•Wax adsorption at oil-water interface has been observed by microscope.•The mechanism of wax adsorption at oil-water interface has been explained.•The rheology of oil-water interface has been change by wax adsorption.In this work, wax adsorption at the water–oil interface was studied. Through a polarizing microscope, interfacial adsorption of wax crystals was observed. The cause of the adsorption was proposed to be the synergistic effect between surfactant and wax. An additional experiment, an interfacial rheology test, validated the synergistic effect, showing that wax adsorption enhances the interfacial storage modulus and changes interfacial viscosity. The interfacial storage modulus increased with the decrease in temperature, which implies that the amount of wax adsorption increased at the interface, in agreement with the findings observed with the microscope. Temperature and wax content are two factors that affect interfacial rheology.Download high-res image (161KB)Download full-size image

•Degree of drop size distribution fluctuation decrease with Tween 80 concentration.•PBE is employed as the predicted method for drop size distribution.•Effect of drop size on turbulence intensity is coupled into the breakage model.•Partial mobile and immobile interface are used to describe the film drainage.The effect of nonionic surfactant (Tween 80) concentration on the evolution of drop size distributions (DSDs) of oil/water dispersion in a stirred tank was investigated experimentally and numerically. It was found that as the surfactant concentration increases, the mean droplet size decreases, the number density of small droplets increases and the dispersion stability is enhanced. It could be attributed to the augmentation of the breakage frequency caused by a dramatic reduction of interfacial tension and the suppression of the coalescence efficiency by an immobilized interface. When applying population balance equation (PBE) to surfactant dispersions, the effect of surfactant concentration was characterized by its influence on both breakage and coalescence process. The combined effect of surfactant molecules, drop size, and the number density of droplets on the turbulence intensity was considered in the breakage model. Models of coalescence time that consider partially mobile and immobile interfaces were employed to describe the film drainage at surfactant concentrations below and above CMC (critical micelle concentration) respectively. The effect of surfactant concentration on the critical film thickness was probed by adjusting the Hamaker constant and measuring the interfacial tension, while its influence on the coalescence efficiency was investigated by further balancing the contact and coalescence time. Both the predicted Sauter mean diameter and cumulative DSD agree with the experimentally measured values at different surfactant concentrations, while the discrepancies that exist could be due to the underestimation of coalescence rate between small droplets.

•A PR-t-UNIFAC model is developed.•The nonlinear calculation of the segment fractions of molecules in γC is introduced.•The improved UNIFAC considers the local characteristic of groups (components effects).•The improved UNIFAC is remarkably superior to the original UNIFAC at high mole fraction of light components.A new modified EOS-GE model is developed for the highly asymmetric paraffinic systems, where the volume translated Peng-Robinson EOS is adopted coupled with the LCVM mixing rule. In the new modified EOS-GE model, the original UNIFAC is replaced by a newly established UNIFAC where the nonlinear calculation of the segment fractions of molecules in γC (the combinatorial activity coefficient) is introduced to modify the traditional assumption that “all groups are isotropic in solution”. A total of 956 vapor-liquid experimental bubble points in highly asymmetric paraffinic systems including binary systems, ternary systems, quaternary systems and multiple systems are used to test the new developed EOS-GE model. Results show that the original UNIFAC and the improved UNIFAC both perform well if the molefractions of light components (CH4 or C2H6) are low; however, with the increase of the light components, the improved UNIFAC is remarkably superior to the original UNIFAC.

Drainage and deformation of the intervening film can arguably represent the dynamic nature of colliding soft matters. The development of an interaction force analysis between soft interfaces helps to probe the drop deformation and the interfacial properties. Based on the SRYL model, the fluid flow inside the droplet and the convection-diffusion of the surfactant at the oil-water interface is coupled to model the distribution of a non-ionic surfactant (Span80) during drop deformation using AFM. This study quantifies the in situ interfacial concentration with a trace amount of surfactant at the interface and indicates its effect on the interaction forces between two immersed oil droplets in an aqueous solution.Download high-res image (129KB)Download full-size image

A thermodynamic model for the prediction of wax precipitation with the new solid solution model is established. For liquid phase, regular solution model and Flory free-volume equation are adopted to consider the two contributions of activity coefficient: enthalpy contribution, the energetic interactions between the components, and entropy contribution, the differences in size and shape between the molecules. For solid phase, the derived solid solution model accounting for the two parts of the solid non-ideality is proposed, where an improved regular solution model is developed for the description of residual part (enthalpy contribution), on the basis of the combination of regular solution theory and local composition theory; while Wilson equation with the consideration of the end effects between molecules is used for combinatorial part (entropy contribution). The improved model is tested against the experimental data of binary, ternary, quaternary and multi-paraffins systems. All experimental data are obtained at atmospheric pressure with temperature varying from 260 K to 350 K. Results show that present model could precisely predict the experimental WATs (wax appearance temperature)/WDTs (wax disappearance temperature) and precipitation curves.

•Interfacial dilational properties are introduced to explain for drop breakage in stirred vessels.•Chord length distributions measured with a FBRM instrument are compared.•Strong relationship between interfacial dilational elasticity and dispersing modality is demonstrated.Surfactants reduce the interfacial tension of oil–water and hence favor emulsification; however, attention has rarely been focused on the effect of interfacial dilational rheology on drop breakage. The aim of this work is to rheologically characterize the interfacial properties of oil–water droplets and the effect on drop breakage during emulsification. The dispersed holdups are fixed in dilute dispersion where breakage dominates during mixing. The total number of droplets, the square weighted (sqr-wt) mean diameter and the chord length distribution (CLD) are measured online using the focus beam reflectance method (FBRM). Variations in the dispersed droplets are further analyzed with the interfacial dilational modulus. For emulsions with surfactant concentrations below the critical micelle concentration (CMC), the interfacial tension decreases as the surfactant concentration increases, whereas the mean drop diameter initially shows an increasing trend and then decreases. However, for concentrations above the CMC, the interfacial tension rarely decreases, whereas the mean diameter of the dispersed phase continues to decrease, and the total number of droplets increases as the concentration of Span 80 continues to increase. This indicates that the interfacial dilational elasticity is a key factor that influences the pure breakage emulsification process and the related dispersed modality.

In this paper, heavy crude oil–water flows are studied in a horizontal stainless steel test section with 25.4 mm ID and overall length of 50 m. Crude oil (viscosity = 628.1 mPa s, interfacial tension with water = 10.33 mN/m at 60 °C) and water, collected from an oilfield, were used as test fluids. Visual observations, local sampling and pressure drop measurements were used to identify the flow patterns and their transitions. It was found that in all conditions studied there was a water-in-oil emulsion present. At low mixture velocities and water fractions this occupied the whole pipe cross section. As the velocity or the volume fraction increased water appeared to segregate. At high water fractions and mixture velocities annular flow appeared with the water-in-oil emulsion in the core surrounded by a water layer. The results were compared with those from a model oil with the same viscosity. At low water fractions there was a similarity between the patterns observed with the two oil systems characterized by water segregation from an oil continuous dispersion with increasing water fraction or mixture velocity. However, at high water fractions an oil-in-water dispersion formed with the model oil that was not seen with the crude oil. Pressure drop was generally higher for the crude oil system compared to the model one, while in both cases it decreased when water started to segregate and form layers in contact with the pipe wall. The differences between the two oil systems are attributed to the natural surfactants present in the heavy crude oil (such as asphaltenes and resins), which tend to accumulate on the water/oil interface, retard film drainage and maintain the stability of water drops in oil.Highlights► Heavy crude oil-water flows are compared with a model oil with the same viscosity. ► Flow patterns are identified through local sampling and monitored online pressure drop. ► A similarity is characterized as water segregation from the dispersion with velocity increases. ► Differences are owed to the natural surfactants which maintain the stability of water drops in oil.

A predictive thermodynamic model for the prediction of wax precipitation in paraffinic mixtures is developed, on the basis of the wax prediction model (RSFV-IRSW) established by Yang et al. (2016), where the liquid phase is described by the combination of regular solution model and Flory free-volume equation, and the solid phase is modelled by the combination of improved regular solution model and Wilson equation. Advancements are adopted in the present work: 1. The influence of transition enthalpy between disorder and order solid phases on liquid-solid fugacity ratio of pure component is considered; 2. A method for the precise calculation of fusion enthalpy and transition enthalpy is suggested; 3. The boundary condition to decide whether one component would precipitate out to form the solid phase is added. The predictive model, RSFV-IRSW, predictive Wilson and regular solution model are tested against the experimental data of complex synthetic paraffinic systems. Results show that the predictive model behaves the best with high prediction accuracy in terms of the characteristics of wax precipitation curve and the composition of solid phase.

•Population balance equations are used to predict drop size distributions in stirred vessels.•A two-region approach is followed with different breakage and coalescence correlations.•Drop size distributions measured with a FBRM instrument is compared.•Both experiments and PBEs modeling are compared up to 40% holdup in unstable systems.The drop diameter distribution (DSD) of dispersed Exxsol D80 oil-in-water in a lab scale stirred tank was predicted numerically using population balance equations (PBEs) modeling. It was assumed that the flow area was split into two zones, around the impeller with high energy dissipation rate and further away from the impeller with low energy dissipation rate. The dispersed phase chord length (CLD) distribution was experimentally measured with a focused beam reflectance method (FBRM) probe, and was then converted to the drop diameter distribution (DSD) numerically using a backward transform. Comparisons between the PBEs model results against experimental data from the current work and from previous literature showed that the PBEs predicted well the data at low dispersed phase fractions but overpredicted them at high ones. The predictions were slightly better when an increased power number was used. In addition, it was found that improved predictions could be obtained when the region of high energy dissipation rate around the impeller was enhanced.

•Modified DLA model has been used to simulate the co-precipitation of wax/asphaltene particle.•Fractal dimension is adopted to explore the geometric property of the aggregates.•The changing law of fractal dimension with concentration and size ratio has been explored.Diffusion-limited aggregation (DLA) model has been widely used to simulate the aggregation processes. In this work, the aggregation of wax and asphaltene particles in crude oil is studied with a modified DLA model. Assuming both wax and asphaltene particles are sphere, the co-precipitating process could be regarded as a simple aggregation process containing two kinds of particles. Three important parameters are discussed, as the sticking coefficient between particles, the ratio of particle size, and the particle concentration. Via analyzing the fractal dimension of the formed aggregates, the aggregation mechanism of wax and asphaltene particles could be explained, and help to further reveal the nature of the disordered growth.