Crude Oil Desalting
INTRODUCTION
Oil desalting is the process of removing water-soluble salts
from an oil stream.
Most crude oil is produced with some entrained water, which
normally contains dissolved salts, principally chlorides of
sodium, calcium, and magnesium.
A large part of this salt water is normally removed in the
separation and treating process; however, when the crude is
finally sent to a refinery some entrained water usually remains.
When this crude oil is heated as part of various refining
processes, the entrained water may be driven off as steam.
The salts in the water, however, do not leave with the steam.
They crystallize and may either remain suspended in the oil or
form scale within the heat-exchange equipment.
INTRODUCTION
Entrained salt crystals may deactivate catalyst beds and plug
processing equipment.
A typical salt specification would be 10 pounds per thousand barrels
(10 PTB).
Crude oils are often contaminated with various solid materials such
as silt, sand, iron oxide, sulfur, and carbon.
The process of desalting will also remove portions of these
solids from the oil.
Deposition of salts in exchangers and furnaces results in loss on
stream time and reduced production along with increased cost from
shutdowns and clean-out operations.
Deposits in furnace tubes cause increased pressure drop as well as
hot spots on the tubes.
Desalting Gives these Benefits
Increased crude throughput by:
Longer runs Less erosion by solids in:
Running at maximum capacity Control valves
Less down time for maintenance Exchangers and furnaces
Pumps
Less water charged to the crude unit
Uniform crude charge without slugs of water
during tank switching. Saving or recovery of oil from:
Slops from waste oil recovery system
Less cost for labor due to: Cleanout of storage tank bottoms
Frequent turnarounds Oil in process water (sour vacuum tower
Worn or corroded equipment condensate)
Fouled exchangers Less dumping of oil to sewer for maintenance
Furnace tube hot spots Less slopping of off specification products.
Less plugging, scaling, coking and slaging of:
Improved products because of:
Exchangers
Better operational control
Furnaces
Removal of catalyst poisons
Less corrosion due to sulfur, salts and
Less salt and solids in residual fuel
organic acidity in:
Exchangers
Fractionators
Receivers and lines.
Single-Stage Desalting
Two-Stage Desalting
Oleophilic Impurities
Sulfur Compounds 0.1 - 5 Wt. % as sulfur
Organo-Metallic Compounds 5.400 ppm as the metal
(Nickel, Vanadium, Iron, Arsenic, etc)
Naphthenic Acids 0.03 - 0.4 Vol. %
Nitrogen Compounds 0.05 - 15 Vol. %
Non-Acidic Oxygen Compounds 0 - 2 Wt % as oxygen
(Resin, etc)
These oleophilic materials cannot be effectively removed by electrical
desalting methods and except to recognize them; they will be given no
further consideration.
Oleophobic Impurities
Salts 10- l000 ptb
Water 0.1- 2 Vol. %
Sediment 1 – 500 ptb.
Ptb = pounds per thousand barrels of oil
These oleophobic constituents are insoluble in crude oil and
are present as a second phase, being carried as discrete
particles by the oil.
High Velocity Desalter
The high velocity type contains the
same elements as the low velocity
type except that the wet oil
containing the dispersed phase
enters the electric field through a
distributor that is placed in
between the electrodes.
This arrangement is particularly
suitable for certain types of very
stable emulsions.
The high velocity is necessary in
some cases to prevent the
emulsion particles from forming a
chain between the electrodes
causing a short circuit.
Low velocity desalter
The low velocity type is used, in oil field production where the
incoming wet oil may contain up to 50% of a dispersed phase, which
is usually salt water, in a continuous phase of crude oil.
