Demulsifier Classification and Selection

Demulsifiers are surfactants that disrupt the structure of emulsified liquids, separating the different phases within the emulsion. Crude oil demulsification refers to the use of demulsifiers to chemically separate oil and water from an emulsified oil-water mixture, achieving crude oil dehydration and ensuring the required water content for export.
I. Demulsifier Classification
Based on current usage, demulsifiers can be broadly classified into water-soluble and oil-soluble types.
Demulsifiers are surfactants; their molecules consist of lipophilic and hydrophilic groups. The lipophilic portion is composed of hydrocarbon groups, particularly long-chain hydrocarbon groups; the hydrophilic portion is composed of ionic or nonionic hydrophilic groups. There are many types of demulsifiers. According to surfactant classification methods, they can be categorized as cationic, anionic, nonionic, and biionic demulsifiers.
1. Anionic demulsifiers, when dissolved in water, form negatively charged hydrophilic groups. Based on their hydrophilic groups, they are further classified into carboxylic acids, sulfonic acids, sulfates, and phosphates. Anionic demulsifiers include carboxylates, sulfonates, and polyoxyethylene fatty acid sulfates, etc., which have disadvantages such as high dosage, poor effectiveness, and susceptibility to electrolyte effects.
2. Cationic demulsifiers, when dissolved in water, form positively charged hydrophilic groups, primarily basic nitrogen atoms, but also including phosphorus, sulfur, and iodine. Cationic demulsifiers mainly include quaternary ammonium salts, which are effective for general crude oils but unsuitable for heavy oils and aged oils.
3. Nonionic demulsifiers, when dissolved in water, do not dissociate into ions and therefore do not carry a charge. Nonionic demulsifiers mainly include block polyethers initiated with amines, block polyethers initiated with alcohols, alkylphenol aldehyde resin block polyethers, phenol-amine aldehyde resin block polyethers, silicone demulsifiers, ultra-high molecular weight demulsifiers, polyphosphate esters, modified products of block polyethers, and zwitterionic demulsifiers represented by imidazoline crude oil demulsifiers.
4. Two-type demulsifiers can generate both positive and negative ions after dissolving in water. It is cationic in acidic solutions and anionic in alkaline solutions.
II. Currently, the nonionic demulsifiers commonly used in oil fields are mainly as follows:
1. SP type demulsifier
The main component of SP type demulsifier is polyoxyethylene polyoxypropylene octadecyl alcohol ether, with the theoretical structural formula R(PO)x(EO)y(PO)zH, where: EO-polyoxyethylene; PO-polyoxypropylene; R-fatty alcohol; x, y, z-degree of polymerization. SP-type demulsifiers appear as a pale yellow paste with an HLB value of 10-12 and are soluble in water. SP-type nonionic demulsifiers have a good demulsifying effect on paraffinic crude oil. Their hydrophobic portion consists of C12-18 hydrocarbon chains, while their hydrophilic groups achieve hydrophilicity through hydrogen bonding between hydroxyl (-OH) and ether (-O-) groups in the molecule and water. Because hydroxyl and ether groups are relatively weakly hydrophilic, one or two hydroxyl or ether groups are insufficient to pull the hydrophobic C12-18 hydrocarbon chain into water; multiple hydrophilic groups are required to achieve water solubility. The larger the molecular weight and the longer the molecular chain of the nonionic demulsifier, the more hydroxyl and ether groups it contains, resulting in greater pulling force and stronger demulsifying ability for crude oil emulsions. Another reason SP-type demulsifiers are suitable for paraffinic crude oil is that paraffinic crude oil contains little or no gum and asphaltenes, has fewer lipophilic surfactants, and a relatively low density. For crude oil with high gum and asphaltenes content (or water content greater than 20%), SP-type demulsifiers have weak demulsifying ability because their molecular structure is simple, lacking branched and aromatic structures.
2. AP-type demulsifiers
: AP-type demulsifiers are polyoxyethylene and polyoxypropylene polyethers initiated by polyethylene polyamines. They are multi-branched nonionic surfactants with the molecular formula: D(PO)x(EO)y(PO)zH, where: EO – polyoxyethylene; PO – polyoxypropylene; R – fatty alcohol; D – polyethylene polyamine; x, y, z – degree of polymerization.

AP-type demulsifiers are more effective than SP-type demulsifiers for demulsifying paraffinic crude oil emulsions. They are more suitable for demulsifying crude oils with a water content higher than 20% and can achieve rapid demulsification at low temperatures. For example, if SP-type demulsifiers settle and demulsify within 2 hours at 55-60℃, AP-type demulsifiers only need to settle and demulsify within 1.5 hours at 45-50℃. This is due to the structural characteristics of AP-type demulsifier molecules. The initiator, polyethylene polyamine, determines the molecular structure: long molecular chains with many branches, resulting in higher hydrophilicity than the simpler SP-type demulsifier. The multi-branched characteristic gives AP-type demulsifiers higher wetting and penetrating properties. When demulsifying crude oil emulsions, AP-type demulsifier molecules can quickly penetrate the oil-water interface film, occupying a larger surface area than the upright monolayer arrangement of SP-type demulsifier molecules. Therefore, a smaller dosage is required, resulting in a more significant demulsification effect. Currently, this type of demulsifier is the best nonionic demulsifier used in Daqing Oilfield.
3. AE type demulsifier
AE type demulsifier is a polyoxyethylene polyoxypropylene polyether with polyethylene polyamine as the initiator. It is a multi-branched nonionic surfactant. Compared with AP type demulsifier, the difference is that AE type demulsifier is a two-stage polymer with small molecules and short branches. The molecular structure is: D(PO)x(EO)yH, where: EO-polyoxyethylene; PO-polyoxypropylene; D-polyethylene polyamine; x, y-degree of polymerization. Although there are great differences in the molecular appearance of AE type demulsifier and AP type demulsifier, the molecular composition is the same. The only difference is in the amount of monomer and the polymerization sequence.
(1) When designing and synthesizing the two nonionic demulsifiers, the amount of materials used at the head and tail are different, and the length of the polymer molecules produced is also different.
(2) AP-type demulsifiers have a two-stage molecular structure, using polyethylene polyamine as an initiator to polymerize with polyethylene oxide and polypropylene oxide to form block copolymers; AE-type demulsifiers also have a two-stage molecular structure, using polyethylene polyamine as an initiator to polymerize with polyethylene oxide and polypropylene oxide to form two-stage copolymers. Therefore, the designed molecules of AP-type demulsifiers should be longer than those of AE-type demulsifiers.
AE-type demulsifiers are two-stage, multi-branched crude oil demulsifiers, and are also suitable for demulsifying asphalt-based crude oil emulsions. The higher the content of lipophilic surfactants in asphalt-based crude oil, the stronger the viscosity, the smaller the oil-water density difference, and the less prone it is to demulsification. AE-type demulsifiers, on the other hand, have a faster demulsification speed and are also better anti-waxing and viscosity-reducing agents. Due to its multi-branched molecular structure, it easily forms tiny networks, causing the paraffin single crystals already formed in the crude oil to fall into these networks. This hinders the free movement of the paraffin single crystals, preventing them from connecting with each other and forming a paraffin network structure. This reduces the viscosity and freezing point of the crude oil, preventing wax crystal aggregation and thus achieving the purpose of preventing wax formation.
4. AR-type demulsifier
AR-type demulsifier is a new type of oil-soluble nonionic demulsifier synthesized by polymerizing alkylphenol resin (AR resin) with polyoxyethylene and polyoxypropylene. Its HLB value is around 4-8, and its demulsification temperature is as low as 35-45℃. The molecular structure is: AR(PO)x(EO)yH, where: EO-polyoxyethylene; PO-polyoxypropylene; AR-resin; x, y, z-degree of polymerization. In the process of synthesizing demulsifier, AR resin acts as both an initiator and enters the demulsifier molecule as a lipophilic group. The characteristics of AR-type demulsifiers are: small molecular size, good dissolution, diffusion, and penetration effects when the freezing point of crude oil is above 5℃, promoting the flocculation and aggregation of emulsion droplets, and the ability to remove more than 80% of the water from crude oil with a water content of 50%~70% within 45 minutes at temperatures below 45℃, which is unmatched by SP-type and AP-type demulsifiers.
III. General Principles for Selecting Demulsifiers
Demulsifiers eliminate factors that stabilize the original emulsion, leading to the aggregation, aggregation, stratification, and demulsification of the emulsion. The main reason for emulsion stability is the formation of an interfacial film by the emulsifier, which has a certain mechanical strength or steric hindrance. Therefore, the main function of demulsifiers is to eliminate the effective effects of emulsifiers, and the selection of demulsifiers should be based on the characteristics of the emulsifiers.
The basic principles for selecting demulsifiers are as follows:
1. Good surface activity, capable of displacing the emulsifier in the emulsion from the interface. Emulsifiers all possess surface activity; otherwise, they could not form an adsorption film at the interface. This adsorption is a spontaneous process. Therefore, demulsifiers must also have strong interfacial adsorption capabilities to replace emulsifiers.

2. The interfacial film formed by demulsifiers at the oil-water interface cannot be robust and is easily broken under external conditions or during droplet collisions, leading to easy droplet aggregation.
3. Ionic emulsifiers can charge and stabilize droplets; using ionic demulsifiers with opposite charges can neutralize the surface charge of droplets.
4. Nonionic or polymeric demulsifiers with large relative molecular masses dissolved in the continuous phase can cause droplet aggregation due to bridging effects, leading to aggregation, stratification, and demulsification.
5. For emulsions stabilized by solid powder emulsifiers, good wetting agents of the solid powder can be selected as demulsifiers to ensure complete wetting of the powder into the aqueous or oil phase.
From these principles, it can be seen that there is often no clear boundary between some emulsifiers and demulsifiers, and the choice depends on the specific system. Of course, some surfactants are only suitable as demulsifiers for a certain type of emulsion and cannot be used as either demulsifiers or emulsifiers for other systems.