Research progress on demulsifiers for heavy oil wastewater

China has abundant heavy oil reserves, and its heavy oil extraction and processing volumes are enormous. The extraction, storage, transportation, and processing of heavy oil generate a large amount of wastewater containing heavy oil, known as heavy oil wastewater. Heavy oil wastewater has a high mineral oil content, primarily consisting of floating oil and emulsified oil, making it highly valuable for recycling. Therefore, the demulsification treatment of heavy oil wastewater must both improve effluent quality and ensure resource recovery.

Currently, common methods for demulsifying oily wastewater include thermal, electrochemical, chemical, microbial, and physicochemical methods, or a combination of several methods. Adding chemical agents is the easiest and most effective demulsification method using existing sedimentation and filtration equipment. There are four types of demulsifiers available: electrolytes, low-molecular-weight alcohols, surfactants, and polymers. Among these, polymers are the preferred choice due to their functions such as charge neutralization, flocculation, and adsorption bridging.

This article introduces the sources of heavy oil wastewater, the research progress and demulsification mechanism of heavy oil wastewater demulsifiers, and provides prospects and suggestions for the development of demulsifiers.

1. Characteristics of heavy oil wastewater

1.1 Complex origins and diverse types

With the development of heavy oil entering the middle and late stages and the implementation of various production enhancement measures, the types and quantities of heavy oil wastewater have increased significantly, the stability of heavy oil wastewater emulsions has become stronger, and the separation difficulty has increased significantly. The main sources of heavy oil wastewater include produced fluid separation water [14], steam-assisted gravity displacement (SAGD) wastewater, tailwater discharged from deep treatment and reuse of heavy oil wastewater (such as scum dewatering, filtration concentrate, ion exchange acid and alkali wastewater, etc.), oil washing pipe wastewater and other miscellaneous wastewater, and the water quality is becoming increasingly complex.

1.2 Severe emulsification and strong stability

The pollutants in heavy oil wastewater mainly consist of mineral oil, inorganic clay minerals, and organic surfactants such as demulsifiers, viscosity reducers, bactericides, scale inhibitors, and corrosion inhibitors added during heavy oil extraction, storage, transportation, and processing. Heavy oil contains colloids, asphaltenes, and organic acids, which have polar groups and possess surface activity. Their viscous structure acts as a natural emulsifier, enabling the formation of emulsions. Naphthenates, especially sodium naphthenate, are highly hydrophilic compounds that readily lead to the formation of oil-in-water emulsions. The stability of the emulsion and the effectiveness of demulsification are strongly correlated with the aromatic hydrocarbon content. Synthetically produced surfactants reduce the strength of the oil-water interfacial film, increasing the stability of the wastewater. The addition of viscosity reducers, emulsifiers, and other surfactants during pipeline transportation, along with the agitation from pumps and pipeline machinery, further stabilizes the heavy oil emulsion, increasing the difficulty of wastewater treatment.

1.3 Small density difference between oil and water, high emulsified oil content, and high difficulty in demulsification.

Heavy oil wastewater is a typical oil-in-water emulsion, characterized by high oil content, high emulsification, high COD, high suspended solids, and complex organic composition. The oil it contains has high viscosity, a density close to water (the density of Du 84 heavy oil from Liaohe Oilfield reaches 0.997 g/cm³), a high pour point, and a content of gums and asphaltenes exceeding 30% of the total crude oil mass. The high content, small particle size, and strong stability of emulsified oil in heavy oil wastewater make demulsification difficult.

2. Research progress on demulsifiers for heavy oil wastewater

China is a major producer of heavy oil in the world, with internationally leading technologies in heavy oil extraction and processing, and its research on heavy oil wastewater treatment is also at the forefront internationally.

2.1 Epichlorohydrin-Dimethylamine Series Polymers

Epichlorohydrin and dimethylamine series polymers are water-soluble cationic polymers with advantages such as high positive charge density, good water solubility, easy control of relative molecular mass, high efficiency, non-toxicity, and low cost. They can be widely used in the water treatment field and have attracted great attention from the water treatment industry both domestically and internationally. These polymers can be used as primary flocculants and coagulants in wastewater treatment, as well as demulsifiers for oily wastewater.

HEY-M, a polycationic quaternary ammonium salt demulsifier, was synthesized using epichlorohydrin, dimethylamine, and the crosslinking agent ethylenediamine. It was used to treat heavy oil wastewater from Huan Silian Oilfield in Liaohe Oilfield, achieving an oil removal efficiency of over 96%.

SAGD (Super-Aged Oil Demulsification) is an effective method for enhancing oil recovery in extra-heavy oil, but the produced water has high oil content, severe emulsification, and strong stability, making it difficult for traditional demulsifiers to meet the requirements. The China Petroleum Exploration and Development Research Institute synthesized chlorinated polyethers using polyols and epichlorohydrin, and then quaternized them with dimethylamine to obtain a series of polyether quaternary ammonium salt demulsifiers, GBED-08, GBEDE-08, and GBEDL-08, with an effective content of approximately 60%. Applying this agent to treat SAGD extra-heavy oil wastewater from the Liaohe Oilfield achieved an oil removal rate of over 99%. This agent carries a strong positive charge and a high relative molecular mass, exhibiting strong neutralization, adsorption bridging, and flocculation functions. It demonstrates good treatment effect and fast oil removal rate in SAGD produced water, and the agent also exhibits strong thermal stability.

Compared with other types of polymers, this type of agent does not react with chlorides when used in aqueous dispersions containing chlorine, thus not reducing the flocculation effect. Furthermore, it produces small sludge volumes and requires less dosage, saving 25% to 30% in costs.

2.2 Dimethyl diallyl ammonium chloride series polymers

Dimethyl diallyl ammonium chloride polymer is a water-soluble cationic polymer material with special functions, which has been widely used in industries such as oil extraction, papermaking, water treatment, pharmaceuticals, textiles, and food. Research on this product began in the United States, West Germany, Poland, Japan, and other countries in the 1960s, and it gradually became a hot topic in the 1980s and 1990s, with many scholars both domestically and internationally conducting in-depth research.

Demulsifiers with relative molecular masses of (6~7) × 10⁶ and cationicity of 40%, 60%, and 80% were synthesized using acrylamide and diallyl dimethyl ammonium chloride. Experiments treating extra-heavy oil wastewater from Liaoning Petrochemical Company showed that, at the same dosage, lower cationicity resulted in higher removal rates. Under the conditions of 40% cationicity and an optimal dosage of 15 mg/L, the oil content after demulsification was below 600 mg/L, and the suspended solids removal rate was greater than 65%.

The polymer of dimethyl diallyl ammonium chloride is a linear water-soluble polyquaternary ammonium salt with many advantages, such as high positive charge density, good water solubility, high efficiency and non-toxicity, low cost, easy control of relative molecular mass, wide pH range, and stable cationic unit structure.

2.3 PAMAM dendritic demulsifier

Zhou Guizhong et al. from Tsinghua University synthesized a highly efficient PAMAM dendritic demulsifier for treating heavy oil wastewater from the Liaohe Oilfield using a protection and anti-protection method. Under conditions of 20 ℃, pH 4.8–10.57, and an addition amount of 20 mg/L, the oil removal rate reached 96.9%.

2.4 Compound type

Different types of demulsifiers have different demulsification mechanisms, so they generally tend to be used in combination.

2.4.1 Combination of two materials

Zhang Zhenhua et al. reacted polyacrylamide with formaldehyde and dimethylamine, then quaternized it with dimethyl sulfate to obtain a cationic flocculant. This flocculant was then compounded with a selected polyol polyoxypropylene polyoxyethylene ether demulsifier to treat extra-heavy oil wastewater from the Shuyi area of the Liaohe Oilfield. The results showed that the flocculant and demulsifier had a synergistic effect, effectively improving the demulsification effect of the demulsifier, reducing the oil content in the organic impurities, and decreasing the amount of reagent required.

The polymeric composite flocculant TC-2, developed by Song Jianping et al., is a copolymer of agent A and agent B under certain pressure and temperature. Its polymeric components mainly consist of anionic, nonionic, and cationic copolymers, as well as a water-soluble polymer containing various functional groups. TC-2 is readily soluble in water, generating various cations, high-valent polyhydroxy complexes, and lipophilic groups. It enhances demulsification and achieves high-efficiency flocculation, while also possessing corrosion inhibition, bactericidal, and desulfurization effects. It demonstrates excellent treatment results for complex wastewater and heavy oil wastewater in the later stages of oilfield development, meeting the requirements for wastewater reinjection.

2.4.2 Compound formulation of 3 or more materials

The high-efficiency demulsifier HL-005 developed by Zhang Wandong et al. comprises three components: A, B, and C. Component A is a scientifically compounded mixture of acrylamide and its derivatives with polyquaternary ammonium and polyether. Components B and C are coagulants with different properties. Results from the treatment of Du 84 extra-heavy oil wastewater from the Shuyi Block of Liaohe Oilfield using this demulsifier show that, under the original process conditions, pretreatment of the wastewater using the compound of components A and C, followed by purification using the compound of components A and B, resulted in effluent meeting production requirements.

2.5 Other

Zhao Lin et al. from Yangtze University invented a water-soluble acrylic acid-modified polymeric demulsifier. Under the conditions of 65 ℃, pH 6.5~7.0, and an addition amount of 80 mg/L, it can reduce the oil content of heavy oil wastewater from 100 mg/L to 20 mg/L and the suspended solids concentration to below 15 mg/L.

Zhu Jianzhao synthesized demulsifier KW-04 using formaldehyde, acetone, polyamines, and other additives as raw materials to treat heavy oil wastewater from Hongqian and Jiuqu in Xinjiang. Experimental results showed that under the conditions of 10–30 mg/L addition, 80 ℃ temperature, and 10–30 min treatment time, the oil removal rate was greater than 90%. The raw materials were readily available, and the synthesis process was simple.

Wang Linhong et al. synthesized a polyether-based polymeric demulsifier with a density of 1.8 g/cm³ and a solid content of 98%. When used to treat heavy oil wastewater from the Du 84 heavy oil field in the Shuyi block of Liaohe Oilfield, the appropriate dosage is 20-40 mg/L. Compared with similar products, this demulsifier requires less dosage and achieves better oil removal.

Wang Ye et al. developed a cationic polymer demulsifier, TJ-1, to treat heavy oil wastewater from Huan Silian Oilfield in Liaohe Oilfield. For wastewater of different water qualities, when the amount of demulsifier added reaches 30 mL/L or more, the oil content in the wastewater after demulsification can be stabilized at 300~350 mg/L.

3. Demulsification Mechanism and Development Trend

Heavy oil wastewater has a negative charge, while demulsifiers are generally high-molecular-weight cationic polyelectrolytes. Upon addition, they neutralize the charge, compress and disrupt the electric double layer, and weaken the interfacial film strength, causing the emulsion droplets to collide and coalesce, thus demulsifying. Then, through bridging and flocculation, large oil droplets are formed, which gradually separate from the water under the buoyancy of the water, thereby achieving the purpose of demulsification.

Organic cationic polymers offer advantages such as high efficiency, rapid processing, and low cost, but they can leave residues in the effluent, affecting subsequent treatment processes, especially biological treatment. Demulsifiers are generally used in combination to enhance demulsification and achieve synergistic effects, thereby improving treatment efficiency and reducing costs. Physical treatment processes such as filtration and hydrocyclone separation should be vigorously developed to reduce the use of chemical agents.

4. Conclusion

Heavy oil wastewater is characterized by its wide range of sources, complex composition, small oil-water density difference, and the presence of both natural and artificially added emulsifiers. The emulsions exhibit strong stability and are difficult to separate. Currently used demulsifiers mainly include epichlorohydrin-dimethylamine series cationic polymers, dimethyl diallyl ammonium chloride series cationic polymers, PAMAM, and their compound formulations. The demulsification mechanism for heavy oil wastewater primarily involves charge neutralization, followed by oil-water separation through bridging and flocculation. Organic demulsifiers tend to remain in the wastewater, adversely affecting subsequent processes, especially biochemical treatment. Therefore, the development and use of environmentally friendly demulsifiers should be strengthened, and physical treatment processes such as filtration and hydrocyclone separation should be vigorously developed.