Demulsifiers are surfactants that break down emulsions. They primarily disrupt emulsions by partially replacing the stabilizing film. As dehydrating agents, they remove water from crude oil and heavy oil, achieving the required water content. In oil wells, they reduce crude oil viscosity, preventing well blockage. They are polymerized from fatty alcohols, propylene oxide, and ethylene oxide. They are readily soluble in water, a pale yellow or milky white viscous liquid with a soapy odor. The freezing point is 25–40℃. The hydroxyl value is ≤60 mg/g of potassium hydroxide. The aqueous solution is milky white.
1. Introduction:
In flotation processes, sometimes emulsifiers are added to emulsify reagents (e.g., collectors) to improve dispersion and flotation efficiency. Some mineral processing reagents are easily emulsified, but emulsified foam in the slurry can affect subsequent processes, causing concentrate overflow losses or tailings wastewater pollution of rivers. Therefore, demulsifiers are added in these situations. When using demulsifiers, the pH value of the emulsion must be adjusted first. For strong alkaline and strong acid emulsions, the pH value generally needs to be adjusted to the range of 1-10. In the wastewater treatment of oil-containing emulsions in industries such as petroleum, chemical, wool textile, and machinery processing, in addition to physical demulsification methods (such as heating, membrane separation, and electric field demulsification), chemical treatment with demulsifiers is also commonly used. Commonly used demulsifiers are highly dispersed mixtures composed of multiple components. For demulsification of oil-in-water emulsions, inorganic substances with cations such as H+, Al3+, and Fe3+ are usually used as demulsifiers, such as inorganic acids and ferric sulfate; while for demulsification of water-in-oil emulsions, anionic and nonionic surfactants or mixtures of both are generally used as demulsifiers.
Because some solids are poorly soluble in water, when one or more of these solids exist in large quantities in an aqueous solution, under hydraulic or external agitation, these solids can exist in the water in an emulsified state, forming an emulsion. Theoretically, such a system is unstable. However, in the presence of surfactants (such as soil particles), the emulsification can become severe, making it difficult to separate the two phases. Typical examples include oil-water mixtures in oil-water separation and water-oil mixtures in wastewater treatment. These two phases form relatively stable water-in-oil or oil-in-water structures, based on the “electric double layer structure.”
In this case, agents are added to disrupt the stable electric double layer structure and stabilize the emulsion system, thereby achieving phase separation. These agents used to disrupt the emulsification process are called demulsifiers.
2. Demulsification Mechanism
Due to the complexity of the composition of oil, oil-water layers, and the natural emulsifiers they contain, and the extreme difficulty in studying the physicochemical processes occurring at the oil-water interface, the demulsification process and mechanism of chemical demulsifiers are still under investigation. However, both demulsifiers and emulsifiers are surface-active substances, but their effects are diametrically opposed. The demulsification mechanisms of various demulsifiers are summarized as follows:
Surface activity. Demulsifiers are all highly active surfactants, exhibiting higher activity than emulsifiers. Some literature suggests that demulsifier activity should be 100 to 1000 times greater than emulsifier activity, allowing it to rapidly penetrate the external phase of the emulsion and disperse at the oil-water interface. This replaces or neutralizes the emulsifier, reducing the interfacial tension and interfacial film strength of the emulsion droplets, making the W/O emulsion highly unstable. The interfacial film is easily ruptured under external forces, allowing water from the internal phase of the emulsion particles to enter the external phase, thus separating oil and water. This not only disrupts the already formed crude oil emulsion but also prevents further emulsification of the oil-water mixture, reducing its viscosity and accelerating separation. However, practice shows that there is no direct correlation between higher demulsifier activity and stronger demulsification ability.
Reverse emulsification occurs when a crude oil emulsion is formed under the action of a hydrophobic emulsifier, commonly known as a W/O emulsion, such as naphthenic acids or asphaltenes. Hydrophilic demulsifiers can transform emulsions into O/W (oil-in-water) emulsions, facilitating oil-water separation through the emulsification process and the instability of O/W emulsions. When the demulsifier promotes the phase transition from oil-in-water to oil-in-water emulsions, the water on the outside easily collides and aggregates into large droplets, which then settle.
This involves both wetting and penetration. Demulsifiers can dissolve natural emulsifiers such as colloids, asphaltenes, and solid powders adsorbed at the oil-water interface, preventing the interfacial film formed by these natural emulsifiers from hindering droplet aggregation. Solid particles such as clay, iron sulfide, and drilling mud are hydrophilic, and demulsifiers can pull these solid emulsifiers from the oil-water interface into the droplets; asphaltenes and high-melting-point wax crystals are oleophilic, and demulsifiers can allow them to leave the oil-water interface and enter the crude oil. This facilitates the coalescence of water droplets during collisions, achieving the goal of droplet settling. (4. Counterion effect. Since the water droplets in the dispersed phase of crude oil emulsion are always negatively charged and adsorb some positive ions on their surface, the dispersed phase is often positively charged. Because the charges are the same, the water droplets in the dispersed phase repel each other and are difficult to merge. If an ionic demulsifier is added to the crude oil, ions with opposite polarities are adsorbed on the surface of the water droplets and neutralize the positive charge, which weakens the electrostatic repulsion between the water droplets, destroys the interfacial film protected by the same charge, and causes the water droplets to merge and settle from the oil.
Although the demulsification mechanism of demulsifiers is not yet perfect, two conclusions have been drawn from long-term practice: ① The molecular weight of the demulsifier must be greater than that of the natural emulsifier to be effective in demulsification; ② If the demulsifier is used as an emulsifier for oil-water mixtures, an inverse emulsion, i.e., an O/W type emulsion, will be generated.)
3. Main Uses:
Demulsifiers are surfactants that disrupt the structure of emulsified liquids, separating the different phases within the emulsion. Crude oil demulsification refers to using the chemical action of demulsifiers to separate oil and water from an emulsified oil-water mixture, achieving crude oil dehydration and ensuring the water content standard for exported crude oil.