and amount of emulsifier, mixing characteristics, and emulsion preparation, all of which affect the emulsion stability. The aim of this study is to characterize oil-in-water (O/W) emulsions (droplet size and stability) in terms of surfactant concentration and surfactant composition (sodium dodecyl benzene sulphonate (SDBS)/Tween 80 mixture). Ultraviolet-visible (UV-vis) transmission spectroscopy has been applied to obtain droplet size and stability of the emulsions and the verification of emulsion stability with the relative cleared volume technique (time required for a certain amount of emulsion to separate as a cleared phase). It is demonstrated that the DSD of the emulsions is a function of the oil concentration and the surfactant composition with higher stability for emulsions prepared with higher SDBS ratio and lower relative cleared volume with the time. Results also show that smaller oil droplets are generated with increasing Tween 80 ratio and emulsifier concentration.     

Spontaneous generation of emulsion droplets by autonomous fluid-pumping using the gas permeability of poly(dimethylsiloxane) (PDMS)

Conventional droplet-based microfluidic systems require expensive, bulky external apparatuses, such as electric power supplies and pressure-driven pumps for fluid transportation. This study demonstrates an alternative way to produce emulsion droplets by autonomous fluid-handling based on the gas permeability of poly(dimethylsiloxane) (PDMS). Furthermore, basic concepts of fluid-handling are expanded to control the direction of the microfluid in the microfluidic device. The alternative pumping energy resulting from the high gas permeability of PDMS is used to generate water-in-oil (W/O) emulsions, which require no additional structures apart from microchannels. We can produce emulsion droplets by simple loading of the oil and aqueous solutions into the inlet reservoirs. During the operation of the microfluidic device, changes in droplet size, volumetric flow rate, and droplet generation frequency were quantitatively analyzed. As a result, we found that changes in the wetting properties of the microchannel greatly influence the volumetric flow rate and droplet generation frequency. This alternative microfluidic approach for preparing emulsion droplets in a simple and efficient manner is designed to improve the availability of emulsion droplets for point of care bioanalytical applications, in situ synthesis of materials, and on-site sample preparation tools.     

脉冲电场O/W乳状液破乳的分子动力学模拟

采用分子动力学方法研究了水包油(O/W)型乳状液体系中重油油滴在脉冲电场中的聚集行为. 通过改变电场占空比的模拟参数, 探讨了一定电场强度下的油滴聚集行为, 以及电场破乳时电场强度参数与占空比参数之间的联系. 同时利用静电势分布、 相互作用势能以及结合构象统计等分析方法, 从微观角度说明在电场作用下油滴的电荷分布与聚集机制. 模拟结果表明, 在近0.40~0.75 V/nm范围内电场强度下, 距离一定的重油滴聚集, 低电场强度可通过增加占空比促使油滴聚集, 且占空比随电场强度的增大而减小; 油滴在电场中发生形变, 油滴电荷出现两极化分布, 带负电沥青质分子引导油滴朝电场反方向移动; 无电场下聚集过程中沥青质处于两油滴界面, 范德华作用力为油滴聚集的主要作用力, 同时油滴界面沥青质分子与周围分子形成π-π结合构象, 增强了油滴间的相互作用力.     

Study on the relationship between emulsion stability and droplet dynamics of a spontaneous emulsion for chemical enhanced oil recovery

Spontaneous emulsion (SE) has attracted increasing attention, especially in the development of low-permeability reservoirs (with an average throat radius of 0.1–2?µm) for enhanced oil recovery. In this work, based on multiple light scattering principles, the relationship between emulsion stability and the droplet dynamics of SEs was investigated. The results showed that the synergistic effect of surfactant and polymer was crucial for oil emulsification in brine, since the stability of the emulsion was greatly improved. The emulsion stability and droplet dynamics depend on the temperature, concentration, and type of emulsifier. The optimal combination system had the lowest Turbiscan stability index value, and the emulsion stability time was more than 2000s. The average droplet size was 1.50?µm, and the droplet migration rate was 7.21?mm/h. The stability of the emulsion was resulted from the microscopic droplet dynamics. By reducing the migration rate of the droplets, stability of the emulsion can be obtained. Finally, the stability and droplet dynamics mechanism of the system were explained by using a schematic representation of the various equilibriums in the spontaneous emulsification flooding system.     

Regarding the effect that different twin-tailed surfactant have on a solid stabilized petroleum emulsion

An industrial petroleum emulsion stabilized by colloidal silica particles was treated with four different twin-tailed surfactants: sodium bis-2-(ethylhexyl) sulfosuccinate (AOT), didodecylammonium bromide (DDAB), calcium oleate (Ca(OL)₂), and dioctadecyldimethylammonium bromide (DODAB). Fourier transform infrared (FT-IR) spectroscopy, optical microscopy, centrifuge test, and conductivity measurement were employed to determine the effect of the amphiphile molecules on the crude oil emulsion. AOT and DDAB produce emulsion breakdown, while Ca(OL)₂ does not alter the emulsion stability and DODAB produces an extra stabilization of it. The AOT adsorption at the oil–water droplet interface is a spontaneous process (ΔH^ads < 0), which promoted the emulsion breakdown through an inter-droplet interaction mechanism. DDAB needs extra energy (via centrifugation) to destabilize the emulsion. Ca(OL)₂ dissolves in oil phase and remains there without altering the emulsion strength, while DODAB increases the emulsion stability.     

Oscillations Produced From Acidity Hydrolysis of Triglyceride in Emulsion

A new type of oscillating reaction was found from the systematic design of the chemical oscillator in water in oil (W/O) emulsions. It is an acidity hydrolysis reaction of long chain triglyceride in W/O emulsion at 25.0 ± 0.1°C in a bath stirring reactor. During the process of reaction, there were periodic and semi‐periodic changes lasting more than 10 hours both in electrolytic conductivity and electric potential. Microscope also revealed that the emulsion structure changed regularly and quickly. Because of the large difference in the solubility of the hydrolyzed products, it could be thought, that the different redistribution in the two phases of water and oil induces the regular changes. Marangoni effect of interface membrane made oscillation to form. The oscillating reaction can be used to explain the periodic change in the living system produced from coupling between reaction and diffusion.     

Experimental study on high-pressure rheology of water/crude oil emulsion in the presence of methane

The crude oil is in most cases accompanied with water and natural gas. For this reason, it is important to understand the rheology of the oil emulsion. There are already many works relating to rheology of the oil/water emulsion. However, studies on high-pressure rheology of water/crude oil emulsion in the presence of CH₄ are rare. In this work, light crude oil with characteristics of high wax content, which is typical in Northwest China, was studied. The rheology of water/crude oil emulsion in the presence of CH₄ under various conditions were fully studied. The results show that the crude oil emulsion showed obvious characteristics of non-Newtonian fluid at a lower temperature. Before water fraction reached a certain limit, the viscosity increases with the increase of water fraction, when water fraction reaches and exceeds the limit the emulsion viscosity drops with the increase of water fraction. The shear stress–shear rate curves become similar as the increase of temperature, indicating the decreasing effect of temperature on the relation between shear stress and shear rate. When the pressure reaches 8 MPa, the shear stress measured with CH₄ in the system surpasses that measured without CH₄. At higher pressure, CH₄ shows obvious influence on the rheology.     

无机盐对支状嵌段聚醚破乳作用的影响

通过阴离子聚合反应合成了一种七支状聚氧丙烯/聚氧乙烯(PPO/PEO)三嵌段聚醚;考察了不同无机盐存在时对原油乳状液的破乳效果的影响;通过界面张力、浊点和界面膨胀流变性的测定探讨了其界面聚集行为和破乳作用对无机盐的依赖性.结果表明,盐溶型无机离子存在时,能提高聚醚的破乳效果,而盐析型无机离子存在时,不利于聚醚的破乳作用;温度升高破乳速度加快,但45℃时脱出的水质最清.     

酯化淀粉乳化剂制备的高效氯氟氰菊酯O/W乳液的稳定机制

通过测定辛烯基琥珀酸淀粉钠的用量、盐离子、pH值和温度等因素对油滴Zeta电位及表面吸附量的影响,分析了以酯化淀粉辛烯基琥珀酸淀粉钠为乳化剂制备的5％高效氯氟氰菊酯水乳剂的稳定机制.结果表明,辛烯基琥珀酸淀粉钠质量分数为7％时,Zeta电位达到最大值,油滴表面吸附量接近饱和;Na+、Mg2+和Al3+压缩油滴表面的双电层,降低Zeta电位,削弱静电排斥作用,增加辛烯基琥珀酸淀粉钠分子柔性,提高辛烯基琥珀酸淀粉钠表面吸附量,且随着Na+、Mg2、Al3+离子强度依次增大,压缩双电层能力依次增强,Zeta 电位降低和表面吸附量增加程度依次增大;pH值影响辛烯基琥珀酸淀粉钠在水中的解离,在碱性范围内解离出较多羧酸根,静电排斥力较大,Zeta电位较高,但表面吸附量有所降低;温度升高,辛烯基琥珀酸淀粉钠在水溶液中溶解度增大,呈舒展状态,且辛烯基琥珀酸淀粉钠从油滴表面逃逸的趋势增加,油滴表面Zeta电位和表面吸附量均随着温度升高而降低,在低温区差别不大,温度越高二者变化越明显.辛烯基琥珀酸淀粉钠通过吸附于油滴表面为其提供较强的静电斥力和空间位阻作用而维持O/W乳液稳定.