Crude oil usually contains water in varying proportions in an emulsion. In most cases the water is salty. It is necessary to demulsify and desalt the crude oil with chemicals at its production site because:

• Buyers have a purchase specification limit on the amount of water in the crude oil
• A higher amount of water in the crude oil always means a lower price

The chemical is injected:

• Injection into the well for higher pressure
• Injection into the gathering system for low to medium pressure
• Injection into the storage tank, slug dosing at low pressure

The most widely used methods are well and gathering system injection, storage tank injection often treats bad pre-injection. Well head injection is recommended because:

• Usually this pump is smaller and less expensive
• The sooner water is removed the lower the corrosion, i. e. less maintenance, lower cost

Considerably reduced costs:

• Demulsification of the crude oil has the side effect of reducing the viscosity meaning easier transportation lower pressure drops  (less kW or hp) and reduced cross section of the pipelines and the appurtenances
• The corrosive contents in the crude oil are reduced meaning less corrosion of the production equipment, a longer service life and lower specs for the pipelines and appurtenances, e.g. regular steel instead of stainless steel or even super duplex

Demulsifiers are compounds that are distributed in the emulsion in very small amounts when injected and defuse to the water/oil interface and act as stabilizer. They have a variety of action mechanisms:

• Reducing or even nullification of the electrostatic charges responsible for the repulsion between the droplets
• Change of the wettability of the solid particles absorbed at the interface
• Change of the viscoelasticity of the interface film

Demulsification progress takes place as follows:

Demulsifier concentrates at the surface of the small water drops. Like a flocculant the demulsifier “collects” small water drops. The droplets form an increasing water concentration. Visible effect is the brightening of the oil layer as the water droplets, defusing light passing through, are removed. Finally the water drops unite and settle out (coalescene).

Why is demulsifier a mixture?

Demulsifiers frequently have a hydrophilic hydrocarbon chain and a hydrophilic group giving them surfactant properties. They are classified into three categories based on the polarity of their hydrophilic groups:

• Amonic (negatively charged)
• Cationic (positively charged)
• Non-ionic (uncharged)

The latter is the most widely used compound. Demulsifiers normally must provide a combination of effects and namely act as:

• Flocculant
• Coalescer
• Wetting agent

These properties combined cannot be offered with a single substance and so commercial demulsifiers are blends of two or more types. Because of various pre-treatment stages single substance demulsifiers can be found in refinery desalting (not in production desalting).