Demulsifiers changing

In 1983, Steinbom and Flock studied the rheology of crude oils and water-in-oil emulsions (58). Emulsions with high proportions of water exhibited pseudoplastic behavior and were only slightly time dependent at higher shear rates. Omar et al. also measured the rheological characteristics of Saudi crude oil emulsions (59). NonNewtonian emulsions exhibit pseudoplastic behavior and followed a power-law model. Mohammed et al. studied crude oil emulsions using a biconical bob rheometer suspended at the interface (60). More stable emulsions displayed viscoelastic behavior and a solid-like interface. Demulsifiers changed the solid-like interface into a liquid one. 

Table 1. Summary of Demulsifier Changes in the Petroleum Industry (from...

In the production of crude oil, the greatest part of the crude oil occurs as a water-in-oil emulsion. The composition of the continuous phase depends on the water/oil ratio, the natural emulsifier systems contained in the oil, and the origin of the emulsion. The natural emulsifiers contained in crude oils have a complex chemical structure, so that, to overcome their effect, petroleum-emulsion demulsifiers must be selectively developed. As new oil fields are developed, and as the production conditions change at older fields, there is a constant need for demulsifiers that lead to a rapid separation into water and oil, as well as minimal-residual water and salt mixtures. 

The imaginary component, "(f), is the dilational viscosity modulus. This arises when the demulsifier in the monolayer is sufficiently soluble in the bulk liquid, so that the tension gradient created by an area compression/expansion can be short circuited by a transfer of demulsifiers to and from the surface. It is 90° out of phase with the area change. 

Two principal approaches for the demulsification of the loaded emulsion are chemical and physical treatments. Chemical treatment involves the addition of a demulsifier to the emulsion. This method seems to be very effective. However, the added demulsifier will change the properties of the membrane phase and thus inhibits its reuse. In addition, the recovery of the demulsifier by distillation is rather expensive. Therefore, chemical treatment is usually not suitable for breaking emulsion liquid membrane, although few examples of chemical demulsification have been reported for certain liquid membrane systems [88]. 

Heat can be used to break some emulsions Changing the pH of the water can often break an emulsion The presence of acid-forming and sulfate-reducing microorganisms may enhance the formation of emulsions treat the fuel with a microbiocide Formulate fuels with a demulsifier to inhibit emulsion formation Ensure that surfactant compounds and additives... 

Experience is a very useful teacher in selecting demulsifiers. The man who is familiar with the history of treating m an area, the demands of the treating plants, and the performance of the chemicals can do a pretty good job ot picking compounds. However, this approach fails when changes occur in emulsion characteristics, new emulsions are encountered, or new chemicals become available. 

For studies with real systems, Isaacs et al. (27) used the simplified approach of examining changes at the oil-water interface without specifying adsorption mechanisms or pathways. Based on measurements of time-dependent interfacial tensions, the following expression (termed the spreading rate parameter) served to characterize the relative adsorption performance of demulsifiers or demulsifier combination ... 

Surface active additives (cosurfactants, demulsifiers, etc.), such as fatty alcohols in the case of ionic surfactants, may affect the emulsifier partitioning between the phases and its adsorption, thereby changing the Gibbs elasticity and the interfacial tension. The surface-active additive may also change the surface charge (mainly by increasing the... 

The exterior phase was analyzed for phenylalanine concentration and pH. All sample volumes were recorded and used for mass balance determination. Phenylalanine was measured spectrophotometrically at lmax = 257.5 nm. Changes in interior phase volume were calculated using material balances. All material balances closed to within 2%. Interior phase concentrations were estimated by the use of material balances and exterior phase concentrations. The interior phase components of several representative emulsions were measured by analyzing the interior phase components after thermally demulsifying the emulsion samples. These measurements agreed with estimates to within 10%. 

Often, in bench studies aimed at understanding emulsion-stabilization mechanisms, a hypothesis is devised. Most often the components of crudes are first separated, and a model emulsion is prepared from various combinations of the components in a model oil and in water of quality similar to that of formation or process water. The stability or instability is traced either by water resolution or by observing flie interfacial film properties under some form of externally applied stress over time. The stress may include temperature increases or solvent changes. The system may then be modified by the demulsifier and the changes in behavior are compared to that without the demulsifier. Deductions are then made about the film mechanics of the system in response to the variables. 

In the following discussions the published experimental findings are presented interrelatedly first in terms of internal oil chemistry at the interface and instabilities based on its composition, secondly in terms of effects of water chemistry, and thirdly in terms of demulsifier interaction. We include the activity of interfacial components involved in the structure of the protective skin, the behavior(s) of this structure with changes to water chemistry or solvency, or the effects of changes in film stmeture itself due to modification of relative proportions of interfacially active components. In some examples, developments in interfacial rheology, which is both a tool for understanding stable films and a means of rationalizing the effects of demulsifiers in demulsification, are discussed interrelatedly. Films may be sensitive to crude oil type, gas content, aqueous pH, salt content, temperature, age, and the presence of demulsifiers. Demulsifier performance is also influenced by many of these variables. 

Singh (143) followed up this study to understand the performance of unidentified demulsifiers with a change in solvent properties. By noting the relative decrease in surface pressures resulting from added demulsifiers in various solvents, he found that benzene was the best in that it helped the demulsifier to lower the surface film pressures. The lowering of interfacial tension was measured at the same time and the results suggested that rapid adsorption occurred. It was concluded that structure, orientation, and film pressmes were the most important factors in demulsifier performance. 

Mukerjee and Kushnick (167) showed that at low frequency the demulsifier behaves as a soluble mono-layer, and at high frequency as an insoluble monolayer. Variation in interfaeial tension from a local change in area is virtually instantaneous. This gradient is short circuited when the demulsifier moleeule moves to and from the surface to bulk or is sufficiently soluble in the bulk phase. 

At low frequency they found that the demulsifier behaved as soluble monolayers and the tension was governed by the bulk concentration and did not change with change in area. At high frequency the demulsifiers behaved as insoluble monolayers and the change in interfacial tension resulting from area change was instantaneous ... 

Wasan and coworkers (63,65,174) extended techniques for studying film rheology of the foam lamella to studies of crude-oil emulsion lamella. Using a capillary balance technique and light interferometry, the film thinning of foams was studied with and without chemical demulsifiers, with solvent properties changed, etc. (182). They confirmed that there were two contributions to emulsion stability - a struc-tiual component that originates from the nature of the bulk phase, and an adsorbed-layer contribution to film stability (170). This will be covered in another chapter in this series. 

C. Demulsifier Creating Surface Changes to Solid Stabilizers... 

Figure 25 Resolution of water and decrease in oil-phase moisture traced by changes in each phase of a bitumen W/ O emulsion after demulsifier B was added at 50°C middle phase is not visible. Top-photograph of its separation, showing clarity and optimum dosage.

The petroleum industry generally solves the emulsion problem by adding demulsifiers in an ad hoc manner, often based on simple bottle tests. There are many problems associated with this solution. First, the chemical composition of a given well changes with time and can in a worst-case scenario result in a composition totally incompatible with the given demulsifier. Second, little is known about the exact interaction between demulsifiers and other chemical additives (e.g., corrosion inhibitors and flow enhancers ). One may, flierefore, create a new problem by solving another. 

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