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- Salts deposit inside the tubes of furnaces and on the tube bundles of heat exchangers creating fouling, thus reducing the heat transfer efficiency;
- Corrosion of overhead equipment;
- The salts carried with the products act as catalyst poisons in catalytic cracking units.
- Water washing: Water is mixed with the incoming crude oil through a mixing valve. The water dissolves salt crystals and the mixing distributes the salts into the water, uniformly producing very tiny droplets. Demulsifying agents are added at this stage to aide in breaking the emulsion by removing the asphaltenes from the surface of the droplets.
Desalter unit washes out salt from the crude oil before it enters the atmospheric distillation unit.
Oil produced in most oil fields is accompanied by water in the form of an emulsion that must be treated. In addition, this water normally contains Dissolved salts, principally chlorides of sodium, calcium, and magnesium. If crude oil is left untreated, when it is processed in a refinery the salt can cause various operating and maintenance problems.
This web-document covers the basic elements of Crude Unit Desalter System design in sufficient detail to allow an engineer, operations or maintenance personnel to understand the design, operation and maintenance criteria for a crude unit Desalter system with the suitable water wash flow, injection water, and settling velocity.
The design of a Desalter unit is influenced by many factors, including process requirements, economics and safety. In this guideline, there are tables that assist in making these factored calculations from the vary reference sources. Include in this guideline is a calculation spreadsheet for the engineering design. All the important parameters use in the guideline are explained in the definition section which help the reader understand the meaning of the parameters or the terms used.
The theory section explains the type of Desalter (single stage, two stage and dual polarity), troubleshooting and emulsion drop theory. The application section of this guideline with the examples will make the user understand a Desalter and its operation.
Crude Oil Desalting
When the crude oil enters the unit, it carries with it some brine in the form of very fine water droplets emulsified in the crude oil. The salt content of the crude measured in pounds per thousand barrels (PTB) can be as high as 2000. Desalting of crude oil is an essential part of the refinery operation. The salt content should be lowered to between 5.7 and 14.3 kg/1000 m3 (2 and 5 PTB). Poor desalting has the following effects:
Types of Salts in Crude Oil
Salts in the crude oil are mostly in the form of dissolved salts in fine water droplets emulsified in the crude oil. This is called a water-in-oil emulsion, where the continuous phase is the oil and the dispersed phase is the water. The water droplets are so small that they cannot settle by gravity. Furthermore, these fine droplets have on their surfaces the big asphaltene molecules with the fine solid particles coming from sediments, sands or corrosion products. The presence of these molecules on the surface of the droplets acts as a shield that prevents the droplets from uniting with each other in what is called coalescence. The salts can also be present in the form of salts crystals suspended in the crude oil. Salt removal requires that these salts be ionized in the water. Hence, wash water is added to the crude to facilitate the desalting process as will be explained later.
Going back to the subject of salt types, these are mostly magnesium, calcium and sodium chlorides with sodium chloride being the abundant type. These chlorides, except for NaCl, hydrolyze at high temperatures to hydrogen chloride.
On the other hand, NaCl does not hydrolyze. Hydrogen chloride dissolves in the overhead system water, producing hydrochloric acid, an extremely corrosive acid.
To remove the salts from the crude oil, the water-in oil emulsion has to be broken, thus producing a continuous water phase that can be readily separated as a simple decanting process. The process is accomplished through the following steps: