Dehydrator/ Desalter

The purpose of a bath heater is to increase oil temperature in order to improve the performances of the following desalting units. Crude oil flowing inside a tube exchanger immersed in the water bath is heated by hot water whose temperature is controlled and maintained by the fuel gas combustion; heat is therefore transferred from the water bath to the coils and then to the process fluid. Both oil and wash water can be heated. Bath Heater is provided with a burner system using natural gas to heat the water bath by means of a firetube. Combustion air can be supplied by natural or by forced draft by blower. Flue gas exiting the chamber are exhausted to the outside air through a vertical stack. The Bath Heaters are atmospheric vessels and each of them is provided with an expansion tank mounted on the top in order to monitor and control water bath volume changes due to thermal expansion. The expansion tank is connected to the atmosphere through an open vent.
Available fuel gas from header is fed to pilot and main burners, normally, after filtration in Fuel Gas Scrubber, flow measurement and pressure reduction.

The task of the Dehydrator is the removal of insoluble and emulsified water from oil.  Both the dehydrator/ desalter are constituted of horizontal vessels in which a series of electrodes are installed inside. Electrostatic field is generated between the electrodes by means of power units, at a voltage function mainly of the types of oil to be treated. The emulsion is introduced under the electrodes by means of distributors.
Crude oil and low saline recycle water from Desalter (see later) are mixed trough a mixing device before entering the Dehydrator. The Dehydrator operates completely full of liquid and pressure and should be normally maintained above certain value to prevent vapour formation inside the unit. The high voltage electrical field induces water droplets coalescence so that they can be separated by gravity to the bottom of the vessel.

The dehydrated oil continues to flow up through the vessel and is collected in a header at the top of the vessel. Demulsifying agent is normally injected upstream mixing point to promote oil/water separation. To prevent solids build-up, the Dehydrator can be fitted with mud wash system to lift the solids off the bottom.

Desalter is identical to Dehydrator, but the task of the Desalter is the removal of water soluble salts (in water). Before entering the Desalter, the partially treated oil is mixed with wash water (dilution water) through the mixing device to dilute the residual saline formation water contained in the crude oil. Inside the Desalter, water drops coalescence takes place in the high voltage electrical field generated from the electrodes installed inside the vessel (identical to what installed in the Dehydrator), then water is removed and recycled upstream the Dehydrator via Recycle Water Pump.
The treated oil leaves the desalter via an outlet header from the top of the vessel.