CRUDE OIL DESALTING

 

◆ CRUDE OIL DESALTING ◆

1. CRUDE OIL DESALTING

The crude oils delivered to refineries always contain a little water (generally 0.1 to 0.6% volume), mineral salts (20 to 300 grams per ton of crude) and sediments. The mineral salts come from the producing wells or are the result of sea water contamination during transportation. They consist principally of sodium, magnesium and calcium chlorides and they have two main drawbacks:    - like sediments they clog the crude oil preheating heat exchangers    - some of them (calcium and magnesium chlorides) undergo hydrolysis at high temperatures, which produces hydrochloric gas(HCl).       As soon as the latter comes into contact with liquid water it becomes soluble and is converted into hydrochloric acid which is very       corrosive where metal material is concerned. This type of corrosion affects the sections where steam is likely to condense, i.e. the       top of corrosion affects the sections where steam is likely to condense, i.e. the top of the atmospheric columns and the condenser. The desalting operation designed to eliminate the water, mineral salts and sediments contained in crudes is an effective mean of combatting the problems they involve. Desalting is carried out in practically all atmospheric distillation installations. It is integrated in the heat exchanger train in the following 3 stages:

1.1 Crude Oil Washing

Crude oil washing with desalting water to extract the mineral salts, resulting in an emulsion of tiny droplets of salt water in the crude.

1.2 Enlarging the Water Droplets by Electrocoalescence

Enlarging the water droplets by electrocoalescence in an electrostatic field inside a horizontal drum called a desalting drum or desalter

1.3 Water-Crude Separation by Decantation

water-crude separation by decantation in the desalting drum. The desalted crude is less dense and is withdrawn at the top of the drum while the water is withdrawn from the lower part.

2. DESALTING OPERATION

An efficient desalting operation requires:    - a fairly temperature (110 °C to 150 °C), particularly if the crude is rather heavy and viscous, so as to facilitate water-crude decantation.    - an injection (from 3 to 8% volume per crude) of water depending on how difficult the crude is to desalt.    - a fairly long crude residence time (20 to 30 minutes) in the desalter, which explains the latter's considerable size. In a unit processes      1000 m3 of crude per hour the cylindrical desalter will be 50 meters long with a 5 meter diameter.    - the possible use of a demulsifying agent.

Nevertheless, desalting is never totally effective and the inevitable formation of hydrochloric acid has to be combatted downstream in the unit. This entails two main injection operations:    - the injection of soda in the heat exchanger train to convert the HCl into stable sodium chloride.    - the injection of ammonia or another neutralizing agent in the sections where water is likely to condense (top of the atmospheric column      and condenser system)

The additional treatment are shown Figure 4.

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