DEWATERING AND DESALTING

Before the separation of petroleum into its various constituents can

proceed, there is the need to clean the petroleum. This is often referred to as
desalting and dewatering, in which the goal is to remove water and the
constituents of the brine that accompany the crude oil from the reservoir to
the wellhead during recovery operations.
Petroleum is recovered from the reservoir mixed with a variety of
substances: gases, water, inorganic salts, suspended solids, and water-
soluble trace metals. Thus, refining actually commences with the production
of fluids from the well or reservoir and is followed by pretreatment
operations that are applied to the crude oil, either at the refinery or prior to
transportation. Pipeline operators, for instance, are insistent on the quality
of the fluids put into the pipelines; therefore, any crude oil to be shipped by
pipeline or, for that matter, by any other form of transportation must meet
rigid specifications with regard to water and salt content. In some instances,
sulfur content, nitrogen content, and viscosity may also be specified.
Field separation, which occurs at a field site near the recovery operation, is
the first attempt to remove the gases, water, and dirt that accompany crude
oil coming from the ground. The separator may be no more than a large
vessel that gives a quieting zone for gravity separation into three
phases: gases, crude oil, and water containing entrained dirt.
Desalting is a water-washing operation performed at the production
field and at the refinery site for additional crude oil cleanup. The desalting
process also removes much of the water-soluble minerals, and suspended
solids that cause equipment corrosion and catalyst deactivation.

If these crude oil contaminants are not removed, they can cause
operating problems during refinery processing, such as equipment plugging
and corrosion as well as catalyst deactivation.
The usual practice is to blend crude oils of similar characteristics, although
fluctuations in the properties of the individual crude oils may cause
significant variations in the properties of the blend over a period of time.
Blending several crude oils prior to refining can eliminate the frequent need
to change the processing conditions that may be required to process each of
the crude oils individually.
However, simplification of the refining procedure is not always the end
result. Incompatibility of different crude oils, which can occur if, for
example , a paraffinic crude oil is blended with heavy asphaltic oil, can

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cause sediment formation in the unrefined feedstock or in the products,
thereby complicating the refinery process.

Water is present in the untreated oil and may vary from 1% to over 90%.
Purchasers, depending on local conditions, accept a range of 0.2 to 3% of
water in oil. When water forms a stable emulsion with crude oil and cannot
be removed in conventional storage tanks, emulsion-treating methods must
be used.
Operating Conditions:
Desalting should include:
- Heating to about 110-140C (depending on CO density)
- Applying pressure (10-11Kg/cm2 – to avoid vaporization of light
components or water)
- Addition of soda
- Addition of demulsifying agents
- application of high-voltage electrostatic charges
- Mixing with freshwater

An emulsion
It is a heterogeneous liquid system consisting of two immiscible liquids
with one of the liquids intimately disposed in the form of droplets in the
second liquid (the water remaining is less than 10% of the oil). A common
method for separating water-oil emulsion is to heat the stream. The use of
heat in treating crude oil emulsions has four basic benefits

1. Heat reduces the viscosity of the oil, resulting in a greater force during
collision of the water droplets.

2. Heat increases the droplets' molecular movement.

3. Heat can enhance the action of treating chemicals, causing the chemical to
work faster to break the film surrounding the droplets of the dispersed phase
of the emulsion.

4. Heat may increase the difference in density between the oil and the water,
thus accelerating settling.
Crude oil is pumped from storage to be heated by exchange against
hot overhead and product side streams in the Crude Unit.
The temperature is limited by the vapor pressure of the petroleum
constituents.

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 Desalting involves the mixing of heated crude oil with water
(approximately 3% to 10% of the crude oil volume).
The mixture enters a desalter drum usually containing an electrostatic
precipitator (Electrical desalting).

Demulsifying chemicals such as soaps, fatty acids, sulfonates, and

long-chain alcohols could also be used to enhance this separation process.
When a chemical is used for emulsion breaking during desalting, it may be
added at one or more of three points in the system. First, it may be added to
the crude oil before it is mixed with fresh water. Second, it may be added to
the fresh water before mixing with the crude oil. Third, it may be added to
the mixture of crud e oil and water.
A high-potential field across the settling vessel also aids coalescence
and breaks emulsions, in which case dissolved salt sand impurities are
removed with the water.
. The water phase from the drum is sent to a sour water stripper to be
cleaned before disposal to the oily water sewer.
It must be understood however that this ‘de-salting’ does not remove the
organic chlorides which may be present in the feed.
The crude oil leaves the desalter drum and enters a surge drum. Some of the
light ends and any entrained water are flashed off in this drum and routed
directly to the distillation tower flash zone (they do not pass through to the
heater). The crude distillation booster pump takes suction from this drum
and delivers the desalted crude under flow control to the fired heater via the
remaining heat exchange train. The desalted crude is then sent to the crude
distillation (fractionating ) tower.

Ammonia is often used to reduce corrosion and alkali or acid may be

added to adjust the pH of the water wash.

Mixing with soda

Objective:
- partial removing of sulphur compounds; H2S, RSH, thiophenes.
- Reaction with HCl produced from thermal decomposition of chloride
salts
Reactions
2NaOH + H2S --- Na2S + H2O
NaOH + RSH ---- RSNa + H2O
NaOH + HCl ---- NaCl + H2O
PH of produced water should be within 6-6.5

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 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. Both methods of desalting are continuous. A third and
less-common process involves filtering heated petroleum.

Whenever elevated temperatures are used when desalting sour (sulfur-

containing) petroleum, hydrogen sulfide will be present. And, depending on
the crude feedstock and the treatment chemicals used, the wastewater will
contain varying amounts of chlorides, sulfides, bicarbonates, ammonia,
hydrocarbons, phenol, and suspended solids. Desalting creates an oily
desalter sludge that may be a hazardous waste and a high temperature
wastewater stream that is usually added to other process wastewaters.
In some cases, it is possible to recycle the desalter effluent water back
into the desalting process, depending upon the type of crude being processed

Metallic Constituents

Metallic constituents are found in every crude oil and the

concentrations have to be reduced to convert the oil to transportation fuel.
Metals affect many upgrading processes and cause particular problems
because they poison catalysts used for sulfur and nitrogen removal as well
as other processes such as catalytic cracking.
The trace metals Ni and V are generally orders of magnitude higher than
other metals in petroleum, except when contaminated with coproduced brine

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salts (Na, Mg, Ca, and Cl) or corrosion products gathered in transportation
(Fe).
The occurrence of metallic constituents in crude oil is of considerably
greater interest to the petroleum industry than might be expected from the
very small amounts present. Even minute amounts of iron, copper, and
particularly nickel and vanadium in the charging stocks for catalytic
cracking affect the activity of the catalyst and result in increased gas and
coke formation and reduced yields of gasoline. In high-temperature power
generators, such as oilfired gas turbines, the presence of metallic
constituents, particularly vanadium in the fuel, may lead to ash deposits on
the turbine rotors, thus reducing clearances and disturbing their balance.
More particularly, damage by corrosion may be very severe. The ash
resulting from the combustion of fuels containing sodium and especially
vanadium reacts with refractory furnace linings to lower their fusion
points and so cause their deterioration.
Thus, the ash residue left after burning of a crude oil is due to the presence
of these metallic constituents, part of which occur as inorganic water-
soluble salts (mainly chlorides and sulfates of sodium, potassium,
magnesium, and calcium) in the water phase of crude oil
emulsions. These are removed in the desalting operations, either by
evaporation of the water and subsequent water washing or by breaking the
emulsion, thereby causing the original mineral content of the crude to be
substantially reduced. Other metals are present in the form of oil-soluble
organometallic compounds as complexes, metallic soaps, or in the form of
colloidal suspensions, and the total ash from desalted crude oils is of the
order of 0.1 to 100 mg/L. Metals are generally found only in the nonvolatile
portion of crude oil (Altgelt and Boduszynski, 1994; Reynolds, 1998).
Two groups of elements appear in significant concentrations in the original
crude oil associated with well-defined types of compounds. Zinc, titanium,
calcium, and magnesium appear in the form of organometallic soaps with
surface-active properties adsorbed in the water or oil interfaces and act as
emulsion stabilizers. However, vanadium , copper, nickel , and part of the
iron found in crude oils seem to be in a different class and are present as oil
soluble compounds. These metals are capable of complexing with pyrrole
pigment compounds derived from chlorophyll and hemoglobin and are
almost certain to have been present in plant and animal source materials. It is
easy to observe that the metals in question are present in such form, ending
in the ash content . Evidence for the presence of several other metals in oil-
soluble form has been produced, and thus zinc, titanium, calcium, and

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magnesium compounds have been identified in addition to vanadium,
nickel , iron, and copper.
Examination of the analyses of a number of crude oil samples for iron,
nickel , vanadium , and copper indicates a relatively high vanadium content ,
which usually exceeds that of nickel , although the reverse can also occur.
Distillation concentrates the metallic constituent s in the residues (Reynolds,
1998), although some can appear in the higher boiling distillates , but the
latter may be due in part to entrainment. Nevertheless, there is evidence that
a portion of the metallic constituents may occur in the distillates by
volatilization of the organometallic compounds present in the petroleum. In
fact, as the percentage of overhead obtained by vacuum distillation of a
reduced crude is increased, the amount of metallic constituents in the
overhead oil is also increased. The majority of the vanadium, nickel , iron,
and copper in residual stocks may be precipitated along with the asphaltenes
by hydrocarbon solvents. Thus, removal of the asphaltenes with n-pentane
reduces the vanadium content of the oil by up to 95% with substantial
reductions in the amounts of iron and nickel .