crude oil desalting

   When crude oil is recovered it contains salts, sand, water and small contents of metals such as copper, nickel, and vanadium. Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration).

  The desalting of crude oil is a process that does not have a high profile, but is vital to the operation of the modern petroleum refinery. Desolaters provide more protection to costly refinery equipment than any other single piece of process hardware. The salts that are most frequently present in crude oil are calcium, sodium and magnesium chlorides. If these compounds are not removed from the oil several problems arise in the refining process. The high temperatures that occur downstream in the process could cause water hydrolysis, which in turn allows the formation of corrosive hydrochloric acid. Sand, silts and salt cause deposits and foul heat exchangers. The need to supply heat to vaporize water reduces crude pre-heat capacity. Sodium, arsenic and other metals can poison catalysts. By removing the suspended solids, they are not carried into the burner and eventually flue gas, where they would cause problems with environmental compliance such as flue gas opacity norms 

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Factors affecting crude oil desalting :

The objective of oil desalting is to remove water-soluble salts and the entrained water, which normally contains dissolved salts. Formation water flows with crude in two types: free and emulsified. The free water is not intimately mixed in the crude and is found in larger drops scattered throughout the oil phase. This kind of water is easy to remove by gravity oil-water separators, surge tanks (wet tanks), and desalting vessels. On the other hand, emulsified waters are intimately mixed and found scattered in tiny drops in the oil phase. This kind is hard to remove by simple settling devices, so further treatment such as chemical injection, freshwater dilution, mixing, heating, and electricity are required. The addition of diluent water, heating, and applying electricity can enhance gravity separation.

Factors affecting desalting performance are:

1.  Settling time: Most desalting/ dehydration equipment relies on gravity to separate water droplets from the continuous oil phase. Gravity difference is the active element in this process; the produced formation water droplets are heavier than the volume of oil they displace. The produced water usually carries some salts and solids coated

2.      Chemical/emulsifier injection: Emulsions can be further treated by the addition of chemical destabilizers. These surface-active chemicals adsorb to the water-oil interface, rupture the film surrounding water drops, and displace the emulsifying agents back into the oil.

Molecular attraction. Time and turbulence aid diffusion of demulsifiers. Breaking the film allows water drops to collide by the natural force of through the oil to the interface. Experience has shown that the mechanism of the chemical process is not explicable by any simple theory. Nevertheless, there is a rule of thumb learned in the field that states that the lower the water percentage in an emulsion the more difficult it is to treat

3.     Heating: Heat causes a decrease in viscosity, thickness, and cohesion of the film surrounding water drops. Heat also reduces the continuous phase (oil) viscosity, helping water drops to move freely and faster for coalescing. Controlling the temperature during operations is a very delicate job. Any excessive heat might lead to evaporation, which results not only in a loss of oil volume, but also in a reduction in price because of a decrease in the API gravity.

4.  Dilution with freshwater: Salts in emulsion sometimes come in solid crystalline form. So, the need for freshwater to dissolve these crystal salts arises and dilution with freshwater has become a necessity in desalting/dehydration processes. Freshwater is usually injected before heat exchangers to increase the mixing efficiency and to prevent scaling inside pipes and heating tubes. Freshwater is injected so that water drops in emulsions can be washed out and then drained off, hence the term ‘‘wash water.’’ The quantity=ratio of freshwater injected depends on the API gravity of the crude, but, generally, the injection rate is 3_10% of the total crude flow,

5.      Mixing: High shear actions form emulsions. Similarly, when dilution

     water (freshwater) is added to an emulsion, one needs to mix them in                           order to dissolve the salt crystalline and to aid in coalescing finely distributed droplets. Mixing works in three steps: (1) helps smaller drops to join together, (2) mixes chemical/ demuslifier with the emulsion, and (3) breaks the free injected volume of wash water into emulsion-sized drops and evenly distributes it.

Importance of crude oil desalting:

The purpose of desalting is to remove contaminants from crude oil before it enters the processing units. By removing the contaminants at the onset it is possible to minimize corrosion and fouling in downstream units. Although there can be other objectives the typical objective in a desalter is to reduce the salt, solids, and metals content while minimizing oil undercarry and water over carry.

Salt Removal

Salt occurs naturally in all crudes but can vary significantly between crudes. A fully sweet synthetic crude that has been hydrotreated can have less than 2ptb (part per thousand barrels) salt while a heavy Canadian crude can easily have 80ptb. The salt comes mainly in the form of CaCl and MgCl though other form of salt can be present in smaller quantities.

If the salt is left in the system it will enter the crude charge furnace it will hydrolyze and will form hydrochloric acid. CaCl will typically hydrolyze first with MgCl requiring higher temperatures. NaCl on the other hand has a very high hydrolyzation temperature not normally reached in a crude charge furnace. For this reason Caustic or NaOH will be injected into the desalter crude stream to lower overhead chlorides.

The basic stoichiometric equations are as follows:

CaCl2 + 2H2O -> 2HCl + Ca(OH)2

CaCl2 + H2O -> 2HCl + CaO

Ca can be substituted for Mg to give the same reactions.

 It is clear from these equations that the amount of salt going into he charge furnace must be controlled to minimize corrosion in the downstream equipment.

Methods of crude oil desalting:

The two most typical methods of crude-oil desalting are chemical and electrostatic separation, and both use hot water as the extraction agent. A third (and rare) process filters hot crude using diatomaceous earth.