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Chapter 4 Oil/Water Separation

4.4 Emulsions

4.4.7 Demulsification

Demulsification is the breaking of a crude oil emulsion into oil and water phases. From a process point of view, the oil producer is interested in three aspects of demulsification: the rate or the speed at which this separation takes place, the amount of water left in the crude oil after separation, and the quality of separated water for disposal. A fast rate of separation,


a low value of residual water in the crude oil, and a low value of oil in the disposal water are obviously desirable. Produced oil generally has to meet company and pipeline specifications.

For example, the oil shipped from wet-crude handling facilities must not contain more than 0.2% BS&W and 10 pounds of salt per thousand barrels of crude oil. This standard depends on company and pipeline specifications. The salt is insoluble in oil and associated with residual water in the treated crude. Low BS&W and salt content is required to reduce corrosion and deposition of salts. The primary concern in refineries is to remove inorganic salts from the crude oil before they cause corrosion or other detrimental effects in refinery equipment. The salts are removed by washing or desalting the crude oil with relatively fresh water.

The interfacial film, which is the most reason for emulsion stability, must be destroyed and the droplets made to coalesce. Therefore, destabilizing or breaking emulsions is linked directly to the removal of this interfacial film. The factors that affect the interfacial film and, consequently, the stability of the emulsions were discussed earlier. The factors that enhance or speed up emulsion breaking are discussed here.

Temperature. Application of heat promotes oil/water separation and accelerates the treating process. An increase in temperature has the following effects.

● Reduces the viscosity of the oil

● Increases the mobility of the water droplets

● Increases the settling rate of water droplets

● Increases droplet collisions and favors coalescence

● Weakens or ruptures the film on water droplets because of water expansion and enhances film drainage and coalescence

● Increases the difference in densities of the fluids that further enhances water-settling time and separation

An economic analysis should be performed that takes into consideration factors such as heating costs, reduced treating time, and residual water in the crude.

Very high shear is detrimental and should be avoided. High shear causes violent mixing of oil and water and leads to smaller droplet sizes. Smaller droplets are relatively more stable than


larger droplets; therefore, measures that increase shearing of the crude oil should be avoided or minimized where possible. However, a certain amount of shear is required for mixing the chemical demulsifier into the bulk of the emulsion.

Solids have a strong tendency to stabilize emulsions, especially if they are present as fines or when they are wetted by both oil and water. Removing the solids or their source is

sometimes all that is required for eliminating or reducing the emulsion problem. Oil-wet solids stabilize water-in-oil emulsions. Water-wet solids can also be made oil-wet with a coating of heavy polar materials and can participate effectively in the stabilization of water-in-oil emulsions.

Because emulsifying agents are necessary in the stabilization of emulsions, controlling them allows for their destabilization and resolution.

Mechanisms Involved in Demulsification, Flocculation or Aggregation

● Water content in the emulsion. The rate of flocculation is higher when the water cut is higher

● Temperature of the emulsion is high. Temperature increases the thermal energy of the droplets and increases their collision probability, thus leading to flocculation

● Viscosity of the oil is low, which reduces the settling time and increases the flocculation rate

● Density difference between oil and water is high, which increases the sedimentation rate

● An electrostatic field is applied. This increases the movement of droplets toward the electrodes, where they aggregate

Coalescence. Coalescence is the second step in demulsification. During coalescence, water droplets fuse or coalesce together to form a larger drop. This is an irreversible process that leads to a decrease in the number of water droplets and eventually to complete


Coalescence is enhanced by the following factors:

High rate of flocculation increases the collision frequency between droplets


The absence of mechanically strong films that stabilize emulsions

High interfacial tension. The system tries to reduce its interfacial free energy by coalescing

High water cut increases the frequency of collisions between droplets Low interfacial viscosity enhances film drainage and drop coalescence

Chemical demulsifiers convert solid films to mobile soap films that are weak and can be ruptured easily, which promotes coalescence

High temperatures reduce the oil and interfacial viscosities and increase the droplet collision frequency

Sedimentation or Creaming. Sedimentation is the process in which water droplets settle down in an emulsion because of their higher density. Its inverse process, creaming, is the rising of oil droplets in the water phase. Sedimentation and creaming are driven by the density difference between oil and water and may not result in the breaking of an emulsion.

Unresolved emulsion droplets accumulate at the oil/water interface in surface equipment and form an emulsion pad or rag layer. A pad in surface equipment causes several problems including the following.

Occupies space in the separation tank and effectively reduces the retention or separation time

Increases the BS&W of the treated oil

Increases the residual oil in the treated water

Increases arcing incidences or equipment upset frequency

Creates a barrier for water droplets and solids migrating down into the bulk water layer

Emulsion pads are caused or exacerbated by ineffective demulsifier (unable to resolve the emulsion); insufficient demulsifier (insufficient quantities to break the emulsion effectively);

other chemicals that nullify the effect of the demulsifier; low temperatures; and the presence of accumulating solids. Because emulsion pads cause several operational problems, their cause should be determined and appropriate actions taken to eliminate them.