Apr-2013

Oil/water separation technologies

Where stable emulsions cannot be removed mechanically, the application of demulsifiers, coagulants and flocculants accelerates the separation process

BERTHOLD OTZISK
Kurita Europe

Article Summary

Emulsions can cause fouling and under-deposit corrosion problems in distillation columns, heat exchangers and reboilers. Commercial methods for breaking emulsions include settling, heating, distillation, centrifuging, electrical treatment, chemical treatment and filtration. These separation technologies can be used in combination to secure optimum results.

Emulsions can be classified as oil-in-water and water-in-oil types. The type of emulsion can be determined by adding a drop of emulsion into a beaker containing water and oil. If the emulsion is of the water-in-oil type, the drop diffuses through the oil but remains in water. The oil-in-water emulsion diffuses through the water, but not through the oil. Both types of emulsions can co-exist in crude oil side by side.

Separation of water-in-oil emulsions
In this type of emulsion, water is the internal dispersed or discontinuous phase, while oil is the external or continuous phase. Separation by the different gravity of the two phases is a very slow process, but can be accelerated by the assistance of chemicals. The chemicals used are termed demulsifiers, emulsion breakers or wetting agents. These additives are surfactants, which migrate to the oil/water interface. They adsorb on the oil films surrounding water droplets and break the oil films. Then, water droplets aggregate to form water drops large enough to gravitationally separate them from the oil. Non-ionic surfactants having both lipophilic and hydrophilic groups are mainly used as demulsifiers. 

Typical applications in refineries
Tank farm treatment
Crude oil, intermediates and finished products are stored in the tank farm. It is the first facility in a refinery where free water can be removed by settling from the oil. Pumped crude oil from the well contains water in emulsified and free states. A crude oil emulsion consists of small globules of water surrounded by oil. Water is the internal phase and oil is the external phase, which can easily be detected by microscope. With the help of gravity, small water droplets coalesce to form bigger droplets. An adequate residence time is essential for separation into two phases. The bigger droplets finally settle down to be removed by drainage. 

Most of the time, emulsified water cannot be separated effectively by gravity settling only, as the emulsion can separate into three phases:
•  Oil on the top
•  Water at the bottom
•  Persistent emulsion in the middle or below the water layer.

To break such a persistent emulsion, chemicals have to be applied. A number of demulsifiers are commercially available with varying degrees of performance and selectivity. Generally, demulsifiers are diluted with an organic solvent and injected into crude oils. The nature of the emulsion changes from crude to crude, which can influence the performance of the emulsion breaker programme. This necessitates the evaluation of cost effectiveness and performance in breaking the emulsion.

Crude oil desalting
Crude oil fed from the tank farm to the crude distillation unit still contains water, salts, sludge and various kinds of impurities. This can cause corrosion, fouling, plugging and catalyst degradation in the downstream refining units. The main purpose of electrostatic desalting is therefore to remove impurities, such as inorganic microparticles, suspended solids and water-soluble contaminants, together with the water.

The major variables and effects on the desalter operation are:
•  Wash water mixing
•  Wash water quality and rate
•  Desalting temperature
•  Electric field
•  Retention time
•  Use of demulsifiers.

Wash water is added in front of the mixing valve to the crude oil to prepare a temporary emulsion. A key point of desalting is an appropriate mixing of crude oil with the wash water to obtain a sufficient desalting rate. Heating lowers the viscosity of crude oil. This promotes demulsification and the formation of large water droplets from the emulsion. An electric field is induced by AC or DC current in the oil/water mixture to improve water coalescence. The electrical field imposes an electrical charge on the small water droplets entrained in the temporary emulsion. The water droplets coalesce into bigger droplets, which can settle by gravity. Therefore, sufficient retention time in a desalter is required for efficient water and oil separation. A suitable demulsifier is commonly used to promote the separation of water and oil. The desalted crude oil is continuously fed from the desalter vessel to the atmospheric crude distillation column. The desalter effluent water is discharged from the desalter vessel to the wastewater treatment facility.

Figure 1 shows the laboratory evaluation of demulsifiers in comparison with an untreated crude oil sample (blank). For the evaluation of a demulsifier, the crude oil was mixed with 4 wt% wash water and agitated with an electric stirrer. This mixture was transferred into several centrifuge glasses. With the exception of the blank sample, 10 ppm of different demulsifiers were added into the centrifuge glasses and thoroughly mixed again. After 24 hours, the water content and salt content was determined to find the best performing demulsifier. In this case, Kurita EB-4110 and Kurita EB-4113 showed the highest desalting and dehydration efficiency for this crude oil. EB-4110 is an oil- soluble demulsifier, which is typically injected into crude oil in front of the desalter mixing valve. EB-4113 is a water-soluble demulsifier, which is typically injected into desalter wash water.

Separation of oil-in-water emulsions
In aqueous systems, the hydrocarbons generally carry a negative charge at their surface. Often, they are steady dispersed into small droplets because of their repellent forces. A cationic charged long-chain polymer neutralises the negatively charged oil droplets. The repellent forces are weakened and oil droplets are brought together. This resolves the emulsion of water and oil. The emulsion-breaking process involves three steps:
•  Agglomeration
•  Creaming
•  Coalescence.