Crude Oil desalter: Purpose, Working, Types, Parts, variables

 

Crude oil produced in the fields heavily consists of water in the form of an emulsion that requires to be treated. Extracted crude oil normally contains salts, Chlorides of sodium, calcium, and magnesium.

If crude oil is left untreated before processing, salt present in crude oil can cause many types of operating failure and maintenance problems. Crude oil Desalter is an important part of the refinery that helps to clean crude oil in its initial processing stage.

Before processing crude oil in heater and stating its separation process in Crude or Vacuum column every oil refinery needs to process crude oil through desalter. In this article “Crude oil Desalter” you will learn:

  1. What is desalter and why it is important.
  2. Working principle of Desalter and Its transformer.
  3. Process scheme of desalter
  4. Type of desalter and its component
  5. Frequently used desalter related terms and
  6. Variable that affect the efficiency of desalter in crude processing.

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What is Crude oil Desalter?

Desalting is a water-cleaning process performed at the initial stage in crude oil refineries. The equipment used to clean crude oil is called desalter.

In other words, Desalter is processing equipment in the refinery that removes salt and other impurities from crude oil. Desalter in a refinery is one of the major components. It is typically the first unit towards cleaning and processing crude oil and its distillation.

This equipment of refinery is of shape like a vessel on which transformers are installed for the purpose of the emulsifying mixture of crude oil and emulsifying agent mixed to it for crude oil cleaning. Desalters generally mounted on the civil or structural base.

Why Desalter is important

The main purpose of desalter is to remove undesirable impurities from crude oil before processing it for distillation. The most concerning points for removing impurities from crude oil are as follows:

  • Inorganic impurities like chlorides of sodium, calcium, and magnesium can be decomposed in the Heating unit and Hydrogen Chloride will be formed which can get condensate to Hydrochloric Acid.
  • This formation of hydrochloric acid may cause corrosion to other equipment and piping systems resulting in less life span than expected.
  • By-products of Salts present in crude oil can cause plugging of the heat exchanger, column trays, coils of heat, and also they can cause corrosion to any surface they contact.
  • Sand or Mud present in crude oil can damage Pumps and piping systems and will require more maintenance.

Advantage of Desalter in oil production

The basic benefits of desalter for crude oil are as follows:

  • Cleaning of crude increases flowability in pipes, Pumps, Heat exchangers, etc.
  • Less plugging and choking of equipment and piping system.
  • No formation of scale in the Heat exchanger and heating unit
  • Less corrosion in the Heat exchangers, distillation towers, Piping systems, etc.
  • Less maintenance for pumps because of removal of mud, sand, and other impurities.
  • Saving of oil from waste crude.

How Desalter Works

Crude oil received after transportation includes many types of salt traps in small droplets of water staggered all locations of the volume. These droplets are so small that self-extraction and settling are not possible. In general, there are 03 types of oleophobic Impurities found in crude oil:

  • Salt: Mainly chlorides and Sulphates of Sodium (Na), Potassium (K), Calcium (Ca), and Magnesium (Mg).
  • Sediments: Such as silt, sands, Mud, Iron oxide, and Sulphides.
  • Water: Present as soluble, emulsified, and finely dispersed form.

Apart from above mention impurities crude oil also have some Oleophilic impurities but Desalter basics work to clear those 03 types of impurities for crude oil cleaning. The basic principle to wash the salt from crude oil is called Desalting. This Desalting process is done in two steps:

  1. 1st Step is to dilute the dispersed water-soluble salt content with fresh water
  2. 2nd Step is to remove diluted dispersed brine from crude oil.

For the First Stage of desalting, The crude oil is heated up to 120ºC to 140ºC then freshwater is added in crude oil in the amount of 4%-8% (Vo/Vo) with demulsifying chemicals or Effluent.

Desalter effluent composition is consists of many things such as:

  • brine washing water used for removing salt, sand and
  • Mud washing water jet used at periodic intervals, connate water produced from the reservoir with crude oil.

After adding Wash water to Crude oil a proper mixing is done that collects all small droplets of staggered brine into bigger drops.

In the Second Stage, Water or brine collected is drained out effluent system, and Cleaned Desalted oil is processed to preheat train for further processing.

Method of Desalting of Crude Oil

This Complete process of desalting can be in 03 ways:

  1. Mechanical: Here heated crude mixed with wash water left in a large vessel for settle down water drops because of gravity. This not so effective way and takes too much time.
  2. Chemical: In this method Surfactant or emulsion breakers are mixed with wash water to expedite the process of settling water. This also works on the gravity settling technique but because of the mixing of Emulsion breakers process is respectively quick but no so effective though.
  3. Electrical Desalting: This works on the method of the electric field where an attractive force is generated because of High voltage field application. As water isbipolar Substance its attractive force development attracts all water molecules to get collected and converted into bigger water drops which further drain out.

Desalter transformer working principle

“When two immiscible liquids in which, One is electrically conductive and another non-conductive pass through a high-intensity electric field, The conductive liquid coalesces into large droplets

This Phase separation fundamental of physics is the basic working principle of transformer is used in Salt and water separation from crude oil. In crude oil-water emulsion water mixed with salt content is an electrically conductive liquid and oil is non-conductive.

When 15000-16500 volts of current passes through oil-water emulsion water droplets begin to accumulate and separate from the oil-water interface.

Process Scheme of Desalter

Sketchmatic diagram of desalting

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