desalter desalting

Muhammad Fahad KhanEngineer Operations

Removal of salts from crude is called Desalting

The equipment used for the removal of salts, suspended solids and water soluble trace metals/slits, iron oxides, sand etc.

To reduce fouling and plugging of crude oil fractionators and exchangers due to salt deposition.

To reduce rate of corrosion of crude oil fractionators.

To avoid lengthy and expensive plant shut downs.

To reduce consumption of Neutralizers.

To remove Metals which may poison Reformer Catalyst.

To achieve more then average Plant Life.

To obtain high degree of operating flexibility due to slug or high tank bottom abnormalities.

These salts are mainly chlorides and sulfates of Calcium and Magnesium like NaCl, MgCl2, CaCl2 and MgCl2.

Chlorides hydrolyzed to hydrochloric acid cause severe corrosion.CaCl2 + H2O → Ca(OH)2 +HCl

Salts act as catalyst to Plugging of Exchangers and formation of coke (hot spots) in heater tubes.

Salts in residue contain high ash content causing degradation of product.

Slats are generally measured in PTB (parts per thousands barrels).

Salts concentration in crude depends upon the Source of Crude.

Salts are inorganic in nature and are soluble in water (inorganic) and insoluble in crude (organic).

Natural Desalter

Chemical Desalter

Electro Chemical Desalter

Desalter in which Settling time is given to salts, sediments, water and sludge which under gravity settled down in the bottom of Vessel and then drained.

Example is drainage of sludge and water from tanks and Settling Vessel F-2 at Lummus.

Desalter in which chemical is added to the crude in order to remove salts, sediments, water and sludge.

Chemicals reduce the surface tension for making removal of salts and water easy.

Both Natural and Chemical Desalter are useful for crude having low salts or for Batch Process.

Desalting process by use of chemical along with electric field.

Demulsifier used as chemical while electrodes connected to step up transformer used for electric field generation.

Preheated crude, fresh water along with small amount of chemical injected to Desalter through mixing valve to form Emulsion.

Small water droplets containing salts converted to form big and heavy drops through Electric Field (Emulsion Breaking) and settle in the bottom.

Provide electric filed. A.C Current provided to Transformer. Helps in emulsion breaking.

Cylindrical in shape having caps on both ends with out holes in it.

Holes are random and flow is laminar.

To keep the required water level in the Vessel. LCV on the effluent drain is connected to float

arrangement.

Rectangular in shape having circular holes in it.

The line is equipped with nozzles which enhance the velocity head of water when operated.

Operated in order to remove sludge.

Having circular holes to drain effluent continually. Containing LCV to maintain required water level.

Sample points used to check water level, interface and Crude condition.

Electrodes are connected to Transformer. Electrodes are in form of bed hanging in the vessel. Bed vibrates when field created through them.

Mechanical Safety of Vessel because the Desalter operate at high Pressure.

Used to form emulsion of water salts and crude at the inlet of Desalter.

Exist above the effluent header.

To drain emulsion with out disturbing water level.

Components of wash water are Fresh water and <50% of recirculation water.

Injected to increase the contact b/w water and Salts because salts are soluble in water.

Wash water should not contain any oxygen, under spec carbonates, bicarbonates and sulfates.

pH should be maintained b/w 6 to 8.

Wash Water

pH 5.5 – 7.5Total Hardness <50ppmTotal CO3

-1/ HCO3-2 <50ppm

Ammonia <100ppmChlorides <2000ppmSulfates <200ppmOxygen <1ppm

Before charge pump

Results maximum contact Reduce scale formation of calcium and magnesium

in the pre heat train.Draw Backs

Pumps capacity reduces. Strong emulsion form due to intense shearing

agitation of impeller in pump body. pH of water should be highly controlled.

Down Stream of Charge Pump Before FCV. No Intense shearing due to impeller.

Braw Backs Emulsion formation Scales formation in the Pre heat train

Upstream of Mixing Valve Controlled mixing and emulsion formation. No Scaling in the pre heat train.

Water settling follows Stroke’s LawV = 2r2g(d-do)/9η

Settling Time

T α (1/ size of droplet)

Droplet size should be >50 micron.

Droplet Size increased through mixing valve.

Min. settling time is 20 to 30 minutes.

Decrease in temperature increase the viscosity making settling of salts and water difficult and vice versa.

T α (1/ Viscosity)

High Desalter Temperature cause increase in solubility of water, formation of tight emulsion and carry over of Salts and Water.

Temperate limit is from 90oC to 150oC varying from crude to crude.

These are Chemicals used for breakage of emulsion.

Types of Emulsion Water/Oil Emulsion

(Refinery)Micro size droplets of

water are surrounded by oil.

Oil/Water Emulsion Micro size droplets of

Oil is surrounded by Water.

Water

Crude Oil

Crude Oil

Water

Demulsifier weaken the oil layer around water to coalesce with other droplets.

In some cases changes the polarity and so the droplets attract each other to coalesced.

Excess demulsifying agent effects adversely.

Soap : migrates to interface and weaken interfacial films.

Polar : Migrates to interface and alter polarity, so particles will coalesce.

H2O Soluble : Acts as Soap or Polar but remain in water phase

Ionic : Increase Electric conductivity, which increase tendency to demulsify.

High voltage Electric field is provided to break Emulsion.

Alternating electric field is provided.

Step up Transformer is used to provide high voltage of 10 to 18KV depending upon the salts in crude.

Electric field break the emulsion layer formed across water and so water coalesce to form large droplets which settled under gravity.

Vibration of water droplets occur due to A.C

High voltage electric field will be provided when salts in crude are low and vice versa.

Used for max mixing of wash water, Demulsifier and crude.

Pressure difference across mixing valve is important varying from few to60 psi depending upon:

Nature of Crude ∆P should be increase with increase in salts

in crude and vice versa.Operating Temperature At high operating Temperature ∆P should

be reduced and vice versa.

Flow Velocity: ∆P should be reduced when flow velocity is high

and vice versa.Emulsifying agents presence in crude: ∆P should be reduced and vice versa.Rules of Thumb: If Emulsion is reported from tri-cock, decrease in

∆P will help to normalize the process Reduce ∆P when salts are carry over with crude. Increase/Decrease ∆P by <5psiwhile maintaining

the cuff 6-12 inches thick. If cuff increased more then 12 inches the ∆P must be lowered. If Cuff decreased less then 6 inches, ∆P must be increased.

High pressure keeps system from boiling. Agitation results from boiling carry over lot of

BS&W.

Operating level of water must be maintained below lover electrode.

Conductivity of water cause electric current flow and could shut down the electric system.

If level is allowed to raise it will carry over the interface with crude to the tower causing tower upset and exchangers fouling as well.

Low level cause carry over of crude with effluent water.

Under design Desalter for current crude oil and through put.

Insufficient attention by operating person Improper operating conditions. Improper chemical dosage. Inadequate water wash supply , rate and

quality.

Desalter efficiency can be calculated as:SE = (Si – So ) * 100 / Si

Where:SE salt removal efficiency (%)Si salt content of raw crude oil (ptb)So salt content of desalted oil (ptb).Ptb stands for Pounds per Thousand BarrelsDesalter efficiency should be 90-95 %