Dehydration and Desalination Plant

Dehydration and desalination plan

As already seen in CRUDE OIL TREATMENT, the separation often is not sufficient to provide oil completely free of water, i.e. dehydrated and desalinated. The equipment normally used to obtain the oil with the desired characteristics are essentially washing tanks (Gun Barrel), heater treaters, and electrostatic watermakers.

Treatment with washing tank

A washing tank (Gun Barrel) is used to increase the separation of water from oil.
The Gun Barrel dehydrates by exploiting the coalescence effect obtained by flowing the oil-containing emulsified water through a layer of fresh water.
In this way, it is also possible to obtain the desalination of crude oils with low salinity. Fig.20 shows the Section of a Gun Barrel, while Fig.21 represents a typical installation diagram on the plant.

Figure 20 - Gun Barrel (washing tank)

 Figure 21 - Typical Installation

Trattamento con heater-treater

When the treatment with the Gun Barrel is insufficient to dehydrate and desalinate the crude oil, will be using a Heater-treater or an electrostatic desalinator.
These equipment are the only ones capable of breaking stable emulsions and reducing the water and salt content to the admissible values. There are many heater-treaters with different sizes, also they can be horizontal and vertical. A typical heater-treater is shown in Fig.22.

The mixture of oil, water emulsion and gas enters the degassing section after being preheated in the heat exchanger. The fluids then descend to the diffuser, and the free water which has undergone coalescence separates and is discharged through the appropriate valve.
The oil containing small quantities of emulsion and water is heated by going back toward the filtering roof. Further coalescence removes more water and finally, the oil goes to the settling section where it remains for some time. The treated oil is finally cooled in the outlet exchanger and then goes to the storage tank.

  Figure 22 - Typical Heater-Treater

Treatment with an Electrostatic Desalter

The Electrostatic Desalter essentially consists of a separator, which inside it generates an electrical current by plates connected to a high-voltage alternating current source (10.000 Volts).

The emulsified crude oil is passed into the desalinator where, under the action of the electric power, the water detaches from the oil and forms larger drops which settle quickly on the bottom of the tank.
To improve the efficiency of the treatment, soft washing water (about 6% of the oil flow rate) and chemical demulsifying agents (5 to 10 ppm) are added to the incoming oil, which helps to break down
the emulsion.

However, when the salinity of the layer water is high (200+250 gr/lt), dehydration alone is not sufficient, therefore it will be necessary to proceed with successive phases to dilute the oil several times with fresh water and subsequently dehydrate several times until upon reaching the established value.
For this purpose, several desalter can be installed in series in the oil field.
Fig.23 shows a schematic of a crude oil desalination plant. Fig.24A represents an electrostatic Desalter without the heater incorporated, while, Figure 24 represents an electrostatic Desalter with Heater. Fig.25 represents a diagram which shows the efficiency of dehydration together with temperature. With electricity treatment, the same efficiency can be reached by working at lower temperatures.

Figure 23 - Crude Oil Desalination Plant

  Figure 24A - Electrostatic Desalter without Heater

 Figure 24 - Electrostatic Desalter with Heater