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Petroleum Wastewater - Desalter
Guest article by Greg Johnson, New Logic Research
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New Logic Research has developed an
innovative and rugged ultrafiltration membrane system for the
treatment of desalter effluent. In the past, desalter effluent has
been something that a refinery has just put up with and tried to
manage as best it can. Now by utilizing the proprietary VSEP
Filtration System, desalter effluent can be treated and mitigated
at the source.
Crude Oil Basics
oils are complex mixtures containing many different hydrocarbon
compounds that vary in appearance and composition from one oil
field to another. Crude oils range in consistency from water to
tar-like solids, and in color from clear to black. An
"average" crude oil contains about 84% carbon, 14%
hydrogen, 1%-3% sulfur, and less than 1% each of nitrogen, oxygen,
metals, and salts. Refinery crude base stocks usually consist of
mixtures of two or more different crude oils.
Crude oils are also defined in
terms of API (American Petroleum Institute) gravity. A crude with
a high API gravity are usually rich in paraffins and tend to yield
greater proportions of gasoline and light petroleum products.
Crude oils that contain appreciable quantities of hydrogen sulfide
or other reactive sulfur compounds are called "sour."
Those with less sulfur are called "sweet."
All crude oils are assayed and
valued depending on their potential yield. Crude Oil with low
assay numbers is referred to as "Opportunity Crude".
This type of oil will be more difficult to process due to higher
levels of contaminants and water. This type of crude will
typically give desalter equipment the most trouble and require the
greatest skill of the operator.
Desalting - The First Step in
Crude Oil Refining
Desalting and dewatering of crude
oil upstream of the crude distillation unit is a key process
operation for the removal of undesirable components from crude oil
before it reaches any of the major unit operations. The operation
of a desalting system can be very challenging due to changing
process variables. At best, it is a process of measuring
trade-offs and compromises.
A delicate balance must be
maintained while controlling mixing intensity, wash water quality,
chemical demulsifier feed and other parameters that can provide
optimal salt removal. On one hand the quality of the crude
overflow must be within specific standards and on the other hand
the under-carry must not be so potent that it compromises the
system's dehydration abilities or fouls up downstream wastewater
treatment. New legislative demands placed on effluent water
quality present the operator with a difficult challenge.
Optimizing the desalting process under constantly varying
conditions is a key ingredient to success of the entire refinery
The main function of the Desalter
is to remove salt and water form the crude oil. However, many
other contaminants such as clay, silt, rust, and other debris also
need to be removed. These can cause corrosion and fouling of
downstream equipment when deposited on heat transfer surfaces.
Also, there are metals that can deactivate catalysts used in the
process of refining.
The Desalter removes contaminants
from crude oil by first emulsifying the crude oil with chemicals
and wash water to promote thorough contact of the water and oil.
The salts containing some of the metals that can poison catalysts
are dissolved in the water phase. After the oil has been washed
and mixed as an emulsion of oil and water, demulsifying chemicals
are then added and electrostatic fields are used to break the
emulsion. Desalters are sized to allow the water and oil to settle
and separate according to Stoke's Law.
In addition, solids present in the
crude will accumulate in the bottom of the desalter vessel. The
desalter must be periodically washed to remove the accumulated
solids. A "mud washing" system is installed in the
bottom of the vessel to periodically remove the solids. Mud
washing consists of recycling a portion of the desalter effluent
water to agitate the accumulated solids so that they are washed
out into the effluent water. These solids are then routed to the
Desalter effluent or "undercarry"
is a combination or many things including the mud wash done at
periodic intervals, produced water that came with the crude, and
the brine wash water resulting from the dilution and removal of
salts and other contaminants.
To accomplish desalting, the crude
is first preheated to 120º-150º with heat exchangers in order to
reach the required viscosity level normally in the range of 5-15
centi-stoke. The temperature is limited by the vapor pressure of
the crude-oil feedstock. About 2-6% wash water is metered in ahead
of the desalter as an extraction agent to help dissolve salts and
sediments. Intense mixing then takes place over a mixing valve.
Once in the pressurized desalter
vessel, the salts and sediment settle with wash water and tend to
form emulsions. The wash water is separated by electrostatic
precipitation using de-emulsifiers and acid. The salts that are
removed are mainly chlorides and carbonates. They can cause
corrosion and fouling downstream in the heat exchangers, furnaces,
and distillation units, if not removed.
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. Other less-common processes involve filtering
heated crude using diatomaceous earth and chemical treatment and
settling. Ammonia is often used to reduce corrosion. Caustic or
acid may be added to adjust the pH of the water wash.
Wastewater and contaminants are
discharged from the bottom of the settling tank to the wastewater
treatment facility. The desalted crude is continuously drawn from
the top of the settling tank and sent to the crude distillation
tower. A properly performing desalter can remove about 90% of the
salt in raw crude.
The desalter effluent is a major
source of contaminated wastewater and a source of hydrocarbons as
oil under carry to the extent that emulsions are not completely
broken. Oil under carry can be the single largest source of oil
losses to the wastewater treatment system. Reduction in the amount
of Oil in the undercarry not only reduces sewer loadings but also
recovers valuable raw material that would otherwise be lost.
Rates vary with the water content
of the crude oil and the degree of difficulty in desalting the
crude, but a representative rate would be around 2-2.5 gallons of
wastewater per barrel of crude oil feed to the unit. Desalter
water contains salt, sludge, rust, clay, and varying amounts of
emulsified oil (oil under carry). Depending on the crude oil
source, it may or may not contain significant levels of hydrogen
sulfide (H2S), ammonia, and phenolic compounds. Relatively high
levels of suspended and dissolved solids are usually observed.
VSEP Membrane Filtration -
Enabling Technology for Desalter Effluent
The most fundamental process in a
refinery operation that of separation. Since the desalter is the
first step in refining of crude oil, a bottleneck at the desalter
is a bottleneck for a billion-dollar refinery asset. In addition,
the desalter will be the predominant source of wastewater in a
refinery and an upset in the desalter can cause upsets in the
wastewater treatment plant and put that system at risk of not
meeting discharge requirements. The current methods of desalter
operation are focused on coping and adjusting with the variations
in process variables that are inevitable. The treatment method
involves a crude gravity separator combined with chemical
manipulation and operator expertise. This process is antiquated
and less than desirable from a redundancy and processing point of
The VSEP polymeric membrane
filtration process offers an innovative, precise, and utterly
redundant separation technique for crude oil refining and
processing. In addition to enabling a range of very fine selective
separations, use of membrane filtration results in reduced
capital, chemical, operating, and energy-consumption costs. The
potential for economic benefit to the user industry is
VSEP Desalter Treatment Process
There are a number of possible
scenarios for filtration. During the evacuation of the desalter
undercarry, the settled solids will be washed out of the system to
prevent build up. When this happens some of the rag layer emulsion
comes with it. This blow down is what can give conventional
chemical wastewater treatment plants trouble. The VSEP can be set
up to handle the entire flow of the desalter effluent including
both the briny salt water as well as the modulating rag layer
blowdown. Or, by using automated valves, the VSEP can be
configured to handle only the rag layer blow down.
If the VSEP will handle the entire
desalter effluent flow, the desalter is set to a continuous mud
wash process to even out the solids loading in the feed to the
VSEP. Then the VSEP will dewater this effluent and send a
concentrated sludge to the Coker. If the VSEP will only handle the
rag layer blowdown, the mud wash operator controls destination
valves accordingly. While the desalter is just draining salt-water
effluent, the waste will be sent to the existing treatment plant.
Then just prior to a mud wash, destination valves will configure
to feed the desalter effluent to the VSEP. After the mud wash, the
valves revert back to the original position feeding the wastewater
Case Study Process Conditions
The oily wastewater is fed to the
VSEP treatment system at a rate of 240 gpm. The VSEP system will
monitor tank level and will adjust based upon demand. The VSEP
feed tank acts as a primary settling tank. It is cone bottomed and
heavy solids settle quickly where they are purged. The feed to the
VSEP is taken out of the side of the tank to eliminate excessive
maintenance cleaning of a protective 60 mesh pre-screen to the
VSEP. As the system initiates, a feed pump will spin up to a
pressure of 70 psig. Then a modulating valve will throttle the
flow of the reject to a rate that will produce the desired
concentration of suspended solids.
Three industrial scale VSEP units,
using ultra-filtration membranes are used to treat the 240 gpm
process effluent. The concentrated reject stream is at a
concentration of about 13.3% TSS and is sent to the Coker for
recovery of the oil and hydrocarbons. VSEP generates a permeate
stream of about 192 gpm which is sent on to the existing
wastewater treatment plant. The permeate contains less than ~ 1
mg/L of total suspended solids (TSS), and reduced levels of total
dissolved solids (TDS). Membrane selection is based on material
compatibility, flux rates (capacity) and concentration
requirements. In this example, the TSS reduction is well over 99%
while the oily waste is concentrated from a starting feed of
1.5-2% to a final concentrate of 13.3% by weight.
Benefits of VSEP when used for
The VSEP is installed as a side
stream to the desalter effluent. It is used on a demand basis and
will initiate automatically based upon tank level. It offers many
kinds of redundancy options in operation that are very important
to refinery operations. First, the VSEP system can be bypassed
completely if desired and the desalter effluent will be treated in
the same manner as it is now. Or the VSEP can handle the entire
desalter flow and help to take the load off of the existing
wastewater treatment, thus increasing its capacity and safety
margin when it comes to discharge. VSEP units are modular and easy
to install in parallel for complete redundancy.
Skill of the operator
The rag layer blow down is the
primary cause of process upsets and the manipulation of this
process requires a very skilled operator or group of operators who
cannot afford to make mistakes. The normal salty water discharge
from the desalter is relatively benign and can be handled easily
by existing processes. The fact that the quality of the crude
changes and that the blow downs are done at specified intervals
means that this process is a constant game of tweaking and
cajoling. Installation of the VSEP with the set purpose of
processing the problematic rag layer blowdown removes this process
headache from the effluent operator's daily chores. When VSEP is
installed, the frequency of blowdown, the effectiveness of the
demulsifiers, and the variations in the crude are unimportant and
no longer pose a process upset threat.
Debottlenecking the facility
Since the desalter is the first
step in refining of crude oil, it is critical that this piece of
equipment be consistent and reliable. Because oil is superheated
and pressurized during refining, it is not a process that you can
just turn off with a switch. Shut downs can take hours or days to
complete. In addition, the profitability of the refinery depends
on getting maximum use out of the refinery asset and personnel on
staff. Process upsets cannot be tolerated. The desalter has been
identified as a very significant threat for bottlenecking the
entire refinery operation including the refining of oil and the
wastewater treatment that comes from it. Installation of VSEP can
mitigate or eliminate the possibility of desalter bottleneck. VSEP
is a redundant and effective tool that can be used to reduce
Reduce the load on wastewater
The desalter can be the primary
source of wastewater to the wastewater treatment plant. The
desalter operator must constantly be careful about the discharge
of the desalter so that the treatment plant is not overloaded or
unable to handle the hazardous materials that come from the
desalter. Sometimes this can mean lesser quality desalted crude is
produced as a trade off to not swamping the treatment plant.
Desalter effluent is the primary source of oil to the treatment
Many times the existing wastewater
treatment plant is at capacity and the amount of overdesign and
safety factor is marginal. VSEP is a very effective tool that can
be used to control the load on the treatment plant. Since the VSEP
can take the rag layer blow down out of the wastewater loop and
send it to the Coker, this alone reduces the load significantly.
The VSEP can also be configured to handle the entire desalter
effluent further reducing the load on the treatment plant.
The VSEP can also be configured to
handle the desalter effluent completely and bypass the existing
treatment plant. These options give process engineers ultimate
ability to maximize the use and capacity of the existing
wastewater treatment facility.
For more information contact our
- Mr. Greg Johnson
New Logic Research Inc.
1295 Sixty Seventh Street
Emeryville, CA 94608
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