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Purpose of crude oil desalting

Crude oil introduced to refinery processing contains many undesirable impurities,

such as sand, inorganic salts, drilling mud, polymer, corrosion byproduct, etc. The

salt content in the crude oil varies depending on source of the crude oil. When a

mixture from many crude oil sources is processed in refinery, the salt content canvary greatly.

The purpose of desalting is to remove these undesirable impurities, especially saltsand water, from the crude oil prior to distillation.

The most concerns of the impurities in crude oil:

The Inorganic salts can be decomposed in the crude oil pre-heat exchangers and

heaters. As a result, hydrogen chloride gas is formed which condenses to liquidhydrochloric acid at overhead system of distillation column, that may causes

serious corrosion of equipment.

To avoid corrosion due to salts in the crude oil, corrosion control can be used.

But the byproduct from the corrosion control of oil field equipment consists of

particulate iron sulfide and oxide. Precipitation of these materials can cause

plugging of heat exchanger trains, tower trays, heater tubes, etc. In addition,these materials can cause corrosion to any surface they are precipitated on.

The sand or silt can cause significant damage due to abrasion or erosion topumps, pipelines, etc.

The calcium naphthanate compound in the crude unit residue stream, if notremoved can result in the production of lower grade coke and deactivation of

catalyst of FCC unit

Benefits of Crude Oil Desalting

Increase crude throughput

Less plugging, scaling, coking of heat exchanger and furnace tubes

Less corrosion in exchanger, fractionators, pipelines, etc.

Better corrosion control in CDU overhead

Less erosion by solids in control valves, exchanger, furnace, pumps

Saving of oil from slops from waste oil

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The desalting process is completed in following steps:

Dillution water injection and dispersion

Emusification of diluted water in oil

Distribution of the emulsion in the electrostatic field

Electrostatic coalescence

Water droplet settling

Crude oil passes through the cold preheat train and is then pumped to the Desalters

by crude charge pumps. The recycled water from the desalters is injected in the

crude oil containing sediments and produced salty water. This fluid enters in the

static mixer which is a crude/water disperser, maximizing the interfacial surface

area for optimal contact between both liquids.

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The wash water shall be injected as near as possible emulsifying device to avoid a

first separation with crude oil. Wash water can come from various sources

including relatively high salt sea water, stripping water, etc. The static mixers are

installed upstream the emulsifying devices to improve the contact between the salt

in the crude oil and the wash water injected in the line.

The oil/water mixture is homogenously emulsified in the emulsifying device. The

emulsifying device (as a valve) is used to emulsify the dilution water injected

upstream in the oil. The emulsification is important for contact between the salty

production water contained in the oil and the wash water. Then the emulsion enters

the Desalters where it separates into two phases by electrostatic coalescence.

The electrostatic coalescence is induced by the polarization effect resulting from an

external electric source. Polarization of water droplets pulls them out from oil-

water emulsion phase. Salt being dissolved in these water droplets, is alsoseparated along the way.

The produced water is discharged to the water treatment system (effluent water). It

can also be used as wash water for mud washing process during operation.

A desalting unit can be designed with single stage or two stages. In the refineries,

the two stages desalting system is normally applied, that consists of 2 electrostatic

Coalescers (Desalter).

Crude Preheating Train

It always gives more advantages to have hot streams transferring their heat to theraw crude oil in the heat exchanger networks (HEN) of preheating train to preheat

crude before entering the fractionation column to produce products; naphtha-minus, bulk distillate fraction, and long residue, where it is heated up from 30 to

about 359 C.

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