Introduction to the principles and functions of tar demulsifiers

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This post was last edited by Green Lotus on 2017-8-28 at 16:43. Introduction to the principles and functions of tar demulsifiers 1. The demulsification process. The demulsification process carried out by a demulsifier generally consists of 3 stages: ① After the demulsifier is added to the crude oil emulsion, it disperses throughout the oil phase and is able to reach the water droplets that are part of the emulsion; ② The demulsifier penetrates into the protective layer surrounding these water droplets, weakening and damaging this layer; once the protective layer is destroyed, the water droplets come closer to each other and come into contact; ③ The water droplets coalesce, and they separate from the continuous phase. 2. Demulsification mechanism. Although there are many factors that stabilize crude oil emulsions, from a thermodynamic perspective, emulsions are unstable systems; even the most stable emulsions will ultimately reach a state of two-phase separation, with emulsion breakdown being an inevitable outcome, differing only in terms of the manner and timing. According to Stokes’ law, for W/O crude oil emulsions, increasing the density difference between oil and water or reducing the viscosity of the dispersion medium facilitates the sedimentation of the water droplets; moreover, the sedimentation speed is proportional to the square of the droplet radius. Therefore, during the dehydration of crude oil, it is necessary to control various factors as much as possible in order to create conditions that allow tiny water droplets to coalesce and grow larger, thereby accelerating the oil-water separation process through the settling of these droplets. For example, increasing the droplet size and the density difference between oil and water, as well as reducing the density of crude oil. The main methods include: heating the emulsion (thermal treatment), adding demulsifiers (chemical treatment), applying an electric field (electrical treatment), as well as mixing, vibration, microwaves, ultrasound, centrifugation, filtration, and adding microorganisms. The demulsification of crude oil generally requires the simultaneous use of two or more of the aforementioned methods. In recent years, research on the demulsification mechanism of crude oil emulsions has focused on detailed studies of the droplet coalescence process and the effect of demulsifiers on interfacial rheological properties. However, due to the complex way in which demulsifiers affect emulsions, despite extensive research in this field, there is still no unified theory regarding the demulsification mechanism. It is generally believed that the breakdown of an emulsion involves processes such as oil separation, flocculation, membrane drainage, and coagulation. After being added, the demulsifier diffuses toward the oil-water interface. Since its surface activity is higher than that of the film-forming substances in crude oil, it can adsorb onto the oil-water interface or partially displace the natural emulsifiers already present there. It then forms a mixed membrane with the film-forming substances in crude oil, one whose strength is lower than that of the original interface membrane; this leads to the breakdown of the interface membrane, releasing the water contained within it. The water droplets coalesce into larger droplets that settle to the bottom, resulting in the separation of the oil and water phases, thereby achieving the purpose of demulsification. Kotsari et al. believe that water-soluble demulsifiers cause demulsification by replacing the interfacial co-emulsifiers, destroying the emulsion interfacial film, or altering the wettability of the interfacial layer to form inert interfacial complexes; whereas oil-soluble demulsifiers, in addition to replacing natural co-emulsifiers, act by neutralizing the substances present at the interface, thereby destroying the interfacial film and leading to the breakdown of the emulsion. Hartland et al. described the effect of demulsifiers using a symmetric plane parallel membrane model. In a system without demulsifiers, the interfacial film of the droplets adsorbs natural emulsifiers such as asphaltenes. The coalescence film between two droplets becomes thinner, resulting in an uneven distribution of natural emulsifier molecules at the interface and thus a negative interfacial tension gradient. This situation reduces the membrane’s drainage capacity. When a demulsifier is added, it diffuses into the areas lacking demulsifier at the interface. Since, at the same interfacial concentration, the demulsifier has a greater ability to reduce interfacial tension than natural emulsifiers, it lowers the interfacial tension within the film, prevents the transfer of asphaltenes, creates a positive interfacial tension gradient, and accelerates the membrane’s drainage process. According to the research findings, the currently recognized demulsification mechanisms are as follows: ① Phase transfer and reverse deformation mechanism; ② Collision-induced breaking of the interfacial film mechanism; ③ Solubilization mechanism; ④ Folding deformation mechanism. The mechanism of chemical demulsification is relatively complex; in summary, it can be outlined as follows: ① Chemical demulsifiers possess higher activity than emulsifiers. When they disperse at the oil-water interface, they can displace the emulsifiers and form a new, fragile interfacial film. Under the action of gravity and electric fields, this type of membrane is more prone to breaking, which facilitates the separation of oil and water into distinct layers; ② Chemical demulsifiers have an inverse effect, enabling W/O emulsions to be reversed into O/W emulsions. During the inversion process, the emulsion film breaks apart; ③ Chemical demulsifiers have a strong dissolving effect on the emulsion film, causing it to break apart through dissolution; ④ Chemical demulsifiers can neutralize the charges on the oil-water interface film, thereby destroying the interface film that is protected by these charges. In the mid-1980s, the common demulsifier brands used in China’s oil fields were types AE, AR, SP, TA, DPA, and PE. Since the mid-1980s to the present, oilfield workers in China have done extensive work on the development and synthesis of demulsifiers. Taking into account the specific characteristics of various oilfields, a series of demulsifiers have been developed, including: ① Polyurethane-based crude oil demulsifiers; ② Phosphate ester-based crude oil demulsifiers; ③ Alkylphenolformaldehyde resin-based low-temperature crude oil demulsifiers; ④ Reverse-phase demulsifiers; ⑤ Ultra-high molecular weight polyether-based crude oil demulsifiers.