Pre-treatment of Crude Oil
Lecture 5
IMPURITIES IN CRUDE OIL
The impurities in crude oils can be classified as
1. Oleophobic
2. Oleophilic
Various oleophobic impurities (insoluble in crude oil) include:
3. salts-mainly chlorides and sulphates of sodium, calcium and magnesium,
4. sediments-such as silt, sand, drilling mud, iron oxide, iron sulphide, etc.
5. water-presents as soluble, emulsified and/or finely dispersed water.
The oleophilic impurities (soluble in crude oils) include:
6. sulphur compounds,
7. organometallic compounds containing Ni, V, Fe, As, etc.,
8. naphthenic acids and
9. nitrogen compounds.
Crude Oil desalting
Definition: removal of salts from crude oil
Desalter: The equipment used for the removal of salts, suspended solids and water soluble trace
metals/slits, iron oxides, and sand
Purpose:
• To remove these undesirable impurities, especially salts and water, from the crude oil prior to distillation.
• To reduce consumption of neutralizers
• To remove metals which may poison Reformer catalyst
• To avoid lengthy and expensive plant shut downs
Why need to remove impurities ?
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. The sand or silt can cause significant
damage due to abrasion or erosion to pumps, pipelines, etc.
The calcium naphthanate compound can result in the production of lower grade coke and deactivation
of catalyst of FCC unit.
Corrosion in the atmospheric distillation overhead system caused by HCI, which is liberated due to
hydrolysis/dissociation of chloride salts;
Increased consumption of ammonia to neutralize the HCI;
Erosion of crude oil pumps, pipelines and valves by suspended matter through abrasive action;
Plugging of equipment and fouling of heat-transfer surfaces;
Product degradation, like high ash content in fuel oil;
Trace metals in distillates, which act as catalyst poisons.
Types of Desalting methods
The two most typical methods of crude-oil desalting are chemical and electrostatic
separation, and both use hot water as the extraction agent.
In chemical desalting, water and chemical surfactant (demulsifies) are added to the
crude, which is heated so that salts and other impurities dissolve or attach to the
water, then held in a tank to settle out.
Electrical desalting is the application of high-voltage electrostatic charges to
concentrate suspended water globules in the bottom of the settling tank.
Surfactants are added only when the crude has a large amount of suspended solids.
A third (and rare) process filters hot crude using diatomaceous earth.
The crude oil feedstock is heated to 65-180°C to reduce viscosity and surface
tension for easier mixing and separation of the water. The temperature is limited by
the vapor pressure of the crude-oil feedstock.
In both methods other chemicals may be added. Ammonia is often used to reduce
corrosion. Caustic or acid may be added to adjust the pH of the water wash.
Desalting Process
Chemical
Desalting
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20000 to 24000 volts
95% salt is removed
Electrical desalting
120-130oC
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PROCESS VARIBLES IN DESALTER PROCESS
1. Pressure drop through mixing valve: optimum operation pressure 0.3 kgf/cm2 of the
maximum
2. Chemical addition: not required
3. Water injection rate: 3-8 volume %
4. Conductivity: to maintain good conductivity there must be no voltage drop
5. Temperature: must be as high as 140oC
6. Desalter pressure : It must be high enough to prevent vaporization
7. pH value: 6.5 to 8.5
8. Solids at water-oil interface: Sludge formed reaches upto electrode and cause trouble
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Typical operating conditions
Process variables Haldia refinery Gujarat refinery
Desalting temperature, oC 125-130 126-130
Desalting pressure, kgf/cm2 8.3-8.5 10
Pressure drop across mixing valve, kgf/cm 2 1.35 1.3-1.5
Water injection rate, vol.% of crude 1.3-1.9 4.5-6.0
Salt content, ppm
Inlet salt content, ppm 20-30 -
PTB - 6-20
Outlet salt content, ppm 4-8 -
PTB - 2-4
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• 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.
• The wash water shall be injected as near as possible emulsifying device to avoid a first
separation with crude oil.
• 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 also separated 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.
Benefit of 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
Any Queries?
