crude oil pretreatment (desalting) typical configuration ... · typical configuration atmospheric...

2/16/2015 NPTEL :: Chemical Engineering Petroleum Refinery Engineering

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CRUDE OIL PRETREATMENT (DESALTING)

Crude oil contains water, inorganic salts, suspended solids, and watersolubletrace metals.First step in the refining process is to remove salt and solids to reduce corrosion,plugging, and fouling of equipment and to prevent poisoning of the catalysts inprocessing units.contaminants must be removed by desalting (dehydration).If the salt content of the crude is greater than 10 lb/1000 bbl (expressed asNaCl), the crude requires desalting to minimize fouling and corrosion caused bysalt deposition on heat transfer surfaces and acids formed by decomposition ofchloride salts.(<1000 lb/bbl).

Typical Configuration Atmospheric Column w/o Preflash[1,2]

Fig:2.1 Typical Configuration Atmospheric Column w/o Preflash[3]



Fig:2.2 Atmospheric Distillation Unit Overview [3]

Most typical methods of crudeoil desalting:

Chemical and Electrostatic separation:Washing of the salt from crude oil withwater oil and water phases are separated in a settling tank by adding chemicalsto assist in breaking up emulsion or by the application of electrostatic field tocolapse the droplets of saltwater more rapidly.Chemical desalting:Water and chemical surfactant (demulsifiers) are added tothe crude, heated so that salts and other impurities dissolve into the water orattach to the water, and then held in a tank where they settle out.Electric desalting:Under charge condition polar molecules get oriented and getseparated. More than 90% of salt can be removed in just less than half an hour.

Desalting of Crude



Fig:2.3 Desalting of Crude

Electric Dehydration and desalting[13]

Electrical desalting is the application of highvoltage electrostatic charges toconcentrate suspended water globules in the bottom of the settling tank.EitherAC or DC fields may be used and potentials from 12,000 to 35,000 volts are usedto promote coalescence.Surfactants are added if the crude has a large amount of suspended solids.The feedstock crude oil is heated to between 150 and 350oF to reduce viscosityand surface tension for easier mixing and separation of the water.The temperature is limited by the vapor pressure of the crudeoil feedstock.

Electric desalting[1]

The dual field electrostatic process provides efficient water separation attemperatures lower than the other processes and as a result,higher energyefficiencies are obtained.The salts are dissolved in the wash water and the oil and water phases separatedin a settling vessel either by adding chemicals to assist in breaking the emulsionor by developing a highpotential electrical field across the settling vessel tocoalesce the droplets of salty water more rapidly.Desired pH <6,at higher pH emulsion forms.Ammonia is used to reduce corrosion. Caustic or acid may be added to adjust thepH of the water wash.Wastewater and contaminants are discharged from the bottom of the settling



tank to the wastewater treatment facility.The desalted crude is continuously drawn from the top of the settling tanks andsent to the crude distillation (fractionating) tower.

Wash water and T requirement in Desalter:[1,2]

(34vol%, T 240260oF (API >30), to 710 vol%,T 280300o F for API<30)For singlestage desalting units 90 to 95% efficiencies are obtained and twostage processes achieve 99% or better efficiency.Efficiencies of up to 99% water removal in a single stage are claimed for the dualfield process. About 90% of desalters use AC field separation only.Heavy naphthenic crudes form more stable emulsions than other crudes. Highertemperature is required.

Crude Electrostatic desalting

Fig:2.4 Crude Electrostatic desalting[1,2]

crude oil pretreatment (desalting) typical configuration ... · typical configuration atmospheric...

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